REGULAR ARTICLE
Heteroatom-bridged pillar[4]quinone: evolutionary active cathode material for lithium-ion battery using density functional theory
JU XIEa,b,* , HUIZHONG SHIa, CHAO SHENa, LONG HUANa, MAOXIA HEcand MING CHENa,*
aSchool of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, Jiangsu, China
bKey Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China
cEnvironment Research Institute, Shandong University, Jinan 250100, Shandong, China E-mail: xieju@yzu.edu.cn; chenming@yzu.edu.cn
MS received 13 May 2020; revised 11 October 2020; accepted 20 October 2020
Abstract. Quinone-based macrocyclic compounds have been proposed as promising electrode materials for rechargeable lithium-ion batteries (LIBs). To improve the electrochemical performance, in this paper, two heteroatom-bridged pillar[4]quinones (namely, oxa- and thia-pillar[4]quinones) are presented as active cathode materials for LIBs. The geometry structures, electronic structural properties, and electrochemical properties of these new species are calculated by Density Functional Theory (DFT) at the M06-2X/6- 31G(d,p) level of theory. Two heteroatom-bridged pillar[4]quinones possess higher theoretical specific ca- pacity (659 mA h g-1 and 582 mA h g-1 for oxa- and thia- pillar[4]quinones, respectively) than that of parental pillar[4]quinone (446 mA h g-1). The electrochemical performances of oxa- and thia-pil- lar[4]quinones are predicted theoretically to be superior to those of pillar[4]quinone as cathode material for LIBs. Compared with oxa-pillar[4]quinone, thia-pillar[4]quinone is predicted to be slightly more suitable as cathode electrode material. These results may provide fresh ideas and guidelines for enhancing the perfor- mance of quinones organic electrode materials for LIBs.
Keywords. Heteroatom-bridged pillar[4]quinone; Electrochemical performance; Lithium-ion battery;
Density functional theory.
1. Introduction
Organic molecules are promising candidates as elec- trode materials for lithium-ion batteries (LIBs) because of their sustainability, structure diversity, and environmental friendliness.1–4 The researchers have made enormous contributions to the progress of organic electrode materials such as organic free radical compounds,5–7 organosulfur compounds,8,9 and organic carbonyl compounds.10–16 However, organic electrode materials are confronted with multiple challenges such as low conductivity and high solu- bility in the aprotic electrolyte.17–19 In recent reports, several efficient strategies have been raised such as polymerization and cyclo-oligomerization of electro- chemical redox-active species to increase the
molecular weight and reduce solubility. As the repre- sentative of the latter, macrocyclic compounds with several quinone units were considered as an encour- aging organic cathode material due to their low solu- bility in organic electrolyte compared to the small molecule quinone analogs. In 2014, Chen’s group utilized calix[4]quinone (C4Q, with theoreti- cal specific capacity Ctheo = 446 mA h g-1) as the cathode material for LIBs, which exhibited 85%
capacity retention after 100 cycles at 0.2 C rate.16 Pillar[5]quinone (P5Q, Ctheo = 446 mA hg-1) was subsequently confirmed to be a high-performance cathode active material.20 Our previous theoretical studies have provided the electrochemical redox mechanism of pillar[4,5]quinones as organic cathode for LIBs.21,22
*For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-020-01863-5) contains supplementary material, which is available to authorized users.
https://doi.org/10.1007/s12039-020-01863-5Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)
However, the researches of macrocyclic quinone- based compounds as electrode materials are still lim- ited at the initial stage for their poor cyclic stability.
Much attention has been paid to enhance the perfor- mance of organic redox-active species, especially in computational chemistry. Yokoji et al.,found that the redox potential of benzoquinone derivatives could be enhanced by the introduction of electron-deficient perfluoroalkyl groups into benzoquinone.23 Assary et al., found that the reduction potential of quinone derivatives could become lower with the modification of electron-donating groups.24These results inspire us to improve the electrochemical performance of qui- none-based macrocyclic compounds by a chemical modifying method. Pillarquinones have a symmetrical pillar-shaped skeleton,25 and their electrochemical active sites mainly locate at the identical top and bottom rims.16,20–22 In contrast, the bridging-methy- lene groups have not been given enough attention due to their insignificant contribution to the electrode performance. In fact, this can be changed by chemical modification. The substitutions of the methylene groups by heteroatoms or some other active groups will change the physicochemical properties and make full use of pillarquinone molecular skeleton. Thus, the geometrical structures and electronic features of the molecule can be designed with a purpose. This strat- egy has been implemented in calixarene and pil- lararene chemistry. Several heteroatom-bridged derivatives have been synthesized experimentally26–31 and investigated theoretically.32,33
Pillararenes are calixarene analogues, and pillar- quinones34 are oxidation products of pillararenes which have exhibited excellent electrochemical prop- erties. In order to achieve a higher theoretical speci- fic capacity and a better cycling performance, the heteroatom-bridged derivatization of pillarquinones deserves further exploration. It is well known, oxygen and sulfur are crucial elements for energy storage devices involving Li-ion, Li-O2, and Li-S batteries.35 Therefore, the oxa- and thia-pillarquinones (P4QO andP4QS, as shown in Scheme1) are first chosen as
the novel organic cathode materials for LIBs. The bridging positions ofP4Qwill be activated by oxygen and sulfur atoms to be the active sites to accept and release lithium atoms. Therefore, the structural sta- bility, electronic structural properties, Li-binding models, and electrochemical redox mechanisms will be different from those ofP4Q to different extent.
2. Computational details
2.1 Structural optimization and property analysis All calculations in this work are performed using the Gaussian 09 software.36 The M06-2X density func- tional37,38 is utilized with the 6-31G(d,p) basis set to calculate molecular geometries, solvent effects, elec- tronic structures, and redox potentials. The optimized geometry structures are verified without any imaginary frequency. The structures for subsequent property analysis are the lowest energy states comparing to their other isomers. The calculation of natural bond orbital (NBO)39 analysis is carried out to understand the electronic features. To take the solvation contri- butions into consideration, the polarizable continuum model (PCM)40 is taken to achieve solvation free energies, which can meet the required speed and accuracy. As a common electrolyte in LIBs, the mixed solvent consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) (1:1:1 by volume) is used in this work. The polarity of the mixed solvent is measured by dielectric constant (e) obtained by the following equation.41 emix ¼xECeECþxDMCeDMCþxEMCeEMC
ð1Þ whereemixis the dielectric constant of the mixed solvent, x andeare the mole fraction and the dielectric constant of each solvent, respectively. Meanwhile, we also calcu- lated the solvent effect ofP4Q,P4QSandP4QOunder different pure solvents such as toluene (e= 2.374), tetrahydrofuran (THF, 7.426), acetone (20.493), dimethylsulfoxide (DMSO, 46.826), and water (78.355).
2.2 Prediction of electrochemical properties
The specific theoretical storage capacity (Ctheo) can be calculated viathe following equation.42
Ctheo¼ nF
3:6M ð2Þ
where ‘M’ is the molecular mass and n refers to the max number of Li atoms combined. Based on the Scheme 1. The construction unit of pillar[4]quinone
(P4Q) and heteroatom-bridged pillar[4]quinones (P4QO andP4QS).
lithiation process (3), the redox potential (Eredox) was calculated by using Eq. (4):
Lin1P4QXþðn2n1ÞLi¼Lin2P4QX
ðn2[n1; X = O,SÞ ð3Þ
Eredox¼DGredox
nF ð4Þ
In Eq. (4),n is the number of electrons transferred, and F is the Faraday constant. DGredox is the Gibbs free energy change based on the process (3) which involved the solvent contribution to each redox spe- cies. The Gibbs free energy for each species in solu- tion (Gsolu) was obtained as follows:
Gsolu ¼GgasþDGsolv ð5Þ
where Ggas is the Gibbs free energy in the gas phase.
DGsolv can be achieved by calculating the difference between the single-point energy in solvent phase (Esolv) and the single-point energy (Egas) in the gas phases.43,44 This simplification is based on the fol- lowing reasons. The zero-point motion, thermal and entropic contributions for the redox molecules are neglect because their geometrical structures are almost the same as those in the implicit solvent environment and all the corresponding contributions will cancel each other out.45
The root means square deviation (RMSD) is a fre- quently used measure of the structure changes. In this work, RMSD is used to illustrate the structural deformation before and after Li-binding process, which is defined as the following:
RMSD¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1
N
n atomX
i
½ðxix0iÞ2þ ðyiy0iÞ2þ ðyiy0iÞ2 vu
ut
ð6Þ where i goes over all the atoms, xi and xi0 are the x- coordinates of theithatom in the initial structure and in the final structure, respectively, y and z and so on.
RMSD values are achieved by VMD software.46 3. Results and Discussion
3.1 Geometry structure and energy analysis
The conformations of P4QX (X = O and S) are sim- ilar to that ofP4QwithD4symmetry. InP4QX, there are 12 active sites interacting with Li atoms and eight Li atoms can be bound to the oxygen atom of car- bonyl at the top and bottom rims, then the rest four Li
atoms will be captured at the bridging positions. To discuss the binding ability with Li-atom of P4QX systematically, three Li-binding stoichiometric ratios are considered (namely Li4P4QX, Li8P4QX, and Li12P4QX), while only two stoichiometric ratios for P4Q (Li4P4Q and Li8P4Q) due to lack of four bridging active sites. The lowest-energy structures of P4Q and P4QX and their Li-binding complexes are given in Figure 1. Cartesian coordinates of the opti- mized structures are given in the Supplementary Information. The energy and the Li-binding energy changes of all structures are listed in Table1.
In Figure 1, conformers ofP4Q,P4QX(X = O, S) and their Li-binding complexes with the lowest ener- gies are presented. The geometries of P4QO and P4QS can maintain symmetrical pillar-shaped struc- ture well. The bridging-bond angle inP4QOis 108.9°, which is very close to the angle inP4Q(106.0°). The molecular skeleton ofP4QSis just like a square shape with the bridging-bond angle of 94.0°. The order of molecular cavity size is P4QO\P4Q\P4QS. The relatively large change in structures will occur when P4Q,P4QOandP4QSaccept four Li atoms (Li4P4Q, Li4P4QOandLi4P4QSin Figure 1). Li atoms always tend to form O-Li-O bridging bonds. The conformers of Li4P4Q and Li4P4QX with four O-Li-O bridging bonds in a staggered arrangement are most stable among their isomers. The single bond between bridging atom and connected quinone unit is easy to rotate leading to the structural changes, which means the deformation of P4Q and P4QX skeleton occurs when four Li atoms are inserted. The lowest-energy structures ofLi4P4QandLi4P4QXalso reveal that Li atoms prefer quinone carbonyl oxygen atoms to the bridging heteroatoms. The formations ofLi4P4Q and Li4P4QX are accompanied by high exothermicity (DEb and DGb in Table1) which indicates their high structural stabilities. WhenP4QandP4QXcombined with 8 Li atoms (Li8P4Q,Li8P4QOandLi8P4QS in Figure1), two uniform Li-O-Li-O rims are formed at the top and bottom. The size of Li-O-Li-O rim is smaller than the molecular cavity ofP4QandP4QX.
Li-O-Li-O rims contribute to the structural stability of Li8P4Q or Li8P4QX, which can be quantified by higher negative values of DEb and DGb in Table1.
The Li-O-Li-O rims ofLi8P4QOandLi8P4QSwill be broken when P4QX combined with 12 Li atoms (Li12P4QOandLi12P4QSin Figure 1). InLi12P4QO structure, it can be found four O-Li-O bonds are formed by Li atom, carbonyl oxygen, and bridging- oxygen. Similarly, the O-Li-S bonds exist in Li12- P4QS. The active sites locate at the bridging positions have a tendency to combine with different orientations
of Li atoms to make the structure stable, and the ori- entations of Li atoms are always up and down alter- nately. The predicted bonding modes are given in Scheme S1 in Supplementary Information. Negative values of DEb and DGb for Li12P4QX confirm their structural stability.
To compare the Li-binding capacities ofP4QXwith P4Q, binding energy changes of per Li atom are listed in parentheses in Table1. The more negative per Li- binding energy changes mean the higher the Li-bind- ing capacity and the higher the structural stability. The Li-binding capacities ofP4QXare slightly higher than Figure 1. The lowest-energy structures ofP4Q,P4QX(X = O and S), and their Li-binding complexes calculated at the M06-2X/6-31G(d,p) level of theory (distance in angstrom and angle in degree). Lithium, oxygen, and sulfur atoms are shown in purple, red, and yellow color, respectively.
that of P4Q. The Li-binding capacities of P4QX decrease when they bind twelve Li atoms with reduced negative values ofDEbper Li atom. The value ofDGb per Li atom has the same trend asDEbper Li atom. For P4Q and P4QX, the mechanisms for binding up to 8 Li atoms are similar to each other. The difference is the additional Li storage in P4QX. To better clarify the additional redox mechanism, Li(9-11)P4QX were supplemented by calculating at the M06-2X/6- 31G(d,p) level of theory (their structures are given in Figure S1 in Supplementary Information). In Li8- P4QX, the Li atoms at the upper and lower rims have reached the saturation state. When combining with 9 Li atoms, the new Li-bridged bonds with heteroatoms are formed followed by some Li-bridged bonds along the upper and lower rims damaging. The molecular skeleton is obviously deformed. Something similar happens when P4QX combined with 10 and 11 Li atoms. When 12 Li atoms were combined, the sym- metry of the molecular skeleton is restored again. In terms of energy, the magnitudes of the binding energy value of Li(9-12)P4QX are about 70 kcal/mol per Li- binding, which are less than those of Li(1-8)P4QX (Table 1). Therefore,Li(9-11)P4QXcan be regarded as the active intermediate states in the lithiating process.
When more than 12 Li atoms were combined, the Li- Li bonds appear indicating the supersaturation state.
As an illustration, structures of Li16P4QO and Li16- P4QS are given in Figure S2 in Supplementary Information. Therefore, only the lithiation states formed between P4QX and Li atoms within 12 are considered in this work. In the case ofP4Q, only 8 Li atoms are combined.
3.2 Electronic structure analysis
The highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of P4Q, P4QX (X = O and S), and their saturated Li- binding states (Li8P4Q, Li12P4QO, and Li12P4QS) are shown in Figure 2. In the case of HOMOs on P4QOandP4QS, the orbital delocalization will occur among the adjacent quinone units accompanied by the introduction of heteroatoms owing to the existence of outermost lone pair electrons on heteroatoms. All LUMOs ofP4QO andP4QS localize on the quinone units uniformly, which means excellent electronic acceptance ability in these areas.16 HOMOs of Li12- P4QOandLi12P4QSdistribute at the locations where the Li atoms gather, which mainly consist of 2 s atomic orbitals of Li atoms. Different from Li8- P4Q, LUMOs of Li12P4QO andLi12P4QSdistribute on Li atoms which interact with one bridging-het- eroatom. The energy difference between HOMO and LUMO, i.e., HOMO-LUMO gap (DELUMO-HOMO in parentheses in Figure2), represents the relative sta- bility. As shown in Figure 2, P4Q,P4QO andP4QS possess large values of DELUMO-HOMO (6.190 eV, 6.476 eV, and 5.409 eV, respectively), which suggest that they have relatively high chemical stability. The value of the energy gap will slightly decrease when they combine with 8 Li atoms (4.935 eV, 5.118 eV, and 5.227 eV for Li8P4Q, Li8P4QO and Li8P4QS, respectively). In the case ofLi12P4QOandLi12P4QS, the gap values are 3.311 eV and 3.116 eV, respec- tively, which means reduced stability compared with their 8-Li-binding structures. The energy gap results Table 1. Energies (EandG) and Li-binding energy changes (DEbandDGb, per Li-binding energy changes in parentheses) of all conformers calculated at the M06-2X/6-31G(d,p) level of theory in the gas phase at 298 K and 1 atm.
Species E(a.u., including zero-point energy) aDEb(kcal/mol) G(a.u.) DGb(kcal/mol)
P4Q -1677.29441 -1677.35118
P4QO -1820.87929 -1820.93543
P4QS -3112.77267 -3112.83616
Li4P4Q -1707.71411 -314.02 (-78.50) -1707.77061 -313.85 (-78.46)
Li4P4QO -1851.32164 -328.23 (-82.06) -1851.37788 -328.29 (-82.07)
Li4P4QS -3143.24523 -347.19 (-86.80) -3143.30374 -344.06 (-86.02)
Li8P4Q -1738.13119 -626.39 (-78.30) -1738.18649 -625.47 (-78.18)
Li8P4QO -1881.72979 -635.00 (-79.38) -1881.78480 -634.29 (-79.29)
Li8P4QS -3173.65903 -657.50 (-82.19) -3173.71732 -654.24 (-81.78)
Li12P4QO -1911.90839 -797.73 (-66.48) -1911.97372 -803.50 (-66.96)
Li12P4QS -3203.82590 -812.87 (-67.74) -3203.89549 -816.70 (-68.06)
Li -7.47982 -7.49322
aDEb= ELinP4QX-EP4QX-nELi.
are consistent with the structure and energy analyses above.
Figure3 shows the electrostatic potential (ESP) maps of all the conformations considered. P4Q, P4QO and P4QS show negative charge distributions symmetrically at the top and bottom rims while posi- tive charge distribution in the molecular cavity. The ESP value of carbonyl oxygen will increase when the CH2 bridging groups are substituted by heteroatoms (O and S atoms). In Li12P4QOandLi12P4QS, the Li atoms which interact with carbonyl oxygen and bridging heteroatom simultaneously show positive
charge distribution, owing to the strong electronega- tivity of heteroatoms.
Natural bond orbitals (NBO) analysis can detail the charge distribution of atoms in the molecular which explains the interaction between Li atom and carbonyl oxygen atoms or bridging heteroatoms. The natural charge (e) distributions in the form of the representa- tive molecular moiety of all considered species are given in Figure4. The natural charge of carbonyl oxygen in P4Q is -0.517, and this value becomes - 0.491 and -0.487 in P4QO and P4QS, respectively.
These negative values will significantly increase after Figure 2. Frontier orbitals of HOMO and LUMO ofP4Q, P4QO andP4QSand their final Li-binding states (Li8P4Q, Li12P4QOandLi12P4QS) with energy gap (DELUMO-HOMOin eV) in parentheses.
Figure 3. The electrostatic potential (ESP) maps (atomic unit) ofP4Q, P4QOandP4QSand their final Li-binding states (Li8P4Q,Li12P4QOandLi12P4QS).
combining with Li atoms. The maximum negative value on carbonyl oxygen (-0.912) is found in Li12- P4QO. Consistently, the natural charges on bridging group or atoms decrease with the increasing number of Li atoms. The Li atoms that interact with bridging- heteroatoms have a relatively high positive charge (0.866 inLi12P4QO, 0.819 inLi12P4QS). The charge
on Li atom of O-Li-O bridging bond is significantly higher than that of Li-O single bond. For example, the natural charge on Li atom of Li-O single bond is only 0.570 inLi12P4QO. The natural charge of a bridging- sulfur atom in P4QS shows positive natural charge (0.352), and this phenomenon still exists in its Li- binding complexes. In contrast toP4QS, the bridging- Figure 4. The natural bond orbital (NBO) analysis of all conformations considered in this work calculated at the M06-2X/
6-31G(d,p) level of theory.
Table 2. The NICS (1) (at points 1 A˚ above the ring centers, in ppm) of the initial state (P4Q,P4QOandP4QS) and their lithiation states based on the optimized structures.
Initial state NICS(1) Lithiation state NICS(1) Lithiation state NICS(1) Lithiation state NICS(1)
P4Q 4.69 Li4P4Q -4.96 Li8P4Q -22.73
P4QO 3.96 Li4P4QO -4.35 Li8P4QO -21.10 Li12P4QO -21.60
P4QS 4.56 Li4P4QS -6.82 Li8P4QS -21.60 Li12P4QS -21.99
oxygen atom ofP4QOhas a negative natural charge (- 0.526). The atomic radius of sulfur is larger than oxygen leading to the relatively weak attractive force of sulfur nucleus to its outermost electrons. Therefore, the bridging-sulfur using its lone pair electrons can interact with the positively charged Li atom. This interaction in Li12P4QS is similar to that between bridging-oxygen atom and Li atom, while the former is weaker. The relatively large change of charge value on carbonyl oxygen represents stronger Li binding capacity, which confirms the relatively weak interac- tion between the bridging site and Li atoms.
The values of Nucleus-independent chemical shift (NICS) are the acknowledged parameter to describe the aromaticity index for planar rings.47 NICS(1) (at point 1 A˚ above ring center of quinone fragment) were calculated using Multiwfn software,48 and the results are shown in Table 2. Significantly negative values of NICS(1) indicate the planar rings possess aromaticity.
In Table2, six-member rings in an initial state (P4Q, P4QO and P4QS) are found without aromaticity.
However, when these initial states accept Li atoms, the rings will be converted into the benzene-like rings and exhibit negative NICS(1) values and possess some aromaticity. This finding indicates that the electronic acceptance and storage capacity of P4Q and P4QX molecules will increase significantly after combining a number of Li atoms.
3.3 Solvent effect analysis
In this section, the solvent effects are studied at the M06-2X/6-31G(d,p) level of theory. The solvation free energies (DGsolv) of P4Q, P4QO and P4QS are pre- sented in Figure5. DGsolv values, as well as other
corresponding data, are given in Table S1 of Supple- mentary Information. Six solvents with different dielectric constant (e) ((toluene (2.37), THF (7.42), acetone (20.49), mixed electrolyte (34.96, calculated based on Eq. (1)), DMSO (46.83), and water (78.35)) are chosen to discuss the energetic changing trend with the dielectric constant changing. The more negative value of DGsolv means a more stable state of the molecule under dielectric environment will be. As depicted in Figure 5, theDGsolvofP4QOis lower than P4QSandP4Qwhich suggest thatP4QOcan achieve an extremely stable state in the solvent. In contrast to P4QOandP4Q, theP4QSpossesses a relatively high value ofDGsolvin various solvents which means poorer solubility than that ofP4QO. InP4QO, the bridging- oxygen atoms can interact with most of the polar sol- vent molecules, especially water molecules to form hydrogen bonds. In the case of organic electrolyte, DGsolvvalue of P4QS (-14.26 kcal/mol) is less nega- tive than that ofP4QO(-69.37 kcal/mol) representing lower solubility in solvents. Thus,P4QSis superior to P4QO as organic electrode active materials. The flat solvation curves in Figure5 also show that solvent polarity has little effect onDGsolvin polar solvents with a dielectric constant greater than 20.
3.4 The Li-binding ability of the bridging-oxygen or -sulfur sites
DFT calculations indicated that one Li atom would like to be bound to carbonyl groups of the quinone derivatives first and then to the other active sites.49 One benzoquinone can undergo two-electron reduc- tions to accept two Li-ions.50 According to the molecular structures, P4Q could bind at most eight lithium atoms. Different from P4Q, P4QS or P4QO can uptake more Li atoms with its bridging O (S) sites.
Each bridging-O (S) atom has two bonds with carbon atoms already, whereas two lone-pair orbitals remained to interact with additional Li atoms. One bridging site can store two more Li atoms theoreti- cally. However, the interaction between bridging-O (S) atom and Li atom is weak, and this Li atom can also interact with the adjacent carbonyl oxygen to finally form O(S)–Li–O bridged bond. Structural deformation caused by the formation of O(S)–Li–O bond hinder bridging site from stabilizing more Li atoms, and only one Li atom is bound at each bridging site. As a result,P4QOandP4QSare able to combine four more Li atoms compared with parent P4Q. This finding can lead to differences in some properties associated with structure and electrochemistry.
Figure 5. The solvation free energies (DGsolv) of P4Q, P4QOandP4QSin different solvents.
3.5 The theoretical prediction for some electrochemical performance
Introduction of heteroatom (bridging-O and S atoms) into the molecular skeleton of P4Q can significantly enhance the theoretical specific capacity. The molec- ular mass of P4QO and P4QS are 488.002 amu and 551.910 amu, respectively. The maximum number (n) of Li atoms combined is 12. According to Eq. (2), the theoretical specific capacity (Ctheo) can reach 659 mA h g-1 (P4QO) and 582 mA h g-1 (P4QS), respectively, which is much higher than that of P4Q (Ctheo = 446 mA h g-1). These Ctheo values corre- spond to the proposed redox mechanism shown in Scheme S2, Supplementary Information.
As the electrode materials, the structure changes during charging and the discharging process should be minimized. Based on Eq. (6), RMSD is calculated to show the structural changes between the initial states (P4Q,P4QOandP4QS) and their saturated lithiation states (Li8P4Q, Li12P4QO and Li12P4QS). The smaller RMSD value is, the better the structure is maintained after Li-binding. RMSD values of P4QO (0.110) andP4QS(0.192) are much lower than that of P4Q (2.250), which indicate that heteroatom-bridged pillar[4]quinones have the advantage in the structural stability of P4Q(O,S) compared withP4Q.
Finally, the redox potentials (obtained by using Eq. (4)) are discussed. In the dielectric environment, the redox potentials of P4QO and P4QS can reach 3.20 V and 3.19 V, and these values are higher than that of pillar[5]quinone (2.84 V) 20 and those of most small molecular quinones. In comparison with P4QO andP4QS, theP4Qhas relatively high redox potential (4.91 V). However, P4Q possesses poor structural stability, which is a very unfavourable factor as elec- trode materials. As a whole, the heteroatom substitu- tion of quinone compounds will be beneficial to the improvement of some electrochemical properties.
4. Conclusions
In this paper, the theoretical investigation of heteroa- tom-bridged pillar[4]quinones (P4QO and P4QS) as cathode materials for LIBs is performed at the M06- 2X/6-31G(d,p) level of theory. The following con- clusions are drawn from this work.
1. The geometries of P4QO and P4QS with the bridging-heteroatoms (O and S) can maintain sym- metrical pillar-shaped structure well in the process of inserting Li atoms. The Li-O-Li-O rims formed at the top and bottom rims ofP4QOandP4QSwhen
forming Li8P4QO and Li8P4QS. The bridging atoms in Li12P4QO andLi12P4QS tend to interact with Li atoms different orientations (up and down alternately) forming the stable structures.
2. The introduction of bridging heteroatoms can reduce energy gap (DELUMO-HOMO) significantly, and the electron delocalization will occur in the molecular skeleton of P4QO and P4QS. The Li atoms bonded in O-Li-O and S-Li-O bonds in Li12P4QOandLi12P4QS show electron deficiency to a large extent.
3. In comparison with P4Q, the heteroatom-bridged pillar[4]quinone possesses higher theoretical speci- fic capacity and better structural stability during Li- binding processes.
4. P4QS is predicted to be slightly superior to P4QO as the results of lower solubility in organic electrolyte.
Supplementary Information (SI)
Cartesian coordinates of the optimized structures, Schemes S1-S2, Figures S1-S2, Table S1 are available atwww.ias.ac.
in/chemsci.
Acknowledgements
This work was supported by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD); Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP, PPZY2015B112);
111 Project, B12015; Graduate Research & Practice Innovation Program of Jiangsu Province (XKYCX18_052, XKYCX19_071).
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