Functionalization of Ca2MnO4–δ by controlled calcium extraction: Activation for electrochemical Li intercalation.

Calcium manganate Ruddlesden–Popper phases Ca 2 MnO 4–δ were prepared by soft chemistry and treated with acid to extract a part of the calcium ions. Combined structural, morphological and spectroscopic analyses by XRD, SEM/EDX, TEM and Raman revealed an amorphization of the treated surface with a preserved inner crystalline phase in the core of the particles. The inner part of the particles consists of Ca 2 MnO 4–δ and the outer part of hydrated amorphous manganese oxide MnO 2 ·xH 2 O. Although pristine Ca 2 MnO 4–δ is not electrochemically active, electrochemical characterization of the acid-treated powders shows a significant change in capacity that correlates to the amount of extracted calcium. The cycling stability of the Ca 2 + extracted compounds was improved by more than a factor of 10 in comparison to pure MnO 2 ·xH 2 O. The acid-treated Ca 2 MnO 4–δ showed enhanced capacity retention of up to 50% after 70 cycles compared to 4% for bare MnO 2 ·xH 2 O due to its crystalline core. [ABSTRACT FROM AUTHOR]