Suppressing the dendrite formation and managing the volume change of lithium (Li) metal anode have been global challenges in the lithium batteries community. Herein, a duplex copper (Cu) foil with an ant-nest-like network and a dense substrate is reported for an ultrastable Li metal anode. The duplex Cu is fabricated by sulfurization of thick Cu foil with a subsequent skeleton self-welding procedure. Uniform Li deposition is achieved by the 3D interconnected architecture and lithiophilic surface of self-welded Cu skeleton. The sufficient space in the porous layer enables a large areal capacity for Li and significantly improves the electrode–electrolyte interface. Simulations reveal that the structure allows proper electric field penetration into the connected tunnels. The assembled Li anodes exhibit high coulombic efficiency (97.3% over 300 cycles) and long lifespan (>880 h) at a current density of 1 mA cm⁻² with a capacity of 1 mAh cm⁻². Stable and deep cycling can be maintained up to 50 times at a high capacity of 10 mAh cm⁻².

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.