Electrochemical performance of Li2MnSiO4/C lithium-ion cathode materials with low-level doping of Ru in the Mn site

Lithium manganese orthosilicate (Li2MnSiO4, LMS), a prospective high-voltage lithium-ion battery cathode material, exhibits poor cycling performance due to structural instabilities during charge and discharge. To alleviate cathode degradation and understand the effects of transition metal dopants on the material properties, Mn was partially substituted by Ru between 0.5 and 3 mol% in carbon-coated LMS (LMS/C) cathode. Undoped LMS/C had orthorhombic LMS structure in the Pmn21 space group. While Ru doping of 0.5 and 1 mol% yielded phase-pure orthorhombic Pmn21 phase, Ru doping of 3 mol% yielded an additional orthorhombic Pmnb phase. The galvanostatic charge-discharge profiles showed a slight improvement in first discharge capacity for 0.5 mol% Ru-doped LMS/C compared to undoped LMS/C, but the discharge capacity decreased with higher doping levels. The capacity retention, however, improved with increasing doping level. In situ XRD results showed that Ru-doped LMS/C was able to retain a higher level of crystallinity than undoped LMS/C during the first charge. By the end of the first charge, in situ XANES of LMS/C revealed that Mn2+ in [MnO4] was partially oxidized to Mn3+ in [MnO6] ligand upon first charging, which coincided with a reduction in crystallinity of LMS/C. As shown by in situ XANES, Ru doping decreased the extent of oxidation of Mn2+ to Mn3+ upon charging. The results revealed that Ru doping of LMS/C between 0.5 and 3 mol% in the Mn site could help maintain Li2MnSiO4 crystallinity during the initial charging step.