Mechanical properties of cathode–electrolyte interphase layers in high-voltage lithium-ion batteries

Abstract

LiNi_0.5Mn_1.5O₄ (LNMO) is a promising cathode material owing to its high operating potential of 4.75 V vs. Li⁺/Li. However, the high potential triggers interfacial instability, such as electrolyte oxidation. A mechanically robust cathode–electrolyte interphase (CEI) is essential for maintaining structural integrity and ensuring reliable performance of high-voltage lithium-ion batteries. CEI layer thickness is often under 20 nm, making the assessment of its elastic properties challenging. We applied an amplitude-modulated/frequency-modulated scanning force microscopy method to enable quantitative mechanical characterization of thin CEI layers on rough composite electrode surfaces. We systematically varied the number of battery cycles and investigated the morphology and elastic modulus of the interphase layers on LNMO cathodes. The pristine crystalline LNMO surface exhibited an elastic modulus of approximately 126 ± 20 GPa, whereas the binder/carbon (b/c) regions had a modulus of 1.9 ± 0.1 GPa. After only 5 cycles the elastic modulus on LNMO decreased to 3.2 ± 1.2 GPa, indicating an LNMO passivation by CEI growth. After 200 cycles, the elastic modulus became homogeneous with the moduli on the LNMO and b/c regions reaching 3.9 ± 0.8 GPa and 3.9 ± 0.4 GPa, respectively. This mechanical convergence is supported by a convergence in chemical composition of the interphase between the LNMO and b/c regions. We also observed a compositional shift from ether-rich oligomers to a more oxidized, carbonyl-rich organic network. The final stabilized modulus of ≈4 GPa reflects an organic-dominated interphase with enhanced intermolecular interactions. Inorganic species are likely buried beneath the organic-rich top layer. This work provides understanding of interfacial stability and establishes a robust and reproducible framework for quantifying the elastic modulus of interphase layers within composite electrodes, providing insights for the design of stable high-voltage battery systems.