Development of Selenium Substituted Lithium Titanium Sulfide Materials (Li₂TiSe_xS_3-x) As Positive Electrodes for Li-Ion Batteries

Rock salt materials are experiencing a comeback and being reconsidered as promising positive materials for lithium ion batteries due to recent advancements. This reappraisal has been accelerated by the latest studies1 which have shown that the overlithiated compositions could provide much higher capacities (> 250 mAh/g) than the stoichiometric compositions. As the material becomes richer in lithium, the probability for Li+ ions finding an available space in the lattice increases due to the percolating network that is formed through the lithium diffusion channels2. Thus, greater material capacities can be achieved. Lithium titanium sulfide (Li2TiS3), which has been reported by Sakuda et al., demonstrated excellent capacity owing to the multi electron redox reactions3. By utilizing both sulfur and titanium redox, more than two lithium ions were reversibly intercalated through the structure and the capacity reached 425 mAh/g upon cycling between 3 V and 1.5 V vs Li+/Li. Although Li2TiS3 possesses such a large reversible capacity, its cycling stability is rather poor. Therefore, new approaches are necessary to enhance its cyclability. Partial substitution mechanisms are promising solutions in this issue. Here, we propose Se-substituted lithium titanium sulfide materials (Li2TiSexS3-x), which have recently been patented. By applying our patented synthesis process, Se substituted materials showed better cycling stability. For comparison, we will provide a comprehensive study of pure and substituted materials through fine characterization tools (XRD, SEM, EDX, ex situ and in situ XRD) in order to examine electrochemical and structural properties as well as the capacity degradation mechanism. References: [1] N. Yabuuchi, Proc. Natl. Acad. Sci., 112 (25), 7650–7655 (2015) [2] J. Lee, Science, 343 (6170), 519-521 (2014) [3] A. Sakuda, Sci. Rep, 4 (1), 4883 (2015) Figure 1