Article pubs.acs.org/cm
Electrochemical Nature of the Cathode Interface for a Solid-State Lithium-Ion Battery: Interface between LiCoO2 and GarnetLi7La3Zr2O12 Kyusung Park,†,# Byeong-Chul Yu,†,# Ji-Won Jung,‡ Yutao Li,† Weidong Zhou,† Hongcai Gao,† Samick Son,§ and John B. Goodenough*,† †
Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea § Environment & Energy Research Team, Central Advanced Research and Engineering Institute, Hyundai Motor Group, Uiwang-si, Gyeonggi-do 437-815, Republic of Korea
Chem. Mater. 2016.28:8051-8059. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/27/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: Garnet-structured solid electrolytes have been extensively studied for a solid-state lithium rechargeable battery. Previous works have been mostly focused on the materials’ development and basic electrochemical properties but not the cathode/electrolyte interface. Understanding the cathode interface is critical to enhance chemical stability and electrochemical performance of a solid-state battery cell. In this work, we studied thoroughly the cathode/electrolyte interface between LiCoO2 and Li7La3Zr2O12 (LLZO). It was found that the high-temperature process to fuse LiCoO2 and LLZO induced cross-diffusion of elements and formation of the tetragonal LLZO phase at the interface. These degradations affected electrochemical performance, especially the initial Coulombic efficiency and cycle life. In a clean cathode interface without the thermal process, an irreversible electrochemical decomposition at > ∼ 3.0 V vs Li+/Li was identified. The decomposition was able to be avoided by a surface modification of LLZO (e.g., Co-diffused surface layer and/or presence of an interlayer, Li3BO3), and the surface modification was equally important to suppress a reaction during air storage. In a LiCoO2/ LLZO interface, it is important to separate direct contacts between LiCoO2 and pure LLZO.
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INTRODUCTION Conventional carbonate-based liquid electrolytes used in lithium-ion batteries are flammable with a low flash point, and plating of a lithium anode from the liquid electrolyte results in the formation of dendrites during charge that grow across the electrolyte to the cathode to short-circuit a cell with incendiary consequences. Therefore, Li-ion batteries are fabricated in a discharged state in order to avoid having any metallic lithium in the anode. Use of a carbon, an alloy, or a conversion-reaction anode having a voltage less than 1.2 V versus Li+/Li requires formation of an anode passivation layer that removes lithium from the cathode, especially in the initial charge, to reduce the capacity of insertion-compound cathodes. Concerns about battery safety and too low a cell energy density with an anode free of metallic lithium has led to increasing interest in a solid electrolyte having a Li+-ion conductivity greater than 10−4 S cm−1 at room temperature, which is not reduced on contact with metallic lithium and can effectively either block lithium-anode dendrites or be wet by metallic lithium so dendrites do not form. One of the solid electrolytes of interest has been the garnet ceramic Li7La3Zr2O12 (LLZO).1 An all-solid-state cell has a solid−solid interface with both the anode and the cathode, and © 2016 American Chemical Society
the conventional cathode is an insertion compound that undergoes large three-dimensional (3D) volume changes over a charge/discharge cycle. Maintenance of a small impedance for Li+ transfer across a solid-electrolyte/solid-cathode interface is particularly difficult when the electrode particles undergo large volume changes. In this work, we have focused on the interface between the LLZO garnet electrolyte and a LiCoO2 cathode. LLZO has the cubic garnet structure with a 3D interstitial space consisting of tetrahedral-site Li(1) sites (24d) bridged by octahedral Li(2) sites (48g or 96h where Li(1)−Li(2) interactions displace a Li(2) ion).1−3 In the LLZO, Li+ conductivity was reported to be σLi > 10−4 S cm−1 at 25 °C with a motional enthalpy of 500 °C): partial diffusions of Co from LiCoO2 to LLZO and, concurrently, La/Zr from LLZO to LiCoO2. Interfacial impurity phases such as La2CoO4 were also suggested, but the thin-film-LiCoO2/LLZO/Li cell delivered reversible capacities of ∼130 mAh/g for 100 cycles.7,8 Recently, direct loading of LiCoO2 powder on a LLZO pellet was approached with a solid-electrolyte glue layer, Li3BO3, because bare LiCoO2 particles have limited physical contacts with LLZO.9,10 Li3BO3 is a Li+ conductor with a room-temperature conductivity of 2 × 10−6 S cm−1 and melts at ∼700 °C.11 Li3BO3 not only conducts Li+ in between LiCoO2 and LLZO but also suppresses the cross-diffusion across the cathode interface at high temperature (>700 °C). The LiCoO2/Li3BO3 cathode composite on LLZO delivered reversible capacities of ∼80 mAh/g for a few cycles.9 Stable charge/discharge properties with garnet-type solid electrolytes have been demonstrated as summarized above, but the previous studies mostly focused on thin/thick film processing and characterization. The chemical and electrochemical nature of the LiCoO2/LLZO interface has not yet been seriously explored. In this report, we first examined the chemical stability of the LiCoO2/LLZO interface and then made four cathode interface models with different degrees of chemical and physical interactions. Our model experiments have unveiled: (1) the cathode interface between pure LLZO and LiCoO2 is not stable electrochemically, (2) surface modification of LLZO is critical to charge/discharge solidstate electrochemical cells, and (3) strong adhesion of LiCoO2 is also necessary to secure high reversible capacities.
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Chemical stability between LiCoO2 and the as-received LLZO was characterized by two different methods: (1) direct formation of LiCoO2 on LLZO and (2) thermal annealing of LiCoO2 and LLZO powder mixture (1:4 wt. ratio). The direct formation of LiCoO2 was done by solid-state reaction of 1/2Li2CO3, Co acetate, and LLZO with various target mole ratios (LiCoO2/LLZO = 0.2−2.0). The reaction temperature in both methods was set to be 700 °C. Crystallinity and phase purity were characterized with X-ray diffraction (Rigaku MiniFlex 600 II, Cu Kα radiation). LLZO particles and LiCoO2/LLZO interfaces were observed with a transmission electron microscope (TEM; JEOL 2010F). Physical and chemical interactions at the interface were systematically interpreted with highresolution TEM images, EDS mapping, and electron-beam diffraction. Surface chemical states of LLZO were characterized with X-ray photoelectron spectroscopy (XPS; Kratos AXIS Ultra DLD). Fresh LLZO and LiCoO2/LLZO were compared before and after air storage for 3 days. High-resolution elemental distributions were obtained with time-of-flight secondary ion mass spectroscopy (TOF-SIMS; TOF.SIMS 5 by ION-TOF GMBH, Germany). Bi32+ ions were accelerated at 30 kV for the analysis, and oxygen was accelerated at 1 kV for the sputtering. Cross-diffusion of elements at LiCoO2/LLZO after thermal annealing and the air-storage property of LLZO were characterized with TOF-SIMS. All the electrochemical tests were performed at 50 °C because room-temperature tests showed short-circuit of test cells. A Li/Au/ LLZO/Au/Li symmetric cell was used to characterize Li+ transport properties of LLZO at a current density of 0.1 mA cm−2. Gold was coated on both sides of a polished LLZO pellet with a sputtering equipment (Pelco Model 3 sputter coater) for 30 s and then sandwiched between Li metal electrodes. A lab-made test cell that was made of 304 stainless steel and sealed with an O-ring was used for the electrochemical characterization. After cell assembly in an Ar-filled glovebox, the cell was transferred to an oven (50 °C) and aged at least for an hour before the test. For half-cell characterization, a Au/ LiCoO2/LLZO/Au/Li configuration was used with four different LiCoO2/LLZO interfaces that were summarized in Table 1. Regardless of the cathode configuration, a gold current collector layer was coated on LiCoO2 via sputtering for 6 min. For the anode side, LLZO/Au/Li configuration was adopted and prepared in the same way for the symmetric cell experiment. Galvanostatic charge/discharge tests were performed at 2.5−4.4 V vs Li+/Li and a current density of C/5 with the LAND battery cycler.
EXPERIMENTAL SECTION
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Crystalline and stoichiometric Li7La3Zr2O12 (LLZO) powder was provided by Hyundai Motor Co. LLZO pellets (diameter = 1 cm) as solid electrolytes for electrochemical cell tests were fabricated at 1150 °C for 20 h in air. LiCoO2 powder was synthesized with a sol−gel method: 0.02 mol of LiNO3 and Co(NO3)2·6H2O, 0.04 mol of citric acid, and 0.08 mol of ethylene glycol were dissolved in distilled water on a hot plate; after drying overnight, a clear red gel was obtained, which was decomposed at 450 °C for 1 h and heat-treated at 800 °C for 6 h to form LiCoO2.
RESULTS AND DISCUSSION High-density LLZO pellets were prepared by a conventional sintering process at 1150 °C. The resulting pellet has Al contamination from the crucible, and the diffraction peak was well-matched to cubic Li6.06Al0.20La3Zr2O12. However, we will use the sample name LLZO instead of the Al-doped composition for convenience. Typical pellet density was 8052
DOI: 10.1021/acs.chemmater.6b03870 Chem. Mater. 2016, 28, 8051−8059
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Chemistry of Materials ∼93% in our experiment. Li-ion conductivity was measured to be 1.0 × 10−4 S cm−1 at room temperature. First, we checked the Li transport properties with Li/Au/LLZO/Au/Li symmetric cells. To lower interfacial impedance, gold layers on both sides of the LLZO pellet were deposited before testing. The same idea was recently published independently by Tsai et al.,12 and the gold layers reduced impedance significantly by (1) uniform and complete coverage of the solid electrolyte and (2) ability to form Li−Au alloy. The same strategy can be used for other materials such as Ag, Si, Sn, which can form alloys with lithium. The gold coating reduced the interfacial impedance dramatically, but in contrast to the previous report, it was found that gold could not suppress the formation of lithium dendrites in a reasonable charge/discharge conditions. Figure 1a clearly demonstrates short circuits in 591 and 28 s at 0.1 and 0.5 mA cm−2, respectively. Lithium dendrites can penetrate through open pores and grain boundaries at room temperature, as has been reported previously and as is shown in Figure 1b.13−15 Cross-sectional area of the short-circuited LLZO turned black, and the SEM image for the corresponding area
showed deposit of a secondary material. The materials should be lithium metal or another grain boundary phase that was reduced by lithium. Because it is essential to get stable and reversible Li+ transport properties across the LLZO electrolyte to study a cathode interface, several experimental parameters were adjusted to minimize the chance of a short-circuit. Here we increased the cell temperature to 50 °C. Figure 1c shows reversible charge/discharge voltage curves of a Li/Au/LLZO/ Au/Li symmetric cell. It shows a reversible cycling at a current density of 0.1 mA cm−2 unlike the room-temperature test results. A relevant theoretical model about the temperature effect on the lithium dendrite growth has been recently reported. It was suggested that a higher atomic mobility of Li atoms at a higher temperature can help reduce the surface area of the metastable dendrites and promote isotropic lithium growth.16,17 All the following electrochemical tests in this work were performed at 50 °C to prevent possible dendrite-induced short circuits of test cells. For the analysis of the cathode/solid-electrolyte interface, thermal stability of LiCoO 2 and LLZO mixture was characterized in air. Li2CO3 and cobalt acetate, which are corresponding to 20−200 mol % of LiCoO2 to the as-received LLZO, were thoroughly mixed with LLZO at room temperature, and the powder blends were heat-treated at 450 °C for 1 h and 700 °C for 6 h to form LiCoO2. Annealing at 800 °C, a typical heat-treatment temperature for LiCoO2 synthesis, brought a significant phase collapse of LLZO (Figure S1 of the Supporting Information), so the annealing temperature was fixed to 700 °C. Figure 2a,b show powder X-ray diffraction patterns of pristine and thermally annealed LiCoO2/LLZO
Figure 2. Powder X-ray diffraction patterns of (a) pristine LLZO powder, (b) thermally formed LiCoO2 on LLZO by a solid state reaction method at 700 °C for 6 h with ratios of 20−200 mol % LiCoO2 to LLZO, and (c) powder mixture of LiCoO2 and LLZO (1:4 wt. ratio) annealed at 700 °C for 1 h. Peaks from LiCoO2 were enclosed by rectangles, and those from the tetragonal LLZO phase were marked with asterisks.
Figure 1. (a) Li-plating voltage curves of Li/Au/LLZO/Au/Li symmetric cells at 0.1 and 0.5 mA cm−2. (b) Cross-sectional photograph of LLZO after short-circuit (left) and the corresponding SEM image for the dark part in the center (right) (c) Reversible charge/discharge voltage curves of Li/Au/LLZO/Au/Li symmetric cell at a current density of 0.1 mA cm−2 and 50 °C. 8053
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TEM images are available in Figure S3. Second, a part of the electron diffraction pattern of the garnet surface in LiCoO2/ LLZO could be assigned to the tetragonal LLZO, and the result is consistent with the XRD data in Figure 2b. Direct formation of LiCoO2 on LLZO has been proven to be not ideal for a chemically stable interface owing to the chemical interaction; we also performed a TEM study on the thermally annealed powder mixture of LiCoO2 and LLZO. We put LiCoO2 powder on a LLZO pellet and annealed it at 700 °C for 1 h to form a cathode interface, and then scratched the surface to collect the LiCoO2/LLZO interface for a TEM experiment. Our sample preparation condition should reflect a real cathode interface configuration. Figure 4 shows how LiCoO2 and LLZO
mixtures, respectively. The (003) peak of LiCoO2 was observed from 20 mol %. In particular, we found diffraction peaks of the tetragonal phase of LLZO as is indicated with asterisks. The peak assignment is further discussed in the Supporting Information. The tetragonal phase is a stable phase at low temperature and shows less Li+ conductivity values (∼10−6 S cm−1 at room temperature).18−20 The presence of the tetragonal phase must increase the interfacial impedance at the cathode/electrolyte interface to some extent, so it should be important to find a condition for minimizing its formation. It was also noted that the XRD peak of LLZO was broadened as the added Li−Co increased (Figure S2). In summary, synthesis of LiCoO2 in the presence of LLZO degrades LLZO and their interface. From the results in Figure 2b, it is clear that it is not desirable to form LiCoO2 directly on LLZO owing to the hightemperature chemical reaction at the interface. To determine if reaction time and precursors can mitigate the problem, asprepared crystalline LiCoO2 powder was mixed with LLZO powder, and the powder blend was annealed at 700 °C for 1 h. Figure 2c shows its powder XRD pattern. The tetragonal phase was still observed, but its intensity was reduced notably. Another advantage is that high crystallinity of LiCoO2 could be retained in this experimental condition. Minimizing annealing time is also important to reduce the interface degradation. To verify the presence of the tetragonal LLZO phase, LLZO particles in bare LLZO and LiCoO2/LLZO (100 mol % LiCoO2 sample in Figure 2b) powders were characterized with TEM experiments, as shown in Figure 3. First, bare LLZO has a
Figure 3. TEM images and electron diffraction patterns of LLZO in (a) pristine LLZO and (b) LiCoO2/LLZO (100 mol % LiCoO2 sample in Figure 2b) powder samples.
less crystalline surface layer while LLZO in contact with LiCoO2 is clearly more crystalline. Since LLZO quickly reacts with air and forms Li2CO3, it is reasonable to expect that the surface of LLZO may have an amorphous structure. In contrast, LiCoO2/LLZO may keep better a crystalline structure, or the secondary interfacial phases were actually observed. Additional
Figure 4. TEM images of LiCoO2/LLZO that was directly obtained from LiCoO2 film on a LLZO pellet and the corresponding EDS elemental maps: (top) combined La (yellow), Co (blue), C (green), and O (red) map and (bottom) individual O, La, Co, and Al maps. 8054
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Figure 5. TOF-SIMS-enabled three-dimensional elemental maps of the LiCoO2/LLZO interface that is displayed in the inset SEM image. Color scales next to the maps show ionic concentrations of each ion: upper side represents a higher concentration.
LLZO at 700 °C for 1 h. This process should give physical bonding as well as chemical interaction, as shown in Figures 4 and 5. Fair physical bonding was confirmed by scratching the LiCoO2 coating layer even though sintering was not completed at this relatively low annealing temperature. (ii) Sample RTLCO/LLZO is made simply by sprinkling LiCoO2 particles on LLZO at room temperature without thermal annealing. In this sample, we expected no physicochemical interaction during interface formation and a high cell impedance. (iii) Sample RTLCO/Co-LLZO was made by (1) polishing the surface of 700LCO/LLZO to remove the LiCoO2 layer but retain the Codiffused surface on the LLZO pellet, as shown in Figure S4, and (2) sprinkling LiCoO2 particles on the Co-diffused LLZO at room temperature. No physical bonding was expected between LiCoO 2 and LLZO while having a chemical surface modification at the interface. (iv) The last sample was 700LCO-LBO/LLZO, which was similar to 700-LCO/LLZO, but the cathode is a mixture of LiCoO2 and Li3BO3, which melts during the heat treatment at 700 °C. This type of cathode interface was expected to have strong physical bonding but restricted chemical interaction between LiCoO2 and LLZO, owing to the presence of the Li3BO3 glassy interlayer. The positive effect of Li3BO3 to minimize the cross-diffusion has been reported previously. The first sample 700-LCO/LLZO had a typical cathode interface. Cathode and solid electrolyte were joined by thermal annealing, and the interface had relatively good adhesion, but chemical degradation as well. In this case, the electrochemical charge/discharge property was characterized as shown in Figure 6. Initial charge capacity was 64.0 mAh g−1, and discharge capacity was 35.0 mAh g−1 with a Coulombic efficiency of 54.7% at C/5. The first cycle efficiency was low, and the reversible capacities decayed during cycling, as shown in Figure 6b. We attributed this poor performance to the structural degradation and the chemical contamination by the hightemperature annealing. The second sample RT-LCO/LLZO was initially expected to have poor electrochemical performance owing to the lack of physical contacts. Figure 7 shows initial charge voltage curve, and it was found that there was an electrochemical reaction below Co(III)/Co(IV) of LiCoO2 starting from ∼2.7 V vs Li+/ Li and more pronounced from > ∼3.0 V. The unexpected
contact and where individual elements are present. It clearly demonstrates that strong physical contact has barely been achieved. No clear signature of sintering such as neck formation and growth was observed during the characterization. Moreover, it was found that there was diffusion of Al from LLZO to LiCoO2: Al was found in every single particle. Aluminum is a common contaminant of the garnet electrolytes. For other elements, it is not obvious to examine the extent of thermal diffusion owing to the low resolution of the EDS maps. Because Al plays an important role to stabilize the cubic LLZO phase and to increase density of LLZO pellets, we further characterized the high-resolution elemental distribution with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Exact understanding of the chemical interaction at the cathode interface is also important to identify the chemical origin for the formation of the tetragonal LLZO. For TOFSIMS characterization, the LiCoO2/LLZO pellet sample used in Figure 4 was adopted. It shows a clear phase boundary between LiCoO2 and LLZO as is displayed in the inset SEM image of Figure 5. Illustrations in Figure 5 represent elemental distributions (280 × 240 × 2 μm3) at the LiCoO2/LLZO interface. There is minor cross diffusion at the interface as appears as dots with less density: Co diffuses into LLZO, and Zr/La diffuses into LiCoO2. There is a very interesting result for the Al distribution. Al diffusion occurred significantly and was not just limited at the interface area but also at the opposite end of the LiCoO2 layer, as shown in Figure 4. The Al concentration change across the interface was dramatic: color codes for elemental concentration were changed from white to red, green, and blue. Al is in the Li-sites of LLZO and stabilizes the cubic phase. As Al is leached out of LLZO and diffuses into the LiCoO2, the cubic LLZO at the interfaces transformed to the tetragonal phase, as shown in the XRD and TEM results. High-temperature processing to form the LiCoO2/LLZO interface causes elemental cross-diffusion across the interface and destabilizes the cubic garnet framework. So far we have checked chemical aspects of the LiCoO2/ LLZO interface. To investigate the electrochemical nature of the LiCoO2/LLZO interface, we have made four samples with different physical and chemical characteristics as summarized in Table 1. (i) Sample 700-LCO/LLZO is a conventional cathode interface that is made by thermal annealing of LiCoO2 and 8055
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XRD experiment, but no notable crystalline impurities were identified, probably as a result of the extremely small cathode/ solid-electrolyte interface area and the negligible total amount of the irreversible reaction in this model interface. Theoretical calculations used perfect crystals for cathode and solid electrolyte and assumed a clean interface, which is very close to the case of RT-LCO/LLZO. All the previous experimental reports on LiCoO2/LLZO used thermal processes to form an intimate and uniform cathode interface, and the high-temperature heating inevitably caused cross-diffusion of elements across the cathode interface and the corresponding structural changes. With our model experiment, it can be demonstrated that a clean LiCoO2/LLZO cathode interface experiences an irreversible phase decomposition at > ∼3.0 V vs Li+/Li, as predicted by theoretical calculations. Moreover, the data also show that (1) electrochemical stability of a solid electrolyte should be characterized in the presence of the cathode material as well as the alkali-metal anode and (2) theoretical modeling should also deal with nonideal and chemically impure interfaces to predict correctly real electrochemical stabilities. To compare the cathode interfaces of RT-LCO/LLZO and 700-LCO/LLZO, the third sample RT-LCO/Co-LLZO was characterized. In this case, 700-LCO/LLZO was first prepared, and then the surface-modified LLZO was isolated by polishing LiCoO2. After this process, the surface color of LLZO became nonuniformly dark (Figure S4). Figure 8 shows charge/ discharge voltage curves in the first cycle. The charge capacity was 28.4 mAh g−1, and discharge capacity was 18.1 mAh g−1 with a Coulombic efficiency of 63.7%. Unlike RT-LCO/LLZO, it shows fairly large reversible capacities and stable cycle life. The results provide two important messages: (1) the poor electrochemical performance of RT-LCO/LLZO was not
Figure 6. (a) Charge/discharge voltage curves and (b) cycle performance of 700-LCO/LLZO. Sample information is summarized in Table 1.
Figure 7. (a) Charge/discharge voltage curves and (b) cycle performance of RT-LCO/LLZO. Sample information is summarized in Table 1.
electrochemical reaction proceeded up to ∼5 mAh g−1. Above ∼3.8 V, a typical voltage curve from LiCoO2 was observed. However, (1) the total charging capacity from LiCoO2 (> ∼3.8 V) is as low as ∼10 mAh g−1, and (2) there was negligible reversible capacity in the following cycles after the first charge. These results suggest that the electrochemical reaction at the low voltage range is irreversible. The irreversibility has not been reported experimentally, but theoretically. There are a few recent theoretical calculations that predict a potential-dependent electrochemical instability of the LiCoO2/LLZO interface.21−24 Those first-principles calculations suggested that the Li1−xCoO2/LLZO interface would experience an electrochemical decomposition to several phases such as La2O3, La 2Zr2O 7, Li 6Zr2O7 , Li2 CoO 3, and Li5 CoO4. However, experimental data have shown no such electrochemical instabilities yet, which makes the sample RT-LCO/LLZO unique. We have tried to identify the decomposition product by
Figure 8. (a) Charge/discharge voltage curves and (b) cycle performance of RT-LCO/Co-LLZO. Sample information is summarized in Table 1. 8056
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adopt solid electrolytes such as polymer-based materials at room temperature for the cathode interface formation.25 In this way, undesirable chemical reactions can be minimized and good physical binding can be achieved. We have covered the importance of physicochemical nature of the LiCoO2/LLZO cathode interface and its effect on electrochemical properties. The characteristics of the cathode interface also significantly affect the chemical stability of the garnet-structured solid electrolyte. Figure 10 shows C 1s XPS spectra of LLZO and LiCoO2/LLZO powder (100 mol % LiCoO2 sample in Figure 2b) before and after 3 days of air storage. The spectra were deconvoluted into three peak components of C−C, C−O−C, and O−CO. Here we focused on the peak intensity ratio between the carbonate (O− CO) component and the direct C−C bonding component as a signature to represent air stability because the surface carbonate species have been attributed to the origin of high interfacial impedance. The ratios, I(O−CO)/I(C−C), for LLZO before and after the air storage are 79.8 and 88.9%, respectively, whereas those for LiCoO2/LLZO are 56.3 and 67.9, respectively. The bare LLZO powder has more lithium carbonate on its surface. Moreover, the LiCoO2/LLZO even after the air exposure for 3 days still had a significantly lower carbonate contamination than the bare LLZO. The surface modification with LiCoO2 indeed benefited air stability. One lesson from this XPS characterization is that a surface modification such as coating or doping can be used to lower the interfacial impedance of the garnet-structured solid electrolytes. We showed an increase in the surface carbonate species with XPS spectra, but there have been no experimental and theoretical reports to show where and how those carbonates exist on the surface. TOF-SIMS characterization was again adopted to characterize the surfaces of fresh and one-year-aged LLZO pellets to demonstrate how the surface of LLZO degraded over time. Figure 11 shows high-resolution elemental maps for aluminum and lanthanum before and after air exposure for 1 year. A fresh sample showed uniform spatial distributions of elements. However, after the aging, Al distribution is not uniform, and it is particularly less populous on the surface. Lanthanum also exhibits a notable concentration gradient between bulk and surface. It is very interesting that those two elements became separated from each other, which suggested a possible surface reconstruction. In addition, the surface becomes very rough, and those empty surface regions in the images represent the surface contamination layer, such as Li2CO3 or lithium oxide without heavy metallic elements. The rough surface image in Figure 11 does not indicate real surface roughness.
because of a lack of physical bonding and (2) a pure LLZO surface is subject to irreversible electrochemical decomposition, but a surface-modified LLZO can be stable, which is also why a typical cathode interface like 700-LCO/LLZO has shown reasonable electrochemical performances in previous works. With this model experiment, we could successfully verify our argument on the electrochemical instability of the pure LiCoO2/LLZO interface. To confirm advantages of an ionically conductive interlayer between LiCoO2 and LLZO, a glass solid electrolyte Li3BO3 was adopted. Li3BO3 reportedly blocks or minimizes chemical interactions at high temperature.9,10 It was shown that Li3BO3 could minimize chemical diffusion by removing the surface LiCoO2 layer: the exposed LLZO layer has its original color, not dark as in the case of RT-LCO/Co-LLZO (Figure S5). Figure 9 shows charge/discharge voltage curves in the first cycle
Figure 9. (a) Charge/discharge voltage curves and (b) cycle performance of 700-LCO-LBO/LLZO. Sample information is summarized in Table 1.
of the sample 700-LCO-LBO/LLZO. The charge capacity was 79.9 mAh g−1, and discharge capacity was 67.2 mAh g−1 with a Coulombic efficiency of 84.1%. This sample shows the best capacity and efficiency. Capacity retention was also stable. From the result, it is obvious that an interlayer material is necessary to improve electrochemical reactions and stabilize the cathode interface. However, it should be noted that the charging voltage has a notable polarization of ∼0.1 V, so the charging started from ∼3.9 V. To reduce the polarization, electrically conductive materials should be added during the formation of the cathode interface since Li3BO3 is an electronic insulator. In parallel, another approach to reduce the amount of Li3BO3 to solve the polarization issue was not successful (results not shown): (1) good physical bonding was not achieved owing to the lack of Li3BO3, (2) the LLZO surface notably got darkened as an evidence of chemical interaction, and (3) the polarization still existed with less than 25 wt % Li3BO3 in the cathode composite. It could also be possible to
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CONCLUSION We have studied chemical and electrochemical properties of the LiCoO2/LLZO cathode interface. It was found that the chemical nature of the cathode interface profoundly influences the electrochemical charge/discharge properties of a solid state battery cell. There were inevitable chemical interactions during the high-temperature cathode formation. Cubic LLZO reacts with LiCoO2 and forms the tetragonal LLZO phase at the interface. Cross-diffusions of elements, especially Al, occurred, and the Al-deficient LLZO surface partially transformed into the tetragonal LLZO. High-temperature surface degradation also deteriorates electrochemical properties even though high temperatures can promote physical bonding to some extent. In 8057
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Figure 10. C 1s XPS spectra of LLZO and LiCoO2/LLZO powder (100 mol % LiCoO2 sample in Figure 2b) before and after 3 days of air storage.
improved physical bonding. Moreover, the surface modification was also important to reduce formation of surface carbonates during air storage. Surface modification of the garnet-LLZO should be further studied in order to stabilize LLZO against side reactions and to improve battery performance.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03870. XRD data, TEM images, and photographs of the LiCoO2 and LLZO interface (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
Figure 11. TOF-SIMS elemental maps of fresh and one-year-aged LLZO pellet samples. Aluminum and lanthanum were chosen here to display surface degradation during sample storage in air. Top faces of these rectangular prism represent the surface of LLZO pellets.
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These authors contributed equally to this work (K.P. and B.C.Y.). Notes
The authors declare no competing financial interest.
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contrast, in the case of LiCoO2/LLZO with a chemically pure interface, an irreversible electrochemical reaction at 3.0−3.8 V versus Li+/Li was identified, and it was detrimental to reversible capacity retention. After modifying the surface of LLZO (i.e., Co-doped LLZO), it was possible to reversibly charge/ discharge the cell, although high capacity could not be achieved. Good physical bonding is important to improve electrochemical reversibility, but it is not as critical as the chemical stability. Surface modification with Li3BO3 between LiCoO2 and LLZO reduced chemical cross-contamination and
ACKNOWLEDGMENTS J.B.G., K.P., and B.C.Y. appreciate financial support from the Hyundai Motor Co. J.B.G. and Y.L. acknowledge support from the Advanced Battery Materials Research (BMR) Program, Department of Energy (Project Number 7223523).
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REFERENCES
(1) Cussen, E. J. Structure and ionic conductivity in lithium garnets. J. Mater. Chem. 2010, 20, 5167−5173.
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Article
Chemistry of Materials
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DOI: 10.1021/acs.chemmater.6b03870 Chem. Mater. 2016, 28, 8051−8059
