Research Overviews Environmental concerns over the use of fossil fuels have spurred the use of energy generated from renewable sources such as wind and solar. Solar and wind energy are among the most abundant and readily available. For example, the solar radiation energy the Earth receives in one hour is sufficient to meet worldwide energy needs for one year. However, one of the major problems of the solar and wind power is that they are variable and uncontrolled, which might cause significant challenges for the electric grid operators because other power plants (usually fossil fueled power plants) need to compensate for the variability of these renewable power sources. An effective approach to smooth out the intermittency and make the renewable sources dispatchable is to use electrical energy storage (EES), which can store excessive energy when they are available and release it when needed. The EES technologies are also demanded to improve the reliability and efficiency of grids. For example, one of such applications is so-called balancing services, which can be used to balance generation and demand in tightly limited situations. EES is perfect for this type of service by absorbing electric energy (charging cycle) whenever there is too much generation and by injecting electric energy into the power grid (discharging cycle) when there is a deficit. Recently, EES technologies are receiving more and more attention by providing fuel (i.e., electricity) for hybrid and electrical vehicles, and the broad applications and market penetration of these vehicle technologies will significantly reduce the world's dependence on fossil fuels. Among various EES technologies, electrochemical energy storage systems or batteries are capable of reversibly storing and releasing electrical energy via electrochemical reactions. The electrochemical energy storage systems do not involve “Carnot” cycles, thus potentially allowing a high efficiency. A number of battery technologies including Li-ion, Na-S, Ni-MH, Ni-Cd, Pb-acid, and RFB have been developed over the last century. Even though systems up to multi MWs/MWhs have been demonstrated for several technologies such as Li-ion, Na-S, Pb-acid, and RFB, most of these technologies still cannot meet the economic requirements for most markets even without accounting for operation/maintenance and replacement costs. In addition to the cost, these technologies may also face safety, environmental, or others issues. In our group, we are focusing on all types of materials & chemistries, components, and systems, and the ultimate goal is to enable next-generation high-performance batteries for various types of applications. We are particularly interested in the following technologies:
Solid-State Na-Metal Batteries Na-ion/metal batteries have been recognized as one of the most promising alternatives to Li-ion batteries for energy storage market because Na has properties similar to those of Li while costing less. Compared to liquid electrolyte Na-ion batteries, solid-state Na-metal batteries (SSNBs) can simultaneously achieve high energy and excellent safety, also providing advantages in fabrication, battery stacking/packaging, and transport. All of these make SSNBs ideal power sources for both stationary and transportation applications. However, one of the major issues with the SSNBs is the poor interfacial contact between the solid-state electrolyte (SSE) and the electrodes, which tends to incur Na dendrite formation and cracks in the SSE. The resistance at both the SSE/anode and the SSE/cathode interfaces needs to be reduced and stabilized, which is extremely challenging at the current stage. In our group, we are exploring a few of approaches such as modification of the interface with others phases and structures, introducing new technologies for battery component/system fabrication, etc. All of these will potentially address the problem and enable high-performance SSNBs.
Advanced Zn-Ion Batteries Current Li-ion batteries utilize an electrolyte consisting of flammable organic solvents (e.g., ethylene carbonate, diethyl carbonate). Under failure and abuse mechanisms such as overcharging, internal shorting, defect, physical damage, or overheating, the batteries can catch fire and explode, which potentially could cause serious safety concerns. Therefore, the use of non-flammable electrolytes is of great interest to improve the battery safety. Zn-based aqueous batteries (ZABs), which utilize low-cost and safe water-based electrolytes, recently have attracted attention particularly for grid-scale applications. For the Zn metal anodes, the advantages include high theoretical capacity (~820 mAh/g), low cost (high abundance, low toxicity, and therefore large-scale production), and high safety. Despite of the above-mentioned advantages, ZABs are facing a number of issues such as electrolyte decomposition, hydrogen evolution, low Zn plating/stripping coulombic efficiency (CE), etc. All of these issues are primarily related to the water-based electrolytes. Due to the narrow potential window of water (∼1.23 V), the electrolyte decomposition and hydrogen evolution could occur during battery operation. Although Zn dendrite formation could be minimized in mild aqueous electrolytes, its low CE remains a concern due to side reactions with the formation of Zn(OH)2 and ZnO. As a result, it leads to substantial underutilization of the theoretical capacity of Zn metal. Currently, we are focusing on the development of new electrolyte materials that can overcome the intrinsic problems of the start-of-the-art electrolytes. The new electrolytes may also open doors for new chemistries that typically do not work for the current systems.
Others Multivalent Metal Batteries Rechargeable aluminum batteries (RABs) based on Al metal anodes have attracted a lot of attention due to its advantages in earth abundance, cost and capacity. For example, Al enables a trivalent electrochemical redox reaction with a specific capacity of 2,980 mAh/g and a volumetric capacity almost four times of Li metal. Al is also advantageous in material handling and safety. An Al/S battery with Al metal and elemental sulfur as the electrodes exhibits a theoretical energy density of 1,340 Wh/kg, which is almost three times that of a LiCoO2/carbon cell (see the right figure). Great efforts have been taken to develop appropriate electrolytes with ideal properties and performance for RABs. Low-cost aqueous electrolytes typically cannot be used because of the inherent hydrogen generation in the Al anode, which is similar to those in the ZABs. Organic solvent electrolytes generally do not provide reversibility for Al plating/stripping due to formation of a surface passivation layer on the Al anode. Further optimization of current electrolyte materials or development of alternatives would be helpful for advancing the RAB technologies.
We are grateful for the financial support of our research from the following sponsors: