In this work, a lightweight, ultrathin and flexible paper-based gel electrolyte is developed for liquid-free Al-air batteries, which employs a natural cellulose paper to store an alkaline gel. The as-fabricated electrolyte can be applied in either mechanical-rechargeable or single-use Al-air batteries, providing an open-circuit voltage (OCV) of 1.5 V and a peak power density around 3.5 mW cm−2. In addition, the battery discharge specific capacity is as high as 900 mA h g−1 even with low-purity Al. Moreover, a flexible Al-air battery can be obtained, which exhibits a stable performance under different bending angles. By optimizing the electrolyte properties including polymer concentration, gel loading and solution casting time, the battery power output can be further improved to 6.4 mW cm−2. Finally, two stacking strategies are investigated for developing Al-air battery packs, including both vertical stacking and planar stacking. Benefited from the complete ionic isolation among the single cells, both stacking strategies are demonstrated viable, with a normal OCV of 5.6 V for a 4-cell stack and a stacking efficiency as high as 87.5%. Such kind of liquid-free Al-air batteries are especially suitable for powering portable electronic devices with small rated power. •A paper-based gel electrolyte is developed for liquid-free Al-air battery.•The battery can provide an OCV of 1.5 V and a peak power density of 6.4 mW cm−2•A high specific capacity of 900 mA h g−1 can be achieved even with low-purity Al.•A flexible Al-air battery is also successfully developed.•A convenient and efficient stacking of Al-air battery pack is demonstrated.