2017 has been proclaimed the year of the electric car revolution. Mass-market electric vehicles (EVs) have reached the 300+ mile range and have garnered support by governments worldwide. Much of this progress stems from advancements in Li-ion battery manufacturing. A deeper look at the metrics and economics of batteries can provide clues on why the EV market is taking off.
The “storage capacity” of a battery is usually reported in units of kiloWatt-hours (kWh). EVs are designed to provide 3-6 hours of steady driving (200-400 miles), but this depends on the power demands, which can vary during the drive. The “energy density” of a battery is another performance metric. Measured in units of Watt-hours per kilogram (Wh/kg), energy density is especially helpful when comparing batteries of different shapes and sizes. A “battery pack” is composed of individual battery “cells” together with the “pack infrastructure” that includes structural components, wiring, cooling loops and power electronics.
Energy is stored only in the cells, while its safe use is enabled by the pack infrastructure. As a result, the pack has half the energy density of the cells, at a price that is typically 25-50% higher per kWh than the price of the cells alone. These factors need to be taken into account when reporting cell or pack values. For example, the estimated pack level energy density of the Tesla Model 3 is ~150 Wh/kg, while the energy density of the individual cells used in the pack, as reported by Panasonic, is ~265 Wh/kg.
Each battery cell contains energy storing cathode and anode materials as well as inactive non-energy storing parts (membrane, current collectors, shell). The “theoretical energy density” of a battery cell is defined by the capacity of cathode and anode materials and the voltage difference between them (“cell voltage”). Some currently available chemistries have energy densities of over 600 Wh/kg. While the gap between the theoretical and actual cell energy density can never be closed, it is gradually shrinking through optimization of cell packing and manufacturing techniques.
Earlier models of fully electric EV battery packs were limited to low storage capacities: Nissan Leaf (24-30 kWh), Chevy Spark 2014 (19-20 kWh), Honda Clarity EV (25 kWh), Ford Focus Electric (23 kWh), Fiat 500e (24 kWh), Smart ForTwo Electric Drive (17 kWh), and the Mitsubishi i-MiEV(20 kWh), Kia Soul EV (27 kWh), Hyundai Ionic (27 kWh), VW E-Golf (36 kWh), BMW i3 (33 kWh), Mercedes E-drive (28 kWh) and Tesla Roadster (56 kWh) all provided ~100±25 miles range per single charge. But progress in the ergonomics of battery pack infrastructure has increased the energy density of individual battery cells, and the storage capacity of EV packs has recently tripled for nearly the same pack volume, enabling ranges of over 300 miles per charge. The Tesla S is now offered with 60 kWh - 100 kWh packs, for 208 - 336 EPA rated miles. It was even demonstrated that a Tesla S with a 100 kWh pack could last for 669.8 miles/charge if driven continuously at 29 miles/hour.
Currently, the two mass-market electric cars with over 200 mile range are the Tesla Model 3 and the Chevy Bolt, with many more to come over the next 5 years. The Tesla Model 3 offers 50 kWh and 75 kWh packs for ranges of 220 and 310 EPA miles respectively. The Chevy Bolt is offered with a 75 kWh pack that provides 238 EPA miles. The energy efficiency in terms of kWh/100 miles, achieved through “lightweight design and lowest in the industry drag coefficient”, is better for the Tesla 3 (22.7-23.8) than for the Bolt (31.5). This difference results in a lower electricity cost per 100 miles for the end user.
Another important characteristic of battery packs is their price. Both Tesla Model 3 and Chevy Bolt are offered at comparable starting prices of $35,000 and $37,495. Usually automakers don’t release figures on battery pack cost, but some information can be extracted from the pricing of short and long range Tesla 3 models. An extra 25 kWh storage for the long range model (75 kWh) translates to a $9,000 price jump or $360 per kWh of energy storage. The price of a full 50 kWh pack for Tesla 3 can be estimated from the cost of the add-on module including a 10-20% margin to be $18,000-22,000, i.e. more than 50% of the total vehicle price. The difference in the curb weight between the standard and the long range models is 265 lbs (120 kg) indicating that the energy density of add-on modules is ~208 Wh/kg. Energy density and price per kWh of add-on modules could be different from those of the total pack, but it is still an impressive advancement from the reported energy density of earlier models (130 Wh/kg). This improvement is enabled by innovation in pack design along with increased energy density of the cells.
This raises another question that is rarely answered - will the EV battery last the entire life of the car? Tesla owners on the Dutch-Belgium Tesla Forum are gathering data from 286 Tesla Model S owners across the world and frequently updating it in a public Google file. The data clearly shows that most Tesla battery packs lose about 5% of their capacity in the first 50,000 miles (or ~200 charge/discharge cycles) and then level off at 90% of initial capacity at about 150,000 miles or ~600 cycles. Similarly, Tesla has reported that a battery pack with a simulated 2,000 cycles (500,000 miles) performed at 80% of the original pack capacity. Use of high rate charging accelerates the degradation of the battery. While the number of cycles per battery life (“cycle life”) is an important metric for comparing EV batteries, current batteries could easily outlast some of the other vehicle components. Nonetheless, cycle life is an important factor for alternative EV applications, such as vehicle-to-grid storage scenarios. For example, if a pack’s initial cost is $400/kWh and its cycle life is 1,000 cycles, the levelized cost of energy storage will be $0.40 per kWh - much higher than the cost of electricity from the grid. However, if the cycle life is extended to 5,000-10,000 cycles, this cost would drop significantly, making electrical storage competitive with traditional means of delivery and providing new added value to EV batteries.
With the shift towards electrification of cars and the optimistic prognosis for the future of EVs, automakers are racing to gain expertise and to invest in all the components of electric vehicles, conducting in-house development for electric motors, drivetrains and battery cell manufacturing. Analysts expect that most automakers will focus on making battery systems in-house, partly to offset the potential loss of thousands of powertrain assembly jobs, but also to better differentiate their vehicles from those of competitors through control of electric power delivery. Currently, partnerships between automakers and battery cell manufacturers play important roles as they build in-house expertise.
Some smaller automakers are considering off-the-shelf vehicle platform solutions, which could be an essential part of future EVs. Williams Advanced Engineering, the technology offshoot of the Williams Formula 1 team, has revealed its own electric vehicle platform called the FW-EVX. The platform includes numerous innovations, such as reinforced suspension components weighing 40 percent less than traditional aluminum wishbones, and an exoskeleton for the battery module that contributes to the platform's structural performance.
Batteries are also competing with other energy storage technologies. Toyota’s vision of the future includes a broad portfolio of hybrid-electric vehicles, battery powered plug-ins and fuel-cell cars and trucks. BMW have previously shown an electric sports car (i5 and i8 series Gran Turismo) equipped with a combination of batteries and fuel cells, which can cover over 300 miles. GM has recently announced that arriving at a “zero emissions future” will require a “two-pronged approach: battery electric and hydrogen fuel cell electric vehicles”. Rational diversification of energy technologies is due to the lack of major breakthroughs in the battery space. While it has been demonstrated that incremental improvements to Li-ion batteries is sufficient to build a competitive electric vehicle, the performance, costs and cycle life of batteries will require further advancements.
Contributors are Elena V. Timofeeva, Carlo. U. Segre and John P. Katsoudas.