Development of Modified Separator Using Vermiculite for Safer Lithium-Ion Batteries
Abstract
Lithium-ion batteries (LIBs) have been used in portable device application for decades because they are lightweight and can store a high amount of energy. Consequently, they pose a risk if mechanically, electrically, or thermally abused. A thermal runaway occurs when the exothermic reactions propagate uncontrollably, usually ending in a cell burst. Commercially, the components used in a LIB do not aid in the thermal runaway and can ultimately act as catalysts. One factor that can result in a short-circuit and thermal runaway is internal contact between the electrodes. Usually, these are separated by a polymer separator made of polypropylene (PP) or polyethylene (PE). These compounds can start shrinking past temperatures of 130°C, providing spaces for the electrodes to touch, resulting in an exothermic reaction. This research explores a fire-retardant mineral, vermiculite, in combination with polyvinylidene fluoride (PVdF), a common binder in electrode and separator fabrication, as a PP separator modification. Results indicated enhanced thermal stability while maintaining comparable electrochemical performance. After performing several characterization experiments on the vermiculite to understand its structure and composition, it was ground into a powder and fabricated into a slurry with PVdF. Initial iterations suggested that a thicker vermiculite ratio and application layer demonstrated high capacity fade and therefore required optimization. It was discovered that a 1:10 ratio of vermiculite to PVdF applied on PP with a thickness of 7μm maintained comparable capacity and ionic conductivity to the pristine PP cells. The voltage profiles demonstrated similar charging and discharging plateaus at ~3.46 V and ~3.40 V respectively. The modified and pristine separators were subjected to thermal stability tests such as differential scanning calorimetry (DSC) and multimode calorimetry (MMC). The DSC results demonstrated a 3.38°C higher melting temperature for the modified separator. When assembled into cells and subjected to MMC testing, the modified separator cell produced less exothermic energy release than the pristine separator cell, indicative of a safer cell during thermal runaways.
Degree
M.Sc.
Advisors
Pol, Purdue University.
Subject Area
Energy|Design|Materials science|Polymer chemistry
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