Nanocrystalline FeSn2 was prepared by chemical reduction of Sn−Fe chlorides in tetraethylene glycol using a “one-pot” method. Structural characterization is carried out by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and 119Sn Mössbauer spectroscopy. The electrochemical reaction of nanocrystalline FeSn2 with Li was examined by electron paramagnetic resonance (EPR) and 57Fe Mössbauer spectroscopy. Nanocrystalline FeSn2 delivers reversible capacities of about 600 mAhg−1 vs lithium after 20 cycles. The mechanism of the electrochemical reaction involves the conversion of FeSn2 into Li x Sn phases and superparamagnetic iron (or tin-doped iron) nanoparticles. The composition and the dimensions of the superparamagnetic particles depend on the depth of discharge. The electrochemically formed superparamagnetic particles are preserved in the course of the reverse electrochemical reaction.

Crystalline particles with a nanometric size and a cubic shape of CoSn 3 have been prepared at a low temperature. The observed maximum reversible capacity in lithium test cells was 690 mAh g −1 . This capacity value is higher than that previously shown by other crystalline CoSn x intermetallic compounds with a lower tin content. The reaction of lithium with CoSn 3 yields first to a lithium insertion into CoSn 3 and second to the irreversible formation of a metallic glass, as studied using X-ray diffraction and 119 Sn Mössbauer spectroscopy.The capacity of Sn to form intermetallics with Li up to Li 4.4 Sn leads to a great interest in its potential use as an active electrode in lithium-ion batteries. The addition of other elements ͑e.g., transition metals and carbon͒ and the control of the grain size are currently needed to obtain an adequate electrochemical performance upon charge-discharge cycling. Crystalline cobalt-tin phases, such as CoSn, CoSn 2 , and Co 3 Sn 2 , and noncrystalline alloys have been studied. 1-10 The ultimate role of the cobalt atoms is to avoid the volume changes of the Li x Sn phases and to help maintain the electrode integrity upon cycling. Carbon atoms can help to achieve this goal but, in addition, carbon grains can contribute to the capacity for lithium intercalation. In contrast, the cobalt atoms decrease the global specific capacity. The study of the tin-rich phases of the cobalttin phase diagram might contribute to developing high-capacity electrode materials.Lang and Jeitschko described the intermetallic compound CoSn 3 ; they found two structural modifications. 11 The ␣-CoSn 3 phase is isotypic with PdSn 3 , while the ␤-CoSn 3 modification is a stacking variant of the other. These authors used the heating of cobalt and tin up to 1000°C and controlled cooling in evacuated silica tubes to obtain CoSn 3 . Very recently, CoSn 3 has been prepared at lower temperatures by using a method based on tetraethylene glycol ͑TEG͒ as a solvent and polymers as surface stabilizers. 12 In this work we prepared CoSn 3 with a very small particle size and the electrochemical behavior in lithium test cells was studied. X-ray diffraction ͑XRD͒ and 119 Sn Mössbauer spectroscopy were used to study the reaction mechanism at the crystalline level and local order, respectively. ExperimentalA previously described one-pot method was used to prepare CoSn 3 . 12 The synthesis was carried out in a glass vessel and under an Ar-flowing atmosphere by sequential reduction of anhydrous SnCl 2 ͑Aldrich͒ and CoCl 2 ·6H 2 O ͑Aldrich͒ with NaBH 4 ͑Panreac͒ in TEG ͑purity higher than 99% from Fluka͒ as a solvent. Polyvinylpyrrolidone ͑PVP, Aldrich, M W = 40,000͒ and poly-͑2-ethyl-2-oxazoline͒ ͑PEO͒ polymers were used as surface stabilizers. It is known that cobalt is an anomalous fast diffuser in tin. 13 First, tin nanoparticles are formed that then act like templates for the formation of CoSn 3 . This intermetallic compound is prepared throughout the reduction of tin and the rapid diffusion of cobalt atoms into the tin part...

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A sonochemical method has been used to prepare negative electrode materials containing intermetallic nanoparticles and polyacrylonitrile (PAN). The ultrasound irradiation is applied to achieve small particle size. After annealing at 490 °C under Ar-flow, the polymer PAN is partially carbonized and the metallic nanoparticles are surrounded by a carbonaceous matrix. The main metallic phase is CoSn(2). The carbonaceous coating and the surface oxides have been explored by using XPS. The resulting CoSn(2)-carbonaceous phase electrode (CoSn(2)@C) shows improved electrochemical behavior (ca. 450 mAh/g after 50 cycles) in comparison with previous reports on pure crystalline CoSn(2). The reaction between CoSn(2)@C and Li has been studied by using XRD and (119)Sn Mössbauer spectroscopy. The formation of large grains of crystalline Li(x)Sn phases after the first discharge is discarded. The small particle size which is achieved by using ultrasonication and the carbonaceous matrix contribute to maintain the Co-Sn interactions during the electrochemical cycling. The aggregation of the nanosized metallic particles upon electrochemical cycling can be suppressed by the carbonaceous matrix (pyrolytic PAN).