Effects of carbon impurities on the performance of silicon as an anode material for lithium ion batteries: An ab initio study

Abstract

Silicon is widely used in the semiconductor industry and has recently become very attractive as a lithium ion battery anode due to its high capacity. However, volume changes associated with repeated lithiation–delithiation cycles expose fresh silicon surfaces to the electrolyte, causing irreversible side reactions. Moreover, silicon suffers from a poor electronic conductivity at a low lithium content. Carbon impurities originating at synthesis or resulting from subsequent contact with other electrode components are often neglected. However, atomistic simulations reveal that dissolved carbon decreases the local potential energy surface by drawing the electron density from silicon to form polar covalent C–Si bonds that are stronger than the non-polar covalent Si–Si bonds they replace. This leads to a higher density and elastic stiffness, regardless of the interstitial lithium concentration. Substitutional carbon also reduces the mobility of silicon self-vacancies and interstitial lithium by increasing their diffusion barriers by 24.7 and 27.3 kJ mol−1, respectively. Moreover, the [carbon, silicon vacancy] complex is basically stable, while the [carbon, lithium] complex is found to become stable against both single defects at a spacing of 4.72 Å. The minimum energy paths ultimately demonstrate that both the interstitialcy and dissociative mechanisms are mainly responsible for carbon diffusion in silicon.

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

“…Beyond that, our ReaxFF-MD calculations predict the Young’s modulus of a-SiC in close agreement with empirical models , fitted from experimental data to our model’s density. Unfortunately, ReaxFF underestimates the high stiffness of 3C-SiC (bulk, shear, and Young’s moduli are 215, 188, and 436 GPa, respectively) compared with experiments published by Thakore et al and previous DFT studies . While ReaxFF and DFT similarly describe isotropic (energy-volume) EoS curves for 3C-SiC, ReaxFF underestimates the mechanical energy under uniaxial strain (lattice length a ) and even more under shear strain (lattice angle α) (see Figure S.10).…”

“…Especially at low Li concentration, e.g ., x < 1, pristine Si 1– x would co-exist with the stable (LiSi) x phase, , as both empirical phase diagrams ,, and first-principles methods ,, predict. The high stiffness and thermodynamic stability of SiC suggest that it might store Li ions while resisting fracturing or pulverization.…”

“…estimates the high stiffness of 3C-SiC (bulk, shear, and Young's moduli are 215, 188, and 436 GPa, respectively) compared with experiments published by Thakore et al 20 and previous DFT studies. 21 While ReaxFF and DFT similarly describe isotropic (energy-volume) EoS curves for 3C-SiC, ReaxFF underestimates the mechanical energy under uniaxial strain (lattice length a) and even more under shear strain (lattice angle α) (see Figure S.10). Hence, particularly the shear elements , but also the normal elements of the elastic stiffness matrix are underestimated by ReaxFF (see formula S.21) so that the elastic moduli (see formulas S.22−S.27) become systematically lower.…”

“…The fact that these C allotropes absorb Li exothermically and reversibly at high kinetic rates without undergoing significant structural changes makes C more effective than O at improving the cyclability of Si anodes. On the other hand, the strength of Si–C sp 3 -hybridized bonds leads to robust, thermodynamically stable SiC compounds. , Recent electrochemical studies showed that crystalline SiC (c-SiC) polymorphs such as 3C-, 4H-, and 6H-SiC likely react with Li ions at their surfaces. − 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 . Although SiC suffers from poor electronic conductivity, our recent density functional theory (DFT) calculations of C and Li point defects in c-Si showed that they may contain local, lithiated regions where electrons are conducted more easily.…”

“…On the other hand, the strength of Si−C sp 3 -hybridized bonds leads to robust, thermodynamically stable SiC compounds. 20,21 Recent electrochemical studies showed that crystalline SiC (c-SiC) polymorphs such as 3C-, 4H-, and 6H-SiC likely react with Li ions at their surfaces. 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.…”