Sodium-ion batteries and capacitors are considered as low-cost energy storage devices, compared to Li-ion counterparts. However, most anodes for sodium-ion devices show sluggish kinetics and poor structural stability caused by the large radius (1.02 Å) of Na+. One candidate anode is MoS2, a 2D atomic layered material with a large interlayer spacing of 6.2 Å, that can take up and release Na+via two working principles: a two-electron intercalation process and a four-electron conversion reaction. Herein, we report a facile method to synthesize a MoS2–amorphous carbon (MoS2–AC) nanocomposite and further study the effect of the two working principles on the structure, interphase, and charge storage properties of MoS2–AC. The two-electron intercalation reaction enables the MoS2–AC electrode to have a higher rate capability and superior stability than that via the four-electron Na+ conversion reaction. This favorable Na+ charge storage performance of MoS2–AC via the two-electron intercalation process is attributed to its pseudocapacitive behavior, a stable solid electrolyte interphase and robust stability of the structure, which enables us to fabricate a sodium-ion capacitor that can deliver high energy density at a high rate. This work underscores the potential importance of realizing fast Na+ charge storage via an intercalation process as a strategy for the fabrication of high-performance sodium-ion capacitors and batteries.
SiO 2 is one of the critical raw materials for preparing Si anode. The direct use of porous Si made from SiO 2 in LIBs is challenging. Although carbon cladding can improve the cycling performance of Si anode materials made from SiO 2 , the surface coating cannot address the comminution inside the material during repeated volume deformation. For the first time, we prepared an in-situ doped Si/carbon anode material that uses SiO 2 and CO 2 as Si and carbon sources and Mg as the reduction medium (Mg 2 Si + CO 2 → MgO + Si + C). The in situ embedded carbon completes the nanosizing of Si (less than50 nm) and forms a three-dimensional (3-D) carbon network that enhances electrical conductivity and acts as a buffer layer. After 500 cycles at 0.5 A g −1 , the discharge specific capacity is 912 mAh g −1 , and the capacity still has 1487 mAh g −1 even at 4 A g −1 . The simple method of preparing Si/carbon composite material in situ provides a new idea for modifying Si-based anode material and provides a valuable reference for carbon doping.
Rechargeable Mg batteries are an advantageous energy-storage technology with low cost and high safety, but the design of high-performance cathode materials is currently the major difficulty. Herein, a new cathode material of Cu 3 VSe 4 is fabricated with a comprehensive consideration of the chemical and electronic structures. The intermediate band semiconductor Cu 3 VSe 4 has a cubic crystal structure containing interlaced 3D tunnels. The V and Se atoms form chemical bonds with high covalent proportions and facilitate the charge delocalization via the V‒Se bonds. Because of these features, Cu 3 VSe 4 provides a high capacity of 251 mAh g ‒1 with co-redox of Cu, V, and Se elements and an outstanding rate performance of 44 mAh g ‒1 at 15 A g ‒1 . Prominently, a high mass load of 3.0 mg cm ‒2 is achieved without obvious rate capability decay, which is quite favorable to pair with the high-capacity Mg metal anode in practical application. The mechanism investigation and theoretical computation demonstrate that Cu 3 VSe 4 undergoes first a Mg-intercalation and then a displacement reaction, during which the crystal structure is maintained, assisting the reaction reversibility and cycling stability. These findings reveal a rational design principle of rechargeable Mg battery cathodes based on a comprehensive consideration of chemical and electronic structures.
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