Molecular Layer Deposition of Titanicone over Lithium Silicide Anode
Authors: Caban-Huertas Zahilia , Settipani Ramirez Daniel , Flox Cristina , Jacas Biendicho Jordi , Kallio Tanja , Morante Joan Ramon 
Published: May 2020

The increasing demands for more advanced consumer electronics, electric vehicles, and stationary facilities call for the next generation of rechargeable batteries with higher specific energy (energy per mass) and energy density (energy per volume) than the current generation of lithium ion batteries (LIBs), which largely use lithium metal oxide (LMO) positive electrodes and graphite negative electrode.

Si has become one of the most highly investigated materials for LIB negative electrodes because of its ability to accommodate 3.75 moles of Li per mole of Si (Li15Si4) for a theoretical capacity of 3579 mA h g−1 at room temperature. Despite Si inherent advantages, progress towards a commercially viable Si anode has been impeded by Si rapid capacity fade, poor rate capability, and low coulombic efficiency (CE). Si exhibits volume changes of ~300% upon lithium alloying and de-alloying, leading to material degradation and presenting a major problem for electrochemical performance. This high volume expansion and contraction is too large to be controlled by currently developed coating technologies.

As the electrode matrix fractures, the continuous exposure of fresh nano-Si surface to the liquid electrolyte causes parasitic formation of a solid-electrolyte interphase (SEI) leading to irreversible charge loss and low CE. The volume fluctuation, also causes a damage of the electrical contact between Si and the current collector.

One of the main front-end strategies established for the realization of such an approach is coating nano-Si particles with flexible materials in order to attempt to accommodate the volumetric changes of the particles. In order to address the challenge of Si dramatic volumetric change, we carried out a surface modification, in which Molecular layer deposition (MLD) was utilized to grow a mechanically robust, flexible coating, producing high-capacity Si nanocomposite anodes.

MLD is a thin film deposition technique where hybrid organic–inorganic films are grown by exposing the substrate to subsequent, self-limiting, metal–organic precursor gases. The processes used here is based on TiCl4 as the titanium precursor. TiCl4 is a relatively small molecule, and when combined with the larger organic reactants it leads to high growth rates which are mainly limited by steric hindrance caused by the organic reactants.

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. The combination of both the carbon related groups and the Ti absorptions provides evidence for the successful deposition of a hybrid organic–inorganic film containing titanium.

The electrochemical performance of the LixSi coated with titanicone used as an anode was investigated by Cyclic Voltammetry (CV). Typical voltammograms of the half cell in the first scan show the formation of the SEI. In the subsequent scans these peaks disappears.

We successfully show the deposition of titanicone on LixSi electrode and an initial electrochemical assessment of this electrode.

Figure 1