Thickness Evaluation of Solid Electrolyte Interphase Layer and Lithium Diffusion Phenomenon on Negative Electrodes for Li-Ion Batteries By Neutron Scattering Study
Authors: Chang Chia Chin , Wu Chun-Ming , Ho Guan-Ting , Chen Hung-Yuan , Pan Ping-I , Fung Kuan-Zong 
Published: February 2020

Investigation of solid electrolyte interphase (SEI) film thickness on a mesophase graphite electrode in the lithium-ion battery is successfully demonstrated by the ex-situ small-angle neutron scattering (SANS) technique for the MGP and the FMGP anodes, after the first charge state and analyzing it precisely by the Guinier–Porod model. SANS data revealed a stable, maximum (150 nm) bi-layer SEI formed on MGP anode (in EC/DMC) at a capacity of 50 mAhg-1 and sluggish above 100 mAhg-1. The SEI formed on the FMGP with and without 3% FEC (in EC/DMC) shows the thickness of 220 nm and 245 nm, respectively, whereas the solvent contains only EC/DEC produce thin SEI of 140 nm. Our primary results make evident that SANS could be employed to better understand the complex microstructure SEI formation and its accurate thickness, on a mesophase graphite anode.

Lithium diffusion phenomenon in negative electrodes for Li-ion batteries were studied by using a cold triple-axis spectrometer in-operando neutron scattering. The study on the lithium diffusion mechanism of Li+ diffusion in LiC6 , LiC12 of the MGP and the electrolyte additives (as venylene carbonate, VC) at low temperature (253K or 268K) were very importance for the lithium plating and long cycle. Therefore, SIKA is the instrument good to conduct an experiment at low temperature to understand the Li+ diffusion effect in the cell of NMC/graphite LIB at different temperature in order to give an ideal to improve the battery design and to resolve the lithium plating issue in the low temperature which regarding to the safety. From the qualitative analysis of the phase transition as a function of time and temperature, considering the (00l) peak intensities of graphite to extract the diffraction peaks corresponds to the relative fractions of LiC6 and LiC12 which in turn were related to the amount of Li diffusion through the SEI and the lithium intercalation takes place in the anode.

From the spectra, we obtained the intensity of LiC6 diffraction plane is increased with increasing charging capacity yielded with the value of 2064 and 1757 mAh at 298 and 268 K, respectively. While, the intensity of graphite phase is firstly shift from 40º to 39.5º then LiC12 and LiC6 phases were observed with obvious increased intensity. The integrated intensity of LiC6, and LiC12 reflections as a function of time varies with different temperature. At initial charging condition in Fig. 1, the intensity of pure graphite plane centred at 40º is large and also observed the generation of small amount of LiCx (37.1º, LiC6 and 38.9º, LiC12) compounds.

This process is observed to be reverse in the case of discharging the cell at 298 and 268 K. At 298 K, decrease in the voltage (3.99 to 2.80 V), the amount of LiC6 is slowly disappears and the amount of LiC12 is maximum, finally observed the graphitic peak with small low angle shift, which confirms the presence of small Li+ into the graphite layer/surface, in the discharge condition. In the case of discharging at 268 K, the LiC6 plane is quickly fading from 3.80 V and disappear. The ratio of capacity of the cell was 86% for 268 K compare with 298 K, which was not consistence with the quantity of LiC6 plane. We also establish the neutron diffraction analysis for the samples prepared with different additives and the results were discussed in detail. In the future, we will also pay attention to the formation of lithium plating which is highly related to the battery safety.