Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Include electrolyte chemically specified and method
Reexamination Certificate
1999-05-19
2003-01-14
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Include electrolyte chemically specified and method
C429S331000
Reexamination Certificate
active
06506524
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to secondary cells and, in particular, to an electrolyte for alkali metal-ion secondary cells.
BACKGROUND OF THE INVENTION
Concerns about the impact of the disposal of batteries on the environment have led to the development and improvement of rechargeable cells, also referred to as secondary cells by those skilled in the art. Non-aqueous alkali metal secondary cells typically include an anode of an alkali metal, such as lithium, potassium, or sodium, an electrolyte prepared from an alkali metal salt dissolved in an organic solvent, and a cathode of an electrochemically active material, typically a chalcogenide of a transition metal. During discharge, alkali metal-ions from the anode pass through the electrolyte to the cathode where the ions are taken up with a simultaneous release of electrical energy. During charging, the flow of ions is reversed so that alkali metal-ions pass from the cathode through the electrolyte and are plated on the anode. During each discharge/charge cycle, small amounts of the alkali metal and electrolyte are consumed by chemical reactions on newly created surfaces on the alkali metal of the anode. This reaction condition is further aggravated by the tendency of the alkali metal to form dendrites as it is plated back onto the anode. The dendrites continue to grow until they eventually contact the cathode, thereby causing the cell to fail. Furthermore, the alkali metal may not cohesively plate onto the anode during the charge cycle, resulting in the formation of spongy deposits near the surface of the anode. The spongy deposits are not in electrically conductive contact with the anode and eventually may adversely affect the capacity of the cell. This consumption of the alkali metal may be minimized by providing a sheet-like microporous separator on the surface of the alkali metal and by applying substantial pressure on the separator and the anode, so that the alkali metal is deposited on the anode in the form of a layer, thereby preventing the growth of dendrites and spongy deposits. Typically, pressure is applied as an interelectrode pressure, also referred to by those skilled in the art as “stack pressure”. However, only cells with cylindrical symmetry are capable of withstanding the stack pressure with a thin metal casing. Rectangular and coin-shaped cells would require very thick metal casings in order to withstand the stack pressure without excessive flexing. However, the cell would then be significantly larger and more expensive to produce. Furthermore, microporous separators which are capable of preventing dendritic penetration and withstanding the applied slack pressure are typically very expensive. However, there is still a risk that the microporous separator will be punctured by dendritic growth. As a result, long recharge times are required to reduce the risk of puncture. Unfortunately the risk of puncture increases with repeated charging even on low rates, thereby limiting the number of discharge/charge cycles which may be obtained during the life of the cell. Even when a microporous separator and the appropriate stack pressure are used, a small percentage of the alkali metal is still consumed during each discharge/charge cycle. Thus in order to obtain a sufficiently long cycle life an excess of the alkali metal is required in the cell, thereby significantly increasing the cost and size of the cell. Moreover, alkali metals are extremely reactive and have low melting points. Accordingly, excess heat generated during extended operation, especially in relatively large cells, may lead to melting of the anode. Such melting may not only render the cell inoperative, but could also lead to an undesirable reaction between the alkali metal and electrolyte and to direct contact between the molten alkali metal and the electrochemically active material of the cathode, resulting in a vigorous reaction that could rupture the cell casing.
Thus, there is a need for a secondary cell which will provide the advantages provided by an alkali metal anode, but which will not have the drawbacks associated with this type of cell. One approach has been to replace the alkali metal anode with a carbonaceous anode formed by a carbonaceous material intercalated with alkali metal-ions to form compounds of the formula M
x
C, wherein M represents an alkali metal. In operation of the cell, alkali metal-ions pass from the intercalated carbonaceous material through the electrolyte to the cathode. When the cell is recharged, the alkali metal-ions are transferred back to the anode for re-intercalation with the carbonaceous material, thereby preventing the formation of dendrites or spongy deposits. Furthermore, melting of the anode cannot occur, even under extended periods of operation, because the alkali metal of the anode is not in a metallic form.
Suitable carbonaceous materials include graphite, coke, carbon fibre, pyrolytic carbon, non-graphitizable carbon and chemically modified carbon. Different forms of carbonaceous material which are at least partially crystalline can be characterized by their respective degrees of graphitization. The term “degree of graphitization” refers to the value g according to the formula:
g
=(3.45
−d
002
)/0.085
wherein d
002
represents the spacing (A) between the graphitic layers of the carbonaceous material in the crystal structure, determined by standard X-ray diffraction techniques, and g represents a dimensionless number with a value between 0 and 1.0. In general, carbonaceous material having a high degree of graphitization, for example graphite, has a more ordered microstructure, whereas carbonaceous material having a low degree of graphitization, for example coke, has a less ordered microstructure. A high degree of graphitization in the carbonaceous material of the anode provides a higher cell capacity in conjunction with less variation of cell voltage.
The voltage profile, reversibility and final stoichiometry of the alkali metal-intercalated carbonaceous material is dependent on the structure of the carbonaceous material. For example, petroleum coke has a turbostratic structure, shows a steep voltage profile, and intercalates up to a stoichiometry of Li
0.5
C
6
. On the other hand, graphite has a nearly perfect layered structure and is able to intercalate up to a stoichiometry of LiC
6
, with a flat voltage curve near zero volts relative to lithium. The theoretical capacity of a graphite anode is 372 mAh/g based on the stoichiometry of LiC
6
, thereby making graphite one of the most desirable candidates for a carbonaceous anode material (Shu, Z. X. et al “Electrochemical Intercalation of Lithium into Graphite” J Electrochem Soc 140: 4: 922-927; 1993). Highly graphitic carbonaceous materials such as graphite are inexpensive, non-toxic and are capable of incorporation into secondary cells having relatively high specific capacities. Canadian Patent Number 1,265,580 (Yoshino, A. et al. Feb. 6, 1990) discloses a secondary cell wherein the anode ions may be intercalated reversibly and the cathode is made of an active material consisting of a sulphide or an oxide of a transition metal.
However, there are numerous problems associated with the use of a carbonaceous anode. In particular, compounds of the formula M
x
C are reactive materials which are difficult to handle in air. Accordingly, the anode is preferably produced in situ in a cell by an initial intercalation step. However, some of the alkali metal-ions and the electrolyte are consumed in an irreversible reaction in the initial intercalation step. This irreversible reaction results in an initial capacity loss for the cell which reduces the overall performance thereof. Without being bound by theory, it is believed by those skilled in the art that the reaction which occurs during the initial intercalation step involves the formation of a passivation film on the bare surfaces of the carbonaceous material by decomposition of electrolyte salt and/or solvent. The ideal passivation film is insoluble in the electrolyte and is a
Davidson Isobel
McMillan Roderick S.
Murray John J.
Shu Zhi Xin
Worsfold Denis James
Anderson J. Wayne
National Research Council of Canada
Ryan Patrick
Wills M.
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