In recent years, with the country's strong support for new energy vehicles, the sales of clean and pollution-free electric vehicles have achieved a spurt growth. However, the current commercial lithium-ion battery anode material graphite can only reach 300~340mAh in practical applications. /g capacity, and it has been difficult to improve, far from meeting the urgent needs of high-performance lithium-ion batteries in new market users.
Therefore, more and more people are committed to the development of high energy density battery materials. Silicon anode materials are favored by researchers because of their high theoretical specific capacity (3752mAh / g), environmentally friendly and low cost, is expected Become the main force of the next generation battery system.
However, there are still many problems in the development of silicon anode materials. For example, the volume expansion effect of elemental silicon during charging and discharging is as high as 300%, which causes structural collapse and pulverization, which seriously restricts the development of silicon as a negative electrode material for lithium ion batteries. Application. To solve the above problems, the problem of suppressing the volume expansion effect in the electrode reaction and improving the conductivity of elemental silicon is the key to the research.
In view of this, the research group of Professor Wang Xianyou of Xiangtan University successfully prepared a double-coated hollow spherical Si@TiO by one-step method. 2@C negative material.
▲ Figure 1 Si@TiO
2(a) preparation diagram of the @C anode material and (b) schematic diagram of the structure
In this work, hollow Si spheres were prepared by template-free method and magnesium thermal reduction method, and then hollow spheres HN-Si were coated with butyl titanate and glucose to prepare Si@TiO with rich pore structure and high stability. 2@C negative material.
▲ Figure 2 SiO
2(a,d-f), HN-Si(b,g-i) and Si@TiO
2Electron micrograph of @C(c,j-l)
First, in the process of charge and discharge, Si nanospheres with hollow structure can self-regulate huge volume expansion; secondly, TiO 2The shell layer can increase the lithium ion transport rate (the volume expansion ratio is only 4%) due to its structural advantages, and further restricts the volume expansion of the Si active material to the inner cavity instead of the outward; finally, the outer C layer is further improved. The electrical conductivity and structural stability of the composite.
The results indicate that the traditional single-layer cladding strategy cannot meet the structural stability requirements of electrode materials in the face of the huge volume expansion effect of Si anode materials, and this new double-cladding-hollow strategy Can effectively improve the volume expansion effect of silicon and improve its conductivity.
The results show that the two-layer stable hollow Si@TiO synthesized by the magnesium thermal reduction method and the sol-gel method 2@C nanosphere anode material, at a current density of 0.2A/g, operating voltage of 0.01-2.5V, the first discharge specific capacity is 2557.1mAh/g, and the coulombic efficiency is 86.06%. At a current density of 1A/g, Si@TiO after 250 cycles 2The reversible specific capacity of the @C anode material is still 1270.3 mAh/g. The undischarged HN-Si anode material has a first discharge specific capacity of 2264 mAh/g and a coulombic efficiency of only 67.3%.
This double-layer cladding-hollow structure design can shorten the transmission path of Li+ and electrons. The rich pore structure can also promote the full wetting of the electrolyte and improve its rate performance, while uniform TiO. 2Shell and C layers greatly enhance Si@TiO 2@C anode material structural stability and conductivity.
▲Figure 3 Si@TiO
2Characterization of Electrochemical Properties of @C Anode Materials
▲Figure 4 Si@TiO
2@C(a) Schematic diagram of the working device, (b) Structural change of charge and discharge under TEM and (c) Schematic diagram of lithiation (delithiation)
▲ Figure 5 cycle performance, rate performance and impedance analysis
In summary, the design of the bistable cavity structure in this study can promote the further research and development of silicon-based anode materials, and also provide a reference for the study of negative electrode materials with serious volume expansion and poor conductivity.
