What is the difference between propylene carbonate (PC) and vinyl carbonate (EC) in lithium-ion batteries?

Lithium ion batteries by lithium ion in lithium transition metal oxides and poor graphite material between the embedding and out energy storage and release. Graphite materials can achieve in the lithium ion battery application depends on the electrolyte in the break down to form the surface of the graphite ion can't guide, electronic conductivity of the solid electrolyte interface (SEI) film. This layer of protective film is far lower than the embedded will restore stability lithium electricity(0.01 V) electrolyte and graphite electrode, thus ensuring the intercalated-li potential under no reduction of electrolyte decomposition, makes lithium-ion reversible embedded in the graphite material.

How is such an important SEI?Why some electrolyte decomposition product can form a stable SEI film, and some of the electrolyte are above the intercalated-li potential under continuous reduction of decomposition, eventually leading to graphite layer structure collapse?The most typical differences between the interface should be lithium ion battery history known as the "propylene carbonate (PC) and ethylene carbonate (EC) differences".PC before the intercalated-li potential (~ 0.7 V) continuous reduction decomposition, eventually making graphite structure collapsed, unable to properly embedded lithium.And EC, its molecular structure is only less than a methyl, PC was slightly higher than 0.7 V potential occurrence break down to form a stable layer of the SEI film, thus inhibiting the decomposition of the electrolyte in a low potential, makes the lithium ion can be embedded in graphite material normally and emergence.

In the past more than 20 years of time, some scientists try to reveal the PC and the reason of the difference of EC behavior, but has not a mechanism model can make people completely convinced.Such as Zhuang, put forward by the PC and the difference of EC on the electrode surface is due to the PC through double electronic reduction, directly generate Li2CO3 and propylene gas, which lead to graphite layer structure damage.The EC is through single electron reduction, to form carbonic acid ester polymer.This mechanism, however, could not explain the Xu and others were detected in the PC and EC reduction reaction single electron reduction product carbonate oligomer experimental results.Tasaki argues that the difference is mainly due to the embedded PC in the total of the formation of graphite content [Li (PC) n] + volume is greater than the distance between layers of graphite layer structure, thus open graphite layer.And EC system were embedded content is less than the volume of graphite layer upon layer spacing, so will not lead to the destruction of the graphite layer.However, the mechanism fails to explain the molecular volume is larger than the PC interface behavior the solvent molecule is similar to EC experimental phenomena.

Recently, Dr XingLiDan from south China normal university, professor li, and the U.S. army laboratory Xu Kang researcher (corresponding author) common cooperation, in the Acc. Chem. Res. Published titled "Deciphering the Ethylene Carbonate? Propylene Carbonate Mystery in Li - Ion Batteries "research paper.They using quantum chemical calculation and experiment method, the combination of lithium ion battery electrolyte is studied in detail to take off the solvation process and its relationship with graphite interface compatibility, found that the lithium salt anionic PF6 - is difference between PC and EC interface behaviors of the most fundamental reason.

When graphite electrode voltage drop (intercalated-li reaction, namely the battery charging process), solvated lithium ion in electric field under the action of migration to the surface of graphite anode.Because the volume than lithium ion solvation layer graphite layer upon layer spacing, therefore needs to happen before embedding solvation.EC lithium ion solvation layer off the base system priority off EC molecular form solvation layer, containing PF6 - PF6 - and the subsequent reduction decomposition, form a stable SEI film that are rich in LiF.

PC based system, however, lithium ion solvation layer off to take off the PC molecules and PF6 - probability is, so to participate in the reductive decomposition of PF6 - content reduce, lead to formation of the decomposition product of low content of LiF.Their subsequent design a series of experiments proved that the low content of LiF is leads to the formation of a PC based electrolyte decomposition product not dense root cause of the stability of SEI.

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