Adrien Soloy

Delphine Flahaut

Jean-Bernard Ledeuil

Joachim Allouche

Dominique Foix

Germain Vallverdu

Emmanuelle Suard

Erwan Dumont

Lucille Gal

François Weill

Germain Salvato Vallverdu

Laurence Croguennec

Unravelling the

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

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INTRODUCTION

The rapid development of the electric and hybrid vehicles industry caused an increasing demand for Li-ion batteries (LiBs) with high energy densities and low cost in the last decade.

Indeed the global Li-ion cell manufacturing capacity was expected to increase between 4 and 6 times by 2021-2022 compared with 2017 levels. [START_REF] Tsiropoulos | Li-Ion Batteries for Mobility and Stationary Storage Applications: Scenarios for Costs and Market Growth[END_REF] The demand for LiBs is still expected to increase exponentially until 2030 thanks to the democratization of electric vehicles. By 2040, 50 times more electric vehicles are expected to be on the road compared to 2018. [START_REF] Tsiropoulos | Li-Ion Batteries for Mobility and Stationary Storage Applications: Scenarios for Costs and Market Growth[END_REF] One of the main challenges remains to find materials that can handle fast charge as the goal of the electric vehicle industry is to manage to decrease the charge duration of the vehicles to enable their use during long journeys. Layered oxides LiMO 2 (M= Ni, Mn, Co) are part of the best candidates as positive electrode materials for LiBs. [START_REF] Noh | Comparison of the Structural and Electrochemical Properties of Layered Li[END_REF] Among all the possible compositions, the LiNi 0.6 Mn 0.2 Co 0.2 O 2 layered oxide (NMC622) was shown to be one of the best compromises to combine the beneficial effects of the Ni, Mn and Co for good energy density, structural and thermal stability and good cyclability respectively. [START_REF] Noh | Comparison of the Structural and Electrochemical Properties of Layered Li[END_REF] Many efforts have been made in the past twenty years to control the synthesis and properties of this material. [START_REF] Liang | Co-Precipitation Synthesis of Ni 0.6 Co 0.2 Mn 0.2 (OH) 2 Precursor and Characterization of LiNi 0.6 Co 0.2 Mn 0.2 O 2 Cathode Material for Secondary Lithium Batteries[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 0.6 Co 0.2 Mn 0.2 O 2[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] Ronduda | 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 0.6 Co 0.2 Mn 0.2 O 2 Cathode Material Synthesized in Different Sintering Conditions[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 0.6 Co 0.2 Mn 0.2 O 2[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] In order to optimize their electrochemical properties, different studies have been focused on preparing them with different morphologies (i.e. particle size or shape), especially as oriented or platelet-shaped particles in order to obtain materials that can handle fast charge. [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] Ju | Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial to Li-Ion Transport Kinetics with Layered LiNi 0.6 Co 0.2 Mn 0.2 O 2[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] Li | The Synergism of Nanoplates with Habit-Tuned Crystal and Substitution of Cobalt with Titanium in Ni-Rich LiNi 0[END_REF][START_REF] Zuo | Hydrothermal Synthesized Rugby-like LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathode Materials with Micro-Nano Structure for High Performance Li-Ion Batteries[END_REF][START_REF] Du | Radially Microstructural Design of LiNi 0.8 Co 0.1 Mn 0.1 O 2 Cathode Material toward Long-Term Cyclability and High Rate Capability at High Voltage[END_REF][START_REF] Oh | Superior Long-Term Energy Retention and Volumetric Energy Density for Li-Rich Cathode Materials[END_REF][START_REF] Hua | Unravelling the Growth Mechanism of Hierarchically Structured Ni 1/3 Co 1/3 Mn 1/3 (OH) 2 and Their Application as Precursors for High-Power Cathode Materials[END_REF][START_REF] Li | LiNi 1/3 Co 1/3 Mn 1/3 O 2 Nanoplates with {010} Active Planes Exposing Prepared in Polyol Medium as a High-Performance Cathode for Li-Ion Battery[END_REF][START_REF] Tian | High-Rate and Cycling-Stable Nickel-Rich Cathode Materials with Enhanced Li + Diffusion Pathway[END_REF][START_REF] Luo | LiMO 2 (M = Mn, Co, Ni) Hexagonal Sheets with (101) Facets for Ultrafast Charging-Discharging Lithium Ion Batteries[END_REF][START_REF] Fu | Synthesis of Single Crystalline Hexagonal Nanobricks of LiNi 1/3 Co 1/3 Mn 1/3 O 2 with High Percentage of Exposed {010} Active Facets as High Rate Performance Cathode Material for Lithium-Ion Battery[END_REF] Indeed, the platelet morphology tends to stabilize the {010} facets, that are the Li + ions diffusion pathways into the structure, so it enhances the alkali diffusion during cycling and increases the performances of the material. [START_REF] Ju | Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial to Li-Ion Transport Kinetics with Layered LiNi 0.6 Co 0.2 Mn 0.2 O 2[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] A further enhancement of the electrochemical performances has been observed when these platelets are radially oriented inside secondary particles as this particular morphology is the best one to maximize the access to the diffusion paths. [START_REF] Du | Radially Microstructural Design of LiNi 0.8 Co 0.1 Mn 0.1 O 2 Cathode Material toward Long-Term Cyclability and High Rate Capability at High Voltage[END_REF] Such morphologies are mostly obtained by coprecipitation [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] Du | Radially Microstructural Design of LiNi 0.8 Co 0.1 Mn 0.1 O 2 Cathode Material toward Long-Term Cyclability and High Rate Capability at High Voltage[END_REF][START_REF] Oh | Superior Long-Term Energy Retention and Volumetric Energy Density for Li-Rich Cathode Materials[END_REF][START_REF] Hua | Unravelling the Growth Mechanism of Hierarchically Structured Ni 1/3 Co 1/3 Mn 1/3 (OH) 2 and Their Application as Precursors for High-Power Cathode Materials[END_REF][START_REF] Luo | LiMO 2 (M = Mn, Co, Ni) Hexagonal Sheets with (101) Facets for Ultrafast Charging-Discharging Lithium Ion Batteries[END_REF][START_REF] Fu | Synthesis of Single Crystalline Hexagonal Nanobricks of LiNi 1/3 Co 1/3 Mn 1/3 O 2 with High Percentage of Exposed {010} Active Facets as High Rate Performance Cathode Material for Lithium-Ion Battery[END_REF] or hydrothermal synthesis [START_REF] Ju | Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial to Li-Ion Transport Kinetics with Layered LiNi 0.6 Co 0.2 Mn 0.2 O 2[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] Zuo | Hydrothermal Synthesized Rugby-like LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathode Materials with Micro-Nano Structure for High Performance Li-Ion Batteries[END_REF] followed by a solid-state reaction at high temperature. Regardless of the synthesis method chosen, the growth of platelet particles requires the use of a surfactant that will adsorb preferentially on specific facets of the particles and impede the growth of the particles in one particular direction. [START_REF] Hua | Unravelling the Growth Mechanism of Hierarchically Structured Ni 1/3 Co 1/3 Mn 1/3 (OH) 2 and Their Application as Precursors for High-Power Cathode Materials[END_REF] The most usual surfactants are cetyltrimethylammonium bromide (CTAB) [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] , sodium dodecyl sulfate (SDS) [START_REF] Ju | Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial to Li-Ion Transport Kinetics with Layered LiNi 0.6 Co 0.2 Mn 0.2 O 2[END_REF][START_REF] Zuo | Hydrothermal Synthesized Rugby-like LiNi 0.5 Co 0.2 Mn 0.3 O 2 Cathode Materials with Micro-Nano Structure for High Performance Li-Ion Batteries[END_REF] , polyvinylpyrrolidone (PVP) [START_REF] Tian | High-Rate and Cycling-Stable Nickel-Rich Cathode Materials with Enhanced Li + Diffusion Pathway[END_REF][START_REF] Luo | LiMO 2 (M = Mn, Co, Ni) Hexagonal Sheets with (101) Facets for Ultrafast Charging-Discharging Lithium Ion Batteries[END_REF][START_REF] Fu | Synthesis of Single Crystalline Hexagonal Nanobricks of LiNi 1/3 Co 1/3 Mn 1/3 O 2 with High Percentage of Exposed {010} Active Facets as High Rate Performance Cathode Material for Lithium-Ion Battery[END_REF] and ammonium hydroxide (NH 4 OH). [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] Du | Radially Microstructural Design of LiNi 0.8 Co 0.1 Mn 0.1 O 2 Cathode Material toward Long-Term Cyclability and High Rate Capability at High Voltage[END_REF][START_REF] Oh | Superior Long-Term Energy Retention and Volumetric Energy Density for Li-Rich Cathode Materials[END_REF][START_REF] Hua | Unravelling the Growth Mechanism of Hierarchically Structured Ni 1/3 Co 1/3 Mn 1/3 (OH) 2 and Their Application as Precursors for High-Power Cathode Materials[END_REF] NH 4 OH is a reactant commonly used during coprecipitation as a chelating agent but it was shown by Hua et al. that it could also play the role of a surfactant when it is used in large amount. [START_REF] Hua | Unravelling the Growth Mechanism of Hierarchically Structured Ni 1/3 Co 1/3 Mn 1/3 (OH) 2 and Their Application as Precursors for High-Power Cathode Materials[END_REF] In this study, NMC622 samples with platelet-shaped particles were synthesized using the NH 4 OH-assisted coprecipitation method. Their electrochemical performances and reactivity were studied and compared with particles with a more conventional morphology. Indeed, despite numerous studies were already performed on NMC622 with platelet morphology. The surface reactivity versus the electrolyte of NMC622 materials, depending on their morphology, still requires to be studied for an optimization of their performance in Li-ion batteries. The aim of this work is thus to get better insight into the relationship between the parti les' orp olo y of NMC622 aterials wit ontrolled o position and stru ture, t eir electrochemical performances and their reactivity in organic liquid electrolyte.

EXPERIMENTAL SECTION

The synthesis of Ni 0.6 Mn 0.2 Co 0.2 (OH) 2 hydroxide was made by coprecipitation using the same synthesis protocol to that described in one of our previous works, [START_REF] Soloy | Effect of Particle Size on LiNi 0.6 Mn 0.2 Co 0.2 O 2 Layered Oxide Performance in Li-Ion Batteries[END_REF] using a 0.36 mol/L NH 4 OH (Sigma-Aldrich, 28-30%) aqueous solution to obtain the transition metal hydroxide precursor of the NMC622 with conventional particles, and using a highly concentrated 5 mol/L NH 4 OH aqueous solution to obtain the transition metal hydroxide precursor of the NMC622 with platelet particles of different thicknesses. Figure S1 given in supplementary information shows the XRD patterns and SEM pictures of the mixed transition metal hydroxide precursors obtained in the two conditions. These mixed transition metal (TM) hydroxides are intimately mixed with lithium carbonate Li 2 CO 3 (Sigma-Aldri , ≥99%) to prepare three different lithiated transition metal oxides NMC622. A thermal treatment of the Ni 0.6 Mn 0.2 Co 0.2 (OH) 2 /Li 2 CO 3 mixture is performed at high temperature, under air, and with a Li/TM ratio of 1.05 (an additional 5 wt.% Li 2 CO 3 excess is added to ensure this ratio despite Li loss durin t e synt esis). T is sli t overlit iation (i.e. Li/TM ≥ 1.05) is re uired to obtain a layered structure with the smallest amount of Ni 2+ ions in the Li sites. Two platelet-type layered oxide samples obtained at 780°C (sample 17-PL) and 810°C (sample 86-PL) are compared to a conventional material obtained at 870°C (sample 870-5-air). [START_REF] Soloy | Effect of Particle Size on LiNi 0.6 Mn 0.2 Co 0.2 O 2 Layered Oxide Performance in Li-Ion Batteries[END_REF] Figure S2 shows how the XRD patterns and morphologies of the samples evolve with the temperature of the thermal treatment, when the mixed TM hydroxide is obtained from the highly concentrated 5 mol/L NH 4 OH solution. This preliminary study revealed that the higher the temperature, the less 2D is the layered structure (as shown for instance by the decreasing (003)/(104) line intensity ratio [START_REF] Soloy | Effect of Particle Size on LiNi 0.6 Mn 0.2 Co 0.2 O 2 Layered Oxide Performance in Li-Ion Batteries[END_REF] ) and the thicker are the platelets. These results explain the temperatures chosen for the thermal treatment to obtain the platelet-like layered oxide samples, i.e. 780°C and 810°C.

The cationic composition of the lithiated layered oxides was determined by Inductively Coupled Plasma -Optical Emission Spectrometry (ICP-OES) (Varian ICP/OES 720 ES) and the carbon content of the powders was determined using a CHNS elemental Thermo Fischer Scientific analyser equipped with an automatic sampler. The detail protocol has already been described in a previous work. [START_REF] Soloy | Effect of Particle Size on LiNi 0.6 Mn 0.2 Co 0.2 O 2 Layered Oxide Performance in Li-Ion Batteries[END_REF] To identify the phases contained in the samples and to determine the crystallographic structure of the layered oxides, powder X-Ray diffraction (XRD) patterns were recorded using a PANalyti al X'Pert PRO MPD diffra to eter e uipped wit a Cu radiation X-ray tube, and the powder neutron diffraction (NPD) patterns using the high-resolution powder diffractometer D2B at Institut Laue Langevin (ILL, Grenoble, France). The XRD patterns were collected in the angular range of 10-80°(2θ) wit 0.008° intervals, for a total a uisition time of around 35 hrs, whereas the NPD patterns were collected at room temperature with a wavelen t λ = 1.5947(1) Å, in the angular range of 0-160° (2θ) wit 0.05° intervals, for a total acquisition time of around 5 hrs. For NPD the powders were contained in a vanadium tube with a diameter of 8 mm. The collected XRD and NPD patterns were refined by the Rietveld method using FullProf software. [START_REF] Rodriguez-Carvajal | Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction[END_REF] T e pea s' profiles were des ribed 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. 25 XRD patterns were also collected to study structural changes in electrode materials recovered from coin cells after cycling, using a Philips PW1820 diffractometer equipped with a Cu radiation X-ray tube. The electrodes were fixed with a Kapton tape in a hygroscopic sample-holder made of aluminium alloy and Kapton windows. The XRD patterns were collected in the angular range of 5-80°(2θ) wit 0.02° intervals, for a total a uisition ti e of around 10 hrs.

The particles morphology and size were observed by scanning electron microscopy (Hitachi S4500 FEG), after being metallized by gold deposition. Transmission electron microscopy (JEOL JEM-2100) and Selected Area Electron Diffraction (SAED) were performed to get information on the nature of the faces. BET surface areas were determined using a Micromeritics 3FLEX analyser, after degassing the powders under vacuum at 90°C for 1 hr and then at 150°C for 10 hrs. X-ray Photoelectron Spectroscopy (XPS) has been used to determine the surface composition of the pristine powders using a Kratos Axis Ultra DLD spectrometer, of the pristine electrodes using an ESCALAB 250 Xi spectrometer, and of cycled electrodes using a Thermo Scientific K-Alpha spectrometer. A monochromatized Al-Kα X-Ray source (hν = 1486.6 eV) has been used with a neutraliser gun to minimize surface charging. For the pristine powders, the spectral calibration was carried out using the C-C/C-H component (285.0 eV) originated from the usual surface carbon contamination in C1s core peaks. For the cycled electrodes, it was carried out using the CF 2 component (291.0 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 surfa e o position was perfor ed usin basis of S ofield's relative sensitivity fa tors corrected by specific values depending on the 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.

Auger Electron Spectroscopy (AES) and Scanning Auger Mapping (SAM) were carried out with a JEOL JAMP 9500F Auger Microprobe spectrometer (JEOL Ltd, Tokyo, Japan) working under UHV conditions (pressure < 2 .10 -7 Pa). The UHV equipment was a Schottky field emission Auger electron spectrometer (FE-AES) dedicated to very high spatial resolution analysis and high brightness. The hemi-spherical electron analyzer coupled with a multichannel detector (7 channeltrons) offered ideal settings for energy resolved Auger analysis. SEM images, SAM images and Auger spectra were recorded using an acceleration voltage of 10 keV, a probe current of 3 nA and a working distance at 23 mm (sample tilt angle = 45°). Spectra (kinetic energy from 0 to 1000 eV) were carried out at a constant retarding ratio (CRR) mode with dE/E = 0.5% (high sensibility). The reference spectra of MnO, LiF and CoO were taken from the JEOL standard materials reference database. The SAM images were performed in constant retarding ratio (CAE) operating conditions to enable defining the useful energy width needed to obtain a significant peak background-intensity difference with respe t to t e Au er transition and t e ba round s ape. An "auto probe tra in " orre tion was also used to control and compensate the drift due to the balance of sample surface charge effects and instrument-dependent fluctuations.

Evaluation of the electrochemical performances was done in CR2032-type coin cells versus a lithium metal foil as negative electrode. The positive electrodes were prepared by coating a slurry on an aluminium foil as current collector. The slurry was prepared by mixing the active aterial wit t e arbon bla and t e polyvinylidene fluoride (PVD , Si a, avera e Mw ≈ 534000 g/mol by Gel Permeation Chromatography (GPC)) binder (92:4:4 wt.%) in N-methyl-2-pyrrolidone (NMP) (Sigma Aldrich, 99%). The electrolyte was 1 mol/L LiPF 6 dissolved in a mixture of dimethyl carbonate (DMC) and fluoro ethylene carbonate (FEC) in a 70/30 volume ratio. The coin cells were assembled in an argon-filled glovebox and were cycled using a VMP (Biologic) in galvanostatic mode. Galvanostatic Intermittent Titration Technique (GITT) experiments were performed at a constant current pulse of C/100 for 30 min and then rest for 15 hrs to stabilize the cell voltage between 2.5 and 4.6 V. Coin cells were dismantled after cycling in an Ar-filled glovebox to recover the electrodes, which were washed 3 times with an excess of DMC and dried under vacuum.

RESULTS AND DISCUSSION

Structural and physico-chemical characterizations

Figure 1 depicts the XRD patterns collected for three different NMC622 layered oxides, two of them with platelet-type morphologies (17-PL and 86-PL) as it will be shown hereafter and the latter with a conventional morphology (870-5-air). The three samples appear from XRD as pure layered oxides; all the diffraction lines can be indexed in an -NaFeO 2 -type layered structure described in the R-3 spa e roup. T e pea s' widt s are si nifi antly lar er for t e platelet-type materials 17-PL and 86-PL, reflecting smaller crystallites compared to the conventional material 870-5-air, as shown by the larger Full Width at Half Maximum (FWHM) values reported in Table 1 for the (003), ( 104) and (110) peaks. The I (003) /I (104) intensity ratios for the three materials are also reported in this table. The lower I (003) /I (104) ratio obtained for the 86-PL sample reflects a less 2D layered structure compared to the two other materials. The experimental stoichiometry in transition metals, as determined by chemical analyses performed by ICP-OES, is close to the expected one for the three samples (Table S1). The slight overlithiation required to obtain the layered structure is obtained for the three materials with Li/TM ratios larger than 1. As in one of our previous works, we notice that the Li/TM ratio varies from 1.00 to 1.10 between the three samples whereas the same Li/TM ratio was used for the synthesis and the loss of Li does not seem to be linear with the increasing temperature of the thermal treatment. [START_REF] Soloy | Effect of Particle Size on LiNi 0.6 Mn 0.2 Co 0.2 O 2 Layered Oxide Performance in Li-Ion Batteries[END_REF] 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] This amount of carbon does not evolve continuously with the thermal treatment temperature nor the Li/TM ratio.

In the layered oxide type structures, it is known that some transition metal ions can be present in the lithium site, reducing the 2D character of the material. [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] In such case, the amount and the nature of the transition metal can be determined by neutron diffraction. In Ni-rich layered oxides such as NMC622, this transition metal was shown to be Ni 2+ ions. [START_REF] Guilmard | Structural and Electrochemical Properties of LiNi 0.70 Co 0.15 Al 0.15 O 2[END_REF][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] A combined structural refinement based on X-ray and neutron diffraction data has been undertaken to determine the cation distribution between the Lithium site (i.e. within the interslab spaces) and the transition metal site (i.e. within the slabs). Figure S3 shows, as an example, the results obtained for the 17-PL sample, whereas Table S2 gathers the structural parameters. A small amount of Ni 2+ ions (2.7%) is found to be present in the Li-3b site. Table 1 compares the lattice parameters and the cationic distributions determined by the Rietveld refinement for the three NMC622 layered oxides studied in this work. The proposed cationic distributions take into account the charge balance, the presence of Ni 2+ ions in the interslab space as evidenced by the neutron diffraction and preferential stabilization of trivalent and tetravalent states for the Cobalt and Manganese ions respectively in these structures and synthesis conditions. [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] It reveals, for the 86-PL sample for example, that 5.2% Ni 2+ ions occupy the Li sites, inducing a theoretical capacity of 273 mAh/g ( ) corresponding to 0.948 Li + ions exchanged per formula unit. The 870-5-air sample consists of primary particles of 280 nm of diameter in average while the two other samples consist of platelet-shaped particles of 17 nm and 86 nm of thickness in average at 780°C and 810°C respectively (Figure 2) and 213 nm and 274 nm of length in average (Figure S4). When lithiations are performed at higher temperatures, the thickness of the platelets is increased while their length evolve less significantly, in accordance with Fu et al. works. [START_REF] Fu | Synthesis of Single Crystalline Hexagonal Nanobricks of LiNi 1/3 Co 1/3 Mn 1/3 O 2 with High Percentage of Exposed {010} Active Facets as High Rate Performance Cathode Material for Lithium-Ion Battery[END_REF] As reported in Figure 2, an increase of the specific surface area is observed when t e parti les' orp olo y is odified fro onventional s apes to t in platelets, fro 3 to 12 m 2 /g.

To confirm the nature of the crystallographic facets of these platelets selected area electron diffraction (SAED) was performed, the example of the 17 nm thick platelets (17-PL) is given in Figure 3. Figure 3a

Surface analyses

To get a better knowledge of the chemical composition of the surface of the materials, analyses by XPS were performed. Figure 4 compares the C1s, Li1s and O1s spectra obtained for the three samples.

From the CO 3 2-component observed at 290.1 eV in the C1s spectra and that of lithium in a carbonate at 55.5 eV in the Li1s spectra, the presence of Li 2 CO 3 at the surface of the particles has been identified and quantified. The results are reported on Figure 4 for the three materials and the complete quantitative analysis is given in Table S3 for the 870-5-air sample. We can note that the contribution of oxygen from the NMC622 material (at 529.7 eV) in the O1s spectra is larger for the platelet-type materials (~ 10.8 ± 0.4 at.%) than for the 870-5-air one (3.2 at.%) with conventional particles. From these results it appears that the platelet-shape particles have less lithium carbonate at their surface than the conventional particles. Ni3p, Co3p, Li1s and Mn3p core peaks have been identified for which the peak maxima are located at 67.8 eV, 61.3 eV, 55.4 eV and 50.0 eV. On the basis of literature, the Ni3p core peak at 67.8 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 Lithium-Ion Battery[END_REF] Even if the spectra corresponding to Ni, Co and Mn contributions are not very intense due to the presence of the carbonate layer, the quantification of these elements leads to a surface composition similar to that of the bulk, with transition metal stoichiometry varying between 6:2:2 and 6:1:3 (Table S4). Note that we used for this quantitative analysis the 3p core peaks since they are in the same binding energy range (45-80 eV) as opposed to the 2p core peaks.

As XRD measurements did not show the presence of any impurity and as only a very small amount of carbon (< 1 wt.%) was detected by gas phase chromatography, we believe Li 2 CO 3 detected by XPS is only located as a thin layer at the extreme surface of the particles. No obvious relationship exists between the quantity of Li 2 CO 3 determined by XPS and, on one side, the amount of Ni 2+ ions present in the interslab space as determined for these three NMC622 materials by Rietveld refinement and, on the other side, their specific surface area.

Nevertheless, the difference appears clearly between the conventional and the platelet-shaped particles showing {001} crystallographic oriented facets (Figure 4). As shown by Quesne-Turin et al., {001} facets of Li 2 MnO 3 layered oxide preferentially consists, for the last layer, of transition metal slabs as terminal slabs rather than of Li ions interslabs. [START_REF] Quesne-Turin | Surface Reactivity of Li 2 MnO 3 : First-Principles and Experimental Study[END_REF] Similarly, we can assume that the {001} facets of our materials mainly consist of transition metal slabs. In the case of platelets with extended {001} facets, Li + ions would thus be less exposed to ambient air than for conventional particles so the formation of Li 2 CO 3 is reduced, in good agreement with a lower amount of Li 2 CO 3 at the surface of the platelets. It was shown by theoretical calculations on NMC compounds of different compositions that CO 2 would adsorb easily on (012) facets, that are the facets making an angle of 19° with the (010) facets, leading to the enhanced formation of carbonates. [START_REF] Fang | Formation of Surface Impurities on Lithium-Nickel-Manganese-Cobalt Oxides in the Presence of CO 2 and H 2 O[END_REF] Despite large carbonate contributions, intense signals coming from oxygen atoms in the NMC network (Figure 4) and transition metals (Table S4) are detected for the platelets. This induces that Li 2 CO 3 may be organised in clusters, as nanoparticles at the surface of the platelets, probably mostly located on the more reactive lateral facets, without a uniform covering of the surface of the primary particles.

Electrochemical analyses

These samples were electrochemically tested in galvanostatic mode in CR2032 type coin cells versus Li. Figure 5 shows the charge-discharge curves obtained for the three materials during the 1 st cycle performed at the cycling rate of C/10 (1 charge or 1 discharge in 10 hrs) in the 2.5-4.3V potential range and Figure S5 shows the corresponding dQ/dV=f(V) and V=f(x Li )

curves. The inset in Figure 5 shows the evolution of the discharge capacities for the three materials when cycled at different rates (C/ [START_REF] Ju | Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial to Li-Ion Transport Kinetics with Layered LiNi 0.6 Co 0.2 Mn 0.2 O 2[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] . This difference may probably be explained by the organization of the agglomerates: in references [START_REF] Ju | Surfactant-Assisted Synthesis of High Energy {010} Facets Beneficial to Li-Ion Transport Kinetics with Layered LiNi 0.6 Co 0.2 Mn 0.2 O 2[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] the platelets are radially organised within spherical agglomerates so that the {010} faces are exposed to the electrolyte promoting fast Li + diffusion through the interslab spaces, [START_REF] Du | Radially Microstructural Design of LiNi 0.8 Co 0.1 Mn 0.1 O 2 Cathode Material toward Long-Term Cyclability and High Rate Capability at High Voltage[END_REF] whereas in our case the platelet type particles are randomly distributed in the agglomerates decreasing the benefit of this particular morphology of the primary particles versus fast diffusion within the layered structure. Other cycling tests were performed at a C-rate of C/5 (1 charge/discharge in 5 hrs) in a larger potential window of 2.5-4.6 V: the higher cut-off voltage potential was accompanied with an increase of the cycling rate in order to mitigate irreversible degradation of the electrolyte above 4.3 V vs. Li + /Li. In these cycling conditions, the material made of 17 nm thick platelet type particles delivers the largest 1 st charge and 1 st discharge capacities among the three materials (Figure 6 and Table 2). But it reveals also the lowest capacity retention compared to the materials made of conventional particles (870-5-air) and of thicker platelet type particles (86-PL), as shown in inset in Figure 6. In addition, as clearly observed in Figure 6 and on the derivative curve dQ/dV given in Figure S6a in supplementary information, an irreversible oxidation is observed at the end of the first charge between 4.4 and 4.6 V vs.

Li + /Li for the material 17-PL with the thinner platelet type particles, this irreversible reaction inducing a large irreversible 1 st discharge capacity (IRC) of 28% (Table 2). As seen by the evolution of this dQ/dV derivative curve upon cycling (Figure S7), this phenomenon is actually partly reversible with a small contribution in discharge at 4.53 V at 1 st cycle and 4.47

V at 2 nd cycle, and it disappears in the following cycles. Furthermore, an increasing overpotential is observed upon charge with a continuous shift towards higher potentials of the oxidation peak at 3.67 V for the 2 nd cycle and at 3.72 V for the 40 th cycle. Similarly, in discharge, the reduction peak observed at 4.25 V for the 2 nd cycle shifts to 4.15 V for the 40 th cycle. This irreversible oxidation could be associated to a drastic degradation of the electrolyte during the 1 st charge promoted by the high specific surface area of the 17-PL material (12 m 2 /g versus less than 4 m 2 /g for the two others), whereas the gradual formation of a resistive layer at the electrode/electrolyte interface would increasingly impede diffusivity of Li + ions within the structure and suppress this phenomenon in the following cycles. The reversible part of the phenomena could be linked to the H2H3 phase transition occurring at this potential for NMC622. [START_REF] Jung | Oxygen Release and Its Effect on the Cycling Stability of LiNi x Mn y Co z O 2 (NMC) Cathode Materials for Li-Ion Batteries[END_REF] To better understand the behaviour observed for the 17 nm thick platelets (i.e. better performance in the 2.5-4.3 V potential window, whereas irreversible reaction at high voltage before fast performance degradation in the 2.5-4.6 V potential range), a comparative characterization of the three materials was performed upon cycling: by XRD to check for structural evolutions and by XPS to identify surface reactivity. In order to be as close as

Electrode/electrolyte interface analyses

Cycling was performed in an electrolyte consisting of LiPF 6 in a mixture of DMC and FEC.

As it was demonstrated that the degradation products of the salt are mostly LiF and Li x PF y O ztype species while carbonaceous and oxygenated species are generated from the decomposition of the solvents, [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] at.% to 10 at.% for the whole O1s signal) due to the presence of oxygenated species coming from the electrode formulation and thus to a partial coverage of the active electrode material.

At first, the electrode prepared with platelets material contain, at the surface, more binder and CB that the electrode prepared with the traditional morphology material. Moreover, at the surface of 870-5-air electrode, we note a large content of lithium carbonate, as already detected in the as-prepared material.

After cycling, an evolution in the core peaks spectra is observed. First, we describe the new environment detected by XPS whatever the cycling step. In the F1s spectra, a third component (in dark blue) appears at 686.6 eV associated to the creation of Li x PF y O z species. A huge increase of the LiF component (in dark green) is also observed at the OCV step. Those species coming from the electrolyte salt decomposition induce the increase of the LiF/Li x PF y O z components (in dark green) at 56.2 eV in the Li1s spectra and at 685.6 eV in the F1s spectra.

In the O1s spectra, we observe an enlargement of the peak due the apparition of oxygen component (located at 534.4 eV) associated to the fluorophosphates species and a large increase of the component at 532.5 eV associated to the C-O environment of the DMC and FEC solvents. [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] The component assigned to the NMC oxygen atoms at 529.6 eV is still present along the first cycle. The area of the peak located at 531.5 eV is increasing as it includes oxygen function linked to solvent degradation like CO 2 or O=C species. In the C1s spectra, the solvents degradation generates a clear increase of the contributions at 285.0 eV, 286.5 eV, 288.9 eV assigned to C-C/C-H, C-O, and CO 2 environments respectively, thus confirming the attributions previously made.

Quantitative analyses were performed to estimate the surface composition of those electrodes as a function of the state of charge in order to study the reactivity of the materials versus the electrolyte depending on their morphology. The results obtained from the quantitative surface analysis of the 870-5-air sample (conventional particles) recovered at 4.3V vs. Li + /Li at the end of the 1 st charge are given in Table S5 as an example. The overall quantitative analyses are reported in Table S6, Table S7 and Table S8 and summarized in Figure 9 First, the pristine electrodes are compared to the OCV electrodes. For the two platelets materials, the decrease of carbon black and PVDF contributions after OCV step (from 31.7 at.% to 7.6 at.% and from 27.3 at.% to 12.8 at.% respectively for the 86-PL sample as example) while keeping roughly the same proportion of active material suggests a selective coverage of electrodes surface by the degradation products from the moment they are exposed to the electrolyte. Especially the important increase of LiF/Li x PF y O z and carbonaceous species proportions (from 2.5 at.% to 15.8 at.% and from 34.0 at.% to 61.0 at.% respectively for the 86-PL sample for example) is in accordance with this observation. On the other hand, to identify a possible diffusion of chemical species especially fluorine within the particle aggregates, AES analyses (Figure S12) were performed on a cross cut section of this sample. Mapping (SAM) elemental maps of the cross cut section were also obtained (Figure S12d).

As shown on

The SAM images, especially the Ni/C overlaid map, show a good junction between the active NMC material and the matrix after cycling. Moreover, a low but significant fluorine detection (in red) on the particles is also displayed on the F map and Ni/F overlaid map in good agreement with the previous observation of AES spectra. For the 870-5-air sample (conventional particles), the electrode surface reactivity is completely different as the at.% of CB and PVDF are not decreasing in the same manner than for the platelets samples. Thus, the degradation products are covering a major part of the active material. Indeed the proportion of PVDF contributions even increases from 14.4 at.% to 24.0 at.% after OCV step reflecting the uncovering of some parts of the electrode. At the same time, a significant decrease of the carbonaceous species proportion is observed from 50.0 at.% to 25.3 at.% (Table S6 Those results also show that the exposition of larger {010} facets by increasing the thickness of the platelets, enables to minimize the proportion of {001} facets leading to a lower reactivity of the material versus the lithium salt and the solvents, so a lower amount of degradation products is generated at the electrode/electrolyte interface compared to the 17 nm platelets and the conventional particles. We can also conclude that the SEI is less than 10 nm thick or does not cover completely the surface as XPS signal associated to NMC material is still detectable at the end of the 1 st cycle.

Structural analysis after cycling

XRD analysis was also performed ex situ on electrodes recovered after cycling at the same states of charge during the 1 st cycle to compare the structural changes occurring for the three samples. The XRD patterns collected for the 870-5-air sample (with conventional particles), the 86-PL sample (86 nm thick platelets) and the 17-PL (17 nm thick platelets) are gathered in Figure S13, Figure S14 and Figure S15 respectively. If no particular evolution, versus those already reported in literature for NMC622 type layered oxides, is observed for the conventional particles (870-5-air) and the 86 nm thick platelets (86-PL), additional reflections clearly appear at the end of charge for the 17 nm thick platelets (17-PL) at 19.9°, 41.8° and 55.4°(2θ) (Figure S15). Figure 10a-c focus on the 17-21°(2θ) an ular ran e w ere t e diffraction lines corresponding to the c hex parameter are observed. Indeed, this cell parameter is very sensitive to structural variations occurring during cycling, as the amount of Li + ions present within the structure controls changes in the iono-covalency of the M-O bonds in the MO 2 slabs and the electrostatic repulsions between the oxygen layers from both sides of the interslab spaces. [START_REF] Ohzuku | Solid-State Redox Reactions of LiCoO 2 (R3m) for 4 Volt Secondary Lithium Cells[END_REF][START_REF] Reimers | Electrochemical and In Situ X-Ray Diffraction Studies of Lithium Intercalation in Li x CoO 2[END_REF] As reported in literature, [START_REF] Jung | Oxygen Release and Its Effect on the Cycling Stability of LiNi x Mn y Co z O 2 (NMC) Cathode Materials for Li-Ion Batteries[END_REF][START_REF] Tian | Depth-Dependent Redox Behavior of LiNi 0.6 Mn 0.2 Co 0.2 O 2[END_REF][START_REF] Quilty | Probing Sources of Capacity Fade in LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622): An Operando XRD Study of Li/NMC622 Batteries during Extended Cycling[END_REF][START_REF] Croguennec | NiO 2 Obtained by Electrochemical Lithium Deintercalation from Lithium Nickelate: Structural Modifications[END_REF][START_REF] Croguennec | Structural Characterisation of New Metastable NiO 2 Phases[END_REF] during delithiation the layered oxide NMC622 undergoes structural changes with first the pristine phase H1 for compositions in Li richer than 0.6 and whose structures are described in the R-3m space group, then the phase H2 isostructural to H1 but with a smaller a hex. parameter and a larger c hex. parameter, and finally for lithium content lower than 0.3 the phase H3 isostructural to H1 and H2 but with an even smaller a hex. parameter and a contracted c hex. parameter. The a hex. parameter decreases continuously all along Li + deintercalation as the transition metal ions are oxidized leading to a higher covalency of the M-O bonds and thus to a decrease of the M-M (= a hex ) distance. The c hex. parameter is the sum of the slab and interslab thicknesses, the first decreases continuously upon Li + deintercalation as the M-O bond becomes more covalent and thus shorter, and the second increases first due to the electrostatic repulsions between the oxygen layers before decreasing due a higher covalency of the M-O bonds and thus a lower negative partial charge on the oxygen anions. The c hex. parameter thus increases first before decreasing.

The phase diagram and crystallographic parameters of the phases identified at each state of charge or discharge were determined from Le Bail refinements of all the XRD data collected ex situ. The evolution of the c hex. parameter as function of the content in lithium is illustrated in Figure 11, the Li content being that associated to the electrochemical experiment and to the number of electrons exchanged. For the materials obtained at 3.8 and 4.3 V (lithium content in the range of about 0.8 to 0.3), whatever the morphology of the layered oxide NMC622, two phases are needed to refine the XRD data. The coexistence of the H1 and H2 phases can be deduced from the chex. values, of 14.16-14.22 and 14.30-14.36 Å for H1 and H2 respectively.

For the electrode materials recovered at 4.6 V, one H2 phase is needed for the 870-5-air while two and three phases are needed to explain the XRD data of the 86-PL and the 17-PL material respectively (Figure 10d-f). The new phase, denoted H4, has a much smaller c hex. parameter of 13.40 Å compared to 13.79 Å for the H3 phase. This type of phase was already observed for the close-to-the-stoichiometry LiNiO 2 and appears only when the amount of Li and extra Ni ions present in the Li site is low as the formation of the phase H4 requires slab gliding to stabilize an AB AB type oxygen packing (S.G P-3m1) from the AB CA BC one observed for H1, H2 and H3, as described in Figure 11. [START_REF] Croguennec | NiO 2 Obtained by Electrochemical Lithium Deintercalation from Lithium Nickelate: Structural Modifications[END_REF][START_REF] Croguennec | Structural Characterisation of New Metastable NiO 2 Phases[END_REF] The diffraction lines broadening observed, especially at high voltage, in comparison with that of the pristine XRD can be explained by the presence of 2 or 3 layered phases at the same time within the particles. These particles are intergrowths of domains of limited sizes showing different Li contents and possibly stacking (O3 vs O1). As highlighted in Figures S14-S15, these phase transitions are reversible, with values of c hex parameter obtained at end of discharge between 14.10 Å and 14.21 Å, close to the ones obtained for the pristine electrodes between 14.21 Å and 14.23 Å (see Figure 11).

The 17 nm thick platelets with less than 2.7% Ni 2+ ions in excess in the Li sites as determined by Rietveld refinement (Table 1) are prone to favor easier Li + ions diffusion and formation of highly deintercalated compositions, and thus stabilization of the phase H4 with an O1-type stacking. The more drastic c hex parameter drop is observed for the 17 nm thick platelets (17-PL), versus that observed for the 86 nm thick platelets (86-PL), with the apparition of the H4 phase: it can be partly explained by the lower amount of Ni 2+ ions present in the Li sites, i.e.

2.7% for 17-PL versus 5.2% for 86-PL as determined by Rietveld refinement (Table 1).

Indeed, the Ni ions present in the interslab spaces play the role of pillars preventing the collapse of the interslab space and the slab gliding required to form the O1 stacking from the O3 one. [START_REF] Croguennec | Structural Characterisation of New Metastable NiO 2 Phases[END_REF] However, it is worth mentioning that the conventional particles with only 3.4% Ni ions in excess in the Li sites, versus 5.2% in the 86 nm thick platelets (86-PL), reveal limited Li deintercalation ratio: at the end of the charge only the phase H2 is observed whereas H3 and/or H4 are observed for the platelets morphologies, inferring that the morphology plays a significant role. These results show that those phase transitions are also dependent on the morphology. O3 oxygen stacking associated to H1, H2 and H3 phases and O1 oxygen stacking associated to the H4 phase are also displayed.

Discussion

Three NMC622 layered oxides with different morphologies were studied in this work: one sample with 280 nm primary particles of conventional morphology and two other samples with platelet-shaped particles of 17 nm and 86 nm thickness. The three materials have a layered structure close to the ideal 2D layered structure, with less than 5.2% Ni 2+ ions in the Li site and the desired composition in transition metals and lithium, both in the bulk and at the surface. Only differences in surface composition were observed with lower amounts of Li 2 CO 3 at the surface of the platelets thanks to a higher proportion of less reactive surfaces to ambient air (see Figure 4). The three materials are then very similar in structure and in chemical composition, their main difference being their morphology. Cycling tests were performed on those three materials to investigate the influence of these different morphologies on the electrochemical performances. The results obtained from the analyses performed after cycling up to 4.6V in GITT conditions helped us to explain the electrochemical performances observed.

Phase transitions and surface composition depending on the delithiation rate/potential

For cyclings performed at different rates from C/10 to 2C in the 2.5-4.3 V potential window, the 17 nm platelets showed slightly better performances than the two other morphologies but the difference with the performances of the conventional particles was not significant, especially at high rates (see Figure 5). In the 2.5-4.3 V potential window, moderate delit iation ratios (X ≈ 0.34-0.38 in Li X (Ni 0.6 Mn 0.2 Co 0.2 )O 2 at C/10 at 1 st cycle, see Figure S5b) are reached so no significant structural changes are expected to occur in the materials and, as observed in the electrochemical tests, no significant capacity loss was observed for the three materials, whatever the cycling rates. Cyclings were also performed in a larger potential window (2.5-4.6 V) at C/5 to study the behaviour of those materials when reaching higher delit iation ratios (X ≈ 0.15-0.26 in Li X (Ni 0.6 Mn 0.2 Co 0.2 )O 2 at C/5 at 1 st cycle, see Figure S6b) that may be critical for the stability of the material during cycling. Indeed, at such potential/delithiation ratios, phase transitions in the layered phase domain such as H2H3 are known to be harmful for the structure and the cyclability of the material because of pronounced c hex parameter drop. [START_REF] Jung | Oxygen Release and Its Effect on the Cycling Stability of LiNi x Mn y Co z O 2 (NMC) Cathode Materials for Li-Ion Batteries[END_REF][START_REF] Tian | Depth-Dependent Redox Behavior of LiNi 0.6 Mn 0.2 Co 0.2 O 2[END_REF][START_REF] Quilty | Probing Sources of Capacity Fade in LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622): An Operando XRD Study of Li/NMC622 Batteries during Extended Cycling[END_REF][START_REF] Croguennec | NiO 2 Obtained by Electrochemical Lithium Deintercalation from Lithium Nickelate: Structural Modifications[END_REF][START_REF] Croguennec | Structural Characterisation of New Metastable NiO 2 Phases[END_REF] In those cycling conditions, as observed in Figure 6 and Table 2, the platelets showed higher 1 st discharge capacities than the conventional particles but the 17 nm platelets showed very reduced capacity retention caused by an irreversible oxidation occurring at the end of the 1 st charge between 4.5 and 4.6 V. The 86 nm platelets and the conventional particles showed similar capacity retentions. As shown in

Relation between the phase transitions and the surface composition

The more intense volume changes observed for the 17 nm thick platelets (17-PL) as well as the larger amount of LiF/Li x PF y O z resistive species found at their surface must explain the lower capacity retention observed for the 17 nm platelets (17-PL) when cycled at C/5 in the 2.5-4.6 V potential window (Figure 6). Contrary to the GITT experiments, the H3H4 phase transition is not expected to intervene in the C/5 cycling conditions because the delithiation ratios reached are not high enough to allow such phase transition. [START_REF] Croguennec | NiO 2 Obtained by Electrochemical Lithium Deintercalation from Lithium Nickelate: Structural Modifications[END_REF][START_REF] Croguennec | Structural Characterisation of New Metastable NiO 2 Phases[END_REF] However, the H2H3 phase transition is very likely to occur, concomitantly with oxygen release due to the stabilization of the M 4+ ions into M 2+ ions. [START_REF] Jung | Oxygen Release and Its Effect on the Cycling Stability of LiNi x Mn y Co z O 2 (NMC) Cathode Materials for Li-Ion Batteries[END_REF] This oxygen release would be limited to the nearsurface region because of the limited diffusion length of oxygen anions in NMC particles at 25°C. [START_REF] Jung | Oxygen Release and Its Effect on the Cycling Stability of LiNi x Mn y Co z O 2 (NMC) Cathode Materials for Li-Ion Batteries[END_REF] This could be associated to the irreversible oxidation observed for the 17 nm platelets at end of charge in Figure 6 and Figure S6. As the surface area is 3 times larger for the 17 nm thick platelets (17-PL) than for the 86 nm thick platelets (86-PL), it would explain why this surface phenomenon is significantly observed only for the 17 nm thick platelets (17-PL) in the C/5 cycling. This oxygen loss in the near-surface region inevitably accompanies with the transformation of the layered structure to spinel and/or rocksalt structure, although those phases were not observed on the XRD patterns obtained after cycling, since they must be limited to a few atomic layers. As this layer forms, the amount of oxygen released decreases because of the limited diffusivity of oxygen anions in the lattice, the oxygen-depleted surface layer gradually stops forming and the active material/electrolyte interface stabilizes. As observed with the evolution of the dQ/dV curves of the 17-PL sample (17 nm platelets) for the C/5 cycling in Figure S7, the irreversible oxidation occurs mainly at 1 st cycle as the oxidation peak at 4.5-4.6 V disappears almost completely from the 2 nd cycle. This release of reactive oxygen may also be responsible for a more important degradation of the electrolyte by electrochemical oxidation. [START_REF] Jung | Oxygen Release and Its Effect on the Cycling Stability of LiNi x Mn y Co z O 2 (NMC) Cathode Materials for Li-Ion Batteries[END_REF] This degradation would create a large amount of degradation products and the lower surface energy of the {001} facets would allow the deposition of those products and facilitate their adsorption at their surface, like it does for specific solvents. [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][START_REF] Hua | Unravelling the Growth Mechanism of Hierarchically Structured Ni 1/3 Co 1/3 Mn 1/3 (OH) 2 and Their Application as Precursors for High-Power Cathode Materials[END_REF][START_REF] Fu | Synthesis of Single Crystalline Hexagonal Nanobricks of LiNi 1/3 Co 1/3 Mn 1/3 O 2 with High Percentage of Exposed {010} Active Facets as High Rate Performance Cathode Material for Lithium-Ion Battery[END_REF][START_REF] Xu | Tailoring Anisotropic Li-Ion Transport Tunnels on Orthogonally Arranged Li-Rich Layered Oxide Nanoplates Toward High-Performance Li-Ion Batteries[END_REF] This would explain the more important covering of the 17-PL (17 nm platelets) electrodes by the SEI, as shown in

Stability of the different facets during cycling

Those results infer that the 17 nm platelets (17-PL), with extended {001} facets, are more prone to undergo phase transitions and associated oxygen loss during cycling. This is in opposition with what was shown by Jung et al. [START_REF] Jung | New Insight into Microstructure Engineering of Ni-Rich Layered Oxide Cathode for High Performance Lithium Ion Batteries[END_REF] They studied doped LiNi 0.92 Mn 0.04 Co 0.04 O 2 platelet-shaped particles of different thicknesses, boron-doped particles of 74 nm thickness and phosphorus-doped particles of 144 nm thickness in average, and they showed that the 144 nm platelets were more prone to oxygen loss and phase transition during cycling compared to the thinnest ones. [START_REF] Jung | New Insight into Microstructure Engineering of Ni-Rich Layered Oxide Cathode for High Performance Lithium Ion Batteries[END_REF] From first-principle calculations, they revealed that the (102) planes, identified as the lateral planes of their particles, were more prone to undergo oxygen release compared to the (003) planes due to a lower oxygen vacancy formation energy. [START_REF] Jung | New Insight into Microstructure Engineering of Ni-Rich Layered Oxide Cathode for High Performance Lithium Ion Batteries[END_REF] However, they also showed that doping had modified the surface composition of their particles and made them less prone to oxygen loss, so the reactivity of the facets was very likely modified by the doping. Zhu and Chen studied NMC333 materials with different morphologies. [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] They compared platelet-like particles with extended {001} surfaces being ~100 nm thick and polyhedral particles of ~100 nm with a majority of {012} (lateral) surfaces. Despite the larger capacities delivered by the polyhedral particles, thanks to the more opened Li + ion diffusion pathways, they also emphasized the higher stability of the platelet-like particles with extended {001} facets, due to their lower surface energy making them less prone to Ni surface reduction, less reactive and more stable during electrochemical cycling. [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] However, in that case, their particles are closer to the morphology obtained for our 86 nm platelets and conventional particles, that showed to be more stable than our 17 nm platelets. Our 17 nm platelets, showing the most unstable behaviour during cycling, are mainly composed of {001} facets that are supposed to be among the most stable during cycling. The main difference between our samples and the ones described in these articles is that, our 17 nm platelets are much thinner so they generate a much larger amount of surfaces, as shown by the BET surface area measurements given in Figure 2. As the oxygen release phenomenon is observed at the surface, it could explain why oxygen release is more intense for this morphology despite the relative stability of the {001} facets compared to the others. The larger specific surface area may play a significant role here and must explain the relative instability of the 17 nm platelets compared to thicker platelets.

Strategies to overcome the exacerbated reactivity of the thin platelets

Increasing the thickness of the platelets to some extent, up to around 100 nm for example, may allow to alleviate the high reactivity of the thin platelets morphology without exposing a too large amount of lateral facets that were shown to be more reactive than the {001}. [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][START_REF] Jung | New Insight into Microstructure Engineering of Ni-Rich Layered Oxide Cathode for High Performance Lithium Ion Batteries[END_REF] Another efficient way could be also to increase the length of the platelets and to organize them radially into the agglomerates. Du et al. synthesized such NMC 811 particles organized radially into the secondary particles and revealed the important role played by the particles dimension and organization in the H2H3 phase transition at end of charge. [START_REF] Du | Radially Microstructural Design of LiNi 0.8 Co 0.1 Mn 0.1 O 2 Cathode Material toward Long-Term Cyclability and High Rate Capability at High Voltage[END_REF] This strategy allowed them to obtain slightly better capacities compared to randomly distributed particles with a 1 st discharge capacity of 222 mAh/g (versus 213 mAh/g for the randomly distributed particles) when cycling at C/5 in the 2.8-4.6 V potential window, their radially grown plateletlike particles giving the Li + ions ideally oriented pathways. They showed that long radiallyoriented platelet-like particles allowed to have gradually distributed Li + ions inside the diffusion channels, acting like a buffer at deep lithiation states, and thus delaying the H2H3 phase transition. This distribution alleviates the decrease of the c hex parameter, impedes the structure collapse at the particle level and the generation of important volume variations and microstresses caused by this phase transition occurring at deep delithiation state. This particular morphology allowed the particles to deliver slightly improved capacity retention with 85% (versus 82% for the randomly distributed particles) after 150 cycles at C/5 and 76% (versus 70%) after 200 cycles at 1C in the 2.8-4.6 V potential window. [START_REF] Du | Radially Microstructural Design of LiNi 0.8 Co 0.1 Mn 0.1 O 2 Cathode Material toward Long-Term Cyclability and High Rate Capability at High Voltage[END_REF] Those results show that platelets dimensions must be chosen carefully because they can have a strong impact on the electrochemical performances and the cycling stability. For cycling at regular potentials such as 4.3 V, thin platelets of 17 nm thickness and thicker platelets of 86 nm thickness can be used because no critical delithiation ratios are reached at this potential in regular cycling conditions. At higher potentials such as 4.6 V, thicker and longer platelets must be preferred in order to improve the structural stability of the particles upon cycling.

CONCLUSION

NMC622 samples with platelet-shaped particles were synthesized by NH 4 OH-assisted coprecipitation of a mixed transition metal hydroxide precursor followed by a solid-state reaction with Li 2 CO 3 at high temperature. Two samples with crystallographic structure close to the ideal 2D layered structure (less than 5.2% Ni 2+ ions in Li + sites) and with plateletshaped particles of 86 and 17 nm thickness were obtained. Those two samples were compared to a sample consisting of 280 nm particles with a regular morphology. Unexpectedly, the platelets didn't s ow better electrochemical performances than the conventional particles, the random orientation of the platelets in large micrometric agglomerates limiting the beneficial effect of oriented particles on the diffusion of Li + ions in the structure. The 17 nm platelets showed particular reactivity when cycled up to 4.6 V, with an irreversible oxidation occurring at end of the 1 st charge. GITT experiments and post cycling analyses were performed in order to explain this phenomenon. XPS analyses revealed an exacerbated degradation of the electrolyte salt versus the 17 nm platelets leading to a more important covering of the electrodes by the SEI. Ex situ XRD analyses after cycling revealed that the H2H3 and H3H4 phase transitions were favored at end of charge for the platelets because their crystallographic oriented growth enhances the Li + ions diffusion into the structure. Those structural changes are promoted when the platelet is thinner and are responsible for numerous anisotropic volume changes, causing lower capacity retention when cycling is performed at high potential. The smaller dimensions of the 17 nm platelets are also likely to enhance lattice oxygen release at the surface of the particles, which catalyzes the degradation of the electrolyte and explains the larger amount of degradation products found at the surface of the particles.

ASSOCIATED CONTENT

Figure 1 .

 1 Figure 1. X-ray diffraction patterns obtained for the platelet-type layered oxide samples

Figure 2

 2 Figure 2 compares scanning electron micrographs typical of the three samples and the

Figure 2 .

 2 Figure 2. On the left, morphologies of the 870-5-air (particles of conventional morphology

  shows two platelets placed on their lateral faces, showing a larger thickness than the average one, of around 70 and 50 nm. The grey circle highlights the area selected to perform electron diffraction. The direction identified in Figure3bcorresponds to the reciprocal c* direction, which demonstrates the thickness of the platelets is along the c direction and that the frontal plane of the particles is the (001) plane. This shows that the surfactant preferentially adsorbs on the (001) planes, stops the growth of the particle along the c axis and favours it in the (a,b) plane orthogonal to the c direction, as illustrated in Figure2and in accordance with the mechanism unravelled by Hua et al.[START_REF] Hua | Unravelling the Growth Mechanism of Hierarchically Structured Ni 1/3 Co 1/3 Mn 1/3 (OH) 2 and Their Application as Precursors for High-Power Cathode Materials[END_REF]

Figure 3 .

 3 Figure 3. TEM image (a) and Selected Area Electron Diffraction (SAED) (b) performed on

Figure 4 .

 4 Figure 4. C1s (a), Li1s (b) and O1s (c) XPS spectra collected for the 870-5-air (conventional

Figure 5 .

 5 Figure 5. Charge-discharge curves obtained during the 1st cycle for the conventional particles

  possible to the equilibrium and to promote all the irreversible reactions, coin cells were cycled in GITT conditions (slow C-rate of C/100 with intermediate relaxation periods) and were stopped at different states of charge and discharge during the 1 st cycle: at 3.8 V, 4.3 V, 4.6 V during the 1 st charge and at 2.5 V at the end of the 1 st discharge. The cycling curves obtained are shown in Figure S8, Figure S9 and Figure S10 respectively for the 870-5-air sample, the 86-PL sample and the 17-PL sample.

Figure 6 .

 6 Figure 6. Charge-discharge curves obtained during the 1 st cycle for the conventional particles

8 .Figure 4 .

 84 Figure 4. In the Li1s spectra, components associated to Li in the NMC material at 54.3 eV

and Figure S11 ,

 S11 as a comparison of evolution observed for the three NMC622 samples. In those tables and figures, the different components reported in the XPS spectra are gathered into 5 chemical groups in order to follow the electrolyte degradation and the electrode covering during cycling. The quantitative results are obtained as following for: LiF/Li x PF y O z species, sum of the atomic percentages of LiF and Li x PF y O z components from the Li1s, F1s, O1s and P2p spectra; for the active material, sum of the at.% of the Ni, Mn, Co, O NMC and Li NMC components in the O1s and Li1s spectra; for the carbon black (at.% of the as-named components in the C1s spectra); for PVDF, sum of the at.% of the PVDF components in C1s and F1s spectra; and for carbonaceous species, sum of the at.% of the remaining components (other than that of CB and PVDF) in C1s spectra, the remaining contributions in O1s spectra (different from O NMC and phosphate species) and the Li 2 CO 3 contribution in Li1s spectra.Only the most relevant values obtained from those quantitative analyses are discussed hereafter.

  S12a, AES spectra were recorded on 3 target dots on the material section shown on the SEM image of the Figure S12b. Spectra of dot 1 and dot 2 exhibit clearly visible Mn LMM (525-640 eV), Co LMM (620-780 eV) and Ni LMM (675-860 eV) transitions. In comparison, the spectrum of dot 3 targeting the PVDF matrix only displays a F KLL (620-660 eV) transition in the same energy range. For dot 1 and dot 2, due to the overlapping of the metal and fluorine transitions, attesting of the presence of fluorine on the NMC particle aggregates from the direct spectra is difficult. However, it is possible through the derivative experimental spectra of the dots 1 and 2 in Figure S12c. Such spectra have been well-fitted by the convolution of MnO, CoO, NiO and LiF reference spectra showing a significant contribution of fluorine in the experimental signals. In addition, Scanning Auger

Figure 7 .

 7 Figure 7. F1s (a-c) and C1s spectra (d-f) obtained from XPS analyses of the 870-5-air sample

Figure 8 .

 8 Figure 8. O1s (a-c) as well as Li1s, Ni3p, Co3p and Mn3p spectra (d-f) obtained from XPS

Figure 9 .

 9 Figure 9. LiF/Li x PF y O z species (a), carbonaceous species (b) and carbon black (c) surface

Figure 10 .

 10 Figure 10. (003) peak evolution obtained from the XRD patterns of the 870-5-air

Figure 11 .

 11 Figure 11. Evolution of the chex parameter as a function of the Li content in the layered

Figures S8 - 10 and Figure 11 ,

 S81011 Figures S8-S10, higher delithiation ratios are reached in GITT conditions at end of charge

Figure 9 .

 9 Moreover, a quick increase of the internal resistance is observed from the shift of the potential at the beginning of discharge reflecting the formation of a resistive surface layer that hinders the reintercalation of Li ions into the structure. This internal resistance increases quickly during the first cycles and less rapidly in the following cycles. Similarly, as observed in Figure6, the capacity drop of the 17 nm platelets (17-PL) is very fast during the 10 first cycles and less rapid afterwards, reflecting a stabilization of the surface of the material.

after 18 36 38 40 42 44 46 48 58 60 62 64 66 68 76 78 80 2q(°), lCu 870-5-air 17-PL 86-PL

  Note t at tra es of arbon (≈1 wt.%) were found by CHNS analyses, probably associated to the presence of Li 2 CO 3 coming either from the Li precursor used in excess during the synthesis or from the reaction of the material with air

	(003)
	(104)
	(a.u.)
	Intensity	(018)	(110)
			LiNiO2-type phase
			PDF 01-070-4310

Table 1 .

 1 Comparison of the lattice parameters, c/a ratios, compositions and cation distributions calculated from the Rietveld refinements of the X-ray diffraction patterns

		a (b) (Å)	c (Å)	c/a	(003)	FWHM (104)	(110)	I(003)/ I(104)	Composition and cation distribution
	86-PL	2.87248(9)	14.225(1)	4.9522(5)	0.16	0.21	0.25	1.52	(Ni 2+	(Li0.948(3)Ni 2+ 0.262(4)Ni 3+ 0.317(5)Mn 4+ 0.210(1)Co 0.052(3)) 3+	0.210(1))O2
	17-PL	2.86658(9)	14.208(1)	4.9564(5)	0.16	0.23	0.25	1.82	(Ni 2+	(Li0.973(3)Ni 2+ 0.232(4)Ni 3+ 0.357(5)Mn 4+ 0.205(1)Co 0.027(3)) 3+	0.205(1))O2
	870-5-air	2.86778(6)	14.2160(5)	4.9571(3)	0.08	0.12	0.14	1.81	(Ni 2+	(Li0.966(3)Ni 2+ 0.241(4)Ni 3+ 0.346(5)Mn 4+ 0.207(1)Co 0.034(3)) 3+	0.207(1))O2

Table 2 .

 2 As shown in FigureS5a, the polarization at 1 st cycle is very similar for the 3 materials. During the 1 st cycle, the material made of 17 nm thick platelets (17-PL) delivers a slightly larger 1 st discharge capacity at C/10 of 156 mAh/g with the lowest IRC of 9% compared to the two other materials. Furthermore, whatever the cycling rate, ranging between C/10 and 2C, it shows the best performance, even if the difference with those delivered by the material made of conventional shape particles (870-5-air) is reduced with increasing cycling rate. The benefit of the platelet morphology is thus not as important as what could have been expected considering previous results published byJiang et al. and Ju et al.

10, C/5, C, 2C and C/10 again, for 5 cycles at each rate) as a function of the cycle number. The 1 st discharge capacities and irreversible capacities (IRCs) are gathered in

Table 2 .

 2 1 st discharge capacities and irreversible capacities (IRC, %) obtained for the three NMC622 materials when cycled at C/10 in the 2.5-4.3 V potential range and at C/5 in the 2.5-4.6 V potential range.

		C/10 ; 2.5-4.3V	C/5 ; 2.5-4.6V
	Sample	1 st discharge capacity (mAh/g)	IRC (%)	1 st discharge capacity (mAh/g)	IRC (%)
	870-5-air (280 nm)	149	18	161	21
	17-PL (17 nm thick platelets)	156	9	171	28
	86-PL (86 nm thick platelets)	150	15	171	18

  ), reflecting the dissolution of a large amount of carbonaceous impurities at the surface of the electrode, including Li 2 CO 3 (from 36.8 to 7.2 at.%) present in large amount at the surface of the 870-5-air sample (conventional particles) as shown previously in Figure 4. Then, the surfa e's o position of ele trodes re overed at different states of ar e or dis ar e durin the 1 st cycle is compared for the three morphologies. A larger amount of LiF/Li x PF y O z species is observed at the surface of the 17 nm thick platelets (17-PL sample) compared to the 2 other morphologies, regardless of the state of charge (Figure 9a) (37.6 at.% for the 17-PL sample at 4.6 V for example versus 20.3 at.% for the 86-PL and 23.4 at.% for the 870-5-air): it reveals the more pronounced LiPF 6 degradation at the surface of those thin platelets. In good agreement with one of our previous works that studied the reactivity at the surface of

NMC622 type samples depending on their particles' size, t is would be due to t e lar er specific surface area observed for the sample made of the thinnest platelets (12 m 2 /g vs. less than 4 m 2 /g for the others).

[START_REF] Soloy | Reactivity at the Electrode-Electrolyte Interfaces in Li-Ion and gel electrolyte lithium Batteries for LiNi 0.6 Mn 0.2 Co 0.2 O 2 with Different Particle Sizes[END_REF]

ACKNOWLEDGEMENTS

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 and Eric Lebraud at ICMCB for their technical support for ICP-OES and XRD, respectively. The authors also 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.

Supporting information

The Supporting information contains Figures, Tables and description to complete and precise some points in the main manuscript, such as: chemical analyses performed by ICP-OES; the synthesis description and characterization of the hydroxide precursors used to obtain the different lithiated layered oxides; the XRD patterns and SEM images obtained for a series of lithiated layered oxides prepared in different synthesis conditions from the hydroxide precursor showing a platelet morphology; as an example, the results of the Rietveld refinements carried out for the XRD and neutron diffraction patterns of one of the studied lithiated layered oxides; the distribution of the platelet lengths in two of the lithiated layered oxides; detailed XPS results (identifications and quantifications); AES analyses, and comparison of the XRD patterns obtained for electrodes recovered after cycling. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding author

* Laurence Croguennec Laurence.Croguennec@cnrs.fr * François Weill François.Weill@cnrs.fr


Aurélien Delphin

Fabien Boux

Clément Brossard

Jan M Warnking

Benjamin Lemasson

Emmanuel Luc Barbier

Thomas Christen

Using 3D realistic blood vessel structures and machine learning for MR vascular Fingerprinting

MR vascular ngerprinting aims at mapping cerebral vascular properties such as blood volume and blood oxygenation. We propose to improve the technique by generating dictionaries based on 3D vascular networks segmented from whole brain high-resolution (3 µm isotropic) microscopy datasets. In order to compensate for the limited number of available data and long computing times, used a machine-learning reconstruction process to generalize results and tested our approach in healthy, stroke and tumor animal models. Our results show high quality maps expected contrast and baseline values in animals as well as expected trends pathological tissues.

Introduction

MR vascular Fingerprinting (MRvF) proposes to use the Magnetic Resonance Fingerprinting (MRF) framework to map cerebral microvascular properties; namely the blood oxygenation saturation (SO2), blood volume fraction (BVf) and vessel averaged radius (R). Dictionaries are generated based on numerical voxels containing vascular structures and physiological parameters. The initial MRvF implementation used 2D synthetic vascular networks. In this study, we pursue previous work and use dictionaries based on 3D geometries, either created in silico (synthetic) or segmented from whole brain high resolution microscopy images. In order to compensate for the limited number of available data and long computing times, we use a machine-learning reconstruction process to generalize our results and test our approach in healthy, stroke and tumor animal models.

Materials & Methods

Animal data: Data were acquired on a 4.7T Bruker system. The MRF acquisition pattern consists on the concatenation of two GESFIDE sequences acquired pre and post injection of USPIOs (P904, Guerbet, France, 200µmol Fe/kg) . 26 animals were imaged: 8 healthy rats (Wistar) 9 rats (Fischer) implanted with 9L tumors, imaged 14 to 16 days after induction 9 rats (Sprague-Dawley) undergoing ischaemic stroke throughout acquisition Dictionary generation: 3 Dictionaries were generated on Matlab (The MathWorks Inc., Natick, Ma, USA) with the MRVOX software . Dictionary 3D micro.: Around 28,000 MRI-size voxels containing realistic vascular structures were extracted from whole brain highresolution (3 µm isotropic) microscopy datasets using ImageJ (Rasband, W.S., NIH, Maryland, USA), Fig. 1a. Erosion transformations were performed to obtain continuous BVf and R distributions (see histograms in Fig. 1c. After assigning to each MRI-size voxel one SO2 and one T2 value in the range [35,90]% and [45, 110]ms, respectively, MR simulations including magnetic eld estimation and water di usion were performed to create the "3D microscopy" dictionary. Dictionaries 2D synth. and 3D synth.: Based on the previous histograms, 2D circles or 3D cylinders with gaussian distribution of radii (Fig. 1b) were generated to create the "2D synthetic" and "3D synthetic" dictionaries respectively.

Reconstruction: All data were processed with Matlab using the MP3 software8. MRvF maps were created either using a standard direct matching ("MRvF Matching") between dictionaries entries and in vivo data, or using a dictionary learning tool ("MRvF Learning"). On top of MRvF results, maps of BVf, Vessel Size Index (VSI) and SO2 were also obtained with analytical models . ROIs were manually de ned in lesions and contralateral tissues. Paired Student's tests were used to assess the signi cance of distributions di erence, with a p-value of 5%. The whole pipeline is presented on Fig. 1d.

Results

Fig 2 shows examples of maps obtained with the di erent approaches in one representative animal of each group. Fig 3, 4 and 5 present the numerical results obtained in all animals for BVf, R and SO2, respectively. It can be seen that without any prior on the estimates, all the MRvf approaches provide high quality maps with expected contrast and baseline values in healthy animals (3% < BVf < 5% , 4µm<R<6µm, and 60% < SO2 < 80% in healthy tissue). Dictionary learning seems however necessary as it reveals lesions that are unseen with direct matching (see SO2 maps in stroke animal with 3D micro approach). After learning, most trends between healthy and pathological tissue are in line with literature reports and similar between dictionaries (increase of BVf and R in tumors, decrease of BVf/SO2 and increase of R in stroke). For all dictionaries, the T2 measurements (not shown) are also in line with expectations and follow the same trend in all dictionaries (about 60ms in healthy tissue, 20ms increase in tumor, 5ms decrease in stroke). The dictionaries yielded di erent results in a few particular cases such as in the 9L tumors that appear hyperoxic with the 3D Micro dictionary, but normoxic with the synthetic geometries. These ndings warrant further investigation with complimentary oxygenation measurements.

Conclusions

This study demonstrates the feasibility of using 3D realistic vascular structures in the MRvF framework and the possibility to obtain high quality maps after machine learning in both healthy and pathological brains. It is possible that adding realistic pathological vascular structures in the dictionaries will further improve the results in lesions. This work also opens the possibility to perform MRvF in voxels containing closed vascular networks, allowing for the simulation of blood ow and oxygen variations along the vessels.

FiguresFigure 1 :

 1 Figures

Figure 2 :

 2 Figure 2: Results obtained with the analytical method and the di erent MRvF reconstructions in one slice of one animal in each group.

Figure 3 :

 3 Figure 3: Statistical analysis of BVf values obtained in the animal groups, for analytical methods and MRvF with di erent dictionaries. Signi cance evaluated at 5% with paired t-test.

Figure 4 :

 4 Figure 4: Statistical analysis of R values obtained in the animal groups, for analytical methods and MRvF with di erent dictionaries. Signi cance evaluated at 5% with paired t-test.

Figure 5 :

 5 Figure 5: Statistical analysis of SO2 values obtained in the animal groups, for analytical methods and MRvF with di erent dictionaries. Signi cance evaluated at 5% with paired t-test.

Acknowledgements

The MRI facility IRMaGe is partly funded by the French program "Investissement d'avenir" run by the French National Research Agency, grant "Infrastructure d'avenir en Biologie et Santé". [ANR-11-INBS-006] Project supported by ANR MRFUSE [ANR-20-CE19-0030]