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Effect of calcination temperature on microstructure and electrochemical performance of lithium-rich layered oxide cathode materials
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Effect of calcination temperature on microstructure and electrochemical performance of lithium-rich layered oxide cathode materials Quanxin Ma, Fangwei Peng, Ruhong Li, Shibo Yin, Changsong Dai ∗ MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 February 2016 Received in revised form 6 April 2016 Accepted 12 April 2016 Available online xxx
a b s t r a c t Lithium-rich layered oxide cathode materials (Li1.2 Mn0.56 Ni0.16 Co0.08 O2 (LLMO)) were synthesized via a two-step synthesis method involving co-precipitation and high-temperature calcination. The effects of calcination temperature on the cathode materials were studied in detail. Structural and morphological characterizations revealed that a well-crystallized layered structure was obtained at a higher calcination temperature. Electrochemical performance evaluation revealed that a cathode material obtained at a calcination temperature of 850 ◦ C delivered a high initial discharge capacity of 266.8 mAh g−1 at a 0.1 C rate and a capacity retention rate of 95.8% after 100 cycles as well as excellent rate capability. Another sample calcinated at 900 ◦ C exhibited good cycling stability. It is concluded that the structural stability and electrochemical performance of Li-rich layered oxide cathode materials were strongly dependent on calcination temperatures. The results suggest that a calcination temperature in a range of 850–900 ◦ C could promote electrochemical performance of this type of cathode materials. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As a power source of portable electronics and electric vehicles, lithium-ion batteries are highly demanded. Cathode materials play a major role in the development of lithium-ion batteries with high performance. The search for high-capacity and safe cathode materials for high-energy-density Li-ion batteries has become one of the most popular subjects in material science in recent years [1–3]. Among all the cathode materials investigated, lithium-ion cathode materials based on layered manganese oxides (LLMO), commonly denoted as xLi2 MnO3 ·(1–x)LiMO2 (M = Co, Ni, Mn, etc.), are the most promising ones because of their low cost and extremely high capacity. This type of materials has been extensively investigated since their first introduction by Lu and Thackeray [4–6]. Their extraordinary capacity is a result of an electrochemical reaction involving transfer of multiple lithium ions and electrochemical activation of Li2 MnO3 . During the activation process, lithium and oxygen ions are simultaneously extracted from the lattice of Li2 MnO3 [7,8] and thus a high capacity over 250 mAh g−1 is released. However, this type of materials suffer from some significant challenges includ-
∗ Corresponding author. Tel.: +86 451 86413751; fax: +86 451 86418616. E-mail address: [email protected] (C. Dai).
ing low initial Coulombic efficiency, long-term cycling instability and poor rate capability [9–11], which impose a great obstacle for the practical application of these layered composite cathode materials. Various approaches, such as partial replacement of nickel and manganese by transition metals [12,13], surface modification [14,15] and optimization of preparation methods [16,17], have been developed to improve the electrochemical properties of these lithium-rich layered oxide cathode materials. Cao et al. [18] synthesized high capacity porous Li[Li0.2 Mn0.534 Ni0.133 Co0.133 ]O2 layered oxides using a novel polymer-thermolysis method, and studied the influence of annealing temperature on electrochemical performance. They concluded that the best electrochemical performance was obtained with a sample synthesized at 850 ◦ C. Chen et al. [19] prepared a variety of Li-rich layered oxide compounds 0.4Li2 MnO3 ·0.6LiNi1/3 Co1/3 Mn1/3 O2 by a coprecipitation method at different temperatures; they also investigated the effects of different annealing temperatures on the structure and electrochemical performance. Their results suggested that samples annealed at 900 ◦ C exhibited the best electrochemical performance. However, the correlation among calcinations temperatures, microstructure and electrochemical performance of Li-rich cathode materials has rarely been thoroughly investigated. This study was designed to explore this area for better strategies to improve the
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electrochemical properties of these lithium-rich layered oxide cathode materials. In this work, a series of Li-rich layered oxide (Li1.2 Mn0.56 Ni0.16 Co0.08 O2 ) samples were prepared at different calcination temperatures via a two-step synthesis method involving co-precipitation. The effects of the calcination temperature on the structural properties and electrochemical performance of Li-rich layered oxide (Li1.2 Mn0.56 Ni0.16 Co0.08 O2 ) cathode materials were thoroughly investigated. The results show that the samples calcined at 850–900 ◦ C performed the best electrochemically. 2. Experimental 2.1. Preparation of Li1.2 Mn0.56 Ni0.16 Co0.08 O2 The Li1.2 Mn0.56 Ni0.16 Co0.08 O2 materials were prepared by a solid-state reaction from lithium hydroxide (LiOH·H2 O) and manganese-nickel-cobalt hydroxide (Mn0.7 Ni0.2 Co0.1 (OH)2 ). Manganese-nickel-cobalt hydroxide was firstly prepared by co-precipitation from an aqueous mixture of MnCl2 ·4H2 O, NiSO4 ·7H2 O, and CoCl2 ·6H2 O (Mn:Ni:Co = 7:2:1, molar ratio; the combined concentration was 2 mol L−1 ) and a mixture of 2.0 mol L−1 NaOH aqueous solutions with a desired amount of NH3 ·H2 O. The solutions were mixed slowly in a nitrogen filled reactor and the pH of the mixed solution was kept in a range of 9.8–10.2 for 12 h during the precipitation process. After the reaction was finished, the precipitated Mn0.7 Ni0.2 Co0.1 (OH)2 particles were filtered, washed using deionized water, and then dried in a vacuum at 120 ◦ C for 24 h. The obtained Mn0.7 Ni0.2 Co0.1 (OH)2 and LiOH·H2 O were then mixed at a molar ratio of 1.00:1.55 by using a ball mill for 5 h with absolute ethyl alcohol, and then the mixture was pressed into pellets. To the mixture 5% excess lithium hydroxide was added to compensate for the lithium evaporation during the calcination process at a high temperature. The pellets were respectively heated at 750, 800, 850, 900, 950 ◦ C for 20 h in air and then quenched to room temperature. Hereafter samples obtained at different calcination temperatures (750, 800, 850, 900, 950 ◦ C) are represented by LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950, respectively.
carbonate (DMC) and diethyl carbonate (DEC) at a volumetric ratio of 1:1:1. Galvanostatic charge and discharge were performed at different current densities in a voltage range of 2.0–4.8 V using an 8-channel battery analyzer (Neware, China). Theoretical capacities of all cathode materials were set to 250 mAh g−1 , i.e., a current density of 250 mA g−1 corresponding to 1.0 C. Electrochemical storage capacities of the working electrodes were calculated based on the mass of active cathode materials. Cyclic voltammograms (CV) of different cathodes were recorded at a scanning rate of 0.1 mV s−1 between 2.0 and 4.8 V using an electrochemical analyzer CHI630b (Chenhua, China). The electrochemical impedance spectroscopy (EIS) investigations were performed on an electrochemical workstation (PARSTAT 2273, AMETEK, USA) with three-electrode system over a frequency range from 100 kHz to 10 mHz after the 3rd cycles in the charged state of 3.0 and 4.4 V. In order to make the test cell reaching a quasi-equilibrium state, the cells were aged for 6 h. To ensure the validity and reliability of the results, in electrochemical measurements, three cells for each sample were test and the average one was selected for discussion. All experiments were carried out at a temperature of 25 ± 0.5 ◦ C.
2.2. Characterizations
3. Results and discussion
The stoichiometric molar compositions of the prepared samples were accurately analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, PerkinElmer Optima 5300DV, PerkinElmer, Waltham, MA, USA). The thermal behavior of Mn0.7 Ni0.2 Co0.1 (OH)2 and LiOH·H2 O mixtures was determined by differential scanning calorimetry/thermal gravimetric analysis (DSC/TGA, TA-SDTQ600, USA) conducted on the dried material at a heating rate of 5 ◦ C min−1 from room temperature to 1000 ◦ C in air. The morphologies of the as-prepared samples were observed with a scanning electron microscope (SEM, HITACHI, S-4700). The crystal structures of the synthesized samples were determined by X-ray diffraction (XRD) using a D/max-␥ X’ pert diffractometer (Rigaku, Japan) with Cu K␣ radiation. The microstructural characteristics of the synthesized samples were observed using a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2010) working at an accelerating voltage of 200 kV.
Fig. 1. The DSC–TGA curves of Mn0.7 Ni0.2 Co0.1 (OH)2 and LiOH·H2 O mixtures.
Chemical compositions of the cathode materials determined by ICP analysis are listed in Table 1. The obtained results are close to their nominal compositions within experimental errors.
2.3. Electrochemical evaluation Two-electrode CR2032-type coin cells were assembled with LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 as the cathodes, respectively, metallic lithium foil as the anode and Celgard-2320 membrane as the separator. The electrolyte used comprised 1 M LiPF6 dissolved in ethylene carbonate (EC), dimethyl
Fig. 2. XRD patterns of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO950 samples.
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The compositions of these cathode materials can be described with a notation of Li1.5 Mn0.7 Ni0.2 Co0.1 O2.5 , which is equivalent to Li1.2 Mn0.56 Ni0.16 Co0.08 O2 after the conventional normalization to two oxygen atoms. Reduction of lithium content was a result of the lithium evaporation at high temperatures. Thermochemical properties of the Mn0.7 Ni0.2 Co0.1 (OH)2 and LiOH·H2 O mixtures investigated by differential scanning
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calorimetry/thermal gravimetric analysis (DSC/TGA) are shown in Fig. 1. The weight loss in a range of 20–305 ◦ C was about 4%, which is attributed to water desorption. A slight weight increase (0.4%) in a range of 305–342 ◦ C was observed which was caused by the precursor oxidation. Another weight loss of 8% occurred in a range of 342–550 ◦ C, which was related to the decomposition of hydroxide and precursor. However, slow weight compensation
Fig. 3. SEM images of the LLMO-750 (a, a ), LLMO-800 (b, b ), LLMO-850 (c, c ), LLMO-900 (d, d ) and LLMO-950 (e, e ) samples at different magnifications.
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Table 1 The chemical composition results of ICP-OES analysis for LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples. Samples
LLMO-750 LLMO-800 LLMO-850 LLMO-900 LLMO-950
ICP analyses (atom ratios) Li
Co
Mn
Ni
1.509 1.506 1.502 1.498 1.495
0.098 0.096 0.096 0.097 0.096
0.695 0.697 0.696 0.696 0.696
0.207 0.207 0.208 0.207 0.208
was observable above 700 ◦ C, which may be due to the integration of oxygen into the crystal lattice. From the DSC curve, an endothermic peak can be seen at 456 ◦ C, which was related to LiOH melting [20]. Furthermore, another endothermic peak at 698 ◦ C was probably due to the completion of the hexagonal lattice ordering. This reasoning is consistent with the proposal by Zhu et al., who suggested that synthesis of Li[Ni1/3 Co1/3 Mn1/3 ]O2 at temperatures higher than 700 ◦ C produced an ordered layered structure [21], and with the conclusion by Jouybari et al., who reported a required calcination temperature of 700 ◦ C during their synthesis of LiNi0.8 Co0.2 O2 [22]. These results indicate that the calcination temperature for synthesis of Li1.2 Mn0.56 Ni0.16 Co0.08 O2 should be above 700 ◦ C. The XRD patterns of samples LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 are shown in Fig. 2. The main diffraction peaks could be indexed with the ␣-NaFeO2 type structure (space group R-3m). With increase in the calcination temperature, the intensities of all the peaks increased and the peaks became sharper reflecting the increasing crystallinity [23]. Distinct splitting (006)/(102) and (108)/(110) reflections can be seen in the XRD patterns, which indicates well-defined layered structures for all the samples [24]. The intensity ratio of I(003) /I(104) is inversely proportional to the degree of cation mixing of a layered structure [25,26]. When the calcination temperature was higher than 800 ◦ C, a high ratio of I(003) /I(104) (larger than 1.8) implies the lower cation mixing
Table 2 The crystallographic parameters of LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples. Samples
a (Å)
c (Å)
c/a
V (Å3 )
I(003) /I(104)
LLMO-750 LLMO-800 LLMO-850 LLMO-900 LLMO-950
2.828 2.847 2.846 2.850 2.831
14.008 14.185 14.190 14.225 14.128
4.954 4.983 4.987 4.992 4.990
96.99 99.19 99.50 100.04 98.07
1.793 1.804 1.828 1.820 1.816
and a more ordered layer structure (Table 2). In addition, according to Ngala et al. [27], an ideal cubic close-packed lattice has a c/a ratio of 4.899, where a larger c/a ratio corresponds to a more ordered layered structure. For all of the synthesized samples in this work, the c/a ratios were larger than 4.954, indicating the wellordered layered structures of all those samples. These results prove that a higher calcination temperature promotes the formation of a well-defined layered structure. Morphology and particle size of cathode materials are of great importance to battery performance. The SEM images of the LLMO750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples at different magnifications are shown in Fig. 3. It can be found that samples LLMO-750 (Fig. 3a, a ) and LLMO-800 (Fig. 3b, b ) were composed of irregular agglomerates with an average size between 200 and 300 nm. On contrast, particles of the LLMO-850 sample (Fig. 3c, c ) were well crystallized with no obvious aggregation observed and the estimated average particle size was ca. 500 nm. With the calcination temperature increasing, significant crystal growth between 900 ◦ C (Fig. 3d, d ) and 950 ◦ C (Fig. 3e, e ) was observed. The particles were more uniformly distributed and the particle size increased suddenly. With further increase in calcination temperature, the crystals grew larger. The crystal size was up to ca. 600 nm at 900 ◦ C and eventually it increased to ca.1000 nm at 950 ◦ C. To thoroughly study the effects of calcination temperature on the bulk and surface structures of the layered Li1.2 Mn0.56 Ni0.16 Co0.08 O2 cathode materials, three representative samples, LLMO-750 (Fig. 4a–d), LLMO-850 (Fig. 4e–h) and
Fig. 4. TEM, HRTEM images and EDS spectra of the samples LLMO-750 (a–d), LLMO-850 (e–h) and LLMO-950 (i–l).
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LLMO-950 (Fig. 4i–l) were investigated by using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). The TEM images in Fig. 4(a, e and i) confirm that the particle size increased with the increase of calcination temperature. The LLMO850 sample was composed of uniformly distributed particles with an average size of 400–500 nm, while LLMO-750 sample showed some agglomeration. The HRTEM images in Fig. 4(c, g and k) show that an amorphous layer was formed on the surface of the LLMO750 and LLMO-850 samples, which may serve as a barrier layer between the bulk structure and the electrolyte. As the calcination temperature increased to 850 ◦ C, the amorphous layer became thinner (Fig. 4g). The LLMO-950 sample showed a smooth edge and there was no other layer on the surface (Fig. 4k), indicating that high temperature may help the formation of a better crystal lattice. Fig. 4(b, f and j) also shows that the amorphous layer gradually disappeared as the calcination temperature increased. The EDS spectra of the samples in Fig. 4(d, h and l) show that the nature of the amorphous layer might be lithium carbonate. The well-defined edges of the particles are in agreement with their high crystallinity that is reflected by the XRD results. The above-mentioned results reveal that the structure of Li-rich layered oxides is strongly related with the calcination temperatures. Fig. 5 shows the cyclic voltammograms (CV) of the LLMO-750, LLMO-850 and LLMO-950 samples in a potential range of 2.0–4.8 V at a scan rate of 0.1 mV s−1 for the first 3 cycles. As shown in Fig. 5, two oxidation peaks were observed at ca. 4.3 and ca. 4.7 V in the first charge process, which correspond to the reversible lithium intercalation/deintercalation in Li1.2 Mn0.56 Ni0.16 Co0.08 O2 and the electrochemical activation of Li2 MnO3 . A reduction peak at ca. 3.75 V was observed in the initial discharge process, which corresponds to the reduction of Ni4+ and Co4+ [9]. The potential difference between oxidation peaks and reduction peaks in the first charge process was increased from 0.5 to 0.69 V with the calcination temperature increase, indicating that the initial reversibility was reduced. Moreover, a new reduction peak at ca. 3.2 V appeared in the 2nd and 3rd cycles, suggesting the phase transformation from a layered one to a spinel one during the subsequent cycling process [28], which could lead to a layered-spinel intergrowth structure and a redox reaction at ca. 3.2 V [29]. In contrast to the initial charge process, the CV curve exhibits a large broad anode peaks in a voltage range of 3.4–4.3 V in the subsequent cycling process. From Fig. 5b it can be seen that the broad anode peaks became narrower and the intensity of peaks increased, which indicate the enhancement in reversibility of the electrode reaction as a result of easier insertion and deinsertion of lithium ions. The anode peaks disappeared subsequently in the LLMO-950 sample (Fig. 5c), indicating the activation of materials becoming more difficult with the calcination temperature increase as a result of more difficult insertion and deinsertion of lithium ions. The electrochemical performance of all the samples was evaluated using coin-cells with each one of the layered Li1.2 Mn0.56 Ni0.16 Co0.08 O2 materials calcinated at different temperatures as the cathode and metallic lithium as the anode to further study the effects of calcination temperature on their electrochemical performance. Fig. 6(a) shows the initial charge/discharge curves of all the samples at different calcination temperatures during cycling at a 0.1 C rate between 2.0 and 4.8 V and the pertinent electrochemical properties are tabulated in Table 3. It can be seen that all the samples exhibited similar charge–discharge characteristics: a sloping voltage profile under 4.5 V followed by a high-voltage plateau at 4.5–4.8 V during charging. The sloping region below 4.5 V arises from the oxidation of Ni2+ to Ni4+ ions and Co3+ to Co4+ , respectively, while the long plateau from 4.5 to 4.8 V can be attributed to the simultaneous release of lithium ions and oxygen according to the previous studies by different groups
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Fig. 5. Cyclic voltammograms (CV) of the Li1.2 Mn0.56 Ni0.16 Co0.08 O2 scanned at 0.1 mV s−1 : (a) LLMO-750, (b) LLMO-850 and (c) LLMO-950.
Table 3 Initial charge/discharge data of LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples. Samples
Charge capacity (mAh g−1 )
Discharge capacity (mAh g−1 )
Coulombic efficiency (%)
LLMO-750 LLMO-800 LLMO-850 LLMO-900 LLMO-950
404.2 399.3 392.4 364.8 340.6
254.8 257.9 268.8 262.7 258.1
63.0 64.6 68.5 72.0 75.8
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Fig. 6. (a) The charge/discharge and (b) dQ/dV curves of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 at 0.1 C (25 mA g−1 ) between 2.0 and 4.8 V.
Fig. 7. (a) Discharge capacities and (b) the mid-point voltage (MPV) profiles as a function of cycle numbers of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 at 0.1 C (25 mA g−1 ) between 2.0 and 4.8 V.
[30,31]. As shown in Fig. 6(a), the initial charge capacity gradually decreased from 404.2 to 340.6 mAh g−1 as the calcination temperature increased. The initial discharge capacity were 254.8, 257.9, 268.8, 262.7, and 258.1 mAh g−1 for the samples calcinated at 750, 800, 850, 900 and 950 ◦ C. The LLMO-850 sample delivered the highest discharge capacity (ca. 268.8 mAh g−1 ) among all the samples. The initial coulombic efficiency of all sample increased as the calcination temperature increased. The corresponding dQ/dV curves of all the samples are shown in Fig. 6(b). The oxidation peaks at ca. 4.0 V during the initial charge process can be attributed to the oxidation of Ni2+ and Co3+ to Ni4+ and Co4+ ions in the layered component, respectively. The intensity of the oxidation peaks at ca. 4.6 V decreased as the calcination temperature increased, indicating that the content of the Li2 MnO3 component, which can be activated in the first cycle, probably decreased as the calcination temperature increased. In addition, a reduction peaks at ca. 3.8 V corresponding to the reduction of Ni4+ and Co4+ were observed in the initial discharge process. In the meantime, the intensity of the reduction peaks at ca. 3.3 V became lower as the calcination temperature increased, suggesting that the activation product MnO2 was reduced during the initial charge process. In order to evaluate the cycling stability of the cathode materials, the cells were cycled at a rate of 0.1 C and the discharge capacities of all the samples are illustrated in Fig. 7(a). Although the LLMO-850 delivered a higher initial discharged capacity than LLMO-900, LLMO-900 exhibited the best cycling stability with a capacity retention rate of 101.5%. LLMO-850 had a capacity retention rate of 95.8% over the same cycling period. According to Li et al. [32], the improvement in capacity with increasing calcination temperatures may be attributed to the improvement in the crystallization and cation-ordering of the cathode materials. To get a more intuitive idea of the voltage evolution, the midpoint voltage (MPV) profiles of these materials are plotted in Fig. 7(b). It can be seen that all these materials showed similar
mid-point voltages in the initial cycle. However, the MPV retention of these lithium-rich materials (Li1.2 Mn0.56 Ni0.16 Co0.08 O2 ) increased as the calcination temperature increased (ca. 84.5% for the LLMO-750, ca. 87.0% for the LLMO-800, ca. 87.4% for the LLMO850, ca. 87.7% for the LLMO-900 and ca. 89.8% for the LLMO-950 after 100 cycles), illustrating again the benefits of high calcination temperature in stabilizing the structure and suppressing the voltage decay of the lithium-rich cathode materials. In addition, the difference in mid-point voltages among materials LLMO-850, LLMO-900 and LLMO-950 was very small, suggesting that there is no need to raise the temperature to sustain the voltage. Fig. 8 gives the rate capability of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples under galvanostatic
Fig. 8. Rate capability performance of the LLMO-750, LLMO-800, LLMO-850, LLMO900 and LLMO-950 samples under galvanostatic charge at a rate of 0.1 C and discharge at different rates from 0.1 C to 2 C (1.0 C = 250 mA g−1 ).
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Fig. 9. Nyquist plots of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 electrodes (a) 3.0 V and (b) 4.4 V after the 3rd cycles (inset shows the equivalent circuit).
Table 4 The impedance parameters of LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 electrodes at different potential after 3rd cycles. Samples
LLMO-750 LLMO-800 LLMO-850 LLMO-900 LLMO-950
3.0 V
4.4 V
Rs ()
Rct ()
Rs ()
Rct ()
2.823 2.572 2.348 2.390 4.418
926.2 482.6 161.4 208.7 245.2
3.333 4.651 4.670 4.034 2.525
415.9 241.5 65.8 117.4 138.8
charge at a rate of 0.1 C and discharge at different rates from 0.1 C to 2 C (1.0 C = 250 mA g−1 ) for every five cycles. From Fig. 8, it can be seen that LLMO-750 and LLMO-800 showed lower rate capabilities than other samples duo to the formation of agglomerates and existence of an amorphous layer on the surface. LLMO-850 showed the best rate capability with a discharge capacity of 182 mAh g−1 at 2 C. The rate performance enhancement can be ascribed to a better crystal structure and small particles with high specific surface areas, which can facilitate faster diffusion of the lithium ions between the active particles and electrolyte [23]. With the calcination temperature increased, the rate capabilities of LLMO-900 and LLMO-950 decreased, which is probably due to the formation of larger particles at a higher temperature. Larger particles can cause prolonged lithium diffusion pathways during charging and discharging. Electrochemical impedance spectroscopy (EIS) has been performed to get insights into the electrochemical performance of the Li1.2 Mn0.56 Ni0.16 Co0.08 O2 materials. All Nyquist plots at 3.0 V and 4.4 V after the 3rd cycle are shown in Fig. 9 and the corresponding equivalent circuits are presented in the inset. In this equivalent circuit, Rs represents the resistance of the electrolyte and cell components, Rct corresponds to charge transfer resistance in the electrode–electrolyte interfaces, and Zw refers to the diffusion impedance of lithium ions in the solid phase [33,34]. Each impedance spectrum is fitted well with the suggested equivalent circuit model and the corresponding resistance parameters are summarized in Table 4. Obviously, the LLMO-850 electrode showed the lowest Rct compared with the others electrodes at 3.0 V and 4.4 V. An exchange current density i0 is inversely proportional to Rct , and a low Rct means a large i0 . Therefore, it can be derived from the above data that optimal calcination temperature could enhance the electrochemical activity of LLMO. 4. Conclusion In summary, a series of Li-rich layered oxides (LLMO) was successfully prepared via a two-step synthesis method involving co-precipitation and high-temperature calcination. The morphology and structures of the synthesized LLMO samples were
characterized by XRD, SEM and TEM to evaluate the effects of calcination temperature on the cathode materials’ structures. The results show that particles size became larger and an amorphous layer on the cathode material surface gradually disappeared with the increase of calcination temperature, indicating that high temperatures may promote formation of more ordered layered structures. Electrochemical experiments revealed that the LLMO850 cathode material delivered a high initial discharge capacity of 266.8 mAh g−1 at a 0.1 C rate and a capacity retention rate of 95.8% after 100 cycles as well as excellent rate capability; the LLMO-900 sample exhibited better cycling stability with a capacity retention rate of 101.5% over the same cycling period. The above-mentioned results indicate that calcination of the cathode material in a range of 850–900 ◦ C can promote the electrochemical performance tremendously, which can be attributed to good morphology, structural stability and the improved surface film resistance in the cathode materials. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 51274075), the National Environmental Technology Special Project (No. 201009028), and Guangdong Province-Department University-Industry Collaboration Project (Grant no. 2012B091100315). References [1] J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2010) 587–603. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243–3262. [3] M. Hu, X. Pang, Z. Zhou, J. Power Sources 237 (2013) 229–242. [4] M.M. Thackeray, S.H. Kang, C.S. Johnson, J.T. Vaughey, S.A. Hackney, Electrochem. Commun. 8 (2006) 1531–1538. [5] H.X. Deng, I. Belharouak, R.E. Cook, H.M. Wu, Y.K. Sun, K. Amine, J. Electrochem. Soc. 157 (2010) A447–A452. [6] G.M. Koenig, I. Belharouak, H.X. Deng, Y.K. Sun, K. Amine, Chem. Mater. 23 (2011) 1954–1963. [7] P. Lanz, H. Sommer, M. Schulz-Dobrick, P. Novak, Electrochim. Acta 93 (2013) 114–119. [8] N. Yabuuchi, K. Yoshii, S.T. Myung, I. Nakai, S. Komaba, J. Am. Chem. Soc. 133 (2011) 4404–4419. [9] M.M. Thackeray, S.H. Kang, C.S. Johnson, J.T. Vaughey, R. Benedek, S.A. Hackney, J. Mater. Chem. 17 (2007) 3112–3125. [10] J.R. Croy, D. Kim, M. Balasubramanian, K. Gallagher, S.H. Kang, M.M. Thackeray, J. Electrochem. Soc. 159 (2012) A781–A790. [11] X. Yu, Y. Lyu, L. Gu, H. Wu, S.M. Bak, Y. Zhou, K. Amine, S.N. Ehrlich, H. Li, K.W. Nam, X.Q. Yang, Adv. Energy Mater. 4 (2014) 1300950. [12] M. Sathiya, K. Ramesha, G. Rousse, D. Foix, D. Gonbeau, A.S. Prakash, M.L. Doublet, K. Hemalatha, J.M. Tarascon, Chem. Mater. 25 (2013) 1121–1131. [13] L. Ban, Y. Yin, W. Zhuang, H. Lu, Z. Wang, S. Lu, Electrochim. Acta 180 (2015) 218–226. [14] J.Q. Zhao, Y. Wang, Nano Energy 2 (2013) 882–889. [15] J. Lu, Q. Peng, W.Y. Wang, J. Am. Chem. Soc. 135 (2013) 1649–1652. [16] J. Cho, Chem. Mater. 12 (2000) 3089–3094.
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Effect of calcination temperature on microstructure and electrochemical performance of lithium-rich layered oxide cathode materials Quanxin Ma, Fangwei Peng, Ruhong Li, Shibo Yin, Changsong Dai ∗ MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 February 2016 Received in revised form 6 April 2016 Accepted 12 April 2016 Available online xxx
a b s t r a c t Lithium-rich layered oxide cathode materials (Li1.2 Mn0.56 Ni0.16 Co0.08 O2 (LLMO)) were synthesized via a two-step synthesis method involving co-precipitation and high-temperature calcination. The effects of calcination temperature on the cathode materials were studied in detail. Structural and morphological characterizations revealed that a well-crystallized layered structure was obtained at a higher calcination temperature. Electrochemical performance evaluation revealed that a cathode material obtained at a calcination temperature of 850 ◦ C delivered a high initial discharge capacity of 266.8 mAh g−1 at a 0.1 C rate and a capacity retention rate of 95.8% after 100 cycles as well as excellent rate capability. Another sample calcinated at 900 ◦ C exhibited good cycling stability. It is concluded that the structural stability and electrochemical performance of Li-rich layered oxide cathode materials were strongly dependent on calcination temperatures. The results suggest that a calcination temperature in a range of 850–900 ◦ C could promote electrochemical performance of this type of cathode materials. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As a power source of portable electronics and electric vehicles, lithium-ion batteries are highly demanded. Cathode materials play a major role in the development of lithium-ion batteries with high performance. The search for high-capacity and safe cathode materials for high-energy-density Li-ion batteries has become one of the most popular subjects in material science in recent years [1–3]. Among all the cathode materials investigated, lithium-ion cathode materials based on layered manganese oxides (LLMO), commonly denoted as xLi2 MnO3 ·(1–x)LiMO2 (M = Co, Ni, Mn, etc.), are the most promising ones because of their low cost and extremely high capacity. This type of materials has been extensively investigated since their first introduction by Lu and Thackeray [4–6]. Their extraordinary capacity is a result of an electrochemical reaction involving transfer of multiple lithium ions and electrochemical activation of Li2 MnO3 . During the activation process, lithium and oxygen ions are simultaneously extracted from the lattice of Li2 MnO3 [7,8] and thus a high capacity over 250 mAh g−1 is released. However, this type of materials suffer from some significant challenges includ-
∗ Corresponding author. Tel.: +86 451 86413751; fax: +86 451 86418616. E-mail address: [email protected] (C. Dai).
ing low initial Coulombic efficiency, long-term cycling instability and poor rate capability [9–11], which impose a great obstacle for the practical application of these layered composite cathode materials. Various approaches, such as partial replacement of nickel and manganese by transition metals [12,13], surface modification [14,15] and optimization of preparation methods [16,17], have been developed to improve the electrochemical properties of these lithium-rich layered oxide cathode materials. Cao et al. [18] synthesized high capacity porous Li[Li0.2 Mn0.534 Ni0.133 Co0.133 ]O2 layered oxides using a novel polymer-thermolysis method, and studied the influence of annealing temperature on electrochemical performance. They concluded that the best electrochemical performance was obtained with a sample synthesized at 850 ◦ C. Chen et al. [19] prepared a variety of Li-rich layered oxide compounds 0.4Li2 MnO3 ·0.6LiNi1/3 Co1/3 Mn1/3 O2 by a coprecipitation method at different temperatures; they also investigated the effects of different annealing temperatures on the structure and electrochemical performance. Their results suggested that samples annealed at 900 ◦ C exhibited the best electrochemical performance. However, the correlation among calcinations temperatures, microstructure and electrochemical performance of Li-rich cathode materials has rarely been thoroughly investigated. This study was designed to explore this area for better strategies to improve the
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electrochemical properties of these lithium-rich layered oxide cathode materials. In this work, a series of Li-rich layered oxide (Li1.2 Mn0.56 Ni0.16 Co0.08 O2 ) samples were prepared at different calcination temperatures via a two-step synthesis method involving co-precipitation. The effects of the calcination temperature on the structural properties and electrochemical performance of Li-rich layered oxide (Li1.2 Mn0.56 Ni0.16 Co0.08 O2 ) cathode materials were thoroughly investigated. The results show that the samples calcined at 850–900 ◦ C performed the best electrochemically. 2. Experimental 2.1. Preparation of Li1.2 Mn0.56 Ni0.16 Co0.08 O2 The Li1.2 Mn0.56 Ni0.16 Co0.08 O2 materials were prepared by a solid-state reaction from lithium hydroxide (LiOH·H2 O) and manganese-nickel-cobalt hydroxide (Mn0.7 Ni0.2 Co0.1 (OH)2 ). Manganese-nickel-cobalt hydroxide was firstly prepared by co-precipitation from an aqueous mixture of MnCl2 ·4H2 O, NiSO4 ·7H2 O, and CoCl2 ·6H2 O (Mn:Ni:Co = 7:2:1, molar ratio; the combined concentration was 2 mol L−1 ) and a mixture of 2.0 mol L−1 NaOH aqueous solutions with a desired amount of NH3 ·H2 O. The solutions were mixed slowly in a nitrogen filled reactor and the pH of the mixed solution was kept in a range of 9.8–10.2 for 12 h during the precipitation process. After the reaction was finished, the precipitated Mn0.7 Ni0.2 Co0.1 (OH)2 particles were filtered, washed using deionized water, and then dried in a vacuum at 120 ◦ C for 24 h. The obtained Mn0.7 Ni0.2 Co0.1 (OH)2 and LiOH·H2 O were then mixed at a molar ratio of 1.00:1.55 by using a ball mill for 5 h with absolute ethyl alcohol, and then the mixture was pressed into pellets. To the mixture 5% excess lithium hydroxide was added to compensate for the lithium evaporation during the calcination process at a high temperature. The pellets were respectively heated at 750, 800, 850, 900, 950 ◦ C for 20 h in air and then quenched to room temperature. Hereafter samples obtained at different calcination temperatures (750, 800, 850, 900, 950 ◦ C) are represented by LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950, respectively.
carbonate (DMC) and diethyl carbonate (DEC) at a volumetric ratio of 1:1:1. Galvanostatic charge and discharge were performed at different current densities in a voltage range of 2.0–4.8 V using an 8-channel battery analyzer (Neware, China). Theoretical capacities of all cathode materials were set to 250 mAh g−1 , i.e., a current density of 250 mA g−1 corresponding to 1.0 C. Electrochemical storage capacities of the working electrodes were calculated based on the mass of active cathode materials. Cyclic voltammograms (CV) of different cathodes were recorded at a scanning rate of 0.1 mV s−1 between 2.0 and 4.8 V using an electrochemical analyzer CHI630b (Chenhua, China). The electrochemical impedance spectroscopy (EIS) investigations were performed on an electrochemical workstation (PARSTAT 2273, AMETEK, USA) with three-electrode system over a frequency range from 100 kHz to 10 mHz after the 3rd cycles in the charged state of 3.0 and 4.4 V. In order to make the test cell reaching a quasi-equilibrium state, the cells were aged for 6 h. To ensure the validity and reliability of the results, in electrochemical measurements, three cells for each sample were test and the average one was selected for discussion. All experiments were carried out at a temperature of 25 ± 0.5 ◦ C.
2.2. Characterizations
3. Results and discussion
The stoichiometric molar compositions of the prepared samples were accurately analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, PerkinElmer Optima 5300DV, PerkinElmer, Waltham, MA, USA). The thermal behavior of Mn0.7 Ni0.2 Co0.1 (OH)2 and LiOH·H2 O mixtures was determined by differential scanning calorimetry/thermal gravimetric analysis (DSC/TGA, TA-SDTQ600, USA) conducted on the dried material at a heating rate of 5 ◦ C min−1 from room temperature to 1000 ◦ C in air. The morphologies of the as-prepared samples were observed with a scanning electron microscope (SEM, HITACHI, S-4700). The crystal structures of the synthesized samples were determined by X-ray diffraction (XRD) using a D/max-␥ X’ pert diffractometer (Rigaku, Japan) with Cu K␣ radiation. The microstructural characteristics of the synthesized samples were observed using a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2010) working at an accelerating voltage of 200 kV.
Fig. 1. The DSC–TGA curves of Mn0.7 Ni0.2 Co0.1 (OH)2 and LiOH·H2 O mixtures.
Chemical compositions of the cathode materials determined by ICP analysis are listed in Table 1. The obtained results are close to their nominal compositions within experimental errors.
2.3. Electrochemical evaluation Two-electrode CR2032-type coin cells were assembled with LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 as the cathodes, respectively, metallic lithium foil as the anode and Celgard-2320 membrane as the separator. The electrolyte used comprised 1 M LiPF6 dissolved in ethylene carbonate (EC), dimethyl
Fig. 2. XRD patterns of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO950 samples.
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The compositions of these cathode materials can be described with a notation of Li1.5 Mn0.7 Ni0.2 Co0.1 O2.5 , which is equivalent to Li1.2 Mn0.56 Ni0.16 Co0.08 O2 after the conventional normalization to two oxygen atoms. Reduction of lithium content was a result of the lithium evaporation at high temperatures. Thermochemical properties of the Mn0.7 Ni0.2 Co0.1 (OH)2 and LiOH·H2 O mixtures investigated by differential scanning
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calorimetry/thermal gravimetric analysis (DSC/TGA) are shown in Fig. 1. The weight loss in a range of 20–305 ◦ C was about 4%, which is attributed to water desorption. A slight weight increase (0.4%) in a range of 305–342 ◦ C was observed which was caused by the precursor oxidation. Another weight loss of 8% occurred in a range of 342–550 ◦ C, which was related to the decomposition of hydroxide and precursor. However, slow weight compensation
Fig. 3. SEM images of the LLMO-750 (a, a ), LLMO-800 (b, b ), LLMO-850 (c, c ), LLMO-900 (d, d ) and LLMO-950 (e, e ) samples at different magnifications.
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Table 1 The chemical composition results of ICP-OES analysis for LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples. Samples
LLMO-750 LLMO-800 LLMO-850 LLMO-900 LLMO-950
ICP analyses (atom ratios) Li
Co
Mn
Ni
1.509 1.506 1.502 1.498 1.495
0.098 0.096 0.096 0.097 0.096
0.695 0.697 0.696 0.696 0.696
0.207 0.207 0.208 0.207 0.208
was observable above 700 ◦ C, which may be due to the integration of oxygen into the crystal lattice. From the DSC curve, an endothermic peak can be seen at 456 ◦ C, which was related to LiOH melting [20]. Furthermore, another endothermic peak at 698 ◦ C was probably due to the completion of the hexagonal lattice ordering. This reasoning is consistent with the proposal by Zhu et al., who suggested that synthesis of Li[Ni1/3 Co1/3 Mn1/3 ]O2 at temperatures higher than 700 ◦ C produced an ordered layered structure [21], and with the conclusion by Jouybari et al., who reported a required calcination temperature of 700 ◦ C during their synthesis of LiNi0.8 Co0.2 O2 [22]. These results indicate that the calcination temperature for synthesis of Li1.2 Mn0.56 Ni0.16 Co0.08 O2 should be above 700 ◦ C. The XRD patterns of samples LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 are shown in Fig. 2. The main diffraction peaks could be indexed with the ␣-NaFeO2 type structure (space group R-3m). With increase in the calcination temperature, the intensities of all the peaks increased and the peaks became sharper reflecting the increasing crystallinity [23]. Distinct splitting (006)/(102) and (108)/(110) reflections can be seen in the XRD patterns, which indicates well-defined layered structures for all the samples [24]. The intensity ratio of I(003) /I(104) is inversely proportional to the degree of cation mixing of a layered structure [25,26]. When the calcination temperature was higher than 800 ◦ C, a high ratio of I(003) /I(104) (larger than 1.8) implies the lower cation mixing
Table 2 The crystallographic parameters of LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples. Samples
a (Å)
c (Å)
c/a
V (Å3 )
I(003) /I(104)
LLMO-750 LLMO-800 LLMO-850 LLMO-900 LLMO-950
2.828 2.847 2.846 2.850 2.831
14.008 14.185 14.190 14.225 14.128
4.954 4.983 4.987 4.992 4.990
96.99 99.19 99.50 100.04 98.07
1.793 1.804 1.828 1.820 1.816
and a more ordered layer structure (Table 2). In addition, according to Ngala et al. [27], an ideal cubic close-packed lattice has a c/a ratio of 4.899, where a larger c/a ratio corresponds to a more ordered layered structure. For all of the synthesized samples in this work, the c/a ratios were larger than 4.954, indicating the wellordered layered structures of all those samples. These results prove that a higher calcination temperature promotes the formation of a well-defined layered structure. Morphology and particle size of cathode materials are of great importance to battery performance. The SEM images of the LLMO750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples at different magnifications are shown in Fig. 3. It can be found that samples LLMO-750 (Fig. 3a, a ) and LLMO-800 (Fig. 3b, b ) were composed of irregular agglomerates with an average size between 200 and 300 nm. On contrast, particles of the LLMO-850 sample (Fig. 3c, c ) were well crystallized with no obvious aggregation observed and the estimated average particle size was ca. 500 nm. With the calcination temperature increasing, significant crystal growth between 900 ◦ C (Fig. 3d, d ) and 950 ◦ C (Fig. 3e, e ) was observed. The particles were more uniformly distributed and the particle size increased suddenly. With further increase in calcination temperature, the crystals grew larger. The crystal size was up to ca. 600 nm at 900 ◦ C and eventually it increased to ca.1000 nm at 950 ◦ C. To thoroughly study the effects of calcination temperature on the bulk and surface structures of the layered Li1.2 Mn0.56 Ni0.16 Co0.08 O2 cathode materials, three representative samples, LLMO-750 (Fig. 4a–d), LLMO-850 (Fig. 4e–h) and
Fig. 4. TEM, HRTEM images and EDS spectra of the samples LLMO-750 (a–d), LLMO-850 (e–h) and LLMO-950 (i–l).
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LLMO-950 (Fig. 4i–l) were investigated by using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS). The TEM images in Fig. 4(a, e and i) confirm that the particle size increased with the increase of calcination temperature. The LLMO850 sample was composed of uniformly distributed particles with an average size of 400–500 nm, while LLMO-750 sample showed some agglomeration. The HRTEM images in Fig. 4(c, g and k) show that an amorphous layer was formed on the surface of the LLMO750 and LLMO-850 samples, which may serve as a barrier layer between the bulk structure and the electrolyte. As the calcination temperature increased to 850 ◦ C, the amorphous layer became thinner (Fig. 4g). The LLMO-950 sample showed a smooth edge and there was no other layer on the surface (Fig. 4k), indicating that high temperature may help the formation of a better crystal lattice. Fig. 4(b, f and j) also shows that the amorphous layer gradually disappeared as the calcination temperature increased. The EDS spectra of the samples in Fig. 4(d, h and l) show that the nature of the amorphous layer might be lithium carbonate. The well-defined edges of the particles are in agreement with their high crystallinity that is reflected by the XRD results. The above-mentioned results reveal that the structure of Li-rich layered oxides is strongly related with the calcination temperatures. Fig. 5 shows the cyclic voltammograms (CV) of the LLMO-750, LLMO-850 and LLMO-950 samples in a potential range of 2.0–4.8 V at a scan rate of 0.1 mV s−1 for the first 3 cycles. As shown in Fig. 5, two oxidation peaks were observed at ca. 4.3 and ca. 4.7 V in the first charge process, which correspond to the reversible lithium intercalation/deintercalation in Li1.2 Mn0.56 Ni0.16 Co0.08 O2 and the electrochemical activation of Li2 MnO3 . A reduction peak at ca. 3.75 V was observed in the initial discharge process, which corresponds to the reduction of Ni4+ and Co4+ [9]. The potential difference between oxidation peaks and reduction peaks in the first charge process was increased from 0.5 to 0.69 V with the calcination temperature increase, indicating that the initial reversibility was reduced. Moreover, a new reduction peak at ca. 3.2 V appeared in the 2nd and 3rd cycles, suggesting the phase transformation from a layered one to a spinel one during the subsequent cycling process [28], which could lead to a layered-spinel intergrowth structure and a redox reaction at ca. 3.2 V [29]. In contrast to the initial charge process, the CV curve exhibits a large broad anode peaks in a voltage range of 3.4–4.3 V in the subsequent cycling process. From Fig. 5b it can be seen that the broad anode peaks became narrower and the intensity of peaks increased, which indicate the enhancement in reversibility of the electrode reaction as a result of easier insertion and deinsertion of lithium ions. The anode peaks disappeared subsequently in the LLMO-950 sample (Fig. 5c), indicating the activation of materials becoming more difficult with the calcination temperature increase as a result of more difficult insertion and deinsertion of lithium ions. The electrochemical performance of all the samples was evaluated using coin-cells with each one of the layered Li1.2 Mn0.56 Ni0.16 Co0.08 O2 materials calcinated at different temperatures as the cathode and metallic lithium as the anode to further study the effects of calcination temperature on their electrochemical performance. Fig. 6(a) shows the initial charge/discharge curves of all the samples at different calcination temperatures during cycling at a 0.1 C rate between 2.0 and 4.8 V and the pertinent electrochemical properties are tabulated in Table 3. It can be seen that all the samples exhibited similar charge–discharge characteristics: a sloping voltage profile under 4.5 V followed by a high-voltage plateau at 4.5–4.8 V during charging. The sloping region below 4.5 V arises from the oxidation of Ni2+ to Ni4+ ions and Co3+ to Co4+ , respectively, while the long plateau from 4.5 to 4.8 V can be attributed to the simultaneous release of lithium ions and oxygen according to the previous studies by different groups
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Fig. 5. Cyclic voltammograms (CV) of the Li1.2 Mn0.56 Ni0.16 Co0.08 O2 scanned at 0.1 mV s−1 : (a) LLMO-750, (b) LLMO-850 and (c) LLMO-950.
Table 3 Initial charge/discharge data of LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples. Samples
Charge capacity (mAh g−1 )
Discharge capacity (mAh g−1 )
Coulombic efficiency (%)
LLMO-750 LLMO-800 LLMO-850 LLMO-900 LLMO-950
404.2 399.3 392.4 364.8 340.6
254.8 257.9 268.8 262.7 258.1
63.0 64.6 68.5 72.0 75.8
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Fig. 6. (a) The charge/discharge and (b) dQ/dV curves of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 at 0.1 C (25 mA g−1 ) between 2.0 and 4.8 V.
Fig. 7. (a) Discharge capacities and (b) the mid-point voltage (MPV) profiles as a function of cycle numbers of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 at 0.1 C (25 mA g−1 ) between 2.0 and 4.8 V.
[30,31]. As shown in Fig. 6(a), the initial charge capacity gradually decreased from 404.2 to 340.6 mAh g−1 as the calcination temperature increased. The initial discharge capacity were 254.8, 257.9, 268.8, 262.7, and 258.1 mAh g−1 for the samples calcinated at 750, 800, 850, 900 and 950 ◦ C. The LLMO-850 sample delivered the highest discharge capacity (ca. 268.8 mAh g−1 ) among all the samples. The initial coulombic efficiency of all sample increased as the calcination temperature increased. The corresponding dQ/dV curves of all the samples are shown in Fig. 6(b). The oxidation peaks at ca. 4.0 V during the initial charge process can be attributed to the oxidation of Ni2+ and Co3+ to Ni4+ and Co4+ ions in the layered component, respectively. The intensity of the oxidation peaks at ca. 4.6 V decreased as the calcination temperature increased, indicating that the content of the Li2 MnO3 component, which can be activated in the first cycle, probably decreased as the calcination temperature increased. In addition, a reduction peaks at ca. 3.8 V corresponding to the reduction of Ni4+ and Co4+ were observed in the initial discharge process. In the meantime, the intensity of the reduction peaks at ca. 3.3 V became lower as the calcination temperature increased, suggesting that the activation product MnO2 was reduced during the initial charge process. In order to evaluate the cycling stability of the cathode materials, the cells were cycled at a rate of 0.1 C and the discharge capacities of all the samples are illustrated in Fig. 7(a). Although the LLMO-850 delivered a higher initial discharged capacity than LLMO-900, LLMO-900 exhibited the best cycling stability with a capacity retention rate of 101.5%. LLMO-850 had a capacity retention rate of 95.8% over the same cycling period. According to Li et al. [32], the improvement in capacity with increasing calcination temperatures may be attributed to the improvement in the crystallization and cation-ordering of the cathode materials. To get a more intuitive idea of the voltage evolution, the midpoint voltage (MPV) profiles of these materials are plotted in Fig. 7(b). It can be seen that all these materials showed similar
mid-point voltages in the initial cycle. However, the MPV retention of these lithium-rich materials (Li1.2 Mn0.56 Ni0.16 Co0.08 O2 ) increased as the calcination temperature increased (ca. 84.5% for the LLMO-750, ca. 87.0% for the LLMO-800, ca. 87.4% for the LLMO850, ca. 87.7% for the LLMO-900 and ca. 89.8% for the LLMO-950 after 100 cycles), illustrating again the benefits of high calcination temperature in stabilizing the structure and suppressing the voltage decay of the lithium-rich cathode materials. In addition, the difference in mid-point voltages among materials LLMO-850, LLMO-900 and LLMO-950 was very small, suggesting that there is no need to raise the temperature to sustain the voltage. Fig. 8 gives the rate capability of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 samples under galvanostatic
Fig. 8. Rate capability performance of the LLMO-750, LLMO-800, LLMO-850, LLMO900 and LLMO-950 samples under galvanostatic charge at a rate of 0.1 C and discharge at different rates from 0.1 C to 2 C (1.0 C = 250 mA g−1 ).
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Fig. 9. Nyquist plots of the LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 electrodes (a) 3.0 V and (b) 4.4 V after the 3rd cycles (inset shows the equivalent circuit).
Table 4 The impedance parameters of LLMO-750, LLMO-800, LLMO-850, LLMO-900 and LLMO-950 electrodes at different potential after 3rd cycles. Samples
LLMO-750 LLMO-800 LLMO-850 LLMO-900 LLMO-950
3.0 V
4.4 V
Rs ()
Rct ()
Rs ()
Rct ()
2.823 2.572 2.348 2.390 4.418
926.2 482.6 161.4 208.7 245.2
3.333 4.651 4.670 4.034 2.525
415.9 241.5 65.8 117.4 138.8
charge at a rate of 0.1 C and discharge at different rates from 0.1 C to 2 C (1.0 C = 250 mA g−1 ) for every five cycles. From Fig. 8, it can be seen that LLMO-750 and LLMO-800 showed lower rate capabilities than other samples duo to the formation of agglomerates and existence of an amorphous layer on the surface. LLMO-850 showed the best rate capability with a discharge capacity of 182 mAh g−1 at 2 C. The rate performance enhancement can be ascribed to a better crystal structure and small particles with high specific surface areas, which can facilitate faster diffusion of the lithium ions between the active particles and electrolyte [23]. With the calcination temperature increased, the rate capabilities of LLMO-900 and LLMO-950 decreased, which is probably due to the formation of larger particles at a higher temperature. Larger particles can cause prolonged lithium diffusion pathways during charging and discharging. Electrochemical impedance spectroscopy (EIS) has been performed to get insights into the electrochemical performance of the Li1.2 Mn0.56 Ni0.16 Co0.08 O2 materials. All Nyquist plots at 3.0 V and 4.4 V after the 3rd cycle are shown in Fig. 9 and the corresponding equivalent circuits are presented in the inset. In this equivalent circuit, Rs represents the resistance of the electrolyte and cell components, Rct corresponds to charge transfer resistance in the electrode–electrolyte interfaces, and Zw refers to the diffusion impedance of lithium ions in the solid phase [33,34]. Each impedance spectrum is fitted well with the suggested equivalent circuit model and the corresponding resistance parameters are summarized in Table 4. Obviously, the LLMO-850 electrode showed the lowest Rct compared with the others electrodes at 3.0 V and 4.4 V. An exchange current density i0 is inversely proportional to Rct , and a low Rct means a large i0 . Therefore, it can be derived from the above data that optimal calcination temperature could enhance the electrochemical activity of LLMO. 4. Conclusion In summary, a series of Li-rich layered oxides (LLMO) was successfully prepared via a two-step synthesis method involving co-precipitation and high-temperature calcination. The morphology and structures of the synthesized LLMO samples were
characterized by XRD, SEM and TEM to evaluate the effects of calcination temperature on the cathode materials’ structures. The results show that particles size became larger and an amorphous layer on the cathode material surface gradually disappeared with the increase of calcination temperature, indicating that high temperatures may promote formation of more ordered layered structures. Electrochemical experiments revealed that the LLMO850 cathode material delivered a high initial discharge capacity of 266.8 mAh g−1 at a 0.1 C rate and a capacity retention rate of 95.8% after 100 cycles as well as excellent rate capability; the LLMO-900 sample exhibited better cycling stability with a capacity retention rate of 101.5% over the same cycling period. The above-mentioned results indicate that calcination of the cathode material in a range of 850–900 ◦ C can promote the electrochemical performance tremendously, which can be attributed to good morphology, structural stability and the improved surface film resistance in the cathode materials. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 51274075), the National Environmental Technology Special Project (No. 201009028), and Guangdong Province-Department University-Industry Collaboration Project (Grant no. 2012B091100315). References [1] J.B. Goodenough, Y. Kim, Chem. Mater. 22 (2010) 587–603. [2] V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Energy Environ. Sci. 4 (2011) 3243–3262. [3] M. Hu, X. Pang, Z. Zhou, J. Power Sources 237 (2013) 229–242. [4] M.M. Thackeray, S.H. Kang, C.S. Johnson, J.T. Vaughey, S.A. Hackney, Electrochem. Commun. 8 (2006) 1531–1538. [5] H.X. Deng, I. Belharouak, R.E. Cook, H.M. Wu, Y.K. Sun, K. Amine, J. Electrochem. Soc. 157 (2010) A447–A452. [6] G.M. Koenig, I. Belharouak, H.X. Deng, Y.K. Sun, K. Amine, Chem. Mater. 23 (2011) 1954–1963. [7] P. Lanz, H. Sommer, M. Schulz-Dobrick, P. Novak, Electrochim. Acta 93 (2013) 114–119. [8] N. Yabuuchi, K. Yoshii, S.T. Myung, I. Nakai, S. Komaba, J. Am. Chem. Soc. 133 (2011) 4404–4419. [9] M.M. Thackeray, S.H. Kang, C.S. Johnson, J.T. Vaughey, R. Benedek, S.A. Hackney, J. Mater. Chem. 17 (2007) 3112–3125. [10] J.R. Croy, D. Kim, M. Balasubramanian, K. Gallagher, S.H. Kang, M.M. Thackeray, J. Electrochem. Soc. 159 (2012) A781–A790. [11] X. Yu, Y. Lyu, L. Gu, H. Wu, S.M. Bak, Y. Zhou, K. Amine, S.N. Ehrlich, H. Li, K.W. Nam, X.Q. Yang, Adv. Energy Mater. 4 (2014) 1300950. [12] M. Sathiya, K. Ramesha, G. Rousse, D. Foix, D. Gonbeau, A.S. Prakash, M.L. Doublet, K. Hemalatha, J.M. Tarascon, Chem. Mater. 25 (2013) 1121–1131. [13] L. Ban, Y. Yin, W. Zhuang, H. Lu, Z. Wang, S. Lu, Electrochim. Acta 180 (2015) 218–226. [14] J.Q. Zhao, Y. Wang, Nano Energy 2 (2013) 882–889. [15] J. Lu, Q. Peng, W.Y. Wang, J. Am. Chem. Soc. 135 (2013) 1649–1652. [16] J. Cho, Chem. Mater. 12 (2000) 3089–3094.
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