As technology behind portable electronics, grid-scale energy storage, and electric vehicles advances, there is an increased dependence on rechargeable battery systems. Today, this market is dominated by high power and energy-dense Li-ion batteries (LIBs). However, despite their many advantages, the limited natural abundance of lithium raises concerns of sustainability. As such, it may be necessary to develop similar, intercalation-based rechargeable battery systems that rely on the reversible cycling of more cost-effective, abundant charge-carrying ions. While many researchers are focusing on Na-ion batteries (SIBs) as an alternative to LIBs, few studies to date have focused on novel K-ion batteries (PIBs).

Despite their larger ionic radii and atomic mass, K⁺ ion possess many advantages as a Li⁺ ion substitute, the foremost being that potassium is highly abundant, making up 2.0% of the earth’s crust and 0.04% of oceanic waters. Secondly, the voltage difference between Li⁺ and K⁺ ions is smaller than that between Li⁺ and Na⁺ ions, making K-ion batteries a more attractive alternative than Na-ion batteries for achieving high energy density. Thirdly, PIBs can utilize low-cost graphite as an anode material, with capacities as high as 240 mAh g^-1, which is not possible for SIBs. As such, the bottle-neck in the development of highly performing PIBs currently is the cathode material.

Recently, bilayered vanadium oxide (δ-V₂O₅) has emerged as a high capacity cathode material for beyond-Li ion battery systems, including Na-ion and Mg-ion batteries. However electrochemical activity of this material has never before been studied in K-ion electrochemical storage system. We have previously reported on the use of our wet chemical pre-intercalation technique to synthesize bilayered vanadium oxide phases with various positively charged ions within its interlayer spacing.[1,2] This method can be used for two aims. First, the preintercalation charge-carriers into the interlayer spacing can be done in order to predefine diffusion pathways and intercalation sites within the structure. Previously, we have demonstrated the potential of this technique through chemical pre-intercalation of Na⁺ ions into the structure of δ-V₂O₅ as charge-carriers in SIBs, which resulted in record high initial capacities above 350 mAh g^-1.[1] Alternatively, electrochemically inactive ions can be preintercalated using this method in order to stabilize the structure and mitigate effects of lattice breathing during cycling. In our research into stabilization of the δ-V₂O₅ phase in Li-ion cells using monovalent and divalent cations, it was found δ-Mg_xV₂O₅ achieved the highest capacity retentions over cycling. This improved stabilization was attributed to the increased interlayer spacing of 13.44 Å.[2]

In this work, we will report on the use of chemically preintercalated δ-K_xV₂O₅ phase as a cathode active materials for PIBs. Bilayered vanadium oxide (δ-V₂O₅) preintercalated with both K⁺ ions and H₂O molecules into the interlayer space was prepared using the chemical pre-intercalation synthesis approach developed previously.[1,2] The chemical formula of synthesized materials was determined to be δ-K_0.42V₂O₅·nH₂O (n = 0.25). X-ray diffraction and transmission electron microscopy measurements confirmed the formation of the bilayered phase with an interlayer spacing of 9.65 Å. We for the first time demonstrate electrochemical performance of δ-K_0.42V₂O₅·nH₂O in non-aqueous K-ion cells. This material demonstrated a record high discharge capacity, 268 mAh·g^-1 at C/50 and 226 mAh·g^-1 at C/15 current rates, for K-ion battery cathodes. δ-K_0.42V₂O₅·nH₂O electrodes retained 74 % of their initial capacity after 50 cycles at a constant current of C/15, and 57 % of initial capacity when the current rate was increased from C/15 to 1C. Analysis of the kinetics of charge storage revealed that diffusion-controlled intercalation dominates over non-faradaic capacitive contribution. This work demonstrates the viability of chemically preintercalated bilayered vanadium oxide phases to demonstrate high electrochemical performance in beyond lithium ion intercalation batteries. We will also report on the use of stabilized δ-Mg_xV₂O₅ and δ-Li_xV₂O₅ phases in K-ion cells.

1. Clites, M. et al, E. Journal of Materials Chemistry A 2016, 4, (20).
2. Clites, M. et al. Energy Storage Mateirals 2018, 11.