Twenty Years After the EV1, What’s Happening with EV Batteries?

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by Paul Pickering, Technical Contributor

 

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2016 marks twenty years since the introduction of General Motors’ iconic EV1, the first mass-produced purpose-designed electric vehicle (EV) from a major automaker. By the time production ceased in 1999, it included a 102 kW (134 hp) motor and a 26.4 kW battery—specifications that compare well today. After the EV1, though, manufacturers abandoned EVs in favor of an interim solution: hybrid vehicles, which combine a conventional internal combustion engine with an electric motor in order to increase fuel economy.

Recently, there’s been a resurgence in pure electric vehicles with the Nissan Leaf, Chevrolet Spark, and others garnering attention. The viability of an EV as a transportation option is intimately related to the performance of its battery, but how far has battery technology advanced since the EV1 went away?

No Moore’s Law For Batteries

Sadly, the answer is “not very far.” Sales of pure electric vehicles continue to be hobbled by the lack of progress in battery technology. One plus for EVs: the efficiency of an electric motor is far greater than that of an internal combustion engine. The latest Toyota Prius claims an efficiency of 44 percent, but a traditional gasoline-powered vehicle averages 25 percent efficiency and dissipates the other 75 percent through friction and heat. Pure electric cars, however, have an average of 80 percent efficiency and some get close to 95 percent.

Figure 1: some popular Evs and a comparison of their battery performance. (Source: Vehicle Manufacturers)

Figure 1 shows a comparison of some EV vehicle specifications. Li-Ion batteries dominate. They have higher energy densities than lead-acid or nickel-metal hydride (NiMH) batteries, resulting in a compact and lightweight battery. Although Li-Ion energy density has almost doubled since 2000, EVs are far from matching the range of gasoline-powered vehicles. Gasoline is extremely energy dense—around 12,000 Wh/kg. In comparison, current Li-Ion batteries can store about 250 Wh/kg without including the weight of equipment such as the battery cooling system.

Even considering that EVs are three times more efficient, the energy density of batteries must improve by a factor of 20 to give a comparable range. The existing range of fewer than 100 miles has spawned a new term “range anxiety”—the fear that an EV has insufficient range to reach its destination and will, therefore, strand the vehicle’s occupants.

The Tesla Model S 70, while not comparable in range to a traditional vehicle, causes much less range anxiety. Unfortunately, it also comes with a starting price of $70,000. While we wait for the Tesla Model 3 with its 215-mile range and $35,000 MSRP, what are designers doing to improve Li-Ion battery performance?

Improving Energy Density

Improving energy density is the subject of intensive research. One avenue being explored is to tweak the cell chemistry by adding silicon to the anodes, which are primarily made of graphite at present. Silicon allows the anode to absorb more lithium ions, but in doing so, its volume increases by as much as 300 percent, compared to about 7 percent for graphite. The repeated expansion and contraction during charging and discharging drastically reduces the battery life. Possible solutions include better anode additives and changes to the electrolyte formulation.

EV manufacturers are inching ahead anyway. Tesla recently announced a 90 kWh battery pack with silicon-graphite anodes and is targeting 5 percent per year improvement in energy density by increasing the silicon content.

Charging and Discharging Performance

Another stubborn issue is degradation of Li-Ion battery capacity with repeated charging—no surprise to anybody who owns a smartphone. Researchers at several national DoE laboratories have studied the process with a transmission electron microscope (TEM), which passes a beam of electrons through a sample hundreds of nanometers thick. Their results show that reductions in capacity occur both during charging and discharging and are caused by two different processes.

Figure 2: ion flow in a Li-Ion battery. (Source: batteryuniversity.com)

In a Li-Ion battery, lithium ions move from the cathode to anode during charging and in the reverse direction during discharging via a non-aqueous electrolyte. Both processes cause irreversible changes in the electrodes. During charging, the ions cause crystallization along the Li-ion reaction channels, which reduces capacity. A higher charging voltage, so-called “supercharging,” reduces charging time but also causes more rapid degradation.

During discharging, a different process occurs: the ions cause minute breaks in the anode material, reducing its capacity. These findings point the way to possible solutions—depositing coatings on the cathode that resist crystallization, for example—but it’s expected to be years before production-ready batteries make their appearance.

Alternative Battery Chemistries

Longer term, much work is focusing on alternatives to Li-Ion, notably chemistries that replace the anode with atmospheric air. A Li-Ion battery moves ions between the anode and cathode; they then reside between the atomic layers of the electrode, so the battery capacity is a function of electrode volume. In contrast, the reaction in a lithium-air battery occurs on the surface of the electrode between the ions and oxygen, forming lithium peroxide (Li2O2) during discharging. Recharging reverses the reaction. The battery capacity, therefore, depends on the surface area of the electrode, so even a very light electrode can exhibit high energy density.

Racing Battery Development

“Racing improves the breed,” as the saying goes. How is that working for batteries? One of the primary goals of the Formula-E all-electric racing series is to improve EV technology by transferring knowledge from the track to the highway. To encourage innovation, the plan is gradually to relax the rules during successive seasons and allow more variation between vehicles. In season one, all cars were identical; for season two, teams have developed their own e-motor, gearbox, and inverter.

Figure 3: the Williams Formula E battery. (Source: fiaformuale.com )

Battery-wise, all cars must use a standard battery pack from Williams Advanced Engineering, which consists of a carbon-fiber composite housing containing more than 150 off-the-shelf Li-ion polymer cells. It weighs 320 kg (705 lbs), a third of the minimum allowed weight of 896 kg (1975 lbs).

The battery has a maximum output of 170 kW during a race and 200 kW during qualifying. Although it has a total energy capacity of 28 kWh (sufficient to power an average U.S. home for a day) it’s not enough for a race, so drivers must stop midway to change cars. The goal is to eliminate the car swap, so the FIA, Formula E’s governing body, has started looking for a new sole battery supplier for season five with a requirement of 54 kWh of usable energy and a maximum power of 250 kW. At least three suppliers, including Williams, are expected to bid.

How do the Tesla and Formula E batteries stack up? The two batteries must meet different requirements, but on a basic comparison, the Tesla battery comes out ahead with an energy density of 140 Wh/kg versus Formula E’s 80 Wh/kg. The maximum tested output is 691 W/kg, against 625 W/kg for Formula E.

Perhaps in this case, the technology transfer will be in the opposite direction!