Yi-Chun Lu

Platinum-gold nanoparticles: a highly active bifunctional electrocatalyst for rechargeable lithium-air batteries.

PtAu nanoparticles (NPs) were shown to strongly enhance the kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in rechargeable Li-O(2) cells. Li-O(2) cells with PtAu/C were found to exhibit the highest round-trip efficiency reported to date. During ORR via xLi(+) + O(2) + xe(-) --> Li(x)O(2), the discharge voltage with PtAu/C was considerably higher than that of pure carbon and comparable to that of Au/C. During OER via Li(x)O(2) --> xLi(+) + O(2) + xe(-), the charge voltages with PtAu/C fell in the range from 3.4 to 3.8 V(Li), which is slightly lower than obtained with Pt. It is hypothesized that PtAu NPs exhibit bifunctional catalytic activity, having surface Au and Pt atoms primarily responsible for ORR and OER kinetics in Li-O(2) cells, respectively.

Lithium–oxygen batteries: bridging mechanistic understanding and battery performance

Rechargeable energy storage systems with high energy density and round-trip efficiency are urgently needed to capture and deliver renewable energy for applications such as electric transportation. Lithium–air/lithium–oxygen (Li–O2) batteries have received extraordinary research attention recently owing to their potential to provide positive electrode gravimetric energies considerably higher (∼3 to 5×) than Li-ion positive electrodes, although the packaged device energy density advantage will be lower (∼2×). In light of the major technological challenges of Li–O2 batteries, we discuss current understanding developed in non-carbonate electrolytes of Li–O2 redox chemistry upon discharge and charge, oxygen reduction reaction product characteristics upon discharge, and the chemical instability of electrolytes and carbon commonly used in the oxygen electrode. We show that the kinetics of oxygen reduction reaction are influenced by catalysts at small discharge capacities (Li2O2 thickness less than ∼1 nm), but not at large Li2O2 thicknesses, yielding insights into the governing processes during discharge. In addition, we discuss the characteristics of discharge products (mainly Li2O2) including morphological, electronic and surface features and parasitic reactivity with carbon. On charge, we examine the reaction mechanism of the oxygen evolution reaction from Li2O2 and the influence of catalysts on bulk Li2O2 decomposition. These analyses provide insights into major discrepancies regarding Li–O2 charge kinetics and the role of catalyst. In light of these findings, we highlight open questions and challenges in the Li–O2 field relevant to developing practical, reversible batteries that achieve the anticipated energy density advantage with a long cycle life.

The Influence of Catalysts on Discharge and Charge Voltages of Rechargeable Li–Oxygen Batteries

This study revealed the strong influence of carbon, Au/C, and Pt/C catalysts on the charge and discharge voltages of rechargeable Li―O 2 batteries. Li―O 2 single-cell measurements showed that Au/C had the highest discharge activity, while Pt/C exhibited extraordinarily high charging activity.

The discharge rate capability of rechargeable Li–O2 batteries

The O2electrode in Li–O2cells was shown to exhibit gravimetric energy densities (considering the total weight of oxygen electrode in the discharged state) four times that of LiCoO2 with comparable gravimetric power. The discharge rate capability of Au-catalyzed Vulcan carbon and pure Vulcan carbon (VC) as the O2electrode was studied in the range of 100 to 2000 mA gcarbon−1. The discharge voltage and capacity of the Li−O2 cells were shown to decrease with increasing rates. Unlike propylene carbonate based electrolytes, the rate capability of Li−O2 cells tested with 1,2-dimethoxyethane was found not to be limited by oxygen transport in the electrolyte. X-Ray diffraction (XRD) showed lithium peroxide as the discharge product and no evidence of Li2CO3 and LiOH was found. It is hypothesized that higher discharge voltages of cells with Au/C than VC at low rates could have originated from higher oxygen reduction activity of Au/C. At high rates, higher discharge voltages with Au/C than VC could be attributed to faster lithium transport in nonstoichiometric and defective lithium peroxide formed upon discharge, which is supported by XRD and X-ray absorption near edge structure O and Li K edge data.

Electrocatalytic Activity Studies of Select Metal Surfaces and Implications in Li-Air Batteries

Rechargeable lithium-air batteries have the potential to provide ≈3 times higher specific energy of fully packaged batteries than conventional lithium rechargeable batteries. However, very little is known about the oxygen reduction reaction (ORR) and oxygen evolution in the presence of lithium ions in aprotic electrolytes, which hinders the improvement of low round-trip efficiencies of current lithium-air batteries. We report the intrinsic ORR activity on glassy carbon (GC) as well as polycrystalline Au and Pt electrodes, where Au is the most active with an activity trend of Au ≫ GC > Pt. Rotating disk electrode (RDE) measurements were used to obtain the kinetic current of the ORR and the reaction order with respect to oxygen partial pressure in 1 M LiClO 4 propylene carbonate: 1,2-dimethoxyethane (1:2 v/v). In addition, air electrodes with Vulcan carbon or Au or Pt nanoparticles supported on Vulcan were examined in Li-O 2 single cells, where the observed discharge cell voltages follow the catalytic trend established by RDE measurements. The ORR mechanism and the rate-determining steps were discussed and contrasted with the ORR activity trend in acid and alkaline solutions.

Self-assembly of hierarchical MoSx/CNT nanocomposites (2<x<3): towards high performance anode materials for lithium ion batteries.

Two dimension (2D) layered molybdenum disulfide (MoS2) has emerged as a promising candidate for the anode material in lithium ion batteries (LIBs). Herein, 2D MoSx (2 ≤ x ≤ 3) nanosheet-coated 1D multiwall carbon nanotubes (MWNTs) nanocomposites with hierarchical architecture were synthesized via a high-throughput solvent thermal method under low temperature at 200°C. The unique hierarchical nanostructures with MWNTs backbone and nanosheets of MoSx have significantly promoted the electrode performance in LIBs. Every single MoSx nanosheet interconnect to MWNTs centers with maximized exposed electrochemical active sites, which significantly enhance ion diffusion efficiency and accommodate volume expansion during the electrochemical reaction. A remarkably high specific capacity (i.e., > 1000 mAh/g) was achieved at the current density of 50 mA g−1, which is much higher than theoretical numbers for either MWNTs or MoS2 along (~372 and ~670 mAh/g, respectively). We anticipate 2D nanosheets/1D MWNTs nanocomposites will be promising materials in new generation practical LIBs.

Probing the Reaction Kinetics of the Charge Reactions of Nonaqueous Li–O2 Batteries

Understanding the reaction mechanism of nonaqueous oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is key to increase the low round-trip efficiency and power capability of rechargeable Li-air batteries. Here we show that the ORR kinetics are much faster than OER kinetics and OER occurs in two distinct stages upon Li-air battery charging. The first OER stage occurs at low overpotentials (<400 mV) with a slopping voltage profile, whose kinetics are relatively insensitive to charge rates and catalysts. This OER stage could be attributed to the delithiation of the outer part of Li2O2 forming lithium-deficient Li2-xO2, which is chemically disproportionate to evolve O2. The second stage takes place at high overpotentials (400-1200 mV), whose kinetics are sensitive to discharge/charge rates and catalysts, which can be attributed to the oxidation of bulk Li2O2 particles. Our study provides insights into bridging current two schools of thought on the OER mechanism.

In Situ Ambient Pressure X-ray Photoelectron Spectroscopy Studies of Lithium-Oxygen Redox Reactions

The lack of fundamental understanding of the oxygen reduction and oxygen evolution in nonaqueous electrolytes significantly hinders the development of rechargeable lithium-air batteries. Here we employ a solid-state Li4+xTi5O12/LiPON/LixV2O5 cell and examine in situ the chemistry of Li-O2 reaction products on LixV2O5 as a function of applied voltage under ultra high vacuum (UHV) and at 500 mtorr of oxygen pressure using ambient pressure X-ray photoelectron spectroscopy (APXPS). Under UHV, lithium intercalated into LixV2O5 while molecular oxygen was reduced to form lithium peroxide on LixV2O5 in the presence of oxygen upon discharge. Interestingly, the oxidation of Li2O2 began at much lower overpotentials (~240 mV) than the charge overpotentials of conventional Li-O2 cells with aprotic electrolytes (~1000 mV). Our study provides the first evidence of reversible lithium peroxide formation and decomposition in situ on an oxide surface using a solid-state cell, and new insights into the reaction mechanism of Li-O2 chemistry.

Sulphur-impregnated flow cathode to enable high-energy-density lithium flow batteries

Redox flow batteries are promising technologies for large-scale electricity storage, but have been suffering from low energy density and low volumetric capacity. Here we report a flow cathode that exploits highly concentrated sulphur-impregnated carbon composite, to achieve a catholyte volumetric capacity 294 Ah l(-1) with long cycle life (>100 cycles), high columbic efficiency (>90%, 100 cycles) and high energy efficiency (>80%, 100 cycles). The demonstrated catholyte volumetric capacity is five times higher than the all-vanadium flow batteries (60 Ah l(-1)) and 3-6 times higher than the demonstrated lithium-polysulphide approaches (50-117 Ah l(-1)). Pseudo-in situ impedance and microscopy characterizations reveal superior electrochemical and morphological reversibility of the sulphur redox reactions. Our approach of exploiting sulphur-impregnated carbon composite in the flow cathode creates effective interfaces between the insulating sulphur and conductive carbon-percolating network and offers a promising direction to develop high-energy-density flow batteries.

Critical Role of Redox Mediator in Suppressing Charging Instabilities of Lithium–Oxygen Batteries

Redox mediators have been widely applied to reduce the charge overpotentials of lithium-oxygen (Li-O2) batteries. Here, we reveal the critical role of redox mediator in suppressing the charging instability of Li-O2 batteries. Using high temporal resolution online electrochemical mass spectrometry, we show that charging with redox mediators (using lithium bromide as a model system) significantly reduces parasitic gas evolution and improves oxygen recovery efficiency. Using redox mediator transforms the charge reactions from electrochemical pathways to chemical pathways, which unexpectedly bypasses the formation of highly reactive intermediates upon electro-oxidation of lithium peroxide (Li2O2). Such transformation reduces self-amplifying degradation reactions of electrode and electrolyte in Li-O2 cells. We further show that the improved stability associated with the redox mediator is much more pronounced at higher charging rates, owing to fast charge-transfer kinetics of the redox mediator. Together, we show that employing redox mediator not only reduces the charge overpotential but also suppresses side reactions of Li-O2 cells with improved charging rate. Our work demonstrates that transforming electro-oxidation of Li2O2 to chemical oxidation of Li2O2 is a promising strategy to simultaneously mitigate charging side reactions and achieve low overpotential for the Li-O2 batteries.