Formation of NaF-Rich Solid Electrolyte Interphase on Na Anode through Additive-Induced Anion-Enriched Structure of Na+ Solvation.

High-capacity sodium (Na) anodes suffer from dendrite growth due to the high reactivity, which can be overcome through inducing a stable NaF-rich solid electrolyte interphase (SEI). Herein, we propose an additive strategy for realizing the anion-enriched structure of Na+ solvation to obtain a NaF-rich SEI. The electron-withdrawing acetyl group in 4-acetylpyridine (4-APD) increases the coordination number of PF6 - in the Na+ solvation sheath to facilitate PF6 - to decompose into NaF. Thus, the NaF-rich SEI with high mechanical stability and interfacial energy is formed to repress the growth of Na dendrites. With the 4-APD-contained electrolyte, the symmetric Na||Na cells show excellent cycling performance over 360 h at 1.0 mA cm-2 . Meanwhile, excellent stability is also achieved for Na||Na3 V2 (PO4 )2 O2 F full cells with high Coulombic efficiency (97 %) and capacity retention (91 %) after 200 cycles.

Optimizing Electrode/Electrolyte Interphases and Li-Ion Flux/Solvation for Lithium-Metal Batteries with Qua-Functional Heptafluorobutyric Anhydride.

The safety and electrochemical performance of rechargeable lithium-metal batteries (LMBs) are primarily influenced by the additives in the organic liquid electrolytes. However, multi-functional additives are still rarely reported. Herein, we proposed heptafluorobutyric anhydride (HFA) as a qua-functional additive to optimize the composition and structure of the solid electrolyte interphase (SEI) at the electrode/electrolyte interface. The reduction/oxidation decomposition of the fluorine-rich HFA facilitate uniform inorganic-rich SEI and compact cathode electrolyte interphase (CEI) formation, which enables stable lithium plating during charge and suppresses the dissolution of transition-metal ions. Moreover, HFA optimizes the Li-ion solvation for stable Li plating/stripping and serves as the surfactant to enhance the wettability of the separator by the electrolyte to increase Li-ion flux. The symmetric Li∥Li cell with 1.0 wt % HFA electrolyte had an excellent cycling performance over 340 h at 1.0 mA cm-2 with a capacity of 0.5 mAh cm-2 while the Li∥NCM622 cell maintained high capacity retention after 250 cycles and outstanding rate performance even at 15 C.

Gradient Solid Electrolyte Interphase and Lithium-Ion Solvation Regulated by Bisfluoroacetamide for Stable Lithium Metal Batteries.

The structures and components of solid electrolyte interphase (SEI) are extremely important to influence the performance of full cells, which is determined by the formulation of electrolyte used. However, it is still challenging to control the formation of high-quality SEI from structures to components. Herein, we designed bisfluoroacetamide (BFA) as the electrolyte additive for the construction of a gradient solid electrolyte interphase (SEI) structure that consists of a lithophilic surface with C-F bonds to uniformly capture Li ions and a LiF-rich bottom layer to guide the rapid transportation of Li ions, endowing the homogeneous deposition of Li ions. Moreover, the BFA molecule changes the Li+ solvation structure by reducing free solvents in electrolyte to improve the antioxidant properties of electrolyte and prevent the extensive degradation of electrolyte on the cathode surface, which can make a superior cathode electrolyte interphase (CEI) with high-content LiF.

Construction of Localized High-Concentration PF6 - Region for Suppressing NCM622 Cathode Failure at High Voltage.

High-voltage Li||LiNi0.6 Co0.6 Mn0.2 O2 (NCM622) batteries have obtained great interest owing to their high energy density. However, some obstacles hinder their practical applications, e.g., the structural failure of NCM622 and corrosion of the Al current collector, which lead to limited cycling life. Herein, an electrolyte additive strategy is proposed for constructing localized high-concentration PF6 - zone near the cathode to form an efficient cathode electrolyte interphase (CEI) for protecting NCM622 and preventing Al current collector from the corrosion. Potassium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide is used as the additive to regulate the sheath structure of Li+ solvation to force PF6 - anions away from the solvated Li+ . During the charge process, the nonsolvated PF6 - anions gather on NCM622 surface to form a localized high-concentration PF6 - zone to facilitate the formation of F-rich CEI on NCM622 for protecting its structural stability and Al current collector.

Constructing Highly Li+ Conductive Electrode Electrolyte Interphases for 4.6 V Li||LiCoO2 Batteries via Electrolyte Additive Engineering.

To improve voltage is considered to effectively address the energy-density question of Li||LiCoO2 batteries. However, it is restricted by the instability of electrode electrolyte interphases in carbonate electrolytes, which mainly originates from Li dendrite growth and structural instability of LiCoO2 at high voltage. Herein, an electrolyte additive strategy is proposed for constructing efficient LiNx Oy -contained cathode electrolyte interphase for 4.6 V LiCoO2 and LiF-rich solid electrolyte interphase for Li anode to enhance the stability of Li||LiCoO2 battery using 4-nitrophthalic anhydride as the additive. As expected, the Li||LiCoO2 battery can stably operate up to 4.6 V, with a high specific capacity of 216.9 mAh g-1 during the 1st cycle and a capacity retention of 167.1 mAh g-1 after 200 cycles at 0.3 C. This work provides an available strategy to realize the application of high-voltage Li||LiCoO2 battery.

Stabilizing the cycling stability of rechargeable lithium metal batteries with tris(hexafluoroisopropyl)phosphate additive.

The application of rechargeable lithium metal batteries (LMBs) has been hindered by the fast growth of lithium dendrites during charge and the limited cycling life because of the decomposition of the electrolyte at the interface. Here, we have developed a non-flammable triethyl phosphate (TEP)-based electrolyte with tris(hexafluoroisopropyl)phosphate (THFP) as an additive. The polar nature of the C-F bonding and the rich CF3 groups in THFP lowers its LUMO energy and HOMO energy to help form a stable, LiF-rich solid electrolyte interphase (SEI) layer through the reduction of THFP and increases the binding ability of the PF6- anions, which significantly suppresses lithium dendrite growth and reduces the electrolyte decomposition. Moreover, THFP participates in the formation of a thin, C-F rich electrolyte interphase (CEI) layer to provide the stable cycling of the cathode at a high voltage. The symmetric Li||Li and full Li/NCM622 cells with THFP additive have small polarization and long cycling life, which demonstrates the importance of the additive to the application of the LMBs.

Separator-Wetted, Acid- and Water-Scavenged Electrolyte with Optimized Li-Ion Solvation to Form Dual Efficient Electrode Electrolyte Interphases via Hexa-Functional Additive.

The performance of lithium metal batteries (LMBs) is determined by many factors from the bulk electrolyte to the electrode-electrolyte interphases, which are crucially affected by electrolyte additives. Herein, the authors develop the heptafluorobutyrylimidazole (HFBMZ) as a hexa-functional additive to inhibit the dendrite growth on the surface of lithium (Li) anode, and then improve the cycling performance and rate capabilities of Li||LiNi0.6 Co0.2 Mn0.2 O2 (NCM622). The HFBMZ can remove the trace H2 O and HF from the electrolyte, reducing the by-products on the surface of solid electrolyte interphase (SEI) and inhibiting the dissolution of metal ions from NCM622. Also, the HFBMZ can enhance the wettability of the separator to promote uniform Li deposition. HFBMZ can make Li+ easy to be desolvated, resulting in the increase of Li+ flux on Li anode surface. Moreover, the HFBMZ can optimize the composition and structure of SEI. Therefore, the Li||Li symmetrical cells with 1 wt% HFBMZ-contained electrolyte can achieve stable cycling for more than 1200 h at 0.5 mA cm-2 . In addition, the capacity retention rate of the Li||NCM622 can reach 92% after 150 cycles at 100 mA g-1 .

Unveiling the Role of Li+ Solvation Structures with Commercial Carbonates in the Formation of Solid Electrolyte Interphase for Lithium Metal Batteries.

Solid electrolyte interphase (SEI), determined by the components of electrolytes, can endow batteries with the ability to repress the growth of Li dendrites. Nevertheless, the mechanism of commercial carbonates on in situ-generated SEI and the consequential effect on cycling performance is not well understood yet, although some carbonates are well used in electrolytes. In this work, quantum chemical calculations and molecular dynamics are used to reveal the formation mechanisms of SEI with carbonate-based electrolyte additives on the atomic level. It is confirmed that the Li-coordinated carbonate species are the leading participant of SEI formation and their impact on battery performance is clarified. Fluoroethylene carbonate (FEC) exhibits a completely different behavior from vinyl ethylene carbonate (VEC), ethylene carbonate (EC), and vinylene carbonate (VC). High reduction potential Li+ -coordinated additives, e.g. FEC and VEC can dominate the formation of SEI by excluding propylene carbonate (PC) and LiPF6 from the decomposition, and the corresponding Li||Li symmetric cells show enhanced long-term performance compared with those with pure PC electrolyte, while the low reduction priority additives (e.g., EC and VC) cannot form a uniform SEI by winning the competitive reaction.

Electrolytes enriched by potassium perfluorinated sulfonates for lithium metal batteries.

Lithium (Li) metal is widely considered as a promising anode for next-generation lithium metal batteries (LMBs) due to its high theoretical capacity and lowest electrochemical potential. However, the uncontrollable formation of Li dendrites has prevented its practical application. Herein, we propose a kind of multi-functional electrolyte additives (potassium perfluorinated sulfonates) from the multi-factor principle for electrolyte additive molecular design (EDMD) view to suppress the Li dendrite growth. The effects of these additives are revealed through experimental results, molecular dynamics simulations and first-principles calculations. Firstly, K+ can form an electrostatic shield on the surface of Li anode to prevent the growth of Li dendrites. Secondly, potassium perfluorinated sulfonates can improve the activity of electrolytes as co-conductive salts, and lower the electro-potential of Li nucleation. Thirdly, perfluorinated sulfonate anions not only can change the Li+ solvation sheath structure to decrease the desolvation energy barrier and increase the ion migration rate, but also can be partly decomposed to form the superior solid electrolyte interphase (SEI). Benefited from the synergistic effects, an outstanding cycle life over 250 h at 1 mA cm-2 is achieved in symmetric Li||Li cells. In particular, potassium perfluorinated sulfonate additives (e.g., potassium perfluorohexyl sulfonate, denoted as K+PFHS) can also contribute to the formation of high-quality cathode electrolyte interphase (CEI). As a result, Li||LiNi0.6Mn0.2Co0.2O2 full cells exhibit significantly enhanced cycling stability. This multi-factor principle for EDMD offers a unique insight on understanding the electrochemical behavior of ion-type electrolyte additives on both the Li metal anode and high-voltage cathode.

Roadmap on Ionic Liquid Electrolytes for Energy Storage Devices.

Ionic liquids are considered to be promising electrolyte solvents or additives for rechargeable batteries (i. e., lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, aluminum-ion batteries, etc.) and supercapacitors. This is related with the superior physical and electrochemical properties of ionic liquids, which can influence the performance of rechargeable batteries. Therefore, it is necessary to write a roadmap on ionic liquids for rechargeable batteries. In this roadmap, some progress, critical techniques, opportunities and challenges of ionic liquid electrolytes for various batteries and supercapacitors are pointed out. Especially, properties and roles of ionic liquids should be considered in energy storage. Ionic liquids can be used as electrolyte salts, electrolyte additives, and solvents. For optimizing ionic liquid-based electrolytes for energy storage, their applications in various energy storage devices should be considered by combing native chemical/physical properties and their roles. We expect that this roadmap will give a useful guidance in directing future research in ionic liquid electrolytes for rechargeable batteries and supercapacitors.

Construction of Ag-modified TiO2/ZnO heterojunction nanotree arrays with superior photocatalytic and photoelectrochemical properties.

The work involves the preparation of TiO2/ZnO heterojunction nanotree arrays by a three-step: hydrothermal, sol-gel, and secondary hydrothermal method, and then modification of Ag quantum dots (QDs). In the above process, the ZnO nanoparticles attached to the TiO2 surface were subjected to secondary growth by a hydrothermal method to form a unique nanotree structure with TiO2, followed by Ag quantum dot modification by quantum dot deposition. In summary, TiO2/ZnO nanotree arrays are cited for the first time. The prepared Ag-modified TiO2/ZnO heterojunction nanotree arrays were found to exhibit enhanced photoelectrochemical and photocatalytic properties. The photocurrent of the Ag-modified TiO2/ZnO heterojunction nanotree arrays is increased by 8-fold compared to the pure TiO2 nanorod arrays, the photocatalytic degradation rate within 180 minutes increased from 37% to 77% and the kinetic rate constants for the degradation of methyl orange were three times higher than the pure TiO2 nanorod arrays. The improved performance is partly due to the introduction of the TiO2/ZnO heterojunction nanotree arrays which provide Ag QDs with greater adhesion area. Localized surface plasmon resonance (LSPR) leads to an increase in the intensity of absorbed light due to the modification of Ag QDs. On the other hand the generation of the TiO2/ZnO heterojunction decreases the forbidden band width, resulting in the redshift of the light absorption edge. Therefore, TiO2/ZnO heterojunction nanotree arrays are expected to play an important role in solar cells and photocatalytic materials.

Antimony- and Bismuth-Based Chalcogenides for Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have received much attention, owing to their great potential for large-scale application. A lack of efficient anode materials with high reversible capacity is one main challenge facing the development of SIBs. Antimony- and bismuth-based chalcogenides materials can store large amounts of Na+ ions, owing to the alloying/dealloying reaction mechanism within a low potential range, and thus, are regarded as promising anodes for SIBs. However, these materials face great challenges of poor ion diffusion rate, multiple phase transformations, and severe morphology pulverization. Herein, recent developments in antimony- and bismuth-based chalcogenides materials, mainly rational structural design strategies used and the electrochemical reaction mechanisms involved, are summarized. Perspectives for further improving antimony- and bismuth-based chalcogenides anodes are also provided.