In this work, we developed an in situ one-step, scalable, high-yield (98.2%) method for the preparation and surface functionalization of boron nitride nanosheets (BNNSs) with amino and carboxyl groups by ball milling l-glutamine and hexagonal boron nitride. The obtained l-glutamine-grafted BNNSs (BNNSs-g) can be well-dispersed in water at concentrations above 30 mg/mL, which is a value that falls on the high end of the reported literature. The functionalized BNNSs were further cross-linked with hydroxyl-rich cellulose nanofibers (CNF) in a low concentration to form a durable and porous aerogel. This aerogel was then applied as a 3D skeleton for the polyethylene glycol (PEG) phase change materials (PCMs) used for thermal storage. These BNNSs-g/PEG-based PCMs have a high phase change enthalpy of 150.1 J/g (reaching ∼ 94% that of pure PEG), as well as high thermal diffusivity. The highest thermal diffusivity (0.148 mm²/s) is about 4.5 times higher than that of a pure PEG. In addition, the BNNSs-g/CNF/PEG-based PCMs have excellent shape stability, facilitating the broad application of BNNSs composite materials.