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Cathodes in lithium‐ion batteries with anionic redox can deliver extraordinarily high specific capacities but also present many issues such as oxygen release, voltage hysteresis, and sluggish kinetics. Identifying problems and developing solutions for these materials are vital for creating high‐energy lithium‐ion batteries. Herein, the electrochemical and structural monitoring is conducted on lithium‐rich cathodes to directly probe the formation processes of larger voltage hysteresis. These results indicate that the charge‐compensation properties, structural evolution, and transition metal (TM) ions migration vary from oxidation to reduction process. This leads to huge differences in charge and discharge voltage profile. Meanwhile, the anionic redox processes display a slow kinetics process with large hysteresis (≈0.5 V), compared to fast cationic redox processes without any hysteresis. More importantly, a simple yet effective strategy has been proposed where fine‐modulating local oxygen environment by the lithium/oxygen (Li/O) ratio tunes the anionic redox chemistry. This effectively improves its electrochemical properties, including the operating voltage and kinetics. This is also verified by theoretical calculations that adjusting anionic redox chemistry by the Li/O ratio shifts the TM 3d—O 2p bands and the non‐bonding O 2p band to a lower energy level, resulting in a higher redox reaction potential.

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Hybrid capacitors, especially sodium-ion capacitors (SICs), which combine the complementary merits of high-energy batteries and high-power capacitors, have received increasing research interest and have been expected to bridge the gap between the rechargeable batteries and EDLCs. The biggest challenge for SICs is to overcome the kinetics discrepancy between the sluggish faradaic anode and the rapid nonfaradaic capacitive cathode. To boost the Na + reaction kinetics, robust and conductive Na 2 Ti 2 O 5−x nanowire arrays have been constructed as an accessible and affordable SIC anode. It is found that the utilization of oxygen vacancies (OVs) can endow Na 2 Ti 2 O 5−x high electrical conductivity, introduce intercalation pseudocapacitance, and maintain the crystal structure integrity. It exhibits high reversible discharge capacity (225 mAh g −1 at 0.5C), superior rate capability, and ultralong lifespan when utilized as self-supported and additive-free anode for SIB, remaining almost 100% capacity retention after 20 000 cycles at 25 C. When assembled as flexible hybrid SIC (4.5 cm 3 ) with rGO/AC film cathode, a high-level energy density of 70 Wh kg −1 at power density of 240 W kg −1 based on active materials can be achieved, and high volumetric energy density (15.6 Wh L −1 ) and power density (120 W L −1 ) based on the whole packge volume can be delivered with superior cycle stability (5000 cycles, 82.5%).