FLOW PHYSICS AND HEAT TRANSFER ASSOCIATED WITH VENTING OF SMALL FORMAT LITHIUM-ION CELLS

The safety issues triggered by thermal runaway is one of the most crucial problems to be considered for the development of Li-ion batteries.  The series exothermic reactions during thermal runaway created a lot of heat and gas.  The vented gas may ignite and cause excessive heat transfer to nearby surfaces, thus creating an extreme risk for propagating failures. The objective of this dissertation was to study the effect of safety vent geometry on the venting flow physics and subsequent heat transfer to nearby surfaces.  

Four commercially available Li-ion safety vents (MTI, LG MJ1, K2, and LG M36) were considered in this work.  The safety vents were inspected, through computed tomography (CT) scanning, to understand the design and construction of each vent assembly.  Experiments and simulations were conducted to compare several key venting parameters among the four designs: current interrupt device (CID) activation pressure, vent-activation pressure, and discharge flow coefficient.  A semi-empirical model, based on flow through a sharp-edged orifice, was proposed for estimating mass flowrate through the safety vent.  The model required a single geometric parameter, the sharp-edged equivalent area, which is inversely proportional to flow resistance.  

Computational fluid dynamics (CFD) was used to study the venting jet flow physics for each safety vent design.  The geometric design of the CID mechanism strongly influenced the flowfield structure.  Spot-weld CID designs, when compared to notch-groove CID designs, showed elevated turbulence levels within the safety vent assembly and showed a strong recirculation zone directly above the safety vent assembly.   Heat transfer resulting from impingement of the vented jets onto a surface placed above the safety vent was measured experimentally and simulated using CFD.  In general, heat transfer increased with increasing sharp-edged equivalent area.  Also, safety vents with notch-groove CID designs exhibited elevated heat transfer rates when compared to spot-weld CID designs.  

Based on lessons learned from the flowfield and heat transfer analysis, an optimized safety vent design was proposed.  The gas flow restriction was lowered by enlarging the equivalent area.  The current collector component was modified to include a lobed mixing nozzle for the purpose of enhancing turbulence to mix the jet with the surrounding air, thereby reducing the jet temperature striking the target surface.  Compared to the LG M36 design, the optimized safety vent increased the equivalent area by 58% without increasing the heat transfer rate.