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Yao H.R.,Jiangxi Normal University | Wang Z.Q.,Jiangxi Normal University | Wu M.S.,Jiangxi Normal University | Liu G.,Jiangxi Normal University | And 5 more authors.
International Journal of Electrochemical Science | Year: 2014

Li ion migration dynamics in the bulk and at the surface of Li-Si alloys are investigated from first principles calculations. Vacancy mediated Li migration and interstitial Li migration are considered. Among various phases of Li-Si alloys, we selected Li1Si1 and Li2Si1 as two typical examples in this work. The vacancy formation energies and the site energies of the interstitial Li are calculated in advance in order to figure out reasonable migration pathways, which are then optimized with nudged elastic band methods. Results shown that Li vacancy diffusion in the bulk phases can be fast, while the intrinsic vacancy formation is difficult. At the Li-Si alloy surfaces, Li migration can be assisted either by Li vacancy or interstitial Li. Li migration at the surface creates vacancies at the surface region and the vacancies can diffuse into the bulk, which in turn enhances the Li diffusion in the bulk region © 2014 by ESG.

Yan H.J.,Jiangxi Normal University | Xu B.,Jiangxi Normal University | Xu B.,Key Laboratory for Advanced Functional Materials of Jiangxi Province | Shi S.Q.,Key Laboratory for Advanced Functional Materials of Jiangxi Province | And 2 more authors.
Journal of Applied Physics | Year: 2012

Recently, Yoo and Zhou [ACS Nano 5, 3020-3026 (2011)] reported that graphene can be directly used as electrode for Li-air batteries, suggesting that graphene itself can be served as catalyst for O2 dissociation reaction. In this work, we show from density functional theory calculations that the O2 dissociation reaction energy barrier is substantially decreased on the graphene surface comparing to that of in vacuum. Furthermore, N-doping can further decrease the energy barrier from 2.39 eV (undoped case) to 1.20 eV. The O2 molecule physical adsorption, O atom chemical adsorption at the graphene, and N-doped graphene surfaces are also simulated. © 2012 American Institute of Physics.

Gong X.,Jiangxi Normal University | Huang J.,Jiangxi Normal University | Chen Y.,Jiangxi Normal University | Wu M.,Jiangxi Normal University | And 9 more authors.
International Journal of Electrochemical Science | Year: 2013

Although density functional theory has played important role in designing electrode materials for lithium ion batteries, it fails in correctly prediction of temperature dependant parameters of performance of the battery system that is more relevant in application. To simulate the temperature dependence of the thermodynamic parameters, the lattice vibrational energy should be included. In this paper, we calculated the lattice vibrational dynamic properties of the LiCoO2/Li half battery system. We found that there is no imaginary frequency appeared when all Li atoms are removed, indicating that the bulk structure is stable upon Li removal. Furthermore, the vibration frequency of Co atom along the ab-plane is substantially increased after lithium is removed, due to the strengthened Co-O bonds. The vibrational entropy of the LiCoO2 and its delithiated state Li□CoO2 is very close to each other. We show that the intercalation potential of the battery is altered noticeable when the lattice vibrational contribution is included. © 2013 by ESG.

Huang J.,Jiangxi Normal University | Wang Z.,Jiangxi Normal University | Gong X.,Jiangxi Normal University | Wu M.,Jiangxi Normal University | And 10 more authors.
International Journal of Electrochemical Science | Year: 2013

Silicon is one important candidate as anode materials for next generation lithium ion batteries because of its extremely high specific capacity (4200mAh/g). In this paper, we show from first principles calculations that vacancies play important role in the beginning of the lithiation process of crystalline silicon. Without vacancy, the Li binding energy is lower than that of Li metal, indicating that lithium can not be intercalated into crystalline Si. On the other hand, vacancies can enhance the binding energy substantially and make the lithium intercalation process thermodynamically favorable during the initial stage of the discharge process. © 2013 by ESG.

Xiong F.,Jiangxi Normal University | Yan H.J.,Jiangxi Normal University | Chen Y.,Jiangxi Normal University | Xu B.,Jiangxi Normal University | And 4 more authors.
International Journal of Electrochemical Science | Year: 2012

The physical origin of the degradation on the cycling performance of LiCoO2 upon deep lithium extraction is studied from first principles calculations. Results show that the structural stability is strongly associated with the electronic structures of Co-3d, which is very flexible and can be exhibited as different electronic configurations. In LiCoO2, Co3+ is non-magnetic and holds the (t2g↑)3(t2g↑)3 electronic configuration. Upon lithium deintercalation, some Co3+ ions loss one electron and become Co4+ with (t2g↑)3(t2g↑)2, configuration. Both structures are stable since the distortion of the CoO6 octahedral is small, and thus these structures do not contribute much to the instability. On the other hand, oxygen vacancy is one important reason to the structural instability. We found that spin flip occurs to the electronic structure of Co-3d close to oxygen vacancies. Co3+ holds the (↑)4(↑)2 electronic configuration and magnetized with 2 μB magnetic moment. Furthermore, some Co3+ obtains one electron and becomes Co2+ near the oxygen vacancy. In these cases, the charge distribution around the Co atom is not symmetric and the local structure is distorted obviously, which can further accelerate the process of the structural degradation. ©2012 by ESG.

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