Transition Metal Ordering Optimization for High ... - ACS Publications

Article pubs.acs.org/JPCC

Transition Metal Ordering Optimization for High-Reversible Capacity Positive Electrode Materials in the Li−Ni−Co−Mn Pseudoquaternary System Roberto C. Longo,*,† Fantai Kong,† Chaoping Liang,† Dong-Hee Yeon,‡ Jaegu Yoon,‡ Jin-Hwan Park,‡ Seok-Gwang Doo,‡ and Kyeongjae Cho*,† †

Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States Energy Lab., Samsung Advanced Institute of Technology, Samsung Electronics, Yongin 446-712, Republic of Korea

‡

S Supporting Information *

ABSTRACT: The phase diagram of the Li−Ni−Co−Mn layered oxide pseudoquaternary system is used as starting point to elucidate the influence of transition metal (TM) ordering on the structures and electrochemistry of positive electrode materials with LiNi1−y−xCoyMnxO2 composition. Whereas our obtained phase diagram shows a comprehensive search of the most suitable target compositions for singlephase layered materials with large structural stability and reversible capacity, a detailed analysis of the transition metal ordering tries to fill the gap existing in the literature between transition metal composition and ordering, showing the effect on the electrochemical performance. Our results demonstrate that, in order to achieve high reversible capacities, control of the TM arrangement at the atomic scale seems to be crucial to enhance both ionic and electronic mobilities, thus maximizing the rate capability and structural stability during cycling.

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INTRODUCTION Current Li-ion batteries use layered oxides such as LiCoO2 or LiNiCoMnO2 (NCM) as cathode materials, and graphite as anode.1−5 Co layered oxides show good reversibility and rate capability with a capacity retention of 130 mAhg−1 after 40 cycles of charge and discharge. However, these oxides suffer from the high cost and toxicity of cobalt, together with certain instability at high operational temperatures. With an aim to overcome these difficulties, the synthesis of cobalt-free or lowcobalt layered oxides with different sets of transition metals (TM) has become the most successful way to avoid the particular drawbacks of every single-oxide family.6−11 The uniformity of the chemical species at the atomic level can significantly mitigate the observed voltage fade and energy degradation of NCM cathode materials. Indeed, novel synthesis techniques like hydrothermal assisted methods can produce a more homogeneous cation distribution at the atomic level in the cathode material,12 thus avoiding intrinsic structural changes that arise as a consequence of the nonuniform distribution of the TM cations, specially Ni ions. To date, only two NCM cathode materials have been already successfully commercialized in Li-ion batteries: LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM111) and LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523).13,14 Although layered materials can deliver larger capacity than other families of cathode materials (e.g., phosphates), the energy density has yet to be increased in © XXXX American Chemical Society

order to match the expectations deposited on the NCM layered oxides. In order to deliver high capacities, NCM cathode materials need to be cycled at high operational voltage (∼4.5 V),13 resulting in the well-known problems of voltage and capacity fade over a large number of charge and discharge cycles. In recent years, much research efforts have been devoted to address these problems. There seems to be a general agreement in that the origin of the voltage and capacity fading is motivated by structural changes. 13,15 Indeed, phase transitions occur in layered oxides through the reduction and subsequent rearrangement of TM ions,16 accompanied by changes in the electronic density of states within the material. Previous work (see, e.g., ref 15 and references cited therein) consistently showed that the Li/TM ordering disappears with electrochemical cycling at high voltage (4.8 V), indicating a significant cation rearrangement in the bulk of the cathode material. The degradation mechanisms proposed differ depending on the stoichiometry of the NCM cathode material. For instance, the capacity fade in NCM111 was attributed to a phase transformation between the standard O3 layered structure to the O1 phase for a highly charged (delithiated) state, via the generation of stacking faults between the two Received: March 2, 2016 Revised: April 5, 2016

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DOI: 10.1021/acs.jpcc.6b02240 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C phases,17−19 whereas the NCM523 cathode material does not show any evidence of the O1 phase,13 probably due to the larger amount of Ni. Li−Ni site exchange would prohibit the sliding of the slabs to create the O3−O1 stacking faults. Whether these structural changes and subsequent phase transformations are originated at the surface of the cathode nanoparticles and then propagated into the deeper regions of the bulk is still under dispute, although the degradation mechanisms for the NCM523 electrodes seem to indicate13 that a phase transformation from layered to spinel mainly occurs at the surface of the cathode material. Further operation of the cell at high voltages can also cause oxygen evolution and the appearance of the ionically blocking rock salt structure,13 thus increasing the charge transfer resistance and accelerating the voltage and capacity fading. Recent experimental work in Dahn’s group9−11 has shown an extensive search and mapping of the Ni−Co−Mn phase diagram in an attempt to synthesize layered oxides with optimal capacity and structural stability. Although some of the conclusions were drawn for Li-rich layered oxides, others are of general validity for layered oxides. For instance, most of the materials with low-irreversible capacity are single-phase layered with metal-site vacancies and exhibit an O3 type structure. Only for Li-deficient materials beyond a certain limit, and depending on the Ni/Co/Mn ratios (more specifically, Ni-rich compositions), a spinel phase appears on the samples. None of the recent theoretical studies addresses the fundamental questions raised previously, and the peculiarities of the NCM layered oxides have only been investigated partially. For instance, Luo et al. reported recently the reversible electrode potentials, crystal parameters, and Li diffusion barriers for a set of NCM stoichiometries within a random TM solid solution model.20 Previously, Koyama et al. used an ordered superlattice structure to obtain the structural and electronic properties of the NCM111 cathode material.21 Other works focused on Co replacement by different TM in an attempt to improve the electrochemical properties22 and/or guide the synthesis conditions.23 Guided by the experimental search9−11 of ideal compositions for NCM layered materials, we have determined the thermodynamic stability of different NCM-stoichiometries, comparing our results with the available data. Furthermore, by means of density functional theory (DFT) calculations for a well-known structure (NCM111), we investigated the effect of TM ordering on the structural and electronic properties and electrochemical performance of NCM layered oxides, including ionic and electronic mobilities. We propose that by controlling the uniformity of the chemical species the operational voltage (and the energy density) can be substantially enhanced due to the different electrochemical activation of the TM ions. Therefore, our results not only provide valuable information about the atomic ordering effect on the electrochemical properties of layered oxides but also reasonable targets for the synthesis of NCM compounds with desired properties depending on the specific application.

Perdew−Burke−Ernzerhof (PBE) functional of the generalized gradient approximation (GGA).27 For all the compounds studied, a k-point mesh within the Monkhorst−Pack scheme28 was used to ensure a convergence of 1 meV per unit cell. All the structural relaxations were performed without any constraint until a convergence of 10−4 eV in the total energies and 0.01 eV Å−1 in the forces on every atom was reached. All the calculations were spin-polarized and both ferromagnetic (FM) and interlayer and intralayer antiferromagnetic (AFM) configurations were considered. Our results show that for most cases AFM configurations within the same TM layer (intralayer) give rise to lower formation energies, and that is the magnetic structure we adopted to build the phase diagrams. Intralayer TM AFM configurations lead to frustrated hexagonal magnetic interactions (between TM nearest neighbors with the same spin orientation) but the calculations show that, given the distance between TM layers, magnetic exchange interactions lower the energy of the system with respect to interlayer TM AFM configurations. The GGA+U approach was employed to account for the electron localization around the TM ions and to calculate the oxidation states of Ni, Co and Mn accurately. The U parameters were obtained by a linear response method29,30 and in this work we used the values 6.88, 5.95, and 5.2 eV for Ni, Co, and Mn, respectively. The oxidation state and the magnetic moment of each ion were determined by integrating the corresponding charge density (up and down components) in a sphere centered around the TM ion. To accurately reproduce the oxygen molecule binding energy when obtaining the phase diagrams, we introduced a correction term in the calculations (the room temperature enthalpy of the O2 molecule was determined to be −8.95 eV per formula unit) to account for the addition of two electrons to the oxygen porbitals to form O2− from O2.31 LiMnO2, the parent structure of NCM, was reported to have a trigonal R3̅m structure,32 and it is composed of a repeating sequence of TM, oxygen, Li, oxygen, and TM layers. The atomic sites within the TM layer are arranged in a hexagonal pattern and each one of them has the same probability of being occupied by any TM (Ni, Co, or Mn) with no specific ordering. In this work, our supercell models were generated from 2 × 2 × 2 nonorthogonal unit cells of LiMnO2. Thus, the total number of atoms in the system is 192. It is important to note that even a 192-atoms supercell might not be large enough to model some of the configurations considered, but it is a practical choice for DFT calculations, especially given the large number of supercell structures studied.20 On a pseudoquaternary phase diagram (the one formed by the three TM layered oxides and the oxygen molecule as an independent phase), there are obviously almost infinite choices of different LiNi 1−y−x Co y Mn x O 2 compositions. Although this work takes the NCM111 stoichiometry as an example to show the TM ordering effect on the structural, electronic, and electrochemical properties of NCM layered oxides, our second goal is to map the entire NCM-phase diagram in the search of ideal synthesis target compositions. Most of the experiments reporting practical capacities higher than 160 mAhg−1 use samples13,20 located around the lines with Mn/Co ≈ 1:1 or Mn/Ni ≈ 1:1 on the phase diagram. Therefore, in order to select a representative and realistic group of NCM compositions to sample the entire phase diagram, here we chose compositions with Mn/Co = 1:1, Mn/Ni = 1:1, and Co/Ni = 1:1. To construct the phase diagram of the obtained NCM stoichiometries, we calculated the normalized Gibbs free energies using the TM fcc and Li bcc

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COMPUTATIONAL METHODS The results reported in this work were obtained using DFT with plane wave basis sets and projector augmented wave (PAW) pseudopotentials,24 as implemented in the VASP code.25,26 The electronic wave functions were represented by plane wave basis with a cutoff energy of 500 eV. The exchange correlation interactions were included by using the semilocal B


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The Journal of Physical Chemistry C Table 1. Characterization of NCM111 Superstructures by the Trace of the Local Order Matrixa Trace energy/f.u. a,c (Å) β (degrees) V (Å3) gap (eV)

Group 1

Group 2

Group 3

Group 4

Group 5

0 −22.56 10.13,10.22 109.26 1718.94 1.0/1.6

4 −22.50 10.09,10.20 108.65 1727.36 1.06/0.5

6 −22.49 10.07,10.21 108.51 1728.95 1.2/1.2

10 −22.35 10.01,10.13 107.13 1738.86 0.5/0.4

18 −22.30 10.00,10.12 107.03 1743.98 0.6/0.5

a Energy per formula unit (in eV), supercell lattice parameters, non-orthogonal lattice vector angle, supercell volume, and electronic band gap (spin up and spin down components) are also given.

bulk phases and O2 gas phase as reference states. Then, we obtained the pseudoquaternary phase diagram by taking the convex hull (the smallest set containing the Gibbs free energies) and projecting it onto a tetrahedron.31,33 In this work, the Li and O compositions share one of the axis in order to allow a clear visualization of the computational phase diagram. Furthermore, the electrochemical stability of some selected NCM compositions has been assessed through the phase diagram in the chemical potential space, by obtaining meaningful limits for the respective chemical potentials.31,34 In order to investigate the TM ordering effect on the properties of layered oxides, the cation distribution in the TM layers is another relation that needs to be appropriately handled. Previous ab initio calculations35 have shown that TM ions (Ni, Co, and Mn) tend to be homogeneously distributed within the TM oxide slabs rather than forming NiO2, CoO2, and MnO2 separated phases. Between these two limits, a perfect solid-solution (SS) and a phase separation (PS), the cation distribution can form a large amount of superstructures (e.g., for a 36 atoms supercell, that is, 9 TM ions, there are 1680 different ways to distribute the TM cations in the supercell). In order to develop a concise picture of the possible ways to characterize such superstructures, we follow the methodology proposed in ref 36. The tendency of the TM ions to group together forming clusters within the TM layer can be quantified by a local order matrix. By counting the number of TM ions in the nearest-neighbor positions around each lattice site in the TM layers, we can introduce the order matrix, which measures TM clustering patterns. If we obtain the trace of such order matrix (i.e., the number of nearest-neighbors of the same type averaged over the total number of TM ions), all the possible superstructures can be classified into only five different groups,36 as shown in Table 1 and Figure 1. Of course this local matrix is merely a simple measure of the topology of the NCM superstructures and does not offer any information about the TM interaction and thermodynamic stability but because different models in the same trace group have similar patterns in the TM arrangement only a limited number of superstructures is necessary to model the NCM111 cathode material without loss of generality. The kinetic barriers, transition states, and reaction paths were obtained using the climbing image-nudged elastic band method (CI-NEB).37−39 This method allows us to obtain the minimum energy path (MEP) between a set of two different states. To do that, the reaction path is divided into a set of images “connected with a spring”. During the relaxation, the initial and final states are kept frozen while the images move according to the constraint of the “elastic band”. The MEP is then found when the components of the forces perpendicular to the “elastic band” vanish with the relative positions of the

Figure 1. Structural arrangement of one representative of each group of NCM111 superstructures studied in this work with their corresponding Traces (see text for details). Color code: Li, green; oxygen, red; Ni, gray; Co, blue; and Mn, purple balls.

images and the barrier being determined by the parallel components of the forces.38,39

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RESULTS AND DISCUSSION TM Ordering Effect and Thermodynamic Stability of NCM Layered Oxides. As outlined in the Computational Methods, most of the NCM superstructures can be classified into five different groups, depending on the local order matrix. For obvious reasons, such ordering is only valid for the NCM111 supercell, that is, structures with the same amount of each TM ion. Table 1 shows the obtained energies per formula unit of the NCM111 cathode material, as a function of the Trace of the local order matrix, and Figure 1 shows the structural arrangement of one representative of each group of superstructures (for the two limit cases, with Trace 0 and 18 there is obviously only one possible structure). The energies decrease with the Trace of the layered material, thus indicating that a perfect TM solid solution is always the thermodynami-

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process, it supports the well-established idea that a perfect SS of TM ions can deliver higher capacity during the cycling process. Figure 2 shows the density of states (DOS) of the five different groups of NCM structures, according to the Trace of

cally favorable configuration for the arrangement of TM cations in a layered oxide NCM structure. Questions like disorder or TM diffusion upon charging may change the energy ordering and will be discussed later. Table 1 also shows the lattice parameters, together with the supercell volume and electronic band gap of the different superstructures. The volume of the supercell and the c/a ratio also increase with the Trace of the local order matrix. It is commonly accepted that a greater volume cell results in larger capacity for a given crystal structure of a specific cathode material.40 However, this might not be entirely true, and other factors should also be taken into account. Our results show that the interlayer spacing of the SS (Trace 0) structure is the largest among the five different groups of NCM configurations considered in this work (see Table 1) but the supercell volume always increases with the Trace of the layered material. The reason for such volume enlargement is the in-plane stretching of the TM distance along the y-axis (b-axis in Figure 1). Indeed, for the SS NCM configuration, the TM average distances are 2.95, 2.93, and 2.89 Å for Mn−Co, Co−Ni, and Mn−Ni, respectively, whereas for the PS NCM structure the distances are 3.04, 3.02, and 3.03 Å for Mn−Mn, Co−Co, and Ni−Ni, respectively. The average TM−O bond distance and the volume of the TM octahedra also increase with the Trace (see Table 2) with the exception of

Figure 2. DOS of the NCM111 superstructures studied in this work with their corresponding Traces.

their local order matrix. Although all of them present similar features, there are a few differences that might affect aspects like the electronic conductivity of the material. First, as corresponds to its almost full electronic d-shell (s1d9), the valence band maximum (VBM) is always in the Ni ions, which are to a large extent hybridized with the oxygen 2p states (see the projected DOS in the Figure S1 of the Supporting Information). Second, for the symmetric SS, the conduction band minimum (CBM) lies on the empty Co d-states, ∼1.0 eV above the Fermi level, whereas Mn and Ni empty d-states are 3 and 4 eV above the Fermi level, respectively. This fact implies that the main contribution to the electronic current arises from the Co dorbitals. As in a perfect SS there are no TM first nearestneighbors of the same type, this might negatively affect the charge transport with respect to the pure LiCoO2. If the Trace of the local order matrix increases, there will always be TM first nearest-neighbors of the same type. This interaction decreases the ligand-field splitting energy of the empty molecular orbitals and, consequently, they move toward the Fermi level, as can be noted in the Mn PDOS of the Trace 4 and the Ni PDOS of the Trace 6 NCM structures, respectively (see Figures 2 and S1 in the Supporting Information). For the PS NCM (Trace 18), the two effects are combined together: the Co subnetwork shows available empty states for charge transport at ∼1.5 eV above the Fermi level but on the other hand the Mn and Ni subnetworks can also show a significant contribution to the electronic conductivity. Thus, for the PS NCM structure the separation of the three TM subnetworks do not necessarily constitute a handicap. The different TM local environment is also reflected in the charge states of the TM ions, as was previously mentioned. A simple Bader charge analysis41 for the two limit cases (Trace 0 and 18) gives very different results: the charge states of the Ni, Co, and Mn ions in the SS model (Trace 0) are 8.72, 7.30 and 4.89e−, respectively. Thus, the valence state of the TM ions is Ni2+, Co3+ and Mn4+, as expected. There is one Co atom with a slightly different charge state, 7.47e−, as can be noted in the gap state of the DOS shown in Figure 2 (the formation of polarons will be discussed later). On the contrary, for the PS (Trace 18) model, some of the Ni ions show a charge state of 8.60e−, whereas most of the Mn atoms have a

Table 2. TM-Oxygen Average Bond Distance (in Å) and Average Volume of the TM Octahedra Oh (in Å3) for the SS (Trace 0) and PS (Trace 18) NCM111 Superstructuresa SS (Trace 0) Ni−O Co−O Mn−O (Ni−O)Oh (Co−O)Oh (Mn−O)Oh VLi (eV)

2.08 2.01 1.95 11.79 10.79 9.85 4.09 (Ni), 4.07 (Co), 4.13 (Mn)

PS (Trace 18) 2.01 2.03 2.05 10.72 11.06 11.49 3.41 (Ni−Co), 3.46 (Ni−Mn), 3.65 (Co−Mn)

a The three different Li vacancy formation energies are also shown (see text for details).

Ni, resulting in the larger volume of the PS NCM structure, even though the interlayer distance slightly decreases with the (increase of) Trace of the local order matrix. This is consequence of the different charge states of the TM ions, which depend on the local environment, as will be described at the end of this section. On the contrary, the different behavior along the other two orthogonal directions (a- and c-axis in Figure 1) can be easily explained from simple geometry considerations of the O3 LiMO2 structure: the TM tetrahedra form linear chains along the y direction but they are arranged in zigzag chains along the x- and z-directions. Recent experimental reports12 have emphasized the importance of improving the uniformity of chemical species at the atomic level in order to mitigate the voltage fade and energy degradation of layered structures. Modern hydrothermal assisted synthesis methods allow to control the uniformity of the chemical species and prevent surface segregation of one of the TM ions (particularly Ni), as compared to standard sol−gel or coprecipitation techniques. Our results clearly support this picture and, although the atomic structure of the NCM layered material is only the first step of the whole electrochemical D


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Figure 3. Tetrahedral phase diagram of the LiNi1−y−xCoyMnxO2 cathode material, showing the most thermodynamically stable phases at room temperature. Li and O share one of the edges.

charge of 5.10e−. Not surprisingly, this fact indicates that most of the Mn ions are in a Mn3+ valence state (resembling the LiMnO2 layered structure) and, consequently, some of the Ni ions must be already oxidized to a Ni3+ valence state. The change in the valence state has important consequences in the electrode potential (and the energy density of the cathode material), as we will discuss later. The one question that still remains open is that there are two different ways to model a NCM layered material with a TM phase separation structure: with each type of TM in separate layers, thus having three completely separated layered oxide systems with Ni, Co, and Mn in a TM3+ valence state and with the TM ions segregated but sharing at least one boundary within the same layer. This is the model that we adopted throughout this study, because it can show different features with respect to previously reported works on layered oxides.42 However, the lack of experimental evidence on the dependence of the electronic conductivity on the degree of uniformity of the chemical species does not allow to draw any definitive conclusion on this issue. Additional mechanisms for charge transport, like hole polaron formation in the VB will be examined later. So far, we have shown the effect of the TM ordering on the energetics and electronic structure of NCM111 layered materials. Obviously, the 111 stoichiometry is the only one that can lead to a perfect TM SS (Trace 0) but in order to find the thermodynamically stable NCM stoichiometries we need to sample the entire Ni1−y−xCoyMnx composition. Starting from the NCM111 SS, we modified the TM stoichiometry along the Mn/Co, Mn/Ni, and Co/Ni = 1:1 composition lines, always maximizing the TM mixing and then calculated the Gibbs free energies of all the structures. Finally, taking the convex hull of the obtained free energies, we plotted the corresponding pseudoquaternary phase diagram, as shown in Figure 3. The phase diagram shows the stable phases under certain conditions (O2 chemical potential at room temperature in this work and a 1:1 Li/TM ratio). On the other hand, Table S1 of the Supporting Information lists the total energies of all the NCM structures included in the calculations. The pseudoquaternary phase diagram shown in Figure 3 is projected onto a tetrahedron. Li and O2 compositions share

one of the edges, whereas Ni, Co, and Mn are represented in the other three axis of the tetrahedron. The phase diagram shows that among all the NCM stoichiometries studied in this work only 17 appear as thermodynamically stable phases. Besides the single compounds (LiNiO2, LiMnO2, and LiCoO2), two binary compounds are also stable phases at room temperature, LiNiMnO2 and LiCoMnO2. Finally, among the ternary compounds, obviously LiNi1/3Co1/3Mn1/3O2 but also (with all the TM subscripts normalized to the unit) L iNi 3 . 7 5 Co 2 . 5 Mn 3 . 7 5 O 2 , L iNi 4 . 2 5 Co 1 . 5 Mn 4 . 2 5 O 2 , L iNi2.5Co2.5Mn5O2, LiNi4.5Co1Mn4.5O2, LiNi1Co1Mn8O2, LiNi1Co4.5Mn4.5O2, LiNi1.25Co1.25Mn7.5O2, LiNi2Co2Mn6O2, LiNi0.5Co0.5Mn9O2, LiNi2Co4Mn4O2, and LiNi0.5Co9Mn0.5O2 are thermodynamically stable phases at room temperature. With these results in mind, few considerations are in order. First, the phase diagram has been obtained from the NCM SS (Trace 0) structure. There are entire families of NCM structures that do not appear as stable phases, like Ni-rich NCM (with compositions of Ni > 0.5). This is likely because there must be some segregation of Mn and Co ions from the Ni host structure in order to be thermodynamically stable at room temperature. Second, all the NCM stable phases are slight variations of the single or binary structures. Our results show that Mn content is always predominant (with the exception of LiNi0.5Co9Mn0.5O2). This can be understood as follows: along the Ni/Co = 1:1 composition line, the volume of the supercell decreases with increasing Mn content. This is usually correlated with a lower Li capacity of these materials.20,40 Generally, Mn is electrochemically inactive in the NCM superstructures, but it contributes to the structural stability of the LiMnO2 phase and its derivatives. On the contrary, increasing Ni content also increases the supercell volume and thus the capacity but it also might have adverse effects on the structural stability, leading to the segregation of Mn and Co ions or even inducing secondary phase transformations.13 Moreover, the ease with which Ni2+ ions occupy Li+ sites43 and the promptness with which it forms two-phase compounds9,10 significantly affects the cycle life and rate properties of these cathode materials. Then, according to this phase diagram, all the LiNi4±xCo2±yMn4±xO2 structures (with x and y ≈ 1) should be reasonable synthesis targets in terms of delivered capacity and thermodynamical and structural E


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Figure 4. Phase diagram in the chemical potential space of the most thermodynamically stable LiNi1−y−xCoyMnxO2 configurations.

stability. For larger capacity requirements, Ni content should be increased but it does not guarantee a good uniformity of the chemical species at the atomic level and, more importantly, the structural stability with cycling and rate capability are issues that should still be addressed. The results shown in Figure 3 perfectly complement the experimental work on the search for suitable layered materials with high reversible capacity aforementioned.11 Although such a report was primarily focused on “Li-rich” layered materials, the triangular area bounded by LiCoO2, LiMnO2, and LiNi0.5Mn0.5O2 encompasses most of the thermodynamically stable NCM stoichiometries in our study and the “Li-rich” region used as synthesis target.11 In absence of Ni-rich compounds, Mn-rich samples were shown to deliver the highest capacities. Also, “Li-deficient” materials (Li/TM ratio ≈ 1) with higher Co content can remain as single-phase layered materials, whereas Ni-rich materials readily form a coexisting spinel phase with the same degree of Li deficiency.11 Our obtained phase diagram clearly supports this picture. To further assess the electrochemical stability of NCM layered materials, Figure 4 shows the phase diagram in the chemical potential space of the convex hull-thermodynamically stable obtained configurations (shown in Figure 3). The plot shows the correlation between the NCM stoichiometries as a function of both the chemical potential of Li, μ(Li), and oxygen, μ(O). Metallic bcc Li, bulk fcc Mn, Co, and Ni, and O2 gas phase at 0 K are used as reference states, respectively. Therefore, the chemical potential of Li gives the negative of the voltage against Li|Li+ extraction/insertion.34 On the other hand, as the chemical potential of the oxygen gas phase strongly depends on pressure and temperature, the dotted line indicates the chemical potential, μ(O), at room temperature and ambient pressure, conditions of most of the synthesis experiments. The lines mark the equilibrium between two different phases, whereas three different phases coexist at the points where two lines intersect. The compounds with a stability window above the chemical potential reference state (μ*(O)) of oxygen are thermodynamically not stable at 0 K. The phase diagram shows several interesting features. All the NCM configurations are stable at relatively high voltages, but their temperature-oxygen partial pressure stability window differ significantly. LiCoO2 and LiCo0.5Mn0.5O2 show the widest domain of stability close to room temperature and atmospheric oxygen partial pressure conditions. Further increase of the oxygen chemical potential leads to LiNiO2 and all the NCM ternary configurations as stable compounds under high voltage conditions. LiMnO2

appears as the predominant phase at high temperature and oxygen partial pressure, which is consistent with the superior thermal stability and larger temperatures required for its synthesis, as compared to LiNiO2 and LiCoO2.42 Also, it agrees well with the fact than Mn ions constitute a source of stability for all the NCM compounds, as stated previously. Finally, delithiation leads to the formation of the different TM oxides. In the upper region of the phase diagram, overlithiation leads to the formation of both Li oxides: Li2O2 (under oxidizing conditions) and Li2O (upon reduction), which are the main reaction products of the activation mechanisms (Li vacancies and O evolution) of the redox activity of layered oxide cathode materials.31,44 Li Vacancy Formation and Reversible Electrode Potentials. The electrochemistry (especially the rate capability and the structural stability during cycling) of any family of cathode materials is always described in terms of the ability to successfully and rapidly extract and insert Li ions without affecting the host structure. In this section, we report the Li vacancy (VLi) formation energies and reversible electrode potentials (or operational voltages) of some of the NCM111 superstructures. Li vacancy formation can be regarded as the first voltage step of the charging process. We have focused our attention on the two limit cases of the NCM111 system (SS and PS, corresponding to Traces 0 and 18, respectively), as all the compounds with intermediate Traces show similar features to the NCM111 SS. Li vacancy formation energies strongly depend on the local structure, that is, on the TM nearestneighbors to the Li ion that has to be extracted. We have grouped all the possibilities for the SS and PS NCM materials into three different Li vacancy formation energies, depending on the local environment. For instance, as in our PS model the three TM are primarily distributed in different layers, there are three different possibilities for the local environment of the Li vacancy: Ni−Co, Ni−Mn, and Co−Mn. However, for the SS model the local environment is always formed of different TM. In order to simplify the description and compare the results with those of the PS model, we grouped the Li vacancy formation energies in three different cases depending on the predominant TM nearest-neighbor couple: Ni, Co, or Mn. Our results show that the obtained energies are in practice not affected by this choice. Table 2 lists the calculated Li vacancy formation energies for the SS and PS NCM111 compounds. The formation energies are substantially larger for the SS model, 0.5−0.7 eV, regardless of the TM local environment. The reason must be found on the difference in the charge states F


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which all the Ni ions will have been oxidized to the Ni3+ charge state. Then, the slope of the Trace 18 and Trace 6 NCM models drastically increases, because most (Trace 18) or some of the Mn ions (Trace 6), which are in Mn3+ valence state, oxidize to Mn4+, whereas all the Mn ions are already in Mn4+ valence state in the SS model (Trace 0). The difference in the voltage decreases to ∼0.3 V in this region (with 50−60% of the Li ions extracted). Finally, Ni and Co ions are oxidized to the TM4+ valence state. The voltage difference is about 0.25 V between the PS model (Trace 18) and the two solid solution models (Traces 0 and 6). The control of the uniformity of the chemical species at the atomic limit is then of fundamental importance to optimize the charging and discharging cycles of the cathode material. A similar 0.2 V voltage difference with increasing TM uniformity has been shown for Lithium-rich NM layered oxide cathode materials, see Figure 2 of ref 12. Finally, although the sloping regions correspond to the oxidation of TM ions, Figure 5 also shows two plateau regions at ∼4.3 V for the SS NCM materials (Traces 0 and 6). These plateaus are usually attributed to electrochemical reactions dictated by oxygen release46 or oxygen oxidation.47 Given that oxygen release should a priori be observed only in “Li-excess” materials, we can conclude that those plateaus emerge from additional oxygen oxidation. Indeed, our calculations show a strong distortion of the oxygen 2p electron cloud below a 30% of Li concentration, similarly to results previously found for Li-excess NM layered materials.15 Neither the plateau nor the distortion of the oxygen electron cloud are observed for the PS NCM material. Therefore, the TM ordering can drastically affect the delivered capacity and the rate capability of the layered material, making unnecessary the extraction of certain amount of Li ions (30%, according to our results) to achieve the highest capacity, thus helping to maintain the structural stability of the NCM material. Ionic and Electronic Mobility. Li+ diffusion in a cathode material is one of the key factors of the rate capability and cycling stability of the rechargeable Li-ion battery. Although the electrons travel through the external circuit, their mobility is also of fundamental importance as they merge with the Li+ ions in the cathode during the operation (discharging) of the battery. Thus, the electronic conductivity depends on two factors: the availability of charge carriers and their mobility. The ionic diffusion of Li+ ions has been widely studied in the literature (see, for example, refs 20 and 48−50). For single layered materials, all the reported kinetic barriers agree in considering LiCoO2 as the layered oxide with lower activation energy for Li+ diffusion, ∼0.30 eV. Then, a slightly higher barrier has been reported for LiNiO2, ∼0.40 eV, due to the shorter TM−O bond in the intermediate diffusion state. Finally, the activation energy for Li+ diffusion in LiMnO2 is the largest one, ∼0.6 eV in our calculations. The reason is, obviously, the strong structural distortion due to the Jahn− Teller effect of Mn3+ ions. As the activation energies for Li+ diffusion primarily depend on the local environment, especially on the closest TM to the migration path, the ionic mobility on the NCM SS model depends on the closest TM ion to the initial, final, and intermediate position of the diffusing Li+ ion (see Figure 6 for a sketch of the Li+ diffusion trajectory). Our results (see Table S2 in the Supporting Information) basically agree with the work by Luo et al.,20 and any single TM basically enhances the effect of the diffusion properties of the corresponding single layered oxide, for example, if the diffusing Li+ ion has a Co ion as first nearest-neighbor in the initial

of the surrounding TM atoms and, more specifically, whether they are electrochemically active (i.e., show redox activity) or not. For the SS model, there is no change in the redox state of the surrounding TM ions, that is, none of them are electrochemically activated and the removed electron is taken from the quasi-2D electron gas of the oxygen network. It is energetically very demanding to break the perfect symmetry of the Ni2+Co3+Mn4+ SS model, resulting in a relatively high voltage to remove the first Li atom. More Li ions must be extracted in order to electrochemically activate one of the TM redox couples. On the contrary, as shown previously, in the PS model some of the TM metals are already oxidized, resulting in a lower voltage for the extraction of the first Li atom. More specifically, depending on the position from which the Li atom is extracted (see Table 2), Mn3+ can be oxidized to Mn4+ (the VLi formation energies are 3.41 and 3.46 eV) and Ni2+ to Ni3+ (VLi is 3.65 eV). The reversible electrode potential to extract Lithium atoms from the host structure can be approximately calculated as the average potential of LixNCMO2 between two different compositions x1 and x2.45 Figure 5 shows the reversible

Figure 5. Charge plot of the NCM111 cathode material with three different TM arrangements: Trace 0 (SS), Trace 6 and Trace 18 (PS). The data has been interpolated with a cubic spline.

electrode potentials for three of the NCM111 materials shown previously: the two limit TM arrangements (SS and PS with Traces 0 and 18, respectively) and one of the intermediate cases (Trace 6). The extraction of Lithium ions was done in small steps of 10% of the total amount of Li atoms with the partially delithiated configurations obtained as the average of two different extraction pathways: the first one corresponds to the extraction of Li along the linear chains of the layered structure (the simplest pathway), and for the second one Li ions were gradually extracted starting from the sites with smaller Li vacancy formation energy (lower voltage), as described in the previous paragraph. The picture shows the electrode voltage as the average of both potentials. The results are extremely interesting and show clearly the effect of the atomic arrangement of the chemical species on the delivered electrochemical properties. The first part of the charging voltages correspond to the Ni oxidation (from Ni2+ to Ni3+) and, as described above, the required voltage is much higher for the SS (Trace 0) NCM material than for the PS (Trace 18) with the Trace 6 model showing an intermediate behavior. There is an ∼0.5 V voltage difference between the two limit cases during the initial 30−40% of the charging process, after G


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Table 3. Hole Polaron Formation in the SS (Trace 0) and PS (Trace 18) NCM111 Superstructuresa SS (trace 0) no perturbation Ni perturbation

Figure 6. Li diffusion trajectory in the NCM111 cathode material. Color code: Li, green; oxygen, red; Ni, gray; Co, blue; and Mn, purple balls.

−

PS (trace 18)

0.18*e , 0.00 eV, Co

0.19e , 0.18e−, 0.00 eV, Mn, Co

0.12*e−, −0.04 eV, Co

0.18*e−, 0.12e−, + 0.81 eV, Co, Ni

Co perturbation

0.30e−, + 0.63 eV, Co

Mn perturbation

0.18e−, 0.12e−, −0.03 eV, Co, Ni

−

(0.19e−, 0.18*e−, 0.19e−, + 0.17 eV, Mn, Co, Ni) 0.19e−, 0.18*e−, + 0.35 eV, Mn, Co (0.17e−, 0.19e−, 0.18*e−,+0.16 eV, Mn, Co, Co) 0.18e−, 0.18*e−, + 0.32 eV, Mn, Co (0.19e−, 0.24*e−, + 0.42 eV, Mn, Co)

a

The amount of charge carried by the polaron and the relative energies are shown (in units of the electron charge and eV, respectively), and the TM indicates the ion where the polaron is trapped. The results correspond to the lattice relaxation with or without an initial perturbation given to a specific TM, as shown. For the PS model, the initial perturbation can be introduced in a TM at the boundary between phases or within the corresponding TM-layered oxide phase (results shown in parentheses). The asterisk indicates a charge transfer process, rather than polaron formation.

position of the migration path, both Ni (0.05 eV) and Mn (0.25 eV) increase the activation energy if they are the closest TM to the transition state of the diffusion path. Similarly, Co ions decrease (0.05 eV) and Mn ions increase (0.20 eV) the kinetic barrier of a diffusion path starting close to Ni ions and both Co (0.22 eV) and Ni (0.10 eV) decrease the activation energy of the Li mobility if the path has Mn ions as first nearestneighbors of the initial position. Of course, the activation energies are only a small part of the whole picture of the ionic mobility in a cathode material, but at least they provide an idea about the difference between Li+ diffusion in the SS (Trace 0) and PS (Trace 18) NCM layered materials. The ionic conductivity in the PS NCM model is basically that of the corresponding single-layered oxide, whereas Li+ mobility will be substantially increased for a SS NCM cathode material. Indeed, the larger activation barrier (Mn oxide region) will always be lowered due to neighboring Ni or (specially) Co ions. Much more interesting is the effect of Li extraction on the electronic mobility. As lithium ions are always in a Li+ oxidation state, an extra hole is created in the system after the removal of one electron. This hole can be trapped by the lattice distortion and form a hole polaron, which is the quasiparticle formed by the hole itself and the lattice distortion induced in its surroundings.51 The charge transport (holes in this case) is therefore coupled with the migration of the hole polarons, which implies that the charge motion drags a series of lattice distortions that might ultimately increase the resistance to the motion and thus decrease the electronic conductivity. In order to investigate the possibility of polaron formation in NCM111 layered materials, we removed one electron of the two limit cases studied in this work, the SS (Trace 0) and PS (Trace 18) NCM materials. Although the semilocal character of the DFT GGA+U potential makes difficult the complete localization of the created hole in one of the TM ions, some general trends can still be obtained. Table 3 summarizes our results. For the SS NCM, no hole polaron is created after the extraction of one electron, that is, no additional charge carriers are trapped in the potential well of any of the TM ions and the hole is delocalized in the VBM of the system. However, if an initial perturbation is introduced in the TM−O bonds, a hole polaron can be formed in some of the TM ions. This initial perturbation can be regarded as a small thermal vibration with few millielectronvolts of activation energy, given the small differences with respect to the initially nonperturbed NCM111 material (see Table 3). Our results show that for the SS one of the Co ions can be further oxidized but with a relatively high cost in energy (0.63

eV), making the process very unlikely to happen. If the initial perturbation is given to the electrochemically inactive Mn−O bonds, a weak polaron is introduced in the form of a shallow state in the VB, primarily localized in two Co and Ni ions, at no cost of energy. The polaron is very mobile (with an activation energy of 0.21 eV, according to our calculations) and it should not deteriorate the overall electronic mobility. As an example, Figure 7 shows the polaron formation in the Co ion (charge

Figure 7. Hole polaron formation in a Co ion of the SS NCM111 structure and the corresponding DOS. Color code: Li, green; oxygen, red; Ni, gray; Co, blue; and Mn, purple balls.

density difference before and after the extraction of the electron) and the corresponding DOS. The picture shows the presence of a gap state on the band gap of the SS NCM compound after the removal of the electron. Such defect state is highly mobile (it is located close to the CBM) and it can be easily detected by means of photoelectron spectroscopy (PES).52 However, for the PS NCM111 material the situation is completely different. The boundaries between different TM regions facilitate the formation of hole polarons. If no initial H


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The Journal of Physical Chemistry C perturbation is introduced in any of the TM−O bonds, one of the Mn ions almost oxidizes to a Mn4+ valence state and another of the Co ions is partially oxidized from the initial Co3+ valence state. Both ions are located in the Mn−Co boundary region. To introduce a small perturbation in the TM−O bonds, we differentiate two options: a TM from the boundary region and a nearest-neighbor that does not belong to the boundary with another chemical species. For instance, if an initial perturbation is given to one of the Mn−O bonds, a hole polaron is formed in both situations with the subsequent oxidation of the corresponding Mn ion. For Co, the polaron cannot be formed at the boundary, only in the inner region of the Co oxide. Finally, Ni polarons can also be formed at the boundary and the inner region of the Ni oxide. The mobility of these polarons is slightly lower than that of the SS NCM material with activation energies of 0.36 and 0.41 eV for Mn and Ni, respectively. Polaron transport across TM boundaries still needs to be investigated carefully. These results clearly highlight the importance of the chemical composition at the atomic level. Boundaries are almost impossible to avoid during a standard synthesis process, and the electronic transport of some carriers (holes in the present study) can be severely hindered through any particular boundary within the cathode material, depending on the TM species. For the specific material studied in this work, NCM111, only a perfect solid solution mixing of the chemical species at the atomic level could avoid the problem. For this reason, coating materials (to avoid decomposition of the cathode materials and TM dissolution into the electrolyte) should also be chosen carefully.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +14697321856; . *E-mail: [email protected]. Phone: +19728832845. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Samsung GRO project. The authors also acknowledge the Texas Advanced Computing Center (TACC) for providing the computational resources. The software used to generate the phase diagram shown in Figure 3 is based on the original Matlab application developed by S. P. Ong.33

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REFERENCES

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CONCLUSIONS In conclusion, we have shown that the TM arrangement strongly affects the structural, electronic, and electrochemical properties of NCM layered materials. A perfect SS (Trace 0) is the most thermodynamically stable phase. Our obtained phase diagram shows the most reasonable targets for the synthesis of NCM cathode materials. Mn ions always enhance the structural stability of the NCM structures, although most of them are electrochemically inactive. If NCM materials for larger capacity applications are desired, such as Ni-rich NCM, this will result in the segregation of Mn and Co ions. Besides, the electrochemical activity of Ni2+ ions may induce structural instability and secondary phase transformations, producing two-phase materials.11 The reversible electrode potentials strongly depend on the arrangement of the chemical species at the atomic level. By optimizing the TM mixing, the voltage can be raised up to 0.5 V with the corresponding increase of the energy density (which depends on both delivered capacity and operational voltage). Finally, ionic and electronic mobility can also be increased with an appropriate tuning of the TM mixing. Then, the control of the chemical species, and particularly the TM distribution, seems to be crucial to obtain NCM layered materials with the desired properties in terms of structural stability and rate capability during the operational life of the Li-ion battery. Modern hydrothermal assisted synthesis methods12 point to the right direction to achieve this objective.

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Total energies of the NCM structures, PDOS of the NCM111 layered materials, and kinetic barriers for TM diffusion. (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02240. I


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