IBM Almaden June 27, 2017 Seongmin Ha, Dongho Koo, Kyu Tae Lee * Chemical and Biological Engineering Seoul National University (ktlee@snu.ac.kr)
1) Introduction 2) Failure mechanism of a redox mediator in Li O 2 batteries 3) LiNO 3 as a scavenger of oxidized redox mediators in Li O 2 batteries Melting earth
Failure mechanism of Li O 2 batteries with a redox mediator
10-Methylphenothiazine (MPT) Reversible redox reaction in Ar and O 2 atmospheres (LiSO 3 CF 3 + MPT in tetraglyme) 1st Redox center -e - 2nd Redox center + -e - +e - +e - + + Ar O 2 K. Kochi et al. J. Am. Chem. Soc. (2004) 126 1388-1401 A. A. Golriz et al., Nanoscale (2011) 3 5049-5058
Full discharging and charging w/o MPT (2) Smaller polarization for MPT Reversible formation/decomposition of Li 2 O 2 ~ 1.6 V (1) Ex situ XRD patterns w/ and w/o MPT w/ MPT ~ 1.0 V (4) (3) (4) (3) (2) (1) Li 2 O 2 Pristine
Constant capacity: 1000 ma h g -1 w/o MPT w/o MPT Improved cycle performance, but still fading w/ MPT w/ MPT
carbon Li metal Reversible redox reaction of MPT in O 2 atmosphere w/o Li metal Failure mechanism of MPT attributable to Li metal RM Degradation of MPT in Li- O 2 batteries with Li metal Li 2 O 2 O 2 RM + 2Li + RM
Li/Li symmetric cells in Ar atmosphere (LiSO 3 CF 3 w/ and w/o MPT in tetraglyme) w/o MPT w/ MPT Stable voltage profiles for Li/Li cells containing no MPT Unstable voltage profiles and voltage fluctuation for Li/Li cells containing MPT Decomposition of MPT when in contact with Li metal
MPT stability when in contact with Li metal ~ 1.3 V additional peak reductive decomposition of MPT
Decomposition of MPT when in contact with Li metal new peaks Appearing new 1 H NMR peaks after the storage with Li metal decomposition of MPT
Li/Li symmetric cells in O 2 atmosphere (LiSO 3 CF 3 w/ and w/o MPT in tetraglyme) w/o MPT w/ MPT Unlike Ar, stable voltage profiles despite the addition of MPT in O 2 Suppressed MPT decomposition in O 2
Role of O 2 for Li metal electrodes w/ O 2 w/o O 2 Electrolyte (THF) Average cycling efficiency (5~10 cycles) Bare Oxygen saturated 0.2 M LiClO 4 ~ 50 % 60~65 % 0.5 M LiClO 4 75 % > 85 % 0.5 M LiAsF 6 95 % 95 % Dissolved O 2 in the electrolyte formed the lithium oxides-based protective layer on the Li metal surface Lithium oxides-based protective layer 1) Suppressing dendrite growth 2) enhancing electrochemical performance D. Aurbach et al., J. Electrochem. Soc. (1989) 119 3198-3205
XPS analysis of the Li metal surface after the storage in electrolytes In Ar In O 2 More lithium oxides (Li 2 O, Li 2 O 2 ) were formed in O 2 atmosphere compared to Ar atmosphere
carbon Li metal oxides Li metal What happen on the Li metal surface in Li-O 2 batteries? RM Li/Li symmetric cell with MPT in O 2 stable cycle performance Reduced form of MPT Li 2 O 2 O 2 RM + 2Li + stable when in contact with lithium oxides-protected Li metal surface in O 2 RM Role of MPT + in the Li O 2 batteries Oxidized form of MPT+ related to the degradation of Li metal and MPT
Designed five electrodes cell to form MPT + in the electrolyte for Li/Li symmetric cells The Li/Li cell was degraded as soon as MPT + was formed The degradation of the Li/Li cell is attributable to the formation of oxidized MPT + MPT + broke the lithium oxide protective layer on Li
Role of MPT + in the degradation of Li metal 1) MPT + decomposes the lithium oxides protection layer on the Li metal 2) MPT is exposed to fresh Li metal, thus being decomposed on the Li metal 3) Thick passivation layer forms on the Li metal surface, resulting in the failure of Li metal
LiNO 3 as an MPT + scavenger
Constant capacity: 1000 ma h g -1 1.0 M LiNO 3 in tetraglyme Improved cycle performance when LiNO 3 was used instead of LiSO 3 CF 3 1.0 M LiNO 3 in DMA Best for LiNO 3 in DMA stable solvent against oxygen radical stable cycle performance over 120 cycles
LiSO 3 CF 3 LiNO 3 The Li/Li cell was stable even after the formation of oxidized MPT+ NO 3- protected the Li metal against the attack of MPT +
Reaction between LiNO 3 and Li metal 0.2 M LiTFSI + 20 mm NaNO 2 in diglyme The reaction between NO 3- and Li metal forms soluble NO 2- and lithium oxides on Li metal NO 2 - A redox couple with a redox potential of ~ 3.6 V vs. Li/Li + Daniel Sharon et al., ACS Appl. Mater. Interfaces (2015) 7 16590-16600
CV of MPT with Li metal (MPT + tetraglyme, Li metal reference) LiSO 3 CF 3 LiNO 3 At fast scan rate: the reduction peak was observed Redox potential: NO 2- : ~ 3.6 V MPT : ~ 3.7~3.8 V At fast scan rate: the reduction peak disappeared MPT+ was consumed slowly by NO 2 - NO 2- + MPT + NO 2 + MPT
CV of MPT with Li metal (MPT + tetraglyme, Li metal reference) LiSO 3 CF 3 + NaNO 2 Even at slow scan rate: the reduction peak disappeared when NO 2- from NaNO 2 was added MPT+ was consumed by NO 2 -
Redox potential: NO 2- : ~ 3.6 V MPT : ~ 3.7~3.8 V NO 2- + MPT + NO 2 + MPT (2) The reaction between NO 2- and MPT + LiO x (1) Formation of the protection layer on Li metal and NO 2- scavenger
Failure mechanism of Li-O 2 batteries with MPT 1) MPT + decomposes the lithium oxides protection layer on the Li metal 2) MPT is exposed to fresh Li metal, thus being decomposed on the Li metal 3) Thick passivation layer forms on the Li metal surface, resulting in the failure of Li metal
carbon Li metal To overcome the failure of MPT and Li metal Protection layer carbon Li metal RM RM RM + RM + O 2 O 2 Li 2 O 2 2Li + Li 2 O 2 2Li + RM RM 1) Stable protective layer on Li metal anode against redox mediators 2) Stable redox mediators against Li metal anode
Thanks for your attention