High nickel NCA is gaining a foothold in the high-end lithium-ion battery market due to its high capacity, but the high-nickel NCA material brings higher capacity and higher requirements for lithium-ion battery production process. Due to the high content of Ni, the NCA material is easy to produce a surface layer of LiOH / Li2CO3 on the surface of the material particles, resulting in a higher residual ratio of the NCA material particles, thus providing higher humidity control for electrode homogenization and coating The homogenization and coating as lithium-ion battery production of key aspects of the electrochemical properties of lithium-ion battery has an important impact, so in this article, Xiaobian and we talk about the electrode production process on Influence of electrochemical performance of Li-ion battery.
Li-ion battery production process, the slurry uniformity, stability, coating thickness, rolling density and porosity of the electrode have an important impact on the electrochemical properties of lithium-ion batteries, a good production process to play The electrochemical properties of NCA materials are of great importance, but this part often belongs to the core technology of the enterprise. The technology blockade is usually done in a confidential and patent protected mode, so this part of the content is rarely accessible to us. More Is relying on the technical accumulation of the designers in their daily work. Today we want to introduce the contents to everyone from the ZSW Research Center in Germany and organize and analyze them by Hai Yen Tran, Corina Täubert, Magret Wohlfahrt-Mehrens and others.
In this article, Hai Yen Tran shares with us the effects of electrode thickness, tap density, and conductivity type on the cycling properties, calendar life and thermal stability of NCA materials.
Effect of coating thickness
The amount of coating has an important effect on the cycling performance of the Li-ion battery. Manabu et al. Believe that the coating thickness of the positive electrode of the Li-ion battery should be controlled between 80 and 250 μm in order to ensure the high energy density and long-term cycle performance of the Li-ion battery. The picture shows the cross-sectional view and performance test result of Hai Yen Tran coating the slurry of NCA material (LiNi0.8Co0.15Al0.05O2) on the surface of Al foil according to different coating thickness (before drying).
From the magnification test results, it can be seen that at lower magnification, the coating thickness has no significant effect on the capacity of the NCA material, but at higher magnifications, the thin electrode can exert more capacity. Because the lower coating can shorten the distance Li + diffusion, reducing electrode polarization, thereby enhancing the material capacity to play in the design of lithium-ion batteries, lithium-ion batteries to take full account of the use of energy-based batteries can be coated The higher the amount, in order to improve the active material in the battery as much as possible, but the power of the battery will have to reduce the amount of coating to reduce the spread of Li +, lithium-ion battery to improve rate performance.
The effect of compaction density
Lithium-ion battery cathode coated and dried, the electrode porosity is generally between 60% -70%, after rolling the porosity will be reduced to 30-40%. The electrode rolling not only can improve the lithium ion Battery volume energy density, but also improve the conductivity of the electrode, reducing the resistance.But the low porosity will affect the infiltration of electrolyte in the electrode, affecting the diffusion of Li + in it, while the pressure is too easy to cause rolling activity Material particles broken and so on, so the electrode is not laminated the higher the better density.
The following figure shows the coating thickness of 250um (before drying) of the electrode in the 0-867MPa pressure roller interface after the electrode surface and electrical properties test results can be seen from the figure, a certain pressure roller is not only significant Increased the density of the electrode and also increased the bond strength between the active material layer and the current collector of Al foil, but when the pressing pressure was higher than 694 MPa, we observed that part of the active material particles were embedded in the Al foil, which May have a certain impact on the battery cycle performance.
Electrical performance test results can be seen that the roller NCA electrode after electrode magnification performance has been significantly improved, mainly because the roller NCA process makes the contact between particles better, reducing the contact resistance, At the same time, the contact between the active material and the Al foil current collector is also enhanced, thereby increasing the capacity of the electrode at a high magnification. However, after the pressure exceeds 694 MPa, the particles of the NCA material undergo some crushing and the excessively low porosity Also affected the diffusion of Li +, so the electrode rolled at 520MPa showed the best rate performance from the above analysis shows that the rolling pressure is too small and too large are not conducive to the performance of NCA materials, Hai Yen Tran It is suggested that the pressure of rolling should be controlled between 520MPa and 694MPa so as to ensure good contact between the electrode and the Al foil and to reduce the contact resistance and to prevent the active material particles from being broken and excessive deformation of the Al foil to reduce the loss of the active material , In order to improve the electrode rate performance and cycle performance.
Effect of conductive agent
Cathode material as a transition metal oxide material, its conductivity is significantly lower than the graphite anode material, so in the preparation of lithium-ion battery cathode, usually need to add part of the conductive agent, in order to be able to build a between the active material particles Conductive network to reduce the contact resistance and enhance the rate and cycle performance of the electrode currently used in lithium-ion battery conductive agent are: 'zero-dimensional' conductive agent, such as SP and so on, 'one-dimensional' conductive agents such as carbon fiber, carbon nano Tubes, etc. Two-dimensional conductive agents, such as graphene-based materials, etc. Different materials may have different effects on lithium-ion batteries due to their structural characteristics.
The figure below shows the cycle performance and rate performance of electrodes with different proportions of SuperP and conductive graphite SFG6 added to the NCA electrode. It can be seen from the figure that only the conductive graphite electrode is added, and the capacity is only 100 mAh / g, which is about the other three The addition of 63% of the SP conductive agent electrode is mainly due to the fact that the conductive graphite material has a relatively large particle size (D50 = 3.5 μm) and has a lamellar structure so that good contact between the NCA particles can not be ensured and small particles Of SP (particle diameter of about 40nm) can be well dispersed in the electrode to improve the conductivity between the NCA particles from the cycle performance, add more than 4% of SP, you can get good cycle performance NCA electrode , But 4% of the SP still can not meet the electrode discharge requirements at high current from the rate performance point of view, you can see from the figure below, at 5C rate, the addition of 8% SP electrode, the rate performance is significantly better than Other Add less SP electrodes.
From the above analysis we can see that the choice of conductive agent should be based on the design of the battery to choose from, such as energy-efficient battery, only need to add more than 4% of the SP to meet the requirements, but the power-type battery needs as much as possible Add more SP conductive agent.
As a key part of lithium-ion battery production, homogenization and coating process have a decisive influence on the performance of lithium-ion batteries. Especially NCA material as a new high-capacity cathode material needs more Conduct in-depth research to maximize the performance benefits of NCA materials.
