Effect of Lithium Deficiency on Lithium-Ion Battery Cathode LixNi0

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Effect of Lithium Deficiency on Lithium-Ion Battery Cathode LiNi Mn O Hyun Woo Choi, Su Jae Kim, Young-Hoon Rim, and Yong Suk Yang

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06501 • Publication Date (Web): 13 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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The Journal of Physical Chemistry

Effect of Lithium Deficiency on Lithium-Ion Battery Cathode LixNi0.5Mn1.5O4

Hyun Woo Choi,† Su Jae Kim,‡ Young-Hoon Rim,ξ and Yong Suk Yang*,§

†

Department of physics, Pusan National University, Busan 609-735, Korea

‡

KOMAC (Korea Multi-Purpose Accelerator Complex) KAERI, Gyeongju 780-904, Korea

ξSchool §

of Liberal Arts, Semyung University, Chechon, Chungbuk 390-711, Korea

Department of Nano Fusion Technology, Pusan National University, Busan 609-735, Korea

ABSTRACT We have investigated Li effects on the structural property change in LixNi0.5Mn1.5O4 (LNMO, x=1, 0.7, 0.5, 0.3, 0.1, 0) by using Rietveld refinement with neutron diffraction measurements. The polycrystalline LNMO samples were synthesized using the sol-gel process and calcinated at 1000 ℃ for 10 h. It was found that all the structures of LNMO belong to the face centered cubic spinel structure Fd 3 m, irrespective of Li amount and there exists a small amount of the NiO secondary phase for x=1. As lithium is extracted from LNMO, oxygen reduction also follows and the amount of oxygen released (y) from the sample changes in the manner to keep both the amount and oxidation state of Ni/Mn cations, and the total chemical formula can be written as (Lix)x+(Ni0.5Mn1.5)6.25+(O4-y)-(6.25+x) for x ≤ 0.7, and LiNi0.5-δMn1.5O4-y-δ + δNiO with δ=0.012 for x=1. Both the lattice constant of a unit cell and the nearest neighbor bond distances of ions in LNMO continuously decrease with the increase of Li content, indicating that the variation of those parameters are much dependent on the strength of coulomb interactions between ions.

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PACS number: 61.05.cp, 61.05.fm, 61.50.-f Keywords: Lithium deficiency, Lithium ion battery, Rietveld refinement, Neutron diffraction

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■ INTRODUCTION There is an increasing demand for improving energy storage systems to extend the use of lithium-ion batteries to electric vehicles and smart grid systems. The development of new electrode materials for the next generation of lithium-ion batteries requires systematic studies of the structural changes during charge-discharge cycling performance and thermal decomposition.

Li-Mn (Co, Fe, Ni)-O compounds are of great interest as cathode materials

in Li ion batteries.1-4 The transition metals in the compounds can be replaced or doped with each other, or mixed together.5-8 Among these, Li-Co-O is one of the widely used cathode materials due to its high storage capacity and cyclic performance.9,10 But there exist some barriers on cobalt containing materials such as high cost, and low stability caused by degradation or failure when overcharged, which lead to a rapid decrease in cell capacity. One of the causes of degradation is thought to be from the dissolution of cobalt into the electrolyte.11-14 To date, LiMn2O4, Co replaced by Mn, has focused on due to the advantage of low cost, environmental sustainability, and high ionic and electronic conductivity with good thermal stability. However, this material shows fading capacity during the operation of charging and discharging, possibly caused by the structural dissolution, an oxidation of electrolyte, an ionic dissolution and the Jahn-teller distortion.15-19 Techniques to overcome this problem are the introduction of cation defects as a form of Li1+zMn2-zO4 or doping different transition metals on the manganese sites. Among these, 4V LiMn2O4-modified composition LiNizMn2-zO4 has been focused on due to the high operating voltage of ~4.7 eV for dense energy cathode material.20,21 Various studies have been reported on LiNizMn2-zO4 such as synthetic method, thermal stability, effects of ordered and disordered local structure, cation ordering, particle size and composition change.22-26 There exist two kinds of LNMO (LiNi0.5Mn1.5O4 ) spinel crystal structure. One is the face-

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centered cubic (FCC) with the space group Fd 3 m and the other is the primitive simple cubic (SC) with the space group P4332. For the FCC structure, the unit cell consists of the Li ionoccupied tetrahedral 8a sites, Mn/Ni ion-occupied octahedral 16d sites and O-occupied cubic close-packed 32e sites. The Mn/Ni ions in 16d sites are randomly distributed and the FCC unit cell can be described with the general chemical structural formula (Li)tet(MnNi)oct(O)ccp. For the primitive SC structure, the Li ions are located in the 8a sites, Mn ions the 12d sites, Ni ions the 4a sites, oxygen ions the 24e and 8c sites, and the Ni/Mn ions are ordered regularly. The production whether the crystal structure belongs to FCC or SC is much dependent on the synthesizing process with annealing temperature.27,28 In the LiNizMn2-zO4 spinel structure, during battery operation, the Li ions transport from one tetrahedral site to the next through an unoccupied octahedral site. Since the site configuration of Ni and Mn ions is different for the FCC and SC structures, the activation energy barriers for the migration of Li ions, which are much dependent on the Ni and Mn repulsive energy, are also different for those two structures, affecting a cycling performance.29 Therefore, it is clear that structurerelated information in LiNizMn2-zO4 with the variation of Li and Mn/Ni contents is one of the most important factors to develop the improved electrochemical performing Li batteries. It is known that doping of the transition elements such as Al, Fe, Zn, Cu, Cr, Cr on LiNizMn2-zO4 can improve the structural stability during Li intercalation/deintercalation process and many investigations on this subject are undergoing.30 In spite many structure-related studies on LixNizMn2-zO4 such as Monte Carlo simulations,29 neutron diffraction,27,29,31-33 X-ray diffraction22,24,34 have been reported, but there has been no study that has shown the effect of Li content on crystal structure with the as-synthesized samples in the wide range of Li variation. Throughout the years, it has been known that the crystal structure and stoichiometry of LiNi0.5Mn1.5O4 are much dependent on the sample preparation conditions such as calcination 4


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and annealing temperature, cooling rate and gas environment during synthetic process.4,24,35 And the previous reports of the ex situ studies for LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4-y show that the materials undergo structural phase transitions on Li+ deintercalation.16 These phenomena of structural changes depending on (a) the sample synthetic process and (b) the Li+ extraction during cycling in the battery cell, motivated us to characterize the structure properties in detail for the samples prepared under the same synthesizing condition with various Li contents. We thus provide the as-synthesized LixNi0.5Mn1.5O4 samples with various concentrations of Li. In particular, we establish structure-related information of the delithiated LiNizMn2-zO4 for the fixed value of z=0.5, the as-synthesized LixNi0.5Mn1.5O4 (x=1, 0.7, 0.5, 0.3, 0.1, 0) samples prepared at 1000 oC in air, mainly by using the structural analysis of Rietveld refinement with the data from neutron diffraction experiments. Through the systematic investigation of as-synthesized disordered LNMO samples with various concentrations of Li, we characterize the condition for the total chemical formula of oxidation state when the Li is extracted from LNMO materials. New findings on the oxygen releases of the as-synthesized LiNizMn2-zO4-y (z=0.5) may provide some insight on understanding the (de)-lithiation mechanism of developing advanced cathode materials.

■ EXPERIMENTS Different compositions of LixNi0.5Mn1.5O4 (x=1, 0.7, 0.5, 0.3, 0.1, 0) powder were prepared by

the

sol-gel

process.

Ni(NO3)2·6H2O,

citric

acid,

Li(CH3COO)2·H2O

and

Mn(CH3COO)2·4H2O were mixed together. After stirring in distilled water for 1 h, the mixture was dried and calcinated in air at 1000 ℃ for 10 h. The sample was cooled down to room temperature slowly to avoid a possible phase change during the thermal process.

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Structures are identified firstly by X-ray diffraction (XRD) with the CuKα radiation of λ=1.5406 Å at room temperature. The neutron diffraction data were collected at the HRPD diffractometer (KAERI, Korea), with the wavelength λ=1.8343 Å, in the diffraction angle 2θ range 15-140o. The Rietveld refinements were conducted by using the FullProf program. The neutron scattering lengths of Li, Ni, Mn and O used for the refinement were -1.90, 10.30, 3.73 and 5.80 fm, respectively.

■ RESULTS AND DISCUSSION In the structural study of LixNi0.5Mn1.5O4 (x=1, 0.7, 0.5, 0.3, 0.1, 0) (LNMO), we consider a few different effects such as the Li deficit, the Ni/Mn oxidation states, the charge neutrality condition, the variation of oxygen content by changing Li amount, the site occupancy of constituting ions. Most of the previous ex-situ and in-situ studies of neutron and X-ray diffractions for the Li battery-related materials have been performed for the samples prepared from battery cells. For ex-situ measurements in the other studies, typically, Li-containing cathode materials are charged or discharged in the battery cells, and the cells are dismantled in an inert gas environment and then sealed into desired experimental tubes at room temperature. It is expected that, with this ex-situ method of sample preparation, the sample condition corresponds to the situation of a typical moment of the cycling process, maintaining the number density of constituting elements with the same values as initially determined except the amount of intercalation and deintercalation-dependent Li atoms. Meanwhile, the procedure of sample preparation in this study is different from those cases. We prepared each sample with the different Li mole ratio of LixNi0.5Mn1.5O4 (x=1, 0.7, 0.5, 0.3, 0.1, 0) through the sol-gel route in air, not via a battery cell, as explained previously in the experimental section. The amount of oxygen is no longer constant with the change of Li mole ratio because

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the sample was calcinated at 1000 ℃. The site locations of Ni and Mn atoms can be changed to keep the system’s charge neutrality and to lead the system to the most stable thermodynamic state. Our refinement process of the neutron diffraction data is rather complicated for these reasons and the structure-related information for the samples prepared under the process in this study have not been checked previously. Figure 1(a) shows the XRD patterns of LixNi0.5Mn1.5O4 (x=1, 0.7, 0.5, 0.3, 0.1, 0, LNMO) measured at room temperature. The weight of each compound for different x in LNMO was the same on XRD measurements and the diffracted intensities are therefore normalized. The XRD patterns in the figure correspond to the diffraction from the typical face centered cubic spinel structure (space group Fd 3 m) irrespective of Li amount, where nonzero Bragg peaks exist only for the (hkl) values, all even or all odd, and the structure factor becomes zero for the mixed (hkl) values. As can be seen in Fig. 1(a), with the increase of Li amount, some XRD peak intensities decrease (e.g. (220)) or increase (e.g. (111)), depending on the Miller indices, in spite of the fact that there is no composition change in the Ni and Mn atoms. Considering the small value of the X-ray scattering cross section of Li atoms, this anomalous intensity change indicates that not only Li effect but also some other factors are involved in X-ray diffraction. And, as a matter of fact, with the change of Li content, the oxygen content changes as well, which gives rise to an intensity change on the XRD patterns, as can be seen in the neutron diffraction refinement results. The LixNi0.5Mn1.5O4 compound can be thought of as the partial replacement of the Mn ions by Ni ions in LixMn2O4 and it is normally assumed that the replacement occurs at the Mn sites. But, since the X-ray scattering cross sections of Mn and Ni atoms (atomic numbers Mn; 25 and Ni; 28) are so close, no clear Ni substitution effect can be found in the XRD peak intensities. In the LixMn2O4 spinel structure, the Li, Mn and O ions are expected to locate at 8a, 16d and 32e sites, respectively. Under this site-fixed condition, as an example, the structure factor at the (220) reflection from Mn (16d) 7



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sites with the ionic positions (1/2,1/2,1/2), (1/4,1/4,0), (3/4,0,1/4) and (0,1/4,3/4) becomes zero, and the Bragg peak intensity change would be much dependent on the Li amount. But in the figure, in spite of the fact that there is no Li ions, the intensity of (220) Bragg peak for x=0 in LixNi0.5Mn1.5O4, is larger than any other peak intensities containing Li ions. This suggests that not only Li but also Mn/Ni ions may occupy 8a sites. The refinement of neutron diffraction measurements in this study clarifies the site locations of the transition metal ions. Figure 1(b) is a magnified view of the (111) and (440) XRD peaks taken from Fig. 1(a). The shift of the Bragg peak positions toward a lower angle with the decrease of Li amount indicates a volume expansion with the extraction of Li ions. These brief explanations of the XRD pattern change with the change of Li amount will be further discussed in detail with the Rietveld refinement analysis of the neutron diffraction data. The neutron diffraction and X-ray diffraction patterns of LixNi0.5Mn1.5O4 for x=1 are shown in Figure 2. These two different diffraction patterns are plotted together for a comparison. In the diffraction measurements, we used the neutron and X-ray wavelengths of 1.8343 Å and 1.5406 Å, respectively, but in this figure the neutron diffraction angle is converted into the same angle as scattered with X-ray wavelength, and in this way the Bragg peak positions from neutron diffraction and XRD can be put at the same diffraction angles for the given (hkl). The intensity ratios between all Bragg peaks are clearly different between the neutron diffraction and XRD data, caused by the different scattering lengths for given atoms between neutron and X-ray. Furthermore, there appear extra Bragg peaks in the neutron diffraction patterns, marked with arrows, which are not seen in the XRD data. These small extra peaks can be indexed by diffractions from the NiO (cubic) crystalline phase. The scattering cross section of X-ray is linearly dependent to the electron charge density of the atoms, so the XRD is not sensitive to the Li+ positions. Meanwhile the scattering cross

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section of neutron is more or less random for the atomic numbers, since the neutron is sensitive to the nucleus. Thus neutron scattering cross section from Li-ion in the sample is observable. Kim et al. presented the Rietveld refinement profiles of XRD data for ordered LNMO and disordered LNMO materials,34 and our XRD patterns of LixNi0.5Mn1.5O4 do not show the peaks from ordered structure of LNMO. Therefore we performed the Rietveld refinement of neutron data for the disordered LNMO. The neutron diffraction patterns of LNMO with the different Li mole ratios are shown in Figure 3. All the diffracted Bragg peaks can be indexed with the Miller indices corresponding to a face centered cubic spinel structure. We refine all of these diffraction patterns with the Rietveld refinement method by using the FullProf program. It should be reminded that the small amount of extra NiO phase exist for x=1 in LixNi0.5Mn1.5O4 and for the rest of the compositions x=0.7, 0.5, 0.3, 0.1 and 0, only single spinel phase appears. The Rietveld refined neutron diffraction data for LixNi0.5Mn1.5O4 (x=1, 0.7 and 0) are shown in Figure 4. The Bragg peak positions from the spinel structure are indicated, including the reflections from the NiO phase for x=1, as vertical bars. The line at the bottom of each figure represents the difference between the measured and refined values. The spinel structure is refined in the face centered cubic Fd 3 m, under an assumption in the beginning, with the Li ions occupying the tetrahedral 8a sites and the Mn/Ni ions occupying the octahedral 16d sites. But the refinement results indicate that there are considerable site occupancies of Mn ions at the 8a sites. The structure-related parameters are summarized in Table 2. Figure 5 shows the change of the lattice parameter and bond lengths of Li-O, Mn/Ni-O and O-O as a function of Li amount in LNMO. It should be noted that the lattice parameter of a unit cell decreases even though the number of Li ions increases. In general, a few key factors can be considered to affect a change in the lattice parameter. 1) The number of atom; a

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reduced number of atoms in a unit cell causes the contraction of a unit cell with a low packing fraction. 2) The size of cations; for a given atomic species, the ionic sizes depend on the oxidation states, and the lattice parameter will be decreased with the occupation of smaller atoms consisting of a unit cell, e.g., the ionic sizes of manganese and nickel are so different depending on the location sites and oxidation states as Mn2+=80 (97), Mn4+=53 (67) fm in the tetrahedral (octahedral) sites, and Ni2+=69 (83) fm in the tetrahedral (octahedral) sites. 3) The configuration of ion locations; Coulomb interactions of the repulsive and the attractive will be much dependent on the location of ions. The strength change of such Coulomb interactions between the same or different charges will change the cell volume. As can be seen in Fig. 5, not only the lattice constant but the nearest neighbor bond distances Li-O, Mn/Ni-O and O-O, irrespective of the connected charge species, continuously decrease in spite of the increase in the number of Li ions. This indicates that the variation of the lattice constant is much dependent on the strength of Coulomb interactions between ions, as aforementioned, rather than the number of atoms occupying unit cells or the change of ionic sizes of manganese and nickel. This effect clearly appears in Fig. 5 (b), where the bond distances between any nearest neighbor ions increase with the subtraction of Li ions, indicating that the attractive force of cation-anion decreases, and the repulsive force of anionanion increases with the decrease of Li ions. It should be reminded that, in this study, the number of O ions in the as-synthesized LNMO samples changes according to the variation of the Li mole ratio. Different behaviors of the lattice constant change can be found from other reports, when the number of O ions is kept constant. Some neutron diffraction and X-ray diffraction studies of Li intercalation and deintercalation effects on the structural evolution of LNMO at room temperature, when the number of O ions is kept constant, as the case when the samples of different Li contents are prepared with working battery cells, show that there

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exist different cubic phases, depending on Li content, and accordingly the behavior of the lattice parameter change is different for each phase. 31,34,35 From the results of neutron diffraction Rietveld refinement, we have found that oxygen loss is accompanied by the extraction of Li in LNMO. The oxygen deficiency of LNMO with respect to the change of Li amount is shown in Fig. 6. The oxygen loss behavior can be divided into two regions, depending on the single or mixed crystalline phases of LNMO. The oxygen deficient compounds maintain the single phase pure cubic spinel structure for x ≤ 0.7 and the oxygen deficiency linearly decreases with the increase of Li amount. But the linearity slightly changes above x=0.7, where the LNMO compounds have mixed phases of the spinel and NiO structures. The appearance of a NiO phase may be related to the process of providing the samples. The stoichiometry of LNMO is not perfect because when it is calcinated at 1000 ℃, side reactions of generating oxygen voids is unavoidable.36 From the refinement results, we have found that there is a close relationship between the Li and O mole ratio. With the extraction of Li from LNMO, the amount of oxygen lost from the sample changes in a way that keeps the oxidation states of Ni/Mn cations as Ni2+Mn3.5+, irrespective of Li content, including x=1. The total oxidation states of the compositions can be written as Lix+(Ni,Mn)6.25+O-(6.25+x), and with the general LixNi0.5Mn1.5O4-y formula as written in the figure, the relationship 6.25+x=2(4-y) holds for x≤ 0.7 and is shown as a straight line in the figure. This result indicates that, when the Li amount varies, the introduction of oxygen vacancy is preferable to keep the charge neutrality of the system, rather than changing the oxidation state of Ni or Mn ions. And the simple calculation of the oxidation state under that charge balance condition is consistent with the refined results of the neutron diffraction data in the figure, represented as symbols. For x=1, according to the results of Rietveld refinement, the molar ratio between two phases of the spinel LNMO and NiO is about 97.6 : 2.4, considering the amount of Ni/O contents. And the spinel structure of Fd 3 m sustains its 11



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stability with this small subtraction of NiO (refer Fig. 2). Therefore, the chemical formula of the coexisting two phases for x=1 can be written as LiNi0.5-δMn1.5O4-y-δ + δNiO with δ=0.012. And, considering the change of oxygen content with x, the chemical formula of the spinel structure at room temperature can be written as LixNi0.5-δMn1.5O4-y-δ instead of LixNi0.5Mn1.5O4 with δ=0 and 0.012 for x ≤ 0.7 and 1, respectively. As a consequence, our results from a free energy point of view indicate that the system, sustaining its spinel structure, keeps in the lower energy state for the case of decreasing O content with the decrease of Li than for the case of a changing in oxidation state of the transition metals with keeping O content, since we only extract Li and leave other parameters to be changed free. The results also imply that the higher energy state of the system can be one of the reasons for the structural phase transitions when the oxygen content is fixed during the delithiated process in the electrochemical cycling. Figure 7 shows a unit cell of the spinel cubic structure for x=1 (solid line), obtained from the refinement results. For the case of x=0 (dotted line), the displacement is exaggerated 10 times larger for eyes. The lattice constants are 8.427 and 8.178 Å for x=0 and 1, respectively, which is ~3% change, but the dotted line is ~30% magnified. The Rietveld refined results for the spinel phase are summarized in Table 1. At the initial step of simulation, the quantity and position for Li at 8a-site were fixed and the displacement parameters for Ni/Mn at the 16d-site and O at the 32e-site were refined, where the Ni- and Mn-occupations were constrained to correspond to the formulation LiNi0.5Mn1.5O4. In a subsequent refinement, the position of Li atom was tested, while the site occupancies of other elements were varied in the model. The disordered states of LNMO crystalline with space group Fd 3 m were successfully refined for the annealed samples at 1000 ℃. Meanwhile, there is no texture/preferential direction for the cubic structured sample that was carefully

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prepared by controlling temperature in the furnace. From this we can obtain the important and crucial structure-related parameters and information as follows: 1) Occupation sites of ions; the Li ions occupy the 8a tetrahedral sites at the position (1/8,1/8,1/8) irrespective of the number of Li ions. The Ni ions always occupy the 16c octahedral sites at the position (1/2,1/2,1/2). The Mn ions can occupy both 8a and 16c sites, and the occupancy ratio of those sites is dependent on the Li mole ratio. The Mn occupancy of the 8a sites increases with the decrease of the Li mole ratio from 0.01 for x=1 to 0.33 for x=0. The O ions always occupy 32e sites. 2) Oxygen deintercalation; the amount of oxygen ions decreases according to the Li mole ratio. There is a deintercalating tendency in such a way that keeps the charge neutrality, where the oxidation states of nickel and manganese are Ni2+ and Mn3.5+ on average, and that of oxygen is O2-, irrespective of Li content. The bond lengths resulting from the Rietveld refinement are listed in Table 2, with comparisons between the determined nearest neighbor distances and those of the theoretically calculated values shown in the parenthesis. Table 2 shows the change of bond lengths between ions with x, and as also shown and explained in Fig. 5(b) the lengths decrease with the increase of x. Table 3 shows the change of oxygen content and the oxidation state with x, and the corresponding results are represented in Fig. 6. Two numerical values in the table, one the normal and the other in parenthesis, are obtained from the refinement and the calculation satisfying 6.25+x, respectively.

■ CONCLUSIONS We have characterized the room temperature structural properties of LixNi0.5Mn1.5O4 (x=1, 0.7, 0.5, 0.3, 0.1, 0) (LNMO), mainly by using the structural analysis of Rietveld 13



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refinement with the data from neutron diffraction experiments. The powder samples were prepared by the sol-gel process and calcinated in air at 1000 ℃ for 10 h. It is found that all the structures of LNMO belong to the face centered cubic spinel structure Fd 3 m, irrespective of Li amount and there exists a small amount of NiO secondary phase for x=1. The refinement results show that the Li, Ni and O ions locate at the tetrahedral 8a, octahedral 16d and 32e sites, respectively. The Mn ions are not only located at the 16d sites but also considerable site occupancies of 8a exist. With the extraction of Li from LNMO, the amount of oxygen released from the sample changes in a way that keeps both the amount and oxidation state of Ni/Mn cations and the total chemical formula can be written as (Lix)x+(Ni0.5,Mn1.5)6.25+(O4-y)-(6.25+x) and a relationship 6.25+x=2(4-y) holds for x ≤ 0.7. And for x=1, because there are two phases, the formulae are LiNi0.5-δMn1.5O4-y-δ + δNiO with δ=0.012. Considering the results of Rietveld refinement on the change of oxygen content with x, the chemical formula of the spinel structure at room temperature should be written as LixNi0.5-δMn1.5O4-y-δ with δ=0 and 0.012 for x ≤ 0.7 and 0, respectively. Both the lattice constant of a unit cell and the nearest neighbor bond distances Li-O, Mn/Ni-O and O-O continuously decrease with the increase in the number of Li ions, indicating that the variation of those parameters are much dependent on the strength of Coulomb interactions between ions, rather than the number of atoms occupying unit cells or the change of ionic sizes of manganese and nickel.

■ AUTHOR INFORMATION Corresponding Author *Address: Department of Nano Fusion Technology, Pusan National University, Busan 609735, Korea. TEL: +82-51-510-2796, FAX: +82-51-514-2358; E-mail: [email protected] 14


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Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2010-0024388). This research was also supported by a national nuclear R&D program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013M2B2A4041435 and 2015M2B2A4033329).

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■ REFERENCES (1) Fisher, C. A. J.; Prieto, V. M. H.; Islam, M. S. Lithium Battery Materials LiMPO4 (M = Mn, Fe, Co, and Ni): Insights into Defect Association, Transport Mechanisms, and Doping Behavior. Chem. Mater. 2008, 20, 5907-5915. (2) Xu, J.; Dou, S.; Liu, H.; Dai, L. Cathode Materials for Next Generation Lithium Ion Batteries. Nano Energy 2013, 2, 439-442. (3) Duncan, H.; Hai, B.; Leskes, M.; Grey, C. P.; Chen, G. Relationships between Mn3+ Content, Structural Ordering, Phase Transformation, and Kinetic Properties in LiNixMn2−xO4 Cathode Materials. Chem. Mater. 2014, 26, 5374-5382. (4) Liu, G.; Park, K.; Song, J.; Goodenough, J. B. Influence of Thermal History on the Electrochemical Properties of Li[Ni0.5Mn1.5]O4. J. Power Sources 2013, 243, 260-266. (5) Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nano-Sized TransitionMetal Oxides as Negative-Electrode Materials for Lithium-Ion Batteries. Nature 2000, 407, 496-499. (6) He, P.; Yu, H.; Li, D.; Zhou, H. Layered Lithium Transition Metal Oxide Cathodes towards High Eenergy Lithium-Ion Batteries. J. Mater. Chem. 2012, 22, 3680-3695. (7) 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, 1339-1350. (8) Ebner, M.; Geldmacher, F.; Marone, F.; Stampanoni, M.; Wood, V. X-ray Tomography of Porous, Transition Metal Oxide Based Lithium Ion Battery Electrodes. Adv. Energy mater. 2013, 3, 845-850. (9) Oh, S.; Lee, J. K.; Byun, D.; Cho, W. I.; Cho, B. W. Effect of Al2O3 Coating on Electrochemical Performance of LiCoO2 as Cathode Materials for Secondary Lithium Batteries. J. Power Sources 2004, 132, 249-255. (10) Jung, S. K.; Gwon, H.; Hong, J.; Park, K. Y.; Seo, D. H.; Kim, H.; Hyun, J.; Yang, W.; Kang, K. Understanding the Degradation Mechanisms of LiNi0.5Co0.2Mn0.3O2 Cathode

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Material in Lithium Ion Batteries. Adv. Energy Mater. 2014, 4, 1300787. (11) Yamamoto, K.; Minato, T.; Mori, S.; Takamatsu, D.; Orikasa, Y.; Tanida, H.; Nakanishi, K.; Murayama, H.; Masese, T.; Mori, T. et al. Improved Cyclic Performance of Lithium-Ion Batteries: An Investigation of Cathode/Electrolyte Interface via In Situ Total-Reflection Fluorescence X‑ray Absorption Spectroscopy. J. Phys. Chem. C 2014, 118, 9538–9543. (12) Wu, L.; Nam, K. W.; Wang, X.; Zhou, Y.; Zheng, J. H.; Yang, X. Q.; Zhu, Y. Structural Origin of Overcharge-Induced Thermal Instability of Ni-Containing Layered-Cathodes for High-Energy-Density Lithium Batteries. Chem. Mater. 2011, 23, 3953-3960. (13) Shikano, M.; Kobayashi, H.; Koike, S.; Sakaebe, H.; Saito, Y.; Hori, H.; Kageyama, H.; Tatsumi, K. X-ray Absorption Near-Edge Structure Study on Positive Electrodes of Degraded Lithium-Ion Battery. J. Power Sources 2011, 196, 6881-6883. (14) Smith, A. J.; Burns, J. C.; Xiong, D.; Dahn, J. R. Interpreting High Precision Coulometry Results on Li-Ion Cells. J. Electrochem. Soc. 2011, 158, A1136-A1142. (15) Fu, Y.; Jiang, H.; Hu, Y.; Zhang, L.; Li, C. Hierarchical Porous Li4Mn5O12 Nano/Micro Structure as Superior Cathode Materials for Li-Ion Batteries. J. Power Sources 2014, 261, 306-310. (16) Chen, R.; Knapp, M.; Yavuz, M.; Heinzmann, R.; Wang, D.; Ren, S.; Trouillet, V.; Lebedkin, S.; Doyle, S.; Hahn, H. et al. Reversible Li+ Storage in a LiMnTiO4 Spinel and Its Structural Transition Mechanisms. J. Phys. Chem. C 2014, 118, 12608-12616. (17) Tang, D.; Sun, Y.; Yang, Z.; Ben, L.; Gu, L.; Huang, X. Surface Structure Evolution of LiMn2O4 Cathode Material Upon Charge/Discharge. Chem. Mater. 2014, 26, 3535-3543. (18) Roder, P.; Stiaszny, B.; Ziegler, J. C.; Baba, N.; Lagaly, P.; Wiemhofer, H. D. The Impact of Calendar Aging on the Thermal Stability of a LiMn2O4-Li(Ni1/3Mn1/3Co1/3)O2/Graphite Lithium-Ion Cell. J. Power Sources 2014, 268, 315-325. (19) Zhao, S.; Bai, Y.; Ding, L.; Wang, B.; Zhang, W. Enhanced Cycling Stability and Thermal Stability of YPO4-Coated LiMn2O4 Cathode Materials for Lithium Ion Batteries. Solid State Ionics 2013, 247-248, 22-29.

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(20) Park, S. H.; Oh, S. W.; Kang, S. H.; Belharouak, I.; Amine, K.; Sun, Y. K. Comparative Study of Different Crystallographic Structure of LiNi0.5Mn1.5O4−δ Cathodes with Wide Operation Voltage (2.0–5.0 V). Electrochim. Acta 2007, 52, 7226-7230. (21) Liu, G.; Zhang, L.; Sun, L.; Wang, L. A New Strategy to Diminish the 4V Voltage Plateau of LiNi0.5Mn1.5O4. Mater. Res. Bull. 2013, 48, 4960-4962. (22) Kunduraci, M.; Amatucci, G. G. Effect of Oxygen Non-Stoichiometry and Temperature on Cation Ordering in LiMn2−xNixO4 (0.50≥x≥0.36) Spinels. J. Power Sources 2007, 165, 359-367. (23) Sakunthala, A.; Reddy, M. V.; Selvasekarapandian, S.; Chowdari, B. V. R.; Christopher, P. Synthesis of Compounds, Li(MMn11/6)O4 (M = Mn1/6, Co1/6, (Co1/12Cr1/12), (Co1/12Al1/12), (Cr1/12Al1/12)) by Polymer Precursor Method and Its Electrochemical Performance for Lithium-Ion Batteries. Electrochim. Acta 2010, 55, 4441-4450. (24) Hu, E.; Bak, S. M.; Liu, J.; Yu, X.; Zhou, Y.; Ehrlich, S. N.; Yang, X. Q.; Nam, K-W. Oxygen-Release-Related Thermal Stability and Decomposition Pathways of LixNi0.5Mn1.5O4 Cathode Materials. Chem. Mater. 2013, 26, 1108-1118. (25) Zhong, G. B.; Wang, Y. Y.; Zhao, X. J.; Wang, Q. S.; Yu, Y.; Chen, C. H. Structural, Electrochemical and Thermal Stability Investigations on LiNi0.5-xAl2xMn1.5-xO4 (0≤2x≤1.0) as 5 V Cathode Materials. J. Power Sources 2012, 216, 368-375. (26) Breger, J.; Kang, K.; Cabana, J.; Ceder, G.; Grey, C. P. NMR, PDF and RMC Study of the Positive Electrode Material Li(Ni0.5Mn0.5)O2 Synthesized by Ion-Exchange Methods. J. Mater. Chem. 2007, 17, 3167-3174. (27) Cabana, J.; Casas-Cabanas, M.; Omenya, F. O.; Chernova, N. A.; Zeng, D.; Whittingham, M. S.; Grey, C. P. Composition-Structure Relationships in the Li-Ion Battery Electrode Material LiNi0.5Mn1.5O4. Chem. Mater. 2012, 24, 2952-2954. (28) Song, J.; Shin, D. W.; Lu, Y.; Amos, C. D.; Manthiram, A.; Goodenough, J. B. Role of Oxygen Vacancies on the Performance of Li[Ni0.5−xMn1.5+x]O4 (x = 0, 0.05, and 0.08) Spinel Cathodes for Lithium-Ion Batteries. Chem. Mater. 2012, 24, 3101-3109.

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(29) Lee, E.; Persson, K. A. Solid-Solution Li Intercalation as a Function of Cation Order/Disorder in the High-Voltage LixNi0.5Mn1.5O4 Spinel. Chem. Mater. 2013, 25, 28852889. (30) Shin, D. W.; Bridges, C. A.; Huq, A.; Paranthaman, M. P.; Manthiram, A. Role of Cation Ordering and Surface Segregation in High-Voltage Spinel LiMn1.5Ni0.5−xMxO4 (M = Cr, Fe, and Ga) Cathodes for Lithium-Ion Batteries. Chem. Mater. 2012, 24, 3720-3731. (31) Pang, W. K.; Sharma, N.; Peterson, V. K.; Shiu, J. J.; Wu, S. H. In-Situ Neutron Diffraction Study of the Simultaneous Structural Evolution of a LiNi0.5Mn1.5O4 Cathode and a Li4Ti5O12 Anode in a LiNi0.5Mn1.5O4∥Li4Ti5O12 Full Cell. J. Power Sources 2014, 246, 464472. (32) Berg, H.; Thomas, J. O.; Liu, W.; Farrington, G. C. A Neutron Diffraction Study of Ni Substituted LiMn2O4. Solid State Ionics 1998, 112, 165-168. (33) Patoux, S.; Daniel, L.; Bourbon, C.; Lignier, H.; Pagano, C.; Cras, F. L.; Jouanneau, S.; Martinet, S. High Voltage Spinel Oxides for Li-Ion Batteries: From the Material Research to the Application. J. Power Sources 2009, 189, 344-352. (34) Kim, J. H.; Myung, S. T.; Yoon, C. S.; Kang, S. G.; Sun, Y. K. Comparative Study of LiNi0.5Mn1.5O4-δ and LiNi0.5Mn1.5O4 Cathodes Having Two Crystallographic Structures: Fd3m and P4332. Chem. Mater. 2004, 16, 906-914. (35) Wang, L.; Lee, H.; Huang, X.; Baudrin, E. A Comparative Study of Fd-3m and P4332 “LiNi0.5Mn1.5O4”. Solid State Ionics 2011, 193, 32-38. (36) Wang, J.; Lin, W.; Wu, B.; Zhao. J. Porous LiNi0.5Mn1.5O4 Sphere as 5 V Cathode Material for Lithium Ion Batteries. J. Mater. Chem. A 2014, 2, 16434-16442.

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Page 20 of 32

TABLES Table 1 Sample

x=1

x=0.7

x=0.5

x=0.3

x=0.1

x=0

a (Å)

8.17805

8.23754

8.28836

8.33355

8.39528

8.42726

atom

x

y

z

Occupancy

Biso

Rwp

χ2

Li/Mn1

1/8

1/8

1/8

1 / 0.01

0.2

6.38

3.31

Ni/Mn2

1/2

1/2

1/2

0.24/0.74

0.5

O

0.26300

0.26300

0.26300

0.90

1.5

Li/Mn1

1/8

1/8

1/8

0.7 / 0.06

0.3

8.77

5.55

Ni/Mn2

1/2

1/2

1/2

0.25/0.69

0.5

O

0.26340

0.26340

0.26340

0.87

1.5

Li/Mn1

1/8

1/8

1/8

0.5 / 0.13

0.4

6.01

3.30

Ni/Mn2

1/2

1/2

1/2

0.25/0.62

0.8

O

0.26386

0.26386

0.26386

0.84

1.4

Li/Mn1

1/8

1/8

1/8

0.3 / 0.21

0.5

8.53

4.30

Ni/Mn2

1/2

1/2

1/2

0.25/0.54

0.8

O

0.26381

0.26381

0.26381

0.82

1.9

Li/Mn1

1/8

1/8

1/8

0.1 / 0.26

0.5

7.89

4.23

Ni/Mn2

1/2

1/2

1/2

0.25/0.49

0.7

O

0.26395

0.26395

0.26395

0.79

1.7

Li/Mn1

1/8

1/8

1/8

0 / 0.33

0.6

6.79

3.56

Ni/Mn2

1/2

1/2

1/2

0.25/0.42

0.9

O

0.26384

0.26384

0.26384

0.78

1.6

20


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Table 2

x=0

x=0.1

x=0.3

x=0.5

x=0.7

x=1

2.0215 Å

2.0036 Å

1.9935 Å

1.9747 Å

1.9548 Å

(2.0196 Å)

(2.0097 Å)

(1.9919 Å)

(1.9741 Å)

(1.9540 Å)

1.9970 Å

1.9897 Å

1.9750 Å

1.9639 Å

1.9552 Å

1.9440 Å

(2.0103 Å)

(1.9889 Å)

(1.9741 Å)

(1.9629 Å)

(1.9527 Å)

(1.9419 Å)

2.6496 Å

2.6383 Å

2.6208 Å

2.6055 Å

2.6002 Å

2.5907 Å

(2.6501 Å)

(2.6350 Å)

(2.6199 Å)

(2.6020 Å)

(2.5980 Å)

(2.5820 Å)

Li-O

Ni/Mn-O

O-O

Table 3

Li content (x)

Amount of oxygen (%) R

Ionic charge of oxygen R

(C)

(C)

1

90.6 (90.6)

→

-7.25 (-7.25)

0.7

86.8 (86.9)

→

-6.94 (-6.95)

0.5

84.6 (84.4)

→

-6.77 (-6.75)

0.3

82.5 (81.9)

→

-6.60 (-6.55)

0.1

80.0 (79.4)

→

-6.40 (-6.35)

0

77.3 (78.1)

→

-6.19 (-6.25)

Structural formula Ni

Mn

Spinel; LixNi0.5-δMn1.5O4-y-δ Spinel

2+

21

3.5+


+ δNiO

Spinel

(δ=0.012)

(δ=0)


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 CAPTIONS Fig. 1. (Color online) (a) XRD patterns of LixNi0.5Mn1.5O4 (x=1, 0.7, 0.5, 0.3, 0.1, 0) measured at room temperature. The weight of each sample for different x on XRD measurements is the same and the diffracted intensities are therefore normalized. The XRD patterns correspond to the diffraction from the face centered cubic spinel structure with space group Fd 3 m irrespective of Li amount. (b) The magnified view of some Bragg peaks to show the pronounced changes in the intensity and peak position.

Fig. 2. (Color online) Neutron diffraction and X-ray diffraction patterns of LixNi0.5Mn1.5O4 for x=1 are plotted together for a comparison of the distinct difference of intensities between those two patterns. The inset is the magnified view of the (400) Bragg peak where the secondary phase NiO appears on the neutron diffraction but not in the XRD pattern. In this figure, the neutron diffraction wavelength 1.8343 Å is converted into the X-ray wavelength 1.5406 Å for the consistency of diffraction angles.

Fig. 3. (Color online) (a) Neutron diffraction patterns of LixNi0.5Mn1.5O4 with different Li mole ratio. All the diffracted Bragg peaks can be indexed with the Miller indices corresponding to the face centered cubic spinel structure. A small amount of extra NiO phase exists for x=1, but for the rest of the compositions x=0.7, 0.5, 0.3, 0.1 and 0, the only single spinel Fd 3 m phase appears. (b) The magnified view of some Bragg peaks, the same peaks presented in Fig, 1, shows the change of the intensity and peak position.

Fig. 4. (Color online) Rietveld refined neutron diffraction patterns of LixNi0.5Mn1.5O4 for (a) x=1, (b) x=0.7, (c) x=0. The Bragg peak positions from the spinel structure refined in the face centered cubic Fd 3 m are indicated as vertical bars, including reflections from NiO for x=1, as vertical bars. The line at the bottom of each figure represents the difference between the measured and refined values.

Fig. 5. (Color online) The change of (a) the lattice parameter and (b) the nearest neighbor bond lengths of Li-O, Mn/Ni-O and O-O in LixNi0.5Mn1.5O4 as a function of x. The filled symbols refer to refined values and the unfilled symbols refer to theoretically calculated values. Lines are a guide for the eye only. 22


Page 22 of 32

Page 23 of 32

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Fig. 6. (Color online) Oxygen deficiency of LixNi0.5Mn1.5O4 with respect to the change of Li amount, obtained from the neutron diffraction Rietveld refinement. As written in the figure, the structural chemical formula should be written as LixNi0.5Mn1.5O4-y. The oxygen loss behavior can be divided into two regions, depending on the single or mixed crystalline phases. As explained in the text, a linear relationship y=-0.5x+0.875 holds in the single phase region.

Fig. 7. (Color online) A schematic view of one unit cell for the spinel cubic structure, obtained from the refinement results. For the case of x=0 (dotted line), the expansion from the unit cell for x=1 (solid line) is exaggerated 10 times larger for eyes.

TABLE CAPTIONS

Table 1. Structural parameters of LixNi0.5Mn1.5O4, obtained from the neutron diffraction Rietveld refinement at room temperature. All the structures belong to the space group Fd 3 m, except a small amount of NiO phase for x=1. The atomic positions in the table correspond to 8a, 16d and 32e sites for Li/Mn1, Ni/Mn2 and O, respectively.

Table 2. Bond lengths between atoms in LixNi0.5Mn1.5O4, obtained from the neutron diffraction Rietveld refinement at room temperature.

Table 3. Variation of oxygen content and ionic charge with respect to the change of Li amount in LixNi0.5Mn1.5O4, obtained from the neutron diffraction Rietveld refinement at room temperature. (R: refined value, C: calculated value)

23



FIGURES

(440) (531)

(511)

(422)

(331)

(400)

(311) (222)

Intensity (a.u.)

(220)

(a)

(111)

Fig. 1(a)

x= 1 x= 0.7 x= 0.5 x= 0.3 x= 0.1 x= 0

10

20

30

40

50

60

70

80

2θ (degree)

(111)

(b)

(440)

Fig. 1(b)

x= 1

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 24 of 32

x= 0.7 x= 0.5 x= 0.3 x= 0.1 x= 0

18.0

18.5

19.0 61

62

63

64

2θ (degree) 24


65

66

Page 25 of 32

(222)

α : NiO

(622)

(440)

α (620)

0.480 0.495

(531)

(511)

(331)

(311)

(220)

α

(422)

(400)

α

Neutron (111)

(400)

Fig. 2

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


XRD 0.2

0.4

0.6

1/d ( Å -1 )

25


0.8


Fig. 3(a)

(553)

(622)

(444) (551)

(620)

(440) (531)

(422) (511)

(400) (331)

(311)

(220)

Intensity (a.u.)

(111)

(222)

(a)

x= 1 x= 0.7 x= 0.5 x= 0.3 x= 0.1 x= 0

20

40

60

80

100

120

140

2θ (degree)

(111)

(b)

(622)

Fig. 3(b)

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 26 of 32

x= 1 x= 0.7 x= 0.5 x= 0.3 x= 0.1 x= 0 17

18

19

74

76

2 θ (degree)

26


78

Page 27 of 32

Fig. 4(a) Li1Ni0.5Mn1.5O4 Yobs Ycalc Yobs-calc Li(Ni0.5Mn1.5)O4 NiO

Intensity (a.u.)

(a)

20

40

60

80

100

120

2θ (degree) Fig. 4(b) Li0.7Ni0.5Mn1.5O4 Yobs Ycalc Yobs-calc Li0.7(Ni0.5Mn1.5)O4

(b) 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


20

40

60

80

2θ (degree)

27


100

120


Fig. 4(c) LixMn1.5Ni0.5O4 (x=0) Yobs Ycalc Yobs-calc Ni0.5Mn1.5O4

(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

Page 28 of 32

20

40

60

80

2θ (degree)

28


100

120

Page 29 of 32

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


Fig. 5(a)

Fig. 5(b)

29



Fig. 6

Oxygen deficiency (y)

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 32

0.9

0.6

0.3

0.0 0.0

Single phase

Mixed phases

Spinel; LixNi0.5Mn1.5O4-y

Spinel + NiO

0.2

0.4

0.6

x

30


0.8

1.0

Page 31 of 32

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


Fig. 7

31



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TABLE OF CONTENTS (TOC) GRAPHIC For table of contents only.

32


Page 32 of 32

Effect of Lithium Deficiency on Lithium-Ion Battery Cathode LixNi0

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