International audienceUnderstanding the failure mechanism of silicon based negative electrodes for lithium ion batteries is essential for solving the problem of low coulombic efficiency and capacity fading on cycling and to further implement this new very energetic material in commercial cells. To reach this goal, several techniques are used here: post mortem7Li MAS NMR and SEM, electrochemical impedance spectroscopy (EIS) and three-electrode-based electrochemical analysis. 7Li MAS NMR analyses of the charged batteries demonstrate that the major part of the lithium lost during the charge of batteries is not trapped in LixSi alloys but instead at the surface of the Si particles, likely as a degradation product of the liquid electrolyte. Observed by SEM, a dead electrode has a thick "SEI" layer at its surface. EIS and incremental capacity analyses demonstrate that the growth of this layer is responsible for the failure of the electrode through a continuous decrease of its active surface area associated with a rise of the electrode polarization. It is demonstrated that the main cause of capacity fade of Si-based negative electrodes is the liquid electrolyte degradation in the case of nano Si-particles formulated with the carboxymethyl cellulose (CMC) binder. This degradation results in the formation of a blocking layer on the active mass, which further inhibits lithium diffusion through the composite electrode

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Understanding the aging mechanism of silicon-based negative electrodes for lithium-ion batteries upon cycling is essential to solve the problem of low coulombic efficiency and capacity fading and further to implement this new high-capacity material in commercial cells. Nevertheless, such studies have so far focused on half cells in which silicon is cycled versus an infinite reservoir of lithium. In the present work, the aging mechanism of silicon-based electrodes is studied upon cycling in a full Li-ion cell configuration with LiCoO2 as the positive electrode. Postmortem analyses of both electrodes clearly indicate that neither one of them contains lithium and that no discernible degradation results from the cycling. The aging mechanism can be explained by the reduction of solvent molecules. Electrons extracted from the positive electrode are responsible for an internal imbalance in the cell, which results in progressive slippage of the electrodes and reduces the compositional range of cyclable lithium ions for both electrodes.

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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 SiA (commercial powder), SiU (home-made powder) and SiUd (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.

Among candidates at the positive electrode of the next generation of Liion technology and even beyond post Li-ion technology as all-solid-state batteries, spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) is one of the favorites. Nevertheless, before its integration into commercial systems, challenges still remain to be tackled, especially the stabilization of interfaces with the electrolyte (liquid or solid) at high voltage. In this work, a simple, fast, and cheap process is used to prepare a homogeneous coating of Al 2 O 3 type to modify the surface of the spinel LNMO: the supercritical fluid chemical deposition (SFCD) route. This process is, to the best of our knowledge, used for the first time in the battery field. Significantly improved performance was demonstrated vs those of bare LNMO, especially at high rates and for highly loaded electrodes.