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Computational Screening for Design of Optimal Coating Materials to Suppress Gas Evolution in Li-Ion Battery Cathodes Kyoungmin Min,† Seung-Woo Seo,† Byungjin Choi,‡ Kwangjin Park,*,‡ and Eunseog Cho*,† †

Platform Technology Lab and ‡Energy Lab, Samsung Advanced Institute of Technology, 130 Samsung-ro, Suwon, Gyeonggi-do 16678, Republic of Korea S Supporting Information *

ABSTRACT: Ni-rich layered oxides are attractive materials owing to their potentially high capacity for cathode applications. However, when used as cathodes in Li-ion batteries, they contain a large amount of Li residues, which degrade the electrochemical properties because they are the source of gas generation inside the battery. Here, we propose a computational approach to designing optimal coating materials that prevent gas evolution by removing residual Li from the surface of the battery cathode. To discover promising coating materials, the reactions of 16 metal phosphates (MPs) and 45 metal oxides (MOs) with the Li residues, LiOH, and Li2CO3 are examined within a thermodynamic framework. A materials database is constructed according to density functional theory using a hybrid functional, and the reaction products are obtained according to the phases in thermodynamic equilibrium in the phase diagram. In addition, the gravimetric efficiency is calculated to identify coating materials that can eliminate Li residues with a minimal weight of the coating material. Overall, more MP and MO materials react with LiOH than with Li2CO3. Specifically, MPs exhibit better reactivity to both Li residues, whereas MOs react more with LiOH. The reaction products, such as Li-containing phosphates or oxides, are also obtained to identify the phases on the surface of a cathode after coating. On the basis of the Pareto-front analysis, P2O5 could be an optimal material for the reaction with both Li residuals. Finally, the reactivity of the coating materials containing 3d/4d transition metal elements is better than that of materials containing other types of elements. KEYWORDS: Li residue, Li-reactive coating, phase diagram, metal phosphate, metal oxide applied to LiCoO2 and LiNi0.9Co0.1O2 cathode materials.7,8 In addition to simply preventing degradation, enhancing Li-ion diffusion during cycling has been considered; the direct use of Li-ion-conductive materials such as LiAlO2,9 Li2SiO3,10 and Li2TiO311 as coatings has been proposed recently to obtain an excellent rate capability and enhanced capacity retention rate. Another effect from coating materials of Li-containing phases such as Li2TiO3 and Li2SiO3 is that they can slightly decrease the initial capacity without changing the voltage compared to that of the pristine cathode.12,13 In addition, in terms of impedance, the charge transfer resistance for a Li-containingcoated material is decreased. As expected, they still exhibit great rate capability as well as cycling retention. Apart from experimental works, high throughput screening methods have been applied to search for optimal cathode coating materials. Aykol et al. introduced a first-principles-based framework to find promising coating materials among 81 MO structures that can scavenge hydrofluoric acid (HF) for

1. INTRODUCTION Mitigating degradation phenomena in lithium-ion batteries (LIBs) with nickel (Ni)-based layered-oxide cathodes has been a key challenge to exploit their high reversible capacities of over 200 mA h/g.1 For example, cation disordering, oxygen evolution, and phase transformations are well-known phenomena that degrade the electrochemical performance of LIBs.1 To alleviate the resulting difficulties and improve the electrochemical performance of the cathode, surface coating (also referred to as surface modification) has been suggested; in this method, an additional physical barrier is provided at the surface of the cathode, which can limit direct contact between the cathode surface and electrolytes. For example, coating with Al2O3 has been extensively explored and proven to improve the capacity retention rate of layered-oxide-based cathode materials like LiCoO2 and Li1.05Ni0.4Co0.15Mn0.4O2.2,3 Other types of metal oxide (MO) materials such as SiO2,4 TiO2,5 and V2O56 have been widely investigated and have also been demonstrated to be effective in preventing capacity fading. In addition, metal phosphates (MPs) such as AlPO4 and FePO4 are another type of promising coating material, and they have been found to improve the cycle performance and thermal stability when © 2017 American Chemical Society

Received: January 6, 2017 Accepted: May 5, 2017 Published: May 5, 2017 17822

DOI: 10.1021/acsami.7b00260 ACS Appl. Mater. Interfaces 2017, 9, 17822−17834

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ACS Applied Materials & Interfaces

Figure 1. (a) A schematic view of removing Li residuals at the surface of the cathode during the coating process. (b) Screening process for finding optimal coating materials for removing Li residuals.

improved electrochemical performance.14 A similar approach was used by the same group for investigating the reactive stability at the interface between coating and cathode as well as for finding coating materials with superior solubility of transition metals (TMs) in the coating materials.15 Aykol et al. further developed their computation model for examining the HF scavenging capability and reaction stability between coatings and cathode materials by utilizing their extended materials database.16 Although these surface coating methods have been shown to enhance the electrochemical performance of cathodes, they do not entirely prevent side reactions on the cathode surface. This is because Li residues remain on the surface and are the main source of gas generation there.17−19 These remaining impurities originate from the excess Li provided during synthesis to realize adequate cathode capacity.20,21 This Li can react with O2, H2O, or CO2 in the air and form LiOH and Li2CO3 on the surface, leading to undesirable gas generation by their decomposition reactions, mainly with electrolytes.18,22,23 This gas evolution during cycling causes the battery pack to swell and can permanently degrade the performance of the battery.22 More specifically, a previous study suggested that the main gas species generated in the reaction between residual Li compounds and an electrolyte is CO2, and other species such as O2, N2, and CO can also be formed when the cathode is charged.23 In addition, Li residues can react with slurry components during electrode fabrication, leading to gelation of the cathode.19 Li residues also cause the formation of undesirable LiF precipitates on the surface of electrode particles, subsequently impeding the Li diffusivity.24 More importantly, more Li residue exists on the surface of Ni-rich layered oxides than on oxides with low Ni content (the amount is more than 10 times higher for 95% Ni content than that for a Ni content of 33%).25 Thus, eliminating

Li residues by physically washing them off with water has been proposed as a general approach to lessening this potential risk. Although this method can significantly reduce the amount of Li residue, this additional step complicates the synthetic process and makes it more costly; most importantly, it considerably degrades the capacity retention rate of the cathode.18,22,23 Therefore, the use of Li-reactive coating materials is an ideal way to overcome these problems simultaneously, that is, to both remove Li residues to prevent gas evolution and leave the Li-containing phases for better Li diffusion. The amount of Li residues largely increases as more Ni content is introduced for a larger capacity of the cathode materials;20 thus, the coverage of residual Li area on the cathode material becomes large. Hence, we expect that coating materials could react with the Li residues first and unreacted oxide/phosphate materials would remain at the surface of the cathode as a chemically inert island, which could then also work as the conventional coating material. A schematic view of the coating process is shown in Figure 1; here, Li−M−P and Li−M−O represent the Li-containing MP and oxide phases, respectively. This concurrent coating methodology is expected to effectively reduce the generation of gas species as well as the formation of LiF, which results from the decomposition reaction between residual Li compounds and electrolytes. Representative reaction equations obtained from previous studies are shown as follows.17,23,26 3LiOH + 2C3H6O3(DMC) + EMC + LiPF6 → C3PH 9O4 + C5H10O3(DEC) + 2HF + 4LiF + H 2O + CO2

17823

(1)

LiOH + 2HF → LiF + H 2O

(2)

Li 2CO3 + LiPF6 → 3LiF + POF3 + CO2

(3)


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Figure 2. Calculated coating materials of MPs and MOs. Red, violet, black, and blue circles are present if the reaction product exists. From the screened materials, MP and MO indicate calculated materials for those species only, respectively, and MP/MO means both materials are considered.

elements such as Mg and Ca are considered only in calculations for MP materials, whereas Si and Zr are considered only for MOs. This is because we chose only coating materials whose Li-containing compounds are available in the form of Li−M−P or Li−M−O for MPs and MOs, respectively, in the inorganic crystal structure database (ICSD). In addition, because the same material can have different crystal structures (e.g., the space group of Al2O3 can be R3c̅ or C2/m), we chose the structures satisfying the following conditions: (a) the structures are available in the ICSD and (b) their energy above the hull is close to zero according to the Materials Project database.27 All of the data (crystal structures and their formation energies) are tabulated in the Supporting Information (SI). To calculate the formation energy accurately, we used the Heyd−Scuseria− Ernzerhof (HSE06) hybrid functional, although it is computationally expensive.28,29 Once the database was compiled, the phase diagrams of the MPs and MOs in reaction with residual Li compounds (LiOH/ Li2CO3) and O2 were constructed to find the equilibrium phases of the products of the reactions between Li and the coating materials. The additional phases generated from an extra O2 axis can provide information that is useful when the reaction occurs under an O2 environment at various values of pressure, which could be a control parameter during cathode coating. From the phase diagram, the reliable reaction equations can be extracted using the information on the products and energetics. Finally, a design chart with axes of ΔHLi−M and GC was constructed to identify the optimal coating materials that can remove residual Li compounds by reacting with them. 2.2. First-Principles Calculation. Computations based on density functional theory (DFT) were performed using the projector augmented wave method and the Vienna ab initio simulation package.30,31 Structure files were obtained from the Materials Project library.27 The HSE06 hybrid functional was used to calculate the energy of the system without further geometry optimization.28,29 It is well known that the generalized gradient approximation (GGA) poorly describes the highly localized d orbitals in TM compounds, introducing considerable error into the calculation of the energy; thus, it is necessary to use the U energy (GGA + U) for calibration.32,33 However, U values are available only for certain TMs. More importantly, it is essential that the U values are calibrated

Li 2CO3 + C3H6O3(DMC) + HF → C3H5O3Li + LiF + H 2O + CO2

(4)

Li 2CO3 + 2HF → 2LiF + H 2O + CO2

(5)

Despite increasing demands for ideal Li-reactive materials, it is difficult to select the proper coating material from among the many candidates because a limited number of previous reports are available for this purpose. A study performed by Kim and Cho found that a Co3(PO4)2 nanoparticle coating can react with Li impurities on the cathode material LiNi0.8Co0.16Al0.04O2 during annealing and form the Li-containing phase LiCoPO4 on the bulk surface.18 In addition, Jo et al. showed that phosphoric acid (H3PO4) can react with residual Li compounds on a Ni-rich layered cathode, forming a Li3PO4 surface layer that significantly improves the electrochemical performance.17 Herein, we implement a novel computational framework for searching through numerous types of MPs and MOs to identify optimal coating materials that can react with residual Li compounds such as LiOH and Li2CO3. In this study, 16 MPs and 45 MOs are considered. By constructing phase diagrams, we compare the reaction energy (ΔHLi−M) and gravimetric capacity (GC) of equilibrium Li-containing phases obtained from the reaction equations of the coating materials and Li residues. ΔHLi−M refers to the change in the reaction energy when the reactants are transformed to products, and GC is the weight of the coating material required to remove 1 mol of Li residue. This study reveals that a first-principles-based computational approach is an effective tool for screening and suggesting ideal coating materials.

2. COMPUTATIONAL DETAILS 2.1. Screening Process. To discover the optimal MP and MO coating materials for removing residual Li compounds, we proposed the screening process illustrated in Figure 1. First, a database containing the formation energies of all of the relevant materials was created. A total of 431 structures were calculated, including 16 MP and 45 MO materials, as well as other pure and compound materials consisting of carbon, lithium, oxygen, hydrogen, and metals. (The results for the interaction of ZrO2 with LiOH and Li2CO3 are adopted from a previous work.19) Figure 2 shows an overview of the calculated types of coating materials as well as their reactivity with residual Li. Some 17824


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Figure 3. Overall design chart for the reaction between MP/MO and LiOH/Li2CO3.

configurations used in the Materials Project were applied. Moreover, although the free energy for gaseous species can be largely affected by the entropy contribution at finite temperature, we did not include this effect to provide the complete list of possible reaction phases that the target materials can have as the phases only disappear at elevated temperature (SI).

according to the oxidation state of the TMs, and at the same time, the identical U values should be used to compute the reaction energy.34 Although some of the GGA and GGA + U problems are resolved by adding correction factors with respect to the experimental results, these corrections still need further work because not all experimental enthalpy information is available for many other structures.35,36 These problems limit the use of U values because the oxidation state of TMs in the reaction can vary depending on their oxide form. Therefore, in this work, the hybrid-functional approach is adopted to address these problems, even though its computational cost is more than 100 times greater than that of the conventional GGA approach. The energy values calculated using the hybrid functional were found to show greater agreement with the experimental results than those obtained by GGA calculation, as shown in Figure S1. The mean absolute error in the experimental results was computed as 0.052 and 0.218 eV for the hybrid and GGA calculations, respectively. The plane-wave cutoff was set to 500 eV. For k-point generation, the

3. RESULTS AND DISCUSSION 3.1. Design Chart. Figure 2 shows all of the metal elements in the MP and MO materials considered in this study and also shows the possibilities of reactions between the coating materials and Li residues. (For the 16 MP and 45 MO materials, 14 and 28 metal elements were calculated, respectively.) For example, Ti-, V-, and B-based phosphate and oxide materials are all reactive to both Li residues. However, only the phosphate forms of Cu- and Zn-based materials are reactive to LiOH. To provide a general overview of the materials’ capabilities, Figure 3 shows the design chart for all of the Li-reactive MP and MO coating materials. This 17825


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Figure 4. Descending order of (a) ΔHLi−M and (b) GC, and (c, d) comparison within materials having the same metal element for the reaction between MPs and LiOH/Li2CO3. Each number in (a) and (b) denotes the reaction equation number.

realize higher energy density in the cathode. However, it would leave unreacted LiOH and Li2CO3 residues on the cathode surface, potentially leading to side reactions. Therefore, it is critical to understand this trade-off and identify materials having both capabilities, which will be discussed in depth in the following sections. 3.2. Reactions between MPs and LiOH/Li2CO3. We extracted and analyzed the data in the design chart presented above in more detail. First, all of the equations for reactions between the coating materials and residual Li are shown in the SI. Note that two types of data analysis are performed; ΔHLi−M and GC values are each given in descending order with their corresponding GC and ΔHLi−M values (denoted as GC′ and ΔHLi−M′). To sort by descending order, the data are arranged starting with the largest value of each quantity among the possible reactions for each material. The corresponding values are the GC value obtained for the reaction with the largest ΔHLi−M (and so on, in descending order of ΔHLi−M), and vice versa. All of these orders are also shown in the SI. First, the ΔHLi−M and GC values of the 16 MP materials for reactions with either LiOH or Li2CO3 are presented in descending order in Figure 4. The reaction of the MPs with LiOH was calculated to be energetically the most preferred when Mn3(PO4)2 is used; the remaining materials follow in the order W(PO4)3, Fe3(PO4)2, CoPO4, Co3(PO4)2, and so forth. Further, BPO4 yields the largest GC value, [Figure 4b], followed by FePO4, Fe3(PO4)2, TiPO4, AlPO4, and the rest. The overall reaction equations for the most energetically preferred reaction for all of the MP materials are shown in the SI to show the mechanism in detail. Regarding the order of GC values for the reactions of MP with LiOH, it is important to note that when the coating material is selected according to the magnitude of ΔHLi−M, the corresponding order of GC can be completely different from

information indicates the strengths and weaknesses of each material in terms of its reactivity (ΔHLi−M) and efficiency (GC). Several interesting points can be observed. First, a comparison of the average ΔHLi−M values of the MPs and MOs reveals that for the MPs, reaction with LiOH (−0.634 eV) is more energetically favorable than that with Li2CO3 (−0.596 eV), whereas for the MOs, reaction with Li2CO3 (−2.397 eV) is more energetically favorable than that with LiOH (−0.564 eV). Regarding the GC values, MP materials require a slightly smaller weight for removal of both Li compounds (0.0192 and 0.0124 mol Li/g for LiOH and Li2CO3, respectively) than that of the MO coating materials (0.0160 and 0.0082 mol Li/g for LiOH and Li2CO3, respectively). In addition, Li2CO3 is clearly more energetically favorable than LiOH for reaction with the MP and MO coating materials. For example, the average ΔHLi−M value for reaction of the MPs and MOs with Li2CO3 is −1.496 eV, which exceeds that for their reaction with LiOH (−0.599 eV). However, for the Li2CO3-reactive materials, their average GC value (0.010 mol Li/g) is smaller than that for their reaction with LiOH (0.017 mol Li/g), which indicates that a greater weight of coating material is required to eliminate the same amount of residual Li in the form of Li2CO3. According to these observations, two strategies for selecting coating materials can be proposed depending on whether the primary focus is on (1) removing a large amount of Li residue or (2) using less of the coating material. If the goal is primarily to reduce the amount of Li, MO coating materials will be an ideal choice considering their higher reactivity to Li2CO3. However, this choice requires sacrificing the specific capacity of the cathode because a larger weight of the coating material is necessary. (Note that increasing the amount of the coating usually reduces the specific capacity.) To satisfy condition (2), removal of LiOH with MPs is an ideal strategy because this process requires less coating material, making it possible to 17826


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Figure 5. Descending order of (a) ΔHLi−M and (b) GC, and (c, d) comparison within materials having the same metal element for the reaction between MOs and LiOH/Li2CO3. Each number in (a) and (b) denotes the reaction equation number.

that in Figure 4b for some materials. For example, the GC value of Fe3(PO4)2 appears in the third place in Figure 4b, but Fe3(PO4)2 is found to be the least efficient when the GC values for reaction 1 are arranged in descending order. (Note that the reaction in the first place is that calculated to be the most energetically preferred.) This is because each coating material has several possible equilibrium reaction phases with respect to the Li residues, depending on the molar ratio of the reactants. In contrast, BPO4 and FePO4 are the most gravimetrically efficient materials irrespective of which of the several possible reactions occur. (Reactions 5 and 2 are available for BPO4 and FePO4, respectively.) In addition, VPO4 exhibits the smallest GC value for all of its equilibrium phases, but it appears in second place among all of the materials for reaction 1. Table S7 lists the order of GC′ for reaction 1 for all of the materials. Interestingly, unlike the order of the GC values, the order of ΔHLi−M in Figure 4a is generally unchanged when the values are arranged according to the gravimetric efficiency of the reactions; for example, Mn3(PO4)2, W(PO4)2, and CoPO4 are still energetically preferred regardless of the order of their GC values. Overall, this analysis indicates that the amount of coating material should be carefully determined to achieve the best performance. Another important finding is that the metal Fe can exist in two different compounds, FePO4 and Fe3(PO4)2. This suggests that FePO4 is energetically less reactive, but its GC value is larger than that of Fe3(PO4)2, as shown in Figure 4c,d. Because FePO4 can be transformed into Fe3(PO4)2 during annealing above 500 °C, it could be critical to control the temperature during the coating process depending on the purpose.37 The two Co compounds [CoPO4 and Co3(PO4)2] are comparable according to both criteria. Information on the reactions between the MP coating materials and Li2CO3 is also presented in Figure 4. The reaction is most energetically preferred when TiPO4 is used;

the remaining compounds follow the order Mn3(PO4)2, Fe3(PO4)2, Zn3(PO4)2, Ni3(PO4)2, and so forth. The reaction equations for the most energetically preferred reaction of each material are tabulated in the SI. When the compounds are arranged in descending order of GC, BPO4 exhibits the best efficiency, followed by Ni3(PO4)2, Mg3(PO4)2, Mn3(PO4)2, Ca3(PO4)2, and so forth, [Figure 4b]. Interestingly, unlike the case for reaction with LiOH, each of the MP materials has fewer equilibrium phases for reaction with Li2CO3; for example, there are five BPO4−LiOH reactions available, but only one exists for BPO4−Li2CO3. Even though a larger ΔHLi−M requires the use of a greater weight of the coating material for reactions of the same materials, as observed in the MP−LiOH interactions, the descending order of the GC values is almost unchanged, unlike that of ΔHLi−M, except for Fe3(PO4)2 and Zn3(PO4)2. As in the MP−LiOH case, two of the Co−P compounds can react with Li2CO3, but in this case, their ΔHLi−M and GC values are comparable [Figure 4c,d]. Because two types of Li residue are present on the cathode surface, it is important to find coating materials that react with both of them. Twelve of the sixteen MP materials considered here meet this condition [all except for YPO4, AlPO4, FePO4, and Cu3(PO4)2]. Another important condition is that it would be more useful to find a coating material that reacts better with Li2CO3 to reduce the total amount of Li residue. This is because the amount of Li2CO3 reportedly occupies a larger portion of Li residue than that of LiOH.19,25 Considering these two criteria, Mn3(PO4)2, TiPO4, Fe3(PO4)2, and Zn3(PO4)2 could be good candidates for reducing the total amount of both types of Li residue according to their ΔHLi−M values. Alternatively, if it is necessary to use the minimum weight of the coating material, then using BPO4 and Mg3(PO4)2 would be an ideal option on the basis of their GC values. The above analysis reveals important points regarding the use of MP coating materials. (1) For the material having the 17827


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Figure 6. Phase diagram for (a) Co3(PO4)2−LiOH/Li2CO3−O2 and (b) P2O5−LiOH/Li2CO3−O2.

its ΔHLi−M value), but their corresponding GC′ values are much lower, in the 19th and 24th positions, respectively, out of the 28 materials (Table S8). On the other hand, B2O3 (1st) and Fe3O4 (2nd) are highly efficient in terms of GC, as shown in Figure 5b, but their corresponding ΔHLi−M′ values are in 22nd and 24th place, respectively. Further, P2O5 is superior under both conditions (considering ΔHLi−M and GC); that is, its ΔHLi−M value is in first place when both ΔHLi−M and ΔHLi−M′ are arranged in descending order. In addition, it has the thirdhighest GC value and the fifth-highest GC′ value. This means that P2O5 possesses multiple desirable features and can be used according to either criterion to maximize LiOH removal. Similarly, B2O3 has the fourth-highest ΔHLi−M value and the highest GC among the values for all of the possible reactions of the materials, indicating that this material can also be used for multiple purposes depending on the amount that is used. Further, both TiO (second) and V2O3 (third) are highly reactive to LiOH when their ΔHLi−M′ values are considered (fourth and second for TiO and V2O3, respectively). This result indicates that these materials can be used in situations where the application of lightweight coating materials is less important. Materials exhibiting outstanding GC values generally show poorer reactivity when their values are sorted in terms of ΔHLi−M (e.g., B2O3 has the highest GC value but drops to 17th place for GC′). Among them, SiO2 and TiO2 demonstrate moderate performance, as they are ranked fifth and eighth when all of the available reactions are considered and seventh and first for reaction 1, respectively. Because MO materials can form different oxidation states even though the metal element is the same, it is important to compare their performance to that in reaction with LiOH, which provides meaningful information for their synthesis. Four metal elements (Ti, V, Nb, and Sn) have various types of oxides with different oxidation numbers, as shown in Figure 5c,d. (Mn is not considered here because of its inertness to Li residues;

most energetically preferred reaction among all its possible equilibrium phases, the largest weight of the coating material is generally required; in other words, a coating material with a larger ΔHLi−M value is not necessarily gravimetrically efficient. (2) More MP materials can react with LiOH than with Li2CO3, with more possible equilibrium phases, indicating that it is more important to choose an appropriate MP−Li2CO3 reaction to obtain the maximal reactivity and efficiency. 3.3. Reactions between MOs and LiOH/Li2CO3. The ΔHLi−M and GC values obtained for reactions of the MOs with LiOH are presented in Figure 5. Note that among the 45 screened MO materials, 28 of them can reach thermodynamically stable phases after reaction with LiOH. Among the remaining 17 MOs, 10 (MnO, Mn3O4, ZnO, Y2O3, CuO, FeO, Bi2O3, In2O3, PdO, and NiO) are shown to be nonreactive to LiOH. Further, the other 7 MOs (Pd2O, CrO2, Fe2O3, Sn3O4, Nb4O5, Mn2O3, and CoO) are thermodynamically unstable and decompose to more stable oxide forms. For example, Nb4O5 is more energetically stable when it exists as NbO + NbO2. Hence, we omit these 7 MOs when calculating the reactivity to Li residues. In terms of the ΔHLi−M values in Figure 5a, P2O5 was calculated to be the most energetically favorable coating material, followed by TiO, V2O3, B2O3, VO, and so forth. The reaction equations for reaction 1 for each material can be studied to show the detailed reaction mechanism (SI). In terms of the GC values, B2O3 is shown to be the most efficient, followed by Fe3O4, P2O5, MoO3, SiO2, and the rest [Figure 6b]. As was observed for the MP−LiOH reactions, the MO−LiOH reaction with a large value of ΔHLi−M (TiO, ranked 2nd) is positioned 15th in terms of GC. There are a number of interesting points regarding the relationship between the ΔHLi−M and GC values. First, coating materials TiO (2nd) and V2O3 (3rd) are highly reactive to LiOH for reaction 1 (the number in parentheses is the order of 17828


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heavier coatings must be applied for MOs exhibiting larger ΔHLi−M values for reactions with LiOH and Li2CO3. 3.4. Equilibrium Phases of MPs/MOs and LiOH/Li2CO3. In the previous sections, we determined which of the MO/MP materials are reactive to residual Li compounds in terms of their ΔHLi−M and GC values. Because the reaction products always include Li-containing phases, it is also important to validate those products by comparing them to the results of previous studies. As confirmed by the reaction products in Table I, MP coating materials normally form Li−M−PO4 or Li−M−P2O7 phases

only MnO2 is reactive.) For example, Ti has three oxide forms, TiO, Ti2O3, and TiO2, each of which exhibits different reactivity to LiOH. Among them, TiO is the most reactive, but its GC value is the smallest and ranks 15th among those of all of the materials; thus, one might want to use TiO2 instead (its GC value is ranked 8th) to obtain better efficiency when using a Tibased oxide. Applying Ti2O3 as a coating would not be advisable because it is less reactive than TiO and less gravimetrically efficient than TiO2. For the V and Nb oxides, the materials with the largest reactivity, V2O3 and NbO, also have the largest GC values; thus, these materials are satisfactory for use. Compared to that of the other materials, both Sn-based oxides (SnO and SnO2) exhibit very poor reactivity, but the GC value of SnO2 is almost twice that of SnO, indicating its moderate efficiency (ranked 16th out of 28). Many fewer preferred MO−Li2CO3 reactions are possible compared to the number of possible MO−LiOH reactions, as shown in Figure 5, indicating that MO materials are generally not reactive to Li2CO3. Only 8 of the 45 MO materials can react and form Li-containing stable phases. The reaction equations for the most energetically preferred reaction of each material are shown in SI. The most reactive material was calculated to be V2O3, followed by VO, NbO, TiO, Ti2O3, and so forth [Figure 5a]. Interestingly, P2O5 is the most reactive to LiOH, but here it is ranked seventh out of eight materials. However, considering that only a few materials react with Li2CO3, P2O5 is still a decent material for removing both types of Li residue. More importantly, P2O5 is the most efficient material in terms of GC, followed by NbO, V2O3, B2O3, VO, and so forth, as shown in Figure 5b. This is because P2O5 has only one possible reaction, unlike NbO and V2O3, which have five and three phases, respectively. Note that although NbO and V2O3 have several stable phases, their GC values are still large (second and third places, respectively), and they are also highly reactive (third and first places, respectively) to Li2CO3. This result reveals that these materials can be used for multiple purposes depending on the amount used. As we saw for the MO−LiOH interactions, three metal elements (V, Nb, and Ti) can form oxides with different oxidation numbers and react with Li2CO3 [Figure 5c,d]. The ΔHLi−M and GC values of the V oxides (V2O3 and VO) are comparable. For the Nb and Ti oxides, it is interesting that their oxide forms with higher reactivity also exhibit a larger GC value. As discussed for the MPs, it is also important to find MO coating materials that are reactive to both Li residues. Eight out of the 45 MO materials satisfy this condition. (Note that this is the same as the total number of MOs that are reactive to Li2CO3. Among them, seven of the MO materials, all except for NbO2, are highly reactive to LiOH. They are all ranked seventh among all of the reactions with LiOH.) In terms of their GC values, three of the MO materials (B2O3, P2O5, and NbO) exhibit superior efficiency for the reaction with LiOH. (They are ranked first, third, and seventh, respectively.) This result demonstrates that although many potential MO materials are available, only a few candidates are practical for removing both Li residues. In the analysis of the MP−LiOH/Li2CO3 reactions in the previous section, we discussed important findings on the relationships between ΔHLi−M, GC, and the number of possible reaction equations. Here, for the MO−LiOH/Li2CO3 reactions, those findings are also applicable; that is, MOs preferably react with LiOH over Li2CO3, with more equilibrium phases, and

Table I. Product Phases from the Reaction between MP and LiOH/Li2CO3 Li residues coating material AlPO4 CoPO4 Co3(PO4)2 Mn3(PO4)2 FePO4 Fe3(PO4)2 Mg3(PO4)2 Zn3(PO4)2 VPO4 Ni3(PO4)2 Cu3(PO4)2 Ca3(PO4)2 YPO4 BPO4 W(PO4)2 TiPO4

LiOH

Li2CO3

Li3PO4, LiAlO2 LiCoPO4, Li3PO4 LiCoPO4, Li3PO4 Li2MnP2O7, Li3PO4 LiFeO2, Li5FeO4, Li3PO4 Li2Fe3(P2O7)2, LiFePO4, LiFeO3, Li3PO4 Li3PO4 LiZnPO4, Li3PO4 LiVO2, Li3V2(PO4)3, Li3PO4 Li3PO4 Li3PO4 Li3PO4 Li3PO4 Li3B7O12, Li2B4O7, LiBO2, Li3PO4 Li2WO4, Li3PO4 Li2TiO3, Li3PO4

n/a LiCoPO4 LiCoPO4 Li2MnP2O7, Li3PO4 n/a Li2Fe3(P2O7)2, LiFePO4, Li3PO4 Li3PO4 LiZnPO4, Li3PO4 LiVC2O6, Li3PO4, Li3V2(PO4)3 Li3PO4 n/a Li3PO4 n/a Li3B7O12, Li3PO4 Li3PO4 LiTi2(PO4)3, Li3PO4

after reacting with residual Li, whereas MO materials form the Li−M−O phase. It is also important to note that the reactions of the coating materials with the Li residues occur during the coating process at a high temperature of around 700 °C after synthesizing the pristine cathode. Hence, the detrimental gas species generated during this process will be evaporated during subsequent ventilation processes. To confirm the phase information, we compared the predicted phases with those from a few experimental studies. First, we compared the equilibrium phases shown in the Co3(PO4)2−Li residue−O2 phase diagram, as shown in Figure 6a; the representative phases are LiCoPO4 and Li3PO4. A previous study of Co3(PO4)2 found that it can react with Li residues and form the LiCoPO4 phase on the cathode surface during annealing.18 It is unclear from the experiment which of the Li residues can react, but the present calculation verifies that this LiCoPO4 phase can be formed in reaction with either of the Li residues. Additional validation of the present calculated phases of Li3PO4 can also be obtained from the previous report; it suggests that Li residues can react with diammonium phosphate [DAP, (NH4)2HPO4], which is commonly used as a phosphorous source in MP synthesis, and form the Li3PO4 phase.38 It is also known that DAP can be thermally decomposed to P2O5 above 400 °C.39 Hence, the actual product that reacted with the Li residues is P2O5, not DAP. The P2O5−Li residue−O2 phase diagram in Figure 6b 17829


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ACS Applied Materials & Interfaces clearly indicates that Li3PO4 can be formed, which further validates the experimental product found by the present approach. Further, for the reactions of the MOs, the predicted products of Li2SiO3 and Li2Si2O5 from the reaction of SiO2 with LiOH are also supported by previous experiments. (The phase diagram is shown in Figure S2.) Zhao et al. suggested that Li2SiO3 can be formed by the reaction of LiOH with SiO2, which is the same reaction observed in this study.10 In addition, Li2Si2O5 can reportedly be generated by the reaction between LiOH and SiO2.40,41 In the case of B2O3, it has been shown that its precursor, H3BO3, can react with LiOH and generate Li2O− LiBO2−Li3BO3 phases on the surface of Li(Li0.18Ni0.15Co0.15Mn0.52)O2.42 As H3BO3 is decomposed to B2O3 at around 300 °C,43 this reaction shows agreement with the current calculation, which demonstrates that the reaction of B2O3 with LiOH generates the Li3BO3 phase. Another example is V2O3, it has been shown that ammonium metavanadate (NH4VO3) can react with LiOH and Li2CO3 on the surface of LiNi0.8Co0.15Al0.05O2, thus producing LiVO2, V2O3, VO2, LiV2O5, and V2O5.44 Although it is not clearly described what reactants form LiVO2, current calculations clearly confirm that LiVO2 can be generated from the reaction of V-oxide materials such V2O5 and VO2 with LiOH and VO2 to form Li2CO3. Finally, for NbO, it has been shown that LiNbO3 can be coated on the top of a LiMn2O4 cathode when Nb2O5 is provided with the extra amount of Li2CO3 under 750 °C for 24 h.45 It is known that Nb2O5 can gradually dissociate to NbO2 and NbO after around 150 °C46 and the current prediction agrees that NbO can react with Li2CO3 and form the LiNbO3 phase. In summary, the experimentally observed Li-containing products, which are generated in Li residue removal by reaction with the MP or MO coating materials, are in reasonable agreement with the results calculated here. According to the analysis of the product phases in the reactions of the MPs with the Li residues, Li3PO4 is a frequently generated phase, as shown in Table I. (It appears in 25 out of 28 available reactions; the three exceptions are the reactions of Co−P and V−P with Li2CO3.) In addition, the Li−M−PO4, Li−M−P2O7, and Li−M−O phases can be generated as forms of Li-containing material. Note also that some of the MPs produce Li-containing products only after the reaction, for example, reactions 1−3 for VPO4, reaction 2 for AlPO4, and reactions 2−5 for BPO4 with LiOH. (All of the reactions are shown in the SI.) This result means that when the surface of the cathode is coated through this type of reaction with those materials, Li-ion diffusion is expected to be enhanced compared to that after reactions that produce MO phases, such as CoO in reactions 1 and 2 of Co3(PO4)2−LiOH. Further, for the products of the reactions between the Li residues and the MO materials, all of the Li-containing phases are categorized as a form of Li−M−O except for P2O5, whose possible product phase is Li3PO4, as shown in Table II. Similar to the finding for the MP materials, the products of the reactions with the MO coatings can also result in the appearance of Li-containing materials only on the cathode surface, including H2O impurities; examples include reactions 3−6 for B2O3 and reaction 1 of MnO2 and WO3 with LiOH. 3.5. Comparison between Subgroups. 3.5.1. MP versus MO Coating Materials. We compared the reaction energies of the reactions of the Li residues with the MP and MO coating materials having the same metal elements, as shown in Figure 7. The reactions of eight and three element-based coating materials with LiOH and Li2CO3 were compared. Unlike the

Table II. Product Phases from the Reaction between MO and LiOH/Li2CO3 Li residues coating material Al2O3 TiO2 TiO Ti2O3 Co3O4 ZrO2 B2O3

LiOH

Li2CO3

MnO2 WO3 SiO2 Nb2O5 NbO

LiAl5O8, LiAlO2 Li2TiO3 LiTiO2, LiTi2O4, Li2TiO3 LiTi2O4, Li2TiO3 LiCoO2 Li2ZrO3 Li3B7O12, Li2B4O7, LiBO2, Li3BO3 Li2MnO3 Li2WO4 Li2Si2O5, Li2SiO3 LiNb3O8, LiNbO3, Li3NbO4 LiNbO2, Li3NbO4

NbO2

LiNbO2, Li3NbO4, LiNbO3

Ta2O5 MoO3 V2O3 VO2 V2O5 VO SnO SnO2 RuO2 Fe3O4 Cr2O3 Ga2O3 GeO2 Rh2O3 P2O5

LiTa3O8, LiTaO3, Li3TaO4 Li2MoO4, Li4MoO5 LiVO2 LiVO2, Li3VO4 LiVO3, Li3VO4 LiVO2 Li2SnO3 Li2SnO3 Li2RuO3 LiFeO2, Li5FeO4 LiCrO2 LiGa5O8, LiGaO2 Li4Ge5O12, Li2GeO3 LiRhO2 Li3PO4

n/a n/a LiTi2O4, Li2TiO3 LiTi2O4, Li2TiO3 n/a n/a Li3B7O12 n/a n/a n/a n/a LiNbO2, Li3NbO4, LiNbO3 LiNbO2, LiNbO3, LiNb3O8 n/a n/a LiVO2 n/a n/a LiVO2 n/a n/a n/a n/a n/a n/a n/a n/a Li3PO4

MP materials, the Cu-, Zn-, and Y-based oxides are not reactive to any of the Li residues. Thus, for LiOH removal, the Ti-, V-, B-, and Al-based oxides work better, whereas the W-, Fe-, Mn-, and Co-based phosphates are more reactive. Note that the ΔHLi−M value depends strongly on which of the oxide forms is used; that is, TiO and Ti2O3 have higher values than TiPO4, but TiO2 is less able to remove LiOH. Regarding the GC values, the MP materials are generally more efficient than the MO materials except for the V-, Mn-, and B-based phosphates in reaction with LiOH. In contrast, for reaction with Li2CO3, the MO materials are generally better. It is interesting that for boron, B2O3 and BPO4 are the most gravimetrically efficient for reaction with LiOH, and BPO4 is best and B2O3 is second-best for removing Li2CO3. For further clarification and to consider the combination effect from the ΔHLi−M and GC values, Pareto-front analysis was performed.47 This analysis shows the optimal point by sorting the data sets to satisfy both values without sacrificing one or the other. In other words, for coating materials which do not meet this criterion, they either have poorer ΔHLi−M or GC values compared to those on the line of the Pareto-front. In Figure 7a, the materials on the Pareto-front for the reaction of MP/MO and LiOH are shown to be P2O5, BPO4, and B2O3. Meanwhile, for the reaction of MP/MO and Li2CO3, V2O3, VO, NbO, Mn3(PO4)2, and P2O5 were found to be the optimal materials. Overall, P2O5 could be the ideal material as it can react with 17830


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Figure 7. Comparison of reactivity to Li residues with respect to their (a) ΔHLi−M and (b) GC values for MPs and MOs whose metal elements are the same. MP and MO materials on Pareto-front for the reaction with (c) LiOH and (d) Li2CO3. Numbers in the parenthesis indicate the corresponding number of their reaction equations.

except for the TM3d and TM4d materials, as shown in Figure 8c. Note that these materials exhibit exceptional ΔHLi−M values compared to those of the others. In contrast, a comparison of the GC values shows that the MTLs and PNM have superior efficiency, and the rest are not noteworthy [Figure 8d]. To summarize, on the basis of the reactivity and efficiency of the coating materials, using materials from the TM3d and TM4d subgroups will be an ideal option for removing residual Li compounds.

both residual Li materials. Oxide materials exhibit better performance as 9 out of 11 of the optimal cases are MOs. Among the MPs, BPO4 could be used for the purpose of removing LiOH whereas Mn3(PO4)2 is more effective for Li2CO3 removal. 3.5.2. Subgroups in the Periodic Table. Finally, we compared the reactivity and gravimetric efficiency of the MP and MO coating materials in terms of their metals’ subgroups in the periodic table, as shown in Figure 8. A total of five subgroups can be identified: alkaline earth metals (AEMs, 2), 3d to 5d TMs (TM3d−TM5d, 18), post-TMs (PTMs, 5), metalloids (MTLs, 3), and polyatomic nonmetals (PNMs, 1). The number after the abbreviation is the number of screened metals in that subgroup. For the reaction with LiOH in Figure 8a, the TM-, MTL-, and PNM-based materials have more energetically preferred reactions than those of the AEMs and PTMs. The GC values show that the MTL, TM3d, and TM4d subgroups include materials exhibiting superior efficiency [Figure 8b]. Further, there are many fewer reactions with Li2CO3 than with LiOH,

4. CONCLUSIONS We presented a novel computational approach to designing optimal coating materials for removing Li residues on the surface of cathode materials. We examined 16 MPs and 45 MOs in terms of the ΔHLi−M and GC values for their reactions with LiOH and Li2CO3. A phase diagram was constructed to find the equilibrium phases of their reactions using a database obtained by hybrid-functional-based DFT for improved accuracy. It was found that the greatest weight of coating 17831


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Figure 8. Comparison between reactivity to Li residues with respect to their (a, c) ΔHLi−M and (b, d) GC of MPs and MOs which are located in the same group of the periodic table. Each label in the x axis represents the periodic table groups as follows. AEM: alkaline earth metal, TM3d ∼ 5d: TM with d-block number, PTM: post-transition metal, MTL: metalloid, and PNM: polyatomic nonmetal.

ORCID

material is generally required for materials with the most energetically preferred reaction considering all of the available reaction products of that material. In addition, more MP/MO materials can react with LiOH than with Li2CO3 by various reaction pathways; the reactions with LiOH exhibit better gravimetric capacity, but the reactions with Li2CO3 are generally preferred. This indicates that it is more important to choose an appropriate MP/MO−Li2CO3 reaction to maximize the reactivity and efficiency. To validate this approach, the reaction products of the Li-containing phosphate and oxide materials were compared with the results of previous experimental studies. In addition, the reactivity of coating materials containing 3d/4d TM elements was found to be better than that of materials in the other subgroups in the periodic table. We believe that this screening framework provides an efficient computational method to search for optimal coating materials for reaction with Li residues.

■

Eunseog Cho: 0000-0001-5308-8278 Notes

The authors declare no competing financial interest.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00260. Formation energy comparison between hybrid functional, GGA, and experiment; phase data; reaction equations for MP−LiOH, MP−Li2CO3, MO−LiOH, and MO−Li2CO3; the descending and corresponding order of ΔHLi−M and GC for MP−LiOH, MP−Li2CO3, MO−LiOH, MO−Li2CO3; phase diagram of SiO2− LiOH−O2 and Co3(PO4)2−LiOH−O2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.P.). *E-mail: [email protected] (E.C.). 17832


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