Lithium Dendrite Suppression and Enhanced ... - ACS Publications

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Lithium Dendrite Suppression and Enhanced Interfacial Compatibility Enabled by an Ex-situ SEI on Li Anode for LAGP-based All-Solid-State Batteries Guangmei Hou, Xiaoxin Ma, Qidi Sun, Qing Ai, Xiaoyan Xu, Lina Chen, Deping Li, Jinghua Chen, Hai Zhong, Yang Li, Zhibin Xu, Pengchao Si, Jinkui Feng, Lin Zhang, Fei Ding, and Lijie Ci ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01003 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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ACS Applied Materials & Interfaces

Lithium Dendrite Suppression and Enhanced Interfacial Compatibility Enabled by an Ex-situ SEI on Li Anode for LAGP-based All-Solid-State Batteries Guangmei Hou,† Xiaoxin Ma,† Qidi Sun,† Qing Ai,† Xiaoyan Xu,† Lina Chen,† Deping Li,† Jinghua Chen,† Hai Zhong,‡ Yang Li,‡ Zhibin Xu,‡ Pengchao Si,† Jinkui Feng,† Lin Zhang,† Fei Ding,*,‡ Lijie Ci *,† †

SDU& Rice Joint Center for Carbon Nanomaterials, Key Laboratory for Liquid-Solid Structural

Evolution & Processing of Materials (Ministry of Education), School of Materials Science and En gineering, Shandong University, Jinan 250061, China ‡

National Key Lab of Power Sources, Tianjin Institute of Power Sources, Tianjin 300384, P.R.

China Corresponding author: Lijie Ci*

E- mail: [email protected]

Fei Ding*

E- mail: [email protected]

ABSTRACT The electrode–electrolyte interface stability is a critical factor influencing cycle performance of All-solid-state lithium batteries (ASSLBs). Here, we propose a LiF&Li3N-enriched artificial solid state electrolyte interphase (SEI) protective layer on metallic Lithium (Li). The SEI layer can stabilize metallic Li anode and improve the interface compatibility at the Li anode side in ASSLBs. We also developed a Li1.5Al0.5Ge1.5(PO4)3−poly(ethylene composite

solid

electrolyte.

The

oxide)

(LAGP-PEO)

symmetric

concrete

Li/LAGP-PEO/Li

structured cells

with

SEI-protected Li anodes have been stably cycled with small polarization at a current density of 0.05 mA cm-2 at 50 oC for nearly 400 h. ASSLB based on SEI-protected Li

ACS Paragon Plus Environment

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anode, LAGP-PEO electrolyte, and LiFePO4 (LFP) cathode exhibits excellent cyclic stability with an initial discharge capacity of 147.2 mA h g-1 and a retention of 96 % after 200 cycles. KEYWORDS: artificial SEI, electrochemical pre-cycle, Lithium metal anode, interfacial stability, buffer layer, solid state electrolyte INTRODUCTION High-energy-density batteries are urgently demanded with the booming advances in high-end portable electronics and electric vehicles. Lithium (Li) metal, with the highest specific capacity (3,860 mA h g-1) and the lowest potential (−3.04 V vs the standard hydrogen electrode), has long been considered as the ultimate anode material for high-energy Li battery systems.1-3 Despite all these advantages, issues with safety and low coulombic efficiency (CE) caused by undesired dendrite growths or mossy Li generation greatly impede the practical application of Li-metal-based energy-storage systems using liquid electrolyte.4-8 Exploring ASSLBs is a promising solution to address the issue of battery safety by replacing flammable organic liquid electrolytes with solid Li-ion conductors. 9 Over the past decades, various types of inorganic SSEs have been reported and successfully employed in ASSLBs, such as sulfide-type, oxide-type Li+ ion conductors.10-12 Among them, NASICON-type Li1.5Al0.5Ge1.5(PO4)3 (LAGP) inorganic Li+-conducting materials is considered as one of the promising candidates for solid electrolytes due to their high Li+ conductivity, wide electrochemical window, air-stability, broad applicable temperature range and low cost.13-17 However, there are


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still some challenging issues need to be solved before the practical application of inorganic LAGP in ASSLBs. Firstly, dense and uniform LAGP thin sheet is hard to fabricate although complicated integrally molding processes have been involved. Secondly, the interface contact performance is usually poor due to the rigid nature of solid LAGP and electrode material. Therefore, composite solid electrolyte easy of film-forming and wettability by introducing poly (ethylene oxide) (PEO) is explored in this study. In addition to the issues mentioned above, a stable Li-metal anode without dendrite formation and adverse reaction with electrolyte is of crucial importance for long term stability of the rechargeable Li-metal battery (LMB) with high-density storage.18 However, more and more reports have indicated that using solid electrolyte only is not sufficient to suppress Li dendrite.19-21 Moreover, LAGP is found to be unstable during contact with metallic lithium.22-24 High valence state germanium in LAGP is inclined to be reduced (Ge4+→Gex+) accompanied by the formation of mixed conducting interphase (MCI) at the lithium/electrolyte interface.22-23 Consequently, rapid capacity loss has been reported in ASSLBs based on LAGP.19, 25-26 Adopting the strategy of introducing a buffer layer to modify the interface at the Li metal side has been verified to solve the problem effectively. Various Li-ion conductors with sufficient stability against Li metal such as Li3PO4, PEO, lithium phosphorous oxynitride (LiPON) have been suggested as interlayer materials to mitigate untoward side reactions. Jadhav et al. proposed a Li-O2 cell exhibited reasonable cycling life of 52 cycles through separating Li metal from LAGP using a thin amorphous LiPON layer as a protective interlayer.24 Hu et al. reported a



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modified interface at the anode side by introducing ultrathin aluminium oxide (Al2O3) by atomic layer deposition which was converted to ion-conductive Li-Al-O layer later. The coated interlayer maintains interface stability and excellent electrochemical properties.27 However, sophisticated thin-film preparation equipments are needed and high cost limits their large-scale applications. Moreover, untoward side reactions may occur during high temperature processing which is likely detrimental to the Li+ transport property of the interface.28 Solid electrolyte interphase (SEI) is a well-known concept in battery system using liquid electrolytes, which is an electrically insulating but ionically conductive film formed on the electrode surface through spontaneous reactions between electrolyte and electrode materials during the initial charging cycles.29 An ideal SEI would be expected to have chemical stability, extraordinary Li-ion conductivity and high modulus simultaneously, and SEI on Li metal can be a suitable buffer layer to stabilize the Li metal and block its contact with the electrolyte. Several strategies have been proposed to construct an optimized SEI to protect Li metal anode. Since a coating layer with single ingredients cannot satisfy the all demands, SEI formed in working cells exhibits overwhelming advantages for its nature of complex structure and composition. 30-31 In this work, we propose an effective strategy to stabilize Li metal by constructing an ex-situ SEI film via electroplating method involving precycling the Li anode in an advanced electrolyte of Li bis-trifluoromethanesulfonimide (LiTFSI, 1.0 M)-LiNO3 (1.0

wt%)-1,3-dioxolane

(DOL)/1,2-dimethoxyethane


(DME)/

fluoroethylene

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carbonate (FEC) (9:9:2, volume ratio). The introduced SEI is expected to be electronic insulator film with satisfactory ionic conductivity. On one hand, FEC is beneficial for robust SEI formation and the resulting LiF is important in uniform and compact lithium deposits.32 33 On the other hand, the reduced product of Lithium nitride (Li3N) from LiNO3 is superior lithium ion conductor with a high Li ion conductivity of 6*10-3 S cm-1.34-35 Together with other elastic organic composites, the SEI is supposed to be an effective protective layer for Li anode in ASSBs. Unexpectedly, in a proof-of-concept experiment, the SEI-protected Li anodes exhibited interfacial compatibility with significantly reduced voltage hysteresis (~200 mV) in solid state symmetric Li-Li cells at the current density of 0.05 mA cm-2 for nearly 400 h. Consequently, by applying the SEI-protected Li anode, improved cell performance with superior long-term electrochemical stability was realized in LiFePO4 (LFP)/Li ASSBs based on LAGP-PEO composite electrolyte as illustrated in Figure 1. Our new finding of using implantable SEI to stabilize the Li metal anode in ASSBs could open a way to improve the metallic Li/solid electrolyte interface stability by searching for novel SEI initiators through modifying the electrolyte composition.

Figure. 1. Schematic of the ASSLB with optimized cell configuration.



EXPERIMENTAL SECTION Preparation of LAGP LAGP was prepared as follows. Stoichiometric amounts of lithium carbonate Li2CO3, Al2O3, GeO2, and NH4H2PO4 were thoroughly mixed by planetary ball milling at 450 rpm for 12 h. The mixture was then heated in an alumina crucible at 700 °C for 2 h. Subsequently, the synthesized powders were reground and reheated to 1450 °C for 2 h to produce the melt. The melt was then quenched on a preheated stainless-steel mold to form a transparent glass pellet. The pellet was immediately shifted to a furnace at 900 °C for 12 h to annealing to relieve thermal stress and ensure the formation of glass ceramic LAGP. The glass ceramic was ground into powder and sieved to obtain fine powder for further use. Preparation of LAGP/PEO Composite Solid Electrolyte Membrane PEO (M.W. = 500000) and LiTFSI (99.9%) were purchased from J&K Scientific Ltd and carefully vacuum-dried before use. PEO and LiTFSI were mixed at a molar ratio of EO:Li =8:1 and dissolved in tetrahydrofuran (THF) to form a solution. The PEO (LITFSI) solution was added to LAGP powder at a weight ratio of 1.5:98.5 (PEO:LAGP) after stirring at 70 °C for 12 h. Then the mixture was intensely agitated to achieve satisfactory homogenization and slowly dried at room temperature for 48h. Uniform composite solid electrolyte membranes were obtained through cold-pressing and punched into required shapes and sizes. In case of any residual solvent, the electrolyte membrane was heated at 70 °C for several hours before further use. All the above experiments were processed in an argon-filled glovebox (H2O, O2

Lithium Dendrite Suppression and Enhanced ... - ACS Publications

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