Search

The safety and overall operation of lithium metal batteries depends largely on the prevention of lithium dendrite build. Numerous methods have been proposed for dendrite suppression such as use of external pressure, modification of the nucleation sites for dendrite initiation and the use of solid electrolytes. Each of these approaches shows promise, but neither has led so far to a practical solution. A conceptually different design approach involves the use of high-surface area, 3D structures. Electrodes built on the principle of porous or three-dimensional substrate enable low current densities, while still offering high overall current. The low current density in turn enables long Sands time and prolonged lithium deposition without dendrite build. The entire concept relies on the creation of a high-surface area current collector in direct contact with both the electrolyte and the active material. This is the, often forgotten, principle of the three-phase boundary. The principles of creating large three-phase boundary are used to design the pore structure and to balance the distribution of the electrode components. While the effective design of the three-phase boundary is critical for both charge and discharge reactions in the lithium metal battery, it’s principles are more obvious during the discharge reaction. It is demonstrated how the effective three-phase boundary affects the cell performance. A high-surface area porous silicon current collector is used for the metal silicon anode as well as for the air cathode. The study depicts the fundamental behavior and dependence of lithium dendrite build on the current density at the anode. Cycling of a symmetrical cell demonstrates stable voltage and current, with no capacity fade, i.e., no dendrite build, for C rates below a critical value. Figure 1 depicts stable capacity during cycling, while figure 2 shows reduction in impedance as the cycling progresses. Further suppression in dendrite creation is demonstrated through the geometrical effects of the structure as well as by modifying the nature of the current collector. The application of this 3D structure for the air cathode is also evaluated and modeled.

In order to develop electric vehicles (EVs), electrochemical scientists and engineers have paid great attention to the study of batteries in recent years. Now, while new types of batteries are still in development, the valve-regulated lead-acid (VRLA) battery is a near-term candidate for EV applications in mass markets. The recharge time for valve-regulated lead-acid batteries has been reduced and is now less than 5, 15, and 240 min for 50, 80, and 100% charge. However, many other problems arise from pulse charging such as reduced cycle life of the battery, corrosion of the positive-electrode grid, loss of electrolyte, and thermal management. The kinetic processes of both PbSO{sub 4} oxidation and the oxygen evolution reactions were studied under pulse charging conditions. Three kinds of diffusion processes appeared in the positive active mass. The time intervals required to relax the respective concentration polarizations were about 0.1, 300, and more than 10,000 s. At the beginning of the pulse charge, the passivation layer formed between the grid and active mass at the positive plates exhibited a poor electronic conductivity and impeded the diffusion of O atoms. Thus, the electrode polarization increased and oxygen evolution accelerated. Following the initial pulse, the diffusion of Omore » atoms became rapid while the concentration of ions such as HSO{sub 4}{sup {minus}} and SO{sub 4}{sup 2{minus}} at the reaction surface increased gradually so that their diffusion became dominant in the solution near the reaction surface. For longer times, the charge rate was controlled by the diffusion of these ions in the micropores of the active mass. The electrode polarization, oxygen evolution, and loss of water can be inhibited by using a pulse discharge or/and prolonging the off-time of pulse charge.« less

The performance of lithium ion battery cells is influenced by microscopic and macroscopic parameters. The microscopic parameters are generally responsible for the electrochemical, thermal, safety and lifetime performance of a battery. These parameters are cell chemistry, particle size, diffusion coefficient, equilibrium potential, reaction rate, thermodynamic parameters, thermal and electrical conductivity, heat capacity of active materials and different components of a cell etc. The macroscopic parameters are cell form, aspect ratio, thickness of active materials current collector and separator, tab size and its location. They are used in the cell design process and are optimized to enhance the performance and lifetime of a given cell. When considering upscaling the cell size, the macroscopic parameters attain significance in determining the overall performance of large format cells, which may be largely ignored in short format cells. Increase in the cell size has certain advantages such as, increased cell capacity, fewer connections to the battery pack (parallel connection), low assembly cost, high weight/volume ratio and high reliability of the components of a battery pack. But on the other hand it increases the cell cost, increased safety risk and difficult thermal management etc. But the main disadvantages in terms of performance and lifetime are related to the inhomogeneities occurring in the cell due to increase in cell size. The inhomogeneities can be listed as temperature, current density and state of the charge (SOC) distribution. The current distribution in the cell is affected by tab configuration, current collector thickness and cell aspect ratio. Recent studies show that increasing the cell size increases inhomogeneous current density distribution. Despite the optimized design, the current flow near the tabs is constricted and the location near cell tabs experience higher current density compared to surfaces far away from the tabs. The current distribution influences the local heat generation and introduces temperature inhomogeneity, especially near cell tabs. It also introduces SOC inhomogeneity as SOC in the integral of current over time. These inhomogeneities are related to one another and will have a cascading effect on the overall cell performance. A long term exposure to these inhomogeneities leads to localized aging of the cell. The localized aging creates stronger and weaker regions in cell and further accelerating the aging and leads to premature end of life (EOL) of the cell. With the existing methodologies it is difficult to evaluate cell inhomogeneity as it is completely a localized process. However, recently some experimental and modelling techniques are able to evaluate cell inhomogeneity. These methods can be classified into two as, direct measurement such as, spatial temperature measurements both inside and on the cell surface, multiple or segmented electrode measurements, in-situ and ex-situ diffraction or neutron imaging tools and the second classification is modelling and simulation techniques, which is the focus of this study. This paper shows two modelling techniques, first with spatially distributed battery model developed in Matlab/Simulink and second with Multiphysics based modelling in COMSOL. Both models are able to evaluate inhomogeneity in a large format cell. This work also compares these modelling techniques with respect to complexity, computation and accuracy of the simulation results. Figure 1

Lithium-rich layered-layered oxide materials aLi[Li1/3Mn2/3]O2·(1-a)Li[NixCoyMn1-x-y]O2 (LMO-NCM) are seen as promising candidates for cathode material in next generation Li-ion batteries [1]. This is due to their high specific capacity (> 250 mAh g-1) [2] and low price due to lower cobalt content. On the other hand, the major obstacles for implementation of these materials are high irreversible capacity in first cycle, voltage fade upon cycling due to structural changes, and poor rate capability [1]. For cathode material, thermal stability is particularly relevant as it influences safety. In addition to well-known tendency of layered oxides to decompose exothermally in charged state and emit O2 [3], the oxygen in above mentioned group of materials also participates in charge-discharge reaction [4]. This raises additional safety concerns. We have shown for 0.5Li[Li1/3Mn2/3]O2·0.5Li[Ni1/3Co1/3Mn1/3]O2 that presence of oxidized oxygen in lattice after first cycle increases decomposition enthalpy of the cathode [5]. On the other hand, reversible oxygen and manganese participation in reactions of first cycle also stabilize the structure, which results in a higher onset of decomposition reaction [5]. It has been found that during the activation of Li[Li1/3Mn2/3]O2 (LMO) component in the first cycle, oxygen is emitted from the surface of the material, and oxygen in the bulk is oxidized to compensate for the removal of lithium [6]. Furthermore, the crystal structure changes while the material is held at higher voltage for longer time [7]. These phenomena affect not only performance, but also thermal stability. Knowing that the LMO component decisively impacts the thermal stability of the cathode, the effects of cycling regime, cycling history, ratio between LMO/NCM component, and active material particle size/surface area on thermal stability are investigated. In this research, cathodes are made using LMO-NCM as active material. They are built into half-cells vs. lithium metal with 1 M LiPF6 in flouroethylenecarbonate/diethylcarbonate (1:1) as electrolyte. Cells are conditioned electrochemically at certain current. At certain state of charge, the cathodes are taken out, washed and dried. Thermal stability is investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Evolved gasses are characterised by mass spectrometry. Stability of cathode-electrolyte system is addressed by accelerating rate calorimetry (ARC). Results of the investigations will be presented at the meeting and are expected to elucidate the safety behaviour of LMO-NCM cathodes and help define the required synthesis conditions and microstructure of the cathode active material with regards to safety. Furthermore, appropriate charging regime and cut-off voltage can be derived from the findings. [1] H. Yu, H. Zhou, J. Phys. Chem. Lett. 4 (2013) 1268-1280 [2] A. Ito, Y. Sato, T. Sanada, M. Hatano, H. Horie, Y. Ohsawa, J. Power Sources 196 (2001) 6828-6834 [3] D.D. MacNeil, J.R. Dahn, J. Electrochem. Soc. 148 (2001) A1205-A1210 [4] H. Koga, L. Croguenec, M. Ménétrier, K. Douhill, S. Belin, L. Bourgeois, E. Suard, F. Weill, C. Delmas, J. Electrochem. Soc. 160 (2013) A786-A792 [5] J. Geder, J.H. Song, S.H. Kang, D.Y.W. Yu, ICMAT 2013, Symposium A, Presentation no. ICMAT13-A-1712, Proceedings in preparation [6] H. Koga, L. Croguenec, M. Ménétrier, P. Mannessiez, F. Weill, C. Delmas, J. Power Sources 236 (2013) 250-258 [7] D. Mohanty, A. Sefat, S. Kalnaus, J. Li, R. Meisner, E. Payzant, D. Abraham, D. Wood, C. Daniel, J. Mater. Chem. A 1 (2013) 6249-6261

The energy of a Li-ion cell is in average about 3,250 kJ/kg. About ¼ of this energy is related to electrochemical energy (chemical energy convertible into electrical energy via normal use or short circuit) and ¾ to thermal energy (chemical energy convertible only in thermal energy released at suitable stimulation; e.g. short circuit). The main safety related events are overcharge, external heating, external and internal short circuits, and mechanical deformations of the cell/battery case. These unpredictable events may leads to a temperature increase and at T > 120 °C by internal chemical processes (SEI layer breakup, anode-electrolyte reaction) with gas development and internal pressure increase to cell container rupture at predetermined breaking points. Gas and electrolyte will escape through the rupture holes into the environment and, eventually, inflame by an external ignition source. A further temperature increase till about 200 °C triggers high rate chemical reactions (cathode decomposition with oxygen development, oxygen reaction with the electrolyte, electrolyte decomposition) leading to a very rapid temperature rise (thermal runaway) followed by flames caused by reaching the self-ignition temperature of the electrolyte (~ 450 °C) and possibly explosions. The specific occurrence of most of the above described safety related events, that may occur during the Li-ion battery life cycle phases of transportation, storage, and disposal, will be discussed in this presentation. During transportation and storage, external heating, external and internal short circuits, and mechanical deformations, are the events most likely to happen. Measures which can prevent them (e.g. reliable and low flammable packaging, thermal barriers) and transport related standards (e.g. UN 38.3) are described. The safety hazard in the disposal phase of the cell/battery are also associated with external heating, external and internal short circuits, and mechanical deformations. After collections it is to proof whether the battery is defective (not all functions properly) or damaged (loss of physical integrity). Defective batteries with capacity ≤ 80 % of the nominal value (end-of-life by definition) could be still used in lower demanding applications, e.g. stationary storage in PV houses. Damaged batteries and defective batteries with << 80 % capacity and other malfunctions have to be recycled. In order to reduce the safety risk before the recycling process (including transport), it is advisable to de-energize the battery by low current discharge and finally short it permanently.

The tubular positive plate formation in lead-acid batteries has been studied by an in situ electrochemical scan technique to measure the current and potential distributions. The distributions of the current density, potential, and polarization resistance were uneven in the earlier stage of the formation. This is attributed to the conductivity of the positive paste. The formation of the tubular positive plate can be divided into three stages according to the polarization. The first stage is before inputting ∼50% the charge amount, in which the conductivity of the paste is dominant. After that, the positive polarization depends on both the conductivity and the electrochemical reactions in the formation process between about 50 and 80% charge amount. Finally, the electrochemical reactions become dominant. The charge amount of different parts was also uneven in the entire formation, which was determined by the distribution of the current density in the initial stage of high current charge.

A method for preparing membrane electrode assemblies (MEA) for direct methanol fuel cells (DMFCs) by a layer-upon-layer fabrication onto a porous substrate is presented. This preparation technique is based on a single method for the fabrication of the whole MEA, e.g., spraying one functional layer onto the other, and therefore simplifies its preparation considerably. It also permits new fuel cell designs with in-plane serial connection of single cells by appropriate fabrication procedures. It is therefore especially suited for low power microfuel cells operated at ambient conditions. This method offers high flexibility in the choice of cell geometry as well as superior control of the structural parameters of the layer. Parameters like layer thickness or composition, catalyst and ionomer loadings can easily be varied and adapted to the specific needs of the ionomer and catalysts used. Here process parameters enabling such a layer-upon-layer MEA preparation have been developed successfully. Initial results with single cells prepared with the new technique are presented and compared with conventionally fabricated MEAs. Power densities of 5-7 mW/cm 2 at a cell voltage of 0.4 V have been achieved with single cells at ambient conditions, 0.5 M MeOH solution, and self-breathing cathode. This result is promising since conventionally prepared MEAs with prefabricated membranes show similar power densities. A planar multicell configuration is proposed and issues concerning serial connection in this concept are discussed and future work proposed.

A polymer lithium-ion battery, formed by a Li 4/3 Ti 5/3 O 4 -LiFePO 4 electrode combination and a poly(vinylidene fluoride) (PVdF)-based gel electrolyte, is presented and discussed. The electrochemical characterization demonstrates that this battery is capable of delivering appreciable capacity values at rates ranging from C/32 (160 mAh g -1 ) to 0.75C (130 mAh g -1 ), this being accompanied by a remarkable cycle life. In addition, because the two electrodes are based on common and nontoxic materials and operate within the stability window of the electrolyte, the battery is expected to be safe, inexpensive, and compatible with the environment. All these properties make the battery of prospective interest for application in the hybrid and electric vehicle field.

The effects of different factors such as the state of charge (SOC), the temperature, the polarization potential, the H 2 SO 4 concentration, and the saturation level of the negative plate on oxygen recombination in valve-regulated lead-acid batteries were investigated. The increase of the temperature and of the negative polarization as well as the decrease of the saturation level of the negative plate can all promote oxygen recombination, yet the SOC has almost no effect on the oxygen recombination as long as the capacity of the negative plate is high enough. Furthermore, it is found that the oxygen recombination rate increases and then drops when the H 2 SO 4 concentration at the negative plate increases from 0 to 5 M. The experiments also show that considering all factors, the saturation level of the negative plate is the dominant factor affecting the oxygen recombination rate.

Center for Solar Energy and Hydrogen Research, D-89081 Ulm, GermanyIn the oxygen cycle of valve-regulated lead-acid~VRLA! batteries, there are two ways in which oxygen can move from thepositive to the negative plates, namely, either horizontally to penetrate the absorptive glass mat ~AGM! separator, and/or transportvertically via the gas space. It is found that the oxygen transport depends on the passageway with big void space in the AGMseparator and its rate is proportional to the oxygen partial pressure. The rate constant of vertical transport is about three ordershigher than that of horizontal transport because of the large void space between the AGM separator and plates. However, in thehorizontal direction, the area is very large and the transport path is very short. So the way and the rate of oxygen transport actuallydepend on the level of saturation in VRLA batteries. The horizontal transport is dominant when the saturation is less than 93%,while the vertical transport becomes dominant when it is higher than 93%. The experiments also indicate that with decreasingsaturation, the recombination of more oxygen at the negative plate may oxidize more active H