Synthesis and electrochemical characterization of Ge⁴⁺, Sn⁴⁺ doped spinel LiMn₂O₄

Spinel LiMn₂O₄ materials doped with tetravalent cations Ge⁴⁺ and Sn⁴⁺ were synthesized through solid-state reaction. Analysis of the materials by X-ray diffraction (XRD) and scanning electron microscopy (SEM) suggested that Ge⁴⁺ ions occupied octahedral sites by substituting Mn⁴⁺ ions in the spinel structure to form the solid solution LiMn_2-xGe_xO₄ (x=0.02, 0.04, 0.06), while Sn⁴⁺ ions were present at the surface of the spinel LiMn₂O₄ as SnO₂. The substitution of Mn⁴⁺ with Ge⁴⁺ could suppress the long-range ordering of the Li⁺ ions in the spinel LiMn₂O₄, enhancing its stability. SnO₂ on the surface of LiMn₂O₄ could reduce the acidity of the liquid electrolyte, suppressing acid etching of the LiMn₂O₄ active material. Galvanostatic charge/discharge tests showed that both Ge⁴⁺ and Sn⁴⁺-modified spinel LiMn₂O₄ materials exhibited significantly higher capacity retention than LiMn₂O₄. The increased capacity retention should benefit the application of spinel LiMn₂O₄ as a cathode material for lithium-ion batteries.