61，NQ。7（1993） 729

一Communication一

Charge／Discharge Behavior of Metal Hydride Mixed with Conduetive Bin〔ler for the Anode of Ni－MH Batteries．Influences of Conduetive Bin〔ler on Diseharge Charaeteristies of Metal Hydride Eleetrodes

Masamichi YAMASHITA＊，Hayato HIGUC田， Hideaki TAKEMURA and Kazuyoshi OKUNO†

Received February15，1993，Accepted March22，1993

11NTRODUCTION composition ratio is class面ed CB1，CB2and CB3 In recent years，nicke1＿metal hydride（MH） as shown in Table14）．Moreover，the electrode batteries have attracted great attention as clean， was covered with another nickel mesh in order to high capacity and long life secondary batteries， prevent the disintegration．MH electrode was and have been widely investigated．MH powder tested in an acrylic ce1L A Ni（OH）2counter electrodes are suitable fbr Ni－MH batteries in elecもrode and a Hg圧｛gO reference electrode were order to show good discharge capability and high used．Cycle tests were carried out at25。C in8M capacity1）．It’s very important for MH powder KOH electrolyte solution by repeating charge at electrodes to select proper binder。When PTFE 50mA／g for6h，rest fbr10minutes and discharge mixed with acetylene black in a special machine at50mA！g歓｝一〇，7Vus，Hg／HgO， is produced a new binder materials becoming a fiber form2）．This new composite material has Table l Composition ofCB（Conductive binder）4）。 been called CB（Conductive Binder）． Adding some surfactants to the CB fbature remarkable PTFE Acetylene black Surfacねnt wettability of the IBA（International Battery CB1 34％ 66％ 0％ Material Association）MH electrodes， CB2 33％ 66％ 1％ CB3 32％ 64％ 4％ 2 EXPERIMENTAL MH alloy MmNi3．55C（》o、75Mno，4Alo．33）as re－ The discharge characteristics of MH elec－ ceived IBA MH sample No．1（MH1）was mecha－ trodes have been studied with cyclic voltammetry nically pulverized to250～170mesh（63～88μm）， （CV）and frequency response analyzer（FRA）AC Then少MH powder100mg was mixed with CB50 impedance method．The impedance spectra at mg and pressed on either side ofa nickel mesh as each discharge depth were measured in frequency a current collector（10mmφ）．CB is composed of region from10mHz to10kHz and AC amplitude PTFE，acetylene black and su㎡actαnt，and its of10mV．

Department of Industrial Chemistry，Faculty of Engineer、 3 RESULTS AND DISCUSSlON ing，Doshisha University（Karasuma－Imadegawa， Figure l shows the discharge capacity os， Kami師o－ku，Kyoto602，Japan）。 †Okuno Chemical Industries Co．，Ltd．（1－10－25， cycle number釦r MHI electrodes mixed with Hanaten－Higashi，Tsurumi－ku，Osaka538，Japan）・ various CB。Pressure ofthe prepared electrode is KεンWor幽：Conductive Binder，Meね1Hydride Electro4es， 30kg／cm2．CB3electrode showed a maximum FRA AC Impedance Spectra，Discharge Characteristics capacity of240mAh／g，while those of both the 730 DENKI KAGAKU

CBI and CB2were160mA／g．Figure2shows the assigned to the reaction resistance and double cyclic voltammograms of Ni mesh and MHl layer capacitance，respectively． electrode mixed with various CB a乱er30cycles． Figure4shows the Cole－Cole plots of MHl The anodic peaks，which are assigned to the electrode mixed with various CB at di£ferent oxi（la七ion of atomic H，with the exception of Ni discharge depths after 30cycles，and Table 2 mesh。CB3electrode had the highest anodic peak shows the fitting parameters of impedance inthoseofthreeCBs，resultinginMHelectrode spectra、The R3values of the CB3electrode were mixed with CB3absorbed the largesもamount of found tobe considerably smaller than those of七he hydrogen． Further，the sharp peak of the CB3 other electrodes and the C3values increased one electrode indicated reversibility causedも（》addi－ orderofmagnitude． tion ofsurfゑctant，and the broad peak ofthe CBl The（legree ofθas shown in Fig．3b is at－ was irreversibility，respectively． tributable to distribution of relaxation timeτ（τ Figure3shows the e級uivalent circuit and the ニC3×θ）5），i．e．，θ＞O deno匂es tha，t heterogeneous Cole＿Cole plot for the MH electrodes．RI was charge（listribution may be occurred due to the assigned to the electrolyte resistance。The semi－ rQughness of sur£ace， The resultingθvalues of circle in the range of high frequency，represented the CB3electrode were smaller than those ofthe by R2and C2，was varied with pressure of the other electrodes as shown in Table2．CBI causes prepared electrode and it was constant with the decrease of reaction surface area． D置usion discharge depth。R2and C2were assigned to the rate Qf the atQmic H through the MH electrodes， contact resistance and capacitance of pellet， estimatedfromWarburgimpedanceW，increased respectively。The semicircle in the range of low under the condition oflower amount ofsu血ctant frequency，represented by R3and C3，was varied duetodecreasingthediffusioadistancefrom with degree of discharge depth．R3an（l C3were alloysur飴cetoreactioninter£ace．

0 0 0 0 15幅 ハ鯉 0 ズ＼ 茗2。・

『 あ ／漕㌧c鴎 ボ O 『6 鱒 匿 8 じ 慧 ・ ＼、∠！CB2 100 1ノ ’ 、 畠 彗 一〇一CB1 ノ1．CBl り ・夕 十 魯 じ B2 よ！ ・一 ・一 、望 一ン／

一・ ト・一CB3 o 伊 ／ 0 0 10 20 30 Ni mesh 十 Cycle number

Fig。1 Discharge capacities Ps，cycle number for MHl electrode mixed witb various CB． 一tO －O．5 0 Potentia1（Vvs．Hg／HgO） Fig，2 CyclicvoltammogramsofNimeshandMHl elecもrQde mixed with variG“s CB after the30th cycle discharge． 61，No．7（1993） 731

C2 C3 Table2 ParametersofCole－Cole plots（after a） Fig．4）fbr MHI electrode mixed with various CB at d欝erent《1ischarge depth， R1 R2 R3 W Di8charge depth（％） b） 0 1m W 60～70 90～95 R3 CBl 1．68 1．04 1．56 CB2 （Ω） 1．76 1．05 1。51 CB3 0，68 0．44 0．88

㌧ C3 CBl 23，8 24．2 64．5 8 Re CB2 28．7 30．2 66，4 （mF） CB3 186 228 453 R1 R2 R3 0 CBl 7．5 13．0 22．0 CB2 7．5 （。） 10．0 19．0 CB3 6．5 6．5 13．5 Fig。3 （a） An equivalent circuit and （b） a schematic figure of Cole－Cole plot for the MH electrodes． 4 CONCLUSION The results indicate the MH electrode mixe（l with CB3has best discharge characteristics of all because of the presence of a suitable amount of CB3 d the surfactant。The wettability，contact resist－ ance and capacitance ofthe MH electrodes mixed C with CB are important factors aff㏄ting high rate b a dischargeperfbrmance． CB2 d REFERENCES C 1）T．Sakai，K．Muta，H．Miyamura，N。Kuriyam駄

b a and H．Ishikawa，“HYDROGEN STORAGE MATERIALS，BATTERIES，AND ELECTRO・ CB1 CHEM1STRY”，The Electrochem．Soc，Inc。，p．59 （，書．0 d α （1992）． C 2）A。Kozawa，M．Yoshio，H．Noguchi，G，Piao and 一〇．5 5 A。Uchiyama，ProgreεS勉Bα漉r‘2s＆Bα∫陀ry b a Mα‘εrめ1s，11，235（1992）． ロ ロさ でド まロら ロ 3）JEC Battery Newsletter No．3（May－June）シ28 Re （Ω） （1992）． 4）JEC Battery Newsletter No．3（May－June），25 （1992）． Fig．4 Cole－Cole plots of MHI electrode mixed 5）T。Hirai and T．Ohzuku，“Complex Plane with various CB atdif箪erentdischarge depths． Analysis fbr the Electrochemical Studies’，The Discharge depth a： 0％ b160～70％ U．S．O伍ceoftheJapaneseTheElectrochem、 c＝ 90～959ら d： ～100％ Soc．Inc．，p。13（1981）。