Multiple Printing-Enabled All-Solid-State Flexible Bipolar Lithium-Ion Batteries
Authors: Kim Se-Hee , Kim Ju-Myung , Kim Jung-Hwan , Lee Dong-Gue , Lee Sang-Young 
Published: February 2020

Enormous commercial interest in upcoming new energy storage application fields such as electric vehicles (EVs) and smart portable electronics has continuously pushed us to search for high-energy density rechargeable power sources. Bipolar cell configuration (in particular, lithium-ion battery applications) has garnered a great deal of attention as a promising way to achieve this goal. In comparison to the simple connection of monopolar cells in series, the bipolar batteries can offer several advantageous performances, including low internal resistance through the reduced terminal connections in a cell pack assembly and, more importantly, high volumetric energy density due to the minimal use of electrically-inert cell components such as current collectors and packaging substances.
A key components to realize the bipolar batteries is solid-state electrolytes, which play a crucial role as a separator membrane and also an electrolyte in electrodes. In case the adjacent cells inside the bipolar batteries are ionically connected, they lose the advantageous features (in particular, voltage build-up arising from the accumulated cells), eventually behaving like a single unit cell. Therefore, solid-state electrolytes without fluidic characteristics are urgently needed to secure reliable electrochemical performance. To date, inorganic electrolytes, including LiSICON, perovskite, garnet and sulfide types, have been extensively investigated for potential use in bipolar batteries, however, their intrinsic limitations such as low ionic conductivity, grain boundary resistance and mechanical brittleness have posed a formidable challenge in their practical development. To overcome these problems, recently, a few works suggested inorganic/organic hybrid electrolytes, wherein inorganic electrolytes were mixed with polymeric (gel) electrolytes. Some possibility for resolving the grain boundary issue was suggested, however, numerous technical issues have been still unresolved. Most notably, taking into consideration continuous manufacturing process and also application versatility, the mechanical fragility and stiffness of bipolar batteries should be urgently overcome. To the best of knowledge, no works have reported bipolar batteries with mechanical flexibility and safety tolerance, in addition to offering reliable/sustainable electrochemical performance.
Here, as a facile and eﬃcient strategy to address the aforementioned longstanding issues, we demonstrate a new class of printed flexible bipolar lithium-ion batteries (referred to as “PFBB”). A distinctive feature of the PFBBs, compared to previously-reported bipolar batteries, is the use of printable inorganic/organic (I/O) solid-state electrolytes (fabricated via ultraviolet (UV) curing process) with good electrochemical performance, robust thermal stability and mechanical deformability. Due to such unusual printable characteristic, the I/O solid-state electrolyte can be seamlessly integrated with electrodes comprising the I/O solid-state electrolyte, active materials and conductive additives, eventually leading to the simple and facile fabrication of all-solid-state bi-polar batteries without interfacial resistance concerns. Specifically, within an extremely short time (less than 30 min), PFBBs comprising 5 cells can be successfully prepared. The I/O electrolytes were composed of LSTP (Li2O-SiO2-TiO2-P2O5) inorganic electrolytes and gel polymer electrolyte (GPEs) containing glyme-based ionic liquids. The GPEs act as an ionic bridge between the LSTP particles, thereby alleviating the grain boundary resistance. Also, due to their ionic liquid-like behavior, excellent thermal stability can be achieved. More notably, the GPEs exploit semi-IPN (interpenetration polymer network) matrix based on UV-cured triacrylate (as a crosslinked polymer network) and PVdF-HFP (as a linear polymer), thus enabling exceptional mechanical flexibility. Meanwhile, to further improve the mechanical flexibility of the electrodes, current collectors with a diversity of pattern were employed. Owing to the well-defined microscale pattern, the electrode components (i.e., active materials, conductive additives and I/O electrolytes) were tightly adhered to the pattern current collectors, contributing to the improvement of mechanical deformability. Driven by the material/architecture uniqueness mentioned above, the PFBBs provided unprecedented improvements in mechanical flexibility and thermal stability with reliable/sustainable electrochemical performance (above operating voltage of 10.0 V) which lie far beyond those accessible with conventional bipolar battery technologies.