Hydroiodic-Acid-Initiated Dense yet Porous Ti3 C2 Tx MXene Monoliths toward Superhigh Areal Energy Storage.

The assembly of 3D structured materials from 2D units paves a royal road for building thick and dense electrodes, which are long sought after for practical energy-storage devices. 2D transitional metal carbides (MXene) are promising for this due to their capabilities of solution-based assembly and intrinsic high density, yet face huge challenges in yielding high areal capacitance electrodes owing to the absence of porous ion-transport paths. Here, a gelation-densification process initiated by hydroiodide acids (HI) is proposed, where the protons break the electrostatic balance of MXene nanosheets to trigger gelation, while HI serves as a spacer to prevent nanosheets from restacking during capillary shrinkage. More promising, the controlled evaporation of reductive HI leaves superiorly shrinking yet porous network for ion transport, and the produced monoliths exhibit a high density of 2.74 g cm-3 and an unprecedented areal capacitance of 18.6 F cm-2 .

A Permselective Coating Protects Lithium Anode toward a Practical Lithium-Sulfur Battery.

Lithium metal is a desirable anode for high-energy density lithium-sulfur (Li-S) batteries. However, its reliability is severely limited by dendrite growth and side reactions with polysulfides, which are yet challenging to solve simultaneously. Herein, we report a protective layer that works the same way as the ion-permselective cell membrane, yielding a corrosion-resistant and dendrite-free Li metal anode specially for Li-S batteries. A self-limited assembly of octadecylamine together with Al3+ ions on a Li metal anode surface produces a dense, stable yet thin layer with ionic conductive Al-Li alloy uniformly embedded in it, which prevents the passage of polysulfides but regulates the penetrated Li ion flux for uniform Li deposition. As a result, the assembled batteries show excellent cycling stability even with a high sulfur-loaded cathode, suggesting a straightforward but promising strategy to stabilize highly active anodes for practical applications.

Bidirectional Lithiophilic Gradients Modification of Ultralight 3D Carbon Nanofiber Host for Stable Lithium Metal Anode.

Using 3D host is an effective way to solve the dendrite growth problem and accommodate volume changes of lithium (Li) metal anode. However, the preferred Li deposition on the top surface leads to the Li metal agglomeration at the surface. In addition, the large weight of the 3D host also greatly decreases the capacity based on the whole anode. Herein, a bidirectional lithiophilic gradient modification, including a top-down ZnO gradient and a bottom-up Sn gradient, is applied to an ultralight 3D carbon nanofiber host (density: 0.1 g cm-3 ) and ensures the evenly filling lithium deposition in the 3D host. ZnO transforms into highly ionic conductive Li-Zn alloy and Li2 O during cycling, enhancing the Li-ion transportation from top to bottom. The metallic Sn also lowers the Li nucleation potential, guiding the preferential Li deposition from the bottom. With such a host, a stable CE of 97.5% over 100 cycles at 1 mA cm-2 and 3 mAh cm-2 is achieved, and the full battery also delivers good cycling stability over 300 cycles with a high CE of 99.8% coupled with high loading LiFePO4 cathode (10 mg cm-2 ) and low N/P ratio (≈3).

The Catalyst Design for Lithium-Sulfur Batteries: Roles and Routes.

Lithium-sulfur battery is a promising candidate for next-generation high energy density batteries due to its ultrahigh theoretical energy density. However, it suffers from low sulfur utilization, fast capacity decay, and the notorious "shuttle effect" of lithium polysulfides (LiPSs) due to the sluggish reaction kinetics, which severely restrict its practical applications. Using the electrocatalyst can accelerate the redox reactions between sulfur, LiPSs and Li2 S and suppress the shuttling of LiPSs, and thus, it is a promising strategy to solve the above problems, enabling the battery with high energy density and long cycling stability. In this personal account, we discuss the catalyst design for lithium-sulfur batteries according to the sulfur reduction reaction (SRR) and sulfur evolution reaction (SER) in the discharging and charging processes. The catalytic effects for each step in SRR and SER are highlighted and the homogenous catalysts, the selective catalysts, and the bidirectional catalysts are discussed, which can help guide the rational design of the catalysts and practical applications of lithium-sulfur batteries.

l-Cysteine-Modified Acacia Gum as a Multifunctional Binder for Lithium-Sulfur Batteries.

A binder plays an important role in stabilizing the electrode structure and improving the cyclic stability of batteries. However, the traditional binders are no longer satisfactory in lithium-sulfur (Li-S) batteries because of their failure in accommodating the large volume changes of sulfur and trapping soluble intermediate polysulfides, thus causing severe capacity decay. In this work, we prepared a multifunctional binder for Li-S batteries by merely modifying the acacia gum (AG), a low-cost biomass polymer, with l-cysteine under mild conditions. Owing to the introduced amino and carboxyl branches by the l-cysteine, the modified AG shows enhanced polysulfide trapping ability and can effectively restrain the shuttling of polysulfides. In addition, the introduction of branches can help form a cross-linked 3D network with better mechanical strength and flexibility for adhering sulfur and accommodating the volume changes of cathode materials. As a result, compared with the normally used polyvinylidene fluoride binder and the unmodified AG binder, the l-cysteine-modified AG binder effectively enhanced the rate capability and cycling stability of the Li-S batteries directly using sulfur as the cathode, showing a promising way to prompt the practical use of Li-S batteries.

A Lightweight 3D Cu Nanowire Network with Phosphidation Gradient as Current Collector for High-Density Nucleation and Stable Deposition of Lithium.

Lithium metal anodes with high energy density are important for further development of next-generation batteries. However, inhomogeneous Li deposition and dendrite growth hinder their practical utilization. 3D current collectors are widely investigated to suppress dendrite growth, but they usually occupy a large volume and increase the weight of the system, hence decreasing the energy density. Additionally, the nonuniform distribution of Li ions results in low utilization of the porous structure. A lightweight, 3D Cu nanowire current collector with a phosphidation gradient is reported to balance the lithiophilicity with conductivity of the electrode. The phosphide gradient with good lithiophilicity and high ionic conductivity enables dense nucleation of Li and its steady deposition in the porous structure, realizing a high pore utilization. Specifically, the homogenous deposition of Li leads to the formation of an oriented texture on the electrode surface at high capacities. A high mass loading (≈44 wt%) of Li with a capacity of 3 mAh cm-2 and a high average Coulombic efficiency of 97.3% are achieved. A lifespan of 300 h in a symmetrical cell is obtained at 2 mA cm-2 , implying great potential to stabilize lithium metal.

Electrochemically activated spinel manganese oxide for rechargeable aqueous aluminum battery.

Aluminum is a naturally abundant, trivalent charge carrier with high theoretical specific capacity and volumetric energy density, rendering aluminum-ion batteries a technology of choice for future large-scale energy storage. However, the frequent collapse of the host structure of the cathode materials and sluggish kinetics of aluminum ion diffusion have thus far hampered the realization of practical battery devices. Here, we synthesize AlxMnO2·nH2O by an in-situ electrochemical transformation reaction to be used as a cathode material for an aluminum-ion battery with a configuration of Al/Al(OTF)3-H2O/AlxMnO2·nH2O. This cell is not only based on aqueous electrolyte chemistry but also delivers a high specific capacity of 467 mAh g-1 and a record high energy density of 481 Wh kg-1. The high safety of aqueous electrolyte, facile cell assembly and the low cost of materials suggest that this aqueous aluminum-ion battery holds promise for large-scale energy applications.

High-Voltage and Noncorrosive Ionic Liquid Electrolyte Used in Rechargeable Aluminum Battery.

As a promising post-lithium battery, rechargeable aluminum battery has the potential to achieve a three-electron reaction with fully use of metal aluminum. Alternative electrolytes are strongly needed for further development of rechargeable aluminum batteries, because typical AlCl3-contained imidazole-based ionic liquids are moisture sensitive, corrosive, and with low oxidation voltage. In this letter, a kind of noncorrosive and water-stable ionic liquid obtained by mixing 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM]OTF) with the corresponding aluminum salt (Al(OTF)3) is studied. This ionic liquid electrolyte has a high oxidation voltage (3.25 V vs Al3+/Al) and high ionic conductivity, and a good electrochemical performance is also achieved. A new strategy, which first used corrosive AlCl3-based electrolyte to construct a suitable passageway on the Al anode for Al3+, and then use noncorrosive Al(OTF)3-based electrolyte to get stable Al/electrolyte interface, is put forward.