Imaging Sodium Dendrite Growth in All‐Solid‐State Sodium Batteries Using Na T2‐Weighted Magnetic Resonance Imaging

Abstract

Two‐dimensional, Knight‐shifted, T2‐contrasted ²³Na magnetic resonance imaging (MRI) of an all‐solid‐state cell with a Na electrode and a ceramic electrolyte is employed to directly observe Na microstructural growth. A spalling dendritic morphology is observed and confirmed by more conventional post‐mortem analysis; X‐ray tomography and scanning electron microscopy. A significantly larger ²³Na T2 for the dendritic growth, compared with the bulk metal electrode, is attributed to increased sodium ion mobility in the dendrite. ²³Na T2‐contrast MRI of metallic sodium offers a clear, routine method for observing and isolating microstructural growths and can supplement the current suite of techniques utilised to analyse dendritic growth in all‐solid‐state cells.

Full text

Batteries
Imaging Sodium Dendrite Growth in All-Solid-State Sodium Batteries
Using 23Na T2-Weighted Magnetic Resonance Imaging
Gregory J. Rees, Dominic Spencer Jolly, Ziyang Ning, T. James Marrow,
Galina E. Pavlovskaya,* and Peter G. Bruce*
Abstract: Two-dimensional, Knight-shifted, T2-contrasted
23Na magnetic resonance imaging (MRI) of an all-solid-state
cell with a Na electrode and a ceramic electrolyte is employed
to directly observe Na microstructural growth. A spalling
dendritic morphology is observed and confirmed by more
conventional post-mortem analysis; X-ray tomography and
scanning electron microscopy. A significantly larger 23Na T2
for the dendritic growth, compared with the bulk metal
electrode, is attributed to increased sodium ion mobility in
the dendrite. 23Na T2-contrast MRI of metallic sodium offers
a clear, routine method for observing and isolating micro-
structural growths and can supplement the current suite of
techniques utilised to analyse dendritic growth in all-solid-state
cells.
All-solid-state batteries (ASSB) with a ceramic electrolyte
and an alkali metal anode could deliver a step-change in
energy storage and safety. The use of solid-state electrolytes
has numerous advantages over the conventional organic
electrolytes such as the ability to use metal anodes, removal of
volatile and flammable electrolyte organics, and they open up
the possibility of Li-Air and Li-Sulphur cathodes (which have
higher volumetric density).[1] These advantages are protracted
when coupled with sodium anodes which allow the use of
aluminium current collectors (whereas more-expensive Cu is
required for Li), sodium also has a significantly higher natural
abundance (2.36% abundance in the earths crust) compared
to that of conventionally used lithium (<0.002%) and,
therefore, offers more security against a volatile Li
market.[1,2] One of the greatest barriers to the progress of
ASSBs is the formation of dendrites (filaments of alkali
metal) on charging that penetrate the ceramic leading to
a short-circuit and cell failure.[3] Dendritic growths in ASSB
systems have different morphologies to their solution coun-
terparts, which can be correlated to the electrochemistry.
These growths have been categorized into four discrete
morphologies; straight, branching, spalling, and diffuse.[4, 5]
Imaging such dendrites is essential to understand their
growth and to develop mechanisms to prevent their forma-
tion. Magnetic resonance imaging (MRI) can provide non-
destructive, isotope specific, structural, time-resolved, and
quantifiable multi-dimensional information.[6] Both 1H and
7Li MRI and magnetic resonance spectroscopy (MRS) have
been utilised to probe dendrites in liquid electrolyte electro-
chemical cells,[7] and 7Li chemical shift imaging has explored
Li microstructural growth in ASSBs.[8] Recent work by Bray
et al. shows that in-operando 23Na MRI and MRS studies on
sodium cells with organic liquid electrolytes were able to
determine the sodium speciation upon galvanostatic cycling.[9]
In-situ 23Na nuclear magnetic resonance (NMR) during
electro-deposition of Na, shows that reversible high-surface-
area mossy and/or dendritic structures can be observed and
attributed to a nucleation mechanism.[12] Direct T2contrast
MRI on any battery material (1H, 6/7Li and 23Na) has never
been investigated. However, NMR relaxometry measure-
ments (spin-lattice relaxation; T1, spin-spin relaxation; T2)
have been vital in understanding ion dynamics in a range of
battery systems.[13]
Imaging solid-state electrolytes is technically more diffi-
cult than their solution counterparts as the NMR linewidths
are substantially broader, this causes the T2/T2*to be short
and hence the signal to dephase during the application of the
imaging gradients. Conventional 1H MRI is not possible in
ASSBs due to the lack of protons in the system and if the
[*] Dr. G. J. Rees, D. Spencer Jolly, Z. Ning, Prof. T. J. Marrow,
Prof. P. G. Bruce
Department of Materials, University of Oxford
Parks Road, Oxford, OX1 3PH (UK)
E-mail: peter.bruce@materials.ox.ac.uk
Prof. P. G. Bruce
Department of Chemistry, University of Oxford
South Parks Road, Oxford, OX1 3QZ (UK)
and
The Henry Royce Institute
Parks Road, Oxford, OX1 3PH (UK)
Dr. G. J. Rees, Prof. P. G. Bruce
The Faraday Institution
Harwell Campus, Didcot, OX11 0RA (UK)
Prof. G. E. Pavlovskaya
Sir Peter Mansfield Imaging Centre, School of Medicine, University of
Nottingham
Nottingham, NG7 2RD (UK)
and
NIHR Nottingham Biomedical Research Centre, University of
Nottingham
Nottingham, NG7 2RD (UK)
E-mail: galina.pavlovskaya@nottingham.ac.uk
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202013066.
 2020 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
A
ngewandte
Chemi
e
Communications
How to cite: Angew. Chem. Int. Ed. 2021,60, 2110–2115
International Edition: doi.org/10.1002/anie.202013066
German Edition: doi.org/10.1002/ange.202013066
2110  2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021,60, 2110 –2115

system did contain protonated groups these would have broad
intrinsic linewidths (kHz) which would dephase during the
application of gradients. Therefore, work in this field has
focussed on the narrower linewidths of 7Li and its distribution
within the solid-electrolyte.[14] Likewise, 23Na MRI is consid-
ered more challenging than 7Li, as it has a significant chemical
shift, greater quadrupole moment, and lower sensitivity than
7Li.[15] However, these larger properties can be utilised to
achieve greater image contrast. Specifically, with a time-
incremented series of MRI experiments, spin-spin (T2)
relaxation can be measured at each spatial element of the
object and an MRI image based on relaxation rather than spin
density can be produced. The achieved spatial contrast
supplies greater information to the images and allows one
to comment on the dynamics or local site symmetry of
individual pixels. Here we report for the first time, T2contrast
23Na MR images of metallic Na electrodes in a pristine state
and after short-circuiting. The T2maps of the electrochemical
cells allow us to comment on the Na-ion dynamics of the
formed features. As 23Na is significantly more difficult than
7Li, this methodology can readily be adapted to more
conventional Li-ASSBs and liquid electrolyte systems.
Two-dimensional spin-echo Knight-shift 23Na MRI images
of the symmetrical all-solid-state Na jNa-b’’-Alumina jNa
cell are shown for the pristine cell in Figure 1c and a cell after
passing a current in Figure 1d, with the dendrite apparent in
the latter. A thorough description of the experimental setup is
given in the SI (Figure S1). These intensity images show that
the amount of signal originating from the dendritic feature is
limited due to the deficiency of 23Na nuclear spins in this
structure, despite this, the dendrite is still observable in
Figure 1 d. Herein lies the major obstacle with materials MRI;
to increase the resolution (or to produce 3D representations)
a smaller area of space is needed to be sampled, however, this
means that the number of nuclear spins contributing to the
signal, in this given area, is reduced and, therefore, the
experimental times increase dramatically. MRI has circum-
vented this issue by employing contrast driven sequences
which can isolate the area of interest and, thus, does not
require such high-resolution images.
The microstructural growth observed in Figure 1d follows
a spalling morphology previously observed by Kazyak and co-
workers, other dendritic morphologies which form in ASSBs
are also discussed.[4] A spalling morphology is formed when
a dendritic crack propagates back to the surface, forming
a conical surface fracture which often precedes the dendrite
fully traversing the solid-electrolyte and causing a short
circuit. Although the dendrite must have extended from
counter to working electrode at the moment of short-circuit
(voltage =0 V in Figure 1a), at the time of imaging there
remains no observable dendrite transversing the electrodes
and only the spallation section of the dendrite is observed.
This is due to the high current passing through the thinnest
parts of the dendrite causing Joule heating, which burns away
the thinnest sections of the dendrite. Growth of a metallic Na
dendrite parallel to the magnetic field causes the NMR signal
to shift to a higher frequency by 5 ppm (from 1126 up to
1131 ppm),[9,12] this is due to orientation dependant nature of
the Knight shift.[16] The shift is smaller than that observed in
7Li as Na has a reduced bulk magnetic susceptibility.[2] The
spalling nature of the microstructural growth seen here has no
distinct directionality, with respect to the magnetic field (B0,
Figure 1b), therefore no obvious
secondary peak formation is
observed in the MRS spectra
(Figure S2b).
The skin depth of sodium is
11 mm at 105 MHz (9.4 T), there-
fore, only the top and bottom
11 mm of metallic Na is observed
in 2D projections. At 9.4 T the
lithium-7 skin depth is compara-
ble, 12 mm. As dendrites are
narrow thin filaments of metallic
species, their formation should
increase the number of 23Na
nuclei visible in the MRS and
MRI experiments as more nuclei
are moved into areas below the
skin-depth and are, therefore,
accessible to the radio-frequency
pulse. There are two possible
reasons for not notably detecting
an increase in 23Na signal, the
first is that large spherical spal-
ling morphology of the dendritic
growth does not reduce the skin
depth and secondly the size of
the parallel component of the
dendrite may be below the limit
Figure 1. a) Galvanostatic cycling of a Na jNa-b’’-Alumina jNa cell under 1 MPa stack-pressure and
a current density 0.5 mA cm2. A charge of 9.5 mAh cm2is passed on the first 1
=
2cycle during which
increased polarisation is observed, consistent with the formation of voids at the electrode as Na is
stripped, as shown previously.[10] On current reversal, the cell short-circuits rapidly consistent with the
formation of dendrites. b) The arrangement of the cell (imaged in the z,yplane) within the magnetic
field (B0), this orientation is conserved throughout all the images. The traditional, intensity, Knight shift
23Na MRI images of the c) pristine cell and d) cell after short-circuit containing a dendritic growth
(highlighted) at the edge of the cell, which is consistent with ion migration along the electric field
lines.[11] The equivalent gradient-echo intensity images are given in Figure S4.
A
ngewandte
Chemi
e
Communications
2111Angew. Chem. Int. Ed. 2021,60, 2110 –2115  2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org

of detection. The lack of chemical shift change and/or
increase in signal intensity means that conventional 23Na
NMR cannot be utilized to observe dendritic growth in these
ASSBs.
More established ex-situ post-mortem dendrite character-
isation techniques, X-ray computed tomography and scanning
electron microscopy (SEM), were completed to confirm the
nature of the dendrites morphology. The X-ray computed
tomography image for the pristine cell is shown in Figure 2a
and the cell after shorting is given in Figure 2b. A spalling
morphology dendritic formation is observed in the shorted
cell, which is in good agreement with the 23Na MRI. The pixel
size of the tomography images is 4.66 mm and no clear
evidence of a dendrite or crack is observed within the
electrolyte. Likewise, the lack of dendrite observed penetrat-
ing the length of the electrolyte agrees with the MRI results
and suggests that Joule heating has burnt away the dendrite.
The corresponding SEM images are presented in Figure 2c
and an increased resolution image focusing on part of the
spalling feature is given in Figure 2d, with the corresponding
EDX images in Figure 2e. The post-mortem cross-sectional
images show the formation of a spallation crack. This crack is
filled with a metallic material assumed to be Na, however, this
was not confirmed with certainty as although there is a weak
Na signal from the dendritic area, the response is dominated
by a strong C signal, likely due to the reaction of adventitious
carbon species with the highly reactive and newly exposed Na
surfaces.
These images highlight the
difficulties with definitively
imaging dendrites in ASSBs.
The nano-scale of the crack and
dendrite makes visualizing these
formations challenging by X-ray
tomography and although higher
resolution can be achieved, these
are not routinely accessible for
post-mortem cell-failure analysis
or industrial use. In addition,
SEM/EDX is often imperfect as
its cross-sectional nature makes
it a destructive technique, only
able to show a 2D area of a 3D
dendrite and EDX is often not
able to achieve the desired spe-
cificity of chemical information.
The effect of T2weighting the
MR images is shown in Figure 3 a
for the pristine and Figure 3c for
the cell after short-circuit, with
their respective T2histograms
given in Figure 3 b and d. With
current experimental parame-
ters, a full T2map (5 echo incre-
ments) can be achieved in
11.5 hours, using a spin-echo
acquisition scheme. Although
this timescale is too long for
in-situ or in-operando measurements of dendritic growth,
making this method only applicable for post-mortem analysis,
one needs to recall that these measurements were completed
on commercially available equipment (suitable for medical
and biological applications) and optimisation of the coil size,
probe power handling, gradient strength, and magnetic field
strength will all significantly reduce the experiment time. The
superior contrast of the dendrite (Figure 3 c, red region) is due
to a longer T2of the Na nuclei in the growth, this is an isolated
peak in the corresponding distribution histogram (Figure 3 d;
labelled, yellow peak).
The T2exponential decay curves for a range of pixels in
the bulk electrode (Figure 4 c) and the dendrite (Figure 4 a),
with respect to the background noise (Figure 4; given in dark
blue), illustrates the appreciable differences between the T2
relaxation characteristics of the dendrite and the bulk metal
electrodes. The noise level in these images is minor even at
extended echo delays. In the dendrite pixels, a signal to noise
ratio of 4 is achieved despite the lack of 23Na nuclei in these
sites. The T2relaxation of the electrodes varies from 5–10 ms
in both the pristine and cell after short-circuiting, with an
increased full width at half maximum height (FWHM,
Figure 3 b,d) being observed in the cell after short-circuiting.
This FWHM increase is attributed to increased local disorder
of the 23Na nuclei due to the formation of voids in the
electrode.[10,17] The T2relaxation times in the dendritic growth
are significantly longer (>12 ms) with a cluster of T2s at
16 ms (Figure 4 b).
Figure 2. The X-ray tomography images (4.66 mm resolution) of the a) pristine cell and b) cell after
short circuit showing the spalling morphology. Scanning electron micrographs illustrating the dendritic
crack formation with increasing resolution, c) 100 and d) 10 mm scale. e) The corresponding energy-
dispersive X-ray (EDX) images highlighting the distributions of (i) C, (ii) O, (iii) Na, and (iv) Al.
A
ngewandte
Chemi
e
Communications
2112 www.angewandte.org  2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021,60, 2110 –2115

Spin-spin (T2) relaxation is a time constant which
describes the signal decay in the transverse (x,y) plane and
is routinely utilized in conventional in-vivo medical 1H MRI
to generate image contrast.[18] The T2relaxation mechanism,
in solids, is attributed to numerous factors such as dipole-
dipole coupling, anisotropies, quadrupole effects, and local
motion. The dominant T2relaxation effect in metals is Pauli
paramagnetism coupled with strong dipole-dipole coupling.[19]
The 100% natural abundance 23Na spins directly couple
through their nuclear magnetic moments, and indirectly
through the intermediary of the conduction electron spins,
the so-called pseudo-dipolar and pseudo-exchange coupling,
respectively.[19] The T2weighted map of the cell after short-
circuit shows significant contrast between the Na electrodes
(T2span =5–12 ms) and the increased relaxation of the
dendrite (T2span =16–17 ms). As the dendrite growth has
reduced dimensions with reduced local symmetry, compared
to the electrode, then one would expect the T2to be shorter
than the bulk Na metal electrode, due to increased quad-
rupolar/anisotropic effects. In metallic NMR, the signals are
broadened when particle sizes are lower than 10 nm.[20] Below
10 nm the local symmetry experienced by the 23Na nuclei will
distort away from cubo-octahedral, causing systematic line-
width broadening as the particle size decreases. This dis-
tortion of the local symmetry will also affect the difference
between the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO), resulting
in a change in the Knight shift.[21] As there is no observed shift
or broadening in the 23Na peak position from either the
electrode and the dendrite (Figure S2b), then it may be
assumed the dendrite is larger than 10 nm. Incidentally,
tomography images were col-
lected with a 4.66 mm pixel size
and no dendrite was observed in
these images. Therefore, we pro-
pose that the dendrite width
ranges from tens to hundreds of
nanometres. The spalling micro-
structural growth observed in the
23Na images is 23–46 mm in size
(one—two pixels along the z-
axis).
The absence of broadened
features in the NMR suggests
that the increased spin-spin relax-
ation must be dominated by
another effect. We attribute the
substantially increased T2to
increased mobility of the 23Na
nuclei in the dendritic filament,
this mobility would reduce the
dipole-dipole coupling and,
hence, increase the spin-spin
relaxation. The mechanisms gov-
erning self-diffusion in locally
disordered materials are driven
by their defects. Point defects,
such as vacancies or interstitials,
give increased cation self-diffu-
sion.[22] Dislocations, grain boundaries, phase boundaries, and
free surfaces are other types of defects found in solids cause
significant clusters of local defects and act as a diffusion
barrier.[23] Therefore, the observed increase in T2within the
dendrite could be due to a higher concentration of point
defects or a lower concentration of dislocations, grain
boundaries, phase boundaries or free surfaces within the
dendrite compared to the bulk metal electrode, likely due to
the unique conditions of plating Na at a fast rate into a thin
crack with a small aperture.
Regardless of the mechanism, the effect of the different T2
values between the bulk electrode and dendrite is the ability
to resolve one from another and to discretely determine
where a dendrite begins and the electrode stops. The isolation
of the minor peak at a T2of 16 ms (Figure 3d) of the cell
after short-circuit shows that the 23Na nuclei in the dendrite is
in a significantly different dynamic environment to the bulk
metal in the electrode, and offers the opportunity to develop
T2resolved experiments to solely image the dendrite. A
broader T2distribution (Figure 3d) in the electrode region
(5–12 ms) of the shorted cell is evident and can be attributed
to a greater range of local Na site symmetries and the
formation of voids.
The application of T2weighted 23Na MRI is a promising
technique to directly observe the formation and determine
the structural dynamics of dendrites in ASSBs. The drive for
contrast imaging is to remove resolution limits and here,
despite the dendrite being smaller than the resolution of both
23Na MRI and tomography, one can still observe the dendrite
by MRI. The T2of the dendrite is significantly longer than the
T2of the bulk metal electrode. Due to a lack of broadening
Figure 3. The T2weighted contrast maps (with the same orientation and scale as Figure 1c and d) of
a) the pristine symmetrical cell and c) the cell after short-circuit. The dendrite is highlighted and has
a significantly increased T2. The respective total T2distributions are given in the histograms (b) and
(d), with the isolated dendrite distribution highlighted. The individual 23Na images which were used to
produce the T2weighted maps are given in Figure S3.
A
ngewandte
Chemi
e
Communications
2113Angew. Chem. Int. Ed. 2021,60, 2110 –2115  2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org

observed in the corresponding NMR spectra, the longer T2
must be attributed to increased Na dynamics in the dendrite.
This contrast driven methodology can also be utilized on
liquid electrolyte cells, which have different dendritic mor-
phologies to ASSBs.[24] One limitation of 23Na MRI is the
inability to image the crack development in ASSBs, therefore
we recommend a multimodal imaging approach, combining
X-ray computed tomography to track morphological changes
in the cells, high-resolution elemental content can be achieved
from ex-situ SEM, and dynamic information can be attained
from contrast-driven MRI. This multimodal approach allows
one to image crack formation, microstructural growth, ion-
dynamics, and any dendritic formations.[25]
Acknowledgements
P.G.B. is indebted to the Engineering and Physical Sciences
Research Council (EPSRC), including Next Generation
Solid-State Batteries [EP/P003532/1], Henry Royce Institute
for Advanced Materials [EP/R00661X/1, EP/S019367/1, EP/
R010145/1], and the Faraday Institution All-Solid-State
Batteries with Li and Na Anodes [FIRG007, FIRG008] for
financial support. G.E.P. thanks the Medical Research
Council for funding (Grant # MC_PC_15074) of sodium
imaging methodology development. The X-ray tomography
facilities were funded by an EPSRC Grant [EP/M02833X/1];
“University of Oxford: experimental equipment upgrade”.
Conflict of interest
The authors declare no conflict of interest.
Keywords: all-solid-state electrolytes · batteries ·
magnetic resonance imaging · NMR spectroscopy · sodium
[1] C. Vaalma, D. Buchholz, M. Weil, S. Passerini, Nat. Rev. Mater.
2018,3, 18013.
[2] W. M. Haynes, D. R. Lide, T. J. Bruno, CRC Handbook of
Chemistry and Physics: A Ready-Reference Book of Chemical
and Physical Data, 97th Ed., CRC Press, Boca Raton, FL, 2016.
[3] J. Janek, W. G. Zeier, Nat. Energy 2016,1, 16141.
[4] E. Kazyak, R. Garcia-Mendez, W. S. LePage, A. Sharafi, A. L.
Davis, A. J. Sanchez, K.-H. Chen, C. Haslam, J. Sakamoto, N. P.
Dasgupta, Matter 2020,2, 1025 – 1048.
[5] J. Garche, C. K. Dyer, P. T. Moseley, Z. Ogumi, D. A. J. Rand, B.
Scrosati, Encyclopedia of Electrochemical Power Sources, Elsev-
ier Science, Amsterdam, 2013.
[6] B. Blumich, NMR imaging of materials, Oxford University Press,
Oxford, 2000.
[7] S. Chandrashekar, N. M. Trease, H. J. Chang, L.-S. Du, C. P.
Grey, A. Jerschow, Nat. Mater. 2012,11, 311 –315 ; A. J. Ilott, M.
Mohammadi, H. J. Chang, C. P. Grey, A. Jerschow, Proc. Natl.
Acad. Sci. USA 2016,113, 10779 – 10784.
[8] L. E. Marbella, S. Zekoll, J. Kasemchainan, S. P. Emge, P. G.
Bruce, C. P. Grey, Chem. Mater. 2019,31, 2762 – 2769.
[9] J. M. Bray, C. L. Doswell, G. E. Pavlovskaya, L. Chen, B.
Kishore, H. Au, H. Alptekin, E. Kendrick, M.-M. Titirici, T.
Meersmann, M. M. Britton, Nat. Commun. 2020,11, 2083.
[10] D. Spencer Jolly, Z. Ning, J. E. Darnbrough, J. Kasemchainan,
G. O. Hartley, P. Adamson, D. E. J. Armstrong, J. Marrow, P. G.
Bruce, ACS Appl. Mater. Interfaces 2020,12, 678 – 685.
[11] R. Failla, M. Bologna, B. Tellini, J. Power Sources 2019,433,
126675.
[12] P. M. Bayley, N. M. Trease, C. P. Grey, J. Am. Chem. Soc. 2016,
138, 1955 – 1961.
[13] L. Zhou, M. Leskes, T. Liu, C. P. Grey, Angew. Chem. Int. Ed.
2015,54, 14782 – 14786 ; Angew. Chem. 2015,127, 14995 – 14999;
A. Grenier, P. J. Reeves, H. Liu, I. D. Seymour, K. Mrker, K. M.
Wiaderek, P. J. Chupas, C. P. Grey, K. W. Chapman, J. Am.
Chem. Soc. 2020,142, 7001 – 7011; A. Dunst, V. Epp, I. Hanzu,
S. A. Freunberger, M. Wilkening, Energy Environ. Sci. 2014,7,
2739 – 2752.
[14] S. Chandrashekar, O. Oparaji, G. Yang, D. Hallinan, J. Electro-
chem. Soc. 2016,163, A2988– A2990 ; P.-H. Chien, X. Feng, M.
Tang, J. T. Rosenberg, S. ONeill, J. Zheng, S. C. Grant, Y.-Y. Hu,
J. Phys. Chem. Lett. 2018,9, 1990 – 1998.
Figure 4. The spin-echo decay curves showing the determined T2
parameters for a range of pixels from a) the dendritic growth and
c) the bulk electrode. The curves are colour coded to the pixels shown
in the expanded 2D T2-contrast image presented in (b). Each pixel
intensity normalised curve is fitted to a mono-exponential with
a constant offset determined by the noise level (given in dark blue).
A
ngewandte
Chemi
e
Communications
2114 www.angewandte.org  2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH Angew. Chem. Int. Ed. 2021,60, 2110 –2115

[15] E. G. Sadykhov, Y. A. Pirogov, N. V. Anisimov, M. V. Gulyaev,
G. E. Pavlovskaya, T. Meersmann, V. N. Belyaev, D. V. Fomina,
Appl. Magn. Reson. 2018,49, 925 – 957.
[16] R. Bhattacharyya, B. Key, H. Chen, A. S. Best, A. F. Hollen-
kamp, C. P. Grey, Nat. Mater. 2010,9, 504 – 510.
[17] M.-C. Bay, M. Wang, R. Grissa, M. V. F. Heinz, J. Sakamoto, C.
Battaglia, Adv. Energy Mater. 2020,10, 1902899.
[18] G. B. Chavhan, P. S. Babyn, B. Thomas, M. M. Shroff, E. M.
Haacke, RadioGraphics 2009,29, 1433 – 1449.
[19] C. P. Slichter, Surf. Sci. 1981,106, 382 – 396.
[20] G. J. Rees, S. T. Orr, L. O. Barrett, J. M. Fisher, J. Houghton,
G. H. Spikes, B. R. C. Theobald, D. Thompsett, M. E. Smith, J. V.
Hanna, Phys. Chem. Chem. Phys. 2013,15, 17195 – 17207.
[21] J. J. van der Klink, H. B. Brom, Prog. Nucl. Magn. Reson.
Spectrosc. 2000,36, 89 – 201.
[22] S. Kumarakrishnan, N. L. Peterson, T. O. Mason, J. Phys. Chem.
Solids 1985,46, 1007 – 1014.
[23] H. Mehrer, S. V. Divinski, Defect Diffus. Forum 2009,289 – 292,
15 – 38.
[24] L. Frenck, G. K. Sethi, J. A. Maslyn, N. P. Balsara, Front. Energy
Res. 2019,7, 115.
[25] Supporting research data has been deposited in the Oxford
Research Archive and is available under this DOI: https://doi.
org/10.5287/bodleian:1amymAzYZ.
Manuscript received: September 27, 2020
Accepted manuscript online: October 6, 2020
Version of record online: November 24, 2020
A
ngewandte
Chemi
e
Communications
2115Angew. Chem. Int. Ed. 2021,60, 2110 –2115  2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH www.angewandte.org