Invited: Critical Role of Si Nanoparticles Surface on Lithium Cell Electrochemical Performance

The surface of Si particles is strongly involved in different phenomena of crucial importance for the electrochemical performance of lithium batteries with Si-based composite electrodes. Indeed, Si electrodes suffer from poor cyclability due to the large volumetric expansion of Si particles upon cycling. The successive swelling and shrinkage upon alloying/dealloying of the stack of Si particles within the confined space of the electrochemical cell leads to major geometrical change of granular texture with a vast redistribution of inter-particle contacts, the formation of cracks within the composite electrode and loss of electrical contacts at the current collector interface. The repetition of all these processes with frictional sliding of the particles leads to a damaging of the SEI layer that exposes again the Si particles surface to the liquid electrolyte. It results in an irreversible capacity loss at each cycle by the reduction at low potential of some liquid electrolyte on the exposed surface [1].

            Silicon is always recovered by a native silicon oxide layer. An electrochemical reduction of the SiO₂ surface layer occurs at the first cycle with the formation of lithium silicate, Li₄SiO₄, and Li₂O as degradation products as proven by XPS [2]. Moreover, the silicon oxide layer is composed of hydroxylated silanol (SiOH) groups which likely dramatically increases the reactivity compared to the dehydroxylated siloxane phase (Si-O-Si) [3]. As a consequence various efforts have been made to reduce the thickness of SiO₂ layer on silicon nanoparticles to decrease the first irreversible capacity and to enhance electrochemical performance.

            Although the surface of Si particles is playing a major role in the electrochemical performance, it has rarely been characterized in depth. A combination of techniques is here used to finely describe the surface of Si nanoparticles: Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy combined with TGA, Raman spectroscopy, solid-state high-resolution Nuclear Magnetic Resonance (MAS NMR), TEM coupled with Electron Energy Loss Spectroscopy (EELS), X-Rays Photoelectron Spectroscopy (XPS), and finally Broad Band Dielectric Spectroscopy (BDS).

            With respect to previous works and common belief, we demonstrate, on the electrochemical performance, a favorable effect of a particular thin layer silicon oxide with a well-defined SiO₂ composition at its extreme surface.

References

[1] B.P.N. Nguyen, J. Gaubicher, B. Lestriez, “Analogy between electrochemical behaviour of thick silicon granular electrodes for lithium batteries and fine soils micromechanics”, Electrochimica Acta, 2014, 120, 319.

[2] B. Philippe, R. Dedryvère, J. Allouche, F. Lindgren, M. Gorgoi, H. Rensmo, D. Gonbeau, K. Edstrom, “Nanosilicon Electrodes for Lithium-Ion Batteries: Interfacial Mechanisms Studied by Hard and Soft X-Ray Photoelectron Spectroscopy”, Chem. Mater. 2012, 24, 1107.

[3] S.F. Lux, I.T. Lucas, E. Pollak, S. Passerini, M. Winter, R. Kostecki, “The mechanism of HF formation in LiPF₆ based organic carbonate electrolytes”, Electrochem. Commun. 2012 14, 47.

 Figure 1. Discharge capacity for Si_A (commercial powder), Si_U (home-made powder) and Si_Ud (home-made powder with designed durface) in LP30+FEC-VC electrolyte vs Li metal electrode in Swagelock cell. Composite electrode with CMC binder and prepared at pH=3.