Investigation On Novel Lithium Fluorosulfonimides As Conducting Salts For Lithium-Ion Batteries

As one of the key materials in lithium-ion (Li-ion) batteries, nonaqueous liquid electrolyte is mainly functionalized as Li-ion conductors between the anode and cathode electrodes, and as tailors of electrolyte/electrode interphases, which has an important impact on the cycling life, high- and low-temperature, current-rate and safety performances of Li-ion batteries (LIBs). In today's commercial LIBs, the nonaqueous liquid electrolyte is solutions, comprised of LiPF6 as conducting salt, a mixture of linear and cyclic carbonates as dipolar aprotic solvents, together with small amount of functional ized additives. This is mainly attributed to several key natures of LiPF6-carbonate electrolytes, including high ionic conductivities, ability to passivate Al current collector in the high potential region of 3-5 V vs. Li/Li+, and excellent resistance toward oxidation of 4 V class cathodes. More importantly, the great success of LiPF6 as conducting salt for state-of-art LIBs should also thank for its compatibility with graphite anode, i.e., the ability of forming Li-ion conductive but electronically insulating solid-electrolyte-interphase (SEI) films on graphitized carbon anodes, which is initiated by kinetically electrochemical reduction of both LiPF6 and carbonate solvents (including additives) with small irreversibility during the first few cycles, and can protect both graphitized carbon anode and organic electrolyte components from further detrimental reactions, thus allowing LIBs to work with relatively long cycle life and desirable safety in ambient temperatures (<55?).However, the electrolyte of LiPF6 suffers from thermal and hydrolytic instability. It is widely demonstrated that the LiPF6-based electrolyte undergoes very complicated decompositions, either at elevated temperature or in the presence of small amounts of protic impurities (e.g., H2O and alcohols), accompanied with the formation of HF. Further, the HF impurity in the electrolyte can cause metal ion dissolution from cathode electrodes, and destroy SEI films formed on the carbon-based anodes, thus causing the rapid capacity fading and safety problem, which has been one of the main technology bottlenecks for developing the long-life LIBs used in large-scale applications, such as electric vehicles (EVs) and grid storage. Therefore, the novel electrolyte systems, including new lithium salts, solvents and additives, are highly desired to overcome above drawbacks of LiPF6-based electrolyte, which also is the main object of a prolonged endeavor, both in China and abroad.On the basis of our continuous interest to investigate the relationship between the structure and electrochemical performance of fluorinated sulfonylimide anion as counterpart of lithium salt for LIBs, over the past 20 years, several series of novel lithium salts based on fluorinated sulfonylimide anions (e.g., Li[(FSO2)(n-CmF2m+1SO2)N], Li[(CF3CH2OSO2)(n-CmF2m+1SO2)N], m= 0,1,2,4,6,8, etc.), have been designed and synthesized in our research group. It is demonstrated that lithium(fluorosulfonyl)(n-nonafluorobutanesulfonyl)imide (Li[(FSO2)(n-C4F9SO2)N], LiFNFSI), as single conducting salt in carbonates, possesses the prerequisites for a conducting salt for LIBs, including absence of HF contamination, good thermal and electrochemical stabilities, excellent ability to passivate Al in the high potential region of 3-5 V vs. Li/Li+, and enough anodic stability toward oxidation of 4 V class cathodes. More importantly, LiFNFSI is advantageous over LiPF6 in the cycling test for both graphite/LiCoO2 and MCMB (mesocarbon microbead)/LiMn2O4 cells at room and elevated temperatures. These outstanding properties of LiFNFSI would make it promising to overcome the bottleneck of rapid capacity fading of LiPF6-based LIBs at elevated temperatures. On the other hand, the in-depth investigation on the mechanism underlying this improvement has been not conducted in these preliminary work for LiFNFSI used in Li-ion cells.Based on the above research background and the progresses, being made in earlier work, this dissertation aims to improve the stabilities of LIBs at high temperatures. LiFNFSI is chosen as conducting salt, to replace conventional used LiPF6, or as a co-salt in electrolytes of LiPF6, through comparatively characterizations of the key properties required for evaluating new salts for LIBs. The fundamental physicochemical and electrochemical properties of the LiFNFSI- and LiPF6-based electrolytes have been comparatively investigated at both room (25?) and elevated temperatures (60 and/or 85?). Moreover, the electrochemical performances for graphite/LiCoO2 cells, and electrode/electrolyte interphases, have been comparatively characterized, with particular attention on how the salt identity (i.e., the structure of anions) impact on the properties of electrode/electrolyte interphases (i.e., SEI films), and the electrochemical performances of Li-ion cells. The main results could be summarized as below:(1) For improving our understanding on the decomposition mechanism of conventional LiPF6-based electrolyte, and its effect on electrochemical performances of LIBs, the decomposition behaviors of the LiPF6-based electrolyte have been systematically characterized at both room (25?) and elevated temperatures (60 and 85?) by liquid nuclear magnetic resonance (NMR), On the base of characterization and analysis for the distribution percentages of the representative decomposition products in the LiPF6-based electrolytes under different storage conditions (i.e., different storage temperatures and storage time), a new catalytic cycle pathway for the chemical decompositions of both LiPF6 and carbonate solvents in the as-prepared electrolyte, has been proposed in the second chapter, i.e., trace amounts of HF and protic impurities (e.g., H2O, CH3OH and C2H5OH, unavoidably occur in the LiPF6-based electrolyte), are the key factors responsible for initiating the decompositions of PF6 anions and carbonate solvents, and the regeneration of HF and alcohols during the decomposition process, cause the long-term, continuous decompositions of both PF6- anions and carbonate solvents. This proposed mechanism provides a reasonable explanation for the continuous decompositions occurring in LiPF6-based electrolyte at room temperature, compared with the reported mechanism, wherein the decomposition of LiPF6-based electrolyte is suggested by the thermal dissociation of LiPF6 (i.e., LiPF6?LiF+PF5).(2) Five kinds of novel lithium fluorosulfonylimides (Li[(FSO2)(n-CmF2+1SO2)N], m= 0,1,2,4,6), as co-salts of LiPF6 in carbonates, are investigated on the effect and mechanism for improving the chemical stability of LiPF6-based electrolyteat elevated temperature by liquid NMR. Analyses of NMR indicate that, the five kinds of lithium fluorosulfonylimides are demonstrated to effectively suppress the decompositions of LiPF6-based electrolyte during storage at the high temperature of 85?. It is suggested that 1) the HF-removing effect of the fluorosulfonylimide anions blocks the reactions of HF with carbonates to generate alcohols; 2) while the O=PF3-removing effect of the fluorosulfonylimide anions removes the reactive intermediate, O=PF3, thus blocking the continuous generation of HF and protic species (e.g., alcohols) in the electrolyte, both of which play the key role in preventing the decompositions of both the PF6-anions and carbonate solvents.(3) The fundamental physicochemical and electrochemical properties of LiFNFSI in ethylene carbonate (EC)/ethyl-methyl-carbonate (EMC),3:7, v/v) are characterized systematically, in terms of ionic conductivity, Li-ion transference number, Al corrosion behavior, and electrochemical behavior on Pt electrode. It is demonstrated that LiFNFSI-based electrolyte displays high Li-ion transference number (0.5), good electrochemical stability (Eox= 5.7 V vs. Li/Li+), and does not corrode Al collector even at 4.5 V (vs. Li/Li+) and 60?. The electrochemical performances of LiFNFSI for graphite/LiCoO2 Li-ion cells have been comparatively investigated with those of LiPF6, including the high-temperature storage, current-rate, and cycling performances at both room (25?) and elevated temperatures (60?), with particular attention to characterizing the electrode/electrolyte interphases formed on both graphite anode and LiCoO2 cathode by using electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). It is demonstrated that LiFNFSI is advantageous over LiPF6 in the high-temperature storage, current-rate and cycling tests, particularly at elevated temperature, which are mainly attributable to the improved chemical and electrochemical stability of SEI films formed on graphite anode, together with the superior stability and absence of HF contamination, higher Li-ion transference number, and better wetting ability toward battery materials, for the LiFNFSI-based electrolyte. However, it is noted that the impedances are all larger for the LiFNFSI-based cells than for the LiPF6-based ones, mainly due to the thicker SEI films formed on graphite anode. Analyses of XPS reveal that the chemical compositions of electrode/electrolyte interphases formed on both electrodes are highly dependent on the types of lithium salts. Particularly, on graphite anode, the SEI films formed in the LiFNFSI-based electrolyte are majorly dominated by the reductive products of FNFSI" anions, and are relatively stable at elevated temperature, while those formed in the LiPF6-based electrolyte are largely governed by the reductive products of carbonate solvents, and occur significant dissolutions and regrowth at elevated temperature. All above results suggest that the improved capacity retention for the cells with LiFNFSI is mainly attributable to the robust nature of the SEI films formed on graphite anode, and the superior stability and absence of HF contamination for the LiFNFSI-based electrolyte, while the rapid capacity fading of the cells with LiPF6 is essentially due to the decompositions and regrowth of SEI films on graphite anode, and the detrimental impact of HF and protic residues in the electrolyte of LiPFg.(4) LiFNFSI has been investigated systematically as a functionalized co-salt of LiPFg in EC/EMC (3:7, v/v), including the fundamental physicochemical and electrochemical properties of electrolytes, high-temperature storage, current-rate and cycling performances at both room (25?) and elevated temperatures (60?) for graphite/LiCoO2 cells, with particular attention to investigating the effect of mixed lithium salts on the cell impedances, and electrode/electrolyte interphases by using EIS and XPS. It is demonstrated that the electrolytes with themixed LiPF6-LiFNFSI salts are advantageous over the ones with the single salt of LiPF6 or LiFNFSI in cycling test, e.g., the capacity retention after 100 cycles in 0.5 M LiPF6+0.5 M LiFNFSI-based electrolyte:92.1%(25?) and 85.0%(60?). This would be attributed to the following three factors as below:1) the thermal stabilities of electrolytes have been improved,due to the present of LiFNFSI; 2) FNFSF anions remove the detrimental HF in the electrolytes, thus reducing chemical corrosion of SEI films formed on the graphitized carbon anodes, and dissolution of metal ion from cathode materials, induced by HF; 3) the robust SEI films have been formed on graphite anode, due to cooperative participation of PF6- and FNFSI- anions. On the other hand, EIS results indicate that the capacity fading for the cells based on the electrolytes with the mixed LiPF6-LiFNFSI salts is mainly attributable to very small scale of redox decompositions of electrolytes unavoidably occurring on both electrodes during cycling, together with the corresponding impedance increase, which is pronounced at elevated temperature (60?).Finally, the perspectives are suggested for the electrolytes used in Li-ion cells, from several aspects, such as the chemical and electrochemical stabilities of electrolytes, and the properties of forming stable electrode/electrolyte interphases. The design of novel electrolyte systems, with high chemical and electrochemical stabilities, and good compatibility with battery materials, would be the main direction in future, for effectively promoting the application process of LIBs, especially for developing long-life LIBs used in EVs. The mixture of two or more kinds of lithium salts (e.g., the mixed LiPF6-LiFNFSI salts presented here) as conducting salts for LIBs, would be one of the promising strategies, for enhancing the performances of electrolytes used in LIBs, especially at elevated temperatures.