Structural modification and performance optimization of red phosphorus nanomaterials as anodes for lithium/sodium-ion batteries

Developing electrode materials for high-performance secondary batteries is one of the most effective approaches to alleviate energy and environmental crises. Nowadays, graphite anodes, which are widely used in commercial lithium-ion batteries, cannot satisfy the ever-growing energy needs of humans owing to their relatively low theoretical capacities and nearly no capacity in sodium-ion batteries. Therefore, developing new anodes with high capacity and energy density is necessary for next-generation large-scale energy systems. Red phosphorus has become an interesting topic in alkali-ion battery research and is expected to be commercially used as anode material in the next generation of secondary batteries owing to their intrinsic properties, such as their high activity, high theoretical specific capacity (2596 mA∙h∙g⁻¹), suitable oxidation–reduction potential, highly abundant earth resources, and low cost of lithium/sodium-ion batteries. However, red phosphorus exhibits poor electrical conductivity and large volume expansion when used as electrode material, resulting in low utilization of active material, serious electrode pulverization, and poor electrode cycling stability, which seriously hindered their commercial application in next-generation rechargeable batteries. Recent studies have shown that the cycle stability and electronic conductivity of red phosphorus can be improved by rational structural design, which promotes the electrochemical performance of red phosphorus anodes. For example, reducing the material size to the nanoscale can effectively shorten the diffusion path, enhancing the ion diffusion rate while alleviating the volume expansion and pulverization of the active substance. Additionally, the size reduction changes the band energy of the red phosphorus, which can transform indirect into direct bandgap semiconductors. Besides, the external characteristics of the active materials affect the performance by reducing the internal stress generated by the phase transformation in charging and discharging cycles. By modifying the morphology and structure of red phosphorus to form porous, layer, hollow, or composite structures, the cyclability and chargeability of batteries could be optimized because the internal stress generated by the volume change of the active material can be effectively released, and the generation probability of cracks or fractures in the electrode is drastically reduced. Therefore, these strategies help alleviate electrode pulverization and promote the commercial application of red phosphorus in lithium/sodium-ion batteries. Herein, we review the recent research progress in controllable synthesis, structural design, and performance optimization mechanisms of red phosphorus-based nanocomposites. Finally, we summarize the challenges in current research on red phosphorus anode materials, propose potential solutions, and provide an outlook on the future development of red phosphorus-based anode materials in the energy storage system.