https://www.cell.com/joule/pdf/S2542-4351(20)30094-5.pdf

Scientific Challenges for the Implementation of Zn-Ion Batteries

Over the past two decades, a series of aqueous rechargeable metal-ion batteries (ARMBs) have been developed, aiming at improving safety, environmental friendliness and cost-efficiency in fields of consumer electronics, electric vehicles and grid-scale energy storage. However, the notable gap between ARMBs and their organic counterparts in energy density directly hinders their practical applications, making it difficult to replace current widely-used organic lithium-ion batteries. Basically, this huge gap in energy density originates from cell voltage, as the narrow electrochemical stability window of aqueous electrolytes substantially confines the choice of electrode materials. This review highlights various ARMBs with focuses on their voltage characteristics and strategies that can effectively raise battery voltage. It begins with the discussion on the fundamental factor that limits the voltage of ARMBs, i.e., electrochemical stability window of aqueous electrolytes, which decides the maximum-allowed potential difference between cathode and anode. The following section introduces various ARMB systems and compares their voltage characteristics in midpoint voltage and plateau voltage, in relation to respective electrode materials. Subsequently, various strategies paving the way to high-voltage ARMBs are summarized, with corresponding advancements highlighted. The final section presents potential directions for further improvements and future perspectives of this thriving field.

Voltage issue of aqueous rechargeable metal-ion batteries

Aqueous Zn batteries that provide a synergistic integration of absolute safety and high energy density have been considered as highly promising energy-storage systems for powering electronics. Despite the rapid progress made in developing high-performance cathodes and electrolytes, the underestimated but non-negligible dendrites of Zn anode have been observed to shorten battery lifespan. Herein, this dendrite issue in Zn anodes, with regard to fundamentals, protection strategies, characterization techniques, and theoretical simulations, is systematically discussed. An overall comparison between the Zn dendrite and its Li and Al counterparts, to highlight their differences in both origin and topology, is given. Subsequently, in-depth clarifications of the specific influence factors of Zn dendrites, including the accumulation effect and the cathode loading mass (a distinct factor for laboratory studies and practical applications) are presented. Recent advances in Zn dendrite protection are then comprehensively summarized and categorized to generate an overview of respective superiorities and limitations of various strategies. Accordingly, theoretical computations and advanced characterization approaches are introduced as mechanism guidelines and measurement criteria for dendrite suppression, respectively. The concluding section emphasizes future challenges in addressing the Zn dendrite issue and potential approaches to further promoting the lifespan of Zn batteries.

Dendrites in Zn-Based Batteries.

The dendritic issue in aqueous zinc-ion batteries (ZBs) using neutral/mild electrolytes has remained an intensive controversy for a long time: some researchers assert that dendrites severely exist while others claim great cycling stability without any protection. This issue is clarified by investigating charge/discharge-condition-dependent formation of Zn dendrites. Lifespan degradation (120 to 1.2 h) and voltage hysteresis deterioration (134 to 380 mV) are observed with increased current densities due to the formation of Zn dendrites (edge size: 0.69-4.37 µm). In addition, the capacity is also found to remarkably affect the appearance of the dendrites as well. Therefore, at small current densities or loading mass, Zn dendrites might not be an issue, while the large conditions may rapidly ruin batteries. Based on this discovery, a first-in-class electrohealing methodology is developed to eliminate already-formed dendrites, generating extremely prolonged lifespans by 410% at 7.5 mA cm-2 and 516% at 10 mA cm-2 . Morphological analysis reveals that vertically aligned Zn dendrites with sharp tips gradually become passivated and finally generate a smooth surface. This developed electrohealing strategy may promote research on metal dendrites in various batteries evolving from passive prevention to active elimination, rescuing in-service batteries in situ to achieve elongated lifetime.

Do Zinc Dendrites Exist in Neutral Zinc Batteries: A Developed Electrohealing Strategy to In Situ Rescue In-Service Batteries.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/eem2.12067

Issues and Future Perspective on Zinc Metal Anode for Rechargeable Aqueous Zinc-ion Batteries

While α-MnO2 has been intensively studied for zinc batteries, δ-MnO2 is usually believed to be more suitable for ion storage with its layered structure. Unfortunately, the extraordinary Zn ion storage performance that δ-MnO2 should exhibit has not yet been achieved due to the frustrating structural degradation during charge-discharge cycles. Here, we found the Na ion and water molecules pre-intercalation can effectively activate stable Zn ion storage of δ-MnO2. Our results reveal that the resulted Zn//pre-intercalated δ-MnO2 battery delivers an extraordinarily high-rate performance, with a high capacity of 278 mAh g-1 at 1 C and up to 20 C, and a high capacity of 106 mAh g-1 can still be measured. The capacity retention is as high as 98% after charged-discharged up to 10,000 cycles benefiting from smooth Zn ion diffusion in the pre-intercalated structure. Further in situ/ex situ characterization confirms the superfast Zn ion diffusion in the pre-intercalated structure at room temperature. In addition, utilizing the well-chosen electrode material and modified polyurethane shell, we fabricated a quasi-solid-state healable Zn-δ-MnO2, which can be self-healed after multiple catastrophic damages, emphasizing the advanced features of aqueous Zn ion battery for wearable applications.

Jianbo Liang

Papers

A Superior δ-MnO2 Cathode and a Self-Healing Zn-δ-MnO2 Battery.

Porous single-crystal NaTi2(PO4)3 via liquid transformation of TiO2 nanosheets for flexible aqueous Na-ion capacitor