Ni-rich layered LiNi1–x–y Co x Mn y O2 (NCM, x + y ≤ 0.2) is an intensively studied class of cathode active materials for lithium-ion batteries, offering the advantage of high specific capacities. However, their reactivity is also one of the major issues limiting the lifetime of the batteries. NCM degradation, in literature, is mostly explained both by disintegration of secondary particles (large anisotropic volume changes during lithiation/delithiation) and by formation of rock-salt like phases at the grain surfaces at high potential with related oxygen loss. Here, we report the presence of intragranular nanopores in Li1+x (Ni0.85Co0.1Mn0.05)1–x O2 (NCM851005) and track their morphological evolution from pristine to cycled material (200 and 500 cycles) using aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and time-of-flight secondary ion mass spectrometry. Pores are already found in the primary particles of pristine material. Any potential effect of TEM sample preparation on the formation of nanopores is ruled out by performing thickness series measurements on the lamellae produced by focused ion beam milling. The presence of nanopores in pristine NCM851005 is in sharp contrast to previously observed pore formation during electrochemical cycling or heating. The intragranular pores have a diameter in the range between 10 and 50 nm with a distinct morphology that changes during cycling operation. A rock-salt like region is observed at the pore boundaries even in pristine material, and these regions grow with prolonged cycling. It is suggested that the presence of nanopores strongly affects the degradation of high-Ni NCM, as the pore surfaces apparently increase (i) oxygen loss, (ii) formation of rock-salt regions, and (iii) strain-induced effects within the primary grains. High-resolution STEM demonstrates that nanopores are a source of intragranular cracking during cycling.

The layered oxide LiNiO2 (LNO) has been extensively investigated as a cathode active material for lithium‐ion batteries. Despite LNO's high gravimetric capacity, instability issues hinder its commercialization. It suffers from capacity loss during electrochemical cycling and is difficult to synthesize without defects. This is related to poor structural stability, leading to decomposition into the parent rock‐salt‐type oxide. In order to understand such phase transformations and to develop measures to inhibit them, the development of techniques able to image all atoms is crucial. In this study, the use of a fast, pixelated detector and 4D imaging in scanning transmission electron microscopy are explored to tackle this challenge. Selecting specific angular regions in the diffraction patterns and calculating virtual annular bright‐field images significantly enhances the contrast of the lithium atoms, such that all atoms are visible even in realistic samples. The developed technique is applied to image the layered‐to‐rock salt phase transition region. The data show that in this region, nickel atoms are in tetrahedral positions and the oxygen atoms are asymmetrically distributed. Taken together, the results shed light on the phase transformation mechanism at the atomic scale and can guide future research toward stabilizing LNO.

Considering the high theoretical energy density and improved safety, thiophosphate-based all-solid-state batteries (ASSBs) have become one of the most promising candidates for next-generation energy storage systems. However, the intrinsic electrochemical instability of thiophosphate-based solid electrolytes in contact with oxide-based cathodes results in rapid capacity fading and has driven the need of protective cathode coatings. In this work, for the first time, a fumed lithium titanate (LTO) powder-based coating has been applied to Ni-rich oxide-based cathode active material (CAM) using a newly developed dry-coating process. The LTO cathode coating has been tested in thiophosphate-based ASSBs. It exhibits a significantly improved C-rate performance along with superior long-term cycling stability. The improved electrochemical performance is attributed to a reduced interfacial resistance between coated cathode and solid electrolyte as deduced from in-depth electrochemical impedance spectroscopy analysis. These results open up a new, facile dry-coating route to fabricate effective protective CAM coatings to enable long-life ASSBs. This nondestructive coating process with no post-heat-treatment approach is expected to simplify the coating process for a wide range of coatings and cathode materials, resulting in much improved cathode/electrolyte interfacial stability and electrochemical performance of ASSBs.

No abstract

Due to their high theoretical energy densities and superior safety, thiophosphate‐based all‐solid‐state batteries (ASSBs) are considered as promising power source for electric vehicles. However, for large‐scale industrial applications, interfacial degradation between high‐voltage cathode active materials (CAMs) and solid‐state electrolytes (SSEs) needs to be overcome with a simple, cost‐effective solution. Surface coatings, which prevent the direct physical contact between CAM and SSE and in turn stabilize the interface, are considered as promising approach to solve this issue. In this work, an Al2O3/LiAlO2 coating for Li(Ni0.70Co0.15Mn0.15)O2 (NCM) is tested for ASSBs. The coating is obtained from a recently developed dry coating process followed by post‐annealing at 600 °C. Structural characterization reveals that the heat treatment results in the formation of a dense Al2O3/LiAlO2 coating layer. Electrochemical evaluations confirm that the annealing‐induced structural changes are beneficial for ASSB. Cells containing Al2O3/LiAlO2‐coated NCM show a significant improvement of the rate capability and long‐term cycling performance compared to those assembled from Al2O3‐coated and uncoated cathodes. Moreover, electrochemical impedance spectroscopy analysis shows a decreased cell impedance after cycling indicating a reduced interfacial degradation for the Al2O3/LiAlO2‐coated electrode. The results highlight a promising low‐cost and scalable CAM coating process, enabling large‐scale cathode coating for next‐generation ASSBs.