Ion‐Sieving Carbon Nanoshells for Deeply Rechargeable Zn‐Based …liu.chbe.gatech.edu/paperPDF/2018_AEM_Wu.pdf · 2018-10-31 · Sireesha Aluri, and Nian Liu* DOI: 10.1002/aenm.201802470

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Ion-Sieving Carbon Nanoshells for Deeply Rechargeable Zn-Based Aqueous Batteries

Yutong Wu, Yamin Zhang, Yao Ma, Joshua D. Howe, Haochen Yang, Peng Chen, Sireesha Aluri, and Nian Liu*

DOI: 10.1002/aenm.201802470

between safety and performance has been a challenge due to the use of intrinsically flammable organic electrolyte materials. In addition to the high level of recent interest in all-solid-state LIBs,[5–8] an alter-native is to develop battery technologies that use aqueous electrolytes which are intrinsically safe.[9–11]

Among candidate anode materials for aqueous batteries, Zn is the most active metal that is stable in water and also has one of the highest specific capacities. As an anode Zn has roughly three times the volumetric capacity (5854 mAh cm−3) com-pared to Li (2062 mAh cm−3).[12,13] When paired with an oxygen cathode, the theoret-ical volumetric energy density of a Zn-air battery (4400 Wh L−1) approaches that of a Li-S battery (5200 Wh L−1). Additional advantages of the Zn-air cell compared to the Li-S cell are that Zn is much more economical than Li[14–16] and the battery is safer due to absence of flammable organic liquid, making Zn-based batteries attrac-tive candidates for electric vehicles and large-scale energy storage. There has been recent progress on rechargeable Zn anode materials in neutral or mildly acidic con-

ditions that eliminate concerns of ZnO passivating the Zn surface.[17–19] In order for Zn-based aqueous batteries to have higher specific energy than state-of-the-art LIBs, however, an oxygen cathode must be used,[16] which favors alkaline elec-trolytes (e.g., KOH) to facilitate the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Although developing efficient ORR and OER electrocatalysts could lower the polarization and improve the round trip energy efficiency of Zn-air batteries, their reversibility is mainly limited by the Zn anode, which has received far less attention.[20–25]

A deeply rechargeable zinc anode in lean alkaline electro-lyte (a cell utilizing the minimum amount of electrolyte) is a critical step toward zinc air battery that has not been achieved yet. A few attempts have been made before.[26–28] However, to the best of our knowledge, all of the electrochemical data in past reports were obtained in beaker cells (Figure 1) with ZnO saturated electrolyte or a low depth of discharge (DOD), which raises several problems: (1) the amount of electrolyte exceeds the amount of electrode materials by ≈1000 times, which lowers the overall energy density and covers the problem of

As an alternative to lithium-ion batteries, Zn-based aqueous batteries feature nonflammable electrolytes, high theoretical energy density, and abundant materials. However, a deeply rechargeable Zn anode in lean electrolyte configuration is still lacking. Different from the solid-to-solid reaction mechanism in lithium-ion batteries, Zn anodes in alkaline electrolytes go through a solid-solute-solid mechanism (Zn-Zn(OH)4

2−-ZnO), which introduces two problems. First, discharge product ZnO on the surface prevents further reaction of Zn underneath, which leads to low utilization of active material and poor rechargeability. Second, soluble intermediates change Zn anode morphology over cycling. In this work, an ion-sieving carbon nanoshell coated ZnO nanoparticle anode is reported, synthesized in a scalable way with controllable shell thickness, to solve the problems of passivation and dissolution simultaneously. The nanosized ZnO prevents passivation, while microporous carbon shell slows down Zn species dissolution. Under extremely harsh testing conditions (closed cell, lean electrolyte, no ZnO saturation), this Zn anode shows significantly improved performance compared to Zn foil and bare ZnO nanoparticles. The deeply rechargeable Zn anode reported is an important step toward practical high-energy rechargeable aqueous batteries (e.g., Zn-air batteries). And the ion-sieving nanoshell concept demonstrated is potentially beneficial to other electrodes such as sulfur cathode for Li-S batteries.

Y. Wu, Y. Zhang, Y. Ma, Prof. J. D. Howe, H. Yang, P. Chen, S. Aluri, Prof. N. LiuSchool of Chemical and Biomolecular EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332, USAE-mail: [email protected]. J. D. HoweDepartment of Chemical EngineeringTexas Tech UniversityLubbock, TX 79409, USA

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201802470.

Zinc Aqueous Batteries

Energy storage technologies have the potential to change the energy infrastructure from relying heavily on fossil fuels to mostly using temporally intermittent renewable energy sources.[1] Lithium-ion batteries (LIBs), for instance, have been extremely successful in portable electronic devices.[2–4] In scaling up intermittent energy sources to be broadly utilized to power devices such as electric vehicles, the delicate balance

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electrolyte side reactions; (2) the open cell configuration covers the problem of gas evolution and cell swelling; (3) the electrolyte is usually saturated with ZnO to extend the cycle life. Yet, the capacity from the ZnO dissolved in the electrolyte is 250-folds of the active material used (assuming 10 mL of electrolyte and ZnO solubility of 0.256 mol L−1 measured by inductively cou-pled plasma), and the mass of dissolved ZnO is not counted when calculating the specific capacity. The true performance of the active material was not evaluated; (4) the utilization of Zn is usually low (<50%), which extends the cycle life, but lowers the overall energy density.

Testing under extremely harsh conditions (lean electrolyte, deep cycling, no ZnO in electrolyte) is not only necessary to evaluate the practicality of zinc anodes, but also places high demand on their performance. We test our battery material in coin cell which only uses 100 µL of electrolyte without any ZnO dissolved in the electrolyte from which the active material per-formance can be directly reflected in battery cycling. Also, we test to check if the active material can survive the harsh condi-tion in the coin cell with problems such as electrolyte drying, gas evolution induced pressure build up, and loss of material contact. The testing condition is of 100% DOD and at 1C. A comparison of testing conditions between this work and most of the past reports is summarized in Table 1.

The typical electrochemical reactions of Zn anode in an alka-line electrolyte are shown in Equations (1)–(3). Rather than a solid-to-solid direct transition, Zn metal first electrochemi-cally oxidizes to a soluble zincate ion, then precipitates to solid ZnO[29]

Zn s 4OH aq Zn OH aq 2e4

2( ) ( )( ) ( )+ +− − − (1)

Zn OH aq ZnO s H O 2OH aq4

22( ) ( )( ) ( )+ +− − (2)

Overall : Zn s 2OH aq ZnO s H O 2e2( )( ) ( )+ + +− − (3)

Three major challenges currently exist for the rechargeable Zn anode for aqueous batteries as a result of the solid-solute-solid mechanism, the insulating nature of discharge product

(ZnO), and the water stability window: (1) ZnO (the soluble and insulating discharge product) passivates the surface of unre-acted Zn, which leads to low utilization of active material and a poor rechargeability (Figure 2A); (2) ZnO dissolution causes Zn deposition to happen in random locations, which leads to elec-trode morphology change and dendrite growth after continuous cycling. In a lean electrolyte configuration, the Zn dendrite can penetrate the separator to short-circuit the battery. (3) H2 evolu-tion on the Zn anode compromises Coulombic efficiency. Espe-cially in a sealed cell with a limited amount of electrolyte (rather than the often-used beaker cell with saturated ZnO), H2 evolu-tion dries out the electrolyte, enhances internal pressure of the battery, and gas bubbles block the ionic pathway, which leads to a low Coulombic efficiency (≈60%) and even sudden battery failure. The H2 evolution reaction is shown in Equation (4)

+ → +− −2H O 2e H 2OH2 2 (4)

Bulk foil is the most commonly used Zn anode in aqueous batteries. Figure 2B shows the voltage profile of a Zn foil (0.25 mm thick) anode cycled with an excess-capacity NiOOH cathode in a 2032 coin cell. The current density is 2.55 mA cm−2 and the capacity is limited to 2.55 mAh cm−2 with voltage cut-offs of 2 V in charging and 1.4 V in discharging. The capacity decays quickly over cycling and completely diminishes after only seven cycles, suggesting that an irreversible and insulating ZnO layer quickly builds up during cycling. Such a passivation layer blocks the materials underneath this layer from further reaction. To confirm the surface passivation and morphology change, scanning electron microscopy (SEM) images of Zn foil before and after the test are shown in Figure 2C,D, respectively. Initially, the Zn foil has a flat surface (Figure 2C), but sub-micrometer particles appeared on the surface (Figure 2D) after cycling. 3D electrodes with micropores on the order of 50 µm have demonstrated promising performance,[30,31] yet the fea-ture size is still above the critical passivation size of ≈2–3 µm (Figure S1, Supporting Information), so ZnO can build up and impair battery performance.

Starting with a nanomaterial with high-surface area could avoid the passivation layer problem. However, the dissolution


Figure 1. Comparison between the coin cell used in our work and the beaker cell used in most of the past reports. The electrolyte volume exceeds the amount of electrode materials by ≈1000 times, and the ZnO added in the electrolyte contributes to majority of the capacity and battery performance.

Table 1. Comparison of testing conditions between the coin cell used in our work and the beaker cell used in most of the past reports. The dissolved ZnO stores ≈250 times more capacity than the active material on the electrode. Also, the electrochemical data may be varied based on particular cycling conditions.

Harsh testing condition (this work)

Mild testing condition (most past reports)

Electrolyte volume 0.1 mL (limited OH−) ≈10 mL (infinite OH−)

ZnO in electrolyte None Saturated ZnO in electrolyte

(extra capacity source)

Mass of ZnO in

electrolyte

0 mg 208 mga)

Extra capacity from

electrolyte

0 mAh 137 mAh (≈250 times of

active material)

Depth of discharge 100% <50%

a)Assuming 10 mL electrolyte. Zincate solubility (0.256 mol L−1) determined by ICP.



problem is more severe. On the other hand, a nonporous coating could prevent ZnO dissolution but would also block the OH− transport necessary for the zinc redox reaction to occur. Therefore, it has been a challenge to simultaneously solve the dilemmas of passivation and dissolution.[32–35]

The concept of separating ions and molecules by size using selective membranes has been widely used in water purifica-tion, gas and fuel separation, and biomedical dialysis.[36,37] A variety of materials such as graphene, graphene oxide, polymer, and metal carbide membranes with a controllable pore size and permeability have been demonstrated to have ion-sieving capa-bilities, in various applications.[38–42] Applying the ion-sieving concept to Zn anode, to confine larger zincate ions and allow

smaller hydroxide ions to permeate, is expected to prevent ZnO dissolution and electrode shape change.

In this work, we propose an optimized structure to solve Zn anodes’ passivation and dissolution problems simultane-ously (Figure 2E). Specifically, the structure features (1) a sub-micrometer ZnO particle as the core, and (2) an ion-sieving carbon coating as the shell. First, unlike the bulk Zn foil which is several hundreds of micrometers thick, sub-micrometer particles will not have a passivation problem and will remain active in extended cycling. We choose to start from ≈100 nm ZnO nanoparticles (discharged state) rather than Zn nanopar-ticles (charged state) because on the synthesis and scalability aspect, compared to Zn, ZnO is much easier to make into


Figure 2. A) Schematic of passivation layer formation in the discharging process of a Zn metal anode. The passivation layer is not fully reversible upon charging, indicated by a thin layer of ZnO remaining after charging. B) Voltage profile of a 2032 coin cell with a Zn foil anode and a NiOOH cathode. The battery failed to function after seven cycles with 2.55 mAh cm−2 at 1C. C) SEM image of a Zn foil before cycling, showing a flat surface. D) SEM image of the Zn foil after cycling, sub-micrometer level spherical particles are present, indicating a morphology change of the material. E) Schematic of the ion-sieving nanoshell design for the Zn anode. The carbon nanoshell coated onto the ZnO nanoparticle allows hydroxide ions to freely pass through while slowing down zincate ion escape.



nanostructures which serves as the starting material for this study. On the battery performance aspect, starting from Zn to ZnO will rupture the carbon shell due to volume expansion. It is of note that nanoparticles with even smaller diameter offer no further benefit to reversibility but have much more severe dissolution concerns. We deposit the carbon shell on these ZnO nanoparticles through carbonization of a uniform polydo-pamine coating. The ability of polydopamine coatings to form a uniform shell with controllable thickness has been confirmed before.[43–45] The two-step synthesis method we report here is simple and scalable, and the carbon shell thickness is con-trollable by simply adjusting the dopamine mass during syn-thesis; details of the synthesis can be found in the Supporting Information. Second, the pore size of the carbon nanoshell is tailored to allow hydroxide ions to pass through while blocking transport of zincate ions. During charging, the zincate inter-mediate is trapped inside the carbon shell and reacts with Zn within the shell, preventing deposition of Zn in another loca-tion. In contrast, the OH− by-product can diffuse out freely through the micropores in the shell due to their smaller size. During discharging, the trapped Zn oxidizes to form ZnO with the participation of OH− coming from outside the shell.

The uniform polydopamine shell is first coated onto ZnO nanoparticles by stirring the particles with dopamine hydro-chloride in Tris buffer (pH 8.5) for 24 h at room temperature in the presence of air. After carbonization at 600 °C, ZnO@C nanoparticles are obtained. The SEM image of bare ZnO nano-particles is shown in Figure 3A; the particles are of short rod-like shapes. SEM image shows a quasi-spherical morphology of ZnO@C (Figure 3B and Figure S3, Supporting Information), which indicates the coating is successful, and the particles are of slightly larger size than bare ZnO. The transmission elec-tron microscopy (TEM) image in Figure 3C shows a single

ZnO@C nanoparticle with an amorphous carbon shell coated uniformly on the surface and the thickness of the carbon shell is 20–30 nm. Notice that the coating thickness is tunable by simply changing the dopamine hydrochloride mass. TEM images of single coated particles with different coating thick-ness are shown in Figure S4 (Supporting Information). We choose 600 °C as the carbonization temperature because the ZnO core is reduced to Zn vapor and escape at above 680 °C, determined from the thermogravimetric analysis (TGA) results (Figure S5, Supporting Information). TEM results confirm par-tial ZnO loss inside the carbon shell at 700 °C (Figure S6, Sup-porting Information), and complete loss at 800 °C (Figure S7, Supporting Information). This phenomenon is also confirmed by the fact that 100 mg ZnO@polydopamine becomes ≈20 mg after 800 °C carbonization, and ≈70 mg after 600 °C treat-ment, respectively. Complete and self-supporting hollow carbon nanoshells can be obtained after etching away ZnO using HCl (Figure 3D). The carbon mass fraction in the ZnO@C nanoparticles is determined to be 41% by TGA in ambient air (Figure 3E). The X-ray diffraction (XRD) patterns in Figure 3F for both the bare ZnO and ZnO@C have the same peak posi-tions, indicating the retention of ZnO hexagonal wurtzite crys-talline structure after coating. No signature of crystallinity is observed for the carbon shell.

To investigate the nature of the zincate and hydroxide anion species, density functional theory (DFT) calculations are performed to study the structure and size of each species. Details of the DFT calculations are provided in the Supporting Information.[46–49] The sizes of hydroxide ion and zincate ion, without solvation shells, are simulated to be 2.42 and 6.09 Å, respectively (Figure 4A). We note that these “rigid molecule” sizes computed here are substantially smaller than the sizes of these ions under thermal motion, but that our calculations


Figure 3. A) SEM image of commercial ZnO nanoparticles with short rod-like shape. B) SEM image of carbon-coated ZnO nanoparticles (ZnO@C). C) TEM image of a ZnO@C nanoparticle. The uniform carbon coating is ≈20 nm thick. D) TEM image of HCl etched ZnO@C particles, resulting in a uniform hollow carbon nanoshell. E) TGA results used to determine the carbon content in ZnO@C nanoparticles. F) XRD results for bare ZnO and ZnO@C nanoparticles.



support the much smaller size of the hydroxide anion relative to the zincate anion species. An effective ion-sieving nanoshell should be uniform and have a pore size between the sizes of hydroxide and zincate ions. X-ray photoelectron spectroscopy (XPS) results confirm the uniformity of our carbon coating. As shown in Figure 4B, bare ZnO has strong Zn and O sig-nals, while ZnO@C only has a C signal. A comparison between both samples’ high-resolution Zn spectra further confirms the complete coverage of carbon on ZnO in ZnO@C (Figure 4C). The Brunauer–Emmett–Teller (BET) method is used to ana-lyze the pore size of carbon-coated ZnO (Figure 4D). The pore width is calculated from the adsorption/desorption isotherm to be around 5–8 Å (Figure 4E), between the sizes of hydroxide and zincate ions plus the solvation shell. We therefore expect hydroxide species to be more mobile and zincate species to be less mobile when diffusing through the nanoshell. In com-parison, uncoated ZnO particles do not have pores in the same range. To directly verify the ability of the sample to prevent ZnO@C from dissolving into the alkaline electrolyte, both ZnO and ZnO@C powders are soaked in KOH (4 m) for 5 min, 1 d, and 10 d and the dissolved Zn(OH)4

2− are quantified using inductively coupled plasma atomic emission spectroscopy (ICP-AES). As shown in Figure 4F, although Zn still dissolves into the electrolyte, the dissolved Zn(OH)4

2− from bare ZnO is much lower and slower than that from ZnO@C, which con-firms that the carbon nanoshell functions as a barrier to slow down the zincate escape.

To evaluate the electrochemical performance of ZnO@C nanoparticle anodes, we pair them with commercial Ni(OH)2 counter electrodes with largely excess areal capacity.

2032 coin-type batteries are used to limit the amount of elec-trolyte and mimic practical application conditions. A Ni(OH)2 counter electrode is used rather than an air electrode to evaluate our anode because Ni(OH)2 has simpler electrochemistry, fewer factors influencing its battery performance, and it is compatible with sealed coin cells. The cells are galvanostatically charged to the theoretical capacity (658.5 mAh g−1 ZnO) and fully dis-charged to 1.5 V at 1C rate and 100% DOD. An upper voltage cutoff of 2 V is set to avoid electrolyte decomposition. Figure 5A compares the specific capacity of ZnO@C anode with a bare ZnO anode at similar mass loading. Performance of Zn foil, which degrades quickly in only seven cycles due to severe ZnO passivation is also shown in the same plot. The bare ZnO anode lasts for 20 cycles before severe capacity degradation is evident, indicating the success of decreasing feature size in mitigating passivation. Without nanoshell encapsulation, zincate is able to dissolve and diffuse prior to redeposition, and electrode mor-phology changes over cycling. The ZnO@C anode outlasts the bare ZnO anode with ≈1.6 times longer life in terms of cycles, we attribute the increased performance to reduction in the mobility of zincate.

Noticeably, bare ZnO quickly decays to half of the initial energy storage capacity, while ZnO@C has a significantly slower decay and longer cycle life. Another set of cycling data shown in Figure S9 (Supporting Information) demonstrates similar battery performance. This supports our hypothesis that the carbon nanoshell has suitable pore sizes to reduce the trans-port of zincate ions while allowing the hydroxide ions to pass freely. The carbon shell also increases the conductivity of the anode material, which is helpful for preventing the formation


Figure 4. A) Structure and size of hydroxide ion and zincate ion calculated by DFT. B) XPS spectra for bare ZnO and ZnO@C nanoparticles. C) XPS high-resolution Zn spectra for bare ZnO and ZnO@C nanoparticles. D) N2 adsorption/desorption isotherms of ZnO and ZnO@C nanoparticles. E) BET pore width distribution of ZnO and ZnO@C nanoparticles. F) ICP-AES quantification of ZnO dissolution in KOH electrolyte from bare ZnO and ZnO@C nanoparticles for 5 min, 1 d, and 10 d. Bare ZnO dissolves much faster than ZnO@C.



of a passivation layer, also with the help of the carbon nanoshell the overpotential for every single cycle of the bat-tery with ZnO@C is lower than that of bare ZnO (Figures S10 and S11, Supporting Information). The nitrogen doping also facilitates the conductivity of the carbon layer and charge transfer at the interface.[50] Figure S12 (Supporting Informa-tion) shows the voltage versus specific capacity during the charging and discharging processes. The performances of the charging processes are similar, indicating a stable performance. Figure 5B–E shows the SEM images of both bare and coated ZnO anodes before cycling and after three cycles. Noticeably, the surface of bare ZnO anode has holes after cycling, labeled with yellow arrows in Figure 5C, which is a result of ZnO

dissolution. In contrast, the ZnO@C anode maintained the electrode morphology, which confirms the ability of the carbon shell on ZnO@C to mitigate Zn anode dissolution and passiva-tion. It is also confirmed from the TEM image in Figure S13 (Supporting Information) that after charging the active material is still confined in the nanoshell.

To compare the battery performance under harsh testing conditions reported in this work to the performance under mild testing conditions in most of the past reports, we test a ZnO@C pouch cell using electrolyte saturated with ZnO (Figure S15, Supporting Information), and the battery reaches 100 cycles with >90% efficiency under 100% DOD at 1C. Another ZnO@C pouch cell using electrolyte saturated with ZnO shows a performance of ≈95% efficiency and 100% retention for 500 cycles (Figure S16, Supporting Information), but the bat-tery is cycled at 12C. The comparison is clear evidence that a deeply rechargeable anode in lean electrolyte configuration is in demand and necessary to reflect the true performance of battery active material in Zn-based aqueous batteries while the drastically enhanced battery performance in most of the past reports is attributed to the ZnO saturated in the electrolyte and low utilization of active material.

In summary, we have simultaneously solved the dissolu-tion and passivation problems of Zn anode materials by applying an ion-sieving carbon nanoshell coating onto ZnO nanoparticles which are well below the critical passivation thickness. The carbon nanoshell is uniform and complete. The micropores successfully slow down ZnO dissolution and limit zincate ion transport but allow hydroxide ions to pass freely, and the nanoshells’ rigidity prevents anode shape change and dendrite growth. The battery lifetime is greatly enhanced with the ZnO@C anode; the coated anode outper-forms bare ZnO and Zn foil with ≈1.6 and 6 times longer cycle life in a coin cell with harsh testing conditions, respec-tively. The synthesis is simple and scalable with a control-lable nanoshell thickness. This work is expected to provide guidance to the future design of Zn anodes for high-energy rechargeable aqueous batteries, and other high-energy battery electrodes.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsN.L. acknowledges support from faculty startup funds from the Georgia Institute of Technology, and thanks Sankar Nair, Pradeep Agrawal, and Ryan P. Lively for sharing lab facilities while the Liu lab was under construction. This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which was supported by the National Science Foundation (Grant No. ECCS-1542174). J.D.H. was supported by the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0012577.


Figure 5. A) Specific capacity and Coulombic efficiency of bare ZnO (1.03 mg), ZnO@C (0.94 mg), and bulk Zn foil anodes during discharge process, the inset shows a typical 2032 coin cell fabricated during the experiment. B) SEM image of bare ZnO anode before cycling. C) SEM image of bare ZnO anode after three cycles, showing a dramatic shape change. Pores are indicated by yellow arrows. D) SEM image of ZnO@C anode before cycling. E) SEM image of ZnO@C anode after three cycles, the nanoparticles maintained a spherical shape as before cycling.



Conflict of InterestThe authors declare no conflict of interest.

Keywordsanodes, aqueous batteries, core–shell, microporous carbon, zincate, ZnO nanoparticles

Received: August 8, 2018Revised: September 29, 2018

Published online:

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