- Email: [email protected]
Li3Ti4CoCrO12, a new substituted lithium titanium compound as anode material for lithium ion batteries
Journal of Solid State Chemistry 279 (2019) 120970
Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Li3Ti4CoCrO12, a new substituted lithium titanium compound as anode material for lithium ion batteries Huihui He a, Qian Guo a, Dongyun Zhang a, **, Chengkang Chang a, b, * a b
School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai, 201418, China Shanghai Innovation Institute for Materials, Shanghai University, Shanghai, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Anode material XRD refinement Specific capacity Rate performance Impedance
A new compound Li3Ti4CoCrO12, used as an anodic material for LIBs, was successfully prepared by nano-milling enhanced solid-state reaction method. Retrieved refinement of the X-ray diffraction (XRD) profile indicated that the material crystallizes in an ordinary Fd-3m lattice space group in which the substituted ions Co2þ and Cr3þ occupy the original Liþ and Ti4þ lattice positions, respectively. Upon cyclic voltammetry (CV) measurement, the sample showed a pair of redox process peaking at 1.44 V/1.53 V, implying that only Ti3þ/Ti4þ redox pair was involved in the electrochemical process. Electrochemical impedance spectroscopy (EIS) test suggested that both the low interfacial resistance (8.56Ω) and the high Li ion diffusion (9.86 1010 cm2⋅S1), caused by the mesoporous nature of the anode powder, could be regarded as the important factors for the outstanding electrochemical behavior of the new anode material. The results imply that the synthesized compound Li3Ti4CoCrO12 has high potential for the application in lithium secondary batteries.
1. Introduction Li4Ti5O12 (LTO) has been commonly regarded as a potential candidate for next-generation anode materials (LIB) due to its safe performance and long lifetime [1–3]. LTO presents almost no volume change during the process of Lithium ion insertion/extraction with a high working voltage (1.55 V vs. Li/Liþ ), and therefore excellent electrochemical performance could be ensured by avoiding the deposition of lithium dendrites on the surface rather than graphite electrodes [4–8]. Based on the long lifespan mentioned above, the material has found its application in energy storage field. Despite the advantages listed above, LTO shows some problems in electronic and lithium ion conductivity [9] that cause slow capacity fading during the repeated cycling. So far, several methods have been developed to solve this problem, among which the composition adjustment and the surficial modifications are frequently employed [10,11]. Recently, several methods have been attempted to modify the chemical composition of the LTO compound. One way to change the composition deals with the substitution of Liþ with M2þ (e. g., Ca2þ) [12]. Another method involves replacing Ti4þ with M5þ /M6þ (for example, V5þ, Mo6þ) [13,14]. These methods produce charge-compensated electron defects, which result in an improvement in
electron conductivity so that an promotion in electrochemical performance was observed. This single metal ion substitution has been proven to be a useful method to increase the powder conductivity and ionic diffusion coefficient within the lattice (DLi) [15]. When metal ions holding different valence states are introduced into the tetrahedral 8a lattice site of Liþ site or the Ti4þ octahedron 16d lattice site [16], the electronic conductivity and lithium diffusion coefficient can be enhanced. H. El-Shinawi et al. [17] proposed a double-cation replacement to produce new anode material Li3Ti4NiCrO12 and Li3Ti4MnCrO12 based on the structure of Li4Ti5O12. When Ti4þ and Liþ ions in the Li4Ti5O12 lattice are replaced by Ni2þ /Cr3þ and Mn2þ/Cr3þ ions, new compound of Li3Ti4NiCrO12 and Li3Ti4MnCrO12 can be obtained according to the following formula: Li4 Ti5 O12 þ M2þ þ Cr3þ ¼ Li3 Ti4 MCrO12 þ Ti4þ þ Liþ
(1)
Through this substitution, the newly developed anodes Li3Ti4NiCrO12 and Li3Ti4MnCrO12 have a spinel structure similar to Li4Ti5O12, and very similar electrochemical behavior is observed [17]. However, to our knowledge, no detailed report on the crystalline structure, electrochemical performance of Co2þ/Cr3þ substituted spinel compound was released up to now.
* Corresponding author. School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai, 201418, China. ** Corresponding author. E-mail addresses: [email protected] (D. Zhang), [email protected] (C. Chang). https://doi.org/10.1016/j.jssc.2019.120970 Received 19 July 2019; Received in revised form 18 September 2019; Accepted 18 September 2019 Available online 20 September 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
Table 1 Crystal structure from Rietveld refinements for Li3Ti4CoCrO12 sample. Composition
System and space group Cubic Fd3m
Crystal cell parameters
Atom Li1 Co Li2 Ti Cr O
y/b 0.000 0.000 0.625 0.625 0.625 0.389
R-factor
a (Å) V(Å3) Rp 8.3489(5) 581.96 8.34% Fraction coordinate and total number of equivalent atoms in unit cell
Rwp 9.94%
ƒ 8a
NTotal 2.0001 0.9999 1.0000 4.0002 0.9998 12.0000
Li3Ti4CoCrO12
16d
32e
x/a 0.000 0.000 0.625 0.625 0.625 0.389
z/c 0.000 0.000 0.625 0.625 0.625 0.389
components. The obtained slurry was casted onto an Aluminum foil and then completely dried in a vacuum oven at 120 C. The tested half cells were finally assembled in a glove box filled with Ar. An 1.0M LiPF6 solution, dissolved in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 in volumic ratio), was used as the working electrolyte. Cyclic voltammetry (CV) measurement was performed on an electrochemical workstation (CHI604D) at a voltage range of 0.8–2.5 V (vs. Li /Liþ) with a scan rate of 0.01 mV/s. The constant current charging and discharging cyclic test was performed on a Land testing system with a potential range of 1.0–2.0 V vs.Liþ/Li. Electrochemical impedance spectroscopy (EIS) measurements was carried out with AutoLab PGSTAT302 in the frequency range from 102–105 Hz.
Fig. 1. Observed, calculated and error for XRD data for Li3Ti4CoCrO12 material.
In this paper, Co2þ/Cr3þ co-substitution LTO in the form of Li3Ti4CoCrO12 was prepared by a nano milling enhanced solid reaction method. The phase composition, microstructure and electrochemical behaviors of Li3Ti4CoCrO12 compound were studied. The measured specific capacity of 155.1mAh/g (theoretical capacity: 156 mAh/g), together with a high capacity retention of 97.7% at the end of 100th cycle for the spinel type Li3Ti4CoCrO12 compound, strongly suggested high potential for the application in the field of LIBs.
3. Result and discussion
2. Experimental
3.1. The structure and morphology of the sample Li3Ti4CoCrO12
2.1. Preparation and physical characterization of materials
The phase composition of the synthesized anode material was investigated by XRD, and the result was shown in Fig. 1. The diffraction profile of the prepared anode sample was in good consistence to the standard diffraction pattern of the cubic spinel Li4Ti5O12 phase with a space group of Fd-3m. No additional peak representing other phases was observed from the XRD profile, indicating that the substituted ions (Co2þ, Cr3þ) did not cause the formation of other phases during the high temperature synthesis. Strong diffraction peaks were observed for the sample, implying the establishment of good crystalline materials. All the information obtained from the XRD pattern imply that, well developed pure phase Li3Ti4CoCrO12 anodic powder was successfully prepared through the solid phase reaction under 850 C in air. Rietveld refinement of the XRD profile was employed with software Jade9.5 and the results provide a clear understanding of the crystal structure for the new compound. The structural model of Li4Ti5O12 was taken as the starting structure. According to the ionic radius reported in the document [18], the ionic radii of Liþ, Ti 4þ, Co 2þ and Cr 3þ ions are regarded as following: r(Liþ) ¼ 0.076 nm, r(Ti 4þ) ¼ 0.0605 nm, r(Co2þ) ¼ 0.0745 nm and r(Cr3þ) ¼ 0.0615 nm. The introduced Co2þ and Cr3þ ions were arranged on the original Liþ and Ti4þ lattice sites. Based on the above assumption, whole pattern refinement was conducted. Fig. 1 shows the collected, calculation and error profile of the Li3Ti4CoCrO12 sample, where good agreement can be observed. Table 1 shows the results of crystal structure analysis. Where, Rp is profile residual, Rwp is weighted profile residual, and ƒ is site occupancy. When the value of Rwp less than 10%, the result of the refinement usually means reasonable and reliable [19], x/a, x/b, x/c is the fractional coordinates for the different atoms in the unit cell, NTotal cell is equivalent to the sum of the atoms. From the Rietveld refinement, it can be seen that the Co2þ ions introduced into the crystal structure do replace the original Liþ sites, while the Cr3þ ions were found at the original Ti4þ sites. Such results strongly confirmed that the ionic replacements stated in equation (1) did work in our case. That is, 1 Co2þ ion and 1 Cr3þ ion replace 1 Liþ ion at the 8a lattice site and 1 Ti4þ ion at 16d lattice site, respectively.
Li3Ti4CoCrO12 anodic powder was prepared by a nano milling enhanced solid reaction method. The starting materials used in the experiments are lithium hydroxide (LiOH⋅H2O, 99.5%, Xilong Chemical Co. Ltd., China), titanium dioxide (TiO2, 99.5%, Ishihara industry co. Ltd. Japan), cobalt oxide (CoO, 99%, Adamas) and chromium oxide (Cr2O3, AR, Adamas). An excess of LiOH⋅H2O (5mol%) was provided for the compensation of Li loss during the high-temperature calcination. The chemical composition of target anode material was designed as Li3.15CoCrTi4O12. All the starting materials were completely mixed and then ground in a nano ball mill for 4 h with ZrO2 grits in 0.2 mm diameter as the grinding media. A black ink like slurry was achieved which could be kept for long time without sinking due to the small particle size (around 150 nm confirmed by a laser particle analyzer) after the so called nano milling. The obtained suspension is then dried with a spray dryer, which was operated under 180 C with a fluid supplement of 10 mL/min and precursor powder was obtained. Finally, the spherical precursor powder was calcined at 850 C for 12 h in air and the target compound Li3Ti4CoCrO12 was obtained. Powder X-ray diffraction with Cu radiation (0.15418 nm) for the anode material was carried out using a TD3200 X-ray diffractometer (Dangdong Tongda Co., Ltd). The XRD signals were collected between 10 and 70 (2θ) at a step width of 0.02 . Characterization of morphology and the grain size of the material were completed by a scanning electron microscopy (SEM, JEOL, JSM-6700F). 2.2. Electrochemical measurements The electrochemical behaviors of the compound were evaluated with half cells. The anode electrodes were obtained according to the doctor blade method in which an active slurry was obtained by mixing the active Li3Ti4CoCrO12 powder, conducting carbon and plastic binder (polyvinylidene fluoride, PVDF) in a organic solvent N-methyl-2-pyrrolidine. A weight ratio of 8:1:1 was set during the mixing of the different 2
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
spheres denote lithium (16d), titanium and chromium ions; red spheres denote oxygen ions. The spinel structure viewed from the [010] direction clearly provides certain lattice vacancy (16c) for intercalation of Lithium ions released from the cathode material, and therefore good electrochemical performance similar to Li4Ti5O12 was expected. SEM micrographs of Li3Ti4CoCrO12 sample are presented in Fig. 3. As can be seen from Fig. 3a, some spherical agglomerates were observed, with a size in the range of 1–5 μm, close to the average size of 3 μm measured by a particle size analyzer shown in Fig. 3b. The spherical agglomerates were generated in the spray drying process. In the enlarged micrograph, as shown in Fig. 3c, detailed microstructure of an individual agglomerate is presented. It is clear from the micrograph that the agglomerate is composed of well-developed tiny crystalites with uniform grain size distributed in the range of 50–100 nm. These tiny nano scale crystalites were generated during the nano-milling process, where small ZrO2 grits with diameter of 0.2 mm were used. Fig. 3d presented the size distribution of the slurry sample before spray drying, with an average size about 80 nm, further showed good consistence to the crystalite size of the Li3Ti4CoCrO12 anode material observed in Fig. 3c. In some early reports, nano crystalite s will increase the interfacial surface area between the anodic powder and the liquid electrolyte, reduce the migration pathway for lithium-ions and finally the electrochemical behavior of the prepared electrode was enhanced [20–22]. In addition, mesopores were also observed between the primary crystalites in Fig. 3c. The presences of these mesopores are also favorable for the storage of liquid electrolyte, and therefore the delivery of Li cations within the microspheres was increased. Such nano-micro structure of the spherical anode powder provide both short migration pathway and increased surface area for the redox process and thus better electrochemical properties could be expected.
Table 2 Elements concentration of Li3Ti4CoCrO12 compound. Element
Li
Ti
Co
Cr
Weight percentage Atomic ratio
0.0645 3. 005
0.5921 3.998
0.1822 1.000
0.1612 1.003
Fig. 2. Crystal structure of Li3Ti4CoCrO12 build on software Ball & Stick.
The results of chemical analysis with inductively coupled plasmaoptical emission spectrometry (ICP-OES) are displayed in Table 2. The mole ratios among the four cations (Li: Ti: Co: Cr ¼ 3.005: 3.998: 1.000: 1.003) are in good agreement with the results from XRD refinement, which is the same values we expect. The simulated crystal structure viewed from [010] direction is shown in Fig. 2, where yellow spheres denote lithium (8a) and cobalt ions; green
3.2. Electrochemical performance of Li3Ti4CoCrO12 sample To show the polarization between the discharged and the charged electrodes of the Li3Ti4CoCrO12 anode material, cyclic voltammetry (CV)
Fig. 3. SEM micrographs and size distribution of Li3Ti4CoCrO12 material. 3
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
Fig. 4. Cyclic voltammogram of the Li3Ti4CoCrO12 electrode.
Fig. 5. Electrochemical discharge profile.
behavior
of
Li3Ti4CoCrO12
material
Fig. 6. Electrochemical performance.
behavior
of
Li3Ti4CoCrO12
material
cycling
charge/ Fig. 7. Electrochemical behavior of Li3Ti4CoCrO12 material rate performance.
crystal structure of parent compound LTO. The specific capacity of the redox process is estimated as 155.1mAh/g, very close to the theoretical capacity (156 mAh/g) of the nominal compound. The total charging process offered a capacity of 152.3 mAh/g, which in turn corresponds to a coulombic efficiency of 98.3% for the first run. Such high electrochemical performances do suggest that the newly synthesized compound showed excellent Li storage performance, and practical application in LIBs could be expected. Inspired by the above findings, 100 more runs were finished to show the cyclic behavior of the Li3Ti4CoCrO12 anode powder. The voltage profiles from 2nd to the 100th are similar to that of the first run, and therefore not presented in Fig. 6. The measured specific capacities were shown in Figure 5b, in which the coulombic efficiencies at each run were also displayed. It can be observed from the figure that both the capacity retention and the coulombic efficiency for the tested Li3Ti4CoCrO12 sample were kept at a very high level, 97.7% and 99.5% respectively at the 100th run, indicating high potential of the prepared anodic material. Rate performance was further investigated and the results were presented in Fig. 7. The cells were firstly cycled at 0.2C (current density of 32 mAg1) for 20 more cycles, then the current densities were increased stepwise to 10C, and finally the cells were recovered at 0.2C. The rate capacities were recorded as 154.2, 148.8, 142.4, and 136.1 mAh/g at 0.2, 0.5, 2, 5C, respectively. Even at the rate of 10C, a discharging capacity of 129.6 mAh/g, 83.6% of the initial value, could be reserved for the Li3Ti4CoCrO12 anode material. Furthermore, the electrochemical
test was carried out with a scanning rate of 0.01 mv/s and a voltage range of 0.8–2.5 V. The result was shown in Fig. 4. The sample showed a pair of reversible oxidation/reduction peaks at 1.44 V and 1.53 V, which indicates good electrode behavior of the prepared anode material [23]. No other redox peaks were observed, indicating the reversible electrochemical performance was attributed to the Ti4þ/Ti3þ pairs, not the introduced Co and Cr ions. It also should be pointed out that the observed voltage difference (ΔV) for the oxidation/reduction peaks of the Li3Ti4CoCrO12 electrode is very important to the evaluation of the electrochemical properties of the anodic materials. In our case, the voltage difference for Li3Ti4CoCrO12 material is only 90 mV, which implies very slight polarization during the redox process. The voltage profile at the first cycle between 1.0 and 2.0 V for the Li3Ti4CoCrO12 sample is presented in Fig. 5, which was conducted with a constant current density of 32 mAg1 (0.2 C). During the first discharging process, the material showed a fast decrease in voltage before 1.5 V within several minutes and then a steady plateau about 1.5 V was observed for the tested sample. For the first charging process, the anodic electrode experienced a rapid increase in voltage before 1.5 V, followed by a very stable plateau around 1.58 V. A very small voltage gap of 73 mV was recorded, very close to the value of 90 mV obtained from the CV measurement. Such a redox process could be regarded as the typical reduction/oxidation of Ti4þ/Ti3þ pair. We found that the CV curve of Li3Ti4CoCrO12 is similar to the report from H. El-Shinawi et al. [17], which suggests that Co/Cr replacement does not verify the original 4
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
frequency region of 0.1 Hz–0.01 Hz in Fig. 8. After the calculation, a diffusion coefficient of 9.86 1010 cm2/S for the Li3Ti4CoCrO12 was determined. The obtained diffusion coefficients for Lithium ions (DLi) are summarized in Table 3, together with the data of the previous works in our group and the reported for similar materials from other researchers [29–32] which were also obtained from EIS measurements. When taking an intensive examination of the data in the table, it is found that the electrochemical performance of LTO based anodes can be related to the values of interfacial resistance (Rct) and the diffusion process (DLi). When small Rct with high DLi are observed, the anodic material will display excellent redox behavior. Such a role works in our case. It can be found from Table 3 that the size of the material synthesized by the nano-grinding method is smaller than that of other synthetic methods. Since the Li3Ti4CoCrO12 anode material were composed of mesoporous agglomerates constructed by nano scaled primary crystalites, the increased surface area and reduced diffusion pathway will offer the anode small interfacial resistance and enhanced diffusion coefficient. The lithium-ion diffusion coefficient (DLi) of LTO and Li3Ti4CoCrO12 were calculated as 1.02 1012 cm2 s1 and 9.86 1010 cm2 s1, respectively. It is clear that the lithium-ion diffusion coefficient of the Li3Ti4CoCrO12 is increased by an order of magnitude compared to that of the LTO. This result shows that Co2þ/Cr3þ co-substitution LTO could effectively improve lithium-ion diffusion coefficient. Compared with LTO, Li3Ti4CoCrO12 exhibits much better rate performance. The improved electrochemical performance could be explained mainly by the higher lithium-ion diffusion coefficient. Therefore, in summary, a new anodic material, Li3Ti4CoCrO12, was prepared via a nano milling assisted method for the first time. The material presented excellent electrochemical behavior with high capacity retention and good rate performance caused by the mesoporous nature of the anode powder. All the results indicate great potential of the Li3Ti4CoCrO12 compound as the anode candidate for lithium ion secondary batteries.
Fig. 8. Electrochemical behavior of Li3Ti4CoCrO12 material EIS measurement.
performance can be recovered to 152.6 mAh/g by reducing the current density to 0.2C, indicating a good reversibility for the prepared Li3Ti4CoCrO12 anode material. Such electrochemical performance indicated that the new compound displayed a similar rate behavior to that of parent compound LTO, showing the high potential in fast charging area. By measuring the electrochemical impedance spectrum of the anode material, the reasons for the excellent electrochemical properties are revealed, as shown in Fig. 8. The observed Nyquist plot consists of a quasi-circular among the high frequency region and a quasi-linear behavior at the low frequency region. Such a semi-circle represents a charge transfer process at the interface of the anodic material, whereas the low frequency slanted line represented the Warburg impedance dealing with the lithium diffusion within Li3Ti4CoCrO12 crystal lattice. The EIS spectrum could be expressed with an equivalent circuit as shown in Fig. 8, Where Rs is the solution resistance; CPE indicates the capacitance from the passivation film; Zw stands for Warburg impedance [24–26]. After the simulation, the charge transfer resistance (Rct) for the Li3Ti4CoCrO12 electrode was obtained as small as 8.56 Ω, which can be caused by the increased surface area offered from the mesoporous nature of the prepared anode powder. The lithium ion diffusion coefficient (Dþ Li) can be obtained by the formula as following [27,28]. DLiþ ¼
0:5R2 T 2 A2 F 4 C 2 σ 2W
4. Conclusions In this work, pure phase material Li3Ti4CoCrO12 was prepared using a nano milling assisted solid-state reaction method. XRD refinement suggested that Co/Cr entered the lattice of LTO by replacing the Li and Ti cations respectively. Such replacement creates a new compound with crystal structure similar to LTO. The prepared anode material displayed mesoporous morphology with primary particle sizes of 50–100 nm, which offers excellent performance for LIBs. At 0.2C, the anode material displayed a high initial charging capacity of 155.1 mAh/g , close to its theoretical capacity. At the end of 100th cycle, capacity retention of 97.7% was recorded. The small grain size and mesoporous structure of the prepared anode material were regarded as the important factors since they offer short pathway and increased surface area for the redox process. Small interfacial resistance of 8.56 Ω and high Li diffusion coefficient of 9.86 1010 cm2/s were confirmed for Li3Ti4CoCrO12 anode material from EIS measurement. The work we presented provides a new anodic material with stable electrochemical performance, offers an alternative for lithium secondary batteries.
(2)
wherein R is the gas constant, T is the absolute temperature, c is the concentration of Liþ in the material, F is the Faraday constant, A is the surface area of the electrodes, and σw is the Warburg factor obeying the equation as follows: ZRe ¼ Rs þ Rct þ σω0:5
(3)
ZRe is the real part of the experimental impedance data, therefore σw can be calculated from the linear fitting of ZRe vs. ω0.5 in the low Table 3 Diffusion coefficients and resistance values from the equivalent circuits. Sample
Capacity mAh/g
Capacity retention (%)
Columbic efficiency(%)
Rate Perfor-mance (%)
Rct (Ω)
DLi (cm2/S)
Method and partical size
Reference
Li3Ti4CoCrO12 G-LTO R-LTO LTO LTO-F LTOS3 LTOS4 LTO LTO-ZnO
155.1 172.4 165.1 155.0 170.4 170.2 172.2 162.3 165.2
97.7% 96% 86.2% / / 91.0% 92.6% 73.1% 90.3%
98.3% 98.6% 96.9% 97.4% 96.8% 96.3% 92.2% / /
83.6%(10C) 59.22%(10C) / 37.4%(10C) 60.1%(10C) 84.6%(20C) 85.8%(20C) 72.8%(10C) 76.4%(10C)
8.56 / / 80.6 56.4 37.5 22.7 75.9 46.7
9.86 1010 1.02 1012 1.61 1013 1.69 1010 2.51 1010 4.4 1010 9.8 1010 2.6 1010 5.7 1010
Nano-milling, 50–100 nm Nano-milling, < 300 nm Nano-milling, < 300 nm Solid-state reaction, 1 μm Solid-state reaction, 1 μm ionothermal method, 200–500 nm ionothermal method, 200–500 nm Hydrothermal Hydrothermal
This work 29 29 30 30 31 31 32 32
5
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
Conflicts of interest
[15] R. Cai, S.M. Jiang, X. Yu, B.T. Zhao, H.T. Wang, Z.P. Zhao, A novel method to enhance rate performance of an Al-doped Li4Ti5O12 electrode by post-synthesis treatment in liquid formaldehyde at room temperature, J. Mater. Chem. 22 (2012) 8013–8021. https://doi.org/10.1039/c2jm15731d. [16] G.B. Xu, L.W. Yang, X.L. Wei, J.W. Ding, J.X. Zhong, P.K. Chu, Highly-crystalline ultrathin gadolinium doped and carbon-coated Li4Ti5O12 nanosheets for enhanced lithium storage, J. Power Sources 295 (2015) 305–313. https://doi.org/10.1016/j .jpowsour.2015.06.131. [17] H. El Shinawi, M. Falk, C. Greaves, Li3Ti4NiMnO12 and Li3Ti4NiCrO12: new substituted lithium titanium oxides, Solid State Sci. 22 (2013) 65–70. https://doi.org/10.1016/j.solidstatesciences. 2013.05.012. [18] S. James, Lange’s Handbook of Chemistry, sixteenth ed., McGraw-Hill Professional Publishing, New York, 2004. [19] J.K. Liang, Powder diffraction pattern fitting correction crystal structure, J. Struct. Chem. 4 (1985) 241–260. [20] G.R. Hu, X.L. Zhang, Z.D. Peng, Preparation and electrochemical performance of tantalum- doped lithium titanate as anode material for lithium-ion battery, Trans. Nonferrous Metals Soc. China 21 (2011) 2248–2253. https://doi.org/10.1016/S1003-6326(11)61003-0. [21] Q. Zhang, C. Zhang, B. Li, D. Jiang, S. Kang, X. Li, Y. Wang, Preparation and characterization of W-doped Li4Ti5O12 anode material for enhancing the high rate performance, Electrochim. Acta 107 (2013) 139–146. https://doi.org/10.1016/j.el ectacta.2013.05.151. [22] Z. Zhang, L. Cao, J. Huang, S. Zhou, Y. Huang, Y. Cai, Hydrothermal synthesis of Zndoped Li4Ti5O12 with improved high rate properties for lithium ion batteries, Ceram. Int. 39 (2013) 6139–6143. https://doi.org/10.1016/j.ceramint.2013.01 .032. [23] Q.Y. Zhang, Michael G. Verde, J.K. Seo, X. Li, Y.S. Meng, Structural and electrochemical properties of Gd-doped Li4Ti5O12 as anode material with improved rate capability for lithium-ion batteries, J. Power Sources 280 (2015) 355–362. https://doi.org/10.1016/j.jpowsour.2015.01.124. [24] A.Y. Shenouda, H.K. Liu, J., Electrochemical behaviour of tin borophosphate negative electrodes for energy storage systems, J. Power Sources 185 (2008) 1386–1391. https://doi.org/10.1016/j.jpowsour.2008.08.042. [25] A.Y. Shenouda, K.R. Murali, Electrochemical properties of doped lithium titanate compounds and their performance in lithium rechargeable batteries, J. Power Sources 176 (2008) 332–339. https://doi.org/10.1016/j.jpowsour.200 7.10.061. [26] B. Li, C. Han, Y.B. He, C. Yang, H. Du, C. Yuan, F. Kang, Facile synthesis of Li4Ti5O12/C composite with super rate performance, Energy Environ. Sci. 5 (2012) 9595–9602. https://doi.org/10.1039/c2ee22591c. [27] X. Li, H. Xin, Y. Liu, D. Li, X. Yuan, X. Qin, Effect of niobium doping on the microstructure and electrochemical properties of lithium-rich layered Li [Li0.2Ni0.2Mn0.6]O2 as cathode materials for lithium ion batteries, RSC Adv. 5 (2015) 45351–45358. https://doi.org/10.1039/c5ra01798j. [28] D. Luo, S. Fang, L. Yang, S.i. Hirano, Improving the electrochemical performance of layered Li-rich transition-metal oxides by alleviating the blockade effect of surface lithium, J. Mater. Chem. 4 (2016) 5184–5190. https://doi.org/10.1039/c 5ta10779b. [29] Y.W. Shi, D.Y. Zhang, C.K. Chang, Enhanced electrochemical performance of oxygen-deficient Li4Ti5O12-x anode material induced by graphene oxide, J. Alloy. Comp. 639 (2015) 274–279. [30] X. Bai, W. Li, A. Wei, Q. Chang, L. Zhang, Z. Liu, Preparation and electrochemical performance of F-doped Li4Ti5O12 for use in the lithium-ion batteries, Solid State Ion. 324 (2018) 13–19. https://doi.org/10.1016/j.ssi.2018.06.005. [31] X. Li, Y. Meng, S. i Liu, H. Jin, Z. Xiao, Ionothermal synthesis of sponge-like nano Li4Ti5O12 for high rate lithium-ion batteries, J. Solid State Electrochem. 19 (2015) 1745–1753. https://doi.org/10.1007/s10008-015-2810-2. [32] C. Han, Y. He, H. Li, B. Li, H. Du, X. Qin, F. Kang, Suppression of interfacial reactions between Li4Ti5O12 electrode and electrolyte solution via zinc oxide coating, Electrochim. Acta 157 (2015) 266–273. https://doi.org/10.1016/j.el ectacta.2014.12.080.
The authors declare that they have no conflict of interest. Acknowledgement This work was supported by Science and Technology Commission of Shanghai Municipality [grant numbers 14520503100, 201310-JD-B2009]; the Shanghai Municipal Education Commission [grant number 15ZZ095]. References [1] K.C. Hsiao, S.C. Liao, J.M. Chen, Microstructure effect on the electrochemical property of Li4Ti5O12 as an anode material for lithium-ion batteries, Electrochim. Acta 53 (2008) 7242–7247. https://doi.org/10.1016/j.electacta.2008.05.002. [2] H.L. Zhao, Y.L. Li, Z.M. Zhu, J. Lin, Z.H. Tian, R.L. Wang, Structural and electrochemical characteristics of Li4xAlxTi5O12 as anode material for lithium-ion batteries, Electrochim. Acta 53 (2008) 7079–7083. https://doi.org/10.1016/j.el ectacta.2008.05.038. [3] C.H. Jiang, M. Ichihara, I. Honma, H.S. Zhou, Effect of particle dispersion on high rate performance of nano-sized Li4Ti5O12 anode, Electrochim. Acta 52 (2007) 6470–6475. https://doi.org/10.1016/j.electacta.2007.04.070. [4] A.D. Robertson, L. Trevino, H. Tukamoto, J.T.S. Irvine, New inorganic spinel oxides for use as negative electrode materials in future lithium-ion batteries, J. Power Sources 81 (1999) 352–357. https://doi.org/10.1016/S0378-7753(98)00217-1. [5] T. Ohzuku, A. Ueda, N. Yamamato, Zero-strain insertion material of Li[Li1/3Ti5/3]O4 for rechargeable lithium cells, J. Electrochem. Soc. 142 (1995) 1431–1435. https:// doi.org/10.1149/1.2048592. [6] S. Scharner, W. Weppener, P. Schmid-Beurmann, Evidence of two-phase formation upon lithium insertion into the Li1.33Ti1.67O4 spinel, J. Electrochem. Soc. 146 (1999) 857–861. https://doi.org/10.1149/1.1391692. [7] F. Ronci, P. Reale, B. Scrosati, S. Panero, V.R. Albertini, P. Perfetti, M.D. Michiel, J.M. Merino, High-resolution in-situ structural measurements of the Li4/3Ti5/3O4 “Zero-Strain” insertion material, J. Phys. Chem. B 106 (2002) 3082–3086. https:// doi.org/10.1021/jp013240p. [8] K.M. Colbow, J.R. Dahn, R.R. Haering, Structure and electrochemistry of the spinel oxides LiTi2O4 and LiTiO4, J. Power Sources 26 (1989) 397–402. https://doi.org/10.1016/0378-7753(89)80152-1. [9] X.C. Sun, P.V. Radovanovic, B. Cui, Advances in spinel Li4Ti5O12 anode materials for lithium-ion batteries, New J. Chem. 39 (2015) 38–63. https://doi.org/10. 1002/chin.201510292. [10] W.W. Lee, J.M. Lee, Novel synthesis of high performance anode materials for lithium-ion batteries (LIBs), J. Mater. Chem. 2 (2014) 1589–1626. https://doi.org /10.1039/c3ta12830j. [11] G.N. Zhu, Y.G. Wang, Y.Y. Xia, Ti-based compounds as anode materials for Li-ion batteries, Energy Environ. Sci. 5 (2012) 6652–6667. https://doi.org/10.1039/c 2ee03410g. [12] Q. Zhang, C. Zhang, B. Li, S. Kang, X. Li, Y. Wang, Preparation and electrochemical properties of Ca-doped Li4Ti5O12 as anode materials in lithium-ion battery, Electrochim. Acta 98 (2013) 146–152. https://doi.org/10.1016/j.electacta.2013.03.006. [13] Z. Yu, X. Zhang, G. Yang, J. Liu, J. Wang, R. Wang, J. Zhang, High rate capability and long-term cyclability of Li4Ti4.9V0.1O12 as anode material in lithium ion battery, Electrochim. Acta 56 (2011) 8611–8617. https://doi.org/10.1016/j.electacta.20 11.07.051. [14] T.F. Yi, Y. Xie, L.J. Jiang, J. Shu, C.B. Yue, Advanced electrochemical properties of Mo-doped Li4Ti5O12 anode material for power lithium ion battery, RSC Adv. 2 (2012) 3541–3547. https://doi.org/10.1039/c2ra00981a.
6
Contents lists available at ScienceDirect
Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Li3Ti4CoCrO12, a new substituted lithium titanium compound as anode material for lithium ion batteries Huihui He a, Qian Guo a, Dongyun Zhang a, **, Chengkang Chang a, b, * a b
School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai, 201418, China Shanghai Innovation Institute for Materials, Shanghai University, Shanghai, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Anode material XRD refinement Specific capacity Rate performance Impedance
A new compound Li3Ti4CoCrO12, used as an anodic material for LIBs, was successfully prepared by nano-milling enhanced solid-state reaction method. Retrieved refinement of the X-ray diffraction (XRD) profile indicated that the material crystallizes in an ordinary Fd-3m lattice space group in which the substituted ions Co2þ and Cr3þ occupy the original Liþ and Ti4þ lattice positions, respectively. Upon cyclic voltammetry (CV) measurement, the sample showed a pair of redox process peaking at 1.44 V/1.53 V, implying that only Ti3þ/Ti4þ redox pair was involved in the electrochemical process. Electrochemical impedance spectroscopy (EIS) test suggested that both the low interfacial resistance (8.56Ω) and the high Li ion diffusion (9.86 1010 cm2⋅S1), caused by the mesoporous nature of the anode powder, could be regarded as the important factors for the outstanding electrochemical behavior of the new anode material. The results imply that the synthesized compound Li3Ti4CoCrO12 has high potential for the application in lithium secondary batteries.
1. Introduction Li4Ti5O12 (LTO) has been commonly regarded as a potential candidate for next-generation anode materials (LIB) due to its safe performance and long lifetime [1–3]. LTO presents almost no volume change during the process of Lithium ion insertion/extraction with a high working voltage (1.55 V vs. Li/Liþ ), and therefore excellent electrochemical performance could be ensured by avoiding the deposition of lithium dendrites on the surface rather than graphite electrodes [4–8]. Based on the long lifespan mentioned above, the material has found its application in energy storage field. Despite the advantages listed above, LTO shows some problems in electronic and lithium ion conductivity [9] that cause slow capacity fading during the repeated cycling. So far, several methods have been developed to solve this problem, among which the composition adjustment and the surficial modifications are frequently employed [10,11]. Recently, several methods have been attempted to modify the chemical composition of the LTO compound. One way to change the composition deals with the substitution of Liþ with M2þ (e. g., Ca2þ) [12]. Another method involves replacing Ti4þ with M5þ /M6þ (for example, V5þ, Mo6þ) [13,14]. These methods produce charge-compensated electron defects, which result in an improvement in
electron conductivity so that an promotion in electrochemical performance was observed. This single metal ion substitution has been proven to be a useful method to increase the powder conductivity and ionic diffusion coefficient within the lattice (DLi) [15]. When metal ions holding different valence states are introduced into the tetrahedral 8a lattice site of Liþ site or the Ti4þ octahedron 16d lattice site [16], the electronic conductivity and lithium diffusion coefficient can be enhanced. H. El-Shinawi et al. [17] proposed a double-cation replacement to produce new anode material Li3Ti4NiCrO12 and Li3Ti4MnCrO12 based on the structure of Li4Ti5O12. When Ti4þ and Liþ ions in the Li4Ti5O12 lattice are replaced by Ni2þ /Cr3þ and Mn2þ/Cr3þ ions, new compound of Li3Ti4NiCrO12 and Li3Ti4MnCrO12 can be obtained according to the following formula: Li4 Ti5 O12 þ M2þ þ Cr3þ ¼ Li3 Ti4 MCrO12 þ Ti4þ þ Liþ
(1)
Through this substitution, the newly developed anodes Li3Ti4NiCrO12 and Li3Ti4MnCrO12 have a spinel structure similar to Li4Ti5O12, and very similar electrochemical behavior is observed [17]. However, to our knowledge, no detailed report on the crystalline structure, electrochemical performance of Co2þ/Cr3þ substituted spinel compound was released up to now.
* Corresponding author. School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai, 201418, China. ** Corresponding author. E-mail addresses: [email protected] (D. Zhang), [email protected] (C. Chang). https://doi.org/10.1016/j.jssc.2019.120970 Received 19 July 2019; Received in revised form 18 September 2019; Accepted 18 September 2019 Available online 20 September 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
Table 1 Crystal structure from Rietveld refinements for Li3Ti4CoCrO12 sample. Composition
System and space group Cubic Fd3m
Crystal cell parameters
Atom Li1 Co Li2 Ti Cr O
y/b 0.000 0.000 0.625 0.625 0.625 0.389
R-factor
a (Å) V(Å3) Rp 8.3489(5) 581.96 8.34% Fraction coordinate and total number of equivalent atoms in unit cell
Rwp 9.94%
ƒ 8a
NTotal 2.0001 0.9999 1.0000 4.0002 0.9998 12.0000
Li3Ti4CoCrO12
16d
32e
x/a 0.000 0.000 0.625 0.625 0.625 0.389
z/c 0.000 0.000 0.625 0.625 0.625 0.389
components. The obtained slurry was casted onto an Aluminum foil and then completely dried in a vacuum oven at 120 C. The tested half cells were finally assembled in a glove box filled with Ar. An 1.0M LiPF6 solution, dissolved in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 in volumic ratio), was used as the working electrolyte. Cyclic voltammetry (CV) measurement was performed on an electrochemical workstation (CHI604D) at a voltage range of 0.8–2.5 V (vs. Li /Liþ) with a scan rate of 0.01 mV/s. The constant current charging and discharging cyclic test was performed on a Land testing system with a potential range of 1.0–2.0 V vs.Liþ/Li. Electrochemical impedance spectroscopy (EIS) measurements was carried out with AutoLab PGSTAT302 in the frequency range from 102–105 Hz.
Fig. 1. Observed, calculated and error for XRD data for Li3Ti4CoCrO12 material.
In this paper, Co2þ/Cr3þ co-substitution LTO in the form of Li3Ti4CoCrO12 was prepared by a nano milling enhanced solid reaction method. The phase composition, microstructure and electrochemical behaviors of Li3Ti4CoCrO12 compound were studied. The measured specific capacity of 155.1mAh/g (theoretical capacity: 156 mAh/g), together with a high capacity retention of 97.7% at the end of 100th cycle for the spinel type Li3Ti4CoCrO12 compound, strongly suggested high potential for the application in the field of LIBs.
3. Result and discussion
2. Experimental
3.1. The structure and morphology of the sample Li3Ti4CoCrO12
2.1. Preparation and physical characterization of materials
The phase composition of the synthesized anode material was investigated by XRD, and the result was shown in Fig. 1. The diffraction profile of the prepared anode sample was in good consistence to the standard diffraction pattern of the cubic spinel Li4Ti5O12 phase with a space group of Fd-3m. No additional peak representing other phases was observed from the XRD profile, indicating that the substituted ions (Co2þ, Cr3þ) did not cause the formation of other phases during the high temperature synthesis. Strong diffraction peaks were observed for the sample, implying the establishment of good crystalline materials. All the information obtained from the XRD pattern imply that, well developed pure phase Li3Ti4CoCrO12 anodic powder was successfully prepared through the solid phase reaction under 850 C in air. Rietveld refinement of the XRD profile was employed with software Jade9.5 and the results provide a clear understanding of the crystal structure for the new compound. The structural model of Li4Ti5O12 was taken as the starting structure. According to the ionic radius reported in the document [18], the ionic radii of Liþ, Ti 4þ, Co 2þ and Cr 3þ ions are regarded as following: r(Liþ) ¼ 0.076 nm, r(Ti 4þ) ¼ 0.0605 nm, r(Co2þ) ¼ 0.0745 nm and r(Cr3þ) ¼ 0.0615 nm. The introduced Co2þ and Cr3þ ions were arranged on the original Liþ and Ti4þ lattice sites. Based on the above assumption, whole pattern refinement was conducted. Fig. 1 shows the collected, calculation and error profile of the Li3Ti4CoCrO12 sample, where good agreement can be observed. Table 1 shows the results of crystal structure analysis. Where, Rp is profile residual, Rwp is weighted profile residual, and ƒ is site occupancy. When the value of Rwp less than 10%, the result of the refinement usually means reasonable and reliable [19], x/a, x/b, x/c is the fractional coordinates for the different atoms in the unit cell, NTotal cell is equivalent to the sum of the atoms. From the Rietveld refinement, it can be seen that the Co2þ ions introduced into the crystal structure do replace the original Liþ sites, while the Cr3þ ions were found at the original Ti4þ sites. Such results strongly confirmed that the ionic replacements stated in equation (1) did work in our case. That is, 1 Co2þ ion and 1 Cr3þ ion replace 1 Liþ ion at the 8a lattice site and 1 Ti4þ ion at 16d lattice site, respectively.
Li3Ti4CoCrO12 anodic powder was prepared by a nano milling enhanced solid reaction method. The starting materials used in the experiments are lithium hydroxide (LiOH⋅H2O, 99.5%, Xilong Chemical Co. Ltd., China), titanium dioxide (TiO2, 99.5%, Ishihara industry co. Ltd. Japan), cobalt oxide (CoO, 99%, Adamas) and chromium oxide (Cr2O3, AR, Adamas). An excess of LiOH⋅H2O (5mol%) was provided for the compensation of Li loss during the high-temperature calcination. The chemical composition of target anode material was designed as Li3.15CoCrTi4O12. All the starting materials were completely mixed and then ground in a nano ball mill for 4 h with ZrO2 grits in 0.2 mm diameter as the grinding media. A black ink like slurry was achieved which could be kept for long time without sinking due to the small particle size (around 150 nm confirmed by a laser particle analyzer) after the so called nano milling. The obtained suspension is then dried with a spray dryer, which was operated under 180 C with a fluid supplement of 10 mL/min and precursor powder was obtained. Finally, the spherical precursor powder was calcined at 850 C for 12 h in air and the target compound Li3Ti4CoCrO12 was obtained. Powder X-ray diffraction with Cu radiation (0.15418 nm) for the anode material was carried out using a TD3200 X-ray diffractometer (Dangdong Tongda Co., Ltd). The XRD signals were collected between 10 and 70 (2θ) at a step width of 0.02 . Characterization of morphology and the grain size of the material were completed by a scanning electron microscopy (SEM, JEOL, JSM-6700F). 2.2. Electrochemical measurements The electrochemical behaviors of the compound were evaluated with half cells. The anode electrodes were obtained according to the doctor blade method in which an active slurry was obtained by mixing the active Li3Ti4CoCrO12 powder, conducting carbon and plastic binder (polyvinylidene fluoride, PVDF) in a organic solvent N-methyl-2-pyrrolidine. A weight ratio of 8:1:1 was set during the mixing of the different 2
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
spheres denote lithium (16d), titanium and chromium ions; red spheres denote oxygen ions. The spinel structure viewed from the [010] direction clearly provides certain lattice vacancy (16c) for intercalation of Lithium ions released from the cathode material, and therefore good electrochemical performance similar to Li4Ti5O12 was expected. SEM micrographs of Li3Ti4CoCrO12 sample are presented in Fig. 3. As can be seen from Fig. 3a, some spherical agglomerates were observed, with a size in the range of 1–5 μm, close to the average size of 3 μm measured by a particle size analyzer shown in Fig. 3b. The spherical agglomerates were generated in the spray drying process. In the enlarged micrograph, as shown in Fig. 3c, detailed microstructure of an individual agglomerate is presented. It is clear from the micrograph that the agglomerate is composed of well-developed tiny crystalites with uniform grain size distributed in the range of 50–100 nm. These tiny nano scale crystalites were generated during the nano-milling process, where small ZrO2 grits with diameter of 0.2 mm were used. Fig. 3d presented the size distribution of the slurry sample before spray drying, with an average size about 80 nm, further showed good consistence to the crystalite size of the Li3Ti4CoCrO12 anode material observed in Fig. 3c. In some early reports, nano crystalite s will increase the interfacial surface area between the anodic powder and the liquid electrolyte, reduce the migration pathway for lithium-ions and finally the electrochemical behavior of the prepared electrode was enhanced [20–22]. In addition, mesopores were also observed between the primary crystalites in Fig. 3c. The presences of these mesopores are also favorable for the storage of liquid electrolyte, and therefore the delivery of Li cations within the microspheres was increased. Such nano-micro structure of the spherical anode powder provide both short migration pathway and increased surface area for the redox process and thus better electrochemical properties could be expected.
Table 2 Elements concentration of Li3Ti4CoCrO12 compound. Element
Li
Ti
Co
Cr
Weight percentage Atomic ratio
0.0645 3. 005
0.5921 3.998
0.1822 1.000
0.1612 1.003
Fig. 2. Crystal structure of Li3Ti4CoCrO12 build on software Ball & Stick.
The results of chemical analysis with inductively coupled plasmaoptical emission spectrometry (ICP-OES) are displayed in Table 2. The mole ratios among the four cations (Li: Ti: Co: Cr ¼ 3.005: 3.998: 1.000: 1.003) are in good agreement with the results from XRD refinement, which is the same values we expect. The simulated crystal structure viewed from [010] direction is shown in Fig. 2, where yellow spheres denote lithium (8a) and cobalt ions; green
3.2. Electrochemical performance of Li3Ti4CoCrO12 sample To show the polarization between the discharged and the charged electrodes of the Li3Ti4CoCrO12 anode material, cyclic voltammetry (CV)
Fig. 3. SEM micrographs and size distribution of Li3Ti4CoCrO12 material. 3
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
Fig. 4. Cyclic voltammogram of the Li3Ti4CoCrO12 electrode.
Fig. 5. Electrochemical discharge profile.
behavior
of
Li3Ti4CoCrO12
material
Fig. 6. Electrochemical performance.
behavior
of
Li3Ti4CoCrO12
material
cycling
charge/ Fig. 7. Electrochemical behavior of Li3Ti4CoCrO12 material rate performance.
crystal structure of parent compound LTO. The specific capacity of the redox process is estimated as 155.1mAh/g, very close to the theoretical capacity (156 mAh/g) of the nominal compound. The total charging process offered a capacity of 152.3 mAh/g, which in turn corresponds to a coulombic efficiency of 98.3% for the first run. Such high electrochemical performances do suggest that the newly synthesized compound showed excellent Li storage performance, and practical application in LIBs could be expected. Inspired by the above findings, 100 more runs were finished to show the cyclic behavior of the Li3Ti4CoCrO12 anode powder. The voltage profiles from 2nd to the 100th are similar to that of the first run, and therefore not presented in Fig. 6. The measured specific capacities were shown in Figure 5b, in which the coulombic efficiencies at each run were also displayed. It can be observed from the figure that both the capacity retention and the coulombic efficiency for the tested Li3Ti4CoCrO12 sample were kept at a very high level, 97.7% and 99.5% respectively at the 100th run, indicating high potential of the prepared anodic material. Rate performance was further investigated and the results were presented in Fig. 7. The cells were firstly cycled at 0.2C (current density of 32 mAg1) for 20 more cycles, then the current densities were increased stepwise to 10C, and finally the cells were recovered at 0.2C. The rate capacities were recorded as 154.2, 148.8, 142.4, and 136.1 mAh/g at 0.2, 0.5, 2, 5C, respectively. Even at the rate of 10C, a discharging capacity of 129.6 mAh/g, 83.6% of the initial value, could be reserved for the Li3Ti4CoCrO12 anode material. Furthermore, the electrochemical
test was carried out with a scanning rate of 0.01 mv/s and a voltage range of 0.8–2.5 V. The result was shown in Fig. 4. The sample showed a pair of reversible oxidation/reduction peaks at 1.44 V and 1.53 V, which indicates good electrode behavior of the prepared anode material [23]. No other redox peaks were observed, indicating the reversible electrochemical performance was attributed to the Ti4þ/Ti3þ pairs, not the introduced Co and Cr ions. It also should be pointed out that the observed voltage difference (ΔV) for the oxidation/reduction peaks of the Li3Ti4CoCrO12 electrode is very important to the evaluation of the electrochemical properties of the anodic materials. In our case, the voltage difference for Li3Ti4CoCrO12 material is only 90 mV, which implies very slight polarization during the redox process. The voltage profile at the first cycle between 1.0 and 2.0 V for the Li3Ti4CoCrO12 sample is presented in Fig. 5, which was conducted with a constant current density of 32 mAg1 (0.2 C). During the first discharging process, the material showed a fast decrease in voltage before 1.5 V within several minutes and then a steady plateau about 1.5 V was observed for the tested sample. For the first charging process, the anodic electrode experienced a rapid increase in voltage before 1.5 V, followed by a very stable plateau around 1.58 V. A very small voltage gap of 73 mV was recorded, very close to the value of 90 mV obtained from the CV measurement. Such a redox process could be regarded as the typical reduction/oxidation of Ti4þ/Ti3þ pair. We found that the CV curve of Li3Ti4CoCrO12 is similar to the report from H. El-Shinawi et al. [17], which suggests that Co/Cr replacement does not verify the original 4
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
frequency region of 0.1 Hz–0.01 Hz in Fig. 8. After the calculation, a diffusion coefficient of 9.86 1010 cm2/S for the Li3Ti4CoCrO12 was determined. The obtained diffusion coefficients for Lithium ions (DLi) are summarized in Table 3, together with the data of the previous works in our group and the reported for similar materials from other researchers [29–32] which were also obtained from EIS measurements. When taking an intensive examination of the data in the table, it is found that the electrochemical performance of LTO based anodes can be related to the values of interfacial resistance (Rct) and the diffusion process (DLi). When small Rct with high DLi are observed, the anodic material will display excellent redox behavior. Such a role works in our case. It can be found from Table 3 that the size of the material synthesized by the nano-grinding method is smaller than that of other synthetic methods. Since the Li3Ti4CoCrO12 anode material were composed of mesoporous agglomerates constructed by nano scaled primary crystalites, the increased surface area and reduced diffusion pathway will offer the anode small interfacial resistance and enhanced diffusion coefficient. The lithium-ion diffusion coefficient (DLi) of LTO and Li3Ti4CoCrO12 were calculated as 1.02 1012 cm2 s1 and 9.86 1010 cm2 s1, respectively. It is clear that the lithium-ion diffusion coefficient of the Li3Ti4CoCrO12 is increased by an order of magnitude compared to that of the LTO. This result shows that Co2þ/Cr3þ co-substitution LTO could effectively improve lithium-ion diffusion coefficient. Compared with LTO, Li3Ti4CoCrO12 exhibits much better rate performance. The improved electrochemical performance could be explained mainly by the higher lithium-ion diffusion coefficient. Therefore, in summary, a new anodic material, Li3Ti4CoCrO12, was prepared via a nano milling assisted method for the first time. The material presented excellent electrochemical behavior with high capacity retention and good rate performance caused by the mesoporous nature of the anode powder. All the results indicate great potential of the Li3Ti4CoCrO12 compound as the anode candidate for lithium ion secondary batteries.
Fig. 8. Electrochemical behavior of Li3Ti4CoCrO12 material EIS measurement.
performance can be recovered to 152.6 mAh/g by reducing the current density to 0.2C, indicating a good reversibility for the prepared Li3Ti4CoCrO12 anode material. Such electrochemical performance indicated that the new compound displayed a similar rate behavior to that of parent compound LTO, showing the high potential in fast charging area. By measuring the electrochemical impedance spectrum of the anode material, the reasons for the excellent electrochemical properties are revealed, as shown in Fig. 8. The observed Nyquist plot consists of a quasi-circular among the high frequency region and a quasi-linear behavior at the low frequency region. Such a semi-circle represents a charge transfer process at the interface of the anodic material, whereas the low frequency slanted line represented the Warburg impedance dealing with the lithium diffusion within Li3Ti4CoCrO12 crystal lattice. The EIS spectrum could be expressed with an equivalent circuit as shown in Fig. 8, Where Rs is the solution resistance; CPE indicates the capacitance from the passivation film; Zw stands for Warburg impedance [24–26]. After the simulation, the charge transfer resistance (Rct) for the Li3Ti4CoCrO12 electrode was obtained as small as 8.56 Ω, which can be caused by the increased surface area offered from the mesoporous nature of the prepared anode powder. The lithium ion diffusion coefficient (Dþ Li) can be obtained by the formula as following [27,28]. DLiþ ¼
0:5R2 T 2 A2 F 4 C 2 σ 2W
4. Conclusions In this work, pure phase material Li3Ti4CoCrO12 was prepared using a nano milling assisted solid-state reaction method. XRD refinement suggested that Co/Cr entered the lattice of LTO by replacing the Li and Ti cations respectively. Such replacement creates a new compound with crystal structure similar to LTO. The prepared anode material displayed mesoporous morphology with primary particle sizes of 50–100 nm, which offers excellent performance for LIBs. At 0.2C, the anode material displayed a high initial charging capacity of 155.1 mAh/g , close to its theoretical capacity. At the end of 100th cycle, capacity retention of 97.7% was recorded. The small grain size and mesoporous structure of the prepared anode material were regarded as the important factors since they offer short pathway and increased surface area for the redox process. Small interfacial resistance of 8.56 Ω and high Li diffusion coefficient of 9.86 1010 cm2/s were confirmed for Li3Ti4CoCrO12 anode material from EIS measurement. The work we presented provides a new anodic material with stable electrochemical performance, offers an alternative for lithium secondary batteries.
(2)
wherein R is the gas constant, T is the absolute temperature, c is the concentration of Liþ in the material, F is the Faraday constant, A is the surface area of the electrodes, and σw is the Warburg factor obeying the equation as follows: ZRe ¼ Rs þ Rct þ σω0:5
(3)
ZRe is the real part of the experimental impedance data, therefore σw can be calculated from the linear fitting of ZRe vs. ω0.5 in the low Table 3 Diffusion coefficients and resistance values from the equivalent circuits. Sample
Capacity mAh/g
Capacity retention (%)
Columbic efficiency(%)
Rate Perfor-mance (%)
Rct (Ω)
DLi (cm2/S)
Method and partical size
Reference
Li3Ti4CoCrO12 G-LTO R-LTO LTO LTO-F LTOS3 LTOS4 LTO LTO-ZnO
155.1 172.4 165.1 155.0 170.4 170.2 172.2 162.3 165.2
97.7% 96% 86.2% / / 91.0% 92.6% 73.1% 90.3%
98.3% 98.6% 96.9% 97.4% 96.8% 96.3% 92.2% / /
83.6%(10C) 59.22%(10C) / 37.4%(10C) 60.1%(10C) 84.6%(20C) 85.8%(20C) 72.8%(10C) 76.4%(10C)
8.56 / / 80.6 56.4 37.5 22.7 75.9 46.7
9.86 1010 1.02 1012 1.61 1013 1.69 1010 2.51 1010 4.4 1010 9.8 1010 2.6 1010 5.7 1010
Nano-milling, 50–100 nm Nano-milling, < 300 nm Nano-milling, < 300 nm Solid-state reaction, 1 μm Solid-state reaction, 1 μm ionothermal method, 200–500 nm ionothermal method, 200–500 nm Hydrothermal Hydrothermal
This work 29 29 30 30 31 31 32 32
5
H. He et al.
Journal of Solid State Chemistry 279 (2019) 120970
Conflicts of interest
[15] R. Cai, S.M. Jiang, X. Yu, B.T. Zhao, H.T. Wang, Z.P. Zhao, A novel method to enhance rate performance of an Al-doped Li4Ti5O12 electrode by post-synthesis treatment in liquid formaldehyde at room temperature, J. Mater. Chem. 22 (2012) 8013–8021. https://doi.org/10.1039/c2jm15731d. [16] G.B. Xu, L.W. Yang, X.L. Wei, J.W. Ding, J.X. Zhong, P.K. Chu, Highly-crystalline ultrathin gadolinium doped and carbon-coated Li4Ti5O12 nanosheets for enhanced lithium storage, J. Power Sources 295 (2015) 305–313. https://doi.org/10.1016/j .jpowsour.2015.06.131. [17] H. El Shinawi, M. Falk, C. Greaves, Li3Ti4NiMnO12 and Li3Ti4NiCrO12: new substituted lithium titanium oxides, Solid State Sci. 22 (2013) 65–70. https://doi.org/10.1016/j.solidstatesciences. 2013.05.012. [18] S. James, Lange’s Handbook of Chemistry, sixteenth ed., McGraw-Hill Professional Publishing, New York, 2004. [19] J.K. Liang, Powder diffraction pattern fitting correction crystal structure, J. Struct. Chem. 4 (1985) 241–260. [20] G.R. Hu, X.L. Zhang, Z.D. Peng, Preparation and electrochemical performance of tantalum- doped lithium titanate as anode material for lithium-ion battery, Trans. Nonferrous Metals Soc. China 21 (2011) 2248–2253. https://doi.org/10.1016/S1003-6326(11)61003-0. [21] Q. Zhang, C. Zhang, B. Li, D. Jiang, S. Kang, X. Li, Y. Wang, Preparation and characterization of W-doped Li4Ti5O12 anode material for enhancing the high rate performance, Electrochim. Acta 107 (2013) 139–146. https://doi.org/10.1016/j.el ectacta.2013.05.151. [22] Z. Zhang, L. Cao, J. Huang, S. Zhou, Y. Huang, Y. Cai, Hydrothermal synthesis of Zndoped Li4Ti5O12 with improved high rate properties for lithium ion batteries, Ceram. Int. 39 (2013) 6139–6143. https://doi.org/10.1016/j.ceramint.2013.01 .032. [23] Q.Y. Zhang, Michael G. Verde, J.K. Seo, X. Li, Y.S. Meng, Structural and electrochemical properties of Gd-doped Li4Ti5O12 as anode material with improved rate capability for lithium-ion batteries, J. Power Sources 280 (2015) 355–362. https://doi.org/10.1016/j.jpowsour.2015.01.124. [24] A.Y. Shenouda, H.K. Liu, J., Electrochemical behaviour of tin borophosphate negative electrodes for energy storage systems, J. Power Sources 185 (2008) 1386–1391. https://doi.org/10.1016/j.jpowsour.2008.08.042. [25] A.Y. Shenouda, K.R. Murali, Electrochemical properties of doped lithium titanate compounds and their performance in lithium rechargeable batteries, J. Power Sources 176 (2008) 332–339. https://doi.org/10.1016/j.jpowsour.200 7.10.061. [26] B. Li, C. Han, Y.B. He, C. Yang, H. Du, C. Yuan, F. Kang, Facile synthesis of Li4Ti5O12/C composite with super rate performance, Energy Environ. Sci. 5 (2012) 9595–9602. https://doi.org/10.1039/c2ee22591c. [27] X. Li, H. Xin, Y. Liu, D. Li, X. Yuan, X. Qin, Effect of niobium doping on the microstructure and electrochemical properties of lithium-rich layered Li [Li0.2Ni0.2Mn0.6]O2 as cathode materials for lithium ion batteries, RSC Adv. 5 (2015) 45351–45358. https://doi.org/10.1039/c5ra01798j. [28] D. Luo, S. Fang, L. Yang, S.i. Hirano, Improving the electrochemical performance of layered Li-rich transition-metal oxides by alleviating the blockade effect of surface lithium, J. Mater. Chem. 4 (2016) 5184–5190. https://doi.org/10.1039/c 5ta10779b. [29] Y.W. Shi, D.Y. Zhang, C.K. Chang, Enhanced electrochemical performance of oxygen-deficient Li4Ti5O12-x anode material induced by graphene oxide, J. Alloy. Comp. 639 (2015) 274–279. [30] X. Bai, W. Li, A. Wei, Q. Chang, L. Zhang, Z. Liu, Preparation and electrochemical performance of F-doped Li4Ti5O12 for use in the lithium-ion batteries, Solid State Ion. 324 (2018) 13–19. https://doi.org/10.1016/j.ssi.2018.06.005. [31] X. Li, Y. Meng, S. i Liu, H. Jin, Z. Xiao, Ionothermal synthesis of sponge-like nano Li4Ti5O12 for high rate lithium-ion batteries, J. Solid State Electrochem. 19 (2015) 1745–1753. https://doi.org/10.1007/s10008-015-2810-2. [32] C. Han, Y. He, H. Li, B. Li, H. Du, X. Qin, F. Kang, Suppression of interfacial reactions between Li4Ti5O12 electrode and electrolyte solution via zinc oxide coating, Electrochim. Acta 157 (2015) 266–273. https://doi.org/10.1016/j.el ectacta.2014.12.080.
The authors declare that they have no conflict of interest. Acknowledgement This work was supported by Science and Technology Commission of Shanghai Municipality [grant numbers 14520503100, 201310-JD-B2009]; the Shanghai Municipal Education Commission [grant number 15ZZ095]. References [1] K.C. Hsiao, S.C. Liao, J.M. Chen, Microstructure effect on the electrochemical property of Li4Ti5O12 as an anode material for lithium-ion batteries, Electrochim. Acta 53 (2008) 7242–7247. https://doi.org/10.1016/j.electacta.2008.05.002. [2] H.L. Zhao, Y.L. Li, Z.M. Zhu, J. Lin, Z.H. Tian, R.L. Wang, Structural and electrochemical characteristics of Li4xAlxTi5O12 as anode material for lithium-ion batteries, Electrochim. Acta 53 (2008) 7079–7083. https://doi.org/10.1016/j.el ectacta.2008.05.038. [3] C.H. Jiang, M. Ichihara, I. Honma, H.S. Zhou, Effect of particle dispersion on high rate performance of nano-sized Li4Ti5O12 anode, Electrochim. Acta 52 (2007) 6470–6475. https://doi.org/10.1016/j.electacta.2007.04.070. [4] A.D. Robertson, L. Trevino, H. Tukamoto, J.T.S. Irvine, New inorganic spinel oxides for use as negative electrode materials in future lithium-ion batteries, J. Power Sources 81 (1999) 352–357. https://doi.org/10.1016/S0378-7753(98)00217-1. [5] T. Ohzuku, A. Ueda, N. Yamamato, Zero-strain insertion material of Li[Li1/3Ti5/3]O4 for rechargeable lithium cells, J. Electrochem. Soc. 142 (1995) 1431–1435. https:// doi.org/10.1149/1.2048592. [6] S. Scharner, W. Weppener, P. Schmid-Beurmann, Evidence of two-phase formation upon lithium insertion into the Li1.33Ti1.67O4 spinel, J. Electrochem. Soc. 146 (1999) 857–861. https://doi.org/10.1149/1.1391692. [7] F. Ronci, P. Reale, B. Scrosati, S. Panero, V.R. Albertini, P. Perfetti, M.D. Michiel, J.M. Merino, High-resolution in-situ structural measurements of the Li4/3Ti5/3O4 “Zero-Strain” insertion material, J. Phys. Chem. B 106 (2002) 3082–3086. https:// doi.org/10.1021/jp013240p. [8] K.M. Colbow, J.R. Dahn, R.R. Haering, Structure and electrochemistry of the spinel oxides LiTi2O4 and LiTiO4, J. Power Sources 26 (1989) 397–402. https://doi.org/10.1016/0378-7753(89)80152-1. [9] X.C. Sun, P.V. Radovanovic, B. Cui, Advances in spinel Li4Ti5O12 anode materials for lithium-ion batteries, New J. Chem. 39 (2015) 38–63. https://doi.org/10. 1002/chin.201510292. [10] W.W. Lee, J.M. Lee, Novel synthesis of high performance anode materials for lithium-ion batteries (LIBs), J. Mater. Chem. 2 (2014) 1589–1626. https://doi.org /10.1039/c3ta12830j. [11] G.N. Zhu, Y.G. Wang, Y.Y. Xia, Ti-based compounds as anode materials for Li-ion batteries, Energy Environ. Sci. 5 (2012) 6652–6667. https://doi.org/10.1039/c 2ee03410g. [12] Q. Zhang, C. Zhang, B. Li, S. Kang, X. Li, Y. Wang, Preparation and electrochemical properties of Ca-doped Li4Ti5O12 as anode materials in lithium-ion battery, Electrochim. Acta 98 (2013) 146–152. https://doi.org/10.1016/j.electacta.2013.03.006. [13] Z. Yu, X. Zhang, G. Yang, J. Liu, J. Wang, R. Wang, J. Zhang, High rate capability and long-term cyclability of Li4Ti4.9V0.1O12 as anode material in lithium ion battery, Electrochim. Acta 56 (2011) 8611–8617. https://doi.org/10.1016/j.electacta.20 11.07.051. [14] T.F. Yi, Y. Xie, L.J. Jiang, J. Shu, C.B. Yue, Advanced electrochemical properties of Mo-doped Li4Ti5O12 anode material for power lithium ion battery, RSC Adv. 2 (2012) 3541–3547. https://doi.org/10.1039/c2ra00981a.
6