Relationship between mechanical strain and chemical composition in LiFePO 4 via 4D-scanning transmission electron microscopy and scanning transmission X-ray microscopy

Abstract

Phase separation is an important factor for many Li-ion battery materials as it significantly impacts the capacity and cycle life of a battery. Forming Li-rich and Li-poor domains, phase separation induces changes to the composition and microstructure of these materials [1]. Scanning transmission X-ray microscopy (STXM) and X-ray ptychography are used to identify chemical composition changes in the Li-ion battery systems [2]. However, to understand the mechanisms of phase transformation, which effects the capacity loss and Li-insertion/desertion kinetics of the Li-ion battery systems, the relationship between Li-distribution and mechanical strain at phase separation interfaces must be delineated. Four-dimensional scanning transmission electron microscopy (4D-STEM) uses a focused electron beam that is rastered across an electron transparent sample while a diffraction pattern is acquired at each scan position. Individual convergent beam electron diffraction (CBED) patterns provide comprehensive structural information, i.e., orientation, localized lattice strain, and other material properties [3]. These datasets, in combination with computational frameworks, enable high throughput analysis of localized lattice strain and ordering across micro-length scales. Over thirty 4D-STEM datasets of LiFePO4 particles at varying stages of delithiation (LiFePO4, 50% delithiated LiFePO4, and fully delithiated FePO4) were acquired using a FEI Titan-class transmission electron microscope at an accelerating voltage of 300 kV. Maps of the infinitesimal strain matrix were produced using py4DSTEM, an open-source python based data analysis package, and contain ~5,000 CBED patterns each (Fig. 1) [4]. These maps show a clear variation in strain behavior as LiFePO4 transforms via delithiation to FePO4. Defined regions of compressive or tensile strain for the 50% delithiated particles is also observed. Segmentation into two distinct regions for the 50% delithiated LiFePO4 strain map is expounded upon with position-averaged probability distributions of lattice vector lengths, a and c, in which bimodal distribution is apparent in the distribution of a for the 50% delithiated particle (Fig. 1g and 1h). Using strain and lattice parameter data acquired by 4D-STEM (~2 nm resolution) with Li-distribution data acquired by STXM and X-ray ptychography (~10 nm resolution), a phase separation interface can be isolated and the chemo-mechanical relationship between strain and Li-distribution can be investigated for LiFePO4 particles at varying stages of delithiation. 2068