(Invited) Prussian Blue Analogues for Na- and K-Ion Batteries
Authors: Nakamoto Kosuke , Watanabe Kosei , Hosaka Tomooki , Komaba Shinichi 
Published: December 2024

Integrating large amount of intermittent renewable energy sources requires the cost-effective, high-power, and long-life stationary batteries. In this pursuit, there has been a renewed interest in sodium- and potassium-ion batteries since they can considerably mitigate the escalating expenses associated with battery and provide a viable solution for the establishing a widely distributed energy system.1,2

As promising electrode active materials for sodium- and potassium-ion batteries, Prussian blue analogues (PBAs) have attracted much attention due to their large capacity, high redox potential, and three dimensional open-framework structure which can accept sodium and potassium ions in electrochemical event using the redox couples of transition metals of PBAs like Fe2+/3+ and Mn2+/3+ redox.3 Generally optimal PBA shows high capacity of > 100 mAh/g and high voltage of 3 – 4 V and fast diffusion of carrier ions of Na+ and K+, then the stoichiometry and crystal structure of PBAs are crucial to maximize their electrode properties in Na and K cells. PBAs containing various transition metals can be synthesized through simple precipitation route at ambient temperature, thus PBAs containing high-spin Mn exhibited higher voltage and PBAs containing redox-inactive Ni without large structural change exhibited higher cycleability.4,5

Because of electronic insulating nature of PBAs, controlling particle size/morphology and coating/mixing with carbon are also quite important for high performance of PBAs electrode. In the coprecipitation synthesis method used for PBA synthesis, we found that by ion-exchanging from precursor and using chelating agents, the particle size and defect of PBAs could be controlled, and the parameters could affect the electrode performances.6 Additionally, PBA-carbon composites synthesized by mechanochemical solid-state synthesis exhibited high-rate performance.7

From our achievements on PBAs electrode materials, we will show our perspectives on the materials design of PBAs based on our previous fundamental understanding varying the alkali/transition metal species, amount of crystal water/anion vacancies, and morphology/synthesis route. In addition, we will also discuss possible applications of PBAs in aqueous Na- and K-ion batteries as an alternative strategy addressing the issues derived from non-aqueous electrolyte.8-11

References:

(1) K. Kubota, S. Komaba et al., Chem. Rec., 18, 459 (2018).
(2) T. Hosaka, S. Komaba et al., Chem.
Rev., 120, 6358 (2020).
(3) T. Hosaka, S. Komaba et al., Acc. Mater. Surf. Res., 6, 139 (2021).
(4) X. Bie, S. Komaba et al., J. Mater. Chem. A, 5 4325 (2017).
(5) X. Bie, S. Komaba et al., J. Power Sources, 378, 322 (2018).
(6) T. Hosaka, S. Komaba et al., ChemSusChem, 14, 1166 (2021).
(7) T. Hosaka, S. Komaba et al., Electrochemistry, 92, 23-00139 (2024).
(8) K. Nakamoto, S. Okada et al., Electrochemistry, 85, 179 (2017).
(9) K. Nakamoto, S. Okada et al., Small Methods, 3, 1800220 (2019).
(10) T. Hosaka, S. Komaba et al., J. Power Sources, 548, 232096 (2022).
(11) K. Nakamoto, S. Okada et al., RSC Adv., 13, 22070 (2023).