Interaction of High Flash Point Electrolytes and PE-Based Separators for Li-Ion Batteries

Abstract

In this study, promising electrolytes for use in Li-ion batteries are studied in terms of interacting and wetting polyethylene (PE) and particle-coated PE separators. The electrolytes are characterized according to their physicochemical properties, where the flow characteristics and the surface tension are of particular interest for electrolyte-separator interactions. The viscosity of the electrolytes is determined to be in a range of η = 4-400 mPa∙s and surface tension is finely graduated in a range of γL = 23.3-38.1 mN∙m(-1). It is verified that the technique of drop shape analysis can only be used in a limited matter to prove the interaction, uptake and penetration of electrolytes by separators. Cell testing of Li|NMC half cells reveals that those cell results cannot be inevitably deduced from physicochemical electrolyte properties as well as contact angle analysis. On the other hand, techniques are more suitable which detect liquid penetration into the interior of the separator. It is expected that the results can help fundamental researchers as well as users of novel electrolytes in current-day Li-ion battery technologies for developing and using novel material combinations.

Keywords: Li-ion battery; contact angle; electrolytes; safety; separators.

Figure 1

Chemical structures of ethylene carbonate (EC), dimethyl carbonate (DMC), dimethyl sulfone (DMSN), 3-methyl-1-propylpyrrolidinium bis (trifluoromethanesulfonyl) azanide (MPPyrr-TFSA), glutaronitrile (GN) and sulfolane (SL).

Figure 2

Temperature-dependent values of the density of mixtures M-n (n = 1–6).

Figure 3

DSC measurements of mixtures M-n (n = 1–6) in closed cup during cooling (a) and heating (b) at 10 K·min⁻¹ (exo down).

Figure 4

Temperature-dependent values of the viscosity (a) and conductivity (b) of mixtures M-n (n = 1–6) between 20–100 °C (viscosity) and 20–90 °C (conductivity). See Table S3 for detailed values.

Figure 5

Scanning electron microscopy (SEM) picture of the surface of separator COD-20 (a) and COATED (b).

Figure 6

SEM picture of the cross section of separator COATED prepared by ion beam cutting.

Figure 7

Time-dependency of the contact angle between separator COATED and mixture M-2 as a representative for all electrolyte mixtures and both separators.

Figure 8

Time-dependent contact angles between separator and mixtures. (a) Contact angles between mixture M-1 and both separators are depicted within 10 min; (b) Contact angles between mixture M-n (n = 2–6) and separator COD-20 are shown within 24 h; and (c) Contact angles between mixture M-n (n = 2–6) and separator COATED are compared within 24 h.

Figure 9

Representative photographs of drops onto separator foils 5 h after dropping (inside glove box) with selected mixtures: (Above) separator COD-20; and (Below) separator COATED.

Figure 10

Cell cycling of NMC|Li (NMC = LiNi_1/3Mn_1/3Co_1/3O₂) half cells with separator GF/B, COD-20 and COATED for each electrolyte mixture without additional additives. After a short-term jump charge (3.5 V vs. Li/Li⁺), the cells are rested at open circuit for 24 h. Afterwards, the cells are cycled between 3–4.2 V vs. Li/Li ⁺. Parameters: Coin cells CR 2032, T = 25 ± 1 °C, active mass (NMC) = 12.2 ± 0.5 mg∙cm⁻², d (separator) =17 mm, d (Li, NMC) = 16 mm. All separators were soaked in the electrolyte for 4 h additionally.

Figure 11

Backside wetting of separator COD-20 by two selected mixtures before (left) and after (right) pulling the separator off the tape. Above: No wetting is observed in case of mixture M-2; Below: Wetting of the backside can be observed in case of mixture M-1 (t = 5 min). Arrows and circles are painted for a better illustration of the drops.