Insights into the first multi-transition-metal containing Ruddlesden–Popper-type cathode for all-solid-state fluoride ion batteries

Abstract

Promising cathode materials for fluoride-ion batteries (FIBs) are 3d transition metal containing oxides with Ruddlesden–Popper-type structure. So far, the multi-elemental compositions have not been investigated, but it could alternate the electrochemical performance similar to what has been found for cathode materials for lithium-ion batteries. In this study, we investigate RP type La₂Ni_0.75Co_0.25O_4.08 as an intercalation-based active cathode material for all-solid-state FIBs. We determine the structural changes of La₂Ni_0.75Co_0.25O_4.08 during fluoride intercalation/de-intercalation by ex situ X-ray diffraction, which showed that F⁻ insertion leads to transformation of the parent phase to three different phases. Changes in the Ni and Co oxidation states and coordination environment were examined by X-ray absorption spectroscopy and magnetic measurements in order to understand the complex reaction behaviour of the phases in detail, showing that the two transition metals behave differently in the charging and discharging process. Under optimized operating conditions, a cycle life of 120 cycles at a critical cut-off capacity of 40 mA h g⁻¹ against Pb/PbF₂ was obtained, which is one of the highest observed for intercalation electrode materials in FIBs so far. The average coulombic efficiencies ranged from 85% to 90%. Thus, La₂Ni_0.75Co_0.25O_4.08 could be a promising candidate for cycling-stable high-energy cathode materials for all-solid-state FIBs.