Battery Division Research Award: From Insertion-compound Electrodes to Conversion-reaction Electrodes for Energy Storage

The current lithium-ion technology is based on insertion-compound cathodes and anodes. Layered and spinel oxides and olivine phosphates are the leading insertion-cathode hosts while graphite is the sole insertion-anode host at the present time. Each of the three cathode systems in play offers advantages and disadvantages while exhibiting rich but complex structural/chemical behaviors, and their adoption in practical cells is determined largely by the type of application. This presentation will focus on the challenges and prospects of transitioning from insertion-compound electrodes to next-generation, high-capacity, conversion-reaction electrodes.

First, an overview of how low-temperature synthesis and processing (e.g., chemical lithium extraction and microwave-assisted solvothermal synthesis) approaches have assisted to develop a fundamental understanding of some of the complex behaviors of the three classes of cathodes. For example, with the use of low-temperature synthesis processes, the roles of the position of the transtion-metal:3d energy relative to the top of the oxygen:2p band, degree of metal-oxygen covalence, morphology and surface crystal planes, surface compositions, isovalent and aliovalent cationic and anionic doping on the electrochemical performances (e.g., capacity, irreversible capacity loss in the first cycle, rate capability, and cycle life) will be pointed out with representative examples.  

Second, the potential of alternative, conversion-reaction, nanocomposite alloy anodes to overcome the safety issues of the graphite anode will be presented. Specifically, the use of facile mechanochemical reactions to realize nanoengineered alloy anodes both for lithium-ion and sodium-ion batteries will be discussed.

Third, novel electrode architectures as well as innovative cell designs to overcome the persistent problems of the conversion-reaction sulfur cathode will be presented. Specifically, approaches to enhance the electrochemical utilization of sulfur and suppress the migration of dissolved polysulfide ions to the anode will be discussed both with lithium-sulfur and ambient-temperature sodium-sulfur cells. Also, approaches to stabilize the lithium-metal anode surface in a polysulfide-rich environment will be outlined.

Fourth, to overcome the formidable challenges of aprotic lithium-air cells, the possibility of hybrid lithium-air cells in which a solid electrolyte separates the lithium-metal anode in an aprotic electrolyte from the air electrode in an aqueous catholyte will be briefly presented. Specifically, the design and development of inexpensive, efficient catalysts for the oxygen reduction reaction and oxygen evolution reaction will be discussed.