The industrial revolution helped humans run faster than animal species. As the train passed the horses, the human world stepped into a new era that relies more and more on fossil fuels every day. In the first half of the past century, most people considered fossil fuels as the indication of human intelligence and a gift from nature; a few of us noticed the actual environmental implications as the consumption of fossil fuels accelerated yearly. According to a report compiled by the University of Oxford ^[1], the global fossil fuel consumption was 5,972.23 TWh in 1900, and in 2017, this number was 133,853.38 TWh.

In recent years, we have noticed that such vast consumption not only depletes fossil fuels but could also lead to severe issues including global warming, which can impact the society, the environment, and can also lead to a critical energy crisis. Environmentalists set a standard of “an increase of 2 degrees Celsius of warming limit”, which aims to lower the utilization of fossil fuels, and thus slowing the speed of global warming and its potential catastrophic impacts. However, such standard implementation actually means that more than 80 percent of the existing fossil fuels have to be left unused in the underground ^[2].

The United States Environmental Protection Agency reported that 29% of total greenhouse gas emissions are from conventional internal combustion engine vehicles powered by fossil fuels. Transformation of our daily transportation to zero emission cars powered by renewable energies can help solve this problem. As projected in the Electric Vehicle Outlook 2020 compiled by Bloomberg NEF, there will be 1.7 million of Electric Vehicle (EV) annual sales in 2020, while in 2040, this number is projected to be 54 million, surpassing the number for internal combustion engine vehicles sales. Such a great demand attracted everyone’s attention to a technology that can be traced back to 1800s – Battery.

However, not all battery technologies are suitable for this task. Lead-Acid and Ni-MH batteries suffer from low energy density, low operation voltage and memory effects, which limit their performance. In the late 20^th century, three researchers made a great effort in developing a new type of battery – rechargeable Lithium-ion Batteries, which have much higher energy density, higher operational voltage, lower self-discharge rate and lower maintenance requirements compared to earlier battery technologies. After several decades, their achievements have been recognized via the Nobel Prize in Chemistry: Whittingham, Goodenough and Yoshino. Today, the most commonly used battery type in cellphones, portable devices, electric vehicles /plug-in hybrid electric vehicles (PHEV) is the lithium-ion battery.

However, in the transportation field, internal combustion engines have served the society for many decades with rapid refilling time and robust quality. The charging times required and the corresponding safety issues remain to be the main challenges that most of EVs/PHEVs are facing today. The Standard J1772, which was developed by the Society of Automotive Engineers in the United States, provides regulations for the charging power specifications for EVs/PHEVs. As defined in J1772, level 1 and level 2 charging have been designated with a maximum charging power at 1.9 kW for 120V and alternative current, and 19.2 kW for 240V and alternative current, respectively. Most of the existing EVs/PHEVs under this standard have recharging times ranging from 1 hour to 10 hours, which means that on an average, EVs/PHEVs need to be charged overnight from a fully discharged status, while for internal combustion vehicles, the refueling time is usually only about 5-10 minutes during a gas station stop. Such limited charging speed for EVs/PHEVs render their usage mostly applicable for daily short commuting, instead of long–distance transportation and personal travel, which mitigates the process for EVs to replace the conventional internal combustion engine vehicles.

Overall, there are around 78,500 charging outlets and 24,800 charging stations across the United States, as of March 2020. Typically located in urbanized areas, there may be one or more charging outlets at each public electric station. California is ranked as the leading state in the United States for the most charging outlets and stations. Therefore, the U.S. Advanced Battery Consortium has launched the goal for the charging speed of EV batteries in 2023 as “80% of the battery pack capacity in 15 minutes”. However, in order to achieve this goal, the development of appropriate facilities that can generate enough power is just one of the factors that needs to be considered. It is of utmost importance to understand the degradation mechanisms in batteries under fast charging, since increasing the charging speed for lithium-ion batteries could develop higher risks of safety issues. For most of the commercial lithium-ion batteries after 1990, the cathode is usually a type of lithium transition metal oxide, while the anode is graphite.

For EV applications, the cathode material varies across different carmakers. One of the most mature cathode materials is LiFePO₄ (LFP), which is commonly used in short range EVs and PHEVs due to its high tolerance to fast charging. LFP, thanks to its robust olivine crystal structure, can endure up to 20–30 C charging speed. However, the energy density of LFP is usually extremely low (90–50 Wh/kg). Therefore, when the vehicle industry is looking for high energy density cathode materials to increase the driving range, LiNiMnCoO₂ (NMC) and LiNiCoAlO₂ (NCA, more specifically LiNi_0.8Co_0.15Al_0.05O₂), which have 2 to 4 times higher energy density compared to LFP, have gained more market share each year. In 2019, the Tesla Model 3, which uses NCA as the cathode material, had 14% sales among all EVs in that year. However, NCA and NMC materials are unlike LFP, their high nickel and cobalt content, and the rock salt structure make them very sensitive to the charging speed; higher than nominal charging speeds could lead to gas evolution, thermal runaway effects, or even explosions. There is a significant need in achieving a better understanding of the degradation mechanisms of NCA and NMC chemistries under fast charging conditions, which still remains a limitation.

In our recent published work, we have looked at the internal resistance versus state of charge (SOC) for commercial Panasonic NCR 18650B cylindrical cell batteries to investigate the degradation mechanisms of NCA cathodes under constant current fast charging conditions, and proposed an adaptive fast charging method that can mitigate the gas evolution and reduce safety hazards ^[3]. In our research, the values of internal resistance at various SOC (state of charge) were first calculated through the Galvanostatic Intermittent Titration Technique (GITT). Internal resistance is a critical parameter that can characterize the change of the kinetic and thermodynamic properties of lithium-ion batteries at various time scales. Therefore, by looking at the internal resistance change during the charging process, the overall operational voltage range can be divided into several different regions. The degradation mechanism for each region under fast charging conditions was then investigated, and the most sensitive region was targeted for the design of the optimized charging method.

All batteries were divided into 2 groups, and each group had 3 batteries for validation purposes. Group 1 batteries were tested under a standard 2C constant current fast charging algorithm, and GITT was implemented every 30 cycles to monitor the internal resistance change, and Electrochemical Impedance Spectroscopy (EIS) was implemented every 10 cycles to further investigate the degradation mechanisms. For all three batteries in group 1, severe cracks of the battery shells occurred around the 60^th cycle. In order to pinpoint the degradation mechanisms, internal resistance spectrum and EIS at different cycles were analyzed, which indicated that under fast charging conditions, the most charging current sensitive region is around 0%–40% SOC. Unlike several previous studies which considered that the graphite anode suffers most under fast charging conditions, our results indicate the presence of reversible rate–sensitive reactions on the NCA cathode which are responsible for gas evolution and the formation of mechanical cracks, especially within the 0%–40% SOC region.

The main component of the solid electrolyte interface (SEI) on the NCA cathode side is Li₂CO₃, for which the formation reaction is reversible during charging and discharging processes. It is well known that the quality of SEI layers is directly related to the safety and the robustness of lithium-ion batteries. And the decomposition of Li₂CO₃ occurs mainly in the 0%–40% SOC region. Under normal charging rates, this reaction is totally reversible, and, instead of generating CO₂ or O₂ gases, Li₂CO₃ decomposes into intermediate products of C₂O₄^{− •}, which will revert back to Li₂CO₃ during discharging. Under fast charging conditions, the formation of Li₂CO₃ will surpass that of intermediate products and result in generating CO₂ or O₂ gases, and the severe gas evolution increases the internal pressure of the batteries, which finally leads to battery failure and potentially more severe safety issues.

To investigate the degradation mechanisms, batteries first were disassembled and characterized using scanning electron microscopy (SEM) and x-ray diffraction (XRD). The results indicated that a built-up internal high pressure pressed the anode, cathode and separator against each other, which quickly depleted the electrolyte and resulted in short circuiting inside the batteries. Figure 1 summarizes the observed drop in capacity after each fast charging cycle using the industrial charging method versus our internal resistance adjusted fast charging method. Mechanical damage caused by industrial fast charging on the outside and inside of Panasonic NCR 18650B cylindrical cell batteries have been clearly demonstrated.

Based on this characterization and analysis, group 2 batteries were tested under a newly designed adaptive fast charging method, which charges the batteries at 1C rate from 0% SOC to 40% SOC, and then after a 5-minute rest, the batteries were charged again at 2C rate. The results showed that all group 2 batteries not only indicated better performance in capacity compared to group 1 batteries, but more significantly, they did not indicate any physical damage in their outer shells. Furthermore, the shape of the internal resistance spectrum remained nearly identical during cycling, which indicates better stability for the internal structure and potentially minimized any irreversible reactions. For comparing the internal structures, group 2 batteries were also disassembled at the same fast charging cycle number, for which SEM imaging indicated more stable electrode surfaces and interfaces ^[3].

In summary, using the internal resistance parameter at various SOCs as the better fast charging indicator, we have studied the degradation mechanisms of commercial Panasonic NCR 18650B cylindrical batteries’ NCA cathode under constant current industrial fast charging methods. An in-depth analysis was implemented through electrochemical characterization techniques, including EIS and GITT, as well as analytical materials characterization via SEM and XRD. Our work concludes that the 0% to 40% SOC region is the most critical region for fast charging conditions, and battery degradation is found mainly due to the destructive and wild reversible reactions on the NCA cathode. Our work also indicates that for current lithium-ion battery technologies, the degradation mechanisms under fast charging conditions may vary based on the different materials or chemistries for anode, cathode and electrolyte components. Individual specific analysis for each battery chemistry is necessary instead of applying a universal fast charging method. The results of our work could help on future development of personalized fast charging technologies applicable for different battery chemistries, especially for the EV industry to achieve better safety overall. Currently, more EV companies are striving for optimized fast charging technologies, including TESLA and KIA; similar battery testing including added coolant conditions should be conducted and the potential failure mechanisms should be further investigated for each of the battery chemistries or technologies.

Figure 1. The increasing demand on fast charging technologies demonstrated with the increase in the number of publications and patents over the years. Capacity decay in Panasonic NCR 18650B cylindrical batteries under industrial fast charging is demonstrated. Observations of external and internal mechanical or physical damage are indicated. Internal resistance adjusted fast charging method shows improved capacity retention and better chemical stability [3].

References

[1] Hannah Ritchie and Max Roser (2020) - "Fossil Fuels". Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/fossil-fuels' [Online Resource]

[2] Tracker, C. (2011). Unburnable Carbon: Are the World’s Financial Markets Carrying a Carbon Bubble?’ London: Carbon Tracker Initiative.

[3] Sebastian, Sandeep S., Bo Dong, Taner Zerrin, Pedro A. Pena, Amirali S. Akhavi, Yige Li, Cengiz S. Ozkan, and Mihrimah Ozkan. Adaptive fast charging methodology for commercial Li-ion batteries based on the internal resistance spectrum. Energy Storage. 2020; e141. doi.org/10.1002/est2.141

Authors: Bo Dong, Department of Electrical and Computer Engineering, Mihrimah Ozkan, Department of Electrical and Computer Engineering/Materials Science and Engineering Program/Department of Chemistry, Cengiz S. Ozkan, Materials Science and Engineering Program/Department of Mechanical Engineering/Department of Chemistry | University of California, Riverside
Image: University of California, Riverside | www.ucr.edu

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