title: Hexacyanometallates for sodium-ion batteries: insights into higher redox potentials using d electronic spin configurations.
abstract: A fundamental understanding of anomalous redox mechanisms in hexacyanometallate compounds, compared with conventional NaMO2systems (M: transition metals), is presented based on first-principles calculations and experimental validations. From theoretical calculations, we identified low-spin and high-spin states of Fe ions coordinated by the cyanide group (-CN) with the same oxidation state (Fe2+) in Na2Fe2(CN)6. Considering the site dependency of d electronic spin configurations based on the crystal field theory (CFT) of transition metals (TMs), we calculated the thermodynamic mixing energy using Na2Fe2(CN)6and Na2Mn2(CN)6for obtaining a thermodynamically stable phase of Na2FeMn(CN)6. The phase stabilities of Na2Fe2-xMnx(CN)6among many atomic configurations and lattice parameters originating from octahedral structures (i.e., Fe(CN)6and Mn(NC)6) are highly dependent on the electronic structures of TMs with spin states. From partial density of states (PDOS) and spatial electron distributions, it was observed that Fe2+in the low-spin state (t) and Mn2+in the high-spin states (t and e) in the stable phase lead to higher redox potentials (∼3.55 V vs. Na/Na+) with the removal of Na+as compared to that of Na2Fe2(CN)6. In addition, lattice parameters from x = 0 to x = 1 in Na2Fe2-xMnx(CN)6are increased due to the larger ionic radius of Mn2+in the high-spin states. On the other hand, Fe2+in the high-spin states (t and e) and Mn2+in the low-spin state (t) in the most unstable phase of Na2FeMn(CN)6would have lower redox potentials. Based on the fundamental correlation between redox potentials and CFT with spin configurations of TMs, we suggest a material design concept for intercalation compounds with higher energy densities for rechargeable battery systems.
authors.affiliation: Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul 08826, Republic of Korea. mhcho@snu.ac.kr.
authors.forename: Duho
authors.initials: D
authors.lastname: Kim
authors.forename: Taesoon
authors.initials: T
authors.lastname: Hwang
authors.forename: Jin-Myoung
authors.initials: JM
authors.lastname: Lim
authors.forename: Min-Sik
authors.initials: MS
authors.lastname: Park
authors.forename: Maenghyo
authors.initials: M
authors.lastname: Cho
authors.forename: Kyeongjae
authors.initials: K
authors.lastname: Cho