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Research Article pubs.acs.org/journal/ascecg

Improving the Electrochemical Performance of Li2ZnTi3O8 by Surface KCl Modification Huan Yang,† Xiao-Han Wang,† Yong-Xin Qi,† Ning Lun,† Yu-Mei Cao,*,‡ and Yu-Jun Bai*,† †

Key Laboratory for Liquid−Solid Structural Evolution & Processing of Materials (Ministry of Education), Shandong University, 17923 Jingshi Road, Jinan 250061, Shandong P. R. China ‡ Qilu Hospital of Shandong University, 107 Wenhua West Road, Jinan 250014, Shandong P. R. China S Supporting Information *

ABSTRACT: Inorganic salt of KCl was first employed as an effective modifier to modify Li2ZnTi3O8 anode material via simply mixing in KCl solution followed by sintering at 800 °C in air. The Li2ZnTi3O8 modified with 1.0 wt % KCl exhibited splendid rate capabilities (retaining reversible capacities of 225.6, 195.4, 178.0, 162.4, and 135.6 mAh g−1 at 100, 200, 400, 800, and 1600 mA g−1, respectively) and excellent long-term cycling stability (maintaining a capacity of 201.6 mAh g−1 after 700 cycles). Combining structural characterization with electrochemical analysis, the KCl modification leads to simultaneous doping of K+ and Cl− in Li2ZnTi3O8, contributing to enhance the electronic and ionic conductivities of Li2ZnTi3O8. KEYWORDS: Modification, Doping, Li2ZnTi3O8, KCl

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electrode materials and guarantee electrochemical stability.17−19 Molten KCl as the reaction media to synthesize LIB electrode materials of LiCoO2,20 LTO,21 and LiMn1/3Ni1/3Co1/3O222 could influence the particle morphology, structure, and size. The KCl coating on graphite electrode is capable of suppressing the initial irreversibility and enhancing the kinetics.23 Inspired by these investigations available, KCl was first chosen to modify LZTO anode material by one-step mixing of LZTO in KCl solution, followed by high-temperature sintering. The KCl-modified LZTO demonstrates eminent rate capability and cycling stability compared to the pristine LZTO, so the simple fabrication with low expenditure enables the KClmodified LZTO to be promising for industrial application in LIBs.

INTRODUCTION Recently, Li2ZnTi3O8 (LZTO) as anode material for lithiumion battery (LIB) is receiving more attention because of a series of advantages: (1) The three-dimensional network of (Li0.5Zn0.5)tet[Ti1.5Li0.5]octO4 facilitates Li+ intercalation/deintercalation during charge/discharge.1 (2) The theoretical specific capacity of LZTO (229 mAh g−1) increases by 30% compared with that of Li4Ti5O12 (LTO, 175 mAh g−1), but the lithium content decreases by 34.5%, thus reducing the material cost. (3) LZTO could be fabricated simply and environmentally friendly, so it is appropriate for practical production. Even so, the application of LZTO is still greatly affected by its poor electronic conductivity and imperfect high rate performance. To date, some explorations have been done to improve the performance of LZTO, including reducing particles size,2−4 coating conducting materials,5−9 doping with other ions,10−13 and surface modification.14−16 Among these, the surface modification could be realized easily and controllably, and modifiers, such as LiCoO2,14 Li2MoO4,15 La2O3,16 have been reported to ameliorate the electrochemical performance of LZTO, but further investigations are still needed to realize the utilization of LZTO in industry. KCl is a common inorganic salt and has some uses in electrochemistry field. The neutral KCl aqueous as the electrolyte of the capacitor could avoid side reaction with © 2017 American Chemical Society

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EXPERIMENTAL SECTION

Fabrication of LZTO. All the reagents are analytically pure grade and were used without further processing. LZTO was synthesized 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 mixtures at 800 °C for 5 h. Received: March 30, 2017 Revised: June 3, 2017 Published: June 14, 2017 6099

DOI: 10.1021/acssuschemeng.7b00974 ACS Sustainable Chem. Eng. 2017, 5, 6099−6106

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ACS Sustainable Chemistry & Engineering

Figure 1. (a) Cycling performance at 100 mA g−1, as well as Coulombic efficiency of LZTO/KCl, (b) rate capabilities, and (c) long-term cycling performance at 500 mA g−1 for LZTO/KCl.

Table 1. Discharge Capacities (mAh g−1) of the 10th Cycle at Each Current Density (mA g−1) and Capacity Retention (%) capacities/capacity retention at varied current densities sample

100

200

400

800

1600

100

P-LZTO LZTO/KCl

176.6/100 225.6/100

141.7/80.2 195.4/86.6

108.1/61.2 178.0/78.9

57.3/32.4 162.4/71.9

9.8/5.5 135.6/60.1

136.1/77.3 230.9/102.3

Fabrication of KCl-Modified LZTO. The KCl-modified LZTO was prepared according to the LZTO/KCl mass ratio of 1:0.01 by magnetically mixing the as-prepared LZTO (1.5 g) in KCl solution (0.015 g KCl dissolved in 20 mL deionized water) for half an hour, the dried product (at 120 °C for 12 h in an oven) was sintered in a horizontal tube furnace at 800 °C for 5 h in air. The final product was labeled as LZTO/KCl, and the as-prepared LZTO as P-LZTO. Material Characterization. The microstructure was examined 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 energydispersive X-ray spectroscopy (EDS) was adopted on an FEI Tecnai G2 F20 S-TWIN microscope (V = 200 kV). Raman spectra were measured on a Renishaw confocal Raman microspectroscopy with a laser excitation wavelength of 780 nm. Extended X-ray absorption finestructure (EXAFS) measurements for the K K-edge were performed on the XAFS station in Beijing synchrotron radiation facility. K-edge (3608 eV) data were collected at the 4B7A beamline of the spectra in the fluorescence mode with a Si (Li) detector. X-ray photoelectron spectra (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). The crystal structure was confirmed 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. N2 adsorption−desorption isotherms were tested to calculate the specific surface area by Brunauer−Emmett− Teller (BET) method. Assembly of Li-Ion Cells. The assembly of 2025 coin-type halfcells is used to test the electrochemical performance of electrode material. First, LZTO/KCl anode material was fabricated by mixing 80 wt % active material with 10 wt % acetylene black and 10 wt % polyvinylidene difluoride in N-methyl-2-pyrrolidine. Second, the gained slurry was coated onto copper current collector followed by drying at 120 °C for 12 h in a vacuum oven and punched into

electrodes of 14 mm in diameter with the mass loading of active material above 3.0 mg. Last, the half-cells were assembled in an argonfilled glovebox with the moisture and oxygen less than 5 ppm. Lithium foil was used 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. Electrochemical Test. Galvanostatic charge and discharge tests were performed between 0.02 and 3.0 V at 25 °C by a Land CT2001A battery test system. An IviumStat electrochemical workstation was employed to measure the cyclic voltammograms (CV) in a potential range of 0.02−3.0 V (vs Li/Li+) at a scanning rate of 0.3 mV s−1, and the electrochemical impedance spectra (EIS) with a signal amplitude of 5 mV in the frequency range from 100 kHz to 1 mHz.

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RESULTS AND DISCUSSION

Electrochemical Performance. The electrochemical performance of the as-prepared products was tested by galvanostatic discharging/charging (Figure 1). From the cycling curves at 100 mA g−1 (Figure 1a), the initial discharge/charge capacities of LZTO/KCl are 285.1/190.7 (66.9% for Coulombic efficiency), those of P-LZTO are 267.5/166.1 mAh g−1 (62.0% for Coulombic efficiency). The discharge capacity of LZTO in the first cycle is higher than the theoretical value (229 mAh g−1) because the decomposition of electrolyte and formation of solid electrolyte interface (SEI) film consume some Li+.24 With further cycling, P-LZTO displays a drastic capacity fading, and only a capacity of 108.8 mAh g−1 is retained after 100 cycles. In contrast, the cycling of LZTO/KCl is quite stable, and a reversible capacity of 243.3 mAh g−1 is achieved with the Coulombic efficiency of 99.0% after 100 cycles. 6100

DOI: 10.1021/acssuschemeng.7b00974 ACS Sustainable Chem. Eng. 2017, 5, 6099−6106

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ACS Sustainable Chemistry & Engineering

full lithiation was realized in the large particles, the polarization and capacity reached a stable state (Figures 1c and S3). To elucidate the origin of the outstanding electrochemical performance of LZTO/KCl, the detailed characterizations were conducted as described in the following sections. TEM Examination. Microstructure examination was performed by HR-TEM (Figure 3). The interplanar spacings of 0.34 and 0.48 nm correspond to those of (211) and (111) planes of LZTO, respectively. In comparison with the TEM image of P-LZTO (Figure 3a), the lattice fringes adjacent to the surface of LZTO/KCl (Figure 3b) are slightly different from the inner ones, forming a lattice distortion region of about 7 nm in thickness as marked by the red lines.The distortion is related to the superficial doping occurred during sintering at 800 °C, and the doping could be evidenced by HAADF-STEM, EXAFS, Raman spectra, XPS, and XRD. HAADF-STEM Examination, EDS Mapping, and Line Scan. To determine the doping in LZTO/KCl, individual LZTO particles were examined by HAADF-STEM, EDS mapping, and line scan (Figures 4 and 5). From the EDS mappings, besides the uniform distribution of Ti (Figure 4c) and Zn (Figure 4d), Cl (Figure 4e) and K (Figure 4f) were also detected homogeneously on the surface of LZTO particles. The EDS line scan (Figure 5) reveals the counts of K and Cl for 50 evenly distributed points along the white arrow on the surface of LZTO particle (Figure 5a). The counts of K are slightly lower than those of Cl, and in the number from 26 to 32, a weak Cl peak presents at the edge of LZTO particle (Figure 5b), indicating that Cl tends to situate at the outermost surface of LZTO particle because the larger ionic radius of Cl− (1.81 Å) than K+ (1.38 Å)27 restricts the diffusion of Cl− in LZTO. From the Cl peak, the thickness of doping layer is about 10 nm, coinciding with the estimation from TEM examination in Figure 3. X-ray Absorption Spectra. The coordination state of element K in LZTO/KCl was measured by EXAFS. Figure 6 reveals the normalized K-edge absorption spectra for LZTO/ KCl, KCl, and K2CO3. Evidently, the two absorption peaks at 3612 and 3616 eV for K/LZTO/KCl are quite different from those for K/KCl.28 However, the low-energy absorption peak of K/LZTO/KCl locates at the same energy as that of K/ K2CO3.29 The comparative results suggest that element K no longer presents in the form of KCl in the KCl-modified LZTO, but in the form of K2O because the reaction of KCl+CO2+O2 → K2CO3+Cl2 (ΔG > 0) to produce K2CO3 is thermodynamically impossible. The alteration in the relevant coordination state confirms the K+ doping in the LZTO/KCl to some degree. Raman Spectra. Raman spectrum could reflect the variation of an internal molecular structure by virtue of the peak shift. The Raman spectrum of LZTO (Figure 7a) in the region of 200−1000 cm−1 reveals peaks at 238, 267, 359, 403, 440, 528, and 726 cm−1. The bands at 403, 440, and 726 cm−1 correspond to the stretching vibrations of Zn−O (ZnO4),30,31 Li−O (LiO4),32 and Ti−O (TiO6),31 respectively. With respect to the peaks in P-LZTO, the Zn−O (Figure 7b) vibration for LZTO/KCl shifts to higher frequencies, and the Li−O (Figure 7c) and Ti−O (Figure 7d) vibrations barely change, suggesting the substitution of K+ for Zn2+, and rare replacement for Li+ and Ti4+. Consequently, the K+ doping in the KCl-modified LZTO occurs by the substitution of K+ for Zn2+. XPS Analysis. The binding energy of surface elements acquired from XPS could provide further information for

Because of the prominent cycling performance of LZTO/ KCl, the rate capabilities were also assessed at varied current densities from 100 to 1600 mA g−1 each for 10 cycles (Figure 1b). The discharge capacities of the 10th cycle at each density and the capacity retention are summarized in Table 1. Apparently, LZTO/KCl reveals much higher reversible capacity and capacity retention than P-LZTO at the corresponding current densities. The long-term cycling property of LZTO/KCl was also evaluated at a high current density of 500 mA g−1 for 700 cycles (Figure 1c). The initial discharge/charge capacities are 172.4/ 91.1 mAh g−1 (52.9% for Coulombic efficiency), which increase slowly to 200.2/198.9 mAh g−1 (99.4% for Coulombic efficiency) after 100 cycles, and then are approximately stable. The variation of specific capacity is ascribed to the following reasons. (1) The directly cycling at the high rate of 500 mA g−1 would inhibit the formation of stable SEI film or lead to unstable, loose, and inhomogeneous SEI film, thus resulting in the low initial capacity. When the cycle number is over 100, the capacity is almost stable due to the formation of stable SEI film. If the cell was cycled at a low current density of 100 mA g−1, the capacities in the initial cycles are normal without the presence of marked capacity increase (Figure 1a). If the rate performance was tested first from low rate to high rate (100, 200, 400, 800, 1600, and 100 mA g−1 each for 10 cycles) before the long-term cycling at 500 mA g−1, the capacity is also steady at 500 mA g−1 (Figure S1), further demonstrating that a low current rate in the initial cycles is necessary for the formation of steady SEI film and to achieve the stable performance. (2) When directly cycling at 500 mA g−1, the large particles in LZTO/KCl (Figure S2) could not be fully lithiated within a markedly shortened duration (from 171 min at 100 mA g−1 to 20 min at 500 mA g−1 for the discharge time in the first cycle), triggering severe electrochemical polarization at 500 mA g−1 in the initial cycles, but the polarization decreases gradually with cycling and reaches the similar level to that at 100 mA g−1 after about 100 cycles (Figure S3). The weakening polarization gives rise to the gradual increase in capacity for LZTO/KCl. The lithiation in the large particles upon cycling at 500 mA g−1 could be schematically illustrated in Figure 2. In the first cycle,

Figure 2. Schematic illustration for the lithiation in a large LZTO particle upon cycling at 500 mA g−1.

only a certain thickness of LZTO adjacent to the surface was lithiated due to the limited discharge time, resulting in some lattice distortions in the lithiated region, and the defects are conducive to Li+ diffusion,25,26 so the lithiated region would move constantly to the center of particle with cycling accompanying with the further weakening of polarization and the increase in capacity. After about 100 cycles at 500 mA g−1, 6101

DOI: 10.1021/acssuschemeng.7b00974 ACS Sustainable Chem. Eng. 2017, 5, 6099−6106

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Figure 3. HR-TEM images of (a) P-LZTO and (b) LZTO/KCl.

presented in P-LZTO and LZTO/KCl,33 Cl 2p peak also displays in LZTO/KCl. The signal of K 2p could hardly be distinguished in the survey spectrum due to the less amount of K on the surface of LZTO particle and the overlap of K 2p peak (292.93 eV) with C 1s peak (284.78 eV) resulted from the similar binding energies of them, but could be revealed by the core level spectrum of K 2p (Figure 8e). The binding energies of the elements in LZTO/KCl are tabulated in Table S1. The core level Cl 2p spectrum contains two peaks of Zn−Cl34 and Ti−Cl35 with an atomic ratio of 1:0.47 for Zn−Cl: Ti−Cl calculated by the integral area of the two peaks (Figure 8f). Certainly, the Zn−Cl36 and Ti−Cl37 also occur in Zn 2p (Figure 8c) and Ti 2p (Figure 8d) peaks, respectively. Accordingly, Cl− occupies some O2− sites to combine with Zn2+ and Ti4+. The two deconvoluted peaks around 295.7 eV for K 2p1/2 and 292.9 eV for K 2p3/2 further confirm the K+ doping in LZTO/KCl (Figure 8e), as detected in the Raman spectra. XRD Structure Analysis. Besides the Raman spectra and XPS analysis, XRD could supply structure information (Figure 9). The diffraction peaks in Figure 9a match well with those for LZTO (JCPDS no. 86-1512, P4332 space group). The average crystallite size calculated by the Scherrer equation (d = Kγ/ B cos θ)38 adopting the (311) plane is 54.7 nm for P-LZTO and 37.0 nm for LZTO/KCl. The specific surface area is 8.08 m2 g−1 for LZTO and 8.95 2 −1 m g for LZTO/KCl (Figure S4), which could be employed to estimate the average particle size.39 DBET = 6000/ρS

(1)

where DBET is the average particle size (nm), ρ the powder density (which is 3.96 g cm−3 for LZTO, as calculated in SI), and S the specific surface area (m2 g−1) acquired by the BET method. The particle size estimated by eq 1 is 187 nm for LZTO and 168 nm for LZTO/KCl, which is larger than the average crystallite size because one particle is likely agglomerated by several crystallites, as depicted in Figure S2. From the HAADF-STEM observation, the crystallite size is in the range of 25 to 170 nm. The enlarged (311) peak in Figure 9b reveals a slight shift to a lower angle in the KCl-modified LZTO owing to the substitution of K+ (1.38 Å) for Zn2+ (0.74 Å) as well as Cl− (1.81 Å) for O2− (1.4 Å).27 Both the decreased crystallite size and augmented lattice constant (Table S2) in the KCl-modified LZTO are beneficial to enhance the Li+ diffusion in LZTO. Electrochemical Reaction. The Electrochemical reactions occurred in the active materials could be reflected by

Figure 4. (a) HAADF-STEM image of LZTO/KCl and EDS mappings, (b) mixed color map, (c) Ti, (d) Zn, (e) Cl, and (f) K maps.

structural change (Figure 8). In the survey spectra (Figure 8a), the C 1s peak (284.78 eV) is used as a reference for identifying other elements, besides the Li 1s, O 1s, Zn 2p, and Ti 2p peaks 6102

DOI: 10.1021/acssuschemeng.7b00974 ACS Sustainable Chem. Eng. 2017, 5, 6099−6106

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Figure 5. (a) HAADF-STEM image of LZTO/KCl and (b) the counts of K and Cl along the white arrow in panel a acquired from the line scan of EDS.

The discharge/charge curves at 100 mAh g−1 for the second cycle are displayed in Figure 10c to compare the polarization. Evidently, the doping of K+ and Cl− relieves the polarization of LZTO due to the simultaneously improved electronic and ionic conductivities, as will be confirmed by EIS. EIS Analysis. The Nyquist plots and fitting plots obtained by the corresponding equivalent circuit are shown in Figure 11. The fitting plots accord well with the measured ones, and the profiles display two arcs in high-medium frequency range and a sloping line in low-frequency region. In the equivalent circuit, Re is the electrolyte resistance, RSEI the SEI film resistance, RKCl the resistance introduced by KCl modification, Rct the charge transfer resistance, CPE the constant phase element related to the interfacial capacitance, and Zw the Warburg impedance related to Li+ diffusion kinetics (represented by the inclined line at low frequency). The fitted data are listed in Table 2. From Figure 11 and Table 2, except for the similar Re values for P-LZTO and LZTO/KCl, two main aspects could certify the highly enhanced kinetics of LZTO/KCl. On one hand, despite the introduction of RKCl in the KCl-modified LZTO, the lower RSEI, Rct, and RT (RT = Re + RSEI + RKCl + Rct) values for LZTO/KCl than those for P-LZTO manifest the improved charge transfer in the KCl-modified LZTO. On the other hand,

Figure 6. Normalized K-edge absorption spectra of element K for KCl, K2CO3, and LZTO/KCl.

galvanostatic discharge/charge curves. The curves for the first, second, 50th, and 100th cycles at 100 mA g−1 between 0.02 and 3 V are revealed in Figure 10. From Figure 10a and 10b, the main discharge/charge plateaus around 1.1/1.5 V in the initial cycle (migrating toward 1.2 V for discharge and 1.7 V for charge in the subsequent cycles) are related to the transition between Ti3+ and Ti4+.40 The discharge plateaus around 0.7 and 0.5 V in the first cycle are ascribed to the formation of SEI film41 and the multiple restorations of Ti4+,42,43 respectively.

Figure 7. Raman spectra (a), and the enlarged spectra of Zn−O (b), Li−O (c), Ti−O (d) for P-LZTO and LZTO/KCl. 6103

DOI: 10.1021/acssuschemeng.7b00974 ACS Sustainable Chem. Eng. 2017, 5, 6099−6106

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Figure 8. XPS survey spectra (a), high resolution spectra of Li 1s (b), Zn 2p (c), Ti 2p (d), K 2p (e), and Cl 2p (f) peaks for LZTO/KCl.

Figure 9. (a) XRD patterns and (b) the enlarged (311) peaks of P-LZTO and LZTO/KCl.

KCl (Table S3), consistent with the tendency of line slope in Figure 11. Combining the TEM, HAADF-STEM, EDS mapping, line scan, EXAFS, Raman spectra, XPS, and XRD results with the EIS analysis, the enhanced electrochemical performance for LZTO/KCl is attributable to the following reasons. (1) At the sintering temperature of 800 °C, molten KCl (melting point 770 °C) could easily diffuse into LZTO in the form of K+ and Cl−, realizing dual-doping. (2) The simultaneous doping of K+ and Cl− could ameliorate the electronic conductivity of LZTO. (3) The augmented lattice constant in the KCl-modified LZTO is conductive to the Li+ diffusion in LZTO.

the larger line slope at the low frequency for LZTO/KCl denotes the meliorated Li+ diffusion kinetics in LZTO/KCl. The Li+ diffusion coefficient (D) could be assessed by eq 244 Ip = 2.69 × 105n3/2AD1/2ν1/2ΔC0

(2)

where Ip represents 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 = 0.3 mV s−1), and ΔC0 the change of Li+ concentration during electrochemical reaction. (The calculation of ΔC0 is provided in Supporting Information.) Choosing the Ip value for the stable third CV cycle (Figure S5), the calculated D values from eq 2 are 1.4 × 10−13 for P-LZTO, 2.0 × 10−13 for LZTO/ 6104

DOI: 10.1021/acssuschemeng.7b00974 ACS Sustainable Chem. Eng. 2017, 5, 6099−6106

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Figure 10. Discharge/charge curves of P-LZTO (a) and LZTO/KCl (b) and the comparison for the second cycle at 100 mAh g−1 (c).

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nitrogen adsorption−desorption isotherms of P-LZTO and LZTO/KCl, CV plots for P-LZTO and LZTO/KCl, surface composition of LZTO/KCl, lattice parameters for P-LZTO and LZTO/KCl, peak current value and other parameters for calculating the Li+ diffusion coefficient, and calculation for the change in Li + concentration and density of LZTO (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Yu-Mei Cao: Tel/Fax: +86 0531 82169097. E-mail: [email protected]. *Yu-Jun Bai: Tel/Fax: +86 0531 88392315. E-mail: byj97@ 126.com.

Figure 11. Nyquist plots. The inset is the equivalent circuit.

Table 2. Impedance Values Acquired from the Fitted Curves samples

Re (Ω)

RSEI (Ω)

P-LZTO LZTO/KCl

4.3 6.1

852.7 510.8

RKCl (Ω)

Rct (Ω)

RT (Ω)

ORCID

510.8

2437.9 956.0

3294.9 1983.7

Yu-Jun Bai: 0000-0002-8013-9437 Notes

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The authors declare no competing financial interest.

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CONCLUSION The modification of LZTO via inorganic salt KCl during sintering at 800 °C for 5 h results in the simultaneous doping of K+ and Cl− in LZTO, giving rise to the enhanced electronic conductivity along with the meliorated Li-ion diffusion kinetics. Therefore, the KCl-modified LZTO exhibits boosted cycling performance and outstanding rate capability. The surface modification of LZTO by KCl opens up a new route for easy fabrication of green anode materials with low cost and high performance that are applicable in LIBs.

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ACKNOWLEDGMENTS This work was supported by Natural Science Foundation of Shandong Province, P. R. China (ZR2015EM016 and ZR2016EMM18), and by Key Research and Development Program of Shandong Province, P. R. China (2015GGX102005 and 2016GGX102031).

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REFERENCES

(1) Hernandez, V. S.; Martinez, L. M. T.; Mather, G. C.; West, A. R. Stoichiometry, structures and polymorphism of spinel-like phases, Li1.33xZn2−2xTi1+0.67xO4. J. Mater. Chem. 1996, 6 (9), 1533−1536. (2) Hong, Z.; Wei, M.; Ding, X.; Jiang, L.; Wei, K. Li2ZnTi3O8 nanorods: a new anode material for lithium-ion battery. Electrochem. Commun. 2010, 12 (6), 720−723. (3) Hong, Z.; Zheng, X.; Ding, X.; Jiang, L.; Wei, M.; Wei, K. Complex spinel titanate nanowires for a high rate lithium-ion battery. Energy Environ. Sci. 2011, 4 (5), 1886−1891. (4) Wang, L.; Wu, L.; Li, Z.; Lei, G.; Xiao, Q.; Zhang, P. Synthesis and electrochemical properties of Li2ZnTi3O8 fibers as an anode

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00974. Rate performance followed by long-term cycling at 500 mA g−1 for LZTO/KCl, HAADF-STEM image of LZTO/KCl, discharge/charge curves of LZTO/KCl, 6105

DOI: 10.1021/acssuschemeng.7b00974 ACS Sustainable Chem. Eng. 2017, 5, 6099−6106

Research Article

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DOI: 10.1021/acssuschemeng.7b00974 ACS Sustainable Chem. Eng. 2017, 5, 6099−6106