While electric car batteries are evolving, their energy density is still lower than traditional fuels
The increasing global fleet of electric cars, which is expected to account for 16% of the total number of automobiles on the planet by 2030, is generating a rush to research and develop new batteries to power them. A study by investment bank Goldman Sachs revealed that global demand for this type of battery is expected to reach US$40 billion per year by 2025. The challenge is to develop a cheaper, more durable, safer model that is capable of storing more energy, thus increasing the distance electric vehicles can travel without needing to recharge (see Pesquisa FAPESP, issue No. 258). Lithium-ion batteries are considered the state-of-the-art technology in the sector, allowing motorists to travel an average of 250 kilometers before they need to recharge. The aim is to double this distance, giving electric vehicles the same range as cars powered by fossil fuels or ethanol.
“The efforts made by battery manufacturers, the automotive industry, and research centers in recent years have produced batteries with higher energy densities [the amount of energy stored in a given mass or volume],” says electrical engineer Raul Beck, coordinator of the Electric and Hybrid Vehicles Technical Commission at the Automotive Engineering Society (SAE Brasil), and head of the Energy Systems Department at the Telecommunications Research and Development Center (CPqD) in Campinas, São Paulo.
Despite this, even the most advanced models are still a long way from offering the same energy density as ethanol or gasoline. While lithium cells store about 690 watt-hours (Wh) per liter, 1 liter of hydrous ethanol provides approximately 6,260 Wh of energy, and 1 liter of gasoline provides about 8,890 Wh. “These figures show that the energy contained in 1 liter of ethanol or gasoline is much higher than that stored in a battery with a volume of 1 liter,” says physicist and energy expert José Goldemberg, professor emeritus at the University of São Paulo (USP) and president of FAPESP. Almost 13 liters of battery would be needed to replace 1 liter of gasoline.
Gasoline and ethanol also perform better than batteries—although to a lesser extent—in terms of energy conversion rate for powering the wheels of the vehicle and the space that each energy system occupies in the car (fuel tank, hoses, pipes etc., for gasoline and ethanol; battery packs and cooling and ventilation systems for batteries). “The energy conversion efficiency of a lithium battery is around 90%, meaning 1 liter of battery volume provides the wheels of the vehicle with approximately 430 Wh,” says Beck, from the CPqD. “The energy conversion efficiency of gasoline and ethanol is much lower, at around 20%, but even so, 1 liter of gasoline provides the wheels with 1,420 Wh, while 1 liter of ethanol provides 1,000 Wh. This calculation is based on lithium cells occupying 70% of the total volume of the battery, and gasoline and ethanol accounting for 80% of the volume of the fuel system in a conventional vehicle. A 50-liter tank of gasoline would thus need to be replaced by a battery with a volume of about 165 liters, while a 50-liter tank of ethanol would need to be replaced by a battery with a volume of around 115 liters.
José Goldemberg notes that as well as the limited energy provided by batteries, vehicles also need to overcome other obstacles, such as the lack of a recharge network and the fact that in many countries, especially in Europe, electricity is generated by burning fossil fuels, which reduces the environmental advantage of electric cars. He believes the most effective way to tackle the pollution caused by exhaust fumes in large urban areas is to use internal combustion engines powered by a clean, renewable (non-fossil) fuel such as ethanol, produced from sugarcane in Brazil and from corn in the USA.
Raul Beck believes it would be foolish for Brazil to use ethanol as an excuse not to follow the global trend of replacing combustion-engine cars with battery-powered models. “Electric vehicles are now a reality, and many countries are investing a lot of resources into improving the performance of current battery technologies,” he says. He explains that the advantage of lithium-ion batteries over other models is that lithium has a high electrochemical potential (the ability to generate energy from oxidation-reduction reactions), and it is the lightest and least dense metal of all the solid elements of the periodic table.
Lithium is about half as dense as water, meaning a 1-liter block of lithium weighs 0.534 kg. “This allows us to make smaller and lighter batteries with a high energy density,” says chemist Maria de Fátima Rosolem, a researcher at the CPqD Energy Systems Department. “What’s more, lithium batteries are made of environmentally friendly materials and have a high cycle life [the number of recharges and discharges a battery can undergo before it needs to be replaced].”
The evolution of battery technology over recent years shows that the conventional lead-acid batteries used in ordinary cars have the lowest gravimetric (mass) and volumetric energy densities; they are heavier and larger than other batteries (see image), followed by nickel-cadmium batteries, mainly used in rechargeable power tools, nickel-metal hydride batteries, used in electric vehicles in the 1990s when commercial lithium batteries did not yet exist, and finally, various lithium-ion technologies.