Solid state NMR structural studies of the lithiation of nano-silicon:: Effects of charging capacities, host-doping, and thermal treatment

The electrochemical lithiation of undoped, P-doped and B-doped nano-silicon particles (100–200 nm diameter) has been studied during the first cycle by ex-situ 6Li and 7Li magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Samples were charged within pouch cells up to capacities of 4000 mAh/g, at C/40 followed by NMR analysis. The spectra reveal important quantitative information on the local lithium environments during the various stages of the intercalation process. Approximate Li/Si ratios of the lithium silicides present in the nanoparticles can be deduced, based on the initial formation of the SEI layer, which accounts for an irreversible capacity of up to 500 mAh/g. Surface lithium silicide environments with high Li concentrations (corresponding to the composition Li15Si4) are preferentially formed at charging capacities near 1000 mAh/g. At higher charging capacities, irreversible capacity losses are lower and a wide distribution of lithium silicide environments is found, resembling those present in the crystalline phases Li12Si7, Li7Si3, and Li13Si4. At a charging capacity of and above 2000 mAh/g the large majority of silicon is converted to lithiated silicide particles. Boron-doped nano-Si materials behave generally similar, while phosphorus-doping reveals clear beneficial effects, in particular concerning the initial lithiation stages. Both irreversible capacity losses and surface “over-lithiation” are significantly diminished in these samples. Exposure of lithiated nano-Si samples to elevated temperatures (400–440 K) results in the crystallization of Li7Si3 in all of those nano-Si samples charged with at least 1500 mAh/g.