In the context of energy crisis and environmental pollution, lithium-ion batteries have attracted more and more attention as an ideal energy source for the development of the 21st century. However, lithium-ion batteries will have some failures during production, transportation and use. Failure of a single battery can affect the performance and reliability of the entire battery pack, and can even cause the battery pack to stop working or other safety issues.
In recent years, there have been many battery-related fire and explosion accidents at home and abroad: the US Tesla Model S electric car fire accident, the Samsung Note7 mobile phone battery fire accident, the Wuhan Fute electronic factory fire, the Tianjin Samsung SDI factory fire, etc...
1 lithium battery failure classification
In order to avoid the above-mentioned performance degradation and battery safety problems, it is imperative to carry out lithium battery failure analysis. Lithium battery failure refers to battery performance degradation or performance abnormality caused by certain specific essential reasons, which is divided into performance failure and safety. Sexual failure.
Classification of common lithium battery failures
2 Lithium battery failure reasons
The reasons for the failure of lithium batteries can be divided into internal and external causes.
Internal factors mainly refer to the physical physics of failure, the nature of chemical changes, and the research scale can be traced back to the atomic, molecular scale, thermodynamics and dynamics of the failure process.
External factors include external factors such as impact, acupuncture, corrosion, high temperature combustion, and vandalism.
Internal situation of lithium battery failure
Analysis of common failure performance and failure mechanism of 3 lithium batteries
Capacity attenuation failure
'In the standard cycle life test, the discharge capacity should be no less than 90% of the initial capacity when the number of cycles reaches 500. Or the discharge capacity should not be lower than 80% of the initial capacity when the number of cycles reaches 1000, if in the standard cycle range 4. The sharp decline in capacity is a failure of capacity attenuation.
The root cause of battery capacity attenuation is the failure of materials, and it is closely related to objective factors such as battery manufacturing process and battery use environment. From the material point of view, the main causes of failure are structural failure of the positive electrode material, and SEI transition growth of the negative electrode surface. Decomposition and deterioration of electrolyte, current collector corrosion, trace impurities in the system, etc.
Structural failure of the positive electrode material: Failure of the positive electrode material structure includes particle breakage of the positive electrode material, irreversible phase transition, material disorder, etc. LiMn2O4 will be distorted due to the Jahn-Teller effect during charging and discharging, and even particle breakage may occur. The electrical contact between the particles fails. LiMn1.5Ni0.5O4 material undergoes a 'tetragonal-cubic system' phase transition during charge and discharge. LiCoO2 material enters Li during the charge and discharge process due to the transition of Li. Layer, causing the layered structure to be chaotic, restricting its capacity to play.
Negative anode material failure: The failure of the graphite electrode mainly occurs on the graphite surface, and the graphite surface reacts with the electrolyte to produce a solid electrolyte interface phase (SEI). If excessive growth causes the lithium ion content in the internal system of the battery to decrease, the result is capacity decay. The failure of silicon anode materials is mainly due to the cyclic performance problems caused by its huge volume expansion.
Electrolyte failure: LiPF6 has poor stability and is easy to decompose to reduce the transportable Li+ content in the electrolyte. It also easily reacts with trace water in the electrolyte to form HF, which causes corrosion inside the battery. Poor air tightness causes electrolyte deterioration. The viscosity and chromaticity of the electrolyte change, which eventually leads to a sharp decline in the transport ion performance.
The failure of the current collector: the fluid collector corrosion, the current collector adhesion is reduced. The HF generated by the above electrolyte failure will corrode the current collector, resulting in poor conductivity of the compound, resulting in increased ohmic contact or active material failure. Cu during charge and discharge After the foil is dissolved at a low potential, it is deposited on the surface of the positive electrode. This is called 'clearing copper'. The common form of fluid collection failure is that the binding force between the current collector and the active material is insufficient to cause the active material to peel off, which cannot be provided for the battery. capacity.
Increased internal resistance
The increase of internal resistance of lithium battery is accompanied by the decrease of energy density, voltage and power drop, heat generation of battery, etc. The main factors leading to the increase of internal resistance of lithium ion battery are the key materials of battery and the environment of battery use.
Key materials of the battery: microcrack and fracture of the positive electrode material, damage of the negative electrode material and surface SEI is too thick, the electrolyte is aged, the active material is separated from the current collector, and the contact between the active material and the conductive additive is deteriorated (including the loss of the conductive additive), The diaphragm shrinkage hole is blocked, the battery ear is abnormally welded.
Battery environment: ambient temperature is too high / low, overcharge and over discharge, high rate charge and discharge, manufacturing process and battery design structure.
Internal short circuit
Internal short circuit often causes self-discharge of lithium-ion battery, capacity attenuation, local thermal runaway and safety accidents.
Short circuit between copper/aluminum current collector: Metal foreign body puncture diaphragm or electrode that is not trimmed during battery production or use. The displacement of pole piece or tab in battery package causes positive and negative current collector contact.
Short circuit caused by diaphragm failure: diaphragm aging, diaphragm collapse, diaphragm corrosion, etc. will cause diaphragm failure, failure of the diaphragm loses electrical insulation or voids become positive, the anode is slightly contacted, then local heat is severe, continue to charge and discharge will spread to the surrounding , causing the heat to run out of control.
Impurities lead to short circuit: The transition metal impurities in the positive electrode slurry are not cleaned, which may lead to piercing the diaphragm or causing the lithium dendrites to form an internal short circuit.
Short circuit caused by lithium dendrites: Lithium dendrites appear in places where local charges are not uniform during long cycling, and dendrites pass through the diaphragm to cause internal short circuits.
In the design of the battery or the assembly process of the battery pack, the design is unreasonable or the partial pressure is too large, which may lead to internal short circuit. The inner short circuit may also occur under the induction of battery overshoot and overdischarge.
Gas production
The gas production phenomenon that occurs when the electrolyte is formed during the battery formation process to form a stable SEI film is normal gas production, but the phenomenon of transitional consumption of electrolyte release gas or release of oxygen from the positive electrode material is abnormal venting. It is often found in soft pack batteries. , will cause the internal pressure of the battery is too large and deformed, breaking the aluminum film of the package, the internal cell contact problems.
Gas composition analysis of normal cells and failed cells
The trace moisture in the electrolyte or the electrode active material is not dried, causing the lithium salt in the electrolyte to decompose to produce HF, corrode the current collector Al and destroy the binder, generating hydrogen. The chain/ring in the electrolyte caused by the inappropriate voltage range The esters or ethers will undergo electrochemical decomposition, which will produce C2H4, C2H6, C3H6, C3H8, CO2, etc.
Thermal runaway
Thermal runaway means that the temperature of the internal or whole part of the lithium-ion battery rises rapidly, the heat cannot be dissipated in time, and a large amount of heat accumulates inside, and induces further side reactions. The factor that induces thermal runaway of the lithium battery is abnormal operating conditions, that is, abuse. Short circuit, high rate, high temperature, extrusion and acupuncture.
Common thermal behavior inside the battery
Lithium
Lithium deposition is the precipitation of metallic lithium on the negative electrode surface of the battery. It is a common phenomenon of aging of lithium batteries. Lithium deposition will reduce the internal active lithium ions in the battery, cause capacity failure, and will form dendritic piercing diaphragm, which will lead to Local current and heat production are too large, eventually causing battery safety problems.
Decomposed battery common lithium picture
China's failure analysis has been systematically developed in the field of machinery and aviation, but has not been systematically studied in the field of lithium batteries. Battery companies and materials companies have carried out research on failure analysis of lithium-ion batteries, but more emphasis on battery manufacturing processes. And research and development of materials to improve battery performance and reduce battery cost as a direct goal. Future research institutes and related companies can strengthen cooperation and exchange, and strive to establish and improve the failure tree and failure analysis process of lithium-ion battery failure.
