Lithium‐based batteries are the most potential state‐of‐the‐art energy storage device for flexible electronics. The flexible lithium batteries have the advantages of high energy density, robust mechanical durability, and stable power output even under dynamic deformation. Among them, the synergies of flexible free‐standing electrodes, solid electrolytes, and electrode–electrolyte interfaces are crucial to achieving the goal of high energy density and safety performance for flexible lithium batteries. Therefore, a thorough understanding of the interface formation mechanism and influencing factors is crucial for the design of flexible electrodes and solid electrolytes. In this review, the interface challenges in flexible lithium‐based batteries including interface formation, electrodes‐electrolyte interface, and interparticle interface characteristics are presented. Then, strategies of interface optimization are summarized and discussed. Following this, the interface of flexible lithium‐based batteries with novel architecture is introduced, including the interface between each component and unit of the battery. Finally, the perspectives for the future development of flexible lithium‐based batteries are also given.

Layered lithium transition metal (TM) oxides LiTMO₂ (TM = Ni, Co, Mn, Al, etc.) are the most promising cathode materials for lithium-ion batteries because of their high energy density, good rate capability and moderate cost. However, the safety issue arising from the intrinsic thermal instability of nickel-based cathode materials is still a critical challenge for further applications in electric vehicles and energy storage power stations. The main reasons include side reactions between the highly reactive Ni^3+/4+ and liquid electrolyte, oxygen release accompanied by structural phase transition, and internal microcrack propagation owing to the low strength of spherical secondary particles. Great efforts have been invested to modify nickel-based cathode materials such as stabilization of bulk structure by element doping, surface engineering, nanostructure design, and particle mono-crystallization. In this review, we summarize these advances and try to give an in-depth insight into the origin of the thermal instability of nickel-based cathode materials. More importantly, some effective strategies to improve thermal stability are outlined, expecting to accelerate the future development of layered TM oxides with high safety.

LiNi_xCo_yMn_zO₂ (LNCM)-layered materials are considered the most promising cathode for high-energy lithium ion batteries, but suffer from poor rate capability and short lifecycle. In addition, the LiNi_1/3Co_1/3Mn_1/3O₂ (NCM 111) is considered one of the most widely used LNCM cathodes because of its high energy density and good safety. Herein, a kind of NCM 111 with semi-closed structure was designed by controlling the amount of urea, which possesses high rate capability and long lifespan, exhibiting 140.9 mAh·g⁻¹ at 0.85 A·g⁻¹ and 114.3 mAh·g⁻¹ at 1.70 A·g⁻¹, respectively. The semi-closed structure is conducive to the infiltration of electrolytes and fast lithium ion-transfer inside the electrode material, thus improving the rate performance of the battery. Our work may provide an effective strategy for designing layered-cathode materials with high rate capability.