Efficient Battery Heating, Not Cooling, is Key for Future Electric Vehicles
Researchers at the Penn State Electrochemical Engine Center, in collaboration with EC Power — the maker of the “All-Climate Battery,” — published an article in Nature Energy that puts forward a battery design which simultaneously alleviates the dreaded “range anxiety,” is intrinsically safe, delivers sporty performance metrics, and, -and this is the big and,- AND, could bring the price of electric vehicles below price-parity with internal combustion engine cars (that is, off-the-lot price parity as electric vehicles are already reported to have a lower lifetime cost). The key, they say, is high temperature resilient components and efficient heating so that the battery can be operated at 60°C.
The largest bane for lithium-ion batteries has always been safety. While thermal runaway events and cell-to-cell propagating failures are relatively rare for lithium-ion batteries, the vast amount of energy released in these events and the difficulty to extinguish them once they begin has made them notorious. Combined with the common knowledge that higher battery temperatures leads to more rapid capacity fade, this has engendered an intense focus on battery cooling systems to remove the waste heat from the battery during driving of electric vehicles, generally ensuring lithium-ion batteries never go above 45°C while attempting to maintain a temperature of 30°C or below. This stands in stark contrast to the operational scheme proposed for this novel battery.
The Penn State-EC Power team, led by Prof. Chao-Yang Wang, proposes the use of a relatively small (~50 kWh) battery pack with cells composed of lithium iron phosphate cathodes and low surface area graphite anodes. It contains resistive heaters internal to the cells, which are packaged in the fashion being popularized by BYD as the “blade” battery. The blade battery is comprised of elongated footprint, high-capacity cells which are used as structural components within the pack to give extremely high volumetric and gravimetric cell-to-pack ratios; nearly double that of a traditional battery pack.
Lithium iron phosphate sacrifices cell-level specific energy due to its lower voltage and lower gravimetric capacity, but it is the safest and least expensive cathode for lithium-ion batteries, already enabling sub $100/kWh battery packs, while the industry average remains above $130/kWh. It maintains the greatest resiliency to high temperatures, alleviating concerns of thermal runaway, which allows the blade battery configuration. The blade battery configuration does not incorporate any active cooling mechanism.
The low surface area graphite anode increases charge transfer resistance in the battery. Increasing the charge transfer resistance may seem counter-intuitive, but it serves to increase overall safety level of the battery, and critically, it significantly increases the calendar life of the battery and improves high temperature resiliency. These positive effects occur because the majority of the time+temperature degradation of the battery occurs via parasitic reactions at the graphite-electrolyte interface.
The resistive heater that is internal to the cell, which was published in Nature in 2016, provides an efficient and cost-effective (both from an $$ perspective and pack level specific energy perspective) method of providing heat directly to the electrochemical components. Past research works and vehicle demonstrations have shown that this route to internal heating allows the battery to self-heat from sub-zero temperatures up to the desired operating temperature at rates higher than 1°C/s and at nearly 100% of the theoretical efficiency for heating only the electrochemical components (not the surrounding mass).
The operating principle behind the proposed battery is that while the car is not in operation, the battery is essentially “turned off” given the low-level of activity, but upon initiating operation, the battery automatically self-heats (or charger-heats if still attached) to 60°C. The temperature is then sustained via normal waste heat from operation. Every component within the battery experiences a large drop in resistance at these temperatures, ultimately leading to a peak power of 340 kW for a 50 kWh pack, representing a whopping 455 horsepower (the author is not a powertrain expert, or even laymen, so how much of that 455 is actually transferred, I do not know). Combining that horsepower with the lighter weight of the proposed 50 kWh (360 kg) battery could lead to economy car acceleration comparable to that of a high-end sports car (Porsche Taycan Turbo S’ sports 616 hp, from a 630 kg battery). In fact, due to the temperature independence of the power output from the proposed battery, at the outset of driving on a cold winter morning, we should expect higher acceleration rates from the economy car! Quite the impressive feat from a battery that is built from the cheapest components available and projected to survive over a million miles of driving.
The impressive power output also means an equally impressive charging rate. Even with the lower surface area graphite, calibrated models are used to demonstrate that this battery can safely (no lithium-plating) support a pure constant voltage charge (as opposed to the industry standard Constant Current-Constant Voltage charge technique) without fear of lithium plating, providing an 80% charge in 10.3 minutes. As was presented by Dr. Halle Cheeseman of ARPA-E, fast-charging with a small battery pack beats conventional charging of a large battery pack in terms of combatting range anxiety. Ultimately, the smaller battery pack makes the car more affordable, is more appropriately sized for everyday driving, and when combined with fast charging, is compatible with the occasional road trip.
High temperature battery design and efficient heating present an opportunity for an economical, high performance, electric vehicle. The battery industry knows the time is coming for high-temperature electric vehicle battery operation. All-solid-state batteries, which promise superior energy density to today’s state of the art, are widely considered the next big step forward in battery technology. Solid-state batteries will inherently have improved resilience to high temperatures as the issue in current state-of-the-art systems largely stems from the electrolyte and separator; both of which will be replaced with a lithium-ion conducting solid. Moreover, solid-state batteries are expected to require higher temperatures for operation due to the higher resistance of the solid electrolytes. Therefore, it is understood that solid-state batteries will flip the battery thermal management paradigm on its head, favoring heating over cooling. But while this technology is still several years away from mass adoption, it seems the time for EV battery heating, rather than cooling, may have already arrived.