Effect of the presence of Si-oxide/sub-oxide surface layer(s) on ‘micron-sized’ Si wires towards the electrochemical behavior as anode material for Li-ion battery

Abstract

We report here a systematic study concerning the effects of different Si oxides and sub-oxides (viz., SiO2, Si2O3, SiO, Si2O), in varying contents, as present on the surface of amorphous Si shell of ‘micron-sized’ Si wires (SiMWs), towards the electrochemical behavior as anode material for Li-ion batteries. This has been carried out by exposing the SiMWs, as stand-alone electrodes sans any conducting additive/binder/interlayer, to different surface treatments and air exposure/non-exposure prior to cell assembly. The 5–6 μm long SiMWs, having crystalline core (∼25 nm in diameter) and amorphous shell (∼0.22 μm thick), were grown on stainless steel substrate via hot-wire chemical vapour deposition technique. The optimal combination of ‘native’ SiO2, SiO and Si2O, as present on the surface of the ‘as-deposited’ SiMWs, yields the best possible 1st cycle coulombic efficiency (>98%), near-theoretical reversible Li-storage capacity (∼3780 mAh g−1) and also superior cyclic stability, as compared to the SiMWs subjected to either HF treatment/etching or additional annealing or extra care to totally prevent any air exposure (post deposition and prior to characterization/cycling). Electrochemical impedance measurements also supported the above results by showing nearly negligible build-ups of the resistances due to SEI layer and towards charge transfer for the ‘as-deposited’ SiMWs (i.e., the ones showing the best electrochemical performances) upon electrochemical cycling, unlike for the other investigated SiMW-types (having greater/lesser O-containing surface layers). Li-ion ‘full cell’ developed with the as-deposited SiMW as anode and home-made LiFePO4 as cathode showed appreciable capacity retention over 100 cycles, despite the anode being made of micron-sized Si, sans any conducting additive or buffer interlayer.