Theoretical investigation of pillar[4]quinone as a cathode active material for lithium-ion batteries

A Review of Macrocycles Applied in Electrochemical Energy Storge and Conversion

Macrocycles composed of diverse aromatic or nonaromatic structures, such as cyclodextrins (CDs), calixarenes (CAs), cucurbiturils (CBs), and pillararenes (PAs), have garnered significant attention due to their inherent advantages of possessing cavity structures, unique functional groups, and facile modification. Due to these distinctive features enabling them to facilitate ion insertion and extraction, form crosslinked porous structures, offer multiple redox-active sites, and engage in host-guest interactions, macrocycles have made huge contributions to electrochemical energy storage and conversion (EES/EEC). Here, we have summarized the recent advancements and challenges in the utilization of CDs, CAs, CBs, and PAs as well as other novel macrocycles applied in EES/EEC devices. The molecular structure, properties, and modification strategies are discussed along with the corresponding energy density, specific capacity, and cycling life properties in detail. Finally, crucial limitations and future research directions pertaining to these macrocycles in electrochemical energy storage and conversion are addressed. It is hoped that this review is able to inspire interest and enthusiasm in researchers to investigate macrocycles and promote their applications in EES/EEC.

Discovery of lead quinone cathode materials for Li-ion batteries

Organic cathode materials are attractive candidates for the development of high-performance Li-ion batteries (LIBs). The chemical space of candidate molecules is too vast to be explored solely by experiments; however, it can be systematically explored by a high-throughput computational search that incorporates a spectrum of screening techniques. Here, we present a time- and resource-efficient computational scheme that incorporates machine learning and semi-empirical quantum mechanical methods to study the chemical space of approximately 200 000 quinone-based molecules for use as cathode materials in LIBs. By performing an automated search on a commercial vendor database, computing battery-relevant properties such as redox potential, gravimetric charge capacity, gravimetric energy density, and synthetic complexity score, and evaluating the structural integrity upon the lithiation process, a total of 349 molecules were identified as potentially high-performing cathode materials for LIBs. The chemical space of the screened candidates was visualized using dimensionality reduction methods with the aim of further downselecting the best candidates for experimental validation. One such directly purchasable candidate, 1,4,9,10-anthracenetetraone, was analyzed through cyclic voltammetry experiments. The measured redox potentials of the two lithiation steps, , of 3.3 and 2.4 V, were in good agreement with the predicted redox potentials, , of 3.2 and 2.3 V vs. Li/Li+, respectively. Lastly, to lay out the principles for rational design of quinone-based cathode materials beyond the current work, we constructed and discussed the quantitative structure property relationships of quinones based on the data generated from the calculations.

Extension of the TraPPE Force Field for Battery Electrolyte Solvents

Optimizing electrolyte formulations is key to improving performance of Li-/Na-ion batteries, where transport properties (diffusion coefficient, viscosity) and permittivity need to be predicted as functions of temperature, salt concentration and solvent composition. More efficient and reliable simulation models are urgently needed, owing to the high cost of experimental methods and the lack of united-atom molecular dynamics force fields validated for electrolyte solvents. Here the computationally efficient TraPPE united-atom force field is extended to be compatible with carbonate solvents, optimizing the charges and dihedral potential. Computing the properties of electrolyte solvents, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and dimethoxyethane (DME), we observe that the average absolute errors in the density, self-diffusion coefficient, permittivity, viscosity, and surface tension are approximately 15% of the corresponding experimental values. Results compare favorably to all-atom CHARMM and OPLS-AA force fields, offering computational performance improvement of at least 80%. We further use TraPPE to predict the structure and properties of LiPF₆ salt in these solvents and their mixtures. EC and PC form complete solvation shells around Li⁺ ions, while the salt in DMC forms chain-like structures. In the poorest solvent, DME, LiPF₆ forms globular clusters despite DME's higher permittivity than DMC.

Ethynyl and Furyl Functionalized Porphyrin Complexes as New Organic Cathodes Enabling High Power Density and Long-Term Cycling Stability

Organic cathode materials have recently attracted abundant attention due to their flexible structural tunability and recyclability. However, the low intrinsic electrical conductivity and high solubility in electrolytes of organic electrode materials have significantly limited their practical application. Herein, we present [5,15-bis(ethynyl)-10,20-difurylporphinato] copper(II) (CuDEOP) as a new cathode for rechargeable organic lithium batteries (ROLBs). The combination of both ethynyl and furyl groups of the CuDEOP cathode with a nanorod structure renders it with enhanced structural stability and an extended delocalized π-electron system to deliver excellent cycling stability (capacity retention of 76% after 6000 cycles) and a high power density (16 kW kg^-1). The furyl electroactive groups participate in charge storage contribution to achieve a reversible six-electron-transfer redox reaction in a specific voltage range. The mechanism characterizations indicate that the nitrogen atoms on the porphyrin ring act as active sites to alternatively store both PF₆^- anions and Li⁺ cations, and the charge storage process is a pseudocapacitive-dominated reaction. This observation will offer a new avenue for designing functionalized molecules for electrochemical energy-storage (EES) systems.

Theoretical prediction on photoelectric and supramolecular properties of benzoquinone-tetrathiafulvalene macrocyclic molecules

Benzoquinone has the ability to serve as an electron acceptor, and tetrathiafulvalene has the ability to serve as an electron donor. Based on the facts above, this work creatively cycles the benzoquinone unit and the tetrathiafulvalene unit alternately into macrocyclic molecules, the cyclopolymers of benzoquinone-tetrafluorene (C[n]QTTF, n = 3~6). To explore their structure and properties, the M06-2X functional of density functional theory (DFT) with 6-311g(d) basis set was used to optimize the ground-state structures of C[n]QTTF. Based on the stable configurations of the ground states, the electronic structure property is analyzed systematically. The results show that these macrocyclic molecules have excellent electron transport capability and electrochemical activity. Then, the electron absorption spectra of each system are carried out by using time-dependent density functional theory (TD-DFT) at the M062X/6-311+G(d) level. It turns out that their maximum absorption wavelengths are all in the visible range. Further calculation suggests that C[n]QTTF can also be characterized with one-dimensional self-assembly, double-walled assembly, and the host-guest inclusion performance, based on which it gains a variety of supramolecular structures. In summary, the benzoquinone-tetrafluorofurene macrocyclic molecules predicted by DFT calculations may be of assistance to the potential applications in organic electronics and supramolecular chemistry.

Effect of salt concentration on properties of mixed carbonate-based electrolyte for Li-ion batteries: a molecular dynamics simulation study

In this work, a computational framework is proposed by utilizing molecular dynamics simulation to explore the existing relation between molecular structure and ionic conductivity of the electrolyte system [LiPF₆+(EC+DMC 1:1)] consisting of a mixture of cyclic ethylene carbonate (EC) and acyclic dimethyl carbonate (DMC) solvents and lithium hexafluorophosphate (LiPF₆) salt to propose as a novel mixed organic solvent-based electrolytes to promote the performance of lithium-ion batteries (LIBs). To acquire a clear understanding of the structural and transport properties of the designed electrolytes, quantum chemistry (QC) calculations and molecular dynamics (MD) simulation are used. In the first step, the accurate molecular structures of the studied electrolytes in addition to their corresponding atomic partial charges are evaluated. The MD simulations are performed at 330 K varying the LiPF₆ concentration (0.5 M to 2.2 M). Analysis of the obtained results indicated that ionic diffusivity and conductivity of the electrolytes are dependent on the structure of solvated ions and lithium salt (LiPF₆) concentration. It is found that the obtained MD simulation results are in reasonable agreement with experimental results. Graphical abstract A representation of dependence of transport properties of electrolyte system [LiPF6 +(EC+DMC 1:1)] as function of salt concentration to be used in Lithium-ion batteries (LIBs).

Theoretical study on lithiation mechanism of benzoquinone-based macrocyclic compounds as cathode for lithium-ion batteries

Benzoquinone (BQ)-based macrocyclic compounds have shown great potential as cathode materials for lithium-ion batteries (LIBs) owing to their high redox potential and specific capacity. However, such materials usually have complex structures, which impede the investigation of lithiation mechanisms. Herein, we take Calix[4]quinone (C4Q) molecule as an example to develop a viable mechanism investigation method for such materials. The lithiation profile of C4Q is determined by condensed Fukui function which provides the reaction sites and orders. A correction of redox potential is proposed by leaving out the ion-transfer effect during the redox reaction based on Gibbs free energy change. The redox potential obtained by this approach shows high consistency with the experimental results. Moreover, this method can also be well extended to study the lithiation mechanism of another BQ-based macrocyclic compound (Pillar[5]quinone). Our results are promising to more deeply understand the reaction mechanism and predict the redox potential of new BQ-based macrocyclic compounds for LIBs.