Crocodile skin inspired rigid-supple integrated flexible lithium ion batteries with high energy density and bidirectional deformability

Flexible lithium ion batteries (FLIBs), considered as the most promising energy source for flexible planar electronic devices especially for portable phones and laptops, have attracted great attention. However, it is a critical challenge to achieve high energy density and high flexibility simultaneously. Herein, we demonstrate a novel and rational rigid-supple integrated FLIB inspired by crocodile skin. The thick, rigid parts for storing the most energy correspond to the osteoderm of crocodile skin, while the thin, supple parts are regarded as dermis of crocodile skin, providing outstanding bidirectional flexibility for planar flexible electronics. The fabricated battery, containing LiNi_0.5Co_0.2Mn_0.3O₂-based cathode and graphite-based anode, shows a record-setting volumetric energy density of 400.3 Wh L⁻¹. More importantly, it can maintain stable electrochemical performance with 92.3% capacity retention and 0.038% capacity decay per cycle even undergo 30,000 times bending and after 200 charge/discharge cycles. Furthermore, the electrochemical property of the battery is confirmed by the tough dynamic bending test, including over 13,000 bidirectional bending times. Meanwhile, mechanical simulation results reveal its flexible mechanism. Accordingly, the reported bidirectional FLIB provides great promise for future practical applications in flexible devices.