Synthesis and characterization of a tri-layered solid electrolyte and oxygen permeable solid air cathode for lithium-air battery cells were carried out in this investigation. Detailed fabrication procedures for solid electrolyte, air cathode and real-world lithium-air battery cell are described. Materials characterizations were performed through FTIR and TGA measurement. Based on the experimental four-probe conductivity measurement, it was found that the tri-layered solid electrolyte has a very high conductivity at room temperature, 23。C, and it can be reached up to 6 times higher at 100。C. Fabrication of real-world lithium-air button cells was performed using the synthesized tri-layered solid electrolyte, an oxygen permeable air cathode, and a metallic lithium anode. The lithium-air button cells were tested under dry air with 0.1 mA - 0.2 mA discharge/ charge current at elevated temperatures. Experimental results showed that the lithium-air cell performance is very sensitive to the oxygen concentration in the air cathode. The experimental results also revealed that the cell resistance was very large at room temperature but decreased rapidly with increasing temperatures. It was found that the cell resistance was the prime cause to show any significant discharge capacity at room temperature. Experimental results suggested that the lack of robust interfacial contact among solid electrolyte, air cathode and lithium metal anode were the primary factors for the cell’s high internal resistances. It was also found that once the cell internal resistance issues were resolved, the discharge curve of the battery cell was much smoother and the cell was able to discharge at above 2.0 V for up to 40 hours. It indicated that in order to have better performing lithium-air battery cell, interfacial contact resistances issue must have to be resolved very efficiently.
Emission from conventional fossil fuel based combustion engine vehicles has prompted introduction of electric vehicles (EV) or hybrid electric vehicles (HEV) in the transportation sector in order to reduce greenhouse-gas emission and environmental pollution [
In general, in the Li-air battery, the Li-metal is oxidized with the help of oxidation catalyst at the anode during discharge and released Li-ion into the electrolyte as shown in equation (1).
However, at the cathode, O2 from the atmospheric air enters into the porous cathode and complete the chemical reaction as
and form the product Li2O2. Even though it seems that the chemical reaction mechanism in the Li-air battery cell is simple but still the Li-air battery system encounters several issues those must need to be resolved such as (i) very low stability of the conventional electrolytes against Li/O2 reaction products, (ii) the high cell polarization with consequent low energy efficiency and (ii) the short cycle life [
In this study, we synthesized a three layered solid electrolyte by combing high performance polymeric-ce- ramic electrolytes. A high performing oxygen permeable air cathode is also fabricated for application in high energy lithium-air battery. A real-world lithium-air battery cell is fabricated employing developed three layered solid electrolyte, solid metallic lithium anode and high performance oxygen permeable solid air cathode. Multi-layered solid electrolyte and oxygen permeable air cathode materials are characterized by using FTIR, TGA and other associated measurement techniques. Fabricated lithium-air battery cell was experimentally tested under different real-world operating conditions. Detailed description of synthesis of different components of three layered electrolyte, air cathode material and lithium-air battery performance results are presented in the subsequent sections in this paper. The solid configuration of the entire lithium-air battery cell and the absence of volatile and flammable components in the developed battery cell are expected to strongly limit or eliminate possible safety issues.
In order to prepare tri-layered solid electrolyte and oxygen permeable solid air cathode, we first synthesis all the required polymers and ceramic discs by using solid-state reaction processes. We synthesized two fluoro-HBPCS polymer materials in order to prepare an oxygen permeable cathode membrane. Syntheses of these two materials are described below.
A 1000 mL 3-neck flask fitted with a mechanical stirrer and a 12.5 mL pressure-equalized addition funnel was charged with hexadecafluorodecanediol (10.0 g, 21.7 mmol), NaOH pellets (2.0 g, 50.0 mmol), benzyltrimethylammonium bromide (0.72 g, 3.2 mmol), and 50 mL of anhydrous tetrahydrofuran (THF). The contents were heated in an oil bath and stirred until the NaOH pellets dissolved. The addition funnel was charged with allyl bromide (7.7 mL, 10.7 g, 8.8 mmol), and the flask was returned to the oil bath, which was maintained at 62˚C - 68˚C. The allyl bromide was added dropwise to the reaction mixture over the course of 1 hour under an N2 flow. Heating and stirring were continued overnight for 17 hours. After cooling, the reaction mixture was poured into a 100 mL beaker filled to 50 mL with water, and this suspension was extracted three times each with 17 mL of ethyl acetate. The combined organic extracts were dried over MgSO4, filtered, and the solvents were removed by rotary evaporation. The viscous liquid was distilled at 123˚C - 124˚C under a 5 - 10 mm Hg vacuum, and 10.4 g (19.2 mmol, 88% yield) of product was recovered in two fractions as hexadecafluorodecanediol diallyl ether. The product hexadecafluorodecanediol diallyl ether was characterized by the Fourier transform infrared spectroscopy (FTIR) spectrum and found a band associated with the vinyl groups.
Hexadecafluorodecanediol diallylether (3.0 g, 5.53 mmol) was weighed into a round-bottom flask equipped with a magnetic stirring bar and a cooling condenser. Trifluoropropyltris (dimethylsiloxy) silane (3.0 g, 8.56 mmol) was added followed by 2 mL of toluene and 2 drops of platinum divinyl tetramethyldisiloxane complex (Karstedt catalyst, −2% in xylene). The solution was then heated in an oil bath at 50˚C for 5 hours at 70˚C for 18 hours, and then 100˚C for 3 days until the FTIR spectrum of the reaction mixture indicated the disappearance of bands associated with the vinyl groups. Material or compound 1 was obtained as a colorless liquid after unreacted trifluoropropyltris(dimethylsiloxy)silane was removed by Kugelrohr distillation at 120˚C. Schematic of synthesis of material 1 is shown in
Synthesized fluorinated-HBPCS 1 was weighed (2.01 g) into a round-bottom flask equipped with a magnetic stir bar, followed by vinyltrimethoxysilane (0.58 g, 3.9 mmol) and 1mL of dry toluene. Residual platinum catalyst from the previous step was assumed to be dispersed in the polymer and still active. The reaction mixture was stirred at room temperature for 17 hours overnight in order to get rid of Si-H band completely. Material or compound 2 was obtained as a viscous colorless liquid after the excess vinyltrimethoxysilane was removed under vacuum at 60˚C. Schematic of synthesis of material 1 is shown in
The synthesized fluorinated-HBPCS 2 (i.e. materials 2) was characterized by the FTIR spectrum in order to make sure that there is no strong Si-H band. The FTIR spectrum of material 2 is shown in
Synthesized fluorinated HBPCS 1 and 2 were mixed in a 75/25 volume ratio in a test tube and the resulting mixture was diluted with an equal volume of isopropyl alcohol, followed by the addition of a catalytic amount of dibutyltin dilaurate. The resulting mixture was transferred onto a glass slide with a pipet and was cured for 5 days at room temperature and thermal analysis of the cured material was performed using Thermal gravimetric analysis (TGA) method as shown in
130˚C. Weight loss was increasing slightly with increasing temperature as can be seen from
Three individual electrolyte discs were prepared to fabricate a three-layered solid ceramic electrolyte laminate. Synthesis method of each individual electrolyte disc is described below.
Polyethylene oxide (PEO) and lithium bistrifluoromethane sulfonimidate (LiTFSI) were dried at 70˚C for 48 hours under N2 and transferred to the glove box after drying. PEO (4.25 g), LiTFSI (0.5g), and boron nitride (BN) (0.047 g, 1 wt%) were weighed in the glove box and mixed in a milling jar. The jar was sealed and taken out of the glovebox for milling. After being milled for 1 hour, the mixture was brought back into the glove box and transferred to a vial. The milled mixture (200 mg) was weighed and transferred into a 12 mm die which was placed on a hot plate set at 95°C (the temperature was not uniform) for 10min and then pressed with a 19.6 KN force to transform it into a polymer ceramic (PC) membrane. The PC membrane was removed from the die after it had cooled down to room temperature after approximately 1 hour. Four PC(BN) discs were prepared. Once the PC (BN) disc were prepared we measured the thickness of the discs and it was found in the range of 1.21 - 1.28 mm.
PEO (4.25 g), LiTFSI (0.5 g), and Li2O (0.047 g, 1 wt%) were weighed in the glove box and mixed in a milling jar. The jar was sealed and taken out of the glove box for milling. After being milled for 1 hour, the mixture was brought back into the glove box and transferred to a vial. Four PC (Li2O) discs were prepared using a similar procedure as above using 200 mg of the milled mixture.
Lithium Aluminum Germanium Phosphonate (LAGP) ceramic powder (0.45 g) was weighed, loaded into a 19 mm die, and pressed with 9 ton force to obtain a ceramic disc. However, the discs always cracked and it was difficult to obtain a complete un-cracked disc. Part of the reason might be that the pressure on the discs was not uniform and another part of the reason may be due to the material itself (components, particle size, and particle size distribution). Reduction of the LAGP amount from 0.45 g to 0.30 g didn’t help much although the disc thickness became thinner (from 0.65 mm to 0.45 mm). Preparation of smaller LAGP discs with a 12 mm die was relatively easier. A lot of effort was applied to improve the quality of the ceramic discs. It was found that addition of a small amount of water (15 - 20 µL) to 0.30 g of ceramic powder helped to form a complete disc in the die. In addition, the die was pressed with 88.2 KN force several times after turning it 50˚C - 90˚C after each pressing in order to minimize the effect of uneven pressure on the disc in a single press. Using this improved procedure, some nice 19 mm ceramic discs were obtained. These discs were placed on a ceramic plate and transferred to the electric furnace and sintered at 850˚C for 12 hours. After sintering, the discs were ground down with sandpaper in order to fit in the battery case (2032 button cell). The grinding process was very time consuming (over 1 hour for each disc) and needed to be done very carefully since the discs are very brittle and easy to break. The thickness of the discs was measured and found it would be in the range of 0.42 - 0.48 mm. The ground discs were placed in a beaker with 30mL ethanol and sonicated for 20 min. The discs were then dried in air and transferred to the argon filled glove box for the electrolyte preparation.
A three-layer solid electrolyte was prepared using PC (BN), PC (Li2O) and LAGP ceramic discs.
In order to prepare a high performance air cathode, we first prepared a cathode paste. To prepare a cathode paste, 0.75 g of LAGP (Lithium Aluminum Germanium Phosphonate) powder, a lithium ion conductive ceramic powder-obtained from the University of Dayton Research Institute, 0.3 mL of Darvan CN (an ammonium polymethacrylate), and 4 mL of DI water were mixed and milled for one hour. An activated carbon mixture (0.15 g of PWA and 0.1 g of Ketjen Black) and 0.4 mL of PTFE were added to the milled mixture and milled for another hour. The mixture paste, called cathode paste, was transferred to a vial and stored for use for fabricating the
air cathode. Once cathode pastes were prepared, fabrication of a complete air cathode was followed afterwards. For this, four Ni foam (Novamet, 110 PPI, thickness 1.6 mm, density 420 g/m2) discs were punched from a sheet of material using a 19 mm punch. The cathode paste, described above, was applied to the nickel foam on one side using a spatula. This Ni foam with the cathode paste was covered with aluminum foil and sandwiched between two stainless steel plates. Then the entire stainless plate was covered with the previously prepared oxygen permeable membrane comprising of fluoro HBPCS and pressed with a 27.4 KN force using a laboratory hydraulic press. Thereafter, these cathode samples were dried at room temperature for 3 days and at 70˚C for 14 hours. The dried cathodes were weighed and the thicknesses were measured. The cathodes were sintered in an argon atmosphere at 300˚C for six hours to remove the dispersant and obtain a mechanically stable cathode structure (oven purge rate: 30 SCFH for the first 2 hours, then decreased to 10 SCFH for the rest of the process). After sintering, the cathodes were transferred to an argon purged glove box. The properties of prepared air cathodes are presented in
In order to prepare a real-world working lithium-air battery, we purchased IEC standard CR-2032 button cell case from the market place. The top cap of the CR-2032 button cell case needs to have holes present to allow for the introduction of air and/or oxygen to interact with the cathode. The top caps of the CR-2032 button cell cases were drilled with 29 holes using a #60 drill bit (1.016 mm diameter) using a drill press. A design template was created in Photoshop to uniformly place the holes in the area available. Each point was first indented using a nail and hammer to guide the drill bit to the proper location. The top part of the cap was then smoothed out using a grinding attachment on a Dremel tool.
Button cells (type CR-2032) were assembled in a glove box under an argon atmosphere. The cathode was first placed in a button cell battery case cap with 29 holes drilled in the cap to enable air/oxygen entry. A wave spring, a spacer, and an aluminum foil disc were put in the bottom portion of the battery case in that order. A lithium foil plate (20 mm by 20 mm) was cut out from a lithium foil roll and scrubbed with a sharp blade to get rid of the surface lithium oxide layer on both sides. After the surface was cleaned, the lithium foil plate was punched to form a 15 mm diameter disc and placed on the top of the aluminum foil disc. The lithium foil plate will act as an anode of the button cell battery. The previously prepared three-layer electrolyte laminate was then placed on the top of the lithium disc and the cap with the air cathode was placed on top to cover the bottom portion of the case. The whole assembly was placed into a ziploc bag and taken out of the glove box. Finally, the button cell battery assembly was taken out of the ziploc bag and placed on a button cell crimping machine die and crimped
Sample number | Ni foam weight (g) | Ni foam + wet paste (g) | Ni foam + dry paste (g) | Dry paste weight (mg) | Weight after sintering (g) | Weight lost uring sintering (mg) | Thickness (mm) |
---|---|---|---|---|---|---|---|
JC806-89-01 | 0.1070 | 0.2108 | 0.1542 | 47.2 | 0.1498 | 4.4 | 0.280 |
JC806-89-02 | 0.1142 | 0.2395 | 0.1677 | 53.5 | 0.1627 | 5.0 | 0.260 |
JC806-89-03 | 0.1046 | 0.2501 | 0.1557 | 51.1 | 0.1512 | 4.5 | 0.270 |
JC806-89-04 | 0.1163 | 0.2750 | 0.1789 | 62.6 | 0.1737 | 5.2 | 0.280 |
JC806-89-05 | 0.1130 | 0.2549 | 0.1665 | 53.5 | 0.1613 | 5.2 | 0.260 |
JC806-89-06 | 0.1155 | 0.3022 | 0.1710 | 55.5 | 0.1661 | 4.9 | 0.250 |
JC806-89-07 | 0.1142 | 0.2618 | 0.1746 | 60.4 | 0.1686 | 6.0 | 0.270 |
JC806-89-08 | 0.1152 | 0.2376 | 0.1670 | 51.8 | 0.1621 | 4.9 | 0.260 |
JC806-89-09 | 0.1160 | 0.2568 | 0.1715 | 55.5 | 0.1659 | 5.6 | 0.250 |
quickly to fabricate lithium-air button cell battery. Once fabrication was completed the lithium-air button cell was available for performance evaluation.
To evaluate the overall conductivity of the tri-layered solid electrolyte, we first calculated the activation energy of each component of the tri-layered solid electrolyte in order to understand the temperature and conductivity relationship. To calculate activation energy of the individual component, we used Arrhenius equation as shown in Equation (3).
where Ea, R and T are the activation energy, gas constant, and absolute temperature, respectively, and
This is a linear equation of y = mx + b, where x = 1/T and m = −Ea/R = slope.
Lithium-air button cells were fabricated utilizing the Ni/C/LAGP based cathode, the PC (Li2O)/LAGP/PC (BN) based solid electrolyte, and a lithium metal anode. The cells were tested under dry air with 0.1 mA discharge/charge current at elevated temperatures.
In a lithium-air cell, the cathode is comprised of ionic and electronic conductors. In order to optimize the cell performance, the composition of ionic and electronic conductors must be in a proper ratio. It is very evident that the ratio may affect both physical properties such as surface area, pore volume, and porosity of the cathode as well as electrochemical properties such as cell capacity, cell impedance, energy density, and power density of the cell. To assess the effect of the cathode composition, three different cathodes were prepared with different LAGP (ionic conductor)/C (electronic conductor) ratios 75/25, 50/50 and 25/75. In addition, it is known that catalysts can enhance the Li-air cell performance since the oxygen reduction reaction is normally very slow. Thus, commercial Co3O4 nano-powder was explored as a catalyst. In total, five air cathodes were fabricated and five lithium-air button cells had been assembled using these cathodes while keeping other parameters constant.
Cell number | Cell 4 | Cell 5 | Cell 6 | Cell 806-188-2 | Cell 806-188-3 | Cell 816-19-1 | |
---|---|---|---|---|---|---|---|
LAGP/C ratio in the cathode | 75/25 | 75/25 | 75/25 | 50/50 | 50/50 | 25/75 | |
Catalyst | none | none | Co3O4 | none | Co3O4 | none | |
Capacity (mAh)* | 1c | 1.4/3.7 | 1.3/1.6 | 1.6 | 2.0/11.0 | 7.0/20.0 | Submitted for testing |
2c | 0.4/0.7 | 0.7/0.9 | 1.5/1.7 | 5.0 | 4.5 | ||
3c | 0.2/0.4 | 0.3/0.4 | 0.7/0.9 | 2.1 (2 V) | Increased current to 0.2 mA, and the cell died | ||
4c | 0.4/0.7 | 0.2/0.2 | 0.5 /0.7 | 1.0 (2 V) | |||
5c | 0.1/0.2 | 0.2/0.2 | 0.2/0.3 | 1.0 (2 V) | |||
6c | 0.1/0.2 | 0.2/0.2 | 0.3/0.3 | - | |||
Specific capacity (mAh/gC) | 152/402 | 125/154 | 160 | 103/567 | 407/1163 | Submitted for testing |
the button cells. From
In order to know the effect of oxygen availability or concentration in the air cathode of lithium-air battery, the battery cell was tested in a partially sealed Ziploc plastic bag which was connected to a dry air cylinder. The Ziploc bag was placed in a temperature controllable oven and the temperature of the oven was kept at the desired value during the test.
As shown in
As shown in
Sample | Cathode | Electrolyte | Anode | Temp. (˚C) | Resistance (ohm) |
---|---|---|---|---|---|
JC806-126-T1 | Li | PC(BN)/LAGP/PC(Li2O) | Li | 24 60 70 80 | −2000 170 120 72.6 |
JC806-126-T2 | Li | PC(BN)/LAGP/PC(Li2O) | Li | 24 60 70 80 | −2000 184 96.0 70.0 |
JC806-126-S1 | Li | Single layer PC(Li2O) | Li | 24 60 70 80 | −3000 190 145 105 |
JC806-126-S2 | Li | Single layer PC(Li2O) | Li | 24 | −2000 |
Cell 6 | 75/25 LAGP/C | PC(BN)/LAGP/PC(Li2O) | Li | 23 60 80 | 3350 320 84 |
perature while performing much better at elevated temperature as shown in Figures 9-11. Surprisingly, the replacement of the tri-layer electrolyte with only one polymer layer didn’t decrease the resistance, although the thickness of the electrolyte was reduced to less than one-third as can be seen from
In order to verify the interfacial contact resistance problem, a cell was fabricated with a 75/25 LAGP/C cathode without Co3O4 catalyst, a PC(Li2O)/LAGP/PC(BN) solid electrolyte, and a Li metal anode − just similar to the cell 6 but with no Co3O4 catalyst. The interface of Li and the electrolyte was wetted with a silicon containing electrolyte (1.0 M LiTFSI in MeC(O)OCH2CH2SiMe2O(CH2CH2O)3Me, conductivity: 7.0 × 10−4 S/cm) to make better contact with the electrolyte. The properties of the new cell, JC806-188-1, is presented in
A tri-layered solid electrolyte and air cathode for lithium-air battery was synthesized using high performance ionic and electronic conductive materials. Characterization of solid electrolyte and air cathode materials were performed through FTIR and TGA measurement. Detailed fabrication procedures for solid electrolyte, air cathode
Cell number | Cathode composition | Electrolyte | Anode | Capacity (mAh) | Specific capacity (mAh/gC) |
---|---|---|---|---|---|
JC806-188-1 | 75/25 LAGP/C No catalyst | PC (Li2O)/LAGP/PC (BN) | Li | 5.1 | 331 |
and real-world lithium-air battery cell were discussed in this paper. Based on experimental four-probe conductivity measurement, it was found that the tri-layered solid electrolyte possesses a very high conductivity of
This work is accomplished under the funding support provided by the U.S. Department of Energy (DOE) −Re- newable Energy Program, grant award number DE-EE 0003109.
Susanta K. Das,Jianfang Chai,Salma Rahman,Abhijit Sarkar, (2016) Synthesis, Characterization and Performance Evaluation of an Advanced Solid Electrolyte and Air Cathode for Rechargeable Lithium-Air Batteries. Journal of Materials Science and Chemical Engineering,04,74-89. doi: 10.4236/msce.2016.41012