GB2029081A

GB2029081A - Lithium molybenum disulphide battery cathode

Info

Publication number: GB2029081A
Application number: GB7928926A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-08-21
Filing date: 1979-08-20
Publication date: 1980-03-12
Also published as: FR2434491A1; GB2029081B; DE2933738C2; NL187943B; NL187943C; IT7950081A0; IT1164041B; NL7906294A; DE2933738A1; FR2434491B1

Abstract

A cathode material for use in lithium batteries has the formula:- Lix MoS2 where 0 <x<3

Description

SPECIFICATION Lithium molybdenum disulphide battery cathode This invention relates generally to storage cells ("batteries") and specifically to a materi al for use as a secondary cell cathode. The invention provides a material which has the advantage, when incorporated as a cathode in a secondary cell, that it allows a high degree of reversibility as the cell is repeatedly charged and discharged. Another advantage of the material of the invention is that it is relatively inexpensive and easily prepared for use as a cell cathode.

It has been discovered that a lithium molybdenum disulphide (LixMoS2) compound exhibits several distinct stages ("phases") of operation, when used as a cathode in a battery having a lithium anode.

During discharge of a newly-constructed battery, (termined "phase 1" operation), lithium cations intercalate into the cathode, thus raising the concentration of lithium in the cathode. It has been found that the battery voltage decreases during battery discharge to a particular point at which a plateau is reached. The plateau represents a region in which the battery voltage remains constant, while the concentration of lithium in the cathode continues to increase. Once a particular concentration of lithium in the cathode is achieved, the battery continues to discharge, in what may thus be termed the "phase 2" region. In this region, the battery potential may be reversibly increased or decreased within certain limits, as the concentration of lithium in the cathode correspondingly decreases or increases.The phase 2 region overlaps the plateau on which the phase 1 to phase 2 transition occurs, in that cathode concentrations of lithium are observed in phase 2 equal to those observed during the transition from phase 1 to phase 2, but at voltages higher than that at which the transition occurs. The transition between phase 1 and phase 2 does not appear to be reversible along the plateau. Phase 2 is a preferred phase of operation, because of the excellent reversibility observed in batteries constructed with cathodes which have been conditioned to operate in phase 2.As discussed in more detail hereinafter, while the initial discharge into phase 2 may be done at room temperature (e.g. about 20 C), if the conversion is done relatively quickly, it is generally preferred (and, in the case of relatively thick cathodes, may become necessary) to operate at a lower temperature (e.g., 0 C).

If the potential of a battery which is operating in phase 2 is allowed to decrease to a particular level, a second plateau is reached along which the cathode concentration of lithium increases at constant battery potential until a third phase ("phase 3") is reached, in which the battery potential may again be reversibly varied as the cathode concentration of lithium increases and decreases. In phase 3 operation, the cathode concentration of lith ium may be decreased, so as to overlap values of cathode lithium concentration found in phase 2 and in the plateau along which the phase 2 to phase 3 transition occurs. A battery operating in phase 3 does not appear to be as highly reversible as a battery in phase 2 and tends to lose capacity more rapidly on repeated charge-discharge cycling.However, in some applications, phase 3 operation may be considered preferable to phase 2 operation, because energy density in phase 3 is considerably higher. The term "reversible" is used herein on the understanding that it does not means perfect or 100% reversibility.

In accordance with one aspect of this invention, an electrolytic cell is provided having a lithium anode, a non-aqueous electrolyte and a cathode which (a) comprises material having the chemical formula: LixMOS2, where 0 < x < 3, and (b) has been conditioned for reversible discharging operation in the cell by discharging the cell to a first cell voltage plateau, further discharging the cell on such first cell voltage plateau and further discharging the cell to a voltage below such first cell voltage plateau, but not less than 0.6 volts.

In accordance with another aspect of the invention, an electrolytic cell is provided, having a lithium anode, a non-aqueous electrolyte and a cathode which (a) comprises a material having the chemical formula: LxMOS2, where O < x < 2, and (b) permits reversible recycling of cell potentials between 2.7 and 0.8 volts, xhaving a value approaching less than 2 and greater than 1 on discharge to cell potentials of 0.8 volt.

In accordance with yet a further aspect, an electrolytic cell is provided, having a lithium anode, a non-aqueous electrolyte and a cathode which (a) comprises a material having the chemical formula: LixMOS2, where 0 < x < 3, and (b) permits reversible recycling of cell potentials between 2.4 and 0.5 volts, x having a value approaching near or about 3 on discharge to cell potentials of 0.5 volt.

In order that the invention may be fully understood and appreciated, reference is made to the single figure of the accompanying drawing and also to the following description and examples.

The figure is a graph showing representative characteristics of a battery having a cathode prepared by coating molybdenum disulphide (MoS2) on to an aluminium foil sub strate, a lithium foil anode and an electrolyte comprising 1 M LiC104 in propylene carbonate. Battery voltage (measured in volts) is plotted as the ordinate vs. and abscissa "x", where "x" represents the concentration of lithium in the cell cathode having the general formula LixMOS2. The quantity x increases as lithium cations intercalate into the cathode during battery discharge. As will become more clearly apparent hereinafter, while the figure is typical it is to be understood that the characteristics shown may vary somewhat in actual practice depending upon various parameters.

The battery is shown to discharge from an initial voltage of above 3 volts along a path AB (3.3 volts is generally typical). Along this path, lithium cations intercalate into the cathode as the battery potential decreases, thus increasing the concentration of lithium in the cathode as indicated. The path AB shows voltage decreasing from the initial value of about 3.3 volts to a plateau (path BD), while x correspondingly increases from 0 to about 0.2. This has been found typical at room temperature. However, at low temperatures (e.g., O"C), the path AB has been found to become much steeper than illustrated, the point B being located so that x is only slightly greater than 0. In some cases, the point B has been observed to lie as high as about x = 0.5 during a room temperature discharge.

However, it is speculated that some electrolyte decomposition may have occurred on discharge or that some impurity may have been present. The term "phase 1" is used to denote the physical structure of a cathode which exhibits a variation of battery potential with cathode lithium concentration governed by the path AB, where the point B lies in the range of x slightly greater than 0 to x ~ 0.5.

If the potential of a battery which is operating in phase 1 is allowed to decrease along the path AB, then a plateau represented by the path BD in the figure is reached. The plateau is shown at about 1.0 volt, in practice, it will typically fall in the range of 0.9 to 1.1 volts at room temperature, but may go as low as 0.7 volt at very low temperatures (e.g., down to - 20'C). Although the plateau is shown as beginning as xs 0.2, it is to be understood on the basis of the immediately preceding discussion that in practice it may begin in the region where x is only slightly greater than 0 up to about x = 0.5. The plateau path BD is shown in the figure as ending at about a point where x = 1.0. In practice, this end point has been observed to occur as high as about x = 1.5.The reason for such variation is not clear, but may be attributable to unknown impurities in the cathode. The plateau path BD shown in the figure indicates a region in which the battery operates at a relatively constant potential of about 1.0 volt, while the concentration of lithium in the cathode represented by x increases during battery discharge.

As shown in the figure, if a battery operating on the plateau path BD is allowed to continue to discharge, then once a cathode lithium concentration represented by a value of about x = 1.0 has been achieved, the battery is observed to discharge along a path DE. The battery discharge may be halted at any point along this path DE and the battery may then be substantially reversibly recharged along a path EC. The term "phase 2" is used to denote the physical structure of a cathode which exhibits a variation of battery potentional with cathode lithium concentration governed by the path CE shown in the figure and this term describes the process of reversibly charging and discharging the battery along the path CE which can thus be regarded as "phase 2 operation". Once phase 2 operation has been achieved, the battery will not reenter the BD plateau directly from phase 2.

However, again it is to be noted that while the figure is representative, variations are observed in practice. As shown, the path DE depicts voltage decreasing from an initial value of about 1.0 volt to about 0.55 volt, while the value of x increases from about 1 to about 1.5. Similarly, the path CE depicts voltage decreasing from about 2.7 volts to about 0.55 volt as xincreases from about 0.2 to about 1.5. In practice, for given voltages, the observed value of x has been somewhat variable. For example, the point C (voltage about 2.7 volts) may vary from a location where xis only slightly greater than 0 (at low temperatures) to about x = 0.5. The point D (about 1.0 volt) may range from about x = 1.0 to about x = 1.6. The point E (voltage about 0.55 volt) may range from about x = 1.3 to about x = 2.0.However, in all cases, the path CE maintains a slope generally downwardly to the right. The reason for such variations is not clear, but again may be attributable to unknown impurities in the cathode. Also it is to be noted that such observations are at room temperature. At lower tem- peratures, measured voltages for a given value of xtend to be somewhat lower. The 0.55 volt value depicted in the figure for the point E (and the path EG discussed hereinafter) is typical at room temperature, but generally the voltage may range from about 0.4 volt to about 0.6 volt.

It has been observed that the most reliably reversible battery operation occurs in a battery having a cathode which has been conditioned to operate in phase 2. Also, the most reliably reversible phase 2 operation has been observed to occur along the path CD. In has been found that, if a battery which is operating in phase 2 is discharged along the path CE, reversibility degrades as the battery potential drops below about 1 volt. Because battery reversibility in phase 2 degrades as the bat tery is discharged below approximately 1 volt, phase 2 operation should desirably be confined to the path CD, by monitoring battery voltage to prevent recharging above a battery potential of about 2.7 volts and by preventing battery discharge below about 1 volt.

If the potential of a battery having a phase 2 cathode is allowed to decrease along the path CE to approximately 0.55 volt (this voltage being typical as noted above), then a second plateau represented by the path EG is reached (at a cathode lithium concentration of about x = 1.5 in the representative figure shown), along which a transition from phase 2 to a third phase occurs at a relatively constant potential, while the concentration of lithium in the cathode represented by x increases to about x = 2.8. It is preferably to maintain the value of xequal to or below approximately 3 in practising the present invention.

If a battery which is operating on the EG plateau is allowed to continue to discharge, then once a cathode lithium concentration represented by a value of about x = 2.8 has been achieved, the cell is observed to discharge further along a path GH. The cell discharge may be halted at any point along the path. The battery may then be recharged along a path HF. The physical structure of a cathode which exhibits a variation of battery potential with cathode lithium concentration governed by the path FH shown in the figure is termed "phase 3" and the process of reversibly charging and discharging the battery along the path FH may be described as "phase 3 operation". Once phase 3 operation has been achieved, the battery will not reenter the EG plateau directly from phase 3.

The point H in the figure does not represent the lower limit of battery discharge capability.

However, a significant degradation in the performance of a battery which is discharged in phase 3 below about 0.3 volts has been observed. This degradation is thought to be related to a diffusion of lithium ions into the cathode aluminium substrate, resulting in the formation of a lithium-aluminium alloy.

It is believed that phase 3 operation is not as reliably reversible as phase 2 operation.

Further, battery potentials cannot normally be achieved in phase 3 as high as those achieved in phase 1 or phase 2 operation of the battery. However, as indicated previously, this does not means that phase 3 operation is invariably undesirable. The energy density of a battery in phase 3 is considerably higher than the energy density of a battery in phase 2. Thus, it is contemplated that in some applications, where energy density requirements are more important than improved reversibility and voltage characteristics, phase 3 operation may be selected as more desirable than phase 2 operation.

It has been found that, if a battery operation in phase 3 is slowly recharged, then a transition from phase 3 to phase 2 occurs when a battery potential of about 2.3 volts is achieved.

A degradation of battery reversibility in phase 2 and phase 3 operation has been observed when battery potential is allowed to fall below about 1 volt. It is thought that this degradation may be due to decomposition of the battery electrolyte. X-ray diffraction analysis reveals the phase 2 structure to be a layered compound having a crystal symmetry distinct from that exhibited by a phase 1 structure.

Example 1.

A battery was constructed as follows: The cathode consisted of approximately 6 cm2 of aluminium foil on which was deposited 3 mg/cm2 of MoS2. The anode was a similar sized piece of lithium foil. The electrodes were separated by a polypropylene separator soaked with an electrolyte of 0.7M LiBr in propylene carbonate (PC). The battery was discharged at 1 mA through phase 1 down through a first voltage plateau of about 1.1 V until the cathode was converted to phase 2.

The battery was then repeatedly charged and discharged more than one hundred times at 10 mA in phase 2 between voltages up to 2.7V, which corresponds to a fully charged phase 2 cathode and 1.0 V. Battery capacity corresponded to 1 electron per MoS2, mole cult (box = 1).

Example 2 A battery was made of similar construction to the cell in Example 1, except that the cathode had only 0.3 mg/cm2 of MoS2 deposited on it. The cathode was converted into phase 3 by discharging the battery through both the first voltage plateau into phase 2 and through a second voltage plateau of about 0.55 V into phase 3. The discharge current was 1 mA. The battery was then repeatedly charged and discharged between 2.4 V (which corresponds to a fully charged phase 3 cathode) and 1.0 V more than one hundred times at 1 mA. Battery capacity corresponded to 1.5 i 0.2 electrons per MoS2 molecule (Ax ~ 1.5 i 0.2). The battery was repeatedly charged and discharged a few times between 2.4 V and 0.5 V.Battery capacity in this case corresponded to 2 i 0.2 electrons per MoS2 molecule (Ax ~ 2 + 0.2).

Example 3.

The battery of Example 2 was recharged slowly (at about 100 microamps) to 2.7 V. It was found that the cathode was reconverted to phase 2 operation after being recharged in this manner.

Example 4 A battery was constructed as follows: The cathode consisted of 1.3 cm2 of aluminium foil on which was deposited 0.5 mg/cm2 of MoS2. The anode consisted of a similar area of lithium foil pressed on to an expanded nickel grid. The electrodes were suspended in an electrolyte of 0.7M LiBr and PC contained in a 50 ml glass beaker. An argon atmosphere was contained within the beaker with a neoprene stopper. This battery was conditioned by a 100 microamp initial discharge, then repeatedly cycled in phase 2 eighty-two times between 2.7 V and 1 V at 100 microamps. The battery was then further discharged into phase 3, where it cycled repeatedly ten times between 2.4 V and .5 V.

Although the initial conditioning discharges in the foregoing examples were done at room temperature (about 20 C) and good results were achieved, it is generally considered desirable to cool a cell for the conditioning discharge. Otherwise, problems with electrolyte decomposition may be encountered. The cathodes of the above examples were relatively thin and it was possible to perform the conditioning discharges relatively quickly at room temperature, without development of significant temperature gradients in the cathode.

However, with thicker cathodes, the desirability of cooling becomes important. At 10 mg/cm2 of MoS2, it appears that cooling is essential. Although cooling temperatures as low as - 20"C have been used, a temperature of O"C has been found quite satisfactory for cathode thicknesses ranging up to about 20 mg/cm2 of MoS2.

Phase operation may also be achieved where the MoS2is partially oxidized to MoO2.

Partial oxidization to MoO2 can improve conductivity without serious loss of capacity.

Example 5.

A battery having a cathode which included MoS2 partially oxidized to MoO2 was constructed as follows: (a) MoS2 powder having an average particle diameter of about 20 microns was mixed in a 1 to 1 volume ratio with propylene glycol and a film of the resulting slurry applied to the aluminium foil substrate.

(b) The substrate with applied film was baked at 580 C in an atmosphere containing about 0.4 mole percent oxygen in nitrogen or about 10 minutes to form a cathode containing approximately 20 mole percent MoO2 and approximately 80 mole percent MoS2.

A cell was constructed using two stainless steel flanges separated by a neoprene O-ring sealer. The anode consisted of a 6 cm2 sheet of lithium. A 6 cm2piece of the prepared cathode (on which had been deposited approximately 43 milligrams of the partially oxidized MoS2) was used as the cell cathode. A porous polypropylene separator sheet which had been soaked in a 1 M solution of lithium perchlorate in propylene carbonate was inserted between the anode and the cathode.

The newly constructed cell was conditioned by initially discharging it at 4 mA to a lower cutoff voltage of about 0.85 V. During this initial discharge, the cell voltage dropped in about 20 minutes to a plateau of about 1 V and then decreased approximately linearly to about 0.85 V in a further 2 hours. The cell thus prepared and conditioned was cycled through 66 discharge-charge cycles at about 4 mA between a minimum voltage of about 0.85 V and a maximum voltage of about 2.7 V.

Claims

1. An electrolytic cell, having a lithium anode, a non-aqueous electrolyte and a cathode which (a) comprises material having the chemical formula: LixMoS2, where 0 < x < 3, and (b) has been conditioned for reversible discharging operation in the cell by discharging the cell to a first cell voltage plateau, further discharging the cell on such first cell voltage plateau and further discharging the cell to a voltage below such first cell voltage plateau, but not less than 0.6 volts.

2. A cell according to claim 1, wherein the first cell voltage plateau is in the range from 0.7 to 1.1 volts.

3. A cell according to claim 2, wherein the first cell voltage plateau is in the range from 0.9 to 1.1. volts.

4. A cell according to any preceding claim, which has been recharged following conditioning to a maximum voltage of 2.7 volts.

5. A cell according to any preceding claim, wherein the cathode has been further conditioned following discharging to the first cell voltage plateau by discharging the cell to a second cell voltage plateau below the first cell voltage plateau, further discharging the cell on such second voltage plateau and further discharging the cell to a voltage below such second cell voltage plateau, but not less than 0.3 volts.

6. A cell according to claim 5, wherein the first cell voltage plateau is in the range from 0.7 to 1.1 volts and the second cell voltage plateau is in the range from 0.4 to 0.6 volt.

7. A cell according to claim 6, wherein the first cell voltage plateau is in the range from 0.9 to 1.1 volts and the second cell voltage plateau is in the range from 0.45 to 0.55 volt.

8. A cell according to any preceding claim, wherein the discharging is done while the cell is below room temperature.

9. A cell according to claim 8, wherein the discharging is done at a temperature in the range from 0 to - 20'C.

10. An electrolytic cell having a lithium anode, a non-aqueous electrolyte and a cathode which (a) comprises a material having the chemical formula: Li,MoS2, where 01 < x < 2, and (b) permits reversible recycling of cell potentials between 2.7 and 0.8 volts, xhaving a value approaching less than 2 and greater than 1 on discharge to cell potentials of 0.8 volt.

11. An electrolytic cell having a lithium anode, a non-aqueous electrolyte and a cathode which (a) comprises a material having the chemical formula: LixM S2, where 0 < x < 3, and (b) permits reversible recycling of cell potentials between 2.4 and 0.5 volts, xhaving a value approaching. near or about 3 on discharge to cell potentials of 0.5 volts.

12. A cell according to any preceding claim, wherein the MoS2 is partially oxidized to MoO2.

13. A cell according to claim 1, substantially as herein discribed with reference to the Examples.

14. A cell according to claim 10, substantially as herein described with reference to the Examples.

15. A cell according to claim 11, substantially as herein described with reference to the Examples.