In Situ Neutron Depth Profiling of Lithium Metal–Garnet Interfaces for

Sep 17, 2017 - The garnet-based solid state electrolyte (SSE) is considered a promising candidate to realize all solid state lithium (Li) metal batter...
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In Situ Neutron Depth Profiling of Lithium Metal−Garnet Interfaces for Solid State Batteries Chengwei Wang,†,‡,§ Yunhui Gong,†,‡,§ Jiaqi Dai,† Lei Zhang,†,‡ Hua Xie,† Glenn Pastel,† Boyang Liu,† Eric Wachsman,*,†,‡ Howard Wang,*,† and Liangbing Hu*,†,‡ †

Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States University of Maryland Energy Research Center, University of Maryland, College Park, Maryland 20742, United States

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S Supporting Information *

ABSTRACT: The garnet-based solid state electrolyte (SSE) is considered a promising candidate to realize all solid state lithium (Li) metal batteries. However, critical issues require additional investigation before practical applications become possible, among which high interfacial impedance and low interfacial stability remain the most challenging. In this work, neutron depth profiling (NDP), a nondestructive and uniquely Li-sensitive technique, has been used to reveal the interfacial behavior of garnet SSE in contact with metallic Li through in situ monitoring of Li plating−stripping processes. The NDP measurement demonstrates predictive capabilities for diagnosing short-circuits in solid state batteries. Two types of cells, symmetric Li/garnet/Li (LGL) cells and asymmetric Li/garnet/carbonnanotubes (LGC), are fabricated to emulate the behavior of Li metal and Li-free Li metal anodes, respectively. The data imply the limitation of Li-free Li metal anode in forming reliable interfacial contacts, and strategies of excessive Li and better interfacial engineering need to be investigated.

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INTRODUCTION With the rapidly increasing demands of high energy density batteries for portable electronics and electrical vehicles applications, lithium (Li) metal anode, considered the “Holy Grail” in Li-ion batteries, is one of the most attractive strategies to further improve the energy density of batteries. However, due to uncontrolled Li dendrite growth and the flammability of the traditional organic liquid electrolytes, Li metal anodes cannot be directly used in traditional batteries without causing serious safety issues. Due to the nonflammable and improved mechanical properties inherent with solid state electrolytes (SSEs), these materials are promising to replace the traditional liquid electrolytes and work with Li metal safely. Among many of the developed SSEs,1−8 cubic garnet phase SSEs,9−14 especially Li7La3Zr2O12 (LLZO), are promising candidates because of their high ionic conductivity (10−4−10−3 S/cm)6,9,10 and outstanding chemical stability.15−18 Nevertheless, there are still critical problems to be solved, including the high interfacial impedance against electrodes and the resistance to dendrite growth.19 As reported by several groups, the poor contact between the electrodes and SSEs can cause locally focused ionic currents and limit the maximum applied current densities to SSEs before failure by short-circuit.20−23 In our previous work, we improved the contact between lithium metal anodes and garnet SSEs to significantly lower the interfacial resistance.24−26 This work focused on the lithium metal−garnet interface, which is important for the lithium metal solid state batteries with Li-free cathodes, such as Li−S and Li−O2 batteries. However, for Li-rich cathodes, such as LiFePO4, LiCoO2, etc., a © 2017 American Chemical Society

Li-free lithium metal anode, normally a 3D current collector that can host lithium metal and prevent dendritic lithium formation, is required in liquid organic electrolytes.27−29 A similar strategy applies to solid state Li metal batteries with Lirich cathodes. A conductive 3D framework for Li metal anodes is necessary to overcome the large volume changes of lithium metal anodes during cycling and provide additional conductivity as a current collector. In these cases, the Li-free Li metal anode/garnet interfacial properties have not been systematically and effectively studied yet. In this work, carbon nanotube (CNT) films are used to mimic the Li-free lithium metal anode and study its interface with garnet SSE using in situ neutron depth profiling (NDP). As one of the lightest and most important elements for energy storage applications, lithium cannot be easily characterized by conventional techniques such as energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Due to the sensitivity of neutrons to light elements such as Li, B, Na, etc.,30−32 NDP is a powerful tool to analyze the Li distribution and transport in solid state battery systems.33−38 When a neutron beam passes through the Li-rich sample, the neutrons (4 meV) react with the 6Li isotope following the reaction 6Li + n →4He (2055 keV) + 3H (2727 keV).30,31 After the reaction, the generated particles lose energy to the matrix at a specified rate, which can be used to identify the initial location of the reaction and the Received: August 4, 2017 Published: September 17, 2017 14257

DOI: 10.1021/jacs.7b07904 J. Am. Chem. Soc. 2017, 139, 14257−14264

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Figure 1. Setup and cell configurations for the NDP measurement. (a) Schematic of the NDP system. (b) Cell configuration of the asymmetric cell. (c) Digital image of a typical Li/garnet/CNT asymmetric cell, where the top surface was coated with 50 nm of Ni as the current collector. (d) Schematic of Li plating−stripping in a Li/garnet/Li symmetric cell, where the plating and stripping processes are reversible. (e) Schematic of Li plating−stripping in a Li/garnet/CNT asymmetric cell, where partially plated Li cannot be stripped back because of the poor contact between the CNT host and garnet solid state electrolyte.

counts indicate the abundance of Li at the corresponding depth.31 With in situ NDP measurements, Li plating−stripping behavior near the electrode−electrolyte interface can be monitored in real time, enabling a better understanding of the interfacial behavior in cycling solid state Li batteries.

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as both a mask and current collector. In the asymmetric cells, a 1−2 μm thick CNT film was coated on the garnet pellet facing the detector using a solution-based process. To ensure good electrical contact, 50 nm of nickel was deposited by electron beam (e-beam) evaporation on the exposed CNT film as well as on the Kapton tape packaging. Both cell configurations use a thin Kapton film (7.6 μm) to block α particles and improve the depth resolution of the NDP measurement (Figure 1b). The Morphologies of Li Plated on Porous CNT Film. The digital image in Figure 2a depicts nonuniform Li plating on top of the CNT layer. From the cross-sectional SEM image of the CNT-coated garnet (Figure 2b), there is a gap between the CNT film and garnet pellet. In contrast, all the interfaces between pure Li and garnet are continuous and uninterrupted (Figure 2c), which leads to a significantly lower local current density for improved Li ion transport and charge transfer. When Li was plated onto the CNT film, the film became a 3D porous Li/CNT composite structure, and the voids and gaps still exist (Figure 2d). The cross-sectional SEM image in Figure 2e, also marked as (2) in Figure 2a, shows how uneven Li chunks are plated on the CNT layer. The zoomed-in picture in Figure 2f shows the vertically aligned wires of Li dendrites that connect the Li/CNT composite layer and the Li chunks on top of the asymmetric cell. The points marked as (3) in Figure 2a, and the corresponding cross-sectional and top-view SEM images in Figure 2g and Figure S1, respectively, show many mushroom-like Li particles growing on top of the garnet SSE. From the SEM images at higher magnifications (Figure 2h,i, the carpet-like Li/CNT composite layer can still be identified underneath the mushroom-like Li particles. The Li plating process on the CNT film is summarized in Figure 2j, in which the CNT layer is first lithiated to form a 3D Li/CNT composite layer. After further plating Li tends to accumulate on the top of the composite layer by forming

RESULTS AND DISCUSSION

Figure 1a depicts the setup for the in situ NDP measurement, in which the cell is attached to a temperature-controlled aluminum plate and disc in the vacuum chamber. A Si detector detects 3H and α particles generated as the neutron beam enters the chamber and reacts with the sample. To ensure a detectable amount of Li transfer during short timespans, tests were conducted at 90 °C to permit higher applied current densities. Figure 1b and c depict an asymmetric cell configuration and digital image prior to the in situ NDP measurement. For comparison, two cell configurations (Figure 1d,e) were used in this NDP study to mimic architectures with a Li metal anode and sulfur-type Li-free cathode versus a Li-free lithium metal anode and Li-rich cathode, respectively. The Li/ garnet/Li (LGL) symmetric cell has continuous contact on both sides, while the Li/garnet/CNT asymmetric cell has poor point-contact on the garnet/CNT interface. The Li plating− stripping process shows much better reversibility in the symmetric cell than in the asymmetric cell, where partially plated Li cannot be stripped back due to poor contact. Near the garnet/CNT interface, Li can be reversibly plated and stripped back, but beyond this layer, reversibility is lost (Figure 1e). As a result, Li metal will continuously accumulate and grow into the CNT network, resulting in poor Columbic efficiency. In LGL symmetric cells, Li was melted onto both sides of the surface-treated garnet SSE with excellent contact and low interfacial resistance. A titanium strip with a punched hole was attached to the top lithium electrode during melting to function 14258

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Figure 2. Morphologies of the Li-plated CNT film. (a) Digital image of the Li-plated CNT film on the surface of the garnet pellet. (b) Crosssectional SEM images of CNT/garnet interfaces, where the voids and gaps indicate a poor point-contact. (c) Cross-sectional SEM image of Li/ garnet interface, indicating a continuous and tight contact. (d) Cross-sectional SEM image of the porous lithiated CNT film on garnet surface. (e, f) Cross-sectional SEM images of a Li chunk on top of the lithiated CNT film at different magnifications. (g−i) Cross-sectional SEM images of Li particles on top of the lithiated CNT film at different magnifications. (j) Schematic of Li plating onto the CNT film.

In Situ NDP Measurement of a Li/Garnet/CNT Asymmetric Cell. The poor point-contact between the CNT film and garnet pellet is also reflected in the Li plating−stripping behavior during cycling, which is carefully observed by NDP measurements. The typical NDP spectra of a Li/garnet/CNT asymmetric cell before and after Li plating on the CNT film, without or with the Kapton film cover, are shown in Figure S3a and b, respectively. Without the Kapton film cover, the 4He signal overlaps the 3H signal at a low channel range (Figure S3a) and limits the depth resolution of the NDP measurement. A thin Kapton film can block the 4He signal to make the spectra only include 3H peaks and shift toward smaller channels (Figure S3b). The energy of the surface-emitted 4He (2055 keV) and 3H (2727 keV) signals in Figure S3a are used to calibrate and convert the channel number to the energy scale. Figure 3a depicts the typical NDP spectra of a Li/garnet/CNT asymmetric cell during a plating− stripping cycle at different times with the background signals omitted. The energy between ∼2150 and 2500 keV corresponds to the surface of the CNT film. As the plating−

mushroom-like structures or big chunks (Figure 2j). This can be attributed to the discontinuous contact between the CNT film and garnet pellet, which causes nonuniform Li deposition. The mushroom-like lithium particles seem to have root-like point contacts with the garnet electrolyte through the Li/CNT composite layer (Figure S2a,b). In the zoomed-in SEM image in Figure S2b, Li grows along the grain boundary into the garnet electrolyte. It is believed that the focused nonuniform Liion diffusion in garnet electrolyte leads to the growth of mushroom-like lithium. This can lead to catastrophic failure, as dendrites propagate through the grain boundaries and eventually short the cell. Therefore, the high local current density imposed by the poor contact between the Li metal host and garnet electrolyte needs to be addressed to successfully implement garnet-based solid state Li metal batteries. Moreover, these results clearly show how poor interfacial contact can lead to dendrite formation, whereas a smooth continuous Li− garnet interface does not, and can therefore help explain discrepancies in the literature about dendrite formation and reported limiting current densities.20−22,39−41 14259

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Figure 3. In situ NDP measurement of a Li/garnet/CNT asymmetric cell while cycling. Typical NDP spectra of the Li/garnet/CNT asymmetric cell with the background collected before cycling omitted, at different times (a), and during one plating−stripping cycle (b). (c) 2D projection of the NDP spectra collected at 5 min intervals during cycling. (d) The corresponding voltage profile (blue), charge curve after multiplying a constant of variation (green), and the integrated NDP count curve (red) of an asymmetric cell cycling at different current densities. The inset illustrates the Li plating−stripping behavior in the reversible surface layer. (e) The integrated NDP data within several small energy (keV) ranges indicate a depthdependent Li deposition during cycling.

stripping time increases, the counts within the surface region continue to increase, therefore indicating that Li accumulates in the CNT film. Within one plating−stripping cycle, the nearsurface peaks increase to a maximum at the end of the plating stage and then start to decrease during the stripping stage, as shown in Figure 3b. The surface accumulation process of Li becomes more obvious from the 2D spectra (Figure 3c), where the counts around 2375 keV increase the most. The voltage profile of the Li/garnet/CNT asymmetric cell is shown in Figure 3d (blue curve). The real-time NDP spectrum in Figure 3c is integrated in 5 min intervals to indicate the total change in Li on the CNT film. The integrated counts are also plotted in Figure 3d (red curve). Since the CNT electrode did not have Li initially, the first three cycles were set to 2 h of plating and 1 h of stripping at low current density to better

demonstrate the gradual accumulate of Li on the CNT frame. The voltage increases and reaches the cutoff value of 0.1 V during the first stripping cycle, which indicates more than half of the plated Li cannot be reversibly stripped back (Figure 3d). For the second and the third cycles, the stripping voltage does not reach the cutoff point due to the improved contact between the accumulated Li and garnet. In the following 15 cycles, the current density was doubled to 80 μA/cm2, and the plating/ stripping time was changed to 30 min/30 min. However, the stripping stage reaches the cutoff voltage in approximately 25 min and does not finish the allocated time for each cycle. This behavior indicates that a similar amount of irreversible Li accumulates in the CNT film. During cycling, the NDP integrated count varies as the current changes directions (Figure 3d). The NDP curve follows the same trend as the 14260

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Figure 4. In situ NDP measurement of a Li/garnet/Li symmetric cell during Li plating and stripping. A typical NDP spectra of the symmetric cell after background calibration at different times (a) and during one plating−stripping cycle (b). (c) 2D projection of NDP spectra collected every 5 min during cycling. (d) Corresponding voltage profile (blue), charge curve (green), and the integrated NDP count curve (red) of symmetric cell cycling at different current densities. The reversibility of the NDP curve has a good agreement with the voltage profile. (e) Integrated NDP data within several small energy ranges.

2014 and 2230 keV, both the count increment and variation are relatively small in comparison with the outer layers between 2230 and 2446 keV, which indicates the electrochemical stability of garnet SSE during cycling. The outer layers corresponding to the CNT layer at 2230−2446 keV have an almost linear increase of NDP counts. The layers corresponding to 2230−2374 keV contain the most periodic variation because of the reversible Li plating−stripping, while the outermost layer mainly just has a linear increment but periodic variation. The results indicate formation of a reversible layer near the CNT/garnet interface contributing the most reversible Li plating−stripping process. If plated beyond this reversible layer, Li will be mostly left behind and accumulate (inset of Figure 3d). In Situ NDP Measurement of a Li/Garnet/Li Symmetric Cell. A Li/garnet/Li symmetric cell with excellent Li/garnet

charge curve (the green curve in Figure 3d), which further demonstrates the linearity between NDP counts and the amount of Li transferred. The counts have almost a linear increase at the current density of 80 μA/cm2 besides the variation with the sign change of current, which is attributed to the reversible layer of Li near the CNT/garnet interface. When Li is deposited beyond this layer, the process is irreversible due to the loss of contact with electrolyte and insufficient transfer through the CNT host (inset of Figure 3d). When the current density is increased to 120 μA/cm2, the voltage profile drops to 0 V and becomes noisy, and the NDP curve becomes flat, both of which indicate the formation of a short-circuit. To better investigate the properties of the near-surface reversible layer, the NDP data were integrated within several small energy ranges (every 72 keV, corresponding to 100 channels) as shown in Figure 3e. For the inner layers between 14261

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Figure 5. In situ NDP measurement for diagnosing short-circuit in a Li/garnet/Li symmetric cell during Li plating and stripping. The zoom-in images of the voltage profile (blue), charge curve (green), and the integrated NDP count curve (red) of symmetric cell cycling at (a) the predicted and (b) the “dynamic short-circuit” stages, respectively. (c) Schematic to illustrate the “dynamic short-circuit” mechanism during Li plating and stripping.

short-circuit is formed at about 21 h, when the NDP counts start to deviate from the charge curve dramatically. As mentioned above, due to the continuous conformal contact on both sides of the symmetric cell, there is no Li accumulation at the end of each cycle before short-circuit. Li plating−stripping is fully reversible, as confirmed by the NDP spectra for several small energy ranges (Figure 4e). Prior to short-circuit formation at 21 h, the integrated NDP counts between 2014 and 2302 keV remain almost constant. The energy corresponding to the outer layers from 2302 to 2446 keV does not show a net count increase as observed in the asymmetric cell. When the cell becomes shorted at 21 h, the NDP counts of the outer layers increase significantly, especially in the range of 2302−2374 keV (Figure 4e). It is interesting to point out that the inner layers from 2014 to 2302 keV gradually gain more counts as time elapses after the short-circuit. The onset of a count increase for each curve is connected by a black dashed line in Figure 4e. The slope of the dashed line further indicates that Li gradually accumulated into the inner layer of the garnet pellet when the unstable short-circuit formed. Predictive Diagnosis for Short-Circuit in a Li/Garnet/Li Symmetric Cell. It is interesting to point out that a small nonzero NDP count is observed at the end of a cycle (15 h) and becomes more apparent between measurements at 18 and 20 h (Figure 4d), which can be seen more clearly in the zoomin image (Figure 5a). The net increase in the NDP count during this period is attributed to a small amount of irreversibility at 15 h; however, the voltage profile cannot confirm this behavior. In the following cycle at about 20 h 45 min, a short-circuit is formed, and the voltage profile drops sharply. Therefore, the nonzero NDP measurement prior to this cycle from 15 to 20 h foreshadowed the short-circuit formation. This predictive capability is infeasible through conventional electrochemical measurements. In the following cycles starting at 22 h, the voltage is noisy but close to the normal value during plating (negative voltage) and drops due

contact on both sides as shown in Figure 2c was also fabricated to conduct in situ NDP measurements at 90 °C. The typical NDP spectra without background signals at different times are shown in Figure 4a. The spectra are plotted in pairs (0 and 0.5 h, 7 and 8 h, 19 and 19.5 h) consistent with the beginning and the end of each plating cycle; corresponding voltage profiles are shown in Figure 4d. Due to the good reversibility of the symmetric cell, the peaks in the NDP spectra are close to zero at the beginning of each cycle. During one plating−stripping cycle, Li was plated and stripped back and forth reversibly, which causes the NDP peak to change accordingly (Figure 4b). The full spectra are collected in 5 min intervals and are shown in Figure 4c as the 2D colored projection. The counts do not increase significantly until the last 6 h of cycling (Figure 4c). Moreover, the range of the NDP peaks start to expand into the inner layer of garnet as the intensity increases (Figure 4c), which indicates that Li accumulated at the Li/garnet interface and diffused toward the garnet surface. The shift in NDP peaks becomes unambiguous when combined with the real-time voltage profile (Figure 4d). The plating/stripping cycles were initially set to 30 min/30 min for the first five cycles, but were then extended to 1 h/1 h limits for the subsequent two cycles at a current density of 100 μA/cm2. Due to the continuous and conformal Li/garnet contact on both sides of the symmetric cell, the voltage profile is a smooth and symmetric curve. The charge (green) and the integrated NDP (red) curves vary with the change in sign of the current and fit each other well. At the end of each cycle, both the net change in charge and the integrated NDP counts are close to zero, which demonstrates fully reversible Li plating/ stripping. When the current density is further increased to 200 μA/cm2, the voltage profile remains smooth and symmetric but experiences a step increase (Figure 4d). The charge and the integrated NDP curves still fit well with each other at 200 μA/ cm2 for the symmetric cell. In the following cycles at a current density of 400 μA/cm2, the voltage profile drops sharply, and a 14262

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contact area, shorten the Li diffusion distance, and overcome the anticipated volume change.

to short-circuiting during stripping (positive voltage) (Figure 5b). This unstable electrochemical behavior leads to the accumulation of Li and causes the incremental increase in NDP counts (Figure 5b). The NDP curve only decreases a little at each stripping stage but increases significantly during each plating stage, which is akin to a unilateral short-circuit. To explain this phenomena, a “dynamic short-circuit” mechanism is proposed where the short-circuit path consists of the electrically conductive Li-rich phase and forms from the bottom surface to the top, through the garnet pellet (Figure 5c). Therefore, when Li becomes stripped at the top surface, the accumulation of Li onto the Li-rich phase makes it grow toward the top surface, which causes the cell to be mostly shorted and results in little change of the NDP count. When the cell is plated, Li in the Lirich phase is stripped back toward the top surface and the short-circuit path is temporarily eliminated (Figure 5c). According to the voltage profile and the NDP data, the unstable unilateral short-circuit state was brief before the cell was fully shorted and the NDP count ceased (Figure 5c). The SEM morphologies and in situ NDP results provide new insight for future designs of garnet-based solid state Li metal batteries. The depletion of the Li metal near the CNT/garnet interface causes significant Li irreversibility because of the loss of contact with the electrolyte. Due to poor conductivity and rigid particle morphology, Li-rich cathode active materials can have even worse contact with the surface of the garnet pellet. According to the NDP results, Li ions can only reversibly transport between the cathode and garnet SSE in a small range near the interface. To solve this problem, ionically conductive materials, such as liquid- or polymer-based electrolytes, may be required to mix with the active materials and improve Li-ion transport. Alternatively, a 3D mixed ion−electron conductive structure can be designed to host the active materials, which will increase the contact area and shorten the Li diffusion distance. A 3D mixed ion−electron conductive structure can also decrease the amount of “dead” Li as well as overcome the large volume change encumbered by the Li metal anode.

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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/jacs.7b07904. Experimental details, additional figures of SEM images, and NDP spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID

Eric Wachsman: 0000-0002-0667-1927 Liangbing Hu: 0000-0002-9456-9315 Author Contributions §

C. Wang and Y. Gong contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of the Center for Research in Extreme Batteries from the Joint Research and Innovation Seed Grant of the University of Maryland (UMD), College Park, the U.S. Army Research Laboratory (ARL), and the National Institute of Standards and Technology (NIST). We also acknowledge the support of the Maryland NanoCenter and its AIMLab. We would like to thank Dr. Robert Gregory Downing from NIST for the use of the NDP facility and helpful discussions.

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CONCLUSION We successfully used an in situ NDP technique to monitor the Li distribution and transport of garnet-based solid state cells during cycling. Due to the poor point contact between a conventional Li metal host and solid state electrolyte, several interesting Li morphologies including the 3D porous lithiated CNT, Li chunks, and mushroom-like Li structures were observed in the Li plated Li/garnet/CNT asymmetric cell. To explain this phenomenon, a reversible layer is distinguished near the CNT/garnet interface where Li can be plated and stripped, reversibly. When Li plates beyond the reversible layer, it becomes “dead” Li and starts to accumulate, which agrees well with NDP measurements and corresponding voltage profiles of asymmetric cells. For Li/garnet/Li symmetric cells, both the voltage profiles and NDP measurements show better reversibility with higher maximum current densities because of the conformal contact between Li and garnet SSE. The diagnostic capability of in situ NDP measurement was demonstrated in a short-circuit prediction. A unilateral “dynamic short-circuit” mechanism is proposed according to the increasing NDP count in symmetric cells prior to apparent short-circuiting in the voltage profile. It is concluded that ionically conductive materials are necessary for implementation of a solid state battery. A 3D mixed electron−ion conductive framework is preferred as a Li metal host to increase the 14263

DOI: 10.1021/jacs.7b07904 J. Am. Chem. Soc. 2017, 139, 14257−14264

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DOI: 10.1021/jacs.7b07904 J. Am. Chem. Soc. 2017, 139, 14257−14264