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The effect of LBO coating method on electrochemical performance of LiMn2O4 cathode material
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Solid State Ionics 178 (2008) 1837 – 1842 www.elsevier.com/locate/ssi
The effect of LBO coating method on electrochemical performance of LiMn2O4 cathode material Halil Şahan, Hüseyin Göktepe, Şaban Patat ⁎, Ahmet Ülgen Department of Chemistry, Faculty of Art and Science, Erciyes University, 38039 Kayseri, Turkey Received 23 May 2007; received in revised form 11 November 2007; accepted 23 November 2007
Abstract The effect of the Li2O–2B2O3 (LBO) glass coating on the charge–discharge cycling performance of spinel powder (LiMn2O4) was investigated in the range of 3.5–4.5 V. The LBO glass coating on the surface of the spinel powder was carried out using the solid-state and solution methods. The lithium manganese oxide, coated with LBO glass via the solution method retained 100% of its original capacity after 30 cycles, showing much better cyclability than both bare lithium manganese oxide and lithium manganese oxide coated with LBO via the solid-state method. The structure and morphology of surface-treated and bare LiMn2O4 were characterized by elemental analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). SEM result showed that LBO glass film coated via solution based method, on the surface of LiMn2O4 powder was more homogenous distribution than that coated via the solid-state based method. The electrochemical results indicated that the solution method is a better way than the solid-state method for coating LBO on the surface of LiMn2O4 powder. © 2007 Elsevier B.V. All rights reserved. Keywords: Surface coating; Li-ion battery; Cathode material; LiMn2O4; LBO glass
1. Introduction Cubic spinel LiMn2O4 is of great interest for the replacement of LiCoO2 in Li-ion batteries due to its high voltage, natural abundance, low cost, and environmental benignity [1–3]. However, LiMn2O4 exhibits serious capacity fading during charge and discharge. The reason for capacity fading is attributed to several factors: Jahn–Teller distortion [4–6], spinel dissolution [7,8], and electrolyte oxidation [9]. On the other hand, some researchers insisted that the Jahn–Teller effect is not an important factor for the capacity losses of spinel oxide in 4 V, and the capacity loss is caused by the simple dissolution of Mn3+ [10,11]. Myung et al. [12] reported that the capacity fading is not related to the structural change during the cycling. Much effort has been made by the international lithium battery community to minimize the capacity fading of spinel electrode. Two main approaches have been used to address this ⁎ Corresponding author. Tel.: +90 352 4374901x33162. E-mail address: [email protected] (Ş. Patat). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.11.024
problem: (i) partial substitution of manganese ions by tri, di or mono valent cations like Co, Cr, Al, Mg and Li [13,14], or (ii) coating the spinel particles with a protective layer such as a LiCoO2 [15], Al2O3 [16], or ZnO [17]. Some studies showed the substitution of Mn ions at 16d sites with ions with a valence ≤ + 3 to give doped spinels LixMyMn2 − yO4 (M = Ti, Fe, Ni, Co, Zn, Al, and Mg), which could enhance the cycling stability at room temperature [18,19]. Although this approach has improved the structural stability, it seriously reduces initial capacity, depending on the kind of substituted metals (M) and their content. Recent studies showed that the dissolution of manganese and electrolyte decomposition are suppressed by surface modification of cathode electrode [20–22]. Because the side reactions caused Mn2+ dissolution occur mainly at the surface of cathode powder during the cycling test, surface modification in both cathode and anode electrode could improve the electrochemical property of Li-ion batteries. According to the reports of Amatucci and coworkers [23,24], lithium boron oxide (LBO, Li2O–2B2O3) glasses are particularly
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suitable for surface treatment. There are several reasons. The first of these is that molten LBO compositions exhibit good wetting properties with respect to the ceramics. The combination of good wetting properties and relatively low viscosity in the molten state allow easy processing and result in even coverage with the use of a minimal amount of material. Secondly, LBO compositions have already been investigated as solid lithium ionics conductors which exhibit good ionic conductivity [25,26]. Furthermore, electrochemical studies have shown that these materials are stable against the high oxidation potentials of the 4 V positive electrode materials used in Li-ion batteries today. Chan et al. [27] has reported that spinel lithium manganese oxide cathode materials coated with LBO via solid-state method exhibited relatively good cycling performance but the capacity fade was still 2.63% after 10 cycles at a current rate of 0.1C. This study examines the effect on the electrochemical cycling performance of LiMn2O4 by coating its surface with the Li2O– 2B2O3 via solution method. The treatment is expected to affect the cycleability. The preparation, structure and electrochemical performance of the surface-treated LiMn2O4 cathode material are discussed in comparison with the bare one. This is the first time that LiMn2O4 spinel coated with LBO glass via solution based method has been reported to exhibit no capacity loss after 30 cycles. 2. Experimental LBO glass-coated LiMn2O4 was prepared using a solution method described by Ying et al. [28]. The precursor of LBO glass, LiOH H2O (Acros) and H3BO3(Fluka), were mixed with a molar ratio of 1:2 in methanol. The bare LiMn2O4 powders were then added to the solution, and the mixture was stirred and gently heated to 70–80 °C until the solvent was completely evaporated. The powders were then heated at 500 °C for 10 h. The melting points of H3BO3 and LiOH.H2O are 186 and 445 °C, respectively. Therefore, during heating the liquid H3BO3 and LiOH H2O can wet the surface of LiMn2O4 and H3BO3 reacts with LiOH H2O to produce LBO. After the mixture cooled to room temperature, the solid Li2O–B2O3 glass was coated on the surface of LiMn2O4 powders. For the sake of comparison, LBO-coated LiMn2O4 spinel was also prepared by using a solid-state method described as follows [27]. The mixture of the precursor of LBO glass with LiMn2O4 powders was thoroughly mixed with mortar and pestle, and the mixed powders were then calcined at 500 °C for 10 h. In order to prepare the precursor of LBO glass, LiOH 2H2O and H3BO3 were mixed with a molar ratio of 1:2 in methanol, and then heated and gently stirred at 70–80 °C until the solvent was completely evaporated. The weight ratio of the precursor of LBO glass to the LiMn2O4 powders for both method was 1 wt.%. The LiMn2O4 spinel was prepared with stoichiometric amounts of raw materials, LiNO3 (Riedel-de Haen) and Mn (CH3COO)2 4H2O (Sigma). Firstly the raw materials were dissolved in distilled water. Glycine (Merck) was added in the solution either as a solid or as a water solution. Its role was to serve both as a fuel for combustion and as a complexant to prevent inhomogeneous precipitation of individual components prior to combustion. Finally, nitric acid with the same mole of acetate anions was added to the solution. The molar ratio of glycine to
nitrate was 1:4. The solution was heated continuously without any previous thermal dehydration. Afterwards the solution became transparent viscous gel which auto-ignited automatically, giving a voluminous, black, sponge-like ash product of combustion. The resulting ash was heated at 800 °C for 12 h. The cation composition of the base and surface-treated LiMn2O4 powders was determined by flame atomic absorbtion spectrometer (AAS, Perkin Elmer 3110) and flame photameter (FP, Jenway PFP7) after dissolving the powders in dilute nitric acid. The valence of Mn was determined by chemical titration. The samples were dissolved in an excess of 20 ml Na2C2O4 (Merck) and 2 ml H2SO4 at ~65 °C (maintained by a water bath) to reduce all Mnn+ to Mn2+ (2 b n ≤ 4), and then the excess C2O42− ions in the solution were determined by titration at 65 °C with a standard solution of KMnO4 (Fluka). The phase identification and the evaluation of lattice parameters of the base and surface-treated LiMn2O4 powders were carried out by powder X-ray diffraction (XRD) using copper CuKα radiation (Bruker AXS D8). The diffractometer was equipped with a diffracted beam graphite monochromator. The diffraction data were collected at 40 kVand 40 mA over a 2θ range from 10° to 70° with a step size of 0.02° and a count time of 10 s per step. The DiffracPlus and Win-Index programs were used to obtain the lattice parameters of the powders. The particle morphology of the powders was examined by means of scanning electron microscopy (LEO 440), operated at 20 kV. The electrochemical studies were carried out in two-electrode teflon cells. The cells were fabricated by using the bare and the surface-treated LiMn2O4 as a cathode and lithium foil as anode. A glass fiber separator soaked in electrolyte separated the two electrodes. The electrolyte consisted of 1 M solution of LiPF6 dissolved in an ethylene carbonate (Aldrich)/diethyl carbonate (Merck) (EC/DEC, 1:1 ratio by volume). For the preparation of the cathode composite, a slurry mixed with 86 wt. % of cathode active material, 9 wt.% of acetylene black conductor (Alfa Aesar) and 5 wt. % of polyvinylidene fluoride (PVDF, Fluka) binder in 1-methyl-2-pyrolidinone (NMP, Merck) was pasted on aluminium foil current collector with a diameter of 13 mm, followed by vacuum drying at 120 °C for overnight in a vacuum oven and uniaxial pressing between two flat plates at 2 t for 5 min. The electrode loading was about 4 mg of cathode active material. Diethyl carbonate, ethylene carbonate, and acetylene black were used after being purified according to the methods given in the literature [29]. Diethyl carbonate: 100 ml DEC was washed with an aqueous 10% Na2CO3 (20 ml) solution, saturated CaCl2 (20 ml), then water(30 ml). After drying by standing over solid CaCl2 for 1 h (note that prolonged contact should be avoided because slow combination with CaCl2 occurs), it should be fractionally distillated. Ethylene carbonate: It was dried over P2O5 then fractionally distillated at 10 mm Hg pressure and crystalized from dry ethyl ether respectively. Acetylene black: It was leached for 24 h with 1:1 HCl to remove oil contamination, then washed repeatedly with distillated water. Dried in air, and eluted for 1 day each with benzene and acetone. Again dried in air at room temperature, then heated in a vacuum for 24 h at 600 °C to remove adsorbed gases.
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Charge–discharge tests were performed galvanostatically at a current rate of 1C with cut-off voltages of 3.5–4.5 V (versus Li/ Li+) at room temperature in a multi-channel battery tester. Cyclic voltammetry data of the bare and LBO-coated LiMn2O4 spinels were collected between 4.5–3.4 V at a sweep rate of 0.02 mV/s. The experiments were carried out in a three-electrode cell. Metallic lithium was used as a counter- and reference electrode. All processes of assembling and dismantling the cells were carried out in an argon-filled dry glove box. In order to verify the effect of LBO coating layer upon decreasing the dissolution of spinel LiMn2O4, the Mn dissolution experiment was also carried out in this study according to the method given in literature [30,31]. About 10 mg each of uncoated and the LBO-coated spinel materials was immersed in 10 ml 1 M LiPF6–EC/DEC (1:1 by volume) electrolyte for 3 days. Then, after separating the spinel powder via filtering through filter paper, the dissolved Mn+2 in the electrolyte was extracted into water phase using 10 ml 0.1 M diluted HCl acid and the Mn contents were quantitatively determined by atomic absorption spectroscopy (AAS). 3. Results and discussion The chemical analysis of the bare and surface-treated lithium manganese oxide indicated that the stoichiometry of the elements was very close to the targeted formula. The XRD patterns of the base and surface-treated LiMn2O4 powders are presented in Fig. 1. The XRD pattern of the base LiMn2O4 powders shows that the material is pure spinel phase
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with the crystal lattice parameter of a = 8.2388 Å. It can be seen from Fig. 1, that the spectrum of the surface-treated LiMn2O4 powders is almost the same as that of the base one. The absence of any other peaks in the patterns indicates that amorphous LBO glass coated the surface of the base LiMn2O4 and did not penetrate the spinel matrix. Fig. 2(a) to (c) shows the SEM micrographs of the base and surface-treated LiMn2O4 powders. The scanning electron microscope investigation reveals that the particles of LiMn2O4 are distributed uniformly before surface treatment. But as seen from Fig. 2b and (c), the surface of LiMn2O4 powders possess agglomerated particles, after surface treatment. It can also be observed from Fig. 2(b) and (c) that the LBO coating via solution method has homogenous distribution on the surface of LiMn2O4, but LBO glass coating via solid-state method has heterogeneous distribution on the surface of LiMn2O4. From the SEM morphology of the LBO-coated and the fact that no other phases appeared in the XRD pattern, it is concluded that the LBO glass layer was amorphous rather than crystalline [27,28]. Fig. 3 shows the continuous charge and discharge profiles of the bare and surface-treated LiMn2O4 by applying current of 148 mA g−1 (1C-rate) for the potential range 3.5–4.5 V (vs Li/ Li+) at room temperature. It can obviously be seen that charge/ discharge curves of all the samples had two voltage plateaus at approximately 4.1 and 4.0 V, originating from the Mn3+/4+ redox couple, which indicated a remarkable characteristic of well defined LiMn2O4 spinel. Two voltage plateaus indicate that the insertion and extraction of lithium ions occur in two stages [32]. The first plateau at 4.0 V is associated with the single phase
Fig. 1. X-ray diffraction patterns of (a) the bare LiMn2O4, (b) LBO-coated LiMn2O4 via solid-state method and (c)LBO-coated LiMn2O4 via solution method.
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Fig. 2. SEM micrograph of (a) the bare LiMn2O4, (b) LBO-coated LiMn2O4 via solid-state method and (c)LBO-coated LiMn2O4 via solution method.
reversible reaction of LiMn2O4→Li0.5Mn2O4 + 0.5Li+, while the second one at 4.1 V is attributed to the two phase reaction of Li0.5Mn2O4→Mn2O4 + 0.5Li+. As can be seen in Fig. 3, the two potential plateaus for all the samples were maintained after 30th cycling. The cycling profiles of LBO-coated LiMn2O4 via solution based method given in Fig. 3(c) are very interesting since there is no capacity loss over 30 cycles between 3.5 and 4.5 V. The lithium ion from counter electrode, the lithium foil, can leave from and insert again to the spinel reversibly without any evidence of structure damage of the material. The electrochemical cycle performance of the bare and LBOcoated LiMn2O4 are shown in Fig. 4. The discharge capacity after first and 30 cycles are evaluated and presented in Table 1. Also listed in Table 1 is the calculated capacity fading rate after 30 cycles for the base cathode as well as the LBO-coated LiMn2O4.
As seen in Table 1 and Fig. 4, after 30 cycles between 3.5 and 4.5 V at 1C charge–dicharge rate, the discharge capacity of the bare LiMn2O4 faded from 115.4 to 92.8 mAh g− 1 with the retention of 80.4% of its initial capacity. However, under the same conditions, the discharge capacity of the LBO-coated spinel electrodes clearly show an improved cycling behavior compared with the bare one. The initial discharge capacities (112 mAh g− 1) of LBO-coated spinel materials using different coating method are almost identical, but the capacity retention among the LBOcoated electrodes is significantly different. The capacity retention of LBO-coated LiMn2O4 via solid-state method is 7.5% after 30 cycles which is consistent with the results found by Chan et al. [27] LBO-coated LiMn2O4 electrode via solution method has an excellent cycling behavior without any capacity loss even after 30 cycles at room temperature and a 1C-rate. The result may indicate that the solution based coating method is a better alternative to the
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Fig. 4. Cyclability of (a) the bare LiMn2O4, (b) LBO-coated LiMn2O4 via solidstate method and (c) LBO-coated LiMn2O4 via solution method. The applied current density is 148 mA g− 1 (1C-rate) at room temperature. Li metal was used as the anode.
Fig. 3. Continuous charge discharge curves during 30 cycles: (a) the bare LiMn2O4, (b) LBO-coated LiMn2O4 via solid-state method and (c) LBO-coated LiMn2O4 via solution method. The applied current density is 148 mA g− 1 (1C-rate) at room temperature. Li metal was used as the anode.
solid-state coating method for coating lithium ion battery cathode materials with LBO. Cyclic voltammogram (CVs) for the bare LiMn2O4 showed a distinct shift and broadening of the oxidation and reduction peaks on cycling, indicative of a change in the surface structure and composition of the spinel electrode. These changes are likely associated with the rapid capacity loss that is observed with stoichiometric LiMn2O4 electrodes. By contrast, the CVs of LBO-coated LiMn2O4 showed significantly less peak shift and broadening, but that of solution based LBO-coated LiMn2O4 showed smaller peak shift and broadening than solid-state based LBO-coated LiMn2O4, consistent with our conclusion that the LBO coating plays the important role of protecting the electrode surface from chemical attack by acidic electrolyte species, such as HF, thereby maintaining the structural and chemical character of the electrode surface and that the solution method gives more homogeneous coating than the solid-state method.
AAS results for the Mn dissolution of the bare and LBOcoated LiMn2O4 spinel cathode materials are given in Table 2. As shown in this table, the dissolved Mn content is the lowest for the solution based LBO-coated spinel and the highest for the bare LiMn2O4. As a result it is demonstrated that both solution and solid-state based LBO coatings could effectively reduce the dissolution of LiMn2O4 spinel into the electrolyte, but the solution based LBO coating is more effective than the solidstate based coating in reduction of the dissolution of LiMn2O4 into the electrolyte. Chan et al. [31] used almost identical procedures to prepare the electrodes and cycle the cells, they found substantial capacity fade in the LBO-coated cells after 25 cycles. In this work, almost no capacity fade for solution based LBO-coated LiMn2O4 was observed after 30 cycles at 1C. The reason for this difference in capacity fade may be that Chan et al. used lithium manganese oxide different, from this work, in stoichiometry and preparation method. Another reason may be the impurity levels of EC, DEC or dimethyl carbonate (DMC) and acetylene black, and composition and preparation method of cathode, effecting capacity retention. Chan et al. [31] did not defined impurity levels of EC and DMC and composition and preparation method of cathode. As Xia et al. [33] reported, capacity loss caused by dissolution of manganese accounted 23% of overall capacity loss at room temperature. Jang et al. [34] confirmed that HF generated during cycling when using LiPF6 based electrolyte was responsible for the dissolution of manganese. In fact, preparation of H2O-free electrolyte containing LiPF6 in organic solvent Table 1 Discharge capacity and percentage of capacity fading in 3.5–4.5 V cycling window for the bare and surface-treated LiMn2O4 Sample
Initial discharge Discharge capacity Capacity fading capacity after 30 cycles after 30 cycles (mAh g− 1)
115.4 Base LiMn2O4 LBO-coated LiMn2O4 111.8 (solid-state method) LBO-coated LiMn2O4 111.3 (solution method)
(mAh g− 1)
(%)
92.8 103.4
19.6 7.5
111.3
0.0
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Table 2 AAS results of base and LBO-coated LiMn2O4 materials immersed in LiPF6 with a 1:1 (by volume) mixture of EC/DEC Base Solid-state based Solution based LBOLiMn2O4 LBO-coated LiMn2O4 coated LiMn2O4 The amount of Mn 12.0 dissolution (ppm)
2.6
Acknowledgements This study was financially supported by the Research Foundation of Erciyes University (Kayseri, Turkey). The authors thank Mr. I.Akşit for the SEM observation.
0.8
is difficult. The small amount of water (though the amount is less than 20 ppm) facilitates decomposition of electrolytic salt, LiPF6. Thus, HF is formed as a by-product by the following reaction [35]. LiPF6 þ H2 O→LiF þ POF3 þ 2HF The generated acid, HF continuously attacks the active material and active material decomposes as the cycle goes by, causing capacity fading 2LiMn2 O4 þ 4Hþ →3k MnO2þ Mn2þ þ 2Liþ þ 2H2 O Therefore, it is possible that Mn dissolution could be reduced by decreasing HF contents at the cathode surface. The electrochemical data obtained in this study demonstrated that the cycling performance of LiMn2O4 spinel electrode is significantly improved by surface coating of LBO glass. This is attributed to the surface modifications accustomed in the glassy film offering high diffusion rates and sintering of the particulate surfaces, thereby decreasing the interface area between the cathode and the electrolyte as well as the HF content at the cathode surface. LBO glass coating prevents the direct contact between the spinel and the electrolyte and therefore reduce the dissolution of manganese and the oxidation of electrolyte. Coating the spinel LiMn2O4 with LBO glass layer via solution based method resulted in a tremendous improvement in retention of capacity at room temperature. 4. Conclusions The effect of coating method on electrochemical performance was investigated. The discharge capacity of the LBO-coated spinel electrodes shows an improved cycling behavior compared with the bare one. The improved performance of the surfacetreated sample is ascribed to LBO coating on the surface of LiMn2O4 material, which prevents the direct contact between LiMn2O4 particles and electrolyte, and therefore reduces the dissolution of manganese and the oxidation of electrolyte. The initial discharge capacities of LBO-coated spinel materials using different coating method are almost identical, but the capacity retention among the LBO-coated electrodes is significantly different. The capacity retention of LBO-coated LiMn2O4 via solid-state method is 7.5% after 30 cycles. LBO-coated LiMn2O4 electrode via solution method has an excellent cycling behavior without any capacity loss even after 30 cycles at room temperature and a 1C-rate. Therefore, to improve the electrochemical performance of LiMn2O4 cathode material, the use of solution method for LBO coating is an effective way. Also, capacity fading is not related with the structural change in our study.
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Solid State Ionics 178 (2008) 1837 – 1842 www.elsevier.com/locate/ssi
The effect of LBO coating method on electrochemical performance of LiMn2O4 cathode material Halil Şahan, Hüseyin Göktepe, Şaban Patat ⁎, Ahmet Ülgen Department of Chemistry, Faculty of Art and Science, Erciyes University, 38039 Kayseri, Turkey Received 23 May 2007; received in revised form 11 November 2007; accepted 23 November 2007
Abstract The effect of the Li2O–2B2O3 (LBO) glass coating on the charge–discharge cycling performance of spinel powder (LiMn2O4) was investigated in the range of 3.5–4.5 V. The LBO glass coating on the surface of the spinel powder was carried out using the solid-state and solution methods. The lithium manganese oxide, coated with LBO glass via the solution method retained 100% of its original capacity after 30 cycles, showing much better cyclability than both bare lithium manganese oxide and lithium manganese oxide coated with LBO via the solid-state method. The structure and morphology of surface-treated and bare LiMn2O4 were characterized by elemental analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). SEM result showed that LBO glass film coated via solution based method, on the surface of LiMn2O4 powder was more homogenous distribution than that coated via the solid-state based method. The electrochemical results indicated that the solution method is a better way than the solid-state method for coating LBO on the surface of LiMn2O4 powder. © 2007 Elsevier B.V. All rights reserved. Keywords: Surface coating; Li-ion battery; Cathode material; LiMn2O4; LBO glass
1. Introduction Cubic spinel LiMn2O4 is of great interest for the replacement of LiCoO2 in Li-ion batteries due to its high voltage, natural abundance, low cost, and environmental benignity [1–3]. However, LiMn2O4 exhibits serious capacity fading during charge and discharge. The reason for capacity fading is attributed to several factors: Jahn–Teller distortion [4–6], spinel dissolution [7,8], and electrolyte oxidation [9]. On the other hand, some researchers insisted that the Jahn–Teller effect is not an important factor for the capacity losses of spinel oxide in 4 V, and the capacity loss is caused by the simple dissolution of Mn3+ [10,11]. Myung et al. [12] reported that the capacity fading is not related to the structural change during the cycling. Much effort has been made by the international lithium battery community to minimize the capacity fading of spinel electrode. Two main approaches have been used to address this ⁎ Corresponding author. Tel.: +90 352 4374901x33162. E-mail address: [email protected] (Ş. Patat). 0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.11.024
problem: (i) partial substitution of manganese ions by tri, di or mono valent cations like Co, Cr, Al, Mg and Li [13,14], or (ii) coating the spinel particles with a protective layer such as a LiCoO2 [15], Al2O3 [16], or ZnO [17]. Some studies showed the substitution of Mn ions at 16d sites with ions with a valence ≤ + 3 to give doped spinels LixMyMn2 − yO4 (M = Ti, Fe, Ni, Co, Zn, Al, and Mg), which could enhance the cycling stability at room temperature [18,19]. Although this approach has improved the structural stability, it seriously reduces initial capacity, depending on the kind of substituted metals (M) and their content. Recent studies showed that the dissolution of manganese and electrolyte decomposition are suppressed by surface modification of cathode electrode [20–22]. Because the side reactions caused Mn2+ dissolution occur mainly at the surface of cathode powder during the cycling test, surface modification in both cathode and anode electrode could improve the electrochemical property of Li-ion batteries. According to the reports of Amatucci and coworkers [23,24], lithium boron oxide (LBO, Li2O–2B2O3) glasses are particularly
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suitable for surface treatment. There are several reasons. The first of these is that molten LBO compositions exhibit good wetting properties with respect to the ceramics. The combination of good wetting properties and relatively low viscosity in the molten state allow easy processing and result in even coverage with the use of a minimal amount of material. Secondly, LBO compositions have already been investigated as solid lithium ionics conductors which exhibit good ionic conductivity [25,26]. Furthermore, electrochemical studies have shown that these materials are stable against the high oxidation potentials of the 4 V positive electrode materials used in Li-ion batteries today. Chan et al. [27] has reported that spinel lithium manganese oxide cathode materials coated with LBO via solid-state method exhibited relatively good cycling performance but the capacity fade was still 2.63% after 10 cycles at a current rate of 0.1C. This study examines the effect on the electrochemical cycling performance of LiMn2O4 by coating its surface with the Li2O– 2B2O3 via solution method. The treatment is expected to affect the cycleability. The preparation, structure and electrochemical performance of the surface-treated LiMn2O4 cathode material are discussed in comparison with the bare one. This is the first time that LiMn2O4 spinel coated with LBO glass via solution based method has been reported to exhibit no capacity loss after 30 cycles. 2. Experimental LBO glass-coated LiMn2O4 was prepared using a solution method described by Ying et al. [28]. The precursor of LBO glass, LiOH H2O (Acros) and H3BO3(Fluka), were mixed with a molar ratio of 1:2 in methanol. The bare LiMn2O4 powders were then added to the solution, and the mixture was stirred and gently heated to 70–80 °C until the solvent was completely evaporated. The powders were then heated at 500 °C for 10 h. The melting points of H3BO3 and LiOH.H2O are 186 and 445 °C, respectively. Therefore, during heating the liquid H3BO3 and LiOH H2O can wet the surface of LiMn2O4 and H3BO3 reacts with LiOH H2O to produce LBO. After the mixture cooled to room temperature, the solid Li2O–B2O3 glass was coated on the surface of LiMn2O4 powders. For the sake of comparison, LBO-coated LiMn2O4 spinel was also prepared by using a solid-state method described as follows [27]. The mixture of the precursor of LBO glass with LiMn2O4 powders was thoroughly mixed with mortar and pestle, and the mixed powders were then calcined at 500 °C for 10 h. In order to prepare the precursor of LBO glass, LiOH 2H2O and H3BO3 were mixed with a molar ratio of 1:2 in methanol, and then heated and gently stirred at 70–80 °C until the solvent was completely evaporated. The weight ratio of the precursor of LBO glass to the LiMn2O4 powders for both method was 1 wt.%. The LiMn2O4 spinel was prepared with stoichiometric amounts of raw materials, LiNO3 (Riedel-de Haen) and Mn (CH3COO)2 4H2O (Sigma). Firstly the raw materials were dissolved in distilled water. Glycine (Merck) was added in the solution either as a solid or as a water solution. Its role was to serve both as a fuel for combustion and as a complexant to prevent inhomogeneous precipitation of individual components prior to combustion. Finally, nitric acid with the same mole of acetate anions was added to the solution. The molar ratio of glycine to
nitrate was 1:4. The solution was heated continuously without any previous thermal dehydration. Afterwards the solution became transparent viscous gel which auto-ignited automatically, giving a voluminous, black, sponge-like ash product of combustion. The resulting ash was heated at 800 °C for 12 h. The cation composition of the base and surface-treated LiMn2O4 powders was determined by flame atomic absorbtion spectrometer (AAS, Perkin Elmer 3110) and flame photameter (FP, Jenway PFP7) after dissolving the powders in dilute nitric acid. The valence of Mn was determined by chemical titration. The samples were dissolved in an excess of 20 ml Na2C2O4 (Merck) and 2 ml H2SO4 at ~65 °C (maintained by a water bath) to reduce all Mnn+ to Mn2+ (2 b n ≤ 4), and then the excess C2O42− ions in the solution were determined by titration at 65 °C with a standard solution of KMnO4 (Fluka). The phase identification and the evaluation of lattice parameters of the base and surface-treated LiMn2O4 powders were carried out by powder X-ray diffraction (XRD) using copper CuKα radiation (Bruker AXS D8). The diffractometer was equipped with a diffracted beam graphite monochromator. The diffraction data were collected at 40 kVand 40 mA over a 2θ range from 10° to 70° with a step size of 0.02° and a count time of 10 s per step. The DiffracPlus and Win-Index programs were used to obtain the lattice parameters of the powders. The particle morphology of the powders was examined by means of scanning electron microscopy (LEO 440), operated at 20 kV. The electrochemical studies were carried out in two-electrode teflon cells. The cells were fabricated by using the bare and the surface-treated LiMn2O4 as a cathode and lithium foil as anode. A glass fiber separator soaked in electrolyte separated the two electrodes. The electrolyte consisted of 1 M solution of LiPF6 dissolved in an ethylene carbonate (Aldrich)/diethyl carbonate (Merck) (EC/DEC, 1:1 ratio by volume). For the preparation of the cathode composite, a slurry mixed with 86 wt. % of cathode active material, 9 wt.% of acetylene black conductor (Alfa Aesar) and 5 wt. % of polyvinylidene fluoride (PVDF, Fluka) binder in 1-methyl-2-pyrolidinone (NMP, Merck) was pasted on aluminium foil current collector with a diameter of 13 mm, followed by vacuum drying at 120 °C for overnight in a vacuum oven and uniaxial pressing between two flat plates at 2 t for 5 min. The electrode loading was about 4 mg of cathode active material. Diethyl carbonate, ethylene carbonate, and acetylene black were used after being purified according to the methods given in the literature [29]. Diethyl carbonate: 100 ml DEC was washed with an aqueous 10% Na2CO3 (20 ml) solution, saturated CaCl2 (20 ml), then water(30 ml). After drying by standing over solid CaCl2 for 1 h (note that prolonged contact should be avoided because slow combination with CaCl2 occurs), it should be fractionally distillated. Ethylene carbonate: It was dried over P2O5 then fractionally distillated at 10 mm Hg pressure and crystalized from dry ethyl ether respectively. Acetylene black: It was leached for 24 h with 1:1 HCl to remove oil contamination, then washed repeatedly with distillated water. Dried in air, and eluted for 1 day each with benzene and acetone. Again dried in air at room temperature, then heated in a vacuum for 24 h at 600 °C to remove adsorbed gases.
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Charge–discharge tests were performed galvanostatically at a current rate of 1C with cut-off voltages of 3.5–4.5 V (versus Li/ Li+) at room temperature in a multi-channel battery tester. Cyclic voltammetry data of the bare and LBO-coated LiMn2O4 spinels were collected between 4.5–3.4 V at a sweep rate of 0.02 mV/s. The experiments were carried out in a three-electrode cell. Metallic lithium was used as a counter- and reference electrode. All processes of assembling and dismantling the cells were carried out in an argon-filled dry glove box. In order to verify the effect of LBO coating layer upon decreasing the dissolution of spinel LiMn2O4, the Mn dissolution experiment was also carried out in this study according to the method given in literature [30,31]. About 10 mg each of uncoated and the LBO-coated spinel materials was immersed in 10 ml 1 M LiPF6–EC/DEC (1:1 by volume) electrolyte for 3 days. Then, after separating the spinel powder via filtering through filter paper, the dissolved Mn+2 in the electrolyte was extracted into water phase using 10 ml 0.1 M diluted HCl acid and the Mn contents were quantitatively determined by atomic absorption spectroscopy (AAS). 3. Results and discussion The chemical analysis of the bare and surface-treated lithium manganese oxide indicated that the stoichiometry of the elements was very close to the targeted formula. The XRD patterns of the base and surface-treated LiMn2O4 powders are presented in Fig. 1. The XRD pattern of the base LiMn2O4 powders shows that the material is pure spinel phase
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with the crystal lattice parameter of a = 8.2388 Å. It can be seen from Fig. 1, that the spectrum of the surface-treated LiMn2O4 powders is almost the same as that of the base one. The absence of any other peaks in the patterns indicates that amorphous LBO glass coated the surface of the base LiMn2O4 and did not penetrate the spinel matrix. Fig. 2(a) to (c) shows the SEM micrographs of the base and surface-treated LiMn2O4 powders. The scanning electron microscope investigation reveals that the particles of LiMn2O4 are distributed uniformly before surface treatment. But as seen from Fig. 2b and (c), the surface of LiMn2O4 powders possess agglomerated particles, after surface treatment. It can also be observed from Fig. 2(b) and (c) that the LBO coating via solution method has homogenous distribution on the surface of LiMn2O4, but LBO glass coating via solid-state method has heterogeneous distribution on the surface of LiMn2O4. From the SEM morphology of the LBO-coated and the fact that no other phases appeared in the XRD pattern, it is concluded that the LBO glass layer was amorphous rather than crystalline [27,28]. Fig. 3 shows the continuous charge and discharge profiles of the bare and surface-treated LiMn2O4 by applying current of 148 mA g−1 (1C-rate) for the potential range 3.5–4.5 V (vs Li/ Li+) at room temperature. It can obviously be seen that charge/ discharge curves of all the samples had two voltage plateaus at approximately 4.1 and 4.0 V, originating from the Mn3+/4+ redox couple, which indicated a remarkable characteristic of well defined LiMn2O4 spinel. Two voltage plateaus indicate that the insertion and extraction of lithium ions occur in two stages [32]. The first plateau at 4.0 V is associated with the single phase
Fig. 1. X-ray diffraction patterns of (a) the bare LiMn2O4, (b) LBO-coated LiMn2O4 via solid-state method and (c)LBO-coated LiMn2O4 via solution method.
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Fig. 2. SEM micrograph of (a) the bare LiMn2O4, (b) LBO-coated LiMn2O4 via solid-state method and (c)LBO-coated LiMn2O4 via solution method.
reversible reaction of LiMn2O4→Li0.5Mn2O4 + 0.5Li+, while the second one at 4.1 V is attributed to the two phase reaction of Li0.5Mn2O4→Mn2O4 + 0.5Li+. As can be seen in Fig. 3, the two potential plateaus for all the samples were maintained after 30th cycling. The cycling profiles of LBO-coated LiMn2O4 via solution based method given in Fig. 3(c) are very interesting since there is no capacity loss over 30 cycles between 3.5 and 4.5 V. The lithium ion from counter electrode, the lithium foil, can leave from and insert again to the spinel reversibly without any evidence of structure damage of the material. The electrochemical cycle performance of the bare and LBOcoated LiMn2O4 are shown in Fig. 4. The discharge capacity after first and 30 cycles are evaluated and presented in Table 1. Also listed in Table 1 is the calculated capacity fading rate after 30 cycles for the base cathode as well as the LBO-coated LiMn2O4.
As seen in Table 1 and Fig. 4, after 30 cycles between 3.5 and 4.5 V at 1C charge–dicharge rate, the discharge capacity of the bare LiMn2O4 faded from 115.4 to 92.8 mAh g− 1 with the retention of 80.4% of its initial capacity. However, under the same conditions, the discharge capacity of the LBO-coated spinel electrodes clearly show an improved cycling behavior compared with the bare one. The initial discharge capacities (112 mAh g− 1) of LBO-coated spinel materials using different coating method are almost identical, but the capacity retention among the LBOcoated electrodes is significantly different. The capacity retention of LBO-coated LiMn2O4 via solid-state method is 7.5% after 30 cycles which is consistent with the results found by Chan et al. [27] LBO-coated LiMn2O4 electrode via solution method has an excellent cycling behavior without any capacity loss even after 30 cycles at room temperature and a 1C-rate. The result may indicate that the solution based coating method is a better alternative to the
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Fig. 4. Cyclability of (a) the bare LiMn2O4, (b) LBO-coated LiMn2O4 via solidstate method and (c) LBO-coated LiMn2O4 via solution method. The applied current density is 148 mA g− 1 (1C-rate) at room temperature. Li metal was used as the anode.
Fig. 3. Continuous charge discharge curves during 30 cycles: (a) the bare LiMn2O4, (b) LBO-coated LiMn2O4 via solid-state method and (c) LBO-coated LiMn2O4 via solution method. The applied current density is 148 mA g− 1 (1C-rate) at room temperature. Li metal was used as the anode.
solid-state coating method for coating lithium ion battery cathode materials with LBO. Cyclic voltammogram (CVs) for the bare LiMn2O4 showed a distinct shift and broadening of the oxidation and reduction peaks on cycling, indicative of a change in the surface structure and composition of the spinel electrode. These changes are likely associated with the rapid capacity loss that is observed with stoichiometric LiMn2O4 electrodes. By contrast, the CVs of LBO-coated LiMn2O4 showed significantly less peak shift and broadening, but that of solution based LBO-coated LiMn2O4 showed smaller peak shift and broadening than solid-state based LBO-coated LiMn2O4, consistent with our conclusion that the LBO coating plays the important role of protecting the electrode surface from chemical attack by acidic electrolyte species, such as HF, thereby maintaining the structural and chemical character of the electrode surface and that the solution method gives more homogeneous coating than the solid-state method.
AAS results for the Mn dissolution of the bare and LBOcoated LiMn2O4 spinel cathode materials are given in Table 2. As shown in this table, the dissolved Mn content is the lowest for the solution based LBO-coated spinel and the highest for the bare LiMn2O4. As a result it is demonstrated that both solution and solid-state based LBO coatings could effectively reduce the dissolution of LiMn2O4 spinel into the electrolyte, but the solution based LBO coating is more effective than the solidstate based coating in reduction of the dissolution of LiMn2O4 into the electrolyte. Chan et al. [31] used almost identical procedures to prepare the electrodes and cycle the cells, they found substantial capacity fade in the LBO-coated cells after 25 cycles. In this work, almost no capacity fade for solution based LBO-coated LiMn2O4 was observed after 30 cycles at 1C. The reason for this difference in capacity fade may be that Chan et al. used lithium manganese oxide different, from this work, in stoichiometry and preparation method. Another reason may be the impurity levels of EC, DEC or dimethyl carbonate (DMC) and acetylene black, and composition and preparation method of cathode, effecting capacity retention. Chan et al. [31] did not defined impurity levels of EC and DMC and composition and preparation method of cathode. As Xia et al. [33] reported, capacity loss caused by dissolution of manganese accounted 23% of overall capacity loss at room temperature. Jang et al. [34] confirmed that HF generated during cycling when using LiPF6 based electrolyte was responsible for the dissolution of manganese. In fact, preparation of H2O-free electrolyte containing LiPF6 in organic solvent Table 1 Discharge capacity and percentage of capacity fading in 3.5–4.5 V cycling window for the bare and surface-treated LiMn2O4 Sample
Initial discharge Discharge capacity Capacity fading capacity after 30 cycles after 30 cycles (mAh g− 1)
115.4 Base LiMn2O4 LBO-coated LiMn2O4 111.8 (solid-state method) LBO-coated LiMn2O4 111.3 (solution method)
(mAh g− 1)
(%)
92.8 103.4
19.6 7.5
111.3
0.0
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Table 2 AAS results of base and LBO-coated LiMn2O4 materials immersed in LiPF6 with a 1:1 (by volume) mixture of EC/DEC Base Solid-state based Solution based LBOLiMn2O4 LBO-coated LiMn2O4 coated LiMn2O4 The amount of Mn 12.0 dissolution (ppm)
2.6
Acknowledgements This study was financially supported by the Research Foundation of Erciyes University (Kayseri, Turkey). The authors thank Mr. I.Akşit for the SEM observation.
0.8
is difficult. The small amount of water (though the amount is less than 20 ppm) facilitates decomposition of electrolytic salt, LiPF6. Thus, HF is formed as a by-product by the following reaction [35]. LiPF6 þ H2 O→LiF þ POF3 þ 2HF The generated acid, HF continuously attacks the active material and active material decomposes as the cycle goes by, causing capacity fading 2LiMn2 O4 þ 4Hþ →3k MnO2þ Mn2þ þ 2Liþ þ 2H2 O Therefore, it is possible that Mn dissolution could be reduced by decreasing HF contents at the cathode surface. The electrochemical data obtained in this study demonstrated that the cycling performance of LiMn2O4 spinel electrode is significantly improved by surface coating of LBO glass. This is attributed to the surface modifications accustomed in the glassy film offering high diffusion rates and sintering of the particulate surfaces, thereby decreasing the interface area between the cathode and the electrolyte as well as the HF content at the cathode surface. LBO glass coating prevents the direct contact between the spinel and the electrolyte and therefore reduce the dissolution of manganese and the oxidation of electrolyte. Coating the spinel LiMn2O4 with LBO glass layer via solution based method resulted in a tremendous improvement in retention of capacity at room temperature. 4. Conclusions The effect of coating method on electrochemical performance was investigated. The discharge capacity of the LBO-coated spinel electrodes shows an improved cycling behavior compared with the bare one. The improved performance of the surfacetreated sample is ascribed to LBO coating on the surface of LiMn2O4 material, which prevents the direct contact between LiMn2O4 particles and electrolyte, and therefore reduces the dissolution of manganese and the oxidation of electrolyte. The initial discharge capacities of LBO-coated spinel materials using different coating method are almost identical, but the capacity retention among the LBO-coated electrodes is significantly different. The capacity retention of LBO-coated LiMn2O4 via solid-state method is 7.5% after 30 cycles. LBO-coated LiMn2O4 electrode via solution method has an excellent cycling behavior without any capacity loss even after 30 cycles at room temperature and a 1C-rate. Therefore, to improve the electrochemical performance of LiMn2O4 cathode material, the use of solution method for LBO coating is an effective way. Also, capacity fading is not related with the structural change in our study.
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