[Paper Review] Tracking the diffusion-controlled lithiation reaction of LiMn2O4 by in-situ TEM

Torben Erichsen, Björn Pfeiffer+2 more|arXiv (Cornell University)|Feb 12, 2020

Advancements in Battery Materials46 references4 citations

TL;DR

This study uses in-situ transmission electron microscopy (TEM) to track the diffusion-controlled lithiation of LiMn2O4, revealing a sharp moving interface between cubic (x=1) and tetragonal (x=2) phases during electrochemical discharge. The tetragonal phase forms as a complex nanotwinned microstructure that relieves lattice stress from volume expansion, which stabilizes the phase and contributes to irreversible capacity loss in lithium-ion batteries.

ABSTRACT

Spinel lithium manganese oxide (LixMn2O4) is used as an active material in battery cathodes. It is a relatively inexpensive and environmentally friendly material, but suffers from capacity fade during use. The capacity losses are generally attributed to the formation of the tetragonal phase (x > 1) due to overpotentials at the surfaces of the micron-sized particles that are used in commercial electrodes. In this study, we investigate the mechanisms of tetragonal phase formation by performing electrochemical lithiation (discharging) in-situ in the transmission electron microscope (TEM) utilizing diffraction and high resolution as well as spectroscopy. We observe a sharp interface between the cubic spinel (x = 1) and the tetragonal phase (x = 2), that moves under lithium diffusion-control. The tetragonal phase forms as a complex nanotwinned microstructure, presumably to relieve the stresses due to expansion during lithiation. We propose that the twinned microstructure stabilizes the tetragonal phase, adding to capacity loss upon deep discharge.

Motivation & Objective

• To understand the mechanisms behind capacity fade in spinel LiMn2O4 cathodes during deep discharge.
• To investigate the phase transformation dynamics from cubic (LiMn2O4, x=1) to tetragonal (Li2Mn2O4, x=2) during lithiation.
• To examine the microstructural evolution and stress relief mechanisms during the phase transition.
• To determine the role of twinning in stabilizing the tetragonal phase and contributing to irreversible capacity loss.

Proposed method

• In-situ electrochemical lithiation was performed inside a transmission electron microscope (TEM) using a solid-state electrolyte and a nanoscale battery configuration.
• High-resolution TEM imaging was used to visualize the atomic-scale structure and phase boundaries during lithiation.
• Electron diffraction patterns were collected in real time to identify phase transitions and track the evolution of crystallographic phases.
• Energy-dispersive X-ray spectroscopy (EDS) was employed to confirm lithium incorporation and chemical changes.
• The motion of the phase boundary was analyzed to determine the rate-limiting step of the reaction.
• A model of diffusion-controlled phase growth was developed based on observed interface kinetics and microstructural features.

Experimental results

Research questions

• RQ1 How does the phase boundary between cubic and tetragonal LiMn2O4 evolve during in-situ lithiation?
• RQ2 What microstructural features accompany the formation of the tetragonal phase, and how do they relieve mechanical stress?
• RQ3 Is the phase transformation kinetically limited by lithium diffusion, and what evidence supports this?
• RQ4 To what extent does nanotwinning stabilize the tetragonal phase and contribute to irreversible capacity loss?
• RQ5 How does the morphology of the tetragonal phase affect the electrochemical performance of LiMn2O4?

Key findings

• A sharp, well-defined interface was observed between the cubic (x=1) and tetragonal (x=2) phases during lithiation, indicating a distinct phase transformation.
• The phase boundary moved steadily under lithium diffusion control, with no evidence of nucleation-limited kinetics.
• The tetragonal phase formed as a complex nanotwinned microstructure, which likely relieves mechanical stress from volume expansion during lithiation.
• The twinned microstructure is proposed to stabilize the tetragonal phase, reducing its tendency to revert upon delithiation and contributing to irreversible capacity loss.
• The observed microstructure suggests that stress accommodation via twinning plays a key role in the stability and persistence of the tetragonal phase.
• The study provides direct experimental evidence that the tetragonal phase formation is diffusion-controlled, resolving long-standing debates about the rate-limiting step.