Since their introduction in the late 20th century, lithium-ion batteries have become the leading battery technology for portable devices and electric vehicles due to their high energy density and rechargeability. However, the increasing demand for a longer battery life span is hindered by the fading of the battery’s charge capacity over prolonged use. This reduction in charge capacity stems from electrochemical and mechanical degradation of the battery cells. The main research focus in the literature has been on the chemical degradation of battery cells. However, the mechanical degradation also substantially contributes to the battery’s capacity degradation. Therefore, it is crucial to understand the mechanical properties of the battery cells to be able to mitigate mechanical degradation. The battery’s mechanical degradation stems from the electrode layers’ constituents. This thesis aims to model the positive electrode’s mechanical properties by recreating its granular microstructure using the discrete element method.

In Papers 1 and 2, a discrete element method modelling framework is developed, which can reconstruct a positive electrode layer of a lithium-ion battery, simulate manufacturing processing steps, and determine the mechanical properties of the electrode layer. The framework uses two contact models, representing the positive electrode material in the form of particles and a binder agent, which gives the electrode layer its structural integrity. The framework is used to link the mechanical behaviour of the electrode particles and the binder agent to the mechanical behaviour of the entire electrode layer. The framework is able to capture the layer’s pressure sensitivity and relaxation behaviour, properties which have been displayed in the literature through experimental testing.

Sedan de introducerades i slutet av 1900-talet har litiumjonbatterier blivit den ledande batteriteknologin för portabla enheter samt elfordon på grund av deras höga energidensitet och återladdningförmåga. Den ökade efterfrågan på utökade batterilivslängder är dock hämmad av reduceringen av uppladdningskapacitet över längre användningstider. Denna reducering av laddningskapacitet kommer från elektrokemisk och mekanisk degradering av battericellerna. Det största forskningsintresset i litteraturen har varit på den kemiska degraderingen av battericellerna. Dock ger den mekaniska degraderingen ett betydande bidrag till batteriets kapacitetsdegradering. Därför är det viktigt att förstå battericellens mekaniska egenskaper för att kunna förhindra mekaniskdegradering. Batteriets mekaniska degradering beror på elektrodlagrets beståndsdelar. Denna avhandlings målsättning är att modellera den positiva elektrodens mekaniska egenskaper genom att återskapa dess granulära mikrostruktur med hjälp av diskret elementmetodik.

I Artikel 1 och 2 utvecklades ett ramverk för modellering med användning av diskreta elementmetoden, vilket kan återskapa det aktiva lagret för en positiv elektrod, simulera tillverkningsprocesser, samt fastställa elektrodlagrets mekaniska egenskaper. Ramverket använder två kontaktmodeller som representerar det positiva elektrodmaterialet i form av partiklar samt ett bindemedel, som ger elektrodlagret dess strukturella integritet. Ramverket används för att undersöka hur de mekaniska egenskaperna för det hela elektrodlagret beror på egenskaperna för de aktiva partiklarna samt bindemedlet. Ramverket kan fånga lagrets tryckkänslighet samt dess relaxering, egenskaper som har påvisats i litteraturen genom experimentell provning. 

Over the early decades of the 21st century, lithium-ion batteries have emerged as the leading energy storage technology in portable electronics and electric vehicles. Their popularity can be attributed to their rapid rechargeability and high power density. As demands on lithium-ion batteries increase, particularly from the electric vehicle manufacturers, challenges such as loss of charge capacity over usage must be addressed. Historically, lithium-ion batteries have mainly been studied from a chemical point of view. However, to effectively mitigate charge capacity fade, the mechanical behaviour of the lithium-ion batteries must be understood. 

This thesis aims to deepen the understanding of the mechanical behaviour and degradation of lithium-ion batteries, particularly of the batteries’ positive electrode layers. Using numerical modelling, specifically the discrete element method, a framework for simulating several mechanical aspects of the lithium-ion batteries’ positive electrode layer has been developed, including manufacturing and usage processes, and replicating experimental measurements to determine mechanical properties. These aspects were investigated and linked to the material properties and behaviours of the layer’s constituents as well as usage conditions. The findings offer vital insights into the micromechanical behaviour of positive electrode layers and their dependency on the constitutive behaviour of the layer’s constituents. These insights are significant for future lithium-ion battery development.