Surface depth-dependent and microstructural analyses of high performance lithium iron silicate with 15% vanadium modification

Abstract

The surface, interface and microstructure studies were carried out for the nominal 15% vanadium modified carbon coated lithium iron silicate composite in an effort to understand the enhanced electrochemical performance and decayed cycle performance. The empirical formula was determined to be Li2.09Fe0.78V0.22Si1.29O6.29. It was confirmed that the surface of the as-prepared composite was coated by amorphous carbon with the thickness of approximately 50 nm. Furthermore, the predominant crystal phase changed from monoclinic P21 to orthorhombic Pmn21 after the vanadium modification. The simultaneous presences of multivalent iron (Fe) and vanadium (V) ions were identified by performing multi-peak fitting analyses in the depth-dependent X-ray photoelectron spectroscopic spectra of Fe2p and V2p. The detailed microstructure, interface and surface characterizations provided strong evidence that the partial substitution of divalent iron (Fe2+) with divalent vanadium (V2+) in the structure and the surface modification by vanadium might become possible, leading to 29.1% increase in total carbon content, and 35.8% and 41.3% decreases in the interfacial charge transfer resistance and lithium–ion diffusion coefficient, respectively. The faster interfacial charge transfer kinetics greatly enhanced the initial discharge capacity, while the slower lithium–ion diffusion process apparently degraded the cycle performance.