Talking about the safety problems of lithium-ion power lithium batteries under mechanical abuse conditions

by:CTECHi     2021-07-31

The background of this article is the rapidly rising electric vehicle market and the accompanying battery safety issues. As the number of electric vehicles continues to increase, when the number of electric vehicle accidents is equal to that of traditional fuel vehicles, the collision safety problem of electric vehicle batteries will become more prominent. In a car collision accident, the battery pack may be severely deformed by being squeezed, or impact overload may occur without obvious deformation, which may cause a certain risk of thermal runaway. A well-known case in this regard is the accident caused by the impact of ground objects on the TeslaModelS during high-speed driving in 2014. The battery pack was severely deformed and caused a thermal runaway fire. In the foreseeable future, the battery capacity of electric vehicles will continue to increase, and the potential risks are greater. The collision safety of power lithium batteries has become an urgent problem to be solved.

Compared with the research on battery safety under thermal and electric abuse conditions, there are relatively few studies on battery safety under mechanical abuse conditions. This article summarizes the current research on the deformation and failure of battery cells, battery modules and battery packs under mechanical loads. From the perspective of research scale, battery collision safety research includes battery component materials, battery cells, battery modules and protective structures, and battery packs, as shown in the figure below. The important goals of battery collision safety research are: (1) Understand the deformation and failure characteristics of battery cells under mechanical load and the correlation with internal short circuit triggers, and finally establish the damage criterion and damage capacity of cells, modules or battery packs. (2) Establish a finite element simulation model that takes into account calculation accuracy and calculation efficiency to guide the design of battery pack protection structure.

Study on the dimensions of component materials. The laminated structure composed of positive electrode, separator and negative electrode is the basic unit of different forms of batteries. The positive and negative electrodes are composed of a metal current collector and a coating on the surface. The mechanical properties of the battery cell component materials, including the mechanical properties of the metal current collector, the positive and negative electrode coatings and the separator, and the interface properties between the coating and the current collector, directly determine the mechanical behavior of the battery cell. Similar to traditional metal materials, the mechanical behavior of metal current collectors includes its plasticity, ductile fracture, anisotropy and rate dependence, and the important difficulty lies in the acquisition of experimental data. This is because the thickness of the metal current collector used in the battery is relatively thin (10-25um), which brings certain difficulties to the preparation, clamping, loading and measurement of the test piece. The separator is mainly used to isolate the positive and negative electrodes, so its mechanical behavior, especially the fracture behavior, also directly affects the occurrence of internal short circuits of the battery. The diaphragm is usually a polymer material (PE, PP), and the characterization of its mechanical behavior is more complicated, including the influence of factors such as elasticity, plasticity, fracture, and material orientation, temperature and time dependence. There are relatively few studies on coating materials and the properties of the interface between the coating and the current collector.

Study on battery cell size. The battery cell is the smallest component unit of the battery pack. In terms of experiments, it is to study the mechanical failure mode of the cell and its correlation with the occurrence of internal short circuits, and considering the actual collision accident, the battery cell is subjected to more loading conditions It is complicated, and it is necessary to carry out experiments of different forms of loading conditions. The following figure lists typical loading conditions of battery cells, including in-plane extrusion, out-of-plane extrusion (spherical loading head, cylindrical loading head), and three-point bending. In addition to the loading form, the influence of the single charge and the loading speed must also be considered.

Battery module size. Similar to battery cells, battery modules also need to consider mechanical failures and thermal runaway phenomena under different loading modes, charge levels and loading speeds. However, battery modules are usually composed of groups of monomers and other accessories, with more structure and higher overall energy, so the cost of experiment and simulation is increased. At present, there are few battery module tests and simulations publicly published in the literature. The following figure shows the experimental results published by Professor Qing Zhou’s team at Tsinghua University. It can be seen that the mechanical damage and thermal runaway of battery modules under different impact directions are quite different. . We see that the battery management system and the power lithium battery pack together form the battery pack as a whole. The two components that have a communication relationship with the battery management system, the vehicle controller and the charger. The battery management system, upwards, communicates with the electric vehicle controller through CANbus, reports the battery pack status parameters, receives instructions from the complete vehicle controller, and determines the power output according to the requirements of the complete vehicle; downwards, monitors the operating status of the entire battery pack, Protect the battery pack from over-discharge, overheating and other abnormal operating conditions; during the charging process, interact with the charger, manage the charging parameters, and monitor the normal completion of the charging process.

Large power lithium battery pack

In general, the battery management system consists of the main control module and the collection Modules or called slave control modules jointly constitute. The functions of individual voltage acquisition, temperature acquisition and equalization are generally allocated to the slave control module; the collection of total voltage, total current, internal and external communication, fault recording, and fault decision-making are all functions of the master control module.

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