In recent years, with the rapid development of new energy vehicles, the power battery capacity has gradually surpassed the traditional 3C lithium-ion battery products. While bringing greater market demand, the new application direction also proposes new performance for lithium-ion batteries. The requirements, such as new energy vehicles, especially plug-in hybrid vehicles, put forward higher requirements on the power discharge capacity of power batteries.
The factors affecting the rate performance of lithium-ion batteries are mainly polarization. Polarization will cause the working state of lithium-ion batteries to deviate from steady state. In practice, the voltage platform of the battery decreases with increasing polarization (discharge), discharge The capacity is reduced. Generally speaking, we believe that the main factors causing the polarization of lithium-ion batteries are: 1) ohmic polarization, that is, the contact between the active material particles and the current between the active material and the current collector, with the current Increasing the voltage drop caused by these factors increases significantly; 2) Concentration polarization, the positive and negative electrodes of the lithium ion battery are porous structure electrodes, and the complex porous structure inside the electrode causes the diffusion rate of Li+ to be slow, resulting in a concentration gradient. In addition, the slow diffusion rate of Li+ in the solid phase is also likely to be a limiting link. Today we mainly introduce how to reduce the contact resistance between the active material and the current collector, and improve the rate performance of the lithium ion battery.
At present, the production process of lithium-ion batteries is basically born out of the process adopted by Sony Corporation in the first commercial lithium-ion battery in 1992. The positive and negative active material pastes are transferred to the current collector made of metal foil by coating equipment. On the upper side (the positive electrode is generally made of Al foil, the negative electrode is generally made of Cu foil), crushed, and after slitting, various shapes of lithium ion batteries are formed by winding or lamination. The electrons inside the positive electrode active material particles in the electrochemical reaction are required. After being transferred between the particles, it is confluent to the current collector and then conducted to the negative electrode through an external circuit to complete a complete electrochemical reaction. Therefore, electron conduction between the active material and the current collector becomes an important part of the electrochemical reaction. Recently, Hiroki Nara (first author, correspondent author) and Tetsuya Osaka (corresponding author) of Waseda University in Japan analyzed the contact between active material and current collector by compaction density and conductive coating on Al foil surface by EIS. The effect between the resistors, studies have shown that the appropriate compaction density and carbon coated Al foil will significantly improve the LCO electrode rate performance.
In the experiment, Hiroki Nara uses LCO as the positive electrode material and graphite as the negative electrode material. The LCO decreases the thickness by 0%, 10%, 20% and 30% respectively by adjusting the pressure and the crack of the joint (calculated that the electrode porosity is 49%, respectively). 42%, 37% and 27%), then the electrode was punched and made into a soft pack battery for electrochemical testing.
The following figure shows the equivalent circuit of the soft pack battery designed by Hiroki Nara (where b is the TLM of Figure a, indicating the parallel circuit in the thickness direction of the electrode), where ZL is the inductive reactance and RS is the ion diffusion impedance of the electrolyte. Ril is the contact impedance between the active material and the current collector, and the capacitor Cd, the charge exchange impedance Rct and the capacitor Cct connected in parallel with it, the Li+ diffusion resistance Ri in the electrode, and the diffusion impedance Cdiff, which are performed by using MATLAB software. Fitting, HirokiNara's fitting result error is less than 1%, which can truly reflect the reaction mechanism inside the lithium ion battery.
Figure a below shows the rate-discharge performance of LCO cathodes with different compaction densities. It can be seen that the electrode with higher compaction density as the magnification is improved shows excellent rate performance without LCO electrode magnification. Poor performance, almost no capacity at 2C rate. Figure b below shows the EIS spectrum of cathode materials with different compaction densities. It can be seen from the figure that the electrode impedance without rolling (0% reduction in thickness) is the largest. The semicircular diameters of the high frequency region and the intermediate frequency region are 4.5 ohms and 1.0 ohms, respectively, and the impedance of the electrode with a 10% decrease in electrode thickness after rolling is significantly reduced. The two semicircles are 1.5 ohms and 0.2 ohms, respectively. Increasing the compaction density and reducing the thickness of the electrode by 20% can further reduce the impedance of the electrode (as shown in the figure below). Hiroki Nara believes that the main reason for the significant decrease in the diameter of the semicircle in the high frequency region is that the compaction density increases gradually. , LCO particles and current collectors, LCO particles and contact with the conductive agent are significantly improved, thereby reducing the contact resistance. When we continue to increase the compaction density, the LCO electrode thickness is reduced by 30 At %, we see that the semicircle in the high frequency region almost disappears, indicating that the contact resistance inside the electrode is minimized at 30% compaction density, but this does not mean that the larger the compaction density, the better. We carefully analyze the EIS results. It can be seen that the intersection of the LCO positive electrode with the compacted density of 30% and the X axis shifts to the right obviously, which indicates that as the compaction density increases, the diffusion resistance Rion of Li+ in the electrode increases significantly, which is not conducive to the rate performance. Improvement. From the test results, the 20% thickness drop is a compaction density that allows the LCO positive electrode to balance the electronic contact impedance and the ion diffusion impedance. Continue to increase the compaction density. The contact impedance is limited, but it leads to ion diffusion. The impedance is significantly increased.
From the cross-section of the electrode, at a lower compaction density, there is a large amount of pores between the active material layer and the current collector, resulting in poor contact between the LCO active material and the current collector, which may result in low pressure solid density. The main reason why the electrode has a large radius in the high frequency region.
In order to further verify the LCO positive performance of different compacted densities, HirokiNara made LCO positives with thickness reductions of 0%, 10%, 15% and 20% respectively after compaction, and metal Li was used for the negative electrodes to make soft pack batteries. It can be seen from the charge-discharge curve of the following figure a that the LCO during the charging process is not polarized, and the polarization is very large during the charging process. The charging voltage has reached the cut-off voltage of 4.3V, and the density is compacted. The electrode polarization decreased by 15% and 20%, and the metal Li negative electrode in the soft pack battery was removed, leaving only the LCO positive electrode (eliminating the influence of the metal Li negative electrode). Then the EIS test is carried out. From the test results, as the compaction density increases, the diameter of the semicircle characterizing the contact impedance in the high frequency region is significantly reduced, indicating that the contact resistance inside the electrode is significantly reduced, thereby effectively reducing the polarization of the electrode.
In order to further reduce the contact resistance between the LCO and the Al foil current collector, Hiroki Nara was tested by using carbon coated Al foil instead of ordinary Al foil. The following figure a shows the use of ordinary Al foil (blue) and carbon coated Al foil (red). The charge-discharge curve of the LCO electrode (the electrode thickness decreased by 15% after compaction). It can be seen from the figure that the charging voltage platform of the battery is significantly reduced after the carbon-coated Al foil is used, and the discharge voltage platform is significantly increased, indicating that the internal polarization of the battery is significant. The reduction also makes the discharge capacity of the battery significantly improved. It can also be seen from the AC impedance spectrum that the contact resistance of the electrode in the high frequency region is significantly reduced after the carbon-coated Al foil is used at the same compaction density. It is impossible to distinguish the semicircle of the high frequency region from the figure, leaving only a charge exchange impedance semicircle in the intermediate frequency region, indicating that the carbon coated Al foil has a very significant effect on reducing the contact resistance.
Hiroki Nara used the equivalent circuit described at the beginning of the article to fit the EIS results of the LCO anode (Fig. a) coated with carbon-coated Al foil and the LCO electrode of the common Al foil (bottom b) at different temperatures. The results are in good agreement with the experimental results, and the error is within 1%. The fitting results are shown in the histogram below. It can be seen that the contact resistance Ril between the active material and the current collector using the carbon coated Al foil is significantly smaller than that of the ordinary The electrode of Al foil, and the ion diffusion resistance and charge exchange impedance are not much different, indicating that the carbon coated Al foil mainly improves the lithium ion battery rate performance by reducing the contact resistance between the active material and the current collector.
Hiroki Nara's work indicates that proper compaction density (about 20% reduction in thickness) is necessary to improve the rate performance of the LCO electrode. Appropriate compaction density can improve the contact between the LCO particles and the LCO particles and the current collector. Therefore, the contact resistance is effectively reduced, and the rate performance of the electrode is improved. In addition, the carbon coated Al foil can significantly reduce the contact resistance between the LCO active material and the current collector, and reduce the polarization of the battery, which has a significant effect on improving the rate performance of the LCO battery. .
