Functionality Selection Principle for High Voltage Lithium-ion Battery Electrolyte Additives

Abstract

A new class of electrolyte additives based on cyclic fluorinated phosphate esters was rationally designed and identified as being able to stabilize the surface of a LiNiMnCoO (NMC532) cathode when cycled at potentials higher than 4.6 V vs Li/Li. Cyclic fluorinated phosphates were designed to incorporate functionalities of various existing additives to maximize their utilization. The synthesis and characterization of these new additives are described and their electrochemical performance in a NMC532/graphite cell cycled between 4.6 and 3.0 V are investigated. With 1.0 wt % 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP) in the conventional electrolyte the NMC532/graphite cell exhibited much improved capacity retention compared to that without any additive. The additive is believed to form a passivation layer on the surface of the cathode via a sacrificial polymerization reaction as evidenced by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonsance (NMR) analysis results. The rational pathway of a cathode-electrolyte-interface formation was proposed for this type of additive. Both experimental results and the mechanism hypothesis suggest the effectiveness of the additive stems from both the polymerizable cyclic ring and the electron-withdrawing fluorinated alkyl group in the phosphate molecular structure. The successful development of cyclic fluorinated phosphate additives demonstrated that this new functionality selection principle, by incorporating useful functionalities of various additives into one molecule, is an effective approach for the development of new additives.

Fluorine and Lithium: Ideal Partners for High‐Performance Rechargeable Battery Electrolytes

“…[88c] This class of additives suffers from severe capacity fading, as their reduction occurs at low potentials and then interferes with the intercalation chemistry.This problem is,however,easily resolved by fluorination of the molecule. [91] Fluorine substitution in the a-position results in the formation of an effective SEI and leads to an improved cycling performance. [92] Thei nfluence of fluorine on the single electron transfer decomposition mechanism of ethyl methyl carbonate (EMC) and (2,2,2-trifluoroethyl)methyl carbonate (FEMC), as examples,i ss hown in Scheme 9, supported by DFT calculations performed by Xing et al [88b,93] Thei mproved cycling performance is related to the composition of the CEI layer as well as the decreased overall impedance.…”

“…This class of additives suffers from severe capacity fading, as their reduction occurs at low potentials and then interferes with the intercalation chemistry. This problem is, however, easily resolved by fluorination of the molecule . Fluorine substitution in the α‐position results in the formation of an effective SEI and leads to an improved cycling performance …”

Understanding the Effectiveness of Phospholane Electrolyte Additives in Lithium‐Ion Batteries under High‐Voltage Conditions

“…It has been previously proposed that the beneficial effect of phospholane functional additives stems from the formation of an effective cathode electrolyte interphase (CEI) [17] layer by means of a polymerization reaction on the cathode surface (Scheme 1). [15,16,18] In the first step, the phospholane molecule is oxidized to the corresponding phosphate i , [19] which makes it susceptible to a nucleophilic attack of a hydroxy group from the active material surface [20] . The formed intermediate ii stabilizes itself by reformation of the P−O double bond and opening of the five membered ring.…”

“…However, if we understand the fundamental properties of both the electrolyte and battery cell system, we can significantly reduce the haystack's size and thus accelerate the search toward identification of “the right needle”. Following this line of thought, we analyzed the proposed working mechanism of phospholane functional additives [15,16] by means of density‐functional theory (DFT) calculations and chemical reasoning and designed a series of phospholane molecules featuring small, systematic changes to their structure (Figure 1). The proposed functional additives were synthesized (3,3,3‐trifluoro‐n‐propylethylene‐phosphite (TFnPrEPi) actually for the first time) and each structure's impact on the overall cycling performance of NMC111‖graphite cells was studied.…”

Regulation of Cathode‐Electrolyte Interphase via Electrolyte Additives in Lithium Ion Batteries

“…The above reasons eventually lead to poor cycling stability of NCM electrode and thus limits their practical application. [63,64,65,66] Previously research has proved that cells with phospholane containing electrolyte have less capacity decay than the cells with counterpart baseline electrolyte due to the preferential oxidize decomposition and the aid to form an robust CEI membrane by participating in polymerization on the surface of cathode [67,68,69] Follow this line, two fluorinated cyclic phosphorus(iii)-based containing compounds, namely, 2-(2,2,3,3,3-pentafluoropropoxy)-1,3,2-dioxaphospholane (PEPOE-Pi) and 2-(2,2,3,3,3-pentafluorpropoxy)-4-(trifluormethyl)-1,3,2dioxaphospholane (PFPOEPi-1CF 3 ) were designed and synthesized by Aspern et. al.…”