Boos ing he capaci y o Mg-s abilized
Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
ca hodes ia pa icle
size con ol in an emulsion-based syn hesis ou e†
Sa´
ul Rubio,
a
E a M. P´
e ez-So iano,
a
C is ina A ´
e alo,
b
Xiaoqiong Du,
c
Xuyun Guo,
c
F ancisco J. Ga cia-Ga cia,
a
Isabel Mon ealeg e-Mel´
endez,
a
Ana M. Bel ´
an,
a
Vale ia Nicolosi
c
and Juan G. Lozano *
a
In his wo k, he p oduc ion o ul a-high efficien Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
ca hodes syn hesized ia an
emulsion-based o ganic syn hesis ou e, along wi h a comp ehensi e a omic-scale cha ac e iza ion using
ad anced elec on mic oscopy echniques, is p esen ed. I is demons a ed ha inc easing he a io o he
su ac an o hyd ophobic and hyd ophilic componen s in he emulsion leads o op imized pa icle size and
a significan ly mo e uni o m pa icle size dis ibu ion. As a esul Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
exhibi s
supe io elec ochemical pe o mance, deli e ing an ini ial discha ge capaci y o 260 mA h g
−1
and
main aining a discha ge capaci y o 170 mA h g
−1
a e 100 cycles, wi h 99% coulombic efficiency. This
enhancemen is a ibu ed o he syne gis ic effec o Mg-induced s uc u al s abiliza ion and he
op imiza ion o pa icle size and dis ibu ion. These ac o s collec i ely acili a e he accommoda ion o
s ain induced by epea ed cha ge–discha ge cycles wi hou subs an ial s uc u al deg ada ion while
p ese ing efficien sodium de-in e cala ion pa hways.
In oduc ion
Ene gy s o age sys ems based on elec ochemical me hods a e
conside ed a solu ion o he challenges posed by he highly
sough ansi ion away om ossil uels. Since hei de elop-
men and comme cializa ion in he 1990s, echa geable
li hium-ion ba e ies (LIBs) ha e e olu ionized he eld o
po able elec onics and played a signican ole in he ea ly
de elopmen o elec ic ehicles. Howe e , LIBs s ill ace majo
challenges, including insufficien ene gy densi y o he ull
elec ica ion o au omobiles and issues ela ed o he sca ci y,
cos , and geog aphic dis ibu ion o aw ma e ials.
1,2
Sodium-ion ba e ies (SIBs) a e ega ded as p omising
candida es o la ge-scale ene gy s o age due o he na u al
abundance and low cos o sodium, as well as hei ope a ional
simila i ies o LIBs.
3,4
Among he mos ex ensi ely s udied
ca hode ma e ials o SIBs a e P ussian blue analogues, poly-
anionic compounds, and ansi ion me al oxides (TMOs).
5
The
la e a e pa icula ly in e es ing due o hei high heo e ical
specic capaci ies, especially hose wi h a P2- ype o O3- ype
s uc u e, whe e “P”and “O” e e o he p isma ic and oc a-
hed al coo dina ion o Na, espec i ely, and “2”o “3”indica es
he epea ing sequence in he s acking o de o he oxygen
laye s.
6,7
The laye ed P2- ype compound Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
has a ac ed a en ion due o i s g ea e a mosphe ic s abili y
and high specic capaci y.
8,9
Howe e , he esul s epo ed so
a emain subop imal and u he esea ch is wa an ed. This
is p ima ily due o a de imen al phase ansi ion ha occu s
du ing cha ging, in which he ma e ial ans o ms om i s
o iginal P2 s uc u e o an O2- ype s uc u e, due o he sliding
o TMO laye s when Na
+
is ex ac ed.
10,11
This ansi ion hinde s
he accommoda ion o Na
+
ions du ing discha ge and leads o
olume changes which induce excessi e in e nal s ess, po en-
ially causing mechanical ailu e o he ca hode. Addi ionally,
he s uc u e suffe s om i e e sible oxygen loss a ol ages
abo e 4 V, esul ing in capaci y ading, ol age d op, and
consequen ly, a apid decline in ene gy densi y.
12
The o he s able polymo phic phase ha his compound can
exhibi is O3. I s main ad an age is ha , unlike P2, i is no
sodium-decien , esul ing in a signican ly highe specic
capaci y.
13,14
Howe e , i has lowe a mosphe ic s abili y and
educed s uc u al s abili y, as i unde goes de imen al phase
ansi ions du ing he cha ging p ocess.
15
Addi ionally, he
highe sodium con en leads o mo e ab up olume changes,
a
Depa amen o de Ingenie ´
ıa y Ciencia de los Ma e iales y del T anspo e, Escuela
Poli ´
ecnica Supe io y Cen o Andaluc´
ıa-Tech Escuela Poli ´
ecnica Supe io
(CATEPS), Uni e sidad de Se illa, 41011 Se illa, Spain. E-mail: [email p o ec ed]
b
Depa amen o de Ingenie ´
ıa y Ciencia de los Ma e iales y del T anspo e, Escuela
T´
ecnica Supe io de Ingenie ´
ıa, Uni e sidad de Se illa, 41092 Se illa, Spain
c
School o Chemis y, Cen e o Resea ch on Adap i e Nanos uc u es and
Nanode ices, Ad anced Ma e ials Bio-Enginee ing Resea ch Cen e (AMBER), T ini y
College Dublin, Dublin, I eland
†Elec onic supplemen a y in o ma ion (ESI) a ailable. See DOI:
h ps://doi.o g/10.1039/d5 a02682b
Ci e his: DOI: 10.1039/d5 a02682b
Recei ed 4 h Ap il 2025
Accep ed 22nd June 2025
DOI: 10.1039/d5 a02682b
sc.li/ma e ials-a
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which gene a e mechanical s ess and, e en ually, he o ma-
ion o mic oc acks ha con ibu e o ca hode deg ada ion.
16
I has been sugges ed ha magnesium doping can offe an
effec i e s a egy o imp o e he s uc u al s abili y o his
ma e ial.
17–19
The smalle ionic adius o Mg
2+
a ou s he
s abiliza ion o he s uc u e and may also o m high-densi y
p ecipi a es on he alkali me al laye du ing high ol age
cycling, which can u he imp o e he mechanical s eng h o
he ca hode and inhibi c ack o ma ion du ing Na
+
in e cala-
ion p ocesses.
20
The use o Mg as a doping agen has been
p e iously explo ed in simila sys ems.
12,21
I has been shown
ha Mg, by subs i u ing Mn in he TM laye s, helps mi iga e
oxygen loss and enhances s uc u al s abili y. Mul i-elemen
doping
22
no only e ains he bene s o each dopan bu also
minimizes hei espec i e d awbacks, allowing g ea e exi-
bili y in uning he c ys al and elec onic s uc u es o ca hode
ma e ials. Fo his eason, Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
has been
s udied in his wo k.
Toge he wi h doping, ano he s a egy o s abilise he Na-
TMO s uc u e and achie e highe capaci ies in ol es pa icle
size con ol.
23–27
I is conside ed ha a small pa icle size
imp o es he inse ion and ex ac ion kine ics o sodium ions
due o a highe su ace- o- olume a io, p o iding supe io
ini ial specic capaci y. On he o he hand, la ge pa icles a e
be e able o accommoda e olume a ia ions associa ed wi h
la ice pa ame e changes du ing cha ge and discha ge cycles,
p o iding be e s uc u al s abili y and g ea e long- e m
capaci y e en ion. Howe e , i he pa icle size is oo la ge, he
olume ic changes will no be uni o m due o he g ea e
inefficiency in sodium inse ion and ex ac ion, leading o
c acking and mechanical a igue. The e o e, possible design
s a egies should also aim a achie ing an op imal pa icle size,
whe e all aspec s a e balanced.
28
In his sense, emulsion-based syn hesis me hods p o ide an
effec i e way o ob aining ma e ials wi h imp o ed mo phology
con ol.
29,30
Mic oemulsions a e sys ems composed o a mix u e
o hyd ophilic and hyd ophobic uids, s abilized by su ac an s
o o m a he modynamically s able and iso opic dispe sion.
When he hyd ophobic componen cons i u es he majo i y o
he sys em, d ople s o he hyd ophilic liquid become encap-
sula ed by su ac an s, known as e e se micelles, o ming
nano essels ha pe mi he syn hesis o a ious
nanopa icles.
31,32
While unde s anding he exac effec ha he balance o
hyd ophobic, hyd ophilic and su ac an componen s has in
pa icula micella sys ems is ela i ely complica ed, i is known
ha a ying many ac o s, such as he na u e o sol en s, size o
he pola headg oup, chain leng h o he su ac an o
empe a u e, among many o he s, makes i possible o ne- une
he composi ion o he mic oemulsion and hence he size and
s uc u e o he e e se micelle
33,34
and, ul ima ely, he size o
he nanopa icles.
Conside ing all he ac o s disclosed abo e, in his wo k, we
ha e used a a ian de eloped by ou esea ch g oup
35
o he
emulsion-based me hod desc ibed by F. J. Ga cia-Ga cia e al.
36
This modied me hod has se e al ad an ages o e he mo e
con en ional ones such as hyd o he mal, sol–gel o d y-milling
me hods: (i) i uses o ganic p ecu so s, which lead o he
p oduc ion o non-con aminan eac ion side-p oduc s; (ii) his
me hod has been p o en o be as e han con en ional d y-
milling o sol–gel me hods, since he mixing imes equi ed o
ob ain he p e-hea - ea men mix u e o oxides is d as ically
educed o 5 minu es wi h he use o a shea mixe , om he
ex ended imes anging om 90 minu es o se e al hou s
equi ed in con en ional me hods such as d y-milling o sol–
gel, and (iii) i holds a b oad po en ial o upscaling o indus ial
le els. Mo e impo an ly, we show ha he size and dis ibu ion
o Mg-doped Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
pa icles can be
modied by a ying he hyd ophobic : hyd ophilic :
su ac an componen a io, leading o an op imum alue,
which p o ides s cha ge capaci ies e y close o he
maximum heo e ical capaci y o Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
and excellen capaci y e en ion up o 100 cycles, ela ed o he
s uc u al s abili y achie ed by bo h doping and op imisa ion o
he pa icle size and dis ibu ion.
The p ocess in ol es he use o a nonpola o ganic liquid
ha is no miscible wi h wa e . In his case, aseline oil was
chosen as i has no known heal h conce ns compa ed wi h
indus ial mine al oils. Since he me al ions used o ab ica e
Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
ha e a hyd ophilic na u e, hey can
be dissol ed in wa e . Addi ionally, oleic acid is used as
a su ac an , due o i s amphipa hic na u e. I consis s o ela-
i ely long ca bon chains, wi h a hyd ophilic end and a hyd o-
phobic end. When he su ac an is added a a gi en
concen a ion, e e se micelles a e o med. In hese micelles,
he non-pola ends p e e en ially o ien hemsel es owa ds he
ex e io , while he pola ends poin owa ds he in e io o he
e e se micelle, in con ac wi h he wa e con aining he me al
ions in solu ion. These micelles ac as he ounda ion o
a mic oemulsion, which is u he hea - ea ed o emo e he
o ganic componen s and ul ima ely achie e he nal Na
0.67
-
Ni
0.27
Mg
0.06
Mn
0.66
O
2
pa icles. Since he size and s abili y o he
e e sed micelles o med in he mic oemulsion will ha e
a di ec impac on he nal size o he syn hesised pa icles, in
his wo k, we ha e in es iga ed he effec o a ying he
concen a ions o wa e , aseline oil and oleic acid on he size,
size dis ibu ion, mo phology, s uc u e and elec ochemical
p ope ies o Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
pa icles.
Ma e ials and me hods
Syn hesis
S oichiome ic amoun s o Na(OCOCH
3
)$3H
2
O,
Mg(OCOCH
3
)
2
$4H
2
O, Ni(OCOCH
3
)
2
$4H
2
O and Mn(OCOCH
3
)
2
-
$4H
2
O om Sigma Ald ich we e mixed wi h dis illed wa e , oleic
acid (Sigma Ald ich, 90%) and aseline oil (ITW Reagen s) in
a ying p opo ions inside a Teon con aine . The quan i ies
used o each syn hesis a e summa ised in Table 1, which
co espond o a syn hesis wi h a high hyd ophobic componen
con en (HV), a syn hesis wi h a high-wa e con en (HW),
a syn hesis wi h a high oleic acid con en (HO), and a nal
sample syn hesised wi h s anda d quan i ies al eady epo ed
in he li e a u e (ST).
36
The mix u es we e p ocessed o 5
minu es using a shea mixe ope a ing a 15 000 pm. To a oid
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po en ial decomposi ion, he esul ing emulsion was pou ed
immedia ely ae mixing in o an alumina c ucible and placed
inside an o en wi h a hea ing amp o 5 °C min
−1
wi h a a ge
empe a u e o 900 °C. I is wo h no ing ha , du ing he
hea ing amp, a a empe a u e o app oxima ely 100 °C, all he
H
2
O and OH g oups we e elimina ed, and a app oxima ely 450
°C he calcina ion o he undesi ed o ganic g oups ook place.
Finally, once he empe a u e o 900 °C was eached, he
p oduc was leinside he o en o 10 hou s and allowed o cool
down o oom empe a u e.
Elec ochemical cha ac e isa ion
The elec ode p epa a ion consis ed o mixing he ac i e
ma e ial (80 w %), ca bon black (10 w %), and PVDF (poly-
inylidene uo ide) (10 w %) in N-me hyl-2-py olidone
(Ald ich, 99.5%) o o m a homogeneous slu y. This pas e was
coa ed on an aluminium cu en collec o and acuum d ied a
120 °C o a minimum o 2 hou s o ob ain a wo king elec ode
loaded wi h ca. 2.5 mg cm
−2
ac i e ma e ial.
The cell assembly was comple ed using coin- ype cells 2032
in an a gon-lled Vigo glo e box unde con olled O
2
and H
2
O
aces (< 0.1 ppm). Fo he sodium-ion hal -cells, he coun e
elec ode consis ed o a me allic sodium disk (Good ellow,
99.8%). The elec oly e was a 1 mol L
−1
solu ion o NaPF
6
(Ald ich, >99%) and die hylene glycol dime hyl e he (Sigma-
Ald ich, 99.7%) as a sol en . Glass b e disks (GF/A-Wha man)
we e soaked in his ion-conduc ing solu ion and used as sepa-
a o s be ween elec odes in he hal -cell. The elec ochemical
cha ac e isa ion o hal -cells was conduc ed on an I ium-n-S a
ba e y es sys em.
Ma e ials cha ac e isa ion
To s udy he c ys allini y and pu i y o he samples, X- ay
diff ac ion (XRD) was employed. XRD pa e ns we e scanned a
0.02° s
−1
be ween 10 and 70° (2qdeg ee) on a B uke D8
Disco e A25 diff ac ome e equipped wi h Cu K adia ion and
a g aphi e monoch oma o . Rie eld enemen s we e pe -
o med using he Topas-32 6 sowa e.
To analyse he chemical composi ion o he samples, wa e-
leng h dispe si e X- ay uo escence (XRF) expe imen s we e
pe o med on a Ze ium spec ome e equipped wi h a Rh ube.
To assess he mo phology and a e age size o he syn he-
sized pa icles, scanning-elec on mic oscopy (SEM) expe i-
men s we e ca ied ou using a FEI Teneo SEM.
Fo he analysis o he nanos uc u e ia ansmission elec-
on mic oscopy (TEM) and ela ed echniques, he TEM
samples we e p epa ed by dispe sing he pa icles in e hanol,
ollowed by sonica ion, and d op cas ing on lacey ca bon coppe
g ids. Scanning- ansmission elec on mic oscopy (STEM) and
ene gy dispe si e X- ay spec oscopy (EDX) we e pe o med
using an FEI Talos F200S mic oscope ope a ing a an accele -
a ing ol age o 200 keV. Fu he STEM analyses wi h a omic
esolu ion we e ca ied ou using abe a ion co ec ed (S)TEM
FEI Ti an Cubed Themis 60-300 and JEOL ARM200F mic o-
scopes and a dedica ed Nion Ul aSTEM200, all ope a ing a 200
kV. In o de o minimize damage and co ec o d iand scan
dis o ions, sequences o as STEM images we e eco ded and
subsequen ly aligned and a e aged using a non- igid egis a-
ion algo i hm.
37
Low-dose in eg a ed diffe en ial phase
con as (iDPC) expe imen s we e also pe o med on he FEI
Ti an Cubed Themis 60-300 since i is equipped wi h a ou -
quad an segmen ed de ec o .
Resul s and discussion
Fig. 1a shows he XRD pa e ns o Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
(ST, HO, HW and HV) powde samples. Rie eld enemen (see
Sec ion 1 o he ESI†) o gu es o me i , uni cell pa ame e s
and phase ac ions) e eals ha all samples p ima ily c ys al-
lize in a hexagonal P2- ype phase wi h space g oup P6
3
/mmc
(PDF 04-009-8579) acco ding o he indexed majo peaks, indi-
ca ing ha Mg doping does no change he P2 s uc u e o
Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
(Table S1†). Smalle amoun s o he
hombohed al O3- ype phase (PDF 04-016-7123), wi h space
g oup R
3ma e also p esen . This was u he con med ia
high- esolu ion STEM (Fig. S4†).
To e i y he s abilizing effec o magnesium on he P2
s uc u e, addi ional XRD pa e ns we e collec ed ae he s
cha ge and du ing he s discha ge (Fig. 1b). Al hough
eec ions co esponding o he O2 s uc u e a e obse ed in all
cases, i is e iden ha he P2 (and O3) s uc u e emained
p edominan du ing bo h cha ge and discha ge. This con ms
he ole o Mg in mi iga ing he de imen al P2- o-O2 phase
ansi ion.
To e i y he chemical composi ion o he pa icles syn-
hesised ia he o ganic ou e, XRF measu emen s we e pe -
o med (Table S2†). In all cases, he measu ed alues ag ee well
wi h he expec ed composi ions, conside ing he ins umen al
e o (5% o Na, Mn, and Ni and 10% o Mg). The homoge-
nei y o he elemen al dis ibu ion wi hin he pa icles was
assessed using EDX mapping in STEM mode (Fig. S1†).
Seconda y elec on SEM was used o inspec he size and
opog aphy o he ab ica ed Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
. I can
be seen om Fig. 2 ha mos o he pa icles ha e a a
hexagonal py amid-like shape wi h edges con aining s eps due
o he lamina s uc u e. A s a is ical analysis o he pa icle size
dis ibu ion (his og ams, Fig. S2,†and me hodology can be
ound in Sec ion 3 o he ESI†) allowed us o d aw some
p elimina y conclusions: in he case o he HV sample,
a bimodal pa icle dis ibu ion is obse ed, wi h mo e han 70%
o he pa icles ha ing a nanome ic size (Fig. 2b), wi h a e age
alues o he longes axis o 205 ±60 nm, and he es being
much la ge , wi h a e age alues o he longes axis o 2.4 ±0.9
Table 1 Mass o he p ecu so s (in g ams) used o he syn hesis o
diffe en samples
Sample Na Ac Ni Ac Mg Ac Mn Ac H
2
O Oleic acid Vaseline oil
ST 2.33 1.64 0.32 4.02 5 7 14
HV 5 7 32
HW 32 7 14
HO 5 32 14
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mm. The HW sample (Fig. 2b) exhibi s a simila bimodal
beha iou , wi h a e age alues o 198 ±60 nm and 2.1 ±1.0 mm.
Howe e , he numbe o nanome ic pa icles dec eases
no iceably, and hey become less equen ly obse ed in he
SEM images.
Howe e , o high concen a ions o oleic acid (sample HO,
Fig. 2d) a educ ion in he a e age size o pa icles is obse ed,
oge he wi h a much mo e homogeneous size dis ibu ion,
ollowing a Gaussian- ype cu e, wi h an a e age alue o 1.6 ±
0.7 mm. This diffe ence in pa icle size and dis ibu ion can be
a ibu ed o he s abilizing effec ha an inc eased oleic acid
concen a ion has on e e se micella sys ems.
38,39
An insuffi-
cien concen a ion o oleic acid (HW and HV) leads o an
uns able emulsion, esul ing in a sepa a ion be ween he
micella sys em and wa e con aining dissol ed ions, e en ually
leading o he uncon olled o ma ion o pa icles wi h highly
a ied sizes. Howe e , he high su ac an concen a ion in he
HO sample esul s in a mic oemulsion wi h much mo e s able
micelles. Al hough he nal pa icle size is la ge han ha ex-
pec ed o e e se micelles, his can be a ibu ed o he ac
ha , du ing he calcina ion s age, he su ac an molecules
e apo a e, and he p ima y pa icles ha nuclea ed begin o
o m union necks be ween hem, which o e ime lead o he
o ma ion o la ge pa icles. Howe e , his size is mo e
uni o m, as he g ea e s abili y o he emulsion esul s in
a highe simila i y in he size and dispe sion o he e e se
micelles in he mic oemulsion. This, in u n, ensu es ha he
nuclea ion o p ima y pa icles occu s wi h equal p obabili y
h oughou he emulsion and ha hei g ow h p oceeds a
simila a es du ing he subsequen holding s age a 900 °C.
Mo e impo an ly, i is con med ha his diffe ence in
pa icle size dis ibu ion has a signican impac on he elec-
ochemical beha iou o he syn hesized samples. Fig. 3 shows
ypical cha ge and discha ge p oles o ST- (Fig. 3a), HV-
(Fig. 3b), HW- (Fig. 3c) and HO-Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
(Fig. 3d), espec i ely, in he ol age window om 2.0 o 4.2 V a
50 mA g
−1
in he s cycle. The samples exhibi he s cha ge
capaci y o 220 (ST), 219 (HV), 274 (HW) and 323 (HO) mAh g
−1
.
Fig. 3e shows he e en ion capaci y p oles o he samples. I
can be obse ed ha he HO sample p esen s he bes e en ion
capaci y ae 100 cycles, wi h discha ge alues o 170 mA h g
−1
and a coulombic efficiency o 99%. Ra e capabili y es s we e
pe o med cycling be ween 2 V and 4.4 V s. Na
+
/Na o kine ics
om 0.5 o 10C and e u ning o 0.5C, as shown in Fig. 3 . All
samples show good capaci y and capaci y e en ion e en a high
kine ics such as 10C wi h a capaci y o a ound 50 mA h g
−1
, he
HO sample showing he bes pe o mance a diffe en cu en
densi ies. When he cha ge and discha ge cu en densi ies a e
educed om 10C o 0.5C, (1C =100 mA g
−1
) he capaci y can
eco e almos 100%, which is a u he indica ion ha Mg
doping can s abilize he s uc u e o P2- ype. Finally, Fig. 3 also
shows he cyclic ol amme y cu es o he ca hodes labelled as
ST (Fig. 3g), HV (Fig. 3h), HW (Fig. 3i), and HO (Fig. 3j) a a scan
a e o 0.1 mV s
−1
. All samples exhibi anodic and ca hodic
bands a ibu able o he edox couples in ol ed in he p ocess
Fig. 1 (a) Rie eld- efined XRD spec a o he
Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
pa icles syn hesized using s anda d
condi ions (ST), high- aseline con en (HV), high-wa e con en (HW)
and high-oleic acid con en (HO). A he bo om, simula ed spec a o
he P2 and O3 c ys al s uc u es a e p esen ed. (b) Ex si u XRD pa e ns
o he ST, HV, HW and HO samples a e ull cha ge and subsequen
discha ge. A he bo om, simula ed spec a o he P2, O3 and O2
c ys al s uc u es a e p esen ed.
Fig. 2 SEM mic og aphs o he Na
0.67
Ni
0,27
Mg
0,06
Mn
0.66
O
2
pa icles
syn hesized using (a) s anda d condi ions (ST), (b) high- aseline
con en (HV), (c) high-wa e con en (HW) and (d) high-oleic acid
con en (HO).
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du ing cycling. A lowe ol ages, he edox couples loca ed a
2.77/2.67 V (ST), 2.39/2.23 V (HV), 2.39/2.26 V (HW), and 2.38/
2.22 V (HO) co espond o he Mn
4+
/Mn
3+
edox p ocess. Addi-
ionally, in he ol age ange be ween 3 and 4 V, all ca hodes
p esen edox pai s ha could be a ibu ed o he Ni
2+
/Ni
3+
ansi ion. A hi d con ibu ion is obse ed a app oxima ely 4.4
V, which could be assigned o oxygen edox couples. These
cyclic ol amme y esul s a e consis en wi h he pla eau
obse ed in he co esponding gal anos a ic cu es.
To analyze he impac o he a e age size and dis ibu ion o
he pa icles p omo ed by he use o diffe en oleic acid– ase-
line oil–wa e a ios on he esis ance a he elec ode–elec o-
ly e in e ace du ing hal -cell ope a ion, elec ochemical
impedance spec oscopy (EIS) spec a we e eco ded be ween 1
mHz and 1 MHz, and plo ed as Nyquis diag ams (Fig. 4a and c)
ae he s cha ge and s discha ge, espec i ely.
Thei spec a p oles highligh wo cha ac e is ic semi-
ci cles, om which i is possible o di ec ly compa e he in e nal
esis ances a he elec oly e, he su ace laye , and cha ge-
Fig. 3 Gal anos a ic fi s cha ge/discha ge cu e o he (a) ST, (b) HV, (c) HW and (d) HO samples. (e) and ( ) Re en ion capaci y p ofiles o e 100
cycles and he a e pe o mance a oom empe a u e o he ou samples. (g)–(j) Cyclic ol ammog ams ob ained in he 2–4.4 V po en ial
window wi h a scan a e o 0.1 mV s
−1
o ca hodes ST, HV, HW and HO, espec i ely.
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ans e eac ion a he elec ode/elec oly e in e phase. The
la e wo alues a e mean o p o ide in o ma ion abou he
kine ic ba ie imposed a he elec ode in e phase. Thei
spec al p oles also highligh he p esence o he Wa bu g
elemen in se ies wi h he cha ge- ans e esis ance, since i is
known ha hese wo p ocesses oen occu in sequence. In
o he wo ds, he Na
+
ions s unde go a cha ge ans e a he
in e ace, which is ollowed by he diffusion o he ions in o he
bulk elec ode ma e ial. The e o e, he Wa bu g elemen would
ep esen he impedance due o Na
+
ion diffusion. In laye ed
oxides, whe e diffusion is a c i ical p ocess, his is impo an ,
since Na
+
ions migh need o mig a e h ough in e laye spaces,
which could be slow a low equencies.
F om he spec a in Fig. 4, i is e iden ha all samples
p esen he same elec ochemical mechanisms o he cha ge/
discha ge. This is somehow expec ed conside ing ha all ha e
he exac same composi ion and ab ica ion me hodology.
Thus, he main diffe ence among hem is in diffe en esis-
ances, which a e ela ed o size, in ag eemen wi h Fig. 3a–din
which diffe en capaci ies a e ob ained. Fo ins ance, bo h
cha ge and discha ge spec al p oles e ealed ha he sample
labeled as HV has he highes esis ance alues, sugges ing ha
he whole p ocess is limi ed by he in e nal impedance o he
cell. This is in ag eemen wi h he p edominan a adaic
diffusion con ibu ion. This highe esis ance is also e idenced
by he sligh ly lowe capaci ance alues ob ained o he HV
sample in bo h long cycling and C- a e (Fig. 3e and ).
In con as , he esul s clea ly show lowe esis ance alues
o he HO sample, which was ob ained by using a ela i ely
high concen a ion o su ac an , con ming he eliabili y o
his a io oge he wi h his syn he ic p ocedu e o build op i-
mized elec o in e phases o sodium mig a ion.
Al hough all samples exhibi conside able capaci ies,
coulombic efficiencies, and e en ion capabili ies due o he
s abilizing ole o Mg, he HO sample s ands ou wi h ema k-
ably high capaci ies (see Table S3 in he ESI† o a compa ison
wi h p e iously epo ed esul s on Mg-doped Na-TMOs). The
only diffe ence e iden among all samples is he pa icle size
and dis ibu ion. The obse ed end is ha bo h he a e age
size o he la ge pa icles and he numbe o smalle pa icles,
which appea o be less elec ochemically ac i e, inc ease a
lowe oleic acid concen a ions. This could explain he diffe -
ence in specic capaci y among a ious samples. Addi ionally,
o u he isola e he effec o pa icle size on he elec o-
chemical pe o mance o he samples, a Na
0.67
Ni
0.27
Mg
0,06
-
Mn
0.66
O
2
sample was syn hesized ia a con en ional d y milling
ou e o compa ison (see he discussion in Sec ion 4 o he
ESI†). This con en ionally syn hesized sample also exhibi s
a p edominan ly P2- ype s uc u e and a homogeneous
elemen al dis ibu ion compa able o hose ob ained h ough
Fig. 4 Impedance spec a, ep esen ed as Nyquis plo s, eco ded on ST, HV, HW and HO elec odes subjec o (a) he fi s cha ge and (c) he fi s
discha ge. (b) and (d) display, o cla ifica ion, he amplified egion co esponding o high equencies in (a) and (c), espec i ely.
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ou o ganic syn he ic ou e. Howe e , i p esen s a la ge
pa icle size, wi h an a e age alue o 3.0 ±1.1 mm. This esul is
consis en wi h he obse ed end in which a la ge a e age
pa icle size leads o lowe capaci ies du ing he s cha ge and
discha ge cycles.
I can be he e o e concluded ha in he HO sample mo e
op imal pa icle size and size dis ibu ion ha e been achie ed—
la ge enough o accommoda e he s esses associa ed wi h
la ice pa ame e changes du ing cha ge and discha ge cycles,
ye small enough o enable easy and efficien pa hways o
sodium ion inse ion and ex ac ion. To co obo a e his
hypo hesis, mo e in-dep h s udies we e conduc ed on he
a omic- and nanome ic-scale beha iou o he HO sample in
diffe en cha ge s a es using abe a ion-co ec ed STEM
echniques.
I is well known ha he main mechanisms o capaci y loss
and, ul ima ely, s uc u al ailu e in laye ed Na-TMOs include
he abo e-men ioned phase ansi ions, induced by laye
gliding, su ace es uc u ing, c ack o ma ion, and ion mig a-
ion o he alkali laye s.
40
The phase ans o ma ions, along wi h
he expansion and con ac ion associa ed wi h sodium inse -
ion and ex ac ion, a e also esponsible o c ack o ma ion
wi hin he pa icles.
13,41
Su ace es uc u ing mainly esul s
om in e ac ions be ween he ma e ial and he elec oly e, as
well as s uc u al modica ions due o oxygen loss.
12,42
Finally,
ansi ion me al ions can mig a e o nea by laye s, occupying
Na
+
acancies gene a ed du ing sodium ex ac ion. These
ansi ions a e gene ally i e e sible, leading o unexpec ed
capaci y loss and s uc u al deg ada ion.
43
In o de o isualize he s uc u al e olu ion o he samples
wi h cycling, abe a ion-co ec ed STEM measu emen s we e
pe o med on a e age sized pa icles o he sample wi h he bes
elec ochemical p ope ies (HO) and, o compa ison, on
compa a i ely la ge pa icles (long axis o e 3 mm) o he one
syn hesized wi h s anda d concen a ions (ST), bo h in he
p is ine s a e and ae he s cha ge–discha ge cycle and 100
cha ge–discha ge cycles. The esul s a e summa ized in Fig. 5.
In he p is ine s a e, bo h samples exhibi cha ac e is ic s ack-
ing o he P2 s uc u e wi h an ABBA oxygen sequence when
obse ed along he [010] di ec ion (Fig. 5a, e and S4† o all ou
samples).
Ae he s cha ge cycle, he HO sample (Fig. 5b) e ains
pa o he o iginal P2 s uc u e, al hough O2 phase domains
a e obse ed due o laye slipping du ing cha ging, in ag ee-
men wi h he XRD esul s (Fig. 1b). Addi ionally, plane bending
(∼10°) is obse ed, a ibu ed o possible elec oly e inl a ion,
along wi h a hin su ace laye (∼2 nm hick) o diso de ed ock-
sal made o Na-TMOs. These ac o s likely con ibu e o he
capaci y ade obse ed ae he s cha ge cycle, as hey
pa ially block sodium einse ion by obs uc ing he channels.
In he ST sample, howe e , his effec is signican ly mo e
p onounced (Fig. 4 ), whe e, in addi ion o a simila plane
bending angle, he ock-sal laye is much hicke , a ound 10
nm.
Ae 100 cha ge–discha ge cycles, in he HO sample
(Fig. 5c), plane bending inc eases (∼21°), and he hickness o
he diso de ed ock-sal laye g ows o app oxima ely 5 nm.
Rema kably, howe e , he ma e ial s ill main ains a laye ed
s uc u e in he bulk. In con as , in he STEM- isible egions o
he ST sample pa icles (Fig. 5g), a comple e ans o ma ion
in o a ock-sal s uc u e has occu ed, and in he ou e mos
egion, he c ys alline o de has been en i ely los , leading o
i e e sible ca hode deg ada ion in subsequen cycles.
Fig. 5 High- esolu ion STEM image o he HO sample (a) in he p is ine s a e, (b) a e 1 cycle and (c) a e 100 cycles; o he ST sample (e) in he
p is ine s a e, ( ) a e 1 cycle and (g) a e 100 cycles. The inse in (a) ep esen s he P2 modelled s uc u e ( ansi ion me als a e shown in g een,
oxygen a oms in ed and sodium a oms in yellow). In (d) and (h) a e shown low magnifica ion STEM images o he HO and ST pa icles,
espec i ely, a e 100 cycles.
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I is he e o e e iden ha in addi ion o he s abilizing effec
o Mg, pa icle size plays a c i ical ole, as he HO sample can
be e accommoda e he olume changes associa ed wi h
sodium ex ac ion and einse ion wi hou unde going s uc-
u al damage as se e e as ha obse ed in he ST sample.
This was u he con med a he mic oscopic le el h ough
con en ional STEM measu emen s on bo h samples. As can be
obse ed, he HO sample (Fig. 5d) main ains i s in eg i y ae
100 cycles, al hough su ace damage wi h c ack ini ia o s
appea s. Howe e , he ST sample (Fig. 5h) suffe s ca as ophic
damage due o c ack o ma ion and p opaga ion, leading o
pa icle pul e iza ion.
Finally, o ule ou ion mig a ion o he alkaline me al laye
in he HO sample, low-dose iDPC measu emen s we e ca ied
ou bo h in he p is ine s a e and a a ious cha ge poin s. This
echnique yields an image ha is di ec ly in e p e able as he
p ojec ed elec os a ic po en ial. The esul ing con as is
oughly p opo ional o he a omic numbe Z, which d as ically
imp o es he de ec abili y o ligh elemen s, such as sodium o
oxygen, among hea y ones in he same image.
44
I has been
shown ha iDPC-STEM images ha e a highe signal- o-noise
a io han o he STEM echniques.
45
Addi ionally, much lowe
beam cu en s can be used, since effec i ely all he elec ons
ha each he de ec o plane a e collec ed. These las wo
aspec s a e c i ical in ma e ials sensi i e o he elec on beam,
such as cha ged pa icles, since i has been demons a ed in
o he simila sys ems ha s uc u al changes induced by he
elec on beam a e simila o hose p oduced by cha ge and
discha ge cycles, leading o e oneous conclusions.
46,47
Fig. 6 shows econs uc ed phase images ob ained ia iDPC
o he sample in he p is ine s a e (Fig. 6a), cha ged a 3.7 V
(Fig. 6b), and cha ged a 4.2 V (Fig. 6c). In all h ee cases, he
con as wi hin he sodium laye is homogeneous, al hough i is
lowe in he sample cha ged a a highe ol age due o a highe
le el o desodia ion. Highe con as s ha would co espond o
he ansi ion me al mig a ion o adjacen alkaline laye s a e
no obse ed in any case, u he con ming he s abilizing ole
o Mg wi hin he Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
la ice.
Conclusions
In conclusion, we ha e shown ha an easy and ul a as emul-
sion-based syn hesis ou e can be used o ab ica e Na
0.67
-
Ni
0.27
Mg
0.06
Mn
0.66
O
2
wi h supe io elec ochemical p ope ies
(260 mA h g
−1
s discha ge and 140 mA h g
−1
ae 100 cycles
and coulombic efficiency o 99% ae 100 cycles). Low
concen a ions o oleic acid, ac ing as he su ac an , lead o he
o ma ion o uns able emulsions and he subsequen nucle-
a ion and g ow h o pa icles wi h uncon olled size. Howe e ,
an inc ease in oleic acid con en p oduces much mo e s able
micelles, which a e he o igin o pa icles wi h op imal size and
a na owe pa icle size dis ibu ion. Thus, he syne gis ic
combina ion o he app op ia e size and he s abilizing ole o
Mg esul s in pa icles ha can be e accommoda e olume ic
a ia ions induced by cha ge and discha ge cycles while also
p o iding channels h ough which sodium can be easily
ex ac ed and in e cala ed.
Ad anced STEM echniques ha e con med ha compa ed
o pa icles syn hesized wi h lowe amoun s o su ac an s,
pa icles syn hesized wi h highe oleic acid con en la gely
e ain he P2 s uc u e ae he s cha ge–discha ge cycle and
a e also capable o main aining he laye ed s uc u e and
s uc u al in eg i y ae 100 cycles. Mo eo e , no mig a ion o
ansi ion me als o he alkali me al laye s is obse ed du ing
he s cha ge, which could o he wise cause i e e sible
capaci y loss.
Al hough u he in es iga ion is needed o ully unde s and
he exac mechanism ha go e ns he su ac an a io–micelle
s abiliza ion ela ionship and o de e mine he op imal
p opo ions, ou esea ch ep esen s an impo an s ep owa d
achie ing he a ional design o Na
0.67
Ni
0.27
Mg
0.06
Mn
0.66
O
2
pa icles o sodium-ion ba e y ca hodes wi h op imized sizes
h ough a simple, as , cos -effec i e, and indus ially scalable
syn he ic ou e.
Fig. 6 Low-dose iDPC econs uc ed phase image o he HO sample
in (a) he p is ine s a e and (b) cha ged a 3.7 V and (c) 4.4 V. The inse in
(a) ep esen s he P2 modelled s uc u e ( ansi ion me als a e shown
in g een, oxygen a oms in ed and sodium a oms in yellow).
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Da a a ailabili y
Da a o his a icle, including scanning/ ansmission elec on
mic oscopy images and elec ochemical da a a e a ailable a
idUS h ps://hdl.handle.ne /11441/161680.
Au ho con ibu ions
Sa´
ul Rubio: in es iga ion, o mal analysis, isualiza ion, w i ing
– e iew and edi ing; E a M. P´
e ez-So iano: in es iga ion,
w i ing – e iew and edi ing; C is ina A ´
e alo: in es iga ion,
o mal analysis, isualiza ion, w i ing – e iew and edi ing;
Xiaoqiong Du: in es iga ion, w i ing – e iew and edi ing;
Xuyun Guo: in es iga ion, w i ing – e iew and edi ing; F an-
cisco J. Ga cia-Ga cia: concep ualiza ion, w i ing – e iew and
edi ing; Isabel Mon ealeg e-Mel´
endez: in es iga ion, w i ing –
e iew and edi ing; Ana M. Bel ´
an: in es iga ion, w i ing –
e iew and edi ing; Vale ia Nicolosi: in es iga ion, esou ces,
w i ing – e iew and edi ing; Juan G. Lozano: concep ualiza ion,
in es iga ion, o mal analysis, unding acquisi ion, p ojec
adminis a ion, w i ing –o iginal d a, w i ing – e iew and
edi ing.
Conflic s o in e es
The e a e no conic s o decla e.
Acknowledgemen s
Financial suppo was p o ided by he G an PID 2020-
113108RB-I00 unded by MICIU/AEI/10.13039/501100011033
and, as app op ia e, by “ERDF A way o making Eu ope”,by
“ERDF/EU”, by he “Eu opean Union”o by he “Eu opean
Union Nex Gene a ionEU/PRTR”. Financial suppo was also
p o ided by he G an CNS2022-135432 unded by MICIU/AEI/
10.13039/501100011033 and, as app op ia e, by “ESF In es ing
in you u u e”,by“ESF+”o by “Eu opean Union Nex Gene -
a ionEU/PRTR”. The au ho s a e also g a e ul o he Uni-
e sidad de Se illa o he use o he esea ch acili ies a
CITIUS. The au ho s acknowledge he use o (S)TEM ins u-
men a ion p o ided by he Spanish Na ional Facili y ELECMI
ICTS (“Di isi´
on de Mic oscop´
ıa Elec ´
onica”, Uni e sidad de
Cadiz, DME-UCA), he Da id Cockayne Cen e o Elec on
Mic oscopy a he Depa men o Ma e ials, Uni e si y o
Ox o d, and he Ad anced Mic oscopy Labo a o y a he Cen e
o Resea ch on Adap i e Nanos uc u es and Nanode ices,
T ini y College Dublin. V. N. X. D. and X. G. wish o hank he
suppo o he Science Founda ion I eland- unded AMBER
Resea ch Cen e (G an No. 12/RC/2278_P2) and he F on ie s
o he Fu u e awa d (G an No. 20/FFP-A/8950). X. D. would like
o hank Ma ie Skłodowska-Cu ie Ac ions G an Ag eemen No.
101107396. Fu he mo e, V. N. and X. G. wish o hank he
Ad anced Mic oscopy Labo a o y in CRANN o p o ision o
hei acili ies.
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