Lithium-ion batteries are mainly composed of a positive electrode, an electrolyte, a separator, etc. During the charging process, lithium ions are released from the positive electrode and embedded in the negative electrode. The discharge process is just the opposite. Ideally, the lithium ion should be pulled out from the positive electrode during charging. Li+, all return to the positive electrode during the discharge process, and evenly embedded in the positive electrode material. However, in practice, due to the problems of interface side reactions and polarization, not only all of the Li+ returns to the positive electrode, Even Li+ is uniformly embedded in the positive electrode material. Inhomogeneous lithium intercalation of the positive electrode material causes uneven stress distribution inside the positive electrode material particles (the positive electrode material causes lattice in the process of Li+ insertion and extraction). The expansion and contraction, causing the volume change of the material), causing cracks inside the secondary particles, causing the internal corrosion of the electrolyte to cause accelerated decay of the positive electrode material.
Lithium inhomogeneity of the positive electrode material is inevitable, but how to analyze and detect it becomes the key. Li is a light element. Conventional EDS tools cannot analyze the distribution of Li element. In order to solve this problem, people first try to use neutrons. The diffraction method detects the distribution of Li inside the lithium ion battery. The neutron is small in volume and uncharged, so it has strong penetrating power and can easily pass through the outer casing of the lithium ion battery, while the light elements such as H and Li Neutron mass is close, so it is easier to interact with neutrons, so neutron diffraction technology is very sensitive to Li distribution and electrolyte distribution in batteries. Helmholtz Institute of Chemical Energy Storage (HIU) and Karlsruhe Institute of Technology, Germany MJ Mühlbauer et al. used neutron diffraction techniques to analyze the distribution of Li in a lithium-ion battery at the end of its life at the end of its life ("Influence of battery aging on the internal distribution of Li in lithium-ion batteries"). At the end of the period, not only the active Li inside the lithium ion battery is reduced, but more importantly, the remaining active Li is also unevenly distributed inside the lithium ion battery. Like.
However, the resolution of the neutron diffraction technique is low, and the uniformity of the Li distribution can only be analyzed at the battery level, and the internal stress accumulation of the positive electrode material is uneven distribution of the lithium ion battery inside the single particle. The uneven distribution of internal Li distribution, Shuyu Fang of the University of Wisconsin and others introduced Raman spectroscopy. In order to observe the Li distribution of NCM materials in the process of lithium insertion, Shuyu Fang prepared a special structure of button cell. The figure below shows the Raman spectrum of the NMC532 material at different charging voltages. It can be seen from the figure that the intensity of the A1g peak around 595/cm will decrease as the potential of the NMC material increases (the delithiation increases). As the positive electrode re-inserts lithium, the intensity of the A1g peak rises again, so we can estimate the Li concentration distribution inside the positive electrode material by using the intensity of the A1g peak.
Shuyu Fang's analysis and calculations show that there is also a phenomenon of unevenness inside a single NMC particle. For example, when particle 1# in the following figure a is 3.88V, the A1g peak of most regions of the particle is at a negative electrode of 540/cm. The top area is at 590/cm negative, indicating that the lithium intercalation reaction in this part is lagging. Compared with different particles, there is also a large non-uniformity between the 1# and 3# particles, for example, the 3# particles reach 3.84V. When the 1# particle has reached 4.01V, the potential difference between the two particles reaches 0.2V, which indicates that there is a large intercalation between the particles inside the positive electrode of the lithium ion battery and between different regions inside the particle. Uniform phenomenon. Inhomogeneous lithium intercalation between particles will cause partial granules to be overcharged. Inhomogeneous lithium intercalation inside the particles will cause internal stress accumulation in the particles, causing cracks in the particles, which will result in long-term cyclic stability of the positive electrode material. Have a negative impact.
With the development of technology, there are more and more means that people can analyze the distribution of lithium resources in cathode materials. For example, Susumu Imashuku (first author, Corresponding author) of Tohoku University of Japan uses laser-induced decomposition spectra. The (LIBS) technique analyzes the Li distribution in the LiCoO2 electrode. LIBS works by using a pulsed laser to vaporize the sample to be detected. The gasified atoms are excited and emit photons, and the emission of these atoms is detected. The spectrum can analyze the element composition and content in the sample. Generally, if LIBS is detected in an air atmosphere, the emission spectrum of Li atoms will strongly absorb, so the intensity of the emission spectrum of Li atoms and the concentration of Li atoms will be caused. It is not proportional, so the LIBS analysis used before is basically qualitative analysis. One of the methods to solve this problem is to test LIBS in an argon atmosphere. The low pressure argon can increase the temperature of the plasma generated by the laser pulse. Thereby increasing the number of Li atoms in the excited state, thereby increasing the intensity of the emission spectrum of Li atoms.
The test system used by Susumu Imashuku in the test is shown in the figure below. The laser source is Nd: YAG, the laser wavelength is 532nm, the pulse time is 16-18ns, and the single pulse energy is 20mJ. The system contains two sets of spectrum analyzers, one of which The set is capable of collecting a wide wavelength range (200-895 nm) for collecting the emission spectrum of Li, and the other set is more sensitive to short waves (13.3 nm) for collecting short-wavelength spectral signals.
Susumu Imashuku first tested the spectra of standard samples with Li/Co ratios of 0, 0.01, 0.10, 0.30, 0.51, 0.62, 0.80 and 0.99 as baselines for subsequent analysis. The following figure shows the LCO positives obtained by the authors using LIBS quantitative analysis. Li/Co ratio distribution map (Fig. a is after 30 cycles, and Figure b is after 50 cycles). It can be seen from the figure that the distribution of Li is relatively uniform after 30 cycles, but after 50 cycles. In the middle of the LCO positive electrode, the Li concentration is lower, and the edge Li concentration is higher. Even at some positions on the edge, the Li/Co ratio is greater than 1 (red dot). It is found by EDS analysis that the edge position of the LCO positive electrode is F. The concentration of P and P are higher (F and P are common electrolyte decomposition products), and the concentration of Co at the edge is slightly lower than the center position. The author believes that this is due to overcharge of LCO at the edge of the positive electrode, resulting in partial Co element. Dissolution occurred, resulting in a Li/Co ratio at the edge position higher than 1.
In practice, the uneven insertion and removal of Li from the cathode material is a common phenomenon. This phenomenon is common in the interior of the particles, or between the particles, and even inside the electrodes. Inhomogeneous lithium intercalation can lead to the accumulation of internal stresses in the particles, resulting in the generation of cracks. The unevenness of Li insertion and extraction between the particles and inside the electrodes can cause problems such as overcharge of the local active substances, which will lead to lithium ion batteries. The reversible capacity continues to decline, so it is particularly important to detect the non-uniformity of Li between the electrode and the active material by means of corresponding means and to take measures to improve the unevenness.
