Engineering interfacial layers to enable Zn metal anodes for aqueous zinc-ion batteries
Introduction
Lithium-ion batteries (LIBs), as the most widely used energy storage devices, are now powering our world owing to their high operating voltages, competitive specific capacities, and long cycle lives [1], [2], [3]. However, the increasing concerns over limited lithium resources, high cost, and safety issues of flammable organic electrolytes limit their future applications in large-scale energy storage systems (ESS) [4], [5], [6]. Hence, researchers have started studying Na-ion and K-ion batteries with relatively high crust abundance [7], [8], [177]. Nevertheless, the issues of low energy density, highly toxic, and flammable organic electrolytes still remain unsettled [6,9,10]. The drawbacks of these nonaqueous-based systems motivate us to explore alternative battery chemistry with lower cost, higher safety, and longer life.
Aqueous zinc-ion batteries (ZIBs) have gained remarkable attention as a promising energy storage technology, especially in mild/neutral aqueous electrolytes. This is due to the unparalleled advantages originated from the adopted Zn metal anodes and aqueous electrolytes. For Zn metal anodes, they have a high theoretical capacity (820 mAh g − 1 and 5854 Ah L − 1), low redox potential (−0.762 V vs. standard hydrogen electrode, denoted as SHE), and good natural abundance [11], [12], [13], [14], [15], [16], [17]. For using a nonflammable aqueous electrolyte, it is of great significance to achieve intrinsic safety, environmental protection, and cost-saving. Importantly, aqueous electrolytes can offer two orders of magnitude higher ionic conductivity (∽ 1 S cm−1) than nonaqueous ones (∽ 1–10 mS cm−1), benefiting from fast reaction kinetics and high-power density in ZIBs [6,15,[18], [19], [20]].
However, due to the employment of Zn metal anodes in an aqueous environment, it is easy for the hydrogen evolution reaction (HER) to take place at the interface between active Zn metal electrode and aqueous electrolyte during the battery operation, along with surface passivation, nonconductive by-products formation, and growth of Zn dendrites [21], [22], [23], [24], [25], [26]. The zinc dendrites with increased surface area further facilitate the corrosion and hydrogen evolution, leading to low Coulombic efficiency (CE) [27]. The severe dendrite growth can penetrate the separator, resulting in an internal short-circuit of the batteries [18,28]. Therefore, it is urgent to build a stable and rechargeable zinc anode, which requires a high zinc utilization, high CE, and durable cycling life.
Numerous works have been done to address the aforementioned problems, such as the cathode design, [29], [30], [31], [32], [33], [34], [35], [36], [37], [38] anode improvement, [5,23,24,[39], [40], [41]] electrolyte optimization, [23,38,[42], [43], [44], [45]] mechanism revealing and others [46], [47], [48], [49], [50]. As a result, the number of the published works on ZIBs has gradually increased in the last few years (the Year 2016 − 2021), and is expected to rise more quickly in the future (Fig. 1a). Aggressive efforts have been devoted to the viable intercalation cathode [29,31,49,51] with the proportion of 55.65%, far exceeding that of other parts (Fig. 1b). It is reasonable that the cathode is the big focus because commercial Zn foils are fixed as the anode study object. However, an ideal zinc anode is essential to ensure good cycling stability and a long lifespan for ZIBs [12,24,52]. It should be noticed that the large-scale application of ZIBs would be hampered if the investigations on the zinc anodes are overlooked. So, we would like to call for more attention to the anode development, whether from academic researchers or industrial organizations.
Currently, the employed Zn anodes in the metal state are mainly categorized into three types as follows (Fig. 1c): (i) Commercial Zn foils. They can be directly used as anodes for ZIBs with different thicknesses or be used after further surface modification in a cell (ii) Zinc powders. It should be made into electrode slurry by adding conductive agent and binder, and then painted onto a conductive and stable substrate in an aqueous electrolyte (such as Ti foil) [53]. (iii) Electroplated zinc. In this case, zinc was electroplated on a zinc host which served as the structural and conductive network to limit zinc growth [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67]. Among the three kinds of Zn metal anodes, commercial Zn foils are the most promising ones, often used as zinc metal anodes in their pristine or modified state due to their easy operation and good tolerability when need further treatment. Moreover, commercial zinc foils have great application potential in the large-scale EES owing to their low production cost and easy replacement by fresh Zn foil during the whole life cycle if the cell failure is caused by zinc anode deterioration. Furthermore, ZIB energy density can be highly improved by shaping Zn foil as thin as Al or Cu foil in the commercial lithium-ion battery industry. Last but not least, zinc foil can hopefully be extended to the field of flexible and wearable devices because of its flexible nature [68], [69], [70]. However, zinc deposition is highly reactive in aqueous electrolytes because hydrogen or oxygen evolution from water decomposition cannot precipitate onto zinc anode surface, which results in an unstable solid electrolyte interphase (SEI) [71,72]. This uneven zinc deposition behavior will further bring a series of parasitic reactions such as dendrite growth and surface passivation with nonconductive by-products, leading to poor rechargeability of Zn anodes in the end [73], [74], [75], [76]. Therefore, it is essential to construct an artificial interfacial layer with a similar role to SEI for inducing homogenous Zn deposition during repeated cycling. Surface engineering strategies are facile and effective in building such robust artificial SEI and tailor the interaction between Zn anode and the electrolyte.
Herein, in this review, surface engineering strategies focusing on commercial Zn foils for dendrite-free Zn metal anodes, and their internal mechanisms are summarized and compared from the perspective of zinc-electrolyte interface optimization to provide clear design ideas for robust zinc anodes. Firstly, we would brief some background knowledge of zinc electrochemistry in a mild acidic aqueous environment and disclose the general degradation mechanism of Zn anodes. Then, the state-of-the-art research progress in the surface and interface engineering on commercial Zn foils was comprehensively reviewed, with a specific and in-depth summary of the mechanisms, and a few examples of advanced characterizations and simulations are presented to help understand the working mechanisms of the introduced surface engineering layers. Lastly, we offer some perspectives on the importance of optimizing commercial Zn foils with respect to zinc utilization, stripping/plating efficiency, depth-of-discharge, and cell energy density in the real battery industry, as well as the future directions and outlooks for constructing high-performance ZIBs assembled with commercial Zn foils. This review may bring new ideas and concepts for the rational design of highly reversible Zn foils and accelerate the commercialization of aqueous zinc-ion batteries in the large-scale EES field.
Section snippets
Zn anode issues
Similar to other alkali-ion battery systems, dendrite growth is the main issue in high-performance ZIB development [73,77]. Compared to the alkaline system, the main reason for the recent boom on ZIBs in mild/neutral aqueous electrolytes is the alleviated issues associated with Zn metal anodes [22,[78], [79], [80]]. Nevertheless, they still suffer from dendrite growth, corrosion/hydrogen evolution, and passivation issues because of the inhomogeneous Zn ions stripping/plating reactions on the
Surface engineering strategies on zinc metals
Up to now, there are two main development directions to improve the reversibility of Zn anodes in an aqueous electrolyte from the perspective of stabilizing the Zn-electrolyte surface. One is from the view of Zn metal anode, on which an artificial interfacial layer is constructed with a similar function to SEI, whether by ex-situ coating or in-situ growth, for tailoring Zn/electrolyte interface reaction with a defined and smooth Zn deposition behavior [53,62,67,87,114]. The surface engineering
Advanced characterizations and simulations for Zn metal anodes
Developing advanced characterizations with the combination of simulations and theoretical calculations are very important for revealing the working mechanisms of the coating layers on Zn metal anodes. In-situ visualized techniques that can monitor real-time changes in morphological structure and surface condition would contribute to a deeper understanding of Zn metal interfaces. Zhi et al. track Zn electrodeposition process and bubbles change during the electrochemical reactions by in situ
Design principle for Zn surface coating layer
Based on the above analysis, an ideal interface layer by surface coating should meet the following requirements during repeated Zn stripping/plating cycles.
- i)
inexpensive raw materials and a cost-effective production process. It is favorable to use cheap raw materials and simple manufacturing processes to introduce surface coating layers, coinciding with the “low-cost” nature of aqueous ZIBs.
- ii)
good adhesion with Zn substrate. Whether the interface layer is introduced by ex-situ coating or in-situ
Comparison of different coating methods
Various coating methods such as doctor-blading, electroplating, ion-exchange, ion/magnetron sputtering, ALD/MLD, spin-coating and 3-D printing, have been developed to engineer interfacial layers on Zn metal anodes. Hence, these methods need to be compared in terms of “scalability, cost, thickness, and protection effect” of the protective coating layer. Doctor-blading seems to be the most commonly used approach because of its simplicity, adjustable thickness and economy. However, the
Summary and outlook
Aqueous ZIBs have been regarded as an ideal alternative to LIBs for large-scale applications due to their advantages of low cost, inherent safety, and environmental benignity. Nevertheless, the current aqueous ZIBs are still far from meeting our ever-increasing energy demands because of the poor reversibility of Zn metal anodes in the water-based electrolytes. Numerous strategies have been devoted to improving battery performance by optimizing the Zn electrodes. Among these strategies, surface
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (21872040), the Excellent Scholars and Innovation Team of Guangxi Universities, Guangxi Major Projects of Science and Technology (Grant No.GXMPSTAA17202032), Guangxi Ba-Gui Scholars Program. Dr. Liu’s work at the University of British Columbia (UBC) is supported by the Nature Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), and BC Knowledge Development Fund (BCKDF).
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