High Specific Energy Li-ion Cells with Si Anodes
for EV Batteries
Girts Vitins
For UK Energy Storage Conference
26th November 2014
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
1
QinetiQ Group
Core business in technology-based services and solutions to the defence, security
and aerospace markets
• Provision of innovative technology development and consultancy to
governmental and commercial customers around the world
• 6,200 knowledge workers worldwide in over 40 specialist areas
• Revenues £1.2bn in year ending March 2014
VIS
© Copyright QinetiQ Limited 2014
IR
QinetiQ Proprietary
2
QinetiQ Advanced Batteries Team
The team offers battery and fuel cell based
technology solutions for a wide range of
applications
Capabilities include:
• Bespoke cell formats
• A dry room facility for
− ink preparation and electrode coatings
− assembly and manufacture of cells
• Multichannel equipment for electrochemical tests of
batteries and fuel cells
• Qualification of commercial cells to defence standards
and supply chain procurement for MoD
• Hazard test facilities and qualification for UN transport
regulations
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
3
Content
Basic pack and cell requirements
High specific energy cathode material options:
• NCA (190 mAh/g) or MNC (250 mAh/g)
Si anode material performance in small test cells
Cell manufacturing
NCA//Si 3.5 Ah test cell performance
Full size 20 Ah cell performance test results
Conclusions
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
4
General Li-ion Cell and Pack Requirements
Nominal cell voltage approximately 3.4 V
Cell capacity range 20-25 Ah
Cell type: High specific energy (aim > 270 Wh/kg) for EV application
Rate requirements
• Discharge rates:
− C/5 continuous
− Up to 3C pulses for up to 30 s
• Recharge rates:
− C/5 continuous; up to 1.7 C peak
Pack configuration 70s-2p comprising 140 Li-ion cells
• 9.5 kWh energy storage
• 70 cells in series, 2 in parallel
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
Cathode Materials: LiNi0.8Co0.15Al0.05O2 (NCA) vs. Li
NCA QX5 (23 gsm) and LCO (86 gsm) vs. Li at C/10, R.T. 2nd cycle; 1.137 cm2
•Area of 1.137 cm2
•Half cell: Li anode as reference and counter electrode
LiCoO2 (LCO)
• Capacity: 140 mAh/g
• Voltage: 3.90 V vs. Li
• Specific energy: 546 Wh/kg per active only
Voltage vs. Li/ V
Laboratory cell tests
4.3
4.1
3.9
3.7
3.5
3.3
3.1
2.9
2.7
2.5
LCO
NCA
0
NCA cathode:
20 40 60 80 100 120 140 160 180 200
Capacity/ mAh/g
• Voltage range: 2.7-4.3 V
• Capacity: 190 mAh/g (at C/10 rate)
• Voltage: 3.80 V vs. Li
• Specific energy: 722 Wh/kg per active only
• Low overvoltage and good cycle life
NCA vs. Li: C/2, 2.8-4.3 V
NCA has 32% higher specific energy than LCO
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
NCA//Graphite Li-ion Cell Evaluation
NCA in Li-ion test cells
At 20°C, C/10
37.5 mAh
• 26.4 cm2 electrode active footprint
• 37.5 mAh at C/10 rate, 2.7-4.3 V
• Average discharge voltage: 3.68 V
When cycled at 1C rate (38 mA) in 2.7-4.2 V
• Discharge capacity:
−32.3 mAh in the 1st discharge
−770 cycles to 80% capacity
26 cm2
−Very good cycle life at RT
NCA is a very competitive cathode material:
• High capacity and energy
• Good cycle life
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
7
High Capacity MNC Cathode vs. Li
120124_LMNC9121_F120116 MNC vs. Li at C/12, R.T.; 1.137 cm2
Laboratory cell data
Voltage vs. Li/ V
4.5
• Commercial high capacity MNC
− Li2MnO3 · x LiMO2 (M= Mn, Ni, Co)
• 250 mAh/g reversible capacity at C/10 rate
• 215-225 mAh/g at C/5 rate at RT
• Voltage 3.6 V vs. Li
− Structural changes, decrease of voltage over cycling
3.5
3.0
2.5
2.0
0
50
100
150
200
Capacity/ mAh/g
• High specific energy: 875 Wh/kg per active
• Good cycle life vs. Li
• Significant hysteresis between charge and discharge even at
C/12 rate
− Material is slow at RT
− Operates faster at higher temperature 45°C
− Delivers up to 280 mAh/g at 45°C
Material is new and needs to be optimised
© Copyright QinetiQ Limited 2014
4.0
QinetiQ Proprietary
250
300
MNC//Graphite Li-ion Cell Evaluation
Li-ion test cells cycled in voltage range 2.0-4.75 V
Both MNC//Graphite and MNC//Si showed
At 20°C, C/6
80% capacity after 50 cycles
• Significant capacity fade
• Gassing observed cycled in voltage range 2.0-4.70 V,
particularly at 40-45°C
− Cathode side reaction products seem to have
impacted anode SEI
Performance of MNC in Li-ion cells has been
below requirements
At 45°C, C/12; 87% after 10 cycles
Finally NCA was selected for full size 20 Ah Li-ion
cells
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
9
Silicon Anode Process
Nanosized nonporous morphology silicon anode
• 0.35 V vs. Li, high capacity around 1000-1200 mAh/g
• At least 3 times more than in graphite anodes (330 mAh/g)
• 25% of full capacity utilised
− For less expansion and better cycle life
• Significant increase in specific energy over conventional Li-ion cells
Electrode production process
• QinetiQ mixing equipment used
−
70% silicon powder
−
Polyacrylic acid based binder; drying at > 120°C under vacuum
Electrode handling in Dry Room at Dew Point -40°C
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
Silicon in Half Cells vs. Li in a Swagelok Test Cell at 20C
• Si composite film loading: 15 g/m2
• 1 M LiPF6 + FEC based electrolyte used.
• 1.4 mAh cell (based on 1200 mAh/g capacity limit)
• Lowest cut-off voltage > 0.1 V
• Li insertion controlled by capacity: 1.4 mAh –
around 25% capacity utilised for best performance
− Good initial cycle efficiency 88%
− Good cycle stability observed
− Hysteresis of 250 mV and drift in the voltage
due to transitions in Si morphology
− Capacity efficiency: around 99.3%
− Less than in graphite anode cells
− Expected performance achieved
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
11
Li-ion Cell: NCA//Si of 3.5 Ah at 20C
Cell was formed at C/10 rate (360 mA)
charge/discharge then degassed
• After formation, cycles at C/10 rate were repeated
• Charges were limited by both
− Voltage maximum 4.3 V and a maximum capacity
3.48 Ah
− In order to not to exceed 1200 mAh/g per Si active
• The cell showed
− Good stability over 5 cycles with capacity of 3.46 Ah
on discharge to 2.4 V
3.5 Ah
− Average discharge voltage 3.38 V
− 10% thickness increase with Si expansion
• Initial Li-ion cell models for 20 Ah cells predicted
270± 5 Wh/kg
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
128 x 90 mm electrode footprint
12
Li-ion Cell: NCA//Si of 3.5 Ah– C/2 Cycle Life at 20C
Cycling at C/2 charge/discharge rate
• Current: 1.7 A
• Voltage range: 2.3 – 4.3 V
• Profiles of the initial two charge/discharge cycles shown
• 3.25 Ah in the 1st discharge
• 80% capacity after 100 cycles
• The fade has been mainly due to irreversible loss of Li
− Cell capacity at C/10 rate after 100 cycles: 2.72 Ah
Relatively good cycle life:
• Acceptable for the demonstrator EV application
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
13
20 Ah Cell Assembly and Formation: 160 off Cells Produced
Automated
cell
stacking
used
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
14
20 Ah Cell: NCA//Si
QinetiQ Li-ion Cell
3.38 V 20 Ah
Size: 160x114 mm incl. margins
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
15
NCA//Si 20 Ah – Performance at C/10 Rate at 22C
3 cycles at C/10 rate – capacity check
Voltage range: 2.3-4.3 V
• Reversible capacity: 20 Ah
− Model: 20-21 Ah
• Discharge energy: 67 Wh (265 Wh/kg)
− Model: 71 Wh (270 Wh/kg)
• Average voltage: 3.35 V
• Hysteresis: 330 mV (90% energy efficiency)
• After formation cells expanded to 11.5 mm in
thickness
− 20-25% increase due to Si anode expansion with
Li insertion
− The expansion was higher than anticipated
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
16
NCA//Si 21 Ah Cells at C/5 Rate to 4.3 V
Cells 66 and 67
Charge/discharge cycles at C/5 rate, 20C
• C/5 is the continuous charge/discharge rate
required
• Reversible capacity: 19.6 Ah in 1st discharge
• Acceptable performance at 20C
− Adding weight load of 20 kg or 0.18 kg/cm2
has no effect on cycle life at 20°C
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
17
NCA//Si 20 Ah Cells at C/5 Rate, in 3.0-4.1 V at 40C
Cells 68 and 69 as formed
• Cycled at RT in range 2.3-4.3V with capacity 20 Ah
at C/10 rate
• Now charge/discharge cycled at C/5 rate, 40C in
range 3.0-4.1 V
− 30-40% less capacity in reduced voltage range as expected
− Rapid capacity fade at 40C without
compression
− Significantly improved stability with 20 kg
weight load as gassing and expansion are
suppressed
− Gassing and cells expansion are suppressed
− Cell integrity improved
− Cell compression normally used in EV battery packs
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
18
20 Ah NCA//Si at 1C Rate at 20C
Cell 68
Acceptable performance at 1C rate
• 10 Ah
• Temperature rise by 14C to 34C
• DC Cell Resistance:
− 7.0 m after 10 s transient
− 13 m after 300 s transient
− Normal cell resistance for a high specific
energy cells
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
19
20 Ah NCA//Si at 3C rate at 20C
Cell 68
Has delivered acceptable performance
at the required 3C rate
• Discharge time more than 30 s required
• 2.5 Ah
• Temperature rise by 16°C to 36C
• DC Cell Resistance:
− 7.1 m after 10 s transient
− 13 m after 300 s transient
− Normal resistance for a high specific
energy cells
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
20
Conclusions
Whilst Si offers high specific capacity (> 1100 mAh/g) and high specific energy in a full
Li-ion cell (265 Wh/kg)
• The cells have exhibited expansion of 20-25% due to LixSi alloys having significantly lower
crystal density
− This has reduced the volumetric energy of cells below that of conventional Li-ion
− More electrolyte was required due to increasing porosity of Si anodes
• Future developments should focus on improving Si particle structure and reducing amount of Si
per cell by engaging a higher capacity, e.g. 1600 mAh/g, offering a higher specific energy
Higher capacity fade than in conventional Li-ion cells is likely due to
• Si particle size changes
− The solid electrolyte interface (SEI) not being fully stable as a result
Constraining cells significantly improves cycle life at 40C, however not to the level of
conventional Li-ion cells
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
21
Acknowledgments
QinetiQ Research Services Advanced Batteries Group:
• Phil Holland, Phil Barnes, Mark Fyrth, Darren Scattergood, Ken Sutcliffe, Kevin Green, Gary Mepsted
Innovate UK for funding
Please also visit QinetiQ Poster:
Energy storage within a residential smart grid demonstrator
Presented by Dr. Kevin Green
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
22
www.QinetiQ.com
© Copyright QinetiQ Limited 2014
QinetiQ Proprietary
23