Invited Presentation: Electrolyte Breakthrough to Advance Flexible Batteries
Authors: Lee Sang-Young 
Published: February 2020

Flexible batteries have garnered a great deal of attention as a promising power source for versatile-shaped electronic devices including bendable smart phones, roll-up displays and wearable electronics, which play crucial roles in facilitating the advent of wearable electronics era. One formidable challenge for development of the flexible batteries arises from difficulties in securing solid-state electrolytes with reliable electrochemical/mechanical properties. Currently available liquid electrolytes show good electrochemical performance suitable for practical applications, however, they often suffer from safety failures due to unwanted leakage problems and flammable characteristics. Moreover, liquid electrolytes limit choices in cell design due to their fluidic attributes, which essentially require the use of separator membranes and packaging canisters with fixed dimension in cell assembly. These shortcomings of liquid electrolytes motivate us to develop advanced solid-state electrolytes that can easily conform to complex-structured substrates such as three-dimensional (3D) electrodes and also ensure sufficient mechanical deformability for reliable use.

Herein, as electrolyte breakthrough to advance flexible batteries, a new class of bendable, shape-conformable and thermally-stable composite polymer electrolyte is demonstrated and also its potential application to flexible batteries featuring shape diversity is explored. More notably, the composite polymer electrolyte can be directly writable and printable onto complicated/contoured electrodes, without the aid of processing solvents. The unique structural/compositional designs and well-tuned rheological properties, in collaboration with direct UV-assisted nanoimprint lithography, allow for successful fabrication of solid-state composite polymer electrolytes in geometries that lie far beyond those accessible with conventional electrolytes. As an alternative electrolyte to exceed conventional carbonate-based liquid electrolytes, plastic crystal electrolytes (PCEs) have been investigated. The PCEs, which are composed of lithium salts and plastic crystals bearing good solvation capability, are characterized with unusual thermal stability and ionic transport behaviors. Among various plastic crystal candidates, succinonitrile (SN) is known to show excellence in thermal stability and ionic transport due to high boiling point and structural defects (i.e., trans-gauche isomerism) of plastic crystal phase. Despite of these advantageous characteristics, application of the PCEs to flexible batteries is staggered due to their poor mechanical properties. Most of PCEs are excessively plastic and even more show liquid-like behavior at room temperature. In this talk, we present a wide variety of material strategies to improve the physicochemical properties (in particular, focusing on mechanical flexibility) of SN-based PCEs, which are mainly devoted to integrating with functional polymer matrix that serves as a mechanical framework. The structural design concept demonstrated herein for the synthesis of plastic crystal polymer electrolytes can be suggested as a versatile and scalable electrolyte platform to bring unprecedented opportunities in progress of next-generation flexible energy storage/conversion devices.

References

Keun-Ho Choi, Sung-Ju Cho, Se-Hee Kim, Yo Han Kwon, Je Young Kim, Sang-Young Lee, "Thin, deformable, and safety-reinforced plastic crystal polymer electrolytes for high-performance flexible lithium-ion batteries", Adv. Funct. Mater. online published.

Sang-Young Lee, Keun-Ho Choi, Yo Han Kwon, Hye-Ran Jung, Heon-Cheol Shin, Je Young Kim, "Progress in flexible energy storage and conversion systems, with a focus on cable-type lithium ion battery", Energy Environ. Sci.
2013, 6, 2414. (featured as a back cover image).

Eun-Hye Kil, Keun-Ho Choi, Hyo-Jeong Ha, Sheng Xu, John A. Rogers, Mi Ri Kim, Young-Gi Lee, Kwang-Man Kim, Kuk Young Cho, Sang-Young Lee, "Imprintable, bendable, and shape-conformable polymer electrolytes for versatile-shaped lithium-ion batteries", Adv. Mater.
2013, 25, 1395. (featured as a back cover image).