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Solid state lithium ion electrolytes are becoming increasingly important in batteries and related technologies. We have used first-principles modeling techniques based on density functional theory and the nudged elastic band method to examine possible Li ion diffusion mechanisms in idealized crystals of the electrolyte material Li 3 PO 4. In modeling the Li ion vacancy diffusion, we find direct hopping between neighboring metastable vacancy configurations to have a minimal migration barrier of E m = 0.6 eV. In modeling the Li ion interstitial diffusion, we find an interstitialcy mechanism, involving the concerted motion of an interstitial Li ion and a neighboring Li ion of the host lattice, that can result in a migration barrier as low as E m = 0.2 eV. The minimal formation energy of a Li ion vacancy-interstitial pair is determined to be E f = 1.6 eV. Assuming the activation energy for intrinsic defects to be given by E A = E m + E f /2, the calculations find E A = 1.0-1.2 eV for ionic diffusion in crystalline ␥-Li 3 PO 4 , in good agreement with reported experimental values of 1.1-1.3 eV.

Recently, there has been significant interest in developing solid-state lithium ion electrolytes for use in batteries and related technologies. We have used first-principles modeling techniques based on densityfunctional theory and the nudged elastic band method to examine possible Li ion diffusion mechanisms in idealized crystals of the electrolyte material Li 3 PO 4 in both the ␥ and ␤ crystalline forms, considering both vacancy and interstitial processes to find the migration energies E m . We find that interstitial diffusion via an interstitialcy mechanism involving the concerted motion of an interstitial Li ion and a neighboring lattice Li ion may provide the most efficient ion transport in Li 3 PO 4 . Ion transport in undoped crystals depends on the formation of vacancy-interstitial pairs requiring an additional energy E f , which results in a thermal activation energy E A = E m + E f / 2. The calculated values of E A are in excellent agreement with single crystal measurements on ␥-Li 3 PO 4 . Our results examine the similarities and differences between the diffusion processes in the ␥ and ␤ crystal structures. In addition, we analyze the zone center phonon modes in both crystals in order to further validate our calculations with experimental measurements and to determine the range of vibrational frequencies associated with Li ion motions which might contribute to the diffusion processes.

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Two recently synthesized crystalline materials which belong to the LiPON materials family, Li12P3N9 and it’s high–pressure polymorph Li4PN3, have been computationally examined as solid electrolytes for the usage in Li–ion batteries through first–principles simulations. The simulations of the idealized electrolyte properties of both materials suggests that these materials are promising solid electrolytes. The simulated crystal structures of both materials found to be in good agreement with experimental data, the phase transition between Li12P3N9 and Li4PN3 at high pressure has been validated through the simulations and found to be consistent with computational and experimental literature. The Li–ion migration analysis predicts that these materials are a very promising conductors for Li–ions with a calculated value for the activation energy comparable and even smaller to those of good solid electrolytes in the same materials family. The Li–ion migration analysis also suggests that the Li–ion migration is dominated with the vacancy migration mechanism which takes place along the three diffusion axes in Li12P3N4 and along the c–axis only in Li4PN3. The simulations of idealized interfaces with metallic Li anode demonstrate that these materials are likely to form a metastable interfaces with Li metal.