In Situ FTIR Spectroscopy Study of Li/LiNi[sub 0.8]Co[sub 0.15]Al[sub 0.05]O[sub 2] Cells with Ionic Liquid-Based Electrolytes in Overcharge Condition

Abstract

We developed a methodology of in situ Fourier transform infrared ͑FTIR͒ measurements of gaseous products formed in an electrochemical cell upon polarization. LiNi 0.8 Co 0.15 Al 0.05 O 2 ͑NCA͒ cathodes were explored at potentials of up to 5.5 V vs Li in the ionic liquid ͑IL͒-based electrolyte solution, LiTFSI/N-butyl-N-methylpyrrolidinium bis͑trifluoromethanesulfonyl͒amide. The polarization of the NCA electrodes formed CO 2 and CO due to the liberation of oxygen and the parallel dissolution of nickel ions, which oxidizes the carbon black in the composite electrode. The oxygen was mostly liberated from the NCA and also due to minor contribution from the surface groups on the carbon black additive.The possible development of internal pressure is one of the most serious safety problems of Li-ion batteries, especially for large applications such as electric vehicles. The development of overpressure due to the gas evolution in side reactions can break the hermetically sealed housing of the battery. Exposure of the reactive battery components to air can then lead to fire and explosions. 1-3 Internal pressure can develop upon storage at elevated temperature 2,4-6 and when the battery is overcharged. 1,7,8 The safety of lithium-ion batteries under overcharge conditions is determined by the thermal behavior and by the nature and quantity of gases that can be generated by them.LiNi 0.8 Co 0.15 Al 0.05 O 2 ͑NCA͒ is a promising cathode material for lithium-ion batteries because of its higher specific capacity and lower cost compared to LiCoO 2 . Gas evolution with NCA cathodes as a function of temperature, voltage, and the presence of additives in conventional electrolyte solutions based on alkyl carbonates has already been studied. 2,4,6 The main gaseous compound that was formed at positive electrodes in conventional electrolyte systems was CO 2 , for which several mechanisms of formation in these systems can be drawn. A main source of CO 2 is the oxidative decomposition of the carbonate solvents or surface films containing carbonate groups on the positive electrode. 4,5 Another possibility for gas evolution in these systems is the degradation of the overcharged cathode material. [6][7][8] To discriminate between these processes during the storage of charged Li-ion cells at elevated temperatures, Onuki et al. employed 13 C-labeled organic carbonate solvents and showed that 37% of CO 2 evolution in extreme situations cannot be related to solvent decomposition, but rather to the carbon present in the composite electrode as a conductive additive, or to the binder. In recent years, there have been efforts to introduce ionic liquids ͑ILs͒ as main solvents or critical solution components into Li-ion batteries because of their apparent high anodic stability and promising safety features: nonvolatility, nonflammability, and expected high thermal stability. 9,10 ILs based on cations that are derivatives of pyrrolidinium are especially important because of their relatively high cathodic stability ͑they may be compatible with Li metal...

Challenges in the development of advanced Li-ion batteries: a review

“…An important class of new solvents that is being examined in connection with Li ion batteries is ionic liquids that offer wide electrochemical windows and good safety features. 80,[106][107][108][109] There are efforts to develop new salts such as LiN(SO 2 F) 2 (LiFSI) 110 and boron coumpounds. 111 There are continuous intensive efforts to introduce and test a large variety of additives.…”

A review of advanced and practical lithium battery materials

“…À anion and LiN(SO 2 CF 3 ) 2 . 35 Hence, in contrast to alkyl carbonate-based solutions in which LiN(SO 2 CF 3 ) 2 has limited usefulness as a salt due to the poor passivation of aluminium in its solutions in the above IL-based systems, the use of N(SO 2 CF 3 ) 2…”

Liquefied Gas Electrolytes for Electrochemical Energy Storage Devices

“…All peaks seen in the non-cycled lithium metal surface in Figure 5B are also present in the cycled lithium metal surface except the C-O peak at 533.4 eV, which is possibly due to a lower formation rate of CH 3 Li decomposition products during cycling. An additional peak is visible at 292.0 eV due to the C-F 3 bonding in the C 1s spectrum, which corresponds to trace LiTFSI salt in SEI surface (59) or possibly CHF 3 from reduced TFSIthe ion (60). Further comparing cycled and non-cycled spectra, there is an apparent increase in Li 2 CO 3 relative to other components and makes up the majority of the SEI.…”

“…There are small peaks in the Li 1s, C 1s and O 1s spectra corresponding to residual salt which was not removed through washing with dimethyl carbonate. Since carbon dioxide is expected to be stable at the potentials seen at this electrode surface and there is no other source of oxygen, the increased peak seen in the O 1s spectra is thought to be due to a change of the surface oxygen of the LiCoO 2 electrode (46,60) and not related to the formation of an additional surface layer on the electrode. There were no other major changes seen in the XPS spectra, indicating very little, if any, SEI layer is formed on the surface of the cathode in the fluoromethane based electrolyte system.…”