Preparation of Layered MnO₂via Thermal Decomposition of KMnO₄and Its Electrochemical Characterizations

Abstract

We report here the preparation of layered MnO 2 and the preliminary results on its cathodic performance in Li secondary batteries. The thermal decomposition of KMnO 4 powder at 250−1000 °C in air produces K x MnO 2+ δ · y H 2 O ( 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 = P 6 3 / mmc, a = 2.84 Å, and c = 14.16 Å), where the K + ions and H 2 O molecules reside at the interlayer trigonal prismatic sites (P2-type structure). Contrary to the previous findings whereby the layered MnO 2 transforms to α-/γ-MnO 2 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 O 2, MnO 4 2- and MnO 4 3-, which are produced during the pyrolyzing process. The materials show a powder density as high as 1.36 g cm - 3 and the Mn 4+ 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 KMnO 4 with an addition of manganese suboxides (Mn 2 O 3, Mn 3 O 4, or MnO). Finally, the preliminary cell tests show that the materials give some promising features as the cathode materials for Li secondary batteries.