Under the background of energy crisis and environmental pollution, lithium-ion battery, as the ideal energy for the development of the 21st century, has attracted more and more attention. However, some failures occur in the process of production, transportation and use. Moreover, the failure of a single battery will affect the performance and reliability of the entire battery pack, and even cause the battery pack to stop working or other safety problems.
In recent years, there have been a number of battery-related fire and explosion accidents at home and abroad: Tesla Model S electric car fire accident in the United States, Samsung Note7 phone battery fire accident, Wuhan Fute electronics factory fire, Tianjin Samsung SDI factory fire...
1. Classification of lithium battery failure
In order to avoid the performance attenuation and battery safety problems mentioned above, it is imperative to conduct lithium battery failure analysis. The failure of lithium battery refers to the degradation of battery performance or abnormal performance caused by some specific essential reasons, which can be divided into performance failure and safety failure.
Performance failures include capacity diving, cycle life attenuation, abnormal voltage, abnormal current, excessive internal resistance, self-discharge, high and low temperature aging, poor rate performance, poor consistency.
Safety failures include thermal runaway, short circuit, fluid leakage, gas expansion, lithium evolution, expansion deformation, puncture (extrusion).
2. Reasons for the failure of lithium batteries
The failure of lithium battery can be divided into internal and external causes.
Internal cause mainly refers to the nature of physical and chemical changes of failure. The research scale can be traced back to atomic and molecular scales, and the thermodynamic and dynamic changes of failure process can be studied.
External causes include impact, acupuncture, corrosion, high temperature combustion, artificial destruction and other external factors.
3. Analysis of common failure performance and failure mechanism of lithium battery
Capacity attenuation failure
"For standard cycle life tests, the discharge capacity should be no less than 90% of the initial capacity at 500 cycles. Or the discharge capacity shall not be lower than 80% of the initial capacity when the number of cycles reaches 1000 ". If the capacity drops sharply within the standard cycle range, it belongs to capacity attenuation failure.
The failure of battery capacity attenuation is caused by material failure, and is closely related to the objective factors such as battery manufacturing technology and battery using environment. From the material point of view, the main causes of the failure are the structural failure of the cathode material, the SEI transition growth of the cathode surface, the decomposition and deterioration of the electrolyte, the collector corrosion, the system trace impurities and so on.
Structural failure of cathode material: structural failure of cathode material includes particle breakage, irreversible phase transformation, material disorder, etc. In the process of charge and discharge, the structure of LiMn2O4 will be distorted due to Jahn-Teller effect, and even particles will be broken, resulting in electrical contact failure between particles. Limn1.5ni0.5o4 material will have a tetral-cubic crystal system phase transition in the charging and discharging process, LiCoO2 material will lead to Co entering Li layer due to the transition of Li, resulting in the layered structure disorder, restricting its capacity play.
Anode material failure: Graphite electrode failure mainly occurs on the graphite surface. The graphite surface reacts with the electrolyte to produce solid electrolyte interface phase (SEI). Excessive growth will lead to the reduction of lithium ion content in the battery internal system, resulting in capacity attenuation. The failure of silicon anode materials mainly lies in the cyclic performance caused by its huge volume expansion.
Electrolyte failure: LiPF6 has poor stability and is easy to decompose, which reduces the content of transportable Li+ in the electrolyte. It also tends to react with trace water in the electrolyte to form HF, causing corrosion inside the battery. Poor air tightness leads to electrolyte deterioration, electrolyte viscosity and chroma change, and ultimately lead to a sharp decline in the performance of transport ions.
Failure of fluid collector: fluid collector corrosion, fluid collector adhesion decreased. HF generated by the failure of the above electrolyte will corrode the collector fluid and generate compounds with poor conductivity, resulting in increased ohmic contact or failure of the active substance. During the charging and discharging process, Cu foil is dissolved at low potential and deposited on the positive electrode surface, which is called "copper extraction". The common form of collector failure is that the insufficient binding force between the collector and the active substance leads to the stripping of the active substance, which cannot provide capacity for the battery.
Internal resistance increases
The increase of internal resistance of lithium battery will be accompanied by the decline of energy density, voltage and power, battery heat generation and other failure problems. The main factors leading to the increase of internal resistance of lithium ion batteries are divided into the key materials and the battery environment.
Key battery materials: micro-crack and breakage of positive material, damage of negative material and SEI too thick surface, electrolyte aging, separation of active substance from collector, poor contact between active substance and conductive additive (including loss of conductive additive), diaphragm shrinkage blockage, abnormal welding of battery electrode ear, etc.
Battery operating environment: high/low ambient temperature, overcharge and overdischarge, high rate charge and discharge, manufacturing process and battery design structure, etc.
In the short circuit
Internal short circuit often causes self-discharge, capacity attenuation, local thermal runaway and safety accidents of lithium ion batteries.
Short circuit between copper/aluminum collectors: Positive or negative collector contact is caused by an unpruned metal foreign object puncturing the diaphragm or electrode, or displacement of the electrode plate or lug in the battery package during battery production or use.
Short circuit caused by diaphragm failure: diaphragm aging, diaphragm collapse, diaphragm corrosion will lead to diaphragm failure, diaphragm loss of electronic insulation or gap ambassador positive and negative micro contact, and then appear serious local heat, continue to charge and discharge will diffuse around, leading to thermal control.
Impurities lead to short circuit: if the transition metal impurities in the positive slurry are not cleaned, it will lead to piercing of the diaphragm or causing the formation of negative lithium dendrite, leading to internal short circuit.
Short circuit caused by lithium dendrite: in the long cycle, the local charge is not uniform where lithium dendrite will appear, and the dendrite will pass through the diaphragm leading to internal short circuit.
In the process of battery design and manufacturing or battery pack assembly, improper design or excessive local pressure can also lead to internal short circuit. Battery overcharge and overdischarge induction will also appear internal short circuit.
gas
In the process of battery formation, the phenomenon of gas production caused by electrolyte consumption to form stable SEI film is normal gas production, but the phenomenon of excessive consumption of electrolyte to release gas or oxygen release of cathode material is abnormal gas release. Often appear in the soft battery, will cause the internal pressure of the battery is too large and deformation, break the packaging aluminum film, internal cell contact problems, etc..
Trace water in the electrolyte or the active electrode material is not dried, resulting in the decomposition of lithium in the electrolyte to produce HF, corrosion of collector Al and destruction of binder to produce hydrogen. Improper voltage range leads to the electrochemical decomposition of chain/ring esters or ethers in the electrolyte, which can produce C2H4, C2H6, C3H6, C3H8, CO2 and so on.
Thermal runaway
Thermal runaway refers to the rapid rise of local or overall temperature inside the lithium ion battery, and the heat can not be dispersed in time, a large amount of accumulation in the interior, and induce further side reactions. The factors inducing thermal runaway of lithium battery are abnormal operating conditions, such as abuse, short circuit, high rate, high temperature, extrusion and acupuncture.
Analysis of lithium
Lithium evolution is the precipitation of lithium metal on the cathode surface of the battery, which is a common phenomenon of lithium battery aging failure. Lithium separation will reduce the active lithium ion in the battery, resulting in capacity failure, and will form dendrites to Pierce the diaphragm, which will lead to local current and heat generation is too large, and ultimately cause battery safety problems.
Failure analysis in China has been systematically developed in the field of machinery and aviation, but has not been systematically studied in the field of lithium batteries. Battery enterprises and material enterprises carry out their own studies on the failure analysis of lithium-ion batteries, but most of them focus on the r&d and preparation of battery manufacturing technology and materials, with the direct goal of improving battery performance and reducing battery cost. In the future, the research institute and relevant enterprises can strengthen cooperation and exchange, and strive to establish and improve the failure tree and failure analysis process of lithium ion battery.
