Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
2000-01-05
2002-07-09
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S303000, C429S306000, C429S307000, C429S313000, C429S321000
Reexamination Certificate
active
06416906
ABSTRACT:
1. FIELD OF THE INVENTION
This invention relates to the performance improvements of lithium electrochemical cells and batteries. More particularly, the invention pertains to an improved electrolyte containing a siloxane additive for lithium-containing electrochemical cells and batteries.
2. DESCRIPTION OF THE PRIOR ART
Lithium battery structures are prepared from one or more lithium electrochemical cells. Such cells include an anode, a cathode and an electrolyte interposed between electrically insulated spaced apart positive and negative electrodes. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, such as nonaqueous aprotic organic solvents. Lithium metal is a preferred anode material for batteries, having superior thermodynamic and kinetic characteristics. Lithium is a good conductor of electricity and heat. Lithium metal is soft and malleable and can be readily extruded into thin sheets. However, it is well known that lithium is reactive with water and other reagents.
The anode includes a current collector, typically of nickel, iron, stainless steel and/or copper foil. The cathode also includes a current collector typically of aluminum, nickel, iron, stainless steel and/or copper foil. Commercially available lithium cells with a pure lithium anode are presently available only as primary cells for the reason of its activity.
Several disadvantages which occur as a result of this reactivity, namely, exothermic reactions and the formation of a passivating film on the anode surface. Liberation of heat from the exothermic reaction, could under some circumstances lead to an explosive release of energy. When primary lithium batteries are subjected to temperatures above recommended levels, exothermic reactions result with the liberation of intense heat.
Rechargeable or secondary cells are more desirable than primary (nonrechargeable)cells since the associated chemical reactions, which take place at the positive and negative electrodes of the battery are reversible. Electrodes for secondary cells are capable of being recharged by the application of an electrical charge thereto. Numerous advanced electrode systems have been developed for storing electrical charge. Concurrently much effort has been dedicated to the development of electrolytes capable of enhancing the capabilities and performance of electro-chemical cells. The most practical electrolytes from the standpoint of functionality, reasonable cost, and ease of handling are organic solutions containing lithium ion salts. Liquid electrolytes while demonstrating receptable ionic conductivity must be free from as many impurities as possible which may affect cell performance.
Lithium ion batteries employ non-aqueous electrolytes Comprising such salts as LiAsF
6
, LiClO
4
, Lithium Triflate LiBF
4
or LiPF
6
and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate and the like and must be free from as many impurities as possible which may affect cell performance. More particularly, the rechargeability of a lithium ion electrode is limited by side reactions of the lithium ion and auto-catalytic reaction within the cell, as disclosed is U.S. Pat. No. 5,830,600 issued to Norang, et al, which is herein incorporated by reference. The lithium ion electrode is limited by side reactions between metallic lithium and impurities, such as acids and water. The water may be present initially or generated in situ. The acids may also be formed in situ by the reaction of water with the components of the electrolyte. When such impurities react with the lithium ion anode there is formed a solid surface layer on the lithium ion anode which increases the impedance of the anode.
Undesired reactions between impurities in electrochemical cells and cell components have essentially formed upon reactivity at the anode. U.S. Pat. No. 5,419,985 issued to Kokshang describes the effect of impurities on the lithium ion anode and the lithium metal anode when water reacts with lithium to form a solid surface layer of high impedance. A lithium ion battery which uses graphite or carbon as negative electrode is also subject to passivation at the carbon electrode by undesired reaction caused by the presence of impurities, especially water and acids.
Liquid electrolytes while demonstrating acceptable ionic conductivity must be free from as many impurities as possible which may affect cell performance.
The problem of lithium reactivity toward the electrolyte has been addressed in various ways. One approach, as disclosed in U.S. Pat. No. 5,830,600, to reduce flammability and increase stability is to dissolve a lithium salt in a solvent such as a phosphate, a phospholene, a cyclophosphalene, a halogenated carbonate, a fluorinated polyether or a complex silane and mixtures thereof. These solvents provide nonflammable, self-extinguishing electrolytes for the lithium batteries. As solvents higher amounts of complex silane and siloxane compounds are required. These higher amounts, however, will cause loss of electrical performance particularly when used with a carbon dioxide generating compound as described in the patent.
Therefore, there is a need to provide more stable primary and secondary lithium batteries with respect to electrolyte performance and decomposition. Such batteries require an electrolyte that is chemically stable with respect to the lithium components and stop the corrosion of the metal current collectors such as aluminum and steel that occurs with certain lithium electrolytes.
This invention provides a selection of alkoxy silane compounds and a method for preventing decomposition of one or more lithium electrochemical cell components comprising a lithium ion anode or lithium ion anode or lithium metal anode, a lithium insertion compound cathode and a nonaqueous electrolyte. Incorporating a small amount of a low molecular weight alkoxy and/or dialkoxy silane to the nonaqueous electrolyte effectively overcomes the problem which arises between the interaction of the cell components and the water present. Such water reacts with the electrolyte which comprises a lithium salt in an organic solvent. This interaction between the salt and water results in the formation of acids. The method of adding select alkoxy silane compounds of this invention to the electrolyte effectively blocks the corrosive effect of the acids by forming reaction products which are relatively inert with respect to the function of the active material in the cell. Preferably the compounds are dissolved in the electrolyte. As a consequence of this reduced corrosion effect, the stability of primary batteries is enhanced and the capacity delivered from secondary batteries is improved even after extended recycling. Recharged or secondary batteries normally exhibit a loss in delivered capacity as a function of charge/discharge cycle. Another unexpected benefit imparted to lithium electrochemical cell by the alkoxy silane compositions of this invention is that it permits aluminum current collectors to be used with certain lithium electrolyte additives which would normally corrode aluminum collectors such as, lithium hexaphosphate, lithium tetrafluoroborate, lithium trifluorate inter alia.
The alkoxy silane compounds useful as additives according to the invention are of the general formula:
(R
1
O)
n
Si R
4−n
where R
1
is an alkyl group and R is selected from hydrogen, alkyl, alicyclic and aryl groups, where each of the R
1
and R constituents may be the same or different from each other, wherein the alkyl groups are straight or branched chains of less than ten carbon atoms and n may be an integer from 1 to 3. Particularly preferred compounds are trimethylethoxy silane, dimethyldiethoxy silane, methyltrimethoxy silane and dimethyldimithoxy silanes. based on the total weight of the electrolyte. These small are necessary to react selectively with small amounts of acid formed by trace amounts of impurities present on the anode, cathode or in the electrolyte. Also, the lower amounts do not interfere with the electrical properties of
McCloskey Joel
Smith W. Novis
Chaney Carol
John Lezdey & Assoc.
Lithdyne Internatioal Inc.
Yuan Dah-Wei D.
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