Application And Mechanism Study Of N-Methylpyrrolidone In Lithium-Ion Battery Electrolytes

Lithium-ion batteries have attracted great attention because of their small size and high energy conversion efficiency.Nowadays,it can be seen in everything from mobile phone batteries to electric vehicles.Every breakthrough in positive and negative electrodes and electrolyte materials will drive the development of lithium-ion batteries.The behavior of lithium ion intercalation in positive and negative electrode materials occurs at the electrode/electrolyte interface,so understanding this interface is crucial.The interface behavior is closely related to the type of electrolyte,the desolvation and film formation of the electrolyte at the interface can also affect the performance of lithium-ion batteries.On the other hand,with the development of material civilization,people are eager for higher performance batteries.This puts higher demands on the electrolyte.In practice,the high-voltage positive electrode needs to be matched with an oxidation-resistant electrolyte,an electrolyte with a low freezing point needs to be matched under low temperature conditions,and an electrolyte with fast migration of lithium ions needs to be matched during fast charging...Based on this,we used N-methylpyrrolidone(NMP),one of the most common solvents in the lithium-ion battery manufacturing process,to study the effect of solvation structure and interfacial film on battery performance.The research results are as follows:1.As the one of the core electrolyte solvents for Li-ion batteries,ethylene carbonate(EC)is still irreplaceable for its balance of ionic conductivity and interfacial stability.However,it also defines the boundary for the low-temperature performance of the battery because of its high melting point(36.4℃).Its homologous series,propylene carbonate(PC),has been proposed as its convenient substitute for its much lower melting point(-48.8℃).Unfortunately,the propylene carbonate/graphite anode interfacial problem has been a puzzle since the days before the advent of the Li-ion battery.Among the various strategies to mitigate this problem,using a solvent that is strongly coordinated with Li+ so that the content of propylene carbonate is reduced in solvated shells has been shown to be effective,but the mechanism from the interfacial chemistry perspective remains unexplored.Herein,we study a new cosolvent,Nmethylpyrrolidone(NMP),for PC-based electrolyte and observe excellent reversibility that approaches the commercial standard,far beyond the similar systems in the past.To understand the mechanism,solvation chemistry analysis and in situ characterizations are undertaken to probe the interfacial chemistry from various standpoints.Based on these results and further theoretical calculation,it is proposed that N-methylpyrrolidone has mediated the reduction process of propylene carbonate to facilitate the growth of a solid electrolyte interphase(SEI)layer akin to ethylene carbonate.Finally,an electrolyte has also been successfully developed based on the NMP/PC couple to outperform the commercial electrolyte by a clear margin when tested in a LiNi0.8Co0.1Mn0.1O2-graphite cell at-30℃.2.Owing to their high specific capacity and low cost as well as their ability to considerably alleviate mileage issues in electric vehicles,nickel-rich layered cathode materials are widely used in state-of-the-art lithium-ion batteries.However,severe material degradation caused by high contents of nickel limits their large-scale commercial utilization.Additives provide a simple and considerably effective means of improving the interfacial instability of nickel-rich cathode materials.Herein,we propose the use of a multifunctional additive N-Methylpyrrolidone(NMP)to modulate the electrode/electrolyte interface of the LiNi0.6Co0.2Mn0.2O2(NCM622)cathodes.In particular,the specific capacity retention of the battery containing this additive was 72.2%after 500 cycles,which was only 31.1%for the baseline electrolyte.Systemic analysis of the electrode surface shows that the improved performance of the additive-containing battery originated from the fact that NMP can scavenge trace water from the cell system,which inhibits the attack of harmful fluorine species that may otherwise attack the electrode.Moreover,the hydrolysis products of NMP form a protective film on the electrode surface,thereby inhibiting the electrode/electrolyte side reactions.In addition,charge/discharge and cyclic voltammetry tests under overcharge conditions reveal that upon introduction of the NMP additive,a highly resistive NMP-derived protective film covered the NCM622 cathode surface,inhibiting a rapid increase in voltage and protecting the battery components to a certain extent.3.Existing lithium-ion battery electrolyte solvents consist of ethylene carbonate(EC)and chain carbonates.EC can form a good solid electrolyte interphase(SEI film)on the surface of graphite anode,and inhibit the co-intercalation of solvents.However,studies have found that nickel-rich materials will release singlet oxygen under conditions such as overcharging and heating,and singlet oxygen is more likely to react with EC to form H2O2,thereby deteriorating battery performance.In addition,below 0℃ EC will form a high-resistance SEI and the electrode kinetics are severely hindered,while the EC-free electrolyte forms a highly stable and low-impedance SEI film through anion decomposition,which improves the capacity retention and eliminates lithium evolution during charging.This has prompted the exploration of ECfree electrolytes.The high-concentration salt electrolyte can effectively broaden the selection range of existing solvents by changing the solvation structure of the electrolyte and adjusting the electrode/electrolyte interface components.Adding an appropriate amount of additives in the solvent can also have a similar effect.However,little research has been done on the differences and connections between the two.Therefore,this chapter takes a common solvent,N-methylpyrrolidone(NMP)as the research object,and found that in 1 M Lithium bis(trifluoromethane sulfonyl)imide(LiTFSI)-NMP system,the intercalation of Li+ to graphite cannot be achieved due to solvent cointercalation.However,adding 2 wt%FEC(Fluoroethylene carbonate)can realize the reversible intercalation of Li+ on the graphite electrode,and the coulombic efficiency in the first cycle is 66.2%;increasing the concentration of salt to 4 M can also have a similar effect,and the coulombic efficiency in the first cycle is 68.4%.The analysis of the solvation structure of the electrolyte and the components of the electrode/electrolyte interface shows that 2 wt%FEC promotes the formation of an SEI film composited by LiF and polycarbonate on the surface of the graphite electrode,and the solvation structure of the electrolyte hardly changes.In 4 M LiTFSI-NMP electrolyte,the free solvent molecules are greatly reduced,TFSI-anions enter the solvation shell of Li+,and the solvation structure decomposes on the graphite electrode surface to form a LiF-rich SEI film.In response to the above research results,a local high-concentration salt electrolyte 2 M LiTFSI-HFE(1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether)+ 2 wt%FEC containing additives was developed in this research to achieve a highly reversible intercalation of Li+on graphite.The optimized electrolyte not only has good low-temperature discharge performance,but also has flame-retardant effect.