Advanced cathodes for aqueous Zn batteries beyond Zn2+ intercalation.

Aqueous zinc (Zn) batteries have attracted global attention for energy storage. Despite significant progress in advancing Zn anode materials, there has been little progress in cathodes. The predominant cathodes working with Zn2+/H+ intercalation, however, exhibit drawbacks, including a high Zn2+ diffusion energy barrier, pH fluctuation(s) and limited reproducibility. Beyond Zn2+ intercalation, alternative working principles have been reported that broaden cathode options, including conversion, hybrid, anion insertion and deposition/dissolution. In this review, we report a critical assessment of non-intercalation-type cathode materials in aqueous Zn batteries, and identify strengths and weaknesses of these cathodes in small-scale batteries, together with current strategies to boost material performance. We assess the technical gap(s) in transitioning these cathodes from laboratory-scale research to industrial-scale battery applications. We conclude that S, I2 and Br2 electrodes exhibit practically promising commercial prospects, and future research is directed to optimizing cathodes. Findings will be useful for researchers and manufacturers in advancing cathodes for aqueous Zn batteries beyond Zn2+ intercalation.

Low-cost and Non-flammable Eutectic Electrolytes for Advanced Zn-I2 Batteries.

As a burgeoning electrolyte system, eutectic electrolytes based on ZnCl2 /Zn(CF3 SO3 )2 /Zn(TFSI)2 have been widely proposed in advanced Zn-I2 batteries; however, safety and cost concerns significantly limit their applications. Here, we report new-type ZnSO4 -based eutectic electrolytes that are both safe and cost-effective. Their universality is evident in various solvents of polyhydric alcohols, in which multiple -OH groups not only involve in Zn2+ solvation but also interact with water, resulting in the high stability of electrolytes. Taking propylene glycol-based hydrated eutectic electrolyte as an example, it features significant advantages in non-flammability and low price that is <1/200 cost of Zn(CF3 SO3 )2 /Zn(TFSI)2 -based eutectic electrolytes. Moreover, its effectiveness in confining the shuttle effects of I2 cathode and side reactions of Zn anodes is evidenced, resulting in Zn-I2 cells with high reversibility at 1 C and 91.4 % capacity remaining under 20 C. After scaling up to the pouch cell with a record mass loading of 33.3 mg cm-2 , super-high-capacity retention of 96.7 % is achieved after 500 cycles, which exceeds other aqueous counterparts. This work significantly broadens the eutectic electrolyte family for advanced Zn battery design.

Triple-Function Electrolyte Regulation toward Advanced Aqueous Zn-Ion Batteries.

The poor Zn reversibility has been criticized for limiting applications of aqueous Zn-ion batteries (ZIBs); however, its behavior in aqueous media is not fully uncovered yet. Here, this knowledge gap is addressed, indicating that Zn electrodes face a O2 -involving corrosion, besides H2 evolution and dendrite growth. Differing from aqueous Li/Na batteries, removing O2 cannot enhance ZIB performance because of the aggravated competing H2 evolution. To address Zn issues, a one-off electrolyte strategy is reported by introducing the triple-function C3 H7 Na2 O6 P, which can take effects during the shelf time of battery. It regulates H+ concentration and reduces free-water activity, inhibiting H2 evolution. A self-healing solid/electrolyte interphase (SEI) can be triggered before battery operation, which suppresses O2 adsorption corrosion and dendritic deposition. Consequently, a high Zn reversibility of 99.6% is achieved under a high discharge depth of 85%. The pouch full-cell with a lean electrolyte displays a record lifespan with capacity retention of 95.5% after 500 cycles. This study not only looks deeply into Zn behavior in aqueous media but also underscores rules for the design of active metal anodes, including Zn and Li metals, during shelf time toward real applications.

Understanding H2 Evolution Electrochemistry to Minimize Solvated Water Impact on Zinc-Anode Performance.

H2 evolution is the reason for poor reversibility and limited cycle stability with Zn-metal anodes, and impedes practical application in aqueous zinc-ion batteries (AZIBs). Here, using a combined gas chromatography experiment and computation, it is demonstrated that H2 evolution primarily originates from solvated water, rather than free water without interaction with Zn2+ . Using linear sweep voltammetry (LSV) in salt electrolytes, H2 evolution is evidenced to occur at a more negative potential than zinc reduction because of the high overpotential against H2 evolution on Zn metal. The hypothesis is tested and, using a glycine additive to reduce solvated water, it is confirmed that H2 evolution and "parasitic" side reactions are suppressed on the Zn anode. This electrolyte additive is evidenced to suppress H2 evolution, reduce corrosion, and give a uniform Zn deposition in Zn|Zn and Zn|Cu cells. It is demonstrated that Zn|PANI (highly conductive polyaniline) full cells exhibit boosted electrochemical performance in 1 M ZnSO4 -3 M glycine electrolyte. It is concluded that this new understanding of electrochemistry of H2 evolution can be used for design of relatively low-cost and safe AZIBs for practical large-scale energy storage.

Studying the Conversion Mechanism to Broaden Cathode Options in Aqueous Zinc-Ion Batteries.

Aqueous Zn-ion batteries (ZIBs) are regarded as alternatives to Li-ion batteries benefiting from both improved safety and environmental impact. The widespread application of ZIBs, however, is compromised by the lack of high-performance cathodes. Currently, only the intercalation mechanism is widely reported in aqueous ZIBs, which significantly limits cathode options. Beyond Zn-ion intercalation, we comprehensively study the conversion mechanism for Zn2+ storage and its diffusion pathway in a CuI cathode, indicating that CuI occurs a direct conversion reaction without Zn2+ intercalation due to the high energy barrier for Zn2+ intercalation and migration. Importantly, this direct conversion reaction mechanism can be readily generalized to other high-capacity cathodes, such as Cu2 S (336.7 mA h g-1 ) and Cu2 O (374.5 mA h g-1 ), indicating its practical universality. Our work enriches the Zn-ion storage mechanism and significantly broadens the cathode horizons towards next-generation ZIBs.

Boosting Zinc Electrode Reversibility in Aqueous Electrolytes by Using Low-Cost Antisolvents.

Antisolvent addition has been widely studied in crystallization in the pharmaceutical industries by breaking the solvation balance of the original solution. Here we report a similar antisolvent strategy to boost Zn reversibility via regulation of the electrolyte on a molecular level. By adding for example methanol into ZnSO4 electrolyte, the free water and coordinated water in Zn2+ solvation sheath gradually interact with the antisolvent, which minimizes water activity and weakens Zn2+ solvation. Concomitantly, dendrite-free Zn deposition occurs via change in the deposition orientation, as evidenced by in situ optical microscopy. Zn reversibility is significantly boosted in antisolvent electrolyte of 50 % methanol by volume (Anti-M-50 %) even under harsh environments of -20 °C and 60 °C. Additionally, the suppressed side reactions and dendrite-free Zn plating/stripping in Anti-M-50 % electrolyte significantly enhance performance of Zn/polyaniline coin and pouch cells. We demonstrate this low-cost strategy can be readily generalized to other solvents, indicating its practical universality. Results will be of immediate interest and benefit to a range of researchers in electrochemistry and energy storage.

Bio-templated fabrication of three-dimensional network activated carbons derived from mycelium pellets for supercapacitor applications.

In this work, a three-dimensional porous mycelium-derived activated carbon (3D-MAC) was fabricated via a facile bio-templating method using mycelium pellets as both the carbon source and the bio-template. After ZnCl2 activation and high-temperature carbonization, the specific thread-like chain structure of mycelium in the pellets can be maintained effectively. The hyphae and junctions of the cross-linking hyphae form nanowires and carbon nanoparticles that link with the neighboring nanoparticles to form a network structure. By adding NH4Cl, foreign nitrogen element doped (N-doped) 3D-MAC was obtained, which has a hierarchical porous structure composed of micropores and macropores. And the multiple pore size distribution benefits from ZnCl2 activation, the specific 3D structure and gas blowing. Meanwhile, the introduction of some hydrophilic groups and abundant N-containing functional groups in extrinsic N-doped 3D-MAC contributes to improving the Faradaic pseudocapacitance, respectively. A specific capacitance of 237.2 F g-1 at 10 mV s-1 was displayed, which is more than 1.5 times that of 3D-MAC. Even at the large scan rate of 500 mV s-1, N-doped 3D-MAC still reveals a nearly symmetric rectangular shape, demonstrating great potential as a high-performance supercapacitor electrode material due to the synergistic effects of its 3D hierarchical porous structure and various functional groups.