WHAT MAKES LITHIUM-ION BATTERY UNSAFE?

What makes lithium battery unsafe

INTRODUCTION

The safety and reliability of lithium-ion batteries has always been the most concerning issue in the green energy industry. As more and more people are getting worried of the safety problems when they are using products with lithium battery, it’s time that we should get to know the scientific definition on lithium ion battery safety and the mechanism of what makes lithium ion batteries dangerous.

WHAT IS THE DEFINITION OF SAFETY ON LITHIUM ION BATTERY?

The word “safety” that comes to our mind, usually refers to the possibility of safety problems that occur when using lithium battery, such as spontaneous combustion, sudden outage, leakage and so on.

However, from the technical point of view, it refers to the severity of the failure reaction of the battery under the condition of abuse, which are in accordance with the national standard “GBT31485-2015 Power battery safety Requirements and Experimental Methods for electric vehicles” implementation. The conditions of abuse are over discharge, overcharge, external short circuit, extrusion etc., while failure reaction includes battery leakage, ignition and explosion.

Indeed, there is a certain correlation with the two definitions mentioned above, but they are not exactly the same. It is technical as to be beyond the ken of the average layman who are hard to distinguish the differences. Thus, some paragraphs are extracted from textbooks or academic reports, cobbled together and published by social media users, causing serious misguidance to ordinary consumers and even many insiders in the industry.

Technically, we can divide the concept of “safety” into two definitions: “reliability” and “severity of failure reaction“. “Reliability” refers to the risk of normal use; “Failure mode severity” is the severity of the reaction in accordance with the national standard test.

AUTOMOTIVE LITHIUM ION BATTERY SYSTEM AND COMPOSITION OF THE CELL

Lithium-ion battery structure

(1)  Automotive lithium battery System is often referred to an automotive battery pack, which is usually composed of a battery (or a module consisting of a small group of single cells), a BMS (power management system), structural components (battery case, wires, connectors, etc.) and a thermal management system.

(2) The cell refers to the monomer of the battery, which is generally 3.2V or 3.6V single lithiumion battery. It is the component of reactions like leakage, ignition and explosion. A single lithiumion battery, composed of a positive plate, a negative plate, a diaphragm, an electrolyte and structural parts.

(3) BMS is a component that monitors the temperature, voltage and status of the cell, interacts with the vehicle controller and further controls the charging and discharging behavior.

(4) The thermal management system is a component that heats or cools the battery to keep the core at the right temperature. The structural part is the carrier or interface of the above components, used to support, install, protect the internal components and provide charging and discharging interfaces.

THE FAILURE REACTION OF THE CELL: "THERMAL RUNAWAY"

Thermal runaway

For lithium-ion batteries, there are three factors that cause failure reaction: internal short circuit, over voltage and high temperature. These three factors cause the temperature to reach a certain level, triggering a series of internal chemical reactions, further heat and gas, resulting in fire and  explosion.

According to relevant experiments, generally the temperature of 120℃ and below is the safe temperature range. 60-120℃ is a safe temperature range but can degrade lithium-ion batteries and only the environment is over 120  can fire breakouts or an explosion occurs.

This all comes down to the temperature, therefore the failure reaction of the cell is called “thermal runaway”.

In addition to the above three factors, no other factors will lead to fire and explosion of lithium-ion batteries, or in another word, other factors of abuse are the deformation of the three.

HOW TO AVOID "THERMAL RUNAWAY"

And here comes the real questions, how do we avoid thermal runaway?

Firstly, we need to understand that it is unlikely to induce violent reactions in lithium-ion batteries through external conditions e.g. press extrusion, pinching or overcharging. In general, in the absence of a serious incident, the root cause of battery pack spontaneous combustion is machining process control. Therefore, to prevent the internal short circuit of the cell and the interlocking problems caused by it is putting top priority in the production control of every battery enterprise.

CONCLUSION

(1) The “reliability” of battery system is mainly related to the control level of metal foreign matter in the process of cell /PACK processing and the reliability of components.

(2) The severity of failure reaction of the cell is mainly related to the design state of the cell, including the capacity/energy density, the choice of cathode material and diaphragm.

(3) The trend of implementing auto equipment in factories may not be able to control the production line better. On the one hand, it can not detect the problems in time, on the other hand, auto equipment with complex system comes with a higher chance of occurring errors, which will increase the possibility of thermal runaway.

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