Exploring the Capacity Limit: A Layered Hexacarboxylate-Based Metal-Organic Framework for Advanced Lithium Storage

Article abstract of DOI:10.1021/acs.inorgchem.7b02939

Our previous work suggested that more carboxylate groups might lead to higher energy density for metal-organic frameworks. In this study, we synthesized a layered metal-organic framework (MOF) Ni-BHC by use of 1,2,3,4,5,6-benzenehexacarboxylic acid. After

Full text of DOI:10.1021/acs.inorgchem.7b02939

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Exploring the Capacity Limit: A Layered Hexacarboxylate-Based
Metal−Organic Framework for Advanced Lithium Storage
Xiaobing Lou, Yanqun Ning, Chao Li, Ming Shen, Bei Hu, Xiaoshi Hu, and Bingwen Hu*
State Key Laboratory of Precision Spectroscopy, Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Materials
Science, East China Normal University, Shanghai 200062, China
S
* Supporting Information
ABSTRACT: Our previous work suggested that more carboxylate groups
might lead to higher energy density for metal−organic frameworks. In this
study, we synthesized a layered metal−organic framework (MOF)
Ni-BHC by use of 1,2,3,4,5,6-benzenehexacarboxylic acid. After evacu-
ation by thermal treatment, this MOF was employed as an anode for
lithium storage. For its rich lithiation sites as well as layered fast-kinetics
structure, it delivers a superior reversible capacity of 1261.3 mA h g−1 at
100 mA g−1, far exceeding the performance of previously reported
MOF-based anode materials. Density functional theory calculation and
O soft X-ray absorption spectroscopy suggest that the luxuriant
carboxylate−metal units play an important part in the electrochemical process.
As a proof-of-concept, a layered metal−organic framework
named Ni-BHC was successfully synthesized here. This MOF is
composed of 1D coordination chains based on two kinds of
6-coordinated nickel(II) clusters, and the chains are further piled
up into 2D layers via hydrogen bonding and finally assembled
into a 3D layered framework structure. After removing the
coordinated water,23,24 we investigated the Li-storage perform-
ance of this material in the potential range of 0.01−3.0 V versus
Li/Li+. This MOF exhibits a high capacity of 1664.4 mA h g−1
after 5 cycles and 1261.3 mA h g−1 within 50 cycles, outper-
forming all the other MOF anodes reported to date as far as we
know. Our work demonstrates the success of designing high-
performance MOF anode for Li-ion batteries by increasing the
content of carboxylate−metal units within a proper topology
choice.
INTRODUCTION
■
Nowadays, the application of rechargeable lithium-ion batteries
(LIBs) is extended to various fields, including small hand-held
electronic devices, electric vehicles, and electrical energy storage.1
However, the commercial anodes, including graphite and lith-
ium titanate, possess only limited theoretical capacities; hence,
exploitation of high-capacity anode materials is of crucial impor-
tance for future needs.
Metal−organic frameworks (MOFs) or coordination polymers
(CPs), being a kind of organic inorganic hybrid, have become an
academic hot point for their changeable components, which can
further help to adjust their structures, chemical activities, and
pore sizes in wide ranges for the critical applications of sensing,
proton conductivity, gas storage, and catalysis.2−6 Recently, the
electrochemical applications of MOFs, especially in LIB anode
materials, have developed rapidly and exhibited tremendous
potential due to their abundant electroactive sites and large
ion diffusion channels,7−11 as summarized in Table S1. In these
MOFs, carboxylate groups (e.g., 1,4-benzenedicarboxylate and
1,3,5-benzenetricarboxylate) are widely employed. It was found
that in the electrochemical process, lithium is reversibly
inserted to the organic components, especially the carboxylate
groups;12−14 meanwhile, the coordinated metal ions might also
be involved in the lithiation processes of these MOFs.15−17
Other functional groups such as imidazole,18 pyridine,19 and
purine20 always lead to low reversible capacity.14,21,22 Conse-
quently, increasing the content of carboxylate groups might be
a feasible method to further improve the capacities of MOFs.
To cater to this strategy, we choose an intriguing linker-
mellitic acid to construct such high-capacity MOFs for its abun-
dant carboxylate groups. Mellitic acid, which is also known
as 1,2,3,4,5,6-benzenehexacarboxylic acid, has six carboxylate
groups, which have rich lithiation sites to boost the capacity.
EXPERIMENTAL SECTION
■
Synthesis. The solvents and reagents were purchased from Innochem
and used without further purification. In an optimized synthesis pro-
cedure, 0.5 mmol mellitic acid and 1.5 mmol Ni(NO3)2·6H2O were
dissolved in 10 mL of deionized water to produce a settled solution.
Then, its pH was in situ measured by a pH meter, and 0.1 M NaOH
was slowly added into the solution with stirring until pH ≈ 3.2. The
mixture was filtered and placed into an autoclave with Teflon-lined
stainless at 130 °C for 1 day. Upon cooling to room temperature, the
precipitates were filtered, and green single crystals (Ni-BHC) were
obtained. The crystal sample was washed with water and allowed to air-dry
(yield 43%). Evacuated Ni-BHC was prepared by annealing Ni-BHC
at 220 °C in air for 3 h. Elemental analysis calcd for C12H36Ni3O30
(Mr = 836.48): C, 17.21; H, 4.3; Ni, 21.05; O, 57.38. Found: C, 16.82; H,
3.83; Ni, 22.29; O, 57.39. Main IR (KBr, cm−1): 3590 m, 3315 m br,
Received: December 7, 2017
© XXXX American Chemical Society
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Figure 1. (a) Coordination mode of the mellitic acid ligand in Ni-BHC. (b) Complex hydrogen-bonding interactions inside and between the layers
of Ni-BHC observed along the c-axis.
1666 w, 1575 s, 1444 s, 1341 s, 1187 w, 991 w, 914 m, 719 m, 599 m,
555 m, 493 m, 421 w.
to deprotonate the mellitic acid. A proper pH of 3.2 can greatly
improve the quality of the Ni-BHC crystal. The morphology of
the obtained Ni-BHC crystal is displayed in Figure S1.
Physical Characterization. Inductively coupled plasma optical
emission spectrometry (ICP/OES) analysis of Ni was conducted using
a Varian 720-ES ICP/OES spectrometer. Elementar Vario ELIII
analyzer was used to analyze elemental composition. Fourier transform
infrared spectroscopy (FTIR) analyses were performed at a Nexus670
infrared spectrometer (Nicolet). X-ray Diffraction (XRD) data were
recorded at an X-ray diffractometer (Rigaku Ultima IV) with Cu Kα
radiation. Thermogravimetric (TG) curves (heating rate: 10 °C min−1)
were obtained on a thermo-analyzer (STA 449 F3 Jupiter) under air
atmosphere. An S-2400 scanning electron microscope (Hitachi, Japan)
was employed to take SEM images. The soft X-ray absorption spec-
troscopy (sXAS) data were measured at Shanghai Synchrotron
Radiation Facility (BL08UA). To conduct an ex situ sXAS test, the
cell was first discharged or further charged to the desired voltages at
100 mA g−1 on the basis of cyclic voltammetry curves (Figure S6).
Then, the cells were disassembled, and the electrodes were washed
with the solvent of dimethyl carbonate (DMC) for a few times in an
argon filled water-free and oxygen-free glovebox before ex situ tests.
Electrochemical Testing. The active material, conducting Super-
P carbon black, and poly(acrylic acid) binder with weight ratios of
7:2:1 were stirred in solvent N-methyl-2-pyrrolidone for several hours
to obtain a homogeneous slurry. Then, a Cu foil was used to load the
obtained slurry before vacuum drying at 110 °C for half a day. We
punched the electrodes into round wafers (diameter: 14.0 mm;
loading: ∼1.2 mg/cm2). The electrolyte is a mixed solvent of dimethyl
carbonate (DMC), ethylene carbonate (EC), and ethyl methyl
carbonate (EMC) (1:1:1, volume ratio) with 1 M LiPF6. Finally, we
assembled coin cells (CR2032) using the electrode, a lithium wafer, a
Celgard 2325 separator, and the as-prepared electrolyte in the water-
free and oxygen-free glovebox filled with argon. A LAND 2001A
battery tester and an electrochemical workstation (CHI660e) were
employed to perform the electrochemical testing.
As isomorphic compound of previous reported Co-BHC,26
orthorhombic Ni-BHC crystallizes in the space group Pbca, and
the whole molecule (containing two asymmetric units) contains
three nickel ions, a mellitate ligand, and 18 molecules of water,
giving a molecular formula of [Ni2(H2O)10][Ni(H2O)2(C12O12)]·
6H2O (CCDC: 1009819).25 Two kinds of Ni(II) node with
octahedral geometries are connected by six oxygen atoms from
mellitate ligands and water molecules to form the secondary
building block, in which the Ni−O distances are in the range of
2.0431(15)−2.0987(15) Å. The coordination environments of
Ni1 and Ni2 are very different. As shown in Figures 1a and 2a,
Ni1 is surrounded by four carboxylate oxygen atoms and two
water molecules, serving as the basic metal node to form 1D
chain, while two Ni2 atoms ligated by one carboxylate oxygen
atom and five water molecules are located on both sides of the
1D chain. In this molecule, 12 water molecules (O1W ∼ O6W)
are connected with the nickel ions to form octahedral NiO6,
and the residual 6 water molecules (O7W ∼ O9W) are uncon-
nected. Due to the abundant water molecules in the MOF,
hydrogen-bonding interactions are found throughout the struc-
ture, as shown in Figure 1b. Oxygens from carboxylate moiety
act as hydrogen atom acceptors, and coordinated water mole-
cules from octahedral NiO6 act as hydrogen atom donors,
forming intrachain (1D) or intralayer (2D) hydrogen bonding
to build a 2D layer. Water molecules coordinated with Ni1 and
Ni2 can also form intralayer hydrogen bonding with each other
(O1W−O4W and O2W−O6W). Thus, as shown in Figure 2b,
the 1D chain, parallel to the b-axis, is piled up to one another
via such hydrogen bonding to form a 2D layer. Eventually, as
shown in Figure 2c, the as-formed 2D layers with a large lamellar
spacing of 8.5285 Å are further connected along the a-axis via
the hydrogen-bonding interactions formed through unconnected
water molecules (O7W ∼ O9W).
RESULTS AND DISCUSSION
■
Structures of the MOFs. In a previous report,25 a room-
temperature synthetic method was employed, which requires a
long reaction time of several weeks, thus limiting its practical
application. In this work, a facile solvothermal method was pro-
posed with reduced reaction time of just 1 day. The solvo-
thermal reaction of Ni(NO3)2·6H2O and mellitic acid in deionized
water would generate green columnar crystals of Ni-BHC. During
the synthesis process, NaOH dilute solution was used as a base
Physical Property. In previous work, we demonstrated the
disadvantages of coordinated solvent in MOFs for Li storage.23,24
The major reasons are (1) solvent, especially water, might irre-
versibly react with the active lithium to form Li2O or serve as
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Figure 2. Hierarchical structure of Ni-BHC. (a) 1D chain parallel to the b-axis (blue: Ni1 octahedral geometries, yellow: Ni2 octahedral geometries).
(b) 2D layer parallel to the b-axis. (c) The whole 3D chain-layer structure with the unit cell edges. Hydrogen atoms are omitted for clarity.
channel fillers, which will remarkably impede the Li-ion diffusion.
(2) Solvent, especially water, might also react with LiPF6 in
electrolyte to form detrimental HF, which will destroy the elec-
trode. (3) A high content of coordination solvent molecules
can lower the content of reversible lithiation sites, thus lowering
the specific capacity. For such reasons, MOFs especially with
high content of solvent should be evacuated before being applied
to Li storage.
Thermal treatment in proper temperature according to the
TGA curve should be the most feasible method to remove sol-
vent from materials. As displayed in Figure 3a, the first weight
loss of ∼40% between 100 and 300 °C in TGA curve of Ni-BHC
can be attributed to the liberating of coordinated water (calcd
38.73%), and the following weight loss of ∼40% at 350 °C can
be ascribed to the pyrolysis of the organic linkers. There is no
further weight loss with further rising of the temperature,
leading to 24.65% NiO proved in Figure S2. To remove the
coordinated water molecules while retaining the MOF skeleton,
a proper temperature of 220 °C was selected to dry the Ni-BHC
in air for 3 h, during which the green hydrous Ni-BHC con-
verted to yellow anhydrous evacuated Ni-BHC, as displayed in
Scheme S1. The TGA curve of the evacuated Ni-BHC was also
plotted in Figure 3b, and there is only weight loss at 300−400 °C,
demonstrating that the water molecules in the framework were
almost thoroughly removed after the thermal treatment. For
the evacuated Ni-BHC, 41.13% NiO is retained after complete
calcination, corresponding to a formula of Ni3C12O12.
bands of mellitic acid at 1250, 1717, and 3010 cm−1 ascribed to
ν(C−O), ν(CO), and ν(OH) of its nonionized −COOH
disappear, while the new bands of Ni-BHC appeared at 1444 and
1575 cm−1 can be assigned to the symmetric stretching vibra-
tions and asymmetric stretching vibrations of −COO−, indicating
the deprotonation of − COOH after coordinating with Ni(II).
For Ni-BHC, there is a strong and broad peak centered at
3300 cm−1, demonstrating the hydrous feature. After the thermal
treatment, the broad peak is almost disappeared, indicating the
successful dehydration of the evacuated Ni-BHC.
Figure 3c shows the XRD patterns of the well-ground
Ni-BHC, consistent with the simulated results. The (200) peak
at 10.364° corresponds well with the interlamellar spacing of
8.5285 Å (a/2). Generally, removing a large amount of coor-
dinated solvent molecules will degrade the crystallinity of MOF,
thus giving an amorphous XRD pattern.21,23,27,28 However, the
Ni-BHC structure can be re-established upon soaking in water
for one day, as illustrated in Scheme S1. Hence, we believe that
the coordination network of evacuated Ni-BHC, mainly metal
oxygen coordination bond, is retained at molecular level after
the thermal treatment process.24 Furthermore, the morphology
of the evacuated Ni-BHC is quite similar to that of the fresh
Ni-BHC, as displayed in Figure S3, supporting the reversibility
of dehydration.
DFT Calculation. To evaluate the redox properties and elec-
tronic conductivities of the Ni-BHC MOFs, we calculated the
energy levels of the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) using the
DFT method with B3LYP/6-311+G(d,p). As shown in Figure 4,
Figure 3b shows the Fourier transform infrared spectra of mel-
litic acid, the fresh and evacuated Ni-BHC. The characteristic
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Figure 3. (a) TG curves, (b) FTIR spectra, and (c) XRD patterns of Ni-BHC and evacuated Ni-BHC, respectively. The FTIR spectrum of mellitic
acid is plotted in (b) for comparison. The XRD patterns of simulated Ni-BHC and re-established Ni-BHC are also displayed for reference.
Li for its highest LUMO value. Furthermore, the gaps (ΔE)
between the HOMO and LUMO levels are closely related with
electronic conductivities, and a lower gap means good electro-
conductivity.29 Thus, the high gap (6.851 eV) demonstrates an
intrinsic insulation feature of mellitic acid. It is well-documented
that the conjugated carboxylate groups, as weakly electron with-
drawing ligands, can be used directly for Li storage.30,31 However,
the low redox activity and electroconductivity restrict its per-
formance. Due to the coordination of Ni(II) and mellitic acid
with consequent 2p-3d orbital overlap and hybridization, the
gaps are effectively reduced, suggesting the higher electronic
conductivities of Ni-BHC and evacuated Ni-BHC. It should
also be noticed that water molecules in the 3D structure can
reduce the activation energy for lithium-ion insertion; hence,
the Ni-BHC with many coordinated water molecules shows a
lower energy gap (0.541 eV) than that of evacuated Ni-BHC
(0.850 eV).32,33 However, the water content is quite high for
Ni-BHC (∼40%), which would lead to limited reversible capacity
and large amount of side reactions.
Li-Storage Performance. A CR2032 coin-type cell config-
uration was used to evaluate the electrochemical performance
Figure 4. Energy diagrams of mellitic acid, Ni-BHC, and evacuated
Ni-BHC implemented in Gaussian 09w suite of program. The left axis
of the evacuated Ni-BHC. The cycle performance was measured
represents the relative voltage (versus Li/Li+, ELi/Li+), and the right axis
at 100 mA g−1, as displayed in Figure 5a. The evacuated Ni-BHC
represents the relative energy level in vacuum (Evac). The two voltages
displays an initial discharge and charge capacity of 2114.5 and
are approximately transformed via NHE (ENHE) by the following
1568.4 mA h g−1, maintaining a Coulombic efficiency of 74.17%.
formula: ELi/Li+ − 3.04 = ENHE = −Evac − 4.5.
While the Ni-BHC and mellitic acid possessed only initial dis-
charge capacities of 1398.7 and 135.8 mA h g−1, far below that
the LUMOs of Ni-BHC (−3.306 eV) and evacuated Ni-BHC
of evacuated Ni-BHC. The evacuated Ni-BHC displayed a
capacity of 1664.4 mA h g−1 after 5 cycles and a capacity of
1261.3 mA h g−1 after 50 cycles. By contrast, Ni-BHC and mel-
litic acid maintained a low reversible capacity of only 910.4 and
(−3.789 eV) are much lower than that of mellitic acid (1.802 eV).
Lower LUMO usually stands for greater electron affinities and
higher redox activity with Li. So, the evacuated Ni-BHC has the
highest redox activity, while the mellitic acid cannot react with
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Figure 5. (a) Cyclic performance of evacuated Ni-BHC at a current density of 100 mA g−1. The discharge curves of mellitic acid and Ni-BHC are
also provided for comparison. (b) Rate performance of the evacuated Ni-BHC at different current densities ranging from 100 to 5000 mA g−1.
(c) CV curves at different scan rates of the evacuated Ni-BHC. (d) log i vs log v plots at different oxidation and reduction states of evacuated Ni-BHC.
Figure 6. Comparison of the performance of metal−organic frameworks as LIB anodes (detailed in the Supporting Information).
82.6 mA h g−1 after 50 cycles. It should be noted that supposing
that this system could absorb 30 Li ions, the evacuated Ni-BHC
could reach a high theoretical capacity of 1570 mA h g−1, while
the Ni-BHC could maintain a low theoretical capacity of only
961 mA h g−1. The initial two discharge/charge profiles plotted
in Figure S4 also declare their different performances. The
overall performances of the three materials are quite in line with
our forecast, showing the alluring promise of designing func-
tional MOFs with luxuriant carboxylate−metal units as high
performance LIB anode. Furthermore, the reversible capacity of
1261.3 mA h g−1 outperforms all the other MOF anodes reported
to date as far as we know (Figure 6).
The discharge capacities corresponding to these current densities
are 1563.1, 1437.2, 1215.1, 983.6, 740.8, and 422.0 mA h g−1,
respectively. After the rate test, the capacity could recover to
1293.0 at 100 mA g−1, demonstrating excellent rate perform-
ance. Figure S5 shows the corresponding rate profiles of the
evacuated Ni-BHC. It should be noticed that there is no plateau
in the profiles, indicating a capacitive-controlled electrochemical
behavior. To measure this behavior, CV measurements were
performed at various sweep rates (from 0.2 to 1.2 mV s−1), as
displayed in Figure 5c. The capacitive effect of the electrode can
be described by the following equation:
i = avb, log i = blog v + log a
To further investigate the high power performance of the
evacuated Ni-BHC, rate capability was evaluated from 100 to
200, 500, 1000, 2000, and 5000 mA g−1, as plotted in Figure 5b.
(where i is the measured peak current, v is the scan rate, and
a and b are adjustable parameters). It is well-known that for a
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diffusion-controlled Faradaic process, b approaches 0.5, while
for a surface-controlled capacitive process, b is close to 1.0.
In this case, as displayed in Figure 5d, the b-values for the 4 oxi-
dation and reduction CV peaks are 1.046, 0.906, 0.817, and
0.680, respectively. The high b-values, especially for cathodic
peaks, demonstrate the ultrahigh capacitive contribution (in some
cases, we use the word pseudocapacitance) in evacuated Ni-BHC.
Such a capacitive behavior can be attributed to the layered fast-
kinetics structure which will notably increase the diffusion rate
of Li ions inside the bulk MOF.
Lithiation−Delithiation Mechanism Investigation. To
demonstrate the participation of carboxylate groups during the
electrochemical process, synchrotron-based soft X-ray total elec-
tron yield (TEY) spectra at O K-edge were also done for the
evacuated Ni-BHC electrode discharged and recharged to the
desired states-of-charge (SOC). As Figure 7 shows, the pre-edge
suggest that the luxuriant carboxylate−metal units play an
important part in the electrochemical process. More work to
improve the cycling stability is still in progress. Our work develops
a new method to improve the energy density of MOFs by
increasing the content of carboxylate groups within a proper
topology choice.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.7b02939.
■
S
Supplementary optical micrograph, the reported MOF
and CP materials for the LIB anode, XRD patterns,
evacuated and re-established processes, SEM micro-
graphs, charge−discharge profiles, and CV curves (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: bwhu@phy.ecnu.edu.cn.
■
ORCID
Xiaobing Lou: 0000-0002-6933-9649
Chao Li: 0000-0002-0153-7825
Ming Shen: 0000-0003-1343-2761
Bingwen Hu: 0000-0003-0694-0178
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by National Natural Science Foun-
dation of China for Excellent Young Scholars (Grant 21522303)
and National Natural Science Foundation of China (Grant
21373086 and 21703068), National High Technology Research
and Development Program of China (Grant 2014AA123401),
and National Key Basic Research Program of China (Grant
2013CB921800). We also acknowledge the sXAS experiments
support from Shanghai Synchrotron Radiation Facility
(BL08UA).
Figure 7. O K-edge sXAS TEY spectra of the evacuated Ni-BHC
discharged or recharged to different states-of-charge at the first cycle.
region between 530 and 537 eV corresponds to the electron
excitation from O 1s to the Ni 3d-O 2p hybrid orbitals, while
the two broad peaks at 539.0 and 539.8 eV represent the elec-
tron excitation from O 1s to Ni 4sp-O 2p hybrid orbitals. The
local environment evolution including Li-ion insertion and
valence transition of coordinated metals will significantly change
the strength and length of the Ni−O coordination bonds, thus
influencing the needed energy for electron excitation in sXAS
analysis. The Ni 3d-O 2p peak of the pristine electrode appears
at 533.8 eV, while the peak moves down to 533.5 eV after dis-
charging to 0.9 V, demonstrating the charge compensation par-
ticipation of carboxylate oxygen. After discharging to 0.01 V,
the Ni 3d-O 2p peak further moves to 533.2 eV, corresponding
to more Li-ions adsorption by the carboxylate−metal units and
further possible reduction of Ni(II) to Ni(0).15−17 The Ni
3d-O 2p peak returns to 533.5 and 533.8 eV after charging to
1.7 and 3.0 V, respectively, demonstrating reversibility of the
local coordination environments upon discharge/charge process.
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CONCLUSIONS
■
In summary, a hexacarboxylate-based layered MOF [Ni2(H2O)10]-
[Ni(H2O)2(C12O12)]·6H2O (Ni-BHC) was synthesized. After
dehydration by thermal treatment, this MOF with abundant
carboxylate groups was applied as an anode for Li storage, exhib-
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of 100 mA g−1. To our knowledge, such reversible capacity is
the highest one ever reported for MOF-based materials. DFT
calculation and O K-edge soft X-ray absorption spectroscopy
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DOI: 10.1021/acs.inorgchem.7b02939
Inorg. Chem. XXXX, XXX, XXX−XXX