Critical lithiation for C-rate dependent mechanical stresses in LiFePO4

Abstract

The prevention of capacity loss after electrochemical cycling is of paramount importance to the development of lithium-ion batteries, especially for applications in the electric vehicle industry. The objective of this research is to investigate C-rate dependent diffusion-induced stresses in electrode materials. LiFePO 4 is selected as the model system in this study since it is one of the most promising cathode materials used in electric vehicle applications. Finite element models incorporating several factors with concentration dependency are developed in this study including concentration-dependent anisotropic material properties, concentration-dependent and Crate-dependent volume expansion coefficients, and concentration-dependent lithium ion diffusivity. Our simulation results show that the effect of concentration dependency on mechanical properties and lithium diffusivities cannot be neglected in mechanical stress predictions. We also observe that C-rate has a great effect on how fast the surface concentration is saturated, suggesting that C-rate dependency of the diffusion-induced stresses occurs at a critical lithiation stage: 47.5, 26.5, 10.1, and 6.8 % lithiation for 1, 2, 6, and 10 C, respectively. Mechanical stresses in perfect and cracked particles are also studied. It is observed that the crack surface orientation plays an important role in the diffusion-induced stress. The existence of the crack surface increases mechanical stresses, suggesting that particles inside the material may undergo fractures faster and may accelerate the material deterioration, leading to capacity loss at higher C-rate (dis)charging.

Multiphysics Coupling in Lithium-Ion Batteries with Reconstructed Porous Microstructures

“…Higher stresses were observed at the connecting areas between particles, suggesting that the higher stresses might result from higher concentration variations in the connecting area. Compared to other previous results where only homogeneous spherical particles were considered, ,,, the current study showed that reconstructed microstructures of electrode particles were very important for stress analysis, and it is hypothesized that it is highly related to crack initiations …”

“…Compared to other previous results where only homogeneous spherical particles were considered, 10,38,42,57 the current study showed that reconstructed microstructures of electrode particles were very important for stress analysis, and it is hypothesized that it is highly related to crack initiations. 58 The rate capability of 14 430 lithium-ion cells performed via a 273A potentiostat/galvanostat at different discharging rates (1.2C, 2.0C, and 3.6C) is shown in Figure 7a, and the capacity lost was recorded. The results showed that 14 430 lithium-ion cells tested at 3.6C have 30% capacity loss compared to cells tested at 1.2C; a corresponding 150% increase in stress was observed from the multiphysic simulations, where both thermal and diffusion-induced stresses were included (Figure 7b).…”

Mechanical stresses at the cathode–electrolyte interface in lithium-ion batteries

“…To adopt the concept of lithiation, a thermal stress analysis approach is used due to the similarity of the partial differential equations of Fourier's law for thermal conduction and Fick's law of diffusion. 8,19,20,34 In this study, due to the similarity of the partial differential equations of Fourier's law for thermal conduction and Fick's law of diffusion, the flux of lithium ions (J) is expressed as heat flux, and the temperature gradient represents the lithium-ion concentration gradient. Since the concepts of heat conduction and temperature are used for ionic diffusion and the concentration of lithium ion, effects of temperature and heat on the half-cell system are disregarded.…”

Diffusion-Induced Transient Stresses in Li-Battery Electrodes Imaged by Electrochemical Quartz Crystal Microbalance with Dissipation Monitoring and Environmental Scanning Electron Microscopy

“…Next-generation electrochemical energy storage devices are designed to provide high energy and power densities during safe and long-term operation under specified conditions. , Unfortunately, high rates of charging–discharging are accompanied by strong concentration-induced stresses causing microcracking of the electrode particles and eventually deteriorating the entire electrode cycling performance. , A variety of coupled electrochemical–micromechanical models (analytical and numerical ones , ) have been proposed to quantify the concentration-induced mechanical stresses in battery electrodes under different cycling conditions. During the last two decades − direct in situ stress measurements of battery electrodes have become available (see recent comprehensive reviews). , …”

“…The data presented here clearly shows the great advantages of acoustic probing of Li-intercalated electrodes cycled at different C-rates. In fact, we have obtained a high-frequency (megahertz range) acoustic reflection (image) of the transient deformations in both single- and multilayered LFP electrodes confirming theoretical predictions of the development of C-rate-dependent diffusion-induced stresses in intercalation electrodes. , In addition, we have experimentally confirmed that multilayered electrodes of similar thicknesses cycled at a slow or fast rate result in either stable cycling with good capacity retention or in a complete deterioration of the electrode capacity, respectively. Cycling the composite electrode containing a stiff binder at high rates of charging clearly demonstrates strong transient stresses, generated initially along the [100] direction with the largest misfit strain between FePO 4 and LiFePO 4 revealed by the mechanical interaction of the rapidly strained LiFePO 4 layer with the stiff binder.…”