High Voltage Potassium Intercalation Cathode

Alkali-ion intercalation compounds are the most common cathode materials for rechargeable batteries, including Li-, Na-, and K-ion batteries. For several years, layered oxide cathode materials have been considered promising cathode materials for Na- and K-ion batteries because of their high reversible capacity and high working voltage in Li-ion technology. However, our recent findings demonstrated that the layered oxides may not be good candidates for Na and K intercalation cathodes.^[1] Na and K transition metal oxide compounds have alkali ion deficient composition (x&lt;1.0 in A_xMO₂, A= Na and K), which leads practical difficulty of realizing Na- and K-ion batteries. This is because all the Na and K ions should come from the cathode in rocking-chair batteries. As the insertion ion size increases, the voltage curve becomes much sloped and the sloped voltage curves cause low specific capacity and low average voltage. Both the problems are attributable to much stronger Na⁺-Na⁺ and K⁺-K⁺ interaction than Li⁺-Li⁺ in the layered oxide structure. In this respect, we propose that polyanion compounds will be better candidates because they have 3 dimensional arrangements of Na and K ions, resulting in longer Na⁺-Na⁺ and K⁺-K⁺ distance and reduced effective interaction between them. We proved this concept using KVPO₄F as a model system of polyanion compounds, which has stoichiometric composition and provides a high average voltage of ~4.3 V (vs. K/K⁺) with a reversible capacity of ~105 mAh/g.^[2]<br/>In this presentation, we will discuss (1) why polyanion frameworks can provide high K intercalation voltages (vs. the layered oxides)^[1-3] and expand our discussion to understand (2) how local structure factors, such as cation and anion substitutions, affect K storage properties and performance with the KVPO₄F model system.^[4] <br/> <br/><b>References</b><br/>[1] H. Kim <i>et al.</i> <b>Chem. Mater.</b> 2018, 30, 6532.<br/>[2] H. Kim <i>et al.</i> <b>Adv. Energy Mater.</b> 2018, 8, 1801591.<br/>[3] H. Kim <i>et al.</i> <b>Trends in Chem.</b> 2019, 1, 682.<br/>[4] Y. -W. Byeon <i>et al.</i> In preparation.