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REPORT OF THE BASIC RESEARCH NEEDS WORKSHOP three length scales, as shown in Figure 2.5.7. At the cell level, 1D heat conduction occurs along the through-cell direction (radial for cylindrical cells). At the electrode pair level, mass and charge transport in the liquid electrolyte are described as 1D diffusion. At the particle level, ion transport is described as diffusion in the solid driven by the electrochemical intercalation reaction and a concentration gradient.44 This model, often called the 1D+1D+1D or the pseudo-3D approach, can describe degradation at charging rates from 0.1 C to 10 C and surface temperatures from 25°C to 60°C, and accounts for energy storage and side reactions, such as SEI formation.44,45 These modeling approaches, however, are at an early stage of development and certainly do not capture the consequences of 3D geometries that are present at multiple length scales in batteries. Separator Active material Electrolyte Li+ Li xz Figure 2.5.7. Three scales for modeling battery operation: macroscopic (cm) at the cell level (left), mesoscopic (100s of μm) at the electrode pair level (upper right), and microscopic (μm) at the particle level (lower right). From Ref. 44. y Solid-electrolyte interphase formation and growth at the graphite anode in Li-ion batteries can be modeled by accounting for initial formation and subsequent growth by slow penetration of solvent molecules through the interphase layer to reach and react with the graphite anode.46 These kinds of models can account for capacity fade due to loss of working ions to continued formation of the interphase at the anode-interphase boundary, a common cause of gradual long-term degradation. One advantage of such models is their inclusion of reaction kinetics at elevated temperatures, allowing them to motivate the design of accelerated aging protocols. However, as 1D models they do not reflect the temporal changes in SEI structure when volume change in the electrode leads to cracking of the SEI and exposure of fresh electrode surface. Positive electrode Li+ Negative electrode e– Fracture mechanics of electrodes due to local stresses arising from volume change or phase separation on cycling can be modeled with continuum phase-field approaches coupled to spatial solute distributions and stress profiles.47 Typically, two-phase regions are intentionally avoided in intercalation cathodes precisely because they lead to local strain and fracture. The downside of avoiding two-phase regions is the significant restriction of theoretical intercalation capacity and reduced practical energy density. However, phase separation is a mesoscale rather than atomistic phenomena, requiring coherence over a finite volume to become energetically favorable. Reducing effective intercalation volumes to tens or hundreds of nanometers dramatically inhibits phase separation. Understanding and deploying this degradation mitigation approach require sophisticated continuum modeling. Modeling the nucleation and growth of dendrites that can short circuit the battery is a major opportunity and challenge for batteries with pure metal anodes.31 Continuum models of dendrite nucleation and growth are at a much earlier stage than models of intercalation.48-50 Nevertheless, they have revealed many features of dendrite nucleation and growth that may lead to effective mitigation strategies, including current density vs. temperature thresholds for diminished dendrite growth, reduced dendrite growth in designed electro-convective flows in liquid electrolytes, and thresholds for dendrite formation as a function of ionic conductivity and current density in solid electrolytes. Modeling dendrite behavior needs to take into account many more phenomena, such as the shear modulus and grain structure of solid-state electrolytes, the conditions for dendrite nucleation, the influence of electrolyte composition and additives, and the role of solid-electrolyte interphases in modulating dendritic nucleation and growth. Self-healing triggered by preferential deposition of inactive cation additives by the higher electric field and potential at a dendrite tip is a promising smart mitigation effect.31 74 PRIORITY RESEARCH DIRECTION – 5PDF Image | Next Generation Electrical Energy Storage
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