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1. Lithium‐Battery Anode Gains Additional Functionality for Neuromorphic Computing through Metal–Insulator Phase Separation

   https://doi.org/10.1002/adma.201907465  

   Gonzalez‐Rosillo, Juan Carlos ; Balaish, Moran ; Hood, Zachary D. ; Nadkarni, Neel ; Fraggedakis, Dimitrios ; Kim, Kun Joong ; Mullin, Kaitlyn M. ; Pfenninger, Reto ; Bazant, Martin Z. ; Rupp, Jennifer L. M. ( January 2020 , Advanced Materials)

   Specialized hardware for neural networks requires materials with tunable symmetry, retention, and speed at low power consumption. The study proposes lithium titanates, originally developed as Li‐ion battery anode materials, as promising candidates for memristive‐based neuromorphic computing hardware. By using ex‐ and in operando spectroscopy to monitor the lithium filling and emptying of structural positions during electrochemical measurements, the study also investigates the controlled formation of a metallic phase (Li₇Ti₅O₁₂) percolating through an insulating medium (Li₄Ti₅O₁₂) with no volume changes under voltage bias, thereby controlling the spatially averaged conductivity of the film device. A theoretical model to explain the observed hysteretic switching behavior based on electrochemical nonequilibrium thermodynamics is presented, in which the metal‐insulator transition results from electrically driven phase separation of Li₄Ti₅O₁₂and Li₇Ti₅O₁₂. Ability of highly lithiated phase of Li₇Ti₅O₁₂for Deep Neural Network applications is reported, given the large retentions and symmetry, and opportunity for the low lithiated phase of Li₄Ti₅O₁₂toward Spiking Neural Network applications, due to the shorter retention and large resistance changes. The findings pave the way for lithium oxides to enable thin‐film memristive devices with adjustable symmetry and retention.

    

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2. Vanadium and Niobium MXenes—Bilayered V 2 O 5 Asymmetric Supercapacitors

   https://doi.org/10.1002/smtd.202201551  

   Saraf, Mohit ; Zhang, Teng ; Averianov, Timofey ; Shuck, Christopher E. ; Lord, Robert W. ; Pomerantseva, Ekaterina ; Gogotsi, Yury ( February 2023 , Small Methods)

   MXenes offer high metallic conductivity and redox capacitance that are attractive for high‐power, high‐energy storage devices. However, they operate limitedly under high anodic potentials due to irreversible oxidation. Pairing them with oxides to design asymmetric supercapacitors may expand the voltage window and increase the energy storage capabilities. Hydrated lithium preintercalated bilayered V₂O₅(δ‐Li_xV₂O₅·nH₂O) is attractive for aqueous energy storage due to its high Li capacity at high potentials; however, its poor cyclability remains a challenge. To overcome its limitations and achieve a wide voltage window and excellent cyclability, it is combined with V₂C and Nb₄C₃MXenes. Asymmetric supercapacitors employing lithium intercalated V₂C (Li‐V₂C) or tetramethylammonium intercalated Nb₄C₃(TMA‐Nb₄C₃) MXenes as the negative electrode, and aδ‐Li_xV₂O₅·nH₂O composite with carbon nanotubes as the positive electrode in 5 mLiCl electrolyte operate over wide voltage windows of 2 and 1.6 V, respectively. The latter shows remarkably high cyclability—capacitance retention of ≈95% after 10 000 cycles. This work highlights the importance of selecting appropriate MXenes to achieve a wide voltage window and a long cycle life in combination with oxide anodes to demonstrate the potential of MXenes beyond Ti₃C₂in energy storage.

    

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3. Free-standing bilayered vanadium oxide films synthesized by liquid exfoliation of chemically preintercalated δ-Li x V 2 O 5 · n H 2 O

   https://doi.org/10.1039/D1MA00085C  

   Houseman, Luke ; Mukherjee, Santanu ; Andris, Ryan ; Zachman, Michael J. ; Pomerantseva, Ekaterina ( April 2021 , Materials Advances)

   null (Ed.)

   A free-standing film composed of bilayered vanadium oxide nanoflakes is for the first time synthesized using a new low-energy process. The precursor powder, δ-Li x V 2 O 5 · n H 2 O, was prepared using a simple sol–gel based chemical preintercalation synthesis procedure. δ-Li x V 2 O 5 · n H 2 O was dispersed and probe sonicated in N -methyl pyrrolidone to exfoliate the bilayers followed by vacuum filtration resulting in the formation of a free-standing film with obsidian color. X-ray diffraction showed lamellar ordering of a single-phase material with a decreased interlayer distance compared to that of the precursor powder. Scanning electron microscopy images demonstrated stacking of the individual nanoflakes. This morphology was further confirmed with scanning transmission electron microscopy that showed highly malleable nanoflakes consisting of ∼10–100 vanadium oxide bilayers. One of the most important consequences of this morphological rearrangement is that the electronic conductivity of the free-standing film, measured by the four-probe method, increased by an order of magnitude compared to conductivity of the pressed pellet made of precursor powder. X-ray photoelectron spectroscopy measurements showed the coexistence of both V 5+ and V 4+ oxidation states in the exfoliated sample, possibly contributing to the change in electronic conductivity. The developed approach provides the ability to maintain the phase purity and crystallographic order during the exfoliation process, coupled with the formation of a free-standing film of enhanced conductivity. The produced bilayered vanadium oxide nanoflakes can be used as the building blocks for the synthesis of versatile two-dimensional heterostructures to create innovative electrodes for electrochemical energy storage applications. 

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4. Electrochemical evaluation of porous CaFe2O4 anode material prepared via solution combustion synthesis at increasing fuel-to-oxidizer ratios and calcination temperatures

   https://doi.org/10.1038/s41598-022-07036-3  

   Strimaitis, Jacob ; Danquah, Samuel A. ; Denize, Clifford ; Pradhan, Sangram K. ; Bahoura, Messaoud ( February 2022 , Scientific Reports)

   The drawbacks of common anodes in lithium-ion batteries (LIBs) and hybrid supercapacitors (HSCs), such as the high voltage plateau of Li₄Ti₅O₁₂(1.55 V vs. Li/Li⁺) and the moderate capacity of graphite (372 mAh-g^-1), have established a need for better materials. Conversion materials, and in particular iron oxide and CaFe₂O₄(CFO), have amassed recent attention as potential anode replacements. In this study, we evaluate the material and electrochemical effects of the solution combustion synthesis (SCS) of porous CFO across novel fuel-to-oxidizer ratios and calcination temperatures. We demonstrate that nearly doubling the amount of fuel used during synthesis increases capacities between 120 and 150% at high current densities (~ 1000 mA-g^-1) and across 500 additional charging-discharging cycles, an effect brought on in part by enhanced compositional purity in these samples. However, in order to ensure long-term cyclic stability, it is necessary to also calcine porous CFO to 900 °C to enhance crystallite size, particle size and spacing, and compositional purity.

    

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5. Defect generation in TiO 2 nanotube anodes via heat treatment in various atmospheres for lithium-ion batteries

   https://doi.org/10.1039/c8cp04368j  

   Savva, Andreas I. ; Smith, Kassiopeia A. ; Lawson, Matthew ; Croft, Sterling R. ; Weltner, Ariel E. ; Jones, Chris D. ; Bull, Hailey ; Simmonds, Paul J. ; Li, Lan ; Xiong, Hui ( January 2018 , Physical Chemistry Chemical Physics)

   In this paper, ordered TiO 2 nanotubes were grown on a Ti substrate via electrochemical anodization and subsequently annealed at 450 °C for 4 h under various atmospheres to create different point defects. Oxygen-deficient environments such as Ar and N 2 were used to develop oxygen vacancies, while a water vapor (WV) atmosphere was used to generate titanium vacancies. Computational models by density functional theory predicted that the presence of oxygen vacancies would cause electronic conductivity to increase, while the presence of Ti vacancies could lead to decreased conductivity. The predictions were confirmed by two-point electrical conductivity measurements and Mott–Schottky analysis. Raman spectroscopy was also conducted to confirm the presence of defects. The annealed samples were then evaluated as anodes in lithium-ion batteries. The oxygen-deficient samples had an improvement in capacity by 10% and 25% for Ar- and N 2 -treated samples, respectively, while the WV-treated sample displayed a capacity increase of 24% compared to the stoichiometric control sample (annealed in O 2 ). Electrochemical impedance spectroscopy studies revealed that the WV-treated sample's increased capacity was a consequence of its higher Li diffusivity. The results suggest that balanced electrical and ionic conductivity in nanostructured metal oxide anodes can be tuned through defect generation using heat treatments in various atmospheres for improved electrochemical properties. 

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