High-Performance Artificial SEI-Capped Nano-SiOx/Graphite Anode for Rechargeable Lithium Batteries

Carbon composites of silicon (Si), silicon monoxide (SiO) or silicon suboxide (SiO_x) are considered as alternative anode materials to graphite for rechargeable lithium batteries, due to the high theoretical capacity of 3579 mAh/g  for Si at room temperature and safer operation voltage above lithium. The Si, however, suffers from capacity fade due to large volume change upon lithiation and delithiation during cycling. This leads to electrochemical and mechanical particle disintegration. SiO and SiO_x are favored with respect to cycling stability despite sacrificing the capacity, as lithium oxide (Li2O) and lithium silicates formed during initial lithiation better accommodates significant volume change of Si.Interfacial processes for Si anode with LiPF₆-containing electrolyte, which is another cause for particle disintegration and performance fade, have been established as LiPF₆ decompose and produce Lewis acids (PF₅, PF₃O) and HF (LiPF₆ → LiF + PF₅;  PF₅ + H₂O → PF₃O + 2HF) that can undergo electrophilic attack to the surface of Si anode and participate in electrochemical reduction leading to Si deactivation. This restricts the formation of a stable SEI and charge-discharge cycling stability. Extensive research efforts have been made to improve interfacial stability and cycling performance of SiOx-based carbon composite.We have been developing a new synthetic method and mechanistic investigation of surface-protected SiO_x, (x < 0.5) nanoparticles and their carbon composites for attaining stabilized electrode performance. Here we report a facile preparation, and material and electrochemical characterization of amorphous nano-SiO_xand its graphite composite with homogeneous compositional distribution.

Artificial SEI-capped SiO_x nanoparticles were synthesized by a chemical reduction of silicon tetrachloride and in-situ capping with a surface protective hydrophobic layer. The hydrophobic surface nature led to an easy and homogeneous dispersion of SiO_x nanoparticles with graphite powders in n-methylpyrrolidinone (NMP) just by room temperature mixing for the preparation of slurry. Artificial SEI-capped nano-SiO_x/graphite (50:50 wt%) electrodes consisted of the active material (70 wt%),  carbon black (15 wt%) and binder (15 wt%) were fabricated onto a copper foil, followed by vacuum drying at 110 °C overnight. Cycling ability of the composite electrode was evaluated using coin half-cell cells with a lithium foil as a counter electrode and the baseline electrolyte of 1M LiPF₆/EC:EMC (3:7 volume ratio) or that with a new carbonate-based solvent, and polyethylene separator, between 0.01 to 1.5 V at a constant current density of 300 mA/g (~0.3C).

SEM and elemental mapping images (Figure 1A) of nano-SiO_x/graphite reveal a uniform distribution of carbon (green) and silicon (red) atoms over bulk composite. Large micron carbon particles are of graphite and relatively small ones are of carbon black, respectively. Figure 1B shows the voltage profiles of nano-SiO_x/graphite electrode measured at 0.3C. The initial charge and discharge capacity are 1370 and 1010 mAh/g, respectively, resulting in the initial coulombic efficiency of 74 %. Coulombic efficiency increases to higher than 98 % after the fifth cycle. Discharge capacity is retained as 931 mAh/g after 100 cycles (Figure 1C), corresponding to capacity retention of 91%. The excellent cycling stability is ascribed to homogenous dispersion of SiO_x and graphite powders, accommodation of volume change of SiO_xby oxygen and graphite, and enhanced interfacial stability by artificial SEI. Further discussion of the high-rate capability, and the SEI formation and stability and their correlation to cycling performance would be presented in the meeting.

Acknowledgments

This work was supported partly by the Korean Ministry of Education and National Research Foundation through the Human Resource Training Project for Regional Innovation (2012026203) and partly by the Converging Research Center Program (2013K000214) through the Ministry of Science, ICT & Future Planning.