Journal of Energy Storage 73 (2023) 108609 Available online 4 September 2023 2352-152X/© 2023 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Journal of Energy Storage journal homepage: www.elsevier.com/locate/est Research papers Coupled electrochemical-thermal-mechanical stress modelling in composite silicon/graphite lithium-ion battery electrodes Mayur P. Bonkile a,e , Yang Jiang b,e , Niall Kirkaldy b,e , Valentin Sulzer c , Robert Timms d,e , Huizhi Wang b,e , Gregory Offer b,e , Billy Wu a,e ,∗ a Dyson School of Design Engineering, Imperial College London, London, SW7 2AZ, United Kingdom b Department of Mechanical Engineering, Imperial College London, London, SW7 2AZ, United Kingdom c Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15213, United States of America d Mathematical Institute, University of Oxford, Oxford, OX2 6GG, United Kingdom e The Faraday Institution, Quad One, Becquerel Avenue, Harwell Campus, Didcot, OX11 0RA, United Kingdom ARTICLE INFO Keywords: Composite negative electrodes Silicon/graphite Stress generation Particle cracking Lithium-ion battery Physics-based model ABSTRACT Silicon is often added to graphite battery electrodes to enhance the electrode-specific capacity, but it undergoes significant volume changes during (de)lithiation, which results in mechanical stress, fracture, and performance degradation. To develop long-lasting and energy-dense batteries, it is critical to understand the non-linear stress behaviour in composite silicon-graphite electrodes. In this study, we developed a coupled electrochemical- thermal-mechanical model of a composite silicon/graphite electrode in PyBaMM (an open-source physics-based modelling platform). The model is experimentally validated against a commercially available LGM50T battery, and the effects of C-rates, depth-of-discharge (DoD), and temperature are investigated. The developed model can reproduce the voltage hysteresis from the silicon and provide insights into the stress response and crack growth/propagation in the two different phases. The stress in the silicon is relatively low at low DoD but rapidly increases at a DoD >∼80%, whereas the stress in the graphite increases with decreasing temperature and DoD. At higher C-rates, peak stress in the graphite increases as expected, however, this decreases for silicon due to voltage cut-offs being hit earlier, leading to lower active material utilisation since silicon is mostly active at high DoD. Therefore, this work provides an improved understanding of stress evolution in composite silicon/graphite lithium-ion batteries. 1. Introduction Lithium-ion batteries have become an integral part of our lives due to their application in electronic devices and electric vehicles. However, improvements in: energy density, safety, and lifetime, are still required. Among the different chemistries, lithium-ion batteries with composite silicon/graphite negative electrodes are a promising near-term option, as silicon is inexpensive, abundant and has a high theoretical specific capacity (3579 mAh/g for Li 15 Si 4 ) vs. graphite (372 mAh/g) [1,2]. However, the significant volume change during (de)lithiation, volt- age hysteresis and non-linear multi-phase interactions are significant challenges. In the case of silicon, the volume change is ca. 300% (theoretically) [3,4] compared to graphite’s 10% [5]. Such a significant expansion/contraction generates a high degree of mechanical stress resulting in particle cracking and accelerated formation/growth of the solid-electrolyte interphase (SEI) layer on the newly exposed surfaces, as well as loss of active material [6,7]. The repercussion of this are: ∗ Corresponding author at: Dyson School of Design Engineering, Imperial College London, London, SW7 2AZ, United Kingdom. E-mail address: billy.wu@imperial.ac.uk (B. Wu). irreversible capacity and power fade, as well as irreversible volume changes. Furthermore, there are differences in silicon’s lithiation and delithiation open circuit potential (OCP) curves, displaying complex potential hysteresis behaviour [8], leading to highly non-linear be- haviour. Therefore, there is a strong need to develop a modelling frame- work which captures the complex coupled electrochemical-thermal- mechanical interactions in these composite electrodes, towards a better understanding of stresses in these systems. Numerous works have investigated composite silicon/graphite neg- ative electrodes and their degradation mechanisms [9–12]. Ko et al. [13], for instance, concluded that the most critical challenge is the severe volume change during cycling, leading to silicon particles poten- tially detaching from the main composite structure and loss of active material [14]. Other authors, such as Shweta et al. [10], observed that the ionic pathway tortuosity increases with cracking, which could be one of the reasons for a power fade. These challenges, amongst others, https://doi.org/10.1016/j.est.2023.108609 Received 2 May 2023; Received in revised form 11 July 2023; Accepted 2 August 2023