Energy Change Pathways in Electrodes during Nonequilibrium Processes

Abstract

It has been discovered that the electrodes of Li-ion batteries have abnormal phase transition phenomena during nonequilibrium processes. As a two-phase coexistence material, LiFePO₄ (LFP) shows a single phase during both high-rate lithiation and delithiation. Furthermore, being far from equilibrium state can make LFP particles display a metastable amorphous structure. Although layered oxides are typically single-phase materials, they exhibit two-phase coexistence during high-rate delithiation. It is well-known that a stable single-phase structure is an important characteristic for electrodes with better high-rate performance since the energy barriers of nucleation and growth for new phases are avoided. Understanding the mechanisms of the abnormal phase transition during nonequilibrium processes is therefore essential for designing electrodes with fast (dis)charging. However, few studies have explained the variation of the phase transitions. Based on nonequilibrium thermodynamics, we formulate the free energy in a multilayered structure with a series of order parameters that represent different states. The variation of phase transition is caused by the pathway altering of free energy change controlled by path factors. Dislocation density is the key parameter for selecting the pathways since the free energy change is dominated by dislocations. The generation/annihilation of dislocations is coupled with the electrochemical reaction of the Li-ion on the surface of electrode particles, which is a necessary condition of altering the pathways under high-rate (de)lithiation. The kinetics of a system’s states during (de)lithiation with different rates is simulated by solving the derived governing equations using the finite difference method. Our simulated X-ray diffraction (XRD) graphs and the order–disorder coexistence phase field agree well with the existing experimental in situ XRD graphs and in situ transmission electron microscopy (TEM) observations for LFP and LiNi_1/3Mn_1/3Co_1/3O₂ (NMC111). Our pathway altering mechanism elucidates the variations of phase transitions for the electrodes of Li-ion batteries and reveals potential strategies for optimizing the (dis)charging pathways and the structural states to obtain stable single-phase characteristics and promote rate capability.