At a Glance
Recent research findings by the Steingart group indicate that a lithium ion battery may be charged considerably faster and with minimal degradation if the charging is done at a higher temperature than usually considered (40 ̊C to 50 ̊C).
The rapid decrease in the cost of lithium ion batteries has increased interest in passenger electric vehicles (EVs) for both consumers and automakers, but it is unclear that cost alone can trigger the mass adoption of electric vehicles. At an average efficiency of 3.5 miles/kWh (Helms et al., 2010), an optimistic target gravimetric pack energy density of 300 Wh/kg (Wood et al., 2015), and a final pack cost of $100/kWh (Ciez & Whitacre, 2016), the average cost of a battery pack for a 400-mile vehicle would be over $11,000 and the average mass over 380 kg. The average daily distance driven is under 50 miles, and most drivers feel that their “range anxiety” is alleviated with a capacity of 200 miles (Neubauer & Wood, 2014). This indicates that except for a rare all-day drive, the typical 400-mile EV battery would be a factor of two to eight times too expensive and massive, adding a cost of $4,000-$7,000 and an excess mass of over 100 kg.
If a 200-mile battery pack could be recharged as fast as the refueling of a petrol-fueled car, this might obviate the need for a 400-mile battery pack. For the average driver making few all-day (400-mile) drives per year, this would simply add one extra recharge stop to the full-day trip. But even with this compromise, there is a paradox. The energy density and cost profile assumes a battery designed to be discharged no faster than in three hours (200 miles/60 mph = 3.33 hours). The rate at which a battery can be safely discharged is the rate at which a battery can be safely charged, so this would indicate that an all-day trip requires two rest stops of three hours per day to fully recover all 200 miles of the battery, or an average recovery rate of one mile per minute of charge. In comparison, petrol refueling provides a range recovery of 100 miles per minute of charge.
For a traditional battery to enable the charge acceptance required to achieve even a recovery rate of 20 miles per minute of charge, the energy density of the battery would likely halve, and the cost would increase by at least a factor of two due to the thinner electrodes required per unit of current collector. And thus the battery capable of an ambient-temperature fast charge costs no less and weighs no less than a battery capable of driving 400 miles. So there would appear to be a fundamental zero gain in the engineering paradigm of practically trading cost for speed.
In the past year of this CMI effort, the Steingart group has been experimentally verifying and dissecting the aforementioned paradox, and they have confirmed this to be true for batteries held in an isothermal condition. But with further experimentation, they feel that there may be a practical methodology for recovery at 10 to 15 miles per minute of charge on a standard high energy-density pack design without the need for significant extra equipment. The key variable is temperature.
In a lithium ion battery, the most significant limitation to the safe and non-destructive recharge operation is the fundamental diffusion behavior of the slowest transport in the cell: the diffusivity of lithium in the solid state. And upon recharge, the diffusion limitation of the typical graphite anode can lead to the unwanted deposition of lithium metal atop the electrode, as opposed to the intercalation of lithium within the electrode (Figure 9.1). This is a lithium cobalt oxide cathode, graphite anode cell with an ethylene carbonate/dimethyl carbonate (EC/DMC) blend electrolyte utilizing lithium hexafluorophosphate (LiPF6). The cell is not designed to store lithium metal—and lithium, being the most reducing compound in existence, will react with its surrounds, at best significantly reducing the capacity of the battery and at worst creating a fire.
The standard method to increase the diffusion rate of a material is simply to raise its temperature, as:
D(T) = D0e(-EA/RT)
If only it were so simple. Within EV batteries, temperature is well controlled and maintained near 25 ̊C because batteries are designed in labs at 25 ̊C, and temperature excursion, high and low, can lead to unwanted behavior. Significant temperature excursions above 90 ̊C can lead to auto-catalytic thermal runaway of the cathode and subsequent ignition of the flammable organic carbonate solvents of the electrolyte (Koch et al., 2018). Slight temperature increases can trigger/enhance non-faradaic side reactions (Gyenes et al., 2015) within the cell, again following Arrhenius behavior
k(T) = Ae(-EA/RT)
Thus it would seem that the damage present in allowing faster lithium transport at higher temperatures would be negated by the damage incurred by the increase in temperature itself.
The Steingart group believes, however, that they have found a window of operation where all of the competing factors above are simultaneously true yet a battery can be recharged quickly without incurring significant damage. Their hypothesis: if a cell is heated to a moderate temperature (40 ̊C to 50 ̊C) for a short period of time (20 minutes or less) and the cell is not held at high potential during this period of time, it can be quickly charge (10 minutes to 80% charge recovery) without incurring damage. This means that if a battery management system raises the temperature of the cell but avoids the constant voltage step, a cell can be charged at a fast rate while avoid voltage-driven degradation.
This is in contrast to traditional charging protocols, known as constant current, constant voltage (CCCV), which hold the cell near room temperature, but charge to a cutoff voltage at constant current, then hold at that constant voltage until a certain time passes or the current drops below given value. The CCCV protocol is designed to preserve the life of the cell while completely charging the battery. Our constant current, high temperature protocol (CCHT) preserves the life of the cell while charging the cell to 80% capacity. We trade maximum range for a fast effective range recovery in this protocol.
In the past year the Steingart group has collected significant evidence to support this hypothesis. The first is the determination of the change in diffusion coefficient as a function of state of charge and state of health. Figure 9.2 indicates that the diffusivity of a given cell (in this case, graphite vs. LCO in LiPF6:EC:DMC) increases by up to two orders of magnitude by raising its temperature from 20 ̊C to 40 ̊C, and that after 1,000 cycles without a high voltage constant-voltage step, the capacity of the cell moderately degrades (less than 10%) from the fresh case at a temperature as high as 60 ̊C.
This behavior was consistent and reproducible for over 100 cells, which included three different chemistries and three different form factors (210 mAh pouch, 1500 mAh pouch, and 2,400 mAh cylindrical).
Figure 9.3 illustrates a comprehensive relationship. When a given cell type, in this case the 2,400 mAh cell mentioned above, is metered on its total throughput, the protocol we have determined stands out.
The cells cycled at 60 ̊C without a constant voltage retain over 90% of their initial capacity at charge rates faster than one hour (as fast as 15 minutes) for over 1,000 cycles.
This is by no means authoritative or complete. Further ex-situ chemical analysis needs to be completed to study other forms of damage that might have occurred but have not yet presented as capacity fade. The implementation of this protocol across an entire pack of cells may be intractable. And the efficacy of this protocol on higher nickel content cathode needs to be confirmed.
From this study, however, the Steingart group is encouraged to further examine and probe the capability of fast charging a cell designed for a three-hour-plus discharge by tuning its charge temperature, and perhaps exploring electrode and cell engineering improvements that might exploit this asymmetric treatment. With further study and analysis, this may be a practical method for solving the paradox of a low-cost, high energy-density, 200-mile battery that can be recharged periodically at a rate recovery of over 15 miles per minute charged.
This work was done by Clem Bommier, funded by the CMI, and Andrew Kim, funded through the ACEE by American Tower. Andrew Kim was exploring damage as a function of environmental conditions such as temperature and humidity.
Ciez, R.E. and Whitacre, J.F., 2016. The cost of lithium is unlikely to upend the price of Li-ion storage systems. Journal of Power Sources, 320: 310–313. doi.org/10.1016/j.jpowsour.2016.04.073.
Gyenes, B., Stevens, D.A., Chevrier, V.L., and Dahn, J.R., 2015. Understanding anomalous behavior in coulombic efficiency measurements on Li-ion batteries. Journal of the Electrochemical Society, 162(3): A278–A283. doi.org/10.1149/2.0191503jes.
Helms, H., Pehnt, M., Lambrecht, U., and Liebich, A., 2010. Electric vehicle and plug-in hybrid energy efficiency and life cycle emissions. 18th International Symposium.
Koch, S., Fill, A., and Birke, K.P., 2018. Comprehensive gas analysis on large scale automotive lithium-ion cells in thermal runaway. Journal of Power Sources, 398: 106–112. doi.org/10.1016/j.jpowsour.2018.07.051.
Neubauer, J. and Wood, E., 2014. The impact of range anxiety and home, workplace, and public charging infrastructure on simulated battery electric vehicle lifetime utility. Journal of Power Sources, 257: 12–20. doi.org/10.1016/j.jpowsour.2014.01.075.
Wood, D.L., III, Li, J., and Daniel, C., 2015. Prospects for reducing the processing cost of lithium ion batteries. Journal of Power Sources, 275: 234–242. doi.org/10.1016/j.jpowsour.2014.11.019.