Analysis of cycle life of lithium ion battery

Feb 11, 2019   Pageview:128

The battery with the use, the feeling is not durable, capacity is not much, these are the embodiment of the cycle life constantly attenuation.

Cycle life of attenuation, actually is the battery current actual usable capacity, relative to its rated capacity of the factory, of a change in the declining trend.

For the ideal lithium ion batteries, within its cycle capacity balance will not change, the initial capacity in each loop are should be a certain value, but in fact it was much more complicated.Any adverse event could produce or consume lithium ion can lead to battery capacity balance changes, once the battery capacity balance changes, this change is irreversible, and can be accumulated through many times cycle, a serious impact on cell cycle performance.

The factors affecting the service life of lithium ion battery cycle has a lot of, but its inherent root causes, or to participate in the energy transfer in decreasing the number of lithium ion.It is important to note that the amount of lithium battery of did not reduce, but the "activation" lithium ion less, they are imprisoned or elsewhere in the activities of the channel is blocked, the participation of not free circulation charge and discharge process.

So, we just to clear up these should have been involved in REDOX reaction of lithium ion, where, can make clear the mechanism of the capacity drops, also can be targeted to take measures to slow down lithium battery capacity of the downward trend, ascension lithium battery the cycle of life.

1. The deposition of metallic lithium

Through the analysis of the front, we know there should be no of lithium ion battery is, in the form of a lithium metal lithium or lithium metal oxide, carbon compounds form, either in form of ions.

Deposition of metallic lithium, generally occurred in the cathode surface.Due to certain reasons, the lithium ions in the migrated to the cathode surface, part of the lithium ion is not in the cathode active material to form stable compounds, but after winning electronic deposit in the cathode surface become metal lithium, and no longer participate in the subsequent cycle, resulting in a decline in capacity.

This kind of situation, generally speaking, there are several reasons: charging more than cut-off voltage;Large charging ratio;Anode materials.When charging or insufficient anode materials, the cathode can't accommodate the positive migrated from lithium ion, cause deposition of metallic lithium.Large charging ratio, due to the amount of lithium ion in a short period of time to reach negative is overmuch, cause congestion and deposit.

Deposition of metallic lithium, will not only cause the loss of the cycle life, serious when still can cause across the short circuit, cause serious security problems.

To solve this problem, we need reasonable ratio of anode materials are strictly limited the use condition of lithium-ion batteries at the same time, avoid over use of limit.Begin from ratio performance, of course, can also partial to improve cycle life.

2. The anode material

As the anode material of lithium metal oxide, although have enough stability, but in the process of long-term use, still continue to decompose, produce some electrochemical inert substances (such as Co3O4, Mn2O3, etc) and some combustible gases, destroy the capacity balance between the electrode and the irreversible capacity loss.

This situation is particularly in the case of charging, sometimes even intense decomposition and gas release occurs, not only influence the battery capacity, also can cause a serious security risk.

In addition to strictly limit the battery charge cut-off voltage, chemical stability and thermal stability of the anode material, is also a feasible way to reduce the cycle life drop speed.

3. The SEI film on the surface of the electrode

Said before, in front of the carbon materials as anode of lithium ion battery, in the process of primary loop, the electrolyte will form a solid electrolyte layer on the surface of the electrode (SEI) film, different anode materials have some differences, but the SEI film composition is mainly composed of lithium carbonate, alkyl lithium, lithium hydroxide, etc, of course, there are salt decomposition product, there are some polymer, etc.

The SEI film formation process of the lithium ion batteries, and SEI is not stable, will constantly in the process of circulation break, uncovered new carbon surface reaction with electrolyte to form new SEI film, it will continue to cause continued loss of lithium ions and electrolytes, resulting in a decline in the capacity of the battery.SEI has certain thickness, although lithium ion can penetrate, but SEI part will cause the cathode surface diffusion channel blockage, the spread of the anode materials for lithium ion in this also causes battery capacity decline.

4. The influence of the electrolyte

In the process of continuous cycle, due to the chemical stability and thermal stability of electrolyte limitation, will continue to decompose and volatilization, accumulated for a long time, causes a decline in the amount of electrolyte, anode materials is not fully infiltrates are, charge and discharge reaction is incomplete, cause the loss of the capacity of actual use.

Electrolyte containing active hydrogen substances and iron, sodium, aluminum, nickel and other metal ions impurities.Because the oxidation potential impurities generally lower than the positive potential of the lithium ion battery, easy in the anode surface oxidation, oxide in the cathode reduction again, consumption is the cathode active material unceasingly, causes the self-discharge, namely change the battery discharge under the condition of the abnormal use.Battery life is charge-discharge cycles, impurities in the electrolyte directly affect cell cycle.

Electrolyte also contains a certain amount of water, the water will react with the electrolyte LiFP6, production LiF and HF, HF, in turn, and destruction of SEI, generate more LiF, cause the LiF deposition, constantly consumption activity of lithium ion, causing the deterioration of the battery cycle life.

By above analysis it can be seen that the electrolyte for lithium ion battery cycle life have very important influence, choosing the appropriate electrolyte, will be able to significantly improve cycle life of the battery.

5. The isolation membrane blocked or damaged

Is the role of isolation membrane is battery cathode separated to prevent short circuit.In the process of lithium ion battery cycle, isolation membrane drying up failure is one of the important reasons for the battery performance early recession.This is mainly due to the isolation membrane itself, electrochemical stability and mechanical properties as well as to the electrolyte to isolation membrane of invasive caused by the variation in the process of being recharged.Due to the isolation of the membrane dries up, battery ohm internal resistance increases, lead to charge and discharge channel blockage, charge and discharge is incomplete, battery capacity unable to reply to the initial state, greatly reduces the capacity and service life of the battery.

6. Anode materials are fall off

Is the cathode active material, is through the adhesive and fixed in the base of the above, in the process of long-term use, due to the failure of adhesive and battery by mechanical vibration, is the cathode active material loss, into the electrolyte solution, it can lead to participate in the electrochemical reaction of the active material shrinking declining cycle life of the battery.

The long-term stability of the binder and the battery good mechanical performance, will be able to delay the decline speed of the battery cycle life.

7. External use factor

Lithium-ion batteries have reasonable use condition and scope, such as charge and discharge by the voltage, charge and discharge rate, operating temperature range, the storage temperature range, etc.But in actual use, beyond the allowed range of abuse is widespread, the unreasonable use of for a long time, can lead to irreversible chemical reaction inside, the mechanism of cell damage, accelerate cell aging, cause the cycle life of rapid decline, serious when, still can cause accidents.

The page contains the contents of the machine translation.