Toward Unraveling the Origin of Lithium Fluoride in the Solid Electrolyte Interphase

Abstract

The solid electrolyte interphase (SEI) is an integral part of Li-ion batteries and their performance, representing the key enabler for reversibility and also serving as a major source of capacity loss and dictating the cell kinetics. In the pervasive LiPF6-containing electrolytes, LiF is one of the SEI’s major components; however, its formation mechanism remains unclear. Electrochemically, two separate reduction pathways could lead to LiF, either via direct anion reduction or electrocatalytic transformation of HF. This work aims to shed light on understanding the role played by these pathways. In a multimodal experimental and theoretical approach, we carried out operando structural characterization on an inert model single crystalline N-doped SiC working electrode during voltammetric scans in LiPF6 baseline electrolytes and complemented these with ex situ chemical characterization. These results were supplemented by cyclic voltammetry measurements using a variety of electrolyte formulations under different cycling rates as well as quantum chemical calculations and Born–Oppenheimer molecular dynamics simulations. Our results reveal that the reductive formation of LiF in these systems is likely a combined mechanism, which concomitantly involves both direct anion reduction and electrocatalytic transformation of HF. Specifically, LiF nucleates via the electrocatalytic transformation of HF followed by significant anion reduction.

Optimized Cycle and Safety Performance of Lithium–Metal Batteries with the Sustained‐Release Effect of Nano CaCO3

“…In STD, inferior SEI can't prevent the decomposition of solvent, resulting in the rapid increase of C content, while in the electrolyte with nano CaCO 3 additive, the proportion of F, Li, and Ca continues to rise. SEI rich in fluorine and alkali metal ions plays a vital role in improving the CE of electrolyte: LiF has good electronic insulation and nano‐sized grain, [ 45 ] which helps to reduce interfacial side reactions and accelerate ion transport; although calcium salts such as CaF 2 are unable to conduct Li + , their higher Young's modulus (Table S1, Supporting Information) will improve the mechanical strength of SEI and act as reinforcing filler. The F‐rich SEI in modified electrolyte could be attributed to the generation of LiPO 2 F 2 and anion enrichment induced by electrostatic adsorption of Ca 2+ near the interface (Figure S17, Supporting Information).…”

Enhancing Li+ Transport in NMC811||Graphite Lithium‐Ion Batteries at Low Temperatures by Using Low‐Polarity‐Solvent Electrolytes

“…The CV curve of the cell with SE shows a small reduction peak at � 1.3 V, followed by two larger reduction peaks at � 1.0 V and � 0.65 V, which are assigned to the reduction of HF impurities and LiPF 6 contact ion pairs (CIPs), DMC and EC solvents, respectively. [27,28] In comparison, the reduction peak of DE starts at a potential of � 1.5 V, which can be attributed to the reduction of the LiFSI and LiF formation. This agrees with the DFT calculations in Figure S3 and previous work.…”

Engineering an Insoluble Cathode Electrolyte Interphase Enabling High Performance NCM811//Graphite Pouch Cell at 60 °C

“…Since the inorganic compounds are less soluble than the organic species, the precipitation of LiF and/or‐like compounds will prevent the further decomposition of electrolytes during the subsequent cycles. Being reported repeatedly, LiF is found to be “good” CEI components [ 22,37 ] owing to that i) enhanced ion conductivity and surface energy of LiF‐containing interface due to its nanoscale nature, [ 37,38 ] as well as the presence of LiF/Li 2 CO 3 or LiF/Li 2 O grain boundaries with nano‐scale level, [ 22,37,39 ] even though bulk LiF is an ionic and electronic insulator (10 −13 to 10 −14 S cm −1 ), [ 22,37,40 ] ii) beneficial interphasial LiF found in cathodes surface layers is believed to be responsible for the stabilization of interfaces in high‐voltage lithium‐ion batteries (LIBs) via reducing hydrogen evolution. [ 41 ] Rationally, the conductive and stable LiF‐contained interface rightly supports the in situ EIS result (Figure 5b).…”

“…The CEI components detected by in situ FTIR (Figure 1a) are basically consistent with those reported in the literatures, which can be divided into inorganic components and organic components. The inorganic species (i.e., LiF, Li x PF y O z ) are believed to be less soluble, [ 22 ] while the Li 2 CO 3 [ 23 ] component and the organic species (i.e., ROCOOLi) [ 5c,24 ] are considered to be decomposed easily. In order to stabilize the easily decomposed components, we proposed a modification strategy of CEI with an electrolyte additive, which is expected to participate in the formation of a highly polymerized interfacial film at the cathode that prevents the CEI from solving, thus resulting in a stable CEI during the practical electrochemical operation.…”