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

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 m(2) 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 beta-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.