The
theoretical energy density of lithium–oxygen (Li–O2) batteries is extremely high, although there are many challenges
that must be overcome to achieve high energy density in a manufactured
cell. For example, little is known about the properties of one of
the key intermediates, lithium superoxide (LiO2), which
until recently had not been stabilized in bulk form. In this work,
lithium superoxide was deposited onto iridium–reduced graphene
oxide (Ir–rGO) cathodes in a Li–O2 system
under a flow of O2. Lithium peroxide (Li2O2) was subsequently produced on the cathode surface in an inert
Ar atmosphere. Based on a detailed analysis of electrochemical impedance
spectroscopy data, it was demonstrated experimentally for the first
time that the charge transport resistance through LiO2 was
much lower than for Li2O2 and correlated with
lower LiO2 charge overpotentials. This result indicates
that LiO2 has good electronic conductivity and confirms
previous theoretical predictions that bulk LiO2 has better
charge transport properties than Li2O2. In addition,
impedance and other characterization of Li2O2 formation from LiO2 in an Ar atmosphere revealed that
when surface-mediated Li2O2 formation occurs,
it has a significantly lower discharge potential than when it forms
through a solution-phase-mediated process. These significant findings
will contribute to the development of Li–O2 batteries
through better understanding of LiO2 properties and formation
mechanisms.
Li–O2 batteries suffer from large charge
overpotentials
due to the high charge transfer resistance of Li2O2 discharge products. A potential solution to this problem
is the development of LiO2-based batteries that possess
low charge overpotentials due to the lower charge transfer resistance
of LiO2. In this report, IrLi nanoparticles were synthesized
and implemented for the first time as a LiO2 battery cathode
material. The IrLi nanoparticle synthesis was achieved by a temperature-
and time-optimized thermal reaction between a precise ratio of iridium
nanoparticles and lithium metal. Li–O2 batteries
employing the IrLi-rGO cathodes were cycled up to 100 cycles at moderate
current densities with sustained low cell charge potentials (<3.5
V). Various characterization techniques, including SEM, DEMS, TEM,
Raman, and titration, were used to demonstrate the LiO2 discharge product and the absence of Li2O2. On the basis of first-principles calculations, it was concluded
that the formation of crystalline LiO2 can be stabilized
by epitaxial growth on the (111) facets of IrLi nanoparticles present
on the cathode surface. These findings demonstrate that, in addition
to the previously studied Ir3Li intermetallic, the IrLi
intermetallic also provides a means by which LiO2 discharge
products can be stabilized and confirms the importance of templating
for the formation process.
Redox
mediators (RMs) are solution-based additives that have been
extensively used to reduce the charge potential and increase the energy
efficiency of Li–oxygen (Li–O2) batteries.
However, in the presence of RMs, achieving a long cycle-life operation
of Li–O2 batteries at a high current rate is still
a major challenge. In this study, we discover a novel synergy among
InX3 (X = I and Br) bifunctional RMs, molybdenum disulfide
(MoS2) nanoflakes as the air electrode, dimethyl sulfoxide/ionic
liquid hybrid electrolyte, and LiTFSI as a salt to achieve long cycle-life
operations of Li–O2 batteries in a dry air environment
at high charge–discharge rates. Our results indicate that batteries
with InI3 operate up to 450 cycles with a current density
of 0.5 A g–1 and 217 cycles with a current density
of 1 A g–1 at a fixed capacity of 1 A h g–1. Batteries with InBr3 operate up to 600 cycles with a
current density of 1 A g–1. These batteries can
also operate at a higher charge rate of 2 A g–1 up
to 200 cycles (for InBr3) and 160 cycles (for InI3). Our experimental and computational results reveal that while X3
– is the source of the redox mediator, LiX
at the MoS2 cathode, In3+ reacts on the lithium
anode side to form a protective layer on the surface, thus acting
as an effective bifunctional RM in a dry air environment. This evidence
for a simultaneous improvement in the current rates and cycle life
of a battery in a dry air atmosphere opens a new direction for research
for advanced energy storage systems.
Electrochemical performance of nanostructured carbon electrodes was evaluated using cyclic voltammetry and a simple simulation model. The electrodes were prepared from soluble precursors by anodic electrodeposition of two sizes of graphene quantum dot assemblies (hexabenzocoronene (HBC) and carbon quantum dot (CQD)) onto a conductive support. Experimental and simulated voltammograms enabled the extraction of the following electrode parameters: conductivity of the electrodes (a combination of ionic and electronic contributions), density of available electrode states at different potentials, and tunneling rate constant (Marcus−Gerischer model) for interfacial charge transfer to ferrocene/ferrocenium (Fc/Fc + ) couple. The parameters indicate that HBC and CQD have significant density of electronic states at potentials more positive than −0.5 V versus Ag/Ag + . Enabled by these large densities, the electron transfer rates at the Fc/Fc + thermodynamic potential are several orders of magnitude slower than those commonly observed on other carbon electrodes. This study is expected to accelerate the discovery of improved synthetic carbon electrodes by providing fast screening methodology of their electrochemical behavior.
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