Biologically Inspired Approach to High-Performance Batteries

New materials are needed to radically transform the efficiencies of energy harnessing, transduction, storage and delivery, yet the synthesis of advanced composites and multi-metallic semiconductors with nanostructures optimized for these functions remains poorly understood and even less well controlled.

UCSB scientists have now developed a revolutionary new, biologically inspired method for the synthesis of semiconductors they believe can address this need. Discovering the secret of the underlying mechanism by which living organisms can make nanostructures of glass at low temperature (to form the skeletons of certain sponges), they developed a revolutionary new method for the catalytic synthesis of a wide range of nanostructured semiconductors and metals that operates at low temperature, and at relatively low cost.

Unlike conventional methods of semiconductor synthesis that operate at high temperature and require costly assembly lines, this new, biologically inspired method produces nanostructured metals and semiconductors by kinetically controlling their growth through the use of catalysts - just as the scientists discovered nature does.

Using this new low-temperature method, they developed a novel composite consisting of nanoparticles of tin uniformly dispersed throughout the compliant and conductive matrix of graphite microparticles. The result¹ is a high-performance anode for lithium ion batteries with 30% higher electrical capacity (on a weight-basis; 50% higher capacity on a volume basis) than the currently used commercial anode of graphite alone, and with rock-solid stability.

In contrast to the efforts of manufacturers that have attempted to make similar composites by mechanically grinding the tin and graphite together, the UCSB team grows the tin nanoparticles catalytically, inside the pores of the graphite, thus achieving a more intimate marriage of the two materials, while retaining the valuable high crystallinity of the graphite (a fragile material, quickly destroyed by grinding).

"The big advantage of this new composite," according to team-leader Professor Daniel Morse, and Dr. Hong-Li Zhang, developer of this anode, "from its higher electrical capacity, is its excellent stability during multiple cycles of battery charging and discharging.

Metals such as tin have long been know to have a significantly higher electrochemical capacity than graphite and other forms of carbon used in commercial batteries today, but they suffer enormous expansion and contraction with each cycle of entry and exit of lithium ions into the metal with each cycle of charging and discharging, quickly causing the metal to disintegrate and lose electrical connectivity, and thus quickly losing power.

In contrast, in the new composite made by the UCSB team, the tin nanoparticles provide their higher electrical capacity, while the conductive and resilient, porous graphite provides a compliant matrix able to buffer and accommodate the large volume changes that accompany the reversible alloying and de-alloying of Li into and out of the tin nanoparticles. Thus, this new composite exhibits a remarkable stability and maintenance of high capacity through multiple cycles of charging and discharging, without the significant loss in capacity typically seen in other composite electrodes.

Cathodes - the other essential electrodes in batteries - made by this method exhibit 70% higher electrical capacity than present commercial levels, also with superior cyclability. The team at UCSB also is developing a novel safety material that will quickly shut off the battery in the event of a short circuit, thus preventing the fires and explosions that continue to plague lithium ion battery manufacturers, causing massive recalls of tens of millions of batteries in the recent past.

The UCSB team's unique, kinetically controlled synthesis method is the key. Conventional processes used by industry today simply cannot make materials with the properties described above.

References

1. Zhang, H.-L. and D.E. Morse. 2009. Vapor-diffusion catalysis and in situ carbothermal reduction yields high performance Sn@C anode materials for lithium ion batteries. J. Mater. Chem. (in press).