GB1599793A - Cathodes for solid electrolyte cells - Google Patents

Abstract

The invention relates to an electrochemical cell of high energy density having a solid negative electrode consisting of an active metal, a solid electrolyte and a solid positive electrode. In this case, the positive electrode contains metal chalcogenides as the active material, which conduct both ions and electrons.

Description

(54) CATHODES FOR SOLID ELECTROLYTE CELLS (71) We, DURACELL INTERNA TIONAL INC. formerly known as P. R.

MALLORY & 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 bv 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 all types of conditions has resulted in a demand for power sources which have characterstics of rigged construction. long shelf life, high reliability, high energy density and an operating capability over a wide range of temperatures, 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 have 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-7 cm-7 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/RbAg4/Rbl5 cell is dischargeable at 40 mA/cm at room temperature, but with about 0.012 Whr/c.c. (0.2 Whr/in3) 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-t ohm-l cm-l. Thus, high energy density cells with an energy density ranging from about ().3 to 0.6 Whr/c.c. (5-10 Whr/in ) and a voltage of about 1.9 volts for a Li/doped-LillPbI.,PbS. Pb cell currently being produced are precluded from having an effective high current capability above 50 ZA/cm-2 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 BaMg5S" and BaMg 5so", which are compatible with a magne sium anode but not a lithium anode. and sodium beta aluminas such as No.0. 11 Al2O3 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 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 operable at room temperature, comprising a solid active metal anode, a solid electrolyte and a solid cathode wherein said cathode consists of at least 90% by weight of one or more metal chalcogenides wherein the ionic and electronic conductivity of said metal chalcogenides range between 10-"' and 102 ohm- cm-l at room temperature.

The present invention involves the formation of the cathode of a solid state cell with 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 cathiodically active as well as being compatible with electrolytes used in high energy density cells include the following metal chalcogenides: CoTe2, Cr2S3, His1. HfSe2, HfTe2, Ire., MoS2, Most., MoTe2, Nubs2, NbSe2, NbTe7, NiTe2, PtS2, PtSe2, PtTe2, SnS2, SnSSe, SnSe2, TaS2, Task2, TaTe2, TiS2, Tis., Title, VS2, Vie2, Vie., WS., Woe2, 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 LiXTiSr 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 such as those with lithium anodes, it should be able to provide a voltage couple with lithium having an open circuit voltage (O.C.V.) of at least 1.5 volts and preferably above 2 volts.

A further criterion for the above cathodic material is that both the ionic and electronic conductivities of the cathode active material should range between 10-"' and 102 ohm-l cm-'. with a preferred ionic conductivity of more than 10-" and an electronic conductivity greater than 10-'. 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 room temperature ionic conductivities greater than 1 x 10- ohm-l cm-l. 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 admixed with lithium hydroxide (LiOH) and aluminium oxide (Al2O3), the latter mixture being referred to as LLA and being disclosed in U.S. patent No. 3,713,897.

High energy density solid electrolyte cells may have as their anodes materials similar to lithium which have high voltage and low electrochemical equivalent weight characteristics. Suitable anodic materials include metals from Groups IA and IIA of the Periodic Table such as sodium, potassium, beryllium, magnesium and calcium as well as aluminium from Group IIIA and other metals above hydrogen in the EMF series.

Cells with other anodes can utilize corresponding salts as electrolytes, such as sodium salts for a cell with a sodium anode. Additionally, electrolyte salts with useful conductivities and having a cation of a metal of a lower EMF than that of the anode metal may also be useful.

It is postulated that the aforementioned ionically and electronically conductive cathode active materials react with the ions of the anode (e.g. 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.

A A limiting factor in solid state cell per- formance is the conductivity of the cell reaction product. A low conductivity product results in large internal resistance losses which effectively terminate cell usefulness. Thus a further advantage of cells having the above ionically and electronically conductive cathode active material is that the complexed reaction product retains conductivity thereby enabling full utilization of the cathode.

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 energy density of the cell with little if any further improvement of current drain capacity.

Accordingly the cathode should include at least 90% by weight of the ionically and electronically conductive cathode active material.

In order that the present invention be more completely understood the following examples are given, with all parts being by weight unless otherwise specified. The examples are only for illustrative purposes and should not be taken as limitations of either cell construction or of materials contained therein.

Example 1 A solid state electrochemical cell was formed using a lithium metal disc having dimensions of about 1.47 cm2 contact surface area and about 0.01 cm thickness: a cathode disc having dimensions of about 1.71 cm2 contact surface area and about 0.02 cm thickness consisting of titanium disulphide (TiS2) and weighing about 100 mg, and a solid electrolyte with the same dimensions as the cathode and consisting of Lil, LiOH, and Al2O3 in a 4:1:2 ratio. The electrolyte was first pressed with the cathode at a pressure of about 6.8 x 108 N/m2 (100.000 psi). The anode was then pressed to the other side of the electrolyte using about 3.4 x 108 N/m2 (50,000 psi).

The resulting cell was discharged at a temperature of 72"C under a load of 10K Q.

The cell produced 14 milliamp hours (mAH) to 2 volts, 21 mAH to 1.5 volts, and about 24 mAH to 1 volt.

The titanium disulphide in the above Example is a good ionic and electronic conductor (10-) ohm-l cm' ionic conductivity and greater than 10-2 ohm-l cm- electronic conductivity at room temperature) and thus constitutes the cathode without conductive additives. The titanium disulphide 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 problem of incomplete cell discharge resulting from nonconductive reaction products choking off further cell reaction.

The ionically and electronically conductive, cathode active materials can be mixed with one another to form a cathode as in the following Examples.

Example 2 A solid state cell was made in accordance with Example 1 but with the cathode having a contact surface area of 1.82 cm2 and comprising a 1:1 mixture of titanium disulphide and molybdenum disulphide weighing about 50 mg. The cell was discharged at 27"C under a load of 18 yA. The cell produced 2.2 mAH to 2 volts, 5 mAH to 1.5 volts and 5.9 mAH to 1 volt.

Example 3 A cell identical to the cell in Example 2 was discharged at 27"C under a load of 36 A. The cell produced about 1 mAH to 2 volts, about 3 mAH to 1.5 volts and about 5 mAH to 1 volt.

It is to be understood that other disclosed conductive metal chalcogenides can function similarly whether without any further conductive enhancers or with a maximum of 10% of conductive materials.

WHAT WE CLAIM IS: 1. A solid state electrochemical cell operable at room temperature, comprising a solid active metal anode, a solid electrolyte and a solid cathode wherein said cathode consists of at least 90rye by weight of one or more metal chalcogenides wherein the ionic and electronic conductivity of said metal chalcoaenides range between 10-1(' and 102 ohm- cm-' at room temperature.

2. The solid state electrochemical cell of claim 1 wherein said active metal anode is lithium.

3. The solid state electrochemical cell of claim 2 wherein said solid electrolyte comprises lithium iodide.

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

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

6. A solid state electrochemical cell as claimed in claim 1 and substantially as herein described.

7. A solid state electrochemical cell substantially as set forth in any one of the foregoing Examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. using about 3.4 x 108 N/m2 (50,000 psi). The resulting cell was discharged at a temperature of 72"C under a load of 10K Q. The cell produced 14 milliamp hours (mAH) to 2 volts, 21 mAH to 1.5 volts, and about 24 mAH to 1 volt. The titanium disulphide in the above Example is a good ionic and electronic conductor (10-) ohm-l cm' ionic conductivity and greater than 10-2 ohm-l cm- electronic conductivity at room temperature) and thus constitutes the cathode without conductive additives. The titanium disulphide 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 problem of incomplete cell discharge resulting from nonconductive reaction products choking off further cell reaction. The ionically and electronically conductive, cathode active materials can be mixed with one another to form a cathode as in the following Examples. Example 2 A solid state cell was made in accordance with Example 1 but with the cathode having a contact surface area of 1.82 cm2 and comprising a 1:1 mixture of titanium disulphide and molybdenum disulphide weighing about 50 mg. The cell was discharged at 27"C under a load of 18 yA. The cell produced 2.2 mAH to 2 volts, 5 mAH to 1.5 volts and 5.9 mAH to 1 volt. Example 3 A cell identical to the cell in Example 2 was discharged at 27"C under a load of 36 A. The cell produced about 1 mAH to 2 volts, about 3 mAH to 1.5 volts and about 5 mAH to 1 volt. It is to be understood that other disclosed conductive metal chalcogenides can function similarly whether without any further conductive enhancers or with a maximum of 10% of conductive materials. WHAT WE CLAIM IS:

1. A solid state electrochemical cell operable at room temperature, comprising a solid active metal anode, a solid electrolyte and a solid cathode wherein said cathode consists of at least 90rye by weight of one or more metal chalcogenides wherein the ionic and electronic conductivity of said metal chalcoaenides range between 10-1(' and 102 ohm- cm-' at room temperature.

2. The solid state electrochemical cell of claim 1 wherein said active metal anode is lithium.

3. The solid state electrochemical cell of claim 2 wherein said solid electrolyte comprises lithium iodide.

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

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

6. A solid state electrochemical cell as claimed in claim 1 and substantially as herein described.

7. A solid state electrochemical cell substantially as set forth in any one of the foregoing Examples.