By Kent Griffith
March 29, 2021 | Undeterred by the hurdles of 2020 and 2021, the longest-running lithium-ion battery conference was held virtually for the second consecutive year. Leaders from industry, national laboratories, and universities came together for presentations with live discussions and panels on the hottest topics in rechargeable batteries.
Coming off the recent news that Professor Jeff Dahn has signed a five-year extension of his initial five-year partnership with Tesla, his keynote presentation was one of the early highlights of the 2021 International Battery Seminar. Dahn, of Dalhousie University in Nova Scotia, piqued the audience’s curiosity with the title, “Can one learn as much about Li-ion battery failure with a micrometer as with a synchrotron?” The former can be purchased for about $1,000 while the latter—in the case of the Canadian Light Source that Dahn’s laboratory typically uses—runs at about $160 million per year.
The context of Dahn’s talk was the ongoing debate surrounding the pros and cons of single-crystal vs. polycrystal particles for Ni-rich layered cathode materials. The team from Dalhousie studied long-term cycling performance of standard lithium-ion cathode formulations including NMC532 and NMC622 in pouch-type full cells against graphite. Dahn later noted that the principles are also applicable to higher nickel compositions such as NMC811. It is well-established that microcracking becomes more severe as the nickel content increases, and high nickel compositions are favored as part of the effort to minimize or eliminate cobalt in lithium-ion batteries. Single-crystal NMC nominally comprises only intermediately sized primary particles without grain boundaries, while polycrystalline NMC has large secondary particles formed by numerous interconnected small primary particles. The grain boundaries between primary particles in polycrystalline NMC are weak points, susceptible to microcracking during cycling-induced strain.
Dahn presented results from 2.5 years of battery cycling data, describing sources of capacity fade and approaches for characterizing battery health. For example, when charging and discharging a battery over only 25% of its usable capacity range (i.e., 25% depth-of-discharge), there is no observable capacity fade. However, the narrow duty cycle hides the fact that the battery is still undergoing degradation, as observed when a full 100% depth-of-discharge ‘check-up’ cycle is performed. He also showed that storage at the top of charge is very harmful, causing similar rates of degradation to continuously cycling the battery. The main modes of degradation in these cells are active material loss, impedance growth, and lithium inventory loss. Positive electrode mass loss is attributed to microcracking in the cathode particles and electrical disconnection within the electrode, which is where the discussion about single crystals versus polycrystals becomes important. Dahn took the microcracking concept a step further and revealed that, under mild cycling conditions where the cell is limited to 4.1 or 4.2 V, virtually all pouch cell expansion comes from cathode thickness growth. Gas generation plays a minor secondary role. Since single-crystal electrodes experience much less microcracking, they also experience less swelling. The implication of pouch expansion and thickness growth is electrolyte unwetting that can lead to cell failure. Dahn pointed out that the “million-mile” batteries from his lab are based on single-crystal NMC, and now we have a better understand of these impressive lifetimes.
Back to his title, pouch thickness can be measured readily with a micrometer while cathode thickness and electrolyte unwetting require complex methods such as X-ray computed tomography and ultrasonic scanning measurements. Now that the origin of the swelling is understood, the Dalhousie team relies on simple micrometer or external pressure measurements for the 800 pouch cells they make each month. In the end, the combined insights of several tools are far greater than any individual technique, and characterization methods like synchrotron tomography have laid the groundwork for interpretation of readily acquired data from a simple micrometer. Looking toward the future of this technology, long-life lithium-ion battery applications such as vehicle-to-grid (V2G) energy transfer could be enabled by single-crystal NMC.
Battery Ecosystem Insights
In a broader keynote address, Dr. David Howell from the Vehicle Technologies Office, US Department of Energy described the “U.S. Lithium-ion Battery Ecosystem: Prospects, Challenges, Opportunities.” The DOE has an established history of supporting vehicle electrification with collaborative initiatives such as the Battery500 program. As motivation, he pointed out that transportation contributes 70% of the US petroleum consumption, is the largest CO2 emitter, and is the second largest household expense (after housing itself). In promising market news, over 325,000 EVs were sold in the US in 2019 and the trend is strong; the projection for 2040 is 9.6 million new EVs. In 2020, two new major programs were announced, the Energy Storage Grand Challenge (ESGC) and the Federal Consortium for Advanced Batteries (FCAB). The FCAB will focus on raw materials production, materials R&D and processing, cell R&D and manufacturing, pack manufacturing, vehicle manufacturing, and recycling. 2025 targets have been set for an EV pack cost of <$100/kWh, a 300-mile driving range, and a charge time of 15 minutes.
Howell described high-level directions and motivations for the leading lithium-ion chemistries. In standard lithium-ion/graphite cells, the field is moving to reduce both cobalt and, eventually, nickel content and to improve recycling. Lithium-ion battery recycling is a central concern both from environmental and supply chain perspectives. The aim of 90% recycling and recovery would reintroduce a domestic supply chain for key raw materials.
In the live discussion between Dahn and Howell, hosted by Brian Barnett of Battery Perspectives, Howell stressed that recycling must be a profitable process at the end of the day. For intermetallic Si-based anodes, targets have been set for >350 Wh/kg, 1000 cycles, and >10-year calendar life. Howell noted relative success in the two former targets, but challenges in the calendar life, which he pinned currently at 2–3 years. Many anodes today are silicon–graphite blends, so another trend is to move toward higher Si content to increase energy density. In the hot areas of Li metal and solid-state batteries, Howell highlighted the importance of practical factors such as low electrolyte loading and low excess Li metal combined with high areal energy density. All of these technologies fit within the larger DOE objective of affordable, reliable, and clean energy for all.