Unraveling the Origin of Instability in Ni-Rich LiNi1–2xCoxMnxO2

Mar 3, 2016 - The good agreement between calculated and fitted mixing energy indicates our bond model is suitable for describing the phase stability o...
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Unraveling the Origin of Instability in Ni-Rich LiNi1−2xCoxMnxO2 (NCM) Cathode Materials Chaoping Liang,† Fantai Kong,† Roberto C. Longo,† Santosh KC,† Jeom-Soo Kim,‡ SangHoon Jeon,§ SuAn Choi,§ and Kyeongjae Cho*,† †

Materials Science & Engineering Department, The University of Texas at Dallas, Richardson, Texas 75080, United States Department of Chemical Engineering, Dong-A University, 37 Nakdong-Daero 550beon-gil, Saha-gu, Busan 604-714, Republic of Korea § L&F Material Co., Ltd., 120, Dalseo-daero 91-gil, Dalseo-gu, Daegu 704-948, Republic of Korea ‡

S Supporting Information *

ABSTRACT: In this work, phase stability of Ni-rich LiNi1−2xCoxMnxO2 (NCM) (x < 0.20) is investigated by means of a bond model based on the effective interaction of transition metal (TM) ions (represented as TM−TM bond), fitted to results obtained within the DFT+U framework. The developed bond model reveals the intrinsic relationship between phase stability and TM−TM bonds, which explains the different roles of TM ions in the phase stability of Ni-rich NCM. A sequence of TM− TM bond strengths (Mn4+Mn4+ > Ni2+Mn4+ > Ni3+Mn4+ > Co3+Mn4+ > Co2+Mn4+ > Ni2+Ni4+) is then predicted by bond model and subsequently used to understand the intricate layered LiTMO2 (TM = Ni, Co, Mn) phase triangle. Our results also show that Co and Mn ions segregate to form clusters within the Ni environment when x < 0.1 (Ni ≥ 80 at. %), and such segregation is responsible for the electrochemical degradation during cycling. The obtained results agree excellently with the validation experiment in the present work and also other experiments in the literature, and could help to clarify the existing controversies about the origin of the instability of Ni-rich NCM compounds. Finally, we show that, by tailoring the TM−TM interaction, i.e., the atomic uniformity of the as-synthesized cathode material, the electrochemical stability of the Ni-rich NCM can be substantially improved.

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An individual TM ion (Co2+, Mn3+, Ni2+, Ni4+, etc.) is usually proposed by experimentalists to be responsible for the instability of not only Ni-rich NCM but also all layered cathode materials.1−10 This is mainly due to the complex valence of TM ions (Ni2+, Ni3+, Ni4+, Co2+, Co3+, Mn3+, Mn4+) in NCM caused by different electronegativities and charge redistribution. As a result, the role of each TM ion in the stability of NCM materials is still poorly understood and under debate. For instance, some experiments showed that Ni2+ in Nirich NCM, which has a similar ionic radius with Li+, would occupy the Li+ sites during the synthesis or cycling, resulting in spinel and rock-salt phase transition and inferior rate performance.5−7 On the contrary, others found out that Ni2+ can stabilize the layered structure of Ni-rich NCM because of its strong bond with oxygen. Ni3+ and Ni4+ which have weak bonds with oxygen are thought be unstable in layered structure.8,9 Besides, Mn4+ is always considered to improve phase stability of Ni-rich NCM, while Mn3+ is detrimental to structural stability due to Jahn−Teller distortion.10

INTRODUCTION

During past decades, LiNi1−2xCoxMnxO2 (x < 0.20) (Ni-rich NCM) has been explored as the most promising alternative to current layered oxide cathode materials on account of its high energy density (>200 mA hg−1 at high voltages 4.3−4.6 eV). The synergistic effect of the three transition metals (TM) and many possible Ni:Co:Mn compositional options make it feasible to inherit the merits of each component material (LiTMO2, TM = Ni, Co, and Mn) and even prevail in the overall performance.1,2 For example, Ni ions are the electronactive species and contribute to a higher capacity, while Mn ions at 4+ help to maintain the structural stability of the αNaFeO2 phase and the Co ion improves the rate performance and processing ability.1,2 To meet the requirement for portable electronics and especially electric vehicles (EVs), Ni contents in LiNi1−2xCoxMnxO2 (NCM) cathode materials have been pushed from NCM333 (x = 0.33) to NCM811 (x = 0.1) for reaching a capacity higher than 200 mA hg−1. Even though increasing Ni content results in an increase of specific discharge capacity, the corresponding capacity retention and safety characteristics, especially the phase stability, sharply deteriorate when Ni ≥ 80 at. % (See Supporting Information for the experimental evidence.)3,4 © 2016 American Chemical Society

Received: January 12, 2016 Revised: February 11, 2016 Published: March 3, 2016 6383

DOI: 10.1021/acs.jpcc.6b00369 J. Phys. Chem. C 2016, 120, 6383−6393

Article

The Journal of Physical Chemistry C

temperature smearing method of Methfessel−Paxton28 was used for the dynamical calculation, and the modified tetrahedron method of Blö chl−Jepsen−Andersen29 was performed for the static calculation. For all the compounds, a k-point mesh was used to ensure a convergence of 1 meV per unit cell and to make sure there is the same density per unit volume for the reciprocal lattice. In each calculation, periodic boundary conditions are added in three directions, and the supercell is allowed to fully relax. The energy criteria are 0.01 and 1 meV for electronic and ionic relaxations, respectively, while 0.01 meV is for the static calculation. All calculations are spin-polarized, and only ferromagnetic (FM) configuration is considered. In first principles calculation, GGA is known to contain errors in the calculation of the electrochemical properties of TM oxides. By explicitly including onsite coulomb, U, and exchange, J, terms in the Hamiltonian (GGA + U approach), one can partially correct the electron overdelocalization (and the self-interaction errors) and accurately calculate the electrochemical properties.30 The U parameter has been obtained by means of a linear response method,30,31 and the U values 6.8, 5.9, and 5.2 eV, which are used in our previous work,32 are adopted for Ni, Co, and Mn ions in the present work, respectively. The J value was set to 0 eV in all cases, as only the U−J difference is relevant for the calculations. b. Structural Model. The crystal structure of NCM materials belongs to the R3̅m space group derived from the protype of α-NaFeO2. Figure 1a−c shows the three atomic

On the other hand, the effective interaction of TM ions has proven to play a more important role in the phase stability of NCM materials. Goodenough and Kanamori have demonstrated that TM ions, in terms of ligand-field theory, interact with each other directly through edge-sharing [TMO6] octahedra and indirectly through 90° or 180° TM−O−TM magnetic superexchange.11−13 These effective interactions strongly correlate with the phase stability of LiNi0.5Mn0.5O2, and a qualitative sequence Ni2+−Mn4+ > Mn4+−Mn4+ > Ni2+− Ni2+ is proposed to explain the stability of some local structures.14,15 Furthermore, using cluster expansion, strong Ni−Mn interaction has been used to explain the “zigzag” and “flower” local structure in LiNi0.5Mn0.5O2 by Ceder et al.16,17 and Ni/Mn ordering (P432 phase) in high voltage spinel LiNi0.5Mn1.5O4 by Lee et al.,18 respectively. As NCM materials are often regarded as a solid solution of LiTMO2 (TM = Ni, Co, Mn), TM−TM interactions should play the same role in the phase stability of Ni-rich NCM.19,20 Therefore, understanding the interaction of TM ions in NCM materials, especially Ni-rich NCM, is critical for the rational design and application of high capacity Ni-rich NCM cathode. The difficulty in understanding the interaction of TM ions in Ni-rich NCM originates mainly from the inaccuracy in determining the valence of TM ions. For instance, by means of X-ray absorption near edge spectra, Lee et al. reported that the valence of TM ions in LiNi1−2xCoxMnxO2 is determined by the relative Ni:Co:Mn ratio, such as Mn ions that are trivalent at x < 0.2 and begin to exist in the tetravalent form from x ≥ 0.2.2,21 Whereas other experimental observations showed that Mn ions in LiNi1−2xCoxMnxO2 always appear as tetravalent over the entire range of x, and the same amount of Ni ions is reduced from Ni3+ to Ni2+ to maintain charge balance.3,22 In addition, due to the charge disproportion of Ni3+, a certain amount of Ni3+ changes into Ni2+ and Ni4+ through lattice distortion23 or defect structure.24 In order to understand the origin of instability in Ni-rich NCM and clarify the above controversies in the literature, the effective interaction of TM ions in Ni-rich NCM has been investigated quantitatively in this study. On the basis of Goodenough and Kanamori’s rule, the effective interaction between the two nearest TM ions is defined as a TM−TM bond in this work, and a bond model is thus proposed for the first time to describe those TM−TM bonds (i.e., effective interactions). The bond strength obtained from our bond model is used to examine the effects of TM−TM bonds on the phase stability of NCM materials and to compare results from the validation experiment in the present work with other available experimental and theoretical results in the literature. Finally, using the developed bond model, we will examine the intrinsic mechanism that leads to the origin of the instability of Ni-rich NCM (Ni ≥ 80 at. %), in an attempt to find an effective way to improve the phase stability of Ni-rich NCM.

Figure 1. Structural model of α-NaFeO2 layered structure (R3̅m) represented in different lattices: (a) conventional R3̅m(H) lattice, (b) primitive R3̅m(R) lattice, (c) C2/m based supercell.

models which are widely used in simulation, conventional cell R3̅m(H), primitive cell R3̅m(R), and C2/m based supercell, respectively. These three atomic models have been tested for the phases like LiCoO2, LiNiO2, and LiMnO2, and the calculated bulk properties of these three structures show almost the same results. To reduce the computing resources, C2/m based supercell is adopted in the present study. The atomic configuration is following the methodology proposed by Luo et al.,33 that NCM superstructures are characterized by the traces of their local order matrix. The clustering tendency (i.e., the arrangements of the TM ions in neighboring sites) is described by the trace which counts the number of TM ions in the nearest-neighbor around each TM ion in the TM layers. Specifically, the solid solution (SS) phase in this study is the one with the smallest trace and uniformly distributed TM ions in the slab (as seen from individual

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COMPUTATIONAL METHODS a. First Principles Calculation. First principles calculation is performed on the well-established Vienna ab initio simulation package (VASP) based on density functional theory (DFT).25 The calculation is based on a plane-wave basis, using the projector-augmented wave (PAW) method.26 Exchange and correlation items are described by generalized gradient approximation (GGA) of Perdew et al.,27 and cutoff energies 500 eV and default value are adopted for plane-wave basis and augmentation charge, respectively. For k space integration, the 6384

DOI: 10.1021/acs.jpcc.6b00369 J. Phys. Chem. C 2016, 120, 6383−6393

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The Journal of Physical Chemistry C transition metal atoms, the first nearest-neighbors are similar for the same TM ion). The structures with higher trace structures or nonuniformly distributed TM ions are all denoted as cluster structures. Specifically, a higher trace means the structure has more severe atomic segregation, and vice versa. Various supercell sizes are chosen to model various traces at the same Ni concentration. For instance, a 3 × 4 supercell, which contains in total 12 formula LiTMO2 units on one slab, is used for modeling a Ni:Co:Mn ratio of 10:1:1, and then a 4 × 6 supercell, which consists of 24 formula LiTMO2 units on one slab, is adopted for modeling a Mn or Co pair interaction at Ni:Co:Mn ratio of 20:2:2. Our tests show that no matter how large the supercell size is, the energies of the same atomic configuration are always the same, while those of different configurations differ from each other. The test results prove the feasibility of our supercell model and the consistency of first principles calculations. c. Bond Model. In the present study, the effective interactions which involve the two nearest TM ions on the TM layer are defined as a TM−TM bond, and a bond model is developed by fitting the mixing energy into a polynomial of all TM−TM bonds. It should be noted that only direct TM−TM and 90° TM−O−TM in-plane interactions are taken into consideration in the bond model. The 180° TM−O−TM interactions, corresponding to cation mixing, are excluded in our bond model for this is often regarded as a defect state which plays a less important role in the phase stability of Nirich NCM. Our main goal of the bond model is to determine the bond strength of TM−TM bonds. The procedures of our bond model are listed below. The energies of selected structures are first calculated using the GGA+U method. These structures are with the lowest trace, highest trace, and middle trace, which may provide the upper and lower limits of the energy. These calculated energies are then expanded in the polynomial of all TM−TM bonds to − TM obtain the TM−TM bond strength ETM i Ω TM − TM

Efit =

∑

Ni × Ei TM − TM

eq 1. (3) A new number of TM−TM bonds is generated by applying a new random vector; this procedure iterates until there is no lower energy appearing in 100 000 iterations. The structures with lower or close energy predicted from the above procedure are chosen to perform another DFT calculation. If those calculated DFT data differ from the predicted values, the newly calculated DFT data together with the old DFT data are put into bond fitting again to update the bond strength. The fitting iterates several times until there is no new low energy structure predicted. A total of 90 DFT calculated energies are used to get the bond strength. Three structures out of 90 with a large energy difference between DFT and the predicted results are eliminated from fitting.

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RESULTS AND DISCUSSION a. Valence Distribution of Ni-Rich NCM. The valences of TM ions are first determined through first principles calculation. The methods used to determine the valence mimic the experimental measures (compare electronic structure, magnetic moment, and bond length of TM ions in NCM with those in reference oxides).22,35 As a typical example, Figure 2 shows the partial density of states (pDOS) for Ni ions

(1)

i − TM ETM i

where Efit and are the fitted energy and the bond strength of the TM−TM bond, respectively, Ω TM−TM represents all TM−TM bonds found in Ni-rich NCM, and Ni is the number of specific TM−TM bonds. The cross validation score (CV) is used as the convergence rule for the fitting34 ⎧1 CV = ⎨ ⎪ ⎩n ⎪

⎫1/2 ∑ (Efit(i) − Emixing(i)) ⎬⎪ ⎭ i=1 n

2⎪

(2)

Figure 2. Partial density of states (pDOS) of Ni ions (Ni2+, Ni3+, and Ni4+) from LiNi0.8Co0.1Mn0.1O2 (NCM811), in comparison to those from reference oxides (Ni2+ from rock-salt NiO, Ni3+ from C2/m LiNiO2, and Ni4+ from C2/m NiO2).

where Emixing(i) is the calculated mixing energy of atomic structure i, while Efit(i) is the fitted value of the mixing energy of structure i obtained from a least-squares fit of the (n − 1) other mixing energies. A CV score less than 0.01 eV/formula unit is used to terminate the polynomial fitting. The TM−TM bond strengths, obtained from the above fitting procedures, are used to predict the energy of the structures which are not included into the bond model fitting. We adopt a method similar to Monte Carlo and machine learning algorithm to predict the energy. The method is programmed using MATLAB, and the main idea is presented as follows: (1) The number of TM−TM bonds is determined by a random vector. (2) The number of TM−TM bonds and the TM−TM bond strengths are used to calculate the energy using

in NCM811 and the reference oxides (Ni2+ from rock-salt NiO, Ni3+ from C2/m LiNiO2, and Ni4+ from C2/m NiO2), respectively. From this figure, the pDOSs of Ni ions in NCM are similar to those in the reference oxides in terms of the location and intensities of the main peaks. The electronic configurations of Ni2+, Ni3+, and Ni4+ are found in the low spin states t62ge2g (S = 1), t62ge1g (S = 1/2), and t62ge0g (S = 0), respectively. The corresponding magnetic moments of Ni2+, Ni3+, and Ni4+ are 0, 1.12, and 1.79 μB, respectively, which are in good agreement with other calculations36 and experimental 6385

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Table 1. Electronic Structure, Magnetic Moment, and TM−O Bonding Length of TM Ions from LiNi1−x−yCoxMnyO2a magnetic moment (μB) TM ions Ni2+ Ni3+ Ni4+ Co3+ Co2+ Mn3+ Mn4+

electronic structure t62ge2g t62ge1g t62ge0g t62ge0g t42ge2g t52ge2g t32ge1g t32ge0g

(S (S (S (S (S (S (S (S

= = = = = = = =

1) 1/2) 0) 0) 2) 3/2) 2) 3/2)

TM−O bond length (Å)

oxides

NCMb

oxidesc

NCMb

exptd

1.79 1.12 0 0 3.18 2.78 3.91 3.40

1.50−1.79 1.00−1.30 0.00−0.50

2.11 1.90 × 4, 2.14 × 2 1.88 1.93 2.02 2.12 1.93 × 2, 2.13 × 4 1.95

2.05−2.11 1.97−2.01 1.87−1.90

2.08 1.99 1.88 1.95 2.02 2.13 2.05 1.94

3.00−3.29 2.50−2.90 3.50−3.91 3.20−3.40

2.01−2.03 2.07−2.12 2.03−2.07 1.92−1.96

a

Those from Ni oxides are also listed for comparison. bThe range of atomic magnetic moment and TM−O bond length is used to determine the valence of TM ions. cThe TM ions from the corresponding oxides are as follows: Ni2+ from rock-salt NiO, Ni3+ from C2/m LiNiO2, Ni4+ from C2/m NiO2, Co3+ from R3̅m LiCoO2, Co2+ from rock-salt CoO, Mn3+ from C2/m LiMnO2, Mn4+ from C2/m MnO2, respectively. dReference 38.

Figure 3. Valence state distribution of LiNi1−2xCoxMnxO2 phases (a) x = 1/3 (33.33 at. % Ni) with solid solution (SS) structure, (b) x = 1/4 (50 at. % Ni) with SS structure, (c) x = 1/4 (50 at. % Ni) with cluster structure, (d) x = 1/5 (60 at. % Ni) with cluster structure, (e) x = 1/8 (75 at. % Ni) with SS structure, and (f) x = 1/10 (80 at. % Ni) with SS structure. 6386

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The Journal of Physical Chemistry C results.37 Bond lengths between TM ions and oxygen ions are another common way to differentiate the valences of TM ions. As listed in Table 1, the TM−O bond lengths from first principles calculations match well with experimental results.38 For example, bond lengths of Ni3+−O, Co3+−O, and Mn4+−O bonds are 1.98, 2.02, and 1.95 Å, which are in excellent agreement with experimental values 1.99, 2.02, and 1.94 Å, respectively.38 The good agreements between calculation and experimental results suggest that first principles are suitable to unveil the valence of TM ion in layered oxide cathode materials. After a series of calculations, the valence distribution of LiNi1−2xCoxMnxO2 has been obtained, and some specific valence distributions are shown in Figure 3. For SS phases, several rules could be discerned from this figure. First, over the whole Ni concentration, Mn and Co always exist in 4+ and 3+, respectively, and the Ni2+:Co3+:Mn4+ ratio stays 1:1:1, unchanged. Figure 3a shows that Ni2+, Mn4+, and Co3+ in NCM333 are distributed in a [√3 × √3]R30° superlattice with Ni2+:Co3+:Mn4+ = 1:1:1, which matches well with experimental observation and other theoretical results.39−41 Second, the number of Ni2+ decreases consecutively from 100% in NCM333 (x = 1/3) to 0% in LiNiO2 (x = 0), while Ni3+ changes in the opposite way. Third, Ni2+ always situates between Co3+ and Mn4+ ions in the first nearest-neighbor at x ≥ 1/8 (Ni ≤ 75 at. %), as shown in Figure 3b,e. At x < 1/8 (Ni > 75 at. %), as shown in Figure 3f, the Ni2+ ion only appears in the first nearest-neighbor of Mn4+ ions, while the Co3+ ion is surrounded by Ni3+ ions, indicating Ni3+ is reduced to Ni2+ in the presence of Mn4+. For cluster structure with higher trace or nonuniformly distributed TM ions, atomic environment and Mn/Co cluster size are found to influence the valence distribution. For example, Figure 3b,c shows that the charge disproportion of Ni3+ happens at x = 1/4 due to the change of atomic environment. In this case, two Ni3+ ions are changed into one Ni2+ and one Ni4+ when they have different atomic environments (Ni2+ is surrounded mainly by Mn and Co3+ ions, while Ni4+ is surrounded mainly by Co and Ni ions) in comparison to the SS phase in Figure 3b. In the present work, charge disproportion of Ni3+ is observed in the cluster structure over the whole Ni concentration range. On the other hand, Figure 3d shows that Mn3+ ions appear when forming a larger Mn cluster. The minimal cluster size for the existence of Mn3+ is found to be five, which means that when a Mn ion is surrounded by more than 4 Mn ions, it is reduced from Mn4+ to Mn3+. As mentioned before, there are controversies regarding the valence of TM ions in Ni-rich NCM; e.g., at x < 0.2, the valence of Mn ions obtained from Lee et al.21 is trivalent, while it is tetravalent from others.3,22 Our results show that Mn ions always appear tetravalent in the SS phases, which agrees well with Mn4+ found in recent experiments,3,22 while it is contradictory to Mn3+ from Lee et al.21 On the other hand, Mn3+, as shown in Figure 3d, does exist in Mn cluster structures. It is noted that Mn3+ has also been observed from experiment in LiNiyCo1−2yMnyO2 (y = 1/3 − 1/2) when forming a Mn-rich cluster.42−44 The above features suggest that the Mn4+ and Mn3+ observed in Ni-rich NCM may be due to the structures being obtained from different experimental conditions.45 All the above evidence implies that the valence of TM ions in Ni-rich NCM is determined by the atomic environment, and such a statement could give a reasonable

explanation to the above-mentioned controversy regarding the valence of TM ions in the literature. The valence state of TM ions influenced by atomic environment indicates that TM ions interact directly with each other. The cooperative distortion of TM−O bond lengths provides additional evidence for the TM−TM interaction in Ni-rich NCM. For instance, Ni3+, a Jahn−Teller active ion, situates at the center of a distorted octahedron with 4 short 1.90 Å and 2 long 2.14 Å Ni3+−O bonds. When it shares an edge with the Co3+ ion in the nearest octahedron, the Co3+ octahedron experiences almost the same distortion. The Co−O bond which shares the O ion with the long Ni−O bond will shorten to certain extent (0−1.2 Å), and those with short Ni−O bonds will likewise stretch. The average Co−O bond length is almost the same as that in the reference oxides (LiCoO2). The Li−O bond length, on the contrary, stays almost a constant value of 2.12 Å, irrespective of the TM ions which share the same oxygen. This indicates TM ions interact with each other mainly on the TM layer, while the Li+ ions have less of an influence on the interaction of TM ions,46 and provide direct support for our assumption that the phase stability of Ni-rich NCM is strongly related to TM−TM interaction. b. Phase Stability and Bond Strength. To understand phase stability of the structures with different valence distribution, the total energies of 90 LiNi1−2xCoxMnxO2 structures are calculated at Ni concentration x = 1/24, 1/12, 1/10, 1/8, 1/6, 1/4, and 1/3. Many TM atomic arrangements generated by the traces of their local order matrix are considered at every concentration. In the present study, the lowest trace with uniformly distributed TM ions is denoted as SS phase. The mixing energy, Emixing, which is widely used to determine phase stability,39,47 is calculated according to the following formula Emixing = Etot − (1 − 2x)E LiNiO2 − xE LiCoO2 − xE LiMnO2 (3)

where Etot, ELiNiO2, ELiCoO2, and ELiMnO2 are the total energies of LiNi1−x−yCoxMnyO2, LiNiO2, LiCoO2, and LiMnO2, respectively. A negative Emixing value means the atomic configuration is stable at this concentration from the point of view of thermodynamics, while a positive value indicates a phase is unstable with respect to LiCoO2, LiNiO2, and LiMnO2 phase segregation. Accordingly, Emixing values of LiNi1−2xCoxMnxO2 structures are derived and plotted in Figure 4. It could be seen that almost all LiNi1−2xCoxMnxO2 structures considered have negative mixing energies, indicating that those structures are stable with respect to phase segregation into LiCoO2, LiNiO2, and LiMnO2. The ground-state structure can be determined by comparing the mixing energies at that concentration. SS phases are energetically favorable from Ni = 33.33 (x = 1/3) to 75 at. % (x = 1/8), while cluster structures become more stable at Ni ≥ 80 at. % (x ≤ 1/10). The mixing energies of ground-state structures almost linearly increase as a function of Ni concentration, which is consistent with phase stability determined by differential scanning calorimetry (DSC).3 It can also be observed that above Ni = 80 at. % the mixing energy of different structures is quite similar in many cases. This behavior will be discussed in the next section. All calculated mixing energies are then used to fit our bond model (details are described in Computational Methods). The fitted mixing energies are also listed in Figure 4. It could be 6387

DOI: 10.1021/acs.jpcc.6b00369 J. Phys. Chem. C 2016, 120, 6383−6393

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Second, Mn4+ has strong bonds with other TM ions. The bonds that involved Mn4+ all have negative bond strength. This feature is consistent with the experimental observation that Mn4+ is beneficial for the stability of layered NCM materials.3,50 The sequence of Mn4+−TM bonds is Mn4+Mn4+ (−22.09 kJ/ mol) > Ni2+Mn4+ (−15.28 kJ/mol) > Ni3+Mn4+ (−9.15 kJ/ mol) > Co3+Mn4+ (−8.95 kJ/mol) > Co2+Mn4+ (−7.17 kJ/ mol). The difference between Mn4+−TM bonds is large, like Ni2+Mn4+ (−15.28 kJ/mol) and Co2+Mn4+ (−7.17 kJ/mol), indicating that the beneficial effect of Mn4+ is not from the individual Mn4+ ions but from the strong bond between Mn4+ and other TM ions. However, the Mn4+Mn4+ bond alone cannot satisfy the condition of charge balance in layered structure, as shown in the previous section; it will be reduced to Mn3+ when forming a Mn cluster. This is the reason that the layered Li2MnO3 phase is very stable and forms ordered “honeycomb” structure by maximizing the number of Mn4+Mn4+ bonds but maintaining charge balance with Li+ on the TM layer.16,17 Third, except those bonds with Mn4+, the bonds which consist of Mn3+ and Co2+ are weak for the positive bond strengths. It could be deduced that the Mn3+ that comes from Mn segregation is detrimental to the stability not only from the Jahn−Teller effect but also from the weak bond with other TM ions. The weak bond between Co2+ and other TM ions (except Mn4+) is consistent with the experimental observation that the existence of Co2+ leads to degeneration of Li[NiCo]O2 cathode materials.51 Fourth, Ni−Ni bonds have different bond strengths when forming a Ni−Ni bond. The Ni2+Ni4+ bond is strong. However, when Ni2+ and Ni4+ do not bond together, the remaining Ni3+ will form a weak bond with a positive value with Ni2+ or Ni4+. The strong Ni2+Ni4+ bond is the reason that charge disproportion of Ni3+ happens when that atomic environment changes. From above analysis, it could be concluded that the phase stability of NCM materials is determined by the TM−TM bonds, instead of the individual TM ion. The strong Mn4+−TM bonds are the reason why Mn4+ can improve the phase stability. The detrimental effect of Mn3+ on structural stability is due to weak Mn 3+ −TM bonds. This is consistent with the experimental observation about the beneficial and detrimental role of Mn4+ and Mn3+, respectively.3,50 The TM−TM bond strength revealed from the present study suggests that the bond between TM ions should have a fundamental effect on the phase stability of Ni-rich NCM, which agrees well with experimental observations, and would probably clarify the above-mentioned controversy regarding the effect of TM ions on the phase stability of Ni-rich NCM in the literature. TM−TM bond strengths are used to predict the phase stability of NCM materials. Figure 6 shows the predicted mixing energies for SS NCM phases using TM−TM bond strengths (details are described in Computational Methods). A detailed comparison between the predicted phase stability and available experimental evidence is performed in order to understand the intricate layered LiTMO2 (TM = Ni, Co, Mn) phase triangle. First of all, Figure 6 shows that LiCoO2, LiNiO2, and LiMnO2 are immiscible around the LiMnO2 area. Specifically, the immiscible area in the phase triangle is from z = 1 to 0.16 in LiMnzCo1−zO2 and x = 0−0.27 in LiNixMn1−xO2, respectively. The solid solubility limit for LiMnzCo1−zO2 (z = 1−0.16) is in good agreement with the experimental value z = 1−0.20.52 The solid solubility of Mn in LiCoO2 is due to the strong Co3+Co3+ (−2.2 kJ/mol) which

Figure 4. Mixing energy of LiNi1−2xCoxMnxO2 (x = 1/24, 1/12, 1/10, 1/8, 1/6, 1/4, and 1/3) from first principles calculation and bond fitting. The solid solution (SS) phases and phase segregation (PS) are singled out for comparison.

seen that the agreement between calculated and fitted mixing energies is good, and the difference is less than 1 kJ/mol (0.01 eV/formula unit). More importantly, bond fitting reproduces the relative mixing energy between different atomic structures at Ni ≥ 80 at. %. The good agreement between calculated and fitted mixing energy indicates our bond model is suitable for describing the phase stability of Ni-rich NCM. Figure 5 shows all TM−TM bond strengths obtained from our bond model. In this figure a negative value means the bond

Figure 5. Bond strength for various transition metal bonds in terms of mixing energy. A negative energy means stronger bonding, and vice versa.

is strong and stable, while a positive value indicates the bond is weak and unstable. Several features can be derived from this figure. First, TM3+TM3+ bond strengths follow Co3+Co3+ > Ni3+Ni3+ > Mn3+Mn3+. The sequence of three TM3+TM3+ bonds indicates that the relative stability of LiTMO2 (TM = Ni, Co, Mn) phases is LiCoO2 > LiNiO2 > LiMnO2. The weak Mn3+Mn3+ bond (4.28 kJ/mol) makes LiMnO2 even unstable in ambient conditions. This is consistent with experimental evidence that LiCoO2 and LiNiO2 crystallize into a stable layered structure, while layered LiMnO2 is unstable and ultimately change into a spinel structure.2,48,49 6388

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phase stability than LiCoO2 when y ≥ 0.2.55−57 All the good agreements above therefore suggest that phase stability is determined by the sequence of TM−TM bond strengths, which could not only help us understand the intricate layered LiTMO2 (TM = Ni, Co, Mn) phase triangle, but also provide helpful guidance on rational design of cathode materials for experimentalists from the viewpoint of phase stability. c. Atomic Segregation of Ni-Rich NCM. As valence distribution and TM−TM bonds have been properly described in the above sections, we now turn to investigate the phase stability of Ni-rich NCM in terms of our bond model. The relative stability of different structures, as shown in Figure 4, displays different features cross Ni = 80 at. %; i.e. the stable structure changes from SS to cluster structure, and the energy difference between various structures narrows when Ni ≥ 80 at. %. We have shown that the valence distribution of SS phases changes consecutively from LiNiO2 (x = 0) to NCM333 (x = 1/3) with a constant ratio Ni2+:Co3+:Mn4+ = 1:1:1. The valence distribution of cluster structures, on the other hand, is determined by atomic environment and Mn/Co cluster size. In order to understand the transition of relative stability at Ni = 80 at. %, it is important to have a detailed examination on the structures around this concentration. Figure 7 shows the mixing energies of SS phases and various cluster structures of LiNi1−2xCoxMnxO2 (x = 1/24, 1/12, 1/10,

Figure 6. Mixing energy predicted from present bonding model for the solid solution LiTMO2 (TM = Ni, Co, Mn) phase triangle.

compensates for the weak Co3+Mn3+ (5.12 kJ/mol). It should be noted that, even though LiCoO2 and LiMnO2 are immiscible in a wide concentration range, LiCoO2 and Li2MnO3 can form a solid solution over the whole concentration range,2 which is a result of the negative bond strength of Co3+Mn4+ (−8.95 kJ/ mol). The immiscible range of LiNixMn1−xO2 (x = 0−0.27) differs a little bit from other calculations (x = 0−0.5).39 Such a difference would mainly be due to the valence of TM ions; i.e., in a present study the valence of Mn ions displays a mixed valence of 4+ and 3 from x = 0.5 to x = 0.0,42−44 while in their study tetravalent Mn ions become trivalent when x < 0.5. With consideration of the bond strengths of Mn3+Mn3+ (4.28 kJ/ mol), Mn3+Ni3+ (4.84 kJ/mol), Mn4+Mn3+ (−4.02 kJ/mol), and Mn4+Ni2+ (−15.28 kJ/mol), it is the large difference between Mn3+−Ni and Mn4+−Ni which brings about the above difference of the immiscible gap between the present study and calculations in the literature. It should be noted that a recent study53,54 on Li1−1.34NixMn1−xO2 shows that a solid solubility limit of Ni can reach around x = 0.30, which is consistent with our immiscible range (x = 0−0.27). In addition, the relative stability with the variation of Ni:Co:Mn composition could also be deduced from Figure 6. Within the whole concentration range, LiNi0.5Mn0.5O2 is the most stable phase for the strong Mn4+Mn4+ (−22.09 kJ/mol) and Ni2+Mn4+ (−15.28 kJ/mol) bonds, which agrees well with experimental and theoretical results.39 When the Co concentration in LiNiyCo1−2yMnyO2 increases from LiNi0.5Mn0.5O2 to LiCoO2, the mixing energies maintain a low value until NCM333 by keeping the Mn4+Mn4+ and Ni2+Mn4+ bonds. From NCM333 to the three LiTMO2 vertexes, the mixing energy increases fast to LiMnO2 as the number of weak Mn3+− TM bonds increases. To LiNiO2 the mixing energy increases slowly, indicating the phase stability of LiNi1−2xCoxMnxO2 from NCM333 to LiNiO2 is promising for cathode materials. The situation from NCM333 to LiCoO2 is a little bit complex. The mixing energy first increases to a maximal value at y = 0.1 in LiNiyCo1−2yMnyO2, and then decreases when it reaches the vertex LiCoO2. This suggests LiNiyCo1−2yMnyO2, in comparison to LiCoO2, is less stable than LiCoO2 at y = 0−0.2, and then becomes more stable at y > 0.2. This result is in excellent agreement with the experimental observation that the phase stability is worse than LiCoO2 when y < 0.2, and shows better

Figure 7. Mixing energies of solid solution (SS) and various cluster structures.

1/8, 1/6), as well as the schematic plots of specific cluster structures at Ni ≥ 80 at. %. When Ni < 80 at. % (x > 1/10), the calculated mixing energies of SS phases are more stable than the corresponding cluster structures. While Ni ≥ 80 at. % (x ≤ 1/10), some cluster structures are energetically more favorable than the SS phases. More specifically, Mn and Co individual cluster structures have lower mixing energies than SS phases, while MnCo cluster structures are energetically unfavorable. Such a feature of Mn and Co cluster structures is in good agreement with similar experimental observations in the literature that Ni and Co cluster structures, instead of SS phases, were indeed formed near the surface at Ni = 80 at. %.51,58−60 In this work, coin cell testing has been performed at Ni = 70, 78, and 80 at. % for the validation of our assumption, and electrochemical properties are listed in Table S1 and Figure S1 (Supporting Information). The experimental results show that when Ni increases from 70 at. % to 78 at. %, the capacity retention at the 30th cycle decreases from 88.92% to 85.60%. A further increase from 78 at. % to 80 at. % causes a sharp drop in 6389

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The Journal of Physical Chemistry C capacity retention (78.19%). The abrupt degradation of Ni-rich NCM from Ni = 78 to 80 at. % matches well with the change of phase stability between SS and the cluster structures at Ni = 80 at. %. It could therefore be concluded that the degradation of Ni-rich NCM when Ni ≥ 80 at. % is due to the Mn and Co segregation at atomic scale. A detailed analysis of TM−TM bonds in Ni-rich NCM has been performed for understanding the transition of phase stability at Ni = 80 at. %. As found in previous sections, Ni2+ bonds with both Co3+ and Mn4+ ions through direct contact in the first nearest-neighbor at Ni ≤ 75 at. % (x ≥ 1/8). This indicates the strong bonds for SS phases are Ni2+Mn4+, Co3+Mn4+, and Ni3+Mn4+. With formation of cluster structures, whether it is a Co, Mn, or MnCo cluster, it will largely reduce the number of Ni2+Mn4+, Co3+Mn4+, and Ni3+Mn4+ bonds, while the number of Ni3+Ni3+, Co3+Co3+, and Mn3+Mn3+ bonds increases. The fact that Ni3+Ni3+, Co3+Co3+, and Mn3+Mn3+ bonds are less stable than Ni2+Mn4+, Co3+Mn4+, and Ni3+Mn4+ bonds leads to the instability of cluster structure in comparison to SS phases at Ni ≤ 75 at. %. On the other hand, a Ni2+ ion only appears in the first nearest-neighbor of Mn4+ ions at Ni ≥ 80 at. % (x ≤ 1/10), which means the strong bonds in this case are Ni2+Mn4+ and Ni3+Mn4+. Under this circumstance, the formation of an individual Mn or Co cluster will substitute Mn4+Mn4+ for Ni3+Mn4+ bonds in the Mn cluster and Co3+Co3+ for Ni3+Ni3+ in the Co cluster, respectively. The stronger Co3+Co3+ and Mn4+Mn4+ bonds can lower the mixing energy, resulting in individual Co or Mn atomic segregation. Forming a CoMn cluster decreases the number of Ni2+Mn4+ and Ni3+Mn4+ bonds, which cannot be counteracted by the small increase of strong Co3+Mn4+ and Mn4+Mn4+ bonds, causing the increase of mixing energy. Therefore, the change of TM−TM bonds from weaker Ni3+Mn4+ (Ni3+Ni3+) to strong Mn4+Mn4+ (Co3+Co3+) results in the formation of individual Mn (Co) clusters. In terms of bond strength, we now turn to having a further discussion about the influence of atomic segregation on the experimental design of Ni-rich NCM. Cation ordering and uniformity in NCM materials are emphasized as the primary factors to the stability and cyclability of Li-ion batteries.61−64 For example, a Ni cluster formed in Li1.2Ni0.2Mn0.6O2 using coprecipitation and sol−gel methods leads to a rock-salt phase transition and poor rate performance,61,62 while a uniformly distributed SS phase obtained using a novel hydrothermal assisted method shows good thermal stability and electrochemical performance.63 The uniformly distributed SS phases can be prepared because Li1.2Ni0.2Mn0.6O2 consists of very strong Ni2+Mn4+ and Mn4+Mn4+ bonds from Li2MnO3 and LiNi0.5Mn0.5O2. Nowadays, at least in the laboratory, most experimentalists claimed they can synthesize uniformly distributed Ni-rich NCM samples, but the deterioration of phase stability can still be observed in Ni-rich NCM when Ni ≥ 80 at. %. Therefore, the origin of instability of Ni-rich NCM could be attributed to the atomic segregation caused by the nature of TM−TM bonds. Because instability is caused by TM−TM bonds, it cannot be solved by the improvement of fabrication methods. A feasible way to improve phase stability is to replace the weak bonds by other strong bonds. For example, as Mn4+ has a stronger bond with TM ions than Co3+, tuning the Mn:Co ratio in Ni-rich NCM will improve the phase stability and keep high capacity.65−68 Using our bond model, we have investigated the effect of Mn:Co ratio on the stability of Ni-rich NCM. Figure 8 shows

Figure 8. Mixing energies of solid solution and various cluster structures of LiNi1−2xCoxMnxO2 calculated from the present bonding model: (a) x = 1/8 (Ni = 75 at. %), (b) x = 1/10 (Ni = 80 at. %). A higher trace means the structure has more severe atomic segregation, while the structure with the lowest trace corresponds to the solid solution (SS) phase.

the mixing energy of represented structures as a function of Mn:Co ratio at 75 at. % and 80 at. % Ni with the variation of Mn:Co ratio. It could be seen that the mixing energy of SS phases decreases linearly with increasing Mn concentration. The relative stability of SS and cluster structures has a transition at Mn:Co = 1. When Mn:Co < 1, the cluster structures become more stable than the SS phase. When Mn:Co > 1, the SS phase shows different features at 75 at. % and 80 at. % Ni. At Ni = 75 at. %, the SS phase becomes less stable than the cluster structure, while the SS phase is more stable in the case of Ni = 80 at. %. The stable SS phase at high Mn:Co ratio matches well with the experimental observations that the increase in Mn:Co ratio can remarkably enhance the stability of NCM811.22,45,69 Another feature is that the energy sequence of various structure is different in the Mn- and Co-rich region. As shown in Figure 8, in the Co-rich region the mixing energy decreases with the increase of trace, while in the Mn-rich region, the situation is reversed. In addition, the energy difference between the structures in the Co-rich side is very small, indicating that Co is easily segregated in the Co-rich region, while the energy difference in the Mn-rich region is large, which makes the clustering hard at this concentration. This result is in excellent agreement with the recent experimental result that when 6390

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acknowledge the Texas Advanced Computing Center (TACC) for providing computational resources.

LiNi0.8Mn0.2O2 with LiNi0.8Co0.2O2 are mixed, they form perfect solid solutions with Mn:Co > 1.68 The good agreements between our bond model and experimental results suggest that the possible way to improve the stability of Ni-rich NCM is through increasing the bond strength of SS phases.

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(1) Liu, W.; Oh, P.; Liu, X.; Lee, M. J.; Cho, W.; Chae, S.; Kim, Y.; Cho, J. Nickel−Rich Layered Lithium Transitional−Metal Oxide for High−Energy Lithium−Ion Batteries. Angew. Chem., Int. Ed. 2015, 54, 4440−4458. (2) He, P.; Yu, H.; Li, D.; Zhou, H. Layered Lithium Transition Metal Oxide Cathodes Towards High Energy Lithium−Ion Batteries. J. Mater. Chem. 2012, 22, 3680−3695. (3) Noh, H.-J.; Youn, S.; Yoon, C.-S.; Sun, Y.-K. Comparison Of The Structural and Electrochemical Properties of Layered Li[Nixcoymnz]O2 (X = 1/3, 0.5, 0.6, 0.7, 0.8 And 0.85) Cathode Material for Lithium−Ion Batteries. J. Power Sources 2013, 233, 121−130. (4) Sun, Y.-K.; Chen, Z.; Noh, H.-J.; Lee, D.-J.; Jung, H. G.; Ren, Y.; Wang, S.; Yoon, C. S.; Myung, S. T.; Amine, K. Nanostructured High− Energy Cathode Materials for Advanced Lithium Batteries. Nat. Mater. 2012, 11, 942−947. (5) Yan, P.; Zheng, J.; Lv, D.; Wei, Y.; Zheng, J.; Wang, Z.; Kuppan, S.; Yu, J.; Luo, L.; Edwards, D.; Olszta, M.; et al. Atomic−Resolution Visualization of Distinctive Chemical Mixing Behavior of Ni, Co, and Mn With Li in Layered Lithium Transition−Metal Oxide Cathode Materials. Chem. Mater. 2015, 27, 5393−5401. (6) Kim, Y.; Kim, D.; Kang, S. Experimental and First-principles Thermodynamic Study of the Formation and Effects of Vacancies in Layered Lithium Nickel Cobalt Oxides. Chem. Mater. 2011, 23, 5388− 5397. (7) Kang, K.; Meng, Y. S.; Bréger, J.; Grey, C. P.; Ceder, G. Electrodes with High Power and High Capacity for Rechargeable Lithium Batteries. Science 2006, 311, 977−980. (8) Nam, K. W.; Bak, S. M.; Hu, E.; Yu, X.; Zhou, Y.; Wang, X.; Wu, L.; Zhu, Y.; Chung, K. Y.; Yang, X. Q. Combining In Situ Synchrotron X−Ray Diffraction and Absorption Techniques with Transmission Electron Microscopy to Study the Origin of Thermal Instability in Overcharged Cathode Materials for Lithium−Ion Batteries. Adv. Funct. Mater. 2013, 23, 1047−1063. (9) Hwang, S.; Kim, S. M.; Bak, S. M.; Chung, K. Y.; Chang, W. Investigating the Reversibility of Structural Modifications of LixNiyMnzCo1−y−zO2 Cathode Materials during Initial Charge/ Discharge, at Multiple Length Scales. Chem. Mater. 2015, 27, 6044− 6052. (10) Goodenough, J. B.; Park, K. S. The Li−Ion Rechargeable Battery: A Perspective. J. Am. Chem. Soc. 2013, 135, 1167−1176. (11) Goodenough, J. B. Magnetism and the Chemical Bond; Interscience: New York, 1963. (12) Kanamori, J. Superexchange Interaction and Symmetry Properties of Electron Orbitals. J. Phys. Chem. Solids 1959, 10, 87−98. (13) Omenya, F. O.; Chernova, N. A.; Nolis, G. M.; Zhou, H.; Li, Z.; Whittingham, M. S. What can we learn about Battery Materials from Their Magnetic Properties? J. Mater. Chem. 2011, 21, 9865−9875. (14) Bréger, J.; Dupré, N.; Chupas, P. J.; Lee, P. L.; Proffen, T.; Parise, J. B.; Grey, C. P. Short− and Long−Range Order in the Positive Electrode Material, Li(NiMn)0.5O2: A Joint X−ray and Neutron Diffraction, Pair Distribution Function Analysis and NMR Study. J. Am. Chem. Soc. 2005, 127, 7529−7537. (15) Chernova, N. A.; Ma, M.; Xiao, J.; Whittingham, M. S.; Breger, J.; Grey, C. P. Layered LixNiyMnyCo1−2yO2 Cathodes for Lithium Ion Batteries: Understanding Local Structure via Magnetic Properties. Chem. Mater. 2007, 19, 4682−4693. (16) Van der Ven, A.; Ceder, G. Ordering In Lix[Ni0.5Mn0.5]O2 and Its Relation to Charge Capacity and Electrochemical Behavior in Rechargeable Lithium Batteries. Electrochem. Commun. 2004, 6, 1045− 1050. (17) Yoon, W. S.; Iannopollo, S.; Grey, C. P.; Carlier, D.; Gorman, J.; Reed, J.; Ceder, G. Local Structure and Cation Ordering in O3 Lithium Nickel Manganese Oxides with Stoichiometry Li[NixMn(2−x/3)Li(1−2x)/3]O2. Electrochem. Solid-State Lett. 2004, 7, A167−A171.

CONCLUSIONS In this study, we have developed a new bond model to unravel the origin of the instability of Ni-rich NCM by combining a first principles calculation with the effective interaction of transition metal (TM) ions (represented as TM−TM bond). Our findings show that the deterioration of Ni-rich NCM when Ni ≥ 80 at. % is due to the Mn and Co segregation at the atomic scale. The Co and Mn atomic segregation formed at Ni ≥ 80 at. % originates from the nature of TM−TM bonds. Specifically, when Ni < 80 at. %, the Ni2+Mn4+, Ni3+Mn4+, and Co3+Mn4+ bonds, which come from the direct contact between Ni, Co, and Mn in the nearest-neighbor, dominate the stability of SS phases. Formation of a cluster structure, whether it is a Co, Mn, or Mn−Co cluster, will substitute strong Ni2+Mn4+, Ni 3+ Mn 4+ , and Co 3+ Mn 4+ bonds for weak Co 3+ Co 3+ , Mn3+Mn3+, and Co3+Mn4+ bonds, making cluster structures energetically less favorable. When Ni ≥ 80%, due to the insufficient amount of Co and Mn atoms in the nearestneighbor of Ni ion, the transition of TM−TM bonds from weaker Ni3+Mn4+ (Ni3+Ni3+) to Mn4+Mn4+ (Co3+Co3+) results in the formation of an individual Mn (Co) cluster. Furthermore, a sequence of bond strength is determined for NCM materials such that Mn4+Mn4+ > Ni2+Mn4+ > Ni3+Mn4+ > Co3+Mn4+ > Co2+Mn4+ > Ni2+Ni4, which has been used to explain the intricate LiTMO2 (TM = Ni, Co, Mn) phase triangle. This phase triangle could not only help us to understand the relationship between phase stability and TM− TM bonds, but also provide helpful guidance on rational design of cathode materials for experimentalists. The results predicted from our bond model are compared with the validation experiment in the present work and also other experiments in the literature, and the agreements between them are very good. The consistency between our bond model and experimental results indicates our bond model not only is effective for Nirich NCM but also could provide a basis for future studies of other layered cathode materials with mixed TM cations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00369. Electrochemical properties of LiNi1−2xCoxMnxO2 (x = 0.15, 0.11, 0.10) show the evidence of cathode degradation when x ≤ 0.1 (Ni ≥ 80 at. %) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work with L&F Material Co., Ltd., was supported by the National Research Foundation and the Fundamental R&D Program for Technology of World Premier Materials funded by the Korean Government (Grant 10037921). The authors 6391

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DOI: 10.1021/acs.jpcc.6b00369 J. Phys. Chem. C 2016, 120, 6383−6393

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DOI: 10.1021/acs.jpcc.6b00369 J. Phys. Chem. C 2016, 120, 6383−6393