- Email: [email protected]
Surface coating of spinel LiMn2O4 cathode electrode with lithium–nickel–manganese-oxide by RF sputtering method for lithium-ion batteries
Journal of Electroanalytical Chemistry 730 (2014) 20–25
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Surface coating of spinel LiMn2O4 cathode electrode with lithium–nickel–manganese-oxide by RF sputtering method for lithium-ion batteries Yujin Chae, Joong Kee Lee, Wonchang Choi ⇑ Center for Energy Convergence Research, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea Department of Energy and Environmental Engineering, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 305-333, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 8 July 2014 Accepted 21 July 2014 Available online 30 July 2014 Keywords: Surface modification Sputtering method Spinel lithium manganese oxide Lithium–nickel–manganese-oxide Lithium-ion battery
a b s t r a c t Lithium–nickel–manganese-oxide was coated using sputtering equipment on spinel LiMn2O4 cathode electrodes prepared by a conventional tape-casting method. Surface modification of the electrodes was confirmed by field emission scanning electron microscopy and field-emission electron-probe microanalysis. Electrochemical evaluations were performed in constant current mode and the galvanostatic intermittent titration technique revealed that surface coating improved the rate capability of electrode in lithium cells. The surface-modified electrode exhibited better capacity retention and suppressed the impedance increase during the storage test at 60 °C in a fully charged state as compared to the bare electrode. Also, the lithium–nickel–manganese-oxide deposition improves the thermal stability of electrodes as confirmed by differential scanning calorimetry. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have been widely used as smaller and lighter power sources for electronic devices owing to their high specific capacity, long cycle life, and light weight [1]. As the demand for LIBs increases, much effort is being directed toward the development of cathode materials. Although lithium cobalt oxide has been successful as a cathode material in commercial LIBs [2], it still suffers from some disadvantages including high cost, toxicity, and the limited supply of cobalt. Among the candidates to replace LiCoO2 in the cathode, LiMn2O4 with a spinel structure is one of the most attractive materials because of its various advantages such as a low cost, low environmental impact, good chemical stability, and limited oxygen loss [3,4]. Despite its advantages, there are still a few drawbacks such as Jahn–Teller distortion on the surface of the cathode, and Mn dissolution at high temperatures [5–7]. The Jahn–Teller distortion associated with 3d Mn3+ ions is especially prevalent during high current discharge processes, accompanied by a crystal structure transformation, from cubic to tetragonal, on the surface of Mn-based cathodes [8]. Moreover, Mn dissolution,
caused by the hydrofluoric acid (HF) generated by residual water and lithium hexafluorophosphate (LiPF6) in the electrolyte solution, is generally accelerated at high temperatures, which results in severe fading of capacity in spinel LiMn2O4 cathodes [9]. As a strategy to solve these problems, cation-substituted spinel LixMyMn2yO4 (M = Ti, Ge, Fe, Co, Zn, Cr, Ni) [10–13] and surfacemodification of the spinel LiMn2O4 with various oxides such as MgO, Al2O3, SiO2, TiO2, ZnO, SnO2, and ZrO2 [14–16] have been widely successful. These approaches have been effective in enhancing the electrochemical performance of spinel LiMn2O4 cathodes. However, in these previous studies either the rate capability of the electrode or the elevated-temperature characteristics were improved, but not both. In this study, we introduced a Li-Ni-Mn-O (LNMO) coating by utilizing a LiNi0.5Mn1.5O4 target and Radiofrequency (RF) magnetron sputtering equipment for simplified modification of a conventionally slurry-casted electrode that consisted of a LiMn2O4 powder, conducting carbon, and polymeric binder on an aluminum foil substrate. A comparison of the electrochemical properties at elevated temperatures, such as rate capability, cyclability, and storage characteristics is also presented. 2. Experimental
Abbreviations: LNMO, Lithium–nickel–manganese-oxide; RF, Radio-frequency. ⇑ Corresponding author. Address: Korea Institute of Science and Technology, Hwarangro 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea. Tel.: +82 2 958 5253; fax: +82 2 958 5229. E-mail address: [email protected] (W. Choi). http://dx.doi.org/10.1016/j.jelechem.2014.07.026 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
2.1. Sample preparation Lithium manganese oxide (LiMn2O4) was synthesized using a mechanical process. LiOH (Aldrich) and MnCO3 (Aldrich) with a
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
1:2 stoichiometric ratio were used as the initial reagents. The precursors were mixed with acetone using a planetary mill (FRITSCH Pulverisette 5) at 350 rpm for 3 h with a ball-to-powder weight ratio of 20:1. The sample was then calcined in air at 800 °C for 72 h in a box furnace with a heating rate of 5 °C min1 and a cooling rate of 1 °C min1. To make the cathode electrode, a slurry was mixed containing 92 wt.% LiM2O4 as active material, 4 wt.% carbon black (DB100) as conducting agent, and 4 wt.% polyvinylidene fluoride (5 wt.% PVdF in the N-methyl-2-pyrrolidone) as binder. The slurries were coated by the doctor blade method on the Al foil (15–20 lm) and dried for 8 h under vacuum before use. The LiMn2O4 electrode was cut into a square sheet of 25 cm2 (5 cm 5 cm) for preparing another sample. All samples were pressed using a roll press machine (roll press type, RohTech, Korea) at room temperature, and calendering speed at 700 rpm, and further dried in a vacuum oven at 80 °C for 24 h to remove the solvent, completely. After tape casting, the thickness of electrode is around 60–70 lm, the thickness after calendaring is around 40 lm. The LNMO was deposited onto the LiMn2O4 electrode mentioned above, using an RF magnetron sputtering system (vertical type, Korea vacuum tech. LTD). An RF power and pressure of 100 W and 1 102 Pa, respectively, were applied to a LiNi0.5Mn1.5O4 (99.9 wt.%) target with 10.16 cm diameter. The sputtering was performed in pure argon and oxygen gases with 93 sccm and 15 sccm flows, respectively. The deposition time was 30 min and the deposition distance between the target and the substrate holder was constant, at 10 cm (see Fig. 1). 2.2. Sample characterization The field emission scanning electron microscope (FE-SEM, NOVA NanoSEM200, FEI Company) was analyzed to measuring LNMO film thickness. Field emission electron probe microanalysis (FE-EPMA, JXA-8500F, JOEL, Japan) was also performed to check the presence of nickel with an accelerating voltage of 30 eV. A probe diameter of 100 nm can be achieved at 100 nA, and the image resolution was 3 nm. 2.3. Electrochemical and thermal measurements 2032-type coin cells were used for the electrochemical and thermal tests. The coin cells were comprised of LiMn2O4 or lithium–nickel–manganese-oxide-deposited LiMn2O4 as the cathode, and lithium foil as the anode. The coin cells were fabricated a polypropylene separator (PP, Celgard 2400) between the anode and cathode electrodes. The electrolyte was 1.3 M LiPF6 dissolved in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a 3:7 volume ratio. The galvanostatic cycle tests were carried out on a Maccor automated battery tester (MACCOR series-4000) in the potential range of 3.5–4.3 V at a constant charge rate of 0.2 C. In this study, 1 C rate corresponds to 140 mA g1. The rate capabilities of LiMn2O4 and LNMO-deposited LiMn2O4 was performed at various C-rates of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 4 C during the discharge process, while the current was fixed at 0.2 C during the charge process. The diffusion coefficients (DLi) were determined using the galvanostatic intermittent titration technique (GITT). This method involves the application of a small current pulse across a cell while monitoring the transient voltage as a function of time. The change in the steady-state voltage then indicates the dependence of the cell voltage on the concentration of the inserted species. To measure using the GITT, a series of current pulses were applied at a low constant current of 2.59 A g1 (i.e. 0.1 C), each followed by a 30 min relaxation time. The storage performance tests were carried out from a fully charged state under an elevated temperature at 60 °C. The open circuit voltage was reported every day.
21
Electrochemical impedance spectroscopy (EIS) experiments were carried out using an impedance/gain-phase analyzer (Solartron SI 1260) equipped with an electrochemical interface (Solartron SI 1260). The ac amplitude was 5 mV over a frequency range from 1 mHz to 100 kHz. The thermal stability of the substituted spinels, LiMn2O4, and LNMO-deposited LiMn2O4, was studied using differential scanning calorimetry (DSC, TA instruments Auto Q20) on fully charged electrodes up to 4.3 V vs. Li/Li+. The charged coin cells were opened in an argon-filled glove box and the cathode materials were recovered from the cells. The recovered cathode materials, including the electrolyte (about 20 wt.% of the recovered cathode), were sealed in a high-pressure stainless steel DSC pan. The DSC scan was carried out at ramp rat of 10 °C min1, from 100 °C to 390 °C under nitrogen flow.
3. Results and discussion A preliminary experiment was performed on the glass in order to determine and control the deposition thickness of lithium– nickel–manganese-oxide by RF sputtering equipment. The thickness identified by SEM data reveals that the LNMO layer, deposited on the glass for 30 min, was approximately 183.6 nm in Fig. 2a. Based on the preliminary experiment, Li-Ni-Mn-O components were deposited onto the conventionally tape-casted electrode, which consisted of an active material (LiMn2O4 with spinel structure), a conductive material (carbon black), and a binder (PVdF) under the sputtering conditions mentioned in the previous section. FE-EPMA analysis in Fig. 2b and c shows the top view and cross-sectional images of the bare and LNMO-modified LiMn2O4 electrodes. As both the LNMO coating layer and bare LiMn2O4 electrode contain manganese and oxygen ions, noticeable differences are not seen in Mn and O mapping images. Nickel mapping for the LNMO-deposited electrode (shown in the top view), however, clearly indicates that a LNMO layer was formed on tape-casted electrode, which may act as a protective or functional layer for the electrode materials. Moreover, the cross-sectional view of the LNMO-modified electrode reveals that the LNMO is deposited or coated uniformly in a certain extent through the porous cathode electrodes by the RF sputtering process. Fig. 3a shows typical galvanostatic charge/discharge profiles of the bare and LNMO-modified LiMn2O4 electrodes for the first cycle between 3.5 and 4.3 V (vs. Li/Li+) at 0.5 C. As presented in Fig. 3(a), both electrodes exhibit similar charge/discharge capacity values and plateaus on the curves near 4.0 V, which correspond to lithium-ion extraction/insertion from/into the tetrahedral sites of
Fig. 1. Schematic diagram of the modification of LiMn2O4 electrode with LiNi0.5Mn1.5O4 target using the RF sputtering system.
22
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
Fig. 2. SEM images and EPMA mapping of the (a) SEM cross-sectional view for the bare and LNMO-modified electrodes, (b) EPMA top view and (c) EPMA cross-sectional view for the bare and LNMO-modified electrodes.
the spinel structure [4,16]. The two distinct potential plateaus around 4.0 V are characteristic of a well-defined spinel structure with high crystallinity and reflect the typical redox processes of LiMn2O4 cathode, which result from the phase transitions concerning lithium ordering during charge/discharge processes. The differential capacity plots as a function of voltage in Fig. 3(b) also certify the well-separated two sets of peaks, corresponding to the plateaus mentioned above. Additionally, the electrochemical evaluation shown in Fig. 3 reveals that there is no additional redox reaction associated with the LNMO modification to the LiMn2O4 electrodes. The cycle performance of the bare and LNMO-modified spinel Li2MnO4 electrodes was evaluated between 3.5 and 4.3 V (vs. Li/Li+) with a current rate of 0.2 C, as demonstrated in Fig. 4. The discharge capacities of the bare and LNMO-deposited electrodes, are 128.6 and 126.2 mA h g1, respectively, in the first cycle, and then gradually decrease in capacity during the cycle test, suggesting that the modification of electrodes does not influence the cyclic performance at low current rates in a room temperature environment. Interestingly, the electrochemical performance in rate capability reveals that the modification of LiMn2O4 electrode with the LNMO material improves the capacity at higher C-rates, as presented in Fig. 5. The data was collected at room temperature between 3.5 and 4.3 V by charging at the same rate of 0.1 C but with different discharge rates from 0.1 C to 4 C. The LNMOmodified electrode retains approximately 125 mA h g1 up to 1 C, whereas the bare electrode delivers 113 mA h g1 at same C-rate. Moreover, at higher C-rates such as 2 and 4 C, the surface-modified electrode provides the better rate capability, exhibiting a capacity value of 117 mA h g1 as compared to the capacity of the bare electrode, which drops to 100 mA h g1 during evaluation at 4 C.
In an effort to better understand the enhancement in rate performance, the GITT is utilized to obtain numerical values concerning Li-ion diffusivity [12,16–19]. Fig. 6a shows a typical current–time step application and resulting voltage–time profiles corresponding to each current step, as well as the DLi values calculated from the profiles.DLi was obtained by the equation:
DLi ¼
2 mB V M DES ps MB S DEt 4
ð1Þ
where s expresses the constant current pulse time, mB is the mass, VM is the molar volume, and MB is the molar mass of the insertion electrode material. Also, S is the area of the electrode–electrolyte interface, DES means change of the steady-state voltage during a single-step GITT experiment, and DEt is the total change of cell voltage during a constant current pulse [16]. While both electrodes show a similar trend in the range up to x = 0.4 for lithium insertion during the discharge process, the LNMO-modified electrode exhibits an increase in DLi in the range of x = 0.4–1.0. Although the DLi values are influenced by various factors such as geometry, surface area, and compositional variation of electrodes, the lithium-containing transition metal oxide layer possibly provides kinetic enhancement in DLi as well as electrical conductivity, resulting in better rate performance of the spinel cathodes. Spinel lithium manganese oxide, in general, exhibits inferior electrochemical performance especially in high temperature environments, as outlined in the introduction. To confirm the effect of the LNMO modification onto and into the spinel LiMn2O4 electrodes, the storage characteristics of the bare and LNMO-modified electrodes were evaluated in a fully charged state at 60 °C for
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
23
Fig. 5. Rate capability of the bare and LNMO-modified LiMn2O4 electrode at various C-rate.
Fig. 3. (a) The charge–discharge voltage profiles and corresponding and (b) differential capacity plots as a function of voltages at first cycle for the bare and LNMO-modified LiMn2O4 electrodes.
Fig. 4. Cycling performances of the bare and LNMO-modified LiMn2O4 electrodes at a charge rate of 0.5 C.
Fig. 6. (a) GITT voltage profiles and (b) diffusion coefficients as a function of x in the Li/LixMn2O4 of the bare and LNMO-modified LiMn2O4 electrodes.
3 weeks. Although the open-circuit voltage (OCV), which is shown in Fig. 7a, exhibited little difference as demonstrated by the flat voltage profile of the spinel cathodes, the retained capacities evaluated at room temperature following the storage test for 3 weeks revealed that the bare LiMn2O4 electrode retained a capacity of approximately 45 mA h g1, while the LNMO-modified electrode retained approximately 55 mA h g1, as shown in Fig. 7b. The improvement in the retained capacity caused by the surface coating indicates that the LNMO layer was effective for protection of
the electrode and suppression of the self-discharge reaction resulting from the side reaction between the electrode and the electrolyte at high temperatures in the highly oxidizing environment. The side reaction and subsequent self-discharge reaction may influence the electrode characteristics, owing to the formation of a solid electrolyte interface (SEI) film and cause consequent degradation in electrochemical performance. To evaluate this, EIS was performed before and after the storage test, and the data was fitted with an equivalent circuit, as shown in the Nyquist plots of Fig 8.
24
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
Fig. 7. Storage characteristics at elevated temperatures: (a) open circuit voltage during the storage and (b) discharge capacity after storage for 3 weeks at 60 °C.
To separate and evaluate each resistance, the Z-Plot software suite was utilized within the measured frequency region. The semicircles in the high frequency region correspond to the SEI film resistance and charge transfer resistance at electrode/electrolyte interface, while the slope in low frequency region, which is referred to as the Warburg impedance, is related to solid-state diffusion of lithium-ions. Before the storage test, both electrodes exhibit similar values in the SEI film and charge transfer resistances, while the LNMO-modified electrode shows a slightly larger semicircle than the bare one. Interestingly, the Nyquist plots after the storage test indicate that the LNMO surface modification leads to suppression of the polarization impedance increase during the high-temperature storage, exhibiting smaller SEI film and charge transfer resistances compared to the bare electrode. The reduced polarization increase under conditions such as a fully charged state or elevated temperature is possibly due to the protective LNMO surface layers, which help suppress the degradation of the electrode resulting from the self-discharge reaction and side reaction between the electrode and electrolyte. Fig. 9 compares the DSC curves of the bare and LNMO-modified LiMn2O4 electrodes in a fully charged state. The exothermic reaction of bare and LNMO-modified LiMn2O4 electrodes commenced minutely at 168.48 and 170.75 °C, respectively. After that, exothermic reactions gradually occur around 250 °C. The total heat generated during DSC measurement for the bare and LNMO-modified LiMn2O4 electrodes, respectively, correspond to 919.3 J g1 and 815.7 J g1. Interestingly, additional shoulder peak around 295 °C is found for the LNMO-modified LIMn2O4, suggesting that the LNMO coating layer is related the exothermic reaction with electrolyte. Although both electrodes exhibit similar onset temperatures for the exothermic reaction with the electrolyte,
Fig. 8. Nyquist plots at fully discharged state for the bare and LNMO-modified LiMn2O4 electrodes (a) before and (b) after the storage test at 60 C.
Fig. 9. DSC profiles of the bare and LNMO-modified LiMn2O4 electrodes.
the LNMO-modified electrode exhibits a reduced exothermic peak intensity at approximately of 300 °C and less total heat flow compared to the bare electrode, as demonstrated in Fig. 9. Based on this, the surface modification of the electrode provides more stable surface characteristics and protects from electrolyte instability at high temperatures, resulting in an enhancement in thermal stability for lithium-ion battery application. 4. Conclusions A tape-casted LiMn2O4 electrode was treated with lithium– nickel–manganese-oxide by a sputtering technique, and the
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
presence of a surface coating was confirmed by FE-SEM and EPMA. The surface modification of the electrodes contributed to electrochemical enhancement of the rate characteristics at room temperature. The storage test conducted at 60 °C in a fully charged state revealed that the surface-coated electrode had better retention of charge capacity, while the bare electrode experienced a severe self-discharge reaction resulting from the side reaction between electrolyte and cathode. Further investigation using differential scanning calorimetry also certified that the surface coating was effective for protecting the electrode from the side reaction with electrolyte, thus providing thermal stability in the cathode of the lithium-ion cell. Conflict of interest There is no conflict of interest among authors for the publication of this article. Acknowledgements This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-2010-0028958), and the KIST Institutional Program.
25
References [1] C.M. Julien, A. Mauger, K. Zaghib, H. Groult, Inorganics 2 (1) (2014) 132–154. [2] H.S. Kim, J. Proell, R. Koheler, W. Pfleging, A. Pique, J. Laser Micro Nano Eng. 7 (2012) 320–325. [3] D.H. Jang, Y.J. Shin, S.M. Oh, J. Electrochem. Soc. 143 (1996) 2204–2211. [4] E. Hosono, T. Kudo, I. Honnma, H. Matsuda, H. Zhou, Nano Lett. 9 (2009) 1045– 1051. [5] M. Satioh, S. Yoshida, H. Yamane, M. Sano, M. Fujita, K. Kifune, Y. Kubota, J. Power Sources 122 (2003) 162–168. [6] J. Vetter, P. Novak, M.R. Wagner, C. Veit, K.-C. Moller, J.O. Bbesenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler, A. Hammouche, J. Power Sources 147 (2005) 269–281. [7] Y. Xia, Y. Zhou, M. Yoshio, J. Electrochem. Soc. 144 (1997) 2593–2600. [8] M.M. Thackeray, Y. Shao-Horn, A.J. Kahaian, K.D. Kepler, E. Skinner, J.T. Vaughey, S.A. Hackney, Electrochem. Solid-State Lett. 1 (1998) 7–9. [9] W. Choi, A. Manthiram, J. Electrochem. Soc. 153 (2006) A1760–A1764. [10] M. Wohlfahrt-Mehrens, A. Butz, R. Oesten, G. Arnold, R.P. Hemmer, R.A. Huggins, J. Power Sources 68 (1997) 582–585. [11] H.J. Bang, V.X. Donepudi, J. Prakash, Electrochim. Acta 48 (2002) 443–451. [12] C. Li, H.P. Zhang, L.J. Fu, H. Liu, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, E. Rahm, R. Holz, H.Q. Wu, Electrochim. Acta 51 (2006) 3872–3883. [13] D. Zhang, B.N. Popov, R.E. White, J. Power Sources 76 (1998) 81–90. [14] J. Cho, T.-J. Kim, Y.J. Kim, Chem. Commun. (2001) 1074–1075. [15] Li. Wang, J. Zhao, S. Guo, X. He, C. Hiang, C. Wan, Investigation of SnO2modified LiMn2O4 composite as cathode material for lithium-ion batteries, Int. J. Electrochem. Sci. 5 (2010) 1113–1126. [16] J.S. Gnanaraj, V.G. Pol, A. Gedanken, D. Aurbach, Electrochem. Commun. 5 (2003) 940–945. [17] Y. Zhu, C. Wang, J. Phys. Chem. C 114 (2010) 2830–2841. [18] E. Deiss, Electrochim. Acta 50 (2005) 2927–2932. [19] M.Y. Saidi, J. Barker, R. Koksbang, J. Solid State Chem. 122 (1996) 195–199.
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Surface coating of spinel LiMn2O4 cathode electrode with lithium–nickel–manganese-oxide by RF sputtering method for lithium-ion batteries Yujin Chae, Joong Kee Lee, Wonchang Choi ⇑ Center for Energy Convergence Research, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea Department of Energy and Environmental Engineering, Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 305-333, Republic of Korea
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 8 July 2014 Accepted 21 July 2014 Available online 30 July 2014 Keywords: Surface modification Sputtering method Spinel lithium manganese oxide Lithium–nickel–manganese-oxide Lithium-ion battery
a b s t r a c t Lithium–nickel–manganese-oxide was coated using sputtering equipment on spinel LiMn2O4 cathode electrodes prepared by a conventional tape-casting method. Surface modification of the electrodes was confirmed by field emission scanning electron microscopy and field-emission electron-probe microanalysis. Electrochemical evaluations were performed in constant current mode and the galvanostatic intermittent titration technique revealed that surface coating improved the rate capability of electrode in lithium cells. The surface-modified electrode exhibited better capacity retention and suppressed the impedance increase during the storage test at 60 °C in a fully charged state as compared to the bare electrode. Also, the lithium–nickel–manganese-oxide deposition improves the thermal stability of electrodes as confirmed by differential scanning calorimetry. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Lithium-ion batteries (LIBs) have been widely used as smaller and lighter power sources for electronic devices owing to their high specific capacity, long cycle life, and light weight [1]. As the demand for LIBs increases, much effort is being directed toward the development of cathode materials. Although lithium cobalt oxide has been successful as a cathode material in commercial LIBs [2], it still suffers from some disadvantages including high cost, toxicity, and the limited supply of cobalt. Among the candidates to replace LiCoO2 in the cathode, LiMn2O4 with a spinel structure is one of the most attractive materials because of its various advantages such as a low cost, low environmental impact, good chemical stability, and limited oxygen loss [3,4]. Despite its advantages, there are still a few drawbacks such as Jahn–Teller distortion on the surface of the cathode, and Mn dissolution at high temperatures [5–7]. The Jahn–Teller distortion associated with 3d Mn3+ ions is especially prevalent during high current discharge processes, accompanied by a crystal structure transformation, from cubic to tetragonal, on the surface of Mn-based cathodes [8]. Moreover, Mn dissolution,
caused by the hydrofluoric acid (HF) generated by residual water and lithium hexafluorophosphate (LiPF6) in the electrolyte solution, is generally accelerated at high temperatures, which results in severe fading of capacity in spinel LiMn2O4 cathodes [9]. As a strategy to solve these problems, cation-substituted spinel LixMyMn2yO4 (M = Ti, Ge, Fe, Co, Zn, Cr, Ni) [10–13] and surfacemodification of the spinel LiMn2O4 with various oxides such as MgO, Al2O3, SiO2, TiO2, ZnO, SnO2, and ZrO2 [14–16] have been widely successful. These approaches have been effective in enhancing the electrochemical performance of spinel LiMn2O4 cathodes. However, in these previous studies either the rate capability of the electrode or the elevated-temperature characteristics were improved, but not both. In this study, we introduced a Li-Ni-Mn-O (LNMO) coating by utilizing a LiNi0.5Mn1.5O4 target and Radiofrequency (RF) magnetron sputtering equipment for simplified modification of a conventionally slurry-casted electrode that consisted of a LiMn2O4 powder, conducting carbon, and polymeric binder on an aluminum foil substrate. A comparison of the electrochemical properties at elevated temperatures, such as rate capability, cyclability, and storage characteristics is also presented. 2. Experimental
Abbreviations: LNMO, Lithium–nickel–manganese-oxide; RF, Radio-frequency. ⇑ Corresponding author. Address: Korea Institute of Science and Technology, Hwarangro 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea. Tel.: +82 2 958 5253; fax: +82 2 958 5229. E-mail address: [email protected] (W. Choi). http://dx.doi.org/10.1016/j.jelechem.2014.07.026 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
2.1. Sample preparation Lithium manganese oxide (LiMn2O4) was synthesized using a mechanical process. LiOH (Aldrich) and MnCO3 (Aldrich) with a
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
1:2 stoichiometric ratio were used as the initial reagents. The precursors were mixed with acetone using a planetary mill (FRITSCH Pulverisette 5) at 350 rpm for 3 h with a ball-to-powder weight ratio of 20:1. The sample was then calcined in air at 800 °C for 72 h in a box furnace with a heating rate of 5 °C min1 and a cooling rate of 1 °C min1. To make the cathode electrode, a slurry was mixed containing 92 wt.% LiM2O4 as active material, 4 wt.% carbon black (DB100) as conducting agent, and 4 wt.% polyvinylidene fluoride (5 wt.% PVdF in the N-methyl-2-pyrrolidone) as binder. The slurries were coated by the doctor blade method on the Al foil (15–20 lm) and dried for 8 h under vacuum before use. The LiMn2O4 electrode was cut into a square sheet of 25 cm2 (5 cm 5 cm) for preparing another sample. All samples were pressed using a roll press machine (roll press type, RohTech, Korea) at room temperature, and calendering speed at 700 rpm, and further dried in a vacuum oven at 80 °C for 24 h to remove the solvent, completely. After tape casting, the thickness of electrode is around 60–70 lm, the thickness after calendaring is around 40 lm. The LNMO was deposited onto the LiMn2O4 electrode mentioned above, using an RF magnetron sputtering system (vertical type, Korea vacuum tech. LTD). An RF power and pressure of 100 W and 1 102 Pa, respectively, were applied to a LiNi0.5Mn1.5O4 (99.9 wt.%) target with 10.16 cm diameter. The sputtering was performed in pure argon and oxygen gases with 93 sccm and 15 sccm flows, respectively. The deposition time was 30 min and the deposition distance between the target and the substrate holder was constant, at 10 cm (see Fig. 1). 2.2. Sample characterization The field emission scanning electron microscope (FE-SEM, NOVA NanoSEM200, FEI Company) was analyzed to measuring LNMO film thickness. Field emission electron probe microanalysis (FE-EPMA, JXA-8500F, JOEL, Japan) was also performed to check the presence of nickel with an accelerating voltage of 30 eV. A probe diameter of 100 nm can be achieved at 100 nA, and the image resolution was 3 nm. 2.3. Electrochemical and thermal measurements 2032-type coin cells were used for the electrochemical and thermal tests. The coin cells were comprised of LiMn2O4 or lithium–nickel–manganese-oxide-deposited LiMn2O4 as the cathode, and lithium foil as the anode. The coin cells were fabricated a polypropylene separator (PP, Celgard 2400) between the anode and cathode electrodes. The electrolyte was 1.3 M LiPF6 dissolved in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a 3:7 volume ratio. The galvanostatic cycle tests were carried out on a Maccor automated battery tester (MACCOR series-4000) in the potential range of 3.5–4.3 V at a constant charge rate of 0.2 C. In this study, 1 C rate corresponds to 140 mA g1. The rate capabilities of LiMn2O4 and LNMO-deposited LiMn2O4 was performed at various C-rates of 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 4 C during the discharge process, while the current was fixed at 0.2 C during the charge process. The diffusion coefficients (DLi) were determined using the galvanostatic intermittent titration technique (GITT). This method involves the application of a small current pulse across a cell while monitoring the transient voltage as a function of time. The change in the steady-state voltage then indicates the dependence of the cell voltage on the concentration of the inserted species. To measure using the GITT, a series of current pulses were applied at a low constant current of 2.59 A g1 (i.e. 0.1 C), each followed by a 30 min relaxation time. The storage performance tests were carried out from a fully charged state under an elevated temperature at 60 °C. The open circuit voltage was reported every day.
21
Electrochemical impedance spectroscopy (EIS) experiments were carried out using an impedance/gain-phase analyzer (Solartron SI 1260) equipped with an electrochemical interface (Solartron SI 1260). The ac amplitude was 5 mV over a frequency range from 1 mHz to 100 kHz. The thermal stability of the substituted spinels, LiMn2O4, and LNMO-deposited LiMn2O4, was studied using differential scanning calorimetry (DSC, TA instruments Auto Q20) on fully charged electrodes up to 4.3 V vs. Li/Li+. The charged coin cells were opened in an argon-filled glove box and the cathode materials were recovered from the cells. The recovered cathode materials, including the electrolyte (about 20 wt.% of the recovered cathode), were sealed in a high-pressure stainless steel DSC pan. The DSC scan was carried out at ramp rat of 10 °C min1, from 100 °C to 390 °C under nitrogen flow.
3. Results and discussion A preliminary experiment was performed on the glass in order to determine and control the deposition thickness of lithium– nickel–manganese-oxide by RF sputtering equipment. The thickness identified by SEM data reveals that the LNMO layer, deposited on the glass for 30 min, was approximately 183.6 nm in Fig. 2a. Based on the preliminary experiment, Li-Ni-Mn-O components were deposited onto the conventionally tape-casted electrode, which consisted of an active material (LiMn2O4 with spinel structure), a conductive material (carbon black), and a binder (PVdF) under the sputtering conditions mentioned in the previous section. FE-EPMA analysis in Fig. 2b and c shows the top view and cross-sectional images of the bare and LNMO-modified LiMn2O4 electrodes. As both the LNMO coating layer and bare LiMn2O4 electrode contain manganese and oxygen ions, noticeable differences are not seen in Mn and O mapping images. Nickel mapping for the LNMO-deposited electrode (shown in the top view), however, clearly indicates that a LNMO layer was formed on tape-casted electrode, which may act as a protective or functional layer for the electrode materials. Moreover, the cross-sectional view of the LNMO-modified electrode reveals that the LNMO is deposited or coated uniformly in a certain extent through the porous cathode electrodes by the RF sputtering process. Fig. 3a shows typical galvanostatic charge/discharge profiles of the bare and LNMO-modified LiMn2O4 electrodes for the first cycle between 3.5 and 4.3 V (vs. Li/Li+) at 0.5 C. As presented in Fig. 3(a), both electrodes exhibit similar charge/discharge capacity values and plateaus on the curves near 4.0 V, which correspond to lithium-ion extraction/insertion from/into the tetrahedral sites of
Fig. 1. Schematic diagram of the modification of LiMn2O4 electrode with LiNi0.5Mn1.5O4 target using the RF sputtering system.
22
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
Fig. 2. SEM images and EPMA mapping of the (a) SEM cross-sectional view for the bare and LNMO-modified electrodes, (b) EPMA top view and (c) EPMA cross-sectional view for the bare and LNMO-modified electrodes.
the spinel structure [4,16]. The two distinct potential plateaus around 4.0 V are characteristic of a well-defined spinel structure with high crystallinity and reflect the typical redox processes of LiMn2O4 cathode, which result from the phase transitions concerning lithium ordering during charge/discharge processes. The differential capacity plots as a function of voltage in Fig. 3(b) also certify the well-separated two sets of peaks, corresponding to the plateaus mentioned above. Additionally, the electrochemical evaluation shown in Fig. 3 reveals that there is no additional redox reaction associated with the LNMO modification to the LiMn2O4 electrodes. The cycle performance of the bare and LNMO-modified spinel Li2MnO4 electrodes was evaluated between 3.5 and 4.3 V (vs. Li/Li+) with a current rate of 0.2 C, as demonstrated in Fig. 4. The discharge capacities of the bare and LNMO-deposited electrodes, are 128.6 and 126.2 mA h g1, respectively, in the first cycle, and then gradually decrease in capacity during the cycle test, suggesting that the modification of electrodes does not influence the cyclic performance at low current rates in a room temperature environment. Interestingly, the electrochemical performance in rate capability reveals that the modification of LiMn2O4 electrode with the LNMO material improves the capacity at higher C-rates, as presented in Fig. 5. The data was collected at room temperature between 3.5 and 4.3 V by charging at the same rate of 0.1 C but with different discharge rates from 0.1 C to 4 C. The LNMOmodified electrode retains approximately 125 mA h g1 up to 1 C, whereas the bare electrode delivers 113 mA h g1 at same C-rate. Moreover, at higher C-rates such as 2 and 4 C, the surface-modified electrode provides the better rate capability, exhibiting a capacity value of 117 mA h g1 as compared to the capacity of the bare electrode, which drops to 100 mA h g1 during evaluation at 4 C.
In an effort to better understand the enhancement in rate performance, the GITT is utilized to obtain numerical values concerning Li-ion diffusivity [12,16–19]. Fig. 6a shows a typical current–time step application and resulting voltage–time profiles corresponding to each current step, as well as the DLi values calculated from the profiles.DLi was obtained by the equation:
DLi ¼
2 mB V M DES ps MB S DEt 4
ð1Þ
where s expresses the constant current pulse time, mB is the mass, VM is the molar volume, and MB is the molar mass of the insertion electrode material. Also, S is the area of the electrode–electrolyte interface, DES means change of the steady-state voltage during a single-step GITT experiment, and DEt is the total change of cell voltage during a constant current pulse [16]. While both electrodes show a similar trend in the range up to x = 0.4 for lithium insertion during the discharge process, the LNMO-modified electrode exhibits an increase in DLi in the range of x = 0.4–1.0. Although the DLi values are influenced by various factors such as geometry, surface area, and compositional variation of electrodes, the lithium-containing transition metal oxide layer possibly provides kinetic enhancement in DLi as well as electrical conductivity, resulting in better rate performance of the spinel cathodes. Spinel lithium manganese oxide, in general, exhibits inferior electrochemical performance especially in high temperature environments, as outlined in the introduction. To confirm the effect of the LNMO modification onto and into the spinel LiMn2O4 electrodes, the storage characteristics of the bare and LNMO-modified electrodes were evaluated in a fully charged state at 60 °C for
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
23
Fig. 5. Rate capability of the bare and LNMO-modified LiMn2O4 electrode at various C-rate.
Fig. 3. (a) The charge–discharge voltage profiles and corresponding and (b) differential capacity plots as a function of voltages at first cycle for the bare and LNMO-modified LiMn2O4 electrodes.
Fig. 4. Cycling performances of the bare and LNMO-modified LiMn2O4 electrodes at a charge rate of 0.5 C.
Fig. 6. (a) GITT voltage profiles and (b) diffusion coefficients as a function of x in the Li/LixMn2O4 of the bare and LNMO-modified LiMn2O4 electrodes.
3 weeks. Although the open-circuit voltage (OCV), which is shown in Fig. 7a, exhibited little difference as demonstrated by the flat voltage profile of the spinel cathodes, the retained capacities evaluated at room temperature following the storage test for 3 weeks revealed that the bare LiMn2O4 electrode retained a capacity of approximately 45 mA h g1, while the LNMO-modified electrode retained approximately 55 mA h g1, as shown in Fig. 7b. The improvement in the retained capacity caused by the surface coating indicates that the LNMO layer was effective for protection of
the electrode and suppression of the self-discharge reaction resulting from the side reaction between the electrode and the electrolyte at high temperatures in the highly oxidizing environment. The side reaction and subsequent self-discharge reaction may influence the electrode characteristics, owing to the formation of a solid electrolyte interface (SEI) film and cause consequent degradation in electrochemical performance. To evaluate this, EIS was performed before and after the storage test, and the data was fitted with an equivalent circuit, as shown in the Nyquist plots of Fig 8.
24
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
Fig. 7. Storage characteristics at elevated temperatures: (a) open circuit voltage during the storage and (b) discharge capacity after storage for 3 weeks at 60 °C.
To separate and evaluate each resistance, the Z-Plot software suite was utilized within the measured frequency region. The semicircles in the high frequency region correspond to the SEI film resistance and charge transfer resistance at electrode/electrolyte interface, while the slope in low frequency region, which is referred to as the Warburg impedance, is related to solid-state diffusion of lithium-ions. Before the storage test, both electrodes exhibit similar values in the SEI film and charge transfer resistances, while the LNMO-modified electrode shows a slightly larger semicircle than the bare one. Interestingly, the Nyquist plots after the storage test indicate that the LNMO surface modification leads to suppression of the polarization impedance increase during the high-temperature storage, exhibiting smaller SEI film and charge transfer resistances compared to the bare electrode. The reduced polarization increase under conditions such as a fully charged state or elevated temperature is possibly due to the protective LNMO surface layers, which help suppress the degradation of the electrode resulting from the self-discharge reaction and side reaction between the electrode and electrolyte. Fig. 9 compares the DSC curves of the bare and LNMO-modified LiMn2O4 electrodes in a fully charged state. The exothermic reaction of bare and LNMO-modified LiMn2O4 electrodes commenced minutely at 168.48 and 170.75 °C, respectively. After that, exothermic reactions gradually occur around 250 °C. The total heat generated during DSC measurement for the bare and LNMO-modified LiMn2O4 electrodes, respectively, correspond to 919.3 J g1 and 815.7 J g1. Interestingly, additional shoulder peak around 295 °C is found for the LNMO-modified LIMn2O4, suggesting that the LNMO coating layer is related the exothermic reaction with electrolyte. Although both electrodes exhibit similar onset temperatures for the exothermic reaction with the electrolyte,
Fig. 8. Nyquist plots at fully discharged state for the bare and LNMO-modified LiMn2O4 electrodes (a) before and (b) after the storage test at 60 C.
Fig. 9. DSC profiles of the bare and LNMO-modified LiMn2O4 electrodes.
the LNMO-modified electrode exhibits a reduced exothermic peak intensity at approximately of 300 °C and less total heat flow compared to the bare electrode, as demonstrated in Fig. 9. Based on this, the surface modification of the electrode provides more stable surface characteristics and protects from electrolyte instability at high temperatures, resulting in an enhancement in thermal stability for lithium-ion battery application. 4. Conclusions A tape-casted LiMn2O4 electrode was treated with lithium– nickel–manganese-oxide by a sputtering technique, and the
Y. Chae et al. / Journal of Electroanalytical Chemistry 730 (2014) 20–25
presence of a surface coating was confirmed by FE-SEM and EPMA. The surface modification of the electrodes contributed to electrochemical enhancement of the rate characteristics at room temperature. The storage test conducted at 60 °C in a fully charged state revealed that the surface-coated electrode had better retention of charge capacity, while the bare electrode experienced a severe self-discharge reaction resulting from the side reaction between electrolyte and cathode. Further investigation using differential scanning calorimetry also certified that the surface coating was effective for protecting the electrode from the side reaction with electrolyte, thus providing thermal stability in the cathode of the lithium-ion cell. Conflict of interest There is no conflict of interest among authors for the publication of this article. Acknowledgements This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-2010-0028958), and the KIST Institutional Program.
25
References [1] C.M. Julien, A. Mauger, K. Zaghib, H. Groult, Inorganics 2 (1) (2014) 132–154. [2] H.S. Kim, J. Proell, R. Koheler, W. Pfleging, A. Pique, J. Laser Micro Nano Eng. 7 (2012) 320–325. [3] D.H. Jang, Y.J. Shin, S.M. Oh, J. Electrochem. Soc. 143 (1996) 2204–2211. [4] E. Hosono, T. Kudo, I. Honnma, H. Matsuda, H. Zhou, Nano Lett. 9 (2009) 1045– 1051. [5] M. Satioh, S. Yoshida, H. Yamane, M. Sano, M. Fujita, K. Kifune, Y. Kubota, J. Power Sources 122 (2003) 162–168. [6] J. Vetter, P. Novak, M.R. Wagner, C. Veit, K.-C. Moller, J.O. Bbesenhard, M. Winter, M. Wohlfahrt-Mehrens, C. Vogler, A. Hammouche, J. Power Sources 147 (2005) 269–281. [7] Y. Xia, Y. Zhou, M. Yoshio, J. Electrochem. Soc. 144 (1997) 2593–2600. [8] M.M. Thackeray, Y. Shao-Horn, A.J. Kahaian, K.D. Kepler, E. Skinner, J.T. Vaughey, S.A. Hackney, Electrochem. Solid-State Lett. 1 (1998) 7–9. [9] W. Choi, A. Manthiram, J. Electrochem. Soc. 153 (2006) A1760–A1764. [10] M. Wohlfahrt-Mehrens, A. Butz, R. Oesten, G. Arnold, R.P. Hemmer, R.A. Huggins, J. Power Sources 68 (1997) 582–585. [11] H.J. Bang, V.X. Donepudi, J. Prakash, Electrochim. Acta 48 (2002) 443–451. [12] C. Li, H.P. Zhang, L.J. Fu, H. Liu, Y.P. Wu, E. Rahm, R. Holze, H.Q. Wu, E. Rahm, R. Holz, H.Q. Wu, Electrochim. Acta 51 (2006) 3872–3883. [13] D. Zhang, B.N. Popov, R.E. White, J. Power Sources 76 (1998) 81–90. [14] J. Cho, T.-J. Kim, Y.J. Kim, Chem. Commun. (2001) 1074–1075. [15] Li. Wang, J. Zhao, S. Guo, X. He, C. Hiang, C. Wan, Investigation of SnO2modified LiMn2O4 composite as cathode material for lithium-ion batteries, Int. J. Electrochem. Sci. 5 (2010) 1113–1126. [16] J.S. Gnanaraj, V.G. Pol, A. Gedanken, D. Aurbach, Electrochem. Commun. 5 (2003) 940–945. [17] Y. Zhu, C. Wang, J. Phys. Chem. C 114 (2010) 2830–2841. [18] E. Deiss, Electrochim. Acta 50 (2005) 2927–2932. [19] M.Y. Saidi, J. Barker, R. Koksbang, J. Solid State Chem. 122 (1996) 195–199.