Engineering topics
Li, Ning
Publications and source records attributed to Li, Ning.
Numerical wave modeling for operational and survival analyses of wave energy converters at the US Navy Wave Energy Test Site in Hawaii
The US Navy Wave Energy Test Site (WETS) is currently operating three grid-connected berths off Marine Corps Base Hawaii in support of technological development through scaled-model testing and pre-commercial prototyping. The location on the east shore of Oahu is open to year-round wind waves and seasonal north swells as well as severe seas from subtropical systems and hurricanes. We have assembled a spectral wave model system comprising WAVEWATCH III and SWAN on a hierarchy of global, regional, and nearshore computational grids. With wind forcing from global forecast and rean-alysis datasets as well as their regional downscaling, the system produces operational 7-day wave forecasts and a long-term hindcast. The daily forecasts, validated with real-time buoy measurements, facilitate safe deployment, operation, and retrieval of wave energy converters (WECs). The wave hindcast from 1979 to 2017 defines the intra- and inter-annual variations for statistical analysis of sea states and energy resources. Due to limited occurrences of hurricanes in Hawaii waters, their wind forcing is determined parametrically with simulated tracks from climate model downscaling. Finally, systematic analysis of the hindcast and simulation datasets provides significant wave heights from trade winds, swells, subtropical systems, and hurricanes with return periods up to 100 years for WECs survival analysis.
Superfluid condensate fraction and pairing wave function of the unitary Fermi gas
The unitary Fermi gas is a many-body system of two-component fermions with zero-range interactions tuned to infinite scattering length. Despite much activity and interest in unitary Fermi gas and its universal properties, there have been great difficulties in performing accurate calculations of the superfluid condensate fraction and pairing wave function. In this paper, we present auxiliary-field lattice Monte Carlo simulations using a lattice interaction which accelerates the approach to the continuum limit, thereby allowing for robust calculations of these difficult observables. As a benchmark test, we compute the ground-state energy of 33 spin-up and 33 spin-down particles. As a fraction of the free Fermi gas energy $E_{\text{FG}}$, we find $E_0/E_{\text{FG}}$ = 0.369(2), 0.372(2), using two different definitions of the finite-system energy ratio, in agreement with the latest theoretical and experimental results. We then determine the condensate fraction by measuring off-diagonal long-range order in the two-body density matrix. We find that the fraction of condensed pairs is α = 0.43(2). Further, we also extract the pairing wave function and find the pair correlation length to be $ζ_pk_F$ = 1.8(3)ℏ, where $k_F$ is the Fermi momentum. Provided that the simulations can be performed without severe sign oscillations, the methods we present here can be applied to superfluid neutron matter as well as more exotic $\textit{P}$-wave and $\textit{D}$-wave superfluids.
Redox Behaviors in Li-Excess Cation-Disordered Mn-Nb-O-F Rocksalt Cathode
Lithium-excess cation-disordered rocksalt oxide (DRX) is a new choice of high energy Li-ion cathode materials. DRX cathode can deliver an exceptionally high capacity of >250 mAh/g, implying the participation of cationic transition metal and anionic oxygen redox in the electrochemistry. Understanding the explicit roles of both redox reactions in the newly developed cathode is of critical importance to further development of these materials. Herein, we synthesize a representative Mn-redox-based fluorinated DRX material, Li1.2Mn0.625Nb0.175O1.95F0.05 (LMNOF). Using a comprehensive suite of electroanalytical methods, we probe the cationic and anionic redox processes and their evolution upon cycling exclusively from the electrochemical point of view. This work sheds light on understanding of cationic and anionic redox and their roles in the capacity loss for high-capacity fluorinated DRX cathode, meanwhile, showcases an explicit approach to study the electrochemistry of novel electrode materials using such non-disruptive electroanalytical methods.
High-Capacity P2-Type Na x Li 0.25 Mn 0.75 O 2 Cathode Enabled by Anionic Oxygen Redox
Sodium-ion battery technology has attracted significant attention due to its substantial cost advantage and similar operating mechanism to Li-ion batteries. P2-type sodium manganese oxide cathode is one of the most promising candidates, demonstrating both high capacity and good cycling stability. Here, we explore the lattice oxygen activity in layered sodium transition metal oxides. We synthesize a series of sodium lithium manganese oxides, NaxLi 0.25 Mn 0.75 O2 (x = 0.75 – 0.833), to optimize Na content. We further investigate the charge compensation mechanism for the best performing Na 0.75 Li 0.25 Mn 0.75 O 2 over an extensive electrochemical cycling window. The large charge and discharge capacity is enabled by reversible lattice oxygen redox in the high voltage region (≥2.5 V), along with Mn redox at the voltages below 2.5 V. Additionally, we reveal a small amount of oxygen gas evolution, 0.04% of the total oxygen in Na 0.25 Li 0.25 Mn 0.75 O 2 . This initial study will trigger an interest in the lattice oxygen activity in layered sodium metal oxide cathode, therefore, leading to better understanding of its correlation with crystal structure and electrochemical performance.
Unraveling the Voltage Decay Phenomenon in Li-Rich Layered Oxide Cathode of No Oxygen Activity
Extensive efforts have been devoted to unraveling the true cause of voltage decay in Li, Mn-rich layered oxides. An initial consensus was reached on structural rearrangement, then leaned towards the newly discovered lattice oxygen activity. It is challenging to differentiate their explicit roles because these events typically co-exist during the electrochemical reaction of most Li-rich layered oxides. Here, we probe the voltage decay behavior in Li 1.2 Ni 0.2 Ru 0.6 O 2 , a structurally and electrochemically relevant compound to Li, Mn-rich layered oxide, but of no oxygen activity. Such intriguing characteristics allow us to explicitly decouple the contribution of transition metal migration and lattice oxygen activity to voltage decay in Li-rich layered oxides. Our results demonstrate that the microstructural evolution, mainly originating from transition metal migration, is a direct cause of voltage decay, and lattice oxygen activity likely accelerates the decay.
High-Resolution Regional Wave Hindcast for Hawaii
This report summarizes modeling efforts for hindcasting of wave climate within the exclusive economic zone (EEZ) around the state of Hawaii. Specifically, the report provides detailed description of model mesh development, sensitivity analysis of the configuration of model parameters, data used for model forcing and validation, model skill assessment, and model outputs for resource characterization. The unstructured, nested-grid modeling approach incorporates a regional high-resolution SWAN (Simulating WAves Nearshore) model within the global-regional WAVEWATCH III® (WW3) wave model. The models were configured to comply with the International Electrotechnical Commission (IEC) Technical Standards for wave energy resource characterization. The SWAN model domain covers the entire EEZ with a spatial resolution of approximately 300 m in the nearshore region. Two levels of WW3 nested grids, from 0.5 arc-degree at global to 7.5 arc-minute at regional scale, were used to provide boundary conditions that drive the high-resolution nearshore SWAN model. The WW3 and SWAN models were forced by 0.5 arc-degree Climate Forecast System Reanalysis wind and 5 km regional Weather Research and Forecasting wind, respectively. Wave resource and bulk wave parameters were simulated for a 32-year period from 1979 to 2010, and were subsequently validated using wave buoy data within the model domain. Model performance was evaluated using a set of error statistics. Overall, the model shows good skills in reproducing the wave climate in the Hawaii EEZ in both spatial and temporal domains. Comparisons of time series and scatter plots for the six IEC resource parameters and the associated error statistics tables, and horizontal two-dimensional plots of annually averaged and monthly averaged IEC parameters are provided.
Unraveling the Cationic and Anionic Redox Reactions in a Conventional Layered Oxide Cathode
Increasing interest in high-energy lithium-ion batteries has triggered the demand to clarify the reaction mechanism in battery cathodes during high-potential operation. Yet, the reaction mechanism often involves both transition-metal and oxygen activities that remain elusive. Here we report a comprehensive study of both cationic and anionic redox mechanisms of LiNiO 2 nearly full delithiation. Selection of pure LiNiO 2 removes the complication of multiple transition metals. Using combined X-ray absorption spectroscopy, resonant inelastic X-ray scattering, and operando differential electrochemical mass spectrometry, we are able to clarify the redox reactions of transition metals in the bulk and at the surface, reversible lattice oxygen redox, and irreversible oxygen release associated with surface reactions. Many results introduced here bring attention to different types of oxygen activities and metal-oxygen interactions in layered oxides, which are of crucial importance to the advancement of a Ni-rich layered oxide cathode for high capacity and long cycling performance.
Surpassing the 10% efficiency milestone for 1-cm 2 all-polymer solar cells
Naphthalenediimide-based n-type polymeric semiconductors are extensively used for constructing high-performance all-polymer solar cells (all-PSCs). For such all-polymer systems, charge recombination can be reduced by using thinner active layers, yet suffering insufficient near-infrared light harvesting from the polymeric acceptor. Conversely, increasing the layer thickness overcomes the light harvesting issue, but at the cost of severe charge recombination effects. Here we demonstrate that to manage light propagation within all-PSCs, a thick bulk-heterojunction film of approximately 350 nm is needed to effectively enhance photo-harvesting in the near-infrared region. To overcome the severe charge recombination in such a thick film, a non-halogenic additive is used to induce a well-ordered micro-structure that inherently suppresses recombination loss. The combined strategies of light management and delicate morphology optimization lead to a promising efficiency over 10% for thick-film all-PSCs with active area of 1 cm 2 , showing great promise for future large-scale production and application of all-PSCs.
Time fractals and discrete scale invariance with trapped ions
We show that a one-dimensional chain of trapped ions can be engineered to produce a quantum mechanical system with discrete scale invariance and fractal-like time dependence. By discrete scale invariance we mean a system that replicates itself under a rescaling of distance for some scale factor, and a time fractal is a signal that is invariant under the rescaling of time. These features are reminiscent of the Efimov effect, which has been predicted and observed in bound states of three-body systems. We demonstrate that discrete scale invariance in the trapped ion system can be controlled with two independently tunable parameters. We also discuss the extension to n-body states where the discrete scaling symmetry has an exotic heterogeneous structure. Furthermore, the results we present can be realized using currently available technologies developed for trapped ion quantum systems.
Galilean invariance restoration on the lattice
We consider the breaking of Galilean invariance due to different lattice cutoff effects in moving frames and a nonlocal smearing parameter, which is used in the construction of the nuclear lattice interaction. The dispersion relation and neutron-proton scattering phase shifts are used to investigate the Galilean invariance breaking effects and ways to restore it. For S-wave channels, 1 S 0 and 3 S 1 , we present the neutron-proton scattering phase shifts in moving frames calculated using both Lüscher's formula and the spherical wall method, as well as the dispersion relation. For the P and D waves, we present the neutron-proton scattering phase shifts in moving frames calculated using the spherical wall method. Furthermore, we find that the Galilean invariance breaking effects stemming from the lattice artifacts partially cancel those caused by the nonlocal smearing parameter. Due to this cancellation, the Galilean invariance breaking effect is small, and the Galilean invariance can be restored by introducing Galilean invariance restoration operators.
Bulk heterojunction organic photovoltaic cells made by glancing angle deposition
A method of making a bulk heterojunction organic photovoltaic cell by glancing angle deposition. As the disclosed method relies on a trajectory of incident vapor flux that is not parallel to the substrate normal, micro and nano-scale columnar structures of thin films can be grown on the substrate. There is also disclosed a method of forming a donor-acceptor heterojunction by depositing at least one additional organic material over the columnar structures.