Silicon Based Anodes and New Electrolytes for Next Generation Lithium-Ion Batteries and Lithium-Sulfur Batteries
Authors: Baasner Anne , Piwko Markus , Doerfler Susanne , Althues Holger , Kaskel Stefan 
Published: February 2020

State of the art lithium-ion batteries containing a transition metal cathode and a graphite anode have nearly reached their energy density limit. Due to its high theoretical gravimetric capacity (3590 mAh g-1 Li15Si4) and its low working potential, silicon is an attractive candidate to at least partially substitute the graphite anode [1]. Lithium-sulfur batteries are a promising battery system because of the high capacitive sulfur cathode (1672 mAh g-1) and low material costs. But current cells mainly suffer from the instable lithium anode. Lithiated silicon could be a more stable alternative preventing dendrite formation and electrolyte decomposition for that next generation battery cell type [2].

During the lithiation process of silicon a large undesirable volume expansion occurs. This volume change leads to pulverisation of the silicon and a steady reformation of the solid electrolyte interface (SEI). The electrolyte is consumed and the capacity declines rapidly. This is why silicon anodes with high quantities of silicon are hindered so far. To overcome these obstacles nanostructured silicon carbon composite anodes are very promising. The volume change is potentially compensated by the free volume between silicon core and carbon shell and a stable SEI is ensured at the surface of the carbon shell preventing electrolyte consumption during cycling. These void structures are mainly generated using silica templates which deliver a precise control of the void structure, but need to be removed by toxic hydrofluoric acid and laborious washing steps. In this work an easily scalable process without etching treatment is presented. The void structure results from the removal of a sacrificial template layer on the surface of commercially available silicon particles during the carbonization of the silicon carbon composite within a concerted process.

For general electrochemical characterization the silicon carbon composite electrodes are galvanostatic cycled vs. metallic lithium in half cells with different electrolyte systems on coin cell as well as on pouch cell level. The silicon carbon composite electrodes reveal a much higher capacity and cycle stability compared to bare silicon nanoparticles and Si-C composites without void structure. High area capacities above 2.5 mAh cm-2 were reached.

In sulfur-lithiated-silicon (SLS) full cells the cycle stability with low lithium excess and the enhanced volumetric energy densities using lithiated silicon anodes was analysed. The electrochemical performance of SLS cells with silicon carbon void structures are compared to that with hierarchical columnar silicon anodes, recently introduced by our group [3]. The feasibility of both SLS cell concepts was successfully demonstrated in coin and in pouch cells. A new electrolyte composition is introduced to enhance the cycle life by stabilization of the SEI and prevention of the electrolyte depletion [4]. Finally, prospects, limits and suggestion for further research directions for SLS cells are envisioned, based on the estimation of the expectable energy density.

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