• Masson, Renaud
• Mathieu, Benoit
• Garajeu, Mihail
• Michel, Bruno
• Dalevedo, Guilherme

Abstract

Lithium ion batteries (LIBs) using silicon as the anode active material have high energy density and areusedinmanyelectronicequipment.AmajorissueforimprovingtheperformanceofLIBsis understanding their degradationmechanisms that lead to capacityfade. In this work,we consider that the anode is composed of spherical nanoparticles of silicon in a soft electrolyte media. We are interested in the mechanical behaviour of a single nanoparticle without interaction with the others.Experimental results [1, 2] indicate that three main phenomena occur during the lithiation of silicon nanoparticles.First,theformationofanadvancinglithiationfrontseparatingtwophases:apure silicon coreand asilicon lithium alloy outer shell. Second,largevolumetransformationof about 300 % and finally, particle fracture.In order to take into account those phenomena, semi-analytical and finite element mechanochemistry models were established. The lithiation is treated similarly to a thermomechanical problem, where the lithium concentration drives the differential swelling within the particle. The semi-analytical model is an extension of the composite model of a sphere subjected to a radial loading in the case of elasto-viscoplastic constituents [3]. The finite element model takes intoaccountlargedeformationviathelogarithmicstrainframework.Thesolutioninbothcases showsthatthelithiationfrontandtheviscoplasticdeformationoftheoutershellareessential ingredients in modelling the lithiation. Moreover, the viscoplastic deformation relaxes the significant internal stresses, induced by the swelling of the silicon lithium alloy, which leads to theformation of a residual tangential traction of the shell. This traction is likely to cause the nanoparticle fracture.Some experiments show that mitigation of nanoparticle mechanical failure can be achieved by using a carbon coating that has several benefits, such as stress alleviation and swelling restriction. In this work,weanalysethemechanicalimpactofcoatingandcalculateitsoptimalthicknessusingthe linear fracture mechanics considering different flaw sizes and geometries

Topics

• nanoparticle
• density
• impedance spectroscopy
• Carbon
• energy density
• experiment
• composite
• Silicon
• Lithium