Electrochemical performance of MnOx obtained through calcination-induced phase transformation of γ- and ε-MnO2 nanomaterials

γ-MnO 2 and ε-MnO 2 nanomaterials, synthesized by coprecipitation from NaMnO 4 -MnCl 2 and KMnO 4 -MnCl 2 precursors, respectively, were thermally treated under controlled conditions to produce manganese oxide nanomaterials with different crystal phases. Their structural, morphological, and electrochemical properties were investigated, highlighting the role of crystal phase and tunnel geometry in determining their capacitance and cycling stability. α-MnO 2 with 2 × 2 tunnels exhibits the highest capacitance (454.4 F g −1 at 1 A g −1 ) but suffers from capacitance fading with a retention of 80.0% after 500 cycles at 10 A g −1 . ε-MnO 2 exhibits predominantly surface-controlled electrochemical behavior, consistent with its compact structure, resulting in high capacitance retention (>90% over 500 cycles) with a capacitance of 352.9 F g −1 at 1 A g −1 . γ-MnO 2 exhibits diffusion-controlled behavior associated with its mixed tunnel structure but suffers from poor cycling stability due to defect-induced metastability. β-MnO 2 exhibits promising preliminary cycling stability despite narrow 1 × 1 tunnels, which may be attributed to reversible structural rearrangement via K + insertion. Overall, the results highlight the interplay between ion-diffusion pathways and surface reactivity in the design of high-performance MnO 2 materials for supercapacitor applications.