Fast charging is seen as a solution for range and recharging time issues for EVs. However, a critical barrier to fast charging is temperature. Fast charging in cold or even cool temperatures brings the risk of lithium plating—the formation of metallic lithium that drastically reduces battery life and even results in safety hazards.
Now, a team from Penn State has devised an approach that enables 15-min fast charging of Li-ion batteries in any temperatures (even at −50 °C) while still preserving remarkable cycle life (4,500 cycles, equivalent to >12 y and >280,000 miles of EV lifetime), thus making EVs truly weather-independent. A paper on their work is published in Proceedings of the National Academy of Sciences (PNAS).
Fundamentally, lithium plating is affected by the rate of ion conduction and diffusion in the electrolyte, lithium diffusion in graphite particles, and reaction kinetics at graphite surfaces. Key parameters governing these processes all follow the Arrhenius law and drop substantially with temperature. As such, a plug-in hybrid EV (PHEV) cell that can withstand a 4-C charge without lithium plating at 25 °C can only allow a 1.5-C charge at 10 °C and C/1.5 at 0 °C to prevent lithium plating, which explains the long recharge time of today’s EVs at low temperatures.
To enhance fast charging ability, research in the literature has been focusing on improving anode materials such as coating graphite with an amorphous silicon nanolayer and developing new materials like lithium titanate and graphene balls, and on developing new electrolytes and additives. LiBs, however, are well known for their trade-off nature among key parameters. Improving one property without sacrificing another is always nontrivial. For instance, electrolyte with superior performance at low temperatures is quite often unstable at high temperatures. … It is extremely difficult, if possible at all, to develop materials with a high rate for charging while preserving durability and safety over a wide range of temperatures.
Here, we make an attempt to free battery science from trade-offs. Specifically, we present a cell structure that can be actively controlled to achieve lithium plating-free (LPF) fast charging in any ambient temperatures, enabling a paradigm shift of the relation between cycle life and temperature, from the Arrhenius correlation of conventional LiBs to a horizontal line insensitive to temperature.—Yang et al.
The Penn State team uses a rapid internal fast-heating step prior to charging. The self-heating LiB structure has thin nickel (Ni) foils embedded inside a cell that can create immense and uniform heating.
The Ni foil becomes an inherent component of a single cell along with electrodes and electrolyte. It serves as an internal heating element, as well as an internal temperature sensor; the introduction of Ni foils only adds 0.5% weight and 0.04% cost to a conventional LiB single cell.
In their study, the team used 9.5-Ah pouch cells with graphite anode, LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode, and a cell-level energy density of 170 Wh/kg.
With the internal Ni foil, the team demonstrated the cell could be charged to 80% state of charge in 15 min even at −50 °C (beyond cell operation limit). Further, the cell sustained 4,500 cycles of 3.5-C charging in 0 °C with <20% capacity loss—a 90× boost of life compared with a baseline conventional cell— and equivalent to >12 y and >280,000 miles of EV lifetime under this extreme usage condition, i.e., 3.5-C or 15-min fast charging at freezing temperatures.
In comparison, cells with identical battery materials charged at 0 °C only sustained 50 cycles before losing 20% capacity.
… the present LPF cell holds great promise for next-generation EVs, as it can modulate cell internal temperature almost instantaneously on demand.—Yang et al.
Xiao-Guang Yang, Guangsheng Zhang, Shanhai Ge, Chao-Yang Wang (2018) “Fast charging of lithium-ion batteries at all temperatures,” PNAS doi: 10.1073/pnas.1807115115