In Situ Construction of a LiF-Enriched Interfacial Modification Layer for Stable All-Solid-State Batteries.

All-solid-state batteries (ASSBs), particularly based on sulfide solid-state electrolytes (SSEs), are expected to meet the requirements of high-energy-density energy storage. However, the unstable interface between the ceramic pellets and lithium (Li) metal can induce unconstrained Li-dendrite growth with safety concerns. Herein, we design a carbon fluoride-silver (CFx-Ag) composite to modify the SSEs. As lithium fluoride (LiF) nanocrystals can be in situ formed through electrochemical reactions, this LiF-enriched modification layer with high surface energy can more effectively suppress Li dendrite penetration and interfacial reactions between the SSEs and anode. Remarkably, the all-solid-state symmetric cells using a lithium-boron alloy (LiB) anode can stably work to above 2,500 h under 0.5 mA cm-2 and 2 mAh cm-2 at 60 °C without shorting. A modified LiB||LiNi0.6Mn0.2Co0.2O2 (NMC622) full cell also demonstrates an improved capacity retention and high Coulombic efficiency (99.9%) over 500 cycles. This work provides an advanced solid-state interface architecture to address Li-dendrite issues of ASSBs.

Dictating the interfacial stability of nickel-rich LiNi0.90Co0.05Mn0.05O2 via a diazacyclo electrolyte additive - 2-Fluoropyrazine.

Nickel-rich (Ni-rich) cathode materials, LiNixCoyMnzO2 (NCM, x ≥ 0.9, x + y + z = 1) hold great promise for developing high energy density lithium ion batteries especially for vehicle electrification. However, the practical application of Ni-rich cathode materials is still suffered from fast structural and interfacial degradation, and the resulted capacity decay. In this study, a diazacyclo type electrolyte additive, 2-fluoropyrazine (2-FP), was explored for the first time to boost the interfacial stabilization of single crystal LiNi0.90Co0.05Mn0.05O2 (NCM90) cathode. The capacity retention of the NCM90 is evidently promoted from 72.3% to 82.1% after 200 cycles at 1C (180 mA g-1) when adding 0.2% 2-FP into the electrolyte. The improvement of the electrochemical performance is ascribed to the generation of a compact and homogeneous cathode electrolyte interphase (CEI) film through ring-opening electrochemical polymerization of 2-FP upon the NCM90 electrode particles. This enhanced CEI layer benefits the suppression of the decomposition of LiPF6 electrolyte and the dissolution of the transition metals (Co and Mn), thus preventing the detrimental side reactions between the NCM90 electrode and the electrolyte.

Stabilizing Ni-Rich LiNi0.83Co0.12Mn0.05O2 with Cyclopentyl Isocyanate as a Novel Electrolyte Additive.

Ni-rich layered structure materials are appealing cathodes for high-energy-density lithium-ion batteries developed for electric vehicles, drones, power tools, etc. However, poor interfacial stability between a Ni-rich cathode and carbonate electrolyte, especially at high temperatures, and fast capacity fading still hinder their mass market penetration. Here, we investigate cyclopentyl isocyanate (CPI) with a single isocyanate (-NCO) functional group as a bifunctional electrolyte additive for the first time to improve the interfacial stability of Ni-rich cathode LiNi0.83Co0.12Mn0.05O2 (NCM83). With an electrolyte containing 2 wt % CPI, the NCM83 cathode shows capacity retention of up to 92.3% after 200 cycles at 1C and 30 °C, much higher than that with the standard electrolyte (78.6%). It is demonstrated that the -NCO of CPI could largely inhibit the thermal decomposition of LiPF6 salt and scavenge water and hydrogen fluoride (HF) species, improving electrolyte stability. More importantly, the additive CPI could be preferentially oxidized, forming a stabilized and protective cathode electrolyte interphase (CEI) layer on the surface of NCM83, which effectively suppresses the parasitic side reactions and maintains the superior interfacial charge-transfer and lithium-ion diffusion kinetics. Both functions enable a significant improvement in electrochemical performance at both 30 and 60 °C.

Highly Efficient Electrochemical Reduction of Nitrogen to Ammonia on Surface Termination Modified Ti3C2Tx MXene Nanosheets.

MXene-based catalysts exhibit extraordinary advantages for many catalysis reactions, such as the hydrogen evolution and oxygen reduction reactions. However, MXenes exhibit inadequate catalytic activity for the electrochemical nitrogen reduction reaction (NRR) because they are typically terminated with inactive functional groups, F* and OH*, which mask the active metal sites for N2 binding. Here we modified the surface termination of MXene (Ti3C2Tx) nanosheets to achieve high surface catalytic reactivity for the NRR by ironing out inactive F*/OH* terminals to expose more active sites and by introducing Fe to greatly reduce the surface work function. The optimally performing catalyst (MXene/TiFeOx-700) achieved excellent Faradaic efficiency of 25.44% and an NH3 yield rate of 2.19 µg/cm2·h (21.9 µg/mgcat·h), outperforming all reported MXene-based NRR catalysts. Our work provides a feasible strategy for rationally improving the surface reactivity of MXene-based catalysts for efficient electrochemical conversion of N2 to NH3.

Surface-Engineered Black Niobium Oxide@Graphene Nanosheets for High-Performance Sodium-/Potassium-Ion Full Batteries.

Nanoscale surface-engineering plays an important role in improving the performance of battery electrodes. Nb2 O5 is one typical model anode material with promising high-rate lithium storage. However, its modest reaction kinetics and low electrical conductivity obstruct the efficient storage of larger ions of sodium or potassium. In this work, partially surface-amorphized and defect-rich black niobium oxide@graphene (black Nb2 O5- x @rGO) nanosheets are designed to overcome the above Na/K storage problems. The black Nb2 O5- x @rGO nanosheets electrodes deliver a high-rate Na and K storage capacity (123 and 73 mAh g-1 , respectively at 3 A g-1 ) with long-term cycling stability. Besides, both Na-ion and K-ion full batteries based on black Nb2 O5- x @rGO nanosheets anodes and vanadate-based cathodes (Na0.33 V2 O5 and K0.5 V2 O5 for Na-ion and K-ion full batteries, respectively) demonstrate promising rate and cycling performance. Notably, the K-ion full battery delivers higher energy and power densities (172 Wh Kg-1 and 430 W Kg-1 ), comparable to those reported in state-of-the-art K-ion full batteries, accompanying with a capacity retention of ≈81.3% over 270 cycles. This result on Na-/K-ion batteries may pave the way to next-generation post-lithium batteries.

Nitrogen-Doped Graphene-Encapsulated Nickel-Copper Alloy Nanoflower for Highly Efficient Electrochemical Hydrogen Evolution Reaction.

Development of high-performance and low-cost nonprecious metal electrocatalysts is critical for eco-friendly hydrogen production through electrolysis. Herein, a novel nanoflower-like electrocatalyst comprising few-layer nitrogen-doped graphene-encapsulated nickel-copper alloy directly on a porous nitrogen-doped graphic carbon framework (denoted as Nix Cuy @ NG-NC) is successfully synthesized using a facile and scalable method through calcinating the carbon, copper, and nickel hydroxy carbonate composite under inert atmosphere. The introduction of Cu can effectively modulate the morphologies and hydrogen evolution reaction (HER) performance. Moreover, the calcination temperature is an important factor to tune the thickness of graphene layers of the Nix Cuy @ NG-NC composites and the associated electrocatalytic performance. Due to the collective effects including unique porous flowered architecture and the synergetic effect between the bimetallic alloy core and graphene shell, the Ni3 Cu1 @ NG-NC electrocatalyst obtained under optimized conditions exhibits highly efficient and ultrastable activity toward HER in harsh environments, i.e., a low overpotential of 122 mV to achieve a current density of 10 mA cm-2 with a low Tafel slope of 84.2 mV dec-1 in alkaline media, and a low overpotential of 95 mV to achieve a current density of 10 mA cm-2 with a low Tafel slope of 77.1 mV dec-1 in acidic electrolyte.

Zener Tunneling and Photoresponse of a WS2/Si van der Waals Heterojunction.

Van der Waals heterostructures built from two-dimensional materials on a conventional semiconductor offer novel electronic and optoelectronic properties for next-generation information devices. Here we report that by simply stacking a vapor-phase-synthesized multilayer n-type WS2 film onto a p-type Si substrate, a high-responsivity Zener photodiode can be achieved. We find that above a small reverse threshold voltage of 0.5 V, the fabricated heterojunction exhibits Zener tunneling behavior which was confirmed by its negative temperature coefficient of the breakdown voltage. The WS2/Si heterojunction working in the Zener breakdown regime shows a stable and linear photoresponse, a broadband photoresponse ranging from 340 to 1100 nm with a maximum photoresponsivity of 5.7 A/W at 660 nm and a fast response speed of 670 µs. Such high performance can be attributed to the ultrathin depletion layer involved in the WS2/Si p-n junction, on which a strong electric field can be created even with a small reverse voltage and thereby enabling an efficient separation of the photogenerated electron-hole pairs.

Composite Transparent Electrode of Graphene Nanowalls and Silver Nanowires on Micropyramidal Si for High-Efficiency Schottky Junction Solar Cells.

The conventional graphene-silicon Schottky junction solar cell inevitably involves the graphene growth and transfer process, which results in complicated technology, loss of quality of the graphene, extra cost, and environmental unfriendliness. Moreover, the conventional transfer method is not well suited to conformationally coat graphene on a three-dimensional (3D) silicon surface. Thus, worse interfacial conditions are inevitable. In this work, we directly grow graphene nanowalls (GNWs) onto the micropyramidal silicon (MP) by the plasma-enhanced chemical vapor deposition method. By controlling growth time, the cell exhibits optimal pristine photovoltaic performance of 3.8%. Furthermore, we improve the conductivity of the GNW electrode by introducing the silver nanowire (AgNW) network, which could achieve lower sheet resistance. An efficiency of 6.6% has been obtained for the AgNWs-GNWs-MP solar cell without any chemical doping. Meanwhile, the cell exhibits excellent stability exposed to air. Our studies show a promising way to develop simple-technology, low-cost, high-efficiency, and stable Schottky junction solar cells.