Understanding the Crystal Chemistry of Phosphate Cathode Materials

Abstract

Thermal batteries are primary reserve batteries that can achieve high power output in a short period of time by melting an insulating salt electrolyte with a pyrotechnic heat source and converting it into a highly conductive eutectic salt electrolyte. The thermal battery exhibits no self-discharge before activation, and the initial capacity of the thermal battery is maintained after long-term storage. Thermal batteries are an attractive option for emergency power supplies in aircraft, ejector seats, space landings and helicopters due to their outstanding robustness, high reliability, and long-term storage properties.FeS2 or CoS2 are the most common cathode materials for thermal batteries.1 However, their high cost and relatively low discharge voltage makes it desirable to find new materials that have higher voltage discharges, higher specific capacities, low cost and that are environmentally friendly. Polyanion cathode materials have attracted much attention in the field of both Li-ion and Na-ion batteries,2–4 as they can be regarded as low-cost materials and environmentally friendly. However, to the best of our knowledge, there have been no studies on polyanion cathode materials for thermal batteries. These materials are formed by tetrahedral units (XO4)n- (X = P, S, Si or B) bonded to MO6 polyhedra (M = transition metal). The tetrahedral anion presents a very strong covalent bond between X-O, giving rise to higher voltages versus Li+/Li or Na+/Na via the inductive effect and their very stable framework and open structure facilitates ion diffusion. Thus, polyanion materials present high structural stability and enable the tuning of the operating potential, making them an excellent choice for developing new cathode materials for thermal batteries. Here we report some of our latest work on the synthesis, structure and electrochemistry of polyanionic cathode materials for thermal batteries. 1 R. Li, W. Guo and Y. Qian, Front. Chem., 2022, in press, doi: 10.3389/fchem.2022.832972 2 Z. Gong and Y. Yang, Energy Environ. Sci., 2011, 4, 3223–3242. 3 C. Masquelier and L. Croguennec, Chem. Rev., 2013, 113, 6552–6591. 4 Q. Ni, Y. Bai, F. Wu and C. Wu, Adv. Sci., 2017, 4, 1600275.