Fluorine-Rich Electrolyte Additive for Achieving Dendrite-Free Lithium Anodes at Low Temperatures.

The practical application of lithium metal anodes is significantly impeded by poor interfacial stability and uncontrolled dendrite growth. Herein, we introduce methyl trifluoroacetate (MTFA), a low-melting-point small molecule, as an electrolyte additive in an ether-based electrolyte. This additive facilitates the formation of an in situ composite solid electrolyte interphase (SEI) layer that is rich in LiF and features an ester-based flexible matrix. The resulting composite layer exhibits high ionic conductivity and mechanical stability, effectively regulating the lithium deposition behavior over a broad temperature range and inhibiting dendrite formation. Based on MTFA, the Li||Li symmetrical cell achieves a lifespan exceeding 5000 h at room temperature and 800 h at -20 °C, both with ultralow overpotential and exceptional cycling stability. In Li||LiFePO4 full cells with a high-area loading (10.52 mg cm-2) and an N/P ratio of 1.68, an average capacity decay of merely 0.096% per cycle is observed over 200 cycles. Even at -20 °C, the Li||LiFePO4 cell shows a CE of over 99% and maintains stable cycling performance. This work provides an innovative approach for optimizing lithium metal anode interfaces and enhancing low-temperature operation capabilities through the use of electrolyte additives.

Simple Framework for Simultaneous Analysis of Both Electrodes in Stoichiometric Lithium-Sulfur Batteries.

A battery is composed of two electrodes that depend on and interact with each other. However, galvanostatic charging-discharging measurement, the most widely used method for battery evaluation, cannot simultaneously reflect performance metrics [capacity, Coulombic efficiency (CE), and cycling stability] of both electrodes because the result is generally governed by the lower-capacity electrode of the cell, namely the limiting reagent of the battery reaction. In studying stoichiometric Li-S cells operating under application-relevant high-mass-loading and lean-electrolyte conditions, we take advantage of the two-stage discharging behavior of sulfur to construct a simple framework that allows us to analyze both electrodes simultaneously. The cell capacity and its decay are anode performance descriptors, whereas the first plateau capacity and cell CE are cathode performance descriptors. Our analysis within this frame identifies Li stripping/plating and polysulfide shuttling to be the limiting factors for the cycling performance of the stoichiometric Li-S cell. Using our newly developed framework, we examine various previously reported strategies to mitigate these bottleneck problems and find modifying the separator with a reduced graphene oxide layer to be an effective means, which improves the capacity retention rate of the cell to 99.7% per cycle.

Asymmetric Coordination Regulating D-Orbital Spin-Electron Filling in Single-Atom Iron Catalyst for Efficient Oxygen Reduction.

The single-atom Fe-N-C catalyst has shown great promise for the oxygen reduction reaction (ORR), yet the intrinsic activity is not satisfactory. There is a pressing need to gain a deeper understanding of the charge configuration of the Fe-N-C catalyst and to develop rational modulation strategies. Herein, we have prepared a single-atom Fe catalyst with the co-coordination of N and O (denoted as Fe-N/O-C) and adjacent defect, proposing a strategy to optimize the d-orbital spin-electron filling of Fe sites by fine-tuning the first coordination shell. The Fe-N/O-C exhibits significantly better ORR activity compared to its Fe-N-C counterpart and commercial Pt/C, with a much more positive half-wave potential (0.927 V) and higher kinetic current density. Moreover, using the Fe-N/O-C catalyst, the Zn-air battery and proton exchange membrane fuel cell achieve peak power densities of up to 490 and 1179 mW cm-2, respectively. Theoretical studies and in situ electrochemical Raman spectroscopy reveal that Fe-N/O-C undergoes charge redistribution and negative shifting of the d-band center compared to Fe-N-C, thus optimizing the adsorption free energy of ORR intermediates. This work demonstrates the feasibility of introducing an asymmetric first coordination shell for single-atom catalysts and provides a new optimization direction for their practical application.

Tandem Catalysis inside Double-Shelled Nanocages with Separated and Tunable Atomic Catalyst Sites for High Performance Lithium-Sulfur Batteries.

Single-atomic catalysts are effective in mitigating the shuttling effect and slow redox kinetics of lithium polysulfides (LiPSs) in lithium-sulfur (Li-S) batteries, but their ideal performance has yet to be achieved due to the multi-step conversion of LiPSs requiring multifunctional active sites for tandem catalysis. Here double-shelled nano-cages (DSNCs) have been developed to address this challenge, featuring separated and tunable single-atom sites as nano reactors that trigger tandem catalysis and promote the efficient electrochemical conversion of LiPSs. This enables high capacity and durable Li-S batteries. The DSNCs, with inner Co-N4 and outer Zn-N4 sites (S/CoNC@ZnNC DSNCs), exhibit a high specific capacity of 1186 mAh g-1 at 1 C, along with a low capacity fading rate of 0.063% per cycle over 500 cycles. Even with a high sulfur loading (4.2 mg cm-2) and a low E/S ratio (6 µL mg-1), the cell displays excellent cycling stability. Moreover, the Li-S pouch cells are capable of stable cycling for more than 160 cycles. These results demonstrate the feasibility of driving successive sulfur conversion reactions with separated active sites, and are expected to inspire further catalyst design for high performance Li-S batteries.

In Situ Construction of Composite Artificial Solid Electrolyte Interphase for High-Performance Lithium Metal Batteries.

Lithium metal is considered as the most promising anode material for high energy density secondary batteries due to its high theoretical specific capacity and low redox potential. However, poor interfacial stability and uncontrollable dendrite growth seriously hinder the commercial application of Li metal anodes. Herein, we constructed a composite artificial solid-electrolyte interphase (ASEI) utilizing the in situ reaction between polyacrylic acid (PAA)/stannous fluoride (SnF2) and lithium metal, which spontaneously generates LiPAA, LiF, and Li5Sn2 alloys. The in situ formed LiPAA as a flexible matrix can accommodate the volume change of the lithium anode. Meanwhile, LiF and Li5Sn2 play the roles for improving the mechanical properties and boosting Li-ion flux in the interfacial layer, respectively. Benefiting from the ingenious design, the PAA-SnF2@Li anodes remain stable and dendrite-free morphology in symmetric cells for over 2000 h and exhibit excellent cycling stability in high-area loading (10.52 mg cm-2) Li||LiFePO4 full cells with a N/P of 1.68, which endures only 0.11% average capacity decay per cycle in 200 cycles. This simple and low-cost method supplies a route for the commercial application of lithium metal anodes with fresh eyes.

Unlocking Layered Double Hydroxide as a High-Performance Cathode Material for Aqueous Zinc-Ion Batteries.

Advanced cathode materials play an important role in promoting aqueous battery technology for safe energy storage. Transition metal double hydroxides are usually elusive as a stable cathode for aqueous zinc-ion batteries (AZIBs) due to their unstable crystal structure, sluggish ion transportation, and insufficient active sites for zinc-ion storage. Here, a trinary layered double hydroxide (LDH) with hydrogen vacancies (Ni3 Mn0.7 Fe0.3 -LDH) is reported as a new cathode material for AZIBs. A reversible high capacity up to 328 mAh g-1 can be obtained and cycle stably over 500 cycles with a capacity retention of 85%. Experimental and theoretical studies reveal that the hydrogen vacancies in LDH can expose lattice oxygen atoms as active sites for zinc-ion storage and accelerate ion diffusion by reducing the electrostatic interactions between zinc ions and the host structure. In addition, the synergy of the trinary transitional metal cations can suppress the Jahn-Teller distortion of manganese (III) oxide octahedron and enable long cycle stability. This work provides not only a series of high-performance cathode materials for AZIBs but also a novel materials design strategy that can be extended to other multi-valence metal-ion batteries.