Test Methodologies for Large Cylindrical Battery Cells in Heterogeneous Conditions

Licentiatavhandling, 2026

This thesis develops and evaluates experimental methods for studying electrochemical and thermal nonuniformity in lithium ion cells. Laboratory cell with varying SiOx content in the graphite electrode (Si-Gr) were used to investigate voltage signatures and the influence of state of charge on ageing. Prototype 4695 cylindrical cells were studied under different current, temperature, and cooling conditions. The methods included slow cycle voltage analysis, capacity measurements, intermittent current interruption, reconstruction of the galvanostatic state of charge window, surface temperature measurements, and computed tomography.

For the laboratory cells, C/20 voltage hysteresis increased systematically with nominal SiOx content and showed the clearest separation at low state of charge. The 4695 cell showed the closest agreement with the nominal 10 % SiOx reference, although its exact silicon content could not be determined. The 15 % Si-Gr cells cycled over 0 - 50 % state of charge reached approximately 80 % state of health (SoH) after 290 equivalent full cycles, compared with about 350 equivalent full cycles for the 0 - 100 % window.

The 4695 cells showed both gradual capacity fade and pronounced knee behaviour in the SoH trajectory. The End of Life (EoL) criteria at 80 % SoH ranged from approximately 350 to 1030 equivalent full cycles, with differences of approximately 150 - 250 equivalent full cycles between nominal replicate cells. The accessible SoC window generally contracted through a downward shift of the upper boundary. Resistance growth, window contraction, and accelerated capacity fade occurred together in several cells, but no universal relation was identified.

The partial cooling system imposed average axial surface temperature gradients of approximately 5 - 8 °C, with peak differences up to approximately 13 °C. The results demonstrate the methodology but do not establish that the imposed gradient alone caused the observed degradation. Computed tomography also revealed apparent core collapse in one aged cell.

Overall, the work shows that capacity, resistance, accessible state of charge window, temperature distribution, and structural observations provide complementary information. The developed methods form a basis for further studies using more mature cells, improved contact and sensor control, larger sample groups, and systematic teardown analysis.