Anion-Assisted Delivery of Multivalent Cations to Inert Electrodes

Understanding the Cathode-Electrolyte Interfacial Chemistry in Rechargeable Magnesium Batteries

Rechargeable magnesium batteries (RMBs) have garnered significant attention due to their potential to provide high energy density, utilize earth-abundant raw materials, and employ metal anode safely. Currently, the lack of applicable cathode materials has become one of the bottleneck issues for fully exploiting the technological advantages of RMBs. Recent studies on Mg cathodes reveal divergent storage performance depending on the electrolyte formulation, posing interfacial issues as a previously overlooked challenge. This minireview begins with an introduction of representative cathode-electrolyte interfacial phenomena in RMBs, elaborating on the unique solvation behavior of Mg2+, which lays the foundation for interfacial chemistries. It is followed by presenting recently developed strategies targeting the promotion of Mg2+ desolvation in the electrolyte and alternative cointercalation approaches to circumvent the desolvation step. In addition, efforts to enhance the cathode-electrolyte compatibility via electrolyte development and interfacial engineering are highlighted. Based on the abovementioned discussions, this minireview finally puts forward perspectives and challenges on the establishment of a stable interface and fast interfacial chemistry for RMBs.

Ca-dimers, solvent layering, and dominant electrochemically active species in Ca(BH4)2 in THF

Divalent ions (Mg, Ca, and Zn) are being considered as competitive, safe, and earth-abundant alternatives to Li-ion electrochemistry, but present challenges for stable cycling due to undesirable interfacial phenomena. We explore the formation of electroactive species in the electrolyte Ca(BH4)2∣THF using molecular dynamics coupled with a continuum model of bulk and interfacial speciation. Free-energy analysis and unsupervised learning indicate a majority population of neutral Ca dimers and monomers with diverse molecular conformations and an order of magnitude lower concentration of the primary electroactive charged species - the monocation, CaBH[Formula: see text] - produced via disproportionation of neutral complexes. Dense layering of THF molecules within ~1 nm of the electrode surface strongly modulates local electrolyte species populations. A dramatic increase in monocation population in this interfacial zone is induced at negative bias. We see no evidence for electrochemical activity of fully-solvated Ca2+. The consequences for performance are discussed in light of this molecular-scale insight.

Toward a quantitative interfacial description of solvation for Li metal battery operation under extreme conditions

The future application of Li metal batteries (LMBs) at scale demands electrolytes that endow improved performance under fast-charging and low-temperature operating conditions. Recent works indicate that desolvation kinetics of Li+ plays a crucial role in enabling such behavior. However, the modulation of this process has typically been achieved through inducing qualitative degrees of ion pairing into the system. In this work, we find that a more quantitative control of the ion pairing is crucial to minimizing the desolvation penalty at the electrified interface and thus the reversibility of the Li metal anode under kinetic strain. This effect is demonstrated in localized electrolytes based on strongly and weakly bound ether solvents that allow for the deconvolution of solvation chemistry and structure. Unexpectedly, we find that maximum degrees of ion pairing are suboptimal for ultralow temperature and high-rate operation and that reversibility is substantially improved via slight local dilution away from the saturation point. Further, we find that at the optimum degree of ion pairing for each system, weakly bound solvents still produce superior behavior. The impact of these structure and chemistry effects on charge transfer are then explicitly resolved via experimental and computational analyses. Lastly, we demonstrate that the locally optimized diethyl ether-based localized-high-concentration electrolytes supports kinetic strained operating conditions, including cycling down to -60 °C and 20-min fast charging in LMB full cells. This work demonstrates that explicit, quantitative optimization of the Li+ solvation state is necessary for developing LMB electrolytes capable of low-temperature and high-rate operation.

Effect of the Electric Double Layer (EDL) in Multicomponent Electrolyte Reduction and Solid Electrolyte Interphase (SEI) Formation in Lithium Batteries

Electrolytes, consisting of salts, solvents, and additives, must form a stable solid electrolyte interphase (SEI) to ensure the performance and durability of lithium(Li)-ion batteries. However, the electric double layer (EDL) structure near charged surfaces is still unsolved, despite its importance in dictating the species being reduced for SEI formation near a negative electrode. In this work, a newly developed model was used to illustrate the effect of EDL on SEI formation in two essential electrolytes, the carbonate-based electrolyte for Li-ion batteries and the ether-based electrolyte for batteries with Li-metal anodes. Both electrolytes have fluoroethylene carbonate (FEC) as a common additive to form the beneficial F-containing SEI component (e.g., LiF). However, the role of FEC drastically differs in these electrolytes. FEC is an effective SEI modifier for the carbonate-based electrolyte by being the only F-containing species entering the EDL and being reduced, as the anion (PF₆^-) will not enter the EDL. For the ether-based electrolyte, both the anion (TFSI^-) and FEC can enter the EDL and be reduced. The competition of the two species within the EDL due to the surface charge and temperature leads to a unique temperature effect observed in prior experiments: the FEC additive is more effective in modulating SEI components at a low temperature (-40 °C) than at room temperature (20 °C) in the ether-based electrolyte. These collective quantitative agreements with experiments emphasize the importance of incorporating the effect of the EDL in multicomponent electrolyte reduction reactions in simulations/experiments to predict/control the formation of the SEI layer.

Ultralow-Temperature Li/CFx Batteries Enabled by Fast-Transport and Anion-Pairing Liquefied Gas Electrolytes

Lithium fluorinated-carbon (Li/CF_x ) is one of the most promising chemistries for high-energy-density primary energy-storage systems in applications where rechargeability is not required. Though Li/CF_x demonstrates high energy density (>2100 Wh kg^-1 ) under ambient conditions, achieving such a high energy density when exposed to subzero temperatures remains a challenge, particularly under high current density. Here, a liquefied gas electrolyte with an anion-pair solvation structure based on dimethyl ether with a low melting point (-141 °C) and low viscosity (0.12 mPa s, 20 °C), leading to high ionic conductivity (>3.5 mS cm^-1 ) between -70 and 60 °C is reported. Besides that, through systematic X-ray photoelectron spectroscopy integrated with transmission electron microscopy characterizations, the interface of CF_x is evaluated for low-temperature performance. The fast transport and anion-pairing solvation structure of the electrolyte are concluded to bring about reduced charge-transfer resistance at low temperatures, which results in significantly enhanced performance of Li/CF_x cells (1690 Wh kg^-1 , -60 °C based on active materials). Utilizing 50 mg cm^-2 loading electrodes, the Li/CF_x still displays 1530 Wh kg^-1 at -60 °C. This work provides insights into the electrolyte design that may overcome the operational limits of batteries in extreme environments.

Toward Bottom-Up Understanding of Transport in Concentrated Battery Electrolytes

Bottom-up understanding of transport describes how molecular changes alter species concentrations and electrolyte voltage drops in operating batteries. Such an understanding is essential to predictively design electrolytes for desired transport behavior. We herein advocate building a structure-property-performance relationship as a systematic approach to accurate bottom-up understanding. To ensure generalization across salt concentrations as well as different electrolyte types and cell configurations, the property-performance relation must be described using Newman's concentrated solution theory. It uses Stefan-Maxwell diffusivity, _ij , to describe the role of molecular motions at the continuum scale. The key challenge is to connect _ij to the structure. We discuss existing methods for making such a connection, their peculiarities, and future directions to advance our understanding of electrolyte transport.

Predicting the Ion Desolvation Pathway of Lithium Electrolytes and Their Dependence on Chemistry and Temperature

To better understand the influence of electrolyte chemistry on the ion-desolvation portion of charge-transfer beyond the commonly applied techniques, we apply free-energy sampling to simulations involving diethyl ether (DEE) and 1,3-dioxoloane/1,2-dimethoxyethane (DOL/DME) electrolytes, which display bulk solvation structures dominated by ion-pairing and solvent coordination, respectively. This analysis was conducted at a pristine electrode with and without applied bias at 298 and 213 K to provide insights into the low-temperature charge-transfer behavior, where it has been proposed that desolvation dominates performance. We find that, to reach the inner Helmholtz layer, ion-paired structures are advantageous and that the Li⁺ ion must reach a total coordination number of 3, which requires the shedding of 1 species in the DEE electrolyte or 2-3 species in DOL/DME. This work represents an effort to predict the distinct thermodynamic states as well as the most probable kinetic pathways of ion desolvation relevant for the charge transfer at electrochemical interphases.

Exchange-Mediated Transport in Battery Electrolytes: Ultrafast or Ultraslow?

Understanding the mechanisms of charge transport in batteries is important for the rational design of new electrolyte formulations. Persistent questions about ion transport mechanisms in battery electrolytes are often framed in terms of vehicular diffusion by persistent ion-solvent complexes versus structural diffusion through the breaking and reformation of ion-solvent contacts, i.e., solvent exchange events. Ultrafast two-dimensional (2D) IR spectroscopy can probe exchange processes directly via the evolution of the cross-peaks on picosecond time scales. However, vibrational energy transfer in the absence of solvent exchange gives rise to the same spectral signatures, hiding the desired processes. We employ 2D IR on solvent resonances of a mixture of acetonitrile isotopologues to differentiate chemical exchange and energy-transfer dynamics in a comprehensive series of Li⁺, Mg²⁺, Zn²⁺, Ca²⁺, and Ba²⁺ bis(trifluoromethylsulfonyl)imide electrolytes from the dilute to the superconcentrated regime. No exchange phenomena occur within at least 100 ps, regardless of the ion identity, salt concentration, and presence of water. All of the observed spectral dynamics originate from the intermolecular energy transfer. These results place the lower experimental boundary on the ion-solvent residence times to several hundred picoseconds, much slower than previously suggested. With the help of MD simulations and conductivity measurements on the Li⁺ and Zn²⁺ systems, we discuss these results as a continuum of vehicular and structural modalities that vary with concentration and emphasize the importance of collective electrolyte motions to ion transport. These results hold broadly applicable to many battery-relevant ions and solvents.

Insights into Spontaneous Solid Electrolyte Interphase Formation at Magnesium Metal Anode Surface from Ab Initio Molecular Dynamics Simulations

Spontaneous chemical reactivity at multivalent (Mg, Ca, Zn, Al) electrode surfaces is critical to solid electrolyte interphase (SEI) formation, and hence, directly affects the longevity of batteries. Here, we report an investigation of the reactivity of 0.5 M Mg(TFSI)₂ in 1,2-dimethoxyethane (DME) solvent at a Mg(0001) surface using ab initio molecular dynamics (AIMD) simulations and detailed Bader charge analysis. Based on the simulations, the initial degradation reactions of the electrolyte strongly depend on the structure of the Mg(TFSI)₂ species near the anode surface. At the surface, the dissociation of Mg(TFSI)₂ species occurs via cleavage of the N-S bond for the solvent separated ion pair (SSIP) and via cleavage of the C-S bond for the contact ion pair (CIP) configuration. In the case of the CIP, both TFSI anions undergo spontaneous bond dissociation reactions to form atomic O, C, S, F, and N species adsorbed on the surface of the Mg anode. These products indicate that the initial SEI layer formed on the surface of the pristine Mg anode consists of a complex mixture of multiple components such as oxides, carbides, sulfides, fluorides, and nitrides. We believe that the atomic-level insights gained from these simulations will lay the groundwork for the rational design of tailored and functional interphases that are critical for the success of multivalent battery technology.