Amorphous Columnar Silicon Anodes with Excellent Cycling Stability for Advanced High Voltage Lithium Ion Full Cells: Dominant Factors Governing Cycling Performance

An excellent cycling stability of monolithic amorphous columnar silicon electrodes with the loqading of 1.3 mg/cm² in fluoroethylene carbonate (FEC)-based electrolyte solutions was demonstrated. The effect of the electrolyte solution composition and the cycling protocol on the long-term cycling performance and surface chemistry of amorphous silicon electrodes was investigated using electrochemical tools, XPS, SEM and EDS. It relates to the ability of FEC to form polyenes, as well as to a high rate of HF formation in its water contaminated LiPF₆ solutions, measured by ¹⁹F NMR spectroscopy. We found that excellent passivation in FEC solutions related to a low content of oxygen containing moieties species (e.g.  organic and inorganic carbonates, alkoxides, oxygen-containing organic polymers) in surface films.

 Galvanostatic tests and spectroscopic analyses revealed also a strong dependence of the composition and properties of the surface films on amorphous Si electrodes on the cycling procedure. Charge limited galvanostatic cycling procedure including repeated deep discharge of Si electrodes down to 10 mV vs. Li/Li⁺in every cycle from the very beginning of the electrodes life, ensures the formation of thin and highly passivating surface films on their surface and good cycling performance. To the contrary, when a limited portion of the capacity of Si electrodes during cycling relates to the higher potential range (discharge limited protocol), surface films enriched with oxygen containing species are formed. Their passivating properties are less effective compared to the other composition discussed above (polyenes + LiF).

Cycling stability of Si electrodes in LiPF₆solutions decreases in the following order FEC/DMC > 3,4-trans-difluoroethylene carbonate (DFEC)/DMC > ethylene carbonate (EC)/DMC > propylene carbonate (PC).

Amorphous columnar silicon electrodes were combined with high voltage LiNi_0.5Mn_1.5O₄ spinel and LiCoPO₄/C composite cathodes and high specific capacity xLi₂MnO₃^.(1-x)LiNi_yMn_zCo_1-y-zO₂ cathodes. These batteries demonstrated excellent performance in term of cycle life, charge-discharge efficiency and good rate capability.