Patent Application
20210288350
The field of the invention is that of electrochemical cells-storage batteries-batteries, in particular those of which the reaction is based on the element lithium.
More precisely, the invention relates to solid polymer electrolytes (SPE) that can be used in these electrochemical devices.
The invention also relates to the method for producing such a SPE.
The applications of these SPE in these electrochemical devices, in particular those of which the reaction is based on the element lithium, constitute other aspects of the invention.
“Storage battery” designates a unitary electrochemical device (cell) comprising two electrodes separated by an electrolyte. “Battery” designates an assembly of storage batteries connected together in order to obtain the desired capacity and voltage. In everyday language, the two terms are often confused.
A storage battery restores energy by converting the chemical energy into electrical energy, through reactions that occur at the electrodes. In the case where the storage battery is the seat of reversible redox reactions, this allows it to be rechargeable with respect to electrical energy with an external source. On the contrary, in an electric battery, the electricity-generating redox reactions are not reversible.
During the discharge of the storage battery, the negative electrode (anode) is the seat of an oxidation that generates an electron in the external circuit and an ion that migrates through the electrolyte. Simultaneously, a reduction takes place at the positive electrode (cathode) thanks to the supply of an electron by the external circuit and of an ion by the electrolyte: this ion can be stored in the material of the positive electrode, called host material. The electrons thus formed are recovered by the collectors and supply the external circuit with electrical current. During the charge, the ions take the reverse path, i.e. they are produced by oxidation at the positive electrode and migrate towards the negative electrode. The electrodes must therefore be both ion and electron conducting. The electrolyte has to be a good ion conductor, but electron insulator in order to force the electrons to pass through the external circuit. Otherwise, the performance of the storage battery deteriorates.
Lithium storage batteries offer the highest specific energy (energy/mass and the highest energy density (energy/volume). These lithium storage batteries have therefore imposed themselves for storing and delivering electrical energy in multiple applications, such as in particular high-energy applications: automobile, aeronautics, intermittent energy storage (solar and/or wind) . . . and applications relating to mobile electronic devices, of which in particular computer or mobile telephones.
There are three main types of lithium storage batteries:
In the framework of the present invention, interest is given more particularly to Lithium Metal Polymer (LMP) storage batteries. These storage batteries or batteries are more specially intended for automobile, aeronautical and intermittent energy applications (solar and/or wind).
Their energy density is lower than that of lithium-ion storage batteries, but LMP storage batteries, fully solid, do not carry the risk of explosion. Their self-discharging is relatively low. They have little or no pollution and no memory effect.
In LMP storage batteries, the elementary electrochemical cell comprises:
This elementary cell, fully solid, also operates reversibly: the anode provides the supply of lithium ions during the discharging and the cathode act as a receptacle where the lithium ions are inserted. The two electrodes are separated by the solid polymer electrolyte, which conducts lithium ions. The conductivity of the ions is assured by the dissolution of lithium salts in the polymer material with a PEO base. This material is generally comprised of random copolymer(s) or blocks, or even PEO/reinforcing polymer composites.
The high viscosity at ambient temperature of this polymer material with a PEO base, provides a mechanical blocking that limits, even eliminates, dendritic growth, a deleterious phenomenon well known in lithium storage batteries. Dendritic growth takes place during the charging of the storage battery. The metal lithium is not deposited uniformly on the surface of the metal electrode, but in the form of dendrites that can short-circuit the electrochemical cell and thus cause the destruction thereof by overheating, even by explosion. In addition, these irregular dendritic deposits can also break into pieces which, not only is detrimental to the performance of the storage battery, but even more gravely, results in the presence of fragments of highly reactive lithium powder in the electrolyte.
The chemical inertia of this polymer material with a PEO base with regards to lithium substantially decreases the risks of explosive reactions, which, also, have tarnished the reputation of lithium storage batteries.
The texture of this polymer material with a PEO base also prevents electrolyte leaks, and its flexibility makes it possible to choose a configuration in sheets, adapted for industrial production and of which the geometrical criteria improve performance (large surface and low thickness of the electrolyte).
However, in order to obtain the required optimum conductivity, in particular in high-energy applications, the temperature of the polymer material with a PEO base, has to be kept between 80° C. and 90° C.
But at these high temperatures of optimum conductivity, the physical properties of the polymer material with a PEO base are degraded. This results in that the latter is no longer capable of mechanically opposing dendritic growth.
Furthermore, operation at these high temperatures of optimum conductivity, assumes that a portion of the energy of the storage battery (battery) is used for this purpose. This substantially decreases the useable energy density of the storage battery.
Moreover, this thermal constraint imposes latency time that delays the turning on of the storage battery, and therefore the supplying of electrical energy, at ambient temperature.
It therefore appears that the ionic conductivity and mechanical resistance properties of the polymer material with a PEO base, are antagonistic.
In this LMP technology based on electrolytes based on PEO, the ionic conductivities obtained at 40° C. are too low for optimum use and only the use of positive electrodes with little “grammage” (<0.5 mA/cm2) (low surface capacity) and low currents (<C/10; charge in 10 h (C=total capacity)) make it possible to recover the capacity at this temperature.
The scientific article A. Lassagne and al “Electrochimica Acta 238 (2017) 21-29: New approach to design solid block copolymer electrolytes for 40° C. lithium metal battery operation”, addresses this issue.
Thus this article discloses solid polymer electrolytes SPE constituted by three-block B-A-B copolymer, based on a central PEO A block and PolyStyrene (PS) PS-b-PEO-b-PS lateral B blocks:
These poly(Styrene-EthyleneGlycolx-Styrene) copolymer blocks are noted as SEGxS with x=1.5 or 2 which indicates the molar mass in kg·mol−1 of the polyethylene glycol (PEG) used in the synthesis of the central block. The SEGxS are obtained in several steps (Diagram 1): 1) the polycondensation of a polyethylene glycol PEG of molar mass of 1.5 or 2 kg·mol−1 and of 3-chloro-2-propene in order to obtain the modified PEO central block, 2) the modification of the ends of the PEO modified by esterification then by intermolecular radical addition with the alkoxyamine MAMA-SG1 in order to obtain the macroinitiator PEO-(MAMA-SG1)2, 3) the radical polymerisation controlled by the nitroxides of the styrene by using the macroinitiator PEO-(MAMA-SG1)2. These SEGxS are then solubilised with a 2 bis-trifluoromethanesulfonylimide lithium salt (LiTFSI) in a dichloromethane/acetonitrile mixture, to form the SEGxS_ϕc. ϕc corresponds to the volume percentage of conductive phase (modified PEO loaded with LiTFSI at an EO/Li ratio=25). This solution of SEGxS_ϕc is subjected to an elimination of the solvent in order to produce SEGxS_ϕc films 100 μm thick.
The SPE described in this prior document can be improved in terms of the ionic conductivity/mechanical properties compromise.
Document FR 2899235 describes a SPE that comprises a three-block copolymer, and in particular a polystyrene-poly(oxyethylene)-polystyrene copolymer, intended for being implemented in lithium storage batteries. The SPE described in this document can however be improved.
In these circumstances, the present invention aims to satisfy at least one of the objectives mentioned hereinafter.
These objectives, among others are achieved by the present invention which first and foremost relates to a Solid Polymer Electrolyte (SPE) comprising:
More particularly, the SPE can include:
This new plasticised SPE material is singularly effective and advantageous in that it offers very good ionic conductivity and very good resistance or very good mechanical reinforcement, favourable to the blocking of the process of the formation of metal dendrites, for example lithium when it entails applications in LMP storage batteries.
The performance, for example at 40° C., of storage batteries that comprise this SPE are greater than or equal to those of storage batteries available on the market and of which the operating temperature is 80° C. This represents gain of more than 40° C., with higher or equal electrical performance. These results are particularly remarkable for “all-solid-state” storage batteries/batteries.
In addition to this performance, “all-solid-state” storage batteries (e.g. LMP) that implement the SPE according to invention, have a fundamental advantage in terms of safety, with respect to storage batteries/batteries that use liquid electrolytes at high saturation vapour pressure and highly flammable.
In addition to its improves conductivity at low temperature, its good mechanical resistance and the reinforcing of safety that it contributes to, this SPE material according to invention also benefits from great ease in implementation.
In a preferred embodiment of the invention, the SPE is at least partially crosslinked.
In another of its aspects, the invention relates to a method for producing a SPE such as described in the present disclosure. This method consists substantially of:
In another of its aspects, the invention relates to an electrochemical storage battery comprising at least one SPE such as described in the present disclosure.
In another of its aspects, the invention aims for an electrode for electrochemical device comprising at least one SPE such as described in the present disclosure.
Throughout the entire present disclosure, any singular designates indifferently a singular or a plural.
The definitions given hereinafter by way of examples, can be used for the interpretation of the present disclosure:
The SPE according to invention is preferably at least partially crosslinked. In this configuration, the three-block A-B-A copolymer 1.1 can be the component involved in this crosslinking.
For this purpose, the A block polymers and/or the B block polymer, can be a carrier or carriers of at least two crosslinking groups CG per molecule, preferably a pendant group, said groups CG being able to react together to form crosslinking bridges, preferably by a thermally-activated crosslinking and/or actinically-activated crosslinking, in particular under UV.
According to a remarkable characteristic of the invention, the crosslinking groups CG can be selected from the group comprising—ideally constituted by—monovalent radicals including at least one unsaturation, advantageously ethylenic and/or alkynilic.
On an interesting alternative of the invention, the crosslinking groups CG are carried by all or a portion of the recurring units of the B block.
In a particular embodiment, each recurring unit of the B block is a carrier of a pendant group CG.
The actinic activation, in particular under UV, of the reaction between the groups CG for the crosslinking is favoured. However, it is possible to consider, as a substitution or as a supplement, other activation modes, for example thermal activation.
The copolymers with PEO blocks used in the solid polymer electrolytes (SPE) can be two-block A-B copolymers or three-block A-B-A copolymers.
The two-block A-B linear polymer can advantageously have the following general formula (I):
(a)n1−(b)m (I)
The three-block A-B-A linear copolymer can advantageously have the following general formula (I-bis):
(a)n′−(b)m′−(a)n′ (I-bis)
The A blocks are advantageously:
The A block is more preferably chosen for its solvation properties of the electrolyte salt. Its chemical nature can therefore depend on the electrolyte salt selected, described in more detail hereinafter.
More preferably, the A blocks are polymers that can be produced from one or more monomers, selected from:
In a preferred embodiment, the A block or blocks are polystyrenes. According to this embodiment, the molar mass of the A block is more preferably comprised between 2,000 and 60,000 g/mol, preferably between 2,000 and 41,600 g/mol, and even more preferably, entre 3,100 and 8,300 g/mol.
Through the definition of copolymer, the monomers that can produce the A blocks are different from the monomers that can produce the B blocks.
The B block is a polymer that can be produced from one or more alkylene-glycol (AG) monomers selected from ethylene oxide (EO), propylene oxide (PO), poly(ethylene-glycol) acrylates, (PEGA), poly(ethylene-glycol) methacrylates (PEGMA), and/or polyoxypropylene diamines, of which in particular those marketed under the brand Jeffamines® diamines.
Preferably, the B blocks are selected from blocks of poly(ethylene oxide) (PEO), blocks of poly(propylene oxide) (PPO) and blocks of PEO/PPO random copolymers.
Preferably, the B block comprises B sub-blocks or molar mass comprised between 0.5 and 5 kg·mol−1, and, even better, between 1 and 3 kg·mol−1.
In a preferred embodiment, the B block is a PEO.
According to an example, the molar mass of the B block is 20,000 g/mol.
According to the characteristics described hereinabove of A and B blocks, the proportion of the A block or blocks of the copolymer can be comprised between 10% and 75% by mass, preferably between 10% and 68% by mass, and even more preferably, between 14% and 30% by mass, with respect to the total mass of the copolymer.
According to an embodiment, the two-block A-B copolymers include a first block formed by a poly(alkyl methacrylate) such as poly(lauryl methacrylate) (PLMA), poly(n-butyl methacrylate) (PnMBA), or poly(methyl methacrylate), and a second block formed by poly(polyethylene glycol methacrylate, 9 units of EO) (PMAPEG). These copolymers can be synthesised radically.
According to another embodiment, one or more silicone polymer, oligomer or monomer segments can be distributed between the A & B blocks. This or these silicone segments can have a glass transition temperature that is strictly less than that of A and B blocks free from silicone segment. The incorporation of one or more segments silicones makes it possible to increase the molecular dynamics of the copolymer. Thus, the ionic conduction of the SPE according to the invention can be improved.
These segments include one or more silicone units of the following formulas (II) and/or (III):
wherein substituents R1 R2 are selected independently from the group comprised of:
Advantageously, the electrolyte salt 1.2 is selected from the alkali metal salts, more preferably from the following compounds: LiSCN, LiN(CN)2, LiClO4, LiBF4, LiAsF6, LiPF6, LiCF3SO3, Li(CF3SO2)2N, Li(CF3SO2)3C, LiN(SO2C2Fs)2, LiN(SO2CF3)2, LiN(SO2CF2CF3)2, lithium alkylfluorophosphates, lithium oxalatoborate, lithium bis(chelato)borates having at least one 5 to 7-membered ring, lithium bis(trifluoromethanesulfoneimide) (LiTFSI), LiPF3(C2F5)3, LiPF3(CF3)3, LiB(C2O4)2, LiPF6, LiSbF6, LiClO4, LiSCN, LiAsF6, NaCF3SO3, NaPF6, Na ClO4, NaI, NaBF4, NaAsF6, KCF3SO3, KPF6, Kl, LiCF3CO3, NaClO3, KBF4, KPF5, Mg(ClO4)2, and Mg(BF4)2 AgSO3CF3, NaSCN, KTFSI, NaTFSI, Ba(TFSI)2, Pb(TFSI)2, Ca(TFSI)2 and the mixtures thereof.
The lithium salts are particularly preferred.
According to a possibility, the electrolyte salt 1.2 according to invention can contain a mineral filler constituted, for example, of particles of ceramic, for example Al2O3, TiO2, and/or SiO2. The size of these particles is advantageously less than or equal to 5 nm.
According to a privileged modality of the invention, allowing for the optimisation of the conductivity of the SPE, the latter has a ratio [MB/M1.2] of the number of moles MB of the constituent monomer or monomers of the B block, over the number of moles M1.2 of the electrolyte salt 1.2, such that—in an increasing order more preferably—:
5≤[MB/M1.2]≤50; 8≤[MB/M1.2]≤40; 10≤[MB/M1.2]≤35; 12≤[MB/M1.2]≤30.
In the case where the B block constituted of monomer ethylene oxide EO and/or the electrolyte salt 1.2 is a lithium salt: 14≤[MB/M1.2]≤28.
The SPE according to the invention is plasticised by means of the plasticiser 1.3. Contrary to the solvents in the production of a SPE, which are generally eliminated during the production of the SPE, for example by evaporation, the plasticiser is here intended for remaining in the SPE. In the framework of the present invention, the production solvents, designated as “solvent”, and the plasticiser are distinguished. The plasticiser has particularly for role to lower the glass transition temperature of the B block in the SPE. Thus, good ionic conductivity of the SPE at low temperature can be obtained. The plasticiser is in particular chosen for its electrochemical stability in the conditions of use of the SPE according to the invention.
The plasticiser 1.3 is, preferably, selected from polar solvents, more preferably from those of molar mass less than or equal to 1,000 g/mol or even better to 500 g/mol, and, more preferably from the group comprising—ideally composed of—
Alternatively, the plasticiser 1.3 can be selected from the group comprising—ideally composed of—
In accordance with a distinctive and interesting characteristic of the invention, the concentration in plasticiser 1.3 is less than or equal to—in % by dry weight with respect to the total mass of the SPE [comprising at least 1.1, 1.2 and 1.3] and according to an increasing order more preferably −45; 40; 35; 30; 25 this concentration being even more preferably comprised between 10% and 40% by dry weight, even between 10 and 32% by dry weight, even between 15% excluded and 40% included in dry weight, preferably between 15 and 30% by dry weight.
It must be observed that this limited quantity of plasticiser 1.3 goes along with a high electrical conductivity, at least greater than or equal to the electrical conductivity of the SPE of the prior art. Furthermore, this limited quantity of plasticiser 1.3 makes it possible to obtain a SPE with good mechanical resistance at ambient temperature, with respect to the SPE comprising a proportion in plasticiser greater than 50%, even greater than 70%, by mass with respect to the total mass of the SPE.
The SPE according to the invention also includes, at least in trace amounts, markers of its method of production, and in particular of the two-block or three-block polymer.
Thus the SPE according to the invention comprises, in a particular embodiment of the invention linked to the production of the SPE:
The material according to invention is singularised by a nano-structuring with polymer domains formed by B blocks (e.g. modified PEO), seat of the ionic conductivity and of the polymer domains formed by A blocks (PS), procuring a mechanical reinforcement. This nano structuring is a key element, among others, for the blocking of the formation of metal dendrites in particular of lithium.
Thus, advantageously, the SPE according to invention is characterised by a nano-separation, entre at least one phase comprising the A blocks and at least one phase comprising the B block more preferably by an SAXS diffraction peak using a copper cathode (λ=1.54 Å) at 20° C., with q1 comprised between 0.05 and 0.4 nm−1, preferably between 0.1 and 0.5 nm−1, even between 0.1 and 0.35 nm−1 and, more preferably between 0.15 and 0.3 nm−1, even more preferably between 0.15 and 0.25 nm−1. The SPE characterised by this nano-separation can thus include a phase comprising the A blocks and at least one phase comprising the B block (e.g. modified PEO), of a period substantially comprised between 20.5 nm and 41 nm, the period corresponding here to the total average size of the unit formed by an A block and a B block.
The nano-separation obtained is more preferably according to a cylindrical or gyroid morphology that makes it possible to minimise the tortuosity of the conducting domains and thus achieve a higher conductivity. This morphology can in particular depend on the volume fraction of the A and B blocks. To obtain the morphology described, the volume fraction of the A block can be comprised between 15% and 40%, preferably between 15% and 35%, even more preferably between 20% and 30%, in relation to the total volume of the copolymer.
In accordance with the invention, the chemical modification of the PEO, by adding unsaturations, allows for the crosslinking of the material which, on the one hand, freezes the nano-structuring, and, on the other hand, allows for the absorption of the plasticiser 1.3, and this with good mechanical resistance of the SPE material.
Another quality of the SPE material according to the invention is that the B block and the plasticiser 1.3 form a homogeneous mixture. B and 1.3 are not subject to any phase separation, dephasing or exudation whatsoever, in usual conditions of use.
Moreover, one of the major interests of the SPE material according to the invention is to have an excellent ionic conductivity/mechanical resistance compromise, at temperatures less than 80° C., for example of about 40° C., even at ambient temperatures less than 40° C.
This this material is characterised by an ionic conductivity at 40° C. greater than or equal to 1.10−4, preferably to 3.10−4, and, even more preferably to 4.10−4, even 4.6±0.5*10−4; and by a Young's modulus (in MPa, at 40° C., and for a mass % of B block between 10 and 40% in the three-block A-B-A 1.1) greater than or equal to 0.05, preferably to 0.1, and, even more preferably to 0.30.
The producing of the SPE according to invention entails a synthesis of two-block A-B or three-block A-B-A copolymers, with modification of the copolymers by introduction of crosslinking functional groups.
This step (i) comprises more preferably the following sub-steps:
The bloc B can be modified by the introduction of an unsaturated function, for example isobutenes, by polycondensation, homogeneously distributed, all along the PEO chain.
In a particular embodiment, it is possible to synthesise a three-block A-B-A, with A: poly(4-methylstyrene) and B: poly(oxypropylene-oxyethylene), the B block able to be designated equivalently Jeffamine® ED-2003. The chemical structure of the copolymer before crosslinking on the double bonds is the following—formula (IV)—:
In another particular embodiment, it is possible to synthesise a three-block A-B-A, with A: poly(4-methylstyrene) and B: PEO. The chemical structure of the copolymer before crosslinking on the double bonds is the following—formula (V)—:
In another particular embodiment, it is possible to synthesise a three-block A-B-A, with A: polystyrene and B: poly(oxypropylene). The chemical structure of the copolymer before crosslinking on the double bonds is the following—formula (VI)—:
In the preferred embodiment, a three-block A-B-A is synthesised, with A: polystyrene and B: PEO.
The PEO is modified by the introduction of an unsaturated function, for example isobutenes, by polycondensation, homogeneously distributed, all along the PEO chain.
The PEGx blocks are obtained by polycondensation between the oligomers of PEO, polyethylene glycol (PEG) and 3-chloro-2-(chloromethyl)-1-propene. They are noted as PEGx with x the molar mass, in kg·mol−1, of the condensed PEG. This synthesis was carried out with PEG of 1.5 kg·mol-1 (PEG1.5) and of 2 kg·mol-1 (PEG2) leading to polymers having double bonds (isobutene) that can be crosslinked distributed in a controlled manner all along the chain (every 34 or 45 EO units respectively). The copolymers are obtained by radical polymerisation controlled by nitroxides (cf. formula hereinbelow). Different PS/PEGx/PS compositions can be produced.
The chemical structure of the copolymer before crosslinking on the double bonds is as follows—formula (VII)—:
The modified AB or ABA copolymers are then doped with at least one electrolyte salt 1.2.
According to a remarkable characteristic of the invention, the electrolyte salt 1.2 is at least partially dissolved in at least one solvent, designated equivalently production solvent, preferably selected from polar solvents, and, even more preferably from the group, comprising—ideally constituted by—the following compounds: substituted ethers, substituted amines, substituted amides, substituted alkyls, substituted PEG, alkyl carbonates, nitriles, boranes and lactones, and the mixtures thereof; tetrahydrofurane, methyl-ethyl-ketone, acetonitrile, ethanol, dimethylformamide, dichloromethane, acetonitrile taken individually or in mixtures thereof, being particularly preferred.
In the preferred embodiment, the PS-PEGx-PS materials obtained are doped [step (ii)] with at least one electrolyte salt 1.2, for example a LiTFSI salt and to which are added [step (iii)] at least one photoinitiator (for example hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone) and/or at least one thermal initiator (for example benzoyl peroxide).
This addition of initiator is carried out at 0.1 to 6% by weight, for example 2% by weight, with respect to the total PS-PEGx-PS/salt 1.2/initiator mixture. This addition is advantageously carried out by dissolution in a solvent, designated equivalently production solvent, e.g. in a dichloromethane/acetonitrile solution.
In a preferred embodiment of the method, the mixture of step (iii) [namely PS-PEGx-PS/salt 1.2/initiator/solvent] comprises from 10 to 45% by weight of solvent, for 90 to 55% by weight of A-B-A or A-B block copolymer, more preferably from 15 to 30% by weight of solvent, for 85 to 70% by weight of A-B-A or A-B block copolymer.
Preferably, the modified A-B or A-B-A are formed, for example transformed into mass objects or into films, membranes, or sheets of a thickness comprised for example between 10 and 200 microns.
Advantageously, this forming consists of a pouring of the solution on a suitable support/containing.
According to variants that make it possible to manufacture a SPE in the form of sheets, films or membranes, it is possible to use known techniques such as spin coating, roll coating, curtain coating, by extrusion etc.
Step (v)
This forming is accompanied by a passing from the liquid state to the solid state, more preferably by elimination of solvent(s), designated equivalently production solvent(s).
In practice, for example an evaporation of the solvent is carried out, designate equivalently production solvent, so as to form a mass object or a film.
The quantity of electrolyte salt 1.2 in the solution is adjusted to produce at the end of the optional step (iv), a solid form of SPE of which the [MB/M1.2] ratio is such as defined hereinabove.
Step (vi)
The crosslinking consolidates the forming and the nano-structuring of the material.
For example, the objects obtained at the end of step (iv), such as the films, are the crosslinked between 80 and 120° C., for example at 100° C. and/or by actinic activation, in particular under UV; for example under a UV P300 MT “Power supply” lamp marketed by the company Fusion UV system Inc” using a UV generator of 15 mW/cm2 (0.25 mJ/cm2), for a λ=200-400 nm.
Step (vii)
The key component 1.3 of plasticising/gelification is finally incorporated.
For example, the crosslinked objects, in particular the films, in step (v) are plasticised with the plasticiser 1.3, for example Tetraethylene-glycol-dimethyl-ether (TEGDME).
The making available of a new effective SPE gives access to new electrochemical devices constituted at least partially by this SPE.
In particular, the invention relates to:
The examples that follow illustrate a preferred embodiment of the production of SPE according to invention, through its composition, its method of production and its physical and chemical characteristics.
These examples are described in reference to the accompanying figures wherein:
This SPE electrolyte copolymer with PS-PEOmodified-PS blocks is loaded with electrolyte lithium salt 1.2 LiTFSI (Bis(trifluoromethane)lithium sulfonimide) with a ratio [MB/M12] or EO/Li=25. It is crosslinked then plasticised with a low quantity of plasticiser 1.2 TEGDME (Tetraethylene-glycol dimethyl ether) (22.9±1.2% by weight TEGDME for 77.1% by weight of (polymer+LiTFSI salt) (Total EO/Li=25).
This SPE is nanostructured with PEOmodified domains that supply the ionic conductivity and PS domains a mechanical reinforcement. The nano-structuring is an important aspect for the blocking of the lithium dendrites. The modification of the PEO allows for the crosslinking thereof thus freezing the nanostructure. It further allows the polymer to absorb the plasticiser without substantial loss of mechanical resistance.
This SPE has very good ionic conductivity at 40° C. with 4.6±0.5×10 S/cm, good mechanical resistance (favourable for blocking dendrites). It makes it possible to manufacture composite electrodes with a base of PEO and LiFePO4, plasticised or not, with high grammages (0.89 and 1.49 mAh/cm2). These electrodes implemented in LMP storage batteries assembled in a button battery. The performance of these storage batteries at 40° C. is greater than or equal (according to the positive electrode) to that of the storage batteries available in the market of which the operating is 80° C., which is a gain of more than 40° C. at equal or greater performance. These results are particularly remarkable for “all-solid-state” systems. The interest with this SPE technology according to invention, is the fundamental gain in safety with respect to lithium-ion batteries using liquid electrolytes at high saturation vapour pressure and highly flammable. Moreover, in comparison with the existing plasticised polymer electrolytes the quality of the plasticiser is here very low (i.e. 22.9±1.2% TEGDME) by weight while still having a conductivity greater than or equal to the state of the art.
The PEO is modified by the introduction of an iso-butene function by polycondensation, distributed homogeneously, all along the PEO chain.
The PEGx blocks are obtained by polycondensation between the oligomers of PEO, polyethylene glycol (PEG) and 3-chloro-2-(chloromethyl)-1-propene. They are noted as PEGx with x the molar mass, in kg·mol−1, of the PEG used. The propene/PEG ratio is set to 0.94 with the purpose of obtaining modified PEO with the hydroxyl ends. This synthesis is carried out with PEG of 1.5 kg·mol-1 (PEG1.5) and of 2 kg·mol−1 (PEG2) leading to polymers having double bonds (isobutene) that can be crosslinked distributed in a controlled manner all along the chain (every 34 or 45 EO units respectively).
The modified PEO, PEG1.5 (Mn=18 kg mol−1, 9 g) is dissolved in 250 mL of tetrahydrofurane in a three-neck flask provided with a coolant, a temperature probe and a septum. The solution is stirred under mechanical stirring and heated to 40° C. using an oil bath preheated to 40° C. When the polymer is completely soluble, 7 mL of triethylamine are added to the polymer solution. The mixture is degassed by argon bubbling for 20 min. Under an argon atmosphere, 4.1 mL of acryloyl chloride are added drop-by-drop to the polymer mixture. When the addition is complete, the reaction mixture is allowed to react under an argon atmosphere, under stirring and at 40° C. for 15 h. The solution is then filtered to eliminate the insoluble salts. The filtrate is reconcentrated by rotative evaporation then precipitated in cold ether. The PEG1.5-diacrylate in the form of a white solid is recovered after filtration and vacuum drying.
The maroalkoxyamine PEG1.5-(MAMA-SG1)2 is obtained by intermolecular radical addition between PEG1.5-diacrylate and the alkoxyamine MAMA-SG1 (BlocBuilder MA, Arkéma). A solution containing PEG1.5-diacrylate (7 g), MAMA-SG1 (1.48 g) and 50 mL of ethanol is introduced into a three-neck flask provided with a coolant and a septum. The solution is degassed by argon bubbling for 30 min then heated under reflux using a heating plate and an oil bath for 4 h. The polymer is then precipitated in cold ether. The PEG1.5-(MAMA-SG1)2 in the form of a white solid is recovered after filtration and vacuum drying at ambient temperature.
The three-block copolymer SEG1.5S_75 is prepared as follows: 1.2 g of PEG1.5-(MAMA-SG1)2 as well as 0.7 g of styrene and 2 g of ethylbenzene are introduced into a three-neck flask provided with a coolant, a temperature probe and a septum. The mixture is degassed by argon bubbling for 20 min. The polymerisation is then carried out under argon atmosphere at 120° C. for 5 h. The copolymer is purified by precipitation in cold ether. After drying, the three-block copolymer SEG1.5S_75 is a white solid.
Different compositions PS/PEGx are produced by following the same protocol and by modifying the PEG1.5-(MAMA-SG1)2/styrene ratio.
Finally, the PS-PEGx-PS 1.1 materials obtained are doped with LiTFSI 1.2 salt with also 2% by weight in thermal initiator (benzoyl peroxide) by dissolution in a dichloromethane/acetonitrile solution, with the quantity of salt adapted to produce after pouring the solution and evaporation of the solvent, designated equivalently production solvent, a film with EO/Li of 25 (number of moles of monomer ethylene oxide over the number of moles of 1.2 LiTFSI salt). The plastic films are then crosslinked at 100° C. for 2 hours in order to obtain films from 15 to 200 μm (SPE “XT”). They are then plasticised with the plasticiser 1.3 TEGDME 1M LiTFSI (equivalent in concentration to EO/Li=25) to obtain from 0 to 40% by weight of plasticiser in the membrane.
SPE noted as “XUV” are also produced under UV activation by proceeding as follows:
The copolymers PS-PEGx-PS 1.1 are doped with 1.2 LiTFSI salt with also 3% by weight of UV photoinitiator (benzoyl peroxide) by dissolution in a dichloromethane/acetonitrile solution, with the quantity of salt adapted to produce after pouring the solution and evaporation of the solvent, designated equivalently production solvent, a film with EO/Li of 25 (number of moles of monomer ethylene oxide over the number of moles of 1.2 LiTFSI salt).
The films are then crosslinked under a UV mercury lamp sold under the commercial name P300 MT Power supply by Fusion UV system Inc. for 30 seconds at 15 mW/cm2 under ambient atmosphere. After having been dried and placed in a glove box, the crosslinked films are then plasticised with the plasticiser 1.3 TEGDME 1M LiTFSI (equivalent in concentration to EO/Li=25) in order to obtain from 15 to 40% by weight of plasticiser in the membrane.
In what follows and in the figures reference is made to the following key:
Initial SPE: SPE before crosslinking.
“XT” SPE: Non-plasticised crosslinked SPE under thermal activation obtained as described in example 1.
“XUV” SPE: “XT” SPE: Non-plasticised crosslinked SPE under UV activation obtained as described in example 1.
One of the main advantages of these materials is the presence of a nanostructuring of the various domains (PS and PEGx), which makes it possible to have a synergy of the two antagonistic properties, ionic conductivity (PEGx) and mechanical resistance (PS). The crosslinking makes it possible to freeze the nanostructure and to further rigidify the material so as to plasticise it without substantial loss of mechanical resistance.
The mesostructural analysis carried out by X-ray diffusion at the small angles according to the wave vector q in nm−1 (SAXS) [by using a copper cathode (λ=1.54 Å)], confirms that the electrolytes have a nano-separation of the PS phases and crosslinked PEGx, by the presence of a diffraction peak [at q1≈0.175±0.01 nm−1 (accompanying
[D=2π/q1=2π/(0.175±0.01 nm−1)≈35.9±2.1 nm].
“XT” is the non-plasticised crosslinked SPE under thermal activation obtained as described in example 1.
To achieve good conductivity at low temperature for SPE, it is necessary to have a low Tg, Tf for the conductive phase, here PEO. The analyse of the thermodynamic properties by DSC (DSC3 Mettler-Toledo, at 10° C./min between −100° C. and 130° C.), shows a sharp drop in the values of Tg, Tf of the PEO phase by the adding of the plasticiser.
The SPE according to invention indeed obey Fox's Law:
where α is the mass proportion of plasticiser (TEGDME), 1−a the mass proportion of PEO phase, TgTEGDME and TgPEO the transition temperatures of the plasticiser TEGDME and of the PEO phase, respectively.
Conductivity is calculated by the following formula:
where S and l are respectively the surface and the thickness of the electrolyte. Rel is the resistance of the electrolyte determined at high frequency by impedance spectroscopy on a symmetrical cell Li/SPE/Li. The temperature is set by means of a climatic enclosure between 10 and 80° C.
The curve of accompanying
The copolymers without plasticiser 1.3 TEGDME have a conductivity of 8·10−5S/cm at 40° C., which is too low for use in a battery, in particular at a high rate and high grammage of the positive electrode (>0.8 mAh/cm2). The plasticising by a low quantity of TEGDME makes it possible to achieve a conductivity of magnitude greater than 4.6±0.5*10−45/cm, without compromising the mechanical stability of the SPE material.
Young's modulus is deduced from curves of tensile stress vs elongation obtained thanks to a dynamic mechanical analyser DMA Q800, marketed by the company TA Instruments, and this at 40° C., under dry air sweeping.
As shown in
To study the dendritic growth of the lithium of the electrochemical cells comprising a SPE according to invention: Li/SPE/Li, were assembled into a button battery. A characteristic constant current density of 180 μA/cm2 is used to displace all the lithium from one electrode to the other. Considering the thickness of lithium and the current density, a duration of 56 h was theoretically expected.
A button battery is manufactured as indicated hereinafter
Electrolyte thickness: 26 μm
Plasticised positive electrode with a base of LiFePO4 and constituted—as % by weight, of 58% LiFePO4, 22.25% polyethylene glycol (PEG), 12% polyvinylidene fluoride, 5.1% LiTFSI, 2.65%, carbon black with a grammage of 0.89 mAh/cm2 coated on a collector made of carbon coated aluminium.
Metal lithium negative electrode
Discs of composite cathode, of SPE and of lithium are cut out respectively to the diameters, 8, 12 and 10 mm. The lithium and SPE discs are laminated at 80° C. to ensure good Li/SPE contacts, then finally the composite cathode is laminated over the Li/SPE unit. The Li/SPE/Cathode sandwich is assembled between two stainless steel shims A spring is placed on the upper stainless steel shim and the whole is crimped in a button battery. The internal pressure on the electrochemical cell is about 1.5 bar.
This button battery is referenced as —1— in the diagrams of
For these tests with a LMP storage battery, the electrolyte PS-PEGx-PS was tested at 22.1%modified-PS crosslinked and plasticised at 22.9±1.2% TEGDME with a content in 1.2 LiTFSI of EO/Li=25.
The discharge curves (i. e. discharged capacity in mAh/g) obtained at 40° C. at different rates (from C/10 to C/0.6) where C in mAh/g of active materials represents the total theoretical capacity and C/n a discharge current that corresponds to the obtaining of the capacity C in n hours, are shown in
The “cyclability” at 40° C. obtained at different discharge rates (the charge always being at a low rate C/10 or C/15) is shown in
Finally, performance at 40° C. in power is shown in
The results clearly show that the SPE according to invention are superior to the commercial electrolytes, in terms of restored capacity in particular at a high rate >C/2 and this despite a lower temperature 40° C.
This example relates to a copolymer electrolyte containing 30% by weight of PS, but still plasticised at 22.9±1.2% by weight of (1.2) TEGDME. In addition to the slightly different content in PS, the main difference is the thickness of the film of electrolyte which here is 100 micrometres, which is nearly 4 times thicker than in the preceding example. For the rest, the assembly, the negative and positive electrodes are identical.
A copper conducting wire —14c— is connected to the lithium —10— and an aluminium conducting wire —14a— is connected to the cathode —11— via the current collector —12—. This sandwich structure is then vacuum heat sealed in an aluminised polyethylene bag 15, from which the collector wires exit in order to conduct the electrochemical tests.
The cathode is comprised of 74% by mass of LiFePO4, 0.5% by mass of carbon black Ketjenblack (EC600-jd, AkzoNobel), 20.1% by mass of co-P(EO)-(OB) (ICPSEB, 115,000 g/mol, Nippon shokubai) and 5.4% by mass of LiTFSI. The grammage is 1.49 mAh/cm2.
The storage battery is manufactured from a cathode standard provided to operate at 80° C. The cathode will be partially plasticised by the TEGDME contained in the plasticised electrolyte. This means that a portion of the TEGDME of the SPE diffuses inside the cathode and plasticises the binder with a PEO base of this cathode until the balance is reached between the quantity of TEGDME in the electrolyte of the cathode, on the one hand, and in the SPE, on the other hand.
The electrolyte selected is the same as the one of example 1. The thickness is however greater (37 μm vs 26 μm for the cell with a plasticised cathode) so as to limit the impact of the loss of plasticiser on the conductivity of the electrolyte.
The results obtained are remarkable, in light of the thickness of the electrolyte (37 μm), of the very grammage of the electrode of 1.49 mAh/cm2, for an electrode not optimised to operate at 40° C., but at 80° C.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1855580 | Jun 2018 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/066376 | 6/20/2019 | WO | 00 |
