Hitherto Unknown Solvent and Anion Pairs in Solvation Structures Reveal New Insights into High-Performance Lithium-Ion Batteries

Solvent-solvent and solvent-anion pairings in battery electrolytes have been identified for the first time by nuclear magnetic resonance spectroscopy. These hitherto unknown interactions are enabled by the hydrogen bonding induced by the strong Lewis acid Li⁺ , and exist between the electron-deficient hydrogen (δ⁺ H) present in the solvent molecules and either other solvent molecules or negatively-charged anions. Complementary with the well-established strong but short-ranged Coulombic interactions between cation and solvent molecules, such weaker but longer-ranged hydrogen-bonding casts the formation of an extended liquid structure in electrolytes that is influenced by their components (solvents, additives, salts, and concentration), which in turn dictates the ion transport within bulk electrolytes and across the electrolyte-electrode interfaces. The discovery of this new inter-component force completes the picture of how electrolyte components interact and arrange themselves, sets the foundation to design better electrolytes on the fundamental level, and probes battery performances.

Switching Electrolyte Interfacial Model to Engineer Solid Electrolyte Interface for Fast Charging and Wide-Temperature Lithium-Ion Batteries

Engineering the solid electrolyte interphase (SEI) that forms on the electrode is crucial for achieving high performance in metal-ion batteries. However, the mechanism of SEI formation resulting from electrolyte decomposition is not fully understood at the molecular scale. Herein, a new strategy of switching electrolyte to tune SEI properties is presented, by which a unique and thinner SEI can be pre-formed on the graphite electrode first in an ether-based electrolyte, and then the as-designed graphite electrode can demonstrate extremely high-rate capabilities in a carbonate-based electrolyte, enabling the design of fast-charging and wide-temperature lithium-ion batteries (e.g., graphite | LiNi_0.6 Co_0.2 Mn_0.2 O₂ (NCM622)). A molecular interfacial model involving the conformations and electrochemical stabilities of the Li⁺ -solvent-anion complex is presented to elucidate the differences in SEI formation between ether-based and carbonate-based electrolytes, then interpreting the reason for the obtained higher rate performances. This innovative concept combines the advantages of different electrolytes into one battery system. It is believed that the switching strategy and understanding of the SEI formation mechanism opens a new avenue to design SEI, which is universal for pursuing more versatile battery systems with greater stability.

Solvent-Diluent Interaction-Mediated Solvation Structure of Localized High-Concentration Electrolytes

The latest developments of localized high-concentration electrolytes (LHCEs) shed light on stabilizing the high-energy-density lithium (Li) metal batteries. It is generally considered that the nonsolvating diluents introduced into the LHCEs improve the viscosity and wettability of high-concentration electrolytes (HCEs) without changing their inner solvation structures, thus maintaining the highly coordinated contact ion pairs (CIPs) and ionic aggregates (AGGs) of the precursor HCEs with limited free solvent numbers and high Coulombic efficiency (CE) of Li metal anodes. Herein, we show an unexpected effect of the diluent amount on the solvation structures of the LHCEs: as the diluent amount increases, the proportions of free solvent molecules and CIPs rise up simultaneously. The latter is probably due to the partial splits of the AGGs via the dipole-dipole interactions between the diluent and solvent molecules. Accordingly, a moderately diluted LHCE shows the best Coulombic efficiency of Li metal anodes (99.6%), compared with the precursor HCE (97.4%) or highly diluted LHCE (99.0%). This work reveals a new criterion of the LHCE chemical formulation for the designing of advanced electrolytes for high-energy-density batteries.

Low-Temperature Electrolyte Design for Lithium-Ion Batteries: Prospect and Challenges

Lithium-ion batteries have dominated the energy market from portable electronic devices to electric vehicles. However, the LIBs applications are limited seriously when they were operated in the cold regions and seasons if there is no thermal protection. This is because the Li+ transportation capability within the electrode and particularly in the electrolyte dropped significantly due to the decreased electrolyte liquidity, leading to a sudden decline in performance and short cycle-life. Thus, design a low-temperature electrolyte becomes ever more important to enable the further applications of LIBs. Herein, we summarize the low-temperature electrolyte development from the aspects of solvent, salt, additives, electrolyte analysis, and performance in the different battery systems. Then, we also introduce the recent new insight about the cation solvation structure, which is significant to understand the interfacial behaviors at the low temperature, aiming to guide the design of a low-temperature electrolyte more effectively.

Electrolyte Chemistry in 3D Metal Oxide Nanorod Arrays Deciphers Lithium Dendrite-Free Plating/Stripping Behaviors for High-Performance Lithium Batteries

Lithium dendrite-free deposition is crucial to stabilizing lithium batteries, where the three-dimensional (3D) metal oxide nanoarrays demonstrate an impressive capability to suppress dendrite due to the spatial effect. Herein, we introduce a new insight into the ameliorated lithium plating process on 3D nanoarrays. As a paradigm, novel 3D Cu₂O and Cu nanorod arrays were in situ designed on copper foil. We find that the dendrite and electrolyte decomposition can be mitigated effectively by Cu₂O nanoarrays, while the battery failed fast when the Cu nanoarrays were used. We show that Li₂O (i.e., formed in the lithiation of Cu₂O) is critical to stabilizing the electrolyte; otherwise, the electrolyte would be decomposed seriously. Our viewpoint is further proved when we revisit the metal (oxide) nanoarrays reported before. Thus, we discovered the importance of electrolyte stability as a precondition for nanoarrays to suppress dendrite and/or achieve a reversible lithium plating/stripping for high-performance lithium batteries.

Trifunctional Electrolyte Additive Hexadecyltrioctylammonium Iodide for Lithium-Sulfur Batteries with Extended Cycle Life

Lithium-sulfur (Li-S) battery with a very high theoretical energy density (∼2500 Wh kg^-1) is a very promising alternative to the commercial lithium-ion battery as the next-generation energy storage device. However, the Li-S battery suffers from shuttle effect and Li dendrites growth due to the solubility of polysulfides in the electrolyte system and the inhomogeneous deposition of Li, resulting in short cycling life span, which is the major obstacle in its practical application. Herein, we report an additive, hexadecyltrioctylammonium iodide (HTOA-I), in the conventional electrolyte system, which shows trifunctional effect on extending Li-S battery cycle life. It can not only help us to form a protective solid-electrolyte interface (SEI) on the surface of Li anode so as to reduce the contact of polysulfides with Li but also hinder the shuttling of polysulfides to the Li anode due to the strong combination of large-sized HTOA⁺ with polysulfide anions (S_n^2-), which retard the migration of S_n^2- and cause homogeneous Li deposition owing to the large size and stronger trend of HTOA⁺ to be absorbed on Li anode as well. A new method of phosphorescence analysis for direct observation of polysulfides shuttling has been put forward for the first time, which can be further developed in future studies. The cell with the HTOA-I-added electrolyte system shows high cycling stability, retaining 83.4% of the initial capacity after 200 cycles at 1 A g^-1 and achieving 689 mAh g^-1 even after 1000 cycles. This cost-effective and facile approach will not increase the complexity of the battery manufacturing process. Compared to other electrolyte additives, the additive in our work, HTOA-I, has better positive effects on extending cycle life. This trifunctional electrolyte additive will inspire the design of other new additives and further promote the development of Li-S batteries.

Electrolyte-Mediated Stabilization of High-Capacity Micro-Sized Antimony Anodes for Potassium-Ion Batteries

Alloying anodes exhibit very high capacity when used in potassium-ion batteries, but their severe capacity fading hinders their practical applications. The failure mechanism has traditionally been attributed to the large volumetric change and/or their fragile solid electrolyte interphase. Herein, it is reported that an antimony (Sb) alloying anode, even in bulk form, can be stabilized readily by electrolyte engineering. The Sb anode delivers an extremely high capacity of 628 and 305 mAh g^-1 at current densities of 100 and 3000 mA g^-1 , respectively, and remains stable for more than 200 cycles. Interestingly, there is no need to do nanostructural engineering and/or carbon modification to achieve this excellent performance. It is shown that the change in K⁺ solvation structure, which is tuned by electrolyte composition (i.e., anion, solvent, and concentration), is the main reason for achieving this excellent performance. Moreover, an interfacial model based on the K⁺ -solvent-anion complex behavior is presented. The electronegativity of the K⁺ -solvent-anion complex, which can be tuned by changing the solvent type and anion species, is used to predict and control electrode stability. The results shed new light on the failure mechanism of alloying anodes, and provide a new guideline for electrolyte design that stabilizes metal-ion batteries using alloying anodes.

Regulating the Solvation Structure of Nonflammable Electrolyte for Dendrite-Free Li-Metal Batteries

High-energy-density Li-metal batteries are of great significance in the energy storage field. However, the safety hazards caused by Li dendrite growth and flammable organic electrolytes significantly hinder the widespread application of Li-metal batteries. In this work, we report a highly safe electrolyte composed of 4 M lithium bis(fluorosulfonyl)imide (LiFSI) dissolved in the single solvent trimethyl phosphate (TMP). By regulating the solvation structure of the electrolyte, a combination of nonflammability and Li dendrite growth suppression was successfully realized. Both Raman spectroscopy and molecular dynamics simulations revealed improved dendrite-free Li anode originating from the unique solvation structure of the electrolyte. Symmetric Li/Li cells fabricated using this nonflammable electrolyte had a long cycle life of up to 1000 h at a current density of 0.5 mA cm^-2. Furthermore, the Li₄Ti₅O₁₂/TMP-4/Li full cells also exhibited excellent cycling performance with a high initial discharge capacity of 170.5 mAh g^-1 and a capacity retention of 92.7% after 200 cycles at 0.2 C. This work provides an effective approach for the design of safe electrolytes with favorable solvation structure toward the large-scale application of Li-metal batteries.

Application of Anomalous X-ray Scattering Method to Liquid Electrolytes Used in a Battery: Local Structural Analysis around a Dilute Metallic Ion

In liquid electrolytes used for a battery, various metal complexes are formed as a result of ion-solvent and ion-ion interactions, which strongly influence the properties of the electrolyte and thus the performance of the battery. Therefore, the structural characterization of such complexes is of great importance. In this study, the anomalous X-ray scattering (AXS) technique was applied to the potassium hydroxide solution including ∼0.3 mol % zinc, which is widely used in various batteries such as the alkaline battery. In spite of the small amount of the metallic ions, we have successfully extracted a local structure around zinc after careful data analysis. The obtained pair distribution function exhibited not only the short-range correlation corresponding to the Zn-O bond within the zincate anion but also a medium-range correlation above 3.5 Å. The present study demonstrates the capability of the AXS technique to detect local structures around dilute metallic ions in liquid electrolytes, which will largely extend the applicable range of this technique, especially to the field related to batteries.

Engineering Sodium-Ion Solvation Structure to Stabilize Sodium Anodes: Universal Strategy for Fast-Charging and Safer Sodium-Ion Batteries

Sodium-ion batteries are promising alternatives for lithium-ion batteries due to their lower cost caused by global sodium availability. However, the low Coulombic efficiency (CE) of the sodium metal plating/stripping process represents a serious issue for the Na anode, which hinders achieving a higher energy density. Herein, we report that the Na⁺ solvation structure, particularly the type and location of the anions, plays a critical role in determining the Na anode performance. We show that the low CE results from anion-mediated corrosion, which can be tackled readily through tuning the anion interaction at the electrolyte/anode interface. Our strategy thus enables fast-charging Na-ion and Na-S batteries with a remarkable cycle life. The presented insights differ from the prevailing interpretation that the failure mechanism mostly results from sodium dendrite growth and/or solid electrolyte interphase formation. Our anionic model introduces a new guideline for improving the electrolytes for metal-ion batteries with a greater energy density.

Reduced-Graphene-Oxide-Guided Directional Growth of Planar Lithium Layers

Rechargeable lithium (Li) metal batteries hold great promise for revolutionizing current energy-storage technologies. However, the uncontrollable growth of lithium dendrites impedes the service of Li anodes in high energy and safety batteries. There are numerous studies on Li anodes, yet little attention has been paid to the intrinsic electrocrystallization characteristics of Li metal and their underlying mechanisms. Herein, a guided growth of planar Li layers, instead of random Li dendrites, is achieved on self-assembled reduced graphene oxide (rGO). In situ optical observation is performed to monitor the morphology evolution of such a planar Li layer. Moreover, the underlying mechanism during electrodeposition/stripping is revealed using ab initio molecular dynamics simulations. The combined experiment and simulation results show that when Li atoms are deposited on rGO, each layer of Li atoms grows along (110) crystallographic plane of the Li crystals because of the fine in-plane lattice matching between Li and the rGO substrate, resulting in planar Li deposition. With this specific topographic characteristic, a highly flexible lithium-sulfur (Li-S) full cell with rGO-guided planar Li layers as the anode exhibits stable cycling performance and high specific energy and power densities. This work enriches the fundamental understanding of Li electrocrystallization without dendrites and provides guidance for practical applications.