Lithium secondary cell with a stannous electrode material

Details Lithium secondary cell with a stannous electrode material Lithium secondary cell with a stannous electrode material

2000-06-06

2002-10-15

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S218100, C429S231950

Reexamination Certificate

active

06465131

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to lithium secondary cells comprising a positive electrode, a negative electrode and a nonaqueous electrolyte, and more particularly to improvements in active substances which undergo a reversible electrochemical reaction with lithium ions for use in the positive electrode or negative electrode of such a lithium secondary cell.
BACKGROUND OF THE INVENTION
For use in portable electronic devices such as compact video cameras, portable telephones and notebook personal computers, attention has been directed in recent years to lithium secondary cells, typical of which are lithium ion cells wherein the positive electrode is prepared from a lithium-containing transition metal oxide such as lithium-cobalt oxide (LiCoO
2
), lithium-nickel oxide (LiNiO
2
) or lithium-manganese oxide (LiMn
2
O
4
), or manganese dioxide (MnO
2
), and the negative electrode active substance used is metallic lithium, lithium alloy, carbon material capable of absorbing and desorbing lithium ions, or the like. Among these cells, lithium secondary cells have already been placed into actual use in which the carbon material serves as the negative electrode active substance.
Various materials of active substances other than those mentioned above are proposed for use in lithium cells. For example, U.S. Pat. No. 4,223,079 proposes lithium cells wherein a metal sulfide is used among these materials. Stannous sulfide (SnS) is used as the metal in the proposed lithium cell in an attempt to give improved characteristics to the cell.
However, the metal sulfide used in the proposed lithium cell as an active substance, i.e., stannous sulfide (SnS), is unstable in crystal structure and therefore has the problem that the cell can not be charged and discharged repeatedly and is consequently unusable as a secondary cell because of poor cycle characteristics.
SUMMARY OF THE INVENTION
An object of the present invention is to stabilize the crystal structure of stannous sulfide (SnS) and provide a lithium secondary cell exhibiting practically useful charge-discharge characteristics.
The present invention provides a lithium secondary cell wherein a compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or a lithium-containing compound sulfide comprising the compound sulfide and lithium contained therein is used as an active substance of one of the positive electrode and the negative electrode of the cell. The compound sulfide comprises at least one transition metal selected from among copper (Cu), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni), tin and sulfur. The compound sulfide is represented by the formula M
x
Sn
1−x
S wherein M is at least one metal selected from among Cu, V, Cr, Mn, Fe, Co and Ni, and 0.02≦x≦0.5.
The compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or the lithium-containing compound sulfide comprising this compound sulfide and lithium contained therein contains the metal element M (Cu, V, Cr, Mn, Fe, Co or Ni) in the crystal lattice of stannous sulfide (SnS), and the crystal structure of stannous sulfide (SnS) is stabilized with the Sn partly replaced by the metal element M. Accordingly, when the compound sulfide or the lithium-containing compound sulfide is used as an active substance for the positive electrode or negative electrode in fabricating a lithium secondary cell, the cell can be given improved charge-discharge cycle life characteristics. Incidentally, X-ray diffraction, for example, reveals that the compound sulfide has substantially the same crystal structure as stannous sulfide (SnS).
It is desired that the metal element M be selected from among copper (Cu), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co) and nickel (Ni) which have been found effective for affording improved charge-discharge cycle life characteristics. It is known that these metal elements M each form a stable compound having a decomposition temperature of at least 1000° C. when combined with sulfur (S) [see, for example, Binary Alloy Phase Diagrams (1986), American Society for Metals, M—S Binary Phase Diagram]. Stated more specifically, a great chemical bond force acts between the metal element M and sulfur (S), causing the element M to occupy part of the crystal lattice of stannous sulfide (SnS) phase to stabilize the crystal structure. This affords improved charge-discharge cycle life characteristics to the lithium secondary cell wherein the compound sulfide or the lithium-containing compound sulfide is used as an active substance.
Other metal elements M, such as cadmium (Cd), indium (In), molybdenum (Mo), lanthanun (La), cerium (Ce), samarium (Sm) and platinum (Pt), which form a compound with sulfur (S) are also expected to be similarly effective for giving improved charge-discharge cycle life characteristics. The upper limit for the proportion x of the metal element M in the compound sulfide is 0.5 because if the proportion x of the metal element M exceeds 0.5, the single phase of metal element M or the sulfide phase will precipitate to impair the effect to improve the cycle life characteristics. In other words, limiting the proportion x of the metal element M to not higher than 0.5 prevents the precipitation of the single phase of metal element M or the sulfide phase to result in a stabilized crystal structure and improved cycle life characteristics.
In the case where the compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or the lithium-containing compound sulfide is used as a positive electrode active substance, preferably as the negative electrode active substance to be used is a carbon material, such as graphite (natural graphite or artificial graphite), coke or organic fired body, which is capable of electrochemically absorbing and desorbing lithium (Li), Li—Al alloy, Li—In alloy, Li—Al—Mn alloy or like lithium alloy, or metallic lithium. In this case, the final charge voltage is about 3.4 V, and discharge voltage is about 2.9 V.
Among these negative electrode active substances, the carbon material, when used, affords a greater effect to improve the cycle life characteristics. This is attributable to the fact that the carbon material, unlike lithium alloys or metallic lithium, will not grow into dentritic crystals due to charging and discharging and leading to short-circuiting. Additionally, the carbon material is free of the likelihood that sulfur (S) dissolving in a very small amount in the electrolyte reacts with the lithium (Li) in the negative electrode of lithium alloy or metallic lithium to form on the negative electrode surface a compound such as Li
2
S [see, for example, Binary Alloy Phase Diagrams, Vol. 2, p. 1500(1986), American Society for Metals, Li—S Binary Phase Diagram] which will cause inactivation.
Further in the case where the compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or the lithium-containing compound sulfide is used as the negative electrode active substance, preferable as the positive electrode active substance is a lithium-containing transition metal oxide such as LiCoO
2
, LiNiO
2
, LiMn
2
O
4
, LiMnO
2
, lithium-containing MnO
2
, LiCo
0.5
Ni
0.5
O
2
or LiNi
0.7
Co
0.2
Mn
0.1
O
2
. Available in this case is a lithium secondary cell which is about 1.2 V in final charge voltage and about 0.7 V in discharge voltage. When the compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) or the lithium-containing compound sulfide is used for the negative electrode, more improved cycle life characteristics result than when the sulfide is used for the positive electrode. This is attributable to a lower charge voltage which suppresses decomposition of the electrolyte.
The compound sulfide having substantially the same crystal structure as stannous sulfide (SnS) can be prepared by firing a mixture of the elements constituting the compound sulfide, compounds of the compo

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