A Gum-Like Electrolyte: Safety of a Solid, Performance of a Liquid
Yu Wang , Bin Li , Jianying Ji , Allen Eyler , and Wei-Hong Zhong *
TION
Lithium ion batteries (LIBs) with fl exibility [ 1,2 ] and safety are
increasingly demanded and have become critical for the devel-opment of various industry sectors including electronics, elec-tric vehicles, aircraft and biomedical electronics. [ 3 ] An ideal electrolyte for safe and fl exible LIBs should possess the high ionic conductivity of liquid electrolyte levels, good safety and mechancial properties of solid electrolyte materials, conform-ability/fl exibility and excellent contact/adhesion with electrodes and so on. However, it is a challenge to integrate these prop-erties in one electrolyte and the existing electrolytes are facing various unsolved issues. [ 3,4 ]
Specifi cally, solid polymer electrolytes (SPEs) are safe in essence; nonetheless, the low ionic conductivity and poor contact/adhesion at the electrolyte-electrode interfaces pre-vent the real applications of SPEs to commerical batteries. For liquid-state electrolytes, including ionic liquid electrolytes, the liquid attribute causes safety concerns, such as leakage or gas-generating reactions at high temperature, [ 5 ] and low mechanical properties. Gel electrolytes are a promising solution for high-performance lithium ion batteries because they can simulta-neously possess high ionic conductivity and good mechanical properties. [ 6 ] However, for existing gel electrolytes, the safety issues still exist because of the high content of liquid electro-lyte and an unstable retention of the liquid electrolyte especially under deformations. [ 7 ]
Addressing these problems is a critical challenge towards unlocking the vast potential of LIBs; in particular, safety is an increasing concern, as demonstrated by the recent grounding of the Boeing 787 due to battery safety problems. Currently, there are two main approaches to improve the safety of LIBs. The fi rst one is to replace the existing liquid electrolytes with safer electrolytes, such as solid polymer electrolytes (SPEs) or ionic liquid electrolytes. The second way is to introduce various sensors or additives for the commonly used liquid electrolytes, such as redox shuttles, [ 8 ] or polymerizable organics. [ 9 ] Unfortu-nately, though these methods could improve the safety of the LIBs, these electrolytes have their intrinsic disadvantages as introduced above. Moreover, as fl exibility is attracting more and more attention for new high-performance LIBs, [ 10 ] excellent performance stability as well as leakage-free behavior under various deformations are the most important factors for their applications. [ 11 ] It is noted that for fl exible/stretchable energy
Y. Wang, Dr. B. Li, Dr. J. Ji, A. Eyler, Prof. W.-H. ZhongSchool of Mechanical and Materials EngineeringWashington State University Pullman , WA 99164 , USA E-mail: [email protected]
Adv. Energy Mater. 2013, 3, 1557–1562
storage devices, not only good mechanical properties of the battery electrolytes are desired, but also a stable and effective electrolyte-electrode interface under deformation are highly required. [ 12 ]
The aim of our work on the gum-like electrolyte reported in this communication is to achieve the ideal electrolyte proper-ties described above. To that end, we specially designed a hybrid electrolyte with multi-network structures (see Figure 1 a). The multi-network structures include: a double percolation network structure, i.e., a percolation network of a liquid electrolyte sup-ported by a packing network of solid particles, which is real-ized by a high loading of particles as shown in Figure 1 b, and a strong entanglement network of polymer electrolyte (ultra-high molecular weight poly(ethylene oxide), PEO, with lithium salt LiClO 4 ). The liquid percolation network can provide an express pathway for the ion transport as shown in Figure 1 c. To construct this percolation network, a new type of core-shell particles with liquid electrolyte (LiClO 4 in propylene carbonate (PC), 1 mol/L) as the shell was obtained by emulsion tech-nique (see Figure 1 d), which means the liquid adheres to the particle surface. At the same time, to improve the safety of the hybrid electrolyte, we chose thermally sensitive particles as the core (wax particles, for example). The core-shell structure can be confi rmed by the concentration of the liquid electro-lyte on the surface of the wax particles as shown by energy-dispersive X-ray spectroscopy (EDS) mappings (see Figure S1, Supporting Information) and also can be confi rmed by the properties of the hybrid electrolytes that will be shown later. The main advantage of the emulsion technique used here is that we can easily control the surface properties, size, and dispersion of the particles in the fi nal electrolytes. As shown in Figure 1 b, a good dispersion of wax particles in the electrolyte sample can be observed (also see Figure S2, Supporting Information). The size and distribution are shown in the inset in Figure 1 b.
We found that the hybrid electrolytes became gum-like (see the inset photograph in Figure 1 a) when the content of the liquid phase exceeded ca. 40 wt%. The hybrid electrolytes with high content of liquid electrolyte (40–60 wt%) and showing gum-like behaviors will be referred to as “gummy the following. Figure 2 a shows that the gummy electrolyte has a liquid-like high ionic conductivity (frequency-independent behavior). [ 13 ] This behavior should result from a liquid-based conductive pathway for the ion transport, that is, the percolation of the liquid shell as designed. This conjecture can be also confi rmed by the percolation phenomenon as observed in the hybrid electrolytes (see Figure S3, Supporting Information; for the methods to determine the ionic conductivity of the hybrid elec-trolytes, see Figure S4, Supporting Information).
Another attractive behavior of the gummy electrolyte is its structural integrity under arbitrary deformations. Figure 2 b
Figure 1. a) Optical microscopy image of the gummy electrolyte. The inset is the photograph of the gummy electrolyte fi lm under stretching. b) SEM image of the surface of the gummy electrolyte with the statistical distribution of wax particle size. c) Schematic of the network structures designed for the gummy electrolyte. d) Details of the designed core-shell particles.
demonstrates how the gummy electrolytes respond to arbitrary deformations. We found that arbitrary compression, twisting and stretching barely affected the ionic conductivity, indicating a good structure integrity or a fast structure recovery against different deformations. Moreover, the mass of the gummy elec-trolyte can remain constant after each kind of deformation (also see Figure 2 b), indicating that the retention of the liquid phase in the gummy electrolyte is very stable. At the same time, the electrochemical stability window of the gummy elecctrolyte was found to be 4.5 V (see Figure 2 c), which is mainly determined by the liquid electrolyte component.
In addition to a high ionic conductivity, mechanical proper-ties, such as modulus, fl exibility or extensibility, of an electrolyte are critical for high-performance energy storage devices with mechanical requirements. The gummy electrolytes can show gum-like mechanical properties as displayed in Figure 3 a. It is noted that a remarkable decrease in the mechanical properties can be observed when the content of the liquid electrolyte is too high (60 wt% for example). This is likely caused by the pen-etration of the liquid electrolyte into the polymer matrix when the content of the liquid electrolyte is much higher than the percolation threshold (ca. 30 wt%, see Figure S3, Supporting Information). Overall, the gummy electrolyte with liquid elec-trolyte content of ca. 50 wt% shows a balance between high
ion conductivity (ca. 3 × 10 −4 S cm −1 at room temperature) and good mechanical properties (the elastic modulus is around 0.1 MPa at a frequency of 5 Hz as determined by the rheolog-ical testing shown in Figure 3 a). It is well-known that, in addi-tion to transporting ions, the electrolyte also serves to form con-tacts with the electrodes and to separate them from each other; therefore, we should also take the adhesion/contact of the electrolyte with the electrode materials into account, especially when battery fl exibility is of interest. As shown in Figure 3 b, the gummy electrolyte demonstrates excellent adhesion prop-erties (sticking to almost any substrate). The average adhesion strength (defi ned as F max / A , where F max is the maximum force that the gummy electrolyte can hold, and A is the contact area for the adhesion test) was found to be 0.34 MPa, which is about two times of that of a gum.
Good contact of the electrolyte with electrodes is a basic requirement for a high-performance electrolyte. For liquid elec-trolytes, there is no concern about the contact as they usually can wet the electrode surface. However, for non-liquid electro-lytes, achieving good contact may become a critical problem. As one can observe in Figure 4 a,b, the gummy electrolyte can form an almost defect-free (no void) contact with the electrode mate-rial (V 2 O 5 for example), implying that the gummy electrolyte can wet the electrode surface well as a liquid does. This excellent
Figure 2. a) AC conductivity of the gummy electrolytes at room temperature. All experiments below were performed at room temperature unless oth-erwise noted. b) Performance stability of the gummy electrolyte against arbitrary deformation. Columns represent the ionic conductivity, white circles are the weight of the gummy electrolyte recorded after each different deformation, and the photographs are the snapshots for the deformations as described. c) Linear sweep voltammetry of the stainless steel/gummy electrolyte/Li cell at room temperature.
interfacial contact also contributes to the strong adhesion shown previously. The identifi cation of the wax particles at the inter-face as well as in the bulk of the gummy electrolyte, which may be critical for us to understand the interfacial properties of the gummy electroltye, has been shown by heating (see Figure 4 c), stretching as well as the size comparison of the particles in the wax emulsion and the gummy electrolyte (see Figure S5, Sup-porting Information). The smooth surface after heating should be caused by the melting and fl owing of the wax particles as wax has a very low viscosity.
It is noted that, for conventional separators, like porous polyolefi n fi lms, mechanical strength is a basic requirement. For gel electrolytes, the requirement of mechanical strength can be lowered as we can avoid the contact of the two elec-trodes by increasing the thickness of the electrolytes, which, however, will sacrifi ce the capacity of the battery. With regard to the gummy electrolyte, although the mechanical strength is much lower than that of a conventional polyolefi n separator, it still can be used as a separator membrane for the following reasons. Firstly, the optimized mechanical strength, like elastic modulus, is around 0.1 MPa, which is at the same level of the gel electrolytes reported. [ 14 ] Secondly, the strong adhesion prop-erties can, to some extent, compensate for the weakness in the mechanical strength as compared with porous polyolefi n fi lm. Particularly, it can lower the possibility for the contact of the two electrodes as a strong adhesion of the gummy electro-lyte may help the formation of a coating layer on the electrode surface.
The above properties of the gummy electrolyte, including the liquid-like ionic conductivity, the gum-like mechanical proper-ties, the structural integrity against various deformations, and the good adhesion/contact with the electrodes, are very critcial for the electrolyte applied to fl exible/stretchable energy storage devices [ 2,15 ] or working with an electrode having complex sur-face morphology structures, such as 3D electrodes. These inte-grated properties can provide batteries with an effective and stable contact at the electrolyte/electrode interface especailly against various deformations or even punching.
More signifi cantly, an additional important property of the gummy electrolyte is the thermal-protection capability for safe design of LIBs. Although the gummy electrolyte is expected to be much safer than liquid electrolytes, the presence of liquid electrolytes will always introduce safety risks. Similar to the safety mechanism of the porous polyolefi n fi lm employed for liquid electrolytes today, [ 16 ] we introduced a high loading of thermally sensitive wax particles into the gummy electrolyte to obtain the thermal-protection capability. Signifi cantly, we found that the ionic conductivity of the gummy electrolyte begins to decrease, instead of increasing as frequently reported, at a high temperature around the melting point of the wax particles as shown in Figure 5 a. This result indicates that the wax particles can form a non-conductive layer (wax layer) between the electro-lyte and electrodes as designed. The ability of the gummy elec-trolytes to form a wax layer on the surface of the electrode can be proven by the contact angle testing of the electrode surface as shown by the inserts in Figure 5 a as well as the SEM image
Figure 3. a) Dynamic mechanical properties and b) adhesion property of the gummy electrolyte as compared with a gum. The inset in (b) shows how the adhesion strength is determined.
of the fracture surface of the thermally destroyed gummy elec-trolyte (see Figure 5 b and Figure S6, Supporting Information).
The above phenomenon is signifi cant for the safety of a bat-tery with the electrolyte. It is well-known that the root of the safety issues, for liquid electrolytes or electrolytes with a liquid component, is the electrochemical reactions between elec-trodes and electrolytes at high temperatures. [ 5,17 ] Therefore, one effective way to solving this problem is to separate the elec-trolytes and electrodes at high temperatures. As demonstrated
Figure 4. SEM images of the contact behavior of the gummy electrolyte onc) SEM image of the interface after heating to identify the wax particles.
in Figure 5 c, as long as we choose particles with the melting point around the electrochemical reaction temperature, T c , the electrochemical reaction will be stopped by the melt layer of the thermally sensitive particles and the temperature of the battery system can remain in a safe range. It is noted that the tempera-ture to activate the thermal-protection can be easily adjusted as we can prepare particles with desired melting point. Regarding the application of this type of gummy electrolytes in LIBs, the study is ongoing in our lab and the results will be reported in the near future.
In summary, the gummy electrolyte provides a very good solution for high-performance electrolytes used in energy storage devices, such as LIBs. The structural integrity under arbitrary deformation, coupled with a strong adhesion and thermal-protection capability in addition to the high ionic con-ductivity, will promote the realization of the advanced batteries with high performance in safety, fl exibility or even extensibility.
Experimental Section Materials : The materials employed for the gummy electrolyte
include lithium salt (LiClO 4 ), poly(ethylene oxide) (PEO, M n = 4 × 10 6 g mol −1 ), paraffi n wax (melting point is 68 °C), propylene carbonate and surfactants (copolymer PEO-PE and sorbitan monostearate). All materials were purchased from Sigma Aldrich.
Sample Preparation : The gummy electrolytes were prepared by wax emulsion technique. Specifi cs are as follows. In the fi rst step, wax emulsion with surfactant PEO-PE or sorbitan was prepared by ultrasonication at 80 °C for 10 min. At the same time, PEO solution in DI water was also prepared. In the second step, we introduced quantifi ed liquid electrolyte PC/LiClO 4 into the wax emulsion and treated them by sonication at room temperature for 10 min. The concentration of the lithium salt in the liquid electrolyte is 1 mol/L. The fi nal loading of the liquid electrolyte in the gummy electrolyte varies from 10 wt% to 60 wt% as determined by the weight after drying. In the third step, the wax emulsion containing the liquid electrolyte was blended with the PEO solution. The weight ratio of the wax particles (including the surfactant) to PEO was fi xed to be two for all the electrolyte samples. After 30 min of stirring at room temperature, the mixture obained in step 3 was solution casted at room temperature for 3 days followed by vacuum drying (15 kPa) for 24 h at 35 °C.
EDS Mapping : To confi rm that the liquid electrolyte has been succesfully located at the surface of the wax particles, that is, the designed core-shell structure, Energy-dispersive X-ray spectroscopy (EDS) mapping was performed on a fi eld-emission scanning electron microscope (FESEM) equipped with an Oxford ISIS energy dispersive X-ray detector. Samples for EDS mapping were specially prepared. First, in order to obtain the mapping of the cation, sodium perchlorate was
mbH & Co. KGaA, Weinheim
an electrode substrate V 2 O 5 at magnifi cation of a) 2000×, b) 10000×, and
Adv. Energy Mater. 2013, 3, 1557–1562
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Figure 5. Thermal-protection capability of the gummy electrolyte. a) Temperature-dependent behavior the ionic conductivity of the gummy electrolyte. The curve shows the melting behavior of the wax particles. The inset photographs are the contact angle testing of the electrode surface before (the one with much lower contact angle) and after the high temperature testing. b) SEM image of the fracture surface of the thermally destroyed gummy electrolyte. c) The proposed mechanism of the thermal-protection capability to improve the safety of lithium ion batteries.
employed since no signal of lithium can be detected by the EDS detector. Secondly, the emulsion mixture for the gummy electrolyte was diluted and dispersed on a carbon-based paper sheet to get a single layer of wax particles. Lastly, after removing the solvent, the paper sheet with wax particles was coated with gold for EDS mapping. The results are shown in Figure S1 (Supporting Information).
Impedance Analysis : The ion conductivity of the gummy electrolyte was obtained by AC impedance spectroscopy measurements (Universal Dielectric Spectrometer BDS 20). The frequency range was chosen to be from 10 −1 Hz to 10 6 Hz. The electrolyte sample was sandwiched between two gold electrodes of 2 cm diameter. The input voltage for the measurement was 1 V. To evaluate the thermal-protection capability, the measurements were carried out at different temperatures (from 20 to 80 °C). Details about the methods to determine the ionic conductivity can be found in Figure S4 (Supporting Information).
Linear Sweep Voltammetry (LSV) : Linear sweep voltammetry was employed to determine the electrochemical stability window of the gummy electrolyte in a stainless steel/gummy electrolyte/Li coin cell, where the stainless steel disk was used as the working electrode and a lithium disk served as the counter electrode. The gummy electrolyte was dried at 50 °C in a vacuum chamber for 2 days to remove the moisture and the coin cell was assembled in the glove box with the protection of argon gas. A voltage sweep was carried out on the material between 2 and 7 V at a constant scan rate of 1 mV s −1 by Autolab PGSTAT101.
Mechanical Testing : Mechanical properties were evaluated by rheological testing (Discovery HR-2, TA). A parallel plate with diameter of 25 mm and a testing gap (the thickness of sample) in the range of 0.5–1 mm was used. Frequency sweep (0.05–100 Hz) was carried out at room temperature to determine the dynamic mechanical properites. For all the rheological testing, the strain was 1%, which is in the linear viscoelastic region of all the samples. At the same time, the mechanical properties of a common gum (Extra, Wrigley Company) were tested with the same conditions.
Adhesion Testing : A home-made setup (see the inserted schematic in Figure 3 b) was used to determine the adhesion strength of the gummy electrolyte. For comparison, a gum was also tested. The fi xed substrate is a fl at plastic plate and levelly fi xed on a table and the weight against the adhesion is controlled by a steel substrate (the effective surface area is 9.42 cm 2 ), the weight of which can be continuously adjusted. The two substrates were cleaned by acetone before the loading of the testing sample. To obtain reliable result, the sample was evenly coated on the plastic substrate fi rst and then the weight (the steel substrate) fi xed with a container was bonded with the sample by a constant weight (ca. 8 kg) for 5 min. The maximum weight that the sample can hold was recorded and repeated 7 times. For gummy electrolyte, only the sample with 52 wt% of liquid electrolyte was tested for example.
PLM and SEM Observation : The morphology of the gummy electrolytes was analyzed using a polar light microscope (Olympus BX51) at room temperature. The morphologies of the normal and thermally destroyed gummy electrolytes were investigated by scanning electron microscopy (SEM). To observe the contact behavior of the gummy electrolyte on electrode material, V 2 O 5 was fi rst prepared by oxidizing vanadium at 500 °C for 4 h. Then, the gummy electrolyte was adhered on the electrode surface without compression for the SEM observation.
Contact Angle Testing : An OCA 15 plus Contact Angle Analyzer was used to perform the contact angle testing. The gold electrode before the loading of gummy electrolyte was cleaned by acetone and then, the contact angle was determined by the average value of fi ve measurements at room temperature. To confi rm that there will be a wax layer on the electrode surface at high temperature (higher than the melting point of the wax particles), the gold electrodes with the gummy electrolyte sandwiched was heated up to 80 °C for 1 min and then separated with the gummy electrolyte at high temperature. The surface separated from the electrolyte was used for the contact angle testing. To investigate the uniformity of the wax layer on the gold electrode, the contact angle was measured at 10 different locations.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors appreciate the fi nancial support from WSU Research Advancement Challenge (RAC) Grant, “Advanced Lithium-ion Batteries Incorporating Bio-and Nano-materials and the Effects on the Agricultural Economy”. The authors also appreciate Dr. David P. Field (Washington State University) for providing the vanadium. The authors are particularly grateful to Drs. John D. Madden, Frank Ko, Ashwin Usgaocar, Eddie Fok and Ms. Yingjie Phoebe Li (The University of British Columbia, Vancouver, BC, Canada) for providing the electrochemical stability testing.
Received: May 8, 2013 Revised: June 26, 2013
Published online: August 9, 2013
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