Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1996-06-11
2001-10-23
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231400
Reexamination Certificate
active
06306541
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of forming electrodes for rechargeable lithium ion batteries and the electrodes formed thereby. These electrodes can be used to form batteries with high capacities.
Lithium ion batteries are known in the art and are widely used as electric sources for lap top computers, cellular phones, camcorders and the like. They are advantageous in that they can provide high voltage, high energy density, small self-discharge, excellent long-term reliability and the like.
Rechargeable lithium ion batteries have a simple mechanism. During discharge, lithium ions are extracted from the anode and inserted into the cathode. On recharge, the reverse process occurs. The electrodes used in these batteries are very important and can have dramatic effects on the batteries' performance.
Positive electrodes known in the art for use in these rechargeable lithium ion batteries include metal chalcogenides, metal oxides, conductive polymers and the like. Negative electrodes (anodes) known in the art for use in rechargeable lithium ion batteries include compounds in which the lithium ion is incorporated into a crystal structure of inorganic materials such as WO
2
, Fe
2
O
3
and the like, and carbonaceous materials such as graphite and conductive polymers.
Properties which are desirable in electrode materials include 1) chemical inertness towards the other battery components such as the lithium ions, the electrolyte salts and the electrolyte medium; 2) the ability to store high quantities of lithium; 3) the ability to reversibly store or bind the lithium; 4) lithium storage that minimizes formation of metallic lithium clusters or agglomerates and, thus, minimizes safety concerns; and 5) a high density which allows for volume efficiency.
The electrodes to date, however, have not maximized these properties. For instance, while lithium metal provides the best electrode potential, large batteries constructed therewith have poor safety behavior. Likewise, while lithium alloys have reasonable electrode potentials and safety profiles, they often crack and fragment with the constant cycling of the battery.
The most desirable anode materials to date have been carbonaceous compounds such as graphite. Graphite is chemically inert, can bind reasonable amounts of lithium (cells with capacities of about 330 mAh/g of anode) with little being irreversible (about 10%), and it has a high density (about 2.2 g/cc
2
, although in the electrode the density is about 1.2 g/cc
2
). Cells with larger capacities, however, are often desired. References which discuss the use of graphite anodes include Dahn et al.; Science, 270, 590-3 (1995), Zheng et al., Chemistry of Materials, 8, 389-93 (1996); Xue et al.; J. of Electrochem. Soc., 142, 3668 (1995), Wilson et al.; Solid State Ionics, 74, 249-54 (1994), Wilson et al.; J. of Electrochem. Soc., 142, 326-32 (1995) and Xue et al.; J. of Electrochem. Soc., 142, 2927 (1995).
It has recently been suggested that the addition of boron, phosphorous or metals such as silicon to carbonaceous anodes can increase the capacity of the resultant batteries. Such batteries, however, have not achieved optimal results.
For instance, Tahara et al. in European publication 582,173 teach the use of a silicon oxide or a silicate as the negative electrode in a lithium ion battery. Similarly, Dahn et al. in European publication 685,896 teach the use of SiC containing materials as anodes in lithium ion batteries. These references, however, do not teach the methods or materials claimed herein.
The present inventors have now discovered that lithium ion batteries containing electrodes made from preceramic polysilanes can have many desirable properties heretofore unobtainable. For instance, such batteries can have large capacities with low irreversible capacity. In addition, these anode materials are chemically inert towards the other battery components, they minimize the agglomeration of lithium and they have a high density. Finally, these materials can be designed to have low hysteresis or a larger hysteresis. The Applicants herein postulate that the hysteresis of these materials may be valuable since it may reduce reaction rates between intercalated lithium and electrolyte under thermal abuse.
SUMMARY OF THE INVENTION
The present invention relates to a method of forming an electrode for a lithium ion battery. The method comprises first pyrolyzing a silane polymer to form a ceramic material. Lithium ions are then incorporated into the ceramic material to form the electrode.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on the unexpected discovery that lithium ion batteries containing anodes derived from polysilanes (also referred to as silane polymers) can provide the batteries with highly desirable properties. For instance, such batteries can have large capacities (the electrodes have the ability to store large quantities of lithium) with low irreversible capacity (the lithium is reversibly stored). In addition, these anode materials are chemically inert towards the other battery components, they minimize the agglomeration of lithium and they have a high density.
The electrodes of the present invention are formed from silane polymers. These polymers may contain units of general structure [R
1
R
2
R
3
Si], [R
1
R
2
Si], and [R
1
Si] where each R
1
, R
2
and R
3
is independently selected from the group consisting of hydrogen and hydrocarbons having 1-20 carbon atoms. The hydrocarbons include alkyl radicals such as methyl, ethyl, propyl etc., aryl radicals such as phenyl, and unsaturated hydrocarbon radicals such as vinyl. In addition, the above hydrocarbon radicals can contain hetero atoms such as silicon, nitrogen or boron. Examples of specific polysilane units are [Me
2
Si], [PhMeSi], [MeSi], [PhSi], [ViSi], [PhMeSi], [MeHSi], [MeViSi], [Ph
2
Si], [Me
2
Si], [Me
3
Si], and the like.
The polysilanes of this invention can be prepared by techniques well known in the art. The actual method used to prepare the polysilanes is not critical. Suitable polysilanes may be prepared by the reaction of organohalosilanes with alkali metals as described in Noll,
Chemistry and Technology of Silicones,
347-49 (translated 2d Ger. Ed., Academic Press, 1968). More specifically, suitable polysilanes may be prepared by the sodium metal reduction of organo-substituted chlorosilanes as described by West in U.S. Pat. No. 4,260,780 and West et al. in Polym. Preprints, 25, 4 (1984), both of which are incorporated by reference. Other suitable polysilanes can be prepared by the general procedures described in Baney, et al., U.S. Pat. No. 4,298,559 which is incorporated by reference.
The polysilane may also be substituted with various metal groups (i.e., containing repeating metal-Si units). Examples of suitable metals to be included therein include boron, aluminum, chromium and titanium. The method used to prepare said polymetallosilanes is not critical. It may be, for example, the method of Chandra et al. in U.S. Pat. No. 4,762,895 or Burns et al. in U.S. Pat. No. 4,906,710, both of which are incorporated by reference.
It should be noted that the term polysilane as used herein is intended to include copolymers or blends of the above polysilanes and other polymers which are also useful herein. For instance, copolymers of polysilanes and silalkylenes [R
1
R
2
Si (CH
2
)nSiR
1
R
2
O] (eg., silethylene), silarylenes (eg., silphenylene [R
1
R
2
Si (C
6
H
4
)nSiR
1
R
2
O]), siloxanes [R
1
R
2
SiO], silazanes, organic polymers and the like can be used herein, wherein R
1
and R
2
are as defined above. Moreover, blends of polysilanes and the above mentioned polymers are also useful herein. Finally, sugars which are modified with polysilanes are also contemplated and useful herein.
Generally, the silane polymer should be capable of being converted to ceramic materials with a ceramic
Dahn Jeffery Raymond
Wilson Alf M.
Xing Weibing
Zank Gregg Alan
Chaney Carol
Dow Corning Corporation
Gobrogge Roger E.
Severance Sharon K.
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