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Crystals 2017, 7(9), 275; doi:10.3390/cryst7090275

Article
The Effects of a Mixed Precipitant on the Morphology and Electrochemical Performance of LiNi0.5Mn1.5O4 Cathode Materials
Yang Shu 1,2,, Wenchao Yan 2,, Haisong Wang 1,*, Jicheng Jiang 2, Deye Sun 2, Xiaodi Ma 2 and Yongcheng Jin 2,*Orcid
1
School of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China
2
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China
*
Correspondence: Tel.: +86-411-8632-3272 (H.W.); +86-532-8066-2703 (Y.J.)
These authors contributed equally to this work.
Academic Editors: Lan Xiang, Jing Wang, Huijun Wu, Guo Gao, Yongcheng Jin and Yi Xia
Received: 2 August 2017 / Accepted: 8 September 2017 / Published: 14 September 2017

Abstract

:
A series of LiNi0.5Mn1.5O4 (LNMO) samples were synthesized by adjusting the molar ratio of (NH4)2CO3 to Na2CO3 in a mixed precipitant for evaluating the effects of ammonia from (NH4)2CO3 as a complexing agent and Na2CO3 as a precipitant on the morphology and electrochemical performances of LNMO. In this research, a rapid precipitation method followed by hydrothermal treatment was used to prepare the precursors of LNMO, and different molar ratios (0:1, 1:2, 1:1, 2:1, 1:0) of (NH4)2CO3 to Na2CO3 were used for mixed precipitants. The test results revealed that the cathode material exhibits the best electrochemical performance when the molar ratio of (NH4)2CO3 to Na2CO3 is set at 1:2, displaying a specific discharge capacity of 129.4 mA h g−1 at 0.5 C and a capacity retention of 82.3% after 200 charge–discharge cycles. In addition, it still shows a high rate performance with a discharge capacity of 112.7 mA h g−1 at 10 C and 98.8 mA h g−1 at 20 C, which is attributed to an accurate Ni/Mn ratio, smaller primary particle sizes and a porous spherical morphology.
Keywords:
rapid precipitation; hydrothermal treatment; mixed precipitants; spherical morphology

1. Introduction

Recently, the development of energy storage technology has played a key role in boosting the popularization of the hybrid electric vehicle (HEV) and electric vehicle (EV). As an excellent energy storage system, the lithium ion battery has increasingly aroused attention due to its high energy density, long cycle life, and environmental friendliness. However, there are still some problems, such as short capacity fading, high costs, and potential safety hazards during the course of practical application [1,2,3,4,5]. It has also been considered that the improvement of energy/power density of lithium ion batteries has been a significant theme, and the development of high performance cathode materials has been the main approach for resolving it [5]. As a promising cathode material for lithium ion batteries, spinel LiNi0.5Mn1.5O4 (LNMO) has caused attracted an increasing amount of attention due to its high discharge plateau around 4.7 V, its low cost, and its high energy density (630 Wh kg−1) [6,7]. Therefore, it is a competitive candidate to be applied in electric vehicles (EVs) and hybrid electric vehicles (HEVs) [8].
Various methods such as solid state [9,10], sol-gel [11,12,13,14,15,16], co-precipitation [17], and hydrothermal [18] have been increasingly reported to synthesize LNMO with different morphologies. The solid state method is the earliest and simplest way to obtain the cathode material. However, it is hard to regulate the morphology and to obtain a pure spinel phase. The sol-gel or co-precipitation method is also a common route used by researchers to prepare cathode materials with excellent electrochemical properties, which is ascribed to a feasible measure with which the materials’ morphology and stoichiometry provided by the two methods can be accurately controlled. Unfortunately, the relatively tedious steps and long time also decrease their glamour [19,20,21]. Specially, as a simple operation, the hydrothermal method can not only regulate the morphology but also guarantee the accurate stoichiometry of LNMO cathode materials [22,23].
It is usually considered that the excellent cycling behavior of cathode materials is partially attributed to the small surface area of spherical particles, which could weaken the unexpected side reaction during the electrochemical process. Meanwhile, what leads to an excellent rate performance is the uniform electrode–electrolyte contact for Li ions fast transfer across the interface, which is more significant in spherical materials with a porous configuration [24,25]. In regulating the morphology and electrochemical performance of LNMO cathode materials, the traditional co-precipitation method is usually performed by adding the NH3·H2O or ammonium carbonate solution drop by drop to control the spherical morphology and pH value under long-term continuous stirring [6,7,26,27], and the hydrothermal treatment is carried out by adjusting the precipitants and hydrothermal parameters. Cheng et al. synthesized LNMO cathode material with sphere-like morphology under hydrothermal conditions when Na2CO3 was used as a precipitant [18]. However, it showed poor electrochemical performance. Uniform LNMO microspheres were successfully prepared by Zhu et al., with NH4HCO3 as a precipitant using the hydrothermal method [22]. The excellent electrochemical performance fully displayed the advantage of spherical LNMO. However, the large amount of consumption and long-term dripping process of NH4HCO3 also needs further optimization.
Herein, a rapid precipitation method followed by a hydrothermal treatment was used to prepare the precursors of LNMO. The precursor samples were synthesized by adjusting the molar ratio of (NH4)2CO3 to Na2CO3 in a mixed precipitant for evaluating the effects of ammonia from (NH4)2CO3 as a complexing agent and Na2CO3 as a precipitant on the morphology and electrochemical performances of LNMO. After the hydrothermal process, the spherical morphology of Ni0.25Mn0.75CO3 precursor was obtained. Then, through the calcination of precursors with a lithium source, the LNMO samples were synthesized.

2. Experimental

2.1. The Synthesis of Ni0.25Mn0.75CO3 Precursors

A rapid precipitation method followed by a hydrothermal treatment is applied to synthesize the spherical Ni0.25Mn0.75CO3 precursor. Firstly, 2.25 mmol NiSO4·6H2O (AR, Sinopharm Chemical Reagent Co, Shanghai, China) and 6.75 mmol MnSO4·H2O (AR, Sinopharm Chemical Reagent Co., Shanghai, China) were ultrasonically dissolved in 25 mL of deionized water followed by the addition of 12.5 mL of ethanol to promote the dispersion of transition metal ions. Meanwhile, (NH4)2CO3 (Aladdin, Los Angeles, CA, USA) and Na2CO3 (AR, Sinopharm Chemical Reagent Co., Shanghai, China) with different molar ratios (0:1, 1:2, 1:1, 2:1, and 1:0) were also ultrasonic dissolved in respectively assigned deionized water (a total of 25 mL) according to the molar ratio of mixed precipitants. The molar ratio between total carbonate and total transition metal ions was set at 1.2:1 to guarantee the complete precipitation of transition metal ions. Secondly, the (NH4)2CO3 solution was poured into the metal salt solution followed by the same operation with an Na2CO3 solution to generate massive sediments. The suspension was directly transferred into 100 mL of PVDF-lined noncorrosive steel and treated at 200 °C for 12 h in the oven. Finally, we obtained the spherical Ni0.25Mn0.75CO3 precursor after it was washed several times and dried at 60 °C overnight.

2.2. The Synthesis of LNMO Cathode Materials

The dried precursor was ground with 5% excess Li2CO3 (Aladdin, Los Angeles, CA, USA) for 1 h. Subsequently, the mixture was transferred to a muffle furnace and calcined at 550 °C for 3 h, 800 °C for 10 h in air (according to the TGA, as shown in Figure S1) at a heating rate of 2 °C/min. The final product, which was named after the molar ratio of (NH4)2CO3 to Na2CO3 (S01, S12, S11, S21, and S10 corresponding to 0:1, 1:2, 1:1, 2:1, and 1:0), could be obtained after it was naturally cooled down to room temperature. The specific experimental process is diagrammatically shown in Figure 1; and more details could be found elsewhere [23,28].

2.3. Materials Characterizations

The phase analysis of as-prepared LNMO cathode material was performed via X-ray powder diffraction (XRD, Microdiffractometer D8 Advance, BrukerAXS, Karlsruhe, Baden-Wurttemberg, Germany). The morphologies and particle sizes were observed with a field emission scanning electron microscope (FESEM, S-4800, Hitachi, Blackwood, NJ, USA). The components of as-prepared materials were confirmed via inductive coupled plasma atomic emission spectrometry (ICP-MS, iCAP-Qc, Thermo Fisher Scientific, Waltham, MA, USA). The Brunauer–Emmett–Teller (BET) surface areas of the materials were determined via a physisorption and chemisorption analyzer (ASAP 2020-M+C, Micromeritics Instrument Corp, Atlanta, GA, USA). The microstructure, the chemical states of Ni and Mn elements, and the thermal gravimetric of the sample with the best electrochemical performance were respectively characterized with transmission electron microscopy (TEM, Tecnai G2 F20, FEI, Hillsboro, OR, USA), X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA), and thermogravimetric analysis (TGA, STA449F3, Netzsch, Selb, Bavaria, Germany), respectively. When conducting the test of TEM, the preparation of the TEM specimen was as follows: firstly, the sample of LNMO was ultrasonically dispersed with ethanol for 40 min; secondly, the sample was filtered onto a copper screen; lastly, after drying, it was tested. XPS was performed under the following conditions: chamber pressure: 8 × 10−8 Pa; take-off angle: 90°; calibration: according to the standard C 1s spectrum with a binding energy of 284.6 eV, the test value of 283.48 eV for C 1 s spectrum was calibrated. The TGA was carried out from 35 to 900 °C at a heating rate of 10 °C/min. The structural parameters and crystallite size were calculated with JADE 6.5 software, which was based on the XRD data.

2.4. Electrochemical Measurements

The electrochemical performance test was carried out by CR-2032-type coin cells including a positive electrode, a Celgard 2500 membrane, an electrolyte, and a lithium negative electrode, which were assembled in an argon-filled glove box. The positive electrode was comprised of 80% as-prepared cathode materials, 10% carbon black (Super P), and 10% polyvinylidene fluoride (PVDF), which was homogeneously mixed by a proper amount N-methyl-2-pyrrolidon (NMP) to form the cathode slurry. Then, the obtained slurry was coated on Al foil and vacuum-dried at 120 °C overnight. The electrolyte was 1M LiPF6 dissolved in EC/DMC (1:1, volume ratio). Galvanostatic charge–discharge tests were carried out between 3.0 and 4.9 V by LAND-2010 automatic battery testers at 25 °C.

3. Results and Discussion

Figure 2 shows the XRD patterns of as-prepared LNMO materials. All the peaks can be assigned to a well-crystallized typical cubic spinel structure with a space group of Fd3m (JCPDS Card No. 80-2162). It is noteworthy that the additional minor XRD reflections at 2θ = 37.4°, 43.5°, and 63.2° can be observed on S01, which is assigned to small amounts of a rock salt phase [29]. We can observe from the pattern that the integrated intensity of S12, which is lower than those of S21 and S10, higher than that of S11, and similar to that of S01, reveals a medium crystallinity among the five samples. Meanwhile, the structural parameters and crystallite sizes are shown in Table 1. The S12 exhibits a lattice parameter of 8.1723 Å and a crystallite size of 256.8 nm, corresponding to the medium crystallinity, which may result in superior electrochemical performances.
Figure 3 presents the SEM images of Ni0.25Mn0.75CO3 precursors prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios. We can observe that the primary particles accumulate increasingly close, and the surface of spheres is increasingly smooth from S01 to S10 at a high resolution (as shown in Figure S2). According to one of SEM images of precursors (as shown in Figure S3), we can find that there was no obvious different in the morphology among the precursors before and after hydrothermal treatment. Additionally, what the images clearly display at low resolution is that the morphologies of the spheres become increasingly complete and uniform-sized. When the molar ratio of (NH4)2CO3 to Na2CO3 was set at 0:1, it can be clearly observed that the precursor shows a high degree of fragmentation. The precursor of S12 and the other three precursors all show complete spherical morphologies with a trend of increasingly uniform primary particles. This interesting phenomenon might be attributed to the complexation of ammonia for (NH4)2CO3 with nickel ions during the hydrothermal process, which can contribute to the uniform precipitation with spherical morphology. The complexation of ammonia with Ni and Mn also slow down the precipitation process and simultaneously favor the dissolution process during hydrothermal treatment when the content of ammonia increases, and therefore increase the compactness of the primary particles, which can influence the morphology and particle size during the calcination process. However, the increased concentration of ammonia will seriously hinder the completed deposition of nickel ions, and lead to a deficiency of nickel [7]. To further confirm the deficiency of nickel, ICP-MS measurement was carried out, and the detailed results regarding the content of nickel and manganese are presented in Table 2, which shows that the Ni/Mn ratio presents a decreasing trend from S01 to S10. The S12 delivered an Ni/Mn ratio of 1.01:3, which is closest to the theoretical value, while the ratio of 0.9:3 for 1:0 reveals that about 10% of nickel ions were lost in the solution.
Figure 4 indicates the SEM images of as-prepared LNMO materials. The low-resolution images indicate that the spherical morphologies of as-prepared samples are basically well maintained and showed a trend that is consistent with the precursors. What we can further observe from the high-resolution images is that the primary sizes of S12 are obviously larger than those of S11 while smaller than those of S21 and S10. In addition, the samples of S01 and S12 show the loose accumulation between primary particles with small pores distributed on the surfaces of microspheres, which may contribute to the improvement of rate performances. Nevertheless, the opposite status, the compact accumulation between particles without pores observed on the surfaces of the microspheres, is displayed by the other samples. This observation result agrees with the data of the BET surface areas, which is enumerated in Table 3. It can be evidently observed from Table 3 that a decreasing trend from S01 (4.3612 m2 g−1) to S10 (2.8820 m2 g−1) is regularly presented.
To further analyze the microstructure and surface chemical states of S12, TEM (Figure 5a), HRTEM (Figure 5b), and XPS (Figure 6) were carried out. Figure 5a presents a typical spherical structure, which corresponds to the SEM image (Figure 4b1). The high-resolution TEM image (Figure 5b) shows obvious lattice fringes (0.47 nm), which matches the (111) plane of spinel LNMO.
Figure 6a shows the XPS spectra of Mn 2p, revealing that the binding energies of 642.1 eV and 642.9 eV, which respectively correspond to Mn3+ and Mn4+, are assigned to the major peak of Mn 2p3/2, near 641.9 eV for Mn3+ and 642.9 eV for Mn4+, which have been reported previously [21]. The coexistence of Mn4+ and Mn3+ observed from the XPS spectra of Mn 2p is well consistent with the discharge capacity curve in Figure 7a. As Figure 6b shows, the major peak assigned to Ni 2p3/2 binding energy is located at 855.1 eV, whose value is close to those reported for LNMO (854.8 eV) [30].
As shown in Figure 7, different electrochemical performances of as-prepared LNMO cathode materials were measured by galvanostatic charge–discharge. According to Figure 7a, the as-prepared cathode materials all exhibit a typical discharge capacity curve of Fd3m-type spinel LNMO with a dominate discharge plateau around 4.7 V attributed to Ni2+/Ni4+ redox couple and a weak plateau around 4 V corresponds to the Mn3+/Mn4+ redox couple [31]. It is worth noticing that some samples show a sloping area below ~3.5 V, which could be an indication of the existence of a layered phase [32]. The highest specific discharge capacity of five samples are 110.7, 129.4, 126.9, 120.3, and 114.7 mAh g−1 with a charge/discharge at 0.5 C, which displays the superiority of S12 in comparison to the other samples. It may be due to the awful spherical morphology that S01 shows the lowest discharge capacity, while the discharge capacity of S11, S21, and S10 present a decreasing trend, which could be ascribed to the deficiency of nickel.
We can observe from Figure 7b that the discharge capacity retention of five samples respectively remain 73.4%, 82.3%, 81.7%, 81.8%, and 93.9% after 200 charge–discharge cycles at 0.5 C. S12 shows favorable cycling stability and S10 exhibits the best result, while S01 shows the worst cycling stability among all five LNMO cathode materials, which may also be ascribed to the broken spherical morphology. The LNMO cathode material with a relatively complete spherical morphology and higher crystallinity can exhibit better cycling stability, which could be attributed to the smaller contact area of the electrode–electrolyte. The small specific surface area will contribute to weakening the unexpected side reaction during the electrochemical process and mitigate the dissolution of transition metal ions [33]. Moreover, an increasing discharge capacity around the first 10 cycles can be observed from samples except S01, which indicates that a longer wetting and electrochemical activation process is necessary for micron-sized spherical particles [34].
The rate performances of LNMO cathode materials from 0.5 to 20 C (the charge rate is fixed at 0.5 C) are shown in Figure 7c. Under high current density, the discharge capacities of S12 still are 112.7 mAh g−1 at 10 C and 98.8 mAh g−1 at 20 C, which displays the best rate performance in comparison to other samples. It indicates that LNMO cathode materials with a porous spherical morphology and smaller primary particle sizes could contribute to the infiltration of the electrolyte and the increase in the contact area between the electrode and electrolyte [35], which provides a relatively short transport length for Li ions. The results of all electrochemical performance tests fully demonstrate that S12 is the most appropriate molar ratio of (NH4)2CO3 to Na2CO3 for synthesizing LNMO cathode material, corresponding to the highest discharge capacity, favorable cycling stability, and the best rate performance.

4. Conclusions

In general, the LNMO with spherical morphology has been successfully synthesized by a rapid precipitation and hydrothermal method with (NH4)2CO3 and Na2CO3 as mixed precipitants followed by a high-temperature calcination with a lithium source. The spherical morphology and crystallinity of the as-prepared samples can be greatly influenced by the molar ratio of (NH4)2CO3 to Na2CO3. The LNMO cathode material prepared with 1:2 for (NH4)2CO3 to Na2CO3 displayed an accurate Ni/Mn ratio, smaller primary particle sizes, and a porous spherical morphology, and delivered the highest discharge capacity, favorable cycling stability, and the best rate performance, corresponding to 129.4 mAh g−1 at 0.5 C, with a capacity retention of 82.3% after 200 charge–discharge cycles, 112.7 mAh g−1 at 10 C, and 98.8 mAh g−1 at 20 C, respectively. Thus, a simple and effective route for the controllable preparation of spherical high-voltage LNMO cathode materials was successfully developed.

Supplementary Materials

The following are available online at www.mdpi.com/2073-4352/7/9/275/s1. Figure S1: TG curve of heating the mixture of precursor and lithium source for mixed precipitants of (NH4)2CO3 and Na2CO3 with molar ratio of 1:2, Figure S2: High resolution SEM images of spherical precursors prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios: (a) 0:1; (b) 1:2; (c) 1:1; (d) 2:1; (e) 1:0. Figure S3: High resolution SEM images of spherical precursors before (a) and after (b) hydrothermal treatment prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with molar ratio of 2:1.

Acknowledgments

The authors appreciate the financial support of the “100 Talents” program of Chinese Academy of Sciences and “YZ201641”.

Author Contributions

Wenchao Yan and Yongcheng Jin conceived and designed the experiments; Yang Shu performed the experiments; Yang Shu and Jicheng Jiang analyzed the data; Haisong Wang, Deye Sun, and XiaoDi Ma contributed reagents/materials/analysis tools; Yang Shu wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Diagrammatic illustration of the synthesis process for LiNi0.5Mn1.5O4 (LNMO) cathode materials.
Figure 1. Diagrammatic illustration of the synthesis process for LiNi0.5Mn1.5O4 (LNMO) cathode materials.
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Figure 2. XRD patterns of LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios.
Figure 2. XRD patterns of LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios.
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Figure 3. Low-resolution SEM images of spherical precursor prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios: (a) 0:1; (b) 1:2; (c) 1:1; (d) 2:1; (e) 1:0.
Figure 3. Low-resolution SEM images of spherical precursor prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios: (a) 0:1; (b) 1:2; (c) 1:1; (d) 2:1; (e) 1:0.
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Figure 4. High- and low-resolution SEM images of (a1,a2) S01; (b1,b2) S12; (c1,c2) S11; (d1,d2) S21; (e1,e2) S10.
Figure 4. High- and low-resolution SEM images of (a1,a2) S01; (b1,b2) S12; (c1,c2) S11; (d1,d2) S21; (e1,e2) S10.
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Figure 5. TEM (a) and high-resolution TEM (b) of S12.
Figure 5. TEM (a) and high-resolution TEM (b) of S12.
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Figure 6. X-ray photoelectron spectra of (a) Mn 2p and (b) Ni 2p of S12.
Figure 6. X-ray photoelectron spectra of (a) Mn 2p and (b) Ni 2p of S12.
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Figure 7. (a) The galvanostatic discharge curves (highest discharge capacity) of the LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios; (b) the cycle performances of the LNMO samples at 0.5 C; (c) the rate performances of the LNMO samples at various C-rates; (d) the discharge curves of S12 at different C-rates.
Figure 7. (a) The galvanostatic discharge curves (highest discharge capacity) of the LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios; (b) the cycle performances of the LNMO samples at 0.5 C; (c) the rate performances of the LNMO samples at various C-rates; (d) the discharge curves of S12 at different C-rates.
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Table 1. The structural parameters for the LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios.
Table 1. The structural parameters for the LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios.
SamplesLattice Parameter (Å)Cell Volume (Å3)Crystallite Size (nm)
S018.1682544.9781263.6
S128.1723545.7992256.8
S118.1692545.1783193.4
S218.1705545.4386287.8
S108.1732545.9796320.5
Table 2. ICP-MS results of the LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios.
Table 2. ICP-MS results of the LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios.
SamplesNi/Mn (Atomic Ratio)Ni/Mn (fix Mn = 3)
S010.5098:1.49021.03:3
S120.5046:1.49541.01:3
S110.4921:1.50790.98:3
S210.4812:1.51890.95:3
S100.4616:1.53850.90:3
Table 3. BET surface areas of the LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios.
Table 3. BET surface areas of the LNMO samples prepared by mixed precipitants of (NH4)2CO3 and Na2CO3 with different molar ratios.
SamplesS01S12S11S21S10
BET/m2 g−14.36123.55223.12173.08372.8820
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