From cathode to anode: Understanding lithium loss in 21700-type Ni-rich NCM||Graphite-SiOx cells

TL;DR

This study investigates lithium loss and degradation mechanisms in large 21700-type Ni-rich NCM||Graphite-SiOx lithium-ion cells during cycling, using advanced in situ and destructive techniques to reveal microscopic and macroscopic aging effects.

Contribution

It provides a comprehensive analysis of lithium loss and electrode degradation in large-format cells, combining multiple advanced characterization methods for the first time.

Findings

Lithium is lost from the cathode and deposited on the anode during cycling.

Active anode material diminishes, and lithium concentration profiles show depletion and accumulation.

Inhomogeneities inside the cell significantly influence aging and capacity fade.

Abstract

Moving to larger cell formats in lithium-ion batteries increases overall useable energy but introduces inhomogeneities that influence aging. This study investigates degradation in 21700-type cells with NCM cathodes and graphite/SiOx anodes under cyclic aging, using in operando neutron diffraction, neutron depth profiling, and X-ray computed tomography. Prolonged cycling causes lithium loss, observed on the cathode side as reduced NCM unit cell change during cycling. On the anode side, this loss appears as diminished formation of the fully lithiated LiC6 phase. Differential voltage analysis during aging reveals not only lithium inventory loss but also active anode material loss. Diffraction data confirm this through shifts in the LiC12 transition and LiC6 onset to lower capacities, requiring less lithium to trigger the transitions. Lithium concentration profiles across electrode positions show depletion in the cathode, while elevated concentrations in the anode indicate increased solid-electrolyte interphase formation, suggesting lithium consumed from the cathode deposits on the anode side. CT measurements show that intrinsic inhomogeneities inside the cells have a stronger influence on the macroscopic structure than aging-induced changes, indicating that the observed capacity fade primarily originates from microscopic degradation processes within the electrodes. Overall, the combined techniques provide direct evidence of lithium loss, active material degradation, and spatially dependent aging mechanisms in large-format cylindrical cells.