Tin Nanoparticles As High-Performance Anode for Sodium-Based Batteries: Preparation and Operando XRD Investigation

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Lithium-ion batteries (LIBs) have been dominating the energy storage market for decades, but the political and economic issues related to raw materials availability and cost is fueling the search for new electrochemical solutions. Sodium-based batteries (SBBs) are the most promising substitute to LIBs, because the similar physico-chemical properties of these two alkali guarantees the knowledge transfer from LIBs to SBBs and facilitate the working phenomena comprehension (so called rocking-chair mechanism).The materials used as cathodes in LIBs often have a sodium-based analogue that offers acceptable performances in SBBs, yet this is not the case for anode materials. Graphite is the most used material as anode in LIBs and its electrochemical behaviour with lithium is based on the intercalation mechanism. Unfortunately, Na does not intercalate into graphite, pushing scientists to find other solutions, such as hard/soft carbons and alloying elements [1]. Tin, in particular, has a high theoretical capacity (847 mAh/g) due to the formation of the high-sodium-content alloy Na15Sn4, making it a promising candidate for the next generation of SBBs [2].In our work tin nanoparticles and tin/carbon composites are prepared and analysed through electrochemical techniques. Being the sodiation of tin an alloying reaction, during the half cells discharge the increasing of sodium amount in tin is observed as the formation of high-sodium-content alloys according to the related phase diagram. We investigated the evolution of the system by operando XRD carried out at low current rate (coulometric titration), to confirm the expected phases based on the phase diagram are indeed observed. The results are corroborated by the electrochemical tests (galvanostatic cycling and voltammetry). Experimental data and theoretical information from literature are used to identify crystalline and amorphous phases [3]. The main issue linked to alloying materials is the high-volume expansion after discharge (420 % for tin), leading to fast capacity fade because of the solid electrolyte interphase (SEI) continuous destruction. For this reason, we prepared Sn/carbon composite structures able to mitigate the expansion and avoid the tin aggregation. In this way the capacity retention is improved, making this material a valid alternative for next generation batteries.