Atomic-Resolution In Situ Exploration of the Phase Transition Triggered Failure in a Single-Crystalline Ni-Rich Cathode.

Single-crystalline cathode materials LiNixCoyMn1-y-zO2 (x ≥ 0.6) are important candidates for obtaining better cyclic stability and achieving high energy densities of Li-ion batteries. However, it is liable to initiate phase transitions inside the grains during electrochemical cycling, and the processes and regions of these phase transitions have remained unknown. In this research, we conducted an intrinsic study, investigating the chemicals and microstructural evolution of single-crystalline LiNi0.83Co0.11Mn0.06O2 using in situ biasing transmission electron microscopy at an atomic scale. We observed that the layered structure on the surface of the single-crystalline material was degraded during the charging process, resulting in continuous phase transitions and the formation of surface oxygen vacancies, which can reduce both the structural and thermal stability of the material. Uneven delithiation led to the formation of high-density defects and discontinuous inactive electrochemical phases, such as local antiphase boundaries and the rock salt phase, in the bulk of the material. The non-uniformity of the structure and the coexistence of active and inactive phases introduce significant tensile stress, which can lead to intragranular cracks inside the grains. As the number of cycles increases, the structural degradation caused by the intragranular phase transition will further increase, ultimately affecting the cycling capacity and stability of the battery. This work has broad implications for creating lithium-ion batteries that are effective and long-lasting.

From Oxygen Redox to Sulfur Redox: A Paradigm for Li-Rich Layered Cathodes.

The utilization of anionic redox chemistry provides an opportunity to further improve the energy density of Li-ion batteries, particularly for Li-rich layered oxides. However, oxygen-based hosts still suffer from unfavorable structural rearrangement, including the oxygen release and transition metal (TM)-ion migration, in association with the tenuous framework rooted in the ionicity of the TM-O bonding. An intrinsic solution, by using a sulfur-based host with strong TM-S covalency, is proposed here to buffer the lattice distortion upon the highly activating sulfur redox process, and it achieves howling success in stabilizing the host frameworks. Experimental results demonstrate the prolonged preservation of the layered sulfur lattice, especially the honeycomb superlattice, during the Li+ extraction/insertion process in contrast to the large structural degeneration in Li-rich oxides. Moreover, the Li-rich sulfide cathodes exhibited a negligible overpotential of 0.08 V and a voltage drop of 0.13 mV/cycle, while maintaining a substantial reversible capacity upon cycling. These superior electrochemical performances can be unambiguously ascribed to the much shorter trajectories of sulfur in comparison to those of oxygen revealed by molecular dynamics simulations at a large scale (∼30 nm) and a long time scale (∼300 ps) via high-dimensional neural network potentials during the delithiation process. Our findings highlight the importance of stabilizing host frameworks and establish general guidance for designing Li-rich cathodes with durable anionic redox chemistry.

A new Li-rich layered cathode with low lattice strain for lithium-ion batteries.

A new layered C2/m oxide, Li2Ni0.2Mn0.4Ru0.4O3 (LNMR), is introduced as a cathode for lithium-ion batteries, which undergoes a low volume variation of 1.5% in the voltage window of 2.0-4.6 V studied via in situ X-ray diffraction. Compared with the contrast sample Li2MnO3, LNMR displays superior capacity, a more stable capacity retention rate, and higher energy density and average discharge voltage.

High-Performance Flexible In-Plane Micro-Supercapacitors Based on Vertically Aligned CuSe@Ni(OH)2 Hybrid Nanosheet Films.

The orientation and hybridization of ultrathin two-dimensional (2D) nanostructures on interdigital electrodes is vital for developing high-performance flexible in-plane micro-supercapacitors (MSCs). Despite great progress has been achieved, integrating CuSe and Ni(OH)2 nanosheets to generate advanced nanohybrids with oriented arrangement of each component and formation of porous frameworks remains a challenge, and their application for in-plane MSCs has not been explored. Herein, the vertically aligned CuSe@Ni(OH)2 hybrid nanosheet films with hierarchical open channels are skillfully deposited on Au interdigital electrodes/polyethylene terephthalate substrate via a template-free sequential electrodeposition approach, and directly employed to construct in-plane MSCs by choosing polyvinyl alcohol-LiCl gel as both the separator and the solid electrolyte. Because of the unique geometrical structure and combination of intrinsically conductive CuSe and battery-type Ni(OH)2 components, such hybrid nanosheet films can not only resolve the poor conductivity and re-stacking problems of Ni(OH)2 nanosheets but also create the 3D electrons or ions transport pathway. Thus, the in-plane MSCs device fabricated by such hybrid nanosheet films exhibits high volumetric specific capacitance (38.9 F cm-3). Moreover, its maximal energy and power density can reach 5.4 mW h cm-3 and 833.2 mW cm-3, superior to pure CuSe nanosheets, and most of reported carbon materials and metal hydroxides/oxides/sulfides based in-plane MSCs ones. Also, the hybrid nanosheet films device shows excellent cycling performance, good flexibility, and mechanical stability. This work may shed some light on optimizing 2D electrode materials and promote the development of flexible in-plane MSCs or other energy storage systems.