Why so much enthusiasm for lithium-ion?
Li-ion batteries are popular because of how much power they can put out for a given size and weight. A typical lithium-ion battery can store 150 watt-hours of power in 1 kilogram of cells, compared to a nickel metal hydride battery pack (100 watt-hours per kilogram) or a lead-acid battery (25 watt-hours per kilogram). A lead-acid battery requires 6kg to store the same amount of energy that a 1kg lithium-ion battery can handle.
However, lithium-ion batteries are extremely sensitive to high temperatures and are inherently flammable. Due to the heat, these battery packs tend to degrade much faster than normal. If a lithium-ion battery pack fails, it can catch fire and cause extensive damage. This requires immediate action and the development of battery safety guidelines.
Recently, fires caused by lithium-ion batteries have occurred frequently. On January 8, 2019, the "COSCO Pacific" ship in the Arabian Sea caught fire due to the spontaneous combustion of lithium-ion batteries. Last April, a 2MW battery exploded at an APS plant in Arizona, injuring four firefighters.
Hans-Otto Schjerven, head of Vestfold's fire department, said rechargeable lithium batteries could cause "hard-to-extinguish fires, where the fire from the battery spreads quickly." These incidents will increase as electric vehicles become more common.
Before we analyze why lithium-ion batteries catch fire, let's understand how they work.
Li-ion battery packs consist of stacked Li-ion battery modules, temperature sensors, voltage taps, and an on-board computer (battery management system) to manage individual cells. Like any other battery, a lithium-ion battery has a positive electrode (cathode), a negative electrode (anode), and a chemical substance called an electrolyte that sits between them. While the anode is usually made of graphite (carbon), the cathode uses a different lithium material – lithium cobalt oxide (LCO), lithium nickel manganese cobalt (or NMC), etc.
When charging current is supplied to the battery, lithium ions move from the cathode to the anode through the electrolyte. Electrons also flow, but take longer paths outside the circuit. The opposite movement occurs during discharge, with the result that electrons power the application the battery is connected to.
When all ions have moved back to the cathode, the battery is fully discharged and needs to be recharged.
