Anomalous decrease in structural disorder due to charge redistribution in Cr-doped Li4Ti5O12 negative-electrode materials for high-rate Li-ion batteries

Abstract

Since one of the main drawbacks of Li₄Ti₅O₁₂ as a negative-electrode material is its low electronic conductivity, several researchers have attempted to improve the conductivity by narrowing the band gap through transition-metal doping. Herein, we report another, more significant effect of doping in addition to the band gap narrowing, namely, an anomalous decrease in the structural disorder in Li₄Ti₅O₁₂ upon Cr³⁺-ion doping. Although it is generally recognized that doping with heterogeneous elements increases the structural disorder, the Cr³⁺-ion doping in Li₄Ti₅O₁₂ demonstrated an unexpected structural phenomenon. From the results of various structural analyses using a synchrotron beam, such anomalous structural changes were revealed to originate from charge redistribution at nearby Ti⁴⁺ ions. Finally, the capacity was markedly enhanced, especially at high C-rates (125 mA h g⁻¹ for 10C charge/10C discharge, 145 mA h g⁻¹ for 1C charge/50C discharge) because of both the band gap narrowing and the increased ionic diffusivity due to the decreased structural disorder, but was decreased instead for too-high doping levels.