Functional Electrolytes For High-voltage LicoO₂Batteries

Abstract:The energy density of lithium-ion batteries has been unable to meet the modern requirements of application in vehicles, power stations and other fields. Therefore, methods to improve the energy density become a research hotspot in recent years. Extending the charging voltage of cathod materials has been proven to be an effective way. However, conventional electrolyte based on carbonate solvents will be oxidized above4.5V. Moreover, transition-metal ions in cathode surface can catalyze the oxide decomposition begins at lower potential and forms a resistive substance on the electrode surface during the high-voltage cycle process. As a result, the cycle performance of lithium-ion batteries would be deteriorated seriously. Electrolyte has become one of the key factors restricting the development of advanced lithium-ion batteries. Therefore, it is meaningful and urgent to research on high voltage electrolytes.Currently, LiCoO2can only take use of half theoretical specific capacity due to the influence of the crystalline structure change by excess lithium extraction at higher charging voltage far beyond4.2V and the oxidation of nonaqueous electrolyte. Charging LiCoO2to4.5V can effectively increase its reversible capacity by approximately30%, but lead to capacity fade in the following cycles. In this work, we focused on the modification of the electrolytes to improve the high-voltage performance of LiCoO2. A class of electrolyte additive for surface film on cathode including thiophene (T),2-methyl-thiophene (DMT),2-thiophenecarbonitrile (TC), benzothiophene (BT) and a kind of fluorinated solvent (FEC) with higher oxidation potential are studied via combination of quantum chemistry calculations, linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and battery performance, charge-discharge test, morphology, composition and structural analysis.Theoretically, thiophene can be preferentially oxidized for higher HOMO value comparing with electrolyte solvents. Introduction of-CN (electron-withdrawing group) can decrease the HOMO value, so the oxidizing of TC is more difficult than thiophene. Introduction of-CH3and phenyl (electron-donating group) can increase the HOMO value so that DMT and BT are more susceptible to oxidation. Ethylene carbonate (EC) can lower the HOMO value and improve its anti-oxidation stability through replacing H by F atoms. FEC combination with the conventional solvents can improve the overall antioxidant capacity of electrolyte.With the addition of0.1%thiophene into the conventional electrolyte, the capacity retention and high temperature performance of LiCoO2cycled in3.0-4.5V are significantly improved. But the additive is likely to cause localized overcharge when charging in high current density. Thiophene electrochemically oxidizes at charge voltage of4.3V and forms a condensity film with less LiF on the cathode surface. Consequently, the oxidation of electrolyte can be suppressed. The charge transfer resistence of cycled cell with thiophene is much lower and the interface is beneficial to migration of Li+. What’s more, thiophene has no adverse effect on the performance of Li/AG battery.The interaction between highly oxidized states of Co and electronegative-CN would cause strong metal-ligand coordination. TC can preferentially polymerize in surface with transition metal ions district and improve rate performance. But there is a certain impact on cycling performance of anode. BT irreversibly consumes a large amount of Li+in the initial charging process and lead to lower specific discharge capacity. But it is benifite to high-voltage cycling stability. Cells with DMT polarize seriously so as to worsen cycling performance. In conclusion, thiophene without substituted groupe has better overall performance for LiCoO2under high voltage operation.Compound additive containing LiDFOB and BT participate the forming of surface film on cathode jointly. It can maintain excellent cycle stability and improve discharge capacity in the mean time. Compound additive will be the development trend of functional electrolyte. The negative impact of this compound additive on cycling performance of graphite anode can not be ignored.FEC as cosolvent can improve the overall stability of electrolyte. Meanwhile, VC, the decomposition intermediate of FEC, can oxidize and polymerize to form an alkyl lithium salt enriched surface film on positive electrode. At the same time, the high-impedance component (LiF) decreases. The cycle and rate performances LiCoO2/Li in high voltage operation are improved significantly with10%FEC. FEC can suppress the increasings of polarization and charge transfer impedance. Also, the capacity retention of LiCoO2/AG battery increases with FEC. It is obviouse to suppress the rise of resistance and volume expansion. Fluorinated solvents have good prospects in high voltage electrolyte.