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Charged and Discharged States of Cathode/SulfideElectrolyte Interfaces in All-Solid-State Lithium-Ion Batteries Masato Sumita, Yoshinori Tanaka, Minoru Ikeda, and Takahisa Ohno J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01207 • Publication Date (Web): 09 Jun 2016 Downloaded from http://pubs.acs.org on June 12, 2016

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The Journal of Physical Chemistry

Charged and Discharged States of Cathode/Sulfide-Electrolyte Interfaces in All-Solid-State Lithium-Ion Batteries

Masato Sumita*,†, Yoshinori Tanaka‡, Minoru Ikeda†, and Takahisa Ohno*,†,‡,¶

——————————————————————————————— *Corresponding author. E-mail: M.S., [email protected] Tel: +81 29 859 2490 ; T.O., [email protected] Tel:+81 29 859 2622 †

National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan,

‡

Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN),

NIMS, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan,

¶

Institute of Industrial Science, University

of Tokyo, Meguro, Tokyo 153-8505, Japan

1

ACS Paragon Plus Environment


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Abstract Interfaces between cathodes and sulfide electrolytes exhibit high resistance in all solid-state lithium ion batteries. In this paper, to elucidate the origin of the high interface resistance we have theoretically investigated the properties of the cathode interfaces with the sulfide-electrolyte and oxide-electrolyte for comparison. From the density functional molecular dynamics simulations of the LiFePO4/Li3PS4 interface in both discharged and charged states, we have demonstrated the instability of the sulfide interface in the charged state, that is, the lithium depletion and oxidation on the sulfide side near the interface, in contrast to the oxide interfaces. The obtained results imply the formation of Li-depleted layer around the sulfide interfaces during charging and support the validity of the insertion of oxide buffer layers at the interface to reduce the interface resistance.

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1. Introductions All solid-state lithium ion batteries (LIB) are expected to be the next-generation energy storage devices because of their high energy-density and good safety performance.1-4 However, all solid-state LIB tends to show low power density1, which comes mainly from the lower ionic conductivity of solid electrolytes than organic liquid electrolytes. Recent intensive studies have succeeded in developing several sulfide and oxide electrolytes5,6 with high ionic conductivity comparable to or even higher than those of liquid electrolytes. Now, the rate-determining process in the solid-state LIB is attributed to the high ionic resistance for at the internal interfaces rather than in the battery components.1,2 Sulfide electrolytes are usually used in the solid-state LIB1,2,7-12. Sulfide electrolytes inherently tend to show fast ionic conduction because of the high polarizability due to sulfide ions. These materials are also soft and deformable enough to form good interface connection, and exhibit low grain boundary resistances without sintering. In spite of these advantages, the interfaces between cathodes and sulfide electrolytes show the high ionic resistance in the solid-state LIB.1,2,13,14 The origin of the high ionic interface resistance has been hypothesized by some mechanisms. One is the formation of a space-charge layer at the interface.1,2,13,14 The high potential of the cathode forms a Li-depleted layer on the electrolyte side of the interface, which is highly resistive due to the absence of the charge carrier. Based on the space-charge layer model, the insertion of insulating buffer layers (LiNbO3, Li4Ti5O12, etc.) at the interfaces has been proposed as a promising method to reduce the interface resistance.1,2,13,14 The space-charge layers are considered to be only about ten nanometers in thickness and have not been observed yet.1,13,14 The other is the formation of a defective layer induced by chemical reactions and inter-diffusion at the interface.1,2,15 At the interface between LiCoO2 and the sulfide electrolytes, the defective layer and the inter-diffusion of Co have been observed by the transmission electron microscopy experiments.15 Understanding of the interface properties between the cathode and the solid electrolytes is 3



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indispensable to the development of the solid-state LIB. However, the details of the interface properties such as the space-charge layer and the defective layer have not been measured yet by experiments, because of the experimental difficulties in observing the buried interfaces on the atomic scale. Therefore, computational science is expected to make a contribution toward elucidation of the interface properties in solid-state batteries. There are some computational studies that report the interface properties. For the interface between LiCoO2 and the sulfide electrolyte Li3PS4, it has been reported that the Li atoms in the Li3PS4 side very close to the interface have low Li vacancy formation energy, which might lead to the formation of space-charge layer.16 On the other hand, for the coherent interface between LiFePO4 and the oxide electrolyte Li3PO4, the authors have reported that although the Li atoms near the interface have low Li vacancy formation energy, only a fraction of them may be extracted at the initial stage of charging which is not enough to lead to the Li depletion at the interface.17,18 These computational studies are limited only to the interfaces at the fully discharged state. But, the interface phenomena such as the formation of space-charge layers and defective layers, which may cause the high interface resistance, often occur at the charging processes. In order to elucidate the origin of the high interface resistance, computational analysis not only for the discharged state but also for the charged state of a battery are necessary. In this study, we have investigated the interface properties between cathode and sulfide electrolytes in both discharged and charged states, using the density functional theory (DFT) calculations. For cathode and electrolyte materials, LiFePO4 19 and Li3PS4

20,21

are adopted, both of

which are typical battery materials used for the solid-state LIB. Based on the obtained results of the atomic structures, electronic properties, and dynamics at the LiFePO4/Li3PS4 interface, we will discuss the formation of Li-depleted layer and defective layer, and the effect of the buffer layers, in comparison with the oxide electrolyte interface.

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2. Computational details DFT calculation implemented in CP2K22 were used for all calculation in this research. We used the PBE functional23 with the local spin density approximation (LSD). The hybrid Gaussian (MOLOPT DZVP) and plane-wave (500 Ry for cutoff energy) basis set24, where valence pseudo-wavefunctions are expanded in Gaussian-type orbitals and the density is represented in a plane wave auxiliary basis, were used with the Goedecker, Teter, and Hutter (GTH) pseudopotentials25 constructed for the PBE functional. To include the electronic correlation within the d orbital of Fe, we applied +U strategy. The value of the effective U was set to 4.3 eV as suggested in the previous research.26 Although LiFePO4 with olivine structure is anti-ferromagnetic as already confirmed theoretically and experimentally, ferromagnetic LiFePO4 and FePO4 were assumed in our calculation because there is no large physical difference between anti-ferromagnetic and ferromagnetic LiFePO4 at the DFT level.27 Total energies were calculated at the Γ point in a super cell approach. The first step of investigations is to build up the reasonable atomic structures of the LiFePO4/Li3PS4 interface, which corresponds to the fully-discharged state in solid-state LIB. The lattice mismatch between LiFePO4 and Li3PS4 is too large to form a coherent interface between them. It is probable that Li3PS4 becomes amorphous near the interface with LiFePO4 since Li3PS4 is more deformable than LiFePO4. We have built up the interface between the LiFePO4 (010) surface and amorphous Li3PS4 by density functional molecular dynamics simulations (DF-MD), using an NPT ensemble in the similar manner of the LiCoO2/Li3PO4 system.28 It is noted that the LiFePO4 (010) surface is the most dominant on LiFePO4 nano-crystals and regarded as active for Li ion intercalation/de-intercalation.29,30 To model the LiFePO4/ Li3PS4 interface, we employed a supercell geometry which is composed of the (1×3×2) LiFePO4 (010) surface slab including 24 LiFePO4 units and the amorphous Li3PS4 5



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slab including randomly-distributed 16 Li3PS4 units. The initial configuration of the LiFePO4/Li3PS4 interface used in DF-MD simulations is shown in Figure S1 of the Supporting Information. The details of the setup of the initial configuration is described in the Supporting Information. Starting from the initial configuration, we have performed DF-MD calculations with an NPT ensemble at the temperature of 400 K under the pressure of 1.0 atm, in which the cell parameter in the direction perpendicular to the interface (the b axis) can be altered flexibly such that the density of the amorphous Li3PS4 slab is automatically adjusted to a suitable value. Since the cell parameters in the directions parallel to the interface (the a and c axes) would depend artificially on the ratio of the thicknesses between the electrode (LiFePO4 or FePO4) and electrolyte (Li3PO4 or Li3PS4) slabs in the calculated supercell geometry, these parameters are fixed to those of the LiFePO4 (010) surface slab to avoid this dependence.17,18 The canonical sampling through velocity rescaling (CSVR) thermostat31 was used to control the temperature. In a typical DF-MD simulation, 80,000 MD steps were carried out with the time step interval of 1.0 fs, and after the first 40,000 steps for equilibration the trajectories over 40,000 steps (40 ps) were used for NPT analysis.

3. Results and Discussion We discuss the LiFePO4 (010)/Li3PS4 interface based on the averaged atomic positions since all atoms only vibrate in the energetic equilibrium condition at the temperature of 400 K. The interface structure averaged over the 40 ps DF-MD trajectories is shown in Figure 1. The average cell parameter along the b axis is 48.58 Å, and the density of the amorphous Li3PS4 slab is estimated at 1.72 g/cm3, which is lower than that of β-Li3PS4 (1.81 g/cm3). The LiFePO4 slab has two surfaces, on each of which one PS4 anion is adsorbed via one Fe—S bond, whose averaged lengths are 2.65 and 2.63 Å (see Figure 2). Since there are four active sites (that is, fivefold-coordinated Fe; Fe5C) on each surfaces of the LiFePO4 slab, the adsorption of one PS4 anion corresponds to one fourth coverage (Θ = 0.25). The one-monolayer adsorption is probably impossible because of the steric 6


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repulsion between bulky PS4 anions.

Figure 1. Atomic structure of the LiFePO4 (010)/Li3PS4 interface obtained by averaging each atomic position over 40 ps DF-MD trajectories. Blue and green balls are Li ions stem from LiFePO4 and Li3PO4 phases respectively. FO6 and PS4 are shown as octahedra and tetrahedra respectively.

Figure 2 shows the contour map of density of states (DOS) for beta spin electrons of the LiFePO4/Li3PS4 interface at the average structure. The middle layers of the LiFePO4 and Li3PS4 regions have almost the same local density of states (LDOS) as the respective bulk materials and thus are regarded as the bulk regions. The band gap of LiFePO4 is estimated to be 3.7 eV from the energy difference between the valence band maximum (VBM) and the conduction band minimum (CBM) in the bulk region, both of which stem from the 3d orbitals of Fe atom. On the other hand, the VBM of Li3PS4 is mainly composed of the 3p orbitals of S atom and the band gap is calculated as 2.7 eV. The snapshot structures of the DF-MD trajectories exhibit the similar DOS properties to the average structure, since the atomic movement is not so significant in the fully-discharged Li-rich state. 7



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From the energy difference between the VBM and CBM of LiFePO4 and Li3PS4 in each bulk regions, the band offsets of the valence bands (VB) and conduction bands (CB) of LiFePO4 and Li3PS4 at the interface are estimated to be 0.7 eV and 0.3 eV, respectively. It is noted that the VBM of the Li3PS4 electrolyte phase is 0.7 eV higher than that of the LiFePO4 cathode phase for the LiFePO4/Li3PS4 interface, which is in strong contrast to the case of the LiFePO4/Li3PO4 interface where the VBM of the Li3PO4 phase is 1.2 eV lower than that of the LiFePO4 phase.18 This is because the S-3p orbitals in Li3PS4 is about 2.0 eV higher than the O-2p orbitals in Li3PO4. The obtained VB offset at the LiFePO4/Li3PS4 interface suggests that the Li3PS4 electrolyte may be oxidized when the LiFePO4 cathode is charged, that is, the Li3PS4 electrolyte is not electrochemically stable against to the LiFePO4 electrode.

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Figure 2. Contour map of density of states (DOS) for beta spin electrons of the LiFePO4 (010)/Li3PS4 interface for the average structure. Fermi energy of the whole system is set to zero. It is noted that the system under consideration has two interfaces at around b = 0 and 18 Å.

In order to verify the above suggestion and investigate the stability of the LiFePO4/Li3PS4 interface in the charged state, we have considered the FePO4/Li3PS4 interface, which is regarded as an ideal fully-charged state of the LiFePO4/Li3PS4 interface. The FePO4/Li3PS4 interface is also considered as an interface system of a battery just after manufactured by using FePO4 as a raw material.32,33 The dynamics of the FePO4/Li3PS4 interface is examined from DF-MD simulations for about 200 ps under the NPT conditions similar to the LiFePO4/Li3PS4 interface. The initial structure of the FePO4/Li3PS4 interface system is obtained by removing all lithium atoms from the LiFePO4 slab of the LiFePO4/Li3PS4 interface. At a relatively early stage of the DF-MD simulations, the FePO4/Li3PS4 interface becomes unstable. Figure 3 shows the contour map of DOS for beta spin electrons of the FePO4/Li3PS4 interface for the snapshot structure at 40.1 ps in the DF-MD simulations. It is evident that both a Li ion and an electron are transferred from the Li3PS4 side to the FePO4 side. The transferred electron reduces one Fe3+ ion near the interface to form a localized level (Fe2+) in the band gap of FePO4, whereas a PS4 anion in the Li3PS4 region is oxidized to generate a hole state. The oxidized PS4 9



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anion tends to be desorbed from the FePO4 surface because S-Fe bond lengths elongate to 3.6 Å. Although the hole is localized on a PS4 anion in this snapshot structure, whether the hole is localized or delocalized on PS4 anions depends on the local atomic structures during the DF-MD simulations. In addition to the electron transfer, one Li ion is transferred from the Li3PS4 side to the FePO4 side. By the transformation, the FePO4/Li3PS4 interface system gains about 0.3 eV (28.88 kJ mol-1), as shown in Figure 4. This energy gain is nearly identical to the difference in the calculated Li chemical potentials between the bulk FePO4 and Li3PS4. (3.59 eV for LiFePO4 and 3.26 eV for β-Li3PS4).

Figure 3. Contour map of density of states (DOS) for beta spin electrons of the FePO4 (010)/Li3PS4 interface for the snapshot structure at 40 ps in the DF-MD simulations, in which both a Li ion and an electron are transferred from the Li3PS4 side to the FePO4 side. Fermi energy of the whole system is set to zero.

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Figure 4. Normalized histograms of energies (kJ mol-1) of the FePO4/Li3PS4 interfaces without Li transfer from the Li3PS4 side to the FePO4 side (green line), with one Li ion transferred (purple line) and with eight Li ions transferred (blue line). The average energy of the interface without Li transfer is set to zero.

The instability of the FePO4/Li3PS4 interface is deduced from the DF-MD simulations. Deliberate migration of eight Li ions to the FePO4 side lowers the total energy of the system with the reduction of twelve Fe3+ to Fe2+. Figure 4 shows that this structure is more stable than no Li migration system by 5.2 eV (506.72 kJ mol-1). Figure 5 shows the contour map of DOS for beta spin electrons of the FePO4/Li3PS4 interface for the snapshot structure at the final stage in the DF-MD simulations. As shown in Figure 5, several localized level (Fe2+) and hole states appear, respectively, in the band gap of the FePO4 and Li3PS4 sides. As a result, the Li-depleted layer is formed in the Li3PS4 side near the interface. In this way, the present DF-MD simulations demonstrate clearly the instability of the FePO4/Li3PS4 interface; the lithium depletion and oxidation in the Li3PS4 side near the interface. The oxidized Li3PS4 tends to undergo a structural transformation and some aggregates such as (PS4)2 and S-S bonds are observed during DF-MD 11



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simulations, as shown in Figure 6.

Figure 5. Contour map of density of states (DOS) for beta spin electrons of the FePO4 (010)/Li3PS4 interface for the snapshot structure at the final stage (about 170 ps) of the DF-MD simulations, in which eight Li ions are transferred from the Li3PS4 side to the FePO4 side. Fermi energy of the whole system is set to zero.

Figure 6. Snapshot structure in the DF-MD simulations, where some aggregates of PS4 are observed in the Li3PS4 phase.

To examine the difference between the sulfide and oxide solid electrolytes, we have also 12


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performed the DF-MD simulations for the FePO4/Li3PO4 interface. The initial structure of the FePO4/Li3PO4 interface is obtained by removing all lithium atoms from the LiFePO4 slab of the LiFePO4 (010)/Li3PO4 (100) interface, which was already obtained in our previous study17,18. Figure 7 shows the contour map of DOS for beta spin electrons of the FePO4/Li3PO4 interface structure averaged over the DF-MD trajectories. Although the VBM of the oxide Li3PO4 side is higher than that of the FePO4 side, similar to the sulfide Li3PS4 case shown in Figure 3, the oxidation of Li3PO4 by FePO4 does not occur during the DF-MD simulations, since the acceptor level (Fe3+) of FePO4 is located too high in energy to accept electrons from the Li3PO4 side.17,18 Furthermore, it is found that when one Li is intentionally transferred from the Li3PO4 side to the FePO4 side, the transferred Li migrates back to the Li3PO4 side during DF-MD simulation. Therefore, the lithium depletion does not occur at the FePO4/Li3PO4 interface, in contrast to the FePO4/Li3PS4 interface. The recent experiment reported that the Li-depletion effect is negligible at the interface between LiCoO2 and Li3PO4-xNx which is fabricated in all-in-vacuum process.34 Because the acceptor level of LixCoO2 is expected to be lower than that of FePO4, this experiment may suggest that Li3PO4 is not oxidized by FePO4, which is in consistent with our results.

Figure 7. Contour map of density of states (DOS) for beta spin electrons of the FePO4 (010)/Li3PO4 (100) interface for the structure averaged during the 40 ps DF-MD simulations. Fermi energy of the whole system is set to zero. 13



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Here, we speculate what happens at the LiFePO4/Li3PS4 interface during charging based on the obtained results. At the beginning of charging (Fig. 8(b)), if the cathode side is oxidized to form acceptor Fe3+ levels, the Fe3+ levels may accept electrons immediately from the sulfide side near the interface even in the applied voltage. This is due to the band alignment at the interface. The cathode side remains neutral and the sulfide side becomes charged. Then, the Li ions in the sulfide side migrate towards the anode side under an electric field. As the result, the sulfide side near the interface works initially like as an active material, which leads to the formation of the Li-depleted layer.1 Actually, recent experiments have reported that Li3PS4 is oxidized and works as an active material in solid-state battery systems.7 When the Li-depleted layer grows thick on the sulfide side during charging, the electron transfer from the sulfide side to the cathode side becomes suppressed and finally electrons and Li ions begin to be removed from the cathode side instead of the sulfide side (Fig. 8(c)). The migration of Li ions from the cathode side to the anode side may not compensate the Li-depleted layer in the sulfide, otherwise the compensated sulfide side would donate electrons to the acceptor Fe3+ levels again according to the same scenario at the beginning of charging. The thickness of the Li-depleted layer may depend on the diffusion of Li ions and electrons in the Li-depleted sulfide. The Li-depleted layer possibly grows up to the defective layer due to the chemical products as shown in Figure 6. The formation of the defective layer is under investigation in our group. Although the LiFePO4/Li3PS4 interface has been investigated in this study, there are many other interfaces between oxide cathodes and sulfide solid electrolytes examined experimentally. For most of those interfaces, the VBM of oxide cathodes, which are mainly composed of the 3d orbitals of transition metals, are lower in energy than the VBM’s of sulfide electrolytes composed of the S-3p orbitals, and the Li chemical potentials of oxide cathodes are lower than those of sulfide electrolytes, which are similar to the LiFePO4/Li3PS4 interface. As described above these features lead to the instability of the interfaces. Therefore, the present charging picture given from the LiFePO4/Li3PS4 14


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interface will be valid for most of the interfaces between oxide cathodes and sulfide electrolytes. In fact, the interface defective layer mainly composed of Co and S atoms has been observed at the interface between LiCoO2 and sulfide electrolytes7. The investigation of the interface reactions is ongoing in our group.

Figure 8: Schematic diagram of charging process of the Li ion batteries with the cathode/sulfide-electrolyte interface: (a) the initial fully-discharged state, (b) the state at the beginning of charging, (c) the state when the charging proceeds, and (d) the final state where the Li-depleted layer is formed.

As discussed above, the electron transfer from the sulfide electrolyte to the cathode at the interface, that is, the oxidation of the sulfide electrolyte during charging may lead to the formation of Li-depleted layer or defective layer, which results in high interface resistance. Therefore, in order to reduce the interface resistance, the electron transfer from the sulfide electrolyte should be suppressed. The oxide electrolytes like Li3PO4 usually have lower VBM’s relative to the sulfide electrolytes and even to the cathode materials. When the oxide layers are inserted at the interfaces, they can act as the buffer layers to suppress the oxidation of the sulfide electrolytes and reduce the interface resistance, as shown in Figure 9. In this way, the present findings support the validity of 15



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the buffer layer insertion at the interface.

Figure 9. Schematic diagram of the band lineups of (a) the cathode/sulfide-electrolyte/Li-metal system and (b) the system with the oxide buffer layer inserted at the cathode/sulfide-electrolyte interface.

4. Conclusions In this paper, we have theoretically investigated the interface properties between cathodes and sulfide electrolytes to elucidate the origin of the high ionic interface resistance, in comparison with the oxide electrolyte interfaces. Based on the DF-MD simulations of the LiFePO4/Li3PS4 interfaces in both discharged and charged states, it has been clearly demonstrated that the sulfide interfaces become unstable in the charged state; lithium depletion and oxidation occurs on the sulfide side near the interface, which is in contrast to the oxide interfaces. The instability is due to the band alignment and the difference in the Li chemical potentials at the sulfides interface. The obtained results imply the formation of the Li-depleted layer in the sulfide interface during charging and support the validity of the insertion of oxide buffer-layers at the interface to reduce the interface resistance. The present picture given from the LiFePO4/Li3PS4 interface will be also valid for most of the interfaces between oxide cathodes and sulfide electrolytes.

Acknowledgment. This work was supported by JST ALCA project. The computations in this work 16


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were carried out on the supercomputer centers of NIMS.

Supporting Information Available: Setup of an initial configuration of the LiFePO4/ Li3PS4 interface is shown. This information is available free of charge via the Internet at http://pubs.acs.org.

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T.

Negligible

“Negative

Space-Charge

Layer

Effects”

at

Oxide-Electrolyte/Electrode Interfaces of Thin-Film Batteries Nano Lett. 2015, 15, 1498-1502.

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Sulfide Electrolyte

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