Stress Evolution on the Phase Boundary in LiFePO₄Particles

Abstract

It is commonly thought that diffusion-induced stress is one of the main factors causing loss of capacity in electrode materials. To understand stress evolution on the phase boundary during the lithiation process, we develop a finite element model adopting lithium ion concentration-dependent anisotropic material properties and volume misfits. Increased mechanical stresses on the phase boundaries are observed during the lithiation process. When the particle is more fully lithiated, larger stresses occur on the free surfaces and these may be related to the cracks on the ac-plane. The C-rate dependent strain energy evolution is also studied. The result shows that with the same amount of lithiation, particles experience different strain energies due to varied C-rate discharging. The high elastic energy from the high C-rate model suggests that the system becomes unstable, and a homogeneous phase transformation path is more plausible for the system. The current study provides a connection between diffusion-induces stresses on the phase boundary and the cracking propensity on free surfaces. Thus, the study could be used to better understand the mechanisms that cause particle fracture and capacity loss.Due to their high energy density, lithium-ion batteries are currently preferred as the main energy storage devices for PHEVs/EVs. 1,2 However, retaining the lithium-ion battery capacity is one of the major challenges facing the electrochemical community today. Capacity loss is found in several conditions, such as at a high (dis)charging rate 2 or with long periods of cycling, 3 and the capacity fade is strongly related to the mechanical stresses inside the materials. 4,5 Among many cathode materials, the olivine-based LiFePO 4 with an orthorhombic crystal structure provides excellent characteristics for application in EVs/PHEVs such as: good thermal stability, abundant iron resource, low raw material cost, and high theoretical energy density (170 mAh/g). 5 However, electronic conductivity and diffusivity for LiFePO 4 are considerably lower than those of LiMn 2 O 4 and LiCoO 2 materials, 6,7 and these intrinsic disadvantages of LiFePO 4 are currently improved by doping, carbon coating, nano-scale particle size, or synthesis controls. 5,8-10 Nevertheless, an understanding of the fundamental mechanisms resulting in capacity loss needs to be obtained, and it will hopefully culminate in breakthroughs in lithium-ion battery technology via synergistic investigative activities that bridge theory, computation, experiment, and manufacturing.It has been reported that the electrochemically cycled LiFePO 4 under low C-rate exhibits a two-phase system: 11-14 lithium-rich (LiFePO 4 ) and lithium-poor (FePO 4 ) phases. These two phases have similar crystal structures and are constrained by a coherent interface during the phase transformation. 15 Because of different lattice constants for the two phases, approximately 7-9% volume misfits are observed. 16 Moreover, computational simulations and experimental observations have identified that ...

Critical lithiation for C-rate dependent mechanical stresses in LiFePO4

“…We used a thermal stress analysis approach [27] to investigate C-rate dependent diffusion-induced stresses during lithiation in LiFePO 4 electrode material. The study on the diffusioninduced stresses based on the thermal analogy has been wildly discussed [26,32,35,42].…”

“…Diffusion-induced stresses in single spherical, cylindrical, or ellipsoidal particles have been extensively discussed by assuming isotropic material properties [23][24][25][26]. In our previous study, we investigated the stress evolution on the phase boundary in a single LiFePO 4 particle [27]. The maximum stress occurs on the particle surface during lithiation, explaining the experimentally observed particle fracture.…”

“…To incorporate the concentration-dependent anisotropy material properties of the LiFePO 4 [27,48], where [C] is the stiffness matrix in the finite element analyses. Orthorhombic elastic constants of both phases C ½ LiFePO 4 and C ½ FePO 4 are obtained from Maxisch and Ceder [49].…”

Computational and Experimental Observation of Li‐Ion Concentration Distribution and Diffusion‐Induced Stress in Porous Battery Electrodes

“…Extensive research has been performed on DIS and electrode fracturing. Prussin [5] was the first to analyze the phenomenon of DIS in wafers by analogy with the stress caused by temperature gradients.S ubsequently,r esearchers studied DIS in LiMn 2 O 4 , [6][7][8] LiFePO 4 , [9][10][11] and Li x CoO 2 [12,13] electrode particles by simulation. Moreover, other investigators analyzed the DIS [14][15][16] and fracturing [17] of Si electrode particles during Li ion insertion.…”

Interaction between dislocation mechanics on diffusion induced stress and electrochemical reaction in a spherical lithium ion battery electrode

“…21 Hao et al studied that DIS of core-shell nanotube electrodes in lithium-ion batteries. 29 Tavassol et al studied the electrochemical surface stress changes during Li deposition in a model system for Li-ion battery anodes. 23 Wu et al presented a coupled thermal-electrochemical modeling of uneven heat generation in lithium-ion battery packs.…”