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

Vaughey – ANL
V.C.7 Three-Dimensional Anode Architectures and Materials (ANL)
Approach
· Design new electrode architectures by
electrodeposition techniques in which a Cu foam
provides an electronically connected substrate onto
which electrochemically active metals can be
deposited.
· Develop methods to build electrodes where copper
metal acts as a binder and conductive additive for high
capacity main group metals, e.g. silicon.
· Develop characterization methods that allow for a
better understanding of the electrochemically active
interfaces with in the electrode architecture.
been used a probe of the system on annealed laminates
processed at various temperatures, Figure V - 96.
Results
The initial focus was to develop a method to encase
micron-size Si particles with a Cu net or mesh formed in
situ. The use of micron-size Si particles (10 to 20 μm)
allows for lower surface area contact with the electrolyte,
thus eliminating losses due to SEI formation and allowing
for easier handling of the raw materials. Initial attention
centered on generating a thin and dispersed Cu coating on
the Si particles. After evaluating various methods, an
electro-less deposition method was chosen for its
simplicity and coating characteristics (see Figure V - 94).
Figure V - 95: Demonstration of how silicon is bound to the copper foil
(edge-on view) after annealing.
Figure V - 96: MAS-NMR study of species formed on annealing of electrode
materials.
Figure V - 94: A SEM image of the copper-coated silicon particle (20-25 μm)
after annealing.
After a detailed annealing study, a set of conditions
were identified where the deposited Cu coating annealed to
form a three-dimensional network over the Si, binding it to
the underlying Cu foil (Figure V - 95). Preliminary XRD
studies showed that below 700°C there was only a minimal
amount of reaction between Cu and the Si particles to form
the intermetallic phase, CuxSi, in agreement with the
reported phase diagram. Using this method, a mesh was
created over the Si in situ, binding it to the current
collector without the need for additional binders or
conductive additives. In an effort to better understand the
interface between the annealed copper mesh and the
electrochemically active silicon, solid state 29 Si NMR has
Early work, in conjunction with X-ray diffraction
studies, indicated that the predominant alloy phases
formed on long term heating are Cu 3 Si and maybe Cu 4 Si.
MAS-NMR studies have indicated that the best cycling
electrodes have very little alloy formation and little silicon
surface oxidation (from annealing process). Electrodes
with larger numbers of Si species did not cycle as well as
other electrodes. Since many silicon oxides are
electrochemically inactive, their presence on the particle
surface from the processing may be blocking lithium
insertion. In addition electrodes annealed at higher
temperatures were found to have peaks that can be
associated with Cu 3 Si. Due to the small amounts of
intermetallic materials present in the laminates, the
identification of the interfacial phases was not achievable
using powder XRD methods.
FY 2011 Annual Progress Report 561 Energy Storage R&D