Study On The Electrolyte Additives For Si-C/Ni-rich System Of Lithium-ion Batteries

With the large-scale application of power batteries and energy storage battery systems,the development of a new generation of lithium-ion batteries with high specific energy has become the focus of the battery industry.High nickel cathode materials and silicon carbon anode materials are the main cathode and anode material systems for the new generation of lithium-ion batteries.The electrolyte in the lithium-ion batteries is an important medium for transferring lithium ions and communicating cathode and anode materials,and plays an important role in the battery performance,including capacity and cycle stability.Electrolyte additive is one of the most important components of the electrolyte,the core of functional electrolyte development,and one of the most economical and effective methods to improve the electrochemical performance of lithium-ion batteries.The main content of this paper is the study of electrolyte additives for high nickel cathode and silicon carbon anode systems.We know that high nickel materials have poor interface stability and are susceptible to damage from acidic substances in the electrolyte,resulting in poor cycle stability of high nickel cathodes.In order to improve the electrochemical performance of high nickel cathodes,we have studied the influence of two film-forming and acid removing additives,1,3,5-tris(3,3,3-trifluoropropyl)methylcyclotrisiloxane(3FO)and 1,3-divinyltetramethyldisiloxane(2SiO)for LiNi0.8Co0.1Mn0.1O2(NCM811)cathodes.Studies have shown that the capacity retention rates of Li/NCM811 cell for 100 cycles with 1wt%3FO or 1wt%2SiO are respectively 93.6%and 89.9%,which are higher than the 82.4%of baseline electrolyte.And 3FO or 2SiO can also increase the capacity retention rate of Li/NCM811cell after 400 cycles(1wt%3FO:63.3%,1wt%2SiO:50.7%,Base:37.5%).3FO and 2SiO additives can suppress the polarization increase and impedance deterioration of the Li/NCM811 cell during the cycle,and increase the cycle stability.In addition,3FO and 2SiO additives can preferentially undergo oxidation reactions,inhibit excessive decomposition of the electrolyte solvent,and can also remove part of the HF in the electrolyte to generate a stable interface structure.Based on theoretical calculations and liquid nuclear magnetic technology,it is speculated that the 3FO additives may decompose to form SiOx and-CF3 compounds during the charge and discharge process,which improves the interface stability.For the silicon-carbon(Si-C)anode material,the huge volume effect in the process of electrochemical insertion and removal limits its application and development.In order to improve the cycle stability of Si-C anode,we studied the effects of two fluorine-containing film-forming additives,3FO and Heptafluoropropyltrifluorovinylether(AFO)on Si-C anodes.Studies have shown that adding 2wt%3FO or 2wt%AFO to the baseline electrolyte can increase the charging specific capacity of Li/Si-C cell for 500 cycles.The charging specific capacity of Li/SiC cell with 2wt%3FO+5wt%FEC and 2wt%AFO+5wt%FEC electrolyte for 500 cycles are 336.1 mAh g-1 and 341.5 mAh g-1,respectively,higher than 307.3 mAh g-1 of 5wt%FEC,and are significantly higher than the 250.7 mAh g-1 of baseline electrolyte.In addition,the Li/Si-C cell can circulate stably for 1200 cycles with 2wt%3FO+5wt%FEC electrolyte.The reduction potential of 3FO or AFO is higher than that of EC,EMC and FEC,which can inhibit the decomposition reaction of the electrolyte and generate a more stable interface component.And the addition of 2wt%3FO or 2wt%AFO can also increase the capacity retention rate of the Si-C/NCM811 full cells.