Enhanced Symmetric Lithium-Ion Batteries: Utilizing Polyhedron Structures Constructed from Ultrafine Li2FeSiO4/C Nanoparticles as Dual-Function Cathode and Anode

Li2FeSiO4 is a promising cathode material for lithium-ion batteries, due to its high capacity, low cost, and superior stability. However, its low conductivity and slow Li⁺ diffusion hinder further development. In this work, polyhedron structures constructed from ultrafine Li2FeSiO4/C nanoparticles (2-10 nm) were successfully synthesized via an in situ confined chelation strategy. Citric acid served as both a carbon source, forming a conductive carbon layer on Li2FeSiO4, and a chelating agent, confining Fe3⁺ within the tetraethyl orthosilicate network. This approach enabled the formation of ultrafine Li2FeSiO4 with enhanced Li⁺ and electron transport pathways. Additionally, the polyhedron structure also exposed more active facets for Li+ diffusion, leveraging reversible Fe0/Fe2+, Fe2+/Fe3+ and Fe3+/Fe4+ redox reactions. The optimized sample showed high capacity (178.7 mAh g-1 at 0.03 A g-1 and 660.9 mAh g-1 at 0.4 A g-1) and superior cycling stability (126.9 mAh g1 at 0.1 A g-1 and 678.1 mAh g-1 at 0.5 A g-1 after 200 cycles). The Li2FeSiO4//Li2FeSiO4 symmetric full battery achieved a reversible capacity of 141.1 mAh g-1 at 0.03 A g-1 with an excellent cycling stability. Electrode kinetics and phase transitions were further analyzed using In-situ EIS. This work presents a simple and efficient method for synthesizing ultrafine nanocrystals and opens new possibilities for symmetric lithium-ion batteries.