Mechanistic analysis on electrochemo-mechanics behaviors of lithium iron phosphate cathodes

Abstract

The cathode in lithium-ion batteries (LIBs) is invariably subjected to mechanical stress due to external packaging constraints, and internal ionic diffusion and particle phase change. The (de)lithiation in lithium iron phosphate (LiFePO₄) occurs through the growth of a two-phase front with a fixed activity, thereby producing a relatively flat (dis)charge curve, posing a grand challenge for the battery status estimation. This knowledge gap not only hinders our understanding of the relationship between mechanics and electrochemical behavior but also limits the potential for leveraging mechanical regulation in the development of high-performance electrode materials and cells. To address this issue, we quantitatively investigate the stress effects on the LiFePO₄ cathode by employing constant current charge/discharge processes to capture the electrochemical performance of LIBs. To further understand the underlying mechanism and establish a new electrochemo-mechanics coupling model, comprehensive electrochemical characterizations upon various external stress statuses are conducted. Our findings reveal that within the 0–0.9 MPa range of external compressive stress, LiFePO₄ cathodes exhibit enhanced ionic diffusion coefficients, improved nucleation kinetics and reversibility, although accompanied by a small reduction in the cathode's equilibrium potential. In-situ X-ray diffraction experiments under diverse stress conditions corroborate the beneficial effects of mechanical stress on electrode reactions. These insights offer critical knowledge and pave the way for improved performance, reliability, and durability.