Mechano-electrochemical Interaction Research Articles

Mechano-Electrochemical Interaction and Degradation in Graphite Electrode with Surface Film

Lithium ion batteries (LIBs) exhibit significant capacity and performance degradation with cycling owing to extensive decrepitation of anodes associated with lithiation-delithiation induced volumetric expansion and contraction. Microcrack formation in the active material and solid electrolyte interphase layer contribute to deleterious effects including hindered diffusion, particle isolation, and loss of cyclable Li inventory, with detrimental performance ramifications. In this work, a stochastic computational methodology, utilizing lattice spring formalism, is extended to probe effect of surface film characteristics on diffusion induced damage in graphite anodes with a direct correlation to LIB performance. Film geometric and mechanical properties of interest, with a direct impact on fracture characteristics, are identified and parametric variations are explored to ascertain the relative influence of each property. Reduction of surface film stiffness is found to substantially ameliorate fracture damage inside both active material and film. Film thickness and fracture threshold energy primarily affect the fracture characteristics inside film with a less discernible impact on the anode active material. Extensive damage density data based regression as well as rate performance study is performed to strengthen the hypothesis and desirable film properties are outlined for improved LIB performance, specifically, low stiffness and high fracture threshold energy film with optimal thickness.

Coupled Mechano-Electrochemical Interaction in Silicon Anode with Solid Electrolyte Interphase Layer

Silicon is an attractive candidate as a negative electrode material for lithium-ion batteries due to its high theoretical capacity (4200 mAh/g). However, high volume change (nearly 300%) in Si during lithiation and delithiation has significant deleterious effects, causing particle fracture beyond a critical size of around 200-300 nm even for low C-rates. The limitations of Si anodes are compounded by the fact that the prohibitively high expansion precludes the formation of a stable solid electrolyte interphase (SEI) layer. The SEI side reactions continue unabated with Si unlike graphite where the SEI film stabilizes after the first few cycles forming a protective layer between the electrode and the electrolyte. Consequently, capacity-fade with the Si anode progresses fast leading to low cycle life. In this regard, it is necessary to delineate the effect of coupled mechanical and chemical degradation modes in Si active particles along with SEI layer. In this work, we focus on investigating the coupled mechano-electrochemical interaction, based on a stochastic modeling and analysis, in Si active particles with SEI layer, as shown representatively in the attached figure including fracture evolution and damage stochastics phase map. Figure 1

Mechanistic Analysis of Mechano-Electrochemical Interaction in Silicon Electrodes with Surface Film

High-capacity anode materials for lithium-ion batteries, such as silicon, are prone to large volume change during lithiation/delithiation which may cause particle cracking and disintegration, thereby resulting in severe capacity fade and reduction in cycle life. In this work, a stochastic analysis is presented in order to understand the mechano-electrochemical interaction in silicon active particles along with a surface film during cycling. Amorphous silicon particles exhibiting single-phase lithiation incur lower amount of cracking as compared to crystalline silicon particles exhibiting two-phase lithiation for the same degree of volumetric expansion. Rupture of the brittle surface film is observed for both amorphous and crystalline silicon particles and is attributed to the large volumetric expansion of the silicon active particle with lithiation. The mechanical property of the surface film plays an important role in determining the amount of degradation in the particle/film assembly. A strategy to ameliorate particle cracking in silicon active particles is proposed.

Mechano-Electrochemical Interaction Gives Rise to Strain Relaxation in Sn Electrodes

Tin (Sn) anode active particles were electrochemically lithiated during simultaneous imaging in a scanning electron microscope. Relationships among the reaction mechanism, active particle local strain rate, particle size, and microcrack formation are elucidated to demonstrate the importance of strain relaxation due to mechano-electrochemical interaction in Sn-based electrodes under electrochemical cycling. At low rates of operation, due to significant creep relaxation, large Sn active particles, of size 1 μm, exhibit no significant surface crack formation. Microcrack formation within Sn active particles occurs due to two different mechanisms: (i) large concentration gradient induced stress at the two-phase interface, and (ii) high volume expansion induced stress at the surface of the active particles. From the present study, it can be concluded that majority of the microcracks evolve at or near the particle surface due to high volume expansion induced tension. Concentration gradient induced damage prevails near the center of the active particle, though significantly smaller in magnitude. Comparison with experimental results indicates that at operating conditions of C/2, even 500 nm sized Sn active particles remain free from surface crack formation, which emphasizes the importance of creep relaxation. A phase map has been developed to demonstrate the preferred mechano-electrochemical window of operation of Sn-based electrodes.

The Effect of Matrix Tension-Compression Nonlinearity and Fixed Negative Charges on Chondrocyte Responses in Cartilage

Thorough analyses of the mechano-electrochemical interaction between articular cartilage matrix and the chondrocytes are crucial to understanding of the signal transduction mechanisms that modulate the cell metabolic activities and biosynthesis. Attempts have been made to model the chondrocytes embedded in the collagen-proteoglycan extracellular matrix to determine the distribution of local stress-strain field, fluid pressure and the time-dependent deformation of the cell. To date, these models still have not taken into account a remarkable characteristic of the cartilage extracellular matrix given rise from organization of the collagen fiber architecture, now known as the tension-compression nonlinearity (TCN) of the tissue, as well as the effect of negative charges attached to the proteoglycan molecules, and the cell cytoskeleton that interacts with mobile ions in the interstitial fluid to create osmotic and electro-kinetic events in and around the cells. In this study, we proposed a triphasic, multi-scale, finite element model incorporating the Conewise Linear Elasticity that can describe the various known coupled mechanical, electrical and chemical events, while at the same time representing the TCN of the extracellular matrix. The model was employed to perform a detailed analysis of the chondrocytes' deformational and volume responses, and to quantitatively describe the mechano-electrochemical environment of these cells. Such a model describes contributions of the known detailed micro-structural and composition of articular cartilage. Expectedly, results from model simulations showed substantial effects of the matrix TCN on the cell deformational and volume change response. A low compressive Poisson's ratio of the cartilage matrix exhibiting TCN resulted in dramatic recoiling behavior of the tissue under unconfined compression and induced significant volume change in the cell. The fixed charge density of the chondrocyte and the pericellular matrix were also found to play an important role in both the time-dependent and equilibrium deformation of the cell. The pericellular matrix tended to create a uniform osmolarity around the cell and overall amplified the cell volume change. It is concluded that the proposed model can be a useful tool that allows detailed analysis of the mechano-electrochemical interactions between the chondrocytes and its surrounding extracellular matrix, which leads to more quantitative insights in the cell mechano-transduction.