Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 30348−30356
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Introducing Artificial Solid Electrolyte Interphase onto the Anode of Aqueous Lithium Energy Storage Systems Moin Ahmed,† Alireza Zehtab Yazdi,† Aly Mitha, and P. Chen* Department of Chemical Engineering and Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L3G1, Canada
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ABSTRACT: Aqueous lithium energy storage systems (ALESSs) offer several advantages over the commercially available nonaqueous systems, and the most noteworthy is that ALESSs have higher ionic conductivity, can be used safely, and are environmental-friendly in nature. The ALESS, however, exhibits faster capacity fading than their nonaqueous counterparts after repeated cycles of charge and discharge, thus limiting their wide-range applications. Excessive corrosion of metallic anodes in the aqueous electrolyte and accelerated growth of dendrites during the charge−discharge process are found to be the main reasons that severely impact the life span of ALESSs. Here, we introduce ultrathin graphene films as an artificial solid electrolyte interface (G-SEI) on the surface of a zinc anode to improve the cycling stability of an aqueous lithium battery system. The G-SEI is fabricated at different thicknesses and areas ranging from ∼1 to 100 nm and ∼1 to 10 cm2, respectively, via a Langmuir−Blodgett trough method and deposited onto the surface of the zinc anode. Electrochemical characterizations show a significant reduction in corrosion current density (0.033 mA cm−2 vs 1.046 mA cm−2 for the control), suppression of dendritic growth (∼50%), and reduction in chargetransfer resistance (222 Ω vs 563 Ω for the control) when the G-SEI is utilized. The aqueous battery system with the G-SEI (100 nm thickness) on the anode exhibits ∼17% improvement in cycling stability (82% capacity retention after 300 cycles) compared to the control system. Comprehensive microscopy and spectroscopy characterizations reveal that the G-SEI not only controls the ion transport between the electrolyte and the anode surface (lower corrosion) but also promotes a uniform deposition (less dendritic growth) of zinc on the anode. KEYWORDS: graphene, dendrite, aqueous electrolyte, artificial solid electrolyte interphase (SEI), rechargeable batteries, zinc anode
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INTRODUCTION The demand for portable and more environmentally friendly energy storage systems, particularly batteries, has been significantly increased over the past decade. Although great progress has been made in the current commercially available batteries, they still suffer from major drawbacks. For example, nickel (Ni)- and lead (Pb)-based battery systems typically utilize toxic heavy metals and/or precursors that are naturally low in abundance. The benchmark lithium ion batteries with organic electrolytes also have toxicity and flammability issues with high costs associated with their assembly at large scale.1−3 In contrast, aqueous lithium energy storage systems (ALESSs) are considered as the most attractive alternative because they can deliver higher power density and use inexpensive and safer electrolytes and their assembly is much simpler than their nonaqueous counterparts.4,5 One major drawback that limits large-scale applications of ALESSs, however, is corrosion of their anodes, followed by the formation of metallic dendrites, leading to a significant capacity fading and ultimately short circuit.6 During the charging process of ALESSs, metal ions (e.g., Zn2+) from the electrolyte adsorb on the anode surface and © 2018 American Chemical Society
convert to metal atoms (e.g., Zn). The surface roughness of the anode localizes the current density and causes a nonuniform deposition of the atoms, leading to the formation of dendrites.7 During successive cycles of charge and discharge, the dendrites gradually grow perpendicular to the anode surface, penetrating through the separator, and eventually come in contact with the cathode, a process so-called short circuit.8 In addition, the formation and growth of dendrites increase the surface area of the anode, leading to higher corrosion rates and accelerated kinetics of detrimental surface-dominant reactions.6,9 Whereas extensive studies have been conducted to suppress lithium dendrites in nonaqueous systems, limited solutions have been proposed for ALESSs. Introducing additives into the aqueous electrolytes and replacing aqueous electrolytes with the gel electrolyte are the two main proposed methods.9−11 One common strategy in both aqueous and nonaqueous systems is to use additives in the electrolyte to suppress the dendritic growth of the anode. These additives form a physical Received: June 4, 2018 Accepted: August 9, 2018 Published: August 9, 2018 30348
DOI: 10.1021/acsami.8b09268 ACS Appl. Mater. Interfaces 2018, 10, 30348−30356
Research Article
ACS Applied Materials & Interfaces
in a G-SEI with a thickness of 10 nm. The anode also shows the reversible capacity of around 95 mA h/g and a capacity retention of 82% after 300 cycles of charge and discharge. We hypothesize that these improvements mainly come from a uniform zinc deposition process on the anode surface during charging. The uniform zinc deposition minimizes the dendritic growth.
barrier at the interface between the anode and the electrolyte. The barrier is supposed to prevent the propagation of dendrites toward the cathode.12−14 However, these physical barriers have limited mechanical strength and chemical stability and may be easily removed after several charge− discharge battery cycles.15,16 Another strategy is to fabricate gel-like structures in the aqueous electrolytes.17 These gel electrolytes mainly consist of fused silica that can inhibit both corrosion of the anode and its dendritic growth. Large-scale fabrication of gel electrolytes, however, is limited and difficult to implement in the ALESS. Facile methods using twodimensional (2D) materials with high mechanical and chemical stabilities to prevent corrosion and dendritic growth of anodes have yet to be developed for ALESS. Our group recently introduced the concept of artificial solid electrolyte interface (G-SEI) in the cathodes of ALESSs.5 Graphene flakes in a continuous film with a thickness of 1−100 nm and a length of 1−10 cm2 were introduced on the cathode surface, where a G-SEI was formed. The G-SEI showed a great improvement in the cycling stability of an aqueous lithium battery through suppression of the Jahn−Teller effect and degradation of carbon materials in the cathode during the charge and discharge process. In fact, the G-SEI nicely controlled both the ion transport and the gas permeation processes on the cathode surface. In this work, we similarly propose the G-SEI on the anode surface to control these processes with the objective to reduce the corrosion rate and dendritic growth of the anode. The graphene flakes in the GSEI have high mechanical strength and chemical stability, which can control the permeation of hydrated lithium and zinc ions into a zinc anode. The Young modulus of a graphene monolayer is found to be ∼200 GPa, more than two times higher than that of zinc (∼90 GPa).18 The high mechanical strength of the graphene layers ensures that zinc dendrites do not penetrate through the layers. Although the mechanical properties of the graphene film can be measured via an atomic force microscope equipped with a nanoindentation probe on a specific substrate,19,20 this measurement would have been challenging on top of an extremely rough surface of the zinc anode with both microscopic and macroscopic features. In this work, Young’s modulus is used as a standard mechanical property to assess the strength of the graphene layers in comparison with the zinc dendrites. The impact of other mechanical properties, such as rupture strength and fracture toughness, of the graphene layers on the suppression of dendrites can also be further explored. It is also reported that graphene oxide (GO) layers allowed ions with hydrated radii of
