4-(Trimethylsiloxy)-3-pentene-2-one (TMSPO) is tested as an electrolyte additive to enhance Coulombic efficiency and cycle retention for the Li/ LiNi 0.5 Mn 1.5 O 4 (LNMO) half-cell and graphite/LNMO fullcell. TMSPO carries two functional groups, siloxane (−Si− O−) and carbon−carbon (CC) double bonds. It is found that the siloxane group reacts with hydrogen fluoride (HF), which is generated by hydrolysis of lithium hexafluorophosphate (LiPF 6 ) by impure water in the electrolyte solution, to produce 4-hydroxypent-3-ene-2-one (HPO). The as-generated HPO, as well as TMSPO itself, is electrochemically oxidized to form a protective surface film on the LNMO electrode, in which it is inferred that the carbon−carbon (CC) double bond initiates radical polymerization. The surface film derived from the TMSPO-added electrolyte shows a superior passivating ability to that generated from the pristine (TMSPO-free) electrolyte. The suppression of electrolyte oxidation enabled by the superior passivating ability offers two beneficial features to the half-cells and full-cells: the suppression of both HF generation and deposition of the resistive surface film on LNMO. As a result, the metal dissolution by HF attack on LNMO appears to be smaller by the addition of TMSPO. The cell polarization is also less significant because of the latter beneficial feature. In short, the bifunctional activity of TMSPO (HF scavenger and protective film former) allows an enhanced Coulombic efficiency and cycle retention to the half-cell and full-cell.

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Tris(pentafluorophenyl)silane (TPFPS) is tested as a film-forming agent for nickel-doped manganese spinel (LiNi0.5Mn1.5O4, LNMO) positive electrode. The surface film derived from the TPFPS-free electrolyte (organic carbonate-based one) is poorly passivating the high-voltage positive electrode to cause continued electrolyte oxidation and film deposition. It leads to an increase in the film resistance and thus electrode polarization eventually causing capacity fading. In contrast, the surface film derived from the oxidation of TPFPS prior to the carbonate-based solvents is highly passivating. As a result, additional electrolyte decomposition is suppressed to give a higher Coulombic efficiency for the Li/LNMO cells. In addition, insignificant film growth/electrode polarization allows a much improved capacity retention.

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High voltage positive electrodes for lithium ion batteries have suffered from continuous oxidation of the electrolyte during cycling, which largely offsets the benefits of high energy and power densities. In this work, the electrolyte oxidation and concomitant film deposition/dissolution behaviors were investigated on Pt electrode by using linear sweep voltammetry (LSV), electrochemical quartz crystal microbalance (EQCM), and X-ray photoelectron spectroscopy (XPS). Two characteristics were identified. First, film deposition is relatively unfavorable at higher potentials (>4.7 V vs. Li/Li + ) because the oxidation products are mostly gaseous or soluble species. Second, the concentration of inorganic species decreases in the surface film as the potential increases, which is likely dissolved by HF or polar species. The dominance of gaseous or soluble products and the partial dissolution of the surface film, are two characteristics which hamper passivation of the electrode surface, leading to severe electrolyte oxidation at the high potentials.