The Effect of Micro-Sized Tin/Hard Carbon Anode Composition on Sodium-Ion Battery Performance
Authors: Amai Joseah , Khirwadkar Ameya , Haines Kenneth , Van Zandt Zachary , Nelson George J. 
Published: December 2024

Sodium-ion batteries (SIBs) are a low-cost alternative to lithium-ion batteries (LIBs) for energy storage. However, their commercial application is currently limited by the unsatisfactory performance of anode materials.
Hard carbon was adopted as the ‘first generation’ anode for SIBs due to the success of graphite as an anode material for LIBs. It has great chemical and thermal stability but suffers from low theoretical capacity and poor rate capability. Tin, another promising anode material, has a high theoretical capacity but is plagued by large volume changes during sodiation/desodiation, leading to faster anode degradation.
To address these limitations a composite anode has been developed by direct mixing of micron-sized tin (Sn) and hard carbon (C) powders. The impact of these composite anodes on SIB performance is being investigated as a scalable approach to creating higher energy density SIBs. Half-cell batteries have been assembled from as-prepared Sn/C composite anodes with varying weight percentages of the active materials. The performance of these anodes has been compared in sodium perchlorate and sodium hexafluorophosphate electrolytes using standard electrochemical testing techniques such as galvanostatic cycling, cyclic voltammetry and electrochemical impedance spectroscopy. Our findings indicate that batteries with Sn/C anodes have a better percentage capacity retention compared to those with bare tin after forty cycles. The specific capacity observed remains over 550 mAh/g at 0.1C after 20 cycles and continues to demonstrate a good rate performance with capacity up to 430 mAh/g after 50 cycles. For high-rate cycling at 1C, the retained capacity was well over 380 mAh/g after 20 cycles. This performance and capacity retention are expected to result from the benefits of hard carbon conductive network and the formation of a more stable solid electrolyte interphase (SEI) on carbon surfaces.
To complement these electrochemical characterization results, investigation of microstructural evolution of the anodes was done. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the morphology and crystal structure of the anodes. SEM images of the as-prepared pristine Sn/C composite anode revealed a uniform distribution of tin and hard carbon in the matrix of carbon black and carboxymethylcellulose (CMC) binder, with no obvious structural change. At a higher magnification, the SEM images show a higher concentration of tin particles around the hard carbon particles, compared to the rest of the matrix. XRD was used to understand the structural features of this composition. The diffraction peaks observed are associated with crystalline tin, and the absence of impurity peaks confirms the metallic nature of the Sn powder used. In addition, no significant peaks were observed for the carbon phases which is consistent with the amorphous nature of the hard carbon. The amorphous structure of the hard carbon aids in mitigation of volume expansion of tin during sodiation/desodiation. By absorbing the repeated induced mechanical stresses, the hard carbon is expected to reduce the overall anode degradation rate thus prolonging the cell performance and capacity retention.
This study demonstrates the significant effect of Sn/C anode composition on SIBs and provides insightful information towards the advancement of practical SIBs. The successful development of high-performance anodes and SIBs will not only transform the landscape of energy storage systems but also alleviate the current challenges and market pressure faced by LIBs.