Interface Re-Engineering of Li10GeP2S12 Electrolyte and Lithium

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Interface Re-engineering of Li10GeP2S12 Electrolyte and Lithium anode for All-Solid-State Lithium Batteries with Ultralong Cycle Life Zhihua Zhang, Shaojie Chen, Jing Yang, Junye Wang, Lili Yao, Xiayin Yao, Ping Cui, and Xiaoxiong Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16176 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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ACS Applied Materials & Interfaces

Interface Re-engineering of Li10GeP2S12 Electrolyte and Lithium anode for All-Solid-State Lithium Batteries with Ultralong Cycle Life Zhihua Zhanga,b, Shaojie Chena*, Jing Yanga, Junye Wanga, Lili Yaoa, Xiayin Yaoa, Ping Cuia, Xiaoxiong Xua* a

Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, 315201 Ningbo, PR China b

University of Chinese Academy of Sciences, 100049 Beijing, PR China

ABSTRACT An ingenious interface re-engineering strategy was applied to in-situ prepare a manipulated LiH2PO4 protective layer on the surface of Li anode for circumventing the intrinsic chemical stability issues of Li10GeP2S12 (LGPS) to Li metal, specifically the migration of mixed ionic-electronic reactants to the inner of LGPS, and the kinetically sluggish reactions in the interface. As consequence, the stability of LGPS with Li metal increased substantially and the cycling of symmetric Li/Li cell showed that the polarization voltage could keep relative stable for over 950 h at 0.1 mA cm-2 within ± 0.05 V. The optimized ASSLiB of LiCoO2 (LCO)/LGPS/Li with interface-engineered structure was able to deliver long cycle life and high capacity, i.e., a reversible discharge capacity of 131.1 mAh g-1 at the initial cycle and 113.7 mAh g-1 at the 500th cycle under 0.1 C with a retention of 86.7%. In addition, the factors effected on the interphases ACS Paragon Plus Environment

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formation of the LGPS/Li interface were analyzed, and the mechanism of the stability between LGPS and Li anode with protective layer was further investigated. Moreover, the probable causes of battery degradation were also explored. Above all, this work would give an alternative strategy for the modification of Li anode in high energy density solid-state lithium metal batteries. KEYWORDS: in-situ preparation, protective layer, Li10GeP2S12, Lithium anode, interface, all-solid-state lithium battery

1. INTRODUCTION The rapid development of All-solid-state lithium batteries (ASSLiBs) is bringing draw on the emerging of the novel batteries with higher energy density, more safety, and longer life span that are regarded as the most promising energy storage technology applied to electric vehicles, smart electricity grid, and digital products1. However, the insufficient conductivity of the electrode materials and solid-state electrolytes, as well as the large interfacial resistance between them need to be overcome urgently for their commercial applications2-3. In recent years, the sulfide-based solid-state electrolytes including Li2S-SiS2, Li2S-B2S3, Li2S-P2S5 and Li10±1MP2S12 (M = Ge, Si, Sn, Al or P)4, especially, the Li10GeP2S12 (LGPS) with extremely high Li+ conductivity of 12 mS cm-1 has partially solved the intrinsic conductivity problem of solid-state electrolytes5. In addition, the lithium (Li) metal with extremely high theoretical specific capacity (3860 mAh g-1), low density (0.59 g cm-3) ACS Paragon Plus Environment

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and the lowest negative electrochemical potential (-3.04 V vs. the standard hydrogen electrode) is considered to be an ideal negative electrode for the high energy density rechargeable batteries5. Nevertheless, the compatibility of LGPS and Li anode has been investigated to be unstable so as to seriously restrict the development of the high energy density ASSLiBs with Li anode. It is reported that attributing to the gradual spontaneous reaction between Li and LGPS, some low ionic conductivity materials including Li2S and Li3P, etc. and ion-electron conductive Li15Ge4 would generate at the interface of Li/LGPS, which have been theoretically calculated7 and definitely verified by experiments8. More sinister is that these interphases with low ionic conductivity but considerable electronic conductivity not only increase the bulk and interface impedance of Li/LGPS, but also would proliferate through the LGPS electrolyte ultimately which causes the sudden death or short circuit of ASSLiBs9. To improve the intrinsic electrochemical stability of LGPS with Li, Yan Sun et al10 reported a method that substituting the Li+ in LGPS with divalent cation Ba2+ for lowering the total energy of LGPS framework and enhance its structural stability. Unfortunately, this novel crystal structure sacrificed the conductivity of Li+ and only providing a short-term stability of LGPS to Li. M Ogawa et al11 employed a 20 nm silicon thin layer at the electrolyte/anode interface of LiCoO2/sulfide/Li cell by pulsed laser deposition (PLD) to stabilize the cycle performance, and the final thin film battery could deliver high specific discharge capacity for 500 cycles. Besides, Li-In alloy or the other alloys are also widely used as the anode for ASSLiBs with sulfide-based ACS Paragon Plus Environment

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electrolytes for their preferable stability with electrolytes5. Indeed, these methods mentioned above can modify the compatibility between LGPS electrolyte and Li anode to a certain extent, however, some drawbacks these methods bring with including the process complexity of doping, high expense of PLD, and the lower energy density of using

Li-In

alloy

as

anode

should

be

noted.

Recently,

a

LGPS/70%Li2S-29%P2S5-1%P2O5 (LGPS/LPOS) bilayer was proposed in our research group to use as the electrolyte in the ASSLiBs for increasing the ionic conduction and stability to Li anode, and this method harvested good results in various battery systems12-14. Whereas, the thick electrolyte bilayer decreases the weight energy density of batteries. Herein, enlightened by the recent progress in Li metal anode of traditional Li-ion batteries15, an ingenious interface re-engineering strategy was applied to in-situ form a LiH2PO4 protective layer in the interface of Li anode and LGPS electrolyte as shown in Figure 1a. This approach is not only in favor of increasing the connect region of protective layer with the Li anode, but also benefits for circumventing the intrinsic chemical stability issues of LGPS to Li metal, specifically the migration of mixed ionic-electronic reactants to the inner of LGPS, and the kinetically sluggish reactions in the interface. Finally, the stability of LGPS with Li metal increased substantially and the ASSLiB of LCO/LGPS/LiH2PO4-Li (Figure 1b) was able to deliver long cycle life and high capacity, i.e., stabilized cycling of symmetric Li/Li cell for over 950h in 0.1mA cm-2 and a reversible discharge capacity maintained at 113.7 mAh g-1 for the 500th cycle at 0.1 ACS Paragon Plus Environment

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C with a retention of 86.7%. The factors that effected on the formation of the LGPS/Li interphases were revealed and analyzed, and the mechanism of the stability between LGPS and Li anode with protective layer was further investigated, moreover, the probable causes of battery degradation are also explored.

Figure 1. (a) Schematic of the preparation process of in-situ LiH2PO4 protective layer and (b) the LCO/LGPS/ LiH2PO4-Li ASSLiB with optimized structure.

2. EXPERIMENTAL SECTION 2.1 Synthesis of electrolyte and cathode materials. LGPS with the Li+ conductivity of 6.22×10-3 S cm-2 at 25 oC (See more detail in Figure S1) prepared by solid-state reaction method which can be found in some early reports4 was used as the solid electrolyte for the assembly of ASSLiB. The LCO material was prepared by solid-state method. In a typical synthesis process, the Co3O4 and Li2CO3 (99.9%, Aladdin Chemistry. Co., Ltd.) were ball-milled in the mortar with a molar ratio of 2/3 for 4 h, followed by a ACS Paragon Plus Environment

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sintering procedure at 850 oC for 20 h, and the as-prepared product was grinded again to obtain a fine powder. Afterwards, the LCO particles was coated with LiNbO3 by the sol-gel method16. (See the SEM and EDX of LCO particles coated with LiNbO3 in Figure S2) The coated Li anodes was prepared in three steps: Firstly, the solid-state phosphoric acid (H3PO4, >99.99%, Sigma-Aldrich) was melted by the vacuum oven at 60 oC for 1 h, and then the 0 wt.%, 20 wt.%, 40 wt.%, 60 wt.%, 80 wt.%, 100 wt.% H3PO4 in Tetrahydrofuran (THF, >99.9%, Sigma-Aldrich) solutions were prepared in glovebox and stirred for 2 h. Secondly, the surface-polished Li foil (80 µm thick, China Energy Li. Co., Ltd.) was punched to pellets and isostatic cool pressed with a pressure of 50 MPa in Al-plastic bags to gain an clear and even surface (Figure S3), after which 0.1 mL of H3PO4 in THF solutions with different concentrations were spin-casted onto the Li foils which were rotating at a speed of 3000 rpm and keeping for 2 min. Finally, the coated Li foils were moved into the vacuum oven at 25 oC for 12 h and respectively marked as 0%-Li, 20%-Li, 40%-Li, 60%-Li, 80%-Li, and 100%-Li for further use. 2.2 Physical characterizations. X-ray diffraction (XRD) measurements were used to characterize the surface structure of Li foils on a Bruker AXS D8 Advance diffractometer with Cu Kα radiation. The Raman spectra of the samples were recorded by a laser beam at a wavelength of 325 nm from an argon ion laser of Raman spectrometer (Renishaw inVia Reflex). The morphology and elemental distribution were analyzed by S-4800 field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray ACS Paragon Plus Environment

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spectroscopy (EDX). The valence state variation of elements on the Li foils surface were identified by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD). The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) of the interphase after cell cycling are measured on JEM-2100F. 2.3 Electrochemical measurements. For the assembly of ASSLiBs, the as-synthesized LCO powder was mixed with the LGPS solid electrolyte in weight ratio of 70:30 using an agate mortar as the composite cathode, and the Li foils were used as the counter electrode. Finally, 7 mg of the homodispersed composite cathode powder (~5.5 mg cm-2 of LCO), 100 mg of the LGPS electrolyte, and the coated Li foil were placed successively and pressed into a Teflon tube of 10 mm in diameter at 250 Mpa to get the solid-state LCO/LGPS/protective layer-Li cell. For comparison, the LCO/LGPS/LPOS/Li cell using 50 mg of LPOS as the protective layer and LCO/LGPS/Li-In alloy cell were also assembled with the similar process. The Galvanostatic charge-discharge cycling of the ASSLiBs were carried out on a standard battery test instrument (Wuhan Rambo Electronics Co., Ltd.) at various C-rates. To exam the interfacial impedance evolution of cathode/LGPS and anode/LGPS, the symmetric cells of Li/electrolyte/Li and LCO/LGPS/anode with different electrodes were assembled and characterized by electrochemical impedance spectroscopy (EIS) measurements using a Solartron 1470E potentiostat electrochemical workstation in the frequency range of 106 Hz - 10-2 Hz with an oscillation voltage of 10 mV.

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3. RESULTS AND DISCUSSION

Figure 2. (a) XRD patterns of the Li foils after be treated with H3PO4 in THF solutions in different concentrations. (b) XRD patterns of the 80%-Li samples which have been heated at various temperatures. (c) Raman spectra of the 80%-Li surface and LiH2PO4.

As shown in the XRD patterns of the Li foils (Figure 2a), except for the 0%-Li, the ACS Paragon Plus Environment

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altered XRD patterns indicates that all of the other samples have reacted with H3PO4 in THF solutions, and in-situ generate reactants on the surface of Li foils. According to the standard XRD pattern of PDF#21-0498, the generated material can be clearly identified to be LiH2PO4. The Raman spectra of 80%-Li and LiH2PO4 are also compared in Figure 2c to verify the identification. Obviously, the Raman spectra of 80%-Li coincides with it of LiH2PO4, and the bands shown in 80%-Li surface at 559, 879, 937, and 1077 cm-1 trace the appearance of the H2PO4- ion according to the references17. It should be noted that the Li is stable with anhydrous THF as reported18, and no other reaction products could be detected in XRD measurement. Closer observation reveals that the 80%-Li and 100%-Li samples only exhibits the peaks of LiH2PO4, whereas, the 20%-Li, 40%-Li, and 60%-Li samples also show the diffraction peaks of Li metal, indicating that the low concentration may be to the disadvantage of formatting a dense enough protective layer to cover the whole Li surface. Notably, the resultants obtained from the reaction between H3PO4 solutions and Li metal was LiH2PO4, instead of the conjectural Li3PO4. The reason may be related to the three steps of ionization process in H3PO4 aqueous solution at room temperature: (1) H3PO4 ⇌ H2PO4- + H+, (2) H2PO4- ⇌ HPO42- + H+, (3) HPO42- ⇌ PO43- + H+, and the hydrogen ionization ability is gradually weakened19. As a result, similar to the ionization process in aqueous solution, the other ionization steps of H3PO4 exhibit much lower ionization equilibrium constants (Ka) in molten state20 compared with the first step ionization (H3PO4 ⇌ H2PO4- + H+), and some studies have found that the organic solvents ACS Paragon Plus Environment

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moderately restrain the ionization of weak acids21. Thus what can be rationally speculated is that the molten H3PO4 in THF solution is difficult to take the further step ionization which resulted in the main resultant is LiH2PO4. On the other hand, the resultant layer of LiH2PO4 on the surface of Li possibly keep the inner fresh Li from the corrosion of H3PO4, so that makes the thickness of the main resultant LiH2PO4 is depend on the H3PO4 concentration within the identical reaction time. In addition, the duration of the reaction can also effect the thickness (Figure S4), whereas, the LiH2PO4 could be generated very quickly and it is difficult to control the thickness via tuning the reactive time. To our knowledge, it is the first time attempting LiH2PO4 to serve as the protective layer of electrolyte/Li interface in ASSLiBs, which is electrical insulator and a Li salt with feasible ionic conductivity22. However, the stability between LiH2PO4 and Li also should be noticed. Analogizing the spontaneous replacement reaction of NaH2PO4 + 2Na = Na3PO4 + H2 (∆rGmθ = -169.3 kJ mol-1)23, the LiH2PO4 layer may react with Li according to the similar following chemical equations: LiH2PO4 + 2Li = Li3PO4 + H2. Moreover, LiH2PO4 also could decompose to Li4P2O7, LiPO3 etc. at high temperature24. Whereupon, since the rising of temperature can accelerate the rate of the elements diffusion and reaction25-26, the 80%-Li samples were heated at various temperatures for 72 h so as to facilitate the possible reactions to verify the stability between LiH2PO4 and Li. Clearly, as the XRD of heated samples shown in Figure 2b, the diffraction peaks of the 80%-Li can remain unchanged until the temperature is increased to 150 oC. The limited peaks shown in the 200 oC ACS Paragon Plus Environment

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sample could be corresponded to Li3PO4 and it turns to amorphous, when the temperature is raised to 250 oC, indicating that Li will react with LiH2PO4 at temperatures higher than 200 oC and generate low crystallinity Li3PO4. Notably, the peaks belonging to Li substrate in the sample of 200 oC and 250 oC resulted from the melt of Li metal which is unstable to LGPS did not show in the samples at the lower temperature. As a result from the above experiments, it can be concluded that this in-situ LiH2PO4 layer would keep more stable at low temperatures of less than 150 oC when used as a protective layer between LGPS and Li. (The charge-discharge tests of symmetric anode/LGPS/anode cells are measured to support this view and shown in Figure S5)


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Figure 3. Surface and cross-section SEM of the Li foils. (a) The pristine Li foil, (b-f) the 20%-Li, 40%-Li, 60%-Li, 80%-Li, 100%-Li samples, (g) the magnification of cracks of 60%-Li, (h-i) the magnification section SEM of 80%-Li and the corresponding EDX element mapping.

The surface and cross-section morphology of the Li foils treated with H3PO4 in THF solvents are shown in Figure 3. Apparently, the pristine Li foil with smooth surface becomes rugged and is covered by a layer with irregular particles after the reaction, moreover, different H3PO4 concentrations lead to different surface morphology and layer thicknesses. Specifically, for the surface profiles of Figure 3b-d, may due to the exhaust ACS Paragon Plus Environment

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of gases and the lower concentration of H3PO4 in THF solvents, the 20%-Li, 40%-Li, and 60%-Li samples show rugged surface with bare Li substrate (dark area in Figure 3g and Figure S6a), whereas the samples treated with high concentration (80%, and 100% H3PO4) present the rod-like LiH2PO4 particles almost coating the Li foils entirely. Besides, it can be clearly found that the coverage density of the LiH2PO4 particles improves as the concentration of the solution increases. As the Figure 3e shows, the surface of 80%-Li is well covered by the uniform interconnected LiH2PO4 network with little voids, moreover, the magnification surface SEM of 80%-Li and the corresponding EDX element mapping (Figure 3h-i) further demonstrate that the LiH2PO4 still can be observed beneath the voids (Figure 3h) and phosphorus (P) or oxygen (O) elements compactly distributed on the entire surface. As for the cross-section view shown in the insert of Figure 3, the thicknesses of surface layers are respectively ~2 µm, ~7 µm, ~10 µm, ~12 µm, and ~15 µm for the samples obtained with increasing concentration solutions treated, meanwhile, P and O elements are also continuously distributed on cross-section of the protective layer of 80%-Li (Figure S 6d-f). All of these well prove that the surface of Li metal has been covered entirely by the LiH2PO4 layer by layer which would adequately prevent the Li from directly contacting with LGPS. It should be noted that the 100%-Li sample presents cocking-up particles with length of 20-30 µm (Figure 3f), and the rough surface resulted

from

the

excessive

H3PO4 concentration

may

not

good

for

the

electrode/electrolyte connection. Given the XRD profiles (Figure 2a) and the leakages of substrate in SEM, it can be reasonably resulted that it is the insufficient and uniform ACS Paragon Plus Environment

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thickness of the surface layer leading to the present of the peaks of Li metal in the XRD pattern27.

Figure 4. The EIS plots and fitting curves of symmetric Li/Li cell with different electrodes at (a) the 1st day and (b) the 14th day at 25oC. (c) Time evolution of total impedance of symmetric Li/Li cells with different electrodes.

To investigate the chemical stability of the interface between LiH2PO4-coated Li and LGPS, EIS tests were carried out on the symmetric Li/Li cells at open circuit voltage to check out the impedance of LGPS with different samples for 14 days, and the full AC impedance spectra ranged from 106 Hz-10-2 Hz at the 1st day and the 14th day are given as shown in Figure 4a-b. Obviously, the plots, composed with an x-intercept (high frequency), an incomplete semi-circle (middle frequency) and an inconspicuous tail (low ACS Paragon Plus Environment

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frequency) are shown in all samples, which can be matched to the bulk impedance of LGPS (Rb), the grain boundaries impedance of LGPS (R1, which can be ignored at this temperature) and the interface impedance of Li/LGPS (R2), respectively. Meanwhile, to get the more accurate impedance values, the line self-inductance (L), the capacitor (CPE) and the Li+ diffusion impedance in electrodes (W) were taken into account28, and the EIS plots were finally fitted via a fitting circuit of LR(RC)(RC)W shown in the insert of Figure 4a. Compared with the impedance in Figure 4a, it can be found that the Rb and R2 of the symmetric Li/Li cell are increasing continuously as the increase of H3PO4 concentration at the 1st day, indicating that the LiH2PO4 layer partially impedes the ionic conductivity and the increase of impedance is positively correlated with the thickness of LiH2PO4 layer, i.e., the H3PO4 concentration. This slight increase in overall impedance which is as a result of the surface treatment could be found in other works of Li anode protection29. Detailedly, the 80%-Li and 100%-Li presents more than 2.5 times (220.2 Ω Vs. 87.5 Ω) and 3 times (287.6 Ω Vs. 87.5 Ω) of the total impedance of 0%-Li owning to the lower ionic conductivity22 and thicker layer of LiH2PO4 as presented in Figure 3b-f. Nevertheless, the total impedance of the symmetric Li/Li cells increase dramatically in the next 14 days except the 80%-Li and 100%-Li samples (Figure 4b) which can be attributed for the reaction between LGPS and Li. Finally, the 0%-Li exhibits the highest impendence and surpasses the 80%-Li and 100%-Li (Figure 4c) reaching about 16 times (3756.8 Ω vs. 236.1 Ω) of 80%-Li cell and 12.5 times (3756.8 Ω vs. 301.3 Ω) of 100%-Li ACS Paragon Plus Environment

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cell, respectively. Base on the aforementioned EIS results, it can be concluded that the 80%-Li with acceptable kinetic limitation exhibits the good chemical stability and the smallest impedance (Figure 4c) when contacting LGPS during the test period.

Figure 5. The surface and cross-section SEM of (a-b) LGPS electrolyte pellet, (c-d) LGPS contacted with Li anode after 14 days. The surface and cross-section SEM of (e-f) LGPS contacted with 80%-Li after 14 days. (g) The XRD of the interface of LGPS/anodes and the pristine LGPS.

As mentioned above, some works have reported that the resultants will be generated at the interface of Li/LGPS8, 30, and our research results are in conformity with these findings. As shown in Figure 5, the stiff and clean surface of LGPS pellet (Figure 5a) becomes uneven and is covered with uniform spherical particles (Figure 5c) after it ACS Paragon Plus Environment

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contacted with Li metal, whereas, Figure 5e shows that the surface of LGPS contacted with 80%-Li has barely changed and is covered with some rod-like particles which come from the 80%-Li surface as displayed in the SEM of Figure 3e. For the cross-section view in Figure 5d, a reacted dark layer with a thickness of 40~50 µm appears at the interface of Li/LGPS compared with the pristine LGPS plate (Figure 5b), however, the LGPS pellet contacted with 80%-Li kept the original cross-section morphology (Figure 5f). Meanwhile, the XRD profile shown in Figure 5g clearly reveals that the interface of LGPS/Li has generated numerous undesirable resultants including Li2S during the fourteen-day contact with Li metal, while, no visible impurity peaks of Li2S can be observed on the surface of LGPS contacted with 80%-Li. It should be noted that the bulges shown in Figure 5g, especially in the LGPS contacted with Li or 80%-Li, are belong to the diffraction signal of polyimide films which are used as the anti-air protection film during the XRD tests. Because of the small amount of LGPS contacted with anodes, the diffraction signal of polyimide film bring stronger influences on the XRD results. The XRD pattern of polyimide film are given in Figure S7.


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Figure 6. The XPS of 80%-Li surface before (red line) and after (blue line) contacted with LGPS for 14 days.

In addition, the elements status of the 80%-Li anode surface were also detected to further confirm whether it has reacted with LGPS. The XPS profiles shown in Figure 6 demonstrate that Li and P are on the surface of 80%-Li which resulted from LiH2PO4 layer. After the fourteen-day contact, germanium (Ge) and sulfur (S) belonging to LGPS appear on the anode surface but the element valence of germanium at oxidation state (+4) has not be lowered, indicating that the LiH2PO4 protective layer on 80%-Li can prevent the reduction of the Ge in LGPS.


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Figure 7. Galvanostatic cycling of the symmetric anode/LGPS/anode cells.

To gain insight into the electrochemical behavior of plating/stripping and dynamic stability of the anodes, the Li and 80%-Li were compared with symmetric anode/LGPS/anode cells via galvanostatic charge-discharge method under a current density of 0.1 mA cm-2 at 25 oC with 60 min for one charge-discharge cycle as shown in Figure 7. In the initial, the electrochemical polarization voltage is enlarging with the increase of cycle numbers since the interfaces is changing inevitably31. Interestingly, the more rampant polarization voltage growing tendency of the Li/LGPS/Li cell makes it catch the 80%-Li/LGPS/80%-Li cell at 40 h and twice the 80%-Li/LGPS/80%-Li cell at 250 h. On the contrary, the polarization voltage increase of 80%-Li/LGPS/80%-Li cell stops at 120 h, and keeps stable for the next 800 h at about ±0.05V (shown in the insert of Figure 7). Taken together, the 14-days EIS test and the charge-discharge test of symmetric cells reveal that the LGPS is capable of showing preferable stability with 80%-Li anode both at static situation and cycling process, and the post-mortem ACS Paragon Plus Environment

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inspection of XRD or XPS further prove that the interphases can be limitedly generated at the interface of LGPS/anode, instead of the original uncontrollable interfacial layer growth. In this work, an ingenious approach is adopted to in-situ form a LiH2PO4 protective layer in the interface of Li metal and LGPS electrolyte in order to increase the connect region of protective layer with the Li anode and circumvent the intrinsic chemical stability issues of LGPS to Li metal. Based on the EIS experimental results, the concentration of H3PO4 in THF solvents were optimized to be 80%, and the ASSLiBs of LCO/LGPS/anodes with four cell constructions: LCO/LGPS/Li, LCO/LGPS/In-Li, LCO/LGPS/LPOS/Li, and LCO/LGPS/80%-Li were assembled to investigate and compare the cell performance of ASSLiBs with different battery structures. Figure 8a shows the charge-discharge curves of ASSLiBs using Li-In alloy, pure Li (with LPOS as protective layer) and 80%-Li as anodes, LCO as cathode material, and LGPS as electrolyte. As the standard electrode potential of In+/In is 0.6 volts higher than that of Li+/Li, the traditional LCO/LGPS/In-Li ASSLiBs exhibits only 83.78% energy density of LCO/Li ASSLiBs (average discharge voltage of 3.1 V vs 3.7 V) which obviously be insufficient of the requirement of high energy density batteries. Meanwhile, similar to the charge-discharge curve of ASSLiBs with LPOS protective layer, the LCO/LGPS/80%-Li cell shows a discharge capacity of 133.1 mAh g-1 at 0.1 C with the voltage window rage of 3-4.2 V and the initial coulombic efficiency of 85.9%, indicating that the LiH2PO4 protective layer does not impede the charge-discharge performance of ACS Paragon Plus Environment

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the ASSLiBs. In addition, although the Li+ conductivity of LiH2PO4 is much lower than LPOS (8 × 10-4 S cm-1 at room temperature)

22, 32

, the battery polarization of the

LCO/LGPS/80%-Li is similar with the LCO/LGPS/LPOS/Li cell since the in-situ LiH2PO4 layer possesses the thin thickness (~1 µm, with the pressure in ASSLiB assembly) which leads to acceptable ohmic impedance.


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Figure 8. (a) The initial charge-discharge curves and (b) galvanostatic cycling of ASSLiBs. The EIS plots of (c) LCO/LGPS/Li cell and (d) LCO/LGPS/80%-Li cell at different cycles. (e) Long cycle and (f) rate performance of LCO/LGPS/80%-Li cell.


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With the same cathode of LCO and the electrolyte materials, as well as confirming that the LCO is basically stable with the LGPS (stability tests are shown in Figure S8a-b), the cycle performance of LCO/LGPS/anodes cells can visually reflect the stability of the interface between electrolyte and anodes as shown in Figure 8b. After 50 cycles at 0.1 C, the discharge capacity retention of the ASSLiBs with 0%-Li, 20%-Li, 40%-Li, 60%-Li, 80%-Li, 100%-Li, and LPOS/Li are 0.2%, 1%, 77.8%, 78.5%, 91.9%, 91.0%, and 91.8%, respectively. From the above findings, it can be concluded that the LiH2PO4 layer with appropriate morphology is adequate to reach the similar modification effect of LPOS protective layer in terms of enhancing the cycle stability of LCO/LGPS/Li ASSLiBs within the test period, and more importantly, it exhibits thinner thickness and lighter weight (~1 µm, 0.13 mg) which is benefit to the increasing of the energy density than the LPOS protective layer (~330 µm, 50 mg). The EIS plots shown in Figure 8c-d demonstrate the impedance changes of the ASSLiBs with Li or 80%-Li anode after cycle various times. In general, the total impedance of ASSLiBs either using 0%-Li or 80%-Li as anode increases with the charge-discharge cycles, since the interphase of LCO/LGPS and LGPS/Li are unavoidably changing during the charge-discharge process33. However, similar to the previous characterization results of Li/Li symmetrical cells, the increase trend of impedance for ASSLiBs with 80%-Li slowed down at the 10th cycle and maintained relatively stable at the 50th cycle. In particular, it is noteworthy that three distinct ACS Paragon Plus Environment

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impedance arcs are presented in the cell with 0%-Li at the 50th cycle (Figure 8c), but only two obvious arcs in the cell with 80%-Li (Figure 8d). According to the references33-35, the resistance observed at the high frequency region (100 kHz) can be assigned to the intrinsic solid electrolyte layer, while the semicircles observed in the medium frequency (500 Hz) can be regarded as the resistances in the positive electrode layer including interfacial resistance of LCO/LGPS and the charge transfer impedance of cathode material, and low-frequency regions (1 Hz) could be the negative electrode layer, i.e., the interface impedance of Li anode and LGPS electrolyte. Thus, the growing total impedance of ASSLiB with 0%-Li in the next 40 cycles and the showing of another large interface impedance arc at low frequency range in the EIS plots well prove that the interphases generated by the reaction of LGPS and Li anode had seriously hindered the carriers transport, whereas the relatively low and constant impedance of ASSLiB with 80%-Li possessed highlights the importance of in-situ LiH2PO4 layer in the interface of LGPS/Li. The long-term cycle and rate performance of LCO/LGPS/80%-Li cell are shown in Figure 8e-f, from which a reversible discharge capacity of 118.7 mAh g-1 at the 500th cycle under 0.1 C with a retention of 86.8% can be clearly identified, and the discharge specific capacities of this cell at different rates (0.1, 0.2, 0.5, and 1 C) are corresponding to 131.1, 119.8, 100.3, and 44.5 mAh g-1, respectively. The long cycle life and good discharge capacities of LCO/LGPS/80%-Li cell at low rates underlines the electrochemical stability and passable migration ability of carriers in this cell structure. ACS Paragon Plus Environment

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However, the low capacity at 1 C indicates that the polarization of ASSLiB should not be neglected, and the issues involved including conductivity of electrode materials and electrolytes, compatibility of electrode/electrolytes are under investigating.

Figure 9. (a) The surface and (b) section-view SEM of LGPS/80%-Li interphase and (c) the corresponding cross-sectional EDX of the interphases after the ASSLiB cycled for 535 cycles.

In order to gain further understanding of the interfacial processes of LGPS/80%-Li and the protection mechanism of LiH2PO4 to the anode interface, the morphology and ingredients of the interphases of LGPS/80%-Li after 535 cycles were explored. Interestingly, as the surface and section-view SEM of LGPS/80%-Li interphase shown in Figure 9, a layer with porous and loose structure (Figure 9a and Figure 9b) has been generated at the interface and replaced the original rod-like particles (Figure 3e). The ACS Paragon Plus Environment

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corresponding EDX of this cross-section (Figure 9c) indicates that most ingredients of the interphases are composed by O and P, while the Ge mostly distributes on the electrolyte beneath. The XRD pattern of the cycled interphases shown in Figure S9a indicates that except for the weakened LGPS electrolyte peaks, there is hardly any other substance can be found. But, it is also possible that the interphases are amorphous which cannot be identified by XRD measurement. To verify this, the TEM and SEAD of the interphases were measured and displayed in Figure S9b. Clearly, the few crystalline lattice fringes in TEM images and the diffuse amorphous rings shown in the SEAD pattern confirm that the interphases are amorphous which agrees with the one measured from the XRD analysis. Thus, it can be reasonably conjectured that it is the volume effect caused by long-term charging/discharging and the non-crystallizing evolution of the interface materials12-13 result in the changed state of the LGPS/Li interface. As results, it could lead to the battery performance degradation once the interface variables transcend their corresponding threshold, such as stress, contact, ionic conductivity, etc.36

4. CONCLUSION In this work, a LiH2PO4 protective layer between LGPS electrolyte and Li anode was in-situ prepared via a manipulated reaction of H3PO4 in THF solvents with Li metal. The experiment results show that the formation of the LiH2PO4 layer is mainly related to the concentration of H3PO4 in THF solvents, and the 80 wt.% concentration H3PO4 resulted in the optimal effect which leaded the ASSLiBs to show superior cell performance. ACS Paragon Plus Environment

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The EIS static monitoring and the cycling test of Li/Li symmetric cells reveal the LiH2PO4 protective layer is capable of playing positive role in the stabilization of the LGPS/Li interface, enhancing both the stability and compatibility of LGPS with Li metal. The post-mortem inspection of XRD and XPS further prove the interphases could be limitedly generated at the interface of LGPS/anode, instead of the original uncontrollable interfacial layer growth. Finally, the optimized cell with interface-engineered structure is able to deliver a reversible discharge capacity of 131.1 mAh g-1 at the initial cycle and 113.7 mAh g-1 at the 500th cycle under 0.1 C with a retention of 86.7%. The research of interfacial processes of LGPS/80%-Li reveals that the volume effect and the slow chemical reaction in the anode interface leaded by the long-term contact and charging/discharging may be the main causes of the cell performance degradation. Moreover, the heating test of the Li metal with LiH2PO4 layer confirm the amorphous substance based Li3PO4 will be generated as the reactant of the reaction between Li and LiH2PO4. Compared with the traditional interface modification such as bilayer electrolyte, physical vapor deposition, and doping of LGPS, this ingenious interface re-engineering strategy between Li10GeP2S12 electrolyte and Li anode is able to reach better protection effect with more facile and economical process via in-situ reaction, not only providing more intimate contact between the protective layer and Li, but also benefits for the stability of LGPS against Li anode, giving an alternative idea for the modification of Li anode in ASSLiBs. However, the low Li+ conductivity of the coating material on Li, the ACS Paragon Plus Environment

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volume effect led by the long-term charging-discharging, and the evolution of the interphases ought to be further investigated.

ASSOCIATED CONTENT Supporting Information Detailed SEM morphologies for LCO surface, lithium surface and cross-section of LiH2PO4-coated lithium. Details for the Arrhenius plots of LGPS, charge-discharge curves of symmetric anode/LGPS/anode cells, and EIS and EDX. The XRD patterns, TEM images and the corresponding SEAD pattern after cycle. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (S. J. Chen) *E-mail: [email protected] (X. X. Xu)

ORCID Zhihua Zhang 0000-0001-6573-4281 Shaojie Chen 0000-0002-8054-9305 Xiaoxiong Xu 0000-0002-8599-4918

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS ACS Paragon Plus Environment

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This work was supported by funding from the Strategic Priority Program of the Chinese Academy of Sciences (Grant No. XDA09010201, XDA09010203), the National Natural Science Foundation of China (Grant No. 51502317), Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ16E020003, LY18E020018, LY18E030011,

LD18E020004), National Key Research

and

Development Program of China (Grant No. 2016YFB0100105) and Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2017342).

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Interface Re-Engineering of Li10GeP2S12 Electrolyte and Lithium

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