Insights into the Na-Alloying Mechanism and Zintl Phase Transition of Lead-Based Anodes

Sodium-ion batteries (SIBs) has been received growing attention in the electrical energy storage fields due to their low cost and earth-abundant sodium. [1] However, there has been a lack of new discoveries, growth directions, and real advancement with respect to Na-storage anodes. Despite the chemical similarities between sodium and lithium as alkali elements, the larger Na ion than Li ions (ionic radii of 0.98 Å and 0.68 Å, respectively) is limited to insertion into host materials and results in different phase transition behavior. [2] Among the available anode candidate materials for SIBs, lead (Pb), which has a large atomic size than other elements (e.g. Si, Sn), provides a big interstitial space to accommodate large Na ions by fast ionic diffusion, enabling reversible Na alloying/dealloying and exhibiting high volumetric capacity. [3] Furthermore, when Pb is used as anode with layered sodium transition metal oxide as cathode, the energy density of the pouch-type cell is estimated to be 549 Wh/L and the cost is lower than 63.5 USD/kWh according to the Argonne BatPac model. [4] Therefore, Pb-based materials have competitive potential as promising anodes and it is crucial to understand the electrochemical process from a fundamental perspective. Here, we investigate a unique Na storage mechanism using a novel Pb-based carbon nanocomposite anode synthesized by a simple high-energy milling method. The electrochemical data show a decent cycle performance with a reversible capacity of 381 mAh/g. Nevertheless, the Na-storage performance of the Pb-based anode was not attractive compared to Li cells. In-situ X-ray diffraction and ex-situ X-ray absorption spectroscopy reveal the reaction mechanism and Zintl-phase formation that limits the Na storage, unlike the Li reaction. We expect these findings provide fundamental knowledge of Na-alloying reaction and guidance for designing anode materials for high-performance SIBs. [1] K. Kubota and S. Komaba Journal of The Electrochemical Society, 2015, 162 (14) A2538-A2550 [2] M. Lao, Y. Zhang, W. Luo, Q. Yan, W. Sun, and S. X. Dou Adv. Mater. 2017, 29 , 1700622 [3] Chia-Yun Chou, Myungsuk Lee, and Gyeong S. Hwang, J. Phys. Chem. C 2015, 119, 27, 14843–14850 [4] P. Nelson, K. Gallagher, I. Bloom, Dennis Dees, and Shabbir Ahmed, BatPaC, Argonne National Laboratory. http://www.cse.anl.gov/batpac.