Synthesis and characterization of urchin-like Mn 0.33 Co 0.67 C 2 O 4 for Li-ion batteries: Role of SEI layers for enhanced electrochemical properties

A new type of hierarchical urchin-like Mn0.33Co0.67C2O4 (HU-Mn0.33Co0.67C2O4) is fabricated using a template-free chemical co-precipitation method. The morphology of this system is highly influenced by the Mn/Co ratio. The obtained urchin-like microspheres are composed of end-connected nanorods and accompanied by structural voids. When tested in lithium-ion batteries, HU-Mn0.33Co0.67C2O4 delivers a high reversible discharge capacity of 924 mAh g−1 at 500 mA g−1 in the 2nd cycle, with 83% capacity retention over 300 cycles. Rate capability testing shows that HU-Mn0.33Co0.67C2O4 can deliver discharge capacities of 734 mAh g–1 at 1 A g–1 and 414 mAh g–1 at 5 A g–1, respectively. Although the transition-metal content and conductivity of HU-Mn0.33Co0.67C2O4 are much lower than that of MnCo2O4, the material still exhibits a high specific capacity, good capacity retention, and excellent rate capability, suggesting that HU-Mn0.33Co0.67C2O4 is a promising anode material for future lithium-ion batteries (LIBs). In addition, a gradual electrochemical activation process is observed to occur in the first several cycles, relating to the gradual generation of SEI layers on the electrode surface. It is found that the SEI layers on the surface of oxalate materials are predominantly composed of lithium alkyl carbonates from the reductive decomposition of electrolyte and the catalytic products of Li2C2O4. We speculate that the formed SEI layers with high lithium ion conductivity play an important role in achieving the observed high specific capacities and excellent rate capabilities of oxalates.