Development of High-performance Carbon and Phosphorus Anode Materials for Sodium-ion Batteries

Abstract Lithium-ion batteries, as dominant power sources for prevailing consumer electronics and electric vehicles, have been plagued by limited lithium resources with the soaring prices. Sodium-ion batteries, benefitted from ubiquitous and inexpensive sodium resources without toxicity and pollution, have emerged as promising alternatives for large-scale applications. Considering their kinetic challenges and disadvantageous energy densities, it is urgently necessary to pursue high-performance electrode materials to remedy these intrinsic defects. For anode materials, the primary choices of sodium metal and graphite are correspondingly denied for safety issues and intercalation incapability. Therefore, the objective for this doctoral work is to develop high-performance anode materials with large reversible capacities, long cycle life, high rate capabilities, and environmental friendliness. Carbon anode materials with the high electrochemical durability and physical/chemical stability have been chosen as objects in the first part. Additionally, phosphorus anode materials with a highest theoretical specific capacity and a desirable operating voltage range have been selected as objects in the second part. Firstly, carbon-based materials with optimized porosities and functionalities have been introduced into an ether-based electrolyte. A novel synergistic mechanism, incorporating the sodium ion insertion into disordered structure with the solvated sodium ions co-intercalation into graphitic structure, has been utilized to boost the sodium storage capabilities of porous carbon blacks. Simultaneously, the controlled emergence of a robust SEI thin film in ether-based electrolyte could maintain the fragile porous structure and further facilitate the sodium ions/solvated sodium ion compounds migrations. Furthermore, the desirable microporosity and the oxygenated functionalities could provide more active sites for sodium storage. Secondly, the effects of storage conditions and binders on electrochemical performances of P/C electrodes have been investigated. Initially, the formation and accumulation of phosphate compounds was identified for P/C electrodes. Even though the cycle stability of an aged electrode would be improved by an oxidation layer toward the smaller volume changes, the formation of these insulating compounds would sacrifice the reversible capacity and rate capability. Surprisingly, the utilization of different binders would determine the oxidation degrees of P/C electrodes. Their electrochemical properties have positive relationship with oxidation of active phosphorus for electrodes using different binders