A solid-electrolyte interphase (SEI) is known to play a significant role on safety and durability of lithium ion batteries (LIBs). Here, the chemical composition of SEI has been usually determined with an ex-situ X-ray photoelectron spectroscopic analysis technique. However, there are few reports on the structural properties of SEI, such as thickness and density, and their variation with cyclic charge and discharge reactions, because of the difficulty for observing SEI with an in-situ manner. This means that we will never reach deep understanding of “alive” SEI and its formation mechanism through the work on “dead” SEI with ex-situ analytical techniques.
Therefore, we have started to use neutron reflectivity (NR), as NR is a powerful technique for estimating thickness and density of an interface between solid and liquid. Furthermore, since the interfacial reaction is expected to occur within a few nanometer-thickness from the initial boundary between electrode and electrolyte, we naturally need a NR technique with more precise spatial resolution for such purpose than the past NRs.
Fortunately, Japan Proton Accelerator Research Complex (J-PARC) is one of the most intense spallation neutron source, leading to the highest depth resolution of NR in the world. Here, we report the first operando measurement of the SEI formation during the initial charge, i.e. lithiation reaction in the carbon electrode of LIB using a combination of in situ NR and electrochemical analyses [1].
Experimental
As an electrode for NR measurements, a carbon/titanium multi-layer film was deposited on a silicon substrate by a magnetron sputtering technique (Fig. 1). Here, the titanium layer is necessary to increase adhesive force between the carbon electrode and Si substrate. In a special electrochemical cell for in-situ NR measurements, the carbon/titanium film was a working electrode, a lithium foil was a counter electrode, 1 mol/L LiPF6 in a 1:1 (v/v) ratio of ethylene carbonate and diethyl carbonate was an electrolyte, and a microporous polyethylene membrane was a separator. Cyclic voltammetry (CV) was performed at a slow sweep rate of 0.2 mV/s between 3.3 and 0.05 V. In situ NR measurements were performed during the repeated charge and discharge, i.e. lithiation and delithiation cycles on the surface profile analysis reflectometer (SOFIA) at BL16 of Materials and Life Science Experimental Facility (MLF) in J-PARC (Proposal No.2015B0016). The NR spectrum was measured as a function of the momentum transfer, Qz = (4π/λ)sin θ, in the Qz range between 0.1 and about 0.45 nm-1. The obtained NR spectra were fitted with Motofit based on a Parratt formalism [2] [3].
Results and Discussion
After the CV measurement, the morphology of the carbon surface was clearly changed as expected. Fig. 2 shows a color map of the reflectivity profiles in the Qz and time plane for the 1st and 2nd charge and discharge cycles in order to demonstrate the overall data for the present NR measurements. NR is found to drastically change at potentials between 1.5 and 0.05 V corresponding to the lithiation reaction, and between 0.05 and 2.5 V corresponding to the delithiation reaction. The change in NR along the Qz axis naturally reflects the formation and variation of SEI. Fig. 3 shows the Scattering length density (SLD) as a function of depth and time. The data were obtained by fitting the NR spectra with a four-layer model. The change in thickness of carbon electrode indicates the volumetric change induced by the lithiation and delithiation reactions into and from carbon. The decrease in SLD of carbon also supports the lithiation reaction into carbon, because Li exhibits a negative coherent scattering length. More correctly, as potential decreases from 1.5 to 0.05 V, the width of the carbon film increases from 67 to 83 nm. On the contrary, the boundary between electrode and electrolyte clearly changes with potential, indicating the formation of SEI. That is, as potential decreases from 1.0 V, SLD increases from 2.4 to 3.2 (x 10-4) nm-2, while the thickness increases from 18 to 36 nm at potential = 0.05 V during the initial lithiation reaction. This clearly demonstrates the growth of SEI during the lithiation reaction.
Consequently, with NR measurements in the most intense neutron source, we have successfully observed the formation and growth of SEI at the boundary of the carbon electrode and liquid electrolyte during electrochemical charge and discharge reactions. Such measurements will provide an insight how to improve the performance of LIBs.
References
[1] H. Kawaura et al, ACS Appl. Mater. Interfaces, 8(2016) 9540.
[2] L. G. Parratt, Phys. Rev., 95(1954) 359.
[3] A. Nelson, J. Appl. Cryst., 39(2006) 273.