V. Focused Fundamental Research - EERE - U.S. Department of ...

V.C.5 Development of High Capacity Anodes (PNNL)
Ji-Guang Zhang and Jun Liu
Pacific Northwest National Laboratory
902 Battelle Blvd., Mail Stop K3-59
Richland, WA 99352
Phone: (509) 372-651; (509) 375-4443
E-mail: jiguang.zhang@pnl.gov; jun.liu@pnl.gov;
Start: Date: October 1, 2010
Projected End Date: September 30, 2011
Objectives
· Develop Si-based anodes with high capacities, cycle
stabilities, and rate capabilities.
· Develop a low-cost synthesis route for Si-based
anodes.
Technical Barriers
Low energy density, limited cycle life, and high cost.
Technical Targets
· Identify good composite structures and good
conductive additives to improve the mechanical and
electrical stability of Si-based anodes
· Develop a low-cost and scalable approach to
synthesizing Si-based nanocomposite materials with
improved capacity and stability.
Accomplishments
· Developed Si and Si-based anodes with a rigid
skeleton support and nanostrucutered carbon coating.
The anode demonstrates high capacity and improved
cycle stability.
· Developed low-cost and scalable synthetic methods,
such as mechanical ball milling, to anchor Si or SiO 2
on a skeleton support and create continuous
conductive paths. Significant progress was made in
developing high-capacity stable Si and SiO x anodes.
A stable capacity of ~600 mAh/g (based on the full
electrode including the carbon additive and binder)
over 90 cycles was obtained. A Si-anode with a
similar structure provided a capacity of ~ 650 mAh/g
(based on the full electrode including the carbon
additive and binder) and 80% capacity retention over
90 cycles.
· In collaboration with Vesta Si, we developed and
tested several batches of micron-sized porous Si for
anodes. The pore sizes ranged from ~5 to 10 nm.
The effect of pore size on battery performance was
investigated. The cycle performance of a porous Si
anode increases with increasing pore-size. A critical
pore size:wall thickness ratio of ~3:1 was estimated to
give stable cycle life.
Introduction
Si and Si-based materials are good high-capacity
anode candidates for Li-ion batteries; however, because of
large volume expansions and phase transformations upon
lithiation and de-lithiation, they often show rapid capacity
fading during cycling. The low conductivity and poor
stability of these materials usually require the addition of
conductive additives and/or coatings to enhance electron
transport and electrical contact of the active materials.
Good capacity retention could be obtained when a much
larger amount of carbon was added to the material, but this
will lead to a decrease in the capacity of the full electrode.
To increase the cycle life of the anode without sacrificing
the capacity, novel structured anode composites, with
capacities more than double that of the state-of-the-art
graphitic anodes need to be developed.
Approach
Using low-cost, scalable methods such as mechanical
ball milling, we developed novel Si and Si-based
composites with rigid skeleton supports and
nanostructured carbon. Micron-sized Si/SiO x were broken
down into nanosized particles and attached to the surface
of rigid mechanical supports. The Si/skeleton support then
was coated with conductive carbon by ball milling to
improve the electrical contact. The three-dimensional
composites have high capacity and improved cycle life.
Different rigid skeletons with high mechanical strengths
can be used. The conductive coating can be graphene
sheets, conductive carbon, metal, conductive polymer, etc.
With an optimized ratio, the composite anode can have a
stable and high capacity.
In another effort, we collaborated with Vesta Si and
developed a simple chemical-etching method to synthesize
micron-sized porous silicon with controllable pore size for
anodes. After CVD coating, porous Si demonstrated high
capacity and reasonable cycle stability. The effects of pore
FY 2011 Annual Progress Report 551 Energy Storage R&D