Enhanced polysulfide conversion kinetics is essential
for realizing
lithium–sulfur batteries with high energy density and rate
performance and promising cyclability. The modification of the local
atomic structure of MN
x
active sites in
single-atom M–N–C catalysts was proposed to improve
their electrocatalytic activity for demanding reactions by fine-tuning
the interaction with reaction intermediates. Here, we demonstrate
that engineering the binding geometry of lithium polysulfides (LiPSs)
by introducing dual binding sites improves the LiPS conversion kinetics.
We use mild oxygen plasma treatment to introduce oxygen species into
the Fe–N–C catalyst. The plasma-treated Fe–N–C
(pFeNG) catalyst with dual sulfiphilic (mononuclear iron) and lithiophilic
(oxygen) binding sites has a lower polysulfide decomposition energy,
especially for Li2S redox, which is known to be the most
sluggish process. The pFeNG cathode shows significant improvement,
especially at high C rates (916.3 mA h g–1 at 5C),
with promising cycling performance.
Unlike cathodes for lithium-ion batteries, oxygen redox (OR) processes at a high voltage (�4.2 V) during the first charge in sodium-ion batteries (SIBs) employ some Li-incorporated Mn oxides that is recovered during subsequent discharge. To determine the intrinsic origin, P2-type Na 0.6 [Li 0.2 Mn 0.8 ]O 2 exhibiting a reversible OR-induced two-phase reaction was investigated using experiments and first-principle calculations. First, operando X-ray diffraction results in reversible P2-Z phase transformations and thermodynamic analysis show the twophase reaction features Li migration into the tetrahedral sites from the transition-metal layer in the latter phase. Second, Liinduced decoupling of the oxygen 2p-electron led to selective anion redox activity depending on the oxygen sites that are Li-rich (redox-active) and Mn-rich (redox-inactive) environments. Third, redox-active oxygen coordinated to the Li vacancy predominantly participates in the formation of peroxo-like dimers with distortion of the MnO 6 octahedron, as observed in the reversible extended X-ray absorption fine structure spectra during the OR reaction. Considering three physicochemical perspectives, we reveal that Li ions play a role in activating OR reactions and control OR participation in the charge-compensation process. Our findings suggest that the Li/Mn ratio is a critical factor for achieving a reversible OR reaction, and broaden the possibilities of exploiting OR to reach high-energy densities in next-generation SIBs.
An
intriguing redox chemistry via oxygen has emerged to achieve
high-energy-density cathodes and has been intensively studied for
practical use of anion-utilization oxides in A-ion batteries (A: Li
or Na). However, in general, the oxygen redox disappears in the subsequent
discharge with a large voltage hysteresis after the first charge process
for A-excess layered oxides exhibiting anion redox. Unlike these hysteretic
oxygen redox cathodes, the two Na-excess oxide models Na2IrO3 and Na2RuO3 unambiguously exhibit
nonhysteretic oxygen capacities during the first cycle, with honeycomb-ordered
superstructures. In this regard, the reaction mechanism in the two
cathode models is elucidated to determine the origin of nonhysteretic
oxygen capacities using first-principles calculations. First, the
vacancy formation energies show that the thermodynamic instability
in Na2IrO3 increases at a lower rate than that
in Na2RuO3 upon charging. Second, considering
that the strains of Ir–O and Ru–O bonding lengths are
softened after the single-cation redox of Ru4+/Ru5+ and Ir4+/Ir5+, the contribution in the oxygen
redox from x = 0.5 to 0.75 is larger in Na1–x
Ru0.5O1.5 than that in Na1–x
Ir0.5O1.5.
Third, the charge variations indicate a dominant cation redox activity
via Ir(5d)–O(2p) for x above 0.5 in Na1–x
Ir0.5O1.5.
Its redox participation occurred with the oxygen redox, opposite to
the behavior in Na1–x
Ru0.5O1.5. These three considerations imply that the chemical
weakness of Ir(5d)–O(2p) leads to a more redox-active environment
of Ir ions and reduces the oxygen redox activity, which triggers the
nonhysteretic oxygen capacity during (de)intercalation. This provides
a comprehensive guideline for design of reversible oxygen redox capacities
in oxide cathodes for advanced A-ion batteries.
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