Z.Y. Liu

Triphenylamine-Based Metal–Organic Frameworks as Cathode Materials in Lithium-Ion Batteries with Coexistence of Redox Active Sites, High Working Voltage, and High Rate Stability

Through rational organization of two redox active building block, a triphenylamine-based metal-organic framework (MOF) material, Cu-TCA (H3TCA = tricarboxytriphenyl amine), was synthesized and applied as a cathode active material for the first time in lithium batteries. Cu-TCA exhibited redox activity both in the metal clusters (Cu(+)/Cu(2+)) and organic ligand radicals (N/N(+)) with separated voltage plateaus and a high working potential vs Li/Li(+) up to 4.3 V, comparing with the current commercial LiCoO2 cathode materials. The electrochemical behaviors of this MOF electrode material at different states of charge were carefully studied by cyclic voltammetry, X-ray photoelectron spectroscopy, and photoluminescence techniques. Long cycling stability of this MOF was achieved with an average Coulombic efficiency of 96.5% for 200 cycles at a 2 C rate. Discussing the electrochemical performances on the basis of capacity contributions from the metal clusters (Cu(+)/Cu(2+)) and organic ligands (N/N(+)) proposes an alternative mechanism of capacity loss for the MOF materials used in lithium batteries. This improved understanding will shed light on the designing principle of MOF-based cathode materials for their practical application in battery sciences.

Elucidating the alkaline oxygen evolution reaction mechanism on platinum

Understanding the interplay between surface chemistry, electronic structure, and reaction mechanism of the catalyst at the electrified solid/liquid interface will enable the design of more efficient materials systems for sustainable energy production. The substantial progress in operando characterization, particularly using synchrotron based X-ray spectroscopies, provides the unprecedented opportunity to uncover surface chemical and structural transformations under various (electro)chemical reaction environments. In this work, we study a polycrystalline platinum surface under oxygen evolution conditions in an alkaline electrolyte by means of ambient pressure X-ray photoelectron spectroscopy performed at the electrified solid/liquid interface. We elucidate previously inaccessible aspects of the surface chemistry and structure as a function of the applied potential, allowing us to propose a reaction mechanism for oxygen evolution on a platinum electrode in alkaline solutions.

Simulation of the electrohydrodynamic instability process used in the fabrication of hierarchic and hollow micro/nanostructures

This article demonstrates that the electrohydrodynamic patterning process, a novel technique for the manufacturing of micro- and nano-scale structures, also allows the one-step realization of hierarchical structures and hollow structures. Through numerical simulation, it is shown that multilevel structures can be obtained if process time and applied electric voltage are optimized. As an example, the growth of structures with a width of around 187 nm and depth of 95 nm has been successfully simulated alongside structures with width of around 0.4 μm and depth of 0.8 μm. The width of the protrusive mask patterns is shown to determine whether hollow structures with single or multiple shapes can be formed using electric field assisted capillarity. The numerical simulation process effectively demonstrates that the realization of micro/nano-structures with hierarchic and multilevel shapes can be considered as an innovative manufacturing process for MEMS or micro/nanofluidic structures.

The long life-span of a Li-metal anode enabled by a protective layer based on the pyrolyzed N-doped binder network

Attempts to utilize lithium metal in secondary batteries are seriously restricted by its uncontrollable side reactions with the electrolyte solvent. Here we utilize a protective porous structure based on the pyrolyzed PAN binder to stabilize the electrolyte/lithium interface to prolong its working life. With the increase of pyrolysis temperatures, the treated PAN fibers possess two mutational points in mechanical properties located at ∼300 & ∼700 °C, and exhibit carbon-like characteristics at ∼400 °C and higher. Compared to the control electrode, the cyclic life-span of the treated electrodes can increase 1.8 times at the first mutational point, and surprisingly rise to 12 & 7 times for the samples pyrolyzed at 400 & 500 °C, then fall back to 1.6 times at the second mutational point. These results reveal that the stable operation of lithium plating/stripping could be provided by the internal interwoven SEI layer grown on the carbon-like binder network with appropriate rigidity. Among the investigated systems, the protective structure treated at 400 °C can stably operate for 350 cycles with an average coulombic efficiency as high as ∼98%, which is the best efficiency recorded for carbonate-based electrolytes to date.

Stability of Li2CO3 in cathode of lithium ion battery and its influence on electrochemical performance

Lithium carbonate is an unavoidable impurity at the cathode side. It can react with LiPF6-based electrolyte and LiPF6 powder to produce LiF and CO2, although it presents excellent electrochemical inertness. Samples of Li2CO3-coated and LiF-coated LiNi0.8Co0.1Mn0.1O2 were prepared to compare their influence on a cathodes behavior. After 200 cycles at 1C, in contrast to 37.1% of capacity retention for the Li2CO3-coated material, the LiF-coated LiNi0.8Co0.1Mn0.1O2 retained 91.9% of its initial capacity, which is similar to the fresh sample. This demonstrates that decomposition of Li2CO3 can seriously deteriorate cyclic stability if this occurs during working.

Influence of Enhanced O2 Provision Achieved with Fluoroether Incorporation on the Discharge Performance of Li-air Battery

As the first step during discharge, the mass transfer of oxygen should play a crucial role in Li-air batteries to tailor the growth of discharge products, however, not enough attention has been paid to this issue. Herein, we introduce an oxygen-enriching cosolvent, 1,2-(1,1,2,2-tetrafluoroethoxy) ethane (FE1), into the electrolyte, and investigate its influence on the discharge performance. The incorporation of this novel cosolvent consistently enhances the oxygen solubility of the electrolyte, and improves the oxygen diffusivity following a volcano-shape trend peaking at 50 % FE1. It is interesting that the discharge capacities obtained with the investigated electrolytes share the similar volcano trends as the oxygen transport under 50 mA gcarbon-1 and higher current densities. The improved oxygen diffusion could benefit the volumetric utilization of the air cathode, especially at the separator side, probably owing to the fast oxygen transport to moderate its concentration gradient. Our results demonstrate the importance of oxygen provision, which easily becomes the capacity-determining factor.

A high power Li–air battery enabled by a fluorocarbon additive

The mass transfer of O2 is a critical factor to determine the rate capability of Li–air batteries (LABs). Here, with improved O2 transport enabled by a solution-phase fluorocarbon additive 3-[2-(perfluorohexyl)ethoxy]-1,2-epoxypropane, the LABs discharge capacity was significantly elevated to 16 368 mA h gcarbon−1@500 mA gcarbon−1 and 1792 mA h gcarbon−1@5000 mA gcarbon−1 in N2–O2 (78 : 22) with power density comparable with state-of-the-art Li-ion batteries.