Nano-phase tin hollandites, K₂(M₂Sn₆)O₁₆ (M = Co, In) as anodes for Li-ion batteries

Abstract

The nano-phase tin hollandites, K₂(M₂Sn₆)O₁₆ (M=Co, In) of particle size <10 nm are prepared by high energy ball-milling of pre-synthesized compounds and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) techniques. The Li-cycling behavior of M=Co (nano-(K-Co)) and M=In (nano-(K-In)) is evaluated by galvanostatic cycling and cyclic voltammetry (CV) with Li-metal as counter electrode in the voltage range, 0.005-0.8 V (or 1.0 V). When cycled at 60 mA g⁻¹ (0.12 C) in the voltage range, 0.005-0.8 V, a stable capacity of 500 (±5) mA h g⁻¹ up to 60 cycles is noticed for nano-(K-Co), whereas nano-(K-In) showed an initial capacity of 570 (±5) mA h g⁻¹, which dropped to 485 (±5) mA h g⁻¹ (15% loss) at the end of 60 cycles. At 1 C-rate, the nano-(K-Co) showed a capacity of 410 (±5) mA h g⁻¹ stable up to at least 100 cycles. Under similar cyclic conditions, the heat-treated electrode (300 °C, 12 h, Ar) of nano-(K-In) showed a significant improvement and gave a stable capacity of 570 (±5) mA h g⁻¹ in the range of 5-50 cycles. The Coulombic efficiencies in both the compounds increased to 96-98% in the range of 10-60 cycles. For both the nano-phases, the average discharge potential is 0.13 V and average charge potential is 0.5 V vs. Li, as determined by the galvanostatic and CV data. Electrochemical impedance spectroscopy (EIS) data on nano-(K-Co) as a function of voltage are presented and discussed. The apparent Li-diffusion co-efficient (D_Li+), estimated from EIS data, is 2.0-2.6(±0.2)×10⁻¹⁴ cm² s⁻¹ between 0.25 and 0.45 V during the first-cycle. The observed Li-cycling data have been interpreted in terms of the alloying-de-alloying reaction of Sn in the nano-composite, 'Sn-K₂O-Co/In-Li₂O'.