P.P. Ferguson

An In Situ Study of the Electrochemical Reaction of Li with Nanostructured Sn30Co30C40

The reaction of lithium with nanostructured Sn 30 CO 30 C 40 alloy prepared by mechanical attrition was studied using in situ X-ray diffraction (XRD) and in situ 119 Sn Mossbauer effect spectroscopy. During the first discharge (insertion of Li into the alloy) of Li/Sn 30 Co 30 C 40 cells, similarities between the structures of fully lithiated Sn 30 Co 30 C 40 and lithiated Sn were observed by XRD. Both XRD patterns showed evidence for the Sn tetrahedra, with neighboring Li atoms, as found in Li 22 Sn 5 . This implies that fully lithiated Sn 30 Co 30 C 40 is composed of nanoscale regions of Li 22 Sn 5 , nanoscale Co, and lithiated disordered carbon. In situ 119 Sn Mossbauer spectroscopy results were consistent with this interpretation and showed the presence of unlithiated Sn atoms at the bottom of the discharge. This observation agrees well with the fact that attrited Sn 30 Co 30 C 40 samples only reach about two-thirds of the theoretical capacity. During the first recharge, in situ results from both techniques show that Co is rebonded to the Sn atoms in the CoSn grains as Li is removed. Overall, the insertion of lithium in nanostructured Sn 30 Co 30 C 40 is believed to proceed via the following reaction where only the fully charged (left) and fully discharged (right) states are indicated: [132(1 - z) + 40y]Li + Sn 30 Co 30 C 40 ↔ (1 - z)(6Li 22 Sn 5 + 30Co) + zCoSn + 40Li y C, where y ≈ 0.5.

Comparison of Mechanically Milled and Sputter Deposited Tin–Cobalt–Carbon Alloys Using Small Angle Neutron Scattering

Small angle neutron scattering (SANS) is used to compare nanostructured Sn-Co-C alloys produced by vertical axis mechanical attriting to those produced by magnetron sputter deposition. The attrited materials have grain sizes that vary with composition and are on the order of 60 A in size. The sputter deposited materials are either amorphous or have a grain size of approximately 10 A, depending on the composition. The SANS results are used to further understand the electrochemistry of these materials when used as negative electrodes for lithium-ion batteries and to understand why mechanically alloyed Sn-Co-C alloys are far from reaching their expected theoretical specific capacity while sputtered alloys achieve capacities much closer to the expected value.

(Sn0.5Co0.5)(1-y)C-y Alloy Negative Electrode Materials Prepared by Mechanical Attriting

Samples of (Sn 0.5 Co 0.5 ) 1-y C y for 0 ≤ y ≤ 0.8 were prepared in increments of y=0.1 using a vertical-axis attritor. The effect of the carbon content on the structure and performance of the Sn-Co-C nanocomposites was examined by X-ray diffraction (XRD), 119 Sn Mossbauer effect spectroscopy, and electrochemical methods. Thermal stability aspects of these nanocomposites were inferred from differential scanning calorimetry (DSC) and surface area measurements. XRD experiments show diffraction patterns characteristic of nanostructured materials, except for the sample without carbon, which shows broad Bragg peaks of Co 3 Sn 2 . Mossbauer effect spectroscopy shows that the samples are best described as Sn-Co grains surrounded by a carbon matrix. DSC of the samples in air showed crystallization of CoSn for samples with low carbon content and combustion of carbon for samples with high amounts of carbon. The specific surface area of the samples was less than 1 m 2 /g for samples with y ≤ 0.6. Excellent charge-discharge capacity retention was observed for samples with y ≥ 0.3. Samples with acceptable electrochemical performance and low reactivity with air at elevated temperature were found in the range 0.3 ≤ y ≤ ≤0.6.