Sodiation vs. lithiation phase transformations in a high rate – high stability SnO2 in carbon nanocomposite

Abstract

We employed a glucose mediated hydrothermal self-assembly method to create a SnO₂–carbon nanocomposite with promising electrochemical performance as both a sodium and a lithium ion battery anode (NIBs NABs SIBs, LIBs), being among the best in terms of cyclability and rate capability when tested against Na. In parallel we provide a systematic side-by-side comparison of the sodiation vs. lithiation phase transformations in nano SnO₂. The high surface area (338 m² g⁻¹) electrode is named C–SnO₂, and consists of a continuous Li and Na active carbon frame with internally imbedded sub-5 nm SnO₂ crystallites of high mass loading (60 wt%). The frame imparts excellent electrical conductivity to the electrode, allows for rapid diffusion of Na and Li ions, and carries the sodiation/lithiation stresses while preventing cycling-induced agglomeration of the individual crystals. C–SnO₂ employed as a NIB anode displays a reversible capacity of 531 mA h g⁻¹ (at 0.08 A g⁻¹) with 81% capacity retention after 200 cycles, while capacities of 240, 188 and 133 mA h g⁻¹ are achieved at the much higher rates of 1.3, 2.6 and 5 A g⁻¹. As a LIB anode C–SnO₂ maintains a capacity of 1367 mA h g⁻¹ (at 0.5 A g⁻¹) after 400 cycles, and 420 mA h g⁻¹ at 10 A g⁻¹. Combined TEM, XRD and XPS prove that the much lower capacity of SnO₂ as a NIB anode is due to the kinetic difficulty of the Na–Sn alloying reaction to reach the terminal Na₁₅Sn₄ intermetallic, whereas for Li–Sn the Li₂₂Sn₅ intermetallic is readily formed at 0.01 V. Rather, with applied voltage a significant portion of the material effectively shuffles between SnO₂ and β-Sn + NaO₂. The conversion reaction proceeds differently in the two systems: LiO₂ is reduced directly to SnO₂ and Li, whereas the NaO₂ to SnO₂ reaction proceeds through an intermediate SnO phase.