Reduced Graphene Oxides Decorated Nise Nanoparticles As High Performance Electrodes for Na/Li Storage

materials

Article Reduced Graphene Oxides Decorated NiSe Nanoparticles as High Performance Electrodes for Na/Li Storage

Yan Liu and Xianshui Wang *

Information and Engineering School, Wuhan University of Engineering Science, Wuhan 430200, China; [email protected] * Correspondence: [email protected]; Tel.: +86-027-81820304

Received: 14 October 2019; Accepted: 5 November 2019; Published: 10 November 2019

Abstract: A facile, one-pot hydrothermal method was used to synthesize Nickel selenide (NiSe) nanoparticles decorated with reduced graphene oxide nanosheets (rGO), denoted as NiSe/rGO. The NiSe/rGO exhibits good electrochemical performance when tested as anodes for Na-ion batteries 1 (SIBs) and Li-ion batteries (LIBs). An initial reversible capacity of 423 mA h g− is achieved for 1 1 SIBs with excellent cyclability (378 mA h g− for 50th cycle at 0.05 A g− ). As anode for LIBs, it 1 1 delivers a remarkable reversible specific capacity of 1125 mA h g− at 0.05 A g− . The enhanced electrochemical performance of NiSe/rGO nanocomposites can be ascribed to the synergic effects between NiSe nanoparticles and rGO, which provide high conductivity and large specific surface area, indicating NiSe/rGO as very promising Na/Li storage materials.

Keywords: NiSe/rGO; hydrothermal method; anodes materials; sodium ion battery; lithium ion battery

1. Introduction Batteries are considered as very promising electrical energy storage technologies for integration of large-scale renewable energy. However, both the commercial Li-ion batteries (LIBs) and the newly developing Na-ion batteries (SIBs) cannot meet the metrics of electrical energy storage, in terms of lifetime, cost, and safety. In order to satisfy the demand, intensive research has been concentrated on developing new type electrode materials with better cyclability and higher capacity. [1–7] With respect to the anodes, graphite is a popular candidate for lithium storage anode. However, the low theoretical capacity limit the application to high energy density LIBs. What is more, graphite is not able to serve as sodium storage anode due to the mismatch of lattice parameters and sluggish kinetics of large Na+ in graphite host. [8,9] Therefore, developing high performance Na/Li storage anodes is of great significance. Great efforts have been concentrated on seeking high theoretical capacity anode materials beyond graphite, including disordered carbon, [10–12] metal/alloy, [13–16] oxide [17–19], and sulfides. [20,21] Transition metal sulfides have been investigated as hopeful Na/Li Storage anode materials owing to their high theoretical capacity, such as FeS2,[22,23] MoS2,[24–32] Co3S4,[33,34] Ni3S2,[35–38] WS2 [39,40], 1 and TiS2 [41,42] with the theoretical capacity of 895, 670, 704, 446.5, 432.3, and 958 mAh g− , respectively. Transition metal selenides have very similar properties as transition metal sulfides, however there are seldom report about transition metal selenides as anode materials for LIBs and SIBs. For example, 1 Zhang et al. prepared FeSe2 microspheres which delivered a stable capacity of 372 mA h g− after 2000 1 cycles at 1 A g− for sodium storage. [43] Yang et al. have reported porous hollow carbon spheres 1 decorated with MoSe2 nanosheets, which have a capacity of about 684 mA h g− after 100 cycles at

Materials 2019, 12, 3709; doi:10.3390/ma12223709 www.mdpi.com/journal/materials Materials 2019, 12, 3709 2 of 12

1 1 1 1 A g− for lithium storage and a capacity of about 580 mA h g− after 100 cycles at 0.2 A g− for sodium storage. [44] Zhang et al. synthesized ZnSe-rGO nanocomposites by a simple hydrothermal process 1 1 for lithium storage which exhibited a capacity of about 778 mA h g− after 400 cycles at 1 A g− .[45] NiSe, a p-type semiconductor, has been widely used for photovoltaic devices. To the best of our knowledge, there is only Zhang et al. have reported core-shell NiSe/C composites used for LIBs and 1 1 1 NIBs which delivers a capacity of about 280 mA h g− /428 mA h g− after 50 cycles at 0.1 A g− for sodium/lithium storage. [46] Nevertheless, the reversible capacity and rate performance still need to improve. Yang et al. synthesized carbon-supported nickel selenide (Ni0.85Se/C) hollow nanowires by 1 two-step hydrothermal process for sodium storage which exhibited a capacity of about 390 mA h g− . However, the capacity still need to be upgraded considerably. Moreover, the complex synthesis route are not suitable for large-scale applications [47]. Like other transition metal selenides, NiSe suffers from its low electronic conductivity and the huge volume change during Na+/Li+ insertion/extraction process. So as to solve these problems, herein, the nanostructured NiSe anchored on the rGO nanosheets are synthesized by a simple hydrothermal approach. Owing to the synergic effects between NiSe nanoparticles and rGO, the NiSe/rGO nanocomposites exhibit a good sodium-storage and lithium-storage performance, offering a low cost and high performance anode material for Na/Li-ion batteries.

2. Experimental Section

2.1. Material Synthesis GO was synthesized from natural graphite nanoflakes using a modified Hummer’s method. [48] NiSe/rGO was synthesized via a facile hydrothermal process. Typically, GO powder (72 mg) was dispersed in 36 mL distilled water using ultrasonication for 3 h. After ultrasonication, 290 mg Ni(NO ) 6H O and 110 mg SeO were added and mechanically stirred for 10 min, followed by the 3 2· 2 2 addition of 15 mL NH H O and 30 mL N H H O. After more than 30 min mechanically stirring, the 3· 2 2 4· 2 mixture precursor solution was transferred to a 100 mL Teflon-lined autoclave and heated at 200 ◦C for 10 h. The resultant slurry was centrifuged and washed several times with distilled water and absolute ethanol, respectively. Then, the products were collected after being dried in a vacuum oven at 80 ◦C overnight. For comparison, bare NiSe and rGO were synthesized via the same method, except without the addition of GO powders and Ni/Se sources, respectively.

2.2. Material Characterization The samples were characterized by X-ray diﬀraction (XRD, PANalytical, Cu Kα Radiation), Scanning electron microscopy (SEM, FEI, Sirion 200, Hillsboro, OR, USA), Tansmission electron microscopy (TEM, FEI, Tecnai G2 F30S, Hillsboro, OR, USA), Raman spectra (Renishaw Invia, 514.5 nm, London, UK), Nitrogen adsorption–desorption isotherm (Micromeritics, TriStar II 3020, GA, USA), and X-ray photoelectron spectroscopy (XPS, Kratos, Axis Ultra DLD, Manchester, UK), thermogravimetric/diﬀerential scanning calorimetry (TG/DSC, Netzsch, STA 449 F5, Selb, Germany). The detailed material characterization is shown in Electronic Supplementary Information (ESI).

2.3. Electrochemical Measurements

Except for using the NaPF6/LiPF6 as Na/Li salt in electrolyte solution, the working electode was fabricated as same to the previous report. [49] Cyclic voltammogram (CV) measurements and electrochemical impedance spectra (EIS) were performed on a CHI 660E electrochemistry workstation. The galvanostatic charge/discharge measurements were carried out at room temperature with cutoﬀ voltages of 0~3.0 V using a LAND battery test system. The detailed electrochemical measurements is exhibited in ESI. Materials 2019, 12, 3709 3 of 12

3. Results and Discussion Materials 2019, 12, x FOR PEER REVIEW 3 of 12 The crystalline phases of NiSe/rGO and bare NiSe were identified by XRD, as shown in Figure1a. 0892,The main P63/mmc phase space of NiSe group, is indexed a = 0.366 as anm, structure b = 0.366 with nm hexagonal and c = 0.533 symmetry nm). [46,50] unit cell The (JCPDS: peaks 02-0892,at 29.8, 37.3,P63/ mmc42.9, spaceand 53° group, in the a XRD= 0.366 pattern nm, b can= 0.366 be assigned nm and to c =NiSe0.5332 (JCPDS: nm). [46 41,50‐1495).] The peaks[51] Compared at 29.8, 37.3, to that42.9, of and bare 53 NiSe,◦ in the there XRD is patterna broad canpeak be observed assigned at to 20°~30° NiSe2 (JCPDS: indicating 41-1495). the disordered [51] Compared stacking to of that rGO of sheets,bare NiSe, as shown there is in a broadFigure peak S1. observed[30] according at 20◦ ~30to the◦ indicating Debye Scherrer the disordered equation, stacking the crystal of rGO size sheets, of NiSe/rGOas shown inand Figure bare NiSe S1. [30 are] according calculated to to the be Debye~14 nm Scherrer and 16 nm, equation, respectively. the crystal A typical size of XPS NiSe survey/rGO patternand bare of NiSeNiSe/rGO are calculated is shown in to Figure be ~14 1b nm and and reveals 16 nm, the respectively. characteristic A peaks typical of XPSC, O, survey N, Ni, and pattern Se. Nof is NiSe derived/rGO from is shown N‐dope in Figureof rGO1 bwith and the reveals source the of characteristicN2H4∙H2O. Figure peaks 1c of shows C, O, the N, high Ni, and‐resolution Se. N is derived from N-dope of rGO with the source of N H H O. Figure1c shows the high-resolution (HR) Ni 2p spectrum, which exhibit two intensive peaks2 4 at· around2 856.5 and 874 eV corresponding to(HR) the Ni Ni 2p 2p3/2 spectrum, and Ni which 2p1/2 exhibitlevels, respectively. two intensive Meanwhile, peaks at around there 856.5are two and weak 874 eV peaks corresponding at binding energiesto the Ni of 2p3 862.3/2 and and Ni 880.5 2p1 /eV2 levels, for Ni respectively. 2p3/2 satellites Meanwhile, and Ni 2p1/2 there satellites, are two respectively. weak peaks at[51] binding After refinedenergies fitting, of 862.3 the and two 880.5 peaks eV of forNi Ni2p at 2p3 856.0/2 satellites and 874 and eV are Ni 2p1indexed/2 satellites, to Ni2+, respectively.while the fitting [51 ]peaks After 2+ atrefined 856.5 fitting,and 874.9 the eV two are peaks indexed of Ni to 2p Ni at4+, 856.0 which and are 874 in good eV are agreement indexed to with Ni the, while XRD the result. fitting The peaks HR 4+ Seat 856.53d spectrum and 874.9 (Figure eV are S2a) indexed exhibits to two Ni intensive, which are peaks, in good one agreementpeak located with at 54.8 the XRDeV attributed result. The to SeHR2‐ of Se NiSe 3d spectrum and NiSe (Figure2, [52] S2a)and exhibitsanother twopeak intensive located at peaks, 59.5 eV one indexed peak located to Se4+ at of 54.8 the eV impurity attributed of 2- 4+ SeOto Se2. [53]of NiSeAs for and the NiSe high2,[‐resolution52] and another C 1s spectrum peak located (Figure at 59.5S2b), eV the indexed peaks at to 283 Se eV,of the286 impurity eV, and 288.5of SeO eV2.[ were53] As assigned for the to high-resolution C–C/C=C, C–O, C 1sand spectrum C=O bond, (Figure respectively. S2b), the [51] peaks The at dominant 283 eV, 286 C 1s eV, peak and at288.5 283 eVeV wereindicates assigned the reduction to C–C/C =ofC, GO C–O, during and the C= Ohydrothermal bond, respectively. process [51 with] The N2 dominantH4∙H2O. C 1s peak at 283 eV indicates the reduction of GO during the hydrothermal process with N H H O. 2 4· 2

Figure 1. (a) X-ray diﬀraction (XRD) patterns of bare NiSe and NiSe/rGO composite. (b) X-ray Figurephotoelectron 1. (a) X spectroscopy‐ray diffraction (XPS) (XRD) survey patterns and (cof) high-resolutionbare NiSe and NiNiSe/rGO 2p spectra composite. of Nickel (b selenide) X‐ray photoelectron(NiSe)/reduced spectroscopy graphene oxide (XPS) nanosheets survey and (rGO) (c composite.) high‐resolution (d) thermogravimetric Ni 2p spectra of (TG) Nickel curves selenide of the (NiSe)/reducedbare NiSe and NiSe graphene/rGO. oxide nanosheets (rGO) composite. (d) thermogravimetric (TG) curves of the bare NiSe and NiSe/rGO. Figure S3 presents the Raman spectra of GO and NiSe/rGO. The intensity ratio of D/G for the NiSeFigure/rGO composite S3 presents and the GO Raman were 0.95 spectra and of 1.26, GO respectively, and NiSe/rGO. also provesThe intensity the reduction ratio of of D/G GO for to rGO the NiSe/rGO composite and GO were 0.95 and 1.26, respectively, also proves the reduction of GO to rGO with more defects and disordered structure. [51] Compared to rGO, the D and G bands of NiSe/rGO composite shift to lower wave number, which could be ascribed to the interaction between rGO and NiSe. Figure S4 shows the N2 adsorption‐desorption isotherms of bare NiSe and

Materials 2019, 12, 3709 4 of 12 with more defects and disordered structure. [51] Compared to rGO, the D and G bands of NiSe/rGO Materialscomposite 2019, shift 12, x FOR to lower PEER waveREVIEW number, which could be ascribed to the interaction between rGO4 of and 12 NiSe. Figure S4 shows the N2 adsorption-desorption isotherms of bare NiSe and NiSe/rGO. Both of bare NiSe/rGO. Both of bare NiSe and NiSe/rGO are identified as type IV isotherm (IUPAC), NiSe and NiSe/rGO are identified as type IV isotherm (IUPAC), indicating the mesoporous structure. indicating the mesoporous structure. Base on the BET analysis, the specific surface areas of the Base on the BET analysis, the specific surface areas of the bare NiSe and NiSe/rGO were calculated bare NiSe and NiSe/rGO2 1 were calculated to be 4.6 and 15.6 m2 g−1, respectively. The large surface to be 4.6 and 15.6 m g− , respectively. The large surface area of the NiSe/rGO not only increases the areacontact of areathe NiSe/rGO of the electrode not /onlyelectrolyte, increases but alsothe facilitatescontact area the rapidof the insertion electrode/electrolyte, and diffusion of but Na +also/Li+ . + + facilitatesThe content the of rGOrapid in insertion NiSe/rGO isand measured diffusion by TGAof Na as/Li shown. The in Figurecontent1d. of For rGO bare in NiSe, NiSe/rGO 46.32% ofis measuredweight is lost by atTGA about as 520shown◦C due in Figure to the transformation 1d. For bare NiSe, of NiSe 46.32% and NiSe of weight2 to NiO is inlost the at air about atmosphere. 520 °C dueIn contrast, to the transformation NiSe/rGO exhibits of NiSe a larger and weight NiSe2 lossto NiO of 63.26%, in the air which atmosphere. is equal to In the contrast, total weight NiSe/rGO loss of exhibitsNiSe and a rGO.larger As weight depicted loss in of the 63.26%, Equation which below: is equal to the total weight loss of NiSe and rGO. As depicted in the Equation below: A 63.26% = A X 46.32% + A (1 X) A× × 63.26% = A× × X× × 46.32% + A × (1−−X) A:A: The The mass of NiSe/rGO. NiSe/rGO. X: The pecentage of NiSeNiSe inin NiSeNiSe/rGO/rGO composite.composite. Therefore, Therefore, the the accurateaccurate loading loading of of NiSe NiSe in in the the NiSe/rGO NiSe/rGO is calculated to be as high as 68.44%. TheThe morphology morphology of of bare bare NiSe NiSe and and NiSe/rGO NiSe/rGO are observed by field field-emission‐emission scanning electron electron microscopymicroscopy (FESEM), (FESEM), as as shown shown in in Figure 22.. thethe barebare NiSeNiSe mainlymainly formform hollowhollow microspheresmicrospheres withwith diametersdiameters of of 2–3 2–3 μµm,m, which which are are assembled assembled by by NiSe NiSe nanoparticles nanoparticles with with the the sizes sizes of of about about 20 20 nm nm (Figure(Figure 22a,b,a,b, FigureFigure S5a).S5a). For NiSeNiSe/rGO/rGO composite, composite, NiSe NiSe particles particles are are distributed distributed on on the the graphene nanosheetsnanosheets with with some some agglomeration agglomeration consisting consisting of of some some nanoparticles nanoparticles with with sizes sizes of of about about 17 17 nm nm (Figure(Figure 22c,d,c,d, FigureFigure S5b).S5b).

FigureFigure 2. fieldField-emission‐emission scanning scanning electron electron microscopy microscopy (FESEM) (FESEM) images images of of (a (a,b,b) )bare bare NiSe NiSe and and ( (cc,d,d)) NiSe/rGONiSe/rGO composite. composite.

Furthermore,Furthermore, the microstructuremicrostructure and and particle particle size size of of the the NiSe NiSe/rGO/rGO are are analyzed analyzed by TEM by TEM and highand highresolution resolution tansmission tansmission electron electron microscopy microscopy (HRTEM) (HRTEM) (Figure3 , Figure(Figure S6). 3, TheFigure low S6). magnification The low magnificationTEM image (Figure TEM 3imagea) presents (Figure a typical 3a) presents specimen a typical of NiSe@graphene specimen of nanosheet.NiSe@graphene It can nanosheet. be seen that It canNiSe be nanocrystals seen that NiSe are anchorednanocrystals on the are graphene anchored nanosheets, on the graphene which nanosheets, indicates that which the rGO indicates nanosheets that thegreatly rGO influence nanosheets the greatly morphology influence of NiSe. the morphology The SAED patternof NiSe. (Figure The SAED S6, Table pattern S1) (Figure presents S6, the Table ring S1)features, presents which the ring can features, be well indexed which can as be di ffwellerent indexed crystal as planes different ((100), crystal (101), planes (102), ((100), (103) (101), (110), (102), and (103)(203)) (110), of hexagonal and (203)) structure, of hexagonal confirming structure, the formation confirming of well-crystallized the formation of NiSe. well Figure‐crystallized3b shows NiSe. the FigureHRTEM 3b result shows of the NiSe HRTEM with lattice result spacingof NiSe with of 0.287 lattice nm spacing agrees well of 0.287 with nm the agrees (101) planeswell with of NiSethe (101) and planesthe crystal of NiSe size and of NiSe the crystal/rGO is size about of NiSe/rGO 20 nm which is about is basically 20 nm consistentwhich is basically with XRD consistent and SEM with results. XRD and SEM results.

Materials 2019, 12, x FOR PEER REVIEW 5 of 12 Materials 2019, 12, 3709 5 of 12 Materials 2019, 12, x FOR PEER REVIEW 5 of 12

Figure 3. (a) TEM image of the NiSe/rGO; (b) high resolution TEM image of the NiSe/rGO.

FigureFigure 3. 3. (a(a) )TEM TEM image image of of the the NiSe/rGO; NiSe/rGO; (b (b) )high high resolution resolution TEM TEM image image of of the the NiSe/rGO. NiSe/rGO. Based on the SEM, TEM analysis, and the previous literatures, [45,54] a likely synthesis mechanismBased on of the the SEM, NiSe/rGO TEM analysis, nanocomposite and the previous is illustrated literatures, in [Figure45,54] a 4. likely Firstly, synthesis under mechanism vigorous Based on the SEM, TEM analysis, and the previous literatures, [45,54] a likely synthesis ofultrasonication the NiSe/rGO in nanocomposite water, the precursor is illustrated of GO inis Figureeffectively4. Firstly, exfoliated under into vigorous GO sheets, ultrasonication and then Ni in2+ mechanism of the NiSe/rGO nanocomposite is illustrated in Figure 4. Firstly, under vigorous water,ions are the electrostatically precursor of GO combined is eﬀectively with exfoliated the negatively into GO charged sheets, and GO then sheets Ni 2through+ ions are vigorous electrostatically stirring ultrasonication in water, the precursor of GO is effectively exfoliated into GO sheets, and then Ni2+ combinedin aqueous with solution. the negatively The intermediate charged GO active sheets species through of vigorous Se2− ions stirring which in are aqueous derived solution. from SeO The2 ions are electrostatically combined with the negatively charged GO sheets through vigorous stirring 2 2+ intermediateaccording to activeEquations species (1) ofand Se (2)− ions during which the are hydrothermal derived from process2− SeO2 according and then to react Equations with Ni (1) andions (2)to in aqueous solution. The intermediate active species 2+of Se ions which are derived from SeO2 duringform NiSe the hydrothermal(Equation (3)), process meanwhile and thenGO is react reduced with to Ni rGOions by to N form2H4∙H NiSe2O. Obviously, (Equation (3)),graphene meanwhile oxide according to Equations (1) and (2) during the hydrothermal process and then react with Ni2+ ions to GOnanosheets is reduced greatly to rGO influence by N2 theH4 morphologyH2O. Obviously, of NiSe. graphene In the absence oxide nanosheets of graphene, greatly NiSe nanoparticles inﬂuence the form NiSe (Equation (3)), meanwhile· GO is reduced to rGO by N2H4∙H2O. Obviously, graphene oxide morphologyare assembled of in NiSe. the form In the of hollow absence nanospheres. of graphene, With NiSe the nanoparticles graphene oxide, are assembledNiSe particles in the disperse form nanosheets greatly influence the morphology of NiSe. In the absence of graphene, NiSe nanoparticles ofon hollow graphene nanospheres. nanosheets With with the some graphene agglomeration. oxide, NiSe As particlesa matter disperse of fact, Figure on graphene 4 shows nanosheets an ideal are assembled in the form of hollow nanospheres. With the graphene oxide, NiSe particles disperse withgrowth some process. agglomeration. The NiSe nanoparticles As a matter exhibit of fact, some Figure agglomeration4 shows an ideal rather growth than dispersed process. individual The NiSe on graphene nanosheets with some agglomeration. As a matter of fact, Figure 4 shows an ideal nanoparticlesnanoparticles exhibiton the rGO some sheets agglomeration in our experiment. rather than dispersed individual nanoparticles on the rGO growth process. The NiSe nanoparticles exhibit some agglomeration rather than dispersed individual sheets in our experiment. nanoparticles on the rGO sheets in our experiment. SeO2 + N2H4 H2O Se + N2 + 3H2O (1) Se𝑂 𝑁𝐻· ∙𝐻𝑂→𝑆𝑒𝑁→ 3𝐻𝑂 (1) 2 2 3Se + 6OH− 2Se− + SeO− + 3H2O (2) 3SeSe𝑂 𝑁 6O𝐻𝐻→2𝑆𝑒→∙𝐻𝑂→𝑆𝑒𝑁𝑆𝑒𝑂3 3𝐻3𝐻𝑂𝑂 (1)(2) 2+ 2 Ni + Se− NiSe (3) 3Se 6O𝐻𝑁𝑖→2𝑆𝑒𝑆𝑒 𝑆𝑒𝑂→→𝑁𝑖𝑆𝑒 3𝐻 𝑂 (2)(3) 𝑁𝑖 𝑆𝑒 →𝑁𝑖𝑆𝑒 (3)

Figure 4. The illustration on growth processes of NiSe/rGO nanocomposites. Figure 4. The illustration on growth processes of NiSe/rGO nanocomposites.

The Na-storageFigure performance 4. The illustration of as-prepared on growth processes NiSe/rGO of NiSe/rGO nanocomposites, nanocomposites. bare NiSe and rGO are The Na‐storage performance of as‐prepared NiSe/rGO nanocomposites, bare NiSe and rGO are presented in Figure5. The cyclic voltammetry measurements of NiSe /rGO are carried out in the range of presented in Figure 5. The cyclic voltammetry measurements of NiSe/rGO are carried out in the range 0~3 VThe as Na shown‐storage in Figure performance5a. In the of first as‐ cathodicprepared scan, NiSe/rGO a major nanocomposites, peak located at 0.75 bare V NiSe could and be observed rGO are of 0~3 V as shown in Figure 5a. In the first cathodic+ scan, a major peak located at 0.75 V could be presentedowing to the in Figure conversion 5. The reaction cyclic voltammetry process (NiSe measurements+ 2Na + 2e− of NiNiSe/rGO+ Na2Se) are and carried formation out in of the the range solid observed owing to the conversion reaction process (NiSe + 2Na→ + + 2e− → Ni + Na2Se) and formation ofelectrolyte 0~3 V as interphase shown in (SEI)Figure films. 5a. In The the reduction first cathodic peak atscan, 0.75 a V major disappears peak afterlocated the at first 0.75 cycle, V could and two be + − → 2 observed owing to the conversion reaction process (NiSe + 2Na + 2e Ni + Na Se) and formation

Materials 2019, 12, x FOR PEER REVIEW 6 of 12 Materials 2019, 12, 3709 6 of 12 of the solid electrolyte interphase (SEI) films. The reduction peak at 0.75 V disappears after the first cycle, and two new peaks located at around 1.0 V and 1.35 V appear. During the anodic scan, two new peaks located at around 1.0 V and 1.35 V appear. During the anodic scan, two peaks centered at peaks centered at 1.45 V and 1.80 V occur corresponding to the oxidation of metallic Ni to NiSe. [46] 1.45 V and 1.80 V occur corresponding to the oxidation of metallic Ni to NiSe. [46] The overlapping of The overlapping of the CV curves since the second scan, indicates the high reversibility of NiSe/rGO the CV curves since the second scan, indicates the high reversibility of NiSe/rGO composite electrode. composite electrode. Figure 5b shows the galvanostatic charge/discharge curves of NiSe/rGO anode Figure5b shows the galvanostatic charge /discharge curves of NiSe/rGO anode at a current density of at a current density of 0.05 A g−1 with voltage range of 0 and 3 V. The initial discharge curve displays 0.05 A g 1 with voltage range of 0 and 3 V. The initial discharge curve displays an irreversible plateau an irreversible− plateau at 0.75 V, in agreement with the CV features. In the following cycles, the at 0.75 V, in agreement with the CV features. In the following cycles, the charge/discharge proﬁles are charge/discharge profiles are nearly overlapping, implying that the good stability of NiSe/rGO nearly overlapping, implying that the good stability of NiSe/rGO electrode. Comparatively, the CV electrode. Comparatively, the CV curve and galvanostatic charge/discharge curves of bare NiSe curve and galvanostatic charge/discharge curves of bare NiSe electrodes are very similar to NiSe/rGO, electrodes are very similar to NiSe/rGO, as shown in Figure S7, indicating composited with rGO do as shown in Figure S7, indicating composited with rGO do not change the electrochemical reaction not change the electrochemical reaction process of NiSe. process of NiSe.

Figure 5. Sodium storage performances of as-prepared electrodes. (a) cyclic voltammograms of 1 NiSeFigure/rGO 5. at Sodium a scan rate storage of 0.1 performances mV s− ;(b) the of initial as‐prepared three cycles electrodes. of NiSe /(rGOa) cyclic composite voltammograms electrode at of 1 −1 0.05NiSe/rGO A g− ;( cat) cyclinga scan rate performance; of 0.1 mV ands ; ( (bd)) the rate initial performance three cycles of NiSe of NiSe/rGO/rGO composite, composite bare electrode NiSe and at rGO0.05 electrodes. A g−1; (c) cycling performance; and (d) rate performance of NiSe/rGO composite, bare NiSe and rGO electrodes. Figure5c presents the cycling performance of as-prepared NiSe /rGO, bare NiSe and rGO electrodes 1 at 0.05Figure A g− .5c After presents two cycles the cycling activation, performance the NiSe /rGOof as composite‐prepared NiSe/rGO, electrode delivers bare NiSe a reversible and rGO 1 1 capacityelectrodes of 423 at mAh0.05 A g −g−,1 which. After maintains two cycles at activation, 378 mA h g the− after NiSe/rGO 50 cycles composite with a capacity electrode retention delivers of a 89.4%reversible (compared capacity with of the 423 second mAh cycle),g−1, which demonstrating maintains at the 378 good mA cycling h g−1 after performance 50 cycles of with NiSe a/rGO. capacity In comparison,retention of bare 89.4% NiSe (compared and rGO with electrodes the second exhibit cycle), much demonstrating inferior performance. the good Bare cycling NiSe performance has decent 1 initialof NiSe/rGO. capacity butIn comparison, obvious fade bare with NiSe only and capacity rGO ofelectrodes 200 mA hexhibit g− after much 50 cycles.inferior The performance. rGO electrode Bare 1 exhibitsNiSe has excellent decent cycling initial capacity performance, but obvious but the fade capacity with is only only capacity 105 mA of h g200− . mA h g−1 after 50 cycles. TheThe rGO rate electrode performances exhibits of excellent NiSe/rGO, cycling bare NiSe performance, and rGO electrodes but the capacity have been is only evaluated 105 mA at h various g−1. 1 currentThe densities rate performances ranging from of 0.05 NiSe/rGO, to 3.2 A bare g− , whichNiSe and are presentedrGO electrodes in Figure have5d. been At theevaluated current at 1 densitiesvarious ofcurrent 0.05, densities 0.1, 0.2, 0.4, ranging 0.8, 1.6, from and 0.05 3.2 to A 3.2 g− A, g the−1, which NiSe/rGO are compositepresented in electrodes Figure 5d. deliver At the 1 reversiblecurrent densities capacities of of0.05, 410, 0.1, 375, 0.2, 364, 0.4, 0.8, 314, 1.6, 227, and 152, 3.2 and A g− 1101, the mAh NiSe/rGO g− , respectively. composite electrodes The reversible deliver 1 1 capacityreversible can capacities quickly recover of 410, to 375, 320 mA364, h 314, g− when227, 152, the currentand 110 density mAh g is−1, returned respectively. to 0.05 The A g reversible− at. In 1 comparison,capacity can bare quickly NiSe andrecover GO to deliver 320 mA capacities h g−1 when of 497, the 394, current 280, 177,density 86, 32, is returned and 15 mA to h0.05 g− Aand g−1 112,at. In 1 92,comparison, 70, 51, 36, 24, bare and NiSe 19 mA and h gGO− atdeliver the above-mentioned capacities of 497, current 394, 280, densities, 177, 86, respectively. 32, and 15 These mA h results g−1 and indicate112, 92, much 70, 51, better 36, 24, high-rate and 19 performancemA h g−1 at the is achievedabove‐mentioned by the compositing current densities, of NiSe respectively. with rGO. These results indicate much better high‐rate performance is achieved by the compositing of NiSe with rGO.

Materials 2019, 12, x FOR PEER REVIEW 7 of 12 Materials 2019, 12, 3709 7 of 12 To extend the applications, the Li storage capability of as‐prepared materials are also investigated with the coin‐type half cells, as shown in Figure 6. Figure 6a exhibits the initial four To extend the applications, the Li storage capability of as-prepared materials are also investigated cycles of CV for NiSe/rGO composite electrode, which are analogous to those of sodium storage. with the coin-type half cells, as shown in Figure6. Figure6a exhibits the initial four cycles of CV There are two cathodic peaks at around 1.3 and 1.7 V, and two anodic peaks at around 1.45 and 2.1 for NiSe/rGO composite electrode, which are analogous to those of sodium storage. There are two V. Figure 6b shows the galvanostatic charge/discharge curves of the NiSe/rGO at 0.05 A g−1. cathodic peaks at around 1.3 and 1.7 V, and two anodic peaks at around 1.45 and 2.1 V. Figure6b shows Compared with those of sodium storage, the voltage plateaus for lithium storage is relatively higher. the galvanostatic charge/discharge curves of the NiSe/rGO at 0.05 A g 1. Compared with those of In addition, the same to sodium storage, bare NiSe and NiSe/rGO electrodes− have the similar CV sodium storage, the voltage plateaus for lithium storage is relatively higher. In addition, the same curve and galvanostatic charge‐discharge profiles as shown in Figure S8. Figure 6c presents the to sodium storage, bare NiSe and NiSe/rGO electrodes have the similar CV curve and galvanostatic cyclability of different electrodes cycled at 0.05 A g−1. During the first cycle, the NiSe/rGO anode charge-discharge profiles as shown in Figure S8. Figure6c presents the cyclability of di fferent electrodes delivers 1023.2 and 622.8 mA h g−1 for discharge and charge processes with an initial coulombic cycled at 0.05 A g 1. During the first cycle, the NiSe/rGO anode delivers 1023.2 and 622.8 mA h g 1 for efficiency of 61%.− In the following cycles, there is a gradual increase of reversible capacity within− the discharge and charge processes with an initial coulombic efficiency of 61%. In the following cycles, initial 50 cycles, indicating a long‐lasting activation process. This phenomenon has been observed for there is a gradual increase of reversible capacity within the initial 50 cycles, indicating a long-lasting transitional metal oxide electrodes in the literatures. [19,55,56] After 50 cycles, the NiSe/rGO anode activation process. This phenomenon has been observed for transitional metal oxide electrodes in the exhibits very stable cycling performance with reversible capacity of 1125 mAh g−1. In comparison, the literatures. [19,55,56] After 50 cycles, the NiSe/rGO−1 anode exhibits very stable cycling performance bare NiSe firstly exhibits a capacity of 1668 mA h g . Although there is a slight increase of the capacity with reversible capacity of 1125 mAh g− . In comparison, the bare NiSe firstly exhibits a capacity of 668 for NiSe1 electrode in the initial 30 cycles, the capacity quickly decays after 30 cycles, which is only mA h g− . Although− there1 is a slight increase of the capacity for NiSe electrode in the initial 30 cycles, around 290 mA h g after 70 cycles. 1 the capacity quickly decays after 30 cycles, which is only around 290 mA h g− after 70 cycles.

FigureFigure 6. 6. ElectrochemicalElectrochemical lithiumlithium storagestorage performancesperformances ofof as-prepared as‐prepared electrodes. electrodes. (a(a)) cycliccyclic 1 voltammetryvoltammetry (CV) (CV) curvescurves of NiSe/rGO NiSe/rGO at at a a scanning scanning rate rate of of 0.1 0.1 mV mV s−1 s; −(b;() theb) theprofiles proﬁles of initial of initial three 1 threecycles cycles of NiSe/rGO of NiSe/rGO at 0.05 at 0.05 A Ag−1 g;− (c;() ccycling) cycling performance; performance; (d (d) )rate rate performance; performance; andand ((ee)) long-term long‐term cyclingcycling performance performance of of NiSe NiSe/rGO/rGO composite, composite, bare bare NiSe, NiSe, and and rGO rGO electrodes. electrodes. To test the rate performance, the NiSe/rGO composite, bare NiSe and rGO are tested at various To test the rate performance, the NiSe/rGO composite, bare NiSe and rGO are tested at various current densities ranging from 0.05 to 3.2 A g 1 in 0~3.0 V as shown in Figure6d. The NiSe /rGO anode current densities ranging from 0.05 to 3.2 A −g−1 in 0~3.0 V as shown in Figure 6d. The NiSe/rGO anode demonstrates the best rate performance among all the tested electrodes, which delivers capacities of demonstrates the best rate performance among all the tested electrodes, which delivers capacities of 573, 596, 602, 552, 448, 302, and 164 mA h g 1 at 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, and 3.2 A g 1, respectively. 573, 596, 602, 552, 448, 302, and 164 mA h −g−1 at 0.05, 0.1, 0.2, 0.4, 0.8, 1.6, and 3.2 A g−−1, respectively.

Materials 2019, 12, 3709 8 of 12

1 The capacity can recover close to its initial capacity (589 mA h g− ) when the current densities switch 1 1 from 3.2 to 0.05 A g− . Furthermore, the discharge capacity of 250th cycle of NiSe/rGO at 0.4 A g− 1 is 491mAh g− , with the capacity retention of 86% compared with the second cycle, indicating the long-term cycling stability of NiSe/rGO electrode (Figure6e). Comparatively, the bare NiSe anode 1 1 delivers an initial capacity of about 566 mA h g− , but quickly decays to 65 mA h g− after 250 cycles with capacity retention of only 11.5%. Although rGO exhibits very stable cyclability, it only delivers a 1 capacity of only 171 mA h g− . To understand why the NiSe/rGO anode exhibits superior electrochemical performances compared with the bare NiSe anode, electrochemical impedance measurements are carried out. A simulated equivalent electric circuit model is included. As shown in Figure7 and Table1, it can be easily found that NiSe/rGO exhibits smaller SEI surface resistance (RSEI) and charge-transfer resistance (Rct) than bare NiSe in both NIBs and LIBs, owing to the conducting frame of rGO in NiSe/rGO. The lower values of RSEI and Rct are considered to be key factors for improving the electrochemical performance of NiSe/rGO. It is noteworthy that the electrochemical impedance of bare NiSe and NiSe/rGO in LIBs are smaller than bare NiSe and NiSe/rGO in SIBs, indicating the easily inserted/extracted of lithium ion in the bare NiSe and NiSe/rGO electrodes. The fast diffusion kinetics is attributed to the small radius of lithium ions. The XRD pattern of NiSe/rGO electrode of SIB at fully-charged-state after 50 cycles, is shown in Figure S9. In this XRD pattern, the diffraction peaks of the NiSe/rGO electrodes can be well assigned to NiSe, NiSe2, rGO, and Cu (from the substrate of Cu foil), which are consistent to the NiSe/rGO composite before cycling (Figure1a), indicating the NiSe /rGO electrodes with very good cycling stability. Moreover, SEM and TEM analysis are carried out to observe the microstructure changes of the bare-NiSe and NiSe/rGO electrodes (before and after 50 cycles) in SIBs. In Figure S10, the bare NiSe electrodes crash and agglomerate during the repeated cycling processes. In contrast, no significant morphology changes can be detected from the cycled NiSe/rGO electrodes, except for a little more agglomeration of NiSe particles compared with the pristine NiSe/rGO composite (Figure3), further revealing the rGO can effectively buffer the volume change of NiSe nanoparticles. Table S2 comparison of the electrochemical performance of NiSe/rGO with those of other anode materials in the literature. As shown in this table, the electrochemical performance of NiSe/rGO is superior to most of other anode materials. The good electrochemical performance of NiSe/rGO can be explained by the following three reasons: Firstly, the highly conductive rGO nanosheets would shorten the Na+/Li+ ions diffusion distance and improve the electron transport during the processes of charging and discharging. Secondly, being composited with rGO effectively enhances the specific surface area, which would offer more active sites for the accommodation of Na+/Li+. Finally, this composite structure of NiSe and rGO would effectively buffer the large volume changes during the sodium/lithium insertion/extraction processes. Materials 2019, 12, x FOR PEER REVIEW 9 of 12 Materials 2019, 12, 3709 9 of 12

Figure 7. Electrochemical impedance spectroscopy (EIS) for the NIBs (a) and Li-ion batteries (LIBs) Figure 7. Electrochemical impedance spectroscopy (EIS) for the NIBs (a) and Li‐ion batteries (LIBs)1 (b) with NiSe/rGO and bare NiSe electrodes after 10 cycles at a constant current density of 0.05 A g− ; (b) with NiSe/rGO and bare NiSe electrodes after 10 cycles at a constant current density of 0.05 A g−1; (c) equivalent circuit model of the EIS spectra. (c) equivalent circuit model of the EIS spectra. Table 1. Fitting results of the Nyquist plots using the equivalent circuit. Table 1. Fitting results of the Nyquist plots using the equivalent circuit. Samples RS (Ω)RSEI (Ω) CPES (F) Rct (Ω) CPEdl (F) Chi-Squared 6 5 Chi‐ 3 Bare NiSe(Na)Samples 3.0RS(Ω) R 25.3SEI (Ω) 5.4CPE10S(F)− Rct171.7(Ω) CPE1.3dl(F)10 − 3.0 10− × 6 × 5 Squared× 3 NiSe/rGO(Na) 2.8 22.4 7.3 10− 67.2 3.1 10− 1.0 10− × 6−6 ×−5 5 ×−3 3 BareBare NiSe(Li) NiSe(Na) 4.33.0 6.725.3 1.65.4 ×10 10− 171.736.5 1.3 ×3.1 10 10 − 3.0 ×3.0 10 10 − × 6 × 5 × 3 NiSeNiSe/rGO(Na)/rGO(Li) 2.12.8 4.122.4 3.97.3 ×10 10− −6 67.218.2 3.1 ×8.5 10−510 − 1.0 ×1.9 10−310 − × × × Bare NiSe(Li) 4.3 6.7 1.6 × 10−6 36.5 3.1 × 10−5 3.0 × 10−3 4. ConclusionsNiSe/rGO(Li) 2.1 4.1 3.9 × 10−6 18.2 8.5 × 10−5 1.9 × 10−3 In summary, NiSe/rGO nanocomposites are successfully synthesized by a one-step hydrothermal 4. Conclusions method and used for sodium and lithium storage. As a result, the as-prepared NiSe/rGO shows a high 1 1 1 reversibleIn summary, capacity of NiSe/rGO 378 mA h g−nanocomposites, and high rate capabilityare successfully of 110 mA synthesized h g− at 3.2 by A g−a forone SIBs.‐step 1 1 Forhydrothermal LIBs, it demonstrates method and a remarkable used for reversiblesodium and capacity lithium of 1125storage. mA hAs g− a atresult, 0.05 A the g− as, good‐prepared rate 1 1 −1 1 −1 capabilityNiSe/rGO of shows 164 mA a high h g− reversibleat 3.2 A g −capacity, as well of as 378 superior mA h g capacity, and high retention rate capability of 491 mA of h 110 g− mAat 0.4 h Ag 1 −1 −1 g−at after3.2 A 250 g cycles.for SIBs. The For enhanced LIBs, it electrochemicaldemonstrates a performanceremarkable reversible of the NiSe capacity/rGO nanocomposites of 1125 mA h owes g at to0.05 the A synergic g−1, good eﬀ rateects capability between NiSe of 164 nanoparticles mA h g−1 at and3.2 A rGO. g−1, as The well results as superior indicate capacity that the retention NiSe/rGO of nanocomposites491 mA h g−1 at have0.4 A greatg−1 after potential 250 cycles. as anodes The enhanced for sodium electrochemical and lithium batteries. performance of the NiSe/rGO nanocomposites owes to the synergic effects between NiSe nanoparticles and rGO. The results Supplementaryindicate that the Materials: NiSe/rGOThe followingnanocomposites are available have online great at potential http://www.mdpi.com as anodes for/1996-1944 sodium/12 and/22/ 3709lithium/s1. Authorbatteries. Contributions: X.W. designed the work. Y.L. carried out the experiments. Y.L. and X.W. writed the manuscript. Funding:SupplementaryThis research Materials: received The no following external funding.are available online at www.mdpi.com/xxx/s1.

Acknowledgments:Author Contributions:The X.W. authors designed thank the Analytical work. Y.L. andcarried Testing out the Center experiments. of HUST Y.L. for and XRD, X.W. SEM writed and the FETEM measurements. manuscript. Conﬂicts of Interest: The authors declare no conﬂict of interest. Funding: This research received no external funding. ReferencesAcknowledgements: The authors thank Analytical and Testing Center of HUST for XRD, SEM and FETEM measurements. 1. Goodenough, J.B.; Park, K.S. The Li-ion rechargeable battery: A perspective. J. Am. Chem. Soc. 2013, 135, Conflicts1167–1176. of Interest: [CrossRef The][ authorsPubMed declare] no conflict of interest. 2. Goodenough, J.B.; Kim, Y. Challenges for rechargeable Li batteries. Chem. Mater. 2010, 22, 587–603. [CrossRef]

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