High capacity positive electrodes for secondary Mg-ion batteries

Composites of layered structured Birnessite-MnO ₂ and tunnel structured Hollandite-MnO ₂ in presence of acetylene black were synthesized as positive electrode materials for rechargeable Mg-ion batteries. Reversible insertion/extraction of Mg-ion in the host structures was examined in the potential range of -1.8 to 1.0 V vs. Ag/Ag ⁺. Results indicated that Mg-ion exchanged Birnessite/acetylene black composite showed the highest discharge capacity (109 mAh g ^-1) at 1st discharge, when compared to other microstructures of Birnessite. Meanwhile, the composite comprising of 65 wt% Hol-MnO ₂ and 35 wt% acetylene black showed very high insertion of Mg-ion (0.87 Mg/Mn) corresponding to discharge capacity of 475 mAh g ^-1 when tested at 60°C in galvanostatic mode. The layered and tunneled framework of the MnO ₂ was retained with minor displacive adjustments even after substantial Mg-ion insertion/extraction after several cycles. However, large specific capacity loss was observed after 20 cycles in all of the microstructures probably due to Mg-ion trapping in the host lattice. Furthermore, the effect of the cation (K ⁺) present in the tunnel of Hollandite on Mg-ion diffusion was analyzed as well and it was concluded that tunnel cation could impede the movement of Mg-ion in host structure.