Oxygen Vacancy Engineering in Tin(IV) Oxide Based AnodeMaterials toward Advanced Sodium-Ion BatteriesDingtao Ma,[a] Yongliang Li,*[a, b] Peixin Zhang,*[a, b] and Zhiqun Lin[c]

1. Introduction

The increasing demand for fossil fuels and dramatic changes in

the climate have greatly motivated the rapid exploration and

development of renewable energies since entering the21st century. Among those alternatives, rechargeable lithium-

ion batteries (LIBs) have been studied in depth and have beencommercialized in recent years. However, as lithium resources

are finite, it seems as though they will ultimately fail to meetever-growing social demands, especially for hybrid electric ve-

hicles (HEVs) and electric vehicles (EVs). In this case, it is highly

desirable and necessary to develop new candidates with theadvantages of low cost and high energy density. Sodium-ion

batteries (SIBs) are emerging storage systems and show greatdevelopment potential owing to their attractive merits, which

include decent energy density, good sustainability, and abun-dant and low-costing sodium resources.[1] Furthermore, theyshare a working principle that is similar to that of LIBs, which

implies that the well-developed anode materials used for LIBscan possibly be directly utilized for SIBs. Until now, a significantamount of work has been done to prove this hypothesis. How-ever, relative to the challenges associated with LIBs, those as-

sociated with SIBs are more significant. Given that the radius

of Na+ (102 pm) is larger than that of Li+ (76 pm), severe

volume expansion and sluggish reaction kinetics are expectedduring the (de)sodiation process, and this results in a low spe-

cific capacity, poor cycling stability, and insufficient rate capa-bility. Besides, commercial graphite anodes for LIBs have been

demonstrated not to match with SIBs, and the theoretical ca-pacity of graphite anodes is clearly not sufficient to satisfy the

energy demands.[2] Therefore, to develop high-performance

SIBs, the key is to explore robust anode materials with highdischarge capacity, outstanding rate capability, and long

cycling life.Recently, tin dioxide (SnO2) has garnered much attention be-

cause it is more abundant, more environmentally friendly, andhas a higher theoretical capacity for Na+ storage than interca-lated graphite and other metal oxides.[2a, 3] However, despite

such superior attributes, the practical application of SnO2

anodes for SIBs is still a huge challenge, because these anodessuffer from low intrinsic conductivity, large volume expansion,and sluggish reaction kinetics. The suppression of volume ex-

pansion and the acceleration of surface electron transport canusually be achieved by binding with conductive substrates.[4]

However, their high rate capability cannot be effectively guar-

anteed owing to poor intrinsic properties. Given this issue, adefect-creating strategy should be a feasible and promising al-

ternative. As an attractive property-tailoring tool, defects havebeen widely employed for energy-storage systems, photocatal-

ysis, sensors, and many other research fields.[5] Among the dif-ferent types of defects, the engineering of oxygen vacancies

(OVs) is considered the most attractive method to tune the in-

trinsic properties of semiconductor oxides. Usually, OVs canunlock the potential to manipulate the energy band structure

and carrier migration, which leads to the alteration of the in-trinsic properties of semiconductor oxides. In terms of energy

storage, the introduction of OVs in semiconductor oxides usu-ally reduces the band gap and raises the density of states

A high theoretical capacity of approximately 1400 mA h g@1

makes SnO2 a promising anode material for sodium-ion batter-

ies (SIBs). However, large volume expansion, poor intrinsic con-

ductivity, and sluggish reaction kinetics have greatly hinderedits practical application. The controlled creation of oxygen va-

cancy (OV) defects allows the intrinsic properties of SnO2 to beeffectively modulated, but related work concerning SIBs is still

lacking. In this Minireview, the mechanism of failure of SnO2

electrodes is discussed and an overview of recent progress in

the general synthesis of OV-containing SnO2 materials and the

feasible detection of OVs in SnO2 is presented. The use ofOV-containing SnO2-based anode materials in SIBs is also

reviewed. Finally, challenges and future opportunities toengineer OVs for semiconductor oxides are examined.

[a] D. Ma, Dr. Y. Li, Prof. P. ZhangCollege of Chemistry and Environmental EngineeringShenzhen UniversityShenzhen, Guangdong, 518060 (P. R. China)E-mail : liyli@szu.edu.cn

pxzhang@szu.edu.cn

[b] Dr. Y. Li, Prof. P. ZhangGuangdong Flexible Wearable Energy Tools Engineering Technology Re-search CentreShenzhen UniversityShenzhen, Guangdong, 518060 (P. R. China)

[c] Prof. Z. LinSchool of Materials Science and EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332 (USA)

The ORCID identification number(s) for the author(s) of this article canbe found under:https://doi.org/10.1002/cssc.201801694.

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(DOS) ; this gives rise to enhanced intrinsic conductivity, whichhas been revealed by previous experiments and density func-

tional theory (DFT) calculations.[5g, 6] In addition to the bandstructure, the presence of OVs can also influence the surface

thermodynamics of semiconductor oxides, which thus facili-tates phase transition, especially at the electrode–electrolyte

interface.The OV strategy has been widely used in many research

fields, but there are only a few reports on the use of such a

strategy for SIBs, and in this minireview, we specifically presentrecent progress in the defect engineering of OVs in SnO2-based anode materials for SIBs. We start with a brief discussionof the reaction mechanism and origin of failure of the SnO2

anode. Approaches for the synthesis of oxygen-defective SnO2-based materials as well as the general characterization of the

OVs are summarized. Then, we discuss the basic effect of OVs

in improving the electrochemical performance on the basis ofpublished work. Finally, we propose remaining challenges and

provide an outlook for the development of oxygen-defectiveSnO2-based anode materials for SIBs.

2. Mechanism of Failure of the SnO2 Electrodein SIBs

As an n-type semiconductor with a rutile structure (spacegroup P42/mnm), SnO2 has been widely used in various fields,

including in sensors, solar cells, photodetectors, and energy

storage.[3e, 6b,c, 7] Specifically, great progress in the research ofSnO2-based anode materials for LIBs has been achieved. As re-

gards sodium-ion storage, studies are still lacking and have yetto be developed. With the aim to explore the reaction mecha-

nism, Gu et al. use in situ transmission electron microscopy(TEM) to probe the structural and electrochemical evolution of

pure SnO2 nanowires in SIBs, as shown in Figure 1 a. Together

with density functional theory calculations, they propose thereaction and evolution of morphology as shown in Figure 1 b.

Along with Na+ insertion, SnO2 successively undergoes the fol-lowing transitions: SnO2!Na2O, amorphous NaxSn!Na2O, and

crystallized Na15Sn4 ; upon dealloying of NaxSn, Sn tends toform a hollow structure and is confined in the Na2O matrix. It

is noted that the reversible formation of the SnO2 phase

cannot be clearly observed, but the reversible alloying of theSn phase is apparent during the experiment.[8] Besides, Wang

et al. employ the in situ TEM technique to study the sodiatedbehavior of Sn nanoparticles (Figure 1 c). They demonstrate

that crystalline Sn is initially sodiated by two-phase sodiationto form an amorphous NaSn2 phase with 56 % volume expan-

sion; it then forms the crystalline Na15Sn4 phase by single-

phase sodiation with nearly 420 % expansion (Figure 1 d).[9]

On the basis of previous studies, it is widely accepted that

the reaction between SnO2 and Na can be described by thefollowing two-step sodiation process [Eqs. (1) and (2)]:

SnO2 þ 4 Naþ þ 2 e@ $ Snþ 2 Na2O ð1Þ

Snþ x Naþ þ x e@ $ Nax Sn ð0 , x , 3:75Þ ð2Þ

Dingtao Ma received the M.S. degree

from Shenzhen University, China, in

2016. He is currently a Ph.D. candidate

at Macau University of Science and

Technology, China. His research inter-

ests focus on high-performance

energy-storage devices and the photo-

thermal and photoelectric properties

of two-dimensional materials.

Yongliang Li received his Ph.D. degree

in Mechanical and Materials Engineer-

ing at the University of Western Ontar-

io, Canada, in 2013. He is currently an

associate researcher at Shenzhen Uni-

versity, China. His research interests

focus on nanomaterials for clean

energy techniques, such as Li-ion bat-

teries and Li-air batteries. He is the

author or co-author of 80 papers in

refereed journals.

Peixin Zhang received his Ph.D. degree

from Northeastern University, China, in

1995. Currently, he is the Dean of the

College of Chemistry and Environmen-

tal Engineering at Shenzhen University,

the Director of Shenzhen Key Labora-

tory of Environmental Chemistry and

Ecological Remediation, and the Direc-

tor of Guangdong Flexible Wearable

Energy and Device Engineering Tech-

nology Research Center. His research

interests focus on the design, synthe-

sis, and applications of nanomaterials

and related applications.

Zhiqun Lin received his Ph.D. degree

in Polymer Science and Engineering

from the University of Massachusetts

Amherst in 2002. He is currently a pro-

fessor of materials science and engi-

neering at the Georgia Institute of

Technology. His research interests in-

clude materials for energy conversion

and energy storage, the synthesis of

wholly soft polymeric materials for

novel nanocomposite design, the syn-

thesis of wholly hard inorganic materi-

als for novel nanocomposite design, the design and synthesis of

novel hybrid (hard/soft) nanocomposite materials, materials charac-

terization, and nanoscale assembly.

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In the first step, the conversion reaction from SnO2 into Sn

contributes a theoretical capacity of 710 mA h g@1. Similar tothe LIB system, this part of the capacity usually fades quickly

after the initial cycle and becomes the irreversible capacity. Inthe second step, crystalline Sn delivers a theoretical capacity of

847 mA h g@1 by alloying with up to 3.75 mol of Na+ . As shownin Figure 2, the amorphous Na2O phase is immiscible with crys-talline Sn (second panel in Figure 2). To minimize the surface/

interface energy, the Na2O and Sn tend to separate and self-ag-gregate. Furthermore, such a tendency becomes more distinctin the subsequent alloying process (third and fourth panels in

Figure 2). Moreover, although a high reversibility of the alloy-

ing reaction has been demonstrated, a large volume variationof more than 400 % during the sodiation/desodiation process

also causes structural collapse. Specifically, such a generatedvolumetric stress in the electrode not only results in the forma-

tion of an unstable solid–electrolyte interface (SEI) film butalso greatly reduces the Sn/Na2O reaction interface and, thus,leads to poor reversibility of the conversion reaction.

Combined with the analysis mentioned above, the electrodestructure should be designed to release greater and morestable capacity. Here, we put forward several basic rules for de-signing high-performance SnO2-based anode materials. First,nanocrystallization of SnO2 is very necessary because of its fastreaction kinetics. Then, binding with a conductive substrate

usually provides fast electron/ion transport and buffers thevolumetric stress during cycling. Apart from external factors,the internal structure is equally important. In this respect,stable thermodynamics of the crystal grains, high intrinsicconductivity, and a low reaction energy barrier are desired.

Interestingly, such desirable internal properties can usually beachieved by controlling the OVs in SnO2.

3. Approaches for the Synthesis of Oxygen-Defective SnO2-Based Materials

Low cost, environmentally friendly, and easy to scale up are

three vital requirements in practical applications. To date,many effective strategies have been developed to prepare

Figure 1. a) Schematic drawing showing the setup of the experiment, and TEM images showing pristine, sodiated, and desodiated states of the same SnO2

nanowire. b) Schematic drawing showing the evolution of the morphology of the SnO2 nanowire upon Na insertion and extraction. Reproduced with permis-sion from Ref. [8] . Copyright 2013, American Chemistry Society. c) Experimental setup and typical morphological evolution of Sn NPs during sodiation andtypical morphological evolution of Sn NPs during electrochemical sodiation. d) Schematic illustration of the structural evolution of Sn NPs during sodiation.Reproduced with permission from Ref. [9] . Copyright 2012, American Chemistry Society.

Figure 2. Proposed model for the mechanism of failure of SnO2 electrodes inSIBs.

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oxygen-defective metal oxides. However, most of them usually

require either complicated synthesis processes or harsh reac-tion condition. In these cases, a comprehensive discussion of

those techniques is definitely necessary. In this section, we

mainly focus on introducing and summarizing the approachesdeveloped for the synthesis of oxygen-defective SnO2-based

materials. Besides that, several methods for the characteriza-tion of these OVs in SnO2 are also given with the aim to

provide a basic understanding (Table 1).

3.1. Thermal annealing treatment

It is known that OVs can be created by heating semiconductor

oxides in an oxygen-deficient environment under temperaturecontrol. Unlike the reduction reaction, in such a process local

oxygen atoms are captured, which creates a defect on the sur-

face without changing the original structure. Typically, thermalannealing treatment can be divided into mainly reductive and

nonreductive treatments, according to the selection of the ex-posure atmosphere. For the former, the OVs are easily created

owing to the existence of reducing hydrogen gas (H2). Actually,such a method has been widely used in various semiconductor

oxides for OV modification.[10] For example, Desimone et al.anneal as-prepared SnO2 under a reductive (H2) atmosphere,

and they note that a high concentration of vacancy clusters isgenerated on the surface of the H2-treated sample. Comparedwith the O2-treated sample, the richly defective one exhibits

higher conductivity.[11] Although this is an effective way tocreate OVs in semiconductor oxides, such a method is not

favorable for large-scale applications owing to the highlyexplosive nature of hydrogen gas.

On the other hand, researchers have found that OVs can

also be created upon exposing samples to a nonreductiveatmosphere, including N2, Ar, and vacuum environments.[12] For

example, by modulating the vacuum level in the synthesissystem, Liu et al. report that the amount of vacancies in the in-

terface of SnO2@x/In2O3@y heterostructured photocatalysts canbe effectively controlled. Through optimization of the OVs,

they observe that the composite produces 60 % more hydro-

gen than the defect-free sample.[12b] Xu et al. note that OV-con-taining amorphous SnO2 ordered arrays can be obtained by

annealing the precursor under a N2 atmosphere (Figure 3). Rel-

ative to the O 1s core-level spectrum of the air-annealedsample, a new peak at 531.6 eV is detected in the spectrum of

the N2-annealed sample, and furthermore, relative to the Sn 3dpeak of the air-annealed sample, that of the N2-annealed

sample is shifted towards lower binding energies by approxi-mately 0.45 eV. Besides that, an emerging hump at approxi-

mately 470 cm@1 in the Raman spectrum as well as an electron

spin resonance (ESR) signal at g = 2.003 are detected for theN2-annealed sample, and these findings correspond to the

electrons trapped in the OVs.[13] Thus, together with XPS,Raman spectroscopy, and ESR analysis, the existence of OVs is

decisively confirmed.Given that OVs can be created by both reductive and

nonreductive treatments, a study of the effect of different

atmospheres in the creation of oxygen-vacancy defects incrystalline SnO2 is necessary. With this consideration, Macchiet al. describe the use of positron annihilation lifetime spec-troscopy (PALS) to study the vacancy defects in SnO2. To avoid

the influence of any other factors, the authors treat all thesamples under the same conditions, except for air, N2/H2, and

vacuum atmospheres. As a result, they show that there aretwo types of vacancy-like defects in SnO2. The first type is va-cancy clusters formed by Sn and O vacancies on the surface of

the grains, whereas the other type is Sn-based vacancies insidethe grains. Relative to those found for two other samples, the

H2-treated sample has a higher concentration of vacancy clus-ters on the surface, which is indicative of lower and narrower

potential barriers.[14]

3.2. Hydrothermal treatment

Hydrothermal treatment is another approach that can be used

to synthesize oxygen-defective semiconductor oxides (e.g. ,TiO2,[15] Fe2O3,[5f, 16] MoO3,[5b] SnO2,[17] and WO3

[18]), owing to its

Table 1. Normal techniques for the characterization of the oxygen vacancies in SnO2.

Characterization technique Phenomenon Ref.

macro color change the generated OVs usually change the optical properties of the semiconductor oxides, which then changetheir macro color

[20]

X-ray diffraction (XRD) the generated OVs in the crystal lattice usually lead to reduction of the lattice parameters, thus resultingin a shift in the diffraction peaks

[5b, 21]

X-ray absorption near-edge struc-ture (XANES) spectra

the unsaturated coordination of the surface Sn ions generates certain resonance signals in the Sn M5,4

edge XANES spectra[17b]

aberration-corrected TEM technique due to the differences in the scattering powers of cations and sublattice oxygen, it is possible to imagethe cation columns by adjusting the aberration coefficient

[5g]

UV/Vis spectroscopy the generated OVs usually reduce the band gap of the semiconductor oxides, which then change theirabsorption range

[20]

X-ray photoelectron spectroscopy(XPS)

Sn 3d spectra are bathochromically shifted as a result of the existence of OVs [5b, 13,20, 21]

Raman spectroscopy the Eg mode is sensitive to defects such as OVs [13, 20]positron annihilation lifetime spec-troscopy (PALS)

the existence of OVs gives rise to different positron lifetime characteristics relative to the complete crystal-line state

[14]

electron spin resonance (ESR) spec-troscopy

for OV-rich oxides, the signal for electron trapped in the OVs can be detected [5h, 13]

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easy operation and environmental friendliness. For example,

Yang et al. report SnO2 with abundant oxygen vacanciesthrough a one-pot hydrothermal process. Relative to the pho-tocatalytic activity of normal SnO2, that of defect SnO2 under

visible-light irradiation is remarkable. Besides, it is noted thatsuch oxygen vacancy defects in the SnO2 nanoparticles can be

eliminated by calcination at 900 8C for 4 h in air.[5c] Wu et al.disclose the synthesis of a SnO2@x/N-doped reduced grapheneoxide (N-rGO) composite through a combination of electrostat-ic adsorption-induced self-assembly and a thermal reduction

process (Figure 4 a). The OVs are generated on the surface ofnanocrystalline SnO2 during thermal treatment.[17a] Similarly, Liet al. use SnCl2·2 H2O as a precursor to synthesize SnO2@d nano-

particles (Figure 4 b). Despite the facts that, at the time of thestudy, OVs had only been investigated at the qualitative level

and the quantitative detection of the concentration of OVs inSnO2 remained absent, for the first time, this group was able

to realize the direct observation OVs and to estimate their con-

centration (&3.3 at %) in SnO2@d nanoparticles by using aberra-tion-corrected TEM (Figure 4 c). This achievement is based on

the difference in the scattering powers of cations and sublat-tice oxygen, which makes it possible to image the cation col-

umns directly by adjusting the aberration coefficient. There isno doubt that such an achievement can help us to understand

deeply the influence of the local oxygen concentration on the

bulk electronic properties.[5g] Unlike the normal hydrothermalmethods, Wang et al. have developed a one-step microwave-assisted hydrothermal method (MAHM) to synthesize a SnO2/

graphene composite (Figure 4 d). In this case, the surfaces ofthe SnO2 nanoparticles are rich in OVs, as revealed by X-ray ab-

sorption near-edge structure (XANES) analysis. Electrochemicalstudies also indicate the great potential of this composite to

be used as an anode material for LIBs.[17b]

3.3. Other methods

In addition to the methods mentioned above, oxygen vacan-

cies in SnO2 can also be created by many other effectiveways.[4e,f, 19] For example, Dong et al. use a molten-aluminum re-

duction method to prepare SnO2@x nanoparticles. As shown inFigure 5 a, the color of SnO2 changes from white to black after

the formation of the SnO2@x nanoparticles. Such a phenomen-

on is caused by intense ultraviolet/visible–near infrared (UV/Vis-IR) absorption by the oxygen vacancies (Figure 5 b). These

authors also study the Sn 3d X-ray photoelectron spectrum ofthe SnO2@x nanoparticles, and the bands are blueshifted by

0.4 eV relative to those of the SnO2 NPs (Figure 5 c). Togetherwith the crystal-structure characterization, the authors demon-

Figure 3. Characterization of N2- and air-annealed samples: a) O 1s X-ray photoelectron spectra, b) Sn 3d X-ray photoelectron spectra, c) Raman spectra, d) ESRspectra. Reproduced with permission from Ref. [13] . Copyright 2018, Elsevier.

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Figure 4. a) Schematic illustration of the process for the preparation of the 3 D hierarchical SnO2@x/N-rGO network. PDDA = polydiallyldimethylammoniumchloride, GO = graphene oxide. Reproduced with permission from Ref. [17a]. Copyright 2018, Royal Society of Chemistry. b) photographs of i) SnO2@d (yellow)and ii) SnO2 (white) powders. CB = conduction band, VB = valence band. c) HRTEM image of a rutile SnO2@d particle along the [11 0] zone axis, which revealsalternating oxygen and tin atoms (arrows) and the deficiency of O columns. Contrast at the center of the O columns is indiscernible, as indicated by a dashedcircle. Reproduced with permission from Ref. [5g]. Copyright 2013, Royal Society of Chemistry. d) Illustration of the microwave-assisted hydrothermal systemand synthesis process of SnO2/graphene nanocomposites. GNs = graphene nanosheets. Reproduced with permission from Ref. [17b] . Copyright 2012,American Chemistry Society.

Figure 5. a) SnO2 powder before (left) and after (right) reduction, forming black SnO2@x. b) UV/Vis absorption spectra showing enhanced absorption in blackSnO2@x (red) compared to white SnO2 (blue). c) Sn 3d X-ray photoelectron spectra of black SnO2@x and white SnO2. d) Electrical conductivity of pressed pelletsof SnO2, SnO2@x, and SnO2@x :RGO. Reproduced with permission from Ref. [20]. Copyright 2017, Wiley-VCH.

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strate that the black SnO2@x nanoparticles are rich in oxygenvacancies but still maintain the pristine structure. Benefiting

from the existence of OVs, the electrical conductivity of theSnO2@x nanoparticles is three orders of magnitude higher than

that of pristine SnO2 (Figure 5 d).[20] Li et al. employ the atomiclayer deposition (ALD) technique to deposit SnO2 onto gra-phene nanosheets (GNS). Owing to the intrinsically isotropiccharacteristics of amorphous SnO2, the sandwich structure andamorphous SnO2-GNS nanocomposite show better ability toovercome electrochemical and mechanical degradation (Fig-ure 6 a, b) than the crystalline counterpart.[3a] Very recently, ourgroup reported a SnO2@x/C composite by homogeneously con-fining SnO2 nanoparticles in polyacrylonitrile (PAN) nanofibers

through an electrospinning technique (Figure 6 c, d). SnO2 istransformed into oxygen-vacancy-containing SnO2@x nanoparti-

cles during carbonization of the PAN nanofibers under the N2

atmosphere. As an anode material for SIBs, such a SnO2@x/Ccomposite has superior electrochemical performance (a de-

tailed discussion is given in the next section).[21]

Apart from that, pulsed laser deposition (PLD),[5e, 22] ionic

doping,[5h] and solution precursor plasma spray (SPPS)[5d] arealso effective approaches. In a typical example, Sinha et al.

note that the crystallinity of SnO2 deposited on the Si (1 0 0)

substrate can be controlled from the amorphous state to thecrystalline state and that the generation of OVs can be easily

modulated by tuning the deposition temperature.[22a] SPPS is aprocess with an ultrahigh heating temperature and cooling

rate and is expected to produce a large amount of OVs, espe-cially for semiconductor oxides. In view of this, Geng et al. de-

scribe the preparation of a black SnO1@a@ZnO1@b@SnO2@g sensi-

tive coating to enhance the sensing performance by the SPPStechnique with stoichiometric white oxide powders.[5d] In addi-

tion, Wu et al. use an ion-doping strategy for this purpose.They demonstrate that Al-doping has a profound influence on

the OVs of SnO2 and that the OV content can be regulated bychanging the Al/(Al + Sn) ratio.[5h]

4. Rational Design of Oxygen-DefectiveSnO2-Based Materials for SIBs

To understand current research progress on oxygen-defective

SnO2-based materials in SIBs, we have conducted a literaturesurvey through a retrieval platform, and to our surprise, the re-

sults showed that only a few studies have been performedand published to date, which also implies great research po-tential for this topic. It is well acknowledged that a high dis-

charge capacity, a long lifespan, and good rate capability arethe three main requirements for high-performance electrode

materials in practical applications. For SnO2 anode materials,the discussions in Section 2 have paved the way for their ra-

tional design. Considering the limited amount of published ar-ticles related to either LIBs or SIBs, we summarize and discuss

the role of OVs in improving the electrochemical performance

of SnO2-based materials, especially for SIBs.Although SnO2 has a high theoretical capacity of

1398 mA h g@1, there are still no experimental values close to it.Seemingly, low reversible conversion reactions seem to be the

primary problem, as they usually lead to large capacity fadingafter the initial cycle. On this basis, Hu et al. have constructed

a unique ternary SnO2-Mn-graphite composite by scalable ball

milling (Figure 7). For the ternary composite, the nanosized Mnadditive plays two important roles. One is that such a non-

electrochemical additive prevents coarsening of Sn therefore arich amount of Sn/Li2O interfaces are retained. The other is

that a small amount of Mn atoms exists in the form of dopingatoms, and they thus benefit the formation of OVs in Mn-

doped SnO2. Besides, the corresponding DFT calculations also

indicate that such OVs favor an enhancement in the reversibili-ty of the conversion reaction in SnO2. After effective wrapping

of graphite nanosheets, such ternary SnO2-Mn-graphite com-posite shows a high average initial Coulombic efficiency of ap-

proximately 77 % as well as a high capacity with a long life-span, even at a high rate (2 A g@1 at an active-material loading

of 1.0–1.5 mg cm@2).[23] On the other hand, to tackle the large

volumetric expansion and low conductivity, Wu et al. report astrategy to wrap hollow-structured SnO2@x nanospheres with a

nitrogen-doped graphene network. During the Li+ insertionprocess, the conductive matrix not only alleviates aggregation

and volume expansion of SnO2@x nanosphere but also acceler-ates the electron/ion transport.[17a]

The role of OVs in boosting the electrochemical performanceof amorphous oxide anodes has also been explored. Xu et al.report OV-containing amorphous SnO2 ordered arrays used as

free-standing anodes by using the nanoimprinted anodic alu-minum oxide (AAO) template and ALD technique (Figure 8).[13]

Notably, the OVs can be easily generated by annealing the as-deposited sample under a N2 atmosphere. The thickness of the

SnO2 layer deposited on the Ni nanorods is approximately

10 nm. As a binder-free and conductive-additive-free anodematerial for SIBs, the electrochemical tests show that such an

OV-containing sample exhibits high reversible capacity, goodcyclic stability, and good rate capability. Especially, the N2-

annealed sample shows a smaller voltage polarization and ahigher rate capability than the non-annealed sample. In terms

Figure 6. a) SEM and b) HRTEM observations of amorphous SnO2-GNS. Re-produced with permission from Ref. [3a] . Copyright 2012, Wiley-VCH. c) SEMand d) TEM images of the SnO2@x electrode. Reproduced with permissionfrom Ref. [21] . Copyright 2018, Wiley-VCH.

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Figure 7. a) Schematic showing the structure and phase evolution of a SnO2 electrode during initial discharge and charge, featuring the formation of nano-Snand Li2O during discharge. b) Schematic of the high reversibility reactions in ternary SnO2-Mn-graphite hybrids. c) Initial discharge–charge curves for fourLi/SnO2-Mn-G cells. d) Summary of the first Coulombic efficiencies for 12 cells with SnO2-Mn-G electrodes. e) Comparison of the cycling performance of theSnO2-Mn-G nanocomposite with those of SnO2, SnO2-G, and SnO2-30Mn at 0.2 and 2 A g@1 between 0.01 and 3 V. Reproduced with permission from Ref. [23] .Copyright 2017, Royal Society of Chemistry.

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of cycling stability, a capacity of 220 mA h g@1 at 1 A g@1 is re-tained after 800 cycles (This capacity is calculated on the basis

of the content of the SnO2 layer). Besides, to further confirmthe enhanced kinetics induced by the OVs, the authors exam-ine various scan rates ranging from 0.1 to 20 mV s@1. Good rate

capability with fast charge-storage kinetics is observed as aresult of the creation of the OVs, which greatly promotes the

surface-controlled process. Briefly, this work reveals the role ofOVs in enhancing charge transfer/transport of amorphous

SnO2 anode materials for SIBs.

As mentioned above, poor intrinsic conductivity, unstableSn/Na2O interface, and volume effects are three major obsta-

cles that impede the development of SnO2-based materials forSIBs. Our group has recently designed SnO2@x/C composites by

the electrospinning method (Figure 9). In this work, to solvethe volume effect problem, a flowable sulfur template is uti-

lized to create a well-defined void space around the SnO2

nanoparticles. The other two problems of poor intrinsic con-

ductivity and unstable Sn/Na2O interface can be effectively ad-dressed by introducing OV defects within the SnO2 nanoparti-cles. Ex situ XRD tests show that the SnO2@x/C composites ex-

hibit high reversible conversion of Sn$SnO2. This is becausethe homogeneous confinement of SnO2@x nanocrystals in the

porous carbon nanofibers stabilizes the reaction interface. Be-sides, the carbon matrix also functions as an artificial barrier to

form a stable SEI film, thus realizing the maximum utilization

of the OVs. Additionally, electrochemical tests demonstrate thelong-term cycling stability and excellent rate capability of the

SnO2@x/C composites (this capacity is calculated on the basis ofthe SnO2@x content). Typically, a discharge capacity of

565 mA h g@1 at 1 A g@1 can be maintained even after2000 cycles. Benefiting from the design of such an intriguing

Figure 8. Characterization of N2-annealed ordered arrays: a) SEM images and b) HRTEM image. Sodium storage performances of the N2-annealed sample:c) comparison of the rate performance from 0.1 to 20 A g@1 and d) cycling performance at 1 A g@1. e) Cyclic voltammetry curves of the N2-annealed samplerecorded at 1.0–20 mV s@1. f) Determination of the b value from the peak currents of the two samples. Reproduced with permission from Ref. [13] . Copyright2018, Elsevier.

ChemSusChem 2018, 11, 3693 – 3703 www.chemsuschem.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3701

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architecture, maintenance of the intrinsic advantages of SnO2@x

nanocrystals and the restraint of the volume effect can be

simultaneously achieved.[21]

5. Conclusion and Outlook

To construct high-performance SIBs, the development of facile

methods for the synthesis of high-performance anode materi-als is critical. Unlike normal metal and oxide anode materials,SnO2 has a high theoretical capacity owing to its complicatedreaction mechanisms, which include both alloying and conver-

sion reactions. Thus, the application of this material for SIBs isstill a great challenge. The OV engineering strategy has been

widely demonstrated as an effective tool to modulate the elec-tronic and optical properties of semiconductor oxides, and thishas made performance modification in various applications

possible. However, relevant research on SnO2-based anode ma-terials for SIBs, and even for LIBs, is still scarce and should be

developed. This minireview summarizes recent progress in theOV engineering strategy for SnO2-based anode materials in

SIBs. After a brief analysis of the failure of SnO2 electrodes for

sodium-ion storage, we introduced general approaches for thesynthesis of oxygen-defective SnO2-based materials, and this

was coupled with a discussion of several feasible spectral char-acterization methods for OVs in SnO2, including XPS, Raman

spectroscopy, UV/Vis-IR spectroscopy, ESR, and PALS. On thisbasis, several published articles detailing the OV engineering

strategy were taken as examples, and we analyzed the role ofOVs in improving electrochemical performance in detail.

Previous work has shown that creating OVs in semiconduc-tor oxides can usually improve their electrochemical per-

formance. However, only a few of them mentioned the reac-tion mechanism. In this case, along with the fast developmentof modern techniques, more advanced detection techniques

need to be employed to explore the reaction mechanism ofsurface defects in future research. It is very necessary to clearlyclarify the relationship between OV defects and electrochemi-cal performance. To do that, we need first to solve the follow-

ing problems: 1) how can the position, type, and correspond-ing concentration of oxygen vacancies in SnO2 be experimen-

tally confirmed; 2) how can the interactions between differenttypes of oxygen vacancies be combined with theoreticalmodel calculations; 3) how do the oxygen vacancies change

the intrinsic conductivity, carrier diffusion kinetics, surface ther-modynamics, and interface energy barriers during Na+ inser-

tion. Only by solving the abovementioned issues can we fur-ther effectively utilize the OV engineering strategy to realize

improved electrochemical performance. Besides, although OV

defects can modify the intrinsic physical and chemical proper-ties of SnO2 nanocrystals, such advanced characteristics might

also be damaged because of a change in the solid–liquid inter-face originating from decomposition of the electrolyte and for-

mation of a SEI film during electrochemical cycling. Therefore,to maximize the positive effects of OV defects, the design of

Figure 9. a) Ex situ XRD patterns of SnO2@x/C electrode after the 20th cycle at 0.05 A g@1 between 0.01 and 2.5 V. b) Proposed reaction mechanisms of the SnO2

and SnO2@x/C electrodes during the charge–discharge process. c) Rate capabilities of the SnO2 and SnO2@x/C electrodes. d) Cycling performances of SnO2@x/C,SnO2, and porous carbon nanofiber (PCNF) electrodes at 1 A g@1. Reproduced with permission from Ref. [21] . Copyright 2018, Wiley-VCH.

ChemSusChem 2018, 11, 3693 – 3703 www.chemsuschem.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3702

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high-performance SnO2-based anode materials should be morecomprehensive.

Despite the great potential of the OV defects strategy inregulating the properties of SnO2 for energy-storage systems,

related studies are still at an early stage. The purpose of thiswork is going to provide basic knowledge of the OV defect en-

gineering strategy as applied to SnO2-based anode materialsand will also guide attention to this research topic and pro-mote the development of high-performance anode materials

for SIBs.

Acknowledgements

This work was financially supported by the National Natural Sci-

ence Foundation of China (No. 51774203), Shenzhen Science andTechnology Project Program (Nos. KQJSCX20170327151152722

and JCYJ20160422112012739), and the Natural Science Founda-tion of SZU (No. 827-000039) and the Energy Conservation and

Environmental Protection Industry Development Special Fund ofShenzhen.

Conflict of interest

The authors declare no conflict of interest.

Keywords: anode materials · batteries · conducting materials ·electrochemistry · tin

[1] a) J.-Y. Hwang, S.-T. Myung, Y.-K. Sun, Chem. Soc. Rev. 2017, 46, 3529 –3614; b) M. D. Slater, D. Kim, E. Lee, C. S. Johnson, Adv. Funct. Mater.2013, 23, 947 – 958; c) C. Delmas, Adv. Energy Mater. 2018, 8, 1703137;d) N. Yabuuchi, K. Kubota, M. Dahbi, S. Komaba, Chem. Rev. 2014, 114,11636 – 11682; e) V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J.Carretero-Gonz#lez, T. Rojo, Energy Environ. Sci. 2012, 5, 5884 – 5901;f) S. Y. Hong, Y. Kim, Y. Park, A. Choi, N.-S. Choi, K. T. Lee, Energy Environ.Sci. 2013, 6, 2067 – 2081; g) H. Zhang, I. Hasa, S. Passerini, Adv. EnergyMater. 2018, 8, 1702582.

[2] a) Z. Li, J. Ding, D. Mitlin, Acc. Chem. Res. 2015, 48, 1657 – 1665; b) D.Saurel, B. Orayech, B. Xiao, D. Carriazo, X. Li, T. Rojo, Adv. Energy Mater.2018, 8, 1703268; c) J. Song, B. Xiao, Y. Lin, K. Xu, X. Li, Adv. EnergyMater. 2018, 8, 1703082; d) M. Goktas, C. Bolli, E. J. Berg, P. Novak, K.Pollok, F. Langenhorst, M. v. Roeder, O. Lenchuk, D. Mollenhauer, P.Adelhelm, Adv. Energy Mater. 2018, 8, 1702724; e) X. Dou, I. Hasa, D.Saurel, M. Jauregui, D. Buchholz, T. Rojo, S. Passerini, ChemSusChem2018, 11, 3276 – 3285.

[3] a) X. Li, X. Meng, J. Liu, D. Geng, Y. Zhang, M. Norouzi Banis, Y. Li, J.Yang, R. Li, X. Sun, M. Cai, M. W. Verbrugge, Adv. Funct. Mater. 2012, 22,1647 – 1654; b) K. Wang, Y. Huang, X. Qin, M. Wang, X. Sun, M. Yu,ChemElectroChem 2017, 4, 2308 – 2313; c) S.-M. Paek, E. Yoo, I. Honma,Nano. Lett. 2009, 9, 72 – 75; d) S. Wei, S. Chu, Q. Lu, W. Zhou, R. Cai, Z.Shao, ChemistrySelect 2018, 3, 4015 – 4022; e) X. W. Lou, C. M. Li, L. A.Archer, Adv. Mater. 2009, 21, 2536 – 2539.

[4] a) Y.-X. Wang, Y.-G. Lim, M.-S. Park, S.-L. Chou, J. H. Kim, H.-K. Liu, S.-X.Dou, Y.-J. Kim, J. Mater. Chem. A 2014, 2, 529 – 534; b) R. S. Kalubarme,J.-Y. Lee, C.-J. Park, ACS Appl. Mater. Interfaces 2015, 7, 17226 – 17237;c) X. Zhou, L.-J. Wan, Y.-G. Guo, Adv. Mater. 2013, 25, 2152 – 2157; d) X.Han, Y. Liu, Z. Jia, Y.-C. Chen, J. Wan, N. Weadock, K. J. Gaskell, T. Li, L.Hu, Nano. Lett. 2014, 14, 139 – 147; e) B. Ahmed, D. H. Anjum, Y. Gogotsi,H. N. Alshareef, Nano Energy 2017, 34, 249 – 256; f) V. Aravindan, K. B.Jinesh, R. R. Prabhakar, V. S. Kale, S. Madhavi, Nano Energy 2013, 2, 720 –725; g) X. Xie, S. Chen, B. Sun, C. Wang, G. Wang, ChemSusChem 2015,

8, 2948 – 2955; h) J. Han, D. Kong, W. Lv, D.-M. Tang, D. Han, C. Zhang,D. Liu, Z. Xiao, X. Zhang, J. Xiao, X. He, F.-C. Hsia, C. Zhang, Y. Tao, D.Golberg, F. Kang, L. Zhi, Q.-H. Yang, Nat. Commun. 2018, 9, 402.

[5] a) Y. Zhao, C. Wei, S. Sun, L. P. Wang, Z. J. Xu, Adv. Sci. 2015, 2, 1500097;b) H.-S. Kim, J. B. Cook, H. Lin, J. S. Ko, S. H. Tolbert, V. Ozolins, B. Dunn,Nat. Mater. 2017, 16, 454; c) Y. Yang, Y. Wang, S. Yin, Appl. Surf. Sci.2017, 420, 399 – 406; d) X. Geng, C. Zhang, Y. Luo, H. Liao, M. Debliquy,Ceram. Int. 2017, 43, 5990 – 5998; e) Y. Porte, R. Maller, H. Faber, H. N. Al-Shareef, T. D. Anthopoulos, M. A. McLachlan, J. Mater. Chem. C 2016, 4,758 – 765; f) P. Zeng, Y. Zhao, Y. Lin, X. Wang, J. Li, W. Wang, Z. Fang,Nanoscale Res. Lett. 2017, 12, 13; g) N. Li, K. Du, G. Liu, Y. Xie, G. Zhou, J.Zhu, F. Li, H.-M. Cheng, J. Mater. Chem. A 2013, 1, 1536 – 1539; h) J. Wu,Q. Huang, D. Zeng, S. Zhang, L. Yang, D. Xia, Z. Xiong, C. Xie, Sens. Ac-tuators B 2014, 198, 62 – 69.

[6] a) K. G. Godinho, A. Walsh, G. W. Watson, J. Phys. Chem. C 2009, 113,439 – 448; b) S. Gubbala, V. Chakrapani, V. Kumar, M. K. Sunkara, Adv.Funct. Mater. 2008, 18, 2411 – 2418; c) Z. Zhu, Y. Bai, X. Liu, C.-C. Chueh,S. Yang, A. K.-Y. Jen, Adv. Mater. 2016, 28, 6478 – 6484.

[7] a) J. Qian, P. Liu, Y. Xiao, Y. Jiang, Y. Cao, X. Ai, H. Yang, Adv. Mater. 2009,21, 3663 – 3667; b) Q. Kuang, C. Lao, Z. L. Wang, Z. Xie, L. Zheng, J. Am.Chem. Soc. 2007, 129, 6070 – 6071; c) Y. Wang, X. Jiang, Y. Xia, J. Am.Chem. Soc. 2003, 125, 16176 – 16177.

[8] M. Gu, A. Kushima, Y. Shao, J.-G. Zhang, J. Liu, N. D. Browning, J. Li, C.Wang, Nano Lett. 2013, 13, 5203 – 5211.

[9] J. W. Wang, X. H. Liu, S. X. Mao, J. Y. Huang, Nano Lett. 2012, 12, 5897 –5902.

[10] a) J.-Y. Shin, J. H. Joo, D. Samuelis, J. Maier, Chem. Mater. 2012, 24, 543 –551; b) X. Peng, X. Zhang, L. Wang, L. Hu, S. H.-S. Cheng, C. Huang, B.Gao, F. Ma, K. Huo, P. K. Chu, Adv. Funct. Mater. 2016, 26, 784 – 791.

[11] P. M. Desimone, C. G. D&az, J. P. Tomba, C. M. Aldao, M. A. Ponce, J.Mater. Sci. 2016, 51, 4451 – 4461.

[12] a) L. Chen, X. Jiang, N. Wang, J. Yue, Y. Qian, J. Yang, Adv. Sci. 2015, 2,1500090; b) W.-T. Liu, B.-H. Wu, Y.-T. Lai, N.-H. Tai, T.-P. Perng, L.-J. Chen,Nano Energy 2018, 47, 18 – 25; c) S. Xia, J. Ni, S. V. Savilov, L. Li, NanoEnergy 2018, 45, 407 – 412; d) Y. Tong, P. Chen, M. Zhang, T. Zhou, L.Zhang, W. Chu, C. Wu, Y. Xie, ACS Catal. 2018, 8, 1 – 7.

[13] Y. Xu, M. Zhou, C. Zhang, C. Wang, L. Liang, Y. Fang, M. Wu, L. Cheng, Y.Lei, Nano Energy 2017, 38, 304 – 312.

[14] C. Macchi, M. A. Ponce, P. M. Desimone, C. M. Aldao, A. Somoza, Philos.Mag. 2018, 98, 673 – 692.

[15] a) C. Li, T. Wang, Z.-J. Zhao, W. Yang, J.-F. Li, A. Li, Z. Yang, G. A. Ozin, J.Gong, Angew. Chem. Int. Ed. 2018, 57, 5278 – 5282; Angew. Chem. 2018,130, 5376 – 5380; b) L. Xu, X. Ma, N. Sun, F. Chen, Appl. Surf. Sci. 2018,441, 150 – 155.

[16] Z. Chen, U. Cvelbar, M. Mozetic, J. He, M. K. Sunkara, Chem. Mater. 2008,20, 3224 – 3228.

[17] a) N. Wu, W. Du, X. Gao, L. Zhao, G. Liu, X. Liu, H. Wu, Y.-B. He, Nanoscale2018, 10, 11460 – 11466; b) D. Wang, X. Li, J. Wang, J. Yang, D. Geng, R.Li, M. Cai, T.-K. Sham, X. Sun, J. Phys. Chem. C 2012, 116, 22149 – 22156.

[18] a) H. Elbohy, K. M. Reza, S. Abdulkarim, Q. Qiao, Sustainable Energy Fuels2018, 2, 403 – 412; b) K. H. L. Zhang, G. Li, S. R. Spurgeon, L. Wang, P.Yan, Z. Wang, M. Gu, T. Varga, M. E. Bowden, Z. Zhu, C. Wang, Y. Du, ACSAppl. Mater. Interfaces 2018, 10, 17480 – 17486.

[19] G. Choi, L. Satyanarayana, J. Park, Appl. Surf. Sci. 2006, 252, 7878 – 7883.[20] W. Dong, J. Xu, C. Wang, Y. Lu, X. Liu, X. Wang, X. Yuan, Z. Wang, T. Lin,

M. Sui, I.-W. Chen, F. Huang, Adv. Mater. 2017, 29, 1700136.[21] D. Ma, Y. Li, H. Mi, S. Luo, P. Zhang, Z. Lin, J. Li, H. Zhang, Angew. Chem.

Int. Ed. 2018, 57, 8901 – 8905; Angew. Chem. 2018, 130, 9039 – 9043.[22] a) S. K. Sinha, R. Bhattacharya, S. K. Ray, I. Manna, Mater. Lett. 2011, 65,

146 – 149; b) Z. W. Chen, H. J. Zhang, Z. Li, Z. Jiao, M. H. Wu, C. H. Shek,C. M. L. Wu, J. K. L. Lai, Acta Mater. 2009, 57, 5078 – 5082.

[23] R. Hu, Y. Ouyang, T. Liang, X. Tang, B. Yuan, J. Liu, L. Zhang, L. Yang, M.Zhu, Energy Environ. Sci. 2017, 10, 2017 – 2029.

Manuscript received: July 25, 2018

Accepted manuscript online: September 12, 2018Version of record online: October 18, 2018

ChemSusChem 2018, 11, 3693 – 3703 www.chemsuschem.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3703

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