GB2311410A - Non-aqueous safe secondary cells - Google Patents

Abstract

A non-aqueous safe secondary cell (10) is provided. The cell can be repeatedly charged and discharged retaining the excellent safety features. This cell comprises as main components a negative electrode which is Lithium or Lithium alloy, a positive cathode (20) which includes MnO 2 and an electrolyte (22) which is 1,3-Dioxolane with Lithium hexafluoroarsenate (LiAsF 6 ) and a polymerization inhibitor. The cell is made by (i) mixing a particulate cathode active material with a binder, carbon and a saturated olefin solvent; and (ii) heating the mixture to evaporate the solvent. The operational life of an electrochemical secondary cell which is to be exposed to charging conditions of greater than 150 mA is enhanced by initially exposing the cell to a plurality of charge and discharge cycles, each such cycle having a charge rate of no greater than 108 mA and a discharge rate of no smaller than 200 mA.

Description

A METHOD OF ENHANCING THE OPERATIONAL LIFE OF AN ELECTROCHEMICAL SECONDARY CELL FIELD OF THE INVENTION This invention relates to a method of enhancing the operational life of an electrochemical cell.

BACKGROUND TO THE INVENTION Rechargeable electrochemical cells, also known as secondary cells, typically include an anode, a cathode and an electrolyte. In many commercially available secondary cells the anode includes an alkali metal; the electrolyte is a solution containing an electrolytic salt which is usually an alkali metal as an anode; and the cathode includes an electrochemically active material, such as compound of a transition metal. During use, electrons pass from the anode through exterior connecting circuitry to the cathode and alkali metal ions from the anode pass through the electrolyte to the cathode where the ions are taken up, with the release of electrical energy. During charging, the flow of electrons and ions are reversed.

In the design of secondary cells two issues are of importance. On the one hand the cell must be safe; on the other hand the cell must have good performance, meaning that it must be able to produce energy and be capable of being cycled (charged and discharged) numerous times.

SUMMARY OF THE INVENTION According to the present invention there is provided a method of enhancing the operational life of an electrochemical secondary cell which is to be exposed to charging conditions of greater than 150 mA, comprising the step of initially exposing the cell to a plurality of charge and discharge cycles, each such cycle having a charge rate of no greater than 108 rnA and a discharge rate of no smaller than 200 mA.

A preferred embodiment of the invention is described in more detail below, with reference to the drawings, and takes the form of a secondary electrochemical cell comprising a Lithium based negative electrode; a positive electrode including the compound My03; and an electrolyte including an ionic salt and a solvent which is stable at temperatures below 100"C, but which polymerizes at temperatures greater than 100"C and at voltages higher than 4 Volts. This polymerization increases the intemal resistivity of the cell and thereby decrease the flow of current and, simultaneously, the temperature within the cell.

Typically, the ionic salt is a Lithium salt and has concentration in the dioxolane solvent from 0.5 mole per liter of solvent up to the saturation point. Preferably the salt is LiAsF6 at concentrations between 0.8M to 1.5M per liter.

The positive electrode or cathode can be Mono2 or lithiated MnO2 and is preferably of the general formula LixMnO2, with x = 0.30 to 0.40.

The electrolyte is preferably a member of the dioxolane family and should include a stabilizer which acts as a polymerization inhibitor. Typically the electrolyte includes 1,3 dioxolane and the stabilizer is a member of the tertiary amine group and typically is one of the group consisting of: Triethylamine, Tributhylamine, Tripropylamine, Tribenzylamine, Trioctylamine, Triphenylamine, methylpiperidine.

Stabilizers such as triethylamine. tributylamine and triphenylamine, are preferably added at concentrations of between 50ppm (v/v) to 5 percent (v/v) while concentrations of 100 ppm (v/v) to 5000ppm (v/v) of triethylarnine and tributylamine were found to be the most effective.

A cell of this type has been found to be particularly advantageous as it is a high density lithium based secondary cell which can be safely cycled many times and which is also appropriately safe under the abusive operating conditions of high temperature, overcharging and short circuiting.

The preferred embodiment is described below and its details will no doubt become apparent to those skilled in the art after having read the following description which is illustrated in the several figures of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The preferred embodiment of the invention will now be described in more detail, by way of example, with reference to the drawings, in which: Fig. 1 is a schematic representation of an electrochemical secondary cell made by a method embodying the invention; Fig. 2 graphs the behaviour of the cell under short circuit conditions; Fig. 3 illustrates a short circuit test of a prior art AA size, rechargeable Li/LixMnO2 cell; Fig. 4 graphs the maximum temperature of a cell, as a function of cycling life, for cells exposed to short circuit conditions.

Fig. 5 plots the behaviour of the above cell under conditions of overcharge at a current of 1A; Fig. 6 illustrates an overcharge test of a prior art rechargeable Lithium AA size cell; Fig. 7 is a graph plotting the results of an experiment conducted with a secondary cell which was repeatedly cycled; Fig. 8 compares the performance of cells with or without the preferred electrolyte; Fig. 9 graphically illustrates discharge tests done on the secondary cells; and Fig. 10 graphically illustrates the capacity of secondary cells tested by discharging at low temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In Fig. 1 of the accompanying drawings, an electrochemical cell made by a method embodying the invention is generally indicated by 10. The cell 10 is shown to include an outer casing 12, a cap 14 and a base 16. Inside the casing 12 an anode 18, cathode 20, electrolyte 22 and separator 24 are located in the form of a tightly wound roll 26.

The cap 14 is of standard industrial manufacture and includes an upstanding protrusion 30 which serves as the positive terminal of the cell. The protrusion 30 is the top of a downwardly extending molybdenum pin 32 which is supported by a glass insulator 34.

As is indicated the pin 32 is connected to the cathode 20 by way of a tab 36 and the anode 18 connected to the casing 12 by a tab 38.

A typical AA size cell of this type of construction would be 50mm long and have an outer diameter of 14 to 1 Smm. The anode 18 and cathode 20 consist of 40mm wide strips respectively about 300mm and 250mm long. Typically the strip making up the anode 18 is 160pm thick and that making up the cathode 20 is 250pm thick. The separator 24 is made of a porous strip about 700mm long, 48mm wide and about 25zm thick.

During manufacture of the cell, the separator 24 is folded lengthwise over the cathode 24 and this combination together with the anode 18 is wound tightly to produce a roll 26.

The roll 26 is then covered in insulation and inserted into the casing 12. As the separator 24 is wider than the cathode, it protrudes beyond both the cathode and the anode and, in use, serves to prevent short circuiting between them. Once the roll 26 is located in the casing, the tabs 36, 38 are respectively connected to the pin 34 and casing 12, the remaining portion of the tab 36 insulated to prevent short circuit with the anode 18 and the cap 14 secured to the top of the casing. Electrolyte 22 is then vacuum injected into the casing 12 through an aperture (not shown) in its base 16 which is thereafter sealed off.

As the electrolyte, which will be described in greater detail below, has a very low viscosity, it fills all the voids, including the pores in the separator and the cathode.

Turning now to the relevant components of the cell, these are described in greater detail below: 1. The Negative Electrode (anode) The anode 18 is typically a thin laminate foil consisting of a copper layer sandwiched between two lithium layers. In the example of the AA cell given above, the copper layer is typically 20KLm thick and the lithium layers each 70pm thick. This type of lithium based anode is known in the art and can be obtained commercially. Furthermore, it is also possible to use a pure Lithium anode.

2. The Positive Electrode (cathode) The cathode 20 preferably contains an active material which is manganese dioxide (MnO2) or lithiated MnO2. Many different ways of producing this cathode active material are known.

For example, one way of producing an MnO2 active material is disclosed in United States Patent 4,133,856 (Ikeda, et al.). This patent teaches that ganuna-phase manganese dioxide is heated for a period of at least 2 hours at a temperature of between 350" and 430 C so that the MnO2 is effectively dried and its structure changed from the gamma-phase to the beta-phase. This method is also referred to in U.S. Patent 5,279,972 (Moses).

As indicated, the cathode active material can also include lithium, in which case it is preferable for the material to be lithiated manganese dioxide of the formula LixMnO2.

One way of preparing this compound is disclosed in Matsushita's Japanese Kokai 62-20250 (1987) which suggests the synthesis of a lithium potassium permanganate of the general formula (l-x) Kx LiMnO4 followed by the thermal decomposition of the permanganate. This publication teaches that the decomposition products resulting from this process have stoichometric formulas of between Liy 3 MnO2 and Liy 8 Mn02.

Yet another method of producing a lithiated manganese dioxide cathode active material is described in Sanyo's U.S. Patent 4,758,484 which was issued to Furukawa et. al. on July 19, 1988. This method is described as selecting particulate MnO2 having a particle size not greater than 30rum, mixing it with a suitable lithium salt and heating the mixture in air at 375 cm. Typical lithium salts mentioned are: LiOH; LiNO3; Lip3; Li2CO3 and Li20. This patent discloses a number of different ratios to define the mixture of MnO2 and lithium salt.

Although this patent indicates that this method results in an active material of the formula, Li2MnO3 tests have shown that it in fact produces a material of the formula of liy MnO2, where x is 0.3 to 0.4, a material which is also appropriate for use in cells embodying this invention There are thus many ways of producing an appropriate cathode active material. This material, whatever the form, is typically in granular/powder form which must be rnixed with a conductive agent such carbon black or graphite in amounts of between 5% to 10% by weight, and with a commercial binder. Many different types of binders are known.

One type is an emulsion of Teflon powder and water. This mixture can then be heated to make a putty which can be moulded under heat and/or pressure.

It has also become common to mix a solvent in with the mixture of binder and cathode active material. As a result of the heat applied during the moulding step, this solvent evaporates. Even so, it has the advantage of producing a better combination between the binder and active material and thus improves the performance of the cell in use.

Generally, the solvent is added in quantities of about 70% w/w solvent to binder plus cathode material. The solvent can, for example, be propanol, ethanol, isopropanol or one of the family of alkanes such as C1 0H22 (decane), C9H20 and C 1H24.

Finally, during the manufacture of the cathode, the mixture is rolled onto an aluminum grid and baked at about 250 to 320"C for 0.2 to 4 hours. The temperature and timing of this step is, to a large degree, a matter of choice but it is important that sufficient heat be applied so that the binder sinters and binds with the active material, and that the solvent evaporates. In use the aluminum grid acts both as a support for the cathode material and as a collector for electrons.

The Electrolvte The electrolyte 22 is important to the safe operation of the cell. It should include a solvent which is stable and which, together with a suitable ionic salt, forms a conductive solution at typical cell operating conditions. The solvent should, however, be such that it polymerizes in the cell so as described, above about 100"C and at high voltages of above about 4V. These conditions would typically be reached when the cell is overcharged at high currents or short circuited. This polymerization is very effective in that it substantially increases the resistivity of the cell and thereby terminates its operation and precludes the possibility of further hazardous reactions between the lithium and the electrolyte itself.

In a preferred embodiment of the invention, the electrolyte is constituted by a solution of a member of the dioxolane family and most preferably by 1,3-dioxolane, a lithium salt and a tertiary amine polymerization inhibitor (stabilizer). The concentration of the lithium salt in the dioxolane, can vary from 0.5 mole per liter of dioxolane up to the saturation point. However, the lithium salt of choice is lithium hexafluoroarsenate (LiAsF6) at concentrations between 0.8 to 1.5 moles per liter.

It has been found that 1 ,3-dioxolane in conjunction with an MnO2 based cathode active material produces very good results. As will be shown in the experiments below, 1,3-dioxolane with the LiAsF6 polymerizes at high voltages (above about 4.0V) and/or under high temperatures (above about 100"C).

It should be noted that although this advantageous result has only been observed in 1,3 dioxolane in the presence of a MnO2 based cathode material, and more particularly when the lithium based ionic salt is LiAsF6, it is conceivable that other combinations of electrode, solvent and salt may produce the same result. For example a cathode containing Ni used with a LiC104 salt in dioxolane could be used. Alternatively a different metal oxide such as a Cobalt oxide could be used in the cathode. Similarly ethers such as 2-methyl tetrahydrafuran could be used. An alternative anode could be lithium ion type of material. Whatever the combination of components used it is important that the electrolyte in combination with the other components will be stable at normal operating temperatures and when the Voltage across the cell is within the operating voltage window of the cell. In addition to this, the electrolyte, in combination with the other elements, must polymerize when the conditions are beyond the normal operating parameters.

The performance of the electrolyte can be enhanced by the use of a stabilizer which inhibits the polymerization of the electrolyte at operating temperatures. This stabilizer is typically one of the group of tertiary amines and preferably is one of the group consisting of: Triethylamine, Tributhylamine, Tripropylamine, Tribenzylamine, Trioctylamine, Triphenylamine, methylpiperidine. A concentration of 1000ppm of stabilizer has been found to yield satisfactory results.

The Separator The separator 24 functions to keep the cathode and anode apart to prevent short circuiting but is porous to allow the flow of ions across it. Many different separator materials are available commercially, for example the polypropylene separator which is sold under the designation 3402 by Hoechst Celanese.

Operation of the Cell: The operation ofthe secondary cell 10 can be illustrated in the following examples which report the results of the experiments conducted: Experiment 1: A number of AA size cells containing a Li metal anode; a LixMnO2 cathode; an electrolyte of 1,3-dioxolane, 1 mole per liter LiAsF6 and 1000ppm v/v tributhylamine; and a polypropylene separator were used in this experiment.

These cell were exposed to the short circuit condition of a 0.01 ohm resistance and the results plotted on Fig. 2. It can be seen from this figure that the temperature rise in the cells was limited to a maximum temperature of about 115"C thereby preventing any hazardous venting.

In contrast to this Fig. 3 illustrates the results of similar tests concluded on prior art cells.

This figure shows very clearly how the temperature within the cell increased dramatically after reaching the 140"C mark, thereby destroying the cell.

Experiment 2: Charged cells similar to those used in Experiment 1 were exposed to short circuit conditions of 0.01 ohm resistance. Prior to the experiment, the cells were cycled (under extreme conditions) by discharging them at 40 to 250mA and charging them at 60 to 250mA. The internal temperature of each cell was monitored during the short circuit conditions. The readings are plotted in Fig. 4, against the number of cycles that the cell had previously been exposed to.

As can be seen the maximum temperature reached decreased with the number of cycles.

this indicates that the cell is safer the more it has been cycled.

Experiment 3: Fig. 5 describes the behaviour of the same type of cell as in Experiment 1 but this time exposed to overcharge conditions at a current of 1A. The cell had previously been cycled 50 times.

As can be seen, the charging voltage increased rapidly to more than 1 0V when the cell voltage reached 4.1 V. This situation was reached after the cell was overcharged for 3 hours. Importantly, the temperature and the current (not shown) decreased at the same time the voltage increased. The cell was tested for a further 20 minutes and remained intact after this test.

In contrast to this Figure 6 shows the results of a similar experiment conducted on prior art cells. From this figure the failure of the cell and hazardous increase in temperature are clearly apparent.

Experiment 4 Cells similar to those used in Experiment 1 were tested by repeated cycling. The results of this experiment are plotted on the graph in Fig. 7 which shows that the cells exhibited a long cycling life of more than 250 cycles. Moreover, even at cycle number 250 the cells were still able to deliver more than 75% of their nominal capacity.

Experiment 5: Li/LixMnO2 Cells similar to those used above, but with two different electrolytes A and B, were tested for comparison purposes. Electrolyte A was a dioxolane/LiAsF6/1000 ppm tributhylamine solution and electrolyte B a prior art EC/PC/LiAsF6 solution.

Cells of both types were initially cycled at a charge rate of 60mA and discharged at a rate of 250mA. This type of cycling is typical of normal operating conditions. Thereafter the cell containing electrolyte A was charged at 250mA and discharged at the same rate. The cell containing electrolyte B was charged at the more advantageous rate of 120mA and also discharged at 250mA per hour. Both cells were cycled between 2.0 and 3.4V at these respective rates until they reached about 65% of their capacity.

As can be seen from the results plotted in Fig. 8, cell A reached the end of its life, with 65% of the capacity it had after the second cycle, after 120 cycles. However, cell B was cycled only 50 times before developing short circuits and failing as a result of formation of dendrite bridges in the electrolyte.

It should be noted that this experiment also shows that the cell can be treated to increase its life expectancy when it is to be exposed to conditions of rapid charging (such as the tested charging rate of 250mA). This can be achieved by first cycling the cell at the usually recommended normal rate, of 60mA charge and 250mA discharge, for about two to ten times. The optimum number of these cycles has yet to be determined, but it is believed that they should be sufficient to allow lithium to redeposit on the anode.

After this cycling, the cell can be rapidly charged as shown. Although this reduces its potential life from over 250 cycles to about 120 cycles, it is a substantial improvement over rapid charging without the initial slower charging steps (which would yield a life of less than 50 cycles even in the cell embodying this invention). It is furthermore anticipated that this method may also be applied to other secondary cells as well.

Experiment 6 The same cells as used in Experiment 1 were tested by discharging at the relatively high rates of 0.25A, 0.5A, 1A and 2A, respectively. As can be seen from the results of these tests, which are plotted on the graph of Fig. 9, the cells perform very well and retain a large percentage of their capacity.

Experiment 7 In this experiment the same cells as used in Experiment 1 were discharged at the three different "low" temperatures ofO"C, -20 C and -30 C. As can be seen from the results plotted in Fig. 10 the cells performed well at low temperature. The cells used delivered more than 60% at their room temperature (RT) capacity at -300C at a 60rnA rate of discharge.

Experiment 8: A cell identical to the one used in Experiment 1 was tested at high temperature. The cell was placed inside an oven, in which a thermocouple was attached to the outer casing of the cell. The oven was heated at rate of 3 "C per minute. When the cell's can reached 128"C, it vented without noise, fire or any explosion. The cell had previously been cycled 120 cycles at 250mA discharge and 60mA charge.

In a manner similar to that of Experiment 1, a variety of cells were tested, using other members of the tertiary amine family defined above. Similar results were obtained. The amines were tested at various concentrations, and proved to be effective over a wide range of concentrations, from about 0.005 percent to about 5 percent. The best results were obtained at concentrations in the 0.01% to 0.3% range.

Furthennore, various concentrations ofthe lithium hexafluoroarsenate were tested. Good results were obtained using a concentration in the 0.5 mole/liter to 2.5 mole/liter range, with the best results at about 1 mole/liter.

From these experiments it can be concluded that the secondary cell illustrated performs well under the adverse conditions of high and low temperature, short circuit and overcharging. In addition, the cell is shown to be capable of being cycled many times.

Also, the addition of the stabilizers gives the cell an added shelf life, even at temperatures ofas high as 80"C.

Although the present invention has been described above in terms of a specific embodiment, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications as fall within the scope of the following claims.

The present application is divided out of application number 9626315.7, dated 1 March 1994, serial number 2,306,762, which is directed to a method of producing a cathode for an electrochemical cell, the method comprising the steps of: mixing a particulate cathode active material with a binder, carbon and a saturated olefin solvent; and heating the mixture to evaporate the solvent; which is itself divided out of application number 9403881.7, serial number 2,275,818, which is directed to an electrochemical cell comprising: (i) a cathode including a metal oxide; (ii) an anode; and (iii) an electrolyte solution which rapidly polymerizes at temperatures exceeding 100 C, and at voltages greater than the operating voltage range of the cell, for safely terminating operation of the cell before any venting occurs, by increasing the internal resistivity of the cell.

Claims (2)

1. A method of enhancing the operational life of an electrochemical secondary cell which is to be exposed to charging conditions of greater than 150 mA, comprising the step of initially exposing the cell to a plurality of charge and discharge cycles, each such cycle having a charge rate of no greater than 108 mA and a discharge rate of no smaller than 200 mA.

2. A method of enhancing the operational life of an electrochemical secondary cell, substantially as herein described with reference to the accompanying drawings.