Field of the invention

The present invention relates to liquid compositions based on ionic liquids for the protection of lithium metal parts, in particular anodes in lithium electrochemical generators.

It also relates to a method of coating a metal part implementing such a composition, the subsequent polymerization of the composition, and an electrochemical storage system with anodes thus coated.

Technological background

Lithium metal is the anode material of choice for lithium batteries because of its highest theoretical capacity and lowest electrochemical potential of all candidates. However, the charging and discharging cycles of lithium metal anode batteries cause the formation of dendrites and other surface defects, which reduces battery life but can also lead to short circuits and thus serious problems of safety (thermal runaway, explosion, fire).

Due to the highly negative electrochemical potential of the Li /Li redox couple, current liquid electrolytes are reduced on the surface of lithium to form a solid electrolyte interface (SEI) [1]. This passivation allows the operation of the electrochemical cell. The SEI must be an ionic conductor and an electrical insulator of homogeneous composition and morphology. It must also have good properties of flexibility and elasticity [2]. Additives are used in electrolytes. They decompose and participate in the formation of the SEI to improve the properties and therefore the performance of the electrochemical cell.

A very high concentration of lithium salt in the electrolyte can also suppress the growth of dendrites. However, this solution is very lithium salt consuming and therefore very expensive.

One of the approaches is the deposition of a protective layer on lithium before cycling (artificial SEI), by various techniques of thin film deposition.

The development and use of solid electrolytes is a mean of physically preventing the growth of dendrites. They can be organic (polymers), inorganic (ceramics) or hybrid.

EP3049471B1 discloses lithium ion conductive polymer compositions for lithium electrochemical generator, containing at least one non-ionic polymer.

WO2016205653A "Multi-layered polymer coated Li anode for high-density Li metal battery" discloses two layers of polymers.

US9627713B2 "Composite electrolyte including polymeric ionic liquid matrix and embedded nanoparticles, and method of making the same" discloses a composite electrolyte comprising inorganic nanoparticles, hence a hybrid organic / inorganic composition. The layer is both protective of lithium and electrolyte. The battery formed does not contain liquid electrolyte.

US20160164102A1 discloses a protective coating of a metal, organic / inorganic hybrid (inorganic part = ceramic nanoparticles). The coating contains ionic liquids and is obtained by UV polymerization.

US5961672B discloses a stabilized anode for lithium polymer batteries. The technique of depositing the protective film of lithium is a vacuum deposit.

WO2018/122428A1 discloses a coating composition comprising an ionic liquid and a cross-linked polymeric ionic liquid, wherein the cross-linked polymeric ionic liquid and the ionic liquid are not joined via covalent bonds, and wherein the cross-linked polymeric ionic liquid is joined to a surface of the substrate.

The object of the present invention is to overcome these disadvantages by providing a liquid composition to protect lithium metal, which is simpler and less costly to implement than currently available protection compositions.

Summary of the invention

This objective is achieved with an ionic liquid-based composition for the protection of lithium metal anodes in a lithium based electrochemical energy storage system, comprising: a polymerizable ionic liquid (or ionic liquid monomer), the cation or the anion of which carries at least one polymerizable function, a non-polymerizable ionic liquid, an ionic liquid of the "crosslinker" type, the cation or the anion of which carries at least two polymerizable functions, a lithium salt, and an ionic polymer.

Such ionic polymer can contribute to improve the mechanical properties of the anodes.

The liquid composition according to the invention may also advantageously comprise a UV or thermal polymerization initiator. This polymerization initiator is degraded during the polymerization and present in negligible amount

According to another aspect of the invention, a method is provided for the coating of a metal piece of lithium, such as a lithium anode of an electrochemical generator, implementing a liquid protective composition obtained by deposition and polymerization of a liquid formulation of the invention, comprising the steps of: depositing a liquid solution having said composition on said metal part, polymerizing said liquid solution thus deposited, under the action of UV radiation or heat. The deposition step may be performed by applying a film of the liquid solution. Or by soaking the metal part in the liquid solution.

The polymer coating thus obtained differs from the prior art in that all the components of this formulation are ionic.

According to yet another aspect of the invention, there is provided a lithium based electrochemical storage system (such as lithium sulfur battery, lithium metal battery, lithium- ion battery, lithium-ion capacitor) comprising a lithium anode covered with such deposited layer having an ionic composition according to the invention.

In the current invention, there is only one polymer layer (between the metal Li and the electrolyte). These are only ionic based polymers and components.

The composition according to the invention is only organic. It is a protective layer for lithium and this "protected" lithium can then form the anode of a battery containing a liquid electrolyte.

The coating used in the present invention is simply deposited, whereas in the prior art, this coating is covalently bonded to the surface of the lithium metal. All components are ionic and therefore participate in the ionic conductivity of the whole.

The protective coating thus obtained constitutes a lithium ion conductive membrane thanks to the combination of the ionic elements and the lithium salt.

This membrane is mechanically, chemically and electrochemically stable in contact with metallic lithium.

It has a very good ionic conductivity (6 x lO mS / cm at room temperature and 4.9 x 10 mS / cm at 80°C), an order of magnitude higher than that of a membrane disclosed in the prior art document EP3049471B1 (5x 10 mS / cm at 80°C).

All components of the membrane are ionic, which provides good conductivity, while a neutral component is still present in other formulations of the prior art.

The protective composition according to the invention can lead to many combinations of materials, in order to optimize the composition as a function of the cathode material to be chosen

Description of the figures

The figures will detail some examples of embodiment of the invention, in particular:

• Figure 1 represents the coating process of the polymeric composition based on ionic liquids on the surface of the metallic lithium foil;

• Figure 2 represents the stripping and deposition tests on symmetrical cells with bare lithium and protected lithium with ionic liquid-based polymer at a current density of 0.5 TTiA cm for 4 h on the first cycle and later with 0.5 mA cm for 2 h. • Figure 3 represent SEM images of a) top-down view and b) cross-section of bare lithium surface; c) top-down view and d) cross-section of lithium surface covered with the polymeric protective composition.

• Figure 4 represents a schematic view of a lithium metal based electrochemical cell of the invention.

• Figure 5 illustrates impedance spectroscopy measurements in symmetrical cells at OCV after 30 min and 1240 min.

• Figure 6 shows charge-discharge voltage profile for LFP assembled in the combination with bare lithium or protected by the polymer composition as negative electrodes.

Detailed description

A practical example of a formulation for a polymeric protective composition according to the invention will now be described.

All the components of the formulation are known from the prior art.

The deposition of the liquid solution can be carried out by different liquid solution deposition techniques on a solid surface (film applicator, soaking, etc.).

The polymerization of the liquid solution layer can be carried out by UV or heating.

The electrochemical cells according to the invention can be manufactured according to known techniques. The use of this protected lithium anode can make it possible to use electrolytes that are not modified by the various types of additives mentioned above or simply to serve as electrolyte and separator at the same time.

The polymerizable ionic liquid, the cation or anion of which carries at least one polymerizable function, can have the following form (Table 1), as a non-limitative example according to a concentration between 50 wt. % and 70 wt. %, typically 60 wt. %. Table 1: Example of different cations which carries at least one polymerizable function and possible associated anions

As exemplified in table 1, different anions can be chosen from the group consisting of hexafluorophosphate (PF ), perchlorate (CIO,), tetrafluoroborate (BF ), hexafluoroarsenate (ASF ), trifluoromethanesulfonate (CFS0 ), bis(trifluoromethanesulfonyl)imide (known by the abbreviation TFSI) N[S0 CF] , bis(fluorosulfonyl)imide (known by the abbreviation FSI) LiN[S0F] , bis(pentafluoroethanesulfonyl)imide, N(CF _s S0 ) (known by the abbreviation BETI), 4,5-dicyano-2-(trifluoromethyl)imidazolide (known by the abbreviation TDI) and mixtures thereof, with preference being given to LiTFSI, LiFSI and LiTDI.

Other examples may include other types of cation such as imidazolium, pyrrolidinium, ammonium, pyridinium, pyridazinium, pyrimidinium, pyrazinium, pyrazolium, thiazolium, oxazolium, triazolium, phosphonium, sulfonium. Some of such cations are illustrated below:

The non-polymerizable ionic liquid may have the following form, as a non-limitative example according to a concentration between 30 wt. % and 50 wt. %, typically 40 wt. %.

Table 2: Example of different cations and anions that are associated to get non- polymerizable ionic liquids

Other examples may include other types of cation such as imidazolium, pyrrolidinium, ammonium, pyridinium, pyridazinium, pyrimidinium, pyrazinium, pyrazolium, thiazolium, oxazolium, triazolium, phosphonium, sulfonium. As exemplified in Table 2, different anions can be chosen from the group consisting of hexafluorophosphate (PF ), perchlorate (CIO,), tetrafluoroborate (BF ), hexafluoroarsenate (AsF ), trifluoromethanesulfonate (CFS0 ), bis(trifluoromethanesulfonyl)imide (known by the abbreviation TFSI) N[S0CF] , bis(fluorosulfonyl)imide (known by the abbreviation FSI) LiN[S0F] , bis(pentafluoroethanesulfonyl)imide, N(CF _s S0 ) (known by the abbreviation BETI), 4,5- dicyano-2-(trifluoromethyl)imidazolide (known by the abbreviation TDI) and mixtures thereof, with preference being given to LiTFSI, LiFSI and LiTDI.

The ionic liquid of "crosslinker" type, the cation of which carries at least two polymerizable functions, can have the following form, as a non-limitative example according to a concentration between 1 mol. % and 5 mol. %, typically 3 mol. %, versus the polymerizable ionic liquid: l,4-butanediyF3,3’-bis-l-vinylimidazolium cation:

The lithium salt may be chosen from the group consisting of hexafluorophosphate (PF ), perchlorate (CIO,), tetrafluoroborate (BF ), hexafluoroarsenate (AsF ), trifluoromethanesulfonate (CFS0 ), lithium bis(trifluoromethanesulfonyl)imide (known by the abbreviation LiTFSI) LiN[S0CF] , lithium bis(fluorosulfonyl)imide (known by the abbreviation LiFSI) LiN[S0F] , lithium bis(pentafluoroethanesulfonyl)imide LiN(CF _s S0) (known by the abbreviation LiBETI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (known by the abbreviation LiTDI) and mixtures thereof, with preference being given to LiTFSI, LiFSI and LiTDI..

The lithium salt may be present in the composition in a molar ratio ranging as a non- limitative example from 1:9 molar ratio to 2:3 molar ratio vs the non-polymerizable ionic liquid.

The ionic polymer included in the polymer composition according to the invention may have the following form, as a non-limitative example according to a concentration between 1 wt. % and 5 wt. % versus the polymerizable ionic liquid: poly(diallyldimethylammonium bis (trifluoromethylsulfonyl)imide), or poly(diallyldimethylammonium bis

(fluorosulfonyl)imide), as shown below:

The polymerization initiator may be chosen from the following materials or compositions: Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide is given as an example that as successfully been used, according to a concentration between 1 mol. % and 5 mol. % versus the polymerizable ionic liquid

Example of a membrane preparation:

The polymeric composition based on ionic liquids was prepared inside an argon filled glove box. To a mixture of 40 wt.% of non-polymerizable ionic liquid N,N-diethyl-N-methyl-N-(2- methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEMETFSI) and 60 wt.% of a polymerizable ionic liquid l-ethyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide (EVIMTFSI) was added an ionic liquid of "crosslinker" type l,4-butanediyl-3,3'-bis-l- vinylimidazolium di-bis(trifluoromethylsulfonyl)imide (BVIMTFSI) in 3 mol% vs. EVIMTFSI. After all the components are dissolved, lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) is added in 1:9 mol. ratio vs. DEMETFSI. At the end, poly(diallyldimethylammonium bis(trifluoromethylsulfonyl)imide) (polyDDATFSI) is added in 2 wt.% vs. EVIMTFSI. For the cross-linked polymerization of the polymeric ionic liquid mixture, the UV curing agent phenyl bis (2,4,6-trimethylbenzoyl) phosphine oxide was added in a 3.5 wt % vs. EVIMTFSI.

After all the components are dissolved forming a liquid viscous slurry, metallic lithium may be covered with such formulation using a doctor blade with a height between 10-250 pi m inside an Ar filled glove-box at room temperature (figure 1). The lithium covered with such formulation was exposed to UV light for 5 min forming a cross-linked polymerized protected layer on the lithium surface. The prepared protection was tested in a symmetrical two electrode pouch cell. Stripping and deposition cycles were done at 0.5 mA / cm for 4 hours on the first cycle and 0.5 mA / cm for 2 hours for the rest of the cycles. In all the experiments, electrodes of 2 cm of area were used.

Figure 2 represents cycling for symmetrical cells of a bare and a protected lithium surface with the ionic liquid-based polymer protecting layer. The shape and the way the peaks evolve with cycling is different for both surfaces. For both surfaces, the over-potential of the cell is increasing with cycling. After 15* cycles some sharp peaks associated with HSAL formation and short-circuits appeared. The bare lithium surface after the 25* cycle go in short-circuits and after that, the cycling ends for potential for the next cycles. For the protected Li with the polymeric composition based on ionic liquids, the cycling is never ending for potential cut off, since never reaches the potential limit, even after 70 cycles the cell is working. It was observed that the coating does not avoid completely formation of dendrites but can retard the short-circuits induced for it.

Membrane homogeneity and thickness

The polymeric protective composition was characterized with a field-emission scanning electron microscopy (FE SEM) Supra 35 VP (Zeiss, Germany). Samples were prepared and attached to custom-made vacuum transfer holder in argon-filled glovebox, which is opened in the SEM chamber under reduced pressure. SEM images of bare lithium surface and lithium surface covered with the polymeric protective composition are shown in Figure 3.

The bare lithium surface is rough and non-uniform (3a, b). When polymeric protective composition is applied on the Li surface (3c, d) this uneven surface is covered with a smooth, compact and very homogenous layer with some pinholes due to a direct polymerization on the lithium surface. On the cross-section view (3d), it can be observed that the polymeric protective composition is well adhered to the lithium surface. An estimated thickness of 60 pi m was determined.

The ionic liquid-based composition according to the invention is applied to one of the two faces of a lithium sheet, by deposition techniques known to those skilled in the art (example: doctor blade coater, such as reference K Control Coater from RK Print, Figure page 14). Then the polymerization is performed, resulting in a protective film of lithium. When mounting the battery, the protected side of lithium faces the second electrode (cathode), as shown in Figure 4.

Conductivity of the polymeric protective composition For determination of ionic conductivity, a self-standing polymeric protective composition membrane with a thickness of 87 pi m and a surface of 0.78 cm was sandwiched between two cupper foils. Nyquist plots were obtained at different temperatures. The ionic conductivity (s) values where obtained from the impedance measurements using the formula: s= t/(S-R), where t and S are the thickness and surface of the membrane respectively and R is the ohmic resistance. The obtained ionic conductivity is 3.6 x 10 mS.cm at room temperature and 4.9 x 10 mS.cm at 80°C.

Stability and compatibility of the protective polymer composition with lithium metal Stability and compatibility of ionic liquid-based polymer membrane with metallic Li was measured by impedance spectroscopy in the symmetrical cells at OCV after 30 min and 1240 min. Measured spectra shown in the Figure 5 were fitted with the equivalent circuit shown in the insert image. For this circuit R1 corresponds to the electrolyte resistance and the sum of R2 and R3 to the resistance of the lithium surface: resistance to the charge transfer and resistance of the SEI at the lithium electrodes.

For bare lithium, the sum of R2 and R3 increases from 49 W for the cell after 30 min to 76 W after 1240 min after the assembly. This increment of 55% is related to the formation of the SEI layer, due to the exposure of metallic lithium surface to the electrolyte when the cell is stored at OCV. In contrast, the resistance of Li-symmetrical cell with ionic liquid-based polymer@Li changes from 37 W for 30 min to 35 W after 1240 min after the assembly.

The membrane protects the metal surface from the continuous consumption of the electrolyte, thus preventing the growth of the SEI over time when the cell is stored at OCV. In consequence, impedance is almost not changing during the measurement. Once that lithium surface is covered with the protective ionic liquid-based polymer membrane, the growth of passive film is not as fast as in the case of the bare lithium surface.

Charging and discharging profile of a complete cell

Figure 6 shows charge-discharge voltage profile for LFP assembled in the combination with bare lithium or protected by the polymer composition as negative electrodes. The cathode is made of LFP and the anode is made of lithium metal and coated with the protective polymer composition.

There is a small difference in the voltage profile, but the capacity is almost identical. The evolution of voltage at the beginning of the half-cycle (charge or discharge) is slower for the cell with lithium protected by the polymer composition as anode than for one with bare Li and, in consequence, the cell LFP/ Li-protected needs more time to reach the plateau of voltage. These can be attributed to the effect caused for the retarding mass transport that was discussed previously for the lithium protected by the polymer composition samples. Nevertheless, it seems this has not negative effect on the cycling of a full cell with LFP as cathode at room temperature using low current densities. Both, charge and discharge profiles as also cell capacity (155 mAh g _' ) are almost the same for the both cell independent of negative electrode selection in this study. However, with the lithium protected by the polymer composition as negative electrode, more stable cycling is obtained with better coulombic efficiency. Of course, the invention is not limited to the embodiments which have just been described and many other embodiments of a polymer composition according to the invention can be envisaged. In particular, it is possible to provide in this composition several polymerizable ionic liquids, whose cation or anion carries at least one polymerizable function, a plurality of non-polymerizable ionic liquids, several ionic liquids of “crosslinker” type, the cation or the anion of which carries at least two polymerizable functions, several lithium salts and several ionic polymers.

References

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