[Paper Review] Flexo-diffusion effect: the strong influence on lithium diffusion induced by strain gradient

Xu Gao, Feng Hao+4 more|arXiv (Cornell University)|Feb 13, 2020

Advancements in Battery Materials4 citations

TL;DR

This study introduces the flexo-diffusion effect, demonstrating that strain gradients in bilayer graphene drastically reduce lithium diffusion barriers—lowering them significantly under positive strain gradients and increasing them under negative gradients—resulting in orders-of-magnitude changes in lithium diffusion coefficients at 300 K, with implications for enhancing ionic conductivity and rate performance in lithium-ion batteries through strain gradient engineering.

ABSTRACT

Lithium ion batteries (LIBs) work under sophisticated external force field and its electrochemical properties could be modulated by strain. Owing to the electro-mechanical coupling, the change of micro-local-structures can greatly affect lithium (Li) diffusion rate in solid state electrolytes and electrode materials of LIBs. In this study, we find that strain gradient in bilayer graphene (BLG) significantly affects Li diffusion barrier, which is termed as the flexo-diffusion effect, through first-principles calculations. The Li diffusion barrier substantially decreases/increases under the positive/negative strain gradient, leading to the change of Li diffusion coefficient in several orders of magnitude at 300 K. Interestingly, the regulation effect of strain gradient is much more significant than that of uniform strain field, which can have a remarkable effect on the rate performance of batteries, with a considerable increase in the ionic conductivity and a slight change of the original material structure. Moreover, our ab initio molecular dynamics simulations (AIMD) show that the asymmetric distorted lattice structure provides a driving force for Li diffusion, resulting in oriented diffusion along the positive strain gradient direction. These findings could extend present LIBs technologies by introducing the novel strain gradient engineering.

Motivation & Objective

• To investigate the influence of strain gradients on lithium diffusion in solid-state electrolytes and electrode materials.
• To explore how electro-mechanical coupling in bilayer graphene alters micro-local structures and affects lithium transport.
• To compare the impact of strain gradients with uniform strain fields on diffusion kinetics.
• To understand the role of asymmetric lattice distortion in directing lithium ion diffusion.
• To evaluate the potential of strain gradient engineering for improving ionic conductivity and rate performance in lithium-ion batteries.

Proposed method

• Employed first-principles calculations to compute lithium diffusion barriers in bilayer graphene under varying strain gradients.
• Applied ab initio molecular dynamics (AIMD) simulations to analyze dynamic lithium diffusion behavior and lattice distortions.
• Quantified changes in lithium diffusion coefficients using the Arrhenius equation based on calculated activation barriers.
• Systematically varied the strain gradient magnitude and sign to assess its influence on energy barriers and diffusion directionality.
• Analyzed the structural response of bilayer graphene to strain gradients, focusing on asymmetric lattice distortions.
• Compared results under strain gradient conditions with those under uniform strain fields to highlight relative effects.

Experimental results

Research questions

• RQ1 How does a strain gradient in bilayer graphene affect the activation energy barrier for lithium diffusion?
• RQ2 What is the magnitude of change in lithium diffusion coefficient induced by strain gradients at room temperature?
• RQ3 How does the strain gradient influence the directionality and anisotropy of lithium diffusion?
• RQ4 How does the flexo-diffusion effect compare quantitatively to the influence of uniform strain fields?
• RQ5 What is the underlying mechanism linking asymmetric lattice distortion to enhanced directional lithium diffusion?

Key findings

• The lithium diffusion barrier in bilayer graphene decreases substantially under positive strain gradient, with the effect being several orders of magnitude more significant than under uniform strain.
• Under negative strain gradient, the lithium diffusion barrier increases significantly, indicating tunable diffusion kinetics via strain gradient control.
• At 300 K, the lithium diffusion coefficient varies by several orders of magnitude due to strain gradient effects, demonstrating strong tunability of ionic conductivity.
• Ab initio molecular dynamics simulations confirm that asymmetric lattice distortion induced by strain gradient provides a driving force for directional diffusion along the positive strain gradient direction.
• The flexo-diffusion effect is markedly stronger than the influence of uniform strain fields, offering a more effective pathway for modulating ionic transport.
• The original material structure remains only slightly altered, indicating that strain gradient engineering can enhance performance without compromising structural integrity.