Patent Application
20140134501
The present invention relates to a non-aqueous electrolytic solutions and secondary (rechargeable) electrochemical energy storage devices comprising the same. Such electrolytic solutions enhance electrochemical performance in devices charged to higher voltages, reduce capacity degradation during cycling at these voltages and during high temperature storage and in general improve the overall electrochemical stability of a device made therewith.
Lithium compound containing electric cells and batteries containing such cells are modern means for energy storage devices. For example, lithium ion batteries have been major power sources for cell phones, laptop computers and a host of other portable electronic consumer products. Penetration of this technology into the transportation market and other large scale applications continues to place ever-increasing demands for higher energy density, higher power density and better cycle life. High voltage cathode materials such as 5V LiNi0.5Mn1.5O4 (LNMO) material have attracted much attention because higher operating voltage facilitates a substantial increase in the energy density of the lithium ion batteries incorporating these cathodes compared with the present 4 V counterpart. Such cathodes require an electrolyte system that can remain stable in that high potential range and can effectively support the cell chemistry in prolonged cycles.
Electrolytes for lithium compound containing energy storage devices are mixtures comprised of one or more highly soluble lithium salts and inorganic additives dissolved in one or more organic solvents. Electrolytes are responsible for ionic conduction between the cathode and the anode in the battery and thus essential to the operation of the system. The conventional carbonate-based electrolytes have been successfully applied in commercial 4V lithium ion batteries. Electrolytes having high level performance are desired when the cell operating window is extended to higher potential to avoid decomposition on the highly oxidative surface of charged cathodes. It is also desired that there be improved the interaction between electrolyte and cathode at high voltage to avoid possible transition metal dissolution from cathodes that can attack the Solid Electrolyte Interphase (SEI) layer on the anode surface and, thus, cause further capacity loss of the cell.
This invention pertains to non-aqueous electrolytic solutions that comprise (a) one or more ionic salts; (b) one or more solvents; (c) at least one solid electrolyte interphase former; (d) at least one fluorinated compound; and (e) optionally, at least one high temperature stability compound; wherein (c), (d) and (e) are each different compounds and each are different from the ionic salts (a) and solvents (b).
In one embodiment this invention involves electrolytes suitable for use in energy storage devices that operate at voltages greater than 4.0 volts.
Embodiment two involves non-aqueous electrolytic solutions suitable for use in electrochemical energy storage devices capable of being charged to up to 5.0 volts.
Embodiment three involves non-aqueous electrolytic solutions suitable for use in electrochemical energy storage devices capable of being charged from about 4.2 up to about 5.0 volts.
Embodiment four involves non-aqueous electrolytic solutions suitable for use in electrochemical energy storage devices (e.g., lithium metal batteries, lithium ion batteries, lithium ion capacitors and supercapacitors) that include one or more SEI (Solid Electrolyte Interphase layer on the anode surface) additives, one or more fluorinated compounds, and optionally, one or more additives that promote improved high temperature performance. One member from each family is preferably present to obtain the desired high voltage performance.
Embodiment five provides non-aqueous electrolytic solutions which include (a) one or more lithium salts, (b) one or more carbonate solvents, (c) one or more compounds selected from SEI formers, (d) one or more compounds selected from fluorinated compounds (organic or inorganic based), and (e) optionally, one or more compounds that promote high temperature stability. A member from each of these components is preferably present in the electrolytic solution and wherein (c), (d) and (e) are each different compounds and each are different from the ionic salts (a) and solvents (b).
Embodiment six provides an electrolytic solution useful in a lithium or lithium-ion batteries.
Embodiment seven provides batteries that include an anode and cathode. The major components, including salts, solvents, high voltage additives, anodes, and cathodes, are each described in turn herein below.
Embodiment eight provides non-aqueous electrolytic solutions that have high voltage stability during room temperature and high temperature cell cycling as well as good performance under high temperature storage conditions.
Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways.
Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.
Salts. The solute of the electrolytic solution of the invention is an ionic salt containing at least one positive ion. Typically this positive ion is lithium (Li+). The salts herein function to transfer charge between the negative electrode and the positive electrode of the battery system. The lithium salts are preferably halogenated, for example, LiPF6, LiBF4, LiSbF6, LiAsF6, LiTaF6, LiAlCl4, Li2B10Cl10, Li2B10F10, LiClO4, LiCF3SO3, Li2B12FxH(12-x) wherein x=0-12; LiPFx(RF)6-x and LiBFy(RF)4-y wherein RF represents perfluorinated C1-C20 alkyl groups or perfluorinated aromatic groups, x=0-5 and y=0-3, LiBF2[O2C(CX2)nCO2], LiPF2[O2C(CX2)nCO2]2, LiPF4[O2C(CX2)nCO2], wherein X is selected from the group consisting of H, F, Cl, C1-C4 alkyl groups and fluorinated alkyl groups, and n=0-4, LiN(SO2CmF2m+1)(SO2CnF2n+1), and LiC(SO2CkF2k+1)(SO2CmF2m+1)(SO2CnF2n+1), wherein k=1-10, m=1-10, and n=1-10, respectively, LiN(SO2CpF2pSO2), and LiC(SO2CpF2pSO2)(SO2CqF2q+1) wherein p=1-10 and q=1-10, lithium salts of chelated orthoborates and chelated orthophosphates such as lithium bis(oxalato)borate [LiB(C2O4)2], lithium bis(malonato)borate [LiB(O2CCH2CO2)2], lithium bis(difluoromalonato)borate [LiB(O2CCF2CO2)2], lithium(malonato oxalato)borate [LiB(C2O4)(O2CCH2CO2)], lithium(difluoromalonato oxalato)borate [LiB(C2O4)(O2CCF2CO2)], lithium tris(oxalato)phosphate [LiP(C2O4)3], and lithium tris(difluoromalonato)phosphate [LiP(O2CCF2CO2)3], and any combination of two or more of the aforementioned salts. Most preferably the electrolytic solution comprises LiPF6 as the ionic salt.
Solvents. The solvents to be used in the secondary batteries of the invention can be any of a variety of non-aqueous, aprotic, and polar organic compounds. Generally, solvents may be carbonates, carboxylates, ethers, lactones, sulfones, phosphates, nitriles, and ionic liquids. Useful carbonate solvents herein include, but are not limited to: cyclic carbonates, such as propylene carbonate and butylene carbonate, and linear carbonates, such as dimethyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate. Useful carboxylate solvents include, but are not limited to: methyl formate, ethyl formate, propyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and butyl butyrate. Useful ethers include, but are not limited to: tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane, methyl nonafluorobutyl ether, and ethyl nonafluorobutyl ether. Useful lactones include, but are not limited to: γ-butyrolactone, 2-methyl-γ-butyrolactone, 3-methyl-γ-butyrolactone, 4-methyl-γ-butyrolactone, β-propiolactone, and δ-valerolactone. Useful phosphates include, but are not limited to: trimethyl phosphate, triethyl phosphate, tris(2-chloroethyl)phosphate, tris(2,2,2-trifluoroethyl)phosphate, tripropyl phosphate, triisopropyl phosphate, tributyl phosphate, trihexyl phosphate, triphenyl phosphate, tritolyl phosphate, methyl ethylene phosphate, and ethyl ethylene phosphate. Useful sulfones include, but are not limited to: non-fluorinated sulfones, such as dimethyl sulfone and ethyl methyl sulfone, partially fluorinated sulfones, such as methyl trifluoromethyl sulfone, ethyl trifluoromethyl sulfone, methyl pentafluoroethyl sulfone, and ethyl pentafluoroethyl sulfone, and fully fluorinated sulfones, such as di(trifluoromethyl)sulfone, di(pentafluoroethyl)sulfone, trifluoromethyl pentafluoroethyl sulfone, trifluoromethyl nonafluorobutyl sulfone, and pentafluoroethyl nonafluorobutyl sulfone. Useful nitriles include, but are not limited to: acetonitrile, propionitrile, butyronitrile and dinitriles, CN[CH2]nCN with various alkane chain lengths (n=1-8). An ionic liquid (IL) is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as 100° C. (212° F.). ILs are largely made of ions and short-lived ion pairs. Common anions of ILs areTFSi, FSi, BOB, DFOB, PF6-xRx, BF4, etc and cations of ILs are imidazolium, piperidinium, pyrrolidinium, tetraalkylammonium, morpholinium, etc. Useful ionic liquids include, but not limited to: Bis(oxalate)borate (BOB) anion based ionic liquids, such as N-cyanoethyl-N-methylprrrolidinium BOB, 1-methyl-1-(2-methylsulfoxy)ethyl)-pyrrolidinium BOB, and 1-methyl-1-((1,3,2-dioxathiolan-2-oxide-4-yl)methyl)pyrrolidinium BOB; tris(pentafluoroethyl)trifluorophosphate (FAP) anion based ionic liquids, such as N-allyl-N-methylpyrrrolidinium FAP, N-(oxiran-2-ylmethyl)N-methylpyrrolidinium FAP, and N-(prop-2-inyl)N-methylpyrrolidinium FAP; bis(trifluoromethanesulfonyl)imide (TFSI)anion-based ionic liquids, such as N-propyl-N-methylpyrrolidinium TFSI, 1,2-dimethyl-3-propylimidazolium TFSI, 1-octyl-3-methyl-imidazolium TFSI, and 1-butyl-methylpyrrolidinium TFSI; Bis(fluorosulfonyl)imide (FSI) anion-based ionic liquids, such as N-Butyl-N-methylmorpholinium FSI and N-propyl-N-methylpiperidinium FSI; and other ionic liquids such as 1-ethyl-3-methylimidazolium tetrafluoroborate. Two or more of these solvents may be used in the electrolytic solution. Other solvents may be utilized as long as they are non-aqueous and aprotic, and are capable of dissolving the salts, such as N,N-dimethyl formamide, N,N-dimethyl acetamide, N,N-diethyl acetamide, and N,N-dimethyl trifluoroacetamide. Carbonates are preferred, with the most preferred being ethylene carbonate, ethyl methyl carbonate and mixtures thereof.
Solid Electrolyte Interphase (SEI) formers. SEI formers are materials that can be reductively decomposed on surfaces of negative electrodes prior to other solvent components to form protective films that suppress excessive decomposition of the electrolytic solutions. SEI has important roles on the charge/discharge efficiency, the cycle characteristics and the safety of nonaqueous electrolyte batteries. Generally, SEI formers can include, but not limited to, vinylene carbonate and its derivatives, ethylene carbonate derivatives having non-conjugated unsaturated bonds in their side chains, halogen atom-substituted cyclic carbonates and salts of chelated orthoborates and chelated orthophosphates. Specific examples of SEI additives include vinylene carbonate(VC), vinylethylene carbonate (VEC), methylene ethylene carbonate (or 4-vinyl-1,3-dioxolan-2-one) (MEC), monofluoroethylene carbonate (FEC), Chloroethylene carbonate (CEC), 4,5-divinyl-1,3-dioxolan-2-one, 4-methyl-5-vinyl-1,3-dioxolan-2-one, 4-ethyl-5-vinyl-1,3-dioxolan-2-one, 4-propyl-5-vinyl-1,3-dioxolan-2-one, 4-butyl-5-vinyl-1,3-dioxolan-2-one, 4-pentyl-5-vinyl-1,3-dioxolan-2-one, 4-hexyl-5-vinyl-1,3-dioxolan-2-one, 4-phenyl-5-vinyl-1,3-dioxolan-2-one, 4,4-difluoro-1,3-dioxolan-2-one and 4,5-difluoro-1,3-dioxolan-2-one, lithium bis(oxalate)borate (LiBOB), lithium bis(malonato)borate (LiBMB), lithium bis(difluoromalonato)borate (LiBDFMB), lithium (malonato oxalato)borate (LiMOB), lithium (difluoromalonato oxalato)borate (LiDFMOB), lithium tris(oxalato)phosphate (LiTOP), and lithium tris(difluoromalonato)phosphate (LiTDFMP). Particularly useful solid electrolyte interphase formers (c) are selected from the group consisting of vinylene carbonate, monofluoroethylene carbonate, methylene ethylene carbonate, vinyl ethylene carbonate, lithium bis(oxalate)borate and mixtures thereof.
Fluorinated compounds. Fluorinated compounds can include organic and inorganic fluorinated compounds.
Organic fluorinated compounds—Compounds in the organic family of fluorinated compounds can include fluorinated carbonates, fluorinated ethers, fluorinated esters, fluorinated alkanes, fluorinated alkyl phosphates, fluorinated aromatic phosphates, fluorinated alkyl phosphonates, and fluorinated aromatic phosphonates. Exemplary organic fluorinated compounds include fluorinated alkyl phosphates, such as tris(trifluoroethyl)phosphate, tris(1,1,2,2-tetrafluoroethyl)phosphate, tris(hexafluoroisopropyl)phosphate, (2,2,3,3-tetrafluoropropyl)dimethyl phosphate, bis(2,2,3,3-tetrafluoropropyl)methyl phosphate, and tris(2,2,3,3-tetrafluoropropyl)phosphate; fluorinated ethers, such as 3-(1,1,2,2-tetrafluoroethoxy)-(1,1,2,2-tetrafluoro)-propane, pentafluoropropyl methyl ether, pentafluoropropyl fluoromethyl ether, pentafluoropropyl trifluoromethyl ether, 4,4,4,3,3,2,2-heptafluorobutyl difluoromethyl ether, 4,4,3,2,2-pentafluorobutyl 2,2,2-trifluoroethyl ether, 2-difluoromethoxy-1,1,1-trifluoroethane, and 2-difluoromethoxy-1,1,1,2-tetrafluoroethane; fluorinated carbonates, such as fluoroethylene carbonate, bis(fluoromethyl)carbonate, bis(fluoroethyl)carbonate, fluoroethyl fluoromethyl carbonate, methyl fluoromethyl carbonate, ethyl fluoroethyl carbonate, ethyl fluoromethyl carbonate, methyl fluoroethyl carbonate, bis(2,2,2-trifluoroethyl)carbonate, 2,2,2-trifluoroethyl methyl carbonate, fluoroethylene carbonate, and 2,2,2-trifluoroethyl propyl carbonate. Also suitable are fluorinated esters, such as (2,2,3,3-tetrafluoropropyl)formate, methyl trifluoroacetate, ethyl trifluoroacetate, propyl trifluoroacetate, trifluoromethyl trifluoroacetate, trifluoroethyl trifluoroacetate, perfluoroethyl trifluoroacetate, and (2,2,3,3-tetrafluoropropyl)trifluoroacetate; fluorinated alkanes, such as n-C4F9C2H5, n-C6F13C2H5, or n-C8F16H; fluorinated aromatic phosphates, such as tris(4-fluorophenyl)phosphate and pentafluorophenyl phosphate. Fluorinated alkyl phosphonate, such as trifluoromethyl dimethylphosphonate, trifluoromethyl di(trifluoromethyl)phosphonate, and (2,2,3,3-tetrafluoropropyl)dimethylphosphonate; fluorinated aromatic phosphonate, such as phenyl di(trifluoromethyl)phosphonate and 4-fluorophenyl dimethylphosphonate, are suitable. Combinations of two or more of any of the foregoing are also suitable.
Inorganic fluorinated compounds—Compounds in the inorganic family of fluorinated compounds include lithium salts of fluorinated chelated orthoborates, fluorinated chelated orthophosphates, fluorinated imides, fluorinated sulfonates. Exemplary inorganic fluorinated compounds include LiBF2C2O4 (LiDFOB), LiPF4(C2O4) (LiTFOP), LiPF2(C2O4)2 (LiDFOP), LiN(SO2CF3)2 (LiTFSI), LiN(SO2F)2 (LiFSI), LiN(SO2C2F5)2 (LiBETI), LiCF3SO3, Li2B12FxH(12-x) where 0<x≦12 and combinations of two or more thereof.
Particularly useful fluorinated compounds (d) are selected from the group consisting of (1,1,2,2)-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)-propane, lithium difluoro(oxalate)borate and mixtures thereof, each provided in an amount of 0.1 to 5.0% by weight of the electrolyte solution.
Compounds that promote high temperature stability. When batteries are operated or stored at 55° C. or above, they tend to have poor capacity retention and swelling phenomenon due to gas generation that results from decomposition of the electrolyte at the cathode. This reduced performance becomes more evident when a cell is charged to higher voltages. High temperature stabilizers can enhance charge-discharge characteristics of batteries and effectively reduce the swelling of batteries at elevated temperatures. They can also help to create a protective layer on the surface of the cathode which will further decrease the amount of solvent oxidation and decomposition at the cathode. Compounds that promote high temperature stability typically include: sulfur-containing linear and heterocyclic, unsaturated and saturated compounds; phosphorus containing linear and heterocyclic, unsaturated and saturated compounds; and compounds that act as HF scavengers.
Sulfur containing compounds include linear and cyclic compounds such as sulfites, sulfates, sulfoxides, sulfonates, thiophenes, thiazoles, thietanes, thietes, thiolanes, thiazolidines, thiazines, sultones, and sulfones. These sulfur containing compounds can include various degrees of fluorine substitution up to and including the fully perfluorinated compounds. Specific examples of sulfur-containing linear and cyclic compounds include ethylene sulfite, ethylene sulfate, thiophene, benzothiophene, benzo[c]thiophene, thiazole, dithiazole, isothiazole, thietane, thiete, dithietane, dithiete, thiolane, dithiolane, thiazolidine, isothiazolidine, thiadiazole, thiane, thiopyran, thiomorpholine, thiazine, dithiane, dithiine; thiepane; thiepine; thiazepine; prop-1-ene-1,3-sultone; propane-1,3-sultone; butane-1,4-sultone; 3-hydroxy-1-phenylpropanesulfonic acid 1,3-sultone; 4-hydroxy-1-phenylbutanesulfonic acid 1,4-sultone; 4-hydroxy-1-methylbutanesulfonic acid 1,4 sultone; 3-hydroxy-3-methylpropanesulfonic acid 1,4-sultone; 4-hydroxy-4-methylbutanesulfonic acid 1,4-sultone; a sulfone having the formula R1(═S(═O)2)R2 where R1 and R2 are independently selected from the group consisting of substituted or unsubstituted, saturated or unsaturated C1 to C20 alkyl or aralkyl groups; and combinations of two or more thereof. In a specific embodiment the sulfur containing compunds (iii)(e) are selected from the group consisting propane-1,3-sultone, butane-1,4-sultone and prop-1-ene-1,3-sultone, each provided in an amount of 0.1 to 5.0% by weight of the electrolyte solution.
Phosphorus containing compounds include linear and cyclic, phosphates and phosphonates. Representative examples of the phosphorus containing compounds include: alkyl phosphates, such as trimethylphosphate, triethylphosphate, triisopropyl phosphate, propyl dimethyl phosphate, dipropyl methyl phosphate, and tripropyl phosphate; aromatic phosphates, such as triphenyl phosphate; alkyl phosphonates include trimethylphosphonate, and propyl dimethylphosphonate; and aromatic phosphonates, such as phenyl dimethylphosphonate. Combinations of any of the foregoing are also suitable.
Compounds that promote high temperature stability also include additives that work as a HF scavenger to prevent battery capacity deterioration and improve output characteristics at high temperatures, including acetamides, anhydrides, Pyridines, tris(trialkylsilyl)phosphates, tris (trialkylsilyl)phosphites, tris(trialkylsilyl)borates. Examples of HF scavenger-type high temperature stabilizers include: acetamides such as, N,N-dimethyl acetamide, and 2,2,2-trifluoroacetamide; anhydrides such as phthalic anhydride succinic anhydride, and glutaric anhydride; pyridines such as antipyridine and pyridine; tris(trialkylsilyl)phosphates such as tris(trimethylsilyl)phosphate and tris(triethylsilyl)phosphate; tris(trialkylsilyl)phosphites tris(trimethylsilyl)phosphite, tris(triethylsilyl)phosphite, tris(tripropylsilyl)phosphit; tris(trialkylsilyl)borates such as, tris(trimethylsilyl)borate, tris(triethylsilyl)borate, and tris(tripropylsilyl)borate; alone or as a mixture of two or more thereof.
In a specific embodiment the solid electrolyte interphase former (iii)(c) , fluorinated compounds (iii)(d), high temperature stability compound (iii)(e), each provided in an amount of 0.1 to 20% by weight of the electrolyte solution.
An embodiment of this invention includes a secondary electrochemical energy storage device comprising
I. an anode;
II. a cathode;
III. an electrolytic solution comprising
Anodes. The anode material is selected from lithium metal, lithium alloys, carbonaceous materials, and lithium metal oxides capable of being intercalated and de-intercalated with lithium ions. Carbonaceous materials useful herein include graphite, amorphous carbon, and other carbon materials such as activated carbon, carbon fiber, carbon black, and mesocarbon microbeads. Lithium metal anodes may be used. Lithium MMOs (mixed-metal oxides) such as LiMnO2 and Li4Ti5O12 are also envisioned. Alloys of lithium with transition or other metals (including metalloids) may be used, including LiAl, LiZn, Li3Bi, Li3Cd, Li3Sb, Li4Si, Li4.4Pb, Li4.4Sn, LiC6, Li3FeN2, Li2.6Co0.4N, Li2.6Cu0.4N, and combinations thereof. The anode may further comprise an additional material such as a metal oxide including SnO, SnO2, GeO, GeO2, In2O, In2O3, PbO, PbO2, Pb2O3, Pb3O4, Ag2O, AgO, Ag2O3, Sb2O3, Sb2O4, Sb2O5, SiO, ZnO, CoO, NiO, FeO, and combinations thereof.
Cathodes. The cathode comprises at least one lithium transition metal oxide (LiMO), lithium transition metal phosphate (LiMPO4), or lithium transition metal fluorosilicate (LiMSiOxFy). Lithium transition metal oxides contain at least one metal selected from the group consisting of Mn, Co, Cr, Fe, Ni, V, and combinations thereof. For example, the following LiMOs may be used in the cathode: LiCoO2, LiMnO2, LiMn2O4, Li2Cr2O7, Li2CrO4, LiNiO2, LiFeO2, LiNixCo1-xO2 (0<x<1), LiMnzNi1-zO2 (0<z<1) (which includes LiMn0.5Ni0.5O2), LiMn1/3Co1/3Ni1/3O2, LiMc0.5Mn1.5O4, wherein Mc is a divalent metal, and LiNixCoyMezO2 wherein Me may be one or more of Al, Mg, Ti, B, Ga, or Si and 0<x,y,z<1. Lithium transition metal phosphate (LiMPO4) such as LiFePO4, LiVPO4, LiMnPO4, LiCoPO4, LiNiPO4, LiMnxMcyPO4, where Mc may be one of or of Fe, V, Ni, Co, Al, Mg, Ti, B, Ga, or Si and 0<x,y<1. Furthermore, transition metal oxides such as MnO2 and V2O5, transition metal sulfides such as FeS2, MoS2, and TiS2, and conducting polymers such as polyaniline and polypyrrole may be present. The preferred positive electrode material is the lithium transition metal oxide, especially, LiCoO2, LiMn2O4, LiNi0.80Co0.15Al0.05O2, LiFePO4, LiMnPO4, and LiNi1/3Mn1/3Co1/3O2. The stoichiometry of elements in the above molecular formulations does not need to be integral. For example, the material could be lithium rich or lithium deficient, that is, the lithium number in the above formula could be larger or smaller than one. Mixtures of such oxides may also be used.
Either the anode or the cathode, or both, may further comprise a polymeric binder. In the preferred embodiment, the binder may be polyvinylidene fluoride, styrene-butadiene rubber, alkali metal salts of carboxymethyl cellulose, alkali metal salts of polyacrylic acid, polyamide or melamine resin, or combinations of two or more thereof.
Further additions to the electrolytic solution may include, but are not limited to, one or more of the following performance enhancing additives: overcharge protection agent, non-flammable agents, anti-swelling agent, low temperature performance enhancers. Examples of such compounds include biphenyl, iso-propyl benzene, hexafluorobenzene, phosphazenes, organic phosphates, organic phosphonates, and alkyl and aryl siloxanes, The total concentration of such additives in the solution preferably does not exceed about 5 wt %.
Assembly of a Lithium Ion Secondary Battery. In a dry box under an inert atmosphere, a lithium ion secondary battery was assembled utilizing a prismatic cell design. That is, an arrangement containing a microporous polypropylene separator, a cathode, another microporous polypropylene separator, and an anode were laid on top of one another and then wrapped tightly together. The assembly was then inserted into the opened end of a prismatic aluminum can. Current leads were attached to both the cathode and anode, with proper insulation against each other, and connections made to the outside terminals. The open end of the prismatic cell was then covered except for a small hole. Through this hole the inventive electrolytic solution was added to each of the batteries and allowed to absorb. Finally, a small steel ball was used to seal the cell, completing the assembly of the prismatic type lithium ion secondary battery.
Testing of the Batteries. Evaluation of the aforementioned assembled battery was carried out by an initial charging and discharging process (formation and capacity confirmation), followed by rate discharge, cycle life testing, high temperature storage and cycling, and low temperature discharge.
Cycle life. Cycle life testing is conducted at room temperature or 60° C., which was the “high temperature,” sometimes indicated by “HT,” by repeatedly charging and discharging the prepared cells according to the following schedule: charging the aforementioned initially charged/discharged battery at a constant current rate of C (700 mA) to 4.4 V and then charged at a constant voltage of 4.4 V until the current was less than or equal to 35 mA. The battery was then discharged at a constant current rate of C (700 mA) until the cut-off voltage 3.0 V was reached.
Certain embodiments of the invention are envisioned where at least some percentages, temperatures, times, and ranges of other values are preceded by the modifier “about.” “Comprising” is intended to provide support for “consisting of and “consisting essentially of.” Where ranges in the claims of this provisional application do not find explicit support in the specification, it is intended that such claims provide their own disclosure as support for claims or teachings in a later filed non-provisional application. Numerical ranges of ingredients that are bounded by zero on the lower end (for example, 0-10 vol % VC) are intended to provide support for the concept “up to [the upper limit],” for example “up to 10 vol % VC,” vice versa, as well as a positive recitation that the ingredient in question is present in an amount that does not exceed the upper limit. An example of the latter is “comprises VC, provided the amount does not exceed 10 vol %.” A recitation such as “8-25 vol % (EC+MEC+VC)” means that any or all of EC, MEC and/or VC may be present in an amount of 8-25 vol % of the composition.
Room temperature and 60° C. cycling performance/Capacity retention after 300 charge-discharge cycles.
The electrolyte solution is prepared by dissolving 1M LiPF6 in a mixed solvent of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) at a ratio of 3 to 7 by volume, to which vinylene carbonate (VC) at 2% by weight, (1,1,2,2)-tetrafluoro-3-(1,1,2,2-tetrafluoroethoxy)-propane (FE) at 2% by weight and 1,3-propane sultone (PS) at 1% by weight (all based on the total weight of the solution) is added.
The resulting electrolyte solution was injected into an aluminum can prismatic battery comprising a cathode of LiNi 0.33Co 0.33Mn0.33O2 and an anode of artificial graphite.
A battery is prepared in the same way as example 1 except that, instead of VC, fluoroethylene carbonate (FEC) at 3% by weight is used.
A battery is prepared in the same way as example 1 except that, instead of VC, methylene ethylene carbonate (MEC) at 1.5% by weight, and instead of FE, lithium difluoro(oxalate) borate (LiDFOB) at 1.5% by weight are used.
A battery was prepared in the same way as example 1 except that VC, FE and PS are not used.
A battery was prepared in the same way as example 1 except that, instead of VC, FE and PS, only VC at 2% by weight is used.
A battery was prepared in the same way as example 1 except that, instead of VC, FE and PS, only FEC at 3% by weight is used.
A battery was prepared in the same way as example 1 except that, instead of VC, FE and PS, only LiDFOB at 1.5% by weight is used.
A battery was prepared in the same way as example 1 except that, instead of VC, FE and PS, only FE at 2% by weight and PS at 1% by weight are used.
A battery was prepared in the same way as example 1 except that, instead of VC, FE and PS, only FEC at 3% by weight and FE at 2% by weight are used.
A battery was prepared in the same way as example 1 except that, instead of VC, FE and PS, only MEC at 1.5% by weight and LiDFOB at 1.5% by weight are used.
The batteries using electrolytes prepared according to Examples 1 through 3 and Comparative examples 1 through 7 were charged to 4.4 V at 1C constant current and discharged to 3.0 V at constant current. This cycle was repeated hundreds of times at room temperature or 60° C. and the capacity retention ratio was calculated according to the formula: capacity retention ratio after nth cycles=(discharge capacity at nth cycle/discharge capacity at 1st cycle)×100 (%). The results are shown below in the last column of Table 1.
As shown in Table 1, the addition of three distinct additives that come from three components (c), (d) and (e) (Examples 1 to 3, respectively) greatly improved both room temperature and high temperature cycling performance of batteries. When only one or two of these three components is used in the electrolyte solution (comparable examples), the capacity retention under both room and high temperature cycling is inferior. Therefore there is direct evidence that at least one of each of these three components must be present to ensure optimal cycling performance in these cells cycled to higher voltages.
While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims. Furthermore, various aspects of the invention may be used in other applications than those for which they were specifically described herein.
