Theoretical Studies Of The Formation Mechanism Of SEI And The Process Of The Reductive Dissociation Of The Carbonate Electrolytes In The Lithium-ion Batteries

Lithium-ion batteries are becoming popular power sources for the portableelectronics. The electrolyte solvents of commercial lithium-ion batteries are mixturecarbonates. The SEI that formed by the reductive dissociation of carbonateelectrolytes on the graphite anode has an important effect for the performance oflithium-ion batteries. It is significance for the development of lithium-ion batteries toexploring and understanding the reductive mechanism of the carbonate electrolytes onthe graphite anode.Firstly, the interaction between the Li⁺and the carbonates in the Li⁺（EC）n,Li⁺（PC）n, Li⁺（VC）_n,（n=1⁴） and Li⁺（PC）n（VC）（n=1³ï¼‰ was studied at theB3LYP/6-31+G（d,p） levels. The structures, interaction energies，NPA and AIManalysis are applied for discussing the complex that Li⁺coordinated by carbonates. Ithas been shown that, near the graphite anode, the the number of the carbonates thatcoordinated Li⁺is2or3; the order of measurement for the interaction between Li⁺and carbonates is PC>EC>VC, but the preferentially coordinated is not exist.Secondly, the electron affinity, the relationship between the LUMO of the clusterand the SOMO of the intermediate, the transition state were carried out to explore theprocess of the reductive dissociation of the carbonate; the weak interactions betweenthe solvent molecules of the Li⁺（Solv）₃were studied to understand the mechanism ofVC reduction.The three-step process of the carbonate reductive dissociation is:â‘ The Li⁺（Solv）_nclusters get the electron from graphite anode which is decidedby the measurement of the vatical electron affinity, the order of the preference to getelectron for the carbonate that coordinate Li⁺is VC>EC>PC;â‘¡The Li⁺（Solv）_nclusters change to the intermediates. The structuresintermediates are depend on the LUMOs of the Li⁺（Solv）_nclusters. There are pyramid（P） and ’W’（W） structures of the Li⁺（EC）₃, Li⁺（PC）₃and Li⁺（VC）₃intermediates;â‘¢Li⁺（EC）₃change to the P-intermediate and form the anion radical with a tinybarrier; Li⁺（PC）₃change to the W-intermediate and form the GIC because of its low vertical electron affinity and higher barrier; Li⁺（VC）₃change to the W-intermediateand form the anion radical with a low barrier.VC play its role as an additive of PC-based electrolytes by form theLi⁺（PC）₂（VC） cluster. Li⁺（PC）₂（VC） get the electron from the graphite morepreferentially than Li⁺（PC）₃. It change to the W-intermediate which should reductivedissociation with VC ring-open. The research of weak interaction between thecarbonates in the Li⁺（PC）₂（VC） cluster indicate that planar structure of VC isdestroyed by the interaction between the carbonates, the barrier is reduced. so that theLi⁺（PC）₂（VC） decompose to an anion radical, and the one-electron reduction ofLi⁺（PC）₃is prevented.