Against the backdrop of the energy crisis and environmental pollution, the lithium-ion battery has received increasing attention as an ideal energy source for development in the 21st century. But there are some function invalids in the battery cell production, transportation, and usage. And the failure of a single battery cell can affect the performance and reliability of the whole battery pack, and even cause the battery pack to stop working or lead to other safety problems.
In recent years, there have been many battery fires and explosion accidents at domestic and abroad: Tesla Model electric vehicle fire accidents, Samsung Note7 mobile phone battery fire accident, Wuhan Fute Electronic Factory fire accident, and Tianjin Samsung Sdi factory fire accident, etc.
Next, refer to the statistic data from SmartPropel, I will bring you to take a look at the valid factors of Lithium battery.
Lithium battery invalid classification
To avoid performance degradation and battery safety problems, it is imperative to carry out an analysis of invalid lithium batteries. The invalid of lithium battery refers to the performance degradation or abnormal performance caused by some specific intrinsic reasons, which can be divided into performance failure and safety failure.
Reasons and analysis for lithium battery invalid
The invalid of the lithium battery can be divided into internal and external causes.
The internal cause mainly refers to the failed physical and chemical reaction nature, the research scale can be traced back to the atomic and molecular scale.
External factors include impact, acupuncture, corrosion, high-temperature combustion, human damage, and other external factors.
Common battery invalid behavior and invalid mechanism analysis
Capacity degradation invalid
“During the standard cycle life test, the discharge capacity shall not be less than 90% of the initial capacity at 500 cycles, or 80% of the initial capacity at 1,000 cycles. ” If in the standard circulation scope, the capacity appears the sharp decline phenomenon all belongs to the capacity attenuation failure. The invalid of the capacity is due to the invalid of the material, also closely related to the manufacturing process and battery usage environment. From the material view, the main causes of invalid are anode material structure invalid, Sei transition growth on the surface of the cathode, electrolyte deterioration, current collector corrosion, etc.
Anode electrode material structure invalid
The structural invalid of anode materials includes particle breakage, irreversible phase transformation, and material disorder. In the process of charging and discharging, the structure of LiMn2O4 will be distorted due to the Jahn-Teller effect, and even the particles will be broken, resulting in the failure of electric contact between the particles.
Cathode material invalid
The invalid of the graphite electrode occurs mainly on the graphite surface, which reacts with the electrolyte to produce a solid electrolyte interface phase (Sei). If it grows too much, the lithium-ion content in the inner system of the battery will decrease, the result is a loss of capacity. The invalid of silicon cathode materials is mainly due to the cycle performance problems caused by its huge volume expansion.
Lipf6 has poor stability and easy to decompose, which reduces the content of transportable Li + in the electrolyte. It also readily reacts with trace amounts in the electrolyte to form HF, causing corrosion within the cell. The poor gas tightness caused electrolyte deterioration, electrolyte viscosity, and chromaticity changes, resulting in a sharp decline in ion transport properties.
The corrosion of collecting fluid and the adhesion of collecting fluid decrease. HF produced by the invalid of the electrolyte will corrode the current collector and produce a compound with poor conductivity, resulting in increased ohmic contact or invalid of the active substance.
Increased internal resistance
The increase of internal resistance of lithium batteries will be accompanied by the decrease of energy density, the decrease of voltage and power, and heat generation. The main factors leading to the increase of lithium-ion battery resistance are the key materials of the battery and the environment in which the battery is used.
The key materials of the battery are micro-crack and breakage of the anode material, destruction of the cathode material and over thickness of the surface Sei, electrolyte aging, separation of the active material from the current collector, the shrinkage hole blockage of the diaphragm, abnormal welding of the electrode ear, etc.
Battery Environment: High / low ambient temperature, over-charge and over-discharge, high-rate charge and discharge, manufacturing process and battery design structure, etc.
Internal short circuits
Internal short circuits often cause lithium battery self-discharge, capacity degradation, local thermal runaway, and safety accidents.
During the battery formation process, gas generation is normal when the electrolyte is consumed to form stable SEI film, but abnormal gas generation occurs when the electrolyte is over consumed or oxygen is released from cathode material. It often appears in the pouch battery, which will cause the internal pressure of the battery to be too large and deformation, rupture of the package aluminum film, lead to internal cell contact problems, etc.
Lithium precipitation is the precipitation of metallic lithium on the cathode surface of the battery, which is a common phenomenon of lithium battery aging invalid. Lithium precipitation will reduce the active lithium ions in the battery and lead to the capacity decrease and will form dendrite to pierce the membrane, which will lead to high local current and excessive heat generation, resulting in the battery safety problems.
Thermal runaway refers to a rapid rise in the temperature of the lithium-ion battery, either single cell or the whole pack, so that heat does not dissipate in time, accumulates in the interior, and induces further side effects. The causes of lithium battery thermal runaway are abnormal operation conditions, such as abuse, short circuit, high discharge, high temperature, extrusion, and acupuncture, etc.
To avoid battery pack invalid, technical specialist Mr.Yang suggests, lithium battery manufacturer should pay high attention to incoming material selection and inspection, develop advanced production technology, persist on the scientific and effective manufacturing process, strengthen staff training, insist on 100% inspection before delivery. For users, they should use the battery in a suitable environment with proper usage method, Handle in that way will highly improve battery pack excellent performance and ensure long cycle life.