In the last years, many efforts have been devoted to the development of batteries based on Earth abundant elements such as sodium, magnesium or more recently potassium. Many questions arise concerning the latter of these metals: are K-ion batteries (KIB) competitive with Li-ion batteries (LIB) and Na-ion batteries (NIB)? Are the benefits of K in terms of ionic conductivity in organic electrolytes, energy density (originating from its low standard potential), abundance and cost, higher than the limitations expected from the high atomic mass and Shannon ionic radius of K⁺ compared to Li⁺ and Na⁺?[1] Similarly to LIB and differently from NIB, graphite can be used as negative electrode in KIB. Indeed, K⁺ ions can be electrochemically inserted in graphite up to the formation of KC₈, corresponding to a specific capacity of 279 mAh/g.[2] Other forms of carbon such as hard carbon or carbon fibres can also be used in KIB providing interesting performance.[3] Moreover, as for LIB and NIB, other p-block elements form binary compounds with potassium with general formula K₃M for Bi, Sb, P and KM for Sn and Ge, leading to theoretical gravimetric and volumetric capacities much higher than that of carbon based materials.[4]^,[5] Tin might be an interesting choice for its sustainability and low toxicity, even though its availability could be limited in the future. Unfortunately, limited cycling performance are reported even with Sn/C composite electrodes.[6]

As a proof of concept, the performance and the detailed electrochemical mechanism of different electrode materials based on C, Sn and/or Sb will be presented and compared to those of the same materials in LIB and NIB. The advantages and drawbacks of such materials in KIB will be highlighted, without forgetting that the development of performing KIB has remained a challenge so far because of the lack of efficient electrolytes and the high reactivity of potassium metal. These effects may alter typical test in half-cells influencing both the electrochemical performance and the nature of the solid electrolyte interphase (SEI). The impact of the electrode/electrolyte reactivity on the formation of the SEI in K/Sb half-cells using electrolytes prepared with carbonate or glyme solvents, and KFP₆ or KFSI salts will be discussed (Fig. 1).[7]

[1] M. Okoshi, Y. Yamada, S. Komaba, A. Yamada and H. Nakai, J. Electrochem. Soc., 2017, 164, A54–A60

[2] Komaba, S.; Hasegawa, T.; Dahbi, M.; Kubota, K. Electrochem. Commun. 2015, 60, 172−175.

[3] W. Wang, J. Zhou, Z. Wang, L. Zhao, P. Li, Y. Yang, C. Yang, H. Huang and S. Guo, Adv. Energy Mater., 2017, 1701648, 1701648

[4] V. Gabaudan, R. Berthelot, L. Stievano and L. Monconduit, J. Phys. Chem. C, 2018, 122, 18266−18273

[5] J. Huang, X. Lin, H. Tan and B. Zhang, Adv. Energy Mater., 2018, 8, 1703496

[6] Q. Wang, X. Zhao, C. Ni, H. Tian, J. Li, Z. Zhang, S. X. Mao, J. Wang and Y. Xu, J. Phys. Chem. C, 2017, 121 (23), 12652–12657

[7] L. Madec, V. Gabaudan, G. Gachot, L. Stievano, L. Monconduit, H. Martinez, ACS Applied Materials & Interfaces, 2018, 10 (40) 34116-34122.