Improvement of the rate capability of the Li4Ti5O12 anode material by modification of the surface composition with lithium polysulfide

Abstract

Partial substitution of oxygen by sulfur was attempted using a Li/Ti rich conditions for the nominal compositions Li4Ti5O12-xSx x = 0.125–0.375. However, a much lower sulfur substitution, i.e. at dopant level, was achieved as deduced from an almost negligible increase in unit cell size observed from X-ray powder diffraction. The sulfur in the samples is mainly deposited on the surface as sulfate and Li2S2 as deduced from XPS. However, most of the nominal sulfur is lost. Interestingly, for nominal x = 0.125 a considerable improvement in performance is achieved compared to undoped LTO. The sample with x = 0.125 not only shows ~9% higher capacity at slow cycling at C/20, but also has a remarkably better power rate behavior, with a capacity of 125 mAh g-1 at higher 2 C, compared to 79 mAh g- 1 for LTO at the same rate. The significant improvement is explained by the formation of lithium polysulfide in the high-temperature reaction and the concomitant Li+ deficiency to form the spinel, causing the reduction of Ti4+ to Ti3+ and the creation of oxygen vacancies with the formation of an oxygen and lithium deficient spinel Li4-y (Ti4+)5-y (Ti3+)y O12-y. Li2S2, detected in the sample with nominal x = 0.125 by XPS and at higher nominal sulfur contents by CV, may prevent carbonation of the spinel surface while providing ionic conductivity upon reduction to Li2S, justifying its better performance. On the other hand, the contribution to capacity from both mass diffusion and the capacitive charge storage mechanism is unaffected, and therefore LTO and surface-modified LTO-S-0.125 are regarded as typical battery materials.