Group members collaborating on computer simulations:

1. Simulations of materials related to LiPON electrolytesN. A. W. Holzwarth, Wake Forest University, DMR 0705239 The thin film solid electrolyte, LiPON, developed at Oak Ridge National Laboratory is the most widely used solid electrolyte for thin film batteries and a number of other related technologies. In order to better understand and further develop this material, we have used first-principles simulation methods to study a number of crystalline structures in the LixPOyNz (x=2y+3z-5) family, including both materials reported in the literature and stable and meta-stable new materials. Structural models of two optimized structures are shown on the right. The top figure shows the structure of the 100-atom unit cell structure of “natural” LiPO3 with the infinite chains of twisted phosphate groups oriented perpendicular to the plane of the figure. Starting with this structure, replacing 20 O atoms in the chain with N and adding 20 extra Li ions, computer optimization finds the stable structure of Li2PO2N shown in the bottom figure. What is intriguing about this new stable structure is the regularization of the chain structure and the planar –P-N-P-N– backbone. From our estimates of heats of formation, we predict several possible reaction pathways that can produce the Li2PO2N product. We are anxious to work with experimental colleagues to test these predictions and further study these interesting materials. Ball and stick model of “natural” LiPO3 (top) and “synthetic” Li2PO2N (bottom) with ball colors: Li=grey, P=yellow, O=blue, N=green.

2. First principles computer simulations of materials N. A. W. Holzwarth, Wake Forest University, DMR 0705239 One of the most important aspects and “broader impacts” of university research is the careful training of students and the development of collaborations. Group meetings each week include informal discussions to share information of mutual interest. In addition, one person presents a prepared discussion on a chosen topic. Group members are engaged in both materials based projects (such as the study of LiPON electrolytes) using a variety of computer techniques and/or in technique development projects (such as the development of new computational formalisms and codes). Group members collaborating on computer simulations: From left: Yaojun Du (post-doc), Xiao Xu and Nicholas Lepley (graduate students), David Harris (undergraduate student), N. A. W. Holzwarth (PI) .