Understanding degradation of Sn-based anodes for sodium-ion batteries Carla Albenga , Mark Copley, Ivana Hasa WMG, University of Warwick, UK Electrochemical energy storage systems such as lithium-ion batteries (LIBs) play a primary role in the transition to a low-carbon economy. As the demand for LIBs rapidly expands, the environmental and social challenges associated with their mass production have triggered attention toward alternative energy storage solutions based on materials that can be sourced in a sustainable and responsible way. In this scenario, sodium-ion batteries (SIBs) represent an appealing alternative. 1 Many classes of materials have been proposed as positive electrode in SIBs, including layered metal oxides, polyanionic compounds, and Prussian blue analogues . However,at the negative electrode, only hard carbon (HC) has been so far implemented in upscaled SIBs, which however presents limited capacity. In order to further increase the energy density and cyclability of SIBs, new types of anodes are being explored. Among them, materials interacting with sodium ions through an alloying mechanisms represent the most promising ones as they offer greater capacity and energy density, however they suffer from poor structural stability due to severe volume expansion upon (de-)sodiation. 2 Among alloying materials, tin (Sn) represents the most interesting candidate due to its high theoretical capacity, sustainability, and low cost. The main strategies adopted to improve Sn anodes stability focus primarily on nano-structuration, and morphology optimisation to limit the damages caused by the volume changes which lead to cracking of the electrode and consequent cell failure. However, nano-structuration brings disadvantages in terms of toxicity and high synthesis costs as well as low density and large surface area which affects the electrochemical performance. 3 Recently, micrometric Sn (m-Sn) anodes showed unexpected stability when cycled with a particular class of electrolyte, paving the way for new considerations and approaches in investigating these materials. 4 Herein, we present a comprehensive investigation of Sn-based electrodes by monitoring the effect of particle size and active material composition. HC and Sn have been adopted to prepare composite electrodes aiming at improving cycling stability. To elucidate the degradation mechanisms occurring during the (de-)sodiation process, morphology and crystalline phases changes have been investigated by using X-Ray diffraction (XRD) and cross-sectional Focused Ion Beam (FIB) microscopy. This work highlights the superior performance of m-Sn compared to the corresponding nanosized materials, providing a consistent study of Sn particle size effect during electrochemical (de-)sodiation. Furthermore, HC is proven to be a valid option to stabilize m-Sn anodes while contributing to the overall capacity of the electrode. By means of electrochemical analysis and ex-situ cross- sectional SEM studies, it is found that m-Sn/HC composite electrodes with high Sn content exhibit good stability and long cycle life as a result of the growth of a robust porous network of metallic Sn mitigating volume expansion and ensuring long term cyclability.
References 1. Vaalma, C. et al., Nat. Rev. Mater. 3 , 18013 (2018).Hasa, I. et al., J. Power Sources 482 , 228872 (2021). 2. Zhang, et al., S. Adv. Energy Mater. 8 , (2018).Zhang, B. et al. Adv. Mater. 28 , 9824–9830 (2016).
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