Evaluating the Effects of Carbon Physicochemistry on the Rate Capability of Polyaniline and Phytic Acid-Derived Sodium-Ion Battery Anodes

As inexpensive anodes for sodium-ion batteries (SIBs), hard carbons are a highly tunable class of materials that hold promise to alleviate societal reliance on traditional lithium-based battery chemistries. However, the combination of sodium storage mechanisms, ranging from surface adsorption to pore-filling, has led to convoluted structure-function relationships and debate toward optimal desired material properties. To this end, a suite of nitrogen and phosphorus codoped carbons (NPCs) derived from phytic acid cross-linked polyaniline precursors were systematically evaluated as SIB anodes at practical cycling rates. The addition of calcium or zinc salts to the cross-linked polymerization process before pyrolysis ultimately led to nanoporous hard carbons with varied physicochemical properties and subsequent electrochemical performances. A majority of sodium storage capacity in these polyaniline-derived NPCs occurred in a higher-voltage, sloping region and primarily stemmed from sodium-ion adsorption at defective sites. This storage mechanism was also associated with increased stability compared to lower-voltage mechanisms related to bulk sodium insertion, both in cycle life and rate capability. Best-performing NPCs demonstrated an initial capacity of 212.1 mAh g(-1) with approximately 77% capacity retention over 300 cycles at a cycling rate of similar to 1 C, and high-rate testing revealed a considerable fast charge capacity of 117.8 mAh g(-1) (similar to 8 C, 7.5 min charge time). The superior rate performance and stability of certain NPCs were strongly correlated to the lateral nanocrystalline domain sizes (L-a). Overall, this study outlines a simple and tunable synthetic method for the production of high-performance NPCs for SIBs and sheds light on important considerations for the design of carbon anodes for practical SIBs.