How Can Nanotechnology Help Solve Problems in Energy Storage?

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1 How Can Nanotechnology Help Solve Problems in Energy Storage? From Fundamental Studies to Electrode Design Candace K. Chan Assistant Professor Materials Science & Engineering School for Engineering of Matter, Transport and Energy Arizona State University Tempe, AZ

2 Nanomaterials for Energy Storage Nano-composite Level Cycling performance Electrode design Nanowire Ensemble Level Single Nanowire Level Phase & structural transformations Chemical & compositional changes

3 Electrochemical Energy Storage J.-M. Tarascon & M. Armand. Nature. 414, 359 (2001).

4 Electric Cars ~1-2 kwh ~5-15 kwh >40 kwh

5 Future Goals DOE cost goals: HEV: PHEV: $20/kWh (by 2010) $250/kWh (by 2015) Today PHEV costs are $2000/kWh!

6 - Stores 56 kwh - Delivers 185 kw power (~ 248 hp) of cells wired in series - weighs 450 kg (990 lbs) - full charge time 3.5h at 240V - costs $25,000 Tesla Roadster battery pack

7 How to Improve? It s clear that the main problems are Cost Weight (volume) How can nanotechnology help? Nanotechnology can 1. Enable better performance more efficient materials mean less material is needed 2. Allow for new types of chemistry and reactions 3. Be designed and assembled to exploit these properties

8 What is a Battery? e- e

9 Lithium-ion Battery Key Characteristics Capacity how much charge (Li) can you store? [mah/g] Voltage at what potential does the electrochemical rxn occur? [V vs. Li] Energy density = capacity x voltage [Wh/kg] Power density

10 Potential vs. Li (V) Cathodes Oxygen evolution LiCoO 2 Electrolyte oxidation Graphite Solvent reduction Lithium plating Li metal Anodes Capacity (mah/g)

11 Potential vs. Li (V) Cathodes Oxygen evolution Electrolyte oxidation Graphite Solvent reduction Lithium plating Li metal Anodes Capacity (mah/g)

12 Potential vs. Li (V) Cathodes Oxygen evolution Electrolyte oxidation Silicon Anodes Capacity (mah/g)

13 Example 1: V 2 O 5 nanoribbons

14 Example 1: V 2 O 5 nanoribbons Key science questions: How does nanostructuring affect physical & chemical transformations? What implications in device performance? O 2 Carrier gas Pressure: 1 Torr Substrate temperature: C Source temperature: 900C Growth time: 1h Chan, C.K. et. al Nano Lett. 2007, 7,

15 Example 1: V 2 O 5 nanoribbons Li + V 2 O 5 : Layered structure X Thickness Slowest growth X Fastest growth Length Width Lithiation V 2 O 5 Li x V 2 O 5 Delithiation Chan, C.K. et. al Nano Lett. 2007, 7,

16 Example 1: V 2 O 5 nanoribbons Chemical transformation V 2 O 5 Electron Energy Loss Spectroscopy (EELS) Li x V 2 O 5 STEM image Li map Increased interfacial contact area In collaboration with Hitachi High Technologies and Gatan, Inc. Decreased Li insertion distance Chan, C.K. et. al Nano Lett. 2007, 7,

17 Example 1: V 2 O 5 nanoribbons Structural transformation V 2 O 5 Li ω-li 3 V 2 O 5 Chan, C.K. et. al Nano Lett. 2007, 7,

18 Example 1: V 2 O 5 nanoribbons What is the width dependence on Li insertion? Red: ω-li 3 V 2 O 5 Blue: γ-li 2 V 2 O 5 Yellow: V 2 O 5 More lithiation at shorter insertion lengths Facile phase transformation at nano vs micron scales Chan, C.K. et. al Nano Lett. 2007, 7,

19 Example 1: V 2 O 5 nanoribbons Is there a thickness dependence on Li insertion? X 100 nm thick and 400 nm wide remains V 2 O 5 20 nm thick and 740 nm wide transforms to Li 3 V 2 O 5 Li intercalation also depends on THICKNESS of nanoribbon Need facile phase transformation in all 3 axes for full lithiation Chan, C.K. et. al Nano Lett. 2007, 7,

20 Example 1: V 2 O 5 nanoribbons Conclusions How are the chemical and structural transformations different in nano vs. bulk? Nanoscale dependence on change in width AND thickness Fully reversible process How can they improve device performance? Higher energy density by 30% Higher power density Chan, C.K. et. al Nano Lett. 2007, 7,

21 Example 2: Si nanowires Key science questions: How does nanostructuring affect physical & chemical transformations? What implications in device performance?

22 Example 2: Si nanowires Beyond Graphite: Silicon as a Lithium Alloy Material Graphite LiC 6 Silicon Li 22 Si 5 (Li 4.4 Si)

23 Example 2: Si nanowires Charge storage capacity (mah/g) Poor Cycling Performance of Bulk Si Anodes Lower capacity than expected Capacity fades over time 10 µm Si particles (15% carbon) Cycle number Electrochem. Sol. St. Lett. 7, 10, A306 (2004)

24 Example 2: Si nanowires Si Morphology Changes with Lithiation Initial After electrochemical cycling (400% volume change) Sanyo

25 Critical size (µm) Example 2: Si nanowires Terminal Particle Size Phenomenon Terminal particle size If the particle size is small enough, it will not fracture! Volume mismatch (strain) Dilation strain parameter R.A. Huggins, W.D. Nix, Ionics 6 (2000) 57

26 Example 2: Si nanowires The Problem with (Nano)particles After volume change Initial Initial state needed for ideal final state not practical S. D. Beattie D. Larcher, M. Morcrette, B. Simon, J.-M. Tarascon, J. Electrochem. Soc. 155, A158 (2008)

27 Example 2: Si nanowires Why Use Nanowires? Large contact area w/electrolyte Li + Li + 1D conduction pathway Volume expansion w/o pulverization Short Li + diffusion distances Li + Electrical contact Initial After electrochemical cycling

28 Example 2: Si nanowires Vapor-Liquid-Solid (VLS) Growth of Si Nanowires Metal nanoparticles Metal substrate SiH ºC CVD Metal Nanoparticles Si Nanowires 5 mm

29 Example 2: Si nanowires Electrode Fabrication Cu current collector Li metal Separator soaked with electrolyte Si Nanowires Stainless steel current collector Electrolyte: 1M LiPF 6 in ethylene carbonate/diethyl carbonate (1:1) Chan, C.K. et. al. Nature Nanotech. 3, 31 (2008)

30 Example 2: Si nanowires Structural Changes in SiNWs During Lithiation Initial Lithiation Chan, C.K. et. al. Nature Nanotech. 3, 31 (2008)

31 Example 2: Si nanowires Morphology Changes in SiNWs During Lithiation Initial After cycling 31 Chan, C.K. et. al. Nature Nanotech. 3, 31 (2008)

32 Example 2: Si nanowires Increase in length during lithiation 32 Initial After cycling Chan, C.K. et. al. Nature Nanotech. 3, 31 (2008)

33 Example 2: Si nanowires Step-wise Pore Evolution Cycle number J. W. Choi, et. al. Nano Lett., 10, 1409, (2010)

34 Example 2: Si nanowires Summary of Morphology Changes in SiNWs Lithiation Many cycles Initial X ~400% increased volume ~400% increased surface area Pulverization Observed in bulk Si See also: In Situ Observation of the Electrochemical Lithiation of a Single SnO 2 Nanowire Electrode, Science, pg 1515, Dec. 10, 2010.

35 Example 2: Si nanowires Electrochemical Cycling Performance Chan, C.K. et. al. Nature Nanotech. 3, 31 (2008) 35

36 Example 2: Si nanowires Role of Electrode Architecture / Scale Up Conducting carbon Polymer binder vs Good electron transport Room for volume change Large interfacial area with electrolyte Compatible with roll-to-roll deposition 36

37 Example 2: Si nanowires Electron Transport Pathways Micron/Bulk Particles Rigid Nanowires/Nanorods High Aspect Ratio Nanowires 37

38 Example 2: Si nanowires Silicon Nanowire Synthesis Vapor-Liquid-Solid (VLS) NWs tethered to substrate Low yield (~200 ug/cm 2 ) Clean surfaces Supercritical Fluid-Liquid Solid (SFLS) Korgel Group (UT Austin) Flow-through reactor Higher yields (~1mg/mL, mg/batch) Pressurized organic solvents ( o C) 38 38

39 Example 2: Si nanowires Carbon Coating of Si Nanowires Uncoated SiNWs Carbon coated Chan, C.K. et. al. ACS Nano, 4, 1443 (2010)

40 Example 2: Si nanowires SiNW/MWNT Composite SiNW:MWNT:CMC ratio %wt of 78:12:8.5 with carbon coating 1.5% 40 Chan, C.K. et. al. ACS Nano, 4, 1443 (2010)

41 Example 2: Si nanowires SFLS SiNW Electrode Comparison Electronic transport improved by using MWNTs CMC not sufficient binder for large volume changes Possibly problems with ionic charge transfer into SiNWs Chan, C.K. et. al. ACS Nano, 4, 1443 (2010)

42 Example 2: Si nanowires SiNWs SiNWs/C composite SiNCs Graphite Chan, C.K. et. al. Nature Nanotech. 3, 31 (2008) * SiNC data from Graetz et. al. Electrochem. Sol. State. Lett. 6, A75 (2003).

43 Conclusions Fundamental studies on chemical, structural, and morphology changes in nanomaterials during electrochemical reactions can help understanding and direct design of better devices A nanowire-based electrode architecture was developed and led to significant improvement in electrochemical performance for the Li-Si system compared to bulk/traditional electrodes Nanoscale dimension for relaxation of strain/stress Space for volume expansion Efficient electronic and ionic transport Integrating nanomaterials into conventional composites while retaining the desired properties needs improvement 43

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45 Acknowledgements Future Work: Next Generation Cathode Cathodes? Anodes SiNWs Capacity (mah/g) 45

46 Acknowledgements Prof. Yi Cui (Ph.D. adviser) Prof. Bob Huggins Dr. Riccardo Ruffo (U. Milano-Bicocca) Group members Collaborators: Xiaofeng Zhang (Hitachi), Gao Liu (LBL), Prof. George Gruner (UCLA), Martti Kaempgen (NTU) Funding: NSF Graduate Fellowship, Stanford Graduate Fellowship, KAUST, GCEP, ONR