Space–Charge Layer Effect at Interface between Oxide Cathode

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Space−Charge Layer Effect at Interface between Oxide Cathode and Sulfide Electrolyte in All-Solid-State Lithium-Ion Battery Jun Haruyama,†,‡ Keitaro Sodeyama,†,§ Liyuan Han,∥,⊥ Kazunori Takada,†,‡ and Yoshitaka Tateyama*,†,§,⊥ †

International Center for Materials Nanoarchitectonics (MANA) and ‡Global Research Center for Environmental and Energy Nanoscience (GREEN), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan ∥ Photovoltaic Materials Unit, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan ⊥ PRESTO and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 333-0012, Japan S Supporting Information *

ABSTRACT: We theoretically elucidated the characteristics of the space−charge layer (SCL) at interfaces between oxide cathode and sulfide electrolyte in all-solid-state lithium-ion batteries (ASS-LIBs) and the effect of the buffer layer interposition, for the first time, via the calculations with density functional theory (DFT) + U framework. As a most representative system, we examined the interfaces between LiCoO2 cathode and β-Li3PS4 solid electrolyte (LCO/LPS), and the LiCoO2/LiNbO3/β-Li3PS4 (LCO/LNO/LPS) interfaces with the LiNbO3 buffer layers. The DFT+U calculations, coupling with a systematic procedure for interface matching, showed the stable structures and the electronic states of the interfaces. The LCO/LPS interface has attractive Li adsorption sites and rather disordered structure, whereas the interposition of the LNO buffer layers forms smooth interfaces without Li adsorption sites for both LCO and LPS sides. The calculated energies of the Li-vacancy formation and the Li migration reveal that subsurface Li in the LPS side can begin to transfer at the undervoltage condition in the LCO/LPS interface, which suggests the SCL growth at the beginning of charging, leading to the interfacial resistance. The LNO interposition suppresses this growth of SCL and provides smooth Li transport paths free from the possible bottlenecks. These aspects on the atomic scale will give a useful perspective for the further improvement of the ASSLIB performance. the organic liquid electrolytes recently.6,7 Therefore, the ratedetermining process is no longer in the electrolyte component and the maximum resistance is now observed around the cathode/sulfide electrolyte interfaces.8 Recently, Ohta and co-workers reported that the oxide buffer layers interposed between the LiCoO2 (LCO) cathode and the sulfide electrolyte significantly reduce the interfacial resistance.9 They covered the surface of the LCO particles with a few nanometer buffer layers of Li4Ti5O12,9 LiNbO3 (LNO),10 LiTaO3,8 and Li2SiO311 by spray-coating or sol−gel method. The interfacial Li-ion conductivities were always improved in the presence of the additional buffer layers. Especially, LNOcoated LCO particles exhibited the smallest resistance among them.5 The experimental results indicate that the oxide cathode/sulfide electrolyte interfaces have a universal difficulty

1. INTRODUCTION Development of a large and stable energy storage device is an essential subject to solve the current energy issue, because it can maximize the efficient use of the renewable and sustainable energy sources such as wind and solar power.1 Lithium-ion battery (LIB) that has large energy density is one of the most promising candidates for that purpose.2 However, improvement of the characteristics, e.g., energy density, power, durability, and safety, is still indispensable, and understanding of cathode/ electrolyte and electrolyte/anode interfaces, in particular, has been an issue of the greatest importance for the improvement.3 All solid-state (ASS)-LIBs involving inorganic solid electrolytes have shown good performance of storage stability and long cycle life4,5 and thus can be one of the targets. However, they still have some crucial difficulties for the practical applications, for instance, the low current drains and the low power density. The main origin was regarded as the rather low ionic conductivity of solid electrolytes. In order to overcome this problem, sulfide type electrolytes were developed. The ionic conductivities of these materials become comparable to © XXXX American Chemical Society

Received: May 11, 2014 Revised: June 16, 2014

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2. CALCULATIONS 2.1. Interface Construction. We used first-principles geometry optimization to obtain the stable interfacial structures. Since the collective lateral displacement is difficult to include in those calculations, systematic preparation of the initial structures for the optimization is crucial for the global energy minimum search. In this work, the following procedures were used for the interfaces among LCO, LPS, and LNO. First, we picked up the applicable surfaces of each component. The experimental observations and theoretical studies indicate the evidence of (110)-oriented LCO surface.24,25 Although larger stability of the (104) surface was recently suggested,26 the (110) surface still exists in the LCO particles and provides apparent Li-ion conduction paths along the ⟨110⟩ directions.27 Therefore, we chose the (110) face as a representative surface of LCO. In a similar way, the LPS(010) surface is selected because the Li-ion conduction path in LPS is b direction.28 Few structural reports of the buffer layer LNO are available, despite that the crystal faces of bulk LNO are wellinvestigated.29 On the basis of the evaluation of the lattice mismatch, LCO(110)/LNO(11̅0) and LCO(110)/LNO(110) interfaces were expected to be possible interfaces. The selected surface structures are shown in Figure 1, where the VESTA package is used to visualize.30

in the Li-ion conduction, and the interposed buffer layers solve the problem. The interfacial resistance has been explained by some possible mechanisms.12 One is the space-charge layer (SCL) mechanism.5,8,13,14 Formation of SCL at the interface in the sulfide side where Li-ion concentration lowers may decrease the Li-ion conductivity. Also the interfacial Li-ion depletion may grow during the beginning of charging. These were suggested by the evidence that the potential slope at the initial stage of charging in the voltage profile disappears as the thickness of the buffer layers increases. However, direct measurement of such SCL has not been done yet due to the difficulties in the observation of complicated interfacial Li-ion distribution on the atomic scale. The second is structural disorder induced by interfacial chemical reaction or diffusion. In fact, experiments using scanning transmission electron microscopy with energy dispersive X-ray spectroscopy indicated outstanding Co diffusion from LCO to the sulfide electrolytes. The Co element was observed even at a distance of 50 nm from the interface. They further suggested that the buffer layers suppress the Co diffusion as well.15−17 The last is a mechanism based on the lattice mismatch. This is applied to thin film systems of the oxide cathode/oxide electrolyte interface.18 However, in the typical oxide/sulfide interfaces, the mismatch is not a significant problem because sulfide is soft enough and the interface connection is easily formed. It should be pointed out that the observed interfacial resistance is rather universal property independent of the component ion-conductive materials with different structures.19 In this respect, we focus on the materialindependent SCL concept and explore its presence and characteristics in this work. We selected LCO, β-Li3PS4 (LPS), and LNO as cathode, sulfide electrolyte, and buffer layers for the present investigation. LCO is the most widely used cathode material for LIBs, and some studies on the interface are available.20 LPS is adopted as a typical example of sulfide electrolyte, because a nanoporous LPS has high Li-ion conductivity (1.64 × 10−4 S cm−1 at room temperature21). In addition, the crystal structure of LPS22 is simpler than the other sulfide electrolytes such as Li10GeP2S126 or Li7P3S12.23 Generally, supercell calculations of oxide/sulfide interfaces need a cell dimension almost commensurate for both lattices, which is usually larger than the primitive ones. It is thus important to keep sulfide materials as simple as possible in terms of the computational cost. Finally, since the LNO coating indicates a lowest interfacial resistance, we decided to examine the systems with these three materials. In this study, we calculate interfacial atomic structures and electronic properties of three interfaces, LCO/LPS, LCO/ LNO, and LNO/LPS, using the density functional theory (DFT) + U methods. A systematic procedure is employed to find most stable interfacial matching of the two crystal solids. We further evaluated the energies of Li-vacancy formation and Li-ion transfer, and examined stable Li-ion sites in each interface under equilibrium and the beginning of charging. On the basis of all the results, we discuss probable mechanisms with the SCL formation, comparing with the experiments. We point out that this work is the first DFT calculation study on the LCO cathode/solid electrolyte as well as the buffer layers effect and gives a novel perspective on the microscopic origin of the interfacial resistance in ASS-LIBs.

Figure 1. Surface structures of (a) LCO(110), (b) LNO(11̅0), (c) LNO(110), and (d) LPS(010), examined in this work. The Li, O, P, S, Co, and Nb atoms are depicted as light green, red, purple, yellow, blue, and green spheres, respectively. Blue, green, purple, and light green polyhedrons represent CoO6, NbO6, PS4, and LiS4 complexes, respectively.

Next, for the LCO(110)/LNO(11̅ 0 ), LCO(110)/ LNO(110), LCO(110)/LPS(010),and LNO(110̅ )/LPS(010) interfaces, we determined the bulk lattice structure of the component by using the DFT+U method as Step 1. The calculated lattice constants are in good agreement with experimental values (see Table S1 in Supporting Information). In Step 2, we cut the slab with stoichiometric surfaces from the bulk crystal and relaxed all the atomic coordinates. For the reference calculations of the surfaces, the vacuum region with about 1.5 nm is added to the supercell. The slab thicknesses were 1−2 nm, which is thick enough to show the bulk character in the central region of each slab. Detailed terminations and surface energies are presented in Supporting Information Figure S2 and Table S2. In Step 3, we multiplied the surface B

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free interfaces. We set the system always neutral, and the occupation number is determined by the Gaussian smearing technique with a smearing parameter of 0.001 Ry. It is worth noting that we checked the interface polarization effect using the ESM method.40,41 Maximum differences of the total and the formation energies between the periodic boundary condition and the non-repeated slab approach are about 0.1 and 0.01 eV, respectively. We also examined the spin polarization of LCO surfaces. A previous study reported stabilization of spin-polarized states on the LCO surfaces due to the missing Co−O bonds.42 Our calculations reproduce this tendency. However, we have investigated the Li passivation of the LCO(110) with and without spin polarization and found that the energy difference among the low, intermediate, and high spin states are within 0.2 eV. Thus, we limit the present discussion to the spin-unpolarized case. The argument about the spin states will be reported elsewhere.

slab in the lateral directions for the reasonable interface supercell. Then, we adjusted the lateral lattice constants of the attaching surfaces and combined them to make the initial interfacial structures. Considering the high elastic modulus of LCO, the LPS and LNO lattice constants are set to that of the LCO for the LCO/LNO and LCO/LPS interfaces. On the other hand, the average of the two lattices is taken for the LNO/LPS. The mismatches of the constructed initial interfaces are 3 to 5%, which are listed in Supporting Information Table S3. In the last stage, we carried out systematic lateral slide of the one surface slab with respect to the other before the DFT+U calculations. Taking the interface symmetry into account, we prepared 16, 4, and 9 samples for the LCO/LNO, LCO/LPS, and LNO/LPS interfaces, respectively. The details of these interface constructions are described in the section S3 in the Supporting Information. Note that a vacuum region with about 1.5 nm width is still added in the outside of the interface slab (consisting of two attached surface slabs). Without this vacuum, the supercell approach always involves two interfaces, which are atomically different in most cases. Besides, artificial interaction between the two interfacial polarizations may arise, preventing accurate estimation of the stability of each interface. Therefore, the presence of the vacuum region is quite crucial. In the DFT +U geometry optimization, all the atomic positions and lateral cell parameters were allowed to relax. Finally, we regarded the minimum energy interface as the most probable in ASS-LIBs. Then, the Li-vacancy formation energies and the projected density of states (PDOS) were calculated with the minimum energy interfaces. The vacancy formation energy of Li at site i is defined as Ev (Lii) = {Etot(Lii) + μLi } − Etot

3. RESULTS AND DISCUSSION 3.1. Interface Structure. We show the calculated stable structures of LCO/LNO interfaces in Figure 2a,b. In the vicinity of the intermediate region, pseudotetrahedral CoO4, instead of typical CoO6 octahedrons, appears, and NbO4 tetrahedrons deviate toward the LCO slab. Consequently, the outermost Co atoms form new CoO6 octahedrons using O from the LNO slab (see the insets in Figure 2a,b). Between the possible LCO(110)/LNO(110̅ ) and LCO(110)/LNO(110), the former forms more tightly combined interface and larger adhesion energy (see Supporting Information, Table S3). Its well-fitted nature is the consequence of the similar crystal structures of LCO and LNO. Note that the distortion of the LCO layer in the LCO(110)/LNO(11̅0) is due to the overbalance of the Li atoms. It is confirmed that the relaxed structure after removal of a few interfacial Li atoms recover a structure without distortion. Effectively, the distortion turns out to have little effect on the PDOS and vacancy formation energy. Later on, we focus on this stable LNO(11̅0) interface, although the same characteristics are expected in the LNO(110). The optimized LCO(110)/LPS(010) structure is shown in Figure 2c. In the interface region, a sulfur atom is attracted to a Co atom. As a result, CoO4S pentahedron is partly formed and the corresponding PS4 tetrahedron shifts toward the interspace. However, the interfacial bonding is quite incomplete, compared with the LCO/LNO interface. This character reflects that the sulfide has largely different lattice constants and bond lengths from those of LNO. Interestingly, the Li atoms originated in the LPS slab are strongly attracted to the LCO and adsorb on top of the CoO6 layers and the Li layers. The former actually involves bindings to the oxygen ridge (bridge) on the CoO6 octahedrons. As a consequence, the LPS slab loses its ordered bulk crystal structure, while the atomic network is kept in the LCO slab. We emphasize that the Li adsorption and the breaking of sulfide structure are crucial to explain the resistive oxide/sulfide interface. The detailed discussion is given in Section 3.3 and 3.4. Finally, we examine the stable LNO(11̅0)/LPS(010) structure displayed in Figure 2d. The interface does not form Nb−S bonds, and no Li adsorption sites are observed. On the other hand, ionic attractive interactions of interfacial O−Li and Li−S pairs still exist, and the adhesion energy of the LNO/LPS interface is comparable to the LCO/LPS interface (see Supporting Information, Table S3). Note that the Li atoms in the LNO slab are slightly deviated toward the LPS side, in

(1)

where Ev(Lii) and Etot are the energies of relaxed interfaces obtained by DFT+U calculations with and without defect Lii, respectively. μLi is the chemical potential of Li, which is set at that of lithium metal. 2.2. Computational Details. DFT calculations were carried out within the plane wave basis and pseudopotential framework as implemented in the QUANTUM-ESPRESSO package.31 PBE exchange correlation functional32 and the spinunpolarized scheme were adopted. The core−valence interactions were represented through ultrasoft pseudopotentials.33,34 The electronic configurations are 1s22s1 for Li, 2s22p4 for O, 3s23p3 for P with nonlinear core correction (NCC),35 3s23p4 for S with NCC, 3d84s1 with NCC for Co, and 4s24p64d45s1 with NCC for Nb. We used the DFT+U method,36 and adopted 5.9 eV37,38 for the Hubbard U value of Co 3d state. This DFT+U treatment is essential to describe the electronic properties of LCO.39 The cutoffs of the plane-wave basis are set to 40 and 320 Ry for the smooth part of the wave functions and the augmented charge, respectively. Convergent k-point sampling was adopted for the bulk and surface systems, while the only Γ point was used in the interface systems. The atomic positions and the cell parameters were relaxed until the residual forces and stresses became less than 0.001 Ry/bohr and 0.5 kbar, respectively. The relaxed cell parameters are listed in Table S3 in the Supporting Information. Regarding the PDOS calculations, we conducted non-selfconsistent field calculation with 2 × 1 × 1 (LCO/LPS and LNO/LPS) and 2 × 2 × 1 (LCO/LNO) k-point meshes using the Γ point electron density. In the calculation of the defects, we fixed the cell parameters at the relaxed values of the defectC

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interface. This simple analysis of the outermost layer can be useful for estimation of the interfacial stability. 3.2. Electronic States. Figure 3 shows calculated PDOSs of the LCO(110)/LNO(11̅ 0 ), LCO(110)/LPS(010), and LNO(11̅ 0 )/LPS(010) interfaces. In the LCO(110)/ LNO(11̅0) interface, the LCO states mainly contribute to the valence band top region. The rather localized states around 1 eV mainly arise from the LCO(110) surface facing the vacuum, which appears in the PDOS of the isolated LCO(110) slab as well (Figure S3 in the Supporting Information). These surface localized states are originated from the crystal field splitting of 3d orbitals. A pseudotetrahedral crystal field breaks the degeneracy of both t2g and eg orbitals in the octahedral symmetry.42 The top of the LNO occupied states is lower than the LCO valence band top by about 1 eV. Considering that the LCO is a p-type semiconductor,43 the LNO looks like a good electronic insulator. This energy relationship is related to Livacancy formation energies and experimental charging curves, which will be discussed below. We found that the states from the first LCO layer facing the LNO slab do not exist in the midgap region around 1 eV. This disappearance of the surface state is attributed to the formation of CoO6 octahedron, indicating that crystal structures are smoothly connected at the interface. Note that the LCO(110)/LNO(110) interface shows almost the same PDOS. In contrast to the tight adhesion nature of the LCO/LNO, the electronic structure of the LCO(110)/LPS(010) interface indicates that the midgap states involve the interfacial LCO states. Due to the interaction of Co 3d orbitals with S or Li ions, the midgap LCO surface states show slight delocalization in energy. For instance, the 3d orbitals near the rather negative S ions shift upward, while those near the positive Li ions are stabilized (see the molecular orbitals at the top of the valence bands and the bottom of the conduction bands in Supporting Information Figure S7). This is an indication of the disorder in this interface. Finally, in the LNO(11̅0)/LPS(010) interface, each LNO and LPS PDOS is almost identical to that of the isolated slab (PDOSs of isolated LNO(11̅0) and LPS(010) slabs are depicted in Supporting Information Figure S3). Therefore, the atomic and electronic structures are almost unchanged, and the interfacial adhesion is mainly due to the ionic interactions through the O−Li and Li−S pairs. The calculated valence band offset where the top of LPS is higher than LNO indicates that the LNO buffer layers play a role of electronic insulator against LPS as well. 3.3. Li-Vacancy Formation Energy. We then calculated Li-vacancy formation energies, Ev(Lii), in the optimized structures, which are listed in Table 1. Indices of the Li sites correspond to those shown in Figure 4. The capital labels of LC, LN, and LP mean the Li site originated from the LCO, LNO, and LPS slabs, respectively. We also carried out the Ev calculations in the bulk crystals. These are 4.0, 5.1, and 3.2 eV for LCO, LNO, and LPS, respectively, in good agreement with the values in the previous DFT calculations.44−46 In the LCO(110)/LNO(11̅0) interface, Ev in the LCO region ranges from 3.59 to 3.84 eV, close to the value of the bulk LCO. The calculated energies indicate that the Li sites in the LCO edge region have slightly lower formation energy, which is attributed to the deformed LiO6 octahedrons at the edge. More interesting is the Ev in the LNO region. The difference from the LNO bulk (5.1 eV) is over 1.2 eV. It reflects the band offset between the LCO and LNO. In the bulk LNO,

Figure 2. Optimized interface structures of (a) LCO(110)/ LNO(110̅ ), (b) LCO(110)/LNO(110), (c) LCO(110)/ LPS(010), and (d) LNO (11̅0)/LPS(010). The same shapes and colors are used as in Figure 1. The periodic boundary of the simulation supercell is denoted with the solid lines. The insets show the detailed atomic networks consisting of CoO6, and NbO6 octahedrons and CoO4S pentahedrons.

contrast to the LCO/LPS case where Li shift from the LPS side to the LCO occurs. This can be understood with the number of outermost O atoms in the oxide interface. The ridge connecting two oxygen atoms is exposed in the LCO interface, whereas the apical oxygen appears in the outermost layer of the LNO. The former may have larger electrostatic attraction with Li ions, accounting for the Li adsorption behavior on the LCO D

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Table 1. Calculated Li-Vacancy Formation Energies Ev(Lii) (i = LCn, LNn, LPn; n = 1−6) (eV) LCO(110)/LNO(11̅0) LC1 3.84

LC2 3.60

LC1 3.49

LC2 3.98

LN1 3.13

LN2 3.32

LC3 LN1 3.59 3.53 LCO(110)/LPS(010) LC3 3.18

LP1 3.27 LP4 2.90 LNO(11̅0)/LPS(010) LN3 3.26

LP1 2.90 LP4 2.99

LN2 3.85

LN3 3.86

LP2 1.44 LP5 2.71

LP3 3.03 LP6 2.62

LP2 2.42 LP5 3.07

LP3 3.18 LP6 3.14

Figure 4. Possible Li sites with their indices used in this work, in the (a) LCO(110)/LNO(11̅0), (b) LCO(110)/LPS(010), and (c) LNO (110̅ )/LPS(010) interfaces. The same shapes and colors are used as in Figure 1. Figure 3. PDOSs of the optimized interfaces of (a) LCO(110)/ LNO(11̅0), (b) LCO(110)/LPS(010), and (c) LNO(11̅0)/LPS(010). Black line: total DOS. Blue line: LCO atoms. Green line: LNO atoms. Orange line: LPS atoms. Black dashed line: atoms in the first LCO layer facing to the LNO or LPS slab. The tops of the valence bands (occupied states) are set to the energy origin.

crystal structure is largely deformed here. On the other hand, the LP1 and LP4 sites, which adsorb on the LCO slab, have large formation energies, ca. 3 eV. These results indicate that the Li atoms adsorbed on the LCO surface are more favorable than those in the subsurface region of the LPS slab. In order to confirm this scenario, we calculated the Li transfer energy between the LP2 to another adsorption site neighboring to the LP1, with geometry relaxation. The transfer energy is −1.6 eV, so that Li ion easily occupies this adsorption site on the LCO surface. Here, we point out the Li interstitial defect formation. The relaxed structures were calculated by inserting an additional Li atom to the interspace surrounded by LC1, LC2, and LC3 sites. The results show that the additional Li pushes the surrounding Li atoms and the LC3 Li moves to the adsorption site on the LCO interface. These calculations clearly suggest that the Li adsorption sites are more stable than the interstitial sites in the bulk LPS and LCO. In the LNO(11̅0)/LPS(010) interface, the formation energies in the LPS region are 2.9−3.2 eV except for the LP2

the electron extracted with Li vacancy formation locates at the valence band top of LNO, while in the LCO/LNO interface, the electron is extracted from the LCO band as shown in Figure 3a. This electron transfer effect makes the Ev(LiLNn) similar to the Ev(LiLCn) (n = 1−3). The LCO(110)/LPS(010) interface has smaller Ev at the LC3 site than the LC1 and LC2. This can be attributed to the lack of Li−O bonds. Namely, the LC3 site consists of the LiO4 tetrahedron. The LPS side has two types of Li sites; mobile Li sites where Li ion can easily transfer among them (LP1−3), and immobile Li sites in the tetrahedrons forming the framework of the LPS crystal (LP4−6).28 We found that the LP2 site has considerably low formation energy, because the E

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interface have different Li chemical potentials.47 In the LCO/ LPS interfaces, the Li transfer from the LPS side to the LCO is expected as shown in Figure 5a, because of the typical lower chemical potential in sulfides. The LCO/LNO/LPS distribution in Figure 5b is also deduced from the ion redistribution models. Because of the insulating properties of the interposed LNO buffer, the amount of Li redistribution is expected to be smaller than that of the LCO/LPS interface. This type of spacecharge profile successfully explains the linear dependence of the conductivity on the inverse layer thickness in the case of CaF2/ BaF2 multilayers.48,49 The present calculations provide slightly modified distributions under equilibrium. Figure 5c represents the Li distribution at the LCO/LPS interfaces where the interfacial Li atoms in the LPS side heavily adsorbing to the LCO surface. Our calculations elucidate that the surface adsorptions are the main contributions to the formation of the equilibrium SCL. Note that Uranov discussed the space-charge region at ionic salt interfaces, originated from the charged defect adsorption together with the lattice distortion, in the framework of the Stern model.50 The SCL distribution is concentrated in the interface region and similar to our Li-ion concentration. The distribution on the LCO/LNO/LPS interface is suggested as in Figure 5d. Since the attractive sites on the LCO surface disappear by the attachment of the LNO layers, the SCL at this interface is significantly suppressed. In addition, the distribution on the LNO/LPS is expected to be flat, because of the rather inert interface. Note that the present supercell calculations well describe the nanoscale region in the interface, although longrange variation of the concentration needs to be analyzed by methods involving the long-range electrostatic interaction. We then discuss Li transfer at the initial stage of charging based on the site dependence of the Li chemical potentials, namely, the calculated Ev energies listed in Table 1. In the LCO/LPS interface, the Ev energies are in the ranges from 1.5 to 4.0 eV. The former energy coincides with the experimental voltage where the charging is initiated. Around this voltage, Li ions at the LP2 sites are expected to begin to transfer into the bulk LPS with releasing the electron to the cathode. This can enhance the SCL in the sulfide side. This mechanism is likely to cause the observed voltage profile slope at the beginning of charging.5,8−10,13,14 When the LNO buffer layers are interposed, Table 1 indicates that the Li transfer from both LCO and LNO sides will begin around the voltage corresponding to the Li chemical potential in the LCO, namely, the plateau voltage. Regarding the LNO/LPS interface, the LP2 site has a rather smaller chemical potential than the LP1 and LN3 by 0.5 and 0.8 eV, respectively. However, these differences are smaller than the LCO/LPS interfaces. Thus, the slope in the voltage profile, corresponding to the SCL growth, is suppressed in this interface. The present elucidation supports the mechanism recently proposed by some of the authors.5,8−10,13,14 The stable interfacial structures obtained in this work also give an insight into the interfacial resistance in terms of the Liion transport paths. Highly conductive sulfide electrolytes such as Li10GeP2S1251,52 and Li7P3S1153 have a common property of one-dimensional (1D) Li-ion paths. These 1D paths can be easily clogged by bottlenecks like defects and impurities. Therefore, how to avoid this clogging at the interfaces is an issue. In the LCO/LPS interface, Li atoms adsorbing to the LCO surface can easily become this disturbance, which may be a possible origin of the interfacial resistance on this interface.

site, which almost recover the value of the bulk LPS. Since the LNO/LPS interface keeps the bulk crystal structure, significant decrease of the vacancy formation energy does not occur. The values in the LNO (3.13−3.32 eV) are attributed to the same origin associated with the valence band offset as in the case of the LCO/LNO interface. Here we transferred the LP2 Li to the empty space between LNO and LPS slabs and found that the energy gain is about 0.3 eV. This suggests that slight Li condensation may occur in the LNO/LPS interface. However, the concentration of Li ion in the LCO/LPS interface should be much higher than that of the LNO/LPS. The difference depends on the number of available Li sites and the corresponding energy gains. The LCO has many energetically favorable sites for the Li adsorption, whereas LNO does not. Therefore, we conclude that the Li atoms in the LPS side are easily attracted to the LCO adsorption sites and the highly Liconcentrated layer is formed at the LCO/LPS interface. 3.4. Discussion. Considering that the vacancy formation energy of Li atom with respect to Li metal can be regarded as Li chemical potential, we discuss the detailed behavior of Li atoms under equilibrium and at the initial stage of charging, on the basis of the present calculation results. First we discuss the equilibrium state before charging. Figure 5a shows schematic concentration profiles in the conventional models,47 where we use the Li chemical potential in each bulk region as the reference (horizontal broken line). Ion redistribution occurs when the two materials forming the

Figure 5. Schematic illustrations of the interfacial Li concentration. The equilibrium concentrations expected by the conventional model and indicated by the present calculations for the LCO/LPS interface (a and c) as well as the LCO/LNO/LPS (b and d). The Li concentrations in (e) and (f) describe the expected changes at the initial stage of charging for both interfaces, respectively, proposed in the present calculations. F

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On the other hand, the bulk LNO has the Li-vacancy migration paths along six equivalent ⟨22̅1⟩ directions, which arise from the hexagonal symmetry.54,55 Such three-dimensional (3D) multiple paths can avoid the bottleneck problem. Also the rather smooth connections of Li-ion pathways at the LCO/ LNO and LNO/LPS interfaces contribute to easier Li-ion transportation. These explain the decrease of the interfacial resistance in the presence of the buffer layers. We, however, point out that the diffusion barrier itself in the oxide buffer layers is usually high, and thus the resistance increases when the buffer thickness grows over a critical value. Nevertheless, we expect that the current discussion on the interfacial structures and adsorptions against sulfides can be generally applied to the other oxide components such as LiMn2O4 and LiFePO4 cathodes as well as Li4Ti5O12, LiTaO3, and Li2SiO3 buffer layers.5,8,9,11,14 Finally, we point out that this study did not deal with the significant mixture of Cobalt in the sulfide side. Therefore, the contribution of the interfacial diffusion/reaction is still to be taken into account, which will be examined in a future study.

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-29-859-2626. E-mail: tateyama.yoshitaka@nims. go.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Stefano Fabris for his helpful discussion on DFT+U calculations. Y.T. and K.S. were partly supported by KAKENHI (no. 23340089). This work was also supported by the Strategic Programs for Innovative Research (SPIRE), MEXT, and the Computational Materials Science Initiative (CMSI), Japan. The calculations in this work were carried out at the supercomputer centers in the NIMS, Kyushu University, ISSP, and ITC (Oakleaf-FX) in the University of Tokyo.

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REFERENCES

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4. CONCLUSIONS We have investigated the LCO/LPS and the LCO/LNO/LPS interfaces, most representative in the ASS-LIB consisting of oxide cathode/sulfide electrolyte, by using the state-of-the-art DFT+U calculations coupling with a systematic procedure for matching of solid−solid interface. This is the first DFT-based computational work on the cathode/electrolyte and cathode/ buffer/electrolyte in the ASS-LIB and clarifies the SCL mechanism on those interfaces and the effect of the buffer layers. We calculated the stable structures and the electronic states as well as the energies of Li vacancy formation and transfer. It is found that the optimized LCO/LPS structure shows the Li adsorption at the oxygen ridge sites on the CoO6 and on the Li layer, leading to deformed interface with a kind of SCLs. On the other hand, the interposition of the LNO buffer layers introduces smoothly matched interfaces without Li adsorption space and suppresses the inhomogeneity of Li distribution. The Li chemical potentials based on the vacancy formation energy indicate that subsurface Li in the LPS side can begin to transfer at the under-voltage condition in the LCO/LPS interface, which suggests the SCL growth at the beginning of charging, leading to the interfacial resistance. The LNO interposition suppresses this growth of SCL and provides several Li transport paths free from possible bottlenecks. These findings on the atomic scale consistently account for the experimental observations so far and clearly indicate the general behavior of the oxide cathode/sulfide electrolyte and the effect of the buffer layers. The microscopic aspects presented here may give a useful perspective to design highly conductive oxide/sulfide interfaces for the improvement of ASS-LIBs.

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ASSOCIATED CONTENT

S Supporting Information *

Structural information on bulk materials, surfaces, and interfaces of LCO, LNO, and LPS. Corresponding surface and interface energies as well as projected density of states. This material is available free of charge via the Internet at http:// pubs.acs.org. G

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