## 3 Non-Optical Applications of Functional Materials with 3D Periodic Nanostructures
### Electrodes for Energy Storage Devices  
As the demand for electric vehicles and portable electronic devices has increased rapidly, it is constantly required to increase the performances (e.g., power or energy density and life cycle) of rechargeable batteries. In this regard, innovation of electrode materials for batteries through the introduction of nanotechnology is being studied at the core. In particular, 3D periodic nanostructures defined by optical 3D lithography have several key characteristics that differentiate them from conventional planar electrode structures manufactured through a slurry-based coating process: (1) 3D periodic nanostructures are lightweight and have an exceptionally high surface area, which can contribute to increasing the energy density of batteries. (2) The interconnected porous network of 3D periodic nanostructures provides an efficient mass transfer path, enabling fast charging. (3) 3D periodic nanostructures enhance the cyclability of electrodes through effective stress relaxation in response to the volume expansion/contraction of electrodes that occurs during charging and discharging. (4) 3D periodic nanostructures can be structurally optimized through feedback with the measured performance. Based on these, it is expected that the overall battery performance, such as energy density, life cycle, and charging speed, will be improved if the 3D periodic nanostructure is well utilized as a new electrode architecture. [PERSON] et al. recently developed a microbatter that showed high energy density (\(\sim\)6.5 \(\upmu\)Whcm\({}^{-2}\upmu\)m\({}^{-1}\)) resulting from the 3D mesostructured LiMnO\({}_{2}\) cathodes and Ni-Sn anodes (Figure 7a-c) [53]. To create a circuit for microbatteries, conventional photolithography was used to pattern once more the 3D nanostructured polymer template prepared by MBIL. Through sequential template-assisted electroplating with different electrolytes, a line-patterned array of 3D nanostructured cathodes and anodes was successfully created. They also demonstrated that the structural parameters (e.g., filling fraction, pore shape and size, etc.) of the 3D nanostructured electrodes, which have a significant impact on the energy and power density of microbatteries, can be precisely controlled during the lithographic step of manufacturing polymer templates. Although the electrode thickness was only \(\sim\)10 \(\upmu\)m, the resulting 3D microbattery showed a power of \(\sim\)3.6 \(\times\) 10\({}^{4}\)\(\upmu\)W/cm\({}^{2}\), and unlike thin film microbatteries with a planar electrode structure, both energy and power scales tended to be proportional to the thickness. Such electrode structures can provide both high energy and high power to the battery, and MBIL allows precise control of the structural parameters of electrodes, facilitating the customization of energy and power for target applications. Since the developed process is compatible with conventional semiconductor processing, it can be potentially used for the on-chip integration of microbatteries and semiconductor devices.  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & **Waveform Type** & **Current Density** & \multicolumn{2}{c}{**Temperature**} & \multirow{2}{*}{**Deposited Material**} & \multirow{2}{*}{**Application**} & \multirow{2}{*}{**Ref**} \\  & & & & (\({}^{\circ}\)C) & & & \\ \hline MBIL & Potentiostatic condition & \(-\)2 V & RT & Ni & Battery & [53] \\ MBIL & Pulse & \(-\)0.22 V 0.2 s, 10 s off & RT & Ni-Sn & Battery & [53] \\ MBIL & Pulse & 1.8 V 0.2 s, 10 s off & RT & MnO\({}_{2}\) & Battery & [53] \\ PnP & Pulse & 2 mA/cm\({}^{2}\) (5 s on, off) & RT & Ni & Electrocalyst & [86] \\ PnP & Pulse & \(-\)1.1 V (5 s on, off) & RT & Au-Ag & Electrocalyst & [83] \\ PnP & Pulse & \(-\)0.05 V 4 s, 0.017 V 6 s & RT & Bi-Sb-Te & Thermoelectric & [38] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Selected examples of 3D nanostructured template-assisted electroplating and their process parameters.