Polymer electrolyte for all-solid-state batteries

Abstract

The present invention relates to a polymer electrolyte comprising: a) at least one polymer comprising at least one motif obtained from a reaction of at least one styrenic monomer with at least one polymer P selected from polyethers; b) at least one cyclic polyether; and c) at least one electrolyte salt.

Description

Title of the invention: Polymer electrolyte for all-solid-state batteries technical field

[0001] The present invention relates to the field of all-solid-state batteries.

[0002] More particularly, the present invention relates to a polymer electrolyte thus that a method for preparing said polymer electrolyte. The invention also relates to an all-solid-state battery cell comprising said polymer electrolyte, as well as an all-solid-state battery comprising said cell. Previous techniques

[0003] In a conventional manner, all-solid-state batteries comprise one or more positive electrodes, one or more negative electrodes, a solid electrolyte forming a separator, an anodic current collector and a cathodic current collector.

[0004] The performance of a battery depends on its ionic and electronic transport properties. In the case of an all-solid-state battery, ionic transport at the electrode scale occurs through the network formed by the solid electrolyte, which creates ionic conduction pathways throughout the entire volume of the electrode for the transport of ions to or from all the particles of active material.

[0005] All-solid-state batteries use different types of solid electrolyte materials. In particular, there are two main branches in the field of solid (or solid-state) electrolytes: ceramics (or inorganic electrolytes) and solid polymer electrolytes, which can be called EPS.

[0006] As a general rule, ceramics exhibit superior ionic conductivity and a wider electrochemical stability window, but their chemical stability and mechanical properties are poor. On the other hand, solid polymer electrolytes exhibit superior chemical stability and mechanical properties, but the main drawback is their low ionic conductivity at room temperature.

[0007] In order to improve the ionic conductivity of solid polymer electrolytes, it is known to add a liquid electrolyte or a plasticizer to the polymer matrix.

[0008] However, this approach has significant drawbacks. The liquid electrolyte and the plasticizer are volatile and can undergo side reactions, particularly at high temperatures, approximately above 50°C, leading to reduced battery performance and swelling of the electrochemical cells. This results in decreased electrolyte stability.

[0009] There is therefore a need to develop a new polymer electrolyte for all-solid-state batteries that overcomes the above disadvantages. Description of the invention

[0010] The invention therefore relates to a polymer electrolyte comprising:

[0011] a) at least one polymer comprising at least one motif obtained from a reaction of at least one styrenic monomer with at least one polymer P selected from polyethers;

[0012] b) at least one cyclic polyether; and

[0013] c) at least one electrolyte salt.

[0014] Thanks to the polymer electrolyte according to the invention, improved performance is obtained, in particular better ionic conductivity at room temperature without affecting the thermal, chemical and electrochemical stability properties of the electrolyte.

[0015] The invention also relates to a process for preparing a polymer electrolyte as described above, the process comprising the following steps:

[0016] i) bring at least said styrenic monomer into contact with at least said polymer P;

[0017] ii) polymerize the product obtained at the end of step i), said cyclic polyether being added before or after step ii); and

[0018] iii) recover the polymer electrolyte;

[0019] said electrolyte salt being added during step ii) and / or after step ii).

[0020] Another object of the invention is an all-solid battery cell comprising the solid polymer electrolyte according to the invention, as well as an all-solid battery comprising the cell according to the invention.

[0021] Other advantages and features of the invention will become more apparent upon examination of the detailed description and accompanying drawings, in which:

[0022] [Fig-1] represents the evolution of the ionic conductivity as a function of the temperature of a polymer electrolyte according to the invention, and of two comparative solid polymer electrolytes.

[0023] [Fig.2] is a thermogravimetric analysis curve characterizing the stability thermal conductivity of a polymer electrolyte according to the invention.

[0024] [Fig.3] represents the evolution of the capacity as a function of charging cycles- discharge of electrochemical half-cells comprising a polymer electrolyte according to the invention and a reference electrolyte.

[0025] The expression "between... and..." used in this description of the invention shall be understood as including each of the limits mentioned.

[0026] Furthermore, the expression "at least one" used should be considered equivalent to the expression "one or more".

[0027] For the purposes of the present invention, "styrene monomer" means a monomer having at least the chemical structure of styrene.

[0028] As indicated above, the solid polymer electrolyte according to the invention comprises a) at least one polymer comprising at least one motif obtained from a reaction of at least one styrenic monomer with at least one polymer P selected from polyethers.

[0029] Advantageously, the polyethers usable for the polymer P can be poly(alkylene oxide), such as poly(ethylene oxide), poly(propylene oxide), etc., cyclic polyethers, such as dioxane, 9-crown-3 ether, 12-crown-4 ether, 15-crown-5 ether, 18-crown-6 ether, etc., and their copolymers or mixtures thereof.

[0030] According to a preferred embodiment, said polymer P is selected from polyoxymethylene, poly(ethylene oxide), poly(propylene oxide), their copolymers and their mixture, preferably polymer P is poly(ethylene oxide).

[0031] In one embodiment, the polymer P may comprise a copolymer group comprising at least two polyethers of different natures.

[0032] For example, the polymer P may comprise a poly(ethylene oxide)-poly(propylene oxide) block copolymer group.

[0033] Advantageously, said styrenic monomer is selected from styrene, o-methylstyrene, p-methylstyrene, 2,4-dimethylstyrene, m-chlorostyrene, p-chlorostyrene, vinylanisole, styrene sulfonate and mixtures thereof, preferably selected from styrene and styrene sulfonate.

[0034] Preferably, the weight ratio between polymer P and styrenic monomer ranges from 0.6 to 9, preferably from 0.95 to 7.5, more preferably from 1.3 to 6.5.

[0035] Advantageously, the polymer a) may comprise a molar mass by weight or molecular weight ranging from 150 to 2000 g / mol, preferably from 200 to 700 g / mol, more preferably from 300 to 450 g / mol.

[0036] Preferably, said polymer a) is present in a content ranging from 20 to 85% by weight, preferably from 40 to 60% by weight relative to the total weight of the polymer electrolyte.

[0037] Advantageously, the total styrenic monomer content can range from 10 to 60% by weight, preferably from 12 to 51% by weight, more preferably from 14 to 35% by weight, even more preferably from 18 to 33% by weight relative to the total weight of the polymer a).

[0038] Advantageously, the total polymer P content can range from 40 to 90% by weight, preferably from 49 to 88% by weight, more preferably from 65 to 86% by weight, even more preferably from 67 to 82% by weight relative to the total weight of polymer a).

[0039] The polymer electrolyte according to the invention further comprises b) at least one cyclic polyether.

[0040] Advantageously, said cyclic polyether may comprise a compound selected from: dioxane, 9-crown-3 ether, 12-crown-4 ether, 15-crown-5 ether, 18-crown-6 ether or a mixture thereof.

[0041] Preferably, said cyclic polyether is present in a content ranging from 5 to 50% by weight, preferably from 10 to 30% by weight relative to the total weight of the polymer electrolyte.

[0042] In a preferred embodiment, said motif has the following formula (I):

[0043] where m goes from 3 to 45, preferably from 5 to 14.

[0044] In another preferred embodiment, said motif has the following formula (II): (II),

[0045] where m goes from 3 to 68, preferably from 3 to 23.

[0046] The solid polymer electrolyte according to the invention further comprises c) at least one electrolyte salt.

[0047] Advantageously, said electrolyte salt is chosen from alkaline earth metal salts, alkali metal salts and mixtures thereof, preferably from alkali metal salts, more preferably from sodium salts, lithium salts and mixtures thereof.

[0048] Among the sodium salts that can be used according to the invention, mention may be made of NaCF3SO3, NaSCN, NaBF4, NaN(CF3SO3)2, or even NaC104.

[0049] Even more preferably, said electrolyte salt is chosen from lithium salts.

[0050] Among the lithium salts that can be used according to the invention, mention may be made, in addition to those cited below, of lithium polyanion salts, such as for example lithium polystyrene sulfonate salt.

[0051] Advantageously, said electrolyte salt is selected from LiPF6, LiFSI, LiTFSI, LiClO4, LiAsF6, LiBF4, Lil, LiN(CF3SO2)2, LiCF3SO3, LiN(CF3CF2SO2)2, LiCH3SO3, LiN(CF3 SO2)(CF2HSO2), LiN(RFSO2)2, LiC(RFSO2)3, RF being a group selected from a fluorine atom and a perfluoroalkyl group comprising from one to eight carbon atoms.

[0052] Preferably, said electrolyte salt is lithium bis(trifluoromethanesulfonyl)imide (LiN(CF3SO2)2).

[0053] Preferably, the electrolyte salt is present in a content ranging from 10 to 50% by weight, preferably from 10 to 30% by weight relative to the total weight of the polymer electrolyte.

[0054] The present invention also relates to a method for preparing a polymer electrolyte according to the invention and as mentioned above.

[0055] Said preparation process comprises the following steps:

[0056] i) bring at least said styrenic monomer into contact with at least said polymer P;

[0057] ii) polymerize the product obtained at the end of step i), said cyclic polyether being added before or after step ii); and

[0058] iii) recover the polymer electrolyte;

[0059] said electrolyte salt being added during step ii) and / or after step ii).

[0060] Thus, the electrolyte salt can be added either during step (ii) of polymerization, or after step (ii), or during step (ii) and after step (ii).

[0061] Preferably, the electrolyte salt is added during step ii).

[0062] Preferably, the cyclic polyether is added after step ii) of polymerization.

[0063] Step i) can take place over a period of 1 hour to 72 hours, preferably from 5 to 60 hours, more preferably from 8 to 36 hours.

[0064] Step i) can be carried out in the presence of a solvent, such as for example acetonitrile or THF.

[0065] Step i) can be carried out at a temperature ranging from ambient temperature to 40°C, preferably at ambient temperature. Ambient temperature is a temperature ranging from 20 to 25°C.

[0066] A solvent may be present during step ii), such as ethanol or THF.

[0067] Step iü) of recovery of said solid polymer electrolyte can be carried out by evaporation of the solvent possibly present, followed by drying, for example at 80°C for a period of up to 24 hours.

[0068] In one embodiment, in step ii), the product obtained at the end of step i) can be heated in the presence of a free radical initiator, preferably an azo initiator, in order to carry out a free radical polymerization.

[0069] The heating of step ii) can be carried out at a temperature ranging from 40 to 110°C, preferably ranging from 60 to 90°C, more preferably ranging from 75 to 85°C.

[0070] Preferably, the heating takes place over a period of 1 hour to 15 hours, preferably from 3 to 10 hours, more preferably from 4 to 8 hours.

[0071] Advantageously, the azo free radical initiator can be selected from: azobisisobutyronitrile (AIBN), 1,1'-Azobis(cyclohexanecarbonitrile), 2,2'-Azobis(2-methylbutyronitrile), 4,4'-Azobis(4-cyanovaleric acid), 2,2'-Azobis(2,4-dimethylvaleronitrile) or a mixture of these.

[0072] The invention also relates to an all-solid battery cell comprising at least one polymer electrolyte according to the invention as defined above.

[0073] Another object of the present invention is an all-solid battery comprising at least one cell as defined above.

[0074] The present invention is illustrated in a non-limiting manner by the following examples. Examples

[0075] Example 1: Formation of 4-vinylbenzyl mcthoxynonaïcthylcnc glycol) ether

[0076] 1 molar equivalent (Eq) of monomethoxy polyethylene glycol, having a A molecular mass of 750 g / mol was dissolved in dry tetrahydrofuran (THF), and the solution was then heated to 40°C. The ratio of polymer to THF was 10 mL of THF per 1 g of monomethoxy polyethylene glycol.

[0077] 1.5 Eq of NaH was then slowly added to the mixture under vigorous stirring then the mixture was stirred for 3 hours at 40°C and under an argon atmosphere, until its color turned brown.

[0078] 1.5 Eq. of 4-vinylbenzyl chloride was then added dropwise using a syringe. NaCl precipitated immediately and the mixture was shaken vigorously for 24 hours.

[0079] The mixture was then filtered and the THF removed. After evaporation of the THF, the resulting solution was yellow and viscous. The crude product was purified several times by washing with water and n-hexane. The aqueous solution was collected and the water it contained was removed using a rotary evaporator under vacuum at 60°C.

[0080] The resulting solution was dissolved in toluene to remove all traces of water by azeotropic distillation, and then the toluene was removed using a rotary evaporator. The resulting liquid was dissolved in dichloromethane (DCM) and then dried with magnesium sulfate.

[0081] After removing the DCM using a rotary vacuum evaporator at 40°C, the 4-vinylbenzyl methoxynona(ethylene glycol) ether, in the form of a clear, light yellow liquid, was obtained and its purity was determined by NMR spectroscopy.

[0082] Example 2: Preparation of a polymer electrolyte according to a first embodiment of the invention

[0083] 56.2% by weight of 4-vinylbenzyl methoxynona(ethylene glycol) ether prepared at Example 1, 10.1% by weight of lithium salt LiTFSI, 1.97% by weight of azo initiator AIBN and 31.8% by weight of crown ether 18C6 were dissolved in a small amount of DCM.

[0084] The solution was poured onto a Teflon sheet and then stored in a dry place for 1 to 2 hours, until the DCM evaporated.

[0085] The solution was then heated to 65°C for 6 hours, and then dried under a high vacuum at 80°C overnight in order to remove any residual water molecules.

[0086] A quasi-solid polymer electrolyte according to the invention has been obtained.

[0087] In this first embodiment, the 18C6 crown ether is added before polymerization.

[0088] Example 3: preparation of a polymer electrolyte according to a second embodiment of the invention

[0089] Example 3 differs from example 2 in that the 18C6 crown ether is added after polymerization.

[0090] The 4-vinylbenzyl methoxynona(ethylene glycol) ether prepared in Example 1, the lithium salt LiTFSI, and the azo initiator AIBN were therefore dissolved in a small amount of DCM.

[0091] The solution was poured onto a Teflon sheet and then stored in a dry place for 1 to 2 hours, until the DCM evaporated.

[0092] The resulting solid polymer electrolyte was then heated to 65°C for 6 hours.

[0093] 18C6 crown ether was dissolved in THF and then added dropwise to solid polymer electrolyte.

[0094] The product obtained was stored for 24 hours until a homogeneous system was obtained, then a quasi-solid polymer electrolyte, or gel, according to the invention was obtained.

[0095] In this second embodiment, the 18C6 crown ether is added after polymerization. Performance evaluation a. Ionic conductivity

[0096] The ionic conductivity of the samples was measured by AC impedance spectroscopy.

[0097] A pellet-shaped polymer electrolyte was prepared using a 10 mm diameter punch. The pellet was placed between two flexible graphite sheets (PAPYEX®, Mersen), forming two blocking electrodes.

[0098] The sample was placed in a controlled environment sample holder (CESH, Biology) to maintain an inert gas atmosphere. The temperature was controlled by an intermediate temperature system (ITS, BioLogic).

[0099] The AC impedance spectrum was recorded by an MTZ-35 frequency response analyzer (BioLogic) in a temperature range of -10°C to 80°C.

[0100] The samples are preheated to 80°C for one hour before measurement. The temperature was then slowly increased (IK / min) to the defined temperature and maintained for 30 minutes before a sinusoidal voltage perturbation of 50 mV was applied between 107 and 0.05 Hz.

[0101] The polymer electrolyte according to the invention as obtained in Example 2 is used, and referred to as EPI.

[0102] A solid polymer electrolyte comprising the reference polymer used in Example 2, without the addition of 18C6 crown ether, was used. It is referred to as EP2 hereafter.

[0103] A polymer electrolyte comprising poly(ethylene oxide) and lithium salt LiTFSI in a molar ratio (Li:OE (ethylene oxide) = 1:20) was used. It is referred to as EP3 hereafter.

[0104] The evolution of the ionic conductivity as a function of temperature of the polymer electrolytes EPI, EP2 and EP3 can be observed on [Fig.1].

[0105] Thus, it is clear that the EPI electrolyte according to the invention exhibits better ionic conductivity than that of the EP2 and EP3 electrolytes in the temperature range considered, in particular at room temperature, 25°C, and at 80°C. For the EPI polymer electrolyte according to the invention, an ionic conductivity value of 2.9 x 10⁴ S / cm was obtained at room temperature, 25°C, and of 1.2 x 10² S / cm at 80°C, by the addition of 18C6 crown ether.

[0106] This represents a significant advance for all-solid-state batteries, the main problem of which remains the low ionic conductivity at room temperature.

[0107] The solid polymer electrolyte according to the invention is therefore a high potential candidate for application within an all-solid-state battery. b. Thermal stability

[0108] In order to study the mass loss of the EPI polymer electrolyte under the influence of heat, a thermogravimetric analysis was carried out between ambient temperature and 800°C with an applied temperature ramp of 10 K / min.

[0109] Although the boiling point of 18C6 crown ether is relatively low (116°C), no mass loss was observed before polymer decomposition.

[0110] This shows that the solid polymer electrolyte according to the invention has significant thermal stability up to 180°C.

[0111] The solid polymer electrolyte according to the invention is therefore a promising candidate for improving battery life. Furthermore, it can be integrated into a battery that needs to operate at high temperatures. c. Interfacial stability test with Li metal

[0112] A lithium electrochemical cell was prepared to test the interfacial stability of the EPI polymer electrolyte according to the invention obtained in Example 2, against Li metal during cycling.

[0113] The EPI polymer electrolyte is sandwiched between two Li metal sheets.

[0114] A comparative cell was made with Li6PS5Cl and a ceramic electrolyte exhibiting high ionic conductivity but reduced interfacial stability and prone to dendrite formation in contact with metallic Li during cycling.

[0115] It has been observed that the EPI polymer electrolyte according to the invention possesses high stability properties against lithium metal, and in particular compared to Li6PS5Cl. Furthermore, no short circuits were observed during cycling in the presence of the EPI polymer electrolyte according to the invention.

[0116] The stability of the solid polymer electrolyte according to the invention against lithium metal makes it a promising candidate for integration into lithium batteries.

[0117] d) Preparation of Swagelok-type electrochemical half-cells

[0118] An electrolyte solution A according to the invention, prior to polymerization, was prepared by contacting 56.2% by weight of 4-vinylbenzyl methoxynona(ethylene glycol) ether prepared in Example 1, 10.1% by weight of lithium salt LiTFSI, 1.97% by weight of azo initiator AIBN and 31.8% by weight of 18C6 crown ether, without solvent.

[0119] A first half-cell Cl was prepared from an organic cathode material based on perylene-diimide (PDI), mixed with conductive carbon in a Cathode Material: Conductive Carbon ratio of 7:3, and the electrolyte solution A according to the invention, before polymerization, prepared above.

[0120] A second half-cell C2 was prepared from a perylene-diimide (PDI) based organic cathode material, mixed with conductive carbon in a cathode material: conductive carbon ratio of 5:5, and the electrolyte solution A according to the invention, before polymerization, prepared above.

[0121] A third reference C3 half-cell was prepared from a perylene-diimide (PDI) based organic cathode material, mixed with conductive carbon in a cathode material: conductive carbon ratio of 7:3, and a reference organic electrolyte solution B LP30, based on EC, DMC and LiPF6.

[0122] The cathode materials were placed in powder form in a Swagelok-type half-cell and a fiberglass separator acting as a physical barrier was placed in the half-cell, prior to polymerization.

[0123] The cathode powders and the separator were then impregnated with electrolyte solution A for half-cells Cl and C2 and with electrolyte solution B for half-cell C3.

[0124] A lithium metal disc was placed in the half-cells Cl, C2 and C3 which were then hermetically sealed.

[0125] The in situ polymerization was carried out by placing the Cl and C2 half-cells at 80°C for 24 hours until complete polymerization and stabilization of the electrolyte system. The polymerization of the reference half-cell C3 was carried out for 24 hours at room temperature.

[0126] With reference to [Fig.3], charge-discharge cycles were carried out on half-cells Cl, C2 and C3 at 80°C and at room temperature, 25°C.

[0127] Unlike the reference C3 half-cell, the Cl and C2 half-cells show good operation and remarkable stability at 80°C. Exceptional capacity retention was observed with the polymer electrolyte according to the invention, particularly compared to the reference liquid electrolyte.

Claims

Demands

1. Polymer electrolyte comprising: a) at least one polymer comprising at least one motif obtained from a reaction of at least one styrenic monomer with at least one polymer P selected from polyethers, said motif having the following formula (I): where m ranges from 3 to 45, preferably from 5 to 14, Or said motive being of the following formula (II):

2. where m goes from 3 to 68, preferably from 3 to 23; b) at least one cyclic polyether; and c) at least one electrolyte salt. Electrolyte according to claim 1, wherein said cyclic polyether comprises a compound selected from: dioxane, 9-crown-3 ether, 12-crown-4 ether, 15-crown-5 ether, 18-crown-6 ether or a mixture thereof.

3. A process for preparing a polymer electrolyte according to claim 1 or 2, characterized in that it comprises the following steps: i) contacting at least said styrenic monomer with at least said polymer P; ii) polymerizing the product obtained at the end of step i), said cyclic polyether being added before or after step ii); and iii) recovering the polymer electrolyte; said electrolyte salt being added during step ii) and / or after step ii).

4. A preparation method according to claim 3, wherein in step ii), the product obtained at the end of step i) is heated in the presence of a free radical initiator, preferably an azo initiator, the heating being carried out at a temperature ranging from 40 to 110°C, preferably from 60 to 90°C, more preferably from 75 to 85°C.

5. All-solid battery cell comprising at least one polymer electrolyte as defined in claim 1 or 2.

6. All-solid-state battery comprising at least one cell as defined in claim 5.

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