1.　Li2SO4,　　A. Kvist and A. Lunden, Z. Naturf., 20a, 235 (1965).
　　2.　Li4SiO4,　　A.R. West, J. Appl. Electrochem., 3, 327 (1973).
　　3.　LISCON, (Li14Zn(GeO4)4),　H. Y-P. Hong, Mat. Res. Bull., 13, 117 (1978).
Ambient-temperature ion conductors [Red marks]

　　4.　Li3.6Ge0.6V0.4O4,　　J. Kuwano and A.R. West, Mat. Res. Bull., 15, 1661 (1980).
　　5.　Li1+XTi2-XMX(PO4)3　　H. Aono et al, J. Electrochem. Soc., 136, 590-591 (1989).
　　6.　β-La0.5Li0.5TiO3　　Y. Imaguma et al, Solid State Communications, 86, 689-693 (1993).
　　7.　α-La0.51Li0.34TiO3　  Y. Harada, Y. Hirakoso, H. Kawai and J. Kuwano,

                                                                Solid State Ionics, 121(1–4), 245-251 (1999) 245–251.

 

History of Development of Lithium Superionic Oxides

      

 

      They can be also classified into four groups according the type of compounds:
                  1　and　2: Lithium oxyacid salts
　　　　　3　and　4: gamma-ⅡLi3PO4 solid solutions
　　　　　5: Li-substituted NASICONs
　　　　　6　and　7: Li-ion conducting A-site deficient perovskites (Li-ADPESSs) Lithium-ion Conductors developed by my research group

 

       I have developed two of the conductor group.
　　　　　The first is γ-ⅡLi3PO4 solid solutions: the most famous composition is Li3.6Ge0.6V0.4O4 [4. in the figure: J. Kuwano and A.R. West, Mat. Res. Bull., 15, 1661 (1980)]. This is known to be the first ambient-temperature Li-ion conductors. We also have synthesized about twenty of the gamma-Ⅱsolid solutions based mainly on silicates, germanates and titanates with high ionic conductivity. One of their merits is that their sinterability is very good; the grain boundary resistance of the sinter is comparable to or small than the bulk resistance unlike those for Li-NASCONs and lithium-ion conducting perovskites. In addition, the silicate-based solid solutions shows good chemical stability to Li metal unlike Li-NASCONs and lithium-ion conducting perovskites. Several research groups including groups of Hitachi, NTT and US Army Research Lab applied them to the electrolyte in all-solid-state lithium solid batteries.
　　　　　The second is the disordered α-La0.51Li0.34TiO3 (7. in the figure), which shows the best lithium ion conductivity (1.510-3 Scm-1 at 25℃) of known oxide-based lithium ion conductors. We have reported that the disordered polymorph undergoes an order-disorder (β-α) transition at a temperature of 1150℃. The high-temperature -form has a simple cubic perovskite cell (a=ap, space group: Pm3m) with the disordered arrangement of La3+ ions, while the low-temperature β-form is stable below 1150℃, and has a tetragonal superstructure cell (a=ap, c2ap, space group: P4/mmm), doubled along the c-axis owing to the alternate arrangement of the La3+-rich and (Li+･vacancy)-rich layers along the same axis. We have reported that the conductivity is strongly depends on the degree of the ordering: the conductivity decreases with increasing degree of the ordering, in spite of the formation of the (Li+･vacancy)-rich layers. The solid solution was first discovered in 1987 by Russian researchers; unfortunately this had attracted almost no attention until Inaguma and co-workers (6. in the figure) rediscovered the partially ordered β-form in 1993.

 

Applications

Batteries (Large-scaled batteries for load leveling, energy storage, EV, HEV; thin film batteries; small-scaled batteries for portable electric devices). Sensor (CO2 etc). Ion-exchangers.