"Pillar effect" engineering to achieve the enhanced energy storage for self-supported manganese dioxide

Manganese dioxide (MnO2) is an extremely vital electrode material in aqueous electrochemical energy storage devices. However, due to the dissolution of Mn and the damage caused by the embedded/de-embedding of the energy storage ions during charge/discharge, the exhibited poor stability limits its application. Here, the α-MnO2 lattice is stabilized by the introduction of alkali metal ions (Li+, Na+, and K+) to create a "pillar effect". A series of characterizations indicate that alkali metal ions are embedded in the lattice tunnel of α-MnO2 due to the induced drive of constant negative voltage at the surface of the MnO2 electrode. The embedded alkali metal ions will exchange electrons with surrounding oxygen atoms, thus mitigating the destruction of the MnO2 crystal structure during charge/discharge. In response, the specific capacitances of the optimal Li+, Na+, and K+ ion-embedded samples are increased by about 210%, 197%, and 182% than that of pure-MnO2 samples. Theoretical calculations indicate that the embedding of alkali metal ions can reduce the system energy and promote the migration of carriers, which further enhance the electrochemical properties of MnO2. This work is expected to inspire and contribute to the development of efficient and durable cathode materials for supercapacitors in the future.