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Spinel Li4Ti5O12 as a reversible anode material down to 0 V
Journal of Alloys and Compounds 465 (2008) 375–379
Spinel Li4Ti5O12 as a reversible anode material down to 0 V X.L. Yao a,b , S. Xie a , H.Q. Nian a , C.H. Chen a,∗ a
Department of Materials Science and Engineering, University of Science and Technology of China, Anhui, Hefei 230026, China b Department of Applied Chemistry, Anhui Agricultural University, Anhui, Hefei 230036, China Received 12 July 2007; received in revised form 14 October 2007; accepted 20 October 2007 Available online 4 November 2007
Abstract The electrochemical behavior of Li1.33 Ti1.67 O4 was investigated as an anode material discharged to 0 V using X-ray diffraction (XRD), galvanostatic cell cycling and ac impedance spectroscopy. The XRD results indicate that the lattice framework of Li1.33 Ti1.67 O4 is almost unchanged even after it is discharged to 0 V. The Li1.33 Ti1.67 O4 electrode can be cycled in the voltage range between 0 and 3.0 V with excellent cyclability and a capacity of about 200 mAh/g. During the discharge process, a 0.75 V plateau is also observed in addition to the usual 1.5 V plateau. The capacity associated with the 0.75 V plateau varies with current density and temperature. The possible cause of this low potential plateau is discussed and attributed to a carbon-triggered-capacity (CTC) effect. © 2007 Elsevier B.V. All rights reserved. Keywords: Electrode; Energy storage; Crystal structure; Transport property; Kinetics
1. Introduction Lithium-ion batteries with carbon as the anode material are very commercially successful as the power sources for portable electronic devices such as laptop computers, mobile phones and camcorders. Yet a carbonaceous anode has incurred obvious safety concerns due to its low operating potential close to that of metallic lithium. Thus, the spinel-type Li1.33 Ti1.67 O4 has been widely investigated as an alternative anode material because it has a very flat charge–discharge plateau at around 1.5 V vs. Li+ /Li [1–6]. This flat plateau combined with its zero-strain insertion characteristics [3] and high thermal stability has made Li1.33 Ti1.67 O4 a much safer anode material than the carbonaceous counterpart [4,7]. The origin of the 1.5 V plateau is due to the coexistence of two phases of lithium titanates Li1.33 Ti1.67 O4 and Li2.33 Ti1.67 O4 with an electrochemical equilibrium reaction [8]: (Li)8a (Li0.33 Ti1.67 )16d (O4 )32e + e− + Li+ ⇔ (Li2 )16c (Li0.33 Ti1.67 )16d (O4 )32e
(1)
∗ Corresponding author at: Department of Materials Science and Engineering, University of Science and Technology of China, Anhui, Hefei 230026, China. Tel.: +86 551 3606971; fax: +86 551 3601592. E-mail address: [email protected] (C.H. Chen).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.10.113
where the subscripts stand for the number of equivalent sites with Wyckoff symbols for the space group Fd3m. Hence, Li+ occupies tetrahedral (8a) and octahedral (16c, 16d) sites of the lattice, and the overall insertion capacity is controlled by the number of free octahedral sites. Because the oxidation state of Ti in Li1.33 Ti1.67 O4 is +4, the highest possible valence for Ti, thus Li1.33 Ti1.67 O4 is a very poor electronic conductor. On the other hand, it is a good lithium-ion conductor because of the existence of available free octahedral sites in its lattice. In order to improve its electronic conductivity, either aliovalent cation doping [9–13] or addition of electronic additive [14] can be used. Of course, the usual electronic additive carbon black is also used to enhance the electronic conductivity of the electrode. On the other hand, Li2.33 Ti1.67 O4 is a good electronic conductor because the average oxidation state of Ti is +3.4 in this lithiated phase, meaning the coexistence of Ti3+ (60%) and Ti4+ (40%) in the lattice. However, due to the full occupancy of the 16c octahedral site by Li+ , Li2.33 Ti1.67 O4 must be a very poor lithium-ion conductor. Considering the structure feature of the two-phase electrode systems, the particles of the active material in the Li1.33 Ti1.67 O4 -Li2.33 Ti1.67 O4 electrode should be present in a core-shell structure, i.e. Li2.33 Ti1.67 O4 (shell)/Li1.33 Ti1.67 O4 (core) during the lithiation process and Li1.33 Ti1.67 O4 (shell)/Li2.33 Ti1.67 O4 (core) during the delithiation process. Hence, during the lithiation process, lithium-ion must pass through the poor ionic conducting Li2.33 Ti1.67 O4
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barrier shell to reach the Li1.33 Ti1.67 O4 core and realize the lithiation. Obviously, if the particle size is large, the mass transport through this shell layer can become a rate-limiting step. Consequently, the whole electrochemical process including the capacity may be controlled by the kinetics. One strategy to solve this problem is to minimize the Li1.33 Ti1.67 O4 particle size [15–17]. In the study of this paper, we have found that the full capacity can be realized by lowering the cut-off potential of the lithiation step. Another potential plateau at around 0.75 V in association with the lithium transport through the barrier shell layer is observed. The results of this study may enrich our knowledge of the important Li1.33 Ti1.67 O4 electrode material and increase the opportunity to apply it in practical lithium-ion batteries.
Fig. 1. Voltage profiles of the first and second cycles for a Li1.33 Ti1.67 O4 /Li cell at 0.2 mA/cm2 in LiPF6 /EC + DEC between 0 and 3 V.
2. Experimental 2.1. Synthesis of the spinel compound Li1.33 Ti1.67 O4 A Li1.33 Ti1.67 O4 powder was prepared by a solid-state reaction between Li2 CO3 and TiO2 in a Li:Ti molar ratio of 4:5. The reagents were mixed in hexamethylene and ball-milled for 48 h. The mixture was dried and heated at 800 ◦ C for 12 h in air to decompose the carbonate and then sintered at 900 ◦ C for 24 h in air. The sintered powder sample was characterized by X-ray diffraction (Philips X’Pert Pro Super, Cu K␣ radiation).
2.2. Cell preparation A negative electrode laminate was prepared by casting a slurry containing Li1.33 Ti1.67 O4 (84 wt%), acetylene black (8 wt%) and poly(vinylidene fluoride) (PVDF) (8 wt%) onto copper foil. It was dried at 70 ◦ C in a vacuum oven and then punched into small discs. Coin cells (CR2032 size) were assembled in an argon-filled glove box (Mbraun Lab Master 130) with lithium as the counter electrode and 1 M LiPF6 /EC + DEC 1:1 (w/w) as the electrolyte.
2.3. Ex situ X-ray diffraction of the anode electrode at different depth-of-discharge Five Li1.33 Ti1.67 O4 /Li cells were respectively discharged to 1, 0.7 and 0 V at the first cycle, discharged to 0.7 and 0 V at the second cycle and then the cells were opened in the glove box. The lithium titanate electrodes were washed with dimethyl carbonate solvent and dried in the glove box to remove the electrolyte. They were then sealed in polyethylene (PE) bags and taken out of the glove box for the characterization of by X-ray diffraction. The use of PE bags was to prevent the samples from reactions with air and moisture.
2.4. Electrochemical evaluation of the half-cells The cells were galvanostatically cycled on a multi-channel battery cycler (Neware BTS 2300, Shenzhen) in the voltage range of 0–3 V. The cycling conditions included different current from 0.1 to 1 mA and temperature from 20 to 60 ◦ C. AC impedance measurement was also carried out on these cells with a CHI 604A Electrochemical Workstation. The frequency range and voltage amplitude were set as 100–0.01 Hz and 5 mV, respectively.
3. Results and discussions In this section, Li1.33 Ti1.67 O4 and Li2.33 Ti1.67 O4 are referred as L1TO and L2TO, respectively. Fig. 1 shows the voltage–capacity curves of the first and second cycles for an L1TO/Li cell in the voltage range between 0 and 3 V. Note that, the specific capacity is calculated based on the weight
of L1TO alone. In fact, the lithium insertion in the conducting additive acetylene black (AB) must have also contributed to the total capacity in this low voltage range. According to our previous study [18], the first discharge capacity of the AB is about 250 mAh/g and its reversible capacity in the subsequent cycles is about 150 mAh/g. It can be seen from Fig. 1 that the voltage profile shows that lithium insertion occurs in four distinct steps, i.e. (i) a plateau at approximately 1.55 V, (ii) an inclined curve from 1.55 to 0.75 V, (iii) a plateau between 0.75 and 0.6 V, and (iv) an inclined curve from 0.6 to 0 V. In step (i), the plateau at 1.55 V is due to the two-phase coexistence of L1TO–L2TO in the electrode. The specific discharge capacity at 1.55 V is controlled by the particle size and current density [16,17]. The discharge result at this plateau is usually the formation of L2TO (shell)/L1TO (core) particles that are separated by AB, PVDF and liquid electrolyte. The lithium insertion into L1TO must be completed by the simultaneous combination of lithium-ions and electrons, which are transported through the already formed L2TO shell layer from liquid electrolyte and AB, respectively. The rate of the lithium insertion is dominantly determined by reaction (1). Thus, the electrode potential is kept at 1.55 V. With the progress of the lithium insertion, the thickness of the L2TO layer increases gradually. At a critical shell thickness, the transport of lithium-ions and electrons through the L2TO layer becomes rate-limiting. In order to overcome the ohmic drop caused by the transport in this shell layer, the electrode potential must be brought to below 1.55 V. To maintain the lithium insertion, the electrode potential decreases continuously. This kinetics controlled process matches with the voltage profile step (ii). In the specific electrode composition in this study, AB is used as a conducting additive to provide a fast electronic pathway. In the composite electrode, a substantial portion of the L1TO surface is intimately covered with AB particles. These surfaces are initially inaccessible for the lithium-ions in the liquid electrolyte. However, in the low potential range of 1.5–0 V vs. Li+ /Li, lithium insertion also occurs in the AB particles. Hence, the lithiated AB gradually becomes lithium-ion conducting. As a rule of thumb, the maximum lithium-ion conductivity can be reached at about half of its capacity, or, according to its voltage profile [18], at a potential around 0.75 V vs. Li+ /Li. Accordingly, in
X.L. Yao et al. / Journal of Alloys and Compounds 465 (2008) 375–379
the L1TO composite electrode, the effective surface area for the lithium-ion conduction increases substantially at this potential. The impedance of the L2TO layer is also reduced so that the transport through this layer is no longer the major rate-limiting step for the lithium insertion process. Therefore, a plateau on the discharge voltage profile appears between 0.75 and 0.6 V, i.e. the voltage profile step (iii). This step can proceed until the entire L1TO core is lithiated into L2TO. In this step, the electrode potential is controlled by AB but the capacity is predominantly determined by the remained L1TO to L2TO transition. It may be called a carbon-triggered-capacity (CTC) effect. Thus, the overall discharge capacity above 0.6 V is close to its theoretical value 175 mAh/g. Specifically, it is about 185 mAh/g for the first cycle and 163 mAh/g for the second cycle (Fig. 1). In these two values, less than 15 mAh/g is contributed from the AB lithiation. This around 0.75 V plateau was also observed by Robertson et al. in the Fe-doped Li1+x Fe1−3x Ti1+2x O4 (0.00 ≤ x ≤ 0.33) [9] and Huang et al. in the Ag-doped Li4 Ti5 O12 [14]. Nevertheless, they did not give a clear explanation. When the cell is discharged between 0.6 and 0 V, namely the voltage profile step (iv), more lithium can be inserted in the electrode; an extra capacity of about 65–60 mAh/g is achieved for the first and second cycles (Fig. 1). The lithium insertion in AB in this potential range should contribute to part of the capacity. However, considering the mass of AB in this composite electrode and its specific capacity [18], the contribution from AB must not exceed 25 mAh/g for the first cycle and 15 mAh/g for the second cycle. Therefore, there must be some lithium inserted in the L2TO lattice. Recently, Amine and co-workers attributed the portion of this capacity to possible formation of a quasi-rock-salt phase [19]. On the delithiation runs, only the 1.55 V plateau is observed; the 0.75 V plateau disappears (Fig. 1). The capacity achieved at the 1.55 V plateau (170 mAh/g) is approximately equal to the sum of discharge capacity between 3 and 0.5 V. This result can be explained by the change of the particle microstructure of the lithium titanate. During the charge process, the core-shell structure of the particles becomes L1TO (shell)/L2TO (core). Because L1TO is a good lithium-ion conductor, the lithium-ion transport through the shell layer is no longer rate-limiting. Therefore, the delithiation is predominantly controlled by reaction (1). Therefore, only the 1.55 V plateau is observed. Fig. 2 shows the profile of specific discharge capacity and columbic efficiency vs. cycle number of the L1TO electrode discharged to 0 V. The specific discharge capacity of AB is deducted. When discharged to 0 V, the specific discharge capacity is 210 mAh/g, which is about 35 mAh/g in excess of the theoretical capacity 175 mAh/g for L1TO → L2TO transition. It can also be concluded that the charge and discharge process between 0 and 3 V is highly reversible because the capacityfading rate is very low and the columbic efficiency is nearly 100%. The irreversible capacity loss at low potentials may partly be due to electrolyte solvent reduction below 0.7 V and the formation of solid electrolyte interphase (SEI) film. In order to find if there is a new phase formation between 0 and 1 V, crystallographic characterization of samples discharged to different voltage was carried out. Fig. 3 shows the XRD patterns
377
Fig. 2. Graph of specific discharge capacity and columbic efficiency vs. cycle number for a Li1.33 Ti1.67 O4 /Li cell. Tests were conducted at 0.2 mA/cm2 between 0 and 3 V; (a) specific discharge capacity in which the contribution of the acetylene black additive is not deducted; (b) specific discharge capacity in which the contribution of the carbon black additive is deducted; (c) columbic efficiency.
of the L1TO powder and the lithiated Li1.33 Ti1.67 O4 electrode samples discharged to 1, 0.7 and 0 V at the first cycle, and discharged to 0.7 and 0 V at the second cycle. Note that, compared with the powder sample (a) some extra peaks from polyethylene (protection plastic bag), copper (current collector) and PVDF (binder) are also detected. The positions and relative intensities of the spinel peaks remain virtually unchanged after lithiation, indicating that the general lattice framework is retained. Because the defect spinel L1TO and the fully lithiated L2TO with ordered rock-salt-type structure show nearly identical X-ray diffractions [3], it is not possible to distinguish L1TO and L2TO by the XRD characterization. Nevertheless, it can be concluded from Fig. 3 that there is no evidence of the formation of a new crystalline phase, nor of any important distortion of the prestine lattice.
Fig. 3. The X-ray diffraction patterns of Li1.33 Ti1.67 O4 at different discharged states: (a) Li1.33 Ti1.67 O4 powder; (b) discharged to 1 V in the first cycle; (c) discharged to 0.7 V in the first cycle; (d) discharged to 0 V in the first cycle; (e) discharged to 0.7 V in the second cycle; (f) discharged to 0 V in the first cycle. The diffraction peaks from the spinel Li1.33 Ti1.67 O4 are indexed, while the peaks marked with circles (), squares () and asterisks (*) correspond to the diffraction of Cu (current collector), PVDF (binder) and polyethylene (sample bag).
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Fig. 6. AC impedance spectra of Li1.33 Ti1.67 O4 /Li cell in LiPF6 /EC + DEC at different temperatures. The cell voltage was selected as about 1.0 V.
Fig. 4. Voltage profiles of Li1.33 Ti1.67 O4 /Li cells in LiPF6 /EC + DEC in the third cycle at different current density. Discharge curves (a) and charge curves (b).
Fig. 4 shows the discharge (Fig. 4a) and charge (Fig. 4b) curves of the third cycle for four L1TO/Li cells that are discharged to 0 V at different current density. The specific discharge capacity at the 1.55 V plateau increases with the decrease of the current density while the capacity below 1.55 V down to 0.6 V as well as the capacity at the 0.75 V plateau increases with the current density (Fig. 4a). This result further confirms the feature of the kinetic control and the CTC effect of the discharge voltage profile. The voltage profile of the L1TO/Li cells between 0 and 3 V in the charge process (Fig. 4b) indicates that the electrochemical behavior of Li1.33 Ti1.67 O4 under different current density has almost no difference. When charged from 0 to 1 V,
extra lithium is extracted from the cell and the anion deficient rock-salt species is transformed into the rock-salt-type L2TO. Between 3.0 and 1.0 V the Li/Li2.33 Ti1.67 O4 cell shows nearly its theoretical capacity 175 mAh/g. Fig. 5 shows the discharge voltage profile of an L1TO/Li cell at different temperatures from 20 to 60 ◦ C in 0–3 V at 0.2 mA/cm2 . The capacity of lithium insertion at the 1.55 V plateau increases with increasing temperature. This trend is due to the faster lithium-ion diffusion through the L2TO shell layer at higher temperatures. Thus, it needs to take longer time to reach the turning point to become the kinetics-controlled process, i.e. the voltage decreases with time below 1.55 V. Fig. 6 shows the impedance spectra of the cell that is discharged to 1.0 V at different temperatures. The cell resistance is substantially reduced with increasing temperature. This is consistent with the viewpoint of kinetic control process for this electrode potential. 4. Conclusions This study has identified that, depending on the current density and temperature, the usually observed lithium insertion process at 1.55 V may turn to be kinetically controlled. The carbon-triggered-capacity (CTC) effect leads to a low potential plateau of lithium insertion into Li1.33 Ti1.67 O4 particles from 0.75 to 0.6 V appears. The Li1.33 Ti1.67 O4 electrode can be cycled in the voltage range between 0 and 3.0 V with excellent cyclability and a capacity of about 200 mAh/g. Acknowledgements This study was supported by 100 Talents Program of Academia Sinica and National Science Foundation of China (20471057). We are also grateful to the China Education Ministry (SRFDP No. 2003035057). References
Fig. 5. Discharge voltage profiles of a Li1.33 Ti1.67 O4 /Li cell in LiPF6 /EC + DEC at different temperatures. The current density was at 0.2 mA/cm2 .
[1] K.M. Colbow, J.R. Dahn, R.R. Haering, J. Power Sources 26 (1989) 397. [2] E. Ferg, R.J. Gummov, A. de Kock, M.M. Thackeray, J. Electrochem. Soc. 141 (1994) L147.
X.L. Yao et al. / Journal of Alloys and Compounds 465 (2008) 375–379 [3] T. Ohzuku, A. Ueda, N.J. Yamamoto, J. Electrochem. Soc. 142 (1995) 1431. [4] J. Jiang, J. Chen, J.R. Dahn, J. Electrochem. Soc. 151 (2004) A2082. [5] P.P. Prosini, R. Mancini, L. Petrucci, V. Contini, P. Villano, Solid State Ionics 144 (2001) 185. [6] K. Zaghib, M. Simoneau, M. Armand, M. Gauthier, J. Power Sources 81–82 (1999) 300. [7] X.L. Yao, S. Xie, C.H. Chen, Q.S. Wang, J.H. Sun, Y.L. Li, S.X. Lu, Electrochim. Acta 50 (2005) 4076. [8] S. Scharner, W. Weppner, P. Schmid-Beurmann, J. Electrochem. Soc. 146 (1999) 857. [9] A.D. Robertson, H. Tukamoto, J.T.S. Irvine, J. Electrochem. Soc. 146 (1999) 3958. [10] C.H. Chen, J.T. Vaughey, A.N. Jansen, D.W. Dees, A.J. Kahaian, T. Goacher, M.M. Thackeray, J. Electrochem. Soc. 148 (2001) A102.
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Spinel Li4Ti5O12 as a reversible anode material down to 0 V X.L. Yao a,b , S. Xie a , H.Q. Nian a , C.H. Chen a,∗ a
Department of Materials Science and Engineering, University of Science and Technology of China, Anhui, Hefei 230026, China b Department of Applied Chemistry, Anhui Agricultural University, Anhui, Hefei 230036, China Received 12 July 2007; received in revised form 14 October 2007; accepted 20 October 2007 Available online 4 November 2007
Abstract The electrochemical behavior of Li1.33 Ti1.67 O4 was investigated as an anode material discharged to 0 V using X-ray diffraction (XRD), galvanostatic cell cycling and ac impedance spectroscopy. The XRD results indicate that the lattice framework of Li1.33 Ti1.67 O4 is almost unchanged even after it is discharged to 0 V. The Li1.33 Ti1.67 O4 electrode can be cycled in the voltage range between 0 and 3.0 V with excellent cyclability and a capacity of about 200 mAh/g. During the discharge process, a 0.75 V plateau is also observed in addition to the usual 1.5 V plateau. The capacity associated with the 0.75 V plateau varies with current density and temperature. The possible cause of this low potential plateau is discussed and attributed to a carbon-triggered-capacity (CTC) effect. © 2007 Elsevier B.V. All rights reserved. Keywords: Electrode; Energy storage; Crystal structure; Transport property; Kinetics
1. Introduction Lithium-ion batteries with carbon as the anode material are very commercially successful as the power sources for portable electronic devices such as laptop computers, mobile phones and camcorders. Yet a carbonaceous anode has incurred obvious safety concerns due to its low operating potential close to that of metallic lithium. Thus, the spinel-type Li1.33 Ti1.67 O4 has been widely investigated as an alternative anode material because it has a very flat charge–discharge plateau at around 1.5 V vs. Li+ /Li [1–6]. This flat plateau combined with its zero-strain insertion characteristics [3] and high thermal stability has made Li1.33 Ti1.67 O4 a much safer anode material than the carbonaceous counterpart [4,7]. The origin of the 1.5 V plateau is due to the coexistence of two phases of lithium titanates Li1.33 Ti1.67 O4 and Li2.33 Ti1.67 O4 with an electrochemical equilibrium reaction [8]: (Li)8a (Li0.33 Ti1.67 )16d (O4 )32e + e− + Li+ ⇔ (Li2 )16c (Li0.33 Ti1.67 )16d (O4 )32e
(1)
∗ Corresponding author at: Department of Materials Science and Engineering, University of Science and Technology of China, Anhui, Hefei 230026, China. Tel.: +86 551 3606971; fax: +86 551 3601592. E-mail address: [email protected] (C.H. Chen).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.10.113
where the subscripts stand for the number of equivalent sites with Wyckoff symbols for the space group Fd3m. Hence, Li+ occupies tetrahedral (8a) and octahedral (16c, 16d) sites of the lattice, and the overall insertion capacity is controlled by the number of free octahedral sites. Because the oxidation state of Ti in Li1.33 Ti1.67 O4 is +4, the highest possible valence for Ti, thus Li1.33 Ti1.67 O4 is a very poor electronic conductor. On the other hand, it is a good lithium-ion conductor because of the existence of available free octahedral sites in its lattice. In order to improve its electronic conductivity, either aliovalent cation doping [9–13] or addition of electronic additive [14] can be used. Of course, the usual electronic additive carbon black is also used to enhance the electronic conductivity of the electrode. On the other hand, Li2.33 Ti1.67 O4 is a good electronic conductor because the average oxidation state of Ti is +3.4 in this lithiated phase, meaning the coexistence of Ti3+ (60%) and Ti4+ (40%) in the lattice. However, due to the full occupancy of the 16c octahedral site by Li+ , Li2.33 Ti1.67 O4 must be a very poor lithium-ion conductor. Considering the structure feature of the two-phase electrode systems, the particles of the active material in the Li1.33 Ti1.67 O4 -Li2.33 Ti1.67 O4 electrode should be present in a core-shell structure, i.e. Li2.33 Ti1.67 O4 (shell)/Li1.33 Ti1.67 O4 (core) during the lithiation process and Li1.33 Ti1.67 O4 (shell)/Li2.33 Ti1.67 O4 (core) during the delithiation process. Hence, during the lithiation process, lithium-ion must pass through the poor ionic conducting Li2.33 Ti1.67 O4
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barrier shell to reach the Li1.33 Ti1.67 O4 core and realize the lithiation. Obviously, if the particle size is large, the mass transport through this shell layer can become a rate-limiting step. Consequently, the whole electrochemical process including the capacity may be controlled by the kinetics. One strategy to solve this problem is to minimize the Li1.33 Ti1.67 O4 particle size [15–17]. In the study of this paper, we have found that the full capacity can be realized by lowering the cut-off potential of the lithiation step. Another potential plateau at around 0.75 V in association with the lithium transport through the barrier shell layer is observed. The results of this study may enrich our knowledge of the important Li1.33 Ti1.67 O4 electrode material and increase the opportunity to apply it in practical lithium-ion batteries.
Fig. 1. Voltage profiles of the first and second cycles for a Li1.33 Ti1.67 O4 /Li cell at 0.2 mA/cm2 in LiPF6 /EC + DEC between 0 and 3 V.
2. Experimental 2.1. Synthesis of the spinel compound Li1.33 Ti1.67 O4 A Li1.33 Ti1.67 O4 powder was prepared by a solid-state reaction between Li2 CO3 and TiO2 in a Li:Ti molar ratio of 4:5. The reagents were mixed in hexamethylene and ball-milled for 48 h. The mixture was dried and heated at 800 ◦ C for 12 h in air to decompose the carbonate and then sintered at 900 ◦ C for 24 h in air. The sintered powder sample was characterized by X-ray diffraction (Philips X’Pert Pro Super, Cu K␣ radiation).
2.2. Cell preparation A negative electrode laminate was prepared by casting a slurry containing Li1.33 Ti1.67 O4 (84 wt%), acetylene black (8 wt%) and poly(vinylidene fluoride) (PVDF) (8 wt%) onto copper foil. It was dried at 70 ◦ C in a vacuum oven and then punched into small discs. Coin cells (CR2032 size) were assembled in an argon-filled glove box (Mbraun Lab Master 130) with lithium as the counter electrode and 1 M LiPF6 /EC + DEC 1:1 (w/w) as the electrolyte.
2.3. Ex situ X-ray diffraction of the anode electrode at different depth-of-discharge Five Li1.33 Ti1.67 O4 /Li cells were respectively discharged to 1, 0.7 and 0 V at the first cycle, discharged to 0.7 and 0 V at the second cycle and then the cells were opened in the glove box. The lithium titanate electrodes were washed with dimethyl carbonate solvent and dried in the glove box to remove the electrolyte. They were then sealed in polyethylene (PE) bags and taken out of the glove box for the characterization of by X-ray diffraction. The use of PE bags was to prevent the samples from reactions with air and moisture.
2.4. Electrochemical evaluation of the half-cells The cells were galvanostatically cycled on a multi-channel battery cycler (Neware BTS 2300, Shenzhen) in the voltage range of 0–3 V. The cycling conditions included different current from 0.1 to 1 mA and temperature from 20 to 60 ◦ C. AC impedance measurement was also carried out on these cells with a CHI 604A Electrochemical Workstation. The frequency range and voltage amplitude were set as 100–0.01 Hz and 5 mV, respectively.
3. Results and discussions In this section, Li1.33 Ti1.67 O4 and Li2.33 Ti1.67 O4 are referred as L1TO and L2TO, respectively. Fig. 1 shows the voltage–capacity curves of the first and second cycles for an L1TO/Li cell in the voltage range between 0 and 3 V. Note that, the specific capacity is calculated based on the weight
of L1TO alone. In fact, the lithium insertion in the conducting additive acetylene black (AB) must have also contributed to the total capacity in this low voltage range. According to our previous study [18], the first discharge capacity of the AB is about 250 mAh/g and its reversible capacity in the subsequent cycles is about 150 mAh/g. It can be seen from Fig. 1 that the voltage profile shows that lithium insertion occurs in four distinct steps, i.e. (i) a plateau at approximately 1.55 V, (ii) an inclined curve from 1.55 to 0.75 V, (iii) a plateau between 0.75 and 0.6 V, and (iv) an inclined curve from 0.6 to 0 V. In step (i), the plateau at 1.55 V is due to the two-phase coexistence of L1TO–L2TO in the electrode. The specific discharge capacity at 1.55 V is controlled by the particle size and current density [16,17]. The discharge result at this plateau is usually the formation of L2TO (shell)/L1TO (core) particles that are separated by AB, PVDF and liquid electrolyte. The lithium insertion into L1TO must be completed by the simultaneous combination of lithium-ions and electrons, which are transported through the already formed L2TO shell layer from liquid electrolyte and AB, respectively. The rate of the lithium insertion is dominantly determined by reaction (1). Thus, the electrode potential is kept at 1.55 V. With the progress of the lithium insertion, the thickness of the L2TO layer increases gradually. At a critical shell thickness, the transport of lithium-ions and electrons through the L2TO layer becomes rate-limiting. In order to overcome the ohmic drop caused by the transport in this shell layer, the electrode potential must be brought to below 1.55 V. To maintain the lithium insertion, the electrode potential decreases continuously. This kinetics controlled process matches with the voltage profile step (ii). In the specific electrode composition in this study, AB is used as a conducting additive to provide a fast electronic pathway. In the composite electrode, a substantial portion of the L1TO surface is intimately covered with AB particles. These surfaces are initially inaccessible for the lithium-ions in the liquid electrolyte. However, in the low potential range of 1.5–0 V vs. Li+ /Li, lithium insertion also occurs in the AB particles. Hence, the lithiated AB gradually becomes lithium-ion conducting. As a rule of thumb, the maximum lithium-ion conductivity can be reached at about half of its capacity, or, according to its voltage profile [18], at a potential around 0.75 V vs. Li+ /Li. Accordingly, in
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the L1TO composite electrode, the effective surface area for the lithium-ion conduction increases substantially at this potential. The impedance of the L2TO layer is also reduced so that the transport through this layer is no longer the major rate-limiting step for the lithium insertion process. Therefore, a plateau on the discharge voltage profile appears between 0.75 and 0.6 V, i.e. the voltage profile step (iii). This step can proceed until the entire L1TO core is lithiated into L2TO. In this step, the electrode potential is controlled by AB but the capacity is predominantly determined by the remained L1TO to L2TO transition. It may be called a carbon-triggered-capacity (CTC) effect. Thus, the overall discharge capacity above 0.6 V is close to its theoretical value 175 mAh/g. Specifically, it is about 185 mAh/g for the first cycle and 163 mAh/g for the second cycle (Fig. 1). In these two values, less than 15 mAh/g is contributed from the AB lithiation. This around 0.75 V plateau was also observed by Robertson et al. in the Fe-doped Li1+x Fe1−3x Ti1+2x O4 (0.00 ≤ x ≤ 0.33) [9] and Huang et al. in the Ag-doped Li4 Ti5 O12 [14]. Nevertheless, they did not give a clear explanation. When the cell is discharged between 0.6 and 0 V, namely the voltage profile step (iv), more lithium can be inserted in the electrode; an extra capacity of about 65–60 mAh/g is achieved for the first and second cycles (Fig. 1). The lithium insertion in AB in this potential range should contribute to part of the capacity. However, considering the mass of AB in this composite electrode and its specific capacity [18], the contribution from AB must not exceed 25 mAh/g for the first cycle and 15 mAh/g for the second cycle. Therefore, there must be some lithium inserted in the L2TO lattice. Recently, Amine and co-workers attributed the portion of this capacity to possible formation of a quasi-rock-salt phase [19]. On the delithiation runs, only the 1.55 V plateau is observed; the 0.75 V plateau disappears (Fig. 1). The capacity achieved at the 1.55 V plateau (170 mAh/g) is approximately equal to the sum of discharge capacity between 3 and 0.5 V. This result can be explained by the change of the particle microstructure of the lithium titanate. During the charge process, the core-shell structure of the particles becomes L1TO (shell)/L2TO (core). Because L1TO is a good lithium-ion conductor, the lithium-ion transport through the shell layer is no longer rate-limiting. Therefore, the delithiation is predominantly controlled by reaction (1). Therefore, only the 1.55 V plateau is observed. Fig. 2 shows the profile of specific discharge capacity and columbic efficiency vs. cycle number of the L1TO electrode discharged to 0 V. The specific discharge capacity of AB is deducted. When discharged to 0 V, the specific discharge capacity is 210 mAh/g, which is about 35 mAh/g in excess of the theoretical capacity 175 mAh/g for L1TO → L2TO transition. It can also be concluded that the charge and discharge process between 0 and 3 V is highly reversible because the capacityfading rate is very low and the columbic efficiency is nearly 100%. The irreversible capacity loss at low potentials may partly be due to electrolyte solvent reduction below 0.7 V and the formation of solid electrolyte interphase (SEI) film. In order to find if there is a new phase formation between 0 and 1 V, crystallographic characterization of samples discharged to different voltage was carried out. Fig. 3 shows the XRD patterns
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Fig. 2. Graph of specific discharge capacity and columbic efficiency vs. cycle number for a Li1.33 Ti1.67 O4 /Li cell. Tests were conducted at 0.2 mA/cm2 between 0 and 3 V; (a) specific discharge capacity in which the contribution of the acetylene black additive is not deducted; (b) specific discharge capacity in which the contribution of the carbon black additive is deducted; (c) columbic efficiency.
of the L1TO powder and the lithiated Li1.33 Ti1.67 O4 electrode samples discharged to 1, 0.7 and 0 V at the first cycle, and discharged to 0.7 and 0 V at the second cycle. Note that, compared with the powder sample (a) some extra peaks from polyethylene (protection plastic bag), copper (current collector) and PVDF (binder) are also detected. The positions and relative intensities of the spinel peaks remain virtually unchanged after lithiation, indicating that the general lattice framework is retained. Because the defect spinel L1TO and the fully lithiated L2TO with ordered rock-salt-type structure show nearly identical X-ray diffractions [3], it is not possible to distinguish L1TO and L2TO by the XRD characterization. Nevertheless, it can be concluded from Fig. 3 that there is no evidence of the formation of a new crystalline phase, nor of any important distortion of the prestine lattice.
Fig. 3. The X-ray diffraction patterns of Li1.33 Ti1.67 O4 at different discharged states: (a) Li1.33 Ti1.67 O4 powder; (b) discharged to 1 V in the first cycle; (c) discharged to 0.7 V in the first cycle; (d) discharged to 0 V in the first cycle; (e) discharged to 0.7 V in the second cycle; (f) discharged to 0 V in the first cycle. The diffraction peaks from the spinel Li1.33 Ti1.67 O4 are indexed, while the peaks marked with circles (), squares () and asterisks (*) correspond to the diffraction of Cu (current collector), PVDF (binder) and polyethylene (sample bag).
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Fig. 6. AC impedance spectra of Li1.33 Ti1.67 O4 /Li cell in LiPF6 /EC + DEC at different temperatures. The cell voltage was selected as about 1.0 V.
Fig. 4. Voltage profiles of Li1.33 Ti1.67 O4 /Li cells in LiPF6 /EC + DEC in the third cycle at different current density. Discharge curves (a) and charge curves (b).
Fig. 4 shows the discharge (Fig. 4a) and charge (Fig. 4b) curves of the third cycle for four L1TO/Li cells that are discharged to 0 V at different current density. The specific discharge capacity at the 1.55 V plateau increases with the decrease of the current density while the capacity below 1.55 V down to 0.6 V as well as the capacity at the 0.75 V plateau increases with the current density (Fig. 4a). This result further confirms the feature of the kinetic control and the CTC effect of the discharge voltage profile. The voltage profile of the L1TO/Li cells between 0 and 3 V in the charge process (Fig. 4b) indicates that the electrochemical behavior of Li1.33 Ti1.67 O4 under different current density has almost no difference. When charged from 0 to 1 V,
extra lithium is extracted from the cell and the anion deficient rock-salt species is transformed into the rock-salt-type L2TO. Between 3.0 and 1.0 V the Li/Li2.33 Ti1.67 O4 cell shows nearly its theoretical capacity 175 mAh/g. Fig. 5 shows the discharge voltage profile of an L1TO/Li cell at different temperatures from 20 to 60 ◦ C in 0–3 V at 0.2 mA/cm2 . The capacity of lithium insertion at the 1.55 V plateau increases with increasing temperature. This trend is due to the faster lithium-ion diffusion through the L2TO shell layer at higher temperatures. Thus, it needs to take longer time to reach the turning point to become the kinetics-controlled process, i.e. the voltage decreases with time below 1.55 V. Fig. 6 shows the impedance spectra of the cell that is discharged to 1.0 V at different temperatures. The cell resistance is substantially reduced with increasing temperature. This is consistent with the viewpoint of kinetic control process for this electrode potential. 4. Conclusions This study has identified that, depending on the current density and temperature, the usually observed lithium insertion process at 1.55 V may turn to be kinetically controlled. The carbon-triggered-capacity (CTC) effect leads to a low potential plateau of lithium insertion into Li1.33 Ti1.67 O4 particles from 0.75 to 0.6 V appears. The Li1.33 Ti1.67 O4 electrode can be cycled in the voltage range between 0 and 3.0 V with excellent cyclability and a capacity of about 200 mAh/g. Acknowledgements This study was supported by 100 Talents Program of Academia Sinica and National Science Foundation of China (20471057). We are also grateful to the China Education Ministry (SRFDP No. 2003035057). References
Fig. 5. Discharge voltage profiles of a Li1.33 Ti1.67 O4 /Li cell in LiPF6 /EC + DEC at different temperatures. The current density was at 0.2 mA/cm2 .
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