Separator coatings as a strategy to improve the cycling performances of lithium batteries

YASNÓ, JUAN PABLO; FRANCIA, CARLOTTA; BODOARDO, SILVIA; GAMBA, MARTINA; GÓMEZ, SOFÍA; SUAREZ, GUSTAVO; VISINTIN, ARNALDO; CABALLERO, MARÍA FLORENCIA; COZZARÍN, MELINA; REAL, SILVIA

La Paz

Workshop; IWLiME 2019: 6th International Workshop on Lithium, Industrial Minerals and Energy; 2019

Universidad Católica Boliviana - CeLiMin - YLB

Lithium batteries are essential components of most electrical devices. They are used as the power source in laptops, smartphones, and cameras as well as in the implementation of electrical vehicles.Lithium-sulfur batteries have advantages over current lithium-ion technology and are presented as the future battery for electric cars and portable equipment, since they have theoretical capacities of 2,600 Wh/kg and sulfur cathode is low cost. However, in normal battery operation, complex polysulfide are generated that cause a loss of active material when migrating to the anode. Furthermore, as sulfur is an insulator, the cathode must contain a conductive additive to maintain the electrical contact of the sulfur with the current collector. In addition, the use of metallic lithium anodes would increase the capacity obtained from current batteries up to ten times, although lithium reactivity and dendrite formation and growth represent the main obstacles because they perforate the separators and produces circuit breakers.In Argentine many researchers are focused on preventing some of the problems associated with lithium-sulfur technology. In this work, we used a polymeric and a ceramic coating on the separator as a strategy to avoid dendrites formation. On one hand, the polymeric coating consisted in a montmorillonite (MMT)/polyaniline (PANI) composite. PANI has already been widely used in many fields due to its low density, high electronic conductivity, ease of preparation and unique physical/chemical properties. However, PANI prepared in the absence of a support is usually powder and difficult to serve as an interlayer material alone. In that sense, montmorillonite, a T:O:T phyllosilicate was used as a matrix to polymerize aniline due to its colloidal size, great cation exchange capacity (80-100 mmol/100g), variable and flexible interlayer spacing, good mechanical properties and because it is wettable and permeable to polar solvents, such as alcoholic and ether electrolytes. On the other hand, the ceramic coating was formed by monoclinic lithium zirconate (m-Li2ZrO3) which is an excellent Li+ conductor. In its structure, Li+ are located among the ZrO3 2−layers and offer a three-dimensional tunnel for Li+ diffusion. This feature, in addition to its thermodynamic stability against Li and chemical inertia in common nonaqueous electrolytes makes this material commonly used as coating to suppress the dissolution of cathode material, such as spinel LiMn2O4 or LiNi0.5Mn1.5O4 at high charge potential [1]. PANI/MMT composite was obtained by chemical polymerization whereas a ratio PANI/MMT = 12.4/87.6 (w/w) was used. The powder was characterized by x-ray diffraction (XRD), thermal analyses, zeta potential measurements, and infrared and x-ray photoelectron spectroscopies (FTIR and XPS, respectively). XRD showed that the polymer was inserted between the montmorillonite sheets. Zeta potential measurements and XPS showed the presence of polymer also in the external surface of the clay. The presence ofabsorptions band at wavelengths >2000 cm-1 and ~1240 cm-1 in FTIR let us establish that the form of PANI was the emerladine salt (the conductive form of polyaniline).The m-Li2ZrO3 was prepared by solid-state reaction, using 5% excess of Li2CO3 over ZrO2, followed by grinding and heat treatment at 1000 °C for 12 h [2]. The materials obtained were left at room temperature and ground in a ball mill for 20 h until sizes smaller than #100 mesh were obtained. The solid was characterized by XRD and Rietveld refinement studies and thermo gravimetric analyses. Both techniques showed that the desired phase was obtained with high purity. To evaluate their usage as an interlayer in lithium batteries, the PANI/MMT composite or the mLi2ZrO3 was mixed with polyvinylidene fluoride (PVDF) as a binder (ratio 80:20) and a suspension was formed with n-methylpyrrolidone (NMP). The slurries were cast on a commercial polypropylene separator. The obtained coated separators were then cut into discs of 20 mm. Li/Li symmetric cells were prepared using metallic lithium as both cathode and anode in order to perform lithium plating and stripping galvanostatic cycles (current density of 5mA/cm2 ) to study both the lithium conduction of the samples and their ability to protect metallic lithium anode.As can be seen in Figure 1, in the first 50 charge/discharge cycles the cell with the PANI/MMT coating showed the lowest average overpotential (22 mV) and highest stability. This indicated that in fact there is a performance improvement of lithium plating and stripping when compared to the cell with the separator without coating, which showed an average overpotential of 33 mV. Thismeans that the polymer coating improved Li+ conduction probably because the regular structured configuration of intercalated polymer chains in the interlayer space provided a good 2D channel for Li+ transition and propitiated a more uniform current distribution on metallic lithium surface reducing dendrite formation. Presence of montmorillonite can also act as a mechanical barrier impeding dendrite growth.The ceramic coating showed an increase in the average overpotential up to 80 mV, probably because the dense Li2ZrO3 ceramic coating increases the resistance, which also implies that this ceramic coating could influence the rate capability of batteries although it may improve the safety and long-term cycling performance of the separator. Further studies are being developed with the aim of understanding the processes involved in each system.