The Solvation of Anions in Propylene Carbonate

Co-Solvent Electrolyte Design to Inhibit Phase Transition toward High Performance K+/Zn2+ Hybrid Battery

Manganese hexacyanoferrate (MnHCF) is one of the most promising cathode materials for aqueous battery because of its non-toxicity, high energy density, and low cost. But the phase transition from MnHCF to Zinc hexacyanoferrate (ZnHCF) and the larger Stokes radius of Zn2+ cause rapid capacity decay and poor rate performance in aqueous Zn battery. Hence, to overcome this challenge, a solvation structure of propylene carbonate (PC)-trifluoromethanesulfonate (Otf)-H2O is designed and constructed. A K+/Zn2+ hybrid battery is prepared using MnHCF as cathode, zinc metal as anode, KOTf/Zn(OTf)2 as the electrolyte, and PC as the co-solvent. It is revealed that the addition of PC inhabits the phase transition from MnHCF to ZnHCF, broaden the electrochemical stability window, and inhibits the dendrite growth of zinc metal. Hence, the MnHCF/Zn hybrid co-solvent battery exhibits a reversible capacity of 118 mAh g-1 and high cycling performance, with a capacity retention of 65.6% after 1000 cycles with condition of 1 A g-1. This work highlights the significance of rationally designing the solvation structure of the electrolyte and promotes the development of high-energy-density of aqueous hybrid ion batteries.

Rational Design of Functional Electrolytes Towards Commercial Dual-Ion Batteries

Dual-ion batteries (DIBs) based on anion (de)intercalation into low-cost graphitic carbon cathodes hold great promise in grid-scale energy storage. Different from the electrolyte in rocking-chair batteries, which only serves as a charge transporter, both cations and anions in the electrolyte for DIBs participate in battery reactions. Hence, the impact of the electrolyte formulation on cycle life, energy density, as well as cost has become a subject of vital importance. This review discussed the challenges and recent progress of electrolytes for DIBs, with a particular focus on the exploration of electrolytes with high oxidation stability, high salt concentration, high ionic conductivity, and low cost. Moreover, the influence of varied ion concentrations at different state-of-charge levels on the electrolyte properties such as ionic conductivity and electrochemical stability is analyzed. Finally, perspectives on the current limitations and future research directions of electrolytes for DIBs are provided.

Enhancing magneto-ionic effects in cobalt oxide films by electrolyte engineering

Electric-field-driven ion motion to tailor magnetic properties of materials (magneto-ionics) offers much promise in the pursuit of voltage-controlled magnetism for highly energy-efficient spintronic devices. Electrolyte gating is a relevant means to create intense electric fields at the interface between magneto-ionic materials and electrolytes through the so-called electric double layer (EDL). Here, improved magneto-ionic performance is achieved in electrolyte-gated cobalt oxide thin films with the addition of inorganic salts (potassium iodide, potassium chloride, and calcium tetrafluoroborate) to anhydrous propylene carbonate (PC) electrolyte. Ab initio molecular dynamics simulations of the EDL structure show that K⁺ is preferentially located on the cobalt oxide surface and KI (when compared to KCl) favors the accumulation of positive charge close to the surface. It is demonstrated that room temperature magneto-ionics in cobalt oxide thin films is dramatically enhanced in KI-containing PC electrolyte at an optimum concentration, leading to 11-fold increase of generated magnetization and 35-fold increase of magneto-ionic rate compared to bare PC.

Co-Solvent Electrolyte Engineering for Stable Anode-Free Zinc Metal Batteries

Anode-free metal batteries can in principle offer higher energy density, but this requires them to have extraordinary Coulombic efficiency (>99.7%). Although Zn-based metal batteries are promising for stationary storage, the parasitic side reactions make anode-free batteries difficult to achieve in practice. In this work, a salting-in-effect-induced hybrid electrolyte is proposed as an effective strategy that enables both a highly reversible Zn anode and good stability and compatibility toward various cathodes. The as-prepared electrolyte can also work well under a wide temperature range (i.e., from -20 to 50 °C). It is demonstrated that in the presence of propylene carbonate, triflate anions are involved in the Zn²⁺ solvation sheath structure, even at a low salt concentration (2.14 M). The unique solvation structure results in the reduction of anions, thus forming a hydrophobic solid electrolyte interphase. The waterproof interphase along with the decreased water activity in the hybrid electrolyte effectively prevents side reactions, thus ensuring a stable Zn anode with an unprecedented Coulombic efficiency (99.93% over 500 cycles at 1 mA cm^-2). More importantly, we design an anode-free Zn metal battery that exhibits excellent cycling stability (80% capacity retention after 275 cycles at 0.5 mA cm^-2). This work provides a universal strategy to design co-solvent electrolytes for anode-free Zn metal batteries.

Specific Ion Solvation and Pairing Effects in Glycerol Carbonate

Identifying the driving forces behind the solvation of inorganic salts by nonaqueous solvents is an important step in the development of green solvents. Here we focus on one promising solvent: glycerol carbonate (GC). Using ab initio molecular dynamics simulations, we build upon our previous work by detailing glycerol carbonate's interactions with a series of anions, a lithium ion, and the LiF ion pair. Through these investigations, we highlight the changes in solvation behavior as the anion size increases, the competition of binding shown by lithium for the oxygens of GC, and the behavior of the LiF ion pair in a GC solution. These results indicate the importance of the cation's identity in ion-pairing structure and dynamics and lend insight into the key factors behind the specific ion effects seen in GC.

Quantum Simulations of Hydrogen Bonding Effects in Glycerol Carbonate Electrolyte Solutions

The need for environmentally friendly nonaqueous solvents in electrochemistry and other fields has motivated recent research into the molecular-level solvation structure, thermodynamics, and dynamics of candidate organic liquids. In this paper, we present the results of quantum density functional theory simulations of glycerol carbonate (GC), a molecule that has been proposed as a solvent for green industrial chemistry, nonaqueous alternatives for biocatalytic reactions, and liquid media in energy storage devices. We investigate the structure and dynamics of both the pure GC liquid and electrolyte solutions containing KF and KCl ion pairs. These simulations reveal the importance of hydrogen bonding that controls the structural and dynamic behavior of the pure liquid and ion association in the electrolyte solutions. The results illustrate the difficulties associated with classical modeling of complex organic solvents. The simulations lead to a better understanding of the underlying mechanisms behind the previously observed peculiar ion-specific behavior in GC electrolyte solutions.

Conjugated electrochromic polymers with amide-containing side chains enabling aqueous electrolyte compatibility

This work illustrates an effective side-chain modification approach using amide functional groups to induce aqueous electroactivity to ProDOT-based electrochromic polymers.

Probing the Hofmeister series beyond water: Specific-ion effects in non-aqueous solvents

We present an experimental investigation of specific-ion effects in non-aqueous solvents, with the aim of elucidating the role of the solvent in perturbing the fundamental ion-specific trend. The focus is on the anions: CH₃COO^->F^->Cl^->Br^->I^->ClO₄^->SCN^- in the solvents water, methanol, formamide, dimethyl sulfoxide (DMSO), and propylene carbonate (PC). Two types of experiments are presented. The first experiment employs the technique of size exclusion chromatography to evaluate the elution times of electrolytes in the different solvents. We observe that the fundamental (Hofmeister) series is observed in water and methanol, whilst the series is reversed in DMSO and PC. No clear series is observed for formamide. The second experiment uses the quartz crystal microbalance technique to follow the ion-induced swelling and collapse of a polyelectrolyte brush. Here the fundamental series is observed in the protic solvents water, methanol, and formamide, and the series is once again reversed in DMSO and PC. These behaviours are not attributed to the protic/aprotic nature of the solvents, but rather to the polarisability of the solvents and are due to the competition between the interaction of ions with the solvent and the surface. A rule of thumb is proposed for ion specificity in non-aqueous solvents. In weakly polarisable solvents, the trends in specific-ion effects will follow those in water, whereas in strongly polarisable solvents the reverse trend will be observed. Solvents of intermediate polarisability will give weak specific-ion effects.

Liquid K-Na Alloy Anode Enables Dendrite-Free Potassium Batteries

A K-Na liquid alloy allows a dendrite-free high-capacity anode; its immiscibility with an organic liquid electrolyte offers a liquid-liquid anode-electrolyte interface. Working with a sodiated Na₂ MnFe(CN)₆ cathode, the working cation becomes K⁺ to give a potassium battery of long cycle life with an acceptable capacity at high charge/discharge rates.