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  1. Abstract

    Silicon is regarded as one of the most promising anode materials for lithium-ion batteries. Its high theoretical capacity (4000 mAh/g) has the potential to meet the demands of high-energy density applications, such as electric air and ground vehicles. The volume expansion of Si during lithiation is over 300%, indicating its promise as a large strain electrochemical actuator. A Si-anode battery is multifunctional, storing electrical energy and actuating through volume change by lithium-ion insertion.

    To utilize the property of large volume expansion, we design, fabricate, and test two types of Si anode cantilevers with bi-directional actuation: (a) bimorph actuator andmore »(b) insulated double unimorph actuator. A transparent battery chamber is fabricated, provided with NCM cathodes, and filled with electrolyte. The relationship between state of charge and electrode deformation is measured using current integration and high-resolution photogrammetry, respectively. The electrochemical performance, including voltage versus capacity and Coulombic efficiency versus cycle number, is measured for several charge/discharge cycles. Both configurations exhibit deflections in two directions and can store energy. In case (a), the largest deflection is roughly 35% of the cantilever length. Twisting and unexpected bending deflections are observed in this case, possibly due to back-side lithiation, non-uniform coating thickness, and uneven lithium distribution. In case (b), the single silicon active coating layer can deflect 12 passive layers.

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    Free, publicly-accessible full text available September 14, 2022
  2. Abstract

    Silicon anodes have been demonstrated to provide significant actuation in addition to energy storage in lithium-ion batteries (LIBs). This work studies the optimization of 1D unimorph and bimorph actuators to achieve a target shape upon actuation. A 1D shape matching with design optimization is used to estimate the varied charge distribution along the length for a LIB actuator and thereby the effect of distance between electrodes in charging.

    A genetic algorithm (GA) is used with actuation strain distribution as the design variable. The objective of the optimization is to shape-match by minimizing the shape error between a target shapemore »and actuated shape, both defined by several points along the length.

    The approach is experimentally validated by shape matching a notched unimorph target shape. A shape error of 1.5% is obtained. An optimized unimorph converges to an objective function of less than 0.029% of the length at full state of charge (SOC) for a 5-segment beam.

    A second shape matching case study using a bimorph is investigated to showcase the tailorability of LIB actuators. The optimal bimorph achieves an objective function of less than 0.23% of the length for a design variable set of top and bottom actuation strain of an 8-segment beam. The actuated shape nearly matches the target shape by simultaneously activating top and bottom active layers to achieve the same differential actuation strain (the difference between top and bottom active layer actuation strain).

    The results show that a bimorph actuator can achieve a given shape while also storing significantly more charge than is necessary to maintain a given complex shape. This demonstrates a strength of energy storage based actuators: excess energy can be stored within the actuator and can be expended without affecting the work done or the shape maintained by the actuator.

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    Free, publicly-accessible full text available September 14, 2022
  3. Free, publicly-accessible full text available November 5, 2022
  4. Sections PDFPDF Tools Share Summary The low cost, simplicity, and easy use of battery‐powered multirotor aircraft has led to their adoption in commercial, industrial, agricultural, and military applications. These aircraft, however, have limited payloads and shorter endurance and range than fuel‐powered conventional aircraft. To extend these key performance metrics, a structural battery is developed that uses commercially available battery cells as load bearing and power source elements for weight critical applications. The cylindrical structural battery is tested in three‐point bending and is found to have four times higher stiffness and two times higher yield strength than the structure without batterymore »reinforcement. Simulations of a quadcopter, redesigned with the proposed cylindrical structural batteries, demonstrate 41% longer hover time.« less
  5. Instabilities during metal electrodeposition create dendrites on the plating surfaces. In high energy density lithium metal batteries (LMBs) dendrite growth causes safety issues and accelerated aging. In this paper, analytical models predict that dendrite growth can be controlled and potentially eliminated by small advective flows normal to the surface of lithium metal electrode. Electrolyte flow towards the Li metal electrode lowers the dendrite growth rate, overpotential, and impedance. Flow in the opposite direction, however, enhances the dendrite growth. For every current density, there exists a critical velocity above which dendrite growth can be totally eliminated. The critical velocity increases almostmore »linearly with increasing current density. For typical current densities and inter-electrode separation, the critical velocity is very small, indicating the potential for practical application.« less
  6. Among anode materials for Li-ion batteries, Si is known for high theoretical capacity, low cost, large volume change, relatively fast capacity fade and significant stress-potential coupling. This article shows that a Li(Ni0.5Co0.2Mn0.3)O2-Si battery can store energy, actuate with Si volume change and sense with stress-potential coupling. Experiments are conducted in an electrolyte-filled chamber with a glass window with Li(Ni0:5Co0:2Mn0:3)O2 cathodes and Si composite anodes. The Si anodes are single-side coated on Cu current collector with Si nanoparticles, polyacrylic acid binder and conductive carbon black in a porous composite structure. During charging, the battery stores energy, Li inserts in the cantileveredmore »Si anodes and the cantilevers bend laterally. Discharging the battery releases the stored energy and straightens the Si cantilevers. Imposing deformation on the Si cantilevers at a fixed state of charge causes bending stress in the composite coating and a change in the open circuit potential. Testing at 1Hz confirms that the Si composite responds to dynamic stress variations and with almost no phase lag, indicating the bandwidth of the stress-potential coupling in Si composite anodes is at least 1Hz.« less
  7. The temperature and capacity of individual cells affect the current distribution in a battery pack. Non uniform current distribution among parallel-connected cells can lead to capacity imbalance and premature aging. This paper develops models that calculate the current in parallel-connected cells and predict their capacity fade. The model is validated experimentally for a nonuniform battery pack at different temperatures. The paper also proposes and validates the hypothesis that temperature control can reduce capacity mismatch in parallel-connected cells. Three Lithium Iron Phosphate cells, two cells at higher initial capacity than the third cell, are connected in parallel. The pack is cycledmore »for 1500 Hybrid Electric Vehicles cycles with the higher capacity cells regulated at 40C and the lower capacity cell at 20C. As predicted by the model, the higher capacity and temperature cells age faster, reducing the capacity mismatch by 48% over the 1500 cycles. A case study shows that cooling of low capacity cells can reduce capacity mismatch and extend pack life.« less