By Kent Griffith
August 25, 2020 | Nickel-rich formulations of lithium-ion battery cathodes are increasing the driving range of electric vehicles, lowering the cost of high-performance batteries, and mitigating concerns around the sustainability, toxicity, and ethics of cobalt mining. Solid-state electrolytes and lithium metal anodes are being actively pursued as technology innovations of the future, but nickel-rich cathodes are here today and appear to be here to stay. Of course, then, the topic of nickel-rich cathodes spanned many talks and sessions at the 2020 International Battery Seminar, which was held virtually for the first time in its 37-year tenure.
Lithium cobalt oxide (LCO, LiCoO2) was used as the cathode by Sony in the successful commercialization of lithium-ion batteries nearly 30 years ago. Nickel, and other elements such as manganese and aluminum, can substitute into the layered LCO structure, giving rise to new families of materials such as lithium nickel manganese cobalt oxide (NMC) and lithium nickel cobalt aluminum oxide (NCA). While aluminum and manganese serve primarily as stabilizing agents, substitution of nickel for cobalt changes the fundamental charge storage chemistry of the battery. Electrochemical reactions (reduction and oxidation) now take place on the nickel, which operates at a slightly lower voltage than cobalt. The lower voltage operation facilitates a larger quantity of lithium ions and electrons to be reversibly cycled in the battery on discharge and charge without harming the electrolyte and with only a minor impact on the cell potential. As a result, NMC- and NCA-based batteries have higher energy densities than cells based on LCO. Cobalt cathodes still find widespread use in consumer electronics thanks to their small volume footprint, but NMC and NCA are the premier solution for EVs with long driving range and reasonable cost.
In the opening plenary session of the International Battery Seminar, Stan Whittingham, one of the inventors of the lithium-ion battery and the 2019 Nobel Laureate in Chemistry, predicted that NMC/NCA will dominate for at least the next 5-10 years. He pushed further and suggested that these batteries will soon have close to zero cobalt. Whittingham has recently worked on NMC622 with 60% nickel and NMC811 with 80% nickel through the Battery500 program and produced high energy cells with 350 and 400 Wh/kg based on those formulations, respectively.
Immediately following Whittingham’s presentation, we heard from Jeff Dahn, famed Tesla Research Chair, on the “unavoidable challenge for Ni-rich positive electrode materials” and on the prospect of eliminating cobalt entirely. Dahn has a reputation for performing careful electrochemical studies on NMC and NCA formulations. Due to the recent interest in nickel-rich cathode materials, his group at Dalhousie University has recently re-investigated the ultimate nickel-rich layered oxide, lithium nickel oxide (LNO, LiNiO2) (DOI:10.1149/2.0381813jes).
LNO does not work well as a cathode because it undergoes a complex series of phase transitions as lithium is removed and reinserted into the crystal structure. The phase transitions in LNO show up in structural analysis with diffraction techniques and also in electrochemical measurements, particularly as peaks in the derivative of the charge and discharge curves. The elimination or suppression of these features has become the tangible target for research on substituted nickel-rich materials such as NMC and NCA. Dahn has found that magnesium ions are most effective for this phase transition suppression, followed by manganese and aluminum, which perform similarly, followed by cobalt.
Surprisingly, he found that cobalt does not effectively suppress the phase transitions without a co-substituent and thus cobalt is not a necessary ingredient for nickel-rich cathode materials. Rather, the “unavoidable challenge” that Dahn presented is the large volume change that occurs any time more than about 78% of the lithium ions are removed from the structure, corresponding to a charge storage capacity of about 210 mAh/g. He also noted that Hubert Gasteiger’s group at the Technical University of Munich showed that oxygen and carbon dioxide gas evolution were observed at this same state-of-charge (DOI: 10.1021/acs.jpclett.7b01927). It is an unsolved question as to whether this capacity and lithium extraction can be exceeded without rapid battery degradation.
Nickel Market Updates
From the market perspective, Michael Sanders of Avicenne Energy showed data and predictions on the global picture and on trends. NMC and NCA comprise about 211,000 tons and 66,000 tons, respectively, of the 390,000-ton cathode material market in 2020. However, moving to the one megaton market of 2030, NMC and NCA are expected to take the lion’s share at about 90% adoption. He noted that the global COVID-19 pandemic appeared to disrupt manufacturing for about two months in most cases, but that the effects were widely variable and have not yet fully materialized.
Gigafactories are going up worldwide, particularly in Europe as it attempts to catch up with Chinese manufacturing, and most of these factories will produce batteries with nickel-rich cathodes. The EV battery story in China, presented by Yuan Gao of Pulead, is rather different. Lithium iron phosphate (LFP), rather than a nickel- or cobalt-based material, has been the dominant cathode in China for many years. The driving forces for LFP include cost, safety, and manufacturability. 2020 marks a year of parity between LFP and NMC, each at about 80,000 tons of cathode material produced, but new safety regulations coming out in 2021 may shift the trend back toward LFP. Even among the NMC cathodes used, lower nickel formulations (50% nickel and below) are in the vast majority of cells. Thus, while China is the largest EV market and battery producer and consumer, it is not the epicenter of nickel-rich cathode activity.
As the nickel content of cathode materials increases, so too does the challenge of achieving a long cycle life. One widely used approach to increase the stability of nickel-rich cathode materials is to apply a surface coating. The three primary coating methods are dry coating, solution coating, and gas-phase coating. ForgeNano, based in Colorado, presented on the advantages of their atomic layer deposition (ALD) gas-phase coating technology. Daniel Higgs of ForgeNano, admittedly not without controversy, claimed that ALD can be the cheapest and the most precise coating option for lithium-ion battery materials. They have demonstrated ALD of alumina onto cathode powders at less than $0.50 per kg, including full CapEx and OpEx, at the 20 ton-per-day scale. Higgs points to the low operating temperature, high throughput, absence of solvent, and chemical efficiency as the explanation for the surprisingly low cost.
ForgeNano has tested ALD-coated NMC622 and demonstrated the advantage of the coating, particularly when cycling to high voltage (4.4 V) or under high current density (4C discharge, 1C charge).
Finally, what will happen to all these batteries at the end of life? Large-scale batteries such as those for EVs will reach their end-of-life approximately 10–20 years after they are produced. As manufacturing is taking off at present, we will see an increase in ‘dead’ battery materials from 200,000 tons in 2025 to 3,500,000 tons in 2025, according to Jeff Spangenberger of Argonne National Laboratory. To address this impending challenge, Argonne is leading an innovative recycling research initiative called the ReCell Center. Along with partners such as the National Renewable Energy Laboratory and Oak Ridge National Laboratory, the goal is to make battery recycling profitable and efficient. In conventional battery recycling, complex battery material compositions such as NMC and NCA are converted to their metallic parent elements and nickel and cobalt, in particular, are recovered. A major focus of ReCell is on direct recycling, where the active battery materials would be separated from the inactive components, renewed through relithiation, and could reenter the manufacturing stream to be made into new electrodes, bringing these nickel-rich cathodes full circle.
Editor’s Note: Did you miss the 2020 International Battery Seminar? Because the event was virtual, you can still access the event including all of the recorded sessions, presentations, and materials. Register for PREMIUM POST-EVENT ON-DEMAND.