Using CoS cathode materials with 3D hierarchical porosity and an ionic liquid (IL) as an electrolyte additive for high capacity rechargeable magnesium batteries

Article abstract of DOI:10.1039/c9ta05233j

Developing high capacity, rechargeable magnesium batteries is highly desired to meet increasing energy demands while targeting viable replacements for lithium batteries. In the present work, cobalt sulfide (CoS) spheres with three-dimensional (3-D) hierac

Full text of DOI:10.1039/c9ta05233j

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Using CoS cathode materials with 3D hierarchical porosity and an
ionic liquid (IL) as electrolyte additive for high capacity
rechargeable magnesium batteries
Received 00th January 20xx,
Accepted 00th January 20xx
Mingguang Pan,ab Jianxin Zou,*ab Richard Laine,*c Darvaish Khan,ab Rui Guo,d Xiaoqin Zeng,ab and
DOI: 10.1039/x0xx00000x
Wenjiang Dingab
Developing high capacity, rechargeable magnesium batteries is highly desired to meet increasing energy demands while
targeting viable replacements for lithium batteries. In the present work, cobalt sulfide (CoS) spheres with three-
dimentional (3-D) hierachical porosity, CoSHE, were prepared in a mixed solvent comprising equal volumes of water and
ethylene glycol using a solvothermal method. The resultant CoSHE offer large pore volumes (0.227 cc/g) and appreciable
specific surface areas (SSAs, 27 m2/g) as well as flexible structures that provide efficient Mg2+ transport channels and
accommodate the volume expansion that occurs during the discharge-charge process. Moreover, our data reveal that the
introduction of an IL into the electrolyte consisting of [2-tert-butyl-4-methylphenolate magnesium chloride (TBMPOMgCl)
and aluminium chloride at a molar ratio of 2:1 not only significantly activates the discharge/charge process, but also
enhances the specific capacity, perhaps by altering the thermodynamic and kinetics of the reaction between CoS and Mg.
As a result, a high capacity (up to 370 mAh g-1), good cycling stability (around 340 mAh g-1 after 88 cycles) and considerable
rate performance (around 300 mAh g-1 at 50 mA g-1) were achieved cooperatively by using CoSHE cathode and the IL
electrolyte additive.
To date, various cathode materials for RMBs have been
developed and principally include elemental sulfur (S),5
transition metal oxides (TMOs),6 and transition metal
Introduction
Advanced energy storage systems with high energy densities,
low cost, and reliable safety provide impetus for robust
development of post lithium (Li) battery technologies to meet
the increasing demands of numerous electric devices.1 In this
chalcogenides (TMCs).7–15 The element S, coupled with Mg is
attractive for its high theoretical capacity (1671 mAh g-1 or
3459 mAh cm-3).5 However, Mg-S batteries still suffer from
critical problems such as fast capacity fade on cycling and large
voltage hysteresis because most electrolytes are not
compatible with both the nucleophilic Mg anode and
electrophilic sulphur cathode.16 TMOs are high-voltage
tolerant, but often suffer from sluggish kinetics and poor
charge-discharge reversibility owing to strong electrostatic
attractions with the guest Mg2+ ions.17 Thus recently, “softer”
TMCs including graphene-like molybdenum disulfide (G-MoS2)
and layered titanium disulfide (TiS2) are preferred options as
cathodes for rechargeable Mg batteries.7,10,11 However, the
narrow interlayer spacings of layered TMCs can be envisioned
to greatly hinder diffusion of Mg2+ ions, resulting in either poor
specific capacity or sluggish Mg intercalation/de-intercalation
kinetics. As a result, in most cases, negligible electrochemical
activity is observed at room temperature, impeding the further
development of TMCs.11 To this end, one promising approach
is to expand the interlayer spacings of layered TMCs by
inserting suitable solvents. Another is to create high specific
surface area and large pore volume TMC cathode materials
with three dimensional (3-D) hierarchical structures.
regard,
rechargeable
magnesium
(Mg)
batteries
merit particular attention since the Mg anode can provide the
advantages of high capacity (2205 mAh g-1 and 3833 mAh cm-
3), low reduction potential (-2.37 V vs. SHE), great natural
abundance and a good safety profile.2,3 The first rechargeable
Mg battery (RMB) prototype was demonstrated by Aurbach et
al. in 2000 using the Chevrel phase compound (Mo6S8) as the
cathode, yielding excellent charge-discharge reversibility.4
However, its low capacity (122 mAh g-1, theoretical) and
operating potential (1.1 V vs. Mg2+/Mg) resulted in a poor
energy density <140 Wh kg-1, thus limiting its commercial
utility.
a. National Engineering Research Center of Light Alloy Net Forming and State Key
Laboratory of Metal Matrix Composite, School of Materials Science and
Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail:
zoujx@sjtu.edu.cn
b. Shanghai Innovation Institute for Materials, Shanghai 200444, China
c. Department of Materials Science and Engineering, University of Michigan, Ann
Arbor, Michigan 48109-2136, United States. E-mail: talsdad@umich.edu
d. Shanghai Institute of Space Power-Sources, Shanghai 200245, China
† Electronic Supplementary Information (ESI) available: Supplementary Table S1
and supplementary Fig. S1 to S18. See DOI: 10.1039/x0xx00000x
Currently, developing high capacity RMBs still remains a
great challenge primarily due to incompatibility between the
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Journal Name
electrolyte and the electrodes.18 Therefore, electrolytes with high reversible Mg deposition/dissolution, VieawnAdrticlemOnolirnee
excellent safety, high chemical stability and wide importantly, air-stability.24 Furthermore, the IL, 1-butyl-1-
DOI: 10.1039/C9TA05233J
electrochemical windows, as well as compatibility with the methylpyrrolidinium chloride (BMPyCl) can be used as an
electrodes,
become
highly
desired.
Unfortunately, additive to effectively inhibit Cl- corrosion at high potentials, as
conventional electrolytes such as Mg(ClO4)2, Mg(TFSI)2 or reported previously.37
Mg(PF6)2 salts in carbonate or ether solvents for Li-ion
In the present work, CoS samples were solvothermally
batteries work poorly in Mg-ion batteries because of the synthesized in pure water (H2O), ethylene glycol (EG), and the
formation of a passivated solid-electrolyte interphase (SEI) mixed solvent V(H2O)/V(EG) = 1:1, denoted as CoSH, CoSE, and
layer on the surface of the Mg metal anode.19 It’s known that CoSHE, respectively.38 Their microscopic structures were found
Grignard reagents (RMgX, R = alkyl or aryl group, X = halide in to be solid polyhedral structures, yolk-shell hollow spheres and
ethereal solvents) can enable reversible Mg deposition and hierarchically porous microspheres, respectively. Cathodes
stripping.20 However, their intrinsically poor oxidative stability based on CoSHE displayed superior electrochemical
(<1.5 V vs. Mg/Mg2+) severely limits their further application as performance compared to CoSH and CoSE, probably attributed
practical electrolytes. Thus, a set of strategies has been to the flexible 3D hierarchical structures with large pore
proposed for enhancing the oxidative stability of Grignard volume and appreciable specific surface area. The electrolyte
based electrolytes.21–25 Especially, the introduction of strong is a mixture of TBMPOMgCl and AlCl3 at a 2:1 molar ratio in
Lewis acids such as organohaloaluminates (R3-nAlCln) or THF with the addition of the IL (BMPyCl), termed POC-xIL
aluminium chloride (AlCl3) combined with Mg2+ -containing (Scheme 1). Interestingly, addition of the IL into the POC
Lewis bases has been successful.21 Moreover, the development electrolyte significantly enhances the activation of the
of non-Grignard electrolytes is also a promising approach to discharge/charge process and increases the specific discharge-
improving electrochemical windows. For instance, the strong charge capacity of Mg batteries. Our data reveal that a high
nucleophilicity of R groups in RMgX could be mitigated by reversible capacity is achieved by combining a CoSHE cathode
replacement with amines, boranes, or phenolates.22-24 Thus, a with a POC-xIL component in the electrolyte when the mole
series of air-stable electrolyte systems with high ionic fraction of IL is approximately x=0.2. Particularly, high capacity
conductivity, high reversibility of Mg deposition/dissolution (up to 370 mAh g-1) with considerable stability (around 340
and good anodic stability have been developed based on mAh g-1 after 88 cycles) and high Coulombic efficiency (nearly
phenolate or alkoxide magnesium chloride salts.24,25 100%) at 20 mA g-1 were achieved by using a CoSHE cathode
Furthermore,
chloroaluminates (e.g. carboranes, ionic liquids etc.) are shown best results reported for TMC cathodes (Table 1 and S1).
to have comparatively high oxidative stability.26–30 Recent
The CoSHE magnesiation/demagnesiation mechanism was
a large number of electrolytes beyond and POC-0.2IL electrolyte at room temperature, one of the
advances have witnessed the incorporation of an additional characterized by ex-situ X-ray diffraction (XRD), X-ray
salt in the electrolyte that facilitates the Mg plating/stripping photoelectron spectra (XPS) and high-resolution transmission
process.31–33
In this regard, ionic liquids (ILs), which consist entirely of
electron microscopy (HRTEM) images as discussed below.
ions and are molten salts with melting points below 100 ˚C,
possess many unique properties including negligible volatility,
ultralow flammability, good thermal stability, high ionic
conductivity, a wide electrochemical window and tunable
polarity.34,35 These properties enable them to serve as
appealing electrolyte media for fabricating high capacity RMBs.
For example, an electrolyte system consisting of
(HMDS)2Mg (HMDS = hexamethyldisilazide) in diglyme plus a
TFSI-based IL as a weakly coordinating co-solvent altered the
Results and discussion
The phase and purity of all CoS products were analyzed by
powder XRD (Fig. S1). All diffraction peaks for CoSH can be
unambiguously assigned to hexagonal CoS (JCPDS card no. 75-
0605). No additional peaks from impurities are detected,
revealing a single phase purity. The XRD peaks for CoSH are
sharper and stronger than for CoSE and CoSHE, indicating a
higher degree of crystallinity. The relatively strong diffraction
rings ascribed to (100), (101), (102), (110) planes in selected
area electron diffraction (SAED) images of CoSHE are indicative
thermodynamic and kinetic properties of
S and Mg
interconversion, resulting in large capacity increases for Mg
batteries compared to ones without an IL.36
Given that cathode, anode and electrolyte are essential
components needed to assemble any battery; here, cobalt
sulfide (CoS) was selected as the cathode material for its high
theoretical capacity of 589 mAh g-1, relatively inexpensive raw
material and simple synthetic procedures. The morphologies
such as the specific surface areas (SSAs) and pore volumes of
CoS can be controlled easily by the ratio of solvents (ethylene
glycol and water) in its solvothermal synthesis. The electrolyte,
Scheme 1 Chemical structures of electrolytes or electrolyte
components.
2-(tert-butyl)-4-methylphenolate
magnesium
chloride
(TBMPOMgCl)/AlCl3 complex tetrahydrofuran solution offers
high ionic conductivity, good anodic stability (2.6 V vs Mg),
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Table 1 Recently reported Mg battery systems with superior electrochemical performance.
Current
density
(mA g-1)
Voltage range
(V vs. Mg2+/Mg)
Capacity (mAh g-1)
(Cycle number)
Electrode material
Ref.
Electrolyte
CoSHE
this work 0.25M POC-0.2IL
0-1.8
20
~134 (1)-≥360(68-85)
170 (1)-161 (50)
graphene-like MoS2
VS4
7
Mg(AlCl3Bu)2/THF
0.5-3.0
0.2-2.2
0.3-3.0
0.0-2.0
20
12
9
0.4 M APC in THF
100
50
251(1)-150(180)
WSe2 nanowires
0.25M Mg(AlCl2EtBu)2/THF
0.25 M APC in THF
~220 (1)-203 (160)
~155 (1)-~114(500)
interlayer-expended TiS2 11
240
of polycrystallinity (Fig. S2a). HRTEM images exhibit lattice structures are observed on the edge (Fig. 1d and 1e).
fringes with interplanar distances of 1.92 and 2.55 nm, Moreover, after embedding epoxy resin and sliced to
consistent with d-spacing of the (102) and (101) planes of CoS, thicknesses of ≈100 nm, the CoSHE sample exhibits a cross-
respectively (Fig. S2b).
linked skeletal structure, suggesting a porous hierarchical
SEM and TEM imaging analysis provides insight into the structure (Fig. 1f). Furthermore, smaller CoSHE spheres (~4.5
surface morphologies and internal structures of these μm diameter) are also found that also exhibit porous internal
materials. As revealed in SEM images, CoSHE appears as structures composed of nanosheets (Fig. S3). In contrast, CoSH
honeycomb-like spheres with primary particle sizes ranging and CoSE display irregular, solid polyhedral structures and yolk-
from 4 to 12 μm and what appear to be porous internal shell hollow spheres with primary particle sizes ranging from 2
structures (Fig. 1a–c). The TEM images in Fig. 1d–f further to 12 μm and 0.45 to 1.8 μm (Fig. S4), respectively. It is worth
confirm the formation of porous honeycomb spheres (~7 μm mentioning that, in contrast to CoSH, CoSHE or CoSE presents
diameter) composed of nanosheets. The edge of the CoSHE more regular and loosely porous structures. This is likely a
sphere is thinner than the center so that more exquisite consequence of EG solvent complexing Co2+ ions through a
Fig. 1 SEM (a–c) and TEM (d–f) images of CoSHE spheres. Note that (b) is the enlarged part (red dashed box) of (a), and (c) is the
enlarged part (pink dashed box) of (b), and (e) is the enlarged part (blue dashed box) of (d).
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coordination interaction that controls the release rate of Co2+ Based on the BJH analysis, the pore volumes for CoSH, CoSHE
from the complex, thus lowering the precipitation rate and and CoSE are measured to be 0.004, 0.227 and 0.190 cc/g,
enhancing the porosity and regularity of the CoS product.39
respectively. Briefly, the order of surface areas is
The SSAs and pore-size distribution of electroactive CoSH<<CoSHE<CoSE, while the order of pore volumes is
materials play an important role in batteries.40 Therefore, the CoSH<<CoSE<CoSHE.
SSAs and porosities of CoSH, CoSHE, and CoSE samples were
The electrochemical performance of CoSHE spheres as
characterized by N2 adsorption/desorption (Fig. 2). As cathode material hosting Mg2+ ions was evaluated by
expected, the absorbed N2 volume in CoSH is small and exhibits assembling 2032 coin-type cells with 0.25 M POC-0.2IL in THF
few changes from 0.1 to 2.0 cc/g when the relative pressure as the electrolyte and polished Mg foil as the anode. Fig. 3a
(P/P0) increases from 0.05 to 0.99, indicating a negligible presents typical CV curves for the CoSHE electrode with initial
amount of mesopores. This corresponds to the SEM/TEM scans at a scan rate of 0.1 mV s-1. The voltage range was set
studies noted above showing that CoSH possesses solid between 0 and 1.85 V to avoid strong polarization and
polyhedral structures.
electrolyte decomposition.11 There is a clear difference in the
Unlike CoSH, CoSHE and CoSE both display a hysteresis loop reduction peak between the first cycle and the subsequent
in their isotherms, indicative of mesoporous structures.41,42 cycles, which is attributed to the relatively strong polarization
CoSH, CoSHE and CoSE offer Brunauer–Emmett–Teller (BET) in the first discharge process.12 For the CV curve in the second
SSAs of 4, 27 and 60 m2/g, respectively. Using the Barrett- cycle, a small redox peak at 1.01 V appears (Fig. 3a). In
Joyner-Halenda (BJH) method, two peaks in the pore size subsequent cycles (3th-5th), the reduction and oxidation peaks
distribution for CoSHE are seen for pore diameters of 4.0 and at 1.01 and 1.78 V respectively, increased progressively,
18.2 nm, while those for CoSE are located at 3.7 and 9.5 nm, suggesting that the CoSHE electrode is gradually activated
respectively (Fig. 2, inset), indicating the presence of two kinds during the CV scans and has a reversible redox behavior. The
of pores in CoSHE and CoSE.42 For example, for CoSHE, the small main redox and oxidation peaks at 1.01 and 1.78 V can be
pores may be attributed to self-assembly of the nanosheets attributable to reversible reduction and oxidation of Co.43
during material growth, while the large pores may be ascribed
The galvanostatic discharge-charge curves for the CoSHE
to hierachical structures assembling from these smaller cathode at 20 mA g-1 in 0.25 M POC-0.2IL THF electrolyte are
particles. In contrast to CoSE, CoSHE exhibits a smaller BET SSA displayed in Fig. 3b. The discharge-charge voltage ranges from
of 27 m2/g with a wider pore-size distribution (Fig. 2, inset).
0 to 1.8 V in order to avoid the oxidation of CoS during the
cycling process. As revealed in Fig. 3b, the discharge and
charge capacities in the first cycle are 134 and 52 mAh g-1,
respectively, demonstrating a low Coulombic efficiency (CE) of
38%. The formation of a solid electrolyte interphase (SEI) layer
when discharged to a lower voltage may hinder the recharge
process, thus resulting in a large capacity loss during the first
cycle.44 In the second cycle, the discharge capacity decreased
to 66 mAh g-1, while the charge capacity increased to 112 mAh
g-1, revealing a remarkable activation process.45 Meanwhile,
the plateau voltages emerge at around 1.1 and 1.7 V,
respectively, corresponding to the redox peaks in the CV
curves (Fig. 3a and 3b). Such a dramatical charge capacity
increase with a CE value beyond 170% indicates that the IL
electrolyte additive, BMPyCl significantly promotes activation
of the electrode. The activation process for the CoSHE cathode
in the POC-0.2IL modified electrolyte is much faster than in the
POC electrolyte (Fig. S5). As the discharge-charge cycles
proceed, the capacity gradually increases, and the voltage gap
between the charge and discharge plateaus narrows and the
plateau region gets flatter (Fig. 3b). The discharge and charge
Fig. 2 N2 adsorption (black) and desorption (red) isotherms
and pore size distribution (inset) of CoSH (▼/▼), CoSHE (■/■)
and CoSE (▲/▲).
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Fig. 3 Electrochemical performances of Mg batteries with CoSHE cathode and 0.25 M POC-0.2IL electrolyte: (a) CV curves
measured in a voltage range between 0 and 1.85 V at a scanning rate of 0.1 mV s-1; (b) discharge-charge curves and (c) cycling
performance at a current density of 20 mA g-1; (d) rate performance at the current densities ranging from 20 to 80 mA g-1.
plateau voltages finally shifts to around 1.16 and 1.52 V,
To better understand the activation effect of the IL, the
respectively. Notably, the discharge capacity gradually electrolyte POC with or without IL was run to observe
increases and ultimately reaches a maximum value of 376 mA differences in the CV curves and discharge-charge process (Fig.
g-1 in the 72th cycle and finally becomes stable at ≈360 mAh g-1 4, S6 and S7). The redox peaks of the CoSHE cathode in POC-
with the CE ≈100% (Fig. 3c). Furthermore, CoSHE with POC-0.2IL 0.2IL electrolyte obviously increased, while those in pure POC
exhibited considerable rate capability at current densities electrolyte increased slightly during the same CV cycles,
ranging from 20 to 80 mA g-1 (Fig. 3d).
indicating the effect of IL on redox reactions (Fig. 4a). The
Fig. 4 (a) CV curves (8 cycles) of CoSHE cathode in POC-0.2IL electrolyte (red) and POC electrolyte (black), and partial discharge-
charge curves of CoSHE cathode in (c) POC-0.2IL electrolyte and (d) POC electrolyte during the initial 20 cycles at a current density
of 20 mA g-1.
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discharge-charge capacities of CoSHE cathode in POC-0.2IL than CoSHE in the first discharge (Fig. S13). However, CoSE
electrolyte are higher than those in POC electrolyte for the capacity is low in subsequent cycles and exhibits a poor cycling
same cycles (Fig. 4b and 4c). The plateau regions for the CoSHE life of only 8 cycles. In contrast, CoSHE provides high specific
cathode in POC-0.2IL electrolyte are also flatter and wider than capacity and long cycling life attributable to its flexible
those in POC electrolyte (Fig. 4b, 4c and S6). Therefore, the IL, hierarchical structures with large pore volume, which can
BMPyCl, as electrolyte additive may accelerate the activation provide sufficient Mg2+ transport channels and accommodate
of electrodes, and the activation of CoSHE cathode is likely due the volume expansion during the discharge-charge process.47
to the phase change from CoS to Co9S8.
Ex situ XRD, ex situ XPS and HRTEM spectroscopy were
The electrochemical impedance spectroscopy (EIS) analysis performed on pristine, discharged and charged CoSHE to gain
of CoSHE before and after the same charge-discharge cycles insight into the magnesiation/demagesiation mechanism. As
was run (Fig. S7). The semicircles in the high-frequency regions seen in Fig. 5a, after discharging to 0 V (70th discharge cycle),
and sloping lines in low-frequency regions of the Nyquist plots the peaks assigned to CoS almost vanish, and new peaks
are relevant to the charge-transfer resistance (Rct) at the attributable to Co9S8 (JCPDS card no. 86-2273) and Co (JCPDS
electrode/electrolyte interface and the Mg2+ diffusion card no. 88-2325) appear, implying a conversion reaction
resistance within the electrodes, respectively. As discharge and mechanism. However, the expected MgS phase is not seen in
charge proceed, Rct and Mg2+ diffusion resistance values for the XRD, implying the formation of amorphous MgS. The peak
CoSHE with POC-0.2IL electrolyte decrease dramatically after 20 positions are almost unchanged after charging to 1.8 V (Fig. 5a).
cycles, and are much smaller than those for CoSHE with POC
XPS was used to investigate the composition and chemical
electrolyte, demonstrating that the IL additive facilitates valence change of CoSHE samples during the discharge-charge
electron and Mg2+ diffusion, thus conducive to Mg2+ storage process. Fig. 5b–d show the XPS spectra at Co 2p, S 2p, and Mg
performance.
2p regions for pristine, discharged and charged CoSHE cathodes,
The effect of added IL was also investigated by changing respectively. The Co 2p peaks exhibit considerable changes
the molar ratio of IL to POC from 0.1 to 0.5 (Fig. S8–S10). after discharging and charging, corresponding to the formation
Increasing the amount of added IL enhances capacity but of Co9S8 as previously reported.48 The pristine S 2p peaks
sacrifices cycling life. The molar ratio of IL to POC at ≈0.2 belonging to CoS are deconvoluted to two main peaks at 163.0
exhibits the best electrochemical performance. Thus, and 162.0 eV with a known band split of 1.1 eV. However,
introducing an IL significantly activates the discharge-charge after discharge, only a new peak at 162.1 eV appears,
process and enhances the specific capacity, probably resulting indicating formation of MgS. After charging, the S 2p peaks
from an alteration in thermodynamic and kinetic properties of shifts toward 162.8 and 161.8 eV attributable to the formation
the conversion between CoS and Mg.36 Moreover, the CoSHE of Co9S8. Moreover, after discharge, the Mg 2p peak located at
cathode with POC-0.2IL electrolyte showed distinctly superior ≈50.8 eV should be assigned to MgS.12 The intensity of Mg 2p
Mg storage performance compared to that with the all phenyl band after discharge is higher than that after charge, indicating
complex (APC) and APC-0.2IL electrolytes (Fig. S11). Finally, that the insertion and extraction of Mg2+ were reversible but
when the coin cell assembly was exposed to air, the Mg some Mg2+ ions still remain in the CoS lattices, in agreement
battery still exhibits capacities to 300 mAh g-1 at ≈50 discharge with the EDX results (Fig. S14). The element mapping images
cycles (Fig. S12), demonstrating the tolerance to moisture and also clearly reveal that the magnesiation of the cathode occurs
oxygen.
after discharge and the demagnesiation is not complete after
The effect of morphology of CoSH, CoSHE and CoSE on charge (Fig. S15). Furthermore, as revealed in the HRTEM
specific capacity was also studied. From the BET and BJH images, after discharge, the lattice fringes with spacings of
results, CoSHE spheres show lower SSAs but higher pore 0.172 and 0.302 nm coorespond to the (200) plane of Co and
volumes than CoSE, while CoSH has far lower specific surface the (111) plane of MgS, respectively (Fig. 5e and 5f); and after
area and pore volume than CoSHE and CoSE. Generally, for charge, the lattice fringes with spacings of 0.353 and 0.295 nm
cathode active materials, high SSAs provides abundant active can be assigned to the (220) and (311) planes of Co9S8,
sites for guest ions insertion and deinsertion, and surface respectively (Fig. 5g). We envision that the phase change from
redox reactions; large pore volume with open pore structure CoS to Co9S8 may result from the activation of CoSHE by the IL
can accommodate mechanical strains and facilitate mass during the cycling process. Furthermore, the XRD data clearly
transport, which is critically important for the loading of guest demonstrate that the phase change from CoS to Co9S8 has
species.46 As expected, CoSH has almost no electrochemical already occurred after the first discharge, whose content
activity with a negligible capacity. CoSE exhibits higher capacity
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Fig. 5 Mechanism investigation of the magnesiation/demagnesiation process in CoSHE cathode with POC-0.2IL electrolyte. (a) Ex
situ XRD patterns of CoSHE at the pristine state, after 70th discharge and after 70th charge. Ex situ XPS spectra at the Co 2p (b), S
2p (c), and Mg (d) regions of CoSHE cathode at the pristine state, after 70th discharge and after 70th charge, respectively. HRTEM
images of CoSHE cathode after 70th discharge (e and f) and after 70th charge (g).
increases as the discharge-charge cycles increase (Fig. S16). pore volumes and appreciable SSAs as well as flexible
Thus, a plausible discharge-charge mechanism on the CoSHE structures, which can provide sufficient Mg2+ transport
cathode in POC-0.2IL electrolyte can be presented as below:
channels and accommodate the volume expansion during the
discharge-charge process. On the other hand, the addition of IL
into an air-stable electrolyte consisting of 2-(tert-butyl)-4-
methylphenolate magnesium chloride salt and AlCl3 at the
molar ratio of 2:1 can significantly activate the discharge-
charge process and enhance the specific capacity. As a result, a
high capacity (up to 370 mAh g-1), good cycling stability (~340
mAh g-1 after 88 cycles) and reasonable rate performance
(~300 mAh g-1 at 50 mA g-1) were cooperatively achieved by
using a CoSHE cathode and POC-0.2IL electrolyte. Furthermore,
the magnesiation/demagnesiation mechanism via reversible
reaction between Co9S8 and Co (or Mg2+ ions and MgS), where
Co9S8 was formed by the phase change from CoS, was also
investigated by ex situ XRD, ex situ XPS and HRTEM
spectroscopy.
Co9S8 + Mg2+ + 2e-
MgS + Co
,
where the formation of Co9S8 is due to the phase change of
CoS.
Conclusions
In summary, CoS spheres with 3D hierarchical porosity, CoSHE,
were prepared from cobalt chloride hexahydrate and
thiocarbamide in the mixed solvent V(H2O)/V(EG) = 1:1 by a
solvothermal method. The CoSHE cathode with POC-0.2IL
electrolyte achieves a high reversible magnesium storage
activity (≥360 mAh g-1 from 68 to 85 cycles) at room
temperature, which is amongst the best electrochemical
performances achieved by all reported TMC cathodes for
RMBs. Specifically, on one hand, CoSHE spheres exhibit large
Experimental
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Journal of Materials Chemistry A
Page 8 of 10
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Journal Name
General Procedures: The IL, 1-butyl-1-methylpyrrolidinium (2.0 M solution in THF) under vigorous stirring for 6 h at room
View Article Online
DOI: 10.1039/C9TA05233J
chloride (BMPyCl), was synthesized according to the literature temperature, giving rise to 0.25 M APC electrolyte.
procedures.49 All reagents including cobalt chloride Electrochemical Testing: The coin-type cells (CR2032) were
hexahydrate
(CoCl2∙6H2O),
thiocarbamide
(CN2H4S), assembled in an argon-filled glovebox with water and oxygen
ethylmagnesium chloride (EtMgCl, 2.0 M solution in THF), contents below 0.1 ppm. The working electrode was prepared
phenylmagnesium chloride (PhMgCl, 2.0 M solution in THF), 2- by mixing active material, Super-P carbon black, and PVDF at a
tert-butyl-4-methylphenol (TBMPOH), aluminium chloride weight ratio of 8:1:1 in the NMP solvent to form a slurry, and
(AlCl3), Super-P carbon black, poly(vinylidene fluoride) (PVDF), then the slurry was pasted on the current collector (Cu foil).
N-methyl-2-pyrrolidinone (NMP), ethylene glycol (EG) and Magnesium (Mg) metal was made into round disks of 15 mm
tetrahydrofuran (THF) were reagent grade and used as diameter and 1.1 mm thickness using wire cutting. The Mg
received. All electrolyte preparations were carried out in an disks were polished with sandpaper to remove the oxides and
argon-filled glove box (<1 ppm of water and oxygen) at room impurities on the surface and served as the counter electrode
temperature. Distilled water (H2O) was prepared from an and reference electrode. The separator adopted was Celgard
Aquaplore ultrapure water system (AFX-1001-U).
2400 polypropylene film. The assembled coin cells were ageing
Synthesis of CoS samples: The CoS samples were sythesized for 24 h, then for the electrochemical test. The full Mg ion
using a typical solvothermal method. Here, the synthetic batteries were discharged at first and then charged.
procedures for CoSHE were shown below in detail. First, Galvanostatic charge/discharge tests were measured on a
CoCl2∙6H2O (3.8g, 16.0 mmol) and CN2H4S (7.3 g, 95.9 mmol) LAND CT2001A multichannel battery test system between 0
were added into 160 mL of a mixed solvent of distilled H2O and and 1.8 V versus Mg2+/Mg at room temperature. Cyclic
EG at a proportion of V(H2O)/V(EG) = 1:1. The mixture was voltammetry (CV) curves were acquired with an
stirring continuously for 30 min at room temperature until the electrochemical workstation (CHI660E) at a scan rate of 0.1 mV
starting materials were dissolved completely. Then the s-1. Electrochemical impedance spectra (EIS) was obtained by
resultant solution was transferred into a Teflon-lined stainless- applying small sinusoidal potential with amplitude of 5 mV and
steel autoclave (200 mL) and heated to 180 ˚C for 20 h. frequency ranging from 100 kHz to 0.01 Hz.
Subsequently, the autoclave was cooled down to room Materials characterization: Powder X-ray diffraction (PXRD)
temperature naturally. The dark precipitate powders were was conducted on a Rigaku Smart Lab diffractometer with Cu
collected and washed with distilled water for several times to Kα radiation. The scanning angle was set from 20° to 80° at a
remove water-soluble impurities, and then with ethanol for scanning rate of 5° min-1 or 1° min-1. The morphologies of the
several times to remove most of the water and water- products were examined by field-emission scanning electron
insoluble impurities, and dried at 50 ˚C for 12 h under high microscope (FESEM, Sirion 200), field-emission transmission
vacuum to remove remaining traces of water. Similarly, CoSH electron microscope (FETEM, TALOS F200X), scanning electron
and CoSE samples were also prepared from this solvothermal microscope (SEM, Phenom XL or NOVA NanoSEM 230) and
method above, where the reaction solvent was replaced by transmission electron microscope (TEM, JEM-2100). Energy-
distilled H2O and EG, respectively.
Preparation of POC electrolyte: The TBMPOH (0.3285 g, 2.0 FESEM or SEM. The slices with the thickness of around 100 nm
mmol) was dissolved in THF solvent (1.0 mL) under vigorous were obtained from cryo-ultramicrotome (UC6-FC6).
dispersive X-ray spectrometer (EDS) was attached on the
a
stirring. Then, 1.0 mL of EtMgCl THF solution (2.0 M) was Selected area electron diffraction (SAED) and high resolution
added dropwise to the aboved solution. The reaction was transmission electron microscopy (HRTEM) was performed on
exothermic and released a myriad of tiny bubbles of ethane a Talos F200X microscope or a JEM-2100F microscope with an
gas. The resulting solution was stirred for 6 h at room accelerating voltage of 200 kV. The elemental valence was
temperature and the TBMPOMgCl THF solution (1.0 M) was characterized by X-ray photoelectron spectroscopy equipped
obtained. Meanwhile, AlCl3 (0.1333 g, 1.0 mmol) was dissolved with a monochromated Al-kα X-ray source (XPS, AXIS Ultra
into 2.0 mL of THF solvent under vigorous stirring. Then the DLD). The nitrogen adsorption-desorption isotherm and
obtained AlCl3 THF solution was added slowly into the Barrett-Joyner-Halenda (BJH) methods were analyzed on a gas
TBMPOMgCl THF solution. The resulting solution was stirred sorption analyzer (Autosorb-IQ3).
for 6 h at room temperature and 0.25 M POC electrolyte was
formed. The molar concentration of as-prepared electrolyte is
calculated from the amount of AlCl3.
Conflicts of interest
Preparation of POC-xIL electrolyte: A certain amount of IL,
BMPyCl, ranging from 0.0178 to 0.0890g (x = 0.1~0.5) was
added into the 0.25 M POC electrolyte (4 mL) under vigorous
stirring. Then, the resulting solution was stirred for 6 h at room
temperature and 0.25 M POC-xIL electrolyte was obtained.
Prepation of APC electrolyte: In a typical synthetic method of
“all phenyl” complex (2PhMgCl-AlCl3, APC) salt solution, AlCl3
(0.1333g, 1.0 mmol) was dissolved in 3.0 mL of THF solvent.
The AlCl3/THF solution was added slowly into a 1.0 mL PhMgCl
There are no conflicts of interest to declare.
Acknowledgements
Prof. Zou acknowledges the support of National Natural
Science Foundation of China (No. 51771112) and “Shuguang”
Scholar Project (16SG08) from Shanghai Education
Commission. Prof. Laine would like to thank the support of the
8 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
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26 O. Tutusaus, R. Mohtadi, T. S. Arthur, F. Mizuno, E. G. Nelson
"111" project from China's Ministry of Education (B16032). Dr.
Pan is supported by the Project funded by China Postdoctoral
Science Foundation (2017M621476).
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254x190mm (96 x 96 DPI)