Characterization and electrochemical properties of Li + ion-exchanged products of hollandite-type K y(Mn 1 − xCo x)O 2 for rechargeable lithium battery electrodes

Abstract

Hollandite-type (K 0.01Li 0.42)MnO 2.08 and its Co-doped (K 0.04Li 0.34) (Mn 0.85 Co 0.15)O 2.07 have been synthesized by Li + ion-exchange of α-K 0.14Mn 1 − x Co x O 2 · zH 2O ( x = 0–0.15, z = 0.15–0.21) in a LiNO 3/LiCl molten salt at 300 °C, and characterized by X-ray diffraction (XRD), chemical analysis, N 2-sorptmetric and electrochemical measurements. XRD measurements and chemical analysis indicated that almost all the K + ions and hydrogens of the structural waters in the [2 × 2] tunnels of the precursor α-MnO 2 were exchanged by Li + ions in the molten salt, resulting in Li +-type α-MnO 2 and its Co-doped one including Li + ions as well as Li 2O in the [2 × 2] tunnels. N 2-sorptmetric measurements revealed that the Li + ion-exchanged products have a mesopore structure as well as a micropore one, in the range of 1–10 nm, and the BET surface area of the precursor α-K 0.14Mn 1 − x Co x O 2 decreased with Li + ion-exchanging, probably due to the incorporation of Li 2O molecules in the [2 × 2] tunnels. The chemical diffusion coefficients of lithium for the Li + ion-exchanged products increased by about one-order of magnitude with Li + ion-exchanging. The Li + ion-exchanged products provided higher initial discharge capacities and better recharge efficiencies than the parent materials, probably due to the structural stability with the existence of Li 2O in the [2 × 2] tunnels.