Suppressing Sodium Dendrites by Multifunctional Polyvinylidene Fluoride (PVDF) Interlayers with Nonthrough Pores and High Flux/Affinity of Sodium Ions toward Long Cycle Life Sodium Oxygen‐Batteries

Rechargeable sodium–oxygen (Na–O₂) batteries are of interest due to their high specific capacity, high equilibrium potential output, and the abundance of sodium resources; however, their cycle life is still very poor due to instability of electrolytes and especially the uncontrollable growth of Na dendrites. Herein, as a proof‐of‐concept experiment, a facile and low‐cost strategy is first proposed and demonstrated to effectively suppress growth of Na dendrites by using a fibrillar polyvinylidene fluoride film (f‐PVDF) with nonthrough pore as a multifunctional blocking interlayer. Unexpectedly, the f‐PVDF interlayer endows Na–O₂ battery with superior electrochemical performances, including high rate capability and long cycle life (up to 87 cycles), which is superior to those of the compact PVDF (c‐PVDF), PVDF with through pores (p‐PVDF), polyethylene oxide (PEO), and conventional polytetrafluoroethylene (PTFE) counterparts due to the following combined advantages: (1) the stronger C? F polar function groups provide a better affinity to Na ions, thus enabling a more homogeneous Na deposition than that of C? O function groups in PEO interlayer; (2) compared with c‐PVDF and p‐PVDF interlayers, f‐PVDF holds more electrolyte uptake for higher ion conductivity; (3) the good wettability of the f‐PVDF interlayer with electrolyte benefits Na dendrite suppression compared with PTFE interlayer.     

Eliminating Dendrites and Side Reactions via a Multifunctional ZnSe Protective Layer toward Advanced Aqueous Zn Metal Batteries

The development of aqueous Zn metal batteries (AZMBs) is impeded by severe corrosion, H₂ evolution, and dendrite formation issues. In addition, the inability of AZMBs to achieve a large capacity also hinders their commercialization. Here, a multifunctional ZnSe protective layer is reported to synchronously solve the above issues. The ZnSe layer can efficiently provide anticorrosion while also suppressing hydrogen evolution. Systematic analyses of the mechanism suggest that the low Zn affinity of ZnSe and the unbalanced charge distribution at the interface can promote a uniform distribution of Zn²⁺ and accelerate Zn²⁺ migration, thus realizing dendrite-free behavior. Therefore, the Zn@ZnSe symmetric cell exhibits notable rate performance and cycling stability (1500 h). Moreover, this symmetric cell can still stabilize with a low polarization (50 mV), even at 10 mA cm⁻² with 5 mAh cm⁻². The full cell paired with MnO₂ achieves a long lifespan (1800 cycles) with a Coulombic efficiency near 100%. Therefore, this strategy for eliminating dendrites and side reactions at a high rate with a large capacity provides a promising solution for the development of AZMBs.     

In Situ Defect-Free Vertically Aligned Layered Double Hydroxide Composite Membrane for High Areal Capacity and Long-Cycle Zinc-Based Flow Battery

Rechargeable aqueous zinc-based flow batteries (ZFBs) are promising candidates for large scale energy storage devices. However, the challenges from zinc dendrites and limited areal capacity considerably impede their wide application. Here, an in situ vertical growth of layered double hydroxide membrane (LDH-G) is constructed to enable long-life ZFBs. Owing to the high hydroxide ion conductivity and ion selectivity nature of LDH nanosheets, specifically, the precise control of directional ion transport in vertical arrangement LDHs, a superior battery performance can be realized. Moreover, the defect-free LDHs layer serves as a buffer layer to enable a uniform Zn deposition, which effectively enhances the areal capacity of the battery. As a result, the designed membrane endows an alkaline zinc-iron flow battery with excellent rate performance and cycling stability, maintaining an energy efficiency of 80% at 260 mA cm⁻² for 800 cycles, which is the highest performance ever reported. Most importantly, the LDHs layer enables the battery for 1200 h long-cycle stability with a uniform Zn deposition and high areal capacity of 240 mAh cm⁻². This work realizes an in situ growth of 3D LDHs arrays on the polymer substrate, which provides a strategy toward high areal capacity and dendrite-free Zn deposition for ZFBs.     

Hexafluoroisopropyl Trifluoromethanesulfonate-Driven Easily Li+ Desolvated Electrolyte to Afford Li||NCM811 Cells with Efficient Anode/Cathode Electrolyte Interphases

Solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) with optimized components and structures are considered to be crucial for lithium-ion batteries. Here, gradient lithium oxysulfide (Li₂SO_x, x = 0, 3, 4)/uniform lithium fluoride (LiF)-type SEI is designed in situ by using hexafluoroisopropyl trifluoromethanesulfonate (HFPTf) as electrolyte additive. HFPTf is more likely to be reduced on the surface of Li anode in electrolytes due to its high reduction potential. Moreover, HFPTf can make Li⁺ desolvated easily, leading to the increase in the flux of Li⁺ on the surface of Li anode to avoid the growth of Li dendrites. Thus, the cycling stability of Li||Li symmetric cells is improved to be 1000 h at 0.5 mA cm⁻². In addition, HFPTf-contained electrolyte could make Li||NCM811 batteries with a capacity retention of 70% after 150 cycles at 100 mA g⁻¹, which is attributed to the formation of uniform and stable CEI on the cathode surface for hindering the dissolvation of metal ions from the cathode. This study provides effective insights on the strong ability of additives to adjust electrolytes in “one phase and two interphases” (electrolyte and SEI/CEI).     

Perpendicular MXene Arrays with Periodic Interspaces toward Dendrite‐Free Lithium Metal Anodes with High‐Rate Capabilities

Although lithium metal is an ultimate anode material for lithium‐based batteries owing to its high theoretical capacity, the uncontrollable dendrites and infinite volume change associated with poor rate capabilities are stagnating its practical applications. Here, a new type of perpendicular MXene–Li array is developed with tunable MXene walls and constant space in between as anodes for lithium metal batteries. Such perpendicular MXene arrays possess dual periodic interspaces, i.e., nanometer‐scale interspaces in MXene walls and micrometer‐scale interspaces between MXene walls. The former interspaces are favorable for the fast transfer of lithium ions upon stripping and plating, and the latter enables efficiently homogenization of the electric field, leading to a good high‐rate capability up to 20 mA cm^?2. More importantly, the notorious lightning rod effect and volume change are efficiently inhibited in such perpendicular MXene arrays, giving rise to a dendrite‐free lithium anode with a low potential of 25 mV, a high capacity of 2056 mAh g^?1, and good cycle stability up to 1700 h.     

Advanced Liquid Electrolytes for Rechargeable Li Metal Batteries

Rechargeable lithium metal batteries (LMBs) have attracted wide attention for future electric vehicles and next‐generation energy storage because of their exceptionally high specific energy density. Recently, the development of electrode materials for LMBs has been extensively discussed and reviewed in the literature, but there have been very few reports that systematically review the status and progress of electrolytes for such applications. Actually, the viability of practical LMBs critically depends on the development of suitable liquid electrolytes due to the high reactivity of Li metals toward most solvents. This paper provides a systematic summary of the background and recent advances of the electrolytes for LMBs with an emphasis on the thermodynamic and kinetic stabilities at the interfaces. In addition, the emerging advanced characterization techniques for understanding the electrolyte–electrode interfaces are surveyed. Finally, a perspective for future directions is provided.     

Controlling Li Ion Flux through Materials Innovation for Dendrite‐Free Lithium Metal Anodes

Lithium (Li) metal with high theoretical capacity and the lowest electrochemical potential has been proposed as the ideal anode for high‐energy‐density rechargeable battery systems. However, the practical commercialization of Li metal anodes is precluded by a short lifespan and safety problems caused by their intrinsically high reductivity, infinite volume change, and uncontrollable dendrite growth during deposition and dissolution processes. Plenty of strategies have been introduced to solve the above‐mentioned problems. Among these, controlling Li⁺ flux plays a vital role to directly influence the plating and stripping process. In this work, the fundamental effect of Li⁺ flux distribution on Li nucleation and early dendrite growth is discussed. Then, recent strategies of controlling Li⁺ flux to suppress dendrite formation and growth through materials design are summarized, including homogenizing Li⁺ flux, localizing Li⁺ flux, and guiding gradient Li⁺ distribution. Finally, underexplored materials are proposed and explored to control Li⁺ flux and further directions for dendrite‐free Li anodes. It is expected that this progress report will help to deepen the understanding of Li⁺ flow tuning and morphology control of Li anodes and eventually facilitate the practical application of Li metal batteries.     

Numerical modeling and experimental investigation of the superficial layer of SKD61 steel during laser surface hardening

Abstract

Laser heating of SKD61 steel usually causes the formation of a melting layer on the steel surface, which is of poor thermal conductivity and diffusivity. The modified Ashby‐Easterling heat‐transfer equation was used to simulate the temperature distribution for laser surface hardening of SKD61 steel. The phase transformation temperatures of SKD61 in the quenched and as‐received conditions were compared with each other for different laser energy densities. When the laser was focused on the steel, the temperature of the SKD61 was raised. Due to the effect of superheating, the critical phase transformation temperature in laser hardening became higher than the austenized temperature (1010°C) in traditional quenching. However, the critical phase transformation temperature of SKD61 decreased with increasing laser energy density.     

Distribution of macro-inclusions in low carbon aluminium-killed steel slabs

Macro-inclusions in low carbon, aluminium-killed steel slabs were characterised by step-machining within a 10?mm zone from the slab surface using an ASPEX automatic inclusion analyzer. Dendritic structures within the cross-section of slabs were examined. The results show that alumina clusters and alumina associated with bubbles are the dominant macro-inclusions. Along the slab width direction, macro-inclusions were mostly found at the slab centre because of the deeper hooks and freezing meniscus surrounding the submerged entry nozzle. In terms of slab thickness, inclusions were mainly concentrated within the zone 3.5–6?mm from the top of the slab surface, where the columnar dendrites showed a relatively small inclination angle, indicating small cross-flow velocities at the solidification front. The number density of macro-inclusions were strongly dependent on the washing effect produced by the flow velocity. High speed casting promotes this behaviour and improves the surface quality of the slabs.