High Performance Lithium Silicide Electrode Enable By Molecular Layer Deposition
Authors: Caban Huertas Zahilia , Settipani Ramirez Daniel , Flox Cristina , Jacas Biendicho Jordi , Morante Joan Ramon , Kallio Tanja 
Published: December 2020

The energy demand of our society is increasing without precedents. Lithium-Ion Batteries (LIBs) are and have been an enabler of the present communication revolution; LIBs are the powerhouse of portable communication devices. Consumers are starting to demand more sustainable and more environmentally friendly energy sources, energy production and distribution. To satisfy these demands, the next generation of rechargeable batteries will need higher specific energy and power (per mass) and energy and power density (per volume) than the current generation of LIBs. The current technology uses Lithium Metal Oxide (LMO) as positive electrodes and graphite as a negative electrode. This system has an energy density value ca. 250 WhL-1. To improve the power and energy density of the LIBs devices; new electrode material needs to be developed.

Silicon (Si) has become one of the most investigated materials for LIB negative electrodes because of its ability to accommodate 3.75 moles of Li per mole of Si (Li15Si4), leading to a theoretical capacity of 3,579 mA h/g at room temperature1. While 372 mA h/g is the theoretical capacity for graphite2, this material can only accommodate 1 mole of Li per 6 mol of C.

Despite Si advantages, progress towards a commercially available Si negative electrode has been impeded by their rapid capacity fade, poor rate capability, and low coulombic efficiency. The cause of electrode degradation is the Si volume change of ~300% upon lithium insertion and extraction3, presenting a major problem for electrochemical performance. The volume fluctuation damages the electrical contact between Si and the current collector, being too large to be controlled by currently developed coating technologies.

One of the leading strategies established for the realization of such an approach is coating nano-Si particles with flexible materials to attempt to accommodate the volumetric changes of the particles. In this context, we propose to carry out a surface modification, in which Molecular Layer Deposition (MLD) is utilized to grow a mechanically robust, flexible coating, to undertake the Si expansion and contraction. For it, we have used TiCl4 molecule and as it is a relatively small molecule, when combined with the larger organic reactants, has high growth rates that are mainly limited by steric hindrance caused by the organic reactants. For these reasons, the recipe here developed is based on Glycerol and TiCl4 as the titanium precursor.

The composition of the films was studied using Fourier-Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope-Energy-dispersive X-ray spectroscopy (SEM-EDX). Multiple characteristic absorption modes for both carbon and titanium related groups can be observed in the FTIR spectrum (Figure 1). These techniques confirm that the combination of both the carbon-related groups and the Ti absorptions providing evidence for the successful deposition of a hybrid organic-inorganic film showing that we are able to detect Ti in the surface of the electrode.

Cyclic Voltammetry was used to characterize the electrochemical performance of the LixSi negative electrode coated with titanicone. We have successfully shown the deposition of titanicone on LixSi electrode and an initial electrochemical assessment of this electrode. These electrodes exhibit high capacity, around 2,000 mAhg-1 at 0.10C, and 270 mAhg-1 at 20C corroborating the goodness of the proposed methodology.

Typical voltammogram shows the formation of the solid electrolyte interface, evident in the first cycle. In the subsequent scans, these peaks disappear (Figure 2). The sample exhibit the regular waves are corresponding to the Si redox reaction.

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Figure 1