Electric Car Battery
Electric vehicles have reemerged as a viable alternative means of transportation, driven by energy security concerns, pressures to mitigate climate change, and soaring energy demand. The battery component will play a key role in the adoption of these vehicles as it defines their cost, range, and safety. An electric car battery (known as the electric vehicle battery – EV battery) is a rechargeable battery used to power the electric motors of a battery electric vehicle (BEV) or hybrid electric vehicle (HEV). Electric car batteries differ from starting, lighting, and ignition (SLI) batteries as they are designed to give power over sustained periods of time and are deep-cycle batteries. Batteries for electric vehicles are characterized by their relatively high power-to-weight ratio, specific energy, and energy density; smaller, lighter batteries are desirable because they reduce the vehicle’s weight and improve its performance.
Previous battery technologies failed to provide the required specifications, particularly in terms of driving range. However, recent developments in lithium-ion battery technology have eliminated these limitations. Today, the most common battery type in modern electric vehicles are lithium-ion and lithium polymer because of their high energy density compared to their weight. Advances in lithium-ion battery technology are creating possibilities for electric vehicles to compete with their gasoline counterparts for the first time. However, many challenges remain, the most important of which is cost.
Cell → Module → Pack
The composition of an EV battery might vary slightly depending on the types of electric vehicles, but generally, EV batteries are composed of
- Electrochemical Cells. An electric cell is essentially a source of DC electrical energy. It converts stored chemical energy into electrical energy through an electrochemical process.
- Battery Modules. A battery module is an assembly of battery cells, which is put into the frame by combining a fixed number of cells to protect the cells from vibration, heat, or external hazards. A battery module will always incorporate many discrete cells connected in series and parallel to achieve the module’s total voltage and current requirements.
- Battery Pack. The final shape of an electric vehicle battery installed to an electric vehicle. The collection of data from the pack sensors and activation of the pack relays are accomplished by the pack’s battery monitoring unit (BMU) or the battery management system (BMS).
An electric car battery is composed of many electrochemical cells. The actual battery cells can have different chemistry, physical shapes, and sizes, as preferred by various pack manufacturers. To operate an electric vehicle, an enormous amount of power a thousand times stronger than that of a smartphone is required. That is why EVs need dozens of battery cells up to as many as thousands. The large stack of cells is typically grouped into smaller stacks called modules. Several of these modules are placed into a single pack. The cells are welded within each module to complete the electrical path for current flow. Modules can also incorporate cooling mechanisms, temperature monitors, and other devices.
Chemistry of Electric Car Battery – How it works
An electric battery is essentially a source of DC electrical energy. It converts stored chemical energy into electrical energy through an electrochemical process. This then provides a source of electromotive force to enable currents to flow in electric and electronic circuits. A typical battery consists of one or more voltaic cells.
The fundamental principle in an electrochemical cell is spontaneous redox reactions in two electrodes separated by an electrolyte, which is an ionic conductive and electrically insulated substance.
But how does such a battery work?
In simple terms, each battery is designed to keep the cathode and anode separated to prevent a reaction. The stored electrons will only flow when the circuit is closed. This happens when the battery is placed in a device and the device is turned on.
When the circuit is closed, the stronger attraction for the electrons by the cathode (e.g. LiCoO2 in lithium-ion batteries) will pull the electrons from the anode (e.g. lithium-graphite) through the wire in the circuit to the cathode electrode. This battery chemical reaction, this flow of electrons through the wire, is electricity.
If we go into detail, batteries convert chemical energy directly to electrical energy. Chemical energy can be stored, for example, in Zn or Li, which are high-energy metals because they are not stabilized by d-electron bonding, unlike transition metals. Lithium metal is the lightest metal and possesses a high specific capacity (3.86 Ah/g) and an extremely low electrode potential (−3.04 V vs. standard hydrogen electrode). Therefore lithium is an ideal anode material for high-voltage and high-energy batteries.
During discharge, lithium is oxidized from Li to Li+ (0 to +1 oxidation state) in the lithium-graphite anode through the following reaction:
C6Li → 6C(graphite) + Li+ + e–
These lithium ions migrate through the electrolyte medium to the cathode, where they are incorporated into lithium cobalt oxide through the following reaction, which reduces cobalt from a +4 to a +3 oxidation state:
CoO2 (s) + Li+ + e– → LiCoO2 (s)
Here is the full reaction (left to right = discharging, right to left = charging):
C6Li + CoO2 ⇄ C6 + LiCoO2
These reactions can be run in reverse to recharge the cell. In this case, the lithium ions leave the lithium cobalt oxide cathode and migrate back to the anode, where they are reduced back to neutral lithium and reincorporated into the graphite network.
Batteries convert chemical energy directly to electrical energy. Chemical energy can be stored, for example, in Zn or Li, which are high-energy metals because they are not stabilized by d-electron bonding, unlike transition metals.
Even though many types of batteries exist with different combinations of materials, all of them use the same principle of the oxidation-reduction reaction. In an electrochemical cell, spontaneous redox reactions take place in two electrodes separated by an electrolyte, which is an ionic conductive and electrically insulated substance. The redox reaction is a chemical reaction that produces a change in the oxidation states of the atoms involved. Electrons are transferred from one element to another. As a result, the donor element, which is the anode, is oxidized (loses electrons), and the receiver element, the cathode, is reduced (gains electrons).
For example, the lithium-ion cell consists of two electrodes of dissimilar materials. The cathode is made of composite material and defines the name of the Li-ion battery cell. Cathode materials are generally constructed from LiCoO2 or LiMn2O4. Anode materials are traditionally constructed from graphite and other carbon materials. Graphite is the dominant material because of its low voltage and excellent performance. The electrolyte can be liquid, polymer (with a polymer gel as electrolyte), or solid. The separator is porous to enable the transport of lithium ions and prevents the cell from short-circuiting and thermal runaway.
During the charging process, Li+ ions move from the Li-containing anode and pass through the electrolyte-soaked separator, finally intercalating to the anode host structure. As a result, the electrons pass through the external circuit in the opposite direction.
During discharge, electrons flow through the external circuit through the negative electrode (anode) towards the positive electrode (cathode). The reactions during discharge lower the chemical potential of the cell, so discharging transfers energy from the cell to wherever the electric current dissipates its energy, mostly in the external circuit. During charging, these reactions and transports go in the opposite direction.
