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Influence of Mg2+ doping on the structure and electrochemical performances of layered LiNi0.6Co0.2-xMn0.2MgxO2 cathode materials
Journal of Alloys and Compounds 671 (2016) 479e485
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Influence of Mg2þ doping on the structure and electrochemical performances of layered LiNi0.6Co0.2-xMn0.2MgxO2 cathode materials Zhenjun Huang, Zhixing Wang*, Huajun Guo, Xinhai Li School of Metallurgy and Environment, Central South University, Changsha 410083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2015 Received in revised form 4 February 2016 Accepted 14 February 2016 Available online 16 February 2016
Introducing the Mg ion into host lattice is applied to improving the electrochemical performance of LiNi0.6Co0.2Mn0.2O2. The effect of Mg substitution for Co on the structure, morphology, electrochemical properties and Liþ diffusion coefficients are investigated in details. Rietveld refinement results reveal that Mg is incorporated into the bulk lattice, which results in reduced cation mixing and expand c-lattice parameter. All Mg-doped sample exhibit better cycle and rate performances, although the Mg substitution for Co led to decreasing a part of capacity. The Li diffusion coefficients obtained by galvanostatic intermittent titration technique (GITT) are increased with increases of Mg content. © 2016 Elsevier B.V. All rights reserved.
Keywords: Mg-doping Cathode material Lithium ion battery Liþ diffusion coefficients
1. Introduction Lithium-ion batteries (LIBs) have been widely utilized as a power source for portable electronic devices. In particular, further increasing the energy density and reducing the cost are main directions of developments in LIBs to be used for mobile electronics and transportation [1]. Cathode materials, as one of the most critical components of lithium ion batteries, play a critical role in determining the performance of LIBs [2]. Among of cathode materials, those in the series LiNi1xyCoxMnyO2 have been intensively investigated as high-energy density cathodes [3e6]. Considering the higher specific capacity and lower cost, LiNi0.6Co0.2Mn0.2O2 is considered to be one of the most promising cathode materials for next generation of commercial materials [7,8]. Unfortunately, this material still has significant drawbacks. Due to the similar radius of Liþ (0.76 Å) and Ni2þ (0.69 Å), Ni2þ ions would occupy the sites in Li layer to form non-stoichiometric structures, known as cation mixing. It causes various problems including capacity loss, structural deterioration and block the pathway of lithium diffusion [9]. The drawback should be overcome before LiNi0.6Co0.2Mn0.2O2 realizes the commercial application [10]. These drawbacks could be greatly mitigated by substituting other ions such as Mg2þ [11,12], Al3þ [13], Cr3þ [14,15], Naþ [16].
* Corresponding author. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.jallcom.2016.02.119 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Among them, Mg is one of the most attractive doping elements. Due to the similar ionic radius of Mg2þ ion (0.72 Å) with Liþ (0.76 Å) ion, it can occupy both Li sites and transition metal sites. On the one hand, the Mg2þ ions substitution for transition metal ions can reduce cation mixing. Furthermore, Mg2þ ion plays in a role in screening the O2--O2- repulsions during the charge, thus preventing the interslab collapse [17]. For instance, Pouillerie et al. reported that the substitution of Mg2þ ions could enhance the electrochemical performance in the LiNi1xMgxO2 system, due to the less structural collapses during cycling [18]. Sun et al. proposed that the structural stability and cycling behavior are improved by a small amount of Mg substitution for Ni in the Li [Li0.15Ni0.275Mn0.575]O2 system [19]. Woo et al. proposed that Mg incorporation into LiNi0.8Co0.1Mn0.1O2 can enhance electrochemical and thermal properties [20]. Huang et al. reported that Mg could suppress the phase transition and improve electrochemical properties at higher upper cutoff potentials [21]. What's more, Co is the most expensive and toxic functional element in cathode materials, it should be replaced by low cost and environmental-friendly elements. Therefore, it is expected that a partial substitution of Co with Mg would have a positive effect to improve battery performances. In this paper, aiming to understand the impact of partial substitution of Co with Mg in LiNi0.6Co0.2Mn0.2O2, the effects of substitution Mg for Co on the morphology structure, surface chemical states and electrochemical performance of the materials were investigated systematically.
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Table 1 Designated and observed elemental content of the as-prepared samples, as determined by ICP-AES. Designated chemical formula
Analyzed chemical formula
LiNi0.6Co0.2Mn0.2O2 LiNi0.6Co0.19Mn0.2Mg0.01O2 LiNi0.6Co0.17Mn0.2Mg0.03O2 LiNi0.6Co0.15Mn0.2Mg0.05O2
Li1.01Ni0.603Co0.203Mn0.198O2 Li1.01Ni0.599Co0.189Mn0.201Mg0.010O2 Li1.00Ni0.601Co0.172Mn0.202Mg0.029O2 Li1.01Ni0.601Co0.151Mn0.199Mg0.051O2
materials have been prepared by co-precipitation and hightemperature solid state method. First, the precursors of (Ni0.6Co0.2-xMn0.2Mgx) (OH)2 were synthesized by typical hydroxide coprecipitation method. An aqueous solution of 1 M NiCl26H2O, CoCl26H2O, MnCl24H2O and MgCl26H2O (molar ratio of Ni: Co: Mn ¼ 0.6: 0.2-x: 0.2: x) was added into a continuously stirred flask using a peristaltic pump under Ar atmosphere. At the same time, an amount of NaOH solution (aq.) and NH4OH solution (aq.), which used as a chelation agent, was also slowly pumped into the reactor. The solution was maintained at 55 C with continuous stirring for 2 h and the pH value of the solution was carefully controlled at 11.50 ± 0.05. The hydroxide was centrifuged, washed with distilled water and then evaporated at 80 C. Finally, the precursors and LiOH$H2O was mixed in a ratio of 1: 1.05, and the resultant powders were preheated at 480 C for 5 h, then calcined at 850 C for 15 h under O2 atmosphere. An excess of lithium was utilized to compensate for lithium loss during the calcinations. The prepared LiNi0.6Co0.2xMn0.2MgxO2 samples with different Mg-doping content, namely M0, M1, M3, and M5, respectively.
2.2. Sample characterization
Fig. 1. XRD patterns of all as-prepared materials.
2. Experimental 2.1. Synthesis of materials LiNi0.6Co0.2-xMn0.2MgxO2 (x ¼ 0, 0.01, 0.03, 0.05) cathode
The crystalline phase and structure of prepared powders were characterized by powder X-ray diffraction (XRD, Rigaku, Rint-2000) using a Cu Ka radiation (l ¼ 1.5406 Å). XRD data were obtained in the 2q range from 10 to 90 . The collected XRD intensity data were analyzed by the GSAS program [22,23]. The chemical compositions in the resulting materials were analyzed using an inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS intrepid XSP, Thermo Electron Corporation). The morphology of the samples were analyzed by scanning electron microscope (SEM, JEOL, JSM-5600LV), X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) tests were conducted to get information of the surface chemical valence state of Ni, Co, Mn and Mg. The binding energy of the element was calibrated with the contaminant C 1s (284.6 eV) as a reference.
Fig. 2. Rietveld refinement profile of the XRD data of (a) M0, (b) M1, (c) M3, (d) M-5.
Z. Huang et al. / Journal of Alloys and Compounds 671 (2016) 479e485 Table 2 Rietveld refinement results of XRD data for the as-prepared samples. Atom
Li1 Ni1 Mg1 Li2 Ni2 Co1 Mn1 Mg2 O a (Å) c (Å) c/a Rwp (%) Rp (%)
c2
Site
3a 3a 3a 3b 3b 3b 3b 3b 6c
Occ x¼0
x ¼ 0.01
x ¼ 0.03
x ¼ 0.05
0.945 0.055 0 0.055 0.545 0.2 0.2 0 1 2.86902 (4) 14.2077 (4) 4.952 11.71 8.92 1.319
0.968 0.030 0.002 0.03 0.571 0.19 0.2 0.009 1 2.86608 (5) 14.2119 (5) 4.959 11.26 8.59 1.522
0.961 0.033 0.006 0.039 0.567 0.17 0.2 0.024 1 2.86592 (5) 14.2146 (5) 4.960 10.96 8.36 1.477
0.968 0.021 0.011 0.032 0.579 0.15 0.2 0.039 1 2.86747 (4) 14.2215 (4) 4.960 10.82 8.30 1.347
2.3. Electrochemical measurements Electrochemical performances of the as-prepared samples were measured using CR2025-type coin cells. The positive material was prepared by 80 wt.% active material, 10 wt.% acetylene black and 10 wt.% polyvinylidene fluoride (PVDF) binder, which were mixed together in N-methylpyrrolidone (NMP), then was applied onto Al
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foil. The coated Al foil was dried in a vacuum oven at 60 C for 12 h. The electrolyte was LiPF6 (1 M) dissolved in a 1:1:1 (v/v/v) mixture of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and ethylene carbonate (EC). The electrode weight was measured by microbalance. Typical positive electrode loadings were about 2 mg cm2, and then the electrode was pressed under 15 MPa for 1 min to control the thickness uniformity. The assembly of the cells was performed in a dry Ar-filled glove box. Galvanostatic chargeedischarge tests were conducted using NEWARE battery cycler. Cyclic voltammetry (CV) tests were carried out in the range 2.8e4.3 V at a scan rate of 0.1 mV/s. 3. Results and discussion The chemical compositions of prepared samples are determined by ICP-AES, and the results are shown in Table 1. It is noted that the chemical composition for each element is close to the target values. Fig. S1 shows SEM images of the as-prepared samples. It shows no significant differences in particle size and morphology, indicating that the Mg-doping has no effect on the particle agglomerations. The structures were characterized using XRD. Fig. 1 shows the XRD patterns of the prepared samples. All of the diffraction peaks of the XRD pattern could be confirmed to have a well-defined layered hexagonal structure of a-NaFeO2 with space group R3 m. No impurity related peaks were observed in the XRD patterns for all samples, which indicating that the partial Mg doping do not destroy the layer structure. The clear splitting between the (006)
Fig. 3. XPS spectra of Ni, Co, Mn and Mg for all as-prepared materials.
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Fig. 4. (a) Initial chargeedischarge profiles and (b) 1C charge/discharge profiles of all as-prepared materials.
and (012) peaks as well as between the (018) and (110) peaks reveals a well-ordered layered structure of the samples. In order to understand the influence of Mg substitution on the crystal structure of LiNi0.6Co0.2Mn0.2O2, the crystal structures are analyzed by Rietveld refinements. Rietveld refinements are constrained for the result of ICP-AES. It is assumed that no vacancies exist in any sites, and the total occupation of Li1/Ni2/Mg2 and Li2/ Ni1/Co1/Mn1/Mg1 should be 1. According to the literature, due to the ionic radius of Mg2þ (0.72 Å) is similar to that of Liþ (0.76 Å), the occupation of Mg in the Li layer is highly possible. Rietveld refinement will provide information for understanding the change of the structures by Mg-doping. Fig 2 shows the Rietveld refinement profile of the XRD data of the as-prepared samples. The structural data are summarized in Table 2. It notes that Mg substitution brought about decreased cation mixing in Li layer. With the increase in Mg contents from 0 to 5%, the amount of Ni2þ ions occupied in lithium layer (3a site) is 5.5%, 3.0%, 3.3% and 2.1%, respectively. This result implies that Mg substitution improve the structural integrity. It clearly appears that the unit cell parameter, the parameter a only a slightly decreased, and the c-axes is increased with the amount of Mg contents increasing. The similar results were reported by Sun et al. [20] The expansion of c value can be associated with larger Mg2þ ions replace the smaller Co3þ (0.54 Å) in transition metal sites. As we known, the c-lattice parameter has a remarkably strong effect on the activation barrier
Fig. 5. Rate capability performances of all samples.
for Li migration [24,25], thus the increase in c-lattice parameter is beneficial for the insertion and removal of Liþ. What's more, according to Lange's Handbook of Chemistry, the bond dissociation energy of MgeO (DHf298 ¼ 394 kJ mol1) is larger than that of CoeO (DHf298 ¼ 368 kJ mol1) [26]. Therefore, it can be expected that introduction of Mg2þ in the host structure could enhance the electrochemical performance. XPS provides information about the valence state of Ni, Co, Mn, Mg. Fig. 3 shows Ni XPS spectra and corresponding fitting curves, the main peaks of Ni 2p3/2 peak shifted from lower binding energy of 854.62 to higher binding energy of 854.75 eV with the increasing amount of Mg2þ, suggesting that more presence of Ni3þ ions at the surface by Mg-doping [17,27]. The results show that the ratio of Ni3þ are 60.78% for M0, 61.83% for M1, 64.43% for M3 and 67.50% for M5, respectively. This is for the reason that more Ni2þ is reduced to Ni3þ with the increasing amount of Mg2þ doping to maintain the balance of valance states. Combination of the results of Rietveld refinements, the less Ni2þ may suppress migration of Ni2þ from transition metal slabs to Li slabs. As shown in Fig 3, the Co 2p3/2 main peak are about 780.2 eV, which indicates that the cobalt ions are mainly in an oxidation state of þ3. The Mn 2p3/2 binding energy peak centers are around 642.2 eV equal to that of Mn4þ at MnO2 [28], which indicates that Mn ions are present in þ4. The measured binding energy for Mg1s is around 1302.3 eV, showing the Mg ions exist in the form of Mg2þ in the compound [29]. By increasing the amount of Mg substitution for Co, the intensity of the Mg1s peaks is gradually increased, implying the concentration of Mg2þ ions increased. Fig. 4a compares the first chargeedischarge profiles for the asprepared samples between 2.8 and 4.3 V at 0.1 C (16 mA g1). It can be seen that both the pristine and Mg-substituted electrodes show similar charge/discharge profiles, and the initial discharge capacity of M0, M1, M3 and M5 is 182.13, 179.42, 175.99 and 174.76 mA h$g1, respectively. Obviously, with the increase of Mgdoping concentration, the discharge specific capacity decrease and the initial coulombic efficiency show the similar tendencies. One of the possible reasons for these phenomena is due to the decrease of the electrochemically active amount of the active Co3þ elements in the final products. The similar results have been reported by Sun et al. [30] On the other hand, part of Mg2þ ions are expected to migrate from the transition metal sites to the Li sites during the first cycle. Therefore, Mg-doping causes a lower coulombic efficiency. Fig. 4b shows the cycle performance of the as-prepared samples at 1 C carried out at 2.8e4.3 V. It is observed that all the Mg-doped samples exhibited better cyclic performance than the pristine samples, and the M3 sample exhibits the best cycling performance. The initial discharge capacity of M0, M1, M3 and M5
Z. Huang et al. / Journal of Alloys and Compounds 671 (2016) 479e485
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Fig. 6. aed GITT curves for the 4th cycle between 2.8 V and 4.3 V, eeh scheme for a single titration step of GITT curves, iel linear fit of the cell voltage as a function of the square root of time (t1/2) with different pulse currents.
are 162.1, 160.2, 159.5 and 155.4 mA h$g1, respectively. The discharge capacity gradually decreases with the number of cycles. After 100 cycle, the capacity retention of M0, M1, M3 and M5 are 83.9%, 88.8%, 90.8%, and 87.3%. This improvement in the capacity retention might be related to the reduced degree of cation mixing by Mg-doping. In addition, the stronger MgeO bonding
induced in the host structure would give improved structural stability. Fig. 5 exhibits the rate capability of as-prepared samples from 0.1 C to 5 C in the voltage of 2.8e4.3 V. As it can be seen, the M0 electrode delivers a higher reversible capacity than those of the doped samples at relatively low rate, which might be attributed to the substitution of Co ions by electrochemically inactive Mg ions. However, the M1 and M3 electrode exhibit better rate capability at 2 C. What is more, at a rate of 5 C, a reversible discharge capacity of M0, M1, M3 and M5 are 63%, 65%, 69% and 75% for the initial capacity is achieved at a 0.1 C rate, respectively. These improvements on the rate capability may be attributed to the less Li/ Ni disorder and the improved ionic conductivity. The diffusion constant of Li ion electrode materials is a key factor to influence the rate capability. In order to investigate the effects of Mg content on rate capability, the chemical diffusion coefficient are determined by GITT method using equation (1) [31].
DLiþ
Fig. 7. The chemical diffusion coefficients of Liþ ion for all samples.
0 12 . 4 mB Vm 2 @ DEs A t≪L2 D þ ¼ p MB S Li t dEt=dpffiffitffi
(1)
Where mB is the mass of the active material (g), Vm and MB are the molar volume and molecular weight of the electroactive compound, respectively. S is the area of the electrode, t is the time duration during the current pulse, DEs are the difference in the steady-state voltage at a single-step GITT experiment, L is the
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Fig. 8. CV curves for the first 3 cycles of (a)M0, (b)M1, (c)M3 and (d)M-5.
length of Liþ ions diffusion [32]. If E vs t1/2 shows a straight line behavior over the entire time period of current flux, the equation (1) can be further simplified as
DLiþ ¼
4 pt
mB Vm MB S
2
DEs DEt
2
. t≪L2 D þ Li
(2)
DEs describes the difference in the steady-state potentials measured at the end of two sequential open-circuit relaxation steps, and DEt is the potential changes during charging or discharging at the time of current of flux without the IR drop. Fig 6aed shows the GITT curves of the cell as a function of time for the 4th discharge cycle. Prior the GITT measurement, the cell is subjected to four chargeedischarge cycles at 0.1 C in the voltage window 2.8e4.3 V. The GITT date is collected at a constant current flux of 0.1 C for a time interval of 10 min, and then relaxing 50 min to allow relaxation to steady state. This procedure is repeated until the voltage window of operation, 2.8e4.3 V Fig. 6eeh shows a step of GITT during the discharge process. Fig. 6iel reveals that the variation of cell potential during the titration step shows a straight line behavior against t1/2, therefore, the diffusion coefficients of Liþ can be calculated by equation (2). Fig 7 shows the Liþ diffusion coefficients of the as-prepared samples as a function of cell voltage during processes of discharge. It can be seen that the value of Liþ diffusion coefficients are in the same order magnitude and their variation with voltage shows similar behavior, which are found to vary from 1011 to 109 cm2 s1. The initial value of 7.85 1011 cm2 s1 at 3.5 V for M0, then increases gradually to 1.16 109 cm2 s1 at 3.68 V, a minimum value of 9.2 109 cm2 s1 are observed at 3.62 V, and then increase again to a steady value about 2 109 cm2 s1 in the voltage range of 3.8e4.3 V. It can be seen that all curves show the similar trend. Interestingly, with the increased Mg content, the value of Dþ Li is increase gradually, suggesting the Mg-doping in favour of improving the Liþ migration. What is the reason for the improving of Liþ diffusion coefficients by Mg-doping? One of the reasons can
attribute to the less Li/Ni disorder. And more important, Liþ diffusion coefficients is more sensitive to the Li slab space, combine with Rietveld refinement results, Mg doping result in a larger Li slab space. Therefore, the diffusion coefficients for the Mg-doped samples are larger than the pristine sample. The results are in agreement with those reported by Shaju [33] and Chil-Hoon Doh [34]. Fig. 8 shows the CV plots of as prepared between 2.8 and 4.3 V vs. Liþ/Li at a scan rate of 0.1 mV s1. Only one pairs of peaks can be observed at about 3.8 V during the cathodic sweep and 3.6 V anodic sweep. No other peaks are present, indicating that no phase transition exists from the hexagonal to monoclinic form in the range of 2.8e4.3 V. In addition, it can be observed that Mg-doped electrodes show lower potential difference (DEp) than that of the M0 electrode. The DEp of M0, M1, M3 and M5 are 0.262, 0.143, 0.172 and 0.210 eV, respectively. Therefore, the Mg doping could reduce the potential difference, which indicates the lowest electrode polarization, thus improving the cyclic performance and rate capability of the as-prepared sample.
4. Conclusion LiNi0.6Co0.2-xMn0.2MgxO2 (x ¼ 0, 0.01, 0.03 and 0.05) materials have been synthesized via hydroxide co-precipitation method. The Rietveld refinements of XRD data indicate that the introduce of Mg into host lattice enlarges the inter-slab distance and reduces the Li/ Ni disorder, which is in favour of the diffusion of the Liþ ion. Electrochemical tests results suggest that the Mg-doped samples deliver a higher capacity retention and excellent rate capability. GITT results confirm that Mg substitution for Co site increases the Li diffusion coefficients. Therefore, the improvements of electrochemical performance of Mg-doped samples could be ascribed to the improved structural stability and effectively suppresses the cation mixing.
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Influence of Mg2þ doping on the structure and electrochemical performances of layered LiNi0.6Co0.2-xMn0.2MgxO2 cathode materials Zhenjun Huang, Zhixing Wang*, Huajun Guo, Xinhai Li School of Metallurgy and Environment, Central South University, Changsha 410083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 December 2015 Received in revised form 4 February 2016 Accepted 14 February 2016 Available online 16 February 2016
Introducing the Mg ion into host lattice is applied to improving the electrochemical performance of LiNi0.6Co0.2Mn0.2O2. The effect of Mg substitution for Co on the structure, morphology, electrochemical properties and Liþ diffusion coefficients are investigated in details. Rietveld refinement results reveal that Mg is incorporated into the bulk lattice, which results in reduced cation mixing and expand c-lattice parameter. All Mg-doped sample exhibit better cycle and rate performances, although the Mg substitution for Co led to decreasing a part of capacity. The Li diffusion coefficients obtained by galvanostatic intermittent titration technique (GITT) are increased with increases of Mg content. © 2016 Elsevier B.V. All rights reserved.
Keywords: Mg-doping Cathode material Lithium ion battery Liþ diffusion coefficients
1. Introduction Lithium-ion batteries (LIBs) have been widely utilized as a power source for portable electronic devices. In particular, further increasing the energy density and reducing the cost are main directions of developments in LIBs to be used for mobile electronics and transportation [1]. Cathode materials, as one of the most critical components of lithium ion batteries, play a critical role in determining the performance of LIBs [2]. Among of cathode materials, those in the series LiNi1xyCoxMnyO2 have been intensively investigated as high-energy density cathodes [3e6]. Considering the higher specific capacity and lower cost, LiNi0.6Co0.2Mn0.2O2 is considered to be one of the most promising cathode materials for next generation of commercial materials [7,8]. Unfortunately, this material still has significant drawbacks. Due to the similar radius of Liþ (0.76 Å) and Ni2þ (0.69 Å), Ni2þ ions would occupy the sites in Li layer to form non-stoichiometric structures, known as cation mixing. It causes various problems including capacity loss, structural deterioration and block the pathway of lithium diffusion [9]. The drawback should be overcome before LiNi0.6Co0.2Mn0.2O2 realizes the commercial application [10]. These drawbacks could be greatly mitigated by substituting other ions such as Mg2þ [11,12], Al3þ [13], Cr3þ [14,15], Naþ [16].
* Corresponding author. E-mail address: [email protected] (Z. Wang). http://dx.doi.org/10.1016/j.jallcom.2016.02.119 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Among them, Mg is one of the most attractive doping elements. Due to the similar ionic radius of Mg2þ ion (0.72 Å) with Liþ (0.76 Å) ion, it can occupy both Li sites and transition metal sites. On the one hand, the Mg2þ ions substitution for transition metal ions can reduce cation mixing. Furthermore, Mg2þ ion plays in a role in screening the O2--O2- repulsions during the charge, thus preventing the interslab collapse [17]. For instance, Pouillerie et al. reported that the substitution of Mg2þ ions could enhance the electrochemical performance in the LiNi1xMgxO2 system, due to the less structural collapses during cycling [18]. Sun et al. proposed that the structural stability and cycling behavior are improved by a small amount of Mg substitution for Ni in the Li [Li0.15Ni0.275Mn0.575]O2 system [19]. Woo et al. proposed that Mg incorporation into LiNi0.8Co0.1Mn0.1O2 can enhance electrochemical and thermal properties [20]. Huang et al. reported that Mg could suppress the phase transition and improve electrochemical properties at higher upper cutoff potentials [21]. What's more, Co is the most expensive and toxic functional element in cathode materials, it should be replaced by low cost and environmental-friendly elements. Therefore, it is expected that a partial substitution of Co with Mg would have a positive effect to improve battery performances. In this paper, aiming to understand the impact of partial substitution of Co with Mg in LiNi0.6Co0.2Mn0.2O2, the effects of substitution Mg for Co on the morphology structure, surface chemical states and electrochemical performance of the materials were investigated systematically.
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Table 1 Designated and observed elemental content of the as-prepared samples, as determined by ICP-AES. Designated chemical formula
Analyzed chemical formula
LiNi0.6Co0.2Mn0.2O2 LiNi0.6Co0.19Mn0.2Mg0.01O2 LiNi0.6Co0.17Mn0.2Mg0.03O2 LiNi0.6Co0.15Mn0.2Mg0.05O2
Li1.01Ni0.603Co0.203Mn0.198O2 Li1.01Ni0.599Co0.189Mn0.201Mg0.010O2 Li1.00Ni0.601Co0.172Mn0.202Mg0.029O2 Li1.01Ni0.601Co0.151Mn0.199Mg0.051O2
materials have been prepared by co-precipitation and hightemperature solid state method. First, the precursors of (Ni0.6Co0.2-xMn0.2Mgx) (OH)2 were synthesized by typical hydroxide coprecipitation method. An aqueous solution of 1 M NiCl26H2O, CoCl26H2O, MnCl24H2O and MgCl26H2O (molar ratio of Ni: Co: Mn ¼ 0.6: 0.2-x: 0.2: x) was added into a continuously stirred flask using a peristaltic pump under Ar atmosphere. At the same time, an amount of NaOH solution (aq.) and NH4OH solution (aq.), which used as a chelation agent, was also slowly pumped into the reactor. The solution was maintained at 55 C with continuous stirring for 2 h and the pH value of the solution was carefully controlled at 11.50 ± 0.05. The hydroxide was centrifuged, washed with distilled water and then evaporated at 80 C. Finally, the precursors and LiOH$H2O was mixed in a ratio of 1: 1.05, and the resultant powders were preheated at 480 C for 5 h, then calcined at 850 C for 15 h under O2 atmosphere. An excess of lithium was utilized to compensate for lithium loss during the calcinations. The prepared LiNi0.6Co0.2xMn0.2MgxO2 samples with different Mg-doping content, namely M0, M1, M3, and M5, respectively.
2.2. Sample characterization
Fig. 1. XRD patterns of all as-prepared materials.
2. Experimental 2.1. Synthesis of materials LiNi0.6Co0.2-xMn0.2MgxO2 (x ¼ 0, 0.01, 0.03, 0.05) cathode
The crystalline phase and structure of prepared powders were characterized by powder X-ray diffraction (XRD, Rigaku, Rint-2000) using a Cu Ka radiation (l ¼ 1.5406 Å). XRD data were obtained in the 2q range from 10 to 90 . The collected XRD intensity data were analyzed by the GSAS program [22,23]. The chemical compositions in the resulting materials were analyzed using an inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS intrepid XSP, Thermo Electron Corporation). The morphology of the samples were analyzed by scanning electron microscope (SEM, JEOL, JSM-5600LV), X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) tests were conducted to get information of the surface chemical valence state of Ni, Co, Mn and Mg. The binding energy of the element was calibrated with the contaminant C 1s (284.6 eV) as a reference.
Fig. 2. Rietveld refinement profile of the XRD data of (a) M0, (b) M1, (c) M3, (d) M-5.
Z. Huang et al. / Journal of Alloys and Compounds 671 (2016) 479e485 Table 2 Rietveld refinement results of XRD data for the as-prepared samples. Atom
Li1 Ni1 Mg1 Li2 Ni2 Co1 Mn1 Mg2 O a (Å) c (Å) c/a Rwp (%) Rp (%)
c2
Site
3a 3a 3a 3b 3b 3b 3b 3b 6c
Occ x¼0
x ¼ 0.01
x ¼ 0.03
x ¼ 0.05
0.945 0.055 0 0.055 0.545 0.2 0.2 0 1 2.86902 (4) 14.2077 (4) 4.952 11.71 8.92 1.319
0.968 0.030 0.002 0.03 0.571 0.19 0.2 0.009 1 2.86608 (5) 14.2119 (5) 4.959 11.26 8.59 1.522
0.961 0.033 0.006 0.039 0.567 0.17 0.2 0.024 1 2.86592 (5) 14.2146 (5) 4.960 10.96 8.36 1.477
0.968 0.021 0.011 0.032 0.579 0.15 0.2 0.039 1 2.86747 (4) 14.2215 (4) 4.960 10.82 8.30 1.347
2.3. Electrochemical measurements Electrochemical performances of the as-prepared samples were measured using CR2025-type coin cells. The positive material was prepared by 80 wt.% active material, 10 wt.% acetylene black and 10 wt.% polyvinylidene fluoride (PVDF) binder, which were mixed together in N-methylpyrrolidone (NMP), then was applied onto Al
481
foil. The coated Al foil was dried in a vacuum oven at 60 C for 12 h. The electrolyte was LiPF6 (1 M) dissolved in a 1:1:1 (v/v/v) mixture of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and ethylene carbonate (EC). The electrode weight was measured by microbalance. Typical positive electrode loadings were about 2 mg cm2, and then the electrode was pressed under 15 MPa for 1 min to control the thickness uniformity. The assembly of the cells was performed in a dry Ar-filled glove box. Galvanostatic chargeedischarge tests were conducted using NEWARE battery cycler. Cyclic voltammetry (CV) tests were carried out in the range 2.8e4.3 V at a scan rate of 0.1 mV/s. 3. Results and discussion The chemical compositions of prepared samples are determined by ICP-AES, and the results are shown in Table 1. It is noted that the chemical composition for each element is close to the target values. Fig. S1 shows SEM images of the as-prepared samples. It shows no significant differences in particle size and morphology, indicating that the Mg-doping has no effect on the particle agglomerations. The structures were characterized using XRD. Fig. 1 shows the XRD patterns of the prepared samples. All of the diffraction peaks of the XRD pattern could be confirmed to have a well-defined layered hexagonal structure of a-NaFeO2 with space group R3 m. No impurity related peaks were observed in the XRD patterns for all samples, which indicating that the partial Mg doping do not destroy the layer structure. The clear splitting between the (006)
Fig. 3. XPS spectra of Ni, Co, Mn and Mg for all as-prepared materials.
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Fig. 4. (a) Initial chargeedischarge profiles and (b) 1C charge/discharge profiles of all as-prepared materials.
and (012) peaks as well as between the (018) and (110) peaks reveals a well-ordered layered structure of the samples. In order to understand the influence of Mg substitution on the crystal structure of LiNi0.6Co0.2Mn0.2O2, the crystal structures are analyzed by Rietveld refinements. Rietveld refinements are constrained for the result of ICP-AES. It is assumed that no vacancies exist in any sites, and the total occupation of Li1/Ni2/Mg2 and Li2/ Ni1/Co1/Mn1/Mg1 should be 1. According to the literature, due to the ionic radius of Mg2þ (0.72 Å) is similar to that of Liþ (0.76 Å), the occupation of Mg in the Li layer is highly possible. Rietveld refinement will provide information for understanding the change of the structures by Mg-doping. Fig 2 shows the Rietveld refinement profile of the XRD data of the as-prepared samples. The structural data are summarized in Table 2. It notes that Mg substitution brought about decreased cation mixing in Li layer. With the increase in Mg contents from 0 to 5%, the amount of Ni2þ ions occupied in lithium layer (3a site) is 5.5%, 3.0%, 3.3% and 2.1%, respectively. This result implies that Mg substitution improve the structural integrity. It clearly appears that the unit cell parameter, the parameter a only a slightly decreased, and the c-axes is increased with the amount of Mg contents increasing. The similar results were reported by Sun et al. [20] The expansion of c value can be associated with larger Mg2þ ions replace the smaller Co3þ (0.54 Å) in transition metal sites. As we known, the c-lattice parameter has a remarkably strong effect on the activation barrier
Fig. 5. Rate capability performances of all samples.
for Li migration [24,25], thus the increase in c-lattice parameter is beneficial for the insertion and removal of Liþ. What's more, according to Lange's Handbook of Chemistry, the bond dissociation energy of MgeO (DHf298 ¼ 394 kJ mol1) is larger than that of CoeO (DHf298 ¼ 368 kJ mol1) [26]. Therefore, it can be expected that introduction of Mg2þ in the host structure could enhance the electrochemical performance. XPS provides information about the valence state of Ni, Co, Mn, Mg. Fig. 3 shows Ni XPS spectra and corresponding fitting curves, the main peaks of Ni 2p3/2 peak shifted from lower binding energy of 854.62 to higher binding energy of 854.75 eV with the increasing amount of Mg2þ, suggesting that more presence of Ni3þ ions at the surface by Mg-doping [17,27]. The results show that the ratio of Ni3þ are 60.78% for M0, 61.83% for M1, 64.43% for M3 and 67.50% for M5, respectively. This is for the reason that more Ni2þ is reduced to Ni3þ with the increasing amount of Mg2þ doping to maintain the balance of valance states. Combination of the results of Rietveld refinements, the less Ni2þ may suppress migration of Ni2þ from transition metal slabs to Li slabs. As shown in Fig 3, the Co 2p3/2 main peak are about 780.2 eV, which indicates that the cobalt ions are mainly in an oxidation state of þ3. The Mn 2p3/2 binding energy peak centers are around 642.2 eV equal to that of Mn4þ at MnO2 [28], which indicates that Mn ions are present in þ4. The measured binding energy for Mg1s is around 1302.3 eV, showing the Mg ions exist in the form of Mg2þ in the compound [29]. By increasing the amount of Mg substitution for Co, the intensity of the Mg1s peaks is gradually increased, implying the concentration of Mg2þ ions increased. Fig. 4a compares the first chargeedischarge profiles for the asprepared samples between 2.8 and 4.3 V at 0.1 C (16 mA g1). It can be seen that both the pristine and Mg-substituted electrodes show similar charge/discharge profiles, and the initial discharge capacity of M0, M1, M3 and M5 is 182.13, 179.42, 175.99 and 174.76 mA h$g1, respectively. Obviously, with the increase of Mgdoping concentration, the discharge specific capacity decrease and the initial coulombic efficiency show the similar tendencies. One of the possible reasons for these phenomena is due to the decrease of the electrochemically active amount of the active Co3þ elements in the final products. The similar results have been reported by Sun et al. [30] On the other hand, part of Mg2þ ions are expected to migrate from the transition metal sites to the Li sites during the first cycle. Therefore, Mg-doping causes a lower coulombic efficiency. Fig. 4b shows the cycle performance of the as-prepared samples at 1 C carried out at 2.8e4.3 V. It is observed that all the Mg-doped samples exhibited better cyclic performance than the pristine samples, and the M3 sample exhibits the best cycling performance. The initial discharge capacity of M0, M1, M3 and M5
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483
Fig. 6. aed GITT curves for the 4th cycle between 2.8 V and 4.3 V, eeh scheme for a single titration step of GITT curves, iel linear fit of the cell voltage as a function of the square root of time (t1/2) with different pulse currents.
are 162.1, 160.2, 159.5 and 155.4 mA h$g1, respectively. The discharge capacity gradually decreases with the number of cycles. After 100 cycle, the capacity retention of M0, M1, M3 and M5 are 83.9%, 88.8%, 90.8%, and 87.3%. This improvement in the capacity retention might be related to the reduced degree of cation mixing by Mg-doping. In addition, the stronger MgeO bonding
induced in the host structure would give improved structural stability. Fig. 5 exhibits the rate capability of as-prepared samples from 0.1 C to 5 C in the voltage of 2.8e4.3 V. As it can be seen, the M0 electrode delivers a higher reversible capacity than those of the doped samples at relatively low rate, which might be attributed to the substitution of Co ions by electrochemically inactive Mg ions. However, the M1 and M3 electrode exhibit better rate capability at 2 C. What is more, at a rate of 5 C, a reversible discharge capacity of M0, M1, M3 and M5 are 63%, 65%, 69% and 75% for the initial capacity is achieved at a 0.1 C rate, respectively. These improvements on the rate capability may be attributed to the less Li/ Ni disorder and the improved ionic conductivity. The diffusion constant of Li ion electrode materials is a key factor to influence the rate capability. In order to investigate the effects of Mg content on rate capability, the chemical diffusion coefficient are determined by GITT method using equation (1) [31].
DLiþ
Fig. 7. The chemical diffusion coefficients of Liþ ion for all samples.
0 12 . 4 mB Vm 2 @ DEs A t≪L2 D þ ¼ p MB S Li t dEt=dpffiffitffi
(1)
Where mB is the mass of the active material (g), Vm and MB are the molar volume and molecular weight of the electroactive compound, respectively. S is the area of the electrode, t is the time duration during the current pulse, DEs are the difference in the steady-state voltage at a single-step GITT experiment, L is the
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Fig. 8. CV curves for the first 3 cycles of (a)M0, (b)M1, (c)M3 and (d)M-5.
length of Liþ ions diffusion [32]. If E vs t1/2 shows a straight line behavior over the entire time period of current flux, the equation (1) can be further simplified as
DLiþ ¼
4 pt
mB Vm MB S
2
DEs DEt
2
. t≪L2 D þ Li
(2)
DEs describes the difference in the steady-state potentials measured at the end of two sequential open-circuit relaxation steps, and DEt is the potential changes during charging or discharging at the time of current of flux without the IR drop. Fig 6aed shows the GITT curves of the cell as a function of time for the 4th discharge cycle. Prior the GITT measurement, the cell is subjected to four chargeedischarge cycles at 0.1 C in the voltage window 2.8e4.3 V. The GITT date is collected at a constant current flux of 0.1 C for a time interval of 10 min, and then relaxing 50 min to allow relaxation to steady state. This procedure is repeated until the voltage window of operation, 2.8e4.3 V Fig. 6eeh shows a step of GITT during the discharge process. Fig. 6iel reveals that the variation of cell potential during the titration step shows a straight line behavior against t1/2, therefore, the diffusion coefficients of Liþ can be calculated by equation (2). Fig 7 shows the Liþ diffusion coefficients of the as-prepared samples as a function of cell voltage during processes of discharge. It can be seen that the value of Liþ diffusion coefficients are in the same order magnitude and their variation with voltage shows similar behavior, which are found to vary from 1011 to 109 cm2 s1. The initial value of 7.85 1011 cm2 s1 at 3.5 V for M0, then increases gradually to 1.16 109 cm2 s1 at 3.68 V, a minimum value of 9.2 109 cm2 s1 are observed at 3.62 V, and then increase again to a steady value about 2 109 cm2 s1 in the voltage range of 3.8e4.3 V. It can be seen that all curves show the similar trend. Interestingly, with the increased Mg content, the value of Dþ Li is increase gradually, suggesting the Mg-doping in favour of improving the Liþ migration. What is the reason for the improving of Liþ diffusion coefficients by Mg-doping? One of the reasons can
attribute to the less Li/Ni disorder. And more important, Liþ diffusion coefficients is more sensitive to the Li slab space, combine with Rietveld refinement results, Mg doping result in a larger Li slab space. Therefore, the diffusion coefficients for the Mg-doped samples are larger than the pristine sample. The results are in agreement with those reported by Shaju [33] and Chil-Hoon Doh [34]. Fig. 8 shows the CV plots of as prepared between 2.8 and 4.3 V vs. Liþ/Li at a scan rate of 0.1 mV s1. Only one pairs of peaks can be observed at about 3.8 V during the cathodic sweep and 3.6 V anodic sweep. No other peaks are present, indicating that no phase transition exists from the hexagonal to monoclinic form in the range of 2.8e4.3 V. In addition, it can be observed that Mg-doped electrodes show lower potential difference (DEp) than that of the M0 electrode. The DEp of M0, M1, M3 and M5 are 0.262, 0.143, 0.172 and 0.210 eV, respectively. Therefore, the Mg doping could reduce the potential difference, which indicates the lowest electrode polarization, thus improving the cyclic performance and rate capability of the as-prepared sample.
4. Conclusion LiNi0.6Co0.2-xMn0.2MgxO2 (x ¼ 0, 0.01, 0.03 and 0.05) materials have been synthesized via hydroxide co-precipitation method. The Rietveld refinements of XRD data indicate that the introduce of Mg into host lattice enlarges the inter-slab distance and reduces the Li/ Ni disorder, which is in favour of the diffusion of the Liþ ion. Electrochemical tests results suggest that the Mg-doped samples deliver a higher capacity retention and excellent rate capability. GITT results confirm that Mg substitution for Co site increases the Li diffusion coefficients. Therefore, the improvements of electrochemical performance of Mg-doped samples could be ascribed to the improved structural stability and effectively suppresses the cation mixing.
Z. Huang et al. / Journal of Alloys and Compounds 671 (2016) 479e485
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