Delocalizing the d-electrons spin states of Mn site in MnO2 for anion-intercalation energy storage

MnO2 offers potentially the supercapacitors with high energy density due to its high theoretical capacity. However, the Na+ storage performance of MnO2 is challenged by the sluggish electron/ion transfer kinetics. Herein, we report the engineering of delocalized d-electrons spin states of Mn site through simple Ni doping in MnO2 (Ni-MnO2) to greatly boost its Na+ storage performance. Experimental results confirm that the obtained Ni-MnO2 exhibits a well-defined d-electrons configuration with delocalized spin states. Consequently, the Ni-MnO2 shows a high capacity of 327 F g−1 and an excellent rate capability of 240 F g−1 at 20 A g−1, apparently outperforming the counterpart of MnO2. Moreover, theoretical simulations suggest that the delocalization of d-electrons spin states of Mn site significantly lowers the Na+ transfer energy barrier and improves the electronic conductivity. The accelerated electron/ion transfer kinetics of Ni-MnO2 are further verified by assembling a Ni-MnO2-based asymmetric supercapacitor, which delivers a remarkable energy density of 70.8 Wh kg−1 at a power density of 3600 W kg−1. Our findings not only provide a rational strategy to boost the Na+ storage performance of MnO2 cathode, but also give a deep insight into the relationship of delocalized d-electrons spin states with the energy storage performance.