In-depth analysis of the cause of lithium ion battery gas

Lithium ion batteries because of the high life, high capacity is widely used, but with longer duration of use, the existence of ballooning, safety performance is not ideal and loop attenuation speed problem is increasingly serious, caused the depth analysis and inhibiting the lithium electricity industry. According to experimental research and development experience, the author will lithium battery ballooning reasons fall into two categories, one is the thickness of the battery pole piece ballooning change; The second is due to electrolysis melt oxidation decomposition gas ballooning. In different cell system, the dominant factors of different thickness change batteries, such as lithium titanate battery cathode system, the main factors of tympanites is gas drum; In the graphite anode system, sheet thickness and gas for ballooning all play a role of the battery.

A change, the thickness of the electrode plate

In the process of lithium battery, the thickness of the electrode plate thickness will happen some change, especially the graphite anode. According to the existing data, the lithium battery after high temperature storage and circulation, are prone to bloating, thickness of the growth rate of about 6% ~ 20%, the positive rate of only 4%, the cathode inflation rate above 20%.Lithium battery plate thickness caused by larger ballooning root cause is influenced by the nature of the graphite, graphite when the intercalated-li cathode formation LiCx (LiC24, LiC12 and LiC6, etc.), the lattice spacing changes, lead to formation of the micro stress, the expansion of the cathode. Below is graphite anode plate in place, in the process of charging and discharging structure change.

The expansion of the graphite anode is mainly caused by irreversible expansion after embedded lithium. This part of the expansion of the main and particle size, adhesive agent, and the structure of the pole piece. Core winding deformation caused the expansion of the cathode to make hole is formed between the electrode and the diaphragm, the cathode particles form micro cracks, phase solid electrolyte interface (SEI) film ruptures and restructuring, the consumption of the electrolyte, improves the performance of the cycle. There are many factors that can influence the cathode plate thickening of the nature of the bonding agent and structure parameters of the sheet are the two most important.

Graphite anode used adhesive is SBR and different adhesive elastic modulus, mechanical strength is different, have different effects on the thickness of the sheet. Plate coating after the completion of the rolling force and negative pole piece of the thickness in the battery. Under the same stress, adhesive elastic modulus, the greater the pole piece the smaller physical put back; When charging, as a result of Li + embedded, graphite lattice expansion; At the same time, because of the cathode particles and SBR deformation and internal stress released in full, make the cathode inflation rate rise sharply, SBR in the stage of plastic deformation. This part of the inflation rate is associated with the elastic modulus and fracture strength of SBR, lead to the greater the elastic modulus and fracture strength of SBR, causing irreversible expansion is smaller.

When do not match the amount SBR, pole piece rolling pressure is different, different pressure make the residual stress produced by the pole piece there is a certain difference, the greater the pressure, the greater the residual stress, leading to early physical shelved expansion, full charge state and empty electricity state expansion rate; SBR content is less, the smaller the pressure on the roller when, early physical shelved, full charge state and the less the inflation rate of empty electricity states; The cathode is core winding deformation, the degree of negative intercalated-li and Li + diffusion rate, and a serious impact on cell cycle performance.

Second, the battery caused by the gas ballooning

Battery internal gas is another important cause of the battery to bulge, whether the battery in normal temperature cycle, high temperature cycle, high temperature aside, it will produce different degrees of bloating and gas. According to the research results show that cause batteries bilge gas is the nature of the electrolyte decomposition.

There are two kinds of circumstances, the electrolyte decomposition is an electrolyte impurities, such as the moisture content and metal impurity gas electrolyte decomposition, the other one is electrolyte electrochemical window is too low, causing the decomposition in the process of charging, the electrolyte of EC, DEC, such as solvent after get electronic, all can produce free radicals, free radical reaction in the immediate aftermath of low boiling point hydrocarbon, esters, ethers and CO2, etc.

Lithium battery pack after completion of loading, advance into process will produce a small amount of gas, the gas is inevitable, is also the so-called source batteries irreversible capacity loss. In the process of charge and discharge for the first time, after electronic by the external circuit to the cathode and electrolyte REDOX reaction on the surface of the cathode, the generated gas. In this process, the graphite cathode surface SEI formation, with the increase of the thickness of the SEI, electronic cannot penetrate inhibits continuous oxidative decomposition of the electrolyte.

In the process of battery, the internal gas production will gradually increase, the reasons or because existing in the electrolyte caused by impurities or excess moisture inside the battery. Electrolyte impurities exist need to rule out earnestly, moisture control is lax could be itself, battery encapsulation is lax to introduce water, electrolyte Angle caused damage, the other battery overcharge had put abuse, internal short circuit, could also accelerate cell gas speed, causing the battery failure.

At different levels in different system, battery production bulge. In battery graphite anode system, gas ballooning reason mainly within the SEI film formation, batteries, as mentioned above water exceeds bid, and poor into abnormal process, packaging, etc., and in the anode system, lithium titanate battery bilge gas is more serious than graphite/sliding battery system, in addition to the electrolyte impurities, moisture and technology, another is different from the graphite cathode for lithium titanate can't like battery graphite anode system, form the SEI film on the surface, suppress its reaction with the electrolyte.

Electrolyte in the process of charging and discharging always direct contact with the Li4Ti5O12 surface, resulting in electric continued reduction in Li4Ti5O12 material surface decomposition, this may be the primary cause of Li4Ti5O12 battery bilge gas.Gas is a major component of H2, CO, CO2, CH4, C2H6, C2H4, C3H8, etc.

When soaked in electrolyte of lithium titanate separately, only CO2 generated, with the sliding material preparation into a battery, a gas including H2, CO2, CO, and a small amount of gaseous hydrocarbons, and make a battery, only when circulation charge and discharge, H2 is produced, at the same time produce gas, H2 content more than 50%.This suggests that the charging and discharging process will produce H2 and CO gas.

LiPF6 in the electrolyte balance which were as follows:

PF5 is a kind of strong acid, easy to cause esters carbonate decomposition, and the amount of PF5 increases with the rise of temperature.PF5 helps the electrolyte decomposition, CO2, CO and CxHy gas. According to the study of the production of H2 from the trace water in the electrolyte, but the general level of water content in the electrolyte is about 20 x 10-6, the contribution to H2 yield is very low. Shanghai Jiaotong University berenikeullmann experimental use graphite/NCM111 do batteries, the conclusion is the source of H2 is carbonate decomposition under high voltage.

Current suppression lithium titanate battery bilge gas solution mainly has three, first, the LTO anode materials processing and modification, including improved preparation methods and surface modification, etc.; Second, development and LTO anode that match the electrolyte, including additives, solvent system; Third, improve the battery technology.

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