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Safety Analysis of Li-Ion Battery

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In this Article, we are discussing different types of Safety Analysis that can be taken to protect Li-Ion Battery.

INTRODUCTION

In recent years, there have been frequent reports of fire and even explosion caused by lithium-ion batteries. Lithium-ion batteries are mainly composed of negative electrode material, electrolyte and positive electrode material. Lithium-ion batteries (LIBs) have been widely used as power sources in EV because of their advantages in high energy density, long cycle life, low self-discharge rate, and wide working temperature range:

  • The chemical activities of the negative electrode material (Graphite) are closer to that of metallic lithium in the charged state, and the SEI (solid electrolyte interphase) film is formed on electrode surfaces from decomposition products of electrolytes at high temperature.
  • The electrolyte generally uses an organic solution of alkyl carbonate, which has flammable properties.
  • The positive electrode material (Li-ion metal oxide) has a strong oxidizing property in the charged state. It is easy to decompose and release oxygen at high temperature. The released oxygen undergoes an oxidation reaction with the electrolyte, and then a large amount of heat is released.

Therefore, from the point of view of materials, lithium-ion batteries have strong danger, especially in the case of abuse; the safety problem is more prominent.

This Article gives you complete Safety Analysis of Li-Ion Battery.

1. Thermal stability analysis

The fire hazard of lithium-ion batteries are mainly determined by the amount of heat generated in chemical reactions. So, It depends on the thermal stability of the battery material, which in turn depends on the chemical reactions that take place between its internal parts.
• At present, people mainly rely on differential scanning calorimeter (DSC), thermo gravimetric analyzer (TGA), adiabatic acceleration calorimeter (ARC), etc. to study the thermal stability of battery-related materials.

I. Factors affecting the thermal stability of negative electrode materials:

a) The onset temperature of the exothermic reaction of the anode material increases with the particle size.
b) The reaction rate of carbon anode materials increases with the increase of specific surface area.
c) Carbon materials with different structures produce different heats of reaction, and the graphite structure produces more heat than the amorphous carbon structure.

II. Influencing factors of thermal stability of cathode materials:

The onset temperature of the reaction between the cathode material and the electrolyte increases as the stoichiometric number decreases. The effect of the change of x on the reaction of the cathode materials LiXCoO2, LiXNiO2, and LiXMn2O4 with the electrolyte was investigated and, it is concluded that the initial temperature of the decomposition reaction of the positive electrode material increases with the decrease of X. Hence, Form above example, the higher the Co/Ni/Mn content in the cathode material, the more stable it is.

III. Factors affecting the thermal stability of the electrolyte:

a) The organic solvent (DMC) is an important factor causing the instability of the electrolyte. The higher the DMC content, the more unstable electrolyte will be.
b) The electrolyte allows the cathode to react at a lower temperature. Different solvents and lithium salts in the electrolyte are suitable for different cathode materials.
• The research shows that the decomposition reaction of Li0.5CoO2 powder occurs when the temperature is higher than 200°C, and oxygen is released, while the exothermic reaction with EC/DEC solvent occurs at 130°C. The reaction is inhibited after adding LiPF6 to the solvent.
•  For LiMn2O4 material, the crystal transformation occurs at 160°C and for exothermic, the presence of solvent has no effect on this reaction.
• After adding LiPF6 to the electrolyte, the reaction between LiMn2O4 and the electrolyte intensifies which increase with LiPF6 concentration.

2. The safety analysis of lithium-ion battery abuse

The safety of lithium-ion batteries mainly depends on the thermal stability of battery materials, and is also closely related to abuse conditions such as battery overcharge, needle penetration, extrusion, and high temperature.

1. Analysis of overcharge safety:

a) The overcharge test is to simulate the potential safety hazards of the battery when the charger voltage detection is wrong, the charger fails or the wrong charger is used.
b) The thermal runaway caused by overcharge may come from two aspects:
• The Joule heat generated by the current, and
• The reaction heat generated by the side reactions of the positive and negative electrodes.
c) When the battery is overcharged, the voltage of the negative electrode gradually increases. When the amount of delithiation of the negative electrode is too large, the process of delithiation becomes more and more difficult, which leads to a sharp increase in the internal resistance of the battery, so a large amount of Joule heat is generated, which is in large scale.
*Note: Delithiation means the removal of lithium from an electrode of a lithium-ion battery.
d) The high-voltage positive oxidant in the overcharged state releases a lot of heat, and the negative electrode will also undergo an exothermic reaction with the electrolyte after the temperature rises.

Thermal runaway occurs when the heat release rate is greater than the heat dissipation rate of the battery and the temperature rises to a certain level.

On the basis of research analysis, it is concluded that the overcharge performance of the cell mainly depends on the cathode material and does not change with the increase of graphite content. This shows that the precipitation of metallic lithium in the negative electrode during the overcharge process is not the key to affecting the overcharge performance, but the thermal stability of the excessively delithiated or the oxidation reaction of the electrolyte on its surface.

2. High Temperature Safety Analysis:

a) In the case of thermal abuse, the heat source comes from the positive and negative electrode materials inside the battery and their reaction occurs with the electrolyte.
b) The separator melts and shrinks at high temperature, resulting in a short circuit of the positive and negative electrodes.

The simulated environment high temperature test can be carried out using the hot box test. The hot box test is to simulate the improper use of the battery at high temperature, such as placing a mobile phone in an exposed car, or placing a mobile phone or electronic product in a microwave oven, the temperature can reach 130°C or even 150°C.

When the temperature is between 90°C and 120°C, the metastable layer of the solid electrolyte interface film (SEI) formed on the surface of the carbon negative electrode by multiple charging and discharging (First decomposes and exothermic).

As the temperature increases, the separator absorbs heat and melts successively. When the temperature is between 180°C to 500°C, the positive electrode and the electrolyte undergo a strong exothermic reaction and generate gas. The SEI film can prevent the interaction between the lithium-intercalated carbon and the organic electrolyte. When the temperature is higher than 120°C, the rupture of the SEI film cannot protect the negative electrode. The anode material may start an exothermic reaction with the solvent and generate gas, when the temperature rises to at 240°C to 350°C, the fluorine-containing binder begins to undergo a violent chain growth reaction with the lithium-intercalating carbon, releasing a lot of heat. These conditions are very dangerous for large lithium-ion power batteries, affecting the life and safety of the battery.
• The simulated environment high temperature test can be carried out using the hot box test. The hot box test is to simulate the improper use of the battery at high temperature, such as placing a mobile phone in an exposed car, placing a mobile phone or electronic product in a microwave oven. The temperature can reach 130℃ or even 150℃.
• The Joule heat generated by the short circuit is also an important heat source in the hot box test.

3. Short – Circuit Safety Analysis:

The short circuit of the battery is divided into:
• An external short circuit, and
• An internal short circuit.
a) External short circuit generally refers to the short circuit caused by the direct contact of the positive and negative electrodes.
b) Internal short circuit refers to the short circuit in the area where the battery is affected by foreign objects when the battery is punctured by a sharp object or is collided or squeezed.

External Short-Circuit Safety Analysis

  • The external short-circuit safety research is tested by the method of directly connecting the positive and negative electrodes to the outside.
  • On the basis of 18650 3S2P battery analysis, the positive and negative electrodes are short-circuited with wires, and a thermocouple is attached to the surface of the battery to detect the temperature change of the battery surface.
  • The maximum temperatures of the two groups of batteries are 73.3°C and 65.1°C respectively. Although, such a temperature will not cause the battery to burn and explode, because of its continuous heat release. For large-capacity battery packs, if the heat cannot be dissipated in time, it may cause a fire or even an explosion.

Internal Short-Circuit Safety Analysis

• The safety research of the internal short circuit of the battery is generally tested by acupuncture, extrusion and other methods, and the purpose is to simulate the situation of the battery being punctured, collided, and squeezed by foreign objects.
 Acupuncture causes the battery to short-circuit at the acupuncture point, and the short-circuit area forms a local hot zone due to a large amount of Joule heat. When the temperature of the hot zone exceeds the critical point, thermal runaway will occur, resulting in the danger of smoke, fire or even explosion.
Extrusion is similar to acupuncture in that both cause localized internal short circuits and may cause thermal runaway. The difference is that squeezing does not necessarily cause damage to the battery casing. If the casing is not damaged, it means that the flammable electrolyte will not leak from the hot zone, and the heat dissipation effect of the hot zone is poorer.
• It is often much more difficult to test the local internal short-circuit of the battery through extrusion and acupuncture. This is because when the battery is short-circuited outside the battery, the internal heat of the battery tends to be uniformed, and the heat generated by the externally short- circuited battery will not directly trigger the thermal runaway reaction of the battery.
• The test conditions such as acupuncture and extrusion have a great influence on the test results, because the internal short-circuit conditions caused by the acupuncture and extrusion tests under different conditions are different, and the values of the internal short-circuit resistance has a greater effect on the heat generation power in the short- circuit area.    
• There are 4 forms of internal short circuit in the battery: 
(a) Between the current collector and the negative electrode material (LiC6, C6);
(b) Between the current collector and the Cu current collector 
(c) Between the positive electrode material and LiC6; And
(d) Between the positive electrode material and the Cu current collector.

CONCLUSION

This Article discusses about safety Analysis developed for the risk assessment of advance batteries. Although the focus here is on the batteries used in hybrid, electric, or plug-in vehicles, the methodology itself, called Hazard Modes & Risk Mitigation Analysis, is quite general and can be used in other applications for batteries as well as for other components & parts that maybe considered hazardous. In addition, the methodology quantifies the risk associated with each hazard and becomes a valuable design tool to develop the most effective way of reducing the risk.

More Articles:

BMS Battery Management System,
Battery Electrical Performance Test,
Safety Analysis of Li-Ion Battery,
IEC Battery Safety Standard for Power Batteries,
POWER BATTERY SHELL WATERPROOF DESIGN,
BATTERY SAFETY PERFORMANCE TEST,

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