<p>Rechargeable Li||I₂ batteries based on liquid organic electrolytes suffer from pronounced polyiodides shuttling and safety concerns, which can be potentially tackled by the use of solid-state electrolytes. However, current all-solid-state Li||I₂ batteries only demonstrate limited capacity based on a two-electron I⁻/I₂ polyiodides chemistry at elevated temperatures, preventing them from rivaling state-of-the-art lithium-ion batteries. Herein, we report a fast, stable and high-capacity four-electron solid-conversion I⁻/I₂/I⁺ chemistry in all-solid-state Li||I₂ batteries at room temperature. Through the strategic use of a highly conductive, chlorine-rich solid electrolyte Li_4.2InCl_7.2 as the catholyte, we effectively activate the I₂/I⁺ redox couple. This activation is achieved through a robust I-Cl interhalogen interaction between I₂ and the catholyte, facilitated by an interface-mediated heterogeneous oxidation mechanism. Moreover, apart from serving as Li-ion conduction pathway, the Li_4.2InCl_7.2 catholyte is demonstrated to show a reversible redox behavior and contribute to the electrode capacity without compromising its conductivity. Based on the I⁻/I₂/I⁺ four-electron chemistry, the as-designed all-solid-state Li||I₂ batteries deliver a high specific capacity of 449 mAh g^-1 at 44 mA g^-1 based on I₂ mass and an impressive cycling stability over 600 cycles with a capacity retention of 91% at 440 mA g^-1 and at 25 °C.</p>