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1. Experimental Study of NCM-Si Batteries With Bi-Directional Actuation

   https://doi.org/10.1115/SMASIS2021-67596  

   Shan, Shuhua; Gonzalez, Cody; Rahn, Christopher; Frecker, Mary (September 2021, 2021 ASME SMASIS)

   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 and (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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2. Multifunctional Li(Ni0.5Co0.2Mn0.3) O2-Si batteries with self-actuation and self-sensing

   Jun Ma, Cody Gonzalez (January 2020, Journal of intelligent material systems and structures)

   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 cantilevered 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. 

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3. ANALYTICAL MODELING OF A MULTIFUNCTIONAL SEGMENTED LITHIUM ION BATTERY UNIMORPH ACTUATOR

   https://doi.org/10.1115/SMASIS2018-8123  

   Cody Gonzalez, Jun Ma (January 2018, SMASIS 2018)

   Silicon anodes in lithium ion batteries have high theoretical capacity and large volumetric expansion. In this paper, both characteristics are used in a segmented unimorph actuator consisting of several Si composite anodes on a copper current collector. Each unimorph segment is self-actuating during discharge and the discharge power can be provided to external circuits. With no external forces and zero current draw, the unimorph segments will maintain their actuated shape. Stresspotential coupling allows for the unimorph actuator to be self-sensing because bending changes the anodes’ potential. An analytical model is derived from a superposition of pure bending and extensional deformation forces and moments induced by the cycling of a Si anode. An approximately linear relationship between axial strain and state of charge of the anode drives the bending displacement of the unimorph. The segmented device consists of electrically insulated and individually controlled segments of the Si-coated copper foil to allow for variable curvature throughout the length of the beam. The model predicts the free deflection along the length of the beam and the blocked force. Tip deflection and blocked force are shown for a range of parameters including segment thicknesses, beam length, number of segments, and state of charge. The potential applications of this device include soft robots and dexterous 3D grippers. 

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4. A BF ₃ ‐Doped MXene Dual‐Layer Interphase for a Reliable Lithium‐Metal Anode

   https://doi.org/10.1002/adma.202210111  

   Shang, Mingwei; Shovon, Osman_Goni; Wong, Francis_En_Yoong; Niu, Junjie (December 2022, Advanced Materials)

   Abstract A dual‐layer interphase that consists of an in‐situ‐formed lithium carboxylate organic layer and a thin BF₃‐doped monolayer Ti₃C₂MXene on Li metal is reported. The honeycomb‐structured organic layer increases the wetting of electrolyte, leading to a thin solid electrolyte interface (SEI). While the BF₃‐doped monolayer MXene provides abundant active sites for lithium homogeneous nucleation and growth, resulting in about 50% reduced thickness of inorganic‐rich components among the SEI layer. A low overpotential of less than 30 mV over 1000 h cycling in symmetric cells is received. The functional BF₃ groups, along with the excellent electronic conductivity and smooth surface of the MXene, greatly reduce the lithium plating/stripping energy barrier, enabling a dendrite‐free lithium‐metal anode. The battery with this dual‐layer coated lithium metal as the anode displays greatly improved electrochemical performance. A high capacity‐retention of 175.4 mAh g⁻¹at 1.0 C is achieved after 350 cycles. In a pouch cell with a capacity of 475 mAh, the battery still exhibits a high discharge capacity of 165.6 mAh g⁻¹with a capacity retention of 90.2% after 200 cycles. In contrast to the fast capacity decay of pure Li metal, the battery using NCA as the cathode also displays excellent capacity retention in both coin and pouch cells. The dual‐layer modified surface provides an effective approach in stabilizing the Li‐metal anode. 

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5. First principles and experimental studies of empty Si ₄₆ as anode materials for Li-ion batteries

   https://doi.org/10.1557/jmr.2016.408  

   Chan, Kwai S.; Miller, Michael A.; Liang, Wuwei; Ellis-Terrell, Carol; Chan, Candace K. (December 2016, Journal of Materials Research)

   The objective of this investigation was to utilize the first-principles molecular dynamics computational approach to investigate the lithiation characteristics of empty silicon clathrates (Si 46 ) for applications as potential anode materials in lithium-ion batteries. The energy of formation, volume expansion, and theoretical capacity were computed for empty silicon clathrates as a function of Li. The theoretical results were compared against experimental data of long-term cyclic tests performed on half-cells using electrodes fabricated from Si 46 prepared using a Hofmann-type elimination–oxidation reaction. The comparison revealed that the theoretically predicted capacity (of 791.6 mAh/g) agreed with experimental data (809 mAh/g) that occurred after insertion of 48 Li atoms. The calculations showed that overlithiation beyond 66 Li atoms can cause large volume expansion with a volume strain as high as 120%, which may correlate to experimental observations of decreasing capacities from the maximum at 1030 mAh/g to 553 mA h/g during long-term cycling tests. The finding suggests that overlithiation beyond 66 Li atoms may have caused damage to the cage structure and led to lower reversible capacities. 

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