The solvation structure,transport properties and reduction behavior of carbonate-based electrolytes of lithium-ion batteries

Despite the extensive employment of binary/ternary mixed-carbonate electrolytes (MCEs) for Li-ion batteries, the role of each ingredient with regards to the solvation structure, transport properties, and reduction behavior is not fully understood. Herein, we report the atomistic modeling and transport property measurements of the Gen2 (1.2 M LiPF₆ in ethylene carbonate (EC) and ethyl methyl carbonate (EMC)) and EC-base (1.2 M LiPF₆ in EC) electrolytes, as well as their mixtures with 10 mol% fluoroethylene carbonate (FEC). Due to the mixing of cyclic and linear carbonates, the Gen2 electrolyte is found to have a 60% lower ion dissociation rate and a 44% faster Li⁺ self-diffusion rate than the EC-base electrolyte, while the total ionic conductivities are similar. Moreover, we propose for the first time the anion–solvent exchange mechanism in MCEs with identified energetic and electrostatic origins. For electrolytes with additive, up to 25% FEC coordinates with Li⁺, which exhibits a preferential reduction that helps passivate the anode and facilitates an improved solid electrolyte interphase. The work provides a coherent computational framework for evaluating mixed electrolyte systems.

The different roles of the anion, cyclic and linear carbonates, and additive in mixed-carbonate electrolytes are revealed. The anion–solvent exchange mechanism and factors influencing the solid electrolyte interphase (SEI) formation are deciphered.     

Unravelling free volume in branched-cation ionic liquids based on silicon

The branching of ionic liquid cation sidechains utilizing silicon as the backbone was explored and it was found that this structural feature leads to fluids with remarkably low density and viscosity. The relatively low liquid densities suggest a large free volume in these liquids. Argon solubility was measured using a precise saturation method to probe the relative free volumes. Argon molar solubilities were slightly higher in ionic liquids with alkylsilane and siloxane groups within the cation, compared to carbon-based branched groups. The anion size, however, showed by far the dominant effect on argon solubility. Thermodynamic solvation parameters were derived from the solubility data and the argon solvation environment was modelled utilizing the polarizable CL&Pol force field. Semiquantitative analysis was in agreement with trends established from the experimental data. The results of this investigation demonstrate design principles for targeted ionic liquids when optimisation for the free volume is required, and demonstrate the utility of argon as a simple, noninteracting probe. As more ionic liquids find their way into industrial processes of scale, these findings are important for their utilisation in the capture of any gaseous solute, gas separation, or in processes involving the transformation of gases or small molecules.

The branching of ionic liquid cation sidechains utilizing silicon as the backbone was explored and it was found that this structural feature leads to fluids with remarkably low density and viscosity.     

Single-ion conducting polymer electrolytes as a key jigsaw piece for next-generation battery applications

As lithium-ion batteries have been the state-of-the-art electrochemical energy storage technology, the overwhelming demand for energy storage on a larger scale has triggered the development of next-generation battery technologies possessing high energy density, longer cycle lives, and enhanced safety. However, commercial liquid electrolytes have been plagued by safety issues due to their flammability and instability in contact with electrodes. Efforts have focused on developing such electrolytes by covalently immobilizing anionic groups onto a polymer backbone, which only allows Li⁺ cations to be mobile through the polymer matrix. Such ion-selective polymers provide many advantages over binary ionic conductors in battery operation, such as minimization of cell polarization and dendrite growth. In this review, the design, synthesis, fabrication, and class are reviewed to give insight into the physicochemical properties of single-ion conducting polymer electrolytes. The standard characterization method and remarkable electrochemical properties are further highlighted, and perspectives on current challenges and future directions are also discussed.

A review of the physicochemical properties of single-ion conducting polymer electrolytes is presented. The standard characterization method, remarkable electrochemical properties and perspectives are further highlighted.     

High-performance all-solid-state electrolyte for sodium batteries enabled by the interaction between the anion in salt and Na3SbS4

All-solid-state sodium batteries with poly(ethylene oxide) (PEO)-based electrolytes have shown great promise for large-scale energy storage applications. However, the reported PEO-based electrolytes still suffer from a low Na⁺ transference number and poor ionic conductivity, which mainly result from the simultaneous migration of Na⁺ and anions, the high crystallinity of PEO, and the low concentration of free Na⁺. Here, we report a high-performance PEO-based all-solid-state electrolyte for sodium batteries by introducing Na₃SbS₄ to interact with the TFSI⁻ anion in the salt and decrease the crystallinity of PEO. The optimal PEO/NaTFSI/Na₃SbS₄ electrolyte exhibits a remarkably enhanced Na⁺ transference number (0.49) and a high ionic conductivity of 1.33 × 10⁻⁴ S cm⁻¹ at 45 °C. Moreover, we found that the electrolyte can largely alleviate Na⁺ depletion near the electrode surface in symmetric cells and, thus, contributes to stable and dendrite-free Na plating/stripping for 500 h. Furthermore, all-solid-state Na batteries with a 3,4,9,10-perylenetetracarboxylic dianhydride cathode exhibit a high capacity retention of 84% after 200 cycles and superior rate performance (up to 10C). Our work develops an effective way to realize a high-performance all-solid-state electrolyte for sodium batteries.

A high-performance all-solid-state PEO/NaTFSI/Na₃SbS₄ electrolyte for sodium batteries is realized owing to the electrostatic interaction between TFSI⁻ in the salt and Na₃SbS₄, which immobilizes TFSI⁻ anions and promotes the dissociation of NaTFSI.     

Cucurbiturils mimicked by low polarizability solvents with pre-formed cavities: an empirical model to predict hydrocarbon selectivity

Relative binding affinities of a series of nine rigid hydrocarbons towards the cavity formed by a portion of the inner wall of cucurbit[8]uril (CB[8]) and a positive auxiliary guest were determined by competitive ¹⁹F NMR titrations in deuterium oxide. The corresponding free binding energies were corrected by the hydrocarbon computed solvation energies to obtain their free energies of transfer from the gas phase to the CB[8]/auxiliary guest cavity. These energies correlate linearly with the hydrocarbon static polarizabilities, thereby suggesting that the selectivity is driven, perhaps exclusively, by dispersive interactions between the hydrocarbons and the tailor-made cavity, regardless of the degree of unsaturation of the guests. The free energies of transfer also correlate linearly with the energy released upon introduction of the hydrocarbon into a pre-formed cavity extruded from a solvent (benzene) selected to mimic the polarity and polarizability of the CB[8]/auxiliary probe cavity – and this, with a unity slope. Among other features, this empirical model also accurately predicts the relative binding affinities of various rigid hydrocarbons to CB[6] and CB[7], as well as noble gases to CB[5], when the macrocycles are mimicked with pre-formed cavities in perfluorohexane or perfluorohexane/benzene mixtures, both being notoriously non-polar and non-polarizable environments.

Mimicking cucurbiturils with low polarizability solvents and pre-formed cavities allows the in silico prediction of their selectivities towards hydrocarbons and noble gases in aqueous solution.     

Non-concentrated aqueous electrolytes with organic solvent additives for stable zinc batteries

Rechargeable aqueous zinc batteries (RAZBs) are promising for large-scale energy storage because of their superiority in addressing cost and safety concerns. However, their practical realization is hampered by issues including dendrite growth, poor reversibility and low coulombic efficiency (CE) of Zn anodes due to parasitic reactions. Here, we report a non-concentrated aqueous electrolyte composed of 2 m zinc trifluoromethanesulfonate (Zn(OTf)₂) and the organic dimethyl carbonate (DMC) additive to stabilize the Zn electrochemistry. Unlike the case in conventional aqueous electrolytes featuring typical Zn[H₂O]₆²⁺ solvation, a solvation sheath of Zn²⁺ with the co-participation of the DMC solvent and OTf⁻ anion is found in the formulated H₂O + DMC electrolyte, which contributes to the formation of a robust ZnF₂ and ZnCO₃-rich interphase on Zn. The resultant Zn anode exhibits a high average CE of Zn plating/stripping (99.8% at an areal capacity of 2.5 mA h cm⁻²) and dendrite-free cycling over 1000 cycles. Furthermore, the H₂O + DMC electrolytes sustain stable operation of RAZBs pairing Zn anodes with diverse cathode materials such as vanadium pentoxide, manganese dioxide, and zinc hexacyanoferrate. Rational electrolyte design with organic solvent additives would promote building better aqueous batteries.

Involvement of dimethyl carbonate and trifluoromethanesulfonate anions in a hybrid aqueous electrolyte enables the formation of a new Zn²⁺-solvation structure and a ZnF₂–ZnCO₃-rich interphase that stabilizes the Zn battery chemistry.     

Direct measurement of the genuine efficiency of thermogalvanic heat-to-electricity conversion in thermocells

Harvesting wasted thermal energy could make important contributions to global energy sustainability. Thermogalvanic devices are simple, chemistry-based devices which can convert heat to electricity, through facile redox chemistry. The efficiency of this process is the ratio of electrical energy generated by the cell (in Watts) to the quantity of thermal energy that passes through the cell (also in Watts). Prior work estimated the quantity of thermal energy passed through a thermocell by applying a conductive heat transfer model to the electrolyte. Here, we employ a heat flux sensor to unambiguously quantify both heat flux and electrical power. By evaluating the effect of electrode separation, temperature difference and gelation of the electrolyte, we found significant discrepancy between the estimated model and the quantified reality. For electrode separation, the trend between estimated and measured efficiency went in opposite directions; as a function of temperature difference, they demonstrated the same trend, but estimated values were significantly higher. This was due to significant additional convection and radiation contributions to the heat flux. Conversely, gelled electrolytes were able to suppress heat flux mechanisms and achieve experimentally determined efficiency values in excess of the estimated values (at small electrode separations), with partially gelled systems being particularly effective. This study provides the ability to unambiguously benchmark and assess the absolute efficiency and Carnot efficiency of thermogalvanic electrolytes and even the whole thermocell device, allowing ‘total device efficiency’ to be quantified. The deviation between the routinely applied estimation methodology and actual measurement will support the rational development of novel thermal energy harvesting chemistries, materials and devices.

We report the first genuine quantification of thermogalvanic heat-to-electricity conversion efficiency, for both the electrolyte and for the entire device.     

Active material and interphase structures governing performance in sodium and potassium ion batteries

Development of energy storage systems is a topic of broad societal and economic relevance, and lithium ion batteries (LIBs) are currently the most advanced electrochemical energy storage systems. However, concerns on the scarcity of lithium sources and consequently the expected price increase have driven the development of alternative energy storage systems beyond LIBs. In the search for sustainable and cost-effective technologies, sodium ion batteries (SIBs) and potassium ion batteries (PIBs) have attracted considerable attention. Here, a comprehensive review of ongoing studies on electrode materials for SIBs and PIBs is provided in comparison to those for LIBs, which include layered oxides, polyanion compounds and Prussian blue analogues for positive electrode materials, and carbon-based and alloy materials for negative electrode materials. The importance of the crystal structure for electrode materials is discussed with an emphasis placed on intrinsic and dynamic structural properties and electrochemistry associated with alkali metal ions. The key challenges for electrode materials as well as the interface/interphase between the electrolyte and electrode materials, and the corresponding strategies are also examined. The discussion and insights presented in this review can serve as a guide regarding where future investigations of SIBs and PIBs will be directed.

The importance of the active material structure and the interface/interphase between the electrode and electrolyte in enhancing the electrochemical performance of sodium and potassium ion batteries.     

A stable and high-energy aqueous aluminum based battery

Aqueous aluminum ion batteries (AAIBs) have received growing attention because of their low cost, safe operation, eco-friendliness, and high theoretical capacity. However, one of the biggest challenges for AAIBs is the poor reversibility due to the presence of an oxide layer and the accompanying hydrogen evolution reaction. Herein, we develop a strongly hydrolyzed/polymerized aluminum–iron hybrid electrolyte to improve the electrochemical behavior of AAIBs. On the one hand, the designed electrolyte enables aluminum ion intercalation/deintercalation on the cathode while stable deposition/stripping of aluminium occurs on the anode. On the other hand, the electrolyte contributes to the electrochemical energy storage through an iron redox reaction. These two reactions are parallel and coupled through an Fe–Al alloy on the anode, thus enhancing the reversibility and energy density of AAIBs. As a result, this hybrid-ion battery delivers a specific volumetric capacity of 35 A h L⁻¹ at the current density of 1.0 mA cm⁻², and remarkable stability with a capacity retention of 90% over 500 cycles. Furthermore, the hybrid-ion battery achieves a high energy density of approximately 42 W h L⁻¹ with an average operating voltage of 1.1 V. This green electrolyte for high-energy AAIBs holds promises for large-scale energy storage applications.

A hybrid-ion aqueous aluminium ion battery (HIAAIB) with nickel hexacyanoferrate as the cathode, Al as the anode and a polymerized Al–Fe hybrid electrolyte is reported. During discharge, an Fe–Al alloy forms at the anode, improving performance by relieving corrosion.     

A novel high-energy-density lithium-free anode dual-ion battery and in situ revealing the interface structure evolution

Lithium-free anode dual-ion batteries have attracted extensive studies due to their simple configuration, reduced cost, high safety and enhanced energy density. For the first time, a novel Li-free DIB based on a carbon paper anode (Li-free CGDIB) is reported in this paper. Carbon paper anodes usually have limited application in DIBs due to their poor electrochemical performance. Herein, by using a lithium bis(fluorosulfonyl)imide (LiFSI)-containing electrolyte, the battery shows outstanding electrochemical performance with a capacity retention of 96% after 300 cycles at 2C with a stable 98% coulombic efficiency and 89% capacity retention after 500 cycles at 5C with a stable coulombic efficiency of 98.5%. Moreover, the electrochemical properties of the CGDIB were investigated with a variety of in situ characterization techniques, such as in situ EIS, XRD and online differential electrochemical mass spectrometry (OEMS). The multifunctional effect of the LiFSI additive on the electrochemical properties of the Li-free CGDIB was also systematically analyzed, including generating a LiF-rich interfacial film, prohibiting Li dendrite growth effectively and forming a defective structure of graphite layers. This design strategy and fundamental analysis show great potential and lay a theoretical foundation for facilitating the further development of DIBs with high energy density.

A novel lithium-free anode dual-ion battery is fabricated based on a carbon paper anode. In situ EIS, XRD and OEMS demonstrate the multi-functional effects of LiFSI on the performance of the Li-free CGDIB.     

Constructing nitrided interfaces for stabilizing Li metal electrodes in liquid electrolytes

Traditional Li ion batteries based on intercalation-type anodes have been approaching their theoretical limitations in energy density. Replacing the traditional anode with metallic Li has been regarded as the ultimate strategy to develop next-generation high-energy-density Li batteries. Unfortunately, the practical application of Li metal batteries has been hindered by Li dendrite growth, unstable Li/electrolyte interfaces, and Li pulverization during battery cycling. Interfacial modification can effectively solve these challenges and nitrided interfaces stand out among other functional layers because of their impressive effects on regulating Li⁺ flux distribution, facilitating Li⁺ diffusion through the solid-electrolyte interphase, and passivating the active surface of Li metal electrodes. Although various designs for nitrided interfaces have been put forward in the last few years, there is no paper that specialized in reviewing these advances and discussing prospects. In consideration of this, we make a systematic summary and give our comments based on our understanding. In addition, a comprehensive perspective on the future development of nitrided interfaces and rational Li/electrolyte interface design for Li metal electrodes is included.

In this perspective, we make a systematic summary and give out our comments on constructing nitrided interfaces for stabilizing Li metal electrodes.     

Alchemical absolute protein–ligand binding free energies for drug design

The recent advances in relative protein–ligand binding free energy calculations have shown the value of alchemical methods in drug discovery. Accurately assessing absolute binding free energies, although highly desired, remains a challenging endeavour, mostly limited to small model cases. Here, we demonstrate accurate first principles based absolute binding free energy estimates for 128 pharmaceutically relevant targets. We use a novel rigorous method to generate protein–ligand ensembles for the ligand in its decoupled state. Not only do the calculations deliver accurate protein–ligand binding affinity estimates, but they also provide detailed physical insight into the structural determinants of binding. We identify subtle rotamer rearrangements between apo and holo states of a protein that are crucial for binding. When compared to relative binding free energy calculations, obtaining absolute binding free energies is considerably more challenging in large part due to the need to explicitly account for the protein in its apo state. In this work we present several approaches to obtain apo state ensembles for accurate absolute ΔG calculations, thus outlining protocols for prospective application of the methods for drug discovery.

Molecular dynamics based absolute protein–ligand binding free energies can be calculated accurately and at large scale to facilitate drug discovery.     

Self-assembled cationic organic nanosheets: role of positional isomers in a guanidinium-core for efficient lithium-ion conduction

The growing energy demand with the widespread use of smart portable electronics, as well as an exponential increase in demand for smart batteries for electric vehicles, entails the development of efficient portable batteries with high energy density and safe power storage systems. Li-ion batteries arguably have superior energy density to all other traditional batteries. Developing mechanically robust solid-state electrolytes (SSEs) for lithium-ion conduction for an efficient portable energy storage unit is vital to empower this technology and overcome the safety constraints of liquid electrolytes. Herein, we report the formation of self-assembled organic nanosheets (SONs) utilizing positional isomers of small organic molecules (AM-2 and AM-3) for use as SSEs for lithium-ion conduction. Solvent-assisted exfoliation of the bulk powder yielded SONs having near-atomic thickness (∼4.5 nm) with lateral dimensions in the micrometer range. In contrast, self-assembly in the DMF/water solvent system produced a distinct flower-like morphology. Thermodynamic parameters, crystallinity, elemental composition, and nature of H-bonding for two positional isomers are established through various spectroscopic and microscopic studies. The efficiency of the lithium-ion conducting properties is correlated with factors like nanostructure morphology, ionic scaffold, and locus of the functional group responsible for forming the directional channel through H-bonding in the positional isomer. Amongst the three different morphologies studied, SONs display higher ion conductivity. In between the cationic and zwitterionic forms of the monomer, integration of the cationic scaffold in the SON framework led to higher conductivity. Amongst the two positional isomers, the meta-substituted carboxyl group forms a more rigid directional channel through H-bonding to favor ionic mobility and accounts for the highest ion conductivity of 3.42 × 10⁻⁴ S cm⁻¹ with a lithium-ion transference number of 0.49 at room temperature. Presumably, this is the first demonstration that signifies the importance of the cationic scaffold, positional isomers, and nanostructure morphologies in improving ionic conductivity. The ion-conducting properties of such SONs having a guanidinium-core may have significance for other interdisciplinary energy-related applications.

Self-assembled organic nanosheets (SONs) having a near-atomic thickness (∼4.5 nm) are obtained through exfoliation. Among two positional isomers of the guanidinium-core analogue used for SONs, one shows greatly improved Li⁺ ion conductivity.     

How many more polymorphs of ROY remain undiscovered

With 12 crystal forms, 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecabonitrile (a.k.a. ROY) holds the current record for the largest number of fully characterized organic crystal polymorphs. Four of these polymorph structures have been reported since 2019, raising the question of how many more ROY polymorphs await future discovery. Employing crystal structure prediction and accurate energy rankings derived from conformational energy-corrected density functional theory, this study presents the first crystal energy landscape for ROY that agrees well with experiment. The lattice energies suggest that the seven most stable ROY polymorphs (and nine of the twelve lowest-energy forms) on the Z′ = 1 landscape have already been discovered experimentally. Discovering any new polymorphs at ambient pressure will likely require specialized crystallization techniques capable of trapping metastable forms. At pressures above 10 GPa, however, a new crystal form is predicted to become enthalpically more stable than all known polymorphs, suggesting that further high-pressure experiments on ROY may be warranted. This work highlights the value of high-accuracy crystal structure prediction for solid-form screening and demonstrates how pragmatic conformational energy corrections can overcome the limitations of conventional density functionals for conformational polymorphs.

Crystal structure prediction suggests that the low-energy polymorphs of ROY have already been found, but a new high-pressure form is predicted.     

The underlying mechanism for reduction stability of organic electrolytes in lithium secondary batteries

Many organic solvents have very desirable solution properties, such as wide temperature range, high solubility of Li salts and nonflammability, and should be able but fail in reality to serve as electrolyte solvents for Li-ion or -metal batteries due to their reduction instability. The origin of this interfacial instability remains unsolved and disputed so far. Here, we reveal for the first time the origin of the reduction stability of organic carbonate electrolytes by combining ab initio molecular dynamics (AIMD) simulations, density functional theory (DFT) calculations and electrochemical stability experiments. It is found that with the increase of the molar ratio (MR) of salt to solvent, the anion progressively enters into the solvation shell of Li⁺ to form an anion-induced ion–solvent-coordinated (AI-ISC) structure, leading to a “V-shaped” change of the LUMO energy level of coordinated solvent molecules, whose interfacial stability first decreases and then increases with the increased MRs of salt to solvent. This mechanism perfectly explains the long-standing puzzle about the interfacial compatibility of organic electrolytes with Li or similar low potential anodes and provides a basic understanding and new insights into the rational design of the advanced electrolytes for next generation lithium secondary batteries.

By theoretical and experimental evidence, the underlying mechanism for the enhanced reduction stability of the HMRE is revealed, suggesting that the interfacial stability of the electrolyte can be adjusted through the modulation of the anion-induced ISC structure.

The state-of-the-art electrolytes in Li-ion batteries (LIBs) are mostly based on 1.0 mol L⁻¹ LiPF₆/ethylene carbonate (EC)-based carbonate due to the surface passivation of the graphite anode by forming a stable solid electrolyte interphase (SEI). However, these electrolytes cannot operate well for new electrode materials and battery systems that are expected to have higher voltage, better safety and wider temperature range than current commercial LIBs.^1–3 For example, EC-based carbonate electrolytes are easily oxidized on a high voltage cathode at or above 4.3 V, resulting in depletion of electrolytes, gas evolution and low coulombic efficiency, which reduce the cycle life and create safety hazards for LIBs.⁴ These problems of the conventional electrolyte significantly hinder the development of new generation lithium batteries and limit these batteries for high voltage and/or high capacity applications and operation in a wide temperature range.To overcome these problems, great efforts have been devoted in recent years to the development of new electrolytes, such as solid state electrolytes,⁵ ionic liquids,^6–8 highly-concentrated electrolytes (HCEs),⁹ electrolyte stabilizing additive,^10–13 and so on. Among them, the HCEs or high-molar-ratio electrolytes (HMREs) of salt to solvent have received particular attention, owing to their unusual electrochemical stability, nonflammability, and good compatibility with a wide range of anode and cathode materials.^14–17 These desirable properties are apparently attributed to the solution structure of HCEs, where there exist almost no free solvent molecules, and the parasitic side reactions of solvents are thereby greatly reduced. Due to the lack of solvent molecules in HCEs, anions have to enter into the solvation shell of Li⁺, in order to meet the Li⁺ coordination number of 4–6, to form an ion–solvent-coordinated (ISC) structure.¹⁸ Several studies have shown that the unique ISC structure of HCEs leads to the shift of the lowest unoccupied molecular orbital (LUMO) from solvent to salt, which makes anions preferentially reduced or decomposed to produce a robust anion-derived SEI.^14,19 In recent years, the anion-derived SEI structure has been regarded as the “holy grail” of electrolyte chemistry for understanding the interfacial stability and compatibility of HCEs. However, recent studies have showed that some HCEs containing non-film-forming salts and solvents can still achieve excellent reversible Li⁺ insertion reactions.²⁰ Therefore, an intrinsic origin for the interfacial stability of HCEs still remains unrevealed. In our previous studies on HCEs or HMREs, their interfacial stability was found to depend predominately on the molar ratio (MR) of salt to solvent rather than the molar concentration.^2,21,22 Thus, the HMREs instead of the HCEs in the following study could more clearly describe the nature of electrolyte stability.In this work, we reveal the correlation between the solvation microstructures and the LUMO energy levels of typical ISC structures in the electrolytes at various MRs with non-film-forming lithium salt (LiClO₄) and organic carbonate solvents (PC, DMC, EMC and DEC) by ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations. The choice of non-film-forming lithium salt and solvent in this study was aimed to exclude the contribution of the formation of the SEI film to the interfacial stability of the electrolytes. It is found from this study that the LUMO energy level of the ISC structure formed at a low MR is lower than that of pure solvent. With the increase of the MR, anions gradually enter into the first solvation shell of Li⁺ to form the anion-induced ISC (AI-ISC) structure, resulting in the increase of the LUMO energy level that enhances the reduction stability of the electrolyte. Also, it is revealed that the LUMO levels of ISC structures at different MRs are always situated at the coordinated solvent molecules, i.e., the strong reduction stability of HMREs is dominated by the modulation of solvent molecules rather than only the formation of the anion-derived SEI. Such a theoretical insight is further unequivocally evidenced by chemical compatibility experiments in this work. These findings reveal the origin of the greatly improved interfacial stability of HMREs and provide a mechanistic insight into the rational design of stable electrolytes for new generation alkali or alkaline metal based batteries.To investigate the specific ISC microstructures of the electrolytes with different MRs, AIMD simulations were first performed (see computational details in the ESI^†). Taking non-film-forming DEC solvent as an example, three types of electrolytes with MRs of LiClO₄ to DEC = 1 : 10, 1 : 5 and 1 : 2 are considered (Table S1^†). After long-time AIMD simulation, the representative images of the equilibrium structures are shown in Fig. 1a–c. To characterize the solution structures, the radial distribution function g(r) of the electrolyte with different MRs is analyzed (Fig. 1e–g), and the changes in the Li⁺ coordination number with the O atoms of solvents and anions are listed in Fig. 1d. In addition, it should be noted that the total coordination number of Li⁺ always remains around 4, which implies that the stable tetragonal solvation shell structure of Li⁺ does not change in the different MR electrolytes; meanwhile, both the coordination numbers of Li⁺ contributed by the solvent and anion change oppositely. This phenomenon can be corroborated experimentally through infrared spectroscopy (IR) because the C O bond of the carbonate group has a strong IR absorption in the carbonyl region (1650–1850 cm⁻¹) and its IR peak position shifts sensitively with its coordination environment. As shown in Fig. 1h, the IR band of carbonyl groups in pure DEC is located at ∼1741 cm⁻¹, which is shifted to ∼1710 cm⁻¹ in a LiClO₄/DEC (MR = 1 : 10) electrolyte due to the coordination of the O atom in C O with Li⁺. With the increase of the MR of Li⁺/DEC, its IR peak at ∼1741 cm⁻¹ gradually disappears, reflecting a gradual decrease in the number of free DEC molecules. In addition, the IR band of free ClO₄⁻ in a LiClO₄/DEC (MR = 1 : 10) electrolyte is located at ∼931 cm⁻¹, which is shifted to ∼942 cm⁻¹ in the 1 : 2 LiClO₄/DEC electrolyte due to the ionic association of Li⁺ and ClO₄⁻ (Fig. S1^†). Combining AIMD simulations and IR experiments, it can be concluded that with the increase of the MR of the electrolyte, the anions gradually enter into the solvation shell of Li⁺, which modulates the chemical stability of the electrolyte.Open in a separate windowFig. 1Snapshots of typical equilibrium trajectories from DFT-MD simulations: (a) 1 : 10 LiClO₄/DEC solution (2-LiClO₄/20-DEC), (b) 1 : 5 LiClO₄/DEC solution (3-LiClO₄/15-DEC) and (c) 1 : 2 LiClO₄/DEC solution (7-LiClO₄/14-DEC). (d) Typical ISC structure extracted from DFT-MD. (e–g) Radial distribution function of lithium–oxygen interaction (short dashed lines) and relationship between the coordination number and bond distances (full lines). (h) FTIR spectra of the carbonyl group in LiClO₄/DEC solution. Atom color: H, white; Li, purple; C, cyan; O, red; Cl, green.Coordination numbers (n(r)) of atom pairs of Li–O(DEC) and Li–O (ClO₄⁻) (cut-off length of r = 2.5 Å)

Reducing the internal reorganization energy via symmetry controlled π-electron delocalization

The magnitude of the reorganization energy is closely related to the nonradiative relaxation rate, which affects the photoemission quantum efficiency, particularly for the emission with a lower energy gap toward the near IR (NIR) region. In this study, we explore the relationship between the reorganization energy and the molecular geometry, and hence the transition density by computational methods using two popular models of NIR luminescent materials: (1) linearly conjugated cyanine dyes and (2) electron donor–acceptor (D–A) composites with various degrees of charge transfer (CT) character. We find that in some cases, reorganization energies can be significantly reduced to 50% despite slight structural modifications. Detailed analyses indicate that the reflection symmetry plays an important role in linear cyanine systems. As for electron donor–acceptor systems, both the donor strength and the substitution position affect the relative magnitude of reorganization energies. If CT is dominant and creates large spatial separation between HOMO and LUMO density distributions, the reorganization energy is effectively increased due to the large electron density variation between S₀ and S₁ states. Mixing a certain degree of local excitation (LE) with CT in the S₁ state reduces the reorganization energy. The principles proposed in this study are also translated into various pathways of canonically equivalent π-conjugation resonances to represent intramolecular π-delocalization, the concept of which may be applicable, in a facile manner, to improve the emission efficiency especially in the NIR region.

The reorganization energies may be significantly reduced by molecular symmetry effect.     

Evolutionary exploration of polytypism in lead halide perovskites

The regular ABX₃ cubic perovskite structure is composed of close-packed AX₃ layers stacked along the 〈111〉 axis. An equivalent hexagonal close-packed network can also be formed, in addition to a series of intermediate polytype sequences. Internally, these correspond to combinations of face- and corner-sharing octahedral chains that can dramatically alter the physical properties of the material. Here, we assess the thermodynamics of polytypism in CsPbI₃ and CsPbBr₃. The total energies obtained from density functional theory are used to paramaterize an axial Ising-type model Hamiltonian that includes linear and cubic correlation terms of the pseudo-spin. A genetic algorithm is built to explore the polytype phase space that grows exponentially with the number of layers. The ground-state structures of CsPbX₃ polytypes are analysed to identify features of polytypism such as the distinct arrangements of layers and symmetry forbidden sequences. A number of polytypes with low ordering energies (around thermal energy at room temperature) are predicted, which could form distinct phases or appear as stacking faults within perovskite grains.

Beyond the regular perovskite structure based on cubic-close packing exists a range of possible polytypes that we explore using computational chemistry.     

Nanostructured liquid-crystalline Li-ion conductors with high oxidation resistance: molecular design strategy towards safe and high-voltage-operation Li-ion batteries

Nanostructured, uncharged liquid-crystalline (LC) electrolyte molecules having bicyclohexyl and cyclic carbonate moieties have been developed for application in Li-ion batteries as quasi-solid electrolytes, which suppress leakage and combustion. Towards the development of safe and high performance Li-ion batteries, we have designed Li-ion conductive LC materials with high oxidation resistance using density functional theory (DFT) calculation. The DFT calculation suggests that a mesogen with a bicyclohexyl moiety is suitable for the high-oxidation-resistance LC electrolytes compared to a mesogen containing phenylene moieties. A tri(oxyethylene) chain introduced between the cyclic carbonate and the bicyclohexyl moiety in the core part tunes the viscosity and the miscibility with Li salts. The designed Li-ion conductive LC molecules exhibit smectic LC phases over a wide temperature range, and they are miscible with added lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt up to 5 : 5 in molar ratio in their smectic phases. The resulting LC mixtures with LiTFSI show oxidation resistance above 4.0 V vs. Li/Li⁺ in cyclic voltammetry measurements. The enhanced oxidation resistance improves the performance of Li half-cells containing LC electrolytes.

Ion-conductive liquid-crystalline molecules with high-oxidation resistance, which were designed with density functional theory calculation, improved charge–discharge reactions in Li-ion batteries.     

Comparing interfacial cation hydration at catalytic active sites and spectator sites on gold electrodes: understanding structure sensitive CO2 reduction kinetics

Hydrated cations present in the electrochemical double layer (EDL) are known to play a crucial role in electrocatalytic CO₂ reduction (CO₂R), and numerous studies have attempted to explain how the cation effect contributes to the complex CO₂R mechanism. CO₂R is a structure sensitive reaction, indicating that a small fraction of total surface sites may account for the majority of catalytic turnover. Despite intense interest in specific cation effects, probing site-specific, cation-dependent solvation structures remains a significant challenge. In this work, CO adsorbed on Au is used as a vibrational Stark reporter to indirectly probe solvation structure using vibrational sum frequency generation (VSFG) spectroscopy. Two modes corresponding to atop adsorption of CO are observed with unique frequency shifts and potential-dependent intensity profiles, corresponding to direct adsorption of CO to inactive surface sites, and in situ generated CO produced at catalytic active sites. Analysis of the cation-dependent Stark tuning slopes for each of these species provides estimates of the hydrated cation radius upon adsorption to active and inactive sites on the Au electrode. While cations are found to retain their bulk hydration shell upon adsorption at inactive sites, catalytic active sites are characterized by a single layer of water between the Au surface and the electrolyte cation. We propose that the drastic increase in catalytic performance at active sites stems from this unique solvation structure at the Au/electrolyte interface. Building on this evidence of a site-specific EDL structure will be critical to understand the connection between cation-dependent interfacial solvation and CO₂R performance.

Site-specific vibrational probes were used to elucidate the interfacial solvation structure between catalytic active sites and inactive sites on a Au electrode to reveal a unique, opposing cation-dependent double layer structure at active sites.