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The effects of LaPO4 coating on the electrochemical properties of Li[Ni0.5Co0.2Mn0.3]O2 cathode material
Solid State Ionics 225 (2012) 532–537
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
The effects of LaPO4 coating on the electrochemical properties of Li[Ni0.5Co0.2Mn0.3]O2 cathode material Han Gab Song a, Kyu-Sung Park b, Yong Joon Park a,⁎ a b
Department of Advanced Materials Engineering, Kyonggi University, Gyeonggi-do 443-760, Republic of Korea Battery Group, Samsung Advanced Institute of Technology (SAIT), Yongin 446-712, Republic of Korea
a r t i c l e
i n f o
Article history: Received 5 September 2011 Received in revised form 21 December 2011 Accepted 22 December 2011 Available online 14 January 2012 Keywords: Surface coating Electrochemical properties Cathode Lithium battery
a b s t r a c t A new coating material, LaPO4, was introduced for the surface modification of an Li[Ni0.5Co0.2Mn0.3]O2 cathode. The surface morphology, rate capability, cyclic performance, and concentration profile of the LaPO4-coated cathode were characterized. The LaPO4-coated sample demonstrated improved electrochemical properties as compared with a pristine sample. Glow discharge-optical emission spectrometer (GD-OES and X-ray diffraction (XRD) confirmed that the LaPO4 coating layer suppressed the dissolution of transition metals (Ni, Mn, Co) into the electrolyte and prevented phase transformation during cycling, results which can lead to stable cyclic performance. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The lithium ion battery has been considered as a possible power source for electric vehicles (EVs), hybrid electric vehicles (HEVs), and electric power tools [1–3]. Among the different components of a lithium ion battery, the cathode material is a key factor in determining the battery's capacity, cyclic performance, and thermal stability. In recent years, Li[Ni, Co, Mn]O2 cathode material has been widely studied for use in a Li-ion battery as a replacement for the currently popular LiCoO2 [4–6]. However, several problems with this material remain to be overcome, such as its inferior rate capability and thermal stability. One approach to resolve these problems is to substitute transition metal ions for other elements [7,8]. These elements may stabilize the layered structure, and improve the electrochemical properties. Another way is to substitute fluorine for oxygen to stabilize the host structure [9,10]. This method was very effective for enhancing thermal stability of cathode materials. However, such approaches could cause a lowering of capacity and Li ion diffusion because the substituents are usually electrochemically inactive elements. In contrast, surface coating with stable materials has shown greatly enhanced electrochemical properties and thermal stability [11–16] without sacrificing the original capacity. In this paper, we introduce a solid electrolyte (LaPO4) as a coating material to enhance
⁎ Corresponding author at: Department of Advanced Materials Engineering, Kyonggi University, San94-6, Yiui-dong, Yeongtong-gu, Suwon, Gyeonggi-do 443-760, Republic of Korea. Tel.: + 82 31 249 9769. E-mail address: [email protected] (Y.J. Park). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.12.014
the ionic conductivity of the coating layer and increase the rate capability. LaPO4 has good ionic conductivity and a strong P_O bond, which facilitates the movement of Li ions through the interface between electrolyte and electrode and protects the cathode surface from reactive electrolytes. In this work, we prepared a Li[Ni0.5Co0.2Mn0.3]O2 cathode where the surface was modified with LaPO4, and then examined the electrochemical and structural properties of the coated electrode.
2. Experimental The pristine Li[Ni0.5Co0.2Mn0.3]O2 powder was a commercial product from ECOPRO. For the synthesis of the LaPO4 coating, La(NO3)3·6H2O (Aldrich, 99.99%), and (NH4)2HPO4 (JUNSEI, 99%) were dissolved in ethanol with continuous stirring for 24 h at 25 °C. Next, Li[Ni0.5Co0.2Mn0.3]O2 powder was added to the coating solution, which was mixed thoroughly for 24 h at 25 °C. The slurry was dried in an oven at 100 °C for 10 h and heat-treated in a furnace at 400 °C for 3 h. The starting ratios of Li[Ni0.5Co0.2Mn0.3]O2 to each of the coating materials were 99:1, 97:3 and 95:5 by weight. X-ray diffraction (XRD) patterns of the powders were obtained using a Philips X-ray diffractometer, and the microstructure of the powder was observed using a field-emission scanning electron microscope (Nova Nano 200). For electrochemical testing, a cathode slurry was prepared by mixing the oxide powder, carbon black (Super P), and poly(vinylidene) fluoride (PVDF) in a weight ratio of 80:12:8. The active material mass was always assumed to be 80% in the capacity calculations without consideration of weight of coating materials. The cells were subjected to galvanostatic cycling using a WonATech system. The electrolyte
H.G. Song et al. / Solid State Ionics 225 (2012) 532–537
was 1 M LiPF6 with ethylene carbonate/dimethyl carbonate (EC/DMC) (50:50 vol.%). Impedance measurements were carried out using an electrochemical workstation (CH instrument, CHI 660A) by applying an AC voltage at an amplitude of 5 mV over a frequency range of 0.1 Hz to 100 KHz. Pristine and coated samples were tested prior to and after storage using a radio frequency glow discharge-optical emission spectrometer [(GD-OES), (JY 10000 RF, KBSI-PA314) Korea Basic Science Institute (Busan Center)]. The samples were charged to 4.6 V and then stored with the electrolyte at 50 °C for 7 days prior to the test. 3. Results and discussion Fig. 1 shows SEM images of the pristine and LaPO4-coated Li [Ni0.5Co0.2Mn0.3]O2 powders. In the images, morphological changes to the surfaces of the coated samples can be clearly observed. The surface of the pristine powders was smooth, without any heterophase particles, while the coated samples showed a rough coating layer. It seems that the coating layer of the 1 wt.% coated sample did not cover the entire surface of the pristine powder. However, the 3 and 5 wt.% coated samples both displayed a homogeneously covered coating layer. The electrochemical properties of the pristine and coated Li [Ni0.5Co0.2Mn0.3]O2 electrodes were characterized in order to examine the effect of the coating on the capacity, cyclic performance, and rate capability of the cathode. Fig. 2(a) presents the discharge capacity of the pristine and coated electrodes at different C rates. Each of the electrodes was tested in the voltage range of 3.0–4.6 V at 0.5, 1, 2, 3, and 6 C rates. The pristine and coated samples showed a similar discharge capacity at 0.5C. However, as the C rate increased, the 1 and 3 wt.% coated samples exhibited a noticeably higher discharge capacity than the pristine sample, which implies enhancement of the rate capability. It is worth noting the dependence upon the amount of coating material. When the amount of coating material was more or less than 3 wt.%, the positive effect of the coating on the rate capability was lower than that of the 3 wt.% coating. In particular, the rate capability of the 5 wt.% coated sample with a thick coating layer was
533
not superior to that of the pristine sample. The stable coating layer could protect the cathode from the reactive electrolyte and suppress the formation of an unwanted interface layer. This facilitates the rapid movement of electrons and Li ions during the charge–discharge process. However, the 1 wt.% LaPO4 coated sample may not have formed a stable coating layer on the entire surface of the powder due to an insufficient amount of coating material. In contrast, the 5 wt.% coated sample may have had too thick a coating layer on the surface. In this case, the coating layer itself acts as an obstacle to the rapid movement of electrons and Li ions during the charge– discharge process, which could explain the relatively low rate capability of the 5 wt.% coated sample. Fig. 2(b) presents the cyclic performance of the pristine and coated samples in the voltage range of 4.8–3.0 V. The upper cut-off voltage was increased to 4.8 V in order to investigate the coating effect on the cyclic performance under a severe condition. It is known that for an Li[Ni, Co, Mn]O2 cathode, chemical or structural instability begins in the high voltage range (i.e., above 4.6 V). As expected, the discharge capacity of the pristine sample dropped rapidly over the course of 50 cycles. However, the 1 and 3 wt.% coated samples exhibited a much enhanced cyclic performance, which may be due to the protective effect of the coating layer with regard to the reactive electrolyte. However, the 5 wt.% coated sample did not prove to be very effective in improving the cyclic performance. In general the coating material is likely to diffuse easily into the surface and react with elements of the bare material such as Li, Co, Ni, and Mn because of the high surface free energy of nano particles. Too thick coating layer could largely change the composition of active material, which may deteriorate the electrochemical property of the thick coated electrode. Fig. 2(c) presents the Nyquist plots of the pristine and coated samples before the electrochemical test. As shown in Fig. 2c, the coated samples have lower resistance than pristine samples. In particular, LaPO4 3 wt.% coated sample present the lower resistance than 1 wt.% and 5 wt.% coated samples. And, the 5 wt.% coated sample showed higher resistance than 1 and 3 wt.% coated sample. This result corroborates the findings regarding the enhanced rate capability of the
Fig. 1. SEM images of Li[Ni0.5Co0.2Mn0.3]O2 powder. (a) Pristine; (b) 1 wt.% LaPO4-coated powder; (c) 3 wt.% LaPO4-coated powder; and (d) 5 wt.% LaPO4-coated powder.
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the samples was still remained after cycling, which explains that coating materials continuously protected active material for cycling test. To investigate the coating effect on the surface protection under a severe condition, the pristine and coated samples were charged to 4.6 V and stored in the electrolyte at 50 °C for 7 days. The stored samples could have been seriously degraded due to the dissolution of the transition metals (Ni, Mn, and Co) in a charged state and at a high temperature. To obtain information about the dissolution of the transition metals during storage, the concentration profile vs. depth of the electrode was investigated using glow discharge optical emission spectroscopy (GD-OES). GD-OES is an essential and convenient technique for elemental surface analysis and depth profiling [17,18]. In this method, a pristine and a coated sample were sputtered from the surface under glow discharge conditions and the optical emission of the sputtered atoms in Ar plasma was analyzed by spectroscopy. Fig. 4a shows the optical emissions of the constituent elements over the sputtering time for the pristine electrode prior to storage. A strong emission was detected for Ni, on account of its high sensitivity to GD-OES analysis, and emissions due to Co and Mn were also detected. The observed deviation of the emission intensity at the surface may be due to the roughness of the electrode. It was notable that for the pristine sample, the emission intensity of the transition metals significantly weakened after storage (Fig. 4b). This result clearly demonstrates that transition metals such as Ni, Co, and Mn dissolved into the electrolyte from the pristine electrode during storage. XRD patterns of the pristine and coated samples before and after storage were also recorded to supplement the GD-OES data. As shown in Fig. 4c, the XRD pattern of the pristine sample was significantly changed after storage. This structural change of the electrode may due to the dissolution of the transition metals. In contrast, the dissolution of transition metals was dramatically improved by LaPO4 coating. Fig. 5a and b shows the GD-OES profile of the 3 wt.% LaPO4-coated sample. It confirms the presence of the coating layer, as La and P emissions were detected, though their intensity was not high due to the small amount of coating material. As shown in Fig. 5a and b, the GD-OES profile of the coated electrode also changed after storage. However, the intensity of the emissions due to transition metals after storage decreased to a much lesser degree than was the case for the pristine electrode. Moreover, as shown in Fig. 5c, the structural change in the coated sample during storage was much smaller than that of the pristine sample. These results indicate that the LaPO4 coating layer successfully suppressed the dissolution of transition metals and related structural changes. This protective effect of the coating layer from the reactive electrolyte may lead to enhanced electrochemical properties of a coated electrode. 4. Conclusion
Fig. 2. Discharge capacity and cyclic performance of pristine and coated Li[Ni0.5Co0.2Mn0.3]O2 electrodes. (a) Comparison of rate capability of electrodes in the voltage range of 4.6–3.0 V at C rates of 0.5, 1, 2, 3, and 6; (b) Cyclic performance of electrodes in the voltage range of 4.8–3.0 V at 1 C rate. ; (c) Nyquist plots of pristine and LaPO4 coated electrodes before the electrochemical test.
coated electrodes in Fig. 2a. Fig. 3 presents SEM image of coated materials after cycling (50 cycles at 1 C rate) in the voltage range at of 4.8–3.0 V. The SEM images clearly show that the coating layer of
The surface of an Li[Ni0.5Co0.2Mn0.3]O2 cathode was modified by LaPO4 coating. A 1 and a 3 wt.% LaPO4-coated electrode both exhibited noticeably enhanced rate capability and cyclic performance as compared to the pristine electrode. GD-OES results showed that the LaPO4 coating successfully suppressed the dissolution of transition metals on the cathode during storage at a high temperature (50 °C). XRD analysis also confirmed that the structural change of the coated sample during storage was much smaller than that of the pristine sample. These results were evidently associated with the enhanced electrochemical properties of the coated sample. Acknowledgment This research was supported by the Converging Research Center Program through the Ministry of Education, Science and Technology (2011K000688).
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Fig. 3. The SEM images of the coated electrodes after cycling (50 cycles). (a)LaPO4 1 wt.% coated sample, (b) LaPO4 3 wt.% coated sample and (c) LaPO4 5 wt.% coated sample.
References [1] N. Yabuuchi, Y. Koyama, N. Nakayama, T. Ohzuku, J. Electrochem. Soc. 152 (2005) A1434. [2] I.B. Weinstock, J. Power Sources 110 (2002) 471. [3] Y.J. Park, M.M. Doeff, J. Power Sources 165 (2007) 573. [4] N. Yabuuchi, Y. Makimura, T. Ohzuku, J. Electrochem. Soc. 154 (2007) A314. [5] J. Choi, A. Manthiram, J. Electrochem. Soc. 152 (2005) A1714. [6] K.M. Shaju, P.G. Bruce, J. Power Sources 174 (2007) 1201. [7] J. Guo, L.F. Jiao, H.T. Yuan, L.Q. Wang, H.X. Li, M. Zhang, Y.M. Wang, Electrochim. Acta 51 (2006) 6275. [8] B. Lin, Z. Wen, Z. Gu, X. Xu, J. Power Sources 174 (2007) 544. [9] G.-H. Kim, J.-H. Kim, S.T. Myung, C.S. Yoon, Y.-K. Sun, J. Electrochem. Soc. 152 (2005) A1707.
[10] L. Liao, X. Wang, X. Luo, X. Wang, S. Gamboa, P.J. Sebastian, J. Power Sources 160 (2006) 657. [11] K.S. Ryu, S.H. Lee, B.K. Koo, J.W. Lee, K.M. Kim, Y.J. Park, J. Appl. Electrochem. 38 (2008) 1385. [12] Y.K. Sun, D.H. Kim, H.G. Jung, S.T. Myung, K. Amine, Electrochim. Acta 55 (2010) 8621. [13] H.J. Lee, K.S. Park, Y.J. Park, J. Power Sources 195 (2010) 6122. [14] J. Liu, A. Manthiram, J. Electrochem. Soc. 156 (2009) A833. [15] G.T.K. Fey, C.Z. Lu, J.D. Huang, T.P. Kumar, Y.C. Chang, J. Power Sources 146 (2005) 65. [16] S.H. Kang, M.M. Thackeray, Electrochem. Commun. 11 (2009) 748. [17] K. Ui, K. Yamamoto, K. Ishikawa, T. Minami, K. Takeuchi, M.K. Itagaki, K. Watanabe, N. Koura, J. Power Sources 183 (2008) 347. [18] Y. Saito, M.K. Rahman, J. Power Sources 174 (2007) 877.
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Fig. 4. Depth profiles of optical emissions from constituent elements depending on sputtering time, in GD-OES measurements of the pristine Li[Ni0.5Co0.2Mn0.3]O2 electrode, which was charged at 4.6 V, (a) before storage, (b) after storage at 50 °C for 7 days, and (c) XRD patterns of the electrode before and after storage.
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Fig. 5. Depth profiles of optical emissions from constituent elements depending on sputtering time, in GD-OES measurements of the 3 wt.% LaPO4-coated Li[Ni0.5Co0.2Mn0.3]O2 electrode, which was charged at 4.6 V, (a) before storage, (b) after storage at 50 °C for 7 days, and (c) XRD patterns of the electrode before and after storage.
Contents lists available at SciVerse ScienceDirect
Solid State Ionics journal homepage: www.elsevier.com/locate/ssi
The effects of LaPO4 coating on the electrochemical properties of Li[Ni0.5Co0.2Mn0.3]O2 cathode material Han Gab Song a, Kyu-Sung Park b, Yong Joon Park a,⁎ a b
Department of Advanced Materials Engineering, Kyonggi University, Gyeonggi-do 443-760, Republic of Korea Battery Group, Samsung Advanced Institute of Technology (SAIT), Yongin 446-712, Republic of Korea
a r t i c l e
i n f o
Article history: Received 5 September 2011 Received in revised form 21 December 2011 Accepted 22 December 2011 Available online 14 January 2012 Keywords: Surface coating Electrochemical properties Cathode Lithium battery
a b s t r a c t A new coating material, LaPO4, was introduced for the surface modification of an Li[Ni0.5Co0.2Mn0.3]O2 cathode. The surface morphology, rate capability, cyclic performance, and concentration profile of the LaPO4-coated cathode were characterized. The LaPO4-coated sample demonstrated improved electrochemical properties as compared with a pristine sample. Glow discharge-optical emission spectrometer (GD-OES and X-ray diffraction (XRD) confirmed that the LaPO4 coating layer suppressed the dissolution of transition metals (Ni, Mn, Co) into the electrolyte and prevented phase transformation during cycling, results which can lead to stable cyclic performance. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The lithium ion battery has been considered as a possible power source for electric vehicles (EVs), hybrid electric vehicles (HEVs), and electric power tools [1–3]. Among the different components of a lithium ion battery, the cathode material is a key factor in determining the battery's capacity, cyclic performance, and thermal stability. In recent years, Li[Ni, Co, Mn]O2 cathode material has been widely studied for use in a Li-ion battery as a replacement for the currently popular LiCoO2 [4–6]. However, several problems with this material remain to be overcome, such as its inferior rate capability and thermal stability. One approach to resolve these problems is to substitute transition metal ions for other elements [7,8]. These elements may stabilize the layered structure, and improve the electrochemical properties. Another way is to substitute fluorine for oxygen to stabilize the host structure [9,10]. This method was very effective for enhancing thermal stability of cathode materials. However, such approaches could cause a lowering of capacity and Li ion diffusion because the substituents are usually electrochemically inactive elements. In contrast, surface coating with stable materials has shown greatly enhanced electrochemical properties and thermal stability [11–16] without sacrificing the original capacity. In this paper, we introduce a solid electrolyte (LaPO4) as a coating material to enhance
⁎ Corresponding author at: Department of Advanced Materials Engineering, Kyonggi University, San94-6, Yiui-dong, Yeongtong-gu, Suwon, Gyeonggi-do 443-760, Republic of Korea. Tel.: + 82 31 249 9769. E-mail address: [email protected] (Y.J. Park). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.12.014
the ionic conductivity of the coating layer and increase the rate capability. LaPO4 has good ionic conductivity and a strong P_O bond, which facilitates the movement of Li ions through the interface between electrolyte and electrode and protects the cathode surface from reactive electrolytes. In this work, we prepared a Li[Ni0.5Co0.2Mn0.3]O2 cathode where the surface was modified with LaPO4, and then examined the electrochemical and structural properties of the coated electrode.
2. Experimental The pristine Li[Ni0.5Co0.2Mn0.3]O2 powder was a commercial product from ECOPRO. For the synthesis of the LaPO4 coating, La(NO3)3·6H2O (Aldrich, 99.99%), and (NH4)2HPO4 (JUNSEI, 99%) were dissolved in ethanol with continuous stirring for 24 h at 25 °C. Next, Li[Ni0.5Co0.2Mn0.3]O2 powder was added to the coating solution, which was mixed thoroughly for 24 h at 25 °C. The slurry was dried in an oven at 100 °C for 10 h and heat-treated in a furnace at 400 °C for 3 h. The starting ratios of Li[Ni0.5Co0.2Mn0.3]O2 to each of the coating materials were 99:1, 97:3 and 95:5 by weight. X-ray diffraction (XRD) patterns of the powders were obtained using a Philips X-ray diffractometer, and the microstructure of the powder was observed using a field-emission scanning electron microscope (Nova Nano 200). For electrochemical testing, a cathode slurry was prepared by mixing the oxide powder, carbon black (Super P), and poly(vinylidene) fluoride (PVDF) in a weight ratio of 80:12:8. The active material mass was always assumed to be 80% in the capacity calculations without consideration of weight of coating materials. The cells were subjected to galvanostatic cycling using a WonATech system. The electrolyte
H.G. Song et al. / Solid State Ionics 225 (2012) 532–537
was 1 M LiPF6 with ethylene carbonate/dimethyl carbonate (EC/DMC) (50:50 vol.%). Impedance measurements were carried out using an electrochemical workstation (CH instrument, CHI 660A) by applying an AC voltage at an amplitude of 5 mV over a frequency range of 0.1 Hz to 100 KHz. Pristine and coated samples were tested prior to and after storage using a radio frequency glow discharge-optical emission spectrometer [(GD-OES), (JY 10000 RF, KBSI-PA314) Korea Basic Science Institute (Busan Center)]. The samples were charged to 4.6 V and then stored with the electrolyte at 50 °C for 7 days prior to the test. 3. Results and discussion Fig. 1 shows SEM images of the pristine and LaPO4-coated Li [Ni0.5Co0.2Mn0.3]O2 powders. In the images, morphological changes to the surfaces of the coated samples can be clearly observed. The surface of the pristine powders was smooth, without any heterophase particles, while the coated samples showed a rough coating layer. It seems that the coating layer of the 1 wt.% coated sample did not cover the entire surface of the pristine powder. However, the 3 and 5 wt.% coated samples both displayed a homogeneously covered coating layer. The electrochemical properties of the pristine and coated Li [Ni0.5Co0.2Mn0.3]O2 electrodes were characterized in order to examine the effect of the coating on the capacity, cyclic performance, and rate capability of the cathode. Fig. 2(a) presents the discharge capacity of the pristine and coated electrodes at different C rates. Each of the electrodes was tested in the voltage range of 3.0–4.6 V at 0.5, 1, 2, 3, and 6 C rates. The pristine and coated samples showed a similar discharge capacity at 0.5C. However, as the C rate increased, the 1 and 3 wt.% coated samples exhibited a noticeably higher discharge capacity than the pristine sample, which implies enhancement of the rate capability. It is worth noting the dependence upon the amount of coating material. When the amount of coating material was more or less than 3 wt.%, the positive effect of the coating on the rate capability was lower than that of the 3 wt.% coating. In particular, the rate capability of the 5 wt.% coated sample with a thick coating layer was
533
not superior to that of the pristine sample. The stable coating layer could protect the cathode from the reactive electrolyte and suppress the formation of an unwanted interface layer. This facilitates the rapid movement of electrons and Li ions during the charge–discharge process. However, the 1 wt.% LaPO4 coated sample may not have formed a stable coating layer on the entire surface of the powder due to an insufficient amount of coating material. In contrast, the 5 wt.% coated sample may have had too thick a coating layer on the surface. In this case, the coating layer itself acts as an obstacle to the rapid movement of electrons and Li ions during the charge– discharge process, which could explain the relatively low rate capability of the 5 wt.% coated sample. Fig. 2(b) presents the cyclic performance of the pristine and coated samples in the voltage range of 4.8–3.0 V. The upper cut-off voltage was increased to 4.8 V in order to investigate the coating effect on the cyclic performance under a severe condition. It is known that for an Li[Ni, Co, Mn]O2 cathode, chemical or structural instability begins in the high voltage range (i.e., above 4.6 V). As expected, the discharge capacity of the pristine sample dropped rapidly over the course of 50 cycles. However, the 1 and 3 wt.% coated samples exhibited a much enhanced cyclic performance, which may be due to the protective effect of the coating layer with regard to the reactive electrolyte. However, the 5 wt.% coated sample did not prove to be very effective in improving the cyclic performance. In general the coating material is likely to diffuse easily into the surface and react with elements of the bare material such as Li, Co, Ni, and Mn because of the high surface free energy of nano particles. Too thick coating layer could largely change the composition of active material, which may deteriorate the electrochemical property of the thick coated electrode. Fig. 2(c) presents the Nyquist plots of the pristine and coated samples before the electrochemical test. As shown in Fig. 2c, the coated samples have lower resistance than pristine samples. In particular, LaPO4 3 wt.% coated sample present the lower resistance than 1 wt.% and 5 wt.% coated samples. And, the 5 wt.% coated sample showed higher resistance than 1 and 3 wt.% coated sample. This result corroborates the findings regarding the enhanced rate capability of the
Fig. 1. SEM images of Li[Ni0.5Co0.2Mn0.3]O2 powder. (a) Pristine; (b) 1 wt.% LaPO4-coated powder; (c) 3 wt.% LaPO4-coated powder; and (d) 5 wt.% LaPO4-coated powder.
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the samples was still remained after cycling, which explains that coating materials continuously protected active material for cycling test. To investigate the coating effect on the surface protection under a severe condition, the pristine and coated samples were charged to 4.6 V and stored in the electrolyte at 50 °C for 7 days. The stored samples could have been seriously degraded due to the dissolution of the transition metals (Ni, Mn, and Co) in a charged state and at a high temperature. To obtain information about the dissolution of the transition metals during storage, the concentration profile vs. depth of the electrode was investigated using glow discharge optical emission spectroscopy (GD-OES). GD-OES is an essential and convenient technique for elemental surface analysis and depth profiling [17,18]. In this method, a pristine and a coated sample were sputtered from the surface under glow discharge conditions and the optical emission of the sputtered atoms in Ar plasma was analyzed by spectroscopy. Fig. 4a shows the optical emissions of the constituent elements over the sputtering time for the pristine electrode prior to storage. A strong emission was detected for Ni, on account of its high sensitivity to GD-OES analysis, and emissions due to Co and Mn were also detected. The observed deviation of the emission intensity at the surface may be due to the roughness of the electrode. It was notable that for the pristine sample, the emission intensity of the transition metals significantly weakened after storage (Fig. 4b). This result clearly demonstrates that transition metals such as Ni, Co, and Mn dissolved into the electrolyte from the pristine electrode during storage. XRD patterns of the pristine and coated samples before and after storage were also recorded to supplement the GD-OES data. As shown in Fig. 4c, the XRD pattern of the pristine sample was significantly changed after storage. This structural change of the electrode may due to the dissolution of the transition metals. In contrast, the dissolution of transition metals was dramatically improved by LaPO4 coating. Fig. 5a and b shows the GD-OES profile of the 3 wt.% LaPO4-coated sample. It confirms the presence of the coating layer, as La and P emissions were detected, though their intensity was not high due to the small amount of coating material. As shown in Fig. 5a and b, the GD-OES profile of the coated electrode also changed after storage. However, the intensity of the emissions due to transition metals after storage decreased to a much lesser degree than was the case for the pristine electrode. Moreover, as shown in Fig. 5c, the structural change in the coated sample during storage was much smaller than that of the pristine sample. These results indicate that the LaPO4 coating layer successfully suppressed the dissolution of transition metals and related structural changes. This protective effect of the coating layer from the reactive electrolyte may lead to enhanced electrochemical properties of a coated electrode. 4. Conclusion
Fig. 2. Discharge capacity and cyclic performance of pristine and coated Li[Ni0.5Co0.2Mn0.3]O2 electrodes. (a) Comparison of rate capability of electrodes in the voltage range of 4.6–3.0 V at C rates of 0.5, 1, 2, 3, and 6; (b) Cyclic performance of electrodes in the voltage range of 4.8–3.0 V at 1 C rate. ; (c) Nyquist plots of pristine and LaPO4 coated electrodes before the electrochemical test.
coated electrodes in Fig. 2a. Fig. 3 presents SEM image of coated materials after cycling (50 cycles at 1 C rate) in the voltage range at of 4.8–3.0 V. The SEM images clearly show that the coating layer of
The surface of an Li[Ni0.5Co0.2Mn0.3]O2 cathode was modified by LaPO4 coating. A 1 and a 3 wt.% LaPO4-coated electrode both exhibited noticeably enhanced rate capability and cyclic performance as compared to the pristine electrode. GD-OES results showed that the LaPO4 coating successfully suppressed the dissolution of transition metals on the cathode during storage at a high temperature (50 °C). XRD analysis also confirmed that the structural change of the coated sample during storage was much smaller than that of the pristine sample. These results were evidently associated with the enhanced electrochemical properties of the coated sample. Acknowledgment This research was supported by the Converging Research Center Program through the Ministry of Education, Science and Technology (2011K000688).
H.G. Song et al. / Solid State Ionics 225 (2012) 532–537
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Fig. 3. The SEM images of the coated electrodes after cycling (50 cycles). (a)LaPO4 1 wt.% coated sample, (b) LaPO4 3 wt.% coated sample and (c) LaPO4 5 wt.% coated sample.
References [1] N. Yabuuchi, Y. Koyama, N. Nakayama, T. Ohzuku, J. Electrochem. Soc. 152 (2005) A1434. [2] I.B. Weinstock, J. Power Sources 110 (2002) 471. [3] Y.J. Park, M.M. Doeff, J. Power Sources 165 (2007) 573. [4] N. Yabuuchi, Y. Makimura, T. Ohzuku, J. Electrochem. Soc. 154 (2007) A314. [5] J. Choi, A. Manthiram, J. Electrochem. Soc. 152 (2005) A1714. [6] K.M. Shaju, P.G. Bruce, J. Power Sources 174 (2007) 1201. [7] J. Guo, L.F. Jiao, H.T. Yuan, L.Q. Wang, H.X. Li, M. Zhang, Y.M. Wang, Electrochim. Acta 51 (2006) 6275. [8] B. Lin, Z. Wen, Z. Gu, X. Xu, J. Power Sources 174 (2007) 544. [9] G.-H. Kim, J.-H. Kim, S.T. Myung, C.S. Yoon, Y.-K. Sun, J. Electrochem. Soc. 152 (2005) A1707.
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Fig. 4. Depth profiles of optical emissions from constituent elements depending on sputtering time, in GD-OES measurements of the pristine Li[Ni0.5Co0.2Mn0.3]O2 electrode, which was charged at 4.6 V, (a) before storage, (b) after storage at 50 °C for 7 days, and (c) XRD patterns of the electrode before and after storage.
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Fig. 5. Depth profiles of optical emissions from constituent elements depending on sputtering time, in GD-OES measurements of the 3 wt.% LaPO4-coated Li[Ni0.5Co0.2Mn0.3]O2 electrode, which was charged at 4.6 V, (a) before storage, (b) after storage at 50 °C for 7 days, and (c) XRD patterns of the electrode before and after storage.