An electrode comprising a binder obtained by polymerization of a polyol, a cross-linker and lignin, as well as a method and slurry for its production

Abstract

The present invention relates to the field of electrodes for lithium-ion batteries comprising a polymer binder obtained by polymerization of lignin or modified lignin, a cross-linker and a polyol. The present invention relates to an electrode comprising a current collector and a layer, wherein the layer comprises an active material, a conductive additive and a binder, wherein the binder is obtained by polymerization of a polyol, a cross-linker comprising at least two carboxylic acid groups, and lignin. Additionally, it relates to a slurry comprising a polyol, a cross-linker comprising at least two carboxylic acid groups, lignin, a conductive additive, an active material, and optionally a solvent. Additionally, the present invention relates to a method for producing an electrode.

Description

[0001] An electrode comprising a binder obtained by polymerization of a polyol, a cross-linker and lignin, as well as a method and slurry for its production

[0002] Field of the invention

[0003] The present invention relates to the field of electrodes for lithium-ion batteries encompassing a polymer binder obtained by polymerization of lignin or modified lignin, a cross-linker and a polyol. The present invention provides alternatives to conventional binders.

[0004] Background of the invention

[0005] Lithium-ion batteries are continuously developed to increase energy and power density as well as to reduce production cost.

[0006] Current electrode production of lithium-ion batteries for the automotive industry is based on the preparation of electrode slurries comprising active materials, conductive additives, binders, and solvents. During the electrode production, the slurries are coated on metal foils, which serve as current collectors in the battery, followed by drying to evaporate the solvents. The electrodes with current collectors having a layer comprising active materials, conductive additives and binders are subsequently used to assemble battery cells.

[0007] Naturally, the capacities of the active materials used for the cathode and anode are important factors for the battery performance.

[0008] Cathodes use active materials such as lithium nickel manganese cobalt oxide (NMC), lithium iron phosphate, lithium nickel cobalt aluminum oxide, and lithium manganate. In the slurry preparation, the active materials are typically mixed with carbon black as the conductive additive, polyvinylidene fluoride (PVDF) as the binder, and N-methylpyrrolidone (NMP) as the solvent. The cathode slurries are typically coated on aluminum current collectors, before the solvent is evaporated. The emphasis in the upcoming generation of electrodes will be placed on the advancement of cathodes with high Ni and Li content, alongside the reduction of Co content. Nonetheless, the utilization of a PVDF binder in electrode production necessitates the use of hazardous solvents, such as NMP. Its use is accompanied by requirements of closed-loop handling with solvent recovery systems, therefore increasing the complexity of industrial electrode production.

[0009] Anodes conventionally use active materials based on graphite. In the slurry preparation, the active materials are typically mixed with carbon black as the conductive additive, a combination of carboxymethylated cellulose (CMC) and styrene butadiene rubber (SBR) as the binder, and water as the solvent. The anode slurries are applied onto copper current collectors and subsequently dried to evaporate the solvent. Furthermore, the active materials of the anode function at a voltage where the electrolyte tends to undergo decomposition. Decomposition of electrolyte results in the formation of layers of decomposition products between the electrolyte and the particles of the anode, which is referred to as the solid electrolyte interface (SEI). Stable SEI layers are associated with good cyclability of the anodes. A promising approach to increase the anode performance is to include silicon in the anode. While graphite has a theoretical capacity of 372 mAh / g, silicon has a capacity in the order of 10 times higher than graphite (3579 mAh / g) and a relatively low operating potential (<0.4 V vs Li+ / Li). However, the charging and discharging of silicon is accompanied by large volume changes (=280%). These volume changes result in rapid capacity loss, linked to the continuous formation of the SEI layer consuming the electrolyte, as well as causing pulverization and loss of contact with silicon active material. Furthermore, conventional binders like CMC / SBR are not well-suited to manage the substantial volume changes of the active material. However, they are suitable for anodes that utilize graphite as the active material, as graphite undergoes limited volume expansion during lithiation, approximately 10%. Several synthetic polymer binders with different structures and functionalities have been patented for enhancing the performance such as self-healing. However, the most common polymer used as a binder for Si anodes is still polyacrylic acid (PAA). The carboxylic acid group in PAA reacts with electrolytes to form a thin protective SEI layer, and promotes hydrogen bonding or crosslinking, adhesion, and rheological properties. It has been shown that the vacuum heating during the drying of the electrodes promotes esterification and decarbonylation reactions in the PAA matrix. However, Si anodes using PAA binders exhibit gradual decrease in capacity during cycling.

[0010] Thus, there are drawbacks associated with binders traditionally used.

[0011] Summary of the invention

[0012] The objective of the present invention is to overcome the drawbacks of the prior art and improve the performance of lithium-ion batteries.

[0013] In the first aspect, the present invention concerns an electrode comprising a current collector having a layer comprising an active material, a conductive additive and a binder. The binder is obtained by esterification of a polyol, a cross-linker comprising at least one carboxylic acid group, and lignin.

[0014] In another aspect, the present invention concerns a slurry comprising polyols, lignin, and a cross-linker comprising at least two carboxylic acid groups, a conductive additive, an active material, and a solvent. Electrodes of the first aspect may be produced, by coating a current collector and evaporating the solvent. In yet another aspect, the present invention concerns a method for producing the electrode according to the first aspect.

[0015] Brief description of figures

[0016] Figure 1 illustrates ATR-FTIR spectra of a) example 2 and b) comparative example 2, respectively.

[0017] Figure 2 illustrates thermogravimetric analysis (TGA) curves for example 2 and example 6, respectively.

[0018] Figure 3 illustrates differential scanning calorimetry (DSC) curves for example 2 (top) and example 6 (bottom), respectively.

[0019] Figure 4 a) illustrates the specific discharge capacity as a function of cycle number for example 1 and comparative example 1, respectively.

[0020] Figure 4 b) illustrates the Coulombic efficiency of example 1 and comparative example 1, respectively.

[0021] Figure 5 a) illustrates differential capacity (dQ / dV) curves for the 2ndand 50thcycle for example 1.

[0022] Figure 5 b) illustrates differential capacity (dQ / dV) curves for the 2ndand 50thcycles for comparative example 1.

[0023] Figure 6 a) illustrates the specific discharge capacity as a function of cycle number for example 3 (top squares) and comparative example 1 (bottom circles), respectively.

[0024] Figure 6 b) illustrates the Coulombic efficiency of example 3 (squares) and comparative example 1 (circles), respectively.

[0025] Figure 7 illustrates specific discharge capacity as a function of cycle number for example 4.

[0026] Detailed description of the invention

[0027] According to the first aspect, the present invention concerns an electrode comprising a current collector having a layer comprising an active material, a conductive additive and a binder. The binder is obtained by esterification of lignin, a cross-linker comprising at least one carboxylic acid group, and a polyol.

[0028] The current collector is a material with electronic conductivity and can support the layer comprising active material, conductive additives and binder.

[0029] The weight ratio between lignin and the sum of cross-linker and polyol, i.e. Iignin:(cross-linker+polyol), is between 10-90 wt-%. The molar ratio between the carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol is between 0.1-2.5.

[0030] Lignin is a macromolecule and can contain a variety of functional groups such as aliphatic hydroxyl, phenolic hydroxyl, methoxy groups, carboxyl, carbonyl and sulfonate groups. Lignin act as a rigid segment in the binder and is involved in the cross-linking with the cross-linker comprising carboxylic acid groups. Additionally, it improves the compatibility with the carbon compounds, such as the conductive additive and active material, in the electrode.

[0031] Lignin contributes to the interaction with active materials, such as silicon, graphite and metal oxides. It also contributes to the interaction between the electrode and electrolyte for the formation of solid electrolyte interface (SEI) layer.

[0032] The cross-linker comprising at least two carboxylic acid groups is involved in the cross-linking with hydroxyl groups of polyols and the lignin through esterification. Additionally, it is involved in interaction with active materials, conductive additives as well as electrolytes. The cross-linker can be selected from aromatic or aliphatic carboxylic acids with various chain length preferably short chain length with at least two carboxylic acid groups, preferably more than two carboxylic acid groups.

[0033] The polyol promotes cross-linking through esterification with the cross-linker. Additionally, it serves as a softening agent and improves the bonding to the active material via non-covalent bonding. Polyols with longer chain can be integrated into the polymer binder matrix for creating well defined soft segments and promoting the elastic behavior of the binder.

[0034] A combination of the weight ratio of lignin and the sum of cross-linker and polyol between 10-90 wt-% and a molar ratio between the carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol between 0.1-2.5 enable polymerization via esterification reactions without using any catalyst at temperatures around 120°C. This allows the lignin, cross-linker, and polyol to be esterified at temperatures traditionally employed in the electrode production for lithium-ion batteries.

[0035] Additionally, the weight ratio of lignin and the sum of cross-linker and polyol between 10-90 wt-% and a molar ratio between carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol between 0.1-2.5 yields mechanical properties of the binder obtained after esterification to achieve desired elasticity and strength, so that electrodes can be repeatedly cycled, i.e. charged and discharged, with stable and high specific charge capacity.

[0036] This results in superior performance for the binders of the present invention compared to state-of-the- art binders such as PAA for Si-based anodes. In one embodiment, the lignin is selected from lignosulfonate, kraft lignin, modified lignin or a combination thereof. Lignosulfonate from sulfite pulping is water soluble, while kraft lignin is water soluble in alkaline media or organic solvents. Therefore, the lignin may be chosen to match the solvent employed in the production of cathodes (i.e. NMP) or anodes (i.e. water).

[0037] Modified lignin can be prepared by fractionation, depolymerization, liquefaction and functionalization of lignin. Several approaches can be used to produce modified lignin by increasing the number of the hydroxyl or carboxyl groups, or increasing the number of aliphatic hydroxyl groups of the lignin. Examples are esterification of lignin using anhydrides or carboxylic acids, or etherification, carboxymethylation, and carboxylation of lignin. Other viable approaches are to liquify lignin by oxyalkylation using cyclic carbonates such as propylene carbonate, or liquefaction using polyhydric alcohols such as PEG and glycerol. Modification of lignin with isocyanate is another approach. Another approach to produce modified lignin is to react hydroxyl groups of lignin with isocyanate to introduce urethane groups on the lignin. This can be achieved by integrating lignin into the polyol or directly reacting lignin with isocyanate to produce iso-cyanate-terminated lignin.

[0038] In one embodiment, the cross-linker is selected from a group consisting of citric acid, succinic acid, itaconic acid, malic acid, glutaric acid, azelaic acid, adipic acid, or a combination thereof. The chemical structure and ratios of hydroxyl and carboxyl groups result in good rheological and thermomechanical properties of the binder as well as favorable interactions with the active material, electrolyte and lithium ions.

[0039] In one embodiment, the polyol is a diol or a triol, preferably selected from a group consisting of butanediol, 1,6-hexanediol, 1,8-octanediol, trimethylolpropane, glycerol, sorbitol, diethylene glycol, or a combination thereof. The choice of polyol influence the strength and elasticity of the material, wherein for aliphatic dihydroxy alcohols longer chain length of the polyol increases elasticity and shorter chain length increases strength.

[0040] One approach is to synthesize longer chains through the condensation of polyols (like diols) and carboxylic acids, involving the use of di- or tri-carboxylic acids, for instance. An example is polymerizing a diol like 1,8-octanediol, diethylene glycol (DEG) or polyethylene glycol (PEG) with a carboxylic acid, while the molar ratio of the acid is higher so that the polymer chains with carboxylic end groups are derived.

[0041] Another approach is to use relatively short polyols such as glycols wherein the molar ratio of the polyol is in excess with respect to the carboxylic acid groups of the cross-linker. Relatively short polyols are synthesized using carboxylic acid and glycols with excessive molar ratio of the glycol. In one embodiment, the cross-linker is adipic acid or citric acid and the polyol is glycols such as diethylene glycol (DEG) or polyethylene glycol (PEG) or glycerol, so that the adipic or citric acid is reacted with DEG or PEG or glycerol, while the molar ratio of the glycol is twice that of the acid.

[0042] In one embodiment, the polyol is glycerol. Glycerol comprises two primary and one secondary hydroxyl group. Glycerol is susceptible to the formation of hyperbranched polyesters with abundant carboxyl and hydroxyl end groups. End group functionality affects the interaction with active materials as well as the interaction with electrolyte and lithium ions.

[0043] In one embodiment, the polyol is selected from polyether or polyester polyols with terminal hydroxyl groups. The polyester polyols with longer chains are synthesized by condensation of the diol with carboxylic acids, or ring-opening polymerization of cyclic esters. A longer chain of the polyol results in longer soft segments and formation of well-defined hard and soft microphases in the polymer chains. Therefore, resulting in polymers with higher elasticity. The tensile strength and elongation at break of the polymer is manipulated by changing the monomer and the cross-linker.

[0044] In one embodiment, an equimolar ratio of the acid and polyol is esterified while the chain length of the polyol can be selected from a short chain to a long chain glycol, a polyether such as PEG or a polyester polyol.

[0045] In another embodiment, a longer polyol is used for preparing carboxyl terminated prepolymer and a second short chain glycol is used for cross-linking the polymer chains and polymerization.

[0046] In one embodiment, the polyol is 1,8-octanediol, PEG400 or a mixture thereof, the cross-linker is citric acid and the lignin is lignosulfonate.

[0047] In one embodiment, the polyol is produced by condensation of a mixture of citric acid (CA) and polyethylene glycol 400 (PEG400) with a molar ratio of 1 / 2 (CA / PEG400) until the acid number of the mixture reach a value of around 110.

[0048] The active material is capable of electrochemically react with lithium ions in the electrolyte. The type of material determines the operating potential as well as the lithium storage capacity.

[0049] In one embodiment, the active material is silicon, graphite, hard carbon, conductive carbon, or combinations thereof.

[0050] In yet another embodiment, the active material is preferably silicon, graphite, or a combination thereof. Active materials such as silicon, graphite, or a combination thereof have high capacity, i.e. above 300 mAh / g, and operate at low potentials, i.e. below 1.5 V vs Li+ / Li, and are therefore well suited as active material in anodes for lithium ion batteries. More preferably wherein the active material is silicon. In one embodiment, the active material is lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium manganate, or combinations thereof.

[0051] In another embodiment, the conductive additive is selected from a group consisting of carbon black, conductive graphite, carbon fibers, carbon nanotubes or a combination thereof. The conductive additive increases the electrical contact of the active materials with the current collector as well as decreases the impedance. More preferably, the conductive additive is carbon black.

[0052] In a preferred embodiment, the weight ratio between the lignin and the sum of the cross-linker and polyol is between 15-70 wt-%.

[0053] More preferably, the weight ratio between the lignin and the sum of the cross-linker and polyol is between 20-50 wt-%.

[0054] In a preferred embodiment, the molar ratio between the carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol is between 0.1-2.5. The molar ratio of the carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol determine the rheological and thermomechanical properties. Preferably, the molar ratio between the carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol is between 0.15-2.3, more preferably between 0.2-2.2.

[0055] In a preferred embodiment, the cross-linker is a tricarboxylic acid and the polyol is a triol.

[0056] In a preferred embodiment, the weight ratio of the active material with respect to the sum of the active material, conductive additive, lignin, cross-linker and polyol is between 50-98 wt-%.

[0057] In one embodiment of the present invention, the electrode comprises a current collector, wherein the current collector is copper, having a layer comprising an active material, selected from graphite, silicon or a combination thereof, a conductive additive, wherein the conductive additive is carbon black, and a binder, wherein the binder is obtained by esterification of lignin, wherein the lignin is lignosulfonate or kraft lignin, a cross-linker, wherein the cross-linker is citric acid, and a polyol, wherein the polyol is glycerol.

[0058] According to another aspect, the present invention relates to a slurry comprising polyols, esterified polyols, cross-linkers, lignin, a conductive additive, an active material, and optionally a solvent. The weight ratio between the lignin and the sum of cross-linkers and the polyols is between 10-90 wt-%, preferably between 15-70 wt-%, more preferably between 20-50 wt-%. The molar ratio between the carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol is between 0.1-2.5.

[0059] A slurry according to the present invention may be cast on a current collector. Subsequent drying at temperatures traditionally used in the electrode production of lithium-ion batteries results in an electrode with the capability of being charged and discharged with Coulombic efficiency >99% after 100 cycles.

[0060] In one embodiment, the slurry comprises lignin selected from lignosulfonate, kraft lignin, modified lignin or a combination thereof.

[0061] In one embodiment, the slurry comprises a cross-linker selected from a group consisting of citric acid, succinic acid, itaconic acid, malic acid, glutaric acid, azelaic acid, adipic acid, or a combination thereof.

[0062] In one embodiment, the slurry comprises a polyol selected from a group consisting of butanediol, 1,6- hexanediol, 1,8-octanediol, trimethylolpropane, glycerol, sorbitol, diethylene glycol, polyethylene glycol or a combination thereof.

[0063] In one embodiment, the slurry comprises an active material selected from a group consisting of silicon, graphite, hard carbon, conductive carbon, lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and lithium manganate. Preferably wherein the active material is silicon, graphite or a combination thereof. More preferably wherein the active material is silicon.

[0064] In one embodiment, the solvent is water, 1,4-dioxane, or N-methylpyrrolidone. More preferably the solvent is water.

[0065] In one embodiment, the conductive additive in the slurry is selected from a group consisting of carbon black, conductive graphite, carbon fibers, carbon nanotubes, or a combination thereof. In a preferred embodiment the conductive additive is carbon black.

[0066] In one embodiment, the weight ratio between lignin and the sum of cross-linker and polyol is between 10-90 wt-%, preferably between 15-70 wt-%, more preferably between 20-50 wt-% in the slurry.

[0067] In one embodiment, the molar ratio between the carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol is between 0.1-2.5, preferably between 0.15-2.3, more preferably between 0.2-2.2, in the slurry.

[0068] In one embodiment, the weight ratio of the active material with respect to the sum of the active material, conductive additive, lignin, cross-linker and polyol is between 50-98 wt-% in the slurry.

[0069] In one embodiment of the present invention, the slurry comprises an active material, selected from graphite, silicon or a combination thereof, a conductive additive, wherein the conductive additive is carbon black, lignin, wherein the lignin is lignosulfonate or kraft lignin, a cross-linker, wherein the crosslinker is citric acid, a polyol, wherein the polyol is glycerol, and a solvent, wherein the solvent is water or 1,4-dioxane. According to another aspect, the present invention relates to a method of producing the electrode according to the first aspect of the invention. The method comprises the steps of providing a slurry according to the present invention, applying the slurry on a current collector surface, and evaporating the solvent at a first temperature.

[0070] The current collector supports the formation of a layer comprising an active material, a conductive additive and a binder. Evaporation of the water formed in the condensation reaction between the polyol, cross-linker and lignin and / or evaporation of the optional solvent at a first temperature result in the formation of a layer on the current collector.

[0071] In one embodiment of the present invention, the electrode comprises a current collector and a layer, wherein the layer is arranged on the current collector, wherein the layer comprises an active material, a conductive additive and a binder, wherein the active material is graphite, silicon or a combination thereof, wherein the conductive additive is carbon black, wherein the binder is obtained by polymerization of a polyol, a cross-linker and lignin, wherein the polyol is glycerol, wherein the crosslinker is citric acid, and wherein the lignin is lignosulfonate or kraft lignin.

[0072] In one embodiment of the present invention, the method of producing the electrode, according to the first aspect of the invention, comprises providing a slurry comprising an active material, wherein the active material is selected from graphite, silicon or a combination thereof; a conductive additive, wherein the conductive additive is carbon black; lignin, wherein the lignin is lignosulfonate or kraft lignin; a cross-linker, wherein the cross-linker is citric acid; a polyol, wherein the polyol is glycerol; and a solvent, wherein the solvent is water or 1,4-dioxane; wherein the slurry is applied to a current collector, wherein the current collector is copper; and evaporating the solvent at a first temperature.

[0073] In one embodiment of the present invention, the method of producing the electrode, according to the first aspect of the invention, comprises providing a slurry according to the present invention, wherein the slurry is provided by dissolving lignin in the optional solvent to form a first mixture, adding the cross-linker to the first mixture to form a second mixture, adding the polyol to the second mixture to form a third mixture, and wherein the conductive additive and the active material is added to the third mixture to provide the slurry. Furthermore, the electrode is produced by applying the slurry on a current collector surface and evaporating water and / or the optional solvent at a first temperature. Providing a slurry by the addition of polyol to the second mixture as specified above is beneficial to achieve good elasticity in combination with good strength, so that electrodes can be repeatedly cycled, i.e. charged and discharged, with stable and high specific charge capacity.

[0074] In another embodiment of the present invention, the method of producing the electrode, according to the first aspect of the invention, comprises providing a slurry according to the present invention, wherein the slurry is provided by dissolving lignin in the optional solvent to form a first mixture. Wherein the lignin is dissolved by heating the optional solvent and / or lignin. Adding the cross-linker to the first mixture to form a second mixture, adding the polyol to the second mixture to form a third mixture, and wherein the conductive additive and the active material is added to the third mixture to provide the slurry. Furthermore, the electrode is produced by applying the slurry on a current collector surface and evaporating water and / or the optional solvent at a first temperature.

[0075] As recognized by a skilled person, the second mixture may additionally comprise a catalyst for the esterification. In some embodiments of the present invention, the catalyst for the esterification is p- toluenesulfonic acid.

[0076] Examples

[0077] Example 1 - Lignosulfonate-based binder for Si anode

[0078] Lignosulfonate (Borregaard, LignoTech) (LS) was dissolved in water in a glass dish with a stirring magnet. Citric acid (CA) and glycerol (GL) with a molar ratio of 1 / 3 (ncA / nci) were mixed in water. The CA and GL mixture was added to the solution containing LS. The weight ratio of LS to the other two components was 50 wt-%. Carbon black super C (CB) was milled for 3 h in a planetary ball mill (Retsch PM400). Silicon nanoparticles with a mean diameter of =100 nm (Sigma Aldrich) and CB were added to the solution under stirring to form a slurry. The mass ratio of the Si, CB, and binder (=LS+GL+CA) was 60:20:20. The slurry was stirred for 2 h. Copper foil with a thickness of 8 pm was employed as a current collector. After stirring, the slurry was cast on the copper foil and dried in an oven at 70°C for 2 h. After drying, disks with a diameter of 15 mm were punched out from the coated copper foil. The disks were transferred to a vacuum oven for additional drying at 120°C for 12 h. The total material mass loading was =1.0 mg / cm2, wherein the mass loading of active material (i.e. Si) was 0.6 mg / cm2. After vacuum drying, the electrode disks were transferred to a glovebox and heated at 90°C for 1 h.

[0079] LIR2032 type coin cells were assembled with the electrode disks, together with a glass fiber separator (Whatman), lithium foil as a counter electrode, and 1.0 M LiPFs in a mixture of ethylene carbonate (EC) and diethylene carbonate (DC) (EC:DEC, 1:1 by weight) with 10 wt-% fluoroethylene carbonate (FEC) as electrolyte.

[0080] The cells were cycled between 0.001 V and 1.2 V at a rate of C / 5 after a first formation cycle at C / 10. The C-rate was calculated based on a theoretical capacity of 3579 mAh / g for Si and a mass loading of 0.6 mg / cm2.

[0081] Comparative example 1 - PAA binder for Si anode Polyacrylic acid (PAA) was dissolved in water in a glass dish with a stirring magnet. Carbon black super C (CB) was milled for 3 h in a planetary ball mill (Retsch PM400). Silicon nanoparticles with a mean diameter of =100 nm (Sigma Aldrich) and CB were added to the solution under stirring to form a slurry. The mass ratio of the Si, CB, and binder (=PAA) was 60:20:20. The slurry was stirred for 2 h. Copper foil with a thickness of 8 pm was employed as a current collector. After stirring, the slurry was cast on the copper foil and dried in an oven at 70°C for 2 h. After drying, disks with a diameter of 15 mm were punched out from the coated copper foil. The disks were transferred to a vacuum oven for additional drying at 120°C for 12 h. The total material mass loading was =1.0 mg / cm2, wherein the mass loading of active material (i.e. Si) was 0.6 mg / cm2. After vacuum drying, the electrode disks were transferred to a glovebox and heated at 90°C for 1 h.

[0082] LIR2032 type coin cells were assembled with the electrode disks, together with a glass fiber separator (Whatman), lithium foil as a counter electrode, and 1.0 M LiPFs in a mixture of ethylene carbonate (EC) and diethylene carbonate (DC) (EC:DEC, 1:1 by weight) with 10 wt-% fluoroethylene carbonate (FEC) as electrolyte.

[0083] The cells were cycled between 0.001 V and 1.2 V at a rate of C / 5 after a first formation cycle at C / 10. The C-rate was calculated based on a theoretical capacity of 3579 mAh / g for Si and a mass loading of 0.6 mg / cm2.

[0084] Example 2 - Characterization of lignosulfonate-based binder

[0085] Lignosulfonate (Borregaard, LignoTech) (LS) was dissolved in water in a glass dish with a stirring magnet. Citric acid (CA) and glycerol (GL) with a molar ratio of 1 / 3 (ncA / nci) were mixed in water. The CA and GL mixture was added to the solution containing LS. The weight ratio of LS to the other two reactants was 50 wt-% to form a binder slurry. The binder slurry was stirred for 2 h. The binder slurry was transferred to a vacuum oven for drying at 120°C for 12 h.

[0086] Comparative example 2 - Lignosulfonate powder

[0087] As a comparative example, the pristine lignosulfonate (Borregaard, LignoTech) (LS) powder was used.

[0088] Example 3 - Kraft lignin-based binder for Si anode

[0089] Industrial kraft lignin (LignoBoost) (LB) was dissolved in 1,4-dioxane in a glass dish with a stirring magnet. Citric acid (CA) and glycerol (GL) with a molar ratio of 0.33 (ncA / nci) were mixed and added to the dish with LB dissolved in 1,4-dioxane. The weight ratio of LB to the other two components was 50 wt-%. Carbon black super C (CB) was milled for 3 h in a planetary ball mill (Retsch PM400). Silicon nanoparticles with a mean diameter of =100 nm (Sigma Aldrich) and CB were added to the solution under stirring to form a slurry. The mass ratio of the Si, CB, and binder (=LB+GL+CA) was 60:20:20. The slurry was stirred for 2 h. Copper foil with a thickness of 8 pm was employed as a current collector. After stirring, the slurry was cast on the copper foil and dried in an oven at 70°C for 2 h. After drying, disks with a diameter of 15 mm were punched out from the coated copper foil. The disks were transferred to a vacuum oven for additional drying at 120°C for 12 h. The total material mass loading was =1.0 mg / cm2, wherein the mass loading of active material (i.e. Si) was 0.6 mg / cm2. After vacuum drying, the electrode disks were transferred to a glovebox and heated at 90°C for 1 h.

[0090] LIR2032 type coin cells were assembled with the electrode disks, together with glass fiber separator (Whatman), lithium foil as counter electrode, and 1.0 M LiPFs in a mixture of ethylene carbonate (EC) and diethylene carbonate (DC) (EC:DEC, 1:1 by weight) with 10 wt-% fluoroethylene carbonate (FEC) as electrolyte.

[0091] The cells were cycled between 0.001 V and 1.2 V at a rate of C / 5 after a first formation cycle at C / 10. The C-rate was calculated based on a theoretical capacity of 3579 mAh / g for Si and a mass loading of 0.6 mg / cm2.

[0092] Example 4 - Lignosulfonate-based binder for Si-graphite anode

[0093] Lignosulfonate (Borregaard, LignoTech) (LS) was dissolved in water in a glass dish with a stirring magnet. Citric acid (CA) and glycerol (GL) with a molar ratio of 0.33 (ncA / nci) were mixed in water. The CA and GL mixture was added to the solution containing LS. The weight ratio of LS to the other two components was 50 wt-%. Carbon black super C (CB) was milled for 3 h in a planetary ball mill (Retsch PM400). Silicon nanoparticles with a mean diameter of =100 nm (Sigma Aldrich), graphite and CB were added to the solution under stirring to form a slurry. The mass ratio of the Si, graphite, CB, and binder (=LS+GL+CA) was 30:30:20:20. The slurry was stirred for 2 h. Copper foil with a thickness of 8 pm was employed as a current collector. After stirring, the slurry was cast on the copper foil and dried in an oven at 70°C for 2 h. After drying, disks with a diameter of 15 mm were punched out from the coated copper foil. The disks were transferred to a vacuum oven for additional drying at 120°C for 12 h. The total material mass loading was =1.0 mg / cm2, wherein the mass loading of active material was 0.6 mg / cm2, wherein the mass loading of Si and graphite was 0.3 mg / cm2and 0.3 mg / cm2. After vacuum drying, the electrode disks were transferred to a glovebox and heated at 90°C for 1 h.

[0094] LIR2032 type coin cells were assembled with the electrode disks, together with glass fiber separator (Whatman), lithium foil as counter electrode, and 1.0 M LiPFs in a mixture of ethylene carbonate (EC) and diethylene carbonate (DC) (EC:DEC, 1:1 by weight) with 10 wt-% fluoroethylene carbonate (FEC) as electrolyte.

[0095] The cells were cycled between 0.001 V and 1.2 V at a rate of C / 5 after a first formation cycle at C / 10. The C-rate was calculated based on a theoretical capacity of 3579 mAh / g for Si and 372 mAh / g for graphite and a mass loading of 0.3 mg / cm2for both Si and graphite, i.e. a theoretical capacity of 1975.5 mAh / g for the Si and graphite mixture.

[0096] Example 6 - Characterization of kraft lignin binder

[0097] Industrial kraft lignin (LignoBoost) (LB) was dissolved in 1,4-dioxane in a glass dish with a stirring magnet. Citric acid (CA) and glycerol (GL) with a molar ratio of 0.33 (ncA / nci) were mixed and added to the dish with LB dissolved in 1,4-dioxane. The weight ratio of LB to the other two components was 50 wt-% to form a binder slurry. The binder slurry was stirred for 2 h. The binder slurry was transferred to a vacuum oven for drying at 120°C for 12 h.

[0098] Example 7 - Carboxyl terminated polyols

[0099] A binder comprising lignosulfonate (Borregaard, LignoTech) (LS) was prepared using a mixture of carboxyl terminated polyols (CTP), LS and glycerol (GL). The CTP was prepared by polycondensation of either polyethylene glycol 400 (PEG400) or 1,8-octanediol and citric acid (CA) with a molar ratio of 2.1 / 1 (CA / PEG400) or (CA / l,8-octanediol). To synthesize the CTP, the ingredients are weighted and transferred to a four-neck round bottom flask equipped with nitrogen gas inlet and outlet, as well as vacuum outlet. The flask is then positioned within an oil bath and gradually heated to 160°C while being stirred. Following the complete melting of the mixture, the temperature is reduced to 140°C, and the heating process is sustained for a duration of 50 minutes. The polymer from a mixture of the CTP, GL and LS cured at 120°C shows elastic behavior. The molar ratio of CA to GL in the binder is 2.1 / 1 (CA / GL) and the weight ratio of the LS to the weight of the GL is 2 / 3 (LS / GL). A mixture of the CTP, GL and LS was used for preparing the electrode slurry in the same manner as in example 1. Electrodes and LIR2032 type coin cells were produced according to example 1. The specific capacity was assessed within half-cells comprising the silicon anode and lithium foils as electrodes.

[0100] Example 8 - Isocyanate functionalized lignin

[0101] Kraft lignin was dissolved in acetone and filtered, after which the collected solid material is subjected to drying in an oven at 80°C for a duration of two days. Subsequently, the dried material was ground using a planetary ball mill. A quantity of 5 g of the ground lignin was dissolved in tetrahydrofuran (THF) and then moved into a round-bottom flask equipped with inlet and outlet openings. The flask, nestled within an oil bath, was gradually heated to 60°C while being stirred with a stirrer under a continuous flow of nitrogen gas. Then 20 mmol of MDI isocyanate was gradually introduced into the container, and the heating process continued for a duration of 120 minutes. Aliphatic polyester polyols were used as second part of the polymer binder. Aliphatic polyester polyols with a molecular weight in the range of about 500-2000 Da were synthesized by polycondensation of adipic acid and diethylene glycol. The monomers' blend was subjected to heat within a round bottom flak containing inlet for nitrogen gas, vacuum, a stirrer, a condenser and a temperature probe. The temperature was gradually raised to 120°C, allowing for thorough mixing of the monomers. Subsequently, the temperature was further increased to 140°C and maintained at this level for a duration of 2 hours. Condensation initiated at around 140°C, and the temperature was progressively increased to reach 180°C. the reaction persisted for an additional 2 hours at 180°C under vacuum. Thereafter, the temperature was elevated 200°C, and this temperature was sustained for a period of 40 hours under nitrogen gas flow.

[0102] Polyol was from a mixture of citric acid (CA) and polyethylene glycol 400 (PEG400). The mixture was CA and PEG400 with a molar artio of 1 / 2 (CA / PEG400) was added to a round flask placed in an oil bath and equipped with stirrer and opening for nitrogen gas and vacuum. The condensation reaction was carried out at the temperature of 140°C until the acid number of the mixture reached a value of around 110.

[0103] An electrode slurry was prepared with isocyanate functionalized kraft lignin and aliphatic polyester polyols, together with Si and carbon black (CB) in the same manner as in example 3. Ethylene-glycol (EG) was added to the mixture before starting the heating for 30 minutes at 80°C. the ratio of NCO groups to the total OH groups in the binder was 1.1 / 1 (NCO / OH). The procedure for creating the electrode and conducting the electrochemical testing was consistent with example 3, except that the vacuum heating was performed at 105°C.

[0104] Example 9 - Lignin-based Si electrode prepared by initially mixing lignin and cross-linker followed by the addition of polyol

[0105] LignoBoost kraft lignin was dissolved in a minimum amount of dimethylformamide in a round-bottom flask equipped with an inlet for nitrogen gas, a vacuum tube, a stirrer, a condenser, and a temperature probe. The temperature was increased to 80°C and stirring was applied to fully dissolve the lignin to form a first mixture. Citric acid was gradually added to the round-bottom flask under stirring. The mixture was stirred for 2 hours while the temperature was gradually increased from 80°C to 110°C to form a second mixture. Subsequently, p-toluenesulfonic acid, at a weight ratio of 1 wt% was added to the second mixture, and stirring was continued for an additional 4 hours. In the final step, glycerol was added to the round-bottom flask, wherein the amount of glycerol was equimolar to the amount of citric acid, and the mixture was stirred for an additional 5 hours to form a third mixture.

[0106] The weight ratio of lignin with respect to the citric acid and the glycerol was 50 wt%. To fabricate the battery electrodes, silicon nanoparticles with an average diameter of approximately

[0107] 100 nm, carbon black, and the third mixture were mixed under stirring to form a slurry.

[0108] The mass ratio of silicon nanoparticles, carbon black, and binder was 60:20:20, wherein binder denotes the sum of lignin, citric acid and glycerol.

[0109] Copper foil with a thickness of 8 pm was used as current collector. The slurry was cast onto the copper foil and dried in an oven at 70°C for 2 hours. After drying, disks with a diameter of 15 mm were punched from the coated copper foil. These disks were then transferred to a vacuum oven for additional drying at 120°C for 12 hours.

[0110] The total material mass loading was approximately 1.0 mg / cm2, with the active material (Si) mass loading of 0.6 mg / cm2. After vacuum drying, the electrode disks were transferred to a glovebox and heated at 90°C for 1 hour. The electrodes were assembled into half cells to analyze capacity and cyclability.

[0111] Results

[0112] Figure 1 depicts ATR-FTIR spectra of a) example 2 and b) comparative example 2, respectively. The intense sharp peak at 1723 cm'1originates from the carbonyl group of the ester and the acid, and is visible for example 2 but not for comparative example 2. The sharp peak around 1183 cm'1originates from the C-0 stretching vibrations of the ester. The wide band around 3400 cm'1is attributed to the hydroxyl groups of the components, indicating incomplete conversion of these moieties. Thus, the characteristic bands in the AT-FTIR spectra confirms the formation of polyester for example 2.

[0113] Figure 2 depicts thermogravimetric analysis (TGA) curves for example 2 (intersects x-axis at =275°C) and example 6 (intersects x-axis at =340°C), respectively. For both materials, thermal decomposition occurs at temperatures higher than 200°C. These decomposition temperatures are relatively high, thereby verifying the cross-linking of the copolymer.

[0114] Figure 3 depicts differential scanning calorimetry (DSC) curves for example 2 (top) and example 6 (bottom), respectively. For both materials, the glass transition temperature is around -18°C, which is significantly lower than the glass transition temperature of PAA.

[0115] Figure 4 a) depicts specific discharge capacity as a function of cycle number for example 1 (top squares) and comparative example 1 (bottom circles), respectively. The initial specific discharge capacity for example 1 is significantly higher than for comparative example 1, >3000 mAh / g compared to 2750 mAh / g, respectively. Furthermore, the specific discharge capacity after 100 cycles was 1910 mAh / g for example 1, considerably higher than 1480 mA / h obtained for comparative example 1. Figure 4 b) depicts the Coulombic efficiency of example 1 (squares) and comparative example 1 (circles), respectively. After 100 cycles, the Coulombic efficiency is higher than 99% for both example 1 and comparative example 1.

[0116] Figure 5 depicts differential capacity (dQ / dV) curves for the 2ndand 50thcycle for a) example 1 and b) comparative example 1. The cathodic peak around 0.09 V is attributed to the I ithiation and transition between a-Liz.oSi <-> a-l.i3.5Si, and the anodic peak around 0.25-0.30 V is attributed to the delithiation and transition between a-l.i3.5Si <-> a-Liz.oSi. The cathodic peak around 0.20-0.25 V is attributed to the lithiation and transition between Si <-> a-Liz.oSi and the anodic peak around 0.45-0.50 V is attributed to the delithiation and transition between a-Liz.oSi <-> Si. For example 1, it can be seen that the shape of the dQ / dV is similar for the 2ndand 50thcycle, i.e. the relative size of the peaks are similar. For comparative example 1, it can be seen that the cathodic and anodic peaks attributed to the transition between a-Liz.oSi <-> a-l.i3.5Si, i.e. from the intermediate to the fully lithiated silicon phases, are significantly lower for the 50thcycle compared to the 2ndcycle. For comparative example 1, the loss of capacity during cycling is associated with the loss of alloying of the intermediate to the fully lithiated silicon phases. For example 1, the higher capacities obtained during cycling can be explained by the formation of a more stable SEI layer.

[0117] Figure 6 a) depicts specific discharge capacity as a function of cycle number for example 3 (top squares) and comparative example 1 (bottom circles), respectively. The initial specific discharge capacity for example 3 is significantly higher than for comparative example 1, =3100 mAh / g compared to 2750 mAh / g, respectively. Furthermore, the specific discharge capacity after 100 cycles was 2370 mAh / g for example 3, considerably higher than 1480 mA / h obtained for comparative example 1.

[0118] Figure 6 b) depicts the Coulombic efficiency of example 3 (squares) and comparative example 1 (circles), respectively. After 100 cycles, the Coulombic efficiency is higher than 99% for both example 3 and comparative example 1.

[0119] Figure 7 depicts specific discharge capacity as a function of cycle number for example 4. The initial specific discharge capacity is around 1700 mAh / g gradually decreasing to around 1500 mAh / g after 30 cycles.

[0120] The polymer from a mixture of CTP, GL and LS cured at 120°C according to example 7 showed an elastic behavior. The specific capacity of electrode of example 7 was 2420 mAh / g after completing 50 cycles.

[0121] The specific capacity of the electrode according to example 8 was 2280 mAh / g after completing 50 cycles. The specific capacity of the electrode according to example 9 was 2540 mAh / g after completing 50 cycles.

Claims

CLAIMS1. An electrode comprisingA current collector and a layer, wherein the layer is arranged on the current collector, wherein the layer comprises an active material, a conductive additive and a binder; wherein the binder is obtained by polymerization of: a polyol, a cross-linker comprising at least two carboxylic acid groups and lignin wherein the weight ratio between lignin and the sum of cross-linker and polyol is between 10-90 wt-%, wherein the molar ratio between the carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol is between 0.1-2.5.

2. The electrode according to claim 1,Wherein the lignin is selected from lignosulfonate, kraft lignin, modified lignin, or a combination thereof; and / or wherein the cross-linker is selected from a group consisting of citric acid, succinic acid, itaconic acid, malic acid, glutaric acid, azelaic acid, adipic acid, or a combination thereof; and / or wherein the polyol is selected from a group consisting of butanediol, 1,6-hexanediol, 1,8- octanediol, trimethylolpropane, glycerol, sorbitol, diethylene glycol, polyethylene glycol or a combination thereof; and / or wherein the active material is selected from a group consisting of silicon, graphite, conductive carbon, hard carbon, lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium manganate, or a combination thereof; preferably wherein the active material is silicon, graphite or a combination thereof, more preferably wherein the active material is silicon.

3. The electrode according to claim 1 or 2, wherein the conductive additive is selected from a group consisting of carbon black, conductive graphite, carbon fibers, carbon nanotubes or a combination thereof, preferably wherein the conductive additive is carbon black.

4. The electrode according to claim 1, 2 or 3, wherein the weight ratio between lignin and the sum of cross-linker and polyol is between 15-70 wt-%, preferably between 20-60 wt-%; and / or wherein the molar ratio between the carboxylic groups of the cross-linker and the hydroxyl groups of the polyol is between 0.15-2.3, preferably between 0.2-2.2; and / or wherein the weight ratio of the active material with respect to the sum of the active material, conductive additive, lignin, cross-linker and polyol is between 50-98 wt-%.

5. A slurry comprising: a polyol, a cross-linker comprising at least two carboxylic acid groups, lignin, a conductive additive an active material, and optionally a solventwherein the weight ratio between lignin and the sum of cross-linker and polyol is between 10-90 wt-%, wherein the molar ratio between the carboxylic acid group of the cross-linker and the hydroxyl group of the polyol is between 0.1-2.5.

6. The slurry according to claim 5, wherein the lignin is selected from lignosulfonate, kraft lignin, modified lignin, or a combination thereof, and / or wherein the cross-linker is selected from a group consisting of citric acid, succinic acid, itaconic acid, malic acid, glutaric acid, azelaic acid, adipic acid, or a combination thereof; and / or wherein the polyol is selected from a group consisting of butanediol, 1,6-hexanediol, 1,8- octanediol, trimethylolpropane, glycerol, sorbitol, diethylene glycol, polyethylene glycol or a combination thereof; and / or wherein the active material is selected from a group consisting of silicon, graphite, conductive carbon, hard carbon, lithium nickel manganese cobalt oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium manganate, or a combination thereof, preferably wherein the active material is silicon, graphite or a combination thereof, more preferably wherein the active material is silicon, and / or wherein the solvent is water, 1,4-dioxane, N-methylpyrrolidone, dimethylformamide preferably wherein the solvent is water or dimethylformamide, and / or wherein the solvent optionally comprises p-toluenesulfonic acid.

7. The slurry according to claim 5 or 6, wherein the conductive additive is selected from a group consisting of carbon black, conductive graphite, carbon fibers, carbon nanotubes or a combination thereof, preferably wherein the conductive additive is carbon black.

8. The slurry according to claim 5, 6 or 7, wherein the weight ratio between lignin and the sum of cross-linker and polyol is between 10-90 wt-%, preferably 15-70 wt-%, more preferably between 20-50 wt-%; and / or wherein the molar ratio between the carboxylic acid groups of the cross-linker and the hydroxyl groups of the polyol is between 0.15-2.3, preferably between 0.2-2.2, and / or wherein the weight ratio between active material and the sum of the active material, conductive additive, lignin, cross-linker and polyol is between 50-98 wt-%.

9. A method of producing the electrode of claim 1, comprising the steps of: a. Providing the slurry of claim 5, b. Applying the slurry on a current collector surface, c. Evaporating water and / or the optional solvent at a first temperature.

10. A method according to claim 9, wherein the slurry is provided by:Dissolving lignin in the optional solvent to form a first mixture, Adding the cross-linker to the first mixture to form a second mixture, Adding the polyol to the second mixture to form a third mixture, and Wherein the conductive additive and the active material is added to the third mixture to provide the slurry.

Images