Nature - USA (2020-01-23)

506 | Nature | Vol 577 | 23 January 2020

Article

supporting the structural models obtained from DFT. We considered a
range of other Mn-disordered structures, and all were higher in energy
than the vacancy cluster reported here. Also, the other disordered con-
figurations could not produce a voltage as close to the experimentally
observed voltage. Similar voltage calculations were carried out for the
ribbon structure, Na0.6[Li0.2Mn0.8]O 2 , where, assuming that no change

in in-plane ordering occurs, a value of 4.1 V is obtained in close agree-

ment with the observed discharge plateau.

To explore experimentally the nature of the oxidized oxygen species

on charge, we used RIXS spectroscopy of a higher resolution than in

past studies^4 ,^12 ,^19 ,^24 –^27. Our data reveal the underlying fine structure of

the elastic peak, labelled B in Fig. 3a, and show that it is composed of

–10

–8

–6

–4

–2

0

2

4

6

0

Energy (eV)

Density of states

Density of states

Density of states

Spin up Spin down

O 2Mn 3pd

–10

–8

–6

–4

–2

0

2

4

6

0

Energy (eV)

Density of states

Density of states

Density of states

Spin up Spin down

O–MnO–Mn 2

Mn 3d^3

–10

–8

–6

–4

–2

0

2

4

6

0

Energy (eV)

Spin up Spin down

a Honeycomb-ordered Na0.75[Li0.25Mn0.75]O 2 b c Ribbon-ordered Na0.6[Li0.2Mn0.8]O 2

–10

–8

–6

–4

–2

0

2

4

6

0

Energy (eV)

Spin up Spin down

O 2

O 2p

Mn 3d

–10

–8

–6

–4

–2

0

2

4

6

0

Energy (eV)

Spin up Spin down

Ionic O

O 2Mn 3pd

–10

–8

–6

–4

–2

0

2

4

6

0

Energy (eV)

Spin up Spin down

O–MnO–Mn 2

Mn 3d^3

O–MnO–Mn 2

Mn 3d^3

Pristine

P2

Charged

O2

Discharged

P2

O 2

1.2 Å O^2

2.6 Å

O–Mn 2

O–Mn 3

O–Mn 0

Frontier Na+–O 2 p–Li+

states from which e–

are removed

Frontier Na+–O 2 p–Li+

states from which e–

are removed

Very ionic

O–Mn 0

Electron holes

on O2–

Localized

electron holes

at higher

energy on O 2

S*

S*

S

S

Fig. 4 | Electronic and structural changes accompanying O redox. a, Density
of states (DoS) plots from DFT for honeycomb-ordered Na0.75[Li0.25Mn0.75]O 2 at
different states of charge. Pristine state: honeycomb-ordering with frontier
Na+–O 2p–Li+ oxygen states that are oxidized on charge. Charged state: Li sites
in the TM layer are vacant, in-plane migration of Mn (along arrows) forms
vacancy clusters triggered by formation of molecular O 2 (1.2 Å). Electron holes
localize in molecular π spin-down states of O 2 , represented by red shading in
the DoS. Discharged structure: Electrons populate the antibonding π and σ*
states, breaking the O–O bond and forming two O2− (2.6 Å apart). These oxide

ions are surrounded by alkali ions in the structure and thus are at the top of the

valence band (purple in DoS). b, Layer stacking at different states of charge for

both Na0.75[Li0.25Mn0.75]O 2 and Na0.6[Li0.2Mn0.8]O 2. c, DoS plots from DFT for

ribbon-ordered Na0.6[Li0.2Mn0.8]O 2. Pristine state: there are two unique O

environments, O–Mn 2 (blue) and O–Mn 3 (green). Charged state: electron holes

localize on O–Mn 2 close to the bottom of the conduction band (blue in DoS).

Discharged state: ribbon ordering maintained, structure unchanged from

pristine. In all DoS plots, the Fermi energy level is set to zero.