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

Zaghib – HQ
V.B.6 Cell Analysis-Interfacial Processes: SEI Formation and Stability on Cycling (HQ)
Approach 140
5
4
3 @ 4.9Volt
Our approach is to develop an appropriate method to 120
stabilize the interface reaction of the high-voltage oxide
100
(Mn-Ni based like LiMn 1.5 Ni 0.5 O 4 ) by surface coating with
LFP effect
a more stable material like olivine. The emphasis is to
80
improve electrochemical performance at high voltage. The
binder type, electrolyte composition and separator will be
60
investigated. The high-capacity Si-based anode
40
composition will be optimized by varying the graphite and
20 0323A LFP 80% LiMn1.5Ni0.5O4 coated Li1Mn1,5Ni0,5O4 20% LiFePO4
SiOx content.
1060K LiMn1.5Ni0.5O4
0
Results
0,0 0,1 1,0 10,0 100,0
Stabilize interface of LMNO. A promising result
was observed with the spinel cathode coated with C­
LiFePO 4 by a dry process, i.e., good cycle life (between 3
and 4.9V) in standard electrolyte EC-DEC-1M LiPF 6 ,
compared to bare material. A comparison of the first few
formations cycles shows a coulombic efficiency (CE) of
91% with a reversible capacity of 108 mAh/g of the bare
cathode, and 85% with a reversible capacity of 123 mAh/g
for the coated cathode (Figure V - 26).
Voltage / V
4,5
Capacity (mAh/g)
Rate (C)
Figure V - 27: Rate capability of LMNO compared to LiFePO4(LMNO) in EC­
DEC-LiPF6.
Si-anode. The SiO anode material was evaluated
using a binder of poly(acrylonitrile butadiene). The first
cycle showed a reversible capacity of 1000 mAh/g and
coulombic efficiency of 80%, and 98% in the second
cycle. The cycle life was determined at a cycling rate of
C/6 between 2.5 and 0.010 V. The capacity dropped from
800 mAh/g to less than 400 mAh/g after the first 60 cycles.
We found that 1-hour floating reduces the capacity fade,
and it remains stable at 800 mAh/g during cycling (Figure
V - 28).
Fe
3,5 1h float
Mn
0312A
800
0312C
3
P
LiMn Ni O
3/2 1/2 4
LiFPO (LiMn Ni O )
4 4
3/2 1/2
2,5
0 20 40 60 80 100 120 140 160
Capacity mAh/g
Figure V - 26: First few cycles of Li/EC-DEC 1M LiPF6 / LiFePO4(LMNO).
Inset: elemental mapping of LFP-coated spinel LMNO.
In Figure V - 27, the Ragone plots show the discharge
capacity as a function of rate. At low rate, a comparable
capacity was obtained, but when the rate is higher than
10C, the C-LFP coated cathode differs from the bare one.
At 15C, 60 mAh/g and 40 mAh/g were delivered by the
cells, respectively, with and without the coated powder
material. At 20C, no capacity was obtained with the spinel
oxide cathode, however even at 40C rate, a low capacity of
20 mAh/g was still obtained with the coated cathode. The
improved rate capability is due to the presence of facile Liions
in LFP, which could serve as a buffer of high gradient
ions when the rate is high.
(mAh/g1000
Discharge Capacity )
600
400
200
No float
0
0 20 40 60 80 100
Cycle number
Figure V - 28: Cycling of Li/EC-DEC-1M LiPF6/SiOx:Gr (1:1) cell at C/6.
Different anode compositions of C-SiOx/Si/graphite
were evaluated to increase the CE and the reversible
capacity. When the anode composition is 50%-50% (C­
SiOx-graphite), the 3 rd cycle CE was 81.6% with 996
mAh/g reversible capacity. When 25% Si-nano is
introduced, replacing the SiOx (25/50/25), the discharge
capacity is 2000 mAh/g, which drops to 50% in the second
charge with low CE of 57%. On other hand, the
composition without Si, shows the highest CE (98%). A
small effect on the reversible capacity was noticed when
the graphite content is reduced; 976 mAh/g and 1005
FY 2011 Annual Progress Report 493 Energy Storage R&D