Improved Cycling Stability of Na-doped Cathode Materials Li1.2Ni0

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Improved Cycling Stability of Na-doped Cathode Materials Li1.2Ni0.2Mn0.6O2 via a Facile Synthesis Yunjian Liu, Dongming Liu, Hong-Hui Wu, Xiaojian Fan, Aichun Dou, Qiaobao Zhang, and Mingru Su ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02552 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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ACS Sustainable Chemistry & Engineering

Improved Cycling Stability of Na-doped Cathode Materials Li1.2Ni0.2Mn0.6O2 via a Facile Synthesis

Yunjian Liu,1 Dongming Liu,1 Hong-Hui Wu,3 Xiaojian Fan,1 Aichun Dou,1 Qiaobao Zhang,2,* Mingru Su1,* 1

School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China

2

Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen, Fujian 361005, China

3

Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588, United States

*Corresponding author: E-mail: [email protected] Phone:+86-511-88790190; (Mingru Su) *Co-Corresponding author: E-mail: [email protected] Phone: +86-15750725796. (Qiaobao Zhang)

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Abstract：Lithium-ion battery cathode materials Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10) were synthesized by introducing Na ions into the Li layer through a facile ball-milling method. XRD results reveal that the cathode materials Li1.2-xNaxNi0.2Mn0.6O2 display a typical layered structure. The enlarged Li layer spacing was confirmed by the characterization of morphology and structure. The Li1.12Na0.08Ni0.2Mn0.6O2 electrode shows an excellent electrochemical performance including high reversible discharge capacity (257 mAh g-1), enhanced rate capability (112 mAh g-1 at 5 C) and superior cycling stability (100% capacity retention after 50 cycles, 96% capacity retention after 100 cycles). The improved electrochemical performance of the Na-LNMO sample compared to the pristine LNMO sample mainly stems from the Na-doping which stabilizes the host layered structure by suppressing the phase transformation from layered to spinel structure during cycling. Moreover, the EIS results also confirm that Na-doping effectively decreases the charge transfer resistance and facilitates the Li diffusion of the as-prepared cathode material. This method provides novel insights into enhancing the electrochemical performance and preventing the high-performance layered electrode materials from structural degradation.

Keywords: lithium-ion battery, cycling stability, Na-doping, electrochemical performance

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Introduction Rechargeable lithium ion batteries have been developed to power an extensive range of applications, from electric vehicle (EV) to cell phones, owing to their high energy density and low cost.1-2 Recently, the layered lithium-rich manganese-based cathode materials xLi2MnO3·(1-x)LiMO2 (M= Ni, Co, Mn, etc.), which offer fascinating advantages over the conventional commercial cathode materials like LiCoO2, LiMn2O4 and LiFePO4,3-7 have attracted extensive interest as one of the most prospective candidates for the next generation commercial cathode materials. However, there are several serious issues hindering their practical commercialization: (1) low initial coulombic efficiency, due to the generation of oxygen vacancies and irreversible capacity loss during the first cycle;8 (2) poor rate performance, attributed to the poor surface conductivity which is caused by the solid electrolyte interface (SEI) film and its low electronic conductivity;9 (3) insufficient capacity retention, owing to layered-to-spinel phase transformation during long cycling.10-11 To solve these problems, various strategies are proposed to improve the electrochemical performance. The first possible attempt is surface coating. As reported, a series of compounds have been applied to coat the cathode materials, such as inert oxides12-14 (ZrO2, Al2O3, TiO2), metal phosphate15-16 (AlPO4, CoPO4) and metal fluoride17-19 (AlF3, CaF2). Coating constitutes a surface with higher electrical conductivity to improve the poor rate capability.20-21 In addition, cation doping is another feasible strategy to stabilize the crystal structure, resulting in an improvement of the electrochemical performance. Up to now, researchers have successfully introduced some cations into the bulk lattice,

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such as Al, Cr, Zr and Fe to replace lithium or substitute the transition metal like Ni, Co or Mn, leading to an enhanced coulombic efficiency and a stable structure.22-26 Considering the different efficiency and cost of the various solutions above, a befitting modified method is very crucial for the breakthrough of the cathode materials. Recently, a better coulombic efficiency, higher discharge capacity and enhanced lithium diffusivity have been achieved by introducing cations with similar radius to the Li ion such as Na, K and Mg into Li sites. Cao et al. synthesized Na-stabilized layered Li1.2[Co0.13Ni0.13Mn0.54]O2 with the polymer-pyrolysis method.27 Lim et al. fabricated Li1.167-xNaxNi0.18Mn0.548Co0.105O2 by spray pyrolysis28 and Wang et al. synthesized Na-doped Li1.4[Mn0.6Ni0.2Co0.2]O2 using a solvothermal method.29 Though the initial coulombic efficiency and reversible capacity of the as-prepared Na-doped lithium rich cathode materials can be improved to some extent, the cycle stability especially the long cycle stability of these cathode materials is still far from the commercial application. Besides, these Na-doping processes are complex, costly. Furthermore, there is a discovery should not be neglected. Kim et al. observed unidentified impurities during synthesizing a lithium-rich nickel-manganese oxide compound using Na2CO3 and Li2CO3.30 Liu et al. also identified a trace of impurity phases

when

they

increased

the

doped

Na+

contents

in

the

Li1.2-xNaxNi0.13Co0.13Mn0.54O2 cathode materials with NaOH and LiOH.31 Meanwhile, these impurities have been confirmed to play an important role on the electrochemical performance. In light of these concerns, it is necessary to prepare the Na-doped Li-rich cathode materials with a well-defined layered structure by a facile and

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industrialized method, such as ball milling. Moreover, the quantitative effects of Na contents on the electrochemical performance and structural stability of the layered lithium-rich Mn-based oxide materials are still very lacking and desirable. Since a lower melting-point is beneficial for the decomposition and crystallization at lower temperature, the lithium acetate (280 °C) and sodium acetate (300 °C), which have lower melting-point than the reported ingredients like lithium carbonate (720 °C), lithium hydroxide (462 °C), sodium nitrate (324 °C) and sodium carbonate32 (851 °C) are chosen as ingredients in the current work. Na was introduced into the Li layer via a facile ball-milling method following high temperature solid state reaction to synthesize a series of layered Li-rich Mn-based cathode materials Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10), the microstructure, Li diffusion, structural stability and electrochemical performance were also systematically investigated.

Experiment Synthesis of materials A simple ball-milling procedure followed by a high temperature solid state reaction was used to fabricate the pristine sample Li1.2Ni0.2Mn0.6O2 (LNMO) and Na-doped samples Li1.2-xNaxNi0.2Mn0.6O2 (Na-LNMO). The synthetic process was as follows: (1) the co-precipitated hydroxides of Ni and Mn were synthesized by adding dissolved transition metal acetates into a 0.1 M NaOH solution drop by drop, and then the hydroxides were dried at 110 °C for 12 h to get the final co-precipitation precursor Ni0.5Mn1.5(OH)4; (2) stoichiometric amounts of C2H3O2Li·2H2O (5% excess), 5



CH3COONa·3H2O and Ni0.5Mn1.5(OH)4 were dispersed into alcohol to generate a homogeneous slurry; (3) the as-prepared homogeneous mixture was then milled for 6 h, with a revolving of 300 r min-1, the obtained slurry was dried at 100 °C for 24 h; (4) the obtained powder was decomposed at 500 °C for 4 h, and finally the particles were annealed at 900 °C for 9 h in air to form the final Li1.2-xNaxNi0.2Mn0.6O2 (x= 0, 0.03, 0.05, 0.08, 0.10) cathode materials.

Characterization of materials The structure of as-prepared materials were characterized by powder X-ray diffraction (XRD, D/MAX2500PC) using a Cu Kα radiation source in the two-theta range of 10 ° to 90 °. The morphology of the powders was captured by a field-emission scanning electron microscope (SEM, JEOL-2100F). Transmission electron microscope (TEM, Hitachi-7650), X-ray Photoelectron Spectrum (XPS, 250XI) and Energy dispersive spectroscopy (EDS) were also employed for further analysis. To prepare a positive electrode film, 80 wt% of prepared active material powders were mixed with 10 wt% of acetylene black and 10 wt% of polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP) until slurry was obtained. Then, the blended slurry was coated onto an aluminum foil, followed by drying at 120 °C in vacuum. The cell for tests was constituted by a positive electrode and lithium foil negative electrode. The positive and negative electrodes were separated by a porous polypropylene film. The solvent including 1 mol L-1 LiPF6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / dimethyl carbonate (DMC) (1:1:1 in

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volume) was selected as the electrolyte. The electrode was cut into rounded pieces with an area of 1.13 cm2 and pressed under 15 MPa for 1 min. The electrode weight was measured by microbalance. Assembled Cells were examined between 2.0 and 4.8 V at room temperature by NEWARE battery circler. The electrochemical impedance spectroscopy (EIS) results were collected in the frequency range of 0.01 Hz – 100 kHz with a CHI660D electrochemical analyzer.

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Results and discussion The XRD patterns of Li1.2-xNaxNi0.2Mn0.6O2 (x = 0, 0.03, 0.05, 0.08, 0.10) materials are shown in Fig. 1(a). These main diffraction peaks of all patterns are well indexed to the standard diffraction pattern of hexagonal a-NaFeO2 structure (R-3m spacegroup), which correspond to the LiMO2 (M = Ni and Mn) phase. There are a few small additional peaks around 2θ = 20-23° appearing in all patterns, which is related to superstructure reflections of Li2MnO3 (C2/m space group). These reflections indicate the cation ordering of Li, Ni and Mn ions in the transition metal layers. It is obviously that both the (006)/(012) and (018)/(110) peaks are completely spilt, which indicates a well-defined layered structure in the lattice. In comparison with the patterns of pristine LNMO, no other peaks can be detected in the Na-LNMO samples, suggesting that Na ions are successfully introduced into the crystal lattice without any destruction to the layered structure. A careful dissection of the XRD patterns is shown in the Fig. 1(b) and (c). As shown in the enlarged sections, the (003) peak and (104) peak shift slightly toward the lower angle area with Na-doping. This observation implies that the Na ions are successfully doped into the bulk lattice so as to enlarge the Li slab space, which is caused by the difference of ionic radii between the Na ion (rNa+= 1.02 Å) and Li ion (rLi+= 0.76 Å).

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(a)

(b)

(d)

(e)

(c)

Fig. 1 (a) - (c) XRD patterns of Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10); (d) - (e) Rietveld refinement patterns of Li1.2Ni0.2Mn0.6O2 and Li1.12Na0.08Ni0.2Mn0.6O2. with the composition of x=0 and 0.08, respectively.

Table 1 Rietveld refinement results of Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10) based on R-3m structure. Atom

Site

Li1 Ni1 Na1 Li2 Ni2 Mn1 O a c c/a Rw (%) Rp (%)

3a 3a 3a 3b 3b 3b 6c

Occ X=0

X = 0.03

X = 0.05

X = 0.08

X = 0.10

0.983 0.017 0 0.217 0.183 0.600 1 2.85509 14.24394 4.9889 8.03 4.74

0.954 0.016 0.030 0.216 0.184 0.600 1 2.85775 14.24529 4.9847 8.74 4.32

0.936 0.014 0.050 0.214 0.186 0.600 1 2.85988 14.24802 4.9820 8.73 4.32

0.910 0.010 0.080 0.210 0.190 0.600 1 2.86231 14.26211 4.9827 8.74 4.32

0.876 0.024 0.100 0.224 0.176 0.600 1 2.87974 14.27730 4.9578 8.44 4.32

Rietveld refinement results of the as-prepared Li1.2-xNaxNi0.2Mn0.6O2 (x = 0, 0.03,

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0.05, 0.08, 0.10) samples are tabulated in Table 1. The Li/Ni mixing level is reduced with the Na doping process. The downward degree of Li/Ni mixing could be explained that the larger driving force generated by Na+ ions make the Li+ ions separate from the transition metal layer, further hindering the Li/Ni disorder. In addition, it is found that Na-LNMO samples have larger volume than the pristine one. The a-axis constants, which are ascribed to the metal-metal interslab distance and the c-axis constants, which are usually related to the Li slab space, both increase with the increasing amount of doped Na ions. The bigger ionic radius of the Na ion (1.02 Å) compared with the Li ion (0.76 Å) is responsible for the expansion of the lattice parameters. The enlargement of lattice constants is beneficial for the diffusion of lithium ions, contributing to an enhanced electrochemical performance. And all the c/a ratios are more than 4.9, further demonstrating that a high ordered layer structure are obtained for all samples.

Fig. 2(a) presents the SEM images of the Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10) powders. All the samples show a similar morphology as nano-particles with the unified size distribution of 200 - 800 nm without noticeable differences between pristine LNMO and Na-LNMO samples. To further investigate the effect of Na ions on morphology and particle size, the BET test was carried out for the Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10) powders. As the BET results revealed, the surface area of Na-doped samples was decreasing with the increase of Na content. Compared with the surface area of pristine sample (14.337 m2·g-1), the smaller surface area of 5% Na-LNMO (12.733 m2·g-1) and 8% Na-LNMO (10.714

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m2·g-1) sample is considered as one of the reasons for the improved electrochemical properties. In addition, as displayed in Fig. 2(c) and (d), the 5% Na-LNMO and 8% Na-LNMO samples appear as homogeneously distributed particles with well-defined morphologies, whereas the 10% Na-LNMO sample exhibits fuzzy shape to some extent. EDS spectrum of Li1.2-xNaxNi0.2Mn0.6O2 indicates that Ni and Mn uniformly distributed in all samples. Sodium signal appeared in the samples of x = 0.03, 0.05, 0.08 and 0.10, which indicates Na was doped in the bulk structure. EDS analysis of element amount of Mn/Ni/Na are given in the Table 2. Ignoring the allowable error of the EDS tests and maintaining a constant Mn molar coefficient as 0.600, the calculated molar ratios are 0.600/0.226/0, 0.600/0.196/0.024, 0.600/0.201/0.046, 0.600/0.196/0.071 and 0.600/0.188/0.092 for Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10) respectively. These values are in good agreement with the expected coefficients ratios. As shown in Table 3, the chemical composition of all these as-prepared samples was analyzed by inductive coupled plasma mass spectrometry (ICP-MS). According to the ICP results, all samples exhibit similar element content to the theoretical stoichiometry. Compared with pristine sample, the Na content is increasing from 0 to 0.108 with the Na-doping process. Meanwhile, the molar number of Li is decreasing regularly. The ICP results are in good agreement with the EDS results and the amount of NaAC in the doping process.

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Table 2 EDS analysis of element amount of Mn/Ni/Na in Li1.2-xNaxNi0.2Mn0.6O2 (x = 0, 0.03, 0.05, 0.08, 0.10) Atomic%

Molar Ratio

Samples O

Mn

Ni

Na

(Mn/Ni/Na)

X=0

80.41

14.23

5.37

-

0.600/0.226/0

X=0.03

79.11

16.11

5.27

0.64

0.600/0.196/0.024

X=0.05

79.75

14.64

4.91

1.12

0.600/0.201/0.046

X=0.08

79.07

15.52

5.08

1.83

0.600/0.196/0.071

X=0.10

77.43

15.45

4.84

2.38

0.600/0.188/0.092

Table 3 Results of the ICP-MS analysis for Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10) Measured stoichiometry (Ref. O=2) Sample

Target stoichiometry

Li

Ni

Mn

O

Na

0

Li1.2Ni0.2Mn0.6O2

1.185

0.205

0.616

2

0

0.03

Li1.17Na0.03Ni0.2Mn0.6O2

1.161

0.210

0.619

2

0.027

0.05

Li1.15Na0.05Ni0.2Mn0.6O2

1.143

0.216

0.610

2

0.052

0.08

Li1.12Na0.08Ni0.2Mn0.6O2

1.110

0.207

0.608

2

0.077

0.10

Li1.10Na0.10Ni0.2Mn0.6O2

1.089

0.211

0.613

2

0.108

12


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(b)

(a)

(c)

(d)

(e)

Fig. 2 (a) SEM images and the corresponding EDS elemental mapping of Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10); TEM images and HRTEM images with corresponding indexed FFT of samples (b, c) LNMO; (d, e) 8% Na-LNMO

To confirm the detailed structure and accurate morphologies of samples, TEM analysis was carried out. The detailed structure of the pristine LNMO and Na-LNMO nano-particles are demonstrated in the Fig. 2(b - e). Obviously, both the shape of the LNMO and Na-LNMO particles is regular. In addition, clear lattice fringes can be 13



measured from the high resolution TEM images, these fringes with d-spacing of 0.47 nm are well matched to the interplanar distance of the (003) plane of the α-NaFeO2 layered structure inferred from XRD patterns. The inserted FFT images correspond to the selected area in Fig. 3(c) and (e). The ordered electron diffraction spots also demonstrate that a single-crystalline structure with a high crystallinity is obtained for the individual particles of the LNMO and Na-LNMO samples. This observation is in good accordance with the XRD results.33 X-ray Photoelectron Spectrum (XPS) technology was employed to determine the chemical states of the constituent elements. The XPS spectra of C 1s, O 1s, Mn 2p, Ni 2p and Na 1s orbitals for the pristine LNMO and doped 8% Na-LNMO samples are exhibited in the Fig. 3. It can be seen from Fig. 3(b) that the Ni 2p3/2 binding energies of the two samples are in accordance with the standard Ni2+, which are theoretically located at 854.4 eV. The observed binding energies for Mn 2p3/2 (642.2) revealed that the oxidation states of Mn in the pristine LNMO and Na-LNMO samples are 4+. Furthermore, as shown in Fig. 3(d), the peak of Na 1s can be observed in the sample of 8% Na-LNMO. The peak position appears at a binding energy of approximately 1072.89 eV, which is close to the peak position of the binding energy of Na+ oxide, suggesting that the Na element appearing as Na+ in the Na-LNMO samples. The XPS results indicate again that the Na element has been successfully introduced into the bulk phase as Na+, and the introduction of Na+ has no influence on the oxidation state of the transition metal ion.34

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(a)

(b)

(c)

(d)

Fig. 3 XPS spectra of (a) survey spectrum, (b) Ni 2p, (c) Na 1s, and (d) Mn 2p for the LNMO and 8% Na-LNMO samples. (a)

(b)

(c)

(d)

Fig. 4 (a) Initial charge-discharge curves (b) Cycling performance and (d) Rate performance of the Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10); (c) Cycling performances of the LNMO and 8%

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Na-LNMO electrodes

Table 4 Initial charge/discharge capacity of the Li1.2-xNaxNi0.2Mn0.6O2 (x = 0, 0.03, 0.05, 0.08, 0.10) Samples

Initial charge capacity

Initial discharge capacity

Coulombic efficiency

0

301

241

77.4

0.03

311

246

79.1

0.05

315

250

79.4

0.08

320

257

80.3

0.10

289

226

78.2

The initial charge and discharge profiles at 0.1 C are plotted in Fig. 4(a). The corresponding initial charging/discharging data were summarized in Table 4. The pristine sample LNMO delivers a high initial charge/discharge capacity of 301 mAh g-1 and 241 mAh g-1with an initial coulombic efficiency of 77.4%. With the amount of Na increase, the discharge capacity at 0.1 C increases to 246 mAh g-1 for 3% Na-LNMO sample, 250 mAh g-1 for 5% Na-LNMO sample, 257 mAh g-1 for 8% Na-LNMO sample, respectively. The initial charge profiles of all samples similarly present a slope below 4.5 V and another high potential plateau at 4.5 V. During the first charge process, the first voltage ramp is related to the reversible extraction of Li+ from the Li-layers and the second long potential plateau can be attributed to the Li2O extraction from Li2MnO3 components. Generally, the initial charge/discharge capacities for the 3%, 5%, 8% Na-LNMO samples are better than the pristine sample. The reason for the improved coulombic efficiency of Na-doped samples can be explained in two aspects. One is attributed to the highly ordered and stable structure 16


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caused by the Na-doping process. The unchangeable doped Na ions with larger ionic radius can suppress collapse of the lattice structure, resulting in a mitigation of irreversible removal of Li2O in the first cycle. Another is that the incorporation of Na leads to a further activation of the reduction of manganese component. Furthermore, there is an obvious enlargement of the reversible capacity in the range of 3.25 V to 2.0 V for the Na-doped samples during the first discharging. To evaluate the effects of Na doping on the cycling stability, all the cathode materials with different Na amounts were measured in the voltage range from 2.0 to 4.8 V at 25 °C. An activation process was carried out at 0.1 C for 5 cycles before all the samples were tested at 0.2 C. The discharge capacities of Na-doped electrodes (3%, 5%, 8%) in Fig. 4(b) are higher than the pristine sample. Among these samples, the 8% Na-LNMO electrode delivers the best cycling capacity of 235 and 235 mAh g-1 at the 1st and 50th cycles, respectively. And its retention ratio is about 100%. Contrastively, the pristine LNMO sample exhibits a lower discharge capacity of 206 mAh g-1 after 50 cycles with a capacity retention ratio of 92.7%. Furthermore, sample LNMO and 8% Na-LNMO were selected for long-term cycling test at 0.2 C and 1 C, respectively. The long-term cycle profiles are plotted in Fig. 4(c). When these cells were tested at 0.2C, the discharge capacity of 8% Na-LNMO sample after 100 cycles approached as high as 224 mAh g-1 with high capacity retention of 96% up from 184 mAh g-1 and 82% for LNMO sample. Surprisingly, when these cells were tested at 1 C, the 8% Na-LNMO sample could still deliver a high discharge capacity of 180 mAh g-1 within 50 cycles. The reason for the greatly enhanced cycle stability for the Na-doped electrodes (a low

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capacity reduction of 0.11 mAh g-1 per cycle at 0.2 C) is that the doped Na ions in the Li sites can steady availably the lattice structure to inhibit the phase transformation during the lithium insertion and extraction. The superior cycling performance especially the capacity retention of the 8% Na-LNMO mentioned above is apparently better than the previous modified layered lithium-rich materials prepared via other approaches

in

literatures.27-29

Fig.

4(d)

displays

the rate

capability

of

Li1.2-xNaxNi0.2Mn0.6O2 (x = 0, 0.03, 0.05, 0.08, 0.10). The prepared samples with different Na contents were discharged at different current densities (0.1 C, 0.2 C, 0.5 C, 1.0 C, 2.0 C 5.0 and 0.1 C) after charging at a constant current density (0.1 C). Obviously, the discharge capacities of Na-doped samples at different current densities are higher than those of the pristine sample, especially at comparatively high densities. Specifically, the 8% Na-LNMO sample exhibits the highest discharge capacities of 257, 234, 206, 182, 156 and 112 mAh g-1 at 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 C, respectively, compared with 241, 222, 174, 133, 73 and 13 mAh g-1 of the pristine sample. In addition, these cells were also tested at 0.1 C after cycled at high current densities. As seen in the Fig. 4(d), the capacity at 0.1 C of Na-doped electrodes (3%, 5%, 8%) after cycled at high current densities are apparently higher than the pristine sample even better than their initial capacity at 0.1 C at first, which demonstrates that Na-doped samples could maintain a good structural stability at high current densities. It should be noted that the 10% Na-LNMO sample presents an insufficient initial charge/discharge capacity, a lower capacity retention ratio (85.4% after 50 cycles) and poor rate capability. The bad performance maybe related to the excessive amounts of

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Na-doping. When excessive Na was added in, the primary particle size of the sample increased to a high level which may prevent the diffusion of Lithium ions and reduce the charge/discharge capacity. In addition, the excessive doping could lead to a severe cation mixing arrangement in the TM layer, which is unfavorable for developing a stable and high capacity LNMO material. This conclusion is in a good consistence with the structural parameters obtained from Rietveld refinement in Table 1.35 The improved rate capability is attributed to the expanded Li space of Na-doping process, which decreases the resistance for Li diffusion and accelerates the electrons migration. Voltage decay upon cycling, which is caused by the layer-spinel structure conversion, is a main obstruction for the application of layered lithium-rich materials. Fig. 5(a) demonstrates the discharge mean voltage plots of all the samples during 50 cycles. Note that the discharge voltage of the LNMO sample sharply decreases from 3.42 V to 2.94 V after 50 cycles (∆E = 0.48 V). As for the 8% Na-LNMO sample, the discharge voltage only varies from initial 3.52 V to 3.18 V upon 50 cycles (∆E = 0.34 V), corresponding to a low voltage reduction of 0.0068 V at one cycle. As expected, the voltage fading rate for LNMO, 3% Na-LNMO, 5% Na-LNMO and 8% Na-LNMO samples efficiently slow down as increasing the amount of Na. The stable operating voltage for Na-doped samples is due to the suppression of cation mixing and the enhancement of structural stability. In addition, a much faster voltage decline was observed for LNMO sample in Figure 5(b) and (c), accompanying rapid capacity fading. To further reveal the variation of voltage decay and phase transformation of

19



Na-doping, the discharge dQ/dV plots for LNMO and 8% Na-LNMO samples during 1st, 10th, 30th, 50th cycles at 0.2C are illustrated in Fig. 5(d-g). This is another way to detect the structural conversion from layered to spinel structure in the bulk phase. The peaks shifting toward the lower potential range area indicate the aforementioned phase transition. As for the first discharge process, the peak at 3.25 V is ascribed to the redox reaction of Mn4+/Mn3+. Fig. 5(e-g) shows the differential capacity curves of various cycles (10th, 30th, 50th) for the two samples. The variation of these voltage peak positions is employed to evaluate the electrochemical stability during 50 cycles. Qualitatively, a narrow reduction peak usually indicates a better electrochemical stability. As revealed, a change of 0.44 V for the LNMO and 0.27 V for the 8% Na-LNMO samples are observed respectively. This difference of cathodic peak resulting from the existence of doping Na ions in the bulk phase restrains the conversion of layered to spinel structural.36

20


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(a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 5 (a) Mean voltage of of the Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10); Discharge curves of 1, 10, 20, 30 and 50 cycles at 0.2C for samples (b) LNMO and (c) 8% Na-LNMO; Differential capacity vs. voltage (dQ/dV) curves of LNMO and 8% Na-LNMO for (d) 1st, (e) 10th,

21



Page 22 of 33

(f) 30th, (g) 50th cycle at 0.2C.

(a)

(b)

(c)

(d)

(e)

Fig. 6 (a) XRD patterns of Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10) materials after 50 cycles; (b, c) detailed XRD patterns of the LNMO and 8% Na-LNMO samples with an interval of 50 cycles; (Na0: pristine LNMO sample; Na0-50: LNMO sample after 50 cycles; Na0.08-0: pristine 8% Na-LNMO sample; Na0.08-50: 8% Na-LNMO sample after 50 cycles); HR-TEM images of cycled (d) pristine LNMO sample and (e) 8% Na-LNMO sample; (d1), (d2), (e1) and (e2) corresponding fast flourier transformation (FFT) patterns and indexing results of FFT patterns.

A detailed analysis of the XRD pattern for 8% Na-LNMO sample after 50 cycles compared with the pristine are shown in the Fig. 6. Both the (003) peak and (104)

22


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peak move slightly toward the lower angle region after 50 cycles. The (003) peak and the (104) peak of the pristine sample display a change of 0.24° and 0.40°, respectively. Whereas the (003) peak (104) peak of the Na-doped sample display a smaller change of 0.12° and 0.16°, respectively. This phenomenon suggests that a stronger structural stability has been achieved for the Na-doped electrodes, which contributes to the excellent cycling performance for the Na-doped samples during the long-term cycling. And the better structural stability for the Na-doped samples mainly results from the existence of the doped Na ions in the Li sites. The unchangeable doped Na ions with larger ionic radius can suppress collapse of the lattice structure, which effectively stabilize the crystal structure by prohibiting the layered-spinel conversion. HR-TEM and fast fourier transform (FFT) technology were employed to explore the difference of cycling stability between the pristine Li1.2Ni0.2Mn0.6O2 and 8% Na doped Li1.12Na0.08Ni0.2Mn0.6O2 sample. As shown in Fig. 6(d), both layered structure and spinel structure were observed in pristine material after 50 cycles. The FFT patterns for selected areas d1 and d2 were indexed to the spinel space group. Furthermore, a noticeable region d3 could be observed on the edge of the pristine particle which was indexed to the layered structure. Obviously, the spinel phase occupies most of the image, indicating an obvious structural conversion from layered to spinel structure during the long cycling. Different from the pristine sample, from the HR-TEM images of 8%Na-LNMO material after 50 cycles in Fig. 6(e), it can be observed that the 8%Na-LNMO material reserved most of the original layered structure with straight fringes during the long cycle. Furthermore, the FFT patterns

23



were indexed to the layered structure without any destruction, and the HR-TEM analysis of different cathodes after cycling also evidenced that the Na-doping could restricts the phase transition effectively, leading to an improved electrochemical performance. EIS spectra of the all samples are plotted in Fig. 7. The plots of all five samples show a semicircle in the high-frequency region and a subsequent sloped line in the low-frequency region. The intercept on the x-axis at the highest frequency is related to the electrolyte resistance (Rs). The diameter of the semicircle in the high frequency region is ascribed to the charge-transfer resistance (Rct) exhibiting the Li insertion reaction. And the slope line in the low frequency region linked to the Warburg impedance (Zw) representing the process of Li diffusion in the bulk lattice.37 On the basis of the equivalent circuit model shown in the inset Fig. 7, the Rct and Rs values are simulated and tabulated in Table 5. Obviously, Rct values for both samples reduce with the Na content increase except the 10% Na-LNMO, implying that the Na-doping could significantly improve the dynamics of lithium-ion diffusion. Hence a noticeable increase in the cycling performance and rate capability has been observed between the pristine and Na-doped electrode by reducing the resistance. The higher Rct value for the 10% Na-LNMO sample further confirmed the conclusion above. In addition, the

Rct of the samples after 50 cycles for the LNMO and 8% Na-LNMO samples are tabulated in Table 4. The Rs and Rct values (2.236 and 109.9 Ω, respectively) of the pristine LNMO electrode are found to be higher than those (1.189 and 46.3 Ω, respectively) of the 8% Na-LNMO electrode in the first cycle. Although the Rs and Rct

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values of both the pristine LNMO electrode and 8% LNMO electrode increased after 50 cycles, the Rs and Rct values of the 8% Na-LNMO electrode are still lower than those of the pristine LNMO sample, proving that the Na-doping process could effectively suppress phase transformation upon cycling. The lower Rct value of the 8% Na-LNMO electrode also contributed to a high rate capability, as shown in Fig. 4(d).

(b)

(a)

(c)

Fig. 7 Nyquist plots for (a) the Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10) after the 1st cycle; (b) the LNMO after the 50th cycle and (c) 8% Na-LNMO after the 50th cycle. The inset in (a) shows the equivalent circuit model

Table 5 Impedance parameters of equipment circuit Samples

Rs(Ω)

Rct(Ω)

0

2.236

109.9

0.03

2.066

83.7

0.05

1.652

60.35

0.08

1.189

46.25

0.10

0.569

197.2

25



Page 26 of 33

To get deeper insight into the improved electrochemical performance, the Li diffusion coefficient was evaluated through the EIS spectra. The lithium ion diffusion coefficient can be calculated by following equation: D = R2T2/2A2n4F4C2σ2

(1)

where A is the superficial area of the electrode, n is the quantity of the electrons per molecule participate in the electronic transfer reaction, F is the Faraday constant, C is linked to the concentration of lithium ion, R is the gas constant, T is the room temperature in our experiment, σ is the slope of the line Z’ ∼ ω−1/2. As shown in Table 6, the Li diffusion coefficient of 8% Na-LNMO sample (1.36×10-13 cm2 s-1) is larger than the LNMO sample (9.04×10-14 cm2 s-1). And the Li diffusion coefficient of the 8% Na-LNMO sample after 50 cycles (9.95×10-14 cm2 s-1) is also higher than those of the pristine LNMO sample (2.54×10-14 cm2 s-1) at the same circumstances. This phenomenon also verfied that the expanded Li slab space which has been evidenced above promotes the diffusion and migration of the lithium ion.

Table 6 Electrolyte resistance (Rs) charge transfer resistance (Rct) and diffusion coefficient (DLi) of the pristine LNMO and 8% Na-LNMO in the 1st and 50th cycle LNMO

8% Na-LNMO

1st charge

50th charge

1st charge

50th charge

Rs(Ω)

2.236

5.330

1.189

4.265

Rct(Ω)

109.9

277.4

46.3

127.3

DLi (cm2 s-1)

9.04×10-14

2.54×10-14

1.36×10-13

9.95×10-14

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4. Conclusion In summary, the layered lithium rich cathode materials Li1.2-xNaxNi0.2Mn0.6O2 (x=0, 0.03, 0.05, 0.08, 0.10) were synthesized by a facile ball-milling and a subsequent high temperature solid state reaction. All the Na-doped samples and pristine sample exhibit the same layered structure, and the Na ion substitution expands the Li slab space. In comparison with the pristine sample, the Na-doped electrodes exhibit higher reversible capacities, better rate performance and excellent cycling stability during the charge/discharge test. The significantly improved electrochemical performances especially the high discharge capacities and superior cycling stability for the Na-LNMO electrodes mainly result from the existence of the doped Na ions in the Li sites, which effectively enhance the layered structural stability. The dQ/dV data of the LNMO and 8% Na-LNMO samples indicate that Na-doping stabilize the crystal structure by prohibiting the layered-spinel conversion. Enhanced lithium ion diffusion coefficients after Na-doping have been proved by the EIS results. The XRD data after 50 cycles also show Na-doping plays an important role in stabilizing structure. Therefore, Na-doping with lithium acetate and sodium acetate by ball-milling is a facile way to prepare a layered lithium rich cathode material with long cycle life, high rate capabilities and enhanced lithium ion diffusion for Li-ion batteries.

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Acknowledgments The authors gratefully acknowledge the National Natural Science Foundation of China (Grants No. 51774150, 51604124, 51604125 and 21703185), Natural Science Foundation of Jiangsu Province (BK20150506, BK20140558, BK20150535) and Fundamental Research Funds for the Central Universities (Xiamen University: 20720170042).

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Alloys Compd. 2016, 685, 523-532, DOI 10.1016/j.jallcom.2016.05.329.

Table of Content Graphic

A facile Na doping strategy is introduced to improve the structural stability and rate performance of the layered lithium-rich manganese-based cathodes for advanced commercial lithium-ion batteries.

33


Improved Cycling Stability of Na-doped Cathode Materials Li1.2Ni0

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