Next Generation Batteries: Aim for the Future

Next generation electrochemical energy storage devices are of great interest for applications in both research and industry. Here, under the help of Dr. Carolina Novo da Silva, we proposed this special issue designed to complete the special issue in Advanced Materials for the Next Generation Battery Symposium which was held in 2016 at the University of Wollongong. This issue contains different length reviews to highlight research hot spots in next generation batteries, including high energy Si anode for Li-ion batteries, carbon based anode and cathode materials for sodium-ion batteries, Li-Sulphur batteries, Na-Sulphur batteries, Li-Air batteries, and Potassium-ion batteries. Silicon is the most promising next generation anode material for Li-ion batteries owing to its highest known theoretical capacity and abundance on earth. The current commercial anode has only a limited amount (≈5%) of Si in the anode side. However, the challenge is still there for a high Si loading anode including low initial coulombic efficiency and a low cycle life due to the large volume changes, unstable solid electrolyte interphase (SEI) layer, and poor electric and ionic conductivity. Luo summarizes the current progress for using surface and interface engineering strategies to solve these issues. Many types of functional shells, such as silicon oxide, metal, metal oxide, carbon, and polymer were used to create the core-shell, yolk-shell and/or sandwiched structures to boost the lithium storage performance. The future work on understanding the fundamental Si electrode and materials chemistry should more focus on real batteries and not coin cells or half cells. The review also points out that the high production cost of nanostructured silicon is also a major obstacle for the commercialization of silicon anode. More work should be focused on developing a commercially available low-cost synthesis method for a high performance nanostructured silicon anode with environmentally friendly route as well. Binder only takes a small partial of weight (<5%) comparing to anode or cathode materials in a real commercial battery, but it plays a critical role in maintaining the integrity of the electrode to achieve a good cycle life. Sun's group summarizes the recent progresses on the development of novel eco-friendly, low-cost and water-soluble binders which were observed with enhanced chemical/physical interactions with the electrode materials and stronger mechanical adhesion for better durability for large volume variation. A significant improvement of electrochemical performance has been reported for Si-based anodes, spinel/layered oxide cathodes and sulfur cathodes. It is also noted that the electrochemical performances for most of the water soluble binders reported in the literature were investigated in lab scales at mild conditions in half-cell testing systems. Long term stability testing in a real full battery system under extreme conditions is yet to be done. This will bring the collaborative work from both academic and industry. Li-sulfur batteries, which can provide high specific energy for next generation batteries beyond portable electronic devices with low-cost feature, have become a hot topic in research. Zhang's group highlight the recent advance in high-sulfur-loading Li-S batteries enabled by hierarchical design principles at multiscale including developing novel host materials for sulfur, using advanced binders, designing rational electrode architectures, optimizing high-efficiency and high current anodes, applying multifunctional separators, and inventing novel cell configurations. Although great progress has been achieved in the laboratory, the performance of Li-S pouch cells is still below the great expectation from the market. Tremendous challenges such as safety issue with the involvement of lithium metal need to be solved before Li-S could be the next commercial success after Li-ion batteries. Organic electrode materials could be the promising active materials for next generation green and sustainable lithium/sodium ion batteries owing to their low cost, environmental benignity, renewability, redox stability and structural diversity. Quinone based organic materials are considered to be the most promising electrode materials for both Li/Na ion batteries because of their high theoretical capacity, good reaction reversibility and high resource availability. The challenges include the solubility in the electrolyte leading to short cycle life, poor electric conductivity leading to poor rate capability, low working potential (<3.0 V) leading to low specific energy, low density leading to low volumetric energy density. Wu summarizes the recent progress on Quinone based organic active materials for both lithium and sodium ion batteries. Introducing a functional group and a conductive/protection coating could effectively prevent the dissolution of active materials, increase the discharge plateaus and enhance the electron transfer. The future looks bright for Quinone based active materials for Li/Na-ion batteries. Li-O2 battery research is also one of the focuses in this special issue as Li-O2 batteries theoretically offer the highest specific energy among all the electrochemical energy storage systems. So far, many research efforts have been made to address the challenges including low round trip efficiency, low specific capacity, and poor cycling stability. Luo highlights the recent progress associated with novel air electrode optimization and understanding the reaction process. It is worth noting that a deeper understanding of the mechanisms behind the electrochemical reactions will also lay the foundation for exploring ideal air electrodes and more stable electrolytes. Therefore, Li and Chen present the perspectives on the development of Li-O2 batteries based on the discussion of various reaction mechanisms at the cathode side, including the direct electrochemical formation and decomposition of Li2O2, LiOH and the redox mediator. Li also points out transforming Li2O2 or LiO2 to LiOH in water-containing electrolytes is an effective method to reduce the discharge/charge potential gap. The mechanism is still under debating. However, the decomposition of LiOH to regenerate O2 was not observed by Nazar's recent work (ACS Energy Lett. 2016, 1, 747). Shen suggests that the different mechanism is due to the huge difference of experiment setup from one report to another and evaluation method rationally borrowed from the Li-ion batteries. Therefore, how to objectively evaluate the electrochemical performance of Li-O2 batteries is worth to be discussed. Shen points out that the specific capacity based on only the weight of cathode materials is misleading and should not be used, because the amount of the electrolyte greatly influences the capacity and thus the specific capacity should be based on the total weight of electrolyte and cathode materials. As for the cycling stability evaluation, the amount of cathode reactant (either O2 or Li+) should be limited to eliminate the contribution from side reaction. On the prototype cell level, high specific energy Li-O2 batteries are still irreversible. Therefore, we are still in the early stage of the development of Li-O2 batteries. Hopefully, the revolutionary cathode chemistry or device configuration will come to improve the practical performance of Li-O2 batteries. Rechargeable Mg-Air batteries are a promising alternative to Li-air batteries owing to their excellent safety, low-cost originating from the abundant resource, and high theoretical volumetric density (3832 A h L−1 for Mg vs 2062 A h L−1 for Li). However, the fundamental scientific difficulties slow down the development of secondary Mg-air batteries. Sun highlights the major progress and the reaction mechanisms of secondary Mg-air batteries. It is noted that the mechanism involved the MgO2 as the cathode product shows high reversibility. Therefore, the future work could focus on how to control the reaction pathway to further improve the energy density and cycling stability. Porous carbon composites have attracted intense attention due to their unique properties, including high surface area, large pore volume, and unique pore size distribution, which open up many applications. Liu summarizes the progress on porous carbon composites for next generation lithium-based batteries such as high energy Li-ion batteries, Li-sulfur batteries, and Li-O2 batteries. Generally speaking, the porous carbon can act as conductive host to shorten the lithium diffusion pathway, to depress the dissolution of polysulfide molecules (Li-S) and to enhance the catalytic property of nanocrystalline catalysts (Li-O2). Carbon materials are also considered to be the most promising candidate as the anode for sodium ion batteries which could be the next generation low cost batteries to store renewable energy. Ji's group systematically summarizes the progress of the sodium storage performances for carbonaceous materials, including graphite, hard carbon, amorphous carbon, heteroatom-doped carbon, and biomass derived carbon. For large-scale application, the biomass derived hard carbon should be preferred owing to the low cost. Although some decent results have been reported, the low initial coulombic efficiency and low rate capability of carbon based materials should be improved. Most of the reports show low initial coulombic efficiency as low as 70%. A practical application would require an initial coulombic efficiency over 90%. Future work would also focus on lowering the processing cost and further optimizing the electrolyte and additive to tailor the formation of stable solid electrolyte interphase. Further understanding of the mechanism for sodium storage will also help us to further improve the performance. Cathode materials plays a decisive role to determine the energy density. Our group focuses on the commercial prospects of existing cathode materials for Na-ion batteries in terms of environmental friendliness, manufacturing cost, and electrochemical performance. The current cathode materials including transition metal oxides, phosphates, Prussian blue analogues, sulphates and fluorides are compared using the ratio of the cost to energy density. The cost here includes the raw materials cost and production cost. The calculation results show that the promising cathode includes O3-Na0.9[Cu0.22Fe0.30Mn0.48]O2, NaMn[Fe(CN)6], and Na4Fe3(PO4)2P2O7. Room temperature Na-Sulfur (RT Na-S) batteries are a promising low-cost energy system with potential application in large-scale stationary energy storage owing to the low-cost and abundance resources for both anode and cathode materials. Wang reviews the progress on the current RT-Na-S batteries which is still in its infancy stage. This system suffers from limited cycle life due to the high solubility of reaction product and serious self-discharging. The knowledge from Li-S batteries could help to develop the cathode of Na-S batteries in some extent. However, the different mechanism route suggests us that the RT Na-S is a whole new system to develop cathode materials, anode, cell design and electrolytes. In situ techniques such as Synchrotron and Neutron could help us to better understand the reaction mechanism and kinetics in batteries. Gu highlights that using large facility technologies,in situ techniques with high resolution and intensity, will become increasingly popular to develop next generation batteries. Potassium-ion batteries were also reviewed with focus on the recent advance by Sharma. The current reports on Potassium full cells indicate an exciting future for K-based batteries. However, the price of Potassium is still high and the high reactivity for K is also a big safety concern. A suitable application in the future is still under digging as soon as the accepted performance can be achieved. The future of next generation batteries is bright and can only be successful with a close collaboration between academic and engineering. If we would have to predict the commercialization for different battery systems, the time scale would be in the order of high energy Li-ion batteries < Na-ion batteries < Li-S batteries < other systems. Last but not least, we would like to thank Dr. Carolina Novo da Silva again for handling the manuscripts and making all the hard decisions, and Dr. Julia Ranzinger for her contribution and kind help, too. We hope this special issue could help researchers to further make contributions to achieve next generation batteries for a better future of our tomorrow world. Shulei Chou is a Senior Research Fellow in ISEM at University of Wollongong (UOW). He obtained his bachelor (1999) and master degree (2004) in Nankai University, China. His research focuses on energy storage materials for battery applications, especially on novel composite materials, new binders, and new electrolytes for Li-ion and Na-ion batteries. Yan Yu is a Professor of material science at the University of Science and Technology of China (USTC). She received her Ph.D. in material science at USTC in 2006. From 2007 to 2008, she worked as a postdoc at the Florida International University. After that, she received a Humboldt Research Fellowship and worked at the Max Planck Institute for Solid State Research in Stuttgart, Germany. Her current research interests mainly include the design of novel nanomaterials for clean energy, especially for batteries and the fundamental science of energy storage systems.