Alkaline rechargeable Ni-Co batteries

Alkaline rechargeable Ni/Co batteries:Cobalt hydroxides as negative electrode materials ?

Xue-Ping Gao,*Su-Mei Yao,Tian-Ying Yan and Zhen Zhou

Received 29th January 2009,Accepted 13th March 2009

First published as an Advance Article on the web 23rd March 2009DOI:10.1039/b901934k

It is demonstrated that b -Co(OH)2has a high discharge capacity and good high-rate discharge ability as a negative electrode mate-rial.A new rechargeable battery system with higher energy density,consisting of a -phase nickel hydroxides as the positive electrode material and b -cobalt hydroxides as the negative electrode material,is proposed on the basis of multi-electron reactions.

Alkaline rechargeable batteries are considered to be one of the most promising power sources for the application in portable electronic devices,electric tools,electric vehicles (EV),and hybrid electric vehicles (HEV).Many electrochemical couples of positive and negative electrode materials have been developed,including nickel/cadmium (Ni/Cd),nickel/iron (Ni/Fe),nickel/zinc (Ni/Zn),and nickel/metal hydride (Ni/MH)alkaline rechargeable Ni-based batteries.1Among these systems,Ni/MH batteries are widely used owing to their outstanding features of high energy density,high power,environmental issue,and safety.Recently,numerous investi-gations have been conducted to propose new battery systems without ?ammable organic electrolytes on the basis of safety concerns,such as aqueous rechargeable lithium batteries,2polymer-based batteries,3and all-solid-state rechargeable batteries.4Moreover,high-rate discharge capability and high discharge capacity are also crucial for electrode materials in such rechargeable batteries.

Rechargeable batteries are mostly based on faradaic reactions,and multi-electron reactions on electrode materials are an effective way to provide a high energy density.a -Phase nickel hydroxide as the positive electrode material with 1.6–2.0exchanged electrons per Ni atom has a higher discharge capacity,in contrast to the capacity limitation of commercial b -phase nickel hydroxide with only one electron reaction.5On the other hand,negative electrode materials with multi-electron reactions have been explored recently.For example,more than one electron is involved in the electrochemical reaction of metallic cobalt or cobalt hydroxide in alkaline electro-lyte.6,7Moreover,cobalt hydroxide has capacitive characteristics with potential applications as an electrochemical supercapacitor with a high energy density.8

Cobalt hydroxide has a hexagonal layered structure and crystal-lizes into two polymorphic forms,a and b .9The a -form is isostructural with hydrotalcite-like compounds that consist of hydroxyl-de?cient Co(OH)2àx layers and charge balancing anions (NO 3à,CO 3à,Cl à,etc.)in the interlayer gallery.On the other hand,the b -form is burcite-like [Mg(OH)2]and consists of a hexagonal packing of hydroxyl ions with Co(II )occupying alternate rows of octahedral sites.9–11The a -cobalt hydroxide is susceptible to losing interlayer water and anions.In contrast,the b -phase Co(OH)2is stable and thus is anticipated to be a more promising electrode material.In the present study,we synthesized single-crystal hexagonal platelets of b -Co(OH)2by homogeneous precipitation,10and inves-tigated the discharge capability of b -Co(OH)2at different discharge rates.It is found that b -Co(OH)2has an excellent high-rate discharge ability and high discharge capacity.In addition,a -Ni(OH)2micro-spheres with 1.8exchanged electrons per Ni atom have a large discharge capacity,excellent high-rate ability and long cycle life as reported in our previous work.12We describe herein a battery system,

Institute of New Energy Material Chemistry,Nankai University,Tianjin,300071,China.E-mail:xpgao@https://www.360docs.net/doc/6e7168199.html,;Fax:+8622-23500876;Tel:+8622-23500876

?Electronic supplementary information (ESI)available:Complete experimental details of syntheses,XRD patterns,SEM images and cyclic voltammograms.See DOI:

10.1039/b901934k

COMMUNICATION https://www.360docs.net/doc/6e7168199.html,/ees |Energy &Environmental Science

D o w n l o a d e d o n 02 O c t o b e r 2011P u b l i s h e d o n 23 M a r c h 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 901934K

consisting of a-Ni(OH)2as the positive electrode material and b-Co(OH)2as the negative electrode material on the basis of multi-electron reactions(Scheme l).

The XRD pattern(see ESI,Fig.S1?)of the as-prepared b-Co(OH)2particles matches with those reported earlier.10All diffrac-tion peaks can be indexed as the hexagonal structure of brucite-like b-Co(OH)2(JCPDS74-1057),and the sharp re?ections indicate the highly crystalline nature of the pink b-Co(OH)2product.A thin platelet-like morphology is observed in the b-Co(OH)2sample from the SEM image(see ESI,Fig.S2?),which is bene?cial to the elec-trochemical reaction on the active surface.

Fig.1shows the dependence of electrochemical discharge capacity of the b-Co(OH)2electrode on the cycle number after activation at different discharge rates.Obviously,there is an activation process to reach the maximum discharge capacity of the b-Co(OH)2electrode. The maximum discharge capacities of455,453,406and338mA h gà1 are achieved at1,2,5and10C rate,respectively,indicating that b-Co(OH)2exhibits excellent durability at high discharge rate as compared to a-Co(OH)2(see ESI,Fig.S3?).Moreover,the discharge capacities of373and258mA h gà1are retained at1C and10C rates after50cycles for b-Co(OH)2,higher that that of313,and 240mA h gà1for a-Co(OH)2,respectively.The typical discharge curves for the as-prepared b-Co(OH)2at different discharge rates after activation are shown in Fig.2.The electrode presents a?at potential plateau aroundà0.78V(vs.Hg/HgO)in the discharge curve at a1C rate,which is in good agreement with the discharge potential plateau of a metallic cobalt-based composite electrode,7and the discharge plateau remains unchanged with increasing cycle numbers for both the a-Co(OH)2and b-Co(OH)2electrodes(see ESI, Fig.S4?).Although both a-Co(OH)2and b-Co(OH)2can be used as a negative electrode,b-Co(OH)2is more favorable due to the struc-ture transformation from unstable a-Co(OH)2to stable b-Co(OH)2 during cycling(see ESI?).

To further investigate the structural change after charging and discharging,XRD patterns of the b-Co(OH)2electrode at the charge/ discharge state in the second cycle are shown in Fig.3.The pattern of the as-prepared b-Co(OH)2is also included for comparison.It can be found that the diffraction peaks of metallic Co are detected at the fully charged state in the second cycle.Meanwhile,most of the diffraction peaks can be indexed as b-Co(OH)2and the diffraction peaks of the metallic Co disappear almost after fully discharging in the second cycle.This means that b-Co(OH)2can be reduced

to Fig.1Cycle performance of the b-Co(OH)2electrode at different

discharge rates after charging at a1,2,5and10C rate for1.5,0.75,0.3

and0.15h,

respectively.

Scheme1Schematic representation of the electrochemical reaction of

Ni/Co

batteries.

Fig.3XRD patterns of the b-Co(OH)2electrode at the charge and

discharge state in the second

cycle.

Fig.2Discharge curves of the b-Co(OH)2electrode at different

discharge rates after activation(charging at a1,2,5and10C rate for1.5,

0.75,0.3and0.15h,respectively).

D

o

w

n

l

o

a

d

e

d

o

n

2

O

c

t

o

b

e

r

2

1

1

P

u

b

l

i

s

h

e

d

o

n

2

3

M

a

r

c

h

2

9

o

n

h

t

t

p

:

/

/

p

u

b

s

.

r

s

c

.

o

r

g

|

d

o

i

:

1

.

1

3

9

/

B

9

1

9

3

4

K

metallic Co during the charge process,and the discharge capacity of the electrode is mainly attributed to the electrochemical oxidation of metallic Co.When a -Co(OH)2is used as a negative electrode,the structure transformation from a -Co(OH)2to b -Co(OH)2is found at the fully charged state,showing higher structure stability of b -Co(OH)2during electrochemical cycling (see ESI,Fig.S5?).

To elucidate the electrochemical reaction process occurring on the b -Co(OH)2electrode,cyclic voltammograms (CVs)of b -Co(OH)2electrode from à0.4V to à1.2V (vs .Hg/HgO)at different scan rates are presented (see ESI,Fig.S6?).A pair of redox peaks is detected,indicating that the reversible capacity is mainly based on the faradaic redox mechanism.The cathodic peak is due to the reduction of Co(OH)2to metallic Co and the anodic peak is attributed to the oxidation of metallic Co to Co(OH)2.An excellent electrochemical reaction activity and a good high rate capability are veri?ed by the results of CV at higher scan rate.For the b -Co(OH)2electrode material,it is well accepted that the surface faradaic reaction with 2electrons is dominant and can be expressed as 6,13

Co eOH T2t2e à

5reduction

oxidation

Co t2OH

à

(1)

According to the above two-electron reaction process,the theo-retical electrochemical capacity of Co(OH)2is 576mA h g à1based on Faraday’s law.The discharge capacity of Co(OH)2at 1C discharge rate is 455mA h g à1and the utilization of Co(OH)2is 79%.From XRD patterns,it can be observed that b -Co(OH)2is still detected as the coexisting phase with metallic Co at the end of the charging state.It implies that the partial irreversible conversion between b -Co(OH)2and metallic Co is involved in the faradic reaction,resulting in the incomplete utilization of Co(OH)2.Therefore,on average, 1.58electrons are involved in the practical electrochemical process of the b -Co(OH)2electrode.It is reported that Co(OH)2undergoes disso-lution in alkaline electrolytes and the amount of cobalt formed from the dissolved Co(OH)2is 8mg L à1in 5M KOH.14In addition,Co(OH)2can be transformed to HCoO 2àto some extent 13and the equilibrium concentration of HCoO 2àis >10à4M in 6M KOH solution at 25 C.15Thus,the partial dissolution of Co(OH)2also accounts for the decay of the discharge capacity during cycling.7

The promising values with respect to both discharge capacity and high rate capability of the b -Co(OH)2in the half-cell clearly demonstrate a suitability for fabricating alkaline rechargeable batteries.A novel alkaline rechargeable battery system is designed with b -Co(OH)2as the negative electrode.The electrode was prepared by mixing b -Co(OH)2with carbonyl nickel powder (NICO 255)in a weight ratio of 1:3and then pressed into a small pellet.As a -Ni(OH)2microspheres with 1.8exchanged electrons per Ni atom have a large discharge capacity,excellent high-rate ability and long cycle life,12they were used here as the positive electrode material (see ESI,Experimental and Fig.S7?).a -Ni(OH)2,cobalt oxide powders,and Ni powders were mixed with a weight ratio of 65.6:7.7:26.7,and pasted into a nickel foam.Then,the resultant positive electrode was coupled with a b -Co(OH)2negative electrode in 6M KOH.The fabricated Ni/Co prototype cell exhibits a ?at discharge voltage plateau,high power density and long-term endurance at a 1C rate as presented in Fig.4.The output energy density reaches 165W h kg à1(both a -Ni(OH)2and b -Co(OH)2as the active materials)and stabi-lizes at 158.8W h kg à1after 50cycles,revealing an energy retention of

96.2%.In the Ni/Co prototype cell,the overall battery reaction is expressed as

2Ni eOH T2tCo eOH T2

5charge

discharge

2NiOOH tCo t2H 2

O

(2)

The operating principle is similar to that of Ni/Cd batteries,16and the Ni/Co cell can be expected as a new member in alkaline rechargeable battery systems.In addition,if the weight percentage of 75–80%is considered in a practical battery,the energy density of 120–130W h kg à1would be obtained for the Ni/Co cell,which is higher than that of commercial Ni/Cd and Ni/MH batteries.(see ESI,Table S1?).1Moreover,cobalt is an element that occurs naturally in many different chemical forms throughout the envi-ronment in air,water,soil,rocks,plants and animals with trace amounts.Small amounts of cobalt are accessible and essential for health because cobalt is a part of vitamin B 12.17Therefore,compared to the toxic features of a Ni/Cd battery,the Ni/Co battery can be denoted as a green battery.

In summary,b -Co(OH)2presents good electrochemical perfor-mances at high discharge rates based on the faradic redox reaction with 1.58exchanged electrons on average.A new alkaline recharge-able battery system,consisting of a -Ni(OH)2as the positive electrode material and b -Co(OH)2as the negative electrode material is proposed on the basis of multi-electron reactions.The fabricated Ni/Co prototype cell shows outstanding features of long durability and high power.The strategy adapted in the present study could be helpful to explore and develop new power sources with high energy density based on multi-electron reactions.

Financial support from the 973Program (2009CB220100)and NSFC (50671049)of China is greatly appreciated.

Notes and references

1U.K €o hler,C.Antonius and P.B €a uerlein,J.Power Sources ,2004,127,45.

2G.J.Wang,L.J.Fu,N.H.Zhao,L.C.Yang,Y.P.Wu and H.Q.Wu,Angew.Chem.,Int.Ed.,2007,46,295.

3H.K.Song and G.T.R.Palmore,Adv.Mater.,2006,18,1764.

4P.H.L.Notten,F.Roozeboom,R.A.H.Niessen and L.Baggetto,Adv.Mater.,2007,19,4564.5W.K.Hu and D.Nor e us,Chem.Mater.,2003,15,

974.

Fig.4Cycling performance of the fabricated Ni/Co prototype cell at a 1C rate after charging at a 1C rate for 1.5h.Inset is the discharge curve of the Ni/Co prototype cell in the second cycle at a 1C rate.

D o w n l o a d e d o n 02 O c t o b e r 2011P u b l i s h e d o n 23 M a r c h 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 901934K

6P.Elumalai,H.N.Vasan and N.Munichandraiah,J.Power Sources ,2001,93,201.

7Z.W.Lu,S.M.Yao,G.R.Li,T.Y.Yan and X.P.Gao,Electrochim.Acta ,2008,53,2369.

8L.Cao,F.Xu,Y.Y.Liang and H.L.Li,Adv.Mater.,2004,16,1853.

9Z.P.Xu and H.C.Zeng,Chem.Mater.,1999,11,67.

10Z.P.Liu,R.Z.Ma,M.Osada,K.Takada and T.Sasaki,J.Am.Chem.Soc.,2005,127,13869.

11P.V.Kamath,G.H.A.Therese and J.Gopalakrishnan,J.Solid State Chem.,1997,128,38.12Q.D.Wu,X.P.Gao,G.R.Li,G.L.Pan,T.Y.Yan and H.Y.Zhu,J.Phys.Chem.C ,2007,111,17082.

13B.S.Haran,B.N.Popov and R.E.White,J.Electrochem.Soc.,1998,145,3000.

14V.Pralong, A.Delahaye-Vidal, B.Beaudoin, B.Gerand and J.M.Tarascon,J.Mater.Chem.,1999,9,955.

15H.Senoh,M.Ueda,N.Furukawa,H.Inoue and C.Iwakura,J.Alloys Compd.,1998,280,114.

16A.K.Shukla,S.Venugopalan and B.Hariprakash,J.Power Sources ,2001,100,125.

17T.Brock and W.Stopford,J.Environ.Monit.,2003,5,71N.

D o w n l o a d e d o n 02 O c t o b e r 2011P u b l i s h e d o n 23 M a r c h 2009 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/B 901934K