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Battery technologies for electric vehicles

Globally, the total number of electric vehicles (EVs) on the road surpassed five million worldwide in early 2019. The growth of EVs has largely been driven by government policy, including public procurement programmes, financial incentives reducing the cost of purchase of EVs, tightened fuel-economy standards and regulations on the emission of local pollutants, low- and zero-emission vehicle mandates and a variety of local measures, such as restrictions on the circulation of vehicles based on their pollutant emission performances.

The rapid uptake of EVs has also been helped by progress made in recent years to improve the performance and reduce the costs of lithium-ion batteries. According to Bloomberg New Energy Finance, lithium-ion battery prices have tumbled in recent years, a remarkable 79% drop in the last even years. The average energy density of EV batteries is also improving at around 5-7% per year. However, further battery cost reductions and performance improvements are essential to improve the uptake of EVs. These are achievable with a combination of improved chemistries, increased production scale and battery sizes.

EVs require large amounts of battery power, thousands of times stronger than that of a smart phone, hence they require from dozens to thousands of battery cells. The composition of an EV battery might vary slightly depending on the types of vehicle, but generally EV batteries are composed of cells, modules that compose of the overall battery pack. A cell that is the basis of a battery must possess high capacity per unit volume in order to provide maximum performance in a restricted area inside a vehicle and should have much longer lifespan compared to batteries used in general mobile devices. Furthermore, cells must endure shocks transmitted during the drive and possess high reliability and stability to the extent of being able to withstand high and low temperatures. A module is classified when a number of cells are placed into a frame to protect them better from external shocks such as heat or vibration. When a number of modules are assembled together with a BMS (Battery Management System) and a cooling device that control and manage battery’s temperature, voltage, etc., this forms the complete battery pack. The typical series and parallel configuration of each defines the total capacity and voltage of the battery pack.

Lithium-ion (and similar lithium polymer) batteries, widely known via their use in consumer electronics, dominate the most recent group of EVs. The traditional lithium-ion chemistry involves a lithium cobalt oxide cathode and a graphite anode which provides impressive energy density and good power density, and 80 to 90% charge/discharge efficiency. Newer EVs are using new variations on lithium-ion chemistry that sacrifice energy and power density to provide fire resistance, environmental friendliness, very rapid charges (as low as a few minutes), and very long lifespans.

EV batteries are typically charged through AC charge points or DC fast chargers. The AC charger powers range across 3.7kW, 7.4kW, 11kW and 22kW, with overall power limitations being the on-board charger within the EV. DC fast chargers directly interface with the EV battery pack and provide a variety of power delivery options from 50kW to 350kW in ultra-fast chargers. The growth of technologies within EV batteries and the charging systems allow faster charge rates for EVs. Typically, a 50kW fast charger will charge an EV within 15-20 minutes to 80% capacity.