Capacity degradation mechanism of thick electrodes in lithium-ion batteries

The demand for high endurance new energy vehicles now forces the energy density of batteries to become increasingly high. The use of thick electrodes with high load density active materials is one of the most practical strategies. However, during their long cycle use, they are accompanied by severe degradation of electrochemical performance, unsatisfactory power performance, and deteriorating capacity retention. So, what is the bottleneck that leads to the deterioration of performance?
Kyu Young Park et al. investigated the key processes that constrain battery degradation by designing thick electrodes with different surface areas.
1. Experimental design
NCM622: carbon black: PVDF 97:1.5:1.5 ratio was mixed with NMP to form a slurry. After coating, drying, and rolling, two thicknesses of piezoelectric half cells (2032) with different surface densities (20 and 28 mg/cm-2) were prepared, compacted between 2.8-2.9 to ensure good porosity; Multi channel device charging and discharging cycles, with a charging and discharging range of 2.8-4.3V and a rate of 1C of approximately 150mA/g. After every 20 cycles, EIS, chemical composition, and morphology characterization analysis are performed.

2. Results and Discussion
The following are cross-sectional views of electrodes with two thicknesses, 70 and 100 μ m (standard electrode, thick electrode), respectively. The remaining design parameters such as porosity and 1C current density are basically the same. Then, 1C cycling tests were conducted. In Figure c, it was found that although the 100 μ m thick electrode only increased the capacity by 40% compared to 70 μ m, after 100 cycles of the battery, the thick electrode only had a capacity retention rate of 36%, while the standard electrode still had a capacity retention rate of 76%. Even considering the volume to capacity ratio, the attenuated thick electrode in Figure c is still much lower than the electrode. Interestingly, even in the initial cycling process, the cycling curves of the thick electrode and the standard electrode are close, and the degree of attenuation is similar. As the cycling increases, in Figure c, The performance of thick electrodes is getting worse and worse.

When explaining the observed poor electrochemical performance, the author noted that thick electrodes may be subject to kinetic limitations, which are caused by the speed of carrier migration. During the electrochemical process, the reaction migration rate is either controlled by lithium ion transport or electron transport accumulated along the electrode. And, in each case, assuming that the main supply sources of electrons and lithium ions to the electrode come from the interface between the electrode/collector and the interface between the electrode/electrolyte, there will be a clear spatial distribution of both after the reaction.
In Figure A, the process by which the slow migration of lithium ions determines the reaction rate is explained. On the side closer to the current collector, the electrochemical reaction rate is lower due to the inability of long-distance lithium ions to migrate quickly, while on the side closer to the electrode liquid, both lithium ions and electrons maintain high concentrations, allowing the reaction to proceed well; In Figure B, the process of slow electron migration determining reaction rate is explained. On the side closer to the current collector, due to the migration of lithium ions, the electrochemical reaction rate in this area is higher. However, on the side closer to the electrode liquid, electron migration is hindered, resulting in lower concentration and poorer reaction rate. The darker the color in the figure, the better the reaction. To verify the two hypotheses, the author characterized the SOC state characteristics of electrodes with different thicknesses during the reaction process and charge discharge process. Figures c and d show the tests every 25 cycles during the standard electrode charge discharge process. The larger the x-axis coordinate, the closer the distance to the current collector. From Figure c It can be seen that for the standard electrode, the amount of lithium embedded in SOC at different positions is almost the same during the charging and discharging process, indicating that even at a 1C rate, the electrochemical reaction is uniform. In Figure e, after 25 cycles, the SOC of the electrode sheet is still relatively uniform. After 50 cycles, during the battery charging process, lithium ions diffuse from the positive electrode to the negative electrode, and the difference at different positions becomes larger, which is consistent with the previous cycle retention curve. The amount of lithium embedded near the current collector is higher, indicating that the reaction is slower. In Figure f, it was found that the difference in the discharge process of thick electrodes was not particularly obvious. The author believes that this is due to the previous charging process, which resulted in more lithium embedded near the current collector side and closer to the electrolyte. There are few lithium ions at the diaphragm, so during the discharge process, the concentration of lithium ions at the diaphragm is high and will not show too much difference, From the SOC characteristics of thick electrodes, it can be analyzed that slow lithium ion migration is an important reason for capacity decay.

XRD characterization also confirmed the speculation of uneven distribution of lithium insertion. From the standard electrodes in Figures a and b, it can be seen that there is no significant difference in XRD peaks at the bottom and top after different cycles. Supporting data shows that after 100 cycles, the SOC at different positions differs by up to 5%, while there is a serious peak position shift at the bottom and top in Figures d and f.

Due to the non-uniformity of SOC caused by material transport, the electrode undergoes a relatively high proportion of electrochemical reactions near the top of the separator, while the reaction is scarce near the bottom of the current collector. As the cycle progresses, thick electrodes exhibit poorer performance, and the reaction current density increases at the top, forming a current hotspot area that destroys the active material, causing cracks or irreversible phase transitions. High current density also forms a gradient of solid-phase lithium ion concentration distribution in the secondary particles, generating a stress field that causes particle breakage, exposes new interfaces, and leads to side reactions. A thick organic layer is formed at the new interface, causing impedance increase and ultimately resulting in battery failure. The above explanation can be verified from the thick electrodes c, g and d, h in the following figure, indicating an increase in C=O content.

Based on the above work, the author proposes a mechanism model for the attenuation of thick electrodes in batteries
a: In the initial cycling process, lithium ions were able to diffuse to the current collector side across the entire electrode distribution, and the electrochemical reaction was relatively uniform, without the occurrence of a large concentration gradient distribution of lithium ions;
b: As the cycle progresses, the limitation of lithium ion transport begins to lead to uneven electrochemical reactions in the upper and lower regions, which means that voltage drop IR and concentration polarization become increasingly important in thick electrodes. Subsequently, the active particles located in the upper region bear higher effective current densities;
C: High current density leads to the rupture of active particles, exposing new interfaces, which in turn increases the porosity of the electrode sheet. This makes it more difficult for lithium ions to migrate to the current collector side, resulting in higher current density occurring at the upper particles and further particle rupture, forming negative feedback.
d: Seriously, electrochemical reactions only occur in the upper part and form current hotspots, which can pose significant risks to battery management.

The author also conducted verification using different cycling methods. For the same standard electrode and thick electrode, the CCCV method can significantly reduce the SOC difference of thick electrodes under the same current density rate cycling.
3. Conclusion
The author verified through batteries designed with different electrode thicknesses that lithium ion diffusion transfer is the limiting factor for charge transfer, rather than electron transfer. This is also the reason why in batteries designed with thick electrodes, uneven SOC at different positions, increased voltage drop IR, particle breakage, and even battery diving occur during charging and discharging. At the same time, it is suggested that for thick electrodes, electrode plates should be designed based on ion transfer characteristics to avoid the phenomenon of high local current density and improve battery service life.