Abstract:

The applications of Li-ion batteries (LIBs) vary widely from the transport sector (viz. electric vehicles, aircraft, robotics, drones, etc.) to battery energy storage technologies supporting and strengthening the power grid to mitigate the problem of intermittency of renewable energy sources. However, the pace of this energy storage and transition greatly relies on the energy density as well as the fire safety of the rechargeable LIBs. Despite significant advancement in other characteristic requirements (viz. cycle life and high-rate capability), the insufficient energy density of existing LIBs to meet the ever-increasing demand for energy in human society, as well as the fire safety threats caused by thermal runaway, persist.
The fire safety of LIBs is generally increased by incorporating flame retardants (FR) in mainly the form of electrode/electrolyte additives, and coating layers on the surface of current collectors and separators. A typical FR increases the fire safety of batteries primarily by inhibiting the chain reactions, including the rise of internal temperature, pyrolysis of organic substances (such as electrolyte solvent and solid-electrolyte interfaces), ignition, and combustion that occur during the thermal runaway of the batteries. However, recently there have been many concerns about the toxicity, environmental, and recycling issues associated with conventional halogenated FR. Due to the advantages of eco-sustainability and easy recyclability, biobased flame retardants have emerged as potential alternatives to the halogenated FR. Therefore, in this work, we introduce biobased flame retardant phytic acid lithium (PALi) as an additive to improve the fire safety of polymer electrolytes.
This dissertation aims to demonstrate a biobased polymer electrolyte integrated with flame-retardant materials. A solid polymer electrolyte was synthesized by using a polyethylene oxide/α-Cyclodextrin (PEO/α-CD)-based complex matrix with a unique polyrotaxane supramolecular assembly. Due to the polyrotaxane crosslinking, the as-prepared solid polymer electrolyte exhibited a higher ionic conductivity of 5.0 × 10–5 S cm–1 compared to the control solid polymer electrolytes without forming polyrotaxane. The higher Li+ ion conductivity of the solid polymer electrolyte is attributed to a greater fraction of amorphous PEO induced by polyrotaxane. The higher percentage of the amorphous PEO in the polyrotaxane solid polymer electrolyte was confirmed by X-ray diffraction (XRD) peak characterization, and differential scanning calorimetry (DSC). The origin of the higher ionic conductivity of the polyrotaxane solid polymer electrolyte was further corroborated by scanning electron microscopy (SEM) characterization. A LiǁLFP cell with the polyrotaxane solid polymer electrolyte exhibited stable cycling performance at 60 °C. The cell delivered an initial reversible specific capacity of 156 mAh g–1 during the first cycle of charge/discharge and retained 143 mAh g–1 after 60 cycles with an average capacity decay rate of 0.14% per cycle. Moreover, AP exhibited stable Coulombic efficiencies of greater than 98% during charge/discharge cycling.
After adding PALi into the AP electrolyte, the results of carbon residue formation after the burning test reveal the synergistic effect of polyrotaxane supramolecular units and phytate lithium salt. According to the EDS analysis for the carbon residue, the even phosphorous distribution can be proved. The presence of PALi offers a beneficial influence on the solid polymer electrolyte in terms of a relatively high flame resistivity by forming carbon residue containing P. The flame-retardant mechanism for this system can be found in some intumescent flame-retardant systems. The polyphosphate chain will form once the AP electrolyte containing PALi is heated, promoting cyclodextrin dehydration and producing carbon residue. In addition, the electrochemical performance of the AP-PALi electrolyte is still stable after adding 20% PALi. The initial capacity of the Li-LFP cell with AP-PALi20% is 160 mAh g-1.