Hyungkyu Han

Nitridated TiO2 hollow nanofibers as an anode material for high power lithium ion batteries

TiO2 nanofibers, TiO2 hollow nanofibers, and nitridated TiO2 hollow nanofibers were synthesized using a simple electrospinning method and subsequent nitridation treatment. The nitridated TiO2 hollow nanofibers showed twice higher rate capability compared to that of pristine TiO2 nanofibers at 5 C. This improvement is mainly attributed to shorter lithium ion diffusion length and high electronic conductivity along the surface of nitridated hollow nanofibers.

Si/Ge Double-Layered Nanotube Array as a Lithium Ion Battery Anode

Problems related to tremendous volume changes associated with cycling and the low electron conductivity and ion diffusivity of Si represent major obstacles to its use in high-capacity anodes for lithium ion batteries. We have developed a group IVA based nanotube heterostructure array, consisting of a high-capacity Si inner layer and a highly conductive Ge outer layer, to yield both favorable mechanics and kinetics in battery applications. This type of Si/Ge double-layered nanotube array electrode exhibits improved electrochemical performances over the analogous homogeneous Si system, including stable capacity retention (85% after 50 cycles) and doubled capacity at a 3C rate. These results stem from reduced maximum hoop strain in the nanotubes, supported by theoretical mechanics modeling, and lowered activation energy barrier for Li diffusion. This electrode technology creates opportunities in the development of group IVA nanotube heterostructures for next generation lithium ion batteries.

A Ge inverse opal with porous walls as an anode for lithium ion batteries

Germanium holds great potential as an anode material for lithium ion batteries due to its large theoretical energy density and excellent intrinsic properties related to its kinetics associated with lithium and electrons. However, the problem related to the tremendous volume change of Ge during cycling is the dominant obstacle for its practical use. The previous research has focused on the improvement in mechanics associated with lithium without consideration of the kinetics. In this study, we demonstrate that the configuration engineering of the Ge electrode enables the improvement in kinetics as well as favorable mechanics. Two types of Ge inverse opal structures with porous walls and dense walls were prepared using a confined convective assembly method and by adjusting Ge deposition parameters in a chemical vapor deposition system. The Ge inverse opal electrode with porous walls shows much improved electrochemical performances, especially cycle performance and rate capability, than the electrode with dense walls. These improvements are attributed to a large free surface, which offers a facile strain relaxation pathway and a large lithium flux from the electrolyte to the active material.

TiO2 nanotube branched tree on a carbon nanofiber nanostructure as an anode for high energy and power lithium ion batteries

The inherently low electrical conductivity of TiO2-based electrodes as well as the high electrical resistance between an electrode and a current collector represents a major obstacle to their use as an anode for lithium ion batteries. In this study, we report on high-density TiO2 nanotubes (NTs) branched onto a carbon nanofiber (CNF) “tree” that provide a low resistance current path between the current collector and the TiO2 NTs. Compared to a TiO2 NT array grown directly on the current collector, the branched TiO2 NTs tree, coupled with the CNF electrode, exhibited ∼10 times higher areal energy density and excellent rate capability (discharge capacity of ∼150 mA·h·g−1 at a current density of 1,000 mA·g−1). Based on the detailed experimental results and associated theoretical analysis, we demonstrate that the introduction of CNFs with direct electric contact with the current collector enables a significant increase in areal capacity (mA·h·cm−2) as well as excellent rate capability.

Silicon nanowires with a carbon nanofiber branch as lithium-ion anode material

Si nanowires (SiNWs)–carbon nanofibers (CNFs) branched structures with variation in carbon densities were synthesized. SiNWs with critical density of CNFs show the best electrochemical performance, which is attributed to increase in free volume around the SiNWs as well as a buffering role of the branched CNFs against large volumetric change during cycling.

Electrospun Sn-doped LiTi2(PO4)3/C nanofibers for ultra-fast charging and discharging

Sn-doped LiTi2(PO4)3/C composite nanofibers are synthesized by a facile electrospinning process. The unique one dimensional nanostructure combined with a uniform electrically conductive carbon matrix allows high-rate transportation of lithium ions and electrons. Besides, Sn-doping could further decrease the electrochemical resistance. Sn-doped LiTi2(PO4)3/C composite nanofibers exhibit excellent electrochemical performance, especially ultra-fast charging/discharge capability. At a charging rate of about 600 C (64 A g−1, 6 s), 66.2% capacity (68.9 mA h g−1) could be obtained when matched with a Li metal counter electrode. They also exhibit excellent electrochemical properties as an anode material for aqueous rechargeable lithium batteries. Sn-doped LiTi2(PO4)3/C composite nanofibers are promising electrode materials for both nonaqueous and aqueous lithium ion batteries.

Electrospun porous lithium manganese phosphate–carbon nanofibers as a cathode material for lithium ion batteries

Porous LiMnPO4/C composite nanofibers between 150 and 250 nm have been synthesized as a cathode material for lithium ion batteries via electrospinning. The porous LiMnPO4/C composite nanofibers show excellent electrochemical performance including a high reversible capacity of 112.7 mA h g−1 at 0.2C, a stable cycle retention of 95% after 100 cycles at 1C, and excellent rate capability (73.1 mA h g−1 at 5C) in constant current mode between 2.0 and 4.5 V. These improved electrochemical properties are attributed to the one-dimensional geometry and porous structure, which significantly enhanced kinetics associated with electrons and Li ions.