Rechargeable lithium-ion batteries are among the most promising battery technologies to meet energy conversion and storage requirements. However, this technology is still limited due to its high cost, which prevents its use on a large scale. Therefore, cheaper energy storage systems with long cycle life, high power/efficiency, and safety are in great demand and interest. Prussian blue analogs are promising candidates for use in large-scale energy storage applications due to their long-life cycles, high power density, and high energy efficiency [1]. Along with the change of the transition metal, some structural parameters and some fundamental properties of these compounds can be changed, affecting some parameters such as the battery's storage capacity.

This work studied the electrochemical insertion of lithium ions into Copper Hexacyanoferrate (CuHCF). This compound was synthesized by a hydrothermal process using a factorial design for determining the significance variables, i. e., precursor concentrations, temperature, and reaction time, on the electrochemical performance of the battery. The samples obtained were characterized by FESEM, TEM, XRD, and TGA. The results show cubic structures with a high degree of crystallinity and an evident dependence of the crystallite size (50 - 140 nm) with the variation of the synthesis conditions. Finally, lithium-ion batteries were measured using a CuHCF cathode (theoretical charge 85.1 mAhg^-1). The capacity values depended on the crystallite size, obtaining the best results in the synthesis under lower concentrations, reaction time, and temperature. This sample was submitted to deep charge/discharge cycles at a rate of C/20, where the capacity value was close to 60 mAhg^-1 (right image). This demonstrates the influence of the crystallite size on the material's performance, managing to find the optimal synthesis conditions to maximize the battery capacity.

References.

[1] 1. Marzhana Omarova, Aibolat Koishybay, Nulati Yesibolati, Almagul Mentbayeva, Nurzhan Umirov, Kairat Ismailov, Desmond Adair, Moulay-Rachid Babaa, Indira Kurmanbayeva, Zhumabay Bakenov, Electrochim. Acta, 184 (2015) 58-63.