Electrochemical Activation of High-Capacity Li2MnO3 Cathode Analyzed By in Situ Neutron Reflectometry

Abstract

Lithium-rich layered rocksalt oxides have attracted increasing attention as high-capacity cathode materials for lithium batteries. These cathodes undergo a structural transformation to an electrochemically active phase at the initial cycle. The reversible capacity and electrochemical stability of the active phase depends on the composition and the structure of pristine materials. Recently, we have reported that a model Li2MnO3(001) film electrode delivers a high discharge capacity of over 300 mAh g-1, and the capacity kept remained during 50 cycles [1]. The discharge capacity increases with decreasing film thickness from 47.8 nm (120 mAh g-1) through 29.8 nm (199 mAh g-1) to 12.6 nm (318 mAh g-1). Thus, we speculated that the electrochemical activation proceeds in the surface region of the model electrode, but there has been no direct information of the structure changes. In this work, direct observation of the reaction distribution in Li2MnO3 was conducted on the initial battery operation using in situ neutron reflectometry. The 30-50 nm-thick Li2MnO3(001) film electrodes were synthesized on a SrRuO3(111)/Nb:SrTiO3(111) substrate by pulsed laser deposition [1], for elucidating depth dependence of the electrochemical activation. In situ neutron reflectometry spectra were collected using SOFIA (BL-16, MLF, J-PARC), which is a time-of-flight reflectometer. An electrochemical cell was composed of the Li2MnO3/SrRuO3/SrTiO3 cathode, a Li anode and a deuterated propylene carbonate electrolyte containing 1 mol dm-3 LiPF6. The neuton scattering length density (SLD) profiles were refined with the observed spectra using the Parratt32 program. After the first charging, the SLD profile shows an increase in the 20 nm region from the top surface of the Li2MnO3(001) film, corresponding to deintercalation of Li with a negative scattering length. The decrease in the SLD is observed after the first discharging. Highly delihiated and lithiated phases form near the electrochemical interface with organic electrolytes, which result in the high charge-discharge capacities observed for nanosized Li2MnO3. A combination analysis with the neutron reflectometry and ex situ X-ray photoelectron spectroscopy suggests that the first lithium deintercalation proceeds with the charge compensation by lattice oxygen, accompanied with no significant oxygen extraction. The lithium intercalation into the delithiated phase does not lead to formation of the original Li2MnO3, indicated by detection of Mn3+ in the first lithiated phase. The structural reconstruction through no oxygen extraction could generate a high capacity phase with high electrochemical stability in the surface region of Li2MnO3. In situ neutron reflectometry gives direct information of the lithium ion distribution on battery operation, which contributes the further understanding of electrochemical reactions at the electrode/electrolyte interface. [1] S. Taminato, M. Hirayama, K. Suzuki, N. L. Yamada, M. Yonemura, J. Y. Son, R. Kanno Chem. Commun. 2015, 51, 1673-1676.