Predicting Structure and Electrochemistry of Dilithium Thiophene-2,5

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C: Energy Conversion and Storage; Energy and Charge Transport

Predicting Structure and Electrochemistry of Dilithium Thiophene-2,5-Dicarboxylate Electrodes by Density Functional Theory and Evolutionary Algorithms Cleber Fabiano N. Marchiori, Daniel Brandell, and C. Moyses Araujo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11341 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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The Journal of Physical Chemistry

Predicting Structure and Electrochemistry of Dilithium Thiophene-2,5Dicarboxylate Electrodes by Density Functional Theory and Evolutionary Algorithms

Cleber F. N. Marchiori1* Daniel Brandell2* and C. Moyses Araujo1* 1 Materials

Theory Division, Department of Physics and Astronomy, Ångström Laboratory, Uppsala University, Box 516, 75120 Uppsala, Sweden. 2Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden.

Abstract Organic electroactive materials are promising candidates to be used as lithium insertion electrodes in the next generation of environmentally friendly battery technologies. In this work, evolutionary algorithms at interplay with density functional theory (DFT) calculations have been employed to predict the crystal structure for both delithiated and lithiated phases of dilithium thiophene dicarboxylate (Li2TDC). Based on the resulting crystals, electronic structure modifications and voltage profiles for the lithiation process have been calculated. The obtained structure for the delithiated phase showed a well-defined salt layer intercalating the organic components, forming a so-called lithium organic framework (LOF). Upon lithiation, new structures appear which deviate from the LOF as a consequence of the reduction of the S atoms, which coordinate the additional Li ions. The calculated average potential of 1.00 V vs. Li/Li+ is found to be in good agreement with experimental findings. An additional study at molecular level has also been conducted aiming at gaining insight into the importance of the crystallographic environment on the structural and thermodynamics properties. This strategy is suitable for an initial assessment of the electrochemical process that underlies the lithiation mechanism of electrode materials. Moreover, the employed evolutionary algorithm emerges as a promising tool to predict crystal structures during lithiation, which are otherwise difficult to resolve experimentally.

Corresponding authors: [email protected], [email protected] and [email protected] 1 ACS Paragon Plus Environment

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Introduction The development of suitable electrical energy storage (EES) systems is a key component for the implementation of a new world-wide energetic matrix that is renewable, secure and environmentally friendly. In this context, lithium-ion batteries (LIBs) has arisen as a promising candidate due to the fact that lithium is the lightest of all metals, displays the greatest electrochemical potential and also provides high gravimetric energy density. The development of rechargeable LIBs has enabled, for instance, the market growth of portable electronic devices such as smartphones and lap-tops. As a consequence, this kind of energy storage devices have been massively produced, in an impressive number of billions of units per year. 1 Up to the present, inorganic materials, mainly transition metal oxides, have been employed as positive electrodes for LIBs, while the negative electrodes are made of graphite. Despite the good performance of such electrodes, these materials have some drawbacks, such as volume expansion during charge/discharge cycling which compromises devices stability. Additionally, these materials are obtained from mining processes that are environmentally harmful and consuming vast amounts of energy and resources, as do the recycling processes at the end of the devices’ lifetime. 2,3 To overcome these shortcomings, organic electroactive materials (OEMs) are currently being considered as possible substitutes thanks to several advantages. If water soluble, OEMs have clear advantages regarding processability and recyclability, and can also from a synthesis viewpoint display a structure versatility to tune specific electrochemical properties. Moreover, some OEMs can be obtained from abundant biomass raw material sources 4–7 or even from waste. 8 These features, along with the fact that organic materials are composed by highly abundant chemical elements, basically C, H, N, O, S, could contribute to low-cost manufacture of devices, making this class of materials a very promising option for sustainable development. Since the first application of organic compounds as electrodes, 9 different kinds of small molecules and conducting polymers (see Ref.10,11 and references therein) have been investigated for applications in lithium-ion batteries. Quinone-based molecules have been extensively investigated as candidates for application as both anodes and cathodes in Li or Na ion batteries.12–16 After the work of Tarascon and co-workers,4 a great deal of attention has turned to organic carboxylates and especially those containing conjugated chains or cyclic functionalities.17–20 For instance, the dilithium therephthalate (Li2TP),4 in this context represents an interesting material because it can be easily obtained by recycling the polyethylene therephthalate plastic (PET), and also displays a useful anode potential of 0.8 V vs Li/Li+ and a reversible capacity of 300 mA h g -1 with decent cycling performance. In both carboxylate and quinone based materials, the C=O (carbonyl) group plays the role of primary redox active center during the discharge/charge process and the conjugated moiety acts as an electron reservoir.10 Here, the first lithiation step leads to the stabilization of the reduced structure by the formation of an enolate 21, while the sp2 carbons can be reduced in a second step, making also the conjugated chains active in the redox process.19,22 In order to maximize specific capacity, it is desirable to have as many redox active centers as possible, reducing any “dead weight” in the molecular structure. In this sense, the insertion of heteroatoms in the conjugated system can be an alternative to improve the electrochemical activity of the organic material. Sulfur, for instance, is an interesting such candidate with well-known redox activity.23–26 Organosulfur compounds can undergo two different redox mechanisms where the S atom can be either oxidized or reduced. 27 In fact, thiophene based compounds have already been reported as redox materials for different applications.28–33 Dilithium-thiophene dicarboxylate (Li2TDC) has previously been reported as a potential anode for LIBs.17 Using Li2TDC as an analogue of Li2TP, Lee and co-workers investigated the 2 ACS Paragon Plus Environment

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influence of the cyclic structure on the capacity of conjugated dicarboxylates. They have found an impressive high discharge capacity of 1143 mA h g-1, indicating that it is prone to undergo so called ‘superlithiation’, where unsaturated C-C bonds are redox-active. 17,19 Despite this intriguing feature, no further explanation on the role of the S heteroatom on the redox process was provided. Despite the great interest in OEMs, it was only recently that theoretical and computational investigations of these started to appear in literature. 34–41 Hernández-Burgos et al. have in this context investigated how insertion of heteroatoms could tune the potential and affect the stability of the material. They found that by changing the heteroatom in a fivemembered ring carbonyl molecule, the potential could be controlled. In other theoretical studies, the evolutionary algorithm has been employed to access the crystal evolution with Li/Na in organic compounds. 42,43 In this study, the authors validate the methodology by studying the Li insertion in the dilithium terephthalate, the archetypal molecule for anode applications and further predicted the effect induced by the insertion of an aliphatic side chain on the lithiation process. The use of computational tools for crystal structure determination could indeed provide a powerful support for experimental findings as well as for structural prediction. This is experimentally challenging, not least during cycling, due to the progressive amorphization often displayed by this class of materials. In this sense, evolutionary algorithmbased codes emerge as a versatile option to access the structural changes upon the redox/metal insertion process. In this study, we report on how the structure of the di-lithium thiophene dicarboxylate (Li2TDC) change during the lithiation process. Using an evolutionary algorithm interplayed with density functional theory (DFT) calculations, we predict the crystal structure for both delithiated and lithiated phases. The predicted structure for the initial (delithiated) phase showed a well-defined salt layer intercalating the organic units, which can be considered a lithium organic framework (LOF). Upon insertion of the reducing equivalents (Li+ + e–), new structures formed which deviated from the LOF as a consequence of the S reduction, which in turn became coordinated to the additional Li atoms. Using the generated structures, the voltage profile was calculated and provided a theoretical average potential of 1.00 V vs. Li/Li+, in good agreement with experimental data.17 Additionally, a similar study at molecular level was conducted aiming at gaining insight on the importance of the crystal environment on the charge distribution and thermodynamics of the lithiation process. We have found that the outcomes from the molecular description mostly follow the trends obtained for the solid-state calculations, indicating that this strategy is suitable for an initial assessment of the underlying mechanism of the lithiation process in OEMs. Computational Method The crystal structures of the three lithiation states (viz. Li2TDC, Li3TDC and Li4TDC) have been resolved in a joint calculation using Density Functional Theory (DFT), as implemented in Viena ab-initio simulation package (VASP), and the evolutionary algorithm based on the USPEX code. 44–47 The main idea behind the latter code is to select the structures aiming to approach the global minimum in the free energy landscape by knowing only the chemical composition of the system. This approach started with the random generation of 300 structures (first population), which have subsequently been fully relaxed. Thereafter, the thirty lowest energy structures were selected (second generation) to undergo a genetic transformation giving rise to the next generations (all thirty structures). The next generations were created by applying three different variation operators: heredity, soft mutation and permutation. The ‘heredity’ operator takes two coherent slabs from pre-existing structures and combines them to form the new structure, which carries a heritage 3 ACS Paragon Plus Environment


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from their predecessors. ‘Soft mutation’ creates mutation in spatial coordinates by moving atoms along the eigenvectors of the softest modes, i.e., the lowest energy barrier pathways. Finally, the ‘permutation’ operator acts to exchange different types of atoms, allowing the system to find the correct atomic ordering. 47 After the creation of each generation, the new structures are fully relaxed within the density functional theory framework using the projector-augmented wave (PAW) method 48,49. For this study, we have considered 20 generations and every structure was relaxed using the Perdew, Burke and Ernzerhof (PBE) 50 functional for description of the exchange-correlation along with the Grimme approach (D2) 51 for dispersive interactions. For the relaxation, 5 steps were considered: step 1 and 2 performed an initial (and crude) relaxation of atomic positions and cell parameters, keeping the volume constant and using a small energy cut-off value (400eV). In step 3, the relaxations were done with medium precision and finally, the more accurate relaxations were done in step 4. In both step 3 and 4 an energy cut-off of 550eV was used. (For further detail on the crystal structure determination, please see Ref. 44–47). For each relaxation step, different reciprocal space resolution generation was used (in units of 2Å-1), starting with a crude resolution of 0.16 and increasing the density to 0.12, 0.10 and 0.08 in the two final steps. This resolution will define the k-point mesh used in each calculation. After the relaxations, the best structures were ranked according to their free energies. Finally, an additional structure relaxation was performed on the obtained structures by employing a higher energy cut-off of 700 eV and a 6×6×6 Monkhorst−Pack k-point mesh. For each lithiation step, the crystal structures were determined considering two molecular units per unit cell. Here, both the PBE-D2 and the hybrid functional HSE06 52 have been used in order to further understand their thermodynamics and electronic structure. The use of the hybrid functional is important since the pure GGA has a spurious contribution of the electron’s selfinteraction that tends to overestimate the electron delocalization, consequently affecting the total electronic energy. This can be avoided by incorporating the exact exchange contribution to the correlation-exchange functional, resulting in the so-called hybrid functional. The lithiation process is described by the following reaction: 𝐿𝑖𝑥0 𝑇𝐷𝐶 + (𝑥1 − 𝑥0 )𝐿𝑖 → 𝐿𝑖𝑥1 𝑇𝐷𝐶

(1)

where the reaction energy is used to estimate the voltage profile as a function of the lithium insertion, such that:

𝑉(𝑥) = −

𝐸(𝐿𝑖𝑥1 𝑇𝐷𝐶) −𝐸(𝐿𝑖𝑥0 𝑇𝐷𝐶) −(𝑥1 − 𝑥0 )𝐸(𝐿𝑖) 𝑥1 − 𝑥0

(2)

with E(Lix1TDC) and E(Lix0TDC) corresponding to the total energies of the structures containing x1 and x0 Li ions per formula unit, respectively. It is worth to mention that the potential is actually determined by the variation of Gibbs free energy (G = U +PV - TS) during the Li insertion. Since we are considering the G for lithium insertion reactions in solidstate, we have approximated G by the variation of the electronic energy (G  E) 53 which is expected to be the dominant term to the reactions free energy variation. Additionally, one should keep in mind that the lowest value for x0 is 2, which corresponds to the so-called delithiated structure. The formation energy of the Li3TDC with respect to the Li2TDC and Li4TDC lithiation stages was calculated using the relation:

𝐸𝐹 = 𝐸(𝐿𝑖3 𝑇𝐷𝑃) −

𝐸(𝐿𝑖4 𝑇𝐷𝑃) +𝐸(𝐿𝑖2𝑇𝐷𝑃) 2

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(3)

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In Eq. (3), E(Li2TDC), E(Li3TDC) and E(Li4TDC) are the total energies of the respective systems. This concept is frequently used for alloys to estimate the stability of a given system. The formation energy as a function of the alloy composition follows a convex hull curve indicating which composition could be stable (see 54–59). In the context of the present work, such a formation energy is used to investigate whether the lithiation process undergoes a stepwise mechanism, having a stable structure after the insertion of the first (Li+ + e–) pair (resulting in the Li3TDC), or follows a two electrons process, directly resulting in the Li4TDC structure. In order to investigate the relevance of the crystal environment on the electrochemical properties of LixTDC, we have also carried out calculations on molecular models. This has been achieved by embedding the molecular structures in a supercell (lattice parameters: a=27 Å, b=25 Å and c=22 Å) containing a vacuum region along all three directions. The structures for Li2TDC, Li3TDC and Li4TDC have been selected from a molecular dynamics simulation performed at a temperature of 400 K and 5 ps time interval. Here, snapshots of the structures have been selected from the simulation trajectory in intervals of 0.5 ps and subsequently quenched through atomic relaxation. The lowest energy configurations have then been chosen for further studies. The same theory level as described to the solid-state calculations has been employed. For Li3TDC, a spin polarized calculation has been considered since this involves formation of a radical. Results and Discussions The crystal structures for three the lithiation stages, Li(2+x)TDC (x=0,1,2), have been theoretically resolved. The obtained lattice parameters are shown in Table 1. For the three cases, a monoclinic crystal structure with P21 space group was found to be the most favorable crystal structure. The lithiation process slightly reduces the crystal volume in the first lithiation step, while no change in volume was observed in the second step. Table 1 – Lattice parameters of Li(2+x)TDC structures predicted using an evolutionary algorithm.

Space Group a (Å) b (Å) c (Å)  (°)  (°)  (°) Volume (Å3)

Li2TDC P21 7.909 5.093 8.471 90 93.45 90 340.6

Li3TDC P21 8.513 7.976 4.960 90 90 93.58 336.1

Li4TDC P21 8.448 4.863 8.196 90 86.9 90 336.3

As depicted in Figure 1(a), the delithiated stage showed a well define salt layer between the organic phase, resulting in the so-called lithium organic framework (LOF) where the Li atoms form a tetrahedral center coordinated by four oxygen atoms. For this structure, the average Li – O bond length is 1.975 Å (displaying good agreement with the experimental value of 1.972Å 60) and the average distance between the thiophene rings is around 3.3 Å. It is worth mentioning that the thiophene moieties adopt an out-of-plane conformation instead of a stacked conformation, which could be expected due to – interactions.



Figure 1 – Crystal structure of a) Li2TDC, b) Li3TDC and c) Li4TDC predicted using evolution algorithm, d) voltage profile for the Li(2+x)TDC systems obtained by DFT calculations and e) the formation energy.

Already after the first lithiation step (Li3TDC structure, shown in Figure 1(b)), the additional Li ion starts to coordinate the S heteroatoms, forming a trigonal-like structure deviating from the initial LOF structure. In order to evaluate the stability of the first lithiated structure, the formation energy was determined as described in Eq. (3). As depicted in Figure 1(d), the Li3TDC energy is lower than the average energy calculated with respect to the initial and final lithiation stage, indicating that this structure is stable. The same trend was observed in the following lithiation step leading to the Li4TDC structure, where the S atoms interacts with 3 Li ions, as seen in Figure 1(c). This effect induces a more drastic structural change since the Li atoms are prone to form both trigonal and tetrahedral structures with O and S atoms at the vertices. As a result, an additional salt-like layer is formed orthogonal to the initial one. This outcome differs from the observed for the lithiated structures of dilithium therephthalate 42 in which the oxygens atoms act as the main redox center. The main reason is that the S atoms have a more accessible redox activity, taking part on the electrochemical process and being reduced upon Li insertion. Moreover, Lee and coworkers reported a progressive amorphization of this material after the initial cycles,17 which could be a consequence of this additional Li layer interacting with the organic environment. In both Li3TDC and Li4TDC, the Li–S distance is around 2.7 Å. It is worth to note that despite the Li insertion induced breaking of the LOF structure, the space group symmetry has not been changed. 6 ACS Paragon Plus Environment

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Table 2 shows the bond-lengths obtained from the crystal structures. It possible to see that the variation of the dC=C and dC–C bonds indicates a transition from an aromatic to a quinoidal structure. Additionally, the Li-S coordination leads to an increase on the dS–C bond length, from 1.72 Å for Li2TDC to 1.76 Å for Li3TDC and 1.80 Å for Li4TDC, indicating a destabilization of the organic heterocycle. Table 2 – Bond-legth variation with the Li insertion for the crystal model structures.

dC = C (Å) dC – C(Å) dC – COOLi (Å) dS – C (Å)

Li2TDC 1.39 1.42 1.44 1.72


Li3TDC 1.40 1.39 1.37 1.76

Li4TDC 1.44 1.37 1.38 1.80


Figure 2 – Total and Projected Density of States for Li2TDC, Li3TDC and Li4TDC crystals calculated employing both GGA and Hybrid functionals.

The redox potential was calculated as described above, using Eq. (2), and the voltage profile is plotted in Figure 1(e). For the first lithiation step, from Li2TDC to Li3TDC, a potential of 1.11 V vs. Li/Li+ was obtained (1.01 V vs. Li/Li+ for the hybrid functional) while the estimated potential for the second step, from Li3TDC to Li4TDC, was 0.88 V vs Li/Li+ (0.93 V vs. Li/Li+ for the hybrid functional). The estimated average potential is 0.99 V vs. Li/Li+ obtained using the GGA functional and 0.97 V vs. Li/Li+ for the hybrid functional, in good agreement with the experimental value (around 1.00 V) 17. Li2TDC is expected to exhibit similar electrochemical properties as terephthalate (Li2TP), which has a benzene ring instead of the thiophene heterocycle. In this sense, the average potential obtained for Li2TDC is also in agreement with previous experimental (0.8 V 4,17) and theoretical (0.94 V 42) outcomes. The projected density of states (DOS) was calculated, aiming to gain insights on how the lithium insertion changes the electronic structure during the electrochemical reaction. As seen in Figure 2, the electron donation in the first Li insertion partially populates a localized band resulting from a hybridization of atomic orbitals from carbon, oxygen and sulphur. After the second lithiation step, this state is completely occupied. 8 ACS Paragon Plus Environment

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Figure 3 – Charge density isosurfaces for Li(2+x)TDC obtained using both GGA (left) and hybrid functionals (right). The isosurfaces were obtained by integrating the DOS in the energy range corresponding to the first unoccupied band of the Li2TDC, the partially occupied band of Li3TDC and the last occupied band of Li4TDC.

This result suggests that the electrons transferred to the dicarboxylate unit tend to delocalize over the entire organic molecule, which also could be expected for a conjugated dicarboxylate. 10 To confirm this, the charge density was calculated by integrating the DOS in the energy range corresponding to the localized empty band, which is further populated with the Li+ + e- insertion. As depicted in Figure 3, the charge density is distributed over the entire organic moiety, showing that the incoming electrons will actually populate an electronic state which is composed by the  states of the thiophene ring as well as p-like states from both oxygen and sulphur. Interestingly, almost no difference can be observed for the charge densities obtained from calculations performed with GGA and hybrid functionals. Furthermore, the net charge per atom was calculated using Bader analysis, in order to monitor the charge variation during the lithiation process (see Table S1 in the supporting information). In the delithiated stage, the oxygen atoms displayed an electron abundance (1.31 obtained with the hybrid functional). The thiophene ring has three different carbon atoms, labelled C(C=C), C(C-S) and C(carbonyl). While the C(C=C) carbons are almost neutral, the C(C-S) atoms are electron-rich (-0.2) and the C(carbonyl) atoms are clearly electron deficient (1.6). Also, the S atoms showed an electron deficiency (0.3). After the insertion of lithium, the O atoms displayed a very low charge variation (around 4 %) even after insertion of 2 Li ions, equivalent to donation of two electrons. Moreover, the C atoms forming the C=C bond were reduced (from almost neutral for Li2TDC, to -0.12 for Li4TDC), as well as the C-S atoms. The S atom, which are electron deficient in Li2TDC (0.3), are also reduced after the insertion of one Li ion (0.06) and become electron rich after insertion of the second Li ion (-0.20). These 9 ACS Paragon Plus Environment


results corroborate the outcomes from DOS and charge density, confirming that the excess electrons are actually transferred to the organic heterocycle which acts as an electron reservoir. The reduction of the organic material also results in an aromatic-quinoidal transition and a destabilization of the C-S bond. Another point is that the excess of electrons due to the reduction of the S atoms together with its lone pair attracts the Li+ ions, resulting in the break of the LOF discussed above. Additionally, the lithiation of the dilithuim thiophene-dicarboxylate has been investigated in a molecular perspective. The main goal here is to gain insights into the influence of the crystal environment on the electronic structure as well as on the calculated electrochemical properties. The calculated DOS shown in Figure 4 resembles the DOS obtained by solid-state calculations (for the spin-polarized DOS, see Fig S1). The empty band, which is further populated by the incoming electrons, in fact results from the hybridization of the C, O and S atomic orbitals, leading to a delocalization of the charge over the whole organic moiety. This can also be observed by looking into the charge variation in each atom (see Table S2). The Bader analysis for the molecular calculations, in agreement with the results obtained for solidstate, revealed the reduction of the C atoms that compose the organic cycle, as well as a strong reduction of the S atoms (from electron deficient for Li2TDC (0.3) to electron rich for Li4TDC (-0.1)). Again, the reduction of the thiophene ring is reflected in a change from aromatic to quinoidal structure (see the bond-length variations shown in Table S3) as well as in an increase of the S-C bond length (from 1.72 Å to 1.81Å). As stated before, these results indicate a sort of destabilization of the organic moiety. The voltage profile, however, does not follow the trend observed for the solid-state calculations. A voltage of 0.23 V and 0.57 V have been obtained for the first and second lithiation steps, respectively (0.15 V and 0.64 V considering the spin polarization). This is an indication that the intermediate structure, the Li3TDC, is not stable probably due to the unpaired electron (a radical state) but can be stabilized by the crystalline environment in the solid state. It is important to point out that an even more pronounced discrepancy was obtained using the hybrid functional, with voltages of -0.08 V for the Li2TDC + Li → Li3TDC reaction and 0.9 V for the Li3TDC + Li → Li4TDC. Note that the obtained negative voltage supports that the intermediate structure containing 3 Li ions is not stabilized in a molecular calculation.


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Figure 4 - Total and Projected Density of States for Li2TDC, Li3TDC and Li4TDC isolate molecules calculated employing both GGA and hybrid functionals.

On the other hand, the average potential when considering the direct insertion of two Li ions results in a voltage of around 0.4 V for both GGA and hybrid functionals. This value is in the same rage of the values obtained via molecular calculations for the dilithium benzenedipropiolate. 19 The lower potential values obtained in the molecular calculations in comparison with the solid-state ones, are likely a result of both the influence of a dielectric environment and the coordination number of the Li ions. To clarify the influence of the surrounding environment on the voltage profile, some small cluster models containing two and four molecular units have been built-up. As depicted in Figure 5(a), the S – Li coordination becomes similar to the obtained for the crystals as the number of molecular units increases. As a consequence, the calculated potentials also increase, getting close to the values obtained from solid-state calculations, shown in Figure 5(b). For instance, for a cluster model containing four molecules, an average voltage of 0.99 V was obtained (result obtained from GGA calculations). Moreover, the stepwise lithiation of the crystal was qualitatively reproduced, with a voltage of 1.01 V for the Li2TDC → Li3TDC step 11 ACS Paragon Plus Environment


and 0.98 V for the Li3TDC + Li → Li4TDC reactions, resulting in an average potential of 0.99 V.

Figure 5 – a) Molecular and cluster model strutures for the unlithiathed and lithiated phases and b) voltage evolution for the two lithiation steps obtained from molecular/cluster model calculations as a function of the number of molecular units. The voltages were obtained using the PBE-D2 functional.

These results indicate that a molecular approach can bring a fair qualitative first assessment of both electronic structure and electrochemical potentials of organic electrode materials. Conclusion The evolutionary algorithm coupled with density functional theory calculations was employed to theoretically determine the crystal structure of dilithium thiophene-2,5-dicarboxylate, as well as its lithiated phases. Li2TDC, which corresponds to the delithiated phase, displayed the expected lithium organic framework structure with a well-defined organic layered structure with an intercalated salt-like region. The lithium atoms are coordinated by four oxygens from the carbonyl groups. The obtained structures for the lithiated phases containing one and two extra lithium ions unveiled some interesting features of the lithiation process. In the very first lithiation step, the inserted Li ion is coordinated by the sulfur heteroatom deviating from the initial LOF structures. The structure evolves, after the insertion of another Li, to a lithiated stage where the two additional Li ions both are coordinated by the S atom. The calculated voltage for the electrochemical reaction is in good agreement with experimental data as well as with previous theoretical values reaffirming this as a powerful methodology for assessment of both structural and electrochemical properties. Especially the use of evolutionary algorithms to determine the crystalline structure without any previous knowledge except of the chemical composition of the crystal is indeed a promising tool, especially in cases were the crystalline structure is difficult to resolve experimentally. The usage of the evolutionary algorithm coupled with solid state ab-initio calculation could, however, be a computational demanding strategy. For this reason, the same study was carried out at a molecular level in order to gain insight on the influence of the crystalline structure on the structural and electronic properties of the dilithium thiophene dicarboxylate and its lithiated phases. The trends for the crystals during Li insertion were also obtained in the molecular approach; for instance, the changes in the bond length associated to the aromatic– quinoidal transition, as well as the reduction of the C, O and especially S atoms. The changes on the electronic structure, observed in the DOS, reproduces quite well the outcomes from the 12 ACS Paragon Plus Environment

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solid-state calculations, except for the more localized nature of the bands which could be expected for a molecular system. Despite the good agreement of the structural properties as well as the DOS, the calculated voltage for the Li insertion reaction does not exactly follow the same tendency as seen using solid-state calculations. The molecular model fails to describe the stepwise process since the Li3TDC structure cannot be stabilized in a molecular calculation in vacuum, indicating that the dielectric environment is indeed necessary to properly describe its energy. On the other hand, if the interest is only the average potentials of the initial and final step (or equivalently, if modelling the process as a two-electron electrochemical process) the result qualitative agrees with the solid-state calculation. The low value obtained is related to the fact that in these calculations were performed in vacuum without the stabilization of a dielectric environment provided by the crystal itself. In this sense, the molecular approach could be used as a first assessment of the lithiation process, not least in special cases were multiple steps should be considered. Associated Content Supporting Information The Supporting information file contains tables with Bader charge analysis for the Li(2+x)TDC crystals calculated using GGA and hybrid functional (Table S1), and for the molecules calculated using GGA functional (Table S2). Table S3 presents the bond-length variation with the Li insertion for the molecular model structures. Figure S1 is showing the Total and Projected Density of States for isolated Li2TDC, Li3TDC and Li4TDC molecules calculated employing GGA and considering spin polarization. Atomic coordinates for the optimized structures are also available.

Notes The authors declare no conflicts of interest. Acknowledgments This project has support from the Swedish Research Council (VR) (Grant number: 621-20145984), Formas (Grant number: 2016-00838), Swedish Energy Agency (Grant number: 454201) and STandUP for Energy, with infrastructure provided by the Swedish National Infrastructure for Computing (SNIC) at the PDC Center for High Performance Computing and National Supercomputer Centre at Linköping University (NSC). CFNM thanks Carl Tryggers Stiftelse for the financial support. References (1) (2)

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Predicting Structure and Electrochemistry of Dilithium Thiophene-2,5

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