Silicon is a very promising candidate to replace graphite as the anode in Li-ion batteries because of its very high theoretical capacity. It has not yet made its way into commercial cells because of severe problems with the charge and discharge cycling of the material. It seems that amorphous silicon and amorphous silicon-containing alloys exhibit much improved cycling performance. Therefore, it is desirable to fully understand the reaction of Li with a-Si. To this end, an in situ X-ray diffraction study of the reaction of lithium with a-Si has been performed. The results confirm that a new crystalline Li 15 Si 4 phase is formed below 30 mV vs. Li/Li ϩ as first reported by Obrovac and Christensen in an article published in Electrochemical and Solid-State Letters. However, the crystalline phase only forms for films of a-Si above a critical thickness of about 2 m.

Many intermetallic materials deliver poor capacity retention when cycled vs. Li. Many authors have attributed this poor capacity retention to large volume expansions of the active material. Here we report the volume changes of continuous and patterned films of crystalline Al, crystalline Sn, amorphous Si ͑a-Si͒, and a-Si 0.64 Sn 0.36 as they reversibly react with Li measured by in situ atomic force microscopy ͑AFM͒. Although these materials all undergo large volume expansions, the amorphous phases undergo reversible shape and volume changes. The crystalline materials do not. We attribute this difference to the homogeneous expansion and contraction that occurs in the amorphous materials. Inhomogeneous expansion occurs in the crystalline materials due to the presence of coexisting phases with different Li concentrations. Thin films of a-Si and a-Si 0.64 Sn 0.36 show good capacity retention with cycle number.

Oven exposure testing is a standard benchmark that Li-ion cells must pass in order to be approved for sale by regulating bodies. In order to test the safety of new cell designs or electrode materials, manufacturers must make small test batches of cells. This can be both costly and time consuming. Using reaction kinetics that have been developed for electrode materials with electrolyte exposed to high temperature, and thermal properties of cells from the literature, a predictive model for oven exposure testing has been developed. The model predictions are compared to oven exposure test results for E-One/Moli Energy, Canada, 18650 LiCoO2 /graphite cells and shown to be in good agreement. The model can predict the response of new cell sizes and electrode materials to oven exposure testing without actually producing any cells. This is illustrated with a number of examples: (i) increasing the specific surface area of the graphite electrode; (ii) using LiMn2O4 or other cathode substitutes instead of LiCoO2 ; (iii) varying the diameter of cylindrical cells; and (iv) varying the thickness of prismatic cells. © 2001 The Electrochemical Society. All rights reserved.

The electrochemistry and the structural changes that occur during sodium insertion and removal from tin are studied by in-situ X-ray diffraction at 30 • C. The Sn vs. Na voltage curve has four distinct plateaus, corresponding to four two-phase regions during sodiation, and indicating that four Na-Sn binary alloys are formed. The alloy formed at full sodiation was found to be Na 15 Si 4 , as expected from the Na-Sn binary system at equilibrium. The three intermediate Na-Sn phases that form during sodiation have X-ray diffraction patterns that do not correspond to any known equilibrium phase of Na-Sn. More work is needed to characterize these new binary Na-Sn phases.

A multitarget sputtering machine with a water-cooled rotating substrate table has been modified to produce films on 75 mm × 75 mm wafers which map large portions of ternary phase diagrams. The system is unconventional because the stoichiometries of the elements sputtered on the wafer vary linearly with position and in an orthogonal manner. Subsequent screening of film properties is therefore quite intuitive, since the compositional variations are simple. Depositions are made under continuous rotation, so either intimate mixing of the elements (fast rotation) or artificial layered structures (slow rotation) can be produced. Rotating subtables mounted on the main rotating table hold the 75 mm × 75 mm substrates. Stationary mask openings over the targets and mechanical actuators that rotate the subtables in a precise manner accomplish the linear and orthogonal stoichiometry variations. Deposition of a film spanning the range SiSn x Al y (0 < x, y < 1), with Sn content increasing parallel to one edge on the wafer and Al content increasing in a perpendicular direction, is given to illustrate the effectiveness of the method. Since the system was easily and inexpensively built, it has enabled our research program in combinatorial materials synthesis to begin without large scale funding.