Abstract text (incl. figure legends and references)

Layered cathode materials are commonly used in lithium and sodium ion batteries, but their structures prone to degradation during battery operation. The rigid fracture is the one of the key factors for degradation of functional materials, in particular the layered materials. Recently, the microstructure and defect engineering have broken a new ground in decreasing the rigid fracture and facilitating electrochemical performance of cathode materials. Herein, we design a buffer structure (tenon-and-mortise structure) at the biphasic boundary of Na0.67Fe0.1Cu0.1Mn0.8O2 layered oxide with a P'2/P2 biphasic structure (denoted as BP), which can inhibit the destructive evolution of layered oxides. We observe that the biphasic boundaries overlap by atomic-resolution scanning transmission electron microscopy (STEM) with spherical aberration correction. As shown in Figure 1, this phases overlap is deemed to a tenon-and-mortise structure that has been widely served as the buffer structure in traditional Chinese wooden architecture [1]. The structure Rietveld refinement combined with neutron powder diffraction (NPD) and X-ray diffraction (XRD) determine that the ration of P'2/P2 phase in BP is about 70%:30%. In addition, equipped with X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and electrochemical tests, we find that BP with low vacancy concentration is able to experience reversibly deeper desodiation/sodiation benefitted from the enhanced biphasic structure. Consequently, BP provides a competitive specific capacity of 200 mAh g-1 at 0.05C (1C = 150 mA g-1) and a superior cycling life for the retention of 84.2% after 1000 cycles at 8C. Ex-situ HAADF-STEM characterizations of cathode materials after different cycles illustrate that the dense tenon-and-mortise structures are stable under the operating condition. Furthermore, in-situ XRD demonstrates that the competition between the oxidation/reduction of transition metals and desodiation/sodiation in dense tenon-and-mortise structures buffers the interlayer spacing variation of BP, resulting in a mild snake-shaped evolution. This work provides an artful phase engineering to strengthen the original layered structure of SIBs cathodes without sacrificing the intrinsic properties.

Figure 1. (a,b) HAADF and ABF-STEM images of biphasic boundary for BP along [010] zone axis. The line scan profiles along the corresponding rectangles are shown on the right of (a). (c, d) HAADF and ABF-STEM images of biphasic boundary with clear tenon-and-mortise structure. (e) Schematic diagram of the biphasic boundary reinforced by tenon-and-mortise structure.

[1] Q. N. Liu, Z. Hu, W. J. Li, C. Zou, H. L. Jin, S. Wang, S. L. Chou, S. -X. Dou, Sodium transition metal oxides: the preferred cathode choice for future sodium-ion batteries? Energy Environ. Sci. 14, 158-179 (2021).