Tin Alloys and Intermetallic Compounds

Early reports revealed the electrochemical formation of Li-Sn intermetallics, which could provide theoretical capacities close to 1000 mAh g in lithium cells [27]. A second impulse was received by a material patented by Fuji tin composite oxides in which tin oxides were the starting material [28]. More recently, the commercialization of the Nexelion battery by SONY gave new attraction to tin-based electrodes, with the special feature of being noncrystalline, which adds new value to the use of spectroscopic techniques. Since the late 1990s, a large number of papers have been published on the application of Sn MS in the study of tin-based electrode materials. MS was found to be extremely useful in the analysis of the different steps commonly found during the electrochemical reaction of tin compounds with lithium (i) reduction of tetra- or divalent tin atoms to the metallic state, followed by the most important step, (ii) a reversible formation of Li-Sn intermetallics. 

An additional effect of great relevance in the electrochemical response of tin intermetallics is the role of nanodispersion. In this way, the maximum capacity achievable for CoSn electrodes was shown to increase significantly on decreasing the particle size (Fig. 28.8). For 10-20 nm, the capacity of the first discharge surpassed the theoretical capacity for CoSn. Moreover, for nanocrystalline CoSn, the reaction with lithium produces irreversible amorphization of the intermetallic compound. The presence of Co atoms avoids the formation of crystalline phases of Li ,Sn, thus reducing the risks of enhanced volume changes. The two quadrupolar doublets found for micro-CoSn at about OV, 

Changes in isomer shift and specific volume of the Li-Sn intermetallics as a function of lithium content. Adapted from Ref. 52 with permission of Elsevier. 

IS = 1.85(3) and IS2 = 2.05(3) mm s were replaced by a single doublet signal in nano-CoSn with IS= l.93(3)mms , indicative of a noncrystalline ternary Li CoSn phase as the main reaction product [33]. 

Barycenter position of Sn Mossbauer spectra for tin-containing alloys and intermetallic compounds annealed at different temperatures. Reproduced from Ref. 32 with permission of Elsevier. 

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