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

V.C.6 Advanced Binder for Electrode Materials (LBNL)
Liu – LBNL
good electronic connectivity throughout the electrode. The
electrodes made with these binders have significantly
improved the cycling capability of Si.
Approach
Use functional polymer design and synthesis to
develop new conductive polymers with proper electronic
properties, strong adhesion and improved flexibility to
provide electric pathways in the electrode, and to
accommodate large volume change of the Si alloy active
material during lithium insertion and removal. The rational
design of binder is assisted with advanced diagnostic
techniques such as XAS at Advance Light Sources and
with advanced molecular computation at National Energy
Reseearch Secientific Computing Center – both are DOE
national user facilities.
Results
Conductive Polymer Binder with Tailored
Electronic Properties. We developed new conductive
polymers through a combination of material synthesis, X-
ray spectroscopy, density functional theory, and battery
cell testing (Figure V - 88). Contrasting other polymer
binders, the tailored electronic structure of the new
polymer enables lithium doping under the battery
environment. The polymer thus maintains both electric
conductivity and mechanical integrity during the battery
operation.
dope the polymer to achieve adequent electronic
conductivity. Mechanically, it is also crucial that the
polymer is intimately adhered to Si particle surface. Both
electrical and mechanical integrity of the electrodes will
then be maintained throughout the battery operation
The molecular structures of the developed polymers,
PFFO and PFFOMB are based on polyfluorene (PF)-type
polymers (Figure V - 88) Two key functional groups, carbonyl
C=O and methylbenzoic ester -PhCOOCH 3 (MB), were
introduced for tailoring the LUMO electronic states and
for improving the polymer adhesion respectively, as
elaborated below.
In order to achieve a properly tailored electronic
structure, we have extensively applied synchrotron based
soft x-ray absorption spectroscopy (XAS) on a series of
polymers to monitor the unoccupied conduction states.
XAS provides a simple but direct probe of the excitations
of core level electrons to the unoccupied states, i.e., the
lowest-energy XAS peaks directly correspond to the
LUMO states. Figure V - 89 shows the XAS data collected on
three selected PF type polymers and the traditionally used
p-type PAN. It is evident that the carbonyl groups in PFFO
and PFFOMB generate a new LUMO state at 284.7eV in
XAS (blue arrow), much lower than that of the PF (black
arrow) and PAN (purple arrow). The additional MB units
in PFFOMB do not change the low energy position of this
LUMO. Therefore, the carbonyl is the key function group
that lowers the LUMO energy level.
Figure V - 88: Molecular structure of conductive polymer binder.
The technical challenge stems from the reducing
enviroment when the Si anode is lithiated. For example,
the typically used p-type polyaniline (PAN) conductive
polymer will not stay p-doped below 1 V (Li/Li + ),
therefore losing electronic conductivity when used in the
Si anode, which is operated between 0.01 V–1 V (Li/Li + ).
In order to solve the crucial problem on electric
connectivity of the binder, we focus on developing
polymer binders that could be cathodically (n-type) doped
for high electronic conductivity under the reducing
environment for anodes. Our strategy for accomplishing
the goal is to tailor the energy levels of the polymer
conduction state, i.e., the lowest unoccupied molecular
orbital (LUMO), so that the electrons could cathodically
Figure V - 89: Carbon- 1s XAS spectra collected on a series of polymers.
Performance of Si/Conductive Polymer Composite
Electrode. A commercial Si material was used to fabricate
electrodes with the PFFOMB conductive binders. The Si
material is NOT carbon coated. A standard slurry making
process was used to fabricate all the composite
Si/conductive binder electrodes. Figure V - 90 presents the
performance of the PFFOMB/Si based anode that is cast
with a Si to polymer weight ratio of 2:1. The electrodes
based on PAN and polyvinylidine difluoride (PVDF)
Energy Storage R &D 556 FY 2011 Annual Progress Report