Abstract
We review herein several important aspects of surface chemistry in Li-ion batteries, and discuss the use of ionic liquids (ILs) for rechargeable Li batteries. We explored the suitability of ILs for 5 V cathodes and Li-graphite anodes. Some advantages of the use of ILs to attenuate the thermal behavior of delithiated cathode materials are demonstrated. We also report briefly on a comparative study of the following cathode materials: LiNi 0.5Mn 0.5O 2; LiNi 0.33Mn 0.33Co 0.33O 2; LiNi 0.4Mn 0.4Co 0.2O 2; LiNi 0.8Co 0.15Al 0.05O 2 and LiMnPO 4, in standard electrolyte solutions based on mixtures of alkyl carbonates and LiPF 6. We also discuss aging, rate capability, cycle life and surface chemistry of these cathode materials. The techniques applied included electrochemical measurements, e.g., XRD, HRTEM, Raman spectroscopy, XPS and FTIR spectroscopy. We found that ILs based on cyclic quaternary alkyl ammonium cations may provide much better electrolyte solutions for 5 V cathodes than standard electrolyte solutions, while being quite suitable for Li-graphite electrodes. All the lithiated transition metal oxides studied (as mentioned above) develop unique surface chemistry during aging and cycling due to the acid-base and nucleophilic reactions of their surface oxygen anions. LiMn 0.33Ni 0.33Co 0.33O 2 has the highest rate capability compared to all the other above-mentioned cathode materials. Cathodes comprising nanometric size carbon-coated LiMnPO 4 produced by HPL demonstrate a better rate capability than LiNi 0.5Mn 0.5O 2 and LiNi 0.8Co 0.15Al 0.05O 2 cathodes. The former material seems to be the least surface reactive with alkyl carbonates/LiPF 6 solutions, among all the cathode materials explored herein.
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