How Electrodes Charge and Discharge

The electrochemical reactions inside the porous electrodes of batteries and fuel cells have been described by theorists, but never measured directly. 

Now, a team at MIT has figured out a way to measure the fundamental charge transfer rate — finding some significant surprises.

The study found that the Butler-Volmer (BV) equation, usually used to describe reaction rates in electrodes, is inaccurate, especially at higher voltage levels. 

Instead, a different approach, called Marcus-Hush-Chidsey charge-transfer theory, provides more realistic results — revealing that the limiting step of these reactions is not what had been thought.

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This illustration shows a battery electrode made of lithium iron phosphate (left side of image) coated with carbon, and in contact with an electrolyte material. As the battery is discharged, lithium ions (shown in purple) jump across the coating and insert themselves into the crystal structure, while electrons (shown as circles with minus signs) in the carbon-coating tunnel into the material and attach to iron ions (shown in red). (Phosphate groups are left out of this diagram for clarity.) Illustration courtesy of Peng Bai and Martin Bazant