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

V.C.5 Development of High Capacity Anodes (PNNL)
Zhang, Liu – PNNL
size on battery performance were investigated
systematically. Porous Si shows improved cycle stability
with increasing pore size. This finding provides a guide
for rational design of stable silicon anodes.
Results
SiO x Anode with a Three-Dimensional Rigid
Skeleton Support. We made significant progress in
developing high-capacity stable SiO x anodes. We obtained
a stable capacity of ~600 mAh/g (based on the full
electrode) over 90 cycles. This achievement was made by
anchoring SiO x on a rigid structural skeleton support and
coating a conductive layer on outside of SiO x to provide
continuous electrical contact (Figure V - 84). We tested coin
cells using this composite as the working electrode and Li
metal as the anode between 0.02 and 1.5 V at a current
density of ~100 mA/g.
Capacity / mAh g -1
1200
1000
800
600
400
200
100mA/g
Charge
Discharge
0
0 20 40 60 80 100
Cycle Number / n
Figure V - 84: Stable cycling of SiOx-based anodes with a rigid structural
skeleton and continuous conductive carbon coating.
Si Anode with a Three-Dimensional Rigid
Skeleton Support. Progress has been made in producing
silicon anodes with rigid skeletons and continuous
graphene coatings. A stable capacity of ~800 mAh/g
(based on the active materials) over 90 cycles was obtained
by anchoring Si on a rigid skeleton support and coating a
conductive layer on the Si/skeleton support to provide
continuous electrical contact (Figure V - 85a). We tested the
coin cell between 0.02 and 1.5 V at a current density of ~1
A/g (three formation cycles are tested at a 100-mA/g
current density). The Si anode also exhibited very good
rate performance. Figure V - 85b showed the cycling
performance under different current densities ranging from
0.5 to 8A/g. The capacity is ~500 mAh/g at 8 A/g.
We used the same Si anodes to investigate the effect
of an electrolyte additive, fluoroethylene carbonate (FEC).
With the FEC additive (the standard electrolyte is 1-M
LiPF 6 in EC:DMC (1:2)), the cycling stability
of the Si-based anode is improved significantly. As shown
in Figure V - 85c, there is almost no capacity fading over 70
cycles with the addition of 10% FEC. As a comparison,
the capacity is only ~50% of the initial capacity after 70
cycles when no FEC additive was used.
Porous Si Anodes and Pore-Size Effect. A porous Si
sample with 10-nm pore size has ~25 wt% carbon after
CVD coating. This carbon coating is thicker than those
coated on porous silicon Si samples with smaller pore sizes
of 7.5 nm (15 wt% CVD carbon) and 5 nm (5 wt% CVD
carbon). TEM anlaysis showed that the porous Si (10-nm
pore size) was coated with partially graphitized carbon
both outside the particle and inside the pores. The primary
silicon nanocrystallines have the size of tens of nanometers
(see Figure V - 86a). We analyzed the pore-size change
before and after CVD carbon coating using the Barrett­
Joyner-Halenda pore-size distribution method. The pore
size decreases from 10 to 7.5 nm after CVD C coating
(Figure V - 86b). The pore volume decreases from 0.4 to
0.14 cm 3 /g. The TEM and BJH results show that the
carbon is coated both on the outside surface and also inside
the pores. The continuous carbon coating greatly improves
the conductivity of the silicon anode.
Battery test results showed that the porous Si with
larger pore sizes have better cycle stability than those with
smaller pore sizes (Figure V - 87a). Porous Si (10-nm pore
size) has a capacity of ~2750 mAh/g calculated from the Si
weight at a current density 100 mA/g. The cycle stability
is much better than the porous Si samples with small pore
sizes. At high current density, the porous Si samples with
10-nm pore size also show higher capacity and better cycle
stability than the samples with small pore sizes. It has a
capacity of ~800 mAh/g based on the whole electrode
weight, including the carbon additives and binder (Figure V -
87b).
Conclusions and Future Directions
Si- and SiO x -based anodes with rigid skeletons
showed significantly improved capacity and stability.
Investigation on the porous Si suggested that improved
performance could be obtained by using large pore-size
material and good carbon coatings. In situ characterization
revealed that the accumulated damage on the Si layer is a
major factor leading to the capacity fading. In future
work, we will focus on the following three aspects:
1. Identifying the best structure-supporting and
conductive-coating materials
2. Optimizing the ratios among the active material, the
structure supporting material, and the conductive
coating to balance the high capacity and cyclability
3. Developing new conductive additives and electrolyte
additives to further improve cycling stability.
Energy Storage R &D 552 FY 2011 Annual Progress Report