Abstract:
A method of making a lithium electrochemical cell includes treating the cathode active material with an agent that includes lithium but not sodium. A cathode including the cathode active material, an anode, a separator, and an electrolyte are assembled in a housing to provide a cell containing less than 1500 ppm by weight of sodium.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of and claims priority to U.S. Ser. No. 11/516,084, filed on Sep. 6, 2006, now U.S. Pat. No. 7,566,350 which is a divisional application of U.S. Ser. No. 10/085,303, filed on Feb. 28, 2002, now abandoned which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to non-aqueous electrochemical cells. 
     BACKGROUND 
     Batteries are commonly used electrical energy sources. A battery contains a negative electrode, typically called the anode, and a positive electrode, typically called the cathode. The anode contains an active material that can be oxidized; the cathode contains or consumes an active material that can be reduced. The anode active material is capable of reducing the cathode active material. 
     When a battery is used as an electrical energy source in a device, electrical contact is made to the anode and the cathode, allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge. 
     It is desirable for batteries to have good capacity, after they are produced and/or after they have been stored for extended periods of time. 
     SUMMARY 
     The invention relates to non-aqueous electrochemical cells. 
     In one aspect, the invention features a lithium electrochemical cell including an electrolyte having a mixture of solvents including propylene carbonate and dimethoxyethane, and a salt mixture including lithium trifluoromethanesulfonate and lithium trifluoromethanesulfonimide. The cell contains less than 1500 ppm by weight of sodium. 
     Embodiments of the invention may include one or more of the following features. The cell contains less than 1200 ppm, e.g., less than 1000 ppm, less than 800 ppm, or less than 600 ppm, by weight of sodium. The cell includes a mixture of solvents having 40-80%, e.g., 50-75%, by weight of dimethoxyethane, and 20-60%, e.g., 25-50%, by weight of propylene carbonate; and the salt mixture has a concentration between 0.4 M and 1.2 M in the mixture of solvents. 
     In another aspect, the invention features a lithium electrochemical cell including an electrolyte having a mixture of solvents including ethylene carbonate, propylene carbonate, and dimethoxyethane, and a salt mixture having lithium trifluoromethanesulfonate and lithium trifluoromethanesulfonimide. The cell contains less than 1500 ppm, e.g., less than 1200 ppm, less than 1000 ppm, less than 800 ppm, or less than 600 ppm by weight of sodium. 
     Embodiments of the invention may include one or more of the following features. The cell includes a mixture of solvents having less than 30%, e.g. 10-20%, by weight of ethylene carbonate, 40-85%, e.g., 50-70%, by weight of dimethoxyethane; and the salt mixture has a concentration between 0.4 M and 1.2 M in the mixture of solvents. The cell includes a mixture of solvents including 5-15% by weight of ethylene carbonate, 70-80% by weight of dimethoxyethane, and 10-20% by weight of propylene carbonate. The cell contains less than 500 ppm by weight of sodium. 
     In another aspect, the invention features an electrolyte for a lithium electrochemical cell consisting essentially of a mixture of solvents including propylene carbonate and dimethoxyethane, and a salt mixture having lithium trifluoromethanesulfonate and lithium trifluoromethanesulfonimide. The electrolyte can be used in a cell, e.g., one that contains less than 1500 ppm, e.g., less than 1000 ppm, or less than 500 ppm, by weight of sodium. 
     The electrolyte mixture can include a mixture of solvents including 40-80%, e.g., 50-75%, by weight of dimethoxyethane, and 20-60%, e.g., 25-50%, by weight of propylene carbonate; and the salt mixture can have a concentration between 0.4 M and 1.2 M in the mixture of solvents. 
     In another aspect, the invention features an electrolyte for a lithium electrochemical cell consisting essentially of a mixture of solvents including ethylene carbonate, propylene carbonate, and dimethoxyethane, and a salt mixture having lithium trifluoromethanesulfonate and lithium trifluoromethanesulfonimide. The electrolyte can be used in a cell, e.g., one that contains less than 1500 ppm, e.g., less than 1000 ppm, or less than 500 ppm by weight of sodium. 
     The electrolyte may include a mixture of solvents including less than 30%, e.g., 10-20%, by weight of ethylene carbonate, and 40-85%, e.g., 50-70%, by weight of dimethoxyethane; and the salt mixture may have a concentration between 0.4 M and 1.2 M in the mixture of solvents. 
     The electrolyte may include a mixture of solvents including 5-15% by weight of ethylene carbonate, 70-80% by weight of dimethoxyethane, and 10-20% by weight of propylene carbonate. 
     Embodiments may have one or more of the following advantages. The cells can have good capacity, after production and/or after storage for extended periods of time. 
     Other features and advantages of the invention will be apparent from the description of the preferred embodiments thereof and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of a nonaqueous electrochemical cell. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an electrochemical cell  10  includes an anode  12  in electrical contact with a negative current collector  14 , a cathode  16  in electrical contact with a positive current collector  18 , a separator  20 , and an electrolyte. Anode  12 , cathode  16 , separator  20 , and the electrolyte are contained within a case  22 . The electrolytic solution includes a mixture of solvents and a salt that is at least partially dissolved in the mixture of solvents. 
     The electrolyte can be in liquid, solid or gel (polymer) form. The electrolyte can contain an organic solvent such as propylene carbonate (PC), ethylene carbonate (EC), dimethoxyethane (DME), dioxolane (DO), tetrahydrofuran (THF), acetonitrile (CH 3 CN), gamma-butyrolactone, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) dimethylsulfoxide (DMSO), methyl acetate (MA), methyl formiate (MF), sulfolane or combinations thereof. The electrolyte can alternatively contain an inorganic solvent such as SO 2  or SOCl 2 . The solvents can be selected to provide a mixture having a combination of physical and chemical properties. For example, the electrolyte can include a first solvent, such as PC or EC, that is polar, sluggishly reactive, and/or high boiling; and a second solvent, such as DME, THF, or DMC, that is relatively less polar, volatile, relatively non-viscous, and/or unreactive. 
     The electrolyte can also contain a lithium salt such as lithium trifluoromethanesulfonate (LiTFS) or lithium trifluoromethanesulfonimide (LiTFSI), or a combination thereof Additional lithium salts, for example, lithium iodide, that can be included are listed in U.S. Pat. No. 5,595,841, which is hereby incorporated by reference in its entirety. 
     In some embodiments, cell  10  includes an electrolyte formed of a mixture of solvents having DME and PC, and a salt mixture of LiTFS and LiTFSI. Cell  10  can further have a low sodium concentration. The concentration of DME in the mixture of solvents can range from about 30% to about 85%. The concentration of DME in the mixture of solvents can be equal to or greater than 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80%; and/or equal to or less than 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, or 35%. The concentration of PC in the mixture of solvents can be equal to 100% minus the concentration of DME. For example, if the concentration of DME in the mixture of solvents is 75%, then the concentration of PC in the mixture of solvents is 25%. If the concentration of DME in the mixture of solvents is 50%-75%, then the concentration of PC in the mixture of solvents is 25%-50%. 
     For the LiTFS and LiTFSI salt mixture, the total concentration of salt in the mixture of solvents can range from about 0.4 M to about 1.2 M. The total concentration of LiTFS and LiTFSI in the mixture of solvents can be equal to or greater than 0.40 M, 0.45 M, 0.50 M, 0.55 M, 0.60 M, 0.65 M, 0.70 M, 0.75 M, 0.80 M, 0.85 M, 0.90 M, 0.95 M, 1.00 M, 1.05 M, 1.10 M, or 1.15 M; and/or equal to or less than 1.2 M, 1.15 M, 1.10 M, 1.05 M, 1.00 M, 0.95 M, 0.90 M, 0.85 M, 0.80 M, 0.75 M, 0.70 M, 0.65 M, 0.60 M, 0.55 M, 0.50 M, or 0.45 M. Of the total concentration of salt, the concentration of LiTFS in the mixture of solvents can be equal to or greater than 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95%; and/or equal to or less than 95%, 90%, 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, or 5%. The concentration of LiTFSI in the mixture of solvents can be equal to 100% minus the concentration of LiTFS in the mixture of solvents. For example, if the total concentration of salt in the mixture of solvents is 0.5 M, and the LiTFS concentration in the mixture of solvents is 90% (i.e., 0.45 M), then the LiTFSI concentration in the electrolyte mixture is 10% (i.e., 0.05 M). In embodiments, other types of salts can be added to the electrolyte. 
     The sodium concentration described herein applies to contents inside cell  10  plus any sodium on the inner side of case  20  and/or cap  24 . For example, the sodium content may apply to the electrolyte and a “jelly roll” inside cell  10 . The jelly roll is formed of anode  12 , cathode  16 , current collectors  14  and  18 , separator  20 , and any protective tapes used in cell  10 . 
     The sodium concentration in cell  10  can be between about 100 ppm to 1500 ppm by weight. The sodium concentration can be equal to or greater than 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, or 1400 ppm by weight; and/or equal to or less than 1500, 1400, 1300, 1200, 1100, 1000, 900, 800, 700, 600, 500, 400, 300, 200, or 100 ppm by weight. Generally, the concentration of sodium in cell  10  can be controlled, e.g., minimized or reduced, by controlling the manufacturing of the cell. For example, in embodiments in which electrolytic manganese dioxide (EMD) is used in cathode  16 , the EMD can be washed and/or neutralized after electrodeposition with an agent that does not contain sodium or have reduced amounts of sodium, such as LiOH. Case  20  can be cleaned with a solution that does not contain sodium or carefully washed. Generally, other components of cell  10 , such as anode  12  and separator  20 , can be similarly specified not to contain sodium or to contain reduced amounts of sodium. 
     Other materials can be added to the electrolyte mixture. For example, in certain embodiments, cell  10  includes an electrolyte formed of a mixture of solvents including EC, DME and PC, and a salt mixture of LiTFS and LiTFSI. The concentration of EC in the mixture of solvents can be between about 5% and 30%. The concentration of EC in the mixture of solvents can be equal to or greater than 5%, 10%, 15%, 20%, or 25%; and/or equal to or less than 30%, 25%, 20%, 15%, or 10%. The concentration of DME in the mixture of solvents can range from about 30% to about 85%. The concentration of DME in the mixture of solvents can be equal to or greater than 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80%; and/or equal to or less than 85%, 80%, 75%, 70%, 65%, 60%, 55%, 50%, 45%, 40%, or 35%. The concentration of PC in the mixture of solvents can be equal to 100% minus the concentration of EC and DME. For example, if the concentration of EC in the mixture of solvents is 15%, and the concentration of DME in the mixture of solvents is 60%, then the concentration of PC in the mixture of solvents is 25%. Examples of an EC:DME:PC solvent mixture are 14:62:24 and 10:75:15 by weight. 
     The LiTFS and LiTFSI concentrations in the electrolyte, e.g., 0.4-1.2 M, can be generally similar to those described herein. In embodiments, other types of salts can be added to the electrolyte. 
     The sodium concentration in cell  10 , e.g., 100-1500 ppm by weight, can be generally similar to those described herein. Without wishing to be bound by theory, it is believed that as the concentration of EC increases, the concentration of sodium can be increased without substantially affecting cell  10  adversely. Accordingly, in embodiments, the sodium concentration in cell  10  having EC can be between about 100 ppm to 3,000 ppm by weight. 
     The sodium concentration can be equal to or greater than 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1600, 1800, 1900, 2100, 2300, 2500, 2700, or 2900 ppm by weight; and/or equal to or less than 2900, 2700, 2500, 2300, 2100, 1900, 1800, 1600, 1500, 1400, 1300, 1200, 1100, 1000, 900, 800, 700, 600, 500, 400, 300, 200, or 100 ppm by weight. 
     Cathode  16  includes an active cathode material, which is generally coated on the cathode current collector. The current collector is generally titanium, stainless steel, nickel, aluminum, or an aluminum alloy, e.g., aluminum foil. The active material can be, e.g., a metal oxide, halide, or chalcogenide; alternatively, the active material can be sulfur, an organosulfur polymer, or a conducting polymer. Specific examples include MnO 2 , V 2 O 5 , CoF 3 , MoS 2 , FeS 2 , SOCl 2 , MoO 3 , S, (C 6 H 5 N) n , (S 3 N 2 ) n , where n is at least 2. The active material can also be a carbon monofluoride. An example is a compound having the formula CF x , where x is 0.5 to 1.0. The active material can be mixed with a conductive material such as carbon and a binder such as polytetrafluoroethylene (PTFE). An example of a cathode is one that includes aluminum foil coated with MnO 2 . The cathode can be prepared as described in U.S. Pat. No. 4,279,972. 
     Anode  12  can consist of an active anode material, usually in the form of an alkali metal, e.g., Li, K, or an alkaline earth metal, e.g., Ca, Mg. The anode can also consist of alloys of alkali metals and alkaline earth metals or alloys of alkali metals and Al. The anode can be used with or without a substrate. The anode also can consist of an active anode material and a binder. In this case an active anode material can include carbon, graphite, an acetylenic mesophase carbon, coke, a metal oxide and/or a lithiated metal oxide. The binder can be, for example, PTFE. The active anode material and binder can be mixed to form a paste that can be applied to the substrate of anode  12 . 
     In some embodiments, the sodium content in a lithium anode is less than about 500 ppm by weight, e.g., less than 400 ppm, less than 300 ppm, less than 200, or less than 100 ppm. The sodium content can be controlled by carefully controlling anode processing (e.g., lithium extrusion), cell processing, cell aging, cell predischarge, cell storage, and/or cell discharge. 
     Separator  20  can be formed of any of the standard separator materials used in nonaqueous electrochemical cells. For example, separator  20  can be formed of polypropylene (e.g., nonwoven polypropylene or microporous polypropylene), polyethylene, layers of polypropylene and polyethylene, and/or a polysulfone. 
     To assemble the cell, separator  20  can be cut into pieces of a similar size as anode  12  and cathode  16  and placed therebetween as shown in  FIG. 1 . Anode  12 , cathode  16 , and separator  20  are then placed within case  22 , which can be made of a metal such as nickel, nickel plated steel, stainless steel, or aluminum, or a plastic such as polyvinyl chloride, polypropylene, polysulfone, ABS or a polyamide. Case  22  is then filled with the electrolytic solution and sealed. One end of case  22  is closed with a cap  24  and an annular insulating gasket  26  that can provide a gas-tight and fluid-tight seal. Positive current collector  18 , which can be made of aluminum, connects cathode  16  to cap  24 . Cap  24  may also be made of aluminum. A safety valve  28  is disposed in the inner side of cap  24  and is configured to decrease the pressure within battery  10  when the pressure exceeds some predetermined value. Additional methods for assembling the cell are described in U.S. Pat. Nos. 4,279,972; 4,401,735; and 4,526,846. 
     Other configurations of battery  10  can also be used, including, e.g., the coin cell configuration. The batteries can be of different voltages, e.g., 1.5V, 3.0V, or 4.0V. 
     Other embodiments are within the claims.