Preparation of Layered MnO2 via Thermal Decomposition of KMnO4 and Its Electrochemical Characterizations

We report here the preparation of layered MnO2 and the preliminary results on its cathodic performance in Li secondary batteries. The thermal decomposition of KMnO4 powder at 250−1000 °C in air produces KxMnO2+δ·yH2O (x = 0.27−0.31, δ = 0.07−0.13, and y = 0.47−0.89) with a product yield of 67−79% based on the Mn molar quantity. It can be judged from the Rietveld refinement on the X-ray diffraction pattern that the 800 °C-prepared sample has a layered structure (hexagonal unit cell, space group = P63/mmc, a = 2.84 Å, and c = 14.16 Å), where the K+ ions and H2O molecules reside at the interlayer trigonal prismatic sites (P2-type structure). Contrary to the previous findings whereby the layered MnO2 transforms to α-/γ-MnO2 phases or manganese suboxides at >450 °C, such impurities are negligible in this synthesis even at higher temperatures. The success of synthesis is ascribed to the high population of K+ ions in the pyrolyzing media that act as pillaring cations to stabilize the layered framework. In addition, the absence of a suboxide transition is indebted to the highly oxidizing species such as O2, MnO42- and MnO43-, which are produced during the pyrolyzing process. The materials show a powder density as high as 1.36 g cm-3 and the Mn4+ fraction of >85%, which gives a theoretical capacity of 210−230 mA h g-1 based on a one-electron charge/discharge reaction. A higher product yield up to >98% is achieved by pyrolyzing KMnO4 with an addition of manganese suboxides (Mn2O3, Mn3O4, or MnO). Finally, the preliminary cell tests show that the materials give some promising features as the cathode materials for Li secondary batteries.