High-Temperature Ca²⁺ Conduction in NASICON-Type Ca_(1+x)/2In_xZr_2−x(PO₄)₃

Calcium-based batteries are an interesting technology because of its abundance and advantageous electrochemical properties (low reduction potential, high volumetric density).^1,2 While there are numerous studies regarding the development of liquid Ca batteries at ambient conditions, little attention has been given to solid-state batteries so far.
Naturally, due to higher activation energies, Ca solid-state batteries must be operated at elevated temperatures. A high-temperature Ca batteries using calcium-silicon alloy and molten salts in the anode compartment and transition metal doped Ca β’’-alumina as the cathode has been studied by Sammells and Schumacher.³ It was operated at 580 °C and used Ca β’’-alumina as the separator, which has been studied extensively in the 1980s.
Apart from Ca β’’-alumina, little research has been done on Ca solid electrolytes. NASICON-type Ca_0.5Zr₂(PO₄)₃ and (Ca_0.05Hf_0.95)_4/3.9Nb(PO₄)₃ were investigated in 1992 and 2017, respectively, and exhibited maximum ionic conductivities in the range of 10⁻⁶ to 10⁻⁵ S×cm⁻¹ at high temperatures.^4,5 While an electronic transference number of 0.013 was measured for Ca_0.5Zr₂(PO₄)₃ , (Ca_0.05Hf_0.95)_4/3.9Nb(PO₄)₃ was reported to be a purely cationic conductor. Thus, aliovalent substitution can increase the ionic conductivity leading to the suppression of electronic conductivity at elevated temperatures.
In this study, the aliovalent substitution of Ca_0.5Zr₂(PO₄)₃ with In was investigated and the structure-transport relations will be discussed in this presentation. The materials were synthesized via a co-precipitation method and highly dense pellets for transport measurements were prepared, adopting the method of Limaye et al.⁶ The structure was confirmed to be NASICON-type by Rietveld refinement. An increase of the ionic conductivity was determined by impedance spectroscopy and can be correlated with the increased charge carrier density and widening of the bottlenecks in the NASICON-structure.⁷

Bibliography
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(4) Nomura, K.; Ikeda, S.; Ito, K.; Einaga, H. Framework Structure, Phase Transition, and Transport Properties in M^IIZr₄(PO₄)₆ Compounds (M^II = Mg, Ca, Sr, Ba, Mn, Co, Ni, Zn, Cd, and Pb). Bulletin of the Chemical Society of Japan 1992, 65 (12), 3221–3227. https://doi.org/10.1246/bcsj.65.3221.
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(6) Limaye, S. Y.; Agrawal, D. K.; Roy, R.; Mehrotra, Y. Synthesis, Sintering and Thermal Expansion of Ca_1-xSr_xZr₄P₆O₂₄ - an Ultra-Low Thermal Expansion Ceramic System. J. Mater. Sci. 1991, 26 (1), 93–98. https://doi.org/10.1007/BF00576037.
(7) Glück, H.; Zeier, W. High-temperature Ca²⁺ conduction in NASICON-type Ca_(1+x)/2In_xZr_2−x(PO₄)₃. 2024 submitted.