Study On Functional Electrolytes For Rechargeable Lithium Battery

This paper reviewed the history and its significance in the new green energy system of rechargeable lithium battery and the challenges for the related materials. At present, the safety issue is one of the principal risks facing its development. As a key component of a battery, electrolyte has a potent influence on its safety, operating temperature, charge/discharge performance and cycle life. Therefore, the development of safe electrolyte for lithium battery has become a hot research topic.With its high electrochemical and thermal stability, ionic liquid has been regarded as one of the most promising alternative to the carbonates used as electrolyte in lithium battery, about which worldwide research has been launched. In this paper, we carried out several ionic liquid/co-solvent electrolyte systems for various applications and obtained main achievements and results as follows:(1) Novel electrolytes based on ionic liquid for wide operating temperature range. Novel electrolytes were prepared based on (N-methoxyethyl-N-methylpyrrolidinium bis (trifluoromethanesulfonyl)-imide, Pyr1,2oiTFSI) and sulfites as co-solvents with lithium difluoro (oxalate) borate (LiODFB) as lithium salt in order to solve the safety and the narrow operating temperature range issues for lithium batteries. The LiODFB-Pyr1,2o1TFSI/sulfite electrolyte showed high conductivity, ability for film-forming, wide operating temperature range and safety. Compared to the ring-like ethylene sulfite (ES), the chain-like dimethyl sulfite (DMS) improved more effectively the electrochemical performance of the bulk electrolyte. The conductivity and Li+transference number of the LiODFB-Pyr1,2o1TFSI/DMS electrolyte was8.163×10-3S cm-1and0.28, respectively. The Li/MCMB and Li/LiFePO4cells containing this electrolyte had good cycle performance and normal charge curve in the temperature range of-40℃to60℃. Besides, the electrolytes with30wt%sulfites showed non-flammability.(2) Novel electrolytes based on ionic liquid with wide electrochemical window. The ring-like ionic liquid N-butyl-methyl piperidinium bis(trifluoromethylsulfonyl)imide (PP14-TFSI) was picked to be the main component of the electrolyte with wide electrochemical window. The ring-like co-solvent tetramethylene sulfone (TMS) was added and LiODFB was used as lithium salt. In charging process the antioxidant (CO2BF2)2was formed by oxidation of LiODFB and accumulated on the surface of the cathode material to form the solid electrolyte interphase (SEI); In discharging process the inorganic inner layer and organic outer layer forming from [ox]BOS·C4H8and TMS(-H)· radical group was obtained to be SEI on the surface of the anode material. The wide electrochemical window of the electrolyte brought possibility to be compatible with high-potential cathode and the SEI has an improvement on the capacity retention of the electrode materials. Li/Li1.2Ni0.2Mn0.6O2cells with the LiODFB-PP14-TFSI/TMS had the capacity of278.2mAh g-1in the first cycle and above220mAh g-1after50cycles. Li/MCMB cells with this electrolyte showed the capacity of338.6mAh g-1and the coulombic efficiency of above99%after50cycles. As a result, the LiODFB-PP14-TFSI/TMS system is expected as safe electrolyte for rechargeable lithium battery with high voltage.(3) Novel electrolytes based on ionic liquid for lithium/sulfur battery. A series of electrolytes for Li/S battery were prepared based on ionic liquid of (N-methoxyethyl-N-methylpyrrolidinium bis (trifluoromethanesulfonyl)-imide, Pyr1,2o1TFSI) with tris(ethylene glycol) dimethyl ether (TEGDME) as co-solvents in different proportions. LiTFSI was used as lithium salt. The electrolyte systems took advantage of the availability of the two compotents, and exhibited the conductivity in order of10-3S cm-1. Experimental results showed that the excessive Pyr1,2o1TFSI would lead to serious polarization of the cells and the inadequate utilization of the active material; while the excessive TEGDME would result in the "shuttle effect" and the consequent severe decay of the cell capacity. Li/S cells with LiTFSI-(70wt%)Pyr1,2o1TFSI/(30wt%)TEGDME electrolyte exhibited excellent cycle and rate performance, whose capacity was1212.8mAh·g-1in the first cycle and693.5mAh·g-1after100cycles at current density of0.1C, and820mAh·g-1with the coulombic efficiency of above99%at current density of1C. In addition, the cells showed good high temperature performance due to the outstanding thermal stability of the electrolyte, which could operate at80℃with the initial cycle capacity of1005.3mAh g-1. This kind of ether-functionalized ionic liquid electrolyte is a promising candidate material for safe Li/S batteries.(4) Modification of LiODFB to electrolytes based on ionic liquid for lithium/sulfur battery. In order to reduce the irreversible capacity loss during cycling, improve the capacity retention and safety performance of Li/S cells with LiTFSI-(70%) Pyr1,2o1TFSI/ (30%) TEGDME as electrolyte, LiODFB was introduced for the modification. However, the addition of LiODFB as additives would lead to extremely high viscosity of electrolyte and inferior performance of the cells. The electrolyte containing LiODFB and LiTFSI mixture as lithium salt in Pyr1,2o1TFSI/TEGDME showed excellent electrochemical properties. The0.24M LiTFSI/0.16M LiODFB-Pyr1,2o1TFSI/TEGDME system was obtained after optimizing ratio, which would protect Li anode from suffering lithium dendrites by effective SEI forming on the surface and avoid the "shuttle effect" of polysulfide without reducing the utilization of the active material in the S cathode. The Li/S cells containing this electrolyte system showed excellent capacity and cycle performance, whose initial discharge capacity was1264.4mAh g-1, and retained71.9%capacity retention after50cycles with the coulombic efficiency of more than95%.(5) A summary of representative electrolyte systems of ionic liquid with organic co-solvent for lithium battery and the corresponding characteristics and properties based on the investigations above and the previous studies are illustrated. The structure-activity relationship is concluded arising form the trend of most electrolytes and it might be proposed to offer a useful starting point to design the mixed electrolyte in lithium batteries.