EFFECT OF ADDITIVES ON LITHIUM CYCLING EFFICIENCY

Lithium cycling efficiency was evaluated for LiAsF6-ethylene carbonate/2-methyltetrahydrofuran mixed-solvent electrolyte (LiAsF6-EC/2MeTHF) with several additives: tetraalkylammonium chlorides with a long n-alkyl chain and three methyl groups. The ammonium chlorides with n-alkyl group longer than n-C-12H25- increased lithium cycling efficiency. Cetyltrimethylammonium chloride (CTAC) produced the best improvement in lithium cycling efficiency. A figure of merit (FOM) of lithium for 0.01M CTAC was 46, which was 1.5 times the FOM for the corresponding additive-free electrolyte. The LiAsF6-EC/2MeTHF with CTAC showed an increase in FOM with stack pressure, but the effect was less than that for the additive-free LiAsF6-EC/2MeTHF. Scanning electron microscope observation showed that the addition of CTAC decreased the needle-like lithium deposition and increased particulate lithium deposition. This deposition morphology may be the main cause of the increase in FOM. The additive had no effect on rate capability for cell cycling at 3 mA/cm2 discharge and 1 mA/cm2 charge.