WO1999050925A1

WO1999050925A1 - Method of recharging a secondary electrochemical cell

Info

Publication number: WO1999050925A1
Application number: PCT/CA1999/000269
Authority: WO
Inventors: Lynn S. Marcoux
Original assignee: Bluestar Advanced Technology Corp.
Priority date: 1998-03-27
Filing date: 1999-03-26
Publication date: 1999-10-07

Abstract

A method of operating a secondary electrochemical cell comprises the steps of (a) measuring the quantity of charge produced by the cell during discharge, (b) recharging the cell by imposing a substantially constant charging current across the cell until the cell voltage reaches a predetermined cutoff voltage, and (c) continuing to charge the cell at the predetermined cutoff voltage until the total charge returned to the cell is equal to the charge produced by the cell during the preceding discharge. The method substantially completely charges lithium ion and other similar secondary cells, under a variety of operating conditions, without causing measurable damage due to overcharge. Discharge capacity remains substantially unchanged over numerous cycles employing the technique. Charging times necessary to achieve full recharge are also shorter than for conventional, prior art techniques. The method also increases the capacity available from secondary cells operated at low temperatures. Adjusting the cutoff voltage to compensate for measured internal resistance and charging to a coulombic cutoff can be practiced separately to advantage, the combination of the two results in improved charge retention and reduction of charging time while protecting the cell from undesirable over-discharge reactions.

Description

METHOD OF RECHARGING A SECONDARY ELECTROCHEMICAL CELL

Field of the Invention

This invention relates to battery charging systems and, in particular, to battery charging systems for lithium ion batteries.

Background of the Invention

Electrochemical storage batteries are well known in commerce. The term battery is used to refer to a device comprising one or more electrochemical cells. These electrochemical cells comprise two active electrodes and an ionically conductive electrolyte interposed between the electrodes. Such cells are designed to convert the energy stored within the cell, in the form of reactable chemical materials, into an electric current. The reactable chemicals, one of which is capable of oxidizing the other, are kept separate from one another, typically within the structure of the two electrodes, to prevent the direct reaction of these chemicals. The driving force for the spontaneous reaction of these chemicals appears as a voltage at the interface between each electrode and the electrolyte, and through electrical connections within the electrodes, to the external terminals of the cell. The reaction of the chemical materials takes place spontaneously when an external circuit connecting the two electrodes is closed, permitting current to flow from one electrode to the other, and the oxidation of one material by the other. - 2

Electrochemical cells and batteries fall into two distinct categories. In some batteries the reactions taking place within the cells during discharge produce products which, due to their chemical or physical nature, cannot be reconverted into the original reactable chemical materials . Such batteries are called primary or non-rechargeable batteries. The common alkaline battery is an example of a primary battery. When the chemical energy stored originally within such primary cells is consumed, the cells are of no further use.

In some electrochemical cells, the chemical and physical nature of the products of cell discharge are such that , by forcing a current through the discharged cell in a direction opposite to that produced by the cell during discharge, the products can be reconverted into the original reactable chemical materials. The cell is then capable of a further discharge. Such cells and batteries are called secondary or rechargeable batteries. The common lead-acid battery is an example of a secondary battery.

In an ideal battery, the terminal voltage decreases only slightly during discharge and then decreases abruptly, as shown in FIG. 1, as the reactable chemical materials are completely consumed. The capacity of the cell is defined as the charge withdrawn from the cell, expressed in coulombs or amp hours, before a predefined cutoff voltage is reached. The cutoff voltage may be selected based upon the requirements of the device being powered, but is typically chosen, in the case of a cell which behaves as shown in FIG. 1, as a voltage in the near vertical region of the discharge curve. Note that that the measured capacity would not differ significantly whether voltage A or voltage B were chosen as the cutoff voltage.

It should be noted that measured capacity for a cell will vary depending upon the rate of discharge (the current being drawn from the cell) and the temperature of the cell. This is due in part to voltage drops across resistive components within the cell, such as the terminals, the tabs connecting the terminals to the electrodes, the electrode structures and the electrolyte. At a high discharge current, a greater fraction of the voltage being generated at the two electrodes will not be seen at the cell terminals due to IR drop across resistive components. The IR drop across the electrolyte may become particularly significant at lower temperatures.

The rates of the electrochemical reactions at the two electrodes can also limit the capacity. When the current being demanded by the external circuit exceeds the capability of one or both electrodes, the voltage at the electrode/electrolyte interface decreases and thus the terminal voltage decreases. Because the reaction rates at the electrodes decrease with temperature, capacity generally decreases at lower temperatures. When an electrochemical cell is recharged, a driving voltage must be supplied to the terminals of the cell in order to force current backward through the cell and to carry out the reconversion of the discharge products. This charging voltage will be the sum of voltages at the two electrode/electrolyte interfaces, driving the electrochemical charging reactions, and the IR drops across the various resistive components of the cell. In an ideal secondary cell, the voltage necessary to force a significant charging current through a cell will rise rapidly, as shown in FIG. 2, when the reconversion - 4

process is completed and substantially all of the product has been transformed back into reactable chemical material. Also, in an ideal cell, the charge necessary to reform the reactable material will exactly equal the charge withdrawn from the cell during discharge.

In general, it is difficult to achieve complete recharge and also to determine that recharge is complete. Often, the last few percentages of the charging process take place at electrode/electrolyte interface voltages at which side reactions also take place. In the case of the lead/acid battery, final charging takes place at a voltage at which water is also broken down into its components, hydrogen and oxygen. Fortunately, in the lead/acid battery, the consumption of water can either be remedied by periodic addition of water or by the incorporation of catalysts to recombine the hydrogen and oxygen to reform the water. In the case of the lead/acid battery, overcharge and the presence of side reactions during charging do not significantly affect the lifetime performance of the battery. In many other battery systems, however, particularly those high energy density battery systems incorporating reactive negative electrodes, such as lithium, and non-aqueous electrolyte solvents, overcharging side reactions tend to cause irreversible damage to the solvent or to irreversibly consume one or both of the reactable chemical materials in the electrodes. In such battery systems, it becomes necessary to limit the charging voltage to a value at which such damage is minimized. This mode of charge termination may, however, result in incomplete charging. In general, a characteristic of lithium ion intercalation compound/non-aqueous solvent cells and batteries (hereinafter referred to - 5

as lithium ion cells and batteries) is a progressive loss of capacity with increasing numbers of cycles. This capacity loss can seriously limit the applicability of these cells. The present invention is directed to providing a method and a device for substantially completely charging lithium ion batteries, under a variety of operating conditions, without causing measurable damage due to overcharge. In this way, discharge capacity remains substantially unchanged over hundreds, and perhaps thousands, of cycles. A further advantage of the present method is that the charging times necessary to achieve full recharge are surprisingly short . Finally, the application of this method significantly increases the capacity available from such lithium ion cells and batteries when operated at low temperatures. While the present method was developed specifically for the lithium ion battery, it is also applicable to other similar secondary battery systems.

A conventional, prior art technique for recharging secondary batteries is disclosed in Lewyn U.S. Patent No. 5,670,862. Lewyn' s technique attempts to reduce charging times by compensating for the expected IR drop across the tabs connecting the electrodes to the terminals. In Lewyn, however, a specific resistor, representing that tab resistance, is wired into the charging circuit . Lewyn does not teach or suggest the importance of measuring the electrolyte resistance and its variation with temperature, nor does Lewyn teach or suggest the variation of resistance within the cell with age.

In the prior art, secondary cells are usually charged by devices that initially supply to the cell the maximum current of which the device is capable. As this initial constant current can easily be 6 -

accepted by the discharged cell, the voltage which the charging device must supply to force this current into the cell is significantly less that the voltage at which concern of over charge damage arises. As the cell becomes more fully charged, the voltage needed to force more current into the cell increases and eventually reaches the predetermined cutoff voltage. At this point, the charging device will continue to supply whatever current the cell can accept at the constant cutoff voltage. Such a process gives rise to charging current and voltage curves, such as those shown in FIGs . 3A and 3B. Note that the charging current in the constant voltage region decreases approximately exponentially with time. There being no abrupt change in cell voltage or current to signal completion of charging and based upon a general concern for not overcharging cells, an arbitrarily convenient time or current value is selected at which charging is stopped._ When such a charging regime is applied to a typical lithium ion cell, a 4.10 V limit is normally employed. This is slightly, but only slightly, above the typical open circuit voltage of such a cell of about 4.07 V. With an arbitrary end of charging time, the observed discharge capacity decreases with cycle number as shown by the data up to cycle 210 in FIG. 4. At cycle 210 the charging limit was changed from a specific time to the time at which the charge (number of coulombs or amp hours) which had been withdrawn from the cell in the previous discharge cycle was exactly replaced. It would have been expected that this change might have slowed the capacity degradation somewhat at the cost of significantly increasing the charging time. It would also have been expected that some coulombic inefficiencies due to side reactions would still - 7

result in capacity loss with cycling. Instead, it was found here that capacity loss was substantially completely arrested, within measurement accuracy, and continued to remain constant over more than 200 cycles.

Equally surprising was that the additional time required to achieve this substantially complete recharging of the cell was not great, and though it has increased with time, the increase is not great, as shown in FIG. 5. It has also been observed that cells which had been undercharged and had lost capacity with cycling could be revived to a large extent by gently overcharging the cells, that is, by returning a few percent more charge than had been withdrawn in the previous discharge cycle, while maintaining the same cutoff voltage.

It was further discovered that the use of a fixed cutoff voltage is undesirable in that it is responsible for the gradually increasing charging time to a coulombic cutoff, and that a fixed cutoff voltage significantly degrades cell capacity, particularly when fixed time charging is used, at low temperatures. These observations can be understood with reference to FIG. 6. Note that, when a current is forced through a cell, a portion of the terminal voltage applied to the terminal will appear as an IR drop across the various resistive components in the cell. The IR drops across the terminal, the electrode tabs, the electrode structures, and the electrolyte solutions will vary, depending upon the design of the cell, the nature of the electrolyte solution, the operating temperature, state of charge, cycling history, and the age of the cell. The difference between the voltage at the terminals and the sum of the IR drops will appear at the two electrode/electrolyte interfaces as the voltage - 8

driving the recharge reactions. It is this effective charging voltage which must be controlled to prevent damaging overcharge reactions.

Consider, however, what happens when a cell with a non-aqueous electrolyte solution, which are typically more resistive than aqueous solutions, is operated at low temperatures. As the temperature is lowered, the resistance of such a cell increases significantly. At a fixed charging current, the IR drop across the electrolyte solution will be significantly greater than it is at room temperature. The presence of the electrolyte IR drop will mean that the charging voltage will reach the cutoff limit sooner at low temperature and that, at that cutoff voltage, the effective charging voltage will be less at low temperature. Both effects mean that when the cutoff voltage is reached, less charge will have been introduced into the cell at low temperature than into a similar cell at room temperature. During the subsequent constant voltage charge to a fixed time cutoff, the effective charging voltage will continue to be less by the electrolyte IR drop in the low temperature case, so that the charging current will also continue to be less than in the room temperature case. During both the constant current and the constant voltage phases, the low temperature cell receives less charge than a room temperature cell and thus will have reduced capacity in the subsequent discharge ._

Summary of the Invention

It has been found that, by measuring the actual IR drop of the resistive components within the cell, and particularly the electrolyte IR drop, at low temperature and then setting the voltage cutoff limit at the desired cutoff voltage plus the product of the charging current and the measured internal resistance (specifically including the electrolyte resistance) , the constant current phase of the charging process is increased in time at low temperature, as shown schematically in FIG. 7B and approaches that at room temperature, while the effective charging voltage at the electrode surfaces does not exceed the specified limit. As shown in FIG. 7A, the voltage during the now nearly constant phase of the charging process varies slightly as the current decreases and the IR drop across the internal resistance decreases.

The measurement of the internal resistance and varying of the cutoff voltage also recognizes the effect of increased internal resistance with age found in some batteries, particularly those with reactive negative electrodes, such as lithium, and improve capacity retention.

While adjusting the cutoff voltage to compensate for measured internal resistance and charging to a coulombic cutoff can be practiced separately to advantage, the combination of the two results in improved charge retention and reduction of charging time while protecting the cell from undesirable over- discharge reactions.

Brief Description of the Drawings

FIG. 1 is a plot of cell terminal voltage as a function of time, showing that, in an ideal battery, the terminal voltage decreases only slightly during discharge and then decreases abruptly as the reactable chemical materials are completely consumed.

FIG. 2 is a plot of cell terminal voltage as a function of time, showing that in an ideal secondary cell, the voltage necessary to force a significant charging current through a cell will rise rapidly when the reconversion process is completed and - 10

substantially all of the product has been transformed back into reactable chemical material.

FIGs . 3A and 3B are companion plots of cell terminal voltage as a function of time and cell current as a function of time, respectively, showing that the charging current in the constant voltage region of secondary cells decreases approximately exponentially with time.

FIG. 4 is a plot of cell capacity (in amp hours) as a function of the number of cycles, showing the observed discharge capacity decreases with cycle number up to cycle 210.

FIG. 5 is a plot of total charging time (in hours) as a function of the number of cycles, showing that the additional time required to achieve substantially complete recharging of a lithium ion secondary cell is not great, and though it increases with time, the increase is not great.

FIG. 6 is a schematic diagram of a secondary cell, showing that, when a current is forced through a lithium ion secondary cell, a portion of the terminal voltage applied to the terminal will appear as an IR drop across the various resistive components in the cell. FIGs. 7A and 7B are companion plots of cell terminal voltage as a function of time and cell current as a function of time, respectively, showing the effects of IR compensation. The curves labeled "a" are identical to those in FIG. 3 and show the conventional, prior art approach. The curves labeled

"b" show the changes in voltage and current when the cutoff voltage is adjusted to account for the internal IR due to resistive cell components.

FIG. 8 is a plot of cell capacity (in amp hours) as a function of the number of cycles for a secondary cell in which the positive electrode material was a 11 -

lithiated manganese dioxide (LiMn₂0₄) rather than lithiated cobalt oxide (LiCo0₂) as in the secondary cell of FIG. 5.

FIG. 9 is a composite plot of cell capacity (in amp hours) as a function of the number of cycles, showing that a significant improvement in cell capacity was achieved when the voltage limits included compensation for cell internal resistance.

Detailed Description of the Preferred Embodiments

The present method substantially completely charges lithium ion batteries, under a variety of operating conditions, without causing measurable damage due to overcharge, as shown in the following examples. In particular, the examples show that the discharge capacity of lithium ion secondary cells remains substantially unchanged over numerous cycles. The examples also show that the charging times necessary to achieve full recharge of lithium ion secondary cells are surprisingly short. The examples further show that the present method significantly increases the capacity available from lithium ion secondary cells and batteries when operated at low temperatures .

Example 1

A 40-Ah fully-welded electrochemical cell was assembled and activated and subjected to cycle life testing. The positive electrode for this cell was prepared by coating a 0.001 inch (0.00254 cm) Al foil on both sides with a mixture of lithiated cobalt oxide (LiCo0₂) , conductive carbon and an organic binder. Following coating, this material was further densified, slit to width and fitted with metal tabs for connection to the internal terminals of the cell. 12 -

The negative electrode for this cell was prepared by coating a 0.0004 inch (.00116 cm) Cu foil on both sides with a mixture of synthetic graphite, conductive carbon and an organic binder. Following coating, this material was also further densified, slit to width and fitted with metal tabs for connection to the internal terminals of the cell. The electrodes were then combined with alternating layers of microporous polyolefin separator and concentrically wound to form a cylindrical element. This cylindrical element was placed within a closed- end stainless steel tube with a length of 7.00 inch (17.78 cm) and a diameter of 2.48 inch (6.2992 cm). A circular cell cap was prepared by welding two glass insulated electrical connectors, a 175-psi (1.21 MP (1.21*10⁶ N/m²) ) rupture disk and stainless steel fill tube into a pre-punched stainless steel cap. The tabs from the positive electrode of the cylindrical cell element were welded to one of the insulated electrical connectors and the tabs from the negative electrode of the cylindrical cell element were welded to the remaining connector. The cap was then fitted into the cylindrical cell container and welded in place. An organic electrolyte solution containing ethylene carbonate, diethyl carbonate and lithium hexafluorophosphate was then introduced under vacuum through the fill tube. The fill tube was then welded closed completing the assembly process.

After 10 initial charge-discharge cycles designed to form the active materials, the cell was subjected to charge-discharge cycle testing at room temperature. The cycling regime involved the constant current discharge of the cell at 20 A until a voltage of 3.0 V was achieved at which point the current was reversed and charging initiated. Cell charging consisted of an initial constant current - 13 -

charging at 8.0 A until a voltage of 4.1 V was achieved. At this point the voltage was held constant at this value and charging, under conditions of decreasing current was allowed to continue for a period of 1.5 hours. The cycling regime was maintained from cycle 11 to cycle 216 and, as may be seen in FIG. 4, during this time the discharge capacity continuously declined from a value of 38 Ah to a value of 32 Ah. After cycle 216, the discharge regime was allowed to remain the same and the charging methodology was modified. The modified charging program consisted of an initial constant current charge to 4.1 V followed by constant voltage charging at 4.1 also identical to that of the initial cycles. The change effected involved the criterion for charge termination. Rather than terminating the charging process after a fixed time interval, a coulombic criterion for charge termination was employed. During the charging process the total charge returned to the cell was determined by continuous integration and the charging process was not terminated until the amount of charge returned was equal to the amount of charge extracted during the discharge segment of the cycle. This process was continued until a total of 485 cycles had been accumulated at which point the test was terminated for convenience. During these latter 269 cycles, the capacity of the cell remained constant as may be seen in Figure 4 and the charging time increased from 2.2 hours to 3.5 hours, as shown in FIG. 5.

Example 2

In this example, the applicability of the present method to other positive electrode 14

chemistries is demonstrated. A cell with a diameter of 1.31 inch (3.3274 cm) and a length of 4.38 inch (11.1252 cm) was assembled using techniques similar to those described in Example 1. In this case the positive electrode material was a lithiated manganese dioxide (LiMn₂0₄) rather than LiCo0₂ and the coulombic charge termination was employed for all cycles following the formation cycles. This cell was discharged at a rate of 1.0 A to a voltage of 3.00 V and then charged at this current until a voltage of 4.1 V was achieved at which point the voltage was held constant at this value and charging was allowed to continue with decreasing current until and amount of charge had been returned equivalent to that extracted during the discharge segment of the cycle. Cycling was continued until a total of 75 cycles had been accumulated and then terminated for convenience. During this time cell capacity remained constant at 4.75 Ah, as shown in FIG. 8, and the total charging time increased from 5 hours to 10 hours.

Example 3

In this example the value of voltage limit compensation for achieving good low temperature performance is demonstrated using 6 -Ah cells with a LiCo0₂ positive electrode. The cell had a diameter of 1.31 inch (3.3274 cm), a length of 4.38 inch (11.1252 cm), and was assembled with components and processes similar to those described in Example 1. The tests were performed at -30 °C with a single 3- cell pack with the cells connected in a parallel configuration. Results were reported as single cell capacities (that is, the pack capacity was divided by 3) for comparison purposes. In one test, the cell pack was cycled at the C/5 rate (1.2A/cell) between 15 -

two sets of voltage limits. In one case, the standard voltage range of 3.0 to 4.1 V was employed and in the other case a range of 2.92 to 4.18 V was employed. The latter range included compensation for the internal resistance of the cell at this temperature. The cells were discharged to the lower voltage limit at the C/5 rate and the current was then reversed and the cells were charged at this rate until the upper voltage cutoff value was achieved. Charging was then continued in the constant voltage mode for a period of 2.5 hours. This test sequence was also repeated at the C/2 rate (2.0 A) in which case the voltage limits were 2.79 V and 4.31 V. Cells were cycled for at least 4 cycles at each set of charge/discharge conditions. The results for these tests are shown in FIG. 9. In both cases, a significant improvement in cell capacity was observed when the voltage limits included compensation for cell internal resistance. While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated by the appended claims to cover such modifications as incorporate those features that come within the spirit and scope of the invention.

Claims

16What is claimed is:

1. A method of operating a secondary electrochemical cell comprising a positive electrode, a negative electrode, and an ion-conductive electrolyte, the method comprising the steps of: measuring the quantity of charge produced by the cell during discharge, recharging the cell by imposing a substantially constant charging current across the cell until the cell voltage reaches a predetermined cutoff voltage, and continuing to charge the cell at the predetermined cutoff voltage until the total charge returned to the cell is equal to the charge produced by the cell during the preceding discharge.

2. The method of claim 1 wherein the negative electrode is lithium metal or a lithiated intercalation compound.

3. The method of claim 2 wherein the negative electrode is lithiated carbon.

4. The method of claim 1 wherein the positive electrode is a transition metal compound capable of reversibly intercalating lithium.

5. The method of claim 4 wherein the positive electrode is derived from lithiated cobalt oxide (LiCo0₂) .

6. The method of claim 4 wherein the positive electrode is derived from lithiated manganese oxide (LiMn₂0₄) . 17

7. A method of charging a secondary electrochemical cell comprising a positive electrode, a negative electrode, a cell terminal electrically connected to each electrode, and an ion-conducting electrolyte, the method comprising the steps of: imposing a substantially constant charging current between the cell terminals, measuring the actual IR drop of the resistive components of the cell, measuring the cell voltage, discontinuing the constant current charging when the cell voltage equals the sum of a predetermined cutoff voltage and the actual IR drop across the resistive components of the cell, continuing to charge the cell at a voltage equal to the sum of the predetermined cutoff voltage and the actual IR drop across the resistive components of the cell until a predetermined end of charge event occurs.

8. The method of claim 7 wherein the predetermined end of charge event is a time limit.

9. The method of claim 7 wherein the predetermined end of charge event is the return of an amount of charge substantially equal to amount of charge removed from the cell during the preceding discharge .

10. The method of claim 7 wherein the negative electrode is lithium metal or a lithiated intercalation compound.

11. The method of claim 10 wherein the negative electrode is lithiated carbon. 18

12. The method of claim 7 wherein the positive electrode is a transition metal compound capable of reversibly intercalating lithium.

13. The method of claim 12 wherein the positive electrode is derived from lithiated cobalt oxide (LiCo0₂) .

14. The method of claim 12 wherein the positive electrode is derived from lithiated manganese oxide (LiMn₂0₄) .