NaSn2(PO4)3/C composites obtained
by solid-state and pyrolysis reactions present high capacity retention
and high-rate capability as anode materials for Li-ion batteries.
The structure of NaSn2(PO4)3 was
analyzed by combining X-ray diffraction and density functional theory
to confirm the R3̅ space group. The composite
is formed by submicrometer particles with a cube-like shape coated
by pyrolytic carbon that improves the electronic percolation. The 119Sn Mössbauer spectroscopy shows the existence of
Sn4+ with a more ionic character than SnO2,
which can be related to the inductive effect of the phosphate groups.
This technique was used in the operando mode to follow the reaction
with lithium during the first discharge that is a crucial step for
improving the performance. A two-step mechanism has been identified
that consists of the irreversible transformation of the pristine material
into Sn0 species for the first half of the discharge followed
by reversible Li–Sn alloying reactions. The Mössbauer
spectra of the Sn0 species differ from the spectrum of
βSn because of their nanosize and the existence of chemical
bonds with the sodium phosphate matrix formed during the conversion
reaction. The first part of the discharge should be considered as
a restructuration of the pristine material leading to the dispersion
of Sn0 small particles in strong interaction with the phosphate
matrix and providing good cyclability. Such a mechanism strongly differs
from the insertion mechanism of NaTi2(PO4)3/C that contains a transition metal with the same oxidation
state as Sn4+.
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