Functional polymer materials for use in the prevention of thermal runaway in lithium-ion batteries

Functional polymers integrated with battery materials have been investigated as a potential safety measure in the prevention of thermal runaway within lithium-ion batteries. To this end a positive temperature coefficient of resistivity (PTCR) was explored to increase resistance of charge carriers at elevated temperatures prior to a thermal runaway event. Studies into the effectiveness of these PTCR responses and the impact of integrating the polymers on key cell performance metrics were performed. PTCR was identified for two different polymer systems. Electrodeposited polyacrylonitrile (PAN) on graphite-composite anodes was one explored system. This system was intended to prevent thermal runaway via a shutdown effect at elevated temperatures that stopped the flow of ions and the resulting electrochemistry of the two electrodes. Chronoamperometry and cyclic voltammetry techniques were used to deposit a thin polymer layer. The techniques were performed with acrylonitrile monomers saturated with oxygen, the polymerisation was initiated via a reactive intermediate of superoxide anions produced from oxygen reduction at the cathode. The choice of electrode surface and exposure to air was found to be of critical importance to the microstructure of the films affecting the continuity of the polymer. Cracks within the polymer films were dependent upon film thickness with precise control of thickness possible by varying deposition times. Post-treatment annealing at 120 oC was also found to remove cracks but caused large changes to the microstructure. Extensive lithium-ion cell testing was performed for electrodeposited PAN films. This was performed within a Swagelok cell using a standard lithium-ion half-cell assembly. Graphite-composites with PAN films formed the working electrode against a lithium metal counter-reference electrode. The electrolyte used was LP57; 1 M LiPF6 in an ethylene carbonate (EC) and ethyl methyl carbonate (EMC) solvent, weight ratio of EC:EMC was 3:7. This was sandwiched between two copper current collectors with glass fibre separators between the electrodes. A trade-off between charge capacity and the effectiveness of the PTCR response was found. Thinner films provided better charge capacity whilst thicker films provided a greater shutdown between 80 oC and 100 oC resulting in reduced charge capacity at elevated temperatures. This temperature is competitive with other commercial alternative solutions to thermal runaway. For example Celgard separators that experience a shutdown around 130 oC and similarly to the PAN films also create a shutdown effect by stopping the flow of ions between electrodes. Both the PAN film response and Celgard response are irreversible leading to the observed shutdown of the cell being permanent. Percolating composites were the second explored system, these were formed from a polymer matrix (polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), high-density polyethylene (HDPE), or low-density polyethylene (LDPE)) and a conductive filler (carbon or TiC). This material system was designed to stop thermal runway by restricting electron flow via a large PTCR response. A solvent cast method was used to form PAN and PVDF based composites, this was where the polymer and filler were dissolved and mixed within 1-methyl-2-pyrrolidinone (NMP). This mixture was subsequently spread onto copper foil and punched out after drying to form 11 mm diameter electrodes. HDPE and LDPE based composites were not easily soluble and so were formed via molten casting, the polymer and filler were mixed and melted at 130 oC onto copper foil on top of a hot plate. The melt was compressed using a glass rolling pin to reduce thickness and punched out after cooling to form 11 mm diameter electrodes. A percolation transition from insulating to conducting was identified at specific filler contents. TiC was found to require 60 % to 70 % content by mass, whilst carbon only needed 15 % to 25 %. The TiC composites formed a more brittle material as a result of the lower polymer content. Transition temperatures of each composite were identified using differential scanning calorimetry (DSC) along with the magnitude of the PTCR responses around these temperatures calculated using impedance measurements. Melting-transitions were found to be far more effective at producing a PTCR than glass transition due to far greater thermal expansion. LDPE-TiC composites were found to produce the greatest PTCR response while maintaining a low room temperature resistivity. The temperature of this response was also at a suitable temperature to prevent thermal runaway, 120 oC. Unlike the PAN films and commercial Celgard, the response of this system was found to be reversible upon return to ambient conditions allowing recovery and re-use of the battery system to be possible. Percolating LDPE-carbon composites were briefly tested in a lithium-ion cell. The electrochemistry was largely unaffected with the exception of the first charge cycle where far greater capacities than typical were drawn. This was a result of the large amount of carbon introduced to the working electrode due to the LDPE-carbon composite resulting in far greater amounts of solid-electrolyte interface (SEI) formation.