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Carbonyl Compounds as Electrochemically Active
Materials for Li-ion and Na-ion Batteries
Mi-Sook Kwon
Department of Energy Engineering
(Battery Science and Technology)
Graduate School of UNIST
2015
Abstract
Li ion batteries (LIB) are currently the dominant energy storage system in portable electronic
devices (EV), electric vehicles and energy storage systems (ESS). With market saturation of battery
for small device, global Li-ion battery market has a great interest in the large energy storage system.
The conventional cathode materials for Li-ion batteries are inorganic materials based on cobalt source.
However, the cobalt source is expensive, limited mineral resources and toxic. Furthermore, the price
of lithium carbonate as a lithium source for the cathode material is fluctuated due to the
geographically-restricted carbonate source. Because of these reasons, current inorganic materials are
inadequate for the post Li ion battery market. So, new alternatives are required and organic electrode
materials are suggested as a promising electrode material. The organic electrode material have
advantages which are cheap, sustainability and structural flexibility. But, they suffer from problems of
dissolution in electrolyte and low conductivity.
In this study, the topics will deal with the solution about the problem of both dissolution and low
conductivity and the utilization about merit of structural flexibility.
To solve drawbacks about dissolution and low conductivity, the carbon composite method was
facilitated by using a mesoporous carbnon having a conductivity and a confinement effect in their
pore. the π-π interaction-dependent vapour pressure of phenanthrenequinone can be used to synthesize
a phenanthrenequinone-confined ordered mesoporous carbon. Intimate contact between the insulating
phenanthrenequinone and the conductive carbon framework improves the electrical conductivity. This
enables a more complete redox reaction take place. The confinement of the phenanthrenequinone in
the mesoporous carbon mitigates the diffusion of the dissolved phenanthrenequinone out of the
mesoporous carbon, and improves cycling performance.
To utilize the structural flexibility of the organic material, the possibility which enables to change
the reduction potential was demonstrated. By correlating the practical reduction potential and the
theoretical LUMO energy, possibility to change the reduction potential was suggested. Experimented
nine organic molecules matched with trend between LUMO level and reduction potential. Results
were categorized into three part. They are affected by the factors of the polar effect and the expansion
of pi conjugation. LUMO level could be decreased through the stronger electron withdrawing group
or the longer pi conjugated and it affect to reduction potential increasing.
Contents
1. Introduction------------------------------------------------------------------------------------------------------1
1.1. Li-Ion Batteries----------------------------------------------------------------------------------------------5
1.2. Organic Electrode Materials-------------------------------------------------------------------------------7
1.3. Strategy to Mitigate Small Molecule Dissolution-----------------------------------------------------11
2. Experiment------------------------------------------------------------------------------------------------------17
2.1. Synthesis----------------------------------------------------------------------------------------------------17
2.2. Electrode Preparation--------------------------------------------------------------------------------------17
2.3. Cell Preparation--------------------------------------------------------------------------------------------19
2.4. Electrochemical Characterization-----------------------------------------------------------------------19
2.5. Instrumental Analysis-------------------------------------------------------------------------------------19
2.6. DFT Calculation-------------------------------------------------------------------------------------------20
3. Results and Discussion---------------------------------------------------------------------------------------22
3.1. Ordered Mesoporous Phenanthrenequinone-Carbon composite-------------------------------------22
3.1.1. Synthesis and Characterization----------------------------------------------------------------------22
3.1.2. Electrochemical Performance-----------------------------------------------------------------------30
3.2. Change in Reduction Potential via Chemical Change----------------------------------------------35
3.2.1. Relationship between LUMO and Reduction Potential------------------------------------------35
3.2.2. Terminal Functional Group Effect------------------------------------------------------------------38
3.2.3. Bridge Functional Group Effect---------------------------------------------------------------------41
3.2.4. Extended pi Conjugation Effect---------------------------------------------------------------------43
4. Conclusion------------------------------------------------------------------------------------------------------46
Reference ----------------------------------------------------------------------------------------------------------47
List of figures
Figure 1. Global Li-ion battery market status and forecast (2011y ~ 2018F): Sale amount
Figure 2. (a) Global cobalt supply and demand (2008 ~ 2013), (b) Li2CO3 cost fluctuation (1990 ~
2010)
Figure 3. Schematic diagram of Li-ion battery.
Figure 4. Schematic diagram of a simplified cycle life of electrochemically active organic materials.
Figure 5. Schematic diagram of the overall electrochemical performance compared between
electrochemically active organic electrode materials
Figure 6. Schematic diagram of the redox potential and specific capacity of conventional Li-ion
battery electrode materials
Figure 7. (a) Skeletal structure of AQ and PAQS, (b) Cycle performance of AQ
(up) and PAQS (down)
Figure 8. (a) Schematic diagram of PTMA brush/SiO2, (b) Cyclic voltammogram (up) of PTMA film
coated on the Au electrode, Cyclic votammogram (down) of PTMA bruch grafted on SiO2.
Figure 9. (a) Skeletal structure of Li2TP, (b) Voltage profile and cycle performance of Li2TP.
Figure 10. (a) Skeletal structure of PTCDA, (b) SEM image of PTCDA/CNT composite, (c) Cycle
performance of PTCDA and PTCDA/CNT composite with various ratio
Figure 11. (a) Schematic diagram of S/CMK-3 composite (black : CMK-3, yellow : Sulfur), (b) Cycle
performance of S/CMK-3composite (red) and composite coated with polymer (black), (c) Elemental
analysis of bare S (red), CMK-3/S composite (blue), S/CMK-3 coated with polymer (black).
Figure 12. A mimetic diagram for the process of a phenanthrenequinone (PQ)-confined ordered
mesoporous carbon (CMK-3) (PQ/CMK-3) synthesis.
Figure 13. SEM images of (a) bare phenanthrenequinone (PQ), (b) CMK-3, (c) ground mixture of PQ
and CMK-3, and (d) composite of PQ/CMK-3.
Figure 14. HR-TEM images at the (110) plane of (a) CMK-3 and (b) PQ/CMK-3 composite, (c)
Barrette-Joyner-Halenda (BJH) adsorption pore size distributions and (d) nitrogen adsorption-
desorption isotherms of CMK-3 (black circles) and PQ/CMK-3 (red squares).
Figure 15. TEM images of (a) CMK-2 and (c) PQ/CMK-3 composite, corresponding to EDS line
profiles of (b) CMK-3 and (d) composite : i) oxygen, ii) carbon, iii) oxygen/carbon*100.
Figure 16. (a) FT-IR spectral profiles, (b) their magnified spectra from 780 cm-1 to 700 cm-1, (c)
thermogravimetric analysis (TGA) profiles, and (d) isothermal analysis : i) bare PQ, ii) CMK-3, and
iii) PQ/CMK-3.
Figure 17. 1H NMR (CDCl3, 400MHz, 300K) spectra of (a) bare PQ and (b) the portion collected
from PQ/CMK-3 via Soxhlet extraction using chloroform
Figure 18. Galvanostatic charge-discharge curves for Li-ion batteries of (a) bare phenanthrenequinone
and (b) PQ/CMK-3 for Li-ion batteries.
Figure 19. Two-stepwise electrochemical reduction reaction mechanism of phenanthrenequinone.
Figrue 20. (a) Cycle retention of PQ/CMK-3, (b) UV-VIS quantitative analysis between bare PQ (blue
circles) and PQ/CMK-3 (red squares) by the divergent absorbance according to the different
dissolution extent at the same amounts of PQ in TEGDME (c) UV-VIS spectrum of the bare
phenanthrenequinone.
Figure 21. (a) Rate capability, (b) their charge-discharge curves of PQ/CMK-3.
Figure 22. The reduction potentials and calculated LUMO energies of nine organic molecules plotted
from table 1.
Figure 23. (a) Voltage profiles in 5th cycle of PTCDA and 2nd cycle of PTCDI and BPE-PTCDI, (b)
the discharge potential and calculated LUMO energies of PTCDA, PTCDI and BPE-PTCDI.
Figure 24. The reactivity effect of common substituted benzenes.
Figure 25 a) Voltage profiles in 2nd cycle of Na-ADB, Na-CDB and Na-BPDC, (b) the discharge
potential and calculated LUMO energies of Na-ADB, Na-CDB and Na-BPDC.
Figure 26. Schematic diagram of molecular orbitals in benzene, naphthalene and anthracene.
Figure 27. a) Voltage profiles in 2nd cycle of Na-PTC, Na-BPDC, Na-NTC and Na-TP, (b) the
discharge potential and calculated LUMO energies of Na-PTC, Na-BPDC, Na-NTC and Na-TP.
List of Tables
Table 1. Name and skeletal formula of nine organic molecules.
Table 2. Conditions of electrode preparation and electrochemical experiment in nine molecules.
Table 3. The calculated LUMO and HOMO energy, and Average discharge potential of nine organic
molecules.
1
1. Introduction
The use of portable devices with modern technology growth has been increasing, at the same time,
and Li-ion battery market also has grown by leaps and bounds. Currently Li-ion battery is an energy
storing device for use in personal handheld electronics, electric vehicles (EV) and energy storage
systems (ESS)1. Global Li-ion battery market has been extremely slower growth almost reached with a
saturation. On the other hand, the need for high-capacity power storage device such as EV and ESS has
been highlighted with a growing market (Figure 1). Recently commercially available cathode materials
for Li-ion batteries are inorganic materials containing cobalt element, for instance, LiCoO22,
LiNi0.8Co0.15Al0.04O23 and LiNi1-x-yCoxMnyO2
4. Price of the cobalt is high and has the potential to be
further improved because the demand is still increasing (Figure 2a). Furthermore, the usage as a mass
storage device is inadequate in terms of the price competitiveness since price fluctuations of lithium
carbonate as a lithium source for the cathode material is severe (Figure 2b). This is also environmentally
improper owing to the toxicity of cobalt and needs alternative to meet their requirements.
Small organic materials are emerging as an alternative because they have advantages such as a low
temperature-synthesis process, needlessness for transition metal capable of polluting the environment
and raising price, recyclability and structural flexibility compared with inorganic materials5.
However, small organic molecules are troubled with several shortcomings such as their dissolution
in nonaqueous electrolytes and low electronic conductivity, giving rise to poor cycle performance and
low reversible capacity.
Recently, some prospective strategies have been showed to alleviate the dissolution of organic
compounds, including polymerization6, grafting7, salt formation8, and composites with carbon9,
resulting in the improved electrochemical performance.
In this study, the ordered mesoporous phenanthrenequinone-carbon composite inspired by the use of
CMK-3 in Li-S batteries was simply synthesized and suggested to improve their drawbacks10. Because
of the π-π interaction between aromatic phenanthrenequinone (PQ) and the ordered mesoporous carbon
(CMK-3), the dissolution was mitigated by small-organic-molucules-confined ordered mesoporous
structural carbon in nonaqueous electrolytes and the electrical conductivity was enhanced owing to the
intimate contact between carbon and PQ. Therefore, the composite showed the highly improved
electrochemical performance.
By using the structural flexibility of the organic material, the possibility which enables to change the
reduction potential was demonstrated through the correlation between the practical reduction potential
and the theoretical LUMO level11. Most organic molecules corresponded with tendency of LUMO level
and reduction potential. Results were categorized into three part and affected by the factors of the polar
2
effect and the expansion of pi conjugation. LUMO level originated from the stronger electron
withdrawing group or the longer pi conjugated is lower and causes rise in the reduction potential.
3
Figure 1. Global Li-ion battery market status and forecast (2011y ~ 2018F): Sale amount
4
Figure 2. (a) Global cobalt supply and demand (2008 ~2013), (b) Li2CO3
cost fluctuation (1990 ~ 2010).
5
1.1. Li-Ion Batteries
Li-ion battery systems are composed with cathode and anode material which are capable of
electrochemical intercalation/deintercalation of lithium, ion conductive electrolyte with dissolved Li-
salts and electronically insulative separator. Typically, the cathode materials are selected as a contributor
to lithium and at the same time as a lithium container. During charge process, Lithium ion came out
from cathode materials goes through electrolyte and then react with anode materials. In contrast, during
discharge process, lithium ion got out from anode materials and reinserted into the cathode materials.
Such a cathode material which is possible to conduct lithium intercalation/deintercalation is mainly
used LiCoO2 as a conventional lithium transition metal oxide material. A negative electrode material
has been used most commonly graphite representing carbon-based materials.
When the battery is charged, lithium ion comes out from cathode material and goes into the interlayer
of the graphite, then voltage increases. On the contrary, in discharging the battery, lithium ion is released
from the anode material and it react with the cathode material spontaneously. A schematic diagram for
the discharge process is shown in Figure 3.
6
Figure 3. Schematic diagram of Li-ion battery.
7
1.2. Organic Electrode Materials
The electrode material of the commercially available lithium ion battery is a lithium oxide-based
inorganic material. The lithium oxide-based material requires a high temperature sintering process,
leads to environmental problems due to the transition metal and has the possibility to increase the cost
owing to limit of the transition metal deposit. In terms of substance, the crystal structure is determined
depending on the reaction temperature and the constituted element and its stoichiometry. Because this
causes a specific electrochemical property, the limited structure of inorganic material is a disadvantage
for searching for a new electrochemical property. In addition, current battery market is interested in
large scale energy storage devices according to market saturation of batteries for portable devices. The
environmental and price consideration needs a greater importance5a, 5b.
The organic electrode materials are being magnified as an alternative. It is able to be synthesized at
the room temperature, has environmental friendly property by needlessness of transition metal, enable
to recycle and facilitate the eletrochemical property change based on the structural flexibility of organic
materials (Figure 4). Because of its advantages, there are many trials to apply organic materials as the
electrode material (Figure 5).
In conducting polymer, Nigrey et al. firstly suggested doped Polyacetylene as a cathode material for
the rechargeable battery12. The electrochemical property of these materials is similar with
pseudocapacitor, which receive anions from the electrolyte. Although this has property of both the good
electronic conductivity and the electrochemical activity, it is not appropriate for electrode material due
to the low specific capacity and energy density and the poor cycling stability caused from over-oxidation
at high voltage.
Nakahara et al. introduced primally poly (2,2,6,6-tetramethyl-1-piperidinyloxy-4yl methacrylate)
(PTMA) as an organic radical compound for lithium ion battery13. These compounds have nitroxyl
radical and store the electrochemical energy through interaction with anion. It is similarly inadequate
to use electrode materials by low cycle performance and specific capacity.
The carbonyl conjugated organic electrode material, dichloroisocyanuric acid (DCA), was firstly
reported in 1969 by Williams et al14. In this paper, DCA as a cathode material for the primary battery
showed the poor cycle performance due to the dissolution and the bad thermal stability. Since then,
many researcher have conducted study, nowadays, conjugated carbonyl compound is considered as a
promising electrode material among other organic materials because of its good electrochemical
property about energy density, cycling stability and power density (Figure 6).
8
Figure 4. Schematic diagram of a simplified cycle life of electrochemically active organic
materials.
9
Figure 5. Schematic diagram of the overall electrochemical performance compared
between electrochemically active organic electrode materials.
10
Figure 6. Schematic diagram of the redox potential and specific capacity of conventional
Li-ion battery electrode materials.
11
1.3. Strategy to Mitigate Small Molecule Dissolution
The organic molecules have some disadvantages, such as the dissolution of active material and the
poor conductivity. Especially, dissolution problem is crucial for the stable cycle performance. To
alleviate dissolution, some strategies have been suggested: polymerization, grafting, salt formation, and
composite with carbon source.
Polymer is hard to dissolve in aprotic and protic solvent compared with monomer. So the
polymerization method is considered as a strategy for the decrease of organic molecules.
Poly(anthraquinonyl sulfide) (PAQS) polymerized from anthraquinone (AQ) monomer was suggested
to reduce the dissolution. PAQS showed the improved cycle retention and the similar specific capacity
compared with AQ (Figure 7)15.
The grafting method is to attach the soluble material on the nonsoluble substrate through the chemical
bonding and makes organic molecule difficult to dissolve in electrolyte by holding. Lin et al. obtained
grafting effect using silica nanoparticles. They grafted nitroxide polymer brushes on the silica surface
and attained the results of good cycle retention over 300 cycles about 96% (Figure 8)7.
Armand et al. reported the lithium terephthalate (Li2TP) which is recyclable from Polyethylene
terephthalate (PET) by using the salt formation method16. This showed good electrochemical property,
such as, low redox potential below 1V, high specific capacity about 300 mAh g-1 and good cycle
retention. These salt form organic molecules show the good cycleability because the conjugated
carboxylate functional group could have the intermolecular interaction with the organic molecules, it
partially alleviated the dissolution (Figure 9).
In case of the strategy of composite with carbon, It gives both electronic conductivity and cycleablility
to insulative and soluble organic molecules. Wu et al. described the enhanced electrochemical property
through the carbon nanotube (CNT) composite with a 3,4,9,10-perylenetetracarboxylic dianhydride
(PTCDA). The composite showed decreased charge transfer resistance and better cycle performance
than bare PTCDA. However, as the cycle proceeded, the cycle of composite also gradually decreased
(Figure 10)9b.
Li-sulfur batteries also has had a analogous dissolution drawback of electrochemically active materials.
Nazar et al. reported the sulfur-confined ordered mesoporous carbon (CMK-3) as a container to capture
the dissolved polysulfides in the mesopores of carbon, giving rise to the enhanced electrochemical
performance (Figrue 11)10.
12
Figure 7. (a) Skeletal structure of AQ and PAQS, (b) Cycle performance
of AQ (up) and PAQS (down).
13
Figure 8. (a) Schematic diagram of PTMA brush/SiO2, (b) Cyclic voltammogram (up) of
PTMA film coated on the Au electrode, Cyclic votammogram (down) of PTMA brush
grafted on SiO2.
14
Figure 9. (a) Skeletal structure of Li2TP, (b) Voltage profile and cycle
performance of Li2TP.
15
Figure 10. (a) Skeletal structure of PTCDA, (b) SEM image of PTCDA/CNT
composite, (c) Cycle performance of PTCDA and PTCDA/CNT composite with
various ratio.
16
Figure 11. (a) Schematic diagram of S/CMK-3 composite (black : CMK-3,
yellow : Sulfur), (b) Cycle performance of S/CMK-3 composite (red) and
composite coated with polymer (black), (c) Elemental analysis of bare S
(red), S/CMK-3 composite (blue), S/CMK-3 coated with polymer (black).
17
2. Experiment
2.1. Synthesis
Mesoporous carbon, CMK-3, was prepared by a reported process17. PQ/CMK-3 composite was
synthesized by using a melt-diffusion-vaporization method. PQ (95 %, Sigma Aldrich) and CMK-3
were measured the weight in the weight ratio of 3:1. And then, PQ and CMK-3 were mixed, pelletized
under pressure and heated at 250 oC for 3h.
PTCDA, PTCDI were purchased from Tokyo Chemical Industry Co. Ltd. (TCI). Na-TP, Na-BPDC,
Na-CDB were synthesized followed by the reported method. BPE-PTCDI, Na-ADB, Na-PTC and Na-
NTC were received from co-workers. Table 1 shows the information of nine molecules, not only name
but also skeletal formula.
2.2. Electrode Preparation
The diversified kinds of powder, which were referred to previous page, were handled manufacturing
electrodes. The PQ/CMK-3 composite slurry and bare PQ slurry were prepared that the composite and
carbon black (Super P) were ground by using mortar and then carboxymethyl cellulose (CMC, Sigma
Aldrich) was added to as-prepared mixture. The weight ratio active material, super P and binder was
8:0.5:1.5 and 3:5:2 weight ratios, repectively, to give the same ratio in PQ : carbon material : CMC. The
mixing machine (Thinky ARE-250, Intertronics Co.) was utilized for blending of the slurry components.
The slurry was casted on the aluminium current collector (20 μm) by using a Dr. Blade and dried
promptly in a convection oven at 80 oC for 10 min to eliminate solvent. The dried one was pressed to
secure electronic conductivity between particles in electrode, punched in circular form of 16 mm size
and dried in the vacuum oven at 120 oC overnight to remove the residual moisture.
The electrodes of electrochemically active organic compounds which were performed the DFT
calculation were prepared by mixing with active material, carbon black and binder according to weight
ratio of 57.1:28.6:14.3. The binder and current collector may be changed according to the active material,
but, the procedure was followed the same with method of preparing PQ electrode. The information of
a current collector and binder, depending on the active material, are given in Table 2.
18
Table 1. Name and skeletal formula of nine organic molecules.
19
2.3. Cell Preparation
The half-cell evaluation of both PQ/CMK-3 composite and bare PQ was implemented to investigate
electrochemical performance. All process related to cell assembly was conducted in the dry box filled
with the inert argon gas. In the lithium ion half-cell, their electrochemical peculiarity was assessed using
2016 coin cells, which was constituted with lithium metal as a anode, film separator affiliated with
polyolefin and electrolyte of 1.3 M lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in a
tetra(ethylene glycol)dimethyl ether (TEGDME) solvent (Panax Etec Co. LTD).
In DFT calculated conjugated carbonyl molecules, the electrochemical property was evaluated by
using 2032 coin cells. These were composed of a disk-shape Na metal, separator of same materials used
in Li cell and electrolyte of 0.8 M sodium perchlorate (NaClO4) (≥98 %, Sigma Aldrich) in a mixed
solvent of ethylene carbonate(EC)/diethyl carbonate(DEC) (v/v=1/1, Panax Etec Co. LTD).
2.4. Electrochemical Characterization
The electrochemical properties were evaluated by the galvanostatic discharge/charge measurements
(WonATech WBCS 3000 battery measurement system). The cycle performance of PQ/CMK-3 was
assessed at 0.1 C (current density : ca. 25.7mA g-1). And the rate capability was characterized through
0.05C, 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C each 3cycles.
The electrochemical analyses of other organic materials were performed according to table #. After
constant current experiments, the reduction potential was determined that the 5th discharge cycle of
PTCDA and the 2nd discharge cycle of other molecules were integrated and then divided in specific
capacity at that cycle. Table 2 shows cut-off voltage of molecules.
2.5. Instrumental Analysis
SEM images were collected by using a Hitachi S-4800 field-emission scanning electron microscope
(FE-SEM). High-resolution TEM samples were obtained using a high resolution transmission electron
microscope (HR-TEM, STEM, JEOL ARM-200F) having a probe Cs aberration corrector (CEOS
GmbH) and an energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments X-je 80 TLE) attached
to the TEM. The surface areas and pore sizes of the samples were analyzed in a nitrogen adsorption
experiment at -196 oC using a BEL BELSORP-Max system and their results were calculated
respectively using the Brunauer-Emmett-Teller (BET) equation and the Barrett-Joyner-Halenda (BJH)
method. Infrared spectra were collected by a varian 670-IR spectrometer equipped with the attenuated
total reflectance (ATR) accessary. Proton nuclear magnetic resonance (H-NMR) were obtained using
an Agilent 400 MHz spectrophotometer (CDCl3: solvent, tetramethylsilane: internal standard). A
thermal analysis was performed using a thermogravimetric analyzer (TGA, SDT Q600) at a heating rate
of 5 oC min-1 in a nitrogen atmosphere. Isothermal analysis was performed at 250 oC using the same
20
condition with a synthesis situation. The quantitative analysis of dissolution between PQ and PQ/CMK-
3 in TEGDME was performed by ultraviolet-visible (UV-VIS) spectroscopy (Varian, Cary 5000).
2.6. DFT Calculation
Nine conjugated carbonyl molecules were calculated using the density functional theory (DFT)
method with the Gaussian 09 program package. And the optimization was performed at the B3LYP / 6-
31G basis set models.
21
Table 2. Conditions of electrode preparation and electrochemical experiment in nine
molecules.
22
3. Results and Discussion
3.1. Ordered Mesoporous Phenanthrenequinone-Carbon composite18
3.1.1. Synthesis and Characterization
PQ-confined CMK-3 (PQ/CMK-3) was synthesized by using a melt-diffusion-vaporization strategy.
In the first place, a nanocasting method employed by ordered mesoporous silica (SBA-15) as a hard
template was used to obtain CMK-3. The two-dimensional hexagonally ordered carbon rods were made,
consisting of an assembly of about 7-nm-thick carbon rods separated by empty channel voids about 6
nm wide. Then, a mixture of PQ and CMK-3 was grounded by a 3:1 weight ratio and heated in air at
250 oC for 3 h, which is the temperature between the melting point (ca. 200 oC) and boiling point (ca.
360 oC) of PQ. The PQ was melted over the melting point, then PQ easily went into CMK-3 pore channel
owing to both of their hydrophobic property and imbibed through capillary forces. But, the continuous
heat treatment during 3 hours made the highly vaporized PQ residue eliminated with the air flow, at the
same time, the PQ was left over on the surface of the ordered carbon rods on account of the π-π
interaction between the aromatic PQ and CMK-3. Thus, PQ-confined CMK-3 composed of carbon-PQ
core-shell nanofibers was made after cooling (Figure 12). The Figure 13 shows scanning electron
microscopy (SEM) images demonstrating the morphological changes in the mixture of CMK-3 and PQ
according to the before and after heating at 250 oC. Figure 13a and b indicate respectively images of
bulk PQ and CMK-3. After hand-grinding, the initial mixture was obtained, as shown in Figure 13c.
But, after the heat treatment at 250 oC, no significant fraction of the bulk PQ was shown on the external
surface of CMK-3 in Figure 13d. Figure 14a and b are the high resolution transmission electron
microscopy (HR-TEM) images ate the (110) plane, which show the carbon channels filled with the PQ.
These images demonstrate the evidence of the decreased pore size of the CMK-3 due to the filing. This
evidence is more supported by the Barrett-Joyner-Halenda (BJH) adsorption pore size distribution and
nitrogen adsorption-desorption isotherms analysis. Figure 14c and d show the decreased pore size and
surface area of the CMK-3 after the imbibition of the PQ. The total surface area decreases from
1117.9m2 g-1 (CMK-3) to 418.18m2 g-1 (PQ/CMK-3). The tendency toward pore volume of CMK-3
(1.0665 cm3 g-1) is identical with composite (0.563 cm3 g-1). A facile Li+ migration into the structure
enable the presence of residual pores in the PQ/CMK-3 to ingest the electrolyte more easily. Figure 15
indicate the TEM EDS line profiles for carbon and oxygen element and it demonstrate the
homogeneously distributed PQ in the carbon framework. The PQ has the keto functional group which
causes a higher oxygen/carbon atomic ratio. So The oxygen/carbon atomic ratio in PQ/CMK-3 was
relatively higher than bare CMK-3. Moreover, FT-IR spectra of PQ/CMK-3 clearly matched with Bare
PQ, demonstrating the existence of the PQ in the PQ/CMK-3 (Figure 16a). The FT-IR spectrum of
PQ/CMK-3 (iii) was compared with those of neat PQ and CMK-3 (i, ii). The vibrational bands
23
correlated with the characteristic absorptions of PQ (ῡ/cm-1; 1674 (C=O, stretch), 1591 (C=C, aromatic
stretch), 1294, 1284, 924 (out-of-plane CH bend), 764 (out-of-plane CH bend), 715 (out-of-plane
CH bend) were observed in the PQ/CMK-319. In particular, the two C-H bending vibrations of
PQ/CMK-3 in the low energy region (715 and 764 cm-1) were red-shifted and blue-shifted, respectively,
compared with the neat PQ (Figure 16b). This demonstrates the evidence of the existence about the PQ
within the PQ/CMK-3 by virtue of non-covalent force (π-π stacking interaction between PQ and CMK-
3). The π-π interaction is further assisted by thermogravimetric analysis (TGA). Figure 16c shows the
PQ in the composite hardly evaporated over 270 oC; on the other hand, almost all of the bare PQ
disappeared before 270 oC. The difference in evaporation behavior between the bare PQ and PQ/CMK-
3 is attributed to the strong π-π interaction between PQ and CMK-3 in PQ/CMK-3. The different vapour
pressure between bare PQ and PQ/CMK-3 is further supported by isothermal analysis (Figure 16d). All
of the bare PQ evaporated at 250 oC in 50 min, whereas the PQ in the PQ/CMK-3 did not evaporate at
250 oC even after 200 min. Furthermore, the TGA plot represents contents of the PQ in the PQ/CMK-3
in the composite range up to 31.5 wt%. In addition, The 1H NMR spectra in Figure 17 demonstrated
that the NMR signal of the PQ fractionated from PQ/CMK-3 composite by using Soxhlet extraction
method corresponds to the bare PQ. It indicates that both the bare PQ and the PQ in the composite have
the same chemical structure, so the infiltrated PQ is stable in spite of the heating at 250 oC in air
condition.
24
Figure 12. A mimetic diagram for the process of a phenanthrenequinone (PQ)-
confined ordered mesoporous carbon (CMK-3) (PQ/CMK-3) synthesis.
25
Figure 13. SEM images. (a) bare phenanthrenequinone (PQ), (b) CMK-3, (c) ground
mixture of PQ and CMK-3, and (d) composite of PQ/CMK-3.
26
Figure 14. HR-TEM images at the (110) plane of (a) CMK-3 and (b) PQ/CMK-3
composite, (c) Barrette-Joyner-Halenda (BJH) adsorption pore size distributions and
(d) nitrogen adsorption-desorption isotherms of CMK-3 (black circles) and PQ/CMK-
3 (red squares).
27
Figure 15. HR-TEM images of (a) CMK-3 and (c) PQ/CMK-3 composite,
corresponding to EDS line profiles of (b) CMK-3 and (d) composite : i) oxygen, ii)
carbon, iii) oxygen/carbon*100.
28
Figure 16. (a) FT-IR spectral profiles, (b) their magnified spectra from 780 cm-1 to 700
cm-1, (c) thermogravimetric analysis(TGA) profiles, and (d) isothermal analysis : i) bare
PQ, ii) CMK-3, and iii) PQ/CMK-3.
29
Figure 17. 1H NMR (CDCl3, 400MHz, 300K) spectra of (a) bare PQ and (b) the portion
collected from PQ/CMK-3 via Soxhlet extraction using chloroform.
30
3.1.2. Electrochemical Performance
Figure 18a and 18b show the voltage profiles of the bare PQ and PQ/CMK-3 electrodes between 1.8
and 3.4 V in the 25.7 mA g-1 current density (0.1 C rate), respectively, which is comparing the
electrochemical performance between the bare PQ and PQ/CMK-3. The PQ/CMK-3 electrode has a
high reversible capacity of approximately 220 mA h g-1 at a rate of 0.1C. However, at the same current
density, the bare PQ electrode showed a very low reversible capacity about 18 mA h g-1, notwithstanding
more carbon additive (super P) was added for the preparation of the bare PQ electrodes compared to
the PQ/CMK-3 (the total amount of carbon in both electrodes were same). But it was previously
reported that bare PQ delivered large reversible capacity. This seems to result from different particle
size of the active material and the composition of electrolyte. Thus, the improved reversible capacity of
the PQ/CMK-3 is attributed to the improved electrical conductivity of PQ/CMK-3. The electrochemical
reaction was occurred at the voltage about 2.8V and 2.4V, demonstrating that the insertion/deinsertion
of Li ions progress the successive two-phase reactions according to each Li ions storing sites related to
electrochemical active carbonyl group. Therefore, we suppose the electrochemical reaction mechanism
of the PQ, indicating that after inserting two Li+ ions to two C=O the carbonyl groups are converted to
two C-O-Li during Li ion insertion and vice versa (Figure 19). Figure 20a showed the stable cycle
performance of the PQ/CMK-3 about 89% of capacity retention compared with the initial capacity over
50 cycles (Figure 20a). This is because the mesoporous structured carbon definitely is conducted as a
PQ container, which is confining the PQ in the CMK-3. Therefore, the dissolution of the PQ in the
nonaqueous electrolyte was alleviated owing to the confinement of the PQ in the CMK-3. The UV-VIS
analysis assist this result. The same amounts of bare PQ and PQ in PQ/CMK-3 were stored in
tetra(ethylene glycol) dimethyl ether (TEGDME) with stirring simultaneously. Small organic molecule
tend to be dissolved in glyme solvents. However, the dissolution of the PQ in composite was greatly
alleviated compared with that of the bare PQ, as shown in the difference in absorbance between bare
PQ- and PQ/CMK-3-dissolved solvents (Figure 20b). Figure 20c shows UV-VIS spectrum of bare PQ
dissolved in the TEGDME solvent. In addition, Figure 21a indicates the rate performance of the
PQ/CMK-3. It delivered roughly 113 mA h g-1 at a 2 C rate, demonstrating that it retain 51% of the
reversible capacity delivered at a rate of 0.1 C. Figure 21b shows the corresponding voltage profiles of
the rate performance.
31
Figure 18. Galvanostatic charge-discharge curves for Li-ion batteries of
(a) bare phenanthrenequinone and (b) PQ/CMK-3.
32
Figure 19. Two-stepwise electrochemical reduction reaction mechanism of
phenanthrenequinone.
33
Figure 20. (a) Cycle retention of PQ/CMK-3, (b) UV-VIS quantitative
analysis between bare PQ (blue circles) and PQ/CMK-3 (red squares) by the
divergent absorbance according to the different dissolution extent at the
same amounts of PQ in TEGDME, (c) UV-VIS spectrum of the bare PQ.
34
Figure 21. (a) Rate capability, (b) their charge-discharge curves of
PQ/CMK-3.
35
3.2. Change in Reduction Potential via Chemical Change
Organic electrode materials have many advantages such as environmental friendly, low cost and
structural flexibility. Especially, diverse structure, which is based on many synthetic methods having
been developed so far, is very closely related to electrochemical performance.
3.2.1. Relationship between LUMO and Reduction Potential
The electrochemical properties are different in accordance with the conjugated backbone structure
and linked substituents. The in/out of electrons are related to HOMO, LUMO level determined by the
molecular orbitals. When it loses electrons, in other words, in oxidation process, HOMO level should
be concerned. On the contrary, in reduction process, LUMO level has to be considered in receiving
electrons. The electrochemically active organic material initiates electrochemical reaction including
insertion of alkali cation with electron. Therefore, The LUMO energy involved in the reduction reaction
plays an important role in this reaction. The more LUMO energy increases, the more reduction is
difficult. As a result, redox potential is decreased. The electrochemical properties have to be considered
for factors about not only thermodynamics but also kinetics. Thermodynamic conditions related to the
energy level are at least to be satisfied with priority.
The relationship between the LUMO energy and the electrochemical characteristics was confirmed
through the evaluated electrochemical property of organic materials and the DFT calculation. Table3
shows most experimented organic materials matches up with the trend of a lower LUMO energy-higher
redox potential. And it is plotted to Figure 22.
36
Table 3. The calculated LUMO and HOMO energy, and Average discharge potential of
nine organic molecules.
37
Figure 22. The reduction potentials and calculated LUMO energies of nine organic
molecules plotted from table 1.
38
3.2.2. Terminal Functional Group Effect
The functional group linked to benzene ring can affect electron density of benzene ring. The
functional group can be effect on aspect of inductive effect induced by sigma bond and resonance effect
generated by pi bond. The effect of pushing or pulling electrons is different in perspective on the
inductive effect or the resonance effect. After these two aspects are all considered, the effect which is
called the polar effect determines final electron density on the benzene ring. This gives rise to the change
in HOMO and LUMO energy levels and affects electrochemical reaction potential. If the electron
donating functional group is introduced into the conjugated structure, this would make it easier releasing
electrons since electron density in the conjugation becomes abundant. Therefore, the oxidation reaction
becomes dominant in opposition to reduction and redox potential decreases. In case of the electron
withdrawing group, the opposite trend appears.
Compared to molecular structures between PTCDA, PTCDI and BPE-PTCDI, all have same perylene
backbone except for the terminal functional group (Figure 23b). After getting a voltage profile (Figure
23a), PTCDA having anhydride-terminal functional group is correlated with PTCDI including a imide
group. LUMO energy goes down and reduction potential increases in accordance with the switch from
PTCDI to PTCDA (Figure 23b). Both anhydride group and imide make electron density deficient.
Because it is hard to evaluate the strength of electron pulling effect directly, it could be demonstrated
by omitting carbonyl group which is a common part of anhydride and imide. After excluding carbonyl,
the factor to give an effect on pi conjugation is only nitrogen (N) and oxygen (O). Oxygen may occur
relatively lower electron density to the conjugation structure because the electronegativity of O is higher
than the electronegativity of N. This tendency correspond to activation effect of –NHR and -OR for
benzene ring, having a higher electron withdrawing effect to show the deactivating effect for benzene
ring (Figure 24).
PTCDI and BPE-PTCDI have different N linked terminal group and showed comparable
electrochemical property (Figure 23b). Any atom with a greater electronegativity could induce an
attractive force on the adjacent bonding electrons of the alkyl group. This enabled phenylethyl
functional group to donate electron to aromatic backbone unlike hydrogen group, as a result, LUMO
energy of BPE-PTCDI increased with decreased reduction potential.
39
Figure 23. (a) Voltage profiles in 5th cycle of PTCDA and 2nd cycle
of PTCDI and BPE-PTCDI, (b) the discharge potential and
calculated LUMO energies of PTCDA, PTCDI and BPE-PTCDI.
40
Figure 24. The reactivity effect of common substituted benzenes.
41
3.2.3. Bridge Functional Group Effect
Na-ADB, Na-CDB and Na-BPDC have a bridge functional group existing intermediate between pi-
conjugation and show dramatic shift of reduction potential (Figure 25a). The azo and carbonyl bridge
pulling electronic distribution could affect both side of benzene ring and shorter backbone than
perylene. This effect contributed strongly to redox potential compared with terminal functional effect.
Figure 25b shows electron withdrawing effect of azo bridge is stronger than carbonyl bridge through
Na-ADB comparing with Na-ACB has higher reduction potential. Especially, reduction potential of
Na-ADB has the increased potential about 0.7V than Na-BPDC.
42
Figure 25. a) Voltage profiles in 2nd cycle of Na-ADB, Na-CDB and
Na-BPDC, (b) the discharge potential and calculated LUMO
energies of Na-ADB, Na-CDB and Na-BPDC
43
3.2.4. Extended pi Conjugation Effect
Molecular orbital is wavefunction (ѱ) of electron which is obtained from schrӧdinger equation
(ĤѰ=EѰ) solved through the linear combination of basis set of atomic orbitals. Atomic orbitals are
overlapped through destructive/constructive interference and then molecular orbitals are generated
amount of the number of atom. Orbital energy states due to many overlapping make the distribution
and spontaneously band gap is decreased. This is applicable to aromatic compound from benzene to
anthracene. Figure 26 shows band gap (LUMO level – HOMO level) shrinks along with expansion of
pi conjugation, from benzene to anthracene. Furthermore, the diminished band gap results from a
increased HOMO and a decreased LUMO because functional group which is effectible to energy level
is identical
This tendency could be confirmed in molecules of Na-PTC, Na-BPDC, Na-NTC and Na-TP. The
electrochemical properties of corresponding molecules were evaluated and enable to do comparisons
of voltage profile in 2nd cycle (Figure 27a). Both Na-BPDC and Na-TP have two carboxylate functional
group in one molecule and Na-BPDC has longer pi conjugation chain. So band gap of Na-BPDC (4.047
eV) is lower than Na-TP (4.431 eV) by the molecular orbital theory, inspired by lowered LUMO level
(Na-BPDC : -1.480 eV, Na-TP : -1.427 eV). This result is coincident with Na-NTC and Na-PTC having
four carboxylate functional group in unit molecule. As pi conjugation get longer, LUMO get lower from
Na-NTC(-1.447 eV) to Na-PTC(-1.557 eV) attributed by the reduced band gap (Na-NTC : -1.447 eV,
Na-PTC : -1.557 eV). In addition, Although Na-NTC and Na-BPDC have a similar length of pi
conjugation, why the LUMO energy difference occurs is due to number of the carboxylate functional
group affected in one benzene. Most molecules satisfy that the lower LUMO level affects the higher
electrochemical potential except for relationship between Na-NTC and Na-BPDC (Figure 27b). It
seems that redox potential is strongly affected by kinetics of steric hindrance by different number
carboxylate functional group, but it needs more study.
44
Figure 26. Schematic diagram of molecular orbitals in benzene, naphthalene and
anthracene.
45
Figure 27. a) Voltage profiles in 2nd cycle of Na-PTC, Na-BPDC, Na-NTC
and Na-TP, (b) the discharge potential and calculated LUMO energies of
Na-PTC, Na-BPDC, Na-NTC and Na-TP.
46
4. Conclusion
This research suggested electrochemically active conjugated carbonyl compounds as electrode
materials for Li-ion and Na-ion batteries.
The small organic molecules have critical problems, such as, low electrical conductivity and high
solubility into the electrolyte. To solve these drawbacks, synthetic method by using a π-π interaction-
dependent vapour pressure of phenanthrenequinone with carbon framework was proposed. This
improves both electrical conductivity according to intimate contact between hydrophobic
phenanthrenequinone and carbon and dissolution problem by confinement effect. The enhanced
electrical conductivity facilitates more absolute redox reaction, which shows reversible capacity of 240
mAh g-1 at a current rate of 0.05C. The alleviated dissolution drawback due to the confinement effect
derived from the mesoporous carbon escalated the cycling performance with less damage of the
electrochemically active material. This enables good capacity retention over 50 cycles.
The organic compounds are possible to modify redox potential based on a strong point of the
structural flexibility. This feasibility was discovered through electrochemical experiments and
theoretical simulation of nine conjugated carbonyl molecules. Most molecules satisfied a tendency
towards relationship between LUMO level and reduction potential. The factors having an effect on the
reduction potential were categorized into three parts: terminal functional group effect, bridge functional
group effect, extended pi conjugation effect.
In terminal functional group effect, three molecules (PTCDA, PTCDI, BPE-PTCDI) having perylene
backbone were compared. The difference in the electronegativity between oxygen and nitrogen affected
the electron density in the benzene ring conjugation and altered the reduction potential. According to
transition from nitrogen to oxygen, the electron density in conjugation was decreased, LUMO level got
lower and reduction potential ascended. On the contrary, this showed an objective aspect in the case of
different electron donating influence between –H group and –ethylphenyl group.
In bridge functional group effect, from Na-BPDC, through Na-CDB, to Na-ADB, reduction potential
rose on account of carbonyl or azo functional group pulling electrons. The results demonstrated that the
azo group has the stronger electron withdrawing force than the carbonyl group.
Finally, in extended pi conjugation effect, the more pi conjugation got, the higher reduction potential
the carbonyl compounds obtained. This is because of molecular orbital theory and only applicable to
the case having a same functional group. Therefore, Na-PTC indicated higher potential than Na-NTC
and Na-BPCD was higher than Na-TP.
47
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