Understanding battery degradation across scales is essential for long lifetime batteries. In this talk, we explore these multi-scale effects, with a particular focus on coupled electrochemical and mechanical effects.
Starting at the particle-level, we highlight how phase field fatigue models can be used to understand electrochemically driven fatigue-fracture behaviour of cathode particles. Here, thresholds between stable and unstable cracking can be determined based on particle-size and C-rate, but we also highlight how this varies with non-idealised particle geometries [1] and the implications for core-shell cathode particle design [2]; identifying failure mode regions where debonding and fracture dominate.

At the electrode-scale, continuum-level models can be used to describe stress-distributions, subsequent particle fracture and crack growth behaviour, leading to capacity and power fade. Here, the complex stress-evolution behaviour is compounded in high energy dense silicon-graphite composite anodes. Motivated by this, we show how physics-based models can explain why the electrochemical activity of the 2 phases is highly state-of-charge dependent due to mismatches in the thermodynamic open circuit voltage curves [3]. Insights from this, then allows us to explain the high sensitivity of lifetime with depth-of-discharge in cells with composite silicon-graphite anodes, towards exploring the load-profile dependent trade-off between energy density and lifetime [4–6]. This modelling framework, which is integrated into the open-source Python Battery Mathematical Modelling (PyBaMM) package, is then extended to describe the reversible and irreversible cell volume change behaviour, where the non-linear and mechanical hysteresis behaviour can be elucidated.

At the battery pack level, we then show how these heterogeneities can be further amplified by a combination of cell-to-cell variation effects and also pack design, through the finite interconnection resistances between cells [7]. Long term degradation studies at the pack level, where individual cell currents are decoupled, then reveals that not only is the well-known solid-electrolyte interphase growth important, but also the often overlooked growth of the cathode electrolyte interphase. Here, this can significantly affect cell-to-cell current distributions through impedance growth [8], leading to convergent or divergent cell-to-cell degradation trajectories depending on operating condition.

Overall, this talk highlights the importance and mechanistic understanding of coupled electrochemical, mechanical and thermal effects across scales in the development of long lifetime and high energy dense battery systems.

References
[1] W. Ai, B. Wu, E. Martínez-Pañeda, A coupled phase field formulation for modelling fatigue cracking in lithium-ion battery electrode particles, J Power Sources 544 (2022) 231805. https://doi.org/10.1016/J.JPOWSOUR.2022.231805.
[2] Y. Tu, B. Wu, W. Ai, E. Martínez-Pañeda, Influence of concentration-dependent material properties on the fracture and debonding of electrode particles with core–shell structure, J Power Sources 603 (2024) 234395. https://doi.org/10.1016/J.JPOWSOUR.2024.234395.
[3] W. Ai, N. Kirkaldy, Y. Jiang, G. Offer, H. Wang, B. Wu, A composite electrode model for lithium-ion batteries with silicon/graphite negative electrodes, J Power Sources 527 (2022) 231142. https://doi.org/10.1016/J.JPOWSOUR.2022.231142.
[4] M.P. Bonkile, Y. Jiang, N. Kirkaldy, V. Sulzer, R. Timms, H. Wang, G. Offer, B. Wu, Coupled electrochemical-thermal-mechanical stress modelling in composite silicon/graphite lithium-ion battery electrodes, J Energy Storage 73 (2023) 108609. https://doi.org/10.1016/J.EST.2023.108609.
[5] M.P. Bonkile, Y. Jiang, N. Kirkaldy, V. Sulzer, R. Timms, H. Wang, G. Offer, B. Wu, Is silicon worth it? Modelling degradation in composite silicon–graphite lithium-ion battery electrodes, J Power Sources 606 (2024) 234256. https://doi.org/10.1016/J.JPOWSOUR.2024.234256.
[6] W. Ai, L. Kraft, J. Sturm, A. Jossen, B. Wu, Electrochemical Thermal-Mechanical Modelling of Stress Inhomogeneity in Lithium-Ion Pouch Cells, J Electrochem Soc 167 (2020) 013512. https://doi.org/10.1149/2.0122001jes.
[7] X. Liu, W. Ai, M. Naylor Marlow, Y. Patel, B. Wu, The effect of cell-to-cell variations and thermal gradients on the performance and degradation of lithium-ion battery packs, Appl Energy 248 (2019) 489–499. https://doi.org/10.1016/j.apenergy.2019.04.108.
[8] M. Naylor Marlow, J. Chen, B. Wu, Degradation in parallel-connected lithium-ion battery packs under thermal gradients, Communications Engineering 2024 3:1 3 (2024) 1–15. https://doi.org/10.1038/s44172-023-00153-5.