Paul R. Abel

Improving the Stability of Nanostructured Silicon Thin Film Lithium-Ion Battery Anodes through Their Controlled Oxidation

Silicon and partially oxidized silicon thin films with nanocolumnar morphology were synthesized by evaporative deposition at a glancing angle, and their performance as lithium-ion battery anodes was evaluated. The incorporated oxygen concentration was controlled by varying the partial pressure of water during the deposition and monitored by quartz crystal microbalance, X-ray photoelectron spectroscopy. In addition to bulk oxygen content, surface oxidation and annealing at low temperature affected the cycling stability and lithium-storage capacity of the films. By simultaneously optimizing all three, films of ~2200 mAh/g capacity were synthesized. Coin cells made with the optimized films were reversibly cycled for ~120 cycles with virtually no capacity fade. After 300 cycles, 80% of the initial reversible capacity was retained.

Sn–Cu Nanocomposite Anodes for Rechargeable Sodium-Ion Batteries

Sn0.9Cu0.1 nanoparticles were synthesized via a surfactant-assisted wet chemistry method, which were then investigated as an anode material for ambient temperature rechargeable sodium ion batteries. The Sn0.9Cu0.1 nanoparticle-based electrodes exhibited a stable capacity of greater than 420 mA h g(-1) at 0.2 C rate, retaining 97% of their maximum observed capacity after 100 cycles of sodium insertion/deinsertion. Their performance is considerably superior to electrodes made with either Sn nanoparticles or Sn microparticles.

Nanostructured Si(1-x)Gex for Tunable Thin Film Lithium-Ion Battery Anodes

Both silicon and germanium are leading candidates to replace the carbon anode of lithium ions batteries. Silicon is attractive because of its high lithium storage capacity while germanium, a superior electronic and ionic conductor, can support much higher charge/discharge rates. Here we investigate the electronic, electrochemical and optical properties of Si(1-x)Gex thin films with x = 0, 0.25, 0.5, 0.75, and 1. Glancing angle deposition provided amorphous films of reproducible nanostructure and porosity. The films composition and physical properties were investigated by X-ray photoelectron spectroscopy, four-point probe conductivity, Raman, and UV-vis absorption spectroscopy. The films were assembled into coin cells to test their electrochemical properties as a lithium-ion battery anode material. The cells were cycled at various C-rates to determine the upper limits for high rate performance. Adjusting the composition in the Si(1-x)Gex system demonstrates a trade-off between rate capability and specific capacity. We show that high-capacity silicon anodes and high-rate germanium anodes are merely the two extremes; the composition of Si(1-x)Gex alloys provides a new parameter to use in electrode optimization.

Tin–Germanium Alloys as Anode Materials for Sodium-Ion Batteries

The sodium electrochemistry of evaporatively deposited tin, germanium, and alloys of the two elements is reported. Limiting the sodium stripping voltage window to 0.75 V versus Na/Na+ improves the stability of the tin and tin-rich compositions on repeated sodiation/desodiation cycles, whereas the germanium and germanium-rich alloys were stable up to 1.5 V. The stability of the electrodes could be correlated to the surface mobility of the alloy species during deposition suggesting that tin must be effectively immobilized in order to be successfully utilized as a stable electrode. While the stability of the alloys is greatly increased by the presence of germanium, the specific Coulombic capacity of the alloy decreases with increasing germanium content due to the lower Coulombic capacity of germanium. Additionally, the presence of germanium in the alloy suppresses the formation of intermediate phases present in the electrochemical sodiation of tin. Four-point probe resistivity measurements of the different compositions show that electrical resistivity increases with germanium content. Pure germanium is the most resistive yet exhibited the best electrochemical performance at high current densities which indicates that electrical resistivity is not rate limiting for any of the tested compositions.

Sub-stoichiometric germanium sulfide thin-films as a high-rate lithium storage material

Nanocolumnar, sub-stoichiometric germanium sulfide thin-films with compositions of Ge0.9S0.1 and Ge0.95S0.05, deposited by glancing angle deposition, were investigated as lithium storage materials. The materials are amorphous and homogeneous as deposited, but lithiation induces phase separation leading to the formation of poorly-crystallized lithium sulfide inclusions during the first cycle. The presence of these inclusions raises the lithium diffusion coefficient above that of pure germanium and provides superior capacity retention at high rates. While the lithium sulfide is non-cycling, the low weight percentage of sulfur necessary for enhanced lithiation/de-lithiation does not significantly reduce the specific lithium storage capacity of the films relative to that of germanium. In addition to high capacity and superior lithium transport, the sub-stoichiometric germanium sulfide thin-films show excellent cycling stability at high rates, retaining 88% of their initial capacity after 500 cycles at a rate of 20 C.