Cryo STEM EDS Tomography Probing of Solid Electrolyte Interphase in Rechargeable Batteries

Solid-electrolyte interphase (SEI) refers to a thin layer formed between electrode and electrolyte in response to electrochemical process, which plays a vital role in electrochemical reactions and battery stability. A class of electrode, often high capacity one, appears to inevitably subject to large volume swing upon ion insertion and extraction, raising a critical question as how the SEI interactively responds to and evolves with the volume change of electrode and consequently controls the stability of the battery. Silicon is typical high-capacity anode for rechargeable lithium-ion battery. It is generally conjectured that capacity fading of Si roots in the intrinsic large volume changes and the building up of SEI layer with repeated cycling. Typically, the volume change will lead to cracking of silicon, crumpling, regeneration and accumulative growth of SEI and attendant loss of electrical contact. This picture appears to be self-consistent for explaining the capacity fading of Si with size of larger than a critical dimension of typically 150 nm, but fails in reconciliation with the capacity fading of nanosized Si wherein the material cracking issue is effectively attenuated by the reduced particle size. Electrode-level failure aside, capacity fading of nanosized Si has been related hypothetically to the thickening of the SEI layer on the material surface, corrosion of Si and electrochemical-mechanical degradation. However, these mechanisms remain far-fetched and call for an explicit picture on how the SEI layer responds to and correlates with the evolution of Si during cycling. Current studies on the SEI on Si are largely based on the two-dimensional projection imaging and energy dispersed X-ray spectroscopy (EDS) mapping of elements (Fig. 1), which is hard to correlate the three-dimensional (3D) structure with the electrochemical performance of the nanosized Si materials. Electron tomography is a technique that retrieves 3D structural information from a tilt series of 2D projections and can be used for nanoscale structures. However, electron tomography studies on the cycled electrode material have been challenging due to the severe damage from high electron dose necessary for electron tomography imaging. Inspired by structural biology and the imaging on Li whiskers, we utilized cryogenic imaging to minimize sample damage from air and the imaging electron beam, in combination with an advanced reconstruction algorithm, we preserve the lightweight materials in the SEI for and during electron tomography without severely damaging the sample [1]. We use a Si nanowire-based coin cell for the current study. The coin cells were disassembled directly inside an Ar-filled glove box. The working electrode was rinsed 3 times by using dimethyl carbonate (DMC) to remove residual electrolyte and then dried in vacuum for ∼15 mins. Then, a lacey carbon Cu mesh TEM grid was placed on top of the working electrode and gently pressed on the electrode to pick up the nanowires. Since our anodes are free of binder and conductive substances, materials that were picked up from it should only contain the Si nanowire and the SEI. The TEM grid was cooled sufficiently in liquid nitrogen under Ar gas protection, loaded onto a pre-cooled Cryo-TEM holder (Gatan Elsa) which thereafter was quickly inserted into the TEM chamber. The total exposure time of the sample to the air is ∼10 seconds at ∼ -170 oC. The reaction between the sample and air is thereby minimized, reserving the material at the “as-cycled” state for TEM characterization. By combining cryogenic scanning transmission electron microscopy (Cryo-STEM) tomography imaging and EDS tomography, we show, explicitly in 3D (Fig. 1), the correlated structural and chemical evolution of the SEI and Si. The structure starts with integral pure Si core with a SEI shell. As the electrolyte starts to permeant along the channels of the damaged Si core, SEI gradually grows inward. With increasing structural deterioration of the Si core, SEI gradually engulfs the segmented Si domain within the Si core, disrupting electron conduction pathways, depriving the engulfed Si domains from electrochemical reactions and hence rendering capacity loss. Oxidation of Si at the interface with SEI also contributes to the capacity loss. The mechanistic understanding points to restraining electrolyte permeation as a key to enable stable cycling and high-capacity retention of large-swelling anodes [2]. Solid electrolyte interphase on silicon as revealed by cryo-STEM EDS. The left panel is the 2D EDS maps and the right panel is the 3D EDS tomography to show the gradual mixing of solid electrolyte interphase layer with silicon upon the battery cycling.