Exploration of a calcium-organic framework as an anode material for sodium-ion batteries

Article abstract of DOI:10.1039/c6cc05365c

In this communication, we designed and synthesized a novel calcium-organic framework and presented it as an anode material for sodium-ion batteries. The results show that it delivers a reversible capacity of higher than 140 mA h g^-1 even after 300 cycles. The remarkable performance is attributed to the high structural stability and extremely low solubility of the calcium-organic framework in electrolytes.

Full text of DOI:10.1039/c6cc05365c

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Exploration of a calcium–organic framework as
an anode material for sodium-ion batteries†
Cite this:DOI: 10.1039/c6cc05365c
Yan Zhang,ab Yubin Niu,ab Min-Qiang Wang,ab Jingang Yang,ac Shiyu Lu,ab
Jin Han,ab Shu-Juan Baoab and Maowen Xu*ab
Received 28th June 2016,
Accepted 13th July 2016
DOI: 10.1039/c6cc05365c
www.rsc.org/chemcomm
In this communication, we designed and synthesized a novel calcium–
Herein, a novel calcium–organic framework (Ca2BTEC)
organic framework and presented it as an anode material for sodium- composed of Ca2+ and 1,2,4,5-benzenetetracarboxylic acid
ion batteries. The results show that it delivers a reversible capacity (H4BTEC) was designed and explored as an anode material
of higher than 140 mA h gÀ1 even after 300 cycles. The remarkable for SIBs for the first time. Ca2BTEC exhibited high capacity and
performance is attributed to the high structural stability and extremely excellent capacity retention. Ca2BTEC was synthesized by a
low solubility of the calcium–organic framework in electrolytes.
facile hydrolysis and cationic exchange method. First, 1 mmol
pyromellitic dianhydride (PMDA, Aladdin, 96%) and 4 mmol
Sodium-ion batteries (SIBs), the most attractive alternative to NaOH (Aladdin) were dissolved in 30 mL of de-ionized water (in
lithium-ion batteries (LIBs), have gained extensive research a beaker) under vigorous stirring for 10 h. Then, 2 mmol CaCl2
attention owing to their low cost and abundant resource.1,2 (dissolved in 10 mL of de-ionized water beforehand) was added
Similar to LIBs, most of the host electrode materials in SIBs into the above clear solutions drop by drop. After about 15 min,
are inorganic compounds, such as transition metal oxides,3,4 the white precipitate appeared. Afterward, the beaker was covered
polyanion compounds,5,6 carbon materials,7,8 alloy materials,9,10 with a glass dish and heated to 80 1C keeping for 12 h to complete
etc. However, these materials are generally prepared through the exchange of Na+ by Ca2+. The precipitate was filtered and
energy intensive processing from limited mineral resources, washed with water several times. Finally, Ca2BTEC was obtained
which enhances their cost greatly. Naturally, organic materials by drying at 180 1C for 5 h. The synthesis procedure is shown in
have attracted our attention due to their structural diversity and Scheme 1.
resource renewability.11 Indeed, many organic–metal compounds
The phase composition and crystallinity of the as-prepared
have been studied as novel electrode materials for LIBs and SIBs. sample were characterised by powder X-ray diffraction (PXRD,
However, these organic lithium/sodium salt electrodes suffer from MAXima-XXRD-7000). Fourier transform-infrared spectra analysis
serious capacity fading because of their solubility in organic
electrolytes.12,13 Fortunately, one approach using calcium instead
of lithium in Li2C8H4O4 to overcome this issue was reported, due
to its higher ionization energy with organic anions than that of
Li+.14,15 Meanwhile, Ca is an abundant element widely distributed
in oceans and the crust of the earth.16 To our knowledge, no
reports have been published on the investigation of calcium
displacement of sodium in organic sodium salts and its electro-
chemical behaviour in SIBs.
a Institute for Clean Energy & Advanced Materials, Faculty of Materials and Energy,
Southwest University, Chongqing 400715, P. R. China.
E-mail: xumaowen@swu.edu.cn; Fax: +86-23-68254969; Tel: +86-23-68254969
b Chongqing Key Laboratory for Advanced Materials and Technologies of Clean
Energies, Chongqing 400715, P. R. China
c Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),
Scheme 1 The synthesis procedure of Ca2BTEC by a hydrolysis and
Nankai University, Tianjin 300071, China
cationic exchange method.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cc05365c
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(FT-IR, Thermo Nicolet, 6700) was performed to further obtain and 1490 cmÀ1 are assigned to symmetric and asymmetric
information about functional groups. The thermal stability of the carboxylate stretching vibrations, respectively. The relatively
sample was evaluated at a heating rate of 5 1C minÀ1 from room weak peaks between 600 and 400 cmÀ1 are ascribed to the O–Ca
temperature to 800 1C in air using a TG thermo-gravimetric bonds.20 The above functional information can also prove the
analyzer (TGA, TA Instruments Q50). For electrochemical testing, formation of a calcium–organic framework. Fig. 1(d) shows the
the anode materials were evaluated in 2032-type coin cells using a TGA curve of the synthesized sample. Below 500 1C, the quality
Na disk as the counter electrode and 1 M NaClO4 in ethylene of the sample remains almost unchanged, indicating the
carbonate/diethyl carbonate (EC/DEC, 50 : 50 vol%) solution as complete evaporation of coordinated water during the dry
the electrolyte. The anode electrodes were prepared by mixing procedure and the high thermal stability of Ca2BTEC. When
a composite of active materials (60 wt%), Super P carbon black heated from 500 1C to 800 1C, the sample shows a sharp weight
(30 wt%) and carboxymethyl cellulose (CMC, 10 wt%) and loss of about 65.75% due to the pyrolysis from Ca2BTEC to CaO
dispersing in de-ionized water to form an electrode slurry. residues. The value of weight loss is almost consistent with the
Then the slurry was cast onto a copper foil collector and dried theoretical value according to the reaction formula. Considering
overnight in a vacuum at 120 1C to remove the dispersant. The that water was used as solvent during electrode fabrication,21 we
typical loading of active materials per electrode was approxi- further investigated the potential formation of Ca2BTECÁ6H2O.
mately 1.5–2 mg. Assembly of the cells was carried out in a dry The TGA results in Fig. S2 (ESI†) show that the hydrated counter-
Ar-filled glove box (DELLIX, LS800S). Galvanostatic charge– part cannot form during the electrode fabrication procedure,
discharge tests were performed on a LAND cycler (Wuhan which ensures that Ca2BTEC, not Ca2BTECÁ6H2O, as an active
Kingnuo Electronic Co., China). Cyclic voltammetry (CV) was material participates in the electrochemical reaction.
performed at a scan rate of 0.1 mV sÀ1 on an Arbin battery
As a consequence of the unique inorganic–organic hybrid
testing system. Electrochemical impedance spectroscopy (EIS) layered structure, Ca2BTEC exhibits excellent electrochemical
experiments were conducted in the frequency range of 0.1–100 kHz performance as an anode material for SIBs. Fig. 2(a) shows the
using a CHI600D electrochemical workstation.
Fig. 1(a) displays the PXRD pattern of the as-prepared tested in the potential range 0.01–2.2 V and at a current density
Ca2BTEC. All main X-ray diffraction peaks are sharp and strong, of 20 mA gÀ1. The first discharge capacity is 306 mA h gÀ1
discharge/charge voltage profiles of the as-prepared Ca2BTEC
,
which indicates as-prepared samples with high phase crystal- while the charge capacity is only 140 mA h gÀ1. The initial
linity. The PXRD pattern of Ca2BTEC is different from that of capacity loss could be attributed to the formation of an SEI film
Ca2BTECÁ6H2O (CCDC 1279417, Fig. S1, ESI†), which can be on the electrode surface. Except the first cycle, a stable discharge/
attributed to the fact that removing coordinated water causes charge capacity of 120 mA h gÀ1 can be achieved. Similar voltage
some distortion in lattices.17,18 The crystal structure of Ca2BTEC is profiles can be found in the sodium/lithium storage behaviour of
schematically shown in Fig. 1(b). The connection of Ca2+ with TiO2 and another MOF, which indicate that the reaction between
[C6H2(COO)4]4À forms an obvious inorganic–organic hybrid Ca2BTEC and sodium species is a surface-confined charge-
layered structure, which is advantageous for the extraction/ transfer process (pseudo-capacitive process).22–24 A couple of
insertion of Na+.19
broad redox peaks in CV curves (Fig. S3, ESI†) also prove it. In
In order to obtain information about functional groups, addition, most of the discharge capacity is delivered below 1 V,
FT-IR analysis was used. Fig. 1(c) displays the FT-IR spectra of
the as-prepared Ca2BTEC. The strong peaks located at 1650 cmÀ1
Fig. 2 (a) Voltage profile at 20 mA gÀ1 between 0.01 and –2.2 V versus
Na+/Na; (b) long-term cycle performance at 20 mA gÀ1; (c) rate perfor-
Fig. 1 (a) PXRD pattern, (b) crystal structure along the a-axis, (c) FT-IR
spectra and (d) TGA curve of the as-prepared Ca2BTEC.
mance at different current densities; (d) Nyquist plots before the cycle test
and after 20, 60, and 100 cycles.
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will further extend the family of anode materials for SIBs to
more metal–organic frameworks.
The work was supported by grants from Fundamental
Research Funds for the Central Universities (SWU 113079,
SWU 114099, XDJK2014C051, XDJK2015C062).
Fig. 3 Proposed electrochemical redox mechanism of Ca2BTEC.
Notes and references
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Ca2BTEC as the anode material.
The long-term cycle performance of Ca2BTEC is demonstrated
in Fig. 2(b). A superior high discharge capacity of 140 mA h gÀ1
(86% of theoretical capacity) can be achieved after 300 cycles and
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leaving. A similar mechanism could also be found in previous
reports.27 Compared with the poor electrochemical performance
(Fig. S7, ESI†) of its hydrated counterpart (Ca2BTECÁ6H2O), it can
be proposed that Ca2BTEC suffers a weaker steric effect during
de/sodiation due to the removal of coordinated water from the
inorganic–organic layered structure in the synthesis procedure,
which is more conducive to the migration of Na+.
A novel Ca-based metal–organic framework was synthesized
by a simple hydrolysis and cationic exchange reaction. Tested
as an anode material for SIBs, it delivers a reversible capacity
higher than 140 mA h gÀ1 after 300 cycles and shows superior
capacity retention and excellent rate performance. This work
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Chem. Commun.