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一Article一

Improvement of High－Temperature Characteristics of the Sintered Nickel Positive Electrode for an Alkaline Storage Battery

Katsuhiko SHINYAMA，＊Yoshifumi MAGARI，Atsuhiro FUNAHASHI， Toshiyuki NOHMA，and Ikuo YONEZU

R＆D Business Unit，Mobile Energy Company，Sanyo Electric Co．，Ltd．（7－3－21bukidai－higashimachi，Nishi－ku，Kobe City，Hyogo651－2242，Japan）

Received August9，2002；Accepted May20，2003

The high－temperature characteristics of sintered nickel positive electrodes for alkaline storage batteries such as nickel・metal hydride batteries and nicke1－cadmium batteries were investigated．Generally，the discharge capacity of an alkaline storage battery charged at high temperature is smaller than that charged at room temperature due to the oxygen evolution reaction．In order to enhance high－temperature characteristics，we coated sintered n量ckel positive electrodes with yttrium hydroxide，calcium hydroxide or cobalt hydroxide．Th6high－temperature characteristics of the sintered nickel positive electrodes coated with y血ium hydroxide and calcium hydroxide by immersing them in sodium hydroxide solution after immersing in nitrate solution were greatly enhanced because of the increased oxy． gen OVerVOltage．

κ¢y防鳩：Sintered Nickel Positive Electrode，High－temperature Characteristics，Alkaline Storage Battery，Oxygen Overvoltage

l lntroduction In the past，the electrolyte composition3）and dissolv－ Cun℃ntly，alkaline storage batteries such as nicke1－ ing such elements as cobalt and calcium into nickel hy－ cadmium batteries and nickel－metal hydride batteries1） dro》dde4－8）were investigated in order to improve the are used for a variety of applications from portable equip－ high－temperature charge ef匠ciency．In recent years，how－ ment to power tools．Nickel hydroxide is used as the ac－ ever，further improvement of the high－temperature tive material for a positive electrode in the alkaline stor－ charge ef且ciency has been required．Powder additives age baUleries．The nickel positive electrode is either a such as calcium compounds，barium compounds and sintered nickel posi丘ve electrode or a non－sintered nickel rare－earth compounds are used for this purpose for a posidve electrode．The advant丑ge of the sintered nickel non－sintered nickel electrode，9｝13）while it is d面cult to positive electrode is its high power characteristics and apply such powder additives to a sintered nickel elec－ that of廿1e non－sintered nickel posidve elec仕ode is its trode丘om the manufacturing Point ofview。 high capacity．Thus，the sintered nickel positive elec－ The purpose of this sUldy is to hnprove the high－ trode is often used in alkaline storage battehes when temperature charge e伍ciency of a nickel－metal hydride high power is required．In recent years，the sintered baUlery with a sintered nickel posi行ve electrode for h三gh nickel posidve elect■ode is considered to be aUrac丘ve for power applications．In our approach，we coat the nickel HEV（Hybrid Electric Vehicle）from this reason．For hydroxide parUcles of the sintered nickel positive elec－ such high power applications the high－temperature char－ trode with hydroxide species in order to elevate the oxy－ acteristics are s廿ongly required because the apPlications 9en evolution potential． are often used in a hot place and in addidon the baUlery temperature tends to rise due to high current． 2 Experimental The discharge capacity of an alkaline storage battery 2．1 P1・eparation of a．coa．ted nickel positive elec－ charged at high temperature，either using a sintered trode nickel electrode or using a non－sintered nickel elec廿’ode， First，we selected calcium from the alkaline earth met－ is usually lower than that charged at room tempera一 als and yttrium from the rare earth elements as a coating 加re．2）The cause of this is consi（1ere（1to be due to the materia1。We also selected cobalt which is commonly oxygen evoludon reaction or a side reaction （Eq．2）of used as an element to be co－precipitated in nickel hy－ the charging reaction（Eq．1）of the positive electrode at dro》dde．The coating method was as follows．After filling high temperature． nickel hydroxide into a porous sintering body by using a chemical impregnation method，we carried out immer－ Charge reaction sion in a cobalt nitrate solution，yttrium nitrate solution Ni（OH）2＋OH一一→・NiOOH＋H20＋e－ （1） or calcium nitrate solution of a specific gravity of1．2．By Side reacdon of oxygen evoludon immersion in a sodium hydroxide solution，we obtained 40H｝一→02＋2H20＋4e一 （2） the sintered nickel positive electrodes surface－coated 71，No．8（2003） 687

temperature charge characteristics were measured again．

E蓋ectrolyte 3 Results and Discussion 3．l Preparation of a coated nickel positive elec－ trode Our concept for improving high－temperature charge ef ficiency is illustrated in Fig．2．The sintered nickel elec－ trode has a three－dimensional porous structure consist－ ing of a sintered nickel body fille（1with nickel hydroxkie as the active material．When the cell temperature is high， the side reacdon in Eq．20ccurs，generating oxygen and water．Cobalt co－precipitation in nickel hydroxide is often used in order to improve the charge acceptImce charac－ teristics．Instead，we focused on the modhication of the active material surface by coating the nickel positive elec－ Sepaダa症or trode with hydro】ddes of other elements as this side re－ Fig．l Test cell configuration using a coated nickel positive action is considered to occur on the active material sur－ electrode and a sintered cadmium electrode． face． The amount ofthe coating materials is appro｝dmately2 with cobalt hydro⊃dde，yttrium hydroxide and calcium hy－ wt％as shown in Table1． droxide，respectively． SEMξmd EPMA images of the surface of the nickel The amount of coated hydroxide was calculated from positive electrode coated with y血ium hydroxide are the weight d逝erence of the electrode before and a［fter shown in Fig．3．The white dots in the EPMA image indi－ coating．The nickel positive electrode coated with yttrium cate the presence ofyttrium。We con丘rmed that the sur－ hydro⊃dde was characterized with scanning electron mi－ face of the nickel hydroxide，which is thought to be the croscopy （SEM）and electron probe micro analysis active oxygen evolution site，was coated with y血ium hy－ （EPMA）． dro）dde． 2．2 Characteristics of a test cell using a co＆ted Figure4shows cross－sectional SEM and EPMA im－ nickel positive electrode ages of the nickel positive electrode coated with yttrium We assembIed test cells with the coated positive elec－ hydrqxide，also indicating that the surface of the nickel trode as the worldng electrode and a sintered cadmium hydro》dde inside the electrode is coated with yttrium hy－ electrode as the counter electrode．We used a three－ droxide． ingredient solution of potassium hydro）dde，sodium hy－ 3．3 Characteristics of test cells droxide an（l lithium hydro】dde as the electrolyte．Figure The effect of the hydroxide coadng on the high－ l shows the test cell configuration．The condition testing temperature charge ef丘ciency of the test cells is shown their high－temperature charge characteristics was as foト in terms of the discharge capacity ratio versus the charg－ lows：（1）Charge the positive electrode up to160％of ing temperature in Fig．5．The charging ef丘ciency is im－ the theoretical capacity for a one－electron reaction at1／5 proved at high temperature for all the three hydroxides． C within an environmental temperature range from25℃ to60℃．（2）Set the environmental temperature at25℃． （3）Discharge the positive electrode up to a cell voltage ofO．8Vat1／3C． 2．3 Characteristics of a cylindrical se＆led－type cell using a hydrogen－absorbing alloy electrode as the negatiVe eleCtrOde We prepared cylindhcal sealed－type cells using the coated nickel positive electrode，a hydrogen－absorbing al－ loy negative electrode and a three－ingre（lient electrolyte consisting of potassium hydroxide，sodium hydroxide ξmd lithium hydro》dd．The composition of the hydrogen－ Fig．2 0xygen evolution reaction at the surface of the sin－ absorbing alloy was MmNi3．6Coo．謝o．3Mno．5．The condi－ tered positive nickel electrode． tions testing their high－temperature charge characteris－ tics was as follows：（1）Charge the cell up to120％of the cell capacity at1／2C at an environmental tempera－ T＆ble l Amount of coating hydroxides on the nickel posi－ ture range from25℃to60℃．（2）Set the environmental tiVe eleCtrOde material． temperature was at25℃．（3）Discharge the cell up to a Coating Amount of coa廿ng hydroxides（wt％〉

cellvoltageof1．OVat1／2C． Cobalthydroxide 1．8 In the cycle tests，the cylindrical sealed－type cells were Yttrium hydroxide 2．4 charged at3C for l6min．and discharged at3C to a Calcium hydro》dde 2．1 cell voltage of 1．O V．After 200 cycles，the high一 688 EJ66ケoohε”露ε」りノ

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20μm Fig．3Su㎡ace SEM and EPMA images ofa nickel positive electrode coated with yttrium hydroxide．

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Sinteri陥9 body 耳臓tema墾魎 60μm Fig．4Cross sectional SEM and EPMA images ofa nickel positive electrode coated with yttrium hydroxide．

105 1．55 Ca 斜oo へ ヘ （1．50 ヤ ヘ ＞ ，’一一一曙一一胴一” ≧く95 8ミ、．r臥、 Y／‘一 お ） n。n⑳ated脅、，、、ざス bO1．45o ！ノ 890 ε 1！ダCOnon．coated 忌 ㌦ 2h． ￥ 宕 Q85 、 、へ 、 ＞ 1．40 ノ ひ 1 α1arge：1／5C×8h へ．＼■ 冒 ∠ 1 響80 噺 、Ca ひ Q 1．35 7 鋸75 Discharge：・／3C・ ＼、C♂ 窃 グ ひ Charge彫1／5C×8h，25。C 070 E．V．＝0。8V，250C ＼ ←41．30 65 ！ 1．25 30 40 50 60 20 100 200 300 400 500 Charge Temperature（？C） 0 Capacity of Charging（mAh） Fig．5 Effect of hydroxide coating on high－temperature Fig．6 Charge characteristics of test cells at25℃． charge efficiency of test cells．

In particular，the electrodes using yttrium hydroxide an（i ξmd calcium hydroxi（ie compared to the non－coated elec－ calcium hydroxide show a large discharge capacity even trode．Coating with yttrium hydroxide and calcium hy－ for charging at60℃．The cobalt hydroぬde shows the droxide supPresses the oxygen evolution reacdon．The same characteristic as the non－coated electrode for timing of the oxygen evolution reaction is delayed for the charging at60℃while it shows a superior capacity com－ electrode coated with cobalt hydroxi（le although the oxy－ paredtothenon－coated electrodeupto55℃． gen evolution overvoltage does not change． In order to interpret the di丘erence depending on the Figure7shows the charge charactehstics of the test coating materials，we analyzed the voltage behavior dur－ cells at60℃．Among the three elements，yttrium hydrox－ ing charging．Figure6shows the charge charactedstics ide and calcium hydro】dde suppress the oxygen evolu－ ofthetestcellsat25℃．Thepotential differencebetween tion reaction more effectively，and high charging e伍一 oxygen evolution and nickel hydroxide oxidation is large ciency is achieved even at high temperatures．Contrary when using the electrodes coated with yttrium hydroxide to this，the electrode with cobalt hydroxide exhibits the 71，No．8（2003） 689

1．40 ing an active material2md an additive in paste making Y＿＿＿＿＿＿一 ググ罰ざ’一一一一 process for a non－sintered electrode．However，蛋or a sin－ ε ノ Co tered electrode，immersing the electrode in a solution in－ 〆’ じ の じ ロ ロ ロ ド ロ ロ ロ ロ ロ の ラ cluding an additive is required in order to disperse the ＆ 1．35 〃■ non－coated ε 礎多 additive in the electrode． も ＞ 〃一 3．4 Characteristics of cylin《lrical sealed－type cells 目 の We mαde cylindrical sealed－type cells using a 1．30 Q hydrogen－absorbing alloy electrode as the negative elec－ お 1 ←o Charge1／5C×8h，600C trode．Figure8shows the effect of the hydroxide coating on the high－temperature charge ef丘ciency of AA cells． 1．25 Similar to the results for the test cells，the AA cells using 0 100 200 300 400 500 the coa』ted electrodes show a large discharge capacity at Capacity of Charging（n孟h） the charging temperature above40℃．In particular，the electrodes using yttrium hydroxide and calcium hydrox－ Fig．7Charge characteristics oftest cells at60℃． ide show a large discharge capacity even when charging at60℃compared to the non－coated electrode． Table3shows the charge charactehstics of the AA same characteristic as the non－coated electrode because cells at60℃after200cycles．The cells with the yttrium the drop in oxygen evolution potential is large．Thus， hydroxide and calcium hydroxide coated electrodes show high－temperature characteristics of a sintered nickel posi－ almost the same high discharge capacity at high tem－ tive electrode are greatly enhanced when coated with yt－ peraUlres as those before the cycle test． The high－ trium hydroxide and calcium hydroxide． temperature charge characteristics after cycles are innu－ Next，we prepare（1a positive electrode where only its ence（l by not only the hy（1roxide coating but also other surface was spread with calcium oxide，yt面um o】dde and factors such as the capaci取change in the posi丘ve and cobalt hydroxide in order to examine in detail the influ－ negative electrodes．However，the result in Table3is ence of the additives on the high－temperature charge considered to indicate that the hydroxide coating is still charactehstics．Here slun・y consisting of these additives effective after cycles since the decrease in the discharge and HPC（hydroxypropylcellulose）aqueous solution was capaci寄a丘er200cycles is only2．6％of the initial capac－ spread on the positive electrode surface．The amount of ityorleSS． calcium oxide and yttrium oxide was approximately3wt Figure9illustrates a very small part of a sintered elec－ ％ of the nickel hydroxide in the electrode and the trode．This sintered body is the ol童ginal nickel metal sin－ amount of cobalt hydroxide was appro】dmately5wt％of tered powder．On this sintered body surface，the nicke1 the nickel hydroxide in the electrode． The d冠ference between the electrode whose active ma－ 850 terial surface is coated and the electrode whose elec－ パ ・なこ、賑、一￥ 一鰍艦←胴 trode su㎡ace is spread isthatthe additive ofthe fomer 雲800 、 、辱 、■ ヘ ▲ ヘ ヤ electrode e》dsts on the surface of the entire active ma－ ・、 Ca ）750む tedal while the additive of the latter electrode exists only ’δ700 ￥、 ご 亀￥Co on the surface of the positive electrode． 含650 Q non＿coated ＼ Table2shows the high－temperature charge character－ ￥、 ⇔oo 600 istics of the spread nickel positive electrode．The high－ 旨 Charge＝05C×2。4h ￥ temperature charge characteristics are not improved． 台 550 ．曽 Discharge：0．5C，E．V．＝1．OV，250C、 This result indicates that the additive on the positive o 500 electrode surface cannot influence the high－temperature 450 charge characteristics and that it is important to coat the surface of the entire positive electrode active material 20 25 30 35 40 45 50 55 60 65

with an additive for the improvement．The most impor－ Charge Temperat皿e（。C） tant point is that an additive should be dispersed in the electro（le．This dispersion can be easily realized by mix一 Fig．8 Efiect of hydroxide coating on high－temperature charge ef五ciency of AA cells．

Table2 High－temperature charge characteristics of the spread electrodes． Table3 Charge characteristics ofAA cells at60℃after200 cycles． Discharge capacity（％） Additives 25℃ 40℃ 45℃ 5Q℃ 60℃ C・・…g面露鷹。悪翻） 』aner ZO｛）cycle（mAh） beIore cyCle teSt（mA切 Yttrium oxide 100 93 86 77 66 Y（OH）2 754 774 Calcium o】dde 100 92 86 79 69 Ca（OH）2 751 757 Cobalt hydroxide 100 93 87 80 69 Charge：0．5C×2．4h（120％） 60℃ No additive 100 91 84 76 67 Discharge：0。5C，E．V．＝1．O V 25℃ 690 EJ60f名06h6”zゑsr勿y

trium hydro》dde and calcium hydro》dde． This improvement method will be able to contribute to the development of high temperature and power appli－ cations using alkaline storage baUleries．

H20 References OH一 Coated 1）M．Nogami，M．Tadokoro，M．Kimoto，Y．Chikano，T．Ise， Ni（OH）・ layer and N，Furukawa，Z）6励づ陥g罐％（presently EJo6吻6h6吻一 づs耽ソ），61，1088（1993） （in Japanese）． NioOHo 2）K．Shinyama，R．Maeda，Y．Matsuura，1．Yonezu，and K． Sinteri盤9 body Nishio，65云h、4”η襯J Moof勉gげ≠h6EJε6ケ06ho”zづαzJ Sooか 6砂夢ノ41り‘zκ，14δs∫7．，P．162（1998） （in Japanese）． 3）M．Oshitani，M．Yamane，and S．Hattori，∫．Poωθ7So％名6εs， 8，471 （1980）． Fig．9Fmction of the surface coating materia1． 4）M。Oshitani，Y．Sasaki，and K．Takashima，1．Po麗7 so既8s，12，219（1984）． hydro】dde active material is maintained to hold the cur－ 5）D．M．Constantin，E．M．Rus，L．Oniciu，and L．Ghergari， rent collectiveness．This time we coated the surface of 1．Pozθ67Soz6〆68s，74，188（1998）． the nickel hydroxide active material with yttrium hydrox－ 6）E．」．casey，A R．Dubois，P．E．Lake，and w．z．Moroz，1． ide and calcium hydro》dde as in Fig．9，resulting in the EJ80ケooh6吻．So6．，112，371 （1965）． increase in the oxygen overvoltHge． 7）M．E．Folquer，J．R．Vilche，and A J．Arvia，1．EJθo加α％α」． C物伽．，172，235（1984）． 4 Conclusion 8）D．F．Pickett and J．T．Maloy，1．．EJθo加oh翻．Soo．，125， In order to improve the charging ef且ciency at high 1026（1978）． temperature of sintered nickel positive electrodes for al－ 9）H．Nakahara，H．Sasaki，T．Murata，and M．Yamachi，OS kaline storage battehes，we investigated the effect of hy－ ハ珍zos7セ6h．1～61》．，57（1），26 （1998） （in Japanese）． droxide coating ofsintered nickel posidve electrodes．We 10）H．Matsuda and M．Ikoma，Z）召癩働即肋（presently conclude EJ66ケ06h8〃2ゑs耽y），65，96（1997） （in Japanese）． 1．The high－temperature charge ef且ciency of a sintered 11）A．Yuan，S．Cheng，J．Zhang，and C．Cao，／．Poω67 So既6s，76，36（1998）． nickel positive electrode is greatly improved when it is 12）C．Shaoan，Y．Anbao，L．Hong，Z．Jianqing，and C． coated with yttrium hydroxide and calcium hydroxide． Chunan，∫．Po側θ7Soz‘κ6s，76，215（1998）． 2．The oxygen overvol田ge is increased by using the sin－ 13）K．Ohta，K．Hayashi，H．Matsuda，and Y．Toyoguchi， tered nickel positive electrode in which the active ma－ 186孟h ／1κπ％α」 ハ48α勉9 の「 云h6 EJ80ケoohθ〃z∫αzJ So6げθ砂， terial surface，which is considered to be the active oxy－ ノ1δsオ7．，p．98（1994）． gen evolution site，is almost completely coated with yt一