Polypropylene Carbonate-Based Adaptive Buffer Layer for StableInterfaces of Solid Polymer Lithium Metal BatteriesHaochen Yang,† Yamin Zhang,† Michael J. Tennenbaum,‡ Zachary Althouse,† Yao Ma,† Yubin He,†

Yutong Wu,† Tzu-Ho Wu,† Anmol Mathur,† Peng Chen,† Yanghang Huang,†

Alberto Fernandez-Nieves,‡,§,∥ Paul A. Kohl,*,† and Nian Liu*,†

†School of Chemical and Biomolecular Engineering and ‡School of Physics, Georgia Institute of Technology, Atlanta, Georgia30332, United States§Department of Condensed Matter Physics, University of Barcelona, Barcelona 08028, Spain∥ICREA-Institucio ́ Catalana de Recerca i Estudis Avanca̧ts, Barcelona 08010, Spain

*S Supporting Information

ABSTRACT: Solid polymer electrolytes (SPEs) have the potential toenhance the safety and energy density of lithium batteries. However, poorinterfacial contact between the lithium metal anode and SPE leads to highinterfacial resistance and low specific capacity of the battery. In this work, wepresent a novel strategy to improve this solid−solid interface problem andmaintain good interfacial contact during battery cycling by introducing anadaptive buffer layer (ABL) between the Li metal anode and SPE. The ABLconsists of low molecular-weight polypropylene carbonate , poly(ethyleneoxide) (PEO), and lithium salt. Rheological experiments indicate that ABL isviscoelastic and that it flows with a higher viscosity compared to PEO-onlySPE. ABL also has higher ionic conductivity than PEO-only SPE. In thepresence of ABL, the interface resistance of the Li/ABL/SPE/LiFePO4 batteryonly increased 20% after 150 cycles, whereas that of the battery without ABLincreased by 117%. In addition, because ABL makes a good solid−solidinterface contact between the Li metal anode and SPE, the battery with ABL delivered an initial discharge specific capacity of>110 mA·h/g, which is nearly twice that of the battery without ABL, which is 60 mA·h/g. Moreover, ABL is able to maintainelectrode−electrolyte interfacial contact during battery cycling, which stabilizes the battery Coulombic efficiency.

KEYWORDS: adaptive interface, all-solid-state battery, solid polymer electrolyte, lithium metal anode, interfacial adhesion,viscoelastic

■ INTRODUCTION

Lithium-ion batteries (LIBs) are valuable in portable devices,electrical vehicles, and electric grids.1−6 However, with thematuration of LIBs, it is challenging to improve the safety andenergy-density. Recently, lithium metals have attracted interestas an alternative anode material because it has a hightheoretical capacity (3860 mA·h/g), which is more than tentimes that of graphite (372 mA·h/g) and the lowest negativeelectrochemical potential (−3.04 V vs standard hydrogenelectrode)7−11 among all possible anodes. The Li−LMObattery (M represents a transition metal, such as Co, Ni, andMn) has a specific energy of ∼440 W·h/kg, which is higherthan that of the state-of-the-art LIBs (∼250 W·h/kg).8 Pairingwith new cathodes such as sulfur and oxygen can increase thetheoretical energy density to 2600 W·h/kg (Li−S)12 and 3500W·h/kg (Li−O2), respectively.

13 Thus, development of the Limetal anode is an enabling technology for future batteries.When used with a Li metal anode, traditional organic liquid

electrolytes have severe safety concerns due to their low flashpoint, toxicity, and complex side reactions with Li.14−16 The

potential safety issues of liquid organic electrolytes restrictsome applications in large-scale systems.17 Because of its low-or nonflammability, the solid-state electrolyte (SSE) is anexciting research direction to mitigate the safety concerns oforganic electrolytes. SSEs generally have two categories:inorganic ceramic electrolytes and solid polymer electrolytes(SPEs).18,19 The list of inorganic ceramic electrolytes mainlyincludes oxide-based ceramic electrolytes such as NASICON-like ceramic electrolytes,20 garnet structure electrolytes,21 andsulfide-based ceramic electrolytes.22 As for SPEs, poly(ethyleneoxide) (PEO) is one of the most popular polymer approachesdue to its relatively high ionic conductivity, reasonablemechanical stability, good compatibility with electrodes, andexcellent film-forming ability.23−25 The Li+ transport mecha-nism in PEO originates from the ether repeat linkages, whichenable lithium salt dissociation and lithium ion mobility.26−28

Received: May 12, 2019Accepted: July 12, 2019Published: July 12, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

© XXXX American Chemical Society A DOI: 10.1021/acsami.9b08285ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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In a lithium battery system with SSE and Li metal, the Li−SSE interfaces are important to the electrochemical reactionbecause only the active materials that are in direct contact withthe electrolyte can be effectively utilized. However, unlikeliquid electrolytes that are able to easily deform to maintaininterfacial contact with the surface of electrodes, it is hard tomaintain a solid−solid interface between the Li anode andSSE, especially during battery cycling because the surfacemorphology of the Li metal continuously changes.19,29

Expansion and contraction of the cathode during cycling canalso be a problem. Thus, the overall interfacial resistance of thebattery with SSE increases during battery cycling, which leadsto a decay in specific capacity. To improve the solid−solidinterfacial contact, many surface modification methods havebeen developed. For inorganic ceramic SSEs, the methods ofball-milling,30,31 pulsed laser deposition,32,33 atomic layerdeposition,21,34 and composite electrolytes35,36 have beenwidely used. As for SPEs, gel polymer electrolytes (GPEs),where the organic liquid electrolyte is incorporated into thematrix of the SPE, have attracted increasing attention.Combining the advantages of liquid electrolytes and SPEs,GPEs have good interfacial contact with Li metal and excellentinterfacial stability.37 However, because of the existence of theorganic liquid electrolyte, the safety performance of GPEs isnot ideal. “Self-healing” additives have been used in batteryelectrodes (e.g., LiMn2O4 and LiTi2(PO4)3 electrodes,38

silicon anode,39 and Li metal anode40) to in situ repairdamaged electrical networks during cycling in liquid electro-lytes.In this work, we have improved the interfacial contact

between Li metal and SPE by designing a polymer-basedadaptive buffer layer (ABL) with the ability to “self-heal”. Asshown in Figure 1, the interfacial contact between Li metal and

SPE in the battery without ABL deteriorates as the Li surfacedeforms during cycling. In contrast, ABL is able to maintaingood interfacial contact by filling the undulations and voids onthe lithium metal formed during battery cycling.40 Exper-imentally, we created this ABL by mixing lithium salt, high-molecular-weight PEO (600 000 Da), and low molecular-weight polypropylene carbonate (PPC) (2000 Da). PPC is acopolymer of propylene oxide and carbon dioxide and has alow degree of crystallinity.41,42 The addition of low molecular-weight PPC gives the ABL fluidity at elevated temperature (i.e.,50 °C). During battery cycling, the ABL is able to adapt to theshape change of Li metal anode, so it maintains good

interfacial contact between the Li metal anode and SPE.Note that an ABL with too much fluidity will result in it beingsqueezed out during battery assembly, and should be avoided.

■ EXPERIMENTAL SECTIONMaterials. PEO (MW 600 000, Sigma-Aldrich), poly(propylene

carbonate) (MW ≈ 2000, Novomer Inc.), bis(trifluoromethane)-sulfonimide lithium salt (LiTFSI, Sigma-Aldrich), acetonitrile (AN)(Alfa Aesar), LiFePO4 powder (LFP, MTI Corporation), Super P(MTI Corporation), and Li foil (thickness 0.75 mm, Alfa Aesar).

Preparation of SPE and ABL. PEO, PPC, and LiTFSI werecarefully dried at 60 °C overnight before use. PEO and LiTFSI weredissolved in AN and stirred for 24 h. The molar ratio of EO:Li was setto 8:1. After stirring, the slurry containing PEO and LiTFSI wascoated onto the stainless steel disk (0.001 in thickness, TBICorporation) or LFP cathode and dried at 60 °C for 12 h to obtainsamples for ionic conductivity or battery tests. The surface area of thestainless steel electrode for the conductivity test is 0.785 cm−2, andthe thickness of SPE membrane is around 150 μm. To prepare theABL, PEO, PPC, and LiTFSI were dissolved in AN and stirred for 24h. The mass ratio of PEO, PPC, and lithium salt is 1:1:1.16. Afterstirring for 24 h, the ABL solution was cast onto stainless steel forconductivity tests or onto the top of the SPE membrane for batterytests. The above electrodes were transferred into an argon-filledglovebox and dried at 60 °C for 12 h. The areal sizes of the cathode,electrolyte, and anode in a working cell are 0.785, 0.785, and 0.503cm2, respectively. The thickness of ABL is around 10 μm in the fullbattery, which was measured by caliper (MITUTOYO Corp., CD-6″ASX) at room temperature, shown in Figure S1.

Ionic Conductivity Measurements of SPEs. The ionicconductivities of the SPEs were measured by electrochemicalimpedance spectroscopy (EIS) with an ac amplitude of 10 mV inthe frequency range of 1 MHz to 0.1 Hz. The measurements wereperformed on a Bio-Logic SAS at various temperatures ranging from30 to 70 °C. The PEO and ABL were separately sandwiched betweentwo stainless steel round disks inside a 2032 type coin cell for testing.The batteries were kept at each test temperature for 30 min to reachthermal equilibrium. The ionic conductivity (σ) of SPEs could becalculated by the resistance (R), the electrode area (S), and theelectrode thickness (L), according to the following equation

L R S/( )σ = · (1)

Material Characterization and Electrochemical Test. Therheology was measured by a cone-plate geometry in an Anton PaarMCR 501. The diameter of the cone, the cone angle, and thetruncation height is 25 mm, 2°, and 0.051 mm, respectively.Frequency sweeps were performed at an applied strain amplitude of0.1 at 50 °C. Differential scanning calorimetry (DSC, Q600) wascarried out to measure the melting point of the polymer electrolytefrom −30 to 80 °C at a heating rate of 5 °C/min under a nitrogenatmosphere.

A composition of 60:20:12:8 of LiFePO4/PEO/Super P/LiTFSIwas used in the cathode slurry. These components were dispersed inAN and stirred overnight. The slurry was cast onto carbon-coatedaluminum (Guangzhou Nano New Material Technology Co., Ltd)round disks with 1 cm diameter. After drying at 70 °C for 12 h, theLFP cathode was obtained. The mass loading of LiFePO4 on thecathode ranged from 2.2 to 2.5 mg/cm2. To get the cathode with SPE,the PEO SPE solution was directly cast onto the LFP cathode. Afterdrying the SPE layer in a vacuum dryer overnight, they weretransferred into an argon-filled glovebox and heated at 70 °C for 12 hto remove the remaining traces of the solvent. For the cathodes withABL, the ABL solution was cast onto PEO SPE in the glovebox anddried for another 12 h. The full battery consisting of the Li metalanode, SPE (with or without ABL), and LFP cathode were assembledinto a 2031 coin cell inside the glovebox. Full cells and Li/Lisymmetric cells were cycled using an 8-channel (Wuhan LANHEelectronics Corporation) battery tester in a temperature chamber(Tenney Environment, Thermal Product Solutions). To visualize the

Figure 1. Schematic of the interface contact between Li metal anodeand SPE before and after repeated cycles, (a) without and (b) withABL.

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change of interfacial contact between the Li metal and SPE, thesymmetric cells, Li/SPE/Li, with and without ABL, were deeplydischarged to 20 mA·h/cm2 at 0.04 mA/cm2. The morphology of theinterfaces between the Li metal and SPE after deep discharge werecharacterized by scanning electron microscopy (SEM, Hitachi 8230).All tests of the batteries were conducted at 50 °C.The total ionic conductivity measurements of full batteries were

performed on a Bio-logic SAS. EIS measurements were performed atfrequencies ranging from 1 MHz to 0.1 Hz before and after batterycycling at various temperatures (from 30 to 70 °C). The bulkresistance (Rb) and interface resistance (Rint) were simulated from theEIS curve. The total ionic conductivity (σt) of the full battery wascalculated from the total resistance (Rt), the electrode area (S), andthe electrode thickness (L), according to the following equation

R R Rt b int= + (2)

L R S/( )t tσ = · (3)

■ RESULTS AND DISCUSSIONFrom a mechanical perspective, our ABL exhibits long-timeflow behavior at 50 °C. To compare the PEO SPE, ABL, andPPC SPE solutions, we poured them into three vials, driedthem for 24 h in the fume hood at 70 °C, for 12 h in thevacuum dryer, and for 12 h in an argon-filled glovebox at 70 °Cto completely remove the solvent. The vials were then laidhorizontally over a time span of 12 h at 50 °C. The PEO SPEshowed nearly no fluidity and remained at the bottom of thevial (Figure 2a). The ABL exhibited flow behavior withrelatively high viscosity, as qualitatively deduced from theslope-free surface developed by the system in the same timescale(Figure 2b). Without PEO, the PPC SPE showed fluid-flow behavior with much lower viscosity compared to ABL, asreflected by the spreading of the material inside the vial(Figure 2c). Besides, the surface condition of the cathode withABL was different from the one without ABL. In this work,LiFePO4 (LFP) was used as the cathode material for the fullbattery. To reduce the interfacial resistance between the SPEand cathode, the SPE solution was directly cast onto theelectrode. The surface of the cathode without ABL was darkand uneven, as shown in Figure 2d; in contrast, the one withABL was smooth and reflective (Figure 2e). Ultimately, it is

the difference in mechanical properties that determines thedifferences between these two situations.To more quantitatively characterize the mechanical proper-

ties of both ABL and PEO SPE, we performed oscillatoryrheology in the linear regime at 50 °C. In these experiments,we apply a harmonic strain and measure the resultant stress,which is generally also harmonic but out-of-phase with respectto the applied strain. The material response thus includes bothliquid-like and elastic-like responses. The loss modulus, G″,and the storage modulus, G′, quantify the relative importanceof viscous dissipation and elastic-energy storage, respectively.We measured these moduli as a function of frequency for lowapplied strain amplitudes. We find that for small frequencies,G″ > G′, whereas at high frequencies, G′ > G″. Hence, there isa crossover at certain frequency, ωc. Although this is true forboth PEO SPE and ABL, as shown in Figure 3a,b, we find that

the crossover is significantly different. Although for PEO SPE,ωc ≈ 0.033 s−1 and for ABL, ωc ≈ 0.12 s−1, indicating that thestructural rearrangement in the latter happens at shorter timescales and that its liquid-like response is more pronounced.43

This is also reflected by the smaller values of the moduli at thecrossover and of G′ at the highest frequencies we can probe.For SPEs, high ionic conductivity and interfacial retention

during battery cycling are very important. At the same currentdensity, batteries with higher ionic conductivities will havelower Ohmic voltage loss, more complete reactions, and higherspecific capacities for the electrode materials. The mechanism

Figure 2. The completely dried (a) PEO SPE, (b) ABL, and (c) PPC SPE. The surface condition of the LFP cathode with PEO SPE (d) withoutand (e) with ABL.

Figure 3. Characterization of the viscoelastic properties usingoscillatory rheology of (a) PEO SPE and (b) ABL.

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of Li+ transport in PPC is similar to that of PEO where ionconduction is assisted by chain-segment mobility.41,44 PEO is asemi-crystalline polymer that includes both amorphous phasesand crystalline phases. As the Li+ transport only occurs in theamorphous regions of the polymer, and the presence of thecrystalline phase is detrimental to Li+ transport,26 PEO SPEhas low ionic conductivity, on the order of 10−6 at ambienttemperature.45 In contrast, the PPC SPE has higher Li+

conductivity due to greater presence of amorphous structureswithin PPC.44 Amorphous polymers do not have a meltingpoint (Tm) when analyzed by DSC. In previous DSC results,PPC SPE did not show a Tm, which confirms its amorphousstructure.41 The DSC curves of PEO SPE and ABL are shownin Figure 4a. The endothermic peak of PEO SPE appears at ca.

67 °C. The ABL used here showed no obvious Tm. The ionicconductivity was measured at various temperatures (rangingfrom 30 to 70 °C), using EIS in 2032 coin cells. The SPE andABL/SPE were sandwiched between two stainless steel disksfor measurement of conductivity. As shown in Figure 4b, ABLhas higher ionic conductivity than PEO SPE at all measuredtemperatures due to the incorporation of low molecular weightPPC.Full-cell battery cycling tests were performed using a lithium

metal anode, lithium iron phosphate cathode, and SPEelectrolyte, as shown in Figure 5. The electrolyte Ohmic lossis related to the mobility of Li+ and the number of carriers inthe solid electrolyte. However, if there is loss of interfacialcontact between the SPE and either electrode, the resistanceincreases because that section of the electrolyte is effectivelyuseless. The purpose of the ABL is to maintain good interfacialcontact between the Li metal anode (or cathode) and SPEkeeping the resistance of the battery constant during cycling.To study the change in interfacial contact before and afterbattery cycling, batteries were assembled and tested at 50 °Cand 1 C rate using Li/SPE/LiFePO4 and Li/ABL/SPE/LiFePO4 full batteries, Figure 5a. The details of assembling thebatteries with ABL are shown in Figure S2. The impedance offull-cell batteries before and after 150 cycles was measuredusing EIS. Figure 5b showed the fitted EIS results and the insetshows the equivalent circuit model used to simulate the EIScurves. Rb and Rint represent the bulk resistance and interfaceresistance, respectively.46 The low frequency resistance, Rint,increased from 202 to 439 Ω (117%) for the battery withoutABL, showing a loss in interfacial surface area between the SPEand electrodes because the fundamental exchange current isassumed to remain about the same. With the ABL, there wasonly a 20% increase in resistance from 156 to 187 Ω.47Furthermore, the EIS tests of full batteries before and after 150

cycles were carried out from 30 to 70 °C to measure the totalresistance (Rt) or low-frequency intercept, which is the sum ofbulk resistance (Rb) and interface resistance (Rint). Totalconductivity based on Rt is shown in Figure 5c,d. The batterywithout ABL had an increase in Rt (ranging from 43 to 63%)after cycling as indicated by the blue arrow in Figure 5c. Incontrast, there is essentially little change in Rt (increase of 6−14%) of the battery with ABL, showing that the ABL iseffective in keeping the overall ionic conductivity fromdecreasing during cycling.The increase in interfacial resistance was mitigated by

incorporation of low molecular weight PPC in the ABL, whichresulted in higher specific capacity after cycling. The ratecapabilities of the Li/SPE/LiFePO4 battery are presented inFigure 6a. The battery with ABL had higher specific capacity atall tested rates. Figure 6b,c show the voltage profiles, which areadditional evidence that the battery with ABL is capable ofdelivering higher specific capacity. At 0.5, 1, 1.5, 2, and 3 Cdischarge rates, the capacities of the battery with ABL are 30,40, 57, 89, and 240% higher than the one without ABL,respectively.The capacity retention and Coulombic efficiency of batteries

at 1 C and 50 °C are shown in Figure 6d,e. Because the ABLmade a good solid−solid interface contact between Li andSPE, the battery with ABL delivered a higher initial specificcapacity (110 mA·h/g) than the one without ABL (60 mA·h/g). Moreover, as the surface morphology of Li metal changesduring battery cycling, the Coulombic efficiency fluctuates dueto the continuously changing interfacial contact between Limetal and SPE. In contrast, the semi-liquid ABL is able todeform and maintain good interfacial contact, thereforedelivering a more stable Coulombic efficiency over 150 cycles.To evaluate the cycling stability with ABL, symmetric cells Li/SPE/Li and Li/ABL/SPE/ABL/Li were assembled and testedat 0.25 mA·cm−2, 50 °C. As shown in Figure S3, the cellwithout ABL exhibits severer voltage polarization during the360 h cycling. This can be ascribed to the unstable interface

Figure 4. (a) DSC curves of PEO SPE and ABL. (b) Arrhenius plotsof the conductivity of PEO SPE and ABL.

Figure 5. (a) Full cell configuration for cycling and EIS measurement.(b) Nyquist plots of full batteries at 50 °C. Inset: Equivalent circuitmodel for the impedance spectra. Arrhenius plots of the conductivitiesof full batteries (c) Li/SPE/LiFePO4 (d) Li/ABL/SPE/LiFePO4before and after 150 cycles.

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between the Li metal and SPE. In contrast, the cell with ABLexhibits less voltage polarization, and interfacial contact ismore stable.35,36,47

To visualize the change in interfacial contact between the Limetal and SPE, symmetric cells (Li/PEO SPE/Li) with andwithout ABL were charged to 20 mA·h/cm2 at low currentdensity (0.04 mA/cm2). Figure 6f shows a cross-sectional SEMimage of the cell without ABL on the Li-stripped side, wherethe obvious detach between Li metal and SPE can be seen. Incontrast, the cell with ABL maintained good interfacial contact(Figure 6g) due to the semi-liquid ABL. The SEM imagessupport the ABL concept of maintaining the intimate solid−solid interface between Li metal and SPE.

■ CONCLUSIONSIn summary, we have successfully designed and fabricated anABL to improve the interfacial contact between the Li metalanode and SPE and to maintain good interfacial contact duringbattery cycling. From the rheological tests, ABL showed betterliquid-like properties at 50 °C, which helped the SPE adapt tothe shape change of the anode during battery cycling. Aftercycling, the increase in interface resistance for the battery withABL (20%) was lower than the one without ABL (117%).Because of the improved interfacial contact between Li andSPE, the initial specific discharge capacity of the Li/ABL/SPE/LFP battery (110 mA·h/g) was nearly twice that of the batterywithout ABL (60 mA·h/g). The battery with ABL was morestable in terms of Coulombic efficiency during battery cycling.

In addition, the SPE with ABL is better matched to highertemperature operation because there are no volatile solventsand the conductivity of the SPE and ABL improve withtemperature. The ABL working temperature can also beadjusted by modifying the structure of PPC to encouragegreater segmental mobility at lower temperatures. This shouldbe the subject of future studies. Our work offers a new way tomaintain stable interfaces between dimensionally changingelectrodes and electrolyte, particularly for all-solid-statebatteries.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b08285.

Measurement of the thickness of ABL with caliper atroom temperature, preparation of the battery with ABL,and voltage−time curves of symmetric Li/Li cells(current density: 0.25 mA·cm−2; capacity: 0.25 mA·h·cm−2) (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: kohl@gatech.edu (P.A.K.).*E-mail: nian.liu@chbe.gatech.edu (N.L.).ORCIDHaochen Yang: 0000-0002-8367-9711Yao Ma: 0000-0002-8283-8645Yutong Wu: 0000-0003-1214-9147Paul A. Kohl: 0000-0001-6267-3647Nian Liu: 0000-0002-5966-0244NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe work was financially supported by Kolon Industries, Inc.through the Kolon Center for Lifestyle Innovation at GeorgiaTech, faculty startup funds from Georgia Institute ofTechnology, and the NSF (DMR-1609841). Material charac-terization was performed in part at the Georgia Tech Institutefor Electronics and Nanotechnology, a member of the NationalNanotechnology Coordinated Infrastructure, which is sup-ported by the National Science Foundation (grant ECCS-1542174).

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Figure 6. (a) Comparison of the rate performance of Li/SPE/LFPbatteries with and without ABLs. (b,c) Charge and discharge voltagevs. discharge specific capacity profiles of Li/SPE/LFP at various rates.(d,e) Capacity retentions and Coulombic efficiency at 1 C. (f,g)Cross-sectional SEM images of symmetric cells (Li/SPE/Li) afterbeing charged to 20 mA·h/cm2 at 0.04 mA/cm2 current density.(b,d,f) Without and (c,e,g) with ABL. The working temperature wasset as 50 °C for all batteries.

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