Dopant Modulated Li Insertion in Si for Battery Anodes: Theory and Experiment

Abstract

Lithium-ion batteries are energy storage devices for portable electronics and are also of interest for more demanding applications, such as transportation. Present day Li-ion batteries feature a transition metal oxide cathode such as LiCoO 2 , LiMn 2 O 4 , or LiFePO 4 and a graphitic carbon anode. 1,2 These anode and cathode materials have Li capacities that are much less than desired. On the anode side, the graphitic anode presently utilized exhibits a low theoretical specific capacity of 372 mAhg À1 . 3 Other anode materials have much greater capacities. 4 Among these, Si is of intense interest due to its 4200 mAhg À1 theoretical specific capacity in its fully lithiated composition, over an order of magnitude greater than graphitic carbon. 5 Currently the extreme volumetric expansion of Si during lithiation limits its practical use owing to the large mechanical strain encountered, which leads to pulverization and loss of active material. 6 Several different methods have been researched to improve the cyclability of silicon, such as limiting the voltage range, 7 making thin film electrodes, 8À11 alloying, 12À15 and creating composites with carbon. 16À20 Limthongkul et al. first reported that crystalline Si undergoes solid-state amorphization upon reaction with Li. 21 Obrovac and Christensen confirmed this amorphization and showed via ex situ XRD that the Li 15 Si 4 phase crystallizes below 50 mV. 22 Raman spectroscopy showed the transition from crystalline to amorphous Si in a carbon composite by monitoring the 520 cm À1 c-Si phonon mode. 23 Recently we used Raman spectroscopy to show that Li inserts anisotropically into single crystalline Si. 24 Work to date on the lithium insertion into, and accompanying amorphization of, crystalline Si has primarily dealt with undoped Si. First principles computational studies have been carried out on bulk crystalline Si showing that interstitial sites are the most favorable for single Li insertion into the crystalline Si lattice. 25 While a few studies exist on the lithiation of doped Si, these involved amorphous Si 26 or polycrystalline Si-graphite mixtures, 27,28 in which the effects of dopants apart from electrical conductivity enhancement may be obscured. Dopants have been shown to change the properties of Si in processes such as electrochemical etching. 29 A first principles study suggests that dopants can have an effect on the binding energy of Li with Si. 30 First principle studies on Si nanowires show nontrivial effects of surface passivating species such as H and halogens on Li binding energies. 31 However, the fundamental effects of dopants on Li insertion into crystalline Si remain unclear, particularly with regard to anisotropy and energetics. The work in this paper is aimed at developing a better fundamental understanding of the role of dopants on the lithiation of crystalline silicon electrodes. We present in situ Raman spectroscopy experiments and density functional theory (DFT) calculations of the effects of dopants on the lithiation of Si. We show that addit...

The influence of carrier density and doping type on lithium insertion and extraction processes at silicon surfaces

“…The cathodic current related to Li insertion for p-type Si increases at ~0.3 V indicating that it is forming the Li2.33Si phase [62], while the single anodic peak at ~0.5 V corresponds to the deformation of this Li2.33Si phase [63]. For the n-type Si, cathodic current does not increase until ~0.1 V, but it increases dramatically, forming a high Li mole fraction Li-Si phase in a narrow insertion potential window.…”

“…To determine the effective carrier concentration at the interface between the Si(100) and 1 mol dm -3 (100) versus (111) for instance) is known to have negligible effect on the chemical potential for Li insertion into Si [63].…”

“…Phonon scattering is significantly reduced in amorphous Si, specifically at a particular frequency such as the TO phonon. [63,87] Raman scattering was previously used to demonstrate the transition from crystalline to amorphous Si in a carbon composite by monitoring the TO c-Si phonon mode [87]. Recently it was shown using Raman scattering that Li inserts anisotropically into single crystalline Si [88].…”

Metal-assisted chemical etching of silicon and the behavior of nanoscale silicon materials as Li-ion battery anodes

“…Long et al [195] showed the effects of boron (B) and phosphorus (P) dopants on the lithiation of crystalline Si. By using Raman scattering spectroscopy, they were able to identify differences in Li insertion potentials for the different dopant types.…”

In situ methods for Li-ion battery research: A review of recent developments

“…Long et al [68] measured the intensity of the characteristic phonon mode corresponding to crystalline silicon as a function of the electrode potential vs a Li/Li þ reference electrode. Since a decrease in the phonon mode intensity is indicative of the transition from crystalline to amorphous silicon, it corresponds to the onset of the lithiation process.…”