In the present study,
an in-depth investigation on the structural
transformation in a mesoporous γ-MnO2 cathode during
electrochemical reaction in a zinc-ion battery (ZIB) has been undertaken.
A combination of in situ Synchrotron XANES and XRD studies reveal
that the tunnel-type parent γ-MnO2 undergoes a structural
transformation to spinel-type Mn(III) phase (ZnMn2O4) and two new intermediary Mn(II) phases, namely, tunnel-type
γ-Zn
x
MnO2 and layered-type
L-Zn
y
MnO2, and that these phases
with multioxidation states coexist after complete electrochemical
Zn-insertion. On successive Zn-deinsertion/extraction, a majority
of these phases with multioxidation states is observed to revert back
to the parent γ-MnO2 phase. The mesoporous γ-MnO2 cathode, prepared by a simple ambient temperature strategy
followed by low-temperature annealing at 200 °C, delivers an
initial discharge capacity of 285 mAh g–1 at 0.05
mA cm–2 with a defined plateau at around 1.25 V
vs Zn/Zn2+. Ex situ HR-TEM studies of the discharged electrode
aided to identify the lattice fringe widths corresponding to the Mn(III)
and Mn(II) phases, and the stoichiometric composition estimated by
ICP analysis appears to be concordant with the in situ findings. Ex
situ XRD studies also confirmed that the same electrochemical reaction
occurred on repeated discharge/charge cycling. Moreover, the present
synthetic strategy offers solutions for developing cost-effective
and environmentally safe nanostructured porous electrodes for cheap
and eco-friendly batteries.
Rechargeable zinc-ion batteries (ZIBs)
with high energy densities
appear promising to meet the increasing demand for safe and sustainable
energy storage devices. However, electrode research on this low-cost
and green system are faced with stiff challenges of identifying materials
that permit divalent ion-intercalation/deintercalation. Herein, we
present layered-type LiV3O8 (LVO) as a prospective
intercalation cathode for zinc-ion batteries (ZIBs) with high storage
capacities. The detailed phase evolution study during Zn intercalation
using electrochemistry, in situ XRD, and simulation techniques reveals
the large presence of a single-phase domain that proceeds via a stoichiometric
ZnLiV3O8 phase to reversible solid–solution
Zn
y
LiV3O8 (y > 1) phase. The unique behavior, which is different
from
the reaction with lithium, contributes to high specific capacities
of 172 mAh g–1 and amounts to 75% retention of the
maximum capacity achieved in 65 cycles with 100% Coulombic efficiency
at a current density of 133 mA g–1. The remarkable
performance makes the development of this low-cost and safe battery
technology very promising, and this study also offers opportunities
to enhance the understanding on electrochemically induced metastable
phases for energy storage applications.
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