Seul Cham Kim

Reversible High‐Capacity Si Nanocomposite Anodes for Lithium‐ion Batteries Enabled by Molecular Layer Deposition

The molecular-layer deposition of a flexible coating onto Si electrodes produces high-capacity Si nanocomposite anodes. Using a reaction cascade based on inorganic trimethylaluminum and organic glycerol precursors, conventional nano-Si electrodes undergo surface modifications, resulting in anodes that can be cycled over 100 times with capacities of nearly 900 mA h g(-1) and Coulombic efficiencies in excess of 99%.

Stable silicon-ionic liquid interface for next-generation lithium-ion batteries

We are currently in the midst of a race to discover and develop new battery materials capable of providing high energy-density at low cost. By combining a high-performance Si electrode architecture with a room temperature ionic liquid electrolyte, here we demonstrate a highly energy-dense lithium-ion cell with an impressively long cycling life, maintaining over 75% capacity after 500 cycles. Such high performance is enabled by a stable half-cell coulombic efficiency of 99.97%, averaged over the first 200 cycles. Equally as significant, our detailed characterization elucidates the previously convoluted mechanisms of the solid-electrolyte interphase on Si electrodes. We provide a theoretical simulation to model the interface and microstructural-compositional analyses that confirm our theoretical predictions and allow us to visualize the precise location and constitution of various interfacial components. This work provides new science related to the interfacial stability of Si-based materials while granting positive exposure to ionic liquid electrochemistry.

Ionic liquid enabled FeS2 for high-energy-density lithium-ion batteries.

High-energy-density FeS2 cathodes en-abled by a bis(trifluoromethanesulfonyl)imide (TFSI-) anion-based room temperature ionic liquid (RTIL) electrolyte are demonstrated. A TFSI-based ionic liquid (IL) significantly mitigates polysulfide dissolution, and therefore the parasitic redox shuttle mechanism, that plagues sulfur-based electrode chemistries. FeS2 stabilization with a TFSI(-) -based IL results in one of the highest energy density cathodes, 542 W h kg(-1) (normalized to cathode composite mass), reported to date.

Hierarchical Porous Framework of Si‐Based Electrodes for Minimal Volumetric Expansion

A tunable hierarchical porous framework is fabricated to house the volumetric changes outputted by Si. The nSi@cPAN/cPAN electrodes only expand by 14.3% at full initial lithiation and remain within 23% expansion from its uncycled state after 20 cycles with remarkable cycling stability and high coulombic efficiencies in excess of 99.5%.

Epitaxial growth and ordering of GeTe nanowires on microcrystals determined by surface energy minimization.

We report self-assembly of highly aligned GeTe nanowires epitaxially grown on octahedral GeTe microcrystals in two well-defined directions by using one-step vapor transport process. The epitaxial relationship of nanowires with underlying microcrystals along with the growth orientations of nanowires were investigated in detail by electron microscopy combined with atomic unit cell models. We demonstrate that maximizing atomic planar density to minimize energy of the exposed surfaces is the determining factor that governs the unique growth characteristics of micro/nanostructures that evolve from three-dimensional octahedral microcrystals to tetrahedral bases to finally one-dimensional nanowires. The crystallographic understanding of structuring of crystalline nanomaterials obtained from this study will be critical to understand, predict, and control the growth orientation of nanostructures in three-dimensions.

Optimized Silicon Electrode Architecture, Interface, and Microgeometry for Next‐Generation Lithium‐Ion Batteries

Optimized performance of silicon-ionic- liquid lithium-ion batteries through the implementation of a new electrode-microgeometry. The incorporation of 1D silicon nanowires into the cyclized-polyacrylonitrile-based electrode-architecture allows for greatly improved active material utilization, higher rate capabilities, and reduced interfacial reactions.

Crystallization and memory programming characteristics of Ge-doped SbTe materials of varying Sb:Te ratio

A phase change memory (PCM) utilizes resistivity changes accompanying fast transitions from an amorphous to a crystalline phase (SET) and vice versa (RESET). An investigation was made on the SET characteristics of PCM cells with Ge-doped SbTe (Ge‐ST) materials of two different Sb:Te ratios (4.53 and 2.08). For the material of higher Sb:Te (4.53), a SET operation was completed within several tens of nanoseconds via nucleation-free crystallization whereas the material of lower Sb:Te (2.08) rendered a slower SET operation requiring several hundred nanoseconds for a nucleation-mediated crystallization. From measurements of nucleation and growth kinetics via laser-induced crystallization, the observed SET characteristics of the former case were found to derive from a growth time about 10 3 times shorter than the nucleation time and those of the latter from a much shorter nucleation time as well as a longer growth time than in the former case. The measured nucleation kinetics of the lower Sb:Te (2.08) material is unexpected from the existing data, which has led us to advance an interesting finding that there occurs a trend-reversing change in the nucleation kinetics of the Ge-ST materials around the eutectic composition (Sb:Te ∼2.6); nucleation is accelerated with the increase in the Sb:Te ratio above Sb:Te of 2.6, but with a decrease in the Sb:Te ratio below it.

A Generic Approach for Embedded Catalyst-Supported Vertically Aligned Nanowire Growth

We demonstrate a general approach for growing vertically aligned, single-crystalline nanowires of any material on arbitrary substrates by using plasma-sputtered Au/Pd thin films as a catalyst through the vapor-liquid-solid process. The high-energy sputtered Au/Pd atoms form a reactive interface with the substrate forming nanoclusters which get embedded in the substrate, thus providing mechanical stability for vertically aligned nanowire growth. We demonstrate that our approach for vertically aligned nanowire growth is generic and can be extended to various complex substrates such as conducting indium tin oxide.

Face-centered-cubic lithium crystals formed in mesopores of carbon nanofiber electrodes.

In the foreseeable future, there will be a sharp increase in the demand for flexible Li-ion batteries. One of the most important components of such batteries will be a freestanding electrode, because the traditional electrodes are easily damaged by repeated deformations. The mechanical sustainability of carbon-based freestanding electrodes subjected to repeated electrochemical reactions with Li ions is investigated via nanotensile tests of individual hollow carbon nanofibers (HCNFs). Surprisingly, the mechanical properties of such electrodes are improved by repeated electrochemical reactions with Li ions, which is contrary to the conventional wisdom that the mechanical sustainability of carbon-based electrodes should be degraded by repeated electrochemical reactions. Microscopic studies reveal a reinforcing mechanism behind this improvement, namely, that inserted Li ions form irreversible face-centered-cubic (FCC) crystals within HCNF cavities, which can reinforce the carbonaceous matrix as strong second-phase particles. These FCC Li crystals formed within the carbon matrix create tremendous potential for HCNFs as freestanding electrodes for flexible batteries, but they also contribute to the irreversible (and thus low) capacity of HCNFs.

The effect of energetically coated ZrOx on enhanced electrochemical performances of Li(Ni1/3Co1/3Mn1/3)O2 cathodes using modified radio frequency (RF) sputtering

To date, most coating layers for electrode materials for Li-ion batteries have been fabricated using the sol–gel method or atomic layer deposition (ALD), which involve complicated processing steps and limited candidates for coating materials. With an emphasis on solving these issues, herein, a new coating methodology based on a sputtering system was developed, and sputtered zirconium oxide was coated on Li(Ni1/3Co1/3Mn1/3)O2 (L333) cathode powders. The continuous movement of the cathode powders during the coating procedure and the high kinetic energy from the sputtering process resulted in a highly uniform coating layer with multiple structures exhibiting a concentration and valence state gradient of Zr, i.e., surface (mainly Zr4+) and doped (mainly Zr2+) layers. The ZrOx-coated L333 powders exhibited an outstanding capacity retention (96.3% at the 200th cycle) and superior rate capability compared with the uncoated version in a coin cell with 1 M LiPF6 in EC:DEC liquid electrolyte. The ZrOx-coated L333 powders also exhibited an enhanced specific capacity in a solid state battery cell with a sulfide-based inorganic solid-state electrolyte. The improved electrochemical performance of ZrOx/L333 was attributed to the synergetic effect from the surface and doped layers: physical/chemical protection of the active material surface, enhancement of Li-ion diffusion kinetics, and stabilization of the interfaces.