Advanced Powder Technology 25 (2014) 647–653 Contents lists available at ScienceDirect Advanced Powder Technology journal homepage: www.elsevier .com/locate /apt Structural and electrochemical characterization of layered 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2 cathode synthesized by spray drying 0921-8831/$ - see front matter � 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights http://dx.doi.org/10.1016/j.apt.2013.10.008 ⇑ Corresponding author. Tel./fax: +86 731 88836633. E-mail address:
[email protected] (Z. Wang). Zhenjiang He, Zhixing Wang ⇑, Lei Cheng, Zhenguo Zhu, Tao Li, Xinhai Li, Huajun Guo School of Metallurgical Science and Engineering, Central South University, Changsha 410083, PR China a r t i c l e i n f o Article history: Received 22 May 2013 Received in revised form 15 August 2013 Accepted 18 October 2013 Available online 5 November 2013 Keywords: Lithium ion batteries Cathode material Spray drying Elements doping a b s t r a c t Cr doped 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2 (x = 0, 0.02, 0.04, 0.06) as a cathode material for Li-ion battery has been successfully synthesized by spray drying and subsequent calcination. The effects of Cr dopant on the structural and electrochemical properties of this material have been investi- gated by XRD, SEM, EDS, charge–discharge measurements, Ac impedance spectroscopy as well as cyclic voltammetry. These results demonstrated that the element Cr distributed uniformly in these materials. With the Cr content increasing, lattice parameters a and c decrease and less Li ion locates in transition metal site. Among the synthesized Cr-doped materials, when x = 0.04, this material shows the best elec- trochemical properties. Between 2.5 and 4.8 V (vs. Li/Li+), the initial discharge capacities of the materials increased from 143 to 168 mA h g�1 at a constant current density of 250 mA g�1. After 50 cycles, the capacity retention of the materials raised from 83% to 93%. � 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. 1. Introduction With the widespread use of lithium ion battery as the power source for portable electronics, lots of researches have been made to improve its properties. To date, there still have some non- ignorable drawbacks for lithium ion battery further on, such as low specific capacity, cost, safety and cycle life. In recent years, a Li-rich layered solid solution system Li2MnO3–LiMO2 (M = Ni, Co, Mn, etc.) has attracted a number of attentions for its higher specific capacity (�250 mA h g�1) and lower cost comparing to the com- mercial cathode materials. Many researchers believed that these two components (Li2MnO3 and LiMO2) exist in the layered oxide materials are mixed smoothly with composition. On the other hand, some researchers believed that these two components exist as a solid solution [1–4]. In this compound, the Li2MnO3 compo- nent (electrochemically inactive between 2.0 and 4.4 V vs. Li/Li+) stabilizes the structure of the trigonal LiMnO2 phase during Li-extraction [4–6]. However, when charged beyond 4.6 V, Li can be extracted from the Li2MnO3 component accompanying by oxy- gen loss (effective removal of Li2O) to form activated MnO2. There- fore, activation of the Li2MnO3 component can result in a reversible capacity greater than 200 mA h g�1 [7–11]. However, due to the insulating Li2MnO3 component and the thick solid-electrolyte interfacial (SEI) layer, this material displays a low electronic conductivity which results in the low rate capacity [12]. Over the past decade, many efforts have been made to enhance the electrochemical performance of Li2MnO3–LiMO2, such as sur- face modification, element doping, designing core–shell, thermal reduction and electrochemical pre-treatment. Surface modification of the material, using oxides [13–15], phosphates [16,17], metal [18], carbon [19], polymerization [20], mildly acidic, fluorinated solutions [21] and other lithium insertion hosts [6,22], improved the electrochemical performance of Li2MnO3–LiMO2 successfully. In addition, some researchers [23] used H2 to partially reduce the Mn4+ to Mn3+ to enhance rate capacity. Other researchers [24] im- proved the cyclic performance through a per-cycling treatment, which initially tested the cells at 4.5 V for several cycles and then increasing the cut-off voltage form 4.5 to 4.8 step by step. More- over, partial substitution of the transition metal with Cr, Zr, Mg and Mo in the layered oxides can enhance the rate capacity and cycling performance due to structural stabilization and can allow faster lithium ion diffusion in the bulk electrode materials [25–27]. Spray drying technology has been widely employed for prepara- tion of lithium ion materials [28–30]. In this study, layered 0.3Li2- MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2 (x = 0, 0.02, 0.04, 0.06) was successfully synthesized via spray drying and subsequent cal- cination, in which the elements are evenly mixed during the spray drying process. Furthermore, we investigated structural and elec- trochemical properties of Cr-doped cathode materials with various doping amount of Cr. reserved. http://crossmark.crossref.org/dialog/?doi=10.1016/j.apt.2013.10.008&domain=pdf http://dx.doi.org/10.1016/j.apt.2013.10.008 mailto:
[email protected] http://dx.doi.org/10.1016/j.apt.2013.10.008 http://www.sciencedirect.com/science/journal/09218831 http://www.elsevier.com/locate/apt Fig. 1. XRD patterns of 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2 powders. Table 1 Lattice parameters and I(003)/I(104) ratio of the materials in different Cr-doped contents. Samples a(Å) c(Å) c/a I(003)/I(104) x = 0 2.8691(8) 14.2698(4) 4.974 1.71 x = 0.02 2.8650(5) 14.2669(1) 4.980 1.78 x = 0.04 2.8598(2) 14.2615(1) 4.987 1.85 x = 0.06 2.8541(6) 14.2570(2) 4.995 1.97 648 Z. He et al. / Advanced Powder Technology 25 (2014) 647–653 2. Experiment 2.1. Synthesis and characterization of 0.3Li2MnO3�0.7LiMn0.35�x/3 Ni0.5�x/3Co0.15�x/3CrxO2 In this experiment, the samples were prepared by the following process. The desired amount of LiCH3COO�2H2O, Co(CH3COO)2� 4H2O, Mn(CH3COO)2�4H2O, Ni(CH3COO)2�4H2O and (CH3COO)3Cr (produced by Sinopharm Chemical Reagent Co., Ltd.)were dissolved in de-ionized water. The prepared precursor solution was dried by spray drier machine (SD-2500, produced by Shanghai Triowin Lab Technology Company). The solution was added into the spray drier machine by peristaltic pump at the 400 ml h�1. The homogenous solution was atomized at the temperature of 200 �C using a two-fluid nozzle with atomizing pressure of 0.2 MPa. Then the as-prepared powder product was collected by Fig. 2. SEM images of 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3Cr the Spiral dust collector. The precursor was initially decomposed at 450 �C in air for 5 h and then ground after cooling. The decom- posed mixture was calcined at 900 �C in air for 12 h. The powder X-ray diffraction (XRD, Rint-2000, Rigaku) using Cu Ka radiation was employed to identify the crystalline phase of the 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2. The xO2 powders, (a) x = 0, (b) x = 0.02, (c) x = 0.04, and (d) x = 0.06. Table 2 Results of the ICP chemical element analysis of the 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3 Co0.15�x/3CrxO2 samples. Samples Expected Mn/Ni/Co/Cr ratio Experimental Mn/Ni/Co/Cr ratio x = 0 0.545:0.350:0.105:0.000 0.546:0.348:0.103:0.000 x = 0.02 0.540:0.346:0.100:0.014 0.538:0.343:0.099:0.016 x = 0.04 0.536:0.340:0.096:0.028 0.537:0.337:0.093:0.027 x = 0.06 0.531:0.336:0.091:0.042 0.533:0.336:0.091:0.043 Z. He et al. / Advanced Powder Technology 25 (2014) 647–653 649 stoichiometric molar composition of the cathode materials was checked by ICP–OES (IRIS IntrepidII, XSP). The SEM images of the particles were observed with scanning electron microscopy (SEM, Sirion 200) and a scanning transmission electron microscope (EDX-GENESIS 60S) was used for the elemental mapping. Fig. 3. Elemental mapping of the Cr-doped sample 2.2. Electrochemical measurement The electrochemical performance was performed using a two- electrode coin-type cell (CR2025) of LijLiPF6 (EC:EMC:DMC = 1:1:1 in volume) j0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2. The working cathode was composed of 80 wt.% cathode powders, 10 wt.% acetylene black as conducting agent, and 10 wt.% poly (vinylidene fluoride) as binder. After being blended in N-methyl pyrrolidinone, the mixed slurry was spread uniformly on a thin aluminum foil and dried in vacuum for 12 h at 120 �C. A metal lith- ium foil was used as the anode. Electrodes were punched in the form of 14 mm diameter disks. A polypropylene micro-porous film was used as the separator. The assembly of the cells was carried out in an argon-filled glove box. The cells were charged and dis- charged over a voltage range of 2.5–4.8 V vs. Li/Li+ electrode at cur- rent densities of 12.5–250 mA g�1 at room temperature. by scanning transmission electron microscope. Fig. 4. Typicalinitialdischargecurvesof0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3 CrxO2electrodes. 650 Z. He et al. / Advanced Powder Technology 25 (2014) 647–653 3. Results and discussion 3.1. Characterization of 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3 CrxO2 The XRD patterns of post-treated 0.3Li2MnO3�0.7LiMn0.35�x/3 Ni0.5�x/3Co0.15�x/3CrxO2 powders with different x values are given in Fig. 1. As it can be seen from Fig. 1, all the reflections of 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2 samples corre- spond to the layered oxide without any peaks for Cr compounds because of the low quantity of Cr. All the peaks can be indexed based on the R�3m structure and the peaks between 20� and 30� are caused by super-lattice ordering of Li, Mn, Ni, Co and Cr atoms in the layered structure with Li2MnO3-type character [31–34]. However, with increases of Cr content from x = 0 to x = 0.06, the super lattice peaks become weaker and more difficult to detect, which indicates that there is less Li in transition-metal layers, and Mn, Ni, Co and Cr are uniformly mixed on the transition-metal sites [3]. In addition, higher values of I(003)/I(104) (which lists in Table 1) also confirms an ideal structure in which less Li ion would locate in transition metal site [35,36]. The lattice constants, a, c and c/a ratio of 0.3Li2MnO3� 0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2 were presented in Table 1. The lattice parameters of 0.3Li2MnO3�0.7LiMn0.35Ni0.5Co0.15O2 are a = 2.8691 Å and c = 14.2698 Å, which are consistent with other re- ports [37–39]. A certain amount of Cr dopant in the host structure decreases lattice parameters. When x values increase from 0.02 to 0.06, lattice parameters a and c decrease from 2.8650, 14.2669 to 2.8541, 14.2570, respectively. The decrease of lattice parameters in Cr doped samples probably due to the ionic radius of Cr3+ (0.615 Å) being smaller than that of the Ni2+(0.69) ions [40]. There- fore, the unit-cell volume will underwent a diminution, which means more stable structure during the charge/discharge process. In addition, the value of c/a, indicating the degree of ordering in the hexagonal structure, increased along with the increase of Cr con- tent. This appears to indicate that Cr dopant can enhance the layer structure on some level. Fig. 2 shows the SEM images of the 0.3Li2MnO3�0.7LiMn0.35�x/3 Ni0.5�x/3Co0.15�x/3CrxO2 positive powders. It can be seen from Fig. 2 that all calcined samples consisting of a number of primary particles have a size of under 2 lm, and the particle agglomeration becomes serious with the Cr content decrease. The stoichiometric molar composition of the cathode materials was checked by ICP-OES, with the results summarized in Table 2. The experimen- tally measured Mn/Ni/Co/Cr ratios are in good accordance with the expected stoichiometries. Besides that, element mapping using scanning transmission electron microscope is performed to show the conforming uniform distribution of the Cr dopant in the sam- ples (take x = 0.04 as an example). It can be seen from Fig. 3 that the Cr dopant was definitely distributed uniformly in the Cr-doped sample, illustrating a homogeneous phase in the materials. This also can ensure that spray drying is an excellent technology to make elements mixed evenly. Fig. 5. Cycle properties of 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2 electrodes. 3.2. Electrochemical properties of 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3 Co0.15�x/3CrxO2 In order to evaluate effects of Cr dopant on materials, galvanostat- ic cell cycling test of 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3 CrxO2 is conducted within the voltage between 2.5–4.8 V vs. Li/ Li+ electrode. As shown in Fig. 4, when x = 0.04, the layered oxide cathode shows the best rate capacity, with the initial discharge capacities are 243, 232, 206, 184 and 168 mA h g�1 at a constant current density of 12.5, 25, 50, 125 and 250 mA g�1, respectively. Fig. 5 compares the cycling performances of the pristine and Cr-doped materials at various C rates. When x = 0.04, this material demonstrates remarkably improved cycle stability over the others, especially at higher current density. At 250 mA g�1, the Cr-doped material delivers higher discharge capacity than the pristine sam- ple (168 vs. 143 mA h g�1). After 50 cycles, these capacities repre- sent 93% and 83% of the initial discharge capacity of the Cr doped and pristine material, respectively. However, when x rise to 0.06, the discharge capacity and capacity retention dropped to 163 mA h g�1 and 62% after 50 cycles. Those data indicate that appropriate amount of Cr doping can enhance the rate capacity Table 3 The impedance parameters of 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2 electrodes. Samples RS (X) Rct (X) rw (X cm2 s�0.5) D (cm2 s�1) i0 (mA cm�2) x = 0 2.40 106.80 95.18 4.61 � 10�16 2.40 � 10�4 x = 0.02 2.60 87.67 51.01 1.60 � 10�15 2.93 � 10�4 x = 0.04 2.43 88.08 24.09 7.19 � 10�15 2.92 � 10�4 x = 0.06 2.31 105.30 59.54 1.18 � 10�15 2.44 � 10�4 Z. He et al. / Advanced Powder Technology 25 (2014) 647–653 651 and cycling performance, which completely accord with the anal- ysis of lattice parameter and particles morphology (Table 1 and Fig. 2). Smaller unit-cell volume causing by Cr doping enhances structure stability during the lithium ion intercalation and de- intercalation, and slightly agglomerated particles benefit lithium ion diffusion in the bulk electrode materials. Therefore, with the increase of Cr content, this material shows better rate capacity and cycling performance. However, further raising Cr content in materials will result in declines of electrochemical performance. This may due to the aggravation of reaction between particles and electrolyte when the particle size decreases to certain limit [41]. In addition, when the intensity of super lattice peaks, reflect- ing the Li2MnO3 component, weakens to a certain extent (Fig. 1), the structure of this material cannot maintain stable enough under the voltage of 4.8 V. In order to further understand the electrode process kinetic behavior, AC impedance measurements are conducted. The ob- tained plots, as shown in Fig. 6(1), consist of a semicircle arc and a straight line, which are attributed to the charge transfer process and the lithium diffusion process in the oxide structure, respec- tively [25,42,43]. These plots are well fitted using the electric equivalent circuit shown in the inset figure of Fig. 6(1), the value of the charge-transfer resistance (Rct) and solution resistance (Rs). CPE is related to the double layer capacitance and passivation film capacitance. ZW represents the Warburg impedance (Z0 is the real Fig. 6. AC impedance measurement of 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3 CrxO2 electrodes. The inset figure shows the equivalent circuit for the plot fitting. (2) Relationship between real impedance with the low frequencies of 0.3Li2MnO3� 0.7LiMn0.35�x/3Ni0.5�x/3Co0.15�x/3CrxO2 electrodes. impedance and Z00 is the imaginary impedance). The parameters of the equivalent circuit are listed in Table 3. The plots of the real axis Zre vs. the reciprocal square root of the lower angular frequen- cies x�0.5 are illustrated in Fig. 6(2). This relation is governed by Eq. (1) [44,45]. According to Fig. 6(1) and Eq. (2), the slope of the straight lines represents the values of Warburg impedance coeffi- cient (rw). The diffusion coefficients (D) of the lithium ions diffus- ing into the bulk electrode materials are calculated using Eq. (2) and recorded in Table 3. Obviously, the solution resistances are very similar and small. However, with the increasing of Cr content, the charge-transfer resistances decrease from 106.8 (x = 0) to 87.7 X (x = 0.02), and then increase to 105.3 (x = 0.06) finally. On the contrary, diffusion coefficients rise from 4.61 � 10�16 (x = 0) to 7.19 � 10�15 cm2 s�1 (x = 0.04) and then fall to 1.18 � 10�15 cm2 s�1 (x = 0.06) in the end. Additionally, those exchange current densities (i0) are calculated by the formula (i0 = RT/nFRct, where n is the number of transferred electrons) and list in Table 3. When x = 0.04, the as-prepared 0.3Li2MnO3�0.7LiMn0.35�x/3 Ni0.5�x/3Co0.15�x/3CrxO2 shows the highest exchange current Fig. 7. Cyclic voltammetry curves for first two cycles in the voltage range of 2.5–4.8 V at a scan rate of 0.1 mV s�1. 652 Z. He et al. / Advanced Powder Technology 25 (2014) 647–653 densities (2.92 � 10�4 mA cm�2) than those of as-prepared materi- als at other x value. Those results indicate that appropriate Cr con- tent doping can improve its electrochemical properties. Zre ¼ Rs þ Rct þ rw �x�0:5 ð1Þ D ¼ 0:5 RT AF2rwC � �2 ð2Þ where x, angular frequency region; R, the gas constant; T, the abso- lute temperature; F, Faraday’s constant; A, the area of the electrode surface; and C, molar concentration of Li+ ions. As can be seen clearly from Fig. 7, for both cathodes, anodic and cathodic peak around �4.1 and �3.7 V is corresponding to the nickel/cobalt oxidation and reduction. The peak about 4.65 V is due to simultaneous removal of lattice oxygen and lithium ion from Li2MnO3 [34]. When x = 0.04, the electrode shows higher and narrower peaks than that with x = 0 in the cyclic voltammo- grams. Additionally, remarkable cathodic peaks are emerging at �3.2 V at 2nd scans, which may corresponding to the several reduction of manganese ion (Mn4+ ?Mn3+) [34,46]. These phe- nomenons may contribute to the influence of Cr dopant, which re- sult in weaken of super lattice peaks, decrease of lattice constants, etc. In addition, this cyclic voltammogram also illustrates the bet- ter in reversibility of the material with x = 0.04 upon de-intercala- tion and intercalation of lithium ions over the potential range. 4. Conclusion Li-rich layered solid solution 0.3Li2MnO3�0.7LiMn0.35�x/3Ni0.5�x/3 Co0.15�x/3CrxO2 is prepared by spray drying followed by calcination, and Cr dopant definitely distributing uniformly in the Cr doped materials is realized easily through the spray drying technology. Whit increase in Cr content, the lattice parameter a, c and particle size decrease gradually which means more stable structure and shorter diffusion path of lithium ions during charge/discharge pro- cess. Furthermore, higher values of I(003)/I(104) and decline of super lattice peaks intensity, and reaction between particles and electro- lyte becomes aggravated. 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synthesized by spray drying 1 Introduction 2 Experiment 2.1 Synthesis and characterization of 0.3Li2MnO3·0.7LiMn0.35−x/3 Ni0.5−x/3Co0.15−x/3CrxO2 2.2 Electrochemical measurement 3 Results and discussion 3.1 Characterization of 0.3Li2MnO3·0.7LiMn0.35−x/3Ni0.5−x/3Co0.15−x/3 CrxO2 3.2 Electrochemical properties of 0.3Li2MnO3·0.7LiMn0.35−x/3Ni0.5−x/3 Co0.15−x/3CrxO2 4 Conclusion Acknowledgements References