Study of lithium defects in lithium phosphate and in the interface with metallic Li
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Study of lithium defects in lithium phosphate and in the interface with metallic Li Santosh KC1, Ka Xiong1, Roberto C. Longo1, and Kyeongjae Cho1, 1
Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA
[email protected] ABSTRACT Using first-principles calculations, we investigate lithium vacancy and interstitial defects in lithium phosphate (γ-Li3PO4) and in its interface with metallic Li. We find that γ-Li3PO4 is good electronic insulator with a wide band gap of 6 eV. The calculated formation energies of Li vacancies are higher than those of Li interstitials, which indicate that the ionic conductivity is determined by the migration of Li interstitial defects in bulk electrolyte. The Li vacancyinterstitial pair defect formation energy in the Li/γ-Li3PO4 interface is comparable to the sum of Li vacancy defect at the electrode and Li ion interstitial defect in the electrolyte. Our calculation indicates that the low ionic conductivity of Li/electrolyte interface is associated with the high Li ion defect formation energy. Our study provides some useful insights on Li defect formation and migration mechanisms at the electrode-electrolyte interface and, hence, a research direction for designing future Li-ion batteries. INTRODUCTION There is a growing need of thin film solid micro-batteries as highly efficient power sources for numerous technological applications. For this purpose, intense research efforts are carried out in the field of Li-ion batteries [1, 2]. Solid electrolyte is the main component that makes an all-solid-state lithium battery. Because of the various advantages like simple design, resistant to variations in shock, vibration, pressure, temperature, natural seal, wider electrochemical stability and safety over the liquid and polymer electrolytes, solid electrolytes have attracted much attention [3, 4]. Among various solid electrolytes, lithium phosphorous oxynitride (LiPON) has been used as an electrolyte in thin-film batteries [5]. Solid electrolyte Li3PO4 is one of the natural and synthetic crystalline forms of the LiPON group. Those electrolytes are used in thin films because of its chemical and physical stability. However, the main drawback of solid electrolytes for its commercial application is its relatively low ionic conductivity. An ideal solid electrolyte requires a high ionic conductivity, negligible electronic conductivity (good insulator), and good stability in contact with cathode and anode over a wide range of operating voltage (0-5V) [6]. The ionic conductivity in crystalline Li3PO4 is approximately 10-6 S/cm [7]. However, there are reports of improving the ionic conductivity with a proper concentration of solid solutions: the study of Li4GeO4-Li3PO4 system shows that the ionic conductivity increases up to 10-4-10-5 S/cm [8], and the solid solution Li4SiO4-Li3PO4 [9] shows that mixing Li4SiO4 into Li3PO4 introduces large amount of Li interstitial defects that
lowers the activation barrier for Li diffusion to adjacent neighboring
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