Accurate and Precise Temperature-Controlled Boxes for the Safe Testing of Advanced Automotive Li-Ion Cells with High Precision Coulometry

Abstract

Lithium-ion cells for electronics and automotive applications have an excellent safety record. However, safety-related events can sometimes occur during routine testing of prototype designs, especially in the case of designs using new electrode materials, new separators and/or new electrolytes. High precision measurements of coulombic efficiency have recently been shown to have great value in predictions of the impact of electrolyte additives on cell lifetime. In order to apply those methods to prototype automotive cells, special compact temperature-controlled boxes were required that could maintain the temperature to ± 0.05 • C precision and ensure the safety of the laboratory. These versatile temperature-controlled boxes were designed to accommodate cells as small as coin cells and as large as 40 Ah automotive pouch cells. The ability of the design to prevent cell-to-cell fire propagation and to channel smoke and flame away from the rest of the laboratory was experimentally verified. It is hoped that the information presented here will be of value to those designing precision testing facilities for large Li-ion cells. The summer 2012 issue of Electrochemical Society Interface is devoted to Li-ion battery safety. Doughty and Roth 1 state that the failure rate, leading to a safety incident, of Li-ion rechargeable battery cells in the field is very small, less than 1 in 10 million. Li-ion cells that are in use in the field have all been subjected to a large number of mandatory tests to demonstrate that they are safe under normal conditions of use and also under some conditions of electrical and mechanical abuse.2 Prototype Li-ion batteries in the R + D stage, that may be hand assembled, with higher energy density, new designs, new electrode materials and/or new electrolytes may be less safe than commercial cells and hence must be handled appropriately in the testing phase in the laboratory.Our research group has recently been applying precision measurements of the coulombic efficiency of Li-ion batteries to the study of cell lifetime and the efficacy of electrolyte additives.3-5 High precision coulometry measurements made over a period of a few weeks can be used to rank electrolyte additives and their combinations for effectiveness in prolonging cell lifetime. Applying such methods to automotive-scale Li-ion batteries is expected to yield similar advantages in cell lifetime predictions.Smith et al. 3 published requirements for testing equipment to perform high precision coulometry on Li-ion cells. These experiments are generally made at low rates, C/10 and slower, but need to be made at a very stable temperature (preferably stable to better than ±0.1• C) for measurements of extremely high precision. Therefore, there is a need for temperature-controlled boxes designed for high precision cycling of automotive cells that can maintain strict temperature stability while ensuring laboratory safety.One of the authors recently visited or contacted many makers of large Li-ion cells (>20 Ah) to learn about the chambers use...

Using High Precision Coulometry Measurements to Compare the Degradation Mechanisms of NMC/LMO and NMC-Only Automotive Scale Pouch Cells

“…Pairs of cells with a piece of foam in-between were compressed between two aluminum plates with spacers designed to maintain a constant gap on the cell assembly. Each pair of cells was placed in a temperature controlled chamber designed to maintain the cells at a given set-point within ± 0.1 • C. 4 Two cells were placed in each of five temperature chambers at the following temperatures: 25, 30, 35, 40 and 50 • C. The cells were allowed to stabilize at the specified temperatures for 24 hours before current was applied. Each of the cells was then subjected to the following test protocol on the high current UHPC chargers at Dalhousie University:…”

Simulation of temperature rise in Li-ion cells at very high currents

“…Without doing expensive and dangerous battery safety tests [58], with the help of numerical simulations, we can explore the cell performance in a wide range, obtain deep understanding of overheating inside the cell, and interpret many experimental phenomena. Mass-transport resistance is a large source of internal resistance determining the heat production.…”

Ultra High Precision Study on High Capacity Cells for Large Scale Automotive Application

“…The Ultra High Precision Charger (UHPC) at Dalhousie University 4,5 was used to collect high quality cycling data on fresh, one-year, and two-year aged cells from a commercial automotive Liion cell manufacturer, as well as fresh cells cycled to higher potential and at higher temperature. Highly accurate and precise measurements of potential and capacity allow small changes in charge and discharge capacity to be observable within just a few cycles.…”