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Article pubs.acs.org/JPCC

A Synergistic Effect in a Composite Cathode Consisting of Spinel and Layered Structures To Increase the Electrochemical Performance for Li-Ion Batteries Zu-Wei Yin,† Zhen-Guo Wu,†,§ Ya-Ping Deng,† Tao Zhang,† Hang Su,† Jun-Chuan Fang,‡ Bin-Bin Xu,‡ Jian-Qiang Wang,∥ Jun-Tao Li,*,† Ling Huang,‡ Xiao-Dong Zhou, ‡,⊥ and Shi-Gang Sun*,†,‡ †

College of Energy, Xiamen University, Xiamen 361005, China State Key Lab of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China § School of Chemical Engineering, Sichuan University, Chengdu 610065, China ∥ Shanghai Synchrotron Radiation Facility, Chinese Academy of Sciences, Shanghai 201204, China ⊥ Department of Chemical Engineering, University of South Carolina, Columbia 29208, United States ‡

S Supporting Information *

ABSTRACT: In this work, a composite consisting of layered Li[Li0.2Ni0.12 Mn0.56Co0.12]O2 (LNMC) and spinel Li[Ni0.5Mn1.5]O4 (LNMO) was synthesized by a modified Pechini method. Extensive analysis was carried out to investigate the synergistic effect between the layered oxide and spinels in the composite by comparing its properties with baseline individual compounds, as well as a physical mixture of LMNC and LNMO. Comparing to the LNMC, the compsoite cathode exhibited a similar initial capacity of ∼250 mA·h/g at 0.1 C, but a much higher first-cycle effeciency, better cyclability and rate capability, attributed to the presence of spinel. The synertistic effect of integrated spinel on the microstructure, crystal strucutre, Mn oxidation states, and Li+/Ni2+ disordering of the composite was studied by X-ray absorption near edge structure (XANES), electron microscopy, and X-ray diffraction (XRD). The presence of a spinel component in the composite cathode is the origin for the improvement of cyclability and rate capability, largely due to a lower Li+/Ni2+ disordering, milder redox reaction of manganese ions, and suppressed converting reaction to form LixMn2O4-like spinel. ∼160 mA·h/g for LiFePO4.1−3 LNMC, however, undergoes a phase transformation to form LixMn2O4 during cycling. The discharge plateau of LixMn2O4, a spinel-type oxide, is only at 3.0 V vs Li/Li+, therefore is responsible for the voltage fading in LNMC that exhibits a discharge plateau of ∼3.8 V.4 Moreover, the Jahn−Teller distortion in Li[Li, Ni, Mn, Co]O2, which originates from a larger ionic radius of Mn3+ (0.645 Å) than that of Mn4+ (0.53 Å) and the disproportionation reaction of Mn3+ → Mn2+ + Mn4+, is responsible for the poor cycle performance

1. INTRODUCTION In plug-in vehicle systems, the specific energy density of a lithium-ion battery corresponds to the mileage range of a vehicle. Currently, the cathode capacity limits the overall cell capacity because most solid cathodes are insertion compounds; therefore the capacity is limited by the solid-solution range of lithium. During the past few decades, increasing the capacity of the cathode has remained as a focal research area across government, industry and academia. Recently, layered Li[Li, Ni, Mn, Co]O2 (LNMC), a solid solution of Li2MnO3 and LiMO2 (M= Ni, Mn, Co), has attracted increasing attention because of its high specific capacity, >250 mA·h/g, in comparison to that of the state-of-the-art cathodes: ∼ 140 mA·h/g for LiCoO2 and © 2016 American Chemical Society

Received: July 18, 2016 Revised: October 23, 2016 Published: October 25, 2016 25647

DOI: 10.1021/acs.jpcc.6b07169 J. Phys. Chem. C 2016, 120, 25647−25656

Article

The Journal of Physical Chemistry C

Figure 1. SEM images of (a) spinel LNMO, (b) Li-rich oxide LNMC, (c) com-8.5/1.5, and (d) mixed-8.5/1.5 materials.

and rate capability because the dissolution of Mn2+ and mismatch between the original cubic (c/a = 1) spinel and distorted tetragonal (c/a = 1.16) spinel.5,27 Additionally, during the first charge process at ∼4.5 V, the Fermi level is located within the oxygen valence band, which results in the oxidation of O2−, accompanied by the extraction of Li+ and decomposition of Li2MnO3 to form MnO2, then leads a low first-cycle efficiency (∼70%).6 Spinel Li[Ni0.5Mn1.5]O4 (LNMO) is considered as a promising cathode material for its high discharge potential (∼4.65 V), good cycling performance (>85% retention after 1000 cycles at 1 C), and rate capability (capacity at 5 C > 80% of that at 0.1 C) due to 3-dimensional Li+ diffusion channels and empty 16c octahedral sites.7,8 Moreover, when discharging to 2 V (vs. Li/Li+), the spinel LNMO can exhibit a first-cycle efficiency >100% because of additional Li+ into the vacant 16 c octahedral sites.9,10 Unfortunately, the LNMO cathode can only deliver a limited capacity of ∼130 mA·h/g. Considering the complementary nature between LNMC and LNMO, it might be possible to synthesize a composite consisting of both compounds to achieve a cathode with a high capacity, efficiency, cyclability, and rate capability. Our recent work reported a one-step solvothermal approach to synthesize layered/spinel heterostructure, Li1.14Mn0.622Ni0.114Co0.124O2, which exhibited a high first-cycle Coulombic efficiency (101% at 0.2 C) and excellent rate capability (303 and 206 mA·h/g at 0.2 and 10 C respectively).11 Other approaches to synthesize layered/spinel composite materials include coprecipitation,12−14 modified solvothermal,15−17 and surface modification.18−20 An optimized electrochemical performance can be obtained by controlling the spinel fraction and synthesis condition.21−23 What remain unknown are (1) how to control the microstructural distribution of these two phases, (2) how the structural and electrochemical properties will change in the composite, comparing with an individual component, and (3) what is the mechanism for these changes. In this study, for the first time, we used a modified Pechini method and subsequent calcination to prepare xLNMC−(1−x)LNMO (0.5 ≤ x ≤ 1) composite cathode, which hereafter will be denoted as “com−x/1−x” and the composition of LNMC is

Li[Li0.2Ni0.12Mn0.56Co0.12]O2. A physical mixture of LNMC and LNMO was used as a comparison and was prepared by mechanically grinding both compounds without further heat treatment. To understand the synergistic effect, X-ray absorption near edge structure (XANES) spectroscopy was used to characterize the valence state of element in materials and their local environment. The structure, Mn K-edge XANES spectra, and electrochemical performance of LNMC and composite cathodes were analyzed. Our results show that the presence of a spinel phase in the composite leads to a similar microstructure, milder Mn ion redox reaction, suppressed phase transformation, and better electrochemical performance. We also studied the degree of Li+/Ni2+ disordering in the composite, which was dependent on the spinel fraction and synthesis conditions. The presence of a spinel component in the composite cathode is the origin for the improvement of cyclability and rate capability, largely due to a lower Li+/Ni2+ disordering, milder redox reaction of manganese ions, and suppressed converting reaction to form LixMn2O4-like spinel.

2. EXPERIMENTAL SECTION 2.1. Synthesis. The xLNMC−(1−x)LNMO (0.5 ≤ x ≤ 1) composite cathodes were synthesized using a modified Pechini method.24,25 A stoichiometric amount of Mn(CH3COO2)· 4H2O, Ni(CH3COO2)·4H2O, and Co(CH3COO2)·4H2O, and an excess (5% in molar ratio) of Li(CH3COO2)·H2O were used as raw materials, together with citric acid and ethylene glycol (molar ratio =1:4) to form a polymeric precursor. After heating at 450 °C for 6 h, the precursor was calcined at different temperatures (650−900 °C) and time (12, 24, and 36 h). The optimization process of materials preparation is shown in Figure S1. A spinel LiNi0.5Mn1.5O4 heated at 700 °C for 24 h is denoted as LNMO. A Li-rich layered cathode heated at 700 °C for 24 h is named as LNMC. A composite of 0.85LNMC−0.15LNMO is denoted as “com-8.5/1.5”. A cathode prepared by mechanical grinding LNMC and LNMO in a 8.5:1.5 molar ratio is named as “mixed-8.5/1.5”. For other materials prepared at different conditions, they were named, based on the order to the spinel ratio, calcination temperature, and time, such as LNMC-80024h, com-8.5/1.5-800-12h etc. 25648


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rate of 1°·min−1. XRD refinement was conducted by Rietveld method using Topas program (Bruker, Topas 4.2). Ex situ X-ray absorption near edge structure (XANES) measurements were carried out at the beamline (BL14W1) on Shanghai Synchrotron Radiation Facility (SSRF). In an Ar-filled glovebox, thin-film LNMC and com-8.5/1.5 electrodes at different cutoff voltages were obtained by disassembling the cells and washing with dimethyl carbonate (DMC), and then the electrodes were reassembled back into CR2025-type coin cells. For XANES studies, a hole of 3 mm in diameter was drilled on the surface and sealed with Kapton membrane. 2.3. Electrochemical Measurements. The electrochemical measurements were performed using CR2025-type coin cells. Electrodes were made of a mixture of the synthesized material, acetylene black and polyvinylidenediuoride (PVDF) binder at a weight ratio of 80:10:10 on an Al foil current collector. N-Methyl-2-pyrrolidone (NMP) was used as solvent. Before each test, the electrodes were dried at 100 °C for 12 h in vacuum. The diameter of the electrodes were 16 mm and the mass loading of active material was kept at 1.25 ± 0.1 mg cm−2. The CR2025-type coin cells were assembled in an argon-filled glovebox. A lithium foil was used as counter electrode and Celgard2400 film as separator. The electrolyte solution was 1 M LiPF6 dissolved in ethylene carbonate/dimethyl carbonate (EC:DMC = 1:1 by volume). The cells were charged and discharged galvanostatically on a Land-V34 battery tester (Wuhan, China) at 30 °C. The current density of 1 C equals to 200 mA/g. All the materials were tested between 2 and 4.8 V (vs. Li/Li+). The calculated capacities were based on the weight of active material. The cyclic voltammetry curves were obtained by a CHI660E electrochemical working station, which was tested between 2 and 5 V (vs. Li/Li+). The scan rate is 0.2 mV/s.

3. RESULTS AND DISCUSSION 3.1. Microstructure and Crystal Structure. Figure 1 shows the microstructures of spinel LNMO, Li-rich oxide LNMC, com-8.5/1.5 and mixed-8.5/1.5 cathodes. The LNMO displays a typical truncated octahedron shape with a particle size of 100−300 nm. The average particle size of layered LNMC is between 100 and 200 nm, while the com-8.5/1.5 exhibits a microstructure with an average particle size ∼200 nm. The mixed-8.5/1.5 material is a mechanical mixture, so its microstructure comprises of a dispersion of LNMC and LNMO particles. Figure 2 shows the XRD patterns of LNMO, LNMC, and com-8.5/1.5. For LNMC, the strong peaks at 18.7° and 44.7° correspond to the reflections of (003) and (104) respectively, indicating the existence of a hexagonal α-NaFeO2 layered structure (R3̅m symmetry). The weak peak near 21° is indexed to (110) and belongs to monoclinic Li2MnO3-like domains with C2/m group symmetry, and each LiO6 octahedral interstice is surrounded by six MnO6 octahedra making a hexagonal LiMn6 neighbor unit in its Mn-rich layer. All of these three peaks can be distinctly observed in the composite material, thus manifests the presence of layered component. For LNMO, the peaks at 18.8° and 44.3° are typical spinel reflections, corresponding to (111) and (400) respectively. For com-8.5/1.5, all of the aforementioned reflections, including (003), (104), (110), (111) and (400), can be observed, suggesting the formation of a layered/spinel heterostructure. The spinel ratio in the composite was obtained by XRD refinement using Topas program (see Figure S2a,b and Table 1). Both the calculated curves of LNMC and com-8.5/1.5 of are fitted well with experimental

Figure 2. XRD patterns of spinel LNMO, Li-rich oxide LNMC, and com-8.5/1.5 materials. (b, c) Local enlarged images of part a.

Table 1. Results of XRD Rietveld Refinement for Li-Rich Oxides LNMC and com-8.5/1.5 lattice parameter (Å)

I003/I104 Ni2+% (Ni2+3b/Ni2+total) spinel % Rwp (%)

ahex chex acub

Li-rich oxide LNMC

com-8.5/1.5

2.8520 14.2335 8.1220 0.9389 31.6 3.01 2.72

2.8508 14.2580 8.1499 1.5097 24.11 14.93 2.24

2.2. Characterization. The microstructure was examined using a scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM2100). The crystal structure was characterized by powder X-ray diffraction (XRD) patterns recorded on an automated Philips X’Pert Pro Super X-ray diffractometer with Cu Kα radiation. Data were collected over the 2θ range of 15−90° with a scan 25649


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Figure 3. Polycrystalline SAED patterns and index of the diffraction rings of (a, b) Li-rich oxide LNMC and (c, d) com-8.5/1.5 materials.

Figure 4. (a) Initial charge−discharge profiles at 0.1 C and (b) cycle performance at 1 C, (c) Coulombic efficiencies, and (d) energy densities versus cycle number plots at 1 C of spinel LNMO, Li-rich oxide LNMC, com-8.5/1.5, and mixed-8.5/1.5 materials.

curves. The calculated spinel ratio of the composite material is 14.9%, so we named this composite as “com-8.5/1.5”. The LNMC has a small fraction of spinel, ∼ 3%.

Figure 3 illustrates the selected area electron diffraction (SAED) pattern and index of the diffraction rings of LNMC and com-8.5/1.5, another evidence of the existence of spinel 25650


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The small ∼2.7 V plateau indicates that a spinel phase exists in LNMC because of the loss of lithium during the heating process for 24 h. For the mixed-oxide electrode, the discharge plateau decreases to a low level of the spinel component at ∼2.7 V. However, for the composite electrode, the discharge plateau at ∼3 V rises, which reflects a role of intergrown spinel phase on the electrochemical property and differs from that of the spinel in a mixed-oxide electrode. The discharge platform according to Ni4+/ Ni2+ couples of LiNi0.5Mn1.5O4 is not so obvious, but the Ni4+/ Ni2+ redox reaction can be observed in CV curve, as illustrated in Figure S4. The CV curve suggests the existence of LiNi0.5Mn1.5O4 composition. Figure 4b depicts the cycle performance at 1 C of these four electrodes. The composite electrode yields the highest capacity of 193.9 mA·h/g and the highest capacity retention of 99.2% after 100 cycles. The discharge capacity of LNMO is 191.2 mA·h/g, but its capacity retention was poor, only about 62.1%. On the other hand, the pure layered LNMC has a high retention ratio of 99.6% and a low capacity of 167.2 mA·h/g, in contrast to 75% and 177.3 mA·h/g for the mixed-oxide electrode. At 1 C, the Coulombic efficiency in the first cycle for the spinel LNMO is 152.05% (Figure 4c), which is greater than 100% because of the insertion of additional Li+ into the vacant 16 c octahedral sites.9,10 On the contrary, the first-cycle efficiency of LNMC electrode is solely 69.5%, which is a low value attributed to the formation of oxygen gas and irreversible decomposition of Li2MnO3 during the charging process.6 The first-cycle efficiencies of both composite and mixed-oxide electrodes are improved comparing to LNMC. It appears that the presence of spinel phase does play a key role in the improvement of Coulombic efficiency. Table 2 summaries the first-cycle efficiencies of these four electrodes and their energy density that is shown in Figure 4d. The composite electrode has a high energy density of 656.8 Wh/kg and retention ratio of 94.7%. The initial energy density of spinel electrode is high, reaching 712.9 Wh/kg, because of its high average discharge voltage as indicated in Figure S6a; however the retention ratio is only 58.1% after 100 cycles. The LNMC has a high energy retention ratio of 99.1% and a low energy density of 529.3 Wh/kg, in comparison to 68.6% and 567.1 Wh/kg for the mixed-oxide electrode. To further understand the interaction between the spinel and layered structures in the composite, a systematic investigation was carried out by changing composite chemistry, as well as calcination temperature and time. Detailed results are shown in Figure S5 and S6 to illustrate the capacity, average discharge voltage, and energy density as a function of cycle number. Indeed, the electrochemical performance was improved by adjusting the LNMO/LNMC ratio and synthesis conditions. Of all the electrodes studied, the com-8.5/1.5 presents the best cyclability and highest energy density. Figure 5 provides a comparison of the rate capacity of LNMO, LNMC, com-8.5/1.5 and mixed-8.5/1.5 electrodes. The cells were tested under 0.2, 0.5, 1.0, 2.0, and 5.0 C in a sequence for three cycles each. The com-8.5/1.5 electrode demonstrates the best rate performance. At 5.0 C, the composite electrode delivers 136.6 mA·h/g, whereas LNMO and LNMC release ∼30 and 80 mA·h/g, respectively. The rate capacity of mixed-oxide electrode is between LNMO and LNMC. One reason why the rate performance of composite electrode is improved is because the spinel phase in the composite can provide a three-dimensional Li+ diffusion path and then improve the Li+ diffusion coefficient.11 Spinel LNMO has a high Li+ diffusion coefficient

Table 2. First Coulombic Efficiencies (%) of Spinel LNMO, Li-Rich Oxides LNMC, com-8.5/1.5, and mixed-8.5/1.5 Materials

first efficiency

spinel LNMO

Li-rich oxide LNMC

com-8.5/1.5

mixed-8.5/1.5

152.05

69.5

86.3

78.1

Figure 5. Rate capability of spinel LNMO, Li-rich oxide LNMC, com8.5/1.5, and mixed-8.5/1.5 materials.

phase at the microscale. The radius of the rings is equivalent to the interplanar spacing. The two rings can only be identified as (220) and (442) planes of the spinel phase (Fd3m ̅ ), not the layered structure (R3̅m or C2/m). The other spinel diffraction rings, (111) and (400), are very close to (003) and (104) rings of the layered structure, respectively. The ring indexed to (110) belongs to the layered structure (C2/m) of Li2MnO3. The SAED result indicates the presence of layered (R3̅m)-layered (C2/m)-spinel (Fd3̅m) local heterostructure, which is consistent with bulk XRD results. The high resolution TEM image of the composite cathode is also shown in Figure S3. The d-spacing of ∼0.47 nm in Figure S3b is ascribed to the plane (003) of layered structure, while the d-spacing of ∼0.33 nm in Figure S3d can only be ascribed to the plane (211) of spinel structure. These results show the presence of layered/spinel heterostructure in the composite cathode. 3.2. Electrochemical Performance. Figure 4a demonstrates the initial charge−discharge profiles at 0.1 C of spinel LNMO, Li-rich oxide LNMC, com-8.5/1.5, and mixed-8.5/1.5 electrodes. An optimization process of synthesis condition for the composites is shown in Figure S1. The mixed-8.5/1.5 powders were prepared by mechanical grinding LNMC and LNMO in an 8.5/1.5 molar ratio for comparison. Both LNMO and LNMC exhibit an initial discharge capacity ∼250 mA·h/g, similar to 251.4 mA·h/g of the mixed-8.5/1.5. However, the discharge curves of these electrodes differ substantially. During the discharging process, LNMO electrode yields three distinctive plateau regions. The plateau at ∼4.7 V originates from Ni4+/Ni2+ couples. The plateau at ∼2.7 V results from a Mn4+/ Mn3+ couple, which coincides with the insertion of additional Li+ into vacant 16c octahedral sites and phase transformation from cubic to tetragonal structure.9 The Mn4+/Mn3+ redox and phase transformation in turn lead to a drastic volume change, as well as Jahn−Teller distortion, resulting in a poor cyclability, as shown in Figure 4b.10 During first discharge process for LNMC, a wide plateau region above 3 V originated from Ni4+/ Ni2+ and Co4+/Co3+ couples in LNMC and a small plateau at ∼2.7 V corresponding to the spinel phase can be observed. 25651


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Figure 6. Ex situ Mn K-edge XANES spectra of (a, c, e) Li-rich oxide LNMC and com-8.5/1.5 at open circuit potential (b, d, f) Li-rich oxide LNMC and com-8.5/1.5 after the first charge. (c, e) Local enlarged images of part a. (d, f) Local enlarged images of part b.

of ∼10−8 cm2 s−1, about two orders greater than that of layered LNMC.9,26 Unfortunately, the severe Jahn−Teller distortion in LNMO leads to a poor rate capacity. In the ∼2.7 V discharging plateau, Mn4+ is reduced to Mn3+, resulting in a phase transition from cubic (c/a = 1) to tetragonal (c/a = 1.16). This distortion and the mismatch of the lattice parameters impede the threedimensional Li+ diffusion pathway, thus a poor rate capability.16,27 Judging from the aforementioned discussion, we know that the role of spinel phase in the composite cathode differs from that in the mixed-oxide electrode. It appears that there exists a synergetic effect between the spinel phase (LNMO) and the layered oxide (LNMC) in the composite, which enhances the cyclability, first-cycle Coulombic efficiency, energy density, and rate capacity.

3.3. Valence, Local Environment of Mn, and Li+/Ni2+ Disordering. We carried out an extensive research to understand the origin of the property improvements in a composite cathode by using XANES. XANES is a fingerprinting technique for the determination of local chemical environment since the absorption edge energy increases with an increase in the oxidation states due to a shorter bond length in the compound with a higher valence. Parts a, c, and e of Figure 6 display the ex situ Mn K-edge XANES spectra of LNMC and com-8.5/1.5 cathodes under open circuit potential (OCP), along with Mn2O3 and MnO2 for a baseline study. The black line is the absorption spectrum of Mn2O3 (Mn3+) and the red line is for MnO2 (Mn4+). It can be seen that the valence state of Mn in LNMC and com-8.5/1.5 25652


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Figure 7. Ex situ Mn K-edge XANES spectra of (a, c, e) Li-rich oxide LNMC and (b, d, f) com-8.5/1.5 at different preset voltages during the first cycle. (c, e) Local enlarged images of part a. (d, f) Local enlarged images of part b.

Table 3. Distances between Mn and Its First Oxygen Neighbors (dMn−O), and Its First Metal Neighbors (dMn‑M) during the First Discharge Process LNMC composite

dMn−O-4.8V (Å)

dMn−O-2V (Å)

dMn‑M-4.8V (Å)

dMn‑M-2V (Å)

1.9156 1.9105

1.9477 1.9166

2.9101 2.9061

2.9243 2.9134

in Figure 4a and CV curve in Figure S4. The absorption zones, A and B, in Figure 6e manifest the existence of Li2MnO3.28 After the first charging process, there exists a positive shift in the absorption spectra for both LNMC and com-8.5/1.5 electrodes, indicating the partial oxidation of Mn3+, as shown in Figure 6, parts b, d, and f. Figure 6d illustrates the half-height energy of LNMC, similar to com-8.5/1.5, but LNMC has a greater energy for the maximum absorption, ∼ 0.7 eV. During

electrodes is between +3 and +4. The half-height energy of ∼6557 eV of LNMC and com-8.5/1.5 is almost the same. But the maximum of absorption edge for com-8.5/1.5 is 0.3 eV higher than that of LNMC, which indicates the valence state of Mn in the composite is greater, likely due to the presence of spinel phase. This fact proves that the Mn valence state of integrated spinel is +4 and the composition is LiNi0.5Mn1.5O4, which is consistent with the Ni4+/ Ni2+ plateau of com-8.5/1.5 25653


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Mn in LNMC. The negative shift of absorption zone A illustrates the formation of LixMn2O4-like spinel,28 just similar to those in Figure 6, parts e and f. Since the image scale in Figure 6 is the same, we can see that the degree of conversion reaction to spinel in LNMC is greater than that in composite material. Besides, the distance between Mn ion and its first oxygen neighbor/first metal neighbor of LixMn2O4 is larger than that of LNMC, which is attributed to the larger size of Mn3+ (0.645 Å) than Mn4+ (0.53 Å). Table 3 lists the distances between Mn ion and its first oxygen neighbor/first metal neighbor, which are determined from the analysis of Fourier transforms of Mn K-edge EXAFS. Comparing to LNMC, the changes of dMn−O and dMn‑M in a composite material is smaller, indicating less production of LixMn2O4-like spinel and suppressed phase transformation. The ex situ Mn K-edge XANES spectra of LNMC and com-8.5/1.5 after first cycle are shown in Figure 8. As seen in Figure 8b, the higher half-height and top-peak energies of com8.5/1.5 indicate a higher Mn oxidation state in com-8.5/1.5 than LNMC. The positive position of absorption zone A confirms a milder convert reaction to LixMn2O4-like spinel in composite material, as illustrated in Figure 8c, which indicates the phase transformation from layered structure to spinel in a composite material is suppressed. XANES studies demonstrate that the spinel phase in a composite electrode results in a higher initial valence of Mn than that in LNMC. As a result, LNMC has a greater oxidation reaction of Mn during the charging process and greater reduction reaction during discharge than that com-8.5/1.5. More LixMn2O4-like spinel is produced during the cycling test in LNMC than composite electrode. As a result, a severer Jahn− Teller distortion occurs in LNMC, but is suppressed in the composite electrode. This is a vital reason why composite material has a better cyclability and rate capability. Besides, the Mn3+ in composite material may not participate oxidation and reduction reaction. According to Li’s report, a suitable Mn3+ can improve the rate performance of Li-rich oxides,29 which is attributed to the bigger size of Mn3+ than Mn4+ and a wider path for Li+ diffusion can be provided. Our previous work indicates that the degree of Li+/Ni2+ disordering plays a critical role in the electrochemical performance of LNMC. A lower Li+/Ni2+ disordering is desirable for the diffusion of Li+ since the size of Ni2+ (0.69 Å) is smaller than Li+ (0.76 Å), and then leads to a better rate capability.26 The degree of Li+/Ni2+ replacement can be evaluated by the ratio of I003/I104 of LMNC (Figure 2a). A higher ratio of I003/ I104 indicates a lower Li+/Ni2+ disordering.26 For the composite electrode, the (003) and (104) reflections still exist in the XRD pattern, which enable us to use this method to study Li+/Ni2+ disordering. Table 1 lists I003/I104 ratios for LNMC (0.94) and com-8.5/1.5 electrodes (1.51). The degree of Li+/Ni2+ disordering in the composite electrode is lower than LNMC. Results of Rietveld refinements for LNMC and com-8.5/1.5 are also listed in Table 1. The value of Ni2+(3b) represents the amount of Ni2+ in the Li+ layer, equivalent to the Li+/Ni2+ replacement ratio.26 The value of Ni2+3b/Ni2+total of com-8.5/1.5 is 24.1%, smaller than 31.6% of LNMC. These results confirm the decrease of Li+/Ni2+ disordering in the composite cathode by integrating a spinel component. The deceased Li+/Ni2+ disordering leads to higher Li+ diffusion rate, which contributes to the improved cyclability and rate capability. To further understand the interaction between the intergrown spinel and layered structures, more investigations were

Figure 8. Ex situ Mn K-edge XANES spectra of (a, b, c) Li-rich oxide LNMC and com-8.5/1.5 after the first discharge. (b, c) Local enlarged images of part a.

the first charging process, the Mn oxidation reaction in composite electrode appears milder than that in pure layered LNMC. After the first charging cycle, the absorption zone A, B in Figures 6, parts e and f, indicates that Li2MnO3 undergoes incomplete decomposition, and is coincided with the activation process of LNMC and composite materials in Figure 4 b. Parts a, c, and e of Figure 7 present the ex situ Mn K-edge XANES spectra of LNMC, and parts b, d, and f of Figure 7 show the ex situ Mn K-edge XANES spectra of com-8.5/1.5 at different preset voltages in the first cycle. As illustrated in parts c and d of Figure 7, the half-height energy in the first discharging process is almost the same for both LNMC and com8.5/1.5 electrodes. The absorption peak of LNMC shifts by −1.5 eV during the first discharging process, more negative than com-8.5/1.5 (∼ −0.4 V), suggesting a greater reduction of 25654



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conducted by studying the effect of spinel ratio and synthesis condition on the Li+/Ni2+ disordering in both LNMC and the composites. Tables S1, S2, and S3 summarize the effects of heating temperature, heating time and spinel fraction on the I003/I104 ratio. As seen from Tables S1 and S3, the increase in heating temperature and spinel fraction do increase the I003/I104 ratio, and thus decrease the level of Li+/Ni2+ disordering. As demonstrated in Table S2, for LNMC heated at the same temperature, the Li+/Ni2+ disordering increases with increasing the time during the initial period. Then the Li+/Ni2+ disordering decreases because of the formation of spinel phase, originating from the loss of lithium in the long term heating process. This is consistent with the small ∼2.7 V discharge plateau in Figure 4a and XRD Rietveld refinement. Hence, the Li+/Ni2+ disordering can be controlled by proper selection of spinel fraction and synthesis condition to improve the electrochemical performance of the cathode.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07169. The optimization process to prepare layered/spinel materials, XRD refinements of pure layered and composite materials, TEM and HRTEM images of layered and spinel particle in composite material, the second CV curve of composite material, comparison of electrochemical performance of layered, composite and mixed materials prepared in different conditions, and I003/I104 ratio of layered and composite materials prepared under different conditions (PDF)

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REFERENCES

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4. CONCLUSIONS In summary, we investigated the electrochemical performance of a composite electrode consisting of layered Li[Li0.2Ni0.12Mn 0 . 56 Co 0 . 12 ]O 2 (LNMC) and spinel Li[Ni 0 . 5 Mn 1 . 5 ]O 4 (LNMO) by comparing it with LNMC, LNMO, and a physical mixture of both. The composite cathode exhibited a superior electrochemical performance, with respect to the cyclability, first-cycle efficiency, rate capability and energy density. Extensive post analyses demonstrated that the improvement was attributed to a lower Li+/Ni2+ disordering, milder redox reaction of manganese ions, and suppressed converting reaction to form LixMn2O4-like spinel. From a practical point of view, the cathode performance of the layered LNMC can be enhanced in a composite electrode by properly choosing the spinel ratio, synthesis temperature, and calcination time.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86 0592-5197067. E-mail: [email protected]. (J.-T.L.). *Telephone: +86 0592-2180181. E-mail: [email protected] (S.-G.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (21321062, 21373008, 21273184) and the Natural Science Foundation of Fujian Province of China (2015J01063). 25655


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A Synergistic Effect in a Composite Cathode ... - ACS Publications

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