WO2011008742A1

WO2011008742A1 - Alternative polymers for lithium ion primary and secondary batteries

Info

Publication number: WO2011008742A1
Application number: PCT/US2010/041806
Authority: WO
Inventors: Murali Sethumadhavan
Original assignee: Rogers Corporation
Priority date: 2009-07-14
Filing date: 2010-07-13
Publication date: 2011-01-20

Abstract

A lithium formulation for use in the manufacture of a lithium battery, the formulation including lithium metal powder, a cross-linkable polymer binder, and a solvent.

Description

ALTERNATIVE POLYMERS FOR LITHIUM ION PRIMARY AND SECONDARY

BATTERIES BACKGROUND OF INVENTION

1. Field of the Invention

[0001] The present invention relates to lithium metal batteries. In particular, it is related to lithium metal batteries containing polymers in lithium ion primary and secondary batteries, as well as to processes for making a battery for lithium ion primary and secondary batteries using polymers.

2. Description of the Related Art

[0002] During the last ten years, lithium batteries of the primary and rechargeable type have been the objects of a considerable number of research and development works. The intent was to develop a battery that is safe, inexpensive, having a large energetic content, and good electrochemical performances. In this context, a plurality of a battery designs were developed to meet different applications, such as microelectronics, telecommunications, portable computers and electrical vehicles, to name only a few.

[0003] Lithium primary and secondary batteries are an important component in developing many energy-related applications ranging from vehicles to consumer electronics. The material properties of lithium, however, present challenges in many applications, as well as in the manufacturing process. In particular, lithium foils can be difficult to work with, particularly when ultra thin layers of lithium are desirable. A general need exists to improve the processability of lithium materials for use in batteries for a wide variety of applications. More particularly, improved materials are desired for use in polymer electrolyte batteries and thin film structures.

[0004] Electrochemical batteries or generators, whether rechargeable or not, are all made of an anode which can consist of a metal such as lithium and alloys thereof, or an insertion compound which is reversible towards lithium, such as carbon, a cathode which consists of an insertion compound which is reversible towards lithium such as transitional metal oxide, a mechanical separator and an electrolytic component placed in between the electrodes. The term "electrolytic component" means any material placed inside the battery or generator and which is used as ionic transport, with the exception of electrode materials in which the ions Li+ can be displaced. During the discharge or charge of the generator, the electrolytic component ensures the transport of ionic species through the entire generator from one electrode to the other and even inside the composite electrodes. In lithium batteries, the electrolytic component is generally in the form of a liquid electrolyte or a dry or gel polymer matrix that can also act as mechanical separator.

[0005] When the electrolytic component is in liquid form, it consists of an alkali metal salt that is dissolved in an aprotic solvent. In the case of a lithium generator, the more common salts are LIPF₆, LiBF₄ and LiN(SO₂ CF₃)₂ and the polar aprotic solvents can be selected from propylene carbonate, ethylene carbonate, Y-butyrolactone and 1,3-dioxolane or their analogs to name only a few. At the level of the separator, the liquid electrolyte is generally impregnated in a porous polymer matrix that is inert towards the aprotic solvent used, or in a fiberglass paper. The use of a liquid electrolyte impregnated in an inert polymer matrix enables a sufficient ionic mobility to reach a level of conductivity of the order of 10-3 S/cm at 25⁰C. At the level of the composite electrodes, when the latter are made of an insertion material that is bound by a polymer matrix that is inert towards aprotic solvents, which have only little interaction with the latter, the liquid electrolyte fills the porosity of the electrode. Examples of batteries utilizing a liquid electrolytic component are found U.S. Pat. Nos. 5,422,203; 5,626,985 and 5,630,993.

[0006] When the electrolytic component is in the form of a dry polymer matrix, it consists of a high molecular weight homo- or copolymer, which is cross -linkable or non cross-linkable and includes a heteroatom in its repeating unit such as oxygen or nitrogen for example, in which an alkali metal salt is dissolved such as LiN(SO₂ CF₃)₂, LiSO₃ CF₃ and LiClO₄.

[0007] Polyethylene oxide is a good example of a polymer matrix that is capable of solubilizing different alkali metal salts. Armand, in U.S. Pat. No. 4,303,748 describes families of polymers that can be used as electrolytic component in lithium batteries. Other families of polymers (cross -linkable or non cross -linkable copolymers and terpolymers) are described in U.S. Pat. Nos. 4,578,326; 4,357,401; 4,579,793; 4,758,483 and in Canadian Patent No. 1,269,702. The use of a high molecular weight polymer enables electrolytes in the form of thin films (of the order of 10 to 100 micrometers) which have sufficiently good mechanical properties to be used entirely as separator between the anode and the cathode while ensuring ionic transport between the electrodes In the composite, the solid electrolyte serves as binder for the materials of the electrode and ensures ionic transport through the composite. The use of a cross -linkable polymer enables a polymer of lower molecular weight, which facilitates the preparation of the separator as well as the composite and also enables to increase the mechanical properties of the separator and, by the same token, to increase its resistance against the growth of dendrites when using a metallic lithium anode. As is well known in the art, repeated charge/discharge cycles can cause growth of dendrites on the lithium metal electrode. These dendrites can grow to such an extent that they penetrate the separator between positive and negative electrodes and create an internal short circuit. For this reason, metallic lithium anodes are used exclusively with solid polymer electrolyte separators sufficiently resistant and opaque to prevent dendrite growth from piercing its layer and reaching the positive electrode. In contrast to a liquid electrolyte, a solid polymer electrolyte is safer because it cannot spill nor be evaporated from the generator. Such electrolytes can have a lower ionic mobility obtained in these solid electrolytes, which restricts their uses at temperatures between 40⁰C and 100⁰C.

[0008] The gel electrolytic component is itself generally constituted of a polymer matrix that is solvating or non-solvating for lithium salts, aprotic solvent, and an alkali metal salt being impregnated in the polymer matrix. The most common salts are LiPF₆, LiBF₄ and LiN(SO₂ CF₃)₂ and the polar aprotic solvents can be selected from propylene carbonate, ethylene carbonate, butyrolactones and 1,3-dioxolane, to name only a few. The gels can be obtained from a high molecular weight homo or copolymer which is cross-linkable or non cross -linkable or from a cross -linkable homo or copolymer. In the latter case, the

dimensional stability of the gel is ensured by cross-linking the polymer matrix. Polyethers including cross -linkable functions such as alkyls, acrylates, or methacrylates are good examples of polymers that can be used in formulating a gel electrolyte, such as described in U.S. Pat. No. 4,830,939. This is explained by their capacity to solvate lithium salts and their compatibility with polar aprotic solvents as well as their low cost, and ease of handling and cross-linking. A gel electrolyte has the advantage of being handled as a solid and of not spilling out of the generator as is the case with liquid electrolyte generators. Ionic transport efficiency is associated with the proportion of aprotic solvent incorporated in the polymer matrix. Depending on the nature of the polymer matrix, the salt, the plasticizing agent, and its proportion in the matrix, a gel can reach an ionic conductivity of the order of 10-3 S/cm at 25°C while remaining macroscopically solid. As in the case of a dry electrolyte, a gel electrolyte can be used as separator between the anode and cathode while ensuring ionic transport between the electrodes. In the composite electrode(s) of the generator, the gel electrolyte is used as binder for the materials of the electrode(s), and ensures ionic transport through the composite electrode(s). However, the loss of mechanical property resulting from the addition of the liquid phase (aprotic solvent) should generally be compensated by the addition of solid fillers, by cross-linking the polymer matrix whenever possible, or in some cases, when the proportion of liquid is too high, by using a porous mechanical separator which is impregnated with the gel which serves as electrolytic component in the separator.

[0009] The low resistance of polyethers towards oxidation is a factor when the voltage in recharge can reach and even exceed 3.5V to 3.7 V. This results in a loss of capacity of the generator, caused by degradation of the polymer matrix during consecutive cycles of discharge/charge.

[0010] Gustafson et al. (U.S. Pat. No. 5,888,672) (the '672 patent) disclose a battery where the anode, the cathode, and the electrolyte each comprise a soluble, amorphous, thermoplastic polyimide. Since the polyimides are pre-imidized prior to the fabrication of the battery, there is no need to further cure them at high temperatures, thus reducing the risk of damaging the battery. The polyimide based electrolyte is resistance to oxidation and capable of high ionic conductivity at or near room temperature. Nor is there a chance of incidental condensation as the battery temperature rises. In addition, since no further polymerization occurs, there are no by-products of the condensation reaction (water) to interact with the lithium salts. The battery of Gustafson et al. is said to be a dry cell.

[0011] In fabricating the battery in the '672 patent, an electrolyte solution comprising a soluble, amorphous, thermoplastic polyimide solution, and a lithium salt is prepared. The thermoplastic polyimide solution is prepared by mixing about 8% to about 20% by weight of a thermoplastic polyimide powder with about 80% to about 92% by weight of a solvent. About 20% to about 35% by weight of a lithium salt is dissolved in about 65% to about 80% by weight of a solvent to form a solution. The solution is then mixed with the thermoplastic polyimide solution to form the electrolyte solution. The electrolyte comprises from about 2% by weight to 10% by weight of soluble, amorphous, thermoplastic polyimide, from about 1% by weight to 12% by weight of the lithium salt and from about 78% by weight to 97% by weight of the solvent. An electrolyte layer is then formed by casting a film of the electrolyte solution which is fully dried in an oven at about 150⁰C for about 30 to 60 minutes to create a dry, opaque, flexible, smooth, tough film. The polyimide based electrolyte solution is dried at the flash point of the solvent for the purpose of removing the solvent such that a dry electrolyte is obtained.

[0012] Gustafson et al. (U.S. Pat. No. 6,451,480 issued Sep. 17, 2002) later disclosed a polyimide-based lithium-ion battery in which the anode and the cathode are prepared from an electrolyte polyimide binder solution comprising from about 9% by weight to about 15% by weight of a pre-imidized soluble, amorphous, thermoplastic polyimide powder dissolved in about 75% by weight to about 85% by weight of a polar solvent; and from about 6% by weight to about 12% by weight of a lithium salt and an electrolyte separator consisting of a typical separator film saturated with a liquid electrolyte solution of lithium salts dissolved in a variety of organic solvents such as ethylene carbonate mixed with dimethyl carbonate. A cell stack is assembled and placed in a container, which is then filled with the electrolyte and sealed.

[0013] Vallee et al. (U.S. Pat, No. 7,390,336 issued June 24, 2008) later disclosed polyimide-based electrolytic component having an electrolyte consisting of at least one solvent and at least one alkali metal salt, with specific amounts of solvents, to optimize the properties of conductivity of the polyimide-based electrolyte and the mechanical properties of the polyimide-based electrolyte separator towards metallic lithium anode to prevent dendrites growth.

[0014] While suitable for their intended purposes, such high temperature polyimides as described in Vallee and Gustafson can be expensive, sometimes requiring custom synthesis. The polymers can have limited solubility in solvents commonly used in screen printing, roll coating, rotary printing, flexographic printing, offset litho printing, and the like. There accordingly remains a perceived need for a polymers for use in lithium ion batteries that are cost effective and/or soluble in solvents commonly used in screen printing, roll coating, rotary printing, flexographic printing, offset litho printing, and the like. It would be a further advantage if the polymers could be formulated to optimum viscosity for both coating and screen-printing.

BRIEF SUMMARY OF THE INVENTION

[0015] The invention generally relates to lithium metal batteries containing polymers, in which the polymer is a low temperature cross-linkable polymer or a high temperature soluble polymer. Exemplary polymers are a polyamide, polyphenylene oxide, polyarylate, polyamide-imide, polyester-imide, polyester-amide-imide, polybenzimidazole,

polybenzoxazoles, polysulfone, polyether sulfone, polysulfonamide, poly(quinoxaline), poly(para-phenylene), poly(arylene ether), including one substituted with a pyridyl group, poly(aryl ether sulfone), polyepoxide or a combination thereof. The crosslinkable polymer can further be a polyimide. The crosslinkable polymers are be functionalized or modified to make them crosslinkable. The foregoing polymers can also be functionalized for use in lithium metal batteries, as is know to those of ordinary skill in the art. Blends of polymers can be used including blends comprising the polymer binder and also a different type of polymer to the extent the application allows. [0016] The polymers can be used in the formulation of inks, for example in screen- printing, and in as coatings, for example to form films.

[0017] Also disclosed are processes for making a battery for lithium ion primary and secondary batteries using the polymers mentioned above, and the batteries made using the polymers.

DETAILED DESCRIPTION OF THE INVENTION

[0018] Described herein are cross -linkable low temperature polymers and soluble high-temperature polymer for use as a polymeric binder in lithium ion primary and secondary batteries. The polymers can be low temperature (i.e., having a Tg of less than 15O⁰C), cross- linkable or soluble, high temperature (i.e., having a Tg of greater than 15O⁰C) polyamides, polyphenylene oxides, polyarylates, polyamide-imides (PAI), polyester-imides, polyester- amide-imides, polybenzimidazoles (PBIs) and poly(benzoxazoles). Polysulfones, polyether sulfones, polysulfonamides, poly(quinoxaline) (PPQ), poly(para-phenylenes), poly(arylene ethers) (PAE) substituted with a pyridyl group, poly(aryl ether sulfones) and polyepoxides. In some cases the polymers binders can be functionalized or modified to make them

crosslinkable and/or suitable for use in lithium metal batteries. These polymers will lend themselves to optimum viscosity for both coating and screen-printing due to its better solubility. The polymers can also be used for supercapacitors and other applications

[0019] Lithium batteries (both primary and secondary) contain three main components - an anode, a cathode, and separator. The cathode contains a current collector and then a coating. The coating is made from a slurry containing a polymeric binder, lithium salt, and solvent. The anode can be either lithium foil or a coating. This coating is made from a slurry containing polymeric binder, lithium metal or salt and solvent. The separator contains a polymeric binder, lithium salt, and solvent. The polymeric binder used in all the three layers is an important component of the battery and helps the battery with transportation of lithium ions, temperature stability of the battery and protection against shorts. The polymeric binder also helps during the processing. In order to do this function, the polymeric binder should have high temperature stability and soluble in solvents which can be used for coating or printing.

[0020] Low temperature cross-linkable polymers include those described above and can be, for example, polyimides, which can be the copolymers described in U.S. Pat. Nos. 6,855,433 and 6,881,820, and the branched the rod-coil polyimide-PEO copolymers that are disclosed in NASA Tech Briefs, LEW- 18205-1, "Room-Temperature-Cured Copolymers for Lithium Batter Gel Electrolytes, the entire contents of which is hereby incorporated by reference. Branched rod-coil polyimide-PEO copolymers can be cured in the film state at ambient temperatures. On curing, a gel network is formed that is capable of conducting lithium ions within lithium ion batteries. Conductivity comes from the PEO portion, while the imide segments and branching provide dimensional stability. Furthermore, this network is able to hold large amounts of liquid (greater than 400% by wt.), while maintaining good dimensional stability. These liquid additives (such as ionic liquids and carbonate solvents) can significantly aid in the conduction of lithium ions, and in some circumstances increase the battery cycles life.

[0021] The branched rod-coil polyimide-PEO polymer is made in three distinct steps. First, a dianhydride is reacted with a PEO oligomer that is terminated with primary aliphatic amines on both ends to make the polyamide acid prepolymer. The stoichiometry of the two chemicals is adjusted so a linear polymer is made that is endcapped with an amine on both ends. The solvent is selected to solubilize the polymer, have a boiling point preferably between 150 and 200 ⁰C, and be inert to lithium metal and other battery components. In the second step the polyamide-acid is imidized in solution. The water that is generated from this reaction step is removed by azeotropic distillation. In the third step, the appropriate additives are dissolved in the polymer solution (such as Li salt and carbonate solvent). A trifunctional molecule is then added that reacts with the polymers amine endcaps at ambient temperatures to form a gel. Gelation time and film properties can be adjusted by changing the length of the polymer chains. Film properties can also be adjusted by altering the dianhydride, PEO chain length, and partially replacing the trifunctional molecule with a difunctional molecule. The film can either be packaged once gelation occurs, or the reaction solvent can be allowed to evaporate. The reaction solvent has a boiling point that is about 100 ⁰C lower than cyclic carbonate solvents (like ethylene carbonate and propylene carbonate), and can be

preferentially evaporated. However, the reaction solvent can be inert toward all battery components and does not need to be removed.

[0022] The resulting branched rod-coil polyimide-PEO that is made that is easy to fabricate with good dimensional stability. The synthesis is very versatile. Gelation time can be varied from minutes to hours. Polymer consistency can be varied from highly rigid to highly tacky (but strong and stretchy). The imide segments still provide mechanical strength, phase separation, and a completely amorphous polymer in the presence of lithium salt. This new synthetic route adds the dimension of low-temperature curing and gelation, and therefore the ability to add organic solvents to increase conductivity.

[0023] Other suitable polymers include crosslinkable low temperature polymers and soluble high temperature polyimides, polyamides, polyphenylene oxides, polyarylates, polyamide-imides (PAI), polyester-imides, polyester-amide-imides, polybenzimidazoles (PBIs) and poly(benzoxazoles). Polysulfones, polyether sulfones, polysulfonamides, poly(quinoxaline) (PPQ), poly(para-phenylenes), poly(aryl ethers) (PAE) substituted with a pyridyl group, poly(aryl ether sulfones) and polyepoxides can also be used. When these polymers are in the form of the low temperature crosslinkable oligomers or polymers, a crosslinkable functional group is present. Blends of polymers can be used including blends comprising the polymer binder and also a different type of polymer to the extent the application allows. Polymers, including high temperature polymers, having functional groups capable of complexing with lithium salts and participating in ionic conduction are particularly preferred.

[0024] In addition to polyimides, other generally glassy amorphous polymers can be used including, for example, high glass transition temperature polymers (e.g., Tg greater than 150⁰C; Tg greater than 180⁰C; Tg greater than 200⁰C; Tg greater than 250⁰C; Tg greater than 350⁰C). In general, the polymer can have polar groups which can complex with lithium salts and participate in ionic conduction. In some instances it is desirable to employ polymers having a Tg in the lower Tg range because they are easier to process. Alternatively, low TG glassy amorphous polymers can be used, for example 100 to 15O⁰C.

[0025] The alkali metal salt(s) can be for example salts based on lithium

trifluorosulfonimide described in U.S. Pat. No. 4,505,997, LIPF₆, LiBF₄, LiSO₃CF₃, LiClO₄, and LiSCN, etc. The nature of the salt is not a limitation of the present invention.

[0026] The solvent(s) can for example be selected from N,N-methylpyrolidinone (NMP), Y-butyrolactone, and sulfamides of formula; RiR₂N-SO₂-NR₃R₄, in which R₁, R₂, R₃ and R₄ are alkyls having between 1 and 6 carbon atoms and/or oxyalkyls having between 1 and 6 carbon atoms or combinations thereof. Preferably the solvent or combination of solvents is (are) polar aprotic solvent(s). The nature of the solvent is not a limitation of the present invention.

[0027] Additional solvents can be typically anhydrous, aprotic polar organic solvents which are sufficiently chemically stable toward lithium metal, and capable of solvating, the lithium powders, polymeric binders and any conductive materials or lithium salts. Suitable polar solvents include, but are not limited to, gamma-butyrolactone (GBL), tetrahydrofuran (THF) and propylene carbonate (PC). GBL is particularly well suited for printing

applications. Other suitable solvents include dioxane, 1,3-dioxolane, 1,2-dimethyloxyethane and dimethylsulfoxide. [0028] The active material of the cathode can be selected from cobalt oxide, nickel oxide, nickel cobalt oxide, nickel cobalt aluminum oxide, manganese oxide (LiMn₂O₄) or their analogs for so-called 4 V cathodes or among cathodes of less hand 4 V such as phosphates or other polyanions of transition metals such as LiFePO₄, Nasicon structures also including V₂O₅, LiV₃Og and MnO₂. The nature of the active material is not a limitation of the present invention.

[0029] The soluble polymers used in the present invention can be in powder in form. In order to produce a film, coating or a slurry from the polymer, the powder must first be dissolved in a solvent such as N,N-methylpyrolidinone (NMP) and gamma-butyrolactone to name a few in order to form a solution. Note that polyimides dissolve in these solvents. In addition, large amounts of lithium salts can be dissolved in these polyimide solutions without disturbing the polymer matrix. The polyimide solution is then partially dried at a temperature suitable to evaporate excess solvent in order to obtain a polyimide solution to form a polyimide-based electrolyte.

[0030] Each cathode layer can comprise an electrochemically active material such as a transitional metal oxide (LiCoO₂; LiMnO₂; LiNiO₂; LiV₃Os; Li₄TIsOi₂; V₆Oi₃; V₂O₅; and LiMn₂O₄ and their equivalents); an electronic conductive filler such as conductive carbon, carbon black, graphite, and graphite fiber; and an ionically conductive electrolyte polymer binder. The ionically conductive electrolyte polymer binder can also comprise a lithium salt and comprises either a polyether based mono, ter- or co-polymer or a pre-imidized soluble, polyimide powder. The lithium salt and the polymer are soluble in any polar solvent known to those of ordinary skill in the art.

[0031] The cathode layer and the polymer based electrolyte separator can also be coated or otherwise applied onto polypropylene support films separately and then laminated together as is well known in the art.

[0032] When preparing the polymer-based electrolyte, a solution is first prepared with an excess of solvent to ensure proper mixing of the polymer powder and the lithium salt. Next, the solution can be either dried to obtain a specific content of solvent (20-40%) prior to coating or laminating onto the cathode layers of the cell or the polymer based electrolyte can be coated or laminated with its excess solvent and thereafter dried to obtain a specific solvent content such as 20-40% by weight.

[0033] In an alternative embodiment of the invention, the polymer powder and a lithium salt are first dissolved in the solvent to form a polymer solution. To the polymer solution is added a cross linkable co-monomer and optionally a cross-linking initiator. The polymer solution is then either partially dried prior to assembly or assembled with excess solvent as described above. Once layered onto each cathode layer, cross-linking of the polymer electrolyte is carried out thermally, by UV radiation or with electron beam (EB). The cross-linked polymer electrolyte has improved mechanical resistance over the non-cross- linked electrolyte.

[0034] In fabricating a battery, a lithium metal-based anode, a separator film, and a cathode are assembled in alternate layers to form a cell stack. The separator film needs to be positioned between the anode and the cathode layers to prevent shorting in the cell. The cell can be mono-faced or bi-face and can be stacked in prismatic, folded, wound, cylindrical, or jelly rolled configuration as is well known to those skilled in the art. Once the cells stack is formed, pressure is preferably applied to the cells stack and maintained. The pressurized cells stack is placed in a container wherein the pressure on the stack is maintained. The cells stack must be assembled using pressure to improve interlayer conductivity. The pressure is maintained when the cells stack in placed into a battery container.

[0035] In a further alternative embodiment of the invention, a separator film can be used as a barrier between each anode and cathode layer. The separator film is a freestanding film comprised of an organic polymer, such as polypropylene. Examples of such films include but are not limited to Kynar FLEX from Atochem North America; and CELGARD 3401 from Polyplastics Co., Ltd. The freestanding separator film is either partially soaked with a solution of the polymer, such as the polyimide based electrolyte, with the specific solvent content within the preferred range or the freestanding separator film is saturated with a solution of polyimide based electrolyte having excess solvent and partially dried to obtain the desired% age by weight of solvent.

[0036] The manufacture of the battery is completed after the cell is placed in the package, as described earlier. At this point the battery can be charged to store an electric charge and it is then ready for use.

[0037] In an alternative embodiment, coating compositions, lithium metal powder based inks, anodes made from the inks, and primary and secondary lithium metal batteries made from the coatings, inks, and anodes include a lithium metal powder, a solvent and a binder, and optionally an electronically conductive material and/or a lithium salt. In most battery applications, the electronically conductive material is present. In particular, this embodiment provides formulation, in particular a coating or lithium ink formulation for use in printing an electrode for a lithium battery, the coating or ink formulation comprising lithium metal powder, a polymer binder, and a solvent. In general, compositions are formulated to function as sophisticated ink formulations rather than mere slurry compositions. For example, the ink spreading properties and viscosity can be carefully controlled, together with particle shape, particle size distribution, and average particle size.

[0038] The coatings and inks can be formulated to be formed or printed onto substrates using a variety of printing techniques, including screen printing, offset litho printing, gravure printing, flexographic printing, pad printing and ink jet printing. As such, the formulations provide significant advantages over lithium powder compositions formulated for application by casting or spin coating. For example, printing techniques, such as offset litho, gravure, flexographic, pad printing and ink jet techniques, allow for the high speed, high volume production of printed substrates (e.g. anodes) that is not possible with casting techniques. In addition, printing allows the inks to be applied in desired shapes and locations, which eliminates the need to punch out appropriately shaped sections from a larger film of cast ink, reducing waste. Printing also makes it possible to form very thin layers of ink on a broad range of substrates, including flexible polymeric substrates and metal substrates. In some cases, layers of ink having an average thickness of no more than about 30 micrometers, or even less, can be produced. This represents a substantial improvement over cast films of lithium powder compositions which typically have an average film thickness of 50 micrometers or greater.

[0039] The relative amounts of the various components that make up the formulations can vary depending a variety of factors, including the viscosity requirements of the chosen printing method and conductivity requirements of the intended application. The viscosity of the formulations can be tailored to a chosen printing technique by changing the nature and amount of binder and/or solvent in the formulation, while the conductivity of the

formulations can be tailored for a selected application by changing the nature and amount of conductive materials and the size of the conductive particles in the formulations. In some embodiments, when the inks are printed onto a substrate, for example using a screen-printing process, they provide an ink layer containing from about 20 to 50% lithium powder and from about 10 to 30% polymer binder on a dry weight basis. This includes embodiments where the lithium powder based ink includes from about 30 to 45% lithium powder, from about 15 to 20% polymer binder, from about 20 to 30% conductive material and from about 15 to 20% lithium salt on a dry weight basis. However, the lithium powder based ink formulations provided herein are not limited to those having components present in amounts falling within these ranges. [0040] The lithium metal powder used to form the formulations can be a finely divided lithium metal powder desirably having an average particle size of 10 nanometers (nm) to 1,000 micrometers, e.g., no greater than about 100 micrometers, no greater than about 50 micrometers, no greater than about 20 micrometers, or no greater than about 1

micrometer. An exemplary average particle size is 1 to 100 micrometers, or 10 nm to 1,000 nm. Because the printing processes are capable of applying very thin layers of ink, in some cases the lithium powder size can be the limiting factor in minimizing the average layer thickness.

[0041] Binders can be included in the formulations to adjust the viscosity of the formulations and to facilitate the adherence of the lithium metal powder particles to a substrate, such as a metal anode current collector. Although a variety of polymer binders can be employed, it has been discovered that polymer binder binders, such as polyimide polymer binders are well suited for use with the inks provided herein. The polyimides can be pre- imidized polyimides and are desirably provided as amorphous, thermoplastic polyimide powders soluble in a polar solvent, such as y-butyrolactone. In some ink formulations, one or more polyimides having a repeat unit weight per imide ring of no more than 350 are employed.

[0042] The solvents used in the lithium powder based inks should be capable of dissolving the polymer binder and should be sufficiently non-reactive with the lithium powders for the application. In addition they should be volatile enough to allow the inks to dry in a reasonable time frame after printing, yet not so volatile that they evaporate prematurely, clogging printer parts, such as ink jet heads. In some instances the solvent can be a solvent mixture. Not all solvents that are typically used in conducting ink formulations are well suited for use as printing solvents for lithium based inks. Generally, suitable solvents will be anhydrous aprotic organic solvents. The inventors have discovered, for example, that gamma butyrolactone (GBL) is particularly well suited for use in lithium powder based inks designed for printing applications, particularly when combined with a polyimide binder.

[0043] A conductive material can also be included in the formations to enhance their conductivity and to ensure good electrical contact between the parts of electronics (e.g.

batteries) into which they are incorporated. Typically these materials will take the form of a conductive powder. Examples of conductive materials that can be added to the formulations include carbonaceous materials, such as carbon powders and/or carbon nano-tubes. In some formulations, the amount of conductive materials can be greater than the amount of lithium powder.

[0044] Lithium salts for use in the formulations include those lithium salts that are commonly used in lithium metal and lithium ion batteries. Many such salts are well known and commercially available. LiPF₆, LiBF₄, lithium perfluoro sulfonate salts, and lithium bis(trifluoromethanesulfonyl)imide (LiTFSi) are examples of a suitable lithium salts.

[0045] Anodes made from the lithium metal powder based formulations include an anode current collector and an electrode layer disposed on the anode current collector. Also provided from the invention is an anode comprising: (a) an anode current collector; and (b) an electrode layer disposed on the anode current collector, the electrode layer comprising lithium powder, and a polymer binder. The electrode layer can be formed from the lithium powder based inks and includes lithium metal powder a polymeric binder and optionally, at least one conductive material and/or at least one lithium salt. The anode can be produced by printing a lithium metal powder based ink onto a current collector, such as a metal foil.

However, more conventional application techniques, such as casting can also be employed, although such techniques generally less efficient and more time consuming.

[0046] Primary and secondary batteries that incorporate the anodes described herein include at least one anode, at least one cathode, and an electrolyte in electrochemical communication with each anode and each cathode. The batteries are characterized by high specific energy densities, high unit cell voltages, high specific capacities, long lifetimes, and improved electrochemical stability. In some embodiments multiple parts, including anodes, cathodes, and electrode layers can be produced by printing. Also provided in this invention is a lithium metal battery comprising: (a) a cathode; (b) an anode comprising a printed electrode layer comprising lithium metal powder and a polymer binder; and (c) a polymer electrolyte sandwiched between the cathode and the anode.

[0047] The electrolyte can be a solid polymer electrolyte, a polymer matrix electrolyte (PME), or a liquid electrolyte generally containing a lithium salt. A battery based on a liquid electrolyte will typically include a separator film disposed between each anode and each cathode with the liquid electrolyte distributed between each anode, each cathode, and each separator film. The liquid electrolyte desirably includes a lithium salt, such as LiPF₆, in an organic solvent and the separator films are typically porous organic polymer films saturated with the liquid electrolyte. Polymer electrolytes are generally gel type electrolytes that trap solvent and salt in the pores of the polymer to provide a medium for ionic conduction. [0048] Polymer matrix electrolytes provide an alternative to more conventional liquid and solid polymer electrolytes. Polymer matrix electrolytes differ from solid polymer electrolytes in that once the polymer matrix electrolyte is formed, the electrolyte is substantially free of non-absorbed solvent or identifiable pores. Instead, the solvent is integrated with the polymer and the lithium salt in a homogeneous and substantially optically clear matrix. In addition, unlike conventional gel polymers where the polymer only provides mechanical support, the polymer, salt, and solvent that make up the PME all participate in ionic conduction. In one exemplary battery construction, the electrolyte is a PME made from a polyimide, at least one lithium salt in a concentration of at least, 0.5 moles of lithium per mole of imide ring provided by said polyimide and at least one solvent intermixed with the polyimide and the lithium salt to provide a polyimide matrix electrolyte which is substantially optically clear.

[0049] Also provided are electronic devices, such as radiofrequency identification devices, Smart Cards, Time Temperature Indicators (TTI) and wearable external medical devices, powered by the batteries provided herein.

[0050] In another embodiment, provided is a lithium salt-based ink for use in printing an anode for a lithium ion battery, the ink comprising an intercalation carbon material, a conductivity enhancing carbon material, a lithium salt, a polymer binder, and a solvent.

[0051] Application techniques for the formulations include both conventional coating techniques and printing techniques. For example, the formulations can be coated onto a surface, such as a current collector by vapor deposition, dip coating, spin coating and brush coating. However, the formulations are preferably formulated to be printed onto a surface. Application of the lithium inks using printing methodologies is particularly advantageous because such methodologies allow for high speed, high volume application of the lithium inks to underlying substrates, such as metal current collectors. This is particularly advantageous in the manufacture of lithium metal batteries because it is simpler and faster than more conventional production techniques where lithium metal foils are laminated to anode current collectors. Moreover, printing the inks eliminates the need for more complicated and expensive coating techniques, such as vapor deposition. In addition, by printing the inks onto a surface, such as a current collector, better interfaces can be produced and the need to spray a lithium composition is eliminated, reducing waste and environmental concerns. Many printing application also make it possible to deposit very thin layers of the inks. This reduces cost and makes thinner electronic components possible. For example, depending upon the particle size of the lithium powder used to formulate the inks, layers of lithium ink having an average layer thickness of no more than about 40 micrometers can be printed. This includes layers of ink having an average layer thickness of not more than about 30 micrometers, further includes layers of ink having an average layer thickness of not more than about 20 micrometers and still further includes layers of ink having an average layer thickness of not more than about 10 micrometers.

[0052] The inks can be printed onto a substrate using a variety of printing methods, including screen printing, offset litho printing, gravure printing, flexographic printing, pad printing and ink-jet printing.

[0053] Screen printing uses a screen coated with a light sensitive emulsion or film, which blocks the holes in the screen. An image to be printed is supplied to the film. The imaged screen is then exposed to ultra-violet light followed by the washing away of any light sensitive emulsion that has not been hardened by the ultra-violet light. This leaves an open stencil that corresponds to the image that was supplied on the film. The screen is fitted on a press and the substrate to be printed is placed in position under the screen while ink is placed on top of the screen. A blade is pulled across the top of the screen, pushing the ink through the mesh onto the substrate to be printed. The lithium ink formulations for screen-printing desirably have viscosities of about 0.5 to 50 Pa-sec and more desirably 1 to 30 Pa-sec at 25⁰C. Using a screen printing process, a layer of lithium powder based ink having an average thickness of no more than about 30 micrometers and desirably no more than about 15 micrometers can be printed onto a substrate, provided the ink is prepared from a powder having a sufficiently small particle size.

[0054] In offset litho printing, generally, images are supplied to printing plates, which are dampened by ink that adheres to the image area. The image is then transferred to a rubber blanket, and from the rubber blanket to a substrate. Typical viscosities for lithium powder based inks for use in offset litho printing are from about 10 to about 100 Pa-sec at 2O⁰C, desirably from about 30 to 80 Pa-sec at 2O⁰C. Offset litho printing is capable of providing very thin layers of ink. In fact the layer thickness of the printed ink can be limited by the particle size of the lithium powder. For example, if lithium powder having small enough particle sizes is employed, lithium powder based inks can be applied in layers having an average thickness of from about 0.5 to 2 micrometers.

[0055] Gravure printing uses, generally, a cylinder onto which an image has been engraved with cells. To print, the cells are filled with ink and the substrate to be printed is passed between the printing cylinder and an impression roller. The lithium powder based inks used with gravure printing typically have viscosities much lower than those used with screen or offset litho printing. For example, some gravure printing inks will have a viscosity from about 0.01 to 0.5 Pa-sec at 25°C, desirably from about 0.05 to 0.2 Pa-sec at 25°C. Like offset litho printing, gravure printing is capable of printing very thin layers of lithium ink. For example, depending on the size of the lithium powders used, gravure printing can be used to print lithium ink in layers having an average thickness of about 0.8 to 8 micrometers.

[0056] In flexographic printing, a screened roller is generally used to apply a thin layer of relatively fluid ink to the surface of a flexible printing plate. A rotating plate roller is then used to bring the inked surface of the printing plate into contact with a web to be printed and an impression roller presses the web against the plate to effect the transfer of the ink. The lithium ink formulations for flexographic printing have viscosities similar to those used for gravure printing, desirably from about 0.01 to 0.7 Pa-sec and more desirably 0.05 to 0.5 Pa-sec at 25°C. Depending on the size of the lithium powders used to formulate the inks, flexographic printing can be used to produce ink layers with an average thickness from about 0.5 to 5 micrometers, desirably from about 0.8 to 2.5 micrometers.

[0057] In pad printing an image to be transferred generally is etched into a printing plate known as a cliche. The cliche is initially flooded with ink and then cleaned to leave ink only in the image area. A silicon transfer pad is then positioned over the cliche and pressed onto it to transfer the ink. The pad is lifted away from the cliche and the ink is allowed to partially dry, rendering it tackier and more viscous. The pad is then pressed onto a substrate where the ink is deposited. Lithium metal powder based ink formulations for pad printing will have viscosities similar to those of the screen printing ink formulations.

[0058] InkJet printers generally use a series of nozzles to spray drops of ink directly on the paper. InkJet printers are typically used with inks having viscosities of about 0.001 to 0.05 Pa-sec at 25°C and can produce ink layers having an average thickness of about 0.1 to 30 micrometers, desirably 0.3 to 20 micrometers.

[0059] After printing, the crosslinkable polymers are crosslinked, for example by the application of radiation or heat, depending on the crosslinkable groups in the polymers. Initiators, curing agents, and crosslinking agents can also be present in the formulations in order to adjust the rate and ° of crosslinking.

[0060] The lithium powder based inks can be provided in a number of different formulations to be used with different printing processes for different applications. Two of the primary considerations when choosing a formulation are viscosity and conductivity. As noted above, the viscosity requirements can vary dramatically based on the printing technique to be used. Viscosity will depend, at least in part, on the amount of solvent and polymer binder present in the composition, and, in particular, on the ratio of the total amount of lithium powder and conductive material to binder in the composition. Another consideration that can be taken together with viscosity considerations is the conductivity requirements for a given application. Exemplary viscosities are from about 0.01 to about 0.5 Pa-sec at 25⁰C, about 0.5 to 50 Pa-sec at 25°C, or about 50 to 100 Pa-sec at 25°C, depending on the coating method.

[0061] In some embodiments, the ratio of (lithium powder + any conductive material + any lithium salt): polymer binder in the lithium powder based inks may be between 3:1 and 6:1. These embodiments are well suited for use in screen-printing applications. In certain embodiments the ink contains from about 20 to 50% lithium powder, from about 15 to 40% conductive material and from about 10 to 25% polymer binder on a dry weight basis, where the dry weight would include the lithium powder and any optional conductive material or lithium salt. This includes embodiments wherein the ink contains from about 30 to 45% lithium powder, from about 20 to 35% conductive material, from about 15 to 23% binder, and optionally from about 10 to 25% lithium salt on a dry weight basis. Typically ink

formulations fitting these descriptions will be dissolved in an appropriate solvent at about 10 to 25% . In some formulations of this type, the binder is a polyimide binder, the conductive material is carbon powder, the lithium salt is LiPF₆ or LiTFSi, and the solvent is GBL.

[0062] Higher viscosity lithium powder formulations may also be produced.

Depending on their viscosities, such formulations may find use as offset litho printing inks. Such inks are typically significantly less dilute that screen-printing inks. In some

embodiments these inks contain from about 40 to 80% lithium powder, from about 30 to 70% conductive material and from about 10 to 20% polymer binder on a dry weight basis, where the dry weight would include the lithium powder and any optional conductive material or lithium salt. This includes embodiments wherein the ink contains from about 50 to 70% lithium powder, from about 30 to 40% conductive material, from about 10 to 15% binder, and optionally from about 5 to 15% lithium salt on a dry weight basis. Typically ink formulations fitting these descriptions will be dissolved in an appropriate solvent at about 30 to 80% , desirably about 40 to 60% . In some formulations of this type, the binder is a polyimide binder, the conductive material is carbon powder, the lithium salt is LiPF₆ or LiTFSi, and the solvent is GBL.

[0063] Lithium powder based inks that may find use as flexographic or gravure may have a ratio of (lithium powder + any conductive material + any lithium salt): polymer binder of between 3:1 and 8:1. In some such embodiments the ink contains from about 30 to 60% lithium powder, from about 20 to 50% conductive material and from about 2 to 15% polymer binder on a dry weight basis, where the dry weight would include the lithium powder and any optional conductive material or lithium salt. This includes embodiments wherein the ink contains from about 40 to 50% lithium powder, from about 20 to 30% conductive material, from about 5 to 10% binder, and optionally from about 10 to 20% lithium salt on a dry weight basis. Typically ink formulations fitting these descriptions will be dissolved in an appropriate solvent at about 5 to 20% , desirably about 10 to 15% . In some formulations of this type, the binder is a polyimide binder, the conductive material is carbon powder, the lithium salt is LiPF₆ or LiTFSi, and the solvent is GBL.

[0064] One application for which the lithium powder based formulations provided herein are useful is in the production of electrodes, such as anodes for lithium metal batteries. Such anodes may be fabricated with the lithium metal powder based formulations provided herein by printing the formulation onto the surface of an anode current collector. The anode current collector is an electrically conductive member made from a metal, such as aluminum, copper, nickel, iron, or stainless steel. The current collector may be a foil or a grid.

[0065] Batteries containing the above-described lithium based formulations are further disclosed. In one embodiment, a battery includes: at least one anode, the anode comprising: an anode current collector; and an electrode layer disposed on the anode current collector, the electrode layer comprising the lithium metal powder and cross -linkable polymer binder as described above; at least one cathode; and an electrolyte in electrochemical communication with the each cathode and each anode. Optionally, the cathode comprises: a cathode current collector; and an electrode layer disposed on the cathode current collector, the electrode layer comprising a polyimide, an electronic conductive filler, and a metal oxide.

[0066] In another embodiment, a lithium metal battery comprises: a cathode; an anode comprising a printed electrode layer comprising lithium metal powder and a cross- linkable polymer binder as described above; and a polymer electrolyte between the cathode and the anode.

[0067] Such batteries find utility in a variety of applications, for example a radiofrequency identification device, for example where the device is a Time Temperature Indicator; a Smart Card; or a wearable medical device; for example an external defibrillator, wearable infusion pump, patient monitor, or electrical stimulation device.

[0068] Various non-limiting, numbered, illustrative embodiments are described in the claims as follows. The above description is only illustrative of preferred embodiments that achieve the features and advantages of the present invention, and it is not intended that the present invention be limited thereto. Any modification that comes within the spirit and scope of the following claims is considered part of the present invention.

Claims

What is claimed is:

1. A lithium formulation for use in the manufacture of a lithium battery, the formulation comprising

lithium metal powder,

a cross -linkable polymer binder, and

a solvent.

2. The lithium formulation of claim 1, further comprising at least one electronically conductive material.

3. The lithium formulation of any of claims 1-2, further comprising at least one lithium salt.

4. The lithium formulation of any of claims 1-3, wherein the formulation has a viscosity from about 0.01 to 50 Pa-sec at 25°C.

5. The lithium formulation of any of claims 1-4, comprising about 20 to 50% lithium powder, about 15 to 40% conductive material and about 10 to 25% polyimide binder on a dry weight basis.

6. The lithium formulation of any of claims 1-5, wherein the solvent comprises gamma- butyrolactone.

7. The lithium formulation of any of claims 1-6, wherein the polymer binder comprises a polyimide.

8. The lithium formulation of any of claims 1-6, wherein the polymer binder comprises a polybenzimidazole .

9. The lithium formulation of any of claims 1-6, wherein the polymer binder comprises a polyamide-imide.

10. The lithium formulation of any claims 1-6, wherein the low temperature cross- linkable polyimide is a branched rod-coil polyimide-PEO copolymer.

11. The lithium formulation of any claims 1-6, wherein the polymer binder is selected from polyamides, polyphenylene oxides, polyarylates, polyester-imides, polyester-amide - imides, poly(benzoxazoles), polysulfones, polyether sulfones, polysulfonamides,

poly(quinoxaline), poly(para-phenylenes), poly(aryl ethers), poly(aryl ethers) substituted with a pyridyl group, poly(aryl ether sulfones), polyepoxides, or a combination thereof.

12. The lithium formulation of any claims 1-6, wherein the polymer binder has a glass transition temperature of greater than 15O⁰C.

13. The lithium formulation of any claims 1-6, wherein the polymer binder has a glass transition temperature of less than 15O⁰C.

14. The lithium formulation of any claims 1-6, wherein the polyimide comprises a pre- imidized, amorphous, thermoplastic polyimide powder soluble in a polar solvent.

15. An anode comprising:

an anode current collector; and

an electrode layer disposed on the anode current collector, the electrode layer formed from any of the lithium formulations of claims 1-14.

16. A battery comprising:

at least one anode, the anode comprising:

an anode current collector; and

an electrode layer disposed on the anode current collector, the electrode layer comprising a crosslinked polymer layer formed from any of the lithium formulations of claims 1-14;

at least one cathode; and

an electrolyte in electrochemical communication with the each cathode and each anode.

17. The battery of claim 16, wherein the cathode comprises:

a cathode current collector; and

an electrode layer disposed on the cathode current collector, the electrode layer comprising a polyimide, an electronic conductive filler, and a metal oxide.

18. A lithium metal battery comprising:

a cathode;

an anode comprising a printed electrode layer formed from any of the lithium formulations of claims 1-14; and

a polymer electrolyte between the cathode and the anode.

19. A method of printing an anode for a lithium battery comprising: printing the formulation of any of claims 1-14 on a current collector.