Effect of Mechanical Pressure on Rate Capability, Lifetime, and Expansion in Multilayer Pouch Cells with Silicon-Dominant Anodes

Abstract

Microscale silicon particles have a higher specific capacity but larger volume expansion than graphite particles, leading to particle decoupling and lifetime limitations. This study investigates a wide range of external mechanical pressures from zero (ZP - 0.00 MPa) to high (HP - 0.50 MPa) pressure to determine the optimal pressure for high rate capability, cyclic lifetime, energy density, low temperature rise, and low cell thickness gain. The cells are characterized by rate tests and impedance spectroscopy, and are aged until 70% state of health (SoH). The post-mortem analysis after 70% SoH and thickness measurements over 360 cycles in a compression test bench offer insights into the thickness gain. Electrochemical results reveal an immediate reduction in discharge capacity upon transitioning from normal pressure (NP - 0.20 MPa) to ZP, with NP and HP exhibiting superior performance over aging. The impedance was reduced initially and over aging for higher mechanical pressures, especially the cathode contact resistance, resulting in lower temperature rises during the rate tests. Overall, applying higher pressures reduced the anode and cell thickness gain. Moreover, the porosity decreased with increasing pressure, as determined by mercury intrusion porosimetry and pycnometer measurements. The increase of the anode mass correlates to the total charge throughput, which is pressure-dependent and the highest for NP.