Last Updated on 01/02/2025 by Bonnen Battery
Lithium Battery Overcharge: How It Affects Lithium Ion Batteries And What You Need to Know
1. What is the overcharging of lithium batteries?
Overcharging occurs when a battery continues to charge even after reaching its full capacity during the charging process. For instance, if a battery is rated at 3.7V, its full charge voltage typically stands at 4.2 ± 0.05V. However, if the charging voltage surpasses 4.25V, it crosses into the realm of overcharging.
2. What are the hazards of overcharging?
When overcharged, the battery voltage rapidly increases with increasing polarization, which can cause irreversible changes in the structure of the positive electrode active material and the decomposition of the electrolyte, producing a large amount of gas and releasing a large amount of heat, causing a sharp increase in battery temperature and internal pressure. The internal separator melts or contracts, leading to short circuits in contact with positive and negative electrode materials, and posing hidden dangers such as explosion and combustion.
3. What is the overcharging mechanism of lithium batteries?
When lithium batteries are overcharged, heat and gas are generated, including Ohmic heat and heat generated by side reactions, with Ohmic heat accounting for the majority. The side reaction of the battery caused by overcharging is firstly the excessive insertion of lithium into the negative electrode, which will lead to the growth of lithium dendrites on the surface of the negative electrode (the N/P ratio will affect the initial SOC of lithium dendrite growth). Secondly, excessive lithium is released from the positive electrode, causing the structure of the positive electrode to collapse, releasing heat and oxygen. Oxygen will accelerate the decomposition of the electrolyte, and the internal pressure of the battery will continue to rise. After a certain degree, the safety valve will open. The contact between active substances and air will further generate more heat.
Slight overcharging will not cause thermal runaway, but it will cause capacity decay. Research has found that when a battery with NCM/LMO hybrid material as the positive electrode is overcharged, there is no significant attenuation of capacity when the SOC is below 120%, while the capacity will decay significantly when the SOC is above 130%.
The following figure shows the voltage and temperature curves of NCM+LMO/Gr system batteries during overcharging. The maximum voltage was reached at 5.4V, followed by a decrease in voltage, ultimately leading to uncontrolled heating. The voltage and temperature curves of overcharged ternary batteries are very similar to them.
4. What are the side effects during overcharging?
The following is an experimental and simulated overcharging performance of a 40Ah pouch battery with NCM111+LMO as the positive electrode, with overcharging currents of 0.33C, 0.5C, and 1C, respectively. The battery size is 240mm * 150mm * 14mm. According to the rated voltage of 3.65V, its volumetric specific energy is about 290Wh/L, which is relatively low. The overcharge currents are 0.33C, 0.5C, and 1C, respectively. The battery size is 240mm * 150mm * 14mm. (Calculated based on the rated voltage of 3.65V, its volumetric specific energy is about 290Wh/L, which is still relatively low.)
The voltage, temperature, and internal resistance changes during overcharging are shown in Figure 1. It can be roughly divided into four stages:
Stage 1: 1<SOC<1.2, no obvious side reactions occurred inside the battery, and the temperature and internal resistance of the battery changed relatively little.
Stage 2: 1.2<SOC<1.4, Mn in the positive electrode dissolves, oxidizes the electrolyte on the positive electrode side, and lithium metal precipitates on the negative electrode surface. The reaction between lithium metal and solvent thickens the SEI film, increases the battery impedance, and the battery temperature begins to slowly rise.
Stage 3: 1.4<SOC<1.6, the temperature of the battery increases rapidly, the battery swells significantly, and the oxidation of the positive electrode electrolyte accelerates, releasing a large amount of heat and gas. The metal lithium on the negative electrode surface
continues to precipitate, and the SEI film begins to decompose. The lithiated graphite reacts with the electrolyte. Due to changes in the structure of the positive electrode material, the battery voltage slightly decreased after reaching its peak value of 5.2V.
Stage 4: If SOC>1.6, the internal pressure of the battery exceeds the limit, the shell ruptures, the diaphragm contracts and deforms, and the battery loses thermal control. A short circuit occurs inside the battery, and a large amount of energy is rapidly released, causing the battery temperature to rise sharply to 780 ℃.
Figure 1
Below Figure 2 is showing side reactions in each stage of overcharging
Figure 2
The heat generated during the overcharging process includes reversible entropy change heat, Joule heat, chemical reaction heat, and heat released from internal short circuits. The heat of chemical reactions includes the dissolution of Mn, the reaction between metallic lithium and electrolyte, electrolyte oxidation, SEI film decomposition, negative electrode decomposition, and the heat released by the decomposition of positive electrodes (NCM111 and LMO).Table shows the enthalpy change and activation energy of each reaction. (This blog ignores the side effects of binders)
Figure 3 shows the comparison of heat generation rates under different charging currents during overcharging. We can get the following conclusion drawn from Figure 3:
1) As the charging current increases, the thermal runaway time advances.
2) The heat generation in overcharging is mainly Joule heat. SOC<1.2, total heat production is basically equal to Joule heat.
3) In the second stage (1<SOC<1.2), three types of side reactions, including Mn dissolution, reaction between metallic lithium and electrolyte, and electrolyte oxidation, begin to react successively. When the current is 1C, the reaction will advance.
4) If SOC>1.45, the heat released by the reaction between lithium metal and electrolyte will exceed Joule heat.
5) When SOC>1.6, the SEI film and negative electrode decomposition reaction begin, and the heat production rate of the electrolyte oxidation reaction increases sharply, with the total heat production rate reaching its peak.
6) During overcharging, the main reactions are the reaction between lithium metal and electrolyte, as well as electrolyte oxidation.
Figure 3
Based on the above analysis, the electrolyte oxidation potential, negative electrode capacity, and initial temperature of thermal runaway are the three key parameters for overcharging. Figure 4 shows the impact of three key parameters on overcharging performance. It can be seen that an increase in the oxidation potential of the electrolyte can greatly improve the overcharging performance of the battery, while the negative electrode capacity has little effect on the overcharging performance. (In other words, high-voltage electrolytes help improve battery overcharging performance, and increasing the N/P ratio has little effect on battery overcharging performance.)
Figure 4
5. What is the method to solve the overcharging problem?
There are currently several main methods available:
1) Set protection voltage in BMS, usually the protection voltage should be lower than the peak voltage during overcharging;
2) Improving the overcharge resistance of batteries through material modification (such as material coating);
3) Add anti-overcharging additives such as redox pairs to the electrolyte;
4) The use of voltage-sensitive membranes significantly reduces the membrane resistance when overcharging occurs in batteries, playing a role in diverting current;
5) OSD and CID designs are commonly used in square aluminium shell batteries to prevent overcharging.
Understanding the overcharging mechanism and its associated hazards is crucial for ensuring the longevity and safety of lithium-ion batteries. For more insights and guidance on battery management and safety practices, continue exploring Bonnen’s blog and stay informed on best practices in battery care.
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