Cascade phase change from highly-ordered salt hydrate composite monolith for Li-ion battery low-temperature resistance enhancement

The rapid growth of electric vehicles raises an unmet need to enhance low temperature resistance of Li-ion battery (LIB). Herein, highly-ordered and porous boron nitride/polyvinyl alcohol (BN/PVA) via directional freeze-drying is fabricated to impregnate with sodium sulfate decahydrate (SSD). The highly-ordered SSD@BN/ PVA (HSBP) that is flexible, mechanically strong and electrically insulated possesses high latent heat and leakage-proof temperature. Being inspired by the traditional nucleation theory, a multi-layer structure with gradient freezing points (f.p.s) was built based on HSBP composite along the highly-ordered direction to achieve cascade solidification and controllably thermal energy release. The highly-ordered structure enhances thermal conductivity by 30 times and largely reduces heat dissipation in layers interface. By studying vital factors as solidified heat, f.p. gradient and layer number, a mechanism for cascade solidification is proposed and trilayer HSBP monolith with 3 degrees C f.p. gradient is proven to be effective for controllable heat release. Consequently, cascade solidification from the multi-layer HSBP monolith could increase temperature holding time by 70 min when battery stops working at -20 degrees C and improve discharge capacity by 0.71 Ah during cold-start. Therefore, cascade phase change from the multi-layer HSBP monolith shows a significant potential in improving low-temperature resistance of LIB.