POLYMER ELECTROLYTE COMPRISING A POLYACRYLAMIDE AND METHODS

FOR THE PRODUCTION THEREOF

TECHNICAL FIELD AND BACKGROUND

[0001] The present invention relates to a polymer electrolyte comprising a polymer network having a polyacrylamide backbone, which is particularly suitable for use with electrolyte compositions comprising a deep eutectic solvent. The present invention further relates to methods for the production of said polymer electrolyte, as well as to uses of the polymer electrolyte, such as in an electrochemical cell.

[0002] Recently, there has been a rapid development of small and lightweight electronic products, electronic devices and communication devices. Similarly, there has been widespread adoption of electric vehicles which have emerged as an environmentally attractive transportation means. These factors have led to a demand for the improved performance of secondary batteries which are used as power sources in said products. Furthermore, lithium secondary batteries have come into the limelight as high-performing batteries due to their high energy density and high reference electrode potential.

[0003] Conventional lithium secondary batteries comprise liquid electrolytes such as organic solvents. A significant disadvantage of liquid electrolytes is that the compositions, in particular the solvents, are inflammable, which poses a large safety risk during normal operation and in particular in case of an incident. Another disadvantage is inherent to the liquid nature of the electrolyte, associated with risks of leakage and with increased risk of environmental pollution in case of a spill or leakage.

[0004] Recently, efforts have been made to develop solid electrolytes which allow the provision of a solid-state lithium-ion battery. Such solid-state batteries have significantly reduced EHS (environmental, health and safety) hazards. Conventional solid electrolytes may include oxide-based solid electrolytes, polymer-based solid electrolytes and sulfide-based electrolytes. Polymer-based electrolytes have been generally used due to their low flammability, good flexibility, excellent thermal stability and high safety.

[0005] The difficulty in developing solid electrolytes with high ionic conductivity, a broad electrochemical window, and mechanical/thermal stability, has led to the concept of solid-liquid composites, such as solid composite electrolytes (SCE). These electrolytes comprise a liquid lithium-ion conducting electrolyte enclosed within a solid backbone or network. Examples include ionic liquids confined within an inorganic (e.g. silica) or a polymeric (e.g. poly(ethylene glycol) diacrylate (PEODA)) solid backbone.

[0006] A major challenge in the production of solid composite electrolytes, is the selection of a polymer backbone which is stable to sol-gel synthesis and can effectively encapsulate a liquid lithium-ion electrolyte. Moreover, it is difficult to develop solid composite electrolytes which are compatible with high-potential cathode materials, such as NMC622 or NMC811 and which exhibit satisfactory anodic stability. For example, the most common solid polymer electrolyte, PEO (polyethylene oxide), has an anodic stability that is limited to potentials around 4.0 V vs. Li ⁺ /Li.

[0007] Energy Environ. Sci., 2021, 14, 931-939 contemplates the use of alkali metal bis(trifluoromethane)-sulfonimide (TFSI) salts in a polymer electrolyte comprising a N-isopropylacrylamide (NIPAM) polymer backbone.

[0008] Chem. Mater. 2020, 32, 3783-3793 contemplates the use of a lithium bis(trifluoromethane)-sulfonimide lithium salt (LiTFSI) and N-methylacetamide (MAc) based deep eutectic solvent in a polymer electrolyte comprising an ethylene glycol 4-acryloylmorpholine (AcMo) backbone.

[0009] US2020/0343586A1 contemplates the use of various deep eutectic solvents in polymer electrolytes comprising various polymeric backbones. A polymer electrolyte comprising a lithium bis(trifluoromethane)-sulfonimide lithium salt (LiTFSI) and N-methylacetamide (MAc) based deep eutectic solvent in a polymer network comprising an acrylate backbone is exemplified.

[0010] It is an object of the present invention to provide a polymer electrolyte comprising a polymeric network which is compatible with deep eutectic solvents. [0011] It is a further object of the present invention to provide a polymer electrolyte which is compatible with high-voltage cathode active materials, in particular NMC622.

[0012] It is a further object of the present invention to provide a polymer electrolyte which has high anodic stability.

[0013] It is a further object of the present invention to provide a polymer electrolyte which has good mechanical flexibility. SUMMARY OF THE INVENTION

[0014] The present inventors have found that polymer electrolytes comprising a polymer network on specific acrylamide monomers and bisacrylamide crosslinkers effectively encapsulate deep eutectic solvents (DES) and are surprisingly compatible with high potential electrodes as described herein. As is shown in the appended examples, it was found that the polymer electrolytes as described herein have excellent cycling stability when combined with a high potential electrode such as NMC622, even outperforming polymer electrolytes based on the same monomer but using a bisacrylate crosslinker. Additionally, the inventors have found that the polymer electrolyte can conveniently be presynthesised but may also be synthesised in the presence of cathode active material, thereby providing a cathode material. One or more objects of the invention are achieved by the difference aspects of the invention as described herein.

[0015] Hence, in a first aspect of the present invention, there is provided a polymer electrolyte comprising an electrolyte composition and a polymer network, wherein the electrolyte composition preferably comprises a deep eutectic solvent (DES), wherein the polymer electrolyte is obtainable by polymerizing a precursor composition comprising the electrolyte composition, a first monomer according to formula (I) and a crosslinker according to formula (II) wherein R ¹ is selected from Ci-Ce hydroxyalkyl, wherein R ² is selected from H, methyl or ethyl, wherein n is an integer within the range of 0-5, wherein R ³ , and R ⁴ are each independently selected from H, methyl or ethyl, wherein R ⁵ and R ⁶ are each independently selected from H or methyl, wherein X is an alkanediyl or polyoxyalkylene, preferably X is (- CH ₂ -)O or -CH2-CH2(-O-CH2-CH ₂ ) _P - wherein o is an integer within the range of 1-10, wherein p is an integer within the range of 1-200.

[0016] In preferred embodiments of the invention, the first crosslinker is comprised in the precursor composition in an amount such that the molar ratio of the total amount of the first monomer comprised in the precursor composition to the total amount of the first crosslinker comprised in the precursor composition is within the range of 99.5:0.5 to 80:20, preferably within the range of 98:2 to 80:20, more preferably within the range of 95: 5 to 85: 15.

[0017] In another aspect, the invention provides a method for the preparation of the polymer electrolyte of the invention, comprising the following steps:

(a) providing a precursor composition comprising an electrolyte composition and a first monomer according to formula (I) as described herein, wherein the electrolyte composition preferably comprises a deep eutectic solvent (DES), and

(b) polymerizing the precursor composition.

[0018] In another aspect, the invention provides a composite cathode comprising the polymer electrolyte of the present invention.

[0019] In another aspect, the invention provides an electrochemical cell comprising the polymer electrolyte according to the present invention.

[0020] In another aspect of the invention, there is provided the use of the polymer electrolyte according to the present invention as an electrolyte for an electrochemical cell.

[0021] In another aspect of the invention, there is provided a battery, more specifically a lithium-ion battery or a lithium metal battery comprising at least one electrochemical cell comprising the polymer electrolyte as described herein, for example two or more electrochemical cells according to the invention.

[0022] In another aspect of the invention, there is provided a method of making or operating cars, computers, personal digital assistants, mobile telephones, watches, camcorders, digital cameras, thermometers, calculators, laptop BIOS, communication equipment, remote car locks, and stationary applications such as energy storage devices for power plants by employing at least one battery or at least one electrochemical cell comprising the polymer electrolyte material as described herein.

[0023] In another aspect of the invention, there is provided the use of the electrochemical cell comprising the polymer electrolyte of the invention in motor vehicles, bicycles operated by electric motor, robots, aircraft (for example unmanned aerial vehicles including drones), ships, satellites or stationary energy stores.

BRIEF DESCRIPTION OF THE FIGURES

[0024] Fig. 1 shows the EIS characterisation of symmetric cells comprising the NMC622 electrodes and the polymer electrolyte of Comparative Example 1.

[0025] Fig. 2 shows the EIS characterization of symmetric cells comprising the NMC622 electrodes and the polymer electrolyte of Example 1.

[0026] Fig. 3 shows the cycling capacity of cells comprising NMC622 and Li electrodes with the Comparative Example and Example 1.

DETAILED DESCRIPTION OF THE INVENTION

[0027] In the following detailed description, preferred embodiments are described in detail to enable the practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. But to the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description.

[0028] The expression "comprise" and variations thereof, such as, "comprises" and "comprising" as used herein should be construed in an open, inclusive sense, meaning that the embodiment described includes the recited features, but that it does not exclude the presence of other features, as long as they do not render the embodiment unworkable.

[0029] The expressions "one embodiment", "a particular embodiment", "an embodiment" etc. as used herein should be construed to mean that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of such expressions in various places throughout this specification do not necessarily all refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. For example, certain features of the disclosure which are described herein in the context of separate embodiments are also explicitly envisaged in combination in a single embodiment.

[0030] The singular forms "a," "an," and "the" as used herein should be construed to include plural referents unless the content clearly dictates otherwise. It should also be noted that the term "or" is generally employed in its broadest sense, that is as meaning "and/or" unless the content clearly dictates otherwise.

[0031] The expression "cathode active material" as used herein is also interchangeably referred to as "positive electrode active material". As will be understood by the skilled person, the cathode polarity can be positive or negative depending on the mode of operation of an electrochemical cell comprising the cathode active material. The term "a cathode active material" or "a positive electrode active material" as used herein and in the claims is defined as a material which is electrochemically active in a positive electrode or a cathode. By active material, it must be understood to be a material capable to capture and release Li ions when subjected to a voltage change over a predetermined period of time.

[0032] The expression "(meth)acrylamide" as used herein should be interpreted as "methacrylamide, acrylamide or combinations thereof". For example, N-dialkyl (meth)acrylamide should be interpreted as "N-dialkyl methacrylamide, N-dialkyl acrylamide, or combinations thereof".

[0033] The parameters x, y, z and a as referred to herein in the context of the composition of cathode active material, are measured by Inductively coupled plasma optical emission spectrometry (ICP-OES).

[0034] The ionic conductivity as referred to herein, refers to the ionic conductivity determined by electrochemical impedance spectroscopy (EIS) (using a Biologic SP-300) of the polymer electrolyte in a symmetric stainless steel | electrolyte | stainless steel Swagelok-type cell at the specified temperature by perturbing the open-circuit potential with an AC sinusoidal potential of 10 mV amplitude over a frequency range of 10 kHz - 100 mHz.

[0035] The "anodic stability limit" as referred to herein is determined by linear sweep voltammetry (preferably using a Bio-Logic, SP-300) of the polymer electrolyte which was sandwiched between a stainless-steel working electrode and a lithium metal reference and counter electrode in a coin cell setup wherein the voltage at the working electrode was swept from the open circuit potential to 6 V vs Li ⁺ /Li in the anodic scan, with a scan rate of 10 mV s’ ¹ . The stability limit was determined as the onset of electrolyte oxidation, which can be observed by a sudden increase in the measured current.

The Polymer Electrolyte of the Invention

[0036] In a first aspect of the present invention, there is provided a polymer electrolyte comprising an electrolyte composition and a polymer network, wherein the electrolyte composition preferably comprises a deep eutectic solvent (DES), wherein the polymer electrolyte is obtainable by polymerizing a precursor composition comprising the electrolyte composition, a first monomer according to formula (I) and a first crosslinker according to formula (II) wherein R ¹ is selected from Ci-Ce hydroxyalkyl, wherein R ² is selected from H, methyl or ethyl, wherein n is an integer within the range of 0-5, wherein R ³ , and R ⁴ are each independently selected from H, methyl or ethyl, wherein R ⁵ and R ⁶ are each independently selected from H or methyl, wherein X is an alkanediyl or polyoxyalkylene, preferably X is (-CH2-)o or - CH2-CH2(-O-CH2-CH ₂ ) _P - wherein o is an integer within the range of 1-10, wherein p is an integer within the range of 1-200.

[0037] The polymer network referred to in the present disclosure is a three- dimensional network, obtainable by polymerisation of one or more monomers according to formula (I) in the presence of a crosslinker according to formula (II). Such three-dimensional polymer networks are also referred to as gels, such that the polymer electrolytes of the present invention may also be referred to as "gel polymer electrolytes". For the purposes of the present disclosure, a gel polymer refers to a polymer network (i.e. a three-dimensional cross-linked system) which exhibits no flow when in the steady-state, but allows for diffusion of a liquid phase through the polymer network. Preferably, the gel is self-standing. Such gels typically exhibit a combination of flexibility, mechanical robustness, low vapour pressure and preferably nonflammability.

[0038] As the skilled person will understand on the basis of the present disclosure, the electrolyte composition is comprised the polymer network. The electrolyte composition is typically confined in the polymer network, meaning that substantially no electrolyte composition spontaneously flows from the polymer electrolyte of the invention when it is placed on a surface (such as a ceramic lab bench) without application of external pressure. In all aspects of the invention, it is highly preferred that the electrolyte composition, in the absence of the polymer network, is liquid at 20 °C.

[0039] In preferred embodiments, the polymer electrolyte of the present invention is provided wherein R ¹ is selected from Ci-Ce hydroxyalkyl, preferably R ¹ is selected from C1-C3 hydroxyalkyl, more preferably from C1-C2 hydroxyalkyl. In more preferred embodiments, the polymer electrolyte of the present invention is provided wherein R ¹ is selected from Ci-Ce hydroxyalkyl wherein the Ci-Ce hydroxyalkyl comprises one or two hydroxyl functional groups, preferably R ¹ is selected from C1-C3 hydroxyalkyl, wherein the C1-C3 hydroxyalkyl comprises one or two hydroxyl functional groups, more preferably from C1-C2 hydroxyalkyl, wherein the C1-C2 hydroxyalkyl comprises one or two hydroxyl functional groups. Preferably R ¹ comprises one hydroxyl functional groups. Most preferably R ¹ is 2- hydroxyethyl.

[0040] In preferred embodiments, the polymer electrolyte of the present invention is provided wherein R ² is selected from H or methyl, most preferably H.

[0041] In preferred embodiments, the polymer electrolyte of the present invention is provided wherein n is an integer within the range of 0-5, more preferably within the range of 0-3, more preferably 0-2 and in highly preferred embodiments, n is equal to 0.

[0042] Hence, in accordance with highly preferred embodiment of the invention :

• R ¹ is selected from Ci-Ce hydroxyalkyl wherein the Ci-Ce hydroxyalkyl comprises one or two hydroxyl functional groups, preferably R ¹ is selected from C1-C3 hydroxyalkyl, wherein the C1-C3 hydroxyalkyl comprises one or two hydroxyl functional groups, more preferably from C1-C2 hydroxyalkyl, wherein the C1-C2 hydroxyalkyl comprises one or two hydroxyl functional group, most preferably R ¹ is 2- hydroxyethyl;

• R ² is selected from H or methyl, most preferably H; and

• n is an integer within the range of 0-5, more preferably within the range of 0- 3, more preferably 0-2, most preferably n is equal to 0.

It will be understood from the above by the skilled person that the first monomer according to formula (I) is preferably selected from N-(2-hydroxyethyl)acrylamide, N-(2-hydroxyethyl)methacrylamide and combinations thereof, most preferably N- (2-hydroxyethyl)acrylamide.

[0043] In preferred embodiments of the invention, the first monomer constitutes at least 80 mol% of all monomers in the precursor composition, preferably at least 90 mol%, more preferably at least 95 mol%. In highly preferred embodiments of the invention, the first monomer comprises at least 98 mol%, 99 mol% or about 100 mol% of all monomers in the composition. For the purposes of determining the total amount of monomers in the precursor composition, any compound which is polymerizable with the first monomer and has a functionality of 1 is considered a monomer, wherein the functionality is determined on the basis of functional groups which are free-radical polymerizable with the acrylamide functionality of the first monomer.

Crosslinkers

[0044] In accordance with the invention, the precursor composition further comprises a first crosslinker according to formula (II) as described herein. Since the first monomer (which is an acrylamide according to formula (I)) is monofunctional, the inclusion of a crosslinker in the precursor composition allows a three-dimensional polymer network to be formed. As is shown in the appended examples, it was found that using a crosslinker according to formula (II) results in markedly improved cycling stability of the polymer networks of the invention compared to when a bisacrylate crosslinker is used.

[0045] In preferred embodiments of the invention, R ³ and R ⁴ are each independently selected from H, or methyl, preferably H. In highly preferred embodiments of the invention, R ³ and R ⁴ are the same and selected from H or methyl, most preferably R ³ and R ⁴ are both H.

[0046] In preferred embodiments of the invention, R ⁵ and R ⁶ are each independently selected from H or methyl. In preferred embodiments, R ⁵ and R ⁶ are the same and selected from H or methyl, most preferably R ⁵ and R ⁶ are both H.

[0047] As will be understood by the skilled person, in case X is -CH2-CH2(-O- CH2-CH2)p- the compound of formula (II) will in practise be provided in the form of a mixture of compounds with varying degrees of ethoxylation and thus varying numbers of p. It is preferred that in case the precursor composition comprises one or more crosslinkers according to formula (II) wherein X is -CH2-CH2(-O-CH2-CH2) _P - the number-average p determined over all compounds of formula (II) in the precursor composition is within the range of 1-200, preferably within the range of 2-20.

[0048] In preferred embodiments of the invention, X is selected from (-CH2- ) ₀ wherein o is within the range of 1 to 10, preferably within the range of 1 to 5, and more preferably o is equal to 1. In other words, it is highly preferred that X is methanediyl.

[0049] Thus, as will be understood by the skilled person based on the above, in preferred embodiments of the invention

• R ³ and R ⁴ are each independently selected from H, or methyl, preferably H;

• R ⁵ and R ⁶ are each independently selected from H or methyl; and

• X is selected from (-CH2-)o wherein o is within the range of 1 to 10, preferably within the range of 1 to 5, and more preferably o is equal to 1.

Accordingly, in preferred embodiments of the invention

• R ¹ is selected from Ci-Ce hydroxyalkyl wherein the Ci-Ce hydroxyalkyl comprises one or two hydroxyl functional groups, preferably R ¹ is selected from C1-C3 hydroxyalkyl, wherein the C1-C3 hydroxyalkyl comprises one or two hydroxyl functional groups, more preferably from C1-C2 hydroxyalkyl, wherein the C1-C2 hydroxyalkyl comprises one or two hydroxyl functional group, most preferably R ¹ is 2- hydroxyethyl;

• R ² is selected from H or methyl, most preferably H;

• n is an integer within the range of 0-5, more preferably within the range of 0- 3, more preferably 0-2, most preferably n is equal to 0;

• R ³ and R ⁴ are each independently selected from H, or methyl, preferably H;

• R ⁵ and R ⁶ are each independently selected from H or methyl; and

• X is selected from (-CH2-)o wherein o is within the range of 1 to 10, preferably within the range of 1 to 5, and more preferably o is equal to 1.

[0050] The precursor composition may further comprise a second crosslinker different from the first crosslinker, and chosen from any which is polymerizable with the first monomer and which has a functionality of 2 or more, wherein the functionality is determined on the basis of functional groups which are free-radical polymerizable with the acrylamide functionality of the first monomer. It is preferred that such a second crosslinker different from the first crosslinker is selected from crosslinkers comprising two or more functional groups selected from the group consisting of allyl (-CH3-CH=CH2), oxiranyl (-C2H3O), glycidyl (-CH2-C2H3O), vinyl ether (-O-CH=CH2), vinyl ester (-C(O)-O-CH=CH2), vinyl amide (-C(O)-NH- CH=CH2), vinyl amine (-NH-CH=CH2), norbornene, maleate, fumarate, itaconate, alkynyl (-C=CH), styrene (-Ph-CH=CH2), acrylamide (-NH-C(O)-CH=CH2), methacrylamide (-NH-C(O)-C(CH3)=CH2), acrylate (-O-C(O)-CH=CH2), methacrylate (-O-C(O)-C(CH3)=CH2) and combinations thereof, preferably selected from crosslinkers comprising two or more functional groups selected from acrylamide (-NH-C(O)-CH=CH2), methacrylamide (-NH-C(O)-C(CH3)=CH2), acrylate (-O-C(O)-CH=CH2), methacrylate (-O-C(O)-C(CH3)=CH2) and combinations thereof. In some embodiments of the invention, the second crosslinker comprises two, three or four functional groups selected from the functional groups described in the preceding sentence, however it is preferred if the second crosslinker comprises two of the same or different functional groups selected from the functional groups described in the preceding sentence.

[0051] In general, it is preferred that the average functionality determined across all crosslinkers in the precursor composition is within the range of 2-3, preferably within the range of 2-2.5, most preferably within the range of 2-2.2. For the purposes of determining this average functionality, any compound which is polymerizable with the first monomer and which has a functionality of 2 or more is considered a crosslinker, wherein the functionality is determined on the basis of functional groups which are free-radical polymerizable with the acrylamide functionality of the first monomer.

[0052] In preferred embodiments of the invention, the first crosslinker constitutes at least 80 wt.% (by total weight of all crosslinkers comprised in the precursor composition) of all crosslinkers in the precursor composition, preferably at least 90 wt.% (by total weight of all crosslinkers comprised in the precursor composition), more preferably at least 95 wt.% (by total weight of all crosslinkers comprised in the precursor composition). In highly preferred embodiments of the invention, the first crosslinker is the only crosslinker present in the precursor composition. Any compound which is polymerizable with the first monomer and which has a functionality of 2 or more is considered a crosslinker, wherein the functionality is determined on the basis of functional groups which are free-radical polymerizable with the acrylamide functionality of the first monomer.

[0053] As will be understood by the skilled person, the amount of crosslinker employed in the precursor composition will influence the mechanical properties as well as the electrochemical properties of the obtained polymer electrolyte. In preferred embodiments of the invention, the first crosslinker is comprised in the precursor composition in an amount such that the molar ratio of the total amount of the first monomer comprised in the precursor composition to the total amount of the first crosslinker comprised in the precursor composition is within the range of 99.5:0.5 to 80: 20, preferably within the range of 98:2 to 80:20, more preferably within the range of 95:5 to 85: 15. In case the precursor composition comprises further additional crosslinkers other than the first crosslinker, it is preferred that the total amount of crosslinkers in the precursor composition is within the range of 99.5:0.5 to 80:20, preferably within the range of 98:2 to 80:20, more preferably within the range of 95:5 to 85: 15. For the purposes of determining this total amount of crosslinkers, any compound which is polymerizable with the first monomer and which has a functionality of 2 or more is considered a crosslinker, wherein the functionality is determined on the basis of functional groups which are free-radical polymerizable with the acrylamide functionality of the first monomer. As explained earlier, it is highly preferred that the first crosslinker is the major (80 wt.% or more of all crosslinkers as explained herein earlier) or only crosslinker present in the precursor composition.

[0054] In accordance with preferred embodiments of the invention, the precursor composition further comprises one or more radical initiators, preferably one or more radical initiators selected from thermal initiators, photoinitiators and combinations thereof.

[0055] Suitable radical thermal initiators include but are not limited to benzoyl peroxide, dibenzoyl peroxide, succinic acid peroxide, dilauroyl peroxide, didecanoyl peroxide, dicumyl peroxide, di-t-butyl peroxide, di-t-amyl peroxide, o,o'-di(t-butyl peroxy)diisopropyl-benzene, 2,5-dimethyl-2,5-di-(t-butylperoxy) hexane, 2,5- dimethyl-2,5-di-(t-butylperoxy) hexyne-3,t-butyl cumyl peroxide, o-cumyl peroxyneodecanoate, o-cumyl peroxyneopheptanoate, t-amyl peroxyneodecanoate, t-butyl peroxyneodecanoate, di-(2-ethylhexyl) peroxydicarbonate, t-amyl peroxypivalate, t-butyl peroxypivalate, 2,5-dimethyl-2,5 bis(2-ethyl-hexanoylperoxy) hexane, dibenzoyl peroxide, t-amyl peroxy-2- ethylhexanoate, t-butyl peroxy-2-ethylhexanoate, l,l-di-(t- amylperoxy)cyclohexane, l,l-di-(t-butylperoxy) 3,3,5-trimethyl cyclohexane, 1,1- di-(t-butylperoxy)cyclohexane, OO-t-amyl-O(2-ethylhexyl)monoperoxycarbonate, OO-t-butyl O-isopropyl monoperoxycarbonate, OO-t-butyl O-(2- ethylhexyl)monoperoxycarbonate, t-amyl peroxybenzoate, t-butyl peroxyacetate, t-butyl peroxybenzoate, ethyl 3,3-di-(t-amylperoxy) butyrate, ethyl 3,3-di-(t- butylperoxy) butyrate, dicumyl peroxide; and azo compounds such as 4,4'- azobis(4-cyanovaleric acid), l,l'-azobis(cyclohexane carbonitrile), azobisisobutyronitrile (AIBN), and 2,2'-azobis(2- methylpropionam id ine)di hydrochloride, 2,2'-azobis[2-imidazolin-2- yl)propane]dihydrochloride, 2,2'-azobis[2-(2-imidazolin-2-yl) propane disulfate di hydrate, 2, 2'-azobis(2-methyl propionam id ine)di hydrochloride, 2,2'-azobis[N-(2- carboxyethyl)-2-methyl propionamidine] hydrate, 2,2'-azobis{2-[l-(2- hydroxyethyl)-2-imidazol in- 2-yl ]propane}di hydrochloride, 2,2'-azobis[2-(2- imidazolin-2-yl)propane], 2,2'-azobis(l-imino-l-pyrrolidino-2- ethylpropane)di hydrochloride, 2,2'-azobis{2-methyl-N-[l,l-bis(hydroxymethyl)- 2- hydroxyethyl] propionam ide}, 2,2 '-azobis[2-methyl-N- (2- hydroxyethyl) propionamide], cumene hydroperoxide, and ammonium persulfate.

[0056] Suitable radical photoinitators include but are not limited to benzophenone (e.g., "IRGACURE 500"), 3-methylbenzophenone, 2- methylbenzophenone, 3,4-dimethylbenzophenone, 3-hydroxybenzophenone, 4- hydroxybenzophenone, 4,4'-dihydroxybenzophenone, 4-benzoylbenzoic acid, 2- benzoylbenzoic acid, methyl 2-benzoylbenzoate, 4,4'-carbonyldiphthalic anhydride, methylbenzoyl formate (e.g., "DAROCUR MBF"), 1-hydroxy-cyclohexyl- phenyl-ketone (e.g., "IRGACURE 184"), 2-hydroxy-2-methyl-l-phenyl-l- propanone (e.g., DAROCUR 1173"), 2-hydroxy-l-[4-(2-hydroxyethoxy)phenyl]-2- methyl-l-propanone (e.g., "IRGACURE 2959"), oxy-phenyl-acetic acid 2-[2 oxo-2 phenyl-acetoxy-ethoxy]-ethylester and oxy-phenyl-acetic 2-[2-hydroxy-ethoxy]- ethyl ester (e.g., "IRGACURE 754"), alpha, alpha-dimethoxy-alpha- phenylacetophenone (also known as 2,2-dimethoxy-2-phenyl-acetophenone (DMPA) e.g., "IRGACURE 651"), 2-benzyl-2-(dimethylamino)-l-[4-(4- morpholinyl)phenyl]-l-butanone (e.g., "IRGACURE 369"), 2-methyl-l-[4- (methylthio)phenyl]-2-(4-morpholinyl-l-propanone (e.g., "IRGACURE 907"), diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide (e.g., "DAROCUR TPO"), phosphine oxide, phenyl bis(2,4,6-trimethyl benzoyl) (e.g., "IRGACURE 819"), bis(eta 5-2,4-cyclopentadien-l-yl)bis[2,6-difluoro-3-(lH-pyrrol-l- yl)phenyl]titanium (e.g., "IRGACURE 784"), 1-hydroxy-cyclohexyl-phenyl-ketone (e.g., ("IRGACURE 184"), 2-hydroxy-2-methyl-l-phenyl-l-propanone (e.g., "DAROCUR 1173"), 2-hydroxy-l-{4-[4-(2-hydroxy-2-methyl-propionyl)-benzyl]- phenyl}-2-methyl-propan-l-one (e.g., "IRGACURE 127"), 2-hydroxy-l-[4-(2- hydroxyethoxy)phenyl]-2-methyl-l-propanone (e.g., "IRGACURE 2959"), phenylglyoxylate, oxy-phenyl-acetic acid 2-[2-oxo-2-phenyl-acetoxy-ethoxy]- ethyl ester, oxy-phenyl-acetic acid 2-[2-hydroxy-ethoxy]-ethyl ester, phenyl glyoxylic acid methyl ester (e.g., "DAROCUR MBF"), 2,4,6-trimethylbenzoyl- diphenylphosphine oxide (e.g., "LUCIRIN TPO"), 2,4,6-trimethylbenzoyl-diphenyl phosphinate (e.g., "LUCIRIN TPO-L"), liquid blend of acylphosphine oxides (e.g., "IRGACURE 2100"), bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (e.g., "IRGACURE 819"), titanocene, bis(r|5-2,4-cyclopentadien-l-yl)bis[2,6-difluoro-3- (lH-pyrrol-l-yl)phenyl] (e.g., "IRGACURE 784"), [l-(4- phenylsulfanylbenzoyl)heptylideneamino]benzoate (e.g., "IRGACURE OXE 01"), [l-[9-ethyl-6-(2-methylbenzoyl)carbazol-3-yl]ethylideneamino ]acetate (e.g., "IRGACURE OXE 02"), 2-methyl-l-[4-(methylthio)phenyl]-2-morpholinopropan-l- one (e.g., "IRGACURE 907"), 2-benzyl-2-dimethylamino-l-(4-morpholinophenyl)- butanone-1 (e.g., "IRGACURE 369"), 2-dimethylamino-2-(4-methyl-benzyl)-l-(4- morpholin-4-yl-phenyl)-butan-l-one (e.g., "IRGACURE 379"), benzyldimethyl ketal, 2,2-dimethoxy-l,2-diphenylethan-l-one (e.g., "IRGACURE 651"), camphorquinone, acetophenone, 4'-hydroxyacetophenone, 3'- hydroxyacetophenone, 4-(dimethylamino)-benzophenone, 4,4'-

Bis(di methylamino)- benzophenone, 4,4'-Bis(diethylamino)-benzophenone, 4,4'- dichlorobenzophenone, 4-phenylbenzophenone, 1,4-dibenzoylbenzene, 4-(p- tolylthio)-benzophenone, dibenzosuberenone, benzil, p-anisil, methyl benzoylformate, 9,10-phenanthrenequinone, 2-benzoyl-2-propanol, 2-Hydroxy-4'- (2-hydroxyethoxy)- 2- methyl propiophenone, 1- Benzoylcyclohexanol, benzoin, anisoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, 2,2-diethoxyacetophenone, benzil dimethylketal, 2-- Methyl-4'-(methylthio)-2-morpholi nopropiophenone, 2- Benzyl -2-

(dimethylamino)-4'-morpholinobutyrophenone, 2-Isonitrosopropiophenone, 9,10- Phenanthrenequinone, 2-ethylanthraquinone, sodium Anthraquinone-2-sulfonate, 2-chlorothioxanthone, l-chloro-4-propoxythioxanthone, 2-isopropylth ioxanthone, 2,4-diethylthioxanthen-9-one, 2, 7-di methyloxythioxanthone, 2,2'-Bis(2- chlorophenyl)4,4',5,5'-tetraphenyl-l,2'-bi imidazole, Di phenyl (2,4, 6-trimethyl- benzoyl)phosphine Oxide, phenylbis(2,4,6-trimethyl-benzoyl)phosphine Oxide, lithium Phenyl-(2,4,6-trimethylbenzoyl)phosphinate, and ferrocene.

[0057] In some embodiments of the invention, the precursor composition further comprises one or more radical initiators selected from 2,2-dimethoxy-2- phenyl-acetophenone (DMPA), azobisisobutyronitrile (AIBN) and combinations thereof.

[0058] The one or more radical initiators are preferably employed in an amount such that the molar ratio of the total amount of the first monomer comprised in the precursor composition to the total amount of the one or more radical initiators comprised in the precursor composition is within the range of 99.8:0.2 to 80:20, preferably within the range of 99: 1 to 85: 15, more preferably within the range of 98:2 to 90: 10.

[0059] As will be understood by the skilled person, in order to obtain a geltype polymer electrolyte, it is preferred that the polymer is synthesized in the presence of an electrolyte composition, thereby effectively encapsulating the electrolyte composition in the polymer network. However, without wishing to be bound by any theory, other methods of obtaining a polymer electrolyte to those described herein may be feasible, such as exchanging another liquid composition (e.g. a solvent) encapsulated in the polymer for the electrolyte composition, absorbing the electrolyte composition into a preformed polymer network, injecting the electrolyte composition into a preformed polymer network, etc. Hence, in accordance with the invention, the polymer electrolyte is obtainable by polymerizing a precursor composition which comprises the polymer electrolyte and other components as discussed throughout the present disclosure (monomer, crosslinker, initiator, etc.). In all aspects of the invention, it is highly preferred that the electrolyte composition, in the absence of the polymer network, is liquid at 20 °C.

[0060] As is shown in the appended examples, the present inventors have found that deep eutectic solvent (DES) loaded polymer electrolytes exhibit exceptional performance, in particular in combination with high-potential cathode active material such as NMC622. Hence, in accordance with highly preferred embodiments of the invention, the electrolyte composition comprises or consists of a deep eutectic solvent (DES). The deep eutectic solvent is preferably liquid at 20 °C.

[0061] Various relative amounts of DES to polymer have been found to result in functional electrolyte materials. The precursor composition preferably comprises from about 45 to 95 vol% (by total volume of the precursor composition) of the deep eutectic solvent (DES), preferably from about 55 to 90 vol%, more preferably from 70 to 90 vol%. It was found that precursor compositions having about 85 vol% DES provided superior ionic conductivity and mechanical properties (increased flexibility). Thus, in highly preferred embodiments of the invention, the precursor composition comprises from about 75 to 90 vol% (by total volume of the precursor composition) of the deep eutectic solvent (DES), preferably from about 80 to 90 vol%, most preferably from about 83 to 87 vol%. The remainder of the precursor composition is made up of the first monomer, optionally further monomers, the first crosslinker, optionally further crosslinkers, one or more radical initiators, and optional further components. In some embodiments the remainder of the precursor composition consists essentially of the first monomer, optionally further monomers, the first crosslinker, optionally further crosslinkers, and one or more radical initiators.

[0062] The deep eutectic solvent (DES) preferably has a eutectic point of less than or equal to 25 °C, preferably the eutectic point is equal to or less than 15 °C, more preferably less than or equal to 0 °C. In highly preferred embodiments of the invention, the deep eutectic solvent (DES) has a eutectic point of less than or equal to -15 °C, most preferably less than or equal to -25 °C. This allows the DES to remain in a liquid state over the typical operating temperature window of an electrochemical cell for regular applications such as automotive. The eutectic point referred to herein is determined at a pressure of about 101 kPa.

[0063] The deep eutectic solvent (DES) preferably comprises at least one hydrogen bond acceptor and at least one hydrogen bond donor. The molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is preferably at least 1 : 1, more preferably at least 1 : 2, more preferably at least 1 :3. In preferred embodiments of the invention, the electrolyte composition comprises or consists of (preferably consists of) a deep eutectic solvent (DES) comprising at least one hydrogen bond acceptor and at least one hydrogen bond donor, wherein the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is within the range of 1 : 1 to 1 :8, preferably within the range of 1 :2 to 1 :6, more preferably within the range of 1 : 3 to 1 :5. A highly preferred molar ratio of hydrogen bond acceptor to hydrogen bond donor (in particular in case the hydrogen bond acceptor is lithium bis(trifluoromethanesulfonyl)imide as described herein elsewhere and/or in case the hydrogen bond donor is /V-methylacetamide as described herein elsewhere) is within the range of 1 :3.5 to 1 :4.5, such as about 1 :4.

[0064] In preferred embodiments of the invention, the hydrogen bond acceptor comprises a lithium salt, a zinc salt or a combination thereof, preferably a lithium salt. In more preferred embodiments, the hydrogen bond acceptor is selected from the group consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiOTf), lithium chloride (LiCI), lithium hexafluorophosphate (LiPFe), lithium polysulfide, lithium perchlorate (LiCICU), lithium bromide (LiBr), lithium iodide (Lil), lithium thiocyanate (LiSCN), lithium tetrafluoroborate (UBF4), lithium hexafluoroarsenate (LiAsFe), lithium bis(oxalate)borate (LiBOB), lithium fluoroalkylphosphate (LFAP [LiPF3(CF2CF3)3]), and combinations thereof, preferably lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and/or the hydrogen bond donor is selected from the group consisting of urea, /V-methylurea, /V/ZV-dimethylurea, N,N'-d methyl urea, /Vz/Vz/V'-trimethylurea, thiourea, /V-methylthiourea, /V/ZV-dimethylthiourea, N,N'-dimethylthiourea, N,N,N'- trimethylthiourea, ethylene glycol, propane-1, 2-diol, propane-1, 3-diol, butane- 1,4-diol, 1,2,3-propanetriol, acetic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, benzoic acid, glycolic acid, citric acid, 2- hydroxy propionic acid, 2-hydroxy isobutyric acid, o-phenylenediamine, choline chloride, acetamide, /V-methylacetamide, trifluoroacetamide, N-methyl trifluoroacetamide, benzamide, benzenesulphonic acid, p-toluenesulphonic acid, o- toluenesulphonic acid, m-toluenesulphonic acid, and combinations thereof, preferably /V-methylacetamide.

[0065] Hence, in some embodiments of the invention, the electrolyte composition consists of a deep eutectic solvent (DES) comprising, preferably consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and N- methylacetamide wherein the molar ratio of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to /V-methylacetamide is within the range of 1 : 1 to 1 :8, preferably within the range of 1 :2 to 1 :6, more preferably within the range of 1 :3 to 1 : 5, such as about 1 :4.

[0066] The electrolyte composition preferably comprises at least 90 wt.% (by total weight of the electrolyte composition) of the DES, preferably at least 95 wt.%, more preferably at least 99 wt.%. In some embodiments, the electrolyte composition comprises water, such as 0.1-30 wt.% (by total weight of the electrolyte composition) of water or 0.1-10 wt.% (by total weight of the electrolyte composition) of water. In other, preferred embodiments, the electrolyte composition comprises less than 5 wt.% of water, preferably less than 0.1 wt.% water, more preferably less than 0.01 wt.% of water. In some embodiments, the electrolyte composition is substantially free of water. The latter is particularly preferred for use in combination with moisture-sensitive electrodes such as Li or graphite. In all embodiments described herein, the electrolyte composition preferably consists of the deep eutectic solvent (DES).

[0067] As will be understood by the skilled person in the context of the present disclosure, the precursor composition mainly consists of the electrolyte composition as described herein in combination with the first monomer, the first crosslinker and the one or more initiators as described herein. Hence, according to preferred embodiments of the invention, the precursor composition comprises at least 90 wt.% (by total weight of the precursor composition) of the combined amount of the electrolyte composition, the first monomer, optionally further monomers, the first crosslinker, optionally further crosslinkers and one or more radical initiators, preferably at least 95 wt.% (by total weight of the precursor composition), more preferably at least 99 wt.% (by total weight of the precursor composition). In some embodiments, the precursor composition comprises at least 90 wt.% (by total weight of the precursor composition) of the combined amount of the electrolyte composition, the first monomer, the first crosslinker, and one or more radical initiators, preferably at least 95 wt.% (by total weight of the precursor composition), more preferably at least 99 wt.% (by total weight of the precursor composition). As explained herein earlier, any compound which is polymerizable with the first monomer and has a functionality of 1 is considered a monomer, and any compound which is polymerizable with the first monomer and which has a functionality of 2 or more is considered a crosslinker, wherein the functionality is determined on the basis of functional groups which are free-radical polymerizable with the acrylamide functionality of the first monomer. Consequently, in accordance with preferred embodiments of the invention, the polymer electrolyte comprises at least 90 wt.% (by total weight of the polymer electrolyte) of the combined weight of the polymer network and the electrolyte composition, more preferably at least 98 wt.% (by total weight of the polymer electrolyte) and most preferably at least 99 wt.% (by total weight of the polymer electrolyte). In some embodiments, the polymer electrolyte consists essentially of the polymer network and the electrolyte composition.

[0068] In preferred embodiments of the invention, the polymer electrolyte is provided which has an anodic stability limit of at least 4.6 V vs Li ⁺ /Li, preferably at least 4.7 V vs Li ⁺ /Li .

Method for the preparation of the polymer electrolyte of the invention

[0069] In another aspect the invention provides a method for the preparation of the polymer electrolyte of the invention, comprising the following steps:

(i) providing a precursor composition as described herein, and

(ii) polymerizing the precursor composition.

[0070] The embodiments described in the present disclosure relating to the polymer electrolyte, apply mutatis mutandis to the method for the preparation of the polymer electrolyte. For example, the various embodiments relating to the identity and amounts of monomer, crosslinker, initiator, electrolyte composition as explained herein in the context of the polymer electrolyte are equally applicable to the method for the preparation of the polymer electrolyte.

[0071] In preferred embodiments of the method for the preparation of the polymer electrolyte, the precursor composition comprises one or more radical initiators as described herein before, and step (ii) comprises activating the radical initiator. The activation is preferably performed by UV irradiation of the precursor composition or by heating the precursor composition to a temperature of at least 50 °C, preferably at least 60 °C. It is preferred that step (ii) is performed under an inert gas atmosphere, preferably an inert atmosphere, such as nitrogen or argon.

[0072] In some embodiments of the invention, step (ii) comprises the steps of (ii)a contacting the precursor composition with cathode active material, and (ii)b polymerizing the precursor composition in the presence of the cathode active material. Step (ii)a preferably comprises mixing the precursor composition with preferably particulate cathode active material, or depositing precursor composition on a surface of porous cathode active material. The contacting preferably takes place for at least 1 minute before polymerisation, such that a thorough mixing or impregnation may be achieved. In this way, a composite cathode comprising the polymer electrolyte of the invention and cathode active material can be obtained.

[0073] The cathode active material may be any cathode active material, preferably a cathode active material suitable for secondary lithium-ion batteries.

[0074] As is shown in the appended examples, the present inventors have found that the polymer electrolytes of the invention exhibit surprisingly good electrochemical performance when used in conjunction with high-potential cathode active material. Hence, the cathode active material preferably has an upper cutoff voltage of at least 4.3 V vs. Li ⁺ /Li, preferably at least 4.4 V vs. Li ⁺ /Li, more preferably at least 4.5 V vs. Li ⁺ /Li. The cathode active material is preferably a cathode active material comprising Li, M and 0, wherein M comprises Ni and one or both of Mn and Co, preferably wherein M comprises:

- Ni in a content x, wherein 50.0 mol% < x < 95.0 mol%, preferably wherein 55.0 mol% < x < 95.0 mol%,

- Mn in a content y, wherein 0.0 mol% < y < 40.0 mol%,

- Co in a content z, wherein 0.0 mol% < z < 40.0 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; more preferably wherein M comprises:

- Ni in a content x, wherein 50.0 mol% < x < 85.0 mol%,

- Mn in a content y, wherein 7.5 mol% < y < 25.0 mol%,

- Co in a content z, wherein 7.5 mol% < z < 25.0 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; more preferably wherein M comprises:

- Ni in a content x, wherein 55.0 mol% < x < 80.0 mol%,

- Mn in a content y, wherein 10.0 mol% < y < 30.0 mol%,

- Co in a content z, wherein 10.0 mol% < z < 30.0 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; more preferably wherein M comprises:

- Ni in a content x, wherein 55.0 mol% < x < 75.0 mol%,

- Mn in a content y, wherein 12.5 mol% < y < 22.5 mol%,

- Co in a content z, wherein 12.5 mol% < z < 22.5 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; most preferably wherein M comprises:

- Ni in a content x, wherein 55.0 mol% < x < 70.0 mol%,

- Mn in a content y, wherein 15.0 mol% < y < 22.5 mol%,

- Co in a content z, wherein 15.0 mol% < z < 22.5 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%.

[0075] As is known to the skilled person, NMC cathode active material can comprise impurities, or be doped or coated resulting in an overall cathode active material comprising one or more elements other than Li, Ni, Mn, Co and 0, which is reflected in the parameter "D" used herein. In preferred embodiments of the invention, D is an element selected from the group consisting of: Al, B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W Zr, and Zn; preferably Al, B, Cr, Nb, S, Si, Ti, Y, Zr and W; more preferably B, Nb, Ti, Zr and W.

[0076] Examples of suitable cathode active materials are NMC532, NMC622, NMC 811, preferably NMC622 or NMC811, more preferably NMC622.

Composite cathode comprising the polymer electrolyte of the invention

[0077] In another aspect of the invention, there is provided a composite cathode comprising the polymer electrolyte described herein and cathode active material as described herein.

[0078] The embodiments described in the present disclosure relating to the polymer electrolyte, apply mutatis mutandis to the composite cathode comprising the polymer electrolyte. For example, the various embodiments relating to the identity and amounts of monomer, crosslinker, initiator, electrolyte composition as explained herein in the context of the polymer electrolyte are equally applicable to the method for the preparation of the polymer electrolyte.

[0079] The cathode active material comprised in the composite cathode may be any cathode active material, preferably a cathode active material suitable for secondary lithium-ion batteries.

[0080] As is shown in the appended examples, the present inventors have found that the polymer electrolytes of the invention exhibit surprisingly good electrochemical performance when used in conjunction with high-potential cathode active material. Hence, the cathode active material preferably has an upper cutoff voltage of at least 4.3 V vs. Li ⁺ /Li, preferably at least 4.4 V vs. Li ⁺ /Li, more preferably at least 4.5 V vs. Li ⁺ /Li. The cathode active material is preferably a cathode active material comprising Li, M and 0, wherein M comprises Ni and one or both of Mn and Co, preferably wherein M comprises:

- Ni in a content x, wherein 50.0 mol% < x < 95.0 mol%, preferably wherein 55.0 mol% < x < 95.0 mol%,

- Mn in a content y, wherein 0.0 mol% < y < 40.0 mol%,

- Co in a content z, wherein 0.0 mol% < z < 40.0 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and 0,

- wherein x + y + z + a is 100.0 mol%; more preferably wherein M comprises:

- Ni in a content x, wherein 50.0 mol% < x < 85.0 mol%, - Mn in a content y, wherein 7.5 mol% < y < 25.0 mol%,

- Co in a content z, wherein 7.5 mol% < z < 25.0 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; more preferably wherein M comprises:

- Ni in a content x, wherein 55.0 mol% < x < 80.0 mol%,

- Mn in a content y, wherein 10.0 mol% < y < 30.0 mol%,

- Co in a content z, wherein 10.0 mol% < z < 30.0 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; more preferably wherein M comprises:

- Ni in a content x, wherein 55.0 mol% < x < 75.0 mol%,

- Mn in a content y, wherein 12.5 mol% < y < 22.5 mol%,

- Co in a content z, wherein 12.5 mol% < z < 22.5 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; most preferably wherein M comprises:

- Ni in a content x, wherein 55.0 mol% < x < 70.0 mol%,

- Mn in a content y, wherein 15.0 mol% < y < 22.5 mol%,

- Co in a content z, wherein 15.0 mol% < z < 22.5 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%.

[0081] As is known to the skilled person, NMC cathode active material can comprise impurities, or be doped or coated resulting in an overall cathode active material comprising one or more elements other than Li, Ni, Mn, Co and O, which is reflected in the parameter "D" used herein. In preferred embodiments of the invention, D is an element selected from the group consisting of: Al, B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W Zr, and Zn; preferably Al, B, Cr, Nb, S, Si, Ti, Y, Zr and W; more preferably B, Nb, Ti, Zr and W.

[0082] Examples of suitable cathode active materials are NMC532, NMC622, NMC 811, preferably NMC622 or NMC811, more preferably NMC622.

[0083] The composite cathode may comprise a homogenous mixture of cathode particles and polymer electrolyte particles. Alternatively, the composite cathode may comprise the polymer electrolyte coated on and/or at least partially embedded in the cathode active material. Such composite cathodes are obtainable by the method for the preparation of the polymer electrolyte described herein earlier wherein step (ii) comprises the steps of (ii)a contacting the precursor composition with cathode active material, and (ii)b polymerizing the precursor composition in the presence of the cathode active material.

[0084] An optional but preferred additional component of the composite cathode material of the invention is a conductivity aid, in particular a carbon-based conductivity aid. The carbon-based conductivity aid may be any carbon-rich material, such as any material comprising at least 95 wt.% carbon, preferably any material comprising at least 99 wt.% carbon. Examples of suitable materials are graphite, carbon black, carbon fibers, carbon nanotubes, graphene and combinations thereof. Carbon black is known to the skilled person and includes variants such as acetylene black or super C65.

[0085] In preferred embodiments, the carbon-based conductivity aid as described herein is present in the solid composite cathode composition of the present invention in an amount of at least 0.5 wt.% (by combined weight of the polymer electrolyte and the cathode active material), preferably at least 1 wt.% (by combined weight of the polymer electrolyte and the cathode active material), more preferably at least 3 wt.% (by combined weight of the polymer electrolyte and the cathode active material). Typically, the carbon-based conductivity aid is present in an amount of less than 12 wt.% (by combined weight of the polymer electrolyte and the cathode active material), preferably less than 9 wt.% (by combined weight of the polymer electrolyte and the cathode active material), more preferably less than 7 wt.% (by combined weight of the polymer electrolyte and the cathode active material).

[0086] In another aspect of the invention, there is provided a composite cathode obtainable by the method for the preparation of the polymer electrolyte described herein earlier wherein step (ii) comprises the steps of (ii)a contacting the precursor composition with cathode active material, and (ii)b polymerizing the precursor composition in the presence of the cathode active material.

[0087] The composite cathode material may be formulated into a cathode using techniques known to the skilled person. In particular, in some embodiments of the invention there is provided a cathode comprising the solid composite cathode material of the present invention in combination with a binder, such as a polymer binder. The binder is not particularly limiting and can be any suitable polymer binder, such as polyimide (PI), polyvinylidene chloride (PVdC), polyethylene oxide (PEO), polyvinylidene fluoride (PVdF) and the like.

Electrochemical cell comprising the polymer electrolyte of the invention [0088] In another aspect of the invention, there is provided an electrochemical cell comprising the polymer electrolyte of the present invention. [0089] The embodiments described in the present disclosure relating to the polymer electrolyte, apply mutatis mutandis to the electrochemical cell comprising the polymer electrolyte. For example, the various embodiments relating to the identity and amounts of monomer, crosslinker, initiator, electrolyte composition as explained herein in the context of the polymer electrolyte are equally applicable to electrochemical cell comprising the polymer electrolyte.

[0090] The electrochemical cell preferably comprises an anode, a cathode and an electrolyte.

[0091] The anode comprises anode active material. Suitable electrochemically active anode materials are those known in the art. For example, the anode may comprise graphitic carbon, metallic lithium or a metal alloy comprising lithium as the anode active material.

[0092] The cathode comprises cathode active material. The cathode active material may be any cathode active material, preferably a cathode active material suitable for secondary lithium ion batteries.

[0093] As is shown in the appended examples, the present inventors have found that the polymer electrolytes of the invention exhibit surprisingly good electrochemical performance when used in conjunction with high-potential cathode active material. Hence, the cathode active material preferably has an upper cutoff voltage of at least 4.3 V vs. Li ⁺ /Li, preferably at least 4.4 V vs. Li ⁺ /Li, more preferably at least 4.5 V vs. Li ⁺ /Li. The cathode active material is preferably a cathode active material comprising Li, M and 0, wherein M comprises Ni and one or both of Mn and Co, preferably wherein M comprises:

- Ni in a content x, wherein 50.0 mol% < x < 95.0 mol%, preferably wherein 55.0 mol% < x < 95.0 mol%,

- Mn in a content y, wherein 0.0 mol% < y < 40.0 mol%,

- Co in a content z, wherein 0.0 mol% < z < 40.0 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; more preferably wherein M comprises:

- Ni in a content x, wherein 50.0 mol% < x < 85.0 mol%,

- Mn in a content y, wherein 7.5 mol% < y < 25.0 mol%,

- Co in a content z, wherein 7.5 mol% < z < 25.0 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; more preferably wherein M comprises:

- Ni in a content x, wherein 55.0 mol% < x < 75.0 mol%,

- Mn in a content y, wherein 12.5 mol% < y < 22.5 mol%,

- Co in a content z, wherein 12.5 mol% < z < 22.5 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%; most preferably wherein M comprises:

- Ni in a content x, wherein 55.0 mol% < x < 70.0 mol%,

- Mn in a content y, wherein 15.0 mol% < y < 22.5 mol%,

- Co in a content z, wherein 15.0 mol% < z < 22.5 mol%,

- D in a content a, wherein 0.0 mol% < a < 2.0 mol%, wherein D is at least one element other than Li, Ni, Mn, Co, and O,

- wherein x + y + z + a is 100.0 mol%.

[0094] As is known to the skilled person, NMC cathode active material can comprise impurities, or be doped or coated resulting in an overall cathode active material comprising one or more elements other than Li, Ni, Mn, Co and O, which is reflected in the parameter "D" used herein. In preferred embodiments of the invention, D is an element selected from the group consisting of: Al, B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W Zr, and Zn; preferably Al, B, Cr, Nb, S, Si, Ti, Y, Zr and W; more preferably B, Nb, Ti, Zr and W.

Examples of suitable cathode active materials are NMC532, NMC622, NMC811, preferably NMC622 or NMC811, more preferably NMC622.

[0095] An optional but preferred additional component of the cathode comprised in the electrochemical cell of the invention is a conductivity aid, in particular a carbon-based conductivity aid. The carbon-based conductivity aid may be any carbon-rich material, such as any material comprising at least 95 wt.% carbon, preferably any material comprising at least 99 wt.% carbon. Examples of suitable materials are graphite, carbon black, carbon fibers, carbon nanotubes, graphene and combinations thereof. Carbon black is known to the skilled person and includes variants such as acetylene black or super C65.

[0096] In preferred embodiments, the carbon-based conductivity aid as described herein is present in the cathode comprised in the electrochemical cell of the present invention in an amount of at least 0.5 wt.% (by total weight of the cathode), preferably at least 1 wt.% (by total weight of the cathode), more preferably at least 3 wt.% (by total weight of the cathode). Typically, the carbonbased conductivity aid is present in an amount of less than 12 wt.% (by total weight of the cathode), preferably less than 9 wt.% (by total weight of the cathode), more preferably less than 7 wt.% (by total weight of the cathode).

[0097] In some embodiments of the invention, the cathode comprised in the electrochemical cell of the present invention further comprises a binder, such as a polymer binder. The binder is not particularly limiting and can be any suitable polymer binder, such as polyimide (PI), polyvinylidene chloride (PVdC), polyethylene oxide (PEO), polyvinylidene fluoride (PVdF) and the like. In preferred embodiments of the invention, the electrochemical cell comprises the polymer electrolyte of the invention placed in contact with the cathode active material.

[0098] For example, the electrochemical cell may comprise the polymer electrolyte of the invention placed between and in contact with the anode and the cathode.

[0099] For example, the electrochemical cell may comprise the polymer electrolyte of the invention in the form of a coating on the anode and/or the cathode.

[00100] For example, the electrochemical cell may comprise the polymer electrolyte of the invention and electrode active material in the form of a composite cathode as described herein earlier. In such embodiments, the electrochemical cell preferably comprises a further electrolyte placed between the composite cathode and the anode, which further electrolyte may be the polymer electrolyte of the present invention, or another electrolyte.

[00101] The polymer electrolyte of the present invention, which has a gel-like consistency, is considered a solid electrolyte for the purposes of the present disclosure. As will be understood by the skilled person, it may also function as a separator in an electrochemical cell.

[00102] Electrochemical cells as described herein are preferably lithium-ion- containing cells wherein the charge transport is effected by Li ⁺ ions. The electrochemical cell may have a disc-like or prismatic shape. The electrochemical cells can include a housing that can be made from steel or aluminium. A plurality of electrochemical cells may be combined into an all solid-state battery, which has both solid electrodes and solid electrolytes.

[00103] In particularly preferred embodiments, the cathode active material described herein is the only cathode active material comprised in the cathode of the electrochemical cell.

Method for the production of the electrochemical cell of the invention

[00104] In another aspect, the invention provides a method for the production of an electrochemical cell, comprising the following steps:

(a) providing a cathode,

(b) providing an anode,

(c) providing an electrolyte,

(d)forming the electrochemical cell by assembling the cathode, the anode, and the polymer electrolyte into an electrochemical cell, wherein the electrolyte comprises the polymer electrolyte of the invention, and/or wherein the cathode is provided in the form of a composite cathode comprising cathode active material and the polymer electrolyte of the invention.

[00105] The embodiments described in the present disclosure relating to the electrochemical cell or the polymer electrolyte, apply mutatis mutandis to the method for the preparation of the electrochemical cell. For example, the various embodiments relating to the identity and amounts of monomer, crosslinker, initiator, electrolyte composition as explained herein in the context of the polymer electrolyte are equally applicable to the method for the preparation of the electrochemical cell.

Uses of the polymer electrolyte of the invention [00106] In another aspect of the invention, there is provided the use of the polymer electrolyte described herein as an electrolyte for an electrochemical cell.

[00107] The embodiments described in the present disclosure relating to the polymer electrolyte, apply mutatis mutandis to the uses of the polymer electrolyte. For example, the various embodiments relating to the identity and amounts of monomer, crosslinker, initiator, electrolyte composition as explained herein in the context of the polymer electrolyte are equally applicable to the uses of the polymer electrolyte.

[00108] The electrochemical cell is preferably an electrochemical cell as described herein in the context of another aspect of the invention.

Batteries comprising the electrochemical cells of the invention and uses thereof [00109] Another aspect of the present invention concerns a battery, more specifically a lithium-ion battery or a lithium metal battery comprising at least one electrochemical cell comprising the gel-polymer electrolyte as described herein, for example, two or more electrochemical cells as described herein.

[00110] Electrochemical cells as described herein can be combined with one another, for example in series connection or in parallel connection. Series connection is preferred. The electrochemical cells or batteries described herein can be used for making or operating cars, computers, personal digital assistants, mobile telephones, watches, camcorders, digital cameras, thermometers, calculators, laptop BIOS, communication equipment, satellites or remote car locks, and stationary applications such as energy storage devices for power plants.

[00111] A further aspect of the present invention is a method of making or operating cars, computers, personal digital assistants, mobile telephones, watches, camcorders, digital cameras, thermometers, calculators, laptop BIOS, communication equipment, satellites, remote car locks, and stationary applications such as energy storage devices for power plants by employing at least one battery or the at least one electrochemical cell as described herein, comprising the polymer electrolyte of the invention.

[00112] A further aspect of the present invention is the use of the electrochemical cell or the battery as described herein, comprising the polymer electrolyte of the invention, in motor vehicles, bicycles operated by electric motor, robots, aircraft (for example unmanned aerial vehicles including drones), ships, satellites or stationary energy stores. [00113] A further aspect of the present invention is a method of providing power to an apparatus, wherein the power is supplied by the electrochemical cell or the battery as described herein, comprising the polymer electrolyte of the invention, wherein the electrochemical cell or the battery as described herein, preferably the electrochemical cell, is operated at a voltage above 4.4 V, preferably above 4.5 V, more preferably above 4.6 V, such as above 4.7 V. The apparatus may be any battery-powered apparatus, but is preferably selected from motor vehicles, computers, personal digital assistants, mobile telephones, watches, camcorders, digital cameras, thermometers, calculators, laptop BIOS, communication equipment, satellites, remote car locks, stationary applications such as energy storage devices for power plants, bicycles operated by electric motor, robots, aircraft (for example unmanned aerial vehicles including drones), ships, satellites, etc.

[00114] The present invention further provides a device comprising at least one battery or electrochemical cell as described herein, comprising the polymer electrolyte of the invention. Preferred are mobile devices such as vehicles, for example automobiles, bicycles, aircraft, satellites, or water vehicles such as boats or ships. Other examples of mobile devices are those which are portable, for example computers, especially laptops, telephones or electrical power tools, for example from the construction sector, especially drills, battery-driven screwdrivers or battery- driven tackers.

EXAMPLES

1. Preparation of material

[00115] A electrolyte composition consisting of deep eutectic solvent (DES) was prepared by mixing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and N- methylacetamide in a 1 :4 molar ratio with vigorous stirring until a homogeneous transparent liquid was obtained. A precursor composition was prepared by mixing the monomer indicated in table 1, the crosslinker indicated in table 1, and the free radical initiator indicated in table 1, into a premix having a (monomer:crosslinker): initiator molar ratio of (90: 10) :5, and combining the DES with the premix in a DES:premix volume ratio of 85: 15. The resulting mixture was stirred to obtain a homogeneous blend and polymerized by means of UV- illumination (365 nm) for 1 h.

[00116] Cathodes comprising NMC622 as cathode active material (LiNi0.6Mn0.2Co0.2O2) were formulated with 80 wt.% LiNi0.6Mn0.2Co0.2O2, 10 wt.% carbon black, and 10 wt.% poly(vinylidene fluoride) (PVDF) by mixing them in N- methyl-2-pyrrolidone (NMP). The well-mixed slurry was tape-casted on aluminium foil and dried in air at 110 °C to obtain electrodes with a thickness of approximately of ~18.7 pm and an active material loading of 0.622 mg cm ^-2 (0.109 mAh cm ^-2 , assuming a theoretical capacity of 175 mAh g ^-1 for the NMC622).

[00117] For use in coin cells, the polymer electrolyte and cathode were cut to appropriate size by means of a hollow punch. The Li | polymer electrolyte | NMC622 cells were assembled by placing the positive electrode in front of a Li foil negative electrode separated by the P-ETG in between.

Table 1

2. Determination of Electrochemical Performance

[00118] The ionic conductivity was determined by electrochemical impedance spectroscopy (EIS) (using a Biologic SP-300) of the polymer electrolyte in a symmetric stainless steel | electrolyte! stainless steel Swagelok-type cell at the specified temperature by perturbing the open-circuit potential with an AC sinusoidal potential of 10 mV amplitude over a frequency range of 10 kHz - 100 mHz.

[00119] The anodic stability limit was determined by linear sweep voltammetry (using a Bio-Logic, SP-300) of the polymer electrolyte which was sandwiched between a stainless-steel working electrode and a lithium metal reference and counter electrode in a coin cell setup wherein the voltage at the working electrode was swept from the open circuit potential to 6 V vs Li ⁺ /Li in the anodic scan, with a scan rate of 10 mV s’ ¹ . The stability limit was determined as the onset of electrolyte oxidation, which can be observed by a sudden increase in the measured current. Without wishing to be bound by any theory, the inventors believe the electrolyte oxidation may originate from the oxidation of the TFST anions.

[00120] The electrochemical compatibility of the polymer electrolyte with high- voltage cathode materials was studied by electrochemical impendance spectroscope (EIS) of NMC622 | polymer electrolyte | NMC622 symmetrical cells. [00121] The cycling performance was determined on the Li | polymer electrolyte | NMC622 cells prepared as explained above using a TOYO battery cycler. The cells underwent a 16 h open-circuit potential (OCP) period prior to galvanostatic cycling between 3.0 and 4.3 vs. Li ⁺ /Li. The electrodes were activated by two galvanostatic charge/discharge cycles at C/20 between 3.0 and 4.3 vs Li ⁺ /Li . Cycling protocol consisted of 5 cycles each at C-rates C/20, C/10, C/5, C/2 1C followed by 100 cycles at C/10. Capacity values are normalized with respect to the weight of cathode active material (NMC) and results were verified with reproducibility.

3. Results

[00122] The results of the electrochemical characterisation of the polymer electrolytes of the present invention are shown in figures 1-3 and Table 2. Table 2 illustrates the excellent ionic conductivity of the polymer electrolyte according to the invention at three different temperatures. Table 2 also highlights the compatibility of the polymer electrolytes of the invention towards NMC622 cathode active material, as can in particular be derived from figure 3. The cycling performance with the crosslinker of Example 1 shows improved performance in comparison to the crosslinker of the comparative example.

Table 2

[00123] As can be seen from Fig. 3, the precursor composition comprising a first crosslinker according to the invention results in improved cycling performance.