Batteries explained– from cell formats to chemistries
Batteries are highly complex electrochemical systems with different cell formats and chemistries being used. The processes inside the battery also deserve particular attention.
- Round cell, prismatic cell, and pouch cell are the most frequently used cell formats
- There are considerable differences in the cell chemistry at the cathode, while graphite is used at the anode in most cases
- Lithium ions diffuse from the cathode to the anode and vice versa charging and discharging the battery respectively
With increasing electrification, the importance of batteries is steadily increasing. Due to the countless fields of application and differing requirements regarding voltage, power, and capacity, there are many different types of batteries on today's market. A Li-Ion battery as a whole is generally a pack which is subdivided into modules. The battery module again represents a serial and/ or parallel connection of several cells. The magic of energy storage happens in these cells so we will take a closer look at these. The Lithium-Ion (Li-Ion) battery cells are explained in more detail with regard to formats, materials, composition, and processes.
Cylindrical, prismatic and pouch are dominant cell formats
The basic functionality of Li-Ion batteries is the principle of a galvanic cell: galvanic cells store chemical energy and can release electrical energy. A galvanic element consists of two metallic electrodes which are submerged in an electrolyte. In case of the Li-ion battery, it is ions which diffuse through the ion-conduction electrolyte (medium) due to the charge conservation. As a result, small, negatively charged particles (electrons) are moving in the outer circuit from the anode to the cathode of the battery, doing the electrical work. There are currently three dominant cell formats for lithium-ion batteries: cylindrical, prismatic and pouch.
Cylindrical cells consist of a solid, metallic outer shell. Active layers are wrapped around the inner electrode. The battery poles of this type are arranged opposite each other. Electrodes and separators are wound up together giving it its cylindrical form. Cylindrical cells have the second highest energy density after prismatic cells. This is due to the high packing density as more active material can be accommodated in the smallest assembly space. Additionally, the production of the cylindrical cell is cost-efficient. However, the disadvantage of this cell format is the complicated assembly process and cooling system as well as the monitoring effort required during operation. Tesla is using cylindrical cells in all their electric vehicles.
Prismatic cells consist of a solid, metallic housing in cubic form. Electrodes and separators are stacked onto each other in layers. The battery poles are typically arranged on the upper flat side at the outer edges. The large surface area specific to the design allows good thermal conductivity for heat transfer and the cells can be stacked to save space. The prismatic format offers a simple assembly procedure but a complicated design and production process, in turn making them more expensive for consumers. BMW uses prismatic cells in their first all-electric vehicle, the i3.
Pouch cells consist of folded active layers enclosed by a flexible, mostly aluminum-based outer foil. In light of this fact, pouch cells are framed and tensed up in the module to create equal pressure and aging behavior. The battery poles are usually led to the outside as thin, metallic current collector. Pouch cells can be manufactured in almost any size due to the lack of massive outer housings. They have a good thermal conductivity for heat transfer (cooling) due to this outer shape but are vulnerable to damages. Pouch cells were the selection of choice for Jaguar, when introducing the i-Pace.
Lithium ions diffuse from the cathode to the anode and vice versa charging and discharging the battery respectively
The central components of a Li-Ion battery are the two electrodes, the electrolyte and the separator. The separator spatially and electrically separates the anode from the cathode but is permeable to lithium ions. The electrolyte, the medium for ion exchange, is a liquid that can move ions from one electrode to another during charge/discharge.
When a battery is discharged, the lithium ions diffuse from the anode into the electrolyte through the separator to the cathode, where they are stored. At the same time, the cathode absorbs one electron for each lithium-ion (Li+) stored to balance the charge. The electrons required by the cathode to store the lithium ions come from the anode to which it is connected by the external circuit. The electrons removed oxidize the anode material and, depending on the number of missing electrons, lithium ions are released into the electrolyte.
Cathode and anode materials define the performance of batteries
No battery type is as versatile as Li-Ion batteries. They can be manufactured with dozens of electrode materials - all with different properties.
Various materials are used on the cathode. Li-Ion cathodes consist of a current collector (usually aluminum foil) on which an active material is coated in which Li-Ion can be stored. Lithium cobalt oxide (LCO) is one possible cathode material. They have a high energy density but are not very resistant to temperatures compared to other materials. The fire hazard and limited load capacity are some of the disadvantages of the LCO material. Lithium Nickel Cobalt Manganese Oxide (NMC) is the most widely used lithium-ion system. It offers a compromise of good general electrochemical performance, high energy densities, and cost. Additionally, Lithium Nickel Cobalt Aluminum Oxide (NCA) is used as cathode material, offering high energy density and durability, similar to NMC, but with drawbacks on the cost and safety side. Lithium iron phosphate (LFP) has better thermal and chemical stability. Because of this, they are very safe batteries, fireproof and more resistant, with a longer life time than other materials. The disadvantage of this material is the low nominal voltage and therefore low energy density.
Li-Ion anodes consist of a current collector (mostly copper foil), on which an active material is coated, in which ions are stored. Graphite (C) as an anode material represents a low electrode potential and only expands slightly during charging. Lithium Titanite Oxide (LTO) enables a higher discharge rate and performance at different operating temperatures than graphite. This electrodes are safe because of their high potential and tolerance against overcharging. This gives them a long service life. However, the disadvantage is the lower energy density and the high costs compared to the graphite anode. Lithium metal (Li) is another possible anode material that has a very high energy density, but it is also very expensive. Additionally, it causes faster degradation.
TWAICE is applicable to any Li-Ion cell and battery type
The TWAICE digital twin based on predictive battery analytics software improves development and deployment of all these batteries. A initial laboratory parametrization and our large battery model library provide the basis to cover the whole range of Li-Ion batteries. During operation standard measurement data from the battery management system, that is current, voltage, and temperature are used to create the new battery status means digital mirror image of the battery and enable determination, prediction, simulation and optimization of battery parameters and battery aging.
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