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Abstract The development of affordable and sustainable Na-ion batteries (NIBs) and K-ion batteries (KIBs) is critical for grid-scale energy storage owing to the low cost and natural abundance of sodium and potassium resources. However, state-of-the-art cathode materials exhibit limitations in terms of cycling stability, power density, and performance under extreme-temperature conditions. To address these challenges, we developed three-dimensional mesoporous polyimides with engineered flexible alkyl chains and rigid conjugated structures to manipulate cross-linked structures and porosity in the polyimides for high structural stability and fast reaction kinetics in NIBs and KIBs. The mesoporous polyimide cathode delivers a high specific capacity of 152.7 mAh g–1 at 500 mA g–1 at 80 °C, fast-charging capability at up to 5 A g–1, and a long cycle life of 10,000 cycles at 1 A g–1 in NIBs, representing one of the most durable organic cathodes to date. The superior performances are extended to KIBs and low-temperature NIBs at −40 °C, demonstrating great promise for practical applications. To gain insight into the mechanism behind the performance, Fourier transform infrared spectroscopy (FTIR), electron paramagnetic resonance (EPR), and solid-state nuclear magnetic resonance (NMR) spectroscopy were leveraged to confirm reversible redox reaction, free-radical intermediate formation, and high structural stability of mesoporous polyimide cathodes, providing guidance for rational structure design of redox-active polymers for fast-charging and wide-temperature-range batteries.