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Posted on July 11, 2016

Li-air battery technology: tough road ahead

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In mid 2016, A. Laforgue, Automotive and Surface Transportation, National Research Council of Canada and collaborators issued a paper "Assessment of the Li-air battery technology for automotive applications through the development of a multi-electrode solid-state prototype". It comes at a time when the US Department of the Environment has told IDTechEx it has de-emphasised research on this option because of fundamental limitations.
 
This report highlights the results of a project carried out at the National Research Council of Canada (NRC), with the overall objective to address the major issues of the Li-air battery technology and assemble a multi-electrode pilot-scale prototype. Several innovative ideas and technologies were investigated along the core concept of a polymer-based solid-state technology. The assessment of the Li-air technology is discussed through the results of the project as well as other sources, especially concerning its readiness and perspectives for automotive applications.
 
The authors say, "All the components of a Li-air battery have been investigated in the project, keeping in mind the overall objective of building a realistic Li-air battery, including 3D porous current collectors, cathode catalysts, carbon supports for the catalysts, and a solid polymer electrolyte".
 
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Design and assembly of a large-scale Li-air battery prototype was carried out because 500 mile range for affordable EVs is the primary objective, they studied not just electrochemistry at the cell level, but also the engineering feasibility of large-scale systems.
 
"All the Li-air large-scale prototypes reported in the literature are based on soft pouch cases with areas open to the atmosphere for O2 intake. These are often 2-electrode designs or 3-electrodes designs (a lithium metal anode sandwiched between two cathodes) for the more elaborated ones 12-13 and unfortunately do not represent real prototypes that can be extrapolated to larger scales. The only way to achieve a realistic Li-air battery prototype is to develop a multi-electrode stack design with integrated gas channels, they advise. Such a design was realized, with the strategy to limit the penetration of the SPE integration layer into the nickel foam, in order to use the upper part of the foam as the gas channel. Selected Li-air components were upscaled. Unfortunately, the NRC foams could not be reproducibly obtained at a larger scale, and commercial INCO foams were used instead. The spray deposition of the active materials onto nickel foams was upscaled to a 12x12 cm2 dimension using an automated scanning spray deposition technique. Methods of integration and component alignment were also designed and fabricated.
 
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The design of a number of stacked air-electrodes requires efficient gas channels to provide oxygen to the electrodes. In the developed design, two main gas tunnels are located at the vertical of the gas inlet/outlets. These vertical tunnels provide gas to each level of the stack. The gas is then channeled through the upper part of the nickel foams and reaches the outlet on the other side of the stack, hence providing oxygen to the whole active area. X-ray microtomography was used for characterization of the quality of the stack assembly and the efficiency of gas channels.
 
The SPE dimension was slightly bigger than the lithium (and cathode) dimension, in order to protect
lithium from the oxygen atmosphere at the stack edges. The prototypes were assembled in an anhydrous room, then inserted into a soft pouch that was thermally sealed on all sides. The last side was vacuum sealed in order to remove all atmospheric contaminants before flushing with oxygen. The prototypes were then transferred to the testing lab".
 
Overall, the best catalysts tested were the lead ruthenium oxide pyrochlore (PbRuO) 7 and the nickel cobalt oxide spinel (NiCo2O4). However the batteries did not perform well despite using such toxic and expensive materials.
 
The authors concluded:
 
"The Li-air technology holds great promise for the development of high energy density batteries needed to extend the range of electric vehicles. However, a number of fundamental challenges need to be overcome to enable this technology. This project aimed at investigating these challenges and possibly find solutions to a number of them. On the anode side, the use of metallic lithium requires the development of a solid-state electrolyte layer that can protect it while preventing the growth of dendrites during the recharge. Such a solid-state electrolyte, formulated with specific additives was successfully developed.
 
The cathode needs to keep a high porosity, while integrating a significant load of active materials and ensure an efficient triple phase (O2-ions-electrons). For this purpose, ultra-porous nickel foams were developed, allowing to decrease the foam weight contribution by 30 % compared to commercially available INCO foams. The formulation of the active layer was extensively studied, with the screening of multiple carbon supports, catalysts, binders and deposition methods. It was found that the use of catalysts did not clearly lead to performance improvements relative to carbon-based cathodes. The loading of active materials was optimized to reach the highest energy per surface area. An optimum of 3 mg/cm2 was established for this cathode technology, higher than the sub-1 mg/cm2 usually reported in the fundamental studies. However, it still needs to be improved for real applications.
 
The development of a cathode-electrolyte compatibilization layer was necessary to enable an efficient triple-phase in the porous structure of the cathode. The melt-integration of a PEO-based electrolyte formulation into the porosity of the cathode proved to be the best way to integrate both components.
 
The overall performances of the solid-state cells did not meet the expectations in terms of energy density and power density, and more fundamentally, in terms of electrochemical stability. A number of studies carried out within the last three years revealed that the major issue of the Li-air technology lies in the reactivity of its lithium peroxide product, which corrodes both the electrolyte layer and the carbon of the cathode's active layer. No definitive answer to that problem has been found so far, but a few recent reports might lead to interesting perspectives, all of them avoiding the formation of lithium peroxide : Liu et al. developed a specific electrochemistry that directly transforms Li2O2 into LiOH, which can then be redecomposed using a specific redox mediator 23. On the other hand, Lu. et al. demonstrated an iridium based catalyst that specifically leads to the production of lithium superoxide (LiO2), which is both more conductive and more stable than lithium peroxide 24. Another interesting avenue is the development of sodium-air batteries, which have also been shown to lead to the superoxide rather than the peroxide 25-27.
 
Finally, a large-scale multi-electrode prototype was designed and fabricated to investigate the Li-air upscaling challenges. The prototypes reached similar performances as the lab-cells in terms of cell capacity, which demonstrated that gas distribution could be possible in the upper part of the cathode foam, suppressing the necessity to use specific gas distribution plates as typically required in fuel cells technology. However, the significant weight added to the stack by the frame and gaskets - even if nonoptimized - pointed out the important challenges still lying at the system level.
 
The overall results of the project concord well with other recent reports to conclude that this technology, despite its important promises, is still far from commercialization 14, 28-29. A number of critical issues and challenges need to find solutions before an application can be envisioned".
 
IDTechEx sees this study confirming its view that Li-air is not one of the most promising post Li-ion options for the next decade. Lithium-sulfur is in the lead but even that is a long way from threatening lithium-ion battery sales to any significant extent, though niche uses are opening up. See the IDTechEx reports, Advanced and Post Lithium-ion Batteries 2016-2026 Technologies, Markets, Forecasts External Link and Lithium-ion Batteries 2016-2026 External Link.
 
References
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