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.
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
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).
Lithium-ion batteries - sometimes
referred to as Li-ion batteries, are a type of rechargeable battery in which
lithium ions move from the negative electrode (cathode) to the positive
electrode (anode) during discharge, and from the cathode to the anode when
charged. Lithium-ion batteries are common in portable consumer electronics
because of their high energy-to-weight ratios, lack of memory effect, slow loss
of charge when not in use, and short lifespan.
"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.
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).
Copyright AZoNano.com, Professor Daniel Morse (University of
Calfornia, Santa Barbara)