Lithium-rich layered rock-salt type Li₂MnO₃ is one of the most attractive cathode materials for lithium batteries because it achieved the high discharge capacities of 250−300 mAh g^-1 after activation during the first cycling [1]. However, the structural deterioration in the following cycles at the cathode/liquid electrolyte interface leads to severe capacity fading. We reported that the Li₂MnO₃ films on solid system exhibit a high capacity of 250 mAh g^–1 and better cycle stability, although liquid system showed severe capacity degradation [2]. To elucidate the activation process and high capacity phase contributing to high cycle stability, crystal structure changes at activation process were analyzed using the different initial structure of Li₂MnO₃. The initial structure was controlled by stacking solid electrolyte using two types of synthesis methods.

The Li₂MnO₃ electrode films were synthesized on SrRuO₃/SrTiO₃(111) substrates by pulsed laser deposition (PLD). Amorphous Li₃PO₄ as the solid electrolyte was synthesized by PLD and magnetron sputtering. Lithium metal was used as the negative electrode, respectively. The crystal structure changes at activation process were analyzed by in situ X-ray diffraction (XRD) measurements.

The Li₂MnO₃ structure had layered rock-salt type structure after stacking Li₃PO₄ by using PLD. On the other hand, in case of stacking by using magnetron sputtering, the Li₂MnO₃ changed disordered structure with the transition metal (TM) layer disordering of lithium and manganese ions in the honeycomb lattice. For the Li₂MnO₃ with layered rock-salt type structure, the discharge capacity increased continuously in the following cycles with persisting plateau region for several cycles. On the other hand, the Li₂MnO₃ with disordered structure showed about 300 mAh g^–1 at the first cycle. The XRD intensity ratio of I₀₂₀/I₀₀₁ was zero after activation at both of initial structures [3]. Since the 020 peak arises from a superlattice structure by a honeycomb-type ordered arrangement of Li and Mn atoms in the TM layer, atomic arrangements in the TM layer transformed to a disordered state. This high capacity phase could contribute to its high capacity and cycle stability.

Acknowledgment: This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 (RISING2) of the New Energy and Industrial Technology Development Organization (NEDO).

References: [1] M. Sathiya et al., Nat. Mater., 2013, 12, 827-835.[2] K. Hikima et al., The 57th battery symposium in Japan, 2016, 2G22. [3] K. Hikima et al., The 58th battery symposium in Japan, 2017, 2C22.