Phosphonate additives for nonaqueous electrolyte in rechargeable electrochemical cells

[0002] The present application is a continuation-in-part of application Ser. No. 09/324,268, filed Jun. 2, 1999, now U.S. Pat. No. 6,200,701 to Gan et al., which claims priority based on U.S. provisional application Ser. No. 60/117,109, filed Jan. 25, 1999.

[0003] The present invention generally relates to an alkali metal electrochemical cell, and more particularly, to a rechargeable alkali metal cell. Still more particularly, the present invention relates to a lithium ion electrochemical cell activated with an electrolyte having an additive provided to achieve high charge/discharge capacity, long cycle life and to minimize the first cycle irreversible capacity. According to the present invention, the preferred additive to the activating electrolyte is a phosphonate compound. A phosphite compound is another name for a phosphonate compound and is also preferred for the present invention.

[0004] Alkali metal rechargeable cells typically comprise a carbonaceous anode electrode and a lithiated cathode electrode. Due to the high potential of the cathode material (up to 4.3 V vs. Li/Li⁺ for Li_1-xCoO₂) and the low potential of the carbonaceous anode material (0.01 V vs. Li/Li⁺ for graphite) in a fully charged lithium ion cell, the choice of the electrolyte solvent system is limited. Since carbonate solvents have high oxidative stability toward typically used lithiated cathode materials and good kinetic stability toward carbonaceous anode materials, they are generally used in lithium ion cell electrolytes. To achieve optimum cell performance (high rate capability and long cycle life), solvent systems containing a mixture of a cyclic carbonate (high dielectric constant solvent) and a linear carbonate (low viscosity solvent) are typically used in commercial secondary cells. Cells with carbonate based electrolytes are known to deliver more than 1,000 charge/discharge cycles at room temperature.

[0005] One aspect of the present invention involved the provision of ethylene carbonate (EC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and diethyl carbonate (DEC) as the solvent system for the activating electrolyte. Lithium ion cells with such electrolyte systems are capable of discharge at temperatures down to as low as −40° C. while exhibiting good cycling characteristics. However, lithium ion cell design generally involves a trade off in one area for a necessary improvement in another. The achievement of a lithium-ion cell capable of low temperature cycleability by use of the above quaternary solvent electrolyte, in place of a typically used binary solvent electrolyte (such as 1.0 M LiPF₆/EC:DMC=30:70, v/v which freezes at −11° C.), is obtained at the expense of increased first cycle irreversible capacity during the initial charging (approximately 65 mAh/g graphite for 1.0 M LiPF₆/EC:DMC:EMC:DEC=45:22:24.8:8.2 vs. 35 mAh/g graphite for 1.0 M LiPF₆/EC:DMC=30:70). Due to the existence of this first cycle irreversible capacity, lithium ion cells are generally cathode limited. Since all of the lithium ions, which shuttle between the anode and the cathode during charging and discharging originally come from the lithiated cathode, the larger the first cycle irreversible capacity, the lower the cell capacity in subsequent cycles and the lower the cell efficiency. Thus, it is desirable to minimize or even eliminate the first cycle irreversible capacity in lithium ion cells while at the same time maintaining the low temperature cycling capability of such cells.

[0006] According to the present invention, these objectives are achieved by providing an organic phosphonate or phosphite in the quaternary solvent electrolyte. Lithium ion cells activated with these electrolytes exhibit lower first cycle irreversible capacities relative to cells activated with the same quaternary solvent electrolyte devoid of the phosphonate additive. As a result, cells including the phosphonate additive present higher subsequent cycling capacity than control cells. The cycleability of the present invention cells at room temperature, as well as at low temperatures, i.e., down to about −40° C., is as good as cells activated with the quaternary electrolyte devoid of a phosphonate additive.

[0007] It is commonly known that when an electrical potential is initially applied to lithium ion cells constructed with a carbon anode in a discharged condition to charge the cell, some permanent capacity loss occurs due to the anode surface passivation film formation. This permanent capacity loss is called first cycle irreversible capacity. The film formation process, however, is highly dependent on the reactivity of the electrolyte components at the cell charging potentials. The electrochemical properties of the passivation film are also dependent on the chemical composition of the surface film.

[0008] The formation of a surface film is unavoidable for alkali metal systems, and in particular, lithium metal anodes, and lithium intercalated carbon anodes due to the relatively low potential and high reactivity of lithium toward organic electrolytes. The ideal surface film, known as the solid-electrolyte interphase (SEI), should be electrically insulating and tonically conducting. While most alkali metal, and in particular, lithium electrochemical systems meet the first requirement, the second requirement is difficult to achieve. The resistance of these films is not negligible, and as a result, impedance builds up inside the cell due to this surface layer formation which induces unacceptable polarization during the charge and discharge of the lithium ion cell. On the other hand, if the SEI film is electrically conductive, the electrolyte decomposition reaction on the anode surface does not stop due to the low potential of the lithiated carbon electrode.

[0009] Hence, the composition of the electrolyte has a significant influence on the discharge efficiency of alkali metal systems, and particularly the permanent capacity loss in secondary cells. For example, when 1.0 M LiPF₆/EC:DMC=30:70 is used to activate a secondary cell, the first cycle irreversible capacity is approximately 35 mAh/g of graphite. However, under the same cycling conditions, the first cycle irreversible capacity is found to be approximately 65 mAh/g of graphite when 1.0 M LiPF₆/EC:DMC:EMC:DEC=45:22:24.8:8.2 is used as the electrolyte. In contrast, lithium ion cells activated with the binary solvent electrolyte of ethylene carbonate and dimethyl carbonate cannot be cycled at temperatures less than about −11° C. The quaternary solvent electrolyte of the previously referenced patent application, which enables lithium ion cells to cycle at much lower temperatures, is a compromise in terms of providing a wider temperature application with acceptable cycling efficiencies. It would be highly desirable to retain the benefits of a lithium ion cell capable of operating at temperatures down to as low as about −40° C. while minimizing the first cycle irreversible capacity.

[0010] According to the present invention, this objectives is achieved by adding a phosphonate additive in the above described quaternary solvent electrolytes. In addition, this invention may be generalized to other nonaqueous organic electrolyte systems, such as binary solvent and ternary solvent systems, as well as the electrolyte systems containing solvents other than mixtures of linear or cyclic carbonates. For example, linear or cyclic ethers or esters may also be included as electrolyte components. Although the exact reason for the observed improvement is not clear, it is hypothesized that the phosphonate additive competes with the existing electrolyte components to react on the carbon anode surface during initial lithiation to form a beneficial SEI film. The thusly formed SEI film is electrically more insulating than the film formed without the phosphonate additive and, as a consequence, the lithiated carbon electrode is better protected from reactions with other electrolyte components. Therefore, lower first cycle irreversible capacity is obtained.

[0011] These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.

[0012] A secondary electrochemical cell constructed according to the present invention includes an anode active material selected from Groups IA, IIA, or IIIB of the Periodic Table of Elements, including the alkali metals lithium, sodium, potassium, etc. The preferred anode active material comprises lithium.

[0013] In secondary electrochemical systems, the anode electrode comprises a material capable of intercalating and de-intercalating the alkali metal, and preferably lithium. A carbonaceous anode comprising any of the various forms of carbon (e.g., coke, graphite, acetylene black, carbon black, glassy carbon, etc.) which are capable of reversibly retaining the lithium species, is preferred. Graphite is particularly preferred due to its relatively high lithium-retention capacity. Regardless of the form of the carbon, fibers of the carbonaceous material are particularly advantageous because the fibers have excellent mechanical properties which permit them to be fabricated into rigid electrodes that are capable of withstanding degradation during repeated charge/discharge cycling. Moreover, the high surface area of carbon fibers allows for rapid charge/discharge rates. A preferred carbonaceous material for the anode of a secondary electrochemical cell is described in U.S. Pat. No. 5,443,928 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated herein by reference.

[0014] A typical secondary cell anode is fabricated by mixing about 90 to 97 weight percent graphite with about 3 to 10 weight percent of a binder material which is preferably a fluoro-resin powder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylenetetrafluoroethylene (ETFE), polyamides and polyimides, and mixtures thereof. This electrode active admixture is provided on a current collector such as of a nickel, stainless steel, or copper foil or screen by casting, pressing, rolling or otherwise contacting the active admixture thereto.

[0015] The anode component further has an extended tab or lead of the same material as the anode current collector, i.e., preferably nickel, integrally formed therewith such as by welding and contacted by a weld to a cell case of conductive metal in a case-negative electrical configuration. Alternatively, the carbonaceous anode may be formed in some other geometry, such as a bobbin shape, cylinder or pellet to allow an alternate low surface cell design.

[0016] The cathode of a secondary cell preferably comprises a lithiated material that is stable in air and readily handled. Examples of such air-stable lithiated cathode materials include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese. The more preferred oxides include LiNiO₂, LiMn₂O₄, LiCoO₂, LiCo_0.92Sn_0.08O₂and LiCo_1-xNi_xO₂.

[0017] Before fabrication into an electrode for incorporation into an electrochemical cell, the lithiated active material is preferably mixed with a conductive additive. Suitable conductive additives include acetylene black, carbon black and/or graphite. Metals such as nickel, aluminum, titanium and stainless steel in powder form are also useful as conductive diluents when mixed with the above listed active materials. The electrode further comprises a fluoro-resin binder, preferably in a powder form, such as PTFE, PVDF, ETFE, polyamides and polyimides, and mixtures thereof.

[0018] To recharge such secondary cells, the lithium ion comprising the cathode is intercalated into the carbonaceous anode by applying an externally generated electrical potential to recharge the cell. The applied recharging electrical potential serves to draw the alkali metal ions from the cathode material, through the electrolyte and into the carbonaceous anode to saturate the carbon comprising the anode. The resulting Li_xC₆electrode can have an x ranging between 0.1 and 1.0. The cell is then provided with an electrical potential and is discharged in a normal manner.

[0019] An alternate secondary cell construction comprises intercalating the carbonaceous material with the active alkali material before the anode is incorporated into the cell. In this case, the cathode body can be solid and comprise, but not be limited to, such materials as manganese dioxide, silver vanadium oxide, copper silver vanadium oxide, titanium disulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide, carbon and fluorinated carbon. However, this approach is compromised by the problems associated with handling lithiated carbon outside of the cell. Lithiated carbon tends to react when contacted by air.

[0020] The secondary cell of the present invention includes a separator to provide physical segregation between the anode and cathode active electrodes. The separator is of an electrically insulative material to prevent an internal electrical short circuit between the electrodes, and the separator material also is chemically unreactive with the anode and cathode active materials and both chemically unreactive with and insoluble in the electrolyte. In addition, the separator material has a degree of porosity sufficient to allow flow therethrough of the electrolyte during the electrochemical reaction of the cell. The form of the separator typically is a sheet which is placed between the anode and cathode electrodes. Such is the case when the anode is folded in a serpentine-like structure with a plurality of cathode plates disposed intermediate the anode folds and received in a cell casing or when the electrode combination is rolled or otherwise formed into a cylindrical “jellyroll” configuration.

[0021] Illustrative separator materials include fabrics woven from fluoropolymeric fibers of polyethylenetetrafluoroethylene and polyethylenechlorotrifluoroethylene used either alone or laminated with a fluoropolymeric microporous film. Other suitable separator materials include non-woven glass, polypropylene, polyethylene, glass fiber materials, ceramics, a polytetraflouroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.), a polypropylene membrane commercially available under the designation CELGARD (Celanese Plastic Company, Inc.) and a membrane commercially available under the designation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.).

[0022] The choice of an electrolyte solvent system for activating an alkali metal electrochemical cell, and particularly a fully charged lithium ion cell is very limited due to the high potential of the cathode material (up to 4.3 V vs. Li/Li⁺ for Li_1-xCoO₂) and the low potential of the anode material (0.01 V vs. Li/Li⁺ for graphite). According to the present invention, suitable nonaqueous electrolytes are comprised of an inorganic salt dissolved in a nonaqueous solvent and more preferably an alkali metal salt dissolved in a quaternary mixture of organic carbonate solvents comprising dialkyl (non-cyclic) carbonates selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC) and ethylpropyl carbonate (EPC), and mixtures thereof, and at least one cyclic carbonate selected from propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC) and vinylene carbonate (VC), and mixtures thereof. Organic carbonates are generally used in the electrolyte solvent system for such battery chemistries because they exhibit high oxidative stability toward cathode materials and good kinetic stability toward anode materials.

[0023] Preferred electrolytes according to the present invention comprise solvent mixtures of EC:DMC:EMC:DEC. Most preferred volume percent ranges for the various carbonate solvents include EC in the range of about 20% to about 50%; DMC in the range of about 12% to about 75%; SMC in the range of about 5% to about 45%; and DEC in the range of about 3% to about 45%. In a preferred form of the present invention, the electrolyte activating the cell is at equilibrium with respect to the ratio of DMC:EMC:DEC.

[0024] The reason an equilibrated mixture of DEC, DMC and EMC is so important is that as the cell is cycled between a discharged and a charged state, DMC and DEC undergo an ester exchange reaction to produce product EMC. Also, EMC disproportionates to DMC and DEC during cell cycling. If the ternary solvents of DMC:EMC:DEC are not filled into the cell in an equilibrated mixture, the cell performance changes as DMC and DEC react to form EMC, and EMC disproportionates to form DMC and DEC. At equilibrium, the rate at which EMC is produced from DMC and DEC by an ester exchange reaction is essentially equal to the rate at which EMC disproportionates into DMC and DEC such that a beginning ratio of DMC, DEC and EMC essentially does not change as the cell discharges.

[0025] An equilibrated mixture is important because it is known that due to the presence of low-potential (anode) materials in a charged cell, an un-equilibrated mixture of DMC:DEC in the presence of lithiated graphite (LiC₆˜0.01 V vs Li/Li⁺) results in a substantial amount of EMC being formed. Then the concentrations of DMC, DEC and EMC change, the cycling characteristics and temperature rating of the cell also changes. Such unpredictability is unacceptable. This phenomenon is described in detail in U.S. patent application Ser. No. 09/669,936, filed Sep. 26, 2000, which is assigned to the assignee of the present invention and incorporated herein by reference. Electrolytes containing the quaternary carbonate mixture of the present invention exhibit freezing points below −50° C., and lithium ion cells activated with such mixtures have very good cycling behavior at room temperature as well as very good discharge and charge/discharge cycling behavior at temperatures below −40° C.

[0026] Known lithium salts that are useful as a vehicle for transport of alkali metal ions from the anode to the cathode, and back again include LiPF₆, LiBF₄, LiAsF₆, LiSbF₆, LiClO₄, LiAlCl₄, LiGaCl₄, LiC(SO₂CF₃)₃, LiNO₃, LiN (SO₂CF₃)₂, LiSCN, LiO₃SCF₂CF₃, LiC₆F₅SO₃, LiO₂CCF₃, LiSO₃F, LiB(C₆H₅)₄, LiCF₃SO₃, and mixtures thereof. Suitable salt concentrations typically range between about 0.8 to 1.5 molar.

[0027] In accordance with the present invention, at least one organic phosphonate additive is provided as a co-solvent in the electrolyte solution of the previously described alkali metal ion or rechargeable electrochemical cell. The phosphonate additive preferably has the general formula (R¹O)P(═O) (OR²) (R³) wherein R³is a hydrogen atom and wherein at least one, but not both, of R¹and R²is a hydrogen atom and the other of R¹and R²is an organic group containing 1 to 13 carbon atoms, or at least one of R¹and R²is an organic group containing at least 3 carbon atoms and having an sp or sp²hybridized carbon atom bonded to an sp³hybridized carbon atom bonded to the oxygen atom bonded to the phosphorous atom, or at least one of R¹and R²is an unsaturated inorganic group.

[0028] Phosphonate compounds coming under the purview of the present invention include those where at least one of R¹and R²has a first bond structure of the type C(sp)—C(sp³) directly connected to the —OPO₂— functional group, those where at least one of R¹and R²has a second bond structure of the type C(sp²)—C(sp³) directly connected to the —OPO₂— functional group, or those where at least one of R¹and R²is an unsaturated inorganic group. Phosphonate compounds having at least one of the first or the second bond structures or which have an unsaturated inorganic group have, for example, R¹and R²selected from the group of benzyl, allyl, nitromethyl, propargyl and naphthyl(methyl). In that case, the other of R¹and R²not being the first bond structure or the second bond structure is either a linear or a cyclic alkyl group having 1 to 13 carbon atoms or an unsaturated inorganic group. Exemplary alkyl groups include: methyl, ethyl, propyl, butyl, pentyl, hexyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononanyl, cyclodecanyl, cycloundecyl, cyclododecyl, phenyl, tolyl and naphthyl.

[0029] Examples of phosphonate compounds having at least one of R¹and R²of the first bond structure of the type C(sp²)—C(sp³) directly connected to the —OPO₂— functional group include:

[0030] dibenzyl phosphonate (R═benzyl) or “alkyl” benzyl phosphonate (R≠benzyl)

[0031] diallyl phosphonate (R═allyl) or “alkyl” allyl phosphonate (R≠allyl)

[0032] “alkyl” nitromethyl phosphonate [unsaturated inorganic group—C(sp³)—O—P (═O) (OR)R³]

[0033] Examples of phosphonate compounds having at least one of R¹and R²of the second bond structure of the type C(sp)—C(sp³) directly connected to the —OPO₂— functional group include:

[0034] dipropargyl phosphonate (R═propargyl) or “alkyl” propargyl phosphonate (R≠propargyl)

[0035] dicyanomethyl phosphonate (R═cyanomethyl) or cyanomethyl “alkyl” phosphonate (R≈cyanomethyl)

[0036] The above described compounds are only intended to be exemplary of those that are useful with the present invention, and are not to be construed as limiting. Those skilled in the art will readily recognize phosphonate compounds which come under the purview of the general formula set forth above and which will be useful as additives for the electrolyte to achieve high charge/discharge capacity, long cycle life and to minimize the first cycle irreversible capacity according to the present invention.

[0037] While not intended to be bound by any particular theory, it is believed that due to the presence of the —OP(═O)(R)O— functional group, the reductive cleavage of at least one of the OR¹and OR²bonds in the phosphonate additives of the present invention has to be weak enough to produce lithium phosphonate or the lithium salt of a phosphonate reduction product of the general formula O═P(O—Li)_n(OR)_mR³(n=1 or 2; m=0 or 1) deposited on the anode surface. If one of the R¹or R²is H, the compound will be acidic and it will easily react with lithium metal or lithiated carbon to form a lithium salt. In that case, at least one, but not both, of R¹and R²is a hydrogen atom and the other of R¹and R²is an organic group containing 1 to 13 carbon atoms with R³being a hydrogen atom. However, if both of the R¹and R²groups are organic groups, the reactivity of the C—O bond depends on the bond strength and the structure of the R group. The reactivity of the phosphonate compound is also increased by having R³being a hydrogen atom with at least one of R¹and R²being an organic group containing at least 3 carbon atoms and having an sp or sp²hybridized carbon atom bonded to an sp³hybridized carbon atom bonded to the —OP(═O)R(O)— functional group or having at least one of R¹and R²being an unsaturated inorganic group. During reduction, the O—R bond in the above described phosphonate compounds breaks to form a product containing the P—O—Li salt group. This anode surface film is ionically more conductive than the film formed in the absence of the additives and is responsible for the improved performance of the lithium-ion cells.

[0038] The concentration limit for the phosphonate additive is preferably about 0.001 M to about 0.40 M. The beneficial effect of the phosphonate additive will not be apparent if the additive concentration is less than about 0.001 M. On the other hand, if the additive concentration is greater than about 0.40 M, the beneficial effect of the additive will be canceled by the detrimental effect of higher internal cell resistance due to the thicker anode surface film formation and lower electrolyte conductivity.

[0039] The assembly of the cell described herein is preferably in the form of a wound element cell. That is, the fabricated cathode, anode and separator are wound together in a “jellyroll” type configuration or “wound element cell stack” such that the anode is on the outside of the roll to make electrical contact with the cell case in a case-negative configuration. Using suitable top and bottom insulators, the wound cell stack is inserted into a metallic case of a suitable size dimension. The metallic case may comprise materials such as stainless steel, mild steel, nickel-plated mild steel, titanium or aluminum, but not limited thereto, so long as the metallic material is compatible for use with components of the cell.

[0040] The cell header comprises a metallic disc-shaped body with a first hole to accommodate a glass-to-metal seal/terminal pin feedthrough and a second hole for electrolyte filling. The glass used is of a corrosion resistant type having up to about 50% by weight silicon such as CABAL 12, TA 23 or FUSITE 425 or FUSITE 435. The positive terminal pin feedthrough preferably comprises titanium although molybdenum, aluminum, nickel alloy, or stainless steel can also be used. The cell header comprises elements having compatibility with the other components of the electrochemical cell and is resistant to corrosion. The cathode lead is welded to the positive terminal pin in the glass-to-metal seal and the header is welded to the case containing the electrode stack. The cell is thereafter filled with the electrolyte solution comprising at least one of the phosphonate additives described hereinabove and hermetically sealed such as by close-welding a stainless steel ball over the fill hole, but not limited thereto,

[0041] The above assembly describes a case-negative cell,, which is the preferred construction of the exemplary cell of the present invention. As is well known to those skilled in the art, the exemplary electrochemical system of the present invention can also be constructed in a case-positive configuration.

[0042] It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the appended claims.