Sulfone As Additives For High-voltage Lithium Ion Battery

With the wide application of lithium ion batteries, high-voltage lithium ion battery systems become research hotspots in recent years. Electrolyte, as an important part of lithium ion batteries, is facing challenges. On one hand, oxidation decomposition will occur on the traditional carbonate electrolyte when the voltage is above 4.5 V. On the other hand, many new electrolytes with higher anodic stability have not been widely used due to high viscosity and low conductivity. There is no doubt that the stability of electrolyte under high voltage is crucial to the lithium ion battery system and also affects the development of high voltage lithium ion batteries. According to literatures, one of the most effective and economic methods is to introduce a certain amount of functional additives into electrolyte, which can stabilize the battery system under high voltage so as to improve the electrochemical performance of batteries.In the present work, we select sulfone as electrolyte additives for high-voltage batteries. Electrochemical performances of LiNi1/3Co1/3Mn1/3O2/Li, graphite/Li and LiNi1/3Co1/3Mn1/3O2/graphite batteries in reference electrolyte (STD), that is 1.0 M LiFP6 in EC/DMC (1/1, by volume), with different additives are investigated and the mechanism behind the improvements is explored. Testing methods used in this paper include constant current-charge/discharge test, linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The main conclusions obtained are as follows:1. The effects of sulfolane (SL) as an electrolyte additive in high-voltage battery systems:(1) The electrochemical window and oxidation stability of the reference electrolyte can be improved. And the reduction decomposition of EC solvent will be restrained after adding 2% SL into the reference electrolyte.(2) The electrolyte with 2% SL has a good compatibility with LiNi1/3Co1/3Mn1/3O2 cathode and graphite anode. The discharge capacity, cycle performance and rate performance of LiNi1/3Co1/3Mn1/3O2/Li, graphite/Li, LiNi1/3Co1/3Mn1/3O2/Li batteries are improved.(3) The mechanisms for enhancing the battery capacity retention are investigated through LSV, EIS, and SEM. Results indicate that SL improves the stability of STD against oxidative decomposition and participates in the formation of interface films on the cathode and anode surfaces. These stable and uniform films display low interfacial resistances, which effectively suppress the subsequent decomposition of electrolyte and prevent electrodes from destruction.2. The effects of ethyl isopropyl sulfone (EIS) and ethyl vinyl sulfone (EVS) as electrolyte additive on high-voltage battery systems:(1) When EIS as an electrolyte additive, the electrochemical window of the reference electrolyte can be improved to 4.5V. The discharge capacity and cycle stability of LiNi1/3Co1/3Mn1/3O2/Li cells under high voltage are improved, but the resistance is increased and electrochemical properties of the graphite anode in the electrolyte containing the additive system are not satisfied.(2) The compatibility of the electrolyte containing ethyl vinyl sulfone with LiNi1/3Co1/3Mn1/3O2 cathode is not satisfied. The cycle performance of LiNi1/3Co1/3Mn1/3O2 is not stable and the coulombic efficiency remains below 100% during cycling. The possible reason for the bad performance is the increase of the irreversible property and the electrochemical polarization of LiNi1/3Co1/3Mn1/3O2 during the charge-discharge process after adding the EVS into the reference electrolyte. Although EVS can be reduced firstly at 1.2 V, it failed to stabilize the battery system. The cycle performance of graphite/Li is not good in the electrolyte with EVS.