Enhanced symmetric lithium-ion batteries: utilizing polyhedral structures constructed from ultrafine Li₂FeSiO₄/C nanoparticles as dual-function cathode and anode materials

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 its further development. In this work, polyhedral structures assembled from ultrafine Li₂FeSiO₄ nanoparticles (2-10 nm) were successfully synthesized via an in situ confined chelation strategy. Citric acid acted 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 nanoparticles with enhanced Li⁺ ions and electron transport pathways. Additionally, the polyhedral structure also exposed more active facets for Li⁺ ions diffusion, leveraging reversible Fe⁰/Fe²⁺, Fe²⁺/Fe³⁺ and Fe³⁺/Fe⁴⁺ redox reactions. The optimized sample showed a high capacity (178.7 mA h g⁻¹ at 0.03 A g⁻¹ and 660.9 mA h g⁻¹ at 0.4 A g⁻¹) and superior cycling stability (126.9 mA h g¹ at 0.1 A g⁻¹ and 678.1 mA h g⁻¹ at 0.5 A g⁻¹ after 200 cycles). The Li₂FeSiO₄//Li₂FeSiO₄ symmetric full battery achieved a reversible capacity of 141.1 mA h g⁻¹ at 0.03 A g⁻¹ with excellent cycling stability. Electrode kinetics and phase transitions were further analyzed using in situ EIS. This work presents a simple and efficient method for synthesising ultrafine nanocrystals and opens new possibilities for symmetric lithium-ion batteries.