Spray‐coated Hard Carbon Composite Anodes for Sodium‐Ion Insertion

Abstract Sodium‐ion batteries are among the most promising alternatives to lithium‐ion batteries. Hard carbon (HC) electrodes have been recognized as suitable active anode material for mono‐valent ion batteries. Here, we present a simple and cost‐effective spray‐coating process to prepare HC composite electrodes on copper current collectors with different binder (sodium carboxymethyl cellulose, CMC) content and different HC particle sizes. The spray‐coated electrodes were evaluated and tested in 1 M sodium perchlorate (NaClO4) in propylene carbonate (PC) in dependence of the CMC content with and without fluoroethylene carbonate (FEC) as additive, and the performance was also compared to doctor bladed HC electrodes. Spray‐coated anodes in Na half‐cells revealed improved capacity during the first cycles compared with doctor bladed anodes with similar thicknesses. Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) studies were performed, which revealed a significant increase of inorganic fluoro‐compounds in the formed solid electrolyte interphase (SEI) when FEC was present as additive. In addition, first single electrode microcalorimetry studies on spray‐coated thin HC composite electrodes yielded an entropy of the sodiation process of 80 J mol−1 K−1 at high state of charge (SoC), comparable to that of bulk Na deposition.