Solid electrolytes are crucial for the development of solid state batteries. Among different types of solid electrolytes, poly(ethylene oxide) (PEO)-based polymer electrolytes have attracted extensive attention owing to their excellent flexibility and easiness for processing. However, their relatively low ionic conductivities and electrochemical instability above 4 V limit their applications in batteries with high energy density. Herein, we prepared poly(vinylidene fluoride) (PVDF) polymer electrolytes with an organic plasticizer, which possesses compatibility with 4 V cathode and high ionic conductivity (1.2 × 10 S/cm) at room temperature. We also revealed the importance of plasticizer content to the ionic conductivity. To address weak mechanical strength of the PVDF electrolyte with plasticizer, we introduced palygorskite ((Mg,Al)SiO(OH)) nanowires as a new ceramic filler to form composite solid electrolytes (CPE), which greatly enhances both stiffness and toughness of PVDF-based polymer electrolyte. With 5 wt % of palygorskite nanowires, not only does the elastic modulus of PVDF CPE increase from 9.0 to 96 MPa but also its yield stress is enhanced by 200%. Moreover, numerical modeling uncovers that the strong nanowire-polymer interaction and cross-linking network of nanowires are responsible for such significant enhancement in mechanically robustness. The addition of 5% palygorskite nanowires also enhances transference number of Li from 0.21 to 0.54 due to interaction between palygorskite and ClO ions. We further demonstrate full cells based on Li(NiMnCo)O (NMC111) cathode, PVDF/palygorskite CPE, and lithium anode, which can be cycled over 200 times at 0.3 C, with 97% capacity retention. Moreover, the PVDF matrix is much less flammable than PEO electrolytes. Our work illustrates that the PVDF/palygorskite CPE is a promising electrolyte for solid state batteries.

Flexible lithium-ion batteries (LIBs) can be seamlessly integrated into flexible devices, such as flexible displays, wearable devices, and smart cards, to provide power for steady operation under mechanical deformation. An ideal flexible battery should have high flexibility, high energy density, and high power density simultaneously, which are often in conflict with each other. In this Perspective, we analyze the flexible batteries based on structural designs from both the component level and device level. Recent progress in flexible LIBs, including advances in porous structures for battery components, superslim designs, topological architectures, and battery structures with decoupling concepts, is reviewed. In the end, perspectives on the future of flexible batteries are presented and discussed.

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