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Asymmetric Behavior of Positive and Negative Electrodes in Carbon/Carbon Supercapacitors and Its Underlying Mechanism Lintong Hu, Daqiang Guo, Guang Feng, Huiqiao Li, and Tianyou Zhai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09898 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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The Journal of Physical Chemistry

Asymmetric Behavior of Positive and Negative Electrodes in Carbon/Carbon Supercapacitors and Its Underlying Mechanism Lintong Hu,‡ Daqiang Guo,‡ Guang Feng,* Huiqiao Li,* and Tianyou Zhai State Key Laboratory of Coal Combustion, School of Energy and Power Engineering; School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

ACS Paragon Plus Environment

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ABSTRACT The existing asymmetric behavior of positive and negative electrodes has been early observed experimentally in carbon/carbon supercapacitors, however, the understanding of its working mechanism is still lacking. In this paper, experiment and molecular dynamics (MD) simulation were integrated to investigate this phenomenon and its underlying origins. Two different electrolytes, tetraethylammonium tetrafluoroborate (TEABF4)/propylene carbonate (PC) and lithium hexafluorophosphate (LiPF6)/PC, were employed in the electrochemical measurements. Regardless of whether anions are smaller than cations or not, the positive electrode where anions adsorbed possesses a larger capacitance than the negative electrode. This asymmetric behavior in electrolyte LiPF6/PC is more distinct than in TEABF4/PC. MD simulations were carried out to render a full-length understanding that the ion motion and desolvation as well as the size of solvated ions are the main factors accounting for the asymmetric behavior. As unbalanced capacitances of two identical electrodes can impose restrictions on the operating potential window of the device and consequently its energy density, this work would shed light on asymmetric behavior of electrodes in carbon/carbon supercapacitors and have the potential to give a new strategy to balance the capacitance of two electrodes in symmetric supercapacitors.

Keywords: Asymmetric Capacitance, Carbon supercapacitor, Organic electrolyte, Ion diffusion, Ion desolvation


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INTRODUCTION As one of the efficient energy storage devices, electrical double layer capacitors (EDLCs) have attracted tremendous attention, owing to their high power density, long cycle life, and excellent performance stability.1-5 However, the wide application of EDLCs is limited by their energy density relatively lower than batteries.6-9 To address this issue, carbon-based materials have been developed extensively as electrodes used for EDLCs, due to their high surface area, high conductivity, excellent stability in different solutions and low cost.10-12 Surprisingly, a common phenomenon that the positive and negative electrodes have unequal capacitance in carbon/carbon supercapacitors has not received the deserved attention and its inherent origins are still unclear, although there is a big concern that the asymmetric capacitance limits the working potential window and hence the cell energy density. In view of energy balance, if the capacitance of one electrode in carbon/carbon supercapacitors is lower than that of the other, the working voltage of the former has to run in a wider range than the latter to balance the charge of two electrodes._ENREF_1313-15 Thus, this asymmetric feature of working voltages between positive and negative electrodes would narrow the operating voltage of the full cell, since the wider potential range at the electrode with lower capacitance would be limited by the electrolyte decomposition voltage, which would lead to a lower utilization efficiency of the stable electrochemical window of electrolytes. Since the energy density of supercapacitors is proportional to the square of its operating voltage, expanding the voltage of the cell is a more effective approach to increase the energy density.6, 1618

By smartly balancing the asymmetric behavior of the positive and negative electrodes, the cell

voltage can be expanded and in sequence, a higher energy density can be expected for


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supercapacitors. Therefore, investigation of the asymmetric phenomenon is of great interest to achieve high-performance supercapacitors. The unequal capacitance of positive and negative electrodes has been observed in 2008 by Gogotsi and coworkers19 when carbide-derived carbon was used as the electrode in a TEABF4/acetonitrile electrolyte. They studied the relationship between pore size and capacitances of positive and negative electrodes, respectively, and found that the capacitances of these two electrodes have different dependences on the pore sizes. Later, they proposed to mix two commercial ionic liquid electrolytes with different anions to equalize the capacitance of the two electrodes, and found that this approach can effectively balance the operating voltage.14 Their results indicated that the asymmetric phenomenon is related to the difference in the electrolyte cation and anion. Besides electrolyte type, the recent work reported that the positive and negative electrodes in carbon/carbon supercapacitors undergo different charge storage mechanisms in TEABF4/acetonitrile by using in situ

31

P and

19

F NMR with electrochemical

quartz crystal microbalance techniques,20-23 which are considered to be associated with the pore/ion size, ion desolvation and kinetic effects. However, the detailed origins to cause the charge storage mechanism underlying such an asymmetric feature are not fully understood yet. To diminish the effects of the asymmetric phenomenon, several approaches have been explored to adjust the unbalanced capacitances in carbon/carbon supercapacitors, such as balancing electrode mass,15, 24-26 controlling electrochemical charge injection,13 and mixing the ionic liquids.14 These efforts would work much better if the inherent reason of asymmetric behaviors is deeply understood. Herein, to study the asymmetric behaviors of positive and negative electrodes in carbon/carbon supercapacitors, two electrolytes with different radii of bare ions, TEA (0.34 nm)-BF4 (0.24 nm) and Li (0.06 nm)-PF6 (0.38 nm) in an organic solvent


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propylene carbonate (PC),27-29 were employed in electrochemical measures. A series of electrochemical experiments have been performed on two identical commercial activated carbon electrodes, and MD simulations were further carried out for different electrolytes under varying ion concentrations. The combination of experiment and simulation is aiming to render a deep understanding on the intrinsic factors of asymmetric behaviors observed in carbon/carbon supercapacitors.

METHODS Experiment. All electrochemical experiments were carried out using a conventional three electrodes system. Activated carbon, acetylene black and poly (tetrafluoroethylene) (PTFE) with the mass ratio of 85: 10: 5 were mixed together by adding an amount of isopropanol. Then the mixture was rolled into a carbon film, which was dried overnight. For three-electrode test, two pieces of carbon films were pressed on titanium mesh current collectors, which were used as the work and counter electrodes. A piece of Li metal and titanium mesh was used as reference and pseudo-reference electrode in LiPF6/PC and TEABF4/PC electrolytes, respectively. The cyclic voltammetry (CV) was carried out using CHI 760E electrochemical workstation (Chenhua, Shanghai, China). Galvanostatic charge-discharge (GCD) was performed on Land cell test system (Wuhan Kingnuo Electronic Co., China). The pore structure of activated carbon was characterized by N2 adsorption and desorption and the pore size distribution was calculated using NLDFT (nonlocal density functional theory). As shown in the Figure S1, the AC shows a typical I sorption isotherm, which exhibit the characterization of monolayer-multilayer adsorption with a hysteresis loop at high pressure. The hysteresis loop results from the capillary condensation in the mesoporous. The AC contains micropores with the range of 1.1-2 nm and mesopores with


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range of 2-4 nm. As shown in Table S1, the AC has a total surface area of 1347 m2 g-1 while 257 m2 g-1 assigned to micropores. Modeling. MD simulations of the bulk solutions were performed in the NPT ensemble using MD package GROMACS.30 For PC, the force field was taken from ref. 31. The parameters of Li+ were modeled using the force fields in ref 32 and the force fields of TEA+ was taken from the General AMBER Force Field (GAFF).33 As to anions, the force fields for BF4− and PF6− anions were respectively taken from ref. 34 and ref. 35, respectively. The temperature was maintained at 298 K using the Nosé-Hoover thermostat with a time constant of 0.1 ps. And the ParrinelloRahman barostat was used to maintain the pressure of 1 bar with a time constant of 1 ps. The initial configurations were obtained by randomly distributing the molecules in a cubic box, periodic in XYZ directions using PACKMOL.36 The electrostatic interactions were computed using the PME method.37 An FFT grid spacing of 0.1 nm and cubic interpolation for charge distribution were used to compute the electrostatic interactions in the reciprocal space. A cutoff length of 1.5 nm was used in the calculation of electrostatic and nonelectrostatic interactions in the real space. And the Verlet leapfrog integration algorithm was used with a time step of 1 fs to solve the equations of motion. Each simulation was started at 1000 K and subsequently annealed at 800K, 600K, 400K and finally to 298 K within 4 ns. After that, 20 ns production run was performed for analysis. To ensure the accuracy of the simulation results, each case was repeated three times with different initial configurations. Beyond these simulation of organic electrolytes without confinements, simulations of nanopores filled with 0.5 M organic electrolytes were performed to study the desolvation of ions due to confinement. As shown in Figure S5a, the simulation system in 3D periodical system consists of a slit pore connected to electrolyte reservoir which is about 6.2 nm wide. The pore wall was modeled by fixing three graphene


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layers separated by a size of 1.2 nm which is chosen based on the pore size distribution showing a peak at 1.2 nm (Figure S1). The slit-pore length was selected as 6.1 nm, which is long enough to eliminate the edge effects from the entrance of slit-pore, and the data analysis is based on the central part (2.1 nm long) of the pore. The force fields for the pore wall atoms (carbon) were taken from Ref. 38. The system temperature as well as other parameters is same as the bulk simulation.

RESULTS AND DISCUSSIONS Firstly, the capacitive performance of carbon electrodes in a supercapacitor was evaluated by cyclic voltammetry (CV), in which the three-electrode system was used and pre-cycled for several cycles to reach a steady state. Figure 1a shows the typical CVs of one carbon electrode in the full potential range in 1.0 M TEABF4/PC and LiPF6/PC, respectively. The curves display representative EDL feature: no redox reaction and no electron transfer process. With an identical potential window, CVs in two different electrolytes exhibit similar potential of zero charge (PZC), which is nearly at 0.0 V vs. NHE (normal hydrogen electrode, 3.0 V vs. Li+/Li). Rapidly increased currents occur near a high positive potential in TEABF4/PC and a low negative potential in LiPF6/PC, which would be attributed to the electrolyte decomposition. Taking the PZC as a boundary, CVs of the carbon electrodes, which are cycled at solely positive (or negative) potential range vs. NHE, are presented in the Figure 1b. One can see an asymmetric CV at positive and negative potential ranges. Moreover, the curves in the Figure 1 deviate from ideal CV curves of carbon/carbon supercapacitors which have good rectangular shapes. With the different deviations in the operating voltage, TEABF4/PC works more stably at negative potential while LiPF6/PC is more stable at the positive potential.


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+

1.5

4.5

TEABF4

(b) 1.5

-1

1.5

Potential (V, vs. Li /Li) 2.0 2.5 3.0 3.5 4.0

Current density (A g )

-1

)

(a) Current density (A g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

LiPF6

1.0 0.5 0.0 -0.5 -1.0

1.0

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+

1.5


4.5

TEABF4 LiPF6

0.5 0.0 -0.5 -1.0

-1.5 -1.0 -0.5 0.0 0.5 1.0 Potential (V, vs. NHE)

1.5


1.5

Figure 1. (a) Cyclic voltammograms at 5 mV s-1 in 1.0 M TEABF4/PC (black line) and 1.0 M LiPF6/PC (red line) from -1.5 V to 1.5 V, (b) cyclic voltammograms in positive and negative potential ranges at 5 mV s-1, respectively. The CVs of 1.0 M TEABF4/PC and LiPF6/PC at different scan rates are displayed in Figure 2 (CVs of electrolytes with ion concentrations of 0.5 M and 1.3 M are given in Figure S2). With the scan rate increasing, a stable voltage of about 2.4 V can be achieved and stay unchanged. As shown in Figure 2a, for TEABF4/PC electrolyte, the shape of CV curves changes little within the negative potential range in spite of the scan rate varying. However, within the positive potential range, the side reaction occurs at a high potential when the scan rate decreases. The stable voltage in the negative potential is larger than in the positive potential. On the contrary, in LiPF6/PC electrolyte (Figure 2b), CVs at the positive potential show more stability than at the negative potential. The stable voltage in the positive potential is larger than in the negative electrode. Interestingly, the asymmetry becomes more obvious with increasing scan rates, which may be related to the different mobility of ions. The electrochemical performance with different ion concentration was shown in Figure 2c and d, which shows that with the ion concentration increasing the capacitance increases as well, while the shape of curves changes slightly and the stable voltage remains almost unchanged.


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+

1.5

300

4.5

1.5 200

-1

1 mV s -1 5 mV s -1 10 mV s -1 20 mV s

200 100 0 -100

150

-1

)

100 0 -100 -200 -300 -1.5 -1.0 -0.5 0.0 0.5 1.0 Potential (V, vs. NHE)

1.5

4.5

0.5 M 1.0 M 1.3 M

50 0 -50 -100

Capacitance (F g )

200


100

-150 (d) 200 150 100 50 0 -50 -100 -150 -200

(b) -1

+

(c) -1

Capacitance (F g

400


Capacitance (F g )

500

-1

)

(a)

Capacitance (F g

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0.5 M 1.0 M 1.5 M


1.5

Figure 2. Cyclic voltammograms at different scan rates in (a) 1.0 M TEABF4/PC and (b) 1.0 M LiPF6/PC. Taking the potential of zero charge as a boundary, the solid arrowed lines point out the stable voltage of the electrolyte. In TEABF4/PC, it shows that the stable voltage in the negative potential is larger than in the positive potential. However, in LiPF6/PC, the stable voltage in the positive potential is larger than that in the negative potential. Cyclic voltammograms with different ion concentrations of (c) TEABF4/PC and (d) LiPF6/PC (scan rate: 10 mV s-1). To examine the asymmetric capacitances of the positive and negative electrodes, galvanostatic charge-discharge (GCD) measurements at a current density of 300 mA g-1 were performed. Based on CVs in Figures 1 and 2, a 1.5 V-voltage window is found to be too large for the carbon electrode to work without electrolyte decomposition. In order to avoid the side reaction, a 1.2 V potential window is chosen at GCD tests. Figure 3a and 3b show GCD curves from 0 V to ± 1.2 V vs. NHE for positive and negative electrodes (details of GCD curves are given in Figure S3). A similar phenomenon was found in these two electrolytes: the capacitance of positive electrode (C+) is much larger than that of negative electrode (C-) in spite of the ion


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concentration varying. This asymmetric capacitance phenomenon is also observed in ionic liquid electrolyte previously.14 Furthermore, the capacitance difference between positive and negative electrodes in LiPF6/PC is much larger than in TEABF4/PC (Figure 3c and 3d). Despite whether the size of the cation is larger than the anion or not (TEA+ vs. BF4- or Li+ vs. PF6-), the capacitance at positive electrode is always larger.

3.0

-0.5

2.5

-1.0

2.0 1.5

20 40 60 80-1 100 Capacitance (F g )

4.0

0.5

3.5 0.5 M 1.0 M 1.5 M

0.0 -0.5

2.5

-1.0

2.0 0

20

(d)

Positive Negative

120

40 60 80 -1 100 120 Capacitance (F g )

1.5

Positive Negative

-1

100

3.0

+

1.0

-1.5

Capacitance (F g )

0

4.5 Potential (V, vs. Li /Li)

0.0

120 )

3.5 0.5 M 1.0 M 1.3 M

Potential (V, vs. NHE)

0.5

(c) -1

4.0

+

1.0

-1.5

(b)1.5

4.5 Potential (V, vs. Li /Li)

Potential (V, vs. NHE)

(a)1.5

Capacitance (F g

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80 60 40 20 0

0.5 M TEABF4 1.0 M TEABF4 1.3 M TEABF

4

100 80 60 40 20 0

0.5 M LiPF6 1.0 M LiPF6 1.5 M LiPF6

Figure 3. Galvanostatic charge-discharge curves at a current density of 300 mA g-1 in (a) TEABF4/PC and (b) LiPF6/PC electrolytes. Capacitance comparison of positive and negative electrodes in (c) TEABF4/PC and (d) LiPF6/PC electrolytes with different concentrations. Previous studies suggested that the ion size has important impacts on the capacitance:14,19,3940

electrodes with same surface area could accommodate more smaller ions, resulting in a larger

capacitance. However, ions in solvents are mostly solvated, surrounded by a certain number of solvent molecules. Thus, it is more reasonable to use the size of solvation shell of an ion instead of the size of a bare ion to evaluate the ion accommodation onto the electrode surface. To


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roughly compare the solvated sizes of these ions, using MD simulation we calculated both the radial distribution functions (RDFs) and the cumulative distribution function (CDFs) between solvated ions (Li+, PF6-, TEA+, BF4-) and PC molecules. Figure 4a exhibits the RDF curves (solid lines) and CDF curves (dash lines), and Figure 4b shows the coordination number of solvent molecules in ion solvation shell with different ion concentrations. Regarding the location of the first minimum in RDF curve as the radius of ion solvation shell, it can be found that TEA+ has a bigger solvated size than BF4- (0.85 nm vs. 0.65 nm, Figure S4 and Table S2), which is responsible for the observation that the positive electrode has a larger capacitance in TEABF4/PC electrolyte (i.e., the asymmetric behavior that C+ is larger than C-). For LiPF6/PC electrolyte, MD simulations show that Li+ and PF6- ions have similar solvation size (0.72 nm vs. 0.71 nm, Figure S4 and Table S2), while in experiment there is a quite obvious difference between C+ and Cobserved. To scrutinize the possible ion desolvation when ions enter the micropore or by other confinements, MD simulation with ions inside a 1.2 nm-sized nanopore was performed and it is found that (Figure S5): some organic solvent molecules in the solvation shell would be lost when ions enter to the micropore (specifically, for TEA+ and BF4- ions, the coordination number changes slightly, while, for Li+ and PF6- ions, the coordination number was reduced about 20%); however, within micropores, the ion solvated size changes slightly, which shows the same trend of the ion size effect as the above analysis from the bulk electrolyte simulation. The accumulation of counter-ions in EDLCs is a kinetic process: the slower kinetics cannot quickly balance the electrode surface charge, leading to a smaller capacitance.39 Very critical to such kinetic process of charging/discharging, the conductivity of electrolytes is found to be proportional to the ion self-diffusion.41 Thus, the ion diffusion is likely to be a factor that results


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in the asymmetric behavior, which could be calculated based on MD simulation using Einstein relation42:

1 < [ r (t0 + t ) − r (t0 )]2 > t →∞ 6t

D = lim

where r is the molecule position. Figure 5 shows the diffusion coefficients of all four ions and PC solvent molecules in different ion concentrations, obtained from MD simulation.

Figure 4. (a) Radial distribution function, g(r), (solid line) and cumulative distribution function, n(r) (dash

line)

of

solvated

ions

in

1.0

M

TEABF4/PC

and

LiPF6/PC.

Here,

n( r ) = ρ 0 ∫ 4π r 2 g ( r )dr , ρ 0 is the bulk number density. (b) The number of solvent molecules in the ion solvation shell with different ion concentrations. In order to verify our hypothesis, we focus on qualitative comparison of the diffusion coefficients of cations and anions. It can be found that, in both 1.0 M TEABF4/PC and LiPF6/PC electrolytes, anions move faster than cations, which indicates that cations cannot balance the


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electrode surface charge as quickly as anions, leading to capacitance asymmetry. Another interesting result revealed by MD simulation is that the difference in diffusion of anions and cations reduces gradually with the increase of ion concentration, which can be ascribed to the ion pair effect that could be demonstrated by the concentration-dependent ion pair ratio (defined as the number of cation-anion pairs divided by the number of all cations or anions, Figure S6). As ion concentration increases, the ion pair ratio increases as well, i.e., more cations and anions prefer to be in pairs. This suggests that with higher ion concentration, not all of ions can be fully solvated, that is, more cation-anion pairs would form, which explicates the reason that cations’ diffusion coefficient is approaching that of anions with increasing ion concentration.

0.5

+

PC(TEABF4)

TEA

PC(LiPF6 )

BF4

0.4

-

+

Li PF6

2

-1

D (x10 , cm s )

0.3

-5

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2 0.1 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Concentration (M)

Figure 5. Diffusion coefficient of TEABF4 and LiPF6 with different ion concentrations. In measurements of the capacitance in the series of carbon materials studied herein (with pores smaller than the size of the solvated ion), there must be at least a partial removal of the solvent shell as a solvated ion is moving into a nanopore.19 The easier an ion loses a solvent molecule, the more partially solvated ions could enter the electrode pore. To illustrate the difficulty of ions’ desolvation, the ion-solvent interaction energy is computed between an ion and its surrounding solvent molecules with different ion concentrations (Figure 6), which consists of two parts: van der Waals interaction energy (Figure S7a) and Coulombic interaction energy


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(Figure S7b). In the TEABF4/PC electrolyte, with all ion concentrations, it can be found that the total interaction energy between one PC solvent molecule and a TEA+ ion is approximately -10 kJ mol-1, which means that the desolvation energy of 10 kJ mol-1 should be averagely paid to lose one solvent molecule. The ion desolvation process is schematized in Figure 7. It can be seen that the desolvation energy of TEA+ cation is less than that of BF4- anion. Hence, the desolvation energy has a positive effect on the capacitance of negative electrode where the cations would adsorb. On the contrary, the desolvation energy between a PC molecule and a Li+ ion is about 50 kJ mol-1, larger than that (25 kJ mol-1) between a PC molecule and a PF6- ion (Figures 6 and 7), implying that it is easier for PC molecules to escape from the solvation shell of PF6- anion, resulting in a higher capacitance of positive electrode. Therefore, the different behavior between TEABF4 and LiPF6 electrolytes is interpreted: the desolvation energy that has different influences on these two organic electrolytes brings a larger capacitance difference of two electrodes in LiPF6/PC electrolyte than in TEABF4/PC electrolyte. What’s more, the average desolvation energy of TEA+ and BF4- is smaller than that of Li+ and PF6- (Figure 6), properly suggesting that in porous carbon supercapacitors, TEABF4/PC electrolyte could offer advantage in capacitance due to their superiority in desolvation as well as diffusivity. 0

+

TEA

-

BF4

+

Li

-

PF6

-1

)

-10 Etot (KJ mol

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-20 -30 -40 -50 0.0

0.4 0.8 1.2 Concentration (M)

1.6


14

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Figure 6. Total interaction energy (Etot) of an ion and a solvent molecule which is in ion’s solvation shell, composed by van der Waals interaction energy (EvdW) (Figure S7a) and Coulombic interaction energy (ECoul) (Figure S7a), respectively.

Figure 7. Schematic representation of desolvation as a solvated ion is moving into a nanopore. Since the solvated ions are too large to enter the pores of electrodes, desolvation would happen (i.e., the solvent shell must be partially removed). The interaction between ions and PC is described as red dash line(s). For example, in a solvated TEA+ ion, energy of 10 kJ mol-1 should be paid averagely for one PC molecule to escape from the solvent shell. The larger desolvation energy means more difficulty for desolvation.

It is worth noting that the asymmetric phenomenon revealed in this work is difficult to be quantitatively explained using the above “size effect”, “motion effect” and “desolvation effect”. Therefore, we analyze this phenomenon qualitatively and synthetically. Firstly, in LiPF6/PC electrolyte although the solvated size of Li+ is nearly the same with that of PF6-, positive


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electrode exhibits a higher capacitance due to the easier accumulating PF6- anions, resulting from the anion’s better motion ability and lower desolvation energy. By contrast, in TEABF4/PC electrolyte, better motion ability and smaller solvated size facilitate BF4- anions to accumulate on positive electrode, whereas higher desolvation energy of BF4- ions weakens the superiority of positive electrode in capacitance, which is responsible for a less asymmetric phenomenon than between two electrodes in LiPF6/PC electrolyte. Therefore, Ion desolvation energy and diffusion as well as ion solvation size are used as vital factors to estimate the capacitance asymmetry. Beyond these points, in Figure 1, TEABF4/PC electrolyte shows its better tendency toward ideal EDL behavior, comparing to that in LiPF6/PC electrolyte. Combining the above several views by experiment and modeling, it can be concluded that TEABF4/PC solution is more appropriately used as electrolytes than LiPF6/PC solution, which is in conformity with the present industrial application situation.

CONCLUSION In summary, the asymmetric behavior of positive and negative electrodes was investigated using experiment and MD simulation. Capacitive performances of two organic electrolytes, LiPF6/PC and TEABF4/PC, were measured by means of CV and GCD. It is found that the capacitance of positive electrode is larger than that of negative electrode, and the difference between positive and negative electrodes in LiPF6/PC is larger in TEABF4/PC. Besides, the stable voltage windows at the positive and negative potential ranges seem also asymmetric in both electrolytes. To give a detailed explanation of these phenomena, MD simulation was carried out to rationalize that ion solvation size, diffusion, and desolvation energy are vital factors for the asymmetric capacitance. Specifically, in LiPF6/PC electrolyte that the solvated size of cation and anion is quite similar, the positive electrode with PF6- ions adsorbed possesses larger capacitance owing


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to the faster diffusion and lower desolvation energy of anions than cations; in TEABF4/PC electrolyte, the smaller solvated size and faster ion diffusion of anions promote their accumulation on positive electrode, but higher desolvation energy weakens this superiority in capacitance, which synergically results in a less asymmetric phenomenon than LiPF6/PC electrolyte. These results and analysis help to get a deep understanding of mechanism involved in the charge storage and provide guidance for designing appropriate carbon-electrolyte system for energy storage.

ASSOCIATED CONTENT Supporting Information. Additional CV, GCD, radial distribution function and cumulative distribution function of solvated ions, the radius and number of solvent molecules in the solvation shell of solvated cations and anions. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H. Q. Li). *E-mail: [email protected] (G. Feng).

Author Contributions ‡These authors contributed equally.

ACKNOWLEDGMENT


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We acknowledge the support from National Basic Research Program of China (2015CB932600), National Natural Science Foundation of China (21571073, 51302099, 51406060), Pro-gram for New Century Excellent Talents in University (NCET-13-0227), Natural Science Foundation of Hubei Province of China (2014CFA089), Program for HUST Interdisciplinary Innovation Team (2015ZDTD038) and Fundamental Research Funds for the Central University. The simulation work was carried out at National Supercomputer Center in Tianjin, and the calculations were performed on TianHe-1 (A).

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Asymmetric Behavior of Positive and Negative ... - ACS Publications

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