When charging
lithium ion battery, Lithium precipitation not only reduces the performance of the battery and greatly shortens the cycle life, but also limits the fast charging capacity of the battery, and may cause disastrous consequences such as combustion and explosion.
In a series of articles we will discuss about the problems from the macro scale of lithium-ion battery, working conditions, gradient existing in the battery, electrochemical test, safety test, etc.), micro scale (electrode, particle, microstructure, etc.) and atomic scale (atom, ion, molecule, activation energy barrier, etc.). Today we are going to discuss about the experimental evidence of side effects of lithium deposition was collected from different angles:
By comparing the lithium-ion batteries with the same model, the researchers found that the side reaction of lithium deposition made the battery have a faster aging rate, and its battery capacity, energy density and energy efficiency were significantly attenuated.
1. Detect the degree of lithium deposition side reaction by analyzing the coulomb efficiency: the battery aging mechanism involving lithium deposition side reaction reduces the coulomb efficiency of the battery. Therefore, it is a feasible method to monitor the degree of lithium deposition side reaction by accurately measuring the coulomb efficiency of lithium-ion battery. The metal lithium generated in the side reaction of lithium deposition reacts with electrolyte to form SEI film, which reduces the coulomb efficiency. It should be noted that the decrease of coulomb efficiency is not entirely caused by the side reaction of lithium deposition. For example, the falling off of electrode active materials, the formation of SEI film and the blockage of microspores on the electrode surface will increase the internal resistance of the battery and cause irreversible capacity loss. These phenomena will reduce the coulomb efficiency.
2. The apparent activation energy of lithium deposition side reaction is obtained by analyzing the Arrhenius curve: the Arrhenius curve can be obtained from the capacity attenuation curve at different temperatures by testing the charge discharge cycle of lithium-ion battery at different temperatures (check the figure below). When the temperature is high, the side reaction of lithium deposition does not occur, the dissolution of positive active materials and the formation of SEI film on the surface of positive and negative electrodes accelerate with the increase of temperature, and the aging rate of battery also accelerates; When the temperature is low, the side reaction of lithium deposition appears on the stage, which suddenly changes the aging mechanism. Because the side reaction of lithium deposition becomes more and more intense with the decrease of temperature, the battery aging rate accelerates with the decrease of temperature. To sum up, the Arrhenius curve of lithium ion battery is V-shaped as shown in Figure 4, and its slope is the negative value (- EA) of apparent activation energy during aging. The side reaction of lithium deposition has negative apparent activation energy.
3. Analyze lithium evolution reaction by voltage curve
3.1 analyze lithium precipitation reaction by using the voltage platform of discharge curve: if lithium deposition side reaction occurs during low-temperature charging, the voltage platform corresponding to lithium dissolution reaction will appear on the subsequent discharge curve. With the increase of lithium dissolution during discharge, the voltage platform becomes longer.
3.2 analyze the side reaction of lithium deposition by using capacity voltage differential curve (DQ / DU) or voltage capacity differential curve (DU / DQ): both DQ / Du or Du / DQ curves can be used to estimate the amount of lithium dissolved during discharge, and Du / DQ curve is more sensitive.
3.3 analyze the side reaction of lithium deposition by using the voltage current curve after relaxation: during charging, lithium ion concentration gradient is formed in the negative electrode material and / or electrolyte. If the current is cut off after charging, the lithium ion concentration distribution in the negative electrode material and / or electrolyte will reach a new equilibrium, and the voltage current curve varying with time can be observed in this process. The information obtained from this curve can be used to analyze the side reactions of lithium deposition.
Note: (1) if there is a voltage plateau in the discharge curve or the change of voltage current curve during relaxation has corresponding characteristics, it indicates that lithium deposition side reaction occurred during charging. However, if none of the above phenomena exist, it does not mean that the side reaction of lithium deposition has not occurred. This may be because the relaxation process is inhibited at low temperature and the variation characteristics of voltage current curve cannot be observed, or the speed of metal lithium embedding into the negative electrode at high temperature is too fast to observe the voltage platform corresponding to lithium dissolution reaction.
(2) The electrochemical method can only measure the average results in a large area, and has nothing to do with the detection of metal lithium in the negative electrode.
Conclusion:
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