GB2235537A

GB2235537A - Solid electrolyte for electrochemical cells and process for making same

Info

Publication number: GB2235537A
Application number: GB9017368A
Authority: GB
Inventors: Andrei Nikolaevich Smirnov; Oleg Vladimirovich Glumov; Dmitry Borisovich Samusik; Tatyana Ivanovna Shakhova
Original assignee: TSNT TVORCHESTVA MOLODEZHI GAL
Current assignee: TSNT TVORCHESTVA MOLODEZHI GAL
Priority date: 1989-08-09
Filing date: 1990-08-08
Publication date: 1991-03-06
Also published as: FR2650918A1; GB9017368D0; JPH03149704A; DE4025161A1

Abstract

A solid electrolyte for use eg. in gas sensors, leakage indicators, ion-selective electrodes, batteries comprises from 85 to 99.7 mol% of a rare-earth metal fluoride or a mixture of rare-earth metal fluorides and 0.3 to 15 mol. % of an alloying additive; the solid electrolyte having a porous polycrystalline structure with a crystal size of not more than 250 mu m, a pore depth not exceeding the crystal size and a relative density of not less than 0.98. A process for producing the electrolyte comprises mixing the starting components (namely, a rare-earth metal fluoride or a mixture of fluorides of rare-earth metals together with an alloying additive); adding a highly-volatile sintering component to the resulting mixture; by melting the resultant mixture with the removal of the highly-volatile sintering component; cooling the resulting melt to solidification; disintegrating the solidified melt; isolating a fraction with a particle size of not more than 25 mu m and compression moulding the isolated fraction with simultaneous heat-treatment.

Description

1 1 1 SOLID ELECTROLYTE AND PROCESS FOR UKI'-TG SAME The present invention

relates to electrochenistry and, more specifically, to a solid electrolyte and a pro cess for making same.

The solid electrolyte, according to the present invention, can be used in electrochemical cells of vari ous devices such as gas sensors, leakage indicators, ion -selective electrodes, dispensing means for fluorine and compounds thereof, halogen actuators, batteries and other instruments.

Solid electrolytes intended for a broad range of electrochemical devices operating in aggessive fluorinecontaining media should have a high conductivity relative to fluorine-ion of 10-4 _ 10-5 Ohm-'-am-', a low solubili- ty in the working medium, impermeability of the electro- lyte for liqui-d and gas products taking part in the elec trochemical process, a high mechanical strength, heat-re sistance, a long service life and to be suitable for the manufacture of various shaped membranes therefrom.

Known in the art is a solid electrolyte having a single-crystalline structure (of., Roos A. Mater. Res.

Bull.,, 1983, Vol.189 p.405) containing 90 - 99.9 %o b.v mast of rare-earth metal fluoride such as LaF 3 and 0.1 - 7o by mass of an alkali-earth metal fluoride such as BAF 2 The process for the production of this electrnlyte comprises mixing the starting components - the rare-earth metal fluoride and the alkali-earth metal fluoride with the addition of CdP 2 in order to purify fluorides from oxy- impurities, heating the resulting mixture to a temperature above its melting point. maintaining the mixt ure at this temperature till homogenization thereof. cooling the resulting melt to room temperature to give a solid electrolyte. The latter is characterized by a low mechanical strength and heat -resistance. likewise by the impossibility of making it in the fnrm of shaped articles such as vessels for working solutions.

Furthermore. a low heat-resistance nf said electro- -lyte hinders the manufacture of shaped membranes, since cracking of an article can occur at the regir)ns of stress concentrations in the presence of temperature gradients. Also known in the art is a solid electrnlyte (US, Aq 4352869) consisting of a matrix - flunrides of rare- earth C> metals, an alloying additive (an alkali-earth metal compnund) and a binding additive necessary fnr nbtaining a -he binding additive said dense-structure electrolyte. As t 20 electrolyte contains an alkali-earth netal compound selected from the group of fluorides, sulphates, chlorides C or carbonates of an alkali inetal in an amount nf from 1 to- mol.

A molar ratio of the matrix to the alloying additive is within the range of from 7:1 to 99:1. Said electrolyte has a porous structure featuring the presence of through pores. The process for the manufacture of this electrelyte comprises mixing the matrix - a rare- earth metal g a fluoride - with the alloying additive (an alkali-earth metal compound) in a molar ratio of frnm 7:1 tn 99:1 and placed into a graphite crucible. Then the resulting mixture is heated in the atmosphere of argon to a temperature exceeding its melting point. i.e. to a temperature nf 1550 - 1600 OC, kept at this temperature for 5 mn till its homogenization, the thus-obtained melt is cooled in argon, whereafter the solidified melt is disintenrcited in :1 the atmosphere of argon, mixed with a binder and mnulded.

The moulding operation comprises compression at room temperature, followed by sintering in vacuum at a temperature within the range of from 800 to 950 'C.

This electrolyte has an insufficient conductivity relative to-flunrineinn (2.10x10-4 _ 1.01x10-5 Ohm-1.0m-1 if fluorine con- which lowers the accuracy p.IL' measurement centration. Furthermore, the presence of the binding additive in the. composition of the above- mentinned electrolyte in the form of compounds of alkali metals well- soluble in aqueous media wherein the electrol,-y-te can be employed reduces the service life of membranes prnduced from this electrolyte and impairs stability o.L their characteristics.

In operation of this thrqugh-pore electrolyte in aqueous eolutinns the stability of an electrochemical cell's potential is not ensured, its service life is shortened due to an electrnchemical corrosion of the non-dense solide electrolyte, mechanical strength and heatresistance of the material are lowered. its chemical composition is changed.

It is the main object of the present invention tn provide a solid electrolyte with a porous polycrystalline structure featuring a high ionic conductivity, a low per- meability for gas and liquid media, a high mechanical strength, a capability of being moulded into shaped articles possessing high mechanical strength and heat-resistance.

It is another object of the present invention to pro- vide a process for making said solid electrolyte.

These and other objects are accomplished by a solid electrolyte containing fluoride of a rare-earth metal or a mixture of fluorides of rare-earth metals and an alloying additive which. according to the present invention.

consists of the following components. molar -o:

-ure fluoride of a rare-earth metal or a mix', of fluorides of rare-earth metals 85-99.7 alloying additive 0.3-15 and has a porous polycrystalline structure with a crystal size of not more than 250/.lm, with a pnre depth not exceeding the cryslal size and a relative density of nnt less than 0.98.

As the rare-earth metal fluoride or a mixture of rare-earth metal fluorides the electrnlyte, according to the present invention, preferably contains lanthanum fluoride or cerium fluoride. or a inixture of both, while as the alloying additive it contains calcium fluoride, or barium fluorideg or lead.fluoride, or aluminium oxide.

The electrolyte, according to the present invention, as compared to the known electrnlyte (US, Aq 4352869) has a higher ionic conductivity (9x105 - 7x10 -4), a lower solubility in aqueous media (by more than 15 times as a compared to the known electrolyte), a lower permeability for gas and liquid media due to the absence of thrnugh. pores.

The electrolyte,, according to the present invention, has a bending strength of about 20 - 35 MPa thus exceed- ing by 2 - 3 times corresponding parameters nf the known electrolyth (US, Aq 4352869). The enntrolled size of crystals (not more than 250,,tim) ensures good mnuldability of articles from the electrolyte, according to the present invention. and their high mechanical strength and heat resistance.

In tests of the heat-resistance nf samples of the electrolyte, according tn the present invention, in enmparison with the known electrolyte (US, A, 4352869) by way of verification of integrity of samples of a standardized size having temperature of 25 OC by the method of their abrupt immersion into boiling distilled water it was found that the electrolyte, according tn the present invention, retained stability, while the knngn electrolyte was broken.

The present invention also relates to a prncess for producing a solid electrolyte which enmprises mixing the starting components - fluoride of a rare-earth metal nr a mixture of flunrides of rare-earth metals 'with an alloying additive, melting the resulting mixture, cooling the obtained melt to solidification thereof, its disintegration and compression-moulding, wherein, according to the present invention, into the mixture of the starting compo5 nents a highly-volatile sintering agent is additionally introduoed, whereafter the thus-produced mixture is melted with the removal of the highly-volatile sintering agent and, after disintegration of the cooled melt, a fraction is recovered with a particle size of not more than 250jim, followed by compre ssion-moul ding of the recovered fraction and a simultaneous heat-treatment thereof. The highly-volatile sintering agent is used in a pre-Perable amount of from 1 to 5 % by mass.

As the highly-vnlatile sintering agent it is advis- able to use elemental fluorine, lead fluoride, sulphur hexafluoride, - carbon tetrachloride, carbon disulphide, polytetrafluoroethylene.

The introduction of a highly-volatile sintering agent

C3 has made it possible to avoid the use of a binding addit- ive and thus increase mechanical strength of the electrolyte, according to the present invention, and its heat-resistance, to make it impermeable for gas and liquid media and to increase its electrical conductivity.

The c ompre ssion-moul ding of the recovered fraction with a simultaneous heat-treatment thereof is preferably effected in vacuum under a pressure of from 70 to 680 MPa and a temperature of from 850 to 1400 OC.

The process,, according to the present invention, 1 makes it possible to obtain a solid electrolyte of a porous polycrystalline structure with a crystal size of not h a pore depth not exceeding the more than 2 50.1m, and will crystal size.

The process, according to the present invention, makes it also possible to avoid the fnrmation of through pores owing to the removal of gaseous products at the temperature of hot moulding and owing to a dense packing nf crystals at a relative density nf the electrolyte above 0.98 and good adhesion of crystals to one another attained through the use of a highly-volatile sintering agent. The absence of through pores, likewise a relative density of the electrolyte of not less than 0.98 results in a substantial decrease of the permeability of the electrolyte for gases and liquid media.

The process, according to the present invention, ensures the production of an electrolyte having a high ionic conductivity. a high mechanical strength, mnuldability into shaped articles, as well as a high heatresist- ance and mechanical strength.

The solid electrolyte. according to the present invention, is a pnlycrystalline material with a crystal size. of not more than 250 lim. The necessity of using crystals with a size of below 250.,km is caused by the require- ments to the electrolyte mechanical strenRth. At a crys tal size of over 25 0/t 1m the area of contact between crys tals is reduced so that the adhesinn forces between them are substantially lowered and, hence, the mechanical strength of the electrolyte. The moulding of an electrolyte from a fraction with a particle size of not more than 250,im makes it possible to produced membranes of sophisticated configurations ensuring a substantially si- milar density over the entire volume of the membrane, thus enlarging the range of functional applications n.."

C) the electrolyte, according to the present invention, and ensuring a high mechanical strength thereof.

The electrolyte, according to the present invention.

has pores with their depth not exceeding the crystal size.i.e. through pores are absent. The absence of. through pores, as Nell as a relative density of the electrolyte, according to the present invention, of not less than 0.98 result in a substantial decrease of the electrolyte permeability for gas and liquid media. At a relative density of the electrolyte below 0.98 cross-sectinns of pores are increased and permeability of the electrolyte is increased.

The process for producing a solid electrolyte, according to the present invention, is effected in the fnllnwing manner.

The starting COMDonents - a rare-earth metal fluoride or a mixture of fluorides of rare-earth metals - are mixed with an alloying additIve In an amount n:f R5 - qq.7 mol. % and 0.3 15 molc re spelitt ively.

As the rare-earth metal fluoride or the mixture of fluorides of rareearth metals it is preferred to use lanthanum fluoride, or cerium fluoride, or a mixture nf both. As the alloying additive use can be made of any alloying additive; it is preferable tn use calcium flueride or strontium flunride, or barium flunride, nr lead fluoride, or aluminium oxide. A highly-vnlatile sintering agent is added to the mixture in a preferable amnunt nf from 1 to 5 % by mass. As the highly-volatile sintering reagent use can be made, fnp example, nf the fnllnwing compounds or mixtures thereof: F.. BF 31 PbP2, CF 4' C2P69 11 sip 4' NF 3' SP69 CiF, XeF 21 C121 'C14' '32 It is advisable to use, as the highly-volatile sintering agent, lead fluoride, sulphur hexaflunride, carbon tetrachloride. carbnn disulphide, polytetraflunrnethylene or elemental fluorine. The intrnductinn of a hi;-, hl.v-volatile sintering agent into the starting mixture, in addition to its main functinn - to facilitate sintering of p-articles of the solid lelectrnlyte and imprnvement of its structure - prevents the fnrmatinn of thrnup:h pores in the solid electrolyte, since the sintering aRent during heating of the mixture in melting thereof enters into reaction with impurities present therein (H 2 0, nxides and the like). whereafter the reaction prnducts tneether with the decomposed sintering reagent pass intn a gas f,.)rm, removed from the compnsitinn and modify the snlid electrolyte in such a manner that its particles during the process of moulding form a polycrystalline material with the density of 0.98. The resulting mixture is charged into a crucible and placed into a furnace; the wnrking space of the furnace is set. under vacuum and the mixture is subjected to melting. To thib end, the mixture is heated to a temperature of 1500 - 1 00 00 and maintained f or two hours for homogenization of the melt. Then, the furnace is de-energized and the resulting homogeneous melt is cooled to room temperature; the cooled melt is extracted, crushed and disintegrated, e.g. in a ball mill. A fraction with a particle size of.not more than 250((m is isolated and a solid electrolyte is moulded therefrom. To this end, the isolated fraction is placed into a mould. The unit is set under vacuum, the temperature in the mould is brought to the hot-compression temperature and subjected 'to compression moulding. Thereafter, the press- V ure is released, temperature in the mould is lowered to room temperature and the final electrolyte is withdrawn from the mould.

It is advisable that the moulding of the isolated fraction be effected simultaneously with its heat-treatment in vacuum under pressure of 70 to 80 Y!Pa at a temperature within the range of from 850 to 1400 OC. Such conditions make it Possible to obtain the above-specified re- lative density of the electrolyte, according tn the present invention, lack of pores therein and the fnrmation of crystals having dimensions corresponding to the size of the shaped particles or below it.

For a better understanding of the present invention, some s-pecific examples illustrating the manufacture of a solid electrolyte are given hereinbelow.

Example 1

3,175 g of LaP 3 were mixed with 150 g of 'BaF 2 and 175 g of PbF2 (the molar ratio of LaP 3 P-RaP2 = 19:1). The resulting mixture was charged into a graphite crucible coated with a layer of pyrographite to prevent enntamination of the processed product. The crucible was covered with a lid, placed into a furnace which was set under vacuum to the pressure of 10-2 mm Hg, and subjected to heating to the temperature of 1585 OC. The crucible with the mixture was maintained at this temnerature fnr twn hnurs, the furnace was cnnled. the solidified melt was withdrawn, crushed and disintegrated in a ball mill. A fraction of a particle size of below r-3tim was isolated 'by screenin7, 1 g of the obtained powder were placed into a mould and compression-moulding was effected in a vacuum nf frnm 10-2 to 10-3 mm Hg under the pressure of 70 M.Pa at the temperature of 1400 OC. Then, the pressure was released and the mould temperature was lowered to room temperature.

Lhe final electrolyte was obtained as a pnrnus pr)ly- crystalline material with a crystal size of not more tYan 60.,m and with a pore depth not exceedinR the crystal size; it had the following composition, mnl. IS:

LaP 3 95 BaF2 5.

Properties and quality characteristics of the resulting electrolyte are shown in the Table bereinl-elnw.

Example 2

E 3,409 g of LaP 3 were mixed with 91 g of CaF 2 gnd 17.5 g of PbP 2 (the molar ratio of LaP 3 /CaP 2 - 15:1), whereafter the process was carried nut in a manner similar to that described in the foregoing ExaMD1e 1. A fraction was then isolated with a particle size nf below 125/vm; 200 g of the thus-nbtained powder were placed into a mnuld and compression was effected in a vacuum of 10-2 t o 10-3 mm Hg under the pressure of 300 IMPa and at the temperature of 1200 'C. The final electrnlyte was in the form of a porous polycrystalline material with a crystal size of not more than 120.qm. the pore depth not exceeding the crystal size; the material had the following composition, mol.

LaF 94 3 CaF 2 Properties and quality characteristics nf the resulting electrolyte are shown in the Table hereinbelnw.

Example 3

CeF were mixed with 87 oP SrF 3,413 g of NJ 3 2 and 35 -in of CeF of PbF2 (the molar rat 3/SrF2:-- 25:1). The process was further carried out in a manner similar to that des cribed in Example 1 hereinbefore. A fractinn was recnvered with a particle size of belrNw 250/bm and subjected to com pression moulding under the pressure of 80 MPa and at the temperature of 850 OIC like in Example 1.

Example 4

CeP 3 and SrF 2 were mixed in the molar ratio of 25:1 in a manner similar to that of Example 3. The resulting mixture was charged into a graphite crucible coated with layer of pyrographite, covered with a lid, placed into furnace whose working space was set under vacuum to reach the pressure of 10-2 mm Hg and gaseous argon was fed thereinto up to the pressure of 103 kPa. Thereafter. gaseous SF6 was admissed intn argon at the rate e)f 0.2 1/mn at the supply rate of argon of 10 1/mn and the pressure of 103 kPa. Ten volumes of the eas mixture were passed through the furnace and the furnace was heated to the temperature of 1500 00 at a continuous flow of the gas mixture. The crucible with the mixture was kept at the above-mentioned temperature for two hours, whereafter, without stopping the gas mixture supply, the furnace was cooled to room temperature, the supply of 3Pg was discontinued, ten volumes of argon were passed thrnugh the furnace, whereafter the unit was opened and the solidified melt was withdrawn from the crucible. The melt was then disintegrated and a fraction with a particle size of less than 250.4m was recovered. The compression moulding was effected in a manner similar to that described in Example 1 hereinbefnre under the pressure of 290 MPa and at the temperature of 950 'C.

The final electrolyte was a porous pnlycrystalline material with a crystal size of not mnre than 250 1m, a 1 pore depth not exceeding the crrstal size and having the following composition. mol. -%: CeF 3 - "; SrF2 - 4.

Properties and quality characteristics of the result. ing electrolyte are shown in the Table hereinbelow.

Example 5

CeP 3 and SrF2 were intermixed in the mnlar ratio of 25:1 and charged into a processing unit in a manner simil ar to that described in Example 4. After setting the unit under vacuum and supplying argnn to the pressure of 103 kPa, argon prior to the supply into the unit was start- ed to be bubbled through liquid carbon tetrachloride at the temperature of 20 00 at the supply rate nf 10 llmn. Then, the process was carried nut as in Example 4 hereinabove to give the final electrolyte enmprising a pornus polyarystall-ine material with a crystal size of not mnre than 250.41m, a pore depth nnt exceeding the crystal size and having the following cnmpositinn, mnl. %: CeP 3 - 99; Stp 2 - 4.

Properties and quality characteristics nf the obtained electrolyte are shown in the Table hereinbelow.

Example 6

CeF 3 and STF2 were mixed in the mnlar ratin of 25:1 and the process was conducted aG in Example 5, except that carbon disulphide was used as the highly-volatile sintering agent.

The obtained electrolyte was a pnrnus pnlycr,vstalline material with a crystal size of nnt more than 250.1im, the pore size not exceeding the crystal size and having the l following composition, mol. %: CeF 3 - 96; SrF2 - 4.

Properties and quality characteristics of the resulting electrolyte are shown in the Table hereinbelnw.

Example 7

A mixture of CeF 3 and SrF2 in the molar ratio of 25:1 was prepared in a manner similar to that described in Example 4 hereinbefore, charged into a graphite crucible and placed into a furnace, wherein in a low- temperature zone, where during melting of the mixture of CeP 3 and SrF2 the temperature did not exceed 500 - 600 OC, 100 g of polytetrafluoroethylene chips -were present.

The unit was set under vacuum to reach the pressure of 10-2 mm Hg and the mass was heated to the temperature of 1585 OC so that polytetrafluoroethylene was subjected for to pyrolysis. The mixture ',Nas kept at this temperature two hours, then cooled, the solidified melt was extracted from the unit and disintegrated in a ball mill.

A fraction of particles with a size of less tl-an 250,/4m was isolated and subjected to compressinn moulding under the pressure of 290 MPa and at the temperature of 950 OC.

The thus-prociuced electrolyte was a pnrous polycrystalline material with a crystal size of not more than 250,I(m, a pore size not exceeding the crystal size and having the following composition, mol. %: CeF 3 - 9e; SrF2 - 4.

"eristics of the thus- Properties and quality charact produced electrolyte are shown in the Table hereinbelow.

Example 8

328 g of LaF 3 -were mixed with 21.6 g of PbF 2 (the molar ratio of 1aF 3 /PbF 2 = 19:1) in a ball mill; the mixture was placed into a container made from fluornplastic throun-h which elemental fluorine was passed at the tempera- r ture of 150 OC for 2 hours at the supply rate of 10 1/mn under the pressure of 103 kPa. On completion of the pro-al fluorine the latter -Nas cess of treatment with element replaced with argon which was passed through the container in the amount of 50 volumes. A fraction with a particle size of less than 6/m was isolated by screening from the obtained mixture and 200 g of the resulting powder were placed into a mould; the compression moulding was effected under conditions similar to those of Example 7 hereinabove. The thus-produced electrolyte was a pornus polycrystalline material with a cx%ystal size of not more than -he crystal size and m, a pore depth not exceeding t having the following compositinn, mol. lo: LaF 95; 3 PbF2 5.

Properties and quality characteristics of the result ing electrolyte are shown in the 'Iable bereinbelr1w.

Example 9

194 g of LaF 3 and 5.33 g of A1203 were mixed in a ball mill (the molar ratio of LaP 3/A1203 = 19:1) and the preparation of a solid electrolyte was further ef-oected as in the foregoing Example P.

The final electrolyte was a porous pnlycrystalline material with a crystal size of not more than 901im, a pore depth not exceeding the crystal size and having the following composition, mnl..'g: LaF 3 - 95; AL:,0 - 5.

2 3 Properties and quality characteristics nf the thus- -produced electrolyte are shown in the Table hereinbelnw.

Example 10

1681 g of LaF were mixed with 192 g of CeF 127 g 3 39 of SrF 2 and 35.4 g of PbF 2 (the molar ratio of LaF 3 /IeF 3/ /SrF2 = 8.05:8.05:1) and the preparation of a solid elec- trolyte was then effected in a manner similar to that described in Example 2 hereinbefore.

The final electrolyte was a porous pnlycrvstalline material with a crystal size of not more than 1?0,fm, a pore depth not exceeding the crystal size and having the following composition, mol. lo: LaF 3 - 47; CeF 3 - 47; SrF2 - 6' Properties and quality characteristics of the tbus-produced electrolyte are shown in the Table hereinbelow.

Example 11

332.5 g of LaF 3 were mixed with 2'.5 g of naF 2 and 3.3 c, of PbF (molar ratio of LaP /BaP 1:191) and then C) 2 3 2 n that desthe process. was effected in a manner similar cribed in Example 2.

The final electrolyte was a porous pnlycrystalline material with a crystal size not exceeding 120lm, a pore depth of not more than the crystal size and having the following composition, mol. - lo: LaP 3 - gin.5; BaP 2 0.5.

- is Properties and quality characteristics nf the thus- -produced electrolyte are shnwn in the Table hereinbelow.

Example 12

947 g of LaP were mixed with 150 g of BaP and 12 3 2 of PbP 2 (the molar ratio of LaP 3 /BaF2 = 1:5.67); then.

the prncess was effected as in Example 2 hereinbefnre.

The thus-produced electrolyte was a pnrous pnlycrys- talline material with a particle size of not more than 120.11m, a pore depth not exceeding the crystal size and havinc, the following enmpnsitinn. mol. -:,: LaR n 3 P5; BaP 2 - 15.

Properties and quality characteristics of the thus- -obtained electrolyte are shown in the Table hereinbelow.

- lq _ Properties and quality characteristics of the solid electrolyte of the present invention compared to the known electrolyte (US, A, 4352969) No. Solid electrolyte Mle 1 t ing Relative Pnre characte _point, OC density ristic 1 Electrolyte of the invention Example 1 1485 0.99 absence of through pores Example 2 1490 0.99 ditto Example 3 1395 0.98 ditto Example 4 1395 0.98 ditto Example 5 1395 0.99 ditto Example 6 1395 0.99 ditto Example 7 1395 0.98 ditto Example 8 1275 0.99 ditto Example 9 1490 0.99 ditto Example 10 1400 0.99 ditto Example 11 _1500 0.99 ditte) Example 12 1450 0.99 ditto 2 Known electrolyte 1375 0.85 presence of of the composition, thrnugh pores mol. %:

LaF 3 - 90.25 SiF 2 - 4.75 LiF - 3 Known electrolyte 1310 of the enmpositiong 1nol. %:

CeF 3 - 94.05 SrF2 - 0.95 PbCl - 5 0.89 presence of throuc?,h pores Table (cont.)

Permea- Bending Innic cnn- Mouldability strength,ductivity, bility MPa Ohm-1.cm-1 No. Solid electrolyte 1 2 6 7 1 Electrolyte of the invention Example 1 absent, 22 9.0x10-5 good Example 2 ditto 30 8.0x10-5 ditto Example 3 33 8.0x10-4 E ditt-o ditto Example 4 ditto 32 4.0x10-4 ditto Example 5 ditto 33 &. ox, 0-4 ditto Example 6 ditto 35 3.5x10-4 ditto 3xample 7 ditto 29 3.8x10-4 ditto Example 8 ditto 20 7.0x10-4 ditto Example 9 ditth 29 9.0x10-5 ditto Example 10 ditto 25.5xl 0-4 ditto Example 11 ditto 33 1.oxio- ditto - Example 12 ditto 20 2.5x10-4 ditto 2 Known electrolyte avail- 7 6.3x10-5 bad of the composition, able.

mol. %:

Lap 3 - 90.25 sip 2 - 4.75 Lip - 5 3 Known electrolyte availof the Composition, able mol. 79 Cep 3 - 94.05 SrF2 - 0.95 PbCl - 5 k 11 5.41 xl 0-5 good c f 21 M&C FOLIO: 230P61846

Claims

CLAIMS:

WANGDOC:1114i 1. A solid electrolyte comprising from 85 to 99.7 mol% of a rare-earth metal fluoride or a mixture of rareearth metal fluorides and 0.3 to 15 mol.% of an alloying additive; the solid electrolyte.htLving a porous polycrystalline structure with a crystal size of not more than 250 gm, a pore depth not exceeding the crystal size and a relative density of not less than 0.98.
2. A solid electrolyte according to claim 1, in which the rare-earth metal fluoride component comprises lanthanum fluoride and/or cerium fluoride.
3. A solid electrolyte according to claim 1 or claim 2 containing, as alloying additivel calcium fluoride or barium fluoride or lead fluoride or aluminium oxide.
4. A process for producing a solid electrolyte according to any one of claims 1 to 3, comprising mixing the starting components (namely, a rareearth metal fluoride or a mixture of fluorides of rare-earth metals together with an alloying additive); adding a highly-volatile sintering component to the resulting mixture; by melting the resultant mixture with the 22 removal of the highly-volatile sintering component; cooling the resulting melt to solidification; disintegrating the solidified melt; isolating a fraction with a particle size of not more than 250gm and compression moulding the isolated fraction with simultaneous heattreatment.
5. A process according to claim 4, wherein the sintering agent is employed in an amount of from 1 to 5% by weight.
6. A process according to claim 4 or claim 5, wherein the highly-volatile sintering agent is elemental fluorine, lead fluoride, sulphur hexafluoride, carbon tetrachloride, carbon disulphide or polytetrafluorethylene.
7. A process adcording to any one of claims 4 to 6, wherein the compression moulding of the isolated fraction with simultaneous heattreatment is effected under a reduced pressure of from 70 to 680 MPa at a temperature of from 850 to 1400C.
8. A solid electrolyte according to claim 1 to 3 substantially as hereinbefore described with reference to the Examples.
9. A process according to claim 4 substantially as hereinbefore described with reference to the Examples.

Published 1991 at The Patent Office. State House. 66/71 High Holborn, LondonWC I R4TP. Further copies may be obtained from Sales Branch. Unit 6, Nine Mile Point Cwmfelinfach. Cross Keys. Newport, NP1 7HZ. Printed by Multiplex techniques ltd. St Mary Cray, Kent.