Li-ion batteries (LIBs) have succeeded in powering small portable electric devices and are currently expanding to large scale applications (i. e. electric vehicles and grid-level energy storage systems) but it is debatable whether the lithium reserve can meet the increasing demands on the emerging large scale applications. Thus, K-ion batteries (KIBs) and Na-ion batteries (NIBs) are considered alternative energy storage systems due to the natural abundance of K and Na reserve. KIB technology is particularly interesting because K has a lower standard redox potential than Na in non-aqueous carbonate-based electrolytes that are commonly used for NIBs.^1-3 It indicates that KIBs can potentially have a higher cell voltage than NIBs when the cathode materials provide the same working voltage as their analogues in Na system. More importantly, graphite, which is a standard anode for LIBs, cannot reversibly intercalate Na ions, thus NIBs require expensive hard carbon anodes. On the other hand, graphite can store and release K ions reversibly, which in fact sparks research interest in KIBs.^3,4

The discovery of novel positive electrodes is a critical step toward realizing KIBs. In this work, we develop new cathode materials with layered-structure (i. e. K_xTMO₂, TM = Co and Mn)^{5, 6} for KIBs and investigate K-storage properties and mechanism in them by in-situ diffraction and electrochemical characterization combined with theoretical first-principles calculations. Here, we also found that the alkali ion species significantly affect electrochemical properties in layered transition metal oxides. When the size of alkali ion increases (Li⁺ < Na⁺ < K⁺), the voltage curves become more sloped. This is because the formation energy of layered compounds changes more significantly according to K concentrations in K_xTMO₂ due to the stronger interaction between K⁺-K⁺ than Na and Li systems. It can deteriorate the specific capacity and operating voltage, and thus energy density. Therefore, it is important to design novel electrode materials that can minimize the interaction between K ions. This presentation will also provide a good guideline how to design electrode materials for KIBs.

References

1. Eftekhari, A. et al. Potassium Secondary Batteries. ACS Appl. Mater. Sci., 9, 4404. (2016).
2. Marcus, Y. Thermodynamic functions of transfer of single ions from water to non-aqueous and mixed solvents: Part 3 - Standard potentials of selected electrodes. Pure and Applied Chemistry 57, 1129. (1985).
3. Komaba, S. et al. Potassium intercalation into graphite to realize high-voltage/high-power potassium-ion batteries and potassium-ion capacitors. Electrochem. Commun. 60, 172. (2015).
4. Jian, Z., et al. Carbon Electrodes for K-Ion Batteries. J. Am. Chem. Soc. 137, 11566. (2015).
5. Kim, H. et al. K-Ion Batteries Based on a P2-Type K_0.6CoO₂ Cathode. Adv. Energy Mater. 7, 1700098. (2017)
6. Kim, H. et al. Investigation of potassium storage in layered P3-type K_0.5MnO₂ cathode. Adv. Mater. 29, 1702480 (2017)