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Synthesis and characterization of Zn-doped LiCo0.3Ni0.4(xMn0.3ZnxO2 cathode materials for lithium-ion batteries
Journal of Alloys and Compounds 476 (2009) 539–542
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of Zn-doped LiCo0.3 Ni0.4(x Mn0.3 Znx O2 cathode materials for lithium-ion batteries Chen Yuhong a,∗ , CHEN Ruizhen a , Tang Zhiyuan b , Wang Liang c a
Department of Chemical and Environmental Engineering, Hebei Chemical & Pharmaceutical Vocational Technology College, Shijiazhuang 050026, China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China c Library of Hebei University of Science & Technology, Shijiazhuang 050018, China b
a r t i c l e
i n f o
Article history: Received 12 August 2006 Received in revised form 6 September 2008 Accepted 10 September 2008 Available online 2 December 2008 Keywords: Lithium-ion batteries Cathode materials Zn-doped Co-precipitation
a b s t r a c t Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 cathode materials were synthesized via co-precipitation method. The structure, electrochemical performance and thermal stability were characterized by X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammograms (CV), electrochemical impedance spectroscopies (EIS) and differential scanning calorimetry (DSC). LiCo0.3 Ni0.4−x Mn0.3 Znx O2 had stable layered structure with a-NaFeO2 type with x up to 0.05. The compounds of x = 0.02 showed the best discharge capacity and cycle performance which was related to the most stable structure and Zn-doping prevented structural transformations during the topotactic reactions. Meanwhile, Zn-doping improved the high rate discharge capability and thermal stability. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Recently many researchers have interested in layer LiCox Niy Mn1−x−y O2 for its higher capacity and better safety compared with LiCoO2 [1,2]. However, this material has some problems, such as a low rate capability arising from the low electronic conductivity and tap density that should be resolved before it can replace LiCoO2 . One approach to improve the electrochemical performance is to partially substitute manganese cobalt nickel oxides, for transition metals such as iron, titanium, molybdenum or chromium and non-transition metals such as aluminum or magnesium, which may stabilize the layered structure with or without participating in the redox processes and prevent unwanted reactions between cathode and electrolyte [3–7]. However, to the best of our knowledge, no studies on the electrochemical performance of the Zn-doped metal oxide had been published. In this study, we employed Zn as an additional dopant to synthesize a series of Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 materials prepared by coprecipitation. The structural and electrochemical performance of the layered LiCo0.3 Ni0.4−x Mn0.3 Znx O2 materials were studied in this paper.
2.1. Synthesis of LiCo0.3 Ni0.4−x Mn0.3 Znx O2 The precursor Co0.3 Ni0.4−x Mn0.3 (OH)2 was prepared by co-precipitation from a solution containing molar ratio of cobalt/nickel/manganese sulfate by the addition NaOH and NH4 OH solution in a specially designed reactor. The Co0.3 Ni0.4−x Mn0.3 (OH)2 particle size was controlled by the reaction time in order to enhance the BET surface area. A longer reaction time in the solution resulted in a larger particle size. The precipitate continued to be rotated at 1000 rpm in the reactor at 45–50 ◦ C. The pH was maintained at 11–11.5 by controlling the amount of NH4 OH. After filtration, rinsing, and drying, the LiCo0.3 Ni0.4−x Mn0.3 Znx O2 was prepared by mixing molar ratio amounts of 1:1:x in LiOH, Mn0.3 Co0.3 Ni0.4−x (OH)2 and ZnO followed by a heat-treatment procedure at 900 ◦ C for 20 h in a stream of dried air.
2.2. Preparation of test cells Positive electrodes slurry for 2032 type simulated cells was prepared by thoroughly mixing active material (85%) with carbon black (10%) and polytetrafluoroethylene (5%) in ethanol. Electrodes with 0.5 cm−2 area and 0.02 mm depth extruding onto aluminum foil were dried for 12 h at 120 ◦ C under vacuum. 2032 type coin cells were then assembled in a argon filled dry box using foils of Li metal as counter electrodes and celgard 2400 saturated with 1 M LiPF6 in EC/EMC/DMC (1:1:1,wt:wt:wt) as separators.
2.3. Structure tests ∗ Corresponding author. Tel.: +86 13032620998. E-mail address: [email protected] (Y. Chen). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.055
Powder X-ray diffraction (XRD) (Philips X’ Pert Pro Pro MPD Co K␣) was done to determine the structure.
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Y. Chen et al. / Journal of Alloys and Compounds 476 (2009) 539–542
Fig. 2. Cyclic voltammogram LiCo0.3 Ni0.4(x Mn0.3 Znx O2 electrodes v = 0.1 mV/s.
Fig. 1. Typical X-ray diffractograms of LiCo0.3 Ni0.4−x Mn0.3 Znx O2 compounds.
slightly changed with the Zn dopant increasing, which would affect the electrochemical property [9]. 3.2. Cyclic voltammetry studies
2.4. Electrochemical tests The cyclic voltammograms (CV) and electrochemical impedance spectroscopies (EIS) were performed by electrochemistry working station (Gamry Instruments). The constant current and constant voltage charge (CCCV charge) was performed galvanostatically at 0.2 C rate or 0.5 C rate and then potentiostatically at 4.3 V until the current dropped to less than 0.01 C and constant current discharge was enforced at different rate to 2.75 V. 2.5. DSC tests For DSC experiments, the cells were finally fully charged to 4.3 V and opened in the Ar-filled dry box. After opening carefully, the extra electrolyte was removed from the surface of the electrode using DMC rinsing and the electrode materials were removed from the current collector. A aluminum sealed pan with a gold plated copper seal was used to collect 3–5 mg samples. The measurements were carried out in differential scanning calorimeter (NETZSCH-204) using a temperature scan rate of 10 ◦ C/min.
3. Results and discussion 3.1. Structural properties The typical X-ray diffraction patterns recorded for the undoped and Zn-doped materials were shown in Fig. 1. All the diffractograms showed patterns indexable in the a-NaFeO2 layered structure assuming a hexagonal lattice setting [2,4]. All the diffraction patterns showed a clear splitting of the hexagonal characteristic doublets (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0). This indicated that the products possessed typical layered characteristics [4]. The hexagonal unit cell parameters calculated for all the products were shown in Table 1. As shown in Table 1, the ratio of the intensity of the (0 0 3) reflection to that of the (1 0 4) reflection increased up to a dopant level of 0.02 and then decreased. The maximum in the value for the ratio showed that at a x value of 0.02, the system exhibited the maximum hexagonal ordering [8]. Comparing with undoped sample, with the Zn dopant increasing, the value of c decreased while the value of a increased and the volume of the crystal lattice increased. This phenomena showed that the sample structure was
Cyclic voltammograms (CV) of the Li/LiCo0.3 Ni0.4−x Mn0.3 Znx O2 cells between 3.0 and 4.3 V for the first cycle was shown in Fig. 2. The CV curves showed one distinct anodic and cathodic peak centred at around 3.83 V, 3.55 V of the undoped sample and 3.86 V, 3.65 V of the Zn-doped sample. Due to Mn4+ /Mn3+ redox processes at 2.9 V [6] and Co3+ /Co4+ redox processes between 4.55 and 4.65 V [8], the redox processes in CV curves was Ni2+ /Ni4+ reaction [5]. This phenomenon suggested that no structural transitions existing from hexagonal to monoclinic during electrochemical cycling between 4.3 and 3.0 V [9,10]. As shown in Fig. 2, the oxidation and reduction peak area of Zn-doped sample was very larger than that of undoped one, this meant that Zn-doping increased the charge–discharge capacity. At the same time, the difference between the cathode and anode peak of the Zn-doped sample was less than that of the bare one. It was clear that the reversibility of Zn-doped sample was larger than that of the bare one due to the fact that Zn-doped enhanced stability and structural order. 3.3. Electrochemical property 3.3.1. First charge–discharge curves The third charge–discharge curves of Li/LiCo0.3 Ni0.4−x Mn0.3 Znx O2 cells were evaluated at a current density of 20 mA/g in the range of 2.75–4.3 V were shown in Fig. 3. It can be seen that an increase of discharge specific capacity with an increase of doped Zinc amount, from 151.75 mAh/g for x = 0 to 171.2 mAh/g for x = 0.02. However, with Zn-dopant amount increasing, the discharge specific capacity decreasing from 171.2 mAh/g for x = 0.02–148.6 mAh/g for x = 0.05.
Table 1 Lattice constants and other crystallochemical parameters of the Zn-dopant concentration. x
c
a
c/a
V
R2
0 0.01 0.02 0.03 0.04 0.05
14.2518 14.2518 14.2492 14.2464 14.2503 14.2467
2.8353 2.8485 2.8471 2.8524 2.8699 2.8617
5.0266 5.0033 5.0004 4.9945 4.9654 4.9784
99.2198 100.1458 100.0291 100.3822 101.6455 101.0399
1.29 1.34 1.39 1.34 1.31 1.27
Fig. 3. Discharge curves of Li/LiCo0.3 Ni0.4(x Mn0.3 Znx O2 cells at current density of 20 mA/g. (a) x = 0, (b) x = 0.02 and (c) x = 0.05.
Y. Chen et al. / Journal of Alloys and Compounds 476 (2009) 539–542
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Fig. 4. Cycle performance of Li/LiCo0.3 Ni0.4(x Mn0.3 Znx O2 cells. (a) x = 0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04 and (f) x = 0.05.
The change of the discharge capacity of the Zn-doped systems can be attributed to the presence of the size-invariant Zn2+ ions in the interslab regions of the products. From a general point of view, layered-type LiMO2 compounds are formed because of the size differences between [LiO6 ] and [MO6 ] octahedra, therefore, the larger Zn2+ cations (the ionic radii for a eight-coordination: Li+ , 0.76 Å; Ni2+ , 0.69 Å; Ni3+ , 0.56 Å; Co2+ , 0.64 Å; Co3+ , 0.545 Å; Mn3+ , 0.62 Å; Mn4+ , 0.59 Å; and Zn2+ , 0.88 Å) are expected to preferentially occupy the lithium sites if the Li/M ratio is less than 1. According to Gao et al [11,12], a value of more than 1.3 for the I0 0 3 /I1 0 4 intensity ratio is an indicator of reduced cation mixing, thus, it looks possible that a part of the dopant ions reside in the interslab spaces [13,14]. Lastly, if the Zn2+ ions occupy the transition metal ion, the capacity of the doped system is expected to be lower than that of the undoped one, however, in the present case, introducing the divalent Zn led to an increase in capacity and cyclability of the system, thus we can reasonably assume that the Zn-dopant will occupy the lithium sites instead of the transition metal sites [15]. The presence of the size-invariant Zn2+ ions in the interslab region prevents structural transformations during the topotactic reactions, which enhances the cyclability of the system. 3.3.2. Cycling studies The cycling performances of the LiCo0.3 Ni0.4−x Mn0.3 Znx O2 materials in the voltage range 2.75–4.3 V at the current density of 20 mA/g were shown in Fig. 4. The discharge capacities in the 1st and 20th cycles of undoped material for x = 0.00 were 158.5 and 129.8 mAh/g, respectively, with capacity retention of 81.9%. The cycling performances of Zn-doped materials remarkably improved. At x = 0.01, 0.03, the capacity retention after 20 cycles were 83.7 and 86.6%, respectively. At x = 0.02, the system shows the best cycling performance, the first discharge capacity was 175.7 mAh/g after 20 cycles remained 156.7 mAh/g, the capacity retention was 89.1%. As shown in Fig. 4, a little of Zn doping in LiCo0.3 Ni0.4 Mn0.3 O2 compounds can improved the discharge capacity and the cycling performance, but the Zn content cannot exceed 0.05. 3.3.3. High rate discharge capacity Rate capability was one of the important electrochemical characteristics of a lithium secondary battery required for power storage application. In the present study, rate capabilities were investigated at different current densities (Fig. 5). It can be seen that the voltage-flat and discharge capacity of both compounds decreased by increasing the discharge current comparing with Fig. 3. Comparing to the discharge capacity at the current density of 50 mA/g and 100 mA/g, the Li/LiCo0.3 Ni0.38 Mn0.3 Zn0.02 O2 battery discharge rate was 93.88% and 88.24 %, respectively, while Li/LiCo0.3 Ni0.4 Mn0.3 O2 cells discharge rate was 90.26% and 69.85 %, respectively. The bet-
Fig. 5. Discharge curves of Li/LiCo0.3 Ni0.4(x Mn0.3 Znx O2 cells at current density of 50 mA/g (a) and 100 mA/g (b).
ter high rate discharge capability with Zn-doping was related to Zn-doping made the layer structure much more stable. 3.4. Electrochemical impedance spectroscopic studies Fig. 6 compared the impedance spectra of the pristine and x = 0.02 Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 samples at a charge potential of 4.25 V, respectively. In Fig. 6, it was clear that a high-frequency semicircle represented the impedance due to a solid-state interface layer formed on the surface of the electrodes, and a low frequency semicircle was related to a slow charge transfer process at the interface and its relative double-layer capacitance at the film/bulk oxide [16]. The Warburg impedance was related to a combination of the diffusion effects of lithium ions on the interface between the active material particles and electrolyte, which was generally indicated by an inclined straight line at the low frequency end [16]. It was clear that the charge-transfer resistance of Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 electrode was much
Fig. 6. Nyquist plots of LiCo0.3 Ni0.4x Mn0.3 Znx O2 electrodes.
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Y. Chen et al. / Journal of Alloys and Compounds 476 (2009) 539–542
4. Conclusion Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 cathode materials were prepared via co-precipitation method. We can obtain the flowing conclusions through XRD, DSC and electrochemical tests.
Fig. 7. DSC curves of LiCo0.3 Ni0.4x Mn0.3 Znx O2 compounds in the charged state.
lower than that of undoped one, which indicated that the material became more conductive with Zn doping. Electrochemical impedance was a major part of internal resistance of a battery, and small impedance was favorable for the insertion and de-insertion of lithium ions during the charge and discharge process. At the same time, the straight line slope of Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 electrode was larger than that of undoped one, it showed that the diffusion rate of Lithium ion was easy with Zn-doped, which was related to the crystal structure variety with Zn-doped. Hence, the electrochemical impedance characteristic of Zn-doped LiCo0.3 Ni0.4 Mn0.3 O2 confirmed the improving electrochemical properties. 3.5. Thermal stability study To study the thermal stability of LiCo0.3 Ni0.4−x Mn0.3 Znx O2 materials, the full charged electrodes were tested by differential scanning calorimetry. As shown in Fig. 7, the onset temperature of thermal decomposition was increased by Zn-doping, and the undoped electrode had a sharp exothermic peak at about 240 ◦ C, while the Zn-doped electrode had a much small peak at about 269 ◦ C. The Zn-doping largely reduced the heat amount associated with exothermic peak, undoped and Zn-doped electrodes produced 45.44 and 21.07 J/g, respectively. The sharp exothermic peak of LiCo0.3 Ni0.4 Mn0.3 O2 electrode was obviously of great safety concern for Li-ion battery using LiCo0.3 Ni0.4 Mn0.3 O2 as the cathode material, while Zn-doped electrode had much better thermal safety characteristics.
(1) LiCo0.3 Ni0.4−x Mn0.3 Znx O2 had stable layered structure with aNaFeO2 type with x up to 0.05 and the compounds of x = 0.02 had the best layered structure. (2) The compound of x = 0.02 improved the cycle performance and high rate discharge capacity, the reason may be that the Zn-dopant occupy the lithium sites instead of the transition metal sites. The presence of the size-invariant Zn2+ ions in the interslab region prevents structural transformations during the topotactic reactions. (3) The onset temperature and peak temperature of thermal reaction were postponed and the quantity of heat was reduced with Zn-doping. Zn doping improved the material thermal stability. Acknowledgments This project was supported by the National Natural Science Foundation of China (No. 20273047). The authors wish to sincerely acknowledge all assistance from teachers, other students and leaders of McNair New Power Co. Ltd. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
N. Yabuuchi, T. Ohzuku, J. Power Sources 146 (2005) 636. N. Yabuuchi, T. Ohzuku, J. Power Sources 119–121 (2003) 171. S.H. Kang, J. Kim, M.E. Stoll, J. Power Sources 112 (2002) 41. S.H. Park, S.W. Oh, Y.K. Sun, J. Power Sources 146 (2005) 622. X. Xiao, X.H. Yang, H.G. Zheng, Solid State Ionics 176 (2005) 1043. Y.S. Meng, Y.W. Wu, et al., J. Electrochem. Soc. 151 (2004) A134. V. Subramanian, G.T.K. Fey, Solid State Ionics 148 (2002) 351. K.M. Shaju, G.V. Subba Rao, B.V.R. Chowdari, Electrochimic. Acta 48 (2002) 145. D.T. Liu, Z.X. Wang, L.Q. Chen, Electrochimic. Acta 51 (2006) 4199. D.C. Li, T. Muta, L.Z. Zhang, J. Power Sources 132 (2004) 150. H. Cao, Y. Zhang, J. Zhang, Solid State Ionics 176 (2005) 1207. C. Pouillerie, L. Croguennec, C. Delmas, Solid State Ionics 132 (2000) 15. Y. Gao, M.V. Yakovleva, W.B. Ebner, Electrochem. Solid-State Lett. 1 (1998) 117. C. Pouillerie, L. Croguennec, P. Biensan, et al., J. Electrochem. Soc. 147 (2000) 2061. R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B: Struct. Sci. 52 (1969) 925. G.T.K. Fey, J.G. Chen, V. Subramanian, et al., J. Power Sources 112 (2002) 384.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of Zn-doped LiCo0.3 Ni0.4(x Mn0.3 Znx O2 cathode materials for lithium-ion batteries Chen Yuhong a,∗ , CHEN Ruizhen a , Tang Zhiyuan b , Wang Liang c a
Department of Chemical and Environmental Engineering, Hebei Chemical & Pharmaceutical Vocational Technology College, Shijiazhuang 050026, China School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China c Library of Hebei University of Science & Technology, Shijiazhuang 050018, China b
a r t i c l e
i n f o
Article history: Received 12 August 2006 Received in revised form 6 September 2008 Accepted 10 September 2008 Available online 2 December 2008 Keywords: Lithium-ion batteries Cathode materials Zn-doped Co-precipitation
a b s t r a c t Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 cathode materials were synthesized via co-precipitation method. The structure, electrochemical performance and thermal stability were characterized by X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammograms (CV), electrochemical impedance spectroscopies (EIS) and differential scanning calorimetry (DSC). LiCo0.3 Ni0.4−x Mn0.3 Znx O2 had stable layered structure with a-NaFeO2 type with x up to 0.05. The compounds of x = 0.02 showed the best discharge capacity and cycle performance which was related to the most stable structure and Zn-doping prevented structural transformations during the topotactic reactions. Meanwhile, Zn-doping improved the high rate discharge capability and thermal stability. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Recently many researchers have interested in layer LiCox Niy Mn1−x−y O2 for its higher capacity and better safety compared with LiCoO2 [1,2]. However, this material has some problems, such as a low rate capability arising from the low electronic conductivity and tap density that should be resolved before it can replace LiCoO2 . One approach to improve the electrochemical performance is to partially substitute manganese cobalt nickel oxides, for transition metals such as iron, titanium, molybdenum or chromium and non-transition metals such as aluminum or magnesium, which may stabilize the layered structure with or without participating in the redox processes and prevent unwanted reactions between cathode and electrolyte [3–7]. However, to the best of our knowledge, no studies on the electrochemical performance of the Zn-doped metal oxide had been published. In this study, we employed Zn as an additional dopant to synthesize a series of Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 materials prepared by coprecipitation. The structural and electrochemical performance of the layered LiCo0.3 Ni0.4−x Mn0.3 Znx O2 materials were studied in this paper.
2.1. Synthesis of LiCo0.3 Ni0.4−x Mn0.3 Znx O2 The precursor Co0.3 Ni0.4−x Mn0.3 (OH)2 was prepared by co-precipitation from a solution containing molar ratio of cobalt/nickel/manganese sulfate by the addition NaOH and NH4 OH solution in a specially designed reactor. The Co0.3 Ni0.4−x Mn0.3 (OH)2 particle size was controlled by the reaction time in order to enhance the BET surface area. A longer reaction time in the solution resulted in a larger particle size. The precipitate continued to be rotated at 1000 rpm in the reactor at 45–50 ◦ C. The pH was maintained at 11–11.5 by controlling the amount of NH4 OH. After filtration, rinsing, and drying, the LiCo0.3 Ni0.4−x Mn0.3 Znx O2 was prepared by mixing molar ratio amounts of 1:1:x in LiOH, Mn0.3 Co0.3 Ni0.4−x (OH)2 and ZnO followed by a heat-treatment procedure at 900 ◦ C for 20 h in a stream of dried air.
2.2. Preparation of test cells Positive electrodes slurry for 2032 type simulated cells was prepared by thoroughly mixing active material (85%) with carbon black (10%) and polytetrafluoroethylene (5%) in ethanol. Electrodes with 0.5 cm−2 area and 0.02 mm depth extruding onto aluminum foil were dried for 12 h at 120 ◦ C under vacuum. 2032 type coin cells were then assembled in a argon filled dry box using foils of Li metal as counter electrodes and celgard 2400 saturated with 1 M LiPF6 in EC/EMC/DMC (1:1:1,wt:wt:wt) as separators.
2.3. Structure tests ∗ Corresponding author. Tel.: +86 13032620998. E-mail address: [email protected] (Y. Chen). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.09.055
Powder X-ray diffraction (XRD) (Philips X’ Pert Pro Pro MPD Co K␣) was done to determine the structure.
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Y. Chen et al. / Journal of Alloys and Compounds 476 (2009) 539–542
Fig. 2. Cyclic voltammogram LiCo0.3 Ni0.4(x Mn0.3 Znx O2 electrodes v = 0.1 mV/s.
Fig. 1. Typical X-ray diffractograms of LiCo0.3 Ni0.4−x Mn0.3 Znx O2 compounds.
slightly changed with the Zn dopant increasing, which would affect the electrochemical property [9]. 3.2. Cyclic voltammetry studies
2.4. Electrochemical tests The cyclic voltammograms (CV) and electrochemical impedance spectroscopies (EIS) were performed by electrochemistry working station (Gamry Instruments). The constant current and constant voltage charge (CCCV charge) was performed galvanostatically at 0.2 C rate or 0.5 C rate and then potentiostatically at 4.3 V until the current dropped to less than 0.01 C and constant current discharge was enforced at different rate to 2.75 V. 2.5. DSC tests For DSC experiments, the cells were finally fully charged to 4.3 V and opened in the Ar-filled dry box. After opening carefully, the extra electrolyte was removed from the surface of the electrode using DMC rinsing and the electrode materials were removed from the current collector. A aluminum sealed pan with a gold plated copper seal was used to collect 3–5 mg samples. The measurements were carried out in differential scanning calorimeter (NETZSCH-204) using a temperature scan rate of 10 ◦ C/min.
3. Results and discussion 3.1. Structural properties The typical X-ray diffraction patterns recorded for the undoped and Zn-doped materials were shown in Fig. 1. All the diffractograms showed patterns indexable in the a-NaFeO2 layered structure assuming a hexagonal lattice setting [2,4]. All the diffraction patterns showed a clear splitting of the hexagonal characteristic doublets (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0). This indicated that the products possessed typical layered characteristics [4]. The hexagonal unit cell parameters calculated for all the products were shown in Table 1. As shown in Table 1, the ratio of the intensity of the (0 0 3) reflection to that of the (1 0 4) reflection increased up to a dopant level of 0.02 and then decreased. The maximum in the value for the ratio showed that at a x value of 0.02, the system exhibited the maximum hexagonal ordering [8]. Comparing with undoped sample, with the Zn dopant increasing, the value of c decreased while the value of a increased and the volume of the crystal lattice increased. This phenomena showed that the sample structure was
Cyclic voltammograms (CV) of the Li/LiCo0.3 Ni0.4−x Mn0.3 Znx O2 cells between 3.0 and 4.3 V for the first cycle was shown in Fig. 2. The CV curves showed one distinct anodic and cathodic peak centred at around 3.83 V, 3.55 V of the undoped sample and 3.86 V, 3.65 V of the Zn-doped sample. Due to Mn4+ /Mn3+ redox processes at 2.9 V [6] and Co3+ /Co4+ redox processes between 4.55 and 4.65 V [8], the redox processes in CV curves was Ni2+ /Ni4+ reaction [5]. This phenomenon suggested that no structural transitions existing from hexagonal to monoclinic during electrochemical cycling between 4.3 and 3.0 V [9,10]. As shown in Fig. 2, the oxidation and reduction peak area of Zn-doped sample was very larger than that of undoped one, this meant that Zn-doping increased the charge–discharge capacity. At the same time, the difference between the cathode and anode peak of the Zn-doped sample was less than that of the bare one. It was clear that the reversibility of Zn-doped sample was larger than that of the bare one due to the fact that Zn-doped enhanced stability and structural order. 3.3. Electrochemical property 3.3.1. First charge–discharge curves The third charge–discharge curves of Li/LiCo0.3 Ni0.4−x Mn0.3 Znx O2 cells were evaluated at a current density of 20 mA/g in the range of 2.75–4.3 V were shown in Fig. 3. It can be seen that an increase of discharge specific capacity with an increase of doped Zinc amount, from 151.75 mAh/g for x = 0 to 171.2 mAh/g for x = 0.02. However, with Zn-dopant amount increasing, the discharge specific capacity decreasing from 171.2 mAh/g for x = 0.02–148.6 mAh/g for x = 0.05.
Table 1 Lattice constants and other crystallochemical parameters of the Zn-dopant concentration. x
c
a
c/a
V
R2
0 0.01 0.02 0.03 0.04 0.05
14.2518 14.2518 14.2492 14.2464 14.2503 14.2467
2.8353 2.8485 2.8471 2.8524 2.8699 2.8617
5.0266 5.0033 5.0004 4.9945 4.9654 4.9784
99.2198 100.1458 100.0291 100.3822 101.6455 101.0399
1.29 1.34 1.39 1.34 1.31 1.27
Fig. 3. Discharge curves of Li/LiCo0.3 Ni0.4(x Mn0.3 Znx O2 cells at current density of 20 mA/g. (a) x = 0, (b) x = 0.02 and (c) x = 0.05.
Y. Chen et al. / Journal of Alloys and Compounds 476 (2009) 539–542
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Fig. 4. Cycle performance of Li/LiCo0.3 Ni0.4(x Mn0.3 Znx O2 cells. (a) x = 0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04 and (f) x = 0.05.
The change of the discharge capacity of the Zn-doped systems can be attributed to the presence of the size-invariant Zn2+ ions in the interslab regions of the products. From a general point of view, layered-type LiMO2 compounds are formed because of the size differences between [LiO6 ] and [MO6 ] octahedra, therefore, the larger Zn2+ cations (the ionic radii for a eight-coordination: Li+ , 0.76 Å; Ni2+ , 0.69 Å; Ni3+ , 0.56 Å; Co2+ , 0.64 Å; Co3+ , 0.545 Å; Mn3+ , 0.62 Å; Mn4+ , 0.59 Å; and Zn2+ , 0.88 Å) are expected to preferentially occupy the lithium sites if the Li/M ratio is less than 1. According to Gao et al [11,12], a value of more than 1.3 for the I0 0 3 /I1 0 4 intensity ratio is an indicator of reduced cation mixing, thus, it looks possible that a part of the dopant ions reside in the interslab spaces [13,14]. Lastly, if the Zn2+ ions occupy the transition metal ion, the capacity of the doped system is expected to be lower than that of the undoped one, however, in the present case, introducing the divalent Zn led to an increase in capacity and cyclability of the system, thus we can reasonably assume that the Zn-dopant will occupy the lithium sites instead of the transition metal sites [15]. The presence of the size-invariant Zn2+ ions in the interslab region prevents structural transformations during the topotactic reactions, which enhances the cyclability of the system. 3.3.2. Cycling studies The cycling performances of the LiCo0.3 Ni0.4−x Mn0.3 Znx O2 materials in the voltage range 2.75–4.3 V at the current density of 20 mA/g were shown in Fig. 4. The discharge capacities in the 1st and 20th cycles of undoped material for x = 0.00 were 158.5 and 129.8 mAh/g, respectively, with capacity retention of 81.9%. The cycling performances of Zn-doped materials remarkably improved. At x = 0.01, 0.03, the capacity retention after 20 cycles were 83.7 and 86.6%, respectively. At x = 0.02, the system shows the best cycling performance, the first discharge capacity was 175.7 mAh/g after 20 cycles remained 156.7 mAh/g, the capacity retention was 89.1%. As shown in Fig. 4, a little of Zn doping in LiCo0.3 Ni0.4 Mn0.3 O2 compounds can improved the discharge capacity and the cycling performance, but the Zn content cannot exceed 0.05. 3.3.3. High rate discharge capacity Rate capability was one of the important electrochemical characteristics of a lithium secondary battery required for power storage application. In the present study, rate capabilities were investigated at different current densities (Fig. 5). It can be seen that the voltage-flat and discharge capacity of both compounds decreased by increasing the discharge current comparing with Fig. 3. Comparing to the discharge capacity at the current density of 50 mA/g and 100 mA/g, the Li/LiCo0.3 Ni0.38 Mn0.3 Zn0.02 O2 battery discharge rate was 93.88% and 88.24 %, respectively, while Li/LiCo0.3 Ni0.4 Mn0.3 O2 cells discharge rate was 90.26% and 69.85 %, respectively. The bet-
Fig. 5. Discharge curves of Li/LiCo0.3 Ni0.4(x Mn0.3 Znx O2 cells at current density of 50 mA/g (a) and 100 mA/g (b).
ter high rate discharge capability with Zn-doping was related to Zn-doping made the layer structure much more stable. 3.4. Electrochemical impedance spectroscopic studies Fig. 6 compared the impedance spectra of the pristine and x = 0.02 Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 samples at a charge potential of 4.25 V, respectively. In Fig. 6, it was clear that a high-frequency semicircle represented the impedance due to a solid-state interface layer formed on the surface of the electrodes, and a low frequency semicircle was related to a slow charge transfer process at the interface and its relative double-layer capacitance at the film/bulk oxide [16]. The Warburg impedance was related to a combination of the diffusion effects of lithium ions on the interface between the active material particles and electrolyte, which was generally indicated by an inclined straight line at the low frequency end [16]. It was clear that the charge-transfer resistance of Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 electrode was much
Fig. 6. Nyquist plots of LiCo0.3 Ni0.4x Mn0.3 Znx O2 electrodes.
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4. Conclusion Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 cathode materials were prepared via co-precipitation method. We can obtain the flowing conclusions through XRD, DSC and electrochemical tests.
Fig. 7. DSC curves of LiCo0.3 Ni0.4x Mn0.3 Znx O2 compounds in the charged state.
lower than that of undoped one, which indicated that the material became more conductive with Zn doping. Electrochemical impedance was a major part of internal resistance of a battery, and small impedance was favorable for the insertion and de-insertion of lithium ions during the charge and discharge process. At the same time, the straight line slope of Zn-doped LiCo0.3 Ni0.4−x Mn0.3 Znx O2 electrode was larger than that of undoped one, it showed that the diffusion rate of Lithium ion was easy with Zn-doped, which was related to the crystal structure variety with Zn-doped. Hence, the electrochemical impedance characteristic of Zn-doped LiCo0.3 Ni0.4 Mn0.3 O2 confirmed the improving electrochemical properties. 3.5. Thermal stability study To study the thermal stability of LiCo0.3 Ni0.4−x Mn0.3 Znx O2 materials, the full charged electrodes were tested by differential scanning calorimetry. As shown in Fig. 7, the onset temperature of thermal decomposition was increased by Zn-doping, and the undoped electrode had a sharp exothermic peak at about 240 ◦ C, while the Zn-doped electrode had a much small peak at about 269 ◦ C. The Zn-doping largely reduced the heat amount associated with exothermic peak, undoped and Zn-doped electrodes produced 45.44 and 21.07 J/g, respectively. The sharp exothermic peak of LiCo0.3 Ni0.4 Mn0.3 O2 electrode was obviously of great safety concern for Li-ion battery using LiCo0.3 Ni0.4 Mn0.3 O2 as the cathode material, while Zn-doped electrode had much better thermal safety characteristics.
(1) LiCo0.3 Ni0.4−x Mn0.3 Znx O2 had stable layered structure with aNaFeO2 type with x up to 0.05 and the compounds of x = 0.02 had the best layered structure. (2) The compound of x = 0.02 improved the cycle performance and high rate discharge capacity, the reason may be that the Zn-dopant occupy the lithium sites instead of the transition metal sites. The presence of the size-invariant Zn2+ ions in the interslab region prevents structural transformations during the topotactic reactions. (3) The onset temperature and peak temperature of thermal reaction were postponed and the quantity of heat was reduced with Zn-doping. Zn doping improved the material thermal stability. Acknowledgments This project was supported by the National Natural Science Foundation of China (No. 20273047). The authors wish to sincerely acknowledge all assistance from teachers, other students and leaders of McNair New Power Co. Ltd. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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