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Lucht – URI
V.D.7 Development of Electrolytes for Lithium-ion Batteries (URI)
Results
Development of Cathode Film Forming Additives.
An investigation of the reactions of electrolyte with the
surface of LiNi 0.5 Mn 1.5 O 4 cathode materials cycled to 4.9
V vs Li or stored at elevated temperature (55°C for two
weeks) has been conducted to develop a better
understanding of the sources of performance fade. In
order to investigate the difference in electrolyte oxidation
on LiNi 0.5 Mn 1.5 O 4 vs Pt, electrodes were stored at 4.75 and
5.30 V vs Li for several days. After subtraction of the
current associated with lithium extraction from
LiNi 0.5 Mn 1.5 O 4 the residual currents are very similar
suggesting that the oxidation reactions are similar for
LiNi 0.5 Mn 1.5 O 4 and Pt (Figure V - 161). Surface analysis
suggests that the surface species on LiNi 0.5 Mn 1.5 O 4 and Pt
are very similar. Thus, electrolyte oxidation reactions
apper to be largely independent of the electrode structure.
smaller impedance suggests thinner surface films. Related
investigations are also being conducted on novel Lewis
basic additives to inhibit Mn leaching form the cathode
and provide similar performance improvements.
Discharge Capacity, mAh/g
200
180
160
140
120
100
STD ELECTROLYTE
1% LiBOB
0.25% LiBOB
Elevated temperature storage experuiments suggest that 80
0 2 4 6 8 10 12 14
the electrolyte reacts with the cathode surface and Mn
Cycle Number
dissolution is a problem for LiPF 6 electrolytes.
Figure V - 162: Specific capacity of LiNi0.5Mn1.5O4 cells containing standard
2.4
4.75V (Pt)
5.30V (Pt) electrolyte and added LiBOB.
4.75V(LNMO)
mAh/cm 2
2.0
1.6
1.2
0.8
0.4
0.0
0 50 100 150
Time(hr)
5.30V(LNMO)
Figure V - 161: C/20 charging to 4.75 & 5.30V and hold at same V,
LNi0.5Mn1.5O4; Scan (5mV/S) and hold (4.75 & 5.30V) on Pt; LiPF6/EC/EMC.
Investigations of cathode film forming additives with
LiNi 0.5 Mn 1.5 O 4 /Li cells cycled to 4.9 V vs Li have been
conducted. The discharge capacities are provided in Figure
V - 162. Cells containing baseline electrolyte showed higher
discharge capacity after the first cycle but gradual capacity
fade was observed after subsequent cycling. Cells
containing LiBOB showed lower discharge capacity after
the first cycle but capacity retention was improved in
following cycles. Cells containing LiBOB retained 93% of
the initial discharge capacity as compared to 71% capacity
retention from baseline electrolyte cells. The cell
efficiency was also improved from 95 % for the STD to
above 99 % for cells containing 0.25 LiBOB (Figure V - 162).
The electrochemical impedance of the cells indicate lower
impedance for cells containing LiBOB than the cells
containing baseline electrolyte. The increased capacity
retention and efficiency suggest that the presence of
LiBOB is decreasing the detrimental reactions of the
electrolyte with LiNi 0.5 Mn 1.5 O 4 cathode materials while the
The XPS spectra of fresh and cycled cathodes were
acquired. The C1s element spectra are very similar and
support the presence of polyethylene carbonate (PEC)
while the F1s spectra of the cathodes extracted from cells
containing LiBOB have a slightly higher concentration of
LiF at 685 eV than the cells cycled with standard
electrolyte. The O1s element spectra contain higher
concentration of the lithium metal oxide at 529.5 eV for
the cell cycled with electrolyte containing LiBOB than for
the standard electrolyte. The greater relative concentration
of metal oxide on the surface supports a thinner cathode
surface film, or less Mn dissolution from the surface, and
suggests that the presence of LiBOB is inhibiting the
reaction of the electrolyte with the surface of the cathodes.
Development an understanding of poor first cycle
efficiency for LiPF 4 (C 2 O 4 ). The reversible capacity
observed on the formation cycles of cells
(MCMB/LiNi 0.8 Co 0.2 O 2 ) containing LiPF 4 (C 2 O 4 )
electrolytes is 62 % compared to 84 % for LiPF 6
electrolytes. The differences were suspected to be due to
lithium oxalate impurities in the LiPF 4 (C 2 O 4 ). Extensive
purification of LiPF 4 (C 2 O 4 ) followed by cycling of the
electrolyte in cells suggests that the lithium oxalate
impurities contribute to the irreversible capacity, but
efficiencies in excess of 65 % could not be obtained via
LiPF 4 (C 2 O 4 ) purification.
However, formation cycle efficiencies vary
significantly as a function of the electrode materials.
Investigation of LiPF 4 (C 2 O 4 ) electrolytes in natural
graphite (NG)/LiNi 0.8 Co 0.2 O 2 cells provided nearly
identical reversible capacity (89 %) to LiPF 6 electrolytes
upon formation (Figure V - 163). In addition, the cell capacity
100
90
80
70
60
50
Efficiency, %
FY 2011 Annual Progress Report 617 Energy Storage R&D