Tuning electronic structure of δ-MnO2 by the alkali-ion (K, Na, Li) associated manganese vacancies for high-rate supercapacitors

Abstract

Cation vacancies can bring numerous surprising characters due to its multifarious electron and orbit distribution. In this work, δ-MnO2 with alkali-ion (K, Na, Li) associated manganese (Mn) vacancies is fabricated by a simple hydrothermal reaction, and the correlation between their electronic structure and pseudocapacitance are systematically investigated. FESEM/TEM images have shown that the morphology of MnO2 is obviously changed after the introducing of cation vacancies. The position of alkali-ion in MnO2 structure can be controlled by adjusting the ion concentration. XRD patterns and Raman spectra demonstrate that the alkali-ion is embedded in Mn vacancies at low concentration, while entered the interlayer of MnO2 at high concentration. The existence of Mn vacancies will resulting in the distortion of neighboring atoms, leading to the electronic delocalization, and thus enhancing the conductivity, pseudocapacitance and rate capability of MnO2. Accordingly, the specific capacitances of optimized 0.4KMO, 0.4NaMO and 0.4LiMO samples are enhanced about 1.9, 1.6 and 1.6 times compared to pure MnO2. Meanwhile, the rate performance has also been improved about 76%, 46% and 42%, respectively. Theoretical calculations further confirm that the Mn vacancies can generate additional occupancy states and cause an increase in carrier concentration, which will improve the conductivity and further boost the pseudocapacitance of MnO2. This result open up a promising approach to explore active and durable electrode materials.