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Tang X.,Hong Kong University of Science and Technology | Liu B.,Hong Kong University of Science and Technology | Lv Z.,Guangzhou HKUST Fok Ying Tung Research Institute | Gao F.,Hong Kong University of Science and Technology | Gao F.,Guangzhou HKUST Fok Ying Tung Research Institute
Applied Energy | Year: 2017

Sensor drifts and modelling mismatches are key factors that influence the accuracy of state of charge (SOC) estimation for LiFePO4 batteries. In this study, an observer robust to these factors is proposed. First, the causes of SOC errors, for example, modelling error and uncertain initial error, are studied. Second, a geometry classifier is designed to categorize these errors into different groups using the information of voltage error between model and measurements. Third, with the classifier, different types of errors are treated differently by switching the gains of the observer. Finally, the method is tested in comparison to the existing methods for both new and aged cells. The test results show that the proposed method can correctly categorize the error causes and take the corresponding countermeasures. The common problems encountered in SOC estimations, such as local model inaccuracy, current sensor drifting and data saturation, could be overcome. The computation time of the proposed method is close to that of the Luenberger observer, making it suitable for real embedded applications. © 2017 Elsevier Ltd.

Gao X.-W.,University of Wollongong | Deng Y.-F.,South China University of Technology | Wexler D.,University of Wollongong | Chen G.-H.,Hong Kong University of Science and Technology | And 4 more authors.
Journal of Materials Chemistry A | Year: 2015

Conductive polypyrrole (PPy)-coated LiNi0.5Mn1.5O4 (LNMO) composites are applied as cathode materials in Li-ion batteries, and their electrochemical properties are explored at both room and elevated temperature. The morphology, phase evolution, and chemical properties of the as-prepared samples are analyzed by means of X-ray powder diffraction, thermogravimetric analysis, Raman spectroscopy, X-ray photoelectron spectroscopy and scanning and transmission electron microscopy techniques. The composite with 5 wt% polypyrrole coating shows a discharge capacity retention of 92% after 300 cycles and better rate capability than the bare LNMO electrode in the potential range of 3.5-4.9 V vs. Li/Li+ at room temperature. At the elevated temperature, the cycling performance of the electrode made from LNMO-5 wt% PPy is also remarkably stable (∼91% capacity retention after 100 cycles). It is revealed that the polypyrrole coating can suppress the dissolution of manganese in the electrolyte which occurs during cycling. The charge transfer resistance of the composite electrode is much lower than that of the bare LNMO electrode after cycling, indicating that the polypyrrole coating significantly increases the electrical conductivity of the LNMO electrode. Polypyrrole can also work as an effective protective layer to suppress the electrolyte decomposition arising from undesirable reactions between the cathode electrode and electrolyte on the surface of the active material at elevated temperature, leading to high coulombic efficiency. © The Royal Society of Chemistry 2015.

Deng Y.,South China University of Technology | Deng Y.,Guangzhou HKUST Fok Ying Tung Research Institute | Wan L.,South China University of Technology | Xie Y.,South China University of Technology | And 3 more authors.
RSC Advances | Year: 2014

The development of new electrode materials for lithium-ion batteries (LIBs) is of great interest because available electrode materials may not meet the high-energy demands for electronic devices, especially the demands for good cyclic and rate performance. Mn-based oxides have received substantial attention as promising anode materials for LIBs due to their high theoretical specific capacities, low charge potential vs. Li/Li+, environmental benignity and natural abundance. Herein, the preparation of Mn-based oxide nanomaterials with various nanostructures and chemical compositions along with their applications as negative electrodes for LIBs are reviewed. The review covers MnO, Mn3O4, Mn2O3, MnO2, CoMn2O4, ZnMn2O4 and their carbonaceous composite/oxide supports with different morphologies and compositions. The aim of this review is to provide an in-depth and rational understanding of the relationships among the chemical compositions, morphologies and electrochemical properties of Mn-based anode materials and, to understand how electrochemical performance can be improved using materials engineering strategies. Special attention has been paid to the discussion of challenges in the practical applications of Mn-based oxides in LIB full cells. © 2014 the Partner Organisations.

Xu H.,Hong Kong University of Science and Technology | Deng S.,Guangzhou HKUST Fok Ying Tung Research Institute | Chen G.,Hong Kong University of Science and Technology | Chen G.,Guangzhou HKUST Fok Ying Tung Research Institute
Journal of Materials Chemistry A | Year: 2014

Li-rich, Mn-based layered material is one of the most promising cathode materials for next-generation lithium ion batteries. However, this material is subject to severe capacity fading and poor rate capability. When Li was replaced with Mg, the electrochemical performance of the material improved. Structural and elemental analyses indicate an expansion of the unit cell as a result of Mg doping, while the crystal structure remained unaffected. Higher specific capacity and better rate capability with more stable cycling performance were obtained from a sample of Li1.17Mg0.03Mn 0.54Ni0.13Co0.13O2 as compared with other samples that were either Mg-free or with Mg atomic content beyond 0.03. © the Partner Organisations 2014.

Xu H.,Guangzhou HKUST Fok Ying Tung Research Institute | Deng Y.,Guangzhou HKUST Fok Ying Tung Research Institute | Deng Y.,South China University of Technology | Shi Z.,Guangzhou HKUST Fok Ying Tung Research Institute | And 5 more authors.
Journal of Materials Chemistry A | Year: 2013

Relatively uniform sized graphene-encapsulated sulphur (GES) composites with a core (S)-shell (graphene) structure were synthesized in one pot based on a solution-chemical reaction-deposition method. These novel GES particles were characterized by XRD, Raman spectrometry, SEM, TGA, EDS and TEM. The electrochemical tests showed that the present GES composites exhibit high specific capacity, good discharge capacity retention and superior rate capability when they were employed as cathodes in rechargeable Li-S cells. A high sulphur content (83.3 wt%) was obtained in the GES composites. Stable discharge capacities of about 900, 650, 540 and 480 mA h g-1 were achieved at 0.75, 2.0, 3.0 and 6.0 C, respectively. The good electrochemical performance is attributed to the high electrical conductivity of the graphene, the reasonable particle size of sulphur particles, and the core-shell structures that have synergistic effects on facilitating good transport of electrons from the poorly conducting sulphur, preserving fast transport of lithium ions to the encapsulated sulphur particles, and alleviating the polysulfide shuttle phenomenon. The present finding may provide a significant contribution to the enhancement of cathodes for the lithium-sulphur battery technology. © 2013 The Royal Society of Chemistry.

Fang C.,Hong Kong University of Science and Technology | Deng Y.,South China University of Technology | Xie Y.,South China University of Technology | Su J.,Hong Kong University of Science and Technology | And 2 more authors.
Journal of Physical Chemistry C | Year: 2014

Graphene oxide/polydopamine-coated Si nanocomposite (GO/PDA-Si) was synthesized by a novel facile solution-based chemical method at room temperature. The nanocomposite with a PDA coating layer of ∼1.5 nm exhibits a high reversible specific capacity and excellent cycling stability (1074 mAh g-1 after 300 cycles at 2100 mA g-1) as an anode material for lithium ion batteries. The synergistic effect of the PDA coating layer and GO plays an important role in improving the electrochemical lithium storage performances. Both of them can serve as a cushion to buffer the volume change of Si nanoparticles (NPs) during the charge/discharge process and prevent Si NPs from direct contact with a liquid electrolyte. The surface property of Si NPs was also modified by introducing secondary amine groups, which can form amide groups with carboxyl groups and hydrogen bonds with hydroxyl/carboxyl groups on GO. These chemical interactions firmly anchor Si NPs to GO so that aggregation of Si NPs can be mostly prevented. Moreover, the good lithium ion conductivity of PDA is beneficial for rate performance. The experimental results should be very useful in guiding the preparation of long-cycle-life Si-based anode materials with good rate performance using a simple surface-modification process. © 2015 American Chemical Society.

Zhang Q.,Dalian University of Technology | Zhang Q.,Guangzhou HKUST Fok Ying Tung Research Institute | Shi Z.,Dalian University of Technology | Shi Z.,Guangzhou HKUST Fok Ying Tung Research Institute | And 7 more authors.
Journal of Power Sources | Year: 2012

Unique hollow Fe3O4/C spheres are prepared by a simple one-pot solvothermal method, with spinel structure and 750 nm in diameter identified by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The hollow Fe 3O4/C spheres exhibit excellent cycling and rate performance as anode material for lithium ion batteries, delivering reversible specific capacities of 984 mAh g-1 even after 70 cycles at 0.2 C, 620 mAh g-1 at 2 C, and 460 mAh g-1 at 5 C, respectively. Lithium insertion mechanisms are also proposed in terms of the ex situ XRD analysis of the electrodes after discharged and charged to certain voltages together with cyclic voltammetry (CV) and voltage profile. © 2011 Elsevier B.V. All rights reserved.

Ng C.W.W.,Guangzhou HKUST Fok Ying Tung Research Institute | Ng C.W.W.,Hong Kong University of Science and Technology | Liu J.,Hohai University | Chen R.,Harbin Institute of Technology | Xu J.,Hohai University
Waste Management | Year: 2015

As an extension of the two-layer capillary barrier, a three-layer capillary barrier landfill cover system is proposed for minimizing rainfall infiltration in humid climates. This system consists of a compacted clay layer lying beneath a conventional cover with capillary barrier effects (CCBE), which is in turn composed of a silt layer sitting on top of a gravelly sand layer. To explore the effectiveness of the new system in minimizing rainfall infiltration, a flume model (3.0. m. ×. 1.0. m. ×. 1.1. m) was designed and set up in this study. This physical model was heavily instrumented to monitor pore water pressure, volumetric water content, surface runoff, infiltration and lateral drainage of each layer, and percolation of the cover system. The cover system was subjected to extreme rainfall followed by evaporation. The experiment was also back-analyzed using a piece of finite element software called CODE_BRIGHT to simulate transient water flows in the test. Based on the results obtained from various instruments, it was found that breakthrough of the two upper layers occurred for a 4-h rainfall event having a 100-year return period. Due to the presence of the newly introduced clay layer, the percolation of the three-layer capillary barrier cover system was insignificant because the clay layer enabled lateral diversion in the gravelly sand layer above. In other words, the gravelly sand layer changed from being a capillary barrier in a convention CCBE cover to being a lateral diversion passage after the breakthrough of the two upper layers. Experimental and back-analysis results confirm that no infiltrated water seeped through the proposed three-layer barrier system. The proposed system thus represents a promising alternative landfill cover system for use in humid climates. © 2014 Elsevier Ltd.

Zheng J.,Hong Kong University of Science and Technology | Ni L.M.,Hong Kong University of Science and Technology | Ni L.M.,Guangzhou HKUST Fok Ying Tung Research Institute
UbiComp 2013 - Proceedings of the 2013 ACM International Joint Conference on Pervasive and Ubiquitous Computing | Year: 2013

Understanding a user's social interactions in the physical world proves important in building context-aware ubiquitous applications. A good way towards that objective is to categorize people to whom a user is socially related into what we call as social circles. In this note, we propose a novel unsupervised approach that learns from the Bluetooth (BT) sensed data recording one's dynamic proximity relations with others to identify her social circles, each of which is formed along a semantically coherent aspect. For each circle we learn its members as well as the temporal dimensions along which it is formed. Our method is innovative in that it well over- comes data sparsity by information sharing, and allows for circle overlaps which is common in reality. Experiments on real data demonstrate the effectiveness of our method, and also show the potentials of relational mobile data in sensing personal behaviors beyond personal data. Copyright © 2013 ACM.

Guangzhou Hkust Fok Ying Tung Research Institute | Date: 2014-12-12

A method for indoor localization using nomadic access points, comprising the following steps: a target object transmits a modulated signal over wireless channels to nearby access points, wherein the access points include both static access points and nomadic access points; the access points measure channel state information via the signal from the target object, after that, the static access points export the channel state information to a server, the nomadic wireless access point report the channel state information along with their coordinates of the current sites to the server; and based on the channel state information, the server determines the target objects relative proximity to two arbitrary access points, then based on the result of the relative proximity determination and the coordinates, the server estimates a feasible region for the target object with a space partition-based algorithm.

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