Patent Publication Number: US-2004053136-A1

Title: Lithium carbide composition, cathode, battery and process

Description:
BACKGROUND  
       [0001] Lithium ion rechargeable batteries (for example, the battery disclosed in U.S. Pat. No. 5,989,744) are a commercially successful source of portable electric power for cell phones and other electronic devices. The anode of a fully charged lithium ion rechargeable battery is usually graphite intercalated with metallic lithium. The cathode of such a battery is usually a mixture of graphite (or other electrically conductive carbonaceous material) and, for example, a cobalt oxide compound. The anode and cathode are usually immersed in a non-aqueous solution of lithium salt and separated by a porous polymer separator. During the discharge of such a battery, the metallic lithium of the anode gives up electrons to produce lithium ions that diffuse toward the cathode where lithium ions react with the cobalt oxide compound and the electrons to form a lithium cobalt oxide compound. During the recharging of such a battery, the lithium cobalt oxide compound gives up electrons to produce lithium ions that diffuse toward the anode where the lithium ions react with the electrons to produce metallic lithium.  
       [0002] Many improvements have been made to lithium ion batteries. Currently available lithium ion batteries using cobalt oxide material in the cathode provide excellent power to weight, cell voltage and cycle life characteristics. However, the cobalt oxide materials used in the cathode are relatively expensive, toxic and flammable. It would be an advance in the lithium ion battery art if a material were discovered to replace the cobalt oxide material that was less expensive, less toxic and non-flammable.  
       SUMMARY OF THE INVENTION  
       [0003] The central theme of the instant invention is the use of lithium carbide in the cathode of a lithium ion battery. It has been discovered that lithium carbide can be used to replace the prior art materials (such as a lithium cobalt oxide material) used in the cathode of a lithium ion battery to electrochemically release electrons and lithium ions. Lithium carbide is relatively inexpensive, non-toxic and non-flammable. A preferred cathode of the instant invention for use in a lithium ion rechargeable battery comprises graphite intercalated with a mixture of lithium carbide and a lithium salt such as lithium tetrafluoroborate.  
       [0004] In one embodiment, the instant invention is a composition of matter, comprising: graphite, the layers of covalently bonded carbon atoms of the graphite being intercalated with lithium carbide, the mole ratio of carbon of the graphite to the carbon of the lithium carbide being less than one hundred.  
       [0005] In another embodiment, the instant invention is a process for making a composition of matter comprising graphite, the layers of covalently bonded carbon atoms of the graphite being intercalated with lithium carbide, the mole ratio of carbon of the graphite to the carbon of the lithium carbide being less than one hundred, the process comprising the step of: contacting graphite with molten lithium carbide, the mole ratio of carbon of the graphite to the carbon of the lithium carbide being less than one hundred.  
       [0006] In yet another embodiment, the instant invention is a process for making a composition of matter comprising graphite, the layers of covalently bonded carbon atoms of the graphite being intercalated with lithium carbide and a lithium salt, the mole ratio of carbon of the graphite to the carbon of the lithium carbide being less than one hundred, the mole ratio of carbon of the graphite to the lithium of the lithium salt being less than one hundred, the process comprising the step of: contacting graphite with a molten mixture of lithium carbide and lithium salt, the mole ratio of carbon of the graphite to the carbon of the lithium carbide being less than one hundred, the mole ratio of carbon of the graphite to the lithium of the lithium salt being less than one hundred.  
       [0007] In another embodiment, the instant invention is an improved lithium ion secondary battery of the type comprising an anode, a graphite cathode, a porous separator between the anode and the cathode and an electrolyte in ion conducting contact with the anode, the cathode and the porous separator, wherein the improvement comprises: the layers of covalently bonded carbon atoms of the graphite of the cathode being intercalated with lithium carbide when the improved battery is in the discharged state, the mole ratio of carbon of the graphite to the carbon of the lithium carbide being less than one hundred.  
       [0008] In another embodiment, the instant invention is an improved cathode for a lithium ion secondary battery, the cathode comprising an electrically conductive carbonaceous material and a precursor dispersed in the electrically conductive carbonaceous material, which precursor reacts with lithium ion to produce a lithium compound when the lithium ion secondary battery is being discharged, wherein the improvement comprises: that the lithium compound is lithium carbide, the mole ratio of carbon of the electrically conductive carbonaceous material to the carbon of the lithium carbide being less than one hundred.  
       [0009] In another embodiment, the instant invention is a process for producing electricity, comprising the steps of: (a) conducting electrons from metallic lithium to produce lithium ions; and (b) reacting lithium ions with lithium depleted lithium carbide and the electrons to form lithium carbide.  
       [0010] In another embodiment, the instant invention is a process for storing electricity, comprising the steps of: (a) conducting electrons from lithium carbide to produce lithium ions; and (b) reacting lithium ions with the electrons to form metallic lithium. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIG. 1 is a cross-sectional schematic side view of a prior art lithium ion rechargeable battery in its recharge mode;  
     [0012]FIG. 2 is a cross-sectional schematic side view of a prior art lithium ion rechargeable battery in its discharge mode;  
     [0013]FIG. 3 is a cross-sectional schematic side view of a lithium ion rechargeable battery of the instant invention in its recharge mode; and  
     [0014]FIG. 4 is a cross-sectional schematic side view of a lithium ion rechargeable battery of the instant invention in its discharge mode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0015] Referring now to FIGS. 1 and 2, therein is shown a cross-sectional schematic side view of a prior art lithium ion rechargeable battery  10  having a case  11  containing a non-aqueous solution or gel  12  of lithium salt (such as LiPF 6  or LiBF 4  dissolved in ethylene or propylene carbonate). The anode  13  of a recharged battery  10  is typically graphite intercalated with metallic lithium (but the anode can simply be an electrode made of lithium metal). The anode  13  is shown in schematic form with the crystalline layers of the graphite depicted as being connected at one edge thereof. The cathode  15  is typically an electrically conductive carbonaceous material such as graphite having a lithium compound  16  dispersed therewith. The cathode  15  is also shown in schematic form as graphite with the crystalline layers of the graphite depicted as being connected at one edge thereof. The lithium compound  16  is typically a lithium cobalt oxide material. An optional porous separator  17  is used to prevent contact between the anode  13  and the cathode  15 . The separator  17  is typically a porous polymer such as porous polyethylene or porous polypropylene.  
     [0016] Referring now to FIG. 1, when the battery  10  is recharged, electrons are conducted from the lithium compound  16  by the cathode  15  to produce lithium ions  21 . The electrons flow through the generator  22  (or other such source of electricity) to the anode  13 . Lithium ions diffuse through the separator  17  to the anode  13 . The lithium ions react with the electrons in the anode  13  to form metallic lithium  14  intercalated in the graphite anode  13 .  
     [0017] Referring now to FIG. 2, when the battery  10  is discharged, electrons are conducted from the metallic lithium  14  by the anode  13  to produce lithium ions  18 . The electrons flow through the motor  19  (or other load) to the cathode  15 . Lithium ions diffuse through the separator  17  to the cathode  15 . Lithium ions react with a precursor material (such as a cobalt oxide) and the electrons to form the lithium compound  16 . The relative voltage difference between the anode  13  and the cathode  15  is typically about 3.6 volts.  
     [0018] The following United States Patents (herein fully incorporated by reference) will provide a person skilled in the art with a review of the lithium ion rechargeable battery art: U.S. Pat. Nos. 4,687,716; 4,828,834; 5,053,297; 5,168,019; 5,273,842; 5,292,601; 5,370,710; 5,427,874; 5,427,875; 5,437,945; 5,451,477; 5,474,752; 5,561,005; 5,580,684; 5,629,107; 5,639,575; 5,683,672; 5,691,620; 5,705,292; 5,709,969; 5,714,281; 5,763,119; 5,773,165; 5,804,333; 5,834,138; 5,972,536; 5,989,744; 6,022,641; 6,064,182; 6,066,414; 6,083,646; 6,093,505; 6,120,938; 6,124,700; 6,127,065; 6,146,790; 6,277,516; 6,300,013; 6,335,122; 6,395,428; and 6,440,609.  
     [0019] In one embodiment the instant invention is a composition of matter comprising graphite wherein the layers of covalently bonded carbon atoms of the graphite are intercalated with lithium carbide and wherein the mole ratio of carbon of the graphite to the carbon of the lithium carbide is less than one hundred. Preferably, the mole ratio of carbon of the graphite to the carbon of the lithium carbide is less than thirty. More preferably, the mole ratio of carbon of the graphite to the carbon of the lithium carbide is less than ten. Even more preferably, the mole ratio of carbon of the graphite to the carbon of the lithium carbide is less than three. The maximum amount of lithium carbide that can be intercalated in graphite is probably a mole ratio of carbon of the graphite to the carbon of the lithium carbide of about one half. Lower mole ratios of the carbon of the graphite to the carbon of the lithium carbide result in a higher capacity for a given volume or weight of cathode but compositions having the maximum amount of lithium carbide intercalated in the graphite are not preferred because it is believed that such compositions will probably show relatively slower lithium ion conductivity.  
     [0020] It is also preferable that a lithium salt or mixture of lithium salts also be intercalated into the graphite, the mole ratio of carbon of the graphite to the lithium of the lithium salt(s) being less than one hundred. The presence of the lithium salt(s) reduces the maximum amount of lithium carbide that can be used but increases the lithium ion conductivity of the composition. Preferably, the mole ratio of carbon of the graphite to the lithium of the lithium salt(s) is less than thirty. More preferably, the mole ratio of carbon of the graphite to the lithium of the lithium salt(s) is less than ten. Most preferably, the mole ratio of carbon of the graphite to the carbon of the lithium carbide is less than three and the mole ratio of lithium salt(s) to lithium carbide is about one to three. Lithium salts that can be used for this purpose include LiClO 4 , LiPF 6 , LiAsF 6 , LiBF 4 , LiCF 3 SO 3 , and LiN(CF 3 SO 2 ) 2  and probably even LiCl and LiF. Most preferably, the lithium salt used consists essentially of lithium tetrafluoroborate (LiBF 4 ). The term “consists essentially of lithium tetrafluoroborate” means the commercial grade of lithium tetrafluoroborate.  
     [0021] Graphite is the preferred matrix material for the improved cathode of the instant invention. However, it is believed that in the full scope of the instant invention it is possible to disperse the lithium carbide in any electrically conductive carbonaceous material such as the prior art electrically conductive carbonaceous materials for the cathode of a lithium ion rechargeable battery disclosed in the patent references above.  
     [0022] Crystalline Lithium carbide can be made by reacting lithium metal with carbon at 800-900 degrees Celsius (Juza, et al., Zeitschrift fur Anorganische und Allgemeine Chemie, 352, pp252-257, (1967)) or by reacting lithium carbonate with carbon at 800-950 degrees Celsius (Kroger, et al., Zeitschrift fur Anorganische und Allgemeine Chemie, 212, pp 269-283 (1933)). Crystalline lithium carbide is reported to melt at about 450 degrees Celsius (Inorg. Mater. (Transl. Of Neorg. Mater.) (1997), 33,(11), 1103-1105. Lithium carbide intercalates into graphite when molten lithium carbide is exposed to graphite. A mixture of lithium carbide and lithium salt(s) intercalates into graphite when a molten mixture of lithium carbide and the salt(s) is exposed to graphite. For example, lithium carbide and lithium tetrafluoroborate intercalates into graphite when a molten mixture of lithium carbide and lithium tetrafluoroborate is exposed to graphite. Prior to such exposure, the graphite is preferably heated to four hundred degrees Celsius under vacuum for one hour to remove adsorbed gasses and other adsorbed impurities.  
     [0023] In another embodiment, the instant invention is an improved lithium ion rechargeable battery, i.e., a “secondary battery”, of the type comprising an anode, a graphite cathode, a porous separator between the anode and the cathode and an electrolyte in ion conducting contact with the anode, the cathode and the porous separator. The constitution of the conventional components of the battery of the instant invention such as the anode, the separator, the electrolyte solvent, the battery case and shape is not limited to a particular type. The improvement of the instant invention is to use the above-described composition of matter as the cathode.  
     [0024] Referring now to FIGS. 3 and 4, therein is shown a cross-sectional schematic side view of a lithium ion rechargeable battery  30  according to the instant invention having a case  31  containing a non-aqueous solution  32  of lithium salt(s) (such as LiBF 4  dissolved in ethylene or propylene carbonate). The anode  33  is typically graphite to be intercalated with metallic lithium  33   a.  The cathode  34  is graphite intercalated with lithium carbide  40  (or lithium carbide dispersed in another electrically conductive carbonaceous material). An optional porous separator  37  is used to prevent inadvertent contact between the anode  33  and the cathode  34 .  
     [0025] The term “intercalated” used herein with regard to graphite means that a material has entered between the crystal lattice planes of the graphite. For example, it is well known in the lithium ion rechargeable battery art that lithium ions can diffuse between the crystal lattice planes of graphite and react with electrons to produce a metallic form of lithium, i.e., lithium in the neutral charge state, with a maximum metallic lithium loading of about one lithium per six carbons of the graphite. Lithium carbide can also enter between the crystal lattice planes of graphite to produce graphite intercalated with lithium carbide.  
     [0026] Referring now to FIG. 3, when the battery  30  is recharged, electrons are conducted from the lithium carbide  40  by the cathode  34  to produce lithium ions  41 . The electrons flow through the generator  42  (or other such source of electricity) to the anode  33 . Lithium ions diffuse through the separator  37  to the anode  33 . The lithium ions react with the electrons in the anode  33  to form metallic lithium  33   a  intercalated in the graphite anode  33 .  
     [0027] Referring now to FIG. 4, when the battery  30  is discharged, electrons are conducted from the metallic lithium  33   a  by the anode  33  to produce lithium ions  38 . The electrons flow through the motor  39  (or other load) to the cathode  34 . Lithium ions diffuse through the separator  37  to the cathode  34 . Lithium ions react with lithium depleted lithium carbide  40   a  and the electrons to form lithium carbide. The voltage difference between the anode  33  and the cathode  34  provides the electrical driving force for powering the motor  39  or other electrical load.  
     [0028] In the above discussion the term “lithium depleted lithium carbide” is used to describe the material that is left behind when lithium ions and electrons are removed from the lithium carbide  40 . The exact nature of lithium depleted lithium carbide is not known and does not need to be known to make and use the instant invention. However, lithium depleted lithium carbide is probably a mixture of Li 2 C 2 , LiC 2  and perhaps C 2  (plus the lithium salt, if used) in various ratios depending on the state of charge of the cathode. Or, perhaps lithium depleted lithium carbide is a hybrid solid-state glassy material of formula Li x C 2  (plus the lithium salt, if used) where the value of x varies (perhaps from zero to two) depending on the state of charge of the cathode. Preferably, the amount of lithium depleted from the lithium carbide of a fully recharged cathode of the instant invention is less than one half of the theoretical maximum amount that is available.  
     EXAMPLE 1  
     [0029] Crystalline lithium carbide is synthesized and purified as described by Juza, et al., Zeitschrift fur Anorganische und Allgemeine Chemie, 352, pp252-257, (1967). Twelve grams of 200-400 mesh sized graphite is heated to four hundred degrees Celsius in a vacuum for one hour. 11.72 grams of lithium tetrafluoroborate and 14.22 grams of crystalline lithium carbide are mixed, melted and added to the graphite. After the graphite absorbs the molten mixture of lithium tetrafluoroborate and lithium carbide the resulting product is cooled to room temperature and wetted with a saturated solution of lithium tetrafluoroborate in propylene carbonate in a dry box.  
     [0030] A discharged prior art lithium ion rechargeable battery having an electrolyte of lithium tetrafluoroborate in propylene carbonate is disassembled in the dry box. The wetted graphite/lithium carbide/lithium tetrafluoroborate composition is pressed into the same shape as the cathode removed from the prior art lithium ion rechargeable battery. The prior art lithium ion rechargeable battery is reassembled using all of its original components but replacing its original cathode with the cathode pressed from the graphite/lithium carbide/lithium tetrafluoroborate composition to produce a lithium ion rechargeable battery according to the instant invention.  
     EXAMPLE 2  
     [0031] Crystalline lithium carbide is synthesized and purified as described by Kroger, et al., Zeitschrift fur Anorganische und Allgemeine Chemie, 212, pp 269-283 (1933). Twelve grams of 200-400 mesh sized graphite is heated to four hundred degrees Celsius in a vacuum for one hour. Four grams of lithium tetrafluoroborate and five grams of crystalline lithium carbide are mixed, melted and added to the graphite. After the graphite absorbs the molten mixture of lithium tetrafluoroborate and lithium carbide the resulting product is cooled to room temperature and wetted with a saturated solution of lithium tetrafluoroborate in propylene carbonate in a dry box.  
     [0032] A discharged prior art lithium ion rechargeable battery having an electrolyte of lithium tetrafluoroborate in propylene carbonate is disassembled in the dry box. The wetted graphite/lithium carbide/lithium tetrafluoroborate composition is pressed into the same shape as the cathode removed from the prior art lithium ion rechargeable battery. The prior art lithium ion rechargeable battery is reassembled using all of its original components but replacing its original cathode with the cathode pressed from the graphite/lithium carbide/lithium tetrafluoroborate composition to produce a lithium ion rechargeable battery according to the instant invention.  
     EXAMPLE 3  
     [0033] The example of Example 1 is repeated except that no lithium tetrafluoroborate is used and two grams of crystalline lithium carbide is used.  
     EXAMPLE 4  
     [0034] The example of Example 1 is repeated except that 4.7 grams of crystalline lithium carbide is used.  
     EXAMPLE 5  
     [0035] The example of Example 1 is repeated except that 2.4 grams of crystalline lithium carbide is used.  
     EXAMPLE 6  
     [0036] This example is of an improved lithium ion rechargeable battery of the instant invention in the shape of a coin. Crystalline lithium carbide is prepared and purified as described by Kroger, et al., Zeitschrift fur Anorganische und Allgemeine Chemie, 212, pp 269-283 (1933). Twelve grams of 200-400 mesh sized graphite is heated to four hundred degrees Celsius in a vacuum for one hour. 11.72 grams of lithium tetrafluoroborate and 14.22 grams of crystalline lithium carbide are mixed, melted and added to the graphite. After the graphite absorbs the molten mixture of lithium tetrafluoroborate and lithium carbide the resulting product is cooled to room temperature and wetted with a saturated solution of lithium tetrafluoroborate in propylene carbonate in a dry box. A portion of the wetted graphite/lithium carbide/lithium tetrafluoroborate composition is pressed in the dry box into a disk shaped cathode one millimeter thick and ten millimeters in diameter.  
     [0037] Twelve grams of 200-400 mesh sized graphite is heated to four hundred degrees Celsius in a vacuum for one hour, cooled to room temperature and then wetted with a saturated solution of lithium tetrafluoroborate in propylene carbonate in the dry box. A portion of the wetted graphite is pressed in the dry box into a disk shaped anode two millimeters thick and ten millimeters in diameter.  
     [0038] A one half millimeter thick and ten millimeter diameter porous polypropylene disk shaped separator is wetted with a saturated solution of lithium tetrafluoroborate in propylene carbonate in the dry box. The cathode, separator and anode are stacked together and sealed in a close fitting polypropylene case having sealed in electrical leads to the anode and to the cathode.