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irreversibly distorts to a CdI2-like structure. This new polymorph is completely incapable of Li-ion insertion because the Li site shares a face with the CoO6 octahedra, an environment that is energetically unfavorable due to the strong coulombic repulsion between the Li and Co ions.18 Similarly, the framework of spinel LiMn2O4 is known to undergo a structural distortion that causes an elongation along the c-axis of the cubic cell and lowers the symmetry to the tetragonal polymorph of λ−MnO2.19 While polyanionic materials also experience distortions to their frameworks, the complex ways that their rigid oxoanionic subunits can pack offers an endless number of unique structural topologies for designing new intercalation hosts. The many advantages of polyanionic electrodes include robust structures that provide long-term stability over thousands of cycles, the ability to tune redox potentials through inductive effects, and rich compositional phase diagrams.20 These materials also offer advantages over oxides because polyanionic compounds do not typically release O2 on thermal decomposition, which can exacerbate thermal runaway when cells fail. Charge-storing materials (carbons, metal oxides, etc.) are now available in a wide range of nanoscale forms and have been incorporated in complex electrode architectures that have shown improved electrochemical performance. With these new materials, the traditional lines between capacitor-like and battery-like behavior are being blurred, both in terms of the current–voltage characteristics and the charge/discharge time scale.21,22 For example, conventional battery materials, such as LiCoO2, may exhibit a capacitor-like (pseudocapacitive) current–voltage response when synthesized in crystallites sized <10 nm.23 Alternatively, nanocrystalline LiMn2O4, when incorporated as a thin coating on an ultraporous conductive carbon architecture, exhibits energetically well-defined Li+-insertion peaks as would be found in a typical battery, but the resulting LiMn2O4–carbon electrode architecture delivers charge on a few-seconds timescale that is typical of supercapacitors.24 There are many other such examples, including individual materials exhibiting multiple charge-storage mechanisms that operate at different time regimes, from few-second pulses (supported by double-layer capacitance or redox pseudocapacitance) to long-term/high-capacity operation (battery-like ion insertion). These new materials and electrode architectures promise to provide new functionality to the electrochemical devices into which they are ultimately incorporated. Yet, a sufficient understanding of the structural characteristics that give rise to such behaviors is lacking. A Holy Grail: Lithium Metal as a High Capacity Negative Electrode: The development of Li metal anodes is generally considered critical to enable rechargeable battery systems with high energy density, including those that go beyond Li-ion, such as Li-S. However, Li metal anodes suffer from well-known challenges such as low coulombic efficiency, instability against most electrolytes, and formation of dendrites. Many reports focus on approaches that mitigate the symptoms of poor performance (i.e., suppression of dendrites) without addressing the underlying root cause of why they form and how they evolve. By looking beyond performance-based metrics, a more fundamental understanding of why a certain methodology is successful (or not) can be gained. Recent advances in operando analysis, combined with computational modeling, allow for an understanding of the coupled morphological, electrochemical, chemical, and mechanical behavior of Li metal anodes during cycling. This coupling between multiple variables (i.e., morphology, electrochemical activity, and mechanical stresses) is critical to understand the origins of poor performance, which allows for rational design of new solutions. In the example shown in Figure 3.4.2, operando video microscopy of Li metal morphology is coupled with galvanostatic voltage vs. time traces.25 This correlation between morphology and electrochemical properties provides insights into dendrite and pit formation. Another concern with Li-metal and Li-alloy materials is the effect of the solid-electrolyte interphase generated due to unwanted reactions at the electrode surface. These materials undergo significant mechanical volume expansion and contraction during lithiation-delithiation.26 Due to this volume change, the solid-electrolyte interphase continuously breaks and reforms during cycling. This repeated breakage and reformation consume both the active materials and the electrolyte, resulting in low cycling efficiency and poor cycling life. A number of efforts have been made to stabilize the solid-electrolyte interphase.27-33 Some of the more common approaches include optimizing solid-electrolyte interphase compositions by tuning electrolyte components, developing a protective layer on the material surface to circumvent the poor interfacial chemistry between Li-alloy materials and electrolyte, and using surface modification techniques to construct an artificial solid-electrolyte interphase layer to protect both electrode material and electrolyte. Despite these efforts, the solid-electrolyte interphase NEXT GENERATION ELECTRICAL ENERGY STORAGE PANEL 4 REPORT 117PDF Image | Next Generation Electrical Energy Storage
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