The synergic effects of Ca and Sm co-doping on the crystal structure and electrochemical performances of Li4-xCaxTi5-xSmxO12 anode material

Abstract

Li4Ti5O12 as the well-known “zero strain” anode material for lithium ion batteries (LIBs) suffers from low intrinsic ionic and electronic conductivity. The strategy of lattice doping has been widely taken to relieve the intrinsic issues. But the roles of the dopants are poorly understood. Herein, we propose to modulate the crystal structure and improve the electrochemical performance of Li4Ti5O12 by substituting Li and Ti with Ca and Sm, respectively. The roles of Ca and Sm on the crystal structure and electrochemical performances have been comprehensively investigated by means of X-ray diffraction (XRD), neutron diffraction (ND) and electrochemical analysis. The Rietveld refinement of ND data indicate that Ca and Sm prefer to take 8a site (tetrahedral site) and 16d site (octahedral site), respectively. Li3.98Ca0.02Ti4.98Sm0.02O12 has the longer Li1-O bond and shorter Ti-O bond length which reduces Li+ migration barrier as well as enhances the structure stability. Ca-Sm co-doping also alleviates the electrode polarization and enhances the reversibility of oxidation and reduction. In compared to bare Li4Ti5O12 and Li3.95Ca0.05Ti4.95Sm0.05O12, Li3.98Ca0.02Ti4.98Sm0.02O12 electrode shows the lower charge transfer resistance, higher Li+ diffusion coefficient, better rate capability and cycling performance. The proposed insights on the roles of dopants are also instructive to design high performance electrode materials by lattice doping.