Multi-functional Polymer Composite Insulations Reinforced with Boron Nitride Nanosheets

Polymers are widely used in insulation applications, including cable insulation, battery casing, and polymer electrolytes, due to their significant advantages of low-cost availability and ease of manufacturing processability. However, their poor mechanical, thermal, and electrochemical properties pose substantial material safety, stability, and performance challenges. We show that two-dimensional (2D) boron nitride (BN) nanosheets can alleviate these concerns if appropriately designed and embedded into the polymer matrix. To achieve this, the challenges stemming from such design and fabrication include BN compatibility and molecular interactions with the host polymer matrix, alignment, and assembly of the BN nanosheets in the host polymer matrix should be addressed. This thesis focuses on investigating the effect of BN nanosheets in polyethylene (PE) insulative polymer materials. Two types of BN nanosheets, pristine and silane-modified, were chosen for this study to reveal the effect of interfacial interactions between BN nanosheets and PE molecular chains on their mechanical, thermal, and tribological properties. The results indicate that the silane functionalization increases molecular interactions at the PE and BN nanosheets interface, thus increasing mechanical reinforcement and thermal conductivity. As a result, the wear resistance of the polymer composite is also improved, as evident by experimental results. The effect of functionalized BN nanosheets was further investigated for poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF) based insulative polymer electrolytes. High-energy density LMBs suffer from thermal runaway and dendritic lithium growth, limiting long-term cycling. This thesis employed PVdF-BN composite as the battery electrolyte to address these concerns, with highly aligned BN nanosheets. Utilizing a custom-designed direct ink writing (DIW) process, highly aligned BN nanosheets were embedded in PVdF polymer composite electrolytes, enabling novel architectural designs for safe LMBs. The PVdF-BN composite electrolyte possesses higher thermal conductivity, enabling faster heat dissipation than the polymer electrolytes without BN nanosheets. The improved heat dissipation regulates uniform heat distribution and prevents hotspot formation and thermal runaway. On the other hand, dendritic lithium is mechanistically suppressed by aligned BN nanosheets. As a result, PVdF-BN composite electrolyte containing symmetric lithium cells exhibits stable Li plating/stripping over 2000 cycles without short-circuiting. This DIW printed electrolyte could be used as a model for other electrolytes or electrodes, thus enabling new chemistry and improved performances in energy-storage devices.