Studies on nano-CaO·SnO₂ and nano-CaSnO₃ as anodes for Li-ion batteries

Abstract

The nanocomposite "CaO·SnO₂" and nano-CaSnO₃ are prepared by the thermal decomposition of CaSn(OH)₆ precursor and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HR-TEM) along with selected area electron diffraction (SAED) and density measurements. Nanosize (3-6 nm) grains of CaO and SnO₂ in the X-ray amorphous CaO·SnO₂ and particles of ~60 nm size in nano-CaSnO₃ are obtained. Galvanostatic cycling of both the phases vs Li metal is performed in the voltage ranges 0.005-1.0 V and 0.005-1.3 V at the current rate, 60 mA g⁻¹ (0.12 C). Stable and reversible capacities of 490 (±5) and 550 (±5) mA h g⁻¹ are observed for nano-CaO·SnO₂ respectively up to 50 cycles in the above voltage windows. These values correspond to 3.8 and 4.2 mol of cyclable Li per mole of CaO·SnO₂ in comparison to the theoretical value of 4.4 mol of Li. A capacity of 420 (±5) mA h g⁻¹ is observed at a rate of 0.4 C. Nano-CaSnO₃ showed a stable capacity of 445 (±5) mA h g⁻¹ (3.4 moles of Li) up to 50 cycles when cycled in the voltage window, 0.005-1.0 V. The average discharge and charge potentials are 0.2 V and 0.5 V, respectively, for both the phases. The reasons for the superior Li-cycling performance of nano-CaO·SnO₂ in comparison to nano-CaSnO₃ are discussed. Ex situ XRD, TEM, and SAED studies are carried out to support the reaction mechanism. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) data as a function of voltage are presented and discussed to complement the galvanostatic results. The "apparent" Li-ion diffusion coefficient (D_Li+) estimated from EIS is ~1.0 × 10⁻¹⁴ cm² s⁻¹ at V ≤ 1.0 V during the first cycle and 11th discharge cycle.