Ultra High Resolution Soft X-Ray Microscopy Studies of Intercalation Mechanism in LiFePO4 Cathodes

Abstract

With the rising demand in renewable energy storage devices for portable electronic applications and vehicle electrification, the development of novel techniques to study and characterize materials’ functions and their impact on performance of energy storage systems has become crucial. Olivine structured LiFePO4, currently cathode of choice in state-of-the art commercial Li ion batteries, has been specifically and extensively studied to elucidate its charge/discharge mechanism, kinetics and sources of capacity fade through various characterization methods in recent years. While these studies have resulted in significant improvements in the rate capabilities of this compound, the exact lithium intercalation mechanism remains controversial. Various studies on single-particle, many-particle and bulk LiFePO4 through a multitude of characterization techniques, have mainly pointed to two different possible intercalation pathways: non-equilibrium solid solution or a two-phase separation. 1 To follow phase, structural and chemical transformation in these cathodes, a technique with an increased resolution conducive to the study of nano-particles is required.In this work, the intercalation pathway and phase transition mechanism in single particle nano-LiFePO4 is studied utilizing an ultra high-resolution soft X-ray ptychographic microscope. The high penetration depth and short exposure times of soft X-rays enables scanning transmission X-ray microscopes (STXM) to probe the structural and chemical states of materials in nano scale with high resolution. X-ray ptychography is a coherent diffraction imaging technique that extracts structural information from inversion of diffraction data, hence, offering a spatial resolution that is not limited by the X-ray spot size.2 Measurements on chemically delithiated single nanoplates were conducted at Fe L3 edge in both normal scanning X-ray microscopy mode and in ptychography mode on beamlines 5.3.2.1. and 11.0.2 of the Advanced Light Source. In this study, a spatial resolution better than 10 nm was achieved on Beamline 11.0.2, utilizing a 60 nm outer zone-width zone plate and a CCD detector for diffraction measurements. Particles were chemically mapped through linear combination fitting of partially delithiated particles with LiFePO4 (LFP) and FePO4 (FP) as end member standards. While normal STXM measurements on these particles appeared to point to the existence of solid solution reaction, increased resolution in ptychography mode clearly showed the existence of a two-phase equilibrium within a single particle.Increased spatial resolution offered by ptychography also allows for studying intercalation and phase transformation inhomogeneities in bulk electrodes in operando conditions. In-situ experiments will be conducted on bulk LiFePO4 electrodes to examine the intercalation mechanism, lithium/electron mobility and phase boundary growth in the bulk as charge/discharge cycles progress. With such high spatial resolution ptychography can be the ideal technique for characterizing mesoscale chemical and structural transformations in electrochemical systems. Figure 1. STXM (right) and Ptychography STXM (left) images of partially delithiathed particle outlined in yellow; ptychography mode allows for chemical mapping of the particle at higher spatial resolution than STXM. Linear combination fitting of the ptychgraphic image points to the clear existence of a two phase equilibrium of lithium-rich and lithium poor regions, with a 70.9:29.1 LFP:FP ratio in the red colored region and 2.5:97.5 LFP:FP in the blue colored region.