Tracing the Impact of Hybrid Functional Additives on a High-Voltage (5 V-class) SiO_x-C/LiNi_0.5Mn_1.5O₄ Li-Ion Battery System

The development of next generation high energy density lithium ion batteries (LIBs) adopting electrode materials with higher specific capacity or higher working voltage has attracted great interest. In this paper, fluoroethylene carbonate (FEC) and 1,3-propanediolcyclic sulfate (PCS) are unprecedentedly combined as hybrid functional additives to significantly improve the performances of a very challenging next-generation high voltage (5 V-class) SiO_x-C/LiNi_0.5Mn_1.5O₄ battery system, where SiO_x-C composite shows a high specific capacity of 450 mAh g^-1 and LiNi_0.5Mn_1.5O₄ has a high working voltage plateau (∼4.7 V vs Li⁺/Li). Combining in situ differential electrochemical mass spectrometry (DEMS) technology, theoretical calculations, and conventional ex situ characterizations, it is revealed that small amounts of lithium-containing species (such as LiF, sulfate species, and organic sulfite species) with excellent electronic-insulating, ionic-conducting, and compact properties are derived from prior decomposition of additives and incorporated into the solid electrolyte interface (SEI) layer of SiO_x-C electrode, suppressing the reductive decomposition of carbonate solvents as well as the gas generation (C₂H₄, CO₂, and H₂). Moreover, hybrid functional additives are beneficial for forming a compact and homogeneous cathode SEI layer, alleviating dissolution of transition metals, structure degradation, and loss of active lithium. This manuscript provides a very useful research method for understanding the working mechanism of functional additives and will also help us to depict the SEI layer formation mechanism more accurately.