Mechanism of the Na-substituted spinel phase generation in a li4Ti5O12 electrode via sodium-ion battery cycling

Abstract

Spinel lithium titanate (LTO; Li4Ti5O12) attracts much attention as a negative electrode material for a sodium-ion battery (SIB), while large volume changes in Na-insertion and extraction processes prevent practical applications of LTO-based electrodes. It is desirable to form a Na-substituted LTO phase as (Na3)8a(LiTi5)16d(O12)32e, which is expected to show excellent performance in a SIB, due to a small volume change from a Na-inserted phase, (Na6)16c(LiTi5)16d(O12)32e, analogous to a strain-free LTO electrode in a LIB. In this work, we have discovered that such a Na-substituted phase is really formed via the discharge (Na-extraction) process from a Na-inserted LTO electrode consisting of two phases as (Na6)16c(LiTi5)16d(O12)32e and (Li6)16c(LiTi5)16d(O12)32e. The Na-substituted phase generation occurs by the discharge with a high current density about 10 C rate, which induces high electrochemical polarization, exceeding the Li-extraction electrochemical potential in the discharging cell voltage. Thus both the Na-substituted LTO and pure LTO phases are formed due to the extraction of both Na+ and Li+ ions as an electrochemical equilibrium process. The present finding is a significant step toward practical application of the LTO-based electrode in a SIB.