Directional Freezing Assisted 3D Printing to Solve a Flexible Battery Dilemma: Ultrahigh Energy/Power Density and Uncompromised Mechanical Compliance

Abstract Flexible lithium‐ion batteries (LIBs) have been in the spotlight with the booming development of flexible/wearable electronics. However, the dilemma of simultaneously balancing excellent energy density with mechanical compliance in flexible electrodes impedes their practical applications. Here, for the first time, a directional freezing assisted 3D printing strategy is proposed to construct flexible, compressible, and ultrahigh energy/power density LIBs. Cellulose nanofibers (CNFs) and carbon nanotubes (CNTs) are entangled with each other to form an interwoven network and uniformly wrap the active materials, ensuring fast electron transfer and stress release through the entire printed electrode. Furthermore, vertical channels induced by directional freezing can act as high‐speed ion diffusion paths, which effectively solve the sluggish ion transport limitation of flexible 3D printed electrodes as the mass loading increases. As expected, the 3D printed LIB delivers a record‐high energy density (15.2 mWh cm –2 ) and power density (75.9 mW cm –2 ), outperforming all previously reported flexible LIBs. Meanwhile, the printed flexible full LIBs maintains favorable electrochemical stability in both bending and compression states. This work suggests a feasible avenue for the design of LIBs that resolves the long‐standing issue of high electrochemical performance and mechanical deformation in wearable and smart electronics.