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Synthesis and Electrochemical Properties of LiNi0.5Mn1.5O4 for Li–ion Batteries by the Metal–Organic Framework Method Chengjie Yin, Hongming Zhou, Zhaohui Yang, and Jian Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02553 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Materials & Interfaces

Synthesis and Electrochemical Properties of LiNi0.5Mn1.5O4 for Li–ion Batteries by the Metal–Organic Framework Method Chengjie Yin,† Hongming Zhou, †,‡,* Zhaohui Yang, † Jian Li†,‡,*

†

School of Materials Science and Engineering, Central South University, Changsha,

Hunan 410083, China

‡

Hunan Zhengyuan Institute for Energy Storage Materials and Devices, Changsha,

Hunan 410083, China

*Corresponding Author:

*E-mail: [email protected]. Telephone: +86-731-82075517. Fax: +86-731-82075517 (H.M. Zhou).

*E-mail: [email protected]. Telephone: +86-731-88877173. Fax: +86-731-88877173 (J. Li).

KEYWORDS: cathode material, LiNi0.5Mn1.5O4, crystal surface orientation, metal– organic framework, lithium–ion batteries

ABSTRACT: A LiNi0.5Mn1.5O4 cathode material with high surface orientation was prepared via a complexing reaction coupled with the elevated-temperature solid-state method. First, a bimetal–organic framework containing Ni2+ and Mn2+ ions was 1

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synthesized via a self-assembly route using pyromellitic acid (PMA) as a dispersant and complexing agent. This step was followed by calcination with lithium acetate using PMA as a structure-directing agent. The resulting LiNi0.5Mn1.5O4 (M-LNMO) cathode material was investigated using X-ray diffraction, transmission and scanning electron microscopies, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and charge/discharge tests. For comparison, LiNi0.5Mn1.5O4 samples were prepared by co-precipitation and the solid-phase method under the same conditions. M-LNMO was highly crystalline with low impurity, uniform grain size and a preferred orientation in the (111) and (110) planes. Owing to these advantages, the M-LNMO cathode material exhibited overwhelmingly high cyclic stability and rate capability, the M-LNMO delivered a capacity of 145 mAh g–1 at a discharge rate of 0.1 C, and a discharge capacity retention of 86.6% at 5 C after 1000 cycles. Even at an extremely high discharge rate (10 C), the specific capacity was 112.7 mAh g−1, and 78.7% of its initial capacity was retained over 500 cycles. The superior electrochemical performance, particularly during a low-rate operation, was conferred by improved crystallinity and the crystal orientation of the particles.

1. INTRODUCTION

To meet the continuous development of electric vehicles (EVs) and hybrid-electric vehicles (PHEVs), a new generation of lithium-ion batteries (LIBs) is urgently required. The development of LIBs crucially depends on the research and 2


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development of promising cathode materials.1,2 Spinel LiNi0.5Mn1.5O4 (LNMO) has attracted considerable attention owing to its high operating voltage (4.7 V vs. Li/Li+) and theoretical capacity (147 mAh g–1), as well as a high rate capacity derived from three-dimensional (3D) channels for lithium-ion transfer and high conductivity.3,4 Therefore, LNMO has been extensively researched for applications in novel LIBs with high energy density.5–7

Depending on the heat treatment process, LNMO exists in two crystalline forms: face–centered cubic (Fd-3m) and cubic (P4332).8 The P4332 structure is stoichiometric with a Mn valence of +4. The Fd-3m structure is nonstoichiometric and contains trace amounts of Mn3+. Mn3+ is generated when oxygen escapes from the LNMO lattice at a high temperature (>800°C) and is not replaced by inhalation during rapid cooling.9 Because of its disordered structure, Mn3+-containing LiNi0.5Mn1.5O4 has a higher lithium-ion diffusion coefficient, greater conductivity, and stronger electrochemical performance in comparison with P4332.10 However, the structurally unstable Fd-3m type is vulnerable to specific-capacity attenuation and poor stability. This deterioration is mainly due to Mn3+ ions (generated as a result of the Jahn–Teller effect), decomposition of the catalytic electrolyte, and dissolution of Mn2+.11,12 Moreover, the poor crystallinity and the LixNi1–xO impurities accelerate the structural collapse of the Fd-3m type during the charging/discharging process. As revealed in recent studies, the crystalline orientations of the particle surfaces significantly affect a material’s electrochemical performance. LNMO usually presents a variety of crystal surfaces, among which (111), (110), and (100) are most closely associated with the 3



electrochemical properties of the material.13,14 The (111) facets possess the lowest surface energy, and increasing the number of 3D Li+ transmission channels can improve the rate capacity and stability. However, the (100) surface facets also accelerate the Mn dissolution, causing specific-capacity attenuation, oxidation decomposition of the electrolyte, and structural collapse.15–17 Hence, it is important to improve the crystallinity and crystal orientation of the particles for enhancing the electrochemical performance, particularly when the cell is operated at a low rate.

Recently, several researchers have enhanced the electrochemical performance of materials by improving the synthetic methods. Co-precipitation is generally considered as a suitable procedure for synthesizing materials from mixed metal oxides and solid solutions. Cui et al.18 used this method to explore the influence of two precipitants (Na2C2O4 and Na2CO3) on the electrochemical performance of materials. The Na2CO3 precipitator yielded a material with high specific capacity (123.8 mAh g– 1

) at a rate of 0.25 C and an interface resistance of 441.8Ω. Atomic dispersion in

co-precipitation has been achieved by various strategies. Adding organic compounds to the co-precipitation process can effectively improve the dispersion degree, purity, crystallinity, and crystal orientation of LNMO. Arrebola et al.,19 Lin et al.,20 and Zhang et al.21 added PEG400, PVP, and PEG with different molecular weights to the co-precipitation process, and investigated the electrochemical properties of the resulting LNMO material. The organic surfactant guided the dispersant and provided a template for the synthetic process. By adjusting the particle size at the same time, the nucleation and evolution of the crystal materials can be controlled for preferential 4


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growth, thereby improving the electrochemical performance of the material. Such growth crucially affects the morphology, structure, and electrochemical performance of the as-prepared materials with different organic molecular weights, chain lengths, and structures. However, in most cases, the carbonate or hydroxide precipitated by an inorganic precipitator partially dissolves the OH− and CO32− during the washing process. The nonstoichiometric loss of metal ions introduces structural defects in the as-prepared materials.22

Metal–organic frameworks (MOFs) have been traditionally applied in energy storage devices such as supercapacitors, and as anode materials for lithium ion batteries. These applications profit from the adjustable pore structure, uniform pore-size distribution, high specific surface area, topological structure, and high compositional diversity of MOFs.23–25 More importantly, MOFs have been fully testified as a prospective precursor in the preparation of mixed metal oxides with stoichiometric, morphology-inherited, and uniform element distribution. MOF-based preparations of cathode electrode materials have been applied as precursors of dual metal oxides, and calcined at high temperature. These oxides exhibit high specific capacity and rate capability.26–28 However, these methods cannot control the crystal orientation and size of the prepared materials. Moreover, the presence of Li+ impedes the mixing with oxides and the entry of Li+ into the lattice during the calcination process. Therefore, herein, a cathode material with an MOF precursor is prepared by a one-step calcining method. The organic ligand in the preparation of homogeneous Ni–Mn–MOFs is pyromellitic acid (PMA). In this approach, the organic matter serves not only as an 5



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organic precipitant and dispersant during the co-precipitation process, but also as a Li+ adsorbent and a structure-directing agent. The prepared M-LNMO exhibits considerably higher lithium-ion storage performance than S-LNMO and C-LNMO, which samples were prepared using solid-state method (marked as S-LNMO) and normal coprecipitation method (marked as C-LNMO) as well. To the best of our knowledge, the fabrication of mixed metal-oxide cathode materials using MOF precursors has been rarely investigated.

2. EXPERIMENTAL SECTION

2.1 Material Preparation

M-LNMO powder was synthesized via the calcination with Ni–Mn-MOFs as precursors (see Figure 1).

2.2 Synthesis of Ni–Mn-MOFs by a Hydrothermal Method

In

a

typical

process,

Ni(CH3COO)2·4H2O

(7

mmol,

AR,

99.0%)

and

Mn(CH3COO)2·4H2O (21 mmol, AR, 99.0%) were dissolved in 50 mL deionized water, Furthermore, 14 mmol PMA (Alfa, 99.0%) was dissolved in 50 mL of 1.26 M NaOH aqueous solution (nNaOH: nPMA = 4.5:1). The mixture solution of metal ions was added dropwise to the PMA solution. The mixed solution was mechanically agitated for 1 h, and then transferred into a Teflon-lined stainless steel vessel at 120 °C for 24 h. After cooling to ambient temperature, the brownish yellow precipitate was filtered 6


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and washed 10 times with deionized water to remove the residual Na+ content, after which it was dried at 80 °C overnight.

2.3 Synthesis of Spinel LNMO Cathode Materials

The mixture of Ni–Mn-MOFs and 10% excess (relative to the stoichiometric amount) LiCH3COO·2H2O was first calcinated at 400 °C for 2 h and then at 800 °C for 10 h in air to obtain M-LNMO materials. For comparison, the S-LNMO and C-LNMO samples were prepared using the solid-state method and by the conventional co-precipitation procedure with the Na2CO3 precipitant, respectively. All samples were calcinated at 800 °C for 10 h in air at a heating rate of 5 °C min–1.

2.4 Material Characterizations

The fast-Fourier infrared (FTIR) spectra were recorded on a Nicolet 6700 FTIR spectrometer with a wavenumber range of 400–4000 cm–1. The Raman spectra of the three samples were collected on a laser confocal micro-Raman spectroscope using an excitation light of 520 nm from an Ar ion laser. The crystallite structures of the as-obtained samples were characterized by X-ray diffraction (XRD) using Cu Kα radiation (λ =1.54056 Å) operated at 40 kV and 250 mA. The structural characteristics of LNMO were investigated by field-emission scanning electron microscopy (SEM), Hitachi SU8010), energy-dispersive X-ray spectroscopy (EDS) (Bruker Quantax) and high-resolution transmission electron microscopy (TEM) (Tecnai G2 F20). The surface chemical states of M-LNMO were determined by X-ray photoelectron 7



spectroscopy (XPS) (Thermo ESCALAB 250XI) with Al Kα radiation (1486.6 eV) as the X-ray source. The surface areas and pore volumes of the samples were measured by the N2 adsorption/desorption technique using a TriStar 3000 system. The elemental contents of the materials were characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES).

2.5 Electrochemical Performance Test

The cathode electrodes were fabricated by forming homogenous slurry of the active materials, acetylene black and polyvinylidene fluorides in N–methylpyrrolidinone at a weight ratio of 80:10:10. The prepared slurry was coated onto aluminum foil and dried at 80 °C for 5 h in a vacuum. The mass loading of LNMO was ~3 mg cm–2. The anode, separator, and electrolyte were lithium metal in a CR2032 coin cell, Celgard 2500, and 1M LiODFB, respectively, at an EC:DEC:DMC ratio of 1:1:1. All cells were assembled in a Mikrouna glovebox under an argon atmosphere (H2O≤ 0.1 ppm, O2≤ 0.1 ppm). Galvanostatic tests were performed at a voltage of 3.5–4.9 V (vs. Li/Li+) using a battery test system (XINWEI, CT 4008) at different rates (1 C = 147 mAh g–1). The cyclic voltammetry (CV) curves were measured on an electrochemical workstation (CHI660D, Shanghai Chen Hua Instrument Co., Ltd) in the range 3.5–4.9 V at different scan rates. The electrochemical impedance spectroscopy (EIS) data were acquired at 5 mV from 100 KHz to 10 mHz.

3. RESULTS AND DISCUSSION

8


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3.1 Morphology and Structural Analysis

Figure 2 shows the SEM images of LNMO obtained via different synthesis procedures. As shown in Panels a–c’, the three samples exhibited different morphologies and size distributions. The C-LNMO and S-LNMO exhibited irregular polyhedral shapes with a wide particle size distribution (Figures 2b, and 2c, respectively). Owing to the high calcination temperature (>800 °C), these compounds mainly comprised (100) crystal surfaces.29 The (100) surface exhibited higher electrochemical performance than the (111) surface because ethylene carbonate (EC) was oxidized more easily on the (100) surface, thereby weakening the Mn–O ionic bonding network.30 In contrast, M-LNMO comprised microsized (1.5-µm) regular polyhedral crystals with a preferred orientation of the (111) surface (Figure 2 a′). This structure is more stable (has lower surface energy) than those of C-LNMO and S-LNMO,31 so it delivers high performance and provides a generous 3D channel for quick Li+ transmission. These properties are mainly derived from the molecular guide template and size-tailoring agent, which can control the crystal orientation and size of the material. It is also evident that the material surface is roughened by a coating of Li2CO3. The possible reaction is Li2O + CO2 = Li2CO3 (where Li2O and CO2 are produced as result of the decomposition of excess CH3COOLi and organic matter, respectively at high temperatures). During the cooling process, the moisture in the surrounding atmosphere,32,33 as confirmed by TEM and XPS characterization ( Figures 3a and 5). The surface layer of Li2CO3 probably suppressed the side reactions of the active cathode material (M-LNMO) with the electrolyte by reducing 9



the contact interface, providing cyclic stability. Furthermore, in the detailed TEM images of the particles, the particle size of M-LNMO appears more uniform than those of S-LNMO and C-LNMO (Figure 3), and M-LNMO exhibits higher crystallinity and an enriched (111) crystal surface. The Li2CO3 coating on the M-LNMO surface was ~8 nm thick, and exhibited discernible neighboring lattice fringes with approximate widths of 0.471 nm, which match the interspaces of the (111) crystal surface of the cubic spinel LiNi0.5Mn1.5O4 (see Figure 3a′). The neighboring lattice fringes of the (111) crystal surface are not obvious in Figures 3b′ and 3c′.

Figure 4a shows the XRD pattern of the as-prepared M-LNMO sample. The XRD patterns of C-LNMO and S-LNMO are also shown for comparison. By referring to the standard card (JCPDS no. 80-2162), all peaks of the three samples can be attributed to the spinel LNi0.5Mn1.5O4 phases (111), (311) and (400).34 This finding proves that spinel LiNi0.5Mn1.5O4 was successfully synthesized from the Ni–Mn– MOF precursors. As observed in Figure 4a, all three samples contained a LixNi1–xO impurity phase owing to the formation of Ni prolapse from the lattice, caused by oxygen loss during the high-temperature calcination.35 However, the peak intensity of the LixNi1–xO impurity was significantly weaker in the M-LNMO pattern than in the other patterns. This phenomenon demonstrates the significant role of PMA, which strongly coordinates with Ni2+ and Mn2+. Aided by the strong adsorbability of MOF, PMA enables homogeneous adsorption of Li+, which encourages nucleation, improves the even mixing, and maintains a strictly stoichiometric precursor. Confirming this result, the chemical constitutions of the M-LNMO, C-LNMO, and S-LNMO samples 10


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were

estimated

as

Li1.010Ni0.499Mn1.504O4,

Li0.998Ni0.485Mn1.505O4,

and

Li0.997Ni0.491Mn1.496O4, respectively, in the ICP test. Therefore, the oxygen deficiency and LixNi1–xO impurities were largely prevented in M-LNMO. The intensity ratio I111/I311 indicates the anti-site disorder between Li+ and the transition metal ion in spinel LiNi0.5Mn1.5O4. As summarized in Table 1, the I111/I311 ratio of M-LNMO was 2.165, higher than that observed for the other two samples and much more proximate to the standard JCPDS (80–2162) data. By providing a smooth route for Li+ transport, M-LNMO should achieve a superior rate capability.36 On the contrary, the intensity ratio I311/I400 demonstrates the a degree of tetragonal distortion from the cubic spinel structure of LiNi0.5Mn1.5O4 and some degradation of structural integrity. As summarized in Table 1, M-LNMO more closely matched the standard JCPDS (80-2162) data37 than the other samples. The deviation of the I311/I400 ratio from the standard data may be related to the presence of Mn3+. The lattice parameters of the M-LNMO, C-LNMO, and S-LNMO samples, calculated from the XRD patterns using the Rietveld refinement method, were 8.175(1) Å, 8.184(7) Å, and 8.172(4) Å, respectively. The increased lattice constants of C-LNMO and S-LNMO can be explained by the larger amount of Mn3+ in these samples in comparison with that in M-LNMO. In particular, Mn3+ (0.65 Å) has a larger ionic radius than Mn4+ (0.54 Å). This conjecture is consistent with the discharge performances discussed later.38

The XRD data cannot easily distinguish the Fd-3m and P4332 space groups, because of the identical scattering factors of Mn and Ni. However, Raman spectroscopy can distinguish the ordered P4332 space group from the disordered Fd-3m space group of 11



spinel LiNi0.5Mn1.5O4.39 Figure 4b shows the Raman spectra of the three samples. The Raman peak at approximately 638 cm–1 is assignable to the symmetric Mn–O stretching mode of the MnO6 octahedral, and the peaks around 420 and 495 cm–1 are associated with the Ni2+–O stretching mode in the spinel structure. The division of the T2g3 band near 585–620 cm–1 and 164 cm–1 proves the existence of ordered-phase P4332 in the spinel. The M-LNMO, C-LNMO, and S-LNMO samples all show the evidence of the coexisting Fd-3m and P4332 space groups. The apparent shoulder bands at 620 cm–1 and an intensity of 164 cm–1 in M-LNMO suggest a higher extent of Ni/Mn ordering in this sample than in the other samples. In comparison, the spectra of S-LNMO sample are characteristic of a typical disordered Fd-3m spinel. This suggests that the suitable content of redox-active Mn3+ in M-LNMO increases the electronic conductivity of the material, thereby improving its specific capacity, rate capability, and cyclic stability.40

To further understand the surface chemical composition of M-LNMO, the M-LNMO sample was investigated by XPS, the results of which are shown in Figure 5. The spectrum displays four distinguishing features in the Ni2p region (Figure 5a). The main peaks of Ni2p3/2 and Ni2p1/2, which appear at 855.4 and 872.6 eV, respectively, are assigned to Ni2+ cations. The observed satellite peaks at 861.4 and 878.5 eV can be ascribed to the multiple splitting of the energy levels of nickel oxide (NiO).41 The two Mn2p peaks at 642.1 and 654.3 eV are assignable to Mn2p3/2 and Mn2p1/2, respectively. The peaks at 641.7 and 653.4 eV are attributed to Mn3+, whereas those at 643.1 and 654.4 eV arise from Mn4+. The O1s spectrum (Figure 5c) mainly comprises 12


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M–O (where M is the metal ion) existing in the spinel lattice and carbonate species (– CO32–).42 The peak at 531.2 eV can be assigned to C=O bonds in the carbonate species. The EDS, TEM, and ICP results testify the existence of Li2CO3. Inhibiting the dissolution of Mn3+ can improve the cyclic stability of LNMO during the charge/discharge process. To investigate the surface properties of the material, we conducted an N2 adsorption/desorption analysis (Figure S1, Supporting Information). The BET specific surface areas of M-LNMO, C-LNMO, and S-LNMO were 9.7, 2.6, and 1.3 m2 g–1, respectively, demonstrating an electrochemically active surface that effectively improves the electrochemical performance in batteries.

Figure 1 shows the possible formation mechanism of LiNi0.5Mn1.5O4 in the one-step calcining method. Owing to the strong coordination complexation of PMA with the transition metal ions, banded Ni–Mn–MOFs are easily self-assembled at room temperature, forming a bright-yellow precipitate. Self-assembly begins at when the mixed acetate solution of Ni and Mn is added into the PMA solution dropwise under mechanical agitation (Figure S2, Supporting Information). Excessive OH– acts as an auxiliary complexing agent in the reaction between the ligand PMA (C10H6O8) and metal ion (denoted as M). The chemical reactions are as follows:

C10H6O8 + 4NaOH

C10H2O8Na4

(1)

1.5nC10H2O8Na4 + 4nM(Ac)2 + 2nNaOH

nM4(C10H2O8)1.5(OH)2 + 8nNaAc (2)

The Ni–Mn–MOF monomers provide a guidance template, on which the polymer 13



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laterally overgrows to form a banded precursor. The strong adsorbability of MOF ensures the even dispersal of Li+ ions in the precursor. In the final calcination procedure, the organic ligands play three major roles: a molecular self-elimination role which guides the preferred crystal growth on the template; a size-tailoring role which controls the size of the material; and a crystal-forming assistance role which acts through the heat liberated by combustion (Figure S3, Supporting Information).

Table 1. Comparison of physical properties of the LNMO materials Samples Lattice % of BET surface 3+ parameter I111/I311 I311/I400 M area (m2 g–1) a b α (Å) (%) M-LNMO 8.175(1) 2.165 0.921 10.3 9.7 C-LNMO 8.184(7) 1.847 0.961 16.1 2.6 S-LNMO 8.178(4) 1.864 0.927 11.1 1.3 JCPDS(80-2162) 8.171(5) 2.660 0.876 0 a

Rp/% 8.98 8.46 8.57

Lattice parameters are calculated by the Rietveld refinement of the XRD results.

b

Relative Mn3+ content, evaluated by dividing the initial discharge capacity between

4.3 and 3.8 V by the theoretical capacity (147 mAh g−1).

3.2 Electrochemical Characterization.

The phase transformation or oxidation/reduction processes during electrode reactions were investigated via CV. Figure 6 compares the CV curves of the three LNMO cathode materials between 3.5 and 4.9 V scanned at a 0.05 mV s–1. The curves exhibit the typical reversible peaks of the disordered Fd-3m material. The redox-current peaks in the 4.6–4.8 V region correspond to Ni2+/Ni3+ and Ni3+/Ni4+, demonstrating a two-stage Li+ extraction–insertion process from and into the spinel framework.43 14


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Among the three samples, M-LNMO exhibits the smallest potential difference between the cathodic and anodic current peaks of Ni2+/Ni3+ and Ni3+/Ni4+ (0.09 and 0.10 V, respectively), indicating fast lithium extraction/insertion kinetics.34 Thus, M-LNMO is expected to deliver superior high-rate performance. The weak redox-current peaks at around 4.0 V are attributed to the Mn3+/Mn4+ redox reaction. As observed in the enlarged region around 4.0 V, the intensity of this peak increases with the increasing amount of Mn3+ in the LNMO material. This finding is consistent with the XRD result.

The CVs of the three LNMO materials recorded at different voltage scanning rates are shown in Figure S4 (Supporting Information). As the scanning rate (v) increases, the peak current (ip) also increases and the potential separation of each redox couple widens. Assuming that the intercalation reaction is controlled by the solid-state diffusion of Li+, the diffusion coefficient of the Li ions (DLi) can be calculated using the Randles−Sevcik equation:44

ip = 2.69 × 105n3/2 ADLi1/2ν1/2C0Li,

(3)

where ip is the peak current (A), n is the number of electrons per reaction species, A is the total surface area of the electrode (1.13 cm2 in this case), DLi is the apparent diffusion coefficient of Li+ (cm2 s−1), ν is the voltage scanning rate (V s−1) and C0Li is the bulk concentration of Li+ in the electrode (0.02378 mol cm−3) based on the Rietveld refinements.45 Note that ip is proportional to ν1/2, confirming an ion

15



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diffusion-controlled behavior. The diffusion coefficients of the three LNMO materials, determined from the slopes of linear ip vs. ν1/2 plots, are listed in Table 2. The DLi values of M-LNMO, C-LNMO, and S-LNMO were 3.26 × 10–9, 6.0 × 10–10, and 8.0 × 10–11 cm2 s–1, respectively. The DLi of M-LNMO was 2–4 orders of magnitude higher than those reported in previous reports,45,46 implying that the present M-LNMO sample intrinsically provides a fast lithium diffusion pathway through the disordered Ni and Mn ions in the Fd-3m phase. The large number of (111) crystal surfaces in M-LNMO provide a rich transmission channel for Li+ ions. In contrast, diffusion through C-LNMO and S-LNMO is likely hindered by the relatively large impurity phase.

Table 2. Lithium diffusion coefficients (DLi) of the three LNMO materials, calculated from CV measurements DLi (×10–9 cm2 s–1) sample

O1

O2

R1

R2

Da

M-LNMO C-LNMO S-LNMO

2.79 0.52

4.13 0.82

3.6

2.5

3.26 0.60 0.08

0.45 0.06

0.10

The average peak values Da are listed for comparison.

Figure 7a shows the initial galvanostatic charge/discharge profiles of the M-LNMO, C-LNMO, and S-LNMO materials between 3.5 and 4.9 V at a current rate of 0.1 C (1 C = 147 mAh g−1). The initial discharge capacities of the three samples were 145, 106, and 119 mAh g–1, respectively. The three materials exhibit two discharge plateaus around 4.7 and 4.0 V, consistent with the above CV curves, and with the results obtained from the XRD and Raman characterizations. The large specific surface area 16


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of M-LNMO provides a highly electrochemically active surface that contact with the electrolyte. The cyclic performances of the obtained LNMO materials are shown in Figure 7c. At 30 °C, after 500 charge/discharge cycles at a discharge rate of 5 C, the M-LNMO, C-LNMO, and S-LNMO samples delivered specific discharge capacities of 136, 83, and 76 mAh g–1, respectively, and retained 92.8%, 64.7%, and 31.6% of their largest capacities, respectively. After 500 cycles at 10 C, the specific-capacity retention rate of M-LNMO was 78.7%. The reasons for this excellent cyclic performance are threefold. First, the cathode materials are obtained with uniform distribution of elements, high stoichiometry, high crystallinity, and few impurities due to the strong complexation between the organic ligands and metal ions. Second, the organic ligands provide a molecular guiding template with a stable (111) crystal surface that optimizes the crystal growth, while hindering the formation of the cyclical (100) crystal surface. Finally, the uniformly coated Li2CO3 on the M-LNMO surface effectively prevents the dissolution of Mn2+ during the charging/discharging process. These three factors cooperate to enhance the performance of M-LNMO.

To evaluate the high-rate capability, the three samples were cycled at different discharge rates (0.1, 1, 2, 5, 10, 15 and 20 C) and a constant charge rate (1 C). The discharge voltage platform decreased continuously with increasing current rate (Figure 8). This phenomenon is expected because a battery typically operates near the equilibrium condition at low rates, but the electrode over-potential and internal ohmic (IR) drop increase at high rates.47 The M-LNMO plateaus remained around 4.2 V even at the highest rate (20 C). The C-LNMO and S-LNMO samples exhibit a more 17



significant negative shift of the discharge potential in comparison with M-LNMO, and the plateaus are limited at 3.6 and 3.9 V at 15 C, respectively. At 20 C, the M-LNMO delivered a discharge capacity of 105 mAh g–1. This result confirms the much higher rate capability of M-LNMO compared with C-LNMO and S-LNMO, conferred by the largely reduced polarization and high Li+ diffusivity. The crystal surface orientation significantly influences the properties of the material. As discussed in the structural analysis section, Li+ diffusion is inhibited on the (100) surfaces of spinel, but enhanced on the (111) surfaces. Because the C-LNMO and S-LNMO materials are mostly composed of (100) surfaces, they (unsurprisingly) display poor rate capability. In summary, the rate capability of the LNMO spinel material is more critically affected by the surface orientation of the cathode material than the crystal morphology and size. Besides the reduced crystal dimensions (decreased ionic-diffusion pathways), the crystal structure is more stable in M-LNMO than in C-LNMO and S-LNMO. Consequently, the stability of the material is improved.

To determine the structural tolerance and kinetic behavior of Li+ transfer in the LNMO materials, we compared the EIS results of the samples (see Figure 9). The electrolyte resistances (Rs) of the samples in fresh cells were below 3Ω, and remained almost constant during the charging/discharging process. This result can be attributed to the slightly increased electrolyte concentration and conductivity of LiODFB.48 On fresh cells, the M-LNMO sample is expected to have lower surface film resistance (Rf) and charge transfer resistance (Rct) than the other samples, probably because of the expanded lattice. After cycling, the surface film resistance (Rf) decreased in all three 18


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samples, indicating a dynamic optimization process in the surface film and lowered resistance to Li+ migration. The charge transfer resistance (Rct) increased after cycling, mainly because the solid/electrolyte interphase (SEI) formed on the material surface hindered Li+ transmission. Based on the lowered interfacial resistance and charge transfer resistance of M-LNMO, we found that the preferred (111) crystal plane orientation and smooth particle surface probably favor the formation of a high-quality SEI under benign interfacial reaction conditions. The Li2CO3 coating layer also encourages the formation of SEI, affecting Li+ insertion and extraction in the bulk material. Consequently, the ohmic polarization and activation polarization during repeated lithiation/delithiation is lower in M-LNMO in comparison with those in the other samples, corroborating the aforementioned superior rate capability and high structural tolerance of M-LNMO.

To visually investigate the interfacial stability between the electrodes and electrolyte in truncated octahedral LNMO materials, we obtained the SEM images and XRD patterns of M-LNMO after 1000 cycles at 5 C and 500 cycles at 10 C. The results are shown in Figure S5 (Supporting Information). After 500 cycles at 5 C (Figure S5b), M-LNMO maintained its octahedral morphology, and the roughened surface indicates the formation of an SEI. Figure S5d provides a comparison of the XRD patterns of the mixtures taken from the cycled electrode. The main LNMO peak was only slightly shifted in the cycled M-LNMO, and the (111) peak remained the strongest-intensity peak. This result proves the extraordinary stability of the (111) crystal surface, which confers the excellent cyclic stability of M-LNMO. 19



Table 3. Comparison of EIS fitting results of the three samples Fresh Specimen Rs (Ω) Rf (Ω) Rct (Ω) Rs (Ω) M-LNMO 1.8 82.4 21.7 2.6 C-LNMO 2.6 125.8 34.7 3.5 S-LNMO 1.5 141.2 40.8 2.9

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500th Rf (Ω) 25.6 40.3 75.6

Rct (Ω) 50.8 110.7 114.8

4. CONCLUSIONS

Spinel LiNi0.5Mn1.5O4 was synthesized using a novel one-step calcining method based on MOFs. This synthesis route is more convenient than the normal co-precipitation and solid-phase methods. Moreover, it greatly restrains the formation of LixNi1–xO impurities and oxygen deficiencies, and effectively controls the orientation of the crystal surface via an artificial process. The LiNi0.5Mn1.5O4 particles become coated by a surface layer of Li2CO3 with nanometer thickness. The resulting M-LNMO material demonstrated an excellent specific capacity of 145 mAh g–1 at a discharge rate of 0.1 C, and 105 mAh g–1 at a discharge rate of 20 C. Even over 1000 cycles at 5 C, the capacity retention was 86.6%. With their (111) crystal surfaces and in situ Li2CO3 coating layer, the synthesized materials are suitable for LIBs with high power and long lifetime, which are feasibly installable in EVs and PHEVs.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. 20


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Nitrogen adsorption isotherms curve, SEM images, FT-IR, XRD patterns, TG curve, CV curves, structure and morphology of after electrochemical cycles (PDF).

AUTHOR INFORMATION

Corresponding Author:

*E-mail:

[email protected].

Telephone:

+86-731-82075517.

Fax:

+86-731-82075517 (H.M.Z).

*E-mail:

[email protected].

Telephone:

+86-731-88877173.

Fax:

+86-731-88877173 (J.L).

ORCID

Hongming Zhou: 0000-0002-9194-7325

Chengjie Yin: 0000-0002-2318-8486

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest. 21



Page 22 of 36

ACKNOWLEDGMENTS

Financial supports from the National Science Foundation of China, Granted No. 51371198 and Technology Project of Changsha, Granted No. K1202039-11 is gratefully acknowledged. The authors would like to thank Enago (www.enago.cn) for the English language review.

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Figure 1. A schematic illustration of the preparation of M-LNMO

Figure 2. SEM images of (a) and (a’) samples M-LNMO, (b) and (b’) samples C-LNMO, (c) and (c’) samples S-LNMO, and (d) EDS mapping images of M-LNMO.

31



Figure 3. TEM and HRTEM images of three LNMO materials: (a and a’) M-LNMO, (b and b’) C-LNMO, and (c and c’) S-LNMO.

Figure 4. (a) XRD patterns of the three LNMO materials M-LNMO, C-LNMO and S-LNMO, The dashed rectangle region is enlarged and shown on the right, with the strongest LixNi1-xO impurity peaks being marked by*. (b) Raman spectroscopy of the 32


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three LNMO materials.

Figure 5. XPS spectra for a) the Ni2p, b) the Mn 2p, and c) the O1s regions. The spectra correspond to the M-NMO sample.

Figure 6. Cyclic voltammetry (CV) curves of the M-LNMO, C-LNMO, and S-LNMO materials at a scan rate of 0.05 mV s−1 between 3.5 and 4.9 V. Inset shows the enlarged region around 4.0 V.

33



Figure 7. (a) Initial charge/discharge profiles measured at 0.1 C in the voltage range of 3.5−4.9 V. (b) Rate capability investigation with a constant charge at 1 C followed by discharging at various C-rates ranging from 0.1 to 20 C. (c) Cycling performances of LNMO-MF and LNMO-NP at a 5 C discharge rate at 30 °C. (d) Long-term cycling stability of the M-LNMO material measured at a high current density of 10 C between 3.5 and 4.9 V at 30 °C.

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Figure 8. Discharge curves (a), (b), and (c) of the synthesized M-LNMO, C-LNMO, and S-LNMO samples on cycling sequentially from 0.1 C to 20 C, (d) Evolution of normalized capacity as a function of discharge rate for the three LNMO materials.

Figure 9. Nyquist plots of the three LNMO materials (a) before cycling and (b) after 35



500 cycles at 5 C at 30 °C. The inset is an equivalent circuit.

Table of Contents Graphic

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Synthesis and Electrochemical Properties of LiNi0 ... - ACS Publications

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