Image from reference 1
The positive and negative active materials of lithium ion batteries are granular, so there are many tiny curved channels inside the electrodes of lithium ion batteries. During charging and discharging, lithium ions are separated from the electrodes on one side and spread to the electrodes on the other side , Diffused from these tortuous channels to the inside of the electrode and then reacted with the active material particles and embedded into the active material particles resulted in concentration polarization due to the limitation of Li + diffusion rate during charging and discharging, Lithium intercalation throughout the distribution process is not uniform, especially for the positive electrode, due to the poor conductivity of the cathode material, easier to make part of the poor contact with the uneven lithium intercalation phenomenon, which will cause positive particles of local SoC is different, so there will be greater stress in the particle, resulting in particle crushing, loss of part of the active material with the conductive network connection, the transition metal element dissolution and electrolyte oxidation and other issues, making the battery capacity irreversible decline .
In general, in order to reduce the imbalance of intercalation lithium in the active material, people will try to prolong the standing time of the battery after charging, hoping to eliminate the local SoC unevenness through the internal balance of the electrodes, However, a study by William E. Gent found that nonuniformity of Li in an NMC secondary particle with a diameter of 1 to 3 μm was as high as 10% even after standing for as long as 170 h, and the local SoC value was too high , Will lead to overcharge at this place, accelerating the failure of this part, so that the capacity of the material will decrease.In addition, the aging of the battery will also have a great influence on the distribution of Li in the Li-ion battery.MJ Mühlbauer's research shows that with the battery Of the aging, not only the internal active Li battery resources continue to decrease, but also makes the distribution of Li in the lithium-ion battery has also produced a great heterogeneity in the electrode near the ear Li part of the higher concentration, far away from the pole The lower Li concentration at the ear location may be due to uneven current distribution and insufficient electrolyte infiltration.
From the above study, we can easily see that the decay of Li-ion battery must be accompanied by the uneven distribution of Li. In order to study the distribution of Li in Li-ion battery, many methods have been developed, the simplest is The battery is disassembled and visually observed whether there is partial lithium on the pole piece. EDS is used to analyze the distribution of Li element on the pole piece. A little complicated point is to use neutron diffraction method to perform non-destructive testing on Li-ion battery and analyze Li In Li-ion battery, while Shuyu Fang et al. From the University of Wisconsin designed a method to observe the imbalance of lithium intercalation in lithium-ion battery electrodes by Raman spectroscopy.
To meet the optical conditions required for the measurement of Raman spectroscopy, Shuyu Fang et al. Used a 2032 SS coin cell to design a cell capable of Raman spectroscopy detection. The button cell lid was opened with a 1/8 " It was covered with MgO and sealed with epoxy resin. A layer of 300 nm thick Al was deposited on the MgO window as a positive current collector by electron beam (Al was not deposited in the center of the MgO window by 2 mm) This layer is coated with NMC532 Al layer material, the coating amount of 12-18mg / cm2, the battery structure as shown below
Image from reference 4
The electrochemical performance of the battery test as shown below, from the results, the modified button cell electrochemical performance and the general button cell electrochemical performance no significant difference.
With the insertion or exfoliation of Li, the vibration of the MO bond will be changed. Therefore, the intensity and frequency of Raman spectroscopy will change with the degree of lithium intercalation of NMC material. Shuyu Fang Man spectrum was measured, the measurement results as shown below.
According to Raman spectra, Shuyu Fang analyzed the intercalation state (SoC) of the NMC electrode in the 35'35um region. In order to analyze the intercalation state of NMC particles, Shuyu Fang mainly focused on the A1g peak near 595 / cm, As the voltage of NMC material increases from 2.3V to 4.2V, the intensity of A1g peak gradually decreases. When the potential of NMC material drops back to 3.16V again, the intensity of A1g peak rises again. Therefore, NMC particles embedded lithium state analysis.The following figure is based on the A1g peak intensity distribution map from the figure we can see that with the NMC potential changes in the field of view of the three particles 1 #, 2 # and 3 # embedded Lithium state also changed with the NMC material cell once again dropped to 3.16V, 1 # and 2 # particles embedded lithium state have been restored, but 3 # particles embedded lithium state did not return to the original state, indicating that Li Particles Part 3 did not return to the inside of the particles, which is also a sign of material degradation.
Since the intensity of the Raman peak is affected by factors such as particle morphology and position, the above simple A1g peak intensity data can not accurately reflect the lithium intercalation state of the particles, for which Shuyu Fang fitted the above data , Matched and calculated to get a more accurate local SoC analysis of the NMC material as shown in the figure below.The results show that there is a large inhomogeneity in the re-intercalation of lithium, for example for # 1 particles at 3.88V , The A1g peak in most of the particles is near 540 / cm, but the top region is around 590 / cm, which indicates the lithium insertion lag in this part of the region. The tracking of particles 1 and 3 shows that when 3 # Particles reached 3.84V, 1 # particles have reached 4.01V, 0.2V potential difference shows that lithium intercalation in the process, there is a large non-uniformity between the particles, and this non-uniformity is likely to lead to Local particles 'overcharge' or 'over' cause the capacity to decay.
Shuyu Fang's research shows lithium imbalance lithium ion battery electrode plate is not only occurred in the macro-scale, also occurred in the microscopic scale, the different parts of an active material particles and different active material particles are larger Of lithium embedded inhomogeneity, this non-uniformity will lead to partial over-discharge of particles, resulting in an irreversible loss of battery capacity.
