Designing High-Performance Nanostructured P2-type Cathode Based

Jul 25, 2018 - Herein, nano P2-Na2/3(Mn0.54Ni0.13Co0.13)O2 (NCM) materials are ... and energy density of 148 mAh g–1 and 555 Wh kg–1, respectively...
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Article Cite This: ACS Omega 2018, 3, 8309−8316

Designing High-Performance Nanostructured P2-type Cathode Based on a Template-free Modified Pechini Method for Sodium-Ion Batteries Karthikeyan Kaliyappan,† Wei Xiao,‡ Keegan R. Adair,† Tsun-Kong Sham,‡ and Xueliang Sun†,* †

Department of Mechanical and Materials Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B9 Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7

‡

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S Supporting Information *

ABSTRACT: Layered oxides are promising cathode materials for sodiumion batteries because of their high theoretical capacities. However, many of these layered materials experience severe capacity decay when operated at high voltage (>4.25 V), hindering their practical application. It is essential to design high-voltage layered cathodes with improved stability for high-energydensity operation. Herein, nano P2-Na2/3(Mn0.54Ni0.13Co0.13)O2 (NCM) materials are synthesized using a modified Pechini method as a prospective high-voltage sodium storage component without any modification. The changes in the local ionic state around Ni, Mn, and Co ions with respect to the calcination temperature are recorded using X-ray absorption fine structure analysis. Among the electrodes, NCM fired at 850 °C (NCM-850) exhibits excellent electrochemical properties with an initial capacity and energy density of 148 mAh g−1 and 555 Wh kg−1, respectively, when cycled between 2 and 4.5 V at 160 mA g−1 along with improved cyclic stability after 100 charge/discharge cycles. In addition, the NCM-850 electrode is capable of maintaining a 75 mAh g−1 capacity even at a current density of 3200 mA g−1. In contrast, the cell fabricated with NCM obtained at 800 °C shows continuous capacity fading because of the formation of an impurity phase during the synthesis process. The obtained capacity, rate performance, and energy density along with prolonged cyclic life for the cell fabricated with the NCM-850 electrodes are some of the best reported values for sodium-ion batteries as compared to those of other p2-type sodium intercalating materials.

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INTRODUCTION Energy storage devices for large-scale applications require power packs fabricated with environmentally friendly raw materials.1 When taking into consideration the benefits of sodium such as low cost, natural abundance, and low toxicity, sodium-ion batteries (SIBs) can serve as an excellent alternative to lithium-ion batteries (LIBs), which contain toxic and expensive materials.1,2 In recent times, several materials have been identified as potential candidates for application in SIBs.3−7 Among them, layered oxides with different structural types (O3 or P2 types) show high electrochemical activity and are capable of delivering high-rate performance.8 Since P2-type materials exhibit high sodium-ion diffusion rates and low phase changes during the charge/discharge process as compared to those of O3-type materials, intensive research has been conducted to develop high-performance P2-type layered materials.8−16 Although parent P2-type layered NaMO2 (M = Fe, Co, Ni, Ti, V, Cr, and Mn) has already been studied extensively for decades and exhibited reasonable Na-ion intercalation/ deintercalation properties, toxicity and inherent electrical conductivity of these materials resulted in poor cycling performance at high current rates.7,14,17 Hence, constructing © 2018 American Chemical Society

layered materials with multitransition metal oxides has been demonstrated to improve the electrochemical performance of parent NaMO2.3,4,9,11,12 Numerous multitransition materials have been adopted as energy-storage hosts for SIB applications.2−4,8−12,15,16,18−27 All of the aforementioned electrodes had showed stable cyclic behavior at a lower cutoff voltage; however, these materials have shown poor-rate performances even at a low cutoff voltage.3,8,9,11,16,19−27 In contrast, P2-type layered materials possess large capacity fading at high-voltage cycling.28 This capacity fading is attributed to the irreversible phase transition and dissolution of active species into the electrolyte, restricting the complete utilization of cathode materials for energy storage reactions.8−10 Very few studies on the charge/discharge performance of P2-type layered materials at high cutoff voltage over 4.4 V have been conducted.8−10,28 None of these reports display stable performance at high-voltage cycling. Of late, Wang et al. studied the cyclic performance of P2-Na2/3[Ni1/3Mn2/3]O2, which yielded a capacity of ∼140 mAh g−1 at 0.1 C rate when cycled between Received: February 2, 2018 Accepted: March 19, 2018 Published: July 25, 2018 8309

DOI: 10.1021/acsomega.8b00204 ACS Omega 2018, 3, 8309−8316

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ACS Omega Scheme 1. Preparation of Nano-NCM Using PM

enhance the contact between the particles, which assists in enhancing the electrochemical sodium storage performances.33 In this study, we have made a novel attempt to design nanostructured NCM with enhanced electrochemical performance using PM (Scheme 1). Because the Na-ion diffusion in micrometric materials larger than 1 μm has very low rate capability, the utilization of nano-NCM reduces the ion migration path, thereby increasing the ability to storing more ions within the structure.10,16,23,29−31 In this article, highperformance nano-NCM with high energy density is prepared and studied for its Na-ion storage ability within 2−4.5 V at various current rates ranging from 0.16 to 3200 mA g−1 (1 C = 160 mA g−1). Moreover, the impact of temperature on the structural and electrochemical performance of nano-NCM is simultaneously studied using X-ray absorption fine structure (XAFS) and charge/discharge studies, respectively, and discussed in detail. Preparation of Nano-NCM Powders. Nano-NCM powders were prepared using a PM in which appropriate amounts of metal acetates are dissolved in 50 mL of distilled water in one beaker. In another beaker, 2.5 g of poly(ethylene glycol) (nonionic surfactant) and 5.72 g of citric acid are dissolved in 50 mL of ethanol. Then, the two solutions are mixed together and stirred at 110 °C to encourage the esterification reaction. Ethylene glycol (2 mL) and concentrated nitric acid (2 mL, 68−70%) were slowly added into the above solution during the esterification process. It can be seen that the solution quickly turns clear pink during mixing. After evaporating excess water and ethanol, the resultant precursor is decomposed at 400 °C for 4 h to remove organic moieties. Finally, the black powders obtained are calcined at different temperatures ranging from 800 to 900 °C for 12 h in air. The nano-NCM powders obtained at 800, 850, and 900 °C are labeled as NCM-800, NCM-850, and NCM-900, respectively. The structural characterization of NCM nanomaterials was carried out by X-ray diffraction (XRD) with Cu Kα radiation using a Bruker D8 Discover Diffractometer. The morphological features such as particle size and distribution are observed by field emission scanning electron microscopy (SEM, Hitachi S4800) coupled with energy-dispersive spectroscopy. The influence of temperatures on structural integrity was investigated using X-ray adsorption fine structure (XAFS) measurement at the Canadian Light Source, Saskatoon, Saskatchewan,

2.0 and 4.5 V along with capacity retention of 29% after 100 cycles.28 Lu et al. achieved a discharge capacity of 161 mAh g−1 for P2-Na2/3[Ni1/3Mn2/3]O2 at a very low current rate of C/85 within the 2−4.5 V cycling range.8 Guo et al. prepared P2- and O3-type composite electrodes as high-performance electrode materials for SIBs within a potential range of 4.5−1.5 V. Despite delivering a discharge capacity of ∼190 mAh g−1, they exhibited low-rate performance.26 It was proposed that doping of other transition metals such as Ti, Mg, Zn, and Al into the structure could improve the structural stability of sodium-based P2-type cathodes.10,16,18,29 Nevertheless, such compounds have shown poor-rate performance, which is one of the prerequisite for large-scale energy storage devices. Recently, it was reported that the design of cathode materials with off-stoichiometry could deliver better structural and cycling behavior than that with their stoichiometric counterparts.30,31 By keeping this concept in mind, we synthesize P2type Na2/3(Mn0.54Ni0.13Co0.13)O2 (NCM) as high-performance cathode materials for SIB applications. In our previous work, we succeeded in developing NCM by the sol−gel method with excellent electrochemical stability by tailoring the surface of NCM through atomic layer deposition.30 However, the sol− gel-based NCM displayed a low capacity value (120 mAh g−1 at 1 C rate) as well as poor electrochemical rate performance.32 Generally, the low electrochemical rate capability of the P2-type cathode results from its poor morphological and structural properties.9,11,12,23 It is well known that morphological factors such as particle size and distribution have great impacts on enhancing electrochemical storage behavior.30,31 Nevertheless, the synthesis of nanoscale sodium-based layered materials with even size distribution is scarce because of particle agglomeration at high-temperature calcination. Recently, the modified Pechini method (PM) has been widely utilized to synthesize oxide nanocrystals.33 The method involves an intensive blending of positive ions in a solution and controlled transformation of the mixed metal solution into a polymer gel. This is followed by the removal of the polymer matrix at a desired temperature for the development of an oxide precursor with a high degree of homogeneity.33 In addition, the coupling of the polymer and nanoparticle during the synthesis has great advantages over the conventional sol−gel method. The polymeric materials can tailor the nanoparticle surface to 8310

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Figure 1. (a) XRD patterns of nanostructure NCM particles prepared using the Pechini method at different temperatures for 12 h, and SEM images of (b) NCM-800, (c) NCM-850, and (d) NCM-900 nanoparticles.

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Canada. XAFS measurements are taken using a 06ID superconducting wiggler sourced hard X-ray microanalysis beamline with a premirror−double crystal monochromator− postmirror configuration by Si(111) crystals and Rh mirrors. The monoenergy calibration has been done using Co, Ni, and Mn foils at their corresponding edges. The beamline current and wiggler were running at 250 mA and 1.9 T, respectively. All of the XAFS measurements are taken at room temperature in transmission and fluorescence modes using a 32-element Ge detector. The cathodes with desired size are punched out from the slurry containing 70% active material (nano-NCMs), 20% acetylene black, and 10% poly(vinylidene fluoride) in Nmethyl-2-pyrrolidone coated on Al foil. The cells for electrochemical testing are constructed in an argon-filled glove box using CR2032 configuration. The cells are fabricated by sandwiching a slurry-coated cathode, a polypropylene separator (Celgard 2400), and a Na-foil anode. NaClO4 (1 M) dissolved in ethylene carbonate and diethyl carbonate (1:1, v/v) is used as the electrolyte. The cyclic voltammetry (CV) and Nyquist plots are recorded in an electrochemical analyzer (Biologic SP150, France). The charge/discharge measurements are conducted in an Arbin BT-2000 Battery Test System between 2 and 4.5 V at different current densities ranging from 16 to 3200 mA g−1 (1 C = 160 mA g−1).

RESULTS AND DISCUSSION

Figure 1a shows the XRD patterns of nano-NCM powders prepared using the PM at different calcination temperatures. All diffraction peaks can be assigned to the hexagonal structure with a P63/mmc space group (JCPDS PDF No: 194).11,12,23 Small, unindexed peaks associated with spinel NiMn2O4 impurity (PDF 01-1110) could be observed between 2θ = 35 and 38° in the XRD pattern of the NCM-800 sample.34 However, the impurity phases are gradually suppressed as the calcination temperature is increased.35,36 The suppression of impurity phase with the increasing temperature also concludes that the mixed phase turns into the hexagonal layered rock-salt phase.30 In addition, lattice parameters (ah and ch) are also increased with the increasing calcination temperature, as shown in Table 1, which is an ideal indication of increasing structural integrity and particle size of the nano-NCM material with respect to the firing temperature. Because the lattice content is strongly related to the atomic distribution, differences in lattice Table 1. Lattice Parameters and I(002)/I(104) Ratio of NanoNCM Electrodes Prepared at Different Calcination Temperatures

8311

electrode material

ah (Å)

ch (Å)

I(002)/I(104) ratio

NCM-800 NCM-850 NCM-900

2.261 2.267 2.275

11.255 11.273 11.280

3.41 4.03 1.60 DOI: 10.1021/acsomega.8b00204 ACS Omega 2018, 3, 8309−8316

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ACS Omega content reveal that the samples are synthesized with different structural integrity.31 The integrated intensity ratio I(002)/I(104) is an identification of cation mixing between the transition metal oxide and alkali metal sites.13 The I(002)/I(104) ratios for NCM-800, NCM-850, and NCM-900 are calculated as 3.41, 4.03, and 1.60, respectively. Higher ratios suggest that there is low cation mixing between the sites.13 Among the samples synthesized, NCM-900 materials show a lower I(002)/I(104) intensity ratio, implying high cation mixing in the NCM-900 sample and hence poor electrochemical performance could be expected from the NCM-900 sample. It is believed that fractional cation mixing and highly crystalline nature of cathode materials are essential characters for exhibiting good electrochemical cycling performance and structural stability during cycling.13,30,31 The Rietveld refinement patterns of samples are presented in Figure S1, and their crystallographic data are listed in Table S1. As seen in Figure S1, the difference between experimental and calculated patterns is very small. Moreover, the low reliability factor values in Table S1 (Rwp and Rp) demonstrate a successful refinement. The morphologies of the NCM samples are presented in Figure 1. All particles are appearing to consist of aggregates of sub-micrometer-sized particles with different morphologies. As expected, the particle size and crystallinity are increased with the increasing temperature. It can be observed from Figure 1 that the particle sizes gradually increased from 150 to 650 nm with unchanged particle shapes. However, particle sizes rapidly increased at 900 °C and their shapes changed greatly, which may be due to the agglomeration of particles during hightemperature calcination.10,13,19,31,37 In addition, the lattice and I(002)/I(104) intensity ratio parameters are listed in Table 1, clearly showing some structural changes when the calcination temperature reached 900 °C. It has been reported that electronic migration and ionic migration are significantly influenced by the morphological features of the cathode materials.10,17,22,28 Because the nanostructured materials with uniform distribution can minimize the pathway for Na-ion diffusion, enhanced Na-ion storage is expected for the NCM850 sample.30,31 Although the cathode powder calcined at 900 °C displays the best layered hexagonal structure and crystallinity as observed from the XRD patterns, it is not the best material in terms of energy storage capability, which will be discussed in the following sections. The inductively coupled plasma (ICP)-atomic emission spectroscopy results of NCM samples are presented in Table S2, implying that the experimental values are close to the calculated values. X-ray absorption near-edge spectroscopy (XANES) analysis is used to demonstrate the changes of the local ionic state around the Co, Mn, and Ni atoms in nano-NCM powders calcined at various temperatures, and the corresponding K edges together with Li(Ni0.33Mn0.33Co0.33)O2 (LNMC) standard compounds are illustrated in Figure 2. All spectra display a pre-edge (denoted #) weak adsorption peak, corresponding to the electronic excitation of the 1s core to unoccupied 3d orbitals of the transition metal ions. This is also ascribed to the orbital mixing between 3d and 4p, which results from the uneven centrosymmetric nature of the distorted octahedral 3a site in the rhombohedral R3m space group.38 Consequently, information about the site symmetry, degree of distortion in the octahedra, and the oxidation state of the core atoms could be attained from these pre-edge peaks.39,40 The main absorption peaks in Figure 2 (denoted *) are from dipole-allowed transitions of 1s core electrons to unoccupied 4p states without

Figure 2. XANES of (a) Mn K edge, (b) Co K edge, and (c) Ni K edge peaks of NCM materials before the cycling process. The spectrums are compared with standard LNMC spectrums.

any shakedown processes. As seen from Figure 2b, Co K edge spectra of all samples are unchanged and almost identical to the Co3+ standard LNMC spectrum. This states that the local ionic structure around the Co atom is unchanged greatly with the calcination temperature and mostly exists in the +3 state.40 Similarly, the Mn K edge peak position of nano-NCM powders in Figure 2a does not shift much and predominantly consists of Mn4+ ions.41,42 However, it can be seen that the pre-edge intensity (inset in Figure 2b) slightly increased for the samples calcined at 900 °C, signifying increased local structural distortion in the Mn−O6 octahedra for the NCM-900 sample.11,39 In contrast to the Co and Mn K edge spectrums, the Ni K edge spectra of NCM-800 and NCM-900 powders show an obvious peak shift toward higher energy, whereas the spectrum of NCM-850 does not exhibit any energy shift compared with 8312

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Figure 3. (a) CV curves of nano-NCM electrodes between 2 and 4.5 V at 0.1 mV s−1 scan rate, (b) Nyquist plots of nano-NCM electrodes measured at open circuit voltages, and (c) equivalent circuit that was used to fit the Nyquist plots presented in (b).

the electrochemical reaction, thereby decreasing Na-ion storage capability, agreeing well with the XAFS studies.13 The superior electrochemical behavior of the NCM-850 electrode originates from the small particle size with large exposed reaction sites as well as negligible cation mixing during the preparation process.30,31 On the other hand, uniform distribution of NCM-850 particles increases the contact at the particle− particle and the particle−current collector interfaces and hence charge storage characteristics are improved.9,13,31,37 Figure 3b illustrates the Nyquist plots of nano-NCM electrodes recorded between 200 kHz and 10 mHz frequency range at open circuit voltage. The Nyquist plots are fitted according to the equivalent circuit presented in Figure 3c, and the parameters are listed in Table 2. As seen from Figure 3b,

the standard spectrum. The shift in energy demonstrates that the oxidation state of Ni in NCM-800 and NCM-900 electrode materials is slightly increased at high temperatures.40,42 Moreover, an obvious increase in the pre-edge peak for the sample calcined at 900 °C is observed, which results from increased local structural distortion of the Ni−O6 octahedra because of the increased amount of Jahn−Teller active Ni3+ ions. On the other hand, powders calcined at 850 °C have a low pre-edge intensity and exhibit lower cation disordering among other samples, as shown in Figure 2a−c. This outcome is correlated well with the data obtained from XRD analysis. The XANES results reveal that the sample prepared at 900 °C would deliver low discharge capacity because of the higher initial oxidation state of Ni ions and high structural disorder, which limit the use of the Ni2+/4+ redox couple during the charge/discharge process.39−42 To study the Na-ion storage mechanism, CV studies of nanoNCM electrodes have been carried out between 4.5 and 2 V at a 0.1 mV s−1 scan rate, and the curves are presented in Figure 3a. All CV curves display three redox peaks at around 2.3, 3.6, and 4.3 V, which can be attributed to the oxidation/reduction reaction of Mn3+/4+, Co3+/4+, and Ni2+/4+ couples, respectively.11,20 This confirms that the valence states of Co, Ni, and Mn in NMC powders would be at +3, +2, and +4, respectively, which will also be confirmed in charge/discharge experiments. Furthermore, half-cells fabricated with the NCM-850 electrode have strong current response and capacity, whereas the NCM-900 electrode shows poor electrochemical storage behavior. In addition, NCM-800 electrodes show a predominant peak shift during positive and negative sweeps because of the polarization of the Ni2+/Ni4+ redox couple. Consequently, the NCM-900 electrode displays suppressed peaks associated with the Ni2+/Ni4+ redox reaction, as presented in Figure 3a. This restricts full participation of the Ni2+/Ni4+ redox couple in

Table 2. Interfacial Resistance (Rsf), Charge Transfer Resistance (Rct), Discharge Capacities, and Energy Densities of Nano-NCM Electrodes electrode material

Rsf (Ω)

Rct (Ω)

discharge capacity (mAh g−1)

energy density (Wh kg−1)

NCM-800 NCM-850 NCM-900

65.45 78.72 157.24

938.6 406.05 2376.48

108 148 88

405 555 330

the Nyquist spectrum contains three predominant regions including two semicircles and an inclined line. The semicircle in the high- and intermediate-frequency regions is related to the Na-ion migration resistance (Rsf) through the electrode− electrolyte interfacial (EEI) film and the charge transfer resistance (Rct) in the EEI film, respectively. An inclined line in the low-frequency region is related to the Na-ion diffusion in the bulk of the electrode active material. It is clear that the NCM-900 electrode has high Rsf compared to that of other 8313

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electrode results from the uniformity of the nanostructured material, which enables faster ionic movement and also enhances the contact between the active materials, thereby improving the capacity value.30,43 The cycling performance of Nano-NCM electrodes recorded at 160 mA g−1 is presented in Figure 4b. Although all electrodes show a linear decrease in capacity value during the initial few cycles, the cyclic performance becomes stable during subsequent cycles. This capacity fading during initial cycles is mainly associated with small structural distortion and/or side reactions during the charge/discharge process.13,26,31 It is clear from Figure 4b that the NCM-850 electrode has enhanced cyclic stability along with high capacity compared to those of other nano-NCM electrodes. It is apparent that the electrochemical stability is irrevocably linked with the particle size and the degree of agglomeration. Suryanarayana et al. reported that nanocrystalline materials prepared with uniform grain size and distribution could demonstrate enhanced ion diffusivity and higher thermal coefficients than those of conventional materials.45 The cell fabricated with NCM-800 suffers from significant capacity fading at 160 mA g−1, whereas the sample calcined at 900 °C possesses much better cycle retention. The enhanced electrochemical performance of the NCM-850 electrode even at high cutoff operation can be assigned to its improved structural integrity with low cation mixing.13,30 The difference in cycling performance results from the cation mixing as well as from the degree of crystallinity, as determined from the XRD and XANES studies. Although the NCM-900 electrode delivers low capacity of 88 mAh g−1 at 160 mA g−1, it shows the highest capacity retention rate. This is because of its well-developed layered structure and highly crystalline nature, which assist in maintaining the structural stability during charge/discharge studies. Figure 4c shows the rate performance of the cells at various current densities. The NCM-850 electrode delivers excellent rate performance with the capacities of 168, 145, 115, 95, and 66 mAh g−1 at 0.16, 160, 800, 1600, and 3200 mA g−1 current densities, respectively. On the other hand, the NCM-800 and NCM-900 electrodes deliver only about 25 and 14 mAh g−1 at 3200 mA g−1 current densities, respectively. In addition, when the current density is reverting to 16 mA g−1, almost 95% of original capacity is retained, as presented in Figure 4c. It is clear that the NCM-850 electrode is capable of achieving more than twice the capacity values of the other nano-NCM electrodes, especially at high current density. The comparison of the discharge capacity of NCM-850 with that of various P2 layered materials is presented in Figure S3. To the best of our knowledge, the electrochemical behaviors such as cycling stability and rate performance at high cutoff voltage obtained from nano-NCM in this study are the best among those reported thus far for sodium-based P2-type intercalation electrodes.2−4,8−13,15,16,19,23,24,28,38,46−48 A maximum capacity of ∼185 mAh g−1 at C/30 was obtained from the layered NaNi0.5Mn0.5O2 electrode along with 16% capacity retention after 20 cycles.49,38 P2-Na2/3[Ni1/3Mn2/3]O2 (1/3 < x < 2/3) material delivered 161 and 141 mAh g−1 at C/85 and C/30 C within 2−4.5 V and experienced large capacity fading after the cycling process.8,28 Interestingly, the capacity achieved for nano-NCM at high current densities (75 mAh g−1 at 3200 mA g−1) in the present investigation has also outperformed the rate performance of Al-, Ti-, Mg-, and Zn-doped P2-type layered cathode materials. For instance, layered materials such as Na x Mg 0.11 Mn 0.89 O 2 (35 mAh g −1 at 610 mA g −1 ),

materials, which may be due to the change in the interfacial structure that influences the properties of the EEI layer such as thickness and density.30 Among the electrodes, NCM-850 exhibits a lower Rct value of 406.05 Ω, as shown in Table 2. This is owing to its uniform morphological feature, facilitating faster ionic migration. It is worth mentioning here that the electrode with a lower Rct value would have exhibited excellent electrochemical performance.43 Because lowering the resistance directly increases the current flow on the electrode surface, the diffusion rate of Na ions toward the electrode is enhanced and thus improves the sluggish kinetic behavior and increases the charge/discharge properties of NCM-850 electrodes.30,43 The initial charge/discharge curves of nano-NCM electrodes conducted within the 2−4.5 V window at a current density of 160 mA g−1 are presented in Figure 4a. The curves show three

Figure 4. (a) Charge−discharge studies and (b) cycle life of nanoNCM materials at a current density of 160 mA g−1 within 2−4.5 V and (c) rate performance of the electrodes at different current densities ranging from 16 to 3200 mA g−1 (1 C = 160 mAh g−1).

redox plateaus during the charge and discharge processes. The plateaus around ∼2.3, 3.6, and 4.2 V are ascribed to the redox reaction of Co3+/4+, Mn3+/4+, and Ni2+/4+ couples, correlating well with CV results.12,15 It can be seen that irreversible capacity losses of 75, 22, and 30 mAh g−1 could be observed for the NCM-800, NCM-850, and NCM-900 electrodes, respectively. This demonstrates the pre-existence of metal ions in the Na ion layer, which leads to the formation of inactive regions during the first charge/discharge process.13,26,31 However, the columbic efficiency of all cells reach more than 99% after 100 cycles (Figure S2). As seen from Table 2, the NCM-900 electrode delivers low capacity value of 88 mAh g−1 at 160 mA g−1 current density because of the large charge transfer resistance, as listed in Table 2. On the other hand, NCM-800 and NCM-850 electrodes exhibit discharge capacities of 108 and 148 mA g−1 under the same testing conditions. The discharge capacity obtained from the NCM-850 electrode is one of the highest reported values for layered P2-type cathodes in SIBs, especially at high current values.11,12,15,16,21,23,26,39,40,44 The enhanced electrochemical behavior of the NCM-850 8314

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ACS Omega Na0.67[Mn0.65Co0.2Ni0.15]O2 (65 mAh g−1 at 400 mA g−1), Na2/3Co2/3Mn2/9Ni1/9O2 (60 mAh g−1 at 252 mA g−1) Na0.45Ni0.22Co0.11Mn0.66O2 (59 mAh g−1 at 610 mA g−1), Na0.66Ni0.33−xZnxMn0.67O2 (55 mAh g−1 at 748 mA g−1), and Na2/3Ni1/3Mn1/2Ti1/6O2 (93 mAh g−1 at 484 mA g−1) had exhibited lower rate performance compared to that in the present investigation.9−11,15,16,50 In addition, the NCM-850 electrode delivers an energy density of 555 Wh kg−1 (calculated on the basis of an average voltage of 3.75 V), which is much higher than by many other SIB cathode materials and also surpasses the energy density of commercial lithium-ion battery cathodes like LiFePO4 (530 Wh kg−1) and LiMn2O4 (450 Wh kg−1).9,40,51,52,25 The outstanding electrochemical Na-ion storage behavior of NCM-850 can be attributed to the following reasons: (i) the highly crystalline nature and structural integrity along with low cation disorder of the NCM-850 material, which can facilitate the ionic and electronic migration especially at high current densities; (ii) the nanoparticles with preferred surface morphology enhance the availability of electrochemical reaction sites and the contact between the particles and the particles/ current collector, improving the electrical conductivity and capacity; (iii) evenly distributed particles also shorten the diffusion length for Na-ion diffusion, stabilize the EEI layer, and enhance its stability; and (vi) highly interconnected particles can accommodate more electrolyte within its structure and eliminate the inherent mechanical stress during the electrochemical process, thus improving the rate performance. This justification is well agreed with the results obtained from the charge/discharge and rate performance studies. These results clearly demonstrated that the PM could significantly improve the electrochemical reaction kinetics, energy density, and cyclic stability of the P2-NCM cathode material for high-performance SIB applications.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Research Chair (CRC) Program, the Canada Foundation for Innovation (CFI), the Ontario Research Fund (ORF), the Canadian Light Source (CLS), the University of Western Ontario, and the National Natural Science Foundation of China (NSFC) (grant no. 51672189). K.K. was supported by the MITACS.

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CONCLUSIONS Nanostructured Na2/3(Mn0.54Ni0.13Co0.13)O2 (NCM) electrode materials with improved sodium-ion storage capability have been prepared by a modified Pechini method. XAFS and XRD results reveal that the NCM material calcined at high temperatures has high structural disorder as well as higher Ni oxidation states, which results in poor electrochemical performance. Of the samples tested, NCM prepared at 850 °C delivers the highest capacity of 147 mAh g−1 along with a stable cyclic performance at 160 mA g−1 in the 2−4.5 V range. Although the material prepared at 900 °C exhibits a lower capacity of 88 mAh g−1 at 160 mA g−1, it displays good capacity retention during charge/discharge studies because of its superior structural integrity. We observed that the P2 cathode fabricated through the modified Pechini method shows favorable sodiumion storage performance with much alleviated degradation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00204. XRD refinement, ICP results, columbic efficiency of the electrodes, and lattice parameters (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 8315

DOI: 10.1021/acsomega.8b00204 ACS Omega 2018, 3, 8309−8316

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DOI: 10.1021/acsomega.8b00204 ACS Omega 2018, 3, 8309−8316