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A new strategy for synthesis of lithium zinc titanate as an anode material for lithium ion batteries
Electrochimica Acta 159 (2015) 102–110
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A new strategy for synthesis of lithium zinc titanate as an anode material for lithium ion batteries Baokuan Chen a , Chaojun Du b , Yezhen Zhang a , Ruixue Sun a , Li Zhou a , Lijuan Wang a, * a b
College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China School of Biochemical and Chemical Engineering, Nanyang Institute of Technology, Nanyang 473000, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 December 2014 Received in revised form 30 January 2015 Accepted 31 January 2015 Available online 2 February 2015
Lithium zinc titanate (Li2ZnTi3O8) anode materials have been firstly synthesized via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 as eutectic molten salts. The effects of sintering temperature and sintering time on the structures and physicochemical properties of the Li2ZnTi3O8 materials are also studied in detail. It is found that Li2ZnTi3O8 obtained by sintering at 700 C for 3 h exhibits a typical cubic spinel structure with P4332 space group. Nano-sized particles are presented and the particles are homogeneous for the Li2ZnTi3O8 prepared by sintering at 700 C for 3 h. Electrochemical tests demonstrate that the sample possesses large capacities. The largest capacities of 167.8 and 142.4 mAh g 1 are delivered at 2 and 3 A g 1, respectively. 137.8 and 113.3 mAh g 1 are kept for the sample at the 100th cycle at the two current densities, respectively. The large discharge specific capacities may be attributed to the good crystallinity, small particle size and low charge-transfer resistance of Li2ZnTi3O8. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Lithium zinc titanate Anode material Molten-salt method Lithium ion batteries
1. Introduction Rechargeable lithium ion batteries have been regarded as promising power sources for hybrid electric vehicles (HEVs) and electric vehicles (EVs) due to their large specific capacity, high power density, high safety and environmental friendliness. Safety is one of the paramount factors in large-format lithium ion batteries applications [1]. Generally speaking, graphite is usually used as an anode material for lithium ion batteries because of its low cost and good cyclic performance. Nevertheless, the potential safety problem limits the wide applications of carbon materials because the operating potential is so close to 0 V (vs. Li/Li+) at the end of Li+ insertion that the dendritic lithium easily grows on the surface of carbon materials during fast-charge or over-charge [2]. In order to solve the crucial safety concern, Li4Ti5O12 has been proposed as a substitute anode material for lithium ion batteries due to its high safety and long cycle life. However, the small theoretical capacity of 175 mAh g 1, poor electronic conductivity of 10 13 S cm 1 and high intercalation potential of 1.5 V (vs. Li/Li+) prevent the practical applications of the Li4Ti5O12 material. Therefore, it is urgent to find out a new alternative anode material.
* Corresponding author. Tel.: +86-377-63513540; fax: +86-377-63168316. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.electacta.2015.01.206 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Cubic spinel structure lithium zinc titanate (Li2ZnTi3O8) with a space group of P4332 was firstly researched as an anode material for lithium ion batteries by Hong etc. in 2010 [3]. According to the crystal structure [4–6], Li2ZnTi3O8 can be described as (Li0.5Zn0.5)tet[Li0.5Ti1.5]octO4, in which Zn exists in the tetrahedral sites and 1:3 cation ordering of Li:Ti is located in the octahedral sites. Hence, a three dimensional network is formed in such a structure, where Li and Zn atoms are located in tetrahedral sites forming tunnels, benefiting to the Li+ reversible intercalation and deintercalation during charging and discharging. Compared with Li4Ti5O12, Li2ZnTi3O8 has larger theoretical capacity of 227 mAh g 1 [3], lower preparation cost and discharge voltage plateau around 0.5 V (vs. Li/Li+), which can enhance the energy density of a full cell when Li2ZnTi3O8 is used as an anode material [7,8]. So far, Li2ZnTi3O8 has been synthesized via conventional solidstate reaction method [9–11], modified solid-state reaction route [3,12], combining the sol-gel chemistry and electrospinning technique [13]. High temperature solid-state reaction is the primary industrial method. However, long ball-milling or grinding time, high sintering temperature and long sintering time are needed and result in a large particle size and the aggregation of the final product. In addition, it is difficult to mix the raw materials homogeneously by the solid-state process and impurity phases can also be found in the final product. In a word, the electrochemical performance of the product obtained via a high temperature solidstate route will be adversely influenced by the factors mentioned
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above. For a solution route, although high reaction temperatures are not required and large capacities can be delivered, long reaction time and some additional processes, such as stirring and heating, are also needed. Therefore, a simple and convenient route to synthesize Li2ZnTi3O8 with good electrochemical performance is highly desired. Molten-salt synthesis has been proven to be a simple one-pot method to obtain single-phase cathode or anode for lithium ion batteries [14,15]. In comparison with the conventional solid-state reaction, the molten-salt method shows an accelerated reaction rate and controllable particle morphology because the molten salt has a high ion diffusion rate and strong dissolving capability. Using eutectic molten mixed lithium salts allows the salts to be uniformly mixed with solid oxide particles at a relative low temperature. As a liquid ion diffuses much more quickly than a solid ion, molten lithium salts can easily adhere to the surface of the solid particles and infiltrate the inside of the solid particles, accelerating the exchanges and reactions between the ions [14]. In the work, we firstly synthesize the Li2ZnTi3O8 anode materials via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 (with a melting point of 175.6 C) as eutectic molten salts. The effects of sintering temperature and sintering time on the
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structures and physicochemical properties of the Li2ZnTi3O8 materials are also studied in detail. 2. Experimental 2.1. Synthesis of Li2ZnTi3O8 materials The Li2ZnTi3O8 anode materials were synthesized by a moltensalt method using 0.38LiOHH2O-0.62LiNO3 (with a melting point of 175.6 C) as eutectic molten salts. In a typical process, TiO2 (anatase, A.R.), LiOHH2O (A.R.), LiNO3 (A.R.) and Zn (CH3COO)22H2O (A.R.) were ground in an agate mortar for 0.5 h at a Li/Zn/Ti = 2.2:1:3. The molar ratio of LiOHH2O to LiNO3 was 0.38:0.62. The mixture was dried at 120 C for 12 h in vacuum, preheated at 250 C for 3 h and subsequently heated at 600 C for 4 h in air. Finally, the pre-heated powder was divided into four parts and sintered at 650, 700, 750 and 800 C for 1 h, respectively. The obtained materials were denoted as LZTO-650-1, LZTO-700-1, LZTO-750-1, LZTO-800-1, respectively. In addition, the samples of LZTO-700-3 and LZTO-700-5 with the third-stage sintering time for 3 and 5 h were also fabricated, respectively. 2.2. Physical characterization and electrochemical measurements The thermal behavior of the precursor was characterized by thermogravimetry (TG) and differential thermogravimetric (DTG) analyses in air from room temperature to 900 C using a RD496 thermal analyzer at a heating rate of 10 C min 1. Structural and crystallographic analyses of the products synthesized were taken from the powder X-ray diffraction data obtained using a Bruker D8 Advance X-ray Diffractometer with Cu Ka radiation (l = 1.54 Å). The diffraction patterns were recorded in the 2u range from 5 to 85 with a scanning speed 6 min 1. The morphologies and particle sizes of the samples were observed by a SU8020 (Japan) scanning electron microscope (SEM). The specific surface areas and pore size distributions were measured by a specific surface area and pore size distribution analyzer (3H2000PS2) via nitrogen adsorption. The specific surface areas were measured by the Brunauer–Emmett–Teller (BET) technique. The pore size distributions were analyzed by Barrett–Joyner–Halenda (BJH) method. The nanoscale microstructure was examined by a high-resolution transmission electron microscope (HR-TEM) (Tecnai G2 F20). The electrochemical measurements were performed using CR2025 coin-type cells. For the fabrication of the anode electrodes, 85 wt.% Li2ZnTi3O8 was mixed with 10 wt.% acetylene black and
Fig. 1. TG–DTG curves of the precursor of Li2ZnTi3O8 (a) and schematic model of the synthetic procedure for Li2ZnTi3O8 (b).
Fig. 2. X-ray diffraction patterns of the as-prepared Li2ZnTi3O8 anode materials synthesized at different sintering temperatures of 650–800 C.
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Fig. 3. SEM images of the as-prepared Li2ZnTi3O8 anode materials synthesized at 650 C (LZTO-650-1) (a), 700 C (LZTO-700-1) (b), 750 C (LZTO-750-1) (c) and 800 C (LZTO-800-1) (d).
Fig. 4. N2 adsorption–desorption isotherms of the as-prepared Li2ZnTi3O8 anode materials synthesized at 650 C (LZTO-650-1) (a), 700 C (LZTO-700-1) (b), 750 C (LZTO-750-1) (c) and 800 C (LZTO-800-1) (d).
B. Chen et al. / Electrochimica Acta 159 (2015) 102–110 Table 1 Specific surface areas, total pore volumes and average pore diameters of LZTO-6501, LZTO-700-1, LZTO-750-1 and LZTO-800-1. Samples
LZTO-6501 LZTO-7003 LZTO-7505 LZTO-8003
Specific surface area (m2 g
Total pore volume Average pore diameter (nm) (mL g 1) 1
)
18.3390
0.1132
20.01
18.0029
0.1138
20.68
11.5093
0.0745
20.58
7.3473
0.0462
20.38
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Celgard2300 as the separator, and 1 M LiPF6 dissolved into a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with the volumetric ratio of 1:1 as the electrolyte. The active material loading was about 4.3 mg cm 2. The charge-discharge measurements were conducted on a Land Battery Test system in the range of 0.05–3.0 V under different current densities. Cyclic voltammograms (CVs) and the electrochemical impedance spectroscopies (EIS) were performed by a Gamry PCI4-750 electrochemical workstation. The CV tests were done between 0.05 and 3.0 V at a scanning rate of 0.5 mV s 1, and the EIS were measured by applying an ac voltage of 5 mV in the frequency range of 10 mHz to 100 kHz. All tests were carried out at room temperature. 3. Results and discussion
5 wt.% polyvinylidene difluoride (PVDF) in an appropriate amount of N-methyl-2-pyrrolidine for 1 h to form slurry. The slurry was pasted onto the copper current collector. Then the electrodes were dried at 120 C in vacuum for 12 h. The coin-type cells were assembled in a glove box filled with high purity argon. The lithium metal foil was served as the counter and reference electrode,
Fig. 1a shows the TG-DTG curves of the precursor for Li2ZnTi3O8 at a heating rate of 10 C min 1 under air atmosphere. It can be seen that there are three obvious weight losses during heating the precursor from room temperature to 900 C, corresponding to the three peaks on the DTG curve. The weight loss in the range from room temperature to 100 C may be related to the evaporation of
Fig. 5. Pore size distributions of the as-prepared Li2ZnTi3O8 anode materials synthesized at 650 C (LZTO-650-1) (a), 700 C (LZTO-700-1) (b), 750 C (LZTO-750-1) (c) and 800 C (LZTO-800-1) (d).
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Fig. 6. Initial charge–discharge curves of the Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C at a constant current density of 0.1 A g 1 from 0.05 to 3.0 V (vs. Li/Li+).
absorbed water from the precursor. Subsequently, the weight loss from 190 to 320 C may originate from the decomposition of zinc acetate dehydrate. In the range of 480–560 C, a sharp weight loss associated with the decomposition of LiOH and LiNO3 appears on the TG curve. When the temperature exceeds 560 C, a platform appears on the TG curve, indicating zero weight loss and the formation of a relatively stable material after 560 C. The melting point of the eutectic molten salts for 0.38LiOHH2O-0.62LiNO3 is 175.6 C. However, there is no sharp weight loss on the corresponding TG curve. It indicates that the eutectic molten salts start to melt at this low temperature of 175.6 C and then infiltrate the surface and interior of the precursor [14]. Based on the analyses, we confirm the optimal three-stage sintering process. The synthetic procedure is schematically depicted in Fig. 1b. Through the heat treatment at 250 C, zinc acetate dehydrate is decomposed into ZnO, and the solid lithium salts turn into liquid and mix intimately with TiO2 and ZnO. The following produced Li2O adheres to the surfaces and then infiltrates the inside of the TiO2 and ZnO particles, which highly shortens the diffusion paths of the ions and accelerates the reaction rate. 3.1. Effects of sintering temperature on Li2ZnTi3O8 The XRD patterns of the Li2ZnTi3O8 anode materials synthesized at different sintering temperatures of 650–800 C are shown in Fig. 2. The diffraction peaks of all samples can be indexed to a cubic spinel structure of Li2ZnTi3O8 with P4332 space group (JCPDS#441037). It suggests that the single-phase Li2ZnTi3O8 can be formed at a relative low temperature of 650 C via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 as eutectic molten salts. The diffraction intensities of the peaks increase with the increase of the sintering temeperature, indicating an increase of crystallinity. It is well known that good crystallinity is beneficial to the electrochemical performance of an electrode material. SEM images are used to analyze the morphologies of the Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C and shown in Fig. 3. It can be seen that the Li2ZnTi3O8 particles show good uniformity and have no obvious coalescence when the sintering temperatures are 650 and 700 C. When the sintering temperature is over 700 C, agglomerations occur and the particle sizes become large for the Li2ZnTi3O8 materials. Small particle size and low degree of particle agglomeration are advantageous to increasing contact area between active material particles and electrolyte, and then making the Li+ intercalation/de-intercalation efficient [10–12].
Fig. 7. Cyclic performance of Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C at various current densities in the range of 0.05–3.0 V.
To further study the LZTO-650-1, LZTO-700-1, LZTO-750-1 and LZTO-800-1, the specific surface areas and pore size distributions are investigated and obtained from N2 adsorption–desorption isotherms shown in Fig. 4. The specific surface areas, the total pore
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Fig. 8. XRD patterns of the Li2ZnTi3O8 anode materials synthesized at 700 C for 1– 5 h.
volumes and average pore diameters of the four samples are listed in Table 1. It can be seen that the specific surface area becomes small with the increase of the sintering temeperature. The specific surface area of LZTO-650-1 and LZTO-700-1 is close. As shown in Fig. 5 and Table 1, the LZTO-700-1 has the largest pore volume and average pore diameter. The large surface area can increase the contact area between active particles and electrolyte, and then more active sites can be used for Li+ intercalation/de-intercalation, which will be advantageous to the reversible capacity. Moreover, the large pores are in favor of electrolyte penetrating into the active materials and the diffusion of Li+ ions. The initial charge–discharge curves of the Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C at a constant current density of 0.1 A g 1 from 0.05 to 3.0 V (vs. Li/Li+) are depicted in Fig. 6. It can be seen that there is a charge plateau about 1.38 V and a corresponding discharge plateau about 0.66 V on the curves for each sample, which is the characteristic of the electrochemical reaction for Li2ZnTi3O8 [9–12]. The initial discharge specific capacities are 191.4, 212.9, 221.3 and 192.3 mAh g 1 and the charge specific capacities are 167.4, 186.4, 193.5 and 178.9 mAh g 1 for LZTO-650-1, LZTO-700-1, LZTO-750-1 and LZTO-800-1, respectively. The small specific capacities of sample LZTO-650-1 may be related to the poor crystallinity. The large specific capacities of sample LZTO-700-1 may originate from its good crystallinity and morphology. The good crystallinity of sample LZTO-750-1 benefits its large specific capacities. The
Fig. 9. Initial charge–discharge curves of the Li2ZnTi3O8 materials synthesized at 700 C for 1–5 h at a constant current density of 0.5 A g 1 from 0.05 to 3.0 V (vs. Li/ Li+).
Fig. 10. Cyclic performance of the Li2ZnTi3O8 materials synthesized at 700 C for 1– 5 h at 0.5 and 0.8 A g 1 in the range of 0.05–3.0 V.
agglomeration and large particles may decrease the specific capacities of sample LZTO-800-1. They are in good agreement with the XRD and SEM results. In addition, the irreversible capacity losses are 24, 26.5, 27.8 and 13.4 mAh g 1 for the four samples mentioned above, respectively. The high irreversible capacity losses should be attributed to the formation of uniform and dense protective layer, which can reduce the side reaction and will be advantageous to the cyclic performance of the samples [12]. Cyclic performance of the Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C at different current densities is shown in Fig. 7. At 0.1 A g 1, the initial discharge specific capacities are 191.4, 212.9, 221.3 and 192.3 mAh g 1 and the discharge specific capacities are 197.1, 226.4, 224.7 and 204.4 mAh g 1 at the 50th cycle for LZTO-650-1, LZTO-700-1, LZTO-750-1 and LZTO-800-1, respectively. At the low current density of 0.1 A g 1, good cyclic performance is presented for the four samples. It may be related to the small electrode polarization for the four samples at low current density. When the current density is increased to 0.2 A g 1, the initial discharge specific capacities are 179.1, 191.8, 201.9 and 191.4 mAh g 1 and the discharge specific capacities are 163.0, 199.1, 192.8 and 190.7 mAh g 1 at the 100th cycle for LZTO-650-1, LZTO-700-1, LZTO-7501 and LZTO-800-1, respectively. At the current density, the sample LZTO-700-1 possesses the best cyclic performance. At 1 A g 1, the initial discharge specific capacities are 165.8, 172.2, 169.9 and 150.3 mAh g 1, and 121.1, 146.0, 142.9 and 134.2 mAh g 1 are delivered at the 100th cycle for LZTO-650-1, LZTO-700-1, LZTO750-1 and LZTO-800-1, respectively. The largest specific capacities are delivered for the sample LZTO-700-1 at the high current density of 1 A g 1. The discharge specific capacities are larger and
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Fig. 12. Impedance spectra of LZTO-700-1, LZTO-700-3 and LZTO-700-5 electrodes, and corresponding equivalent circuit (inset).
the cyclic performance is better than the previous reports [9,11] for the sample LZTO-700-1. However, compared with the low current densities of 0.1 and 0.2 A g 1, the cyclic performance of the four samples gets worse at the high current density of 1 A g 1. The phenomenon occurs in previous reports [9,11]. It is related to the low electronic conductivity of Li2ZnTi3O8, which leads to the poor high rate performance [9–12]. 3.2. Effects of sintering time on Li2ZnTi3O8 Based on the work above, 700 C is the optimal sintering temperature to prepare Li2ZnTi3O8 via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 as eutectic molten salts. The XRD patterns of the Li2ZnTi3O8 anode materials synthesized at 700 C for 1–5 h are depicted in Fig. 8. The diffraction peaks of all samples can be indexed to a cubic spinel structure of Li2ZnTi3O8 with P4332 space group (JCPDS#44-1037). Additionally, no other secondary phases are detected from the XRD patterns. The results are in good agreement with the previous reports by Tang et al. [9–12], Hong et al. [3,4,7,8], Wang et al. [13]. It means that the single-phase Li2ZnTi3O8 can be formed at relatively short sintering time of 1 h via a molten-salt method using 0.38LiOHH2O– 0.62LiNO3 as eutectic molten salts. It is obvious that the diffraction intensities of the peaks increase with the increase of the sintering time, indicating an increase of crystallinity. It is well known that the crystallinity can influence the electrochemical performance of an electrode material. The initial charge–discharge curves of the Li2ZnTi3O8 materials synthesized at 700 C for 1–5 h at a constant current density of 0.5 A g 1 from 0.05 to 3.0 V (vs. Li/Li+) are illustrated in Fig. 9. It can be seen that there is a charge plateau about 1.44 V and a corresponding discharge plateau about 0.54 V on the curves for each sample, which is the characteristic of the electrochemical reaction for Li2ZnTi3O8 and agrees with the literatures [9–12]. The initial discharge specific capacities are 183.5, 204.5 and 192.6 mAh g 1 and the charge specific capacities are 165.4, 192.4 and 174.0 mAh g 1 for LZTO-700-1, LZTO-700-3 and LZTO-700-5, respectively. The small specific capacities of the sample LZTO700-1 may be related to the poorer crystallinity compared with the
Table 2 Impedance parameters calculated from equivalent circuit model.
Fig. 11. Cyclic voltammograms of LZTO-700-1 (a), LZTO-700-3 (b) and LZTO-700-5 (c) electrodes from the 1st to the 6th cycle at a rate of 0.5 mV s 1 in the range of 0.05 3.0 V; cyclic voltammograms of LZTO-700-1, LZTO-700-3 and LZTO-7005 electrodes for the 2nd cycle at a rate of 0.5 mV s 1 in the range of 0.05 3.0 V (d).
Samples
Re (V)
Rct (V)
LZTO-700-1 LZTO-700-3 LZTO-700-5
8.82 4.14 3.14
63.91 42.45 56.99
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Fig. 13. Cyclic performance of the LZTO-700-3 electrode at high current densities of 2 and 3 A g 1 in the range of 0.05–3.0 V.
LZTO-700-3 and LZTO-700-5. The large specific capacities of the samples LZTO-700-3 and LZTO-700-5 may be ascribed to their good crystallinity. They are in good agreement with the XRD results. Fig. 10 shows the cyclic performance of the Li2ZnTi3O8 materials synthesized at 700 C for 1–5 h at 0.5 and 0.8 A g 1 in the range of 0.05–3.0 V. At 0.5 A g 1, the initial discharge specific capacities are 183.5, 204.5 and 192.6 mAh g 1, and 95.8%, 98.4 and 96.2% of the initial capacities are kept at the 100th cycle for LZTO-700-1, LZTO700-3 and LZTO-700-5, respectively. When the current density is increased to 0.8 A g 1, the initial discharge specific capacities become 175.7, 203.4 and 190.0 mAh g 1, and 166.6, 199.4 and 176.0 mAh g 1 are kept at the 100th cycle for LZTO-700-1, LZTO700-3 and LZTO-700-5, respectively. Among the three samples, LZTO-700-3 presents the largest capacities and the best cyclic performance. It may be ascribed to the good crystallinity of LZTO700-3. To further understand the charge–discharge behaviors of the LZTO-700-1, LZTO-700-3 and LZTO-700-5 materials, the CV curves of the three samples have been recorded at a scanning rate of 0.5 mV s 1 in the range of 0.05–3.0 V and the results are shown in Fig. 11. It can be seen that there is a pair of cathodic and anodic peaks for each sample in the potential range of 1.0–2.0 V, which is based on the Ti4+/Ti3+ redox couple. In addtion, from the second cycle, the cathodic peak shifts to high potential for each sample. It might be related to the phase transition between spinel and rock salts [3]. Furthermore, it can be seen that a cathodic peak below 0.5 V is detected for each sample. This might be attributed to multiple restoration of Ti4+, which is in good agreement with the previous reports by Borghols et al. and Ge et al. [16,17]. As shown in Fig. 11d, the sample LZTO-700-3 has the largest curve area and the highest redox peak current, implying that the LZTO-700-3 has the largest capacities and the fastest kinetics for the reversible intercalation and de-intercalation of Li+, which agrees with the charge and discharge results. It can be seen that the three samples have different electrochemical properties. Electrochemical impedance is a major part of internal resistance for a battery, and the in series uncompensated ohmic resistance allied to a high charge transfer resistance and a high mass transport resistance is not benefit for the overall performance exhibited by the overall capacitive behavior. In order to further investigate the different electrochemical behaviors of the LZTO-700-1, LZTO-700-3 and LZTO-7005 electrodes, the electrochemical impedance spectra (EIS) were measured and are presented in Fig. 12. The data were collected on
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as assembled cells at the potential of 2.5 V, respectively. It is remarkable that similar EIS patterns are demonstrated and composed of a small intercept in high frequency, a semicircle in high to medium frequency and a straight line in low frequency. Among, the small intercept represents the ohmic resistance consisting of the contact resistance between electrolyte, separator and electrode; the high frequency semicircle is assigned to the charge-transfer impedance on the electrode/electrolyte interface and the straight line is ascribed to the diffusion of Li+ into the bulk of the electrode material, namely the Warburg resistance. The impedance spectra were fitted by the equivalent circuit model as shown in Fig. 12 (inset), where Re represents the combined impedance of the electrolyte and cell components; CPE is used as a constant phase element; Rct represents the charge-transfer resistance, and W represents Warburg impedance. Table 2 lists the parameters calculated from equivalent circuit model for the samples LZTO-700-1, LZTO-700-3 and LZTO-700-5. As is well known, it is important to have a small charge-transfer resistance for an electrode material with good electrochemical performance. Among the LZTO-700-1, LZTO-700-3 and LZTO-700-5 electrodes, the LZTO-700-3 electrode has the smallest charge-transfer resistance of 42.45 V, which is advantageous to its electrochemical performance. This is good consistent with the charge and discharge results. It is well known that the rate capability is very important if commercially viable systems are to be developed [18]. Fig. 13 shows the cyclic performnce of the LZTO-700-3 electrode at high current densities of 2 and 3 A g 1 in the range of 0.05–3.0 V. Owing to the activation of the LZTO-700-3 electrode at high current densities, the largest capacities of 167.8 and 142.4 mAh g 1 are reached after several cycles at 2 and 3 A g 1, respectively. 137.8 and 113.3 mAh g 1 are kept for the two current densities at the 100th cycle, respectively. The specific capacities are larger or the cyclic performance of the electrode is better than the previous reports [9,11,12]. However, the cyclic performance of the electrode gets worse at the high current densities of 2 and 3 A g 1 than that at the low current densities of 0.1 and 0.2 A g 1. The phenomenon occurs in previous reports [9,11,12]. It is because that the low electronic conductivity worsens the high rate performance of Li2ZnTi3O8 [9–12]. As is well known, small particle size could decrease the lithium ions diffusion and electron-transportation distances and then is beneficial to the rate capability of an electrode. In order to further explain the good rate capability of the LZTO-700-3 electrode, TEM investigation was conducted and the TEM image is displayed in Fig. 14. It is clear that nano-sized particles are presented and the particles are homogeneous with the size about 100 nm.
Fig. 14. TEM image of LZTO-700-3 material.
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4. Conclusions Li2ZnTi3O8 anode materials have been firstly synthesized via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 as eutectic molten salts. It is found that the pure phase of Li2ZnTi3O8 can be successfully synthesized even at low sintering temperature of 650 C and short sintering time of 1 h by the molten-salt method. The Li2ZnTi3O8 material prepared by sintering at 700 C for 3 h delivers large capacities. The largest capacities of 167.8 and 142.4 mAh g 1 are delivered at 2 and 3 A g 1, respectively. 137.8 and 113.3 mAh g 1 are kept for the two current densities at the 100th cycle, respectively. The large specific capacities may be attributed to the good crystallinity, small particle size and low charge-transfer resistance of Li2ZnTi3O8. Acknowledgments We acknowledge financial support from Henan Province Project Education Fund (14A150020), Henan Province Project Education Fund (14A150032) and Scientific and Technological Project of Henan Province (0611023600). References [1] Q. Kuang, Y.M. Zhao, J.T. Xu, Synthesis, structure, electronic, ionic, and magnetic properties of Li9V3(P2O7)3(PO4)2 cathode material for Li-ion batteries, J. Phys. Chem. C 115 (2011) 8422–8429. [2] J. Liu, C.Q. Du, Z.Y. Tang, M. Yang, X.H. Zhang, In situ nickel/carbon coated lithium titanium oxide anode material with improved electrochemical properties, Electrochim. Acta 143 (2014) 56–62. [3] Z.S. Hong, M.D. Wei, X.K. Ding, L.L. Jiang, K.M. Wei, Li2ZnTi3O8 nanorods: A new anode material for lithium-ion battery, Electrochem. Commun. 12 (2010) 720–723. [4] Z.S. Hong, X.Z. Zheng, X.K. Ding, L.L. Jiang, M.D. Wei, K.M. Wei, Complex spinel titanate nanowires for a high rate lithium-ion battery, Energy Environ. Sci. 4 (2011) 1886–1891.
[5] T. Trendafilova, K. Ivanova, D. Kovacheva, Electrical characteristics of Li2MM‘3O8, (M = Mg Zn; M‘ = Ti, Sn), J. Optoelectron. Adv. M. 9 (2007) 271–274. [6] V.S. Hernandez, L.M. Torres Martinez, G.C. Mather, A.R. West, Stoichiometry, structures and polymorphism of spinel-like phases Lil.33xZn2–2xTil + 0.67xO4, J. Mater. Chem. 6 (1996) 1533–1536. [7] Y.X. Xu, Z.S. Hong, L.C. Xia, J. Yang, M.D. Wei, One step sol–gel synthesis of Li2ZnTi3O8/C nanocomposite with enhanced lithium-ion storage properties, Electrochim. Acta 88 (2013) 74–78. [8] Z.S. Hong, T.B. Lan, Y.Z. Zheng, L.L. Jiang, M.D. Wei, Spinel Li2MTi3O8 (M = Mg, Mg0.5Zn0.5) nanowires with enhanced eletrochemical lithium storage, Funct. Mater. Lett. 4 (2011) 65–69. [9] H.Q. Tang, Z.Y. Tang, C.Q. Du, F.C. Qie, J.T. Zhu, Ag-doped Li2ZnTi3O8 as a high rate anode material for rechargeable lithium-ion batteries, Electrochim. Acta 120 (2014) 187–192. [10] H.Q. Tang, J.T. Zhu, C.X. Ma, Z.Y. Tang, Lithium cobalt oxide coated lithium zinc titanate anode material with an enhanced high rate capability and long lifespan for lithium-ion batteries, Electrochim. Acta 144 (2014) 76–84. [11] H.Q. Tang, J.T. Zhu, Z.Y. Tang, C.X. Ma, Al-doped Li2ZnTi3O8 as an effective anode material for lithium-ion batteries with good rate capabilities, J. Electroanal. Chem. 731 (2014) 60–66. [12] H.Q. Tang, Z.Y. Tang, Effect of different carbon sources on electrochemical properties of Li2ZnTi3O8/C anode material in lithium-ion batteries, J. Alloys Compd. 613 (2014) 267–274. [13] L. Wang, L.J. Wu, Z.H. Li, G.T. Lei, Q.Z. Xiao, P. Zhang, Synthesis and electrochemical properties of Li2ZnTi3O8 fibers as an anode material for lithium-ion batteries, Electrochim. Acta 56 (2011) 5343–5346. [14] Z.R. Chang, Z.J. Chen, F. Wu, X.Z. Yuan, H.J. Wang, The synthesis of Li(Ni1/3Co1/ 3Mn1/3)O2 using eutectic mixed lithium salt LiNO3–LiOH, Electrochim. Acta 54 (2009) 6529–6535. [15] M.M. Rahman, J.Z. Wang, M.F. Hassan, D. Wexler, H.K. Liu, Amorphous carbon coated high grain boundary density dual phase Li4Ti5O12-TiO2: A nanocomposite anode material for Li-ion batteries, Adv. Energy Mater. 1 (2011) 212–220. [16] W.J.H. Borghols, M. Wagemaker, U. Lafont, E.M. Kelde, F.M. Mulder, Size effects in the Li4+xTi5O12 spinel, J. Am. Chem. Soc. 131 (2009) 17786–17792. [17] H. Ge, N. Li, D. Li, C. Dai, D. Wang, Electrochemical characteristics of spinel Li4Ti5O12 discharged to 0.01 V, Electrochem. Commun. 10 (2008) 719–722. [18] M.Y. Saidi, J. Barker, H. Huang, J.L. Swoyer, G. Adamson, Performance characteristics of lithium vanadium phosphateas a cathode material for lithium-ion batteries, J. Power Sources 119–121 (2003) 266–272.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A new strategy for synthesis of lithium zinc titanate as an anode material for lithium ion batteries Baokuan Chen a , Chaojun Du b , Yezhen Zhang a , Ruixue Sun a , Li Zhou a , Lijuan Wang a, * a b
College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China School of Biochemical and Chemical Engineering, Nanyang Institute of Technology, Nanyang 473000, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 December 2014 Received in revised form 30 January 2015 Accepted 31 January 2015 Available online 2 February 2015
Lithium zinc titanate (Li2ZnTi3O8) anode materials have been firstly synthesized via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 as eutectic molten salts. The effects of sintering temperature and sintering time on the structures and physicochemical properties of the Li2ZnTi3O8 materials are also studied in detail. It is found that Li2ZnTi3O8 obtained by sintering at 700 C for 3 h exhibits a typical cubic spinel structure with P4332 space group. Nano-sized particles are presented and the particles are homogeneous for the Li2ZnTi3O8 prepared by sintering at 700 C for 3 h. Electrochemical tests demonstrate that the sample possesses large capacities. The largest capacities of 167.8 and 142.4 mAh g 1 are delivered at 2 and 3 A g 1, respectively. 137.8 and 113.3 mAh g 1 are kept for the sample at the 100th cycle at the two current densities, respectively. The large discharge specific capacities may be attributed to the good crystallinity, small particle size and low charge-transfer resistance of Li2ZnTi3O8. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Lithium zinc titanate Anode material Molten-salt method Lithium ion batteries
1. Introduction Rechargeable lithium ion batteries have been regarded as promising power sources for hybrid electric vehicles (HEVs) and electric vehicles (EVs) due to their large specific capacity, high power density, high safety and environmental friendliness. Safety is one of the paramount factors in large-format lithium ion batteries applications [1]. Generally speaking, graphite is usually used as an anode material for lithium ion batteries because of its low cost and good cyclic performance. Nevertheless, the potential safety problem limits the wide applications of carbon materials because the operating potential is so close to 0 V (vs. Li/Li+) at the end of Li+ insertion that the dendritic lithium easily grows on the surface of carbon materials during fast-charge or over-charge [2]. In order to solve the crucial safety concern, Li4Ti5O12 has been proposed as a substitute anode material for lithium ion batteries due to its high safety and long cycle life. However, the small theoretical capacity of 175 mAh g 1, poor electronic conductivity of 10 13 S cm 1 and high intercalation potential of 1.5 V (vs. Li/Li+) prevent the practical applications of the Li4Ti5O12 material. Therefore, it is urgent to find out a new alternative anode material.
* Corresponding author. Tel.: +86-377-63513540; fax: +86-377-63168316. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.electacta.2015.01.206 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
Cubic spinel structure lithium zinc titanate (Li2ZnTi3O8) with a space group of P4332 was firstly researched as an anode material for lithium ion batteries by Hong etc. in 2010 [3]. According to the crystal structure [4–6], Li2ZnTi3O8 can be described as (Li0.5Zn0.5)tet[Li0.5Ti1.5]octO4, in which Zn exists in the tetrahedral sites and 1:3 cation ordering of Li:Ti is located in the octahedral sites. Hence, a three dimensional network is formed in such a structure, where Li and Zn atoms are located in tetrahedral sites forming tunnels, benefiting to the Li+ reversible intercalation and deintercalation during charging and discharging. Compared with Li4Ti5O12, Li2ZnTi3O8 has larger theoretical capacity of 227 mAh g 1 [3], lower preparation cost and discharge voltage plateau around 0.5 V (vs. Li/Li+), which can enhance the energy density of a full cell when Li2ZnTi3O8 is used as an anode material [7,8]. So far, Li2ZnTi3O8 has been synthesized via conventional solidstate reaction method [9–11], modified solid-state reaction route [3,12], combining the sol-gel chemistry and electrospinning technique [13]. High temperature solid-state reaction is the primary industrial method. However, long ball-milling or grinding time, high sintering temperature and long sintering time are needed and result in a large particle size and the aggregation of the final product. In addition, it is difficult to mix the raw materials homogeneously by the solid-state process and impurity phases can also be found in the final product. In a word, the electrochemical performance of the product obtained via a high temperature solidstate route will be adversely influenced by the factors mentioned
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above. For a solution route, although high reaction temperatures are not required and large capacities can be delivered, long reaction time and some additional processes, such as stirring and heating, are also needed. Therefore, a simple and convenient route to synthesize Li2ZnTi3O8 with good electrochemical performance is highly desired. Molten-salt synthesis has been proven to be a simple one-pot method to obtain single-phase cathode or anode for lithium ion batteries [14,15]. In comparison with the conventional solid-state reaction, the molten-salt method shows an accelerated reaction rate and controllable particle morphology because the molten salt has a high ion diffusion rate and strong dissolving capability. Using eutectic molten mixed lithium salts allows the salts to be uniformly mixed with solid oxide particles at a relative low temperature. As a liquid ion diffuses much more quickly than a solid ion, molten lithium salts can easily adhere to the surface of the solid particles and infiltrate the inside of the solid particles, accelerating the exchanges and reactions between the ions [14]. In the work, we firstly synthesize the Li2ZnTi3O8 anode materials via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 (with a melting point of 175.6 C) as eutectic molten salts. The effects of sintering temperature and sintering time on the
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structures and physicochemical properties of the Li2ZnTi3O8 materials are also studied in detail. 2. Experimental 2.1. Synthesis of Li2ZnTi3O8 materials The Li2ZnTi3O8 anode materials were synthesized by a moltensalt method using 0.38LiOHH2O-0.62LiNO3 (with a melting point of 175.6 C) as eutectic molten salts. In a typical process, TiO2 (anatase, A.R.), LiOHH2O (A.R.), LiNO3 (A.R.) and Zn (CH3COO)22H2O (A.R.) were ground in an agate mortar for 0.5 h at a Li/Zn/Ti = 2.2:1:3. The molar ratio of LiOHH2O to LiNO3 was 0.38:0.62. The mixture was dried at 120 C for 12 h in vacuum, preheated at 250 C for 3 h and subsequently heated at 600 C for 4 h in air. Finally, the pre-heated powder was divided into four parts and sintered at 650, 700, 750 and 800 C for 1 h, respectively. The obtained materials were denoted as LZTO-650-1, LZTO-700-1, LZTO-750-1, LZTO-800-1, respectively. In addition, the samples of LZTO-700-3 and LZTO-700-5 with the third-stage sintering time for 3 and 5 h were also fabricated, respectively. 2.2. Physical characterization and electrochemical measurements The thermal behavior of the precursor was characterized by thermogravimetry (TG) and differential thermogravimetric (DTG) analyses in air from room temperature to 900 C using a RD496 thermal analyzer at a heating rate of 10 C min 1. Structural and crystallographic analyses of the products synthesized were taken from the powder X-ray diffraction data obtained using a Bruker D8 Advance X-ray Diffractometer with Cu Ka radiation (l = 1.54 Å). The diffraction patterns were recorded in the 2u range from 5 to 85 with a scanning speed 6 min 1. The morphologies and particle sizes of the samples were observed by a SU8020 (Japan) scanning electron microscope (SEM). The specific surface areas and pore size distributions were measured by a specific surface area and pore size distribution analyzer (3H2000PS2) via nitrogen adsorption. The specific surface areas were measured by the Brunauer–Emmett–Teller (BET) technique. The pore size distributions were analyzed by Barrett–Joyner–Halenda (BJH) method. The nanoscale microstructure was examined by a high-resolution transmission electron microscope (HR-TEM) (Tecnai G2 F20). The electrochemical measurements were performed using CR2025 coin-type cells. For the fabrication of the anode electrodes, 85 wt.% Li2ZnTi3O8 was mixed with 10 wt.% acetylene black and
Fig. 1. TG–DTG curves of the precursor of Li2ZnTi3O8 (a) and schematic model of the synthetic procedure for Li2ZnTi3O8 (b).
Fig. 2. X-ray diffraction patterns of the as-prepared Li2ZnTi3O8 anode materials synthesized at different sintering temperatures of 650–800 C.
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Fig. 3. SEM images of the as-prepared Li2ZnTi3O8 anode materials synthesized at 650 C (LZTO-650-1) (a), 700 C (LZTO-700-1) (b), 750 C (LZTO-750-1) (c) and 800 C (LZTO-800-1) (d).
Fig. 4. N2 adsorption–desorption isotherms of the as-prepared Li2ZnTi3O8 anode materials synthesized at 650 C (LZTO-650-1) (a), 700 C (LZTO-700-1) (b), 750 C (LZTO-750-1) (c) and 800 C (LZTO-800-1) (d).
B. Chen et al. / Electrochimica Acta 159 (2015) 102–110 Table 1 Specific surface areas, total pore volumes and average pore diameters of LZTO-6501, LZTO-700-1, LZTO-750-1 and LZTO-800-1. Samples
LZTO-6501 LZTO-7003 LZTO-7505 LZTO-8003
Specific surface area (m2 g
Total pore volume Average pore diameter (nm) (mL g 1) 1
)
18.3390
0.1132
20.01
18.0029
0.1138
20.68
11.5093
0.0745
20.58
7.3473
0.0462
20.38
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Celgard2300 as the separator, and 1 M LiPF6 dissolved into a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with the volumetric ratio of 1:1 as the electrolyte. The active material loading was about 4.3 mg cm 2. The charge-discharge measurements were conducted on a Land Battery Test system in the range of 0.05–3.0 V under different current densities. Cyclic voltammograms (CVs) and the electrochemical impedance spectroscopies (EIS) were performed by a Gamry PCI4-750 electrochemical workstation. The CV tests were done between 0.05 and 3.0 V at a scanning rate of 0.5 mV s 1, and the EIS were measured by applying an ac voltage of 5 mV in the frequency range of 10 mHz to 100 kHz. All tests were carried out at room temperature. 3. Results and discussion
5 wt.% polyvinylidene difluoride (PVDF) in an appropriate amount of N-methyl-2-pyrrolidine for 1 h to form slurry. The slurry was pasted onto the copper current collector. Then the electrodes were dried at 120 C in vacuum for 12 h. The coin-type cells were assembled in a glove box filled with high purity argon. The lithium metal foil was served as the counter and reference electrode,
Fig. 1a shows the TG-DTG curves of the precursor for Li2ZnTi3O8 at a heating rate of 10 C min 1 under air atmosphere. It can be seen that there are three obvious weight losses during heating the precursor from room temperature to 900 C, corresponding to the three peaks on the DTG curve. The weight loss in the range from room temperature to 100 C may be related to the evaporation of
Fig. 5. Pore size distributions of the as-prepared Li2ZnTi3O8 anode materials synthesized at 650 C (LZTO-650-1) (a), 700 C (LZTO-700-1) (b), 750 C (LZTO-750-1) (c) and 800 C (LZTO-800-1) (d).
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Fig. 6. Initial charge–discharge curves of the Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C at a constant current density of 0.1 A g 1 from 0.05 to 3.0 V (vs. Li/Li+).
absorbed water from the precursor. Subsequently, the weight loss from 190 to 320 C may originate from the decomposition of zinc acetate dehydrate. In the range of 480–560 C, a sharp weight loss associated with the decomposition of LiOH and LiNO3 appears on the TG curve. When the temperature exceeds 560 C, a platform appears on the TG curve, indicating zero weight loss and the formation of a relatively stable material after 560 C. The melting point of the eutectic molten salts for 0.38LiOHH2O-0.62LiNO3 is 175.6 C. However, there is no sharp weight loss on the corresponding TG curve. It indicates that the eutectic molten salts start to melt at this low temperature of 175.6 C and then infiltrate the surface and interior of the precursor [14]. Based on the analyses, we confirm the optimal three-stage sintering process. The synthetic procedure is schematically depicted in Fig. 1b. Through the heat treatment at 250 C, zinc acetate dehydrate is decomposed into ZnO, and the solid lithium salts turn into liquid and mix intimately with TiO2 and ZnO. The following produced Li2O adheres to the surfaces and then infiltrates the inside of the TiO2 and ZnO particles, which highly shortens the diffusion paths of the ions and accelerates the reaction rate. 3.1. Effects of sintering temperature on Li2ZnTi3O8 The XRD patterns of the Li2ZnTi3O8 anode materials synthesized at different sintering temperatures of 650–800 C are shown in Fig. 2. The diffraction peaks of all samples can be indexed to a cubic spinel structure of Li2ZnTi3O8 with P4332 space group (JCPDS#441037). It suggests that the single-phase Li2ZnTi3O8 can be formed at a relative low temperature of 650 C via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 as eutectic molten salts. The diffraction intensities of the peaks increase with the increase of the sintering temeperature, indicating an increase of crystallinity. It is well known that good crystallinity is beneficial to the electrochemical performance of an electrode material. SEM images are used to analyze the morphologies of the Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C and shown in Fig. 3. It can be seen that the Li2ZnTi3O8 particles show good uniformity and have no obvious coalescence when the sintering temperatures are 650 and 700 C. When the sintering temperature is over 700 C, agglomerations occur and the particle sizes become large for the Li2ZnTi3O8 materials. Small particle size and low degree of particle agglomeration are advantageous to increasing contact area between active material particles and electrolyte, and then making the Li+ intercalation/de-intercalation efficient [10–12].
Fig. 7. Cyclic performance of Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C at various current densities in the range of 0.05–3.0 V.
To further study the LZTO-650-1, LZTO-700-1, LZTO-750-1 and LZTO-800-1, the specific surface areas and pore size distributions are investigated and obtained from N2 adsorption–desorption isotherms shown in Fig. 4. The specific surface areas, the total pore
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Fig. 8. XRD patterns of the Li2ZnTi3O8 anode materials synthesized at 700 C for 1– 5 h.
volumes and average pore diameters of the four samples are listed in Table 1. It can be seen that the specific surface area becomes small with the increase of the sintering temeperature. The specific surface area of LZTO-650-1 and LZTO-700-1 is close. As shown in Fig. 5 and Table 1, the LZTO-700-1 has the largest pore volume and average pore diameter. The large surface area can increase the contact area between active particles and electrolyte, and then more active sites can be used for Li+ intercalation/de-intercalation, which will be advantageous to the reversible capacity. Moreover, the large pores are in favor of electrolyte penetrating into the active materials and the diffusion of Li+ ions. The initial charge–discharge curves of the Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C at a constant current density of 0.1 A g 1 from 0.05 to 3.0 V (vs. Li/Li+) are depicted in Fig. 6. It can be seen that there is a charge plateau about 1.38 V and a corresponding discharge plateau about 0.66 V on the curves for each sample, which is the characteristic of the electrochemical reaction for Li2ZnTi3O8 [9–12]. The initial discharge specific capacities are 191.4, 212.9, 221.3 and 192.3 mAh g 1 and the charge specific capacities are 167.4, 186.4, 193.5 and 178.9 mAh g 1 for LZTO-650-1, LZTO-700-1, LZTO-750-1 and LZTO-800-1, respectively. The small specific capacities of sample LZTO-650-1 may be related to the poor crystallinity. The large specific capacities of sample LZTO-700-1 may originate from its good crystallinity and morphology. The good crystallinity of sample LZTO-750-1 benefits its large specific capacities. The
Fig. 9. Initial charge–discharge curves of the Li2ZnTi3O8 materials synthesized at 700 C for 1–5 h at a constant current density of 0.5 A g 1 from 0.05 to 3.0 V (vs. Li/ Li+).
Fig. 10. Cyclic performance of the Li2ZnTi3O8 materials synthesized at 700 C for 1– 5 h at 0.5 and 0.8 A g 1 in the range of 0.05–3.0 V.
agglomeration and large particles may decrease the specific capacities of sample LZTO-800-1. They are in good agreement with the XRD and SEM results. In addition, the irreversible capacity losses are 24, 26.5, 27.8 and 13.4 mAh g 1 for the four samples mentioned above, respectively. The high irreversible capacity losses should be attributed to the formation of uniform and dense protective layer, which can reduce the side reaction and will be advantageous to the cyclic performance of the samples [12]. Cyclic performance of the Li2ZnTi3O8 materials synthesized at different sintering temperatures of 650–800 C at different current densities is shown in Fig. 7. At 0.1 A g 1, the initial discharge specific capacities are 191.4, 212.9, 221.3 and 192.3 mAh g 1 and the discharge specific capacities are 197.1, 226.4, 224.7 and 204.4 mAh g 1 at the 50th cycle for LZTO-650-1, LZTO-700-1, LZTO-750-1 and LZTO-800-1, respectively. At the low current density of 0.1 A g 1, good cyclic performance is presented for the four samples. It may be related to the small electrode polarization for the four samples at low current density. When the current density is increased to 0.2 A g 1, the initial discharge specific capacities are 179.1, 191.8, 201.9 and 191.4 mAh g 1 and the discharge specific capacities are 163.0, 199.1, 192.8 and 190.7 mAh g 1 at the 100th cycle for LZTO-650-1, LZTO-700-1, LZTO-7501 and LZTO-800-1, respectively. At the current density, the sample LZTO-700-1 possesses the best cyclic performance. At 1 A g 1, the initial discharge specific capacities are 165.8, 172.2, 169.9 and 150.3 mAh g 1, and 121.1, 146.0, 142.9 and 134.2 mAh g 1 are delivered at the 100th cycle for LZTO-650-1, LZTO-700-1, LZTO750-1 and LZTO-800-1, respectively. The largest specific capacities are delivered for the sample LZTO-700-1 at the high current density of 1 A g 1. The discharge specific capacities are larger and
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Fig. 12. Impedance spectra of LZTO-700-1, LZTO-700-3 and LZTO-700-5 electrodes, and corresponding equivalent circuit (inset).
the cyclic performance is better than the previous reports [9,11] for the sample LZTO-700-1. However, compared with the low current densities of 0.1 and 0.2 A g 1, the cyclic performance of the four samples gets worse at the high current density of 1 A g 1. The phenomenon occurs in previous reports [9,11]. It is related to the low electronic conductivity of Li2ZnTi3O8, which leads to the poor high rate performance [9–12]. 3.2. Effects of sintering time on Li2ZnTi3O8 Based on the work above, 700 C is the optimal sintering temperature to prepare Li2ZnTi3O8 via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 as eutectic molten salts. The XRD patterns of the Li2ZnTi3O8 anode materials synthesized at 700 C for 1–5 h are depicted in Fig. 8. The diffraction peaks of all samples can be indexed to a cubic spinel structure of Li2ZnTi3O8 with P4332 space group (JCPDS#44-1037). Additionally, no other secondary phases are detected from the XRD patterns. The results are in good agreement with the previous reports by Tang et al. [9–12], Hong et al. [3,4,7,8], Wang et al. [13]. It means that the single-phase Li2ZnTi3O8 can be formed at relatively short sintering time of 1 h via a molten-salt method using 0.38LiOHH2O– 0.62LiNO3 as eutectic molten salts. It is obvious that the diffraction intensities of the peaks increase with the increase of the sintering time, indicating an increase of crystallinity. It is well known that the crystallinity can influence the electrochemical performance of an electrode material. The initial charge–discharge curves of the Li2ZnTi3O8 materials synthesized at 700 C for 1–5 h at a constant current density of 0.5 A g 1 from 0.05 to 3.0 V (vs. Li/Li+) are illustrated in Fig. 9. It can be seen that there is a charge plateau about 1.44 V and a corresponding discharge plateau about 0.54 V on the curves for each sample, which is the characteristic of the electrochemical reaction for Li2ZnTi3O8 and agrees with the literatures [9–12]. The initial discharge specific capacities are 183.5, 204.5 and 192.6 mAh g 1 and the charge specific capacities are 165.4, 192.4 and 174.0 mAh g 1 for LZTO-700-1, LZTO-700-3 and LZTO-700-5, respectively. The small specific capacities of the sample LZTO700-1 may be related to the poorer crystallinity compared with the
Table 2 Impedance parameters calculated from equivalent circuit model.
Fig. 11. Cyclic voltammograms of LZTO-700-1 (a), LZTO-700-3 (b) and LZTO-700-5 (c) electrodes from the 1st to the 6th cycle at a rate of 0.5 mV s 1 in the range of 0.05 3.0 V; cyclic voltammograms of LZTO-700-1, LZTO-700-3 and LZTO-7005 electrodes for the 2nd cycle at a rate of 0.5 mV s 1 in the range of 0.05 3.0 V (d).
Samples
Re (V)
Rct (V)
LZTO-700-1 LZTO-700-3 LZTO-700-5
8.82 4.14 3.14
63.91 42.45 56.99
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Fig. 13. Cyclic performance of the LZTO-700-3 electrode at high current densities of 2 and 3 A g 1 in the range of 0.05–3.0 V.
LZTO-700-3 and LZTO-700-5. The large specific capacities of the samples LZTO-700-3 and LZTO-700-5 may be ascribed to their good crystallinity. They are in good agreement with the XRD results. Fig. 10 shows the cyclic performance of the Li2ZnTi3O8 materials synthesized at 700 C for 1–5 h at 0.5 and 0.8 A g 1 in the range of 0.05–3.0 V. At 0.5 A g 1, the initial discharge specific capacities are 183.5, 204.5 and 192.6 mAh g 1, and 95.8%, 98.4 and 96.2% of the initial capacities are kept at the 100th cycle for LZTO-700-1, LZTO700-3 and LZTO-700-5, respectively. When the current density is increased to 0.8 A g 1, the initial discharge specific capacities become 175.7, 203.4 and 190.0 mAh g 1, and 166.6, 199.4 and 176.0 mAh g 1 are kept at the 100th cycle for LZTO-700-1, LZTO700-3 and LZTO-700-5, respectively. Among the three samples, LZTO-700-3 presents the largest capacities and the best cyclic performance. It may be ascribed to the good crystallinity of LZTO700-3. To further understand the charge–discharge behaviors of the LZTO-700-1, LZTO-700-3 and LZTO-700-5 materials, the CV curves of the three samples have been recorded at a scanning rate of 0.5 mV s 1 in the range of 0.05–3.0 V and the results are shown in Fig. 11. It can be seen that there is a pair of cathodic and anodic peaks for each sample in the potential range of 1.0–2.0 V, which is based on the Ti4+/Ti3+ redox couple. In addtion, from the second cycle, the cathodic peak shifts to high potential for each sample. It might be related to the phase transition between spinel and rock salts [3]. Furthermore, it can be seen that a cathodic peak below 0.5 V is detected for each sample. This might be attributed to multiple restoration of Ti4+, which is in good agreement with the previous reports by Borghols et al. and Ge et al. [16,17]. As shown in Fig. 11d, the sample LZTO-700-3 has the largest curve area and the highest redox peak current, implying that the LZTO-700-3 has the largest capacities and the fastest kinetics for the reversible intercalation and de-intercalation of Li+, which agrees with the charge and discharge results. It can be seen that the three samples have different electrochemical properties. Electrochemical impedance is a major part of internal resistance for a battery, and the in series uncompensated ohmic resistance allied to a high charge transfer resistance and a high mass transport resistance is not benefit for the overall performance exhibited by the overall capacitive behavior. In order to further investigate the different electrochemical behaviors of the LZTO-700-1, LZTO-700-3 and LZTO-7005 electrodes, the electrochemical impedance spectra (EIS) were measured and are presented in Fig. 12. The data were collected on
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as assembled cells at the potential of 2.5 V, respectively. It is remarkable that similar EIS patterns are demonstrated and composed of a small intercept in high frequency, a semicircle in high to medium frequency and a straight line in low frequency. Among, the small intercept represents the ohmic resistance consisting of the contact resistance between electrolyte, separator and electrode; the high frequency semicircle is assigned to the charge-transfer impedance on the electrode/electrolyte interface and the straight line is ascribed to the diffusion of Li+ into the bulk of the electrode material, namely the Warburg resistance. The impedance spectra were fitted by the equivalent circuit model as shown in Fig. 12 (inset), where Re represents the combined impedance of the electrolyte and cell components; CPE is used as a constant phase element; Rct represents the charge-transfer resistance, and W represents Warburg impedance. Table 2 lists the parameters calculated from equivalent circuit model for the samples LZTO-700-1, LZTO-700-3 and LZTO-700-5. As is well known, it is important to have a small charge-transfer resistance for an electrode material with good electrochemical performance. Among the LZTO-700-1, LZTO-700-3 and LZTO-700-5 electrodes, the LZTO-700-3 electrode has the smallest charge-transfer resistance of 42.45 V, which is advantageous to its electrochemical performance. This is good consistent with the charge and discharge results. It is well known that the rate capability is very important if commercially viable systems are to be developed [18]. Fig. 13 shows the cyclic performnce of the LZTO-700-3 electrode at high current densities of 2 and 3 A g 1 in the range of 0.05–3.0 V. Owing to the activation of the LZTO-700-3 electrode at high current densities, the largest capacities of 167.8 and 142.4 mAh g 1 are reached after several cycles at 2 and 3 A g 1, respectively. 137.8 and 113.3 mAh g 1 are kept for the two current densities at the 100th cycle, respectively. The specific capacities are larger or the cyclic performance of the electrode is better than the previous reports [9,11,12]. However, the cyclic performance of the electrode gets worse at the high current densities of 2 and 3 A g 1 than that at the low current densities of 0.1 and 0.2 A g 1. The phenomenon occurs in previous reports [9,11,12]. It is because that the low electronic conductivity worsens the high rate performance of Li2ZnTi3O8 [9–12]. As is well known, small particle size could decrease the lithium ions diffusion and electron-transportation distances and then is beneficial to the rate capability of an electrode. In order to further explain the good rate capability of the LZTO-700-3 electrode, TEM investigation was conducted and the TEM image is displayed in Fig. 14. It is clear that nano-sized particles are presented and the particles are homogeneous with the size about 100 nm.
Fig. 14. TEM image of LZTO-700-3 material.
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4. Conclusions Li2ZnTi3O8 anode materials have been firstly synthesized via a molten-salt method using 0.38LiOHH2O–0.62LiNO3 as eutectic molten salts. It is found that the pure phase of Li2ZnTi3O8 can be successfully synthesized even at low sintering temperature of 650 C and short sintering time of 1 h by the molten-salt method. The Li2ZnTi3O8 material prepared by sintering at 700 C for 3 h delivers large capacities. The largest capacities of 167.8 and 142.4 mAh g 1 are delivered at 2 and 3 A g 1, respectively. 137.8 and 113.3 mAh g 1 are kept for the two current densities at the 100th cycle, respectively. The large specific capacities may be attributed to the good crystallinity, small particle size and low charge-transfer resistance of Li2ZnTi3O8. Acknowledgments We acknowledge financial support from Henan Province Project Education Fund (14A150020), Henan Province Project Education Fund (14A150032) and Scientific and Technological Project of Henan Province (0611023600). References [1] Q. Kuang, Y.M. Zhao, J.T. Xu, Synthesis, structure, electronic, ionic, and magnetic properties of Li9V3(P2O7)3(PO4)2 cathode material for Li-ion batteries, J. Phys. Chem. C 115 (2011) 8422–8429. [2] J. Liu, C.Q. Du, Z.Y. Tang, M. Yang, X.H. Zhang, In situ nickel/carbon coated lithium titanium oxide anode material with improved electrochemical properties, Electrochim. Acta 143 (2014) 56–62. [3] Z.S. Hong, M.D. Wei, X.K. Ding, L.L. Jiang, K.M. Wei, Li2ZnTi3O8 nanorods: A new anode material for lithium-ion battery, Electrochem. Commun. 12 (2010) 720–723. [4] Z.S. Hong, X.Z. Zheng, X.K. Ding, L.L. Jiang, M.D. Wei, K.M. Wei, Complex spinel titanate nanowires for a high rate lithium-ion battery, Energy Environ. Sci. 4 (2011) 1886–1891.
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