Shape Influence of Active Material Micro-Structure on Diffusion and Contact Stress in Lithium-Ion Batteries

Davide Clerici,Aurelio Somà,Francesco Mocera

doi:10.3390/en14010134

Abstract

Electrochemical-mechanical modelling is a key issue to estimate the damage of active material, as direct measurements cannot be performed due to the particles nanoscale. The aim of this paper is to overcome the common assumptions of spherical and standalone particle, proposing a general approach that considers a parametrized particle shape and studying its influence on the mechanical stresses which arise in active material particles during battery operation. The shape considered is a set of ellipsoids with variable aspect ratio (elongation), which aims to approximate real active material particles. Active material particle is divided in two domains: non-contact domain and contact domain, whether contact with neighbouring particles affects stress distribution or not. Non-contact areas are affected by diffusion stress, caused by lithium concentration gradient inside particles. Contact areas are affected simultaneously by diffusion stress and contact stress, caused by contact with neighbouring particles as a result of particle expansion due to lithium insertion. A finite element model is developed in Ansys™APDL to perform the multi-physics computation in non-spherical domain. The finite element model is validated in the spherical case by analytical models of diffusion and contact available for simple geometry. Then, the shape factor is derived to describe how particle shape affects mechanical stress in non-contact and contact domains.

Highlights

Lithium-ion batteries (LIB) play a strategical role in the actual green energy conversion, they are the most widespread storage system of electrical energy so far, with a huge field of application: from small electronics up to heavy-duty vehicles [1,2].Nowadays, the main challenge is to increase the amount of energy stored in the battery, working with simulations [3] on new materials which guarantee a greater nominal energy density and ensuring that the nominal properties, such as capacity, power and internal resistance, could be kept through the whole life cycle as well
Besides the electrochemical ageing mechanisms, the mechanical ones have a crucial role in battery damage [5], and they can be identified at the active material particles scale
The validation of FE multiphysics model with analytical models in spherical geometry is shown the results in contact and non-contact domain are presented along with the shape factor which highlights the effect of particle shape on mechanical stress

Summary

Introduction

Lithium-ion batteries (LIB) play a strategical role in the actual green energy conversion, they are the most widespread storage system of electrical energy so far, with a huge field of application: from small electronics up to heavy-duty vehicles [1,2].Nowadays, the main challenge is to increase the amount of energy stored in the battery, working with simulations [3] on new materials which guarantee a greater nominal energy density (amount of energy stored per unit of weight) and ensuring that the nominal properties, such as capacity, power and internal resistance, could be kept through the whole life cycle as well. Electrode manufacturing [6] consists in the mixing of active material powder (major content), binder, conductive components and additives to get the so-called wet slurry, which is deposited on current collector and dried. In this manner, particles of the active material are embedded in a solid porous matrix, which is filled with electrolyte when battery is assembled. Insertion and extraction of lithium ions in active material during battery operation cause an inhomogeneous ions concentration in the active material particles which results in swelling and differential strain, causing mechanical stress. The analysis of flaws size in electrode microstructure [11]

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