Basic theory of battery safety
The safety of the battery mainly involves fire and explosion problems, which are caused by various chemical reactions caused by abnormal energy conversion and temperature rise in the battery. During normal operation, the charging and discharging process of the battery is controllable, but under abusive conditions, rapid energy conversion will produce by-products.
In order to ensure the safety of the battery, the temperature rise inside the battery must be suppressed, because most of the dangerous situations such as heat generation, heat dissipation, fire and explosion are caused by heat. When the temperature rises, the lithium-ion battery will have a self-heating reaction, increasing the temperature. Improper use or unbalance of heat dissipation and heat generation caused by manufacturing defects may cause the lithium-ion battery to catch fire.
The relationship between heat dissipation/heat generation and battery safety. If the heat dissipation rate is higher than the heat generation rate, thermal runaway can be avoided and the battery is stable. If the rate of heat generation exceeds the rate of heat dissipation, energy will accumulate and the temperature of the battery will rise over time and become unsafe.
We represents the energy entering the battery from outside (J/s); Wiis the heat spontaneously generated inside the battery (J/s); Wd is the heat lost (J/s); Cb is the heat capacity of the battery [(J/ s) /T]; 1 is the time (s); T0 is the external temperature: k is the heat dissipation constant (1/s). The battery temperature (T) can be obtained by the following formula:
dT/dt= (We+ Wi)/Gb– Wd/Cb= (We+ Wi)/Cb-k(T- T0)
Since the minimization of the (We + Wi)/Cb value is very important to improve safety, let us first look at the spontaneous response (Wi) in the above equation. The temperature of the battery is largely determined by the heat generated inside the battery and the heat lost to the external environment. Above a certain temperature, the exothermic decomposition reaction is accompanied by self-heating, and the increase in temperature will trigger more decomposition reactions, resulting in thermal control.
Generally, Wi is proportional to the battery capacity, and Wd and We are proportional to the surface area of the battery. The problem of thermal runaway occurs more frequently in large batteries, and the heat generated (We + Wi) is often larger than the heat lost (Wd). The main exothermic reactions in the battery include: the reduction reaction of the electrolyte at the negative electrode, the decomposition reaction of the electrolyte, the oxidation reaction of the electrolyte at the positive electrode, the thermal decomposition reaction of the positive electrode, and the deoxygenation decomposition reaction of metal oxides under high pressure. The short circuit caused by the melting of the diaphragm also generates heat. The melting point of the PE diaphragm is 135°C, and the melting point of the PP diaphragm is 165°C.
At 60°C, the decomposition of the electrolyte causes thickening of the negative and positive surface films, which affects battery performance. When the temperature exceeds 100°C, SEI (Solid Electrolyte Interface Membrane) decomposes and generates heat. The melting of the separator will cause a short circuit of the battery. At this time, electrons are rapidly transferred from the negative electrode to the positive electrode, and the resistance generates iR heat. The reaction between the electrolyte and the negative electrode will generate more heat and catalyze and accelerate the explosive thermal reaction of the positive electrode. At this stage, the most important thing is to ensure the thermal stability of the cathode material. DSC analysis results of charged LiCoO2 positive electrode, charged graphite negative electrode and electrolyte. The positive electrode (LiCoO2) has an exothermic reaction at a lower temperature (180-260°C), while the negative electrode has an exothermic reaction at 100~150°C due to the decomposition of the SEI film, and there is a large exothermic peak at 360°C. Caused by the decomposition of LiC6. The electrolyte shows an exothermic reaction at 250~300 ℃. Due to so many complex reactions, it is difficult to accurately analyze the heat generation of an actual battery.
Battery safety and cathode materials
The safety factors of the battery can be divided into three categories: overcharge/safety at high temperature/short circuit
Overcharge refers to charging the battery beyond its operating voltage, usually caused by the failure of the charger. The generation of heat occurs at the same time as the abnormal electrochemical reaction in the battery, and the short circuit is caused by the manufacturing defect or abuse of the battery. These reactions may have different causes, but they are all closely related to heat generation, and they all lead to violent thermal reactions on the negative electrode. Some related processes are the surface reaction between the electrolyte and the positive electrode, the pyrolysis reaction of the positive electrode (generation of oxygen), and the oxidation reaction and pyrolysis reaction of the electrolyte. The respective reactions and energies are as follows:
1) Positive electrode/electrolyte surface reaction;
2) Thermal decomposition reaction of the positive electrode (generation of oxygen);
3) Electrolyte oxidation reaction;
4) Thermal decomposition reaction of electrolyte, △H=-0.139kJ/g (electrolyte).
The electrolyte does not decompose under normal conditions, because the decomposition potential of the electrolyte is higher than that of the cathode material. However, when the positive root potential rises above the decomposition potential of the electrolyte, that is, in the overcharged state, the electrolyte undergoes an oxidation reaction and releases heat at the same time. Transition metal oxides, as cathode materials, undergo thermal decomposition reactions at high temperatures, and release a large amount of heat and oxygen during the process of structural changes. For different active materials, the initial decomposition temperature is also different, rising in the order of LiNiO2<LiCoO2<LiMn2O4.