Research
Laboratory for Energy & Electronic Nanomaterials
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Research Overview
Lithium-Ion Batteries
Lithium-ion battery separators are critical safety components that prevent internal short circuits while enabling selective lithium-ion transport between electrodes. However, commercially used polyolefin-based separators, such as polyethylene (PE), suffer from severe thermal shrinkage at elevated temperatures and insufficient mechanical compliance under deformation, which raises significant safety concerns for next-generation batteries operating under high-temperature or mechanically demanding conditions. In particular, for emerging flexible and stretchable battery systems, conventional separators fail to simultaneously satisfy thermal stability and mechanical stretchability, as most previous approaches have focused on improving only one of these properties at the expense of the other.
To address these limitations, our research group is conducting research on heat-resistant and stretch-tolerant lithium-ion battery separators based on electrospun core–shell nanofiber architectures. Specifically, we developed a coaxially electrospun polyimide@polyimide/thermoplastic polyurethane (PI@PI/TPU) nanofiber separator, in which a thermally and chemically robust polyimide shell directly interfaces with the electrolyte and electrodes, while an elastic PI/TPU core accommodates mechanical deformation. This core–shell design enables the separator to maintain dimensional integrity at high temperatures while simultaneously absorbing tensile strain, thereby overcoming the intrinsic trade-off between thermal robustness and flexibility observed in conventional separators.
The resulting nanofiber separator exhibits exceptionally low thermal shrinkage even at 150 °C, along with significantly enhanced stretchability compared to conventional polyimide and polyethylene separators. Owing to its high porosity and excellent electrolyte affinity, the separator also achieves favorable ionic conductivity and stable electrochemical performance, not only at room temperature but also under elevated temperatures of 65–90 °C. Furthermore, by employing a pre-stretching methodology, we systematically isolated the effect of separator mechanics from electrode deformation and clarified the relationship between strain history, separator microstructure, and electrochemical performance.
Through this work, our research group is advancing the design of nanofiber-based lithium-ion battery separators that simultaneously provide thermal stability, mechanical compliance, and reliable ion transport, with the ultimate goal of enabling safe and durable stretchable energy storage systems for next-generation electronic devices.
Piezoelectric Vibration Sensors
Low-frequency vibrations, typically ranging from a few hertz to several hundred hertz, can disrupt precision industrial processes such as semiconductor manufacturing and are also critical signals to monitor in wearable devices and structural health monitoring systems. Accordingly, there is a growing demand for high-sensitivity vibration sensors capable of converting mechanical vibrations into electrical signals without external power sources. Conventional piezoelectric ceramic–based sensors can generate strong electrical outputs; however, their intrinsic brittleness and the use of toxic lead-based materials significantly limit their applicability in flexible and wearable electronics. In contrast, piezoelectric polymers such as polyvinylidene fluoride (PVDF) offer excellent flexibility and mechanical robustness, but they generally suffer from low piezoelectric sensitivity due to a limited fraction of the electroactive β-phase, resulting in insufficient signal output.
To address these limitations, our research group has conducted research on high-performance vibration sensors based on structure-engineered piezoelectric nanofibers fabricated by coaxial electrospinning. Specifically, we designed a core–shell nanofiber architecture in which PVDF serves as the core material, while a PVDF matrix incorporating tetragonal-phase barium titanate (BaTiO₃, BTO) nanoparticles forms the shell. This configuration selectively positions the piezoelectric ceramic near the fiber surface, enabling more efficient transfer of mechanical stress to the piezoelectrically active components while preserving the mechanical flexibility of the polymer core.
In addition, the strong electric field and elongational forces inherent to the electrospinning process, combined with uniaxial fiber alignment, effectively promote the formation of the β-phase in PVDF, thereby significantly enhancing the overall piezoelectric response. The aligned core–shell nanofiber mats were integrated with metal electrodes to fabricate vibration sensor devices. As a result, the sensors exhibited markedly enhanced output voltages under low-frequency vibration conditions (e.g., 10 Hz) compared to sensors based on pristine PVDF fibers, while maintaining stable performance over repeated vibration cycles.
Through this work, our research group is advancing the development of flexible, lead-free, and high-sensitivity piezoelectric vibration sensors through nanoscale structural design and coaxial electrospinning strategies, with the aim of enabling reliable low-frequency vibration detection for wearable electronics, industrial monitoring, and structural diagnostics.
Stretchable Electronics
Stretchable and transparent conductors are key components in next-generation optoelectronic devices, including wearable electronics, flexible displays, and artificial skin systems. However, conventional transparent electrode materials have faced fundamental limitations in practical device applications due to inherent trade-offs among high electrical conductivity, optical transparency, and mechanical stretchability. In particular, widely used transparent electrodes such as indium tin oxide (ITO) exhibit excellent electrical performance but suffer from severe mechanical brittleness, while metal nanowire- or nanofiber-based electrodes can achieve stretchability but often experience degraded electrical reliability due to contact resistance at internal junctions and structural instability under repeated mechanical deformation.
To overcome these challenges, our research group has pursued stretchable and transparent electrode designs based on geometric engineering of two-dimensional mesh structures, rather than relying on modifications of intrinsic material properties. Specifically, we proposed a junction-free unibody hierarchical (web-in-web) gold (Au) mesh electrode, in which symmetric macro-scale meshes are integrated with asymmetric submicron-scale meshes to maximize mechanical strain accommodation. Using computational fluid dynamics (CFD) simulations, we systematically analyzed stress distribution and structural stability under tensile deformation, and identified diagonal mesh architectures as an effective strategy for mitigating stress concentration during stretching.
Furthermore, we developed a process-friendly fabrication strategy by integrating photolithography with electrospinning, enabling large-area manufacturing of stretchable and transparent electrodes without complex nanopatterning processes. Electrospun polymer nanofibers were employed as etching masks to readily form hierarchical mesh structures. The resulting electrodes maintained low sheet resistance even under tensile strains exceeding 70%, while simultaneously achieving over 90% visible-light transmittance and excellent durability over more than 1,000 stretching cycles. These results demonstrate superior electrical and mechanical stability compared to conventional nanowire-based electrodes or single-level mesh structures.
Through this work, our research group is advancing the development of next-generation stretchable and transparent electrodes that simultaneously satisfy mechanical compliance, optical transparency, and electrical reliability, with the ultimate goal of enabling high-performance wearable and flexible optoelectronic devices.
Particulate Matter Filter
Particulate matter (PM) is a major indoor air pollutant that significantly degrades air quality, and ultrafine particles such as PM2.5 are particularly hazardous because they can penetrate deep into the human respiratory system, increasing the risk of respiratory diseases and mortality. Consequently, the demand for high-performance air purification filters has continued to grow. However, conventional high-efficiency filters typically achieve high PM removal efficiency at the expense of large pressure drops and low air permeability. While such characteristics are acceptable for systems driven by forced air circulation, they are not suitable for window-mounted filters or low-energy air purification systems that must operate under natural convection conditions. In particular, filters based on purely mechanical filtration mechanisms suffer from a fundamental trade-off between particle removal efficiency and airflow resistance, which limits their effectiveness in capturing ultrafine PM2.5 particles.
To address these challenges, our research group has conducted research on functional nanofiber filters that capture particulate matter through electrostatic interactions rather than relying solely on mechanical filtration. Specifically, we introduced tetragonal BaTiO₃ nanoparticles with self-polarization properties into a low-resistance electrospun nanofiber network to realize an electrostatic PM capture mechanism without the need for external power sources or additional energy input. Owing to its ferroelectric nature, tetragonal BaTiO₃ can form spontaneous polarization under ambient conditions, and when uniformly dispersed within the nanofiber matrix, it generates a persistent electrostatic field on the filter surface.
In this work, surface-modified BaTiO₃ nanoparticles were incorporated into polyimide-based electrospun nanofibers to fabricate a transparent air filtration membrane capable of efficiently removing PM2.5 while maintaining high optical transparency and a low pressure drop. Furthermore, density functional theory (DFT) calculations and Kelvin probe force microscopy (KPFM) analyses were employed to elucidate the spontaneous polarization behavior of BaTiO₃ nanoparticles and the resulting surface charge characteristics of the filter. These analyses confirmed that the enhanced PM removal performance originates from electrostatic interactions. As a result, the developed filter simultaneously achieves high particulate removal efficiency and extremely low airflow resistance under natural convection conditions, demonstrating its potential as a low-energy, high-performance air purification filter suitable for window applications.
We are becoming the frontiers in the area of energy & electronics with nano-material applications.
Our key research topics are mainly focused on, but not limited to, stretchable electronics, including:
sensors (deformation, light, vibration, etc.),
energy devices (supercapacitors, batteries, power generators, etc.)
stretchable conductor structure, and
nano-material analysis and characterization for those above applications.
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