Polyacrylonitrile-Nanofiber-Based Gel Polymer Electrolyte for Novel Aqueous Sodium-Ion Battery Based on a Na4mn9o18 Cathode

Article Polyacrylonitrile-Nanoﬁber-Based Gel Polymer Electrolyte for Novel Aqueous Sodium-Ion Battery Based on a Na4Mn9O18 Cathode and Zn Metal Anode

Yongguang Zhang 1, Zhumabay Bakenov 2 ID , Taizhe Tan 1,* and Jin Huang 1,* 1 School of Materials and Energy, Synergy Innovation Institute of GDUT, Guangdong University of Technology, Guangzhou 510006, China; [email protected] 2 Institute of Batteries LLC, National Laboratory Astana, School of Engineering, Nazarbayev University, 53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan; [email protected] * Correspondence: [email protected] (T.T.); [email protected] (J.H.)

Received: 7 June 2018; Accepted: 31 July 2018; Published: 2 August 2018

Abstract: A gel polymer electrolyte was formed by trapping an optimized Na+/Zn2+ mixed-ion aqueous electrolyte in a polyacrylonitrile nanoﬁber polymer matrix. This electrolyte was used in a novel aqueous sodium-ion battery (ASIB) system, which was assembled by using a zinc anode and Na4Mn9O18 cathode. The nanorod-like Na4Mn9O18 was synthesized by a hydrothermal soft chemical reaction. The structural and morphological measurement conﬁrmed that the highly crystalline Na4Mn9O18 nanorods are uniformly distributed. Electrochemical tests of Na4Mn9O18//Zn gel polymer battery demonstrated its high cycle stability along with a good rate of performance. The battery delivers an initial discharge capacity of 96 mAh g−1, and 64 mAh g−1 after 200 cycles at a high cycling rate of 1 C. Our results demonstrate that the Na4Mn9O18//Zn gel polymer battery is a promising and safe high-performance battery.

Keywords: aqueous sodium-ion battery; cathode; gel polymer electrolyte; Na4Mn9O18 nanorod; polyacrylonitrile nanoﬁber

1. Introduction The lithium-ion battery (LIB) is the most preferred technology for application in portable electronic devices [1,2]. However, facing the ever-increasing demand for energy storage devices, there is a huge stress on the lithium supply chain, resulting in a rather unsustainable increase in the raw material cost [3–5]. Additionally, the use of liquid organic electrolytes in the existing LIB systems usually brings forward safety concerns due to their high flammability and toxicity [6]. Polymer electrolytes with high ionic conductivity and energy density can overcome the disadvantages of the liquid electrolyte, therefore, they have received much attention due to their potential applications in electrochemical devices [7,8]. In 1975, Feuillade and Perche developed a polymer electrolyte by adding polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF) to increase the ionic conductivity at room temperature [9]. The PAN-based electrolytes have advantages over other polymer electrolytes due to their good mechanical properties and high ionic conductivity [10]. Therefore, gel polymer electrolytes have attracted considerable attention as an alternative to the liquid systems [11]. Along with the safety enhancement, gel polymer membranes serve both as conducting media and as separators, a combination which provides advantages of much simplified fabrication and modularity in design of batteries [12]. J. Prakash et al. found that the battery thermal runaway in LIBs occurs with increase of temperature during cycling corresponding to the SEI film breakdown and thermal decomposition of its components [13]. J. R. Dahn and E. W. Fuller showed by thermal gravimetric analysis that the positive electrodes (LiCoO2, LiNiO2 or LiMn2O4) of commercial LIBS are metastable and liberate oxygen when

Polymers 2018, 10, 853; doi:10.3390/polym10080853 www.mdpi.com/journal/polymers Polymers 2018, 10, 853 2 of 10 they are heated in air or in inert gas rising remarkable safety risks [14]. J. Prakash et al. expound contributions of reversible and irreversible heats to the overall heat generated during charge and discharge cycling by a continuum model [15]. To solve the thermal runaway problem, C. W. Lee et al. proposed the electrolyte containing a flame-retardant additive hexamethoxycyclotriphosphazene [NP(OCH3)2]3 to improve electrochemical performance of the cell [16]. Such approaches can also improve thermal stability and hence nonflammability of the electrolyte. However, the use of such additives and other techniques, for example, the use of ceramic separators (which are usually fragile and reduce the system’s conductivity), the flammability of the organic electrolytes cannot be completely avoided. Aqueous electrolyte could eliminate flammability issue, although there are some disadvantages of these electrolytes such as the lower operating potential which needs to be solved for enhancing the power density and resolve the tradeoff between safety and battery performance. These considerations lead to an exciting way to build a novel aqueous sodium-ion gel polymer electrolyte battery system (ASGB), minimizing flammable liquid leakage, while maintaining high ionic conductivity. In this work, we report on preparation of a polyacrylonitrile (PAN)-based gel polymer electrolyte via electrospinning at high voltage, which transformed the polymer solution into uniform and slender nanofibers [17]. A previous study in this field had reported a novel aqueous lithium-ion//zinc (Zn/LiMn2O4) rechargeable battery built without relying on the typical rocking-chair mechanism of LIB’s cathode and anode. The cathode reaction involves intercalation/de-intercalation of lithium-ion 2+ into/from the LiMn2O4, while the anode reaction consists of dissolution/deposition of Zn on the metallic Zn foil. This aqueous system exhibits an excellent cyclability and rate capability [18,19]. + 2+ Inspired by these results, we herein built a novel Zn/Na4Mn9O18 battery with an optimized Na /Zn mixed-ion electrolytes. In this battery system, a nanorod-like Na4Mn9O18 prepared by a hydrothermal soft chemical reaction is used as a cathode, demonstrating a long cycle span as well as a good rate capability.

2. Materials and Methods

2.1. Materials Preparation

Na4Mn9O18 was prepared by the hydrothermal soft chemical reaction (HSCR) with a high NaOH concentration. At first, 25 mL of aqueous solution containing 3.0 mol L−1 NaOH and 0.1 mol L−1 −1 KMnO4 was prepared. Then, an equal volume of 0.28 mol L MnSO4 aqueous solution was added to the previous solution with continuous stirring which quickly resulted in the formation of a dark brown precipitate. The precipitate was separated by filtration and washed by deionized water, and then dried in air at room temperature for 24 h to obtain the Na-birnessite precursors. To synthesize Na4Mn9O18, 3.0 g wet Na-birnessite precursor was added into 75 mL of a high concentration (15 mol L−1) NaOH solution and stirred for 20 min to obtain a brown suspension. Further, the suspension was poured into a 100 mL stainless steel autoclave with a Teflon liner (Zhongkaiya, Jiangsu, China), and heated at 180 ◦C for 18 h. Finally, the product was washed repeatedly with deionized water to eliminate the excessive NaOH followed by drying at 60 ◦C. The PAN-nanofiber membranes were prepared by the electrospinning method. PAN was vacuum dried at 60 ◦C for 24 h prior to use. Firstly, a certain amount of PAN (Sigma-Aldrich, St. Louis, −1 MO, USA, MW = 150,000 g mol ) was dissolved in the N,N-Dimethylformamide (DMF, ≥99%, Aladdin, Shanghai, China) to form a homogeneous 12.5 wt. % solution under stirring for 24 h at room temperature. Then the polymer solution was electrospun by loading into a 10 mL plastic syringe, having a flow rate of 4 mL h−1 under a high voltage of 20 kV at room temperature. The distance between the syringe tip (having a diameter of 0.51 mm) and the collector plate of aluminum was 21 cm. The electrospun membranes were finally obtained on the aluminum foils after drying in a vacuum oven at 80 ◦C for 12 h. Finally, the prepared PAN-nanofiber membranes were activated by immersing in an aqueous sodium electrolyte solution consisting of 1 M Na2SO4 and 0.5 M ZnSO4 in water with Polymers 2018, 10, 853 3 of 10 pH = 4 at room temperature for 12 h. The PAN-nanofiber-based gel polymer electrolyte was obtained after wiping the surface of the membranes from the excess of the liquid electrolyte.

2.2. Physical Characterization

The crystalline structure of Na4Mn9O18 nanorods was analyzed using X-ray diffraction (XRD, D8 Discover, Bruker, Karlsruhe, Germany, Cu-Ka, λ = 0.154 nm). Brunauer-Emmett-Teller (BET, V-Sorb 2800P, Gold APP Instrument Corporation China, Beijing, China) tests were performed to analyze the speciﬁc surface area and porosities of the samples. The morphologies and structure of the samples were observed using scanning electron microscopy (SEM, Hitachi S-4800, Hitachi Limited, Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM, JEOL2100, JEOL, Tokyo, Japan), respectively. Surface elemental analysis was done by the energy dispersive spectroscopy (EDS) module of the HR-TEM apparatus. Thermal runaway of the cells was investigated by means of differential scanning calorimetry (DSC Q20, TA Instruments, New Castle, DE, USA) with an open pan system under nitrogen purge at 30–40 mL min−1. Approximately 6 mg of PAN-based gel polymer electrolyte was placed inside the DSC pan for thermal analysis. The non-isothermal studies were performed at a scan heating rate of 10 ◦C min−1. Tensile strain of the samples was measured by dynamic mechanical analysis (DMA, Q800, TA Instruments, New Castle, DE, USA).

2.3. Electrochemical Characterization

The cathodes were made by casting the slurry of Na4Mn9O18, polyvinylidene ﬂuoride and acetylene black in N-methyl-2-pyrrolidinone on carbon foil following a weight ratio of 8:1:1, and air dried at 75 ◦C for 12 h. The carbon foil and zinc foil were cut into disc electrodes (15 mm) and used as the cathode and anode, respectively. The CR2025 coin-type batteries were assembled by placing the PAN-nanoﬁber-based gel polymer electrolyte between the anodes and cathodes. The galvanostatic charge and discharge tests were carried out using a multichannel battery tester (BTS-5V5mA, Neware, Shenzhen, China) between 1.0 and 1.85 V (vs. Zn/Zn2+). The VersaSTAT electrochemical workstation (Princeton, VersaSTAT 4, Ametek, PA, USA) was used for cyclic voltammetry (CV) measurements of the cells. CV was conducted at different scan rates ranging from 1 to 2 V (vs. Zn/Zn2+).

3. Results and Discussion

The electrochemical mechanism of Zn/Na4Mn9O18 battery operation is illustrated in Figure1. When battery is charged, sodium ions are removed from Na4Mn9O18 cathode and dissolved in the electrolyte accompanied by liberation of electrons. This is accompanied by the deposition of zinc from the electrolyte on the surface of zinc anode. During the discharge process, Zn is oxidized and dissolved into the electrolyte, while sodium ions are inserted into the cathode to reversibly form Na4Mn9O18 [3]. The electrochemical reactions in the battery are schematically can presented by the following Equations (1)–(3): The reaction at cathode:

+ − Na4Mn9O18 ⇔ Na4−xMn9O18 + xNa + xe (1)

The reaction at anode: Zn2+ + 2e− ⇔ Zn (2)

The total reaction:

2+ + 2Na4Mn9O18 + xZn ⇔ 2Na4−xMn9O18 + 2xNa + xZn (3)

The XRD patterns of Na4Mn9O18 synthesized by HSCR method are displayed in Figure2. The diffraction peaks are in good agreement with the JCPDS (The Joint Committee on Powder Polymers 2018, 10, x FOR PEER REVIEW 4 of 10

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Diffraction Standards) Card number 27-0750 [20]. No impurity peaks were observed. These results Polymersagree well 2018, with 10, x FOR the previousPEER REVIEW reports [21]. 4 of 10

Figure 1. Schematics of mechanism of Zn/Na4Mn9O18 aqueous battery.

The XRD patterns of Na4Mn9O18 synthesized by HSCR method are displayed in Figure 2. The diffraction peaks are in good agreement with the JCPDS (The Joint Committee on Powder Diffraction Standards) Card number 27-0750 [20]. No impurity peaks were observed. These results agree well with the previous reports [21]. FigureFigure 1.1. SchematicsSchematics ofof mechanismmechanism ofof Zn/NaZn/Na4MnMn99OO1818 aqueousaqueous battery. battery.

FigureFigure 2. XRDXRD pattern pattern of of Na Na44MnMn99OO1818. .

The morphology and structure of the prepared Na4Mn9O18 were confirmed by SEM and TEM, The morphology and structure of the prepared Na4Mn9O18 were conﬁrmed by SEM and TEM, as displayed in Figure 3. Figure 3a shows that the Na4Mn9O18 grew anisotropically into nanorod as displayed in Figure3. Figure3a shows that the Na 4Mn9O18 grew anisotropically into nanorod crystal structure. The TEM image of Na4Mn9O18 in Figure 3b shows a rod-shaped structure with a crystal structure. The TEM image of Na4Mn9O18 in Figure3b shows a rod-shaped structure with smooth surface. The HR-TEM image in Figure 3c acquired from the red box in Figure 3b shows the a smooth surface. The HR-TEMFigure image 2. in XRD Figure pattern3c acquired of Na4Mn from9O18. the red box in Figure3b shows vertical lattice fringe to be 0.45 nm, which corresponds to the (200) crystallographic plane of the vertical lattice fringe to be 0.45 nm, which corresponds to the (200) crystallographic plane of Na4Mn9O18. Interestingly, EDS spectrum of the Na4Mn9O18 nanorods (shown in Figure 3d), suggests Na4MnThe9O morphology18. Interestingly, and EDSstructure spectrum of the of prepared the Na4Mn Na9O4Mn18 nanorods9O18 were (shownconfirmed in Figure by SEM3d), and suggests TEM, a Na/Mn molar ratio of ~0.44. asa Na/Mndisplayed molar in Figure ratio of 3. ~0.44. Figure 3a shows that the Na4Mn9O18 grew anisotropically into nanorod crystalThe structure. XRD patterns The TEM of the image PAN-nanoﬁber of Na4Mn9 membranesO18 in Figure are 3b shown shows in a Figurerod-shaped4. A strong structure diffraction with a smoothpeak around surface. 17 ◦Thebelonging HR-TEM to image the (110) in Figure crystalline 3c acquired plane of from a hexagonal the red box structure in Figure could 3b shows be clearly the verticalseen. Furthermore, lattice fringe the to sample be 0.45 displays nm, which another co weakrresponds diffraction to the peak (200) around crystallographic 29◦, which isplane related of Na4Mn9O18. Interestingly, EDS spectrum of the Na4Mn9O18 nanorods (shown in Figure 3d), suggests a Na/Mn molar ratio of ~0.44.

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to the (200) plane. The two obvious diffraction peaks suggest a semi-crystalline structure of the PolymersPAN-nanoﬁber 2018, 10, x membranesFOR PEER REVIEW due to mixing of crystalline and amorphous phases [22,23]. 5 of 10

Figure 3. (a) SEM and (b) TEM image of Na4Mn9O18; (c) HR-TEM image and (d) EDS spectrum of

Na4Mn9O18.

The XRD patterns of the PAN-nanofiber membranes are shown in Figure 4. A strong diffraction peak around 17° belonging to the (110) crystalline plane of a hexagonal structure could be clearly seen. Furthermore, the sample displays another weak diffraction peak around 29°, which is related to the (200) plane. The two obvious diffraction peaks suggest a semi-crystalline structure of the

Figure 3. (a) SEM and (b) TEM image of Na4Mn9O18; (c) HR-TEM image and (d) EDS spectrum of PAN-nanofiberFigure 3. membranes(a) SEM and due (b) TEM to mixing image of of Na crystalline4Mn9O18;( cand) HR-TEM amorphous image andphases (d) EDS [22,23]. spectrum Na4Mn9O18. of Na4Mn9O18.

FigureFigure 4. 4.XRDXRD pattern pattern of of the PAN-nanoﬁberPAN-nanofiber membranes. membranes.

The TheSEM SEM images images of ofthe the PAN-nanofiber PAN-nanofiber membrane membraness prepared prepared by by electrospinning electrospinning are shownare shown in in FigureFigure 5a. 5Thea. The electrospun electrospun membranes membranes are composedcomposed of of a a three-dimensional three-dimensiona crosslinkingl crosslinking network network structurestructure with with extremely extremely regularregular nanofibers. nanofibers. The nanofibersThe nanofibers are smooth are and smooth uniform, and which uniform, positively which affects the mechanical properties (strength) of the membrane [24,25]. positively affects the mechanicalFigure 4. properti XRD patternes (strength) of the PAN-nanofiber of the membrane membranes. [24,25].

The SEM images of the PAN-nanofiber membranes prepared by electrospinning are shown in

Figure 5a. The electrospun membranes are composed of a three-dimensional crosslinking network structure with extremely regular nanofibers. The nanofibers are smooth and uniform, which positively affects the mechanical properties (strength) of the membrane [24,25].

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Figure 5. (a,b) SEM images of the PAN-nanofiber with different magnification. Figure 5. ((aa,,bb)) SEM SEM images images of of the the PAN-nanofiber PAN-nanofiber with different magnification.magnification. Figure 6 shows the DSC thermogram of PAN-based gel polymer electrolyte (before immersed in sodiumFigure electrolyte 66 shows shows solution) thethe DSCDSC thermograminthermogram the temperatur ofof PAN-basedPAN-basede range gelofgel 40–350 polymerpolymer °C. electrolyteelectrolyte From DSC, (before(before one immersedimmersed endothermic peakin could sodium be electrolyte observed solution) around in 117 thethe temperature°Ctemperatur (Tg) correspondinge range of 40–350 to ◦glass°C.C. From transition DSC, one temperature endothermicendothermic and ◦ anotherpeak endothermic couldcould be be observed observed peak around couldaround 117 be 117 C(observed °CTg) ( correspondingTg) correspondingaround 305 to glass °Cto transition(glassTm) correspondingtransition temperature temperature andto the another andmelting ◦ temperatureendothermicanother endothermicof the peak PAN-nanofi could peak be observedcouldber bemembranes. aroundobserved 305 around WhenC(Tm )305 correspondingthe °C prepared (Tm) corresponding toPAN-nanofiber the melting to temperaturethe membranesmelting wereoftemperature immersed the PAN-nanofiber inof anthe aqueousPAN-nanofi membranes. sodiumber When membranes. electrolyte the prepared When solution, PAN-nanofiber the preparedthe liquid membranesPAN-nanofiber uptake (%) were was membranes immersed determined were immersed in an aqueous sodium electrolyte solution, the liquid uptake (%) was determined usingin the an aqueousrelation sodium(W2 − W electrolyte1) × 100/W solution,1, where the W1 liquid and W uptake2 denote (%) the was weights determined of PAN using before the relation and after using the relation (W2 − W1) × 100/W1, where W1 and W2 denote the weights of PAN before and after absorbing(W2 − theW1) sodium× 100/W solution,1, where W respectively.1 and W2 denote The the result weights shows of PAN that before the PAN-nanofiber and after absorbing membrane the sodiumabsorbing solution, the sodium respectively. solution, The respectively. result shows The that result the PAN-nanofibershows that the membranePAN-nanofiber exhibited membrane a high exhibited a high ability to absorb liquid electrolyte exceeding 65 wt. %. abilityexhibited to absorba high liquidability electrolyteto absorb li exceedingquid electrolyte 65 wt. exceeding %. 65 wt. %.

Figure 6. DSC curve of PAN-based gel polymer electrolyte. FigureFigure 6. 6.DSCDSC curve curve of of PAN-based PAN-based gelgel polymer polymer electrolyte. electrolyte. One can see from Figure 7 that ultimate failure of the PAN-based gel polymer electrolyte does notOne occur Onecan canseeeven seefrom when from Figure it Figure is extremely 77 that ultimateultimate stretched failure failur up to ofe 6%. theof theIn PAN-based the PAN-based same geltime, polymer gelthe polymertensile electrolyte modulus electrolyte does of notthe does not occuroccurmesh even network whenwhen is itonly it is is extremely 24.5extremely MPa stretched within stretched 3.8% up to upstrain, 6%. to In6%. suggesting the In same the time, samea high the time, elasticity tensile the modulus tensile of the system.modulus of the mesh It isof the meshnetworkgenerally network is accepted onlyis only 24.5 that24.5 MPa bothMPa within low within 3.8% modulus strain, 3.8% and suggestingstrain, larg suggestinge elongation a high elasticity a of high the of elasticityelectrolytes the system. of are Itthe is extremely generallysystem. It is acceptedimportant that for batteries. both low modulus and large elongation of the electrolytes are extremely important generally accepted that both low modulus and large elongation of the electrolytes are extremely for batteries. important for batteries. Cyclic voltammograms (CV) of Na4Mn9O18 and Zn electrodes are shown in Figure8a. The upper cut-off potential was limited to 2.0 V to prevent decomposition of water [26]. The Na4Mn9O18 electrode shows redox peaks at ~1.35 V and ~1.59 V vs. Zn/Zn2+ in its discharge and charge curves, respectively, conforming to the de-insertion/insertion of Na+ ions in the aqueous electrolyte.

Polymers 2018, 10, 853 7 of 10

PolymersThe Na20184Mn, 109, Ox FOR18 electrode PEER REVIEW possesses an excellent electrochemical reversibility which is reﬂected by its7 of 10 capability to withstand the electric current and the redox peak potentials in the same position upon the consequent cycles.

FigureFigure 7. 7. Stress-strainStress-strain curvecurve of of the the PAN-based PAN-based gel gel polymer polymer electrolyte. electrolyte.

CyclicFigure voltammograms8b presents the cyclability(CV) of Na data4Mn for9O18 the and cell, Zn which electrodes shows are a lowshown initial in dischargeFigure 8a. The −1 uppercapacity cut-off of 96potential mAh g was, and limited a low to initial 2.0 V coulombic to prevent efficiency decomposition around 90%. of water Such [26]. low coulombicThe Na4Mn9O18 electrodeefficiency shows could redox be due peaks to some at ~1.35 irreversible V and ~1.59 side reactionsV vs. Zn/Zn in the2+ in first its chargingdischarge process. and charge A cell curves, with the PAN-nanofiber-based gel polymer electrolyte shows an excellent cycling performance respectively, conforming to the de-insertion/insertion of Na+ ions in the aqueous electrolyte. The (red colored curves). Figure8c presents the data on an excellent rate performance of a cell with the Na4Mn9O18 electrode possesses an excellent electrochemical reversibility which is reflected by its PAN-nanofiber-based gel polymer electrolyte. At a low rate of 1 C, the electrode provides a reversible capabilitydischarge to capacitywithstand of ~88the mAhelectric g− 1current. At the and higher the cycling redox ratespeak ofpotentials 2 C, 3 C andin the 4 C, same the discharge position upon thecapacities consequent of ~65,cycles. 52 and 44 mAh g−1 were sustained, respectively. Furthermore, the electrode recoversFigure most8b presents of its initial the cyclability capacity of data 84 mAh for gthe−1 whencell, which the cycling shows rate a low was initial modulated discharge back capacity to −1 of 961 C, mAh demonstrating g , and a an low excellent initial high coulombic current abuseefficiency tolerance around of our 90%. Na4 MnSuch9O 18lowcathode. coulombic To further efficiency coulddemonstrate be due to the some rate capability irreversible of the side system, reactions the charge/discharge in the first charging profiles atprocess. various A C ratescell with are the PAN-nanofiber-basedshown in Figure8d. Angel increase polymer in electrolyte current densities shows from an 1excellent C to 4 C causecycling a decreaseperformance of discharge (red colored −1 −1 curves).capacity Figure from 888c mAh presents g to 44the mAh data g .on The electrodean excellent exhibits rate a good performance cycle stability, of maintaininga cell with a the −1 PAN-nanofiber-basedspecific capacity of 33 mAhgel polymer g after 500electrolyte. cycles at 4At C asa shownlow rate in Figure of 18 e.C,The the electrode electrodemaintains provides a 60% of its initial capacity after 500 cycles. reversible discharge capacity of ~88 mAh g−1. At the higher cycling rates of 2 C, 3 C and 4 C, the Table1 presents the comparative performance data from the literature on the relevant/similar discharge capacities of ~65, 52 and 44 mAh g−1 were sustained, respectively. Furthermore, the materials, and our PAN-nanofiber-based gel polymer electrolyte for novel aqueous sodium-ion battery electrode recovers most of its initial capacity of 84 mAh g−1 when the cycling rate was modulated exhibits a superior electrochemical performance compared with other reported systems [27–29]. back to 1 C, demonstrating an excellent high current abuse tolerance of our Na4Mn9O18 cathode. To further Tabledemonstrate 1. Literature the data rate comparison capability on the of electrochemical the system, performancesthe charge/discharge of gel polymer profiles electrolytes at various C rates arefor shown sodium-ion in Figure batteries. 8d. An increase in current densities from 1 C to 4 C cause a decrease of discharge capacity from 88 mAh g−1 to 44 mAh g−1. The electrode exhibits a good cycle stability, Cycle Capacity Remaining Current Material −1 Reference maintaining a specific capacity of 33 mAhNumber g after 500(mAh cycles g−1 )at 4 C asDensity shown in Figure 8e. The electrodePMMA-based maintains gel 60% polymer of its electrolyte initial capacity 100th after 500 cycles. 68 1 C [27] TablePhosphonate-based 1 presents gel the polymer comparative electrolyte performance35th data from 117.8 the literature 1 Con the relevant/similar [28] materials,Glass-fiber-based and our gelPAN-nanofiber-based polymer electrolyte 100thgel polymer electrolyte 70 for novel 1 C aqueous [29 ]sodium-ion PAN-based-based gel polymer electrolyte 100th 74 1 C This study battery exhibits a superior electrochemical performance compared with other reported systems [27–29].

Table 1. Literature data comparison on the electrochemical performances of gel polymer electrolytes for sodium-ion batteries.

Cycle Capacity Remaining Material Current Density Reference Number (mAh g−1) PMMA-based gel polymer electrolyte 100th 68 1 C [27] Phosphonate-based gel polymer electrolyte 35th 117.8 1 C [28] Glass-fiber-based gel polymer electrolyte 100th 70 1 C [29] PAN-based-based gel polymer electrolyte 100th 74 1 C This study

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4 -1 120 100 1st cycle 2nd cycle 2 rd PAN based gel polymer electrolyte 80 3 cycle 100 Liquid electrolyte 60 0 80 40 60 Current/ mA Current/ -2 20 (a) 40 (b) 1 C Coulombic efficiency/ % Discharge capacity/ mAh g 0 1 1.5 2 0 100 200 Potential /(V vs. Zn/Zn2+) Cycle number

-1 2 100 4 C, 3 C, 2 C, 1 C 1 C 1 C 80 2 C 3 C 60 1.5 4 C 40 Voltage/ V Voltage/ 20 (c) (d) Discharge capacity/ mAh g 0 1 0 10 20 30 40 50 0 20 40 60 80 100 Cycle number -1 Capacity/ mAh g

-1 100 100 80

60 50 40

20 (e) Coulombic efficiency/ % efficiency/ Coulombic Discharge capacity/ mAh g mAh capacity/ Discharge 0 0 0 100 200 300 400 500 Cycle number

Figure 8. Electrochemical performance of a cell with the PAN-nanofiber-based gel polymer electrolyte Figure 8. Electrochemical performance2+ of a cell with the PAN-nanofiber-based−1 gel polymer of Na4Mn9O18 electrode (vs. Zn/Zn )(a) CV curves at a rate of 0.1 mV s ;(b) Cycling performance electrolyte of Na4Mn9O18 electrode (vs. Zn/Zn2+) (a) CV curves at a rate of 0.1 mV s−1; (b) Cycling of liquid electrolyte cell (the blue) and the PAN-nanofiber-based gel polymer electrolyte cell (the red); performance of liquid electrolyte cell (the blue) and the PAN-nanofiber-based gel polymer (c) Rate capability and (d) Charge/discharge profiles at various rates; (e) Prolonged cycling performance electrolyte cell (the red); (c) Rate capability and (d) Charge/discharge profiles at various rates; (e) of the PAN-nanofiber-based gel polymer electrolyte cell at 4 C. Prolonged cycling performance of the PAN-nanofiber-based gel polymer electrolyte cell at 4 C.

4.4. ConclusionsConclusions

InIn thisthis work, Na Na44MnMn9O9O1818 nanorodsnanorods were were successfully successfully synt synthesizedhesized by the by theHSCR HSCR method. method. A Arechargeable rechargeable hybrid hybrid aqueous aqueous battery battery employing employing metallic metallic Zn Zn as as a a negative negative electrode electrode and and Na Na44MnMn99OO1818 asas thethe positivepositive electrodeelectrode hashas beenbeen developed.developed. The as-prepared Na4MnMn99OO1818 electrodeelectrode exhibited exhibited a a −1 dischargedischarge capacitycapacity of of 64 64 mAh mAh g g−1 atat1 1 C C even even after after 200 200 full full cycles. cycles. The The excellent excellent cycle cycle stability stability and and rate

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performance of our Na4Mn9O18 material makes it a strong candidate for cathodes for rechargeable hybrid aqueous battery systems.

Author Contributions: Formal Analysis, Z.B., Y.Z. and J.H.; Investigation, Y.Z.; Writing-Original Draft Preparation, T.T. and Y.Z.; Writing-Review & Editing, Z.B. and J.H.; Supervision, J.H.; Project Administration, T.T.; Funding Acquisition, Y.Z. Funding: This research was funded by the China Postdoctoral Science Foundation (2018M630924); Program for the Outstanding Talents of Guangdong sailing plan, Human Resources and Social Security of Guangdong Province (201634007). Z.B. acknowledges support from a research project AP05136016 by the Ministry of Education and Science of Kazakhstan. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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