Preparation and investigation of anode materials with highly conductive materials for high-performance lithium-ion batteries

Abstract

With the increased demand for developing energy storage technologies, lithium-ion batteries have been considered as one of the most promising candidates due to its high energy density, excellent cyclic performance, and environmental benignity. Indeed, extensive applications of lithium-ion batteries are witnessed in the market, for example, in portable electronic equipment. However, the commercialized graphite anodes for lithium-ion batteries exhibiting low theoretical specific capacity is far from meeting the tremendous demands created by the fast-growing market. Therefore, enormous efforts have been devoted to developing desirable electrode materials with better recyclability and advanced capacity for next-generation lithium-ion batteries. Although alloy anode materials like silicon have the highest gravimetric and volumetric capacity, its huge volume change and low electron and ions conductivity still hinder the broad application in other fields, such as large-scale energy storage systems. Similar challenges also impede the wide implementation of conversion materials in Li-ion batteries. This work is focused on employing different highly conductive materials to improve the electrical conductivity of the entire electrode. At the same time, the formation of the conductive framework is beneficial to accommodate the substantial volume change of the active materials. In Chapter 3, copper nanowires and multi-wall carbon nanotubes coated on the surface of Cu foils built a porous substrate to support the active materials. Silicon was deposited on the porous substrate by the template of copper nanowires and multi-wall carbon nanotubes. The formation of copper nanowires/silicon and multi-wall carbon nanotubes/silicon core-shell structures intrinsically reduces the volume expansion of active materials. Meanwhile, the poles created by the intertwined copper nanowires and multi-wall carbon nanotubes further accommodate the stress from volume change. In addition, the copper nanowires/silicon and multi-wall carbon nanotubes/silicon core-shell structure provide the highly efficient electrons and Li+ diffusion pathways. As a result, we have demonstrated that multi-wall carbon nanotubes/copper nanowires/silicon delivers a high specific capacity of 1845 mAh g-1 in a half cell at a current density of 3.5 A g-1 after 180 cycles with a capacity retention of 85.1 %. In Chapter 4, a free-standing silicon-based anode was developed by preparing a three-dimensional copper nanowires/silicon nanoparticles@carbon composite using freeze-drying. Silicon nanoparticles were uniformly attached along with the copper nanowires, which was reinforced by the carbon coatings. The three-dimensional conductive structure allows the silicon nanoparticles to distribute evenly as well as enhance the electrical and ionic conductivity of the whole electrode. Similarly, considerable interspace produced by the three-dimensional structure can relieve the stress produced by the vast volume expansion of silicon nanoparticles, which is also restricted by the carbon coating layers during the charge and discharge processes. Moreover, the outer layers strengthen the stability of the three-dimensional framework and the contact between the copper nanowires and silicon nanoparticles. The electrochemical performance of copper nanowires/silicon nanoparticles@carbon composite electrode has been measured, which exhibits excellent cycling performance. In Chapter 5, a new highly conductive material, MXene nanosheets, was introduced to promote the electrochemical performance in lithium-ion batteries. In this chapter, the cobalt oxides were chosen as the active material for its controllable and facile synthesis methods. Meanwhile, cobalt oxides, one of the conversion materials as anodes for lithium-ion batteries, face similar issues with silicon. Therefore, an anode involving cobalt oxides nanoparticles mixed with MXene nanosheets on Ni foams has been developed. Small-size cobalt oxides nanoparticles were uniformly distributed within the MXene nanosheets leading to high lithium ions and electrons transmission efficiency, as well as preventing restacking of MXene nanosheets and colossal volume change of the cobalt oxides nanoparticles. As shown in Chapter 5, cobalt oxides /MXene composite electrode remains a stable capacity of 307 mAh g-1 after 1000 cycles when the current density approaches 5 C, which indicates the enormous potential of cobalt oxides/MXene composite as an anode for the high-performance lithium-ion batteries.