Sulfurization synthesis of a new anode material for Li-ion batteries: understanding the role of sulfurization in lithium ion conversion reactions and promoting lithium storage performance

Abstract

Development of electrode materials with high capacity, good cycling stability and excellent rate performance is of great importance to promote the applications of Li-ion batteries (LIBs) in electric vehicles and many other electronic devices. Transition metal oxides (TMOs) with metal ions at high valence states are promising anode materials which deliver high energy capacity, since they allow for the storage of a high number of Li ions. They, however, suffer from low electrical conductivity. Here, MnOS@NSC with MnOS microspheres encapsulated in N, S co-doped carbon shells with a core–shell structure synthesized by a sulfurization method has been reported to be a potential anode material for LIBs with high capacity, good cycling stability and excellent rate performance. Specifically, it can deliver a stable reversible capacity of 1400 mA h g⁻¹ at 100 mA g⁻¹. Even at a high current rate of 1.0 A g⁻¹, it still deliver a stable reversible capacity of 1052.3 mA h g⁻¹. The rate performance investigation indicates that the MnOS@NSC can deliver reversible capacities of 1339.3, 1204.1, 1073, 928.2, 734.7, 506.5 and 290.1 mA h g⁻¹ at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g⁻¹, respectively, much higher than those of most transition metal based anodes reported. The in-operando XRD and CV investigations show that the lithiation process of the MnOS@NSC includes the insertion of Li⁺ ions into the lattice of MnOS and the reduction of MnOS to MnO/MnS and further to Mn⁰, clearly demonstrating that Mn is at a high valence state of +4. The DFT calculations demonstrate that sulfurization is crucial for the high performance of the MnOS@NSC. First, it increases the valence state of Mn, allowing for the storage of a high number of Li atoms. Second, it improves the electrical conductivity of MnOS, facilitating charge transfer and reducing the energy losses caused by the polarization resistance. Third, it decreases the energy barrier for Li diffusion, which promotes fast lithiation and delithiation, allowing for good rate performance.