A facile preparation of 3D flower-shaped Ni/Al-LDHs covered by β-Ni(OH)2nanoplat

时间：2024-05-22

State Key Laboratory of Chemical Resource Engineering,Beijing Engineering Center for Hierarchical Catalysts,Beijing Advanced Innovation Center for Soft Matter Science and Engineering,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Nanostructures Nickel hydroxide Cathode material Ultrafast charge-discharge Ni-MH batteries

ABSTRACT In the present study,we propose a novel electrode material of β-nickel hydroxide covering nickel/aluminum layered double hydroxides via a facile complexation-precipitation method.The as-obtained materials with 3-dimensional nanostructures are further utilized as highly capable electrode material in nickel-metal hydride batteries.The electrochemical test results demonstrated the β-nickel hydroxide covering nickel/aluminum-layered double hydroxides with 28%of β-nickel hydroxide provided a superior specific capacity value of 452 mA·h·g-1 in a current density of 5 A·g-1 using 6 M KOH as electrolyte as compared with other materials.In addition,the optimized sample displays an outstanding cyclic stability along with a huge specific capacity value of 320 mAh·g-1,and very small decay rate of 3.3%at 50 A·g-1 after 3000 cycles of charge/discharge test.These indicate that the newly designed material with nanostructures not only provides an efficient contact interface between electrolyte and active species and facilitates the transport of electrons and ions,but also protects the 3-dimensional nickel/aluminum layered double hydroxides,achieving a high specific capacity,fast redox reaction and excellent long-term cyclic stability.Therefore,the β-nickel hydroxide covering nickel/aluminum layered double hydroxides with superior electrochemical performance is predictable to be a gifted electrode material in nickel-metal hydride batteries.

1.Introduction

Recently increasing the energy demand encourage scientist to extensive research on the preparation of flexible and co-friendly energystorage devices like nickel-based batteries,supercapacitors,lithiumion batteries,and fuel cells[1-6].Among them,Ni-MH batteries have received significant concentration because of their fast charge-discharge ability,great power density with long-term cycle life when compared with that of existing secondary batteries[4-12].Alkaline batteries are one of the most capable power sources due to their extensive application in electric vehicles and hybrid electric vehicles for safety purposes[13-16].Ni-MH batteries are the dominant batteries for application in hybrid vehicles such as TOYOTA Prius[17].In general,Ni-MH batteries contained metal hydride(MH)as an anode,nickel hydroxide(Ni(OH)2)as a cathode in 6 M KOH as the alkaline electrolyte.The hydrogen is generated and stored in a metal alloy to form metal hydride;meanwhile,the oxidation reaction occurs on the positive electrode,in which Ni2+transfers into Ni3+during the charge process.Compared with the larger specific capacity(SC)of the MH electrode(360 mA·h·g-1)with higher charge-discharge speed,the Ni(OH)2cathode provides a much lower SC of 230 mA·h·g-1,limiting the improvement of the electrochemical properties of Ni-MH batteries.According to the Bode Cycles,Ni(OH)2possesses two polymorphic structures like α-Ni(OH)2,and β-Ni(OH)2.In addition,these α-Ni(OH)2and β-Ni(OH)2are easily changed into γ-NiOOH and β-NiOOH,correspondingly in the time of oxidation process[18-22].The β-Ni(OH)2is largely utilized in commercial batteries,owing to its immense current density,and great durability in very strong alkaline solution[23-26].Nonetheless,the theoretical SC value of β-Ni(OH)2is very low(289 mA·h·g-1),because of one-electron transfer reaction occurred between the β-Ni(OH)2and β-NiOOH.This capacity value is mainly attained by the capacities of commercially available alkaline secondary batteries.Besides,the β-NiOOH is partially transformed into γ-NiOOH in case overcharging happens,resulting in wide volume extension of the electrode which causes the quick capacity loss in the time of charge/discharge cycling test[27-31].

At the same time,the α-Ni(OH)2is able to accomplish two-electron exchange between the α-Ni(OH)2and γ-NiOOH via electrochemical conversion.Due to the average oxidation level of Ni ion is around 3.3-3.7 in the γ-NiOOH[32].In consequence,the superior discharge capacity value was gained from α-Ni(OH)2based batteries than the β-Ni(OH)2.Apart from that,the α-Ni(OH)2does not demonstrate the perceptible volume change by the repeated charge/discharge(C/D)cycles between the α-Ni(OH)2and γ-NiOOH,owing to the parallel lattice nature of α-Ni(OH)2 and γ-NiOOH.Although,the stability of pure α-Ni(OH)2based batteries is still limited in strong alkaline solution,because it is easily changed to β-Ni(OH)2[33-37].In order to resolve this issue,it is a necessary action to enhance the stability of α-Ni(OH)2in case of a concentrated alkaline solution.Therefore,numerous attempts have been made,one of which is that most of the researchers are focused on the development of stable α-Ni(OH)2through fractional replacement of Ni ion by other metal ions,like Al[38,39],Co[40],Mn[41],and Zn[42]in the α-Ni(OH)2.Among them,the Al ion has been considered as the most interesting metal ion owing to its outstanding stability at trivalent state with less expansion.Unexpectedly,the substitution of Al in α-Ni(OH)2has drawbacks.Due to the electrochemically inert nature of Al element at the time of electrochemical redox reaction,the discharge capacity typically decreases as the substitution content of Al increases.Thus,it is important to control the appropriate substitution amount of Al ion in α-Ni(OH)2.In addition,the chemical as well as phase compositions of the α-Ni(OH)2material are largely changed by Al substitution,which affects the electrochemical properties of the electrode[43].In general,Al-substituted α-Ni(OH)2is called as Ni/Al-LDHs.Usually,an optimum amount of α and βphases in the Ni(OH)2product was attained via accurately controlling the proportion of Al metal substitution.

Until now,there are many reports available on Ni/Al-LDHs based batteries in the literature[44,45].However,only few studies have studied the structure,and electrochemical characteristics of the Ni(OH)2product with α/β-mixed structures.For example,Tu et al.[46]developed the α/β-Ni(OH)2mixed phase with different amounts of α-Ni(OH)2,and noted that the α/β-Ni(OH)2containing 10 wt%α-Ni(OH)2showed a good stability with higher discharge capacity when compared with that of pure β-Ni(OH)2.This study indicates that the β-Ni(OH)2has superior structural stability and also,the α-Ni(OH)2possesses a superior discharge capacity.Hence,the Ni(OH)2with a mixture of α and βphases is expected to achieve an excellent electrochemical activity.Li et al.[47]also synthesized the Al-substituted α/β-Ni(OH)2by a facile co-precipitation method.However,the observed SC value of this material is not satisfactory at 0.2 A·g-1current density.Yao et al.[48]also prepared a β-Ni(OH)2covering Ni/Al-LDH sample using chemical precipitation method,however that material only provided a low SC value of 300 mA·h·g-1at a low current density of 0.5 A·g-1.

Therefore,in this study,we have proposed a novel,facile method for preparation of a new material of 3D flower-shaped Ni/Al-LDHs covered with β-Ni(OH)2nanoplates,denoted as β-Ni(OH)2@Ni/Al-LDHs using a two-step reaction of complexation-precipitation in one-pot for ultra-high performance Ni-MH power batteries.The as-synthesized β-Ni(OH)2@Ni/Al-LDH material achieved a higher SC value of 452 mA·h·g-1in a high current density of 5 A·g-1using 6 mol·L-1KOH aqueous solution than the previously reported Ni(OH)2-based materials.Furthermore,it provided excellent cyclic stability with a very low loss rate of 3.3%after 3000 cycles.So,this new material is clearly indicative of outstanding active material for manufacturing advanced Ni-MH batteries for next generation electric vehicles with excellent performance.

2.Experimental

2.1.Chemicals and reagents

Chemicals and reagents with AR grade presented in this work were employed without any cleansing process.Nickel sulfate hexahydrate(NiSO4·6H2O),sodium hydroxide(NaOH)and ammonia solution(NH3·H2O)were acquired from Beijing Jingxi Refined Chemicals Regent,Beijing,China.Aluminum hydroxide(Al(OH)3)was provided by Tianjin Zhiyuan Chemical Reagent Co.,Ltd.,China.Graphite powder(99.9%)was procured from Qingdao Laixi Fine Graphite Co.Ltd.,China.Potassium hydroxide with battery grade(KOH,99.8%)was provided by Beijing Huarong Chemical Factory,Beijing,China.Polytetrafluoroethylene(PTFE,60 wt%)was purchased from Guangzhou Songbai Chemicals,China.

2.2.Preparation of β-Ni(OH)2@Ni/Al-LDHs

The β-Ni(OH)2@Ni/Al-LDHs were prepared by using a two-step controllable complexation-precipitation method in single-pot.Firstly,100 ml of 1.2 mol·L-1of NH3·H2O solution was placed in a 250 ml round bottom flask,followed by heating at 50°C using a super thermostatic water bath.Then,25 ml of alkaline solution containing 2.4 mol·L-1of NaOH,2.4 mol·L-1of Al(OH)3and 2 mol·L-1of NH3·H2O was denoted as solution A,and 25 ml of 1.2 mol·L-1of NiSO4·6H2O was denoted as solution B.Afterwards,these two solutions were simultaneously dripped into the flask reactor by two peristaltic pumps at a rate of 0.25 ml per minute.The pH of this reaction solution was controlled in 11.5±0.03 through maintaining the pour rate of solutions A and B at about(0.25±0.01)ml·min-1which was noted via a HANNA pH meter(pH 211).The obtained reaction mixture was continuously stirred at 50°C for about 16 h of the aging process,resulting in obtaining a flower-like Ni/Al LDH precipitate.After that,the different ratios(16%,23%,28% and 33%)of 1.2 mol·L-1of NiSO4·6H2O,and 2.4 mol·L-1of NaOH were separately dripped into the above reactor containing Ni/Al LDH precipitate using two peristaltic pumps at a rate of 0.25 ml·min-1in order to form β-Ni(OH)2/Ni/Al-LDHs.Afterward,this obtained suspension was continuously stirred at 50°C for 16 h as the aging process.Then,the resultant material was collected by washing with deionized(DI)water for many times until the filtrate became neutral.Lastly,the obtained β-Ni(OH)2@Ni/Al-LDH material was dried at 60°C for 4 h.Also,the pure Ni/Al-LDHs were collected without the addition of β-Ni(OH)2for comparison purposes.

2.3.Characterization techniques

The morphologies of the as-obtained materials were studied via a Hitachi S-4700 model scanning electron microscope(SEM)at an operated voltage of 20 kV and an H-800 JEOL JEM-2100F model highresolution transmission electron microscope(HR-TEM)at an accelerating voltage of 300.0 kV.The crystalline natures of the synthesized materials were assessed using X-ray diffraction(XRD)with a Bruker D8 Advance model X-Ray Diffractometer in the 2θ range of 10°-90°at a scan rate of 10(°)·min-1with a Cu Kαradiation source.The chemical state and element compositions of the as-obtained materials were confirmed via a Thermo Fisher ESCALAB 250 model X-ray photoelectron spectrometer using mono-chromated Al Kαradiation.

2.4.Electrochemical characterizations

0.2 g of as-prepared material,0.06 g of graphite powder and 0.05 g of PTFE were well-ground with 0.1 ml DI water for 30 min using an agate mortar in order to attain a paste.Subsequently,this obtained paste was rolled into a thin film with a thickness of 50 μm.This thin film with an area of 1 cm2was pressed on same sized nickel foam to make an electrode by a roller pressing machine for 3 min at 10 MPa.

The electrochemical characterizations were investigated through a 3-electrode system with as-prepared electrode serving as a working electrode,Hg/HgO electrode serving as a reference electrode and a nickel plate acting as counter electrode.The 6 mol·L-1KOH was used as an electrolyte for performing the electrochemical study.Then,the galvanostatic charge-discharge(GCD)experiments were conducted with a LAND-CT2001A battery test system.Also,the cyclic voltammetry(CV)and Tafel polarization analysis were performed using a CHI 760D model electrochemical workstation(CH Instrument,Shanghai,China).All the above electrochemical analyses were carried out at room temperature.

3.Results and Discussion

3.1.Structural and morphological studies

Fig.1(a,b)illustrates the schematic representation for the synthesis stages of β-Ni(OH)2@Ni/Al-LDH nanostructure material and also,the phase conversion of the Ni(OH)2during the C/D processes.Fig.1(a)indicates the β-Ni(OH)2was formed gradually on the surface of Ni/Al-LDHs through increasing the β-Ni(OH)2ratios from 16 to 33%.As per the Bode-Cycle,the α-Ni(OH)2is transformed to γ-NiOOH,and the β-Ni(OH)2is transformed to β-NiOOH,respectively during the charging process,which indicates the oxidation of Ni atom valence from+2 into+3,even+4.At the same time,the γ-NiOOH is transformed to α-Ni(OH)2,and the β-NiOOH is transformed to β-Ni(OH)2,correspondingly during the charging process,resulting in the reduction of Ni atom valence from+3 into+2,as given in Fig.1(b)[49,50].Although,the α-Ni(OH)2can be freely changed into β-Ni(OH)2,the γ-NiOOH can be freely changed into β-Ni(OH)2under the C/D process[51].

The XRD patterns of pure α-Ni(OH)2,and the β-Ni(OH)2@Ni/Al-LDHs along with various ratios of β-Ni(OH)2are displayed in Fig.1(c,d).The pure α-Ni(OH)2exhibits the diffraction peaks at 2θ=11.89°,23.56°.33.76°,39.56°,and 62.22°which can be indexed to the lattices of(003),(006),(012),(015)and(113)crystal planes of rhombohedral phase Ni/Al LDHs,respectively with JCPDS card No.38-0715[52].After addition of the different ratios of β-Ni(OH)2into Ni/Al-LDHs,some new diffraction peaks appear in regard into β-Ni(OH)2at 2θ=11.89°,19.25°,22.76°,33.06°.38.53°,52.09°,and 62.72°,corresponding to(003),(001),(006),(101),(102),and(111)diffraction planes,respectively,which are well in agreement with the hexagonal phase of β-Ni(OH)2(JCPDS card No.03-0177)[53].This is indicative of the formation of the mixed phase structure of α and β-Ni(OH)2.However,the β-Ni(OH)2peak intensities are increased while the Ni/Al LDH peak intensities are decreased when increasing ratios of β-Ni(OH)2from 16%to 33%.These detected changes in XRD pattern suggested that the surface of Ni/Al LDHs has been uniformly covered by β-Ni(OH)2nanoplates via appropriately increasing the ratio of β-Ni(OH)2.Overall,these attained XRD results proved the construction of β-Ni(OH)2Ni/Al-LDH material,which is expected to drastically decrease the electron transfer resistivity,thus increasing the specific discharge capacity value and also considerably enhancing the obtained capacity of the active electrode material.In addition,no other diffraction peaks are noted,indicating the high purity of the as-obtained material.Furthermore,the lattice parameters of pure Ni/Al-LDHs and the β-Ni(OH)2@Ni/Al-LDHs of different ratios of β-Ni(OH)2are estimated by the obtained XRD results using Scherer's equation and listed in Table S1.It clearly demonstrates the as-prepared β-Ni(OH)2@Ni/Al-LDHs have a good crystalline nature.

Furthermore,to study the surface changes in the β-Ni(OH)2@Ni/Al-LDH material,the SEM analysis was conducted,and the obtained SEM images of pure Ni/Al-LDHs and the β-Ni(OH)2@Ni/Al LDHs with various ratios of β-Ni(OH)2are presented in Fig.2(a-e).The pure Ni/Al LDHs exhibit that the uniform thin nano-layers contain a flower-like morphology.Additionally,the different β-Ni(OH)2@Ni/Al-LDHs also show that the uniform thin layers contain flower-like morphologies.However,it indicates that the thin layer of Ni/Al-LDHs is uniformly covered by β-Ni(OH)2,and also,the thin layer thickness is gradually decreased with the increase of the ratios of β-Ni(OH)2.Additionally,a gradual creation of β-Ni(OH)2on the surface of the Ni/Al-LDHs is favorable towards the movement of OH-and Ni2+ions from solution into the surface and inside the pores of the Ni/Al-LDHs to produce more β-Ni(OH)2,which is beneficial for achieving better electrochemical activity[54].In general,the Ni(OH)2with regular shape possesses a large specific surface area,which can make more active sites,and consequently improve the contact between the electrolyte and active material[55,56].Therefore,the covering of β-Ni(OH)2on the surface of Ni/Al-LDHs can avoid the changes of α-Ni(OH)2into β-Ni(OH)2,which is helpful in producing a high SC and greatly improving the cyclic stability.The element mapping images were taken for pure Ni/Al-LDHs and β-Ni(OH)2@Ni/Al-LDHs with various ratios of β-Ni(OH)2and as displayed in Fig.3(a-e).It demonstrates that the Ni,Al and O elements are detected for all samples after the addition of various ratios of β-Ni(OH)2.At the same time,the Al element contents are found to be greatly reduced.These element mapping results are perfectively matched with the obtained XRD patterns[57,58].

Fig.1.(a)Schematic diagram for the synthesized β-Ni(OH)2@Ni/Al-LDHs with the different ratios of β-Ni(OH)2,(b)phase conversion of Ni(OH)2during the charging-discharge experiment;(c)XRD patterns of pure Ni/Al-LDHs and β-Ni(OH)2@Ni/Al-LDHs with the different ratios of β-Ni(OH)2and(d)XRD patterns of pure Ni/Al-LDHs and optimized 3D β-Ni(OH)2@Ni/Al-LDHs with their corresponding(hkl)values.

Fig.2.SEM images at different magnifications of the obtained(a)pure Ni/Al-LDHs and(b-e)β-Ni(OH)2@Ni/Al-LDHs with the different ratios of β-Ni(OH)2of(b)16%,(c)23%,(d)28%and(e)33%.

Additionally,an XPS test was conducted to study the element composition and the chemical states of the materials.The XPS spectra of pure Ni/Al-LDHs,and optimized β-Ni(OH)2@Ni/Al-LDHs:(a)full spectra,(b)Al 2p,(c)Ni 2p,and(d)O 1s are shown in Fig.4(a-d).As seen from Fig.4(a),the full XPS spectra exhibit the three elements of Al,Ni,and O.Fig.4(b)shows the typical peak of Al 2p located at 532.1 eV with a satellite peak due to the substitution of Al3+.Also,Fig.4(c)reveals that there are two typical peaks of Ni 2p centered at 856.2 and 873.9 eV along with two satellite peaks,which correspond to Ni 2p3/2and Ni 2p1/2,correspondingly,indicating the presence of high-spin Ni2+.Fig.4(d)shows the O 1s contains two peaks at 531.7 and 532.7 eV that can be owing to binding energies of Ni--O--H and Ni--O--Ni,correspondingly[59,60].This result indicates the successful development of Ni/Al LDH material.Furthermore,the XPS peak intensities of the β-Ni(OH)2@Ni/Al-LDHs are reduced than pure Ni/Al-LDHs,which is ascribed to the Ni/Al-LDHs covered by β-Ni(OH)2and also these observed XPS results are well matched with XRD results.

For detailed investigation of the surface morphology of as-prepared β-Ni(OH)2@Ni/Al-LDHs,HR-TEM analysis was also done for pure Ni/Al LDHs and the optimized β-Ni(OH)2@Ni/Al-LDH samples.The obtained HR-TEM images and the corresponding SEAD patterns are shown in Fig.5(a-f).As seen in Fig.5(a-c),the pure Ni/Al-LDHs exhibit a spherical-shaped flower-like morphology.Whereas,Fig.5(d-f)illustrates that the morphology of optimized β-Ni(OH)2@Ni/Al-LDH sample exhibits the flower-like spherical-shaped Ni/Al LDHs covered by β-Ni(OH)2.Moreover,the spherical-shaped Ni/Al LDHs densely covered with β-Ni(OH)2,which may create a good interface between Ni/Al-LDHs and β-Ni(OH)2,is helpful for charge transfer to the radial direction from β-Ni(OH)2to Ni/Al-LDHs.The SEAD pattern of pure Ni/Al-LDHs only contains Ni/Al-LDH crystalline planes,whereas,the optimized β-Ni(OH)2@Ni/Al-LDH sample contains both Ni/Al-LDHs and β-Ni(OH)2crystalline planes in their structure.Both,XRD and HR-TEM results well confirmed the presence of crystalline planes of β-Ni(OH)2and Ni/Al-LDHs together in the β-Ni(OH)2@Ni/Al-LDH sample.Therefore,the above obtained results confirm the construction of β-Ni(OH)2@Ni/Al-LDHs by addition of β-Ni(OH)2.In addition,the surface area of the assynthesized pure Ni/Al-LHD and β-Ni(OH)2@Ni/Al-LDH samples were analyzed through BET analysis,as shown in Fig.S1.As seen in Fig.S1,the specific surface area of the pure Ni/Al-LHDs and β-Ni(OH)2@Ni/Al-LDH samples are determined to be 3.73 and 5.12 m2·g-1,respectively.It is proved that the β-Ni(OH)2@Ni/Al-LDHs have a large surface area,resulting in the better electrochemical performance due to having more active sites.

Fig.3.Element mapping images of the obtained(a)pure Ni/Al-LDHs and(b-e)β-Ni(OH)2@Ni/Al-LDHs with the different ratios of β-Ni(OH)2of(b)16%,(c)23%,(d)28%and(e)33%.

Fig.4.XPS spectra of the obtained pure Ni/Al-LDHs and optimized 3D β-Ni(OH)2@Ni/Al-LDHs:(a)full survey spectra,(b)Al element,(c)Ni element and(d)O element.

Fig.5.TEM images of the obtained(a-c)pure Ni/Al-LDHs and(d-f)optimized 3D β-Ni(OH)2@Ni/Al-LDHs with their corresponding SEAD patterns.

3.2.Electrochemical performance

Fig.6.(a)CV curves of the β-Ni(OH)2@Ni/Al-LDHs with different ratios of the β-Ni(OH)2at a scan rate of 20 mV·s-1,(b)the calculated specific capacity of the β-Ni(OH)2@Ni/Al-LDHs with different ratios of β-Ni(OH)2during the CV tests by integral method,(c)CV curves of the optimized 3D β-Ni(OH)2@Ni/Al-LDHs at different scan rates from 5 to 100 mV·s-1 and(d)Tafel plots of the β-Ni(OH)2@Ni/Al-LDHs with different ratios of β-Ni(OH)2.

To assess the capacity properties of as-fabricated β-Ni(OH)2@Ni/Al-LDH electrodes for Ni-MH batteries,the numerous electrochemical tests were performed using 6 mol·L-1KOH electrolyte.The CV curves for the as-prepared β-Ni(OH)2@Ni/Al-LDHs with the addition of different ratios of β-Ni(OH)2at a scan rate of 20 mV·s-1are displayed in Fig.6(a).It is revealed that the intensities of oxidation and reduction peaks are increased towards the negative and positive directions,respectively as the ratio of β-Ni(OH)2is increased.This mainly ascribed to the enhancement in reversibility under C/D process as a result of“Fusion Effects”as well as the development of better conductivity medium after addition of β-Ni(OH)2.Additionally,the SC values for the β-Ni(OH)2@Ni/Al-LDHs with different ratios of β-Ni(OH)2are calculated via attained CV results and are illustrated in Fig.6(b).From Fig.6(b),it is seen that the SC values are gradually increased with the increase of the added β-Ni(OH)2up to 28%.However,the SC value further starts to decrease after the addition of 33%β-Ni(OH)2,because of the occurrence of an excessive amount of β-Ni(OH)2content,due to the fact that the β-Ni(OH)2can only give single-electron in theory,which is much lower than Ni/Al-LDHs(1.67 electrons).The reversibility character of the optimized β-Ni(OH)2@Ni/Al-LDHs was studied by CV measurement at various scan rates of 5 to 100 mV·s-1,as shown in Fig.6(c).It indicates that the β-Ni(OH)2@Ni/Al-LDHs exhibit a typical redox pair of peaks,which proves its excellent reversibility property.In order to further assess the reaction kinetics of the as-obtained samples,the electrode polarization experiments were performed for the pure Ni/Al-LDH and β-Ni(OH)2@Ni/Al-LDH electrodes,the attained Tafel plots are displayed in Fig.6(d).It is revealed that the exchange current density was increased when increasing the β-Ni(OH)2ratios.This implies the full consumption of Ni(OH)2towards the electrochemical reactions under the C/D process.Moreover,the β-Ni(OH)2supports the construction of electrically conductive medium in the electrode for effective movement of electrons from the Ni(OH)2to electrolyte during the C/D process.As a result,it is concluded that the reaction rate is increased via more active sites available in Ni(OH)2electrode.

Furthermore,the GCD curves for the as-synthesized β-Ni(OH)2@Ni/Al-LDHs with various ratios of β-Ni(OH)2in 5 A·g-1using 6 mol·L-1KOH electrolyte are displayed in Fig.7(a).This shows that the curves present two relatively flat charge and discharge platforms,indicating that the β-Ni(OH)2@Ni/Al-LDHs displayed a typical redox peak during the C/D process.Afterward,the SC values for the fabricated electrodes are calculated by the following Eq.(1),

where,i,t and m denote the discharge current(mA),the charge/discharge time(h)and the mass of active electrode sample(g),respectively.

The SC value of the pure Ni/Al-LDHs is 400 mA·h·g-1,and the SC values of the β-Ni(OH)2@Ni/Al-LDHs with various ratios of 16%,23%,28%and 33%β-Ni(OH)2are 413,421,452 and 438 mA·h·g-1,respectively and as presented in Fig.7(b).It shows the SC values of β-Ni(OH)2@Ni/Al-LDHs are increased with the increase of the β-Ni(OH)2ratios from 16%to 28%,and decreased when β-Ni(OH)2ratio was beyond 28%.So,the optimal amount of β-Ni(OH)2in the β-Ni(OH)2@Ni/Al-LDHs to achieve a superior electrochemical activity is 28%.Afterwards,the GCD curves of the optimized β-Ni(OH)2@Ni/Al-LDHs in various current densities of 5 to 50 A·g-1are presented in Fig.7(c)and(d),respectively.That expressed that the SC of the sample is decreased when it increases the current density from 5 to 50 A·g-1.Also,the electrode reached the highest SC value of 452 mA·h·g-1at a current density of 5 A·g-1.Apart from that,this SC value is still retained around 70.8%(320 mA·h·g-1)after current density was increased up to 50 A·g-1,which reflected an excellent rate property.Fig.8(a)illustrates the cyclic stability of the pure Ni/Al-LDHs and β-Ni(OH)2@Ni/Al-LDHs with various ratios of β-Ni(OH)2at a high current density of 50 A·g-1during the 3000 cycles.It is shown that the SC value of pure Ni/Al-LDH electrode is considerably decreased after 3000 cycles.Moreover,the β-Ni(OH)2@Ni/Al-LDH electrodes exhibit very stable SC values after 3000 cycles.However,among the different electrodes,β-Ni(OH)2@Ni/Al-LDHs with 28%β-Ni(OH)2offers the highest stability after 3000 cycles with a very low decay rate of 3.3%,indicating that the optimized sample displays very long durability under an ultra-heavy load.In addition,the changes in structure and morphology of the electrode were investigated using XRD and SEM analysis before and after the cyclic test,and the obtained result is illustrated in Figs.S2 and S3,respectively.The attained XRD and SEM results show that the β-Ni(OH)2@Ni/Al-LDHs remain unchanged after the 3000 cycle test in an ultra-high current density of 50 A·g-1.

Fig.8.(a)GCD cycle stability of the samples with different ratios of β-Ni(OH)2at a current density of 50 A·g-1;(b)comparison of the specific capacity and(c)cycling stability of the optimized sample with the previously reported Ni(OH)2in the literatures.

Fig.8(b)displays the comparison for the SC of optimized β-Ni(OH)2@Ni/Al-LDHs with previous reports on Ni(OH)2-based materials available in literature at various current densities.It demonstrates that our β-Ni(OH)2@Ni/Al-LDHs gave an outstanding electrochemical activity regarding capacity and power performance as compared with previously reported Ni(OH)2-based materials[21,31,35,44,52].Besides,the comparison of the current density versus a cyclic number of the β-Ni(OH)2@Ni/Al LDH electrode as with previously reported Ni(OH)2-based materials in literature is shown in Fig.8(c).This proves that this new material provides much superior SC and power density when compared with reported materials[23,31,35,44,52,53].Therefore,the as-synthesized β-Ni(OH)2@Ni/Al-LDHs can be the greatest active material to achieve a high-performance Ni-MH battery.

4.Conclusions

In conclusion,the β-Ni(OH)2@Ni/Al-LDHs were prepared using a simple two-step complexation-precipitation method in one-pot with accurately controlling pH and temperature.The obtained XRD results confirm the construction of β-Ni(OH)2@Ni/Al-LDHs through the addition of β-Ni(OH)2.SEM images indicate the β-Ni(OH)2@Ni/Al-LDHs exhibit the 3D thin-layered flower-shaped morphology.In addition,the CV and GCD experiments also show that the β-Ni(OH)2@Ni/Al-LDHs with 28% β-Ni(OH)2display better SC values up to 452 and 320 mA·h·g-1at 5 and 50 A·g-1,correspondingly.Due to the good layered structure and robustly-arranged nanoplates,the β-Ni(OH)2@Ni/Al-LDHs display a higher SC,better rate performance along with longer cycling durability as an electrode material for Ni-MH batteries than other Ni(OH)2based materials reported in the literature.Hence,these optimized β-Ni(OH)2@Ni/Al-LDHs will be a promising cathode material to make highly efficient Ni-MH power batteries.