Nickel Hydride No. 5 battery.CT technology as a characterization method for lithium-ion batteries

　　CT technology as a characterization method for lithium-ion batteries

　　Lithium-ion batteries are increasingly favored by the market and consumers due to their high gravimetric and volumetric energy density, long cycle life, and no memory effect. With the rapid development of lithium-ion batteries, many lithium-ion related characterization methods are becoming more and more familiar to people. These include many in situ characterization techniques, such as in situ X-ray diffraction (in situ XRD), in situ transmission electron microscopy (in situ TEM), in situ Raman spectroscopy (in situ Raman), in situ scanning electron microscopy (in situ situ SEM) etc. Today, the in-situ characterization method that I will briefly share with you is CT technology, which is called Computed Tomography in English.

　　01CT technology introduction

　　When it comes to CT technology, I believe many people will immediately think of medical imaging. Indeed, the invention of CT has many relations with medicine. The invention of CT was a revolution in the history of medical impact science.

　　In 1917, the Austrian mathematician J. Radon proposed a theory that a two-dimensional or three-dimensional reconstructed image could be mathematically calculated through projection from all directions. Later, Cormac completed mathematical problems related to CT image reconstruction in 1967. Hounsfield conducted relevant computer and reconstruction technology research at the British EMI Experimental Center to reconstruct the image. The first CT device was born in 1971.

　　CT uses an X-ray beam to scan a layer of a certain thickness. The detector receives the X-rays that pass through the layer, converts them into visible light, converts them from photoelectric to electrical signals, and then converts them into digital signals through an analog/digital converter. Enter computer processing. In the field of materials science, the imaging principle of CT is to analyze the different components based on the density of different phases and components inside the sample to be tested and the differences in atomic coefficients. . The CT machine structure includes the X-ray generating part (high-voltage generator, X-ray tube, cooling system, collimator and wedge filter/plate), Moving part (scanning frame, slip ring), computer part (host and array processor), image display and storage part (monitor, memory), workstation, etc.

　　02CT application fields

　　CT has a wide range of applications. As mentioned above, CT is widely used in life sciences and medicine. The shadow of CT can also be seen everywhere in the security inspection field. With the continuous occurrence of various terrorist attacks around the world, the field of explosives detection is becoming an important direction for the application of CT technology. In addition, CT is increasingly used in the industrial field for problem detection, failure analysis, assembly inspection of complex structures, advanced material research, etc. In recent years, CT has also gradually opened up new applications in new energy fields, such as lithium-ion batteries, fuel cells, and solid-state batteries.

　　03Application of CT technology in lithium-ion batteries

　　Lithium-ion positive and negative electrode materials and the microstructure of the electrodes significantly affect the performance of the battery. With the help of high-resolution CT, we can have a deeper understanding of the internal structure of materials or batteries and detect changes in the interface area.

　　3.1 CT technology is used to characterize the pores of lithium iron phosphate/carbon cathode materials

　　Jiao et al. used nano-CT technology to explore the three-dimensional morphology of the pores of the lithium iron phosphate/carbon cathode material and achieve intuitive observation of the interior of the material. The research process mainly includes fine sample preparation, data collection and data preprocessing. Through software ImageJ and Avizo processing, quantitative calculation of pore content is achieved. With the help of the three-dimensional rendering distribution map, it is known that there are a small number of independent pores inside, and such pores do not contribute to the battery capacity.

　　Lithium iron phosphate electrode sheets with different compaction densities have different electrochemical properties. In view of this, the researchers conducted CT characterization of lithium iron phosphate/carbon electrode sheets with four different compaction densities to study the changing patterns of their pores. Research results show that under larger compaction density, the pores inside the electrode are more uniformly distributed. Uniform pore distribution is conducive to the smooth transmission of electrolyte inside the material, which can fully improve the utilization of active materials. As compaction increases, the porosity gradually decreases, and the proportion of pores with smaller pore sizes increases.

　　3.2 CT technology to study the structure of silicon carbon walnut

　　Jiao et al. used nano-CT technology to study the structure of silicon-carbon walnuts. Using the threshold segmentation method, the Si material is selected. Use Avizo's built-in continuous smoothing algorithm to obtain beautiful three-dimensional renderings. Use the Extract Subvolume function to get an intuitive distribution map of internal Si. The blue part is the material Si. It can be seen that silicon is distributed in the center and outer surface of the particles in a scattered state, and is mainly coated on the outside. Statistical calculations were performed through Material Statistics and the volume percentage of Si in a single particle was found to be 30%.

　　3.3 CT technology to study the capacity fading mechanism of soft pack batteries containing silicon alloy negative electrodes

　　Berckmans used X-ray CT technology to study the reasons for the rapid capacity decay of soft-pack batteries containing silicon alloy negative electrodes, and found that obvious mechanical deformation occurred between the electrode layers. It is speculated that the main reason is that during the electrochemical cycle, FEC decomposes to produce more CO2 gas. FEC reacts with the electrolyte to generate LiF, which helps form a more stable SEI film and increases cycle stability, but it also produces CO2 gas. CO2 gas will hinder the transmission of lithium ions, causing loss of active surface area and reducing battery capacity. The gas production is too large, and the negative effects of using FEC exceed the positive effects.

　　The author then tried to reduce the decomposition rate of FEC by applying external pressure and found that the discharge capacity increased by 19%. In addition, no bulging of the battery was seen after the test, indicating that applying external pressure can reduce the consumption rate of FEC. Therefore, external pressure is one of the key parameters to consider when designing batteries containing silicon materials.

　　3.4 CT technology is used to distinguish active and inactive phases in lithium-ion batteries

　　Litster et al. used nano-X-ray CT technology to construct a three-dimensional image of lithium cobalt oxide, the cathode material of lithium-ion batteries, to distinguish the independent volumes of active materials, conductive agents, binders and pores. The morphological parameters of additives and active materials are known from the three-dimensional reconstruction, including the distribution of the contact area between the two. Using this method, the current distribution and structural integrity of the electrode can be better understood.

　　3.5 CT technology is used to analyze battery aging

　　Figgemeier et al. used nano-X-ray CT technology to conduct comparative analysis of new batteries and aged batteries to determine the changes in results after cycling. X-ray CT imaging shows that organic residues and sediments are the main reasons for the decrease in porosity of the negative electrode of aging batteries. Particle crushing and current collector corrosion phenomena were observed on the cathode side. The above phenomena may be the reasons for the reduction of battery capacity and the increase of impedance. Quantitative analysis of lithium ion distribution shows that battery capacity fading is attributed to the loss of recyclable lithium, which is enriched on the surface of the negative electrode.

　　The current collector thickness of the aging cathode becomes larger. Since the presence of micron-sized Al particles has been detected on the negative electrode side, and considering that micron-sized particles are difficult to pass through the separator, it is speculated that the dissolved aluminum ions migrate from the positive electrode to the negative electrode and are reduced to metallic Al at a reduced electrochemical potential.

　　In short, the author used X-ray CT method to examine the structural changes of the electrode and found that the particle cracks on the positive electrode side will not affect the battery capacity. At the negative electrode, the pores were found to be unevenly filled with the covering layer. The coating on the electrode surface increases the thickness of the electrode during cycling. Inside the porous structure of the electrode, an electrode deposited layer was also observed. The growth process of the overlay is highly non-uniform, resulting in blocked pores and a reduced number of pores. In addition, corrosion of the positive electrode current collector was also found. Based on the above phenomenon, the author proposes that the following points should be paid attention to in electrode design:

　　(1) The aluminum current collector should be protected from corrosion. Possible methods include coating with carbon or adding special electrolyte additives; (2) Further increasing the strength of the positive electrode material particles, because cracks will increase the volume of the battery and cause current Uneven distribution; (3) A strong and uniform SEI layer on the negative side is beneficial to the long cycle of the battery.

　　3.6 CT technology is used to study thermal runaway of lithium-ion batteries

　　Finegan et al. used X-ray CT technology, combined with thermal and electrochemical testing technology, to investigate the thermal runaway mechanism caused by overcharging of commercial lithium-ion soft-pack batteries. The battery uses LiCoO2 as the positive electrode and graphite as the negative electrode. The battery is overcharged at 3A (18.75C) at 100% SOC (4.2V) until failure.

　　Thermal runaway soft-pack batteries appear segmented Al phases, which are agglomerated at the top and bottom of the electrode layer respectively. LiCoO2 and the electrolyte decompose to produce gas, and the gas can diffuse from the inner layer to the outer layer, possibly sending molten Al to the end of the coiling needle. The porous structure of Al droplets appears at both ends of the battery, proving that gas plays an important role in the diffusion of molten metal. Uneven gas production will cause a creeping effect in the closely adherent wound layer, causing the molten Al to spread to both ends. Using CT, area A in Figure 19(b) was sampled and Co was found on the surface. However, when sampling in area B in Figure 19(b), there is no Co on the particle surface. It may be that the local temperature in the outer region is lower (due to enhanced heat rejection) or the electrolyte is insufficient, preventing the Co reduction reaction from occurring.

　　The Al current collector after thermal runaway exhibits a highly porous morphology, and the distance between the LiCoO2 layers becomes farther, which helps gas escape during the thermal runaway process. The high specific heat and thermal conductivity of Al can increase the heat diffusion of local exothermic reactions before and during thermal runaway.

　　CT images of dissected LiCoO2 particles show severe microstructural degradation. There is a Co metal surface layer in the electrode particles. The translucent three-dimensional rendering shows the presence of Co on the surface of the particles and the presence of Co channels inside the particles. The density of Co metal is about twice that of LiCoO2, and the Co layer detaches from the bulk particles. During thermal runaway, delamination causes the material surface to be further exposed, allowing more exothermic reactions to occur.

　　The particle size distribution of LiCoO2 particles in the fresh state is concentrated at 3.87um. The average particle size of the particles after thermal runaway is reduced to 1.99um and 1.97um respectively, and the particle size distribution width is obviously different. The particle size distribution of the internal sample shows a single peak below 2um, which is attributed to the particle shrinkage during the phase transition process and obvious fragmentation (including broken particles and delaminated Co). The particle size distribution of the external sample appears bimodal, with most of it distributed in the smaller particle diameter range. Particles below 1um in diameter show a higher frequency distribution, while the second peak may be small fragments of broken larger particles.

　　04 Summary

　　X-ray CT technology has great application value in studying electrode materials or batteries. Compared with destructive methods, the use of CT methods can obtain more information about changes in materials or electrode structures. CT technology helps researchers design and analyze materials and batteries in a more efficient and precise manner.

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