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 138602 2Energy Research Institute @ NTU, 50 Nanyang Drive, Singapore 637553 2013/05/16 ECS TORONTO 2013
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
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
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? 3.0 2.8 2.6 2.4 50 ma/g c 2.2 1000 ma/g c 2.0 50 ma/g c 0 400 800 1200 1600 2000 2400 Ali Rinaldi 4 Discharge Capacity / mah/g c
Introduction Electrochimica Acta 55, (2010), 4086. Li 2 O 2 Nucleation Growth sudden death Ali Rinaldi 5
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
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
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
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
Experiments: Impedance Spectroscopy 100kHz-10mHz, AC 10mV, DC 2.6V (vs Li/Li + ). Ali Rinaldi 10
Experiments: Impedance Spectroscopy Argon O 2 Ali Rinaldi 11
Experiments: Impedance Spectroscopy log ( Z / ) O 2 Nyquist Plot O 2 Bode Plot Z Im / 1400 1200 1000 800 600 400 O 2 _OCV O 2 _2600mV 4.5 4.0 3.5 3.0 2.5 0-20 -40-60 Phase / o 200 0 2.0-80 0 200 400 600 800 1000 1200 1400 Z Re / -2-1 0 1 2 3 4 5 log (freq /Hz) The 2 nd semicircle correspond to the formation of Li 2 O 2. Ali Rinaldi 12
Results: History effect in Li-O 2 cathode (i) 2.3 V 2.6 V nucleation Ali Rinaldi 13
Results: History effect in Li-O 2 cathode (i) Slow initial growth Fast initial growth 14
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
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
Results: Impedance Spectroscopy measurements E (vs Li/Li + ) / V Eeq = ~ 2.85 V E EIS = 2.6 V E app time Ali Rinaldi 17
Results: Impedance Spectroscopy measurements Ali Rinaldi 18
Results: Impedance Spectroscopy measurements Ali Rinaldi 19
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
Results: History effect in Li-O 2 cathode (ii) Fresh Carbon VS Impedance Spec (Rct, Cdl) Ali Rinaldi 21 1.7 V
Results: History effect in Li-O 2 cathode (iii) 1 st step (mah/g) 2 nd step (mah/g) 0 (fresh carbon) 400 500 3 rd step (mah/g) 400 (8 monolayer) 400 1100 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
Proposed mechanism: J Chem Phys 138, (2013), 034703. 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
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.
Acknowledgments Thank you