Cross-linking matters: Building hard carbons with enhanced sodium-ion storage plateau capacities

Hard carbon is a promising anode material for sodium-ion batteries (SIBs). However, its commercialization is hampered by its relatively low capacity. This limitation primarily arises from the insufficient closed pore structures developed within the hard carbon. In this contribution, we precisely control the crosslinking structures of phenol-formaldehyde resin precursors by adjusting the catalyst amount, producing carbon materials with varied microstructures. We explore the relationship between the microstructures of the resin-derived hard carbon and its sodium storage performance. Our findings indicate that increasing the crosslinking degree of phenol-formaldehyde resins enhances the number of closed pores formed during high-temperature carbonization, thereby improving the plateau capacity. Notably, the hard carbon exhibits superior rate performance, due to its smaller closed pore diameter. This study provides an in-depth exploration of the relationship between precursor structural characteristics and the microstructures of hard carbons. By calculating the closed pore parameters, we elucidate the structure-performance relationship, demonstrating that optimizing the closed pore structures significantly enhances the electrochemical performance of hard carbons for SIBs. This research offers valuable insights for the development of high-performance hard carbon anodes, paving the way for the commercialization of SIBs.