Electrolyte Reactivity in the Double Layer in Mg Batteries: An Interface Potential-Dependent DFT Study

Binary Cation Matrix Electrolyte and Its Effect on Solid Electrolyte Interphase Suppression and Evolution of Si Anode

An unstable solid electrolyte interphase (SEI) has been recognized as one of the biggest challenges to commercializing silicon (Si) anodes for high-energy-density batteries. This work thoroughly investigates a binary cation matrix of Mg2++Li+ electrolyte and its role in SEI development, suppression, and evolution of a Si anode. Findings demonstrate that introducing Mg ions dramatically reduces the SEI growth before lithiation occurs, primarily due to the suppression of solvent reduction, particularly ethylene carbonate (EC) reduction. The Mg2+ alters the Li+ cation solvation environment as EC preferably participates in the oxophyllic Mg2+ solvation sheath, thereby altering the solvent reduction process, resulting in a distinct SEI formation mechanism. The initial SEI formation before lithiation is reduced by 70% in the electrolyte with the presence of Mg2+ cations. While the SEI continues to develop in the postlithiation, the inclusion of Mg ions results in an approximately 80% reduction in the postlithiation SEI growth. Continuous electrochemical cycling reveals that Mg2+ plays a crucial role in stabilizing the deep-lithiated Si phases, which effectively mitigates side reactions, resulting in controlled SEI growth and stable interphase while eliminating complex LixSiy formation. Mg ions promote the development of a notably more rigid and homogeneous SEI, characterized by a reduced dissipation (ΔD) in the Mg2++Li+ ion matrix compared to the solely Li+ system. This report reveals how the Mg2++Li+ ion matrix affects the SEI evolution, viscoelastic properties, and electrochemical behavior at the Si interface in real time, laying the groundwork for devising strategies to enhance the performance and longevity of Si-based next-generation battery systems.

Structure and Dynamics of Aqueous 2-Aminothiazole/NaCl Electrolytes at Electrified Interfaces

A computational study was performed to investigate the dynamics of aqueous electrolytes containing organic corrosion inhibitors near electrified interfaces by using the constant-charge model in classical molecular dynamics simulations. The results showed that when inhibitors form films at the interface, the surface charge of the electrode causes displacement of the molecules, referred to as electroporation. The hydrophobicity of the inhibitor molecules affects both the stability of the films and their recovery time. This study highlights the value of computational investigations of the dynamics within inhibitor films as a complementary approach to the traditional focus on inhibitor-substrate interactions, leading to deeper insights into the mechanisms of corrosion inhibition mechanisms.

Revealing the Structural Evolution of Electrode/Electrolyte Interphase Formation during Magnesium Plating and Stripping with operando EQCM-D

Rechargeable magnesium batteries could provide future energy storage systems with high energy density. One remaining challenge is the development of electrolytes compatible with the negative Mg electrode, enabling uniform plating and stripping with high Coulombic efficiencies. Often improvements are hindered by a lack of fundamental understanding of processes occurring during cycling, as well as the existence and structure of a formed interphase layer at the electrode/electrolyte interface. Here, a magnesium model electrolyte based on magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2 ) and MgCl2 with a borohydride as additive, dissolved in dimethoxyethane (DME), was used to investigate the initial galvanostatic plating and stripping cycles operando using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D). We show that side reactions lead to the formation of an interphase of irreversibly deposited Mg during the initial cycles. EQCM-D based hydrodynamic spectroscopy reveals the growth of a porous layer during Mg stripping. After the first cycles, the interphase layer is in a dynamic equilibrium between the formation of the layer and its dissolution, resulting in a stable thickness upon further cycling. This study provides operando information of the interphase formation, its changes during cycling and the dynamic behavior, helping to rationally develop future electrolytes and electrode/electrolyte interfaces and interphases.

Research development on electrolytes for magnesium-ion batteries

Magnesium-ion batteries (MIBs) are considered strong candidates for next-generation energy-storage systems owing to their high theoretical capacity, divalent nature and the natural abundancy of magnesium (Mg) resources on Earth. However, the development of MIBs has been mainly limited by the incompatibility of Mg anodes with several Mg salts and conventional organic-liquid electrolytes. Therefore, one major challenge faced by MIBs technology lies on developing safe electrolytes, which demonstrate appropriate electrochemical voltage window and compatibility with Mg anode. This review discusses the development of MIBs from the point-of-view of the electrolyte syntheses. A systematic assessment of promising electrolyte design strategies is proposed including liquid and solid-state electrolytes. Liquid-based electrolytes have been largely explored and can be categorized by solvent-type: organic solvent, aqueous solvent, and ionic-liquids. Organic-liquid electrolytes usually present high electrochemical and chemical stability but are rather dangerous, while aqueous electrolytes present high ionic conductivity and eco-friendliness but narrow electrochemical stability window. Some ionic-liquid electrolytes have proved outstanding performance but are fairly expensive. As alternative to liquid electrolytes, solid-state electrolytes are increasingly attractive to increase energy density and safety. However, improving the ionic conductivity of Mg ions in these types of electrolytes is extremely challenging. We believe that this comprehensive review will enable researchers to rapidly grasp the problems faced by electrolytes for MIBs and the electrolyte design strategies proposed to this date.

Fine tuning of electrosynthesis pathways by modulation of the electrolyte solvation structure

Electrosynthesis is a method of choice for designing new synthetic routes owing to its ability to selectively conduct reactions at controlled potentials, high functional group tolerance, mild conditions and sustainability when powered by renewables. When designing an electrosynthetic route, the selection of the electrolyte, which is composed of a solvent, or a mixture of solvents, and a supporting salt, is a prerequisite. The electrolyte components, generally assumed to be passive, are chosen because of their adequate electrochemical stability windows and to ensure the solubilization of the substrates. However, very recent studies point towards an active role of the electrolyte in the outcome of electrosynthetic reactions, challenging its inert character. Particular structuring of the electrolyte at nano- and micro-scales can occur and impact the yield and selectivity of the reaction, which is often overlooked. In the present Perspective, we highlight how mastering the electrolyte structure, both in bulk and at electrochemical interfaces, introduces an additional level of control for the design of new electrosynthetic methods. For this purpose, we focus our attention on oxygen-atom transfer reactions using water as the sole oxygen source in hybrid organic solvent/water mixtures, these reactions being emblematic of this new paradigm.

An Electrostatically Embedded QM/MM Scheme for Electrified Interfaces

Atomistic modeling of electrified interfaces remains a major issue for detailed insights in electrocatalysis, corrosion, electrodeposition, batteries, and related devices such as pseudocapacitors. In these domains, the use of grand-canonical density functional theory (GC-DFT) in combination with implicit solvation models has become popular. GC-DFT can be conveniently applied not only to metallic surfaces but also to semiconducting oxides and sulfides and is, furthermore, sufficiently robust to achieve a consistent description of reaction pathways. However, the accuracy of implicit solvation models for solvation effects at interfaces is in general unknown. One promising way to overcome the limitations of implicit solvents is going toward hybrid quantum mechanical (QM)/molecular mechanics (MM) models. For capturing the electrochemical potential dependence, the key quantity is the capacitance, i.e., the relation between the surface charge and the electrochemical potential. In order to retrieve the electrochemical potential from a QM/MM hybrid scheme, an electrostatic embedding is required. Furthermore, the charge of the surface and of the solvent regions has to be strictly opposite in order to consistently simulate charge-neutral unit cells in MM and in QM. To achieve such a QM/MM scheme, we present the implementation of electrostatic embedding in the VASP code. This scheme is broadly applicable to any neutral or charged solid/liquid interface. Here, we demonstrate its use in the context of GC-DFT for the hydrogen evolution reaction (HER) over a noble-metal-free electrocatalyst, MoS₂. We investigate the effect of electrostatic embedding compared to the implicit solvent model for three contrasting active sites on MoS₂: (i) the sulfur vacancy defect, which is rather apolar; (ii) a Mo antisite defect, where the active site is a surface bound highly polar OH group; and (iii) a reconstructed edge site, which is generally believed to be responsible for most of the catalytic activity. According to our results, the electrostatic embedding leads to almost indistinguishable results compared to the implicit solvent for the apolar system but has a significant effect on polar sites. This demonstrates the reliability of the hybrid QM/MM, electrostatically embedded solvation model for electrified interfaces.

Interfacial Engineering of Magnesiophilic Coordination Layer Stabilizes Mg Metal Anode

Rechargeable magnesium batteries (RMBs) are seriously plagued by the direct exposure of the Mg anode to the electrolyte components, leading to spontaneous and electrochemical side reactions and interfacial passivation. Herein, a benign coordination layer is constructed at the Mg/electrolyte interface where aniline with a strong magnesiophilic amine group and high stability to Mg is chosen as representative, which has higher adsorption energy than DME (1,2-dimethoxyethane) and trace water. This Mg coordination environment mitigates side reactions, forming a non-passivating interface consisting of aniline and much fewer by-products after several cycles. Therefore, the Mg symmetrical cell operates with a low overpotential and uniform Mg0 deposition. This interfacial coordination can also be adopted for Mg anode protection in various electrolyte cases of Mg(TFSI)2 electrolyte systems.

Chloride-Free Electrolytes for High-Voltage Magnesium Metal Batteries: Challenges, Strategies, and Perspectives

The demand for high-energy-density and safe energy storage devices has spurred increasing interest in high-voltage rechargeable magnesium batteries (RMB). As electrolytes are the bridge connecting the cathode and anode materials, the development of high-voltage electrolytes is the key factor in realizing high-voltage RMBs. This concept presents an overview of three chloride-free electrolyte systems with wide electrochemical windows, together with the degradation mechanisms and modification strategies at the anode/electrolyte interphase. Finally, future directions in stabilizing Mg anodes and realizing high-voltage RMBs are highlighted.

Adaptive Electric Fields Embedded Electrochemical Barrier Calculations

Electrochemical interfaces are grand canonical ensembles of varying electrons. Simulating them by standard first-principles methods is a challenging task, since the number of electrons (or charge) is fixed in the calculation. Under the constant charge framework, we developed a constant potential simulation method realized by adding an adaptive electric field to a charge neutral system. Electric field is the controlling variable. In addition, we defined an internal reversible hydrogen electrode potential (ϕ_IRHE), which can ensure the model independence of our method. To validate our method, the reaction energies of some electrochemical reactions are calculated, the results are comparable with the computational hydrogen electrode model and experiments. At last, the evolution of transition state structures and charge transfer coefficients of some electrochemical reactions on Ag(111) surface were discussed by our method.

Organic Electrolyte Design for Rechargeable Batteries: From Lithium to Magnesium

Rechargeable magnesium batteries (RMBs) have been considered as one of the most viable battery chemistries amongst the "post" lithium-ion battery (LIB) technologies owing to their high volumetric capacity and the natural abundance of their key elements. The fundamental properties of Mg-ion conducting electrolytes are of essence to regulate the overall performance of RMBs. In this Review, the basic electrochemistry of Mg-ion conducting electrolytes batteries is discussed and compared to that of the Li-ion conducting electrolytes, and a comprehensive overview of the development of different Mg-ion conducting electrolytes is provided. In addition, the remaining challenges and possible solutions for future research are intensively discussed. The present work is expected to give an impetus to inspire the discovery of key electrolytes and thereby improve the electrochemical performances of RMBs and other related emerging battery technologies.

Electrolyte and Interphase Design for Magnesium Anode: Major Challenges and Perspectives

The rechargeable magnesium battery (RMB) is regarded as a high-energy, safe, and cost-effective alternative for conventional batteries. Unfortunately, the passivation and uneven Mg growth not only raise the voltage hysteresis but also shorten the cycle life of RMBs. In this review, Mg passivation induced by electrolytes/contaminants, growth patterns of high dimensional Mg⁰ , and mechanisms of Mg anode degradation are discussed. The recent efforts on suppressing electrolyte decomposition and uneven Mg growth including electrolyte/interphase modifications through additives, weakly coordinating anions, artificial interphases, and 3D magnesiophilic hosts are summarized. Finally, the future directions in stabilizing Mg anode and realizing high-performance RMBs are highlighted.

Investigating the abnormal conductivity behaviour of divalent cations in low dielectric constant tetraglyme-based electrolytes

Solutions made of tetraglyme (G4) containing Ca(TFSI)₂ have been studied as models to understand the solvation structure and the conductivity properties of multivalent ions in low dielectric constant ethereal electrolytes. These solutions have been characterised using electrochemical impedance spectroscopy, rheological measurement, and Raman spectroscopy. The ionic conductivity of these electrolytes shows an intriguing non-monotonic behaviour with temperature which deviates from the semi-empirical Vogel-Tammann-Fulcher equation at a critical temperature. This behaviour is observed for both Mg(TFSI)₂ and Ca(TFSI)₂, but not LiTFSI, indicating a difference in the solvation structure and the thermodynamic properties of divalent ions compared to Li⁺. The origin of this peculiar behaviour is demystified using temperature-controlled Raman spectroscopy and first-principles calculations combined with a thermodynamic analysis of the chemical equilibrium of Ca²⁺ ion-pairing versus solvation. As long-range electrostatic interactions are critical in solutions based on low dielectric ethereal solvents, a periodic approach is here proposed to capture their impact on the solvation structure of the electrolyte at different salt concentrations. The obtained results reveal that the thermodynamic and transport properties of Ca(TFSI)₂/G4 solutions stem from a competition between enthalpic (ionic strength) and entropic factors that are directly controlled by the solution concentration and temperature, respectively. At high salt concentrations, the ionic strength of the solution favours the existence of free ions thanks to the strong solvation energy of the polydentate G4 solvent conjugated with the weak complexation ability of TFSI^-. At elevated temperatures, the configurational entropy associated with the release of a coordinated G4 favours the formation of contact ion-pairs due to its flat potential energy surface (weak strain energy), offering a large configuration space. Such a balance between ion-pair association and dissociation not only rationalises the ionic conductivity behaviour observed for Ca(TFSI)₂/G4 solutions, but also provides valuable information to extrapolate the ionic transport properties of other electrolytes with different M(TFSI)_n salts dissolved in longer-chain glymes or even poly(ethylene oxide). These findings are essential for the understanding of solvation structures and ionic transport in low-dielectric media, which can further be used to design new electrolytes for Li-ion and post Li-ion batteries as well as electrocatalysts.

Tailoring Coordination in Conventional Ether-Based Electrolytes for Reversible Magnesium-Metal Anodes

Rechargeable magnesium (Mg) batteries based on conventional electrolytes are seriously plagued by the formation of the ion-blocking passivation layer on the Mg metal anode. By tracking the Mg²⁺ solvation sheath, this work links the passivation components to the Mg²⁺ -solvents (1,2-dimethoxyethane, DME) coordination and the consequent thermodynamically unstable DME molecules. On this basis, we propose a methodology to tailor solvation coordination by introducing the additive solvent with extreme electron richness. Oxygen atoms in phosphorus-oxygen groups compete with that in carbon-oxygen groups of DME for the coordination with Mg²⁺ , thus softening the solvation sheath deformation. Meanwhile, the organophosphorus molecules in the rearranged solvation sheath decompose on the Mg surface, increasing the Mg²⁺ transport and electrical resistance by three and one orders of magnitude, respectively. Consequently, the symmetric cells exhibit superior cycling performance of over 600 cycles with low polarization.

Ion Motor as a New Universal Strategy for the Boosting the Performance of Zn-Ion Batteries

The quiescent electrolyte causes serious concentration polarization and dendrite problems during the charging and discharging of the battery, which restricts the development of metal secondary batteries and flow batteries. Herein, we report a new concept of ion motors, with which the directional driving and uniformity of the electrolyte are realized to eliminate the concentration polarization and dendritic phenomenon for secondary metal batteries and flow batteries without additional external energy. In this study, a dendrite-free secondary metal battery with ion motors is constructed to eliminate a considerable concentration polarization voltage by a tiny induced counter electromotive force generated by Lorentz force, significantly improving the output power and energy efficiency of the battery. An actual pump-free flow battery with an ion motor is also assembled, which overcomes the problems of low energy efficiency and the complex structure caused by the traditional flow battery requiring 1-2 pumps to drive the electrolyte. The efficiency of ion motors to drive the electrolyte is hundreds of times higher than that of the mechanical pump. Therefore, the ion motor provides a universal strategy for designing more pump-free flow batteries and metal secondary batteries without the risk of dendrites in the future.

On the Practical Applications of the Magnesium Fluorinated Alkoxyaluminate Electrolyte in Mg Battery Cells

High-performance electrolytes are at the heart of magnesium battery development. Long-term stability along with the low potential difference between plating and stripping processes are needed to consider them for next-generation battery devices. Within this work, we perform an in-depth characterization of the novel Mg[Al(hfip)4]2 salt in different glyme-based electrolytes. Specific importance is given to the influence of water content and the role of additives in the electrolyte. Mg[Al(hfip)4]2-based electrolytes exemplify high tolerance to water presence and the beneficial effect of additives under aggravated cycling conditions. Finally, electrolyte compatibility is tested with three different types of Mg cathodes, spanning different types of electrochemical mechanisms (Chevrel phase, organic cathode, sulfur). Benchmarking with an electrolyte containing a state-of-the-art Mg[B(hfip)4]2 salt exemplifies an improved performance of electrolytes comprising the Mg[Al(hfip)4]2 salt and establishes Mg[Al(hfip)4]2 as a new standard salt for the future Mg battery research.

Defect-Free Metal-Organic Framework Membrane for Precise Ion/Solvent Separation toward Highly Stable Magnesium Metal Anode

Metallic magnesium batteries are promising candidates beyond lithium-ion batteries; however, a passive interfacial layer because of the electro-reduction of solvents on Mg surfaces usually leads to ultrahigh overpotential for the reversible Mg chemistry. Inspired by the excellent separation effect of permselective metal-organic framework (MOF) at angstrom scale, a large-area and defect-free MOF membrane directly on Mg surfaces is here constructed. In this process, the electrochemical deprotonation of ligand can be facilitated to afford the self-correcting of intercrystalline voids until a seamless membrane formed, which can eliminate nonselective intercrystalline diffusion of electrolyte and realize selective Mg²⁺ transport but precisely separate the solvent molecules from the MOF channels. Compared with the continuous solvent reduction on bare Mg anode, the as-constructed MOF membrane is demonstrated to significantly stabilize the Mg electrode via suppressing the permeation of solvents, hence contributing to a low-overpotential plating/stripping in conventional electrolytes. The concept is demonstrated that membrane separation can serve as solid-electrolyte interphase, which would be widely applicable to other energy-storage systems.

Amin M, Yamauchi Y, Xu M, Pan L. MoS2 Nanosheets with Expanded Interlayer Spacing for Ultrastable Aqueous Mg-Ion Hybrid Supercapacitor

Aqueous magnesium ion supercapacitors (MISs) have attracted attention due to their safety, low cost and environmental friendliness. However, the cycling stability of MISs is usually not ideal due to magnesium...

Investigation of alkali and alkaline earth solvation structures in tetraglyme solvent

This study compares molecular calculations performed with molecular and periodic codes through an investigation of the solvation structures of alkali and alkaline earth metal ions in tetraglyme solution. The two codes are able to produce equivalent structural and energetic information at the same level of theory, and in the presence of the implicit solvation model or not. This comparison reveals that molecular optimisations can be performed with periodic codes and used directly as input models for interface or electrochemistry calculations in order to preserve the solvent-solute interaction and the cavitation energy. By a rigorous comparison, we have demonstrated that equivalent energetic values can be obtained with the conventional PBE-D3 and the newly developed SCAN-rVV10 functionals. Nevertheless, as far as the vibrational features are concerned and when the molecule possesses a highly conjugated system, the SCAN-rVV10 functional is required to describe the vibrational modes properly. The computed IR/Raman spectra can thus be used as essential information to determine the first solvation shell of metal ions in glyme-based solutions. In tetraglyme solution, the alkali and alkaline earth metal ions exhibit a diverse solvation structure. Small ions like Li⁺ and Mg²⁺ tend to adopt a coordination number of five or six, while larger ions, Na⁺, K⁺, and Ca²⁺, prefer an eight-coordinated environment, and the metal-ligand interaction increases in the order K⁺-O < Na⁺-O < Li⁺-O < Ca²⁺-O < Mg²⁺-O. The solvation spheres play a significant role in the stability and the reactivity of the solvated ions, and can thus be used as input models to construct the solvation structure in more sophisticated electrolytes, such as polyethylene oxide, or perform electrochemical calculations.

Modeling of Electron-Transfer Kinetics in Magnesium Electrolytes: Influence of the Solvent on the Battery Performance

The performance of rechargeable magnesium batteries is strongly dependent on the choice of electrolyte. The desolvation of multivalent cations usually goes along with high energy barriers, which can have a crucial impact on the plating reaction. This can lead to significantly higher overpotentials for magnesium deposition compared to magnesium dissolution. In this work we combine experimental measurements with DFT calculations and continuum modelling to analyze Mg deposition in various solvents. Jointly, these methods provide a better understanding of the electrode reactions and especially the magnesium deposition mechanism. Thereby, a kinetic model for electrochemical reactions at metal electrodes is developed, which explicitly couples desolvation to electron transfer and, furthermore, qualitatively takes into account effects of the electrochemical double layer. The influence of different solvents on the battery performance is studied for the state-of-the-art magnesium tetrakis(hexafluoroisopropyloxy)borate electrolyte salt. It becomes apparent that not necessarily a whole solvent molecule must be stripped from the solvated magnesium cation before the first reduction step can take place. For Mg reduction it seems to be sufficient to have one coordination site available, so that the magnesium cation is able to get closer to the electrode surface. Thereby, the initial desolvation of the magnesium cation determines the deposition reaction for mono-, tri- and tetraglyme, whereas the influence of the desolvation on the plating reaction is minor for diglyme and tetrahydrofuran. Overall, we can give a clear recommendation for diglyme to be applied as solvent in magnesium electrolytes.

Reversible Magnesium Metal Anode Enabled by Cooperative Solvation/Surface Engineering in Carbonate Electrolytes

Magnesium metal anode holds great potentials toward future high energy and safe rechargeable magnesium battery technology due to its divalent redox and dendrite-free nature. Electrolytes based on Lewis acid chemistry enable the reversible Mg plating/stripping, while they fail to match most cathode materials toward high-voltage magnesium batteries. Herein, reversible Mg plating/stripping is achieved in conventional carbonate electrolytes enabled by the cooperative solvation/surface engineering. Strongly electronegative Cl from the MgCl₂ additive of electrolyte impairs the Mg…O = C interaction to reduce the Mg²⁺ desolvation barrier for accelerated redox kinetics, while the Mg²⁺-conducting polymer coating on the Mg surface ensures the facile Mg²⁺ migration and the effective isolation of electrolytes. As a result, reversible plating and stripping of Mg is demonstrated with a low overpotential of 0.7 V up to 2000 cycles. Moreover, benefitting from the wide electrochemical window of carbonate electrolytes, high-voltage (> 2.0 V) rechargeable magnesium batteries are achieved through assembling the electrode couple of Mg metal anode and Prussian blue-based cathodes. The present work provides a cooperative engineering strategy to promote the application of magnesium anode in carbonate electrolytes toward high energy rechargeable batteries.

Ab initiomodelling of interfacial electrochemical properties: beyond implicit solvation limitations

First-principles calculations are an important tool to investigate the complex processes occurring at solid/liquid interfaces which are at the heart of modern technologies. Currently, capturing the whole electrochemical environment at an interface, including the applied potential and solvation, still remains challenging as it necessitates to couple different approaches whose interactions are not fully understood. In this work, a grand canonical density functional theory approach is coupled with solvation models to investigate the electrochemical interfaces under applied potential. We show that a parametrized polarizable continuum model (PCM) which represent solvation in a mean field approach by a continuous polarizable media, possesses catastrophic limitations for the modelling of ionic and charged interfaces. We reveal the origin of PCM instabilities under chemical or electrochemical strong oxidation to be the consequence of a phase transition in the surface Li electronic structure. Thus, PCM undergoes an unphysical response to this phase transition by penetrating within the atomic radius of surface Li atoms. To recover a physical response, an explicit first solvation shell has to be included in addition to the PCM in order to properly describe the electrochemistry of the interface. The Fukui functions show that the first solvation shell becomes involved in the redox process as solvent electron doublet is transferred to the acidic Li⁺. If another explicit solvent layer is added, the interface electrochemical properties become independent of the PCM parameters: in particular capacitance can then be computed from a parameter-free electrochemical approach. This is an important conclusion as the experimental electrochemical capacitance are not easily found and thus the parametrization of the PCM for electrochemical interface can be difficult. This approach can easily be applied to investigate electrochemical properties at the atomic scale and generalized to any electrochemical device for which interfaces play a crucial role.

Stability of Calcium Ion Battery Electrolytes: Predictions from Ab Initio Molecular Dynamics Simulations

Multivalent batteries, such as magnesium-ion, calcium-ion, and zinc-ion batteries, have attracted significant attention as next-generation electrochemical energy storage devices to complement conventional lithium-ion batteries (LIBs). Among them, calcium-ion batteries (CIBs) are the least explored due to difficult reversible Ca deposition-dissolution. In this work, we examined the stability of four different Ca salts with weakly coordinating anions and three different solvents commonly employed in existing battery technologies to identify suitable candidates for CIBs. By employing Born-Oppenheimer molecular dynamics (BOMD) simulations on salt-Ca and solvent-Ca interfaces, we find that the tetraglyme solvent and carborane salt are promising candidates for CIBs. Due to the strong reducing nature of the calcium surface, the other salts and solvents readily decompose. We explain the microscopic mechanisms of salt/solvent decomposition on the Ca surface using time-dependent projected density of states, time-dependent charge-transfer plots, and climbing-image nudged elastic band calculations. Collectively, this work presents the first mechanistic assessment of the dynamical stability of candidate salts and solvents on a Ca surface using BOMD simulations, and provides a predictive path toward designing stable electrolytes for CIBs.

Building Ab Initio Interface Pourbaix diagrams to Investigate Electrolyte Stability in the Electrochemical Double Layer: Application to Magnesium Batteries

Insights into the electrochemical processes occurring at the electrode-electrolyte interface are a crucial step in most electrochemistry domains and in particular in the optimization of the battery technology. However, studying potential-dependent processes at the interface is one of the biggest challenges, both for theoreticians and experimentalists. The challenge is pushed further when stable species also depend on the concentration of specific ligands in the electrolyte, such as chlorides. Herein, we present a general theoretical ab initio methodology to compute a Pourbaix-like diagram of complex electrolytes as a function of electrode potential and anion's chemical potential, that is, concentration. This approach is developed not only for the bulk properties of the electrolytes but also for electrode-electrolyte interfaces. In the case of chlorinated magnesium complexes in dimethoxyethane, we show that the stability domains of the different species are strongly shifted at the interface compared to the bulk of the electrolyte because of the strong local electric fields and charges occurring in the double layer. Thus, as the interfacial stability domains are strongly modified, this approach is necessary to investigate all interface properties that often govern the reaction kinetics, such as solvent degradation at the electrode. Interface Pourbaix diagram is used to give some insights into the improved stability at the Mg anode induced by the addition of chloride. Because of its far-reaching insights, transferability, and wide applicability, the methodology presented herein should serve as a valuable tool not only for the battery community but also for the wider electrochemical one.

Modeling of Ion Agglomeration in Magnesium Electrolytes and its Impacts on Battery Performance

The choice of electrolyte has a crucial influence on the performance of rechargeable magnesium batteries. In multivalent electrolytes an agglomeration of ions to pairs or bigger clusters may affect the transport in the electrolyte and the reaction at the electrodes. In this work the formation of clusters is included in a general model for magnesium batteries. In this model, the effect of cluster formation on transport, thermodynamics and kinetics is consistently taken into account. The model is used to analyze the effect of ion clustering in magnesium tetrakis(hexafluoroisopropyloxy)borate in dimethoxyethane as electrolyte. It becomes apparent that ion agglomeration is able to explain experimentally observed phenomena at high salt concentrations.

Modeling interfacial electrochemistry: concepts and tools

This paper presents a grand canonical formalism and provides tools to investigate electrochemical effects at interfaces.