Cross-reference to related applications

[0001] This application claims priority to US application No.

62/869106 filed on July 4, 2019, the whole content of this application being incorporated herein by reference for all purposes.

Technical Field

[0002] The present invention generally relates to salified polyamide- imide (PAI-Salt) polymers and their use as binders in negative electrodes for lithium ion batteries.

Background Art

[0003] Current lithium ion batteries are limited in their storage of

electrical charge by the capacity of the negative electrode. There is a general view that the most direct path to creating the next generation of energy storage systems is to significantly increase the storage capacity of lithium ion batteries by incorporating silicon into the graphite negative electrode. Silicon is able to reversibly store far more lithium than graphite, and presently small quantities are blended into the negative electrode resulting in marked capacity increases, but current binders only accommodate limited silicon loading (< 20 wt %) before battery lifetime is significantly reduced because of reduced charge cycle stability. Further capacity improvements are limited because the silicon particles swell and shrink significantly as a result of the large quantities of lithium stored and released during charging and discharging. This produces mechanical stress that results in cracking and particle attrition, impeding the path of ions in the cell, which in turn diminishes battery performance with battery cycling. [0004] One approach has been to focus on improving performance of the binder in the negative electrode. The binder holds the graphite and silicon particles together in a continuous layer in contact with both the current collector and the separator membrane that partitions the negative electrode and positive electrode. Traditionally, all graphite negative electrodes have relied on polyvinylidene fluoride (PVDF) as a binder. PVDF interacts favorably with graphite particles but does not adhere well to silicon particles, making the binder susceptible to failure resulting from mechanical stress caused by swelling and shrinking of the silicon during charge cycles.

[0005] New binders based on polyacrylic acid (PAA), and carboxymethyl cellulose with styrene butadiene (CMC-SBR) have been studied, however they are brittle and have been found to create failure points within the binder matrix itself.

[0006] Polyamide-imides (PAI) exhibit better adhesion to silicon, yet their widespread commercial use has been hindered, due in part to their unique processing requirements. Most PAIs are only soluble in organic solvents such as N-methyl-2-pyrrolidone (NMP) and to exploit their full potential as binders, they must be cured to high temperatures (e.g. >150-300°C) in a gradual step wise fashion. This processing requirement is costly and can be problematic for the overall battery manufacturing process.

[0007] Producing PAI via an acid chloride process generates hydrogen chloride as a byproduct. Left unremoved, the hydrogen chloride can cause corrosion if the PAI is used in a battery electrode. JP 2015145483 A2 (UNITIKA), addresses this problem by adding 1 to 1.02 mol per 1 mol of diamine of a lithium salt such as lithium hydroxide or lithium carbonate to the reaction mixture used to make the PAI, thereby converting the hydrogen chloride byproduct to lithium chloride. In the methods described by this reference, all or most of the lithium ions from the lithium salt are bound to the chloride and are therefore not available to form salts with any amic acid groups in the PAI. In addition, this reference does not disclose making a negative electrode with an aqueous solution of the PAI.

[0008] US 2012/0088150 (SAMSUNG) is generally directed to an

electrode including a PAI-based binder for a lithium-ion secondary battery where a 1 ,3-benzenediamine peak is not observed when a composition including components extracted from the electrode by a solvent capable of dissolving the polyamideimide (PAI)-based binder is analyzed by pyrolysis-gas chromatography. The reference does not disclose, among other things, lithiated PAI.

[0009] US 2015/0044578 (El DU PONT DE NEMOURS AND CO.) is directed to binder precursor compositions containing certain PAIs for use in cathodes of lithium-ion batteries. The reference generally discloses that acid sites in the unimidized or partially imidized binder precursor compositions can optionally be exchanged with cations such as lithium by contact, preferably with a non-aqueous solution of lithium salt, but no specific amounts are provided. Long lists of possible dianhydrides and diamines are described; however trimellitic anhydride monoacid chloride (TMAC) is not disclosed, and only a non-lithiated pyromellitic dianhydride (PMDA) - oxydianiline (ODA) copolymer is actually described and exemplified. The reference also provides no suggestion, inter alia, that PAIs could be used as binders in negative electrodes, with their far different chemistry.

[0010] JP 2013-089437, A (TORAY IND. INC.) discloses a binder

material for the anode of a lithium-ion battery which, in some embodiments, can include a PAI and a lithium salt. In particular, the reference provides examples of a solution including a PAI, a negative electrode active material, and a lithium salt; however, the PAI is highly imidized and the binder solutions are non- aqueous.

[0011] US 8,734,989 (SAMSUNG SDI) is generally directed to a

negative electrode for a rechargeable lithium battery including a high-strength binder layer that is distinct from a negative active material layer to reduce the expansion rate of the electrode and improve cycle-life. In some embodiments, the high-strength binder can include a lithium salt in addition to a polymer selected from a long list of diverse polymers. One of the polymers is generically described as an“amide-imide-based polymer;” however, the reference states that the high strength binder must have a crystallinity of at least 10% and a glass transition temperature (Tg) of not more than 100°C, and no PAI is exemplified.

[0012] US 2017/0155151 discloses water-soluble lithiated polyamic acids for use in binders for lithium-ion batteries to prevent a decrease in initial efficiency. This reference does not disclose PAIs, which have a different chemical structure and properties. Moreover, the lithiated polyamic acids described are highly imidized, have an acid equivalent of less than 300 g/eq., have a molecular weight greater than 10,000 g/mol, and are prepared specifically with lithium hydroxide.

[0013] Accordingly, a need exists for PAI binders for negative electrodes that are soluble in low cost and environmentally-friendly solvents such as water, and that preferably achieve high cycle stability without the requirement of high temperature curing.

Summary of invention

[0014] It has now surprisingly been found that the capacity retention of lithium-ion batteries can be significantly improved by the use of salified polyamide-imide (PAI-Salt) polymers as electrode binders. Moreover, the inventive electrodes can be made using an environmentally-friendly aqueous solution, and, in some embodiments, achieve unexpectedly good capacity retention without the need for high temperature curing of the PAI-Salt.

[0015] Thus, in a first aspect, the present invention relates to a salified polyamide-imide polymer (PAI-Salt) comprising more than 50% by moles of recurring units RPAI selected from the group consisting of units of any of general formulae (RpAi-a) (RpAi-b)

provided that RPAI-C represents at least 30 % by moles of recurring units in the salified polyamide-imide (PAI-Salt),

wherein:

- Ar is a trivalent aromatic group; preferably Ar is selected from the group consisting of the following

structures:

and corresponding optionally substituted structures, wherein X is selected from the group consisting of -0-, -C(O)-, - Chh-, -C(CF3)2-, -(CF2)n-, with n being an integer from 1 to 5;

X is selected from the group consisting of -0-, -C(O)-, -CFI2-, - C(CF3)2-, and

-(CF ₂ )p-;

n is an integer from 1 to 5;

R is a divalent aromatic group selected from the group consisting of:

and corresponding optionally substituted structures,

Y is selected from the group consisting of -0-, -S-, -SO2-, -CH2-, - C(O)-, -C(CF3)2-, -(CF2)q-, q being an integer from 0 to 5; and Cat ⁺ is a monovalent cation preferably selected from Na ⁺ and K ⁺ , preferably is Na ⁺ .

[0016] In a second aspect, the present invention provides a process for preparing PAI-Salt as above defined comprising contacting a polyamide-imide (PAI) with a sodium or potassium salt in a solvent, preferably water, to form the salified polyamide-imide (PAI-Salt).

[0017] In a third aspect, the present invention pertains to an electrode forming composition comprising :

a) the salified polyamide-imide (PAI-Salt) as above defined, b) an electrode active material,

c) optionally at least one electro-conductive additive, and d) water. [0018] In a further aspect, the present invention relates to a process for preparing a negative electrode comprising:

(i) contacting at least one surface of a metal substrate with the electrode forming composition as above defined,

(ii) drying the electrode-forming composition at a temperature less than or equal to 150°C, and

(iii) compressing the dried electrode-forming composition on the metal substrate to form the negative electrode.

[0019] In a further aspect, the present invention relates to a negative electrode comprising, based on the total weight of the electrode:

0.5 to 15 wt %, preferably 0.5 to 10 wt % of the PAI-Salt binder,

60 to 95 wt %, preferably 70 to 85 wt % of the carbon-based material,

3 to 50 wt %, preferably 10 to 50 wt % of the silicon-based material, and

0 to 10 wt %, preferably 0.5 to 2.5 wt %, more preferably about 1 wt % of the electro-conductive additive.

[0020] In still a further aspect, the present invention relates to an

electrochemical cell comprising the negative electrode as above defined.

Description of embodiments

[0021] In the context of the present invention, the term“weight percent” (wt %) indicates the content of a specific component in a mixture, calculated as the ratio between the weight of the component and the total weight of the mixture. When referred to the recurring units derived from a certain monomer in a polymer/copolymer, weight percent (wt %) indicates the ratio between the weight of the recurring units of such monomer over the total weight of the polymer/copolymer. When referred to the total solid content (TSC) of a liquid composition, weight percent (wt %) indicates the ratio between the weight of all non-volatile ingredients in the liquid.

[0022] By the term "electrochemical cell", it is hereby intended to denote an

electrochemical cell comprising a positive electrode, a negative electrode and a liquid electrolyte, wherein a monolayer or multilayer separator is adhered to at least one surface of one of said electrodes.

[0023] Non-limitative examples of electrochemical cells include, notably, batteries, preferably secondary batteries, and electric double layer capacitors.

[0024] For the purpose of the present invention, by "secondary battery" it is

intended to denote a rechargeable battery. Non-limitative examples of secondary batteries include, notably, alkaline or alkaline-earth secondary batteries.

[0025] The salified polyamide-imide polymer (PAI-Salt)

[0026] In the formulae that follow, the floating amide bond indicates that the amide can be bonded to either of the closest carbons to the floating amide bond on the ring. In other words,

[0027] In some embodiments, recurring units RpAi-a in the PAI-Salt are selected from at least one recurring unit of formula:

[0028] In some embodiments, recurring units RpAi-b in the PAI-Salt are selected from at least one recurring unit of formula:

[0029] In some embodiments, recurring units RPAI-C in the PAI-Salt are selected from at least one recurring unit of formula:

[0030] In some embodiments, the recurring units Rp _Ai -a, Rp _Ai -b, and RPAI-C in the PAI-Salt are respectively units of formulae: [0031] Preferably, recurring units RPAI-C in the PAI-Salt are units of formula:

[0032] In some embodiments, the recurring units RpAi-a, RpAi-b, and RPAI-C in the PAI-Salt are respectively units of formulae:

[0033] In some embodiments, the recurring units RpAi-a, RpAi-b, and

RPAI-C in the PAI-Salt are respectively units of formulae:

[0034] In some embodiments, the PAI-Salt comprises more than one, for example two, of each of recurring units RpAi-a, RpAi-b, and RPAI-C.

Accordingly, in some aspects the PAI-Salt comprises:

a) recurring units RpAi-a of formulae:

and

c) recurring units RPAI-C of formulae:

[0035] In some embodiments, the PAI-Salt includes less than 50 % by moles, preferably less than 49 % by moles, 45 % by moles 40 % by moles 30 % by moles 20 % by moles, 10 % by moles, 5 % by moles, 2 % by moles, 1 % by moles of the Rp _Ai -a recurring units. In some embodiments, the PAI-Salt is free of recurring units RPAI- a.

[0036] In some embodiments, the PAI-Salt includes less than 70 % by moles, preferably less than 60 % by moles, 50 % by moles, 40 % by moles, 30 % by moles, 20 % by moles, 10 % by moles, 5 % by moles, 2 % by moles, 1 % by moles of recurring units Rp _Ai -b.

[0037] Preferably, the PAI-Salt includes at least 30 % by moles, 35 % by moles, 40 % by moles, 45 % by moles, 50 % by moles, 60 % by moles, 70 % by moles, 80 % by moles, 90 % by moles, 95 % by moles, 99 % by moles of recurring units RPAI-C. Most preferably, all of the recurring units in the PAI-Salt are recurring units RPAI-C.

[0038] In some embodiments, the mole ratio Rp _Ai -a / (Rp _Ai -b+ RPAI-C) is 1.0 or less, preferably 0.9, 0.8. 0.7, 0.6, 0.5, 0.4. 0.3, 0.2, 0.1 or less.

[0039] In some embodiments, the mole ratio RPAI-C/ (Rp _Ai -a + Rp _Ai -b) is 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 or more. For example, the mole ratio RPAI-C/ (Rp _Ai -a + Rp _Ai -b) is preferably greater than 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 99.

[0040] In a preferred embodiment, the amount of recurring units RpAi-b ranges from 0 to 50 % by moles, and the amount of recurring units RPAI-C ranges from 50 to 100 % by moles.

[0041] Determination of the relative amounts of recurring units RpAi-b and RPAI-C in the PAI-Salt can be performed by any suitable method. For example the amount of recurring units RpAi-a

(degree of imidization) can be assessed by NMR and the amount of recurring units RpAi-b and RPAI-C can be assessed by NMR, elemental analysis, or titration.

[0042] The PAI-Salt has an acid equivalent greater than 300 grams per equivalent (g/eq) of acid. Preferably, the PAI-Salt has an acid equivalent greater than 325 g/eq, more preferably greater than 350 g/eq, and most preferably at least 375 g/eq or more.

[0043] The PAI-Salt is water soluble. As used herein“water soluble” or“ soluble in water” means that at least 99 wt % of the PAI-Salt, based on the total weight of the PAI-Salt, dissolves in deionized water to form a homogenous solution at 23°C with moderate stirring.

[0044] In some embodiments, the PAI-Salt has a number average

molecular weight (Mn) of at least 1000 g/mol, preferably at least 2000 g/mol, more preferably at least 4000 g/mol. In some embodiments, the PAI-Salt has a number average molecular weight (Mn) of at most 10000 g/mol, preferably at most 8000 g/mol, more preferably at most 6000 g/mol.

[0045] The PAI-Salt used in the present invention can be prepared from the corresponding polyamide-imide (PAI) by neutralizing amic acid groups with a corresponding alkali metal salt in a solvent. [0046] The PAI-Salt used in the present invention can be prepared from the corresponding polyamide-imide (PAI) by neutralizing amic acid groups with a sodium or potassium salt in a solvent.

[0047] As used herein,“polyamide-imide (PAI)” means any polymer comprising:

0 to 50 % by moles of at least one recurring unit RpAi-a, of formula:

(RpAi-a)

of at least one recurring unit RpAi-b of

(RpAi-b), provided that recurring units RpAi-a and RpAi-b collectively represent more than 50 % by moles, preferably at least 60 % by moles, 75 % by moles, 90 % by moles, 95 % by moles, 99 % by moles of recurring units in the PAI, and Ar and R are as defined above.

[0048] Polyamide-imide polymers are available from Solvay Specialty Polymers USA, L.L.C. under the trademark, TORLON ^® PAI.

[0049] PAI can be manufactured according to known methods in the art.

For example, processes for preparing PAI polymers are disclosed in detail in British Patent No. 1 ,056,564, U.S. Pat. No. 3,661 ,832 and U.S. Pat. No. 3,669,937. [0050] PAI can be manufactured by a process including the

polycondensation reaction between at least one acid monomer chosen from trimellitic anhydride and trimellitic anhydride monoacid halides and at least one comonomer chosen from diamines and diisocyanates. In some embodiments, the molar ratio of the at least one acid monomer to the comonomer is 1 :1.

[0051] Among the trimellitic anhydride monoacid halides, trimellitic

anhydride monoacid chloride (TMAC) is preferred:

TMAC

[0052] When polymerized, the acid monomers can exist in either an imide form or an amic acid form.

[0053] The comonomer can comprise one or two aromatic rings.

Preferably, the comonomer is a diamine. More preferably, the diamine is selected from the group consisting of 4,4'- diaminodiphenylmethane (MDA), 4,4'-diaminodiphenylether (ODA), m-phenylenediamine (MPDA), and combinations thereof :

MDA ODA MPDA

[0054] The sodium or potassium salt can be any salt capable of

neutralizing amic acid groups.

[0055] In some embodiments for preparing PAI-Salt, the sodium salt is selected from the group consisting of sodium carbonate, sodium hydroxide, sodium bicarbonate, and combinations thereof.

[0056] In some embodiments for preparing PAI-Salt, the potassium salt is selected from the group consisting of potassium carbonate, potassium hydroxide, potassium bicarbonate [0057] The solvent can be any solvent capable of dissolving the sodium or potassium salt and the resulting PAI-Salt.

[0058] In some embodiments for preparing PAI-Salt the solvent is

preferably selected from at least one of water, NMP, and alcohols, such as, for example, methanol, isopropanol, and ethanol.

[0059] Preferably the solvent is selected from at least one of water,

NMP, and alcohols, such as, for example, methanol, isopropanol, and ethanol.

[0060] Preferably, the solvent includes less than 5 wt %, preferably less than 2 wt %, preferably less than 1 wt % of NMP. More

preferably, the solvent is free of NMP. Most preferably, the solvent is water.

[0061] Preferably the concentration of the salt in the solvent ranges from 0.1 to 30 wt %, preferably from 5 to 30 wt %, more preferably 5 to 15 wt %, based on the total weight of the solvent and the sodium or potassium salt.

[0062] The PAI-Salt used in the present invention are prepared by using the concentration of the sodium or potassium salt in the solvent that allows providing at least 0.75 eq, 1 eq, 1.5 eq, 2 eq, 2.5 eq, 3 eq, 4, eq of sodium or potassium to acid groups.

[0063] The concentration of the sodium or potassium salt in the solvent preferably provides at most 5 eq, preferably at most 4 eq. of sodium or potassium to acid groups.

[0064] The solution of the sodium or potassium salt and the PAI (or PAI- Salt) is preferably heated to a temperature ranging from 50°C to 90°C, preferably from 60°C to 80°C, most preferably from 65°C to 75°C, preferably for a time ranging from few seconds to 6 hours.

[0065] The pH of the PAI-Salt obtained as above detailed is preferably lowered by adding to the reaction mixture after salification at least one source of acid, for example, as a mineral acid or as an organic acid such as acetic acid, formic acid, oxalic acid, benzoic acid, or as an acid generating species, such as a polymer having acidic sites.

[0066] Following salification, the concentration of the PAI-Salt in the solution preferably ranges from 1 to 20 wt %, preferably 5 to 15 wt %, most preferably 5 to 10 wt %, based on the total weight of the PAI-Salt and the solvent.

[0067] In some embodiments, the method further includes heating the solution of the lithium salt and the PAI (or PAI-Salt) to a temperature ranging from 50°C to 90°C, preferably from 60°C to 80°C, most preferably from 65°C to 75°C, preferably for a time ranging from 15 min to 6 hours, preferably 1 to 2 hours.

[0068] In some embodiments, the method further includes lowering the pH of the PAI-Salt obtained as above detailed by adding to the reaction mixture after salification at least one source of acid, for example, as a mineral acid or as an organic acid such as acetic acid, formic acid, oxalic acid, benzoic acid, or as an acid generating species, such as a polymer having acidic sites.

[0069] The PAI-Salt can be isolated as a solid from the salification

solution and optionally stored for later use. The solid PAI-Salt can also be dissolved (or re-dissolved) in water to prepare the aqueous electrode-forming composition described below.

Preferably, however, the solution including the PAI-Salt is an aqueous solution that can be used directly, optionally with further dilution with water, in preparing the electrode-forming

composition as described below.

[0070] Electrode-Forming Composition Including the PAI-Salt

[0071] The PAI-Salt can be desirably incorporated into an aqueous

electrode-forming composition for negative electrodes of lithium- ion secondary batteries. [0072] The electrode-forming composition comprises the PAI-Salt as described above, an electrode active material, optionally at least one electro-conductive additive, and water. The electrode forming composition is preferably in the form of an aqueous dispersion, preferably a homogeneously-dispersed aqueous dispersion.

[0073] The electrode active material comprises a carbon-based material and a silicon-based material.

[0074] In some embodiments, the carbon-based material may be, for example, graphite, such as natural or artificial graphite, graphene, hard carbon or carbon black.

[0075] These materials may be used alone or as a mixture of two or more thereof. The carbon-based material is preferably graphite.

[0076] The silicon-based material may be one or more selected from the group consisting of silicon, alkoxysilane, aminosilane, silicon carbide and silicon oxide. Preferably, the silicon-based material is silicon.

[0077] The optional electro-conductive additive may be selected from carbonaceous materials such as carbon black, graphite fine powder, carbon nanotubes, graphene, or fiber, or fine powder or fibers of metals such as nickel or aluminum. The optional electro- conductive additive is preferably carbon black. Carbon black is available, for example, under the brand names, Super P ^® or Ketjenblack ^® . When present, the electro-conductive additive is different from the carbon-based material described above.

[0078] In some embodiments, the electrode-forming composition (and/or the electrode described below) includes an excess amount of lithium salt ranging from 0.75 to 5 eq., preferably 2 eq. to 5 eq., 2.5 eq. to 5 eq., 3 eq. to 4 eq., to acid groups.

[0079] Further, the electrode-forming composition of the invention can contain at least one thickener; when present, the amount of thickener (also designated as rheology modifier) is not

particularly limited and generally ranges between 0.1 and 10 wt %, preferably between 0.5 and 5 wt %, with respect to the total weight of the composition. The thickener is generally added in order to prevent or slow down the settling of the powdery electrode material from the aqueous composition of the invention, while providing appropriate viscosity of the composition for a casting process.

[0080] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0081] Non-limitative examples of suitable thickeners include, notably, organic thickeners such as partially neutralized poly(acrylic acid) or poly(methacrylic acid), carboxylated alkyl cellulose like carboxylated methyl cellulose and inorganic thickeners such as natural clays like montmorillonite and bentonite, manmade clays like laponite and others like silica and talc.

[0082] Method of Making Negative Electrodes

[0083] The electrode-forming composition can be used in a method for making a negative electrode. The method comprises:

(i) contacting at least one surface of a metal substrate with the electrode forming composition,

(ii) drying the electrode-forming composition at a temperature less than or equal to 150°C, and

(iii) compressing the dried electrode-forming composition on the metal substrate to form the negative electrode.

[0084] In some embodiments, contacting at least one surface of the

metal substrate with the electrode forming composition includes casting, printing, or roll-coating the electrode forming composition on at least one surface of the metal substrate.

[0085] The metal substrate is generally a foil, mesh or net made from a metal, such as copper, aluminum, iron, stainless steel, nickel, titanium or silver.

[0086] The compression step (iii) may include calendering to achieve a target porosity and density for the negative electrode. In some embodiments the dried electrode forming material on the metal substrate is hot pressed at a temperature ranging from 25°C to 130°C, preferably about 60°C.

[0087] The preferred target porosity for the negative electrode ranges from 15% to 40%, preferably from 25% to 30%. The porosity of the negative electrode is calculated as the complementary to unity of the ratio between the measured density and the theoretical density of the electrode, where:

the measured density is given by the mass divided by the volume of a circular portion of the negative electrode having diameter equal to 24 mm and a measured thickness; and

the theoretical density of the negative electrode is calculated as the sum of the product of the densities of the components of the electrode multiplied by their mass ratio in the electrode formulation.

[0088] Negative Electrodes Including the PAI-Salt

[0089] Some embodiments are directed to negative electrodes obtained by the method as described above.

[0090] The negative electrode preferably comprises, based on the total weight of the electrode:

0.5 to 15 wt %, preferably 0.5 to 10 wt % of the PAI-Salt binder,

60 to 95 wt %, preferably 70 to 85 wt % of the carbon-based material, 3 to 50 wt %, preferably 10 to 50 wt % of the silicon-based material, and

0 to 10 wt %, preferably 0.5 to 2.5 wt.%, more preferably about 1 wt % of the electro-conductive additive.

[0091] In some embodiments, the negative electrode includes 0.5 to 5 wt %, preferably 0.5 to 3 wt % of the PAI-Salt, based on the total weight of the electrode.

[0092] In some aspects, the negative electrode includes 20 to 50 wt %, preferably 30 to 50 wt % of the silicon-based material.

[0093] In some embodiments, the negative electrode of the invention is uncured. As used herein“uncured” means that the PAI-Salt (e.g., in the electrode or electrode-forming composition) has not been exposed to a curing temperature of greater than 150°C.

[0094] In alternative embodiments, the negative electrode of the

invention is a cured. As used herein“cured” means that the PAI- Salt (e.g., in the electrode or electrode-forming composition) has been exposed to a curing temperature of greater than 150°C, preferably from 150°C to 300°C.

[0095] The Applicant has surprisingly found that the negative electrodes of the present invention, in particular the uncured negative electrodes, exhibit significantly greater capacity retention than negative electrodes using comparative negative electrode binders.

[0096] Battery Including the Negative Electrode

[0097] The negative electrode of the invention is particularly suitable for use in lithium-ion in secondary batteries, which can be prepared by standard methods known to those of skill in the art.

[0098] Accordingly, an embodiment of the invention is directed to a lithium ion secondary battery comprising the negative electrode of the present invention together with a positive electrode, and electrolyte, and a separator. [0099] Exemplary embodiments will now be described in the following non-limiting examples.

[00100] EXAMPLES

[00101] Raw Materials

[00102] The following raw materials were used in the examples below:

[00103] Torlon ^® PAI 4000T and AI-50 available from Solvay Specialty Polymers USA, LLC;

[00104] T rimellitic acid chloride (TMAC) and oxydianiline (ODA), available from Aldrich;

[00105] N-methylpyrrolidone (NMP) available from VWR International or Sigma Aldrich;

[00106] Sodium carbonate available from Sigma-Aldrich;

[00107] Silicon/carbon, commercially available as BTR 450-B from BTR (a mixture of Si and graphite with a silicon content of about 6- 7%). The theoretical capacity is 450 mAh/g;

[00108] Carbon black, available as SC45 from Imerys S.A;

[00109] Carboxymethylcellulose (CMC), available as MAC 500LC from Nippon Paper;

[00110] Styrene-Butadiene Rubber (SBR) suspension (40 wt.% in water), available as Zeon ^® BM-480B from ZEON Corporation;

[00111] Poly(amic acid) aqueous solution (35% w/w) available from

Sigma Aldrich;

[00112] Ethylene carbonate : dimethyl carbonate = 1 :1 in weight percent, available as Selectilyte™ LP 30 from BASF;

[00113] Fluoroethylene carbonate (F1 EC) available from Sigma Aldrich; and

[00114] Vinylene carbonate available from Sigma Aldrich.

[00115] Example 1 : Preparation of TMAC-ODA (50-50) PAI Copolymer

[00116] ODA monomer (60.0 g, 0.3 moles) was charged into a 4-neck jacketed round-bottom flask fitted with overhead mechanical stirrer. NMP (250 mL) was charged to the flask and the mixture was cooled to 10 ^° C with mild agitation under a nitrogen

atmosphere. The flask was fitted with a heated addition funnel to which TMAC (64.0 g, 0.3 moles) was charged and heated to a minimum of 100 ^° C. The molten TMAC was added to the solution of diamine in NMP at a rate sufficient not to exceed 40 ^° C with vigorous agitation. Once the addition was complete, external heating was applied to maintain 35-40 ^° C for 2 hours. Additional NMP (50 ml_) was added and the reaction mixture discharged into a 500 ml_ beaker. The polymer solution was slowly added to water (4000 ml_) in a stainless steel high-shear mixer. The precipitated polymer was filtered and washed multiple times with water to remove residual solvent and acid by-product.

[00117] Example 2 : TMAC-ODA (50-50) PAI Copolymer - 5 Wt.%

Polymer and 3 eq. Sodium (NaPAI)

[00118] Deionized water (184ml_) was charged to a 4-neck jacketed

round-bottom flask fitted with overhead mechanical stirrer. 5.3 g, 0.050 mol, of sodium carbonate was added and the solution was heated to 70 ^° C. With vigorous agitation, the PAI resin obtained as in example 1 (60.5 g at 20.7 % solids) was added in step-wise fashion, allowing each portion to dissolve prior to further addition. After the entire polymer was charged to the reactor, heating was continued for 1 - 2 hours, at which time the homogenous solution was discharged. The solution was further diluted during electrode slurry preparation to achieve desired binder loading in electrode.

[00119] Preparation of Electrode-Forming Compositions and Negative Electrodes

[00120] Electrode-forming compositions and negative electrodes were prepared as detailed below using the following equipment:

Mechanical mixer : planetary mixer (Speedmixer) and mechanical mixer of the Dispermat ^® series with flat PTFE lightweight dispersion impeller; Film coater/doctor blade : Elcometer ^® 4340 motorised / automatic film applicator;

Vacuum oven : BINDER APT line VD 53 with vacuum; and Roll press : precision 4" hot rolling press/calender up to 100°C.

[00121] Example 3: Preparation of Negative Electrode Including NaPAI resin

[00122] An aqueous composition was prepared by mixing 25 g of a 5 wt % solution of a NaPAI obtained as in Example 2, in water, 1.25 g of deionized water, 23.5 g of silicon/graphite, and 0.25 g of carbon black. The mixture was homogenized by moderate stirring in planetary mixer for 10 min and then mixed again by moderate stirring for 2h.

[00123] A negative electrode was obtained by casting the binder

composition thus obtained on a 20 pm thick copper foil with a doctor blade and drying the coating layer in an oven at temperature of 60°C for about 60 minutes. The thickness of the dried coating layer was about 90 pm. The electrode was then hot pressed at 60°C in a roll press to achieve target porosity (30%). The resulting negative electrode (E1 ) had the following

composition: 94 wt % of silicon/carbon, 5 wt % of NaPAI and 1 wt % of carbon black.

[00124] Comparative Example 1 : Negative Electrode Including Torlon ^® PAI AI50

[00125] Torlon ^® PAI AI50 was first made water-soluble by neutralizing its amic acid groups with butyldiethanolamine. An aqueous composition was prepared by mixing 19.44 g of a 9 wt % solution of the Torlon ^® PAI AI50 in 17.31 g of deionized water, 32.9 g of silicon/carbon, and 0.35 g of carbon black.

[00126] The mixture was homogenized by moderate stirring in a planetary mixer for 10 min and then mixed again by moderate stirring for 2h giving a binder composition. [00127] A negative electrode was obtained by casting the binder composition on a 20 pm thick copper foil with a doctor blade and drying the coating layer in an oven at temperature of 60°C for about 60 minutes. The thickness of the dried coating layer was about 90 pm. The electrode was then hot pressed at 60°C in a roll press to achieve target porosity (30%).

[00128] The negative electrode had the following composition: 94 wt % of silicon/carbon, 5 wt % of the PAI and 1 wt % of carbon black.

[00129] Electrode E2 was thus obtained.

[00130] Comparative Example 2 : Negative Electrode Including Torlon ^® PAI 4000T

[00131] An NMP composition was prepared by mixing 25.00 g of a 5 wt % solution of Torlon ^® PAI 4000T in NMP, 1.25 g of NMP, 23.5 g of silicon/carbon and 0.25 g of carbon black. The mixture was homogenized by moderate stirring in planetary mixer for 10 min and then mixed again by moderate stirring for 2h giving a binder composition.

[00132] A negative electrode was obtained by casting the binder

composition on a 20 pm thick copper foil with a doctor blade and drying the coating layer in an oven at temperature ramp from 80°C to 130°C for about 60 minutes.

[00133] The thickness of the dried coating layer was about 90 pm. The electrode was then hot pressed at 90°C in a roll press to achieve the target porosity (30%).

[00134] The negative electrode had the following composition: 94 wt % of silicon/carbon, 5 wt % of the PAI and 1 wt % of carbon black.

[00135] Electrode E3 was thus obtained.

[00136] Comparative Example 3 : Negative Electrode Including Styrene- Butadiene Rubber (SBR) and Carboxymethyl Cellulose (CMC)

[00137] An aqueous composition was prepared by mixing 29.17 g of a 2 wt % solution of CMC, in 5.25 g of deionized water, 32.9 g of silicon/carbon and 0.35 g of carbon black. The mixture was homogenized by moderate stirring. After about 1 h of mixing, 2.33 g of SBR suspension was added to the composition and mixed again at low stirring for 1 h, to give a binder composition.

[00138] A negative electrode was obtained by casting the binder

composition on a 20 pm thick copper foil with a doctor blade and drying the coating layer in an oven at temperature of 60°C for about 60 minutes. The thickness of the dried coating layer was about 90 pm. The electrode was then hot pressed at 60°C in a roll press to achieve target porosity (30%).

[00139] The negative electrode had the following composition: 94 wt % of silicon/carbon, 1.66 wt % of CMC, 3.33 wt % of SBR, and 1 wt % of carbon black.

[00140] Electrode E4 was thus obtained.

[00141] Comparative Example 4 : Negative Electrode Including Poly(amic acid)

[00142] An aqueous composition was prepared by mixing 5.2 g of a PAA aqueous solution (35% w/w), 30.2 g of deionized water, 34.2 g of silicon/graphite, and 0.36 g of carbon black. The mixture was homogenized by moderate stirring in planetary mixer for 10 min and then mixed again by moderate stirring for 2h to give a binder composition.

[00143] A negative electrode was obtained by casting the binder

composition on a 20 pm thick copper foil with a doctor blade and drying the coating layer in an oven at temperature of 60°C for about 60 minutes. The thickness of the dried coating layer was about 90 pm. The electrode was then hot pressed at 60°C in a roll press to achieve target porosity (30%).

[00144] The negative electrode had the following composition : 94 wt % of silicon/carbon, 5 wt % of PAA, and 1 wt % of carbon black.

[00145] Electrode E5 was thus obtained. [00146] Manufacture of Batteries

[00147] Lithium coin cells (CR2032 type, 20 mm diameter) were prepared in a glove box under an Ar gas atmosphere by punching a small disk of the electrode prepared according to Example 3 and Comparative Examples 1-4 together with lithium metal as a counter and reference electrode. The electrolyte used in the preparation of the coin cells was a standard 1 M LiPF ₆ solution in Selectilyte™ LP 30, with 2 wt % of VC and 10 wt % of F1 EC additive; polyethylene separators (commercially available from Tonen Chemical Corporation) were used as received.

[00148] Capacity Retention Testing

[00149] After initial charge and discharge cycles at a low current rate, each of the two cells was galvanostatically cycled at a constant current rate of C/10 - D/10 with positive cut off of 1.5V and negative cut off of 0.05V.

[00150] Capacities were measured in triplicate and the results are shown in Table 1 below:

Table 1