Investigation of Ruddlesden Popper-Type Oxide as an Intercalation Based Cathode Material for All-Solid-State Fluoride-Ion Batteries

Increasing use of Lithium-ion batteries (LIBs) has led to and will further lead to the depletion of lithium reserves. In response, investigations are being done on other cations (Na⁺) and anions (F^-, Cl^-)¹ as shuttling ions for battery systems. Recently, fluoride ion batteries (FIBs) are considered to as an alternative for all-solid-state batteries², for which cells based on conversion-based cathode materials can provide high specific capacity at the cost of fast capacity fading on cycling^{3, 4}. This can be understood from the fact that the conversion mechanism involves large degree of atom organisation, changes in chemical bonds and massive volume changes upon cycling. This has been shown to be prevented by using intercalation-based cathode materials, which drastically reduce the volume changes due to the possible intercalation and de-intercalation of ions from the host lattice⁵. This not only led to higher cycling stability but also facilitates the lowering of overpotentials⁶. In this respect, different second-generation intercalation-based materials have been derived from initially studied materials^5-7, among which Ruddlesden-Popper type La₂Ni_0.75Co_0.25O₄ have been identified for improved cycling performance⁸ and further LaSrMnO₄ / La₁Sr₂Mn₂O₇ are under investigation.
In this study, we explore the structural changes of La₂Ni_0.75Co_0.25O₄ / Pb-PbF₂, LaSrMnO₄ / Pb-PbF₂ and La₁Sr₂Mn₂O₇ / Pb-PbF₂ cells during fluoride intercalation and de-intercalation by using X-ray diffraction (XRD) and electrochemical characterization methods. By X-ray diffraction analysis of cells cycled to different cut-off conditions, reveal an increase of the unit cell along the c-axis and contraction in the ab-plane. The detailed complex reaction behaviour of the phase, focusing on changes in the oxidation states and co-ordination environments of Ni and Co in La₂Ni_0.75Co_0.25O₄ was studied via X-ray absorption spectroscopy (XAS). Under optimized operating conditions, we achieved a cycle life of 120-cycles at a critical cut-off capacity of 40 mAh.g^-1 and over 400-cycles under a pressure of 452 MPa. The average Coulombic efficiencies ranged from 85% to 90% for the cell operated without pressure and 95 % to 99 % for the cell operated under pressure for La₂Ni_0.75Co_0.25O₄ / Pb-PbF₂ cells. Therefore, La₂Ni_0.75Co_0.25O₄ stands out as one of the promising cycling-stable high-energy cathode materials for all-solid-state FIBs, offering improved capacity ^{8, 9}.

References
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