Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
1994-08-02
2001-11-27
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S218100, C423S593100
Reexamination Certificate
active
06322927
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a cathode active material and cathodes for electric current producing and storage cells and method of making same.
BACKGROUND OF THE INVENTION
Lithium-based cells or batteries often comprise cathodes of transition metal oxides which are used as intercalation compounds. The intercalation reaction involves the interstitial introduction of a guest species, namely, lithium into the host lattice of the transition metal oxide, essentially without structural modification of the host lattice. Such intercalation reaction is essentially reversible because suitable transition states are achieved for both the forward and reverse of the intercalation reaction.
The basic components of a lithium cell typically include a lithium anode, a separator, and a metal oxide intercalation cathode active material such as a vanadium oxide compound also referred to as vanadates or vanadate compounds. The cathode is usually a mixture of such oxide compound and other components such as graphite and an electrolyte/binder which provide ionic transport. During cell operation, incorporation of lithium in the metal oxide occurs. Some vanadates have high initial capacities, which, however, rapidly decline especially in the first cycles. Many metal oxides are prepared in a complex process by mixing precursor components containing an alkali metal with vanadium pentoxide and then baking the mixture to a temperature in the range of about 700° C. (centigrade) to 800° C. to cause formation of the product. The molten product is then cooled and ground up into a powder. The melt process has certain disadvantages because it is difficult to handle molten metal oxides at high temperatures and special procedures are required; there is a reaction between the molten product and containers used for conducting the reaction which thereby causes contamination of the product; and a significant amount of mechanical energy is required to grind the cooled, solidified products into a powder for inclusion in a cathode composition of an electrochemical cell. Despite these difficulties, typical melt processes, as described in U.S. Pat. No. 5,013,620, continue to be used to obtain positive electrode active material, such as LiV
3
O
8
. Recently, it has been suggested to form vanadium oxide compounds by reaction of a precursor oxide with an alkali hydroxide such as LiOH (Pistoia U.S. Pat. No. 5,039,582). Still another approach relies on reaction of an alkali with metavanadate (VO
3
) in an acidified solution. This approach has been used with the alkali being potassium or sodium. (
Solid State Ionics
40/41 (1990) 585-588;
J. Power Sources
43/44 (1993) 561-568.) Despite the many available compounds and methods, it is desirable to have a new active material which has a high specific energy, high cycle life and high rate capability; and a method for preparing such active material which is relatively simple and economical, which does not require handling metal oxide constituents in a molten state, and which achieves good conversion of the starting materials to the final metal oxide product.
SUMMARY OF THE INVENTION
The present invention provides a cathode active material having as its major component, or consisting essentially entirely of an oxide of vanadium of the nominal general formula A
y
Z
x
V
3
O
8
where A and Z are each selected from the group of alkali metals, excluding lithium, x and y are each greater than 0 and where the sum of x and y is less than 2; and where such active material in a lithiated state is represented by the nominal general formula Li
a
A
y
Z
x
V
3
O
8
where a is greater than 0 and up to about 4. A preferred active material of the invention is represented by the nominal general formula Na
y
K
x
V
3
O
8
where the sum of x+y is greater than or equal to about one and less than two. Preferably, x+y is greater than one.
Preferably, the vanadium oxide based active material (A
y
Z
x
V
3
O
8
, Na
y
K
x
V
3
O
8
) is prepared for use in cells with an anode active material made of lithium or a compound which includes lithium. The cells also include an electrolyte which is electrochemically stable with respect to the cathode active material and the lithium, and which allows lithium ions from the anode (negative electrode) to move through the electrolyte to react electrochemically with the cathode (positive electrode) active material of the invention. The electrolyte may be liquid, solid, polymeric, and in the case of a liquid electrolyte, typically includes a separator.
A preferred lithium cell comprises the positive electrode active material of the invention, a negative electrode which is metallic lithium, and an electrolyte which is in the form of a polymeric network containing an electrolyte solution comprising a metal salt of lithium.
In one embodiment, the cathode active material of the nominal general formula (A
y
Z
x
V
3
O
8
, Na
y
K
x
V
3
O
8
) is prepared in a series of steps. In the first step, a mixture comprising sodium hydroxide and potassium hydroxide is prepared with the relative amounts of the hydroxides being sufficient to provide Na
y
and K
x
, where the sum of x+y is greater than 1 and less than 2. It is preferred that the hydroxide mixture be prepared with relative amounts of the hydroxides to provide, on the basis of mole equivalent in total of sodium (Na) and potassium (K), Na
1−x
K
x
, where x is greater than or equal to zero and less than or equal to one (0≦×≦1). In the next step, progressive amounts of an oxide of vanadium having the general formula V
2
O
5
(vanadium pentoxide) is added preferably while stirring the mixture. It is preferred that vanadium pentoxide be added so as to provide about three moles equivalent of vanadium for each mole equivalent of the Na
1−x
K
x
. Either before adding the vanadium pentoxide or while adding the vanadium pentoxide the temperature of the mixture is maintained in a range of up to about the boiling point of the mixture and preferably no less than about room temperature (i.e. 10° centigrade). The vanadium pentoxide and hydroxides react and such reaction is manifest by a change in the color from the characteristic yellow/yellow-red of vanadium to a red/brown color and the formation of a solid precipitate. The solid precipitate is separated from the mixture and dried to obtain a powder of an oxide of vanadium having the nominal general formula Na
(1−x)
K
x
V
3
O
8
. It should be noted that the value of oxygen in the final product may be slightly different from 8 if the sum of x+y exceeds 1 and if V
+4
is present together with V
+5
. Accordingly, the general formula may also be written as: Na
y
K
x
V
3
O
8±z
; where x+y is greater than 1 and less than 2, and the value of z normally varies between about −0.1 and +0.1, so that the amount of oxygen normally varies between about 7.9 and 8.1.
It is preferred that the mixture contains a stoichiometric amount of the hydroxides and pentoxides. This corresponds to about two moles total of the hydroxide for each three moles of vanadium pentoxide; this will yield a product having about one mole equivalent of total alkali (Na
1−x
K
x
) for each three moles of vanadium, since each mole of vanadium pentoxide contains two moles of vanadium. It is preferred that the mixture be heated to an elevated temperature in a range to about 80 to 90° centigrade for up to about 24 hours. The progress of the reaction as stated is monitored by observing a color change. It is preferred that the product precipitate, typically in the form of a gel, be separated from the mixture and dried at a temperature of about 300° centigrade.
The vanadium oxide product of the invention is in the form of a fine powder having a surprising small particle size on the order of one micron. It was tested in a cell to determine the behavior of specific capacity at an increasing number of charge and discharge cycles and showed markedly improved characteristics as compared to a vanadium oxide having only a single alkali constituen
Chaney Carol
Harness & Dickey & Pierce P.L.C.
Valence Technology Inc.
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