Nonaqueous Hybrid Electrolyte for Sodium-Ion Batteries

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Aqueous/Non-aqueous Hybrid Electrolyte for Sodium-ion Batteries Huang Zhang, Bingsheng Qin, Jin Han, and Stefano Passerini ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00919 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Energy Letters

Aqueous/Non-aqueous Hybrid Electrolyte for Sodium-ion Batteries Huang Zhang,†,‡ Bingsheng Qin,†,‡ Jin Han,†,‡ and Stefano Passerini*,†,‡ †

Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany

‡

Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

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ABSTRACT: Here, we report an aqueous/non-aqueous hybrid electrolyte based on sodium trifluoromethanesulfonate with an expanded electrochemical window up to 2.8 V and high conductivity (~25 mS cm−1 at 20 °C). The hybrid electrolyte inherits the safety characteristic of aqueous electrolytes and the electrochemical stability of non-aqueous systems enabling the stable and reversible operation of the Na3V2(PO4)3/NaTi2(PO4)3 sodium-ion battery.

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ACS Energy Letters

Sodium-ion batteries are of particular interest in large-scale energy storage applications due to their competitive cost advantage. Rather than the energy density for portable electronics, cost and safety are of more importance for grid-energy storage.1 With this respect, aqueous electrolytes promise increased operational safety and reduced manufacturing cost. However, the narrow electrochemical window of aqueous solution and the poor electrode materials stability in the presence of water limit the achievement of a successful device.2-3 Recently, a concept of superconcentrated “water-in-salt” electrolyte (WiSE) was reported that can highly reduce the electrochemical activity of water thus enabling the realization of high energy aqueous batteries.48

However, the NASICON (Na3V2(PO4)3) compound, a promising cathode for sodium-ion

batteries owing to its high structural stability and specific energy in nonaqueous system, suffers from the degradation in aqueous electrolytes, resulting in the severe energy-loss.9-11 Here,

we

report

an

aqueous/nonaqueous

hybrid

electrolyte

based

on

sodium

trifluoromethanesulfonate (NaOTf) with an extended electrochemical window up to 2.8 V, i.e., enabling the use of Na3V2(PO4)3, and high conductivity (~25 mS cm−1 at 20 °C). This study first demonstrates the use of concentrated aqueous/nonaqueous hybrid electrolyte for sodium-ion batteries, enabling the stable operation of the Na3V2(PO4)3 (NVP) cathode and NaTi2(PO4)3 (NTP) anode combination in aqueous environment. Note that NVP and NTP are both excellent high rate, high stability electrode materials in nonaqueous system while their combination could contribute to suitable output voltage in aqueous system. The hybrid electrolyte was prepared by mixing the same weights of 7 m (molality) NaOTf in water and 8 m NaOTF in propylene carbonate (PC). This hybrid electrolyte offers a wide electrochemical stability window up to 2.8 V, from −1.25 to 1.55 V vs. Ag/AgClKCl at 20 °C on stainless steel (Figure 1a). Normally, a higher concentration is required to widen the


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electrochemical window of the aqueous sodium solution due to the lower charge density of Na+ resulting in a weak interaction with the solvent molecules. Compared to the reported 9.26 m NaOTf WiSE,12 the electrochemical stability of the hybrid electrolyte is improved, which can be attributed to the presence of PC. In Figure 1a the typical cyclic voltammograms of NTP and NVP electrodes on stainless steel in aqueous (or non-aqueous) electrolytes are presented together with the electrochemical stability window of the hybrid electrolyte. Such an electrode combination would result in cells with a ~1.2 V output.11 Figure 1b displays the temperaturedependent conductivities of the hybrid electrolyte. The conductivity strongly depends on the operating temperature, which decreases from ~60 mS cm−1 at 55 °C to ~25 mS cm−1 at 20 °C. However, the electrolyte still maintains a conductivity of ~5.0 mS cm−1 at −15 °C, which is comparable to that of nonaqueous electrolyte (4.0 mS cm−1 of 1 M NaOTf in PC). To probe the solution’s structure, Raman spectroscopy was performed as shown in Figure 1c. The change of the SO3 stretching band (1010-1060 cm−1) supports for the ion pairing in NaOTF solution.12 In fact, the large blueshift observed in the concentrated aqueous/nonaqueous hybrid electrolyte indicates the highly increased fraction of contact ion pairs and aggregated cation–anion pairs when compared to the low concentrated electrolyte as well as the WiSE. This intensified cationanion association is beneficial for the formation of an interphase at the electrode/electrolyte interface, which stabilizes water molecules against the reducing surfaces of the electrode materials.12,13


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ACS Energy Letters

Figure 1. Electrochemical and structural characterization of aqueous/nonaqueous hybrid electrolyte based on NaOTf: (a) electrochemical stability on stainless steel evaluated using linear sweep voltammetry at a scan rate of 10 mV s−1. Typical cyclic voltammograms of NTP and NVP electrodes are presented within the electrochemical window. (b) Temperature-dependent conductivities. (c) Raman spectra in the wavenumber region of the SO3 stretching modes. A full sodium-ion cell using the NVP cathode and NTP anode was assembled to evaluate the viability of the aqueous/nonaqueous hybrid NaOTf electrolyte (Figure 2). The cyclic voltammogram of NTP/NVP cell at 0.1 mV s−1 in Figure 2a exhibits anodic and cathodic peaks at 1.4 V and 1.1 V with a voltage hysteresis of 0.3 V. During the 2nd scan, no increased polarization is observed. The galvanostatic charge/discharge profiles at different rates are presented in Figure 2b. The initial charge and discharge capacities at 0.2 C are 115 and 91 mAh g−1, corresponding to an initial coulombic efficiency of 79% and an energy density of 45 Wh


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kg−1 (based on both electrode masses). On the other hand, the cell exhibited a rather poor initial coulombic efficiency (21%) in WiSE. Figure 2c depicts the rate capability and cycling performance of the cell, where the capacity drops from 65 to 22 mAh g−1 as increasing from 1 C to 10 C, but the coulombic efficiency reaches to 99.8%. The initially irreversible processes apparently diminish and eventually disappear, which is the typical behavior associated to the SEI formation and its subsequent stabilization.14 Moreover, the cell underwent 100 cycles at 10 C (1.2 A g−1) without any capacity fade, indicating the improved reversibility of NVP and favorable suppression of electrolyte decomposition on the anode surface. Electrochemical impedance spectra of the cell were recorded before cycling and after 30 cycles (Figure 2d). The results show the neat increase of the charge transfer resistance after cycling, suggesting that a passivation layer is formed on the electrode in combination with the hybrid electrolyte which could be highly interesting for the stable operation of aqueous batteries.13 Experiments were carried out to evaluate the cell self-discharge, in which a cell was fully charged to 1.5 V, rested in open circuit (OC) for different amounts of time and then fully discharged (Figure S2, Supporting Information). These experiments reveal that some capacity fading occurs upon storage time. The comparison of the capacity retentions detected during the discharge after the rest period and the following charge indicate that the initial fading is due to some irreversible process such as the dissolution of Vn+ species,12,13 while, for the longer rest periods, the fading seems to be related with a different process, e.g., water decomposition, which does not involve electrode degradation.


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ACS Energy Letters

Figure 2. Electrochemical performance of NTP/NVP in aqueous/nonaqueous hybrid electrolyte: (a) CV curves at 0.1 mV s-1, (b) initial galvanostatic charge-discharge profiles at various C-rates (1 C=120 mA g-1), (c) rate and cycling performance, and (d) EIS spectra at open-circuit voltage and after 30 cycles. In summary, the use of an aqueous/nonaqueous hybrid electrolyte providing a potential window up to 2.8 V has been demonstrated for the first time. Such a hybrid electrolyte enables the stable and reversible operation of a sodium-ion battery using Na3V2(PO4)3 as the cathode and NaTi2(PO4)3 as the anode. The widened stability window ensures an improved cell voltage and, consequently, higher energy density. The hybrid electrolyte concept herein reported opens an avenue for the exploration of sodium-ion batteries with high safety and electrochemical stability. ASSOCIATED CONTENT


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX Experimental methods; XRD and SEM of NVP and NTP materials. AUTHOR INFORMATION ORCID Huang Zhang: 0000-0002-7695-261X Stefano Passerini: 0000-0002-6606-5304 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the Chinese Scholarship Council. Financial support from the Helmholtz Association is also acknowledged. REFERENCES (1)

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Nonaqueous Hybrid Electrolyte for Sodium-Ion Batteries

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