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High chemical and mechanical stability of cathode surface are the prerequisites enabling high‐performance rechargeable battery. Surface facet is among the surface properties that dictate surface stability and cycling performance, while its underlying mechanism remains elusive. Herein, it is reported that surface stability is closely related to the surface facet for a variety of layered cathodes. The investigation shows that surface structure of P2 layered cathode undergoes sequential transformation upon cycling, which results in severe surface degradation. This study finds that the surface facets perpendicular to the (002) planes experience severe cracking and corrosion, while other surface facets are much more stable. The surface stability difference mainly comes from a geometric effect on strain release, which determines the mechanical stability of surface. Chemically, transition metal condensation forms a passivation layer to effectively prevent the inward propagation of surface degradation. Therefore, the surface facets oblique to the layered‐planes are intrinsically more resistant to mechanical cracking and chemical corrosion, which is further verified as a common effect in several O3‐type layered cathodes. This work not only deepens the understanding of the mechanism how surface facet affects surface stability, but also validates surface facet regulation can be a promising strategy for optimizing battery materials. [ABSTRACT FROM AUTHOR]

Abstract Electrochemically charging induced phase transition is a common and thermodynamically-driven phenomenon for variety of cathode materials, which couples with chemical and mechanical effects leading to performance degradation. Phase transition is particularly complex for layered sodium transition metal oxides and its related detrimental effects remain elusive. Herein, we take P2-type Na 2/3 Ni 1/3 Mn 2/3 O 2 (P2-NNM) as an example to scrutiny the detrimental consequences upon high voltage cycling. We find that repeated P2-O2 phase transition breaks down cathode primary grains by generating high density of intragranular cracks, which is qualitatively proved to be the main cause of performance decay. Intriguingly, the nucleation and growth of intragranular crack is through loss of atoms rather than cleavage, resembling the stress corrosion cracking mechanism which preferentially nucleates at P2/O2 phase boundary. Moreover, we find the P2-structured cathode is not sensitive to surface degradation, which explains the superior performance of P2-NNM cathode when cycling at low voltage. Graphical abstract Layered cathode for sodium ion battery, P2-type Na 2/3 Ni 1/3 Mn 2/3 O 2 (P2-NNM), suffers fast performance decay when cycling at high charge cutoff voltages. Our findings unveil that P2-O2 phase transition induced cracking degradation is the main cause of performance decay instead of surface degradation, which explains the sharp drop of cyclability at high voltage cycling. The crack nucleation and growth mechanism is studied using advanced transmission electron microscopy. fx1 Highlights • High voltage cycling induced P2-O2 phase transition leads to massive cracks, causing break-down of primary grains. • Qualitatively proved cracking is the main cause of performance decay of the P2-structured cathode. • The crack nucleation and growth follow the stress corrosion cracking mechanism in P2-structured cathode. • P2-structured cathode is not sensitive to surface degradation, explaining its superior performance at low cycling voltage. [ABSTRACT FROM AUTHOR]

Exploring the structure transformation mechanism of the spent cathodes during their regeneration process is the key to optimizing the processing protocol. The spent cathode materials are in different state of health (SOH) due to their different operation history, which leads to the challenge of restoring them in one-pot. Herein, to address the effect of SOH on the direct regeneration protocol, we systematically investigated the spent LiCoO 2 (LCO) cathode with low and high SOH. We find that lithium-deficiency and Al impurity are the two important factors in affecting the regeneration quality of the spent LCO. Lithium-deficiency can cause void defects and disordered lattice structure. Al impurity is brought to the LCO surface during lithium replenishment process, which further diffuses into the LCO subsurface layer during the following high temperature sintering. Based on our understanding of the regeneration process, the regeneration protocol is optimized accordingly, which can successfully restore the spent LCO with different SOH. [Display omitted] • State of health (SOH) of spent LiCoO 2 (LCO) affects the direct regeneration quality. • Poor regeneration protocol leads to surface residue, void defect, and disordered layered structure. • Al impurity is brought to LCO particle surface during lithium replenishment. • Lithium replenishment and impurity control are critical for direct regeneration of spent LCO. • We develop a direct regeneration protocol capable of restoring spent LCO with different SOH. [ABSTRACT FROM AUTHOR]

Electrodeposited copper samples composed of columnar grains subdivided by alternating twin/matrix (T/M) lamellae have been cold rolled to 30–85% reduction in thickness. The thickness of the T/M lamellae varies over a wide range from a few nanometres to about 1??m. The deformation microstructure has been characterized systematically. In thin T/M lamellae (below 50–100?nm) the deformation behaviours differ significantly from that of thick T/M lamellae, as the dislocation activity is concentrated at the T/M boundaries illustrated by the observations of stacking faults and Shockley partial dislocations. In thick T/M lamellae (100–1000?nm), the deformation microstructure is related to the grain orientation as also observed previously in deformed single crystals and polycrystals with a grain size at the micrometre scale. The experiment therefore suggests that the universal structural characteristics of deformation microstructure can be extended one order of magnitude from about 5??m to the sub-micrometre scale (about 0.5??m). [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) organic-inorganic hybrid perovskite materials have received substantial attention because of their exceptional optoelectronic properties. Although the applications of 2D perovskite nanosheets are promising in various optoelectronic devices, which all face harsh working conditions of light exposure, little is known about the photo-stability and degradation mechanisms of these 2D perovskite nanosheets. In this work, degradation of (C4H9NH3)2PbBr4 (BA2PbBr4) nanosheets when exposed to ultraviolet (UV) light and white light is explored. The morphology, optical properties, and microstructure of the nanosheets, under different conditions of light exposure, was studied in detail. UV light is more destructive compared to white light, which both led to a nanosheet breakdown. A combination of transmission electron microscopy (TEM) imaging and electron diffraction revealed that the organic moieties are most sensitive to light exposure and partial disorder toward complete disorder takes place during light exposure. Moreover, excessive light exposure further causes a [PbBr6]4− octahedron tilt and re-ordering within the perovskite structure. This study could enrich the understanding of 2D perovskite nanosheets and their photostability, offer a new perspective in interpreting the light–perovskite interaction, and further help the design of robust and light-tunable 2D perovskite-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

Abstract During the growth process of a plasma electrolytic oxide (PEO) coating on an AZ-31 Mg alloy, electric breakdown inevitably occurs and causes local disintegration of the coating, thus forming breakdown pores that result in an increased coating porosity and suppressed corrosion resistance. Here, we report for the first time that the self-repairing capability of the PEO process can restore the coating after breakdown damage by decreasing the porosity, tuning the components and improving the corrosion resistance via the deposition of electrolyte anions into the breakdown pores. Based on detailed scanning electron microscopy and transmission electron microscopy characterizations of the PEO coatings processed in alkaline silicate electrolytes, it was found that the pores were repaired by silica fillings formed by the plasma-induced deposition of anions (SiO 3 2−) in the electrolyte. The plasma-deposited silica possesses short Si O bond lengths that are manifested by an Si-L 2,3 edge shift by ∼1.3 eV to higher energies. In addition, the presence of some amount of Mg together with a minor admixture of K indicates the contribution of ionic migration and plasma collisions to the self-repairing process in the PEO coating. Graphical abstract Image 1 Highlights • Self-repairing capability of PEO process could recover the coating from breakdown damage. • Breakdown pores are repaired by silica fillings. • Self-repairing process contributes to decreasing porosity and improving corrosion resistance of PEO coating. [ABSTRACT FROM AUTHOR]