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A persistent challenge with the development of electric vertical take-off and landing vehicles (eVTOL) to meet flight power and energy demands is the mass of the load and thermal management systems for batteries. One possible strategy to overcome this problem is to employ optimization techniques to obtain a lightweight battery pack while satisfying structural and thermal requirements. In this work, a structural battery pack with high-energy-density cylindrical cells is optimized using the level-set topology optimization method. The heat generated by the batteries is predicted using a high-fidelity electrochemical model for a given eVTOL flight profile. The worst-case scenario for the battery's heat generation is then considered as a source term in the weakly coupled steady-state thermomechanical finite element model used for optimization. The objective of the optimization problem is to minimize the weighted sum of thermal compliance and structural compliance subjected to a volume constraint. The methodology is demonstrated with numerical examples for different sets of weights. The optimized results due to different weights are compared, discussed, and evaluated with thermal and structural performance indicators. The optimized pack topologies are subjected to a transient thermal finite element analysis to assess the battery pack's thermal response., Comment: AeroBest 2023 II ECCOMAS Thematic Conference on Multidisciplinary Design Optimization of Aerospace Systems

Lithium fluorinated carbon is one of the most promising chemistries for high-energy-density primary energy storage systems in applications where rechargeability is not required. Though Li/CFx demonstrates high energy density under ambient conditions, achieving such a high energy density when exposed to subzero temperatures remains a challenge, particularly under high current density. Here, we report a liquefied gas electrolyte with an anion-pair solvation structure based on dimethyl ether with a low melting point and low viscosity, leading to high ionic conductivity between a wide temperature range. Besides that, through systematic X-ray photoelectron spectroscopy integrated with transmission electron microscopy characterizations, we evaluate the interface of CFx for low-temperature performance. We conclude that the fast transport and anion-pairing solvation structure of the electrolyte bring about reduced charge transfer resistance at low temperatures, which resulted in significantly enhanced performance of Li/CFx cells. Utilizing 50 mg/cm2 loading electrodes, the Li/CFx still displayed 1530 Wh/kg at reduced temperature. This work provides insights into the electrolyte design that may overcome the operational limits of batteries in extreme environments.

Structural and ion-ordering phase transitions limit the viability of sodium-ion intercalation materials in grid scale battery storage by reducing their lifetime. However, the combination of phenomena in nanoparticulate electrodes creates complex behavior that is difficult to investigate, especially on the single nanoparticle scale under operating conditions. In this work, operando single-particle x-ray diffraction (oSP-XRD) is used to observe single-particle rotation, interlayer spacing, and layer misorientation in a functional sodium-ion battery. oSP-XRD is applied to Na$_{2/3}$[Ni$_{1/3}$Mn$_{2/3}$]O$_{2}$, an archetypal P2-type sodium-ion positive electrode material with the notorious P2-O2 phase transition induced by sodium (de)intercalation. It is found that during sodium extraction, the misorientation of crystalline layers inside individual particles increases before the layers suddenly align just prior to the P2-O2 transition. The increase in the long-range order coincides with an additional voltage plateau signifying a phase transition prior to the P2-O2 transition. To explain the layer alignment, a model for the phase evolution is proposed that includes a transition from localized to correlated Jahn-Teller distortions. The model is anticipated to guide further characterization and engineering of sodium-ion intercalation materials with P2-O2 type transitions. oSP-XRD therefore opens a powerful avenue for revealing complex phase behavior in heterogeneous nanoparticulate systems., Comment: 23 pages, 4 main figures, 9 supplemental figures

The relationship between charge and structure dictates the properties of electrochemical systems. For example, reversible Na-ion intercalation - a low-cost alternative to Li-ion technology - often induces detrimental structural phase transformations coupled with charge compensation reactions. However, little is known about the underpinning charge-structure mechanisms because the reduction-oxidation (redox) reactions within coexisting structural phases have so far eluded direct operando investigation. Here, we distinguish x-ray spectra of individual crystalline phases operando during a redox-induced phase transformation in P2-Na2/3Ni1/3Mn2/3O2 - an archetypal layered oxide for sodium-ion batteries. We measure the resonant elastic scattering on the Bragg reflection corresponding to the P2-phase lattice spacing. These resonant spectra become static midway through the sodium extraction in an operando coin cell, while the overall sodium extraction proceeds as evidenced by the X-ray absorption averaging over all electrochemically active Ni atoms. The stop of redox activity in the P2-structure signifies its inability to host Ni4+ ions. The coincident emergence of the O2- structure reveals the rigid link between the local redox and the long-range order during the phase transformation. The structure-selective x-ray spectroscopy thus opens a powerful avenue for resolving the dynamic chemistry of different structural phases in multi-phase electrochemical systems.

Graphite fluoride (CFx) cathodes coupled with lithium anodes yield one of the highest theoretical energy densities (>860 Wh/g) among primary batteries. In practice, the observed discharge voltage (~2.5 V) is significantly lower than thermodynamic limits (>4.5 V), the discharge rate is low, and so far Li/CFx has only been used in primary batteries. Understanding the discharge mechanism at atomic length scales will improve practical CFx energy density, rate capability, and rechargeability. So far, purely experimental techniques have not identified the correct discharge mechanism or explained the discharge voltage. We apply Density Functional Theory calculations to demonstrate that a CFx-edge propagation discharge mechanism based on lithium insertion at the CF/C boundary in partially discharged CFx exhibits a voltage range of 2.5 to 2.9 V -- depending on whether solvent molecules are involved. The voltages and solvent dependence agrees with our discharge and galvanostatic intermittent titration technique measurements. The predicted discharge kinetics are consistent with CFx operations. Finally, we predict Li/CFx rechargeability under the application of high potentials, along a charging pathway different from that of discharge. Our work represents a general, quasi-kinetic framework to understand the discharge of conversion cathodes, circumventing the widely used phase diagram approach which most likely does not apply to Li/CFx because equilibrium conditions are not attained in this system., Comment: 31 pages, 7 figures

Lithium phosphorus oxynitride (LiPON) is an amorphous solid-state lithium ion conductor displaying exemplary cyclability against lithium metal anodes. There is no definitive explanation for this stability due to the limited understanding of the structure of LiPON. We provide a structural model of RF-sputtered LiPON via experimental and computational spectroscopic methods. Information about the short-range structure results from 1D and 2D solid-state nuclear magnetic resonance experiments investigating chemical shift anisotropy and dipolar interactions. These results are compared with first principles chemical shielding calculations of Li-P-O/N crystals and ab initio molecular dynamics-generated amorphous LiPON models to unequivocally identify the glassy structure as primarily isolated phosphate monomers with N incorporated in both apical and as bridging sites in phosphate dimers. Structural results suggest LiPON's stability is a result of its glassy character. Free-standing LiPON films are produced that exhibit a high degree of flexibility highlighting the unique mechanical properties of glassy materials.