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More efficient batteries

The electric car's biggest technological limitation is its energy storage system, which is directly related to travel range

Imagem: GLOBAL EV OUTLOOK 2017 / IEA CPQD power tests on lithium-ion batteries used in electric vehiclesImagem: GLOBAL EV OUTLOOK 2017 / IEA

Research consortiums from Asia, Europe, and the United States, comprising battery manufacturers, car manufacturers, universities, and innovation centers, are in a race to find a safer battery with a greater energy density (the amount of energy stored by its volume), a longer service life, and a lower cost. Today, the cost of the battery accounts for up to 50% of the price of the car, but the value has been falling consistently. The current price of a battery, in terms of its storage capacity, is close to US$300 per kilowatt-hour (kWh). Seven years ago, it was three times higher.

At the same time, batteries have become more energy efficient, giving vehicles a greater range—the more energy the battery stores, the greater distance the vehicle can travel before it needs recharging. Their energy density has more than tripled in the last seven years, reaching roughly 350 watt-hours per liter (see infographic). A watt-hour (Wh) corresponds to one watt of power expended for one hour of time, and is the unit used for measuring electric power, while the liter (L) refers to the volume of the battery.

Lithium-ion batteries are the state-of-the-art technology, similar to those used in cell phones and laptops. “The great advantage of these batteries is their high energy density. Because lithium is a light metal, it can store more energy in less space,” says chemist Maria de Fátima Rosolem, a researcher at the CPQD Energy Systems department. “It also has a high electrode potential, meaning it can easily gain or lose electrons, which is the basic principle of the generation of an electric current.”

The batteries consist of one or more lithium-ion cells interconnected in one battery pack. The most common types are cylindrical cells, measuring 18 millimeters (mm) in diameter by 65 mm in height. Another model, known as the pouch cell, is rectangular and flat. And finally, there are prismatic cells, which resemble a rectangular box about the size of a book. As each cell has an average voltage of 3.6 volts (V) and an electric car needs 300 to 600 V to run, hundreds or thousands of lithium cells are used in the batteries for these vehicles. Tesla’s models, for example, have 7,400 cylindrical cells, packaged in smaller modules and fitted into the chassis of the vehicle.

The problem with lithium is its operating safety. “The batteries work well up to a certain temperature. Above 25 degrees Celsius, they need to be cooled. Overheating can cause them to explode,” says electrical engineer Celso Novais, from Itaipu Binacional’s Electric Vehicle Program. The solution to this issue is an electronic circuit that manages the battery temperature, current, and voltage, and keeps it running under the proper conditions.

Battery imbalances occur when the cells work at an uneven capacity, decreasing their potential cycle count (the total number of discharges and recharges) and reducing their service life, which is currently around eight years. “Beyond this period, they lose up to 30% of their initial capacity. But they can be reused in other applications, such as storing photovoltaic energy in homes and telecommunications centers,” says Maria de Fátima. “Because they do not contain heavy metals, they can be recycled.” When the battery dies, electric car owners have to replace the battery. This is another factor that makes electric cars expensive. Owners may choose to simply buy a new vehicle, since the battery represents half of its value.

Modern batteries use lithium metal oxides to make the positive plate (cathode) and graphite for the negative plate (anode) (see infographic). “The composition of the plates is the key to improving battery performance,” says Maria de Fátima. Scientists are trying to identify new metals and the best combination of lithium with other materials in order to optimize their energy density. Two possibilities still under development are zinc-air and lithium-air batteries, which can store almost twice as much energy as lithium-ion models.

Induction charging
In May this year, smartphone chip manufacturer Qualcomm presented a new technology that charges the car while it is in motion. The Dynamic Electric Vehicle Charging system (DEVC) charges the battery by induction (with no contact between the car and the charger, which is embedded in the ground) as the vehicle travels on a special road fitted with a kind of electric rail. The advantage of DEVC is that cars could use smaller batteries without decreasing their range.

In Brazil, there are very few basic studies on battery chemistry for electric cars. The most common involve testing existing technologies and incremental research on existing cells. “We have conducted characterization and aging studies on cells and we have developed complete batteries. Although we used commercial lithium-ion cells, we designed the electronic control unit, the cooling system, and the mechanical packaging,” says researcher Raul Beck from the CPQD.

In Foz do Iguaçu, Paraná, researchers from the Itaipu Electric Vehicle Program were able to create a 100% recyclable battery from sodium, nickel, and chlorine. “Our model is equivalent to lithium in terms of storage capacity and power,” explains Celso Novais. “However, it is the shape of a monobloc battery, which cannot be divided into smaller modules. It is therefore better suited to larger electric vehicles such as buses, trains, and trucks.”

In 2012, Itaipu began a project to produce its sodium battery in Brazil. “Now we are working with Swiss and German companies on an advanced version of the model. Characterized by flat and compact cells, it can be divided into smaller modules. We expect it to be more competitive than lithium-based batteries and to start production in 2019,” says Novais.