(435b) Sustainable Potassium-Ion Batteries with Their Thermal Safety

Lithium-ion batteries (LIBs) are currently the flagship electrochemical energy storage system due to their high energy density, cycling efficiency, and long cycle life. However, scarcity and high prices of raw materials needed for LIB production impedes their long-term viability.^1,2,3 To alleviate this issue, various charge-carrying metals are being explored, with potassium-ion batteries (KIBs) being a notable alternative. Unlike sodium, potassium is compatible with traditional graphite-based anodes and offers a lower redox potential compared to lithium. However, KIBs currently face challenges regarding their energy density and cycling stability due to the unfavorable mass-to-charge ratio and larger size of K⁺ cations. Additionally, the safety of KIBs is called into question because of the higher reactivity and lower melting point of K metal. Hence, strategies to mitigate these drawbacks are of paramount importance in realizing the full potential of KIBs as a safer alternative to LIBs.

Prussian blue analogues (PBAs) are a promising class of cathode materials for KIBs due to their 3D open framework and high redox potentials. Further, unlike layered transition metal oxides typically used in LIBs, PBAs do not contain any oxygen and exhibit strong organometallic coordination, increasing their thermal safety.¹ We quantified the thermal safety of PBAs, evaluating overall heat generation and identifying the mechanisms of electrolyte thermal decomposition. Due to the presence of cyanide ligands in the PBA cathodes, CN-based gases are released into the electrolyte at ~200 °C, furthering thermal runaway. Yet, while the initiation temperature of thermal runaway for PBA-graphite KIBs (127 °C) was lower than that of LiCoO₂-graphite LIBs (162 °C), the total heat generated was greatly reduced from 773 J/g to 357 J/g. Thus, on the metric of heat generation under thermal stresses, oxygen-free KIB cathodes are an improvement on conventional oxygen-containing LIB cathodes.

Graphite anodes for KIBs also exhibit this same trend as compared to graphite anodes for LIBs. Using a 0.8M KPF₆ in EC/DEC electrolyte, the onset temperature for thermal runaway in a K-graphite system was found to be ~100 °C compared to ~125 °C in a Li-graphite system, yet the overall heat generated was suppressed from 1048 J/g down to only 395 J/g between 50-450 °C.² This is attributed to the lessened heat generation during thermal decomposition of the K-based SEI as well as an SEI shielding effect between intercalated potassium and the electrolyte. Further, we have reported the use of an electronically conducting PEDOT:PSS binder for graphite anodes, with remarkable success in both mitigating capacity fade and enhancing thermal stability.³ Compared to the standard PVDF/carbon black electrode matrix, utilizing PEDOT:PSS as a binder increases the onset temperature from 120 °C to 162 °C while simultaneously decreasing heat generated from 722 J/g to 567 J/g. Taken together, we have demonstrated the exceptional thermal safety performance of KIB cathodes and anodes against their lithium-based counterparts. Moving into the future, additional synergistic materials engineering and development can further unlock the great potential of KIBs as a safer, high-performing alternative to LIBs.

References

1) Li, M. Dadsetan, J. Gao, S. Zhang, L. Cai, A. Naseri, M. Jimenez-Castaneda, T. Filley, J. T. Miller, M. J. Thomson, V. G. Pol, "Revealing the Thermal Safety of Prussian Blue Cathode for Safer Nonaqueous Batteries", Adv. Energy Mater. 2021, 2101764.

2) A. Adams, A. Varma, V. G. Pol, "Mechanistic Elucidation of Thermal Runaway in Potassium-Ion Batteries", J. Power Sources, 2018, 375,131-137.

3) D. A. Gribble, Z. Li, B. Ozdogru, H. J. Kim, E. McCulfor, Ö. Ö. Çapraz, V. G. Pol, "Mechanistic Elucidation of Electronically Conductive PEDOT:PSS Binder for a Potassium-ion Battery Graphite Anode: Electrochemical, Mechanical, and Thermal Safety Aspects", Adv. Ener. Mater. 2022, 2103439.