CA1095117A

CA1095117A - Cells with solid electrolytes and electrodes

Info

Publication number: CA1095117A
Application number: CA300,021A
Authority: CA
Inventors: Charles C. Liang; Ashok V. Joshi
Original assignee: PR Mallory and Co Inc
Current assignee: Duracell Inc USA
Priority date: 1977-04-25
Filing date: 1978-03-30
Publication date: 1981-02-03
Also published as: CH637789A5; FR2389245A1; GB1603154A; DK177178A; DE2817701A1; JPS53133729A; SE7804645L; BE866319A; DE2817701C2; NL7804333A

Abstract

CELLS WITH SOLID ELECTROLYTES AND ELECTRODES

Abstract of the Disclosure A solid state electrochemical cell comprising a solid active metal anode wherein the metal is above hydro-gen in the EMF series. The cell also comprises a solid electrolyte and a solid cathode wherein the cathode com-prises a compound having an ionic and electronic conduc-tivity ranging between about 1 x 10-10 to 1 x 102 ohm-1 cm-1 at room temperature and being selected from the group con-sisting of metal oxides, metal hydroxides, metal iodides, non-metal chalcogenides, and non-stoichiometric compounds of the metal oxides, hydroxides, iodides and non-metal chal-cogenides with the anode metal.

Description

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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 wa-tches, calculators, cameras, pacema]~ers 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 temperatures as well as concomi-tant miniaturization of the power source. These requirements pose problems for conventional cells having solution or even paste type electrolytes especially with re~ard 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. ~here 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. Increasing closure ' ' ~vl~ }_g S~

reliability is both bulky and!costly and will not eliminate the problem of self discharge. Additionally, solution cells have a limitedOperating temperature range dependent upon the rree~ing and boiling'points of the con-tained solutions.
' Success in meetin~ 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, self discharge on,long standing or have 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 conducti-vity 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 require-ments for a solid electrolyte , is that it must be virtually solely an ionic conductor. Conductivity,due to the mobility' of electrons must be neglible because otherwise the resulting partial internal short clrcuiting would result ln the consumption ! of electrode materials even under apen circult conditions.

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Solution electrolyte cells include an electronically non-conductive separator between the electrode elements to prevent such a short circuit, whereas solid s-tate cells u-tilize the solid electrolyte as both electronic separator and 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 RbAg4I5 (.27 ohm~l cm 1 room temperature conductivity). However these conductors are only useful as electrolytes in cells having low voltages and energy densities.
As an example, a solid state Ag/RbAg4I5/Rb~ cell is dischargeable at 40 mA/cm2 at room temperature but with about 0.2 whr/in3 and an OCV of 0.66V. High energy density and high volta~e 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 conductivity of 5 x 10-5 ohm~l cm~l at room temperature. Thus, high energy density cells with an energy density ranging from about 5-10 whr/in3 and a voltage at about 1.9 volts for a Li/doped-~iI/PbI, PbS,Pb cell currently being produced are precluded from having an effective high current capability above 50~ A/cm 2 at room temperature. A further ag~ravation of the reduced current capability of high energy density cells is the lcw conductivity (both electronic and _3_ . _ . .. .. . .
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ionic) of active cathode materials. Conductivity enhancers such as graphite Eor electronic conductivity and electrol~te 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 cGnsidered in the construction of solid state cells. Thus, the physical properties of electrolytes such as BaMg5S5 and BaMg5Se6, which are compatible with a magnesium but not a lithium anode, and sodium beta aluminas such as Na20-11 A12O3, 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 makiny them dif-ficult to work with especially in manufacturing - with processes involving grinding and pelletization,~such processes re~uiring a firing step for structural integrity. Furthermore, the glazed material so formed inhibits good surface contact with the electrodes with a result of poor conductivity leading to poor cell performance. These electrolytes are thus t~pically used in ce].ls with molten electrodes.
Xt 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 compat-ible electrolytes such that there is an increase in energy density ! without current capability losses, while maintaining chemical stability between the cell components.

~4-_ ... .. .. . .. . _ .. _ _ ... . _ _ _ . .. . .. . . . _ . _ . _ . _, _ _ _ According to the a~orementioned object, from a broad aspect, the present invention provides a solid state electrochemical cell comprising a solid active metal anode wherein the metal is above hydrogen in the EMF series. The cell also comprises a solid electrolyte and a solid cathode wherein the cathode comprises a c-ompound having an ionic and electronic conductivity ranging between about 1 x 10 10 to 1 x 10 ohm 1 cm at room temperature and being selected from the group consisting of metal oxides, me-tal hydroxides, metal iodides, non-metal chalcogenides, and non-stoichio-metric compounds of the metal oxides, hydroxides, iodides and non-metal chalcogenides with the anode metal.

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' Generally the present invention involves the incorpora-tion into the cathode of a solid state cell of a material which has the characteristics of being both ionically and electron-ically conductive as well as being able to ~unction as an active cathode material. Normally~ cathodes re~uire 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 generally been cathode active materials with the result of significant capacity loss. Additionally, cathode active materials which are poor electronic conductors as well re~uire 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~lla higher energy density and current capability is attained with the need for space wasting conductors being obviated.
Examples of materials having the requisite character-istics 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 compounds:
metal oxides such as TiO2, MoO3, Ta205, V205 and W03; metal iodides such as CdI2, FeI2j GeI2, MnI~, TiI2, TlI2, VI2, ~nd YbI2;

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metal hydroxides such as Cd(OH)2, Fe(OH)2, Mn(OH)2, and Ni(OH)2, and non-metal chalcogenides such as SiTe and CSn wherein n is 0.001 to 1.0 and is made in the manner des-cribed in Canadian Patent Application Serial No. 300,032 filed March 30, 1978.
Also included are the non-stoichiometric compounds such as NaxWO3 where x < 1, which to some extent contain the complexed form of one of the cathode materials with the ano-dic cation and which are believed to be intermediate reac-tion products during cell discharge.
In order for the ionically-electronically conductive cathode active material to be commercially useful in high voltage cells such as those with lithium anodes it should pre-ferably be able to provide a voltage couple with lithium of at least an O.C.V. of 1.5 volts and most preferably above 2 volts.
A further criteria for the above cathodic material is that both the ionic and electronic conductivities of the c~thode active material should range between 10 10 and 10 ohm cm with a preferred ionic conductivity of more than 10 6 and an electronic conductivlty greater than 10 1, all at room temperature.
In addition, and most importantly, the ionically-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 9 ohm 1 cm 1 at room temperature. These salts can either be in the pure form or combined with conduc-. ~ , .

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tivity enhancers such that the current capability is improved therebyO Examples of lithium salts having the requisite conductivity for meaningful cell utilization include lithium iodide (LiI) and - 6a -_ ~S~
lithium iodide admixed with llthium hydroxide (I.iOH) and aluminum oxide (A1203) with the latter mixture being referred to as LLA
and disclosed in U.S. patent No. 3,713,897.
~ igh 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 aluminum 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 ccn-ductivities 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-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 eomplexing 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 eonduetivlty.
A limiting factor in solid state cell performance is the conductivity of the cell reaction product. ~ low eonductivity produet results in large internal resistance losses which~
effectively terminate cell usefulness. Thus,in cells having the above ionically-electronically conductive,cathode active material the complexed reaction product retains conductivity ~S~

thereby enabling full utilization oE unreac-ted cathocle materials which are in proximity therewith.
A ~mall 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 should the ionically conductive cathode active material have a lower con-ductivity than that of the electrolyte. This inclusion however, if made, should preferably not exceed 10% by weight since greater amounts would merely decrease the energy density of the cell with little if any further tradeoff in terms of current drain capacity. Additionally small amounts of an electronic con-ductor may also be added to increase electronic conductivity.
However, the total percentage by weight of conductivity enhancers should not exceed 20% of the cathode.
In order that the present invention may be more completely understood the following example is given in which all parts are parts by weight unless otherwise specified. The example is for illustrative purposes only and should not be interpreted as providing limitations to the present invention.
EXAMPLE
A solid state cell made from a lithium metal disc having the dimensions of about 1.~7 cm2 surface area by~about , 0.01 ~m thickness; a cathode disc of the dimensions of about 1.71 cm2surface area by about 0~02 cm thickness consisting of ; -8-. _ _ . , ... ,, : ....... . ,, . __ .
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85% WV3, 5% carbon black and 10% LLA, and weighing 200 mg; and a solid electrolyte therebetween with the same dimensions as the cathode and consisting of LiI, LioH~ and A12O3 in a 4:1:2 ratio is assembled in the following manner. The electrolyte is first pressed with -the cathode at a pressure of about 100,000 psi~
Then the anode pressed thereto using about 50,000 psi. The resulting cell is discharged at 95C under a load of 20k~. The cell realizes 8 milliamp hours (mAH) to 2 volts, about 15 mAH to 1.5 volts, and about 30 mAH to 1 volt.
It is understood that changes can be made without departing from the scope of the present invention as defined in the following claims.

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Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:-

1. A solid state electrochemical cell comprising a solid active metal anode wherein said metal is above hydro-gen in the EMF series, a solid electrolyte and a solid cathode wherein said cathode comprises a compound having an ionic and electronic conductivity ranging between about 1 x 10-10 to 1 x 102 ohm-1 cm-1 at room temperature and being selected from the group consisting of metal oxides, metal hydroxides, metal iodides, non-metal chalcogenides, and non-stoichiometric compounds of said metal oxides, hydroxides, iodides and non-metal chalcogenides with said anode metal.

2. The solid state electrochemical cell of claim 1 wherein said group consists of TiO2, MoO3, Ta2O5, V2O5, WO3, CdI2, FeI2, GeI2, MnI2, TiI2, TiI2, VI2, YbI2, Cd(OH)2, Fe(OH)2, Mn(OH)2, Ni(OH)2, CSn wherein n is 0.001 to 1 and SiTe2.

3. The solid state electrochemical cell of claim 2 wherein said anode is comprised of lithium.

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

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

6. The solid state electrochemical cell of claim 5 wherein said compound is WO3.

7. The solid stata electrochemical cell of claim 1 wherein said non-stoichiometric compound is NaxWO3 wherein x < 1 and said anode is comprised of sodium.

8. The solid state electrochemical cell of claim 1 wherein said ionic conductivity is more than 10-6 ohm-1 cm-1 and said electronic conductivity is greater than 10-1 ohm-1 cm-l.