Li-Binding Thermodynamics and Redox Properties of BNOPS-Based

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Li-Binding Thermodynamics and Redox Properties of BNOPSBased Organic Compounds for Cathodes in Lithium-Ion Batteries Dae Kyeum Lee, Chae Young Go, and Ki Chul Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09947 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 8, 2019

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ACS Applied Materials & Interfaces

Li-Binding Thermodynamics and Redox Properties of BNOPSBased Organic Compounds for Cathodes in Lithium-Ion Batteries Dae Kyeum Lee, Chae Young Go and Ki Chul Kim* Computational Materials Design Laboratory, Division of Chemical Engineering, Konkuk University, Seoul 05029, The Republic of Korea Keywords: lithium-ion battery; cathode; borole; redox potential; charge capacity; energy density; electron affinity; solvation energy

ABSTRACT: Cyclic organic compounds with pentagon-rings have been paid less attention for cathodes in lithium-ion batteries as compared with aromatic compounds. In this study, we investigate the Li-binding thermodynamics, redox property, and theoretical performance for a selected set of hetero-atom-containing, pentagon-shaped, organic compounds, namely borole, pyrrole, furan, phosphole, thiophene, and their derivatives to assess their potential for organic cathode materials. This investigation provides us with three important findings. First, the Libinding thermodynamics and redox properties for the organic compounds would be systematically tailored by the type of the incorporated hetero-atom and backbone length, exhibiting both the strongest Li-binding and the highest redox potential for borole. Second, it is highlighted that the borole can store up to two Li atoms per molecule exhibiting the exceptionally high charge capacity (839 mAh/g) despite the absence of any well-known redox-active moieties (e.g., carbonyl). Third, dibenzothiophene exhibits weak and comparable Li-binding strengths at multiple feasible binding configurations with an indication of its low Li-storage capability, while the others dominantly bind with Li at their most stable binding configurations. All these findings will provide insight on guidelines for the systematic design of promising hetero-cyclic organic compounds (i.e., borole-based, insoluble polymeric forms) for cathodes in secondary batteries.

improved the charge capacity. They also employed a combined approach of experimental and computational methods to investigate the potential of carbon nanotubes spontaneously coated by self-polymerized dopamines as organic cathode materials.9 They verified that the redox-active carbonyl groups in the dopamines could be the main resource to consistently sustain their redoxactivity at high cell voltages during the discharging process. Kim et al. explored the redox properties for a given set of quinone derivatives.10 They found that the redox properties and theoretical performance of the organic molecules could be tuned by the structural and electronic variations. Song et al. synthesized anthraquinone polymers, namely poly(1,4-anthraquinone) (P14AQ) and poly(1,5-anthraquinone) (P15AQ) for organic cathodes.13 They reported that both the polymers displayed high charge capacities of ~263 mAh/g with the high capacity retention arising from the insoluble polymeric characteristics.

1. Introduction Nowadays, lithium-ion batteries are widely utilized as electrochemical storage devices for electric vehicles owing to their high charge capacities and energy densities.1 However, several challenges, including the poor power density arising from the slow ionic diffusivity in highly-packed inorganic cathode materials as well as the high cost of the inorganic cathode materials, impede their explosive utilization.2-4 Therefore, cost-effective organic materials with the structural flexibility, which would facilitate the ionic diffusion in the materials, have attracted as alternative candidates to replace the inorganic cathode materials.5-16 Gall et al. studied the specific charge capacity and cyclic stability of poly(2,5dihydroxy-1,4-benzoquinone-3,6-methylene) as an organic cathode material in lithium-ion batteries.5 They reported that the polymerized cathode material achieved moderately good performance. Lee et al. developed layer-by-layer assembled carbon nanotubes as nanostructured cathode materials.6-7 They highlighted that this approach could provide the binder-free redoxactive organic materials with high cell performance. Liu et al. employed a hydrothermal reduction method to develop closepacked reduced graphene oxide materials for cathodes in lithiumion batteries.8 They demonstrated that the reduction in the density of redox-inactive hydroxyl functional groups on graphene layers

Organic cathode materials usually experience the electrochemical reduction followed by the accommodation of Li cations with the materials during the discharging process. This makes their redox properties strongly rely on the interactions between the organic materials and Li. Many studies have therefore focused on investigating the Li binding properties of organic materials.9-10, 17-18 Liu et al. verified that carbonyl-containing

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dopamine molecules could bind strongly with Li through the electrostatic interaction between the carbonyl and Li.9 Kim et al. investigated the Li binding properties for a series of quinone derivatives and highlighted that carbonyl moiety could be the main redox-active site for the Li binding.10 Reddy et al. studied Li binding mechanisms in purpurin-based cathode materials during the discharging process.18 They demonstrated that the binding of Li to purpurin would be formed by the shared binding of carbonyl and neighboring hydroxyl group with Li. However, despite such significant efforts, only a limited number of organic compounds have been explored and most studies have primarily focused on materials with redox-active carbonyl moiety. Comprehensive understanding of the Li-binding thermodynamics and electrochemical properties for a broad array of organic compounds is urgently needed to establish guidelines on the systematic design of promising organic cathode materials with structural diversity.

Here, 𝑬𝒄𝒐𝒎𝒑𝒍𝒆𝒙, 𝑬𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒆, and 𝑬𝑳𝒊 represent the MP2-calculated energies for a complex of compound and Li, compound, and Li, respectively.

In efforts to search for other promising redox-active moieties in addition to the well-studied, redox-active, oxygen-containing moieties, organosulfur compounds containing sulfuric moiety, such as thiophene, might be a chemically intuitive family for cathodes in lithium-ion batteries, owing to the equality in the oxidation number between oxygen and sulfur atoms. However, unlike redox-active oxygen-containing compounds, the characteristics of organosulfur compounds has not been fully explored and thus their potential for cathodes is still veiled. Likewise, deep understanding on the redox properties of boron and nitrogen in organic materials is still lacking despite special attention to boron- and nitrogen-doped organic materials.

𝑷𝒊 = ∑ 𝑭 × 𝟏𝟎𝟎 𝒊 𝒊

We employed the Boltzmann factor to compute the relative thermodynamic stability for multiple Li-binding configurations involved in an organic compound. The Boltzmann factor (𝑭𝒊) for a Li-binding geometry, i, of a given compound is computed by 𝑬𝒊

𝑭𝒊 = 𝒆

(2)

𝑭𝒊

(3)

where 𝑷𝒊 is the probability of a Li-binding geometry, i, and ∑𝒊𝑭𝒊 is the summation of the Boltzmann factors for all the Li-binding geometries. The geometrically optimized structures for the organic compounds were prepared to examine their redox potentials (∆𝑬𝒓𝒆𝒅) which would be defined by ∆𝑬𝒓𝒆𝒅 =

―∆𝑮𝒓𝒆𝒅(𝑹, 𝒔𝒐𝒍𝒏) 𝒏𝑭

―𝟏.𝟑𝟗 𝑽

.

(4)

Here, ∆𝑮𝒓𝒆𝒅(𝑹, 𝒔𝒐𝒍𝒏) is the change in the Gibbs free energy in solution during the reduction of a species, R, at 298.15 K, n is the number of electrons transferred during the reduction, and F is Faraday constant. The constant, 1.39 V, is introduced to convert the redox potential into the Li/Li+ reference electrode. We employed an approach developed by Truhlar and coworkers to evaluate the change in the Gibbs free energy in solution during the reduction.22-23 Through their thermodynamic cycle, the change in the Gibbs free energy in solution during the reduction of the species, R, (∆𝑮𝒓𝒆𝒅(𝑹, 𝒔𝒐𝒍𝒏)) would be calculated by ∆𝑮𝒓𝒆𝒅(𝑹, 𝒔𝒐𝒍𝒏) = ∆𝑮𝒓𝒆𝒅(𝑹, 𝒈𝒂𝒔) + ∆𝑮𝒔𝒐𝒍𝒗(𝑹 ― ) ― ∆𝑮𝒔𝒐𝒍𝒗(𝑹) (5) where ∆𝑮𝒓𝒆𝒅(𝑹, 𝒈𝒂𝒔) is the change in the Gibbs free energy in gas phase during the reduction, ∆𝑮𝒔𝒐𝒍𝒗(𝑹 ― ) is the solvation free energy for the species in the anionic state, and ∆𝑮𝒔𝒐𝒍𝒗(𝑹) is the solvation free energy for the species in the neutral state. The solvation free energies were calculated using Poisson-Boltzmann implicit solvation model to consider the contribution of the solvation to the free energy.24-25 A dielectric constant of 16.14, which would reliably describe the polarity of the solvents in mixture (ethylene carbonate and dimethyl carbonate (3:7 v/v)), was used to perform the solvation free energy calculations.

2. Computational Methods The interactions between Li and organic compounds were investigated using well-designed cluster models with the aim of calculating the binding energies of Li to the organic compounds. All atoms in the clusters were allowed to relax to find minimum energy configurations. The geometry optimizations were performed at the MP2 level of theory using the GAUSSIAN 09 software package.19 In the calculations, a 6-31+G(d,p) basis set was used for all the atoms. The basis set superposition errors were corrected in the binding energy calculations using the counterpoise method.20-21 The binding energy (𝑬𝒃) of Li to an organic compound can be defined by .

𝑩𝑻

where, 𝑬𝒊 is the MP2-calculated energy of the Li-binding geometry, 𝒌𝑩 is the Boltzmann constant, and T is 298.15 K. Based on the calculated Boltzmann factors for all the Li-binding geometries of the given compound, their thermodynamic probabilities in the unit of percentage could be calculated using

In this study, we employ an advanced computational modeling approach to investigate the Li-binding thermodynamics and redox properties for a selected set of boron-, nitrogen-, oxygen-, phosphorus-, and sulfur-containing organic compounds with the aim of assessing the potential of the compounds for organic cathodes in lithium-ion batteries. The designed compounds are varied with the identity of a chosen hetero-atom as well as the backbone length. Their Li-binding thermodynamics and redox properties are discussed in two aspects: (1) the change in the identity of the hetero-atom (boron vs. nitrogen vs. oxygen vs. phosphorus vs. sulfur) and (2) the change in the number of the aromatic rings in the backbone. The theoretical performance parameter is further discussed for borole with the highest redox potential to assess its potential for organic cathodes in lithium-ion batteries.

𝑬𝒃 = 𝑬𝒄𝒐𝒎𝒑𝒍𝒆𝒙 ― (𝑬𝒎𝒐𝒍𝒆𝒄𝒖𝒍𝒆 + 𝑬𝑳𝒊)

―𝒌

The theoretical charge capacities and energy densities for a selected set of organic molecules were computed based on the number of stored Li atoms and described as a function of the (calculated) redox potential. The theoretical charge capacity (𝑸) of a molecule was calculated by 𝒏𝑭

𝑸 (𝒎𝑨𝒉/𝒈) = 𝟑.𝟔𝑴

𝒘

.

(6)

Here, Mw depicts the molecular weight of the molecule and n describes the number of stored electrons which would rely on the

(1)


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ACS Applied Materials & Interfaces cyclic backbone, depending on the electronic structures illustrated in Figure 3. In details, three furan derivatives show strong interactions between the redox-active oxygen and Li, while the other organic compounds have Li atoms stably positioned on top of either pentagon or hexagon rings. The unique Li-binding behaviors of furan derivatives can be rationalized by their electronic structures in Figure 3. The electrostatic potentials mapped through the organic compounds in Figure 3 can be interpreted by one condition: More negative electrostatic potential indicates higher electron density showing higher Li-binding preference. Notably, electrons in the furan derivatives are highly localized to have redoxactive hetero-atoms (i.e., oxygen) with high electron densities, leading to the presence of Li bound to the oxygen atoms. In contrast, the other organic compounds generally have electrons delocalized over the structures, resulting in the presence of Li stably positioned on top of cyclic rings. All these highlight the electronwithdrawing nature of oxygen superior to the other hetero-atoms.

(calculated) redox potential. The theoretical energy density (𝑾) of the molecule was computed by 𝑸

𝑾 (𝒎𝑾𝒉/𝒈) = ∫𝟎 𝑽(𝒒)𝒅𝒒

.

(7)

Here, 𝑽 is the (calculated) redox potential with respect to Li electrode. 3. Results and Discussion As stated earlier, organic cathode materials undergo the Li-involved redox reactions during the discharging and recharging processes. This indicates that the redox properties of the organic materials would vary with the interactions between organic materials and Li. For instance, Kim et al. studied the redox properties of quinone derivatives with different numbers of bound Li atoms.10 They verified that 2,6-diaminoanthraquinone and anthraquinone-2,6dicarboxylic acid would have the redox potentials ranged within 0.8 ~ 3.5 V vs. Li/Li+ and 1.6 ~ 2.2 V vs. Li/Li+, respectively, depending on the Li-binding geometries. Understanding the Libinding thermodynamics of organic compounds and their geometries is therefore essential to assess their potential for organic cathodes. In subsequent sections, the designed structural models are used to study two primary properties, namely (1) Li-binding thermodynamics and (2) redox properties (including theoretical performance).

Three borole derivatives show a distinctive feature in their electronic distributions: Electrons are uniformly distributed and highly dense over the whole structures except boron. This indicates the electronic transfers from boron to cyclic rings. As shown in Figure 2, this could assist Li to strongly bind to the cyclic rings in the borole derivatives, exhibiting the most negative Li-binding energies (-214.86, -167.90, and -98.76 kJ/mol for borole, 1benzoborole, dibenzoborole, respectively) as compared with their analogues.

The core parameters employed in this study are the Li-binding energy and redox potential calculated by the computational modeling approach. Previous studies have verified that the computational approach at the MP2 level of theory would provide reliable predictions on the binding properties of organic compounds.26-27 Likewise, the redox potentials of various organic compounds have been accurately predicted by the computational modeling approach, exhibiting an uncertainty of ~0.3 V as compared with electrochemical measurements, indicating the reliability of our computational protocol.9-10, 28 Our preliminary calculations at the MP2 level of theory also provide highly accurate redox potentials for six quinones as listed in Table 1. Hence, these suggest that our robust computational protocol would enable us to draw meaningful conclusions from a comprehensive discussion of the calculated Li-binding thermodynamics and redox properties for targeted organic compounds.

From further analysis of the Li-binding thermodynamics, the order in the Li-binding strength for the five families is found to be borole > phosphole > pyrrole > furan > thiophene. A couple of notable features are also observed from this analysis. First, organic compounds containing a hetero-atom with the oxidation number of 3, namely boron, nitrogen, and phosphorus, show stronger binding with Li rather than those containing a hetero-atom with the oxidation number of 2, namely oxygen and sulfur. This observation is somewhat surprising because this does not follow a general consensus: “Oxygen and sulfur with more electronegative characteristics would be more beneficial to the Li-binding thermodynamics than boron, nitrogen, and phosphorus”. This indicates that the Li-binding thermodynamics would be affected by the local coordination geometry (e.g., carbonyl vs. epoxide for oxygen) of the hetero-atom as well as its electronic characteristics. Second, for boron-, nitrogen-, and phosphorus-containing compounds, the Li-binding strength is generally weakened leading to the reduction in the Li-storage ability as increasing the number of the aromatic rings in the backbone (see Figure 4a). Note that the change in the Li-binding strength with the number of the aromatic rings in the backbone for oxygen- and sulfur-containing organic compounds is negligible, exhibiting the Li-binding energies ranged within -14.41 ~ -13.75 kJ/mol and -5.93 ~ -3.39 kJ/mol, respectively.

3.1 Li-Binding Thermodynamics We selected a series of organic compounds, namely borole, 1benzoborole, dibenzoborole, pyrrole, 1-benzopyrrole, dibenzopyrrole, furan, 1-benzofuran, dibenzofuran, phosphole, 1benzophosphole, dibenzophosphole, thiophene, 1-benzothiophene, and dibenzothiophene (see Figure 1). They can be categorized into five families, each of which contains either borole, pyrrole, furan, phosphole, or thiophene as the simplest compound. For each family, either one or two aromatic ring(s) would be added into the simplest compound to generate benzo- or dibenzo-based compound, respectively.

To further understand the Li-binding thermodynamics for the organic compounds, we studied the total density of states (TDOS) for the organic compounds without and with bound Li atoms, as shown in Figure S3 in Supporting Information. As seen in the figures, the binding of Li to each compound would generate another peak between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels. This would lead to a shift of the HOMO energy level

The Li-binding properties of the fifteen organic compounds are shown in Figure 2 as well as Figures S1 and S2 in Supporting Information. It can be summarized from these figures that Li tends to be bound either near redox-active hetero-atom or on top of



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Note that a computation methodology utilizing Li-binding mechanisms to predict redox potentials has been also reported to study the redox potentials of inorganic materials. However, this method usually requires well-defined Li-binding configurations to reliably predict the redox potentials. In addition, the electrochemical reduction occurs before organic compounds accommodate lithium cations during the discharging process. Thus, the computational protocol estimating the intrinsic electrochemical reduction potentials would be appropriate for organic compounds whose lithiated or crystalline structures are not well defined.

towards a less negative value, making the resultant compound-Li complex less stable than the pristine compound. Simultaneously, the LUMO energy level would be slightly shifted towards a less positive value after binding with Li. These shifts would commonly result in the reduction in the HOMO-LUMO energy gap, leading to the Li-induced increase in the reactivity. For example, after the binding of the borole with Li, its HOMO energy level would change from -8.53 to -7.44 eV while its LUMO energy level would be relatively moderately reduced from 0.58 to -0.01 eV. Another interesting feature to be highlighted is that the HOMO energy levels of the compound-Li complexes are predicted to follow the order: borole < phosphole < pyrrole ≈ furan ≈ thiophene. Considering that lower HOMO energy level would lead to higher thermodynamic stability, this order explains why borole and phosphole show the strongest and second-strongest bindings with Li, respectively.

It is unambiguous from Figure 6 that the redox potential would significantly vary with the type of the incorporated hetero-atom, following the order: borole > phosphole > furan ≈ pyrrole ≈ thiophene. Note that borole and phosphole exhibit positive values in the redox potential but vice versa for furan, pyrrole, and thiophene. Another interesting observation from this figure is that the correlation between the redox potential and the number of the aromatic rings in the backbone would strongly rely on the type of the hetero-atom and the sign of the redox potential for the simplest compound in a given family. Specifically, the redox potentials for borole and phosphole having positive redox potentials would decrease as increasing the number of the aromatic rings, exhibiting the significant changes from 2.20 and 1.36 V vs. Li/Li+ to 1.93 and 0.56 V vs. Li/Li+ for one-aromatic-ring case or 1.68 and 0.46 V vs. Li/Li+ for two-aromatic-ring case, respectively. On the other hand, the redox potentials for furan, pyrrole, and thiophene having negative redox potentials would be improved by the incorporation of aromatic ring(s). Such a variation in the redox potential can be further explained by the intrinsic electronic properties (e.g., electron affinity and solvation energy) of the compounds, as shown in Figure S4 in Supporting Information and previous studies.8-10, 2932 For instance, a compound having more negative value in electron affinity is predicted to show higher redox potential owing to the improved reductive ability. On the other hand, the redox potential seems to be insensitive to solvation energy which describes the solvation free energy change (∆∆𝑮𝒔𝒐𝒍𝒗) of a compound through its state change from the neutral to anionic state which is defined by ∆∆𝑮𝒔𝒐𝒍𝒗 = ∆𝑮𝒔𝒐𝒍𝒗(𝑹 ― ) ― ∆𝑮𝒔𝒐𝒍𝒗(𝑹), where ∆𝑮𝒔𝒐𝒍𝒗(𝑹 ― ) and ∆𝑮𝒔𝒐𝒍𝒗(𝑹) are depicted by the solvation free energies of the anionic and neutral states, respectively.

The Li-binding geometries and thermodynamic energies discussed above correspond to the most stable Li-binding scenarios. However, the diffusion of Li ions in organic cathodes might be limited by many other factors, such as the steric hindrance by neighboring organic compounds, making the most stable Libinding sites unavailable. In this scenario, Li ions would be placed in metastable binding sites, forming metastable Li-binding configurations. It is therefore meaningful to understand the relative thermodynamic stability of metastable Li-binding configurations for each organic compound (see Figures S1 and S2 in Supporting Information). Figure 5 shows the relative thermodynamic stability (based on the Boltzmann factor) of all feasible Li-binding configurations for each of eleven organic compounds having multiple Li-binding geometries. The thermodynamic Li-binding probability for each configuration is further computed on the basis of the relative thermodynamic stability. Six of them, namely 1benzoborole, dibenzoborole, 1-benzophosphole, furan, 1benzofuran, and dibenzofuran, are predicted to dominantly bind with Li at their most stable configurations, exhibiting the Libinding probabilities over 97%, mainly due to the exceptionally strong Li-bindings at their most stable configurations. Four compounds, namely 1-benzopyrrole, dibenzopyrrole, phosphole, and dibenzophosphole, are predicted to bind with Li not only at their most stable configurations with the Li-binding probabilities over 61% but also at their metastable binding sites with appreciable Li-binding probabilities ranged within 0.3 – 39%. It is worthwhile to note that dibenzothiophene has a total of four Li-binding configurations with appreciable probabilities ranged within 6 – 49%, suggesting any of the configurations would not be a primary Libinding configuration. This is primarily owing to the comparable Li-binding energies (ranged within -5.93 ~ -0.86 kJ/mol with a maximum deviation of 5.07 kJ/mol) among the feasible Li-binding configurations.

We extended our study to further assess the theoretical performance, namely Li-storage capability (or charge capacity), for the borole with the highest open-circuit redox potential. The performance parameter can be predicted by investigating the change in the redox potential of the borole as increasing the number of bound Li atoms during the discharging process and identifying the number of Li atoms stored in the borole until the compound is cathodically deactivated exhibiting a negative value in the redox potential (see Figure 7 and Figure S5 in Supporting Information). As expected, the redox potential (2.20 V vs. Li/Li+) of the borole continues to decrease as increasing the number of bound Li atoms, owing to the decrease in the reductive ability of the compound. To be noted, the borole with two bound Li atoms exhibits a negative value in the redox potential, indicating the loss of the cathodic activity and no additional Li-binding to the borole2Li complex (i.e., the Li-storage capability of two Li atoms per molecule). The Li-storage mechanism of the borole is described in Figure S6 in Supporting Information. Considering that the borole

3.2 Redox properties In addition to the afore-mentioned Li-binding thermodynamics, the redox activity would be another critical factor to describe the electrochemical Li-binding behaviors of organic cathode materials during the discharging process. We therefore investigated the redox properties of the fifteen organic compounds through analyzing the change in the Gibbs free energy during the reduction (see Figure 6).


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does not have any well-known redox-active moieties (e.g., carbonyl) in its structure, such a high Li-storage capability is somewhat surprising. This may be due to the absence of unshared electron pairs in boron as compared with the other hetero-atoms. As shown in Figure 7 describing the profile of the charge capacity as a function of the redox potential within the window of 0 – 4.5 V vs. Li/Li+, the first electron is electrochemically stored at the redox potential of 2.20 V vs. Li/Li+, followed by the electrochemical storage of the second electron at the redox potential of 1.72 V vs. Li/Li+. The capability of the borole storing two Li atoms per molecule is equivalent to the charge capacity of 839 mAh/g, which is much higher than the performance for any other typical organic and inorganic cathode materials (e.g., LiCoO2:33 ~160 mAh/g, LiFePO4:34-35 ~170 mAh/g, 1,4-benzoquinone:10 490 mAh/g, 9,10anthraquinone:10 216 mAh/g, reduced graphene oxides:36 ~170 mAh/g).12 Such an exceptionally high performance for borole as compared with other organic compounds, such as quinones, can be explained by two primary factors. First, borole, which contains only light elements (B, C, and H), has a relatively low molecular weight as compared with any other organic compounds introduced in this study as well as qunones. Second, borole has an electrochemical Listorage capability comparable to quinones even though borole has no redox-active carbonyl moiety in its structure. Considering that the specific charge capacity is defined by the electrochemical Listorage capability per unit mass, the afore-mentioned factors would be the main resources of the exceptional performance of borole. Similar to the charge capacity profile, the theoretical energy density profile for borole also reveals an exceptionally high value (1644 mWh/g). In addition to the performance parameters, sustaining the structural integrity of organic cathode materials during the Liinvolved discharging process is another important factor to improve their cyclic stability. It is highlighted from our computational investigation that the organic compounds of our interests would be structurally stable during both the Li-binding and electrochemical reduction events, suggesting their structural integrity (see Figures 2 and 7). All these findings suggest a guideline for the systematic design of high-performance cathode materials with cyclic stability, more in details compounds in borolebased, insoluble polymeric forms (see Figure 7c).

positive redox potentials are predicted to decrease as increasing the number of the aromatic rings, and vice versa for the other three compounds. Another important observation from this study is that most of the organic compounds are predicted to dominantly bind with Li at their most stable binding configurations, except dibenzothiophene which shows weak and comparable Li-binding strengths at multiple feasible binding configurations with an indication of its low Li-storage capability. Further investigation on the theoretical performance for the borole with the highest redox potential suggests that the compound would be able to store up to two Li atoms per molecule, implying the exceptionally high charge capacity of 839 mAh/g. All these findings would shed light on efforts of systematically designing hetero-atom-involved organic cathode materials (i.e., borole-based, insoluble polymeric forms) for high-performance lithium-ion batteries.

4. Conclusions

(1) Zu, C. X.; Li, H. Thermodynamic Analysis on Energy Densities of Batteries. Energy Environ. Sci. 2011, 4, 2614-2624, DOI: https://doi.org/10.1039/c0ee00777c.

ASSOCIATED CONTENT Supporting Information. Li-binding properties, Li-storage mechanisms, and correlations of redox potential with electronic properties and solvation energy. Figures S1–S5.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (K. C. Kim)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2017R1C1B5017482).

REFERENCES

In this study, we investigated the Li-binding characteristics, redox properties, and theoretical performance for boron-, nitrogen-, oxygen-, phosphorus-, and sulfur-containing organic compounds to assess their potential for redox-active cathode materials in lithiumion batteries. The structural models introduced in this study were systematically designed by varying the backbone length as well as the identity of the hetero-atom. An advanced computational modeling approach was employed to reliably study the thermodynamic properties of the organic compounds. It is highlighted that the Li-binding thermodynamics and redox property would be systematically tailored by the type of the incorporated hetero-atom and the backbone length, exhibiting both the strongest Li-binding and highest redox potential for borole. More in details, organic compounds containing a heteroatom with the oxidation number of 3 show stronger binding with Li than those containing a hetero-atom with the oxidation number of 2. In addition, the redox potentials for borole and phosphole having

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Figure 1. Chemical structures of fifteen organic compounds, namely borole, benzoborole, dibenzoborole, pyrrole, benzopyrrole, dibenzopyrrole, furan, benzofuran, dibenzofuran, phosphole, benzophosphole, dibenzophosphole, thiophene, benzothiophene, and dibenzothiophene. The atoms in gray, white, and black depict carbon, hydrogen, and redox-active fragment, respectively.



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Figure 2. The most stable Li-binding geometries and energies (BE) for the fifteen organic compounds, namely (a) borole, (b) pyrrole, (c) furan, (d) phosphole, and (e) thiophene derivatives, described in Figure 1. The atoms in gray, white, magenta, blue, red, orange, yellow, and violet depict carbon, hydrogen, boron, nitrogen, oxygen, phosphorus, sulfur, and lithium, respectively. A full version of all feasible Li-binding geometries and their thermodynamics for the fifteen organic compounds is shown in Figures S1 and S2 in Supporting Information.

Figure 3. The electrostatic potential maps for the organic compounds, namely (a) borole, (b) pyrrole, (c) furan, (d) phosphole, and (e) thiophene derivatives, described in Figure 1. The atoms in gray, white, magenta, blue, red, orange, and yellow depict carbon, hydrogen, boron, nitrogen, oxygen, phosphorus, and sulfur, respectively.

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Figure 4. Correlations between the global-minimum Li-binding energy and the number of the aromatic rings in the backbone for (a) boron-, nitrogen-, phosphorus-, (b) oxygen-, and sulfur-containing molecular families.

Figure 5. The Boltzmann-factor-based Li-binding probabilities for (a) boron-, nitrogen-, phosphorus-, (b) oxygen-, and sulfur-containing molecular families which have multiple numbers of Li-binding configurations. The bars in red, green, blue, and dark yellow represent complex 1 (the most stable Li-binding configuration), complex 2, complex 3, and complex 4, respectively.



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Figure 6. Redox potentials for the fifteen organic compounds described in Figure 1 as a function of the number of the aromatic rings in the backbone.


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Figure 7. (a) The profile of the redox potential for borole as a function of bound (stored) Li atoms and (b) the resultant theoretical charge capacity and energy density profiles as a function of redox potential within the window of 0 – 4.5 V vs. Li/Li+. A suggested design direction (borole-based organic compounds in polymeric forms) for promising organic cathodes is illustrated in (c). The atoms in gray, white, magenta, and violet depict carbon, hydrogen, boron, and lithium, respectively. Table 1. The redox potentials for six quinone derivatives predicted by both the MP2 calculation (this work) and experimental measurement (ref. 10; DOI: 10.1021/jacs.5b13279). Molecule

Redox potential (V vs. Li/Li+) MP2 calculation (this work)

Experiment10

1,4-Benzoquinone

2.9

3.1

1,4-Naphthoquinone

2.6

2.6

9,10-Anthraquinone

2.3

2.2

2-Aminoanthraquinone

2.1

2.1

2,6-Diaminoanthraquinone

1.8

2.0

Anthraquinone-2-carboxilic acid

2.4

2.3



SYNOPSIS TOC


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Li-Binding Thermodynamics and Redox Properties of BNOPS-Based

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