In the simplest terms, a lithium-ion battery refers to a battery with a negative electrode (anode) and a positive electrode (cathode) that transfers lithium ions between the two materials. Lithium ions move from the anode to the cathode during discharge and deposit themselves (intercalate) into the positive electrode, which is composed of lithium and other metals. During charge, this process is reversed.
Within the cells, there are many layers of anode and cathode with a separator in between. Between the two plates, there is also an electrolyte solution, typically LiPF6 mixed with a liquid solution. This combination of materials can either be stacked (prismatic cells) or wound in a spiral (cylindrical cells). Cells vary in size and shape; some are encased in plastic while others are in aluminum cases. The casing is dependent on the environment they are going into and the size is determined by the amount of capacity needed for the application.
Each lithium-ion cell has a safe voltage range that it can be operated in. This range is dependent on the chemistry used in the battery. For example, an LFP battery at 0% State of Charge (SOC) is 2.5V and at 100% SOC is 3.6V. This is considered the safe operating range of this battery. Going below the stated 2.5V SOC can cause degradation of the electrodes. This is considered an over-discharge. If a cell is repeatedly over-discharged it can cause many issues that permanently damage the battery. The same is true for an over-charge, going above the stated 100% SOC. These two failures have led battery manufacturers to develop safety devices and features.
A battery is typically comprised of many cells working in conjunction with one another. Let’s consider an LFP cell with a nominal voltage of 3.2V and a capacity of 100 Ah. Most applications require a higher voltage and capacity, how would this be done? In order to increase the voltage of a battery, multiple cells must be connected in series. To increase the capacity, cells must be connected in parallel. For example, let's say we want a 12V battery with a capacity of 300 Ah. With the given LFP cell we would need 4 cells in series with 3 modules in parallel. This would produce a system that is 12.8V with a capacity of 300 Ah.