PRIMARY AND SECONDARY BATTERIES

HIGH CAPACITY MATERIALS FOR Li-ION and Li-S BATTERIES

Li-ion batteries have transformed energy mobility since their commercialization in the 1990s.  Until now, conventional materials (graphite anodes and spinel oxide cathodes) have been sufficient since the energy density of these batteries has been more than enough to enable the desired applications.  However, the power demand of mobile devices is rapidly increasing as the public’s demand for enhanced functionality shows no sign of stopping.  The automotive industry is also putting considerable pressure on traditional materials as increased energy storage is needed to enable an acceptable vehicle range.  Both these applications demand lightweight, small footprint solutions that can only be accomplished by transitioning both the anode and cathode to newer materials with much higher energy density. 

At the anode, our primary focus has been the replacement of graphite to transition metal oxides (TMOs).  TMOs are a promising class of materials because they allow for multiple electron transfer steps per mol of active material.  Unfortunately, most metal oxides undergo alloying or chemical conversion reactions that change the bonding of active materials, and sometimes even forming electrochemically inactive materials, limiting both cycle retention and rate capability.  Our primary work in this area has revolved around understanding the roles of structure and conductivity on the reaction reversibility of TMOs during charge and discharge, with NiO being our primary probing compound, though we have done a lot of work with Co, Mn and Sn oxides as well.  To date, We have been able to achieve anodes with > 700 mAh/g capacity over 1000's of cycles and excellent rate capability (up to a 10C rate) .  We are further pushing these materials to more realistic formats and systems, with the hopes of reducing their cost and helping to commercialize their use in the next generation of Li-ion batteries.

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At the cathode, we have focused on the development of Li-S cathodes.  Sulfur is a potentially game-changing cathode.  However, most of the Li-S cells that have been produced to date suffer from either lower-than-expected capacity or rapid capacity fading with cycling.  Our focus has been working to understand the role of the S microstructure on mechano-electro-chemical stability.  This includes the design of novel S structures.  However, making fancy new materials is not the only way to make progress in electrochemical cells.  We have also used our background in electrode optimization for a number of applications and have applied it to this system.  The result are Li-S cells that are stable over 100's of cycles.  

PRIMARY ALKALINE BATTERIES

In alkaline batteries, one of the main issues remains the evolution of hydrogen gas during cell life.  Hydrogen gassing occurs both on the shelf and during cell rest - from thermodynamically driven corrosion of the anode material.  Hydrogen gassing also occurs in rechargeable alkaline cells - significantly reducing the faradaic efficiency below the desired levels.  Hydrogen gassing is even worse on so-called "next generation" materials like aluminum because the driving force is greater in Al-anode alkaline batteries than commercial Zn-anode batteries.  Our work in this area focuses on understanding the material structures that are responsible for corrosion and developing new approaches to mitigate it - not only to make existing batteries safer and more reliable, but also to enable the use of higher energy density materials to be used and rechargeable ultra-low cost alkaline battery platforms to be realized.

NEW METHODS DEVELOPMENT

The behavior of materials in operating devices is very complex.  During charge and discharge, cells are cycling between (at least) two completely distinct chemical phases, which impacts electronic conductivity, ion diffusion, etc.  In our group, we work to create new methods to study the operation of materials and cells in-situ and operando.  We have developed several tools in our lab that fall into this category.  For batteries, we have recently created new techniques that allow for multiphase diffusion decoupling, identical location TEM imaging and direct observation of the active materials during the discharge of operating alkaline AA cells.