Cation-disordered Li₂FeTiO₄ nanoparticles with multiple cation and anion redox for symmetric lithium-ion batteries

Symmetric secondary batteries are expected to become promising storage devices on account of their low cost, environmentally friendly and high safety. Nevertheless, the further development of symmetric batteries needs to rely on bipolar electrodes with superior performance. Cation-disordered rocksalt (DRX) Li₂FeTiO₄ shows promising properties as symmetric electrodes, based on the ability of iron to undergo multiple electrochemical reactions over a wide voltage window. Unfortunately, this cation-disordered structure would not provide a cross-path for the rapid migration of Li⁺, ultimately resulting in inferior electrochemical dynamics and cycle stability. Herein, Li₂FeTiO₄ nanoparticles assembled by ultrafine nanocrystals are synthesized via a sol-gel method through an orderly reaction regulation strategy of precursor reactants. Such ultrafine nanocrystals increase the active sites to promote the reversibility of multi-cationic (e.g., stable Fe²⁺/Fe³⁺, Ti³⁺/Ti⁴⁺ and moderated Fe³⁺/Fe⁴⁺) and anionic redox, and maintain the DRX structure well during the cycling process. The half cells with nano-sized Li₂FeTiO₄ as the cathode/anode exhibit a high reversible capacity of 127.8/500.8 mAh/g, respectively. Besides, the Li₂FeTiO₄//Li₂FeTiO₄ symmetric full cell could provide a reversible capacity of 95.4 mAh/g at 0.1 A/g after 200 cycles. This hierarchical self-assembly by nanocrystal strategy could offer effective guidance for high-performance electrode design for rechargeable secondary batteries.