Understanding the Role of Zinc Hydroxide Sulfate and its Analogues in Mildly Acidic Aqueous Zinc Batteries: A Review

On the hidden transient interphase in metal anodes: Dynamic precipitation controls electrochemical interfaces in batteries

The solid-electrolyte interphase (SEI) formed on a battery electrode has been a central area of research for decades. This structurally complex layer profoundly impacts the electrochemical deposition morphology and stability of metal anodes. Departing from conventional approaches, we investigate metal dissolution-the reverse reaction of deposition-in battery environments using a state-of-the-art electroanalytical system combining a rotating-disk electrode and operando visualization. Our key finding is the presence of a transient SEI (T-SEI) that forms during fast discharging at high dissolution rates. We attribute T-SEI formation to local supersaturation and resultant electrolyte salt deposition. The T-SEI fundamentally alters the dissolution kinetics at the electrochemical interface, yielding a flat, clean surface. Unlike a classical SEI formed due to electrolyte decomposition, the T-SEI is "relaxable" upon removal of the enforced dissolution current; that is, the T-SEI dissolves back into the electrolyte when rested. The formation of T-SEI plays an unexpected critical role in the subsequent electrodeposition. When the metal is redeposited on a fully relaxed T-SEI surface, the morphology is remarkably different from that deposited on pristine or low-rate-discharged metal electrodes. Electron backscatter diffraction analysis suggests that the deposition occurs via growth of the original grains; this is in stark contrast to the isolated, new nuclei seen on standard metal electrodes without T-SEI formation. Using 3D profilometry, we observe a 42% reduction in surface roughness due to T-SEI formation. Our findings provide important insights into the kinetics at ion-producing electrochemical interfaces, and suggest a new dimension for engineering next generation batteries.

Imaging the 4D Chemical Heterogeneity of Single V2O5 Particles During Charging/Discharging Processes

Microparticle cathode materials are widely used in secondary batteries. However, obtaining dynamic chemical heterogeneities of these microparticles is challenging, hindering in-depth mechanistic investigation of the underlying processes. For example, although vanadium pentoxide shows promise as an electrode material for zinc ion batteries, its poor performance's root cause is elusive. Herein, a fluorescence/scattering dual-mode spinning disk confocal microscopy-based approach is developed to visualize the 4D chemical heterogeneity of single V2O5 particles during cycling. Dual-mode in situ imaging identifies valence state changes of vanadium ions with high spatiotemporal resolution. A unique difference is observed between the scattering intensities of a particle's bottom electric contact points and the rest parts during the discharging process. In contrast, fluorescence intensity variation suggests high consistency across the particles. Correlative Raman, UV-Vis spectroscopy, and electrochemical impedance spectroscopy analyses suggest the precipitation of V3+ species at the bottom interface of the V2O5 electrode, leading to increased electron transfer resistance and compromised overall performance. A coordination strategy between ethylene diamine tetraacetic acid and V3+ is proposed for inhibiting V3+ precipitation, and its effectiveness is further verified by imaging and electrochemical impedance spectroscopy analyses. Insights from the imaging approach presented herein will enable the rational design of high-performance batteries.

Electrolyte Regulation toward Cathodes with Enhanced-Performance in Aqueous Zinc Ion Batteries

Enhancing cathodic performance is crucial for aqueous zinc-ion batteries, with the primary focus of research efforts being the regulation of the intrinsic material structure. Electrolyte regulation is also widely used to improve full-cell performance, whose main optimization mechanisms have been extensively discussed only in regard to the metallic anode. Considering that ionic transport begins in the electrolyte, the modulation of the electrolyte must influence the cathodic performance or even the reaction mechanism. Despite its importance, the discussion of the optimization effects of electrolyte regulation on the cathode has not garnered the attention it deserves. To fill this gap and raise awareness of the importance of electrolyte regulation on cathodic reaction mechanisms, this review comprehensively combs the underlying mechanisms of the electrolyte regulation strategies and classifies the regulation mechanisms into three main categories according to their commonalities for the first time, which are ion effect, solvating effect, and interfacial modulation effect, revealing the missing puzzle piece of the mechanisms of electrolyte regulation in optimizing the cathode.

Improved Performances of Zn//MnO2 Batteries with an Electrolyte Containing Co-Additives of Polyethylene Glycol and Lignin Derivatives

Multi-component electrolyte additives may significantly contribute to improving the performance of rechargeable aqueous zinc-ion batteries. Herein, we propose a mixed electrolyte system employing polyethylene glycol 200 (PEG200) and quaternized kraft lignin (QKL) as co-additives in Zn//MnO2 batteries. Reduced corrosion and the suppression of the hydrogen evolution reaction on the zinc electrode were achieved when 0.5 wt.% of PEG200 and 0.2 wt.% of QKL were added to the reference aqueous electrolyte. This optimized electrolyte, 0.5% PEG200 + 0.2% QKL, was conducive to improving Zn reversibility in Zn//Zn symmetric batteries and resulted in higher cycling stability, with a coulombic efficiency of 98.01% under 1 mA cm-2 and 1 mAh cm-2 for Zn//Cu cells. Furthermore, Zn//MnO2 full batteries with 0.5% PEG200 + 0.2% QKL presented good overall electrochemical performance and exhibited a decent discharge capacity of around 85 mAh g-1 after 2000 cycles at 1.5 A g-1. As confirmed by X-ray diffraction and scanning electron microscopy, a dominant (002) oriental dendrite-free Zn deposition was achieved on the zinc anode of the battery using 0.5% PEG200 + 0.2% QKL, and the byproducts were also reduced significantly. This study has contributed to the development of electrolyte co-additives for zinc-ion batteries.

Transforming Detrimental Crystalline Zinc Hydroxide Sulfate to Homogeneous Fluorinated Amorphous Solid-Electrolyte Interphase on Zinc Anode

The formation of non-ion conducting byproducts on zinc anode is notoriously detrimental to aqueous zinc-ion batteries (AZIBs). Herein, we successfully transform a representative detrimental byproduct, crystalline zinc hydroxide sulfate (ZHS) to fast-ion conducting solid-electrolyte interphase (SEI) via amorphization and fluorination induced by suspending CaF2 nanoparticles in dilute sulfate electrolytes. Distinct from widely reported nonhomogeneous organic-inorganic hybrid SEIs that exhibit structural and chemical instability, the designed single-phase SEI is homogeneous, mechanically robust, and chemically stable. These characteristics of the SEI facilitate the prevention of zinc dendrite growth and reduction of capacity loss during long-term cycling. Importantly, AZIB full cells based on this SEI-forming electrolyte exhibit exceptional stability over 20,000 cycles at 3 A/g with a charging voltage of 2.2 V without short circuits and electrolyte dry-out. This work suggests avenues for designing SEIs on a metal anode and provides insights into associated SEI chemistry.

Carbon Dioxide Evolution in Aqueous Zinc Metal Batteries

Gas evolution reactions in aqueous zinc metal batteries (AZMBs) cause gas accumulation and battery swelling that negatively affect their performance. However, previous work often reported hydrogen as the main, if not the only, gas species evolved in AZMBs; the complexity of gas evolution has been overlooked. For the first time, this work found the CO2 evolution reaction (CER) in AZMBs, pinpointed its sources, and identified electrolyte modulation strategies. Using differential electrochemical mass spectrometry, CER was detected in V2O5||Zn full cells, instead of in asymmetric Cu||Zn cells, and it became substantial when being charged to 2.0 V. By using a carbon isotope tracing method, the primary origin of CER was identified as the electrochemical corrosion of conductive carbon at the cathode. Among six representative electrolytes, the weakly solvating electrolyte (3 m Zn(OTf)2 in acetonitrile/water) presented a high CER resistance by reducing water solvating and disturbing hydrogen bonding. This work sheds light on interfacial parasitic reactions for practical aqueous metal (Zn and Al) batteries.

Building a Rechargeable Voltaic Battery via Reversible Oxide Anion Insertion in Copper Electrodes

Voltaic pile, the very first battery built by humanity in 1800, plays a seminal role in battery development history. However, the premature design leads to the inevitable copper ion dissolution issue, which dictates its primary battery nature. To address this issue, solid-state electrolytes, ion exchange membranes, and/or sophisticated electrolytes are widely utilized, leading to high costs and complicated cell configuration. Herein, we build a rechargeable zinc-copper voltaic battery from simple and cheap electrolyte/separator materials, thus eliminating the need to use the above components. Notably, our battery leverages the Zn4SO4(OH)6·xH2O precipitation in ZnSO4 electrolytes, a common side reaction in zinc batteries, to provide a "locally alkaline" environment for copper electrodes. Consequently, oxide (O2-) anion insertion takes place and readily transforms copper to copper(I) oxide (Cu2O) without any copper ion dissolution issue. Therefore, this battery realizes a high capacity of ∼370 mA h g-1 and a long cycling of ∼500 cycles. Our work provides an innovative approach to stabilize anion insertion in metal electrodes for energy storage.