Stabilization of Tunnel Manganese Oxide Electrodes in Li-Ion and Na-Ion Batteries

Abstract

Tunnel structured manganese oxides, built from corner and edge sharing MnO6 octahedra arranged around stabilizing cations and water molecules, are of significant interest for energy storage applications such as Li-ion and Na-ion batteries (LIBs and SIBs, respectively) due to their low cost, environmental friendliness, and high electrochemical activity. Moreover, the structural tunnels in these materials act as 1D diffusion channels and provide crystallographic sites for ion insertion. However, tunnel manganese oxide phases face challenges in regards to long term cycling stability in intercalation-based batteries, which has been attributed to structural instabilities resulting from Jahn-Teller distortions induced by manganese reduction [1, 2]. Therefore, it is imperative to stabilize the structure of tunnel manganese oxides for improved electrochemical stability as cathode materials in intercalation-based batteries. Here, we demonstrate stabilization of a newly reported tunnel manganese oxide phase, 2xn-MnO2, with the crystal structure consisting of 2x2, 2x3, and 2x4 octahedra tunnels containing Na+ stabilizing cations and water molecules. Building off of previous work by C.S. Johnson et al. [3], where the smaller tunnel structured α-MnO2 was stabilized with Li2O, we modified the structure and chemical composition of 2xn-MnO2 via mixing in methanol and lithium hydroxide, resulting in insertion of Li2O into the structural tunnels of 2xn-MnO2. In LIBs, the Li2O-stabilized 2xn-MnO2 demonstrates both higher initial capacity and superior capacity retention after 100 cycles. The initial capacity is improved from 93 mAh g-1 for the pristine material to 168 mAh g-1 for the stabilized 2xn-MnO2. Moreover, the galvanostatic discharge-charge profile for the stabilized 2xn-MnO2 exhibits a better pronounced plateau-shaped region compared to the original 2xn-MnO2 phase, indicating that the stabilized material contains more well-defined insertion sites. Similarly, in SIBs, the capacity of the stabilized 2xn-MnO2 was improved by 35% to 81 mAh g-1, and capacity retention increased from 43% to 75% after 100 cycles, demonstrating the effectiveness of this stabilization approach in both Li-ion and Na-ion battery systems. Thus, we show that stabilization with Li2O can be applied to various tunnel manganese oxide phase to improve their electrochemical performance as intercalation-based battery electrodes. References Y. Yuan et al., Nano Energy (2016) 19, 382A. Tompsett and M.S. Islam, Chem. Mater. (2013) 25, 2515S. Johnson et al., J. of Power Sources (1997) 68, 570