Predicting the Electrochemical Properties of MnO2 Nanomaterials Used in Rechargeable Li Batteries: Simulating Nanostructure at the Atomistic Level
ABSTRACT Nanoporous beta-MnO2 can act as a host lattice for the insertion and deinsertion of Li with application in rechargeable lithium batteries. We predict that, to maximize its electrochemical properties, the beta-MnO2 host should be symmetrically porous and heavily twinned. In addition, we predict that there exists a "critical (wall) thickness" for MnO2 nanomaterials above which the strain associated with Li insertion is accommodated via a plastic, rather than elastic, deformation of the host lattice leading to property fading upon cycling. We predict that this critical thickness lies between 10 and 100 nm for beta-MnO2 and is greater than 100 nm for alpha-MnO2: the latter accommodates 2 x 2 tunnels compared with the smaller 1 x 1 tunnels found in beta-MnO2. This prediction may help explain why certain (nano)forms of MnO2 are electrochemically active, while others are not. Our predictions are based upon atomistic models of beta-MnO2 nanomaterials. In particular, a systematic strategy, analogous to methods widely and routinely used to model crystal structure, was used to generate the nanostructures. Specifically, the (space) symmetry associated with the nanostructure coupled with basis nanoparticles was used to prescribe full atomistic models of nanoparticles (0D), nanorods (1D), nanosheets (2D), and nanoporous (3D) architectures. For the latter, under MD simulation, the amorphous nanoparticles agglomerate together with their periodic neighbors to formulate the walls of the nanomaterial; the particular polymorphic structure was evolved using simulated amorphization and crystallization. We show that our atomistic models are in accord with experiment. Our models reveal that the periodic framework architecture, together with microtwinning, enables insertion of Li anywhere on the (internal) surface and facilitates Li transport in all three spatial directions within the host lattice. Accordingly, the symmetrically porous MnO2 can expand and contract linearly and crucially elastically under charge/discharge. We also suggest tentatively that our predictions for MnO2 are more general in that similar arguments may apply to other nanomaterials, which might expand and contract elastically upon charging/discharging.
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ABSTRACT: Central to porous nanomaterials, with applications spanning catalysts to fuel cells is their (perceived) "fragile" structure, which must remain structurally intact during application lifespan. Here, we use atomistic simulation to explore the mechanical strength of a porous nanomaterial as a first step to characterizing the structural durability of nanoporous materials. In particular, we simulate the mechanical deformation of mesoporous Li-MnO(2) under stress using molecular dynamics simulation. Specifically, such rechargeable Li-ion battery materials suffer volume changes during charge/discharge cycles as Li ions are repeatedly inserted and extracted from the host beta-MnO(2) causing failure as a result of localized stress. However, mesoporous beta-MnO(2) does not suffer structural collapse during cycling. To explain this behavior, we generate a full atomistic model of mesoporous beta-MnO(2) and simulate localized stress associated with charge/discharge cycles. We calculate that mesoporous beta-MnO(2) undergoes a volume expansion of about 16% when Li is fully intercalated, which can only be sustained without structural collapse, if the nanoarchitecture is symmetrically porous, enabling elastic deformation during intercalation. Conversely, we predict that unsymmetric materials, such as nanoparticulate beta-MnO(2), deform plastically, resulting in structural collapse of (Li) storage sites and blocked transport pathways; animations revealing elastic and plastic deformation mechanisms under mechanical load and crystallization of mesoporous Li-MnO(2) are presented at the atomistic level.ACS Nano 10/2009; 3(10):3308-14. DOI:10.1021/nn9009592 · 12.03 Impact Factor
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ABSTRACT: High power rechargeable lithium batteries are a key target for transport and load leveling, in order to mitigate CO(2) emissions. It has already been demonstrated that mesoporous lithium intercalation compounds (composed of particles containing nanometer diameter pores separated by walls of similar size) can deliver high rate (power) and high stability on cycling. Here we investigate how the critical dimensions of pore size and wall thickness control the rate of intercalation (electrode reaction). By using mesoporous beta-MnO(2), the influence of these mesodimensions on lithium intercalation via single and two-phase intercalation processes has been studied in the same material enabling direct comparison. Pore size and wall thickness both influence the rate of single and two-phase intercalation mechanisms, but the latter is more sensitive than the former.Journal of the American Chemical Society 01/2010; 132(3):996-1004. DOI:10.1021/ja905488x · 11.44 Impact Factor
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ABSTRACT: A bioinorganic nanohybrid glucose-responsive membrane is developed for self-regulated insulin delivery analogous to a healthy human pancreas. The application of MnO2 nanoparticles as a multifunctional component in a glucose-responsive, protein-based membrane with embedded pH-responsive hydrogel nanoparticles is proposed. The bio-nanohybrid membrane is prepared by crosslinking bovine serum albumin (BSA)–MnO2 nanoparticle conjugates with glucose oxidase and catalase in the presence of poly(N-isopropyl acrylamide-co-methacrylic acid) nanoparticles. The preparation and performance of this new nanocomposite material for a glucose-responsive insulin release system is presented. The activity and stability of immobilized glucose oxidase and the morphology and mechanical properties of the membrane are investigated. The enzymatic activity is well preserved in the membranes. The use of MnO2 nanoparticles not only reinforces the mechanical strength and the porous structure of the BSA-based membrane, but enhances the long-term stability of the enzymes. The in vitro release of insulin across the membrane is modulated by changes in glucose concentration mimicking possible fluctuations of blood-glucose level in diabetic patients. A four-fold increase in insulin permeation is observed when the glucose concentration is increased from normal to hyperglycemic levels, which returns to the baseline level when the glucose concentration is reduced to a normal level.Advanced Functional Materials 05/2010; 20(9):1404 - 1412. DOI:10.1002/adfm.200901581 · 10.44 Impact Factor