Longer-lasting batteries to permit longer driving times with electric vehicles (EVs) are desperately needed to revolutionize transportation, Dr. Yang-Kook Sun, world-renowned expert in advanced battery technology, told the Twenty-Seventh International Conference on the Unity of the Sciences, held virtually April 23–24, 2021.
Noting that cutting greenhouse gas emissions is essential to halt rising global temperatures, which have already increased by 1.29°C since 1880, Dr. Sun outlined the current dilemma and then highlighted the immediate need for improved electric vehicle technology.
EVs today, such as the Tesla Model 3 and Hyundai Kona, typically have a driving range between 300 and 400 km (180 to 250 mi) per charge. However, the next major benchmark—driving ranges in excess of 500 km per charge—will require more advanced battery components.
In his presentation, Dr. Sun said that high capacity nickel-rich cathodes—such as LiMO2 (M = Ni, Co, Mn, called NCM or A1 called NCA)—have been prime candidate materials for next generation EV batteries. Over time, researchers have been able to steadily increase the fraction of nickel (Ni) in these cathodes to increase the capacity of current. Unfortunately, as Dr. Sun explained, this approach is limited by the deterioration of capacity retention and thermal stability caused by excessive Ni enrichment. During H2-H3 phase transition (when lithium ions are extracted, or the battery is in use), the notable volume change of the cathode causes microcracks in the structure. The resulting buildup of impurities ultimately accelerates the degradation of the internal structure of the battery. Cycle ability, thermal stability, and rate capacity are all adversely affected.
To overcome the tradeoff relationship between reversible capacity and cycling stability, Dr. Sun presented two approaches. One approach is to use a concentration gradient of rod-shaped particles that extends radially from the center of the cathode. This can be done in boron-doped Ni-rich cathodes (where boron is added to change the properties of the cathode). Boron plays a vital role in producing highly oriented and elongated primary particles, which can effectively dissipate the internal strain resulting from H2-H3 phase transitions, realizing a significant improvement in cycling stability.
Another strategy involves optimizing the crystal structure and shape of particles by introducing ions with high valency (>5+), such as tantalum (Ta). The substituted Ta not only induces the ordered occupation of Li sites by Ni ions but also produces radially oriented primary particles, as demonstrated by Li[Ni0.9Co0.09Ta0.01]O2 with a capacity retention of 90% after 2000 cycles.
Commenting on Dr. Sun’s presentation, Dr. Walter van Schalkwijk, affiliate professor of the Clean Energy Institute of the University of Washington, “Dr. Sun has shown that microstructure engineering of cathode particles using gradient composition modification and doping combined with protective coatings may provide a long-sound method of harnessing the high capacity of Ni-rich layered cathodes without sacrificing the cycling stability.”
For more information on Dr. Sun’s latest research: https://hanyang.elsevierpure.com/en/persons/yang-kook-sun
*Dr. Yang-kook Sun is a Professor in the Department of Energy Engineering at Hanyang University in Seoul, Korea.