Michel Latroche

Didier Blanchard

Abdelouahab Fermin Cuevas

El Kharbachi

Bjorn C Hauback

Torben R Jensen

Petra E De Jongh

Sangryun Kim

Nazia S Nazer

Peter Ngene

Fermín Cuevas

Abdelouahab El Kharbachi

Shin-Ichi Orimo

Dorthe B Ravnsbaek

Volodymir A Yartys

Full-cell hydride-based solid-state Li batteries for energy storage

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

Introduction

Dense energy packing is a key issue for future development of renewable energy production and will lead to increasing needs for mass storage. Energy can be stored in many ways, but chemical and electrochemical ones are foreseen as very promising in the future. In that field, hydrogen-based energy storage offers a good alternative to other methods and has been extensively studied in the frame of the IEA-HIA task 32. Beside solid-state storage of gaseous hydrogen with metallic or complex hydrides, electrochemical storage is one of the most advanced commercial applications developed so far, for hydrides with well-known alkaline Ni-Metal Hydrides (NiMH) [START_REF] Cuevas | Intermetallic compounds as negative electrodes of Ni/MH batteries[END_REF][START_REF] Feng | Electrochemical behaviour of intermetallic-based metal hydrides used in Ni/metal hydride (MH) batteries: a review[END_REF][START_REF] Notten | Secondary Batteries: Nickel Batteries Metal Hydride Alloys[END_REF][START_REF] Notten | Rechargeable nickel-metal hydride batteries: a successfull new concept[END_REF]. Recently, a breakthrough was realized by demonstrating that metal or complex hydrides can be also used for lithium-based electrochemical cells either as anodes or electrolytes. Lithium batteries are considered as one of the most advanced technologies for electrochemical energy storage. However, their energy densities are still too low to respond to the highly demanding mass storage and they suffer from serious safety issues, mainly related to the use of organic liquid-based electrolytes. Using hydrides for such batteries can bring smart solutions to those drawbacks as they can be implemented either as high-capacity anodes through a conversion reaction with lithium or/and as high-ionic conductivity solid electrolyte allowing fast lithium transport between both electrodes.

Several review papers [START_REF] De Jongh | Complex hydrides as room-temperature solid electrolytes for rechargeable batteries[END_REF][START_REF] Matsuo | Lithium Fast-Ionic Conduction in Complex Hydrides: Review and Prospects[END_REF][START_REF] Sartori | Metal hydrides used as negative electrode materials for Li-ion batteries[END_REF] have been already published on this topic by IEA-HIA task 32 partners but, beyond the materials properties of complex or metallic hydrides, the most important challenge consists of building practical batteries. The present paper focusses on the recent development of half-and full-cells § using the combination of solid hydride-based electrolytes and metal hydride anodes that pave the way to future hydride-based efficient electrochemical storage systems, and further expand the concept of Li-ion battery technology. It also addresses the needs for operando or in-situ characterization of working batteries using penetrating neutron beams to probe reaction mechanisms within the electrochemical cells. § In this paper, half-cell signifies a cell made of lithium as counter-electrode facing a "low potential active material" than can be considered as a negative electrode (or anode) while full-cell defines a cell made of lithium (or metallic hydrides) facing a "high potential active material" that can be considered as a positive electrode (or cathode) in a real battery.

Half-cell configurations combining metal hydride anode and complex hydride electrolyte

Mg 2 FeH 6 |LiBH 4 |Li

The complex hydride Mg 2 FeH 6 was recently employed as conversion type anode with a solid state LiBH 4 electrolyte at 120 °C [START_REF] Huen | All-solid-state lithium batteries -The Mg 2 FeH 6 -electrode LiBH 4 -electrolyte system[END_REF]. This configuration provided a factor-3 increase in the capacity retention compared to the conventional counterpart employing standard liquid electrolyte. The initial columbic efficiency also improved from the use of the solid-state electrolyte by a factor of approx. 2.5. Characterization of the charge-discharge mechanism in the MgH 2 and LiBH 4 system showed that it follows the expected conversion reactions according to eq. 1. [START_REF] Provost | XAS and XRD studies of the Thermal and Li-Driven Electrochemical Dehydrogenation of Nanocrystalline Complex Hydrides Mg 2 MH x (M=Co, Ni)[END_REF] Mg 2 FeH 6 + 6Li → 2 Mg + Fe + 6LiH

(1)

However, cyclic voltammetry, galvanostatic intermittent titration and PXD suggested that MgH 2 may form as an intermediate during both charge and discharge.

MgH 2 +CoO|LiBH 4 |Li

Ongoing research on MgH 2 shows that the capacity retention can be enhanced by addition of an electrochemically active CoO conversion-type anode to MgH 2 [START_REF] Kharbachi | MgH 2 -CoO: a conversion-type composite electrode for LiBH 4based all-solid-state lithium ion batteries[END_REF]. This approach consisted first to assess the performance of the improved single MgH 2 anode (without any added catalyst) and then after CoO addition in solid-state Li-ion battery configuration using While using LiBH 4 as solid electrolyte, the need for heating at 120 °C to get good ionic conductivities will limit its usage to some applications where the source of heat is accessible at low cost. Tentative to lower the temperature, down to room temperature, of the Li + conducting solid-electrolyte in the MgH 2 |LiBH 4 |Li cell by halide-substitution and further (Li 2 S) 0.80 (P 2 S 5 ) 0.20 solid electrolyte incorporation has been reported recently by Hauback's group [START_REF] Kharbachi | Reversibility of metal-hydride anodes in all-solid-state lithium secondary battery operating at room temperature[END_REF]. The study demonstrates a possible improvement (1 st cycle almost full reversible capacity) at moderate temperature while the optimization of the configuration is needed.

An apparent increase of the capacity can be seen during re-charge. This is may be due to the complexity of the conversion mechanism in a solid-state configuration during the phase separation (MgH 2 > Mg + 2LiH), along with the changes in the structural, electronic and surface variations between the (dis)charged states [START_REF] Kharbachi | Reversibility of metal-hydride anodes in all-solid-state lithium secondary battery operating at room temperature[END_REF]. Only few studies reported on the conversion mechanism in solid-state batteries; one can expect that future works can bring more insights on this type of combination and potential prospects. Particles with sizes in the order of 150 nm can be seen (Figure 1b), though some larger agglomerates are still present. The calculated d-spacings from the spots agree with tetra- This electrolyte system was tested first with TiS 2 electrode at low applied currents at RT in the configuration TiS 2 |Li(BH 4 ) 0.75 I 0.25 +(Li 2 S) 0.75 +(P 2 S 5 ) 0.25 |Li (Figure 1c). The capacity of the initial discharge corresponds to 89% of the theoretical one with a reversibility yield of around 90%. Up to one Li can be inserted assuming eq. 2:

xLi + TiS 2 → Li x TiS 2 (2)
The discharge/charge profiles were then recorded for the same cell configuration when TiS 2 electrode is substituted by MgH 2 tape electrode (Figure 1d) [START_REF] Kharbachi | Reversibility of metal-hydride anodes in all-solid-state lithium secondary battery operating at room temperature[END_REF]. It presents flat dischargecharge plateaus and low polarization-hysteresis. The obtained discharge capacity reflects only a part of the total active material; most probably the loaded mass is not fully involved in the process. Interestingly, the obtained 1 st cycle reversibility exceeds significantly the one obtained with liquid electrolyte for the same MgH 2 tape electrode [START_REF] Kharbachi | Morphology effects in MgH 2 anode for lithium ion batteries[END_REF]. The results point to a means of guided cell assembly and possible enhancement at the electrode/electrolyte level and fabrication process.

Full-cell configurations using Li metal anode and complex hydride electrolyte

Li|LiBH 4 +LiCl|S or TiS 2

Solid-state Li-S (working voltage = 2.2 V) [START_REF] Unemoto | Pseudo-binary electrolyte, LiBH 4 -LiCl, for bulk-type all-solid-state lithium-sulfur battery[END_REF][START_REF] Unemoto | Development of bulk-type all-solid-state lithium-sulfur battery using LiBH 4 electrolyte[END_REF] and Li-TiS 2 (working voltage = 2.1 V) [START_REF] Unemoto | Stable Interface Formation between TiS 2 and LiBH 4 in Bulk-Type All-Solid-State Lithium Batteries[END_REF] employing LiBH 4 as solid electrolyte present high performances over repeated dischargecharge cycles. The Li-S battery delivered an initial discharge capacity of 1140 mAhg -1 at a rate of 0.05C (C = 1672 mA g -1 ) at 393 K, which corresponds to 68.2% of the theoretical capacity (1672 mAhg -1 ) [START_REF] Unemoto | Development of bulk-type all-solid-state lithium-sulfur battery using LiBH 4 electrolyte[END_REF] (Figure 2a). In addition, 64.0% of the initial capacity was retained after 45 cycles. In the case of Li-TiS 2 battery cycled at a rate of 0.2C at 393 K, the discharge capacity was 205 mAhg -1 (theoretical capacity = 239 mAhg -1 ) in the second cycle, and this value dropped only to 180 mAhg -1 after 300 cycles, corresponding to an 87.8% capacity retention [START_REF] Unemoto | Stable Interface Formation between TiS 2 and LiBH 4 in Bulk-Type All-Solid-State Lithium Batteries[END_REF] (Figure 2b). A combined experimental and computational analysis revealed that the Li 2 B 12 H 12 interfacial phase, which displays superior oxidative stability compared to LiBH 4 , was formed between the TiS 2 cathode and the LiBH 4 solid electrolyte, enabling the reversible and stable lithium ion transfer across the interface.

The main challenge of the LiBH 4 solid electrolyte is the high phase transition temperature, requiring high temperature operations. In this regard, the substitution of halide ions such as Cl -, Br -, and I -for the BH -complex anions in LiBH 4 significantly lowers its phase transition temperature [START_REF] Maekawa | Halide-Stabilized LiBH 4 , a Room-Temperature Lithium Fast-Ion Conductor[END_REF], leading to considerable progress in all-solid-state batteries using LiBH 4based solid electrolytes. When the Cl --substituted LiBH 4 (Li[BH 4 ,Cl]), which displays the phase transition at approx. 345 K, was used as solid electrolyte, solid-state Li-S batteries showed reversible discharge-charge reaction even at 383 K [START_REF] Unemoto | Pseudo-binary electrolyte, LiBH 4 -LiCl, for bulk-type all-solid-state lithium-sulfur battery[END_REF] (Figure 2c). However, the battery using Li[BH 4 ,Cl] showed worse cyclability compared to that using LiBH 4 , due to the alleviated electrochemical stability of the solid electrolyte by the Cl --substitution.

Figure 2. All-solid-state batteries using LiBH 4 -based complex hydride solid electrolytes. Dischargecharge profiles of (a) Li|LiBH 4 |S [START_REF] Unemoto | Development of bulk-type all-solid-state lithium-sulfur battery using LiBH 4 electrolyte[END_REF], (b) Li|LiBH 4 |TiS 2 [START_REF] Unemoto | Stable Interface Formation between TiS 2 and LiBH 4 in Bulk-Type All-Solid-State Lithium Batteries[END_REF], and (c) Li|LiBH 4 +LiCl|S batteries [START_REF] Unemoto | Pseudo-binary electrolyte, LiBH 4 -LiCl, for bulk-type all-solid-state lithium-sulfur battery[END_REF].

Li|LiBH 4 @SiO 2 |S Solid-state lithium-sulfur batteries with LiBH 4 @SiO 2 nanocomposite as electrolyte were also realized by Das et al. [START_REF] Das | All-Solid-State Lithium-Sulfur Battery Based on a Nanoconfined LiBH 4 Electrolyte[END_REF]. In these Li|LiBH 4 @SiO 2 |S+C batteries, Li metal on stainless steel current collector is used as negative electrode while the solid-electrolyte is a composite of LiBH 4 and ordered mesoporous silica (MCM-41) prepared via melt infiltration under hydrogen pressure [START_REF] Blanchard | Nanoconfined LiBH 4 as a Fast Lithium Ion Conductor[END_REF][START_REF] Ngene | LiBH 4 /SBA-15 Nanocomposites Prepared by Melt Infiltration under Hydrogen Pressure: Synthesis and Hydrogen Sorption Properties[END_REF]. The solid electrolyte contained 54 wt.% LiBH 4 which corresponds to 160 % of the pore volume of the silica, allowing the extra LiBH 4 to provide a percolating ion pathway between the two electrodes. The S+C electrode are fabricated by ball-milling mixture of elemental sulfur and conductive carbon (45:55, S:C weight ratio) followed by melt-diffusion at 155 °C to improve the dispersion of sulfur in the carbon matrix [START_REF] Ji | A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries[END_REF]. The conductive carbon is a mixture (1:1 weight ratio) of Ketjen Black and activated carbon, a compromise between high electronic conductivity and large surface area, with 10-15 wt% polyvinylidene fluoride (PVDF) added as binder.

The authors showed that the nanoconfined LiBH 4 exhibits very good properties as solid electrolyte to be used in lithium-sulfur batteries. They reported a conductivity of 10 -4 Scm -1 at room temperature, three orders of magnitude larger than that of unmodified LiBH 4 and investigated the nature of the conduction in LiBH 4 @SiO 2 nanocomposite by measuring the electronic (t e ), ionic (t ion ) and cationic transport numbers (t + ) which are crucial properties for effective electrolytes in battery applications. They showed that the composite has a cationic transport number of 0.96 and negligible electronic conductivity, hence the nanocomposite is close to a pure cationic conductor. The stability of the nanocomposite in contact with lithium metal was investigated by galvanostatic plating-stripping cycles using symmetric Li electrode cells. Only a slight increase (4.5%) in cell resistance was observed in 40 cycles. This is excellent for solid electrolytes, which are known to face challenges, reacting with lithium metal or with maintaining good interfacial contact with electrode materials during cycling. ) of a solid-state Li|LiBH 4 @SiO 2 |S+C battery based on a LiBH 4 -SiO 2 nanocomposite electrolyte [START_REF] Das | All-Solid-State Lithium-Sulfur Battery Based on a Nanoconfined LiBH 4 Electrolyte[END_REF].

The solid-state Li-S battery realized with LiBH 4 @SiO 2 nanocomposite electrolyte showed a very good performance, delivering high capacities, typically 1220 mAhg -1 after 40 cycles at moderate temperature (55 °C) and working voltage of 2 V, albeit at a relatively low chargedischarge rates (0.03 C). Except for the first cycle, the cyclic voltammogram of a solid-sate Li-S battery (Figure 3), exhibited clear peaks at around 1.4 V during the cathodic scan and around 2.4 V during the anodic scan. These peaks correspond respectively to the formation of lithium polysulfides and their conversion to elemental sulfur and lithium, i.e. dischargecharge of the battery. The first cycle showed a different behavior with a cathodic peak at around 2.2 V, no well-defined peaks at lower voltage but a higher cathodic current. This peculiar behavior, during the first cycle was also found in the charge-discharge voltage versus capacity curves (Figure 4). During the first discharge, the battery yielded a capacity of around 3100 mAhg -1 of sulfur. This is almost twice the theoretical discharge capacity (1675 mAhg -1 for sulfur). Furthermore, a voltage plateau at around 2.4 V was also visible; the difference in voltage with the 2.2 V observed during the cyclic voltammetry is due to the difference in current density used in the two different measurements. During the following cycles the capacity stabilized around the theoretical value and the high voltage plateau faded with the number of cycles. With liquid electrolytes, at least two voltage plateaus must be observed. A first one following the dissolution of the high order polysulfides (Li 2 S 8 ), the discharge occurring in the liquid phase and a second one, with the precipitation of lower order polysulfides (Li 2 S x , 1≤ x <8), the discharge occurring between liquid-solid phases [START_REF] Zhang | Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions[END_REF].

For solid-state Li-S batteries, only one plateau is expected for the solid-state electrochemical reactions [START_REF] Wang | Sulfur-Based Composite Cathode Materials for High-Energy Rechargeable Lithium Batteries[END_REF]. Therefore, the large capacity obtained during the first cycle together with the fading high voltage plateau were attributed to parasitic electrochemical reactions between the solid electrolyte and the sulfur electrode, possibly forming a stable cathode-electrolyte interface (CEI). The nature of these reactions as well as of the interface is the subject of an ongoing research. Possible reaction of the electrolyte with Li metal was excluded following the galvanostatic plating-stripping studies performed on symmetric cells.

During the 3 rd cycle, the capacity stabilized to around 1570 mAhg -1 of sulfur, (94 % of the theoretical capacity) and gradually decreases to 78% of this value after 40 cycles. The authors attributed the small loss in capacity upon cycling to an increase in the overall resistivity of the cell (from 1.2 kΩ to 1.8 kΩ after the first cycle to about 5 kΩ after the 40 th cycle). The increased resistance could arise from the formation of an insulating CEI and partial loss of contact between the electrodes and electrolyte, as observed for other type of solid-state batteries based on lithium borohydride solid-electrolytes [START_REF] Kobayashi | All solid-state battery with sulfur electrode and thio[END_REF][START_REF] Nagao | All-solid-state Li-sulfur batteries with mesoporous electrode and thio-LISICON solid electrolyte[END_REF][START_REF] Sveinbjörnsson | The LiBH 4 -LiI Solid Solution as an Electrolyte in an All-Solid-State Battery[END_REF]. The rate capability measurements on the full Li-S at different charge-discharge rates (0.03 C to 0.12 C) indicates that the capacities are lower at higher C-rates, but full capacity is regained when the charge rate is lowered again to 0.03 C. The low capacity at high chargedischarge rate is attributed to polarization of the sulfur electrode. At high C-rates, full sulfur utilization is impeded by the large potential drop in the C+S cathode, indeed sulfur and lithium-sulfur compounds have low electronic conductivities; this could be remediated by using more highly conductive carbon additives and improved dispersion of sulfur with smaller particle size. The recovering of the full capacity after lowering the rate underscores the good stability of the solid-state Li-S battery based on LiBH 4 @SiO 2 nanocomposite as electrolyte. Current effort is devoted to improving the sulfur utilization at high rates via reengineering of the S+C cathode, to improve electronic conductivity and interfacial contact with the electrolyte during cycling. Further work is devoted in obtaining a better fundamental understanding of the ionic transport in this type of composite solid electrolyte and at the interfaces with the electrodes [START_REF] Lefevr | Lithium Conductivity and Ions Dynamics in LiBH 4 /SiO 2 Solid-Electrolytes studied by Solid-State NMR and Quasi Elastic Neutron Scattering and applied in Lithium-Sulfur Batteries[END_REF].

Li|Li(BH 4 ) 0.75 I 0.25 +(Li 2 S) 0.75 (P 2 S 5 ) 0.25 |TiS 2

Progress has also been made at the solid electrolyte level by proceeding to the optimization of the composition in the electrolyte system Li(BH 4 ) 0.75 I 0.25 +Li 2 S+P 2 S 5 [START_REF] Kharbachi | Lithium ionic conduction in composites of Li(BH 4 ) 0,75 I 0,25 and amorphous 0.75Li 2 S•0.25P 2 S 5 for battery applications[END_REF]. Indeed, the highest RT ionic conductivity (~10 -3 S.cm -1 ) was found for the system with the approximate nominal composition Li(BH ], likely as it does for the [I -] anions [START_REF] Miyazaki | Room temperature lithium fast-ion conduction and phase relationship of LiI stabilized LiBH 4[END_REF]. This allows less hindered effect regarding Li mobility and consequently facilitating the Li ion conduction in the mixed system at lower temperatures RT-150 °C. This composite electrolyte seems to show an electrochemical window up to 5 V and stability in contact with Li metal and battery tests using TiS 2 electrodes show notable initial reversibility for further work on advanced battery tests (Figure 5). optimal coulombic efficiency at two different rates. Primarily, these tests are very promising as it shows repeatable cycling, mainly during the delithiation (superimposed curves) for the experienced cycling rates, meaning good response can be expected at fast recharge solicitation. Secondly, the lithiation emphasizes the good stability of the TiS 2 electrode toward the solid electrolyte with no significant side reaction (inset in Figure 5) can be observed as the system remains almost fully reversible during cycling for the same current rate. ) have been investigated intensively due to the high ion conductivities approaching 10 -1 S cm -1 in their high-T phases [START_REF] Kim | Fast Lithium-Ion Conduction in Atom-Deficient closo-Type Complex Hydride Solid Electrolytes[END_REF][START_REF] Tang | Liquid-Like Ionic Conduction in Solid Lithium and Sodium Monocarba-closo-Decaborates Near or at Room Temperature[END_REF][START_REF] Tang | Unparalleled lithium and sodium superionic conduction in solid electrolytes with large monovalent cage-like anions[END_REF]. When LiCB 11 H 12 was used as solid electrolytes, the all-solid-state Li-TiS 2 battery delivered high initial discharge capacity of ~230 mAhg -1 for a rate of 0.2C at 403 K [START_REF] Tang | Unparalleled lithium and sodium superionic conduction in solid electrolytes with large monovalent cage-like anions[END_REF], as shown in Figure 6a. Despite the high initial capacity at a relatively high rate, they exhibited large capacity loss during cycling and the 5 th discharging retained only 75% of the initial capacity (Figure 6a). One possible reason of this poor performance is high reactivity of carbon in complex anions with the lithium metal. It is noted that carbon is the commercial anode material in the conventional liquid-based lithium-ion batteries [START_REF] Tarascon | Issues and challenges facing rechargeable lithium batteries[END_REF].

Li|LiCB
Figure 6. All-solid-state batteries using closo-type complex hydride solid electrolytes. Dischargecharge profiles of (a) Li|LiCB 11 H 12 |TiS 2 [START_REF] Tang | Liquid-Like Ionic Conduction in Solid Lithium and Sodium Monocarba-closo-Decaborates Near or at Room Temperature[END_REF] and (b) Li|atom-deficient Li 2 B 12 H 12 |TiS 2 batteries [START_REF] Miyazaki | Room temperature lithium fast-ion conduction and phase relationship of LiI stabilized LiBH 4[END_REF].

The important feature of closo-type complex hydrides that differentiates them from LiBH 4based compounds is that complex anions consist of not only multiple H atoms but also multiple B (and C) atoms. In such closo-type complex anions, the B and H atoms form a particular covalent bonding pattern (B-B and B-H), leading to the robust cage-type polyanionic structure. Recent experimental and theoretical study on the basis of the given peculiar structure of closo-type complex anions reveals that lithium and hydrogen could be simultaneously extracted from Li 2 B 12 H 12 by applying a small external energy, enabling the formation of atom-deficient closo-type compounds [START_REF] Kim | Fast Lithium-Ion Conduction in Atom-Deficient closo-Type Complex Hydride Solid Electrolytes[END_REF]. These atom deficiencies lead to an increase in carrier concentration, improving lithium-ion conductivity by 3 orders of magnitude (2 10 -5 S cm -1 at 30 °C) compared to that of a pristine material (2.5 10 -8 S cm -1 at 30 °C). An all-solid-state Li-TiS 2 battery employing atom-deficient Li 2 B 12 H 12 as a solid electrolyte exhibited a first discharge capacity of 228 mAhg -1 and good capacity retention over 20 cycles for a rate of 0.05C at 353 K (Figure 6b). These results suggest that atomdeficient closo-type materials can be used as the solid electrolyte in all-solid-state batteries.

Full-cell configurations combining metal hydride anode and complex hydride electrolyte

MgH 2 -TiH 2 |LiBH 4 |S+Li 2 S+C

A metal-hydride (MH; M=Mg, Ti)) nanocomposite made from MgH 2 and TiH 2 counterparts (with 8:2 molar ratio) and a complex borohydride solid electrolyte (LiBH 4 ) were integrated in a complete solid-state battery with a sulfur+carbon composite as cathode [START_REF] López-Aranguren | An allsolid-state metal hydride -Sulfur lithium-ion battery[END_REF]. LiBH 4 was used as the solid electrolyte due to its high Li + conductivity of 10 -3 S cm -1 at 120 °C, suitable mechanical properties and outstanding performance in LIBs [START_REF] Matsuo | Lithium Fast-Ionic Conduction in Complex Hydrides: Review and Prospects[END_REF][START_REF] Unemoto | Development of bulk-type all-solid-state lithium-sulfur battery using LiBH 4 electrolyte[END_REF][START_REF] Takahashi | All-solid-state lithium battery with LiBH 4 solid electrolyte[END_REF][START_REF] Unemoto | Complex Hydrides for Electrochemical Energy Storage[END_REF].

The anode electrode was prepared from a mixture of the MH composite with C65 conductive carbon black and LiBH 4 . The positive electrode was made from a mixture of Li 2 S with LiBH 4 and carbon Ketjen Black (KB). This mixture was obtained from the first discharge of a S+LiBH 4 +KB│LiBH 4 │Li half-cell. The redox reaction (eq. 3) for the full-cell can be written as:

Li 2 S + (yMgH 2 +(1-y)TiH 2 ) → S + (yMg+(1-y)Ti) + 2LiH (3)
with theoretical reduction potentials at 1.7 V and 2.0 V vs Li/Li + , for MgH 2 and TiH 2 , respectively, the charge/discharge profiles for the complete cell are shown in Figure 7. On the first charge, three plateaus are observed: a large sloping first plateau at ~1.8 V, followed by a second smaller one at approx. 2.2 V and an even smaller third one at approx. 2.35 V.

The first and second plateaus correspond to the conversion reaction of MgH 2 and TiH 2 phases with lithium, respectively. The third plateau at ~2.35 V is attributed to minor lithiation of Mg metal. The reached capacity (1720 mAhg -1 ) suggests the complete lithiation of the metal hydride mixture. Despite this, the capacity was successfully recovered to ca. 780 mAhg -1 at C/50 after 20 cycles (Figure 8). Coulombic efficiency remained over 97 % after the initial cycles.

The present work comes out from the smart concept of merging for the first time in a complete all-solid-state cell, a high-performance nanocomposite hydride anode with sulfur as cathode. The electrochemical performance of the complete cell demonstrates the potential of metal hydrides for high-energy all-solid-state Li batteries.

Operando study of Li cell by neutron diffraction

In-situ and operando studies of working electrodes have been developed by several groups in the past using either conventional lab or SR X-ray radiations both for diffraction or absorption [START_REF] Huen | All-solid-state lithium batteries -The Mg 2 FeH 6 -electrode LiBH 4 -electrolyte system[END_REF][START_REF] Provost | XAS and XRD studies of the Thermal and Li-Driven Electrochemical Dehydrogenation of Nanocrystalline Complex Hydrides Mg 2 MH x (M=Co, Ni)[END_REF][START_REF] Kharbachi | Lithium ionic conduction in composites of Li(BH 4 ) 0,75 I 0,25 and amorphous 0.75Li 2 S•0.25P 2 S 5 for battery applications[END_REF][START_REF] Zaïdi | Reactivity of complex hydrides Mg 2 FeH<sub>6</sub, Mg<sub>2</subCoH<sub>5</sub and Mg<sub>2</subNiH<sub>4</sub with lithium ion: Far from equilibrium electrochemically driven conversion reactions[END_REF][START_REF] Zhang | XAS Investigations on Nanocrystalline Mg 2 FeH 6 used as Negative Electrode of Li-ion Batteries[END_REF][START_REF] Teprovich | Li-Driven Electrochemical Conversion Reaction of AIH 3 , LiAlH 4 , and NaAlH 4[END_REF], leading to valuable results for comprehension of the electrochemical reaction mechanisms. This was mainly done with half-cells using specially designed devices like the electrochemical cell of Leriche et al. dedicated to operando studies [START_REF] Leriche | An electrochemical cell for operando study of lithium batteries using synchrotron radiation[END_REF]. However, such devices are not well designed for bulk analysis as X-ray beam can hardly go through matter containing metallic elements because of strong absorption. This impedes bulk analysis of full-cells as the crossing all the components (casing, current collectors, electrodes, electrolyte, separators…) is not easily achievable. To overcome these difficulties, use of neutron beam is very interesting as it can penetrate deeply in materials without significant absorption for most of the elements. In addition, with the high hydrogen content it will appear a heavy background due to large incoherent scattering. This last point can be solved by using deuterium instead of hydrogen or by limiting the hydrogen content in the cell.

Recently, we used neutrons as an efficient probe for Li-ion batteries to investigate the structural evolution of the electrode materials in an ICR 10440 commercial cylindrical cell shown in Figure S1 of the Supplementary Information file (A3 type rechargeable battery with a diameter of 10 mm and length of 45 mm; capacity 360 mAh; operating voltage 3.7 V [START_REF] Nazer | In operando neutron diffraction study of a commercial graphite/(Ni, Mn, Co) oxide-based multi-component lithium ion battery[END_REF]).

The battery showed a practical capacity of 360 mAh operating at 3.7 V [START_REF] Nazer | In operando neutron diffraction study of a commercial graphite/(Ni, Mn, Co) oxide-based multi-component lithium ion battery[END_REF]. The multicomponent cathode material was identified as a combination of three phases:

Li(Ni,Mn,Co)O 2 , LiCoO 2 and LiMn 2 O 4 whereas the anode was made of commonly used graphite. The high neutron flux of 10 14 n cm -2 s -1 available at SINQ, PSI, allows studies of the transformation processes at a relatively short time scale. Importantly, neutron scattering is interacting with nuclei of the elements and is thus independent of the atomic number of the elements and allows studies of both atomic structures and macroscopic characterization of Li distribution in the studied batteries as related to their state of charge, rate of battery charge and discharge and temperature of the system studied. In the present study, accumulated high quality neutron diffraction data sets collected in just 3-5 min were suitable to accomplish Rietveld refinements.

Upon cycling, the data analysis showed that graphite underwent structural changes to form a various insertion-type lithiated LiC x , with a maximum volume expansion of 12.7% for the compound LiC 6 . For the cathode, the charge process was related to fractional lithium depletion from the Li-saturated compounds, leading to a small volume contraction for the layered oxides. Successful Rietveld fitting of the diffraction pattern allowed identification of all individual constituents and showed that there were as many as 7 components accounted during the refinement (Figure 9).

Upon cycling, the data analysis showed that graphite underwent structural changes to form a various insertion-type lithiated LiC x , leading to 12.7% volume expansion for the compound LiC 6 . For the cathode, the charge process was related to fractional lithium depletion from the Li-saturated compounds, leading to volume contraction for Li(Ni,Mn,Co)O 2 , in contrast to (Ni,Mn)-free LiCoO 2 . For the discharge completed at fast rate (2C) for various temperatures (5, 25 and 45 °C), the capacity followed the trend of an increased diffusion rate of Li + in the electrolyte and Li in the two electrodes, especially at 45 °C. Successful refinement of the diffraction patterns requires identification of all individual constituents which contribute to the scattering of the diffraction beam. Together with constituents of the cathode, anode, the material of the current collectors and casing should be properly accounted to quantitatively assess the phase-structure transformations taking place. Figure 9 shows that there were as many as 7 components accounted during the Rietveld refinement of the data. During operation, swelling and contraction of the active material on cycling was manifested by change of the level of electrolyte as related to the state of charge (0, 50 and 100%;

Figure S2).

This work demonstrates that operando and in-situ neutron scattering studies of a full-cell battery lead to a valuable information on the complex mechanisms involved upon cycling, related to structural changes in the anode and cathode materials. Moreover, in the frame of this paper, it might also bring new insights on the solid electrolyte, if any structural changes occur (as it is typically the case for LiBH 4 for which superionic properties can be followed through the structural transition observed only for the hexagonal high temperature modification of the complex hydride).

Discussion

Recently, metal hydrides have been successfully used as electrodes in half-cell solid state configurations using solid LiBH 4 and Li 2 S+P 2 S 5 electrolytes [START_REF] Ikeda | Anode properties of Al 2 O 2 -added MgH 2 for all-solid-state lithium-ion batteries[END_REF][START_REF] Ikeda | Anode properties of magnesium hydride catalyzed with niobium oxide for an all solid-state lithium-ion battery[END_REF][START_REF] Kawahito | Electrochemical Performance of Titanium Hydride for Bulk-Type All-Solid-State Lithium-Ion Batteries[END_REF][START_REF] Zeng | Bulk-Type All-Solid-State Lithium-Ion Batteries: Remarkable Performances of a Carbon Nanofiber-Supported MgH 2 Composite Electrode[END_REF][START_REF] Zeng | Metal hydride-based materials towards high performance negative electrodes for all-solid-state lithium-ion batteries[END_REF]. In some cases, these have proved to be superior to their liquid-electrolyte counterparts. A solid-state battery MgH 2 |LiBH 4 |Li has been demonstrated in Li-ion half-cell configuration by Kojima and coworkers where lithium metal was used as auxiliary/reference electrode [START_REF] Zeng | Metal hydride-based materials towards high performance negative electrodes for all-solid-state lithium-ion batteries[END_REF]. The active material was prepared by mixing MgH 2 and LiBH 4 (2:1 wt. ratio). In addition to carbon black additive, the MgH 2 anode was doped with Nb 2 O 5 (1 mol.%) to improve the electrochemical performance [START_REF] Ikeda | Anode properties of magnesium hydride catalyzed with niobium oxide for an all solid-state lithium-ion battery[END_REF]. This hydride-optimized half-cell yielded high reversible capacity (1586 mAhg -1 ) with low polarization of 0.05 V. Unfortunately, the capacity retention decreased significantly to 924 mAhg -1 within 50 cycles. Possible formation of Mg(BH 4 ) 2 , because of interaction of MgH 2 with LiBH 4 , may occur at higher voltage (2 V). Besides, similarly to the alloying mechanism happening in carbonate-based liquid electrolytes at low voltage [START_REF] Oumellal | Metal hydrides for lithium-ion batteries[END_REF], the formation of Mg-Li alloys may occur also in solid-state configuration. This both unwanted processes, happening at extreme compositions, can be avoided by limiting the voltage to 0.3-1 V [START_REF] Zeng | Metal hydride-based materials towards high performance negative electrodes for all-solid-state lithium-ion batteries[END_REF]. The reversible capacity (~1200 mAh.g -1 ) of the MgH 2 |LiBH 4 |Li cell has been improved using carbon nanofiber-supported MgH 2 composite electrode, thanks to the enhancement in the electronic conductivity of MgH 2 [START_REF] Zeng | Bulk-Type All-Solid-State Lithium-Ion Batteries: Remarkable Performances of a Carbon Nanofiber-Supported MgH 2 Composite Electrode[END_REF]. Rate capability tests showed a capacity retention of 1100 mAhg -1 at high current density (4C rate). Different cells configurations were assembled by incorporating an additional layer of 80Li 2 S+20P 2 S 5 solid electrolyte (Li side) for possible use in full cells. Comparable performance between cells with and without 80Li 2 S+20P 2 S 5 solid electrolyte, were reached in this study [START_REF] Zeng | Bulk-Type All-Solid-State Lithium-Ion Batteries: Remarkable Performances of a Carbon Nanofiber-Supported MgH 2 Composite Electrode[END_REF]. In quite similar cell configuration with LiBH 4 solid electrolyte, the in-situ incorporation of the highly dispersive CoO/Co nanoparticles to MgH 2 led to improvement of the cyclability and reversibility, and mitigated kinetics at high rate between the charge and discharge. The

present results may open up to possibilities for making a bulk-type solid-state full battery with cathode materials such as LiCoO 2 [START_REF] Zeng | Bulk-Type All-Solid-State Lithium-Ion Batteries: Remarkable Performances of a Carbon Nanofiber-Supported MgH 2 Composite Electrode[END_REF].

Very recently, a new series of rare earth metal borohydrides, LiM(BH 4 ) 3 Cl, M = La, Ce, Gd, were found to have high Li + ion conductivity, with σ(Li + ) approx. 10 -4 Scm -1 at room temperature [START_REF] Ley | New Li Ion Conductors and Solid State Hydrogen Storage Materials: LiM(BH 4 ) 3 Cl, M = La, Gd[END_REF][START_REF] Ley | LiCe(BH 4 ) 3 Cl, a New Lithium-Ion Conductor and Hydrogen Storage Material Isolated Tetranuclear Anionic Clusters[END_REF][START_REF] Payandeh Gharibdoust | Synthesis, Structure, and Li-Ion Conductivity of LiLa(BH 4 ) 3 X, X = Cl, Br, I[END_REF]. The cerium-based compound, LiCe(BH 4 ) 3 Cl, exhibits the highest stability when employed in Li and Li-In alloy symmetrical cells without formation of resistive layers upon cycling. This compound, LiCe(BH 4 ) 3 Cl, is also demonstrated as an electrolyte in solid-state Li-S batteries using a carbon sulfur composite as cathode and Li-In as anode. A reversible electrochemical reaction between Li and S with an initial discharge capacity of 1186 mAhg -1 and a current density of 13 μA cm -2 (i.e. a rate of charge/discharge of C/100) takes place at 45 °C with a remaining capacity of 510 mAhg -1 after 9 cycles [START_REF] Nguyen | Investigation of the stability of metal borohydridesbased compounds LiM(BH 4 ) 3 Cl (M=La, Ce, Gd) as solid electrolytes for Li-S batteries[END_REF].

The discovery of the electrochemical properties of hydride as conversion anodes or Lisuperconductor electrolytes is quite recent, as first published papers on that matter appeared only ten years ago. Nevertheless, much work has been devoted to the characterization of their electrochemical properties in half-or full-cells. and reversible capacities ranging between 550 and 878 mAhg -1 for at least a few tens of cycles with CE ranging between 47 and 99.5%.

Beside half-cells, the most important challenge remains the development of full-cells with enhanced properties compared to the state-of-the-art. Metal and complex hydrides might be used as anode or electrolyte but, for building a full battery, should also be coupled to a suitable positive electrode. Sveinbjornsson et al. [START_REF] Sveinbjörnsson | The LiBH 4 -LiI Solid Solution as an Electrolyte in an All-Solid-State Battery[END_REF] have used lithium titanate (Li 4 Ti 5 O 12 ). A significant overvoltage and a sloppy voltage were observed, especially during discharge. The battery showed rapid capacity fading over cycling. Another attempt was made by Takahashi et al. [START_REF] López-Aranguren | An allsolid-state metal hydride -Sulfur lithium-ion battery[END_REF] with the classically-used cathode material LiCoO 2 but stabilization of the interface between this oxide and the reductive borohydride remains very challenging. Thus, a coating made of Li 3 PO 4 was necessary to protect the interface. Sulfur-based materials, though working at lower potential (roughly 2.2 V), were more simply used to build full-cell batteries.

Mainly, pure sulfur, Li 2 S and TiS 2 were tested successfully. Again, the use of LiBH 4 as electrolyte implies a working temperature larger or equal to 120 °C but various strategies (substitution by LiI, LiCl, Li 2 S or P 2 S 5 , nanoconfinement in SiO 2 , replacement by closoboranes) has allowed to decrease the temperature down to 100-80 °C and even 55-45 °C, keeping practical capacities and reasonable rates. For batteries using metallic lithium as anode, capacities are given regarding the active materials at the cathode (as Li with 3828 mAhg -1 can be considered as quasi-infinite beside the cathode) except for López-Aranguren et al. [START_REF] Unemoto | Complex Hydrides for Electrochemical Energy Storage[END_REF] that refer to the composite MgH 2 +TiH 2 anode. Thus, comparison is not straightforward though one can notice that full-cells built with TiS 2 are less capacitive than the sulfur ones, because of the extra weight brought by Ti and the lower S/Li ratio. For sulfur, capacities as high as 1140-1570 mAhg -1 are reported. Reversible capacities are also significant, ranging from 510 to 1220 mAhg -1 with CE from 73% to 99,6% for a few tens of cycles. Finally, it is worth to note that only one full-cell combining both metallic hydride as anode and complex hydride as electrolyte has been reported so far. The system MgH 2 +TiH 2 |LiBH 4 |S+Li 2 S can provide more than 720 mAhg -1 for at least 25 cycles at 120 °C [START_REF] Unemoto | Complex Hydrides for Electrochemical Energy Storage[END_REF]. It is important to keep in mind that the use or metallic lithium as anode, though very promising due to the very high capacity of Li, remains challenging in terms of safety.

Dendrites might find a path within the granular morphology of solid electrolytes leading to hazardous shortcuts. Indeed, recovering of a lithium plate negative electrode upon prolonged cycling is not straightforward and anodic host structures could be mandatory keeping increasing interests in the development of conversion materials like metallic hydrides.

Despite proof of concept for the usability of metallic and complex hydrides as anodes and electrolytes to develop full-cell solid-batteries, some drawbacks remain to be solved to go forward into electrochemical applications. Regarding anodes, the conversion reaction between metallic hydrides and lithium is easily observed as far as ∆G f (MH x )/x is greater than ∆G f (LiH). However, the reversibility remains an issue as the recovery of the hydride from the metal and lithium hydride is quite challenging. A recent study has explored how different preparation methods influence the electrochemical reactivity of MgH 2 and what may cause the fast capacity fade [START_REF] Huen | Insight into Poor Cycling Stability of MgH 2 Anodes[END_REF]. Huen et al. [START_REF] Huen | Insight into Poor Cycling Stability of MgH 2 Anodes[END_REF] found that growth of the Mg crystallites upon discharge may limit the Mg-LiH surface contact and thereby hinder the reversibility, and that part of the Mg formed during discharge can be reconverted into MgH 2 during charge, while the reconverted MgH 2 cannot fully react in subsequent discharge. Thus, the hydride conversion may be hampered by limited diffusion of Mg. The results agree with a kinetically limited process tracked by impedance as function of particle size and state of charge [START_REF] Kharbachi | Understanding Capacity Fading of MgH 2 Conversion-Type Anodes via Structural Morphology Changes and Electrochemical Impedance[END_REF].

This issue was also investigated in a recent work on a MgH 2 thin film [START_REF] Berti | Thin films as model system for understanding the electrochemical reaction mechanisms in conversion reaction of MgH 2 with lithium[END_REF]. Basing their study on a 2D thin-film electrode, the authors discarded electronic or morphologic degradations but identified mass-transport limitation as the main factor for hampering the reversibility of the conversion reaction. Future strategies should pay attention to the understanding and enhancement of mass-transport of hydrogen, lithium and metal atoms within the solid-state electrode especially at room temperature.

For solid-state electrolytes, the main challenge remains the working temperature that is still too high for practical applications. Though improvement have been made by playing with substitutions in the anion sublattice or nanoconfinement, liquid-like ionic conductivities at room temperature remains difficult to achieve. In addition, electrode formulations are still to be improved to ensure good interfaces between the active materials and the electrolyte keeping practical mechanic properties for the system.

Finally, a lot is still to be understand on the electrochemical reaction paths, the role of interfaces and the ionic conduction mechanisms in solid-state electrolytes. The development of powerful operando techniques using either photons or neutrons beams is mandatory to fully describe the behavior of the complex chemistry involved in real batteries.

LiBH 4

 4 as solid electrolyte at 120°C. The cycling led to formation of a single-plateau of the 75MgH 2 +25CoO nanocomposite electrode with higher reversibility yield, lowered dischargecharge hysteresis and mitigated kinetic effect. The presence of CoO is clearly contributing to the improvement of the cyclability and reversibility while remaining active and delivering an extra capacity. Reduced diffusion pathways and less polarized electrodes are believed to be at the origin of such beneficial properties, owing to the in-situ formation of highly dispersive Co/CoO nanoparticles in the electrode matrix. MgH 2 |(Li(BH 4 ) 0.75 I 0.25 )+(Li 2 S) 0.80 (P 2 S 5 ) 0.20 )|Li

  Microstructural characteristics (synchrotron radiation powder X-ray diffraction (SRPXD) and transmission electronic microscopy (TEM)) of the MgH 2 powder used in this study are shown in Figure 1a & b), indicating a well crystalline tetra-MgH 2 with reduced grain size (3-7 nm).

MgH 2 .

 2 The solid electrolyte was synthesized by mixing the hexagonal phase Li(BH 4 ) 0.75 I 0.25 with (Li 2 S) 0.80 (P 2 S 5 ) 0.20 precursor followed by annealing at 240 °C and 20 bar H 2 . The final product gives an ionic conductivity of about 10 -4 S cm -1 at room temperature (RT).

Figure 1 .

 1 Figure 1. PXD patterns (a) and bright field TEM micrographs (b) of the 24h-milled MgH 2 under H 2 atm. (Inset: Fourier transform of the bright field TEM image); RT discharge-charge (3 first cycles, 30 °C, 0.01 mA cm -2 )

Figure 3 .

 3 Figure 3. Cyclic voltammogram (at 55 °C and scan rate of 0.1 mV s -1

Figure 4 .

 4 Figure 4. Cycling performance and discharge /charge capacity and Coulombic efficiency of a solid-state Li-S battery based on LiBH 4 -SiO2 nanocomposite as electrolyte, measured at 55 °C, current density of 6.2 µA cm -2 , corresponding to C-rates of 0.03 C, and with cut-off voltages of 1 and 3.5 V [17].

Figure 5 .

 5 Figure 5. Electrochemical galvanostatic discharge/charge cycling with TiS 2 electrode with Li(BH 4 ) 0.75 I 0.25 +(Li 2 S) 0.75 +(P 2 S 5 ) 0.25 solid electrolyte at 50 °C (Inset: dQ/dE curve for the first cycle) [27]. Noticeably, the cell configuration TiS 2 |Li(BH 4 ) 0.75 I 0.25 +(Li 2 S) 0.75 +(P 2 S 5 ) 0.25 |Li shows a first discharge capacity of 91% of the theoretical one for TiS 2 (~239 mAh.g -1 ) with reversibility yield of about 98%. The following three discharge curves indicate a good cyclability and

Figure 7 .

 7 Figure 7. Charge/discharge profiles at different C rates (C/50, C/20 and C/10, five cycles each) of the complete ASS battery Li 2 S+LiBH 4 +KB│LiBH 4 │MH+LiBH 4 +C65 at 120°C and schematic representation of the discharge redox reaction [33]. First discharge of the complete cell exhibits two plateaus at 1.8 and 1.4 V corresponding to the reformation of TiH 2 and MgH 2 , respectively. The cell shows an excellent outcome, with reversible capacities as high as 910 mAhg -1 at C/50. The capacity of the battery gradually decreases with increasing the C rate due to kinetic constraints of the MH redox conversion.

Figure 8 .

 8 Figure 8. Charge/discharge profiles at different C rates (C/50, C/20 and C/10 then C/50 again, five cycles each) of the complete solid-state battery Li 2 S+LiBH 4 +KB│LiBH 4 │MH+LiBH 4 +C65 at 120 °C [35].

Figure 9 .

 9 Figure 9. Neutron diffraction patterns for the fully charged (a) and discharged (b) battery along with the results of the Rietveld refinements. Phase constituents, from bottom to top: (a) Li 0.2 (Ni,Mn,Co)O 2 , Li 0.4 CoO 2 , LiC 12 , LiC 6 , Cu, Fe, Al; (b) Li(Ni,Mn,Co)O 2 , LiCoO 2 , LiC 12 , C, Cu, Fe, Al. Further to neutron diffraction, conventional neutron radiography is an excellent way of imaging various types of battery cells, where the high penetration of neutrons appears to be extremely useful for imaging a battery interior during in-situ studies. Experiments with ICR 10440 battery cell were performed at ICON cold neutron imaging facility at SIN, PSI and some results are shown in Figure 10 [41] and show variations of lithium content as related to the state of the battery.

Figure 10 .

 10 Figure 10. Change in contrast as related to the state of charge of the battery within the yellow marked area (right image) which are caused by the change in lithium content.

  11 H 12 or Li 2 B 12 H 12 |S Recently, beside LiBH 4 -type electrolytes, other closo-type complex hydrides containing socalled closo-type (cage-like) complex anions (such as B 12 H 12

	2-, CB 11 H 12	-, and CB 9 H 10	-

Table 1

 1 FeH 6 ) have been also tested with success. Regarding electrolytes, LiBH 4 -type ones are mainly used and improved with various substitutions (LiI, Li 2 S, P 2 S 5 , …) though other non-hydride systems were also tested (like 80Li 2 S+20P 2 S 5 ). Half-cells were investigated in potential windows between 0.05-0.3 and 1-2 V at T=120 °C according to the enhanced Li conductivity of the high-temperature LiBH 4 phase except for two of them for which a low temperature electrolyte (Li(BH 4 ) 0.75 I 0.25 + (Li 2 S) 0.75 +(P 2 S 5 ) 0.25 ) allowed to work close to RT (30 °C). All systems give rather high capacities at the first discharge (more than 650 mAhg -1 )

	gathers the

Table 1 .

 1 Half-and full-cell systems developed with metallic and/or complex hydrides as electrode or electrolyte. Works done by the IEA-Task 32 partners are highlighted in grey (CE: Coulombic Efficiency; AC: Activated Carbon; VGCF: Vapor Grown Carbon nanoFibers; KB: Ketjen Black; PVDF: PolyVinyliDene Fluoride).ConclusionMuch progress has been achieved in fundamental research to understand the properties of metallic or complex hydrides. Their ability to undergo conversion reactions with lithium to function as capacitive anodes or to transport Li ions to perform as efficient super-ionic conductors has been demonstrated by many research groups worldwide. Those efforts have construction of practical batteries showing better and safer electrochemical properties than classic commercial materials. Though still at its beginning, this research opens a new field for solid-state lithium batteries.

	Cell type	Active material	Conductor/Binder	Electrolyte	Counter	-	E +	Current density	Rate	T	1	st disch.	Rever-sibitity	Cycling	CE	Reference
						(V)	(V)				°C	mAhg -1	mAhg	-1	Cyc.	%
	Half-	TiH 2	AC 30	LiBH 4	Li	0.05	1	400 mAg -1	C/4	120		1052	878		50	86	[44]
	Half-	MgH 2 (Nb 2 O 5 1%)	VGCF nanofiber	80Li 2 S-20P 2 S 5	Li	0.3	1	200-8000 mAg -1	0.1-4C	120	1382-1731	840		1	st	60.8-96.7	[45]
	Half-	MgH 2 (Nb 2 O 5 1%)	VGCF nanofiber	80Li 2 S-20P 2 S 5	Li	0.3	1	200-8000 mAg -1 0.1-4C	120		1214	575		1 st	47.3	[45]
	Half-	MgH 2 (Nb 2 O 5 1%)	AC black	LiBH 4	Li	0.3	1	100-3200 mAg -1	-	120		1650	924		50	99.5	[46]
	Half-	MgH 2 (CoO 25%)	AC black	LiBH 4	Li	0.3	1	-		0.05-0.5C	120		1240	900		20	99	[10]
	Half-	MgH 2 (Al 2 O 3 5%)	AC black	80Li 2 S-20P 2 S 5	Li	0.1	2	0.05 mAcm -2	-	100		1171	861		10	52	[42]
	Half-	MgH 2	AC + PVDF	Li(BH 4 ) 0.75 I 0.25 + (Li 2 S) 0.8 (P 2 S 5 ) 0.2	Li	0.2	1.8	10 µAcm -²	-	30		652	550		3	-	[11]
	Half-	Mg 2 FeH 6	AC MTI Corp.	LiBH 4	Li	0.3	1	-		C/50	120		1200	300		10	95	[8]
	Full-	TiS 2	none	Li(BH 4 ) 0.75 I 0.25 + (Li 2 S) 0.75 (P 2 S 5 ) 0.25	Li	1.6	2.7	10 µAcm -²	C/100 C/20	50		239	200		6	96	[11]
	Full-	TiS 2	none	LiBH 4	Li	1.6	2.7	230 µAcm -²	C/5	120		205	180		300	99	[15]
	Full-	TiS 2	none	LiCB 11 H 12	Li	1.75	2.6	285 µAcm -²	C/5	130		200	175		5	95	[30]
	Full-	TiS 2	none	Li 2 B 12 H 12	Li	1.6	2.7	239 mAg	-1	C/20	80		228	180		20	-	[29]
	Full-	Sulfur	KB + AC +PVDF	LiBH 4 @ SiO 2	Li	1	3.5	25 µAcm -²	C/33	55		1570	1220	40	99.6	[17]
	Full-	Sulfur	Maxsorb + KB	LiBH 4	Li	1	2.4	250 µAcm -²	C/20	120		1140	700		45	99	[14]
	Full-	Sulfur	Maxsorb + KB	LiBH 4 + LiCl	Li	1	2.5	150 µAcm -²	C/33	100		1377	600		5	99	[13]
	Full-	Sulfur	KB 600	LiCe(BH 4 ) 3 Cl	Li-In	0.6	2.4	13 µAcm -²	C/100	45		870	510		9	73	[51]
										C/50
	Full-	Li 2 S/S	C65	LiBH 4	MgH 2 +TiH 2	0.8	2,5	-		C/20	120		910	780		25	97	[35]
										C/10
	Full-	LiCoO 2	Li 3 PO 4 coating	LiBH 4	Li	3	4.2	65-130 µAcm -²	-	120		89	86		30	-	[33]
	Full-	Li 4 Ti 5 O 12	C65+PVDF	Li(BH 4 ) 0.81 I 0.19	Li	1	2.6	12.7 µAcm -²	-	60		142	10		200	[25]

Acknowledgments

Part of this research was funded by the European Marie Curie Action under ECOSTORE grant agreement no. 607040, the Villum Foundation (grant no. VKR023453) and the Danish Council for Independent Research (grant no. 4184-00143A and 4181-00462). This work was also financially supported by Research Council of Norway under the program EnergiX, Project no. 244054, LiMBAT -"Metal hydrides for Li-ion battery anodes" and NordForsk and The Nordic Neutron Science Program via the project FunHy (Project no. 81942). Petra de Jongh and Peter Ngene acknowledge support from NWO-ECHO 712.015.005, and the European Union's Horizon 2020 research and innovation program (ERC-2014-CoG No 648991). All authors are thankful to the International Energy Agency; Hydrogen Implementing Agreement, and especially to all the dedicated and IEA-HIA Task-32 Researchers for their inspired and inspirational work.


Nour- Eddine Filippov

Menad

A Geneyton

L O Filippov
email: lev.filippov@univ-lorraine.fr

N.-E Menad

Effects of atom substitutions and dissolved carbonate species on monazite electrophoretic mobility

Keywords: Rare-earth minerals, Lanthanides, Monazite, Electrophoretic mobility, Zeta potential, Substitution, CO2, carbonate Light rare-earth elements, IEP: Iso-electric point, PZC: Point of zero charge, XRD: X-ray diffraction, EDL: Electrical double layer

Introduction

Electrophoretic mobility and potentiometric titration are two crucial techniques to study mineral surface properties. Significant discrepancies in the IEPs and the PZCs of monazite and other rare-earth semi-soluble salts minerals were found in the literature [START_REF] Owens | Zeta potentials of the rare earth element fluorcarbonate minerals focusing on bastnäsite and parisite[END_REF][START_REF] Houot | Recovery of Rare Earth Minerals, with Emphasis on Flotation Process[END_REF][START_REF] Chelgani | A review of rare earth minerals flotation: Monazite and xenotime[END_REF]. In the case of monazite, published IEPs range mainly from 5 to 6.5, although IEPs outside this range are also reported (see Table 1). Beyond variations in apparatuses and analytical protocols, different explanations, listed below, were proposed to explain such variability in the monazite surface properties.

A common explanation is that the atom substitutions that commonly affect the monazite lattice result in the exposure of the substituting atoms or point defects on the mineral surface changing its properties [START_REF] Cheng | Technical Note -The point of zero charge of monazite and xenotime[END_REF][START_REF] Zhang | Surface chemistry considerations in the flotation of rare-earth and other semisoluble salt minerals[END_REF]. This phenomenon has already been described for certain minerals, such as cassiterite or ilmenite [START_REF] Balachandran | The influence of point defects on the floatability of cassiterite, I. Properties of synthetic and natural cassiterites[END_REF][START_REF] Mehdilo | Effect of chemical composition and crystal chemistry on the zeta potential of ilmenite[END_REF]. In the case of monazite, different types of elements may be naturally incorporated in the monazite lattice. Although cerium, lanthanum, praseodymium and neodymium are generally predominant, heavier rare-earth elements may be present in significant amounts in the monazite chemical composition [START_REF] Zhu | Monazite chemical composition: some implications for monazite geochronology[END_REF]. To a certain extent, trivalent lanthanides may be directly substituted by tetravalent actinides forming lattice vacancies [START_REF] Clavier | Crystal chemistry of the monazite structure[END_REF]. Coupled substitutions of trivalent lanthanides by divalent elements, notably calcium, and tetravalent elements are also reported and explains the observed incorporation of up to 31.5 wt% of ThO2 and 15.6 wt% of UO2 in the monazite lattice [START_REF] Clavier | Crystal chemistry of the monazite structure[END_REF][START_REF] Podor | Experimental study of Th-bearing LaPO4 (780 °C, 200 MPa); implications for monazite and actinide orthophosphate stability[END_REF][START_REF] Gramaccioli | A uranium-and thorium-rich monazite from a south-alpine pegmatite at Piona, Italy[END_REF]. Furthermore, coupled substitution of LREE and phosphate by actinides and silicate may occur and explain the frequent presence of silicon in monazite chemical composition [START_REF] Seydoux-Guillaume | Experimental determination of Thorium partitioning between monazite and xenotime using analytical electron microscopy and X-ray diffraction Rietveld analysis[END_REF]. LREE may also be directly substituted by alkaline earth metals, notably calcium and strontium, leading to oxygen vacancies [START_REF] Clavier | Crystal chemistry of the monazite structure[END_REF].

Monazite surface properties are also suggested as being significantly affected by the crystal defects caused by the radioactive decay chain of the radiogenic actinides isotopes naturally present within the monazite lattice [START_REF] Nduwa-Mushidi | Surface Chemistry and Flotation Behaviors of Monazite-Apatite-Ilmenite-Quartz-Rutile-Zircon with Octanohydroxamic Acid[END_REF]. This assumption is based on the fact that the radioactive decay chains of actinides elements, such as thorium, release recoil nuclei, and the induced collision cascades are known to generate intense atom displacements, which may ultimately induce an amorphisation [START_REF] Was | The Displacement of Atoms[END_REF][START_REF] Devanathan | Molecular dynamics simulation of energetic uranium recoil damage in zircon[END_REF]. However, in the case of monazite, the influence of crystal lattice defects on zeta potential is likely to be limited, since generally only isolated defects are observed [START_REF] Seydoux-Guillaume | Microstructure of 24-1928 Ma concordant monazites; implications for geochronology and nuclear waste deposits[END_REF][START_REF] Seydoux-Guillaume | An XRD, TEM and Raman study of experimentally annealed natural monazite[END_REF]. A recent study highlights that despite exposure to high doses of radiation, a crystalline state is maintained due to the electronic energy loss of released alpha particles that heal monazite lattice defects [START_REF] Seydoux-Guillaume | Why natural monazite never becomes amorphous: Experimental evidence for alpha self-healing[END_REF].

It cannot be excluded that variability in published IEPs is induced by the presence of mineral inclusions in monazite crystal, or the presence of middling grains and non-monazite grains in analysed materials. Indeed, large and pure monazite specimens are scarce and electrophoretic mobility measurements are frequently performed on monazite samples concentrated from heavy minerals sands (see Table 1).

In aqueous media, free lanthanide ions are known to have a strong affinity for dissolved carbonate species [START_REF] Kragten | Hydroxide complexes of lanthanides-VIII: Lanthanum(III) in perchlorate medium[END_REF]. In a previous study, we examined the affinity of monazite surface lanthanide ions with carbonate species using synthetic monazites. It was shown that treatment with significant amounts of sodium carbonate did not result in mineral surface carbonation after thorough rinsing with deionised water [START_REF] Geneyton | Advances in carboxylate collector adsorption on monazite surface: Part 1 -Assessment of the hydroxylation and carbonation of surface lanthanide ions[END_REF]. However, in the literature, the monazite electrophoretic mobility has been reported tending towards zero, for a broad pH range, when measured in a Na2CO3 solution rather than NaCl solution [START_REF] Espiritu | The effect of carbonate ions on the electrokinetic properties of bastnaesite, monazite and xenotime[END_REF]. This may suggest that the monazite surface ions react with dissolved carbonate species, although this interaction is not strong enough to result in detectable surface carbonation after thorough rinsing. This interaction may explain the variability observed in published IEP values and suggest that the aqueous media temperature that controls the solubility of carbon dioxide [START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF], and thus the concentration of carbonate species in the bulk, affect the monazite electrophoretic mobility. Note: H.S. = heavy mineral sands; CB=carbonatite

The aim of this study is to investigate the effect of atom substitutions on the monazite electrophoretic mobility. To this end, a comparative study of the electrophoretic mobility of a natural monazite with a complex composition and some synthetic monazite materials with various chemical compositions was performed. The natural presence of dissolved carbonate species during the treatment of monazite was also considered as a factor affecting electrophoretic mobility.

Materials and methods

Monazite samples

A monazite powder was prepared using a monocrystal from Manangotry (Madagascar).

Further information on the sample origin, crystal size reduction, and impurities removal is detailed by [START_REF] Richard | Monazite Alteration in H2O ± HCl ± NaCl ± CaCl2 Fluids at 150 o C and psat: Implications for Uranium Deposits[END_REF], who analysed and prepared this sample [START_REF] Richard | Monazite Alteration in H2O ± HCl ± NaCl ± CaCl2 Fluids at 150 o C and psat: Implications for Uranium Deposits[END_REF]. Based on 32 electron microprobe analyses, they showed that the composition of this monazite powder was highly homogeneous. A typical electron microprobe analysis is presented in Table 2. Pure cerium, lanthanum and neodymium monazite materials were the three synthetic materials studied. Cerium, lanthanum and neodymium monazite precursors were obtained through a two-step chemical synthesis. Lanthanide nitrates with the formula Ln(NO3)3.6H2O (Ln: lanthanides) and USP grade phosphoric acid were used for the monazite precursors synthesis.

Cerium nitrate (99 wt% purity) and neodymium nitrate (99.9 wt% purity) were supplied by Sigma Aldrich, lanthanum nitrate (99.9 wt% purity) was supplied by Merck. A diluted H3PO4 solution (55 wt% H3PO4) was heated to 90-95 °C, and rare-earth nitrates were added at a weight ratio of 1 to 4 (rare-earth nitrates to diluted phosphoric acid). Once complete dissolution was achieved, which takes a few seconds, the solution was quenched in a large volume of boiling deionised water (18 MΩ cm) resulting in fast precipitation of lanthanide orthophosphates with rhabdophane structure. The suspension was removed from the heat, stirred for 30 minutes, and filtered. The filtrate was then rinsed several times and dried.

In a study which will later be the subject of a dedicated paper, we highlighted that the rhabdophane-to-monazite thermal conversion is a three steps process. First dehydration of the rhabdophane structure occur below 300 °C, then a transitional monazite structure is obtained at around 600 °C, and finally, recrystallisation of the monazite occurs at around 1100 °C. In order to ensure total conversion of hydrated orthophosphates with a rhabdophane structure into dehydrated orthophosphates with a monazite structure, the dried precipitates were successively heated at 350 °C, 850 °C, and 1250 °C for 20 min at each temperature. This thermal treatment was performed in an electric muffle furnace using glassy carbon crucibles. The XRD diffractograms presented in Figure 1 confirmed that the thermal treatment led to the monazite structure. Further details on the synthesis of rhabdophane materials and further explanations on the selected thermal conversion route will be given in a dedicated paper.

In order to perform electrophoretic mobility measurements, the natural and synthetic monazites were pulverised in an agate planetary ball mill (Fritsch Pulverisette) for 30 minutes.

X-ray diffraction (XRD)

The synthetic monazites were analysed at the LIEC laboratory using a Bruker D8-Advance diffractometer equipped with a cobalt anticathode (35 kV -45 mA) as an X-ray emitting source (λ=1.78897 Å), θ-2θ goniometer, and LynxEye energy-dispersive one-dimensional detector.

XRD analyses were performed with a 2θ angle ranging from 3° to 64°, a step size of 0.034° and an acquisition time of 3 seconds. The synthetic materials were identified by comparing their XRD patterns to standard line patterns available from the "Powder Diffraction File 2000" database.

Electrophoretic mobility measurement and zeta potential calculation

Electrophoretic mobility measurements were performed using a ZetaCompact zetameter from CAD Instruments. With this device, particles are illuminated by a laser sheet and their electrophoresis directly observed by camera. This direct observation allowed fine control of the absence of settling particles. A distribution of the particle electrophoretic mobility was obtained, and the Smoluchowski equation was used to calculate a zeta potential distribution [START_REF] Hunter | Zeta Potential in Colloid Science -Principles and Applications[END_REF]. Prior to any series of measurements, the electrodes conductivity was controlled with a solution of 1 x 10 -2 mol L -1 KCl, and the stationary layer setting was controlled with a standard suspension of TiO2 whose zeta potential must be in the range of -30 mV to -40 mV.

In order to see the effect of dissolved carbonate species on the monazite electrophoretic mobility, two distinct treatment protocols were followed. With the first protocol, the electrolyte solution was generated with deionised water (18 MΩ cm) equilibrated with air by shaking, using a mechanical shaker. This equilibrium was achieved at variable room temperature (20 °C or 30 °C). The monazite treatment was performed in an open beaker at the same room temperature.

With the second protocol, the electrolyte solution was generated with deionised water boiled for two hours to decrease the amount of dissolved carbonate species. After the addition of potassium chloride, the electrolyte solution was placed for cooling inside individual sealed vessels. The monazite samples were treated in a closed glass reactor equipped with five inlets in which a pH probe, an air tube, syringes (containing KOH and HCl solutions), and a sampling pipe connected to the zetameter were hermetically inserted (see Figure 2). After pouring the deionised water, the air inside the reactor was flushed with pure dinitrogen, and then a dinitrogen overpressure (circa 15 mmHg) was applied to limit exchanges with the atmosphere and consequent CO2 contaminations. The same procedure was followed to insert the monazite material.

These two treatment protocols shared common steps. 60 mL of the solution of deionised water containing 10 -3 mol L -1 KCl as supporting electrolyte was pH adjusted with hydrochloric acid or potassium hydroxide solutions produced with CO2-depleted or air-equilibrated deionised water.

24 mg of milled monazite was added in the reactor, and the suspension was stirred for 10 minutes under continuous pH adjustment. The mineral suspension was then pumped into the zetameter located in a second room whose temperature was maintained at 20 °C. It is thus important to differentiate the variable treatment temperature and the fixed measurement temperature of 20 °C.

A delay of 10 minutes was required for settling coarse particles inside the zetameter measuring cell and detecting solely fine particles whose motion was Brownian in nature. A population of between 500 and 1500 particles on the stationary layer was typically observed after coarse particles settling. For each pH condition, measurements were repeated five times. Calculated standard deviations typically ranged from 0.1 to 0.3 mV and did not exceed 0.5 mV.

Results and discussion

Zeta potential of monazite treated in an electrolyte solution equilibrated with air

As discussed in the introduction, it is suspected that dissolved carbonate species influence the monazite electrophoretic mobility. Therefore, in the case of monazite, the temperature that affects the carbon dioxide solubility and thus the carbonate species concentration in the bulk may be a determining parameter of zeta potential. A slight increase of the temperature of the electrolyte solutions may indeed significantly decrease the carbon dioxide solubility [START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF]. Therefore, in order to see the influence of LREE substitutions and the treatment temperature, the synthetic monazites were treated at a room temperature of 20 °C and 30 °C and in the pH range of 4 to 9 using an electrolyte solution equilibrated with the atmosphere. The obtained zeta potential values reported in Figure 3 outline the influence of the treatment temperature. Indeed, the zeta potential of the synthetic cerium monazite treated at 20 °C was found closer to 0 mV than when treated at a higher temperature. Since the zeta potential of the standard TiO2 suspension was unchanged and the temperature of each cerium monazite suspensions was stabilized at 20 °C after the settling period inside the zetameter, those zeta potential differences may be explained only by a difference in the treatment temperature, and thus a possible difference in the amount of dissolved carbonate species. The data presented in Figure 3 also highlight that when using electrolyte solutions equilibrated with air, the electrophoretic mobilities of the synthetic monazites are identical. The IEP of the three synthetic monazites treated at 30 °C was observed at pH 5.9. These results suggest that LREE substitutions will not affect the monazite surface properties. In order to see the effect of the other atom substitutions that may occur in the monazite lattice, the electrophoretic mobility of the cerium monazite material was compared to that of a natural monazite sample with a complex chemical composition. For this comparison, an electrolyte solution equilibrated with air was used, and both monazite treatment and measurement were performed at a temperature of 20 °C. As depicted in Figure 4, the electrophoretic mobility of monazite was found to be unaffected by the presence of substituting atoms such as thorium, contrary to what was suggested in the literature.

Zeta potential of monazite treated in CO2 depleted electrolyte solution

In order to confirm the influence of dissolved carbonate species on the monazite electrophoretic mobility, the synthetic cerium and lanthanum monazites were treated at 30 °C, in a controlled atmosphere, using a CO2 depleted electrolyte solution. The obtained zeta potential values are presented in Figure 5. From a general perspective, although erratic zeta potential values were observed, it seems that the presence of lower amounts of dissolved carbonate species led to higher electrophoretic mobilities. Certain zeta potential values obtained when treating monazite under controlled atmosphere are similar to the ones obtained when the electrolyte solutions were equilibrated with air. Therefore, we assume that the inconsistency of the zeta For pH below the IEP, the treatment of monazite under open atmosphere rather than controlled atmosphere, and thus the presence of higher amounts of dissolved carbonate species in the electrolyte solution, decrease its zeta potential. This decrease may be attributed to chemical adsorption on surface lanthanide ions of bicarbonate ions, which are present in relatively low quantities in the bulk according to the speciation diagrams presented in Figure 6. This decrease may also be attributed to a concentration of bicarbonate ions in the EDL that compensates the positively charged monazite surface. However, at pH above the IEP, an increase in the amount of dissolved carbonate species induced higher zeta potential values, which is inconsistent with a simple presence of additional negative charges in the EDL or on the mineral surface. In a previous study, we suggested that the under-coordinated lanthanide ions on the monazite mineral surface may react with dissolved carbonate species [START_REF] Geneyton | Advances in carboxylate collector adsorption on monazite surface: Part 1 -Assessment of the hydroxylation and carbonation of surface lanthanide ions[END_REF]. However, in this study, as mentioned in the introduction section, after treatment with sodium carbonate and thorough rinsing with deionised water, surface probing did not highlight a significant carbonation, suggesting that the interactions between the monazite surface ions and dissolved carbonate species are not strong in nature. It may be proposed that at pH above the IEP, the presence of carbonate species on the mineral surface or within the EDL significantly affect the monazite surface properties or the EDL structure, resulting in a decrease of electrophoretic mobility. At around pH 9, despite higher stability of dissolved carbonate species in aqueous media (see Figure 6), and thus higher kinetics of CO2 solubilisation, the influence of these species on the monazite electrophoretic mobility seems limited. Therefore, it is hard to conclude whether carbonate contamination becomes too significant or hydroxide ions become the major potential determining ions. To conclude on the effect of dissolved carbonate species, it was shown that when equilibrating the electrolyte solutions with air and controlling the room temperature, no erratic zeta potential values were observed. The compilation of all these results highlights that significant erraticness of zeta potential values appeared when attempting to decrease the amount of dissolved carbonate species without fine control of exchanges between the electrolyte solutions and the atmosphere.

Hence, we recommend the use of a glove box and monitoring of dissolved CO2 to study the influence of carbonate species on the monazite zeta potential.

Conclusions

This study demonstrates the limited effects of the monazite chemical composition on its electrophoretic mobility. The electrophoretic mobilities of synthetic cerium, lanthanum and neodymium monazite materials and of a natural monazite sample which present a complex chemical composition were found identical when measured in electrolyte solutions equilibrated with air.

Conversely, it was shown that dissolved carbonate species have a strong influence on the monazite electrophoretic mobility. Indeed, the monazite treatment protocol (controlled and open atmosphere) and the temperature of the electrolyte solution during this treatment significantly affect the monazite electrophoretic mobility. The results underscore that carbonate species reduce the monazite electrophoretic mobility. The decrease of electrophoretic mobility at pH above the IEP may not be easily explained and suggests that the influence of dissolved carbonate species on the monazite surface properties or on the EDL structure is complex.

Electrophoretic mobility measurement is a common method to determine the zeta potential of a mineral and to study the phenomena that occur on the mineral surface, such as the adsorption of surfactants of metal species. Therefore, in the case of monazite, we suggest to strictly control the temperature and amount of dissolved carbonate species in order to achieve reliable zeta potential comparisons and surface mechanism deduction. Furthermore, this control appears essential to avoid erraticness of zeta potential values that decrease the precision of the IEP calculation.

Figure 1 :

 1 Figure 1: X-ray diffractograms of the synthetic monazite materials

Figure 2 :

 2 Figure 2: Schematic drawing of the glass reactor used in this study

Figure 3 :

 3 Figure 3: Effect of the chemical composition of synthetic monazites and treatment

Figure 4 :

 4 Figure 4: Effect of the complex chemical composition of the natural monazite on the

  potential values was caused by carbon dioxide contamination. A trendline of the expected zeta potential in absence of carbon dioxide contamination may be drawn, pointing out the shifting of zeta potential towards the values observed when treating monazite in open atmosphere, and thus in the presence of larger amounts of dissolved carbonate species. Interestingly, this trendline reveals an IEP similar to the one obtained when the electrolyte solutions were equilibrated with air.

Figure 5 :

 5 Figure 5: Effect of the exchanges with the atmosphere during the treatment on the

Figure 6 :

 6 Figure 6: Carbonate species concentrations (left) and related speciation diagram (right)

Table 1 : Measured IEP values of monazite reported in the literature

 1

	V
		Monazite origin	References
	alue
	3.
		Korea (H.S.)	Choi et Whang, 1963 [23]
	5
	4.
		China	Ren et al., 2000 [24]
	7
	5	Bayan Obo, China (CB)	Luo & Xiangyong, 1987 [25]
	5	Namibia (CB)	Espiritu & Waters, 2018 [26]
	5.	São Gonçalo do Sapucai,
			Pavez et al., 1996 [27]
	2	Brazil
	5.
		Australia (H.S.)	Cheng et al., 1993 [28]
	3
	5.		Nduwa-Mushidi & Anderson, 2017
		New Mexico, USA
	3		[13]
	5.
		China	Zhang & Honaker, 2018 [29]
	3
	5.
		Bayan Obo, China (CB)	Li et al., 2018 [30]
	4
	5.
		Near Rosetta, Egypt	Abeidu, 1972 [31]
	5
	5.
		Brazil	Azizi et al.,2016 [32]
	9
	5.
		Itambé, Brazil	Sarvaramini et al., 2016 [33]
	9
	6	Hebei, China	Zhang & Honaker, 2017 [34]
	6.
		China	Zhang et al., 2017 [35]
	5
	6.	Not indicated	Abaka-wood et al., 2017 [36]

Table 2 : Electron microprobe analysis of the monazite monocrystal expressed in weight percent and reproduced from Richard et al. (2015) [38]

 2

	Composition (wt %) P2O5 SiO2 Ce2O3 La2O3 Nd2O3 Pr2O3 Sm2O3 Gd2O3 Dy2O3 CaO Y2O3 ThO2 UO2 PbO Total
	Average	26.58 1.99 28.31 13.76 9.78 2.91 1.05	0.43	0.04 0.95 0.12 12.93 0.28 0.31 99.44
	Standard deviation	0.30 0.13 0.48 0.41 0.39 0.35 0.28	0.27	0.08 0.08 0.07 0.75 0.09 0.17	-

Acknowledgements

This research was funded by the Carnot Institutes ICEEL and BRGM through the TERRAFINE Inter-Carnot Action (Carnot-ICEEL no. 118-IC and IC-BRGM no. 2013-01).

Financial support from LabEx RESSOURCES21 is also acknowledged (contract Investissements d'Avenir no. ANR-10-LABX-0021).

The authors would like to thank Antonin Richard, Jean-Marc Montel and Cecile Fabre for assisting in the preparation and characterisation of the natural monazite sample, and the synthesis of monazite materials. Pierric Hubert is also acknowledged for the XRD analyses.