Moisture-Resistant Mn4+-Doped Core-Shell-Structured Fluoride Red Phosphor Exhibiting High Luminous Efficacy for Warm White Light-Emitting Diodes

K2TiF6:Mn4+ is a highly efficient narrow-band emission red phosphor with promising applications in white light-emitting diodes (LEDs) and wide-gamut displays. Nevertheless, the poor moisture-resistant properties of this material hinder commercialization. A convenient reverse cation-exchange strategy is introduced for constructing a core-shellstructured K2TiF6:Mn4+@K2TiF6 phosphor. The outer K2TiF6 shell acts as a shield for preventing moisture in the air from hydrolyzing the internal MnF62- group, while effectively cutting off the path of energy migration to surface defects, thereby increasing the emission efficiency (especially for the phosphors doped with high concentrations of Mn4+). Employed as a red phosphor, the packaged white LED exhibits an extraordinarily high luminous efficacy of 162 lm W-1, a correlated color temperature (CCT) of 3510 K, and a color rendering index of 93 (R-a). Aging tests performed on this device at 85 degrees C and 85 % humidity for 480 h retain up to 89% luminous efficacy. The findings could facilitate commercial application of K2TiF6:Mn4+@K(2)TiF(6 )phosphor.