Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-01-14
2002-12-03
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231100, C029S623100
Reexamination Certificate
active
06489060
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to non-aqueous rechargeable lithium manganese oxide batteries with greatly improved cycling performance at elevated temperatures, and methods of producing such batteries. Specifically, the invention pertains to using deposits of certain metal compounds, particularly ones containing Y, Bi, Pb and La, on the surface of a spinel cathode as means to stabilize the spinel surface, thereby avoiding the capacity loss.
BACKGROUND OF THE INVENTION
Various types of non-aqueous rechargeable lithium ion batteries are available commercially for consumer electronics applications. Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Presently available lithium ion batteries are high voltage systems based on LiCoO
2
cathode and coke or graphite anode electrochemistries. However, many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO
2
and LiMn
2
O
4
. Also, a wide range of carbonaceous compounds is suitable for use as the anode material. These batteries employ non-aqueous electrolytes comprising LiBF
4
or LiPF
6
salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
The rechargeable lithium battery industry has found that LiMn
2
O
4
can be a more desirable cathode material than LiCoO
2
, because of its low cost and its relative harmless effect on the environment. Therefore, research efforts to use LiMn
2
O
4
as the cathode material of choice have increased.
Typically LiMn
2
O
4
based batteries have good performance at room temperature. However, at elevated temperatures they suffer a gradual loss of delivered capacity with cycle number, herein referred to as capacity fade or the capacity fade rate. Researchers in the art have devoted substantial effort to reducing this loss in capacity.
There are many patents/patent applications and articles in the literature claiming that doping with a foreign metal or a combination of metals during the synthesis of LiCoO
2
or LiMn
2
O
4
improves capacity and/or capacity fade. For instance, U.S. Pat. No. 5,147,738 (Yoshinori Toyoguchi) claims improved cycle life of LiCoO
2
batteries by using cathode active material containing Li
x
Co
1−y
M
y
O
2
, where M═W, Mn, Ta, Ti, Nb; Japanese published application serial number 09134723 (Okada et. al.) uses Li
y
Mn
2−x
M
x
O
4
, cathodes where M═Fe, Ti, Ni, Ta, Cr, W, Pb, etc. to obtain large total discharge capacity; U.S. Pat. No.5,759,720 (Glenn Amatucci) discloses capacity and capacity fade improvement at 55° C. for lithium rechargeable batteries using lithium aluminum manganese oxy-fluoride cathodes.
Addition of Bi is known to improve the stability of manganese oxides used in the so called RAM (Rechargeable Alkaline Manganese) battery technology. In the context of such aqueous alkaline cells, D. Larcher et. al. (J. Electrochem. Soc., Vol. 145, No.10, pp.3392-3400) study the effects of Bi, Pb and Tl doping, on the stability of &lgr;-MnO
2
(the de-lithiated form of LiMn
2
O
4
) when stored in aqueous acidic media. Larcher et al. characterize stability by measuring the rate at which Mn is dissolved from the solid phase, and no capacity fade measurements where carried out in electrochemical cells. Brief mention is made of the storage characteristics of Bi doped LiMn
2
O
4
in non-aqueous acidic electrolyte, and it appears from the reported X-ray data that the stability was not improved by Bi doping.
Matsushita Electric Co. Ltd.'s laid open Japanese application serial number 05047384 (Yamaura et. al.). claims improved overdischarge at high temperature by using their cathode. The cathodes here are made from powder obtained from simply mixing spinel powder with metal oxide powder. The mixed powders are not heat treated before they are made into a slurry for cathode coating. This is definitely not a surface coating technique.
Coating the surface of LiMn
2
O
4
to obtain specific effects has been investigated. For example, in U.S. Pat. No.5,705,291 (Amatucci et al.) the self discharge rate of batteries stored at 55° C. is reduced by treating the surface of the lithium intercalating cathode material with a passivating layer containing an annealed coating composition of boron oxide. The inventors show no evidence that capacity fade is improved by this treatment.
Thus far, the compounds and methods attempted in the prior art either give marginal improvements in capacity fade rate at elevated temperatures or attempt to solve other problems facing lithium manganese oxide batteries.
SUMMARY OF THE INVENTION
Rechargeable batteries exhibit a loss in delivered capacity as a function of the number of charge/discharge cycles. Herein, the fractional loss of capacity per cycle is referred to as the capacity fade rate. The instant invention includes non-aqueous rechargeable lithium manganese oxide batteries having greatly improved capacity fade rates at elevated temperatures and methods for achieving the improved capacity fade rate. Non-aqueous rechargeable spinel batteries generally comprise a lithium manganese oxide cathode, a lithium compound anode, a separator and a non-aqueous electrolyte comprising a lithium salt dissolved in a non-aqueous solvent and are hereinafter called spinel or Li
1+x
Mn
2−x
O
4
(0<=x<0.33) batteries. We have discovered that heating a mixture of a small amount of one or more of certain foreign metal compounds with Li
1+x
Mn
2−x
O
4
powder can result in a reduced capacity fade rate at elevated temperatures in spinel lithium ion batteries. Preferably, the temperature applied to heat the mixture is high enough so that the foreign metal compound is converted to foreign metal products, which cover the surface of the spinel and do not enter the bulk of the spinel structure. Such treated spinel powders serve to reduce the capacity fade of spinel lithium-ion batteries at elevated temperatures. Hereinafter synthesized Li
1+x
Mn
2−x
O
4
powder will be referred to as ready-made spinel. This is to distinguish it from either the actual synthesis of spinel, where precursors such as EMD-MnO
2
and Li
2
CO
3
are heated to obtain Li
1+x
Mn
2−x
O
4
or from doped spinel, where for example, EMD-MnO
2
, Li
2
CO
3
and Bi
2
O
3
are heated to obtain Li
1+x
Mn
2−x−y
Bi
y
O
4
.
During cycling, lithium-ion cells tend to produce alcohols such as methanol or ethanol as a result of reactions between trace amounts of H
2
O and the materials forming the Solid Electrolyte Interface (SEI) layer on the anode (Aurbach et. al., J. Electrochem. Soc. 141, L1 (1994), ibid. 142, 1746 (1995), ibid. 142, 2873 (1995)). The alcohol in turn oxidizes on the surface of the charged or de-lithiated cathode, and produces more water. Water then reacts irreversibly with more SEI material and possibly with some intercalated lithium in the anode. Once intercalated Li has reacted with water to form LiOH, it is permanently inactive and the cell capacity decreases correspondingly. In particular, the effect is most prominent in spinel lithium ion batteries at elevated temperature as shown in Yu Wang et al., co-inventors of this invention (Poster III, The 9th International Meeting on Lithium Batteries, Edinburgh, Scotland, July 1998).
The rate of methanol reaction can be used to gauge whether specific treatments of ready-made spinel powder will improve the capacity fade of spinet batteries at elevated temperatures. That is, the charged or de-lithiated spinel, henceforth known herein as &lgr;-MnO
2
, in question can be soaked in a known amount of methanol, and then the water produced from the oxidation reaction of methanol and the cathode can be measured over time. The less water produced by a surface treated cathode relative to a reference cathode of untreated spinel, the lower the capacity fade rate is expected when the lithium-ion batte
Gee Michael
Reimers Jan Naess
Wang Yu
Zhang Meijie
Dove Tracy
E-One Moli Energy (Canada) Limited
Synnestvedt & Lechner LLP
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