Enhanced Electrochemical Performance of LiNi0.5Mn1.5O4 Cathode

Subscriber access provided by Queen Mary, University of London

Enhanced Electrochemical Performance of LiNi0.5Mn1.5O4 Cathode Material by YPO4 Surface Modification Tinghua Xu, Yaping Li, Dandan Wang, Muying Wu, Du Pan, Huiling Zhao, and Ying Bai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03935 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Enhanced Electrochemical Performance of LiNi0.5Mn1.5O4 Cathode Material by YPO4 Surface Modification Tinghua Xu†, Yaping Li†, Dandan Wang†, Muying Wu‡, Du Pan†, Huiling Zhao†, Ying Bai *,† †Henan

Key Laboratory of Photovoltaic Materials and School of Physics &

Electronics, Henan University, Kaifeng 475004, P.R. China ‡ Department

of Electronic Engineering, Dongguan University of Technology,

Dongguan 523808, P.R. China

*Corresponding author: Ying Bai E-mail: [email protected]

ABSTRACT Cathode material LiNi0.5Mn1.5O4 (LNMO) for lithium ion batteries is successfully synthesized by a sol-gel method and is further modified by a thin layer of YPO 4 (1, 3 and 5 wt.%) through a simple wet chemical strategy. Physical characterizations indicate that the YPO4 nanolayer has little impact on the cathode structure. Electrochemical optimization reveals that the 3 wt.% YPO 4-coated LNMO could still deliver a high specific capacity of 107 mAh g-1 after 240 cycles, with a capacity retention of 77.5%, much higher than that of the pristine electrode. Electrochemical 1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

impedance spectroscopy (EIS) analysis proves that the rapid increase of surface impedance could be suppressed by YPO4 coating layer, thus facilitates the surface kinetics behavior in repeated cycling. Through further material aging experiments, the improvement of electrochemical performances could be attributed to the formation of Lewis acid YF3, converted from the YPO4 coating layer in LiPF6-based electrolyte, which not only scavenges the surface insulating alkaline species with high acidity, but also accelerates ion exchange on material surface and thus helps to generate the solid solution Li-Ni-Mn-Y-O on the surface of YPO4-coated LNMO. KEYWORDS: Spinel LiNi0.5Mn1.5O4 (LNMO), Surface Modification, Lewis acid, Electrochemical performances

INTRODUCTION At present, the shortage of fossil energy sources and the crisis of environmental pollution are becoming increasingly serious. Lithium-ion batteries (LIBs) have been considered as promising sustainable power sources and energy storage devices owing to their high energy densities, long cycle life, and environmental friendliness. Recently, high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) is considered as a promising high energy density cathode material for LIBs, which could be implemented in hybrid electric vehicles (HEVs) and plug-in HEVs (PHEVs) due to high operating voltage (~ 4.7 V), reasonable capacity of 147 mAh g -1, and fast three dimensional Li+ migration channel.1,2 However, it suffers from limited cycle life, particularly at elevated temperature, which is triggered by degradation of electrolyte at high operation 2


Page 2 of 36

Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


voltage3,4 and structure-related Mn/Ni dissolution. 5-8 To meet these challenges, a series of metal oxides, fluorides and phosphates have been applied to modify the surface of LNMO, including RuO2,9 Fe2O3,10 ZnO,11 TiO2,12 MgF2,13 AlF3,14 FePO4,15 AlPO416 and Li3PO417 etc, which have been proved to be effective in enhancing the electrochemical performances. Yttrium orthophosphate (YPO4) belongs to the space group of I1/amd with a tetragonal symmetry (a = b = 6.822 Å). Chains parallel to the c axis of corner-sharing structural units built of a dodecahedron (YO8) and a tetrahedron (PO4) are linked together by an edge,18 guaranteeing its high structural stability. The yttrium (Y) has been doped

into LiNi0.7Co0.3O219

Li1.2Ni0.2Mn0.6O220

and

to improve the

electrochemical properties, which was attributed to the stablized structural by the addition of Y3+ ion. Recently YPO4 has been coated on the surface of LiCoO 2,21 LiNi1/3Co1/3Mn1/3O222 and Li4Ti5O12,23 which proved to be effective in improving the electrochemical properties. In this study, YPO4 was applied to modify the surface chemistry of spinel LNMO for the first time. Electrochemical tests show that appropriate YPO4 coating is beneficial for the cycling stability and rate capability of LNMO electrode. As another highlight of this work, the underlying mechanism of YPO4 surface modification on LNMO was intensively investigated. Systematic aging experiments of the pristine and 3 wt.% YPO4-coated LNMO powders indicate that the ion exchange and the generation of surface solid solution, induced by the formation of YF3 Lewis acid, contribute to the acceleration of surface ion transportation and the enhancement of electrochemical properties. 3



EXPERIMENTAL SECTION Material Synthesis. Pristine LNMO was prepared by a sol-gel method as described in refs.24, 25 All the chemical reagents used were of analytical grade in this work. In a typical experiment, stoichiometric amounts of lithium acetate, nickel acetate, manganese acetate and citric acid were dissolved separately in distilled water. After they were homogeneously mixed, the transparent acetate solution was added in to form a lithium/manganese/nickel mixture by vigorous stirring. Afterwards, ammonium hydroxide was added dropwise to adjust the pH value to about 6.5. Then, the mixture was stirred continuously at 80 °C till a transparent sol was formed, which was transferred into a drying oven for 12 h to obtain the corresponding gel. Next, the gel was annealed at 550 °C in air for 6 h. Finally, the preheated product was further calcined in a muffle furnace at 820 °C for 10 h and naturally cooled down to room temperature to obtain the target LNMO powder. To prepare YPO4 surface modified LNMO, Y(NO3)3·6H2O and NH4H2PO4 were selected as the starting materials for the coating layer. Firstly, stoichiometric amount of LNMO was homogeneously dispersed in distilled water. After that, aqueous solution of Y(NO3)3·6H2O and NH4H2PO4 were successively added into the above mixture drop by drop with vigorously mechanical stirring. Then the filtered precipitation was repeatedly washed with distilled water and dried at 120 °C for 12 h. Finally, the obtained powder was annealed in air at 400 °C for 6 h. The contents of expected (nominal) YPO4 on LNMO are set to be 1 wt.%, 3 wt.% and 5 wt.% for systematic experiments. 4


Page 4 of 36

Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Materials Characterizations. X-ray diffraction (XRD) patterns of the samples were measured on Bruker D8 Advance diffractometer (Bruker, Germany) with the slit size of 0.6 mm and Cu kɑ radiation of λ ~ 0.15418 nm between 10o and 90o at a scan rate of 0.1 s per 0.02o. All the diffraction patterns were refined using the Rietveld method, as implemented in the Fullprof program. The Raman spectra were recorded on a laser Raman Spectrometer (RM-1000, Renishaw) with 633 nm He-Ne light source. The surface morphologies of the as-prepared samples were observed through a field emission scanning electron microscopy (FESEM, JEOL JSM-7001F) operated at 15 kV. Energy dispersive spectroscopy (EDS) was collected on an APOLLO X silicon drift X-ray detector. High resolution transmission electron microscopy (HRTEM) images were collected on a JEOL JEM 2010 equipment to investigate the microstructure. The inductively coupled plasma (ICP) analysis was conducted on a Thermo Fisher ICAP 6300 to determine the exact chemical compositions. X-ray photoelectron spectroscopy (XPS) was performed on a spectrometer (ESCALAB 250, Sigma Probe, Thermo VG Scientific Co. Ltd) with Mg Kα radiation. All binding energies reported were corrected using the signal for carbon at 284.8 eV as an internal standard. Electrochemical Measurement. The working electrodes were prepared with the obtained samples, carbon black, and polyvinglidene fluoride (PVDF) at a mass ratio of 8:1:1. Afterwards, they were dried at 120 °C under vacuum overnight, and cut into circular electrodes with a diameter of 8 mm. CR2032 coin cells were assembled with Li metal as counter electrode and 1 M LiPF6/EC:DMC (with volume ratio of 1:1) as 5



electrolyte. The batteries were fabricated in an argon filled glove-box with H2O and O2 contents less than 1 ppm. Before electrochemical tests, the batteries were aged for 24 h to ensure sufficient soakage. Cycling performances were evaluated by using a Neware battery tester (CT-3008W-5V3A-S4) between 3.5 and 4.9 V. The electrochemical impedance spectroscopy (EIS) investigations were performed on an electrochemical workstation (CHI660C, Shanghai Chenhua) with a three electrode system. After 10, 20 and 50 cycles at a current density of 0.1 C, the cells were charged and aged for two days to reach equilibrium at 4.9 V before the EIS spectra were recorded. The EIS investigations were performed at different cycles with AC amplitude of 5 mV over a frequency range from 5 mHz to 100 KHz. The thermal stabilities of charged LNMO electrodes were examined by differential scanning calorimetry (DSC, TA Q600). The cells were initially charged to 4.9 V at a current density of 0.1 C, and then they were separated in an argon-filled glove-box. The electrodes were carefully fetched out and sealed in an Al pan for DSC experiment measurements, which were performed under a dry Ar atmosphere from room temperature to 350 °C at a heating rate of 5 °C min−1. Before material aging experiment, the pristine and 3 wt.% YPO4-coated LNMO powders, two magnetons, two polytetrafluoroethylene (PTFE) containers were thoroughly dried. Then, the powders with the same weight were soaked into electrolyte (the same with electrochemical cycling) with the same volume in glove box. After the containers were strictly sealed, the mixtures were mechanically stirred at 50 °C. After each one week, the PTFE containers were carefully unpacked and the 6


Page 6 of 36

Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


same amount of mixture was taken out, followed by thorough solid-liquid separation. The acquired liquid was tightly sealed for ICP and acidity tests, while the solid was repeatedly rinsed by DMC and then completely dried in a vacuum chamber for SEM and XPS characterizations. In titration test, the weighted liquid was added in 0 °C water to prevent the decomposition of LiPF 6. Some known amount of 0.1 wt. % bromthymol blue (as an indicator) was dripped into the mixture. Then NaOH solution (0.01 M) was carefully added drop by drop, till the color of the electrolyte was turned from yellow to blue. The volume of consumed NaOH (V) and the weight (W) of the tested electrolyte were recorded. The acidity (calculated as HF) of the electrolyte could be determined as: Acidity = VNaOH× CNaOH× MHF× 1,000,000 ÷ Welectrolyte (ppm).

RESULTS AND DISCUSSION XRD patterns of the pristine and YPO4-coated LNMO are shown in Figure 1a and Figure S1. All the samples exhibit typical diffraction peaks that could be indexed to a simple cubic structure with space group of Fd 3 m.26 The diffraction lines in all the profiles are sharp, indicating that the as-prepared samples are well-crystallized.27 A LixNiyO impurity phase could be observed for all the samples, which is commonly existed in LNMO material Figure S1a.28 For comparison, pure YPO4 was synthesized with the same condition as the modification process without adding LNMO and the diffraction patterns are presented in Figure. S2. No diffraction line could be observed at 400 °C, nevertheless, with higher annealing temperature of 800 °C, the sample exhibits typical diffraction peaks, matching well with YPO 4 (JCPDS card No. 7



11-0254). This result indicates that the YPO 4 coating layers are amorphous at 400 °C, explaining the absence of its signal in Figure 1a and Figure. S1. In addition, the lattice parameters (determined from Rietveld refinement) are 8.1779, 8.1773, 8.1750 and 8.1760 Å for the pristine, 1, 3 and 5 wt.% YPO4-coated LNMO, respectively, indicating that the lattice structure experiences little change after YPO4 surface modification. Given that the ionic radius of Y3+ (0.93 Å)29 is much larger than those of Li+ (0.59 Å), Ni2+ (0.69 Å), Mn3+ (0.65 Å) and Mn4+ (0.53 Å),30 the Y3+ ions have little chance to migrate into the lattice structure of active material in surface modification process at only 400 °C. To detect the surface structure of as-prepared samples, Raman spectra are collected and the results are plotted in Figure 1b. The vibration at 630 cm-1 is assigned to the symmetric Mn-O stretching of MnO6 octahedron31 and the bands at 394, 487 and 586 cm-1 are originated from the Ni-O stretching.32 Careful observation indicates that the Raman vibrations in Figure 1b undergo gradual intensity decay with the increase of YPO 4 modification content. Typically, the Mn-O stretching at 630 cm-1 for the four samples is selected and the fitted intensities are compared in the inset histogram. Obviously, the fitted intensities experience monotonous decline, which could be attributed to the masking effect of the YPO4 coating layers.33 Surface morphologies of the pristine, 1, 3 and 5 wt.% YPO4-coated LNMO samples are displayed in Figure 2a-d. All the samples exhibit classic spinel feature, with an average particle size of ~ 200 nm. As could be observed in Figure 2a, the pristine LNMO particle demonstrates very smooth surface and clear boundary. With the 8


Page 8 of 36

Page 9 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increase of YPO4 content, the particle edge becomes less sharper with no longer plain surface, and when the YPO4 proportion is augmented to 5 wt.%, even a number of fragments appear on the particle surface (Figure 2d). Typically, the microstructures of LNMO before and after 3 wt.% YPO 4 surface modification are compared in Figure 2e and f. Both images present distinct lattice fringes with a uniform spacing of ~ 0.47 nm, corresponding to the (111) plane of the spinel phase.34 As expected, a clean surface and perfect crystallization was observed for the pristine LNMO in Figure 2e. Nevertheless, an amorphous surface modification layer appears outside the particle of 3 wt.% YPO4-coated LNMO, with a homogeneous thickness of ~ 7 nm (Figure 2f). Surface microstructures of the YPO4-modified LNMO particles with low magnification are compared in Fig. S3 (a-b). Amorphous surface modification layers appear outside the particles of 1, 3 and 5 wt.% YPO 4-coated LNMO, with homogeneous thickness of ~ 4, 7 and 15 nm, respectively. Typically, the presence of P and Y elements in 3 wt.% YPO4-coated LNMO could also be evidenced by the EDS analysis in Figure S4 (a-e), which unambiguously implies that the elements of P and Y are evenly distributed on the surface of LNMO particles. Furthermore, ICP investigation provides that the exact contents of YPO4 are 0.92 ± 0.01, 2.70 ± 0.01, 4.24 ± 0.01 wt.% for 1, 3, and 5 wt.% YPO4-coated LNMO. For simplicity, we still call 1, 3 and 5 wt.% YPO4-coated LNMO hereafter. XPS analysis was further carried out to identify the surface chemical environment of the as-prepared samples. Two main peaks at 854.3 and 871.7 eV, corresponding to Ni 2p3/2 and Ni 2p1/2 respectively,35 are observed for the pristine and YPO4-coated 9



LNMO in Figure 3a with identical binding energies, which degrade in their intensities monotonously as the YPO4 content increases. Analogously, the signals of Mn 2p rapidly shrink in their intensities after YPO4 surface modification, both for the Mn 2p3/2 and Mn 2p1/2 at 641.7 and 653.9 eV (Figure 3b).36 The binding energies of P 2p3/2, Y 3d5/2 and Y 3d3/2 in the YPO4-coated samples are detectable at constant 133.2, 157.4 and 159.4 eV in Figure 3c and d, consistent with the values reported for (PO4)337

and Y3+.38 It could also be noticed that along with the attenuation of Ni and Mn

intensities, the P and Y signals enhance remarkably, providing a direct evidence of LNMO/YPO4-core/shell structure for YPO4-coated LNMO. Typical charge-discharge profiles of the pristine and 3 wt.% YPO4-coated LNMO between 3.5 and 4.9 V at a current rate of 0.1 C are demonstrated in Figure 4a. In the charging process, both electrodes exhibit a short slope at ~ 4.0 V, followed by two long and distinct flat plateaus in the region of 4.6 ~ 4.8 V. The former slope is associated with the Mn3+/Mn4+ redox couple,39 while the latter double plateaus are originated from the Ni2+/Ni4+ redox couples, indicating that both samples have a structure of Fd 3 m space group.40 This result is in good agreement with the XRD and Raman analysis and undoubtedly confirms that YPO4 surface modification layer does not change the intrinsic structure of LNMO. Figure 4b graphs the initial dQ/dV curves for the galvanostatic cycling profiles. Not surprisingly, a small hump and two sharp peaks are observed for both the pristine and 3 wt.% YPO4-coated LNMO. In the locally magnified inset of Figure 4b, the peak intensity of 3 wt.% YPO4-coated LNMO is slightly reduced than that of the pristine 10


Page 10 of 36

Page 11 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


material in the ~ 4 V region, indicating minor surface structure transformation from disordered phase to ordered space group, which will in certain extent benefit to the structure stability of bulk material.41 Additionally, it is generally accepted that the difference between redox pairs (D-value) reflects the polarization degree42 and the full width at half maximum (FWHM) of a charge/discharge peak relates to the electrode kinetics behavior.43 In this case, the D-values and FWHM of the pristine and 3 wt.% YPO4-coated LNMO electrodes are fitted and compared in Table S1, which are generally lower for the latter electrode than those of the pristine one, implying that the YPO4 surface modification is insulating and increases surface resistance. Figure 4c reveals the cycling performances among the pristine and YPO4-coated samples at 0.1 C under room temperature. Apparently, the pristine LNMO displays a poor cyclic performance, decaying from 135 to 94 mAh g-1 after 240 cycles, with only a capacity retention of 69.6 %. After appropriate amount of YPO4 surface modification, typically 3 wt.%, the capacity retention is prominently enhanced to 77.5 % in the same process, indicating significantly improved cycling stability. When the YPO4 content further increases to 5 wt.%, the discharge capacity rapidly declines, indicating that excess YPO4 plays a negative role in electrochemical cycling, which will be discussed later. Moreover, rate performances of the pristine and 3 wt.% YPO4-coated LNMO under different current densities are illustrated in Figure 4d. As the current density increases from 0.1 C to 5 C, remarkable capacity decay could be observed for both electrodes. While comparatively, the discharge capacity of 3 wt.% YPO4-coated LNMO degrades more slower than that of the pristine electrode, 11



indicating that YPO4 surface coating effectively improves the rate performance of LNMO. To elucidate the Li diffusion kinetics, EIS measurements of the pristine and 3 wt.% YPO4-coated LNMO were carried out after different cycles, as demonstrated in Figure 5a and b. Clearly, all the plots are consist of two ever-increasing semicircles and a low-frequency tail. The high-frequency semicircle is linked to the solid electrolyte interface (SEI) film (Rsf) that covers cathode particles.44 The middle-frequency semicircle reflects the charge-transfer resistance (R ct) at the electrode/solid electrolyte interface.45 The low-frequency tail arises from Li+ ion diffusion in the bulk LNMO.46 The fitted results based on the equivalent circuit (inset of Figure 5b) are listed in Table 1, in which the Rsf and Rct values experience apparent increase with repeated cycling. For quantitative comparison, the Rsf and Rct values of each electrode after 10 cycles are defined (normalized) as 1. The impedance evolutions of these two materials are clearly displayed in Figure 5c and d. Apparently, the impedances of the pristine electrode increase more rapidly with cycling, indicating that the 3 wt.% YPO4 coating layer somehow suppresses the fast augment of surface impedance, which will facilitate the surface kinetics and partly explains the enhanced electrochemical performances. The apparent Li+ diffusion coefficient (DLi) could be calculated from the EIS profiles in the low-frequency zone by the following equations:[47]

Z img  A  W 0.5

(1)

R 2T 2 2S 2n 4 F 4C 2 2

(2)

D 

12


Page 12 of 36

Page 13 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Where R is the gas constant, T is the kelvin (room) temperature, n is the electron number per molecule participating in the oxidation-reduction reaction, S is the surface area of the electrode (Here it is estimated to be 0.5024 cm2), F is the Faraday’s constant, C is the concentration of Li+, W is the angular frequency and δ is the Warburg factor. From the plot of Zimg as a function of W-0.5, the slop δ could be obtained and D could be calculated. Thus, the diffusivities are determined to be 5.2 × 10-10 cm2 s-1 and 7.8 × 10-10 cm2 s-1 for the pristine and 3 wt.% YPO4-coated LNMO, respectively. Although the mechanism is unclear yet, the enhanced diffusion ability (derived from the YPO4 coating layer) will sure beneficial to the electrochemical properties of LNMO material. Figure S5 unfolds the DSC curves for the pristine and 3 wt.% YPO4-coated LNMO electrodes. It could be observed that the exothermic reaction temperature of the 3 wt.% YPO4-coated LNMO is 13 °C higher than that of the pristine electrode. Additionally, the coated electrode reveals significant reduction of heat release. The striking exothermic reaction delay and heat reduction after YPO 4 surface coating imply that the decisive factor governing the thermal stability is interface reactions. It is believed that the covalent-bond nature of (PO4)3- contributes to its strong resistance to the electrolyte, and (PO4)3- based phosphates have been reported to be thermally stable.48 It could be thus concluded that the YPO4 coating layer, improves the thermal stability of LNMO electrode. To further elucidate the genuine roles of the YPO4 nano layer played in the enhanced electrochemical performances, aging experiments were conducted for the pristine and 3 wt.% YPO4-coated LNMO powders. 13



Firstly, as provided in Table 2, NaOH titration analysis reveals that the acidity of 3 wt.% YPO4-coated LNMO doubled than that of the pristine material under the same condition. Considering the extraordinarily high acidity of pure YPO4 soaked in electrolyte for the same period (1176.30 ppm), the augmented acidity for YPO4-coated LNMO could be attributed to the modification layer. As a common Lewis acid, YF3 is supposed to be formed through the reaction of YPO 4 and trace HF-contained electrolyte, which should be responsible for the greatly increased acidity and will be involved in the surface reaction/diffusion process. Figure S6 shows the surface morphologies of the pristine and 3 wt.% YPO4-coated LNMO powders soaked in the same electrolyte for different times. Generally, the particle edge gets less sharper and the surface becomes more ambiguous with aging, which could be attributed to the spontaneously generated SEI film in chemical soaking process.49, 50 It could also be observed that the surface morphologies change more significantly for 3 wt.% YPO4-coated LNMO compared with the pristine material, indicating more intense surface reactions. Specifically, even some round pits could be found distributed on the surface of 3 wt.% YPO4-coated LNMO in Figure S6d, providing a direct evidence of surface corrosion from YF3 Lewis acid. As widely accepted, some insulating surface species could exist on the surface of oxide electrode material, which are negative for electrochemical performances.51 The appearance of corrosion pit implies that the insulating surface species could be firstly neutralized by the strong acidity, partly contributing to the enhanced surface kinetics in electrochemical cycling. With the process of electrochemical cycling/chemical immersion, it was reported 14


Page 14 of 36

Page 15 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that Mn could be dissolved from LNMO electrode materials.52, 53 The concentration of Mn ions of the pristine and 3 wt.% YPO4-coated LNMO with aging time are measured by ICP and the results are shown in the histogram of Figure 6a. Clearly, the dissolution of Mn ions aggravate with aging time for both systems. And the Mn contents detected in liquid are always higher in 3 wt.% YPO 4-coated system than those of the pristine one, indicating the accelerated migration of Mn ions after YPO 4 surface modification. Combined the previous titration experiment and morphology observation, YPO4, at least in part, is believed to convert to Lewis acid YF 3 in LiPF6-based electrolyte. The greatly increased acidity, locally in particular, will sequentially corrode the surface lattice after neutralize the surface insulating species, leading to the intensified dissolution of Mn ions. From this point of view, the exacerbated Mn dissolution after YPO4 surface modification provides another evidence for the generation of YF3 Lewis acid. Along with the Mn migration from surface lattice, some Y3+ ions have greater chance to be involved in the back-diffusion, owning to concentration gradient and/or valence equilibrium. This will, together with the dissolution of Mn ions, eventually lead to the formation of Li-Ni-Mn-Y-O surface solid solution on the surface of modified LNMO particle, which will undoubtedly enhance the surface kinetics behavior of Li ions and explains the improved rate performance of LNMO electrode (Figure. 4d). This reveals that the negative effect of Mn dissolution deduced by the generated YF3-based Lewis acid and the positive influence of surface solid solution balance well at an appropriate YPO 4 coating content (herein 3 wt.%). With further increased proportion of YPO 4, e.g. 5 wt.%, 15



higher acidity from more generated YF3-based Lewis acid will induce more dissolution of Mn ions and thus deteriorate the bulk structure of LNMO to a larger extent, explaining the aggravated electrochemical performances for 5 wt.% YPO4-coated LNMO in Figure 4. The binding energies of Mn 2p in the pristine and 3 wt.% YPO4-coated LNMO before and after aging experiment are compared by surface sensitive XPS (Figure 6b). As expected, the surface Mn signals degrade after aging experiment for both pristine and 3 wt.% YPO4-coated LNMO, while the latter one demonstrates more severe attenuation, in accordance with the results in Figure 6a. The binding energies of Y 3d in 3 wt.% YPO4-coated LNMO before and after aging test are displayed in Figure 6c, wherein the signals of Y 3d decay along with the peaks shifting towards higher binding energies after aging. This finding not only implies that some Y ions migrates into the inner side of surface lattice (to form surface solid solution), but also proves again the generation of Y-F bond.

CONCLUSION Different contents of YPO4 were successfully coated on the surface of LNMO without influencing its bulk structure. Electrochemical characterizations establish that 3 wt.% YPO4 coating is the most effective to enhance the cycling stability of LNMO. Intensive aging experiments reveal that the Lewis acid YF 3, converted from YPO4 modification material in LiPF6-based electrolyte, increases the acidity of the electrolyte. As a Lewis acid, YF3 corrodes the surface alkaline impurities and 16


Page 16 of 36

Page 17 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


accelerates ion exchange on material surface, thus helps to generate the surface solid solution Li-Ni-Mn-Y-O on the surface of YPO4-coated LNMO, which explains the improvement

of electrochemical performances.

The underlying mechanism

exploration for performance improvement in this work not only unambiguously elucidates the essential roles the YPO 4 plays as a modifications material, but also provides a guidance in surface functionalizaiton for other electrode materials in the community of secondary batteries.

ASSOCIATED CONTENT Supporting Information XRD patterns of all the as-prepared samples, Rietveld refinements and low magnification TEM images of YPO4-coated LNMO materials, EDS mapping images of 3 wt.% YPO4-coated LNMO, DSC curves and the dQ/dV analysis of the pristine and 3 wt.% YPO4-coated LNMO, and surface morphologies of the as-prepared samples soaked in the same electrolyte for different times.

AUTHOR INFORMATION Corresponding Author *Ying

Bai. Tel.: +86 (0371) 23881602. Fax.: +86 (0371) 23881602. E-mail:

[email protected]. Notes The authors declare no competing financial interest. 17



Page 18 of 36

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (50902044, 51672069, 10674041), the 863 Program of China (2015AA034201), the Program for Science and Technology Innovation Talents in Universities of Henan Province (16HASTIT042), the International Cooperation Project of Science and Technology Department of Henan Province (162102410014) and the Natural Science Foundation of Guangdong Province (2016A030313129).

REFERENCES (1) Devaraj, A.; Gu, M.; Colby, R.; Yan, P.; Wang, C.; Zheng, M. J.; Xiao, M.; Genc, J. A.; Zhang, J. G.; Belharouak, I.; Wang, D.; Amine, K.; Thevuthasan, S. Visualizing nanoscale 3D compositional fluctuation of lithium in advanced lithium-ion

battery

cathodes.

Nat.

Commun.

2015,

6,

8014-8022,

DOI

10.1038/ncomms9014. (2) Wang, C. Y.; Zhang, G.; Ge, S.; Xu, T.; Ji, Y.; Yang, X. G.; Leng ,Y. Lithium-ion battery structure that self-heats at low temperatures. Nature 2016, 529, 515-518, DOI 10.1038/nature16502. (3) Manthiram, A.; Chemelewski, K.; Lee, E. S. A perspective on the high-voltage LiMn1.5Ni0.5O4 spinel cathode for lithium-ion batteries. Energy Environ. Sci. 2014, 7 (4), 1339-1350, DOI 10.1039/c3ee42981d. (4) Xu, M. Q.; Zhou, L.; Dong, Y. N.; Chen, Y. J.; Demeaux, J.; MacIntosh, A. D.; Garsuch, A.; Lucht, B. L. Development of novel lithium borate additives for designed 18


Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface modification of high voltage LiNi0.5Mn1.5O4 cathodes. Energ Environ. Sci. 2016, 9 (4), 1308-1319, DOI 10.1039/c5ee03360h. (5) Lin, M.; Ben, L.; Sun, Y.; Wang, H.; Yang, Z.; Gu, L.; Yu, X.; Yang, X. Q.; Zhao, H.; Yu, R.; Armand, M.; Huang, X. Insight into the atomic structure of high-voltage spinel LiNi0.5Mn1.5O4 cathode material in the first cycle. Chem. Mater. 2015, 27 (1), 292-303, DOI 10.1021/cm503972a. (6) Kim, J. H.; Pieczonka, N. P. W.; Lu, P.; Liu, Z.; Qiao, R.; Yang, W.; Tessema, M. M.; Sun, Y. K.; Powell, B. R. In situ formation of a cathode electrolyte interface with enhanced stability by titanium substitution for high voltage spinel lithium ion batteries.

Adv.

Mater.

Interfaces

2015,

2

(10),

1500109-1500122,

DOI

10.1002/admi.201500109. (7) Shkrob, I. A.; Kropf, A. J.; Marin, T. W.; Li, Y.; Poluektov, O. G.; Niklas, J.; Abraham, D. P. Manganese in graphite anode and capacity fade in Li ion batteries. J. Phys. Chem. C 2014, 118 (42), 24335-24348, DOI 10.1021/jp507833u. (8) Lim, J. M.; Oh, R. G.; Kim, D.; Cho, W.; Cho, K.; Cho, M.; Park, M. S. Design of surface doping for mitigating transition metal dissolution in LiNi 0.5Mn1.5O4 nanoparticles. ChemSusChem 2016, 9 (20), 2967-2973, DOI 10.1002/cssc.201600821. (9) Pang, Q. F.; Wang, Q.; Zhang, Y.; Zou, Y.; Du, B.; Chen, F.; Wei, G.; Ying, J. Improved electrochemical properties of spinel LiNi0.5Mn1.5O4 cathode materials by surface modification with RuO2 nanoparticles. Electrochim. Acta 2015, 152, 240-248, DOI 10.1016/j.electacta.2014.11.142. (10) Ming, H. M.; Oh, J.; Tian, S. M.; Zhou, S.; Huang, Q.; Sun, H.; Zheng, Y. K. 19



Page 20 of 36

Surfactant-assisted synthesis of Fe2O3 nanoparticles and F-doped carbon modification toward an improved Fe3O4@CFx/LiNi0.5Mn1.5O4 battery. ACS Appl. Mater. Interfaces 2014, 6 (17), 15499-15509, DOI 10.1021/am504144d. (11) Alcantara, R.; Jaraba, M.; Lavela, P.; Tirado, J. L. X-ray diffraction and electrochemical impedance spectroscopy study of zinc coated LiNi0.5Mn1.5O4 electrodes.

J.

Electroanal.

Chem.

2004,

566

(1),

187-192,

DOI

10.1016/j.jelechem.2003.11.025. (12) Tao, S. K.; Kong, F. J.; Wu, C. Q.; Su, X. Z.; Xiang, T.; Chen, S. M.; Hou, H. H.; Zhang, L.; Fang, Y.; Wang, Z. C.; Chu, W. S.; Qian, B.; Li, S. Nanoscale TiO2 membrane coating spinel LiNi0.5Mn1.5O4 cathode material for advanced lithium-ion batteries. J. Alloys Compd. 2017, 705, 413-419, DOI 10.1016/j.jallcom.2017.02.139. (13) Wu, Q.; Zhang, X.; Sun, S.; Wan, N.; Pan, D.; Bai, Y.; Zhu, H.; Hu, Y. S.; Dai, S. Improved electrochemical performance of spinel LiMn1.5Ni0.5O4 through MgF2 nano-coating. Nanoscale 2015, 7 (38), 15609-15617, DOI 10.1039/c5nr03564c. (14) Wu, Q.; Yin, Y.; Sun, S.; Zhang, X.; Wan, N.; Bai, Y. Novel AlF3 surface modified spinel LiMn1.5Ni0.5O4 for lithium-ion batteries: performance characterization and

mechanism

exploration.

Electrochim.

Acta

2015,

158,

73-80,

DOI

10.1016/j.electacta.2015.01.145. (15) Xiao, B.; Liu, J.; Sun, Q.; Wang, B.; Banis, M. N.; Zhao, D.; Wang, Z.; Li, R.; Cui, X.; Sham, T. K.; Sun, X. Unravelling the role of electrochemically active FePO 4 coating by atomic layer deposition for increased high-voltage stability of LiNi0.5Mn1.5O4 cathode material. Adv. Sci. 2015, 2 (5), 1500022-1500028, DOI 20


Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10.1002/advs.201500022. (16) Shi, J. Y.; Yi, C. W.; Kim, K. Improved electrochemical performance of AlPO4-coated LiMn1.5Ni0.5O4 electrode for lithium-ion batteries. J. Power Sources 2010, 195 (19), 6860-6866, DOI 10.1016/j.jpowsour.2010.02.063. (17) Chong, J.; Xun, S.; Zhang, J.; Song, X.; Xie, H.; Battaglia, V.; Wang, R. Li3PO4-coated LiNi0.5Mn1.5O4: a stable high-voltage cathode material for lithium-ion batteries. Chem. Eur. J. 2014, 20 (24), 7479-7485, DOI 10.1002/chem.201304744. (18) Zhang, L.; Jia, G.; You, H.; Liu, K.; Yang, M.; Song, Y.; Zheng, Y.; Huang, Y.; Guo, N.; Zhang, H. Sacrificial template method for

fabrication of

submicrometer-sized YPO4: Eu3+ hierarchical hollow spheres. Inorg. Chem. 2010, 49 (7), 3305-3309, DOI 10.1021/ic9022739. (19) Rao, G. V. S.; Chowdari, B. V. R.; Lindner, H. J. Yttrium-doped Li(Ni, Co)O2: an improved cathode for Li-ion batteries. J. Power Sources 2001, 97, 313-315, DOI 10.1016/S0378-7753(01)00683-8. (20) Sun, L. N.; Yi, X. W.; Ren, X. Z.; Zhang, P. X.; Liu, J. H. Synthesis and electrochemical performances of Y-doped lithium-rich layered Li1.2Ni0.2Mn0.6O2 cathode

material.

J.

Electron.

Soc.

2016,

163

(5),

A766-A772,

DOI

10.1149/2.1071605jes. (21) Bai, Y.; Yin, Y.; Liu, N.; Guo, B.; Shi, H.; Liu, J.; Wang, Z.; Chen, L. New concept of surface modification to LiCoO2. J. Power Sources 2007, 174 (1), 328-334, DOI 10.1016/j.jpowsour.2007.09.023. (22) Bai, Y.; Chang, Q.; Yu, Q.; Zhao, S.; Jiang, K. A novel approach to improve 21



Page 22 of 36

the electrochemical performances of layered LiNi1/3Co1/3Mn1/3O2 cathode by YPO4 surface

coating.

Electrochim.

Acta

2013,

112,

414-421,

DOI

10.1016/j.electacta.2013.09.002. (23) Li, X.; Xu, J.; Zhu, J.; Zhang, K. J.; Wang, M. S.; Huang, Y. A novel YPO4-coated Li4Ti5O12 composite anode material with suppressed interfacial reactions for lithium ion batteries.

Mater. Lett. 2017, 200, 71-74, DOI

10.1016/j.matlet.2017.04.038. (24) Wang, S.; Li, P.; Shao, L.; Wu, K.; Lin, X.; Shui, M.; Long, N.; Wang, D.; Shu,

J. Preparation of spinel LiNi0.5Mn1.5O4 and Cr-doped LiNi0.5Mn1.5O4 cathode

materials by tartaric acid assisted sol gel method. Ceram. Int. 2015, 41 (1), 1347-1353, DOI 10.1016/j.ceramint.2014.09.067. (25) Cui, X. L.; Li, Y. L.; Li, S. Y.; Li, L. X.; Liu, J. L. Nanosized LiNi0.5Mn1.5O4 spinels synthesized by a high-oxidation-state manganese sol gel method. Ionics 2013, 19 (11), 1489-1494, DOI 10.1007/s11581-013-0897-y. (26) Lin, H. B.; Zhang, Y. M.; Hu, J. N.; Wang, Y. T.; Xing, L. D.; Xu, M. Q.; Li, X. P.; Li, W. S. LiNi0.5Mn1.5O4 nanoparticles: synthesis with synergistic effect of polyvinylpyrrolidone and ethylene glycol and performance as cathode of lithium ion battery. J. Power Sources 2014, 257, 37-44, DOI 10.1016/j.jpowsour.2014.01.089. (27) Wu, Y.; Zhang, J.; Cao, C.; Khalid, S.; Zhao, Q.; Wang, R.; Butt, F. K. LiNi0.5Mn1.5O4 nano-submicro cubes as high-performance 5 V cathode materials for lithium-ion

batteries.

Electrochim.

Acta

2017,

10.1016/j.electacta.2017.01.124. 22


230,

293-298,

DOI

Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Wang, L.; Liu, G.; Wu, W.; Chen, D.; Liang, G. Synthesis of porous peanut-like LiNi0.5Mn1.5O4 cathode materials through an ethylene glycol-assisted hydrothermal method using urea as a precipitant. J. Mater. Chem. A 2015, 3 (38), 19497-19506, DOI 10.1039/c5ta05550d. (29) Kothari, D. H.; Kanchan, D. K. Effect of doping of trivalent cations Ga3+, Sc3+, Y3+ in Li1.3Al0.3Ti1.7 (PO4)3 (LATP) system on Li+ ion conductivity. Phys. B 2016, 501, 90-94, DOI 10.1016/j.physb.2016.08.020. (30) Deng, J. C.; Xu, Y. L.; Li, L.; Feng, T. Y.; Li, L. Microporous LiAlSiO4 with high ionic conductivity working as a coating material and water adsorbent for LiNi0.5Mn1.5O4 cathode. J. Mater. Chem. A 2016, 4 (17), 6561-6568, DOI 10.1039/c6ta02237e. (31) Xue, Y.; Wang, Z.; Yu, F.; Zhang, Y.; Yin, G. Ethanol-assisted hydrothermal synthesis of LiNi0.5Mn1.5O4 with excellent long-term cyclability at high rate for lithium-ion batteries. J. Mater. Chem. A 2014, 2 (12), 4185-4191, DOI 10.1039/c3ta14567k. (32) Qiu, T.; Wang, J.; Lu, Y.; Yang, W. Improved elevated temperature performance of commercial LiMn2O4 coated with LiNi0.5Mn1.5O4. Electrochim. Acta 2014, 147, 626-635, DOI 10.1016/j.electacta.2014.08.122. (33) Bai, Y.; Yan, D.; Yu, C.; Cao, L.; Wang, C.; Zhang, J.; Zhu, H.; Hu, Y. S.; Dai, S.; Lu, J.; Zhang, W. Core-shell Si@TiO2 nanosphere anode by atomic layer deposition for Li-ion batteries. J. Power Sources 2016, 308, 75-82, DOI 10.1016/j.jpowsour.2016.01.049. 23



Page 24 of 36

(34) Cho, H. M.; Chen, M. V.; MacRae, A. C.; Meng, Y. S. Effect of surface modification on nano-structured LiNi0.5Mn1.5O4 spinel materials. ACS Appl. Mater. Interfaces 2015, 7 (30), 16231-16239, DOI 10.1021/acsami.5b01392. (35) Mansour, A. N.; Kwabi, D. G.; Quinlan, R. A.; Lu, Y. C.; Horn, Y. S. Probing the electrode-electrolyte interface in cycled LiNi0.5Mn1.5O4 by XPS using Mg and synchrotron X-rays. J. Electrochem. Soc. 2016, 163 (14), A2911-A2918, DOI 10.1149/2.0331614jes. (36) Yan, X.; Sun, D.; Wang, Y. Q.; Zhang, Z. Q.; Yan, W. C.; Jiang, J. C.; Ma, F. R.; Liu, J.; Jin, Y. C.; Kanamura, K. Y. S. Enhanced electrochemical performance of LiMn0.75Fe0.25PO4 nanoplates from multiple interface modification by using fluorine-doped carbon coating. ACS Sustainable Chem. Eng. 2017, 5 (6), 4637-4644, DOI 10.1021/acssuschemeng.6b03163. (37) Wang, Q. Y.; Liu, J.; Murugan, A. V.; Manthiram, A. High capacity double-layer surface modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode with improved rate capability. J. Mater. Chem. 2009, 19 (28), 4965-4972, DOI 10.1039/b823506f. (38) Chen, Q. L.; Gabaly, F. E.; Akgul, F. A.; Liu, Z.; Mun, B. S.; Yamaguchi, S.; Braun, A. Observation of oxygen vacancy filling under water vapor in ceramic proton conductors in situ with ambient pressure XPS. Chem. Mater. 2013, 25 (23), 4690-4696, DOI 10.1021/cm401977p. (39) Zhong, G. B.; Wang, Y. Y.; Yu, Y. Q.; Chen, C. H. Electrochemical investigations of the LiNi0.45M0.10Mn1.45O4 (M=Fe, Co, Cr) 5 V cathode materials for lithium

ion

batteries.

J.

Power

Sources

2012,

24


205,

385-393,

DOI

Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10.1016/j.jpowsour.2011.12.037. (40) Wang, L.; Li, H.; Huang, X.; Baudrin, E. A comparative study of Fd-3m and P4332 “LiNi0.5Mn1.5O4”.

Solid State Ionics

2011,

193 (1),

32-38, DOI

10.1016/j.ssi.2011.04.007. (41) Xiao, J.; Chen, X.; Sushko, P. V.; Sushko, M. L.; Kovarik, L.; Feng, J.; Deng, Z.; Zheng, J.; Graff, G. L.; Nie, Z.; Choi, D.; Liu, J.; Zhang, J. G.; Whittingham, M. S. High-performance LiNi0.5Mn1.5O4 spinel controlled by Mn3+ concentration and site disorder. Adv. Mater. 2012, 24 (16), 2109-2116, DOI 10.1002/adma.201104767. (42) Maugeri, L.; Simonelli, L.; Iadecola, A.; Joseph, B.; Okubo, M.; Honma, I.; Wadati, H.; Mizokawa, T.; Saini, N. L. Temperature dependent local structure of LiCoO2 nanoparticles determined by Co K-edge X-ray absorption fine structure. J. Power Sources 2013, 229, 272-276, DOI 10.1016/j.jpowsour.2012.11.127. (43) Zhang, X.; Sun, S.; Wu, Q.; Wan, N.; Pan, D.; Bai, Y. Improved electrochemical and thermal performances of layered Li[Li 0.2Ni0.17Co0.07Mn0.56]O2 via Li2ZrO3 surface modification. J. Power Sources 2015, 282, 378-384, DOI 10.1016/j.jpowsour.2015.02.081. (44) Yi, T. F.; Wu, J. Z.; Yuan, J.; Zhu, Y. R.; Wang, P. F. Rapid lithiation and delithiation property of v-doped Li2ZnTi3O8 as anode material for lithium-ion battery. ACS

Sustainable

Chem.

Eng.

2015,

3

(12),

3062-3069,

DOI

10.1021/acssuschemeng.5b00505. (45) Ye, J.; Hao, Q.; Liu, B.; Li, Y.; Xu, C. Facile preparation of graphene nanosheets encapsulated Fe3O4 octahedra composite and its high lithium storage 25



Page 26 of 36

performances. Chem. Eng. J. 2017, 315, 115-123, DOI 10.1016/j.cej.2017.01.023. (46) Mao, J.; Dai, K.; Zhai, Y. Electrochemical studies of spinel LiNi0.5Mn1.5O4 cathodes with different particle morphologies. Electrochim. Acta 2012, 63, 381-390, DOI 10.1016/j.electacta.2011.12.129. (47) Fang, Z. K.; Zhu, Y. R.; Yi, T. F.; Xie, Y. Li4Ti5O12–LiAlO2 composite as high performance anode material for lithium-ion battery. ACS Sustainable Chem. Eng. 2016, 4 (4), 1994-2003, DOI 10.1021/acssuschemeng.5b01271. (48) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. Phospho-olivines as positive-electrode materials for rechargeable lithium batteries. J. Electrochem. Soc. 1997, 144 (4), 1188-1194, DOI 10.1149/1.1837571. (49) Shu, J.; Shui, M.; Huang, F.; Xu, D.; Ren, Y.; Hou, L.; Cui, J.; Xu, J. Comparative study on surface behaviors of copper current collector in electrolyte for lithium-ion

batteries.

Electrochim.

Acta

2011,

56

(8),

3006-3014,

DOI

10.1016/j.electacta.2011.01.004. (50) Wang, Z.; Huang, X.; Chen, L. Characterization of spontaneous reactions of LiCoO2 with electrolyte solvent for lithium-ion batteries. J. Electrochem. Soc. 2004, 151 (10), A1641-A1652, DOI 10.1149/1.1793651. (51) Chen, Z.; Dahn, J. R. Methods to obtain excellent capacity retention in LiCoO 2 cycled

to

4.5

V.

Electrochim.

Acta

2004,

49

(7),

1079-1090,

DOI

10.1016/j.electacta.2003.10.019. (52) Jarry, A.; Gottis, S.; Yu, Y. S.; Rosell, J. R.; Kim, C.; Cabana, J.; Kerr, J.; Kostecki, R. The formation mechanism of fluorescent metal complexes at the 26


Page 27 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LixNi0.5Mn1.5O4-δ/carbonate ester electrolyte interface. J. Am. Chem. Soc. 2015, 137 (10), 3533-3539, DOI 10.1021/ja5116698. (53) Pieczonka, N. P. W.; Liu, Z.; Lu, P.; Olson, K. L.; Moote, J.; Powell, B. R.; Kim, J. H. Understanding transition-metal dissolution behavior in LiNi0.5Mn1.5O4 high-voltage spinel for lithium ion batteries. J. Phys. Chem. C 2013, 117 (31), 15947-15957, DOI 10.1021/jp405158m.

27



Pristine LNMO

Rwp:7.23 Rexp:4.63

20

Exp Cal Difference Brag Position

Intensity (a.u.)

Rp:5.38

Relative intensity

(b)

(a) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

Mn-O 630 cm-1

LNMO 1wt.% 3wt.% 5wt.%

630

487 586 394

300

40 60 80 2 theta (degree)

Pristine 1 wt.%-coated 3 wt.%-coated 5 wt.%-coated

450 600 750 -1 Raman shift (cm )

Figure 1. Rietveld refinement of the pristine LNMO (a) and the Raman spectra of all as-prepared samples (b).

28


Page 29 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. FESEM images of the pristine (a), 1 wt.% (b), 3 wt.% (c) and 5 wt.% (d) YPO4-coated LNMO and the corresponding HRTEM observations of the pristine (e) and 3 wt.% YPO4-coated LNMO (f).

29



(a)

(b) 845.3 eV

Intensity (a.u.)

2p3/2

2p1/2 871.7 eV

852

(c)

Mn


864

870

876

Binding energy (eV)

P

129

2p3/2 133.2 eV

641.7 eV 653.9 eV

640

645

Y

141

650

2p1/2

655

Binding energy (eV)

(d)


132 135 138 Binding energy (eV)


2p3/2

635

Intensity (a.u.)

Intensity (a.u.)

Ni

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 36

3d5/2 157.4 eV 3d3/2


159.4 eV

153 156 159 162 165 168 Binding energy (eV)

Figure 3. Binding energies of Ni 2p (a), Mn 2p (b), P 2p (c) and Y 3d (d) for the pristine and YPO4-coated LNMO samples.

30


Page 31 of 36

(b) -1

dQ/dV (mAh g V )

-1

4.0 3.5 0

(c)

Pristine 3 wt.%-coated

1000 0 -1000

4.65 V


-2000

(d)

150 120

40 80 120 160 200 240

Pristine

0

Cycle number

3 wt.%-coated

0.1 C 0.2 C 0.5 C 1C 2C

0

4.9 4.2 3.5 2.8 4.9 4.2 3.5 2.8

0.1 C 0.2 C 0.5 C

30

4.72 V

3.9 4.2 4.5 4.8 + Potential vs. Li/Li (V)

5C


60

4.66 V 4.70 V

1C

90

4.69 V

3.8 3.9 4.0 4.1 4.2

25 50 75 100 125 150 175 -1 Specific capacity (mAh g )

Voltage (V)

-1

4.5

4.75 V 4.75 V 4.70 V

2000

2C

Discharge capacity (mAh g )

+

Potential vs. Li/Li (V)

(a) 5.0

5C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 40 60 80 100 120 140 -1 Specific capacity (mAh g )

Figure 4. Initial charge-discharge profiles (a) and dQ/dV curves (b) of the pristine and 3 wt.% YPO4-coated LNMO; cycling stabilities of the pristine and YPO4-coated LNMO (c); discharge profiles of the pristine and 3 wt.% YPO4-coated LNMO at various rates (d).

31



(a)

(b) 1200

10 th 20 th 50 th

1350

-z'' (ohm)

-z'' (ohm)

1800 Pristine

900

8

1000 2000 3000 4000 z' (ohm)

Rsf

6


4 2 0

600

10 th 20 th 50 th

10

20 30 40 Cycle number

50

3 wt.%-coated 1000 2000 3000 4000 z' (ohm)

0 0

(d) Normalized resistance

0 0

(c)

900

300

450

Normalized resistance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 36

5 4

Rct


3 2 1 0

10

20 30 40 Cycle number

50

Figure 5. Nyquist plots of the pristine and 3 wt.% YPO4-coated LNMO after different cycles (a) and (b); evolution of the normalized surface resistance R sf (c) and charge transfer resistance Rct (d).

32


Page 33 of 36


-1

Mn dissolution (µg mL )

(a) 100 10 1 1 2 3 Aging time (week)

Intensity (a.u.)

(b) Mn 2p3/2

Pristine Pristine soaked 3 wt.%-coated 3 wt.%-coated soaked

2p1/2

635 640 645 650 655 660 Binding energy (eV)

(c) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Y

3 wt.%-coated 3 wt.%-coated soaked

3d5/2 3d3/2

150

155 160 165 Binding energy (eV)

170

Figure 6. Mn dissolution of the pristine and 3 wt.% YPO4-coated LNMO along with aging time (a); binding energies of Mn 2p (b) and Y 3d (c) in 3 wt.% YPO4-coated LNMO before and after soaked in commercial electrolyte for two weeks.

33



Page 34 of 36

Table 1. Fitted EIS data of the pristine and 3 wt.%YPO4-coated electrodes. Sample

Resistance

10 th

20 th

50 th

Rsf

326

805

1983

Rct

925

1317

3479

Rsf

462

738

761

Rct

714

868

1011

Pristine

3 wt.% YPO4-coated

34


Page 35 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Liquid acidity determined by NaOH titration. Sample

Acidity (ppm)

Commercial electrolyte

20.03

YPO4 soaked in electrolyte for two weeks

1176.30

LNMO soaked in electrolyte for two weeks

104.17

3 wt.% YPO4-coated LNMO soaked in electrolyte for two weeks

250.08

35



SYNOPSIS: YPO4 modification is an effective way to improve the electrochemical performances of LiNi0.5Mn1.5O4.

36


Page 36 of 36

Enhanced Electrochemical Performance of LiNi0.5Mn1.5O4 Cathode

Recommend Documents