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Kinetically Controlled Synthesis of LiNi0.5Mn1.5O4 Micro- and Nanostructured Hollow Spheres as High-Rate Cathode Materials for Lithium Ion Batteries Sheng Li, Guo Ma, Bing Guo, Zeheng Yang,* Xiaoming Fan, Zhangxian Chen, and Weixin Zhang* School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, PR China S Supporting Information *

ABSTRACT: Spinel LiNi0.5Mn1.5O4 hollow spheres with micro- and nanostructures have been successfully synthesized based on the coprecipitation method followed by postheat treatment. A uniform and spherical precursor was obtained by controlling the reaction kinetics in the nucleation−crystallization process of forming nickel and manganese carbonates simply by employing NaHCO3 instead of Na2CO3 as precipitating agent. Single-shelled and double-shelled LiNi0.5Mn1.5O4 hollow spheres were derived of the spherical carbonate precursors via tuning the calcination kinetics. The as-prepared LiNi0.5Mn1.5O4 hollow spheres deliver a discharge capacity of 128.9 mAh g−1 at 0.1 C rate and maintain a capacity retention of 95% at 0.5 C after 100 cycles. Importantly, even at high rate of 30 C, it can still exhibit a discharge capacity of 100 mAh g−1. The excellent rate capability and cycling stability are mainly attributed to its hollow micro- and nanostructure, which could shorten Li+ ions’ diffusion path and buffer the volume change during repeated Li+ insertion−extraction processes.

1. INTRODUCTION With the development of electronic devices, electric vehicles (EVs), and hybrid electric vehicles (HEVs), there exists increasing demand for electrode materials with high energy and power density.1,2 An effective measure of improving the energy density and power density is to look for new cathode materials with higher operating potential and larger specific capacity. As an attractive high energy and power density cathode material, spinel LiNi0.5Mn1.5O4 (LNMO) has drawn significant research interest in recent years because of its high voltage plateau (around 4.7 V vs Li+/Li) and high discharge capacity (146.7 mAh g−1).3,4 However, it is generally accepted that the electrochemical performance of electrode materials is closely dependent on their morphology and microstructure, which are related with synthesis conditions.5−7 Among various synthesis methods for LNMO, the coprecipitation method is the most effective way to prepare micro- and nanostructured LNMO, and the commonly used reagents for coprecipitation are Na2CO3 or NaOH, but the reaction has to be conducted in N2 atmosphere to protect Mn2+ from oxidation under the strong alkalinity of Na2CO3 or NaOH (pH > 10). For example, Gao et al. reported the synthesis of spherical LNMO with diameters of 15 μm based on a coprecipitation reaction through continuously mixing the metal sulfate solution with Na2CO3 solution and simultaneously controlling the pH value of the system precisely at 8 by constantly adjusting the flow velocity of Na2CO3 pumped into a continuous reactor.8 Even so, the electrochemical performance © XXXX American Chemical Society

of the obtained electrode materials is usually unsatisfactory because of difficulties in the accurate manipulation of technological parameters. The as-prepared LNMO solid spheres exhibited only an ordinary rate performance with a discharge capacity of 108.8 mAh g−1 at 5 C, which might be ascribed to the long Li+ insertion−extraction length due to the large size of the solid spheres. Analogously, with NaOH as coprecipitation reagent, it also requires rigorous operation conditions similar to those for Na2CO3.9 To improve the rate capability of LNMO, one possible strategy is to make nanosized electrode materials to enhance Li+ insertion−extraction kinetics by reducing the diffusion pathway of Li+.10 However, LNMO nanoparticles with high surface activity may increase undesirable electrode−electrolyte reactions under high operating potentials and result in poor cycling performance.11,12 Recently, micro- and nanosized hollow structures have been reported as ideal architectures for LNMO materials to achieve high rate capability and long cycling stability because they present the advantages of both nanosized building blocks and microscaled assemblies. Although hollow structures may decrease the tap density of the electrode materials to some extent,13,14 leading to lower volumetric specific capacity, which may limit its use in electric Received: June 27, 2016 Revised: August 14, 2016 Accepted: August 17, 2016

A

DOI: 10.1021/acs.iecr.6b02463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION 2.1. Preparation of Spherical Precursors. The synthesis of spherical carbonate precursor was carried out by a coprecipitation method with different precipitating agents. In brief, 7.5 mmol of MnSO4·H2O and 2.5 mmol of NiSO4·6H2O were dissolved in 100 mL of distilled water, and 100 mmol of NaHCO3 (or Na2CO3) was dissolved in 100 mL of distilled water. Then the NaHCO3 (or Na2CO3) solution was directly put into the MnSO4 and NiSO4 solution mentioned above. The solution became turbid afterward, indicating the carbonate precipitation from the solution. The mixture was filtered after remaining for 3 h at room temperature, and the as-obtained brown precipitates were washed separately several times with distilled water and ethanol and then dried at 65 °C for 10 h in air. For convenience, the as-prepared spherical carbonate precursor with NaHCO3 as a precipitating agent is referred to as precursor A, whereas the sample prepared using Na2CO3 as precipitating agent is referred to as precursor B. 2.2. Preparation of LNMO Samples. To obtain LNMO samples, the as-prepared precursors A and B were mixed with LiOH·H2O (Li:Mn:Ni = 1.05:1.5:0.5 molar ratio). The mixture of as-obtained precursors and LiOH·H2O were ground for 30 min and then calcined at 450 °C for 8 h (2.0−4.0 °C min−1) and at 850 °C for 24 h (2.0−4.0 °C min−1) in air to obtain the final products. 2.3. Characterization of the Samples. The composition and structure of the prepared samples were examined by X-ray diffraction (XRD) on a D/max-γB X-ray diffractometer (Shimadzu International Trading Corporation, Japan) utilizing a Cu Kα radiation source (λ = 0.15406 nm) operated at 40 kV and 80 mA. Raman spectra were collected on a Confocal Laser Micro Raman Spectrometer (LABRAM-HR, Jobin Yvon) at 532 nm excitation. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Thermo ESCALAB 250 spectrometer using an Al Kα X-ray (hν = 1486.6 eV) source operated at 150 W with an analyzer pass energy of 20 eV to investigate the average oxidation of Mn states in the materials. Field-emission scanning electron microscopy (FESEM) measurements were carried out on a field-emission microscope (SU8020, Hitachi Limited Corporation, Japan) operated at an acceleration voltage of 5 kV. Transmission electron microscopy (TEM) images of the samples were taken on an H-800 (Hitachi Limited Corporation, Japan), operated at an accelerating voltage of 200 kV. Elemental mapping and high-resolution TEM images of the samples were taken on a field-emission

vehicles, they are still suitable for solar, wind, and nuclear energy storage applications. For example, spinel LiMn2O4 nanotubes with hollow architecture were synthesized by a self-sacrificing template method and exhibited great rate performance.15,16 As for LNMO, a precursor-templated method was reported to prepare hollow LNMO microspheres and microcubes. MnO2 microspheres and microcubes were adopted as a template precursor, and then Li+ and Ni2+ ions were introduced into the mesopores of the MnO2 precursors by an impregnation method.17,18 The resultant LNMO hollow structures deliver an improved discharge capacity of 104 mAh g−1 at 20 C rate. Similarly, ordered LiNi0.5Mn1.5O4 hollow microspheres have been prepared via a self-templated ion adsorption route followed by a solid-state calcination, which exhibited rate capabilities of 116 mAh g−1 at 5 C and 85 mAh g−1 at 10 C.19 Herein, we report controllable synthesis of spinel LNMO hollow spheres with micro- and nanostructures by tuning the coprecipitation reaction kinetics of preparing nickel and manganese carbonate precursors. Controlling the reaction kinetics during the synthesis of the precursors for electrode materials has a dominant effect on the nucleation; thus, it may affect the morphology of precursors and the final products as well. Meanwhile, tuning kinetics is also helpful to obtain flexible structures, such as single-shelled and double-shelled hollow structures obtained by adjusting the calcination kinetics. We chose NaHCO3 instead of Na2CO3 as coprecipitation agent, which has a relatively mild reaction speed resulting from the small ionization constant of HCO32− (5.6 × 10−11) to form nickel and manganese carbonates. It is beneficial for smooth nucleation and growth of the carbonate precursor crystallites into uniform spheres, so that the morphology of the resulting LNMO product can be controlled perfectly. Furthermore, the as-prepared precursors can eliminate the oxidation of Mn2+ to higher valence under the weak alkalinity of NaHCO3 (pH ≈ 8), even without the protection of N2. Meanwhile, we found that the subsequent calcination kinetics has important effect on the formation of LNMO hollow structures. The formation of LNMO hollow spheres in comparison to nanoparticles is illustrated in Scheme 1. To the best of our knowledge, there are no previous reports of using NaHCO3 as coprecipitation agent to synthesize LNMO. For comparison, we also prepared LNMO nanoparticles with Na2CO3 as the precipitating agent. The results show that LNMO hollow spheres as electrode materials exhibit higher rate and better cycling performance for lithium ion batteries. B

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structure of LNMO adopts two types of crystal structures with space groups of Fd3̅m (F phase) and P4332 (P phase), corresponding to disordered and ordered Ni/Mn location in octahedral sites, respectively.20 The disordered phase is believed to be accompanied by a small amount of oxygen deficiency for compensating for the formation of Mn3+ to maintain charge neutrality, and it may be useful to improve electrochemical performance.21,22 In our study, Raman mapping technology (Figure 2a) was used to detect the coexistence of F and P phases on a selected area of 10 × 10 μm2. Two representative Raman spectra in the mapping (points 1 and 2) correspond to the F phase and P phase (Figure 2b). The stronger Raman signal and the distinguishable peak split around 595 cm−1 rising from the lowered symmetry of cation ordering are the fingerprints of the P phase.23 Therefore, the Raman testing results present the distribution of the two phases in the LNMO hollow sphere sample. X-ray photoelectron spectroscopy (XPS) measurements (Figure 3) were also carried out to confirm the oxidation states of Mn in the LNMO hollow spheres. The Mn 2p3/2 binding energy is negatively shifted to about 642.2 eV, indicating that Mn ions in 16d octahedral sites are partially reduced to Mn3+ for the Mn 2p3/2 binding energies of Mn3+ and Mn4+ are referred to be 641.9 and 643.2 eV (642.2 eV in between), respectively.24,25 Morphologies of the as-prepared carbonate precursors and the derived LNMO samples were examined by FESEM and TEM. As shown in Figure 4a,b, the obtained precursor A exists in uniform spheres with diameter of about 2.8 μm, indicating that the precipitated carbonate clusters were assembled and grown into well-defined solid microspheres during the reaction process, which were further confirmed by the TEM image shown in Figure 4c. Moreover, elemental mapping results for the spherical precursor A are shown in Figure 4d−f. The homogeneous distribution of Mn and Ni elements across the whole sphere indicates that precursor A has grown into microspheres with uniform composition during the nucleation and crystallization. As shown in Figure S1 (Supporting Information), EDS measurements confirm that the atomic Mn/Ni ratio (Mn/Ni = 2.97/1) is close to the designed stoichiometric ratio of 3:1 for LNMO. To obtain LNMO samples, LiOH·H2O was introduced into microsphere precursors via grinding, and then the mixture was calcined to get the final LNMO products. Figure 5 shows FESEM, TEM and HRTEM images of the LNMO product, which was transformed from solid spheres of precursor A. It can be observed that the LNMO sample is composed of porous micro- and nanostructured hollow spheres, i.e. the wall of the hollow microsized spheres is constructed by many nanoscaled building blocks (nanoparticles) and is full of porosities. The shape and size of the LNMO hollow spheres are almost identical to that of precursor A. From the TEM images (Figure 5c), the wall of the LNMO hollow spheres constructed of nanoparticles has a thickness of about 500 nm. The HRTEM image (Figure 5d) and the fast Fourier transform (FFT) analyses (Figure 5e,f; corresponding to the area marked by a circle in Figure 5d) further reveal single-crystalline character of the nanoparticle subunits and confirm (111) crystalline planes of the LNMO sample. For comparison, carbonate precursor B was also prepared under identical conditions with Na2CO3 as precipitating agent, and the morphology is shown in Figure S2. As shown in Figure S2a, the as-prepared precursor B is composed of irregular nanoparticles with diverse diameters from 500 nm to 2 μm,

transmission electron microscope (JEOL-2010, JEOL Limited Corporation, Japan). The ionic conductivity and turbidity of the solution were measured by a DDSJ-308F conductivity meter (Inesa, China) and ET76020 turbidity meter (Lovibond, Germany), respectively. Note that the ionic conductivity results were recorded and obtained (one point per second) in real time. 2.4. Electrochemical Measurements. The cathode was prepared by mixing 80 wt % active material, 10 wt % acetylene black (Beijing Chemical Reagents Corporation, China), and 10 wt % polyvinylidene fluoride (PVDF, Shanghai Chemical Reagents Corporation, China) as a binder in a solvent of Nmethyl-2-pyrrolidone (NMP, Shanghai Chemical Reagents Corporation, China) to form a homogeneous slurry. The mixed slurry was cast onto aluminum foil with the slurry thickness controlled. After the evaporation of the solvent at 65 °C for 2 h in air, the cathode was roll-pressed and cut into pellets of required size for coin-cell fabrication, the pellets were further dried under high vacuum at 120 °C for 5 h. Lithium foil (Energy Lithium Limited Corporation, China) was used as the anode. The liquid electrolyte utilized was 1 mol L−1 LiPF6 in a 1:1 (volume) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC), and the separator was a polypropylene membrane with micropores (Celgard 2400). The coin-type cells (CR2032) were assembled in an Ar-filled dry glovebox. The galvanostatic charge−discharge experiment was conducted using a battery testing system (BTS-5 V/10 mA, Neware Technology Limited Corporation, China) from 3.4 to 4.9 V (versus Li+/Li) at an operating temperature range extending from −20 to 25 °C. Electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical workstation (CHI-660D, Shanghai Chenhua Instrument Limited Corporation, China). The excitation potential applied to the cells was 5 mV, and the frequency ranged from 100 kHz to 10 mHz. Potentiodynamic cycling with galvonostatic acceleration (PCGA) analysis was implemented on a battery testing system utilizing potential steps of 5 mV with a minimum current limit of 20 μA in a potential range of 3.4−4.9 V (versus Li+/Li).

3. RESULTS AND DISCUSSION 3.1. Characterization of the LNMO Samples. The powder X-ray diffraction analysis (Figure 1) of the as-prepared LNMO hollow spheres was taken to identify the crystallographic phase. All of the diffraction peaks can be indexed to the well-crystallized cubic spinel LiNi0.5Mn1.5O4 (JCPDS Card 802162), without any obvious impurities. Normally, the spinel

Figure 1. XRD pattern of LNMO hollow spheres. C

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Figure 2. Raman mapping within a selected 10 × 10 μm2 area of LNMO hollow sphere sample (a) and Raman spectra of the marked two points (1 and 2) in panel a, corresponding to the Fd3̅m and P4332 phases, respectively (b).

Figure 3. XPS spectra of survey spectrum (a) and Mn 2p (b) for LNMO hollow spheres.

Figure 4. FESEM and TEM images of precursor A (a−c) and elemental analysis of precursor A on a single microsphere; TEM image (d); Ni and Mn mapping (e and f, respectively).

which are composed of tiny carbonate clusters (Figure S2b). After the following lithiation of precursor B, the obtained LNMO sample takes the overall morphology of precursor B, also existing in irregular nanoparticles (Figure S9b). 3.2. Kinetics Investigation for Nucleation and Growth of the Precursors. To investigate the underlying mechanism of yielding carbonate precursor A and B with obviously

different morphology, we have studied and compared the kinetics of the precipitation process for carbonate precursor A and B from two aspects of reagent consumption and precursor generation. Considering that ionic conductivity of the solution is linearly related to its concentration of the reactants in a relatively low concentration range,26 we can use ionic conductivity to D

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Figure 5. FESEM (a, b), TEM (c, d), and HRTEM images (e) and the FFT analysis (f) of LNMO hollow spheres.

Figure 6. Ionic conductivity (a) and turbidity change (b) versus time using Na2CO3 (black) and NaHCO3 (red) as precipitating agent, respectively.

kinetics of the precipitation process from the aspect of yielding the carbonate precursors. The results also demonstrate the faster reaction rate using Na2CO3 than using NaHCO3 (Figure 6b), and it takes only about 10 s for the fast rise of turbidity from 0 to 1000 FNU for Na2CO3, but more than 10 min for NaHCO3. The lower reaction rate is beneficial to the comparable crystal nucleation and growth, thus leading to spherical morphology. In fact, the kinetics difference of the two precipitation reactions can be explained by theoretical analysis. As we know, both Na2CO3 and NaHCO3 belong to the strong electrolyte category. For Na2CO3, ionized CO32− could directly react with Mn2+ and Ni2+, but for NaHCO3, the release of CO32− ions are determined by the following ionization reaction rate:

represent concentration to monitor the precipitation reaction in our experimental conditions. Figure 6a shows the ionic conductivity change with time for the two reaction systems (with Na2CO3 or NaHCO3 as precipitating agent) to evaluate the precipitation reaction speed. The ionic conductivity evolution versus time for the two reaction systems demonstrates that the reaction rate for the Na2CO3 system is faster than that for NaHCO3 system, depicting that the nucleation rate for Na2CO3 is greater than that for NaHCO3 at the nascent reaction. The fitted curves of ionic conductivity versus time indicate that it is a second-order reaction for Na2CO3 and zeroorder for NaHCO3 (Figure S3). For the second-order reaction with Na2CO3 as precipitating agent, the nucleation rate at the initial reaction stage is relative greater compared with the following stage, leading to a rate of nucleation that is much greater than that of crystal growth. However, the fast nucleation does not favor the growth of generated nascent carbonate clusters into uniform spheres, resulting in the irregular nanoparticle morphology of carbonate precursor B. However, for zero-order reaction of NaHCO3, the reaction rate is constant, which is independent of the reactant concentration. Therefore, the rates for nucleation and crystal growth are relatively stable and matching; the obtained carbonate precursor A grows into uniform, well-defined spheres. These results demonstrate that tuning the reaction kinetics is an extremely useful approach for controlling morphologies. In addition, we also measured the turbidity evolution of the two reaction systems with reaction time, which reflects the

HCO3− = CO32 − + H+

The equilibrium constant of the above procedure is exactly equal to the secondary ionization constant Ka2 = 5.6 × 10−11 for H2CO3,27 which is pretty small, so that the concentration of CO32− for the two reaction system differs by several orders of magnitude (Na2CO3 > NaHCO3) at the same concentration of precipitation reagents. Therefore, it is reasonable for the difference in the aspect of reaction kinetics. Moreover, we found that the as-prepared samples of precursor A and B exhibit different colors. Precursor A (prepared by NaHCO3) was green, which was consistent with the original color at nascent reaction, while precursor B E

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Figure 7. Charge−discharge curves in the potential range of 3.4−4.9 V at various current rates (here 1 C refers to a capacity of 140 mAh g−1 in an hour) (a), the calculated energy density and power density at various rates (b), cycling performance at different discharge current rates (c), and the rate and cycling performances of LNMO hollow spheres (d).

0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2. Similarly, the SEM images of the carbonate precursor prepared with NaHCO3 (Figure S6a,b) also display a morphology of uniform microspheres, but it exists in flocculent aggregates for Na2CO3 precipitating agent (Figure S6c,d), verifying its considerable generality of adjusting kinetics to morphology control. 3.3. Kinetics Investigation for Synthesizing the LNMO Hollow Micro- and Nanospheres. The morphology evolution related with conversion of carbonate precursors to LNMO products has been observed. The formation mechanism for the LNMO hollow spheres is similar to previous reports on the hollow microspheres of LiNi0.5Mn1.5O4.17,19 This can be attributed to the migration of the mesopores in the intermediate microspheres based on a mechanism analogous to the Kirkendall effect. During the high-temperature calcinations, the fast outward diffusion of Mn and Ni atoms occurs from the inner core, while the slow inward diffusion of O atoms (O2) occurs from the surface. The Kirkendall effect over a long diffusion distance in large precursors could occur here, leading to the formation of hollow interiors in the LNMO microspheres. Interestingly, by simply adjusting the calcination condition of heating rate from 2 °C/min to 4 °C/min, double-shelled morphology of the LNMO product can be obtained (Figure S7). According to the report of Jiang and co-workers, the key to generating multishelled hollow spheres with close double shells is the splitting of the outer functional layer from the inner carbonate core due to the differential shrinking rate of both materials under controlled heating conditions.30 Obviously, in our case, increasing the heating rate from 2 °C/min to 4 °C/ min accelerates the shrinkage kinetically, but with different shrinkage rate to the outer LNMO layer and the inner

(prepared by Na2CO3) was black (Figure S4), indicating that precursor B has been oxidized gradually in air during reaction. The XRD patterns of the two precursors reflect that precursor B has been partially oxidized into MnO2 in the Na2CO3 system, while precursor A retains the state of pure MnCO3 in NaHCO3 system and can still keep intact even after being dried at 80 °C in air, indicating that pH value plays a dominant role in the oxidation of manganese (Figure S5a). Herein, it should be noted that NiCO3 was demonstrated to be amorphous in the two reaction systems (Figure S5b). In order to determine the reason, we measured the pH value of the mixed metal solution, NaHCO3 solution, and Na2CO3 solution, and the corresponding pH values were 4.401, 8.363, and 11.652. The pH value during reaction for NaHCO3 was about 7.256, while it was 10.705 for Na2CO3, indicating that the reaction system for NaHCO3 was close to neutral, while it exhibited strong alkalinity for Na2CO3. As reported previously, Mn2+ tends to be oxidized to high valence by O2 in air under alkaline conditions,28 so that it is reasonable that in the Na2CO3 system, Mn2+ in the carbonate precursor B is partially oxidized into higher valence state, which may also have some effect on the morphology of precursor B, while the carbonate precursor A remains relatively stable in the NaHCO3 system. Due to the formation of mixed phases, not only is the morphology of precursor B beyond control, but also the electrochemical properties of the subsequent calcination product could not be controlled easily and homogeneously. Normally, the reaction environment for Na2CO3 has to be controlled under inert gas protection or in near-neutral pH for fear of the oxidation of Mn2+ as reported in the literature.8,29 As a matter of fact, the efficiency of tuning kinetics on morphology has also been evidenced by synthesizing the carbonate precursor for Li-rich cathode material of F

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cycling performance, it can maintain a capacity retention of 94.3% after 100 cycles at 0.5 C rate. The comparison of single-shelled and double-shelled LNMO electrode materials are shown in Table S2, and we can find that the discharge capacities of double-shelled LNMO electrode materials at various rates are inferior to those of the singleshelled counterparts. This can be ascribed to the great resistance of electrons transporting into the inner shell because the inner layer material cannot be connected by conductive carbon agents which were mixed during the subsequent electrode fabrication to improve the conductivity. Although the electrolyte can penetrate the porous outer shell to reach the inner shell for good ion transfer, it is a poor conductor of electrons. Thus, the capacity of the inner shell material could not be exerted effectively. For comparison, LNMO nanoparticles obtained from the calcination of precursor B were also electrochemically tested for LIBs. From the XRD patterns of LNMO nanoparticles shown in Figure S9a, the diffraction peaks correspond to the well-crystallized cubic spinel LiNi0.5Mn1.5O4 (JCPDS Card 80-2162). However, as shown in Figure S9c, LNMO nanoparticles can deliver a discharge capacity of 113 mAh g−1at 0.1 C rate and only 60 mAh g−1 at 10 C rate. As shown in Figure S9d, after 100 cycles at 0.5 C, LNMO nanoparticles have capacity retention of 67%, significantly lower than that of LNMO hollow spheres. From the experimental results above, it is obvious that the rate and cycling performances of LNMO hollow microspheres are much better than those of LNMO nanoparticles, which can be attributed to unique porous hollow hierarchical micro- and nanostructures. The Li+ diffusion length of LNMO hollow spheres is shorter than that of LNMO nanoparticles, because the primary particle size of LNMO hollow spheres (about 300 nm) is smaller than that of LNMO nanoparticles (about 700 nm) and the primary particles of LNMO hollow spheres attach to each other, which can decrease the charge-transfer resistance; these advantages can lead to better rate performance. Meanwhile, the porous micro and nano hollow structure can provide not only larger electrode−electrolyte contact area for high Li+ flux across the interface but also better accommodation of the volume change during repeated cycles, resulting in enhanced cycling performance (Figure S10).17,18,31 Toward better evaluation of electrochemical properties of the as-prepared LNMO cathode materials, we investigated the lowtemperature electrochemical performance of the LNMO hollow spheres at 0 °C and −20 °C. As shown in Figure 8a, when the temperatures decrease to 0 °C and −20 °C, the discharge capacities drop slightly to 125 and 112 mAh g−1 at 0.1 C, being 97% and 87% of the capacity at room temperature, respectively. In addition, the two voltage plateaus still remain at about 4.7 and 4.0 V, and the electrical polarization as exhibited by the voltage difference (ΔV) between the charge and discharge plateaus increases slowly. Figure 8b illustrates the cycling performance of the LNMO-hollow spheres at 0.5 C under low temperatures. It can be found that the half cells at 0 °C and −20 °C also exhibit excellent cycling stability with almost no capacity fading during 100 cycles. After 100 cycles, the discharge capacity reaches 121.5, 123.3, and 110.7 mAh g−1 with a high capacity retention rate of 95%, 100%, and 100%. Such a good electrochemical performance at low temperature is also attributed to the micro- and nanostructured hollow spheres. Meanwhile, the unexpected uniformity of the sample also contributes to the performance of the materials.32

carbonate core. When the critical stress is reached, there is a strong tendency for the middle part to shrink inward and leave from the preformed outer shell, thus leading to the splitting of different layers and formation of double-shelled hollow spheres. 3.4. Electrochemical Properties of LNMO Micro- and Nanostructured Hollow Spheres and Nanoparticles. Given insight into the application prospects, the as-prepared LNMO hollow spheres were assembled as cathodes in half cell lithium ion batteries. Figure 7a shows the charge−discharge curves of the LNMO hollow sphere electrodes at various current rates from 0.1 to 30 C between 3.4 and 4.9 V versus Li+/Li at room temperature. From the charge−discharge curves, two voltage plateaus can be observed from the charge−discharge curves at around 4.0 and 4.7 V, respectively. The small plateau at 4.0 V can be associated with the oxidation of Mn3+ to Mn4+ reaction, and the main 4.7 V plateau can be associated with the two-step oxidation of Ni2+ to Ni3+ and Ni3+ to Ni4+. At 0.1, 0.2, 0.5, 1, 2, and 5 C discharge rates, the corresponding discharge capacities of LNMO hollow spheres are 128.9, 127.7, 127.6, 127.3, 126.9, 121.6, and 121.0 mAh g−1, respectively. It can still maintain discharge capacities of 120 and 100 mAh g−1 even at higher rates of 20 and 30 C, equivalent to 93% and 78% of the capacity at 0.1 C, respectively. Figure 7b provides the calculated discharge energy densities and power densities at various rate according to the discharge profiles in Figure 7a. The energy densities of as high as about 600 Wh kg−1 and 515 Wh kg−1 were obtained at the rates of 0.1 and 20 C, respectively. It decreased smoothly with the increase of current density when it is smaller than 20 C, but the energy density dropped sharply to 390 Wh kg−1 when the current density reached 30 C, which can be mainly ascribed to the decay of voltage caused by polarization. As for power density, it increased rapidly from 59.7 to 11709 W kg−1 with the increase of current density from 0.1 to 30 C. The property at the rate of 20 C was among the best considering energy density and power density synthetically, because of the sharp decrease of energy density but small increase of power density from 20 to 30 C; details can be found in Table S1). Figure 7c exhibits the room-temperature cycling performance of the half cells at various rates of 0.5 C, 1 C, 2 C, 5 and 10 C, demonstrating that the LNMO hollow spheres have capacity retention of 95% at 0.5 C after 100 cycles. Even at a high rate of 10 C, the specific capacity retention is as high as 80% after 100 cycles. To further investigate the rate and cycling performances of the LNMO materials, the cells are charged at 0.5 C rate and discharged under different rates from 0.1 to 30 C and then back to 0.1 C. The discharge capacity values at various rates are shown in Figure 7d. With the increasing current density, the discharge capacities of the sample remain stable. When the current density is decreased from 30 to 0.1 C at the end of cycling, the discharge capacity of LNMO hollow spheres can still reach 124 mAh g−1, quite close to the initial discharge capacity of 129 mAh g−1, indicating that the hollow LNMO electrode materials are qualified with the ability of resisting the impact force resulting from high current and polarization. Actually, we have also tested the electrochemical properties of double-shelled LNMO electrode materials (Figure S8); it exhibits an initial discharge capacity of 118.3 mAh g−1 with a Coulombic efficiency of 75.6% at 0.2 C. At 0.2, 1, 5, 10, 20, and 30 C discharge rates, the corresponding discharge capacities of double-shelled LNMO electrode material are 118.3, 118.3, 112.0, 108.7, 101.9, and 94.2 mAh g−1, respectively. As for G

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range between 100 kHz and 10 mHz are presented in Figure 9; both of them have a semicircle observed at the high-frequency

Figure 9. Electrochemical impedance spectroscopy (EIS) results of LNMO samples at full charge in the frequency range between 100 kHz and 10 mHz.

region and an oblique line at low frequency. The chargetransfer resistance (Rct) represents the numerical value of the diameter of the semicircle on the Z real axis. From Figure 9, the Rct of the LNMO hollow spheres (189.9 Ω) is much smaller than that of LNMO nanoparticles (584.1 Ω), indicating that the electrons and Li ions transfer more easily in LNMO hollow spheres, which can be ascribed to the better contact of electrode materials as well as electrode−electrolyte contact area for the micro- and nanostructured hollow spheres, compared to the LNMO nanoparticles.

4. CONCLUSIONS We have demonstrated a facile coprecipitation route to synthesize spinel LiNi0.5Mn1.5O4 hollow spheres by using NaHCO3 as precipitating agent. The uniform morphologies of the spherical carbonate precursors and the micro- and nanostructured hollow products were obtained by tuning the kinetics during the preparation of precursors and the subsequent calcination procedure, without using any surfactants or templates. As cathode materials for lithium ion batteries, LNMO hollow spheres display superior rate capability and cycling stability in comparison with the LNMO nanoparticles at room temperature, with discharge capacities of 128.9 mAh g−1 at 0.1 C rate, 120 and 100 mAh g−1 even at 20 and 30 C high rate, respectively. In addition, LNMO hollow spheres also exhibit good electrochemical performance at low temperatures down to 0 °C and −20 °C, which demonstrates that synthesizing hollow electrode materials with micro- and nanostructures is an effective way to improve electrochemical performance for lithium ion batteries. Specifically, the hollow micro- and nanostructure provides short distance for Li+ diffusion and suitable electrode−electrolyte contact area for high Li+ flux insertion and extraction, leading to better rate capability and low-temperature performance. Meanwhile, the structural strain and volume change associated with the repeated Li+ insertion−extraction processes could be buffered effectively by the porosity in the wall and interior void space, thus improving the cycling stability. On the basis of this work, customizing the morphology of electrode materials by adjusting process kinetics for enhanced electrochemical performance in lithium ion batteries can be considered. This facile strategy may

Figure 8. Charge−discharge profiles (a), cycling performance at a charge current density of 0.5 C rate (b), and PCGA profiles (c) of LNMO hollow spheres at different temperatures.

Furthermore, we also investigated the electrochemical performance at low temperatures by the potentiodynamic cycling with galvanostatic acceleration analysis in a potential range of 3.4−4.9 V (versus Li+/Li), as shown in Figure 8c. The ΔV between the anodic and cathodic peaks for redox couple can be easily measured. Two peaks are observed at around 4.6− 4.8 V, corresponding to Ni2+/Ni3+ and Ni3+/Ni4+ redox couples, and another small peak near 4.0 V is due to Mn3+/ Mn4+ redox couple. With decreasing temperature, there is an apparent rightward shift of the cathodic peaks as well as a leftward shift of the anodic peaks, indicating a gradual increase in the polarization of the cells. The ΔV values at 25, 0, and −20 °C are 0.03, 0.06, and 0.11 V, respectively. This is mainly due to the decrease in the ionic conductivity of the electrolyte as well as a deceleration of the electrochemical reactions for the cells at low temperatures. Electrochemical impedance spectroscopy was carried out in the full charge state after 3 cycles at 0.2 C to obtain relatively stable and identical status. The Nyquist plots of the LNMO hollow spheres and LNMO nanoparticles in the frequency H

DOI: 10.1021/acs.iecr.6b02463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

(7) Wang, F.; Xiao, S.; Chang, Z.; Yang, Y. Q.; Wu, Y. P. Nanoporous LiNi(1/3)Co(1/3)Mn(1/3)O2 as an ultra-fast charge cathode material for aqueous rechargeable lithium batteries. Chem. Commun. 2013, 49, 9209. (8) Gao, J.; Li, J. J.; Jiang, C. Y.; Wan, C. R. Controlled preparation and characterization of spherical LiNi0.5Mn1.5O4 cathode material for lithium-ion batteries. J. Electrochem. Soc. 2010, 157, A899. (9) Lee, M. H.; Kang, Y. J.; Myung, S. T.; Sun, Y. K. Synthetic optimization of Li[Ni1/3Co1/3Mn1/3]O2 via co-precipitation. Electrochim. Acta 2004, 50, 939. (10) Wang, Y.; Su, F. B.; Wood, C. D.; Lee, J. Y.; Zhao, X. S. Preparation and characterization of carbon nanospheres as anode materials in lithium-ion secondary batteries. Ind. Eng. Chem. Res. 2008, 47, 2294. (11) Lai, X. Y.; Halpert, J. E.; Wang, D. Recent advances in micro/ nano-structured hollow spheres for energy applications: From simple to complex systems. Energy Environ. Sci. 2012, 5, 5604. (12) Yang, J. G.; Han, X. P.; Zhang, X. L.; Cheng, F. Y.; Chen, J. Spinel LiNi0. 5Mn1. 5O4 cathode for rechargeable lithium ion batteries: Nano vs micro, ordered phase (P4332) vs disordered phase Fd3̅m. Nano Res. 2013, 6, 679. (13) Guo, Y. G.; Hu, J. S.; Wan, L. J. Nanostructured materials for electrochemical energy conversion and storage devices. Adv. Mater. 2008, 20, 2878. (14) Wang, S. L.; Zhang, Z. X.; Deb, A.; Yang, L.; Hirano, S. I. Synthesis, characterization, and electrochemical performance of Cedoped ordered macroporous Li3V2(PO4)3/C cathode materials for lithium ion batteries. Ind. Eng. Chem. Res. 2014, 53, 19525. (15) Ding, Y. L.; Xie, J.; Cao, G. S.; Zhu, T. J.; Yu, H. M.; Zhao, X. B. Single-crystalline LiMn2O4 nanotubes synthesized via templateengaged reaction as cathodes for high-power lithium ion batteries. Adv. Funct. Mater. 2011, 21, 348. (16) Li, X. X.; Cheng, F. Y.; Guo, B.; Chen, J. Template-synthesized LiCoO2, LiMn2O4, and LiNi0.8Co0.2O2 nanotubes as the aathode materials of lithium ion batteries. J. Phys. Chem. B 2005, 109, 14017. (17) Zhou, L.; Zhao, D. Y.; Lou, X. W. D. LiNi0.5Mn1.5O4 Hollow structures as high-performance cathodes for lithium-ion batteries. Angew. Chem. 2012, 124, 243. (18) Wang, Z. Y.; Zhou, L.; Lou, X. W. D. Metal Oxide Hollow Nanostructures for Lithium-ion Batteries. Adv. Mater. 2012, 24, 1903. (19) Wu, W. W.; Xiang, H. F.; Zhong, G. B.; Su, W.; Tang, W.; Zhang, Y.; Yu, Y.; Chen, C. H. Ordered LiNi0.5Mn1.5O4 hollow microspheres as high-rate 5 V cathode materials for lithium ion batteries. Electrochim. Acta 2014, 119, 206. (20) Kim, J. H.; Myung, S. T.; Yoon, C. S.; Kang, S. G.; Sun, Y. K. Comparative study of LiNi0.5Mn1.5O4‑δ and LiNi0.5Mn1.5O4 cathodes having two crystallographic structures: Fd3̅m and P4332. Chem. Mater. 2004, 16, 906. (21) Zheng, J. M.; Xiao, J.; Yu, X. Q.; Kovarik, L.; Gu, M.; Omenya, F.; Chen, X. L.; Yang, X. Q.; Liu, J.; Graff, G. L.; Whittingham, M. S.; Zhang, J. G. Enhanced Li+ ion transport in LiNi0.5Mn1.5O4 through control of site disorder. Phys. Chem. Chem. Phys. 2012, 14, 13515. (22) Pasero, D.; Reeves, N.; Pralong, V.; West, A. R. Oxygen nonstoichiometry and phase transitions in LiNi0.5Mn1.5O4−δ. J. Electrochem. Soc. 2008, 155, A282. (23) Zhang, X. L.; Cheng, F. Y.; Yang, J. G.; Chen, J. LiNi0.5Mn1.5O4 porous nanorods as high-rate and long-life cathode for Li-ion batteries. Nano Lett. 2013, 13, 2822. (24) Xiao, J.; Chen, X. L.; Sushko, P. V.; Sushko, M. L.; Kovarik, L.; Feng, J. J.; Deng, Z. Q.; Zheng, J. M.; Graff, G. L.; Nie, Z.; Choi, D.; Liu, J.; Zhang, J. G.; Whittingham, M. S. High-performance LiNi0. 5Mn1. 5O4 spinel controlled by Mn3+ concentration and site disorder. Adv. Mater. 2012, 24, 2109. (25) Park, J. S.; Roh, K. C.; Lee, J. W.; Song, K.; Kim, Y. I.; Kang, Y. M. Structurally stabilized LiNi0.5Mn1.5O4 with enhanced electrochemical properties through nitric acid treatment. J. Power Sources 2013, 230, 138. (26) Atkins, P.; Paula, J. D. Atkins Physical Chemistry, 8th ed.; Oxford University Press: New York, 2002.

open up a new avenue for tailoring the morphology of various functional materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02463. SEM images and EDS spectrum of precursor A; TEM images of the double-shelled LNMO-hollow spheres; FESEM images of precursor B with Na2CO3 as precipitating agent; XRD patterns of the as-prepared precursors with NaHCO3 and Na2CO3; FESEM images of precursor of 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2 with NaHCO3 and Na2CO3; calculated and fitted curves derived from conductivity and turbidity versus time using Na2CO3 and NaHCO3; electrochemical properties of double-shelled LNMO electrode materials; XRD patterns, SEM images, and charge−discharge profiles for LNMO nanoparticles; illustration of enhanced rate performance and cycle stability for LNMO hollow spheres; calculated energy density and power density at various rates and the comparison of electrochemical properties for single-shelled and double-shelled LNMO electrode materials (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86 551 62901454. Fax: +86 551 62901450. E-mail: [email protected]. *Tel: +86 551 62901454. Fax: +86 551 62901450. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (NSFC Grants 21176054, 21271058, and 91534102) are gratefully acknowledged. We also acknowledge the support from Science and Technology Project of Anhui Province (1501021013) and Intelligent Manufacturing Institute of Hefei University of Technology (IMICZ2015104).

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REFERENCES

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DOI: 10.1021/acs.iecr.6b02463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b02463 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX