Structural Evolution of Layered Manganese Oxysulfides during Reversible Electrochemical Lithium Insertion and Copper Extrusion
2021
The electrochemical lithiation and delithiation of the
layered oxysulfide Sr2MnO2Cu4−δS3 has been investigated by using a combination
of in situ powder X-ray diffraction
and ex situ neutron powder
diffraction, X ray absorption and Li NMR spectroscopy, together with a range of
electrochemical experiments. Sr2MnO2Cu4−δS3 consists of [Sr2MnO2]
perovskite-type cationic layers alternating with highly defective
antifluorite-type [Cu4−dS3]
(d ~ 0.5) anionic layers.
It undergoes a combined displacement/intercalation (CDI) mechanism on
reaction with Li, where the inserted Li replaces Cu, forming Li4S3
slabs and Cu+ is reduced and extruded as metallic particles. For the
initial 2-3% of the 1st discharge process, the vacant sites in the
sulfide layer are filled by Li; Cu extrusion then accompanies further insertion
of Li. Mn2.5+ is reduced to Mn2+ during the first half of
the discharge. The overall charging process involves the removal of Li and
re-insertion of Cu into the sulfide layers with re-oxidation of Mn2+
to Mn2.5+. However, due to the different diffusivities of Li and Cu,
the processes operating on charge are quite different from those operating
during the first discharge: charging to 2.75 V results in removal of most of
the Li, little reinsertion of Cu and good capacity retention. A charge to 3.75
V is required to fully reinsert Cu, which results in significant changes to the
sulfide sublattice during the following discharge and poor capacity retention. This
detailed structure-property investigation will
promote the design of new functional electrodes with improved device
performance.
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