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

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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 carboxy methyl 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 interface (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.

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

Palanisamy,

Daboss,

Schäfer

et al. 2023

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Microscopic and Spectroscopic Analysis of the Solid Electrolyte Interphase at Hard Carbon Composite Anodes in 1 M NaPF₆/Diglyme

Palanisamy,

Daboss,

Romer

et al. 2024

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The formation of the solid electrolyte interphase (SEI) on HC composite electrodes plays a crucial role in enhancing the performance and operational stability of sodium (Na+) ion batteries. It has been demonstrated that for HC anodes improved electrochemical performance, e.g., increase in coulombic efficiency (CE) and improved rate performance have been achieved in ether‐based electrolytes. Here, we investigate spray‐coated HC composite electrodes charged at low and high current rates in 1 M sodium hexafluorophosphate (NaPF6) in diglyme using half‐cell experiments. The pristine and cycled HC anodes were examined in terms of conductivity and their electrochemical properties after cycling. In 1 M NaPF6 ether‐based electrolyte, the spray‐coated HC composite electrodes reached a discharge capacity of 431 mA h g−1 at 0.1C that stays constant for 40 cycles, which is substantially higher than that obtained in carbonate‐based electrolytes. We investigated the formed interphase using scanning (electrochemical) probe microscopy (S(E)PM), X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary‐ion mass spectrometry (ToF‐SIMS), revealing distinct differences for longer cycling and at varying current rates which indicates that the properties of the formed SEI layers are influenced by the formation conditions.

Investigation of Electrolyte Salts in Non‐Flammable Triethyl Phosphate for Sodium‐Ion Batteries

Van Ekeren,

Dettmann,

Tesfamhret

et al. 2024

Five different electrolyte salts, namely NaBF4, NaClO4, NaDFOB, NaFSI and NaPF6, were evaluated in non‐flammable triethyl phosphate (TEP) based electrolyte solutions in sodium‐ion full‐cells using high‐mass loading Prussian white and hard carbon electrodes. Their impact on the viscosity, ionic conductivity and solvation structure was analyzed, revealing that NaFSI‐based electrolytes exhibited a stronger interaction with TEP and less ion‐pairing than the other salts, resulting in the highest ionic conductivity at a concentration of 0.8 m (mol/kg). Galvanostatic cycling experiments showed that none of the electrolyte salts dissolved in TEP forms an efficient passivation layer. However, adding 1 wt.% vinylene carbonate (VC) significantly improved cycling performance for the cells with NaBF4, NaDFOB or NaFSI, but not with NaClO4 or NaPF6. Additionally, NaFSI in TEP with 1 wt.% VC electrolyte solution showed minimal gas evolution during the formation cycling (< 8 mbar). In a 1 Ah multilayer pouch cell, 0.8 m NaFSI in TEP with 1 wt.% VC showed promising results with 88% capacity retention after 200 cycles. X‐ray photoelectron spectroscopy analysis revealed that the addition of VC results in the formation of a thin SEI and minimized TEP decomposition on hard carbon, especially for 0.8 m NaFSI TEP with 1 wt.% VC.