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.
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