Effects of Additives in Dual-Salt/Carbonate Electrolytes on Performances of Lithium Metal Batteries

The lithium (Li) metal battery (LMB) is a promising high-energy-density energy storage system. However, the uncontrollable dendrite formation and low Coulombic efficiency (CE) associated with the Li metal anode have largely hindered the practical application of LMBs [1]. During recent years, extensive efforts have been made to address the main problems of Li metal anode and to improve the electrochemical performance of the related LMBs, and plenty of promising results have been achieved. Among the reported approaches, the reformulation of electrolytes with different Li salts and additives has been regarded as an easy and cost-effective method. The conventional LiPF6/carbonate electrolytes have been found to be detrimental to Li metal anode especially at high charge current densities, which is mainly due to the serious corrosion of Li metal anode, the consumption of electrolyte, the continuous formation of solid electrolyte interphase, and the increase of cell impedance. The highly concentrated electrolytes of LiTFSI or LiFSI in ethers have been reported to be effective to suppress Li dendrites growth and to improve the Li CE [2]. However, the ether-based electrolytes normally are not electrochemically stable at the voltages around or above 4 V. In our previous works, we have systematically investigated the dual-salt electrolytes of LiTFSI-LiBOB in carbonate solvent mixture and their effects on the performances of Li||LiFePO4 [3], Li||LiNi0.80Co0.15Al0.05O2 [4] battery systems. It has been found that the dual-salt electrolytes are favorable to protect Li metal anode and to improve the rate capability of the LMBs. Especially, our recent work discovered that further significant improvements in fast charging capability and long-term cycling stability of the Li||LiNi0.4Mn0.4Co0.2O2 batteries can be achieved by adding an optimal additive amount of LiPF6 to the LiTFSI-LiBOB/carbonate electrolyte [5]. Nevertheless, this electrolyte still leads to relatively low Li CE, which is about 90.6%. In order to further improve the Li CE of the dual-salt/carbonate electrolyte, we systematically investigated different additives such as VC, FEC, VEC, etc. By using certain additives the Li CE in the electrolytes of LiTFSI-LiBOB dual-salt in carbonate solvents can be improved from 90.6% to 98.1%. Moreover, the electrochemical performances of the LMBs with a 4-V Li-ion cathode using the newly developed dual-salt electrolytes were studied, and enhanced battery performances have been obtained. The promising results will be reported at the meeting.