Understanding the high performance of nanosized rutile TiO2 anode for lithium-ion batteries

Abstract

The so-called “nanorutile TiO 2 high performance anode material” is somewhat misleading. Below 1.2 V, rutile undergoes an irreversible phase transformation into disordered rock salt-type c-LiTiO 2 . During repeated charge-discharge cycling in the 3.0–1.0 V range, the cubic structure of LiTiO 2 is retained. It is capable of reversibly storing lithium through a solid solution with a noteworthy little volume change as demonstrated by ex situ XRD in samples taken at selected cycles. Interestingly, its good power rate behaviour is not due to fast diffusion of lithium as, in fact, it is strongly inhibited (D Li+ ≈ 10 −15 cm 2 s −1 ). Most of its capacity arises from capacitive processes. At 0.3 mV s −1 and higher sweep rates it accounts for more than 50% of total specific capacity, and the capacitive contribution is ca. 90% at 1.5 mV s −1 . This work should boost new research on direct synthesis of cubic rock salt-type LiTiO 2. • Nanorutile transforms irreversibly in rock salt-type c-LiTiO 2 upon reaction with Li. • c-LiTiO 2 reversibly stores Li through a solid solution with little volume change. • c-LiTiO 2 and not rutile is the true high performance anode. • High capacity at high current mostly due to capacitive processes. • Significant contribution of faradic processes at low current.