By Kent Griffith
April 16, 2021 | Ramping up the current density and bringing down the charging time was a leading theme at this year’s International Battery Seminar and Exhibit. The conference, held virtually, brought together stakeholders from across the globe representing industry, government funding organizations, universities, and national laboratories. A diverse set of topics were presented covering lithium-ion and “beyond lithium-ion” battery technologies for electric vehicle applications, portable electronic devices, and grid-scale energy storage systems. Highlighting the rising significance of research and development on fast charging for batteries, Dr. Halle Cheeseman, Program Director at the US Advanced Research Projects Agency – Energy (ARPA-E) said, “We must start treating charge time as just as important as energy density, cycle life, or cost.” In the context of electric vehicles, he went on to say, “Charge time arguably is as important as range and potentially has a huge cost advantage.”
Charging time is a multi-scale problem, starting at the atomic level and moving up to particles, electrodes, cells, and packs. There can be limitations related to ionic transport of lithium ions in the electrodes as well as in the electrolyte. As batteries age, transport may be further limited by a resistive surface layer on the cathode after atomic reconstruction, or from a growing surface–electrolyte interphase (SEI) layer growing on the anode. Related the growing SEI, depletion of lithium ions from the electrolyte may further inhibit charge capability. Thus, good mixed ionic–electronic conductive electrode materials and a highly ionically conductive and stable electrolyte are important initial criteria for a fast-charging battery cell. The story does not stop there.
To set the stage for our understanding of fast charging, and limitations thereof, Dr. Andrew Colclasure of the National Renewable Energy Laboratory in Golden, Colorado, presented experimentally validated electrode-scale and electrolyte transport models. Electrode thickness is a fundamental tradeoff in the battery world. As electrode thickness increases, energy density increases because the relative ratio of inactive components such as current collectors and separators decreases. On the other hand, as electrode thickness increases, so too does the distance that lithium ions must travel for a complete charge or discharge. Colclasure focused in on three electrode limitations of fast charge within the context of high-energy EV style cells that would typically employ nickel-rich layered oxide cathodes and graphite anodes, namely, (i) overpotentials scaling as the current density squared, (ii) cathode secondary particle cracking, and (iii) lithium metal plating on the anode surface. He also noted that, with respect to the electrolyte, homogeneity is key because regions of low lithium-ion concentration lead to unutilized or underutilized portions of the anode while regions of high concentration can lead to lithium plating. Lithium plating is well-known as a source of shortened cell lifetime but also dangerous short-circuits that can lead to battery fires.
Colclasure works as part of the eXtreme Fast Charge Cell Evaluation of Lithium-ion Batteries (X×CEL) program sponsored by the Department of Energy with involvement from a number of US national laboratories including Argonne, Idaho, and Lawrence Berkeley. His modeling was paired with experimental data from NMC532//graphite cells produced at the Cell Analysis, Modeling, and Prototyping (CAMP) facility housed at Argonne National Laboratory on the southwest side of Chicago. Among their many published results, the X×CEL team has shown that increased temperature, reduced tortuosity, and improved electrolyte transport can substantially improve fast charging performance. Conversely, increased porosity and increased negative-to-positive (N/P) ratio were not particularly helpful.
Also on the theme of improving the charging time of carbon-anode-based cells, Professor Neil Dasgupta from the University of Michigan presented recent work from his team on ordered, laser-patterned graphite electrodes. He showed that lithium plating occurs on the graphite electrode surface in a conventional NMC532//graphite cell after 4C (15 minute) charging while the identical cell with hole-patterned graphite shows no evidence for lithium plating. In a related manner to the concepts presented by Colclasure, Dasgupta presented modeling results showing that—for the conventional graphite electrode—the lithium ion concentration rapidly decreases from the separator side of the electrode toward the current collector side while the concentration inhomogeneity is far less across the electrode after laser hole patterning. Moving beyond graphite, Dasgupta presented recent results on hybrid graphite/hard carbon composites. These anode materials leverage the higher rate capability of hard carbon with the higher coulombic efficiency of graphite.
Lithium iron phosphate (LFP) is having something of a renaissance as the battery field looks (back) toward cobalt-free and even nickel-free cathode chemistries. A123 Systems, celebrating their 20-year anniversary, discussed their latest LFP products. Compared to ‘Nanophosphate’, Dr. Tomasz Poznar of A123 described ‘Ultraphosphate’ as having more spherical particle shape, a smaller particle size, higher tap density, and lower impedance. An important application of cells that can accept high current densities is mild hybrid vehicles, where Poznar noted that they have 12 V and 48 V LFP packs and can support up to 50C partial fast charging. While they do work on other technologies such as NMC, silicon, and solid-state batteries, LFP is their choice for high power and fast charging.
Conventional cell chemistries such as NMC//graphite and LFP//graphite were not the only cause for conversation at the meeting. Dr. Cheeseman from ARPA-E was interested in fast charging in the context of lithium metal batteries, an under-discussed issue. He noted that realization of the theoretical energy density advantages of lithium metal anodes will be unlikely to make an impact on electric vehicles unless they are capable of supporting high current densities and thus rapid charging. His program is targeting fast charging in under 30 minutes, ideally 5–10 minutes.
As an alternative to lithium, Professor Yong-Sheng Hu gave a tour of sodium-ion battery technology, mentioning electrodes such as sodium vanadium (fluoro)phosphate, Prussian blue analogues, and hard carbon that have all generated interest as possible fast charging sodium-ion hosts. Hu discussed HiNa Battery, founded in 2017, that is producing Ah cells claimed to maintain much of their accessible capacity at 5C (12 min) and even 10 C (6 min) while also being able to cycle under nonambient conditions such as 80 °C owing to the enhanced stability of sodium electrolyte salts.
Fast charging goes beyond the material and electrolyte chemistry and even beyond the electrode and cell engineering. There is an increasing body of work on different charging protocols to decrease battery charging times while maintaining cycle life and safe operation. Perhaps the simplest charging protocol is to apply a constant current until the battery is fully charged. In practice, even simple battery management systems use a method known as constant current–constant voltage or CCCV. In CCCV charging, a constant current is applied until the battery reaches a target voltage, perhaps 4.2 V. Then, the cell is held at that voltage and continues charging as the current naturally exponentially decays. The CCCV method prevents the cell from reaching an unacceptably high voltage that could lead to accelerated degradation or a safety issue.
Professor Rachid Yazami of KVI Battery Intelligence reported his work on a technique he calls ‘non-linear voltammetry’ that relies on not just voltage or state-of-charge but also current, the rate of change in the current, and the state-of-health of the battery in order to devise a fast-charging protocol. The goals of fast charging protocols via battery management are to minimize heat generation, avoid lithium plating, and mitigate material degradation. For those looking for a deeper dive, a recent open-access review from Imperial College London, Tsinghua University, and Shell is recommended.