Zn Electrolyte Research Articles

Designing Interphases in Zn and Al Aqueous Electrolytes

This talk will cover two recent studies led by the Army Research Laboratory investigating interphase formation and cation solvation in zinc and aluminum aqueous electrolytes, respectively. In the former, two co-cations were carefully designed to offer distinct, complementary improvements in aqueous Zn electrolytes: a partially fluorinated pyrrolidinium cation effectively suppresses parasitic reactions such as hydrogen evolution (< 6 µA cm-2), and an ether-functionalized ammonium cation inhibits dendrite formation (almost 10 Ah cm-2 cumulative capacity, > 1 year, Zn||Zn). In the later, transient electrostatic hydrolysis of Al-OH2 ligands, without Al-OTf contact ion pairs, leads to insufficient reductive stability for rechargeable metal anodes. These contrasting systems provide helpful guidance for designing aqueous electrolytes with energy-dense multivalent metal anodes.

Revisiting the Charging Mechanism of α-MnO2 in Mildly Acidic Aqueous Zinc Electrolytes.

In recent years, there have been extensive debates regarding the charging mechanism of MnO2 cathodes in aqueous Zn electrolytes. The discussion centered on several key aspects including the identity of the charge carriers contributing to the overall capacity, the nature of the electrochemical process, and the role of the zinc hydroxy films that are reversibly formed during the charging/discharging. Intense studies are also devoted to understanding the effect of the Mn2+ additive on the performance of the cathodes. Nevertheless, it seems that a consistent explanation of the α-MnO2 charging mechanism is still lacking. To address this, a step-by-step analysis of the MnO2 cathodes is conducted. Valuable information is obtained by using in situ electrochemical quartz crystal microbalance with dissipation (EQCM-D) monitoring, supplemented by solid-state nuclear magnetic resonance (NMR), X-ray diffraction (XRD) in Characterization of Materials, and pH measurements. The findings indicate that the charging mechanism is dominated by the insertion of H3O+ ions, while no evidence of Zn2+ intercalation is found. The role of the Mn2+ additive in promoting the generation of protons by forming MnOOH, enhancing the stability of Zn/α-MnO2 batteries is thoroughly investigated. This work provides a comprehensive overview on the electrochemical and the chemical reactions associated with the α-MnO2 electrodes, and will pave the way for further development of aqueous cathodes for Zn-ion batteries.

Regulating ionic transport and interface chemistry via high-dielectric BaTiO3 porous scaffolds for aqueous Zn-ion batteries

Rechargeable aqueous Zn-ion batteries (AZIBs) are gaining considerable attention as alternatives to Li-ion batteries owing to their excellent safety, high energy density, low cost, and environmental friendliness. However, their Zn anodes suffer from severe dendritic growth, the hydrogen evolution reaction (HER), and corrosion, which limit the commercialization of AZIBs. To address this challenge, we designed a solution involving the development of a high-dielectric hydrophobic BaTiO3 (BTO) porous scaffold layer that is strategically applied to the Zn surface. This BTO layer effectively impedes the dendritic growth of Zn. The distinct polarizability of the BTO layer actively diminishes the electric-field gradient, thereby minimizing the interaction between the H2O molecules and the Zn surface. This phenomenon helps suppress dendritic growth, thus ensuring the uniform deposition of Zn and preventing the HER. The electrochemical performance of a symmetric cell containing the Zn@BTO electrode exhibits low-voltage polarization and stable charge–discharge performance for up to 500 h. In addition, a full-cell AZIB containing a high-capacity α-MnO2 cathode and Zn@BTO anode exhibits good rate capability and a long life of up to 500 cycles. This study demonstrates the development of a cost-effective dendrite-free Zn anode integrated with a highly dielectric hydrophobic BTO porous scaffold layer as a stable Zn–electrolyte interface layer for high-performance AZIBs.

Redox‐Flow Desalination with a Voltage Range Extending up to 2 V Under Dual‐pH Conditions

AbstractThe battery‐like systems utilizing redox materials present a promising avenue for electrochemical desalination with reduced energy consumption. However, several attempts have focused on reducing the operational voltage to minimize energy consumption, and efforts to enhance the performance of such systems as energy storage devices remain limited. Herein, a system with a high energy density capable of sustaining freshwater production was proposed. Furthermore, in this proposed system, the electrolytes with different pH values were utilized to implement an operational voltage of 2 V or higher. Accordingly, an alkaline Zn electrolyte was paired with acidic MnO2 or VOSO4 electrolyte. During the process, potential side reactions such as chlorine evolution and proton crossover were not observed, even without water‐splitting reactions. Ultimately, at operational voltages exceeding 2 V, the system achieved a desalination rate of 0.104 mg/cm2/min and demonstrated maximum energy discharge of up to 2.4 Wh/L. These designed systems pave the way towards a more environmentally friendly and efficient approach to desalination.

ZnO Additive Boosts Charging Speed and Cycling Stability of Electrolytic Zn–Mn Batteries

HighlightsLow pH value of electrolyte suppresses the charge capabilities of electrolytic Zn–Mn batteries.Unique solid phase alkaline properties of zinc sulfate hydroxide hydrate endow the electrolytic Zn–Mn batteries with greatly enhanced charge capabilities.The highly active Zn2Mn3O8·H2O nanorods array deposited during the charge process improve the discharge efficiency and stability of electrolytic Zn–Mn batteries.

Highly Concentration Aqueous Electrolytes for Zinc Metal Batteries

Aqueous zinc batteries (AZB) emerged as a promising technology for renewable and low-cost storage devices.[1] Zinc (Zn) metal features high abundance, intrinsic safety, low cost (~USD $2 kg-1) and high theoretical capacity (820 mA h g-1).[2,3] However, AZBs do suffer from low Coulombic efficiency, dendrite growth and parasitic hydrogen evolution reaction (HER), which restrict their practical applications.[4] To combat this problem, highly concentrated electrolytes (HCEs) are employed to improve the cycling stability of AZBs.[5] In this work, we demonstrate that aqueous HCEs based on Zn and organic salts are a very effective way to address these issues. Aqueous eutectic electrolytes were obtained by simply optimizing the salt ratio, as evidenced by the low liquid-to-solid transition temperature. These unique HCEs feature a wider electrochemical stability window and an improved ionic conductivity. Additionally, the high-density of Zn cations near the electrode surface results in reversible and stable Zn electrodeposition - with relatively low overpotentials in comparison to other reported hybrid Zn electrolytes. Thus, this work presents an efficient, economically attractive and simple approach for the development of HCEs for practical applications of AZBs.

Two-stage method recovery of metals from copper slag: Realize the resource utilization of all components

The recovery of metal from copper slag is of great significance to the resource utilization of copper slag and the sustainable development of copper smelting industry. Currently, hydrometallurgy is the traditional method used to recover mineral metals. However, as copper slag is a silicate mineral, the traditional acid-leaching process of metal results in the leaching of Si, resulting in the formation of silica gel. The silica gel can inhibit the recovery of metal while making metal difficult to separate from Si and causing filtration problems. This study proposed a two-stage method (evaporation + water washing) to selectively recover Fe and Zn from copper slag. Under the optimal conditions (evaporation: H2SO4 = 3 M; time = 60 min; temperature = 120℃; and liquid–solid ratio = 4; water washing: liquid–solid ratio = 5), the leaching efficiency of Fe, Zn, and Si reached 99.94 %, 97.40 %, and 0.15 % respectively. The separation mechanism was determined through mineral characterization (XRD, SEM, HRTEM-EDS, SAED, AFM, etc.) and the application of DLVO theory. It was found that H+ destroyed the Fe2SiO4 structure in copper slag during the evaporation stage, which promoted the release of Fe and Zn, forming soluble sulfate crystals. Simultaneously, evaporation increased the ion concentration, reducing the repulsive force between colloids, resulting in the aggregation and precipitation of silica gel into stable amorphous silica. After washing, soluble sulfate is dissolved in water. The recovered Fe can be processed to get red iron oxide (96.32 % purity), the Zn-containing solution can be recycled for electrolytic Zn, and the residue can be used to prepare geopolymer (52.47 MPa, 28 days). The two-stage method for recovering metals from copper slag is highly efficient and provides valuable insights into resource utilization of metals and Si in silicon-containing solid wastes.

Construction of self-standing MnO2-based films for reversible and practical electrolytic Zn//MnO2 batteries in acidic environment

In acidic environment, electrolytic Zn//MnO2 batteries possess high potential to reach high energy density on the promise of high safety, low cost, and high power density. However, several serious challenges still need to be overcome, mainly including suitable current collector that can be stable in strong acidic and oxidizing environment, optimal cathode that can promote massive and uniform deposition of the MnO2, and Zn-plating efficiency and corrosion resistance improvement. Herein, a free-standing and binder-free film composed of graphene powder (GP) and MnSO4 (GP/MS-F) was developed and used as initial cathode frame. The film with a GP: MnSO4 mass ratio of 2:3 and a total areal mass of 13 mg cm−2 exhibits good conductivity and considerable flexibility without the using of any other additives. Matched with C coated ZnO (ZnO@C) modified anode frame and Carboxy Methyl Cellulose Sodium gelled acidic electrolyte, the film can deliver a high areal capacity of 1.4 mAh cm−2, corresponding to a specific capacity of 184 mAh g−1 based on the total mass of initial GP and deposited MnO2 (∼7.6 mg). At 5 mA cm−2, a high Coulombic efficiency of 92 % can be achieved and maintained for 2,100 cycles. At 30 mA cm−2, the cell can work well for 8,000 cycles. Meanwhile, discharge voltage of the cell can reach 1.9 V, exhibiting high practical application prospect.

Research on Classified Treatment of Electrolytic Zn Anode Slime Based on μ-XRF and Cluster Heatmap

The purpose of this research is to have a clearer understanding of the resource value of electrolytic Zn anode slime, explore its fine structure and treat it in a classified manner. In this paper, the composition, content and distribution of Pb, Mn and Zn in electrolytic Zn anode slime during one production cycle were studied under typical working conditions of enterprises, by using the method of μ-XRF combined with mm-XRF. Based on heatmap and cluster analysis, the resource value of anode slime at different points in the electrolytic Zn silting area and the conventional area was evaluated. The research results were as follows: (1) There were two different areas of electrolytic Zn anode slime: the silting area and conventional area. Between these, the silting area accounted for approximately 15% of the total number of anode plates, and there were significant differences between these areas in terms of surface element content, surface morphology, slime thickness and water content. (2) In the silting area, 3.9 mm away from the Pb-based anode plate surface, there were lumps with fluorescence counting intensity that was close to that of pure Pb. (3) Anode slime at a thickness of 0.1 mm in the silting area could be classified as a type of resource. The resource attributes were categorized using a complete-linkage algorithm and the actual demand of Zn hydrometallurgy enterprises for recycling anode slime from the leaching process, for those with ≥45% Mn content and &lt;20% Pb content. High-Pb resources include 0–1.5 mm (CAP), 0–1.4 mm (SAP) and 6.3–7.0 mm (SAP). A low-Pb, high-Mn and high-Zn resource was identified at 2.9–6.0 mm (SAP). Low-Pb, high-Mn and low-Zn resources include 1.6–6.0 (CAP), 1.5–2.8 mm (SAP) and 6.1–6.2 mm (SAP).

Emergent Solvation Behavior in Dual Anion Electrolytes for Zinc Ion Batteries

The need for a diverse range of battery materials for an ever-growing energy storage landscape motivates the development of new chemistries with higher capacities and more stable supply chains. Zn-ion batteries (ZIB) are a particularly attractive option due to the abundance and low cost of Zn and the associated cathode materials that can be used to create a full cell. Extensive work on both aqueous and non-aqueous ZIBs has required an increasingly complex set of electrolytes – in many cases incorporating multiple solvents and/or anions to improve overall system performance. It is becoming clear that understanding and, more importantly, controlling the bulk and interfacial Zn2+ solvation structure is of paramount importance to enable improved electrolyte stability and electrode reversibility. In this work we discover the presence of emergent solvation behavior in non-aqueous Zn electrolytes containing multiple anions, where bis(trifluoromethane sulfonyl) imide (TFSI-) anions that are fully dissociated in isolation are shown to form contact ion pairs with Zn2+ when combined with more strongly coordinating halides (X = Cl-, Br-, I-). This effect is demonstrated using both X-ray absorption and Raman spectroscopies and is further supported by first-principles density functional theory calculations, all of which indicate the dominant solvation structure is Zn(TFSI)mono(X), where “mono” indicates a monodentate TFSI- coordination. These emergent solvation complexes are shown to directly impact the electrochemical response, where a significant fraction of Zn2+ complexes in Cl-containing electrolytes are electrochemically inactive at low overpotentials. Additional redox species are activated for metal deposition as the halide association strength weakens (i.e., Cl- > Br- > I-) indicating that, depending on the relative anion-cation coordination strength, apparent improvements in Coulombic efficiency can come at the cost of lower accessible charge/discharge rates. This work suggests a completely new framework for electrolyte design, where anion chemistry can be used to tune both the bulk speciation and the interfacial solvation structure, enabling profound changes to the electrochemical behavior of the system._______________________The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan. http://energy.gov/downloads/doe-public-access-plan

Achieving Highly Proton-Resistant Zn-Pb Anode through Low Hydrogen Affinity and Strong Bonding for Long-Life Electrolytic Zn//MnO2 Battery.

High-energy electrolytic Zn//MnO2 batteries show potential for grid-scale energy storage, but the severe hydrogen evolution corrosion (HEC) caused by acidic electrolytes results in subdued durability. Here, we report an all-around protection strategy for achieving stable metal Zn anode. Firstly, a proton-resistant Pb-containing (Pb and Pb(OH)2) interface is constructed on Zn anode (denoted as Zn@Pb), which will in-situ form PbSO4 during H2SO4 corrosion and protect the Zn substrate from HEC. Secondly, to improve the plating/stripping reversibility of Zn@Pb, Pb(CH3COO)2 additive (denoted as Zn@Pb-Ad) is introduced, which will convert to PbSO4 precipitation and release trace Pb2+ that can dynamically deposit Pb layer on Zn plating layer to suppress HEC. The superior HEC resistance stems from the low affinity of PbSO4 and Pb for H+ as well as strong bonding between Pb-Zn or Pb-Pb, which increase the hydrogen evolution reaction overpotential and the H+ corrosion energy barrier. Consequently, the Zn@Pb-Ad//MnO2 battery runs stably for 630 and 795h in 0.2 and 0.1M H2SO4 electrolytes, respectively, which are >40 times than that of bare Zn. The as-prepared A h-level battery achieves the one-month calendar life, opening the door to the next generation of high-durable grid-scale Zn batteries. This article is protected by copyright. All rights reserved.

Carbon nanofibers enabling manganese oxide cathode superior low temperature performance for aqueous zinc-ion batteries

Mn-based aqueous zinc-ion batteries are potential candidates for energy storage owing to the low cost, high efficiency, and safety. Nevertheless, the Jahn–Teller effect induces Mn2+ dissolution and irreversible phase changes, seriously deteriorating the cycling life. Herein, the facile construction and low temperature performance of the core–shell composites of polyimide derived nitrogen-doped carbon nanofibers and δ-MnO2 (δ-MnO2-CNFs) are reported, where ultrafine MnO2 arrays are tightly anchored on carbon nanofibers. The hybrid effect of nitrogen doping, porous structure, and abundant active sites effectively inhibits the structural damage of MnO2. The as-prepared δ-MnO2-CNFs cathode achieves a high specific capacity of 232.9 mAh g−1 and energy density of 450.7 Wh kg−1 at a current density of 100 mA g−1, and could be stably cycled for 500 cycles at a high current rate of 1 A g−1. The density functional theory calculation results show that the synergistic effect in δ-MnO2-CNFs is more conducive to the transfer of zinc ions. Furthermore, a salty ice electrolyte Zn(ClO4)2 has been used for the Zn//δ-MnO2-CNFs cell, which can work at a low temperature of −20 °C, and a stable low temperature (0 °C) cycle of 500 cycles is achieved at 1 A g−1.

Hierarchical porous separator with excellent isotropic modulus enabling homogeneous Zn2+ flux for stable aqueous Zn battery

Low-cost, high-safety aqueous Zn-ion batteries show promise for large-scale energy storage, but the unsatisfactory cycling stability of Zn anodes and the lack of a suitable hydrophilic separator for rechargeable aqueous Zn batteries limit their practical applications. In this work, we report a scalable method for the preparation of a homogenous polyvinylidene fluoride-lithium bistrifluoromethanesulfonimide (LiTFSI) (PVDF-Li)-based separator for aqueous Zn batteries. The homogenously mixed LiTFSI salt interrupted the crystalline framework of PVDF with excellent mechanical strength and led to hierarchical pore structures for the rapid and uniform Zn2+ flux through the separator. Meanwhile, the PVDF-Li separator exhibited excellent wettability toward aqueous Zn electrolytes. With the advanced PVDF-Li separator, the Zn dendrite growth can be suppressed greatly, showing an improved cycle life. The Zn||V2O5 full cell with PVDF-Li separator exhibited a 27% higher initial capacity (324 mA h g−1) and considerably better capacity retention than the traditional glass fiber separator due to the uniform reaction on the V2O5 cathode and Zn anode sides. This work provides new insights into the development of high-performance aqueous batteries by regulating the reactions on the cathode and anode through functional separators.

Nanographene Cathode Materials for Nonaqueous Zn-Ion Batteries

Robust multivalent ion interaction in electrodes is a grand challenge of next-generation battery research. In this manuscript, we design molecularly-precise nanographene cathodes that are coupled with metallic Zn anodes to create a new class of Zn-ion batteries. Our results indicate that while electrodes with graphite or flat nanographenes do not support Zn-ion intercalation, the larger intermolecular spacing in a twisted peropyrene enables peropyrene electrodes to facilitate reversible Zn-ion intercalation in an acetonitrile electrolyte. While most previous Zn-ion batteries utilize aqueous electrolytes, the finding that nonaqueous Zn electrolytes can support intercalation in nanographenes is important for expanding the design space of nonaqueous multivalent batteries, which often possess higher voltages than their aqueous counterparts. Furthermore, because these nanographenes can be synthesized using a bottom-up approach via alkyne benzannulation, this work paves the way for future battery electrodes that contain other molecularly-precise nanographenes with tailored electrochemical properties.

Advanced Buffering Acidic Aqueous Electrolytes for Ultra-Long Life Aqueous Zinc-Ion Batteries.

Mild aqueous Zn batteries have attracted increasing attention for energy storage due to the advantages of high safety and low cost; however, the rechargeability of Zn anodes is one major issue for practical applications. In this work, an effective approach is proposed to improve the reversibility and stability of Zn anodes using advanced acidic electrolytes. A trace amount of acetic acid (HAc) is employed as a buffering agent to provide a stable pH environment in aqueous Zn electrolytes, and thus suppress passivation from precipitation reactions on Zn electrodes. Meanwhile, tetramethylene sulfone (TMS) is introduced as the critical component to stabilize the Zn anodes in the acidic electrolyte. TMS greatly strengthens the hydrogen-bonding network with reduced H2 O activity and extends the electrochemical window of acidic electrolytes. With the optimal 3m Zn(OTF)2 in (H2 O-HAc)/TMS acidic electrolyte (pH 1.6), the Zn electrode exhibits a coulombic efficiency of >99.8% and smooth Zn deposition. The Zn-V2 O5 full cell demonstrates ultra-stable cycling over 20000cycles with a low decay rate of 0.0009% for each cycle at a negative/postive capacity ratio of 6.5. This work provides an insightful perspective to stabilize Zn electrodes by regulating the pH environment and limiting the H2 O activity simultaneously for long-life Zn anodes.

Unraveling H+/Zn2+ Sequential Conversion Reactions in Tellurium Cathodes for Rechargeable Aqueous Zinc Batteries.

Progress toward practical rechargeable aqueous zinc (Zn) batteries is impeded by the low energy density. The use of tellurium (Te) cathodes featuring multielectron redox reactions offers one possible approach to mitigating this dilemma. However, the corresponding energy-storage mechanisms in aqueous electrolytes are not established yet. Here, we uncover a H+/Zn2+-involved sequential conversion reaction with 6e- transfer for Te-based cathodes, which accounts for the outstanding capacity (over 460 mAh g-1 at 50 mA g-1). Two distinct redox processes, corresponding to TeO2 ↔ Te and Te ↔ ZnTe, are explicitly revealed within the electrochemical window of routine aqueous Zn electrolytes. The mechanism elucidated here complements the understanding of energy-dense Te centers and propels the exploration of high-capacity electrodes for multivalent battery chemistries.

Enhancing the Discharge Capacity and Rechargeability of Zn-MnO2 Alkaline Batteries through Partial Inclusion of Al into Zn

Zn-MnO2 alkaline batteries have been a reliable primary battery system for several decades due to their inherent safety, low cost and acceptable performance at low current densities 1. However, the discharge capacity and energy density of primary Zn-MnO2 alkaline batteries has plateaued and there is a high desire to improve these performance metrics to broaden the applications of alkaline batteries. One possible way to enhance the discharge capacity of alkaline batteries is to replace Zn anodes with other anode materials that possess a higher theoretical capacity. Among different elements that could be used in battery systems, Al can offer high theoretical gravimetric and volumetric capacity compared to Zn (2980 mAh g-1 vs. 820 mAh g-1 and 8.05 Ah cm-3 vs. 5.85 Ah cm-3, respectively) 2. Al is also highly naturally abundant and low cost. However, Al is fundamentally more unstable than Zn in alkaline media due to high rates of corrosion and passivation 3. Therefore, the complete replacement of Zn with Al anodes would not be viable and practical.In this work, partial inclusion of Al in Zn anodes was explored through synthesizing hexagonal-shaped Zn-rich electrolytic Zn/Al (e-Zn/Al). In three-electrode cells with excess electrolyte, it was shown that the Al capacity was accessible within these morphologies resulting in higher discharge capacities compared to pure electrolytic Zn (e-Zn) 4. The corrosion of e-Zn and e-Zn/Al was controlled by including ZnO and Al(OH)3 as electrolyte additives. Moreover, the fundamental effects of the two electrolyte additives on corrosion and passivation of Zn surfaces were investigated using an electrochemical quartz crystal microbalance. Finally, in cylindrical full cells with limited volume (i.e. more realistic conditions), it was found that the discharge capacity and energy density of e-Zn were increased by 53% (581 mA g-1 anode) and 56% (~784 Wh kg-1 anode), accordingly, through partial Al inclusion (e-Zn/Al anodes). This was accompanied by a superior cycle performance in secondary full cells (>800 h ~ 200 cycles at C/20 and 10% depth of discharge). References Faegh, E.; Omasta, T.; Hull, M.; Ferrin, S.; Shrestha, S.; Lechman, J.; Bolintineanu, D.; Zuraw, M.; Mustain, W. E., Understanding the Dynamics of Primary Zn-MnO2 Alkaline Battery Gassing with Operando Visualization and Pressure Cells. Journal of The Electrochemical Society 2018, 165 (11), A2528.Faegh, E.; Ng, B.; Hayman, D.; Mustain, W. E., Practical assessment of the performance of aluminium battery technologies. Nature Energy 2020. https://doi.org/10.1038/s41560-020-00728-yFaegh, E.; Shrestha, S.; Zhao, X.; Mustain, W. E., In-depth structural understanding of zinc oxide addition to alkaline electrolytes to protect aluminum against corrosion and gassing. Journal of Applied Electrochemistry 2019, 49 (9), 895-907.Faegh, E.; Ng, B.; Hayman, D.; Mustain, W. E., Design of Highly Reversible Zinc Anodes for Aqueous Batteries Using Preferentially Oriented Electrolytic Zinc. Batteries & Supercaps 2020, 3 (11), 1220-1232.

Enhanced Performance of Ti3C2Tx (MXene) Electrodes in Concentrated ZnCl2 Solutions: A Combined Electrochemical and EQCM-D Study

The need for improved batteries and supercapacitors, which are not based on lithium compounds, promotes significant research efforts to find suitable alternative systems based on various mono and multivalent cations capable of delivering high energy and power density with good long-term stability. The progress in aqueous Zn-ion batteries and supercapacitors obtained over the past years lead to the development of new structures and compounds that enable revisable hosting of Zn-ions while keeping good structural integrity. Yet, as aqueous electrolytes involve also the generation and co-insertion of protons it is necessary to carefully define what is the charge storage mechanism in these Zn insertion compounds. In this work, the use of Ti3C2Tx as an anode for the Zn-ion system was evaluated for the first time in different ZnCl2 concentrations. Remarkable changes in the charge storage mechanism and the performance of the Ti3C2Tx electrodes were observed by moving from dilute Zn electrolytes (1 M) to higher concentrations. The high acidity of the concentrated ZnCl2 solutions results in an additional charge storage mechanism that arises from redox interactions between the MXene and the released protons and hence enhanced capacity values (60 mAh/g with good rate capability and capacity retention of 89.4% after 9000 cycles). In-situ EQCM-D measurements were employed to shed light on the charge storage mechanism of the Ti3C2Tx and the exact identity of the charge carrier inserted into the electrodes. The demonstrated EQCM-D based analysis provides precise quantification of the Zn-ion to proton ratio in the examined system and can be further applied for other aqueous Zn-ion electrodes and multivalent systems as well.

Anti-corrosive Hybrid Electrolytes for Rechargeable Aqueous Zinc Batteries

Aqueous zinc(Zn)-metal cells with cost-effective components and high safety have long been a promising large-scale energy storage system, but Zn anodes are intrinsically unstable with common aqueous electrolytes, causing substantial underutilization of the theoretical capacity. In this work, we report a strictly neutral aqueous Zn electrolyte at a low cost by leveraging the dynamic hydrolysis equilibrium of a dual-salt Zn(Ac)2/NaAc(Ac: CH3COO−) formulation. With the pH regulation, the corrosion and hydrogen evolution encountered in Zn anodes can be suppressed significantly. This hybrid aqueous electrolyte not only enables dendrite-free Zn plating/stripping at a nearly 95% Coulombic efficiency[an increase of 24% compared to that of the single-salt 1 mol/L Zn(Ac)2 electrolyte], but also supports the reversible operation of Zn cells paired with either Na3V2(PO4)3 or iodine cathodes—the former delivers a high output voltage of 1.55 V with an energy level of 99.5 W·h/kg(based on the mass of the cathode), and the latter possesses a high specific capacity of 110.9 mA·h/g while yielding long-term cyclability(thousands of cycles). These findings open up a new avenue of modifying practical electrolytes having targeted properties to stabilize multivalent metal anodes.

Critical Factors Dictating Reversibility of the Zinc Metal Anode

With high energy density and improved safety, rechargeable battery chemistries with a zinc (Zn) metal anode offer promising and sustainable alternatives to those based on lithium metal or lithium ion intercalation/alloying anode materials; however, the poor electrochemical reversibility of Zn plating/stripping, induced by parasitic reactions with both aqueous and non-aqueous electrolytes, presently limits the practical appeal of these systems1. Although recent efforts in rechargeable Zn metal batteries (RZMBs) have achieved certain advancements in Zn metal reversibility2,3, as quantified by the Coulombic efficiency (CE), a standard protocol for CE has not been established, and results across chemistries and systems are often conflicting. More importantly, there is still an insufficient understanding regarding the critical factors dictating Zn reversibility.In this work, a rigorous, established protocol for determining CE of lithium metal anodes4 is transplanted to the Zn chemistry and is used for systematically examining how a series of factors including current collector chemistry, current density, temperature, and the upper voltage limit during stripping affect the measured reversibility of different Zn electrolytes. With support from density functional theory calculations, this standardized Zn CE protocol is then leveraged to identify an important correlation between electrolyte solvation strength toward Zn2+ and the measured Zn CE in the corresponding electrolyte, providing new guidance for future development and evaluation of Zn electrolytes. Figure 1 shows a schematic of proposed method, including initial results for some of the most promising Zn electrolytes reported in the literature. The capability of supporting Zn plating/stripping is differentiated clearly among these selected Zn electrolytes here.Reference1. Blanc, L. E., Kundu, D. & Nazar, L. F. Scientific Challenges for the Implementation of Zn-Ion Batteries. Joule 4, 771–799 (2020).2. Wang, F. et al. Highly reversible zinc metal anode for aqueous batteries. Nat. Mater. 17, 543–549 (2018)3. Naveed, A., Yang, H., Yang, J., Nuli, Y. & Wang, J. Highly Reversible and Rechargeable Safe Zn Batteries Based on a Triethyl Phosphate Electrolyte. Angew. Chemie Int. Ed. 58, 2760–2764 (2019).4. Adams, B. D., Zheng, J., Ren, X., Xu, W. & Zhang, J.-G. Accurate Determination of Coulombic Efficiency for Lithium Metal Anodes and Lithium Metal Batteries. Adv. Energy Mater. 8, 1702097 (2018). Figure 1