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Page 4 of 30 Yoon et al. Energy Mater 2024;4:400063 https://dx.doi.org/10.20517/energymater.2023.146

REACTION MECHANISM
Sb anodes undergo alloying/dealloying with alkali metal ions, especially Li , Na , and K , to form Li Sb, Na 3
+
+
+
3
Sb, and K Sb during charging and discharging. In addition, Sb has a unique puckered structure, and its
3
interlayer space effectively accommodates the insertion of alkali metal ions. The reported electrochemical
reaction mechanism of the Sb anode during discharging and charging in each system (LIBs, SIBs, PIBs, and
ASSLIBs) is summarized below.
Sb-anode reaction mechanism in LIBs
Various studies have examined the electrochemical reaction mechanisms associated with Sb anodes in
LIBs [60-62] . Park et al. demonstrated the reaction mechanism of an Sb anode using ex-situ X-ray diffraction
[60]
(XRD) . During discharging (lithiation), rhombohedral Sb alloys with Li form an intermediate crystalline
hexagonal Li Sb phase and then transform into a fully discharged cubic Li Sb phase. In contrast, during
2
3
charging (delithiation), cubic Li Sb is directly converted into rhombohedral Sb without involving an
3
intermediate phase. Shin et al. demonstrated the same electrochemical reaction mechanism using in-situ
[61]
time-domain thermoreflectance analysis . The asymmetric behavior of the Sb anode during discharging
and charging was also elucidated by Chang et al. using first-principles calculations and nuclear magnetic
[62]
resonance (NMR) spectroscopy . These researchers insisted that the higher nucleation driving force
associated with Sb results in a single-step reaction pathway in which Li Sb is directly recovered to Sb
3
without forming a thermodynamically stable Li Sb intermediate phase during the charging process.
2
Therefore, the suggested reaction mechanism for the Sb anode in LIBs during discharging/charging is
summarized by equations (1) and (2), along with the crystallographic schematic shown in Figure 2.
During discharging:

Sb (Rhombohedral) → Li Sb (Hexagonal) → Li Sb (Cubic) (1)
2
3
During charging

Sb (Cubic) → Sb (Rhombohedral) (2)
Li 3
Sb-anode reaction mechanism in SIBs
The Sb anode in SIBs exhibits a symmetric reaction mechanism during discharging (sodiation) and
charging (desodiation), unlike the behavior observed in LIBs [63-67] . Darwiche et al. reported that
rhombohedral Sb alloyed with Na forms an amorphous intermediate Na Sb (x < 3) phase during the
x
discharge reaction owing to the sluggish kinetics associated with the crystallization of NaSb into the
[63]
monoclinic structure. Subsequently, amorphous Na Sb is transformed into hexagonal Na Sb. Conversely,
3
x
during charging, Na Sb is transformed back into amorphous Na Sb, which is recovered as rhombohedral Sb.
x
3
Although XRD clearly identified Na Sb, the Na Sb phase was not detected owing to its amorphous nature.
x
3
Recent first-principles calculations performed by Caputo and Yu et al. demonstrated that the Na Sb phase
x
corresponds to monoclinic NaSb, which is the most thermodynamically stable phase [64,65] . This theoretical
prediction was further supported by the experimental results, which are demonstrated using extended X-ray
absorption fine structure analysis by Yu et al., and in-situ XRD by Tian et al. [66,67] . Therefore, the suggested
reaction mechanism for the Sb anode in SIBs during discharging/charging is summarized by equations (3)
and (4), along with the crystallographic schematic shown in Figure 3.
During discharging: