Progress and prospective of solid-state lithium batteries
Kazunori Takada ⇑
National Institute for Materials Science,1-1Namiki,Tsukuba,Ibaraki 305-0044,Japan
Abstract
The development of lithium-ion batteries has energized studies of solid-state batteries,becau the non-flammability of their solid electrolytes offers a fundamental solution to safety concerns.Since poor ionic conduction in solid electrolytes is a major drawback in solid-state batteries,such studies have been focud on the enhancement of ionic conductivity.The studies have identified some high performance solid electrolytes;however,some disadvantages have remained hidden until their u in batteries.This paper reviews the development of solid electrolytes and their application to solid-state lithium batteries.Ó2012Acta Materialia Inc.Published by Elvier Ltd.All rights rerved.
Keywords:Lithium batteries;Solid electrolytes;Ionic conduction
1.Introduction
The development of solid-state batteries started with the discovery of fast ionic conduction in solids.Solid-state bat-teries have the following potential advantages:abnce of electrolyte leakage,abnce of problems relating to vapor-ization of liquid electrolytes,abnce of pha transitions at low temperatures improving low-temperature perfor-mance and ea of miniaturization [1,2].In addition,stud-ies revealed that solid-state batteries are highly reliable;that is,they usually show excellent storage stability and very long cycle life.
Battery life is often limited by side reactions,including decomposition of the electrolyte;however,the reactions rarely take place in solid electrolyte systems.Such side reactions are electrochemical,and thus involve the trans-port of reactants to the electrode surface,where charge transfer takes place,in order to proceed.However,only particular species are mobile in solid electrolytes.For example,only lithium ions diffu in solid electrolytes that are ud in solid-state lithium batteries.This means that there are no species that diffu to the electrode surface and take part in the side reactions,and thus solid-state batteries generally tend to have a long life.
The study of solid-state batteries,which started in the middle of the 20th century,was prompted by the develop-ment of lithium-ion batteries.Lithium-ion batteries have rapidly become widespread since their development [3]becau of the following advantages:high voltage,high vol-umetric and gr
avimetric energy density,low lf-discharge rate,no memory effect,quick charge acceptance,excellent cycle life,and wide operating temperature range [4].The high energy densities of lithium-ion batteries come from the high cell voltages.However,this is also a drawback,as high voltages do not allow us to u aqueous solutions as the electrolyte,becau the cell voltage exceeds the decomposition voltage of water.Therefore,organic sol-vents are ud to dissolve the supporting salts.Becau the solvents are flammable substances,they cau safety concerns in lithium-ion batteries.
斜面机械效率
In addition,recent environmental concerns have incread the demand for large-sized batteries as shown in Fig.1.Batteries are now expected to be able to power vehicles [5]for the efficient u of energy,and energy stor-age on a large scale is required to make renewable energy viable [6].However,the increasing battery size increas the amount of combustible electrolyte.It also worns the heat radiation,and the batteries tend to easily heat up toward thermal runaway.Therefore,the increasing bat-tery size makes the safety issues more rious [7].U of solid electrolytes is expected to be a fundamental solution
1359-6454/$36.00Ó2012Acta Materialia Inc.Published by Elvier Ltd.All rights rerved./10.1016/j.actamat.2012.10.034
⇑Tel.:+81298604317;fax:+81298549061.
E-mail address:takada.jp
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Acta Materialia 61(2013)759–770
to the safety issue due to their non-flammability.More-over,such batteries should have a much longer uful life than consumer ones,and solid-electrolyte systems will also meet the requirements.
On the other hand,disadvantages pointed out were vol-ume changes,electrolyte impedance and discharge product impedance [8].Becau the electrode/electrolyte interface in solid-state batteries is between solid and solid,volume changes of electrode or formation of discharge product destroy the interface.The interface problem does not ari in intercalation electrodes.The u of intercalation
chemis-try [9]dramatically improved battery performance as dem-onstrated by lithium-ion batteries,where both electrodes are intercalation compounds.The benefit is significant,particularly in solid-state batteries,becau intercalation reactions neither produce reaction products nor cau large volume changes.
The biggest disadvantage of solid-state batteries histori-cally has been their low current drains,or their low power density –and this is still the ca today.Since the disadvan-tage comes from the low ionic conductivity of the solid electrolytes,studies of solid-state batteries have been mainly focud on the development of highly conductive solid electrolytes.As will be described later,the studies have identified many solid electrolytes having high ionic conductivities;however,not all of them have led to high performance of solid-state lithium batteries.2.History of solid electrolytes
Fast ionic conduction in solids around ambient temper-ature was found for the first time in a -AgI [10].b -or c -pha AgI at room temperature is transformed into the a -pha with higher symmetry at 147°C,in which Ag +ions conduct as fast as ions in liquids.An attempt to stabilize the fast ion conducting pha has led to RbAg 4I 5,who ionic conductivity at room temperature is 0.21S cm À1[11].Analogous material design resulted in a Cu +-ion superconductor,Rb 4Cu 16I 7Cl 13,with an ionic conductivity at room temperature of 0.34S cm À1[12],which is the high-est conductivity ever ob
rved among solid electrolytes.
Solid-state batteries were also constructed with the fast ionic conductors.A solid-state battery with RbAg 4I 5showed very small lf-discharge over 30years [13],while Rb 4Cu 16I 7Cl 13showed highly reversible charge–discharge operation [14]and high exchange current density [15]in combination with a three-dimensional (3-D)intercalation electrode,Cu x Mo 6S 8.
On the other hand,lithium-ion-conductive solid electro-lytes had been poor ionic conductors,as shown in Fig.2.However,the discovery of fast lithium ion conduction in Li 3N [16],which had been previously suggested by 7Li NMR [17],and commercialization of cardiac pacemakers [18],in which LiI is ud as the solid electrolyte,encour-aged the development.
The maximum electric conductivities of organic-solvent electrolytes are of the order of 10À2S cm À1;however,transport numbers of lithium ions are only less than 0.5.Since lithium-ion batteries are “shuttlecock ”or “rocking chair ”cells [19],in which lithium ions go back and forth between the cathode and the anode to operate the battery,the rate capability should be governed by the movement of lithium ions.When taking into account that the lithium transport number in solid electrolytes is unity,ionic con-ductivity of the order of 10À3S cm À1will be enough to make the power density of solid-state batteries comparable to that of commercial lithium-ion batteries with liquid electrolytes.
To date,such conductivities have been achieved in v-eral oxides and sulfides;some have given solid-state lith-ium batteries,and some have not yet.Some solid electrolytes have been ud in batteries in spite of their low ionic conductivity.Before overviewing them,it should be mentioned that fast lithium-ionic conduction was recently discovered in LiBH 4above 110°C [20].It was also reported that the addition of halides suppress the pha transition to maintain the high conduction state down to room temperature [21],which has given a new class of super ionic conductor.
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Requirements of large-sized batteries for a sustainable society.
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3.Solid electrolytes and solid-state lithium batteries
3.1.Oxide electrolyte systems
3.1.1.c-Li3PO4-type oxysalts
Some oxysalts,including Li2SO4,show fast ionic con-duction in their high-temperature phas[22],and many attempts have been made to stabilize the highly conduct-ing phas down to ambient temperature[23].A system-atic study gave various kinds of oxysalts acting as solid electrolytes[24].It revealed that ionic conduction is enhanced when two kinds of oxysalts are formed into solid solutions,and a fast ionic conductor called LIS-ICON(Li superionic conductor),Li14Zn(GeO4)4,was developed in1978[25]on the basis of this study.Studies motivated by the development of LISICON provided many lithium-ion-conducting oxysalts around1980:one group compris solid solutions bad on Li4SiO4,and the other bad on c-Li3PO4.
Another systematic study on x Li4M IV O4–(1Àx)Li3M V O4 systems(M IV=Ge,Ti and M V=As,V)[26]also showed the consistent results that the conductivities of the solid solu-tions always showed maxima at x=0.4–0.6.In addition,the study revealed strong correlation between the unit cell volume and the ionic conductivity;conductivity increas and the activation energy for conduction decreas
with increasing cell volume at constant x of0.5,as demonstrated in Fig.3.Becau the constant x keeps the carrier concentra-tion constant as supported by the constant pre-exponential factor,it can be concluded that the increasing cell volume increas the mobility of lithium ions.That is,the increasing cell volume will open the conduction channel for the lithium
ions and make them more mobile,which lowered the activation energy and incread the conductivity.
3.1.2.Thin-film batteries with c-Li3PO4-type oxysalts
Since we had to wait for the development of highly con-ductive solid electrolytes,studies on solid-state lithium bat-teries began with the thin-film batteries.The thin-film fabrication of the electrolyte layer shortens the distance between the cathode and the anode to reduce the internal resistance to compensate for the low ionic conductivity of solid electrolytes.
Thefirst thin-film battery[27]was constructed with Li and TiS2as anode and cathode,respectively.The electro-lyte layer was Li3.6Si0.6P0.4O4fabricated into the amor-phous state by RF sputtering.Although the conductivity at25°C is only5Â10À6S cmÀ1,the thin-film formation makes the battery operable at a current density of 16l A cmÀ2.
Thermodynamic instability[28]with lithium metal in Li3.6Si0.6P0.4O4was improved in Lipon[29],which is cur-rently the most popular thin-film solid electrolyte.The thin film is deposited by magnetron sputtering of Li3PO4under N2-contained atmosphere,which introduces nitrogen into the Li3PO4and rais the conductivity from7Â10À8to 2Â10À6S cmÀ1.Lipon itlf is amorphous;however,it is regarded as a c-Li3PO4-type oxysalt from the structure of the crystalline counterpart[30].Becau it also has a wide electrochemical window from0to5.5V[31],many thin-film batteries have been constructed with this electrolyte so far.Various kinds of electrode materials have been ud in the batteries:V2O5[29],LiMn2O4[32]and LiCoO2[33] have all been ud as the cathode,and lithium metal [29,32,33],Si3N4[34]and an amorphous Si–Sn oxynitride [35]have been ud as the anode.Although the lithium metal anode promis high energy density of the battery, a battery design for a higher energy density was propod. In“lithium-free batteries”[36],not even lithium metal is ud.The anode is formed by in situ plating of metallic lith-ium on the anode current collector.That is,lithium metal in the anode is supplied from the LiCoO2cathode during the charging process.
Solid-state batteries are inherently highly reliable owing to the single-ionic conduction as described above,and the thin-film batteries also show high reliability.For example, cycling performance obrved for a thin-film solid-state battery with a construction of Li/LiCoO2was remarkable. Results f
rom a cycling test over30,000cycles demonstrated excellent cycling performance with the capacity fading ran-ged from0.0001%to0.002%per cycle[37],as shown in Fig.4.
K.Takada/Acta Materialia61(2013)759–770761
3.1.3.Highly conductive oxide solid electrolytes
In the oxides and oxysalts,ionic conductivity of the order of10À3S cmÀ1has been achieved in LiTi2(PO4)3 [38]and Li3x La2/3Àx TiO3[39].
The crystal structure of LiTi2(PO4)3is NASICON-type,named after a fast sodium ion conductor, Na1+x Zr2P3Àx Si x O12,called NASICON(Na super ionic conductor)[40].In NASICON,3-D skeleton structure is made of ZrO6octahedra linked with PO4tetrahedra, through which sodium ions can be conducted.Since the open structure was considered tofit fast ionic conduction,
many attempts have been made to obtain lithium ion-conducting counterparts.However,simple substitution of Na with Li did not give a good ionic conductor;ionic conductivity of LiZr2(PO4)3is lower than10À9S cmÀ1.
Ionic conduction is strongly correlated with the size of the skeleton network in NASICON[41]as well as c-Li3PO4-type electrolytes;NaM2(PO4)3shows the highest conductivity at M=Zr.On the other hand,since lithium ions are smaller than sodium ions,the skeleton framework consisting of ZrO6octa
hedra is too large for high ionic conductivity.Fig.5shows the clear relationship between activation energies for conduction in the bulk and the cell volume of various kinds of NASICON-type phosphates. Conductivity measurements through different kinds of NASICON-type phosphates from LiGe2(PO4)3to LiHf2(PO4)3revealed that activation energy for conduction is minimized at the cell volume of1310A˚3;that is,the ionic conductivity of LiM2(PO4)3is the highest at M=Ti[42], which is a smaller tetravalent cation than Zr.Fine tuning of the lattice size and optimization of the carrier concentra-tion in the skeleton framework by aliovalent cation substitution gave a bulk conductivity of3Â10À3S cmÀ1 in Li1.3Al0.3Ti1.7(PO4)3[43].
On the other hand,Li3x La2/3Àx TiO3with a perovskite structure was found in the studies on dielectricity and ferroelectricity of perovskite-type alkaline-earth titanates (ATiO3,A=Ca,Sr,Ba).Cosubstituting of the alkaline earth ion with a trivalent rare earth(La)and a monovalent alkali ion(Li)gives Li1/2La1/2TiO3[44].A dielectric anom-aly[45],increasing capacitance upon the heating,large dielectric loss and dielectric relaxation were found in the material,which were attributed to ionic conduction,and the highest conductivity obrved in the system was 1Â10À3S cmÀ1[39].
怎样跳皮筋Li3x La2/3Àx TiO3basically has a simple cubic perovskite structure,where Ti atoms octahedrally coor
dinated with oxygen atoms occupy the corner of the cube(B-site),and the center of the cube(A-site)is occupied by La3+ion, Li+ion or left vacant.Although when the La3+,the Li+ ions and the vacancies are randomly distributed over the A-sites,the lattice belongs to cubic symmetry,the system shows many polymorphs[46].When Li0.5La0.5TiO3is syn-thesized by heating at1350°C and then quenching,the lat-tice is cubic.On the other hand,when it is slowly cooled, Li+ions and La3+ions are somewhat ordered to form alternate Li-rich and La-rich planes.The ordering lowers the symmetry from cubic to tetragonal or orthorhombic. Tilting of the TiO6octahedra also distorts the cubic lattice to hexagonal symmetry.Conductivity among the poly-morphs is mainly governed by two factors:one is bottle-neck size,and the other is site percolation.
Lithium ions conduct from an A-site to the neighboring A-site in the perovskite structure through a bottleneck sur-rounded by four oxygen atoms.Becau the oxide ions attract the lithium ions to act as a potential barrier for the conduction,perovskite-type oxides also show clear cor-relation between the ionic conductivity and the bottleneck size,or the lattice size,as c-Li3PO4or NASICON-type electrolytes.That is,the larger the lattice parameter,the higher is the conductivity and the lower is the activation
762K.Takada/Acta Materialia61(2013)759–770
energy for conduction[47].Therefore,divalent Sr2+ions, which are larger than trivalent cations including La3+ should give higher conductivity as A-site cation.In fact, Sr-substituted[(Li1/2La1/2)1Àx Sr x]TiO3showed a conduc-tivity of1.5Â10À3S cmÀ1[48],which was slightly higher than that in Li3x La2/3Àx TiO3.
Composition influences the conductivity of the perov-skite oxides,not only in respect of the carrier concentra-tion.A-sites on the conduction paths are partially occupied by La3+ions,which block the migration of lith-ium ions;therefore,site percolation plays an important role in the ionic conduction[49].That is,the La content should be low enough to make a group of neighboring lithium ions and vacancies percolate through the system.Sr is prefera-ble to La as the A-site cation in terms of the lattice dimen-sions as described above;however,the divalent state increas the Sr content to increa the percolation threshold.
党章学习小组As described above,ionic conductivity of oxides has become10À3S cmÀ1;however,high conductivity alone is not enough for battery application.The solid electrolytes have two major disadvantages for the application to solid-state batteries.Thefirst one originates from the prence of Ti.Ti in the solid
electrolytes is tetravalent,or3d0state; that is,the solid electrolytes do not have conducting electrons.However,they are easily reduced;Ti4+ions in LiTi2(PO4)3and Li3x La2/3Àx TiO3are reduced to Ti3+below 2.5V[50]and1.5V[51],respectively.Therefore,when they come into contact with low-potential anodes including Li metal or C6Li,the reduction injects electrons to the electrolytes.
The cond disadvantage is that the grain boundary resistance in the solid electrolytes is generally very high [52,39];that is,although the intragrain diffusion of lithium ions is fast,movement between the grains limits the current drain.Of cour,heat treatment at high temperatures reduces the grain boundary resistance to some extent; however,another rious problem aris at the electrode/ electrolyte interface.The solid electrolytes have to be in contact to active materials in batteries,and thus the high-temperature treatment to promote the sintering also induces the interdiffusion between the materials.Such reac-tions between the active material and the solid electrolyte take place to form impurity phas at the interface to increa the electrode resistance[53].
Becau the solid electrolyte alone can be sintered at high temperatures,low-temperature formation of the electrodes will solve the problem.One example is the formation of electrode layers by evaporation process[54], and another is in situ formation of electrodes[55,56],in which parts of the el
ectrolyte in contact with the current collectors are electrochemically converted to electroactive materials at room temperature.
3.1.
4.Garnet-type solid electrolytes
The most-recently developed oxide electrolytes have garnet-type structure[57].The conductivity obrved for Li5La3M2O12(M=Nb,Ta)was of the order of10À6S cmÀ1;it incread to4Â10À5in Li6BaLa2Ta2O12[58], 5Â10À4in Li7La3Zr2O12[59]andfinally8Â10À4S cmÀ1 in Li6.75La3(Zr1.75Nb0.25)O12[60].The conductivities of the garnet-type oxides are still lower that of NASICON-type phosphates and perovskite-type oxides,while they show the following advantages:one is a small grain bound-ary resistance,and the other is stability against lithium metal.
In NASICON-type phosphates and perovskite-type oxides,contribution of grain boundaries to the resistance is much larger than that of the bulks,even at sintering temperatures higher than1200°C.On the other hand, the contribution of the grain boundaries is in the same order of or less than that of the bulk in garnet-type Li6ALa2Ta2O12(A=Sr,Ba),even when the samples were sintered at900°C[58].邹忌修八尺有余
The cond advantage of garnet-type oxides is compati-bility with lithium anode.Although NASICON-type phosphates and perovskite-type oxides are unstable to electrochemical reduction,Li6BaLa2Ta2O12was reported to be stable against lithium metal.No reaction was obrved,even when it was immerd in molten lithium [58].Moreover,the electrochemical window of Li6.75La3 (Zr1.75Nb0.25)O12was demonstrated to be very wide:it is stable from0to9V vs.Li+/Li[60].
女人故事3.2.Sulfide electrolyte systems
3.2.1.Sulfide solid electrolytes
Studies on ionic conduction in sulfides started in glass [61].Becau the high polarizability of sulfide ions weakens the interaction between the anions and the lithium ions, sulfides inherently tend to show fast ionic conduction.In fact,the highest conductivities obrved among sulfide glass, e.g.LiI–Li2S–P2S5[62]and LiI–Li2S–B2S3[63], were already of the order of10À3S cmÀ1in the early 1980s,as listed in Table1.Besides the high ionic conductiv-ity,sulfide electrolytes have the following advantages.The first advantage is that they show high ionic conductivity without prence of transition metal elements that narrow the electrochemical window.Another advantage is the low grain-boundary resistance.
Contrary to oxide or oxysalt solid electrolytes,sulfide solid electrolytes show small grain boundary resistance even in a cold-presd pellet[64].That is,the sintering pro-cess is not necessary to connect ionic path between the par-ticles.This feature is convenient for constructing bulk-type batteries,becau the batteries can be asmbled by press-ing powders of batteries materials into a three-layered structure of anode/electrolyte/cathode.
Becau sulfide systems have advantages for fabricating bulk-type solid-state batteries,the development of lithium-ion batteries triggered many studies on sulfide electrolytes in1990s.Various kinds of sulfide glass doped with oxys-alts were developed[65,66],and in the21st century,con-
K.Takada/Acta Materialia61(2013)759–770763
ductivities of the order of10À3S cmÀ1have been achieved also in crystalline sulfides.
Li4GeS4was found to be a solid electrolyte in2000[67]. Although the conductivity is only2.0Â10À7S cmÀ1,cat-ion substitution in this material has given a lot of solid solutions.They are categorized into thio-LISICON family, becau they have c-Li3PO4-type structure oxysalts typified by LISICON.Aliovalent substitution introducing lithium-ion vacancies or interstitial lithium ions incread the conductivity[68],and the conductivity has reached 2.2Â10À3S cmÀ1at a composition of Li3.25Ge
0.25P0.75S4 [69].
Another high conductivity was found in glass ceramic. Fast ion-conducting pha is precipitated as a primary crystal in the crystallization process of a supercooled Ag+-ion-conducting glass[70].A lithium-ion-conducting sulfide glass also showed a similar phenomenon,although the mechanism may be different.Precipitation of a meta-stable crystalline pha increas the conductivity from 5.4Â10À5S cmÀ1in70Li2S–30P2S5glass to 3.2Â10À3 S cmÀ1with a very low activation energy of conduction of12kJ molÀ1in the glass–ceramic pha[71].
Very recently,the highest ionic conductivity among lith-ium-ion conductive solid electrolytes of1.2Â10À2S cmÀ1 was obrved in Li10GeP2S12[72].Although the paper tells that electric conductivity is comparable to that of organic-solvent liquid electrolyte,lithium-ionic conductivity is much higher,when taking into account that the transport number of lithium ion is unity.
Improvements have been made not only to the conduc-tivity;synthesis methods have also been also improved dur-ing the development.The materials should be heated in a aled container for the synthesis becau of the high vapor pressure of P2S5or B2S3at high temperatures.Synthes under ambient pressure had become possible in LiI–Li2S–SiS2glass[73]owing to low vapor pressure of
SiS2.Since the starting materials do not need to be aled,a twin-roller method is available for the quenching[74].The fast cooling rate in the twin-roller method allows glass-formation at high lithium contents,which is promising for high ionic conductivity due to the high carrier concentration.More-over,preparation of the glass was reported to be possible by mechanical milling[75].That is,even heating process is not necessary,which is beneficial in mass-production of the solid electrolytes.
3.2.2.Solid-state lithium batteries with sulfide electrolytes
One of the main objectives on the development of solid electrolytes other than increasing conductivity is to u electrode materials giving high energy densities.Thefirst sulfide solid electrolytes that showed high ionic conductiv-ities are glass made from P2S5,B2S3,or SiS2as a glass network former and Li2S as a glass modifier to provide ionic conduction to the glass networks.Although the ionic conductivities of the quasi-binary glass are of the order of 10À4S cmÀ1,doping of lithium halides enhances the con-ductivity[76];especially LiI increas the conductivity to the order of10À3S cmÀ1[62,63,73].However,the iodide ions limit the oxidation potential of the glass below3V vs.Li+/Li[62].This problem became critical in the1990s, becau4V cathodes[77–79]became mainstream in lith-ium batteries.In the mid-1990s,the addition of oxysalts was found to allow elimination of LiI while keeping the conductivity of the order of10À3S cmÀ1[65],w
hich enabled the u of4V cathodes in sulfide solid electrolyte systems[80,81].
The4V cathodes enhanced the energy density of the solid-state battery to practical level,and many studies on similar bulk-type batteries followed[82,83].The batteries showed excellent performance besides the high energy den-sities.Self-discharge is negligible,even at a storage temper-ature60°C[84],and cycling performance is also excellent, as shown in Fig.6[85].The features should be attributed to single-ion conduction suppressing side reaction.
Table1
Conductivities at room temperature of sulfide solid electrolytes.
Solid electrolyte Conductivity(S cmÀ1)Preparation method Reference Glassy materials
0.5Li2S–0.5GeS2 4.0Â10À5Water quenching[61]
0.66Li2S–0.33P2S510À4Water quenching[62]
0.45LiI–0.37Li2S–0.18P2S5 1.7Â10À3Water quenching[62]
Li2S–B2S310À4Water quenching[63]
0.44LiI–0.30Li2S–0.26B2S3 1.7Â10À3Water quenching[63]
苦涩的回忆0.5Li2S–0.5SiS2 1.2Â10À4Water quenching[76]
0.3LiCl–0.35Li2S–0.35SiS2 2.7Â10À4Water quenching[76]
感恩老师的文章0.40LiI-0.36Li2S-0.24SiS2 1.8Â10À3Liquid N2quenching[73]
0.6Li2S–0.4SiS2 5.0Â10À4Twin-roller quenching[74]
0.3LiI–0.42Li2S–0.28SiS28.2Â10À4Twin-roller quenching[74]
阎嵩
0.01Li3PO4–0.63Li2S–0.36SiS2 1.5Â10À3Twin-roller quenching[65]
0.6Li2S–0.4SiS2 1.5Â10À4Mechanical milling[75] Crystalline materials
Li3.25Ge0.25P0.75S4 2.2Â10À3Solid-state reaction[69]
70Li2S–30P2S5 3.2Â10À3Glass–ceramic[71]
Li10GeP2S12 1.2Â10À2Solid-state reaction[72]
764K.Takada/Acta Materialia61(2013)759–770
