Ralf Riedel

Prevention of Solid Electrolyte Interphase Damaging on Silicon by Using Polymer Derived SiCN Ceramics

A new composite anode material for lithium ion batteries, based on nano-silicon particles dispersed in a polymer-derived ceramic (PDC) matrix, was produced, characterized and electrochemically analyzed via cyclic voltammetry (CV). For this purpose a commercial preceramic polymer , namely the polysilazane HTT 1800, was mixed with silicon nano powder and pyrolyzed at 900, 1100, 1300 and 1500 °C. It was found that in comparison with pure silicon, the composite presented an enhanced electrochemical stability during lithium insertion/extraction. Moreover, dispersing the silicon into the ceramic matrix avoids a continuous energy loss related to the formation of a solid electrolyte interphase (SEI). By means of cyclic voltammetry measurements we found that for the composites synthesized at temperatures exceeding 1000 °C, no more losses were observable during subsequent insertion/extraction once the stable SEI was formed.

New high pressure nitrides

The light elements of the first row of the periodic table form simple molecular compounds N2, CO2, N2O, C2N2 and CH4 that are characterized by strong covalent bonding between atoms which condense to form high symmetry structures characterized by weak van der Waal’s interactions. With increasing pressure and temperature these solids are being observed to transform to framework or polymeric solids [1,3] and decompose at high temperatures [13].This talk will focus on the results in the case of CO 2 and N2O systems. Although the two systems are isoelectronic and display similar high-pressure phases at room temperature, recent work at high P-T conditions has shown that whereas CO 2 tends to form a covalently bonded structures [1,3], N 2O forms an ionic, aragonite type phase NONO 3 [2]. On the other hand, recent theoretical studies on these two systems have predicted similar high P-T behavior [4]. The talk will focus on presenting the experimental results comparing the high P-T phase diagrams of these two systems, their structural details and the compressibility of the stable phases to be able to understand and rationalize the observed differences in the two systems. Acknowledgements: This work is supported by NASA, NSF-DMR, NSLS, APS and the W. M. Keck foundation. References: [1] V. Iota et al, Science, 283, 1510 (1999). [2] M. S. Somayazulu et al, Phys. Rev. Lett. 87, 135504 (2001). [3] O. Tschauner et al, Phys. Rev. Lett. 87, 75701 (2001). [4] J. J. Dong et al, Phys. Rev. B 61, 5967 (2000).