or Na-ion cells

The present invention relates to the field of Na-ion cells and provides a new Na- based material. The present invention also relates to a method of preparation of said material and its use as a positive electrode material.

The present invention also relates to a positive electrode material comprising said Na-based material, and to a Na-ion cell comprising said positive electrode.

The most appealing alternative to Li-ion batteries regarding chemical element abundance and cost is by all means sodium. Batteries using sodium ions as shuttling ions in place of lithium ions are employed for use in place of lithium in applications where the stored energy density is less critical than for portable electronics or automotive transport, more particularly for the management of renewable energies. Such awareness has prompted the revival of the Na-ion battery concept with intense activity devoted to the search of highly performing electrode material.

The performance of the Na-ion batteries is related to the capacity of the positive electrode material which today mostly uses the Na ₃ V ₂ (P0 ₄ ) ₂ F ₃ material (NVPF). Said material is capable of inserting and de-inserting two sodium ions per formula unit leading to a reversible capacity of 120 mAh/g.

Bianchini et al. ( Chemistry of Materials, 2014, 26(14) : 4238-47) discloses the preparation of Na ₃ V ₂ (P04)2F ₃ material and its use as a positive electrode material, and explains that only the reversible extraction of two sodium ions may be achieved in a Na- ion cell.

Gover et al. ( Solid State Ionics 177 (2006), 1495-1500) discloses that the full extraction of sodium from the Na ₃ V ₂ (P04)2F ₃ material induces some irreversible structural degradation to the Li//Na ₃ V ₂ (P0 ₄ ) ₂ F ₃ cell used for said extraction. Thus, the removal of the third sodium ion was never exploited as the process was thought to be irreversible.

Thus, there is a need to provide a new Na-based material useful in Na-ion cells, said cells exhibiting improved performances in comparison to the Na-ions cells of the prior art.

The aim of the present invention is to provide a Na-ion cell which presents an increased reversible capacity, and an increased energy density in comparison of the Na- cell ion described in the prior art. Another aim of the present invention is to provide a positive electrode material which presents a better cycle life in comparison to the prior art.

Material of formula (!)

Thus, the present invention relates to a material of formula (I):

Na3-xnxV(2- _y )M _y (P0 ₄ )2F(3-z)0z/2 (I)

wherein:

2 < x < 3;

0 < y < 1 ;

0 < z < 1 ;

_□ represents a vacancy; and

M represents a transition or a non-transition metal ion.

Preferably, in the material of formula (I) of the invention, M is selected from the group consisting of magnesium, zinc, iron, aluminum, nickel, cobalt, chromium, titanium, manganese and mixtures thereof, preferably M is aluminum.

Typically, x, y and z are chosen so as to ensure the electroneutrality of the compound.

Advantageously, in the material of formula (I), y = 0 and/or z = 0.

In a preferred embodiment according to the present invention, in formula (I), x is 2.25, 2.35, 2.5 or 2.75.

Preferably, the material of formula (I) can reversibly uptake and remove more than two sodium ions.

Advantageously, the material of formula (I) is disordered, meaning that sodium ions adopt a disordered arrangement. Accordingly, the material of formula (I) presents a structure of tetragonal space group 14/mmm. Said disordering is irreversible. The structure of the material of formula (I) can be spotted by XRD diffraction. The higher the x value, the higher the disordered phase formation is observed.

Method of preparation of the material of formula (I)

The present invention also relates to a method of preparation of the material of formula (I), comprising the following steps:

i) preparation of a Na-ion cell comprising at least one positive electrode, at least one negative electrode, and at least one electrolyte, said positive electrode comprising a Na-based material (A) selected from the group consisting of: o Na ₃ V ₂ (P04)2F3;

o Na3V( _2-y) M _y (P04) ₂ F ₃ ; o Na ₃ V(2-y)M _y (P04)2F _(3-z) 0z/2; and

o mixtures thereof;

y, z and M being as previously defined in formula (I);

ii) galvanostatically and constant-voltage charging of the Na-ion cell obtained at the end of step i) at a potential comprised between 4.3 V and 4.8 V (Na ⁺ /Na°), preferably at 4.8 V, and maintaining said cell at such potential till more than two Na ions are extracted from the material (A), in which the amount of Na ions is determined by coulometry; and

iii) obtaining the material of formula (I).

Advantageously, with such method, up to three sodium ions may be reversibly extracted from the material (A) in order to obtain the material of formula (I).

Preferably, in step ii) of the method of the invention, the amount of the sodium ions extracted is measured by coulometry, especially by controlled-potential coulometry, i.e., the potential applied to the cell is constant. Coulometry enables counting the number of electron which has passed through a material. Specifically, the current is monitored as a function of time so as to calculate the total charges which have passed through the materials. Said technique may be implemented either in a galvanostatic mode (i.e. a current value is fixed) or potentiostatic mode (constant voltage mode). Said charges are proportional to the amount of sodium ions extracted from the material (A).

The potential applied in step ii) is comprised between 4.3 V and 4.8 V (Na ⁺ /Na°). (Na ⁺ /Na°) meaning that the potential is defined with respects to the potential of Na ⁺ /Na°, which is the reference potential.

In step ii) of this method, a galvanostat or a potentiostat such as Bio-Logic© may be used.

In this method, the material of formula (I) is prepared in-situ.

Advantageously, after step iii), the sodium ions (more than 2) that have been extracted from the material (A) to form the material of formula (I) can be re-inserted back into said newly formed material of formula (I) by discharging the potential applied to the Na-ion cell from between 4.3 V and 4.8 V down to 1 V.

The material of formula (I) obtained at the end of step iii) of the method of the invention may be used immediately or stored, preferably under an inert atmosphere. Positive electrode

Advantageously, the positive electrode implemented in the above-mentioned method comprises the material (A), a current collector, a polymer binder and an electronic conductive agent.

Preferably, the positive electrode further comprises a Na-based oxide material (B) such as Na _x -MO ₂ (0<x’<1 ) in which M’ represents at least one metal ion selected from the group consisting of nickel, zinc, cobalt, manganese, iron, chromium, titanium, copper, vanadium, aluminum, magnesium, and mixtures thereof. Typically, the material (B) is

Na2/3Fei/2Mn i/ ₂ 02.

The polymer binder as mentioned above confers mechanical strength to the electrode material, and is preferably a polymer which has a high modulus of elasticity (e.g. of the order of several hundred MPa), and which is stable under the temperature and voltage conditions in which the positive electrode is intended to operate. The binder provides high adhesive strength between the electrode materials and the current collector, maintains the electron conductivity, and facilitates the electrolyte wetting into the electrode materials so as to form a stable interface between the electrodes and the electrolyte. The binder may be selected from polyvinylidene difluoride and poly(tetrafluoroethylene), cellulose fibers, cellulose derivatives such as starch, carboxymethyl cellulose (CMC), diacetyl cellulose, hydroxyethyl cellulose or hydroxypropyl cellulose, styrene butadiene rubber (SBR), and mixtures thereof.

The amount of binder may vary from 0 to 40 wt. %, preferably, from 1 to 10 wt. % with regard to the total weight of the materials of formula (A) and (B).

The electronically conductive agent may be selected from the group consisting of carbon black, Super P® carbon black, acetylene black, ketjen black®, channel black, natural or synthetic graphite, carbon fibers, carbon nanotubes, vapor grown carbon fibers or a mixture thereof.

According to the present invention,“carbon black” is a material having a form of paracrystalline carbon, presenting a high ratio of surface area/volume, and is mostly issued from the incomplete combustion of heavy petroleum products. Acetylene black and channel black are subtypes of carbon black.

According to the present invention, carbon fibers, carbon nanotubes and vapor grown carbon fibers are cylindrical nanostructures of carbon comprising graphene layers arranged as stacks cones, cups or plates. The amount of electronically conductive agent may vary from 0 to 40 w%, preferably, from 1 to 10 wt. % with regard to the total weight of the materials of formula (A) and (B).

The current collector is a material characterized by a high electrical conductivity; a resistance to corrosion, a mechanical strength and flexibility allowing it withstands manufacturing operations (e.g., pasting and rolling), etc. According to the invention, the current collector may be composed of an electron conductive material, more particularly of a metallic material which may be selected from aluminium, copper, nickel, steel, titanium, tungsten, tantalum and carbon foil.

Advantageously, in the positive electrode, the amount of material (A) varies from 0.1 wt. % to 100 wt. %, and the amount of material (B) varies from 0 to 99.9 wt. % with regard to the total amount of the electrode material. Preferably, the amount of material (A) varies from 45 to 100 wt. %, and the amount of material (B) varies from 0 to 55 wt. % with regard to the total amount (weight) of the electrode material.

The positive electrode as defined above and implemented in the method of the invention may be prepared according to the following steps:

a) mixing the material (A) with the binder and the electronic conductive agent, and obtaining a mixture;

b) dispersing the mixture obtained at step a) in a solvent and forming a slurry; c) casting the slurry obtained at the end of step b) onto the current collector, and obtaining a positive electrode;

d) drying the positive electrode obtained at the end of step c) at a temperature comprised between 60°C and 300°C, preferably at 120°C;

e) pressing the dried positive electrode obtained at the end of step d) with a roller machine.

The mixing step a) may be implemented by mixing in a mortar with pestle or by a ball- milling process.

In step b), the mixture obtained at step a) is preferably dispersed in a solvent selected from the group consisting of N-Methyl-2-pyrrolidone (NMP), acetonitrile, ethanol, water and mixtures thereof.. Typically, step c) is carried out using an electrode coating machine.

Advantageously, step e) is carried out with a calendaring machine such as CIS® CLP 2025H. Negative electrode

The above-mentioned negative electrode of the invention may comprise a material (C). Similarly to the positive electrode, the negative electrode may also comprise a current collector, a polymer binder and an electronic conductive agent as previously defined.

Such negative electrode may be prepared according to the following steps:

a') mixing the material (C) with a binder, and an electronic conductive agent, and obtaining a mixture;

b’) dispersing the mixture obtained at step a’) in a solvent and forming a slurry; c’) casting the slurry obtained at the end of step b’) onto a current collector, and obtaining a negative electrode;

d’) drying the negative electrode obtained at the end of step c’) at a temperature comprised between 60°C and 300°C, preferably at 120°C;

e’) pressing the dried negative electrode obtained at the end of step d’) with a roller machine.

The mixing step a’) may be implemented by mixing in a mortar with pestle or by a ball- milling process.

Steps b’)-e’) are similar to steps b)-e) previously defined.

The material (C) of the negative electrode may be selected from the group consisting of hard carbon, antimony, tin, sodium, phosphorus and mixtures thereof.

According to the invention, a "hard carbon” is a carbonaceous compound obeying to a “falling cards model”, in which the graphene layers (2 or 3 layers) are randomly stacked to form nanoscale pores. Said material has been described in Stevens et al. ( Journal of the Electrochemical Society, 147(4) 1271-1273 (2000)) and in Dhan et al. ( Carbon Vol. 35, n°6, pp.825-830 ). The binder and the electronic conductive agent comprised in the negative electrode are similar to the binder and the electronic conductive agent comprised in the positive electrode and as previously defined.

Preferably, the amount of the material (C) varies from 80 to XX 99% with regard to the total amount of the negative electrode material.

The current collector of the negative electrode preferably comprises aluminum.

Electrolyte

The electrolyte implemented in the method of the invention is generally a solution comprising a solvent and a salt, preferably a sodium salt. The solvent may be a liquid solvent, optionally gelled by a polymer, or a polar polymer solvent which is optionally plasticized by a liquid. The liquid solvent may comprise a polar aprotic solvent and may be selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate and esters such as ethyl acetate, ethyl propionate and methyl propionate, and mixtures thereof.

Said liquid solvent may optionally be gelled by addition of a polymer obtained, for example, from one or more monomers selected from ethylene oxide, propylene oxide, methyl methacrylate, methyl acrylate, acrylonitrile, methacrylonitrile, N-vinylpyrrolidone and vinylidene fluoride, said polymer having a linear, comb, random, alternating, or block structure, and being crosslinked or not.

The proportion of liquid solvent in the solvent may vary from about 2% by volume (corresponding to a plasticized solvent) to about 98% by volume (corresponding to a gelled solvent).

Preferably, the electrolyte comprises at least one salt selected from the group consisting of sodium hexafluorophosphate (NaPF ₆ ), sodium perchlorate (NaCIC ), sodium bis (fluorosulfonyl) imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium bis(pentafluoroethanesulfonyl)imide (NaBETI), sodium tetrafluoroborate (NaBF ₄ ), and mixtures thereof.

When the electrolyte is a liquid electrolyte, said liquid electrolyte will be injected into the cell having a separator. The separator may be a conventional polymer-based separator such as a Celgard® separator or a Whatman® borosilicate glass fiber separator, or a cellulose-based separator, such as Dreamweaver® nonwoven nanofiber separator. Advantageously, the electrolyte is a solution comprising a salt of sodium and one or more carbonates selected from ethylene carbonate, propylene carbonate, dimethylcarbonate, ethyl methyl carbonate, diethyl carbonate, vinylene carbonate, and fluoroethylene carbonate.

The electrolyte may further comprise an additive selected from the group consisting of vinylene carbonate (VC) (in an amount ranging from 0.1 to 10 wt.%, preferably from 0.5 to 5.0 wt.%), 1 ,3-Propanesultone (PS) (in an amount ranging from 0.1 to 5 wt.%, preferably from 0.5 to 3.0 wt.%), Succinonitrile (SN) (in an amount ranging from 0.1 to 5 wt.%, preferably from 0.5 to 2.0 wt.%), Sodium difluoro(oxalato)borate (NaODFB) (in an amount ranging from 0.05 to 10 wt.%, preferably from 0.2 to 1 .0 wt.%). Additives can be used individually or mixture of them, in order to achieve the high temperature performance and low self-discharge performance of the Na ion battery.

The electrolyte may be prepared by adding the salt in the solvent under stirring, and then adding the additive into the obtained solution. The adding sequence may be changed, like adding additives into solvent first, then adding salt. The whole preparation is carried out in inert atmosphere (under argon or nitrogen).

Applications

The present invention also relates to the use of the material of formula (I) as defined above, as positive electrode active material for Na-ion batteries.

The present invention also relates to a positive electrode comprising at least one material of formula (I). Said positive electrode may further comprise a current collector.

Na-ion cell

The invention also relates to a Na-ion cell comprising at least one positive electrode as previously defined, at least one negative electrode, and at least one electrolyte as previously defined, and is prepared as previously described.

The Na-ion cell implemented in the method of the invention may be a half-cell using metallic sodium as negative electrode, or a full cell using hard carbon as negative electrode. Typically, the Na-ion cell is a coin cell, a pouch cell, a cylindrical cell or a prismatic cell.

Said Na-ion cell may also comprise at least one separator selected from the group consisting of glass fiber, polyolefin separators, including polypropylene (PP), polyethylene (PE) or a polypropylene/polyethylene/polypropylene film.

Said Na-ion cell may be composed of a single electrochemical cell comprising two electrodes (i.e. one positive electrode and one negative electrode) separated by an electrolyte; or of a plurality of chemical cells assembled in series; or of a plurality of chemical cells assembled in parallel; or of a combination of the two assembly types.

The positive electrode, separator, and negative electrode may be stacked layer by layer (one layer of electrodes by one layer of separator), before being folded and winded to a cell core. The core is then placed inside of the cell shell. After this, the cell is drought at 85°C under vacuum for about 24 hours. The electrolyte is then injected into the cell before said cell is sealed to assembling a Na-ion cell.

Thanks to the fact that the third sodium ion may be reversibly removed of the material of formula (I), the reversible capacity and the energy density of the Na-ion cell comprising said material is increased. FIGURES

Figure 1 concerns the evolution of the voltage (V) with the discharge potential limited to 3 V, as a function of amount of sodium in material of formula (I) for a sodium half-cell having NVPF positive electrode.

Figure 2 concerns the derivative plots of the discharge curves in figure 1 which are used as finger prints to follow the extent of in-situ electrochemical modifications caused in the material of formula (I).

Figure 3 concerns the evolution of the voltage (V), as a function of amount of sodium in the material of formula (I) (cycled between 3 and 4.3 V) for a sodium half-cell having NVPF positive electrode according to the invention.

Figure 4 concerns the evolution of the capacity (in mAh g ^_1 ) as a function of the number of cycles for a for a sodium half-cell (cycled between 3 and 4.3 V) having NVPF positive electrode according to the invention.

Figure 5 concerns the evolution of the voltage (V) with the discharge potential limited to 2 V, as a function of amount of sodium in the material of formula (I) in sodium ion full cells using hard carbon negative electrode and NVPF positive electrode, according to the invention.,

Figure 6 concerns the evolution of the energy (in Wh.Kg ¹ ) as a function of the number of cycles for a for a sodium ion full cell (cycled between 2 and 4.3 V) having hard carbon negative electrode and NVPF positive electrode according to the invention.

Figure 7 concerns the evolution of the voltage (V) with the discharge potential limited to 1 V, as a function of amount of sodium in the material of formula (I) for a sodium half cell having NVPF positive electrode, according to the invention.

Figure 8 concerns the evolution of the voltage (V), as a function of amount of sodium in the material of formula (I) (cycled between 1 - 4.3 V) for a sodium half-cell having NVPF positive electrode according to the invention.

Figure 9 concerns the evolution of the capacity (in mAh g ^_1 ) as a function of the number of cycles for a for a sodium half-cell (cycled between 1 - 4.3 V) having NVPF positive electrode according to the invention.

Figure 10 concerns the evolution of the voltage (V) with the discharge potential down to 0 V, as a function of amount of sodium in the material of formula (I) in sodium ion full cells using hard carbon negative electrode and NVPF positive electrode, according to the invention. Figure 1 1 concerns the evolution of the voltage (V) with the discharge potential limited to 1.8 V, as a function of capacity (in mAh.g ¹ ) for a sodium half-cell having NVPF mixed with layered oxide Na _x M0 ₂ positive electrode, according to the invention.

The present invention is illustrated in more detail in the examples below, but it is not limited to said examples.

Example 1 : Preparation of the material of formula (I).

The materials (A) were modified electrochemically by charging said material to potential higher than 4.3 V and de-inserting more than 2 sodium from the structure.

The electrochemical sodium de-insertion was carried out in half cells with metallic sodium as negative electrode or full cells using Na host material such as hard carbon, antimony, phosphorus, etc. as negative electrode. In both cases, 1 M (mol/litre) NaPF ₆ dissolved in propylene carbonate was used as electrolyte. The charge curves represented in Figure 1 show that the extraction of the third sodium from the structure of material (A) happens through a third plateau at 4.75 V. The charge process was controlled by limiting the amount of sodium that is removed from the NVPF structure on the first charge to 2, 2.25, 2.5 and 2.75 etc. The material of formula (I) thus formed is represented here after as ‘NVPF Dc’ where Dc is the amount of sodium that is removed from the structure of the material (A) on first charge to introduce the electrochemical modifications.

For example, NVPF 2.25 denotes the material of formula (I) wherein x = 2.25.

On subsequent discharge (discharge curves in Figure 1 ), a change in curve shape from biphasic plateau (NVPF 2.0) to S-shape (NVPF 2.25 and so on) is observed and the extent of modification increases with increasing sodium extraction on first charge. Such change in discharge curve shape indicates a structural modification that is caused by the extraction of more than 2 sodium ions from the material (A). The derivative plots of the discharge curves in Figure 2 are used as finger print to follow the extent of structural modifications. The modifications thus produced are irreversible and the cell cycles through S-shape curve on continuous cycling with comparable capacity retention as shown in Figures 3 and 4. Such change in cycling curve from plateau (NVPF 2.0) to S-shape (NVPF >2) offers an advantage to accurately measure and control the cell potential at the given state of charge (SOC) and depth of discharge (DOD), hence helps in simplifying the battery management system in real applications. Example 2: Utilization of the material of formula (I) to improve the energy density of sodium ion cells.

Extraction of more than 2 sodium ions from material (A) is required to prepare the material of formula (I). The excess sodium (higher than 2) thus removed from the material (A) is used to compensate the sodium loss due to solid-electrolyte interface layer formation in the negative hard carbon electrode in the sodium ion full cells. Figure 5 shows the cycling behavior of full cells assembled with same mass balancing between positive NVPF electrode and negative hard carbon electrode. An increase in reversible capacity is observed by moving from NVPF 2 to NVPF 2.35 and it leads to ~10 % raise in overall energy density as shown in Figure 6. The in-situ prepared material of formula (I) also provides better cycle life for the cells (Figure 6) as the cycling happens through a S- shape solid solution instead of maintaining the cell at high voltage (4.2 V) plateau for long time as is the case with NVPF 2.0.

Example 3: Application of the material of formula (I) for over-discharge protection of the sodium ion cells.

The excess sodium (more than 2) that is de-inserted from NVPF to form the material of formula (I) can be re-inserted back into said material by discharging the cells down to 1 V (Figure 7). Thus the in-situ electrochemically modified NVPF structures could be cycled with higher sodium reversibility (Dc= 2.82 for NVPF 2.75) in comparison to the material (A) (Dc= 2.21 for NVPF 2.0) as shown in Figure 8. It leads to increase in specific capacity with all materials showing comparable capacity retention (Figure 9). Further, sodium insertion into the NVPF structure at low potentials (< 1.5 V) could also be used as a protection for the over discharge of the cell. Thus the sodium ion cells with the material of formula (I) could be discharged to 0 V without causing any sodium oxidation of solid electrolytes interphases and associated detrimental issues as shown in Figure 10.

Example 4: Usage of the material of formula (I) for achieving mixed electrodes of high gravimetric and volumetric energy densities.

Na ₃ V ₂ (P0 ₄ ) ₂ F ₃ is mixed with layered Na _x M0 ₂ (x < 1 , M= transition metal ion(s)) (material (B)) by mechanical ball milling and used as a positive electrode for sodium ion cells. The layered sodium transition metal oxides provide higher density (typically 4 to 5 g/cm ³ ) in comparison to that of NVPF (~3.1 g/cm ³ ) material. Hence, the sodium layered oxides offers a better advantage in terms of volumetric energy density. However, the P2, P3 and certain 03 layered oxides (e.g. Na _x CuyFe _z Mni- _y-z 0 ₂ ) are non-stoichiometric and require extra sodium source in the cells for the electrochemical conversion of Na _x M02 to NaiM0 ₂ in order to improve their gravimetric energy density. This was achieved by mixing NVPF with sodium layered oxides and activating the NVPF beyond 2 Na extractions. The sodium thus removed from the NVPF (beyond 2) is used for the electrochemical conversion of Na _x M0 ₂ to NaiM0 ₂ (Figure 1 1 ). The P2 type Na _2/ 3Mgo 3Mn ₀ 70 ₂ is used here to explain the proof of concept; similar principle can be extended to other P2, P3 Na _x M0 ₂ phases and also for 03 phases for which the Na/M ratio is < 1 (e.g. Na _x Cu _y Fe _z Mni- _y-z 0 ₂ ).