Accessing the Lithiation/Delithiation Mechanism of a Li-Ion Battery FeSi2/Si/Graphite Composite Negative Electrode and Differentiating the Si/Graphite Contributions By Simultaneous Operando Synchrotron SAXS/Waxs
Authors: Berhaut Christopher Logan , Zapata Dominguez Diana , Tardif Samuel , Pouget Stéphanie , Lyonnard Sandrine 
Published: February 2020

Combined with renewable energy harvesting technologies, such as solar panels or wind turbines, energy storage is the key to keeping up with the ever-increasing demand for electric energy and reducing the world dependency on fossil fuels. Over the last three decades lithium-ion batteries (LiBs), starting with the first commercialized cell by Sony in 1990 [1], have evolved from the most promising energy-storage technologies [2] to the most widely used nowadays. Not only can LiBs be found in portable devices such as laptops, they are also used in larger scale applications such as electric vehicles.
Since the first commercialization of LiBs, graphite (372 mAh.g-1) has been the usual negative electrode material. Nevertheless, the continuous demand, in the automotive industry in particular, for more performant, light, safe and cheap LiBs drives research towards the design and development of new electrode materials characterized in particular by high energy densities and a longer capacity retention during cycling. Among the candidates for high specific gravimetric capacity is silicon (3579 mAh.g-1) [3]. However, two major problems hamper the use of this material as a LiB electrode material. The first is its high volumetric change (297 % [4]) during lithiation, which causes particle cracking and pulverization during cycling which in turn leads to a quick fading of the cell capacity. The second is its high first cycle irreversible capacity. Reducing the particle size to the nanoscale and thus reducing the mechanical strain on the particle by attenuating its volumetric expansion/contraction helps alleviate pulverization. However, because of poor electrode structural stability during cycling this approach alone does not stop capacity degradation [5]. Using a dual-phase composite material that contains an inactive host matrix in which the active material is well-dispersed also alleviates pulverization. The FeSi2/Si/graphite anode material is particularly interesting due to its low-cost, high reversible capacity and good cycling stability [6]. This material is composed of a dual-phase FeSi2 (inactive)/amorphous Si (active) composite and graphite, a conductive matrix to accommodate the Si volumetric expansion, and is seen as well suited for the development of high capacity LiBs. Be that as it may, it is still unclear how this complex material functions and what makes it so performant.
With the aim of accessing the lithiation/delithiation mechanism of a FeSi2/Si/graphite negative LiB electrode, we performed simultaneous operando Small and Wide Angle X-Ray Scattering (SAXS and WAXS) measurements on a cycling NMC//FeSi2/Si+graphite pouch-cell filled with a 1 mol.L-1 LiPF6 in FEC/EMC (30/70 wt%) electrolyte at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Through WAXS we were able to investigate the local structure (10-1 nm) of the cell materials and in particular observe the lithiation/delithiation of the graphite phase by following its corresponding Bragg peak and those of its different lithiated phases (LiC30, LiC24, LiC18, LiC12 and LiC6). Simultaneously, we gained information on the active silicon state of lithiation/delithiation by observing the morphology changes occurring at the nano-scale (1 nm to 10 nm) during the volumetric expansion/contraction of the silicon through SAXS measurements. The combined information give us insight on the lithiation/delithiation mechanism of the negative electrode and enabled us to dissociate the capacity contribution of the silicon phase from that of the graphite phase as a function of potential, time and capacity. By cycling the cell at several current-rates and by repeating the same experiment on an aged cell cycled 300 times, we were also able to enquire into the impact of fast-charging and ageing on the lithiation/delithiation mechanism of the negative electrode. In this talk, we will present those results after proposing a lithiation/delithiation mechanism for the FeSi2/Si/graphite composite electrode.

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