Electrode-to-electrode monolithic integration for high-voltage bipolar solid-state batteries based on plastic-crystal polymer electrolyte

• The EtE monolithic integration process is proposed for high-voltage bipolar SSBs. • Plastic-crystal polymer electrolyte (PCPE) was infused into a porous electrode. • The infused PCPE enables the fast Li + conduction and intimate interfacial contact. • The microstructural engineering results in enhanced performance of SSB. • The efficacy of the EtE-integration procedure is demonstrated in a bipolar SSB. Solid-state batteries (SSBs) offer a fundamental solution to mitigate the safety and reliability issues of conventional lithium-ion batteries utilizing flammable liquid electrolytes, and enable the bipolar configuration of high-voltage and high-energy storage systems. However, the conventional layer-by-layer (LbL) stacking process using individual electrolyte and electrode layers suffers from poor electrolyte–electrode contacts and challenging process complications for manufacturing multi-layer bipolar SSBs. Herein, we report an electrode-to-electrode (EtE) monolithic integration without a free-standing solid electrolyte layer for high-voltage bipolar SSBs. Positive and negative electrodes seamlessly combined with a thin solid electrolyte are fabricated by the infusion of a plastic-crystal-based polymer electrolyte (PCPE) into porous electrodes with a subsequent ultraviolet-induced solidification process. The infused PCPE in the electrodes forms continuous Li + conduction channels as well as intimate solid–solid interfaces. The thin PCPE film (∼10 μm) formed in situ on the top of the electrodes during the infusion process provides ultra-high Li + conductance (∼3.1 S cm −2 at 45 °C) between the two electrodes. SSBs are constructed via direct integration of the PCPE-infused electrodes: a unit cell-type SSB with LiNi 0.6 Co 0.2 Mn 0.2 O 2 (positive) and Li 4 Ti 5 O 12 (negative) show superior capacity (∼160 mAh g −1 ), rate capability (98 mAh g −1 at 2C), and stable cyclability (81% after 100 cycles) at 45 °C than the SSB fabricated by the conventional LbL stacking process. Moreover, a 10 V-class, bipolar SSB comprising five unit cells stacked in series is constructed via the EtE monolithic integration of multiple PCPE-infused bipolar electrodes, and its stable cycle performance is corroborated with a capacity retention of 84%. This work demonstrates that the suggested EtE integration process offers a promising strategy to addressing the interfacial contact issues of SSBs, thereby being utilized to realize scalable, low-cost, high-voltage bipolar SSBs.