A dissolution–redeposition
reaction mechanism of the MnO2 cathode is directly visualized
in rechargeable aqueous zinc-ion
batteries via in situ Raman microscopy. MnO2 is reduced to Mn3+ during the discharge process, followed
by a disproportionation reaction to form Mn2+ and Mn4+. The dissolved Mn2+ plays an important role in
the battery chemistry. During the following charge process, the redeposition
of Mn2+ forms a species with high Zn-content on the surface
of the MnO2 cathode in the high-potential window. Moreover,
an effective method that allows in operando observation
of Jahn–Teller distortion of manganese is provided for the
first time. This method uses in situ Raman microscopy
to reveal the correlation between Jahn–Teller distortion and
Mn–O bond length change.
Solid
polymer electrolytes (SPEs) have the potential to enhance
the safety and energy density of lithium batteries. However, poor
interfacial contact between the lithium metal anode and SPE leads
to high interfacial resistance and low specific capacity of the battery.
In this work, we present a novel strategy to improve this solid–solid
interface problem and maintain good interfacial contact during battery
cycling by introducing an adaptive buffer layer (ABL) between the
Li metal anode and SPE. The ABL consists of low molecular-weight polypropylene
carbonate , poly(ethylene oxide) (PEO), and lithium salt. Rheological
experiments indicate that ABL is viscoelastic and that it flows with
a higher viscosity compared to PEO-only SPE. ABL also has higher ionic
conductivity than PEO-only SPE. In the presence of ABL, the interface
resistance of the Li/ABL/SPE/LiFePO4 battery only increased
20% after 150 cycles, whereas that of the battery without ABL increased
by 117%. In addition, because ABL makes a good solid–solid
interface contact between the Li metal anode and SPE, the battery
with ABL delivered an initial discharge specific capacity of >110
mA·h/g, which is nearly twice that of the battery without ABL,
which is 60 mA·h/g. Moreover, ABL is able to maintain electrode–electrolyte
interfacial contact during battery cycling, which stabilizes the battery
Coulombic efficiency.
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