Tris(trimethylsilyl) Phosphite (TMSPi) and Triethyl Phosphite (TEPi) as Electrolyte Additives for Lithium Ion Batteries: Mechanistic Insights into Differences during LiNi_0.5Mn_0.3Co_0.2O₂-Graphite Full Cell Cycling

Abstract

Tris(trimethylsilyl) phosphite (TMSPi) has emerged as an useful electrolyte additive for lithium ion cells. This work examines the use of TMSPi and a structurally analogous compound, triethyl phosphite (TEPi), in LiNi 0.5 Mn 0.3 Co 0.2 O 2 -graphite full cells, containing a (baseline) electrolyte with 1.2 M LiPF 6 in EC:EMC (3:7 w/w) and operating between 3.0-4.4 V. Galvanostatic cycling data reveal a measurable difference in capacity fade between the TMSPi and TEPi cells. Furthermore, lower impedance rise is observed for the TMSPi cells, because of the formation of a P-and O-rich surface film on the positive electrode that was revealed by X-ray photoelectron spectroscopy data. Elemental analysis on negative electrodes harvested from cycled cells show lower contents of transition metal (TM) elements for the TMSPi cells than for the baseline and TEPi cells. Our findings indicate that removal of TMS groups from the central P-O core of the TMSPi additive enables formation of the oxide surface film. This film is able to block the generation of reactive TM-oxygen radical species, suppress hydrogen abstraction from the electrolyte solvent, and minimize oxidation reactions at the positive electrode-electrolyte interface. In contrast, oxidation of TEPi does not yield a protective positive electrode film, which results in inferior electrochemical performance. The electrolyte plays a vital role in lithium ion battery (LIB) cells -not only must it enable Li+ ion transport between the electrodes, it should also form passivating layers (such as the solid electrolyte interphase (SEI) on graphite), and remain stable under the oxidizing and reducing conditions at the positive and negative electrodes, respectively. While conventional carbonate-based electrolytes meet the requirements of commercial LIB cells, the next generation of cathode materials will require high-voltage operating conditions (>4.5 V vs. Li/Li + ) in order to meet the demands of higher energy and power. At these higher voltages, the conventional electrolytes experience severe oxidation leading to rapid performance loss and cell failure. Approaches to mitigate electrolyte oxidation at the positive electrode (cathode) include the (i) use of electrolytes with higher oxidation potentials which are intrinsically stable at higher voltages;

Rearrangement on surface structures by boride to enhanced cycle stability for LiNi0.80Co0.15Al0.05O2 cathode in lithium ion batteries

“…Of course, it can be seen from the figure that the P of the three samples will appear inside the secondary particles. We speculate that during the battery process, P in the electrolyte salt LiPF 6 enters the interior of the particle along the gap between the primary particles [52,53] . The side reaction between the active material and the liquid electrolyte can be suppressed by the coating and rearrangement of the surface structure.…”

Surface‐Modified Ni‐Rich Layered Oxide Cathode Via Thermal Treatment of Poly(Vinylidene Fluoride) for Lithium‐Ion Batteries

“…XPS analyses of the recovered NCM811 cathodes also provided informative results that explained these behaviors (Figure S4): the XPS patterns showed similar C 1s spectra for all NCM811 cathodes; however, the peak intensity associated with  C  F  (291.0 eV, the from PVDF binder used in the NCM811 cathode preparation, not for surface coating) 40,41 was higher for the recovered PVDF‐NCM‐1 than for the nontreated NCM811, indicating that less electrolyte decomposition occurs in the PVDF‐NCM‐1. Similar behaviors were observed in the P 2p spectra: the peaks related to Li x PF y (137.3 eV) and Li x PO y F z (135.1 eV) (from electrolyte decomposition) 42,43 were less intense for the PVDF‐NCM‐1, in agreement with the C 1s spectra which meaning the surface stability was markedly enhanced.…”

Anode-Dependent Impedance Rise in Layered-Oxide Cathodes of Lithium-Ion Cells

“…43,44 Other approaches to increasing cell life could include periodic degassing of cells during aging and the addition of compounds to the negative electrode that accelerate the consumption of gases. The ultimate goal would be design cell designs and chemistries that enable and enhance operation over the 15 yr life target set by the Office of Vehicle Technologies at the U.S. Department of Energy.…”