Comparative Analysis of 3D Graphene–LiOH·H2O Composites by Modification with Phytic Acid and Ascorbic Acid for Low-Grade Thermal Energy Storages

3D Graphene (3D-GF) modified with different additive concentrations (phytic acid [PA] and ascorbic acid [AA]) is developed and then combined with different LiOH contents by the hydrothermal method to obtain composite materials. It can be found that the addition of PA and AA will not destroy the carbon skeleton structure of 3D-GF. With increasing additive content, the thermal conductivities of modified 3D-GF first increase and reach their maximum values at 8 mg mL −1 . The modified 3D-GF with the highest thermal conductivity is selected to be combined with different LiOH contents. The addition of PA and AA can broaden the charging temperature range from 60–110 to 30–200 °C and improve the LiOH hydration rate and heat storage density. Compared with the PA-modified composite, the AA-modified composite shows smaller (about 7.6 nm) and more uniform LiOH·H 2 O particles (the particles size standard deviation of 3D-GF-AA-LiOH·H 2 O is 1.1503), a larger specific surface area (119 m 2 g −1 ), higher heat storage density (the highest value of 2763 kJ kg −1 for LiOH⋅H2O, which can be about 4.2 times of pure LiOH), and less influenced by the change of LiOH content (the thermal conductivity standard deviation of 3D-GF-AA-LiOH·H 2 O is 0.123).