Failure Mechanism and Interface Engineering for NASICON

Research Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces 2019, 11, 20895−20904

Failure Mechanism and Interface Engineering for NASICONStructured All-Solid-State Lithium Metal Batteries Linchun He,‡ Qiaomei Sun,‡ Chao Chen,‡,§ Jin An Sam Oh,‡,⊥,# Jianguo Sun,‡ Minchan Li,‡,† Wenqiang Tu,‡ Henghui Zhou,∥ Kaiyang Zeng,‡ and Li Lu*,‡,§ ‡

Department of Mechanical Engineering, National University of Singapore, Singapore 117575 National University of Singapore (Suzhou) Research Institute, Suzhou 215123, P. R. China ∥ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ⊥ Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore 138632 # Singapore Institute of Manufacturing Technology, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Innovis, Singapore 138634

Downloaded via BUFFALO STATE on July 18, 2019 at 07:34:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

§

S Supporting Information *

ABSTRACT: All-solid-state lithium metal batteries (ASSLiMB) have been considered as one of the most promising next-generation high-energy storage systems that replace liquid organic electrolytes by solid-state electrolytes (SSE). Among many different types of SSE, NASICON-structured Li1+xAlxGe2−x(PO3)4 (LAGP) shows high a ionic conductivity, high stability against moisture, and wide working electrochemical windows. However, it is unstable when it is in contact with molten Li, hence largely limiting its applications in ASSLiMB. To solve this issue, we have studied reaction processes and mechanisms between LAGP and molten Li, based on which a failure mechanism is hence proposed. With better understanding the failure mechanism, a thin thermosetting Li salt polymer, P(AAco-MA)Li, layer is coated on the bare LAGP pellet before contacting with molten Li. To further increase the ionic conductivity of P(AA-co-MA)Li, LiCl is added in P(AA-co-MA)Li. A symmetric cell of Li/interface/LAGP/interface/Li is prepared using molten Li−Sn alloy and galvanically cycled at current densities of 15, 30, and 70 μA cm−2 for 100 cycles, showing stable low overpotentials of 0.036, 0.105, and 0.257 V, respectively. These electrochemical results demonstrate that the interface coating of P(AA-co-MA)Li can be an effective method to avoid an interfacial reaction between the LAGP electrolyte and molten Li. KEYWORDS: all-solid-state lithium metal battery, NASICON structure, solid-state electrolyte, failure mechanism, interface battery.15 However, LAGP/LATP is unable to be directly in contact with the Li metal because Ti4+/Ge4+ in LATP/LAGP will be reduced by Li to Ti2+/Ge2+ or even Ti0/Ge0.16,17 Furthermore, the relative high electronic conductivity of pure NASICON-structured SSEs did not also sastify the requirement of a long cycle cell.18−20 To utilize the high ionic conductivity and moisture stability of NASICON-structured LAGP/LATP SSEs and the high-capacity Li metal as the anode, it is essential to design an interface layer that is stable against LAGP/LATP and the Li metal. There are many possible candidates, for example, poly(oxyethylene) (PEO)21 and poly(ethylene glycol) methyl ether acrylate22 films. However, these soft polymers may easily be reduced and penetrated by the Li dendrite when working at an elevated temperature.23,24 Dendrite growth and penetration through a polymer electrolyte still limit the application of polymer electrolyte-based cells. The main reason is associated

1. INTRODUCTION Currently, the Li-ion battery is the main energy storage system for portable electronics and large energy applications such as an electric vehicle, submarine, etc. With the increased efforts in energy density and safety for storage devices, the current commercial electrodes and format of the batteries are however unable to meet these requirements.1 Therefore, designing new energy storage systems with high safety and energy density becomes an urgent need. All-solid-state lithium metal batteries (ASSLiMB) are promising candidates for next-generation energy storage to replace current commercial Li ion batteries.2−5 One of the key differences between the current Li-ion battery and ASSLiMB is the use of solid-state electrolytes (SSEs). Among various SSEs,3,4,6,7 ceramic SSEs such as garnet8−10 and NAICON11−13 structure are outstanding SSEs due to their high ionic conductivity (10−4−10−3 S cm−1), wide working electrochemical windows, and relatively high stability in ambient condition. Because NASICON-structured Li1+xAlxGe2−x(PO3)4/Li1+xAlxTi2−x(PO3)4 (LAGP/LATP) is stable with moisture,14 it has been widely used in the Li−air © 2019 American Chemical Society

Received: March 28, 2019 Accepted: May 15, 2019 Published: May 22, 2019 20895

DOI: 10.1021/acsami.9b05516 ACS Appl. Mater. Interfaces 2019, 11, 20895−20904

Research Article

ACS Applied Materials & Interfaces

molten liquid immediately poured to preheated stainless steel at 500 °C to obtain a dense glass pellet. This glass pellet was maintained at 500 °C for 2 h to release internal stress. It was dry ball-milled again to obtain a small glass particle and further crystallized at 800 °C for 8 h with a heating rate of 5 °C min−1. The obtained LAGP powder was pelletized and sintered at 900 °C for 8 h with the same heating rate as before. After sintering, the pellets were polished to 0.30 mm in thickness. 2.2. Preparation of P(AA-co-MA)Li Solution. Poly(acrylic acidco-maleic acid) (P(AA-co-MA)) solution (MW 3000, 50 wt % in H2O, Sigma-Aldrich) was dissolved in deionized water and magnetically stirred for 12 h to obtain 27.5−40 wt % of P(AA-co-MA) solution. LiOH (98%, Sigma-Aldrich) was added into this solution with a P(AAco-MA) mole ratio of 0.2 to 1.4 to adjust the pH value of poly(acrylic acid-co-maleic acid)lithium (P(AA-co-MA)Li) solution, which was further magnetically stirred for 12 h. 2.3. Preparation of Li/LAGP/Li Cell. To investigate the stability of the LAGP pellet with molten Li or Li−Sn alloy, the LAGP pellet was polished, washed, and vacuum dried for further experiment. Li or Li−Sn alloy was heated to 225 °C until totally molten in the glove box. Because there was a thin impurity such as an oxide layer on the surface of molten Li or Li−Sn alloy, the thin layer on top of the molten Li or Li−Sn alloy was removed before immersing an LAGP pellet into the molten Li or Li−Sn alloy for a given duration. After the reaction, the LAGP pellet was then removed from the glove box and washed to remove unreacted Li by ultrasonic vibration in the deionized water. After vacuum drying at 110 °C for 12 h, the pellets were ready for SPM and XPS characterizations. For the EIS measurement, the Li foil was attached on the two sides of polished LAGP (0.3 mm) in the Swagelok cell. 2.4. Preparation of Li/Interface/LAGP/Interface/Li Symmetrical Cell. The P(AA-co-MA)Li solution (2.5 wt %, pH 7) with or without LiCl was spray-coated on the LAGP pellet at 150 °C for every 3 s and subjected to drying after 60 s. The coating step was repeated eight times to obtain an interface layer with a thickness of 1.5 μm. This LAGP pellet was finally polished to maintain the surface roughness to about 1 μm. After coating and polishing, the LAGP pellets were vacuum-dried at 110 °C for 12 h and then immediately stored in the glove box. The Li foil and Sn powder were mixed with a mass ratio of 1:0.6 and melted at 225 °C. Both sides of surface-modified LAGP were then coated with the Li−Sn alloy. 2.5. Evaluation of Electrochemical Properties. X-ray diffractometry (XRD) of the LAGP powder was conducted using a Shimazu XRD-6000 with Cu Kα radiation from 10° to 80° at 0.5° min−1. The microstructure was characterized using a HITACHI S-4300 field emission scanning electron microscope. Fourier transform infrared spectroscopy (FTIR) was performed by GladiATR from 4000 to 400 cm−1 at a resolution of 2 cm−1 after vacuum drying the P(AA-co-MA) and P(AA-co-MA)Li powders at 110 °C for 12 h. The viscosity of the P(AA-co-MA)Li aqueous solution was measured by a CAP 2000+ viscometer with a rotation speed of 800 rpm immediately after preparation. The thermal behavior of P(AA-co-MA)Li was measured using a thermal gravity analyzer (TA-60WS) and a simultaneous differential temperature and gravity analyzer (DTG-60H) from room temperature to 800 °C at a heating rate of 5 °C cm−1. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanic cycles of LAGP, Li/LAGP/Li, Li/interface/LAGP/interface/Li, and LiCl-added cells were conducted by a Solartron Analytical 1400 cell test system at room temperature (23.8 °C). For the EIS at various temperatures, an oven (WEISS WKL34) was used for controlling the temperature after maintaining each temperature at 30 min. 2.6. Scanning Probe Microscopy (SPM) Measurement. Electrochemical strain microscopy (ESM) and SPM images were conducted using a commercial scanning probe microscope (SPM) system (MFP-3D, Asylum Research, USA) under an ambient atmosphere. A conductive cantilever with a silicon tip coated with a Pt layer (AC240-PP, OPUS, USA) with a first eigenmode spring constant of 2 N/m was used. All the DART-ESM mappings were performed with a drive amplitude of 5 V on a 256 × 256 pixel grid at a scan rate of 0.6 Hz. Then, the ESM parameters were obtained using the simple harmonic oscillator (SHO) fitting method.

with the low mechanical strength of a polymer electrolyte; thus, a higher-shear-strength polymer interface layer was used to decrease or eliminate the penetration of a dendrite.25−28 The pure SSEs such as garnet SSEs (LLZO) also suffer from dendrite growth or Li deposition during the galvanic cycle, which may be caused by its voids inside. As it was reported, the Li dendrite grows from one side voids to the other side one until it results in a short circuit during the galvanic cycle.29−31 The shear modulus of a grain boundary may be also reduced by Li during the cycle, which also results in the dendrite growth along a grain boundary of ceramic SSEs.32 Therefore, introducing a high-mechanicalstrength interface layer may also suppress the growth of the Li dendrite into the voids and grain boundary of ceramic SSEs. Atomic layer deposition (ALD) of LiPON,33 Al2O3,34,35 and artificial solid-state electrolyte25 was used as the physical barriers for direct contact of SSEs with Li. There are two possible ways to integrate Li with SSEs and hence with ASSLiMB. The first way is to deposit Li on SSEs and then a current collector, and the second way is to deposit Li on the current collector and then to press the current collector with the Li layer onto the SSEs. The uneven surfaces of the Li foil and SSEs will cause high impedance due to voids and improper contact if they are directly pressed together. To minimize voids and interfacial resistance, various liquid-state Li alloys such as Li−Sn,36 Li−Ge,37 Li− Nb,38 Li−Au,39 and other alloys3,40,41 had been used in the garnet SSEs. To our best knowledge, the liquid Li alloy metal is still not used in NASICON-structured LAGP. Therefore, deep understanding the reaction process and mechanism of LAGP with molten Li is essential for the successful fabrication of ASSLiMB through molten Li and Li alloys. Among various Li compounds, the lowest molten temperature is about 185 °C, which gives a prerequisite to the interface layer. Thus, the interface layer for ASSLiMB must possess the following four essential properties: (i) thermally stable at least at a temperature of >185 °C, (ii) electrochemically stable with a molten Li metal/ Li compound, (iii) mechanically preventable to Li dendrite penetration, and (iv) highly conductive for Li ions. In this work, we study the failure process and mechanisms of LAGP with molten Li through the characterization of morphology, Li ion distribution, and valence. In addition, we successfully introduce a thermosetting Li salt polymer as an interface layer between LAGP and the molten Li−Sn alloy to avoid a side reaction and eliminate voids. The effect of LiCl on this interface layer is also explored to reduce the interface resistance. Through detailed evaluation of the interface by field emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS), and galvanic cycles, we ascribe the effect of an interface layer as both physical barriers to avoid contact of LAGP with the molten Li compound and Li ionic conductor. The incorporation of LiCl further increases the ionic conductivity of the interfacial layer.

2. EXPERIMENTAL PROCEDURES AND CHARACTERIZATION TECHNOLOGIES 2.1. Preparation of NASICON-Structured LAGP Pellets. Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid-state electrolyte pellets were synthesized by a modified solid-state reaction method. In a typical process, stoichiometric amounts of Li2CO3 (99%, Sigma-Aldrich) (10% excess), Al2O3 (99.98%, Alfa Aesar), GeO2 (99.999%, Alfa Aesar), and NH4H2PO4 (98%, Sigma-Aldrich) were mixed at 150 rpm for 4 h in a zirconia jar filled with ethanol. The mixture was dried at 80 °C for 10 h followed by calcination at 380 °C for 4 h to decompose ammonia and carbon dioxide. After dry milling at 100 rpm for 30 min, this powder was then melted at 1350 °C for 2 h with a heating rate of 10 °C min−1. The 20896


Research Article


Figure 1. Images of an LAGP pellet reacted with the molten Li metal. (a) Surface images of the LAGP pellet changed with time. (b) FE-SEM crosssectional pictures, DART-ESM, (c) topography, (d) deformation amplitude, (e) AC-AFM resonance frequency versus count number, and (f) phases of the primary and reacted LAGP pellet. (g) LAGP particle surface peel off after a heavy reaction. 20897


Research Article


Figure 2. XPS images of (a−f) primary LAGP and (g−l) reacted LAGP.

3. RESULTS AND DISCUSSION

circuit, the ionic conductivity of the LAGP pellet was calculated to be 1.82 × 10−4 S cm−1 at room temperature (23.8 °C). To investigate the stability of the Li metal with the LAGP pellet, a Li metal was melted at 225 °C with the Sn powder in a mass ratio of 1:0.6 to increase the wettability of the Li anode.36 Figure 1a shows that the LAGP pellet surface morphology changes with time when it was in contact with the molten Li−Sn alloy. After contacting the LAGP pellet with the Li−Sn liquid for 5 s, the contacted surface of LAGP changed to black. Continued contacting with molten Li results in the breaking of this pellet. This observation clearly shows that the LAGP pellet was

3.1. Failure Process and Mechanism of LAGP with Molten Li Metal. Figure S1a shows the XRD spectrum of the prepared LAGP pellet without visible impurity, and no ping holes could be observed from the FE-SEM image. Figure S1c displays the Nyquist curve of the LAGP pellet that contains a high-frequency semicircle (1000k to 7.94k) and a long straight line. The high-frequency semicircle comes from the bulk and grain boundary resistance of LAGP, and the long tail relates with an ionic block Au electrode. After being fitted by an equivalent 20898


Research Article


Figure 3. (a) Schematic illustrations of the LAGP/Li interface with and without interface layer. (b) Surface of LiSn0.6 after molten coating on the interface-modified LAGP pellet. (c) TGA-DSC curve of P(AA-co-MA)Li from room temperature to 800 °C.

distribution. For LAGP regions reacted with the Li metal, maximum deformation reached to 4.65 pm (lightly reacted region) and 5.90 pm (heavily reacted region). This implied that the Li ion density increased in these reacted regions. Because the Li metal was the only source of Li ion, the deformation increase based on primary LAGP could be used for the analysis reaction process of LAGP and molten Li. It is noted from the observation that, in the lightly reacted region, some of the grain boundaries exhibit higher deformation than the internal grains or gaps, implying that LAGP and the molten Li reaction started from these grain boundaries. The heavily reacted region of LAGP shows even higher deformation than pristine and lightly reacted regions, which implies that a Li-rich component formed after being heavily reacted. Resonance frequencies are related to contact stiffness by considering the dynamics of a clamped spring-coupled cantilever of the scanned sample (Figure 1e). In the lightly reacted region, the wide resonance frequency range can be attributed to the obvious different stiffness of LAGP and lightly reacted LAGP. Bimodal dual AC images were obtained to analyze the structure and component of LAGP (Figure 1f). Different phases represent different compositions or orientations of the scanned region. For primary LAGP, the phase related with topography was uniform, which proved that this region is composed of one component and one orientation crystal structure. For the lightly reacted region, most relatively flat regions have the same phase, which implied that they have the same component, but regions in the grain boundary have different phases from “grain” regions. These different colors represented that they were different components. For heavily reacted regions, even one small particle composed of different phases at different sides, which implied that the composition or orientation for this one particle was different at different positions. To further scan a small area of the heavily reacted

extremely unstable with the liquid Li metal. As shown in the cross section of the LAGP pellet in Figure 1b, the original grain size of the LAGP pellet is about 1 μm. For the partially reacted region, although big particles could still be seen, no clear sharp grains could be observed with a surface showing an “erosion” type of morphology. On the relatively smooth surface, there are a few cracks. For the heavily reacted region, a lot of nanosized flake-like morphology could be observed. The flake-like morphology seems to be caused by deep corrosion from molten Li. To further explore the reaction process and mechanism of LAGP with molten Li, a scanning probe microscopy (SPM)based technique was employed. Selected regions of the SPM measurement were conducted as shown in Figure S2, which include the lightly reacted region (gray) and the heavily reacted one (black). In contrast, a primary region was also chosen to measure the performance of original LAGP. Electrochemical strain microscopy (ESM) of SPM was used to scan the morphology change and Li ionic density of the original, lightly, and heavily reacted regions. The principle of ESM is based on the relationship between the molar volume change and biasinduced ion movement, which could measure the Li ion movement and diffusion.42 As shown in Figure S3, primary LAGP has a relatively large grain size of about 1 μm, consistent with the FE-SEM image of LAGP (Figure 1b). Lightly and heavily reacted regions were composed of smaller “grain size” values of 400 and 50 nm (Figure 1c), respectively. These topography images clearly show a decrease in grain size after the reaction. The Li ion motion resulted in the deformation of the test surface under an applied voltage (2 V). Thus, the deformation was used for representing the movable Li ion density of these three regions (Figure 1d). The deformation in the primary grain was uniform and relatively low (average 2.5 pm), representing uniform and relatively low density Li ion 20899


Research Article


Figure 4. FE-SEM images of (a) Li/LAGP/Li with P(AA-co-MA)Li as the interface layer and (b−d) P(AA-co-MA)Li added with mole ratios of 0.5, 1, and 3 LiCl as interface layer, respectively. The thickness of the interface layer was ∼1.5 μm with a clear interface boundary.

region, many peel off points could be found (Figure 1g), which may be the reason for the pellet broken in Figure 1a. Although SPM could directly observe the morphology and component change for different reaction regions, the valence of the elements is still unknown. X-ray photoelectron spectroscopy (XPS) of primary and reacted LAGP was measured (Figure 2) to reveal the chemical changes of all elements. To eliminate the effect of the chosen regions, two black regions of reacted LAGP were scanned (Figure S4). After calibration by C 1s of each sample (284.8 eV), XPS results of each element could be compared. The XPS spectra of the two different reacted regions were the same. The results obtained from one example were used to compare with the primary XPS images of LAGP. As can be seen from Figure 2, O 1s (Figure 2b,h) and P 2p (Figure 2e,k) were almost the same after the reaction with molten Li. For Li 1s, the peak intensity increased after the reaction with molten Li (Figure 2c,i). The increase in the intensity of Li resulted from the increasing density of Li atoms, which coincided with the increase in Li ionic activation measured by SPM (Figure 1d). Al 2p (Figure 2d,j) slightly changed after the reaction with LiSn0.6. The position of peaks slightly decreased, and the area ratio of Al 2p1/2 and Al 2p3/2 was different. The change in Al 2p may be from the different binding of the Al component after the reaction, which may imply the formation of a new Al component. Ge 3d also shows an obvious change after the reaction (Figure 2f,l). The splitting of Ge 3d is small (Δ = 0.58 eV) and thus has a symmetric lineshape. For primary LAGP, a sharp peak was observed at 32.76 eV, which corresponds to Ge4+. A peak at 32.62 eV was observed for reacted LAGP but with low intensity. In addition, several high intensity peaks (28.45, 25.62, and 23.54 eV) were observed for reacted LAGP. The three peaks can be ascribed to Ge0, Li−Ge alloy, and O 2s43 (23.2 eV43). A similar peak of the Li−Ge alloy (26.2 eV) was also observed.17 Thus, the reaction of LAGP with molten Li can be expressed as

Ge 4 + + 2Li → Ge 2 + + 2Li+

(1)

Ge 2 + + 2Li → Ge 0 + 2Li+

(2)

Ge 0 + x Li → LixGe

(3)

In summary, LAGP is unstable with molten Li. The reaction started from the grain boundaries resulted in the change in grain size from 1 μm to 100 nm. The Li-rich component was formed during the reaction. The composition of grains changed with a degree of reaction for one refined particle. Because of the fast reaction along the grain boundaries, some grains lost their contact with LAGP after a heavy reaction. Part of Ge4+ near the grain boundaries and surfaces was reduced to Ge0 and even formed the Li−Ge alloy. 3.2. Characterization of Interface-Modified LAGP Pellet and Molten Li. Because the surface of the LAGP pellet and Li foil was not absolutely flat, directly pressing the Li foil on the LAGP pellet may result in gaps leading to the increased interface resistance. To effectively increase the contact area, an interface layer of P(AA-co-MA)Li was introduced between the LAGP pellet and Li, and at the same time, this interface layer can effectively avoid Li to directly contact LAGP. The schematic diagram of this interface modification is shown in Figure 3a. The decomposition temperature of P(AA-co-MA)Li is as high as 370 °C so that it enables it to work with molten Li coating at a temperature below 370 °C (Figure 3c). Figure S5 shows the wettability of Li and Li−Sn alloy on the P(AA-co-MA)Li-coated alumina plate. It is clear that the wettability of pure Li with P(AA-co-MA)Li is poor. Upon alloying Sn into Li, the wettability increased with increasing Sn content. In a mass ratio of Li/Sn at 1:0.6, the molten Li−Sn alloy almost completely wets with the P(AA-co-MA)Li-coated alumina plate. Therefore, the LiSn0.6 alloy was used as the anode for testing. For interface-modified LAGP, a molten Li−Sn liquid was easily coated on its surface as shown in Figure 3b. Unlike bare LAGP without P(AA-co-MA)Li coating, the P(AA-co20900


Research Article


Figure 5. EIS of (a) Li/LAGP/Li without interface, (b) with P(AA-co-MA)Li as the interface layer, and (c) LiCl added in the P(AA-co-MA)Li interface layer. (d) Arrhenius plot of P(AA-co-MA)Li/LAGP/P(AA-co-MA)Li from −25 to 75 °C.

interface layers. The total resistance especially the interface resistance of Li/interface/LAGP/interface/Li decreased a lot compared with Li/LAGP/Li. This decreased resistance could be assigned to a larger contact area between LAGP and Li metal, which could be observed in Figure 4. To increase the conductivity of the interface layer, LiCl was introduced into P(AA-co-MA)Li. The interface layer resistance of the cell with additional 0.5% LiCl (LiCl-0.5) was obviously reduced, and further increase in the content of LiCl to 1% led to further decrease in the resistance of the cell (LiCl-1). It was noted from the Nyquist plots that, when LiCl was increased to 3%, no further decrease in the resistance of the interface layer was found, indicating the saturation of LiCl contribution. Thus, the increase in ionic conductivity by adding LiCl may originate from the increase in the concentration of the Li ion at the interface layer, leading to a reduced Li ion-hopping distance. An excess amount of LiCl cannot contribute to the ionic conductivity of the interface layer. The ionic conductivity relates the interface modified Li/ interface/LAGP/interface/Li symmetrical cell that was studied from −25 to 75 °C to understand impact of interface responses at a wide temperature range (Figure 5d). The activation energy was thus calculated based on

MA)Li-coated LAGP did not react with molten LiSn0.6. The coated LiSn0.6 shows a smooth surface without a color change after prolonged contacting. The cross-sectional image of the Li/ LAGP/Li pellet is shown in Figure S6. It is clear that the interfacial contact is poor, whereas very good adhesion with the help of the P(AA-co-MA)Li interface layer could be observed from Figure 4a−d. The P(AA-co-MA)Li interface layer is 1.52 μm. The grain size and morphology of LAGP near LiSn0.6 were the same as that of primary LAGP (Figure S1b), which implied that LAGP was not reacted with molten Li. To increase the ionic conductivity, we incorporated addition of lithium salt, LiCl, to the interface layer. Three mole ratios of LiCl to P(AA-co-MA)Li, 0.5:1, 1:1, and 3:1, were used, denoted as interface-LiCl-0.5, interface-LiCl-1, and interface-LiCl-3, respectively. The morphologies of three different interfaces are shown in Figure 4b−d. The cross-sectional images of the LiCl-added interface are the same as the one without LiCl. All interface layers were controlled to about 1.5 μm. Electrochemical impedance spectroscopy (EIS) of attached Li on LAGP and molten LiSn0.6 coated on interface-modified LAGP was measured to evaluate the ionic conductivity and interface binding (Figure 5). The thickness of the LAGP pellet was controlled at 0.3 ± 0.02 mm. For the Li/ LAGP/Li symmetrical cell without the interface layer, the Nyquist curve composed of two depressed semicircles. According to the frequency range of LAGP (Figure S1c), the Nyquist plot from a frequency range of 1000k to 7.94k was associated with the bulk and grain boundary resistance of the LAGP pellet and interface layer (P(AA-co-MA)Li). The Nyquist plot in the frequency range from 7.94k to 63.1k in Figure 5a was caused from the interface resistance of LAGP (modified LAGP) and Li, which could also be found from the Li/LAGP/Li symmetrical cell with and without an interface layer. The second depressed semicircle of the Li/LAGP/Li cell was from the Li ionic diffusion in the interface layer, which may form voids in the

σt = A exp( −Ea /KbT )

(4)

where σt is the ionic conductivity at absolute temperature T, A is a preexponential factor, Ea is the activation energy that was treated as a constant in the relatively narrow temperature range, and Kb is the Boltzmann constant. Then, log(σt) related with 1000/T was obtained as shown below. log(σt) = log(A) − Ea log e/(1000·Kb) ·1000/T

(5)

After fitting of the experimental data, the activation energy of this Li/interface/LAGP/interface/Li symmetrical cell was 20901


Research Article


Figure 6. Galvanic cycle of (a) Li/LAGP/Li with P(AA-co-MA)Li as the interface layer, (b) LiCl-added P(AA-co-MA)Li interface layer, and (c) overpotential versus current density of the interface-modified Li/Interface/LAGP/Interface/Li symmetrical cell.

4. CONCLUSIONS The failure mechanism of LAGP with molten Li was systematically investigated. The failure of LAGP started from the grain boundary and expanded to the rest of the area of the grains. Therefore, a reduced grain size was observed. The composition of the LAGP particles was changed with gradual reaction. Some part of the LAGP surface peeled off because of a heavy reaction. The Li-rich component was formed, and Ge4+ was reduced to Ge0 by Li or even formed the Ge−Li alloy after the reaction with molten Li. Based on this, a thermosetting Li salt, P(AA-co-MA)Li, was explored and coated on the LAGP pellet to act as an interface layer for molten LiSn0.6 and LAGP pellet. The obtained Li/interface/LAGP/interface/Li symmetric cell showed neglectable defects in the interface layer and thus had smaller interface resistance. The impedance was further decreased when LiCl was introduced to P(AA-co-MA)Li. The galvanic cycles at different current densities were applied to investigate the interface-modified symmetric cell with and without LiCl added. During 100 galvanic cycles at each current

calculated to be 0.37 eV, which is slightly higher than that of pure LAGP (0.30 eV). The galvanic cycle of the symmetrical cells of Li/interface/LAGP/interface/Li without addition of LiCl and with LiCl-1 was performed from 15 to 70 μA cm−1 at room temperature (23.8 °C) to investigate the stability of interfacemodified symmetrical cells (Figure 6). For each current density, 100 cycles were conducted. The maximum overpotential was low for both cells. The one with LiCl-1 addition showed smaller overpotential compared with pure P(AA-co-MA)Li as the interface layer (Figure 6c), which coincided with the Nyquist plots in Figure 5b,c. The overpotential of Li/interface/LAGP/ interface/Li was not linearly related with current density, which may imply that Li ion diffusion was limited under a high current density (70 μA cm−1). As a comparison, the overpotential of the LiCl-added symmetric cell was linearly related with the applied current density (Figure 6c). 20902


Research Article


(9) Huang, M.; Liu, T.; Deng, Y.; Geng, H.; Shen, Y.; Lin, Y.; Nan, C.W. Effect of sintering temperature on structure and ionic conductivity of Li7−xLa3Zr2O12−0.5x (x=0.5∼0.7) ceramics. Solid State Ionics 2011, 204-205, 41−45. (10) Allen, J. L.; Wolfenstine, J.; Rangasamy, E.; Sakamoto, J. Effect of substitution (Ta, Al, Ga) on the conductivity of Li7La3Zr2O12. J. Power Sources 2012, 206, 315−319. (11) Zhu, Y.; Zhang, Y.; Lu, L. Influence of crystallization temperature on ionic conductivity of lithium aluminum germanium phosphate glassceramic. J. Power Sources 2015, 290, 123−129. (12) Cui, Y.; Mahmoud, M. M.; Rohde, M.; Ziebert, C.; Seifert, H. J. Thermal and ionic conductivity studies of lithium aluminum germanium phosphate solid-state electrolyte. Solid State Ionics 2016, 289, 125−132. (13) Breuer, S.; Prutsch, D.; Ma, Q.; Epp, V.; Preishuber-Pflügl, F.; Tietz, F.; Wilkening, M. Separating bulk from grain boundary Li ion conductivity in the sol−gel prepared solid electrolyte Li1.5Al0.5Ti1.5(PO4)3. J. Mater. Chem. A 2015, 3, 21343−21350. (14) He, K.; Zu, C.; Wang, Y.; Han, B.; Yin, X.; Zhao, H.; Liu, Y.; Chen, J. Stability of lithium ion conductor NASICON structure glass ceramic in acid and alkaline aqueous solution. Solid State Ionics 2014, 254, 78−81. (15) Li, F.; Kitaura, H.; Zhou, H. The pursuit of rechargeable solidstate Li−air batteries. Energy Environ. Sci. 2013, 6, 2302. (16) Zhu, Y.; He, X.; Mo, Y. Origin of outstanding stability in the lithium solid electrolyte materials: insights from thermodynamic analyses based on first-principles calculations. ACS Appl. Mater. Interfaces 2015, 7, 23685−93. (17) Liu, Y.; Li, C.; Li, B.; Song, H.; Cheng, Z.; Chen, M.; He, P.; Zhou, H. Germanium thin film protected lithium aluminum germanium phosphate for solid-state li batteries. Adv. Energy Mater. 2018, 8, 1702374. (18) Fan, X.; Ji, X.; Han, F.; Yue, J.; Chen, J.; Chen, L.; Deng, T.; Jiang, J.; Wang, C. Fluorinated solid electrolyte interphase enables highly reversible solid-state Li metal battery. Sci. Adv. 2018, 4, eaau9245. (19) Fan, X.; Chen, L.; Borodin, O.; Ji, X.; Chen, J.; Hou, S.; Deng, T.; Zheng, J.; Yang, C.; Liou, S.-C.; Amine, K.; Xu, K.; Wang, C. Nonflammable electrolyte enables Li-metal batteries with aggressive cathode chemistries. Nat. Nanotechnol. 2018, 13, 715−722. (20) Han, F.; Westover, A. S.; Yue, J.; Fan, X.; Wang, F.; Chi, M.; Leonard, D. N.; Dudney, N. J.; Wang, H.; Wang, C. High electronic conductivity as the origin of lithium dendrite formation within solid electrolytes. Nat. Energy 2019, 4, 187−196. (21) Fergus, J. W. Ceramic and polymeric solid electrolytes for lithium-ion batteries. J. Power Sources 2010, 195, 4554−4569. (22) Zhou, W.; Wang, S.; Li, Y.; Xin, S.; Manthiram, A.; Goodenough, J. B. Plating a dendrite-free lithium anode with a polymer/ceramic/ polymer sandwich electrolyte. J. Am. Chem. Soc. 2016, 138, 9385−8. (23) Borghini, M. C.; Mastragostino, M.; Zanelli, A. Investigation on lithium/polymer electrolyte interface for high performance lithium rechargeable batteries. J. Power Sources 1997, 68, 52−58. (24) Appetecchi, G. B.; Croce, F.; Mastragostino, M.; Scrosati, B.; Soavi, F.; Zanellit, A. Composite polymer electrolytes with improved lithium metal electrode interfacial properties. J. Electrochem. Soc. 1998, 145, 4133−4134. (25) Hou, G.; Ma, X.; Sun, Q.; Ai, Q.; Xu, X.; Chen, L.; Li, D.; Chen, J.; Zhong, H.; Li, Y.; Xu, Z.; Si, P.; Feng, J.; Zhang, L.; Ding, F.; Ci, L. Lithium dendrite suppression and enhanced interfacial compatibility enabled by an ex situ sei on li anode for LAGP-based all-solid-state batteries. ACS Appl. Mater. Interfaces 2018, 10, 18610−18618. (26) Song, S.; Kotobuki, M.; Zheng, F.; Xu, C.; Savilov, S. V.; Hu, N.; Lu, L.; Wang, Y.; Li, W. D. Z. A hybrid polymer/oxide/ionic-liquid solid electrolyte for Na-metal batteries. J. Mater. Chem. A 2017, 5, 6424− 6431. (27) Song, S.; Wu, Y.; Tang, W.; Deng, F.; Yao, J.; Liu, Z.; Hu, R.; Alamusi; Wen, Z.; Lu, L.; Hu, N. Composite solid polymer electrolyte with garnet nanosheets in poly(ethylene oxide). ACS Sustainable Chem. Eng. 2019, 7, 7163−7170.

density, stability and small corresponding overpotentials (0.036, 0.105, and 0.257 V) were obtained for the symmetric cell with a LiCl-1-added interface layer.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05516.

■

XRD, SEM, and EIS of the prepared LAGP pellet, optical image of selected SPM scanning regions, topography of the original LAGP pellet under SPM scanning, two selected XPS scanning regions of the reacted LAGP surface, pictures of Li and Li−Sn alloy on the P(AA-coMA)Li-coated alumina plate after melting at 225 °C, and FE-SEM image of the Li/LAGP/Li interface (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Linchun He: 0000-0002-4786-9710 Jianguo Sun: 0000-0002-2222-2478 Kaiyang Zeng: 0000-0002-3348-0018 Li Lu: 0000-0002-9670-9430 Notes

The authors declare the following competing financial interest(s): This work is supported by the Natural Science Foundation of China (NSFC 51572182). † Deceased on 20 April 2019.

■

ACKNOWLEDGMENTS This work is supported by the National University of Singapore, the National University (Suzhou) Research Institute, and the Natural Science Foundation of China (NSFC 51572182).

■

REFERENCES

(1) Tu, W.; Xia, P.; Zheng, X.; Ye, C.; Xu, M.; Li, W. Insight into the interaction between layered lithium-rich oxide and additive-containing electrolyte. J. Power Sources 2017, 341, 348−356. (2) Dai, J.; Yang, C.; Wang, C.; Pastel, G.; Hu, L. Interface engineering for garnet-based solid-state lithium-metal batteries: materials, structures, and characterization. Adv. Mater. 2018, 30, 1802068. (3) Gao, Z.; Sun, H.; Fu, L.; Ye, F.; Zhang, Y.; Luo, W.; Huang, Y. Promises, challenges, and recent progress of inorganic solid-state electrolytes for all-solid-state lithium batteries. Adv. Mater. 2018, 30, 1705702. (4) Fan, L.; Wei, S.; Li, S.; Li, Q.; Lu, Y. Recent progress of the solidstate electrolytes for high-energy metal-based batteries. Adv. Energy Mater. 2018, 8, 1702657. (5) He, L.; Chen, C.; Kotobuki, M.; Zheng, F.; Zhou, H.; Lu, L. A new approach for synthesizing bulk-type all-solid-state lithium-ion batteries. J. Mater. Chem. A 2019, 7, 9748−9760. (6) Zheng, F.; Kotobuki, M.; Song, S.; Lai, M. O.; Lu, L. Review on solid electrolytes for all-solid-state lithium-ion batteries. J. Power Sources 2018, 389, 198−213. (7) Bachman, J. C.; Muy, S.; Grimaud, A.; Chang, H. H.; Pour, N.; Lux, S. F.; Paschos, O.; Maglia, F.; Lupart, S.; Lamp, P.; Giordano, L.; Shao-Horn, Y. Inorganic solid-state electrolytes for lithium batteries: mechanisms and properties governing ion conduction. Chem. Rev. 2016, 116, 140−162. (8) Xu, B.; Duan, H.; Xia, W.; Guo, Y.; Kang, H.; Li, H.; Liu, H. Multistep sintering to synthesize fast lithium garnets. J. Power Sources 2016, 302, 291−297. 20903


Research Article

ACS Applied Materials & Interfaces (28) Li, D.; Chen, L.; Wang, T.; Fan, L. Z. 3d fiber-network-reinforced bicontinuous composite solid electrolyte for dendrite-free lithium metal batteries. ACS Appl. Mater. Interfaces 2018, 10, 7069−7078. (29) Wu, B.; Wang, S.; Lochala, J.; Desrochers, D.; Liu, B.; Zhang, W.; Yang, J.; Xiao, J. The role of the solid electrolyte interphase layer in preventing Li dendrite growth in solid-state batteries. Energy Environ. Sci. 2018, 11, 1803−1810. (30) Ren, Y.; Shen, Y.; Lin, Y.; Nan, C.-W. Direct observation of lithium dendrites inside garnet-type lithium-ion solid electrolyte. Electrochem. Commun. 2015, 57, 27−30. (31) Sang, L.; Bassett, K. L.; Castro, F. C.; Young, M. J.; Chen, L.; Haasch, R. T.; Elam, J. W.; Dravid, V. P.; Nuzzo, R. G.; Gewirth, A. A. Understanding the effect of interlayers at the thiophosphate solid electrolyte/lithium interface for all-solid-state li batteries. Chem. Mater. 2018, 30, 8747−8756. (32) Yu, S.; Siegel, D. J. Grain boundary softening: A potential mechanism for lithium metal penetration through stiff solid electrolytes. ACS Appl. Mater. Interfaces 2018, 10, 38151−38158. (33) Guhl, C.; Fingerle, M.; Hausbrand, R. Process related effects upon formation of composite electrolyte interfaces: Nitridation and reduction of NASICON-type electrolytes by deposition of LiPON. J. Power Sources 2017, 362, 299−307. (34) Liu, Y.; Sun, Q.; Zhao, Y.; Wang, B.; Kaghazchi, P.; Adair, K. R.; Li, R.; Zhang, C.; Liu, J.; Kuo, L. Y.; Hu, Y.; Sham, T. K.; Zhang, L.; Yang, R.; Lu, S.; Song, X.; Sun, X. Stabilizing the interface of NASICON solid electrolyte against li metal with atomic layer deposition. ACS Appl. Mater. Interfaces 2018, 10, 31240−31248. (35) Han, X.; Gong, Y.; Fu, K. K.; He, X.; Hitz, G. T.; Dai, J.; Pearse, A.; Liu, B.; Wang, H.; Rubloff, G.; Mo, Y.; Thangadurai, V.; Wachsman, E. D.; Hu, L. Negating interfacial impedance in garnet-based solid-state Li metal batteries. Nat. Mater. 2017, 16, 572−9. (36) Wang, C.; Xie, H.; Zhang, L.; Gong, Y.; Pastel, G.; Dai, J.; Liu, B.; Wachsman, E. D.; Hu, L. Universal soldering of lithium and sodium alloys on various substrates for batteries. Adv. Energy Mater. 2018, 8, 1701963. (37) Luo, W.; Gong, Y.; Zhu, Y.; Li, Y.; Yao, Y.; Zhang, Y.; Fu, K. K.; Pastel, G.; Lin, C. F.; Mo, Y.; Wachsman, E. D.; Hu, L. Reducing interfacial resistance between garnet-structured solid-state electrolyte and li-metal anode by a germanium layer. Adv. Mater. 2017, 29, 1606042. (38) Zhao, N.; Fang, R.; He, M.-H.; Chen, C.; Li, Y.-Q.; Bi, Z.-J.; Guo, X.-X. Cycle stability of lithium/garnet/lithium cells with different intermediate layers. Rare Met. 2018, 37, 473−479. (39) Alexander, G. V.; Patra, S.; Sobhan Raj, S. V.; Sugumar, M. K.; Ud Din, M. M.; Murugan, R. Electrodes-electrolyte interfacial engineering for realizing room temperature lithium metal battery based on garnet structured solid fast Li+ conductors. J. Power Sources 2018, 396, 764− 773. (40) Park, C. M.; Kim, J. H.; Kim, H.; Sohn, H. J. Li-alloy based anode materials for Li secondary batteries. Chem. Soc. Rev. 2010, 39, 3115− 3141. (41) Wang, J.; Wang, H.; Xie, J.; Yang, A.; Pei, A.; Wu, C.-L.; Shi, F.; Liu, Y.; Lin, D.; Gong, Y.; Cui, Y. Fundamental study on the wetting property of liquid lithium. Energy Storage Mater. 2018, 14, 345−350. (42) Li, T.; Zeng, K. Probing of local multifield coupling phenomena of advanced materials by scanning probe microscopy techniques. Adv. Mater. 2018, 30, 1803064. (43) Kim, S.; Won Hwang, S.; Choi, S.-H.; Elliman, R. G.; Kim, Y.-M.; Kim, Y.-J. Formation characteristics and photoluminescence of Ge nanocrystals in HfO2. J. Appl. Phys. 2009, 105, 106112.

20904


Failure Mechanism and Interface Engineering for NASICON

Recommend Documents