3Mn

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P2−Na2/3Ni1/3Mn5/9Al1/9O2 Microparticles as Superior Cathode Material for Sodium-Ion Batteries: Enhanced Properties and Mechanisam via Graphene Connection Xiao-Hua Zhang, Wei-Lin Pang, Fang Wan, Jin-Zhi Guo, Hong-Yan Lü, Jin-Yue Li, Yue-Ming Xing, Jing-Ping Zhang, and Xing-Long Wu* National & Local United Engineering Laboratory for Power Batteries, Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, P. R. China S Supporting Information *

ABSTRACT: As a promising alternative for lithium ion batteries, room-temperature sodium ion batteries (SIBs) have become one significant research frontier of energy storage devices although there are still many difficulties to be overcome. For the moment, the studies still concentrate on the preparation of new electrode materials for SIBs to meet the applicability. Herein, one new P2−Na2/3Ni1/3Mn5/9Al1/9O2 (NMA) cathode material is successfully prepared via a simple and facile liquid-state method. The prepared NMA is layered transition metal oxide, which can keep stable crystal structure during sodiation/desodiation as demonstrated by the ex situ X-ray diffraction, and its electrochemical properties can be further enhanced by connecting the cake-like NMA microparticles with reduced graphene oxide (RGO) using a ball milling method. Electrochemical tests show that the formed RGO-connected NMA (NMA/RGO) can deliver a higher reversible capacity of up to 138 mAh g−1 at 0.1 C and also exhibit a superior high-rate capabilities and cycling stability in comparison to pure NMA. The much improved properties should be attributed to the reduced particle size and improvement of electrical conductivity and apparent Na+ diffusion due to RGO incorporation, which is comprehensively verified by the electrochemical technologies of galvanostatic intermittent titration technique, electrochemical impedance spectroscopy and cyclic voltammetry at various scan rate as well as ex-situ X-ray diffraction studies. KEYWORDS: sodium ion batteries, cathode, layered transition metal oxide, diffusion coefficient, reduced graphene oxide

1. INTRODUCTION The development of lithium-ion batteries (LIBs) during the last two decades is so fast that it has mainly satisfied the needs of portable electronic devices. For the moment, however, the electrical energy demand is still enormous and increasing rapidly in many electric-appliance fields such as smartphone, computer, electric vehicle and large scale power grid, etc. Unfortunately, the use of LIBs in larger scale and longer period will become an insurmountable issue due to the high production cost of lithium resources and its limited and inhomogeneous geographic distribution.1,2 Hence, significant alternatives such as sodium-ion batteries (SIBs) have attracted intense attention because of the similar chemical and physical properties of sodium in comparison to lithium and the more uniform distribution of sodium resources in the world. In comparison to LIBs, so far, the limitation of SIBs’ practical applications focus on the lower operating voltage and specific energy/power densities.3,4 Therefore, there is a need to explore new high-voltage and high-power cathode materials with good stability. On the other hand, modifying the present cathode materials is a promising method to push forward the development and implementation of SIBs, for example, © 2016 American Chemical Society

graphene has been applied to the system of electrochemical energy storage to enhance the properties due to its excellent electrical conductivity and other physical properties.5−8 Until now, a lot of materials such as oxides,9 phosphates,10 pyrophosphates,11 fluorophosphates,12 sulfates13 and cyanides14−16 have been proposed as cathodes for SIBs. Among them, the layered oxides (NaxMeO2, Me = Mn, Ni, Cr, Co, Fe, etc.17−26) is one of the most extensively studied topics in the past few years. Sodium-based layered materials can be categorized into two main groups using the classification proposed by Delmas et al.:27 O3-type or P2-type, in which the sodium ions are accommodated at octahedral and prismatic sites, respectively. It has been generalized that the P2-type materials are more suitable for SIBs in comparison with O3type materials because of its large interlayer spacing and high structural stability.28 At present, the research of P2-type layered oxides mainly focuses on the synthesis of new materials containing different kinds and amounts of transition metals. On Received: April 2, 2016 Accepted: July 25, 2016 Published: July 25, 2016 20650

DOI: 10.1021/acsami.6b03944 ACS Appl. Mater. Interfaces 2016, 8, 20650−20659

Research Article

ACS Applied Materials & Interfaces

2. EXPERIMENTAL SECTION

account of the redox reaction activity of metal Ni, Fe, and Cu at a high voltage (>3 V) and even Mn at voltages less than 2.5 V, for example, P2−NaxNi1/3Mn2/3O2, P2−NaxFe1/2Mn1/2O2, and P2−Na0.67CuxMn1−xO2 were synthesized as cathodes for SIBs by coprecipitation, solid-state reaction, and sol−gel process, respectively.29−31 These materials make a contribution to improving rate performance, reversible capacity and average potential, respectively. However, the various pristine materials are not perfect in terms of meeting the practical application of SIBs, and the detailed electrochemical mechanism is complicated especially poor symmetry of CV curves due to electrochemical activity of all transition metals including even Mn with Jahn−Teller distortion. As a significative achievement in the direction of layered transition metal oxides, in addition, Hu’s group32 designed P2−Na0.6[Cr0.6Ti0.4]O2 which could function as both positive and negative electrodes with average operation voltages of 3.5 and 0.8 V, corresponding to the redox couples of Cr3+/Cr4+ and Ti3+/Ti4+, respectively. Unfortunately, the specific capacity was only 75 mAh g−1 as cathode materials. As a result, there are two aspects of work at least to be further explored. On the one hand, introducing a small amount of inactive metal into layered oxides and making a further decoration with conductive materials may be effective approaches to improve structural reversibility and electrochemical performance, respectively. On the other hand, the aqueous-based synthesis strategy should be more helpful for the homogeneous mixing of precursors in the atom level and hence can promote the crystal growth and structural stability during charge/discharge cycles compared to conventional synthesis procedures including solid-state, coprecipitation and sol−gel methods. Herein, one new P2−Na2/3Ni1/3Mn5/9Al1/9O2 (NMA) material with partial Mn substituted by Al has been successfully prepared via a facile liquid-state method followed by a hightemperature annealing procedure, which should be benefit from the very close ion radius between Al3+ (53.5 pm) and Mn4+ (53 pm). Such substitution can partly inhibit the generation of Mn3+ with Jahn−Teller distortion and thereby improve the stability of crystalline structure upon electrochemical cycling. The pristine P2-NMA showed an initial discharge capacity about 118 mAh g−1 at 0.1 C and a capacity retention of about 82.5% after 150 cycles at 1 C when used as cathode material for SIBs. More importantly, the electrochemical properties of P2NMA can be further enhanced by connecting the cake-like P2NMA microparticles with reduced graphene oxide (RGO). Electrochemical tests show that the formed RGO-connected NMA (NMA/RGO) can exhibit much improved properties in comparison to the pristine NMA. For example, an increased reversible capacity of up to 138 mAh g−1 can be obtained at 0.1 C and there is still a capacity delivery of about 105.4 mAh g−1 even after 100 cycles at 0.2 C. The NMA/RGO composite can even keep a high capacity retention of above 89% after 150 cycles at 1 C. In addition, several electrochemical technologies including galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) at various scan rates had been further used to reveal the improvement mechanism of electrochemical properties, and ex-situ X-ray diffraction (XRD) was employed to study the crystal structure variation of P2− Na2/3Ni1/3Mn5/9Al1/9O2 during Na+ (de)intercalation.

2.1. Preparation of Pristine P2-NMA and NMA/RGO Composite. P2−Na2/3Ni1/3Mn5/9Al1/9O2 was synthesized via a simple liquid-state method followed by a high-temperature annealing process. Typically, four raw materials of NaNO3 (>99.0%), Ni(NO3)2·6H2O (>98.0%), Mn(CH3COO)2·4H2O (>99.0%), and Al(NO3)3·9H2O (Aladdin, 99.0%) were dissolved sequentially in deionized water in a molar ratio of 6:3:5:1 (Na: Ni: Mn: Al). After stirring at room temperature for 5 h, the water in the solution was slowly removed by rotary evaporator at 50 °C, and then dried under vacuum at 80 °C for 2 days. Subsequently, the dried mixture was annealed at 400 °C for 6 h in air, and then, ground and calcined at 800 °C for 15 h. NMA/RGO was prepared through ball-milling the mixture of the pristine NMA and RGO in the weight ratio of 97:3 for 2 h. The RGO was prepared from the ordinary thermal reduction of GO solid prepared via a modified Hummers method.33 2.2. Materials Characterization. The crystalline structures of all materials were characterized by the powder X-ray diffraction (XRD, Rigaku P/max 2200VPC) using the Cu−K (λ = 0.15406 nm) radiation in the 2θ range from 7° to 70°. The morphology and size of pristine P2-NMA and elemental mapping of NMA/RGO composite were observed on the scanning electron microscope (SEM, XL 30 ESEMFEG, FEI Company) and transmission electron microscope (TEM, JEM-2010F). The inductively coupled plasma−atomic emission spectroscopy (ICP-AES, Shimadzu, ICPS-8100) was used to check the elemental stoichiometry ratio of the prepared samples. 2.3. Electrode Preparation and Electrochemical Measurements. The electrochemical properties of the pristine P2-NMA and NMA/RGO samples were studied in the 2032 coin cells. 80% of P2NMA (or NMA/RGO composite), 10% acetylene black, and 10% carboxymethylcellulose (CMC) binder were mixed, then distilled water was added to this mixture to form a slurry, which was then pasted on an aluminum foil and dried under vacuum at 80 °C overnight to prepare a thin film composite cathode. The coin cells were assembled in an argon atmosphere glovebox. Sodium metal was used as the counter electrode, and the electrolyte was 1 mol L−1 NaClO4 in a mixture of anhydrous ethylene carbonate (EC) and propylene carbonate (PC) (1:1 v/v), and glass fiber from Whatman was used as the separator. The mass loading of active materials is about 2 mg cm−2. All of the cells rest at least 10 h before any electrochemical tests to ensure good soaking of electrolyte into separators and electrodes. The cells were cycled in the voltage range of 1.6−4.0 V (vs Na+/Na) at 25 °C using a battery test system (LAND CT2001A). The CV was measured by Versa STAT 3 (Princeton Applied Research) with potential range of 1.6−4.0 V at a scan rate of 0.1 mV s−1. For GITT analyses, the cells were cycled in the voltage range of 1.6−4.0 V (vs Na+/Na) at 0.02 C. The duration time for each applied galvanostatic current and rest was 1 and 5 h, respectively. And the EIS technique were implemented by a Versa STAT 3 (Princeton Applied Research) with the frequency ranging from 1 MHz to 100 mHz with an amplitude voltage of 5 mV.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. As an important layered oxide cathode material, P2−NaxMeO2 (Me represents transition metal) consists of MeO2 slabs made of edge sharing MeO6 octahedra. The sodium ions are sandwiched between these MeO2 slabs so as to occupy trigonal prismatic site (P), when the unit cell consists of two distinguishable MeOx layers it is denoted as P2. Figure 1a shows the XRD patterns of the pristine P2-type Na2/3Ni1/3Mn5/9Al1/9O2 (NMA) and P2-type Na2/3Ni1/3Mn5/9Al1/9O2/RGO (NMA/RGO). All the peaks of the pristine P2-NMA are well-indexed to the hexagonal lattice with space group P63/mmc, which corresponds to the β-type (P2) phase NaxMnO2 as reported by Parant et al.34 No impurity phase is detected. The main peak is very intense indicating a good crystallinity of the material, consequently, 20651

DOI: 10.1021/acsami.6b03944 ACS Appl. Mater. Interfaces 2016, 8, 20650−20659

Research Article

ACS Applied Materials & Interfaces

TEM image of an individual cake-like microparticle which reveals the particle thickness is very homogeneous. The HRTEM image in Figure 2d shows the distinct lattice fringes implying the good crystallinity of the material, and the interplanar crystal spacing of 5.61 Å is attributed to (002) plane. It is in accordance with P2-type Na0.7MnO2 reported by Su et al.36 and indicates the successful preparation of the P2NMA via the liquid-state method in this work. Figure 3 shows the SEM image and the corresponding EDX elemental analysis. Apparently, the homogeneous distribution of carbon element (C) throughout the crystallite demonstrates that RGO nanosheets disperse uniformly among the pristine materials after ball-milling. In addition, Al element distribution is identical with the three metal Na, Ni, and Mn. It indicates that Al atoms take part in the crystalization of as-prepared P2type NMA which is consistent with the results of above XRD. 3.2. Electrochemical Performances. Electrochemical performance of the pristine P2-type NMA and NMA/RGO were examined under the galvanostatic mode within the voltage range of 1.6−4.0 V to avoid side reactions associated with the degradation of the carbonate-based electrolytes at high potential.9 Figure 4a and b illustrates the voltage profiles and CV curves, respectively, recorded during the initial sodium ions removal from and following intercalation into the as-prepared P2-type NMA and NMA/RGO. At a first glance, an effect of the RGO composition on the electrochemical properties of the pristine material is seen. In particular, the potential curve of NMA/RGO appears to be extension toward lower Na content during first discharge, and the cyclic voltage curve of NMA/ RGO distinctly increase to a larger current density. However, the two materials show the same overall profile with three pairs of obvious voltage plateaus in the potential curves and corresponding three couples of large peaks located at about 1.9, 3.3, and 3.6 V in the CV curves. It indicates a similar electrochemical reaction mechanism of both materials during charge and discharge which coincides with the results reported by Zhu et al.37 In details, the voltage vs Na content curves for pristine P2NMA and NMA/RGO (Figure 4a) show both a stair-like profile in first charge and discharge at 0.1 C, indicating a reversible Na-driven structural transition. Note that during the first cycle within the 1.6−4.0 V voltage window, the charge and discharge curves of both samples contain two plateau-like features and three voltage plateaus, respectively. On the other hand, the outline of two charge and discharge curves is identical for pristine P2-NMA and NMA/RGO which indicates a similar structural transition in the process of Na ions insertion and deinsertion. Compared with as-prepared P2-NMA, however, the charge voltage curve extends to lower Na content and the discharge curve simultaneously to both lower and higher Na content for NMA/RGO, which reveals more quantity of Na ions inserted/deinserted within the active materials during charge and discharge, as a result, showing a higher specific capacity for the composite material. It may be attributed to the increased electrical conductivity after decorating pristine P2NMA with RGO. More specifically, the amount of Na+ extraction upon the first desodiation and reuptake during the subsequent sodiation are about 0.3 and 0.52 mol per P2-NMA in NMA/RGO, corresponding to the charge/discharge capacities of 78.8 and 136.9 mAh g−1 respectively. In comparison, all those values for pure P2-NMA are obviously lower, demonstrating the much enhanced Na-storage capacity after RGO connection. At the end of the first charge process,

Figure 1. (a) XRD patterns of the pristine P2-type NMA and NMA/ RGO. (b) Crystal structure of P2-type NMA.

confirming that the liquid-state synthesis method in this work is successful. The present result indicates that the structure of asprepared NMA is P2-type and it is consistent with the reported compound with PDF number of 54-0894.35 And the crystal structure of P2-type NMA remains unchanged after ball-milling with RGO, as shown as the XRD pattern of NMA/RGO in Figure 1a. In the P2 systems of the as-prepared NMA, sodium ions occupy two distinct prismatic sites in which the NaO6 prisms share edges or faces with MeO6 octahedra, called Nae and Naf, respectively. The Nae site is more stable compared to Naf due to its longer Na-Me distance which minimizes cation repulsion (Figure 1b). Moreover, the results of ICP-AES analysis shows that the molar ratio of Na, Ni, Mn and Al is 0.667:0.333:0.556:0.111, which is nearly coincide with the elemental stoichiometry ratio of the raw materials, shown as Table S1. The microstructure of the sample was investigated using scanning electron microscopy (SEM). A typical SEM image of as prepared P2-NMA shown in Figure 2a indicates it is

Figure 2. (a and b) SEM images of pristine NMA at different resolutions. (c) TEM image of pristine P2-type NMA. (d) HR-TEM image of pristine P2-type NMA.

composed of cake-like microparticles with particle size 1−2 μm and thickness 400−500 nm. From the high resolution SEM image in Figure 2b, it is evident that the cake-like microparticles are constituted of the multilayer superposition which is consistent of the layered structure. On the other hand, the as-prepared samples after supersonic treatment presents lamelliform from the TEM image in Figure S1. Figure 2c shows the 20652

DOI: 10.1021/acsami.6b03944 ACS Appl. Mater. Interfaces 2016, 8, 20650−20659

Research Article

ACS Applied Materials & Interfaces

Figure 3. SEM image of NMA/RGO composite and the corresponding EDX elemental analysis.

Figure 4. (a) Voltage profiles of NMA and NMA/RGO tested in sodium ion cells during first charge and discharge at 0.1C; (b) The CV curves of P2-type NMA and NMA/RGO in sodium ion cells at 0.1 mV s−1, respectively. The voltage rage is 1.6−4.0 V.

0.37 Na+ remain in the structure of NMA/RGO, which can give a support to the layered structure, while at the end of the first discharge process, a total of 0.89 Na+ is found in the interlayer spacing. It means that 0.52 Na+ generates the subsequent reversible capacity. Although more Na+ could be inserted into the active materials for NMA/RGO during the discharge, the content of Na+ at the end of discharge state is less than 1. It indicates that the intercalation of more Na+ beyond 0.89 into the crystal is hindered probably due to the effect of Jahn−Teller distortion by reduction of high spin Mn4+ to Mn3+.38 Figure 4b shows the second CV curves of P2-NMA and NMA/RGO vs Na cells in the voltage range of 1.6−4.0 V at a scan rate of 0.1 mV s−1. It reveals three current pulse peaks at 1.918, 3.359, and 3.625 V in the anodic sweep and three peaks (1.702, 3.279, and 3.549 V) as well in the cathodic scan for the pristine P2-NMA, indicating a reversible sodium insertiondeinsertion process. Compared with the layered oxide of NaNi1/3Mn1/3Co1/3O2 reported by A. S. Prakash et al.,39 the less polarization of P2-NMA may be the result of substituting inactive Al for Mn and then it promotes the structural stability during the process of charge and discharge. The assignment of these peaks can be made according to literature data. The peaks located at low voltages (