Effect of Mechanical Pressure on Lifetime, Expansion, and Porosity of Silicon Dominant Anodes in Laboratory Lithium-Ion Cells

Abstract

Silicon (Si) is a promising next-generation anode material for lithium-ion batteries due to its nearly ten times higher specific capacity of 3579 mAh/gSi 1. The high and economic availability of Si due to existing industrial infrastructure is another non-technical advantage.2 The main drawback of Si is its large volume expansion of nearly 300% during full lithiation causing SEI (re-) formation, electrochemical milling, and electrode instability leading to thickness changes resulting in the disruption of the electronic pathways and decoupling of particles.2 These problems still exist in partially lithiated µm-sized Si particles with lower volume expansion.2 In this work, we study the influence of compressive force on electrode stability in full cells composed of NCA3 cathodes and silicon-dominant anodes based on µm-sized particles.2 The different mechanical pressures were applied using spring-compressed 3.34 mAh Swagelok™ T-cells (A = 0.94 cm²) with metallic lithium reference electrodes. The investigated mechanical pressure ranges from 0.02 MPa as low pressure (LP) to 2.00 MPa, as high high pressure (HHP) to determine the optimal pressure for cyclic lifetime. Firstly, we introduce an experimental methodology to precisely adjust the common three-electrode SwagelokTM cell setup. Therefore, we take the compression from the single components into account, which were measured in a universal testing machine4 for the pressure range mentioned above. The cells were built with different mechanical pressures and cycled with C/2 until a SoH of 80% of the initial capacity was reached which was measured periodically in CheckUp tests. In the post-mortem analysis, Si anodes cycled at different mechanical pressures are prepared for scanning electron microscopy using a focused ion beam cross-section polisher. The images are then analyzed regarding electrochemical milling, thickness increase, and porosity decrease at 80% SoH by comparing them to the pristine state.The HHP cells with 2 MPa show a detrimental effect on lifetime during cell cycling as depicted in Figure 1 and with only 100 cycles nearly a halved lifetime compared to a normal pressure (NP) of 0.20 MPa. Moreover, the specific discharge capacity is significantly reduced for HHP or LP compressed cells compared to NP cells (see Figure 1b). The post-mortem analysis showed a dependency of the porosity and thickness of the aged Si anodes on the applied pressure. On the particle level, no electrochemical milling was observed due to the partial lithiation of the Si anodes. Vein-like microstructures were observed for the lithiated probes which might originate from different lithiation pathways of polycrystalline Si.5 Based on these experimental results, multilayer pouch cells with the same electrode setup and balancing are produced and cycled, and the expansion of the cells is measured operando in a compression test bench. Acknowledgments: S. F. gratefully acknowledges the financial support from the BMBF (Federal Ministry of Education and Research, Germany), under the auspices of the ExZellTUM III project (grant number 03XP0255). The authors want to thank the research battery production team of iwb for the manufacturing of the electrodes