How initial nucleation influences discharge capacities of Li-O 2 cells

Transcription

1 How initial nucleation influences discharge capacities of Li-O 2 cells Ali Rinaldi 1, Olivia Wijaya 1, Denis Yu 2, Harry.E. Hoster 1 1TUM CREATE Centre for Electromobility #10-02 CREATE Tower, Singapore Energy Research NTU, 50 Nanyang Drive, Singapore /05/16 ECS TORONTO 2013

2 Introduction Li-O 2 battery has high theoretical energy density. High expectations. A lot of fundamental scientific challenges to be addressed for discharge and charge of Li-O 2 battery; Solvent stability O 2 solubility Catalysis &/or mediator for Li 2 O 2 oxidation Li Anode stability Various power demands within applications. Ali Rinaldi 2

3 Introduction General Battery: Various power demands within applications. Li-ion Battery shows no history effect. At different sequences of varying (high) C-rate the total capacity can be retained when returned to lower C-rate. How is it the case for Li-O 2 Battery? Ali Rinaldi 3

4 Introduction E (vs Li/Li + ) / V Various power demands within applications. Li-ion Battery shows no history effect. At different sequences of varying C-rate the total capacity can be preserve when used at lower C-rate. How is it the case for Li-O 2 Battery? ma/g c ma/g c ma/g c Ali Rinaldi 4 Discharge Capacity / mah/g c

5 Introduction Electrochimica Acta 55, (2010), Li 2 O 2 Nucleation Growth sudden death Ali Rinaldi 5

6 Motivation: The questions raised are: 1. Is there a history effect for Li-O 2 battery? 2. How does the initial discharging profile influence the electrochemical behavior? Ali Rinaldi 6

7 Experiments i. Nucleate (pre-discharge) cathode at varying Overpotentials. ii. Subsequent Discharge at low current density (50 mah/g c ). Ueq nucleation Low current Uapp 400 mah/g c ~8 monolayers of Li 2 O 2 Capacity / mah/g c Ali Rinaldi 7

8 Experiments Li 2 O 2 Li 2 O 2 Li 2 O 2 Fresh Pre-discharge (Nucleation) Fully discharged Investigation with electron microscopy & Impedance Spectroscopy. Ali Rinaldi 8

9 The Li-O 2 Battery Cell O 2 15 ul/min El-Cell GmbH ECC-air setup. Cathode : Graphitized Acetylene Black + 20 wt% PVDF (~1 mg, 10um, coated on Celgard) Electrolyte : 90 ul 0.1M LiCLO 4 in DME (1,2 Dimethoxy ethane). Ali Rinaldi 9

10 Experiments: Impedance Spectroscopy 100kHz-10mHz, AC 10mV, DC 2.6V (vs Li/Li + ). Ali Rinaldi 10

11 Experiments: Impedance Spectroscopy Argon O 2 Ali Rinaldi 11

12 Experiments: Impedance Spectroscopy log ( Z / ) O 2 Nyquist Plot O 2 Bode Plot Z Im / O 2 _OCV O 2 _2600mV Phase / o Z Re / log (freq /Hz) The 2 nd semicircle correspond to the formation of Li 2 O 2. Ali Rinaldi 12

13 Results: History effect in Li-O 2 cathode (i) 2.3 V 2.6 V nucleation Ali Rinaldi 13

14 Results: History effect in Li-O 2 cathode (i) Slow initial growth Fast initial growth 14

15 Extended discharge of cathode at low & high Overpotentials Eeq = ~ 2.85 V E app 2000 mah/g c ~40 monolayers of Li 2 O 2 Capacity / mah/g c Ali Rinaldi 15

16 Extended discharge of cathode at different Overpotentials ~2000mAh/g at 2.6 V ~2000mAh/g at 1.7 V XRD measurements show that the deposits in both cases (the film & flakes) are Li 2 O 2 Different morphology of Li 2 O 2 deposited at different Overpotentials. The history effect is therefore dependent on the nucleation step. Ali Rinaldi 16

17 Results: Impedance Spectroscopy measurements E (vs Li/Li + ) / V Eeq = ~ 2.85 V E EIS = 2.6 V E app time Ali Rinaldi 17

18 Results: Impedance Spectroscopy measurements Ali Rinaldi 18

19 Results: Impedance Spectroscopy measurements Ali Rinaldi 19

20 Results: History effect in Li-O 2 cathode (ii) Fresh Carbon VS Impedance Spec (Rct, Cdl) 1.7 V Fresh Carbon vs low η dis Li 2 O 2 -coated Carbon Ali Rinaldi 20

21 Results: History effect in Li-O 2 cathode (ii) Fresh Carbon VS Impedance Spec (Rct, Cdl) Ali Rinaldi V

22 Results: History effect in Li-O 2 cathode (iii) 1 st step (mah/g) 2 nd step (mah/g) 0 (fresh carbon) rd step (mah/g) 400 (8 monolayer) With Li 2 O 2 seed layer deposited at low Overpotential : higher capacity in the subsequent step. For fast discharge, pre-conditioning of the cathode is necessary. 22

23 Proposed mechanism: J Chem Phys 138, (2013), Li + + e - Li + + e - on Carbon surface on Li 2 O 2 surface Kinetic Overpotential + Crystallization overpotential Kinetic Overpotential = Charge transfer at different available sites. Kinks and steps have the lowest η dis

24 Summary 1. Li-O 2 Battery shows a history effect. 2. The nucleation at different overpotential resulted in different Li 2 O 2 morphology. 3. The nucleation (initial Li 2 O 2 deposits) on carbon alter the electrochemical performance of the cathode. 4. Higher impedance of the cathode at higher Overpotential. 5. Initial Li 2 O 2 from high overpotential, impede the subsequent discharge. 6. Initial Li 2 O 2 from low overpotential stabilizes the subsequent (high overpotential) discharge.

25 Acknowledgments Thank you