Tracing Reversible and Irreversible Li Insertion in SiCO Ceramics with Modeling and Ab-Initio Simulations
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Tracing Reversible and Irreversible Li Insertion in SiCO Ceramics with Modeling and Ab-Initio Simulations Peter Kroll Department of Chemistry and Biochemistry, The University of Texas at Arlington, 700 Planetarium Pl., 76019 Arlington, TX, ABSTRACT We present combined modeling and simulation studies of Li insertion in amorphous SiCO ceramics. Atomistic models of amorphous SiCO with and without so-called “free” carbon have been crafted using a network modeling approach and subsequently relaxed and optimized within density functional theory. In a first series, Li atoms are randomly inserted to test for sites with high binding energy. We find that Li prefers always bonding to O. The enthalpy of insertion, however, depends strongly on electronic states available in the SiCO host matrix. While Li inserted into insulating SiO2 is an energetically unfavorable process, the enthalpy of insertion in SiCO glass is decreased. Some glassy SiCO models exhibit favorable sites for Li insertion, which share a common motif in their local atomic environment. The presence of “free” carbon in SiCO then promotes Li insertion, because carbon-related structural imperfections give rise for low-lying unfilled electronic states. Consequently, strong and irreversible bonding of Li into SiCO is provided, if bonding of Li-cations in Li-O bonds outweighs the promotion energy for the electron to occupy unfilled electronic states. INTRODUCTION Graphite is used as the standard anode material to store Li in Li-ion batteries. Recent experiments show that silicon oxycarbide (SiCO) ceramics provide about 3 times larger storage capacity per gram anode material than graphite [1-3]. The stored lithium falls into two categories: an irreversible part and a reversible part [1,2]. The irreversible part is, once loaded upon initial charging, strongly bonded within the anode material. It is “lost”, because it cannot be used to provide electrical work. It was suggested that this Li is forming Li2O within the oxide anode material, which does constitute a thermodynamic sink. The reversible part, on the other hand, can be continuously discharged; it allows for multiple usages of a battery. While the useful part of inserted Li is still more than twice as large for SiCO than for graphite, it would be very useful and cost efficient to avoid the irreversible insertion of Li in the material. Experimental studies show that Li insertion into SiCO ceramics that contain “free carbon” is very favorable to the extent described above. Structural investigations are obstructed by the nature of these ceramics; they are disordered and resemble amorphous glass, making it very difficult to study local structure and individual bonding. A recent NMR study indicated three contributions to the location of irreversibly stored Li in SiCO [4]. Interpreting the signals of 7 Li-NMR, it was suggested that the major part of Li is stored at the edges of graphitic segregations, while minor parts are stored within the stoichiometric SiCO glass and, in small contributions as well, in open voids in
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