GB1599792A - Cathodes for solid state lithium cells - Google Patents

Abstract

A solid body element of high energy density and having a positive electrode material consisting of ion and electron conducting, dischargeable components in conjunction with non-conductive, positive electrode materials of higher energy density.

Description

(54) CATHODES FOR SOLID STATE LITHIUM CELLS (71) We, DURACELL INTERNATIONAL INC., formerly known as P.R. MAL LORY & CO., INC., a Corporation organised and existing under the Laws of the State of Delaware, United States of America, of 3029 East Washington Street, Indianapolis, Indiana 46206, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described, in and by the following statement: This invention relates to high energy density cells utilizing solid electrolytes, solid active metal anodes and novel solid cathodes, and more particularly to such cells in which the cathodes contain an active material which is both ionically and electronically conductive.

Recently the state of electronics has achieved a high degree of sophistication especially in regard to devices utilizing integrated circuit chips, which have been proliferating in items such as quartz crystal watches, calculators, cameras, cardiac pacemakers and the like.

Miniaturization of these devices as well as low power drainage and relatively long lives under all types of conditions has resulted in a demand for power sources which have characteristics of rugged construction, long shelf life, high reliability, high energy density and an operating capability over a wide range of temperature, as well as concomitant miniaturization of the power source. These requirements pose problems for conventional cells having solution-type or even paste-type electrolytes, especially with regard to shelf life. The electrode materials in such cells may react with the electrolyte solutions and tend therefore to self-discharge after periods of time which are relatively short when compared to the potential life of solid state batteries. There may also be evolution of gases in such cells, which could force the electrolyte to leak out of the battery seals, thus corroding other components in the circuit, which in sophisticated componentry can be very damaging. To increase the reliability of cell closures increases both bulk and cost, and will not eliminate the problem of self-discharge. Additionally, solution cells have a limited operating temperature range, dependent upon the freezing and boiling points of the solutions contained.

Success in meeting the above demands without the drawbacks of solution electrolyte systems has been achieved with the use of solid electrolyte and electrode cells or solid state cells which do not evolve gases, do not self-discharge on long standing, and have no electrolyte leakage problems. These systems however have had their own particular limitations and drawbacks not inherent in solution electrolyte cells.

Ideally a cell should have a high voltage, a high energy density, and a high current capability. Prior art solid state cells have however been deficient in one or more of the above desirable characteristics.

A first requirement and an important part of the operation of any solid state cell is the choice of solid electrolyte. In order to provide good current capability a solid electrolyte should have a high ionic conductivity which enables the transport of ions through defects in the crystalline electrolyte structure of the electrode-electrolyte system. An additional, and one of the most important requirements for a solid electrolyte, is that it must be almost solely an ionic conductor. Conductivity due to the mobility of electrons must be negligible, because otherwise the resulting partial internal short circuiting would result in the consumption of electrode materials even under open circuit conditions. Solution electrolyte cells include an electronically non-conductive separator between the electrode elements to prevent such a short circuit, whereas solid state cells utilize the solid electrolyte both as electronic separator and as the ionic conductive species.

High current capabilities for solid state cells have been attained with the use of materials which are solely ionic conductors such as RbAg4IS (0.27 ohm-l cm-l room temperature conductivity). However these conductors are only useful as electrolytes in cells having low voltages and low energy densities. As an example, a solid state Ag/RbAg4I5/RbI3 cell is dischargeable at 40 mA/cm2 at room temperature, but with about 0.012 Whr/c.c. (0.2 Whr/in5) and an OCV (open circuit voltage) of 0.66V. High energy density and high voltage anodic materials such as lithium are chemically reactive with such conductors, thereby precluding the use of these conductors in such cells. Electrolytes, which are chemically compatible with the high energy density and high voltage anode materials, such as LiI, even when doped for greater conductivity, do not exceed a room temperature conductivity of 5 x 10-5 ohm-l cm-'. Thus, high energy density cells with an energy density ranging from about 0.3 to 0.6 Whr/c.c. (5-10 Whr/in3) and a voltage of about 1.9 volts for a Li/doped-LiI/PbI2,PbS, Pb cell currently being produced are precluded from having an effective high current capability above 50 IsA/cm2 at room temperature. A further aggravation of the reduced current capability of high energy density cells is the low conductivity (both electronic and ionic) of active cathode materials. Conductivity enhancers such as graphite for electronic conductivity and electrolyte for ionic conductivity, while increasing the current capability of the cell to the maximum allowed by the conductivity of the electrolyte, reduce the energy density of the cell because of their volume.

Commercial feasibility in production of the electrolyte material is another factor to be considered in the construction of solid state cells. Thus, the physical properties of electrolytes such as BaMgsS6 and BaMgsSe6, which are compatible with a magnesium anode but not a lithium anode, and sodium beta aluminas such as Na2O.11 A1203, which are compatible with sodium anodes, will preclude the fabrication of cells having a high energy density or current capability even when costly production steps are taken. These electrolytes have ceramic characteristics making them difficult to work with, especially in manufacturing processes involving grinding and pelletization, with such processes requiring a firing step for structural integrity. Furthermore, the glazed material so formed inhibits good surface contact with the electrodes, with the result of poor conductivity leading to poor cell performance. These electrolytes are thus typically used in cells with molten electrodes.

It is therefore an object of the present invention to increase the conductivity of the cathode of the solid state cells in conjunction with high energy density anodes and compatible electrolytes, so that there is an increase in energy density without current capability losses, while maintaining chemical stability between the cell components.

According to the present invention there is provided a solid state electrochemical cell comprising a solid lithium anode; a solid electrolyte comprising one or more lithium salts and having an ionic conductivity in excess of 1 x 10-9 ohm~l cm-l at room temperature, and a solid cathode comprising as active material a mixture of an ionically and electronically conductive metal chalcogenide, whereof the ionic and electronic conductivity range between 10-i and 10-2 ohm-l cm-l at room temperature and which is cathodically active with said lithium anode, and elementary cathode active sulfur, selenium, tellurium, bromine and/or iodine, and 0 to 10% by weight of non-cathode active ionically conductive material.

The present invention involves the incorporation into the cathode of a solid state cell of a material which has the characteristics of being both ionically and electronically conductive as well as being able to function as an active cathode material. Normally cathodes require the incorporation of substantial amounts (e.g. over 20 percent by weight) of an ionic conductor, such as that used as the electrolyte, in order to facilitate ionic flow in the cathode during the cell reaction. This is especially true if the cathodic material is an electronic conductor, since otherwise a reduction product would form at the cathode electrolyte interface which would eventually block off a substantial amount of the ionic flow during discharge. However the incorporated ionic conductors in prior art cells have not in general been cathode active materials with the result of significant capacity loss.

Additionally, cathode active materials which are poor electronic conductors require the further incorporation of electronically conductive materials, which further reduces the cells energy capacity. By combining the functions of electronic and ionic conductivity with cathode activity in accordance with the invention, a higher energy density and current capability are attained, with the need for space wasting conductors being obviated.

Examples of materials having the requisite characteristics of ionic and electronic conductivity and which are cathodically active as well as being compatible with electrolytes used in high energy density cells include the following metal chalcogenides: CoTe2, Cr2S3, HfS2, HfSe2, HfTe2, Irate2, Mops2, Mosey, MoTe2, Nubs2, NbSe2,NbTe2, Nitre2, PtS2, PtSe2, Pete2 SnS2, SnSSe, SnSe2, TaS2, TaSe2, TaTe2, TiS2, TiSe2, TiTe2, VS2, VSe2, VTe2, WS2, WSe2, WTe2, ZrS2, ZrSe2, and ZrTe2, wherein the chalcogenide is a sulphide, selenide, telluride or a combination thereof.

Also suitable are non-stoichiometric metal chalcogenide compounds such as LiXTiS2 where x < 1, which to some extent contain the complexed form of one of the cathode materials with the anodic cation and which are believed to be intermediate reaction products during cell discharge.

In order for the ionically and electronically conductive cathode active material to be commercially useful in high voltage cells with lithium anodes, it should be able to provide a voltage couple with lithium having an open circuit voltage (O.C.V.) of 1.3 volts at least, and preferably above 2 volts.

The operating voltage of the ionically and electronically conductive cathode active material should preferably be roughly equivalent to the voltage of the higher energy density non-conductive cathode active material fixed therewith to avoid detrimental voltage fluctuations.

A further criterion for the above cathodic material is that both the ionic and electronic conductivities of the conductive cathode active material should range between 10-10 and 102 ohm-l cm- with a preferred ionic conductivity of more than 10-6 and an electronic conductivity greater than 10-3, all at room temperature.

In addition, and most importantly, the ionically and electronically conductive, active cathode material must be compatible with the solid electrolytes used in the high energy density cells.

The solid electrolytes used in high energy density lithium cells are lithium salts and have ionic conductivities greater than 1 x 10- ohm-l cm-1 at room temperature. These salts can either be in the pure form or combined with conductivity enhancers such that the current capability is improved thereby. Examples of lithium salts having the requisite conductivity for satisfactory cell utilization include lithium iodide (LiI), and lithium iodide mixed with lithium hydroxide (LiOH) and aluminium oxide (awl203), the latter mixture being referred to as LLA and being disclosed in U.S. patent No. 35713,897.

It is postulated that the aforementioned ionically and electronically conductive, cathode active materials react with the ions of the anode (i.e. lithium cations) to form a non-stoichiometric complex during the discharge of the cell. This complexing of cations allows them to move from site to site thereby providing ionic conductivity. Additionally the above compounds provide the free electrons necessary for electronic conductivity.

The above compounds are mixed with other compounds or elements, specially sulphur, selenium, tellurium, iodine and bromine, which provide a greater density but which cannot be utilized alone because of their inability to function as ionic and/or electronic conductors.

The inclusion of the ionically and electronically conductive cathode active material thereby increases the capacity of the cell by obviating the need for not-dischargeable conductive materials. Furthermore, when the conductive active material is homogeneously admixed with the higher energy density material the reliable utilization of the so formed cells is approximately equal to the theoretical value. A limiting factor in solid state cell performance is the conductivity of the cell reaction product. A low conductivity product results in large internal resistance losses, which effectively terminate cell usefulness. In cells having the above ionically and electronically conductive, cathode active material the complexed reaction product retains conductivity, thereby enabling full utilization of other active cathode materials which are in proximity therewith.

Accordingly, high energy density cathodic materials such as sulphur and iodine as well as other solid chalcogens, Se and Te, and halogens such as bromine can be effectively utilized to greater potential. Solid state cells utilizing sulphur in conjunction with lithium anodes and lithium salt solid electrolytes have shown great promise in terms of voltage obtainable and total energy density. However, one of the drawbacks has been the formation of the low ionically conductive lithium sulphide (Li2S) as the cell reaction product, especially at the cathode/electrolyte interface. This build up has effectively choked off the further utilization of these cells. However the inclusion of the ionically and electronically conductive cathode active materials provides a more uniform distribution of the reaction product throughout the cathode structure because of their ionically conductive characteristics. Since the reaction products of the ionically conductive materials retain conductivity, further utilization of the cell is also possible with the non-conductive active material in conductive proximity to the conductive active material.

A small amount of electrolyte can also be included in the cathode structure in order to blur the interface between cathode and electrolyte, thereby providing more intimate electrical contact between the cathode and the electrolyte. This enables the cell to operate at higher current drains for longer periods of time. Additionally, the electrolyte inclusion can increase the ionic conductivity of the cathode, if the ionically conductive cathode active material has a lower conductivity than that of the electrolyte. This inclusion however, if made, should not exceed 10% by weight since greater amounts would merely decrease the energy density of the cell with little if any further improvement of current drain capacity.

The following examples illustrate the high energy density and utilizability of a sulphur-containing cathode in a solid state cell, in combination with the abovementioned ionically and electronically conductive cathode-active metal chalcogenides. Sulphur cannot be used alone as a cathode in a solid state cell unless it contains substantial amounts of ionic and electronic conductors which constituted 60% or more of the total cathode by weight.

Thus the inclusion, in a cathode of sulphur, of an ionically and electronically conductive metal chalcogenide such as titanium disulphide, enables the use of sulphur without the hitherto concomitant severe losses of energy capacity. Titanium disulphide is a good ionic electronic conductor (10-5 ohm' cm room temperature ionic conductivity and greater than 10-2 ohm' cm' room temperature electronic conductivity) and also functions as a reactive species in the cell reaction with the lithium cations to form the non-stoichiometric LiXTiS2 which is also ionically and electronically conductive, thus further ameliorating the other problem of non-conductive reaction products choking off further cell reaction. In addition TiS2 generally discharges at a voltage similar to that of sulphur i.e. 2.3 volts and thus the cell voltage is steady without cell voltage fluctuations.

In the following examples, as throughout the entire specification and claims, all parts and percentages are by weight unless otherwise specified. The examples are given for illustrative purposes only, and specific details are not to be construed as limitations.

Example I A solid state cell was made from a lithium metal disc having dimensions of about 1.47 cm2 surface area by about 0.01 cm thickness; a cathode disc having dimensions of about 1.82 cm2 surface area by about 0.02 cm thickness, consisting of 80% TiS2 and 20% S, and weighing 100 mg; and a solid electrolyte therebetween with the same dimensions as the cathode and consisting of LiI, LiOH, and Al2O3 in a 4:1:2 ratio. The electrolyte was first pressed to the cathode at a pressure of about 6.8 x 108 N/m2 (100,000 psi), and then the anode was pressed thereto at about 3.4 x 108 N/m2 (50,000 psi). The resulting cell was discharged at room temperature under a load of 100 kQ. The cell provided 26 milliamp hours (mAH) to 2 volts, about 41 mAH to 1.5 volts, and in excess of 56 mAH to 1 volt. The cell had a realizable capacity in excess of 0.73 Whrs/c.c. (12 watt hours/in3).

The following Table illustrates the results obtained from cells generally as set forth in Example 1 but having differing cathode weights, cathode-electrolyte interface surface areas or relative percentages of TiS2 to S, tested under different loads or temperatures, with resulting capacity limits to 2, 1.5 and 1 volt.

TABLE Example % TiS2:S surface area weight discharge Temperature mAH to:2 volts 1.5 volts 1 volt No. (cm) (mg) load k#

1 80:20 1.82 100 100 Room 26 41 46 I # 2 80:20 1.82 100 50 Room 16 34 40 3 80:20 1.82 100 20 Room 6 24 30 4 80:20 1.71 100 50 72 C 32 39 42 # 5 80:20 1.71 100 20 72 C 27 37 40 III 6 80:20 1.71 100 10 72 C 18 35 41 7 60:40 1.82 100 18 A(~100kV) Room 11 28 31 III # 8 60:40 1.82 100 10 72 C 9 19 21 9 80:20 1.82 700 200 37 C 110+ 10 80:20 1.82 700 100 37 C 55 110+ 11 80:20 1.82 700 77.5 37 C 40 90 110+ 12 80:20 1.82 700 50 37 C 25 55 70 IV # 13 80:20 1.82 700 30 37 C 8 26 35 14 80:20 1.82 700 20 37 C 5 21 31 15 80:20 1.82 700 200 Room 72 110+ 16 80:20 1.82 700 100 Room 38 80 102 17 80:20 1.82 700 77.5 Room 28 55 65 18 80:20 1.82 700 50 Room 12 39 50 19 80:20 1.82 700 30 Room 5 15 18 20 80:20 1.82 200 270 37 C 50+ V # 21 80:20 1.82 200 100 37 C 35 52 64 22 80:20 1.82 200 52 37 C 25 45 55 It should be noted from the Table that cells discharged at 370C show even greater capacity than identical cells discharged at room temperature and under the same load.

Thus, since 37"C is human body temperature, cells utilizing the present invention can be used in heart pacemakers with the capability of lasting in excess of 10 years, thereby greatly reducing the need for surgery for implanting fresh batteries.

Additionally, the 80:20 ratio of TiS2 to S by weight, roughly equivalent to a mole-to-mole ratio, provides a greater useful capacity than the 60:40 ratio despite the increased amount of the higher energy density sulphur in the latter. With the mole-to-mole ratio, three lithiums can react stoichiometrically in the cell reactions i.e. 2Li + S < Li2S and Li + TiS2 LiTiS2. These reactions provide a three-electron change with both a high voltage and a high capacity. The mole-to-mole ratio of TiS2 to S provides for complete stoichiometric utilization and is thus highly preferred.

Example 23 A cell made with the materials of Example 1, having the dimensions of 3.195 cm (1.258") external diameter and 0.216 (0.085") thickness, and a cathode of 1.5 grams was made as the back cover of a tritium illuminated liquid crystal display (LCD) watch. A limiting resistor of 330kQ limited the voltage applied to the watch. The operating current for the abovementioned watch ranges between 1 and 3,us. Thus, with a stoichiometric capacity of 750 mAH and assuming a conservative utilizability of 2/3 capacity, the cell is theoretically capable of powering the watch at an average drain rate of 2RA and a voltage in excess of 2.2 volts for about 28.5 years. The lifetime of such cells is in excess of the lifetime of the currently produced watches themselves. Accordingly, with the stability of solid state cells in general and the capacity of the present cell in particular, batteries can be made as integral parts of electrical componentry such as watches, rather than as a part requiring constant replacement.

Example 24 A cell made in accordance with Example 1 was made but with tantalum disulphide (TaS2) in place of titanium disulphide (TiS2) and a weight ratio to sulphur of 87.5:12.5. Upon discharge of the cell at 72"C under a load of 10kQ the cell produced 6 mAH to 2 volts, 18 mAH to 1.5 volts and 24 mAH to 1 volt.

Example 25 A cell made in accordance with Example 24 was discharged at 72"C under a load of 20us1.

The cell produced 14 mAH to 2 volts, 25 mAH to 1.5 volts and about 28 mAH to 1 volt.

Claims (9)

WHAT WE CLAIM IS:

1. A solid state electrochemical cell comprising a solid lithium anode; a solid electrolyte comprising one or more lithium salts and having an ionic conductivity in excess of 1 x 10-9 ohm-l cm-1 at room temperature, and a solid cathode comprising as active material a mixture of an ionically and electronically conductive metal chalcogenide, whereof the ionic and electronic conductivity range between 10-10 and 10-2 ohm-1 cm-1 at room temperature and which is cathodically active with said lithium anode, and elementary cathode active sulphur, selenium, tellurium, bromine and/or iodine, and 0 to 10% by weight of non-cathode active ionically conductive material.

2. The solid state cell of claim 1 wherein said lithium salt is lithium iodide.

3. The solid state cell of claim 2 wherein said electrolyte further includes lithium hydroxide and aluminium oxide.

4. The solid state cell of claim 1, 2 or 3 wherein said metal chalcogenide has a room temperature ionic conductivity in excess of 10-6 ohm~1 cm-1, a room temperature electronic conductivity in excess of 10-3 ohm-1 cm-1, and an open circuit voltage when coupled with said lithium anode in excess of 1.5 volts.

5. The solid state cell of claim 1, 2, 3 or 4 wherein said metal chalcogenide is titanium disulphide.

6. The solid state cell of claim 5 wherein said second cathode active material is sulphur.

7. The solid state cell of claim 6 wherein said titanium disulphide and said sulphur are in a mole to mole ratio.

8. A solid state cell according to claim 1, substantially as herein described.

9. A solid state cell substantially as set forth in any of the foregoing Examples.