Investigation of the Effect of Extra Lithium Addition and Postannealing

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Investigation of the Effect of Extra Lithium Addition and Post-annealing on the Electrochemical Performance of High-voltage Spinel LiNi0.5Mn1.5O4 Cathode Material Yunxian Qian, Yuanfu Deng, LiNa Wan, Hongjie Xu, Xusong Qin, and Guohua Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp503584k • Publication Date (Web): 27 Jun 2014 Downloaded from http://pubs.acs.org on June 30, 2014

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The Journal of Physical Chemistry

Investigation of the Effect of Extra Lithium Addition and Post-annealing on the Electrochemical Performance of High-voltage Spinel LiNi0.5Mn1.5O4 Cathode Material Yunxian Qian,† Yuanfu Deng,* , ‡, § Lina Wan,‡ Hongjie Xu, †

⊥

Xusong Qin,§ and Guohua Chen* , †, §,

⊥

Fok Ying Tung Graduate School, the Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

‡

The Key Laboratory of Fuel Cell Technology of Guangdong province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China §

Center for Green Products and Processing Technologies, Guangzhou HKUST Fok Ying Tung Research Institute, Guangzhou 511458, China

⊥

Department of Chemical and Bimolecular Engineering, the Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

ACS Paragon Plus Environment

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ABSTRACT. The LiNi0.5Mn1.5O4 (LNMO) spinel is an attractive cathode candidate for next generation lithium-ion batteries as it offers high power and energy density. In this paper, the effects of extra amounts of lithium addition and post-annealing process on the physicochemical and electrochemical properties of the spherical LNMO material were investigated. The experimental results show that the amount of lithium and the post-annealing process have significant impacts on the Mn3+ content, phase impurity (rock-salt phase) and phase structures (Fd3m and P4332) of the spherical LNMO cathode materials, so as their electrochemical performance. In particular, the phase transition from Fd3m to P4332 and the Mn3+ content of the LNMO spinels were found to be adjusted by lithium additions and the post-annealing process. With the presence of Mn3+, the absence of the impurity phase (rock-salt phase) and the cation ordering in the spinels, the electrochemical rate performance and capacity retention of the products could be significantly improved. In a half cell test, LNMO cathode material with 5% of lithium access (based on theoretical formula calculation) displays a high specific discharge capacity of 123 mAh g-1 at 2 C rate with excellent capacity retention of 84% after 500 cycles at 55 oC. All these findings show the important roles of the synergic effects of Mn3+ content, phase impurity (rock-salt phase) and phase structures (Fd3m and P4332) on the electrochemical performance improvement of LNMO-based cathode materials, which will guide the preparation of LNMO-based cathode material with excellent electrochemical performance. KEYWORDS: LiNi0.5Mn1.5O4, spinel, lithium addition, post-annealing, phase structure.


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INTRODUCTION

After entering the 21st century, the calling for more energy and a cleaner environment has forced a lot of countries to invest large amounts of money in developing the environmental friendly energy storage systems. Lithium-ion batteries have the advantages of high energy density, long cycle life and memory-free effect, making them widely applied in portable electronic and electric tool market.1 In the near future, they are expected to play even more important roles in EVs and PHEVs area.2 However, currently, they fail in meeting the demands dictated by powering of both hybrid electric vehicles and electric vehicles or by the large-scale storage systems of renewable energy. Hence, it would be necessary to develop lithium-ion batteries with higher energy densities. Cathode materials which could operate at high voltage and deliver high energy and power densities have attracted substantial attention in recent years.3,4 LiNi0.5Mn1.5O4 (LNMO) material, with a theoretical capacity of 147 mAh g-1, has emerged as a promising cathode material to replace LiCoO2 for lithium-ion batteries because of its high operating voltage (4.7 V vs. Li/Li+), which leads to high energy density (~ 700 Wh kg-1) and fast three-dimensional lithium-ion diffusion paths within the cubic lattice.4 Many efforts have been made to optimize the performance of this LNMO spinel material and the correlations among its preparation conditions, physical properties and electrochemical performance have been extensively studied in recent years.5-21 It has also been reported that the preparation approaches could greatly influence the particle size, phase structure and morphologies of the material.22 Depending on the distribution of Ni and Mn ions, the typical spinel LiNi0.5Mn1.5O4 has two different crystal structures with space groups of Fd3m (non-stoichiometric disordered phase) and P4332 (stoichiometric ordered phase).23,24 The non-stoichiometric LiNi0.5Mn1.5O4-δ has a face-


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centered cubic spinel structure with Ni and Mn ions distributed randomly in the 16d octahedral sites, while Li and O atoms occupied 8a tetrahedral sites and 32e sites, respectively. On the contrary, the stoichiometric LiNi0.5Mn1.5O4 has a relatively simple cubic structure with 4a, 12d, and 4c sites occupied by Ni, Mn and Li atoms and 8c and 24e sites occupied by O ions, respectively. In the disordered phase, Mn ions are mainly present in the form of Mn4+, with small amount of Mn3+ present. In the ordered phase, however, Mn ions are only in the form of Mn4+. According to Song et al.16 and Zhong et al.,25 the order-to-disorder transition would occur with the formation of rock-salt LixNi1-xO-type impurities if the LiNi0.5Mn1.5O4 materials were synthesized at temperatures above 700 oC. Previous works suggested that the disordered LNMO spinel would have superior electrochemical performance over the ordered one because of its significantly improved lithium diffusion coefficient and electronic conductivity,14 although superior rate capability was obtained on both samples with the disordered10,14 and ordered20, 26 structures. In previous work, the Mn3+ content has been suggested to play an important role in the performance of LNMO, and a reduction of the Mn3+ content after a post-annealing step would lead to a reduction in the rate capability.14 Considering the complexity of the interior structure and the difficulty to assess the interdependence of the special structure property and the electrochemical performance of this kind of material, the investigations which focus on the relationships between the structural features and the electrochemical properties would be of great importance and beneficial for the preparation of LNMO cathode materials. With the concepts of transition metal ions doping,9,18, 27-30 surface coating,31-34 controlling of the particle morphology913,20,35,36

and surface crystallographic facets,11,19,21,37,38 lots of works have been done to improve

the electrochemical performance and stabilize the charge/discharge cycling behavior of the LNMO-based materials.


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To prepare high voltage LNMO spinel cathode material with good electrochemical performance, the optimization of the synthesis approach and conditions including the types and amounts of raw materials, and calcination temperature and time is critical. Therefore, identifying the key parameters which have significant effects on the structure and performance of this spinel material still remains to be a big challenge. In this work, the influences of the extra lithium addition and the post-annealing process on the physicochemical properties as well as the overall electrochemical properties of the spinel LNMO-based were investigated. The LNMO materials adopted in this paper were prepared by a facile synthesis method using porous MnCO3 microspheres as the self-supporting template and were of sub-micrometer-sized spherical structure and high crystallinity.35 The relationships among the phase structure, Mn3+ content, phase impurities and their roles on the electrochemical performance of the obtained LNMO materials were extensively studied and discussed. The importance for the optimization of the synthesis parameters for LNMO spinel cathode material with superior rate performance and good cyclability at both room and elevated temperatures is also demonstrated.

EXPERIMENTAL SECTION

The LNMO samples with lithium excess of 0, 2, 5 and 8% (based on theoretical formula calculation) were prepared through a facile one-step synthesis route using porous MnCO3 microspheres as precursor, which has been published in our previous communication.35 Sample 0%, 2%, 5% and 8% were used to represent the products with theoretical lithium excess of 0.00, 0.02, 0.05 and 0.08, respectively. Specially, some LNMO samples with lithium theoretical excess of 0.02 and 0.05 were further annealed at 650 oC for additional 24 hours to get the post-annealed products, tagged as A-2% and A-5%. All of the as-prepared samples display spherical morphology of about 800 nm in size (as shown in the Fig. S1).


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The scanning electron microscopy (SEM) images were collected on JEOL 6300F scanning electron microscope. X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance Machine, with Cu Kα radiation (λ= 0.15406 nm) between 10 ° and 80 °. Raman spectra were obtained with a Bio-Rad FTS6000 Raman microscopy with a 532 nm blue laser beam. Fourier transform infrared (FT-IR) spectra were recorded with KBr pellets with a Bruker R 200L spectrophotometer. The compositions of the synthesized samples were tested by inductively coupled plasma (ICP) analysis on Optima 7300DV analyzer (Perkin Elmer). Electrochemical performances of the LNMO cathode materials were evaluated with CR2025 coin half-cells. The LIB cathodes were made from the mixtures of 80wt% LNMO, 10wt% carbon black, 10wt% polyvinylidene fluoride (PVDF) on aluminum foil with typical active material loading of 2.0 ~ 2.5 mg cm-2. The electrolyte was 1.0 mol/L of LiPF6 in 3:7 ethylene carbonate (EC): ethyl methyl carbonate (EMC). The coin cells were electrochemically cycled between 3.5 and 4.9 V at different charge/discharge rates on a multichannel battery test system (Neware CT-3008W). The coin cells were also cycled at 55 oC to evaluate the high-temperature performance of the LNMO cathode materials. The specific capacity was calculated based on the mass of the active material. The cyclic voltammetric (CV) tests were carried out on an electrochemical workstation (Autolab PGSTAT 101) at different scan rates within the range of 3.5-4.9 V vs. Li/Li+. The electrochemical impedance spectroscopy (EIS) data of the electrodes were acquired at the room temperature by a Versa-stat 3 electrochemical workstation (Princeton Applied Research) before cycling and also after a desired number of cycles, respectively, at a constant potential of approximately 4.7 V (vs. Li/Li+) in the frequency range from 100 kHz to 10 mHz by imposing an alternate current with an amplitude of 10 mV on the electrode.

RESULTS AND DISCUSSION


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Table 1 presents the elemental compositions of Mn, Ni and Li for the six as-prepared samples measured by the ICP analysis. The molar ratios of Ni/Mn in the six samples all reside within appropriate range from ICP and are close to the theoretical formula. It is clear that Li contents in these samples increase with the increment of lithium excess. However, even with 8% theoretical excessive amount of lithium, the real lithium ratio is still stays near 1.0 within the LNMO structure. In addition, the lithium contents decrease with the increase calcination time. This result suggested that the lithium loss would occur during the high temperature calcination. In order to figure out the impacts of the extra lithium additions and post-annealing process on the structure of the products, the powder XRD patterns of the six samples are displayed in Figure 1. All of these samples present good crystallinity and the typical spinel structure with (111) peak at ~ 18.8° and the other characteristic peaks of LNMO materials in Figure 1a. Among the XRD patterns of the six samples, Sample 5% shows the sharpest and strongest diffraction peaks, which indicates that the lithium consumption during the preparation process has important effects on the crystallinity of the obtained samples. For Sample 5% and 8%, the weak peaks at 2θ = 37.6 °, 43.7 ° and 63.5 ° were observed (as shown in Figure 1b), which could be attributed to the cationrich rock-salt phase impurities (LiyNi1-yO, ICSD No. 71422). However, there is no obvious impurity phase observed for other four samples. This is an interesting finding as it suggests that the experimental conditions employed here (extra lithium addition and post-annealing process) can simply adjust the phase purity for LNMO spinels. The excessive amount of lithium in the Li1+x[Mn2]O4 system which creates a cation-deficient rock-salt phase with retention of the spinel framework has also been demonstrated in literature.39 For the post-annealed samples, the weak peaks at 2θ = 15.3 °, 39.7 °, 45.7 ° and 57.5 ° (symbolic peaks for the primitive simple cubic crystal P4332 phase) were found. It is generally believed these weak peaks are introduced by the


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phase transition from Fd3m to P4332 structure during the post-annealing process at 600-700 oC. However, these peaks could not be observed clearly from Figure 1a. The broadening and lack of peaks might be caused by several factors, such as the instrumental factors that are related to the resolution and the incident X-ray wavelength, as well as sample factors including the crystallite size and uneven distributions of the microstrain.40 The lattice parameters (a-value/Å) for Sample 0%, 2%, 5%, 8%, A-2% and A-5% were calculated to be 8.152, 8.158, 8.160, 8.152, 8.148 and 8.151, respectively. These values are comparable to values reported in the literatures.41,42 The slight increase in the lattice parameters of Sample 5% proves that it is Mn3+ enriched comparing with other LNMO samples since the ionic radius of Mn3+ (0.65 Å) is larger than that of Mn4+ (0.54 Å). This result is consistent with the fact of the largest interplanar spacing of (111) peak for Sample 5%, based on the Bragg equation 2d111sinθ = λ (as shown in Figure 1c). The reasons for the highest Mn3+ content for Sample 5% might be attributed to the fact that parts of the Mn3+ are converted from certain amount of Mn4+ ions inside the LNMO spinel due to charge compensation. On the contrary, the slight decreases in the lattice parameters for Sample A-2% and A-5% (compared with their counterparts) indicate that the creation of oxygen vacancies in the spinel structure can be compensated and would result in phase transition (from the disordered to ordered phase) during the post-annealing process. [Figure 1] FT-IR and Raman spectra could act as local probes which are sensitive to the crystal symmetry. They are useful tools when the poor chemical contrast of XRD patterns prevents the determination of the internal structure such as cation ordering. In order to further confirm the phase structure of the products, the IR and Raman spectra are presented in Figures 2 and 3, respectively. As reported by Amatucci's group previously,24, 43 the determination of space group


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types could be made by comparing the peaks in the FT-IR spectrum. LNMO with the ordered structure in the P4332 space group typically produces a fingerprint FTIR spectrum of eight welldefined bands at 432, 468, 478, 508, 556, 594, 621 and 649 cm-1. On the contrary, disordered LNMO with Fd3m space group only gives a rather broad spectrum with the absence of the peaks at ~ 560 cm-1 and the peaks at ~ 650 and 430 cm-1. As shown in Figure 2, all the FT-IR spectra of Sample 8%, A-2% and A-5% show eight bands, while only five bands could be found in the spectra of other three samples (0%, 2% and 5%). This phenomenon indicates the ordered nature of Sample 8%, A-2% and A-5% comparing with the other three samples (0%, 2% and 5%). The difference in the intensity ratio between the bands at 594 and 621 cm-1, which has been used to quantify the degree of ordering in LNMO, could also be observed in Sample 8%, A-2% and A5%. The highest ratio in the A-2% suggests the highest degree of ordering in this sample.43 By deep analysis of the FTIR results, it is supposed that the lithium consumption in the preparation procedure of LNMO and post-annealing process can adjust the phase structures of LNMO material. [Figure 2] The local cation ordering in the LNMO structure was further investigated by Raman spectroscopy. Figure 3 shows the Raman spectra of the as-prepared LNMO samples. The peak at 630 cm-1 is assigned to the symmetric Mn-O stretching vibration of MnO6 group and the lines at 490 and 390 cm-1 are assigned to Ni2+−O stretching mode in the structure.16 The Raman spectra of Sample 8%, A-2% and A-5% all exhibit extra peaks at about 150 and 210 cm−1 and a splitting of peaks in the 588−623 cm−1 region, which are characteristic peaks of well separated Ni and Mn sites as a result of the symmetry lowering in the ordered P4332 structure.28,29 [Figure 3]


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As regarding to the electrochemical performance, the variation in cation ordering would make very different charge-discharge profiles, as seen in Figure 4. The discharge capacities are 117.6, 122.0, 121.9, 121.3, 113.4 and 120.1 mAh g-1 for Sample 0%, 2%, 5%, 8%, A-2% and A-5%, respectively. The large difference in the specific discharge capacities of these samples are related to different lithium ion insertion energies into the ordered and disordered spinel phases. The cation-disordered Sample 0%, 2% and 5% spinels all display a distinct two-step plateau in the 4.7 V region (Ni4+/Ni2+ redox couple) and a small plateau of various length in the 4 V region (the Mn4+/Mn3+ redox couple).16 The excessive amount of lithium ions exist in +1 oxidation state and the oxygen vacancies in the LNMO samples, which lowers the average manganese valence (below +4) with the generation of Mn3+. Although both the FT-IR and Raman spectra demonstrate that Sample 8% shows the ordered phase, it exhibits insignificant changes in the 4.7 and 4 V plateaus comparing with other three samples without post-annealing. This may be attributed to the fact that despite the fact that Sample 8% shows the ordered phase, its degree of cation orderring (short-range-ordering domains) in this sample is still quite low comparing with the post-annealed samples.16 In fact, both Sample A-2% and A-5% exhibit a much suppressed low-step process in the 4.7 V plateau (almost a single plateau) and a shorter 4.0 V plateau. The length of the plateau in the 4 V region can be adopted to calculate the relative amount of residual Mn3+ content in the spinels. Detailed analysis was conducted to calculate the capacity percentage contribution from the Mn3+ plateau (4 V) for all the six samples, as displayed in Table 2. The capacity contributed by Mn3+ for Sample 0%, 2%, 5%, 8%, A-2% and A-5% are 4.01, 10.03, 13.49, 10.01, 3.72 and 9.75%, respectively, indicating that the Mn3+ contents increase first and then decrease with the increase of the theoretical amount of lithium ions in the spinels. The highest Mn3+ content could be expected for Sample 5% spinel without post-annealing. For


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Sample 8%, the increase of lithium amount does not result in the occurrence of more Mn3+ content in the final product. This could be explained by the appearance of impurity (lithium enriched rock-salt phase) which had consumed significant amount of the extra lithium in this material. In addition, the ordered structure of Sample 8% also makes less Mn3+ inside the material. It has to be pointed out that the occurrence of Mn3+ ions in theoretical lithium-rich samples is mainly caused by the introduction of Li+ due to charge neutrality. However, the oxygen deficiency still partially contributes to the Mn3+ content as reflected by the extended 4 V plateau in Sample 5% while only a small 4 V plateau shown in Sample 0%.44 [Figure 4] It is therefore could be concluded that for LNMO spinels with theoretical lithium excess amount of 5%, the extra amount of lithium would compensate the lithium loss during calcination. In this regard, little rock-salt phase would appear. Moreover, the increases of lithium content would enhance the Mn3+ content, as seen in Sample 0% and 2%. For samples with more than 5% of theoretical lithium excess, in contrast, the extra amount of lithium would lead to the overdosage of lithium to the products, causing the formation of much rock-salt phase inside the structure. As a consequence, the excessive amounts of impurities in Sample 8% could not only enhance the charge status of manganese (the charge of nickel in the rock-salt phase is +3), producing less Mn3+ and bringing the short-range Ni/Mn ordering inside the material, but also lead to the relatively fast capacity decay during the electrochemical test (as seen from Figure 6). The rate capability of the as-prepared six samples is shown in Figure 5. All these samples show quite similar discharge capacities at low rates, with differences less than 10 mAh g-1. In addition, over 85% of their original discharge capacities still could be retained even at 10 C rate. Moreover, even after high-rate testing, the discharge capacities could still return or get close to


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the original level, revealing the excellent rate performance of the samples. It should be noted that there are still some differences observed for these samples, especially under high rates e.g, 10 C rate. Despite of the similar Mn3+ contents in Sample 2%, 8% and A-5%, Sample 8% and A-5% show worse rate capabilities comparing with Sample 2%. Higher degree of cation ordering in these two samples could be one of the reasons. Similar phenomenon could also be observed in Sample A-2% and 0%. In all of the six samples, Sample A-2% shows the lowest discharge capacity (90 mAh g-1) and the worst rate capability due to the lowest Mn3+ content as well as the highest degree of cation ordering. Although Sample 5% exhibits some little rock-salt phase, it shows relatively good electrochemical rate performance. However, as a matter of fact, it is demonstrated that the poor rate performance of Sample 8% is caused by the large concentrations of rock-salt phase. Therefore, it is supposed that the factor which finally results in the good rate performance of the LNMO sample may be attributed to the synergic effects of high Mn3+ contents, high degree of cation disordering and the absence of rock-salt phase in the spinel lattice. The best rate performance of the Sample 5% is also explained by the smallest difference in potential between the anodic and cathodic peaks in the CV curves (Figure S2). [Figure 5] The cycling performance of the six as-prepared samples at 2 and 10 C rates were plotted in Figure 6. As indicated in Figure 6a, the six samples demonstrate good cycling performance at 25 o

C at 2 C rate with 89.3, 91.2, 92.5, 87.0, 86.2 and 88.8% of initial capacity retained after 500

cycles for Sample 0%, 2%, 5%, 8%, A-2% and A-5%, respectively. In spite of the capacity differences, no significant differences on the capacity retention ratios of these samples are found. However, when they were charged and discharged at higher current densities, for example, 10 C rate, remarkable differences in terms of capacity and capacity retention were observed. Specially,


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the initial discharge capacities are 107.2, 116.0, 115.0, 115.5, 110.0 and 96.5 mAh g-1 for Sample 0%, 2%, 5%, 8%, A-2% and A-5%, respectively, with the corresponding capacity retention ratios of 79.5, 79.2, 86.8, 78.2, 89.4 and 74.2% after 500 cycles. As shown in previous sections, there are no significant differences on particle morphologies and sizes for these six samples. Therefore, these two factors should not have significant impacts on the initial discharge capacity, cycling performance and rate performance. It is suggested that the Mn3+ contents and the degrees of cation ordering play the most important roles on the materials’ initial discharge capacity, cycling performance and rate performance. Moreover, lower Mn3+ content and higher degree of cation ordering lead to relatively lower ionic conductivity, which will lead to low discharge capacity at the high rate. This could explain the fact that Sample A-2% shows the lowest initial discharge capacities, worst cyclability and rate capability among the six samples because of its lowest Mn3+ content and the highest degree of cation ordering. It is quite impressing that Sample 5% shows the best capacity retention, about 10% higher than that of other samples. Therefore, the authors strongly suggests that the coexistence of Mn3+ and the disordered structure is necessary for LNMO cathode materials to achieve improved the cycling performance, especially at high rates. [Figure 6] As well known, the high-temperature performance holds the key for the development of the LNMO materials. Up to date, only few published articles show the high-temperature performance of this kind of material for long cycles.17,29 For Sample 5%, the electrochemical behaviors at high temperature (55 oC) at the 2 C rate after an activation process at 0.5 C for 2 cycles at room temperature (Fig. 7a) were investigated. Its discharge capacity slightly increases to about 123 mAh g-1 after the initial cycles, and a capacity of 103.5 mAh g-1 can still be retained


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even after 500 cycles with a capacity retention of 84%. The coulombic efficiency is above 97% during the whole charge/discharge process. This cyclability could be comparable to the recently reported LiNi0.45Al0.1Mn1.45O4 spinel.29 In contrast, Sample 0%, 2%, 8%, A-2% and A-5% all display much worse capacity retention capabilities than that of Sample 5% with only 59.5, 69.1, 56.4, 63.2 and 53.9% of the initial discharge capacities retained (Figure S3). For these five samples, rapid capacity decays could be observed clearly after about 300 cycles. This phenomenon has also been observed in the recently reported literatures.16,45 The author suggested that this rapid capacity loss was attributed to the electrode polarization and the formation of electrically isolated active material. Therefore, the good cycle ability of Sample 5% at high rates and under high temperature can be explained by the good conductivity and its stable surface structure, which are beneficial to achieve low electrode polarization and ohmic resistance during the charge/discharge cycling (as shown in the following section). By the careful examination of the charge/discharge curves of Sample 5% at the 2 C rate under high temperature (Fig. 7b), one may notice that the 4 V plateau is quite stable and the capacity fading takes place mostly in the 4.7 V region where the Ni2+ ions would be oxidized to Ni4+ during cycling. This reveals that the Mn3+ content in Sample 5% does not decay obviously via the disproportionation reaction of 2Mn3+→ Mn2+ + Mn4+ during the charge/discharge process, which might be originated from the formation of a stable solid electrolyte interface (SEI) film during the initial several cycles. [Figure 7] To further explore the reasons for the excellent performance of Sample 5% at both high rates and high temperature, EIS spectra were adopted to track the impedance changes during cycling. Figure 8 depicts the Nyquist plots of the electrode of Sample 5% after 5th and 500th cycles of charging/discharging at 10 C rate and room temperature. As seen from Figure 8, three semi-


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circles could be detected, with one located at the high frequency region (> 10 kHz), one at the medium high frequency region (158-10 kHz), and the last at the medium low frequency area (< 158 Hz). For a typical EIS curve, the semi-circle at the medium high frequency area indicates the formation of the SEI film, and the semi-circle at the medium frequency region usually represents the contact resistances between the active materials, the electrolyte and the current collector, while that in the medium low frequency area is related with the charge transfer and solid state diffusion resistance of Li+ into the bulk material.46 One may find that even after cycling for long times at the high rate, the impedance of the electrode has not experienced huge changes, which could give a solid support for the outstanding cycling performance of Sample 5% at the room temperature. [Fig. 8] Figure 9 displays the EIS curves of Sample 5% cycled for different times at a 2 C rate under 55 o

C. As depicted in Figure 9, the EIS spectra of this sample at high temperature are quite different

from those at room temperature. The sample shows three semi-curves in the 5th cycle, which are similar to the case at room temperature. However, after cycling for more than 50 times, the two semi-circles which are located in the high and medium high frequency regions have merged into one. The mergence of these two semi-circles in the high and medium high frequency regions is attributed to the intense reactions between the electrolyte and the active materials at high potential and temperature, resulting in a sharp increase of the resistance of the SEI film comparing with the case at room temperature.47 Although the electrode impedance of this sample has continuously and slowly increased during the charge/discharge cycling process, it is still small after 500 cycles, which is consistent with its better capacity retention capability at high temperature.


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[Fig. 9]

CONCLUSION

In this study, the effects of theoretical lithium additions and the post-annealing process were investigated to explore the electrochemical behaviors of LNMO materials, especially at high charge/discharge current densities and high temperature. The amount of lithium addition plays an important role in tuning the Mn3+ content, the degree of cation ordering, the phase composition and purity of the LNMO spinels. The moderate excessive amount of lithium addition in the LNMO spinel compensates the lithium loss during calcination at the high temperature, increasing the Mn3+ content and leading to improved lithium diffusion rate and the ionic conductivity. The post-annealing process reduces the emergence of oxygen deficiency and the rock-salt phase impurity. In addition, it increases the degree of cation ordering of the LNMO spinel. It was found that the LNMO spinel with theoretical lithium excess of 5% exhibits the excellent cycling and rate performance at both room and high temperatures. It delivers a high discharge capacity of ~ 123 mAh g-1 at 2 C rate with excellent capacity retention capabilities of 93% and 84% after 500 cycles at 25 and 55 oC, respectively. The excellent rate and cycling performance of this sample can be attributed to the synergic effects of its disordered structure, high Mn3+ content, uniform spherical structure, the reasonable particle size and the high crystallinity.


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FIGURES (Word Style “VA_Figure_Caption”). Figure 1. XRD patterns of Sample 0%, 2%, 5%, 8%, A-2% and A-5%. Figure 2. IR spectra of Sample 0%, 2%, 5%, 8%, A-2% and A-5%. Figure 3. Raman spectra of Sample 0%, 2%, 5%, 8%, A-2% and A-5%. Figure 4. The charge/discharge curves of Sample 0%, 2%, 5%, 8%, A-2% and A-5% at a 2 C rate and room temperature. Figure 5. (a): The rate performance and (b) the cycling performance of Sample 0%, 2%, 5%, 8%, A-2% and A-5% at different current densities. Figure 6. The cycling performance of Sample 0%, 2%, 5%, 8%, A-2% and A-5% at different current densities and room temperature: (a) 2 C and (b) 10 C rate. Figure 7. (a) The cycling performance of Sample 5% at a 2 C rate and 55 oC, and (b) the corresponding charge/discharge curves at different cycles. Figure 8. EIS curves of Sample 5% after cycling at a 10 C rate and room temperature (25 ℃) for different cycles. Figure 9. EIS curves of Sample 5% after cycling at a 2 C rate and high temperature (55 ℃) for different cycles.


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Figure 1


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Figure 2

594 621

508 466

556

649

432

A-5%

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


A-2% 8% 5% 2% 0%

400

500

600

700 -1

Wavenumber (cm )


19


Figure 3

624 482

148 208

585

392

A-5%

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 20 of 37

A-2% 8% 5% 2% 0%

200

400

600

800

-1

Raman shift (cm )


20

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Figure 4


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Figure 5


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Figure 6


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Figure 7


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Figure 8


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Figure 9


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Table 1. Composition analysis data of the six various LNMO samples by the ICP data. Sample

With the Mn as a standard data (1.50) Ni

Mn

Li

0%

0.52

1.50

0.96

2%

0.53

1.50

0.98

5%

0.53

1.50

1.00

8%

0.53

1.50

1.02

A-2%

0.53

1.50

0.97

A-5%

0.53

1.50

0.98


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Table 2 Capacity percentages from the 4 V plateau in the LNMO samples. Sample Capacity contribution from 4 V plateau [%] 0%

4.01

2%

10.03

5%

13.49

8%

10.01

A-2%

3.72

A-5%

9.75


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ASSOCIATED CONTENT

Supporting Information. The SEM images (Figure S1), cyclic voltammetry curves (Figure S2) and the cyclic performance at 2C rate and 55 oC of the as-prepared LNMO-based samples (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]; [email protected].

ACKNOWLEDGMENT

This work was supported by the special financial grant from the China postdoctoral science foundation (2013T60795), the Science and Information Technology of Guangzhou Municipal (2013J4100112),

the

Fundamental

Research

Funds

for

the

Central

Universities

(SCUT2012ZZ0042), the Guangdong Province Science & Technology Bureau (IndustryEducation-Research Project, grant no. 2012B050300004), and the Fok Ying Tung Foundation (NRC07/08.EG01).


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BRIEFS (Word Style “BH_Briefs”). The different amounts of lithium and the post-annealing process have important effect on the phase structures and electrochemical performance of LiNi0.5Mn1.5O4-based spinel cathode materials. SYNOPSIS (Word Style “SN_Synopsis_TOC”). If you are submitting your paper to a journal that requires a synopsis, see the journal’s Instructions for Authors for details.


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Investigation of the Effect of Extra Lithium Addition and Postannealing

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