Porous MoS2@C heteroshell with a Si yolk structure with improved lithium transport properties and superior cycle stability

Abstract

To solve the problems of the Si anode such as excessive volume expansion and the corresponding destruction of the solid electrolyte interface (SEI) film, we propose a porous MoS₂@C heteroshell with a Si yolk structure. We prepared a porous MoS₂@C heteroshell with a Si yolk structure using polydopamine (PDA) polymerization and a hydrothermal method using sodium molybdate and L-cysteine. In particular, the PDA layer on the Si yolk plays a role as a medium in growing the MoS₂ shell as a MoS₂@C heteroshell. We proposed Si@C@MoS₂ (SCM) structures having different MoS₂ shell thicknesses (SCM1, SCM3, SCM5 in the order of shell thickness) and measured various electrochemical properties. When the MoS₂ shell was thin (SCM1), it exhibited a high specific capacity of 1451 mA h g⁻¹ with a coulombic efficiency of 86.6% after 100 cycles at a current density of 100 mA g⁻¹ (1^st cycle), 200 mA g⁻¹ (2^nd cycle), and 500 mA g⁻¹ (3^rd cycle and following cycles). Furthermore, it shows excellent rate capability even at high current density compared to the Si@C electrode. When the MoS₂ shell was thick (SCM5), ultrahigh electrochemical output properties and rate capability were exhibited although it shows low absolute capacity because of the high MoS₂ weight ratio. From the results, we found that the thin MoS₂ coating on the carbon shell had advantages compared to only the carbon shell because of the remarkable Li ion transport properties of MoS₂. It was also found that there is a trade-off between the MoS₂@C heteroshell thickness and the participation of Si in electrochemical reactions. In addition, the initial coulombic efficiency depended on the MoS₂ shell thickness. This excellent electrochemical phenomenon was attributed to the high specific surface area and high ion conductivity of MoS₂. Therefore, it is expected that these phenomena can also be applied to other anode materials to enable excellent cyclic stability and good rate capability for high performance Li ion batteries.