High Li+ Ionic Flux Separator Enhancing Cycling Stability of Lithium

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High Li+ ionic flux separator enhancing cycling stability of lithium metal anode Rui Jin, Lixin Fu, Hualan Zhou, Zhuyi Wang, Zhengfu Qiu, Liyi Shi, Jiefang Zhu, and Shuai Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02502 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Sustainable Chemistry & Engineering

High Li+ ionic flux separator enhancing cycling stability of lithium metal anode Rui Jina, Lixin Fua, Hualan Zhoub, Zhuyi Wanga,*, Zhengfu Qiua, Liyi Shi a,*, Jiefang Zhuc, Shuai Yuana,* a

Research Center of Nanoscience and Nanotechnology, Shanghai University,

Shanghai 200444, China b

School of Medical Instrument and Food Engineering, University of Shanghai for

Science and Technology, Shanghai 200093, China c

Department of Chemistry - Ångström Laboratory, Uppsala University, 75121

Uppsala, Sweden * Corresponding authors. E-mail addresses: [email protected] (Z. Wang), [email protected] (L. Shi), [email protected] (S. Yuan). ABSTRACT The metallic lithium anode provides unparalleled opportunities for rechargeable batteries with very high energy density. A main problem hindering the development of cells using metallic lithium anodes stems from the electrochemical instability of the interface between metallic lithium and organic liquid electrolytes. This paper reports an approach rationally designing the surface characteristic of separator for stable, dendrite-free operation of lithium-metal batteries. A unique polymer multilayer PEI(PAA/PEO)3 was fabricated on the microporous polyethylene (PE) separator by a simple layer-by-layer (LbL) assembly process, which maintains the pore structure and thickness of PE separator but remarkably enhances the ionic conductivity (from 0.36 1

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mS cm-1 to 0.45 mS cm-1) and Li+ transference number (from 0.37 to 0.48), as well as stabilizes lithium metal anodes against the reaction with liquid electrolytes during storage and repeated charge/discharge cycles, which is responsible for restraining the electrode polarization and the formation of lithium dendrites, and therefore endows lithium metal batteries with long-term cycling at high columbic efficiency and excellent rate capability, as well as the improved safety. KEYWORDS: Lithium metal anode, separator, electrolyte, interface, Li+ ion transference number

INTRODUCTION The ever-growing demand on high energy density lithium batteries, especially for the emerging applications represented by electric vehicles etc., has sharpened the focus on the metallic lithium as anode.1-3 The main problem hindering the development of such cells using metallic lithium anodes stems from the electrochemical instability of the lithium/liquid electrolyte interface, which results in unstable solid-electrolyte-interphase (SEI) layers and uncontrolled dendrite formation over repeated cycles, and consequently increased interfacial resistance, poor Coulombic efficiencies (CEs) and safety concern.1,2,4,5 Effective strategies for stabilizing lithium anodes are now considered to be a prerequisite for the timely advent of upcoming exceptional storage technologies, such as Li-S and Li-O2 batteries.2 More attentions should be paid to address the origin of the problems discussed above from fundamental perspective. The results of modelling and simulations have shown that the key variables governing the stability of Li anodes 2


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include the current density, the interfacial elastic strength, and the Li+ ion transference number and the ionic conductivity of electrolytes, etc.6-10 An electrolyte with high ionic conductivity and high Li+ transference number would dramatically alleviate the concentration gradient to extend the “Sand's time”, i.e., delay or even avoid the formation of Li dendrites.11,12 The ion transport in the electrolyte depends on the size of solvated ions and interactive environment with the surrounding species such as counterions, solvent species etc.13 Compared with the extensive attempts to achieve high ion transport ability by designing and optimizing the electrolyte components,14-17 the effects of separators on the ionic transport behavior of electrolytes lack sufficient attention. In a lithium secondary battery system, the separator not only plays a role in insulating the anode and cathode, but also holds the electrolyte which is responsible for ion transport between the electrodes.18,19 So it is reasonable to believe that the ion transport in electrolyte would be influenced by the interactive environment provided by the separator, which would depend on the porous morphology of the separator as well as the surface chemical characteristics. For example, the interaction of polar groups or sites on the separator with ionic species in the electrolyte via Coulombic interactions can possibly enable a selective change in ionic mobility.20,21 It is expected that tailoring the chemical structure of the separator would facilitate the salt dissociation and ionic mobility of the electrolyte, which would directly enhance the stability of lithium metal anode and improve the battery performance. Among various surface modification methods for activating the inert surface of 3



polyolefin separators, the dip-coating is the most widely used method, but it usually leads to significantly increased thickness and pore blocking of separators due to the use of binders.22,23 As a result, it is difficult to reveal the effects of the chemical structure of separator surface on the behavior of Li+ ions due to the interference of changed morphological characteristics. So it would be important to find a modification method that can tune the surface characteristics of separators without at the expense of the pristine porous structure, which will give us an ideal model to understand the meaning to control the interface of separator/electrolyte. In this paper, a layer-by-layer (LbL) self-assembly process based on the hydrogen bonding force between building blocks PAA and PEO was adopted to fabricate activating layers on the surface of PE separators. Unlike the dip-coating, this method is a process based on the alternate adsorption of various functional building blocks driven by specific intermolecular interactions, and it possesses unique ability to exert molecular-level control over the thickness of modification layer with minimum effects on the pore structure and thickness of the pristine separator.24 The thickness of an assembled monolayer is typically several nanometers. PAA and PEO were selected as the building blocks since the former possesses high affinity with liquid electrolyte arising from the similar chemical structure (carbonyl group) between PAA and the electrolyte components (EC, EMC and DMC),25 and the latter is well-known for its ability to solvate a wide variety of salts through interaction of its ether oxygen with cations.26 The resulting PAA/PEO multilayer-modified PE separators exhibit excellent properties especially in terms of the ionic conductivity and Li+ transference number 4


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due to the altered surface characteristics, leading to the improved stability of lithium metal anodes and superior cell performances.

EXPERIMENTAL SECTION Materials Polyethylenimine (PEI, branched, MW = 25000 g mol-1), poly(acrylic acid) (PAA, Mw = 1800 g mol-1), poly(ethylene oxide) (PEO, Mw = 100000 g mol-1) were purchased from Sigma-Aldrich. All these chemicals were used as received without further purification. Lithium metal foil was purchased from China Energy Lithium Company (Tianjin, China). Liquid electrolyte consisting of LiPF6 (1 mol L-1) in ethylene carbonate (EC)-ethyl methyl carbonate (EMC)-dimethyl carbonate (DMC) (1/1/1 by volume) was obtained from Guotai Huarong Company (Zhangjiagang, China). Commercial polyethylene microporous separators were obtained from SK Innovation (14 µm, 56.7% porosity). Preparation of PEI(PAA/PEO)3 -Modified PE Separators Scheme 1 illustrates the preparation process of PEI(PAA/PEO)3-modified PE separators. In order to get an activated surface to undergo charge or hydrogen bond interactions, PE separators were firstly treated in CO2-plasma (power, 80 W; time, 60 s) to obtain a carboxyl-functioned surface. The resulting PE separators were firstly immersed into the aqueous solution of 1.0 mg mL-1 PEI for 20 min and then rinsed in deionized water to form a primer PEI layer. Subsequently, the PEI-primed PE separators were alternatively dipped into a PAA aqueous solution (1.0 mg mL-1) and a PEO

aqueous

solution

(1.0

mg

mL-1)

with

5


three

cycles

to

obtain


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PEI(PAA/PEO)3-modified PE separators. The pH value of PAA solution and PEO solution was adjusted to 3.0 with 1.0 mol L-1 HCl aqueous solution. The deposition of PAA on the PEI-primed PE separators is driven by the electrostatic force, and the sequential LbL-modification process of PAA and PEO is driven by hydrogen bonding force. The dipping time for each step was 10 min, and each dipping step was followed by the rinse process.

Scheme

1.

Schematic

illustrations

for

the

layer-by-layer

process

of

PEI(PAA/PEO)3-modified PE separator. Characterization A quartz crystal microbalance (QCM) device (Agilent, 53131A) with a frequency counter was used to monitor the self-assembly process of LBL multilayer. The successful self-assembly of LBL multilayer was confirmed by Fourier transformed infrared spectroscopy (FT-IR, Nicolet6700). The surface morphology and thickness of separators before and after modification as well as the elemental distribution on the separator surface were observed using scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscope (EDS) (JEOL, JSM-7500F). The 6


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porosity and pore size distribution of PE separators were obtained by mercury intrusion porosimetry (AutoPore IV 9510). The air permeability of separators was examined with a Gurley densometer (UEC, 1012A) by measuring the time for 100 cm3 air to pass through under a given pressure. The high air permeability corresponds to a low Gurley value. The electrolyte uptake was obtained by measuring the weight of separators before and after immersing in the liquid electrolyte for 1 h via the following formula:

− × 100

% =

Where W0 is the weight of dry separators and Wt represents the weight of the electrolyte-soaked separators. The extra electrolyte on separator surface was wiped with a filter paper before measuring Wt. The electrolyte wettability of separators was observed by adding a certain amount of electrolyte on the surface of separators and confirmed by a contact angle analyzer (KRUSS, DSA100).The thermal shrinkage of separators was determined by measuring the dimensional change of them after exposure to various temperatures for 0.5 h. Electrochemical Measurements The electrolyte-soaked separators were sandwiched between two stainless steel (SS) electrodes of 2.27 cm2 in area and assembled into blocking-type cells to test ionic conductivity by AC impedance spectra using an electrochemical workstation (Chenhua, CHI660E) in the frequency range of 10 mHz to 1 MHz at room temperature. The ionic conductivity was obtained from the following equation:

σ = · S 7



Where σ is the ionic conductivity, Rb is the bulk resistance, S is the geometric area of stainless steel electrodes and d is the thickness of separators. With Li/separator/Li cells, Li+ transference number was evaluated by the combination of chronoamperometry and electrochemical impedance spectra (EIS). The chronoamperometry profile was obtained at a potential difference of 10 mV for 1000 s and the AC impedance spectra were tested before and after polarization. The Li+ transference number was obtained by the following equation:

=

∆ − ∆ − ! !

Where I0 and Is are the initial and steady-state current measured by the chronoamperometry, respectively. ∆V is the constant potential (10 mV), R0 and Rs are the initial interfacial and steady-state resistance measured by EIS, respectively. The electrochemical stability window of separators was determined by the linear sweep voltammetry (LSV) using SS/separator/ Li cells between 3 and 5 V (vs Li+/Li) under a scan rate of 5 mV/s. The lithium metal foil was used as the reference and counter electrodes and stainless steel as the working electrode. The compatibility of electrolyte-soaked separators with lithium electrode was measured by the impedance analysis of Li/separator/Li cells over storage at room temperature. The coin-type cells were assembled by sandwiching the electrolyte-soaked separators between Li/Li in the argon-filled glovebox (MBRAUM, W13006-2) to investigate lithium plating/stripping cycling behavior of symmetric Li/separator/Li cells under constant current density of 2.0 mA cm-2 for 0.5 h or 1.0 mA cm-2 for 1h of 8


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discharge/charge (capacity: 1.0 mAh cm-2) and the internal shorting time during the continuous Li stripping under constant current density of 1.0 mA cm-2. The charge-discharge cycle tests of LiCoO2/Li coin cells were carried out between 3.0 and 4.2 V using a LANHE Battery Testing System (Wuhan, Land, CT2001A). The cathode (LiCoO2 electrodes) was prepared by coating the slurry composing of LiCoO2 powder, carbon black and PVDF (8:1:1, w/w/w) on aluminum foil and dried at 120 °C for 24 h under vacuum. In order to match the dimension of cells, the cast foils were punched into circular pieces (d = 10 mm) and each piece loads about 2.24 mg cm-2 LiCoO2. The C-rates capability was measured from 0.2 to 5.0 C under a voltage range of 3.0 to 4.2 V. For the charge and discharge cycling, all the cells were cycled at a current density of 0.2 C for 400 cycles. The AC impedance analysis of the cells was also tested on the electrochemical working station in the frequency range of 10 mHz to 1 MHz at room temperature.

RESULTS AND DISCUSSION Physical Properties of Separators The self-assembly process of PAA and PEO on PE separators was monitored by the quartz crystal microbalance (QCM) as shown in Figure S1. According to the results, the QCM frequency change (-∆F) presents almost a linear growth with the alternate deposition of PAA and PEO on QCM resonator. The frequency change is 108 ± 4.0 Hz for PAA adsorption and 81 ± 9 Hz for PEO, corresponding to 97.2 ± 3.6 and 72.9 ± 8.1 ng (1 Hz decrease corresponds to a mass increase of 0.9 ng), respectively. These results clearly demonstrate the uniform deposition of PAA and PEO on PE separators. 9



The self-assembly process of PAA and PEO on the surface of PE separators was further confirmed by FT-IR. According to the results shown in Figure S2, the pristine PE separator just shows the stretching and swing vibrations of C-H bond. After the CO2 plasma treatment of PE separators, a new peak assigned to C=O bond appears. The adsorption of PEI primer on the surface of CO2-plasma treated PE separators leads to the appearance of a new peak at 1565.1cm−1 corresponding to N-H bond. The appearance of new peaks ascribed to C-O-C bond (1020-1275cm-1)27 and the increase in the intensity of C=O peak after the alternative deposition of PAA and PEO give the evidence of the successful self-assembly of PAA and PEO on the surface of PE separators.28

Figure 1.SEM images of the surface (a) and cross section (b) for pristine PE, the surface (c) and cross section (d) for PEI(PAA/PEO)3-modified PE. Figure 1a-d presents the surface and cross section SEM images of the pristine and PEI(PAA/PEO)3-modified PE separators. According to the results, the pristine PE separator still remains a uniformly interconnected submicron pore structure after the 10


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self-assembly of PEI, PAA and PEO, and the thickness of PE separator is still kept at around 14µm. The porous structure before and after modification was also quantitatively proved by the measurements of the porosity, pore size distribution and Gurley value (Table 1 and Figure S3). According to the results, the pristine PE separator and PEI(PAA/PEO)3-modified PE separator show almost the same porosity and the pore size distribution of them are almost identical. As for the Gurley value, PE separator shows a small increase in Gurley value after the self-assembly of PEI, PAA and PEO. All these results indicate that the pristine pore structure and thickness of PE separators were well-preserved after this modification process. Table

1.

Physical

and

electrochemical

properties

of

the

pristine

and

PEI(PAA/PEO)3-modified PE. Porosity Sample

(%)

Gurley

Contact Electrolyte

Ionic

Li+

value(s) angle(°) uptake(%) conductivity transference (mS cm-1)

number

pristine PE

56.7

223

114

106

0.36

0.37

PEI(PAA/PEO)3

56.4

267

49.4

201

0.45

0.48

-modified PE It is worth noting that the surface characteristics of PE separators are greatly affected by the self-assembly of PEI/PAA/PEO. The significantly decreased water contact angle (Table 1) of PE separators indicates that the surface of PE separators changes from hydrophobic to hydrophilic, which contributes to the enhanced electrolyte wettability. As shown in Figure S4, the liquid electrolyte forms a droplet 11



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on the pristine PE separator, but is quickly spread on the PEI(PAA/PEO)3-modified PE separator. The enhanced electrolyte wettability leads to a significant increase in the electrolyte uptake of PE separator from 106% to 201% (Table 1). Moreover, the modified PE separator exhibits reduced thermal shrinkage compared with the pristine PE separator, indicating the positive role of the PEI/PAA/PEO modification layer in the thermal stability of PE separators. Electrochemical Properties of Separators The

Nyquist

curves

of

liquid

electrolyte-soaked

PE

separator

and

PEI(PAA/PEO)3-modified PE separator at 25°C were shown in Figure 2(a). The bulk resistance (Rb) is reflected by the high frequency intercept on the real axis. The ionic conductivity

of

the

liquid

electrolyte-soaked

PE

separator

and

PEI(PAA/PEO)3-modified PE separator can be calculated by Rb to be 0.36 and 0.45 mS cm-1 at 25°C, respectively. Obviously, PEI(PAA/PEO)3-modified PE separator shows higher ionic conductivity than the pristine PE separator. The ionic conductivity of PE separators is directly related to the thickness and pore structure of separators, the electrolyte uptake and the separator/electrolyte interaction. According to the results in Figure 2, Figure S3 and Table 1, the thickness and pore structure of PE separators remain almost unchanged after the self-assembly of PEI, PAA and PEO. However, the existence of polymer multilayer PEI(PAA/PEO)3 on the separator surface endows the separator with the different surface characteristics and the significantly increased electrolyte uptake. The enhanced electrolyte uptake is favorable for the ionic conductivity, and the flexible PEO chains also can promote the 12


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ionic transport,28 which synergistically contributes to the increased ionic conductivity of PE separator. The

Li+

transference

number

calculated

by

the

combination

of

chronoamperometry and AC impedance analysis (Figure 2(b)) is shown in Table 1. For the electrolyte-soaked PEI(PAA/PEO)3-modified PE separator, the obtained tLi+ value is 0.48, which is much higher than the pristine PE separator (0.37). From the results in Figure S4, PEI(PAA/PEO)3 polymer multilayer renders the separator excellent electrolyte affinity. Meanwhile, PEO is well-known for its ether coordination sites to promote the dissociation of lithium salts and Li+ ions transport.14 With the combination of improved electrolyte affinity and segmental motions of polymer PEO chains, Li+ transference number of PEI(PAA/PEO)3-modified PE separator was significantly improved as expected.

Figure 2. (a) Nyquist plots of the cells (SS/separator/SS) for liquid electrolyte-soaked PE and PEI(PAA/PEO)3-modified PE at 25°C; (b) Chronoamperometry and EIS of Li/electrolyte-soaked separator/Li cells at 25 °C, Inset: EIS for the same cells before and

after

polarization

assembled

with

(I)

13


pristine

PE

and

(II)


Page 14 of 45

PEI(PAA/PEO)3-modified PE. By testing the impedance variation of Li/liquid electrolyte-soaked separator/Li cells over different storage time, the compatibility of PEI(PAA/PEO)3-modified PE separator with lithium electrode was investigated. A semicircle which represents the Li/electrolyte interfacial resistance (Rint) was observed for both separators as shown in Figure 3. Rint is related to the formation of passivation film on the lithium metal surface due to the reaction with the electrolyte and the charge transfer reaction of Li+ + e−= Li.29 The impedance spectra were fitted using the equivalent circuit to obtain the impedance parameters as shown in Figure S6. The bulk solution resistance Rb is just a few ohms and almost remains constant over storage, and the Rint is several or tens of kΩ and dominates the cell resistance of Li/separator/Li. The initial Rint of Li/separator/Li

cells

employing

the

pristine

PE

separator

and

PEI(PAA/PEO)3-modified PE separator is 462 and 162 Ω, respectively. The former exhibits drastic increase up to 16.2 kΩ within 6 days due to the continuous and rapid growth of a passivation layer on the lithium electrode surface. In contrast, the latter shows a much slower increase of Rint which reaches just 642 Ω after 6 days. This result reflects that the PEI(PAA/PEO)3 polymer multilayer improved the stability of the electrolyte against lithium metal electrodes on continued storage due to the excellent electrolyte uptake,30 which can effectively decrease the interaction between the lithium electrode and electrolyte components due to reduced amount of liquid electrolyte contacting with the lithium electrode, leading to more stable Li/electrolyte interface. 14


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The lithium plating/stripping cycling behavior of symmetric Li/separator/Li cells based on pristine and modified PE separators was investigated to further analyze the effects of PEI/PAA/PEO modification layer on the Li/electrolyte interface. The current density is 1.0 mA cm-2 with a charge-discharge time of 1 h or 2.0 mA cm-2 with a charge-discharge time of for 0.5 h (capacity: 1.0 mAh cm-2) (Figure 4). According

to

the

results,

the

Li/separator/Li

cells

assembled

with

PEI(PAA/PEO)3-modified separators show much smaller overpotential (blue) than the cell with the pristine PE separator (red) after cycling for 370 h at 1.0 mA cm-2 or for 125 h at 2.0 mA cm-2. The Li symmetric cells assembled with the pristine PE separators exhibit a large and irreversible voltage drop when cycling to 408 h at 1.0 mA cm-2 or to 160 h at 2.0 mA cm-2 as a result of dendrite-induced short circuit. In contrast, the Li symmetric cells assembled with PEI(PAA/PEO)3-modified separators keep working until to 450 h at 1.0 mA cm-2 or to 200 h in 2.0 mA cm-2. These results demonstrate that PEI(PAA/PEO)3-modified separators can mitigate the degradation of Li metal during cycling and suppress the electrolyte decomposition and consumption, contributing to a better stability of the Li/electrolyte interface.

15



Figure 3. AC impedance spectra of Li/separator/Li cells based on different separators: (a) pristine PE (b) PEI(PAA/PEO)3-modified PE.

Figure 4．．The voltage-time profiles of symmetrical Li/separator/Li cells based on pristine and modified PE separators at constant current density of (a) 1 mA cm-2, (b) 2 mA cm-2 and capacity of 1 mAh cm-2. Linear sweep voltammetry (LSV) curves of Li/electrolyte soaked separator/SS cells assembled with the pristine and PEI(PAA/PEO)3-modified PE separators were displayed in Figure S7. According to the results, the electrochemical stability of the pristine PE is up to 4.3 V versus Li+/Li, while the PEI(PAA/PEO)3-modified PE separator shows an anodic stability up to 4.5 V. The wider electrochemical stability may benefit from the improved electrolyte affinity of PEI(PAA/PEO)3-modified PE separator because of the introduction of hydrophilic substance PEI, PAA and PEO. Thus, the decomposition of solvent molecules on the cathode may be reduced due to less free solvent molecules contacting with the cathode.31,32 16


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The Li symmetric cells based on pristine and modified PE separators were also assembled to test the internal shorting time during the continuous Li stripping at a constant current density of 1.0 mA cm−2. The time at which a sharp drop-off in the potential was defined as short-circuit time Tsc.33,34 According to the results shown in Figure S8, a sudden drop of potential to nearly zero was observed after 39 h for the pristine PE separator-employed battery, indicating that the battery has been internally shorted. In contrast, the battery life using PEI(PAA/PEO)3-modified PE separator can be extended to as long as 78 h under the same condition, indicating the role of the modified separator in inhibiting the short-circuit failure of the battery. Battery Performance

Figure 5. Discharge profiles of cells assembled with (a) pristine PE, (b) PEI(PAA/PEO)3-modified PE. The discharge profiles of LiCoO2/Li unit cells assembled with the pristine and PEI(PAA/PEO)3-modified PE separators are shown in Figure 5a and b. Figure 6 summarizes the discharge C-rate capabilities of cells employing these two separators. All these profiles were obtained under a voltage range of 3.0-4.2 V ranging from 0.2 17



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to 5 C. According to the results, both cells exhibit decreased voltage and discharge capacity with the increase of discharge current density, but the cell assembled with PEI(PAA/PEO)3-modified PE separator owns much higher discharge capacity over various discharge current density than that with the pristine PE separator. The disparity on discharge capacity becomes bigger with increasing the current densities because the effects of ionic transport on the ohmic polarization (i.e., IR drop) become more obvious.35 Especially, the cell employing the pristine PE separator loses all the capacity at 5 C, but the cell employing the PEI(PAA/PEO)3-modified PE separator still remains a discharge capacity of 49.2%, indicating higher rate capability. This should

be

mainly

due

to

the

higher

Li+

transference

number

of

PEI(PAA/PEO)3-modified PE separator, which can reduce the electrode polarization caused by the accumulation of counter anions especially under high C-rates, and therefore is favorable for the batteries to work at high C-rates.36,37

Figure 6. Comparison of discharge C-rate capabilities between cells assembled with pristine PE and PEI(PAA/PEO)3-modified PE. Figure 7 shows the cycling performance of cells employing these two separators 18


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under a voltage range between 3.0 and 4.2 V at a current density of 0.2 C. Clearly, the discharge capacity retention of PEI(PAA/PEO)3-modified PE separator is 70.0% after the 300th cycle, much higher than the PE separator (48.3%). When cycling to 350th, the cell employing the pristine PE separator almost loses all the capacity, whereas the cell assembled with PEI(PAA/PEO)3-modified PE separator still remains 64.7%. Particularly, the cell employing the modified PE separator remains a capacity retention of 59.4% with the coulomb efficiency of 99.1% even after 400 cycles. The EDS elemental map of PEI(PAA/PEO)3-modified PE separators disassembled from cells after cycling exhibits the homogeneous distribution of N and O elements throughout the separator surface, giving the evidences of the stability of PEI(PAA/PEO)3 multilayer on PE separators upon the repeated charge-discharge cycles (Figure S9).

Figure 7. Cycle performance of cells assembled with the pristine and PEI(PAA/PEO)3-modified PE separator. The Li/electrolyte interfacial stability is a critical factor affecting the cycle 19



performance of Li-metal anodes.38 As shown in Figure 3 and Figure 4, the PEI(PAA/PEO)3 polymer multilayer improves the interfacial stability between Li metal electrode and the electrolyte during storage and electrodeposition. The AC impedance spectra of LiCoO2/Li cells after different cycles were further analyzed to understand the interfacial characteristic of Li metal electrode LiCoO2/Li cells (Figure 8). It can be noted that both Nyquist plots show two semicircles in the high-middle frequency region which corresponds to the resistance through SEI layer (RSEI) and charge-transfer resistance (Rct), respectively, and the sum of RSEI and Rct gives the interfacial resistance (Rint) of cells. The intercept at real axis represents the combination resistance Rb including the ionic resistance of electrolyte, the intrinsic resistance of cathode, separator, and anode, and the interfacial resistance at the electrode/current collector. The sum of Rb, RSEI, and Rct represents the overall internal resistance of cells, and the fitted results of them by proper equivalent circuits were shown in Table 2. According to the results, Rb is negligible in the overall internal resistance of cells, so RSEI and Rct are the leading factors affecting the decay of the discharge capacity with increasing cycle number. It is noteworthy that the cell assembled with the pristine PE separator reveals much higher initial RSEI and Rct than that with PEI(PAA/PEO)3-modified PE separator, and the RSEI and Rct exhibits rapid increase as the battery cycles to the 100th (from 58.9 Ω to 119.1 Ω for RSEI and 90.0 Ω to 229.3 Ω for Rct). In contrast, PEI(PAA/PEO)3-modified PE separator clearly suppresses the increase of RSEI and Rct with cycling. The suppressed increase of RSEI means a more stable SEI-layer formation, the smaller Rct reflects faster charge transfer 20


Page 20 of 45

Page 21 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


at the electrolyte/electrode interface, and the smaller Rint demonstrates much better interfacial stability of Li metal electrodes during repeated charge/discharge cycles.

Figure 8. Variation in AC impedance spectra (1st cycle-50th cycle-100th cycle) of cells assembled with pristine PE and PEI(PAA/PEO)3-modified PE separator. Table

2.

Resistance

data

of

cells

employing

PE

separator

and

PEI(PAA/PEO)3-modified PE separator. Cell

Rb

RSEI

(ohm) (ohm)

Li/PE separator/LiCoO2

Li/PEI(PAA/PEO)3-modified

Rct

Rinta

(ohm)

(ohm)

After 1st cycle

2.68

58.7

90.0

148.7

After 50th cycle

2.72

86.6

97.4

184.0

After 100th cycle

2.81

119.1

229.3

348.4

After 1st cycle

2.41

27.2

45.9

73.1

After 50th cycle

2.66

71.8

47.9

119.7

21



PE separator/LiCoO2 a

After 100th cycle

2.68

Page 22 of 45

85.4

93.7

179.1

Rint represents the sum of RSEI and Rct. For lithium metal anodes, the formation of lithium dendrites is another important

factor leading to the deteriorated cycle performance of lithium metal batteries. The surface morphology of lithium metal electrodes disassembled from cells employing both separators was examined by SEM (Figure S10). The surface of lithium metal electrode employing the pristine PE separator shows very rough surface and a lot of bulges after 100 cycles. The sharp bulges can be found from the enlarged image, which may puncture the separators to make the battery short-circuit. In contrast, the lithium metal electrode of the cell employing PEI(PAA/PEO)3-modified PE separator shows a surface with more smooth area and some deposits. Even the deposits also show

smooth

surface

without

sharp

dendrite,

indicating

that

the

PEI(PAA/PEO)3-modified PE separator can significantly suppress the growth of dendrites during cycling and decrease the short-circuit risk. The results agree well with the short circuit time (Tsc) measurements of symmetrical Li/separator/Li cells in Figure S8. The formation and growth of Li dendrite is caused by the Li+ concentration

gradient

during

the

charge/discharge

process

and

the

Li+

inhomogeneous deposition.1 The enhanced electrolyte uptake and much higher Li+ transference number of PEI(PAA/PEO)3 multilayer-modified PE separators can provide larger amount of available uniform Li-ion flux to reduce the localized lithium deposition on the Li metal surface.38-40 On the other hand, high ionic conductivity and Li+ transference number can reduce the concentration gradient during polarization,41 22


Page 23 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and as well as enhance the uniformity of lithium deposition due to the increased Li+ mass transfer rate between the electrolyte and lithium metal electrode. All these desirable characteristics synergistically contribute to the suppressed Li-dendrite growth.

CONCLUSION In this work, we report a simple and environmentally friendly method to fabricate polymer multilayer PEI(PAA/PEO)3 on PE separator which gives us an ideal model to understand the effects of separator/electrolyte interface on the behavior of Li+ ion. The polymer multilayer remarkably enhances the electrolyte uptake, ionic conductivity, Li+ transference number and electrochemical stability of PE separator. As a result, the lithium metal anodes are stabilized by restraining the electrode polarization and the formation of lithium dendrites during repeated charge/discharge cycles. This research provides a simple and efficient method to solve the intrinsic problems of lithium metal anodes for energy storage applications.

ASSOCIATED CONTENT Supporting Information. QCM frequency change (−∆F); FT-IR spectra; pore-size distributions; water contact angle and liquid electrolyte wettability; the Rb and Rint of Li/separator/Li cells; linear sweep voltammetry (LSV) curves; EDS elemental map; SEM images.

ACKNOWLEDGMENTS The authors acknowledge the National Natural Science Foundation of China (21503131, 51711530162), National Key Technology Research and Development 23



Page 24 of 45

Program of the Ministry of Science and Technology of China (2014BAE12B02), Shanghai

Municipal

Science

and

Technology

Committee

(14520500200,

15DZ1201002, 15DZ2281400), and Shanghai Municipal Education Commission (Peak Discipline Construction Program).

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Page 30 of 45

Page 31 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Captions Scheme

1.

Schematic

illustrations

for

the

layer-by-layer

process

of

PEI(PAA/PEO)3-modified PE separator Figure 1. SEM images of the surface (a) and cross section (b) for pristine PE, the surface (c) and cross section (d) for PEI(PAA/PEO)3-modified PE. Figure 2. (a) Nyquist plots of the cells (SS/separator/SS) for liquid electrolyte-soaked PE and PEI(PAA/PEO)3-modified PE at 25℃; (b) Chronoamperometry and EIS of Li/electrolyte-soaked separator/Li cells at 25 °C, Inset: EIS for the same cells before and

after

polarization

assembled

with

(I)

pristine

PE

and

(II)

PEI(PAA/PEO)3-modified PE Figure 3. AC impedance spectra of Li/separator/Li cells based on different separators: (a) pristine PE (b) PEI(PAA/PEO)3-modified PE. Figure 4．The voltage-time profiles of symmetrical Li/separator/Li cells based on pristine and modified PE separators at constant current density of (a) 1 mA cm-2, (b) 2 mA cm-2 and capacity of 1 mAh cm-2. 31



Page 32 of 45

Figure 5. Discharge profiles of cells assembled with (a) pristine PE, (b) PEI(PAA/PEO)3-modified PE. Figure 6. Comparison of discharge C-rate capabilities between cells assembled with pristine PE and PEI(PAA/PEO)3-modified PE. Figure 7. Cycle performance of cells assembled with the pristine and PEI(PAA/PEO)3-modified PE separator. Figure 8. Variation in AC impedance spectra (1st cycle-50th cycle-100th cycle) of cells assembled with pristine PE and PEI(PAA/PEO)3-modified PE separator. Table

1.

Physical

and

electrochemical

properties

of

the

pristine

and

separator

and

PEI(PAA/PEO)3-modified PE. Table

2.

Resistance

data

of

cells

employing

PEI(PAA/PEO)3-modified PE separator.

32


PE

Page 33 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme

1.

Schematic

illustrations

for

the

layer-by-layer

PEI(PAA/PEO)3-modified PE separator.

33


process

of


Figure 1. SEM images of the surface (a) and cross section (b) for pristine PE, the surface (c) and cross section (d) for PEI(PAA/PEO)3-modified PE.

34


Page 34 of 45

Page 35 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) Nyquist plots of the cells (SS/separator/SS) for liquid electrolyte-soaked PE and PEI(PAA/PEO)3-modified PE at 25°C; (b) Chronoamperometry and EIS of Li/electrolyte-soaked separator/Li cells at 25°C, Inset: EIS for the same cells before and

after

polarization

assembled

with

(I)

PEI(PAA/PEO)3-modified PE.

35


pristine

PE

and

(II)


Figure 3. AC impedance spectra of Li/separator/Li cells based on different separators: (a) pristine PE (b) PEI(PAA/PEO)3-modified PE.

36


Page 36 of 45

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Figure 4．The voltage-time profiles of symmetrical Li/separator/Li cells based on pristine and modified PE separators at constant current density of (a) 1 mA cm-2, (b) 2 mA cm-2 and capacity of 1 mAh cm-2.

37



Figure 5. Discharge profiles of cells assembled with (a) pristine PE, (b) PEI(PAA/PEO)3-modified PE.

38


Page 38 of 45

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Figure 6. Comparison of discharge C-rate capabilities between cells assembled with pristine PE and PEI(PAA/PEO)3-modified PE.

39



Figure 7. Cycle performance of cells assembled with the pristine and PEI(PAA/PEO)3-modified PE separator.

40


Page 40 of 45

Page 41 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Variation in AC impedance spectra (1st cycle-50th cycle-100th cycle) of cells assembled with pristine PE and PEI(PAA/PEO)3-modified PE separator.

41



Table

1.

Physical

and

electrochemical

properties

of

Page 42 of 45

the

pristine

and

PEI(PAA/PEO)3-modified PE. Porosity Sample

(%)

Gurley

Contact Electrolyte

Ionic

Li+

value(s) angle(°) uptake(%) conductivity transference (mS/cm)

number

pristine PE

56.7

223

114

106

0.36

0.37

PEI(PAA/PEO)3

56.4

267

49.4

201

0.45

0.48

-modified PE

42


Page 43 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table

2.

Resistance

data

of

cells

employing

PE

separator

and

PEI(PAA/PEO)3-modified PE separator. Cell

Rb

RSEI

(ohm) (ohm)

Rct

Rinta

(ohm)

(ohm)

After 1st cycle

2.68

58.7

90.0

148.7

After 50th cycle

2.72

86.6

97.4

184.0

After 100th cycle

2.81

119.1

229.3

348.4

After 1st cycle

2.41

27.2

45.9

73.1

Li/PEI(PAA/PEO)3-modified

After 50th cycle

2.66

71.8

47.9

119.7

PE separator/LiCoO2

After 100th cycle

2.68

85.4

93.7

179.1

Li/PE separator/LiCoO2

a

Rint represents the sum of RSEI and Rct.

43



Synopsis A molecular-level polymer multilayer on PE separator enables high Li ion transport capability and improves cycling stability of Li anode. 44


Page 44 of 45

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graphic abstract 237x152mm (300 x 300 DPI)


High Li+ Ionic Flux Separator Enhancing Cycling Stability of Lithium

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