WO2022006666A1

WO2022006666A1 - Anode and method for forming a zinc metal anode using molecular layer deposition

Info

Publication number: WO2022006666A1
Application number: PCT/CA2021/050923
Authority: WO
Inventors: Jian Liu; Huibing He
Original assignee: The University Of British Columbia
Priority date: 2020-07-06
Filing date: 2021-07-06
Publication date: 2022-01-13
Also published as: CA3123894A1

Abstract

Disclosed herein is a zinc ion battery and an anode for use in a zinc ion battery comprising a cathode, an anode and an aqueous zinc containing electrolyte. The anode for use in a zinc ion battery comprises a base formed of zinc having a film of an organic or an inorganic-organic hybrid material applied thereover. Also disclosed is a method for forming an anode for use in a zinc ion battery comprising providing a body of zinc and forming an organic or an inorganic-organic hybrid coating over the body of zinc. The step of forming an organic or an inorganic-organic hybrid coating may comprise molecular layer deposition by locating the body of zinc within an atomic layer deposition device and alternatingly supplying an organometallic compound containing a metal and glycol to the body of zinc through the atomic layer deposition device for a predetermined number of cycles.

Description

ANODE AND METHOD FOR FORMING A ZINC METAL ANODE USING MOLECULAR LAYER DEPOSITION

BACKGROUND

1. Technical Field

This disclosure relates generally to zinc-ion batteries and in particular to an anode, batter and method for forming an anode using molecular layer deposition.

2. Description of Related Art

The ever-growing market of electric vehicles, portable electronics, and smart grid have triggered high demand for reliable energy storage systems with high performance and low cost. As a promising energy storage technology, aqueous zinc-ion batteries (ZIBs) are receiving remarkable attentions owing to the appealing advantages of zinc (Zn) anodes, including high theoretical capacity (820 mAh g^-1), low redox potential (-0.762 V vs. standard hydrogen electrode), cost effectiveness, and environmental benignity. Additionally, using non-flammable aqueous electrolytes is of great significance to achieve intrinsic safety, environmental protection, cost saving and high ionic conductivity. However, the development of aqueous ZIBs has been seriously hindered by the Zn anode, which suffers from dendrite growth, low efficiency, and poor cycle life. The sharp and high-resistant Zn dendrites could not only cause rapid capacity fading by lowering efficiency and elevating internal resistance, but also lead to sudden failure of ZIBs by penetrating the separator.

To circumvent these problems caused by Zn dendrites, several strategies, such as structural design, surface coating, and electrolyte composition optimization, have been proposed to stabilize Zn anodes. Among these strategies, surface coating has been proven to be facile and effective to tailor the interaction between Zn anode and the electrolyte. Various types of surface coatings, including metal nanoparticles, organic polymers, carbon materials and metallic compounds, have been investigated as protective layers on Zn electrodes to render better electrochemical properties.

In particular, Wang et al. obtained a spontaneously reduced graphene oxide coating on Zn foil (Zn/rGO) to promote uniform Zn electrodeposition and improve the cycling stability. This self-assembled, layered rGO on a Zn surface provided a large electroactive area and a soft substrate for Zn electrodeposition, which significantly mitigated Zn dendritic growth by eliminating its driving force. Compared with bare Zn, the Zn/rGO anode exhibited much lower overpotential (~20 mV at 1 mA cm^-2) and excellent long- life cyclability. Furthermore, Kang et al. designed a porous nano-CaCO₃ coating as a protective layer to achieve a uniform and bottom-up Zn stripping/plating. This strategy effectively suppressed the development of Zn dendrites that may cause large polarization and internal short circuit, thus improving the Coulombic efficiency (CE) and cycling stability of Zn batteries.

Nevertheless, these coating strategies by conventional wet-chemical routes usually lead to nonuniform protective coatings with high thickness or loading mass, affecting its function as a protection layer. And the energy density at the cell level would also be reduced if introducing too much non- electrochemically active coating materials. For example, the thickness and mass loading of the rGO layer coated on Zn foil (Φ=1.5 cm) is 1 mm and ~ 3 mg, and the loading mass of the nano-CaCO3 coating was as high as 0.75 mg cm^-2. Such high loading mass of coating material cannot be neglected when considered into large-scale battery electrodes manufacturing. So far, the fabrication of uniform protective coatings on Zn anodes with controllable thickness still remains challenging.

SUMMARY OF THE DISCLOSURE

According to a first embodiment, there is disclosed a zinc ion battery comprising a cathode, an anode comprising a zinc base having a film comprising an organic or an inorganic-organic hybrid material applied thereover and an aqueous zinc containing electrolyte. The film may comprise a metal-based hybrid polymer. The metal-based hybrid polymer may comprise a metalcone material. The metalcone material may be selected from the group consisting of alucone, mangancone, zincone, titanicone, hafnicone and vandicone.

The film may be formed by molecular layer deposition on the zinc base. The film may have a thickness of between 2 to 40 nm.

According to a further embodiment, there is disclosed an anode for use in a zinc ion battery comprising a base formed of zinc having a film of an organic or an inorganic-organic hybrid material applied thereover.

The film may comprise a metal-based hybrid polymer. The metal-based hybrid polymer may comprise a metalcone material. The metalcone material may be selected from the group consisting of alucone, mangancone, zincone, titanicone, hafnicone and vandicone.

According to a further embodiment, there is disclosed a method for forming an anode for use in a zinc ion battery comprising providing a body of zinc and forming an organic or an inorganic-organic hybrid coating over the body of zinc.

The step of forming an organic or an inorganic-organic hybrid coating may comprise molecular layer deposition. The step of forming an organic or an inorganic-organic hybrid coating may comprise locating the body of zinc within an atomic layer deposition device and alternatingly supplying an organometallic compound containing a metal and glycol to the body of zinc through the atomic layer deposition device for a predetermined number of cycles. The number of cycles may be selected to between 10 and 200. The number of cycles may comprise substantially 60.

The body of zinc may be cleaned before being located within the atomic layer deposition device with water and/or alcohol. The organometallic compound may be selected to contain a metal selected from the group consisting of aluminum, manganese, zinc, titanium, hafnium, and vanadium.

Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings constitute part of the disclosure. Each drawing illustrates exemplary aspects wherein similar characters of reference denote corresponding parts in each view,

Figure 1 is a cross sectional view of an exemplary battery according to an embodiment of the present disclosure.

Figure 2 is a cross sectional view of an anode for use in a batter according to a further embodiment of the present disclosure.

Figure 3 is a schematic illustration of the effect of inorganic-organic MLD alucone coating on Zn metal anodes when cycling.

Figure 4 are illustrations of the long term cycling stability of Zn/Zn symmetric cells.

Figure 5 are voltage profiles of bare Zn and 60Alucone@Zn.

Figure 6 are post-cycling characterization of bare Zn and 60Alucone@Zn.

Figure 7 are illustrations of the evaluation of Zn/MnO₂ full cells with bare Zn and 60Alucone@Zn.

Figure 8 are illustrations of Zn and 60Alucone@Zn. Figure 9 is an chart of intensity vs binding energy.

Figure 10 are illustrations of Zn and 60Alucone@Zn.

Figure 11 are illustrations of the voltage profiles for Zn and 60Alucone@Zn during cycling.

Figure 12 are illustrations of the voltage profiles of various coatings.

Figure 13 is an illustration of the Zn|Ti cells.

Figure 14 are illustrations of the rate performance of Zn and 60Alucone@Zn.

Figure 15 are illustrations of the separators recycled from Zn and

60Alucone@Zn cells.

Figure 16 are images of the surfaces of Zn and 60Alucone@Zn.

Figure 17 illustrate contact angle tests for Zn and 60Alucone@Zn.

Figure 18 is a schematic illustration of a Zn/MnO₂ cell configuration.

Figure 19 is a schematic of the equivalent circuit for fitting experimental EIS data.

Figure 20 are illustrations of the physical characterizations of the δ-MnO₂ cathodes.

Figure 21 are images of the cycled MnO₂ cathodes in bare Zn/ MnO₂ and 60Alucone@Zn/ MnO₂.

DETAILED DESCRIPTION

Aspects of the present disclosure are now described with reference to exemplary apparatuses, methods and systems. Referring to Figure 1 , an exemplary battery according to a first embodiment is shown generally at 10. The battery 10 includes an anode 12 and a cathode 14 contained within a housing, which may be formed as top and bottom portions 16 and 18, respectively. The battery 10 further includes an electrolyte 20 and a separator 22 as are conventional. It will be appreciated that the above structure and illustrated layout for the battery 10 is exemplary only and that other designs for the battery may also be utilized. The battery 10 comprises an aqueous zinc-ion battery having a zinc anode and an d-Mh02 cathode. It will also be appreciated that other cathode types may also be utilized, such as, by way of non-limiting example, Mn₂O₃, Mn₃O₄, V₂O₅, VO₂, Zn₂V₂07, ZnHCF and VSs. As described in further detail below, the zinc anode 12 is coated with an organic or an inorganic-organic hybrid material such as alucone or the like. As utilized herein, alucone is part of a broader group of materials referred to herein as metalcones. As used in the present disclosure metalcones refers to a group of compounds based on organometal precursors and various organic precursors to yield a metal alkoxide film. In particular, and by ways of non- limiting example, alcone may be formed through a molecular layer deposition process by alternatingly supplying trimethylaluminum (TMA) and glycol (HO- CH2-CH2-OH) into an atomic layer deposition device as are known. Other metalcones, including mangancone, zincone, titanicone, hafnicone and vandicone are formed in similar manners. In addition, to metalcones, other organic thin films deposited by MLD may also be utilized, such as by way of non-limiting example Poly(3,4-ethylenedioxythiophene) (PEDOT), polyimides, polyazomethines, polyuria and polyamides.

In the present disclosure such films or coating are formed on the surface of the anode by molecular layer deposition (MLD). MLD, is a group of vapour phase think film deposition technique based on self-limiting reactions carried out in sequential manner. In particular, a first precursor material is introduced to the body to be coated which will only permit the molecules of the first precursor to be anchored to the surface in a single layer after which no further surface reaction will occur. Thereafter the first precursor material is removed from the vessel and a second precursor material introduced to react with the anchored molecules of the first precursor material. After all anchored surface molecules have been so reacted, again no further reaction can occur and the second precursor material is removed to permit the cycle to begin again. By alternating the utilized precursor materials, the thickness of the Due to the self limiting nature in the MLD reactions, the thickness of the resulting film is typically formed by repeating the reaction steps a number of times resulting in successively growth of the film 30 as illustrated in Figure 2. The thickness of the film 30 is therefore commonly measured in terms of the number of cycles utilized to form the film. In the present disclosure, it has been found that a number of cycles between 10 and 200 resulting in a film thickness of between 1 and 40 nm of alucone has been useful. In particular it has been found that approximately 60 cycles in the case of alucone has been particularly useful.

An advantage of molecular layer deposition (MLD) is that it is capable of precisely controlling film thickness at the molecular level, owing to its self- limiting nature of the gas-solid reactions during the deposition process The self-limiting reaction mechanisms involved in MLD are ideal for the fabrication of artificial SEI layers with inorganic-organic hybrid components which can prevent side reactions between electrode and electrolyte, thus improving CE and cycling stability.

In particular, in preparing the anode 12 for use in the battery, a portion of zinc, such as by way of non-limiting example, foil is cut or otherwise formed to the desired shape and size. The base body of zinc is then cleaned by sonication sequentially in deionized water and alcohol for 15 minutes, followed with natural drying in the air.

As illustrated herein, MLD metalcone coating was demonstrated as an artificial SEI layer on Zn metals to enable long-life, deeply-rechargeable and dendrite-free Zn anode for aqueous ZIBs. The nanoscale and uniform alucone coating greatly improved wettability and corrosion resistance of Zn anodes in aqueous electrolytes, suppressed the formation of Zn dendrite flakes and non-conductive byproducts, and decreased the charge transfer resistance over repeated cycling. In one example, the optimal alucone coating was found to be ~ 12 nm with 60 MLD cycles. It will be appreciated that for different coating materials, different optimal thickness may also be utilized. Moreover, superior cycling and rate performance were demonstrated in Zn/MnO₂ cells using the optimal alucone-coated Zn anode.

Example 1

Before MLD coating, zinc foil (0.2 mm in thickness) was cut into round plates (Φ = 14 mm), and cleaned by sonication sequentially in Dl water and alcohol for 15 minutes, followed with natural drying in the air. Alucone coating on the Zn round plates was performed at 175 °C by alternatively supplying trimethylaluminum (TMA) and glycol (HO-CH2-CH2-OH) into a commercial ALD system (GEMStar™ XT Atomic Layer Deposition System). Alucone coatings with 10, 30, 60, 100, and 200 MLD cycles were conducted on Zn foils, which were denoted as 10Alucone@Zn, 30Alucone@Zn, 60Alucone@Zn, 100Alucone@Zn and 200Alucone@Zn, respectively. δ-MnO₂ was synthesized by a hydrothermal method. In brief, 6 mmol KMnO4 and 1 mmol MnSO₄-H₂O were dissolved in 70 mL distilled water under stirring. The precursor solution was transferred into a Teflon contained autoclave with a volume of 100 mL, and then heated at 180 °C for 12 h. After cooling to room temperature, the black precipitates were vacuum-filtered until the PH reached to 7, and then dried at 100 °C in the air overnight.

The alucone coating thickness was optimized by conducting different MLD cycles (0 (bare), 10, 30, 60, 100 and 200 cycles) of alucone on Zn foils and testing their cycling performance in Zn|Zn symmetric cells in Figure 12. For fair comparison, 120 μL of 3M Zn(SO₃CF₃)₂ aqueous electrolyte was added into each cell, and the current density and areal capacity was set as 1 mA cm^-2 and 1 mAh cm^-2, respectively. As displayed in the inset of Figure 12a, in the first discharge process, zinc nucleation overpotential (ZNO) is required to initiate Zn plating and stripping process for all the samples after a rest time of 5 min (T=0.083 h). It is obvious that the ZNO value of Zn|Zn cells gradually increases with elevating MLD cycle numbers from 0 to 200. The ZNO value is determined as -128.3, -130.5, -137.6, -141.3, -181.3 and -200.2 mV, for bare Zn, 10Alucone@Zn, 30Alucone@Zn, 60Alucone@Zn, 100Alucone@Zn and 200Alucone@Zn, respectively. This result indicates that higher initial ZNO is required for Zn anodes with alucone coatings to start the Zn plating/stripping process, possibly due to the insulating nature of alucone film. For clearly showing the relationship of the Alucone coating cycle/coating thickness, the statistical data of cycle lifespan and voltage hysteresis over alucone coating cycles/thickness was summarized in Figure 12b. The corresponding voltage profiles of the last cycle before short circuit with labeling the voltage hysteresis was displayed in Figure 12c. Generally, lifespan represents the cycling time before internal short circuit occurs and voltage hysteresis was calculated from the gap between the end point of charge and discharge in the last cycle before short-circuit. As shown in Figure 12b, the voltage hysteresis decreased from 74 mV (0 MLD cycles) to the lowest value of 46.1 mV (60 MLD cycles), and then increased up to 74.6 mV with 200 MLD cycles. The lower voltage hysteresis means a more stable and uniform Zn stripping/plating process and usually leading to a longer cycle lifespan. Consequently, the cycling time increased from 128 h (0 MLD cycles) to the highest value of 498 h (60 MLD cycles), and then drop down to only 56 h when coating with 200 MLD cycles. It is reasonable that the relationship between the voltage hysteresis/cycling time and coating thickness presented to be not absolutely negative/positive, which indicated that a moderate coating thickness of alucone would cause enhanced electrochemical performance because of the protective electrode/electrolyte interface and uniform Zn-ion flux directed by alucone films during Zn stripping/plating process. While too much thicker coating could deteriorate the Zn stripping/plating electrochemical performance which may attributed to the sluggish kinetic reaction at the electrode/electrolyte interface because of the insulating properties of alucone films. In a word, the optimized coating thickness of alucone is ~12 nm (60 MLD cycles) in terms of its longest cycle lifespan (498 h) and lowest voltage hysteresis (46.1 mV).

Structural characterizations. The crystal structure of the Zn foils with and without alucone coatings was confirmed by Powder X-ray diffraction (XRD, Bruker D8-Advance X-ray diffractometer) using Cu Ka radiation (1.54056 A). The morphology was observed by using scanning electron microscope (SEM, Tescan MIRA3 FEG-ESEM) equipped with energy-dispersive X-ray spectroscopy (EDX). The surface chemistry state was characterized by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD). The Zn anodes were thoroughly washed with deionized water to remove residual salt and glass fiber prior to XRD and XPS measurements. The contact angle between Zn with and without alucone coating and the electrolyte was measured by using a contact angle goniometer (Rame Hart 260). The electrolyte droplet for the contact angle test was kept as 5 μL to avoid gravitational effect.

Electrochemical measurements. δ-MnO₂ (active material) was mixed with carbon black (Super P) and polyvinyldifluoride (PVDF) with a weight ratio of 6:3:1 in a N-Methyl-2-pyrrolidone (NMP) solvent. The slurry was cast on a thin titanium foil (100 μm in thickness) by a doctor blade, and then dried under vacuum at 60 °C overnight. After that, the electrode was cut into round disks (Φ = 12 mm) as the cathode in Zn-MnO₂ full cells. The mass loading of MnO₂ was around 1.8 mg cm^-2 on each disk. CR2032 coin cells were assembled in the air to evaluate electrochemical performance. Zn|Zn symmetric cells were assembled with the same two zinc plates (bare Zn or alucone coated Zn), glass fibers (separator, Φ = 5/8 inch) and 3M Zn(SO₃CF₃)₂ aqueous electrolyte. Zn/MnO₂ full cells were constructed with δ-MnO₂ electrode as the cathode, bare Zn or 60Alucone@Zn as the anode, a solution of 3M Zn(SO₃CF₃)₂ and 0.1 M Mn(SO₃CF₃)₂ as the electrolyte, and glass fibers (Φ = 5/8 inch) as the separator. 0.1 M Mn(SO₃CF₃)₂ additive in the electrolyte was used to suppress the dissolution of Mn²⁺ from the cathode and voltage of the cell was limited to 1.80 V for suppressing the decomposition of the aqueous electrolyte. Cyclic voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS) and liner polarization (corrosion test) were performed on a Biologic VSP Potentiostat/Galvanostat Station. CV was tested in the voltage range of 0.8-1.8 V (vs. Zn²/Zn) at a scan rate of 0.1 mV S^-1. EIS was conducted in a frequency range of 100 KHz to 0.01 Hz with a voltage amplitude of 5 mV. For the EIS test before cycling, the fresh Zn/MnO₂ batteries should keep static for at least half an hour to form a stable electrode/electrolyte interface. The EIS before cycling was conducted at an open voltage of -1.30 V and the EIS after cycling (100 cycles) was measured at full charge state (1.80 V). For corrosion test, the working (WE), counter (CE), and reference (REF) electrodes were zinc, platinum, and Ag/AgCI, respectively. Linear polarization technique was applied to the system by scanning between -0.7 and 0.4 V vs. Ag/AgCI from its open circuit voltage (OCV) at the rate of 2 mV s-1 in 3M Zn(SO₃CF₃)₂ solution. The surface area of the working electrode was 1.5 (1 X 1.5) cm², which was used to convert corrosion current (Icorr/mA) to corrosion current density (jcorr/mA cm^-2). All the electrochemical testing was conducted at 25°C. The specific capacities of Zn/MnO₂ cells were calculated based on the MnO₂ loading in each cathode.

Example 2

Here, we report a novel ultrathin inorganic-organic hybrid coating film of “alucone” via MLD as protective layer for Zn-metal anodes to improve the cycle life and stability. By guiding uniform Zn deposition and suppressing dendrite growth on the surface of Zn anodes (Figure 3), a long-life, dendrite- free and deeply rechargeable Zn anode was obtained. As a result, the Zn anode with 60 MLD cycles deposition of alucone (60Alucone@Zn) showed reduced overpotential (~ 84.3 mV) and a prolonged cycle life (over 780 h) at 3 mA cm^-2 in Zn|Zn symmetric cells. Moreover, it demonstrated a deeply rechargeable ability with a lifespan of 300 h at a high areal capacity of 10 mAh cm^-2. Particularly, when this Zn anode was assembled with MnO₂ cathode as a Zn/MnO₂ battery, it delivered a capacity retention of 83.3% after 800 cycles with a CE above 99.5% at 1C rate. The enhanced electrochemical performance was ascribed to the robust Zn metal anode enabled by alucone coating, which improved the surface wettability, enhanced the corrosion resistance of Zn metal, and effectively suppressed the formation of less conductive Zn dendrites.

Alucone coating was performed on fresh Zn foils by alternatively introducing trimethylaluminum (TMA) and ethylene glycol (HO-CH2-CH2-OH) to form polymeric films composed of organic and inorganic chains (Figure 3). Success of alucone coating on Zn foils was confirmed by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDX) analysis (Figures 8 and 9). SEM observation discloses that alucone coating covers the surface and grain boundaries on Zn foils, while maintaining the smooth surface of Zn substrates (Figure 8 a, b). EDX elemental mapping confirms the existence and homogenous distribution of Al, O and C elements, originating from the alucone coating layer on Zn foils (Figure 8d). XPS survey spectra (Figure ) reveals strong Al 2s and Al 2p peaks in 60Alucone@Zn and the absence of Zn peaks, suggesting compact coating of the MLD alucone layer. Decomposition of Al 2p spectrum results in one peak at 75.4 eV assigned to Al³⁺ in the alucone film. For the O 1s spectrum, the decomposed peaks at 532.0 and 533.3 eV correspond to O-AI and O-C bonds, respectively. The C 1s spectrum can be fitted well into two peaks at 286.0 and 290.3 eV, which are attributed to C-H/C-0 and C=0 bonds in alucone molecular structure (Figure 3). Nevertheless, Zn foil and 60Alucone@Zn show identical XRD patterns indexed as hexagonal Zn with P63/mmc space group (JCPDS PDF No. 65-5973) (Figure 8c). No diffraction peaks are detected for alucone in the XRD pattern of 60Alucone@Zn, mainly due to the amorphous nature and nanoscale thickness of the alucone coating. The above results confirm the successful deposition of alucone layer on Zn foils by MLD.

The chemical corrosion test (Figure 10) proves that the alucone coating can greatly reduce the corrosion reaction on the Zn surface, leading to an improved chemical stability in aqueous electrolyte. The effectiveness of alucone coating on the cycling stability of Zn anodes was first investigated in symmetric Zn|Zn cells at a high current density of 3 mA cm^-2 with a constant areal capacity of 1 mAh cm^-2. From Figure 4a, it is evident that 60Alucone@Zn shows reduced overpotentials and significantly prolonged lifetime compared to bare Zn. The overpotential in the 2nd cycle decreases from 243.0 mV for bare Zn to 127.8 mV for 60Alucone@Zn. This trend is observed in the lifespan of both Zn anodes. It should be noted that 60Alucone@Zn shows a higher ZNO than bare Zn in the 1st discharge process, suggesting an activation process required for the alucone coating. Bare Zn undergoes substantial fluctuation in its voltage profile with the increase of cycle time, and short circuit occurs after 70h, as evidenced by the sudden drop of voltage to almost zero. In contrast, 60Alucone@Zn maintains very stable charge-discharge profiles over 780 h, and still shows a low overpotential of 103.6 mV afterwards. The excellent stability of 60Alucone@Zn anode can also be observed from the overlap of charge- discharge curves from the 10th to the 1 ,000th cycle (Figure 11). As a result, the alucone coating achieves 11 -fold enhancement in the lifespan of Zn anodes at a high current density of 3 mA cm^-2. Moreover, it is found that the cycle stability of alucone-coated Zn anodes depends greatly on the alucone coating thickness (Figure 12). 60Alucone@Zn, with an alucone thickness of ~ 12 nm, exhibits the longest lifetime and smallest voltage hysteresis among all the samples. Thinner alucone coatings (10Alucone@Zn and 30Alucone@Zn) have limited improvement in the voltage hysteresis, while a thicker coating (200Alucone@Zn) leads to reduced cycle life of Zn anodes, due to the blocked ion and electron diffusion through the alucone layer.

To quantify alucone coating on the reversibility of Zn anode, bare Zn and 60Alucone@Zn anode were evaluated in Zn|Ti asymmetric cells using Ti foils as the current collector (Figure 13), and the results are illustrated in Figure 4 b-d. It is obvious that compared to bare Zn, 60Alucone@Zn displays enhanced Coulombic efficiency (CE), i.e. the ratio of Zn plating capacity to Zn stripping capacity on Zn anode. Both bare Zn and 60Alucone@Zn show gradual increase of CE in the initial ten cycles, possibly due to the stabilization process of the electrode/electrolyte interface. After 10 cycles, the average CE of 60Alucone@Zn electrode reaches as high as 98.6%, which outperforms that (91 .5%) of bare Zn. Figure 4d and 4e, it is clear that 60Alucone@Zn possesses less irreversible capacity than bare Zn, revealing reduced side reactions and improved Zn plating The chemical corrosion test (Figure 10) proves that the alucone coating can greatly reduce the corrosion reaction on the Zn surface, leading to an improved chemical stability in aqueous electrolyte. The effectiveness of alucone coating on the cycling stability of Zn anodes was first investigated in symmetric Zn|Zn cells at a high current density of 3 mA cm^-2 with a constant areal capacity of 1 mAh cm^-2. From Figure 4a, it is evident that 60Alucone@Zn shows reduced overpotentials and significantly prolonged lifetime compared to bare Zn. The overpotential in the 2nd cycle decreases from 243.0 mV for bare Zn to 127.8 mV for 60Alucone@Zn. This trend is observed in the lifespan of both Zn anodes. It should be noted that 60Alucone@Zn shows a higher ZNO than bare Zn in the 1st discharge process, suggesting an activation process required for the alucone coating. Bare Zn undergoes substantial fluctuation in its voltage profile with the increase of cycle time, and short circuit occurs after 70h, as evidenced by the sudden drop of voltage to almost zero. In contrast, 60Alucone@Zn maintains very stable charge-discharge profiles over 780 h, and still shows a low overpotential of 103.6 mV afterwards. The excellent stability of 60Alucone@Zn anode can also be observed from the overlap of charge-discharge curves from the 10th to the 1 ,000th cycle (Figure 11). As a result, the alucone coating achieves 11 -fold enhancement in the lifespan of Zn anodes at a high current density of 3 mA cm^-2. Moreover, it is found that the cycle stability of alucone-coated Zn anodes depends greatly on the alucone coating thickness (Figure 12). 60Alucone@Zn, with an alucone thickness of ~ 12 nm, exhibits the longest lifetime and smallest voltage hysteresis among all the samples. Thinner alucone coatings (10Alucone@Zn and 30Alucone@Zn) have limited improvement in the voltage hysteresis, while a thicker coating (200Alucone@Zn) leads to reduced cycle life of Zn anodes, due to the blocked ion and electron diffusion through the alucone layer.

To quantify alucone coating on the reversibility of Zn anode, bare Zn and 60Alucone@Zn anode were evaluated in Zn|Ti asymmetric cells using Ti foils as the current collector (Figure 13), and the results are illustrated in Figure 4 b-d. It is obvious that compared to bare Zn, 60Alucone@Zn displays enhanced Coulombic efficiency (CE), i.e. the ratio of Zn plating capacity to Zn stripping capacity on Zn anode. Both bare Zn and 60Alucone@Zn show gradual increase of CE in the initial ten cycles, possibly due to the stabilization process of the electrode/electrolyte interface. After 10 cycles, the average CE of 60Alucone@Zn electrode reaches as high as 98.6%, which outperforms that (91 .5%) of bare Zn. Figure 4d and 4e, it is clear that 60Alucone@Zn possesses less irreversible capacity than bare Zn, revealing reduced side reactions and improved Zn plating process with the alucone coating. The improved Zn plating behavior on the alucone-coated Zn foil is also indicated by the smoother charge curves of 60Alucone@Zn than bare Zn (Figure 4c and 4d). Overall, it can be concluded that alucone coating on Zn anode improve the CE and reversibility of Zn stripping/plating process, leading to the significantly enhanced lifespan of Zn anodes.

The effect of alucone coating on the rate capability (high current density) and deep rechargeability (high areal capacity) of Zn anodes was further investigated using on bare Zn and 60Alucone@Zn. With increasing the current density from 2 to 10 mA cm^-2, voltage polarizations with different amplitudes are observed for both bare Zn and 60Alucone@Zn (Figure 14a). However, 60Alucone@Zn electrode always exhibits substantially lower voltage hysteresis than bare Zn, especially at high current densities (5-10 mA cm^-2). As seen in Figure 5a, when the current density increases from 2 to 10 mA cm^-2, the voltage hysteresis of bare Zn rises from 93.4 to 323.9 mV, while that of 60Alucone@Zn elevates from 65.3 to 163.6 mV. The deep Zn stripping/plating abilities of bare Zn and 60Alucone@Zn were also investigated under various areal capacities (2-10 mAh cm^-2) with a constant current of 1 mA cm^-2 (Figure 5b). The cycle time of bare Zn severely decays from 108h to 75h and 55 h, when the areal capacity, proportional to the amount of Zn stripped/plated, is elevated from 2 mAh cm^-2 to 3 and 5 mAh cnr ² Once the areal capacity is increased to as high as 10 mAh cm^-2, bare Zn shows fluctuating voltage profiles even in the first discharge cycle (Figure 5b). In sharp contrast, 60Alucone@Zn maintains very steady charge-discharge voltage profiles over 300 h with all the areal capacities from 2 to 10 mAh cm^-2 (Figure 5b, Figure 14b, c). The results suggest that the alucone coating significantly improved the Zn stripping/plating performance at high rates and high areal capacities. In comparison to shadow stripping/plating of Zn anode reported (Table 1), the deeply rechargeable Zn anodes endowed by MLD surface engineering is of great importance for practical ZIB applications, because it will increase the utilization of Zn metal and thus the overall energy density of ZIBs.

Table 1. Comparison of the electrochemical performance at high rate and high capacity for this work with recently reported Zn-based symmetric cells

To elucidate the mechanism for the improved performance of Zn anodes by MLD alucone coating, SEM, EDS, and XPS analysis were performed on bare Zn and 60Alucone@Zn recovered from the Zn|Zn symmetric cells after cycling at a current density of 3 mA cm^-2, and the results are compared in Figure 6.

From the top-view SEM images (Figure 6a-b), it can be seen that the cycled bare Zn possesses a rough surface with thin and sharp Zn dendrite flakes grown vertically. These thin and sharp Zn dendrite flakes are approximately 20 μm and have high surface area that might promote side reactions between Zn and the electrolyte, resulting in low CE of bare Zn (Figure 4b). Moreover, these Zn dendrites might easily detach from the Zn substrate over the repeated Zn stripping/plating process, leading to the formation of “dead” Zn and causing internal short circuit. This can be clearly evidenced by the accumulation of many Zn dendrites on the glass-fiber separator recycled from bare Zn symmetric cells (Figure 15 a-c). In contrast, the cycled 60Alucone@Zn presents a relatively clean and flat surface free of Zn dendrite flakes (Figure 6e-h), suggesting the uniform electrochemical plating/stripping of Zn over repeated cycling. Moreover, the glass-fiber separator recycled from 60Alcuone@Zn has much less deposition of Zn dendrites (Figure 15 d-f). Morphology observation shows that alucone coating greatly reduces the formation of Zn dendrites and enables dendrite-free and smooth Zn metal anode.

The phase and chemical composition of cycled bare Zn and 60Alcuone@Zn were analyzed by XRD and XPS, and the results are illustrated in Figure 6i- 6k. As shown in Figure 6i, for both the cycled bare Zn and 60Alucone@Zn, besides the main peaks assignable to Zn metal (JCPDS No. 65-5973), five new diffraction peaks are identified at 13.1 °, 19.7°, 26.3°, 33.1° and 39.9° and can be collectively attributed to Zns(CO₃)₂(OH)₆ (JCPDS No. 19-1458) and Zn(OH)₂ (JCPDS No. 38-0356). The intensity of these new peaks is much weaker for cycled 60Alucone@Zn than bare Zn, revealing suppressed formation of Zn₅(CO₃)₂(OH)₆ and Zn(OH)₂ on Zn anode by alucone coating. This is consistent with the higher Zn/O ratio in the EDS spectrum of 60Alucone@Zn than that for bare Zn after cycling (Figure 16). The chemical species of Zn on bare Zn and 60Alucone@Zn are analyzed and shown in Zn 2p and O 1s XPS spectra (Figure 6j and 6k). For cycled 60Alcuone@Zn, the asymmetric Zn 2p peak is fitted into two peaks at 1020.8 and 1022.7 eV, which is assigned to metallic Zn and Zn-0 bond, respectively. On the contrary, the Zn 2p spectrum of cycled bare Zn is featured with one symmetric peak centered at 1022.7 eV, corresponding to the Zn-O bond in Zh₅(CO₃)₂(OH)₆ and Zn(OH)₂. This is further confirmed with its O 1s spectrum (Figure 6k), which is fitted well into only one peak located at 529.2 eV (O-Zn). While for cycled 60Alcuone@Zn, the O 1s spectrum is composed of a major peak at 529.2 eV (O-Zn) and a minor peak at 531.0 eV (O-AI). The O-AI bond likely comes from the alucone coating layer, as also indicated by the Al 2s (117.3 eV) and Al 2p (72.3 eV) in the full XPS spectrum of cycled 60Alucone@Zn (Figure 9). Nevertheless, the intensity of Al peaks decreases to a significantly lower value than those for 60Alucone@Zn before cycling, which is consistent with the observation in ALD AI2O3 coating on Zn foil and on Li metal. The reason could be due to that Zn plates on the surface of the alucone layer, and alucone coating might undergo mechanical degradation during the Zn stripping/plating process. The above post-cycling analysis suggest that the alucone coating reduces the formation of non-conductive byproducts (Zn₅(CO₃)₂(OH)₆, Zn(OH)₂) on Zn anode and the consumption of active Zn, and suppress the growth of Zn dendrite flakes, thus leading to the significantly improved lifespan, CE, and reversibility of alucone-coated Zn anode.

The influence of the alucone coating on the wettability and corrosion resistance of Zn anode was studied in a 3M Zn(SO₃CF₃)₂ aqueous solution by contact angle measurement and corrosion test (Figure 17). 60Alucone@Zn has a contact angle of 62°, much lower than that of bare Zn (92°), indicating a better surface wettability between the alucone-coated Zn and the aqueous electrolyte. The improved wettability is beneficial for Zn plating/stripping reactions, because it might enable uniform Zn-ion flux across the electrode/electrolyte interface to reduce Zn dendrite formation, and decrease charge transfer resistance in Zn stripping/plating process. Moreover, the corrosion curves (Figure 17c) and Tafel fit corrosion kinetic parameters (Table 2) clearly prove improved corrosion resistance of Zn metal anode by alucone coating. For example, the polarization resistance is determined to be 26.86 and 72.10 W cm^-2 for bare Zn and 60Alcuone@Zn respectively. Moreover, 60Alucone@Zn shows a much lower corrosion current of 0.31 mA cm^-2 than that of bare Zn (1.37 mA cm^-2), with an enhanced corrosion inhibition efficiency of 77.7%. The above studies indicate that the alucone coating layer can act as inhibitor to alleviate corrosion reaction on Zn anode, and the alucone-coated Zn anodes possess greatly improved wettability and corrosion resistance to the aqueous electrolyte. Table 2. Tafel fit corrosion kinetic parameters and corrosion inhibition efficiencies of the bare Zn and 60Alucone@Zn. Samples E I j β β R P

Ecorr: corrosion potential Icorr: corrosion current jcorr: corrosion current density βa and βc: the anodic and cathodic Tafel coefficients, respectively Rp: polarization resistance was calculated by: Rp =βaβc /In( 10)jcorr(βa +βc) Pp: corrosion inhibition efficiency was calculated by: Pp %= j⁰corr - jcorr/ j⁰corr x 100 where jcorr 0 is the value of corrosion current density of bare Zn, and jcorr is the value of corrosion current density of 60Alucone@Zn. Motivated by the development of long-life and dendrite-free Zn anode, we further demonstrate its effectiveness and practicability in a full Zn/MnO₂ battery (Figure 18a).60Alucone@Zn anode was coupled with δ-MnO₂ (Figure 19), a low-cost and high-capacity cathode in aqueous ZIBs. Cycling voltammetry (CV) test (Figure 18b) reveals distinct redox peaks of MnO₂ with high reversibility in both bare Zn/MnO₂ and 60Alucone@Zn/MnO₂ cells), indicating the same mechanism of redox reactions in δ-MnO₂ cathode. Figure 7a displays long-term cycling performance of bare Zn/MnO₂ and 60Alucone@Zn/MnO₂ at 1C.60Alcuone@Zn/MnO₂ exhibits an initial capacity of 250.8 mAh g^-1, and a stable cycling stability over 800 cycles with capacity retention of 83.3% and CE of 99.5%. Although bare Zn/MnO₂ presents a similar initial capacity of 238.9 mAh g^-1, the specific capacity fades dramatically to after 400 cycles, with a capacity retention of only 34.5%. The typical charge-discharge profiles in the 2nd and 400th cycles of bare Zn/MnO₂ and 60Alucone@Zn/MnO₂ cells are compared in Figure 7b. The first discharge plateau above 1.30 V is dominated by the insertion of H+, and the second one below 1.30 V corresponds to the Zn2+ insertion into MnO₂. It can be seen that 60Alucone@Zn/MnO₂ presents much smaller polarization than bare Zn/MnO₂ and shows good overlap in the charge/discharge curves, suggesting the excellent stability by alucone coating. Furthermore, 60Alucone@Zn/MnO₂ cell shows better rate performance than bare Zn/Mn02 one (Figure 7c). Specifically, 60Alucone@Zn/MnO₂ can deliver a reversible capacity of 304.6, 282.5, 250.7, 225.4, and 179.6 mAh g^-1 at 0.1 , 0.3, 0.5, 1 , and 2C, respectively. It can still deliver a capacity of 119.7 mAh g^-1 at a current density as high as 5C. In comparison, bare Zn/MnO₂ cell shows only a discharge capacity of 295.1 , 269.6, 225.3, 197.7, 133.6, and 64.5 mAh g^-1 at 0.1 , 0.3, 0.5, 1 , 2, and 5C, respectively.

Electrochemical impendence spectroscopy (EIS) analysis was performed on bare Zn/MnO₂ and 60Alucone@Zn/MnO₂ before and after cycling to understand the influence of alucone coating on the resistance in the cell (Figure 7d and 7e). By fitting the EIS spectrum (Fig. 18 and Table 3), 60Alucone@Zn/MnO₂ cell exhibits slightly larger Ret (75.9 W) than that of bare Zn/MnO₂ (69.6 W), probably due to the insulting nature of the alucone coating on the Zn anode. However, after 100 cycles, bare Zn/MnO₂ cell shows much larger Ret (800.2 W) than 60Alucone@Zn/MnO₂ counterpart (97.7 W), which should be resulted from the restricted charge diffusivity of the uncoated Zn anode. EIS analysis suggests that alucone coating on the Zn anode reduces the charge transfer resistance at the electrode/electrolyte interface in 60Alucone@Zn/MnO₂ cell, the high Ret in bare Zn/MnO₂ cell after cycling could be due to the accumulation of dead Zn dendrites on the Zn substrate (Figure 6). Furthermore, the morphologies of the cycled Zn anode and MnO₂ cathode from both cells are examined by SEM (Figure 21). The MnO₂ cathodes from both cells maintain the special 3D micro/nano spheres, indicating its good structural stability during repeated Zn insertion/extraction. However, the Zn anode recovered from bare Zn/MnO₂ has a rough surface covered with many Zn dendrites, while that from 60Alcuone@Zn/MnO₂ is found to be dense and dendrite-free. The EIS results and post-cycling SEM observations further prove the effectiveness of alucone-coated Zn anode in enabling long-cycling and high-rate Zn/MnO₂ cells. Table 3. The impedance parameters for bare Zn-MnO₂ and 60Alucone@Zn-

MnO₂ before and after cycling. Before cycling

While specific embodiments have been described and illustrated, such embodiments should be considered illustrative only and not as limiting the disclosure as construed in accordance with the accompanying claims.

Claims

What is claimed is:

1. A zinc ion battery comprising: a cathode; an anode comprising a zinc base having a film comprising an organic or an inorganic-organic hybrid material applied thereover; and an aqueous zinc containing electrolyte.

2. The zinc ion battery of claim 2 wherein the film comprises a metal- based hybrid polymer.

3. The zinc ion batter of claim 2 wherein the metal-based hybrid polymer comprises a metalcone material.

4. The zinc ion batter of claim 3 wherein the metalcone material is selected from the group consisting of alucone, mangancone, zincone, titanicone, hafnicone and vandicone.

5. The zinc ion battery of claim 1 wherein the film is formed by molecular layer deposition on the zinc base.

6. The zinc ion battery of claim 5 wherein the film has a thickness of between 2 to 40 nm.

7. An anode for use in a zinc ion battery comprising a base formed of zinc having a film of an organic or an inorganic-organic hybrid material applied thereover.

8. The anode of claim 7 wherein the film comprises a metal-based hybrid polymer.

9. The anode of claim 8 wherein the metal-based hybrid polymer comprise a metalcone material.

10. The anode of claim 9 wherein the metalcone material is selected from the group consisting of alucone, mangancone, zincone, titanicone, hafnicone and vandicone.

11. The anode of claim 7 wherein the film is formed by molecular layer deposition on the zinc base.

12. The anode of claim 11 wherein the film has a thickness of between 2 to 40 nm.

13. A method for forming an anode for use in a zinc ion battery comprising: providing a body of zinc; and forming an organic or an inorganic-organic hybrid coating over the body of zinc.

14. The method of claim 13 wherein the film comprises a metal-based hybrid polymer.

15. The method of claim 14 wherein the metal-based hybrid polymer comprises a metalcone material.

16. The method of claim 15 wherein the metalcone material is selected from the group consisting of alucone, mangancone, zincone, titanicone, hafnicone and vandicone.

17. The method of claim 13 wherein the step of forming an organic or an inorganic-organic hybrid coating comprises molecular layer deposition.

18. The method of claim 13 wherein the step of forming an organic or an inorganic-organic hybrid coating comprise: locating the body of zinc within an atomic layer deposition device; and alternatingly supplying an organometallic compound containing a metal and an organic precursor to the body of zinc through the atomic layer deposition device for a predetermined number of cycles.

19. The method of claim 18 wherein the number of cycles is selected to between 10 and 200.

20. The method of claim 19 wherein the number of cycles comprises substantially 60.

21. The method of claim 18 wherein the body of zinc is cleaned before being located within the atomic layer deposition device with water and/or alcohol.

22. The method of claim 18 wherein the organometallic compound is selected to contain a metal selected from the group consisting of aluminum, manganese, zinc, titanium, hafnium, and vanadium.