Surface Reforming of Hard Carbon During Battery Rest for Enhanced Sodium Storage

The surface characteristics of hard carbon markedly affect its electrochemical performance. Here, a distinct surface modification of hard carbon (HC), which is achieved by electrolyte additives during cell aging, is reported. Di-sec-Butoxyaluminoxytriethoxysilane (DSB) is selected from a series of alkyl oxides of heteroatoms based on Wiberg bond order of X─O (X = Si, P, S, and B) and the change in Gibbs free energy of hydrolysis reaction. The Si─O─Al bonds in DSB exhibit high reactivity toward the deleterious species in the electrolyte, resulting in the formation of deposits on the surface of hard carbon. These deposits protect the anode surface and alter the pore structures. As a result, hard carbon exhibits an increased initial Coulombic efficiency of 83.5% and a capacity retention of 90.5% after 1500 cycles at 1 A g^-1. The simplicity and versatility of this approach pave the way for the commercial application of hard carbon.