Improving Performance of LiNi0.8Co0.1Mn0.1O2 Cathode Materials

Article Cite This: ACS Appl. Energy Mater. 2019, 2, 4521−4534

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Improving Performance of LiNi0.8Co0.1Mn0.1O2 Cathode Materials for Lithium-Ion Batteries by Doping with Molybdenum-Ions: Theoretical and Experimental Studies Francis Amalraj Susai,† Daniela Kovacheva,‡ Arup Chakraborty,† Tatyana Kravchuk,§ R. Ravikumar,† Michael Talianker,∥ Judith Grinblat,† Larisa Burstein,⊥ Yaron Kauffmann,# Dan Thomas Major,*,† Boris Markovsky,† and Doron Aurbach*,† Downloaded via BUFFALO STATE on July 22, 2019 at 12:14:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Chemistry and Institute for Nanotechnology and Advanced Materials (BINA), Bar-Ilan University, Ramat-Gan 52900, Israel ‡ Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria § Solid State Institute, TechnionIsrael Institute of Technology, Haifa 32000, Israel ∥ Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel ⊥ The Wolfson Applied Materials Research Centre, Tel-Aviv University, Tel-Aviv 69978, Israel # Department of Materials Science and Engineering, TechnionIsrael Institute of Technology, Haifa 32000, Israel S Supporting Information *

ABSTRACT: The work reported herein is an important continuation of our recent experimental and computational studies on Li[NixCoyMnz]O2 (x + y + z = 1) cathode materials for Li-ion batteries, containing minor amounts of multivalent cationic dopants like Al3+, Zr4+, W6+, Mo6+. On the basis of DFT calculations for LiNi0.8Co0.1Mn0.1O2, it was concluded that Mo6+ cations preferably substitute Ni cations in the layered structure due to the lowest substitution energy compared to Li, Co, and Mn. It was established that the electrochemical behavior of LiNi0.8Co0.1Mn0.1O2 as a positive electrode material for Li-ion batteries can be substantially improved by doping with 1−3 mol % of Mo6+, in terms of lowering the irreversible capacity loss during the first cycle, increasing discharge capacity and rate capability, decreasing capacity fade upon prolonged cycling, and lowering the voltage hysteresis and chargetransfer resistance. The latter is attributed to the presence of additional conduction bands near the Fermi level of the doped materials, which facilitate Li-ions and electron transfer within the doped material. This is expressed by a lower charge-transfer resistance of Mo-doped electrodes as shown by impedance spectroscopy studies. We also discovered unique segregation phenomena, in which the surface concentration of the transition metals and dopant differs from that of the bulk. This near surface segregation of the Mo-dopant seems to have a stabilization effect on these cathode materials. KEYWORDS: lithium batteries, Ni-rich cathode materials, Mo6+ doping, electrochemical behavior, computational modeling, dopant segregation

1. INTRODUCTION

the electrode stability is during cycling. Consequently, significant efforts have been devoted to improving intrinsic characteristics of Li[NixCoyMnz]O2 materials by lattice doping with cations or anions.9−13 Amine et al. have shown that small amounts of Al-doping of LiNi0.8Co0.2−xAlxO2 (0 < x < 0.1) significantly stabilized the electrode behavior, lowered their impedance, and improved power performance as well.14 It was also shown that double doping of LiNi0.6 Co0.2 Mn 0.2O 2

The most promising cathode materials for lithium-ion batteries (LIBs) are the lithium intercalated layered transition metal oxides of layered structure having the general formula Li[NixCoyMnz]O2 (x + y + z = 1) (NCM). NCM materials can exhibit a specific capacity >200 mAh g−1 at Ni-content of 80% and higher. They can also demonstrate high rate capability and may reach relatively low costs.1−8 However, these NCM materials have disadvantages like low electronic and ionic conductivities and structural instability; their capacity also decays quickly upon cycling.1 In fact, the higher the Ni-content is, and thus the specific capacity is, the lower © 2019 American Chemical Society

Received: April 16, 2019 Accepted: May 22, 2019 Published: May 22, 2019 4521

DOI: 10.1021/acsaem.9b00767 ACS Appl. Energy Mater. 2019, 2, 4521−4534

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ACS Applied Energy Materials

fraction, XPS, and DFT calculations. The novelty of this research is that, by employing the computational tools, we find out how incorporation of the highly charged Mo6+ dopant into the lattice affects the electronic structure of the material and the relative distribution of the Ni2+-, Ni3+-, and Ni4+-ions therein, aiming at understanding the effects of this doping on the electrochemical behavior of NCM811 electrodes. While there are several recent descriptions in the literature (as cited herein) of Mo-doped NCM cathodes demonstrating their advantage over reference systems, we believe that the present work is important because it supports very strongly the benefit of doping Ni-rich materials by molybdenum cations at low concentration. We consider this study as an important step further in the continuous and extensive efforts to optimize cathodes for advanced Li-ion batteries, in light of the great challenge of the electromobility revolution. Its success depends so strongly on the availability of suitable high energy density, stable, and durable Li-ion battery technology.

(NCM622) by minor contents of aluminum and iron (0.025 at. %) reduced Li+/Ni2+ cation mixing, stabilized the structure, and enhanced electrode behavior.15 Recently, we have demonstrated that substantial improvements in the NCM523 electrode’s performance upon cycling and aging at 60 °C can be achieved by doping of LiNi0.5Co0.2Mn0.3O2 with Al (0.01 at. % at the expense of Ni, Co, and Mn).16 Using density functional theory (DFT), Ni-sites were shown to be the most preferred for Al-doping in this cathode material. Our theoretical studies revealed that one of the main advantages of this dopant is stabilization of the layered structure.17 Importantly, doping the NCM622 cathode material by highcharge state Zr4+ cations resulted in improved electrochemical characteristics as well as in partial inhibition of the structural transformation of layered phase R3̅m to spinel Fd3̅m during cycling.18 Recently, a few works have been devoted to studying the role of Zr4+ cations as dopant in NCM52319 and LiNiO220 electrodes. Although high-charge state W6+ and Mo6+ cationic dopants are promising for NCM materials, their behavior has been less studied.21−24 In a recent study, Kim et al. demonstrated stable Li-ion battery prototypes comprising Nirich (Ni ≥ 80 at. %) cathodes doped with W6+ cations (1 mol %).25 In this work, W-doping was found to enhance rock-salt formation at the particles’ surface. This surface segregation was described as one of the major reasons for the clearly observed stabilization of W6+-doped LiNiO2 cathodes. Konishi et al.22 demonstrated that partial substitution of manganese by Mo6+ in Ni-rich materials (Ni-content 80%) resulted in suppression of the structural transformation from spinel to rock-salt phase and also in improvement of the thermal stability of the doped samples in reactions with electrolyte solutions. In a recently published paper,26 we have shown that doping NCM523 with minor amounts of Mo6+ (1 mol %) pronouncedly affects its structure and surface properties, and electrochemical behavior in terms of increased rate capability and decreased capacity fade during cycling. It was suggested that lower charge-transfer interfacial resistance (Rct) of Mo-doped electrodes can be explained by the formation of additional conduction bands near the Fermi level elucidated from the calculated density of states. In spite of intensive efforts dedicated to Ni-rich layered structure materials doped with foreign cations or anions, more studies with high-charge dopants like Mo6+ are required in order to understand better the effect of doping on capacity, rate capability, structural and cycling stability, and possible surface phenomena, like dopant and transition metal segregation and structural changes induced by the presence of Mo6+ in the lattice. In the work reported herein, we have continued our efforts to improve the long-term stability of Nirich NCM cathode materials, which can be considered as their main problem. It is clear that a promising route for that is doping by multivalent cations like Al3+, Zr4+, and Mo6+ which can reduce Li/Ni cation mixing as was shown in our previous work.16,17,26,27 It is important to note that cation-doped NCM cathode materials are multiparameter systems. Each parameter, like composition (level of Ni content, the choice of dopant, its concentration), the choice of synthetic route and the nature of the electrolyte solution, may dominate the electrochemical behavior. The present study is concentrated on LiNi0.8Co0.1Mn0.1O2 (NCM811, which can be considered as a very important cathode material) and the effect of its doping by Mo6+ cations. The tools included a variety of electrochemical techniques, high-resolution microscopy, XRD, electron dif-

2. EXPERIMENTAL METHODS 2.1. Synthesis of Materials. We synthesized lithiated oxides of transition metals Ni, Co, and Mn of a layered structure with the following undoped composition: LiNi0.8Co0.1Mn0.1O2. Samples include those that are molybdenum-doped: in the cost of Ni (1 and 3 mol %), LiNi 0.79 Mo 0.01 Co 0.1 Mn 0.1 O 2 and LiNi 0.77 Mo 0.03 Co0.1Mn0.1O2; and in the cost of both Ni (1 and 2 mol %) and Mn (1 mol %), LiNi0.79Mo0.01Co0.1Mn0.09Mo0.01O2 and LiNi0.78Mo0.02Co 0.1Mn0.09 Mo 0.01O2 . These were synthesized by a solution combustion method as described previously.18,28,29 The initial compounds (precursors) were analytical grade LiOH·H2O, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Mn(NO3)2·4H2O, ammonium molybdate tetrahydrate (NH4)6Mo7O24·4H2O (doping agent), and sucrose C12H22O11. A small excess of lithium was provided (Li1.02) according to our previous experience. The metal nitrates, LiOH·H2O, and sucrose were dissolved in distilled water until a clear solution was obtained. The latter was placed in a heater for slow evaporation of the water. After the solution dries, an increase of the temperature is observed and the self-ignition starts. The combustion process resulted in a voluminous amorphous mass, which was heat treated in air at 400 °C for 1 h to ensure the burning of residual organic components. The mechanism of self-combustion reactions to form lithiated transition metal oxides was already discussed in detail.30,31 This method is very convenient for fast combinatorial studies of many compositions in lab scales. This allows us to reach very well the desirable stoichiometry of compounds at very uniform compositions. The final stage is calcination, namely, heat treatment under an oxygen atmosphere at high temperatures (750−900 °C), which brings the materials to their final stable structure (confirmed always by crystallographic studies). In the synthesis for the present study, the final calcination stage included heating at 900 °C for 3 h in air, followed by a heat treatment at 800 °C in pure oxygen atmosphere during 15 h. We represent the results of the chemical analysis of NCM811 materials in Table S1. 2.2. Electrochemical Measurements. Preparations of working electrodes (cathodes) containing LiNi0.8Co0.1Mn0.1O2 undoped or Mo-doped materials for electrochemical cells (2325 coin-type configuration, Li anodes), as well as their assemblies, were described in detail previously.16,18,26 We used the electrolyte solutions (Li battery grade) comprising ethyl-methyl carbonate (EMC) and ethylene carbonate (EC), in a weight ratio 7:3 with 1 M LiPF6. For statistical purposes, we studied simultaneously the electrochemical perfromance of at least 2−3 cells of undoped and Mo-doped electrodes, and the results were averaged. For the electrochemical measurements a multichannel Maccor-2000 battery cycler and a battery test unit (BTU-1470), coupled with a frequency response analyzer (FRA-1255) from Solartron, Inc., were used.16 All the potentials in this paper are given vs Li+/Li. 4522


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Figure 1. Schematic structure of NCM811 undoped (a) and Mo-doped (b) materials. Mo-dopant that preferably substitutes Ni is shown as yellow balls. (COHP) as implemented in the LOBSTER code.38−41 This entails postprocessing of the electronic structure to find COHP and integrated COHP (ICOHP). To estimate the bond strength between two TM atoms, we averaged over all possible interactions between pairs of TMs within finite distances (2.0−3.1 Å for Ni−Ni, Ni−Co, Ni−Mn; 4.0−6.0 Å for Co−Co, Co−Mn, Mn−Mn (considering nearest one)). Other experimental methods used in this work [chemical analysis by inductively coupled plasma (ICP), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM); differential scanning calorimetry (DSC); time-offlight secondary-ion-mass-spectroscopy (TOF-SIMS); X-ray photoelectron spectroscopy (XPS)] are presented in the Supporting Information.

2.3. Computational Methods. The electronic structure calculations were performed using plane-wave-based DFT as implemented in the Vienna ab initio simulation package32,33 (VASP) in conjunction with projector augmented wave34 (PAW) potentials. The exchangecorrelation terms are treated within the generalized gradient approximation (GGA) using the Perdew, Burke, and Ernzerhof35 (PBE) functional. A 2 × 2 × 1 k-mesh is applied for the integration in k-space. Further, the effective Hubbard U parameter, i.e., the on-site Coulombic interactions, were chosen to be 5.00, 5.96, 5.00, and 5.10 eV for Mo, Ni, Co, and Mn, respectively.26 Nonbonded van der Waals interactions were incorporated using the D3 dispersion correction approach of Grimme together with PBE+U.26,36 To optimize the geometry of the systems, the coordinates, as well as the volume of the unit cell, were relaxed until the force per atom was less than 0.01 eV/ Å. For our computational model, we adopted NCM811 in R3̅m symmetry, where transition metals (TMs) occupy 3a sites and Li-ions occupy 3b sites in alternating layers along the c-axis. Specifically, we adopted the structure reported by Dixit et al.37 as our initial structure. The unit cell of NCM811 is displayed in Figure 1. In the unit cell, we have a total of 60 TMs, and we substituted one Mo and two Mo atoms in both 3a and 3b sites to reach 1.66% and 3.32% doping concentrations, respectively. To estimate the intercalation voltage from the calculated total energy of the system with different delithiation limits, we applied the following formula.

v=−

[E(Lix + dxNCMO2 ) − E(LixNCMO2 )] + E(Libcc) dx

3. RESULTS AND DISCUSSION 3.1. Structural Analysis and Morphology of Ni-Rich Materials. Structural models of undoped and Mo-doped NCM811 are shown in Figure 1a,b, respectively. Powder X-ray diffraction patterns of the synthesized Ni-rich samples are presented in Figure S1. Undoped LiNi0.8Co0.1Mn0.1O2 and 0.01 Mo-doped samples are single phase, possessing a rhombohedral structure, in the R3̅m space group, while that with 0.03 Mo shows traces of cubic spinel Li(TM)2O4 (small peaks at 18.4°, 30.1°, and 35.4° 2θ) and Li2MoO3 (small peaks at 18.4°, 36°, and 43.6° 2θ). The latter has a disordered NaFeO2 structure (space group R3̅m) with the Mo-ions present as Mo3O13 clusters in the [Li1/3Mo2/3] layer.42 To assess the perfectness of the layered structure of the undoped pristine material, the ratio of (003) to (104) peak intensities was calculated on the basis of the results of the Rietveld analysis of the XRD profile. The calculations have shown that the estimated ratio of intensities of (003)/(104) peaks was about 1.5, which is greater than the value 1.2, thus providing an indication of the pronounced layered character of the structure,43 which means there is a good separation of transition-metal-ions and Li-ions

(1)

Here, E(Lix+dxNCMO2) and E(LixNCMO2) are the energies per formula unit for the system with x + dx and x Li-ions limits. E(Libcc) is the energy per formula unit of bulk Li metal. It is known that PBE can provide accurate trends for intercalation potentials yet it underestimates the absolute voltage values. Therefore, we performed a rigid shift of 0.51 and 0.9 V for the plot of discharging and charging voltage profile to match the experimental values. We further analyzed the strength of chemical bonding between two TM atoms from the calculation of crystal orbital Hamilton population 4523


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ions at different TM-sites. We considered several different Moconfigurations either in the same or in different TM layers. The relative FEs for these configurations are shown in Figure S2b, where a configuration with two Mo-ions replacing two Ni-ions in the same layer is taken as the reference energy level. The calculations of formation energy reveal that Mo-doping at Nisites is energetically more favorable. We note that the relative Mo-doping FE values are independent of the dopant precursor (i.e., (NH4)6Mo7O24·4H2O) used in the experiment. The results from Rietveld refinement of the crystal structure of LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2 are presented in Tables S2 and S3. Further, we calculated the optimized lattice parameters for undoped and 1.66% and 3.32% Mo-doped NCM811 (Table S4). We observe a substantial increase in the c lattice parameter due to Mo-doping, regardless of the detailed computational approach (i.e., PBE and PBE+U+D3), in agreement with the experimental values (Table S4). The increase in unit cell parameters with increasing substituting level is a clear indication that Mo is present in the crystal structure of the layered phase. The increase is likely due to the increasing Ni2+ content in the structure upon partial substitution, as Ni2+ has greater ionic radius than Mo6+ (69 vs 59 pm).44 From the calculations of moment per atomic sites, we can estimate the oxidation state of the atoms. In the case of undoped NCM811, we observed three different types of Niions, corresponding to oxidation states 2+, 3+, and 4+. The distribution of different oxidation states of Ni-ions for undoped and 1.66% and 3.32% Mo-doped at Ni-sites is presented in Figure 2. The Ni3+-ion population is greater than those of Ni2+

onto their respective planes in the layered Li[NixCoyMnz]O2 structure.26 The result of the calculations of lithium occupancy on the 3b site is also in keeping with the conclusion that the analyzed NCM811 pristine material has a distinct layered structure. According to the Rietveld analysis, the contamination of the lithium layer turned out to be minimal: the normalized occupancies of the 3b Li-site by Li- and Ni-ions are, respectively, 0.9640 for Li and 0.0360 for Ni. Rietveld refinement of the crystal structure of LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2 was performed to identify the location of molybdenum-ions in the layered LiNiO2-type structure. The determination of the distribution of Mo-ions between lithium (3b) and 3d-metal (3a) positions on the basis of XRD pattern is a difficult task, since the quantity of Mo in the structure is extremely small. Thus, we performed some refinement with different fixed distributions of Mo over the two positions, allowing Li and Ni occupancies to vary in a constrained manner to fill both cation positions. All variable parameters were refined in a uniform manner for all models. The quality of the solutions was evaluated on the basis of general statistical criteria (the values of the numerical criteria of fit), and the physical reliability of the obtained structural parameters is listed in Table 1. Table 1. Conventional Rietveld R-Factors for Different Models of the Mo-Dopant Distributions in Li or TM-Layers fixed Mo0.01 Mo0.01 Mo0.03 Mo0.03

in in in in

Li-layer TM-layer Li-layer TM-layer

Rp

Rwp

Rexp

χ2

Rf

RBragg

11.6 11.6 13.6 13.6

7.21 7.22 8.25 8.27

3.41 3.42 3.67 3.68

4.48 4.45 5.05 5.04

2.00 1.99 2.24 2.31

2.38 2.37 2.88 2.91

As can be seen from Table 1, the differences between the model placing Mo-ions in the 3b position and that placing Moions in the 3a position are minute. For the Mo0.01 sample, it is not possible to prefer one to the other model. For Mo0.03doped material, the model with Mo in the 3b positions shows systematically lower values for the conventional R-factors. Additionally, the model with Mo0.03 placed in the TM metal layer (position 3a) resulted in negative isotropic temperature factor values for the ions in 3b positions. This result supports the view that Mo-ions likely occupy the 3b position of the crystal structure. Further, we performed DFT calculations to probe the most likely doping position of Mo. To find the energetically favorable position for the Mo in NCM811, we calculated the formation energy (FE) using the following equation:

Figure 2. Distribution of Ni-ions in different oxidation states Ni2+, Ni3+, and Ni4+ for undoped and Mo-doped NCM811 materials.

FE = [E(Mo@NCM811) + E(M) + 2E(Li) + 2E(O2 )] − [E(NCM811) + E(Li 2MoO4 )]

and Ni4+, in both undoped and doped materials. Importantly, the amount of Ni3+ is reduced upon doping due to charge compensation, similar to our earlier observations.26,37 The influence of Mo-doping on reducing the amount of Ni3+ was confirmed by the XPS results presented in Figure 3a,b. The curve fitting of the Ni 2p spectra for undoped (Figure 3a) and Mo-doped (Figure 3b) samples clearly demonstrates an increase in NiO content after Mo-doping, from 59.8% in an undoped sample to 67.0% in a Mo-doped material. The decrease in Ni3+ cannot be revealed from the XPS measurements as the Ni2O3 peak (Ni3+) appears to be in a very close vicinity to the Ni(OH)2 peak (Ni2+). It seems that an increase in the Ni(OH)2 component is dominant and, hence, screens the possible changes/reduction in Ni2O3 quantity with Mo-

(2)

Here E values are the energies of the systems written in parentheses, and x indicates the concentration of Mo-dopant. This equation represents Mo-doping at all metal sites, M. We calculated the FE for four different configurations by replacing Mo at Ni, Co, Mn, and Li-sites. Inspection of the data reveals that Mo-doping is favored at Ni-sites, as was found previously for high-valence doping in other NCM materials.26 We probed six different Mo-configurations at Ni-sites. The calculated FEs of Mo-doping at Co-, Mn-, and Li-sites, relative to Ni-sites, are shown in Figure S2a for 1.66% Mo-doping in NCM811. In the case of 3.32% Mo-doping in NCM811, we substituted two Mo4524


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materials clearly show the increased bond lengths for Modoped samples indicating thus incorporation of the dopant into the lattice (Table S5). SEM and TEM imaging further reveals that Mo-doping slightly influences the morphology and particle size of LiNi 0.79 Mo 0.01 Co 0.1 Mn 0.1 O 2 and LiNi 0.77 Mo0.03 Co0.1Mn0.1O2 (Figure S4). The particle size of the Mo-doped samples is somewhat smaller, probably due to inhibition of the lithiated oxide crystals growth during synthesis. We suggest that even a minute concentration of additives, like Mo-dopant, accumulates at the crystallization front during the synthesis, blocking the interface of the nuclei and slowing down the attachment of further building units.45 This process of dopantinduced inhibition of particle growth in the early stages is a general phenomenon for nanocrystalline materials, and this influences particle size, structure, and morphology.46 3.2. Electronic Structure of Undoped and Mo-Doped NCM811. We further calculated the total and partial density of states (DOS) for undoped and 1.66% and 3.32% Mo-doped NCM811 using PBE (Figure 4a−c, respectively). We observed strong hybridization between Ni-3d states and O-2p states for both undoped and doped materials, similar to Mo-doped NCM523.26 The DOS of Ni2+ near the Fermi level is greatly increased due to Mo-doping, and this is expected to be important for electrochemical performance. From the DOS for undoped NCM811 (Figure 4a), we conclude that t2g of Ni2+ is fully occupied (↑↓|↑↓|↑↓), while eg is partially occupied (↑|↑), yielding ca. 2 μB per Ni (i.e., Ni2+ is in a high-spin (HS) state). Ni3+ is in a low-spin (LS) state, with t2g fully occupied (↑↓|↑↓|↑ ↓) and eg singly occupied (↑|), with a net moment per Ni of ca. 1 μB Ni4+-ions appearing in an LS state, providing zero moment per Ni-site, as t2g is fully occupied (↑↓|↑↓|↑↓), while eg is unoccupied (|). Additionally, t2g of Co-ions is fully occupied (↑↓|↑↓|↑↓) and eg of Co-ions is unoccupied (|), in line with net moment of zero μB. The t2g of Mn-ions is partially filled (↑|↑ |↑), and eg is empty with net moment of ca. 3 μB per Mn (see Figure 3); i.e., Mn is in an HS state. Inspection of the DOS (Figure 4b,c) does not indicate any significant change in the relative position of the t2g and eg bands of Ni, Co, and Mn, with the exception of the appearance of mostly unoccupied Mo-3d states near the conduction band minima. 3.3. Transition Metals and Mo-Dopant Segregation at the Surface in NCM811. We have established that the Modopant and TMs segregate at the surface of NCM811 as is

Figure 3. Results of the XPS studies for Ni 2p3/2 of NCM811 undoped (a) and 3 mol % Mo-doped (b) samples.

doping. The peak at the low-energy knee in both doped and undoped samples, at the binding energy that is slightly lower than the NiO component, is supposed to be the NCM-matrixrelated component, with x < 1. This redistribution of Ni-ions in different oxidation states for Mo-doped materials enhances electrochemical behavior of the doped electrodes, as discussed in detail below. It may be seen that the increase of the unit cell parameters upon Mo substitution is mainly due to the increase of the metal−oxygen bond lengths within the layers. To find the average bond lengths between oxygen atoms and the 3a and 3b sites, we calculated the radial distribution function for the all interatomic distances from the optimized structures (Figure S3). The results using PBE and PBE+U+D3 were similar, and here, we only discuss results using the latter method. From our analysis, we observe a slight increase in both 3a−O and 3b−O bond lengths upon Mo-doping in NCM811 (Table S5). The calculated radial distribution functions for TM(3a)−oxygen and Li(3b)−oxygen distances in NCM811

Figure 4. Calculated density of states for NCM811 undoped (a) and for 1.66% and 3.32% Mo-doped (b, c) materials, using PBE. 4525


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Figure 5. Depth profiles of Ni+, Co+, and Mn+ measured by ToF-SIMS from NCM811 undoped LiNi0.8Co0.1Mn0.1O2 (a) and 1 mol % Mo-doped LiNi0.79Mo0.01Co0.1Mn0.1O2 (b) and 3 mol % Mo-doped LiNi0.77Mo0.03Co0.1Mn0.1O2 (c) materials. Due to high roughness of the powder samples, “Surface” of 15−20 Å (light blue bars) and “Bulk” regions are shown only schematically, as an eye guide. The sputter depth was calculated assuming that the samples are made of Li and using the current of the sputter beam. (d) Typical high-resolution TEM image of the 1 mol % Mo-doped NCM811 material measured by HAADF-STEM at the surface of the particles and in the bulk indicated with the red rectangular areas 1, 3, and 2, respectively. The content of the Mo-dopant in these areas is indicated. The samples for microscopic studies were prepared by the focused ion beam technique.26

ToF-SIMS studies revealed that these TMs segregate at the surface in NCM811, while in the 1 mol % Mo-doped material Ni and Co segregate to a lesser extent and the Mn-segregation is substantially diminished (Figure 5b). Similar results were obtained in the case of 3 mol % Mo-doped material (Figure 5c). The segregation of the Mo-dopant at the surface was confirmed by HAADF-STEM-EDS studies of the doped NCM811 sample, as is evident from the STEM image in Figure 5d. This demonstrates that the atomic concentration of Mo at the surface (areas 1 and 3 marked with the red rectangular) is several times higher compared to that in the bulk of the particle (area 2). These results are in close agreement with our previous work on the Mo-doped NCM523 materials in which the dopant segregation was established using the above techniques.26 We analyzed the strengths of different TM−TM chemical bonds by calculating their ICOHP. A schematic diagram for

evident from the depth profiles recorded using ToF-SIMS in Figure 5a−c. The depth profiles indicate enrichment of TMs due to segregation at the outermost surface layer of 15−20 Å. Inspection of Figure 5b reveals that Mo segregates at the surface, and then its content decreases and levels off, in line with what we observed in our earlier studies on LiNi0.49 Mo0.01Co0.2 Mn0.3O 2 Mo-doped cathode material NCM523.26 However, in contrast to this mildly Ni-rich material, the depth profiles measured from NCM811 samples demonstrate also segregation of Ni, Co, and Mn, in agreement with literature reports of Liang et al.47,48 These authors suggested that the reason for Ni surface segregation is weak Ni−Ni bonding (effective interactions) in Ni-rich NCM materials that breaks, resulting in Ni-segregation. In a proposed new bond model, they attribute the origin of instability and deterioration of Ni-rich NCM (Ni ≥ 80 at. %) to the Co- and Mn-segregation at the surface and formation of clusters. Our 4526


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Figure 6. (a) Results of the rate capability tests obtained at 30 °C from NCM811 undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % Mo and 3 mol % Modoped LiNi0.79Mo0.01Co0.1Mn0.1O2 and Li1Ni0.77Mo0.03Co0.1Mn0.1O2 electrodes, as indicated. (b) Cycling performance of these electrodes at a C/3 rate using the CC−CV mode in the potential range 2.8−4.3 V with potentiostatic steps at 4.3 V for 0.5 h. Capacity retention (Q) values as ratios of the corresponding discharge capacities measured for 100th and 30th cycles are shown. (c) Results of the rate capability tests obtained at 30 °C from NCM811 electrodes: undoped LiNi0.8Co0.1Mn0.1O2 and 2 mol % Mo-doped and 3 mol % Mo-doped (by Ni and Mn partial substitution) LiNi0.79Mo0.01Co0.1Mn0.09Mo0.01O2 and LiNi0.78Mo0.02Co0.1Mn0.09Mo0.01O2, as indicated. (d) Cycling performance of these electrodes at a C/3 rate using the CC−CV mode (2.8−4.3 V range) with potentiostatic steps at 4.3 V for 0.5 h. Capacity retention (Q) values as ratios of the corresponding discharge capacities measured for 100th and 30th cycles are shown.

for Ni atoms in proximity to Mo-dopant atoms. This change in Ni−Ni bond strength is accompanied by a reduction in bond strength of Ni−O bonds near Mo-dopant sites. It should be noted, however, that segregation of Ni, Co, and Mn established in this work reflects an average trend, while the homogeneity of the distribution on the surface of Ni-rich NCM may differ greatly on polar (012), (001), (111), and nonpolar (100), (110), (104) surfaces. 48,49 Taking into account that segregation of TMs originates from the nature of the TM− TM bonds in LiNi0.8Co0.1Mn0.1O2,47 segregation of Ni, Co, and Mn in the undoped material (Ni = 80 at. %) can explain, to some extent, deteriorating performance of these Ni-rich electrodes during cycling, which is a common phenomenon of Ni-rich cathode materials (Ni ≥ 80 at. %).1,50 In contrast, more stable electrochemical behavior of Mo-substituted (doped) electrode materials with Ni = 77−79 at. % shown in the following section can be attributed (according to the

the average ICOHP values between different TM−TM bonds is displayed in Figure S5. We note that more negative values of ICOHP indicate stronger bonds. A main observation is that the greatest changes to bonding are observed for bonds involving Ni-ions, and this may be ascribed to changes in oxidation states on doping. Bonds involving Mn-ions hardly change on doping, as the Mn-oxidation state is unaffected by doping. The average ICOHP values for Ni−Mn and Ni−Co bonds are more negative compared to Ni−Ni bonds in undoped NCM811, consistent with earlier reports on NCM811.47,48 Additionally, Ni−Mn is found to be the strongest bond, and the Mn−Mn bond is the weakest bond among all TM−TM bonds. This indicates that there can be segregation of Ni at the surface. Further, the strength of the Ni−Ni bond enhanced due to Mo-doping and possibly suppresses Ni-segregation at the surfaces. The Mo-doping induced increase in Ni−Ni bond strength is more prominent 4527


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ACS Applied Energy Materials bond theory) to their stabilization by Ni2+−Mn4+, Ni3+−Mn4+, and Co3+−Mn4+ bonds. It can be proposed that the presence of Mo stabilizes TM−TM bonding and, therefore, induces formation of a more stable modified electrochemical interface and improved behavior of the Mo-doped NCM811 electrodes. In the following section, we discuss these issues related to comparative electrode behaviors of undoped and Mo-doped samples. 3.4. Comparative Analysis of the Electrochemical Behavior of LiNi0.8Co0.1Mn0.1O2 Undoped and MoDoped Electrodes. In Figure 6, we demonstrate results of the rate capability studies (a, c) and of cycling behavior (b, d), respectively, of NCM811 undoped and Mo-doped electrodes at 30 °C, as indicated. Note that this figure compares the electrochemical performance of Ni-substituted samples (by 1 and 3 mol % of Mo LiNi0.79 Mo0.01Co0.1 Mn 0.1O 2 and LiNi0.77Mo0.03Co0.1Mn0.1O2, respectively) and of those in which Ni (1 and 2 mol %) and Mn (1 mol %) were both substituted by Mo, namely, LiNi0.78Mo0.02Co0.1Mn0.09Mo0.01O2 and LiNi0.79Mo0.01Co0.1Mn0.09Mo0.01O2. We also computed the voltage profiles using DFT, and inspection of Figure 7a,b

respectively). This can be explained by the increased c lattice parameter for the doped materials, as shown by the Rietveld refinement of their XRD patterns (Table S4). The Mo-doped electrodes deliver increased discharge capacities on prolonged cycling at a C/3 rate and demonstrate higher capacity retention (Q), expressed as ratio of the capacities obtained in the 100th and 30th cycles (Figure 6b,d). We conclude therefore that these results are characteristic of doped electrodes and can be ascribed to the fact that incorporation of highly charged dopant ions like Mo6+ leads to redistribution of charge among TM cations in the NCM lattice, as shown in Figure 2. This, in turn, results in decreasing the relative number of Ni3+-ions and increasing the number of Ni2+ and electrochemically active Ni2+/Ni3+ redox couples responsible mainly for the electrode capacity.18,26,27,50 We also conclude from the results of the cycling behavior that even minor levels of Mo-doping (1 mol %) can be considered as optimal for NCM811 electrodes (i.e., LiNi0.79Mo0.01Co0.1Mn0.1O2), as reflected in the higher discharge capacity and reduced fading upon cycling in Li-cells. This is in agreement with the recent studies of 1% Mo-doped NCM811 cathodes by Su et al.51 In addition, we have established that the voltage hysteresis calculated as the difference of mean voltage in charge and mean voltage in discharge is lower for the Mo-doped samples during cycling as shown in Figure S6. The above parameters were also substantially improved especially for the 3 mol % Mo-doped LiNi0.77Mo0.03Co0.1Mn0.1O2 electrodes during cycling at a C/3 rate at an elevated temperature of 45 °C (Figure 8a,b). As expected, the Ni-rich undoped electrodes typically exhibited an increased capacity degradation during cycling at 45 °C compared to that at 30 °C, likely due to intense structural instability and surface interactions with solution species, in agreement with the literature reports.1,49,52 The results of Figure 8a allow the conclusion that though undoped and 1 mol % and 3 mol % Mo-doped electrodes exhibited similar discharge capacities around 185−195 mAh/g at the beginning of cycling, undoped samples showed an immediate sharp capacity decrease, as expected, while the Mo-doped ones retain their capacity. In addition, the effect of Mo-substitution (doping) is more pronounced at 45 °C for electrodes comprising 3 mol % Mo-doped LiNi0.77Mo0.03Co0.1Mn0.1O2. These electrodes demonstrate similar discharge capacity during ∼70 cycles, yet the capacity retention of Q100/Q30 = 72% is higher compared to that of 1 mol % doped electrodes (Q100/ Q30 = 64%). This can be attributed to a synergetic effect of the following factors: slightly lower Ni-content (77 at. %) and the stabilizing impact of Mo-doping in the lattice. The presence of the Li-rich Li2MoO3 phase (though in minor content) in the above material, as discussed in the section on Structural Analysis, should also be taken into account. Indeed, recent experimental results demonstrated that incorporation of Li2MoO3 in NCM cathodes effectively stabilized their cycling behavior.52 As shown in Figure 8b, voltage hysteresis measured as mean voltage in charge−mean voltage in discharge of the above NCM811 Mo-doped electrodes is much lower compared to that of the undoped samples during prolonged cycling. Cycling and structural stability of the 3 mol % Modoped LiNi0.77Mo0.03Co0.1Mn0.1O2 electrodes is evident from comparison of the differential capacity dQ/dV vs V plots of undoped and Mo-doped samples measured at 45 °C for cycles 5, 50, and 75 (Figure 9). In these plots, the Mo-doped electrodes demonstrate faster electrochemical kinetics, and the redox couple at ∼4.15 V exhibits higher reversibility of these

Figure 7. Voltage profiles of charging (a) and discharging (b) for undoped (black) and 1% (red) and 3% Mo-doped (blue) NCM811 electrodes. Profiles shown with steps are from theoretical calculations, and curves with empty circles are from experiment.

reveals good agreement between theory and experimental results. We established that doped electrodes exhibited 12− 20% higher discharge capacities than the undoped ones, especially at high rates of 4C, in line with our recent results on Zr- and Mo-doped NCM electrodes.18,26,27 We emphasize that all Mo-doped Ni-rich electrodes studied in the present paper, Ni-substituted as well as Ni- and Mn-substituted, outperform the undoped ones in terms of the rate capability (Figure 6a,c, 4528


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electrodes, in which both Ni and Mn were partially substituted by the Mo-dopant. The total doping levels were 2 and 3 mol %, respectively, in these samples. These voltage profiles are typical for NCM electrodes exhibiting a sharp increase of the potential at the beginning of charging from OCV to 3.6−3.7 V and a continuous smooth increase up to cutoff 4.3 V. At potentials ∼4.1−4.2 V they exhibit a plateau-like region that corresponds to structural phase transitions of a hexagonal H2 to H3, typical for Ni-rich electrodes.50,54 It is important to stress that Mo-doped electrodes exhibit significantly less irreversible capacity loss in the first cycle. Additionally, the remaining currents developed during the potentiostatic steps for 3 h after initial charging to 4.3 V are also notably smaller (Table S6), implying lesser side reactions of doped materials with the solution species and formation of a more stable interface. This is evident also from comparison of the total heat evolved during chemical reactions of NCM811 undoped and Mo-doped materials with the battery solutions EC-EMC/LiPF6. Our DSC studies demonstrated that less heat was evolved for the 1 and 3 mol % doped samples (352 and 299 J/g, respectively) compared to the undoped one (433 J/g) (Figure S8). We also established that undoped electrodes exhibited ∼180 mV higher potential (“overvoltage”) upon the Li+ extraction both in the initial cycle at slow rate and by 75 mV in the 12th cycle at 1C rate (Figure S7), while the charging curves of the Mo-doped samples lie at lower potentials compared to the undoped ones, suggesting reduced resistance. This is also reflected in impedance spectra measured at OCV = 4.0 V (during charging) from undoped and doped electrodes after 10, 50, and 100 galvanostatic cycles, as shown in Figure 10a−c, respectively. This finding is one of the advantages of Mo-doped electrodes, which remain less resistive upon cycling compared to the undoped samples. Overall, we conclude that Mo-doping results in lower interfacial charge-transfer resistance, calculated from fitting of the impedance spectra (Nyquist plots) and represented in Figure 10d. The results obtained can be attributed to a modified electrode/solution interface due to dopant segregation at the surface (Figure 5). The lower interfacial resistance results from the additional conduction bands formed near the Fermi level in doped materials,26 as demonstrated in Figure 4. Therefore, the transfer of Li-ions and electrons is facilitated, increasing thus the exchange current density (i0) due to the decreased Rct. Similar results were obtained in Mo6+-doped NCM523 materials26 and in the W-doped TiO2 and spinel materials.55,56 3.5. Structural Analysis by XRD and TEM of Cycled Electrodes Comprising LiNi0.8Co0.1Mn0.1O2 (Undoped) and Mo-Doped LiNi0.79Mo0.01Co0.1Mn0.1O2 Materials. A comparison of XRD profiles of undoped and Mo-doped NCM811 electrodes in conditions before and after 100 charge/ discharge cycles (C/3, 30 °C) is presented in Figure S9. Except the basic rhombohedral phase peaks, the XRD profiles show the presence of additional peaks at 2θ = 26.6° and 2θ = 28.7°. We attribute these two peaks, respectively, to the (002) reflection caused by graphite component of the electrode and to the strongest (110) peak of the tetragonal β-MnO2 phase (pyrolusite), which can be considered as an impurity phase. A strong peak labeled Al220 arises from the Al-current collector of the electrode. The results of Figure S9 show no essential difference between the XRD patterns of the electrodes comprising undoped LiNi0.8Co0.1Mn0.1O2 and Mo-doped LiNi0.79Mo0.01Co0.1Mn0.1O2 materials, as well as between the profiles corresponding to cycled and uncycled conditions. The

Figure 8. (a) Cycling performance at 45 °C of NCM811 electrodes: undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % Mo and 3 mol % Modoped LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2, as indicated. Potential range used was from 2.8 to 4.3 V at a C/3 rate. Every 20th cycle the charge/discharge current was increased corresponding to a C/10 rate. Inset: Discharge capacity measured from the above electrodes every 20th cycle at a C/10 rate. Constant current−constant voltage (CC−CV) mode with potentiostatic steps at 4.3 V for 0.5 h. Capacity retention (Q) values as ratios of the corresponding discharge capacities measured for 100th and 30th cycles are shown. (b) Voltage hysteresis measured as mean voltage in charge − mean voltage in discharge of the above NCM811 electrodes.

electrodes compared to the undoped ones. This indicates diminishing, to some extent, of a possible phase transformation from hexagonal phases H2 + H3 to H3 (at 4.2−4.3 V) that is mainly responsible for structural degradation of Ni-rich materials.1 We note that the question of possible phase transitions in Ni-rich (80 at. %) cathodes upon cycling requires more experimental and theoretical studies. However, as is evident from in situ XRD measurements,53 there is a correlation among evolution of the cell parameters a and c during charge/discharge, dQ/dV plots, and possible transformations of the original phase H1 to another hexagonal phase with a lower Li-ion content, and subsequently to domains comprising both of these two phases and their transformation to H3. The voltage profiles measured at 30 °C in the 1st and 12th cycles of NCM811 undoped and 1% and 3% Mo-doped electrodes LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2, respectively, are demonstrated in Figure S7a,b. Figure S7c,d compares similar profiles of LiNi0.79Mo0.01Co0.1Mn0.09Mo0.01O2 and LiNi0.78Mo0.02Co0.1Mn0.09Mo0.01O2 4529


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Figure 9. Differential capacity plots dQ/dV vs V measured at 45 °C from NCM811 electrodes of undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % and 3 mol % Mo-doped LiNi0.79Mo0.01Co0.1Mn0.1O2 and LiNi0.77Mo0.03Co0.1Mn0.1O2, as indicated, after the 5th, 50th, and 75th cycles. Electrode potentials of the redox couple related to the coexisting H2 and H3 hexagonal phases are indicated.

LiNi0.79Mo0.01Co0.1Mn0.1O2, it was concluded that two models provide similar results for the Mo-ions residing either in the Lilayer or occupying the transition metal sites. In contrast, for the dopant content of 3 mol %, we concluded that Mo-ions are more likely to occupy the 3b position (lithium layer) in the crystal structure. On the basis of the DFT calculations, it was shown that Mo6+ cations preferably substitute Ni in the oxide structure due to the lowest substitution energy compared to Li, Co, and Mn. This conclusion agrees with the relatively low crystallographic mismatch between ionic radii of Ni-host (rNi2+ = 69 pm) and Mo-guest (rMo6+ = 59 pm). Analysis of the unit cell parameters a and c of the undoped and Mo-doped materials that increase with increasing the substituting level allowed us to conclude there was a successful incorporation of the dopant into the lattice. This conclusion was supported by the calculated values of the radial distribution functions for “TM(3a)−oxygen” and “Li(3b)−oxygen” distances in NCM811 materials that clearly show the increased bond lengths for the Mo-doped samples. Studies of the electrochemical behavior of undoped and Mo-doped materials demonstrated that Mo-doped electrodes exhibited much lower irreversible capacity loss in the 1st cycle and lower the remaining currents developed during the potentiostatic steps following the initial charging to 4.3 V. We concluded therefore the formation of a more stable electrode/solution interface and

lattice parameters of undoped and Mo-doped NCM811 electrodes are shown in Table S7. Although the XRD patterns of the cycled doped and undoped NCM-811 samples did not contain direct evidence of the transformation to cubic spinel phase, our TEM examinations of the individual particles of the cycled materials did show the presence of cubic LiMn2O4 (Fd3̅m) spinel. In some cycled Mo-doped samples, in addition to the cubic LiMn2O4 spinel, we also observed completely delithiated spinel Mn2O4. This is demonstrated on examples of nanobeam electron diffractions in Figure 11a−c, which presents spinel patterns in (a) undoped and (b) 1% Modoped NCM-811 material. The pattern in Figure 11c corresponds to delithiated spinel observed in 1% Mo-doped material. Also shown in Figure 11 are examples of the diffraction patterns of the basic rhombohedral phase (Figure 11d) and the tetragonal β-MnO2 impurity phase (Figure 11e), which were observed both in doped and undoped samples.

4. CONCLUSIONS In this work, we have synthesized a family of five Ni-rich lithiated oxides LiNi0.80−xMoxCo0.1Mn0.1−yO2 (x = 0.01−0.03; y = 0.01), in which Ni or both Ni and Mn were partially substituted upon the synthesis with a minor amount of the molybdenum Mo6+-dopant. From the Rietveld refinement analysis of XRD patterns of 1 mol % doped sample 4530


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Figure 10. Impedance spectra measured at OCV = 4.0 V (charged state) after 10 (a), 50 (b), and 100 (c) cycles at a C/3 rate from NCM811 electrodes undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % Mo-doped LiNi0.79Mo0.01Co0.1Mn0.1O2, as indicated. Cycling was carried out at 30 °C using CC−CV mode from 2.8 to 4.3 V with potentiostatic steps for 0.5 h at 4.3 V. Insets: high-frequency semicircles (100−1 kHz) of impedance spectra attributed to the resistance of the Li-ion migration through the surface films formed on the electrodes.26 (d) Charge-transfer (interfacial) resistance measured from impedance spectra (second semicircle at 1 kHz−10 Hz) of the above electrodes as a function of cycle number.

Figure 11. Results of postcycling analysis of NCM811 undoped LiNi0.8Co0.1Mn0.1O2 and 1 mol % Mo-doped LiNi0.79Mo0.01Co0.1Mn0.1O2 materials by electron diffraction in TEM. Patterns of spinel phase (sp) observed in undoped and 1% Mo-doped NCM-811 materials are shown in parts a and b, respectively. (c) Pattern from delithiated spinel observed in 1% Mo-doped material. (d) Example of the diffraction patterns of the basic rhombohedral phase. (e) Example of the diffraction pattern of tetragonal β-MnO2 impurity phase, which was observed both in doped and undoped samples.

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fewer side reactions of doped materials with the solution species. This is evident also from the lower charge-transfer resistance of the above electrodes as well as from decreased the total heat evolved during chemical reactions of undoped (433 J g−1) and 1% Mo-doped (299 J g−1) materials with the battery solutions in the range 30−350 °C, as measured by DSC. The above findings correlate well with the segregation of transition metals and the Mo-dopant at the surface established experimentally, for the first time, in the above Ni-rich materials. We have established that doped electrodes exhibited higher discharge capacities compared to undoped ones, by 12− 20%, especially at high rates of 4C. This is in agreement with our previous results for Zr- and Mo-doped NCM622 and NCM523 electrodes, respectively. It was shown also that Modoped NCM811 electrodes demonstrated higher capacity retention and lower voltage hysteresis during cycling (100 times at C/3 rate) at 30 and 45 °C. The results of the electrochemical studies were attributed to the fact that incorporation of highly charged Mo6+ dopant leads to the redistribution of charge among transition metal cations resulting in decreasing the relative number of Ni3+-ions and increasing the number of Ni2+-ions and electrochemically active Ni2+/Ni3+ redox couples responsible mainly for the electrode capacity. Analysis of the nanobeam electron diffraction patterns of cycled electrodes led us to a conclusion on their structural stability since the initial rhombohedral R3̅m structure was mainly preserved both for the undoped and 1 mol % Mo-doped samples. However, additionally it was suggested the formation of a spinel phase (cubic, Fd3̅m) detected on a few selected particles of cycled electrodes, due to the layered-to-spinel transition in agreement with the literature data.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00767. Additional data and figures and a detailed description of the experimental methods (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Francis Amalraj Susai: 0000-0003-3778-4200 Dan Thomas Major: 0000-0002-9231-0676 Boris Markovsky: 0000-0001-7756-0071 Doron Aurbach: 0000-0001-8047-9020 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Partial support for the work discussed herein was provided by the BASF SE through its Research Network on Electromobility, and the Israeli Committee for Higher Education within the framework of the INREP project. The authors thank Ortal Bruer for her assistance in the materials synthesis and Dr. Vasiliy Rosen for his support in ICP analysis. 4532


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Improving Performance of LiNi0.8Co0.1Mn0.1O2 Cathode Materials

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