Lead-free double perovskites have attracted much attention for their unique optical properties, though achieving efficient broadband near-infrared (NIR) emission with them remains a challenge. In this work, we successfully
synthesized a new microcrystalline structure of lead-free double perovskites Cs2Ag1-xRxBiCl6 (R = Na, K, Rb) using a simple chemical reflux method, achieving NIR emission. Notably, Cs2Ag0.7K0.3BiCl6 demonstrates a broad
emission spectrum spanning from 500 to 1200 nm with a significant half-peak width of 300 nm under 365 nm excitation. This is attributed to the doping of alkali metal ions can break the inherent parity-forbidden transition
and induce strong exciton-phonon coupling through lattice distortion. Temperature-dependent photoluminescence spectroscopy and density functional theory calculations analysis reveal that alkali metal ions can enhance the thermal stability of self-trapped exciton emission by reducing exciton-phonon coupling strength. Finally, we demonstrate the promising applications of Cs2Ag0.7K0.3BiCl6 crystals with broad emission bands for NIR night vision and information encryption. This provides new insights into the realization of near-infrared emission and thermal stability regulation.