Battery knowledge: From A as in anode to C as in cell
How batteries work
Each battery cell consists of four basic elements: two differently charged electrodes (graphite and mixed oxide electrode) and a typically liquid or gel-like electrolyte in which ions (charged atoms such as lithium atoms) can move. The fourth element is a separator that’s permeable for the ion flow between the two electrodes. It protects against short circuits. During the discharge stage, i.e., when the battery is in use, a chemical oxidation process is started. This process releases electrons on the grid structure of the graphite electrode that, like the ions, want to move to the other mixed oxide electrode in order to restore the voltage balance that was upset during the charge stage. However, in contrast to the ions, the electrons cannot directly pass through the separator but must get to the other side by using a bypass. Located between this bypass is an electrical consumer that is supplied with electrical energy by the electron flow. In the case of an electric vehicle, it’s the motor. Once ions and electrons have been recombined at the mixed oxide electrode the current flow stops. During the charging stage another oxidation process is activated but in the opposite direction. That’s why rechargeable batteries are also referred to as redox storage systems. During the operation of an electric vehicle this process can constantly change directions: During acceleration the battery is discharged and during braking events it’s charged by recovering (recuperating) energy. During that process the anode and cathode’s roles change as well: During the discharge stage the negative electrode works as the anode and the positive electrode as the cathode. During the charging stage the anode becomes positive and the cathode negative.
Battery cell
There are different types of cells that are named according to their shape or design. Due to a variety of strengths and weaknesses, no design has won out yet.
Cylindrical or round cells (used by Tesla, BMW and others) have a high energy density per cell and can be manufactured easily and cost-efficiently. The required layers are placed on top of each other, rolled up and covered by a protective metal jacket. High rigidity and tightness. The main disadvantages are difficult cooling, especially in the cell core, and large size due to the spherical shapes. In the case of prismatic cells (used by Audi and others), the various layers are rolled up as well, albeit not in the form of a cylinder but like a box and subsequently stowed in an angular aluminum housing. That increases stability and provides benefits in terms of assembly (minimal loss of space), repair and recycling. Energy density, though, is relatively low. A form of the prismatic cell is the blade cell of Chinese battery and vehicle manufacturer BYD. The enclosure of the lithium-iron-phosphate battery (LFP) is very long and resembles a blade. BYD cites short charging times, long life, high safety due to LFP and low-cost manufacturing as key benefits.
Pouch cells (VW, Smart, Hyundai and others) are rectangular cells welded into soft aluminum composite foil. They’re primarily characterized by their low weight and good thermal conductivity. The soft enclosure enables flexible installation options and efficient space utilization but can inflate and start to leak. Overall, it’s the least stable variant due to its design.
Cells, modules & packs
Currently, the serial arrangement of individual battery cells and combination into modules with separate housings, which simplifies assembly, is common practice. Several modules then form the battery. However, that implies a substantial amount of passive material, i.e., for housings, supporting elements, etc. To increase the share of active material in total weight and volume, the pack architecture is changed so that a larger number of battery cells can be linked directly and accommodated in the battery housing. This design is called cell-to-pack (CTP).
Direct packaging
Battery cells that are directly installed in the chassis – where the chassis then provides a protective function for the battery elements – are referred to as cell-to-body (CTB), cell-to-chassis (CTC) or cell-to-vehicle (CTV) designs. When highly integrative systems like these are optimized the functions of the vehicle components are fused with each other by battery cells assuming structural roles and chassis elements performing thermal roles as part of energy storage. Other benefits include a reduction in the number of components and more efficient space utilization in the vehicle. Repairs and recycling, though, are more than likely a lot more difficult.