Forming a Stable CEI Layer on LiNi_0.5Mn_1.5O₄Cathode by the Synergy Effect of FEC and HDI

Abstract

As a promising additive for film forming on anodes, fluoroethylene carbonate (FEC) has been applied widely in commercial lithium ion batteries (LIBs). However, the influence of FEC in high-voltage Li + energy storage systems are still under disputing for the negative effects on cathodes. Considering a FEC molecule would be electro-oxidized to a radical onium ion, we investigate hexamethylene diisocyanate (HDI) as the additive which can react with the intermediate to form an amide-based polymer via thermodynamic processes. The polymer would be stable at high potential. In the FEC-containing electrolyte with HDI added, a surface layer is formed on the LiNi 0.5 Mn 1.5 O 4 cathode, which would protect the compounds from decomposing and improve the performances of the batteries. The capacity retention of Li/LiNi 0.5 Mn 1.5 O 4 cells cycled between 3.5-4.9 V (vs. Li + /Li) is significantly increased from 59.1% to 76.1% in 200 cycles, and the rate capability is also improved compared with the ones without HDI. The electrolyte system also shows a good compatibility with graphite, presenting a promising prospect for the practical application of high-voltage LIBs. In the development of LIBs, the building of solid electrolyte interphase (SEI) is an important milestone, for which passes Li + cations but blocks the solvent molecules from continual decomposing and/or co-intercalating into anodes, making the "rocking chair" batteries workable. 26 reveals that FEC molecule would be defluorinated and decarboxylated on anodes, then produce a highly cross-linked elastic polymer network, which meets the require of the failure strain in anodes' charging-discharging processes. FEC can also enhance the ionic conductivity of the interphase, 27 widen the operating temperature 17,27,28 and improve the safety 29 of batteries. Thus FEC is widely used as an important multifunctional additive in state-of-the-art LIBs.On the other hand, in order to meet the increasing demand of high energy density, high-voltage cathodes have been proposed for next generation LIBs. 30 However, the effects of the SEI formation additives on cathodes are still under discussing. 31 In the comprehensive considering, the additives for LIBs should improve the performances of both anode and cathode. 32 Unfortunately, as an important additive for SEI forming, FEC exhibited negative effects on LiNi 0.5 Mn 1.5 O 4 cathode even the concentration of the additive is benefit for the anodes. 33 The calculation shows a FEC molecule would be oxidized to a radical cation, and a thick layer on the cathode is formed while the concentration of the additive is 5 wt%, which is not preferential for the electrochemical performance of high-voltage LIBs in the tests. 33,34We have previously reported an onium ionization-polymerized cathode electrolyte interphase (CEI) film formed by HDI and propylene carbonate (PC). A PC molecule would be oxidized to be a free radical-onium ion at high potential (>4.3 V vs. Li + /Li, Reaction 1). The free radical of the intermediate would auto polymerize...

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