<p>Sodium sulfate decahydrate (SSD, Na₂SO₄·10H₂O) possesses high latent heat and an ideal phase transition temperature, making it an excellent candidate for thermal energy storage applications. However, issues such as supercooling and phase separation hinder its practical use. This study employs boron nitride nanosheets (BNNS) to composite with Na₂SO₄·10H₂O, yielding a high-performance Na₂SO₄·10H₂O/BNNS phase change composite material. The results indicate that Na₂SO₄·10H₂O undergoes heterogeneous nucleation on the BNNS surface, effectively suppressing supercooling and phase separation. Additionally, the presence of BNNS enhances the thermal conductivity of the composite material. DSC analysis reveals that the composite material with 2 mass/% BNNS has a latent heat of fusion of 214.39&#xa0;J&#xa0;g⁻¹. The thermal conductivity of the Na₂SO₄·10H₂O/BNNS composite reaches up to 2.172 W·m⁻¹·K⁻¹, which is 5.67 times that of pure Na₂SO₄·10H₂O, and the degree of supercooling is reduced by over 20&#xa0;°C. Furthermore, the Na₂SO₄·10H₂O/BNNS composite material demonstrates excellent cyclic stability, maintaining stable thermal performance after 80 cycles. This study broadens the application prospects of BNNS and other two-dimensional nanomaterials in the field of phase change energy storage.</p>