劉　山，柳絲絲，羅加嚴(yán)

(1.天津大學(xué) 化工學(xué)院，綠色合成與轉(zhuǎn)化教育部重點(diǎn)實(shí)驗(yàn)室，天津300072；2.天津化學(xué)化工協(xié)同創(chuàng)新中心(天津)，天津300072)

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鈉-空氣電池用碳基空氣電極研究進(jìn)展

劉山1,2，柳絲絲1,2，羅加嚴(yán)1,2

(1.天津大學(xué) 化工學(xué)院，綠色合成與轉(zhuǎn)化教育部重點(diǎn)實(shí)驗(yàn)室，天津300072；2.天津化學(xué)化工協(xié)同創(chuàng)新中心(天津)，天津300072)

鈉-空氣電池具有能量密度高、放電平臺(tái)高(2.3 V)及鈉儲(chǔ)量豐富等優(yōu)點(diǎn),被認(rèn)為是一種極具發(fā)展前景的儲(chǔ)能技術(shù)。然而，鈉-空氣電池仍存在諸多問題。本文綜述了鈉-空氣電池近年來的發(fā)展?fàn)顩r，著重探討了炭基空氣電極的研究進(jìn)展，并對(duì)鈉-空氣電池未來發(fā)展方向進(jìn)行了展望。

碳；空氣電極；反應(yīng)機(jī)理；鈉-空氣電池

1　Introduction

The consumption of fossil fuels in the past few decades has resulted in a substantial increase of greenhouse gas emission.Lithium-ion batteries (LIBs) have been widely used in portable electronics and considered as promising energy storage/conversion devices for hybrid or electric vehicles (EVs),which will alleviate the usage of fossil fuels in combustion engines.However,the energy density of LIBs cannot meet the requirements of long-range running of EVs,even the theoretical limit of current electrode materials are reached[1-4].

Rechargeable metal-air batteries have been targeted as a promising technology to meet the energy requirements for future EVs and other energy-demanding devices owing to their high energy densities[5-13].In contrast to most other batteries,which must carry both an anode and a cathode inside a storage system,metal-air batteries are unique in that the active cathode material,oxygen is not stored in the batteries.Instead,oxygen can be absorbed from the environment and reduced by catalytic surface inside the air electrode,which makes them smaller,lighter and more storable than LIBs.Table 1[5-10]lists electrochemical parameters of the various metal-air batteries.As shown in Table 1,the practical voltages of the metal-air pairs are lower than their theoretical values.The large difference between the theoretical voltage and practical voltage of metal-air batteries results from the large polarization at both electrodes under practical operating condition.The zinc-air batteries are relatively stable in both acidic and alkaline electrolytes[6].The corrosion problem of the zinc anode can be controlled by using appropriate inhibitors.But the commercialization is still hindered by various technical problems,including zinc re-plating during the charge process and development of an efficient,high-rate and bi-functional air electrode.The aluminum-air batteries exhibits one of the high theoretical specific energies,considering its high specific capacity (2.98 Ah/g) and high theoretical voltage (2.71 V)[14].However,the practical voltage of the batteries is relatively low (i.e.,1.3 V) because aluminum and air electrodes cannot operate at their thermodynamic potentials due to the side reaction between aluminum and water.Similarly,the theoretical voltage of magnesium-air batteries is 3.1 V,but in practice the open-circuit voltage is 1.3 V[7].Magnesium anode tends to directly react with the electrolyte,forming magnesium hydroxide and generating hydrogen.In the lithium-air batteries,the large charge overpotential leads to a relatively low round-trip energy storage efficiency and significant capacity fading.Even worse,many electrocatalysts can promote the undesired electrolyte decomposition,even the binder is found to be unstable[15].

Table 1　Electrochemical parameters of the various metal-air batteries.

Note:* Specific capacity based on metal alone and specific energy based on the total discharge product weight.

Compared to lithium-air batteries,it is[10,16,17]found that Na-air batteries do not suffer from the negative effects of the electrolyte and energy efficiency as in Li-air batteries.Although lithium and sodium are closely related chemically,they react very differently with oxygen.LiO2is formed when lithium reacts with oxygen,which is highly unstable intermediate in lithium-air batteries,and easily turns into Li2O2.But in the Na-air batteries,sodium and oxygen form a more stable compound,NaO2.Since NaO2would not decompose,the reaction can be reversed during charging,which means that the over-potential of Na-air batteries is much smaller than that of lithium-air batteries.Although Na-air batteries have a lower overall energy density than lithium-air batteries,they are still significantly higher than LIBs.Besides,sodium is abundant and inexpensive because it can be readily extracted from seawater.Thus,development of efficient Na-air batteries is promising owing to their advantages.

Nevertheless,the limited cycling performance hinders the development of rechargeable Na-air batteries[18-24].Precipitation of reaction product (such as NaO2or Na2O2) on the carbonaceous electrodes eventually blocks the oxygen diffusion pathway and limits the capacity of Na-air batteries.So there is a critical need to design an optimum air electrode having a high catalytic property and containing a hierarchical porosity for rapid oxygen diffusion to prevent excessive growth of the discharge products that block the diffusion pathway.Carbonaceous materials have attracted extensive attention either as catalyst supports or metal-free catalysts in electrocatalytic chemistry of oxygen[25-28].They generally provide notable merits including low cost,wide availability,large surface area,high electrical conductivity and excellent stability under harsh environments.Therefore,carbon materials act as the most popular material in Na-air system.Especially great efforts have been made to control the size,shape and uniformity of the porous space to be suitable for the air electrodes of Na-air batteries.

In this review,we focus on the recent progress and technical issues with respect to the carbon-based air electrodes for Na-air batteries.This paper starts with a brief introduction of Na-air battery configuration and operation principle,followed by discussing challenges and obstacles in Na-air batteries.Next,the recent progress of Na-air batteries,especially the carbon-based air electrodes for Na-air batteries will be discussed.Finally,we will provide some perspectives about the future trend of Na-air batteries.

2　The Na-air batteries

In most laboratory researches,the Na-air batteries are actually studied in pure oxygen environment.Thus,in the following technique discussion,Na-oxygen (Na-O2) batteries are used.A typical Na-O2battery is comprised of a porous carbon cathode and Na metal anode.In the cathode,O2diffuses to the porous carbon during discharge.A separator containing aprotic electrolyte is sandwiched between the two electrodes as shown in Fig.1a.The feasibility of rechargeable Na-O2batteries was first investigated by Peled et al[17].The cell was operated at elevated temperatures,using liquid sodium metal as the negative electrode,which differs from the later studies at room temperature.In 2011,Sun et al.[16]reported the first room temperature Na-O2battery,which exhibited a rechargeability up to 20 cycles with a discharge capacity of 1 058 mAh/g and a columbic efficiency of 85%.However,unlike the non-aqueous lithium-oxygen cells,where Li2O2is formed,reports on the nature of the discharge product in sodium-oxygen cells are less consistent and different discharge products have been reported[10,16,21,22].Although Na2O2is thermodynamically favored,NaO2is kinetically preferred because it requires only one electron per formula unit.Hartmann et al.[10]found that NaO2was formed as an exclusive discharge product using NaOT in bis(2-methoxyethyl)ether (DEGDME) as an electrolyte.The formation process was proved by X-ray diffraction (XRD),Raman spectroscopy and monitoring pressure during galvanostatic cycling.Peled et al[17,29].assumed that Na2O2was the product but no experimental proof was given.Liu et al.[30]found that polycrystalline Na2O2as the discharge product using NaPF6or NaClO4in 1,2-dimethoxyethane (DME) as an electrolyte under selected area electron diffraction (SAED).Sun et al.[16]also found the peroxide but with a significant fraction of sodium carbonate (Na2CO3) using NaPF6in ethylene carbonate (EC):DME 1∶1 as an electrolyte,evident from SAED and FTIR spectroscopy.Kim et al.[31]showed that Na2CO3was the main discharge product using a PC based electrolyte (1 M NaClO4),which was identified by XRD and FTIR spectroscopy.Sodium peroxide dihydrate (Na2O2·2H2O) and trace amounts sodium hydroxide using NaClO4in tetraglyme as an electrolyte was detected by XRD.They concluded that the discharge products were dependent on the electrolyte composition,and proposed reaction mechanisms in different electrolytes.Na2O2was also found by Das et al.[32]when discharging the cell in an oxygen atmosphere.In a mixture of oxygen and carbon dioxide,trace oxalate was generated,as identified by XRD and FTIR spectroscopy.

Fig.1　(a) Schematic configuration of Na-O2batteries; (b) XRD patterns of electrochemical formed NaO2[9]; (c)Galvanostatic charge-discharge curves of Na-O2cells operated at the flowing pure and static Ar/O2(80/20 V/V) [10].

Recently the stability of the different product phases was calculated by two groups[33]with somehow controversial results.Lee et al.[33]concluded from first-principle calculations that NaO2and Li2O2are thermodynamically favored discharging products at standard conditions.They also demonstrated that the shape of the particles agreed well with the previously reported experimental data.Kang et al.[32]studied the particle size effect on the relative stability of Na2O2and NaO2.In contradict to Lee et al.,they proposed that Na2O2is the stable bulk phase at standard conditions,while NaO2particles can only become more stable below 10 nm.The authors also concluded that NaO2nucleates much easier than Na2O2,suggesting that the discharge product generated in Na-air batteries be under kinetic control.

Kim et al.[22]observed that the galvanostatic charge/discharge profiles are affected by the condition and duration of the electrochemical operations.And the electrochemically formed NaO2is unstable and degrades into Na2O2·H2O in the absence of an applied current as shown in Fig.1b.With respect to previous work on Na-O2batteries,what determines the nature of the discharge product or how to reproducibly obtain only one of the products,i.e.,NaO2or Na2O2in Na-air batteries? Further work will be required to clear the discrepancies.

Over-potential is a limiting factor for the energy efficiency.The over-potential of Na-O2batteries is only 200 mV (Fig.1c),which is 3-4 times lower than Li-O2batteries[10,34].It is important to note that no catalyst is used in air cathodes.A possible reason for this important observation is the high electrical conductivity of NaO2.Ab initio simulation of the oxygen evolution reaction during charging indicates that NaO2decomposition is more favorable than Na2O2,LiO2,and Li2O2[33].More fundamental studies are needed to clarify these effects.Although the discharge potential of Na-O2batteries is less than that of Li-O2batteries (Fig.1c),the discharge capacity of Na-O2batteries are similar to or higher than Li-O2batteries based on mass of the cathode.The lower over-potential and higher capacities will lead to higher energy density and energy efficiency in reversible Na-O2batteries.

3　The carbon-based cathodes for Na-O2batteries

The use of oxygen from the atmosphere in the Na-O2batteries endowed this technology a great challenge.The ideal air electrode requires a high specific surface area for providing a high utilizing of nanocatalysts,and a high conductivity for electron transfer.Requirements also involve a great chemical and electrochemical stability under the Na-O2battery operating condition.Carbons are employed as typical air cathodes in Na-O2batteries owing to its high conductivity,high surface area and low cost.Great efforts have been made to control the size,shape and uniformity of porous space for gas diffusion and effective electro-catalysis.In the discharging,Na+migrates through the electrolyte and reacts with O2at carbon-electrolyte-gas interface.The insoluble reaction products (NaO2/Na2O2) deposited on the carbon surface eventually block the pores.This results in a steric barrier for Na+/O2diffusion,causing a premature termination of the discharge reaction.The type of carbon might have a major impact on the amount and type of discharge products.Hartmann et al.[10]observed the deposition of micrometer sized (～10 μm) NaO2(Fig.2a) on the carbon cathode.In contrast,other studies showed a nanometer sized (～50-200 nm) or polymer like skin layer of Na2O2on the carbon surface (Fig.2b and c)[23,30].Kim et al.[31]used Ketjen black and found that Na2CO3and Na2O2are the discharge products using a carbonate based electrolyte (2 800 mAh/g) and an ether based electrolyte (6 000 mAh/g),respectively.Liu et al.[30]found that Na2O2is the discharge product (9 268 mAh/g) using graphene nanosheets.Sun et al.[21]detected Na2O2and Na2CO3in their cells using diamond-like carbon (3 600 mAh/g).The carbon nanotube (CNT) air cathode by Zhou et al.[23]exhibited a large discharge capacity of 7 530 mAh/g in the first discharge cycle at a high current density of 500 mA/g and the charge capacity is about 3 300 mAh/g (Fig.3a).These values are 2-3 fold higher than typical carbon (e.g.carbon black) air electrodes.Graphene nanosheets (GNSs) as the air electrode also had a high discharge capacity and high rate capacity (Fig.3b)[30].When discharged at 200 mA/g,it allowed for an extremely high discharge capacity of 9 268 mAh/g.At a high current of 1 000 mA/g,it still maintained a capacity of 1 110 mAh/g.In another investigation by Li et al.[21]nitrogen-doped graphene nanosheets (N-GNSs) displayed better electrochemical performance than pristine graphene nanosheets (GNSs).The discharge capacities of N-GNSs were two times greater than that of GNSs at all investigated current densities (Fig.3c).Additionally,the N-GNS air electrode displayed a lower over-potential than GNS.It is anticipated that nitrogen doping resulted in catalytically active sites in GNSs for O2oxidation and reduction.

To understand whether carbon structure or local current density has any influence on product stoichimetry or cell performance,several carbon materials with a broad range properties were tested[35].Phase-pure NaO2is always found as the discharge product,but capacities values range from 300 to as high as 4 000 mAh/g (Fig.3d).And the electrode using a ketjen black carbon has the best cycling performance,which can steadily yield capacities over 1 666 mAh/g for at least 60 cycles.These studies suggest that ample opportunities exist to enhance the electrochemical performance of Na-O2batteries by tuning the chemistry and microstructure of carbon air cathodes.Nevertheless,so far the results do not show a clear correlation between surface structure/chemistry and products or achievable capacity.Therefore,optimized air cathodes with controlled pore size,pore volume,surface area,and electronic conductivity are essential for enhanced electrochemical performance of Na-O2batteries.

Fig.2　(a)SEM image of NaO2[10]discharge products at higher magnification; (b) TEM images of Na2O2discharge products after the cell initially discharge[30]and (c) SEM image of Na2O2·H2O at deep discharge[23].

Fig.3　(a) Discharge and charge curves of Na-O2battery based on the carbon nanotube cathode [23]; (b) Discharge and charge curves of Na-O2battery use graphene nanosheets as the cathode[30]; (c) Comparative study on graphene and nitrogen doped graphene air electrodes [21]and (d) Capacities values of different carbon materials [13].

4　Conclusions and perspectives

The emergence of rechargeable Na-O2batteries creates exciting opportunities for cost-effective,high energy density and energy efficient electrochemical storage devices.Although many fundamental and engineering challenges remain,the technology has profound strengths that underscore its promise as an alternative to lithium-based electrochemical batteries.The abundance and low cost of Na provide key advantages in large-scale energy storage systems.The preliminary investigations[4,10,16,23]on Na-O2batteries demonstrate three notable features.First,these systems are rechargeable and the charge-discharge curves exhibit an attractive and stable plateaus.Second,the round trip efficiency of Na-O2batteries is very high.Finally,Na-O2batteries can sustain high current rates to deliver high power densities.The energy efficiency and power density can be enhanced significantly by a proper engineering of the air cathodes.

Though the initial studies on Na-O2batteries suggest enormous potential of these systems,the development of Na-O2batteries is still in the infancy[19,24,29].Many critical problems such as the limited cyclic stability,low electrical energy efficiency and poor power density still need to be solved.Different from Li-O2batteries,carbon materials can remain stable in Na-O2system.But the carbon materials do not exhibit catalytic property for reducing the charge polarization.Therefore,exploring new carbon materials via doping heteroatoms or combining with other catalysts to enable them with an oxygen evolution activity is a developmental direction.

Beside cathode materials,additional thorough investigations to understand many fundamental aspects of Na-O2batteries system is required.Firstly,the undergoing electrochemical reaction in Na-O2batteries is one important aspect.It is necessary to unravel the parameters that control the formation of NaO2rather than Na2O2.Secondly,the cycle life is a great challenge.Development of optimized air cathodes is a crucial factor for capacity retention.Finally,the optimization of electrolyte composition and the control of sodium dendrite formation are also pivotal for Na-O2batteries.

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Carbon-based cathodes for sodium-air batteries

LIU Shan1,2,LIU Si-si1,2,LUO Jia-yan1,2

(1.Key Laboratory for Green Chemical Technology of Ministry of Education,School of Chemical Engineering and Technology,Tianjin University,Tianjin300072,China;2.Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),Tianjin300072,China)

Sodium-air batteries are considered promising electrochemical devices for energy storage owing to their high theoretical energy density,high discharge voltage and the abundanceof sodium in nature.The recent progress,current challenges and developing trendsfor Na-air batteries are summarized.The use of carbon materials as their cathode is highlighted.

Carbon; Air electrodes; Reaction mechanism; Sodium-air batteries

date:2016-05-08;Revised date:2016-06-05

National Natural Science Foundation of China (51502197); Chinese Government under “Thous and Youth Talents Program”; State Key Laboratory of Chemical Engineering (SKL-ChE-15B02).

LUO Jia-yan,Professor.E-mail:jluo@tju.edu.cn；LIU Si-si,Ph.D.E-mail:liusisi@tju.edu.cn.

introduction:LIU Shan,Ph.D Candidate.E-mail:lishy63@126.com.

1007-8827(2016)03-0264-07

TQ127.1+1

A

國(guó)家自然科學(xué)青年基金項(xiàng)目(51502197)；國(guó)家青年千人項(xiàng)目；化學(xué)工程聯(lián)合國(guó)家重點(diǎn)實(shí)驗(yàn)室(SKL-ChE-15B02).

羅加嚴(yán)，研究員，E-mail:jluo@tju.edu.cn；柳絲絲，在站博士后,E-mail:liusisi@tju.edu.cn

劉山，博士研究生.E-mail:lishy63@126.com

English edition available online ScienceDirect ( http:www.sciencedirect.comsciencejournal18725805 ).

10.1016/S1872-5805(16)60012-4