Journal of Power Sources 481 (2021) 228832

Available online 11 September 20200378-7753/© 2020 Elsevier B.V. All rights reserved.

Difunctional block copolymer with ion solvating and crosslinking sites as solid polymer electrolyte for lithium batteries

Yubin He, Nian Liu **, Paul A. Kohl *

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• Difunctional block copolymer was syn-thesized as solid polymer electrolyte.

• Comb-shaped second block conductivity (0.6 mS/cm).

• Crosslinked first block enabled dendrite free lithium plating.

• Solid state cell showed stable cycling for 1000 cycles.

A R T I C L E I N F O

Keywords: Lithium batteries Solid polymer electrolytes Block copolymer UV crosslinking

A B S T R A C T

High conductivity, solid polymer electrolytes (sPE) are an enabling technology for safe, high energy-density lithium ion batteries. Unfortunately, polymer architectures with high ion conductivity are usually associated with poor mechanical properties. In this study, a difunctional block copolymer (DFBCP) which addresses the need has been synthesized and demonstrated. The first block, P(DBEA-co-MA), has tethered double bonds and can form a dense, crosslinked network upon UV irradiation to provide mechanical strength. The second block is a pendant type polyethylene glycol (PEG) moiety with low crystallization to provide ion solvation. The PEG moiety is confined on one side of the polymer and has high segmental motion, resulting in an adequate ionic conduc-tivity, 0.6 mS/cm. The amorphous nature of PEG second block also ensures low interfacial resistance, <80 Ω∙cm2, and mechanical adaptability to electrode volumetric changes. The combined advantages of high con-ductivity, low interfacial resistance and good mechanical stability enable full cell durability, >1000 cycles at 2C in a Li–LiFePO4 battery.

1. Introduction

Improved safety and hazard-free operation are particularly impor-tant in future generations of lithium ion batteries (LIB). Compared to organic liquid electrolytes in conventional LIB, solid polymer electro-lytes (sPE) are less flammable than organic solvents because they have a higher ignition temperature. Solid electrolytes are also potentially lower cost because they can be laminated on the solid electrode during manufacturing and do not have to be filled and sealed under vacuum.

There are also claims that sPEs may assist in lithium dendrite preven-tion, however, this is a more speculative benefit [1]. A number of sPE materials have been reported [2]. Although advances have been made, such as improved conductivity ~0.1 mS/cm [3–6], there remains the unsolved problem of obtaining high conductivity and adequate me-chanical and chemical properties.

The reason for the low conductivity of sPE is the sluggish chain mobility originating from either crystallization or chain entanglement. Existing strategies includes developing advanced polymer architectures,

* Corresponding author. ** Corresponding author. ;

E-mail addresses: nian.liu@chbe.gatech.edu (N. Liu), kohl@gatech.edu (P.A. Kohl).

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Journal of Power Sources

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https://doi.org/10.1016/j.jpowsour.2020.228832 Received 1 June 2020; Received in revised form 24 July 2020; Accepted 24 August 2020

Journal of Power Sources 481 (2021) 228832

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such as comb-shaped [7], star-shaped [8], and hyper-branched [9] polymers, to inhibit crystallization or the introduction of plasticizers to enhance ion solvation and segmental motion [10,11]. However, these approaches usually degrade the mechanical strength of the polymer electrolyte, sometime preventing film formation [12]. Consequently, the sPEs cannot be processed in roll-to-roll format [13].

Recently, mechanical reinforcements such as glass fiber [14] and porous cellulose [15] have been explored to prepare sPE composites with improved mechanical properties, but accompanied by a decrease in ion conductivity due to the lower fraction of conductive polymer in the composite. Crosslinking as an effective mechanical reinforcing strategy that can provide covalent bonds to form a thermoset polymer network, enhance the mechanical properties [16,17]. However, current synthetic routes generally use random copolymerization of a crosslinkable monomer and a conductive monomer [18], resulting crosslinking sites in the same polymer segment as the ion solvating moieties (ethylene oxide, carbonate etc.). Immobilization of the ion solvating moieties after crosslinking can increase the glass transition temperature and hinder the conduction of lithium ions [19]. In addition, high crosslink density may also lead to an overly rigid sPE resulting in poor interfacial contact be-tween the sPE and the electrodes [20].

In this report, we demonstrate an integrated sPE for improving the ion conductivity and maintaining the mechanical stability of an sPE. A difunctional block copolymer (DFBCP), with crosslinkable double bonds in the first block and poly ethylene glycol (PEG) pendant structures in its second block, was synthesized by sequential reversible addition− -fragmentation chain transfer (RAFT) polymerization. The double bond undergo a fast crosslinking reaction upon UV radiation to provide me-chanical stability. In the second block, pendant PEG segments decrease the crystallization and facilitate lithium ion solvation and conduction. The pendant PEG moieties have segmental motion leading to a 7-fold higher conductivity when compared with conventional linear PEO- sPE. The lithium ion conductivity was >0.6 mS/cm at room tempera-ture. The low modulus nature of the sPE also enable low interfacial resistance with the electrodes, i.e. <80 Ω∙cm2 for DFBCP-sPE vs. >5500

Ω∙cm2 for PEO-sPE. Li–LiFePO4 cells were successfully cycled for 1000 cycles at 2C and 22 ◦C.

2. Experimental section

Materials: methyl acrylate (MA), 2-hydroxyethyl acrylate (HOEA), poly (ethylene glycol) acrylate (DB-PEG, MW~480), azodiisobutyroni-trile (AIBN), acryloyl chloride (ACl), 2-(dodecylthiocarbonothioylthio)- 2-methylpropionic acid (DDMAT), poly ethylene glycol oligomer (PEG250) plasticizer, succinitrile (SN), bis (trifluoromethane) sulfoni-mide lithium salt (LiTFSI), lithium bis(oxalate)borate (LiBOB), 4-fluoro- 1,3-dioxolan-2-one (FEC), phenylbis (2,4,6-trimethylbenzoyl)phosphine oxide (PPO), poly (ethylene oxide) (PEO, M.W. 600,000), N,N- dimethylformamide (DMF), dichloromethane (DCM), Acetonitrile (CH3CN), tetrahydrofuran (THF), dimethyl ether, potassium carbonate (K2CO3) were purchased from Sigma-Aldrich and used as received. Lithium iron phosphate (LiFePO4, LFP) and Super P powders were purchased from MTI Corporation. Li foil with a thickness of 750 μm was purchased from Alfa Aesar.

Synthesis of the first block by Reversible Addition-Fragmentation Chain Transfer Polymerization (P(HOEA-co-MA)): As shown in Fig. 1A and Fig. S1, MA (5 g, 58.1 mmol), HOEA (6 g, 51.7 mmol), DDMAT (chain transfer agent, 345 mg, 0.95 mmol) and AIBN, (15.5 mg, 0.094 mmol) were dissolved in 20 mL DMF. N2 gas was bubbled through the mixture for 30 min to remove dissolved air. The polymerization was carried out at 70 ◦C for 48 h under N2 atmosphere. The reaction mixture was cooled to − 80 ◦C to stop the polymerization followed by exposure to air at room temperature. The polymer product of P(HOEA-co-MA) was purified by precipitating in dimethyl ether and dried at 60 ◦C for 24 h. The reaction yield and molecular weight was calculated from the 1H NMR (Figs. S1 and S2).

Introducing the second block by RAFT polymerization (P(HOEA-co-MA)- b-PEG): As shown in Fig. 1C and Fig. S3, P(HOEA-co-MA) (11 g), DB-PEG (9 g, 18.8 mmol) and AIBN (15.5 mg, 0.094 mmol) were dissolved in 40 mL DMF. N2 gas was bubbled through the mixture for 30 min to remove

Fig. 1. Synthetic procedures and 1H NMR spectra. A. RAFT polymerization of MA and HOEA to synthesize the P(HOEA-co-MA) first block of DFBCP. DDMAT is the chain transfer agent and AIBN is the free radical initiator. Moieties –R and –Z are the residual of chain transfer agent at the end of the polymer chains. B. 1H NMR spectrum of P(HOEA-co-MA) first block. C. RAFT polymerization of P(HOEA-co-MA)-b-PEG. DB-PEG is the monomer and the P(HOEA-co-MA) is the macro chain transfer agent. D. 1H NMR spectrum of P(HOEA-co-MA)-b-PEG. E. Synthesis of DFBCP by converting the –OH group in P(HOEA-co-MA)-b-PEG to double bonds. F. 1H NMR spectrum of DFBCP.

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dissolved air. The polymerization was carried out at 70 ◦C for 48 h under N2 atmosphere. The reaction mixture was cooled to − 80 ◦C to stop the polymerization followed by exposure to air at room temperature. The polymer product of P(HOEA-co-MA)-b-PEG was purified by precipi-tating into dimethyl ether and dried at 60 ◦C for 24 h. The reaction yield and molecular weight was calculated from the 1H NMR (Figs. S3 and S4).

Introducing double bonds for crosslinking (Difunctional block copolymer, DFBCP): As shown in Fig. 1E, P(HOEA-co-MA)-b-PEG (5 g) and K2CO3 (4.25 g, 30.8 mmol) were added to 50 mL DCM/THF mixture (2:1 in volume) and cooled to 0 ◦C. ACl (2.77 g, 30.6 mmol) was dissolved in 10 mL DCM and added dropwise to the reaction mixture at 0 ◦C. The re-action mixture was then stirred at 22 ◦C for 36 h. The K2CO3 was removed by filtration and the solvent was removed by vacuum evapo-ration. The polymer product of DFBCP was purified by precipitating into dimethyl ether then dried at room temperature for 48 h.

UV crosslinking: DFBCP (0.6 g), PEG250 plasticizer (1 g, 4 mmol), and LITFSI (0.6 g, 2.09 mmol) was dissolved in THF. After forming a ho-mogeneous solution, the THF was removed under vacuum using a rotor evaporator. PPO (photoinitiator, 0.1 wt %) was then added to the ob-tained liquid precursor. FEC (5 wt%) was added to the battery electro-lyte to help form a stable SEI [21]. The precursor solution was cast onto a glass plate (Fig. S9). Another glass plate was placed on top of the solu-tion to flatten the membrane surface. Adhesive tape was used as spacers to control the thickness of prepared membranes. The sample was exposed to 365 nm UV radiation for 10 min (20 mW/cm2, Black-Ray B-100 Series, Analytik Jena). The resulting DFBCP-sPE was peeled off the substrate and stored in the glove box. The thickness of DFBCP-sPE was 340–360 μm.

Synthesis of the comb-shaped PEO by RAFT polymerization (Comb-PEO): As shown in Fig. S5, DB-PEG (10g), DDMAT (112 mg) and AIBN (10.1 mg) were dissolved in 20 mL DMF. N2 gas was bubbled through the mixture for 30 min to remove dissolved air. The polymerization was carried out at 70 ◦C for 48 h under N2 atmosphere. The reaction mixture was cooled to − 80 ◦C to stop the polymerization followed by exposure to air at room temperature. The polymer product of comb-PEO was puri-fied by precipitating into dimethyl ether and dried at 60 ◦C for 24 h. The reaction yield and molecular weight was calculated from the 1H NMR (Figs. S5 and S6).

Fabrication of conventional linear polyethylene oxide solid polymer electrolyte (PEO-sPE): PEO (1g) and LiTFSI (0.41 g) were dissolved in CH3CN (16 mL) and stirred overnight. The resulting viscous solution was then cast onto a PTFE plate and dried at 22 ◦C for 24 h. The PEO-sPE film was then peeled off and transferred to glove box and dried at 60 ◦C for another 12 h. The PEO-sPE was stored in glove box before use.

Material characterization: 1H NMR measurements were performed on a Bruker Avance 400 MHz NMR instrument. The molecular weight of DFBCP were determined by gel permeation chromatography (GPC, Shimadzu) equipped with an LC-20 CE HPLC pump and a refractive index detector (RID-20 A, 120 V). Measurements were performed with THF as the eluent and polystyrene as standard. Electrochemical impedance spectroscopy (EIS) measurements were performed at fre-quencies from 1 MHz to 1 Hz on a Bio-logic SAS. The ionic conductivity (σt) was calculated from the measured values of membrane ionic resis-tance (Rt, ohm), area (S, cm2) and thickness (L, cm), according to Eq. (1).

σt = L/(Rt⋅S) (1)

The Li+ transference number (tLi+) was measured by the potentio-static polarization method [22]. One symmetric cell with the configu-ration Li/DFBCP-sPE/Li were fabricated. A constant polarization of 5 mV (ΔV) was applied on the working electrode. As polarization time increased, the current decreased from initial state current (I0) to steady-state current (Iss). Under small amount of polarization (ΔV), the initial current (I0) and steady state current (Iss) are as follows

I0 =ΔV

R0 + Rbulk(2)

Iss =ΔV

Rss + tLi+∗Rbulk(3)

Rbulk is the initial bulk resistance, R0 is the initial interfacial resis-tance, and Rss is the steady-state interfacial resistance.

From Eqs. (2) and (3), Eq. (4) can be obtained, where the Li+

transference number.

t+ =Iss (ΔV – I0R0)

I0 (ΔV – ISSRSS)(4)

Battery assembly and testing: The composition of the LFP slurry was LiFePO4 (240 mg), PEO (80 mg), Super P (48 mg), LiTFSI (32 mg), and CH3CN (16 mL). The LFP cathode was prepared by casting the slurry onto carbon paper (Fuel Cell Store Co., Ltd) followed by drying at 80 ◦C for 12 h. The mass loading of LiFePO4 was ~1.2 mg/cm2. All cells (2032 type) were assembled in an Ar-filled glove box with water content below 0.1 ppm and oxygen content below 1 ppm. Batteries were cycled using an 8-channel battery tester (LANHE).

3. Result and discussion

Synthesis of Difunctional Block Copolymer: A diblock copolymer with a crosslinkable first block and pendant PEG second block was synthesized, as shown in Fig. 1A and C, via sequential RAFT polymerization reaction. In the first block P(HOEA-co-MA), Fig. 1A, an acrylate monomer with hydroxyl pendant (HOEA) was used as the precursor for forming crosslinking sites. The role of the MA co-monomer was to act as a spacer to adjust the crosslink density and decrease steric hindrance. In 1H NMR spectrum shown in Fig. 1B, Signal 7 is assigned to the residual of chain transfer agent at end of polymer chain, signal 4 is assigned to the –CH3 of MA segments, and signal 1, 2, 3 are assigned to the –CH2-CH2-OH group of HOEA segments. The molecular weight of MA segment was calculated from the integral area ratio between signal 4 (A4) and signal 7 (A7), i.e. MWMA = 86 g/mol × (A4/3)/(A7/3) = 5.3 kDa. The molecular weight of the HOEA segment was calculated by the integral area ratio between signal 3 (A3) and signal 7 (A7), i.e. MWHOEA = 116 × (A3/2)/(A7/3) =6.0 kDa. The total molecular weight of the P(HOEA-co-MA) first block is 11.3 kDa.

In the second step, the block formed in Fig. 1A was used as a macro- chain transfer agent for introducing the second block, poly ethylene glycol (PEG) monomer. PEG is used as a pendant solvating agent for lithium ions. Having PEG pendant on the polymer chain gives it higher chain segment mobility and lowers its tendency to crystalize, compared to linear PEO [19]. The P(HOEA-co-MA)-b-PEG, Fig. 1C, was obtained in high reaction yield, >99% (Table S1, Fig. S3). As shown in the 1H NMR spectrum for P(HOEA-co-MA)-b-PEG (Fig. 1D), signal 7 is assigned to the PEG protons pendant on the second block. Signal 1, 2, 3 and 4 are assigned the protons on the first block. Since signals 3, 4 and 7 overlap in the range of 3.4–3.7 ppm, A7 was calculated by A3,4,7-A3-A4. As shown in Fig. 1B, A3 = 2 × A1, and A4 = 3.56 × A1. Thus A7 = A3,4,7–2 × A1-3.56 × A1. The molecular weight of the second block was calculated based on the integral area ratio between signal 1 (A1) and signal 7 (A7) (i.e., MWPEG = 6.0 kDa × ((A7/36)/(A1/1))/ × (480/116) = 8.0 kDa) The total molecular weight of the P(HOEA-co-MA)-b-PEG was 19.3 kDa. Note that calculating A7 by A7 = A3,4,7 - A3- A4 has a degree of uncer-tainty because the NMR signals tend to widen with interference, but this approach still gives an approximate molecular weight and is helpful in this study.

In the third step, the hydroxyl termination was converted to a ter-minal double bond via electrophilic substitution of acryloyl chloride (ACl, Fig. 1E). As shown in Fig. 1F, the NMR signal for the terminal –OH group were absent and a new peak assigned to the double bond was present between 5.9 and 6.4 ppm, showing the successful conversion of

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the hydroxyl to unsaturated crosslinking sites. Signal 2 and signal 3 are assigned to the –CH2-CH2- group of HOEA segments. The conversion yield from the hydroxyl terminated precursor to double bond termina-tion was estimated by the integral area between signal 9 and signal 2 to be 46% (yield = A9/(A2,3/4) = 46%).

The molecular weight of DFBCP was also measured using GPC (Fig. S10). The number average molecular weight (Mn) was 15.1 kDa and the weight average molecular weight (Mw) was 19.6 kDa. NMR was used to obtain the absolute molecular weight, and GPC gave the relative molecular weight because the same internal standard (polystyrene) was used [23]. The DFBCP in this study has a comb-shaped second block. Thus, its hydrodynamic volume can be different from the linear poly-styrene calibrant. As a result, Mn measured by GPC is lower than that measured by NMR.

DFBCP-sPE Fabrication and Electrochemical Properties: The solid polymer electrolyte was synthesized using DFBCP as the polymer ma-trix, LiTFSI as charge carrier, PEG oligomer as an optional plasticizer, and PPO as the photoinitiator (Fig. 2A). A low concentration of photo-initiator (0.1 wt %) was used in order to avoid any side reactions be-tween electrode and photoinitiator residual. The UV wavelength was chosen to be 365 nm according to the excitation wavelength of PPO

initiator. The reported conversion rate of the acrylate double bond was >95% after 20 s UV exposure [24]. In this study, the irritation time was 10 min which ensured a high degree of crosslinking. The photoinitiator rapidly produces free radicals upon radiation which led to polymeriza-tion of acrylate double bond [25]. The polymerized acrylate group then bonded the DFBCP polymer chains and formed a crosslinked network. After UV exposure, the liquid precursor was converted to solid, free standing films (Fig. S7D). This also shows that a crosslinked network was formed. In previous crosslinked sPEs, the ion solvating groups and crosslinking sites were generally in one same chemical structures, which resulted high glass transition temperature and low conductivity [19,26, 27]. In this study, the pendant PEG moiety was isolated from the double bond via the diblock polymer architecture. The segmental motion of ion solvating groups was not confined by the crosslinking network, thus there was little or no decrease in conductivity due to crosslinking.

The ionic conductivity of as-prepared DFBCP-sPE was compared to conventional PEO-sPEs. The pendant type PEG chains have shorter -[–CH2–CH2-O-]- units, low crystallization, and little chain entangle-ment [19]. In addition, isolating the PEG from the crosslinking sites also ensures high mobility and segmental motion. These combined advan-tages led to a 7-fold higher conductivity compared with PEO-sPE when no plasticizer was introduced (4.4∙10− 6 S/cm vs 5.7∙10− 7 S/cm, Fig. 2B). Afterwards, a PEG250 oligomer was introduced as the plasti-cizer. The PEG250 contained 5 or 6 -[–CH2–CH2-O-]- units. This led to a low degree of crystallization and improved the conductivity of the sPE (0.6 mS/cm) while still maintaining good mechanical stability due to the crosslinked first block (Fig. S7). Note that the PEG250 plasticizer had a high boiling point above 260 ◦C, and TGA curves of DFBCP-sPE in Fig. S8 shows a weight loss <10% below 100 ◦C.

The lithium ion transference number (tLi+) is an important parameter for lithium migration in battery applications which may be influenced by the pendant-type ion solvating moieties due to high chain segment motion. For solvent/solute electrolytes, cations and anions both contribute to the total ionic current. The tLi+ reflects the ratio between Li+ transport and the total ion transport [28]. As shown in Fig. 2C, the tLi+ was measured by the potentiostatic polarization method using a Li/DFBCP-sPE/Li symmetric cell [22]. The initial current was 4.5 μA (I0)

Fig. 2. A. Reaction scheme showing the UV crosslinking of a liquid precursor containing DFBCP, PEG250 oligomer and LiTFSI for the fabrication of DFBCP-sPE. B. Ionic conductivity of PEO/LiTFSI (EO:Li = 16), DFBCP/LiTFSI (EO:Li = 10) and DFBCP/LiTFSI/PEG250 (Mass ratio = 3:5:3) at room temperature. C. Transference number of DFBCP-sPE. I0 and R0 are the initial current and interfacial resistance at open circuit. Iss and Rss are the steady-state current and interfacial resistance after polarization. The effective area of sPE is 0.785 cm2.

Fig. 3. Cyclic voltammetry (CV) profile of DFBCP-sPE. The effective area of sPE is 0.785 cm2.

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and after constant potential (5 mV) for 7 h, the anodic, steady state current was 3.2 μA (Iss). The electrochemical impedance spectroscopy (EIS) plot before and after polarization was recorded and the change of interfacial resistance was negligible (R0 = 1041 Ω and Rss = 1025 Ω). The tLi+ was calculated to be 0.35 (see Eq. (2)). The transference number of conventional PEO-sPE is < 0.2, which has been attributed to crys-tallization of the PEO chains causing lithium ion solvation problems [29]. The higher transference number of DFBCP-sPE further demon-strates the critical role of segmental motion on lithium ion conduction.

The electrochemical stability of DFBCP-sPE was characterized by cyclic voltammetry (CV) using stainless steel as the working electrode and lithium metal as the reference electrode. The Li/DFBCP-sPE/SS cell was scanned from OCV to 4.5 V, then back to − 0.5 V at a scanning rate of 1 mV/s. As shown in Fig. 3, the peak oxidation current for lithium deposition was 0.26 mA at 0.17 V. The oxidation of sPE occurred at voltage >4 V, e.g. the oxidation current was 10 μA at 4.5 V, 1.8 μA at 4 V and 1.6 μA at 3.2 V. This result suggests a stability window ~4 V vs Li+/ Li, which is similar to conventional PEO-sPE [30,31].

In addition to ionic conductivity, interfacial resistance is an impor-tant factor in battery performance. Previous PEO-sPE studies have shown high interfacial resistance, 5500 Ω∙cm2 (Fig. 4A), because of its high degree of crystallization and resulting poor wettability to lithium metal [32]. In contrast, the DFBCP-sPE had lower interfacial resistance, 100 Ω∙cm2 (Fig. 4B), owning to the amorphous nature of the comb-shaped second block [19]. Fig. 4C shows the evolution of bulk resistance (Rbulk) and interfacial resistance (Rint) when cycling at 0.2 mA/cm2 and 22 ◦C. During the first 12 h, the increase in Rbulk and Rint was likely due to the formation of a SEI layer on the electrode, as pre-viously observed [33]. It should be noted that most of the double bonds were converted to inert hydrocarbon groups after UV exposure [24], thus PEG and LiTFSI are the main SEI forming compounds to generate lithium alkoxide species (-ROLi) and LiF, respectively [34]. After 24 h, low interfacial resistance <80 Ω∙cm2 was observed. The slight decrease in Rint could be due to better physical contact between sPE and lithium metal. The Rbulk slightly increased with cycling time, which could be explained by the side reaction between sPE and Li metal.

In a previous study, it was shown that crosslinked sPEs generally have higher interfacial resistance than non-crosslinked ones due to increased surface rigidity [20]. The smaller interfacial resistance of DFBCP-sPE is attributed to the amorphous non-crosslinked second block. As demonstration, a comb-shaped PEO, i.e., the second block of DFBCP, was synthesized by RAFT polymerization (Figs. S5 and S6). Despite a molecular weight (33.0 kDa) higher than DFBCP (Table S1), this polymer showed a semi-solid nature due to absence of crystalliza-tion and lower chain entanglement (Fig. S7). The low modulus nature provides good interfacial contact with the electrodes.

The long term durability of DFBCP-sPE and PEO-sPE in a Li–Li

symmetric cell was evaluated at 0.2 mA/cm2 and 1 h per cycle (Fig. 5). At room temperature, the conventional PEO-sPE showed a high initial overpotential of 460 mV when charged at 0.1 mA/cm2 due to its low conductivity and high interfacial resistance. The overpotential rapidly increased to 5 V within minutes, which is likely due to its poor me-chanical adaptability to the electrode volume changes, especially at the lithium metal anode resulting from the high crystallization of PEO. Loss of intimate electrode/electrolyte contact results in a smaller effective anode area. As a result, the conventional PEO-sPE had to be cycled at 50 ◦C due to its crystalline nature at room temperature. An electrical short circuit occurred at the 350 h point suggesting that its poor me-chanical stability allowed dendrites to form and cause the cell to fail. On the other hand, the DFBCP-sPE had good durability with more than 700 h cycling at 22 ◦C. At the end of 700 h, the cell was still operational. The crosslinked first block is the main reason for the improved cycling sta-bility by preventing lithium dendrites from forming. The SEI morphology and its effect on the interfacial resistance and cycling sta-bility may be the focus of future studies.

In addition to the benefits described above, the DFBCP-sPE was also able mechanically deform so as to accommodate electrode volume changes [35,36]. As shown in Fig. 6, a Li/DFBCP-sPE/Li battery was charged to 3.2 mAh/cm2, i.e., 15.4 μm of lithium. During the initial 8 h, there was a slightly increase in the bulk and interfacial resistance, possibly due to SEI formation or change in surface area [37]. After the initial 8 h, steady-state performance was achieved as shown by the overlapping EIS curves and constant overpotential. This mechanical adaptability is due to the soft nature of second block.

Performance of Solid State Batteries: Li/DFBCP-sPE/LiFePO4 solid state batteries were fabricated to investigate the conductivity, interfacial resistance and electrochemical stability of DFBCP-sPE. The capacity at different current densities was measured, as shown in Fig. 7A and B. The

Fig. 4. (A) Electrochemical Impedance Spectroscopy (EIS) of Li/PEO-sPE/Li cell at 22 ◦C. (B) EIS of Li/DFBCP-sPE/Li cell at 22 ◦C. (C) Evolution of bulk resistance (Rbulk) and interfacial resistance (Rint) of Li/DFBCP-sPE/Li cell when cycled at 0.2 mA/cm2, 1hr per cycle and 22 ◦C.

Fig. 5. Long term cycling stability of Li/DFBCP-sPE/Li cell (0.2 mA/cm2, 1 h per cycle, and 22 ◦C) and Li/PEO-sPE/Li cell (0.2 mA/cm2, 1 h/cycle, and 50 ◦C). Inset: Voltage-time curve of Li/PEO-sPE/Li cell when charged at 0.1 mA/cm2 and 22 ◦C.

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capacity was 125 mAh/g and overpotential was modest at 120 mV at 1C and 22 ◦C. At 5C, the DFBCP-SSB capacity was 74 mAh/g. For com-parison, the PEO-SSB tested at 50 ◦C lost 85% of the initial capacity when the current density increased to 2C. These results highlight the significance of the ion solvating comonomer on the sPE performance.

Fig. 8A shows the cycling performance of DFBCP-SSB at 2C. After 1000 cycles, the capacity decreased from 101.4 mAh/g to 66.6 mAh/g, i. e. ~0.034% decay per cycle, which is only slightly less than the capacity

fade of current lithium ion batteries. The faster capacity fade during the initial cycles was likely due to formation of an SEI passivation layer on the electrode. When cycling at higher current density was 4C, the cell still had reasonable capacity (>70 mAh/g) after 200 cycles (Fig. 8B). The EIS results recorded during cycling at 4C (shown in Fig. S11) shows no obvious change in bulk or interfacial resistance after the initial cycles.

The long term cycling performance of the Li/PEO-sPE/LFP battery at

Fig. 6. Voltage-time profile when charging a Li/DFBCP-sPE/Li cell at 0.2 mA/cm2 for 16 h. Insets: Evolution of impedance at different stages during charging.

Fig. 7. A. Charge-discharge curves of Li/DFBCP-sPE/LFP cell at different current densities. B. Rate performance of Li/DFBCP-sPE/LFP cell at 22 ◦C and Li/PEO-sPE/ LFP cell at 50 ◦C.

Fig. 8. Long term cycling performance of Li/DFBCP-sPE/LFP at 2C (A) and 4C (B).

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50 ◦C and 1C is shown in Fig. S12. The cell had an initial capacity of 58.4 mAh/g at 50 ◦C and 1C. After 250 cycles, the capacity decreased to 49.6 mAh/g. The fluctuation of coulombic efficiency may suggest formation of unrecoverable lithium metal. On the other hand, the Li/DFBCP-sPE/ LFP cell had stable coulombic efficiency (Fig. 8A), suggesting more controlled lithium deposition. The enhanced mechanical stability pro-vided by the crosslinked first block may be the reason for this better controlled growth. Future studies may investigate and optimize the SEI morphology/composition for improved long term cycling stability.

4. Summary

A difunctional block copolymer with isolated crosslinking sites and ion solvating moieties was developed. The second block was a pendant PEG architecture with low crystallization and chain entanglement to help improve lithium ion solvation. The sPE had a 7-fold higher con-ductivity and 55-fold lower interfacial resistance than conventional PEO-sPE. The double bonds in first block were used to form a crosslinked network enabling stable lithium stripping/plating for >700 h as well as good durability >1000 cycles in a Li–LiFePO4 solid state battery.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge funding from Kolon Industries. The authors also gratefully acknowledge the battery assemblies and testing by Haochen Yang, cathodes prepared by Zachary Althouse, and TGA measurements by Youn Ji Min.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2020.228832.

Credit author statement

The authors have no conflict of interest in terms of financial or other issues. They are the sole owners of the content.

The experimental work was done by the first author. Ideas and analysis were provided by the second and third author.

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