Understanding Cation-Disordered Cathode Materials Based on Percolation Theory

and Ligand Field Theory

2016 ECS Prime meeting (10/5/2016, 8:40 – 9:00 am)

Jinhyuk Lee, Dong-Hwa Seo, Alexander Urban, Gerbrand Ceder

UC Berkeley & LBNL

1 These slides can be downloaded at http://ceder.berkeley.edu

High E-density cathodes from “well-ordered” close-packed Li-TM oxides.

2

‘Layered’ - LiCoO2

in Samsung phones

Li TM

‘Spinel’ - LiMn2O4 in 1st gen. Nissan Leaf

Li TM

Oxygen FCC framework

Layered LiCoO2 (> 600 Wh/kg)

Cho et al., Chem. Mater. (2000)

Negligible attention to cation-disordered oxides (i.e. disordered rocksalt), as they tend to cycle poorly with limited Li diffusion.

3

Disordered rocksalt Li-TM oxides

“Random” Li/TM

Oxygen FCC framework

“negligible capacity”

Obrovac et al., Solid State Ionics (1998)

Capacity (mAh/g)

(2) LiVO2: Disorder during cycle

Vo

ltag

e (

V)

Zhang et al., JPS (2007)

(1) Disordered Li-Co-O

Li diffusion can be facile in cation-disordered materials if with Li excess. (e.g. Li1.2TM0.8O2)

4

Lee et al., Science (2014)

Li1.211Mo0.467Cr0.3O2

(1) Yabuuchi et al., PNAS (2015) (2) Wang et al., Electrochem. Commun. (2015)

Li1.3Mn0.4Nb0.3O2

Kitajou et al., Electrochemistry (2016)

Li1.2Mn0.4Ti0.4O2

Chen et al., AEM (2015)

Li2VO2F

> 260 mAh/g

~300 mAh/g > 300 mAh/g

~220 mAh/g

Li diffusion in general rocksalt type materials (e.g. in layered, disordered, spinel-like, γ-LiFeO2-like etc.)

5 J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Science (2014)

o-t-o hopping in Li-TM oxides (layered-, disordered-rocksalt etc.)

Three possible local geometries

ex) Li diffusion in stoichiometric layered oxides

6

1-TM channel (TM3+, 4+, etc)

Ex. ‘Layered’ LiCoO2

1-TM channels can support Li diffusion in the layered structure (i.e. active), as long as the size of an intermediate tetrahedral site stays large enough for the activated Li ion to relax away the strong electrostatic repulsion from the face-sharing high valent TM ion.

Must stay Large !!

J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Science (2014)

In cation-disordered materials, only 0-TM channels can support Li diffusion.

7

“active” “Inactive” Channels exist but too high barriers.

J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Science (2014)

Small tetrahedron size in disordered materials prevents an activated Li+ ion to relax away strong repulsion from high-valent TM species in 2-TM and 1-TM channels.

8

Disordered

Layered

Tetrahedron height (Å ) (Tetrahedron size)

Layered

Li

TM

Disordered

The activated Li+ ion in 0-TM channel do not have high valent TM species to face share with. Therefore, 0-TM channels can support Li diffusion even in the disordered materials with small tetrahedron size.

J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Science (2014)

For macroscopic Li diffusion in disordered materials, 0-TM channels must be percolating in the structure.

9

“Active” in disordered materials

0-TM channels need to percolate.

0-TM channels percolate in layered and disordered structures as soon as their composition becomes highly lithium excess (x> 1.09 in LixTM2-xO2)

10 (1) J. Lee et al., Science (2014), (2) A. Urban, J. Lee, Ceder, Adv. Energy Mater. (2014)

Composition

Li excess

disordered

layered

“0-TM percolating”

Is 0-TM percolation enough for designing high-capacity cation-disordered materials?

11

Fast Li diffusion by 0-TM percolation

Stay active in disordered materials

Li excess No, 0-TM percolation => Li diffusion A good electrode requires enough electrons to be extracted or inserted upon cycling, => We need to understand the redox mechanism.

Controversial “Li-excess” strategy: Improves Li diffusion after sacrificing TM-redox

• Li-excess decreases TM contents and increases the average oxidation state of TM species, reducing TM redox capacity.

12

Li2Mn4+O3 => 2 Li+ + 2 e- + Mn6+O3 (Mn4+/Mn6+ ?) : 0-TM percolation (O), TM-redox (X).

Example: “Li(Li1/3Mn2/3)O2 ”

Reversible O-redox resolve this controversy by making electron capacity unbound to TM redox.

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If oxygen redox can reversible occur, we are no longer bound to TM redox capacity for electron capacity. Moreover, oxygen redox typically delivers high voltage.

O redox is important in Li-excess materials.

Li2Mn4+O2-3 => 2 Li+ + 2 e- + Mn4+O1.333-

3 (O2-/O-)

(1) Reversible O-redox (O2-/O1-)

Can O-redox reversibly occur in Li-excess materials to give extra capacity at a reasonably low voltage?

Shin et al. Chem. Mater. (2016)

Fortunately, both Li-excess and cation-disorder promote oxygen redox which is difficult to occur in stoichiometric layered materials

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M M M

Li Li Li

Local O-coordination In stoichiometric

layered Li-M oxides e.g. LiCoO2

O

TM layer

Li layer

t1u*

a1g*

eg*

t2g

t1ub

a1gb

egb

E

(an

tib

on

din

g)

M s

tate

s (b

on

din

g)

O s

tate

s Too stable for electron extraction

Band structure of stoichiometric layered Li-M oxides

(antibonding) M-states

(bonding) O-states

M d/s/p

O 2p

M d/s/p

O 2p

As M-O covalency increases (e.g. Ni, Co, Ru etc.)

Oxygen electrons that form highly covalent bonding with TM species will be too stable to participate in O-redox, which is the case in the stoichiometric layered materials.

How does Li excess and cation disorder promote oxygen redox?

15

Li-excess layered

Li-excess disordered

O-coordination

M

O

Li

three Li‒O‒M (as in LiCoO2)

O-coordination

M

O

Li

O Li

Li +

+

D.-H. Seo†, J. Lee† et al., Nature Chem. (2016) [†equal contribution]

O Li

Li

(1) O 2p orbitals along the “Li–O–M” or “M-O-M” configurations

(2) O 2p orbitals along “Li–O–Li” configurations

Ligand field theory: Lack of hybridization makes O 2p electrons along Li–O–Li configurations unstable.

16

Li

Li +

Li-excess Layered/disordered Li-M oxides

+ …

D.-H. Seo†, J. Lee† et al., Nature Chem. (2016) [†equal contribution]

Li-excess layered

Li-excess disordered

Oxygen electrons in the labile Li-O-Li state can be easily removed to give extra capacity in Li-excess layered or cation-disordered materials.

t1u*

a1g*

eg*

t2g

t1ub

a1gb

egb

E

Stable bonding O 2p states

Li‒O‒Li (labile; lack of hybridization)

DFT calculations show that indeed the labile O 2p electrons from Li–O–Li states contribute to extra capacity in Li-excess materials.

17 D.-H. Seo†, J. Lee† et al., Nature Chem. (2016) [†equal contribution]

Layered Li1.17Ni0.25Mn0.58O2

Layered Li2Ru0.5Sn0.5O3

Disordered Li1.17Ni0.33Ti0.42Mo0.08O2

Disordered Li1.25Mn0.5Nb0.25O2

‒ 0.83 Li ‒ 1.5 Li ‒ 0.83 Li ‒ 1 Li

O-oxidation from Li-O-Li states leads to extra capacity, therefore we don’t need to worry about

limited TM redox in Li-excess disordered materials.

However, there is one more thing to consider….

Li-O-Li direction

Spin density on oxygen “oxygen hole”

Too much O redox => O loss with densification => destroys 0-TM percolation.

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Li2Mn4+O2-3 => 2 Li+ + 2 e- + [2VLi]Mn4+O1.333-

3

(1) Reversible O redox [Li2MnO3 => Li(Li1/3Mn2/3)O2]

[2VLi]Mn4+O2-2[VO] + 0.5 O2 (g) ↑

“Oxygen loss with O vacancies”

[VLi]Mn4+O2-2

“cation densification”

Now, we are in trouble.

Let’s lower even more !

Let’s lower some energies ! Li:Mn = 2:1 (Li-excess)

Li:Mn = 1:1 (no Li-excess)

0-TM perc. gone

Disordered Li-excess cathodes that lose oxygen show larger polarization upon cycling.

19 J. Lee et al., Energy. Environ. Sci. (2015)

High-capacity cation-disordered Li-excess Ni-Ti-Mo oxides

Consistent with percolation theory, the reversible capacity dramatically improves with Li excess.

20 J. Lee et al., Energy. Environ. Sci. (2015)

Li1.0 Li1.2 Li excess

LiNi1/2Ti1/2O2

(105 mAh/g)

Li1.2Ni1/3Ti1/3Mo2/15O2

(225 mAh/g)

1st cycle

While delivering high capacity, Li1.2Ni1/3Ti1/3Mo2/15O2 still shows large polarization (difference in c/dc profiles).

21

Li1.2Ni1/3Ti1/3Mo2/15O2 GITT upon discharge

J. Lee et al., Energy. Environ. Sci. (2015)

Large polarization in Li1.2Ni1/3Ti1/3Mo2/15O2 appears when charging above ~4.3 V.

22

If charging cut off is set below 4.3 V, polarization become much less.

2 - 4.1 V

J. Lee et al., Energy. Environ. Sci. (2015)

Above 4.3 V, O loss occurs from Li1.2Ni1/3Ti1/3Mo2/15O2, destroying 0-TM percolation at the surface.

23

Surface EELS data on Ti L-edges and O K-edge Substantial reduction of peak intensity ratio between Oxygen : Titanium

Li1.2TM0.8O2 (20 % Li-excess) => Li0.7TM1.3O2 (30 % TM-excess) From these results, we argue that preventing oxygen

loss will be the key to preserve good cycling performances of disordered Li-excess materials.

J. Lee et al., Energy. Environ. Sci. (2015)

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Conclusions

1. If we want facile Li diffusion in cation-disordered materials, we need to introduce Li excess for 0-TM percolation.

2. However, this Li-excess often reduces TM redox capacity, thus O-redox is further necessary for high electron capacity. And fortunately, O-redox from Li-O-Li states can resolve this.

3. We might want to avoid using too much oxygen redox because it can trigger O loss with densification which reduces the Li-excess level and therefore destroys the 0-TM percolation for facile Li diffusion.

25

Acknowledgement

Special thanks to: Dr. Seo, Dr. Urban, and Prof. Gerbrand Ceder

Thank you very much for your attention.

References [1] Lee, Urban, Li, Su, Hautier, Ceder, Science 343 (2014) [2] Seo†, Lee†, Urban, Malik, Kang, Ceder, Nature Chem. 8 (2016) [†equal contribution] [3] Lee, Seo, Balasubramanian, Twu, Li, Ceder, Energy Environ. Sci. 8 (2015) [4] Urban, Lee, Ceder, Adv. Energy Mater. 4 (2014)

These slides can be downloaded at http://ceder.berkeley.edu