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Effect of Surface Modification with Spinel NiFe2O4 on Enhanced Cyclic Stability of LiMn2O4 Cathode Material in Lithium Ion Batteries Feiyan Lai, Xiaohui Zhang, Qiang Wu, Jiujun Zhang, Qingyu Li, Youguo Huang, Zhu Liao, and Hongqiang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02876 • Publication Date (Web): 24 Nov 2017 Downloaded from http://pubs.acs.org on November 26, 2017

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ACS Sustainable Chemistry & Engineering

Effect of Surface Modification with Spinel NiFe2O4 on Enhanced Cyclic Stability of LiMn2O4 Cathode Material in Lithium Ion Batteries Feiyan Lai a,b, Xiaohui Zhang a,b,*, Qiang Wu a, Jiujun Zhang a, Qingyu Li a, Youguo Huang a, Zhu Liaoa, and Hongqiang Wang a,c,* a

Guangxi Key Laboratory of Low Carbon Energy Materials, School of Chemistry and

Pharmaceutical Sciences, Guangxi Normal University, 15 Yucai Road, Qixing District, Guilin 541004, China. b

Guangxi Key Laboratory of Comprehensive Utilization of Calcium Carbonate Resources,

College of Materials and Environmental Engineering, Hezhou University, 18 Xihuan Road, Babu District, Hezhou 542899, China c

Hubei Key Laboratory for Processing and Application of Catalytic Materials, College of

Chemical Engineering, Huanggang Normal University, 146 Xingang 2 Road, Development Zone, Huanggang 438000, China * Corresponding authors: [email protected] (H. Wang), Tel: +86-0773-5856104; [email protected] (X. Zhang), Tel: +86-0774-5271906

ABSTRACT: The spinel NiFe2O4 widely used as anode active material for LIBs was prepared as coating layer on LiMn2O4 particles via a sol-gel route followed by heat treatment. The effect of the surface modification on the electrochemical performances of the cathode material was investigated both in LiMn2O4ǁLi and LiMn2O4ǁgraphite batteries. The well crystallized NiFe2O4 coating layer shows a continuous and uniform film with a thickness of 10-11 nm on the surface of the LiMn2O4 material. The as-prepared LiMn2O4/NiFe2O4 composite with a coating amount of 1% shows an improved rate performance due to the high activity of the coating material. At an elevated temperatures of 55 oC, the initial discharge capacity at 0.1C is 130.8 mAh/g 1

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and the capacity retention is 84.5% after 400 cycles at 1C when it is tested in a LiMn2O4ǁLi cell, while full battery with the modified LiMn2O4ǁgraphite as cathode material shows a capacity retention of 89.1% after 500 cycles. The enhanced cyclic performance can be attributed to the good physical properties of mechanical strength and thermal stability from NiFe2O4 material. The rate performance is also improved due to the higher Li cyclability of the coating layer, which has been proved by the CV and EIS tests. The inherent advantages of natural abundance, eco-friendliness and low cost make it practical to large scale modification of the relevant electrode materials attacked by electrolyte when charged to high potentials and similarly could benefit from surface stabilization. KEYWORDS: Lithium-ion batteries, Cathode materials, LiMn2O4, Surface modification, NiFe2O4 INTRODUCTION Lithium-ion batteries are of great significance in developing hybrid electric vehicles (HEVs) and electric vehicles (EVs), which are expected to alleviate the increasingly serious air pollution.[1-3] Spinel LiMn2O4 is currently one of the most promising cathode materials for lithium-ion batteries because of its low cost, good safety, high power density, and environmental friendliness.[4-6] However, the wide practical use of the spinel LiMn2O4 has been hindered by two key issues: one comes from capacity fading during electrochemical cycling, especially at elevated temperature, caused by the electrochemical reaction, instability of the two-phase structure in the charged state and manganese dissolution.[7, 8] The other is the short 2


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storage time owing to a large polarization on the electrode surface, resulting from a series of complex reactions in lithium-ion batteries.[9] In literatures, the main reason is manganese dissolution and Jahn-Teller effect.[10] A variety of strategies have been proposed to eliminate the problems. For instance, the doping with metal cations[11-13] can enhance the stability of the crystal structure of LiMn2O4 and restrain the Jahn-Teller effect, but the degree of doping cannot be increased more than a certain extent weighed by doping level and specific capacity.[14] Another common approach is to modify the surface of the LiMn2O4 material with stable and inert materials against the produced acid phases in electrolyte, especially for HF. Among the various materials for surface modification,[4, 15-18] metal oxides is an effective choice for solving the high temperature cycle performance of spinel LiMn2O4.[19, 20] However, these amorphous metal oxides impede the Li+ diffusion at the interface between the active bulk and electrolyte and then degrade the rate capability. Although some are transformed into ion conductors after the first few electrochemical reactions, the activation consumes Li and then degrades the discharge capacity partly. In recent years, solid electrolyte has been selected and explored as coating material due to high ion conductivity, but the lattice matching problem between solid electrolyte coating layer and spinel active bulk as well as more issues are still needed to be considered.[21] Spinel transition metal oxides (TMOs) widely used as active materials in supercapacitors,[22] anode materials for LIBs[23] and sensing materials,[24] these materials have attracted enormous research interest owing to their high Li cyclability 3



and their other inherent advantages, such as natural abundance, eco-friendliness and low cost.[25-28] Among these different types of ferrite oxides with the general formula AFe2O4, NiFe2O4 possesses a special inverse spinel structure where Fe3+ ions are located in the tetrahedral site and octahedral site and Ni2+ ions are located in octahedral sites.[29] According to an earlier study by Chen et al.[30], this inverse spinel can incorporate with a larger amount of Li ions compared with the normal and mixed spinels. More important, the anode material did not experience any structural stress and eventual pulverization during lithium cycling and provides an efficient electron conducting pathway.[31-33] In view of the above works, NiFe2O4 is endowed with extreme expectations to become a promising coating material to protect the cathode materials from HF attacks and prevent active-material loss. In this work, NiFe2O4 surface layer was prepared on the spinel LiMn2O4 via a sol-gel method, the as prepared NiFe2O4 layer was uniformly distributed on the surface of the spinel LiMn2O4 with a high Li+ migration ratio. And the film coating makes a contribution to reduce the electrode polarization, resistance and Mn dissolution. The structure, morphology and electrochemical performance of the new-style LiMn2O4-based cathode materials were investigated both in half and full cells. EXPERIMENTAL Synthesis of NiFe2O4-coated LiMn2O4 materials To synthesis NiFe2O4 film coating was used Ni(NO3)2·6H2O (99.9%, AR) and Fe(NO3)3·9H2O (99.9%, AR) ( 1:2.02 in molar ratio) as starting materials for the 4


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sol-gel method. And the compounds were dissolved in deionized water by mechanical stirring at 25 oC for 0.5 h. To modify the surface of LiMn2O4 (CITIC Dameng Mining Industries Ltd. Guangxi), adding the LiMn2O4 into the solution. And the suspension was stirred with a mechanical stirrer at room temperature for 2 h. Ammonium hydroxide, as a gelling, was added into the solution stirred sequentially for 0.5 h and then dry gel was obtained at 80 oC for 12 h in vacuum. Finally, the obtained dry gel powders were heated in a tube furnace at 600 oC for 2 h in air. The pristine LiMn2O4 and the as-prepared LiMn2O4 coated by NiFe2O4 with 0.5, 1 and 2wt% are denoted as LMO, 0.5NLMO, 1NLMO and 2NLMO. Physical characterization The crystalline phase of the synthesized products was characterized by power X-ray diffraction (XRD, D/Max-2500V/PC, Rigaku) using Cu Kαradiation (k=0.15406 nm). Particle morphologies of the prepared powders were observed by field emission scanning electron microscopy (FE-SEM, Quanta 200 FEG, FEI). Electron diffraction spectroscopy (EDS) was applied to determine the elements of the 1N-LMO powders together with SEM in large field of view. Transmission electron microscope (TEM, 2100F, JEOL) investigations were used to analyze electron microscope operating at 200 keV. The surface chemical compositions of the samples were measured by X-ray Photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo) using Al Kαradiation (hv=1487.71 eV) at a power of 75 W. The interaction between NiFe2O4 coating layer and LMO powders was analyzed using Raman Spectroscopy (Jobin Ybon, T64000). 5



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Electrochemical characterization The cathode electrodes were fabricated with the active material, super-P carbon (SP), polyvinylidene fluoride (PVDF) binder in a weight ratio of 85:10:5, which were all dissolved in N-methylpyrrolidinon (NMP). The obtained slurry was coated onto Al foil and dried at 100 oC under vacuum for 12 h prior to use. The dried foil was cool-rolled and cut into wafers with φ12 mm. The electrochemical properties of the prepared LMO and NiFe2O4-coated LMO were measured in tests using R2025 coin-type cells with a Li metal anode and polypropylene film (Celgard 2400). The cells use 1M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 v/v) as the electrolyte. And the cells were assembled in an argon-filled glove box. Electrochemical tests were

carried

out using

an

automatic

galvanostatic

charge-discharge unit (Land CT2001A), with a battery tester between 3.0 and 4.3 V at various C-rates and at different temperatures of 25 and 55 oC. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured between 3.0 and 4.5 V at a scanning rate of 0.1 mV·s-1 and over a frequency range of 0.01 Hz to 100 kHz using a Zahner IM6 electrochemical workstation, respectively.

RESULTS AND DISCUSSION Figure 1a and b show the crystalline structure and phase purity of the NiFe2O4 and the four samples of NiFe2O4-coated LiMn2O4 detected by power XRD patterns. All the peaks can be well indexed as inverse spinel NiFe2O4 of Fd-3m space group (JCPDS No. 54-0964), revealing that NiFe2O4 crystal with high purity and crystallinity can be prepared by the same sol-gel progress and heat treatment at 600 ℃ 6


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for 2 h in air. For the crystal shown by the inset in Figure 1a, Fe3+ ions occupy the 8a sites and Ni2+ and Fe3+ ions equally occupy 16d sites. As a result, the unit cell contains 32 O atoms in the cubic close packing 3D array.[34, 35] The crystal structure provides the interstitial space for electrolyte ion transferring through 16c-18a-16c paths.[36] More important, the strong bonds between metal and oxygen ions that make the cubic spinel more active and the coexistence of Fe and Ni atoms with different oxidation states lead to high electrical conductivity and excellent electrochemical performance.[37, 38] The diffraction patterns of the LMO and coated NLMO samples are indexed to spinel LiMn2O4 of Fd-3m space group (JCPDS No. 35-0782). No impurity is detected as shown in Figure 1b. The lattice parameters of the LMO, 0.5NLMO, 1NLMO and 2NLMO samples were calculated and listed in Table S1. The parameters of the three coated samples have slight change to the ones of the pure LiMn2O4, suggesting that the NiFe2O4 coating does not induce a structural transformation of LiMn2O4 but it may be presented on the surface of active material. According to the elements concentration from the ICP-AES analysis for the coated samples with different coating amount of 0.5, 1 and 2%, the ratio of to LiMn2O4 active material were 0.45: 95.5, 0.96: 90.4 and 1.93: 98.07, respectively. The results are closed to the theoretical value and a high utilization ratio of NiFe2O4 in synthesis process. The low coating concentration may also explain why the NiFe2O4 is not detected in the composites. Further determination of the chemical composition of the LiMn2O4/NiFe2O4 composite, XPS measurements were analyzed, and the results are shown in Figure 1c-f. From the Figure 1c, the full spectrum shows the peaks of O, Mn, 7



Fe and Ni. The value of the sub-peak for the Mn 2p3/2 between 641.4 and 642.1 eV is assigned to Mn3+ and the value at 642.4 and 643.6 eV is assigned to Mn4+. The fraction of Mn4+ in the bare LMO is much less than the coated NiFe2O4-NLMO sample, owing to the diffusion of transition metal ions from the coating layer into the host material and the Mn ions into the surface coating layer during annealing. The cross diffusion is beneficial to the electrochemical performances of the LiMn2O4 electrode material.[39] In Fig. 1e and f, the peaks of Ni 2p3 and Fe 2p3 appeare at 853.35 and 710.89 eV, respectively, signifying the exclusive presence of the Ni2+ and Fe3+ as oxidation state.

8


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Figure 1 XRD patterns and crystal structure of the NiFe2O4 (a) and the NiFe2O4-coated LiMn2O4 samples (b), XPS fully spectra of 1NLMO (c), high-resolution XPS spectra for Mn 2p of the LMO and 1NLMO samples (d), high-resolution XPS spectra for Ni 2p (e) and Fe 2p (f) of the 1NLMO sample.

The morphologies of as-prepared LMO and 1wt% NiFe2O4 coated LMO are quite similar as shown in Figure 2a and c. The particles consist of dispersed crystallites in the size range of 100-500 nm and there are no changes in surface features, which can be explained by the function of the coating layer. From the TEM images, compared with the exposed surface of the pure LMO particles as shown in Figure 2b, the thin NiFe2O4 film with a uniform thickness of 10-11 nm is apparent as shown in Figure 2d. The high-resolution TEM image shown in Figure 2e reveals there is on obvious delineation between the bulk and coating layer. The lattice distance are measured to be 0.478 and 0.293 nm indexed to the (111) and (220) phase of the LiMn2O4 core and shell materials respectively. The suggested well crystallinity of NiFe2O4 coating material is beneficial to good lattice matching to yield lower interface resistance. By EDS analysis during SEM test, the mapping images of the 1NLMO sample shows the homogeneous distribution of O and Mn elements in the particles. It is evident that Fe and Ni elements are evenly distributed on the surface, indicating that the LiMn2O4 bulks are uniformly coated with NiFe2O4. Calculated from the result of the elements concentration obtained from ICP, the mass percent of NiFe2O4 in the 1NLMO samples is 1.04% which is close to the theoretical value. In addition, the as-prepared NiFe2O4 coated electrode material exhibit an increase in Fe and Ni content with an increase in the amount of coating precursor. Likewise, the thickness of the film increases slightly with increasing covering amount (See Figure S1). However, when the amount is 0.5%, 9



the coating layer is not uniform, and some part of the bulk is exposed.

Figure 2 SEM and TEM images of the LMO (a, b) and 1NLMO (c, d and e) samples, SEM elemental mapping images of the 1NLMO sample (f).

The electrochemical properties of the bare LMO and coated NLMO (NiFe2O4-coated LiMn2O4) samples were tested as cathode material by galvanostatic charging-discharging test, CV and EIS in CR2025 coin cells. The initial galvanostatic 10


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charge-discharge curves for the four samples are shown in Figure 3a and tested under 0.1 C rate at 55 oC between 3.0 and 4.3 V. All the three samples with surface modification have less initial discharge capacity and coulombic efficiency than the bare LMO with 124.1 mAh g-1 and 89.5% respectively, due to the capacity consumption of the NiFe2O4 with electrochemical activity.[15, 18] However, as shown in Figure 3b, the rate performance of the four samples presents another change rule under the same conditions of increasing C-rates from 0.1 to 10 C at 55 oC, the detailed values are listed in Table S1. As the charge-discharge rate increases, the capacities of all samples decrease in different degrees. The 1NLMO sample exhibits the highest rate capability with the retention of 74.0% at 10 C (90.1 mAh g-1) of the capacity at 0.1 C (123 mAh g-1), while the LMO is only 55.2%. It indicates that the reaction kinetic of insertion/extortion of Li+ is improved due to the high electrical conductivity and excellent electrochemical activity. To the 2NLMO sample with the coating proportion of 2%, the rate capability decreases slightly. The reason is distributed to the resistance of the thicker coating layer which outweighs the advantage. The results are further proved by the EIS and CV tests, as shown in Figure 3c and d. Figure 3c displays the Nyquist plots of the four electrodes after the initial discharge. All plot curves are consisted of two semicircles in high-to-medium frequency region and an inclined line in the low frequency region. The semicircle at high frequency region represents the impedance owing to SEI film on electrodes surface. The depressed semicircle at the intermediate frequency represents the charge transfer resistance of the interface between electrode and electrolyte. The slope at the low 11



frequency region is due to lithium ion diffusion in the bulk material. Moreover, X-axis offset treated as Rs represents the impedance of electrolyte and the contact resistance at electrode/current-collector. By comparing the four Nyquist plots of the four electrodes, when the potation is 0.5%, the low coating amount of NiFe2O4 does not coat the surface of LiMn2O4 grains perfectly, the slope is bigger than the value of the uncoated sample, suggesting the discontinuously coating has no contribution on the improvement of electron conductivity but the ion transport benefits by partially coating. As the potation increases to 1%, the 1NLMO shows the least Rp for the least semicircle and biggest slope, the uniform coating layer disperses the charges to increase the reactive sites with ions. However, the Rp value rebounds when the film becomes thicker, distributed to long distance of ion transferring from electrolyte to LiMn2O4 core. The 1NLMO shows the lowest Rp value as the amount of the percent increases to 1 wt%, , which is benefit to the transfer of charge and a more uniform current distribution at the LiMn2O4 particle surface due to the continuous and uniform coating film is produced.[40] However, the resistance increases for the 2NLMO, which can be due to the increased thickness of the coating layer. The impendence reduction demonstrates an enhancement in the kinetics of the lithium-ion diffusion through the surface layer and electron conducting. The consequent increases in rate capability can be due to the continuous NiFe2O4 coating film with remarkably efficient pathway for Li-ion and uniformly dispersion for charges.

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Figure 3 Initial charge-discharge capacity (a), rate performance (b), electrochemical impedance spectroscopy (c), cyclic voltammogram (d) and cycling performances at 25 and 55 oC (e) of the bare LMO and as-prepared 0.5NLMO, 1NLMO, and 2NLMO samples. Figure 3d displays the CV curves conducted in the voltage window of 3.4-4.5 V vs. Li/Li+ at a scan rate of 0.1 mV s-1. All four curves show two pairs of redox peaks, indicating two-step reaction of Li ions extracting from and inserting into the spinel LiMn2O4. The oxidation peak of NiFe2O4 at 1.78 V does not appear in the widow, and the coating does not change the insertion/extraction mechanism of lithium ions in 13



spinel structure. The sample with the coating ratio of 1% exhibits the highest peak current and sharper area. This demonstrates the superior electron conductivity and Li reversibility, due to the good diffusion of electron on the surface and ions through the interfaces supplied by the active coating layer. Figure 3e shows the cycling performances of the bare LMO and NiFe2O4-coated LMO cathodes vs. Li/Li+ under 1 C at 25 and 55 oC, respectively. After 1000 cycles under 1 C at 25 oC, the capacity retention of the 1NLMO and 2NLMO samples are 90.7 and 89.4%, while the retention of the bare LMO and 0.5NLMO are less than 30% and the capacity fades severely at the end of the 300th and 400th cyclic times respectively. The improved retention capability suggests that the NiFe2O4 protective layer effectively inhibits the dissolution of Mn by suppressing directive contact with electrolyte to enhance cycling stability. Moreover, the capacity fade is partially attributed to the formation of a cathode/electrolyte interface forming on the cathode surface shown in the insert of Figure 4a, which is roughly double the loss attributed to decomposition of LiMn2O4 as reported by Yamane.[41] As the temperature increases to 55 oC, the LMO sample delivers an initial discharge capacity of 128.5 mAh g-1, and then rapidly decreases to 70.1 mAh g-1 after 400 cycles, with the capacity retention of 54.6% at 1 C. Similarly, the NiFe2O4-coated LMO samples show lower initial discharge capacity but higher capacity retention. And the retentions are 66.6, 84.5 and 79.7% for the 0.5NLMO, 1NLMO and 2NLMO, respectively. The enhanced high-temperature cycling performance is due to the coating NiFe2O4 material with superior mechanical strength, structural and thermal stability during lithium cycling. 14


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Figure 4 SEM images of the LMO (a), 0.5NLMO (b), 1NLMO (c) and 2NLMO (d) samples: o

after 1000 cycles at 1 C and 25 C, TEM image of the 2NLMO sample (the insert).

Figure 4 shows more intuitive observation of the change for the composite after long-term charge/discharge cycling. For the pristine LMO without the protective layer as shown in Figure 4a, the surface structure is destroyed and Mn is dissolved into the electrolyte and then deposited on the electrode, which do harm to the capacity and cycling performances. When being coated by the NiFe2O4 with a coating quantity of 0.5%, the surface is protected in some degree and coalesced partly, while the grains of the 1NLMO with more coating quantity of 1% keep the original shape, the grains touch each other without adhesion, which is beneficial to the capacity release in long-term cycling. But when the quantity increases to 2%, the thick coating layer with 15



high mechanical strength has a different volume effect during in the cycle from the LiMn2O4 bulk, the difference leads to disconnect for the shell from the core. As a result, the originally dispersed particles of 2NLMO are glued together to form blocks, as shown in Figure 4d and the insert.

Figure 5 The initial discharge curves (a, b), cycling performance at 1C (c) and final discharge curve (d) of the LMO and 1NLMO samples in full-cells. The as-assembled 502030 soft-packed cells and the photographs of the cell being used in an electronic clinical thermometer (e).

In order to investigate the effect of NiFe2O4 coating on the electrochemical performances of LiMn2O4 cathode material in practical application, the full-cells with the style of 502030 soft-package were assembled using graphite as anode material, and the cells were tested at various rates in the potential range of 3.0-4.3 V. Figure 5e 16


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exhibits the assembled cell was successfully used in an electronic clinical thermometer. Figure 5a and b show the charge-discharge curves at 0.1 and 10 C rates of the pristin LMO and 1NLMO samples at 25 oC. As shown in Figure 5a and b, the specific discharge capacity of LMO is 109.2 mAh g-1 at 0.1 C and decreases to 8.2 mAh g-1 at 10 C, while the 1NLMO shows an excellent rate capability, it exhibits a higher discharge capacity of 96 mAh g-1 at 10 C, due to the NiFe2O4 improving the ionic and electric conductibility. Moreover, and this active coating material expresses the polarization of the LMO and 1NLMO electrodes. Also the 1NLMO composite exhibits better cycling performance than the LMO as shown in the Figure 5c. The Figure 5d shows the final discharge capacity of LMO is 99.3 mAh g-1 and the capacity retention is 89.1% after 500 cycles, which is better than the most LiMn2O4-based full cells reported in the literature (shown in Figure S2). The remarkable protection effect against HF attack, the improved electrochemical activity of lNiFe2O4 coating as well as other physical advantages for the surface modification of LiMn2O4 cathode material shows superior potential value on practical application.

CONCLUSIONS The spinel NiFe2O4 was coated on LiMn2O4 cathode material via a sol-gel route followed by heat treatment. The coated layer shows well crystallized NiFe2O4 and presents a uniform thin film. The high chemical activity of NiFe2O4 improves the rate performance LiMn2O4, and the composite exhibits an excellent cyclic stability both at room and elevated temperatures due to the good physical properties of mechanical strength and thermal stability of the coating material. The capacity retention is 84.5% 17



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after 400 cycles at 1C when it is tested in a LiMn2O4ǁLi cell and when the temperature increases to 55

o

C, the retention still remains 84.5%, while the modified

LiMn2O4ǁgraphite as cathode material in a full cell shows the capacity retention of 89.1% after 500 cycles at room temperature. And the structure of the coating layer does not change in the long-term cycling.

Supporting Information TEM images of the 0.5NLMO (a) and 2NLMO (b) samples. Comparison of the cycling performance for the 1NLMO sample to the ones reported in literature. The rate discharge capacity of the LMO, 0.5NLMO, 1NLMO and 2NLMO samples.

ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (51762006

and

51774100),

Guangxi

Natural

Science

Foundation

(2016GXNSFDA380023), Hubei Province Key Science and Technology Support Program (2015BCE072), Outstanding Young Science and Technology Innovation Team Program of Hubei Provincial Colleges and Universities (T201514), Scientific Research

and

Technological

Development

Program

of

Guangxi

(Guikegong1598008-14 and GuikeAA16380042).

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For Table of Contents Use Only

Spinel NiFe2O4 usually used as anode and sensing materials was coated on LiMn2O4 and enhance cyclic performance of sustainable Li-ion full cell due to superior physical and electrochemical properties.

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Effect of Surface Modification with Spinel NiFe2O4 ... - ACS Publications

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