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Analysis of common failure performance and failure mechanism of lithium batteries

- Sep 14, 2018 -

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Lithium battery failure


The reasons for the failure of lithium batteries can be divided into internal and external causes.


Internal factors mainly refer to the physical and chemical changes of failure. The research scale can be traced back to the atomic and molecular scales, and the thermodynamics and dynamics of the failure process are studied.


External factors include external factors such as impact, acupuncture, corrosion, high temperature combustion, and vandalism.


Analysis of common failure performance and failure mechanism of lithium batteries


Capacity attenuation failure


"In the standard cycle life test, the discharge capacity shall not be less than 90% of the initial capacity when the number of cycles reaches 500. Or the discharge capacity shall not be less than 80% of the initial capacity when the number of cycles reaches 1000", if in the standard cycle range Inside, 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, SEI transition growth of the negative electrode surface, decomposition and deterioration of the electrolyte, current collector corrosion, and trace impurities in the system.


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, disorder of the material, and the like. LiMn2O4 will be distorted due to the Jahn-Teller effect during charging and discharging, and even particle breakage will occur, causing electrical contact failure between particles. LiMn1.5Ni0.5O4 material undergoes "tetragonal-cubic system" phase transition during charge and discharge. LiCoO2 material will lead to Co into the Li layer due to the transition of Li during charge and discharge, resulting in chaotic layer structure. , 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 degradation. The failure of silicon-based anode materials is mainly due to the cyclic performance problems caused by its huge volume expansion.


Electrolyte failure: LiPF6 has poor stability and is easily decomposed to reduce the transportable Li+ content in the electrolyte. It also easily reacts with traces of water in the electrolyte to form HF, causing corrosion inside the battery. The poor air tightness causes the electrolyte to deteriorate, and the viscosity and chromaticity of the electrolyte change, eventually leading to a sharp drop in the transport ion performance.


The failure of the current collector: the collector corrosion and the current collector adhesion decrease. The HF generated by the above electrolyte failure causes corrosion of the current collector, resulting in a poorly conductive compound, resulting in an increase in ohmic contact or failure of the active material. During the charging and discharging process, the Cu foil is dissolved at a low potential and deposited on the surface of the positive electrode. This is called "clearing copper". A 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 and cannot provide capacity for the battery.


Increased internal resistance


The increase in the internal resistance of a lithium battery is accompanied by failures such as a decrease in energy density, a drop in voltage and power, and heat generation of the battery. The main factors leading to the increase of the internal resistance of lithium-ion batteries are the key materials of the battery and the environment in which the battery is used.


Key materials of the battery: microcrack and fracture of the positive electrode material, damage of the negative electrode material and excessive surface SEI, aging of the electrolyte, detachment of the active material from the current collector, deterioration of contact between the active material and the conductive additive (including loss of conductive additives), The diaphragm shrinkage hole is blocked, the battery tab is abnormally welded, and the like.


Battery use 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 collectors: metal foreign matter puncture diaphragms or electrodes that are not trimmed during battery production or use, and poles or tabs in the battery package are displaced to cause contact between positive and negative current collectors.


Short circuit caused by diaphragm failure: diaphragm aging, diaphragm collapse, diaphragm corrosion, etc. will lead to diaphragm failure, the failure of the diaphragm loses electrical insulation or the gap becomes large, causing the positive and negative poles to contact, and then the local heat is severe, and the charge and discharge will continue to spread around. , causing the heat to get out of control.


Impurities cause a short circuit: the transition metal impurities in the positive electrode slurry are not cleaned, which may cause the piercing of the separator or cause 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.


Unreasonable design or excessive partial pressure can cause internal short circuits in battery design and manufacturing or battery assembly. An internal short circuit can also occur under the induction of battery overshoot and overdischarge.


Gas production


In the process of battery formation, the phenomenon of gas production caused by the consumption of electrolyte to form a stable SEI film is normal gas production, but the phenomenon of transitional consumption of electrolyte release gas or oxygen release of positive electrode material is abnormal deflation. Often appearing in soft-package batteries, it can cause excessive internal pressure of the battery to deform, break the aluminum film of the package, and contact the internal battery.