First-principles calculations of bulk, surface and interfacial phases and properties of silicon graphite composites as anode materials for lithium ion batteries

Abstract

Lithium stabilizes silicon–graphite interfaces, making hollow core–shell structures favorable in the presence of lithium and yolk–shell configurations favorable in its absence.

Development and Validation of a ReaxFF Reactive Force Field for Modeling Silicon–Carbon Composite Anode Materials in Lithium-Ion Batteries

“…On the other hand, Li ions inserted into the Si host subsystem increase its partial volume by breaking Si–Si bonds in favor of softer Si–Li and Li–Li bonds. Moreover, the higher Li concentrations which both surfaces and grain boundaries exhibit results from the lower tension of Li adatoms . In fact, Figures S.5 and S.21 show that the surface tension of Li x 1 Si compounds with respect to their bulk structure decreases proportionally to their Li content, x 1 .…”

“…Indeed, they did not observe crack formation throughout the charge/discharge cycles and proposed the following reversible electrochemical reactions .25ex2ex SiX + 4 Li + + 4 e − ⇔ Si + Li 4 X ⁣ false( normalX = normalC , 0.25em normalO , 0.25em 0.25em ··· false) Si + 4.4 Li + + 4.4 e − ⇔ Li 4.4 Si These equations imply that Li in SiC, as in SiO x anodes, , likely reacts with the more electronegative element, i.e ., C (O in SiO x ) up to a maximum concentration ( e.g ., Li 4 C for SiC and 2Li 2 O for SiO 2 ) before undergoing exothermic mixing with Si up to the Li 4.4 Si phase. Among nongraphitic compounds, only the Li 2 C 2 salt phase (space group Immm ) is known to be thermodynamically stable at STP. ,, By performing DFT calculations, Lin et al showed that Li y C compounds would stabilize themselves up to y = 8 only if a compressive stress about 40 GPa was applied. The capacity of 309 mA h g –1 measured by Zhang and Xu does not match with the electrochemical equations they suggested in eq , which predicts much higher capacities of about 5626 mA h g –1 .…”

“…The high activation energies required for interdiffusion and phase transitions result in slow kinetics. Especially at low Li concentration, e.g., x < 1, pristine Si 1−x would co-exist with the stable (LiSi) x phase, 40,87 as both empirical phase diagrams 84,86,88 and first-principles methods 26,89,90 predict. The high stiffness and thermodynamic stability of SiC 21 suggest that it might store Li ions while resisting fracturing or pulverization.…”

“…Among nongraphitic compounds, only the Li 2 C 2 salt phase (space group Immm) is known to be thermodynamically stable at STP. 26,91,92 By performing DFT calculations, Lin et al 91 showed that Li y C compounds would stabilize themselves up to y = 8 only if a compressive stress about 40 GPa was applied. The capacity of 309 mA h g −1 measured by Zhang and Xu 23 does not match with the electrochemical equations they suggested in eq 4, which predicts much higher capacities of about 5626 mA h g −1 .…”

“…22−24 Capacities exceeding 2000 mA h g −1 have been measured for them in their amorphous state, i.e., as a-SiC, implying that C also participates in the electrochemistry. 25 Although SiC suffers from poor electronic conductivity, our recent density functional theory (DFT) calculations of C and Li point defects in c-Si 26 showed that they may contain local, lithiated regions where electrons are conducted more easily. Moreover, we expect the amorphization of SiC to result in increased ionic and electronic conductivities due to the formation of a-Si and a-C phases with mixed sp 2 /sp 3 bonds and smaller band gaps.…”