Capacity gain in Li-ion cells with silicon-containing electrodes

TL;DR

This paper investigates capacity gain phenomena in silicon-containing lithium-ion batteries, combining simulations and experiments to identify mechanisms that increase capacity through electrode transformations and prelithiation effects.

Contribution

It introduces a quantitative framework explaining capacity gain mechanisms in Si cells, focusing on break-in processes and prelithiation effects, which was previously not well understood.

Findings

Capacity gain linked to electrode transformation processes.

Prelithiation can fully replenish the positive electrode with Li+.

A new framework models capacity gain mechanisms in Si cells.

Abstract

Silicon-containing lithium-ion batteries can exhibit capacity gain early in life, which makes forecasting future cell behavior difficult. We have observed these anomalous trends even in conditions where known mechanisms, such as overhang equalization and excessive electrolyte oxidation, are unlikely to be significant. Here, we combine simulations and experiments to analyze four cases that can produce increased capacity in Si cells. Three of these pathways relate to break-in processes, where improved mass and charge transport can lead to increased access to active electrode domains and decreased cell impedance. The fourth case occurs at high levels of prelithiation, when the positive electrode (PE) is completely replenished with Li+ at the end of cell discharge. We show that the commonality among these mechanisms is that the underlying transformations change the potentials experienced by electrodes at the end of half-cycles, increasing the Li+ inventory available to the cell. A quantitative framework to describe these effects is presented, enabling these ideas to be extended to other battery systems.