High capacity manganese layered-perovskite cathode for fluoride ion batteries involving cationic and anionic redox reaction

Abstract

Developing electrochemical high-energy storage systems is of crucial importance towards a green and sustainable energy supply. A promising candidate is fluoride ion batteries (FIBs), which can deliver a higher energy density than is possible with lithium ion batteries1,2. However, conversion-type reactions with metal fluorides causes a poor electrochemical reversibility1,3,4. Recently, layered perovskite oxides such as LaSrMnO4 have been shown to undergo topotactic electrochemical (de)fluorination, but they have low reversible discharge capacities (25 ~ 100 mAh/g) and poor rate capabilities. Here we show that a double-layered perovskite oxyfluoride La1.2Sr1.8Mn2O7–δF2 exhibits topotactic (de)intercalation reaction inside the rock-salt slabs, achieving a large reversible capacity of 535 mAh/cm3 (0 ≤ x ≤ 2 in La1.2Sr1.8Mn2O7–δFx), with excellent cycle stability and rate capability. Surprisingly, despite the close-packed perovskite-based structure, two extra fluoride ions are (de)intercalated beyond x = 2, leading to a reversible capacity of 1168 mAh/cm3 (0 ≤ x ≤ 4). During the further intercalation, oxygen molecules are formed in the perovskite layer, as in Na0.75[ Li0.25Mn0.75 ]O25, which is responsible for the charge compensation (i.e. anion redox)5,6 and the concomitant formation of oxygen vacancies that allow the incorporation of the excess fluoride ions. These results highlight the layered perovskite oxide/oxyfluorides as a new class of active materials for the construction of high-performance FIBs. More generally, the concept of anion-intercalation through O2 formation in the mixed-anion perovskite materials can be used to develop new functionalities.

The case for fluoride-ion batteries

“…62 The realization of a reversible high-capacity fluoride intercalation cathode was reported in a recent preprint by Miki et al by utilizing oxygen redox instead of a multi-electron transition metal redox. 64 In this case, a reversible intercalation of nearly two fluoride ions per transition metal was demonstrated in a La 1.2 Sr 1.8 Mn 2 O 7 compound with a Ruddlesden-Popper (n = 2) structure (AO(AMO 3 ) 2 ). While Fions are typically intercalated into the rocksalt (AO) layers of Ruddlesden-Popper compounds, the extra fluoride in this case was reported to occupy an oxygen vacancy generated by the formation of O 2 in the Perovskite layer.…”

“…LiCoO 2 ). 58,[61][62][63][64][65][66] These fluoride intercalation electrodes have several established advantages over conversion-type electrodes such as higher cathode potentials and lower volume changes on (de)fluoridation. Intercalation reactions typically also exhibit superior rate capability and improved cycle life as they depend on the diffusion of only one species and do not require the formation and migration of metal/metal fluoride interfaces.…”

“…In fact, oxygen redox has recently been reported in a preprint at ∼4.7 V vs. Li + /Li, in the case of a La 1.2 Sr 1.8 Mn 2 O 7 FIB cathode. 64 The fluoride intercalation electrodes explored thus far in the literature exhibit either the layerstructured Ruddlesden-Popper (AO(AMO 3 ) n ) or the tunnel-structured Schafarzikite (MSb 2 O 4 ) crystal systems (where M is the redox active transition metal). 62,68 Fluoridation of the perovskite BaFeO 2.5 has also been reported.…”

“…67,70 The second system we model represents a hypothetical near-future, best-case scenario employing La 1.2 Sr 1.8 Mn 2 O 7 as a reversible oxygen-redox cathode and the layered electride, Ca 2 NF, as a low-potential, high-capacity anode. 64,71 In terms of energy density, every other potentially favorable case (e.g. La 2 NiO 4 with a two-electron transfer) that is currently known would fall between these two systems.…”