A deeply rechargeable zinc anode with pomegranate-inspired nanostructure for high-energy aqueous batteries

Peng Chen ab, Yutong Wu a, Yamin Zhang a, Tzu-Ho Wu a, Yao Ma a, Chloe Pelkowski a, Haochen Yang a, Yi Zhang ac, Xianwei Hu b and Nian Liu *a
aSchool of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA. E-mail: nian.liu@chbe.gatech.edu
bSchool of Metallurgy, Northeastern University, Shenyang, Liaoning 110819, China
cCollege of Energy and Institute for Electrochemical Energy Storage, Nanjing Tech University, Nanjing, Jiangsu 211816, China

Received 12th August 2018 , Accepted 17th October 2018

First published on 17th October 2018


Rechargeable, Zn-based aqueous batteries because of their advantages of inflammability, high energy density, and low material cost are an attractive alternative to lithium-ion and lead-acid batteries for transportation and grid-scale applications. Historically, zinc anodes have shown low utilization and rechargeability in alkaline electrolytes due to the problems of ZnO passivation and Zn(OH)42− dissolution. Herein, we report a nanoscale, pomegranate-structured Zn anode (Zn-pome) fabricated via a bottom-up microemulsion approach to overcome these problems. In the Zn-pome, primary ZnO nanoparticles (ZnO NPs) assemble into secondary clusters after which they are individually encapsulated by a conductive, microporous carbon framework. The small size of ZnO NPs overcomes the issue of passivation, whereas the secondary structure and ion-sieving carbon shell mitigate the dissolution problem. Inductively coupled plasma (ICP) analysis confirms that Zn dissolution from the Zn-pome anode is effectively suppressed, leading to a considerably prolonged cycle life compared to that of a conventional ZnO anode in an alkaline aqueous electrolyte. The Zn-pome anode even maintains the capacity after long resting. This performance is achieved in harsh yet practical conditions: a limited amount of electrolyte, sealed coin cells, and 100% depth of discharge (DOD). This study represents an important step towards producing aqueous, rechargeable, high-energy batteries. In addition, the design principles reported here can be applied to other battery systems involving passivation or dissolution intermediates.


Introduction

Depletion of fossil fuel resources is leading to steadily increasing energy demands. As a result, it is necessary to develop sustainable electrochemical energy storage (EES) systems that are low-cost, reliable, and eco-friendly.1–3 Extensive research into EES in recent years has prompted the emergence of crucial technologies for applications in portable devices, electric vehicles (EVs) and grid-scale energy-storage systems.4–7 Batteries utilizing faradaic energy storage mechanisms are the most prominent systems among the EES technologies.8,9 Undoubtedly, lithium-ion batteries (LIBs) have been an enormous success in the realms of portable devices and electric vehicles (EVs)10–12 due to their high energy density, light weight, and low self-discharge rate. For these reasons, lithium-ion batteries are still receiving significant attention.13,14 However, LIBs continue to face challenges related to safety, energy density, longevity, and concerns around material availability (such as Li and Co metals). Particularly, battery safety is an increasingly vital concern in electric vehicle applications. These issues seriously limit the popularization of EVs and the development of grid-energy storage.15,16 As a solution, fluorinated organic electrolytes17 and solid-state electrolytes18–20 are being pursued as alternatives to flammable organic solvents. Another approach towards ultra-safe batteries is to develop battery chemistries that are compatible with aqueous electrolytes.21–23 The main obstacles for aqueous batteries include their narrow stable voltage window and evolution of hydrogen and oxygen gases that occurs upon the electrolysis of water.24–26 Thus, the need for ultra-safe, high-energy, and low-cost electrochemical energy storage devices has prompted a search for new energy-storage technologies.

Rechargeable Zn-based aqueous batteries have immense potential in large-scale energy storage systems due to their high capacity (820 A h kg−1 and 5854 A h L−1), cost effectiveness, and high chemical stability in air and aqueous solution.27–31 Without the necessity of flammable organic electrolyte, aqueous Zn-based batteries do not require the comparably complex subsystems required for lithium-based batteries including thermal management, sophisticated electronic controls, and structural protection to manage any catastrophic events.32,33 In contrast to an LIB graphite host anode, which undergoes intercalation and de-intercalation, the zinc anode undergoes dissolution/precipitation, complexation, and reduction/oxidation repetitive processes during the charge/discharge process in aqueous electrolytes. The overall reactions on the zinc anode are shown in eqn (1)–(3):

 
ZnO(s) + H2O(l) + 2OH(aq) ⇌ Zn(OH)42−(aq)(1)
 
Zn(OH)42−(aq) + 2e ⇌ Zn(s) + 4OH(aq)(2)

Overall:

 
ZnO(s) + H2O(l) + 2e ⇌ Zn(s) + 2OH(aq)(3)

As a result of this dissolution/precipitation cycle, the long-standing constraint that has prevented the implementation of Zn in next-generation batteries for large-scale application is its poor rechargeability due to dendrite growth, shape change, and passivation.34 Zn dendrites are formed during the charging process (i.e., electrodeposition of Zn metal) when Zn(OH)42− and/or Zn2+ ions are deposited unevenly, with faster growth occurring along energetically favorable crystallographic directions, resulting in internal short circuit. Furthermore, incomplete reduction of zincate ions coupled with non-uniform redistribution of Zn electrode material during the charging process leads to densification of the electrode at specific regions over many charge/discharge cycles, causing loss of usable capacity. Aside from dendrite formation and shape change of the Zn electrode, the passivation layer on the bulk zinc anode shortens the cycle life because active Zn is transformed into relatively insulating ZnO, which increases the internal resistance of the Zn electrode. This passivation inhibits the discharge process as the insulating ZnO film on the Zn surface blocks the migration of the discharge products and/or hydroxide ions, causing significant loss of energy efficiency for the charge/discharge cycles. In the past decades, the passivation mechanism of Zn anode in alkali electrolytes has been widely investigated.35–37 However, effective methods for resolving this problem have yet to be proposed.

Recently, attempts have been made to mitigate dendrite formation and shape change of the Zn electrode by altering the Zn electrode design.38 A 3D-zinc sponge anode was prepared to improve the rechargeability of Zn-based batteries.39,40 Although the performance of the zinc battery improved dramatically with this design, some problems still persist in the proposed Zn electrode: (1) passivation is still present in the 3D-zinc anodes, especially with high DOD; (2) the larger electrode–electrolyte contact area accelerates the dissolution of zinc, leading to shape change and capacity fading; and (3) the volume capacity decreases because of the porosity of the zinc sponge and the low depth-of-discharge. In another investigation, Zn anodes with a carbon coating were utilized to improve anti-corrosion performance.41–43 However, most of these studies could not overcome the dissolution and passivation problems simultaneously. In these studies, although nanoscale, carbon-coated zinc oxide particles were used as anode materials in rechargeable zinc cells, there is considerable room for improvement to mitigate the dissolution problem. An anode composed of micron-sized ZnO spheres was synthesized by a complicated co-precipitation process or ball milling approach, which increased the tap density of the electrode, but the passivation problem still needs to be resolved.44 Also, some zinc battery systems using mild electrolytes, such as ZnSO4–MSO4 (M = Mn, Co),45,46 Zn(CF3SO3)2–Mn(CF3SO3),47 and Zn(TFSI)2–LiTFSI,48 in which expensive TFSI salts should be replaced with salts having lower costs, were developed to mitigate the zinc dendritic growth effectively. Nevertheless, the reversibility of zinc anode in alkaline electrolyte is a great concern to exploit some highly rechargeable Zn-air batteries with high specific energy density (5200 W h kg−1).49

In terms of battery testing protocols, most previous results of Zn anode performance were obtained using beaker cells rather than closed cells (cylindrical cells or coin cells).38,41,42,50 In these beaker cells, abundant electrolyte significantly decreased the overall specific capacity of batteries. Also, since electrolyte saturated with ZnO was used in these beaker cells, it is difficult to ascribe the contribution of active material and calculate the performance of batteries due to the inevitable reduction of zincate from outsourcing of ZnO in the electrolyte. The performance of these cells cannot reflect real conditions in practical commercial batteries in which the reasonable electrolyte content is a pivotal factor for high volumetric and gravimetric capacity.

Results and discussion

To tackle the long-standing challenges of a completely rechargeable Zn anode in a limited quantity of electrolyte, we designed a ZnO pomegranate (Zn-pome) material in which the zinc oxide nanoparticles (ZnO NPs) are analogous to seeds that are individually encapsulated and held in clusters by a carbon shell diaphragm. The carbon shell coating on the nanoparticles was chosen for its porosity, stability in aqueous alkaline media, and electroconductivity. Fig. 1A shows the schematic of zincate motion during battery cycling of ZnO NPs (Fig. 1A), ZnO@C NPs (Fig. 1B) and Zn-pome (Fig. 1C). There are several distinctive advantages of the Zn-pome electrode. First, multi-layered carbon acts as a conductor, protector, and ion barrier in Zn-pome to adequately constrain the migration of Zn(OH)42− (the discharge product), thus mitigating the dendrite formation and shape change of the Zn electrode. Meanwhile, species with a smaller diameter (e.g., OH and H2O) than zincate can permeate through the carbon shell. Second, the use of nanoscale (<100 nm) primary ZnO particles avoids passivation. Once ZnO reaches a critical passivation thickness, it can no longer fully convert to Zn. The thickness of the passivation layer on the zinc foil in coin cells after complete discharging was determined by scanning electron microscopy (SEM). As shown in Fig. S1 (ESI), the thickness of the passivation layer is ca. 2 μm, which indicates that the Zn anode cannot be consumed entirely in the discharge process if the size of the Zn material is larger than ca. 2 μm regardless of the shape (foil, rod, particle, etc.). Thus, controlling the size of Zn material to nanoscale is a practical approach to fully utilize the Zn material in the charge/discharge process. Therefore, the nanoscale ZnO primary material is chosen to build Zn-pome. Besides, the robust carbon shell on ZnO NPs is both electrically and ionically conducting, which not only allows for effective kinetics, but also improves the mechanical strength of the Zn anode. Third, the Zn-pome has a smaller solid-electrolyte contact area than ZnO@C NPs (as shown in Fig. 1D), which can significantly reduce the dissolution rate during cycling. Hence, the long-standing limitations that have impeded the rechargeability of Zn electrode (i.e., Zn dendrite formation, Zn electrode shape change and ZnO passivation) can be significantly alleviated by the electrode design of Zn-pome.
image file: c8ta07809b-f1.tif
Fig. 1 Cross section schematic of the zinc pomegranate design. (A) ZnO NPs with fast dissolution rate in alkaline aqueous solution; (B) ZnO NPs coated with carbon; (C) Zn-pomegranate in which carbon filled into the free space of ZnO clusters plays a crucial role in ion sieving, conductivity, and structure stabilization of the electrode; (D) calculated surface area in contact with electrolyte and the number of primary nanoparticles in one Zn pomegranate cluster versus its diameter. The smaller the surface contact with the electrolyte, the lower the capacity fading.

The synthesis of Zn-pome is schematically illustrated in Fig. S2 (ESI). Micro-emulsion procedures have been used to synthesize other electrode materials with hierarchical particulate structures for applications in LIB51 and Li–S batteries.52 In this study, ZnO NPs were first dispersed in distilled water by ultrasonication and then mixed with 1-octadecene solution containing an emulsion stabilizer. After removing water and organics, ZnO NPs self-assembled to form close-packed ZnO clusters and then, they were condensed by calcination (Fig. S2A, ESI). The obtained clusters were subsequently coated with a thin layer of dopamine, which was further carbonized under an argon atmosphere to form Zn-pome.

The SEM images of ZnO clusters were investigated under various magnifications (Fig. 2A–C). These clusters with diameters in the range of 1–6 μm primarily contained ZnO NPs. The rounded edge of the Zn-pome (Fig. 2D–F) indicated that the ZnO clusters were adequately coated with a thin layer of carbon framework. The detailed structure of the Zn-pome was investigated using high resolution transmission electron microscopy (HRTEM) and focused ion beam milling image (FIB) analysis. According to TEM images (Fig. 2G and S3A, ESI), the diameter of a typical Zn-pome microparticle was ca. 6 μm. When Zn-pome was treated with 1 M HCl to etch away ZnO, the hollow carbon framework could be clearly observed in TEM images (Fig. 2H and S3C–F, ESI). This indicated that each ZnO NP was individually coated by a thin layer of carbon framework with thickness of ca. 10–15 nm. Moreover, the coated carbon framework was stable even without the solid “seeds”, i.e., ZnO NPs, which is crucial for the structural stability of Zn-pome, especially when the active Zn material is mainly oxidized and dissolved after the deep discharge process. Fig. 2I exhibits the cross-sectional images of the Zn-pome microparticle obtained by focused ion beam milling image (FIB) analysis. A secondary Zn-pome microparticle consists of ZnO NP clusters, in which each ZnO NP with rounded surface was uniformly encapsulated by the carbon framework. More cross-sectional images are illustrated in Fig. S4 (ESI). The abovementioned morphology investigations of Zn-pome reveal that Zn-pome fabricated by bottom-up approach, consisting of robust carbon framework and ZnO nanoparticles, can be used as an anode in zinc-based batteries.


image file: c8ta07809b-f2.tif
Fig. 2 (A–C) SEM images of clusters of ZnO nanoparticles assembled via a microemulsion approach. (D–F) SEM images of Zn-pome (nanoporous carbon-coated ZnO cluster). (G) TEM image of Zn-pome. (H) TEM image of the carbon framework of Zn-pome after etching away ZnO in 1 M HCl for 24 hours. (I) Cross-sectional SEM image of one Zn-pome microparticle obtained by focused ion beam (FIB) analysis.

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the crystal structure and chemical composition of Zn-pome (Fig. 3A–C). The XRD pattern of Zn-pome is similar to that of ZnO NPs and ZnO@C NPs. No characteristic peak of carbon was detected, indicating an amorphous coating of carbon on ZnO NPs. The reduced intensity of ZnO for Zn-pome suggests that ZnO NPs are uniformly covered by the carbon framework; therefore, it shields the diffractive signals of ZnO slightly. Similarly, strong C 1s signal and relatively weak Zn 2p and O 1s signals in the XPS survey spectrum can be observed for Zn-pome in comparison with that for ZnO NPs. Accordingly, Zn-pome is characterized as ZnO NP clusters uniformly coated with an amorphous carbon layer. The content of carbon is found to be about 40% in Zn-pome based on thermogravimetric analysis (TGA) in air, as shown in Fig. 3D. The Brunauer–Emmett–Teller (BET) results reveal that the average pore size of the carbon shell is ca. 10 Å (Fig. 3E). This indicates that the carbon framework can properly mitigate the permeation of zincate through the shell structure.


image file: c8ta07809b-f3.tif
Fig. 3 (A) XRD patterns and (B) XPS spectra of ZnO NPs, ZnO@C NPs and Zn-pome. (C) High-resolution XPS spectra of ZnO NPs and Zn-pome. (D) TGA weight loss curve and (E) BET pore size distribution of Zn-pome. (F) Dissolved and undissolved portions of zinc in 4 M KOH electrolyte for ZnO NPs, ZnO@C NPs and Zn-pome; embedded pictures show the electron microscopy images of ZnO NPs, ZnO@C NPs and Zn-pome.

To verify the ion-sieving ability of the carbon coating, we investigated the dissolution rate of ZnO in an aqueous alkaline electrolyte. Samples of ZnO NPs, ZnO@C NPs and Zn-pome containing equal amounts of zinc were immersed in 1 mL 4 M KOH solutions at the same time. After a certain amount of time, the concentrations of Zn species dissolved in the solutions were analyzed by inductively coupled plasma (ICP). As shown in Fig. 3F, Zn-pome significantly reduced the portion of dissolved Zn in KOH (1.05%) in comparison with ZnO NPs (30.8%) and ZnO@C NPs (11%). This effect is ascribed to the synergistic function of carbon shell and secondary structure in Zn-pome. The diffusion of zincate in alkaline media is confined within the secondary particles, whereas the confined zincate can still be electrochemically reduced. The pomegranate structure is also expected to alleviate Zn dendrite formation and shape change (i.e., localized densification) during the charge/discharge process. Thus, the long-standing constraints for rechargeability of Zn electrodes can be effectively overcome.

The electrochemical performances of Zn-pome and ZnO NPs were evaluated by a full cell configuration consisting of Ni(OH)2 cathode with excess capacity obtained from commercial zinc-nickel batteries. The cells were cycled at 1C in a voltage window between 1.5 and 2.0 V in 2 M KF, 2 M K2CO3 and 4 M KOH.53 It should be noted that the battery testing protocols used in this study were extremely harsh in three aspects: (1) limited electrolyte: we used 2032 coin cells with a limited amount of electrolyte (Fig. S5, ESI) rather than beaker cells with excess electrolyte because coin cells better represent real operating conditions. (2) ZnO-free electrolyte: ZnO-saturated KOH electrolyte is commonly used to provide higher specific capacity and longer cycle life, but zinc species initially present in the electrolyte probably contribute to the capacity of the cell and conceal the actual performance of active zinc material on the electrode. (3) 100% depth of discharge (DOD): under 100% DOD, the full energy density can be delivered. However, <50% DOD is used for Zn anodes because of the passivation problem.

Under such harsh testing conditions, the cells containing Zn-pome anode (Zn-pome/Ni(OH)2) exhibited remarkable capacity and cycle life, which were superior to those of ZnO NP and ZnO NPs@C anodes with Ni(OH)2 cathode, respectively. As shown in Fig. 4A and S6 (ESI), although the specific capacity of Zn NPs/Ni(OH)2 was higher than that of Zn-pome in the first few cycles, the discharge capacity of Zn NPs/Ni(OH)2 decreased sharply over 20 cycles due to the fading of the anode resulting from the high dissolution rate of ZnO in strong aqueous alkali electrolyte. The problem of abrupt capacity decay of the control sample after 25 cycles can best be ascribed to high dissolution of zinc, which subsequently results in the formation of dendrites and substantial electrode shape change after several cycles from repeated redistribution of the active material. Other issues stem from the hydrogen evolution reaction (HER) on the surface of zinc, which not only worsens efficient utilization of zinc but also leads to swelling of the cell, causing the cell to crack and the electrolyte to dry out. The loose contact in the cells inflated by hydrogen further causes abrupt capacity fading. In contrast, the capacity of Zn-pome/Ni(OH)2 is stable for 50 cycles and then gradually decreases, showing better cyclability than that of Zn NPs/Ni(OH)2. This improvement can be ascribed to the ion blocking ability of the carbon shell in the Zn-pome anode and the smaller solid-electrolyte contact area. Fig. 4B presents the typical charge/discharge profiles of the Zn-pome/Ni(OH)2 battery in the 1st, 10th, 20th, 30th and 40th cycles. The average discharge voltage of the Zn-pome/Ni(OH)2 cell is maintained at 1.80 V after 40 cycles, indicating excellent cycling stability.


image file: c8ta07809b-f4.tif
Fig. 4 (A) Specific capacity of ZnO NPs and Zn-pome. (B) Voltage profiles of Zn-pome/Ni(OH)2. (C) Specific capacity of ZnO NPs/Ni(OH)2 and Zn-pome/Ni(OH)2 at 5C discharge rate. (D) Testing of self-discharge of Zn-pome cell cycling at 0.5C for one cycle, resting for 24 h, then cycling at 1C. (E) SEM image of Zn-pome anode before cycling (F and G) and after two cycles.

The improved performance of Zn-pome/Ni(OH)2 cells compared to that of Zn NPs/Ni(OH)2 is due to the ion-sieving ability of the carbon shell and secondary particle structure. The increase in charging voltage in consecutive cycling is possibly due to the accumulation of hydrogen evolved in the reduction of water. Although managing gas generation in sealed cells remains a concern, hydrogen evolution can be effectively suppressed by the adjustments of electrolyte (such as the use of water-in-salt48 or solid state additives54–56). This study mainly focuses on the structure design of Zn anode.

The electrochemical performance of Zn-pome/Ni(OH)2 is also superior to that of Zn NPs/Ni(OH)2 at a higher discharge rate (5C), as shown in Fig. 4C and S7 (ESI). The discharge capacity of Zn-pome/Ni(OH)2 is maintained around 400 mA h g−1 for 45 cycles (e.g., 411 mA h g−1 at the 44th cycle). However, Zn NPs/Ni(OH)2 suffers from a quick decay of discharge capacity after the 3rd cycle (merely 186 mA h g−1 at the 21st cycle). Accordingly, the superior performances (both specific capacity and cyclability) of Zn-pome in comparison with that of ZnO NPs clearly demonstrate the merits of the nano-design of pomegranate-structure ZnO.

To further investigate dissolution-resistivity, coin cells were used for one cycle at 0.5C and then rested for 24 hours before resuming cycling at 1C. During the 24 h resting period, Zn anodes were in the discharged state, and ZnO, the dominant species, rapidly dissolved in the electrolyte if left unprotected (Fig. 3F). As shown in Fig. 4D and S8 (ESI), Zn NPs/Ni(OH)2 exhibited fast capacity fading. On the other hand, Zn-pome/Ni(OH)2 still exhibited high capacity after resting and maintained 84% of capacity even after 40 cycles (on the basis of the 3rd cycle), indicating that Zn-pome anode is effective in retaining Zn active species due to the carbon framework. The morphology evolution of Zn-pome was investigated by SEM (Fig. 4E–G, and S9 (ESI)). The Zn-pome anode maintained the microspheric morphology after ten charge/discharge cycles, indicating the robust morphology of the pomegranate structure. Therefore, Zn-pome is considered to be a novel Zn anode material that can mitigate the Zn dendrite formation, shape change and passivation issues in alkaline Zn-ion batteries.

Summary and conclusions

In summary, we have designed and synthesized a nanoscale pomegranate-inspired hierarchical Zn anode material (Zn-pome) via a bottom-up microemulsion approach. Each Zn-pome microsphere is around 6 μm in size and is composed of ∼105 ZnO nanoparticles individually encapsulated by an amorphous, microporous, and conductive carbon shell that slows down the dissolution of zincate intermediate species during cycling. The secondary structure further suppresses the zinc dissolution by decreasing the electrode–electrolyte contact area. ICP analysis confirms that Zn-pome exhibits significantly suppressed dissolution of zinc compared to ZnO NP nanoparticles and ZnO@C nanoparticles. Because of this design, the Zn-pome anode demonstrates remarkable capacity and cycle stability under extremely harsh testing conditions (limited electrolyte, ZnO-free electrolyte, and 100% DOD); it also retains high capacity after long-term resting in a discharged state, in which ZnO in the electrode has a massive tendency to dissolve. The success of the Zn-pome anode can be ascribed to a few design principles that manage soluble intermediates during repeated electrochemical cycling; this is essential for the future design of Zn aqueous anodes as well as other battery systems involving soluble intermediates (e.g., lithium–sulfur batteries).

Experiment section

Detailed experimental methods can be found in the ESI.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was financially supported by faculty startup fund from Georgia Institute of Technology. Material characterization was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). P. C. gratefully acknowledge the financial support from China Scholarship Council (201706080048) and T.-H. W. thanks the support from Ministry of Science and Technology (107-2917-I-564-040).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta07809b
The authors contributed equally to this work.

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