Against the background of energy crisis and environmental pollution, lithium-ion batteries are receiving more and more attention as an ideal energy source for development in the 21st century. However, lithium-ion batteries are subject to certain failures in the process of production, transportation and use. And after the failure of a single battery will affect the performance and reliability of the whole battery pack, and even lead to the battery pack 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: the U.S. Tesla Model S electric car fire accident, Samsung Note7 cell phone battery fire accident, Wuhan Futer Electronics plant fire, Tianjin Samsung SDI factory fire, etc. ......
1, the classification of lithium battery failure
In order to avoid the above-mentioned performance degradation and battery safety problems, it is imperative to carry out lithium battery failure analysis. Failure of lithium batteries refers to the battery performance decay or use of abnormal performance caused by some specific nature, divided into performance failure and safety failure.
Performance failure includes capacity diving, cycle life decay, abnormal voltage, abnormal current, excessive internal resistance, self-discharge, high and low temperature aging, poor multiplier performance, and poor consistency.
Safety failure includes thermal runaway, short circuit, liquid leakage, flatulence, lithium precipitation, expansion deformation, puncture (extrusion).
2, the causes of lithium battery failure
The causes of lithium battery failure can be divided into internal and external causes.
Internal causes mainly refer to the nature of physical and chemical changes in the failure, the scale of research can be traced back to the atomic and molecular scale, the study of the thermodynamic and kinetic changes in the failure process.
External factors include impact, pinprick, corrosion, high temperature combustion, human damage and other external factors.
3, the common failure performance of lithium batteries and its failure mechanism analysis
Capacity decay failure
"Standard cycle life test, the discharge capacity should not be less than 90% of the initial capacity when the number of cycles reaches 500 times. Or 1000 cycles, the discharge capacity should not be less than 80% of the initial capacity", if in the standard cycle range, the capacity of a sharp decline in the phenomenon are capacity decay failure.
The root cause of battery capacity decay failure lies in the failure of the material, and is closely related to the battery manufacturing process, battery use environment and other objective factors. From the material point of view, the main causes of failure are structural failure of the cathode material, SEI transition growth on the cathode surface, electrolyte decomposition and degradation, collector corrosion, trace impurities in the system, etc.
Structural failure of cathode material: Structural failure of cathode material includes particle fragmentation of cathode material, irreversible phase transition, material disordering, etc. LiMn2O4 will cause structural distortion due to Jahn-Teller effect during charging and discharging, and even particle fragmentation will occur, resulting in failure of electrical contact between particles. LiMn1.5Ni0.5O4 material will occur during charging and discharging "LiCoO2 material will cause Co to enter the Li layer during charging and discharging due to the transition out of Li, resulting in the disorganization of the layer structure and restricting its capacity.
Failure of anode material: The failure of graphite electrode mainly occurs on the graphite surface, which reacts with electrolyte to produce solid electrolyte interface phase (SEI), if overgrown, it will lead to the reduction of lithium ion content in the internal system of the battery, resulting in capacity decay. The failure of silicon based anode materials is mainly due to the cycling performance problems caused by their huge volume expansion.
Electrolyte failure: LiPF6 has poor stability and tends to decompose to reduce the migratable Li+ content in the electrolyte. It also reacts easily with trace water in the electrolyte to produce HF, causing corrosion inside the battery. Poor gas tightness causes electrolyte deterioration and changes in both electrolyte viscosity and chromaticity, which eventually leads to a sharp decline in ion transfer performance.
Failure of collector fluid: corrosion of collector fluid, decrease of collector fluid adhesion. The HF generated by the above electrolyte failure will cause corrosion of the collector fluid and generate compounds with poor conductivity, resulting in increased ohmic contact or failure of the active material. During the charging and discharging process, Cu foil is dissolved at low potential and deposited on the surface of the positive electrode, which is called "copper precipitation". The common form of collector failure is insufficient bonding between the collector and the active material, resulting in stripping of the active material and failure to provide capacity to the battery.
Increase in internal resistance
The increase in internal resistance of Li-ion battery will be accompanied by a decrease in 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 key battery materials and battery use environment.
Battery key materials: micro-cracking and fragmentation of cathode material, destruction of cathode material and excessive surface SEI, aging of electrolyte, detachment of active material from collector fluid, poor contact between active material and conductive additives (including loss of conductive additives), blockage of diaphragm shrinkage, abnormal welding of battery lugs, etc.
Battery use environment: too high/low ambient temperature, overcharge and overdischarge, high rate charging and discharging, manufacturing process and battery design structure, etc.
Internal short circuit
Internal short circuit often causes self-discharge, capacity decay, local thermal runaway and safety accidents of Li-ion batteries.
Short circuit between copper / aluminum collector: the battery production or use process of untrimmed metal foreign body puncture diaphragm or electrode, battery packaging in the pole piece or lug displacement caused by positive and negative collector contact.
Diaphragm failure caused by short circuit: diaphragm aging, diaphragm collapse, diaphragm corrosion, etc. will lead to diaphragm failure, failure of the diaphragm to lose electronic insulation or gap becomes large positive and negative micro-contact, then there is a serious local heat, continue to charge and discharge will spread to the surrounding area, resulting in thermal runaway.
Impurities lead to short circuit: transition metal impurities in the cathode paste is not clean will lead to puncture the diaphragm or prompt the generation of negative lithium dendrites lead to internal short circuit.
Lithium dendrites caused by short circuit: long cycle process where the local charge is not uniform will appear lithium dendrites, dendrites through the diaphragm leading to internal short circuit.
Battery design and manufacturing or battery pack assembly process, the design is not reasonable or the local pressure is too large will also lead to internal short circuit. Internal short circuit can also occur under the induction of cell overshoot and overdischarge.
Gas production
The gas production phenomenon that occurs during the formation process of battery formation by consuming electrolyte to form a stable SEI film is normal gas production, but the phenomenon of gas release from transitional consumption of electrolyte or oxygen release from cathode material is abnormal gas release. It is often found in soft pack batteries and can cause deformation due to excessive pressure inside the battery, bursting of the encapsulated aluminum film, internal cell contact problems, etc.
Trace moisture in the electrolyte or the electrode active material is not dried, which leads to the decomposition of lithium salts in the electrolyte to produce HF, corroding the collector fluid Al as well as destroying the binder and producing hydrogen gas. Electrochemical decomposition of chain/cyclic esters or ethers in electrolyte caused by inappropriate voltage range will generate C2H4, C2H6, C3H6, C3H8, CO2, etc.
Thermal Runaway
Thermal runaway refers to the rapid increase of temperature inside the lithium-ion battery locally or as a whole, and the heat cannot be dissipated in time, accumulating in large quantities inside and inducing further side reactions. The factors that induce thermal runaway of Li-ion battery are non-normal operating conditions, i.e. abuse, short circuit, high multiplier, high temperature, extrusion, and pinprick, etc.
Lithium separation
Lithium analysis is the precipitation of lithium metal on the negative surface of the battery, is a common phenomenon of aging failure of lithium batteries. Lithium analysis will reduce the active lithium ions inside the battery and cause capacity failure, and will form dendrites to pierce the diaphragm, which will lead to excessive local current and heat generation and eventually cause battery safety problems.
Failure analysis in China has been systematically developed in the field of machinery and aviation, while it has not been systematically studied in the field of lithium batteries. Battery companies and material companies each conduct research on lithium-ion battery failure analysis, but mostly focus on battery manufacturing process and material development and preparation, with the direct goal of improving battery performance and reducing battery cost. Future research institutes and related enterprises can strengthen cooperation and communication, and strive to establish and improve the lithium-ion battery failure fault tree and failure analysis process
