Pseudocapacitive Charge Storage at Nanoscale Silicon Electrodes

Current lithium-ion battery anode research involves significant investigations of semiconducting materials, particularly Si as its theoretical specific capacity is >4000 mAh/g¹.  Previous theoretical studies showed that porous Si with a large pore size and high porosity can maintain its structure after Li ion induced alloying and swelling. Metal-assisted chemical (MAC) etching is shown here to form internally mesoporous nanowires in the form of a layer, etched from highly doped Si^2-4. Some porous materials are well known to exhibit pseudocapacitive behaviour in aqueous electrolytes^5,6. Maintaining the structure without stress-induced cracked caused by volumetric changes in material is crucial in achieving a high capacity and long cycle retention. Almost all investigations of nanoscale Si involve their deposition onto a metallic current collector electrode within the battery cell. Here, we demonstrate that pseudocapacitive behaviour can be harnessed when Si nanowires are etched to maximum mesoporosity, forming an electrically dead layer on silicon current collector electrodes. This limits insertion or alloying processes to form Li-Si phases⁷and charge is stored within the electric double layer, even in Li-ion containing electrolytes. Measurements using cyclic and linear voltammetry supported by Raman scattering spectroscopy and electron microscopy confirm surface charge storage mechanisms; pseudocapacitance is not observed when the same nanowires are used on stainless steel current collectors.  In such cases, the rate of lithiation is shown to be related to the degree of porosity and the net surface electronic density of the porous silicon in accumulation mode during charging.

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