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

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.

💡 Research Summary

This work investigates degradation mechanisms in large‑format 21700 cylindrical lithium‑ion cells that combine a Ni‑rich NCM‑831205 cathode with a graphite‑SiOx composite anode. Cells were cycled at 25 °C between 2.5 V and 4.2 V using a protocol that produced three states of health (SOH 100 % pristine, SOH 80 % and SOH 70 %). Electrochemical testing showed a linear capacity fade across C‑rates, with a small capacity recovery during low‑current check‑up cycles, indicating reversible lithium redistribution. Differential voltage analysis (DVA) and incremental capacity analysis (ICA) identified two concurrent aging modes: loss of lithium inventory (LLI) manifested as a systematic shift of the high‑state‑of‑charge (SOC) transition peaks (MaxHi/MinHi) to lower voltages, and loss of active anode material (LAAM) evidenced by the gradual disappearance of the SiOx lithiation peak and a reduction in the charge required to reach the LiC₆ phase.

Operando neutron diffraction (ND) provided real‑time structural insight. The NCM (003) reflection intensity decreased and lattice‑parameter changes were attenuated with cycling, directly indicating reduced lithium insertion in the cathode lattice. Simultaneously, the graphite anode’s LiC₁₂→LiC₆ transition peaks shifted to lower voltages and weakened, confirming that less lithium is available to fully lithiate the anode and that a fraction of the graphite/SiOx becomes electrochemically inactive.

Post‑mortem neutron depth profiling (NDP) measured lithium concentration profiles across the electrode thickness at three radial positions (center, middle, bottom). The cathode showed pronounced lithium depletion, especially near the cell centre, whereas the anode displayed elevated lithium concentrations and a thicker solid‑electrolyte interphase (SEI) in the same regions. This spatial pattern supports a “cathode‑to‑anode lithium migration” model: lithium released from the cathode during cycling re‑deposits on the anode surface, consuming lithium to grow SEI and thereby contributing to irreversible lithium loss.

X‑ray computed tomography (CT) revealed that intrinsic structural inhomogeneities (variations in electrode density, electrolyte distribution, and mechanical packing) persisted after aging and dominated the macroscopic morphology. These pre‑existing gradients correlated with the observed lithium‑profile asymmetries, indicating that height‑wise non‑uniformities amplify local over‑potentials, thermal gradients, and stress, accelerating degradation in the cell centre compared with the bottom edge.

Overall, the combined operando ND, NDP, and CT analyses provide direct, complementary evidence that (1) lithium inventory is depleted via cathode‑to‑anode migration, (2) a fraction of the graphite‑SiOx anode becomes inactive with cycling, and (3) spatial inhomogeneities intrinsic to large‑format cylindrical cells strongly influence the severity and distribution of these processes. The findings underscore the importance of designing more uniform electrode architectures and stabilizing SiOx‑based anodes to mitigate capacity fade in next‑generation high‑energy batteries for electric‑vehicle applications.