Molecular Electrostatic Potential: A New Tool to Predict the Lithiation

Jun 13, 2018 - This work is pioneering to introduce molecular electrostatic potential (MESP) to investigate the interaction between lithium ions and o...
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Energy Conversion and Storage; Plasmonics and Optoelectronics

Molecular Electrostatic Potential: A New Tool to Predict the Lithiation Process of Organic Battery Materials Luojia Liu, Licheng Miao, Lin Li, Fujun Li, Yong Lu, Zhenfeng Shang, and Jun Chen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01123 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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Molecular Electrostatic Potential: A New Tool to Predict the Lithiation Process of Organic Battery Materials Luojia Liu†, Licheng Miao†, Lin Li, Fujun Li, Yong Lu, Zhenfeng Shang* and Jun Chen* State Key Laboratory of Elemento-Organic Chemistry and Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China Corresponding Author *E-mail: [email protected], [email protected]

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Abstract

This work is pioneering to introduce molecular electrostatic potential (MESP) to investigate the interaction between lithium ions and organic electrode molecules. The electrostatic potential on the van der Waals surface of the electrode molecule is calculated, and then the coordinates and relative values of the local minima of MESP can be correlated to the Li binding sites and sequence on an organic small molecule, respectively. This suggests a gradual lithiation process. Similar calculations are extended to polymers and even organic crystals. The operation process of MESP for these systems is explained in detail. Through providing accurate and visualizable lithium binding sites, MESP can give precise prediction of the lithiated structures and reaction mechanism of organic electrode materials. It will become a new theoretical tool for determining the feasibility of organic electrode materials for alkali metal ion batteries.

TOC GRAPHICS

Lithium-ion batteries (LIBs) have played a major role in power supply for portable electronic devices, smart-grid electric energy storage, and electric vehicles.[1-6] Most of them are based on the redox reaction of inorganic compounds containing expensive transition-metal elements,[7-8]

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and high amounts of CO2 are liberated in their production process.[9] Hence, organic electrode materials, due to their low cost, environmental friendliness, lightweight, and abundance in nature, are considered as an alternative to develop eco-friendly and sustainable LIBs.[10-14] Particularly, carbonyl compounds have been extensively studied due to their superior electron transfer and tunable structures and electrochemical properties.[15-19] One of the most important targets of theoretical studies for carbonyl electrode materials is to identify the most favorable Li binding sites and optimize the lithiated structures, based on which accurate theoretical reduction potential and reasonable reaction mechanism of the material can be unraveled through density functional theory (DFT) calculations. The current method to identify the most favorable Li binding sites on organic electrode materials for LIBs is to speculate a number of possible Li binding sites and calculate all corresponding energies of the lithiated structures for comparison. It is time-consuming, and some possible lithiated structures can be easily neglected.[20,21] For example, by speculating five possible active sites, Jang et al. successfully revealed the energetically favored Li ion positions in 2,6-diaminoanthraquinone.[22] Zhang et al. adopted simulation annealing calculation to search for all the possible lithiated structures with lower energies.[23] Therefore, it is urgent to introduce a new method to provide a simple and reliable prediction towards the most stable lithiated structures. Remarkably, the initial interaction between lithium ions and organic molecules in LIBs can be described as electrostatic interaction. Molecular electrostatic potential (MESP), which plays a key role in identifying the electrostatic interaction between molecules, will be useful in unraveling Li binding sequence of organic molecules. MESP, proposed by Eolo Scrocco and Jacopo Tomasi in 1973, can successfully describe the charge distribution around a molecule generated by its nuclei and electrons (see Supporting

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Information for mathematical definition).[24,25] As a critical concept in wavefunction analysis, MESP has been applied to predict nucleophilic and electrophilic reaction sites, suggesting molecular packing modes and adsorption manners of molecules.[26-29] More other applications are still being developed.[8-11] Suresh et al. used MESP to quantitatively examine the substituent effect in π-conjugated molecular systems.[30] Reduction potential of hydrogen evolution catalyst can also be predicted by MESP.[31] Desper et al. emphasized MESP as a way of evaluating selectivity of hydrogen bonding as well as halogen-bond interactions in solid-state architectures.[32,33] To the best of our knowledge, no applications of MESP in prediction of lithiation of organic materials for battery reactions have been reported. The MESP values on an organic electrode molecule’s van der Waals surface can be visualized as a red and blue surface surrounding the molecule, where the positive MESP values are red and the negative ones are blue. Notably, Li atom’s MESP is positive everywhere on its van der Waals surface (the MESP values of all the points on the surface are uniformly 11.3 kcal·mol-1). Thus, it is highly favorable for Li atoms to be attracted by the areas with negative MESP values. In other words, the sites with the most negative MESP value in the blue area, denoted as surface minima, are the most possible sites for Li attraction. This criterion is applied in all the calculations in this study. The aim of this work is to introduce and demonstrate MESP as a new tool for theoretically studying the lithiation process of organic materials for LIBs. The computed electrode materials include six organic small molecules (1,4-benzoquinone (1,4-BQ), 1,4-phenanthraquinone (1,4PQ), 1,7-anthraquinone (1,7-AQ), 1,7-naphthoquinone (1,7-NQ), 5,7,12,14-pentacenetetrone (PT, C22H10O4), and tetra-(phthalimido)-benzoquinone (TPB, C38H16N4O10)), and an organic polymer (poly(anthraquinonyl sulfide) (PAQS)). By investigating the lithiation processes of

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these molecules, their redox mechanisms and the operation process of MESP method are explained in detail. Especially, we provide a new idea for identifying the lithiated sites on periodic organic electrode systems by this MESP approach, taking an organic crystal, 9,10anthraquinone (9,10-AQ), as example. The MESP method proved to be a reliable quantitative tool for predicting Li binding sites, sequence, lithiated structures and theoretical potential of the organic electrode materials, and will be able to give more insights into the design of electrode materials. Small molecules DFT calculations[34,35] were carried out to characterize the structural properties of Li atoms and six organic small molecules, 1,4-BQ, 1,4-PQ, 1,7-AQ, 1,7-NQ, and PT, using Gaussian 09 program package[36] at the B3LYP level of theory[37,38] (see Supporting Information for calculation details). For each small organic molecule shown in Figure 1a, we carried out the following procedures: 1. The MESP plots and the surface minima of the molecules were calculated based on the optimized structures, as shown in Figure 1b. They are depicted as blue points in Figure 1b. 2. When two surface minima with the same MESP value appeared very close to each other, e.g., within 1.0 Å, the Li binding site was determined to be the midpoint of the two minima as rationalized in section 2.1, Supporting Information. This might be due to the nonuniform electron distribution around oxygen atoms. The simplified MESP plots are denoted as MESP-mid, see Figure 1c. 3. The initial guess of the fully lithiated structures of each molecule was performed by placing one Li atom at each surface minimum in the MESP-mid plots.

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Figure 1. a) Optimized structures, b) MESP, c) MESP-mid plots and d) Most stable lithiated configurations of 1,4-BQ, 1,4-PQ, 1,7-AQ, 1,7-NQ, and PT. The atoms in red, pink, grey, and white denote oxygen, Li, carbon, and hydrogen, respectively. The blue numbers in b) are the MESP values of the surface minima, in the unit of kcal·mol-1, similarly hereinafter.

Three general observations are drawn from the results of these procedures. First of all, binding Li ions at the surface minima sites of MESP-mid can provide reliable initial structures for the lithiated molecules. Comparing Figure 1c and 1d, the coordinates of the optimized Li binding sites were in good accordance with those of the surface minima in the MESP-mid plots, illustrating the rationality of regarding the surface minima as initial Li binding sites. This not only saves computation time in screening the most stable configuration, but also has more profound theoretical foundation compared to the trial-and-error speculations. Secondly, when there are more than one surface minima in a small organic molecule with the same MESP value,

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multiple Li atoms are bound to all these surface minima at the same time. The surface minima of 1,4-BQ and PT were identical in number, respectively. We calculated the energy profiles of Li ions binding with 1,4-BQ and PT through a) a simultaneous mechanism, for which all Li atoms are bound to the molecule at the same time; and b) a stepwise mechanism, for which Li atoms are gradually bound to the molecule. The results were presented in Figure S2, Supporting Information, which demonstrated that more stable lithiated structures were obtained from the simultaneous mechanism. Thirdly, when the surface minima of a molecule possess different MESP values, Li atoms are bound to the surface minima through a gradual lithiation process. Apparently, a surface minimum with more negative value reveals a more negative electrostatic potential of the area around it, thus possesses the largest attractive force towards Li ions. To elucidate this point, we take three asymmetrical molecules with unequal surface minima, 1,4-PQ, 1,7-AQ, and 1,7-NQ, as examples to calculate the Li-bounded structures’ single point energies when Li atoms were bound at different surface minima. The results showed that Li atoms tend to first bind at the surface minima with lower MESP values, resulting in lower energies of the lithiated structures (for more detailed discussion, see section 2.2 in Supporting Information), and then bind at the surface minima with higher MESP values, leading to a gradual lithiation process (see Figure S3 in Supporting Information). These three observations verify the feasibility of MESP to be a convenient, accurate, and well-founded means to predict the lithiation mechanism of small organic molecules. We further tested the three observations by applying MESP to a more complicated molecule, namely, tetra-(phthalimido)-benzoquinone (TPB, C38H16N4O10). The TPB molecule, as a newly reported electrode material for organic LIBs, exhibited an initial discharge capacity of 223.2 mAh g-1, corresponding to a 6-electron reaction.[39] Traditional speculation method assigns one

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Li atom to each carbonyl group, declaring a 10-electron transfer in a full discharging process, which fails to describe the actual Li binding mechanism for TPB. By applying MESP, 8 surface minima were found on the TPB molecule, of which minimum 1 and 2, and 3 and 4 were very close to each other, thus these four surface minima were simplified into two minima midpoints 1* and 2*(according to step 2 rationalized above), respectively, corresponding with 6 Li binding sites (Figure 2a). The 6 surface minima had two distinct MESP values. Based on the observation drawn from 1,4-BQ and PT, surface minima possessing the same MESP values accept Li ions at the same time; while based on 1,4-PQ, 1,7-AQ, and 1,7-NQ, surface minima with different MESP values gradually accept Li ions. Thus, there exist two successive steps of Li binding in TPB. Specifically, two Li ions are firstly bound to the minimum 1* and minimum 2* of TPB, which have lower MESP values, displaying the first discharging plateau; then, another four Li ions are bound to the minimum 3*, 4*, 5*, and 6*, which had relatively higher MESP values, presenting the second discharging plateau. The calculated structures of Li2TPB and Li6TPB in the two-step discharging process are shown in Figure 2b. The discharging plateaus calculated from this mechanism agrees well with the experiments.[39]

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Figure 2. (a) MESP plots for TPB molecule. (b) Optimized configurations of TPB, Li2TPB, and Li6TPB.

Polymers Organic polymers are considered as a promising alternative to small organic molecules as electrode materials for LIBs, due to the suppressed solubility in electrolytes relative to small organic molecules. As an example, anthraquinone is linked by alternating -S- to form 1,5poly(anthraquinonyl sulfide) (PAQS) to prevent its dissolution in electrolytes, and the resultant PAQS exhibits outstanding electrochemical performance, a capacity of over 200 mAh·g-1 and good cycle stability.[40] The reaction mechanism of PAQS in the charging/discharging process is further investigated by the MESP to provide profound understanding.

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Figure 3. (a) MESP plot and optimized structure of each PAQS oligomer. (b) Linear regression of the voltage profile of PAQS.

The optimized structure of PAQS was simulated by a series of chain-like oligomers containing 2, 3, 4, and 5 units of anthraquinonyl sulfide (see calculation details in Supporting Information). In regard of the synthesis process of PAQS, the two ends of the chain molecules were saturated by chloride atoms.[40] The MESP plots for these oligomers were calculated and displayed in

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Figure 3a, in which the surface minima are located in the regions around oxygen and sulfur atoms. Similar to small molecules, the lithiated structures of the oligomers were optimized with Li atoms bound to the surface minima as the initial structures. Figure 3a depicts the as-obtained lithiated structures. More initial structures (see Figure S4 in Supporting Information) generated from speculations of Li bonding sites were also optimized. However, no configurations exhibit lower energy than those predicted by the MESP, and the calculation of the speculated structures encountered great difficulties in convergence. Figure 3b describes the relationship between the calculated discharging potential (E) versus Li/Li+ of the oligomers and 1/n, where n is the degree of polymerization of the oligomer (n≥2). Linear regression was performed to predict the theoretical redox potential of the polymer, as rationalized by Ahuja et al.[41] By ordinary least square (OLS) method, the linear correlation between the two variables was estimated to be E = 0.311(1/n) + 2.82, with a square of linearly dependent coefficient (R2) as high as 0.994. This monotonically decreasing linear relationship between E and 1/n reveals the theoretical redox potential of PAQS to be 2.66-2.82 V, in accordance with the experimental value of 2.50 V[40] within a reasonable error.[22] These results demonstrate that the MESP is applicable for the investigation of electrochemical properties of organic polymers by identifying and optimizing the structures of the lithiated oligomers.

Crystals Although small organic molecules and organic polymers are often theoretically studied to understand the electrochemical process, in some cases, the organic electrode materials are in the form of organic crystals. Alternatively, they display a completely different Li storage mechanism, for which the theoretical calculation approaches are insufficient.[20,21,42-44] Thus, application of

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the MESP to the crystalline organic molecules is very necessary. Here we take 9,10anthraquinone (AQ) crystal as an example. The dispersion-corrected density functional theory (DFT-D2)[45] calculations of crystalline AQ were performed using Vienna ab-initio simulation package (VASP).[46,47] Since there is no suitable software to calculate MESP in a crystal system with a periodic boundary condition, a cluster of 23 molecules, in which one AQ unit cell (two AQ molecules) was surrounded by 21 AQ molecules, was directly extracted from the optimized AQ crystal structure to simulate the periodic AQ crystal (see Figure 4a). In particular, in order to obtain the wave function required by the MESP, the single point energy of the cluster was calculated (see calculation details in Supporting Information).[48] Figure 5a shows that there are 4 MESP surface minima around the two center AQ molecules which simulated an AQ unit cell, and minimum 1 is the most possible position to intercalate the first Li atom, of which the MESP value is the lowest. The structure of Li0.5AQ with one Li atom intercalated at minimum 1 was optimized and referred to as structure E1. After optimization, the position of the first Li atom was found between two layers of AQ molecules, located in the middle of two carbonyls which belonged to two adjacent molecules of the AQ crystal (see Figure 5a). In order to verify that this site was the global minimum for the first Li intercalation, we optimized three other possible structures E2, E3, and E4 with Li intercalated at minimum 2, 3, and 4, respectively (see Figure S5a in Supporting Information). By comparing the energies of these structures, the structure E1 was found to be the most stable (see Table S1 in Supporting Information), with a theoretical redox potential of 1.96 V. Meanwhile, to rationally confirm the structure of Li0.5AQ, three other computational methods were also adopted including direct guessing Li intercalation site, molecular dynamics, and simulated annealing. Similar structures (structure Direct, MD, and SA) were obtained but their energies were higher than that using MESP (see Figure S5a in

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Supporting Information). This phenomenon confirmed the rationality of MESP to predict intercalation sites of Li atoms in a crystal. The reasonable initial structure provided by MESP shows that its computational time for optimization is the shortest among the four applied methods (as shown in Table S2).

Figure 4. Calculated MESP plots of the 23-molecule clusters of (a) AQ, (b) Li0.5AQ, and (c) Li1.0AQ. The extreme values of the surface minima were presented below the plots, and the unit is kJ·mol-1. The atoms in red, pink, grey, and white denote oxygen, Li, carbon, and hydrogen, respectively.

Accordingly, the MESP of Li0.5AQ was calculated, as shown in Figure 4b. The three surface minima sites were away from the existing lithium ion, as a result of the positive MESP area generated by the first inserted lithium ion, which reasonably reflected the repulsion between lithium ions. Figure 4b shows that minimum 1 had an obviously more negative MESP value than minimum 2 and 3, so the position of the second Li intercalation was confirmed at minimum 1. After optimization, the lithiated structure was denoted as structure E1, in which the distance between the two inserted lithium ions was 7.04 Å, confirming insignificant lithium repulsion. The energies of two other optimized structures of Li1.0AQ with Li intercalating at minimum 2

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and 3 were calculated to be higher than that of structure E1, as shown in Figure S5b in Supporting Information. This lithiation process showed a redox potential of 1.93 V. Based on structure E1, two surface minima with very close MESP values (within 1.6 kcal·mol-1 difference) were found in Li1.0AQ (see Figure 4c). We optimized the corresponding lithiated structure (Li1.5AQ) E1 and E2 with Li intercalated at minimum 1 and 2, respectively (see Figure S5c in Supporting Information), and find their energies to be exactly identical. Thus, it’s reasonable to regard these two surface minima as equivalent intercalation sites. The slight difference between the two surface minima of Li1.0AQ may be attributed to that the organic crystal cell was simulated by a 23-molecule cluster. According to the observation that Li atoms are inserted into the equivalent surface minima at the same time, the structure of Li2.0AQ was obtained by inserting two Li simultaneously into Li1.0AQ. The theoretical redox potential for this process was calculated to be 1.94 V.

Figure 5. (a)-(c) Optimized structures of LixAQ crystals. (d) Calculated voltage profile in the lithiation process.

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Our calculations indicated a three-step discharging profile for AQ crystal material, in which the discharging voltage remained basically unchanged from 1.96 V to 1.93 and 1.94 V, corresponding to the flat plateau at 2.27 V in experiment. During lithiation, the space group of AQ crystal is changed from P21/C to P1, indicative of phase transition. The computational and experimental X-ray diffraction (XRD) patterns of pristine AQ and Li2.0AQ were compared in Figure S6. Up to now, we have provided a new methodology to calculate MESP for periodic organic electrode systems, paving a brand new way to determine the most stable lithiated structure of organic crystalline materials. For comparison, the theoretical and experimental voltage values for AQ crystal, along with all the small organic molecules studied and PAQS polymer were listed in Table 1 (for calculation details, see Supporting Information). Note that crystallization-related and electrode polarization effects were not included in our computations, which will introduce a common deviation between computational and experimental voltages.49 In addition, the solvation effect is also a primary factor contributing to the charge/discharge voltages.22 In the study, 1,2-dimethoxyethane (DME, dielectric constant 7.2) was used as solvent for all the organic small molecules and polymers, but in the experimental studies in Table 1 different solvents with different dielectric constants were applied. This would give rise to the slight inconsistency between the experimental and calculated voltages. The high consistency between theoretical and experimental voltages indicates the accuracy of both the lithiation mechanisms as well as the computational methods in this work.

Table 1. Theoretical voltages for the organic compounds studied. Redox potential (V vs Li/Li+) System

Theo.

Expt.

Deviation

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1,4-benzoquinone

2.71

2.5411

1,7-naphthoquinone

2.98

\

1,4-phenanthraquinone

2.56

\

1,7-anthraquinones

3.01

\

5,7,12,14-pentacenetetrone

2.14

2.1050

0.04

tetra-(phthalimido)-benzoquinone

3.63 & 2.28

3.05 & 2.0239

0.58 & 0.26

poly(anthraquinonyl sulfide)

2.66-2.82

2.5040

0.16-0.32

9,10-anthraquinone (crystal)

1.94

2.2751

-0.33

0.17

In summary, we introduced MESP to systematically characterize organic electrode materials, including six small organic molecules, one organic polymer, and one organic crystal. The entire procedure of MESP method was also explained in detail for organic LIBs. Instead of trial-anderror speculation, MESP gives visualizable and quantitative prediction towards the Li binding sites and sequence of these materials, leading to reliable lithiated structures and theoretical potentials. This MESP method proves to be a well-founded, accurate, and convenient theoretical method for mechanism clarification of organic electrode materials for lithium ion batteries, and may still be extended to other energy storage systems.

ASSOCIATED CONTENT The supporting information contains detailed computational methods and models, as well as thorough examination of the lithiation mechanism of small organic molecules, polymers and crystal electrode that are not included in the main text. AUTHOR INFORMATION Notes

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[†] These authors contributed equally to this work. The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by Projects of National Nano-Key (2017YFA0206700 and 2016YFA0202500), NSFC (21673243), MOE 111 (B12015), Tianjin Key (16PTSYJC00030) and the Fundamental Research Funds for the Central Universities. The work was carried out at National Supercomputer Center in Tianjin with the calculations on TianHe-1A. REFERENCES (1) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652-657. (2) Lin, F.; Markus, I. M.; Nordlund, D.; Weng, T.-C.; Asta, M. D.; Xin, H. L.; Doeff, M. M. Surface Reconstruction and Chemical Evolution of Stoichiometric Layered Cathode Materials for Lithium-ion Batteries. Nat. Commun. 2014, 5, 3529. (3) Gauthier, M.; Carney, T. J.; Grimaud, A.; Giordano, L.; Pour, N.; Chang, H.-H.; Fenning, D. P.; Lux, S. F.; Paschos, O.; Bauer, C.; et al. Electrode-Electrolyte Interface in Li-Ion Batteries: Current Understanding and New Insights. J. Phys. Chem. Lett. 2015, 6, 4653-4672. (4) Markus, I. M.; Lin, F.; Kam, K. C.; Asta, M.; Doeff, M. M. Computational and Experimental Investigation of Ti Substitution in Li1(NixMnxCo1-2x-yTiy)O2 for Lithium Ion Batteries. J. Phys. Chem. Lett. 2014, 5, 3649-3655. (5) Lee, S. W.; Gallant, B. M.; Byon, H. R.; Hammond, P. T.; Shao-Horn, Y. Nanostructured Carbon-based Electrodes: Bridging the Gap between Thin-film Lithium-ion Batteries and Electrochemical Capacitors. Energy Environ. Sci. 2011, 4, 1972-1985. (6) Wang, D.; Yu, Y.; He, H.; Wang, J.; Zhou, W.; Abruna, H. D. Template-Free Synthesis of Hollow-Structured Co3O4 Nanoparticles as High-Performance Anodes for Lithium-Ion Batteries. Acs Nano 2015, 9, 1775-1781.

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Figure 1. a) Optimized structures, b) MESP, c) MESP-mid plots and d) Most stable lithiated configurations of 1,4-BQ, 1,4-PQ, 1,7-AQ, 1,7-NQ, and PT. The atoms in red, pink, grey, and white denote oxygen, Li, carbon, and hydrogen, respectively. The blue numbers in b) are the MESP values of the surface minima, in the unit of kcal∙mol-1, similarly hereinafter. 85x45mm (300 x 300 DPI)

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Figure 2. (a) MESP plots for TPB molecule. (b) Optimized configurations of TPB, Li2TPB, and Li6TPB. 69x32mm (300 x 300 DPI)

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Figure 3. (a) MESP plot and optimized structure of each PAQS oligomer. (b) Linear regression of the voltage profile of PAQS. 80x118mm (300 x 300 DPI)

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Figure 4. Calculated MESP plots of the 23-molecule clusters of (a) AQ, (b) Li0.5AQ, and (c) Li1.0AQ. The extreme values of the surface minima were presented below the plots, and the unit is kJ∙mol-1. The atoms in red, pink, grey, and white denote oxygen, Li, carbon, and hydrogen, respectively. 61x18mm (300 x 300 DPI)

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Figure 5. (a)-(c) Optimized structures of LixAQ crystals. (d) Calculated voltage profile in the lithiation process. 55x40mm (300 x 300 DPI)

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TOC 49x49mm (300 x 300 DPI)

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