Identifying the rate-limiting processes at the Li-air cathode

Transcription

1 Identifying the rate-limiting processes at the Li-air cathode Tejs Vegge Risø DTU, National Laboratory for Sustainable Energy and Center for Atomic-scale Materials Design, Technical University of Denmark

2 Outline The Li-air battery: identifying the main challenges A computational approach to electrochemistry Density functional theory and free energies Finite bias electron transport calculations (NEGF) Reaction mechanisms from free energies The limiting change and discharge potentials Lithium vacancies and activated processes Electronic transport over Au/Li 2 O 2 and Pt/Li 2 O 2 interfaces Electron conduction mechanism Interfaces states t and defects IV curves Outlook

3 The Li-air battery: what are the main challenges? The overpotentials? Round trip efficiency Catalysts Accessible capacity? Oxygen pressure dependence? Current density dependence? Degradation and cycle life? Capacity loss Electrolyte decomposition Thermal dependence? Electronic conduction? Girishkumar, McCloskey, Luntz, Swanson, Wilcke, J. Phys. Chem. Lett.1, 2193 (2010) Yang and Xia, J. Elec. Soc. 14, 109 (2010) Lu, Kwabi, Yao, Harding, Zhou, Zuin, Shao-Horn, Energy & Environ. Sci. 4, 2999 (2011)

4 Computer simulations and experiments Theory? U eq = ev Experiments? Hummelshøj (2011) Girishkumar, McCloskey, Luntz, Swanson, Wilcke, J. Phys. Chem. Lett.1, 2193 (2010)

5 Density functional theory calculations: lations GPAW Density functional theory (DFT) calculations GPAW: multigrid, real space implementation of Projector Augmented Wavefunction method (PAW) Generalized Gradient Approximation (GGA) RPBE exchange correlation functional Localized LCAO (linear combination of atomic orbitals) basis set Finite bias electron transport calculations using LCAO basis set Non-equilibrium Green s function methods (NEGF) H. Larsen, M. Vanin, J. J. Mortensen, K.S.Thygesen, K. W. Jacobsen, PRB 80, (2009) J. Enkovaraara, et al., J. Phys.: Condens. Matter 22, (2010)

6 Computational Li-air electrodes Model system: aprotic Li-air battery Analyzing the steady state following initial discharge Discharge model: growth of Li 2 O 2 2Li + + 2e - + O 2 (g) Li 2 O 2 (s) (U eq =2.96V) Anode reaction: Li(s) Li + + e - (U=0V) Cathode reaction: 2Li + + 2e - + O 2 (g) Li 2 O 2 (s) Obtaining the reaction free energy Get E from DFT G 0 = E + E zpe -T S 0 Convention: G(U=U eq ) = 0 Potential: G(U) = -neu - ktln[ali + ] Nørskov, Rossmeisl, Logadottir, Lindqvist, Kitchin, Bligaard Jónsson, J. Phys. Chem. B, 108, (2004) Hummelshøj, Blomqvist, Datta, Vegge, Rossmeisl,Thygesen,Luntz,Jacobsen,Nørskov J.Chem. Phys. 132,

7 Li 2 O 2 go growth mechanisms Continuous (i.e. not initial) Li 2O 2 growth/depletion on Li 2O 2 Identifying the relevant surface facets/termination on Li 2 O 2 electrolyte initial growth support, e.g. carbon Hummelshøj (2011)

8 Thermodynamics and Li-O 2 phase diagramam Free energy Li + O2 1/2 Li2O + 3/4 O2 1/2 Li2O2 + 1/2 O2 LiO2 LiO2 Li2O2 Li2O T=0K Hummelshøj (2011) Peng, Freunberger, Hardwick, Chen, Giordani, Bard, Novak, Graham, Tarascon, Bruce, Angew. Chem Int. Ed. 50, 6351(2011)

9 Reconstructed cted (1-100) 100) and (0001) Li 2 O 2 surfaces Relative energies of different surface terminations 0.026eV/Å eV/Å eV/Å 2 Seriani, Nanotechnology 20, (2009). Hummelshøj, Blomqvist, Datta, Vegge, Rossmeisl,Thygesen,Luntz,Jacobsen,Nørskov J.Chem. Phys. 132, (2010) Lu, Kwabi, Yao, Harding, Zhou, Zuin, Shao-Horn, Energy & Environ. Sci. 4, 2999 (2011)

10 LiO2 adsorption Missing row reconstruction of the Li2 O2( 2 (1-100) as a model system Analyzing a stepped surface A 4 electron (2 Li 2O 2 2) ) process leaves the surface unchanged

11 Discharge on steps on Li 2 O 2 (1-100) 100) + LiO2; 2.73 V } 3.2V +LiO LiO2; 3.64V + Li; 3.10V + Li; 2.56V Hummelshøj, Blomqvist, Datta, Vegge, Rossmeisl, Thygesen, Luntz, Jacobsen, Nørskov, J.Chem.Phys.132, (2011)

12 The potential dependent free energy diagram am discharge (Li + +e )+2O U = 0 V 3(Li + +e - )+O2+LiO2* 2(Li + +e - )+2LiO2* (Li + +e - )+LiO2*+Li2O2 2 Li2O2 charge U eq-dft =264V 2.64

13 Identifyinging the limiting potentials 0.38 V 4(Li + +e - )+2O V 3(Li + +e - )+O2+LiO2* 2(Li + +e - )+2LiO2* (Li + +e - )+LiO2*+Li2O2 +Li2O2 Chen, Hummelshøj, Thygesen, Myrdal, Nørskov, Vegge, Catalysis Today 165, 2 (2011)

14 Is the limiting potential also rate determining? Other charging g mechanisms? Li 2 O 2 2Li + + O 2 Li 2 O 2 Li 2 O +½O 2 A lower η charge is needed Non-electrochemical step? Large thermal dependence O * 2 O 2 (solution) Hassoun, Croce, Armand, Scrosati, Angew. Chem 123, 3055 (2011) Girishkumar, McCloskey, Luntz, Swanson, Wilcke, J. Phys. Chem. Lett.1, 2193 (2010) Myrdal et al. (2011)

15 What about the electronic conduction? ction? Li 2 O 2 is an large bandgap insulator 2 2 g g p How are the electrons transported to the Li 2 O 2 surface? electrolyte initial growth e - e - support, e.g. carbon e - Hummelshøj (2011)

16 The role of lithium vacancies The band gap is only 1.8 ev with GGA compared to 4.9 ev with G0 W0 Lithium vacancies pin the Fermi level at the top of the valence band Induce conduction (holes) Surface vacancies ΔE form-bulk 3.0 ev Hummelshøj, Blomqvist, Datta, Vegge, Rossmeisl,Thygesen,Luntz,Jacobsen,Nørskov J.Chem. Phys. 132, (2010)

17 Understanding the role Li-vacancies XRD and XANES experiments indicate defective Li2 O2 The state of the Li-vacancies Charged state at an applied potential Vacancy mobility Localization or delocalization Electronic conduction as a function of [V Li ] Lu, Kwabi, Yao, Harding, Zhou, Zuin, Shao-Horn, Energy & Environ. Sci. 4, 2999 (2011) Lastra and Myrdal (2011)

18 Non-equilibrium i electron transport t Green s function Finite bias electron transport calculations Coherent electron transport through the metal-insulator interface Central device region (C) is connected to two semi-infinite leads (L) and (R), e.g. Au(111) (L) and Li 2 O 2 (0001) slabs (R) are kept at fixed electronic chemical potentials, μ L and μ R, to simulate an applied bias voltage of V = (μ L μ R )/e across the device region Chen, Hummelshøj, Thygesen, Myrdal, Nørskov, Vegge, Catalysis Today 165, 2 (2011)

19 Au/Li 2 O 2 and Pt/Li 2 O 2 model cathode interfaces Modeling a Au and Pt doped porous carbon electrode growth on Au(111) has good lattice match with Li 2 O 2 (0001) Electrolyte-Li 2 O 2 interactions are not considered Valence band: π * (2p x ) and π * (2p y ) Li 2 O 2 -vac Conduction band: σ * (2p z ) Chen, Hummelshøj, Thygesen, Myrdal, Nørskov, Vegge, Catalysis Today 165, 2 (2011)

20 Electronic conduction ction at Au(111)-Li 2 O 2 (0001) The anti bonding σ * (2pp z ) level in the Li2 O2 conduction band is aligned perpendicular to the metal surface. Lithium vacancies pin the Fermi level at the top of the anti bonding peroxide π * (2p x ) and π * (2p y ) Defect free Li vacancy Chen, Hummelshøj, Thygesen, Myrdal, Nørskov, Vegge, Catalysis Today 165, 2 (2011)

21 PDOS at an applied bias Localized σ * (2p z ) states are formed at the Au-Li 2 O 2 interface ( p z ) 2 2 Applying a bias shifts the chemical potentials of μ Li2O2 relative to μ Au The anti bonding σ * (2p z ) level in the Li 2O 2 conduction band is found to couple strongly to the metal substrate and create localized interface states with poor coupling to the Li 2 O 2 bulk states Chen, Hummelshøj, Thygesen, Myrdal, Nørskov, Vegge, Catalysis Today 165, 2 (2011)

22 Transmission curves for Au(111)-Li 2 O 2 (0001) vac Transmission through the interface with lithium vacancies Significant potentials are needed to drive a current Hole based conduction through the valence band Discharge Charge Chen, Hummelshøj, Thygesen, Myrdal, Nørskov, Vegge, Catalysis Today 165, 2 (2011)

23 Calculated lated IV curves for the metal-li 2 O 2 interfaces Very similar results for Au and Pt (good interface matching) The specific onset potentials will depend on the DFT-band gap Localized interface states without coupling to the Li 2 O 2 states can p g 2 2 limit the electronic conduction The obtained current densities are above experimental values Chen, Hummelshøj, Thygesen, Myrdal, Nørskov, Vegge, Catalysis Today 165, 2 (2011)

24 Outlook Li-air holds great potential but face massive challenges The overpotentials depend explicitly on the current density Low accessible capacity and current density Improved electronic conduction at the cathode is needed Dynamic defect generation, e.g. vacancies and dopants Electron-phonon coupling Li 2 O 2 -electrolyte interaction should be considered Non-electrochemical steps and solvation The cathode-li 2 O 2 -electrolyte interfaces (in situ studies) Electrolyte degradation mechanisms Defects and interface species Charge transfer reactions and rates

25 Acknowledgements Stanford (SUNCAT): Jens S. Hummelshøj, Venkat Viswanathan, Alan C. Luntz, Jens K. Nørskov CAMD: Juan Maria Garcia Lastra, Jingzhe Chen, Jan Rossmeisl, Kristian Thygesen Risø DTU: Jon Steinar G. Myrdal, Didier Blanchard, Dadi Sveinbjörnsson, Poul Norby, Søren Højgaard