Transition-Metal Substitution Effects on Ionic Conduction Properties of Solid Electrolyte LiTa2PO8
Authors: Shima Souichi , Yajima Takeshi , Iriyama Yasutoshi 
Published: December 2024

All-solid-state lithium-ion batteries are increasing in interest as the next-generation battery due to their safety, high energy density, and long lifetime in achieving a decarbonized society. However, all-solid-state lithium-ion batteries, using inorganic solid electrolytes instead of liquid electrolytes, face challenges in charging/discharging rates and capacity due to their much lower ion conductivity[1]. Therefore, it is essential to explore novel solid electrolytes with high ionic conductivity.
Solid electrolytes with particularly high ion conductivity are called superionic conductors, such as Li10GeP2S12 (LGPS)[2]and Li1.3Al0.3Ti1.7(PO4)3 (LATP)[3]. Theoretical studies suggest that their fast ionic conduction is governed by a non-trivial conduction mechanism, simultaneous multiple-ion hopping, which is driven by Coulomb interactions of lithium ions instead of classical single-ion hopping[4]. Multiple-ion hopping, namely concerted migration, results in a lower energy barrier for ion migration than single-ion hopping. Thus, triggering concerted migration would be crucial to achieving high ionic conductivity.
To induce concerted migration, chemical substitution may be effective, as seen in LATP. LATP is a superionic conductor with a NASICON-type structure in which the Ti4+ site of the parent compound LiTi2(PO4)3 is partly substituted with Al3+. Though the parent compound has a low ionic conductivity of 3.13×10–6 S/cm, the ionic conductivity in LATP of 5.1×10–3 S/cm increases by approximately three orders of magnitude[5]. A theoretical study suggests the conduction mechanism of LATP is governed by concerted migration, meaning a chemical substitution leads to concerted migration, resulting in such a significant enhancement.
To find ionic conductors with high ionic conductivity, we focused on LiTa2PO8
[6]for the following reasons: First, its ionic conductivity (1.6×10–3 S/cm) is comparable to that of LATP without chemical substitutions. Second, theoretical calculations predict that a substantially weaker Li-Li interaction is expected in LiTa2PO8
[7] than in LATP[4], implying that the conduction mechanism may be classical. Given the increase in ionic conductivity in LATP, high ionic conductivity might be achieved when concerted migration is induced/enhanced in LiTa2PO8. Though the substitutions of Ta5+/P5+ sites with tetravalent ions of typical elements such as Te4+
[8]and Si4+[9] have been studied, no drastic increase in ionic conductivity has been achieved, and the highest ionic conductivity is 1.6×10–3 S/cm. In this study, we performed chemical substitutions of Ta5+ with transition metal ions, Ti4+ and W6+.
Polycrystalline powder of Li1±xTa2–xMxPO8 (M = Ti4+, W6+) (x = 0 ~ 0.2) was prepared by solid-state reaction. The lattice parameters determined by XRD patterns decreased as the substitution amount x increased, indicating successful substitutions of Ti4+ and W6+ since Ti4+ and W6+ have smaller ionic radii than Ta5+. The x dependence of the lattice constants indicates the solubility limit is xTi = 0.08 for the case of Ti4+, while the linear decrease of the lattice constants in the case of W6+ indicates the solubility limit is above xW = 0.20. Elemental analysis via SEM/EDX also supports the above results. The ionic conductivity of xTi = 0.08 and xW = 0.20 was measured by the AC impedance method in the range of 1.0×102~5.0×107 Hz. The results showed that Ti4+ substitution increases ion conductivity, while W6+ substitution decreases compared with non-doped LiTa2PO8, meaning the increase in the number of Li ions is effective in enhancing the ionic conductivity in LiTa2PO8. However, unlike LATP, the increase in ionic conductivity is at most twice as large.
References
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Acknowledgement
This work was supported by JST GteX (JPMJGX23S2), JSPS KAKENHI Grant Number JP21K04636, the Asahi Glass Foundation, Research Foundation for the Electrotechnology of Chubu.