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 ...

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.

To develop lithium-ion batteries with a high rate-capability and low cost, the prevention o f capacity loss is one o f major challenges, which needs to be tackled in the lithium-ion battery industry. During electrochemical processes, lithium ions diffuse from and insert into battery electrodes accompanied with the phase transformation, whereas ionic diffusivity and concentration are keys to the resultant battery capacity. In the current study, we compare voltage versus capacity o f lithium-ion batteries at different current-rates (C-rates) discharging. Larger hysteresis and voltage fluctuations are observed in higher C-rate samples. We investigate origins o f voltage fluctuations by quantifying lithium-ion intensity and distribution via a time-of-flight secondary ion mass spectrometry (ToF-SIMS). The result shows that fo r fully discharged samples, lithium-ion intensity and distribution are not C-rate dependent, suggesting different lithium-ion insertion mecha nisms at a higher C-rate discharging might be solely responsible fo r the observed low frequency voltage fluctuation.

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The application of hot stamping parts has become a dominant approach to achieve lightweight design and increase crashworthiness performance in the automotive industry. Compared to the cold stamping parts, the main feature of the hot stamped component is its high strength and good size stability. However, recent studies have shown that the springback or size deviation of hot stamped parts caused some assembly problems. This paper aims to investigate the influence of forming parameters on the part’s springback via a hot stamping hat-shaped die. Three different forming parameters with the same blank heating temperature (930°C) were conducted in the study: (1) transfer time 8 sec/die quenching 15 sec (2) transfer time 8 sec/die quenching 8 sec and (3) transfer time 18 sec/die quenching 15 sec. The selected forming parameters are used to evaluate the effect of the before-forming blank temperature and die-opening blank temperature on the springback of the hot stamping parts. The results show that the case 3 with a prolonged transfer time causes a small portion of ferrite phase formation in the hot-stamped part, resulting in the decrease of the tensile strength and a more significant springback is observed.