Planar Mesoporous Silicon Flakes with Microns Thicknesses As an Anode Material for Li-Ion Batteries: Fabrication, Characterization and Electrochemical Tests

Abstract

Conventional Li-ion batteries based on a graphite anode with any of Li-Co-(Mn, etc)-oxides cathodes and liquid electrolytes have small specific capacities due to low theoretical capacity of carbon that is of app. 372mAhs/g. Recent research revealed many new perspective materials for using instead of a carbon anode [1] and silicon is one of the best with a theoretical capacity of nearly 4200mAhs/g [2]. This emerging material has been extensively studied in a number of its structural forms, beginning from a simple planar Si wafer to more sophisticated nano-structures embedded into a Si support (wafer) or even as free standing nano-powders of different shapes and geometry, for review see [3]. In this work we studied nano-structured silicon material in form of planar mesoporous flakes with microns thicknesses in an attempt to adjust such Si powder for using it as an anode in Li-ion battery. We report here a fabrication process based on electrochemical etching of Si-wafer in a special regime that allowed not only a formation of meso pores in bulk Si but also a separation of nanostructured material into planar flakes with microns thicknesses and a collection of a final Si powder. We performed an additional coverage of an obtained material with carbon to improve electrical conductivity of the powder. Then we widely characterized both nanostructured materials to know its surface chemical compositions, internal surface areas, pore size distributions and how the performed carbon coating influenced on them. Finally, we prepared anode masses with more then 85% of silicon using standard battery powder technology with conventional binder, solvent and addition of carbon powder. A battery with Li metal as contra electrode was assembled and electrochemical tests fulfilled. Used materials, chemicals and methods: To obtain the nanostructured Si-powder the p+ doped Si wafer of 0.01-0.02 Ohm cm resistivity, 1500 microns thickness and 4cm diameter was used. The electrochemical cell provided the 12.5cm2 area for etching. 100ml of the 1:2 HF (45% conc.):EtOH electrolyte was employed to perform the anodic treatment of a wafer up to 300mkm in-depth in total. The electrochemical regime consisted in application of two anodic steps of different intensity (J1=50mA/cm2; J2=100mA/cm2) and duration (t1=600s; t2=10s) followed by a 20s time interval with zero current for relaxation of a system. This regime was subsequently applied in several cycles (up to 10) without opening the cell and an average depth in a cycle reached app.20mkm. Detailed description of similar regimes is reported elsewhere [4]. After performing all the programmed cycles, the last intensive current (more than 150mA/cm2) was applied for 20s to cut off the obtained mesoporous Si layers (future flakes) from the Si wafer. At this moment all the material was packed together that permitted its exhaustive washing with water and ethanol. Initial crashing of the layers into flakes was done in the laboratory ultrasound bath and the powder was collected into ethanol suspension. After evaporation of solvent at 50oC and additional milling of flakes in the Agatha mortar the dry material was obtained. The carbon coverage was effectuated in the 1:1 acetylene + argon atmosphere at a total gas flow of 0.5L/min and temperature of 850oC during 15 minutes, as described in [5]. Both covered and pristine mesoporous silicon materials were used for preparation of the anodes for battery tests. The weight ratio 85:7:8 between the meso-porous silicon powder, the 15% PVdF binder dissolved in NMP and the C black powder was used for preparation of the anodic masses. The electrochemical examination of the silicon electrodes was performed in double-electrode coin cells CR2032 by using a CT-3008W-5V10 mA battery testing system (Neware). Lithium metal served as the counter electrode. The electrolyte consisted of 1M LiPF6 in a solution mixture of EC/PC/DEC/EMC/PA (ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl-methyl carbonate, propyl acetate) (TC-E810 Tinci).