Reports: ND1050032-ND10: Neutron Scattering Study of Cathode Materials for Li-Ion Batteries

Howard Wang, PhD , State University of New York at Binghamton

Nanoscale morphology has become an increasingly important topic in battery research since much of the transport behavior and structural integrity issues rely on details near surfaces and at nanoscales.  Nanoscale engineering is also considered a key element in addressing the issue of the low power density of batteries as large surface areas and small diffusion lengths assure fast charge/discharge kinetics.  To avoid compromising the system-level energy and powder density, nanoscale materials need to be densely packed into bulk composites when included as active components in batteries.  The structure variation of such materials is best monitored using small angle scattering, SANS.  In particular, SANS measures two aspects, direct structure measurement such as swelling and recovery of nanoscale grain in a bulk assembly, and the contrast changes due to the lithiation-induced scattering length density variation.

SANS has been widely used to measure nanoscale structures in bulk phases.  Time-resolved in situ SANS, TR-SANS has found relatively limited applications in battery research because of the small neturon fluence rate and low scattering.  In recent years, the increase in flux at the reactor-based NIST instruments and deployment of the spallation neutron source at ORNL make it possible to study nanoscale phenomena in LIB in real time.  To explore the applicability of TR-SANS to studying electrode materials, measurements were performed on a model system with the NG-7 30 m SANS instrument at the NIST Center for Neutron Research.

The battery assembly consists of graphite composite vs. Li electrodes for studying lithiation and delithiation processes in the electrode.  A neutron beam passes through a stack of two 1 mm thick quartz plate windows, two 10 mm thick Ni foils as current collectors, 300 mm thick Li anode, 1 mol/L LiPF6 in EC:DMC (1:1 by volume) solution as electrolyte, 25 mm polypropylene separator, and 15 mm graphite composite electrode in the normal direction, with scattering occurring mostly from the graphite electrode, which is composed of graphite particles of order 10 µm, carbon black, and polyvinylidene fluoride (PVDF).  SANS spectra were collected in real time over 10 lithiation/delithiation cycles. The variation of the scattering intensity upon charge/discharge is small compared to the overall intensity.  A dominant scattering feature in the Q-range is the ~Q-4 power law behavior, which is characteristic Porod scattering law due mostly to interfacial contributions.

The charge/discharge current, potential, and electric charge displacement profiles recorded during SANS measurements indicate the typical performance of a graphite anode. The variation of integrated SANS intensity upon cyclic charge/discharge rises and falls in apparently full synchronization with charge states.  This is due to the contrast variation induced by lithiation/delithiation.  As Li intercalates graphite particles, the overall SLD decreases, reducing the contrast with the matrix, hence decreasing the scattering intensity, and vice versa with de-intercalation. However, as the charge transfer becomes shallower at higher cycling rates, the amplitude of the intensity variation becomes even larger. This is contradictory to the prediction that the contrast variation is the only source of the scattering intensity change.  The excess scattering could result from new surfaces created due to fracturing of graphite particles. 

This observation is particularly interesting because fresh surfaces from the fracture will be immediately passivated by new SEI layers, which consume Li in the battery, resulting in the reduction of the reversible energy storage capacity. Lithiation‑induced fracturing of graphite particles in the anode has been one cause of the irreversible capacity loss during LIB operation.  Quantitative in situ monitoring of the generation of new surface areas will assist the development of materials and process recipes for improved cycle life.  There are studies on this topic using ex situ Raman spectroscopy and electron microscopy.  In situ diagnosis by scanning probe acoustic emission measurement has also been developed.  Nevertherless, those techniques offer mostly indirect and qualitative measurements of fracture surfaces.

SANS can be a unique in situ technique for monitoring both the bulk and interfacial activities of individual grains in active components in batteries.  In our study, hydrogenated electrolyte solvent was used; there is little contrast between the electrolyte and the SEI layer formed from the decomposition of the electrolyte molecules because of their similar overall isotopic compositions. However, by selective labeling of molecules in the electrolyte, SANS could be used to determine the reactivity of different species from the resultant interfacial scattering signals upon battery operation.  Such capabilities for surface detection, together with its conventional strength for monitoring the size and shape of nanoscale structures in real time, in situ SANS could be a very powerful tool in understanding the operation of LIBs going through lithiation/delithiation cycles.  We will continue the investigation of electrode materials using TR-SANS.

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