Inorganic Mediator toward Organosulfide Active Material: Anchoring and Electrocatalysis

Article abstract of DOI:10.1002/adfm.202001493

Organosulfides are promising high-capacity cathode materials for rechargeable lithium batteries. However, they are hindered by several key challenges including the electronic/ionic insulation and solubility issues of the discharged products. Herein, TiS₂ nanosheets@carbon nanotubes (TiS₂ NSs@MWCNT) are proposed as a promoter and booster toward phenyl tetrasulfide (PTS). It serves as a bifunctional mediator, not only anchoring active materials in the cathode through chemical adsorption but also facilitating the reaction kinetics. The Li-organosulfide cell yields a reversible discharge capacity of 467.6 mAh g^?1 and a high capacity retention of 81.9% after 200 cycles at 0.5 C rate. When the PTS areal mass loading is 5.8 mg cm^?2 and the electrolyte/PTS ratio is 3.8 μL mg^?1, a high specific capacity of 444 mAh g^?1 at 0.5 C rate can still be achieved. The strong anchoring and electrocatalysis effects of TiS₂ toward PhSLi and polysulfide are revealed using experimental and computational approaches. The study sheds light on metal sulfides as mediators to improve the cycling life of Li-organosulfide batteries and provides deep comprehension of the instinct interaction evolution at molecular level, which is invaluable for fabrication of electrode materials.

Full text of DOI:10.1002/adfm.202001493

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Inorganic Mediator toward Organosulfide Active Material:
Anchoring and Electrocatalysis
Qianqian Fan, Wei Guo, Yubing Si, Xin Wang, Bo Wang, and Yongzhu Fu*
can be achieved due to the low electrolyte/
Organosulfides are promising high-capacity cathode materials for rechargeable
lithium batteries. However, they are hindered by several key challenges including
the electronic/ionic insulation and solubility issues of the discharged products.
Herein, TiS nanosheets@carbon nanotubes (TiS NSs@MWCNT) are pro-
active material ratios. In addition, they
can enhance the discharge voltage by flex-
[9,10]
ibly adjusting the functional groups.
However, many organosulfides have poor
conductivity and unsatisfactory cycling sta-
bility mainly due to the high solubility of
discharged product RSLi and intermediate
2
2
posed as a promoter and booster toward phenyl tetrasulfide (PTS). It serves
as a bifunctional mediator, not only anchoring active materials in the cathode
through chemical adsorption but also facilitating the reaction kinetics. The
LiPSs during the reduction of organo-
−1
[6,11]
Li-organosulfide cell yields a reversible discharge capacity of 467.6 mAh g and
a high capacity retention of 81.9% after 200 cycles at 0.5 C rate. When the PTS
sulfides with lithium.
In particular,
dissolved RSLi and LiPSs may increase
the viscosity and lower the ionic conduc-
tivity of liquid electrolyte, and degrade the
reversibility of the redox reactions of orga-
nosulfides in lithium batteries.
areal mass loading is 5.8 mg cm− and the electrolyte/PTS ratio is 3.8 µL mg ,
2
−1
−1
a high specific capacity of 444 mAh g at 0.5 C rate can still be achieved. The
strong anchoring and electrocatalysis effects of TiS toward PhSLi and poly-
2
sulfide are revealed using experimental and computational approaches. The
study sheds light on metal sulfides as mediators to improve the cycling life of
Li-organosulfide batteries and provides deep comprehension of the instinct
interaction evolution at molecular level, which is invaluable for fabrication of
electrode materials.
In regards to solving these issues, it
is vital to anchor polysulfides within the
cathode by using novel electrode configu-
ration. Various chemical modifications
[
12,13]
[18]
on the electrode surfaces with oxides,
[14,15]
[16,17]
sulfides,
nitrides,
or polymers
have been applied to retain polysulfides
in electrodes in Li-S batteries. However,
related studies on RSLi are still lacking. In addition, suc-
cessful modification should not only show strong adsorption
capability, but also allow favorable reduction reactions. As
we know, two-dimensional layered transition metal disulfides
1
. Introduction
[
19]
Lithium-ion (Li-ion) batteries with high energy densities have
quickly dominated the energy storage market for portable elec-
tronics and electric vehicles.[1–3] However, Li-ion batteries based
on transition metal oxide/phosphate cathodes have reached the
limit of energy density due to the inherent low specific capaci-
ties, which cannot meet the ever-increasing demand of energy
storage market.[ Hence, alternative electrode materials with
higher capacities are needed.
(MS , M = Mo, Ti, V, Co, Fe) possess high conductivity, large
2
[
20–22]
polar surface and high catalytic activity.
is promising
electrical conductivity (5.24 × 10 S m ),
Among them, TiS2
[
23,24]
as it displays high lithium diffusion rate and
3
−1 [25–27]
which are ideal
2]
transport features for facilitating reaction kinetics. Recent
studies also show that TiS2 is able to adsorb polysulfides,
which improves the stability of cathode. Attracted by the multi-
functionality of TiS2 for improving the performance of Li-S
batteries, we construct a hybrid cathode of titanium disulfide
Organosulfides (R-S -R, n ≥ 3) with long sulfur chains in the
n
structure are a class of promising high-capacity cathodes for
[4]
[5]
rechargeable lithium batteries, such as dimethyl trisulfide,
diphenyl trisulfide,[ phenyl tetrasulfide (PTS), phenyl penta-
6]
nanosheets (TiS NSs) embedded in multiwalled carbon nano-
2
[7]
sulfide (PPS), and phenyl hexasulfide (PHS). For example,
they can reduce the formation of high-order lithium polysulfides
tubes (TiS NSs@MWCNT) network with PTS to promote and
2
boost electrochemical performance of battery. The self-woven
(
LiPSs) to a certain extent during charge-discharge processes,
TiS2 NSs@MWCNT shows an effective physical confine-
[5,8]
[28]
reducing the shuttle effect of LiPSs.
High specific energies
ment due to its highly porous structure.
Meanwhile, TiS2
can serve as a bifunctional mediator to adsorb the discharged
product lithium thiophenolate (PhSLi) and intermediate LiPSs
through weak binding and facilitate the reaction kinetics of
Q. Fan, Dr. W. Guo, Dr. Y. Si, Dr. X. Wang, B. Wang, Prof. Y. Fu
College of Chemistry
Zhengzhou University
PhSLi/LiPSs. The interactions between PhSLi and TiS were
2
Zhengzhou 450001, P. R. China
E-mail: yfu@zzu.edu.cn
probed and rationalized using X-ray photoelectron spectroscopy
(
XPS) analysis and density functional theory (DFT) computa-
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202001493.
tion. The TiS NSs@MWCNT-PTS hybrid cathode delivers a
2
−
1
reversible discharge capacity of 467.6 mAh g after 200 cycles
at 0.5 C rate. With a high PTS loading of 5.8 mg cm− and a
2
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Figure 1. a) Synthesis process of PTS along with a visual representation of the PTS liquid. b) Extract ion chromatogram and c) corresponding mass
spectrum of phenyl tetrasulfide.
−
1
low electrolyte/PTS ratio of 3.8 µL mg , PTS exhibits a high
morphology (Figure S1a, Supporting Information) with a thick-
ness of tens of nanometers (Figure S1b, Supporting Informa-
specific energy of 901 Wh kg− at 0.5 C rate.
1
tion). The TiS NSs@MWCNT composite paper was prepared
2
through the vacuum filtration method, the detailed preparation
processes are shown in the experimental section (Supporting
information). MWCNTs were selected as conductive matrix
due to their excellent electrical conductivity and long electron
transport pathway. Based on the elemental analysis shown
in Figure S2 in the Supporting Information, the mass ratio of
2
. Results and Discussion
2
.1. Synthesis of PTS
[
30]
Figure 1a shows the synthesis process of PTS. The phenyl
disulfide/sulfur (PDS/S) molar ratio is 1:2, therefore the target
compound is PTS. The reaction proceeds without the need of
any solvent and catalyst. The synthesized PTS turns out to be a
yellow liquid. After the reaction, there is no solid precipitation
in the PTS liquid indicating complete conversion of sulfur. In
order to verify the success of forming PTS, Waters Xevo G2-XS
QTof mass spectrometer was utilized. The synthesized liquid
was tested in the atmospheric solids analysis probe (ASAP)
positive mode with the first-order mass spectrometry model.
Figure 1b displays the extract ion chromatogram of PTS, a
strong peak appears at the retention time of 1.709 min, and the
corresponding m/z of this peak (Figure 1c) is 281.9665, which
MWCNT to TiS is estimated to be about 7:1.
2
Figure 2a shows the photographs of the TiS NSs@MWCNT
2
composite paper. It can be seen that TiS NSs@MWCNT is
2
freestanding and flexible. To reveal the structure of TiS NSs@
2
MWCNT, scanning electron microscopy (SEM) and high
resolution-transmission electron microscopy (HR-TEM) of TiS2
NSs@MWCNT were performed. As shown in Figure 1b,c, TiS2
NSs are uniformly embedded in the carbon nanotube network,
which can significantly improve the conductivity of cathode
and encapsulate the active materials during the battery cycling.
Figure S3 in the Supporting Information also shows the high
magnification SEM image of the TiS2 NSs@MWCNT to
display its porous structure. The porous structure of
TiS2 NSs@MWCNT can be further proven via Brunauer-
Emmett-Teller (BET) method. The nitrogen adsorption/
desorption isotherms of TiS2 NSs@MWCNT are shown in
+
can be assigned to the molar mass for the ionized [PTS] . The
synthesized liquid could be a mixture of phenyl polysulfides in
which high-order polysulfides such as PPS and PHS may be
present. Because the stoichiometric formula of the product is
Ph-SSSS-Ph, PTS is used in the following discussion.
Figure S4 in the Supporting Information, the TiS NSs@
2
2
−1
MWCNT exhibits a BET surface area of 132 m g and a pore
volume of 0.79 cm g , the pore size distribution is around
3
−1
2
.2. Synthesis and Structure of the TiS NSs@MWCNT
22.5 nm and centered at ≈0.38 nm. The large surface area,
2
Composite Paper
high pore volume, and abundant porous structure of the TiS
2
+
NSs@MWCNT can not only facilitate Li diffusion, but also are
The single-crystal TiS2 nanosheets were synthesized via a
favorable for increasing the electrode/electrolyte contact and
physical confinement for soluble active materials. The energy-
dispersive X-ray spectroscopy (EDS) elemental mapping images
[29]
colloidal chemistry method developed by Zhu et al. Briefly,
appropriate amounts of TiCl and CS were mixed and heated
4
2
at 300 °C for 5 h in the mixture solvent of oleylamine and
shown in Figure 2d–g reveal that TiS NSs are intertwined with
MWCNTs uniformly.
2
octadecen. The as-prepared TiS NSs have uniform hexagonal
2
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Figure 2. a) Photograph of the TiS NSs@MWCNT composite paper showing its free-standing and flexible features. b) SEM image of TiS NSs@
2
2
MWCNT composite paper. c) HR-TEM image of TiS NSs@MWCNT composite. d) EDS images of e) carbon, f) titanium, and g) sulfur. h) XRD patterns
2
of MWCNTs, TiS NSs, and TiS NSs@MWCNT composite. i) Raman spectra of TiS NSs and TiS NSs@MWCNT composite.
2
2
2
2
The X-ray diffraction (XRD) pattern of the TiS2 NSs@
yellow PhSLi and Li S solutions containing TiS NSs become
2 6 2
MWCNT is compared with those of TiS NSs and MWCNTs in
much lighter, whereas the PhSLi and Li S solutions containing
2
2 6
Figure 2h. All the diffraction peaks of the synthesized TiS NSs
MWCNTs still remain yellow. In addition, the UV absorbance
of PhSLi and Li S solutions exposed to TiS NSs become much
lower than those of solutions exposed to MWCNTs, which indi-
2
can be perfectly matched with hexagonal TiS2 (JCPDS
2
6
2
[29]
No. 15-0853), indicating that the obtained TiS NSs are pure
2
without any oxide by-products. Furthermore, the XRD pattern
of the TiS2 NSs@MWCNT exhibits the same peaks as TiS2
NSs except the peak at 26° assigned to the (002) plane peak
cates the TiS NSs has strong ability to adsorb both PhSLi and
LiPSs.
To directly validate the chemical interaction between TiS2
2
of MWCNTs. However, the peak intensities of TiS NSs in the
and PhSLi, XPS analyses of the TiS NSs before and after the
2
2
composite are reduced because the MWCNTs coated on TiS2
PhSLi adsorption experiments were performed. As shown
NSs block the peak signal of TiS . In addition, the Raman
in Figure 3c, the Ti 2p spectra of the pristine TiS shows two
2
2
spectra of TiS NSs and TiS NSs@MWCNT are displayed in
peaks located at binding energies of 456.0 and 462.0 eV with
2
2
Figure 2i. The Raman characteristic peaks of MWCNTs (D and
an energy separation of 6.0 eV, which are attributed to the
−1
−1
[31]
G peaks) and TiS NSs (A 332 cm and E 219 cm ) peaks are
Ti 2p3/2 and Ti 2p1/2 spin-orbit levels of TiS , respectively.
2
1g
g
2
[19]
clearly seen in the composite.
Upon contact with PhSLi, the characteristic Ti 2p3/2 and Ti 2p1/2
peaks of TiS -PhSLi both shift toward higher binding energies
2
of +0.3 eV compared with pristine TiS . The obvious upshifts
2
2
.3. Dual Chemisorption of TiS toward PhSLi and LiPSs
for Ti 2p3/2 and 2p1/2 evidence the polarization of electrons away
from Ti to PhSLi, and the decrease of the electron cloud density
around Ti atoms. The two peaks of Ti 2p at higher binding
energies in the TiS -PhSLi sample are ascribed to Ti in TiO .
2
Although linear organic polysulfides can reduce the formation
of high-order LiPSs to a certain extent,[ a small amount of
LiPSs will inevitably be formed during the discharge process.
8]
2
2
The presence of Ti in TiO environment probably derives from
2
To verify the dual chemisorption ability of TiS toward PhSLi
and LiPSs, visual adsorption experiments and UV–vis analysis
hydrolysis/oxidation process of TiS when it is exposed to
ambience. On the other hand, the upshifts of S 2p3/2 (160.8 eV)
2
2
[
32]
were performed. An equivalent mass of MWCNTs and TiS
and S 2p1/2 (162.0 eV) peaks of TiS in contact with PhSLi are
2
2
NSs was added into 5 × 10− ꢀ PhSLi (Figure 3a) or 5 × 10
3
−3
ꢀ
observed as well. The S 2p peak at ≈161.6 eV belongs to the
3
/2
[
9]
Li2S6 (Figure 3b) solution. After prolonged interaction, the
S in PhSLi. Because of the occurrence of electron transfer
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Figure 3. UV–vis absorption spectra of the a) PhSLi, b) Li S solutions before and after the addition of MWCNT and TiS , the insets in (a) and (b)
2
6
2
show the color changes of the PhSLi and Li S solutions in the adsorption tests. XPS spectra of c) Ti 2p and d) S 2p of TiS before and after contacting
2
6
2
with the PhSLi solution. The fully optimized adsorption configurations of e) PhSLi and f) Li S on TiS with 3D differential charge contour surfaces.
2
6
2
The carbon, hydrogen, lithium, sulfur, and titanium elements are shown in gray, white, purple, yellow and blue, respectively. The electronic attachment
and detachment densities are shown in blue and green surfaces, respectively.
from Ti to PhSLi and the lower electronegativity of S in TiS2
than S in PhSLi as phenyl is an electron-withdrawing group,
the S atoms in TiS2 also lose electron inducing an obvious
shift to the higher binding energy.[ To further verify the
strong chemical adsorption of TiS2 toward PhSLi and Li S ,
2 6
the binding features were studied by first-principles calcula-
tions with the density-functional theory (DFT). Figure 3e,f
shows the final optimized structures of PhSLi and Li2S6
33]
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compounds on the surface of TiS . It is worth to note the
in the Supporting Information, only the UV–vis adsorption
2
adsorption elongate the S-Li bond of PhSLi from 2.20 to 2.39 Å.
Meanwhile, the S-Li distance between TiS2 and PhSLi is in
the range of 2.56–2.64 Å, which is close to bonding distance,
indicating the existence of chemical interaction. The binding
peak of PhS Li can be seen, which indicates that the forma-
x
tion of higher-order polysulfides during cycling is suppressed.
Moreover, the UV–vis absorbance of PhS Li in PTS/TiS
x
2
NSs cathode is higher than that of pure PTS cathode, indi-
[
34]
energy, E , is computed to measure the binding strength.
cating a strong retention of PhS Li in the electrode. In addition,
b
x
The binding energy between PhSLi and TiS is 2.18 eV, which
the morphology of the cycled Li foil extracted after 250 cycles
is shown in Figure S6 in the Supporting Information. It can
be seen that the surface of the Li anode paired with pure PTS
cathode is covered with a layer of black material, which could
be PTS’ discharged products migrated to the Li anode. In con-
2
is much larger than that (1.24 eV) between Li2S6 and TiS2.
The large binding energy indicates the adsorption interaction
is favored. The larger the value is, the stronger the anchoring
effect occurs.[ It is believed that the adsorption interaction
does not only exist between TiS and PhSLi/Li S , but also exist
22]
trast, the surface of the Li anode paired with PTS/TiS cathode
2
2
6
2
between TiS and other possible intermediate and discharged
is smooth, implying the strong affinity and retention of TiS2
NSs for PTS’ discharge products.
2
products such as PhS Li (x > 1), Li S , Li S , Li S , and Li S.
x
2
4
2
3
2
2
2
The strong chemical adsorption of TiS NSs toward LiPSs/
2
PhSLi can be further probed through UV–vis spectroscopy
measurements of the cycled cathodes. The pure PTS and
2.4. Electrocatalytic Effect of TiS NSs toward the Redox
2
PTS/TiS NSs electrodes were both discharged to 1.8 V after
Reaction of PTS Cathode
2
5
cycles at 0.1 C rate, therefore the discharge product PhSLi
was formed. The cycled electrodes were soaked in electrolyte
solvent. The UV–vis spectra are shown in Figure S5a in the
Supporting Information. According to the inset spectrum of
the commercial PhSLi, the peak in the 220–280 cm region
belongs to the characteristic adsorption of PhSLi. For the PTS/
To confirm the electrocatalytic properties of TiS NSs toward
2
redox processes of PhSLi and LiPSs, cyclic voltammetry (CVs)
of symmetric cells were conducted in 0.2 ꢀ PhSLi (Figure 4a)
or 0.2 ꢀ Li S (Figure 4b) electrolyte. Figure 4a shows that TiS
−
1
2
6
2
NSs@MWCNT has a pair of distinct redox peaks at −0.55 and
0.55 V, which are more obvious than pure MWCNTs. It indi-
TiS NSs electrode, the UV–vis absorbance of PhSLi is higher
2
than that of pure PTS, suggesting a strong chemical interaction
cates that the TiS NSs is beneficial for accelerating the conver-
2
between TiS and PhSLi, retaining PhSLi in the carbon paper.
sion reaction of PhSLi. The peak at 0.55 V in the anodic scan
can be assigned to the formation of PDS by the oxidation of
PhSLi on the working electrode, the peak at −0.55 V in cathodic
2
When pure PTS and PTS/TiS NSs cathodes were discharged
2
[7]
to 2.2 V, PhS Li (x ≥ 1) were formed. As shown in Figure S5b
x
Figure 4. CV curves of the symmetric cells with TiS NSs@MWCNT or pure MWCNTs as electrodes in a) 0.2 ꢀ PhSLi or b) 0.2 ꢀ Li S electrolyte at
2
2 6
−
1
3
mV s . c) CV curves of PTS/TiS NSs cathodes in Li cells at different scan rates. d) Plots of CV peak current of PTS/TiS electrodes for the cathodic
2
2
reduction processes (I , I ) and the anodic oxidation process (I ) versus the square root of the scan rate.
C1 C2
A
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scan arises from the reduction of PDS. Similarly, Figure 4b
are very small, indicating the negligible capacity contribution
shows that TiS NSs exhibits a better electrocatalytic effect than
of TiS NSs itself when the cell is cycled between 1.8 and 3.0 V.
2
2
pure MWCNTs for Li S . The TiS NSs@MWCNT symmetric
Pure PTS shows three cathodic peaks. The first peak at ≈2.4 V
corresponds to the breakage of the central S-S bond of PTS, the
second cathodic peak at ≈2.1 V corresponds to the reduction
2
6
2
cell with 0.2 ꢀ Li S exhibits two pronounced reduction and
2
6
oxidation peaks located at −0.38 and 0.48 V, respectively. The
peak at 0.48 V in anodic scan is due to the oxidation of Li2S6
to generate elemental S on the working electrode, the peak at
of LiPSs. Finally, lithium sulfide (Li S) and PhSLi are formed
2
at ≈1.9 V. In the anodic scan, there are two anodic peaks indi-
cating two-step reactions during the charging process. The
main conversion occurs at about 2.5 V, and then the PTS was
−
0.38 V is accompanied by the reduction of S to Li S then
2 6
maybe Li S. A single broad peak is observed in each scan direc-
2
[
7]
tion, which could be due to the catalytic effect of TiS on the
reformed at 2.56 V. Compared to pure PTS cathode, the PTS/
TiS2 NSs cathode displays more negative anodic peaks and
more positive cathodic peaks, indicating smaller polarization
2
charge transfer, which could suppress their redox features in
the narrow voltage range.
To reveal the improvement in electrode kinetics, the lithium
ion diffusion coefficient (DLi+) of PTS/TiS2 NSs@MWCNT
composite (denoted as PTS/TiS2 NSs) (Figure 4c) and pure
PTS (Figure S7, Supporting Information) electrodes were
calculated by a series of CV at different scan rates. As shown
in Figure 4d, the anodic (I ) and cathodic (I , I ) current
and improved kinetics of the PTS/TiS NSs cathode. Figure S10
2
in the Supporting Information shows the EIS spectra of
pure PTS and PTS/TiS2 NSs electrodes before and after
50 discharge–charge cycles. As shown in Figure S10a in the
Supporting Information, the fresh PTS/TiS2 NSs electrode
delivers a smaller R (115.9 Ω) than that (324.1 Ω) of pure PTS
A
C1
C2
ct
peaks of PTS/TiS NSs and pure PTS electrodes both have a
electrode, which is closely related to the excellent conductivity
2
0
.5
linear relationship with the square root of the scan rates (v ),
indicating a diffusion-limited process. Therefore, the diffusion
coefficient of Li ions can be described by the Randles–Sevcik
of TiS @MWCNT composite accelerating the electron transfer
2
of PTS/TiS NSs electrode. After 50 cycles, the R (65.7 Ω) of
2
ct
the PTS/TiS NSs electrode decreases and is still much smaller
2
equation:[ I = (2.69 × 10 ) n S D CLi
34]
5
1.5
0.5
v
0.5
, where I is the
than that of the pure PTS electrode (Figure S10b, Supporting
Information). Figure 5a shows the galvanostatic voltage profiles
p
p
peak current, n is the charge transfer number, S is the area of
the electrode, D is the Li diffusion coefficient, C is the con-
+
of pure PTS and PTS/TiS NSs electrodes at 0.2 C rate. Both
Li
2
centration of lithium ions in the cathode, and v is the scan rate.
cells display three discharge plateaus, which are consistent with
the CV result in Figure S9 in the Supporting Information. The
As n, S, and CLi are unchanged, lithium ion diffusion rate can
0.5
be correlated positively to the slope of the curve (I /v ). It is
PTS/TiS NSs electrode exhibits a higher discharge capacity of
p
2
−
1
−1
−1
worth noting that the slopes for the redox peaks of the PTS/TiS2
527.4 mAh g (1 C = 570 mA g ) than that (438.1 mAh g ) of
the pure PTS electrode. Furthermore, the polarization potential
(ΔE) between the charge plateau and discharge plateau of the
NSs cathode are higher than those of pure PTS electrode, indi-
+
cating that TiS NSs facilitates fast Li transport and reaction
2
kinetics of PTS/TiS NSs cathode. The electrocatalytic effect of
PTS/TiS NSs electrode is much lower than that of the pure
2
2
TiS could enhance the charge transfer kinetics and diffusion
PTS electrode (0.178 vs 0.348 V, respectively). These improved
performances show that the superior catalytic capability of TiS2
NSs boosts the redox reaction kinetics during the discharge/
charge processes in Li-PTS cells.
2
of lithium ions, thus promote the conversion of PhSLi/LiPSs.
Electrochemical impedance spectroscopy (EIS) was also used
to measure charge transfer resistance Rct of both symmetric
cells (Figure S8, Supporting Information). TiS NSs@MWCNT
The rate performances of cells at higher cycling rates of 1 C,
2
exhibits lower Rct than pure MWCNTs, either with PhSLi or
2 C, 3 C, and 4 C are displayed in Figure 5b. The PTS/TiS NSs
2
Li S . Therefore, Higher D along with lower charge transfer
cathode delivers reversible discharge capacities of 570, 512,
451, 438, and 486 mAh g , respectively, which are all higher
than those of the pure PTS electrode, indicating the improved
2
6
Li+
−
1
resistance in EIS verify the accelerated redox kinetics of TiS
NSs in PTS cathode.[
2
35]
rate performance of PTS/TiS NSs cathode. Figure 5c shows
2
the cycling performance of the pure PTS and PTS/TiS NSs
2
−2
2
.5. Electrochemical Performance of the PTS/TiS NSs
cathodes at 0.5 C rate with PTS loading of 1.4 mg cm
2
−1
Composite Cathode
(1 C = 570 mA g ). The PTS/TiS NSs cathode delivers a dis-
2
−1
charge capacity of 467.6 mAh g with high Coulombic effi-
ciency of ≈99.9% after 200 cycles at 0.5 C rate, accounting for
81.9% of the capacity of the 5th cycle, which is much higher
than that (66.1%) of the pure PTS cathode. Because TiS2
is electrochemically active and lithium intercalation reac-
tion occurs when the discharge cut-off voltage is higher
Constructing an “anchoring and electrocatalysis” mediator
toward PTS can serve as an effective strategy to promote and
boost electrochemical performance. To reveal the roles of the
TiS NSs mediator on the actual Li-organosulfide batteries, the
electrochemical performances were collected on coin cells with
2
[24]
the PTS/TiS NSs@MWCNT composite cathode. Figure S9 in
than 1.45 V.
The MWCNTs also contribute irreversible
2
the Supporting Information displays the CV profiles of TiS2
capacity during the first few cycles. Both cause the discharge
capacity of the PTS/TiS2 NSs cathode exceeds the theo-
retical capacity (570 mAh g ) of PTS in the first 5 cycles at
NSs@MWCNT, pure PTS, and PTS/TiS NSs cathodes in the
2
−1
−1
1
st cycle at 0.02 mV s . TiS NSs@MWCNT exhibits only one
2
cathodic peak at ≈2.3 V and one anodic peak at ≈2.4 V, which
are related to the lithium intercalation (Li TiS , 0 < x < 1) and
0.5 C rate. The voltage profiles of TiS @MWCNT composite
2
current collector are presented in Figure S11a, Supporting Infor-
mation. It appears two discharge voltage plateaus at ≈2.3 and
≈1.8 V (Figure S9a, Supporting Information) in the first cycle,
x
2
[36]
de-intercalation process of TiS NSs, respectively. The total
2
areas of the anodic and cathodic peaks of TiS NSs@MWCNT
2
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Figure 5. a) Discharge–charge profiles of pure PTS and PTS/TiS NSs electrodes of the 10th cycle at 0.2 C rate. The theoretical capacity of PTS is
2
−
1
5
70 mAh g . b) Rate performances of pure PTS and PTS/TiS NSs electrodes. c,d) Cycling performances of pure PTS and PTS/TiS NSs electrodes with
2
2
−
2
PTS areal loadings of 1.4 and 5.8 mg cm , respectively, at 0.5 C rate. e) Long cycling stability of PTS/TiS NSs electrode at 2 C rate.
2
2
which are related to the lithium intercalation (Li TiS , 0 < x < 1)
to a high areal capacity of 3.0 mAh cm− (Figure S13, Supporting
x
2
−1
process of TiS NSs and irreversible capacity contribution of
Information) and a high specific energy of 901 Wh kg of PTS.
Furthermore, the cell utilizes an electrolyte to PTS ratio of
3.8 µL mg when the loading of PTS is 5.8 mg cm . Such a
low electrolyte/PTS ratio can help to increase the energy den-
sity of PTS batteries. The cell is stable during 120 cycles with
a capacity retention of 83.5%. In addition, even at 2 C rate, the
2
MWCNT, respectively. In the following cycles, it significantly
−1
−2
decreases. The corresponding absolute capacities of TiS NSs,
2
pure PTS, and PTS/TiS NSs electrodes at same current are
2
shown in Figure S11b in the Supporting Information. The PTS/
TiS2 NSs electrode displays higher discharge capacities than
the sum of the capacities of the TiS and pure PTS electrodes
PTS/TiS NSs cathode still exhibits considerably higher cycling
2
2
except the first cycle, which can verify the improved capacity of
stability with 75.5% of the initial capacity after 300 cycles than
that (60.9%) of the pure PTS cathode after 200 cycles (Figure 5e).
the PTS/TiS NSs cathode is ascribed to the chemical adsorp-
2
tion and electrocatalysis of TiS on PhSLi/LiPSs rather than the
The improved cycling performance of the PTS/TiS NSs cathode
2
2
contributed capacity of TiS itself. Figure S12 in the Supporting
Information presents some selected charge/discharge voltage
is attributed to the immobilization of soluble PhSLi and LiPSs
via an anchoring effect through strong chemical absorption and
2
profiles of the pure PTS and PTS/TiS NSs cathodes for different
accelerated reaction kinetics promoted by TiS NSs.
2
2
cycles at 0.5 C rate. It can be seen that the PTS/TiS2 NSs
cathode displays lower voltage hysteresis and longer discharge
voltage plateaus compared with the pure PTS cathode.
2.6. The Redox Mechanism Analysis of the PTS/TiS NSs
2
To show the feasibility of PTS/TiS NSs in practical applica-
Composite Cathode
2
tion, Figure 5d displays the cycling performance of a high loading
PTS/TiS NSs cell. The PTS/TiS NSs cell delivers a specific
capacity of 444 mAh g at 0.5 C of the 1st cycle, corresponding
As a promising organosulfide cathode material, PTS exhibits a
high capacity in Li batteries. Therefore, understanding its redox
2
2
−1
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Figure 6. a) XPS analysis of the discharge products of PTS/TiS electrode. b) The mass spectrum of the recharged PTS/TiS electrode. c) The proposed
2
2
redox reactions of the PTS/TiS NSs electrode in rechargeable lithium batteries.
2
[
40]
mechanism will be helpful for further optimization of its elec-
be firmly adsorbed on the surface of Li TiS . In following
x 2
trochemical performance. The discharged PTS/TiS electrode
charge process, there is a sloping plateau with two regions.
2
was characterized by UPLC-QTof-MS in the APCI positive
mode and the mass spectrum is shown in Figure S14 in the
Supporting Information. An ion peak at m/z of 110.0206 can
be seen corresponding to the protonated thiophenol, which is
the form of PhSLi in the MS analysis. The discharge products
of PTS/TiS2 cathode were also analyzed by XPS as shown
The first one corresponds to the delithiation of Li S and PhSLi,
which could form S•, SLi•, and PhS• radicals. These radicals
2
are bonded together to form PhS Li and LiPSs, which again are
x
adsorbed on the surface of Li TiS . In the following region, they
x
2
are bonded to form PTS as the primary recharged product. In
the whole process, TiS also facilitates the conversion of these
2
in Figure 6a. The S 2p3/2 peak at binding energy of 169.0 eV
intermediate species via the electrocatalytic effect.
belongs to the S in LiTFSI.[ The S 2p
37]
peak located at
167.4 eV is assigned to the S of S O3 /SO3 , which mainly
3/2
2−
2−
≈
2
results from the oxidization of sulfur species during sample
3
. Conclusion
transfer.[ The S 2p3/2 peaks at ≈161.6 and ≈160.6 eV are
38]
assigned to the S in PhSLi and lithiated TiS , respectively, and
In summary, PTS as a promising organosulfide cathode mate-
rial is synthesized by a scalable method through the melting
2
[37,39]
the S 2p3/2 peak at ≈160.1 eV could be assigned to Li S.
recharged PTS/TiS electrode sample was analyzed by QTof-MS
The
2
reaction of PDS and S without any solvent and catalyst. The
2
8
ASAP probe. As shown in Figure 6b, the m/z of PTS (281.9667)
can be clearly detected in the mass spectrum, indicating that
PTS can be re-formed after recharge.
PTS/TiS NSs@MWCNT cathode provides a facile and practical
2
solution for promoting and boosting the electrochemical perfor-
mance of Li-PTS cell, as well as circumventing the inappropriate
fabrication parameters, such as low amounts of active material
According to the above analysis, we can propose the redox
reactions of PTS/TiS NSs electrode in rechargeable Li batteries,
and excessive electrolyte. The PTS/TiS NSs@MWCNT cathode
2
2
as shown in Figure 6c. The first-half discharge region corre-
sponds to the cleavage and lithiation of S-S bonds in PTS to
features TiS functioning as an anchor toward LiPS and PhS Li,
so as to enhance the electrochemical efficiency and stability of
2
x
form PhS Li (1 ≤ x). At the same time, a small amount of LiPSs
the cell. The catalytic effect of TiS leads to high lithium ion dif-
fusion coefficient, relatively low polarization, and fast reaction
kinetics, which in turn suppresses the dissolution of polysulfide
x
2
are formed which are due to the possible high-order phenyl
polysulfide (e.g., PPS and PHS) in the starting material. These
intermediate species are adsorbed on the surface of lithiated
titanium disulfide (Li TiS , 0 < x < 1). In the following region,
intermediates in ether. The resulting PTS/TiS NSs@MWCNT
2
−1
cathode delivers a reversible discharge capacity of 467.6 mAh g
x
2
these materials are converted to Li S and PhSLi which can also
after 200 cycles at 0.5 C rate. Moreover, with a high PTS loading
2
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of 5.8 mg cm−2 and a low electrolyte/PTS ratio of 3.8 µL mg ,
PTS delivers a high specific energy of 901 Wh kg at 0.5 C rate.
This work opens up a new prospect toward the design of high
performance lithium battery through the effective combination
of the organosulfides and inorganic mediators.
www.afm-journal.de
−1
circuit potential of the cells with a Bio-Logic VMP-3 impedance analyzer
in the frequency range of 100 kHz and 0.1 Hz with an amplitude of 5 mV.
Cell Fabrication and Electrochemical Evaluation: CR2032 type coin cells
were fabricated inside the glove box with 1.0 ꢀ LiTFSI and 0.2 ꢀ LiNO3
in the mixture solvent of DME and DOL (1:1 v/v) as the electrolyte. First,
−1
1
.6–1.7 mg PTS was added into the TiS NSs@MWCNT composite paper
2
as current collector (PTS/TiS NSs) followed by 25 µL of electrolyte. Then
2
a Celgard 2400 separator was placed on the top of the electrode followed
by adding 10 µL electrolyte on the top of the separator and lithium metal
anode. Finally, the cell was crimped and taken out of the glove box for
4
. Experimental Section
Materials: Phenyl disulfide (PDS, C H S , 99%, Sigma-Aldrich),
electrochemical evaluation. The mass ratio of PTS to TiS was calculated
2
12
10 2
to be about 3:1. The pure PTS cell was prepared by following the same
procedure using the pure MWCNT paper as current collector. Cells were
galvanostatically cycled on a LANHE battery cycler at different C rates
lithium thiophenolate (PhSLi, C H Li, 1 ꢀ in THF, Sigma-Aldrich), sulfur
6
5
(
S , ≥99.5%, Aladdin-reagent), titanium chloride (TiCl , 99.99%, Aladdin-
8
4
reagent), carbon disulfide (CS , 99%, J&K chemical), oleylamine (80–90%,
2
−
1
(
1 C = 570 mA g , based on the mass of PTS in the cells). The pure
Aladdin-reagent), octadecene (>90%(GC), Aladdin-reagent), lithium bis
PTS and PTS/TiS NSs cells were cycled between 1.8–3.0 V at 0.1 C and
2
(trifluoromethanesulfonimide) (LiTFSI, LiN(CF SO ) , 99.95%, Sigma-
3 2 2
0
.2 C rates, 1.75–3.0 V at 0.5 C and 1 C rate, 1.7–2.8 V at 2 C rate, and
Aldrich), lithium nitrate (LiNO , 99.99%, Canrd), 1,2-dimethoxyethane
3
1.65–2.8 V at 3 C and 4 C rates. For the pure PTS and PTS/TiS NSs cells,
2
(
DME, 99.5%, Sigma Aldrich), 1,3-dioxolane (DOL, 99.8%, Sigma Aldrich),
the CV was performed with different scan rates between 1.8–3.0 V. EIS
data were collected in the frequency range of 100 kHz–0.1 Hz.
multiwalled carbon nanotubes (MWCNTs, Nanostructure and Amorphous
Materials, Inc.).
Characterizations: The X-ray diffraction (XRD) data of the MWCNT,
TiS2 NSs, and TiS2 NSs@MWCNT composite materials were
collected on a Rigaku MiniFlex600 XRD Instrument equipped with
Synthesis of Phenyl Tetrasulfide: Appropriate amounts of PDS and
sulfur powder (the molar ratio of PDS to sulfur was 1:2) were added into
a quartz tube and sealed by a MRVS-1002 single station vacuum sealing
machine, then heated for about 8 h at 170 °C. The obtained product was
a clear yellow liquid. There was no sulfur powder precipitation.
−
1
Cu Kα radiation. The scanning rate was 0.4° min , and 2θ was set
between 10° and 80°. Raman measurement was conducted on a
HORIBA HR Evolution. N2 adsorption-desorption measurements were
conducted on a Micrometrics ASAP2420-4MP instrument. The nitrogen
adsorption and desorption were performed at 77 K. The morphology
and microstructure of TiS2 NSs and TiS2 NSs@MWCNT composite
materials were investigated by a Phenom FEG SEM and a JEM-2100-F
HR-TEM. The elemental mapping was performed with energy-dispersive
X-ray spectroscopy (EDS) attached to the HR-TEM. X-ray photoelectron
spectroscopy (XPS) were analyzed with a 5000 VersaProbe II XPS
spectrometer with monochromatic Al Kα radiation. Element analysis
was tested by Flash EA 1112 Automatic Element Analyzer. A Waters Xevo
G2-XS QTof mass spectrometer was used to verify the synthesized PTS.
The synthesized PTS and the recharge product of PTS/TiS2 electrode
samples were diluted and soaked in trichloromethane (CHCl3),
respectively, and the solutions were tested in atmospheric pressure
solids analysis probe (ASAP) positive mode with the first-order mass
spectrometry model. The discharge product was verified by a Waters
ACQUITY UPLC I-Class PLUS liquid chromatogram coupled with a
Waters Xevo G2-XS QTof mass spectrometer. The column was ACOUITY
UPLC BEH C18 LC Column (2.1–100 mm, Waters). The discharged PTS/
TiS2 cathode sample was soaked in chromatographic methanol and it
was filtered. 1 µL of the solution was tested in atmospheric pressure
chemical ionization (APCI) positive mode.
Synthesis of TiS2 NSs: TiS2 NSs were synthesized by the procedure
of the synthesis published by Zeng and coworkers[
29]
2 mmol TiCl4
was mixed with 8 mL oleylamine and 30 mL octadecene in a 250 mL
three-neck flask with strong magnetic stirring. The mixture was heated
to 120 °C and then 220 °C under argon atmosphere. Subsequently, CS2
was injected into the mixture and reaction temperature was increased to
300 °C. After 3 h, the reaction was quenched and cooled down to room
temperature. At end, TiS2 nanoplates were collected by centrifugation,
and washed sequentially with cyclohexane/ethanol (1:1 v/v) and pure
ethanol. Finally, the product was dried at 80 °C in a vacuum oven.
Preparation of TiS NSs@MWCNT Composite Paper: The TiS NSs@
2
2
MWCNT composite paper was prepared through a vacuum filtration
method. TiS2 NSs and MWCNT were mixed in 500 mL ethanol with
a mass ratio of 7:1, and the mixture was ultrasonicated for 15 min
in order to interweave MWCNT with TiS2 NSs. Before mixing the
TiS2 with MWCNT, the TiS2 was fully dispersed in 300 mL ethanol
by ultrasonication for about 10 min. The TiS -MWCNT mixture was
2
vacuum filtered onto a 7 cm filter paper to render a free-standing TiS2
NSs@MWCNT composite paper. The composite paper was dried
under vacuum at 80 °C for 24 h before being peeled off and punched
out into circular disks with a diameter of 1.2 cm (1.13 cm , 3.7–3.8 mg
each).
Adsorption of Lithium Polysulfide and Lithium Thiophenolate: Li2S6
solution was prepared by chemically reacting sulfur and an appropriate
amount of Li S in DME/DOL solution (1:1 v/v). The solution was
then stirred at 60 °C in an Ar-filled glove box for 12 h to produce a
brownish-red Li S solution (1.0 ꢀ). The lithium thiophenolate (PhSLi)
powder was obtained by removing THF solvent in the commercial PhSLi
solution in an Ar-filled glove box at 60 °C. The obtained PhSLi powder
was dissolved in DME/DOL solvent (1:1 v/v) to form 0.2 ꢀ PhSLi
solution. The Li S and PhSLi solutions were then diluted to 5 × 10
for the adsorption test. TiS2 or MWCNTs as adsorbents (10 mg) was
added into 2 mL of Li S and PhSLi solutions. After prolonged contact
2
Theoretical Computation: The density function theory (DFT)
calculations were performed within generalized gradient approximation
in the Perde-Burke-Ernzerh (PBE) scheme, as implemented in the
2
Vienna ab initio simulation package (VASP) code.[
41–43]
The valence
electronic states were expanded in plane wave basis sets with energy
cutoff of 400 eV. The energy optimization was completed with force
2
6
−
1
convergence criterion of 10 meV Å . The Monkhorst-Pack k-point mesh
of 3 × 3 × 1 was used for surface structural relaxation and a vacuum
slab of about 15 Å was employed. The TiS2 (00-1) surface was chosen to
calculate the adsorption energies. The adsorption energies (Eads) for the
PhSLi and Li S compounds on the metal sulfide surfaces are defined as
−
3
ꢀ
2
6
2
6
2
6
Eads = Etotal − Ecomp − Esuf, where Etotal is the total energy of the adsorbed
system, Ecomp is the energy of the compound in vacuum and Esuf is the
energy of the optimized clean surface slab. The binding energies (Eb)
with Li S and PhSLi, the supernatant separated by filtration was diluted
2
6
to enable quantitative concentration measurements. Ultraviolet-visible
UV–vis) absorption spectra of these diluted solutions were collected
(
have a negative sign of the Eads
.
using a UV-8453 spectrophotometer (Agilent) with baseline correction.
Catalytic Effect Evaluation: Symmetric cells were assembled with
two TiS2 NS@MWCNT composite or pure MWCNT electrodes. The
electrolyte was 1.0 ꢀ LiTFSI in DME/DOL (1:1 v/v), 0.2 ꢀ Li S or 0.2 ꢀ
PhSLi solution. Cyclic voltammetry (CV) was performed on a Bio-Logic
VMP-3 potentiostat between −1.0 and 1.0 V at the scan rate of 3 mV s .
Electrochemical impedance spectroscopy (EIS) was conducted at open
2
6
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
−
1
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[
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