Mechanical abuse refers to external forces such as impact, compression, and puncture on the battery; electrical abuse refers to internal or external short circuits, overcharging, and over-discharging; and thermal abuse refers to external heating of the battery. In reality, these three types of abuse are not completely independent but rather form a chain relationship. The relationship between the three types of abuse is shown in Figure 1.
As shown in Figure 1, mechanical abuse first leads to electrical abuse, which in turn leads to thermal abuse, ultimately triggering thermal runaway. Mechanical abuse causes electrical abuse because external forces rupture the internal separator of the lithium battery, causing the positive and negative electrodes to connect, thus initiating an internal short circuit—one form of electrical abuse. Electrical abuse leads to thermal abuse because once a short circuit occurs inside the lithium battery, a large amount of heat is released, triggering chemical reactions at even higher temperatures. These reactions further release heat, which is equivalent to an external heat source continuously heating the battery—thermal abuse. Once the heat inside the battery accumulates to a certain level, thermal runaway occurs.
This article provides a comprehensive overview of all chemical reactions occurring inside lithium-ion batteries, focusing on the positive electrode, negative electrode, and electrolyte. Furthermore, it systematically examines the chemical reactions of lithium-ion batteries based on different trigger temperatures. During this review, it was discovered that lithium-ion batteries release a large amount of gas, which differs from the gas released under normal conditions. Based on this, a proposal is made to utilize this gas for thermal diagnostics of lithium-ion batteries.
1. Study on internal chemical reactions of lithium batteries
1.1 Theoretical Basis
The structure of a lithium battery is shown in Figure 2. It consists of a positive electrode, a negative electrode, and an electrolyte. The basic working principle of a lithium battery is as follows:
In the formula: LiMO2 represents the positive electrode, M represents active materials such as Fe, Co, Ni, and Mn; C represents the negative electrode. The reaction proceeding from left to right in formula (1) is the discharge reaction, while the reaction proceeding from right to left is the charging reaction. For the electrolyte, it is composed of two or more solvents and one or more lithium salts. Single solvents are relatively rare in lithium batteries because in the practical application of lithium batteries, a single solvent is difficult to meet a variety of different or even contradictory requirements. We will take a lithium iron phosphate battery with LiCoO2 as the positive electrode, C as the negative electrode, EC and DEC as solvents, and LiPF6 as the solute as an example for explanation. This configuration of positive electrode, negative electrode, and electrolyte is currently the most common.
Inside a lithium battery, a membrane separates the positive and negative electrodes, preventing direct reaction. The primary reactions occur between the positive and negative electrodes themselves and their respective reactions with the electrolyte. Therefore, this article summarizes the chemical reactions from three aspects: the reaction between the positive electrode and the electrolyte, the reaction between the negative electrode and the electrolyte, and the reaction within the electrolyte itself.
1.2 Chemical Reaction Research
(1) Reaction between the positive electrode and the electrolyte
Cathode materials are developed based on transition metal oxides. When heated, cathode materials undergo a decomposition reaction, releasing heat.
Based on this, oxygen will react with the electrolyte, releasing large amounts of carbon monoxide, carbon dioxide, and heat.
As the heat released increases, the internal temperature of the lithium battery also increases significantly, causing the positive electrode active material to decompose further and release oxygen.
In addition, DEC reacts with PF5 from LiPF6, thus accelerating oxygen consumption and releasing carbon dioxide and heat.
From equations (2) to (9), it can be seen that the thermal decomposition of the positive electrode releases a large amount of oxygen, which is an essential factor for thermal runaway. In addition, carbon dioxide will be released in large quantities due to the presence of oxygen, and there will also be smoke C2H5F.
(2) Reaction between negative electrode and electrolyte
The negative electrode surface is coated with a solid electrolyte interface (SEI). SEI formation is a necessary process in battery manufacturing, as it prevents direct reaction between the negative electrode and the electrolyte. The SEI mainly consists of the unstable component CH2OCO2Li and the stable component Li2CO3. As the temperature rises, the unstable component CH2OCO2Li decomposes and converts into the stable component Li2CO3.
or:
The heat generated in this reaction is directly related to the surface area of the negative electrode. Once the SEI decomposes, the negative electrode will be directly exposed to the electrolyte and react with it. If the electrolyte content is low at this point, the reaction is mainly dominated by the negative electrode.
If the electrolyte content is high at this time, then the reaction is mainly dominated by the electrolyte:
It is evident that different negative electrode capacities and electrolyte capacities will induce different chemical reactions. PVDF, the binder, is essential in battery manufacturing; PVDF undergoes dehydrofluorination in the electrolyte. The chemical reaction that occurs is as follows:
(3) Electrolyte reaction
Besides the reactions between the positive and negative electrodes and the electrolyte, the electrolyte itself also decomposes. Firstly, the solute LiPF6 in the electrolyte undergoes a decomposition reaction:
The reaction product PF5 in equations (7) and (8) is obtained through this reaction. Subsequently, the DEC product C2H5OCOOPF4 undergoes a further decomposition reaction:
At the same time, C2H5OCOOPF4 will also undergo further chemical reactions with HF:
Thus, equations (2) to (22) summarize all possible chemical reactions that may occur inside a lithium battery prepared using LiFePO4 and C as electrodes, EC and DEC as solvents, and LiPF6 as solute. As can be seen from equations (2) to (22), different internal chemical reactions in a lithium battery produce different solid and gaseous products and release a large amount of heat. When a large amount of heat accumulates excessively, thermal runaway of the lithium battery will be triggered. The solid products are summarized as: Fe2O3, FeO, C2H5OCOOPF4, CH2OCO2Li, Li2CO3, LiO(C2H5)4OLi, LiO(CH2)4OLi, LiO(C2H5)4F, LiO(CH2)4F, LiF, PF4OH. The gaseous products are summarized as: O2, CO2, C2H4, C4H10, POF3, C2H5F, PF5, H2.
1.3 Chemical Reaction Triggering Sequence
In reality, the chemical reactions inside a lithium battery do not occur all at once, but rather proceed one by one according to different trigger temperatures. To further clarify the order of chemical reactions inside a lithium battery, the chemical reactions given in equations (2) to (22) are summarized.
(1) The solute LiPF6 is unstable, which causes it to decompose at relatively low temperatures of 60–70 °C, corresponding to equation (19). Although the reaction can proceed at low temperatures, the exothermic reaction is relatively low. However, the product PF5 of LiPF6 promotes other reactions.
(2) SEI possesses electronic insulation and ion transport properties, protecting the negative electrode and preventing direct reaction between the negative electrode and the electrolyte. The melting point of SEI is between 100 and 130 °C. At this temperature, it cracks, transforming the unstable components in the SEI into stable components, corresponding to equations (10) and (11). This reaction is exothermic, with a greater heat release than the reaction of the solute LiPF6. Studies have shown that the heat released during the SEI cracking reaction is the source of thermal runaway.
(3) Once the SEI breaks down, the portion of the negative electrode no longer encapsulated by the SEI will be exposed to the electrolyte, and this portion will react directly with the electrolyte. Based on the relationship between the exposed portion of the negative electrode and the electrolyte content, the chemical reaction corresponds to equations (12) to (17). Currently, studies have indicated that the reaction between the negative electrode and the electrolyte will occur at 120–150°C and above 200°C.
(4) In the temperature range of 130 to 220°C, the reactant PF5 of LiPF6 will promote the reaction of EC and DEC in the electrolyte. The reaction product C2H5OCOOPF4 will further decompose in this temperature range, thereby generating a large number of by-products, corresponding to formulas (7), (8), (20) to (22).
(5) The positive electrode decomposes at high temperatures of 170–300°C, undergoing an oxidation reaction and releasing oxygen. These reactions are shown in equations (2), (5), and (6). For a given positive electrode active material, its thermal stability is related to x in equation (2); the lower the x, the worse the thermal stability. For different positive electrode active materials, their decomposition temperatures vary slightly, generally between 180 and 280°C. When the positive electrode material is LiNiO2 or LiCoO2, the decomposition temperature is around 180°C; when the positive electrode material is LiMn2O2, the decomposition temperature is around 200°C; when the positive electrode material is LiFePO4, the decomposition temperature is around 220°C; and when the positive electrode material is LiNi3/8Co1/4Mn3/8O2, the decomposition temperature is around 270°C.
As oxygen is released, a large amount of oxygen reacts with solvents EC and DEC, releasing a large amount of carbon dioxide, corresponding to equations (3), (4), and (9). Studies have shown that the thermal decomposition reaction of the positive electrode itself and the reaction between the positive electrode and the electrolyte release a large amount of heat in a short period of time, causing heat accumulation inside the lithium battery. Therefore, this reaction is the root cause of thermal runaway.
(6) When the temperature rises above 230°C, the PVDF binder will decompose and release a large amount of heat, corresponding to equation (18). Studies have shown that this reaction will further aggravate the degree of thermal runaway and deteriorate the safety performance of lithium batteries.
(7) In addition to the above reactions, a membrane dissolution reaction also occurs in the temperature range of 130~190℃. Although this reaction is endothermic, once the membrane is dissolved, it will cause a short circuit between the positive and negative electrodes, thereby triggering thermal runaway. Therefore, based on the research results of existing literature, the sequence of chemical reactions inside lithium batteries has been summarized according to different trigger temperatures.
2. Early warning and diagnostic strategies
Early warning and diagnosis of thermal runaway in lithium-ion batteries is a crucial prerequisite for ensuring their reliable operation. Analysis of the internal chemical reactions of lithium-ion batteries reveals that various gases are released during the heating process. In other words, different gases are inevitable products of different chemical reactions. Therefore, the released gases can be considered as characteristic quantities for early warning and diagnosis of thermal runaway in lithium-ion batteries. Although battery management systems can measure the current, voltage, and temperature of lithium-ion batteries in real time, the current and voltage information cannot directly reflect the internal chemical reactions. Furthermore, there are significant differences between the temperatures measured inside and outside the battery. Therefore, utilizing gaseous characteristic quantities for differentiation is considered.
The premise of using gas for fault warning in lithium batteries is that the gases evolved by lithium batteries differ between normal operating conditions and elevated temperature conditions. Therefore, it is necessary to further compare the gas evolution under these two conditions. In the previous analysis, the gas studied was the gas evolved from the internal chemical reaction of the lithium battery under elevated temperature conditions, which became the abnormal gas. The following analysis focuses on the gas under normal operating conditions of lithium batteries, i.e., the normal gas.
In lithium-ion batteries, the main gaseous phase is generated during the SEI (Sediment Injection) formation stage. As a necessary process in battery manufacturing, the SEI formation stage primarily involves electronic reactions of the solvent. The solvent includes EC (electrolytic electrolyte) and DEC (deionized electrolyte). Let's first examine the electronic reactions of EC:
Secondly, the electronic reactions of DE:
For ease of comparison, normal and abnormal gases have been categorized. Normal gases include CO, C2H4, and C4H10. Abnormal gases include CO2, CO, C2H4, C4H10, POF3, C2H5F, PF5, and H2. It is evident that abnormal gases differ significantly from normal gases in their types. Furthermore, as temperature increases, internal chemical reactions become more intense, leading to a continuous increase in the content of released gases. Therefore, based on the significant differences between abnormal and normal gases, such as gas content, gas change rate, and gas type, early warning and diagnosis of lithium battery thermal runaway can be achieved. In operation, it is necessary to select appropriate sensors and set corresponding thresholds for judgment.
