Due to an impressive and continuous improvement in performance, the use of lithium ion batteries (LiBs) has greatly increased since this technology was first commercialized by Sony in 1990 and is quickly spreading to more demanding applications. Today, LiBs are not only used to power portable electronic devices such as smartphones or laptops but also larger systems such as electric vehicles. One way of contributing to the continuing success of LiBs in the automotive field is to increase their energy density. Silicon has received considerable attention in recent years due to its high theoretical capacity. However, because of (i) the severe volume expansion/shrinkage of this material during cycling, which leads to the pulverization of the electrode and to a rapid capacity fading, and (ii) the first cycle high irreversible capacity, the implementation of silicon-based negative electrodes in commercial LiBs remains challenging.

Two possible solutions have been approached to overcome these problems: (i) Reducing the size of the silicon particles, and (ii) using dual-phase (a dispersed active phase in an inactive host matrix) composite electrodes. Pulverization is alleviated by reducing silicon particles to nanosized particles or by using a homogenous inactive host matrix which acts as a buffer against volumetric changes. However, because of poor electrode structural stability during cycling, the use of nanoparticles only cannot reduce the capacity degradation and hence, be a practical solution. Even though the total capacity of silicon-based composite electrodes cannot be as high as that of pure silicon electrodes, composite electrodes allow a low first cycle irreversible capacity and good capacity retention.

We have undertaken the study of the aging mechanisms in composite Si based electrodes, by means of small angle X-ray and neutron scattering (SAXS and SANS). These characterization techniques give access to the electrode nanostructure and its evolution upon cycling. Ex-situ measurements provided useful information on the evolution of the morphology of small domains. Indeed, initial results of SANS measurements performed at the Laue Langevin Institute (ILL, Grenoble), presented in Figures a and b, show the existence of two intensity regions at high and low Q values. Those respectively give information on small and large domains within the electrode. Interestingly, upon lithiating the electrode, the intensity in the intermediate and high-Q regions increases, while the low-Q region remains unchanged (Figure b). Moreover, after 100 cycles, the pristine electrode spectrum is recovered whereas it is not the case after 300 cycles (Figure a) thus showing signs of aging from the silicon-based composite electrode.

In order to probe in real-time the effects of lithiation/delithiation on the composite anode nanostructure and, avoid any risk of altering the sample in between the opening of the cell and sample preparation, in-situ and operando measurements were conducted. For this purpose new electrochemical cells (Figure c) were designed using respectively beryllium or titanium windows. Particular care has been taken to ensure that the cycling conditions of the electrochemical system are as close as possible to those of a coin cell, with the use of a separator and applied pressure on the electrodes. In this talk we will present the first results on the evolution of the nanostructure of the composite electrodes.