Multi-Modal Atomic Observation of Oxygen Loss in Layered Oxides Under External Strain
Authors: Kim Dongho , Hwang Jaejin , Byeon Pilgyu , Kim Junseop , Park ChangHyun , Lee Jaekwang , Chung Sung-Yoon 
Published: December 2024

The market share of electric vehicles has more than tripled in the last several years, going from about 4% in 2020 to 14% in 2022. Electric vehicles prevalently utilize lithium transition metal layered oxides as cathode materials, yet they suffer from voltage and capacity fade during cycling. Specifically, the oxygen redox contribution may result in significant structural instability since Li or metal cations are ordered into the octahedral site of the oxygen anion framework in the layered cathode. Recent studies have demonstrated the impact of oxygen redox on various aspects of structural deterioration, including cation migration, surface reconstruction, nanovoid/crack formation, and even trapped O2 molecules in bulk. However, dealing with a cycled sample involves multiple factors, including electrochemical, electrical, and mechanical variables, complicating a comprehensive understanding. In this study, we focused solely on mechanical factors by applying external strain without electrochemical testing.

Utilizing atomic-column-resolved STEM, EELS/EDS spectroscopy, and phase-contrast HRTEM enables direct imaging and chemical analysis. We examined the local structural variation and oxygen content of layered Li transition metal oxides (LiCoO2 and Li2MnO3) under external strain with the characterization techniques. A stepwise phase transition was observed in surface polished LiCoO2, as illustrated in Fig. 1a. While the layered structure remained intact in the bulk region, a region of intermixed Li/Co cations was found near the surface, with the outermost region exhibiting amorphous phase. Nevertheless, layer-by-layer EELS scans of the O K-edge and Co L3
-edges revealed that the phase transformation accompanies the oxygen vacancy formation and metal reduction (Fig. 1b). This result is consistent with previous studies, which suggest the release of lattice oxygen from cycled cathode materials when the surface reconstruction occurs.

In addition to simple surface polishing, the nanoindentation applied the concentrated load in the local region. While surface reconstruction into cation-intermixed and amorphous phase occurred in the same manner, we identified several lattice defects inside the bulk regions in Fig. 1c. Similar to surface polished sample, spectroscopic analyses combined with STEM images demonstrated substantial oxygen deficiency in the cation-disordered and amorphous region. Fig. 1d depicts the HRTEM image, highlighting the amorphous phase's presence in the crack region in addition to ABF-STEM images in Fig. 1c. One thing to note here is that we observed the same phenomena in the Li2MnO3 system, which implies that the hypothesis can be universally applicable to layered oxides. Furthermore, the DFT calculation shows that oxygen vacancy formation is energetically favorable under shear strain. Considering that all of the external strains in this study are shear strains, the calculation theoretically supports the experimental results. The findings in this work emphasize that alleviating lattice strain can effectively mitigate oxygen release in layered cathodes.

Figure 1