Enhanced Electrochemical Properties and Reaction Mechanism of NiTi₂S₄ Ternary Metal Sulfide as an Anode for Lithium–Ion Battery

On account of the fast development of electric vehicles and portable electronics, the need for high performance electrochemical energy storage devices has increased for past few decades. Lithium ion batteries (LIBs) have been considered as one of the most attractive candidates in many electric applications, because of their high energy density and good power performance. In recent years, transition metal sulfides have been considered as promising LIB anode materials owing to their high theoretical capacity and electrical conductivity.<br/>Among many metal sulfides, layered TiS₂ possesses the highest theoretical capacity (960 mAh g^-1). Unfortunately, as other conversion materials, TiS₂ undergoes the extensive volume change, crack formation, pulverization, aggregation, and side reactions with electrolyte during discharge/charge cycling, hence it is difficult to be applied for practical LIBs. To overcome the limitation of the conversion materials, various nanostructures and composites with carbon have been generally explored. Such modifications help the Li ions to diffuse faster and the active material to buffer the volume change and ensure the structural integrity. However, these methods do not alter the intrinsic properties of the materials but only the extrinsic properties.<br/>Ternary metal sulfides with two different metal cations, which is an intrinsic modification of conventional binary metal sulfides, have been investigated as anodes for LIBs. During lithiation/delithiation, sequential redox reactions of two metal cations were observed, resulting in the controlled volume changes. Furthermore, ternary metal sulfides have attracted much attention due to higher electrical conductivity and richer redox chemistry than conventional binary metal sulfides.<br/>NiTi₂S₄ (NTS) ternary metal sulfide had been first reported in 1968, but only its structure, electrical, and magnetic properties have been examined. Its theoretical capacity was calculated to be 759 mAh g⁻¹, assuming the full conversion reaction with Li ions. NTS possesses the highest electrical conductivity compared to other MTi₂X₄ (M = Fe, Co, Ni; X = S, Se) compounds, which is advantageous for the fast electrochemical reactions. The crystal structure of NTS is in the form of Ni atoms embedded between S−Ti−S interlayers of TiS₂, bonding with S atoms. Therefore, when NTS undergoes the conversion reaction, these Ni atoms are expected to improve the electrical conductivity of the active material and suppress the volume change during lithiation/delithiation. Moreover, such an atomic scale distribution of Ni in the active material is speculated to induce better effects than the physically mixed composites of metal nanoparticles.<br/>Herein, we report the lithium ion storage properties of NTS nanoparticles as an anode for LIB for the first time. Lithium ion storage mechanism of NTS has been investigated by ex-situ XRD and TEM analysis. It has been confirmed that Ni remained as inactive metal nanocrystallites while TiS₂ showed the reversible conversion reaction.<br/>Compared to TiS₂ and Ni-2TiS₂ composite, NTS exhibited a stable cycling performance. After 50 cycles at 1000 mA g^-1, it maintained the reversible capacity of 635 mAh g^-1, while a rapid degradation was observed in TiS₂ and Ni-2TiS₂ electrodes because of crack formation and pulverization of active materials. It is speculated that the <i>in situ</i> generated Ni nanocrystallites with fine distribution after 1^st discharge restrained the volume changes during discharge/charge and increased the electrical conductivity, resulting in the enhanced cycle performance and rate capability of NTS.<br/>Moreover, when NTS was combined with 20 wt% graphene, it exhibited an ultra-long cycle stability and excellent rate capability (452 mAh g^-1 after 1000 cycles at 5000 mAh g^-1). The improved electrochemical properties can be attributed to the uniform distribution of NTS nanoparticles on graphene matrix, preventing aggregation of active materials during cycling.