Discussion and research on the safety of lithium-ion power batteries

1. Introduction

Since the birth of lithium-ion batteries, safety has always been an important issue that limits their use scenarios. As early as 1987, Canadian company Moli Energy launched the first commercial metal lithium battery based on metal lithium anode and MoS2 cathode. This battery had multiple explosions in late spring 1989, which directly led to the company's bankruptcy and also prompted the industry to turn to the development of more stable lithium-ion batteries that use intercalation compounds as anodes.

After lithium-ion batteries entered the consumer electronics field, there have been many large-scale recall plans due to battery fire hazards. In 2016, Samsung's Note7 mobile phones experienced multiple fire and explosion accidents around the world. In addition to triggering a global recall plan, "lithium battery safety" has once again become a widely concerned social topic.

In the field of electric transportation, power battery safety accidents have gradually increased with the increase in sales of new energy vehicles. According to statistics, there were more than 200 reported electric vehicle fires and combustion accidents in China in 2021. The safety of electric vehicles has become one of the most concerned issues for consumers and electric vehicle companies.

In the field of energy storage, South Korea experienced more than 30 energy storage power station accidents between 2017 and 2021. On April 16, 2021, the explosion at the Beijing Dahongmen Energy Storage Power Station not only caused the entire power station to burn down, but also caused the deaths of two firefighters and the disappearance of one employee. As the application scenarios of lithium-ion batteries continue to expand, their safety has triggered extensive discussion and research in both industry and academia.

2. Composition of power battery system and battery cells

1. Power battery system

The power battery system is what we often call a vehicle battery pack, which is generally composed of battery cells (or a module composed of a small group of single battery cells), BMS (power management system), structural parts (casing, wires, connectors, etc.) and thermal management system.

A battery cell refers to a single cell of a power battery, generally a single lithium-ion battery of 3.2V or 3.6V, and is a component that can cause leakage, fire, explosion, etc. BMS monitors the temperature and voltage of the battery cell, and uses this to determine the state of the battery cell, interacts with the vehicle controller/charging pile signal, and further controls the charging and discharging behavior.

The thermal management system is a component that heats or cools the battery to ensure that the battery cell is at an appropriate temperature. The structural parts are the carriers or interfaces of the above components, used to support, install, and protect internal components, and provide charging and discharging interfaces.

2. Battery Cell

A battery cell is a single lithium-ion battery, which consists of a positive electrode, a negative electrode, a separator, an electrolyte and structural parts. See the table below for details.

3. Causes of thermal runaway

For lithium-ion batteries, there are three reasons for failure: internal short circuit, high temperature, and overvoltage. These three factors cause the temperature to reach a certain level, triggering a series of internal chemical reactions, further generating heat and gas, leading to fire and explosion. (Ultimately, it all comes down to temperature, so the failure reaction of the battery cell is called "thermal runaway")

It can be found that under normal circumstances, temperatures of 120°C and below are safe temperatures (no fire or explosion will occur), and 60-120°C is a temperature at which lithium-ion batteries deteriorate but are safe. The temperature that reaches above 120°C in an instant is the "cause" in the above figure for fire and explosion to occur.

1) Internal short circuit, that is, foreign matter pierces the diaphragm (only 10 microns thick), causing the positive and negative electrodes to come into direct contact, instantly generating a large amount of heat, which is also the root cause of battery spontaneous combustion;

2) Overcharge (overvoltage), generally when the ternary positive electrode material is overcharged to between 5.0-5.2V, it will have strong oxidizing properties, oxidizing the electrolyte/diaphragm and instantly generating a lot of heat;

3) High temperature: under extreme conditions, the battery pack is heated to above 120°C from the outside, or a part of the battery casing is instantly exposed to a large amount of heat (for example, the external circuit is short-circuited through the casing).

Except for the above three points, other factors will not cause lithium-ion batteries to catch fire or explode; it can also be said that other abuse factors are deformations of these three, which will be explained one by one below.

4. Conditions that cause spontaneous combustion

According to the abuse test in GBT31485, with the current state of battery technology, the following types can generally pass:

With current technical conditions, the following are more difficult to pass:

It can be found that it is relatively difficult to trigger a violent reaction of lithium-ion batteries through external conditions (hydraulic press squeezing, needle puncture or large overcharge). But why does the battery spontaneous combustion still occur? The author sorted out the conditions as follows:

It can be found that in the absence of serious accidents, the root cause of battery pack spontaneous combustion lies in processing control problems (internal short circuit, BMS failure or insulation failure).