In-Situ Electrical STEM Investigation of Interfacial Evolution and Anion Redox Mechanisms in Li-Rich Cathode Materials

High-energy-density Li-rich layered oxide cathodes play a crucial role in next-generation energy storage, with their performance strongly dependent on the dynamic structural changes under electrochemical cycling. Understanding the structural evolution of the cathode-electrolyte interface (CEI) under realistic working conditions is essential for elucidating capacity fading mechanisms. The advent of in-situ electrical transmission electron microscopy (TEM) has made it possible to study the true reaction structures of battery materials under biasing conditions [1-4]. Here, we employed in-situ electrical integrated differential phase contrast (iDPC) STEM to investigate the structural dynamics of Li-rich layered oxides, revealing voltage-dependent restructuring phenomena that provide insights into active site degradation. In this study, we fabricated a micro-battery configuration using Li-rich layered oxide cathodes and solid-state electrolytes. In-situ TEM experiments were performed using a specialized microelectromechanical system (MEMS) chip platform, allowing real-time observation of interfacial evolution under controlled voltage profiles. The atomic structure was characterized using iDPC-STEM, which is capable of imaging light elements (Li, O) with high signal-to-noise ratio under low-dose conditions, revealing excellent crystallinity and clear lithium sublattices in the pristine state. We conducted comprehensive structural characterization of the cathode interface under high-voltage charging conditions, as shown in Figure 1. Low-magnification STEM imaging revealed that while the bulk particle morphology remained intact, a notable feature emerged at the particle surface after charging to high voltage (>4.5 V) (Figure 1 a1-a2). A distinct structural reconstruction layer appeared, which was absent before the reaction. High-resolution iDPC-STEM imaging provided critical insights into this transformation at the atomic level (Figure 1 b1-b2). Before cycling, the transition metal (TM) and lithium layers exhibited a well-ordered alternating arrangement. After exposure to high voltage, the surface lattice underwent a dramatic phase transformation from a layered structure to a disordered rock-salt phase. This restructuring demonstrates the dynamic instability of the surface under strong electrochemical driving forces. The mechanism of this degradation was further elucidated by detailed iDPC observations of the cation migration (Figure 1 c1-c2). Multiple TM ions, originally located in the transition metal slab, were observed migrating into the lithium slab, indicated by the intensity changes in the HAADF signal and the contrast variation in iDPC images. Critically, this migration was accompanied by significant oxygen sublattice distortion. These structural defects were exclusively observed in the surface region (spanning ∼2nm) and were absent in the bulk, indicating that the degradation is initiated at the interface. The widespread distribution of these defects suggests an active mass transport process driven by surface oxygen redox activity. To verify the impact of this reconstruction, we analyzed the evolution of the Li-ion diffusion channels. As shown in Figure 1 d1-d2, the formation of the densified rock-salt layer effectively blocked the 2D diffusion pathways for lithium ions. In the pristine state, the Li-slab channels were open and continuous; however, after reaction, the migrated TM ions occupied these sites, creating a "blocking effect." This stark contrast between the open channels in the bulk and the blocked channels at the surface highlights that the voltage-induced surface reconstruction is the primary cause of the increased interfacial impedance and capacity decay [3]. In summary, our in-situ iDPC-STEM investigation reveals profound voltage-dependent structural evolution in Li-rich cathodes. The material undergoes a potential-induced phase transformation to a disordered rock-salt phase, accompanied by TM migration and oxygen lattice distortion. This degradation is surface-specific, demonstrating that the instability of the anionic redox at the interface drives the dynamic restructuring. These findings suggest that the capacity decay is triggered by the formation of this surface blocking layer, emphasizing the importance of in-situ light-element imaging for visualizing the true working state of battery interfaces [5]. Structural evolution of Li-rich cathodes under in-situ electrical biasing. (a1-a2) Low-magnification STEM images before and after biasing at voltage of 4.8 V. (b1-b2) High-resolution iDPC-STEM images revealing the transformation from a well-ordered layered phase to a disordered rock-salt structure. (c1-c2) Atomic-scale imaging of TM migration into the Li-slab, accompanied by oxygen distortion. (d1-d2) Comparison of Li-diffusion and TM ions channels, showing open pathways in the bulk versus blocked pathways at the reconstructed surface.