Cathode active material for lithium electrochemical cells

Details Cathode active material for lithium electrochemical cells Cathode active material for lithium electrochemical cells

2001-08-20

2004-06-22

Bell, Bruce F. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

Current producing cell, elements, subcombinations and...

Electrode

C429S231900, C429S224000, C423S595000, C423S596000, C423S594150

Reexamination Certificate

active

06753110

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a compound of molecular formula Li
x
Cr
y
Mn
2−y
O
4+z
, wherein 2.2<x<4, O<y<2 and z≧0, and to the use of this compound as a cathode material in secondary lithium and lithium ion cells.
The impetus for this invention was the recent, great increase in demand for rechargeable batteries that combine high specific and volumetric energy density and with low cost and thermal stability.
DESCRIPTION OF THE PRIOR ART
A lithium ion cell is a rechargeable electrochemical cell in which the electrochemically active components in both the cathode and the anode are lithium intercalation compounds. The intercalation compound serves as a host structure for lithium ions, which are either stored or released depending on the polarity of an externally applied potential. A lithium ion cell consists of a lithium intercalation cathode with an oxidizing potential and a lithium intercalation anode with a reducing potential. A lithium intercalation material is able to reversibly store and release lithium ions in response to an electrochemical potential. On discharge, in a lithium ion cell, lithium ions move from the anode to the cathode, and thereby, generate an electrochemical current. On charge, energy is consumed in forcing the lithium ions from the cathode to the anode. The greater the difference in the potentials of the cathode and the anode the greater the electrochemical potential of the resulting cell. The larger the amount of lithium which can be reversibly stored in and released from the cathode and the anode, the greater the capacity. The cell's discharge capacity reflects the time duration for which a cell can deliver a given current. Typical anodes for a lithium ion cell are made from carbonaceous materials such as graphite or petroleum cokes. Typical cathodes are made from transition metal oxides or sulphides. For ease of cell fabrication, lithium ion cells are normally built in the fully discharged state with lithium present only in the cathode and not in the anode. In this state usually both the anode and the cathode materials are air stable. Assembling the cell in the discharged state means that the ultimate capacity of the cell depends on the amount of lithium initially present in the cathode. For example, cathodes based on LiMnO
2
have twice the theoretical capacity of cathodes based on LiMn
2
O
4
.
Previous reports and patents on cathodes for lithium ion cells have proposed using various mixed oxides of lithium, such as LiCoO
2
, LiNiO
2
, LiNi
x
Co
1−x
O
2
and LiMn
2
O
4
as the active material.
The use of phases such as LiMn
2−x
Cr
x
O
4.35
where 0.2<x<0.4, and LiCr
x
Mn
2−x
O
4
where 0<x<1, in secondary lithium batteries which have a metallic lithium anode, are also known. See G. Pistoia et al, Solid State Ionics, 58, 285 (1992) and B. Wang et al, Studies of LiCr
x
Mn
2−x
O
4
for Secondary Lithium Batteries, extended Abstract from the Sixth International Meeting on Lithium Batteries, Munster, Germany, May 10-15, 1992. (See also J. Power Sources, 43-44. 539-546 (1992). The materials are described in the latter case as being of a cubic lattice structure. Also in the latter case, additional Li was inserted electrochemically. However, only an additional 0.4 mole equivalents of lithium could be inserted e.g. to provide an oxide of molecular formula Li
1.4
Cr
0.4
Mn
1.6
O
4
.
Also, a lithium-poor lithium-manganese spinel structure for use in secondary electrochemical cells, having a molecular formula of Li
q
M
x
Mn
y
O
z
where q is 0 to 1.3, is described in U.S. Pat. No. 5,169,736.
More recently, the amount of lithium in such mixed metal oxides has been increased. In our previous U.S. Pat. No. 5,370,949, issued 6 Dec. 1994, a single phase compound of molecular formula Li
2
Cr
x
Mn
2−x
O
4
, wherein 0<x<2, and its use in secondary lithium ion cells is described. These materials were prepared by standard solid state techniques and demonstrated good cycleability with discharge capacities up to 170 mAh/g at low current densities (3.6 mA/g). Higher discharge capacities were found for compositions in which at least half of the transition metal content was manganese. However phases with significantly more manganese than chromium developed a less commercially attractive two-plateau voltage curve on the first or subsequent discharges. Further structural and electrochemical characterization of these cathode materials was published by us in two papers in the Journal of Power Sources (volume 54, pages 205-208, in 1995 and volumes 81-82, pages 406-411, in 1999).
Another related material is described in U.S. Pat. No. 4,567,031 of Riley, issued 28 Jan. 1986. The abstract of this reference describes a mixed metal oxide having the formula Li
x
M
y
O
z
where M is at least one metal selected from the group consisting of titanium, chromium, manganese, iron, cobalt and nickel. It is significant that none of the enabled structures include Mn or Cr. Moreover, there is no enabled disclosure of the use of such compounds as cathodes for secondary cells.
Also, the preparation and characterization of materials of compositions Li
2
Cr
x
Mn
2−x
O
4
in which 1.0≦x≦1.5 was described by Dahn, Zheng and Thomas (J. Electrochem. Soc., 145, 851-859, Mar. 1998). These materials were made using a sol-gel technique followed by heating in an inert atmosphere to temperatures ranging from 500 to 1100° C. In this study a maximum discharge capacity of 150 mAh/g at a low current density (1.5 mA/g) was found for a sample of composition Li
2
Cr
1.25
Mn
0.75
O
4
which had been prepared at 700° C. The same sample cycled at 15 mA/g showed a capacity of about 137 mAh/g. The materials prepared at 700° C. were all found by powder x-ray diffraction to have a layered structure like LiCoO
2
with a hexagonal unit cell symmetry. The volume of the crystallographic unit cells varied from 68.6 to 71.9 cubic angstroms per formula unit of Li
2
Cr
x
Mn
2−x
O
4
.
U.S. Pat. No. 5,858,324 issued Jan. 12, 1999 discloses a process for preparing a compound of formula Li
y
Cr
x
Mn
2−x
O
4+z
where y≧2, 0.25<x<2 and z≧0. The experimental examples provided in the disclosure demonstrated the use of these materials as the active cathodes in rechargeable lithium ion electrochemical cells. The Li
y
Cr
x
Mn
2−x
O
4+z
samples were prepared as in the J. Electrochem. Soc. (145, 851-859, March 1998) publication from chromium nitrate nonahydrate, manganese (II) acetate tetrahydrate and lithium hydroxide monohydrate by a sol-gel process using ammonium hydroxide as the gelling agent. The gels were heat treated in inert atmosphere at temperatures ranging from 500 to 1100° C. to form Li
y
Cr
x
Mn
2−x
O
4+z
compounds.
The compounds were characterized by Reitveld refinement of crystallographic unit cell dimensions from powder x-ray diffraction data. The symmetry and space group of the compounds was found to depend on the composition and processing temperature. The structure refinements and chemical analyses of the compositions studied are summarized in Table 1 of the patent (U.S. Pat. No. 5,858,324). Table 1 of the patent (U.S. Pat. No. 5,858,324) summarizes the structure refinements and chemical analyses for the 37 samples studied. Only two of the examples (samples 22 and 23) of Li
y
Cr
x
Mn
2−x
O
4+z
have compositions in which y is greater than 2.0. The value of x ranges from 0.5 to 1.5 and the value of z as reported in table 6 is zero.
Only samples prepared at the lowest temperatures, 500 or 600° C., have crystallographic unit cell volumes per stoichiometric unit which are substantially smaller than that of LiCrO
2
at 69.9 cubic Angstroms. Electrochemical characterization of these samples is not provided in the (U.S. Pat. No. 5,858,324) patent disclosure. The compositions of Li
y
Cr
x
Mn
2−x
O
4+z
in examples 22 and 23 listed in table 1 of U.S. Pat. No. 5,858,324 have y equal to 2.2 and x equal to 1.25. The crystallographic unit cell volumes

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