Two-Dimensional WS@Nitrogen-Doped Graphite for High-Performance Lithium Ion Batteries: Experiments and Molecular Dynamics Simulations.

As promising candidates for anode materials in lithium ion batteries (LIB), two-dimensional tungsten disulfide (WS) and WS@(N-doped) graphite composites were synthesized, and their electrochemical properties were comprehensibly studied in conjunction with calculations. The WS nanosheets, WS@graphite, and WS@N-doped graphite (N-graphite) exhibit outstanding cycling performance with capacities of 633, 780, and 963 mA h g, respectively. To understand their lithium storage mechanism, first-principles calculations involving a series of ab initio NVT- NPT molecular dynamics simulations were conducted. The calculated discharge curves for amorphous phase are well matched with the experimental ones, and the capacities reach 620, 743, and 915 mA h g for WS, WS@graphite, and WS@N-graphite, respectively. The large capacities of the two composites can be attributed to the tendency of W and Li atoms to interact with graphite, suppressing the formation of W metal clusters. In the case of WS@N-graphite, vigorous amorphization of the N-graphite enhances the interaction of W and Li atoms with the fragmented N-graphite in such a way that unfavorable Li-W repulsion is avoided at very early stage of lithiation. As a result, the volume expansion in WS@graphite and WS@N-graphite is calculated to be remarkably small (only 6 and 44%, respectively, versus 150% for WS). Therefore WS@(N-)graphite composites are expected to be almost free of mechanical pulverization after repeated cycles, which makes them promising and excellent candidates for high-performance LIBs.