Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2014 Symposium (2015)

Challenges in Batteries for Electric Vehicles

SARAH STEWART, JAKE CHRISTENSEN,
NALIN CHATURVEDI, AND ALEKSANDAR KOJIC
Robert Bosch Research & Technology Center North America

There are many reasons why research and development in electric vehicles (EVs) is important. The world needs to reduce its production of greenhouse gases and decrease its dependence on oil (DOE 2014, Energy.gov 2014). The market for electrified vehicles is growing rapidly—Bosch predicts 12 million electrified cars by 2020 (Greimel 2013). And market research suggests that if the United States wants to compete in the future auto industry it will need to become a leader in lithium ion batteries, currently the most promising type of battery for EVs because of their high energy density (Lowe et al. 2010).

By the year 2020, 7 percent of vehicles sold worldwide may be electric (including hybrids) (Hurst and Gartner 2013). The increased use of such vehicles instead of internal combustion engines could reduce greenhouse gas emissions and dependence on oil. Recent improvements in the cost and energy density of lithium ion batteries have provided electric vehicles with a range of more than 265 miles (DOE 2014), but high initial costs limit mass market acceptance.

This paper presents current challenges and recent advances in lithium-ion batteries, and options for making electrified vehicles a more cost-effective choice.

INTRODUCTION

Electrified vehicles are battery powered either entirely (EVs) or partially (plug-in hybrid electric vehicles, PHEVs), but the batteries are expensive ($300–500/kWh). The US Department of Energy (DOE) has therefore called for a reduction in battery cost to $125/kWh by the year 2022, a target that should enable 5-year cost of ownership parity between an internal combustion vehicle and an EV (DOE 2013).

Cost reduction can be accomplished by reducing material and manufacturing costs and/or by increasing energy density (watt hours per kilogram, Wh/kg). We estimate that lithium ion battery energy density can be doubled or tripled through the discovery of new materials and designs; for example, other chemistries, such as lithium air, may offer as much as a fivefold increase in energy density if technical challenges can be overcome.

If the battery industry can hit the DOE cost target before 2020, then a significant portion of the new vehicle market could become electric. If one assumes that most electric vehicles use 100 kWh battery packs (which should support about 400 miles of driving range—a typical distance between refueling for a vehicle powered by an internal-combustion engine), then the potential battery market is $1.4 trillion dollars.

CURRENT CHALLENGES IN EV BATTERIES

In order to achieve greater market penetration, the cost of EV batteries needs to come down. This needed cost reduction will require research and development into new materials.

Cost

Cost is the biggest challenge for EV batteries. Tesla hopes to reduce the cost of a battery pack to less than $210/kWh through economies of scale enabled by its planned “gigafactory,” which is expected to double worldwide lithium ion battery production (Economist 2014).

But further cost reduction is needed to reach the DOE goal. The most promising way to achieve it is by increasing the energy content of the active materials (commonly measured in Wh/kg). The highest-energy lithium ion batteries are now about 250 Wh/kg at the cell level. We estimate that a doubling of energy density is needed to meet the DOE’s cost goal.

Battery Materials: Availability and Chemistry

Significant increases in battery energy density will likely require a disruptive technology involving a lithium anode. We briefly review the materials used in lithium ion cells because these determine energy storage capacity.

Lithium is the primary component of EV batteries, and some people are concerned that there is not enough of it to supply all the batteries needed to fuel transportation. But a study from UC Berkeley into resource availability in 2011 (Wadia et al. 2011) concluded that there was sufficient lithium to replace about 10 percent of the global vehicle fleet of passenger vehicles. The paper shows that battery production is more constrained by the availability of cobalt (often used in cathode materials) than by lithium.

In addition, researchers at the University of Michigan and Ford Motor Company looked into world lithium deposits and concluded that even with rapid adoption of electric vehicles there is enough lithium for the rest of the century (Gruber et al. 2011). They also point out that additional lithium deposits are likely to be discovered. The authors nonetheless encourage responsible use of lithium—although there is enough for the next several decades, the industry will need to conserve this resource.

A typical lithium ion battery stores energy by moving lithium ions from a mixed metal oxide positive electrode (e.g., LiCoO₂, LiMnO₂) to a negative electrode (LiC₆) during charge. When the battery is discharged, lithium ions change direction and move from the graphite to the metal oxide electrode. One can think of charging a lithium ion battery as analogous to storing potential energy as water is moved uphill: when the water is released, it produces work—as the lithium ions do when moving from anode to cathode in a battery being discharged.

To increase the specific energy of the battery (SE, in Wh/kg), one can increase the amount of lithium that can be stored in the electrode materials (the coulombic capacity, C, in Ah/kg) and/or the battery voltage (V), as expressed in Equations 1 and 2:

Table 1 summarizes the coulombic capacities, voltages, and energy densities of some materials of interest. The table shows that replacing a conventional graphitic anode with silicon or lithium increases the anode’s capacity by roughly an order of magnitude. At the practical cell level this translates into a 25–50 percent decrease in total mass when using a conventional cathode material. Using a lithium anode and replacing the cathode with high-energy nickel cobalt manganese (HE-NCM), sulfur, or air results in a theoretical specific energy and energy density that far surpass those of the currently used graphite-NCA (from 3,500 to as much as 10,493 Wh/L).

The new materials, however, come with big challenges. In the negative electrodes, silicon has a high capacity but it experiences large volume changes (~300 percent) during lithiation/delithiation, which leads to rapid capacity fade (BATT 2014). Nanostructured silicon is being explored as a way to manage the volume change, but it is a challenge to achieve high electrode mass loading and volumetric capacity when packing nanostructures in an electrode (Kim et al. 2014).

Using a lithium (Li) negative electrode results in a higher cell voltage and reduces mass significantly, enabling a higher cell-level energy density, but lithium metal has three primary challenges: low electrochemical potential, morphology changes, and dendrite formation. The low potential causes electrolyte decomposition; hence, it is difficult to find a good electrolyte to use. Morphology changes

TABLE 1 Properties of Some Lithium Battery Electrode Materials

Specific energy, density, and volumetric energy density are provided for positive electrode combinations with lithium metal. HE-NCM=high-energy nickel cobalt manganese; NCA=nickel cobalt aluminum.

cause unstable passivation of the electrode and solvent dry-out via continuous solvent decomposition. And lithium dendrites can form and grow through the separator, posing an electrical shorting risk.

Strategies to promote stable cycling of lithium electrodes involve polymer or ceramic solid electrolytes, novel liquid electrolytes (solvents, solids, and additives), and alloying. Each has challenges (Woodford et al. 2012). The solid electrolytes have slow Li⁺ transport and limited chemical and mechanical stability, and the new liquid electrolytes still have significant side reaction rates. Alloy anodes are challenged by large volume changes during cycling that disrupt the solid-electrolyte interphase (passivation layer) and lead to continued reduction of the electrolyte (Woodford et al. 2012). Some of the proposed approaches work well for low power applications (<1 mA/cm²), but not at higher current densities.

Battery vs. Internal Combustion Efficiency

Taking into account the source of the energy used to fuel an electrified car, is a battery really more energy efficient than an internal combustion engine (ICE)? The answer depends on location. A Tesla Motors emissions calculator shows that charging an EV in California has much less carbon impact (i.e., release of carbon dioxide) than an ICE car because the state’s grid generates more than one-half of its energy from natural gas (Tesla 2014). But in states where coal is used predominantly to power the grid, the carbon impact of EVs may be comparable to that of ICE vehicles. As power plants become modernized with more renewable sources of energy, EV-associated emissions will decrease further.

USE OF MODELS TO REDUCE COSTS

Big challenges must be overcome to enable battery technologies such as lithium sulfur and lithium air, but in the meantime improvements can be made in how current technologies are utilized. Physics-based models, for example, can enable more efficient battery use and reduced charge times. They are also useful to optimize the design of the cell and pack (e.g., to retain its energy storage capability while making it smaller and reducing its weight), to understand limitations and failure modes so that they can be avoided, to save money on testing, and to quickly understand the impact of new chemistries.

Battery management systems (BMS) are used to monitor and control batteries in EVs. A well-designed BMS will keep the battery in a safe operating region (e.g., ensuring that it is not overcharged, overdischarged, charged too quickly). Current BMS typically simplify the complex physics inside a battery by assuming that it is a simple RC circuit (a combination of resistors and capacitors) and using only externally available measurements (current, voltage, and temperature) for control. We have developed an approach that uses a physics-based model to predict the internal states in the battery and thus are able to extend the operational region

(Chaturvedi et al. 2010). By increasing the envelope of battery operation, more of the battery is utilized, and it is used more efficiently. This approach is expected to significantly reduce the cost of batteries as well as typical charging times.

SUMMARY

There are some short-term hurdles to overcome (e.g., powering the grid with more renewable energy, reducing ancillary loads/parasitic current draws in electric cars), but trends indicate that developments in energy resources support the likelihood that electric vehicles will be a significant part of the world’s transportation future (Oremus 2013).

Automotive batteries have a huge potential market. New chemistries are needed to achieve significant penetration in the EV market, but they are challenging and will take more years of research. In the meantime, conventional chemistries can be used more efficiently via advanced battery management software.

REFERENCES

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Chaturvedi NA, Klein R, Christensen J, Ahmed J, Kojic A. 2010. Algorithms for advanced battery-management systems. IEEE Control Systems 30(3):50–68.

DOE [US Department of Energy]. 2013. EV Everywhere: Grand Challenge Blueprint. Available at http://energy.gov/sites/prod/files/2014/02/f8/eveverywhere_blueprint.pdf.

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Economist. 2014. Elon Musk’s assault on batteries relies on a big gamble. Business Insider, June 15. Available at www.businessinsider.com/elon-musks-assault-on-batteries-2014-6.

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Gruber PW, Medina PA, Keoleian GA, Kesler SE, Everson MP, Wallington TJ. 2011. 2011. Global lithium availability: A contraint for electric vehicles? Journal of Industrial Ecology 15(5):760–775.

Hurst D, Gartner R. 2013. Electric Vehicle Market Forecasts. Navigant Research.

Kim H, Lee E-J, Sun Y-K. 2014. Recent advances in the Si-based nanocomposite materials as high capacity anode materials for lithium ion batteries. Materials Today 17(6):285–297.

Lowe M, Tokuoka S, Trigg T, Gereffi G. 2010. Lithium-ion Batteries for Electric Vehicles: The US Value Chain. Durham: Duke University Center on Globalization, Governance & Competitiveness.

Oremus W. 2013. How green is a Tesla, really? Slate, September 9. Available at www.slate.com/articles/technology/technology/2013/09/how_green_is_a_tesla_electric_cars_environmental_impact_depends_on_where.html.

Tesla Motors. 2014. Go Electric. Available at www.teslamotors.com/goelectric#electricity.

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