Transition metal carbonates/oxalates for advanced lithium storage: Optimization strategies, further faradic reactions and capacitive/interfacial charge storage

Transition metal carbonates/oxalates (MCO₃/MC₂O₄, M = Mn, Fe, Co, Ni, Cu, etc.) have attracted considerable attention as promising anodes for lithium-ion batteries (LIBs) with high capacities of 1600–1900 mAh g⁻¹, low cost and abundant resources. However, the large bulk particles synthesized by conventional routes possess intrinsic drawbacks of severe volume expansion and sluggish charge transfer kinetics, resulting in poor cycle and rate performance. Here, three efficient strategies to optimize the electrochemical properties of MCO₃/MC₂O₄ are presented. First, the construction of nano, micro-nano hierarchical, porous or hollow structures can shorten the ion transport distance, increase the electrode/electrolyte contact area and accommodate the volume expansion. Second, compositing with functional additives of carbon, polymers or inorganics can buffer the volume changes, improve the conductivity and stabilize the electrode/electrolyte interface. Third, doping with heterometallic ions to form polymetallic solid solutions can promote Li⁺ diffusion through the lattice defects and arouse the synergistic effect between different metal ions. Meanwhile, the novel energy storage mechanism of MCO₃/MC₂O₄ is still unclear enough beyond the originally hypothesized reactions of MCO₃ + 2Li⁺ + 2e^- ⇌ M + Li₂CO₃ and MC₂O₄ + 2Li⁺ + 2e^- ⇌ M + Li₂C₂O₄ with low theoretical capacities of ∼460 and 370 mAh g⁻¹, respectively. Remarkably, four frequently proposed novel lithium storage mechanisms of MCO₃/MC₂O₄ are comprehensively introduced, including further oxidation of M²⁺ ions to M^x+ ions (x > 2), deep lithiation of CO₃^2-/C₂O₄^2- to Li₂O/Li_xC₂ (x = 0, 1 or 2), capacitive contribution and interfacial charge storage. Finally, the future prospects for the rational design and theoretical study of MCO₃/MC₂O₄ anodes for advanced LIBs are highlighted.