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A method is described in which crystalline silicon can be used as a practical anode material for lithium-ion batteries. Commercial lithium-ion cells are typically charged at a constant current to a fixed voltage and then are held by the charger at constant voltage until the current decreases to a certain value ͑also known as constant current/constant voltage or CCCV charging͒. It is first shown that CCCV charging can be used to reversibly cycle crystalline silicon and limit its capacity. A cycling method is then demonstrated in which crystalline silicon is first partially converted to amorphous silicon, in situ, during conditioning cycles. After the conditioning cycles the silicon can be cycled normally, using CCCV cycling limits, with good coulombic efficiency and little overlithiation during the first cycle.

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A set of guidelines is proposed for designing high-energy-density alloy anode materials. It is first shown that the molar volume of lithium is about 9 mL/mol in a wide variety of lithium alloys and is independent of lithium content. Using this property of lithium alloys, simple relationships between the volumetric energy density and the volumetric expansion of an alloy are derived. These relationships are extremely powerful for designing alloys with the maximum possible energy density for a given electrode-coating performance.

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.

The conditions for Li15Si4 formation in Si thin film negative electrodes was studied during charge and discharge cycling in lithium cells. It was found that Li15Si4 formation can be suppressed during cycling of Si thin films by stress induced from the presence of the substrate. Furthermore, Li15Si4 formation was found to be coincidental with capacity fade and delamination of the Si film from the current collector. These results deepen the understanding of the cycling of Si thin films. Moreover, they have profound implications for Si alloy negative electrodes for Li-ion batteries, as the presence of Li15Si4 during cycling can be used as a sensitive indicator for weakly bound Si regions.

Sodium ion batteries have garnered significant research attention in recent years due to the rising demand for large-scale energy storage solutions as well as the high abundance of sodium. P2-type layered oxide materials have been identified as promising positive electrode materials for sodium ion batteries. Previously, P2−Na 2/3 Ni 1/3 Mn 2/3 O 2 was shown to have a high operating voltage and high capacity but suffers from a step-like voltage curve and capacity loss during cycling, potentially due to its P2−O2 transition at high voltages. One strategy to improve cycling performance has been to dope Ni 2+ with other 2+ cations, such as Zn 2+ or Mg 2+ , which improved capacity retention but significantly decreases reversible capacity, since these ions were not electrochemically active. Since Cu 2+ has been shown to be electrochemically active, we replaced Ni 2+ with Cu 2+ , resulting in air-stable Na 2/3 Ni 1/3−x Cu x Mn 2/3 O 2 (0 ≤ x ≤ 1/3). Both Ni 2+ /Ni 4+ and Cu 2+ /Cu 3+ participate in the redox reaction during cycling, capacity retention was greatly improved, and phase changes were suppressed during cycling without sacrificing much capacity. The material retains a P2/OP4 structure even when cycled to high voltages. The doping strategy is a promising approach for the future development of positive electrode materials for sodium ion batteries.