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High energy lithium‐ion batteries have improved performance in a wide variety of mobile electronic devices. A new goal in portable power is the achievement of safe and durable high‐power batteries for applications such as power tools and electric vehicles. Towards this end, olivine‐based positive electrodes are amongst the most important and technologically enabling materials. While certain lithium metal phosphate olivines have been shown to be promising, not all olivines demonstrate beneficial properties. The mechanisms allowing high power in these compounds have been extensively debated. Here we show that certain high rate capability olivines are distinguished by having extended lithium nonstoichiometry (up to ca. 20 %), with which is correlated a reduced lattice misfit as the material undergoes an electrochemically driven, reversible, first‐order phase transformation. The rate capability in several other intercalation oxides can also be correlated with lattice strain, and suggests that nanomechanics plays an important and previously unrecognized role in determining battery performance.

The phase stability and phase transformation kinetics of Li1−x MPO4 olivines are critical to their performance as lithium storage electrodes. In this work, nanoscale (<100 nm primary particle size) Li1−x FePO4 and Li1−x MnPO4 are chosen as model systems for comparison with a coarser-grained LiFePO4 that exhibits a conventional two-phase reaction. The nanoscale materials first exhibit time and state-of-charge dependences of the electrochemical potential and structural parameters which show that stable two-phase coexistence is not reached. The evolution of structural parameters supports the existence of a coherency stress influenced crystal−crystal transformation. However, an additional response, the preferential formation of amorphous phase at nanosize scale, is identified. In Li1−x FePO4, at 34 nm average particle size, at least one amorphous phase of varying Li content coexists with the crystalline phases. In Li1−x MnPO4 of 78 nm particle size, the electrochemically formed delithiated phase is highly disordered. These phenomena are interpreted from the effect of surface and bulk energetics on phase stability of a nanoscale material.

In the drive toward improved electrical energy storage for applications ranging from wireless devices to electric vehicles to grid stabilization, nanoscale materials are of growing interest as ion storage electrodes. Nanoscale olivines based on LiMPO4 (M = Fe, Mn, Co, Ni) are one class of compounds for which recent experimental developments reveal very different phase transition and solid-solubility behavior compared to larger particles. The olivines may be an exemplar for generalized behavior for which metastable crystalline or amorphous phases are produced under the large driving forces incurred during electrochemical reactions. Here we use a diffuse-interface thermodynamic model to assess the conditions under which amorphous phase transitions may occur in nanoscale LiMPO4 particles. There are three central conclusions. First, assuming as with similar solids that the amorphous phase has the lower surface energy, it is found that an initially crystalline phase may undergo amorphization during cycling when the particle size is below a critical value. Second, the effect of applied electrical overpotentials on the phase stability is evaluated for the first time, and is found to strongly influence the phase transition pathways of small particles. Third, the tendency to amorphize is significantly affected by the magnitude of the misfit strain between the lithiated and delithiated crystalline phases. It is shown that there exists a critical misfit strain above which the preferred transformation pathway is amorphization, regardless of the particle size. We use these results to interpret experimentally observed behavior of olivines, including data that up to now have been unexplained.

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Within the aortic valve (AV) leaflet resides a population of interstitial cells (AVICs), which serve to maintain tissue structural integrity via protein synthesis and enzymatic degradation. AVICs are typically characterized as myofibroblasts, exhibit phenotypic plasticity, and may play an important role in valve pathophysiology. While it is known that AVICs can respond to mechanical stimuli in vitro, the level of in vivo AVIC deformation and its relation to local collagen fiber reorientation during the cardiac cycle remain unknown. In the present study, the deformation of AVICs was investigated using porcine AV glutaraldehyde fixed under 0-90 mm Hg transvalvular pressures. The resulting change in nuclear aspect ratio (NAR) was used as an index of overall cellular strain, and dependencies on spatial location and pressure loading levels quantified. Local collagen fiber alignment in the same valves was also quantified using small angle light scattering. A tissue-level finite element (FE) model of an AVIC embedded in the AV extracellular matrix was also used explore the relation between AV tissue- and cellular-level deformations. Results indicated large, consistent increases in AVIC NAR with transvalvular pressure (e.g., from mean of 1.8 at 0 mm Hg to a mean of 4.8 at 90 mm Hg), as well as pronounced layer specific dependencies. Associated changes in collagen fiber alignment indicated that little AVIC deformation occurs with the large amount of fiber straightening for pressures below approximately 1 mm Hg, followed by substantial increases in AVIC NAR from 4 mm Hg to 90 mm Hg. While the tissue-level FE model was able to capture the qualitative response, it also underpredicted the extent of AVIC deformation. This result suggested that additional micromechanical and fiber-compaction effects occur at high pressure levels. The results of this study form the basis of understanding transvalvular pressure-mediated mechanotransduction within the native AV and first time quantitative data correlating AVIC nuclei deformation with AV tissue microstructure and deformation.

For an energy storage application such as electrical vehicles (EVs), lithium-ion batteries must overcome limited lifetime and performance degradation under specific conditions. Particularly, lithium-ion batteries show significant capacity loss at higher discharging rates (C-rates). In this work, we develop computational models incorporating coupled electrochemical–mechanical–thermal factors in order to reveal the relationship between the experimentally observed capacity loss and predicted mechanical stresses during electrochemical (dis)­charging. Specifically, a multiphysics finite element model consisting of electrochemistry, heat generation, mass transport, and solid mechanics is developed to investigate thermal- and diffusion-induced stresses with the reconstructed porous microstructures of commercial LiFePO4 batteries. It has been suggested that porous microstructures in electrodes could mitigate the electrolyte reactivity for an improved battery life and safety. Therefore, the reconstructed porous microstructures from focused ion beam–scanning electron microscopy (FIB-SEM) images are adopted. The integrated experimental measurements and computational simulations show that: (1) Lithium-ion cells electrochemically tested at 3.6C have 30% capacity loss versus cells tested at 1.2C; a corresponding stress increase of 150% is observed from the multiphysic simulations. (2) The thermal models verified by in operando temperature measurement via the fiber Bragg grating (FBG) sensor demonstrate that increasing temperature results in larger thermal stresses during (dis)­charging. However, increases in thermal stress due to higher temperature played a lesser role at higher C-rates. (3) Lithium-ion concentration distribution is location dependent; that is, at any time and at any given C-rate, the outer layer of the particle exhibits a higher concentration than that inside the particle. (4) Higher diffusion-induced stresses are observed at the connecting areas between particles, suggesting that the higher stresses may result from higher concentration variations in the connecting area. This study presents results that include evolutions of lithium-ion concentration and mechanical stresses and could help to provide insight into the decreasing electrochemical performance of lithium-ion batteries at higher C-rates.

A simple biomechanical test with real-time displacement and strain mapping is reported, which provides displacement vectors and principal strain directions during the mechanical characterization of heart valve tissues. The maps reported in the current study allow us to quickly identify the approximate strain imposed on a location in the samples. The biomechanical results show that the aortic valves exhibit stronger anisotropic mechanical behavior than that of the pulmonary valves before 18% strain equibiaxial stretching. In contrast, the pulmonary valves exhibit stronger anisotropic mechanical behavior than aortic valves beyond 28% strain equibiaxial stretching. Simple biochemical tests are also conducted. Collagens are extracted at different time points (24, 48, 72, and 120 h) at different locations in the samples. The results show that extraction time plays an important role in determining collagen concentration, in which a minimum of 72 h of extraction is required to obtain saturated collagen concentration. This work provides an easy approach for quantifying biomechanical and biochemical properties of semilunar heart valve tissues, and potentially facilitates the development of tissue engineered heart valves.