Uniform Surface Modification of Li2ZnTi3O8 by Liquated Na2MoO4 To

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Uniform Surface Modification of Li2ZnTi3O8 by Liquated Na2MoO4 to Boost the Electrochemical Performance Huan Yang, Jiyun Park, Chang-Soo Kim, Yi-Han Xu, Hui-Ling Zhu, Yong-Xin Qi, Longwei Yin, Hui Li, Ning Lun, and Yu-Jun Bai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12208 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Uniform Surface Modification of Li2ZnTi3O8 by Liquated Na2MoO4 to Boost the Electrochemical Performance Huan Yang, 1 Jiyun Park, 2 Chang-Soo Kim,

2

Yi-Han Xu,

2

Hui-Ling Zhu,

3

Yong-Xin Qi, 1

Longwei Yin, 1 Hui Li, 1 Ning Lun, *,1 and Yu-Jun Bai*,1 1

Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials (Ministry of

Education), Shandong University, 17923 Jingshi Road, Jinan 250061, Shandong, P. R. China 2

Materials Science and Engineering Department, University of Wisconsin-Milwaukee,

Milwaukee, WI 53211, USA 3

School of Materials Science and Engineering, Shandong University of Science and

Technology, Qingdao, 266590, P. R. China *Tel/Fax: +86 0531 88392315. E-mail: [email protected] (Y.-J. Bai);[email protected] (N. Lun). Abstract Poor ionic and electronic conductivities are the key issues to affect the electrochemical performance of Li2ZnTi3O8 (LZTO). In view of the water solubility, low melting point, good electrical conductivity, and wettability to LZTO, Na2MoO4 (NMO) was firstly selected to modify LZTO via simply mixing LZTO in NMO water solution followed by calcining the dried mixture at 750 °C for 5 h. The electrochemical performance of LZTO could be enhanced by adjusting the content of NMO, and the modified LZTO with 2 wt% NMO exhibited the most excellent rate capabilities (achieving lithiation capacities of 225.1, 207.2, 187.1, and 161.3 mAh g-1 at 200, 400, 800, and 1600 mA g-1, respectively) as well as outstanding long-term cycling stability (delivering a lithiation capacity of 229.0 mAh g-1 for

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400 cycles at 500 mA g-1). Structure and composition characterizations together with electrochemical impedance spectra analysis demonstrate that the molten NMO at the sintering temperature of 750 °C is beneficial to diffuse into the LZTO lattices near the surface of LZTO particles to yield uniform modification layer, simultaneously ameliorating the electronic and ionic conductivities of LZTO, thus is responsible for the enhanced electrochemical performance of LZTO. First-principles calculations further verify the substitution of Mo6+ for Zn2+ to realize doping in LZTO. The work provides a new route for designing uniform surface modification at low temperature, and the modification by NMO could be extended to other electrode materials to enhance the electrochemical performance. Keywords: Na2MoO4, Li2ZnTi3O8, modification, anode material, first-principles 1. Introduction Further development of lithium-ion batteries (LIBs) in the stationary large-scale energy storage systems and electric vehicles requires persistent exploration for advanced electrode materials.

1

Cubic Li2ZnTi3O8 (LZTO) with a space group of P4332 is a promising anode

material and receiving a great deal of attention due to the safety, nontoxicity, environmental friendliness, decreased lithium content, and increased specific capacity (227 mAh g-1) compared with Li4Ti5O12 (LTO). However, LZTO suffers from poor electronic and ionic conductivities, restricting to fully display the electrochemical performance. To date, some methods have been proposed to ameliorate the performance of LZTO, such as preparing nanowires and nanorods, 2-4 doping foreign ions into the LZTO lattice, materials on the surface of LZTO,

10-14

5-9

coating conductive

and modifying LZTO by some modifiers.15-18

Nanocrystallites are beneficial to shorten the diffusion distance of Li+, doping aliovalent ions


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and coating conductive materials such as carbon might improve the electronic conductivity. Surface modification is a common method to improve the electrochemical performance of electrode materials, but it is difficult to achieve homogenous surface modification layer by coating solid state materials. In general, the coating materials (such as Al2O3, LiAlO2, Li2ZrO3, and Li2SiO3) are of high melting points, even via reaction routes, the reactions are uncertain to just take place around the particles of electrode material to generate uniform coating layer. Accordingly, selecting appropriate modifiers is extremely important for realizing optimal modification effect. In this work, we first chose Na2MoO4 (NMO) as a modifier to prepare NMO-modified LZTO at 750 ºC for 5 h dominantly based on the following considerations. (1) NMO is easily dissolved in water to form homogenous solution, thus could absorb on LZTO nanoparticles to yield uniform NMO coating layer. (2) NMO is of low melting point (687 ºC) among the oxides, so it is easily melted at the sintering temperature of 750 ºC and diffuses into the LZTO lattice to realize doping. (3) Both the dissolved and melted NMO have good wettability on the LZTO particles owing to the common oxide feature, hence yielding homogenous surface modification. (4) What’s more, NMO exhibits good electronic conductivity (~103 Ω-1 cm-1). 19-20

As expected, the NMO-modified LZTO demonstrates remarkably enhanced

electrochemical performance, especially at high current rate. The modification mechanism was proposed by virtue of the detailed characterizations on structure and composition, Firstprinciple calculations, as well as the performance measurement. 2. Experimental Section 2.1. Fabrication of LZTO



LZTO was fabricated by simply mixing lithium carbonate, zinc acetate, and commercial anatase TiO2 with a molar ratio of 2.04: 1: 3 in deionized water, followed by sintering the dried mixture at 800 °C for 5h. The as-prepared product is assigned to P-LZTO. 2.2. Fabrication of NMO-Modified LZTO In terms of the LZTO/NMO mass ratio of 1: 0.02 and 1: 0.04, NMO-modified LZTO were fabricated by magnetically mixing the as-prepared LZTO (1.5 g) with NMO (0.03 and 0.06 g) in 20 mL deionized water for half an hour. The dried mixtures (at 105 °C for 12 h in an oven) were calcined in a horizontal tube furnace at 750 °C for 5 h in air. The final products are labeled as LZTO/NMO and LZTO/NMO-1. Similarly, the NMO-modified LZTO sample with a LZTO/NMO mass ratio of 1: 0.4 was also fabricated, which is designated as LZTO/NMO-2. 2.3. Material Characterization The microstructure was inspected by a JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM), a high angle annular dark field scanning transmission electron microscope (HAADF-STEM) equipped with energy-dispersive X-ray spectroscopy (EDS) on an FEI Tecnai G2 F20 S-TWIN microscope (V = 200 kV). SEM images were acquired by a JSM-6700F field emission scanning electron microscope (FESEM) at an accelerating voltage of 20 kV. The phase and crystal structure were examined by X-ray diffraction (XRD) on a Rigaku Dmax-2500 diffractometer (Ni filtered Cu Kα radiation, λ = 1.540 Å) in the 2θ range of 10-90° with a scanning rate of 4° min-1. Extended X-ray absorption ﬁne-structure (EXAFS) measurements for the Mo K-edge were performed on the XAFS station in Beijing synchrotron radiation facility. Raman spectra were measured on a Renishaw confocal Raman microspectroscopy with a laser excitation wavelength of 780 nm. X-ray photoelectron spectra


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(XPS) were acquired on a KARTOS XSAM800 X-ray photoelectron spectrometer (Kratos Analytical Ltd., Manchester, U.K.) using Al Kα radiation (hv = 1486.6 eV) as the excitation source (V = 12 kV, I = 10 mA). N2 adsorption-desorption isotherms were measured to acquire the specific surface area by Brunauer-Emmett-Teller (BET) method. 2.4. First-Principles Calculation Density-functional theory (DFT) calculations were performed with Dmol3 module implemented in the Materials Studio software package.

21-22

Generalized-gradient

approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional

23

was used with the double numerical plus d-functions (DND) basis set. The

convergence tolerance was enforced to 1×10-5 Ha for energy (1 Ha = 27.21 eV), 2×10-3 Ha/Å for gradient, 5×10-3 Å for maximum displacement. For each computation, four formula units containing 64 atoms (i.e., unit cell) were constructed and their Brillouin zone was sampled with 3×3×3 grid based on the Monkhorst and Pack scheme. 24 2.5. Assembly of Li-Ion Cells The assembly of 2025 coin-type half-cells includes the following several steps, (1) mixing the active material with acetylene black and polyvinylidene difluoride (PVDF) with the mass ratio of 8: 1: 1 in N-methyl-2-pyrrolidine to form homogeneous slurry; (2) coating the slurry onto copper current collector followed by drying at 120 °C for 12 h in a vacuum oven; (3) punching the Cu foil into electrodes of 14 mm in diameter, and the mass loading of active material on each electrode is ca. 3.0 mg; (4) assembling the half-cells in an argon-filled glove box with the moisture and oxygen less than 5 ppm. Lithium foil was utilized as counter electrode, Celgard 2300 as separator, and 1 M LiPF6 dissolved in ethyl carbonate and



dimethyl carbonate (with a volume ratio of 1:1) as electrolyte. 2.6. Electrochemical test Galvanostatic charge and discharge tests were carried out by a Land CT2001A battery test system in a voltage range of 0.02-3.0 V at 26 °C. An IviumStat electrochemical workstation was employed to measure cyclic voltammograms (CV) between 0.02 and 3.0 V (vs. Li/Li+) at a scanning rate of 0.3 mV s-1, and electrochemical impedance spectra (EIS) with a signal amplitude of 5 mV in the frequency range from 1000 kHz to 10 m Hz. 3. Results and Discussion 3.1. Electrochemical Performance

Figure 1. Cycling performance at 100 mA g-1 (a), rate capabilities (b), and cycling performance at 500 (c) and 1000 mA g-1 (d), as well as the corresponding Coulombic efficiency for LZTO/NMO.


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The electrochemical performance of half-cells was tested galvanostatically in the voltage range of 0.02-3.0 V at 26 ºC. From Figure 1a, the initial lithiation/delithiation capacities at 100 mA g-1 are 199.1/115.9, 246.6/167.7, and 244.5/164.8 mAh g-1 for P-LZTO, LZTO/NMO, and LZTO/NMO-1, respectively. The higher lithiation capacity of NMO-modified LZTO than the theoretical value could be ascribed to the comprehensive contribution from the high content of conductive acetylene black (The calculation is provided in Supporting Information, SI), interfacial Li-ion storage (Figure S1 and Table S1), 25-26 and the defects induced by Modoping.

27-28

The initial Coulombic efficiency is 58.2% for P-LZTO, 68.0% for LZTO/NMO,

and 67.4% for LZTO/NMO-1. Despite the comparatively low initial Coulombic efficiency due to the additional consumption of Li+ to form solid electrolyte interface (SEI) film,

29

the

efficiency of LZTO/NMO increases by 9.8% and LZTO/NMO-1 by 9.2% compared to that of P-LZTO, revealing the beneficial modification effect of NMO on LZTO. After 200 cycles, the lithiation/delithiation capacities of P-LZTO decay to 90.3/89.7 mAh g-1, while those of LZTO/NMO and LZTO/NMO-1 increase steadily to 275.4/273.1 and 278.5/277.2 mAh g-1. The gradual increase in capacity is just like what happened in KCl-modified LZTO because of the large particles in the products. 18 Table 1. Lithiation Capacities (mAh g-1) of the 10th Cycle at Each Current Density (mA g-1) capacities at varied current densities

sample P-LZTO LZTO/NMO LZTO/NMO-1

200

400

800

1600

100

175.9 225.1 212.0

162.2 207.2 193.5

121.1 187.1 171.6

53.3 161.3 101.8

216.2 264.0 253.1

Rate capabilities were measured at various current densities (200, 400, 800, 1600, and finally 100 mA g-1) each for 10 cycles (Figure 1b), and the capacities are gathered in Table 1.



Compared to P-LZTO, the rate capabilities of NMO-modified LZTO are remarkably improved, and LZTO/NMO reveals the best performance with the Coulombic efficiency almost close to 100% except for the initial cycle at each current density. In comparison with the rate performance of the cells constructed by other LZTO materials, 6, 13, 16-18 LZTO/NMO also reveals great advantage, as listed in Table 2. Table 2. Comparison of Rate Performance for the Cells Constructed by LZTO Materials with Varied Modifications capacities/current densities (mAh g-1/mA g-1)

sample Li2ZnAg0.15Ti2.85O8 6

210.0/100, 125.0/1600, 206.3/100 (again to 100, each for 10 cycles )

LZTO/C 13

242.5/100, 190.0/500, 165.2/1000, 91.2/2000 (each for 100 cycles)

LZTO/Li2MoO4 LZTO/La2O3

17

16

LZTO/KCl 18 LZTO/NMO

267.5/50, 230/100, 218.8/300, 160.2/500, 151.6/1000 (each for 10 cycles) 222.9/100, 149.3/3000, 203.3/100 (again to 100, each for 10 cycles) 195.4/200, 178.0/400, 162.4/800, 135.6/1600 (each for 10 cycles)

this work

225.1/200, 207.2/400, 187.1/800, 161.3/1600 (each for 10 cycles)

In view of the excellent rate capabilities, the cycling performance of LZTO/NMO was further assessed at 500 and 1000 mA g-1 after cycling 10 times at 100 mA g-1 (Figures 1c and d). The lithiation capacities for the initial/400th cycle are 228.4/229.0 mAh g-1 at 500 mA g-1 and 155.3/132.5 mAh g-1 at 1000 mA g-1, the Coulombic efficiency for the 400th cycle is 99.9% at 500 mA g-1 and 99.4% at 1000 mA g-1, demonstrating the outstanding cyclability of LZTO/NMO. The cycling performance of LZTO/NMO-1 at 500 mA g-1 is slightly inferior to that of LZTO/NMO (Figure S2), namely, the electrochemical performance especially the rate performance decreases with further increasing the NMO content owing to the aggravated polarization and subdued conductivity, as supported by the CV and EIS results. The greatly boosted electrochemical performance of LZTO by the modification of NMO


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was analyzed in detail from the variations in structure, composition, and EIS. 3.2. Microstructure and EDS Mapping

Figure 2. HRTEM images of P-LZTO (a) and LZTO/NMO (b, c, d). The structure and composition of LZTO/NMO were characterized by HRTEM, HAADFSTEM, and EDS mapping. From the HRTEM images (Figure 2), the interplanar spacing of 0.48 nm is from the (111) plane of LZTO (Figures 2a and b), and that of 0.22 nm from the (331) plane of NMO (Figure 2d). In comparison with P-LZTO (Figure 2a), some tiny crystallites with the size less than 5 nm (such as those marked in red circles) adhere uniformly to the surface of LZTO particles in LZTO/NMO, and the lattice fringes near the particle surface are different from those in the interior (Figures 2b and c), suggesting that superficial doping may occur during sintering at 750 ºC for 5 h accompanying with the formation of modification layer about 3 nm in thickness. And the doping will be evidenced by the following XRD, EXAFS, XPS, and Raman spectra. Despite most of the tiny crystallites reveal poor crystallinity, the lattice fringes could be distinguished in some of them, as exemplified in



Figure 2d, confirming the relevance to NMO.

Figure 3. EDS mappings of mixed, Ti, Zn, Na, and Mo (a), HAADF-STEM image (b), and the counts of Mo (c) and Na (d) along the white arrow in panel (b) for LZTO/NMO. From the HAADF-STEM and EDS mappings of LZTO/NMO (Figure 3a), besides the uniform distribution of Ti and Zn, Na and Mo were also detected homogeneously on the surface of LZTO particles. The line scan EDS (Figure 3b) further indicates the distribution of Mo (Figure 3c) and Na (Figure 3d) along the white arrow from point 10 to 70. The much higher counts of Na than Mo is attributable to the Na/Mo molar ratio of 2: 1 in NMO and the easier diffusion of Mo6+ than Na+ in LZTO during calcining at 750 ºC for 5 h, as demonstrated by the following DFT calculations. The full EDS mapping is provided in Figure S3. 3.3. XRD Analysis The phase and structure information could be acquired by XRD results (Figure 4). Except


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for the diffraction peaks for LZTO (JCPDS no. 86-1512), no peaks for NMO was detected due to the less NMO content and the poor crystallinity of NMO in LZTO/NMO (Figure 4a). Given this, we fabricated LZTO/NMO-2 with a high mass ratio of NMO, and the diffraction peaks marked by green rhombus are consistent with those from γ-NMO (JCPDS no. 12-0773), 20

demonstrating from another view the existence of NMO in LZTO. The average crystallite

size estimated by Scherrer formula (d = K λ/ B cos θ) 31 is 44.7 nm for P-LZTO and 57.1 nm for LZTO/NMO, while the average particle size estimated by BET surface area (Figure S4) is 187 nm for P-LZTO and 373 nm for LZTO/NMO. The difference between the particle size and crystallite size lies in that one particle is comprised of some agglomerated crystallites (Figure S5 and S6). Though the individual crystallites could not be distinguished by FESEM, their sizes could be examined by TEM (Figure S5), which are from dozen nanometers to about 500 nm.

Figure 4. XRD patterns (a) and the enlarged (311) peaks (b). To obtain further information about structural variation, the (311) peaks of P-LZTO and LZTO/NMO are enlarged (Figure 4b). In comparison with P-LZTO, the peak for LZTO/NMO shifts to a lower angle, implying the increscent lattice constant (Table S2) resulted from the NMO modification.



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3.4. Raman Spectra

Figure 5. Raman spectra (a) and the enlarged spectra of Zn-O (b), Li-O (c), and Ti-O (d) for P-LZTO and LZTO/NMO. Raman spectrum could reflect the variation in internal molecular structure. Besides the peaks at 234, 262, 355, 400, 441, 520, and 722 cm-1 for P-LZTO,

30

the weak peak at ca. 300

cm-1 is ascribed to the Mo-O vibration in LZTO/NMO 31 (Figure 5a). Concretely, the band at 400 cm-1 corresponds to the stretching vibration of Zn-O (ZnO4), cm-1 to those of Li-O (LiO4),

33

8, 32

the ones at 441 and 520

and the one at 722 cm-1 to that of Ti-O (TiO6).

7, 34-35

With

respect to P-LZTO, the Zn-O and Li-O vibrations (Figures 5b and c) for LZTO/NMO reveal blue shift with different degrees, but the Ti-O vibration (Figure 5d) is nearly invariable, suggesting the substitution of Mo6+ for Zn2+ and Li+, but few replacement for Ti4+. Particularly, the larger shift of Zn-O vibration than Li-O vibration denotes the preferential substitution of


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Mo6+ for Zn2+ in LZTO. Meanwhile, all the Raman peaks broaden (e.g. the full width at half maximum for the Zn-O vibration increases from 39 to 77 cm-1) after the NMO modification, indicative of the structural disordering of LZTO resulted from the Mo6+ doping. 32, 36 3.5. XPS Analysis

Figure 6. XPS survey spectra (a), the core level spectra of Mo 3d (b), Na 1s (c), and Zn 2p (d). The surface elements and the variation in coordination environment could be identified by XPS utilizing C 1s (284.79 eV) spectrum as a reference (Figure 6a). In addition to the peaks of Zn 2p, O 1s, Ti 2p, and Li 1s in P-LZTO, the peak of Mo 3d 37 (234.98 eV for Mo 3d3/2 and 232.16 eV for Mo 3d5/2 in Figure 6b) could also be discriminated in LZTO/NMO. Though Na 1s peak could hardly be distinguished due to the less NMO content, the weak peak derived from the low atomic number of Na, and the peak superposition with Zn, the core level spectrum of Na 1s (Figure 6c) demonstrates the presence of Na in LZTO/NMO. The core



level spectra of Zn 2p (Figure 6d) for LZTO/NMO could be deconvoluted into four peaks of Zn 2p1/2 (1020.61 eV) and Zn 2p3/2 (1043.81 eV) for LZTO, 38 as well as Zn 2p1/2 (1022.54 eV) and Zn 2p3/2 (1045.63 eV) for ZnO. 39-40 The presence of ZnO peaks signifies the replacement of Mo6+ for Zn2+ after the modification of NMO, agreeing well with the Raman spectra. 3.6. DFT Calculations for the NMO-Modified LZTO The feasibility of Mo-doping in LZTO is further supported by DFT calculations. The most stable configuration of LZTO with the lowest energy (Figure S7) was selected to further study the Mo-doping in LZTO. Four different substitutions of Mo atom were assumed to occupy, either the tetrahedral sites (i.e., Li or Zn atoms) or octahedral sites (i.e., Li or Ti atoms). The representative optimized structures and the calculated formation energies for these four Mo-substituted LZTO systems are illustrated in Figure 7 and collected in Table S3, respectively. From the formation energies of -77.99 eV for the standard LZTO, -79.96 and 72.50 eV for the tetrahedral site substitution of Zn↔Mo and Li↔Mo, as well as -73.46 and 71.21 eV for the octahedral site substitution of Li↔Mo and Ti↔Mo, the substitution of Mo for Zn in the tetrahedral sites is more favorable than those for Li and Ti according to energy minimization.

Figure 7. Geometrically optimized structures of Mo-substituted LZTO. (a) Li↔Mo and (b)


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Zn↔Mo in the tetrahedral sites, (c) Li↔Mo and (d) Ti↔Mo in the octahedral sites. Table 3. Average Bond Length between Metal and Oxygen Atom P-LZTO sites

Mo-substituted LZTO

substitution

effective ionic radius (Å)

average bond length (Å)

effective ionic radius (Å)

average bond length (Å)

Zn↔Mo

Zn2+: 0.60

Zn-O: 1.998

Mo6+: 0.41

Mo-O: 1.920

Li↔Mo

Li+: 0.59

Li-O: 2.020

Mo6+: 0.41

Mo-O: 1.940

tetrahedral site Li↔Mo octahedral site Ti↔Mo

+

Li : 0.76 4+

Ti : 0.61

Li-O: 2.146 Ti-O: 1.997

6+

Mo-O: 2.127

6+

Mo-O: 2.027

Mo : 0.59 Mo : 0.59

The average bond lengths between metal atoms and their neighboring oxygen atoms for the geometrically optimized LZTO structures are tabulated in Table 3. Theoretically, if Mo replaces Li, Zn, or Ti, the bond length of Mo-O should shorten compare to the bond lengths of Li-O, Zn-O, or Ti-O due to the smaller ionic radius of Mo6+ (0.41 Å and 0.59 Å in the tetrahedral and octahedral coordination, respectively 41) than Li+, Zn2+, and Ti4+. Nevertheless, from the calculation results, the similar or increscent bond length of Mo-O (Table 3) in the octahedral sites signifies the difficulty of Mo substituting for Li and Ti in the octahedral sites. Additionally, from the formation energy, Mo replacing Li in the tetrahedral sites is also impossible despite the shortened bond length. The puny shift of Li-O vibration in the Raman spectra is likely related to the replacement of Li by Mo at surface defects rather than in the perfect LZTO crystals. Combining the XPS of LZTO, the appearance of additional Zn-O peaks in the XPS of LZTO/NMO suggests the formation of new Zn-O bonds other than those in LZTO, and the higher intensity of the newly formed peaks than those in LZTO implies that more Zn2+ migrates into NMO from the LZTO particle surface to yield Zn-doped NMO. The most possibility is that the Zn2+ in LZTO was substituted by Mo6+ concomitant with the generation of Zn-vacancy in LZTO (One Mo6+ substitution for one Zn2+ accompanies with



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yielding two Zn-vacancies), thus generated lattice defects may compensate the extra charge introduced by Mo6+ to guarantee the electroneutrality of LZTO.

42-43

From both the

calculations for formation energy as well as bond length and the experimental results, Mo6+ substituting for Zn2+ to realize the doping occurs in LZTO/NMO. Besides the simulation of Mo-doping in LZTO, we also investigated the possibility of Na atom replacing for Li, Zn, and Ti atoms in LZTO (Table S4). The substitution of Na for Zn in the tetrahedral site reveals the lower formation energy of -82.79 eV compared to the standard LZTO. However, Na entering into the LZTO lattice results in larger lattice distortion (the volume of unit cell for Zn↔Na increased by 1.49%, and that for Zn↔Mo by 1.26%, as revealed in Table S5), so Na-doping in LZTO is disadvantageous despite the lower formation energy for Zn↔Na, and Na is more likely to stay on the surface of LZTO particles, as confirmed by the EDS mappings in Figure 3. 3.7. X-ray Absorption Spectra

Figure 8. The k3-weighted Fourier transform spectra derived from XAFS of NMO and LZTO/NMO. For further understanding the structural change of LZTO after NMO modification, NMO and LZTO/NMO were characterized by XAFS spectra, and the radial structure function (RSF)


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curves of Mo K-edge are shown in Figure 8. The peak between 1 and 2 Å is ascribed to the interatomic distance of Mo-O shell.

44-45

The Mo-O peak in LZTO/NMO shifts slightly to a

lower R value with respect to that in NMO. Meanwhile, the intensity of Mo-O peak weakens after Mo-doping in LZTO/NMO. These changes might result from the distortion of MoO6 octohedrons. The peak of the second shell (Mo-O-M, M: Na or other metals) in the range of 2-3 Å for LZTO/NMO shifts significantly towards a much lower R value in comparison with that for NMO, manifesting that the bond length of Mo-O-M decreases after Mo-doping in LZTO, consistent with the result of the DFT calculations. This is another confirmation for the Mo-doping in the LZTO lattice, as evidenced by Raman spectra and XPS results. From the analysis on structure and composition together with the DFT calculations, the NMO modification simultaneously results in the formation of NMO modification layer adjacent to the surface of LZTO particles, and the superficial Mo6+ doping, thus contributive to improving the electrochemical performance of LZTO. 3.8. Electrochemical Reaction The CV profiles were measured to compare the electrochemical reactions occurring in PLZTO, LZTO/NMO, and LZTO/NMO-1 during lithiation/delithiation (Figure 9). The similarity of the plots demonstrates that the NMO modification does not lead to the change of electrochemical reactions in LZTO. Taking LZTO/NMO as an example (Figure 9b), the cathodic/anodic peaks of 1.08/1.59 V in the first cycle are related to the Ti4+/Ti3+ redox couple, 46

and the cathodic peak shifts to 1.32 V while the anodic peak to 1.63 V in the following

cycles. The cathodic peak around 0.69 V in the first cycle is related to the formation of SEI film, 47 and those in the range of 0.5-0.2 V to the multiple restoration of Ti4+. 46, 48



Figure 9. CV profiles of P-LZTO (a), LZTO/NMO (b), and LZTO/NMO-1(c) as well as the comparison for the third cycle (d). From the direct comparison of CV curves for the third cycle (Figure 9d), the anodic peak current rises from 0.48 mA for P-LZTO to 1.62 mA for LZTO/NMO and 1.63 mA for LZTO/NMO-1 (Table S6), demonstrating the enhanced specific capacity resulted from the modification by NMO (Figure 1). Meanwhile, the potential differences diminishes from 0.52 V for LZTO to 0.31 V for LZTO/NMO and 0.37 V for LZTO/NMO-1 (Table S7), denoting the attenuated polarization after the NMO modification. In particular, LZTO/NMO-1 reveals aggravated polarization with respect to LZTO/NMO, thus LZTO/NMO-1 exhibits lowered performance, as illustrated in Figures 1b and S2. 3.9. EIS analysis The electrochemical kinetics was analyzed by EIS. The Nyquist plots of P-LZTO,


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LZTO/NMO, and LZTO/NMO-1 were measured when the cells underwent 200 cycles at 100 mA g-1 (Figure 10a). From the corresponding Bode plots (Figure 10b), there are two characteristic frequencies for LZTO and three for NMO-modified LZTO, suggesting that an additional solid interface presents in NMO-modified LZTO besides the two in LZTO,

49

namely, there are two semicircles for LZTO and three semicircles for NMO-modified LZTO. It can be seen that, the fitted plots are in good consistency with the measured ones according to the respective equivalent circuit in the insets of Figures 10c and d. In the circuits, Re represents the electrolyte resistance, RSEI the SEI film resistance, Rct the charge transfer resistance, CPE the constant phase relevant to the interfacial capacitance, and Zw the Warburg impedance associating with Li+ diffusion kinetics. Particularly, RNMO is regarded as the resistance from the NMO modification layer. The data acquired from the fitted plots are collected in Table 4. Table 4. Impedance Values Acquired from the Fitted EIS Plots sample

Re (Ω)

RSEI (Ω)

RNMO (Ω)

Rct (Ω)

RT (Ω)

P-LZTO

9.8

180.0

/

580.0

769.8

LZTO/NMO

5.3

7.6

23.4

260.0

296.3

LZTO/NMO-1

6.4

102.1

17.5

496.0

622.0

Form Figure 10 and Table 4, except for the similar Re, the RSEI, Rct, and RT values (RT = Re + RSEI + RNMO + Rct) for NMO-modified LZTO are smaller than those for P-LZTO even introducing another resistance of RNMO, demonstrating the enhanced electronic conductivity after the NMO modification. However, LZTO/NMO-1exhibits higher RSEI, Rct, and RT than LZTO/NMO despite the smaller RNMO due to the good electronic conductivity of NMO, manifesting that excess NMO is unfavorable to the electronic conductivity for LZTO. Additionally, the decrease of RSEI for LZTO/NMO could be ascribed to the modification of ACS Paragon Plus Environment


surface NMO on the SEI film, especially the high Na+ content on the surface of LZTO particles plays a critical role for the SEI film, just like what has reported in the literature. 50-51 The low RNMO value is due to Mo-doping in LZTO, and the decrease of Rct results from both the Mo-doping and the reduction of actual LZTO particle size by subtracting the modification layer.

Figure 10. Nyquist plots (a, c, d) and Bode plots (b) of P-LZTO, LZTO/NMO, and LZTO/NMO-1. The insets in Figures 10c and d are the corresponding equivalent circuits. Li+ diffusion kinetics is related to the inclined line at low frequency in EIS (Figure 10a), and the Li+ diffusion coefficient (D) could be calculated by Eq. 1. 52 Ip = 2.69×105n3/2 AD 1/2ν1/2∆C0

(1)

where Ip is the peak current, n the number of electrons per molecule during oxidation (n = 3 for LZTO), A the surface area of the electrode (A = 1.5386 cm2), ν the CV scanning rate (ν =


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0.3 mV s-1), ∆C0 the change of Li+ concentration during discharge/charge (The calculation of ∆C0 is presented in SI). Choosing the Ip value for the stable 3rd CV cycle in Figure 9, the calculated D values are 4.33×10-13 for P-LZTO, 4.96×10-12 for LZTO/NMO, and 1.65×10-12 for LZTO/NMO-1 (Table S6), increased by one order of magnitude after the NMO modification. As reported in the literature, 53-54 slight Zn-doping in NMO could ameliorate the ionic conductivity. In this work, the Zn2+ generated by Mo6+ substitution diffuses into the surface NMO to realize doping, resulting in the enhancement of ionic conductivity. However, LZTO/NMO-1 also displays the poor ionic conductivity relative to LZTO/NMO due to the excess NMO content. Combining the structure and composition information (HRTEM, HAADF-STEM, EDS Mapping, XRD, XPS, XAFS, and Raman spectra) with the electrochemical analysis, the modification of LZTO by NMO could be schematically illustrated in Figure 11. When LZTO was mixed in NMO water solution, NMO uniformly absorbed on the surface of LZTO particles with comparatively relaxed surface structure because of the good wettability of NMO on LZTO resulted from the common oxide feature, and after drying, homogenous NMO coating layer formed around the LZTO particles. When heating the NMO-coated LZTO to above the melting point of NMO (say 750 ºC), NMO melted into tiny liquid droplets on the surface of LZTO particles, and the liquated NMO readily ionized and diffused through the liquid/solid interface into the LZTO lattices near the particle surface owing to the loose structure, thus resulting in the superficial Mo6+ doping by substituting for Zn2+ owing to the small ionic radius of Mo6+ and energy minimization. Because the heating time of 5 h at 750 ºC is not enough for the thorough ion diffusion into the LZTO particles, some residual NMO



crystallites with spherical caps could still be discriminated on the surface of LZTO particles, as denoted in the TEM images (Figure 2).

Figure 11. Schematic illustration for the formation of NMO-modified LZTO. On the basis of the above discussion, the remarkably enhanced electrochemical performance of LZTO/NMO is ascribed to the following aspects. (1) The modification of LZTO by appropriate amount of NMO (say 2 wt% NMO in LZTO/NMO) could alleviate the polarization, conducive to improving the electrochemical performance of LZTO. (2) The proper sintering temperature of 750 °C ensures the benign NMO with low melting point in molten state, facilitating the Mo6+ diffusion into the LZTO lattices to yield doping, thus ameliorating the electronic conductivity of LZTO. (2) The good wettability of NMO on the surface of LZTO particles is conductive to forming uniform modification layer, and the high surface Na+ content could further modify the SEI film. (3) The surface modification layer constituted by Zn-doped NMO could enhance the ionic conductivity. (4) The simultaneous improvement in ionic and electronic conductivities gives rise to the attenuated polarization for LZTO/NMO.

4. Conclusions In summary, a new approach was proposed to uniformly and markedly modify LZTO by simply mixing LZTO in NMO water solution followed by sintering the dried mixture at


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750 °C for 5 h. The NMO modification simultaneously results in the formation of modification layer adjacent to the surface of LZTO particles as well as Mo6+ doping by substituting for Zn2+ in the LZTO lattice, conducive to enhancing the rate capability and longterm cycling stability of LZTO. This simple, effective, and uniform surface modification route allows the NMO-modified LZTO applicable as anode material for high-performance LIBs, and could be extended to modify other electrode materials by the similar oxides with relatively low melting points.

Associated Content Supporting Information Calculation for the capacity of active material; CV plots of LZTO/NMO at various scanning rates; Values of peak current and scan rate; Cycling performance of LZTO/NMO and LZTO/NMO-1 at 500 mA g-1; Full EDS mapping of LZTO/NMO; Calculation process of average particle size by BET surface; Low-magnification TEM image of LZTO/NMO; SEM images of P-LZTO and LZTO/NMO; Lattice constant; DFT Calculation for the NMOModified LZTO; Calculated formation energies of Mo-substituted LZTO; Calculated formation energies of Na-substituted LZTO structures; Lattice parameters and volume for structures; Peak current value for the 3rd CV cycle and other parameters; Cathodic and anodic potentials.

Author Information *Corresponding Authors



E-mail: [email protected] (Y.-J. Bai); E-mail: [email protected] (N. Lun). ORCID Yu-Jun Bai: 0000-0002-8013-9437 Huan Yang: 0000-0001-6743-6878 Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by Natural Science Foundation of Shandong Province, P. R. China (ZR2016EMM18), Key research and development program of Shandong Province, P. R. China (2015GGX102005 and 2016GGX102031), and the National Natural Science Foundation of China (51532005).

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(53) Kruglyashov, A.; Skou, E., Ionic Conductivity of Compounds in the System Na2MoO4ZnMoO4. Solid State Ionics 1988, 28, 233-236. (54) Solodovnikov, S. F.; Solodovnikova, Z. A.; Zolotova, E. S.; Yudin, V. N.; Gulyaeva, O. A.; Tushinova, Y. L.; Kuchumov, B. M., Nonstoichiometry in the Systems Na2MoO4-MMoO4 (M=Co, Cd), Crystal

Structures of

Na3.36Co1.32(MoO4)3, Na3.13Mn1.43(MoO4)3 and

Na3.72Cd1.14(MoO4)3, Crystal Chemistry, Compositions and Ionic Conductivity of Alluauditetype

Double

Molybdates

and

Tungstates.

J.

https://doi.org/10.1016/j.jssc.2017.05.031


Solid

State

Chem.

2017.


Table of Contents


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Uniform Surface Modification of Li2ZnTi3O8 by Liquated Na2MoO4 To

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