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Deciphered the differences between propylene carbonate (PC) and vinyl carbonate (EC) in lithium-ion batteries

- Dec 08, 2018 -

Lithium ion battery realizes energy storage and release by lithium ion embedding and releasing between lithium transition metal oxide and lithium graphite material.The reason why graphite material can be used in lithium ion battery is that the solid electrolyte interface (SEI) film formed by the decomposition of electrolyte on the graphite surface is ion-conductive and electronic-non-conductive.The reduction stability of this protective film is much lower than that of lithium imbedded potential (0).01V) electrolyte is isolated from the graphite electrode, so as to ensure that the electrolyte does not undergo reduction decomposition under the lithium implantation potential, so that lithium ions can be reversibly imprinted and removed in the graphite material.

How is such an important SEI film formed?Why is it that some electrolyte decomposition products can form a stable SEI film, while some electrolytes continue to undergo reduction decomposition at a potential higher than that of lithium embedded, leading to the collapse of graphite layer structure?The most typical difference in interface behavior is the well-known "difference between propylene carbonate (PC) and vinyl carbonate (EC)" in the history of lithium-ion batteries.PC embedded lithium potential before (~ 0.7V) continuous reduction decomposition occurs, which eventually leads to the collapse of graphite structure and the failure of normal lithium implantation and deoxidation.EC, on the other hand, has only one less methyl group in its molecular structure than PC, but is slightly higher than 0.The decomposition occurs at 7V potential to form a stable SEI film, which inhibits the electrolyte's decomposition at a lower potential and enables lithium ions to be normally embedded and exhaled in graphite materials.In the past two decades, some scientists have tried to reveal the reasons for the differences between PC and EC behaviors, but no mechanism model has been completely convincing.Zhuang et al., for example, have suggested that the difference between PC and EC is due to the fact that PC directly generates Li2CO3 and propylene gas through double electron reduction on the electrode surface, which leads to structural damage of graphite layer.EC, on the other hand, undergoes one-electron reduction to form carbonate polymers.However, this mechanism is unable to explain the experimental results of single-electron reduction product carbonate oligomer detected by Xu et al. in both PC and EC reduction reactions.Tasaki believed that this difference was mainly due to the fact that the structure volume of PC co-embedded in the graphite layer [Li (PC) n] + was larger than the spacing between the layers of the graphite layer, thus supporting and damaging the graphite layer.However, the volume of the copolymers formed by EC system is less than the spacing between graphite layers, so it will not lead to the destruction of graphite layers.However, this mechanism cannot explain the experimental phenomenon that the molecular interface behavior of solvents larger than PC is similar to that of EC.

Recently, Dr. Lidan xing and professor weishan li from south China normal university collaborated with U.S. army laboratory researcher kang xu (co-corresponding author) in Acc.Chem.Res.Published titled "Deciphering the Ethylene Carbonate?Propylene Carbonate Mystery in li-ion Batteries"They used the combination of quantum chemical calculation and experimental methods to study in detail the dissolvation process of electrolyte of lithium ion battery and its relationship with graphite interface compatibility, and found that PF6-, a lithium anion, was the most fundamental reason for the difference between PC and EC interface behavior.When the voltage of the graphite electrode drops (a lithium immobilization reaction occurs, that is, the battery charging process), the solvated lithium ions migrate to the surface of the graphite cathode under the action of an electric field.At this time, the volume of the lithium ion solvent layer is much larger than the spacing between the graphite layers, so desolvation needs to occur before embedding.EC molecules are preferentially removed from the lithium ion dissolvation layer of the EC base system to form a dissolvation layer containing PF6-, which is involved in subsequent reduction and decomposition to form a stable SEI film rich in LiF.However, the probability of PC molecules and PF6- being removed in the lithium ion dissolvation layer of PC base system is equal to that of PF6-, so the PF6- content involved in reduction decomposition decreases, resulting in low LiF content of decomposition product formed.A series of subsequent experiments designed by them have proved that low LiF content is the fundamental reason why pc-based electrolyte decomposition products cannot form dense and stable SEI films.

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Electrochemical behavior and structure differences between EC and PC based electrolytes

(a) charge and discharge curves of EC and pc-based electrolytes on graphite electrodes

(b) possible lithium ion solvation layer structures embedded in graphite layers in EC and pc-based electrolytes

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The influence of ionic solvation layer on electrochemical behavior of electrolyte

(a) electrospray ionization mass spectrometry for the determination of ionised layer structure

(b) the influence of the proportion of solvents in the electrolyte on its electrochemical properties

(c) the relationship between EC content in ionised layer structure and EC solvent content in electrolyte

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In the solvation layer, the binding energy of lithium ions with solvent molecules and anions is related to the number of solvent molecules

(a) the binding energy of lithium ions in ec-based electrolytes to EC and PF6-

(b) binding energy of lithium ions in pc-based electrolytes to PC and PF6-

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Optimum structure of solvated layer containing PF6- after single electron reduction

5

Electron affinity energy and the frontier molecular orbital energy of the product

(a) electron affinity of solvated layer containing PF6-

(b) the relationship between front-line molecular orbital energy and electron barrier capacity of some major electrolyte decomposition products

Table 1. The content of LiF on the surface of graphite electrode after cycling in different electrolytes

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