Synchronized partial metal leaching and amphoteric N–P modification in MnO2 and VOx pseudocapacitor beyond its energy density limit

Abstract

Pseudocapacitors contributed by surface redox reactions can attain higher capacitances than electrical double-layer capacitors. However, their energy density cannot be fully released when the capacitance and kinetics of the positive and negative electrodes are not balanced. Herein, activated Mo, F-doped MnO₂ (A-MMO) with surface cation vacancies (CVAs) and VO_x coordinated with amphoteric N and P species (N,P–VO_x) are designed with comparable capacitances even at high rates. For A-MMO, surface Mn and Mo vacancies generated from an Ostwald ripening process can lower the coordination number of Mn, modulate electronic structure and buffer Na⁺ insertion/extraction strains. Na⁺ ions can strongly interact with unsaturated [O] dangling bonds nearby CVAs, allowing fast redox kinetics. For N,P–VO_x, the electron-donating property of P to adjacent N atoms can suppress the attraction of electrons by N atoms from V atoms in the V–N bond, which allows high electron density around V and facilitates Na⁺ adsorption. Theoretical calculations, ex situ Raman and XRD results reveal the critical roles of CVAs and amphoteric species in regulating the electronic structure and electrochemical activity of pseudocapacitive materials. Different from using ultrahigh mass loading in the literature, the assembled A-MMO//N,P–VO_x pseudocapacitor with moderate mass loading achieves a high energy density of 15.82 mW h cm⁻³, superior mechanical flexibility, and stable energy output under deformations.