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Apr 30, 2019 - carbonate (EMC) were provided by LG Chem. The base electrolyte was 1 M LiPF. 6. EC/ EMC. (1/2, v/v) (hereafter LE). For the preparation...
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Toward Fast Operation of Lithium Batteries: Ion Activity as the Factor To Determine the Concentration Polarization Dong-Hui Kim,† Sunwook Hwang,† Jeong-Ju Cho,‡ Sunghoon Yu,‡ Soojin Kim,‡ Jongho Jeon,‡ Kyoung Ho Ahn,‡ Chulhaeng Lee,‡ Hyun-Kon Song,§ and Hochun Lee*,† Downloaded via UNIV FRANKFURT on July 26, 2019 at 02:21:35 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.



Department of Energy Science & Engineering, DGIST, Daegu 42988, Republic of Korea Batteries R&D, LG Chem Ltd., Daejeon 34122, Republic of Korea § Department of Chemical Engineering, UNIST, Ulsan 42988, Republic of Korea ‡

S Supporting Information *

ABSTRACT: The concentration polarization, in addition to the activation and ohmic polarizations, limits the fast operation of electrochemical cells such as Li-ion batteries (LIBs). We demonstrate an approach to mitigate the concentration polarization by regulating the effective concentration (i.e., the mean ionic activity) of Li ions. The use of an acrylate-based gel polymer electrolyte (AGPE) improved the rate capability of LIBs compared with its liquid counterpart. Electrochemical and spectroscopic evidence confirms that the unexpected power performance of the A-GPE is ascribed to the unique solvation structure surrounding the Li ions. The solvation structure suppresses an abnormal increase in the activity of Li ions and thus mitigates the concentration polarization during high-rate discharge. Importantly, this study rejects the common wisdom that the solid or semisolid electrolytes discourage the fast charge/discharge of LIBs and suggests an avenue to simultaneously enhance both the safety and high-power performance of rechargeable batteries.

R

than unity (0.3 to 0.4) in most conventional LIB electrolytes,2−7 Li salt accumulates at the anode/electrolyte interface and depletes at the cathode/electrolyte interface during the discharge of LIBs (and vice versa during charging), leading to a concentration gradient of Li salt between the cathode and anode and thus a buildup of the overpotential opposing the net cell voltage.8,9 The concentration difference is possibly as high as 4 M during 3 C discharge (1 C represents the current rate required to discharge the battery in 1 h), which leads to a difference of more than an order of three in the mean ionic activity of Li ions (aLi).4,5,8 Compared with the other two polarizations, ηcon is poorly understood thus far.4,5,8,9 Because ηcon appears only under nonequilibrium conditions of the charge/discharge processes, it is not readily measurable using electrochemical impedance spectroscopy (EIS),9,10 the common tool that provides information on Rohm and Rct under an equilibrium state. The information on ηcon may be collected by EIS measurements at

esolving the chronic safety issues of Li-ion batteries (LIBs) without compromising power density is challenging due to the contradictory nature of the two aspects. Whereas solid and semisolid electrolytes can address the safety concerns of LIBs, the safety merit is achievable only at the expense of power density. Apparently, to achieve high-power LIBs, the total polarization involved in the electrochemical reaction (η total), which includes three components of ohmic, charge transfer, and concentration polarizations,1 needs to be minimized. ηtotal = ηohm + ηct + ηcon

(1)

Ohmic polarization (ηohm), also called IRohm drop (where I and Rohm denote the current and ohmic resistance, respectively), is mainly due to a combination of the ionic resistance of the electrolyte and the electrical contact resistance of the cell components. Charge-transfer polarization (or activation polarization, ηct) is associated with the interfacial charge-transfer resistance (Rct). Concentration polarization (ηcon) arises from a concentration gradient in the Li salt, which develops during LIB operation, especially under fast charge/discharge reactions. Because the transference number of Li ions (t+) is much less © 2019 American Chemical Society

Received: April 4, 2019 Accepted: April 30, 2019 Published: April 30, 2019 1265

DOI: 10.1021/acsenergylett.9b00724 ACS Energy Lett. 2019, 4, 1265−1270

Letter

Cite This: ACS Energy Lett. 2019, 4, 1265−1270

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ACS Energy Letters

Figure 1. (a) Chemical structures of the acrylate monomer and peroxide initiator and a proposed thermal cross-linking polymerization reaction. (b) Vogel−Tammann−Fulcher plots of LE and A-GPE and photo images (insets). (c) Fabrication of A-GPE cells via in situ thermal polymerization process.

Figure 2. Electrochemical performances of LiCoO2/graphite cells. (a) Voltage profiles at 0.2 and 3 C discharge rates of LE and A-GPE cells. (b) Normalized discharge capacity versus C rate and Ragone plots (inset). (c) Nyquist plots measured at state of charge (SOC) 50 and an equivalent circuit for fitting of impedance data (inset), experimental data (symbols), and fitted data (lines). (d) Voltage profiles at 3 C rate with a rest period (30 min) every 5 min. LiCoO2/graphite pouch cells (720 mAh) were employed for the power test, and coin cells (5.8 mAh) were for the impedance measurements.

the extremely low frequency range (10−0.1 mHz),11,12 but it is highly susceptible to the artifacts associated with geometric factors of active materials and electrode structure.13,14 In addition to the complexity of the measurement, the poor understanding of ηcon is also ascribed to the absence of the proper theory of ion−ion and ion−solvent interactions in concentrated (≥1 M) electrolytes; most established theories

on the ionics pertain only to dilute (≤0.01 M) solutions.15,16 According to eq 2, ηcon is governed by both t+ and aLi1 ηcon ∝ 2(1 − t+)RT /F ln(aLi /aref )

(2)

where aref is the activity of Li ions at the reference point in the electrolytes and the others have their well-known meanings. So far, a common practice to suppress the ηcon in LIBs is to maximize t+ by establishing solid-state (inorganic or polymer) 1266

DOI: 10.1021/acsenergylett.9b00724 ACS Energy Lett. 2019, 4, 1265−1270

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ACS Energy Letters

Figure 3. Solvation structure of Li ion in A-GPE. (a) Concentration cell voltage (Ecell) of LE and A-GPE versus Li salt concentration (CLi) and the scheme of the concentration cell (inset). The hypothetical Ecell values assuming the unit activity coefficient of Li ion are presented for comparison (blue line). (b) Concentration of free EC and EMC (Cf) in LE and A-GPE versus Li salt concentration (CLi). (c) FTIR spectra and fitting results of A-GPE (PC/monomer, 7/3 wt/wt) with and without Li salt. (d) Schematic illustration of the solvation structures of Li ion in LE and A-GPE.

electrolyte systems.17−21 However, the cells based on the solid electrolytes suffer from low ionic conductivity and high interfacial resistance, showing inferior power density to the conventional liquid electrolytes. In contrast with the extensive effort to control t+, however, little attention has been paid to address aLi.4,5 We herein report a new approach to mitigate ηcon by suppressing the abnormal rise of aLi during the fast operation of LIBs. The key idea is to modify the solvation structure of Li ions using acrylate-based gel polymer electrolytes (hereafter labeled A-GPEs). The functional groups of A-GPE are expected to participate in the Li-ion solvation, thus liberating organic solvent molecules (cyclic and linear carbonates), which will lower the effective concentration of Li ions (i.e., aLi) and thus ηcon during high-rate operation. The A-GPE was prepared via the in situ radical polymerization of an acrylate monomer (dipentaerythritol penta/hexaacrylate) using a peroxide-based thermal initiator (2,5dimethyl-2,5-di(2-ethylhexanoyl peroxy) hexane). The chemical structures of the monomer and initiator and a proposed polymerization mechanism are presented in Figure 1a. The liquid electrolyte (1 M LiPF6 ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (1/2 v/v), hereafter labeled LE) containing the acrylate monomer (2 wt %) and a thermal initiator (0.02 wt %) was cured at 80 °C for 3 h to obtain AGPE. As presented in Figure S1a, A-GPE displays a slightly lower ionic conductivity than LE (e.g., 8.6 and 9.6 mS cm−1 at 25 °C, respectively). The apparent activation energy for ion conduction (Eaic) in A-GPE deduced from the slope of the Vogel−Tammann−Fulcher (VTF) plot (Figure 1b) is quite similar to that in LE (3.4 and 3.0 kJ cm−1 for A-GPE and LE, respectively), indicating no significant change in the ion conduction mode.22,23 Various features of A-GPE including electrochemical/thermal stabilities and structural character-

ization are given in the Supporting Information (Figures S1− S3 and Table S1). In addition, LIB cells using A-GPE (hereafter A-GPE cells), fabricated via the in situ thermal polymerization process (Figure 1c), exhibited greatly improved safety aspects compared with cells using conventional LE (hereafter LE cell), demonstrating that A-GPE can greatly resolve the safety concerns of the current LIBs (Figure S4). The rate capability of A-GPE cells was examined by measuring the discharge capacity at various current densities. At a slow discharge at 0.2 C current, the A-GPE cell displayed similar discharge behavior to that of the LE cell (Figure 2a). Intriguingly, however, the A-GPE cell delivered much higher discharge capacity than the LE cell at higher discharge rates; the A-GPE cell exhibited 32 and 54% higher capacities at 3 and 4 C, respectively (Figure 2b). A closer look at the voltage profiles at 3 C (Figure 2a) reveals that compared with the LE cell, the A-GPE cell showed a slightly increased overpotential in the initial discharge stage (Region I) but a much smaller overpotential near the end of the discharge (Region II). Although the A-GPE cell displays a lower discharge voltage than the LE cell in the initial stage, the increase in the overall discharge capacity is large enough to overcome the slight voltage loss, resulting in a net gain in the discharge energy, as presented in a Ragone plot (inset in Figure 2b). We also investigated the rate performance of A-GPE cells as a function of the contents of acrylate monomer over 1−10 wt %. As presented in Figure S5, 2 wt % monomer was found to be close to the optimum condition. A superior rate capability is also achieved in other GPEs employing a series of acrylate monomers (Table S2 and Figure S6), which indicates that enhanced power performance is a common feature of A-GPEs; meanwhile, a systematic investigation of the effects of the number of acrylate groups and the nature of the molecular structures on cell performance deserves a future study. 1267

DOI: 10.1021/acsenergylett.9b00724 ACS Energy Lett. 2019, 4, 1265−1270

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In Figure 3a, the Ecell values measured for LE and A-GPE are plotted as a function of log CLi, and the hypothetical Ecell, predicted under ideal conditions, where aLi is the same as CLi (i.e., the unit activity coefficient), is also presented for comparison. At low CLi (0.1 to 0.5 M) in LE, Ecell increased linearly with log CLi, following the ideal relation. At higher CLi values (>0.5 M), however, Ecell showed a large positive deviation from the linear behavior. Importantly, the positive deviation of Ecell is notably diminished in A-GPE; the Ecell value at 4 M CLi is 0.291 V in A-GPE but 0.394 V in LE. Because the t+ values show no significant difference, the lower Ecell in the AGPE cell can be attributed to the smaller aLi (eq 3). Figure S8d compares the activity coefficient of the Li ion (γLi = aLi/CLi) with an assumption that aref is 0.1 (i.e., γLi is 1.0 at 0.1 M CLi).5 The γLi value in LE was close to 1.0 at low CLi (30% higher than that of the cells using a conventional liquid electrolyte at a high-rate discharge (≥3 C) while retaining far better stability in overcharging and high-temperature (90 and 150 °C) storage tests. As evidenced by in-depth electrochemical and spectroscopic analysis, the superior power performance of A-GPE was ascribed to mitigated ηcon, which is associated with the unique solvation structure of Li ions that lowers aLi during high-rate discharge. In addition to the formidable benefits to the safety and power performance, A-GPE has advantages in commercial applications compared with other types of solid electrolytes (a simple fabrication process, low material cost, and intimate electrode−electrolyte contact), which can be readily applicable to various contemporary and next-generation batteries. Additionally, this study underscores the pivotal role of ηcon in determining the power performance of LIBs, which has been rarely appreciated in the field of batteries. In this context, further progress of current LIBs is likely to hinge on our understanding of the ionics (the study of ion−ion and ion− solvent interactions) in concentrated electrolyte solutions.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00724. Ionic conductivity, linear sweep voltammograms, 1H NMR spectra, TGA curve, XRD pattern, safety tests (overcharge test, hot box test, and thickness changes), rate performance of A-GPE (with different monomer contents, different monomer types, and different cathodes) Nyquist plots, structure of concentration cell, self-diffusion coefficients, Raman spectra, dielectric relaxation spectra, and cycle performances (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hochun Lee: 0000-0001-9907-5915 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This research was financially supported by the Mid-Career Researcher Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT) (NRF-2017R1A2B4004470).



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