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

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 (NaClO$_4$) 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.