Figure 1: Yolk@Shell SiOx/C microspheres with semi-graphitic carbon coating on the exterior and interior surfaces for durable lithium storage. The boom of electrical vehicles and large-scale energy storage systems has increased the importance of the energy density of LIBs. LIBs require novel electrode materials with higher capacity and good working potential. Recently, researchers of Liqiang Mai’s group from Wuhan University of Technology in China have developed a novel silicon oxides (SiOx) based anode material, which is promising to high-energy lithium-ion batteries (LIBs) [Liu et al., Energy Storage Materials (2018), doi: org/10.1016/j.ensm.2018.10.011]. Silicon (Si) has attracted significant interest as the prime alternative material due to the high theoretical capacity (4200 mA h g-1) and abundant reserves. "Compared with Si, SiOx possesses much lower production cost and less volume change, and is regarded as "one of the most promising anodes for LIBs." says Prof. Liqiang Mai, the corresponding author of this study.
The researchers creatively designed a yolk@shell structured SiOx/C anode with semi-graphitic carbon coatings on the exterior and interior surfaces (SiOx/C-CVD) through sol-gel process, selective etching, and chemical vapor deposition, as is shown in Figure 1. The as-prepared SiOx/C-CVD composite demonstrates a high reversible capacity (1165 mA h g-1 at 100 mA g-1) as well as outstanding durability (972 mA h g-1 after 500 cycles at 500 mA g-1). "Traditional yolk@shell structured electrode materials widely studied in lithium ion batteries often suffer from structural collapse during cycling due to the thin thickness of the shells. In this work, chemical vapor deposition (CVD) process was used to coat the yolk@shell structured SiOx/C with semi-graphitic carbon on both the exterior and interior surfaces.” says Prof. Huiming Cheng, the academician of Chinese Academy of Sciences. “The CVD-derived carbon coating not only improves the structural stability but also significantly increases the electrical conductivity, therefore contributing to the outstanding electrochemical performance of the SiOx/C anode." In addition, the full cells of SiOx/C-CVD//LiCoO2 show a high energy density of ~ 428 Wh kg-1 with a stable cycling behavior. Without semi-graphitic carbon coating, the shell of the yolk@shell structured SiOx/C microspheres fully collapses after 150 deep discharge-charge cycles. On the contrary, the yolk@shell structure of SiOx/C-CVD remained intact after 150 cycles, indicating that the semi-graphitic carbon coating layers play important roles in buffering the volume change of SiOx, improving the structural integrity, and inducing the formation of stable SEI film.
“It is known that silicon oxide-based materials are promising candidates for LIB anodes and the SiOx/C-CVD anode reported in this work is a typical case. In addition, the CVD-carbon coated yolk@shell design strategy proposed by Mai’s group can be extended to other high-capacity anode materials, which suffer from poor electrical conductivity and large volume variations, to achieve the optimization of electrochemical performance.” says Prof. Qiang Zhang, at Tsinghua University, China.