Anomalous decrease in structural disorder due to charge redistribution in Cr-doped Li_4Ti_5O_12 negative-electrode materials for high-rate Li-ion batteries

摘要

Since one of the main drawbacks of Li_4Ti_5O_12 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 Li_4Ti_5O_12 upon Cr~(3+)-ion doping. Although it is generally recognized that doping with heterogeneous elements increases the structural disorder, the Cr~(3+)-ion doping in Li_4Ti_5O_12 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 Ti~(4+) 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.