Theoretical Study On The Mechanical Behaviors Of Phase-transformation Electrode Materials

The development of vehicles requires the lithium ion batteries to possess high capacity,fast charging rate and long cycle life.Inevitably,stresses generate during the insertion and extraction of lithium ions from the electrodes.Moreover,large stresses arise in phase transformation electrodes,which will lead to plastic deformation and even initiate cracks.Most of theoretical works are dedicated to calculating the amplitude of stress to avoid mechanical degradation.Only a few works focus on the interplay between stresses and kinetics.In this thesis,much effort is devoted to investigating the effect of stress on the lithium diffusion,voltage decay,capacity fade and the phase interface movement.The main works are summarized below:A phase filed model is developed to study the interplay between stress and diffusion in an arbitrarily-shaped particle accompanied with phase transformation and concurrent plasticity.A static order parameter is introduced to characterize the particle shape.The Cahn-Hilliard equation is employed to simulate the phase transformation process.The J2-flow theory is used to describe the plastic deformation.Based on the phase field microelasticity theory,the stress distribution in the arbitrarily-shaped particle is obtained.Here,transition of surface tangential stress from compression to tension during two-phase lithiation is reexamined.Further,it is elucidated that the surface with a large curvature facilitates the transition from surface tangential compression to tension in a particle with complex geometry.A stress coupled Cahn-Hilliard reaction model is obtained to study the effect of stress on the voltage decay and capacity fade in an elastoplastic particle.Based on the generalized Butler-Volmer equation,the voltage-capacity curves can be simulated.During single-phase lithiation,the voltage monotonously decreases with the increasing capacity.The hydrostatic compressive stress causes additional voltage decay and capacity fade.The plastic deformation unloads the hydrostatic compression and even changes it to hydrostatic tension,increasing the voltage and the real capacity.During two-phase lithiation,a voltage plateau appears on the voltage-capacity curve.Both of the amplitude and width of the voltage plateau will decrease due to the hydrostatic compressive stress,leading to the fade of real capacity.The plastic deformation will also mitigate the capacity fade by unloading the hydrostatic compression.A stress coupled nonlinear kinetic model is proposed to reveal the mechanism for the size-dependent slowing down behavior of the phase interface movement in nanosized particles.In the rate limited regime,the phase interface velocity is determined by its driving force.It is demonstrated that the radial compression will decrease the driving force for the phase interface movement,which is responsible for the slowing down behavior.Further,the mechanism is revealed for the size-dependent slowing down behavior of the phase interface movement.On the one hand,the surface/interface stress causes larger radial compression in the smaller particle.On the one hand,for the same given lithiation time,the lithiated phase volume fraction is larger in the smaller particle,which also generates the larger radial compression.As a result,the phase interface velocity is slower in the smaller particle.The mechanism is revealed for transition from deceleration to acceleration of the phase interface movement in hollow sphere electrodes.The plastic zones exist near the inner surface and phase interface.The plastic deformation at the phase interface dissipates extra energy required to drive the inward migration of the phase interface and leads to deceleration.Another plastic zone arises near the inner surface and expands outward as two-phase lithiation proceeds.The coalescence of these two plastic zones significantly decreases the resistance of the phase interface movement and leads to acceleration.Furthermore,we propose to regulate the phase interface movement by tuning the size of the hollow spheres toward high volume energy density and superior rate capability.