New tin-based materials containing cobalt and carbon for lithium-ion batteries

Cobalt–tin and cobalt–tin–carbon composite electrodes with different nominal compositions (CoSn, CoSn2, CoSn3, CoSn4, CoSn15 and [CoSn3]0.6C0.4) have been prepared by annealing of the elements under an argon flow. 119Sn Mössbauer spectra have been recorded. As a consequence of the electronic transfer from tin atoms to the resulting cobalt–tin intermetallic compounds, the isomer shift barycentre is moved to lower values as compared to pure tin (δ = 2.5 mm/s). The lower isomer shift values (δ ∼ 2.0 mm/s) for cobalt–tin samples with cobalt to tin atomic ratio equal to 1:3 are obtained for a preparation temperature between 450 and 750 °C. In contrast, the presence of carbon atoms limits the isomer shift of the tin-based intermetallic phases, with δ = 2.2 mm/s for [CoSn3]0.6C0.4 nominal composition in the 450–750 °C range, and then the electronic transfer from tin atoms to the resulting intermetallic phase is reduced. The carbon atoms exert an effect in the isomer shift value (increase of δ) that is in opposition to the effect exerted by lithium and cobalt atoms (decrease of δ). After treating with a hydrochloric acid aqueous solution, the 119Sn Mössbauer spectra are less broadened and the electrochemical behaviour in lithium cells is greatly improved. Reversible specific capacities of around 700 mA h g−1 are observed between 1 and 0 V for acid-treated cobalt–tin–carbon electrodes. The mechanism of the reaction with Li is explored by using ex situ 119Sn Mössbauer spectroscopy. In the charge process (Li-removing) two phases are segregated from the original lithiated phase.