Single-Ion Polymer Electrolyte Membranes Enable LithiumIon Batteries with a Broad Operating Temperature Range Weiwei Cai,[a] Yunfeng Zhang,[a, b] Jing Li,[a] Yubao Sun,[b] and Hansong Cheng*[a, b] Conductive processes involving lithium ions are analyzed in detail from a mechanistic perspective, and demonstrate that single ion polymeric electrolyte (SIPE) membranes can be used in lithium-ion batteries with a wide operating temperature range (25–80 8C) through systematic optimization of electrodes and electrode/electrolyte interfaces, in sharp contrast to other batteries equipped with SIPE membranes that display appreciable operability only at elevated temperatures (> 60 8C). The performance is comparable to that of batteries using liquid electrolyte of inorganic salt, and the batteries exhibit excellent cycle life and rate performance. This significant widening of battery operation temperatures coupled with the inherent flexibility and robustness of the SIPE membranes makes it possible to develop thin and flexible Li-ion batteries for a broad range of applications.

A long standing issue of lithium-ion batteries is device safety. Solid polymer electrolytes offer a unique advantage over conventional liquid electrolytes of inorganic lithium salts such as low combustibility, no leakage of liquid, and high longevity.[1] However, these electrolytes have consistently displayed much lower lithium-ion conductivities than conventional liquid electrolytes. Recent developments in single-ion conductive polymer electrolytes (SIPEs) have demonstrated that the lithiumion conductivity of SIPE membranes can be substantially improved to the order of 104 S cm1, comparable to that of liquid electrolytes.[2] SIPEs possess the unique advantage of a large lithium-ion transference number, close to unity, and high thermal and electrochemical stability,[3] in addition to the common advantages of polymer electrolytes and thus could enable safer, lighter, and more-flexible lithium-ion batteries to be developed for broader applications. Unfortunately, to date, none of the reported SIPE-equipped lithium-ion batteries has been able to be operative at an ambient temperature, and meaningful battery performance has been demonstrated only at an elevated temperature.[2e, 4] Obviously, the strong dependency on high temperatures severely limits the broad applicabil-

ity of SIPE-based lithium-ion batteries, particularly for portable electronics and electric vehicles,[5] which commonly work at near ambient temperatures. A fundamental issue is whether the high temperature dependency is an intrinsic property of SIPE-based lithium-ion batteries. In this Communication, we attempt to address this issue from a mechanistic perspective by investigating the microscopic ion transfer processes in operating lithium-ion batteries. For the first time, lithium-ion batteries equipped with SIPE membranes are demonstrated to exhibit excellent cycle-life and rate performance at both near-ambient and elevated temperatures via appropriate optimization of the cathodic microscopic structure. The range of operating temperature of these batteries is far broader than that of lithium-ion batteries with conventional liquid electrolytes. An important added potential benefit of the polymeric membranes is to enable thin, flexible, and inexpensive battery cells. Thus, the SIPE-based batteries offer a much wider field of applications with better safety and mechanical flexibility. In Table 1, the experimentally measured conductivity and transference number (tLi-ion) of a commercial liquid lithium-ion electrolyte (CELi, Celgard 2730 saturated with 1.0 mol L1 LiPF6

Table 1. Transference numbers and lithium-ion conductivities of DADS(Li+), Nafion NR 211 (Li + ), and liquid electrolyte (CeLi) at ambient temperature. Sample

Li + conductivity [S cm1]

Li + transference number tLi-ion

CELi[6] DADS(Li+) Nafion(Li+)

2.1  104 3.96  104 2.05  104

0.270 0.901 0.89

EC/DEC solution), a home-made SIPE [aromatic bis(benzene sulphonyl)imide-based lithium polyamide), abbreviated to DADS with molecule structure shown in Scheme 1], and a commercial SIPE (lithium-ion exchanged Nafion NR 211 membrane) are compared. The tLi-ion values of the SIPEs are more than two

[a] Dr. W. Cai, Dr. Y. Zhang, Dr. J. Li, Dr. H. Cheng Department of Chemistry National University of Singapore 3 Science Drive 3, 117543 (Singapore) E-mail: [email protected] [b] Dr. Y. Zhang, Dr. Y. Sun, Dr. H. Cheng Sustainable Energy Laboratory China University of Geosciences Wuhan 388 Lumo RD, Wuhan, 430074 (PR China) Supporting Information for this article is available on the WWW under

 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Scheme 1. Schematic structures of DADS(Li+) and the lithium sulphonamide monomer lithium-ion conductor (SMC).

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times greater than that of the commercial liquid electrolyte, leading to lithium-ion conductivities of the SIPE membranes, surprisingly, close to or even higher than the value of CELi,[6] which is much lower than that of the liquid electrolyte itself[7] due to the insulating nature of the Celgard 2730 separator. Although a definitive mechanism for lithium-ion conduction in batteries has remained a subject of debate,[8] it has become consensus that lithium-ion transport in electrolyte in a battery is mainly composed of concentration diffusion and electromigration under an electric field.[8] A modified Nernst–Planck equation[9] can be used to describe lithium-ion conduction in an electrolyte: dC F dV JLiþ ¼ DLiþ ð Liþ Þ  DLiþ CLiþ tLiþ ð Þð Þ RT dx dx


where ion diffusion is determined by the concentration gradient (dCLi + /dx) and the migration, described by the second term, is driven by the electric force gradient (dV/dx). F, R, and T represent the Faraday constant, gas constant, and operating temperature, respectively. Because (dCLi + /dx) and (dV/dx) are determined by the charging/discharging status of the electrodes, the lithium-ion conductivity is largely dictated by the lithium-ion transference number (tLi + ) as well as the lithium-ion diffusivity (DLi + ), which is higher in the liquid electrolyte than in SIPE. Lithium-ion diffusion inside the condensed solid[8] is significantly slower than in liquid electrolyte[10] with a diffusion coefficient approximately 5 orders of magnitude lower, and thus the lithium-ion conductivity is primarily contributed by lithium-ion migration under an electric field. The high diffusivity in liquid electrolyte is offset by the large ion transference number in SIPE, resulting in comparable lithium-ion conductivities of liquid electrolyte and SIPE as seen in Table 1. Given the remarkably high lithium-ion conductivities of the SIPE membranes, it is apparent that the observed poor performance of SIPE-based batteries at near-ambient temperature is primarily attributable to the substantial interfacial resistance between the electrodes and the SIPE membrane. The cathode of a conventionally fabricated SIPE-based lithium-ion battery can be divided into two layers: the bulk layer (BUL) and the interfacial layer (ITL) where the cathode is embedded in the SIPE membrane, as schematically illustrated in Figure 1 a. Because the triple-phase contact (TPC) boundaries[11] formed by electronic conductor, lithium-ion conductor, and active material, are the active sites for electrochemical reactions (Figure 1 b), the cathodic overpotentional (h), also considered as the cathode/SIPE interfacial resistance, of a conventionally fabricated SIPE lithium-ion battery can then be determined by the Butler– Volmer expression:[12] h¼

RT i C 1 Þ lnð ref ac F i0 CTPC rITL

TPC concentration in ITL with the TPC concentration in the cathode of a liquid electrolyte lithium-ion battery as the reference (Cref). Because no TPC boundary is built in the BUL of the conventionally fabricated SIPE lithium-ion battery (Figure 2 b), a new parameter rITL, which is defined as the volume fraction of ITL in the cathode, is used in Eq. (2). Obviously, the most efficient way to reduce the interfacial resistance (h) in SIPE-based batteries is to increase the rITL value. For this purpose, both


where ac is the cathodic transfer coefficient, and i and i0 stand for the operation current of the battery and the exchange current density of the cathodic reaction. Here, CTPC represents the  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 1. Schematic diagrams of (a) the electrode/electrolyte interface of the SIPE lithium battery, and (b) TPC boundary structure in BUL and ITL.

Figure 2. (a) Nyquist plots (with corresponding equivalent circuit shown in the inset), and (b) galvanostatic charge–discharge curves of batteries Fe1 and Fe-2 at 25 8C.

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CHEMSUSCHEM COMMUNICATIONS a polymeric conductor (DADS) and a lithium sulphonamide monomer lithium-ion conductor (SMC) with structures shown in Scheme 1 were utilized as the supporting electrolyte in the cathode to introduce a lithium-ion conductor to the carbon/ active material boundaries in BUL (Figure 2 b). The volume fraction of ITL is therefore increased to 1, leading to a significantly reduced h value. To further confirm the effectiveness of the supporting electrolyte, we devised several SIPE-based lithium-ion batteries using LiFePO4 as the cathode material. Three DADS-based batteries, denoted as Fe-0, Fe-1, and Fe-2, were assembled with a cathode without a supporting electrolyte, with a DADS-modified cathode and with a SMC-modified cathode, respectively, for electrochemical impedance spectroscopic (EIS) measurements (Figure 2 a) and battery tests (Figure 2 b). With the equivalent circuit[13] shown in the inset of Figure 2 a, the real components of the two semicircles correlate with the interfacial contact resistance (Ri, high frequency) and charge transfer resistance (Rct, middle frequency), respectively. Clearly, both Ri and Rct of the batteries can be significantly reduced upon adding 5 wt % supporting electrolytes into the cathode. In particular, the effect on the SMC-modified cathode is much more pronounced than on the DADS-modified cathode due to the higher lithium-ion exchange capacity of SMC and more uniform porous structure (Figure S1, Supporting Information) of the electrode. As implied in Scheme 1, the lithium-ion exchange capacity of SMC is 7.441 mmol g1, more than 4 times that of DADS (1.657 mmol g1). The higher lithium-ion exchange capacity of SMC enables the supporting electrolyte to be more effective than DADS in enhancing the fraction of ITL, which is a key factor to reduce the interfacial resistance in the battery as shown in Eq. (2). Moreover, unlike the DADS–CLiFePO4 composite, which has a high molecular weight (ca. 80 000 g mol1) and a bulky structure, the SMC–C-LiFePO4 composite is less likely to aggregate upon solvent removal for cathode fabrication. As a consequence, the SMC-modified cathode displays a much lower resistance in the battery cells. Indeed, the Ri value of the battery decreases by almost 90 % simply with the addition of 5 wt % SMC. More importantly, the battery becomes fully functional at room temperature. In particular, by introducing the SMC to the cathode, the charge and discharge capacity densities of Fe-2 are increased to 147 mAh g1 and 137 mAh g1, respectively, with comparable performance to that of the conventional lithium-ion batteries with the liquid electrolyte. In comparison to the DADS-modified cathode, the battery Fe-1 can also be operative at room temperature but the capacities (charge 128 mAh g1 and discharge 108 mAh g1) are considerably lower than the theoretical value of LiFePO4 (170 mAh g1). Moreover, the Coulombic efficiency of 93.2 % for the battery Fe-2 is also significantly higher than the value of 84.4 % for the battery Fe-1. For performance comparison, we further tested two commercial electrolytes: a liquid CELi and a solid lithium-ion-exchanged Nafion membrane in two LiFePO4 batteries. The galvanostatic charge/discharge behavior at 0.1 C is depicted in Figure 3 a. Remarkably, the performance of the DADS-based battery is almost the same as that of the liquid electrolyte  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 3. (a) Galvanostatic charge–discharge curve of SIPE (DADS and Nafion NR 211)-based and liquid electrolyte (CELi)-based lithium-ion batteries at 0.1 C at 25 8C. (b) Cycle life of the DADS-based battery from 0.1 C to 1 C and Nafion and CELi-based batteries at 0.1 C at 25 8C.

based battery with the same SMC-modified cathode at room temperature. Both the capacity and the voltage in the charge– discharge processes of the DADS-based battery are very close to those of the CELi battery with slightly lower Coulombic efficiency (93.2 % for DADS vs. 96.2 % for CELi). The performance of the Nafion-based lithium-ion battery also displays roomtemperature operability of the SIPE-based batteries but with a significantly lower Coulombic efficiency of 88 % and a lower discharge capacity of 123.6 mAh g1. The lower operating voltage and the Coulombic efficiency of the Nafion-based battery, compared to the DADS-based battery, are mainly caused by the lower lithium-ion conductivity (Table 1) and nonporous microscopic structure of the Nafion membrane. We further investigated the cycle life of the SIPE-based batteries with the commercial Nafion NR 211 membrane and the home-made DADS electrolyte membranes. The results are displayed in Figure 3 b. Both batteries exhibit excellent cycling performance at room temperature with the optimized LiFePO4 cathode. The discharge capacity of the DADS-based battery maintains at ca. 140 mAh g1 at 0.1 C. More remarkably, the battery displays excellent rate performance with slight drop of the capacity as the discharging rate increases from 0.1 C to ChemSusChem 2014, 7, 1063 – 1067


CHEMSUSCHEM COMMUNICATIONS 1 C. The battery maintains a 1 C reversible capacity of 105 mAh g1 after 50 charging/discharging cycles. The cycling curve of CELi-based lithium-ion battery, which almost overlaps with that of the DADS-based battery at the same discharge rate as shown in Figure 3 b, confirms the excellent performance of the DADS-based lithium-ion battery at room temperature. A high-temperature battery test at 80 8C was also carried out on the DADS-based battery for cycle-life performance, with the results shown in Figure 4 a. As expected, the temperature increase gives rise to both higher discharge capacity and Coulombic efficiency. The 0.1 C Coulombic efficiency at 80 8C is sistance at room temperature (Figure S3, Supporting Information). The power holding capacity of the DADS based battery at different temperatures, demonstrated by the discharge capacity as a function of discharge rate, is shown in Figure 4 b. Remarkably, the battery retains about 75 % capacity at 0.5 C at room temperature and the capacity retention reaches as high as 86 % at the same discharging rate at 80 8C. Because of the high thermal stability of the SIPE membranes,[2b, f] even better battery performance at higher temperatures is envisaged. In summary, we report a prototype single-ion polymer electrolyte lithium-ion batteries that operates at a wide temperature range, from ambient temperature to 80 8C, with a performance comparable to that of liquid electrolyte batteries. Such a finding is without precedent. The temperature dependence of the SIPE batteries was mechanistically studied, and provided guidance for microscopic structural optimization to activate the electrodes and to minimize the interfacial resistance. The significant widening of battery operation temperatures coupled to the inherent flexibility and robustness of the SIPE membranes makes it possible to develop thin, flexible lithium-ion batteries for a broad range of applications.

Experimental Section Experimental Details and additional data can be found in the Supporting Information.

Acknowledgements The authors gratefully acknowledge support by a start-up grant from NUS, a POC grant from the National Research Foundation of Singapore, Singapore–Peking–Oxford Research Enterprise (SPORE), and the National Natural Science Foundation of China (21233006). Keywords: batteries · electrochemistry · electrode · energy storage · polymer electrolytes

Figure 4. (a) Cycle-life of the DADS based battery at various discharge rates at 80 8C. (b) Discharge capacity according to the discharge rate compared at 25 and 80 8C.

over 97 % (Figure S2, Supporting Information), even slightly higher than that of the CELi-based battery, which would pose serious safety risks for operation at the elevated temperature due to the poor thermal stability of LiPF6 and other small molecular lithium salts.[14] As expected, the cycle performance of the battery at high temperature at different discharge rates is even better than at room temperature with a reversible capacity of 139 mAh g1 at 1 C. The better performance of DADS battery at the higher temperature results from the higher reaction kinetic rate and the 50 % lower overall resistance than the re 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Received: December 18, 2013 Revised: January 21, 2014 Published online on March 12, 2014

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Single-ion polymer electrolyte membranes enable lithium-ion batteries with a broad operating temperature range.

Conductive processes involving lithium ions are analyzed in detail from a mechanistic perspective, and demonstrate that single ion polymeric electroly...
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