Eliminating water molecules through tailored crystal orientation to enhance the lithium storage capacity of iron oxalate†

Abstract

Iron oxalate, a coordination polymer known for its sustainability, is a potential candidate for high-capacity and high-rate anode materials for lithium-ion batteries. However, the inherent crystal water in iron oxalate significantly hampers its electrochemical activity, leading to a reduced lithium storage capability. Therefore, the removal of crystal water is essential for electrochemical performance enhancement. Here, we harnessed the differing binding strengths of water molecules between chains on various crystal planes of iron oxalate dihydrate, using dihydrate crystal plane regulation strategies to enhance the removal of water molecules. Specifically, by introducing ethanol molecules, which have hydroxyl oxygen characteristics similar to those of water molecules, we modified the surface energy of the original crystal planes resulting in the exposure of (202) crystal planes in iron oxalate dihydrate materials. This crystal plane regulation effectively enhances the release kinetics of crystal water. Furthermore, due to the optimal dehydration capability of the (202) facets and the enhanced capacity of lithium-ion diffusion, the dehydrated FeC₂O₄-12 h nanorods with exposed (202) facets exhibited an impressive discharge capacity of 850 mA h g⁻¹ at a high rate of 5 A g⁻¹. This work not only proposes a strategy for removing crystal water from electrode materials but also introduces a crystal facet regulation method, offering new avenues for enhancing the electrochemical performance of transition metal oxalate materials in areas such as proton batteries, electrocatalysis, and solid electrolytes for proton conductors.