Remarkably Improved Electrochemical Performance of Li- and Mn

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Remarkably Improved Electrochemical Performance of Li- and MnRich Cathodes upon Substitution of Mn with Ni Prasant Kumar Nayak, Judith Grinblat, Elena Levi, Tirupathi Rao Penki, Mikhael Levi, Yang-Kook Sun,† Boris Markovsky, and Doron Aurbach* Department of Chemistry, Bar-Ilan University, Ramat-Gan 5290002, Israel † Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea ABSTRACT: Li- and Mn-rich transition-metal oxides of layered structure are promising cathodes for Li-ion batteries because of their high capacity values, ≥250 mAh g−1. These cathodes suffer from capacity fading and discharge voltage decay upon prolonged cycling to potential higher than 4.5 V. Most of these Liand Mn-rich cathodes contain Ni in a 2+ oxidation state. The fine details of the composition of these materials may be critically important in determining their performance. In the present study, we used Li1.2Ni0.13Mn0.54Co0.13O2 as the reference cathode composition in which Mn ions are substituted by Ni ions so that their average oxidation state in Li1.2Ni0.27Mn0.4Co0.13O2 could change from 2+ to 3+. Upon substitution of Mn with Ni, the specific capacity decreases but, in turn, an impressive stability was gained, about 95% capacity retention after 150 cycles, compared to 77% capacity retention for Li1.2Ni0.13Mn0.54Co0.13O2 cathodes when cycled at a C/5 rate. Also, a higher average discharge voltage of 3.7 V is obtained for Li1.2Ni0.27Mn0.4Co0.13O2 cathodes, which decreases to 3.5 V after 150 cycles, while the voltage fading of cathodes comprising the reference material is more pronounced. The Li1.2Ni0.27Mn0.4Co0.13O2 cathodes also demonstrate higher rate capability compared to the reference Li1.2Ni0.13Mn0.54Co0.13O2 cathodes. These results clearly indicate the importance of the fine composition of cathode materials containing the five elements Li, Mn, Ni, Co, and O. The present study should encourage rigorous optimization efforts related to the fine composition of these cathode materials, before external means such as doping and coating are applied. KEYWORDS: Li-ion batteries, Li- and Mn-rich cathodes, Mn substitution by Ni, voltage stabilization, impedance

1. INTRODUCTION In the past decade, there is increasing interest in Li- and Mnrich lithiated layered transition-metal oxide cathode materials (Li1+xMn0.5+yNizCowO2, where x + y + z + w = 0.5) for highenergy-density rechargeable Li-ion batteries because of their high capacity, resulting from activation of their Li2MnO3 component. Activation of these materials by polarization to potentials higher than 4.5 V leads to very high reversible specific capacities that can approach 300 mAh g−1.1−21 Because of their high capacity values, the energy densities of current Liion batteries can be increased by 50% over those employing LiCoO2 if these LixMnyNizCowO2 materials are found as suitable for practical batteries. Despite their high capacity, these cathode materials suffer from capacity fading and average discharge voltage decay upon prolonged cycling, which is considered a major drawback for their commercialization. The lowering of the average discharge voltage is ascribed to structural changes that these materials undergo upon cycling. We suggest based on microscopic and spectroscopic studies that a major structural change of these materials when polarized to high potentials (in order to extract their high specific capacity) involves layered-to-spinel phase transformation. The formation of a spinel phase in these cathode materials seems to originate from the activation of Li2MnO3 as well as the © XXXX American Chemical Society

migration of transition-metal cations to Li sites of the LiMO2 phase.15−17 The redox activity of spinel moieties around 3.0 V shifts the average discharge voltage, which is initially >3.5 V, to values close to 3.3 V.17 Researchers have adopted various methods such as surface coating,22−31 doping of cations,32−47 incorporation of a spinel phase,48−53 and additives to electrolytes54,55 in order to mitigate the capacity fading as well as discharge voltage decay of these cathodes. Surface coating helped to improve stabilization of the capacity by preventing interfacial reactions of electrolyte solution components with these cathode materials.22−31 However, it is difficult to optimize the coating of these powdery materials to obtain uniform coating surface layers on the active mass. Doping of these materials by foreign cations was found to be effective in improving stabilization of the capacity as well as discharge voltage upon cycling. There are many papers reporting on the doping of lithiated transition-metal oxide cathode materials Special Issue: New Materials and Approaches for Beyond Li-ion Batteries Received: June 30, 2016 Accepted: September 12, 2016

A

DOI: 10.1021/acsami.6b07959 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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function was used for the peak-shape approximation. The background was fitted manually by linear interpolation. The morphology of the products was investigated by scanning electron microscopy (SEM; Magellan XHR 400L FE-SEM-FEI). Transmission electron microscopy (TEM) characterization of Li1.2Ni0.27Mn0.40Co0.13O2 was carried out with a JEOL-JEM 2100 electron microscope with a LaB6 emitter operating at 200 kV equipped with a Thermo Scientific Ultra Dry EDS detector. Samples for the TEM studies were prepared by dispersing and sonicating the powdered sample in ethanol. The oxidation states of the transition metals in these cathode materials were measured by X-ray photoelectron spectroscopy (XPS) using a system from Kratos (U.K.). Several series of measurements were carried out, providing ambiguous results. For instance, it was hard to determine from them whether the oxidation state of Ni was 2+ or 3+. What seemed to be clear was that the average oxidation state of Ni in all three materials studied herein was the same. We had to conclude that this surface-sensitive technique is not the right tool for assessing the average oxidation state of the transition metals in these lithiated oxides. Consequently, we do not display here XPS spectra of these cathode materials. The appropriate analytical tool for assessing the oxidation states of the atoms in these compounds is X-ray absorption near-edge spectroscopy (XANES), which was not available for us. In any event, because the stoichiometry of the compounds was precisely obtained by ICP, the information about the exact oxidation states of the transition metals in these cathode materials is not critically important for the conclusions that this study aims to convey. 2.3. Electrode Preparation and Electrochemical Measurements. Slurries of active mass were prepared by mixing 80 wt % active material, 10 wt % conductive super P carbon, and 10 wt % PVDF binder in NMP solvent in a Planetary vacuum mixture. The electrodes were prepared by casting the slurries onto Al foil current collectors using a doctor-blade technique and drying at 80 °C overnight. The coated Al foils were then calendared uniformly and cut into circular electrodes of 14 mm diameter. The electrodes were finally dried at 110 °C for 12 h under vacuum in order to remove absorbed moisture and trace NMP before assembling the cells in a dry argon-filled glovebox. In brief, the electrochemical performance was tested using coin-type cells 2325 (NRC, Canada) assembled in an argon-filled dry glovebox (MBraun). Li-metal foils were used as the counter and reference electrodes. We used ethylene carbonate−dimethyl carbonate (ECDMC) (1:1)/1 M LiPF6 solutions (battery grade) delivered by BASF (their standard commercial product). A typical loading of the active mass was 4−5 mg cm−2. The galvanostatic charge−discharge cycling was performed in the potential range of 2.0−4.7 V at a C/10 rate for the initial two cycles, followed by cycling in the potential range of 2.0− 4.6 V vs Li/Li+ at a C/5 rate in the subsequent cycles using a computerized multichannel battery testing instrument from Arbin Inc. Electrochemical impedance spectroscopy (EIS) spectra of the cathodes were recorded at an equilibrium potential of 3.8 V during charge with an amplitude of 5 mV in the frequency range of 100 kHz to 0.01 Hz, using a Solartron model SI 1287 electrochemical interface and an 1255 HF frequency response analyzer.

with cations such as Na, K, Mg, Al, Cr, Sn, Y, V, etc., and anions such as PO43−.32−47 Recently, we showed that Al doping for Mn resulted in a remarkable stabilization effect on their capacity as well as on the discharge voltage upon prolonged cycling.41,42 Doping processes can be controlled very well during synthesis. Using certain additives such as LiBOB in solutions54 was also found to be useful in stabilizing these cathode materials. The stabilization mechanism is not understood yet. In the present work, we have studied how fine-tuning of the stoichiometry of Li- and Mn-rich cathode materials affects their electrochemical behavior. We found that replacing some Mn ions of Li1.2Ni0.13Mn0.54Co0.13O2 {xLi[Li1/3Mn2/3]O2·(1 − x)Li[Ni1/3Mn1/3Co1/3]O2 (x = 0.6)} materials with Ni ions pronouncedly influences their performance. The rhombohedral component can become Li[Ni2/3Co1/3]O2 upon full substitution of Mn from Li[Ni1/3Mn1/3Co1/3]O2 with Ni. Through full substitution of Mn in the rhombohedral phase of Li1.2Ni0.13Mn0.54Co0.13O2 by Ni, the average oxidation state of Ni changes from 2+ to 3+, which may strongly affect the electrochemical response of these cathode materials. This paper reports on a rigorous study of the effect of the fine stoichiometry of Li1.2Ni0.13+xMn0.54−xCo0.13O2 cathode materials, demonstrating their pronounced stabilization upon an increase in their Ni content.

2. EXPERIMENTAL SECTION 2.1. Materials. Analytical-grade chemicals, namely, Mn(NO3)2 (Fluka), Ni(NO3)2, Co(NO3)2, LiNO3 (Aldrich), sucrose, poly(vinylidene fluoride) (PVDF), and 1-methyl-2-pyrrolidinone (NMP; Aldrich), were used as received. Doubly distilled (DD) water was used to dissolve the metal nitrates and sucrose. Commercially available, Ligrade, ethylene carbonate−dimethyl carbonate (EC−DMC; 1:1)/1 M LiPF6 solutions were used as received (could contain trace water, HF, and PF5 at the ppm level). 2.2. Synthesis and Structural Characterization. Synthesis of Li 1 . 2 Ni 0 . 1 3 Mn 0 . 5 4 Co 0 . 1 3 O 2 , Li 1 . 2 Ni 0 . 2 0 Mn 0 . 4 7 Co 0 . 1 3 O 2 , and Li1.2Ni0.27Mn0.40Co0.13O2. The Li- and Mn-rich cathode materials Li1.2Ni0.13Mn0.54Co0.13O2 were synthesized by self-combustion reactions (SCRs) as reported previously.9,17,18 The metal nitrates and sucrose were dissolved in DD water with continuous stirring and then evaporated by heating at about 100 °C to form a uniform precursor, which upon calcination results in the formation of oxide materials. In a typical preparation of Li1.2Ni0.27Mn0.40Co0.13O2, 1.746 g of Co(NO3)2, 3.49 g of Ni(NO3)2, 4.518 g of Mn(NO3)2, and 3.723 g of LiNO3 were dissolved in 80 mL of DD water by magnetic stirring. Sucrose was then added to the solution with continuous stirring. The ratio of metal nitrates to sucrose was 1:2 in the solution. The solution was heated at about 100 °C in order to obtain a viscous mass, which upon further heating at 350 °C led to the formation of an amorphous product, which was ground to obtain a fine powder sample and then annealed at 450 °C for 2 h in air. This product was further ground and annealed in two steps, 700 °C for 1 h and then at 900 °C for 20 h in air, resulting in the highly crystallized material Li1.2Ni0.27Mn0.40Co0.13O2. The two other Li1.2Ni0.20Mn0.47Co0.13O2 and Li1.2Ni0.13Mn0.54Co0.13O2 materials were synthesized by following the same procedure. Elemental analysis of the synthesized materials was carried out using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Ultima-2 spectrometer from JobinYvon Horiba). X-ray diffraction (XRD) studies were performed with a Bruker Inc. (Germany) AXS D8 ADVANCE diffractometer (reflection θ−θ geometry, Cu Kα radiation, receiving slit 0.2 mm, high-resolution energy-dispersive detector). Diffraction data for Rietveld refinement were collected in the angular range of 10° < 2θ < 140° with a step size of 0.02° and a step time of 6 s/step. The data were analyzed by the Rietveld structure refinement program FULLPROF.57 The structural data for the modeling were taken from ref 58. The Thompson−Cox−Hastings pseudo-Voigt

3. RESULTS AND DISCUSSION The compositions of the materials measured via elemental analysis by ICP were found to be very close to the targeted ones. The elemental analysis results are tabulated in Table 1. Table 1. Relative Molar Content of the Constituent Metallic Elements in the Synthesized Electrode Materials Obtained from ICP Analysis

B

sample

theoretical molar content of Co:Ni:Mn:Li

experimental molar content of Co:Ni:Mn:Li

Li1.2Ni0.13Mn0.54Co0.13O2 Li1.2Ni0.20Mn0.47Co0.13O2 Li1.2Ni0.27Mn0.40Co0.13O2

0.13:0.13:0.54:1.2 0.13:0.20:0.47:1.2 0.13:0.27:0.40:1.2

0.134:0.136:0.55:1.18 0.136:0.204:0.48:1.18 0.135:0.275:0.41:1.18


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Figure 1. Experimental XRD patterns and those derived from Rietveld analysis of Li- and Mn-rich cathodes: (a) Li1.2Ni0.13Mn0.54Co0.13O2; (b) Li1.2Ni0.20Mn0.47Co0.13O2; (c) Li1.2Ni0.27Mn0.40Co0.13O2. The calculated patterns are shown by solid curves, and the red dots show the observed intensities. The differences between the observed and calculated intensities are presented by blue curves.

Table 2. Lattice Parameters and Quality of Fitting Obtained from Rietveld Analysis of the Li- and Mn-Rich Oxide Samples Synthesized by SCRs fitting quality

lattice parameter sample

a (Å)

b (Å)

c (Å)

β (deg)

V (Å3)

Rb (%)

χ2

Li1.2Ni0.13Mn0.54Co0.13O2 Li1.2Ni0.20Mn0.47Co0.13O2 Li1.2Ni0.27Mn0.40Co0.13O2

4.937 4.938 4.943

8.551 8.551 8.558

5.021 5.021 5.022

109.25 109.25 109.24

200.14 200.19 200.58

7.32 8.53 5.28

4.02 2.38 2.09

Li2MnO3-type structure in all of these samples. The typical features of the structure are the presence of Li and its ordering in the transition-metal layers. The (018)/(110) peak splitting at 2θ ∼ 65° clearly indicates the formation of ordered lamellae in the hexagonal structure. The lattice parameters and quality of fitting obtained from Rietveld analysis of these Li- and Mn-rich cathode materials are summarized in Table 2. The substitution of Mn4+ (0.53 Å) with Ni ions in Li- and Mn-rich cathodes resulted in the formation of Ni 3 + (0.56 Å) in Li1.2Ni0.27Mn0.4Co0.13O2, thus leading to a very slight increase in the one of the unit cell parameters and in a very slight increase in the cell volume, compared to the reference sample Li1.2Ni0.13Mn0.54Co0.13O2. The SEM images of the three Li- and Mn-rich materials are shown in Figure 2. All of these materials possess morphologies similar to those of submicron-sized particles in the range of 200−400 nm. The particle size was found to increase upon an increase in the Ni content, which can be clearly seen from the micrographs. The structural information about Li1.2Ni0.27Mn0.40Co0.13O2 was also obtained from TEM studies, the results of which are shown in Figure 3. The electron diffraction study of Li1.2Ni0.27Mn0.40Co0.13O2 indicated the presence of two phases, namely, layered monoclinic Li2MnO3 (C2/m) and rhombohedral LiMO2

The transition-metal elements, namely, Ni, Co, and Mn, usually exist in 2+, 3+, and 4+ oxidation states, respectively, in the reference material Li1.2Ni0.13Mn0.54Co0.13O2 {xLi[Li1/3Mn2/3]O2·(1 − x)LiNi1/3Mn1/3Co1/3]O2 (x = 0.6)}. Thus, it is important to explain the charge balance in the other materials, which were obtained upon substitution of Mn by Ni. It is wellknown that the substitution of transition-metal ions of lithiated transition-metal oxide cathode materials by Li ions results in a higher oxidation state of some of the transition-metal cations in Li 1 + x (Ni 1 / 2 Mn 1 / 2 ) 1 − x O 2 materials. 5 9 Studies of Li[(Ni1/2Mn1/2)0.94Li0.06]O2 cathode materials by XANES revealed indeed the presence of Ni3+ ions.59 Thus, we can assume that substitution of some Mn with Ni ions in the materials that we explore herein may result in an increase in the average oxidation state of the Ni ions therein. Figure 1 shows the XRD Rietveld refinement patterns of all three materials synthesized for the present study. The experimental XRD patterns match well with those of the calculated patterns shown by solid curves when analyzed based on a single monoclinic phase with space group C/2m. The presence of a few broad peaks between 2θ = 20 and 25° can result from the super lattice ordering of Li and Mn in the transition-metal layers,7−9 confirming the existence of the C


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Figure 2. SEM micrographs of (a) Li1.2Ni0.13Mn0.54Co0.13O2, (b) Li1.2Ni0.20Mn0.47Co0.13O2, and (c) Li1.2Ni0.27Mn0.40Co0.13O2 particles. Scale bars are 500 nm.

Figure 3. (a) Typical TEM bright-field image of Li1.2Ni0.27Mn0.40Co0.13O2; (b and c) Indexed NBED patterns showing the presence of monoclinic and rhombohedral phases, respectively. (d) HRTEM image representing the presence of fringes (020) with an interplanar distance of 4.24 Å. (e) Corresponding intensity profile demonstrating the distance 4.24 Å between lattice fringes.

(R3m ̅ ) phases. Figure 3a shows a typical bright-field image of a few agglomerated submicron-sized particles. The examples of indexed nanobeam electron diffraction (NBED) patterns obtained from the particles in Figure 3a clearly demonstrate the presence of monoclinic (Figure 3b) and rhombohedral (Figure 3c) phases. The lattice fringes corresponding to the (020) planes (d = 4.24 Å) of the monoclinic structure are seen

in the high-resolution TEM (HRTEM) image (Figure 3d). It is important to note that both structural components, monoclinic and rhombohedral, were identified on the basis of analysis of several diffraction patterns, taken at different orientations. Several particles and several locations in each particle were measured. Hence, we provide here very representative data. Although some diffractions could be interpreted doubly, on the D


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of Li2MnO3 is present in all three cases. The specific capacities were found to be 352, 344, and 351 mAh g−1, respectively, during the first charge in the first cycle. This is expected because all three materials seem to contain about 60% Li2MnO3 phase {xLi[Li1/3Mn2/3]O2·(1 − x) LiMO2 (M = Mn, Ni, Co; x = 0.6)} in them. This clearly indicates association of the plateau region to the activation of Li2MnO3. The Coulombic efficiencies were found to be 80.7, 70.6, and 60.4% in the first cycle for Li1.2Ni0.13Mn0.54Co0.13O2, Li1.2Ni0.20Mn0.47Co0.13O2, and Li1.2Ni0.27Mn0.40Co0.13O2 electrodes, respectively. A similar tendency of decreasing the Coulombic efficiency was also observed in the literature, when Mn was substituted by Al in Li1.2Ni0.16Mn0.56Co0.08O2.42 This difference in the initial cycling efficiency arises from the fact that the first charging capacity is similar for the three types of electrodes, while the discharge capacity is lower as the Ni content becomes higher. These trends in the capacity are intrinsic to these systems: as the Mn content becomes higher, the specific capacity can reach higher values. However, the high initial capacity of the cathodes containing a high concentration of Mn fades upon cycling, while the capacity retention of the cathodes containing less Mn and more Ni is excellent. For all of these materials, after a few initial cycles, the Coulombic efficiency reaches constant values close to 100%. The specific capacities below 4.4 V during the charge in the first cycle corresponding to the redox activity of LiMO2 are found to be 112, 120, and 148 mAh g−1 respectively for Li 1.2 Ni 0.13 Mn 0.54 Co 0.13 O 2 , Li 1.2 Ni 0.2 Mn 0.47 Co 0.13 O 2 , and Li1.2Ni0.27Mn0.4Co0.13O2. This may indicate a difference in the composition of the initially active rhomohedral phases. The three plateaus in Figure 4 (obtained upon polarization >4.4 V) are different by their length and shape for the three materials. As the Ni content increases, the plateau becomes shorter by

basis of the monoclinic or rhombohedral structures, there were patterns that could only be indexed unequivocally. The voltage profiles of the first cycle measured from Li 1.2 Ni 0.13 Mn 0.54 Co 0.13 O 2 , Li 1.2 Ni 0.2 Mn 0.47 Co 0.13 O 2 , and Li1.2Ni0.27Mn0.4Co0.13O2 electrodes at 25 mA g−1 (C/10 rate) are shown in Figure 4 for comparison. The voltage initially

Figure 4. Comparison of the first galvanostatic charge−discharge cycle (voltage profiles) of (i) Li 1 . 2 Ni 0 . 1 3 Mn 0 . 5 4 Co 0 . 1 3 O 2 , (ii) Li1.2Ni0.2Mn0.47Co0.13O2, and (iii) Li1.2Ni0.27Mn0.40Co0.13O2 electrodes measured at a C/10 rate in the potential range of 2.0−4.7 V versus Li in EC−DMC (1:1)/1 M LiPF6 solutions.

increases gradually to ∼4.4 V, followed by a plateau above 4.4 V. This plateau corresponding to the electrochemical activation

Figure 5. Typical galvanostatic charge−discharge curves (voltage profiles) for 3rd, 25th, 50th, 100th, and 150th cycles of (a) Li1.2Ni0.13Mn0.54Co0.13O2, (b) Li1.2Ni0.2Mn0.47Co0.13O2, and (c) Li1.2Ni0.27Mn0.40Co0.13O2electrodes measured at a C/5 rate in the potential range of 2.0−4.6 V versus Li in EC−DMC (1:1)/1 M LiPF6 solutions. E


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Figure 6. Differential capacity dQ/dE versus E plots for the 3rd and 100th cycles of (a) Li1.2Ni0.13Mn0.54Co0.13O2, (b) Li1.2Ni0.2Mn0.47Co0.13O2, and (c) Li1.2Ni0.27Mn0.40Co0.13O2 electrodes measured at a C/5 rate in the potential range of 2.0−4.6 V versus Li.

transition-metal cations, namely, Mn, Ni, and Co (Figure 6a,b). However, only two redox peaks are observed during charge of Li1.2Ni0.27Mn0.40Co0.13O2 in the third cycle, indicating the absence of redox activity of Mn around 3.2 V. This means that the shoulder due to the redox peak appearing around 3.2 V during charge results from the redox activity of Mn present in the rhombohedral phase. Because the rhombohedral phase of Li1.2Ni0.27Mn0.40Co0.13O2 is Li[Ni2/3Co1/3]O2, the oxidation peak of Mn around 3.2 V was absent in the curves related to this cathode material. However, the three peaks appear during discharge, indicating redox activity of the three transition-metal cations. The peak around 3.4 V can be due to the redox activity of Mn in LixMnO2 formed from activation of Li2MnO3. Thus, the cathode with higher Ni content (without Mn in the LiMO2 phase) exhibits slightly different curves: no additional anodic peaks around 3.3 V and their main cathodic peaks appeared at higher potentials (around 3.4 V). After 100 cycles, it can be seen that the intensity of the shoulder at 3.3 V decreases, indicating a decrease in the specific capacity of Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.2Mn0.47Co0.13O2 upon cycling. Also, another pair of new redox peaks appears at about 3.0 V for Li1.2Ni0.13Mn0.54Co0.13O2, indicating the formation of a new phase (usually assigned to the redox activity of a spinel phase of the LixMn2O4 type) in the active mass upon prolonged cycling, with an upper potential limit beyond 4.5 V. It is well-known that these Li- and Mn-rich cathodes undergo structural layered-to-spinel transformation upon prolonged cycling. Thus, the appearance of the new redox peak can be ascribed indeed to the redox activity of a spinel phase around 3.0 V. However, neither the shoulder redox peak nor the new redox peak around 3.0 V appear in the dQ/dE versus E derivative curves of Li1.2Ni0.27Mn0.40Co0.13O2 after prolonged cycling (a curve after 100 cycles in Figure 6c is a good example). This result indicates suppression of layered-tospinel transformation and stabilization of the structure upon

approximately 20−25%, implying that Li2MnO3 behaves differently. These results may indicate that Mn can be substituted by Ni in both phases. This possibility seems to be reflected also by additional plateaus around 3.9 V (approximately 50 mAh g−1) of the Mn-substituted samples, where the Rh component is active. The discharge capacities are found to be 284, 243, and 212 mAh g−1, indicating a decrease in the capacity upon substitution of Mn with Ni. This again clearly indicates that the active (rhombohedral) phases in the integrated materials are different from each other in composition. A similar result, i.e., a decrease in the discharge capacity, was observed (with similar charging capacity in the first cycle) when Mn in these Li- and Mn-rich cathode materials was substituted with Al.42 In fact, the substitution of Mn (4+) with lower-charged species Al (3+) could have resulted in the formation of some Ni3+, as in the present study that related to substitution of Mn by Ni. Typical charge−discharge voltage profiles measured during prolonged cycling (150 cycles) of these three types of cathodes at a C/5 rate are shown in Figure 5. It can be seen that the specific capacities of Li 1 . 2 Ni 0 . 1 3 Mn 0 . 5 4 Co 0 . 1 3 O 2 and Li1.2Ni0.20Mn0.47Co0.13O2 cathodes decreased gradually upon cycling and then stabilized after 100 cycles. However, the specific capacity of the Li1.2Ni0.27Mn0.40Co0.13O2 cathode was found to be almost stable during 150 cycles. This result is similar to that previously reported in ref 56. The discharge voltage profiles of these cathodes also indicate that the rate of decrease in the average discharge voltage upon cycling is more pronounced in the initial 50 cycles, which then slows to only a moderate decrease upon further cycling. The differential capacity plots for the 3rd and 100th cycles of these cathodes are shown in Figure 6 for comparison. There are thr e e p a i rs o f c l e a r br oa d p e a k s o b s e r ved for Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.2Mn0.47Co0.13O2 in the third cycle corresponding to the redox activity of three F


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210 mAh g−1 over the potential range of 2.0−4.6 V after 150 cycles. The initial average discharge voltages are found to be 3.55, 3.64, and 3.70 V for Li 1.2 Ni 0.13 Mn 0.54 Co 0.13 O 2 , Li1.2Ni0.20Mn0.47Co0.13O2, and Li1.2Ni0.27Mn0.40Co0.13O2 cathodes, indicating that the discharge voltage increases upon this partial substitution of Mn by Ni. The discharge voltages of Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.2Mn0.47Co0.13O2 cathodes are found to decrease from 3.55 and 3.64 V to 3.32 and 3.44 V, respectively, after 150 cycles. On the other hand, the average discharge voltage of Li1.2Ni0.27Mn0.4Co0.13O2 cathodes decreases from 3.70 to 3.50 V after 150 cycles. Thus, the average discharge voltage of Li1.2Ni0.27Mn0.4Co0.13O2 after 150 cycles (3.5 V) is very close to the initial discharge voltage (3.55 V) of the reference Li1.2Ni0.13Mn0.54Co0.13O2 cathodes. Also, this value of the average discharge voltage is even higher in comparison to that obtained for Al-doped Li1.2Ni0.16Mn0.51Al0.05Co0.08O2 cathodes (about 3.48 V) after 100 cycles in our previous study.42 Moreover, the rate of the discharge voltage decay is suppressed after 100 cycles for all of these cathodes. Hence, fine-tuning of the composition of these cathode materials, reported herein, demonstrates clear advantages for the Li1.2Ni0.27Mn0.4Co0.13O2 cathodes in terms of high stability and high energy density of Li-ion batteries, which use them as positive electrodes. The rate capability test of these cathodes was carried out at different current values starting from C/10 to 4C rates with five cycles at each rate and then returned back to the initial rate, as shown in Figure 8. As expected, the specific capacities of all three cathodes decrease with an increase in the current density. On returning back to low rates (C/10), all three types of electrodes show initial high capacities, indicating that high rate cycling is not detrimental to their stability. The effect of the Ni content on their rate capability can be clearly seen at high currents (Figure 8b). Li1.2Ni0.27Mn0.4Co0.13O2 cathodes retain about 50% capacity at a 4C rate when normalized versus a C/10 rate compared to 42% for Li 1.2 Ni 0.13 Mn 0.54 Co 0.13 O 2 cathodes. Hence, Li1.2Ni0.27Mn0.4Co0.13O2 cathodes exhibit advantages in rate capability as well. Yang et al. found by time-resolved X-ray absorption spectroscopy that Mn sites have poor reaction kinetics compared to Ni and Co, thus acting as the kinetic limiting element in Li- and Mn-rich cathodes.60 Also, Abraham et al. observed enhanced rate capability by removing Mn from the rhombohedral phase of these Li- and Mn-rich cathodes.56 Thus, our results presented here correlate well with previously reported work. EIS is very useful in evaluating electrodes’ kinetic and interfacial parameters such as surface films’ resistance, chargetransfer resistance at electrode/electrolyte solution interfaces, double-layer capacitance, and diffusion coefficients of Li+ ions into the active mass in Li-ion batteries. It can also be used as an analytical tool for evaluating the stability of electrode materials by comparing the impedance measured after initial and prolonged cycling. The impedance spectra of the electrodes studied herein were recorded at an equilibrium potential of 3.8 V during charging processes. The impedance spectra presented as Nyquist plots are shown in Figure 9. The EIS measurements of these composite electrodes are analyzed and compared on the qualitative level because it is difficult to assign the spectral features precisely to the various relevant time constants because of the composite nature of these electrodes. The impedance spectra of these electrodes (Nyquist plots) usually comprise two main features: two semicircles, one at high frequencies,

substitution of Mn with Ni in these Li- and Mn-rich cathodes, especially when all Mn ions from the rhombohedral phase are fully substituted by Ni, as is clearly shown in Figure 6c. The specific capacity and average discharge voltage obtained from the galvanostatic charge−discharge measurements of these cathodes are plotted versus cycle number, as shown in Figure 7. We suggest that 150 cycles can be considered as

Figure 7. Plots of (a) the specific capacity in charge (empty symbols) and discharge (filled symbols) and (b) the average discharge voltage versus cycle number for Li1.2Ni0.13Mn0.54Co0.13O2, Li1.2Ni0.2Mn0.47Co0.13O2, and Li1.2Ni0.27Mn0.40Co0.13O2 electrodes measured at a C/10 rate for the initial two cycles followed by a C/5 rate. The potential range was 2.0−4.6 V versus Li.

reasonably long experiments in which the cells reflect indeed the typical behavior of the cathodes. Upon too prolonged cycling in coin-type cells, the side reactions of the electrolyte solutions with Li anodes may pronouncedly affect the overall cell behavior. Both the specific capacity and average discharge voltage of the Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.2Mn0.47Co0.13O2 cathodes decrease upon prolonged cycling (with an upper potential limit of 4.6 V). The decrease in the specific capacity and discharge voltage results in a decrease of the energy density that batteries containing these cathodes can deliver. This capacity and voltage fading is known as a major drawback of these cathodes in their road for commercialization in Li-ion batteries. In contrast, the specific capacity of Li1.2Ni0.27Mn0.40Co0.13O2 remains almost stable at G


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Figure 9. Nyquist plots measured at an equilibrium potential of 3.8 V during charge for (i) Li1.2Ni0.13Mn0.54Co0.13O2, (ii) Li1.2Ni0.2Mn0.47Co0.13O2, and (iii) Li1.2Ni0.27Mn0.40Co0.13O2 electrodes with a voltage amplitude of 5 mV in the frequency range of 100 kHz to 0.01 Hz in EC−DMC (1:1)/1 M LiPF6 solutions.

Finally, EIS was implemented for comparing the stability of Li1.2Ni0.27Mn0.40Co0.13O2 and Li1.2Ni0.13Mn0.54Co0.13O2 electrodes upon cycling. Their impedances after the initial 10 cycles and after 150 cycles are presented in Figure 10. It can be seen

Figure 8. (a) Rate capability tests at several currents (C rates) and (b) normalized capacity retentions for Li 1.2 Ni 0.13 Mn 0.54 Co 0.13 O 2 , Li1.2Ni0.2Mn0.47Co0.13O2, and Li1.2Ni0.27Mn0.40Co0.13O2 electrodes in the potential range of 2.0−4.6 V versus Li. The specific capacities measured in charge (empty symbols) and discharge (filled symbols) are shown in part a.

which is usually assigned to Li+-ion migration through surface films, and another one at medium−low frequencies, assigned to charge-transfer processes (surface and bulk), as reported and discussed previously.12,17,18 It can be clearly seen that the diameter of both semicircles decreased upon increasing Ni content (substituted for Mn), indicating a decrease in both the surface film resistance (Rs) and charge-transfer resistance (Rct). The low surface film resistance can facilitate easy migration of Li+ through surface films, whereas the low Rct can facilitate electrode kinetics, thus enhancing their rate capability. The smaller first semicircle in Nyquist plots of the electrodes with high Ni content can be attributed to lower resistance of the Liion migration through the surface films formed. This, in turn, relates to the surface reactivity of the Ni-containing electrode species with solution and special properties of the films formed (chemical composition, thickness, porosity, etc.). Hence, the relatively low impedance of Li1.2Ni0.27Mn0.40Co0.13O2 electrodes can be correlated well with their high rate capability compared to the more Mn-rich Li1.2Ni0.13Mn0.54Co0.13O2 cathodes.

Figure 10. EIS spectra measured at 3.8 V during charge after 10 and 150 cycles for a comparison between (a) Li1.2Ni0.13Mn0.54Co0.13O2 and (b) Li1.2Ni0.27Mn0.40Co0.13O2 electrodes in EC−DMC (1:1)/1 M LiPF6 solutions.

that the first semicircle in their spectra, related to high frequencies, does not change significantly upon cycling. However, the medium-to-low-frequency response changes pronouncedly after prolonged cycling. The second semicircle disappears, and there is an increase in the third low-frequency part of the spectra, which can be tentatively assigned to the bulk charge-transfer resistance of these electrodes. It is spectacular in Figure 10 that the cathodes containing higher nickel H



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concentration develop lower impedance after prolonged cycling, compared to the reference cathodes. This finding deserves some discussion. It is well-known that Li[NiCoMn]O2 cathode materials are considered to be more surface reactive as the content of Ni becomes higher, while in this work, we find an opposite trend. We explain this apparent discrepancy by the fact that in this work we have explored the result of fine-tuning in the composition of Li- and Mn-rich Li1+x[NiCoMn]O2 cathode materials. The Ni content of the cathodes that we explored is yet small enough, so even the electrodes with the higher content of Ni cannot show the typical behavior of Ni-rich Lix[NiCoMn]O2 materials, but rather show a stabilization effect when the dominant components are Li and Mn.

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4. CONCLUSIONS Li- and Mn-rich Li1+x[NizCowMn0.33+y]1−xO2 (z, w < 0.33) cathode materials are among the most important and promising cathode materials that can take Li-ion batteries further in specific energy density, safety, and cost. Their activation and operation mechanisms are complicated and are not fully resolved. Their main problem relates to both the capacity and average voltage fading during cycling, which relates to very complicated and not fully understood structural changes. Their stability may depend on their fine stoichiometry, which may or may not avoid transition-metal ion lability that may cause detrimental structural changes upon cycling. Thereby much more work in this direction is required in order to collect enough comprehensive data that will guide optimal compositions and structures. In the present work, we have shown that the exact Ni content in these Mn- and Li-rich cathode materials is important and strongly affects their capacity and voltage stability, rate capability, and impact on the energy density. Li1.2Ni0.27Mn0.40Co0.13O2 was found to be superior over Li1.2Ni0.13Mn0.54Co0.13O2 and Li1.2Ni0.2Mn0.47Co0.13O2 as a stable and fast cathode material for advanced high-energydensity Li-ion batteries. A 95% capacity was retained for Li1.2Ni0.27Mn0.40Co0.13O2 cathodes compared to 77% for Li1.2Ni0.13Mn0.54Co0.13O2 cathodes after 150 cycles (at 40 mA g−1, C/5 rate). An increase in the Ni content resulted in a higher average discharge voltage of 3.50 V after 150 cycles compared to 3.32 V obtained for the reference Li1.2Ni0.13Mn0.54Co0.13O2 cathodes. Higher rate capability and lower impedance were measured for Li1.2Ni0.27Mn0.40Co0.13O2 cathodes compared to the reference ones. This study should promote further work on composition optimization. The mechanistic side of the improvement observed herein is being studied further. It is important to allocate the sites at which Ni ions replace Mn ions in these materials by means of various sophisticated techniques.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Israel Science Foundation as part of the INREP project and also by the Israel Ministry of Science and Technology in the framework of the Israel−India binational collaboration program. I


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Remarkably Improved Electrochemical Performance of Li- and Mn

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