Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reissue Patent
1999-01-08
2001-02-27
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S217000, C429S218100, C429S224000, C429S231100, C429S231500, C429S344000
Reissue Patent
active
RE037076
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrochemical devices and, more particularly, to non-aqueous electrochemical cells demonstrating improved performance.
BACKGROUND OF THE INVENTION
Non-aqueous lithium electrochemical cells typically include an anode of metallic lithium, a lithium electrolyte prepared from a lithium salt dissolved in one or more organic solvents and a cathode of an electrochemically active material, typically a chalcogenide of a transition metal. During discharge, lithium ions from the anode pass through the liquid electrolyte to the electrochemically active material of the cathode where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed so that lithium ions pass from the electrochemically active material through the electrolyte and are plated back onto the lithium anode.
Recently, the lithium metal anode has been replaced with a carbon anode such as coke or graphite intercalated with lithium metal to form Li
x
C. In operation of the cell, lithium passes from the carbon through the electrolyte to the cathode where it is taken up just as in a cell with a metallic lithium anode. During recharge, the lithium is transferred back to the anode where it reintercalates into the carbon. Because no metallic lithium is present in the cell, melting of the anode does not occur even under abuse conditions. Also, because lithium is reincorporated into the anode by intercalation rather than by plating, dendritic and spongy lithium growth does not occur. Non-aqueous lithium electrochemical cells are discussed in U.S. Pat. Nos. 4,472,487, 4,668,595 and 5,028,500.
The use of carbon anodes however is not without problems. As Li
x
C is a reactive material which is difficult to handle in air, it is preferably produced in-situ in a cell. In doing so, some of the lithium is consumed in an irreversible process. This irreversible process results in an initial capacity loss for the cell which reduces the cell's overall performance. Furthermore, the cell often exhibits a progressive loss of capacity over numerous charge/discharge cycles. This progressive loss is commonly referred to as “capacity fade.”
In view of the above shortcomings associated with the prior art, there is a need for non-aqueous electrochemical devices that are capable of providing improved performance, including better cycle life, specific electrode capacities, and energy density.
SUMMARY OF THE INVENTION
The present invention is based in part on the discovery that lithium ion electrochemical cells demonstrating expectedly superior performance can be fabricated by employing anodes comprising graphite of extra high crystallinity and purity.
In one aspect, the invention is directed to an electrochemical cell comprising:
an anode comprising a composition having graphite particles that have a BET method specific surface area of about 6 to about 12 m
2
/g and a crystallite height L
c
of about 100 nm to about 120 nm, or higher, and wherein at least 90% (wt) of the graphite particles are less than 16 &mgr;m in size;
a cathode; and
a non-aqueous electrolyte containing a solvent and salt that is interposed between the anode and cathode.
In another aspect, the invention is directed to a method of fabricating an electrochemical cell comprising the steps of:
(a) providing an anode comprising a composition having graphite particles that have a BET method specific surface area of about 6 to about 12 m
2
/g and a crystallite height L
c
of about 100 nm to about 120 nm, or higher, and wherein at least 90% (wt) of the graphite particles are less than 16 &mgr;m in size
(b) providing a cathode; and
(c) forming a non-aqueous electrolyte containing a solvent-containing and salt that is interposed between said anode and said cathode.
Among the important features of the invention are that: (1) the anode has a specific electrode capacity of greater than 300 mAhr/g, (2) the electrochemical cell has a cycle life of greater than 1500 cycles, and (3) the electrochemical cell has a first cycle capacity loss of only about 10% to about 15%, where the first cycle capacity loss (%)=
(
first
cycle
charge
capacity
-
first
cycle
discharge
capacity
)
×
100
%
first
cycle
charge
capacity
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patent: 5352548 (1994-10-01), Fujimoto et al.
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Product Brochure “Graphite for Batteries” Lonza G&T, Ltd., (Sins, Switzerland), unknown.
Technical Documentation for “Graphite KS 6” Lonza G&T, Ltd. (Sins, Switzerland), Apr. 1993.
Technical Documentation for “Graphite SFG 44” Lonza G&T, Ltd. (Sins, Switzerland), Apr. 1993.
Technical Documentation for “Graphite SFG 15” Lonza G&T, Ltd. (Sins, Switzerland), Apr. 1993.
Technical Documentation for “Graphite KS 15” Lonza G&T, Ltd. (Sins, Switzerland), Apr. 1993.
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Deschere Linda M.
Kalafut Stephen
Valence Technology Inc.
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