Adrien Soloy

Delphine Flahaut

Joachim Allouche

Dominique Foix

Germain Vallverdu

Emmanuelle Suard

Erwan Dumont

Lucille Gal

François Weill

Laurence Croguennec

Germain Salvato Vallverdu

Effect of particle size on LiNi0.6Mn0.2Co0.2O2 layered oxide performance in Li-Ion batteries

Keywords: Li-ion battery, Layered oxide, NMC, morphology, reactivity, performances

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The rapid development of the electric and hybrid vehicles industry is increasing the demand for Li-ion batteries with high energy densities and low cost. The best candidates at their positive electrodes are layered oxides LiMO 2 (M= Ni, Mn, Co). [START_REF] Noh | Comparison of the Structural and Electrochemical Properties of Layered Li[END_REF] Among all the possible compositions, the layered oxide LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) was shown to be one of the best compromise to combine the beneficial effects of Ni, Mn and Co for achieving good energy density together with structural and thermal stability as well as good cyclability. [START_REF] Noh | Comparison of the Structural and Electrochemical Properties of Layered Li[END_REF] Indeed, NMC materials with higher Ni contents show higher reversible capacities up to 200 mAh/g for LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811), 2,3 but also higher volume expansion [START_REF] Noh | Comparison of the Structural and Electrochemical Properties of Layered Li[END_REF] and higher instabilities in the charged state due to the presence of a large amount of Ni 4+ ions [START_REF] Lee | Structural and Electrochemical Properties of Layered Li[Ni 1-2x Co x Mn x ]O 2 (X=0.1-0.3Positive Electrode Materials for Li-Ion Batteries[END_REF] . Inversely, at lower Ni contents, lower reversible capacities are obtained (for instance, 150 mAh/g for LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC111) [START_REF] Ohzuku | Layered Lithium Insertion Material of LiCo 1/3 Ni 1/3 Mn 1/3 O 2 for Lithium-Ion Batteries[END_REF] ). In the meantime, as the Co and Mn contents increase, the proportion of Ni 2+ increases versus that of Ni 3+ in the pristine material, leading thus to a lower amount of unstable Ni 4+ ions in the charge state of the battery and to higher cyclability and thermal stability. [START_REF] Lee | Structural and Electrochemical Properties of Layered Li[Ni 1-2x Co x Mn x ]O 2 (X=0.1-0.3Positive Electrode Materials for Li-Ion Batteries[END_REF][START_REF] Ohzuku | Layered Lithium Insertion Material of LiCo 1/3 Ni 1/3 Mn 1/3 O 2 for Lithium-Ion Batteries[END_REF][START_REF] Wang | The Reactivity of Delithiated Li(Ni 1/3 Co 1/3 Mn 1/3 )O 2[END_REF] Many efforts have been made in the past twenty years to control the synthesis and properties of these materials. [START_REF] Liang | Co-Precipitation Synthesis of Ni 0.6 Co 0.2 Mn 0.2 (OH) 2 Precursor and Characterization of LiNi 0[END_REF][START_REF] Zybert | Different Strategies of Introduction of Lithium Ions into Nickel-manganese-cobalt Carbonate Resulting in LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) Cathode Material for Li-Ion Batteries[END_REF][START_REF] Ma | Effect of Metal Ion Concentration in Precursor Solution on Structure and Electrochemical Performance of LiNi 0[END_REF][START_REF] Li | Ni-Rich Layered Oxide with Preferred Orientation (110) Plane as a Stable Cathode Material for High-Energy Lithium-Ion Batteries[END_REF][START_REF] Duda | On the Sensitivity of the Ni-Rich Layered Cathode Materials for Li-Ion Batteries to the Different Calcination Conditions[END_REF][START_REF] Wang | Single-Crystal LiNi 0.6 Co 0.2 Mn 0.2 O 2 as High Performance Cathode Materials for Li-Ion Batteries[END_REF][START_REF] Xia | Morphological and Electrochemical Investigation of LiNi 0[END_REF][START_REF] Li | Synthesis of Single Crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 with Enhanced Electrochemical Performance for Lithium Ion Batteries[END_REF][START_REF] Delmas | An Overview of the Li(Ni,M)O 2 Systems: Syntheses, Structures and Properties[END_REF][START_REF] Delmas | Ni, M)O 2 Systems as the Cathode Material in Lithium-Ion Batteries[END_REF][START_REF] Croguennec | Recent Achievements on Inorganic Electrode Materials for Lithium-Ion Batteries[END_REF] In order to optimize their electrochemical properties, different studies have been also focused on preparing them with different morphologies (i.e. particle size or shape). [START_REF] Li | Ni-Rich Layered Oxide with Preferred Orientation (110) Plane as a Stable Cathode Material for High-Energy Lithium-Ion Batteries[END_REF][START_REF] Duda | On the Sensitivity of the Ni-Rich Layered Cathode Materials for Li-Ion Batteries to the Different Calcination Conditions[END_REF][START_REF] Wang | Single-Crystal LiNi 0.6 Co 0.2 Mn 0.2 O 2 as High Performance Cathode Materials for Li-Ion Batteries[END_REF][START_REF] Li | Synthesis of Single Crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 with Enhanced Electrochemical Performance for Lithium Ion Batteries[END_REF][START_REF] Ju | Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial to Li-Ion Transport Kinetics with Layered LiNi 0[END_REF][START_REF] Jiang | Synthesis of Ni-Rich Layered-Oxide Nanomaterials with Enhanced Li-Ion Diffusion Pathways as High-Rate Cathodes for Li-Ion Batteries[END_REF][START_REF] Zhu | Single-Crystal Based Studies for Correlating the Properties and High-Voltage Performance of Li[Ni x Mn y Co 1-x-y ]O 2 Cathodes[END_REF] Among many others, these layered oxides can be found in two well-known morphologies: the polycrystalline morphology with primary particles of several tens of nanometres organized in spherical aggregates [START_REF] Liang | Co-Precipitation Synthesis of Ni 0.6 Co 0.2 Mn 0.2 (OH) 2 Precursor and Characterization of LiNi 0[END_REF][START_REF] Li | Synthesis of Single Crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 with Enhanced Electrochemical Performance for Lithium Ion Batteries[END_REF] and the single-crystal morphology with primary particles of several micrometres. [START_REF] Wang | Single-Crystal LiNi 0.6 Co 0.2 Mn 0.2 O 2 as High Performance Cathode Materials for Li-Ion Batteries[END_REF][START_REF] Li | Synthesis of Single Crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 with Enhanced Electrochemical Performance for Lithium Ion Batteries[END_REF] The polycrystalline morphology is the most studied and reported in the literature. [START_REF] Liang | Co-Precipitation Synthesis of Ni 0.6 Co 0.2 Mn 0.2 (OH) 2 Precursor and Characterization of LiNi 0[END_REF][START_REF] Ma | Effect of Metal Ion Concentration in Precursor Solution on Structure and Electrochemical Performance of LiNi 0[END_REF][START_REF] Xia | Morphological and Electrochemical Investigation of LiNi 0[END_REF][START_REF] Li | Synthesis of Single Crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 with Enhanced Electrochemical Performance for Lithium Ion Batteries[END_REF][START_REF] Li | Structural and Electrochemical Study of the Li-Mn-Ni Oxide System within the Layered Single Phase Region[END_REF][START_REF] Li | Comparison of Single Crystal and Polycrystalline LiNi 0[END_REF] However, it was shown that polycrystalline materials undergo anisotropic volume variations during cycling at the particle level, resulting in cracks within the spherical agglomerates that modify the grain boundaries and disconnect the primary particles between them. This leads to continuous formation of Solid Electrolyte Interphase (SEI) due to the formation of fresh surfaces and to loss of cyclability and performance at high rates, especially when cycled in temperature. [START_REF] Liu | Single-Crystalline Particles: An Effective Way to Ameliorate the Intragranular Cracking, Thermal Stability, and Capacity Fading of the LiNi 0[END_REF][START_REF] Heenan | Identifying the Origins of Microstructural Defects Such as Cracking within Ni-Rich NMC811 Cathode Particles for Lithium-Ion Batteries[END_REF] Compared to the polycrystalline morphology, the single-crystal morphology enables to limit the risk of thermal runaway by decreasing the amount of heat released during the NMC thermal decomposition. [START_REF] Liu | Single-Crystalline Particles: An Effective Way to Ameliorate the Intragranular Cracking, Thermal Stability, and Capacity Fading of the LiNi 0[END_REF] It was also shown that single-crystal particles had better performances at high rates at 45°C compared to polycrystalline particles, which performances are mainly affected by their large secondary particles and their larger chargetransfer resistance due to the grain boundaries between the primary particles. [START_REF] Liu | Single-Crystalline Particles: An Effective Way to Ameliorate the Intragranular Cracking, Thermal Stability, and Capacity Fading of the LiNi 0[END_REF] However, despite that single-crystal particles show better cyclability than polycrystalline particles when cycled at 45°C, smaller first discharge capacity and higher irreversible capacity (IRC) are obtained when cycled at 30 and 45°C. [START_REF] Li | Comparison of Single Crystal and Polycrystalline LiNi 0[END_REF][START_REF] Liu | Single-Crystalline Particles: An Effective Way to Ameliorate the Intragranular Cracking, Thermal Stability, and Capacity Fading of the LiNi 0[END_REF][START_REF] Li | Synthesis of Single Crystal LiNi 0[END_REF] To get a better insight into the relationship between the powder morphology, electrochemical performance and reactivity versus the organic liquid electrolyte, we managed to obtain NMC622 samples with precisely controlled stoichiometry and 2D structure and with various primary particle sizes. From a complete characterisation of these samples, it will be shown the optimum particle size depends on the choice of the electrochemical cycling conditions. EXPERIMENTAL SECTION Ni 0.6 Mn 0.2 Co 0.2 (OH) 2 hydroxide was made by coprecipitation using similar synthesis protocol to that described by Pajot et al. [START_REF] Pajot | Synthesis of Li and Mn-Rich Layered Oxides as Concentration-Gradients for Lithium-Ion Batteries[END_REF] O (0.6:0.2:0.2 in molar ratio) with a total transition metal (TM) ion concentration of 2 mol/L. This solution was pumped slowly into a continuously stirred tank reactor (CSTR). At the same time, a 4 mol/L aqueous solution of NaOH (Sigma-Aldr , ≥99%), used as precipitant, and a NH 3 (Sigma-Aldrich, 28-30%) aqueous solution used as chelating agent, were separately fed into the reactor in order to maintain the pH value at 11 to avoid the formation of oxy-hydroxides. The CSTR was maintained at 60°C and under stirring at 300 rpm during the overall reaction and the final suspension was aged for 7 hrs. Afterwards, the mixed transition metal hydroxide is washed several times by centrifugation, with hot water to remove residual sodium and sulfate species until the pH of the solution becomes neutral. The hydroxide is then recovered by filtration and dried in an oven at 80°C overnight. To synthesize Li(Ni 0.6 Mn 0.2 Co 0.2 )O 2 layered oxides, the hydroxide is intimately mixed with lithium carbonate refined by the Rietveld method using FullProf software. [START_REF] Rodriguez-Carvajal | Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction[END_REF] The peaks profiles were described by a Pseudo-Voigt function, and the full width at half maximum by a Caglioti function. For the NPD patterns, due to the geometry of the diffractometer (transmission mode), a correction factor µR of 0.79 was applied to take into account the absorption of the signal by the powder and thus the decrease of diffracted intensities. [START_REF] Rodriguez-Carvajal | Https://Www-Llb.Cea.Fr/Fullweb/Powder.Htm[END_REF] The particles morphology and size were observed by scanning electron microscopy (JEOL 6700F & Hitachi S4500 FEG), after being metallized by gold deposition. Average primary particle sizes were determined by measuring 100 particles using the software ImageJ. Granulometry measurements were performed using a Malvern Mastersizer 2000. The powders were previously dispersed in distilled water and sonicated for 1 min. Each sample was measured 3 times with 10 seconds apart. The results given in that paper are thus an average of those 3 measurements. BET surface areas were determined using a Micromeritics 3FLEX analyser. The powders were previously degassed under vacuum at 90°C for 1 hr and then at 150°C for 10 hrs.

The surface composition of the pristine powders, pristine electrodes and corresponding cycled electrodes was determined by X-ray Photoelectron Spectroscopy (XPS) using an ESCALAB 250

Xi spectrometer and a Kratos Axis Ultra DLD spectrometer with a monochromatized Al-Kα Xay sour e ( ν = 1486.6 eV). I bot ases, a eutral ser u was used to ze surfa e charging. For the pristine powders, the spectral calibration was carried out using the C-C/C-H component (285 eV) originated from the usual surface carbon contamination in C1s core peaks.

For the pristine and cycled electrodes, it was carried out using the CF 2 component (291 eV) originated from the PVDF polymeric binder in C1s core peaks. The peak fitting process was carried out using a non-linear Shirley-type background. The peak positions and areas were optimized by weighted least-squares calculation method using 70% Gaussian and 30%

Lorentzian line shapes. The quantification of surface composition was performed using basis of S of eld's relat ve se s t v ty fa tors orre ted by spe f values depe d o t e apparatus. [START_REF] Scofield | Hartree-Slater Subshell Photoionization Cross-Sections at 1254 and 1487 eV[END_REF] The quantification and spectra fitting were carried out using CASA XPS software.

The samples were electrochemically tested in CR2032-type coin cells consisting of a positive electrode and a lithium foil as negative electrode, separated by 2 Celgard 2400 separators. The positive electrodes were prepared by coating a slurry on an aluminium foil as current collector.

RESULTS AND DISCUSSION

Structural and physico-chemical characterizations

Starting from a mixed transition metal hydroxide precursor synthesized by coprecipitation (see Figure S1 given in supporting information for more information on its composition, structure and morphology), lithiations were performed in different conditions. First, lithiations were performed at different temperatures with a 5 wt.% excess of Li 2 CO 3 under air in order to determine the impact of the thermal treatment temperature. Figure S2 depicts the XRD patterns for the corresponding samples . As expected from previous works, [START_REF] Duda | On the Sensitivity of the Ni-Rich Layered Cathode Materials for Li-Ion Batteries to the Different Calcination Conditions[END_REF][START_REF] Wang | Single-Crystal LiNi 0.6 Co 0.2 Mn 0.2 O 2 as High Performance Cathode Materials for Li-Ion Batteries[END_REF][START_REF] Xia | Morphological and Electrochemical Investigation of LiNi 0[END_REF][START_REF] Li | Synthesis of Single Crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 with Enhanced Electrochemical Performance for Lithium Ion Batteries[END_REF][START_REF] Cao | Synthesis and Electrochemical Characteristics of Layered LiNiCoMnO Cathode Material for Lithium Ion Batteries[END_REF] the temperature has a significant impact on the structure of the NMC-type materials. The optimum temperature range, in which the structure is the closer to an ideal 2D packing, with very few divalent nickel ions in the lithium site, was found to be 810 -930°C. Indeed, if all materials crystallize in an -NaFeO 2type layered structure described in the space group, those obtained in that temperature range show XRD patterns with the more separated (018) and (110) diffraction lines, the higher (003)/(104) intensity ratios (> 1.2) (see Figure S2a) and the smaller lattice parameters (see Figure S2b) giving the higher c/a ratios (> 4.95) (see Figure S2c). [START_REF] Xia | Morphological and Electrochemical Investigation of LiNi 0[END_REF][START_REF] Delmas | An Overview of the Li(Ni,M)O 2 Systems: Syntheses, Structures and Properties[END_REF][START_REF] Delmas | Ni, M)O 2 Systems as the Cathode Material in Lithium-Ion Batteries[END_REF][START_REF] Ban | Extremely Durable High-Rate Capability of a LiNi 0.4 Mn 0.4 Co 0.2 O 2 Cathode Enabled with Single-Walled Carbon Nanotubes[END_REF] These factors allow to perform a first evaluation of how far from the ideal cationic distribution the structure of the synthesized sample is. Additional lithiations were performed with a lower amount of Li 2 CO 3 excess (1 wt.%) and/or under O 2 . Among all these materials, 11 were selected to be studied in more details. Their XRD patterns are given in Figure 1. The 11 samples can be sorted into 3 different series: the materials made with 5 wt.% excess of Li 2 CO 3 a r ("5-a r" sa ples), t ose made with 1 wt.% excess of Li 2 CO 3 a r ("1-a r" sa ples) a d t ose ade w t 5 wt.% ex ess of Li 2 CO 3 under O 2 flow ("5-O 2 " sa ples). T y a ou ts of pur t es were dete ted t e 5-air samples, as shown by the low intensity peaks in the 22-23° region in the inset. Those peaks were tentatively attributed to one Mn-rich and one Co-rich oxides but their identification remains uncertain because only the main peaks of their XRD patterns could be observed. As shown by the chemical analyses gathered in Table S1, all the selected samples show compositions in transition metals close to the expected one, i.e. the stoichiometry Ni:Mn:Co of 6:2:2. A slight overlithiation is observed with Li/TM ratios larger than 1 for all the samples, with traces of carbon estimated by gas phase chromatography analyses to be smaller than 1 wt.%.

This was attributed to the presence of Li 2 CO 3 in the powders. This carbonate can be, either a residue of the Li precursor used in excess during the synthesis, or formed by reaction of the lithiated layered oxide with ambient air after the synthesis. [START_REF] Hartmann | Evidence for Li + /H + Exchange during Ambient Storage of Ni-Rich Cathode Active Materials[END_REF][START_REF] Martinez | High Reactivity of the Nickel-Rich LiNi 1-x-y Mn x Co y O 2 Layered Materials Surface towards H 2 O/CO 2 Atmosphere and LiPF 6 -Based Electrolyte[END_REF][START_REF] Busà | The Effects of Ambient Storage Conditions on the Structural and Electrochemical Properties of NMC-811 Cathodes for Li-Ion Batteries[END_REF] Neutron diffraction measurements have been combined to XRD for t e "5-a r" ser es of sa ples to evaluate the possible presence of extra Ni 2+ ions in the interslab spaces. [START_REF] Li | Synthesis of Single Crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 with Enhanced Electrochemical Performance for Lithium Ion Batteries[END_REF][START_REF] Rougier | Optimization of the Composition of the Li 1z Ni 1+z O 2 Electrode Materials: Structural, Magnetic, and Electrochemical Studies[END_REF][START_REF] Wang | Tuning the Li/Ni Disorder of the NMC811 Cathode by Thermally Driven Competition between Lattice Ordering and Structure Decomposition[END_REF][START_REF] Peres | The Relationship between the Composition of Lithium Nickel Oxide and the Loss of Reversibility during the First Cycle[END_REF][START_REF] Wang | Intrinsic Role of Cationic Substitution in Tuning Li/Ni Mixing in High-Ni Layered Oxides[END_REF][START_REF] Yin | X-Ray/Neutron Diffraction and Electrochemical Studies of Lithium De/Re-Intercalation in Li 1-x Co 1/3 Ni 1/3 Mn 1/3 O 2 (x=0→1)[END_REF] Indeed, NPD enables to localize the different transition metals present in the NMC layered structure due to the Fermi lengths different in magnitude and in sign for Li (-0.19.10 -12 cm), Ni (1,03.10 -12 cm), Mn (-0.37.10 -12 cm) and Co (0.25.10 -12 cm). [39][START_REF] Guilmard | Effects of Manganese Substitution for Nickel on the Structural and Electrochemical Properties of LiNiO 2[END_REF][START_REF] Guilmard | Effects of Aluminum on the Structural and Electrochemical Properties of LiNiO 2[END_REF][START_REF] Croguennec | Structural and Electrochemical Characterization of the LiNi 1-y Ti y O 2 Electrode Materials Obtained by Direct Solid-State Reactions[END_REF] Figure 2 shows, as an example, the Rietveld refinement of the neutron and X-ray diffraction patterns recorded for the sample 810-5-air. The small residue observed between the calculated and the experimental patterns proves that the structure of this compound is well described in the space group. The neutron diffraction patterns refinements confirm the presence of 3.5% extra Ni 2+ ions in the interslab spaces and that the transition metals occupancies in the slab are very close to 6:2:2, confirming that the impurities detected in XRD are in very small amounts and do not affect significantly the composition of our layered phases. Details on the structural parameters of this sample can be found in Table S2.

The lattice parameters and the amount of extra Ni 2+ ions in Li + sites as determined for all samples are gathered in Table 1 and compared as a function of the synthesis conditions. These refinements confirm the structures of all the samples are close to the ideal 2D layered one [START_REF] Xia | Morphological and Electrochemical Investigation of LiNi 0[END_REF][START_REF] Delmas | An Overview of the Li(Ni,M)O 2 Systems: Syntheses, Structures and Properties[END_REF][START_REF] Delmas | Ni, M)O 2 Systems as the Cathode Material in Lithium-Ion Batteries[END_REF][START_REF] Ban | Extremely Durable High-Rate Capability of a LiNi 0.4 Mn 0.4 Co 0.2 O 2 Cathode Enabled with Single-Walled Carbon Nanotubes[END_REF] . Thanks to the amounts of Ni 2+ ions in Li sites determined from XRD patterns refinements, the cationic distribution is proposed for each sample, considering the charge balance and the stabilization of Ni in the divalent state in the interslab spaces and of Co and Mn in the trivalent and tetravalent states respectively in the slabs. [START_REF] Yin | X-Ray/Neutron Diffraction and Electrochemical Studies of Lithium De/Re-Intercalation in Li 1-x Co 1/3 Ni 1/3 Mn 1/3 O 2 (x=0→1)[END_REF] Note that the materials synthesized under an O 2 flow have the most layered structures as less than 1.8% Ni 2+ ions are present in Li + sites. On the contrary, the highest amount is observed for the samples obtained under air at 900°C and 930°C (i.e. 3.8% and 3.7% respectively) with 5% of Li 2 CO 3 . These results are in accordance with previous works, in which higher amounts of Ni 2+ ions in Li + sites were obtained for syntheses carried out under air, and even more at higher synthesis temperatures (T > 900°C). Table 1. Comparison of the lattice parameters, c/a ratios, compositions and cation distributions calculated from the Rietveld refinements of the X-ray diffraction patterns. The Average primary particle sizes were determined by measuring 100 primary particles on SEM pictures with the software ImageJ. The corresponding SEM images are given in Figure 3, Figure 4 and Figure S3. 170 nm ure 3 o pares t e orp olo es of four of t e "5-a r" sa ples, the 810-5-air, 870-5-air, 900-5-air and 930-5-air samples, by SEM. The average primary particle size increases with the lithiation temperature, from 170 nm in diameter at 810°C to 1.2 m at 930°C as detailed in Table 1. To estimate the variation of specific surface area generated by this variation of primary part le s ze, surfa e area easure e ts were arr ed out o t e "5-O 2 " sa ples as t ey are representative of the particle size range observed for those samples, from 200 nm to 2 µm. The results are shown in Table S3, the specific surface area decreases from around 3 m²/g for the 240 nm particles of the 810-5-O 2 sample to around 1 m²/g for the 2.1 µm particles of the 930-5-O 2 sample. Despite being very low and at the limit of accuracy of the technique, this evolution is in good agreement with an expected decrease of the surface area for larger particles. The pore volumes are also indicated and they show that the 810-5-O 2 sample with 240 nm particles is more porous than the 930-5-O 2 sample with 2.1 µm particles. The three samples synthesized at 900°C, as well as the corresponding distribution in sizes, are compared in Figure 4: larger primary particles are obtained under O 2 in comparison to those obtained under air for 5% of Li 2 CO 3 . Similarly, the smaller excess of lithium carbonate used during the synthesis gives larger primary particles in the same atmosphere synthesis. The 900-5-air sample has thus a narrow distribution of smaller primary particle size compared to the two others. The morphology of the other samples is shown in Figure S3. For all the materials, the primary particles are embedded into large, non-spherical and porous agglomerates. To go further in the study of the morphology of the materials, laser granulometry measurements were performed. Figure S4 shows very s lar part les' s ze d str but o s, in volume or in number percentages, for the three samples synthesized at 900°C with or without applying 1 min sonication. The appearance of a group of smaller particles on the sizes distribution in volume percentage (Figure S4b) as well as the shift of the distribution in number percentage towards smaller sizes after sonication (Figure S4d) reflect the deagglomeration of some particles and confirm that the primary particles are e bedded to lar e ro etr a lo erates. As s ow by t e part les' s ze d stribution in number percentages given in Figure S4, the average primary particle sizes are in good agreement with the ones determined by the SEM pictures analysis, confirming also that the smaller primary particles are obtained for the 900-5-air sample. The distribution in size expands from 0.1 to 1.0 m for the primary particles observed by SEM, and from 0.1 to 3.0 m for the primary particles and aggregates observed by laser granulometry.

Samples a (Å) c (Å) c/a Composition and cation distribution

Surface analyses

The surface of these samples was analysed by XPS. The complete quantitative analysis of the 900-5-air material is shown in Table S4 as an example.

The chemical surface analyses of the three samples, displayed in atomic percentages, are summarized in Table 2 and in Table 3. Only the most interesting values obtained from those quantitative analyses are shown hereafter. In Table 2, the atomic percentages of the Li 2 CO 3 contributions in the Li1s and C1s spectra, extracted from global quantification, are given, with the contribution attributed to the oxygen atoms in the NMC network (O NMC ) with respect to the rest of the O1s spectra. Thus, we can confirm that the samples 900-1-air and 900-5-O 2 are those

showing the lowest carbonate concentrations on their surface, and thus the highest concentrations of transition metals and oxygen assigned to the layered material. The results in Table 3 (resp.

Table S5) give the transition metal stoichiometry on extreme surface for the materials synthesized at 900°C (resp. all samples) which is closed to 6:2:2, as reported for ICP measurements, showing that the composition does not differ much from the surface to the bulk.

Moreover, for a given atmosphere, the %.at range value of carbonate species at the surface (see Table S6) is directly related to the amount of Li excess during synthesis. For each series of materials (same Li 2 CO 3 and atmosphere conditions), the trends is that a higher proportion of Li 2 CO 3 is detected as the synthesis temperature is increasing which could be related to the particle size increase. This remaining Li 2 CO 3 is prone to be found at the extreme surface as no impurities associated to carbonate species have been identified by XRD measurements and very small amount (<1 wt.%) of carbon were detected in the bulk by gas phase chromatography.

Note that there is no relationship between the quantity of Li 2 CO 3 measured at the surface and the amount of Ni 2+ ions in the interslab spaces as determined by the Rietveld refinement. Indeed, the "5-O 2 " sa ples ave very s lar a ou ts of N 2+ ions in the interslab spaces (between 1.5 and 1.8%, as shown in Table 1) but different amounts of Li 2 CO 3 at their surface.

Electrochemical analyses

These samples were electrochemically tested in CR2032 type coin cells versus Li in galvanostatic mode. Figure 6 o pares t e results obta ed for t e "5-a r" ater als ( ure 6a), t e "5-O 2 " ater als (Figure 6b) and the samples synthesized at 900°C (Figure 6c). They were cycled at C/20 (1 charge-discharge cycle in 20 hours) in the 2.5-4.3V potential range. The 1 st discharge capacities, irreversible capacity rates (IRCs) and capacity retention after 30 cycles are gathered for comparison in Table 4.

The ater als "5-a r" del ver a 1 st discharge capacity between 149 and 165 mAh/g with an irreversible capacity (IRC) between 11 and 21% (Figure 6a and in Table 4). The samples with the largest primary particles, 400 nm and 1.2 µm, show the highest 1 st discharge capacities of 165 mAh/g and the lowest IRCs of 11-13%. In inset of figure 6a, the corresponding derivative curves dQ/dV are compared. They are very similar for all samples. Only the material with 170 nm primary particles shows a slightly larger polarization. Indeed, the corresponding oxidation and reduction peaks are located at 3.79 V and 3.72 V, while they are at 3.76 V and 3.73 V respectively for the other samples. Figure S5 shows the evolution of the discharge capacity delivered by those samples as a function of the number of cycles. The materials 870-5-air and 900-5-air with primary particles of around 280 and 400 nm respectively deliver higher capacities.

Moreover, the sample 810-5-air with 170 nm primary particles shows a better capacity retention (95% after 30 cycles, see Table 4).

All samples of t e "5-O 2 " ser es have very similar performances as shown in Table 4 and Figure 6b, with 1 st discharge capacities between 159 and 173 mAh/g and IRCs between 7 and 11%. samples respectively (Table 4).

The materials synthesized at 900°C deliver a 1 st discharge capacity ranging between 165 and 176 mAh/g with IRCs between 8 and 11% (Table 4 and Figure 6c). From the evolution of the discharge capacities delivered by those three samples, it can be observed that the 900-1-air sample shows the highest 1 st discharge capacity of 176 mAh/g while the 900-5-O 2 shows the best capacity retention of 92% after 30 cycles (see Table 4 and Figure S7), both showing the smallest amounts of Ni 2+ ions in Li + sites (3.8% for 900-5-air versus 1.5% for 900-5-O 2 and 1.9% for 900-1-air). To investigate further the influence of the primary particle size on the electrochemical perfor a es of t ese NMC622 ater als, four of t e "5-a r" sa ples were y led arder conditions: (i) at higher rate C/5 (vs. C/20 as just discussed) and (ii) in a larger potential window 2.5 -4.6 V (vs. 2.5 -4.3 V previously). The 810-5-air, 840-5-air, 900-5-air and 930-5-air samples were chosen to be compared as they allow to scan the particle size range studied in our reference "5-a r" ser es of ater als. As shown by the comparison between Figure 7a and Figure 7b, a higher cycling rate leads to lower performances whatever the particle size. The largest performance drops are observed for samples with either the largest (930-5-air sample) or the smallest particles (810-5-air sample) (Table 5). As shown in Figure S8, the 400 nm particles (900-5-air) offer the best performances with significantly larger capacities compared to the three other samples and good capacity retention. When the samples are cycled at the same cycling rate (C/5) but in larger potential window with a cut-off voltage of 4.6V (Figure 7c), as expected, larger reversible capacities are obtained whatever the particle size, as a larger amount of Li + ions are deintercalated from the layered structure. As for the two previous cycling conditions, the best performances at 1 st cycle are obtained for the 400 nm particles (900-5-air sample) with a 1 st discharge capacity of 185 mAh/g and an irreversible capacity rate of 16% (Table 5). However, as shown in Figure S9, those particles have a declining capacity retention compared to the previous cycling conditions and the best capacities are obtained for the 200 nm particles (840-5-air sample) after 5 activation cycles. To better understand those differences of performances and investigate the impact of those different cycling conditions on the electrode/electrolyte interface composition, XPS analyses were performed on cycled electrodes.

Electrode/Electrolyte interface analysis

XPS analyses were performed on electrodes recovered after cycling of the 900-5-air sample (400 nm primary particles), the one that showed good electrochemical performances regardless of the cycling conditions. One electrode was analysed after cycling at C/20 in the 2.5-4.3V potential window; one after cycling at C/5 in the same potential window to see the impact of the increase of cycling rate and another one after cycling at C/5 in the larger potential window 2.5-4.6V to estimate the impact of higher states of charge (reaching higher delithiated compositions for the active material). Those electrodes were analysed after 50 cycles at end of discharge. A 900-5-air pristine electrode was also analysed as a matter of comparison.

As cycling was performed at 25°C in an electrolyte consisting of LiPF 6 in DMC and FEC, the analysis is focused on some elements, especially C and F, to study the apparition of the degradation products of the solvents and of lithium salts. Indeed, the degradation of LiPF 6 is expected to generate LiF and Li x PF y O z -type species while the decomposition of the solvents will generate carbonaceous and oxygenated species. [START_REF] Shin | Effects of Fluoroethylene Carbonate (FEC) on Anode and Cathode Interfaces at Elevated Temperatures[END_REF][START_REF] Zhang | A Review on Electrolyte Additives for Lithium-Ion Batteries[END_REF][START_REF] Kim | Understanding the Thermal Instability of Fluoroethylene Carbonate in LiPF 6 -Based Electrolytes for Lithium Ion Batteries[END_REF][START_REF] Tasaki | Solubility of Lithium Salts Formed on the Lithium-Ion Battery Negative Electrode Surface in Organic Solvents[END_REF][START_REF] Sinha | Storage Studies on Li/Graphite Cells and the Impact of So-Called SEI-Forming Electrolyte Additives[END_REF][START_REF] Deng | Improving Cyclic Stability of LiNi 0.6 Co 0.2 Mn 0.2 O 2 -SiO x /Graphite Full Cell Using Tris(Trimethylsilyl)Phosphite and Fluoroethylene Carbonate as Combinative Electrolyte Additive[END_REF][START_REF] Dedryvère | Characterization of Lithium Alkyl Carbonates by X-Ray Photoelectron Spectroscopy: Experimental and Theoretical Study[END_REF] It was shown that FEC could be quite unstable in LiPF 6 -based electrolytes but mainly at high temperature (T > 45°C), so the degradation of FEC is expected to be limited in our case. [START_REF] Shin | Effects of Fluoroethylene Carbonate (FEC) on Anode and Cathode Interfaces at Elevated Temperatures[END_REF][START_REF] Zhang | A Review on Electrolyte Additives for Lithium-Ion Batteries[END_REF][START_REF] Kim | Understanding the Thermal Instability of Fluoroethylene Carbonate in LiPF 6 -Based Electrolytes for Lithium Ion Batteries[END_REF][START_REF] Tasaki | Solubility of Lithium Salts Formed on the Lithium-Ion Battery Negative Electrode Surface in Organic Solvents[END_REF] Regarding the DMC, it was shown that it could degrade at high potential leading to the formation of alkyl carbonates and carbonated species and also react with deintercalated Li + ions to form lithium alkyl carbonates ROCO 2 Li. [START_REF] Dedryvère | Characterization of Lithium Alkyl Carbonates by X-Ray Photoelectron Spectroscopy: Experimental and Theoretical Study[END_REF][START_REF] Moshkovich | The Study of the Anodic Stability of Alkyl Carbonate Solutions by in Situ FTIR Spectroscopy, EQCM, NMR and MS[END_REF] The C1s, F1s, Li1s and O1s XPS spectra obtained are shown in Figure 8.

For the pristine electrode, in addition to the contributions previously described for the bare materials and according with the presence of electrode additives, two components in the C1s spectrum located at 286.5 eV and 291.0 eV are attributed to CH 2 , and CF 2 environments of the PVDF binder. The signature of the carbon black in the electrode is well characterized by a set of black peaks corresponding to the XPS signal of our reference. The F1s spectrum is composed of two contributions, one from the PVDF binder located at 688.1 eV attributed to the CF 2 environments respectively and one contribution at 685.1 eV attributed to LiF. [START_REF] Gauthier | Influence of the Cathode Potential on Electrode Interactions within a Li 4 Ti 5 O 12 vs LiNi 3/5 Mn 1/5 Co 1/5[END_REF][START_REF] Karayaylali | The Role of Diphenyl Carbonate Additive on the Interfacial Reactivity of Positive Electrodes in Li-Ion Batteries[END_REF] In the Li1s After 50 cycles and whatever the cycling conditions, we observe an evolution in the core peaks spectra. In the F1s spectra, the cycling is concomitant with the appearance of a third component at 686.1 eV associated to the Li x PF y O z species coming from the electrolyte salt decomposition. A huge increase of the LiF components is also observed due to the electrolyte decomposition during cycling. Those new species induce an increase of the LiF/Li x PF y O z component in the Li1s spectra at 56.1 eV. The O1s and C1s spectra are also modified after cycling, due to the degradation of electrolyte salt and solvents. For O1s spectra, the two peaks of the pristine material are still present, associated to the NMC oxygen atoms and Li 2 CO 3 at 529.6 eV and 531.9 eV respectively. In this last peak, oxygen function linked to solvent degradation like CO 2

or O=C species are also included. Beside them, two new peaks are visible. Indeed, the apparition of fluorophosphates species is characterized by the peak at 534.2 eV, while the last component at 533.0 eV can be attributed to the C-O species. [START_REF] Gauthier | Influence of the Cathode Potential on Electrode Interactions within a Li 4 Ti 5 O 12 vs LiNi 3/5 Mn 1/5 Co 1/5[END_REF][START_REF] Karayaylali | The Role of Diphenyl Carbonate Additive on the Interfacial Reactivity of Positive Electrodes in Li-Ion Batteries[END_REF] contribution in Li1s spectra (Table S8 and Figure 9).

The decrease of carbon black and PVDF proportions from 25.1 at.% to 11.6 at.% and from 26.0 at.% to 15.8 at.% respectively between the pristine electrode and the electrode cycled at C/20 in the 2.5-4.3V potential range reflects the covering of the electrode by degradation products after cycling. This decrease is not observed for the active material contributions because it is already covered by Li 2 CO 3 in the pristine electrode and this Li 2 CO 3 is dissolved during cycling and replaced by a SEI layer at the surface of the cycled electrodes. The proportion of carbonaceous species also decreases from 43.7 at.% to 34.1 at.% because of the important increase of LiF/Li x PF y O z proportion (from 2.7 at.% to 34.9 at.%) due to the electrolyte degradation. To get a better insight into the solvents degradation through the formation of carbonaceous species, their proportion compared to the rest of the C1s spectra was also determined. It increases from 33.8 at.% to 54.0 at.% illustrating the degradation of the solvents after cycling (Figure S10 and Table S8).

A decrease of carbon black and PVDF proportions from 11.6 at.% to 6.1 at.% and from 15.8 at.% to 11.0 at.% respectively (Table S8) is observed when the cycling rate is increased from C/20 to C/5. It shows that the electrode is more covered with SEI. Increasing the cycling rate generates a higher quantity of degradation products, especially LiF/Li x PF y O Z species as shown by the increase of their proportion from 34.9 at.% to 46.2 at.% in accordance with the increase of the LiF/Li x PF y O z contributions in the F1s and Li1s spectra (Table S8 and Figure 8b-c). And even if the global proportion of carbonaceous species slightly decreases from 34.1 at.% to 32.0 at.%, more carbonaceous species are formed when the cycling rate is increased from C/20 to C/5, as their proportion compared to the rest of the C1s spectra (SEI species in C1s) increases from 54.0 at.% to 65.0 at.% (see Figure S10 and Table S8). This increase is actually hidden by the more important increase of the LiF/Li x PF y O Z species contributions.

A significant amount of Li 2 CO 3 (% C CO 3 ) is detected at the surface of our electrode before cycling (4.3 at.%, see Figure S10) (as well as at the surface of our material initially, as shown by the XPS analyses of the powders Table 2 and Table S6), but this amount is less important after cycling at C/20; 2.5-4.3V (1.5 at.%) and at C/5 (1.3 at.%) and at higher potential at 4.6V (1.2 at.%), showing that this carbonate is dissolved in the electrolyte during cycling.

When the potential window is enlarged from 2.5-4.3V to 2.5-4.6V for the C/5 cycling, the amount of degradation products decreases and the electrode is less covered with SEI. Indeed, the carbon black and PVDF proportions increase from 6.1 at.% to 8.4 at.% and from 11.0 at.% to 21.5 at.% respectively, while the proportions of LiF/Li x PF y O z species and carbonaceous species decrease from 46.2 at.% to 37.6 at.% and from 32.0 at.% to 28.9 at.% respectively (Table S8). Finally, to investigate the influence of the primary particle size on the electrode/electrolyte interface composition, XPS analyses were performed on electrodes with the 170 nm primary particles (810-5-air), 400 nm primary particles (900-5-air) and 1.2 µm primary particles (930-5air). The electrodes were analysed after cycling at C/5 in the 2.5-4.6V potential window after 50 cycles at end of discharge. The corresponding pristine electrodes were also analysed as a matter of comparison. The spectra and results of the quantitative analyses are given in Figures 10,S11, S12 and Table S9. The decrease of the relative atomic percentages of carbon black and PVDF contributions between the pristine and the cycled electrodes (from 25.1 at.% to 8.4 at.% and from 26.0 at.% to 21.5 at.% respectively for the 900-5-air sample for example) reflects the covering of the electrodes by the degradation products formed after cycling. Regarding the SEI composition, it is shown that a higher quantity of LiF/Li x PF y O z species is formed at the surface of the 400 nm particles (900-5-air sample) and 1.2 µm particles (930-5-air sample) after cycling (37.6 at.% and 36.7 at.% respectively versus 30.4 at.% for the 810-5-air cycled electrode). As a consequence of this higher amount of LiF/Li x PF y O z species, a lower amount of carbonaceous species is observed at the surface of the cycled electrodes with 400 nm and 1.2 µm particles (28.9 at.% and 28.6 at.% respectively versus 32.7 at.% for the 810-5-air cycled electrode) (Figure 10 and Table S9).

However, the more important proportion of carbonaceous species compared to the rest of the C1s spectra (SEI species in C1s) for the 3 cycled electrodes compared to their pristine counterparts reflects the degradation of the solvents through the generation of carbonaceous products (from 33.8 at.% for the pristine electrode to 49.2 at.% for the cycled electrode of the 900-5-air sample for example, see Figure S12 and Table S9). As previously, a significant amount of Li 2 CO 3 (% C CO 3 ) is detected at the surface of the electrodes before cycling (4.3 at.% for the 900-5-air sample for example, see Figure S12 and Table S9) but this amount is less important after cycling at C/5 in the 2.5-4.6V potential range (1.2 at.% for the 900-5-air sample for example) showing that this carbonate is dissolved in the electrolyte during cycling. However, this beneficial effect of the lower amount of Ni 2+ ions in Li sites is only critical for samples with particles sizes smaller than 810 nm. For larger sizes, the diffusion length becomes the limiting factor, as observed for instance for the 930-5-O 2 sample with only 1.8% Ni 2+ ions in Li sites but 2.1 µm primary particles, and thus lower 1 st discharge capacity (Table 4, Figure S6), and as also reported by Wang et al. [START_REF] Wang | Single-Crystal LiNi 0.6 Co 0.2 Mn 0.2 O 2 as High Performance Cathode Materials for Li-Ion Batteries[END_REF] and by Li et al. [START_REF] Li | Synthesis of Single Crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 with Enhanced Electrochemical Performance for Lithium Ion Batteries[END_REF] Indeed, as the particle size grows, the Li + ion diffusion path gets longer and more Li + ions are trapped at the centre of the particles, becoming electrochemically inactive and leading to a reduction of the reversible discharge capacities. Furthermore, increasing the particle size allows the decrease of the specific surface area to limit the parasitic reactions at the electrode/electrolyte interface, reducing in consequence the amount of resistive degradation products formed in the SEI and the charge transfer resistance at the interface. It also implies less Li + ions lost in the formation of degradation products such as LiF/Li x PF y O z species composing the SEI, and thus more Li + ions remaining electrochemically active. Indeed, a decrease of the irreversible capacity at the end of the first cycle (IRC, %) is observed, from 21% to 11% when increasing the particle size from 170 nm (810-5-air sample) to 400 nm (900-5-air sample), as well as a significant increase in discharge capacities from 149 mAh/g to 165 mAh/g (Table 4). As shown previously, thicker Li 2 CO 3 layers are found at the surface of larger particles (see Table S6). This Li 2 CO 3 tends to react in LiPF 6 -based electrolyte to form LiF. [START_REF] Freiberg | Li 2 CO 3 Decomposition in Li-Ion Batteries Induced by the Electrochemical Oxidation of the Electrolyte and of Electrolyte Impurities[END_REF] This implies an increasing amount of LiF/Li x PF y O z species at the surface of the largest particles after cycling, as shown in Figure 10. As LiF is highly ionically resistive [START_REF] Lin | Surface Reconstruction and Chemical Evolution of Stoichiometric Layered Cathode Materials for Lithium-Ion Batteries[END_REF][START_REF] Aurbach | X-Ray Photoelectron Spectroscopy Studies of Lithium Surfaces Prepared in Several Important Electrolyte Solutions. A Comparison with Previous Studies by Fourier Transform Infrared Spectroscopy[END_REF][START_REF] Aurbach | The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries: II. Graphite Electrodes[END_REF][START_REF] Wang | Impact of Evolution of Cathode Electrolyte Interface of Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2 on Electrochemical Performance during High Voltage Cycling Process[END_REF] , it participates also to an increasing resistivity, to a lower Li + ions diffusion through the electrode/electrolyte interface, and thus to a rapid decrease of the reversible discharge capacities for the largest particles (Table 5).

This study demonstrates that in-depth characterization of the samples (structure, morphology, surface composition) is highly useful to predict and optimize the performance in Li-ion batteries.

Indeed, Ronduda et al. had also studied the impact of the calcination temperature on the structure and morphology of NMC622. [START_REF] Duda | On the Sensitivity of the Ni-Rich Layered Cathode Materials for Li-Ion Batteries to the Different Calcination Conditions[END_REF] They obtained a series of samples with particle sizes ranging from 100 nm to 1.2 µm, but with very different and rather large amounts of Ni 2+ ions in Li sites.

They were thus not able to discriminate between the effect of the particle size and that of the dimensionality of the structure, for cycling conditions such as C/20 between 2.8 and 4.4V.

Effect of the increase of cycling rate on the electrochemical performances and the reactivity versus electrolyte

The detrimental effect of the increase of the Li + ions diffusion path, observed when the particle size is increased up to more than 1 µm, is exacerbated for cycling performed at higher rate. It is shown for instance for the 1.2 µm particles (930-5-air sample) by the large decrease of the 1 st discharge capacity from 165 to 87 mAh/g and the increase of IRC from 13 to 30% when the cycling rate is changed from C/20 to C/5 (Table 5). Such a drastic drop of performance is also observed for the smallest 170 nm particles (810-5-air sample), with the decrease of capacity from 149 to 119 mAh/g and the increase of IRC from 21 to 34% (Table 5). In that case, the formation of a thicker resistive layer is observed due to an exacerbated decomposition of the electrolyte for those particles showing higher specific surface area. Indeed, the XPS results given in Figure 9 and Table S8 show a clear decrease of the carbon black and PVDF contributions (from 11.6 at.% to 6.1 at.% and from 15.8 at.% to 11.0 at.% respectively) for the 900-5-air (400 nm particles) cycled electrodes when increasing the cycling rate from C/20 to C/5. Such result illustrates a more important particles covering with degradation products. The large increase of the LiF/Li x PF y O z proportion from 34.9 at.% to 46.2 at.% (Figure 9 and Table S8) and that of carbonaceous species in C1s spectra from 54.0 at.% to 65.0 at.% (Figure S10 and Table S8) reveal that higher cycling rate favours the degradation of the solvents and, to a greater extent, of the electrolyte salt. [START_REF] Shin | Effects of Fluoroethylene Carbonate (FEC) on Anode and Cathode Interfaces at Elevated Temperatures[END_REF][START_REF] Kim | Understanding the Thermal Instability of Fluoroethylene Carbonate in LiPF 6 -Based Electrolytes for Lithium Ion Batteries[END_REF][START_REF] Tasaki | Solubility of Lithium Salts Formed on the Lithium-Ion Battery Negative Electrode Surface in Organic Solvents[END_REF][START_REF] Sinha | Storage Studies on Li/Graphite Cells and the Impact of So-Called SEI-Forming Electrolyte Additives[END_REF][START_REF] Deng | Improving Cyclic Stability of LiNi 0.6 Co 0.2 Mn 0.2 O 2 -SiO x /Graphite Full Cell Using Tris(Trimethylsilyl)Phosphite and Fluoroethylene Carbonate as Combinative Electrolyte Additive[END_REF] Effect of the increase of cut-off voltage on the electrochemical performances and the reactivity versus electrolyte

In the 2.5-4.6V potential window, the 400 nm primary particles (900-5-air electrodes) deliver the best initial capacity, as in the 2.5-4.3V potential window, but have a declining capacity retention as shown in Figure S9. After 5 activation cycles, larger capacities are delivered by the 200 nm particles (840-5-air electrodes). More reactivity detrimental to the capacity retention could have been expected for smaller particles, but it is not the case as shown by their very good capacity retention. Upon cycling at higher cut-off potential, the materials go through higher deintercalation states at end of charge and are thus prone to undergo more important volume changes due to the successive deintercalation and reintercalation of Li + ions (as discussed in details in Figure S13). Those volume changes are expected to be amplified and thus detrimental to capacity retention upon long range cycling for larger particles (here for the 400 nm particles of the 900-5-air sample versus the 200 nm ones of the 840-5-air sample).

Concerning the nature of the solid-liquid interface, the decrease of the proportion of LiF/Li x PF y O z from 46.2 at.% to 37.6 at.% observed at the surface of the 900-5-air electrodes (400 nm particles) when increasing the cut-off voltage from 4.3V to 4.6V (Figure 9 and Table S8) is in accordance with previous works. [START_REF] Wang | Impact of Evolution of Cathode Electrolyte Interface of Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2 on Electrochemical Performance during High Voltage Cycling Process[END_REF][START_REF] Cherkashinin | The Stability of the SEI Layer, Surface Composition and the Oxidation State of Transition Metals at the Electrolyte-Cathode Interface Impacted by the Electrochemical Cycling: X-Ray Photoelectron Spectroscopy Investigation[END_REF] As the amount of settled LiF would decrease during the charge process, its amount at the surface of the positive electrode decreases when cycling is performed at higher cut-off voltage. [START_REF] Wang | Impact of Evolution of Cathode Electrolyte Interface of Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2 on Electrochemical Performance during High Voltage Cycling Process[END_REF][START_REF] Cherkashinin | The Stability of the SEI Layer, Surface Composition and the Oxidation State of Transition Metals at the Electrolyte-Cathode Interface Impacted by the Electrochemical Cycling: X-Ray Photoelectron Spectroscopy Investigation[END_REF] Indeed, apart from the decomposition of LiPF 6 at the positive electrode surface [START_REF] Tasaki | Solubility of Lithium Salts Formed on the Lithium-Ion Battery Negative Electrode Surface in Organic Solvents[END_REF] , LiF can also be formed at the Li metal negative electrode and migrate from the negative to the positive electrode during discharge so it was shown that LiF would preferentially settle on the positive electrode during discharge. [START_REF] Zhang | Dynamic Evolution of Cathode Electrolyte Interphase (CEI) on High Voltage LiCoO 2 Cathode and Its Interaction with Li Anode[END_REF] Moreover, it was shown that increasing the charge voltage up to 4.5V could cause a decomposition / dissolution of the SEI layer with a vanishing of the spectral features originated from the degradation products of the electrolyte salt and organic solvents, with no noticeable change in the surface chemistry of the NMC material. [START_REF] Cherkashinin | The Stability of the SEI Layer, Surface Composition and the Oxidation State of Transition Metals at the Electrolyte-Cathode Interface Impacted by the Electrochemical Cycling: X-Ray Photoelectron Spectroscopy Investigation[END_REF] This SEI layer dissolution could be caused by weakening or breaking of chemical bonds due to mechanical stresses induced by lattice volume changes, that tend, as just discussed, to be larger when cycling is performed at higher voltages due to the larger amount of deintercalated Li + ions. This is also in accordance with the decrease of the proportion of carbonaceous species from 32.0 at.% to 28.9 at.% observed for these 900-5-air electrodes (400 nm particles) when increasing the cut-off voltage from 4.3V to 4.6V (Figure 9 and Table S8).

As a conclusion, an optimum for the particles size has obviously always to be found in order to find the good balance between the limitation of the reactivity on one side, by increasing the particle size, and the minimization of the diffusion path on the other side, by decreasing the particle size. From those results, an optimum primary particle size of 400 nm can be inferred for cycling performed up to 4.3V, the 900-5-air sample delivering the highest reversible capacities and best capacity retentions regardless of the cycling rate. For cycling performed at higher cutoff potentials up to 4.6V, the optimum primary particle size would rather be 200 nm. Smaller primary particles should prevail upon others for cycling at high potential as they have better capacity retention than larger primary particles, as shown by the very good capacity retention of the 170 nm particles. It thus appears that samples with primary particles between 200 and 400 nm are the most versatile as they deliver the best performances regardless of the cycling conditions. To optimize the performance of the materials, the amount of Ni 2+ ions in the interslab spaces has to be lowered to less than 2 % and that of Li 2 CO 3 at the surface to less than 5 at.%, by performing the lithiation under O 2 atmosphere with an excess of Li limited to 1 wt.%. Those results show that the cycling conditions can affect significantly the reactivity of the materials versus the electrolyte and the composition of the positive electrode/electrolyte interface.

Increasing the cycling rate from C/20 to C/5 in the 2.5-4.3V potential window enhances the formation of degradation products of the solvents (carbonaceous and oxygenated species) and, to a greater degree, degradation products of LiPF 6 (LiF and Li x PF y O z species). Enlarging the potential window from 2.5-4.3V to 2.5-4.6V at the same cycling rate reduces the proportion of LiF/Li x PF y O z species at the surface and increases that of carbonaceous species, resulting from the solvents degradation, DMC and FEC.

CONCLUSION

The parameters of a two-step synthesis consisting of a coprecipitation of the transition metal precursor, followed by a solid-state reaction with Li 2 CO 3 have been investigated to obtain NMC622 layered oxides. Samples with crystallographic structure close to the ideal 2D layered structure (less than 3.8% Ni 2+ ions in Li + sites) and with variable primary particle sizes from 170 nm to 2 µm were obtained. Larger primary particles are obtained when the lithiation temperature is increased up to 930°C, the lithium carbonate excess minimized to 1 wt. % and the thermal treatment carried out under O 2 flow. The amount of Ni 2+ ions in Li + sites of NMC622 can be minimized until less than 1.8% by doing the lithiation under O 2 flow, the oxidant atmosphere disfavouring the stabilisation of nickel ions at the divalent oxidation state. The decrease of the amount of Li 2 CO 3 at the surface of the particles can be achieved by minimizing the excess used for lithiation or by doing the lithiation under O 2 flow. In terms of particle size, an optimum has to be found. On one hand, increasing the particle size enables to decrease the interfacial area with the electrolyte and limit the parasitic reactions. It also decreases the number of grain boundaries that hinders Li + ions diffusion so the performances are significantly enhanced. On the other hand, too large particles are detrimental for the performances because of their larger diffusion path for Li + ions and of their tendency to form higher amounts of highly resistive LiF at the electrode/electrolyte interface during cycling, as a consequence of the larger amount of Li 2 CO 3 located at their surface in their pristine state. An optimum primary particle size of 400 nm can be inferred for cyclings performed up to 4.3V, those particles delivering the highest reversible capacities and best capacity retentions regardless of the cycling rate. For cyclings performed at higher cut-off potentials up to 4.6V, the optimum primary particle size would rather be 200 nm, those particles having better capacity retention than larger primary particles. The cycling conditions have a significant influence on the electrode/electrolyte interface composition after cycling. Increasing the cycling rate from C/20 to C/5 enhances the formation of LiF/Li x PF y O z species at the surface of 400 nm particles which is an evidence of a more important degradation of LiPF 6 . Increasing the potential window from 2.5-4.3V to 2.5-4.6V reduces the proportion of LiF/Li x PF y O z species in favour of carbonaceous species that are degradation products of the solvents used in the electrolyte, DMC and FEC in our case. As a conclusion, particles between 200 and 400 nm must be preferred as they deliver the best performances regardless of the cycling conditions and do not generate a too large amount of degradation products at the positive electrode/electrolyte interface.

For Table of Contents only

Li 2 CO 3 (

 23 Sigma-Aldr , ≥99%). A t er al treat e t s perfor ed at te perature, betwee 780 and 930°C, with different Li 2 CO 3 excesses (1 or 5 wt.% vs. the Li/TM ratio of 1.05) and under air or O 2 flow. T s sl t overl t at o ( .e. L /TM ≥ 1.05) s re u red to obta a layered structure with the smallest amount of Ni 2+ ions in the Li sites. The samples are named according to the synthesis conditions used for their preparation: lithiation temperature (°C) -weight percentage of Li 2 CO 3 excess -atmosphere. The cationic composition of the lithiated layered oxides was determined by Inductively Coupled Plasma -Optical Emission Spectrometry (ICP-OES) (Varian ICP/OES 720 ES) after their complete dissolution into a mixture of hydrochloric acid and nitric acid (HCl/HNO 3 : 2/1 in volume ratio). Each sample was dissolved twice and each solution is measured 5 times, meaning the ICP values given hereafter are averages of 10 measurements. The carbon content of the powders was also determined using a CHNS elemental Thermo Fischer Scientific analyser equipped with an automatic sampler. Powder X-Ray diffraction (XRD) patterns were recorded us a PANalyt al X'Pert MPD Pro d ffra to eter e u pped with a Cu radiation X-ray tube, to determine the purity and structure of the synthesized materials. The XRD patterns were collected in the angular range of 10-80°(2θ) w t 0.008° tervals, for a total a u s t o t e of arou d 35 hrs. Neutron Powder Diffraction (NPD) was performed using the high-resolution powder diffractometer D2B at Institut Laue Langevin (ILL, Grenoble, France). The samples were contained in a vanadium tube with a diameter of 8 mm. The NPD patterns were collected at room temperature w t a wavele t λ = 1.5947(1) Å, in the angular range of 0-160°(2θ) w t 0.05° intervals for a total acquisition time of around 5 hrs. The collected XRD and NPD patterns were

Figure 1 .

 1 Figure 1. X-ray diffraction patterns obtained for the synthesized samples. The samples are named as follows: Lithiation temperature (°C) -excess of Li 2 CO 3 (wt.%) -atmosphere. TM oxides M x O y were detected as impurities in very small amounts in the 5-air samples (in inset, the intensity is multiplied by a factor of ca. 10).

Figure 2 .

 2 Figure 2. Refinements of the neutron (a) and X-ray (b) diffraction patterns recorded for the sample 810-5-air. Axis breaks were used in order highlight better the quality of the refinements, as shown also by the enlargements of the (018)/(110) diffraction lines given on the right of the figure.

Figure 3 .

 3 Figure 3. Morphologies obtained for the 810-5-air, 870-5-air, 900-5-air and 930-5-air samples, selected in the "5-air" series.

Figure 4 .

 4 Figure 4. Morphologies of the samples synthesized at 900°C and the corresponding size distributions determined by Image J considering 100 particles.Surface analyses

  Figure 5 compares as an example the C1s, O1s and Li1s spectra obtained for the samples synthesized at 900°C. The C1s spectra are decomposed into four contributions observed at 285.0, 286.4, 288.6 and 290.1 eV and corresponding to C-C/C-H, C-O, C=O/O-C=O and CO 3 2-environments respectively.

Figure 5 .

 5 Figure 5. C1s (a), O1s (b) and Li1s (c) XPS spectra of the samples synthesized at 900°C. The C CO 3 and Li Li 2 CO 3 peaks are highlighted in blue and the O NMC and Li NMC peaks are highlighted in red. The dots represent the experimental data and the lines the fitted spectra.

Figure 6 .

 6 Figure 6. Charge-discharge curves and corresponding dQ/dV curves obtained at 1 st cycle for the "5-a r" sa ples (a), t e "5-O 2 " samples (b) and the samples synthesized at 900°C (c).

Figure 7 .

 7 Figure 7. Charge-discharge curves and corresponding dQ/dV curves obtained for the 1 st cycle at C/20 2.5-4.3V (a); C/5 2.5-4.3V (b) and C/5 2.5-4.6V (c) for four samples representative of the "5-air" series.

  peak, and, compared to the pristine material, we identify a new component under the Li1s peak according to LiF species at 56.3 eV in accordance with the LiF contribution in the F1s spectrum.In addition to the contribution corresponding to O 2-anions in the NMC crystalline network (O NMC ) at 529.8 eV in the O1s spectra, two others components, mainly corresponding to the Li 2 CO 3 oxygen atoms, are located at 531.9 eV and 533.4 eV. Compared to the Figure5, we can note a decrease of the contribution of the O NMC component in the O1s spectrum corresponding to the presence of oxygenated species coming from the electrode formulation.

Figure 8 .

 8 Figure 8. C1s (a), F1s (b) Li1s, Ni3p, Co3p, Mn3p (c) and O1s (d) XPS spectra of the 900-5-air

Figure 9 .

 9 Figure 9. Evolution of atomic percentages (at. %) of carbon black (CB), PVDF, active material,

Figure 10 .

 10 Figure 10. Evolution of atomic percentage (at. %) of carbon black (CB), PVDF, active material,

  NiSO 4 .6H 2 O, MnSO 4 .H 2 O and CoSO 4 .7H 2 O (Sigma-Aldrich, ≥99%) were used as precursors to prepare an aqueous mixed solution of NiSO 4 .6H 2 O, MnSO 4 .H 2 O and CoSO 4 .7H 2

  at 68.1 eV corresponds to nickel in a mixed valence Ni 2+ /Ni 3+ , the Co3p core peak at 61.3 eV corresponds to Co 3+ and the Mn3p core peak at 50.0 eV corresponds to Mn 4+ in an oxygen environment.[START_REF] Dahéron | Electron Transfer Mechanisms upon Lithium Deintercalation from LiCoO 2 to CoO 2 Investigated by XPS[END_REF][START_REF] Dedryvère | Electrode/Electrolyte Interface Reactivity in High-Voltage Spinel LiMn 1.6 Ni 0.4 O 4 /Li 4 Ti 5 O 12[END_REF] Those peaks were chosen to perform a more precise quantitative analysis as they are in the same binding energy range as opposed to the corresponding 2p core peaks. The Li1s peak is decomposed into two components at 54.4 eV and 55.6 eV attributed respectively to Li atoms in the NMC crystalline network (labelled Li NMC ) and Li 2 CO 3 (labelled Li 2 CO 3 ).First, we can easily observe that, under air atmosphere, limited use of Li 2 CO 3 excess increases the proportion of the O NMC contribution to the O1s core peak. The same observation can be made when the synthesis atmosphere is changed from air to O 2 for a given Li 2 CO 3 content. This

	43,44 The O1s spectra exhibit a component at 529.7 eV corresponding to O 2-anions
	in the NMC crystalline network and two other components located at higher binding energies
	assigned CO 3	2-

/O-C=O/C=O and C-O environments. In the 45-80 eV binding energy range, Ni3p, Co3p, Li1s and Mn3p core peaks have been identified. On the basis of literature, the Ni3p core peak observation is confirmed by the decrease of the components located at 290.0 eV in the C1s spectra and at 55.6 eV in the Li1s spectra, associated both to carbonate species, when the Li 2 CO 3 excess is lowered or when the synthesis atmosphere is O 2 . The less covered particles by adsorbed species and/or carbonate species are obtained for the 900-1-air and 900-5-O 2 samples.

Table 2 .

 2 Comparison of the contribution of the carbonates species in the C1s and Li1s spectra, and the contribution of oxygen from the electrode material in atomic percentages extracted from the global quantitative analyses performed from XPS spectra of the materials synthesized at 900°C.

		Atomic percentages
	Samples	C1s (Li 2 CO 3 )	Li1s (Li 2 CO 3 )	% O NMC in O1s
	900-5-O 2	6.3	12.7	23.5
	900-1-air	6.2	13.6	22.4
	900-5-air	10.0	30.4	4.7

Table 3 .

 3 Transition metal ratio of the materials synthesized at 900°C obtained from Ni, Mn and Co atomic percentages extracted from the global quantitative analyses performed from XPS spectra.

				Atomic percentages
	Samples	Ni 3p	Mn 3p	Co 3p	ΣTM	(Ni:Mn:Co)/ΣTM
	900-5-O 2	5.8	1.3	1.4	8.5	0.68 : 0.15 : 0.16
	900-1-air	4.9	1.1	1.2	7.2	0.68 : 0.15 : 0.17
	900-5-air	1.6	0.4	0.4	2.4	0.67 : 0.17 : 0.17

  The sample 810-5-O 2 with 240 nm primary particles has a 1 st discharge capacity of 173 mAh/g, similar to that of the sample 900-5-O 2 with 810 nm primary particles (170 mAh/g) and much higher than that of the sample 930-5-O 2 (159 mAh/g) with primary particles of 2.1 µm. Larger discharge capacities are reached compared to the previous series of materials, probably in relation with the smaller amounts of Ni 2+ in Li + sites for these materials. As shown in inset of

	Figure 6b, very s lar polar zat o urves are observed for t e "5-O 2 " sa ples dur	t e 1 st
	cycle. Figure S6 shows that the materials 810-5-O 2 and 900-5-O 2 with 240 nm and 810 nm
	primary particles deliver larger specific capacities and capacity retentions. Inversely, the sample
	930-5-O 2 with 2.1 µm primary particles delivers significantly lower capacities with a lower
	capacity retention of 82% after 30 cycles versus 94% and 92% for the 810-5-O 2 and 900-5-O 2

Table 4 .

 4 1 st discharge capacities, irreversible capacities rates (IRC, %) and capacity retentions deter ed for t e "5-a r" sa ples, "5-O 2 " sa ples a d t e sa ples sy t es zed at 900°C.

	C/20 ; 2.5-4.3V

Table 5 .

 5 1 st discharge capacities and irreversible capacities (IRC, %) of four samples

	represe tat ve of t e "5-a r" ser es y led d ffere t o d t o s
		C/20 ; 2.5-4.3V	C/5 ; 2.5-4.3V	C/5 ; 2.5-4.6V
		1 st discharge		1 st discharge		1 st discharge
	Sample	capacity	IRC (%)	capacity	IRC (%)	capacity	IRC (%)
		(mAh/g)		(mAh/g)		(mAh/g)
	810-5-air (170 nm)	149	21	119	34	151	30
	840-5-air (200 nm)	152	19	128	27	165	25
	900-5-air (400 nm)	165	11	158	18	185	16
	930-5-air (1.2 µm)	165	13	87	30	130	38

thank Philippe Legros at PLACAMAT and Marie-Anne Dourges at ISM for their scientific support with SEM and BET analyses.

Funding Sources

This work was funded by Saft and Région Nouvelle Aquitaine.

ACKNOWLEDGMENT

The authors acknowledge Saft and Région Nouvelle-Aquitaine for the funding of the project, as well as the French National Research Agency (STORE-EX Labex Project ANR-10-LABX-76-01) for financial support for their research on the electrochemical energy storage. The authors thank Cathy Denage, Laëtitia Etienne, Eric Lebraud and Dominique Denux at ICMCB for their technical support for ICP-OES, granulometry, XRD and TGA-MS analyses. The authors also

ASSOCIATED CONTENT

Supporting information

The Supporting information contains figures, tables and description to complete and precise some points in the main manuscript. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding author

* Laurence Croguennec Laurence.Croguennec@icmcb.cnrs.fr


Valérie Camel

Marie-Noëlle Maillard

Even Nicolas Descharles

Le Roux

Mathieu Cladière

Isabelle Billault

Even Le Roux

Open digital educational resources for training the chemistry lab safety and security rules

Keywords: General public, Safety / Hazards, Distance Learning / Self Instruction, Internet / Web-Based Learning, Student-Centered Learning

teaching and research institutions in France or abroad, or from public or private research centers.

INTRODUCTION

Security and safety rules should be a prerequisite for students and trainers before chemistry lab sessions. However, in our graduated programs we have experienced that a part of our students arrive in lab sessions without deep knowledge of proper behavior to adopt in a chemistry lab. Indeed, they may have different backgrounds, with several degrees of lab experience. They also mostly originate from biological undergraduate programs, with little specific experience of chemistry lab.

Teaching safety rules in classroom before the lab might be boring and uninteresting for students.

Besides, the time devoted to such course is lacking in our programs. On the other hand, teaching these rules at the beginning of lab sessions is uncomfortable, with subsequent reduced time for the practical experiments. Besides some safety aspects should be trained before arriving in the lab, such as the appropriate dress code for a chemistry lab. In such a context, we have developed digital interactive resources with a view of enabling the students for self-training before the lab sessions.

These open educational resources are easily accessible on any digital consultation device, and attractive due to graphic design and browsing tested previously with students.

BEFORE the lab IN the lab

Directly accessible from the website CHIMACTIV (http://chimactiv.agroparistech.fr/), [START_REF] Camel | CHIMACTIV: an open-source website for student-centered learning in analytical chemistry[END_REF][START_REF] Camel | CHIMACTIV, un site pour se former à l'analyse chimique[END_REF] two digital sheets are dedicated for training the chemistry lab safety and security rules. The first one is dedicated to protection of persons in the lab. The second one is concerned with the chemical hazards possibly present in the lab, including options for managing wastes. A short survey of teachers of different degrees (from high school to university), carried out during a workshop aimed to get them familiar with the CHIMACTIV website, [START_REF] Camel | Intégrer le site CHIMACTIV à ses enseignements[END_REF] clearly showed their great interest in using these two safety sheets with their students. These first returns prompted us to disseminate the use of these digital resources.

After a presentation of the content of these open interactive resources, their alignment with the RAMP approach proposed by the American Chemical Society 4-5 is discussed. Then, consultation data and examples of teaching scenarios for integrating these educational resources are presented, with observed benefits mentioned.

TRAINING BASIC RULES FOR PROTECTING THE OPERATORS

Personal protection

One major point for students lies in adopting adequate behavior when entering the lab. In particular, even though students in graduated programs are very familiar with the lab coat wearing, they sometimes do not have the reflex of closing it properly or removing it when leaving the lab. As illustrated in figure 1, the front page related to personal protective equipment (PPE) clearly shows the proper lab coat dressing. Several icons are interactive, to get more details about face protection, protection gloves, lab coat and proper dress. Face protection is frequently unfamiliar for the students, or neglected. In particular, we have observed that frequently graduate students do not have a positive attitude regarding eye protection, since they are convinced that the operations they will have to do are safe and/or they will operate safely so that there is no risk for their eyes. They must be trained to understand that safety glasses are mandatory in the lab for any person present, even if he/she wears eyeglasses. Protection glasses are presented, with also recalling prohibition of wearing contact lenses in the lab since this is generally unknown by our students. Hair protection is also presented. Using masks in appropriate cases is also detailed.

For protection gloves, very few students know the different types that do exist, and which one they should use depending on the experiment and reagents or solvents they will have to deal with in the lab. So, we detail the main types of gloves they may find in a lab, with their possible applications. In that way, students are more aware of this critical part of their protection.

Adapted dress is also detailed, since we have experienced students coming in the lab sessions with shorts or open shoes, especially during the summer time. Preventing such at risk dress is essential to protect them from possible skin contact with splashes or spills, and this starts with proper education before the lab sessions.

Collective protection

Since safe individual behaviors are mandatory to protect all the group present in the lab, important knowledge is also given relative to collective protection means. As shown in figure 2, we have divided this section in three parts: bench, room and hood.

The room section is useful to remind the students to take care of their bags and personal belongings with appropriate storage. This is sometimes tricky to achieve in practice due to the lack of dedicated spaces in the chemistry laboratories. Yet, this is highly important to prevent incidents or even accidents and injuries given the number of students handling at the same time in the laboratory.

The ban of eating and drinking is also recalled, as well as the ban of running or smoking in the lab.

In the bench part, students are warned of the importance of well organizing the space on the laboratory bench to prevent any spilling or other incident due to lack of space. They are also advised to indicate on any glass container the nature of the filling as well as its concentration to prevent from improper mixing and risk of exothermic reaction.

To complete the collective protection part, indications about fume hood are also presented. Firstly, the simple operation to switch the aspiration on is illustrated. Then, the good laboratory practice (GLP) relative to the correct position of the window pane is also shown thanks to illustrative photographs and short recommendations.

Illustrative videos

To help the students integrating the different recommendations and advices presented previously so as to adopt a safe and secure behavior in the lab and during the experiments, two short videos are proposed. They both show one operator entering the lab, and starting some basic manipulations. The video "A bad day of experimental works" shows unsafe behavior or actions to ban, while the video "A good day of experimental works" shows the safe and secure behaviors and actions to adopt in the lab.

We believe that showing the unsafe behavior is complementary and as important as presenting the proper behavior. Videos are supposed to be humoristic, in order to better mark the memory of the viewers.

HANDLING DANGEROUS CHEMICALS

Recognizing the types of chemicals Obviously, having knowledge about the chemicals is essential to prevent the students to make errors that might result in incident like damage or injury. We have distinguished three essential parts that merge in the front page of this section as shown in figure 4: the hazard pictograms according to the global harmonized system (GHS) of classification and labelling of chemicals, [START_REF]Globally harmonized system of classification and labelling of chemicals (GHS). Eight revised edition[END_REF] the chemicals categories, and their storage conditions.

Training students to get familiar with the hazard pictograms is crucial. In addition to ensuring adapted managing options in the lab, it also contributes to help them having similar safe behavior in other contexts such at home when they use biocides for example. As illustrated in figure 4, pictograms are gathered in three groups, depending on the damages: physical hazards (such as explosion, corrosion, etc.), human health damage (such as reprotoxicity) and chemicals that are toxic for the environment (for example toxic for aquatic life). Whenever adequate, some pictograms merge in two groups. All the pictograms are interactive, so that the lecturer can get into some details about the meaning of the pictogram as well as the hazard statement according to the international labeling regulation (see the example of corrosive substances in figure 4). Information relative to health toxicity is further detailed with all the hazard statements listed and some caution for safe handling given. From the front page the visitor can also access to knowledge of the different types of chemicals (i.e. organic solvents, reagents, toxic products, acids and bases). Each type is further detailed in specific pages. Briefly, most common organic solvents are listed as their categories (e.g. alcohols, esters), and the need of caution for handling them under a fume hood due to their properties (such as volatility)

and toxicities is also clearly exposed. Then essential information relative to acids and bases is given on a separated page, especially that they may be either organic or mineral, present different physical states, and exert strong or weak proton affinity. The need for wearing PPE and manipulating under fume hood is also clearly claimed here.

Caution about the reactivity of some chemicals is detailed in another page of this safety sheet, with examples of chemical reactions concerned as well as details about the relative hazards labelling and statements.

Finally, proper storage conditions are recalled in the last part of this sub-section of the safety sheet. Besides elementary storage conditions in a chemical hold, details are also given for storage conditions in the laboratory (i.e. in the fridge or under the fume hood). A table of chemical compatibility for storage is also given, depending on their labelling pictograms.

Managing liquid wastes

Proper management of liquid wastes is mandatory in the lab. Yet, students are not very familiar with this side of the safety rules. Besides, management actions depend on the type of liquid or mixture, being thus quite uneasy for them.

On the CHIMACTIV website, we alert students to the requirement for being very careful when they throw away a chemical liquid in order to avoid any hazardous reaction upon mixing chemicals (thereby avoiding incident or accident in the lab) as well as prevent environmental contamination. Then we give simple and practical advices or recommendations ("What I must throw in"/ "What I can throw in"/ "What I must not throw in") for the main categories of liquid wastes encountered in chemistry labs as illustrated in figure 5: non-halogenated organic solvents, halogenated organic solvents, toxic liquids, organic and mineral acidic liquids, organic and mineral basic liquids. Finally, practical advices and recommendations relative to the ways of properly and safely throwing the liquid wastes in the adapted containers are detailed.

To help the students getting familiar with the proper waste option depending on the liquid waste considered, an interactive decision tree is proposed as shown in figure 5. It is entirely extractable in a pdf format, enabling the students and their professors to keep this decision tree on their own digital devices or to print it, thereby favoring them to well manage liquid wastes whenever they are in the lab.

Journal of Chemical Education

Managing solid wastes

This sub-section is organized in a similar way as the previous one related to liquid wastes as depicted in figure 6. From the front page an interactive decision tree is accessible to help finding the best management option, which is readily extractable and printable for a convenient use.

5/9/22

Page 11 of 19 Figure 6. The solid waste front page, with some details of the interactive decision tree for choosing the best options for sorting solid wastes.

In practice, we have observed that students have little knowledge about solid wastes, resulting in unsecure behavior or actions (such as to throw their gloves in conventional trash for example). So, we detail the main types of solid wastes they might encounter in a chemistry lab: pointed and/or sharp soiled wastes, non-pointed and/or non-sharp soiled wastes, unsoiled laboratory wastes, chromatography vials, other unsoiled wastes. Here again, simple and practical advices or recommendations are sorted according to "What I must throw in"/ "What I can throw in"/ "What I must not throw in" as illustrated in figure 6. They are implemented by short recommendations for properly and safely throwing the solid wastes in the adapted containers.

Exercising

Two interactive exercises are proposed, to help the students memorizing previous delivered knowledge in this digital sheet. In each one, four different liquid wastes are proposed, and students have to drag-and-drop each of them in the properly labelled container among five that are proposed.

OUR RESOURCES ALIGNMENT WITH THE RAMP APPROACH

This point is considered here relative to the pre-lab instructions only, since this is the major learning objective of our resources as already mentioned. The accessibility of our digital resources enable the students to go back to the information "just-in-time" in the lab whenever they want.

Recognize the hazards

This part of the RAMP approach is the most covered since this is "the key first step to safety". [START_REF] Hill | Recognizing and understanding hazards -The key first step to safety[END_REF] Our digital resources enable the students to get familiar with the GHS and its pictograms before entering in the lab, as well as the different classes of toxic chemicals. They are also introduced to acute and chronic toxicity. Synthetic information is delivered so that they should be able to rapidly understand the labelling of chemical containers, thereby recognizing these chemical hazards.

Assess the risks of the hazards

Along with the GHS pictograms, the CHIMACTIV website introduces the students to the hazard statements. That way, they are aware of the fact that a similar pictogram could reflect different health effects and severity (death, cancer, irritation, allergies, etc.). This knowledge is useful for students to rapidly assess what are the risks with the manipulation of the different chemicals during their lab session.

Minimize the risks of the hazards

Our resources enable students to get knowledge about the protective measures (PPE and collective protection) that should avoid their exposure to the chemical products under inappropriate operating conditions. They also find recommendations for proper use of PPE, especially using gloves adapted to the considered chemicals. In addition, they are also warned about basic rules related to bench preparation before starting the experiment, as well as putting their personal belongings in appropriate place in the room to avoid any incident.

Prepare for emergencies from uncontrolled hazards

The first aid section of the CHIMACTIV website provides the minimal knowledge regarding emergency procedures in case of chemical or thermal burns, or in case of accidental contact with a chemical hazard. In addition, a rapid description of the different emergency devices enables the students to identify them in the lab avoiding to seek for them in emergency case. This is not to replace a complete formation for laboratory first aid, but instead to give everyone the minimum knowledge to ensure safety for everybody.

USING THESE RESOURCES IN PRACTICE

Consultation statistics

Statistics data relative to the CHIMACTIV website consultation are available monthly including all visitors, not only our students. [START_REF] Camel | CHIMACTIV, un site pour se former à l'analyse chimique[END_REF] Based from responding to our survey available on the website, our visitors profile is around 54% students (from high school to master degrees), 15% teachers and 7%

professional outside teaching (unfortunately, the information is missing for 24% of responders).

Over the sole period 1-20 January 2020, there were 6,711 visits on the website, resulting in a total of 21,346 views on the French version and 3,110 on the English version (see figure 7). Looking deeper on the data, the Safety section faces around 2,000 views. Since it offers only two digital sheets while other CHIMACTIV sections gather from 4 to 15 sheets, it is more valuable to assess the view numbers per digital sheet for each section of the website to draw the comparison. Interestingly, figure 7 shows that the Safety section has the highest rank for the French version of the website. It also ranks pretty well for the English version, but due to the lower number of views in this language until now, new data in the next future should comfort this statement. Interestingly, whatever the language (French or English), the CHIMACTIV safety resources are consulted at 60% relative for hazard chemicals and 40% for protection. This is probably linked to the prior knowledge of our graduate students, more familiar with protection equipment.

Back from our teaching experience

We have been using the digital resources with our graduated students from around three years now. Interestingly, several academic programs have been concerned with diverse profiles of students.

In particular, their knowledge and interest in chemistry vary greatly, ranging from biologists with a short experience in chemistry lab up to chemists who are quite familiar with basic safety and security rules. By way of consequence, the time devoted to chemistry lab sessions differ between these academic programs.

Fortunately, the open education resources of the CHIMACTIV website authorize a wide range of teaching and learning experiences. In our case, two major teaching scenarios have been conceived for integrating these resources in chemistry courses that include lab sessions, both for self-training of students before entering in the lab as schematized in figure 8. The simplest scenario lies in advising the students to consult these safety resources: this implies a brief presentation (either oral or written) of the CHIMACTIV website and its browsing tools, 1 as well as an explicitness that they need to have acquired minimal knowledge for their safety in the lab. This concerned rather generalist academic programs. Satisfaction surveys filled by the students at the end of the courses do suggest that they guide their consultation depending on their prior knowledge, spending more time on the resources that are not acquired or brand new for them. Upon their arrival in the lab for the first session, students were asked about main safety and security issues, to make them remember the key messages.

Journal of Chemical Education

A more thorough integration of these safety open resources has been also experienced in a chemistry academic program, by adding a clear request for each student to watch the video "A bad day of experimental works" and to send the teacher at least five unsafe behaviors or actions that he/she has identified in this video. The students were all capable of identifying at least five mistakes, and this exercise was aimed to focus their attention on safety rules before entering the lab.

In addition, these safety resources have been also included in the welcome package for traineeship students in our research lab. They are kindly asked by the security manager to consult these open resources prior their first manipulations in the lab. This allows the trainee in particular to be able to review, at his/her own pace, safety information which have been mentioned during the lab safety visit and which could seem to him/her to be more complex (e.g. management of chemical waste).

Observed benefits for students

We have experienced our students to be more attentive to their protection when arriving in the lab, especially with regards to safety glasses and gloves. In particular, they do care about the different types of gloves that are present, and the most adapted to the experiments they will have to perform.

Videos and photos available in our resources are probably key elements of this effective safety training as already mentioned. [START_REF] Saleh | Testing the effectiveness of visual aids in chemical safety training[END_REF] We also have observed students arriving in the lab more familiar with the lab environment and thus more confident. Engaging the discussion with them at the beginning of the lab sessions revealed them to be more aware of safety rules, and having a deeper related knowledge of chemicals than before thanks to the use of the CHIMACTIV website. Our students did confirm their feeling of being trained to safety aspects partly thanks to the website.

During the lab sessions they were much more aware of the waste management options. On that latter point, having printed in large format both decision trees and posted them in the chemistry lab (see figure 9) is very convenient in practice. Students can refer to these posters at any time during their lab sessions, to make sure they face the correct and safe management option. This is also very helpful for them to understand the principles underlying these decision trees. We also noted that a large proportion of the students prefer to consult the decision tree on their own smartphone during the session in order to ensure that the chemical waste from their experience is emptied into the appropriate container.

Journal of Chemical Education Students were also surveyed for their satisfaction related to these resources (using individual anonymous questionnaires), and their feeling about their usefulness for being prepared for the lab sessions. Results related to two cohorts of graduated chemistry students are displayed in figure 10. In spite of some discrepancy between both cohorts, a great majority of the students perceived these resources as being useful for them to being prepared for the lab sessions. This feeling is even higher when non-chemistry students are surveyed (near 100%), due to their reduced prior knowledge related to safety.

Journal of Chemical Education

CONCLUSION

Open educational resources related to chemical safety are available on the CHIMACTIV website.

Being interactive and responsive, they are attractive for students to train before entering in the lab, or during the lab sessions. Different scenarios have been already implemented for using these resources that can be reproduced by other teachers whenever interested. Our data relative to the students' perception of usefulness of these resources are promising. Teachers' observations in the lab do confirm the valuable aspect of these resources, since students arrive in the lab more familiar with the lab environment (e.g. safety devices) and more aware of safety rules.

Figure 1 .

 1 Figure 1. The PPE front page, with details shown relative to the lab coat.

Figure 2 .

 2 Figure 2. The collective protection front page, with details shown relative to the bench safety recommendations.

Figure 3 .

 3 Figure 3. The first aid front page, with details shown relative to emergency techniques in case of incidents.

Figure 4 .

 4 Figure 4. The chemicals front page, with details relative to hazard pictograms and focus on the corrosive substances pictogram.

Figure 5 .

 5 Figure 5. The liquid waste front page, with some details of the interactive decision tree for choosing the best options for sorting liguid wastes. 175

Figure 7 .

 7 Figure 7. Consultation data of the open resources of the website CHIMACTIV (categorized by their section) over the period 1-20 January 2020: French (FR) version; English (EN) version.

Figure 8 .

 8 Figure 8. Different scenarios tested to train our students with these open educational resources.

Figure 9 .

 9 Figure 9. View of our chemistry lab with both decision trees (liquid and solid wastes) displayed, and details of both trees (credit photos: Even Le Roux).

Figure 10 .

 10 Figure 10. Perception of graduate chemistry students with regards to the usefulness of these safety digital resources for lab sessions.

ACKNOWLEDGMENTS

The authors acknowledge the Université Paris-Saclay for its funding for creating the website CHIMACTIV (IDEX funding attributed on the different calls for projects « Former par le numérique » 2014, and « Initiatives pédagogiques -Transformer » 2016 & 2018).

AUTHOR INFORMATION

Corresponding Author *E-mail: valerie.camel@agroparistech.fr