TECHNICAL FIELD AND BACKGROUND

This invention relates to a negative electrode for a battery comprising a metal substrate and a protective layer disposed directly on at least a part of the metal substrate, a method for preparing the negative electrode, a battery comprising the negative electrode and a use of the protective layer on the negative electrode.

As the development of small and lightweight electronic products, electronic devices, communication devices and the like has advanced rapidly and a need for electric vehicles has widely emerged with respect to environmental issues, there is a demand for improvement of performance of secondary batteries used as power sources for these products. Among these, a lithium metal secondary battery has come into the spotlight as a high-performance battery due to a high energy density and a high reference electrode potential.

Due to the high reactivity of the lithium metal, a side reaction between the lithium and an liquid electrolyte occurs during charge and discharge of the battery thereby promoting decomposition of the electrolyte. In addition, electron density non-uniformization may occur on the lithium metal surface during battery operation due to various reasons. As a result, a branch-shaped lithium dendrite is produced on the electrode surface making the electrode surface very rough and potentially causing separator damages. These side reactions deteriorate coulombic efficiency and/or electrochemical stability of the lithium metal battery and cause a decrease in the lifespan of the lithium battery.

Therefore, there are various attempts conducted to stabilize the lithium metal and prevent or reduce the side reaction between the electrolyte and the lithium metal.

KR.20030042288 A describes a lithium secondary battery capable of suppressing dendrite growth on the surface of a lithium metal negative electrode due to a cross-linked polymer protective film formed on the surface of the lithium metal, wherein the polymer is cross-linked 1,6-hexanediol diacrylate or a copolymer of 1,6- hexanediol diacrylate and oligo(ethylene glycol) diacrylate.

US2016/0372743 Al concerns a coating on a lithium metal anode to prevent direct contact of the lithium metal with the volatile liquid electrolyte. The coating consists of either a single layer polymer consisting of PVDF homopolymer, an ionic liquid and LiFSI or either a double layer polymer coating having a first outer layer of PVDF, an ionic liquid and LiFSI and a second inner layer of poly(styrene-acrylonitrile) and LiFSI.

However, a drawback of the known protective layers remains dendrite penetration. Dendrite penetration occurs on charge as lithium is plated at the anode, wherein dendrites are formed and can break through the polymer film and thereby tearing the polymer film. The reduced resistance due to the cut in the polymer film leads to a localized area of lower resistance and a greater degree of plating in the affected area, which exacerbates dendrite formation. Dendrite formation is largely unavoidable in these systems and due to small thickness of the polymer films (around 5-10 .m), dendrite formation of the polymer protective layer becomes very likely. As a result the coulombic efficiency, the electrochemical stability and/or the lifespan of the lithium metal battery is reduced.

Therefore, there is a need to achieve a protective layer around a negative electrode, which can be reformed around any holes or cuts made by dendrite formation.

It is an object of the present invention to provide a negative electrode for a battery comprising a metal substrate and a protective layer disposed directly on at least a part of the metal substrate.

It is a further object of the present invention to provide a method for preparing a negative electrode for a battery comprising a metal substrate and a protective layer disposed directly on at least a part of the metal substrate.

It is a further object of the present invention to provide a negative electrode obtainable by the method for preparing a negative electrode for a battery according to the invention.

It is a further object of the present invention to provide a battery comprising the negative electrode according to the invention, a positive electrode and an electrolyte composition.

It is a further object of the present invention to provide a use of a protective layer on a negative electrode as a self-healing film, to reduce dendritic growth on the negative electrode and/or improve the cycle efficiency of a battery comprising. SUMMARY OF THE INVENTION

An object of the present invention is achieved by providing a negative electrode for a battery comprising a. a metal substrate, and b. a protective layer disposed directly on at least a part of the metal substrate, wherein the protective layer comprises

• a copolymer obtainable by the reaction between two or more monomers,

• a fluoropolymer additive, and

• a lithium salt.

The present inventors have surprisingly found that the protecting layer comprising the copolymer, the fluoropolymer additive and the lithium salt functions as a self-healing polymer film as demonstrated in the appended examples. The protecting layer according to the invention is a mechanically robust polymer which also has self-healing properties and is the result of a careful balance between polymer chain mobility, the amount of hydrogen bonding motifs and the degree of crosslinking.

Without wishing to be bound by any theory, the inventors believe that the copolymer according to the invention comprises sufficient hydrogen bonds which can be broken and reformed at ambient temperatures resulting in the self-healing properties of the protective layer.

Although self-healing polymers are known in the art (see for example I. Gadwal, Macromol 2021, 1, 18-36) and a self-repairing polymer electrolyte based on dynamic chemical bonds and its application in secondary lithium batteries has been reported (CN111162314A), the present inventors are the first to demonstrate that the protective layer according to the invention displaying self-healing properties also results in an effective anode protection, wherein the dendritic growth of lithium is reduced and electrolyte depletion is minimized. Moreover, the protective layer according to the invention has an improved film conductivity and enhanced mechanical and chemical properties as demonstrated in the appended examples. Finally, the resulting protected negative electrode also significantly extends the cycle life of the cell versus an unprotected lithium metal-based reference cell, as shown in the examples below. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 : SEM (scanning electron microscope) images of the each of the three polymer formulations 1-a, 1-b and 1-c (see also Table 1).

Figure 2: The measured uncompensated resistance (R _u ) for each Li-Li symmetrical coin cells over a 20-hour period after cell construction for an unprotected lithium anode and a protected anode using polymer formulations 1-a, 1-b and 1-c; electrolyte = 4.5 M LiFSI in DME (see also Table 1).

Figure 3: The measured uncompensated resistance (Ru) for each Li-Li symmetrical coin cells over a 20-hour period after cell construction for an unprotected lithium anode and a protected anode using polymer formulations 2-a, 2-b, 2-c and 2-d (see Table 2); electrolyte = 4.5 M LiFSI in DME.

Figure 4: SEM micrographs of polymer films containing varying amounts of HEAA, each polymer film was cut with a scalpel, imaged and then allowed to soak in 4.5 M LiFSI in DME overnight (16 hours) before being rinsed with pure DME and imaged again.

Figure 5: Total specific discharge capacity of Li-NMC pouchcells containing protected anodes with varying PEG: HEAA ratios (cycling regime is C/3-D/1 with a potential hold at the top and bottom of charge).

Figure 6: Coulombic efficiency of Li-NMC pouch cells containing protected anodes with varying PEG: HEAA ratios (cycling regime is C/3-D/1 with a potential hold at the top and bottom of charge).

Figure 7: (a) Polymer film conductivity in (4.5 M LiFSI in DME):TTE (3: 1, v/v) without thermal annealing; (b) Polymer film conductivity in (4.5 M LiFSI in DME):TTE (3: 1, v/v) with thermal annealing.

Figure 8: Total specific discharge capacity of Li-NMC pouch cells containing protected anodes with varying PEG: HEAA ratios subjected to thermal annealing (cycling regime is C/3-D/1 with a potential hold at the top and bottom of charge).

Figure 9: Coulombic efficiency of Li-NMC pouch cells containing protected anodes with varying PEG: HEAA ratios subjected to thermal annealing (cycling regime is C/3-D/1 with a potential hold at the top and bottom of charge).

DETAILED DESCRIPTION

In the drawings and the following detailed description, preferred embodiments are described in detail to enable 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. In contrast, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description and accompanying drawings.

The term "comprising", as used herein and in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a composition comprising components A and B" should not be limited to compositions consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the composition are A and B. Accordingly, the terms "comprising" and "including" encompass the more restrictive terms "consisting essentially of" and "consisting of".

As used herein, the terms "optional" or "optionally" means that a subsequently described event or circumstance can or cannot occur, and that the description includes instances where said event or circumstance occurs and instances where it does not occur.

As used herein the term "alkyl" has the broadest meaning generally understood in the art, and may include a moiety which is linear or branched, or a combination thereof. In particular, the term "alkyl" - alone or in combination - means a straight or branched alkane-derived radical, for example, CF-G alkyl defines a straight or branched alkyl radical having from F to G carbon atoms, e.g. Ci-4 alkyl defines a straight or branched alkyl radical having from 1 to 4 carbon atoms such as for example methyl, ethyl, 1-propyl, 2-propyl (isopropyl), 1 5 -butyl, 2-butyl, 2- methyl-2-propyl (tert-butyl), 2-methyl-l-propyl (isobutyl).

The term "alkanediyl" - alone or in combination - means a straight or branched alkyl derived divalent radical.

As used herein the term "uncompensated resistance (R _u )" is defined as the resistance value which includes the cell resistance and any minor additional resistance. Major contributions to the cell resistance are e.g. derived from ionic conductivity of liquid electrolyte and/or ionic conductivity of the polymer protection layer. Minor resistance contributions are e.g. metallic parts such as cables/contacts etc. Negative electrode

As mentioned above a first aspect of the present invention is to provide a negative electrode (/.e. an anode) for a battery comprising a. a metal substrate, and b. a protective layer disposed directly on at least a part of the metal substrate, wherein the protective layer comprises

• a copolymer obtainable by the reaction between: o a first monomer represented by formula (I),

(I), wherein R ¹ = H or CH3; R ² is a C2-6 alkanediyl, wherein the alkanediyl is optionally substituted with one or more substituents selected from halide, Ci-4 alkyl, CF3 or OR ³ , wherein R ³ is selected from hydrogen or C1-4 alkyl; X ¹ = NH or 0; and n is an integer selected from 5-150, preferably 8-100, more preferably 10-50; and o a second monomer represented by formula (II),

(II), wherein R ⁴ = H or CH3; R ⁵ is a C2-6 alkanediyl, wherein the alkanediyl is optionally substituted with one or more substituents selected from halide, Ci-4 alkyl, CF3 or OR ⁶ , wherein R ⁶ is selected from hydrogen or C1-4 alkyl; X ² = NH or 0; X ³ = SH, NH2 or OH and m is an integer selected from 1- 10, preferably 1-5, more preferably m = 1; and wherein the weight ratio of the first monomer to the second monomer is between 1 :5 and 8: 1 (w/w);

• a fluoropolymer additive selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), a tetrafluoroethylene-perfluoro alkylvinyl ether copolymer, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), a vinylidene fluoride-chlorotrifluoroethylene copolymer, an ethylenetetrafluoroethylene copolymer, a polychlorotrifluoroethylene, vinylidene fluoride-pentafluoropropylene copolymer, a propylenetetrafluoroethylene copolymer, an ethylenechlorotrifluoroethylene copolymer, a vinylidene fluoride- hexafluoropropylene-tetrafluoroethylene copolymer, a vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer and a combination thereof; and

• a lithium salt; wherein the weight ratio of the copolymer to the fluoropolymer additive is between 1 :5 and 8: 1 (w/w).

Metal substrate

An embodiment of the invention is the negative electrode according to the invention comprising a metal substrate. In a preferred embodiment the metal substrate comprises a conductive metal, preferably stainless steel, copper, nickel, iron, cobalt, lithium, lithium alloy or a combination thereof, more preferably stainless steel, copper, nickel, lithium, lithium alloy or a combination thereof. In a more preferred embodiment the metal substrate is a lithium metal substrate or a lithium metal alloy substrate, preferably a lithium metal substrate. As appreciated by the skilled person the metal substrate may be a metal deposited onto a supporting foil such as stainless steel, copper, nickel, iron, cobalt or a lithium or lithium alloy layer made as a foil or as a layer deposited upon a surface.

Copolymer An embodiment of the invention is the copolymer obtainable by the reaction between :

• a first monomer represented by formula (I)

(I), wherein R ¹ = H or CH3; R ² is a C2-6 alkanediyl, wherein the alkanediyl is optionally substituted with one or more substituents selected from halide, Ci-4 alkyl, CF3 or OR ³ , wherein R ³ is selected from hydrogen or C1-4 alkyl; X ¹ = NH or 0; and n is an integer selected from 5-150, preferably 8-100, more preferably 10-50; and

• a second monomer represented by formula (II)

(II), wherein R ⁴ = H or CH3; R ⁵ is a C2-6 alkanediyl, wherein the alkanediyl is optionally substituted with one or more substituents selected from halide, Ci-4 alkyl, CF3 or OR ⁶ , wherein R ⁶ is selected from hydrogen or C1-4 alkyl; X ² = NH or O; X ³ = SH, NH2 or OH and m is an integer selected from 1-10, preferably 1-5, more preferably m = 1.

An embodiment of the invention is the first monomer represented by formula (I) wherein R ¹ is H or CH3, preferably H; R ² is a C2-6 alkanediyl, preferably a C2-4 alkanediyl, more preferably a C2 alkanediyl, wherein the alkanediyl is optionally substituted with one or more substituents selected from halide, Ci-4 alkyl, CF3 or OR ³ , wherein R ³ is selected from hydrogen or C1-4 alkyl; X ¹ = NH or 0, preferably 0; and n is an integer selected from 5-150, preferably 8-100, more preferably 10-50. A preferred embodiment of the invention is the first monomer according the invention, wherein the first monomer is represented by formula (III)

(HI), wherein o is an integer selected from 5-150, preferably 8-130, more preferably 10- 100. In particular, the first monomer represented by formula (III) is poly(ethylene glycol) diacrylate (PEGDA).

A preferred embodiment of the invention is the first monomer according to the invention, wherein the first monomer has a number average molecular weight M _n of more than 50 g/mol, preferably more than 100 g/mol, more preferably more than 200 g/mol, even more preferably more than 550 g/mol, most preferably more than 700 g/mol. A preferred embodiment of the invention is the first monomer according to the invention, wherein the first monomer has a number average molecular weight M _n of less than 20,000 g/mol, preferably less than 7,000 g/mol, more preferably less than 4,000 g/mol, even more preferably less than 2,000 g/mol, most preferably less than 1,000 g/mol. A preferred embodiment of the invention is the first monomer according to the invention, wherein the first monomer has a number average molecular weight M _n between 50 and 20,000 g/mol, preferably between 100 and 7,000 g/mol, more preferably between 200 and 4,000 g/mol, even more preferably between 550 and 2,000 g/mol, most preferably between 700 and 1,000 g/mol. Particular examples are PEGDA having an number average molecular weight M _n of 250 g/mol, 575 g/mol, 700 g/mol, 4,000 g/mol or 6,000 g/mol. Preferred examples of the first monomer according to the invention are PEGDA having a number average molecular weight M _n of 250 g/mol, 575 g/mol or 700 g/mol; preferably 700 g/mol. PEGDA having various number average molecular weights as described above can be commercially bought from suppliers, such as Sigma-Aldrich. As demonstrated in the examples below the first monomer according to the invention, in particular PEGDA, improves the lithium ion conductivity of the copolymer according to the invention and/or the protective layer according to the invention.

An embodiment of the invention is the second monomer represented by formula (II), wherein R ⁴ = H or CH3, preferably H; R ⁵ is a C2-6 alkanediyl, preferably a C2-4 alkanediyl, more preferably a C2 alkanediyl, wherein the alkanediyl is optionally substituted with one or more substituents selected from halide, Ci-4 alkyl, CF3 or OR ⁶ , wherein R ⁶ is selected from hydrogen or C1-4 alkyl; X ² = NH or 0, preferably NH; X ³ = SH, NH2 or OH, preferably OH and m is an integer selected from 1-10, preferably 1-5, most preferably m = 1.

A preferred embodiment of the invention is the second monomer according to the invention, wherein the second monomer is represented by formula (IV)

(IV), in particular the second monomer represented by formula (IV) is /V-(2- hydroxyethyl)acrylamide (HEAA). HEAA can be commercially bought from suppliers, such as Sigma-Aldrich and TCI Chemicals.

As demonstrated in the examples below introduction of the second monomer according to the invention into the copolymer, in particular /V-(2- hydroxyethyl)acrylamide (HEAA), attributes to the self-healing properties of the copolymer. The first monomer according to the invention comprises a hydrogen bond acceptor, e.g. the carbonyl functionality of the first monomer represented by formula (I), preferably the ester functionality of the first monomer represented by formula (III). The second monomer according to the invention comprises a hydrogen bond acceptor, e.g. the carbonyl functionality of the second monomer represented by formula (II), preferably the amide functionality of the second monomer represented by formula (IV). The second monomer according to the invention comprises a hydrogen bond donor, e.g. the hydrogens present in substituent X ³ of the second monomer represented by formula (II), e.g. the thiol, amino or hydroxyl functional group, preferably the hydrogen of the hydroxyl group of the second monomer represented by formula (IV). The present inventors believe that the hydrogen bond donor present in the second monomer can form a hydrogen bond with the hydrogen bond acceptor present in a further second monomer according to the invention and/or the hydrogen bond acceptor present in the first monomer according to the invention. The present inventors believe that the hydrogen bonding properties of the copolymer result in the self-healing properties of the copolymer and/or protective layer. The hydrogen bonds present in the copolymer according to the invention have a relatively weak bond strength (1.9-6.9 kcal. mol' ¹ ) meaning that they can be broken and reformed at ambient temperatures.

As appreciated by the skilled person the copolymer according to the invention is obtainable through a crosslinking step (Z.e. radical polymerization) of the corresponding vinyl monomers, such as the first monomer according to the invention and the second monomer according to the invention. In embodiments of the invention the initiator that may be used for the radical polymerization varies depending on the crosslinking reaction, and known photo initiators or thermal initiators may all be used, preferably a photo initiator is used. Examples of the photo initiator may include benzoin, benzoin ethyl ether, benzoin isobutyl ether, alphamethyl benzoin ethyl ether, benzoin phenyl ether acetophenone, dimethoxyphenyl acetophenone, 2,2- diethoxyacetophenone, 1,1-dichloroacetophenone, trichloroacetophenone, benzophenone, p-chlorobenzophenone, 2,4-dihydroxybenzophenone, 2-hydroxy-4- methoxybenzophenone, 2-hydroxy-2-methylpropiophenone, benzoyl benzoate, anthraquinone, 2-ethylanthraquinone, 2-chloroanthraquinone, 2-methyl-l-(4- methylthiophenyl)-morpholinopropanone-l,2-hydroxy-2-methyl-l -phenylpropan-l- one (Darocure 1173), 2-methyl-4'-(methylthio)-2-morpholinopropiophenone (Irgacure 907), phenyl bis (2,4,6-trimethylbenzoyl)-phosphine oxide (Irgacure 819), 2-benzyl-2-dimethylamino-l-(4-morpholinophenyl)-butanone-l,l - hydroxycyclohexylphenyl ketone (Irgacure 184), Michler's ketone, benzyl dimethyl ketal, thioxanthone, isopropyl thioxanthone, chlorothioxanthone, benzyl disulfide, butanediol, carbazole, fluorenone, alphaacyloxime ester and a combination thereof, and examples of the thermal initiator may include peroxide (-O-O-) series, benzoyl peroxide, cumyl hydroperoxide and the like, and azo-based compound (-N = N-) series azobisisobutyronitrile, azobisovaleronitrile and the like may be used. In a preferred embodiment of the invention the photo initiator is phenyl bis-(2,4,6- trimethylbenzoyl)-phosphine oxide (Irgacure 819) or 2-methyl-4'-(methylthio)-2- morpholinopropiophenone (Irgacure 907), preferably Irgacure 819.

The content of the initiator is not particularly limited in the present invention, and is preferably in a range that does not affect properties of the copolymer according to the invention, of an electrode, such as a negative electrode or a positive electrode, and of a liquid electrolyte present in a battery. In an embodiment of the invention the initiator is used a range of 1 - 15 wt.% by total weight of the copolymer, preferably in a range of 1 -5 wt.%, more preferably in a range of 2 - 4 wt.% by total weight of the copolymer.

In embodiments of the invention the crosslinking step affording the copolymer according to the invention is performed in an organic liquid capable of the dissolving the first monomer according to the invention, the second monomer according to the invention and/or the copolymer according to the invention. Preferably the organic liquid is a non-aqueous organic liquid. In the present invention, known liquids such as a carbonate-based, ester-based, ether-based, ketone-based, alcohol-based or an aprotic liquid may be used as the non-aqueous organic liquid. For example, the aprotic organic liquid is selected from the group consisting of /V-methyl-2- pyrrolidmone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1-butyrolactone, 1,2- dimethoxy ethane, 1,2-diethoxy ethane, tetra hydrofuran, 2-methyl tetra hydrofuran, dimethylsulfoxide, 1,3-dioxolane, 4-methyl-l,3-dioxene, diethylether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, a dioxolane derivative, sulfolane, methyl sulfolane, l,3-dimethyl-2-imidazolidinone, a propylene carbonate derivative, a tetrahydrofuran derivative, ether, methyl propionate, ethyl propionate, and a combination thereof. Preferred examples of the organic liquids are acetonitrile or tetrahydrofuran.

In embodiments of the invention the crosslinking step comprises applying heat or irradiating active energy rays, wherein the crosslinking by heat may use a method for heating, and the active energy rays may be through irradiating far-infrared rays, ultraviolet rays or an electron beam.

In embodiments of the invention the crosslinking step occurs through thermal crosslinking or photo crosslinking, preferably photo crosslinking. In embodiments of the invention, the thermal crosslinking may be carried out at a temperature of 50 °C to 200 °C, preferably at a temperature of 80 °C to 110 °C and the heating time for the crosslinking is from 30 minutes to 48 hours, more preferably from 8 hours to 24 hours. When the heating temperature and time are less than the above-mentioned ranges, crosslinking is difficult to be sufficiently formed and when the heating temperature and time are greater than the above-mentioned ranges, side reactions may occur, or material stability may decrease. In preferred embodiments of the invention, the photo crosslinking including active energy ray irradiation is carried out for 10 seconds to 5 hours, preferably for 1 minute to 1 hour, most preferably for 2 minutes to 10 minutes. In preferred embodiments of the invention, the photo crosslinking occurs at temperature between 5 °C to 60 °C, preferably between 10 °C to 50 °C, most preferably between 20 °C to 40 °C. When the time of active ray irradiation is less than the above-mentioned range, crosslinking is difficult to be sufficiently formed, and when time is greater than the above-mentioned range, side reaction may occur, or material stability may decrease.

As appreciated by the skilled person the copolymer is obtainable by reaction of at least two monomers being the first monomer represented by formula (I) and the second monomer represented by formula (II), but is not limited to be obtainable by reaction between only two different monomers, but may also be obtainable by the reaction between e.g. three monomers, in particular the first monomer according to the invention, the second monomer according to the invention and a third monomer represented by formula (V) or formula (VI), wherein the third monomer is different from the first monomer and the second monomer:

(V), wherein R ⁷ is H or CH3; R ⁸ is a C2-6 alkanediyl, wherein the alkanediyl is optionally substituted with one or more substituents selected from halide, Ci-4 alkyl, CF3 or OR ⁹ , wherein R ⁹ is selected from hydrogen or C1-4 alkyl; X ⁴ = NH or 0; and p is an integer selected from 5-150, preferably 8-100, more preferably 10-50; or

(VI), wherein R ¹⁰ is H or CH3; R ¹¹ is a C2-6 alkanediyl, wherein the alkanediyl is optionally substituted with one or more substituents selected from halide, Ci-4 alkyl, CF3 or OR ¹² , wherein R ¹² is selected from hydrogen or C1-4 alkyl; X ⁵ = NH or 0; X ⁶ = SH, NH2 or OH and q is an integer selected from 1-10, preferably 1-5, more preferably q = 1.

A preferred embodiment of the invention is the copolymer obtainable by the reaction between at most two monomers, wherein the two monomers are the first monomer represented by formula (I) and the second monomer represented by formula (II).

A highly preferred embodiment of the invention is the copolymer obtainable by the reaction between at most two monomers, wherein the two monomers are the first monomer being poly(ethylene glycol) diacrylate and the second monomer being 2-hydrxoyethylacrylamide.

In a preferred embodiment, the weight ratio of the first monomer to the second monomer is between 1 :5 and 8: 1 (w/w), preferably between 1 :3 and 7: 1 (w/w), more preferably between 1 :2 and 5: 1 (w/w), most preferably between 1 :2 and 2: 1 (w/w).

In a preferred embodiment, the copolymer is present in an amount more than 20 wt.% by total weight of the protective layer, preferably more than 30 wt.%, most preferably more than 35 wt.% by total weight of the protective layer. In a preferred embodiment, the copolymer is present in an amount less than 80 wt.% by total weight of the protective layer, preferably less than 70 wt.%, most preferably less than 60 wt.% by total weight of the protective layer. In a preferred embodiment, the copolymer is present in an amount in the range of 20-80 wt.% based on the total amount of protective layer, preferably in the range 30-70 wt.%, more preferably in the range of 35-60 wt.% by total weight of the protective layer.

A particular example of the copolymer according to the invention is represented by formula (VII)

(VII), wherein o is an integer selected from 5-150, preferably 8-100, more preferably 10- 50 and r, s and t are an integer independently selected from 1-10, preferably from 2-8, most preferably from 3-6. However, and as appreciated by the skilled person, the copolymer according to the invention is not limited to formula (VII) and is shown as an exemplary embodiment.

In a highly preferred embodiment of the invention the copolymer according to the invention, and therefore the protective layer according to the invention, has self- healing properties, as demonstrated in the appended examples.

In a preferred embodiment of the invention, the copolymer and/or the protective layer according to the invention is subjected to a thermal annealing step. As appreciated by the skilled person thermal annealing is a common process used in materials chemistry to relieve stress, improve homogeneity and enhance contact with a substrate. In particular, thermal annealing is a process at which the temperature T of the part coated by a coating is at first increased from room temperature to a maximal (operation) temperature Tmax, preferably at a temperature T above recrystallization temperature of the copolymer according to the invention and/or the protective layer according to the invention, and later, after finishing the operation (e.g. cutting), the temperature is decreased back to room temperature. In a highly preferred embodiment the thermal annealing step occurs after the UV-curing of the copolymer at temperature in the range between 40 and 100 °C, preferably in the range between 50 and 90 °C, more preferably in the range between 60 and 80 °C and/or the duration of the thermal annealing is between 5 minutes and 10 hours, preferably between 15 minutes and 5 hours, most preferably between 30 minutes and 90 minutes. In a highly preferred embodiment the thermal annealing step according to the invention occurs in dry air. The present inventors have now surprisingly found that thermal annealing step improves the conductivity of the copolymer. In addition, each copolymer subjected to the thermal annealing step appears more homogenous and has a better contact with the lithium metal surface. Moreover, the inventors have found that after the thermal annealing step the charge transfer resistance (Ret) of the negative electrode according to the invention is reduced.

Fluoropolymer additive

As described above the protective layer according to the invention comprises a fluoropolymer additive, wherein the fluoropolymer additive is selected from the group consisting of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), a tetrafluoroethylene-perfluoro alkylvinyl ether copolymer, poly(vinylidene fluoride-co- hexafluoropropylene) (PVDF-HFP), a vinylidene fluoride-chlorotrifluoroethylene copolymer, an ethylene-tetrafluoroethylene copolymer, a polychlorotrifluoroethylene, vinylidene fluoride-pentafluoropropylene copolymer, a propylenetetrafluoroethylene copolymer, an ethylenechlorotrifluoroethylene copolymer, a vinylidene fluoride- hexafluoropropylene-tetrafluoroethylene copolymer, a vinylidene fluoride- perfluoromethylvinyl ether-tetrafluoroethylene copolymer and a combination thereof, preferably the fluoropolymer additive is polyvinylidene fluoride or poly(vinylidene fluoride-co-hexafluoropropylene), most preferably poly(vinylidene fluoride-co- hexafluoropropylene).

In a preferred embodiment the fluoropolymer additive is present in an amount less than 75 wt.% by total weight of the protective layer, preferably less than 50 wt.%, most preferably less than 20 wt.% by the total weight of the protective layer. In a preferred embodiment the fluoropolymer additive is present in an amount more than 0.05 wt.% by the total weight of the protective layer, preferably more than 5 wt.%, most preferably more than 10 wt.% by total weight of the protective layer. In a preferred embodiment the fluoropolymer additive is present in an amount between 0.05 and 75 wt.% based by total weight of the protective layer, preferably between 5 and 50 wt.%, more preferably between 10 and 20 wt.% by total weight of the protective layer.

In a preferred embodiment of the invention, the weight ratio of the copolymer to the fluoropolymer additive is between 1 :5 and 8: 1 (w/w), preferably between 1 :2 and 5: 1 (w/w), more preferably between 1 :2 and 4: 1 (w/w), most preferably between 2: 1 and 4: 1 (w/w).

In an embodiment of the invention the fluoropolymer additive has a weight average molecular weight M _w of more than 1,000 g/mol, preferably more than 10,000 g/mol, more preferably more than 100,000 g/mol, most preferably more than 300,000 g/mol. In an embodiment of the invention the fluoropolymer additive has a weight average molecular weight M _w of less than 1,000,000 g/mol, preferably less than 800,000 g/mol, more preferably less than 750,000 g/mol, most preferably less than 600,000 g/mol. In an embodiment of the invention the fluoropolymer additive has a weight average molecular weight M _w between 1,000 and 1,000,000 g/mol, preferably between 10,000 and 800,000 g/mol, more preferably between 100,000 and 750,000 g/mol, most preferably between 300,000 and 600,000 g/mol. Preferred examples of the fluoropolymer additive, in particular poly(vinylidene fluoride-co- hexafluoropropylene), have a weight average molecular weight M _w of around 400,000 g/mol or a weight average molecular weight M _w of around 455,000 g/mol. Fluoropolymer additives, such as poly(vinylidene fluoride-co-hexafluoropropylene), can be commercially bought from suppliers, such as Sigma-Aldrich.

In an embodiment of the invention the fluoropolymer additive has a number average molecular weight M _n of more than 250 g/mol, preferably more than 2,500 g/mol, more preferably more than 25,000 g/mol, most preferably more than 75,000 g/mol. In an embodiment of the invention the fluoropolymer additive has a number average molecular weight M _n of less than 250,000 g/mol, preferably less than 200,000 g/mol, more preferably less than 190,000 g/mol, most preferably less than 150,000 g/mol. In an embodiment of the invention the fluoropolymer additive has a number average molecular weight M _n between 250 and 250,000 g/mol, preferably between 2,500 and 400,000 g/mol, more preferably between 25,000 and 190,000 g/mol, most preferably between 75,000 and 150,000 g/mol. Preferred examples of the fluoropolymer additive, in particular poly(vinylidene fluoride-co- hexafluoropropylene), have a number average molecular weight M _n of around 130,000 g/mol or a number average molecular weight M _n of around 110,000 g/mol. Fluoropolymer additives, such as poly(vinylidene fluoride-co-hexafluoropropylene), can be commercially bought from suppliers, such as Sigma-Aldrich.

The present inventors have found that the addition of the fluoropolymer additive improves the conductivity of the protective layer, improves the wetting of the protective layer and allows for printing of homogenous films of the protective layer, as demonstrated in the appended examples.

Lithium salt

As described above the protective layer according to the invention comprises a lithium salt.

In an embodiment of the invention the lithium salt is selected from the group consisting of U2CO3, U2O, U2C2C , LiOH, LiX ⁷ , R.OCO2U, HCOLi, R. ¹³ OLi, U2O, U2C2C , (R. ¹³ OCO ₂ Li) ₂ , (CH ₂ OCO2Li)2, Li ₂ S, LiSCN, LiN(CN) ₂ , LiCIC , LiBF ₄ , LiAsFe, LiPFe, UCF3SO3, LiC(CF ₃ SO ₂ )3, UN(SO2C2F ₅ )2, LiN(SO ₂ CF ₃ )2, LiN(SO ₂ F) ₂ , LiSbFe, LiPF3(CF2CF3)3, LiPF3(CF3)3, LiB(C2O ₄ )2 and a combination thereof, wherein R. ¹³ = a hydrocarbon and X ⁷ = F, Cl, I or Br; preferably selected from the group consisting of LiSCN, LiN(CN) ₂ , LiCIO ₄ , Lil, LiBF ₄ , LiAsFe, UCF3SO3, LiCF ₃ (SO ₃ )2, LiN(SO2C2F ₅ )2, LiN(SO ₂ CF ₃ )2, LiN(SO ₂ F) ₂ , LiSbFe, LiPF3(CF ₂ CF3)3, LiPF ₃ (CF ₃ )3, LiB(C ₂ O ₄ ) ₂ and a combination thereof; more preferably the lithium salt is LiN(SO2CF3)2 or LiN(SO2F)2; most preferably the lithium salt is LiN(SO2F)2.

In a preferred embodiment of the invention the amount of lithium salt is more than 5 wt.% by total weight of the protective layer, preferably more than 10 wt.% by total weight of the protective layer, most preferably more than 40 wt.% by total weight of the protective layer. In a preferred embodiment of the invention the amount of lithium salt is less than 70 wt.% by total weight of the protective layer, preferably less than 65 wt.%, most preferably less than 60 wt.% by total weight of the protective layer. In a preferred embodiment of the invention the amount of the lithium salt is between 5 and 70 wt.% by total weight of the protective layer, preferably between 10 and 65 wt.%, more preferably between 40 and 60 wt.% by total weight of the protective layer.

The present inventors have found that the addition of the lithium salt improves the lithium ion conductivity of the protective layer and prevents the protective layer from depleting the lithium ions present in the electrolyte. Moreover, the addition of higher lithium salt contents to the protective layer results in a reduction of the uncompensated resistance Ru.

Protective layer

In an embodiment of the invention the protective layer has a thickness between 1 and 10 .m, preferably between 2 and 7 .m, more preferably between 3 and 6 .m.

In an embodiment of the invention the protective layer is disposed directly on at least a part of the metal substrate, preferable the protective layer is disposed directly on the entire surface of the metal substrate. As appreciated by the skilled person the negative electrode according to the invention comprises the protective layer according to the invention, wherein the protective layer forms an outer layer on the metal substrate. Worded differently, the protective layer is coated on a part of the surface of the metal substrate, i.e. the protective layer forms a coating on a part of the surface of the metal substrate, preferably on the entire surface of the metal substrate.

In a preferred embodiment of the invention the protective layer according to the invention has an ion-conductivity of more than 0.01 mS/cm, preferably more than 0.015 mS/cm, more preferably more than 0.05 mS/cm. In a preferred embodiment of the invention the protective layer has an ion-conductivity of less than 0.5 mS/cm, preferably less than 0.2 mS/cm, more preferably less than 0.1 mS/cm. In a preferred embodiment of the invention the protective layer according to the invention has ionconductivity in the range of 0.01 - 0.5 mS/cm, preferably in the range of 0.015 -0.2 mS/cm, most preferably in the range of 0.05 - 0.1. mS/cm. The ion-conductivity of the protective layer is measured through quantification of the uncompensated resistance Ru via electronic impedance spectroscopy (EIS) in symmetrical Li-Li cells, wherein the ion-conductivity is measured after cell construction e.g. meaning in a battery comprising the negative electrode according to the invention and an electrolyte composition.

A further aspect of the invention is a method for preparing the protective layer according to the invention comprising the following steps: i) adding the first monomer as defined above, the second monomer as defined above, the initiator as defined above, the lithium salt as defined above and the fluoropolymer additive as defined above to the organic liquid as defined above, ii) subjecting the mixture obtained in step i) to the crosslinking step as defined above, and iii) optionally subjecting the crosslinked mixture of step ii) to the thermal annealing step as defined above.

A further aspect of the invention is the protective layer obtainable by the method for preparing the protective layer in accordance with the invention.

Varia

A further aspect of the invention is the negative electrode according to the invention comprising: a. a metal substrate as defined above, and b. the protective layer obtainable by the method for preparing the protective layer according to the invention the invention disposed directly on at least a part of the metal substrate.

A further aspect of the invention is a method for preparing a negative electrode for a battery comprising the following steps: i) adding the first monomer as defined above, the second monomer as defined above, the initiator as defined above, the lithium salt as defined above and the fluoropolymer additive as defined above to the organic liquid as defined above; ii) coating the mixture obtained in step i) on at least a part of the metal substrate, preferably on the entire surface of the metal substrate; iii) subjecting the coated metal substrate of step ii) to the crosslinking step as defined above, and iv) optionally subjecting the crosslinked coated metal substrate of step iii) to the thermal annealing step as defined above.

In an embodiment of the invention the amount of solids (Z.e. the sum of the first monomer, the second monomer, the initiator, the lithium salt and the fluoropolymer) present in the mixture obtained in step i) as defined above is more than 1 wt.% by total weight of the solids and the organic liquid, preferably more than 2 wt.%, most preferably more than 3 wt.% by total weight of solids and the organic liquid. In an embodiment of the invention the amount of solids (J.e. the sum of the first monomer, the second monomer, the initiator, the lithium salt and the fluoropolymer) is less than 10 wt.% by total weight of the solids and the organic liquid, preferably less than 8 wt.%, most preferably less than 5 wt.% by total weight of solids and the organic liquid. In an embodiment of the invention the amount of solids (Z.e. the sum of the first monomer, the second monomer, the initiator, the lithium salt and the fluoropolymer) is between 1 and 10 wt.% by total weight of the solids and the organic liquid, preferably between 2 and 8 wt.%, most preferably between 3 and 5 wt.% by total weight of solids and the organic liquid.

The coating step may be performed using methods used in conventional wet processes as understood by the skilled person, such as spin coating, spray coating, doctor blade coating, dip coating, and the like.

Optionally, a drying step is performed after the coating step and/or after the thermal annealing step. The drying step may be properly performed at a temperature greater than or equal to a boiling point of the organic liquid and a temperature less than or equal to a glass transition temperature T _g of the copolymer according to the invention and/or the protective layer according to the invention. The drying step may also be performed to remove the liquid remaining on a surface of the metal substrate and simultaneously improve adhesion of the copolymer according to the invention and/or the protective layer according to the invention.

A further aspect of the invention is the negative electrode obtainable by the method for preparing the negative electrode for a battery according to the invention.

Battery

A further aspect of the invention is a battery comprising a. the negative electrode according to the invention or the negative electrode obtainable by the method for preparing the negative electrode for a battery according to the invention, b. a positive electrode, c. an electrolyte composition, preferably the electrolyte composition is disposed between the positive electrode and the negative electrode, and d. optionally a separator.

In a preferred embodiment of the invention the battery is a lithium metal battery, preferably a lithium metal secondary battery. Positive electrode

The material of the positive electrode (/.e. cathode) is not particularly limited, and examples thereof include a transition metal compound having a structure capable of diffusing lithium ions, or a specialized metal compound thereof and an oxide of lithium. In particular, LiCoOz, LiNiOz, LiMnC , LiFePC , etc. can be mentioned. Preferred positive electrode materials are mixed metal oxides comprising lithium, nickel and optionally manganese, cobalt and/or aluminum.

In a preferred embodiment of the invention the positive electrode comprises a positive electrode material selected from the group consisting of lithium nickel- manganese-cobalt oxide, lithium nickel-manganese oxide, lithium nickel-cobalt- aluminum oxide, lithium cobalt oxide, lithium iron phosphate, lithium iron manganese phosphate, lithium iron cobalt phosphate, lithium sulphide, sulphur and aluminum; preferably the positive electrode material is lithium nickel-manganese-cobalt oxide or lithium nickel-cobalt-aluminum oxide.

The positive electrode can be formed by press-molding the positive electrode material as defined above together with a known conductive auxiliary agent or binder, or the positive electrode active material together with a known conductive auxiliary agent or binder into an organic liquid such as pyrrolidone. It can be obtained by applying a mixture and pasting it to a current collector such as an aluminum foil, followed by drying.

Electrolyte composition

In an embodiment of the invention the electrolyte composition is a liquid electrolyte composition.

In an embodiment of the invention the electrolyte composition comprises a further lithium salt and a further organic liquid, preferably a further non-aqueous organic liquid, more preferably a further aprotic non-aqueous organic liquid.

In a preferred embodiment of the invention the further lithium salt is selected from the group consisting of U2CO3, U2O, U2C2O4, LiOH, LiX ⁸ , ROCO2U, HCOLi, R ¹⁴ OLi, U2O, U2C2O4, (R.OCO ₂ Li) ₂ , (CH ₂ OCO2Li)2, Li ₂ S, LiSCN, LiN(CN) ₂ , LiCIC , LiBF ₄ , LiAsFe, LiPFe, UCF3SO3, LiC(CF ₃ SO ₂ )3, LiN(SO2C ₂ F ₅ )2, LiN(SO ₂ CF ₃ )2, LiN LiPF3(CF2CF3)3, LiPF3(CF3)3, LiB(C2O4)2 wherein R ¹⁴ = a hydrocarbon or Br; preferably LiSCN, LiN(CN) ₂ , LiCIC , Lil, LiBF ₄ , LiAsFe, UCF3SO3, LiCF ₃ (SO3)2, LiN(SO2C2F ₅ )2, LiN(SO ₂ CF ₃ )2, LiN(SO ₂ F) ₂ , LiSbFe, LiPF3(CF ₂ CF3)3, LiPF ₃ (CF ₃ )3, LiB(C2O4)2 and a combination thereof; more preferably the lithium salt is LiN(SO2CF3)2 or LiN(SO2F)2; most preferably the lithium salt is LiN(SO2F)2.

In a highly preferred embodiment the further lithium salt and the lithium salt as described above are the same lithium compound.

As appreciated by the skilled person the lithium salt according the invention is present in the protective layer according to the invention, whereas the further lithium salt according to the invention is present in the electrolyte composition according to the invention.

In a preferred embodiment of the invention the further organic liquid comprises a non-fluorinated liquid selected from the group consisting of /V-methyl-2- pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, /-butyrolactone, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, tetra hydrofuran, 2-methyl tetra hydrofuran, dimethylsulfoxide, 1.3-dioxolane, 4-methyl-l,3-dioxene, diethyl ether, formamide, dimethyl formamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, tirmethoxymethane, a dioxolane derivative, sulfolane, methyl sulfolane, l,3-dimethyl-2-imidazolidone, a propylene carbonate derivative, a tetra hydrofuran derivative, ether, methyl propionate, ethyl propionate and a combination thereof; preferably the further organic liquid is 1,2-dimethoxy ethane or 1,2-diethoxy ethane; most preferably 1,2-dimethoxy ethane.

In a preferred embodiment the further organic liquid further comprises a fluorinated liquid, wherein the fluorinated liquid is a fluorinated ether, a fluorinated carbonate or a fluorinated aromatic compound. In a preferred embodiment the fluorinated liquid is selected from the group consisting of 1,1,2,2-tetrafluoroethyl- 2,2,3,3-tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE), 1, 1,2,2- tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE), tris(2,2,2- trifluoroethyl)orthoformate (TFEO), methoxynonafluorobutane (MOFB), ethoxynonafluorobutane (EOFB), di(2,2,2-trifluoroethyl) carbonate (DTFEC), tris(2,2,2-trifluoroethyl)orthoformate (TFEO), tris(hexafluoroisopropyl)orthoformate (THFiPO), tris(2,2-difluoroethyl)orthoformate (TDFEO), bis(2,2,2-trifluoroethyl) methyl orthoformate (BTFEMO), tris(2,2,3,3,3-pentafluoropropyl)orthoformate (TPFPO), tris(2,2,3,3-tetrafluoropropyl)orthoformate (TTPO) and a combination thereof; preferably the fluorinated liquid is 1, 1,2, 2-tetrafluoroethyl-2, 2.3,3- tetrafluoropropyl ether (TTE), bis(2,2,2-trifluoroethyl) ether (BTFE) or 1, 1,2,2- tetrafluoroethyl-2,2,2-trifluoroethyl ether (TFTFE); most preferably 1, 1,2,2- tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE).

In a preferred embodiment of the invention the volume ratio of the nonfluorinated liquid to fluorinated liquid is between 6: 1 to 1 :2 (vol/vol), preferably between 5: 1 to 1 : 1 (vol/vol), most preferably between 4: 1 to 2: 1 (vol/vol).

As appreciated by the skilled person the organic liquid according to the invention is the liquid used in the crosslinking step affording the copolymer according to the invention, whereas the further organic liquid according to the invention is the liquid present in the electrolyte composition according to the invention.

In an embodiment of the invention the concentration of the further lithium salt in the further organic liquid is more than 0.2 M, preferably more than 2 M, most preferably more than 4 M. In an embodiment of the invention the concentration of the further lithium salt in the further organic liquid is less than 10 M, preferably less than 8 M, most preferably less than 6 M. In an embodiment of the invention the concentration of the further lithium salt in the further organic liquid is in the range of 0.2 - 10 M, preferably in the range of 2 - 8 M, most preferably in the range of 4 - 6 M.

Separator

A separator is usually interposed between the positive electrode and the negative electrode in order to prevent a short circuit between the positive electrode and the negative electrode. The material and shape of the separator is not particularly limited, but it is preferable that the electrolyte composition can easily pass therethrough and that the separator is an insulator and a chemically stable material. Examples thereof include microporous films and sheets made of various polymer materials. Specific examples of the polymer material include polyolefin polymers, nitrocellulose, polyacrylonitrile, polyvinylidene fluoride, polyethylene, and polypropylene. From the viewpoints of electrochemical stability and chemical stability, polyolefin polymers are preferred.

Varia

In a preferred embodiment of the invention the battery of the invention has a coulombic efficiency of at least 93%, preferably at least 94%, more preferably at least 95%, even more preferably at least 96%, even more preferably at least 97%, most preferably at least 98%. The coulombic efficiency is measured by using the following general formula: discharge capacity Total number of Li ⁺ ions back to cathode

CE = - = - charge capacity Total number of Li ⁺ ions departing from cathode

Total number of e~back to cathode

Total number of e~ departing from cathode

Uses

It is a further aspect of the invention to provide a use of a protecting layer as a self-healing film on a negative electrode comprising a metal substrate, wherein the protecting layer is according to the invention and the metal substrate is according to the invention.

It is a further aspect of the invention to provide a use of a protecting layer to reduce dendritic growth on a negative electrode comprising a metal substrate, wherein the protecting layer is according to the invention and wherein the metal substrate is according to the invention.

It is a further aspect of the invention to provide a use of a protecting layer on an negative electrode comprising a metal substrate to improve the cycle efficiency of a battery comprising the negative electrode, preferably the cycle efficiency is improved by 20%; wherein the protecting layer is according to the invention, preferably the protection layer is subjected to the thermal annealing step as defined above, and wherein the metal substrate is according to the invention.

EXAMPLES

The invention is further illustrated in the following examples:

Description of cell preparations and testing methods

Tested coin cells were all types CR.2032. The cells where prepared by stacking positive casing, positive electrode (pre-soaked in electrolyte), Cellguard-separator, 50 .L electrolyte droplet, negative electrode, spacer, wave-type spring and negative casing on top of each other in that order. Crimping was done with a manual crimping press from MTI corp, at 80 kg/cm ² pressure. The Li-Li pouch cells were prepared on the following matter: Format = lLi(58x58mm)/lSep(62x62mm)/lLi(50x50mm); 'nominal capacity' = 0.1 Ah; separator = Targray SH220W22 alumina coated; lithium = 100 pm thick (coated or uncoated); electrolyte loading = 3.00 pL mAh’ ¹ (Z.e. 0.3 mL); C-rate = C/5 D/5 (standard regime) or C/3 D/1 with potential holds (new regime); cycling temperature = 25 °C.

The Li-NMC pouch cells were prepared on the following matter: Format = 3Li(58x58mm)/4Sep(62x62mm)/2NMC(56x54mm); nominal capacity = 0.48 Ah (assumes cathode surface capacity of 4 mAh cm’ ² ); separator = Targray SH220W22 alumina coated; lithium = 100 pm thick; electrolyte loading = 2.00 pL mAh’ ¹ (i.e. 0.97 mL) or 1.75 pL mAh’ ¹ ( .e. 0.85 mL); C-rate = C/5 D/5 (standard regime) or C/3 D/1 with potential holds (new regime); cycling temperature = 25°C. The NMC cathode comprises of LiNi0.6Mn0.2Co0.2O2 as positive electrode active material, carbon black and PVDF in a 92:4:4 weight ratio.

The uncompensated resistance R _u is measured through Electrochemical Impedance Spectroscopy (Niquist plot) of Li-Li symmetrical coin cells containing lithium metal anodes which are uncoated or coated with the protective layer. A frequency of 1 MHz to 1 Hz was applied in the testing.

The ion-conductivity of the protective layer is measured through quantification of the uncompensated resistance Ru via electronic impedance spectroscopy (EIS) in symmetrical Li-Li cells, wherein the ion-conductivity is measured after cell construction.

Cross-section of the protective layers were analyzed using a scanning electron microscope (SEM). A JEOL bench top SEM was used, (sample imaged under vacuum within SEM, secondary electron imaging, probe current - set at standard, accelerating voltage of the SEM column.)

The polymer protected anodes are prepared by adding the monomers, the lithium salt, the initiator and the fluoropolymer additive in an organic solvent in amounts as defined in the tables below. These mixtures are coated on the anode and subjected to UV-curing with Irgacure 819 as photo initiator with an UV-curing time of 5 min. Optionally, thermal annealing of the polymer occurs after UV-curing of the polymer protected anodes at 70 °C for 1 hour. Example 1

To evaluate the effect of the amount of fluoropolymer additive the following polymer formulations 1-a, 1-b and 1-c were prepared (see Table 1; PEGDA = poly(ethylene glycol) diacrylate having M _n = 700 g/mol, LiTFSI = LiN(SO2CF3)2, PVDF- HFP = poly(vinylidene fluoride-co-hexafluoropropylene), THF = tetrahydrofuran)

Figure 1 illustrates the effect of decreasing wt.% of the fluoropolymer additive in the polymer formulation on film continuity demonstrating that coating of Formulations 1-a and 1-b form a homogenous film on the lithium substrate, whereas reducing the amount of PVDF-HFP too far (9: 1, Formulation 1-c) results in discontinuous and patchy film which exhibits a much lower uncompensated resistance (Ru). Figure 2 demonstrates the effect on the conductivity: the optimal PEG: PVDF- HFP ratio is 3: 1, providing a continuous polymer film and resulting in a reduction of the _u when compared to Formulation 1-a.

Example 2

To evaluate the effect of the lithium salt the following polymer formulations 2- a, 2-b, 2-c and 2-d were prepared (see Table 2; PEGDA = poly(ethylene glycol) diacryalte having M _n = 700 g/mol, LiTFSI = LiN(SO2CF ₃ )2, LiFSI = LiN(SO ₂ F) ₂ , PVDF- HFP = poly(vinylidene fluoride-co-hexafluoropropylene), THF = tetrahydrofuran)

Table 2: polymer formulation for optimization the Li-salt

The conductivity data from each of the polymer formulations in Table 2 is presented in Figure 3. The issue of low film salt content (10 wt.%) in the polymer formulation is evident, as over the first 20 hours the R _u of the Li-Li coin cell reduces as the resistive polymer formulation takes up salt from the electrolyte, a process which should be prevented. Increasing the salt content of the polymer formulation from 10 wt.% to 50 wt.% by total weight of the formulation results in a reduction of Ru (Formulations 2-b and 2-d display a similar uncompensated resistance of around 45-47 Q) of each of the cells resulting from the improved conductivity of the polymer film in each instance. The stability of the Ru for films containing 50 wt.% salt from 0 to 60 hours indicates that transfer of ions between the polymer film and electrolyte is in equilibrium from cell construction and depletion of the electrolyte salt is avoided.

Example 3

To evaluate the effect of the /V-hydroethyl acrylamide monomer the following polymer formulations 3-a, 3-b, 3-c and 3-d were prepared (see Table 2; PEGDA = poly(ethylene glycol) diacryalte having M _n = 700 g/mol, HEAA = /V-hydroxyethyl acrylamide, PVDF-HFP = poly(vinylidene fluoride-co-hexafluoropropylene), THF = tetra hydrofuran)

Table 3: polymer formulation for optimization the N-hydroethyl acrylamide monomer

Protective layers of each formulation were coated onto a lithium substrate, cured and cut by dragging a scalpel across the coated surface. The cut sample was imaged by SEM and then the sample was soaked in 4.5 M LiFSI in 1,2- dimethoxyethane (DME) overnight (16 hours). The film was rinsed with pure DME to remove the excess of LiFSI salt allowing clearer SEM micrographs, dried and observed under SEM. The results for each of the polymer formulations in Table 3 are displayed in Figure 4.

As expected Formulation 4-a (0 wt.% HEAA) (1 :0) shows no self-healing properties, due to its lack of hydrogen bond donors. The protective layer containing the lowest amount of HEAA (Formulation 4-b) also shows no evidence of self-healing under these conditions. Increasing the HEAA content to a weight ratio PEGDA: HEAA 3: 1 results in a resealing of the polymer film at the site of the incision made in. Finally, Formulation 4-d (weight ratio PEGDA:HEAA 1 : 1 w/w) also shows substantial self- healing properties (see Figure 4). Additionally, it is worth noting that Formulation 4- d appears to have better adhesion to the Li metal surface when observed under SEM as compared to the other formulations.

Each polymer formulation was then tested in Li-NMC cells against reference cells (no polymer). Cells were cycled with (4.5 M LiFSI in DME):TTE (3: 1, vol/vol; TTE = 1,1,2,2-tetra-fluoroethyl 2,2,3,3-tetrafluoropropyl ether) at a loading of 1.75 |j.L mAh' ¹ under a fixed volume pressure of 0.06 MPa using foam (80x80 mm) with cycling regime of C/3-D/1. Each polymer protected anode cell performed similarly to the refence cells (see Figures 5 and 6).

In a next step the conductivity of each polymer formulation was evaluated before and after thermal annealing (1 hour at 70 °C in dry air) and the results are depicted in Figure 7(a) and Figure 7(b), respectively. Clearly, thermal annealing improves the conductivity of the polymer formulation. Increasing film conductivity to a minimum threshold of around 0.1 mS.cnr ¹ is considered vital for successful anode protection. A higher conductivity results in a lower energetic barrier to plating underneath the polymer film, which reduces dendrite penetration, reduces the amount of dead lithium and helps to prolong functionality of the electrolyte and lithium anode. It was also observed that each polymer formulation appeared more homogenous and had better contact with the lithium surface.

Each polymer formulation after annealing was then tested in Li-NMC cells against reference cells (no polymer). Cells were cycled with (4.5 M LiFSI in DME):TTE (3: 1, vol/vol; TTE = 1,1,2,2-tetra-fluoroethyl 2,2,3,3-tetrafluoropropyl ether) at a loading of 1.75 |_iL mAh' ¹ under a fixed volume pressure of 0.06 MPa using foam (80x80 mm) with cycling regime of C/3-D/1. Each thermally annealed polymer protected cell performed similarly and up to 20% better as compared to the refence cells (see Figures 8 and 9).