Elucidating Heterogeneous Li Insertion Using Single Crystalline and Freestanding Layered Oxide Thin Film

Understanding the uniformity of ion insertion in lithium-ion batteries (LIBs) during cycling is crucial for extending battery lifespan. Non-uniform lithium insertion causes localized volume expansion, leading to strain, stress, and subsequent (electro)chemo-mechanical degradation. Solid-solution layered oxides, like Li(Ni,Mn,Co)O₂ (NMC), mitigate this degradation by homogenizing lithium distribution due to fast diffusion. However, NMC exhibits anisotropic diffusion behavior, with higher diffusion barriers between (003) planes compared to within the planes, creating potential phase heterogeneity across channels. This heterogeneity can induce mechanical deformations, affecting the durability of both layered oxides and other materials, such as graphite and transition metal sulfides.

The causes of channel-by-channel phase heterogeneity in lithium-ion batteries remain poorly understood, though anisotropic diffusion likely contributes to variations in lithium insertion kinetics across insertion channels. Factors such as uneven surface reconstruction layers, cathode electrolyte interphase (CEI), or surface coating thickness can locally affect surface kinetics during cycling. To investigate these effects, crystallographically predefined interfacial systems and in-situ lithium mapping are needed to track local insertion kinetics. While previous in-situ imaging techniques like optical and hard X-ray microscopy have revealed inhomogeneous lithium distribution in single-crystalline particles, they struggled to decouple diffusion and insertion effects due to varied crystal facets. Electrochemical impedance spectroscopy (EIS) has provided averaged kinetic data in controlled interfacial systems but lacks the spatial and temporal resolution necessary to capture channel-by-channel variations in lithium insertion kinetics.

Here, we fabricated a freestanding, (104) oriented LiNi_1/3Mn_1/3Co_1/3O₂ (NMC111) single crystal thin film using dissolution-induced release and performed in-situ scanning transmission X-ray microscopy to spatially resolve lithium insertion at well-defined interfaces.^1,2 We observed heterogeneous lithium concentration evolution due to channel-by-channel insertion rate variation, despite the potential for homogeneous lithium distribution via a solid solution phase at equilibrium in NMC111. Increasing current density exacerbates this heterogeneity, highlighting channel-by-channel variation. Our findings provide critical insights into battery electrode utilization and lifetime management, potentially guiding the design of more efficient and durable lithium-ion batteries.

References
1. J. Chung et al. Nano Letters Article ASAP [DOI: 10.1021/acs.nanolett.4c04129]
2. J. Lim et al. Science 353, 566-572 (2016).