Theoretical screening of novel electrode materials for lithium–ion batteries from industrial polymers
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Organic polymers have the potential to be electrode materials for lithium–ion batteries due to their lower solubility, lower self-discharge rates, high mechanical strength, greater flexibility, superior thermal stability, and versatility. In this paper, the density functional theory (DFT) was applied to investigate industrial polymers as electrode materials for lithium–ion batteries. The charge/discharge potentials of reported polymer electrode materials for lithium–ion batteries were collected, and the experimental values were fitted linearly with the values of ΔEpoly (as shown in Eq. (2b)) calculated with a single-molecule model to obtain a semi-empirical formula, which was subsequently applied to predict the charge/discharge potentials of industrial polymers. The results showed that 16th (polypyromellitic diphenyl sulfide), 17th (polypyromellitic diphenyl ether imine), and 23rd (polypyromellitic diphenylmethaneimine) materials have better electrochemical performance than the other materials in this paper, and we also find that the material, such as polypyromellitic diphenylmethaneimine, containing low electronegative heteroatom and electron-donating groups, has a low potential value.
KeywordsLi–ion battery Industrial polymer electrode materials Single-molecule model
Li–ion batteries have already played an important role in powering electronic equipment, grid energy storages, tools, and vehicles [1, 2, 3]. Electrode materials are one of the most important composite materials for improving the performance of lithium–ion batteries . Inorganic electrode materials, including LiNiO2 , LiCoO2 , LiFePO4 , and LiMn2O4 , have been researched in-depth and are extensively applied. However, inorganic materials have two disadvantages: one is limited mineral resources, particularly of Co and Ni, the other is the production or recovery of inorganic compounds that require high temperatures and the release of large quantities of carbon dioxide, which is harmful to the environment .
Recently, due to the unique properties, such as renewable sustainability, lower carbon dioxide emissions [10, 11], abundance of raw materials, and diverse types of structures, organic electrode materials have attracted considerable attention. To date, many organic electrode materials have been reported [12, 13], such as the conductive polymer [14, 15], organic radicals [16, 17], reduced carboxylate, and carbonyl compounds [18, 19, 20, 21, 22, 23, 24, 25, 26]. However, the disadvantages of organic compounds are that they can be easily dissolved and have low thermal tolerance, poor conductivity, and low mechanical stability [27, 28].
Organic polymers can reduce these problems because of their lower solubility, lower self-discharge rates, high mechanical strength, greater flexibility, superior thermal stability, and versatility [19, 29, 30]. These polymers include conducting polymers, radical polymers , polypyrrole , organosulfur derivatives [32, 33], and polymers with carbonyl groups [19, 34]. Recently, many new polymer electrode materials have been developed [25, 35, 36, 37, 38, 39, 40, 41, 42, 43]. In particular, Qichun Zhang et al.  discovered a high-performance all-plastic full battery (both the cathode and anode are poly (2,3-dithiino-1,4-benzoquinone) (PDB)). The electrode is not easily dissolved in the electrolyte, meaning that the electrode has ultra-long cycle stability. These researchers’ study demonstrated that the design and fabrication of high-performance all-plastic batteries is becoming possible.
Poly(ethylene terephthalate) (PET) material, which is highly difficult to degrade in a natural environment, has been reported as an anode material for lithium–ion batteries after a low-temperature solvothermal treatment , which shows that polymer waste is expected to become the electrode material of lithium–ion battery.
For many industrial polymers, their unit structures contain a carbon–oxygen double bond (C=O) in a conjugated carbonyl or carboxyl group, which is a typical characteristic for organic electrode materials of lithium–ion batteries. Thus, a theoretical screening study to find potential electrode materials from industrial polymers, which are produced on a large scale and at low cost, is required.
Previously, the single-molecule model accompanied with DFT calculation has been widely used to study small organic molecule as electrode materials for lithium–ion batteries, but it could not precisely predict the potential [46, 47, 48, 49, 50]. Recently, a high-throughput screening scheme based on the dispersion-corrected density functional theory (DFT-D) and organic crystal structure could accurately calculate the potential of organic electrode materials [16, 35, 48, 50, 51, 52, 53, 54].
For polymer materials, their crystal structures are easily deformed, and the crystallized phases always coexist with an amorphous phase, which means that it is difficult to accurately measure and describe the crystal structure of polymers. In the present study, the charge/discharge potentials of 11 reported polymer electrode materials for lithium–ion batteries was collected, and the experimental values were fitted linearly with the values of ΔEpoly (as shown in Eq. (2b)) calculated with a single-molecule model. After a mathematical treatment, a semi-empirical formula to predict charge/discharge potential of polymer electrode materials was set up, which was further applied to predicted more than 20 industrial polymers as electrode materials. Several materials with high potential and capacity are specified in the paper, and the calculation results demonstrate that the electronegativities of the substituents could adjust the charge/discharge potential of the polymer electrode materials for lithium–ion batteries.
The Amsterdam Density Functional program package (ADF, version 2017) was applied in this study; geometries were optimized using the Perdew−Burke−Ernzerhof (PBE) exchange−correlation (xc) potential as implemented in the Amsterdam Density Functional-BAND (ADF2017) package [35, 36, 53, 55]. The double-ζ polarized (DZP) sets, the zeroth-order regular approximation (ZORA), normal numerical quality, and the large frozen core technology were applied in this work. A conductor-like screening model (COSMO) was used here to describe the acetonitrile solvent environment, and the dielectric constant was selected as 37.5 for the acetonitrile solvent. ADF-BAND uses chain periodicity, DZP basis is set with large frozen core, and the unit cell consists of repeating units. Constructing a one-dimensional chain periodic structure in ADF-BAND based on a single molecular model. The chain periodic structures were optimized.
Results and discussion
The band structure of PDB in the vicinity of the Fermi level is presented in Fig. 3; among the states, α, β, and γ are three different states near the Fermi level. For delithium, the α state was occupied, which was the valence band maximum (VBM); β and γ states are both unoccupied; and β was the conductor band minimum (CBM). The band gap between α and β was approximately 0.8 eV. After lithium was embedded, the electrons go into the β band, and the Fermi level goes to the top of β band. The occupied β state becomes VBM, γ becomes CBM, and the band gap between β and γ decreases to 0.47 eV, which suggests that they are typical semiconductors. The band gaps were all small, but the energy bands (α, β, and γ) were flat, which implies they were all local states.
EA and EA-Lin are calculated with the single molecular model; k and c are unknown and the correction value.
Values of △Epoly and experimental potentials of the 11 types of industrial polymer materials
The verification results of the experiment and calculation are shown in the Fig. 5, where R2 = 0.59, indicating that the experimental and calculated values fit well. The present work is purely theoretical, and the possibilities of 26 industry polymers to be applied as electrode of Li–ion battery have been simulated. The related experiments are out of the present work. Recently, in another published paper (Solid State Ionics 317 (2018) 164–169), PET was reported as electrode material of Li–ion battery, and its experimental potential (about 0.9 V) is very close to the theoretical value (about 0.82 V) reported in the present work, which further verifies the theoretical method.
Thus, three industrial polymers were highlighted: 16th, 17th, and 23rd. The potentials of 16th and 17th were 2.03 V and 2.00 V, respectively, and the potential of the 23rd was 1.95 V. The capacity of the 16th and 23rd were higher than 274 mAh/g, and the capacity of the 17th was 269.4 mAh/g, which was close to 274 mAh/g. The band gaps of the 17th before and after lithium insertion were 0.93 eV and 0.53 eV, respectively. The band gaps of the 16th before and after lithium insertion were 1.69 eV and 0.51 eV, respectively. The band gaps of the 23rd before and after lithium insertion were 2.19 eV and 0.13 eV, respectively. The band gaps show that they have good electronic conductivity and dynamic performance in Li–ion batteries. The high capacity, high potential, and small band gaps were positive factors for the three industrial polymer materials to be electrode materials.
In this work, three polymer electrode materials (with a relatively high potential, high capacity, and small band gap), 16th, 17th, and 23rd, were highlighted from 26 types of industrial polymers. The 16th, 17th, and 23rd materials have similar structures and similar potentials. Therefore, the following studies of the three materials (16th, 17th, and 23rd) with similar structure were made.
Then, the polymer containing a high electronegative heteroatom and electron-absorbing groups has a relatively high potential and small band gap. For capacity, the material with a larger amount of carbonyl groups and a smaller molar mass has a larger theoretical capacity.
The charge/discharge potentials of 11 reported polymer electrode materials for lithium–ion batteries were collected, and the experimental values were fitted linearly with the values of ΔEpoly (as shown in Eq. (2b)) calculated with the single-molecule model. After the mathematical treatment, a semi-empirical formula was employed to predict charge/discharge potentials of more than 20 industrial polymers as electrode materials. The potential energy range of 26 types of industrial polymers was 0.62~2.03 V, the capacity distribution was 79.5~382.9 mAh/g−1, and all band gaps were less than 2.57 eV. The 16th, 17th, and 23rd materials are highlighted from 26 types of industrial polymers which have better electrochemical performance than the other materials in this paper. We also found that the electrochemical properties of materials can be improved by changing the heteroatoms in the structure. More polymer electrode materials can be developed with the present theoretical method. We believe that the results of this study will help in designing better electrode materials.
The study was supported by the fundamental research project of Qinghai province (2017-ZJ-795).
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