In the context of the gradual popularity of electric vehicles (EVs), the development of lithium battery systems with high energy density and power density is regarded as the foremost way to improve the range of EVs. LiNi_1−x−yCo_xMn_yO₂ (NCM) cathodes have been the focus of researchers due to their high energy density, excellent power performance, and low-temperature resistance. However, the elaboration of the decay mechanism of NCM cathode based on lithium metal batteries (LMBs) is still being restricted to the primary level. In the past decades, the development and application of advanced in-situ characterization tools have facilitated researchers' understanding of the internal operation mechanism of batteries during charging and discharging. In this minireview, the latest progress of in-situ observation of the NCM cathode by X-ray diffraction (XRD), fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, atomic force microscopy (AFM), transmission electron microscope (TEM), optical microscope, and other characterization tools is summarized. The mechanisms of structural degradation, cathode-electrolyte interfaces (CEIs) composition, and dynamic changes of NCM, electrolyte breakdown, and gas production are elaborated. Finally, based on the existing research progress, the opportunities and challenges for future in-situ characterization technology in the study of the mechanism of LMBs are discussed in depth. Therefore, the purpose of this minireview is to summarize recent work that focuses on the outstanding application of in-situ characterization techniques in the mechanistic study of LMBs, and pointing the way to the future development of high energy density and power density LMBs systems.

High energy density Li-CO₂ batteries have attracted much attention owing to the “two birds with one stone” feature in fixing greenhouse gas CO₂ and providing renewable energy. However, poor reversibility of the discharge product Li₂CO₃ is one of the main problems that limit its application, resulting in poor cycling stability and severe polarization. Herein, copper indium sulfide (CIS), a semiconducting non-precious metal sulfide, is fabricated as cathode catalysts for high-performance Li-CO₂ batteries. Combined with the synergistic effect of bimetallic valence bonding and coordinated electron transfer, Li-CO₂ batteries using CIS cathodes exhibit high full specific discharge capacity, excellent rate capability and cycle stability, namely it delivers a high specific full discharge capacity of 8878 μAh cm⁻², runs steadily from 10 to 100 μA cm⁻², and performs a stable long-term cycling behavior (>1050 h) under a high energy efficiency of 84% and a low charge voltage of approximately 3.4 V at 20 μA cm⁻² within 100 μAh cm⁻². In addition, a flexible Li-CO₂ pouch cell is constructed to reveal the potential of employing CIS to fabricate flexible high energy storage devices in practical applications. This work shows a promising development pathway toward next-generation sustainable energy storage devices.

Flexible fiber-shaped sodium dual-ion batteries (FSDIBS) as a proof of concept are fabricated by using the hierarchical ReS₂ nanosheets anchored on the carbon nanotube (ReS₂@CNT) fiber as anode and graphite on the CNT as cathode. Owing to large interlayer spacing and weak layer coupling force of the ReS₂ nanosheets and the anion accommodation of the graphite combined with good flexibility of the CNT fiber, the FSDIBS demonstrate outstanding electrochemical performances with high working voltage and high specific volumetric energy density, durable cycling life, and good flexibility. The FSDIBS show a specific discharge capacity of 97.8 mAh cm⁻³ at a current density of 630 mA cm⁻³ and high specific energy density of 25.12 mWh cm⁻³ (based on the whole volume of the two electrodes) and superb stability with a capacity retention of 91.8% even after bending for 2100 cycles. Moreover, a series of ex situ/in situ characterizations are verified that the reversible shuttles of the Na⁺ cations and PF₆⁻ anions between the anode and cathode are simultaneously occurred during the charge/discharge process.