Flexible Lithium Supercapacitors with Water-Processable Solid-State Electrolytes Based on Aromatic Acid-Doped Branched-Poly(ethylene imine) Platforms

Flexible lithium supercapacitors require solid-state electrolytes (SSEs) to secure stability and durability under deformation. This work demonstrates high-performance flexible supercapacitors using water-processable aromatic acid-strengthened SSEs composed of 4-ethylbenzenesulfonic acid (EBSA), branched-poly(ethylene imine) (bPEI), and lithium hydroxide (LiOH). The devices were fabricated by placing the bPEI:EBSA:LiOH (PEL) SSEs between graphite-based active layers on indium-tin oxide (ITO)-coated poly(ethylene naphthalate) (PEN) substrates and bare ITO-PEN substrates. The ion conductivity of PEL exhibited the highest value (13.9 mS/m) at EBSA = 3.0, leading to a maximum specific capacitance of ~310 mF/g (galvanostatic charging/discharging). Wetting and spectroscopic analysis revealed that EBSA plays a crucial role in enhancing ion conductivity via a carbonophilic interlocking mechanism of aromatic groups between EBSA and graphite domains, facilitating the effective transfer of lithium cations from the PEL layers to the graphite electrode layers. Galvanostatic charge/discharge tests showed that the present flexible supercapacitors operate stably across a current density of 0.05–0.8 mA/g and reached 90% capacitance retention over 10,000 cycles. Furthermore, the optimized devices retain ~91% performance even after 500 bending tests. The present PEL SSEs approach represents a significant scientific leap toward developing fully flexible lithium supercapacitors for advanced applications such as humanoid robots and wearable systems for biomedical and aerospace purposes.