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The application of Li(Ni0.8Co0.1Mn0.1)O2 (NCM811) cathode-based lithium-ion batteries (LIBs) has alleviated electric vehicle range anxiety. However, the subsequent thermal safety issues limit their market acceptance. A detailed analysis of the failure evolution process for large-format LIBs is necessary to address the thermal safety issue. In this study, prismatic cells with nominal capacities of 144Ah and 125Ah are used to investigate the thermal runaway (TR) characteristics triggered by lateral overheating. Additionally, TR characteristics under two states of charge (SoCs) (100% and 5%) are discussed. Two cells with 100% SoC exhibit similar characteristics, including high failure temperature, high inhomogeneity of temperature distribution, multi-points jet fire, and significant mass loss. Two cells with 5% SoC demonstrate only a slight rupture of the safety valve and the emission of white smoke. However, there is a difference: the 144Ah cell still has a significant temperature rise inflection point. Moreover, apart from the early voltage drop before the sudden temperature rise, the environmental pressure shows an even earlier rise. The above phenomenon means the pressure signal could also be used as a valid indicator to predict potential thermal failure events. Interestingly, the ablation traces on the aluminum casing of the TR cells show a spreading arch shape on both the side and bottom surface. In summary, the results indicate that the batteries with a high SoC exhibit a more severe mode of failure and a faster failure evolution process. The research provides a comprehensive insight into the TR evolution process and has specific guiding implications for future TR warnings and thermal safety accident traceability work.

Lithium-ion batteries (LIBs) charging at low temperatures will easily accelerate the aging of LIBs and reduce the useful life. This paper applies advanced multi-factors coupling aging model and bi-objective particle swarm optimization (PSO) algorithm to derive suitable charging patterns for LIBs at low temperatures. Based on the results of orthogonal experiments, which consider the effects of charging rate, charging temperature and charging cut-off voltage on battery aging, the low-temperature capacity degradation model was established. Furthermore, two important yet competing charging objectives, including battery health and charging time, are taken into account simultaneously. These two conflicting charging objectives are traded off by solving a bi-objective optimization problem based on battery electrothermal and aging behavior. Combined with PSO algorithm, the optimal low-temperature charging strategy is obtained. As a result, the three-stage constant current and constant voltage (CC-CV) charging strategy is optimized to balance various combinations of charging objectives. Different tradeoffs are compared and analyzed based on the Pareto frontiers. The verification results show that the optimized three-stage CC-CV charging strategy can effectively offer feasible health-conscious charging with desirable tradeoffs among charging speed and battery health under low-temperature charging conditions.

Materials discovery from infinite earth repository is considered the bottleneck of each revolutionary technological progress. The discovery of the new materials is hindered by the labor-intensive and time-consuming process. Although machine learning techniques shown the excellent capability for speeding up materials discovery, it is still challenging to obtain effective material feature representations and making a precise prediction of the material properties are still challenging. This work focuses on developing an automatic material design and discovery framework enabled by data-driven AI models. We utilize the material's thermoelectric (TE) properties prediction as a baseline to demonstrate the investigation logistic. The framework works in a closed-loop mode. Starting from the well-established TE materials databases, we develop a TE material encoding method and TE material screening model by using machine learning technology. The models show excellent accuracy for TE properties prediction. Furthermore, the developed AI models have identified 6 novel promising p-type TE materials and 8 novel promising n-type TE materials. Evaluated by the Density-functional Theory (DFT) calculation, the prediction results have a good agreement with the realistic material's TE property. The proposed framework well accelerates the design and discovery of the new functional materials.

Overcharge is a hazardous abuse condition that has dominant influences on cell performance and safety. This work, for the first time, comprehensively investigates the impact of different overcharge degrees on degradation and thermal runaway behavior of lithium-ion batteries. The results indicate that single overcharge has little influence on cell capacity, while it severely degrades thermal stability. Degradation mechanisms are investigated by utilizing the incremental capacity-differential voltage and relaxation voltage analyses. During the slight overcharge process, the conductivity loss and the loss of lithium inventory always occur; the loss of active material starts happening only when the cell is overcharged to a certain degree. Lithium plating is the major cause for the loss of lithium inventory, and an interesting phenomenon that the arrival time of the dV/dt peak decreases linearly with the increase of the overcharge degree is found. The cells with different degrees of overcharge exhibit a similar behavior during adiabatic thermal runaway. Meanwhile, the relationship between sudden voltage drop and thermal runaway is further established. More importantly, the characteristic temperature of thermal runaway, especially the self-heating temperature (T1), decreases severely along with overcharging, which means that a slight overcharge severely decreases the cell thermal stability. Further, post-mortem analysis is conducted to investigate the degradation mechanisms. The mechanism of the side reactions caused by a slight overcharge on the degradation performance and thermal runaway characteristics is revealed.

Recently, the piezoelectric based sensor coupled with electromechanical impedance (EMI) technique is gaining attention on monitoring the mechanical properties changes in cementitious materials. However, the EMI signals obtained from the sensing system are not only influenced by the development of inherent mechanical properties in the host structure but also affected by the variation of temperature. When implementing a piezoelectric based sensor, both the ambient temperature change and the heat release from newly casted concrete would influence the sensing accuracy. In order to eliminate the biases from temperature effect, the mechanisms of EMI technique for strength sensing were investigated. The experiment work was separated in two parts. The piezoelectric sensors were first used to monitor the strength gaining of the newly casted cementitious samples curing under constant temperature. A strength estimation system was developed based on the experiment results. Later, the temperature variation was induced to affect the sensing performance. A temperature compensation technique was proposed to eliminate the temperature effect. It has concluded that the proposed compensation method can improve the strength sensing accuracy. The new understanding should help to promote the practical applicable EMI sensing technique.

The use of piezoelectric materials based Electromechanical Impedance (EMI) technique for monitoring the hydration of cementitious materials has caught much attention recently. However, very few literatures have explored the feasibility of using this method on monitoring the stiffness development and compressive strength gain at the very early age properties (4th–8th h) of cementitious materials. This research serves as a comprehensive study to verify the reliability of using lead zirconate titanate (PZT) based EMI method in monitoring the compressive strength gain and elastic modulus of mortar at both very early age (4th–8th h) and early age (1st, 3rd, and 7th day). Extensive experiments and data analysis have been done on ten different mixes with various water-to-cement ratio and Type I & III cement. The EMI signatures are measured for each sample at the period of interest, and post-processed with three statistic model including the root mean square deviation (RMSD), correlation coefficient deviation (CCD), and mean absolute percentage deviation (MAPD) as indices. To examine the correlation and linearity between the compressive strength/elastic modulus obtained via conventional cubic testing using ASTM C109 and the EMI indices, a linear least square regression analysis is performed. As the authors postulated, all the mixes display a good linear correlation of R2. Among all three statistical indices, RMSD index is proved as the most accurate statistical index on strength gain monitoring of cementitious materials. The results indicated the feasibility of using piezoelectric-based EMI method for monitoring the cementitious material’s strength gain at very early age, regardless the concrete mix design.

Summary Lithium-ion batteries (LIBs) are widely used as the energy carrier in our daily life. However, the higher energy density of LIBs results in poor safety performance. Thermal runaway (TR) is the critical problem which hinders the further application of LIBs. Clarifying the mechanism of TR evolution is beneficial to safer cell design and safety management. In this paper, liquid nitrogen spray is proved to be an effective way to stop the violent reaction of LIBs during the TR process. Based on extended-volume accelerating rate calorimetry, the liquid nitrogen ceasing combined with non-atmospheric exposure analysis is used to investigate the TR evolution about LiFePO4/graphite batteries at critical temperature. Specifically, the geometrical shape, voltage, and impedance change are monitored during the TR process on the cell level. The morphologies/constitution of electrodes and separators are presented on the component level. Utilizing the gas analysis, the failure mechanism of the prismatic LiFePO4/graphite battery is studied comprehensively.

Piezoelectric sensors have been widely used in the field of infrastructure sensing. However, the materials used for piezoelectric sensor fabrication are dominated by ceramic materials, which has the shortcoming of intrinsic brittleness. Therefore, the application of the piezoelectric sensors is limited to the condition with tiny strain levels. This study set out to develop a piezoelectric nano-sensor with good piezoelectricity and flexibility to address the current bottleneck of piezoelectric-based infrastructure sensing. The experiment program starts from the polymer-based piezoelectric materials, PVDF (Polyvinylidene fluoride or polyvinylidene difluoride) nanofiber, synthesis by using electrospinning process. The materials characterization results reveal the correlation within the materials synthesis methods, nanostructure, and material properties. The materials synthesis techniques for nanostructure control and improve piezoelectric performance are presented in this dissertation. The piezoelectric nano-sensor was fabricated by using an ink-jet printing process. This study further utilized the COMSOL Multiphysics simulation to guide the piezoelectric nano-sensor packaging design from the perspective of energy dissipation. The optimized piezoelectric nano-sensor was then used for civil engineering materials strength sensing and damage detection. The electric response from the piezoelectric nano-sensor is sensitive to the mechanical strength of the sensing structure. The piezoelectric nano-sensor's voltage output can also be a good indicator for damage detection at a decent strain level. A natural progression of this work is to explore the roll-to-roll manufacturing methods for large-scale piezoelectric nano-sensor fabrication. As part of the infrastructure component, the piezoelectric nano-sensor is effective for the mechanical property evaluation and preliminary damage assessment. Moreover, incorporating the artificial intelligence-guided signal process, the piezoelectric nano-sensor could better understand the condition of the infrastructure.

This paper investigated the spatial distribution of steel fibers and air bubbles in UHPC cylinder specimen by using X-ray CT system. Four levels of superplasticizer dosage were adopted to obtain cement mortar mixtures with different rheological properties and then steel fibers were added by volume of 1%, 2% and 3% for every cement mortar mixture respectively. Cylindrical specimens with diameter of 100 mm and height of 200 mm were prepared under the same condition for every mixture and the content of steel fibers and air bubbles at different casting depth was determined by X-ray CT measurement. On the other hand, the cylinder specimen was cut into four pieces for evaluating the splitting tensile strength variation with the casting depth. The results reveal that steel fibers and air bubbles are uniformly distributed in UHPC specimen when the fresh mixture is prepared by adding a lower dosage of superplasticizer. This homogeneity is also proved by the splitting tensile strength for samples at different depths. When the yield stress of fresh mortar was reduced by the higher dosage of superplasticizer, the steel fiber sedimentation and air bubble rise occurred and the splitting tensile strength was significantly decreased at the top portion of cylinder specimen. The influence of steel fiber content on splitting tensile strength can be regressed into a good quadratic equation. Therefore, the balance between homogeneity and flowability should be fully considered for manufacturing UHPC elements with a high depth.

This paper aims to study the distribution of steel fibers in UHPC specimens prepared by different rheological cement mortar. Four levels of superplasticizer dosage were used for every water to binder ratio (0.18, 0.20, 0.22, 0.24) to adjust rheological properties of cement mortar mixtures (without steel fibers). Steel fibers were added by volume of 1%, 2% and 3% for every cement mortar mixture respectively. Cube specimens (70.7 × 70.7 × 70.7 mm3) were cast and cured under the same condition for every mixture and the distribution of steel fibers along casting depth was determined by digital image processing. The experimental results reveal that the flowability of fresh cement mortar increases and the yield stress decreases with the higher water to binder ratio and superplasticizer dosage. The plastic viscosity of fresh mixture is decreased by the higher water to binder ratio and little influenced by the variation of superplasticizer dosage. The fiber distribution along depth mainly depends on rheological parameters of fresh mixture, especially the yield stress. Moderate rheological parameters are necessary for obtaining an uniform distribution of steel fibers in UHPC. On the basis of this study, the optimal yield stress of fresh matrix ranges among 900–1000 Pa, 700–900 Pa and 400–800 Pa respectively for UHPC mixtures with 1%, 2% and 3% fiber volume fraction.

Developing a reliable in-situ testing method to determine the strength of concrete for traffic opening is a critical need for INDOT, due to the fast-pace construction schedule exposes concrete pavements and/or structures undergoing a substantial loading conditions even at its early ages. Nevertheless, the current methods for determining traffic opening times are inefficient and expensive, often causing construction delays or cost overruns. To address this critical need, we propose to develop an in-situ nondestructive testing (NDT) method that enables an accurate and efficient understanding of early age properties of concrete using electromechanical impedance (EMI) method coupled with piezoelectric sensors. Previous literature has indicated that a piezoelectric sensor coupled with electromechanical impedance (EMI) technique can be a promising method to monitor the concrete properties changes of newly casted concrete and evaluate the condition of existed concrete at the laboratory scale. Based on the direct and indirect effect, piezoelectric materials can act both as a transducer and a receiver to capture the properties change of host structures of which it is attached to. The high frequency detection and fast response of this method will provide an accurate and reliable dataset of early age properties of concrete. This data enables us to monitor the in-situ concrete strength for determining the optimal traffic opening time. In this report, the feasibility of EMI sensing technology for monitoring the compressive strength gain of concrete is systematically investigated. The substantial experiments were conducted from cement paste, various mortar, concrete to field test on the interstate highway. Also, computer modeling work was performed to assist the experimental studies. Finally, the novel EMI technology can be delivered to DOT as one of the optional methods for in-situ concrete strength monitoring to determine the optimal traffic opening time.

Electromechanical impedance (EMI) technique coupled with the piezoelectric sensor has been explored as a promising non-destructive testing (NDT) method to determine the quality of cementitious materials. Among the piezoelectric sensors, Lead Zirconate Titanate (PZT) is the most commonly used piezo-ceramic materials for conducting the EMI because of the high sensitivity and high piezoelectric constant. However, the inherent brittleness of PZT limited their potential application as embedded sensors for in-situ monitoring of materials/structure properties. To improve the durability and flexibility of the PZT sensor, we have systematically investigated the effect of different polymer coating methods of PZT on their sensitivity and effectiveness of EMI sensors. Specifically, two polymer coating agents were used to encapsulate the PZT sensor including the flexible organic polymer compound - Polydimethylsiloxane (PDMS) and the rigid polymer compound- Polyepoxides (Epoxy) We have also investigated the feasibility of EMI technique with the surface bonded sensor and embedded sensor to test the compressive strength gain of mortars. The compressive strength test of samples was also conducted using conventional mechanical methods per ASTM C109 as a baseline at the first 5th to 8th hour. The EMI signatures were collected for all the samples at the same time. Our preliminary results have shown a high correlation of EMI methods’ results with the compressive testing results for two different polymers coated PZT sensors, which indicated the polymer-coated PZT sensors can be used as an effective NDT method for in-situ monitoring of concrete compressive strength gain at the very early age.

Occurrence of lithium plating on the anode is a severe side reaction in the lithium-ion batteries, which brings cell capacity degradation and reduces the cell safety. This paper focuses on 37Ah commercial lithium-ion batteries and clarifies the evolution of lithium plating during long-term low temperature (−10 °C) cycling. The tested cells are analyzed at different degradation stages, named “fixed-point analysis”, to evaluate the evolution process of lithium plating. It is found that the capacity fade and the internal resistance increase exhibit a decelerated trend with a turning point around 450 cycles. The loss of cyclable lithium caused by lithium plating is deemed to be the main reason behind the battery degradation. Post-mortem analysis including scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) is conduced to reveal the mechanism. The thickness and morphology change of plated lithium, and the plated lithium covered by solid electrolyte interface film, are disclosed by the surface and cross-section SEM images from the fixed-point analysis. XPS analysis further reveals the composition of solid electrolyte interface film, and the plated lithium existing. In a nutshell, lithium plating is inhomogeneous and has high spatial dependence.

It is critical to develop an accurate non-destructive testing (NDT) methods to evaluate the real-time concrete strength gain due to the fast pace of construction. Among the NDTs for concrete strength monitoring, piezoelectric materials based electromechanical impedance (EMI) method has attracted much attention recently. Some statistical approaches are proposed for impedance signal processing, such as root mean square deviation (RMSD) and mean absolute percentage deviation (MAPD). However, these methods require the baseline value for calculation, which is not convenient, particularly for the very early strength gain monitoring. In this study, three features extracted directly from phase angle spectrum, including resonant frequency of the first local phase angle peak (PARF), local phase angle peak value (PA), and the corresponding width of half-prominence (HPW) are employed as indicators to monitor the strength gain process. Among these three indicators, the first peak of phase angle resonant frequency (PARF) is found the most reliable. The PARF index can directly be obtained from the impedance spectrum without baseline calculation. Five types of different cement mortar with various water-to-cement ratios from 0.38 to 0.46 were prepared. The EMI test and conventional cubic testing (ASTM 109) were conducted concurrently at a very early age (4th to 8th hour) and an early age (1st, 3rd, and 7th day). To examine the correlation between the compressive strength obtained and the EMI-PARF indices, a linear least-squares regression analysis was performed. Furthermore, the finite element analysis (FEA) was conducted to discuss the relationship between the features obtained from the phase angle spectrum and the stiffness of the host structure. The PARF index exhibits a high correlation with the mechanical properties of cementitious materials. Both experiments and FEA results ascertain that piezoelectric-based EMI coupled with the PARF index is a reliable method for in-place strength gain evaluation of cementitious materials.

A novel phenol–formaldehyde carbon with ordered/disordered bimodal mesoporous structure is synthesized by the facile evaporation induced self-assembly strategy under a basic aqueous condition with SiO2 particles as template. The prepared bimodal mesoporous carbons (BMCs) are composed of ordered mesoporous and disordered mesoporous with diameter of about 3.5 nm and 7.0 nm, respectively. They can be employed as supercapacitor electrodes in H2SO4 aqueous electrolyte after the simple acid-treatment. BMC exhibits an exceptional specific capacitance of 344 F g−1 at the current density of 0.1 A g−1, although it has a relatively low surface area of 722 m2 g−1. And the BMC electrode displays an excellent cycling stability over 10,000 cycles.

Highly ordered, body-centered cubic mesoporous phenol–formaldehyde carbon C-FDU-16 is prepared by a facile evaporation induced self-assembly strategy under a basic aqueous condition. Afterwards, in order to increase the active site of surface electrochemical reactions and promote the wettability in aqueous electrolyte, a simple chemical surface modification is carried out on the C-FDU-16 by nitric acid treatment. The C-FDU-16 and the acid-modified C-FDU-16 (N-FDU-16) are used as the active materials for supercapacitors. Although C-FDU-16 has unique 3D mesoporous network combined with high specific surface area, it do not show well supercapacitor properties for its bad wettability with the aqueous electrolyte. However, the N-FDU-16 with acid treatment exhibits the largest specific capacitance of 219 F g−1 at a current density of 0.1 A g−1 in 1 M H2SO4 electrolyte which is 7 times larger than that of the C-FDU-16 and it also shows good cycling stability as well. Therefore, based on the above investigations, ordered mesoporous phenol–formaldehyde carbon such as C-FDU-16 can be a potential candidate for supercapacitors after the simply acid treatment.

The higher specific capacity and richness in natural resources makes SiOx-based become a research focus by international scholar in recent years. A lot volume changes of material in the process of cycling which affects the batteries’ cycling performance, thereby limiting its development and usage. While choosing suitable binder is critical to effectively enhance the performance of the silicon oxide (SiOx) that has high specific capacity and easy pulverization. Researched the lithium insertion/extraction properties of SiOx composite electrode which prepared with polyvinylidene fluoride (PVDF), LA132 (which major copolymerization chain is polyacrylonitrile-polyacrylic acid (PAN-PAA)), polyacrylic acid (PAA). Electrochemical cycling tests with a current density of 160mA/g shows that SiOx electrode with conventional binder PVDF results in rapid capacity loss, while aqueous binder LA132 and PAA could dramatically improve cycle performance of SiOx. SiOx with PAA electrode exhibits reversible capacity still up to 1090mAh/g after 50 cycles. The analysis and characterization methods of SEM, EIS, XRD and differential capacity plots, it is prove that amorphous or little crystalline binder have physical or chemical reaction with –OH of SiOx and formed class network constitute. The expansion pulverization of SiOx power limited in original domain and maintained electronic and ionic conductivity effectively, improved cycle performance of materials. Acknowledgements The project was supported by the Shanghai Science and Technology Development Fund (15DZ2282000). References [1] J. Li, L. Christensen, M. N. Obrovac, K. C. Hewitt and J. R. Dahn. J. Electrochem. Soc., 155(2008): A234-A238. [2] N. S. Hochgatterer, M. R. Schweiger, S. Koller, P. R. Raimann, T. Wöhrle, C.Wurm and M. Winter. Electrochem. Solid-State Lett. 11(2008): A76-A80. [3] X. J. Feng, J. Yang, X. L. Yu, Wang J L, Y. N. Nuli. J. Solid State Electrochem, 17(2013):2461-2469. [4] I. Kovalenko, B. Zdyrko, A. Magasinski, B. Hertzberg, Z. Milicev, R. Burtovyy, I. Luzinov, G. Yushin. Scicence, 334(2011):76-79. Figure 1

As the world into the era of 3G/4G, 3G/4G phones and netbooks gradually popular, mobile Internet, games, video more extensive application of all these applications need to be on the phone for a long time operation, the need of mobile phone battery can support a long time, it puts forward higher requirements on lithium ion batteries, thus to improve the specific energy of lithium ion battery has become one of the problems to be solved1-2. At present mainly in order to improve the specific energy of lithium ion battery by boosting voltage of LiCoO2 material, while it will also bring adverse effect to LiCoO2 materials. So it is necessary to study the evolution of the LiCoO2under high voltage cycling. In this paper, we used the three electrodes to monitor the potential changes of LiCoO2 and graphite from the battery initial state charged to 4.5 V (as shown in figure 1), and before and after cycling. Study the morphology, structure and surface changes of LiCoO2before and after cycling under 4.5V high voltage by using differential capacitance, scanning electron microscopy, X-Ray Diffraction (as shown in figure 2), Raman, IR and differential scanning calorimetry analysis characterization methods. Acknowledgements The project was supported by the Shanghai Science and Technology Development Fund (15DZ2282000) and (14JC1491800) References [1]. A. Aboulaich*, K. Ouzaouit, H. Faqir, A. Kaddami, I. Benzakour, I. Akalay. Materials Research Bulletin 73 (2016) 362-368. [2]. Y. Ito, Y. Sakurai, S. Yubuchi, A. Sakuda, A. Hayashi∗,and M. Tatsumisago, J. Electrochem. Society, 162 (8) (2015) A1610-A1616. Figure 1

With the increasing applications of lithium ion batteries in energy storage, electric vehicles and hybrid electric vehicles, it’s required to improve the energy density, safety, rate capability and cycling stability of the batteries. Spinel Li4Ti5O12 is a promising anode material for lithium-ion batteries which possesses good Li+ insertion and de-insertion reversibility with negligible structure change, high safety and rate capability, excellent cycle stability1-2. While the theoretical capacity of it, 175 mAh g-1, is slightly low. Meanwhile, it’s observed gassing during storage or cycling3, limitting its application.It was found that the surface coating could be an effective method to solve the swelling problem and be beneficial to the electrochemical performance4. In this paper, we investigated the effect of titanate coupling reagent TCA-K38S as electrolyte additive on the performance of Li4Ti5O12. TCA-K38S, isopropyl tri-(dioctylpyrophosphate) titanate, is a liquid monoalkoxy pyrophosphate titanate, usually used as coupling agent not only finorganic fillers with resins, but also fnon-polar materials with polar materials, the structural formula is shown in Fig.1. The electrolyte used in this work are 1.2 mol LiPF6 in EC/ EMC solvents ((3:7 by volume), the TCA-K38S was added at 1wt.% and 3 wt.% to the electrolyte solution. Fig.2 shows the cycling performance of Li4Ti5O12 batteries with different amounts of the TCA-K38S additive at various rates. It can be seen that the specific capacity of batteries with 1wt.% additive is 180 mAh g-1 at the first discharge and stable in 170mAh g-1 at 0.2 C, comparied to 162mAh g-1 and 158mAh g-1 at 0.2 C when it’s without additives. Acknowledgements The project was supported by the National High Technology Research and Development Program of China (863) (2013AA050902) and Shanghai Science and Technology Development Fund (15DZ2282000, 14JC1491800) . References [1]. Z. Huang, D. Wang, Y. Lin, X.Y. Wu, P. Yan, C.M. Zhang, D.N. He, J. Power Sources, 266 (2014) 60-65. [2]. E. Pohjalainen , J. Kallioinen , T. Kallio, J. Power Sources, 279 (2015) 481-486. [3]. K. Wu, J. Yang, Y. Liu, Y. Zhang, C. Y. Wang, J.M. Xu, F. Ning, D.Y. Wang, J. Power Sources, 237 (2013) 285-290. [4]. Z.J. Ding, L. Zhao, L.M. Suo, Y. Jiao, S. Meng, Y.S. Hu, Z.X. Wang, L.Q. Chen, Phys. Chem. Chem. Phys., 2011, 13, 15127–15133. Figure 1