Ultrathin ALD Aluminum Oxide Thin Films Suppress the Thermal Shrinkage of Battery Separator Membranes

Abstract

Thermal runaway is a major safety concern in the applications of Li-ion batteries, especially in the electric vehicle (EV) market. A key component to mitigate this risk is the separator membrane, a porous polymer film that prevents physical contact between the electrodes. Traditional polyolefin-based separators display significant thermal shrinkage (TS) above 100 °C, which increases the risk of battery failure; hence, suppressing the TS up to 180 °C is critical to enhancing the cell's safety. In this article, we deposited thin-film coatings (less than 10 nm) of aluminum oxide by atomic layer deposition (ALD) on three different types of separator membranes. The deposition conditions and the plasma pretreatment were optimized to decrease the number of ALD cycles necessary to suppress TS without hindering the battery performance for all of the studied separators. A dependency on the separator composition and porosity was found. After 100 ALD cycles, the thermal shrinkage of a 15 μm thick polyethylene membrane with 50% porosity was measured to be below 1% at 180 °C, with ionic conductivity >1 mS/cm. Full battery cycling with NMC532 cathodes demonstrates no hindrance to the battery's rate capability or the capacity retention rate compared to that of bare membranes during the first 100 cycles. These results display the potential of separators functionalized by ALD to enhance battery safety and improve battery performance without increasing the separator thickness and hence preserving excellent volumetric energy.

Figure 1

(a) Thermal shrinkage of bare polymer separators as a function of temperature after 30 min exposure. Top-view SEM showing the different microporous structures of (b) PE48 and (c) PE83.

Figure 2

(a) Thermal shrinkage of trilayer separators as a function of the temperature of trilayer separator for different amounts of ALD cycles after 30 min exposure at a given temperature and (b) static water contact angle of the same samples measured 1 s after the deposition of a 2 μL drop.

Figure 3

(a) Thermal shrinkage of PE separators coated by ALD after 30 min exposure at 180 °C and (b) static water contact angle of PE48 separator for the increasing number of ALD cycles with hold step and angle measured 1 s after deposition of a 2 μL drop. The data is an average of four measurements per side. HS indicates a hold step after dosing and before purging the ALD precursors.

Figure 4

(a) Gurley value for the two samples of PE48 separator for an increasing number of ALD cycles and (b) ionic conductivity of the same samples in symmetrical stainless steel coin cells. EIS data is an average from two coin cells tested eight times each.

Figure 5

(a) Electrochemical performance of NMC532/graphite full cells in 1M LiPF6 in a mixture of EC/DMC/DEC 1:1:1 using PE48 bare and with 25, 50, and 100 #cy. Discharge capacity averaged from two batteries at different rates after two forming cycles at 0.2C and (b) subsequent capacity fading performance and Coulombic efficiency at 1C charge and 1C discharge.