Gregory J. Offer

Lithium sulfur batteries, a mechanistic review

Lithium sulfur (Li–S) batteries are one of the most promising next generation battery chemistries with potential to achieve 500–600 W h kg−1 in the next few years. Yet understanding the underlying mechanisms of operation remains a major obstacle to their continued improvement. From a review of a range of analytical studies and physical models, it is clear that experimental understanding is well ahead of state-of-the-art models. Yet this understanding is still hindered by the limitations of available techniques and the implications of experiment and cell design on the mechanism. The mechanisms at the core of physical models for Li–S cells are overly simplistic compared to the latest thinking based upon experimental results, but creating more complicated models will be difficult, due to the lack of and inability to easily measure the necessary parameters. Despite this, there are significant opportunities to improve models with the latest experimentally derived mechanisms. Such models can inform materials research and lead to improved high fidelity models for controls and application engineers.

In-operando high-speed tomography of lithium-ion batteries during thermal runaway

Prevention and mitigation of thermal runaway presents one of the greatest challenges for the safe operation of lithium-ion batteries. Here, we demonstrate for the first time the application of high-speed synchrotron X-ray computed tomography and radiography, in conjunction with thermal imaging, to track the evolution of internal structural damage and thermal behaviour during initiation and propagation of thermal runaway in lithium-ion batteries. This diagnostic approach is applied to commercial lithium-ion batteries (LG 18650 NMC cells), yielding insights into key degradation modes including gas-induced delamination, electrode layer collapse and propagation of structural degradation. It is envisaged that the use of these techniques will lead to major improvements in the design of Li-ion batteries and their safety features.

Battery electric vehicles, hydrogen fuel cells and biofuels. Which will be the winner?

Addressing the economic and environmental sustainability problems of todays road transport requires, inter alia, the rapid introduction of alternative, low-carbon fuels and highly efficient, low-emission powertrains. It is unlikely that the transition from oil-based fuels and conventional internal combustion engines will occur organically at the necessary rate and following an optimum path, hence policy intervention is required. However, in order to design effective policies it is essential that the potential role of the alternative technologies is understood as best as current knowledge allows. Several high-profile studies have compared the potential of alternative road transport technologies, such as hybrids, plug-in hybrids, battery electric vehicles, hydrogen fuel cells and biofuels. The studies, critically reviewed in this paper, have generated a tremendous amount of knowledge. However, we have identified limitations that should be addressed in future comparative studies. These are: a) the complexity of the passenger car market, consisting of many segments characterised by different requirements and use patterns, is not adequately represented; b) future changes in driving behaviour brought about by new policy and technology are generally not considered; c) different studies use different performance indicators for alternative fuels and powertrains, making results difficult to compare and their interpretation difficult for the non-expert. We test the effect of these limitations on the Total Cost of Ownership of each of the alternative technologies mentioned above. We demonstrate that building market segments and behavioural change into a comparative analysis significantly affects its results and we recommend that this is done in future studies.

Online Measurement of Battery Impedance Using Motor Controller Excitation

This paper presents a fast cost-effective technique for the measurement of battery impedance online in an application such as an electric or hybrid vehicle. Impedance measurements on lithium-ion batteries between 1 Hz and 2 kHz give information about the electrochemical reactions within a cell, which relates to the state of charge (SOC), internal temperature, and state of health (SOH). We concentrate on the development of a measurement system for impedance that, for the first time, uses an excitation current generated by a motor controller. Using simple electronics to amplify and filter the voltage and current, we demonstrate accurate impedance measurements obtained with both multisine and noise excitation signals, achieving RMS magnitude measurement uncertainties between 1.9% and 5.8%, in comparison to a high-accuracy laboratory impedance analyzer. Achieving this requires calibration of the measurement circuits, including measurement of the inductance of the current sense resistor. A statistical correlation approach is used to extract the impedance information from the measured voltage and current signals in the presence of noise, allowing a wide range of excitation signals to be used. Finally, we also discuss the implementation challenges of an SOC estimation system based on impedance.

A novel regenerative hydrogen cerium fuel cell for energy storage applications

A novel regenerative hydrogen cerium fuel cell is presented which has the potential to deliver both low cost and high performance. A 5 cm2 prototype is demonstrated, achieving 148 mW cm−2 when fully charged. Rate determining processes within the cell are identified.

Modeling the voltage loss mechanisms in lithium–sulfur cells: the importance of electrolyte resistance and precipitation kinetics

Understanding of the complex electrochemical, transport, and phase-change phenomena in Li-S cells requires experimental characterization in tandem with mechanistic modeling. However, existing Li-S models currently contradict some key features of experimental findings, particularly the evolution of cell resistance during discharge. We demonstrate that, by introducing a concentration-dependent electrolyte conductivity, the correct trends in voltage drop due to electrolyte resistance and activation overpotentials are retrieved. In addition, we reveal the existence of an often overlooked potential drop mechanism in the low voltage-plateau which originates from the limited rate of Li2S precipitation.

A Raman spectroscopic study of the carbon deposition mechanism on Ni/CGO electrodes during CO/CO2 electrolysis

In situ and ex situ Raman analyses of porous Ni/CGO electrodes reveal differences in the amount, location and type of carbon formed during CO/CO2 electrolysis. The results demonstrate the limitations of optical in situ techniques applied to Solid Oxide Cells (SOCs) operated in electrolysis conditions. Increased carbon deposition close to the electrode-electrolyte interface is likely to be the result of high charge-transfer current in that area. The positive effect of a CGO interlayer on reducing carbon formation on the fuel electrode is demonstrated.

Designing a miniaturised heated stage for in situ optical measurements of solid oxide fuel cell electrode surfaces, and probing the oxidation of solid oxide fuel cell anodes using in situ Raman spectroscopy

A novel miniaturised heated stage for in operando optical measurements on solid oxide fuel cell electrode surfaces is described. The design combines the advantages of previously reported designs, namely, (i) fully controllable dual atmosphere operation enabling fuel cell pellets to be tested in operando with either electrode in any atmosphere being the focus of study, and (ii) combined electrochemical measurements with optical spectroscopy measurements with the potential for highly detailed study of electrochemical processes; with the following advances, (iii) integrated fitting for mounting on a mapping stage enabling 2-D spatial characterisation of the surface, (iv) a compact profile that is externally cooled, enabling operation on an existing microscope without the need for specialized lenses, (v) the ability to cool very rapidly, from 600 °C to 300 °C in less than 5 min without damaging the experimental apparatus, and (vi) the ability to accommodate a range of pellet sizes and thicknesses.

Automated vehicles and electrification of transport

Global greenhouse gas (GHG) emission targets can only be met by significantly decarbonising road transport. The only long term way to do this is via the electrification of powertrains combined with the production of low carbon electricity or hydrogen. Current assumptions and models, such as the IEA BLUE Map, demonstrate that this is technically possible, but assume growth in demand for transport services will only double by 2035 and triple by 2050, largely driven by growth in developing economies. However, another transport revolution, automated vehicles, could drive growth in transport services significantly further, which without electrification will have a large negative impact on efforts to curb transport related emissions. In contrast, it is shown in this paper that automated vehicles could significantly improve the economics of electric vehicles, and therefore make the electrification of powertrains more likely, which could help reduce emissions. Despite this uncertainty, little work has been done on understanding how these factors will affect each other, particularly the timing and uptake of automated vehicles and their effect on future transport related GHG emissions and economics, yet the impact on transport policy, infrastructure and society will be profound and should be of interest to policy makers, the automotive and energy industries, and society as a whole.

Chemical Descriptors of Yttria-Stabilized Zirconia at Low Defect Concentration: An ab Initio Study.

Yttria-stabilized zirconia (YSZ) is an important oxide ion conductor with applications in solid oxide fuel cells (SOFCs) and oxygen sensing devices. Doping the cubic phase of zirconia (c-ZrO2) with yttria (Y2O3) is isoelectronic, as two Zr(4+) ions are replaced by two Y(3+) ions, plus a charge compensating oxygen vacancy (Ovac). Typical doping concentrations include 3, 8, 10, and 12 mol %. For these concentrations, and all below 40 mol %, no phase with long-range order has been observed in either X-ray or neutron diffraction experiments. The prediction of local defect structure and the interaction between defects is therefore of great interest. This has not been possible to date as the number of possible defect topologies is very large and to perform reliable total energy calculations for all of them would be prohibitively expensive. Previous theoretical studies have only considered a selection of representative structures. In this study, a comprehensive search for low-energy defect structures using a combined classical modeling and density functional theory approach is used to identify the low-energy isolated defect structures at the dilute limit, 3.2 mol %. Through analysis of energetics computed using the best available Born-Mayer-Huggins empirical potential model, a point charge model, DFT, and a local strain energy estimated in the harmonic approximation, the main chemical and physical descriptors that correlate to the low-energy DFT structures are discussed. It is found that the empirical potential model reproduces a general trend of increasing DFT energetics across a series of locally strain relaxed structures but is unreliable both in predicting some incorrect low-energy structures and in finding some metastable structures to be unstable. A better predictor of low-energy defect structures is found to be the total electrostatic energy of a simple point charge model calculated at the unrelaxed geometries of the defects. In addition, the strain relaxation energy is estimated effectively in the harmonic approximation to the imaginary phonon modes of undoped c-ZrO2 but is found to be unimportant in determining the low-energy defect structures. These results allow us to propose a set of easily computed descriptors that can be used to identify the low-energy YSZ defect structures, negating the combinatorial complexity and number of defect structures that need to be considered.