Lithium-ion batteries are “rocking-chair” batteries, a category to which they belong. When rocking-chair batteries are being discharged, ions, specifically lithium ions, are moved from the negative electrode to the positive electrode, and the process is reversed when the batteries are being charged. In an electrochemical cell, the small red balls represent lithium ions as they move between the negative and positive electrodes.
The positive electrode, made of lithium-iron-phosphate, is located on the left side of the battery (LiFePO4). This should go a long way toward clarifying the origin of this battery type’s moniker. Lithium ions are partially contained by a grid formed by iron and phosphate ions. As the cell is charged, lithium ions go through the central membrane and out through the right-side negative electrode. Lithium ions are allowed to pass through the membrane because it is made of polymer, also known as plastic, and has an abundance of tiny spores.
On the downside, we find a carbon lattice that can trap and store lithium ions that have already crossed over. When you discharge the battery, the operation is precisely the same but in reverse: Lithium ions begin migrating again, this time via the membrane and back towards the iron-phosphate lattice as electrons leave the positive electrode via the negative electrode. They are then saved on the positive side of the battery until the battery is given another charge.
If you have been paying attention, you should now recognize that the battery illustration on the right portrays an LFP battery that is almost fully drained. This is evident due to the nearly depleted battery. Most of the lithium ions are concentrated around the positive electrode. When a battery is completely charged, every one of its lithium ions will be securely ensconced within the carbon of the negative electrode.
In practice, lithium-ion cells are built using alternatingly stacked layers of aluminum, polymer, and copper foils, with the chemicals adhering to the top layer. They are often packaged in a steel canister coiled up like a jelly roll, not unlike the way an AA battery is stored. To increase the voltage and the ampere-hour capacity, many of these cells are combined in series and parallel to create the lithium-ion batteries that you purchase that have a voltage of 12 volts. With an average voltage output of 3.3 V per cell, a series connection of 4 cells yields 13.2 V. This voltage is perfect for swapping out a 12-volt lead-acid battery.
