During the study, PCM-based thermal management for lithium batteries was prevalent but faced issues like leakage and low thermal conductivity, especially in controlling high-performance battery temperatures. This paper introduced a novel composite phase change material (CPCM) with enhanced thermal conductivity and leak resistance. A mesh of expanded graphite (EG) and diatomite significantly boosted PCM's absorption. Tests showed that a CPCM blend of 90% PEG, 10% diatomite, and 6% EG maintained integrity at 50℃ for 5 hours with no surface leakage, proving effective sealing. Under the 2C mode at an initial 30℃, the battery's max temperature stayed below 40℃. CPCM also provided excellent insulation at low temperatures, increasing discharge efficiency by 16.07%, reaching 91.67% under the 1C pattern at 30℃.