Cation-deficient MoySnxO2 oxides as anodes for lithium ion batteries

Fine powders of tin dioxide doped with Mo were prepared under hydrothermal conditions and tested in lithium cells. X-ray and IR data revealed the formation of single-phase products with a rutile-like structure that is maintained upon calcining at 800 degreesC. Mo6+ ions change the habit growth of crystals and are randomly distributed at octahedral positions, thus promoting the formation of cation vacancies. The addition of Mo increases the reversibility of the lithium insertion/de-insertion process, as reflected in the simplified differential specific capacity plots obtained, which result in a single, rather symmetric peak in the anodic and cathodic waves. Furthermore, increasing the Mo content improves retention capacity at the expense of reversible capacity because the transition element acts as an inactive component in the electrochemical process. The following arguments account for the improved performance of these mixed oxides. (i) A diluent effect of Mo atoms that facilitates dispersion of the Sn atoms formed during the reduction process; (ii) hindered formation of large clusters and decreased interfacial strains; (iii) restriction of the number of alloying/de-alloying processes through a decreased reversible capacity during the first few cycles; and (iv) an increased chemical diffusion coefficient for lithium, that results partially from the structural disorder caused by the replacement of tin with molybdenum. (C) 2000 Elsevier Science Ltd. All rights reserved.