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A series of K 2 SiF 6 :0.066Mn4+@Cl-GQDs (KSF:K 2 SiF 6 , Cl-GQDs: Cl containing graphene quantum dots) phosphors were prepared with a H 2 O 2 -free hydrothermal method, and the optimal sample was KSF:0.066Mn4+@Cl-GQDs 8 mg/mol. Enhanced effects on luminescent intensity and thermal stability inducing by coating of Cl-GQDs are observed: (a) Integrated emission intensity is 4.60 times of that of KSF:0.066Mn4+, (b) The luminescent thermal stability is greatly enhanced, which integrated emission intensities at 180 and 240 °C are 282.9 and 135.3% of that at 30 °C, respectively. The mechanisms of the above behaviors are discussed and proposed: Coordination between conjugated large π bond of GQDs and Mn4+ leads to the enhancement of emission intensity and luminescent thermal stability. Thermal stability analysis results show that the optimal samples suitable for high power WLEDs. Warm white light with low correlated color temperature and high color rendering index was produced from prototype WLEDs using the optimal sample as red emitting component. Image 1 • K 2 SiF 6 :Mn4+@Cl-GQDs were synthesized with a H 2 O 2 -free hydrothermal method. • Emission intensity and thermal stability are dramatically improved by coating of Cl-GQDs. • Integrated intensity of PL can be enhanced 4.60 times by coating of Cl-GQDs. • Integrated PL intensities at 180 and 240 °C are 282.9 and 135.4% of that at 30 °C. [ABSTRACT FROM AUTHOR]