Meaning
Ion transport within battery electrodes is hindered when the path through the porous structure becomes increasingly complex. Tortuosity escalation is the rise in the effective path length of lithium ions traveling through the liquid electrolyte within an electrode. This phenomenon occurs as electrodes are made thicker or more densely compacted to increase energy density.
Physical Mechanism
Compaction forces the active material particles closer together. This squeezing reduces the pore size and forces the ions to take a highly winding path around the particles. This winding path increases the resistance to ion diffusion through the electrode thickness.
Electrochemical Penalty
Slow ion transport limits the rate at which the cell can be charged and discharged. High tortuosity leads to large concentration gradients and premature voltage cutoff under high current. This behavior reduces the usable capacity of the battery at high discharge rates.
Electrode Design
Battery engineers must balance the need for high energy density with the need for low tortuosity. They use pore-forming additives or controlled magnetic alignment to create straight channels for ion transport. These channels provide a direct pathway through the dense electrode and minimize the transport resistance.
By optimizing the pore structure, they can achieve high energy density without sacrificing fast-charging performance.
