Core-Shell Nanocomposites for Improving the Structural Stability of Li

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Surfaces, Interfaces, and Applications

Core-Shell Nanocomposites for Improving the Structural Stability of Li-Rich Layered Oxide Cathode Materials for Li-Ion Batteries Roberto C. Longo, Chaoping Liang, Fantai Kong, and Kyeongjae Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03898 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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ACS Applied Materials & Interfaces

Core-Shell Nanocomposites for Improving the Structural Stability of Li-Rich Layered Oxide Cathode Materials for Li-Ion Batteries Roberto C. Longo,∗ Chaoping Liang, Fantai Kong, and Kyeongjae Cho∗ Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States E-mail: [email protected]; [email protected]

Abstract The structural stability of Li-rich layered oxide cathode materials is the ultimate frontier to allow the full-development of these family of electrode materials. Here, FirstPrinciples calculations coupled with Cluster Expansion are presented to investigate the electrochemical activity of phase-separation, core-shell structured xLi2 MnO3 ·(1x)LiNiCoMnO2 nanocomposites. The detrimental surface effects of the core region can be countered by the Li2 MnO3 shell, which stabilizes the nanocomposites. The operational voltage windows are accurately determined, in order to avoid the electrochemical activation of the shell and the subsequent structural evolution. In particular, the dependence of the activation voltage with the shell thickness shows that relatively high voltages can still be obtained to meet the energy density needs of Li-ion battery applications. Finally, activation energies of Li migration at the core-shell interface must also be analyzed carefully, in order to avoid the outbreak of a phase transformation, thus making the nanocomposites suitable from a structural viewpoint.

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keywords: Li-ion batteries, density-functional theory, layered oxides, core-shell nanostructures, electrochemical properties

Introduction Lithium-ion batteries are indispensable in today’s life, as they are the power source for all the portable electronic devices that during the last decade have changed our daily habits. 1 Although the power energy, rate capabilities and fast recharge times of modern batteries seem to be sufficient for the needs of small electronic devices, more impetus in their improvement is required in order to expand their use to applications with larger energy requirements, such as hybrid-electric vehicles or grid storage infrastructure. 2,3 To achieve this goal, Li-rich layered metal oxides, which can accommodate more than one unit of Li per molecule, constitute a very promising alternative cathode material to increase the energy density and rate capabilities of current Li-ion batteries in a reasonable short period of time. 4–7 This family of compounds, with the general formula xLi2 MnO3 ·(1-x)LiMO2 (M = Ni, Co, Mn), can exhibit Li intercalation capacities higher than 200 mAh/g at room temperature and even reach 280 mAh/g at low charge/discharge rates and 60◦ C, i.e., close to the theoretical 1 e− /metal capacity of the transition metal (TM) dioxide systems. The LiMO2 component of the nanocomposite is primarily responsible for Li deintercalation and intercalation at voltages lower than 4.5 V (vs. Li|Li+ ), whilst Li2 MnO3 is an additional capacity reservoir when charging above 4.5 (vs. Li|Li+ ). 4–8 During the first charge of the battery at potentials between 4.3 and 5 V, the Li2 MnO3 phase irreversible activates by releasing Li2 O, thus evolving into the layered LiMnO2 . 4–7 The first cycle of charge is then crucial in order to determine the structural integrity between the Li2 MnO3 and LiMnO2 phases. Besides the irreversible capacity loss during the first charging cycle, additional well-known deficiencies, such as layered to spinel conversion during long-term cycling and low rate capability due to the low electronic conductivity of the LiMnO2 phase, need to be addressed in order to make these

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cathode materials practical. The synthesis of xLi2 MnO3 ·(1-x)LiMO2 or LiMO2 is generally performed via co-precipitation or sol-gel methods, 9 and most of the previous studies found clear evidence of a solid solution between the Li2 MnO3 and LiMO2 phases 10 or, accepting the mixing as “inevitable”, propose the improvement of the atomic level uniformity, 11 i.e., TM metal mixing, as a means to mitigate the voltage fade. However, no remarkable progress has been yet obtained unless very low charging rates (around 0.2 C) were used, and the transformation from the layered to the spinel structure was still observed after long-term cycling. 10,11 Another options for the synthesis of Li-rich layered oxides are given by mechanical treatment of solids, such as ball-milling, which could offer a rather simple and efficient method with lower energy consumption and cost to prepare these electrode materials. 8,12 Even though surface coating can improve the structural and thermal stabilities of cathode materials to prevent such transformations, it is not easy to encapsulate the primary particles with thin coating layers. Therefore, a new concept was introduced to satisfy the requirements for Li- or Ni-rich materials to provide high capacity and good thermal properties. Sun’s group proposed the concept of core-shell cathode materials with a Ni-rich core and a Mn-rich shell. 13 Core (LiMO2 )-shell (Li2 MnO3 ) composite layered oxide cathode materials were shown to exhibit high discharge capacity and cyclability, together with higher thermal stability than layered materials prepared by conventional co-precipitation methods, 8,13–19 although surface transformations into a Mn3 O4 -type spinel structure still persist, 8 most likely due to the dissociation of the Li2 MnO3 shells. However, Li-rich layered surfaces bearing a consistent framework with the corresponding “core” host have shown a reversible capacity of 218.3 mAh/g at 1C, with improved cycle retention (94.1% after 100 cycles). 18 Additional attempts using exfoliated, layered-by-layered self-assembled nanocomposites of MnO2 and Mn1/3 Co1/3 Ni1/3 nanosheets showed promising results in terms of structural stability, although the delivered capacity (around 200 mAg/h) was still far from the theoretical 1 e− /metal limit. 20 Another alternative is given by the use of similar mixtures of TM both in the core (Ni-Mn) and the shell (Ni-Co-Mn), synthesized by standard co-precipitation meth-

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ods. Such composites were shown to deliver a capacity as high as 220 mAg/h, with 98.2% of capacity retention after 40 cycles. 21,22 The use of structurally different shells has also been explored. For instance, spinel-layered core-shell cathode materials also demonstrates a reversible capacity of 200 mAg/h and 95% of capacity retention. 23 However, the use of an spinel-shell structure severely limits the delivered voltage to ∼3.8 V and, hence, the obtained energy density. Here, we intend to investigate the core-shell alternative to the design of Li-rich layered oxide cathode materials in order to propose a solution to both the first charging and long-term cycling stability deficiencies. Using density-functional theory (DFT), ab initio Molecular Dynamics (AIMD) and a ternary Cluster Expansion (CE) in a simple interface model, we propose a phase separation, “core-shell” design of the xLi2 MnO3 ·(1-x)LiNiCoMnO2 (x = 0.1-0.5) electrode material in order to obtain the appropriate operational voltage windows to avoid the aforementioned dissociation of the Li2 MnO3 shell, thus guaranteeing the longterm structural stability of the LiNiCoMnO2 core. Such core-shell design combines the strength of the two different phases: structural stability of the Li2 MnO3 oxide and the superior rate capability of the LiNiCoMnO2 (hereafter NCM). In a first approximation, our model approach can be considered valid, as the surface to volume ratio in common primary electrode nanoparticles is always very small. Previous theoretical studies on the Li-rich cathode material proposed a solid solution model of both Li2 MnO3 and NCM phases. 24 That model basically involves the presence of Li ions in Mn-rich TM layers and seems to agree well with STEM images of the crystal structure, 10,24 which has shown evidence of Li ions in TM layers after synthesis by co-precipitation or sol-gel methods. Thus, Co and Ni atoms would form “broken zigzag chains” in their respective layers (the study aforementioned includes only Li-rich Ni-Mn oxides), in accordance with NMR and electron diffraction data. 25 However, for a Li concentration of less than 0.8 ions per formula unit, the voltage rises up to 4.8 V and some surface TM ions migrate into the Li layer, inducing a surface phase transformation from layered to a defect-spinel structure, which rapidly propagates deeper into the bulk system. 25

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The spinel formation enthalpy appears to be lower (i.e., thermodynamically favorable) below such Li concentration range. On the contrary, our proposed core-shell model shows no thermodynamic driving force for phase transformation, once the operational voltage window is carefully determined.

Methods The calculations were carried out within the framework of the spin-polarized density-functional theory (DFT), using the VASP code. 26,27 The PBE functional 28 of the generalized gradient approximation (GGA) was used for the exchange-correlation interactions and the projector augmented wave (PAW) scheme was applied to describe the interactions between valence electrons and core ions. 29 The wave functions were expanded in plane waves up to a kinetic energy cutoff of 500 eV. In all the calculations, a k -point mesh within the Monkhorst-Pack scheme 30 was used to ensure a convergence of 1 meV per supercell and 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 acting on each atom. Due to the strongly correlated Ni, Co and Mn electronic d bands, the GGA+U approach with U parameters 6.88, 5.95 and 5.2 for Ni, Co and Mn, respectively, was used in all calculations. 6 For the modeling of the core-shell, phase separation LiNiCoMnO2 (NCM) and Li2 MnO3 nanocomposites, we built an interface model between both phases using the trigonal R ¯3m parent structure of LiMnO2 as the host configuration for the NCM part and the monoclinic C2/m structure for Li2 MnO3 . 10 The orientation of both the core and the shell corresponds to the (001) surface, which is the most logical choice, according to its surface energy and the Li migration path in layered materials (perpendicular to the (001) direction). Furthermore, the lattice mismatch between the NCM-core and the Li2 MnO3 -shell is less than 1.5%. In order to study the interlayer migration phenomena and the nanocomposite structural stability, the model comprises 32 layers, 16 with oxygen, 8 with only Li ions and the rest

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with only TM or both Mn and Li atoms, depending on the specific side of the interface (Li2 MnO3 or NCM). Each layer has 6 atoms, for a total of 192 in the supercell and periodic boundary conditions were considered in all directions. For simplicity, we considered only the NCM111 stoichiometry with a TM solid solution mixing in each layer, 31 forming zigzag chains similar to other DFT studies, 32 as can be seen in the Figure S1 of the Supporting Information. We started with a xLi2 MnO3 ·(1-x)LiNiCoMnO2 (x = 0.5) configuration and gradually decreased the thickness of the Li2 MnO3 -shell phase present in the system, until the final 0.125Li2 MnO3 ·0.875LiNiCoMnO2 nanocomposite lower-limit-thickness structure. Interlayer antiferromagnetic (AFM) configurations between TM ions were adopted throughout all the study. Ion diffusion paths and kinetic barriers were obtained using the climbing image-nudged elastic band method (CI-NEB) 33–35 and AIMD. AIMD simulations were performed at room temperature (300 K). The velocity Verlet algorithm coupled with the Nose thermostat was used to solve the equations of motion and a time step of 0.5 fs was used during an equilibration period of 10000 steps, for a total simulation time of 5 ps. The electrochemical stability of some selected systems was studied through the phase diagram in the chemical potential space, by obtaining meaningful limits for the respective chemical potentials. 36 In order to obtain the voltage profiles, avoiding the calculation of every delithiated atomic configuration, we used a ternary CE with the Li/vacancies/(layer of the NCM-core and coreshell interface) degrees of freedom at each Li lattice site (no TM diffusion was considered) to search for stable delithiated configurations. 37–39 Each atomic configuration with any specific Li-vacancy-layer ordering can be expanded into a polynomial as a function of discrete occupation variables σi :

E(σ) = J0 +

X i

Ji σi +

X ij

Jij σi σj +

X ijk

Jijk Jijk σi σj σk ,

(1)

where the indices i, j, k,... correspond to a collection of interstitial sites forming cluster pairs, triplets, etc and the multiple value of σi σj σk is the correlation function for the corresponding 6


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Figure 1: Monoclinic (C2/m) Li2 MnO3 unit cell. The lattice parameters are a = 5.006 Å, b = 8.654 Å, c = 5.091 Å, α = γ = 90◦ , and β = 109.45◦ , according to our DFT calculations. configuration. The coefficients J0 , Ji , Jij are called the effective cluster interaction (ECI). From a practical point of view, the sum must be truncated after some maximum-sized clusters, then using cross-validation score (CV) to select the optimal set of clusters.

CV 2 = n−1

Xn i

[wi (Ei (σ) − EDF T,i (σ))]2 ,

(2)

where Ei (σ) is the energy predicted by the CE, EDF T,i (σ) is the energy obtained with DFT and wi is the weight of the corresponding configuration in the CV score (n represents the total set of clusters considered). Once the ECIs are fitted, they can be used to predict the energy of atomic configurations not included in the fitting. In the current work, six pairs, four triplets, and one quadruplet clusters were included in the fitting of the ECI coefficients (J0 , Ji , Jij ,...). Moreover, 24 configurations, which extend up to 48 formula units, have been adopted for Li-vacancy-layer structures, in order to ensure a CV score of

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less than 0.01 eV/atom. Tables S1 and S2 of the Supported Information show the obtained ECI coefficients and the CE-predicted and DFT-calculated energies of some of the most representative configurations considered, respectively. The CE fitting has been performed in the xLi2 MnO3 ·(1-x)LiNiCoMnO2 (x = 0.125) core-shell structure and all the Li lattice sites have been considered except those of the Li2 MnO3 -shell phase. The obtained ECIs were then used to calculate the voltage profiles of the other configurations (x = 0.125-0.5).

Results and Discussion The monoclinic C2/m space group unit cell (see Figure 1) has been used as building block for the core-shell Li-rich layered oxide. 10 As an example, Figure 2 shows an scheme of our interface model for the xLi2 MnO3 ·(1-x)LiNiCoMnO2 electrode material (see Figure S1 of the Supporting Information for the actual supercell used in the calculations). There are 32 layers in each supercell, with 6 atoms per layer for a total of 192 atoms. The “excess” Li ions are obviously located in the Li2 MnO3 -shell phase of the nanocomposite system. Gradual decrease of the Li2 MnO3 content will allow us to determine the optimal composition in order to guarantee the structural stability while keeping, to the largest extent possible, the energy density and rate capability of the NCM cathode material. It is already well-known that Li2 MnO3 is an electrochemically inert electrode material, 40 which needs an activation mechanism (usually the formation of oxygen vacancies at 4.5 V, although further oxidation of the oxygen ions has also been theoretically suggested). 41–44 After the first charge-discharge cycle, Li2 MnO3 irreversibly evolves into LiMnO2 . Consequently, in order to obtain the optimal xLi2 MnO3 ·(1-x)LiNiCoMnO2 composition, the operational voltage window must also be carefully determined to avoid undesirable Li2 MnO3 phase transformation and possible TM migration from the NCM-core phase during charging (since both NCM and LiMnO2 phases are cycled at similar voltages, 4 ± 0.2 V). 45 In order to estimate the thermodynamic structural stability of our core-shell, phase sep-

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LiNiCoMnO2

Li2MnO3

Figure 2: Diagram of the NCM-core-Li2 MnO3 -shell interface model. aration model, we compare the formation energies of our xLi2 MnO3 ·(1-x)LiNiCoMnO2 (x = 0.125-0.5) model system with a perfect TM solid solution 10 with the same stoichiometry and a hypothetical ternary phase compound showing gradual transformation of the Li2 MnO3 phase into LiMnO2 . All the formation energies (calculated with NCM and Li2 MnO3 as reference states) are slightly positive, as shown in the Figure S2 of the Supporting Information. This result is not unexpected, since most of the formation energies (or mixing enthalpies) previously reported 46 for binary or ternary TM compounds are also positive (only the NCM

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Figure 3: Chemical potential phase diagram of the most thermodynamically stable xLi2 MnO3 ·(1-x)LiNiCoMnO2 nanocomposite configurations, together with the corresponding single oxides. layered electrode material itself shows a negative formation enthalpy), but all of them are achievable given the high temperature regimes during synthesis. It is interesting to note that the formation energies of our core-shell, phase separation model are the lowest ones (less positive) among all the structural models considered, and only a ternary phase compound with similar amounts of Li2 MnO3 and LiMnO2 shows a comparable formation energy. The energy difference obviously decreases with the thickness of the Li2 MnO3 -shell phase. One important question is whether the energy difference between the solid solution and the

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Figure 4: DFT- and CE-Formation energies of all the structures considered for the core-shell xLi2 MnO3 ·(1-x)LiNiCoMnO2 (x = 0.125) composite electrode material, as a function of the Li concentration in the core-NCM phase and at the core-shell (Li2 MnO3 ) interface. The black line indicates the convex hull of the core-data sets. core-shell structural models could be overcome by the mixing entropy, i.e., if the Gibbs (∆U − T∆S) formation energy of the phase separation model is still lower than that of a TM solid solution. Following the methodology described in Ref. 47, the configurational mixing entropy of the TM ions of the NCM phase was calculated. For instance, in the 0.5Li2 MnO3 ·0.5LiNiCoMnO2 compound, for which the TM mixing entropy could really be a factor, the obtained result is ∼0.5Kb , which at room temperature gives T∆S ∼12 meV. Due to the formation energy difference between both phase separation and solid solution models (∼42 meV), we can conclude that the TM mixing entropy lowers such difference to ∼30 meV at room temperature, but still a phase separation between NCM-core and Li2 MnO3 -shell

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phases would be thermodynamically favorable. More important information can be obtained from the chemical potential phase diagram shown in Figure 3. To avoid confusion, only the relevant configurations have been included in the picture, which shows that the core-shell xLi2 MnO3 ·(1-x)LiNiCoMnO2 nanocomposites have a very specific domain of stability, only at relatively large oxidation conditions. Upon reduction (oxygen evolution), the Li2 MnO3 phase transforms into the layered LiMnO2 , as expected, and the xLi2 MnO3 ·(1-x)LiNiCoMnO2 is no longer thermodynamically stable (it evolves into xLiMnO2 ·(1-x)LiNiCoMnO2 ). The figure also shows that a very large overpotential would be necessary to revert the transformation, unphysical under the normal operation conditions of the battery. Finally, the different single oxides (NCM, Li2 MnO3 or LiMnO2 ) also appear as stable phases at highly reductive conditions. Therefore, the phase diagram shows that, after synthesis, the core-shell xLi2 MnO3 ·(1-x)LiNiCoMnO2 nanocomposites show a medium-large stability domain but, during cycling, some phase intermixing can occur in first place, before it transforms into xLiMnO2 ·(1-x)LiNiCoMnO2 . Finally, TM ion diffusion will probably either segregate LiMnO2 from the NCM phase or form a Li-rich, NCM solid solution, with the subsequent well-known phase stability problems. 40,46 To design a reliable strategy for the optimization of the electrochemical performance and structural stability of core-shell xLi2 MnO3 ·(1-x)LiNiCoMnO2 phase separation compounds, we need to accurately determine the first cycle charging plot, in order to delimit the voltage window necessary to keep the Li2 MnO3 phase electrochemically inert, whilst at the same time maintaining the large energy density that makes the NCM family of cathode materials attractive positive electrode compounds. To do that, we mapped the core-shell 0.125Li2 MnO3 ·0.875LiNiCoMnO2 structure chemical space, including Li/vacancies across the NCM phase as well as at the core-shell interface. We did not consider Li/vacancies in the Li2 MnO3 shell, as we intend to preserve its structural integrity. To verify the accuracy of this approach, we obtained the DFT Li vacancy formation energies in the three different regions of the system, NCM, Li2 MnO3 and at the core-shell interface. The re-

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sults range from 3.93 (NCM core) to 4.42 eV (at the Li2 MnO3 shell), thus allowing us to explore delithiated structures only at the core and interface regions of the composite system, in order to obtain ECI coefficients that correctly capture the delithiation trends of the layered electrode material. The obtained results are shown in Figure 4, where the formation energy, defined in the usual way as Ef (x) = E(0.125Li2 MnO3 ·0.875Lix NiCoMnO2 ) − (1-x/2)E(0.125Li2 MnO3 ·0.875NiCoMnO2 ) − (x/2)E(0.125Li2 MnO3 ·0.875LiNiCoMnO2 ), is plotted to clearly differentiate the intermediate stable structures. As mentioned above, we assumed that no phase transformation occurs during delithiation in the NCM core region. Possible phase transformation of the Li2 MnO3 -shell region will be discussed in detail later. The convex hull (solid line in Figure 4) of the layered NCM structures exhibits two intermediate stable states at x = 6 and x = 16, thus leading to three different reaction regions, between x = 0 and x = 6, between x = 6 and x = 16 and between x = 16 and x = 24. However, as can be noted in Figure 4, the amount of states close to the convex hull changes notably in the second region, thus indicating the possibility of having a solid-solution-like behavior, similar to the Li2 MnO3 phase alone, 44 but in the present case of core-shell nanocomposite structures a larger amount of Li has to be extracted. That means that there is a clear path to start the Li extraction, whereas more stable intermediate states might coexist, once the second and third regions have been reached. The ionic configurations of each intermediate stable state (shown in Figure S3 in the Supporting Information) indicate the most likely Li extraction pathway during the first charging cycle: first, a fraction of Li ions is extracted from the two Li middle-layers (there are two, due to the symmetry of the layered model) with respect to the NCM-core-Li2 MnO3 -shell interface. Then, in the second region, few Li ions are extracted from the first and third Li layers (with respect to the interface), together with the remaining Li ions from the middle-layers, forming a specific Li/vacancy ordering and finally, in the third region, the remaining Li ions at the first and third Li layers are extracted. Again, owing to the large number of configurations close to the convex hull in the second and third regions, stable solid-solutions with Li ions being extracted from the first and third layers could easily

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coexist. Finally, Li could also be extracted from the core-shell interface. Figure 4 also shows that a hypothetical Li extraction from the shell (Li2 MnO3 ) phase would correspond to a separate region in the convex hull diagram, with increasing (less favorable thermodynamically) formation energies. Clearly, there is an increasingly strong thermodynamic driving force for the extraction of Li ions from the core-shell interface.

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x = 0.5

x = 0.375 x = 0.25 x = 0.125

Figure 5: Voltage profiles of the core-shell xLi2 MnO3 ·(1-x)LiNiCoMnO2 composite electrode material, obtained from the convex-hull data shown in Figure 4. The shaded areas correspond to Li ions extracted from the core-shell interface. The dashed-dotted line shows the experimental results reported in Ref. 8.

The extra electrons needed to compensate the extracted charge of Li+ ions are primarily originated in the TM cations, especially Ni. Indeed, the evolution of the TM magnetic moments during delithiation (see Figure S4 of the Supporting Information) indicates that Mn remains largely in the 4+ oxidation state throughout the entire first charging cycle, as commonly observed in ternary layered electrode materials. 46 Co ions change their oxidation state from 3+ to 4+, in spite of being always in a high spin state (∼3µB ). On the contrary, Ni ions oxidize from 2+ (∼2µB ) to an average oxidation state of 3.5+ (∼0.8µB ). The reason 15



for not being oxidized completely to a 4+ state must be obviously found in the fact that the core-shell nanocomposite is not completely delithiated, i.e., no Li ions have been extracted from the shell-Li2 MnO3 phase. Another important question is that there could be additional charge compensation from a further oxidation of oxygen anions (from O2− to O− ) or oxygen release. 41,48 Although we will discuss oxygen evolution in detail when analyzing voltage plots, we can anticipate that such mechanisms seem unlikely in our core-shell model. First, careful examination of the magnetic moments shows that the oxidation state of the oxygen anions does not practically change with delithiation and, second, oxygen vacancy formation energies and subsequent oxygen mobility are relatively high for O2− species. 44

Figure 6: Kinetic energy profile corresponding to the Li migration from a Td site at the core-shell interface to an Oh (2b) site at the Li2 MnO3 -shell. The insets show the initial, intermediate and final configurations. Color code: Li, green; oxygen, red; Ni, gray; Co, blue and Mn, purple balls.

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So far, we have shown the most likely delithiation mechanism for the core-shell, Li-rich composite 0.125Li2 MnO3 ·0.875LiNiCoMnO2 nanocomposite electrode material. By taking all the intermediate states along the convex hull (see Figure 4) and obtaining their respective DFT-energies, we calculated the evolving voltage profile of the xLi2 MnO3 ·(1-x)LiNiCoMnO2 (x = 0.125-0.5) nanocomposite structure, selecting the proper intermediate states for each composition (i.e., for x > 0.125) and assuming that the original layered structure remains intact, with the charge compensation corresponding to the extraction of Li enabled by TM oxidation (see Figure S4 of the Supporting Information). The voltage is obtained in the usual way, as the difference in the chemical potential between the cathode and a hypothetical Li − + An metal anode: V = -(µCat Li -µLi )/Ze , where Z = 1 is the valence charge of the carrier (Li ) and

e− the electron charge. 49 The results are shown in Figure 5, together with the voltage profile of a recent experimental work, obtained for a 0.5Li2 MnO3 shell-0.5LiNCMO2 core-structured nanocomposite synthesized by ball-milling. 8 The experiment was performed at 400◦ C for a slightly different NCM composition (Ni0.5 Co0.3 Mn0.2 ), but it still represents a valid example to analyze the trends of our core-shell nanocomposite models. Note that the charging process corresponds to increasing y and, as discussed before, no Li ions have been extracted from the Li2 MnO3 -shell region, to promote the stability of the NCM-core. However, as reflected in Figure 5, the thickness of the Li2 MnO3 shell has a strong effect on the predicted voltages. First, no voltage is obtained above 4.5 V, which corresponds to the activation of the Li2 MnO3 electrode material, 44,50 thus validating our intention of maintaining the Li2 MnO3 -shell region electrochemically inactive. Second, the operational voltage window for the xLi2 MnO3 ·(1x)LiNiCoMnO2 nanocomposites varies between 3.7-4 V (x = 0.125) and 3.9-4.55 V (x = 0.5). A priori, this would make the 0.5Li2 MnO3 ·0.5LiNiCoMnO2 stoichiometry the ideal candidate among all the structures considered, but there are also some important drawbacks that need to be taken into account. For instance, the extraction of Li from the interface can act as the trigger for a phase transformation (as will be discussed in detail later) of the Li2 MnO3 -shell region, first into the LiMnO2 layered oxide and then, into a LiMn2 O4 -defect

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spinel and, also, Li2 MnO3 is not a good electronic conductor. Therefore, a thick Li2 MnO3 shell (with respect to the NCM-core) would be detrimental for the overall performance of the battery. Core-Shell samples with optimal lithium content have shown extremely low irreversible capacity, as well as high capacity and excellent capacity retention. 21,22 Also, the overall rate performance of the core-shell nanocomposite strongly depends on the ionic conductivity of the Li2 MnO3 -shell. Vacancy diffusion is the ionic conductivity mechanism for Li2 MnO3 electrode materials, with an activation energy of ∼0.5-0.6 eV. 51 Such vacancies can be easily formed at the surface of the Li2 MnO3 -shell, where the required voltage to extract Li ions is ∼3.3 eV, much lower than within the Li2 MnO3 bulk, ∼4.6-4.8 eV. 52 Therefore, a thick Li2 MnO3 -shell can also negatively affect the rate performace of the electrode material. The right-hand side of each voltage plot shown in Figure 5 corresponds to Li extraction from the core-shell interface. As such, this should be the upper limit of the voltage operational window if one intends to maintain the structural integrity of the Li2 MnO3 -shell region and, thus, promote the stability of the overall electrode. Finally, all the core-shell nanocomposites show a sloping region in the voltage plots corresponding to the oxidation of TM ions, but no plateau corresponding to electrochemical reactions dictated by oxygen anions (oxygen release or further oxidation) was obtained, similarly to other Li-rich materials. 53 This is obviously not surprising, since these plateaus are usually observed in Li-rich solid solution electrode materials above 4.5 V, 41,48 and can also be noticed in the Figure S4 (see Supporting Information), which shows the evolution of the magnetic moment of the TM ions during the charging process. As discussed previously, Mn ions always remain in a 4+ valence state (the creation of a Mn3+ -oxygen vacancy pair is highly unfavorable from a thermodynamic point of view), whereas Co3+ is oxidized to Co4+ . Ni2+ (∼2µB ) ions are always oxidized to Ni3+ (∼1µB ) but further oxidation (to Ni4+ , ∼0µB ) strongly depends on the thickness of the Li2 MnO3 -shell. The amount of Ni4+ ions (i.e., the extent of Ni oxidation) increases with the thickness of the shell. For the lower-limit-thickness case (0.125Li2 MnO3 ·0.875LiNiCoMnO2 ), some Ni ions close to the core-shell interface remain in a 3+ valence state, in order to maintain

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the overall charge neutrality of the nanocomposite. The core-shell structure of the 0.5Li2 MnO3 ·0.5LiNiCoMnO2 nanocomposite has been experimentally shown to exhibit higher thermal stability than the standard, layered LiNiCoMnO2 , not only retarding the transformation of its structure into a LiM2 O4 -type spinel phase at high temperatures but also slowing down the process. 8 That spinel phase coexists with a Mn3 O4 like phase, formed by dissociation of the Li2 MnO3 shell. Our computational results show that there is no thermodynamic driving force for a structural transformation of the NCM-core into a LiM2 O4 -type spinel phase (see Figure S2 of the Supporting Information). Thus, keeping the structural integrity of the Li2 MnO3 -shell is the key factor to avoid the formation of local regions of Lix M2 O4 at the surface of the layered LiNiCoMnO2 and their subsequent propagation deep into the core region, which would enable partial phase transformations through the well-known TM migration mechanisms. 44,45 One possible limitation of these core-shell structures is the possibility of having boundary cracks between the core and the shell due to different volume expansions during cycling. 17 To overcome this challenge, new core-shell materials with a concentration-gradient-shell have been introduced, in order to produce a continuous composition change from the core to the outer shell. 54 This approach has been very effective to enhance the cycle retention due to the minimized formation of microcracks. Although the modeling of such concentration-gradient-shells is beyond the capabilities of our DFT calculations, it is possible to identify proper core and shell electrode materials to minimize the risk of boundary cracks, i.e., materials and compositions with similar and suitable volume expansions. As an example, Figure S5 (see Supporting Information) shows the evolution of the lattice parameters of the 0.125Li2 MnO3 ·0.875LiNiCoMnO2 nanocomposite during the charging cycle. There is a slight in-plane compression and out-of-plane expansion of the lattice vectors during delithiation. The in-plane compression corresponds to the delithiation of the NCM-core, which reduces even more the core-shell lattice mismatch (from the initial ∼1.5% to ∼0.3%) and the corresponding interface strain. The stretching of the c lattice vector is the result of two opposite effects: the smaller volume of Ni3.5+ ions and the

19



larger coulombic repulsion of the oxygen anions, which is now not screened by Li+ cations. The presence of non-oxidizing Mn4+ in the TM layers in both NCM-core and Li2 MnO3 -shell regions partially mitigates the first effect, leading to a slightly uniform, out-of-plane expansion. Of course the stabilization effect of Mn4+ is not present in Ni-rich NCM core materials, thus making them more prone to suffer boundary cracks. 17 In the low Li content region, we assumed that the structure is still layered, i.e., the Li2 MnO3 -shell phase helps to prevent phase transformations, at least during the first chargedischarge cycle. However, as mentioned above, although there is a kinetic limitation against TM diffusion into the (now) empty Li layers (especially for Mn4+ ions), 55 the extraction of Li+ from the core-shell interface could trigger a transformation of the Li2 MnO3 -shell phase into the LiMnO2 layered oxide. Indeed, our AIMD simulations show a spontaneous migration of the Li ions at room temperature from the Li2 MnO3 region to the tetrahedral empty sites (Td ) of the core-shell interface (see the path in the Figure S6 of the Supporting Information). This path can be regarded as the “first-half” of the well-known “Dumbbell path”, which has been suggested as the probable TM migration path in layered oxides. 44,56 For Li in Li2 MnO3 , the migrating ion (from a 2b lattice site of the Li-Mn layer) aims to occupy an octahedral site (Oh ) of the adjacent Li layer, which would correspond to the 4h or 2c lattice sites. First, it migrates into the nearest Td vacancy site and, then, diffuses through the face of the octahedron into the corresponding Oh site. Direct diffusion from Oh -to-Oh sites shows generally higher activation barriers. 44 Our results show that the “second-half” of the Dumbbell path (from Td to the final Oh site) is both thermodynamically and kinetically blocked, at least at room temperature. Any attempt to increase the kinetic energy of the Li+ ions drives them back to the Td sites. In pristine Li2 MnO3 material, the Oh (2b)to-Oh (2c, 4h) diffusion barrier is ∼0.5 eV, whereas for the reverse process the activation barrier is ∼0.6 eV. 51 Obviously, this effect is well-explained in terms of the high-instability of the Li2 MnO3 compound at low Li concentrations, and the strong thermodynamic driving force for Li2 MnO3 phase transformation into LiMnO2 in the first place and then, upon Mn

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migration, into LiMn2 O4 , where some of the Li ions occupy tetrahedral sites. 44 In our coreshell structure, charging the electrode beyond the voltage shown in the shaded regions in Figure 5 could trigger such phase transformations by driving Li ions from the Li-Mn layers of the shell region into the (now empty) core-shell interface, and thus inducing a chain reaction that would ultimately lead to a LiMnO2 electrochemically active shell. To examine this issue, we have investigated the Li migration from Td sites at the core-shell interface back to Oh sites in the Li-Mn layer of the Li2 MnO3 shell. The result is shown in Figure 6. The “reverse-Dumbbell” path is composed of two separate processes with moderate and mediumhigh kinetic barriers, respectively. First, the Li ion leaves the tetrahedral ligand field moving towards the Td -Oh shared-face. This is not the rate-limiting step of the reaction, with a kinetic barrier of only 0.34 eV. As can be seen in the intermediate structure illustrations shown in Figure 6, in the second step the Li enters in the octahedral ligand field of the final Oh site through the shared-face. The kinetic barrier for this process is 0.82 eV, which is sluggish but still viable at room temperature. The reaction is slightly endothermic, with an activation energy of 0.51 eV. Such energy difference explains the ease of the reverse reaction (Oh -Td Li migration). Therefore, the resulting activation barrier is slightly higher than that of the pristine Li2 MnO3 material. Besides the observed low Li mobility in this material, 57 there is no deformation of the surrounding oxygen environment (all the oxygen ions remain in a O2− valence state) and, consequently, no cooperative ionic motion providing more space for the Li ion to diffuse. Furthermore, even for low Li content, the interlayer distance at the core-shell interface does not change substantially and the tetrahedral Td site is not significantly deformed with respect to the lithiated phase, which could have increased the activation barrier substantially. The kinetic barrier can also be slightly decreased if the core-shell interface is not completely delithiated, approaching the limit of the Li2 MnO3 pristine material (∼0.6 eV). Then, we can conclude by saying that the extraction of Li ions from the core-shell interface is not necessary deleterious for the structural stability of the core-shell electrode material, provided that the operating voltage window is adequately

21



chosen according to the core-shell relative composition, as shown in Figure 5. Although we have found compelling evidence of Li migration from the Li2 MnO3 -shell to the Td sites of the core-shell interface (which could be regarded as the first step of the layered-to-spinel phase transition), the fact that the “second-half” of the Dumbbell path (Td -Oh ) is both thermodynamically and kinetically blocked makes relatively unfeasible the dissociation of the Li2 MnO3 -shell phase. Finally, it is important to remark that diffusion of the cations between the core and shell phases can also occur during certain synthesis processes, especially sintering, which can significantly affect the final core-shell properties. Although not exactly the same core-shell system studied here, interdiffusion coefficients have been recently measured for core-shell composites with mixtures of TM. 58 The activation energy barriers for the Co3+ /Mn4+ and Ni3+ /Mn4+ couples were found to be 3.0 and 5 eV, respectively. 58 As such, it was concluded that a relatively thick initial shell with a high Mn content is required in order to maintain a Mn-rich surface after sintering. Therefore, that result supports our main conclusion that the extraction of Li ions from the core-shell interface would not be a determinant factor for the structural stability of the system. Even though our AIMD results do not show TM spontaneous diffusion from the core region to a partially delithiated core-shell interface (with the aforementioned Li ions occuppying Td sites), in order to support the above argument we have obtained the kinetic barriers for Co and Ni migration from the closest layer of the core to the partially lithiated interface. The obtained results, 0.84 and 1.02 eV for Co and Ni (see Figure S7 of the Supporting Information), are slighlty larger than that of Li diffusion from the shell region to the interface, which constitutes another evidence for the overall stability of the core-shell system.

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Conclusions In this work, we used DFT coupled with a ternary CE to examine the structural stability and voltage characteristics of core-shell xLi2 MnO3 ·(1-x)LiNiCoMnO2 nanocomposites. Our results show appropriate voltage operational windows, as a function of the thickness of the Li2 MnO3 -shell, to maintain the shell region electrochemically inactive, thus avoiding undesired phase transformations. Assuming that the core of the electrode material remains structurally intact (in the original layered structure), we predict three different regions during the charging process, two of them with relatively strong solid-solution-like behavior. Furthermore, no thermodynamic driving force for oxygen release or further oxidation was found, which leaves the charge compensation during delithiation to TM oxidation, especially Ni2+ ions. Finally, if Li+ ions are extracted from the core-shell interface, Li migration from the Li2 MnO3 -shell to the empty Td sites is a spontaneous, although reversible, process, which highlights the importance of determining the operational voltage window carefully, to prevent the dissociation of the Li2 MnO3 -shell and keep the overall structural stability of the nanocomposite. This study opens a new avenue for the structural stabilization of Li-rich layered oxides. Although we have used the well-known layered structure of NCM111 as the core of the nanocomposite, other materials showing more complicated electrochemical drawbacks (for instance, highly-unstable Ni-rich NCM) or even different morphologies (such as vanadates) could also be examined with the methodology presented in this work.

Acknowledgement The authors acknowledge the Texas Advanced Computing Center (TACC) for providing the computational resources. This work was supported by the L&F Co.’s World Class 300 Project of the Korea Institute of Advancement of Technology (KIAT) funded by the Ministry of Trade, Industry, and Energy (No. S2483103). This work was also supported by 23



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the International Energy Joint R&D Program (No. 20168510011350) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy, Korean government.

Supporting Information Available “Tables with the ECI coefficients and CE energies, together with the formation energies, intermediate delithiated configurations and magnetic moment evolution of the compounds studied.” This material is available free of charge via the Internet at http://pubs.acs.org/.

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Core-Shell Nanocomposites for Improving the Structural Stability of Li

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