Journal of The ElectrochemicalSociety

     

High Conductivity, Lithium Ion Conducting Polymer Electrolyte Based onHydrocarbon Backbone with Pendent CarbonateTo cite this article: Yubin He et al 2020 J. Electrochem. Soc. 167 100517

 

View the article online for updates and enhancements.

This content was downloaded from IP address 143.215.38.176 on 09/06/2020 at 00:59

High Conductivity, Lithium Ion Conducting Polymer ElectrolyteBased on Hydrocarbon Backbone with Pendent CarbonateYubin He, Nian Liu, and Paul A. Kohl*,z

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United Statesof America

Solid polymer electrolytes (sPE) offer a pathway for safer, less flammable lithium batteries. However, developing a polymer thatprovides high Li+ mobility as well electrochemical stability remains a challenge, because ion conductive functional units in thepolymer main-chain (e.g., polycarbonate and polyether) usually suffer from poor electrochemical stability at high and lowpotentials. Herein, an sPE with pendent carbonate on a hydrocarbon backbone has been designed and synthesized to overcomeconductivity and electrochemical stability problems. This pendant polycarbonate is different from conventional polycarbonateelectrolytes because the carbonate moiety is in the sidechain, which mitigates polycarbonate backbone stability problems while stillproviding high ionic conductivity when used with a plasticizer. Conductivity as high as 1.1 mS cm−1 at 22 °C was obtained. Stablelithium metal plating and stripping using the sPE was observed for 1,200 h and electrochemical stability up to 4.6 V vs Li+/Li hasbeen demonstrated. The low interfacial resistance (<160 ohm∙cm2 at 22 °C) and reasonable ionic conductivity have enabledacceptable cycling performance in a Li-LiFePO4 battery at 0.98 mA cm−2 for 2,300 cycles.© 2020 The Electrochemical Society (“ECS”). Published on behalf of ECS by IOP Publishing Limited. [DOI: 10.1149/1945-7111/ab9759]

Manuscript submitted April 7, 2020; revised manuscript received May 11, 2020. Published June 8, 2020.

Supplementary material for this article is available online

Lithium ion batteries (LIBs) are useful power sources in mobiledevices including cellphones and electric vehicles.1 However, thereare significant safety concerns due to the volatility and flammabilityof liquid organic electrolytes. Transitioning to a solid electrolyte candecrease flammability concerns, potentially suppress dendritegrowth, inhibit active material shuttling between electrodes2 andpotentially suppress side reactions.3 Compared with inorganicelectrolytes, polymer electrolytes are flexible, low density and cansimplify the manufacturing process.4,5 In addition, the sPE caninclude solid electrode interphase (SEI) forming chemical moieties.Unfortunately, no conventional polymer can provide both highconductivity and sufficient electrochemical stability. For example,polyethylene (PEO) oxide sPEs have low conductivity due tocrystallinity.6 Although various polymer architectures have beendeveloped to improve the conductivity of PEO-sPE,7,8 the oxidationof ethylene oxide units in polymer main-chain leads to polymerbackbone degradation and limits their use with high voltage cathodematerials.9–11 It has previously been reported that the higher uppercut-off potential at the cathode may accelerate dendrite formation atthe PEO-lithium metal interface.12

Another issue of current sPEs is the high interfacial resistancewhich originates from either non-conductive SEI or inadequate ioniccontact between electrode and electrolyte. For example, recent sPEswith high ionic conductivity (i.e., 0.5 to 1 mS cm−1) have shown lowbulk resistance, below 100 ohm∙cm2, however, when in contact withlithium metal, the interfacial resistance increases to 600 to 14,000ohm∙cm2. This large interfacial resistance leads to excessive voltagedrop across the sPE and limits the battery current density.

Polycarbonates (PC) are a promising alternative13 to overcomethe drawbacks of PEO by demonstrating extended electrochemicalstability (>4.2 V),14 decreased crystallinity,15 and higher Li+

transference number.16 Previously, several advanced PC-sPEs havebeen developed by including a plasticizer17 or adopting the polymer-in-salt strategy.18 Although good conductivity (i.e., 0.1 to 0.4 mScm−1) was achieved, these polyelectrolytes were semi-solids and notfree-standing. The instability of PC-sPE at negative potentials is anissue because the hydroxyl termination of the PC moieties readilyreacts with lithium metal, generating nucleophilic –R–O–Li.Polymer backbiting of the ROLi end-group leads to unzippingpolymer degradation.19 The carbonate moiety in the polymer

backbone also leads to chain random scission degradation at hightemperature which generates polymer segments with hydroxyltermination and CO2 by-products.

20

In this work, a pendant-type polycarbonate sPE has beeninvestigated. The sPE has low bulk resistivity, low interfacialresistivity, and high stability at both negative and positive potentials.The hypothesis is that moving the carbonate group to the side-chainand adopting a hydrocarbon backbone mitigates unzipping andrandom scission degradation problems. The polymer synthesizedhere takes advantage of the favorable properties of conventionalpolycarbonate (e.g., polypropylene carbonate and polyethylenecarbonate), including high lithium transference number and extendedelectrochemical stability window compared to PEO. To verify ourhypothesis, a propylene carbonate monomer functionalized with adouble bond (DB-PC) was synthesized. UV polymerization using afree radical mechanism was used to synthesize flexible, free-standingmembranes within minutes. Pendant carbonate groups participate inthe free radical polymerization. The labile hydrogen on thecarbonate ring can be abstracted by the free radical, leading toenhanced chain transfer reaction and produce a crosslinked polymernetwork. A solid plasticizer was used to increase the ionicconductivity, and maintain low interfacial resistance with theelectrodes.

Experimental

Materials.—4-(hydroxymethyl)-1,3-dioxolan-2-one, acryloylchloride, succinitrile (SN), bis (trifluoromethane) sulfonimidelithium salt (LiTFSI), lithium bis(oxalate)borate (LiBOB), phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, poly(ethylene oxide)(PEO, M.W. 600,000), poly(ethylene glycol) diacrylate (DB-PEO,700 Da), conventional liquid electrolyte, 1.0 M LiPF6 in EC/DMC =50/50 (v/v), and acryloyl chloride were purchased from Sigma-Aldrich and used as received. LiFePO4 (LFP) and Super P powerswere purchased from MTI Corporation. Li foil with a thickness of750 μm were purchased from Alfa Aesar. Poly (propylene carbo-nate) (PPC, M.W. 1 kDa, 2 kDa 137 kDa and 263 kDa) was obtainedfrom Novomer Inc. High mass loading LFP cathode was kindlyprovided by GP Batteries (theoretical capacity is 2.2 mAh cm−2).High mass loading LiCoO2 cathode was purchased from MTICorporation (theoretical capacity is 3.2 mAh cm−2).

Characterization and electrochemical tests.—1H NMR mea-surements were performed on a Bruker Avance 400 MHz NMR

zE-mail: kohl@gatech.edu*Electrochemical Society Fellow.

Journal of The Electrochemical Society, 2020 167 1005171945-7111/2020/167(10)/100517/7/$40.00 © 2020 The Electrochemical Society (“ECS”). Published on behalf of ECS by IOP Publishing Limited

instrument. UV light exposures were conducted using a Blak-Ray B-100 Series high powered UV lamp (Analytik Jena). Electrochemicalimpedance spectroscopy (EIS) measurements were performed atfrequencies from 1 MHz to 1 Hz on a Bio-logic SAS. The ionicconductivity (σt) was calculated from the measured values ofmembrane ionic resistance (Rt, ohm), area (S, cm2) and thickness(L, cm), according to Eq. 1.

( · ) [ ]s = L R S 1t t

Transference number evaluation.—The lithium transferencenumber (tLi+) was measured by the potentiostatic polarizationmethod. On symmetric cell with the configuration Li/UVPC-sPE/Li were fabricated. A constant polarization of 10 mV was applied onthe working electrode, and the current was recorded. As polarizationtime increased, the anion current decreased to steady-state (It). TheLi+ transference number was calculated using Eq. 2.

( )( )

[ ]=D -

D -+t

I V I R

I V I R2ss 0 0

0 SS SS

In Eq. 2, I0 and R0 are the initial current and interfacial resistance,respectively, and Iss and Rss are the steady state current andinterfacial resistance, respectively.

Electrode fabrication and battery fabrication.—The composi-tion of the LFP slurry was LiFePO4 (240 mg), PEO (80 mg), Super P(48 mg), LiTFSI (32 mg), and CH3CN (16 ml). The LFP cathodewas prepared by casting the slurry onto porous carbon paper (FuelCell Store Co., Ltd.) followed by drying at 80 °C for 12 h. The massloading of LiFePO4 was ∼1.2 mg cm−2. High area capacity LFP andLCO cathode was pre-soaked in SN/LiTFSI mixture for 2 d to createion conduction pathway within the electrode. All cells (2032 type)were assembled in an Ar-filled glove box with water content below0.1 ppm and oxygen content below 1 ppm. Batteries were cycledusing an 8-channel battery tester (LANHE).

Synthesis of the carbonate monomer (DB-PC).—As shown inFig. S1 (available online at stacks.iop.org/JES/167/100517/mmedia),4-(hydroxymethyl)-1,3-dioxolan-2-one (8 g) was first dissolved in100 mL of dichloromethane. Triethylamine (1.1 eq.) and acryloylchloride (1.1 eq.) were added dropwise at 0 °C. The mixture wasstirred at room temperature for 24 h under N2 atmosphere. Thesolution was then successively washed with 10% HCl, 10%NaHCO3 and saturated NaCl aqueous solution. Dichloromethane

was removed under vacuum to obtain an amber liquid. 1H NMR ofDB-PC was obtained using CDCl3 as the solvent (Fig. S1).

Fabrication of UVPC-sPE via UV polymerization and cross-linking.—Succinitrile (SN), bis (trifluoromethane) sulfonimidelithium salt (LiTFSI) and lithium bis(oxalate)borate (LiBOB) (1 wt% final concentration) was mixed at 60 °C to form a homogeneoussolution. After cooling to room temperature, DB-PC and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (PPO, photoinitiator) wereadded. The precursor solution was cast onto a glass plate. Anotherglass plate was placed on top of the solution to flatten the membranesurface. Adhesive tape was used as spacers to control the thicknessof prepared membranes. The sample was exposed to 365 nm UVradiation for 10 min. The resulting UVPC-sPE was peeled off thesubstrate and stored in the glove box. The thickness of UVPC-sPEwas 340 to 360 um.

Synthesis of non-crosslinked polycarbonate by reversible addi-tion-fragmentation chain transfer polymerization (Raft PC).—Asshown in Fig. S3, DB-PC (5 g), 2-(((ethylthio)carbonothioyl)thio)-2-methylpropanoic acid (chain transfer agent, DDMAT, 81.5 mg) andazodiisobutyronitrile (AIBN) (7.3 mg) were dissolved in 15 mlDMF. N2 gas was bubbled through the mixture for 30 min toremove dissolved air. The polymerization was carried out at 70 °Cfor 48 h under N2 atmosphere. The reaction mixture was cooled to−80 °C to stop the polymerization followed by exposure to air atroom temperature. The polymer product was purified by precipi-tating into methanol and dried at 60 °C for 24 h. The molecularweight of Raft-PC was calculated according to NMR spectrum usingthe Eq. 3.

( ( )) [ ]‐ /= ´MW 172 S S 3 3Raft PC PC CTA

In Eq. 3, SPC and SCTA are the integral area of signal 1 and signal 6in NMR spectrum, respectively (Fig. S3).

Fabrication of UV-crosslinked poly ethylene oxide solidpolymer electrolyte (UVPEO-sPE).—The UVPEO-sPE was synthe-sized under identical conductions as UVPC-sPE. As shown in Fig.S7, SN, LiTFSI and LiBOB were mixed at 60 °C to form ahomogeneous solution. After cooling to room temperature, DB-PEOand PPO were added. The precursor solution was cast onto a glassplate. Another glass plate was placed on top of the solution to flattenthe membrane surface. Adhesive tape was used as spacers to controlthe thickness of prepared membranes. The sample was exposed to365 nm UV radiation for 10 min. The resulting UVPEO-sPE waspeeled off the substrate and stored in the glove box. The thickness of

Figure 1. UVPC: Synthesis, 1H NMR and polymerization mechanism. (a). Reaction scheme for UV polymerization and crosslinking of DB-PC. (b). 1H-NMRspectra of carbonate monomer (DB-PC) with 1% photo initiator (w/w) before and after UV-irradiation. CDCl3 was used as the deuterium solvent. Signals 4, 5and 6 are assigned to the double bonds. Signals 1, 2 and 3 are assigned to the carbonate. (c). Schematic illustration of chain transfer reaction and crosslinkingreaction.

Journal of The Electrochemical Society, 2020 167 100517

UVPEO-sPE was 340 to 360 um. The final LiBOB concentrationwas 1 wt%.

Fabrication of poly propylene carbonate solid polymer electro-lyte (PPC-sPE).—PPC (1 g, 137 kDa) and LiTFSI (0.3 g) weredissolved in DMF and stirred overnight. The resulting viscoussolution was then cast onto a PTFE plate and dried at 60 °C for24 h. The PPC-sPE film was then peeled off and transferred to glovebox and dried at 60 °C for another 12 h. The PPC-sPE was stored inglove box before use.

Results and Discussion

The mechanism of UV polymerization and crosslinking was firstinvestigated. Figure 1a shows the reaction scheme for the synthesisof the pendant type UV polymerized polycarbonate (UVPC). Thephoto-initiation rate was fast and efficient. Exposing the mixture toUV radiation led to the rapid creating of free radicals,21 allowing fastpolymerization of the DB-PC monomer.22 As a result, a highpolymer was realized within several minutes. Figure 1b shows the1H NMR spectra of DB-PC monomer before and after UVirradiation. Signals 1–6 were assigned to DB-PC monomer andsignal 7 was assigned to CHCl3 in CDCl3. After 5 min of UVexposure, the DB-PC monomer signals decreased sharply, while thesignal for CHCl3 remained unchanged. The integral area ratiobetween CHCl3 signal7 and DBPC signal6 changed from 1:8.71 to1:0.10. The conversion rate of monomer was 98.8%. This resultshows the polymerization efficiency and indicates that crosslinking

occurred because the product was insoluble in CDCl3 because awhite precipitate on the NMR tube was observed.

The pendant carbonate group plays a role in the crosslinkingreaction. The labile hydrogen on the carbonate ring can be abstractedby the free radicals, leading to an enhanced chain transfer reaction(Fig. 1c).23 This chain transfer reaction was first demonstrated byEndo and co-worker24 with the radical polymerization of methylmethacrylate in propylene carbonate. After polymerization, the IRspectrum of the poly(methyl methacrylate) showed an absorptionband at 1809 cm−1 corresponding to the 5-membered cycliccarbonate.25 In this study, the chain transfer reaction yields a di-functional character to the DB-PC and enables a crosslinkednetwork. As a result, the as-synthesized UVPC was insoluble in anorganic solvent (Fig. S2). The crosslinked polymer network wascrucial to achieving an sPE with high conductivity and mechanicalstability.26

The high concentration of free radicals and carbonate groups inthis UV polymerization reaction is important for the chain transferand crosslinking reactions. As a demonstration, a non-crosslinkedpolycarbonate was synthesized from DB-PC by reversible addition-fragmentation chain transfer polymerization (Raft-PC), as shown inFig. S3a. The polycarbonate crosslinking reaction was avoided dueto the low free radical concentration and suppressed chain transferreaction in the RAFT polymerization system. Thus, the resultingRaft-PC is soluble in DMF (Fig. S3d). A high molecular weightproduct, 20.6 kDa, was produced based on the NMR analysis (Fig.S3b). This enabled formation of a free-standing film, Fig. S3c.However, once the plasticizer and lithium salt were added, the Raft-PC was converted from solid to liquid form (Fig. S3e). This resultfurther demonstrates the advantages of the UV polymerization &crosslinking strategy. Polymerization and crosslinking wereachieved in a single step, which simplified the synthetic procedure.

A pendant type polycarbonate solid polyelectrolyte (UVPC-sPE)was then prepared using the aforementioned UV polymerization andcrosslinking reaction. Figure 2 shows the chemical structure andpicture of the free-standing sPE membrane. First, a liquid precursorwas prepared by mixing the DB-PC monomer, LiTFSI, succinoni-trile (SN) and photoinitiator without the addition of an organicsolvent. The homogeneous solution was then cast onto a glass plateand exposed to UV radiation for 10 min. Succinonitrile (SN) wasincluded as a solid plasticizer to increase the ionic conductivity.27

The newly created crosslinked polymer network was crucial foroffsetting the loss of mechanical stability due to the plasticizer.Upon UV exposure for 10 min, a densely crosslinked polymernetwork was formed. The free-standing film was flexible evenwith 50 wt% plasticizer. For comparison, the non-crosslinked Raft-PC converted from solid to liquid after mixing with same amount ofplasticizer and lithium salt (Fig. S3e).

At an optimized DBPC:SN:LiTFSI mass ratio of 2:5:3 (Fig. S4),the film had a conductivity of 1.1 mS cm−1. Figure 3 and Table SⅠsummarize the room temperature conductivity of 20 previouslyreported PC-sPE materials. Generally, the plasticizer-free PC-sPEsprepared by mixing a Li salt and polycarbonate exhibit low roomtemperature conductivity, 10−9 to 10−6 s cm−1. Several recentmethods, such as polymer-in-salt,18 in situ polymerization andplasticizer doping17 yield non-free standing PC-sPE with improvedconductivity, 10−6 ∼ 10−4 s cm−1. In this study, free-standingUVPC-sPE with 1.1 mS cm−1 conductivity was obtained through asimple, single-step UV polymerization and crosslinking method.

The flammability of UVPC-sPE was evaluated and comparedwith conventional carbonate liquid electrolyte. Movie S1 and MovieS2 show the flammability of the materials in a laboratory setting. Asexpected, the liquid electrolyte easily ignited, while UVPC-sPE hadlow flammability, which provides an opportunity to improve thesafety of lithium ion batteries.

The mechanical stability of UVPC-sPE was evaluated bymeasuring the membrane resistance change after pressing as afunction of pressure (Fig. S8a). The resistance decreased from17.2 to 15.8 ohm when the pressure was increased from 1.7 MPa

Figure 2. Synthesis of pendant type polycarbonate sPE by UV polymeriza-tion & crosslinking.

Figure 3. Comparison of room temperature conductivity of previouslyreported PC-sPEs and UVPC-sPE.

Journal of The Electrochemical Society, 2020 167 100517

to 7 MPa. After disassembling the coin cell, the UVPC-sPE wasintact (Fig. S8b), showing that it did not crack or fracture duringpressing. For comparison, a UVPEO-sPE synthesized using the sameformulation (20% DB-PEO, 50% SN, 30 liTFSI and 1% LiBOB,Fig. S9) broke into several pieces during similar pressing (Fig. S8c).

Future studies will focus on improving the mechanical properties bytuning the crosslink density and incorporating inorganic fillers.28

Transference number is key parameter for the lithium ion batteryelectrolyte. Cations and anions both contribute to the total ionicconductivity for solvent/solute electrolytes. The Li+ transferencenumber (tLi+) reflects the ratio between Li+ transport and the totalion transport in the absence of a concentration gradient. High tLi+ isdesired because it lessens concentration polarization and suppressesside reactions.29,30 As shown in Fig. 4, the UVPC-sPE had a tLi+ of0.48. PEO-based electrolytes have a transference number <0.231 dueto a lithium ion chelating effect with the multidentate ether linkages9

and relatively weak solvation of TFSI− anions.32 The higher tLi+ inUVPC-sPE is attributed to the absence of significant coordinationbetween the Li+ and polymer chains, as seen in the PEO-basedsystem.18

The electrochemical stability of UVPC-sPE was evaluated usingcyclic voltammetry (CV) (Fig. 5). By moving the carbonate unit tothe side-chain and adopting a hydrocarbon main-chain, the pendanttype UVPC-sPE showed extended electrochemical stability up to4.6 V. For comparison, the CV results with conventional polypro-pylene carbonate sPE (PPC-sPE), Fig. S5, has an electrochemicalstability window at ∼4.2 V. The UVPC-sPE was cycled between3.4 V and 4.4 V for 1000 times, Fig. 6. The higher current responseduring the initial cycle is likely due to the oxidation of impurities (e.g. photoinitiator) and formation of an interface layer. After 200cycles, a low and reproducible oxidation current (2 μA cm−2 at4.4 V) was observed. Figure 7 shows the impedance results beforecycling and after 200, 600 and 1,000 cycles. Little change wasobserved in the electrode impedance.

Figure 4. Current-time profile of Li/UVPC-sPE/Li symmetric battery withan applied potential of 10 mV. The effective area of sPE is 0.785 cm2. I0 isthe initial current (at 0 h), Iss is the steady state current (at 6 h). The inset isNyquist plot of UVPC-sPE before and after potentiostatic polarization.

Figure 5. CV of the UVPC-sPE at 22 °C and 1 mV s−1. The effective areaof sPE is 0.785 cm2.

Figure 6. Selected current-voltage profiles during 1000 scans between 3.4 Vto 4.4 V. The scanning rate is 10 mV s−1. The effective area of sPE is 0.785cm2.

Figure 7. EIS spectra during CV cycling. The constant bulk resistancesuggested excellent high voltage stability of UVPC-sPE.

Figure 8. Evolution of bulk resistance and interfacial resistance of Li/UVPC-sPE/Li cell when cycling at 0.2 mA cm−2. 1 wt% LiBOB was used asthe additive.

Journal of The Electrochemical Society, 2020 167 100517

The chemical stability of the conventional polycarbonate electro-lyte (PPC for example) and UVPC in the presence of lithium metalwas investigated. For a conventional PPC electrolyte, the carbonateunits in main-chain lead to random chain scission degradation andthe hydroxyl ends result in polymer unzipping degradation, Fig.S6.19 PPC degrades into volatile propylene carbonate after being incontact with lithium metal at 130 °C for 10 min, as shown in Fig. S6.There was no degradation of PPC-sPE after being in contact withlithium foil for 2 h at 130 °C, which shows improved chemicalstability and thermal stability.

A Li/UVPC-sPE/Li symmetric cell was cycled at 0.2 mA cm−2

and 1 h per cycle. 1 wt% LiBOB was used as an additive to helpform a stable SEI.33 As shown in the electrochemical impedancespectrum (EIS), Fig. 8, high conductivity (1.1 mS cm−1) leads to alow bulk resistance (i.e. high frequency resistance), 33 ohm∙cm2. Alow interfacial resistance (i.e. low frequency resistance) wasobserved, 118 ohm∙cm2. During cycling, the bulk resistance re-mained essentially constant with cycling which shows that the sPEwas stabile in the presence of lithium metal. The interfacialresistance first increased during the initial 30 h, then becameconstant at 153 ohm∙cm2, suggesting the formation of a stableSEI. The low bulk and interface resistance, compared with pre-viously reported sPEs (600 to 14,000 ohm∙cm2, Table SII) results inlow overpotential, 43.5 mV at 0.2 mA cm−2, Fig. 9.

For comparison, a UVPEO-sPE was synthesized using the sameformulation as UVPC-sPE (20% DB-PEO, 50% SN, 30 liTFSI and1% LiBOB, Fig. S7). When using the crosslinked PEO as thebackbone instead of the pendant type polycarbonate, a much higherinterfacial resistance (422 ohm∙cm2) and overpotential (168 mV)were observed. This could be due to the crystallization of PEO andhigh degree of crosslinking in UVPEO-sPE. The resulting rigiditymade it difficult for the polymer electrolyte to mechanically adapt tothe lithium volume change during plating and stripping. On the otherhand, UVPC-sPE can better adapt to the volume changes of thelithium metal. As shown in Fig. 9, a Li/UVPC-sPE/Li battery wascharged to 3.6 mAh cm−2 at a current density of 0.2 mA cm−2 (i.e.,17.3 um lithium was reacted). During the 18 h of charging, aconstant overpotential of 42 mV was observed, Fig. 9. The bulkresistance and interfacial resistance remained unchanged, suggestingnegligible loss of contact area between electrode and sPE. Thisshows that higher energy density of solid state batteries may bepossible.

The coulombic efficiency (CE) during Li stripping/plating wasmeasured using a Li/UVPC-sPE/Cu cell. The discharge current was0.2 mA cm−2 and the discharge time was 2 h. The charging currentwas 0.2 mA cm−2 and the cut-off voltage was 0.1 V. The CE was>92% on the first cycle and 97% to 98% on subsequent cycles, asshown in Fig. 10. The 2% loss in CE is likely due to the continuousgrowth of a solid electrolyte interphase (SEI) layer on the Li anode

surface, which is reformed after being damaged.34 The measured CEis comparable to that of ether-based polyelectrolytes, which aregenerally considered to be more stable towards lithium metal thanpolycarbonate.35 Figure 11 shows the long term cycling of a Li/UVPC-sPE/Li cell at 0.2 mA cm−2 and 0.4 mAh cm−2 for 1,200 h.At high current density of 0.5 mA cm−2, low overpotential<100 mV and stable cycling for more than 700 h were alsoobserved. The reversibility and durability of Li plating/strippingbehavior in this UVPC-sPE is due to the eliminated degradationreaction through the pendant strategy.

Li/sPE/LiFePO4 cells were fabricated to investigate effect ofimproved conductivity, stability and interfacial resistance of thesPEs synthesized here. Figure 12 shows the capacity-voltage curvesat different current densities. Flat charging/discharging plateauswere observed at 3.45 V, which is typical for the LFP cathodes

Figure 9. Voltage-time profile of Li/UVPC-sPE/Li cell when charged to 3.6 mAh cm−2 at a current of 0.2 mA cm−2. Inserted: evolution of bulk resistance andinterfacial resistance. 1 wt% LiBOB was used as the additive.

Figure 10. Coulombic efficiency of Li/UVPC-sPE/Cu cell measured at0.2 mA cm−2, 0.4 mAh cm−2. 5 wt% FEC was used as the additive.

Figure 11. long term cycling stability of a Li/UVPC-sPE/Li cell at0.2 mA cm−2 and 0.4 mAh cm−2, and 0.5 mA cm−2 and 0.25 mAh cm−2.5 wt% FEC was used as the additive.

Journal of The Electrochemical Society, 2020 167 100517

especially at low C-rate. The high conductivity and low interfacialresistance of UVPC-sPE have resulted low overpotentials (48 mV at0.2 C and 145 mV at 2 C). At 0.2 C, the cathode-based specificcapacity was 145 mAh g−1, which is 85% of the theoretical capacityof LFP (170 mAh g−1). The specific capacity at higher currentdensity was 112 mAh g−1 for 2 C, 85 mAh g−1 for 5 C and 58 mAhg−1 for 10 C.

Long-term cycling performance was measured at 5 C(0.98 mA cm−2) and room temperature. Figure 13 shows thecapacity vs cycle number at two different C-rates. The initialspecific capacity was 91 mAh g−1, after 2300 cycles, and thecapacity retention was 43%. It is worth noting that the capacity fadewas small after the initial 200 cycles: from 76 mAh g−1 to 39 mAhg−1 or 0.023% fade/cycle. For comparison, if a conventional lithiumion battery was to lose 20% capacity in 1,000 cycles, the capacityfade rate would be 0.02% fade/cycle. The capacity fade during theinitial 200 cycles was 0.082% decay/cycle which may be due to theformation of a passivation layer on the cathode side. Previous studiesreport that there was a complex formed between the surface metalatoms of electrode and the –CN groups of the SN plasticizer.36,37

The resulting passivation layer containing the metal-ligand complexsuppressed undesirable interfacial reactions and improved cyclingstability, however, active material was consumed resulting incapacity loss in the initial cycles. The EIS plots during the first600 cycles is shown in Fig. 14. After the first 200 cycles at 22 °C, aconstant bulk resistance and interfacial resistance was observed,suggesting good stability of UVPC-sPE at both high and lowpotentials. At lower current density, 1 C, the Li/UVPC-sPE/LFPcell also showed good durability >4800 cycles, Fig. 14. At 0.2 C,Fig. S9, the capacity decreased from 136 mAh g−1 to 94 mAh g−1

after 287 cycles (0.11% decay per cycle). The capacity decay duringthe initial 20 cycles was much faster (0.29%), which is likely due tothe consumption of active material during the formation of a

passivation layer on the cathode. Figure S10 and Table SIIIsummarize the current density and cycling lifetime of previouslyreported solid polymer electrolytes and the UVPC-sPEs in thisstudy. At both low current density and high current density, theUVPC-sPE had improved cycle life. The large operating currentdensity of UVPC-sPE is due to the high ionic conductivity, lowinterfacial resistance and high transference number. The extendedcycling lifetime is attributed to the high stability at both low andhigh potentials.

In above tests, the area loading of LFP was ∼1.2 mg cm−12. ALi/UVPC-sPE/LiCoO2 battery was assembled using a commercialLiCoO2 cathode and the results are shown in Fig. 15. The battery

Figure 12. Charge-discharge curves of Li-LFP battery at different C rates atroom temperature. The mass loading of LFP is ∼1.2 mg cm−2.

Figure 13. Room temperature cycling of Li/UVPC-sPE/LFP battery at 1C and 5 C. The mass loading of LFP is ∼1.2 mg cm−2.

Figure 14. Evolution of bulk resistance and interfacial resistance of Li/UVPC-sPE/LFP cell when cycling at 5C.

Figure 15. Cycling performance of high area capacityLi/UVPC-sPE/LiCoO2 cell at 50 °C. The theoretical capacity of LCOcathode is 3.2 mAh cm−2.

Journal of The Electrochemical Society, 2020 167 100517

had an initial area capacity of 3.17 mAh cm−2 at 50 °C and0.2 mA cm−2. The area capacity after 27 cycles was 2.84 mAhcm−2, which is 88.7% of the theoretical area capacity (3.2 mAhcm−2). The durability was >800 h, suggesting the good stability ofUVPC-sPE at high voltage, >4 V. Compared with Fig. 13, thecapacity decay during the initial cycles was less obvious due to thehigh mass loading of the LiCoO2 cathode (22.5 mg cm−2). Stabilityat 100 °C was demonstrated using a Li/UVPC-sPE/LFP cell witharea capacity of 2.2 mAh cm−2, Fig. 16. Increasing the currentdensity from 0.1 C to 0.4 C, (i.e., 0.22 to 0.89 mA cm−2) also hadhigh area capacity, >2 mAh cm−2. The cycling time was >320 hand the capacity fade was ∼1.8%, suggesting the good electro-chemical and thermal stability of the UVPC-sPE electrolyte.

Summary

The degradation of conventional main-chain type polycarbonateelectrolytes was addressed by synthesis of a pendant type poly-carbonate sPE using a UV polymerization strategy. Stable lithiumstriping/plating for 1,200 h has been demonstrated. Extended elec-trochemical stability as well as high Li+ transference number wereachieved by taking advantage of the beneficial properties of thepolycarbonate. The UV polymerization mechanism used an en-hanced chain transfer reaction, which formed a crosslinked polymernetwork allowing the fabrication of free-standing sPEs with high ionconductivity (>1 mS cm−1). Low crystallization of the polycarbo-nate led to a low interfacial resistance (<160 ohm∙cm2) and goodadaptability to the electrode volume changes. As a result, a solidstate Li/UVPC-sPE/LFP battery was operated at 0.98 mA cm−2 andshowed long cycling life >2300 cycles.

Acknowledgments

The authors gratefully acknowledge the financial support ofKolon Industries. The authors also gratefully acknowledge thestability tests performed by Yamin Zhang, cathodes prepared by

Zachary Althouse, battery assemblies by Haochen Yang, andconductivity measurements by Youn Ji Min.

ORCID

Paul A. Kohl https://orcid.org/0000-0001-6267-3647

References

1. S. Chu, Y. Cui, and N. Liu, Nat. Mater., 16, 16 (2016).2. C. Li, Z. Guo, B. Yang, Y. Liu, Y. Wang, and Y. Xia, Angew. Chem. Int. Ed. Engl.,

56, 9126 (2017).3. R. Chen, W. Qu, X. Guo, L. Li, and F. Wu, Materials Horizons, 3, 487 (2016).4. J. Lopez, D. G. Mackanic, Y. Cui, and Z. Bao, Nature Reviews Materials, 4, 312

(2019).5. J. Bae, Y. Li, F. Zhao, X. Zhou, Y. Ding, and G. Yu, Energy Storage Mater., 15, 46

(2018).6. J. Bae, Y. Li, J. Zhang, X. Zhou, F. Zhao, Y. Shi, J. B. Goodenough, and G. Yu,

Angew. Chem. Int. Ed. Engl., 57, 2096 (2018).7. Y. Chen, Y. Shi, Y. Liang, H. Dong, F. Hao, A. Wang, Y. Zhu, X. Cui, and Y. Yao,

ACS Appl. Energy Mater., 2, 1608 (2019).8. P.-F. Cao et al., Polymer, 124, 117 (2017).9. J. Mindemark, M. J. Lacey, T. Bowden, and D. Brandell, Prog. Polym. Sci., 81, 114

(2018).10. H. Zhai et al., ACS Appl. Mater. Interfaces, 11, 28774 (2019).11. A. Li et al., Adv. Mater., 32, 1905517 (2020) .12. G. Homann, L. Stolz, J. Nair, I. C. Laskovic, M. Winter, and J. Kasnatscheew, Sci.

Rep., 10, 4390 (2020).13. L. Yue, J. Ma, J. Zhang, J. Zhao, S. Dong, Z. Liu, G. Cui, and L. Chen, Energy

Storage Mater., 5, 139 (2016).14. H. Yue, J. Li, Q. Wang, C. Li, J. Zhang, Q. Li, X. Li, H. Zhang, and S. Yang, ACS

Sustainable Chemistry & Engineering, 6, 268 (2017).15. J. Zhang et al., Adv. Energy Mater., 5, 1501082 (2015).16. K. Kimura, J. Motomatsu, and Y. Tominaga, The Journal of Physical Chemistry C,

120, 12385 (2016).17. C. Li, H. Yue, Q. Wang, J. Li, J. Zhang, H. Dong, Y. Yin, and S. Yang, Solid State

Ionics, 321, 8 (2018).18. Y. Tominaga, K. Yamazaki, and V. Nanthana, J. Electrochem. Soc., 162, A3133

(2015).19. C. Wang, H. Zhang, J. Li, J. Chai, S. Dong, and G. Cui, J. Power Sources, 397, 157

(2018).20. J. An, Y. Ke, X. Cao, Y. Ma, and F. Wang, Polym. Chem., 5, 4245 (2014).21. L. Ma, P. Nath, Z. Tu, M. Tikekar, and L. A. Archer, Chem. Mater., 28, 5147

(2016).22. C. Decker, B. Elzaouk, and D. Decker, J. Macromol. Sci., Part A, 33, 173 (1996).23. S. Rimmer, S. Collins, and P. Sarker, Chem Commun (Camb), 48, 6029 (2005).24. N. Kihara and T. Endo, Die Makromolekulare Chemie, 193, 1481 (1992).25. F. Camara, S. Caillol, and B. Boutevin, Eur. Polym. J., 61, 133 (2014).26. Y. Shi et al., Journal of Materials Chemistry A, 7, 19691 (2019).27. P. J. Alarco, Y. Abu-Lebdeh, A. Abouimrane, and M. Armand, Nat. Mater., 3, 476

(2004).28. M. L. Lehmann et al., Energy Storage Mater., 21, 85 (2019).29. R. Bouchet et al., Nat. Mater., 12, 452 (2013).30. K. D. Fong, J. Self, K. M. Diederichsen, B. M. Wood, B. D. McCloskey, and K.

A. Persson, ACS Cent Sci, 5, 1250 (2019).31. W. Yao et al., Electrochim. Acta, 318, 302 (2019).32. K. M. Diederichsen, E. J. McShane, and B. D. McCloskey, ACS Energy Lett., 2,

2563 (2017).33. Q. Zhao, P. Chen, S. Li, X. Liu, and L. A. Archer, Journal of Materials Chemistry

A, 7, 7823 (2019).34. F. Ding et al., J. Electrochem. Soc., 160, A1894 (2013).35. Q. Zhao, X. Liu, S. Stalin, K. Khan, and L. A. Archer, Nat. Energy, 4, 365 (2019).36. S. H. Lee, J. Y. Hwang, S. J. Park, G. T. Park, and Y. K. Sun, Adv. Funct. Mater.,

29, 1902496 (2019).37. Y.-S. Kim, T.-H. Kim, H. Lee, and H.-K. Song, Energy & Environmental Science,

4, 4038 (2011).

Figure 16. Cycling performance of high area capacity Li/UVPC-sPE/LFPcell at 100 °C. The theoretical capacity of LFP cathode is 2.2 mAh cm−2.

Journal of The Electrochemical Society, 2020 167 100517