Decomposition of Electrolyte and Composite Cathode Materials in High-Voltage Lithium Batteries

Abstract

Introduction High-voltage cathode materials, such as LiNi1/3Mn1/3Co1/3O2 (NMC) and LiNi3/2Mn1/2O4 (LNMO), are promising to enhance the energy density of Li-ion battery due to their high operating voltage. However, the high working voltage causes the decomposition of electrolyte and composite electrode materials, resulting in battery performance degradation.[1] Recently, highly concentrated electrolyte have attracted much attention due to the some specific properties. Our group has reported the increase of oxidative stability in glyme system with high lithium salt concentration (≈3 mol dm−3).[2] Furthermore, the corrosion suppression of Al current collector in highly concentrated lithium bis(trifluoromethanesulfonyl) imide (LiTFSI)[3,4] and lithium bis(fluorosulfonyl) amide (LiFSI)[5] were reported. Additionally, it was also reported that some electrolyte decompose on the carbon black rather than on the active material.[6]The degradation mechanism of electrolyte and composite cathode changes depending on electrolyte type, such as the salt concentration, anion, and the components of composite electrode. In this work, we investigated the decomposition of electrolyte and composite electrode materials in propylene carbonate (PC) system with different Li-salt and its concentration. We also applied highly concentrated electrolytes to single particles of 5V-class cathode materials. Comparison of electrochemical behaviors of the composite electrode and single particle enabled us to elucidate the degradation mechanism of the cathode. Experimental PC and LiClO4, LiBF4, LiTFSI or LiFSI were mixed in a glovebox filled with argon gas. The electrochemical stabilities of the electrolytes were evaluated by linear sweep voltammetry (LSV) with three electrode cells (WE and CE; platinum, RE; Li metal in 1 M Li[TFSA]/G3 with liquid junction). The scanning rate was 1 mV s−1at 30 °C. The interaction of lithium cation and PC was observed by Raman spectroscopy. Single particle measurements can evaluate the electrochemical responses of a single particle of active material without conductive materials and binder. A disk-type platinum microelectrode was attached to a single particles by using a micromanipulator, then electrochemical measurements were performed with two electrode cell (WE; platinum microelectrode, RE; Li metal). The composite electrode was composed of NMC or LNMO (80 wt %), acetylene black (10 wt %) and poly(vinylidene fluoride) (10 wt %). Result and Discussion From the Raman spectra results for LiClO4/PC with concentration dependence, the amount of ‘free’ PC, uncoordinated to Li+, decreased with increase of lithium salt concentration. From the LSV results of 1.4 and 3.9 mol dm−3 LiClO4/PC, the oxidative stability was enhanced by the increase of lithium salt concentration. It is suggested that the small amount of free PC resulted in the high oxidative stability. Figure shows charge-discharge curves of the composite electrode (A and B) and the single particle (C) of LNMO. The side reactions may occur around 4.7 V on the composite electrode (inherent plateau potential of LNMO is around 4.7 V) in 1.4 mol dm−3 LiClO4/PC, whereas it could be charged above 5 V in 3.9 mol dm−3. In addition to this, a single LNMO particle could be also charged even in 1.4 mol dm−3 LiClO4/PC. These results indicated that composite electrode is involved with the decomposition in 1.4 mol dm−3 LiClO4/PC, but the decomposition was suppressed in 3.9 mol dm−3 LiClO4/PC due to its high oxidative stability. From the XPS and SEM results, it was confirmed that the properties of the decomposition products on the positive electrode changed depending on LiClO4concentration. In the case of LiTFSA/PC system, a similar effect of salt concertation on the charge-discharge behavior of composite cathode was observed. However, the main difference between diluted and highly concentrated LiTFSA/PC solutions was the corrosion of Al current collector. The corrosion of Al was suppressed in highly concentrated LiTFSA/PC, leading to the successful operation of LNMO. Acknowledgment This research was supported by Japan Science and Technology Agency (JST) - Advanced Low Carbon Technology Research and Development Program - Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) of Japan.