Is Extreme EV Fast Charging on the Horizon?

One way to increase consumer acceptance of electric vehicles (EVs) is to increase the rate at which they charge. 

 

Image used courtesy of Spk9264, CC BY-SA 4.0, via Wikimedia Commons

 

Using DC fast charging, some of the latest EVs can charge from zero to 80 percent of their capacity in 30 to 40 minutes. While this is quick enough for some customers, others hope charging times can eventually be reduced to 10 minutes—“extreme” fast charging that will provide an 80 percent improvement in charging time and be comparable to the time it takes to fill a conventional gas tank. 

 

How Lithium-ion Batteries Work

A lithium-ion battery consists of a cathode (positive electrode) usually made from a compound made up of nickel, cobalt, and either manganese (NMC) or aluminum (NCA) or from iron and phosphate (LFP) and an anode (negative electrode) typically made from layers of carbon graphite. Between the two electrodes is an electrolyte, made up of organic solvents, that permits the flow of lithium ions between the two electrodes during charging and discharging. A separator membrane between the two electrodes prevents the flow of electrons between the electrodes.

Lithium ions move from the cathode across the electrolyte and meet up with the anode when the battery is charged, where they are inserted into the graphite layers in a process called intercalation. Lithium ions move out of the graphite anode through the electrolyte and are inserted into the lattice structure of the cathode when the battery is discharged.  

If charging of the battery takes place too quickly, intercalation of the lithium ions into the graphite structure cannot occur fast enough, and crystals of lithium metal, called dendrites, plate onto the graphite surface. This can reduce the battery's capacity and further inhibit its fast-charging capacity. In the worst cases, the dendrites can grow large enough to pierce the separator, shorting out the two electrodes and potentially causing a fire.

 

An electric vehicle battery. Image used courtesy of Pixabay.

 

Lithium Plating is Hard to Detect

Detecting initial lithium plating on a graphite anode is difficult. Researchers at the University of California Berkeley and the Lawrence Berkeley National Laboratory recently investigated simple techniques for quantifying irreversible lithium plating on graphite anodes. By observing the effects of energy density, charge rate, temperature, and state of charge, the researchers were able to create and refine a physics-based electrochemical model to explore and predict the onset of lithium plating through an interpretable empirical equation, improving the safety and viability of fast charging. 

 

Improving the Solid Electrolyte Interface

The plating of the lithium metal onto the graphite surface largely depends on the solid electrolyte interface (SEI) that forms between the liquid electrolyte and the surface of the solid graphite. A team of researchers from the Japan Advanced Institute of Science and Technology (JAIST) has found a way to modify that SEI using a bio-derived lithium borate polymer binder material within the graphite anode that promotes lithium ion intercalation of active material. 

 

A lithium borate-type aqueous polyelectrolyte binder for graphite anodes helped improve lithium ion diffusion and lower impedance compared to conventional lithium-ion batteries. Image used courtesy of Noriyoshi Matsumi/JAIST

 

In this study, the improved diffusion of lithium ions across the SEI created through the lithium borate binder enhances charge transfer, promoting intercalation into the graphite and leading to the absence of plating on graphite, even while fast charging. The binder was also observed to form an organoboron SEI with a very low interfacial resistance when compared with ordinary battery cells. 

The biopolymer used for the binder study is derived from plant-based caffeic acid. This material is both sustainable and environmentally safe and has the potential to reduce the carbon dioxide emission that results from the production of battery materials—another advantage beyond the potential for extremely fast charging of future EVs.