Synthesis and Electrochemical Properties of Potassium Cobalt Silicate Cathode Materials for Potassium-Ion Batteries
Authors: Akiyama Saki , Hirayama Takumi , Hosaka Tomooki , Nakamoto Kosuke , Tatara Ryoichi , Kubota Kei , Komaba Shinichi 
Published: December 2024

Potassium-ion batteries (PIBs) have attracted much attention as potential high-voltage and high-power secondary batteries due to a low standard electrode potential of K+/K in carbonate ester-based electrolytes and weak Lewis acidity of K+ (1). In this study, we focused on polyanionic compounds with a three-dimensional open framework structure as a candidate for cathode materials for PIBs. We reported the K+ ion extraction/insertion reactions in polyanionic compounds such as KVOPO4 at a high average working potential of > 4 V vs. K+/K, which should be attributed to the inductive effect of the anions (2)(3). Several sulfates and phosphates such as KFeSO4F and KVPO4F have been reported as polyanionic compounds for cathode materials for potassium ion batteries, but there are few reports on silicate materials for PIBs (3)(4)(5). In this study, we focused on potassium-containing 3d transition-metal silicates and examined their electrochemical behavior in non-aqueous potassium cells.
In previous papers, electrochemical properties in lithium and sodium cells were reported for silicates containing Mn (6)(7), Fe (8)(9), and Co (10)(11), but for potassium, only synthesis of Co was reported (12). K2CoSiO4 is expected to show theoretical capacity of 234 mAh g-1 if 2 mol of K+ ions are extracted/inserted. In a previous paper, K2CoSiO4 was predicted from calculations to have a low diffusion barrier of 0.14 eV for potassium ions and a working voltage of 3.37 V (13). We synthesized K2CoSiO4 through a solid solution method after mixing 5 wt.% excess of K2CO3 and CoO and SiO2 and calcined in an argon atmosphere at 900 °C. K2CoSiO4/C was obtained by adding 10 wt.% Ketjen black (KB) and Charge-discharge tests of resultant K2CoSiO4/C were conducted using coin-type K cells filled with 1.0 mol dm-3 KPF6 / EC:PC (1:1 v/v), and
The Rietveld refinement of the X-ray diffraction pattern of K2CoSiO4 is shown in Figure 1a. Refinement results show that all diffraction peaks are well fitted with the orthorhombic system of space group Pca21, and are consistent with the crystal structure of K2CoSiO4 previously reported (12), showing the successful synthesis of single-phase powder of targeted K2CoSiO4. Figure 1b shows the charge/discharge curves of K2CoSiO4/C electrode with EC: PC-based electrolyte. The electrode exhibited high redox activity in the 2.0-4.0 V, with an initial discharge capacity of about 133 mAh g-1, which would exceed a theoretical capacity of 117 mAh g-1 for a one-electron reaction, suggesting that the extraction/insertion of more than 1 mol of K+ ions proceeded with the reversible redox reaction of Co2+/3+. Furthermore, K2CoSiO4/C showed good reversibility in the first 5 cycles, indicating that 1 mol or more K+ ion extraction/insertion is reversible. We will further discuss the structural changes during the K+ ion extraction/insertion process and redox behavior of Co in the presentation.

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Figure 1