According to Kirchhoff's radiation law, emissivity is equal to absorptivity at thermal equilibrium, so the underlying IR-absorbing aqueous electrolyte will dominate the device's emissivity, screening the influence of the Cu foil. At the stripped state (pristine or +1 V bias versus Cu foil), the device exhibits high emissivity due to the electrolyte, leading to a cooling state that enhances the heat exchange between the buildings and the outer space through the atmosphere transparent window. At -0.7 V (versus Cu foil) bias, the device can switch to the low-e heating state by electrodepositing Cu on the ultra-wideband transparent electrode, thereby suppressing the radiative heat loss of the building. This Cu-aqueous electrolyte possesses three advantages for the wide-range and large-scale applicability for thermal management: (i) nonflammability, (ii) lower price, and (iii) outstanding electrochromic performance (high tunable mid-IR contrast, non-volatile state retention, and long-term durability). A flammability test was conducted to examine the safety of the Cu-aqueous electrolyte compared with Ag-DMSO electrolyte that has been used for mid-IR tuning in prior reports (Figure ). Combustion occurred when the traditional Ag-DMSO electrolyte was exposed to fire, clearly confirming the better safety of Cu-aqueous electrolyte. Cost-effectiveness is another benefit of Cu-aqueous electrolyte. We calculate the price of the Cu-aqueous and Ag-DMSO electrolytes, and the results show that Cu-aqueous is 79.3% cheaper due to the lower cost of solvent and metal salt, as shown in Figure (the detailed price of each component in the electrolyte is provided in the Method).