Spray-dried hard carbon-Sn composites for energy-dense Na-ion batteries

Sustainability and availability of raw materials, besides the usual performance-related metrics, have become crucial aspects for the development of new battery technologies to complement the existing successful Li-ion batteries. Sodium-ion batteries (SIBs) are at the forefront in this respect; however, the development of electrode materials achieving the expected performances is a challenge. Hard carbons (HC) are the most used anode material for SIBs, but poor gravimetric and volumetric capacities have limited the development of energy-dense SIBs. High-density and high-capacity metals that can react with sodium through formation/alloying reactions represent a possible solution, but the huge volume expansion during electrochemical cycling limits their utilization. In this work we explore the synthesis and characterization of sustainable hard carbon-Sn composites as anode materials for Na-ion batteries, with the aim of increasing HC performance without suffering the side effects of Sn volume expansion. Starting from conventional HC synthesis we propose a water-based continuous-flow spray-drying process to prepare our composites, resulting in an increase in HC's gravimetric and volumetric performances, including better long cycling stability. By using a set of analytical tools, we reveal the different physicochemical properties of our composites as a function of the starting cellulose precursors. The amount of Sn in the composites has been carefully evaluated through several techniques and lies at ≈15 or ≈25 wt% depending on the Sn content used for the synthesis. The activation of Sn during electrochemical discharge has been confirmed by operando synchrotron XRD, and the results show the appearance of sodiated Sn phases forming in kinetically driven reactions that do not fully adhere to the expected thermodynamic phase diagram. The electrochemical testing of our materials, carried out using conventional carbonate-based electrolytes, demonstrates excellent performances, with one composite demonstrating 301 mAh g^-1 of capacity after 100 cycles (94% retention). Noteworthy is that the volumetric energy density is also significantly improved. Finally, by synthesizing several HC-Sn composites using alternative methods we demonstrate how spray drying leads to superior performances, especially in terms of capacity retention. Our work establishes the feasibility of spray drying as a scalable and sustainable synthesis route to prepare high-performance negative electrode composites for Na-ion batteries.