In an invited paper presented at the 215th biannual meeting of the Electrochemical Society (ECS 215) being held in San Francisco this week, Professor Esther Takeuchi of the University of Buffalo provided a broad overview of the challenges “in implementing the science we develop” in commercially viable battery solutions, using a current project working with a silver vanadium phosphorous oxide (Ag2VO2PO4) cathode material for an implantable cardiac device (ICD) as an example.
Dr. Takeuchi is the Greatbatch Professor of Advanced Power Sources at the University of Buffalo, and has the distinction of being the most prolific woman inventor in the US, holding 143 patents at last count. Prior to joining the university, she spent 23 years at Greatbatch, Inc., with her final position there being Chief Scientist, Center of Excellence.
Wilson Greatbatch, co-inventor of the first successful implanted pacemaker, founded Greatbatch, Inc. in 1970 to develop long-lived primary batteries to fuel pacemakers. The company has since expanded to develop and produce numerous implantable medical devices, and has expanded to provide technology solutions for critical industrial applications, including customized battery power and wireless sensing systems.
Takeuchi briefly touched briefly on critical applications for Li-ion energy storage, including energy storage for renewable power generation; hybrid, plug-in-hybrid, and electric vehicles; and micro power coupled with sensor technology.
The basic set requirements for all advanced batteries, she noted, includes:
- Higher energy density (volumetric and gravimetric);
- Higher power capacity;
- Extended calendar life;
- Low and stable internal resistance (in most applications, internal resistance changes dramatically, she noted);
- Safety under use and abuse; and
|Mechanical Degradation of Traction Batteries|
|In another invited paper at ECS 215, Mark Verbrugge, Materials and Processes Laboratory, General Motors R&D Center, presented an update of work done in collaboration with Professor Yang-Tse Cheng at the University of Kentucky on the role of mechanical deformation on the life of electrode materials for traction batteries.|
|Insertion-electrodes employed in nickel metal hydride and lithium-ion batteries experience significant volume changes during charging and discharging caused by concentration changes within the host particles. Electrode failure can occur as a result of localized stress and strain energy.|
|In work presented in 2008, Verbrugge and Cheng noted that with decreasing particle size, the effect of the surface becomes significant, since the ratio of surface to volume quantities scales as the inverse of the particle size. In particular, they said, surface energy and surface stress are expected to affect the magnitude and distribution of diffusion-induced stresses when the radius of the particle is at the nanometer scale.|
|In the current talk, Verbrugge discussed the evolution of stress and strain energy within electrode particles when solute transport is dominated by diffusion resistance and mechanical failure criteria of host particles.|
|He noted that elasticity calculations can be used to assess the build-up of tensile stress and strain energy.|
As a result, there are opportunities for fundamental studies in the four primary components of a lithium-ion battery—cathode, anode, electrolyte and solid-electrolyte interphase (SEI). In addition, fundamental work needs to be done on the interactions—functional and parasitic—among those four elements.
An important point is that we need to understand the chemistry not only when first fabricated, but what is happening to active material as a function of discharge—and charge as well for rechargeable batteries—as we are changing the oxidation state of materials. Often many other things happens along with the change.
...there are several things to note, when we are talking about solid state materials, we have the coexistence of multiple oxidation state at any given time. Cathodes or anodes are not at thermodynamic equilibrium at any one given time. We have multiple things present in the cathode...that is some of the complexity that all of us deal with.—Esther Takeuchi
Dr. Takeuchi showed a SEM image of nanofibrils of silver metal forming off the blade-like structure of silver vanadium oxide cathode material. Scientists need to understand what happens in the structure and the physical parameters of the cathode material, Takeuchi said. In the case cited, the changes are quite advantageous, she said,as it improves conductivity by about an order of magnitude.
Takeuchi also stressed the importance of how a given material is actually fabricated, and showed four images of different fabrications of a cathode material, all chemically the same, but structurally and morphologically quite distinct.
How you make the material and what its physical nature is can really impact battery performance...how the material made changes its morphology and physical structure quite substantially, and because electrochemistry is a surface science, these physical parameters impact quite dramatically the behavior of batteries sometimes in a favorable way and sometimes in a not favorable way, depending upon what you are trying to accomplish.
Dr. Takeuchi touched briefly on a concept for a next-generation phosphate-based cathode material designed to improve conduction. Lithium iron phosphate materials, she noted, offer high thermal and chemical stability, but are challenged by low conduction. The novel material concept is a bi-metallic phosphate (MM'POx) in which the metals reduce to form a conduction network inside. The approach offers the stability of the phosphate-based cathode, combined with the possibility for multiple electron transfers. Her current material of interest is the Ag2VO2PO4.
Complete reduction of the silver and vanadium results in capacity of 262 mAh g-1. Her group is currently looking at the long-term stability of the cells.
She concluded by looking and packaging efficiency as another important area of opportunity.
What we have seen deployed is packaging efficiency addressed by mechanical means. Now we are seeing packaging efficiency addressed by new ways of thinking about how to build electrochemical systems.
Takeuchi showed a slide comparing the theoretical specific energy and the practical specific energy of a range of primary and secondary battery systems, and then the percentage of the theoretical specific energy each practical systems could deliver.
Even in the best cases, its someplace around 40% or a little bit above...leaving a lot of opportunity on the table. The opportunity is still to be gained, still out there as a possible challenge in terms of how we think about batteries in a different way.