High capacity positive electrodes for secondary Mg-ion batteries

Abstract

Composites of layered structured Birnessite–MnO2 and tunnel structured Hollandite–MnO2 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 1.0V vs. Ag/Ag+. Results indicated that Mg-ion exchanged Birnessite/acetylene black composite showed the highest discharge capacity (109mAhg−1) at 1st discharge, when compared to other microstructures of Birnessite. Meanwhile, the composite comprising of 65wt% Hol–MnO2 and 35wt% acetylene black showed very high insertion of Mg-ion (0.87Mg/Mn) corresponding to discharge capacity of 475mAhg−1 when tested at 60°C in galvanostatic mode. The layered and tunneled framework of the MnO2 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.