Electricity: battery or capacitor charging or discharging – Diverse charging or discharging rates for plural batteries
Reexamination Certificate
1999-12-21
2001-12-11
Toatley, Jr., Gregory J. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Diverse charging or discharging rates for plural batteries
C320S128000, C320S130000, C320S160000
Reexamination Certificate
active
06329789
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to the field of electrochemical cells. More particularly, the invention pertains to lithium rechargeable cells comprising sulfur-containing cathode materials and to methods of recharging these cells to achieve long cycle life.
BACKGROUND
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
The need for rechargeable batteries with long cycle life, rapid charge capacity, and high energy density for devices such as mobile telephones, portable computers and other consumer electronic devices continues to grow. Rechargeable batteries, such as those based on lithium metal anodes and solid electroactive sulfur-containing cathode active materials, provide one approach to meet this need. For example, U.S. Pat. Nos. 5,529,860, 5,601,947, and 5,690,702 to Skotheim et al., and U.S. Pat. Application Ser. No. 08/995,122 now U.S. Pat. No. 6,201,100 to Gorkovenko et al. of the common assignee, describe electroactive sulfur-containing cathode active materials and lithium/sulfur batteries using these sulfur-containing cathode active materials.
However, one problem encountered in electrochemical cells based on lithium and sulfur-containing cathode active materials is limited cycle life, i.e. the number of rechargings the battery can accept before the battery is no longer able to maintain acceptable levels of charge capacity, such as 50-80% of the initial capacity of the battery.
It has been shown that the charge conditions may directly affect the lithium surface morphology in recharging lithium secondary cells with lithium metal anodes and with transition metal oxide cathodes. It is believed that lithium surface morphology created in the lithium deposition process is one important factor in determining cycle life. For example, Aurbach et al., in
J. Electrochem. Soc
., 1988, 145, 1421-1426, report a much lower cycle life for Li—Li
x
MnO
2
cells, with lithium metal anodes under fast charge rates (1.25 mA/cm
2
) compared with slow charge rates (0.3 mA/cm
2
).
It has also been shown that discharge rates may affect the cycle life of rechargeable batteries. For example, it has been reported that high discharge rates for lithium cells result in longer cycle life than low discharge rates. For example, Saito et al. report, in
J. Power Sources
, 1998, 72, 111-117, that for LiNV
2
O
5
—P
2
O
5
cells, low rate discharging (0.5 mA/cm
2
results in a higher surface area for a lithium metal anode and in much lower cycle life than high rate discharging (5.0 mA/cm
2
).
Cathode performance, at the same time, may also be diminished by application of high charge rates in comparison to lower rates of charge. For example, Tatsuma et al. in
J. Electrochem. Soc
., 1995, 142, L182-L1 84, report that shorter cycle life is achieved when high charge rates, 0.2 mA/cm
2
, are used for polyaniline/dimercaptothiadiazole polymer composite cathodes when compared with low charge rates, 0.05 mA/cm
2
.
Thus in general, these reports indicate that cycle life for rechargeable lithium metal cells is increased by the use of high rates of discharge in conjunction with low rates of charge.
In U.S. Pat. No. 5,550,454 to Buckley charging methods for solid secondary lithium electrochemical cells are reported. Sequences of charging currents, each for a period of time, are applied to discharged lithium secondary cells which extend cycle life or reduce the overall charging time. In U.S. Pat. No. 5,500,583 to Buckley et al. is described a method of extending the cycle life of a solid secondary electrochemical cell by applying a short high magnitude discharge pulse during the charging process.
Charging regimes for nickel based rechargeable batteries, such as nickel-cadmium and nickel metal-hydride, are quite different. For example, in U.S. Pat. No. 5,900,718 to Tsenter, is described a battery charger and methods of charging nickel based batteries in which charging rates are adjusted in response to temperature or open circuit voltage values. There is also provided a summary of the various methods described for use in recharging nickel based batteries.
There is a need in rechargeable lithium metal batteries for both long cycle life and rapid charge times, and for charging methods that maximize the cycle life while shortening charge times. There is also a need for charging regimes designed for rechargeable batteries comprising sulfur-containing cathodes. The present invention addresses the need for rapid charge times while at the same time achieving long cycle life for rechargeable batteries comprising sulfur-containing cathodes.
SUMMARY OF THE INVENTION
The present invention pertains to a method of increasing the cycle life of a discharged lithium electrochemical cell, wherein the cell comprises (i) an anode comprising lithium; (ii) a cathode comprising an electroactive sulfur-containing material; and (iii) a liquid electrolyte interposed between the anode and the cathode, wherein the cell has been discharged at an overall current rate of less than 0.5 mA/cm
2
; wherein the method comprises the steps of: (a) charging the cell at an initial low charge rate of less than 0.2 mA/cm
2
to a cell voltage in the range of 2.1 to 2.3 V; and (b) subsequently charging the cell at a high charge rate of greater than 0.2 mA/cm
2
to a cell voltage of at least 2.4 V.
In one embodiment, the low charge rate in the initial charging step (a) is from 0.03 mA/cm
2
to 0.15 mA/cm
2
. In one embodiment, the high charge rate in the subsequent charging step (b) is from greater than 0.20 mA/cm
2
to 0.75 mA/cm
2
.
In one embodiment, the initial low charge rate charging step (a) comprises two or more sub-steps of less than 0.2 mA/cm
2
in a sequence of increasing charge rate. In one embodiment, the subsequent high charge rate charging step (b) comprises two or more sub-steps of greater than 0.2 mA/cm
2
in a sequence of increasing charge rate.
In one embodiment, the cell is charged to a voltage of 2.2 to 2.3 V in the initial charging step (a).
In one embodiment, the combined sum of the charge provided in the initial charging step (a) and the subsequent charging step (b) is from 105% to 180% of the discharge capacity of the last half discharge cycle. In one embodiment, the combined sum of the charge provided in the initial charging step (a) and the subsequent charging step (b) is from 105% to 120% of the discharge capacity of the last half discharge cycle.
In one embodiment, the overall discharge rate of the cell is from 0.025 mA/cm
2
to 0.25 mA/cm
2
.
In one embodiment, the electroactive sulfur-containing material comprises elemental sulfur. In one embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises one or more polysulfide moieties, —S
m
—, where m is an integer equal to or greater than 3. In one embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises one or more polysulfide moieties, —S
m
−
, where m is an integer equal to or greater than 3. In one embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises one or more polysulfide moieties, S
m
2
−
, where m is an integer equal to or greater than 3.
In one embodiment, the electroactive sulfur-containing material, in its oxidized state, is of the general formula:
&Brketopenst;C(S
x
)&Brketclosest;
n
wherein x ranges from greater than 2.5 to about 50, and n is an integer equal to or greater than to 2.
In one embodiment, the electroactive sulfur-containing material, in its oxidized state, comprises one or more of the polysulfur moieties:
wherein m, the same or different at each occurrence, is an integer and is greater than 2, and y, the same or different at each occurrence, is an integer and is equal to or greater
Gavrilov Alexei B.
Mikhaylik Yuriy V.
Moltech Corporation
Toatley , Jr. Gregory J.
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