Hung K46 LiMn2O4

http://www.ebook.edu.vn 1

MC LC

MC LC 1 Li cm n 3 M u 4 Phn I. Tng quan v phng php nghin cu 5 I. i cng ngun in Lithium 5 I.1. Pin Lithum 10 I.2. c quy Lithum 12 I.3. c quy ion Li+ 14 II. Vt liu ci 15 II.1. Gii thiu chung 15 II.2. C ch ci ion 18 II.3. Vt liu spinel LiMn2O4 20 III. Cc Phng php nghin cu 27 III.1. Phng php qut th vng CV(Cyclic voltammetry) 27

III.1.1. Vi h thng thun nghch 28

III.1.2. Vi h thng bt thun nghch 29 III.2. Phng php o ph tng tr 29 III.2.1. Nguyn l ca ph tng tr 30 III.2.2. Mch tng ng ca ph tng tr 31 III.2.3. Biu din ph tng tr trn mt phng phc 33 III.3. Phng php o ng cong phng in 36 III.1.1. Phng vi dng khng i 36 III.1.2. Phng vi in tr khng i 37

Phn II. Thc nghim v tho lun kt qu 38


I. Qut th tun hon CV 38 I.1. Qut trong dung dch LiClO4 38 I.2. Tnh ton h s khuch tn 43 I.3. Qut th tun hon CV trong dung dch KClO4 44 II. o ph tng tr 45

II.1. o tng tr trong dung dch LiClO4 45 II.2. o tng tr trong dung dch KClO4 55

III. o ng cong phng np 61

Phn III. Kt lun 65


Li cm n

Sau mt thi gian thc hin nghin cu Tt nghip ti phng th nghim b mn Cng ngh in ho & Bo v kim loi n ca em c hon thnh. Trong qu trnh nghin cu, em nhn c s gip tn tnh ca cc thy c, cc cn b phng th nghim v cc bn b ng nghip. Em nhn c mi iu kin tt nht thc hin c n ny. ng thi vic hon thnh n cng nh s gip ca cn b Vin Khoa hc v Cng ngh Vit Nam. c bit l s gip tn tnh ca c gio hng dn TS. Phm Th Hnh em c th hon thnh n ny. Mc d ht sc c gng, nhng do thi gian c hn, trnh hn ch v mt s nguyn nhn khch quan khc nn n ca em khng th trnh khi nhng thiu st. Em mong c s nh gi, nhn xt, gp ca cc thy c v cc bn kin thc ca em c hon thin hn. Em xin chn thnh cm n ! H ni ngy 25 thng 5 nm 2006 Sinh vin thc hin Trn Vn Hng


M u Trong thp k gn y, ngun in Lithium l mt trong nhng ti c

nhc n hng u trong cc ti liu khoa hc v thc nghim.

l do nhng tnh cht u vit v kh nng ng dng cao ca n trong tng

lai nh kh nng tch tr nng lng ln, thn thin vi mi trng, ion Li+ c

kch thc ion nh nn c th s dng trong cng ngh ch to vt liu ci v.v. Vt liu ci spinel LiMn2O4 ang ngy cng c ch a vo ch to

Pin ion Li+ cao cp v c tnh k thut kh thun li nh gi thnh r, khng c

hi v c kh nng phng np hng trm chu k. c mt s n nghin cu v vt liu in cc LiMn2O4 nhng ch yu

ch tp trung vo iu ch vt liu in cc m cha c iu kin nghin cu t m

v tnh in ho ca vt liu. Mc ch ca n ny l nghin cu hnh vi in ho ca vt liu in cc

LiMn2O4 trong dung dch in ly LiClO4.


Phn I

Tng quan v phng php nghin cu

I. i cng ngun in Lithium Lithium l mt kim loi rt nh c khi lng ring l 0.543g/cm3 (nh ch

bng mt na khi lng ca nc), c th in cc chun rt m, oLi/Li+ = -3.04

V so vi NHE. V vy n ng u v hot tnh in ho (d nhng e tr thnh Li+). L vt liu anot, Lithium vi dung lng tch tr nng lng thuc loi cao

nht c 3860 Ah/kg hn hn cc vt liu anot quen thuc (nh ch Pb ~

260Ah/kg; Ag ~500Ah/kg; Cd ~ 480Ah/kg v Zn ~ 820 Ah/kg). Mc d vi tnh cht u vit nh vy, song do hot tnh in ho qu mnh lit nn Li rt d b

oxi ho trong khng kh, phn ng vi rt nhiu hp cht v c v hu c, bng

chy khi gp nc. Ngun in Lithium c bt u nghin cu vo nhng nm

60 ca th k ny, song trnh cng ngh trc y cha iu kin ch ng hot tnh in cc mnh lit ny (lm vic vi Lithium phi khng ch m

< 0.005% ). Bng 1. Mt s tnh cht ca Lithium

Nnglng ion

ha (kJ/mol) K

hiu

I1 I2

Rkl

Rion

Khi

lng

ring

g/cm3

tonc

C tos

C Eo Li

+/Li

V

Li 521 7300 1.519 0.6 0.534 179 1370 - 3.04


Khi bt tay vo nghin cu ngun in lithium trong nhng nm 60 ngi ta

phi i din ngay vi nhng c th v vt liu v k thut m trc khng

mt loi ngun in no gp phi l :

Mi trng in ly lm vic khng c l nc. Vic tm ra h in ly Propylene cacbonat do W.Harris (1958) c th xem nh l mt mc quan

trng, nh du s khc phc nhng tr ngi u tin c th nghin cu

mt cch h thng ngun in Lithium.

Vn an ton ca vt liu anot Lithium v la chn cc vt liu catot thch hp ghp vi Lithium.

Cng ngh ch to ngun in Lithium i hi phi xut pht t nguyn l tch tr v kt cu ch to hon ton mi.

Bn cnh , c quy khi Litthium c a vo s dng cng gp phi nhng

vn cn c khc phc v gii quyt nh:

- Cc mm Li u tin thng b th ng ho do phn ng vi dung dch in

ly to thnh lp b mt cch in vi anot nn, lm cn tr hot ng ca anot

mc d vn dn ion qua lp th ng.

- Trong mt s iu kin lm vic khng kim sot (v d dng phng in qu

mnh, qu np hoc sau nhiu chu k) lp b mt tr nn xp v c th xy ra

phn ng mnh lit gia mm Li mi hnh thnh vi dung mi di dng ta

nhit cc b, nhit ny c th ln hn nhit nng chy ca Li (180o) lm bc la

gy ra hin tng chy n.

- Qu trnh np kt ta in ho, Li kim loi to thnh trn nn anot b th ng,

kh thu c dng bng phng, ngc li thng pht trin g gh thnh dng

hnh cy. Hu qu l d gy ra hin tng chp mch gia vt liu anot v catot.

S chp mch thng dn n pht nhit, bc chy ph hu c quy.

Nhng tn ti trn y gii thch ti sao c quy Lithium cha c thng

mi ho mnh m mc d vi nhng thng s k thut kh u vit so vi cc loi

c quy khc. L do v tui th lm vic cn thp v quan trng hn l an ton


cho ngi s dng cha cao. Chnh v th mun pht trin ngun in Lithium

cn phi khc phc nhng khuyt im ny.

Trong ngun in c in, h in ly l mt phn quan trng ca t bo in

ho, quyt nh s vn chuyn ca ion v ti in bn trong c quy. Mi trng

in ly thng s dng gm dung mi nc v cht in ly (axit, baz, mui).

Nc l dung mi l tng nht v c hng s in mi cao ( = 78,4) v nht

thp ( = 0.890 cP). Nh vy, n thch hp cho vic dn ion trong dung dch v

ho tan tt cc cht in ly. Nhng trong ngun in Lithium v ngun in ion

Li+ th mi trng nc buc phi loi b v cc l do ni trn.

Ngoi ra, nu dng ngun in ion Li+ th anot khng cn l Li kim loi na

m thay vo l vt liu ci anot (VD: LixC6) th kh nng mt an ton ca vt

liu anot l c loi b. Nhng vt liu catot LiMxOy (trong M: Mn, Ni, Co)

hoc V2O5, MoS2 u c in th lm vic 3V, ln hn in th phn hu

ca nc ( 1,23V).

Vic ci tin dung mi cho ngun in Lithium nhm gii quyt nhng kh

khn m ngun ny gp phi. Trc ht dung mi phi l c cc, khng phn ly

proton, nhng li phi c thng s l ha gn ging vi nc. l phi c

mmen lng cc cao, c hng s in mi ln, c nht nh v khi lng

ring thp. Cc thng s ny quan trng v n lin quan n kh nng tan c

ca cc mui dn v nh vy ci thin c dn ca dung mi hu c vn rt

km. Ngoi ra, cc dung mi ny phi lm vic bn theo thi gian, tr vi vt liu

in cc (Li v cc vt liu khc), khng c lm gim linh ng ca ion Li+.

S vn chuyn ca Li+ trong dung mi ny phi t t+ 1,0. Trong thc t, rt

kh c mt dung mi no p ng y cc thng s c bn nh trn. Cc dung

mi c kho st nhiu nht c ngha thc t bao gm: Propylencacbonat

(PC), ethylencacbonat (EC), tetrahydrofuran (THF), methylentetrahydrofuran (2-

MeTHF), - Butyrolacton (-BL), dimethoxyethan (DME), diethyl ether (DEE), Dimethylsunfoxid (DMSO)


Bng 2. Mt vi thng s ho l ca mt s dung

mi hu c v nc

S pht trin ca ngun in Lithium vo nhng nm 70 v 80 din ra vi tc

mnh m trn c s nhng thnh tu v vt liu mi v cng ngh mi. Nhng

sn phm thng mi ho u tin c th trng chp nhn v tnh nng u

vit ca sn phm. l cc lnh vc qun s, in t dn dng v bu chnh

vin thng.

V pin c h Li/SOCl2,lng (do hng SAFT - M sn xut nm 1970); h Li/SO2

lng (cng do hng SAFT - M v Php sn xut nhng nm 80)cc pin Li/catot

lng ni trn c nhu cu s dng ln trong qun s.

Dung mi

Tkttinh

(oC)

T si

(oC)

Hng s

in

mi,

nht,

cP

M men

lng

cc,

debye

Khi

lng

ring,

g/cm3

EC

( 40oC) 39 40 248 89.6 1.58 4.8 1.332

PC -49,0 241 64.4 2.53 5.21 1.19

-BL -43,0 202 39.1 1.75 4.21 1.13

THF -105.8 66 7.59 0.46 1.71 0.88

2-MeTHF -137,0 80 6.24 0.467 - 0.848

H2O 0 100 78,4 0,89 1,86

1,0


Pht trin mun hn l cc h pin phc v cho nhu cu dn dng. Nht l nc

i u trong lnh vc ny vi cc sn phm Li/CFx (hng Matsushita); Li/MnO2

(hng Sanyo); ngoi ra cn c Li/CuO(SAFT- Php),sn lng ln ti nhiu

triu n v ngun in phc v cho cc sn phm vi in t dn dng.

Vic pht trin c quy Lithium (theo nguyn l ca mt ngun in np li

c) gp nhiu kh khn hn nhiu v l do chnh l k thut an ton cho vt

liu anot Li. Cng ngh ch to hin nay mi dng dng c quy kch thc nh

(AA hoc R6) hoc dng khuy vi cng sut c miliwatt (mW). in hnh nh

Li/MnO2 (do Sony Nht hoc Moli Canada ch to); Li/V2O5 (do SAFT

Php ) hoc Li/MoS2 (do Moli - Canada).

Cng vi vic pht trin mnh m ca ngun in Lithium l vic pht hin ra

h vt liu ci (intercalation materials) vo khong nhng nm 70. l hp cht

v c dng oxit hoc oxit phc hp cng nh h Chalcogenit c cu trc cha l

hng hoc xen lp. Nh vy nhng ion c kch thc nh nh Li+ c th khuch

tn vo khung cu trc to thnh hp cht ci. Vt liu ci m ra mt trin

vng ch to ngun in Lithium mi, l c quy ion Lithium. Kim loi qu

hot ng l Li c thay th bng vt liu ci ion Li+, chng hn nh LixCoO2,

LixNiO2 v LixMn2O4. Nh vy cu to ca c quy ion Li+ by gi k c anot ln

catot u l vt liu ci. Khi c quy lm vic, Li+ thot ra t bn cc ny li c

tch vo bn cc kia. Hot ng tch/thot ca ion Li+ chnh l qu trnh phng /

np ca ion Li+.

c quy ion Li+ c in p lm vic cao 3,6V 3,8V; nng lng ring

khong 90 120Wh/kg, nhanh chng c th trng k thut cao n nhn. Vi tr lng khong cha Lithium kh ln cng vi nhu cu tiu th ngy

cng tng v ngun in Lithium hin nay ca th gii nn xu th nghin cu v

ngun in Lithium s cn tip tc gia tng trong th k 21. I.1. Pin Lithium a. Cu to pin Li/MnO2


- Anot l kim loi Li hoc hp kim Li-Al (0.052% Al) - Catot l vt liu oxit kim loi chuyn tip hoc oxit phc hp c cu trc ci

nh MnO2, V2O5, V6O13, CrOx hoc chalcogenua nh MoS2.

H in dch c nht thp v hng s in mi cao. V d hn hp ca dung

mi propylencacbonat (PC) + dimethoxyethan (DME) v h in ly LiClO4 hoc h in ly LiClO4+ LiCF3SO3.

S tng qut:

(-) Li (Al) / PC + DME + LiClO4 1M / MnO2 (C) (+)

Hoc

(-) Li(Al) /PC + DME +LiClO4 0.5M +LiCF3SO3 0.5M/MnO2 (+) b. Qu trnh in cc ca pin Li/MnO2

Trn anot xy ra phn ng:

Li Li+ + 1e-

Phn ng kh trn catot in hnh cho qu trnh ci ion Li+ vo cu trc ch

ca vt liu catot:

x Li+ + MnO2 + x e- LixMnO2

Vt liu ci ion l nhng h vt liu rn c cu trc hng hoc xen lp, khi Li+

thm nhp vo (gi l ion khch) th c trung ho v in tch bi electron v

nh v vo cc v tr rng ca cu trc ch, to thnh hp cht khch- ch theo

s khi qut sau:

x Li+ + MA2 + x e- LixMA2

Trong trng hp lm vic ca pin, ngi ta ch quan tm n qu trnh ci ion

ch khng quan tm n chiu ngc li l kh ci. Chiu ci tng ng vi qu

trnh phng in catot.


lg|i| (mA/cm2)

1,0 1 2

0,5

0

0,5 1,0 1.5 2,0 2,5 3,0 3,5 4,0 E(V)

-0.5

-1,0

Hnh 1. ng cong phng in ca pin Li/MnO2 trong mi trng

in ly PC/LiClO4 (1-Li; 2-MnO2)

T th hnh 1 cho thy qu trnh phng in anot Li gn nh khng b c

ch, cn qu trnh catot b gii hn bi hiu ng ci Li+ vo khung rn ca MnO2.

Cu to ca pin Li/MnO2 c mt trn th trng hin nay thng c dng hnh

tr (loi CR 15400, ng knh 15 mm, cao 40 mm). in cc c b tr dng

cun lp xen k ln lt l anot hp kim Li(Al) dng l / l cch / vt liu catot

MnO2 / l cch tm ca hnh tr l ng cha cht in ly. Ton b c

ng kn trong v bc thp c van an ton. Ngoi cng l v bc kim loi. Ngoi

ra cn c cu to dng khuy. Ngoi vic s dng trong qun s v dn dng, pin Lithium cn c ng dng

rt rng ri trong trong y t nh l nhng thit b cy ghp vo c th ngi. V

d my to nhp tim, my kh hin tng ng si mu theo nguyn l sc in,

my kch hot thn kinh


I.2. c quy Lithium Cc ngun in ho hc nhn chung v nguyn tc c th ch to dng np

li c tn dng v tit kim nguyn vt liu. Mt khc hin nay nhng loi

pin s dng 1 ln vn cha nhng hp cht c hi nh hng vi mi trng

xung quanh. Xu th ca th gii hin nay l bin pin s dng mt ln thnh

ngun np li c. S thnh cng trong h in dch kim Zn/ MnO2 thc

y s pht trin ca c quy Lithium h tng t. in hnh nh h Li/MnO2

hoc LiTiS2, trong : Anot l Li, ngun cung cp e. Catot l oxit hay

chalcoghenit- k hiu khi qut l MX2 (M = Mn, Ti, ; X = O; S ), c tng

hp c cu trc c bit (l trng ng hm hoc xen lp). Nh vy ion Li+ c

th vn chuyn vo/ ra mt cch d dng.

Qu trnh phng / np ca c quy Li/MX2 nh sau:

pphhnngg Anot : x Li x Li+ + x e-

np phng Catot : x Li+ + MX2 + x e

- np LixMX2

Phng trnh tng: phng x Li + MX2 np LixMX2

Bn cht ca qu trnh phng np xy ra anot Li l qu trnh ho tan / kt ta

in ho, cn catot MX2 l qu trnh ci kh ci ion Li+ vo cu trc ch MX2,

hay cn gi l qu trnh tch/thot ion. in t trao i t anot qua mch ngoi

sang catot trung ho - bin i ho tr ca ion Mn+ trong cu trc trong khi ion

Li+ vo/ra. D thy rng ng hc ca qu trnh tng ph thuc vo qu trnh

catot. V vy vic tng hp vt liu catot theo kiu thit k cu trc c tm quan

trng c bit v to ra mt h cc vt liu ci (intercalation compounds) chuyn

dng. Sau y l bng thng k mt s c quy Lithium, kch thc nh (AA hoc

R6 ) c thng mi ho.


Bng 3. Cc thng s k thut c quy Lithium loi (AA, R6)

c so snh vi c quy Ni/Cd cng loi

Mt nng lng H pin in

th(V)

C(mAh)

Wh/AQ Wh/kg Wh/l

Chu k

Li/MnO2 2,80 700 1,96 115 240 >200

Li/ MnS2 1,80 600 1,08 54 140 >300

LiTiS2 2,15 1050 2,25 130 290 >300

Li/NbSe3 1,95 1100 2,15 90 265 >200

Li/V2O5 3,40 1400 4,76 100 175 >100

Li/CuCl2/SO2 3,40 500 1,76 95 210 >100

Li/SO2 3,00 500 1,65 90 200 >100

Ni/Cd 1,20 850 1,02 45 130 >500

T cc s liu bng 3 cho thy c quy Lithium ni chung hn hn v in th

lm vic v mt tch tr nng lng. Ring s chu k phng/ np cn kh thp

so vi c quy Ni/Cd. iu ny lin quan n n nh v an ton lm vic ca

c quy Lithium. S phc tp tp trung anot do s dng kim loi Li kh kim

sot c hot ng an ton. I.3. c quy ion Li+

c quy ion Li+ l xu th pht trin chnh hin nay trong lnh vc ngun in Lithium np li c. Vi nhng c im l khng dng kim loi Li (hoc hp

kim) lm vt liu anot, lm ngun cung cp ion Li+ kiu Li Li+ + e-. Thay

vo l bng vt liu cacbon ci ion Li+, in hnh nh LixC6, cng hot ng

kiu tch thot ion Li+, nh vy c quy ion Li+ s lm vic mt cch an ton hn.


Nh vy cu to c bn ca mt c quy ion Li+ s gm 2 bn cc u l vt

liu ci: anot l vt liu LixC ni trn, cn catot l vt liu c cu trc xen lp

hoc l trng, in hnh l h LixMO2 (M : kim loi chuyn tip nh Co, Ni,

Mn). Hot ng phng / np ca in cc thc cht l qu trnh thot ion Li+

in cc ny ng thi qu trnh tch ion Li+ in cc kia theo kiu :

pphhnngg Anot : LixC6 x Li

+ + 6 C + x e-

np phng

Catot : x Li+ + MO2 + x e- np LixMO2

Phng trnh tng: phng LixC6 + MO2 np 6 C + LixMO2 Ngi ta v s vn chuyn lui ti ca ion Li+ khi phng / np trong mi trng

in ly Propylencacbonat + LiClO4 gia 2 bn cc i in nhau ging nh s

dao ng ca mt gh u. V vy nguyn l lm vic ny c tn l nguyn l gh

u (Rocking Chair).

Vic thay th vt liu anot kim loi bng cacbon hay graphit phi chu thit

thi v mt nng lng v dung lng tch tr l thuyt ca graphit ch bng 1/10

dung lng tch tr ca kim loi Lithium Li

graphit

CC

= 86.3372.0

v in th thp hn

(3 4 V). Song thay vo l s an ton v s chu k lm vic vt tri hn hn, i khi c th t n hng ngn chu k. II. Vt liu ci

II.1. Gii thiu chung

Vt liu ci l mt h vt liu c hnh thnh bng phng php tng hp

pha rn hoc cc phng php c bit khc. Trong c s thm nhp ca cc

tiu phn (ion, phn t) khch c kch thc nh, i vo mt hp cht rn


ch m trong cu trc mng li tn ti nhng v tr trng. C th minh ho

theo hnh di y:

tch

+

Khch Ch thot hp cht khch ch

Hnh 2. C ch ch to vt liu ci

Trong :

Ion hoc phn t khch;

V tr trng trong cu trc ch;

ch chiu vo / ra hay cn gi l chiu tch thot ca cc ion hay

phn t.

V nguyn tc s tch / thot ca cc tiu phn l khng t xy ra. V cc ion

hay phn t c kch thc ng k, hn na li mang in tch cho nn khi c

mt cc trng (v tr trng, ng hm xen lp) c th dn n tng tc ho

tr, lm thay i lin kt trong mng li, gy ra s nhiu lon. Tuy nhin, do c

th ca hp cht ci l qu trnh tch / thot ca cc ion vo mng li rn (cng

c th gi l khuch tn) din ra chm nn khng c s ph v cu trc. Do vy

qu trnh ci/ kh ci c th xem nh i qua mt lot cc trng thi cn bng.

Vt liu ci hu ht l oxit hoc hp cht ca kim loi chuyn tip c dng

tng qut c th l MX2 (M: kim loi chuyn tip ; X: O, S). Trong dng MO2

c tm quan trng hn chalcogenua MS2, v n d tng hp hn, c th tch phn

t nh hn v dung lng tch tr trn n v th tch cng cao hn. Trong qu

trnh in ho cc cation v anion ci / kh ci trong mng oxit to ra cc hp

cht bn vng nh LixTiO2, LixV2O5, LixMnO2


Bng 4. Mt s vt liu ci MO2 in hnh

MO2 Hp cht ci Cu trc

TiO2 LixTiO2 0< x 1 MO6 xp cht, knh rutil

VO2 LixVO2 0< x 1 MO6 xp cht, ng hm

MoO2 LixMoO2 0< x 1 MO6 xp cht, ng hm

MnO2 LixMnO2 0< x 1 MO6 xp cht, ng hm

RuO2 LixRuO2 0< x < 1 MO6 xp cht, ng hm

CrO2 LixCrO2 0< x < 0,2 MO6 xp cht, ng hm

CoO2 LixCoO2 0< x < 1 MO6 xp cht, xen lp

NiO2 LixNiO2 0< x


Hnh 3. Cu trc in hnh ca h MO2

a cu trc xen lp ca LiNiO2

b cu trc knh ca V2O5

c cu trc vt liu ci LiV2O5 II.2. C ch ci ion

mng c bn ca MO2 l mt mng bt din gm ion kim loi chuyn tip chim v tr trng bt din 1/2 bao quanh bi 6 ion O-2 xp cht (XC). V vy c m t bng mng oxit xp cht MO6(XC). Khi thc hin ci c cc qu trnh in ha xy ra gm :

- Electron i vo mng li tinh th dn n s oxy ha M4+ thnh M3+, bn knh ln ln song mng MO6(XC) vn c gi nguyn.

- Li+ ci vo nhng v tr trng bt din cn li, nh vy m khung cu trc khng b ph v.

Phn ng:

tch

Li+ + e_ + M O2 Li M O2

(oct)(oct)(xc) thot (oct) (oct) (xc) oct: v tr bt din xc : mng xp cht

Nguyn nhn hnh hc to nn mng xp cht MO6: Theo nh lut Gonsmit, tnh bn vng ca cu trc tinh th c th m t bng

hnh hc thun tu. Ta c th gi thit rng cu trc cha th coi l bn vng nht nu cc qu cu ion tri du cha tip gip nhau. Vi A l cation, X l anion trong mng cu trc, t s RA/RX l mt yu t hnh hc n thun quyt nh ti s bn vng ca mt cu trc tinh th.


Hnh 4. Cc dng sp xp t cu trc bn vng

Hnh 4 cho thy cc dng sp xp nguyn t t cu trc bn vng vi t s bn RA/RX cho tng trng hp ca phn t AX nh sau:

Kiu cu trc

A B C D

RA/RX 0,150,22 0,22 0,41 0,41 0,73 > 0,73

Xt tng quan kch thc ion trong mng oxy xp cht h MO6 cho thy:

kch thc ca ion kim loi chuyn tip M3+/ M4+ 0,8Ao/ 0,5 Ao cn ion O-2 1,4 Ao. Nh vy t s bn knh M/O trong lin kt phi tr bt din tha mn iu kin to mng oxit xp cht (t 0,41Ao n 0,71Ao). Cc ion kim loi chuyn tip c gi cht trong lin kt M-O v tr bt din. Ngc li cc ion Li+ khi

c ci vo, vi kch thc ion 0,9 Ao ( s phi tr l 6) v 0,73 Ao ( s phi tr l 4), bao quanh cc ion oxy chim cc v tr trng bt din cn li. Nh dao ng mng li v thng ging lin kt ca ion O2- do cc ion kim loi chuyn tip nhn in t, nn ion Li+ c th dch chuyn t v tr ny sang v tr


khc. Hn th, cc v tr trng ca MO2 c ni vi nhau thnh cc ng hm, knh. Nh vy s khuch tn v tch t cc ion Li+ c mng cht rn thc hin. H s khuch tn ca ion Li+(DLi+) trong mng rn ca vt liu ci c xc nh ty thuc vo ch qu in ho v h s ci x nm trong

khong 10-10 10-13 (cm2. s -1). II.3. Vt liu Spinel LiMn2O4

Trong thp k gn y, c nhiu cng trnh nghin cu v cc loi vt liu catot p ng nhng yu cu v ngun in c nng lng cao, khi lng nh. Trong cc vt liu c in p h mch cao nh LiCoO2, LiNiO2, LiMn2O4 c nghin cu rt nhiu. Mc d LiCoO2 c s dng rng ri lm vt liu catot cho acqui Lithium-ion thng mi, tuy nhin vt liu mangan oxit spinel LiMn2O4 ngy cng c ch v nhng c tnh k thut v thng mi kh thun li nh: gi thnh r, in th lm vic cao v mt iu rt quan trng i vi cuc sng hin i ngy nay l khng c vi mi trng. Bng 5 v hnh 5 cho bit mt vi thng s ca 3 loi vt liu trn.

Bng 5. Cc thng s, tnh cht ca cc loi vt liu catot

Tnh cht vt liu LiCoO2 LiNiO2 LiMn2O4

Cu trc tinh th Lc gic

a= 2,82 Ao

c= 14,05 Ao

Lc gic

a= 2,88 Ao

c= 14,18 Ao

Lp phng

a= 8,24 Ao

Dung lng ring l thuyt (mAh/g)

274 275 148

Dung lng ring thc t (mAh/g)

120 130 120 - 150 100 - 120


H s khuch tn (cm2/s) 10-7 - 10-9 10-7 - 10-9 10-9 - 10-11

dn (S/cm) 10-2 10-1 10-6

in th max so vi Li (np vi dng 1A)

4,2 4,1 4,4

in th lm vic

trung bnh 3,6 3,4 3,8

Mt nng lng (Wh/kg)

120 120 90

Hnh 5. in th lm vic v dung lng tch tr ca mt s vt liu catot h LiMO2 ghp vi anot Li2C6

Cu trc ca LiMn2O4 thuc h spinel AB2O4, nhm khng gian Fd3m. Nn tng l nguyn t oxi phn b theo qui lut xp cu lp phng (trng vi mng

Dung lng(mAh)

in

th(

V)

so v

i L

i


lp phng tm mt). Cc nguyn t oxi coi nh cc qu cu xp cht to ra cc v tr trng bt din v v tr trng t din. Cc v tr trng ny c th cho php cha cc ion bn knh ln nht bng gii hn di ca t s bn knh. Theo bn knh ca cc ion cho php vo cc v tr trng phi tha mn:

Rion 8 mt < 0.41. RO-2 = 0,41. 1,4 Ao = 0,5474 Ao

Rion 4 mt < 0.22. RO-2 = 0,22. 1,4 Ao = 0,308 Ao

Ion Li+ ch chim 1/8 v tr trng t din v ion Mn4+ chim 1/2 v tr trng bt din. Xt 2 lp chiu cu ln mt (100) a v b.

Hnh 6. Cu trc ca Li2Mn2O4 trn mt phng (001)

Hnh 6 a,b l 2 lp cu trc lin tc theo trc z. mng c s c th c trnh by di dng tp hp gm 4 lp nh th chng ln nhau. Trong mi lp, cc hnh 8 mt (cha Mn) lin kt qua cc cnh chung thnh nhng chui song song vi mt ng cho mng lp phng. Cc chui ny ti 2 lp k nhau li vung gc vi nhau. Nhng hnh 4 mt cha Li+ c vai tr lin kt cc hnh 8 mt vi nhau. Cc nguyn t Li trn mi lp a hay b li c cao khc nhau,


hoc thp hay cao hn nguyn t oxy (hnh d). Qui lut phn b ca chng ging cc nguyn t C trong kim cng v th vt liu ny cng c nhm i xng khng gian c trng bng mt nh trt Fd3m. V chim 1/8 v tr t din v 1/2 v tr trng bt din nn mng spinel c rt nhiu ch trng c th ci ion vo. Ti nhng v tr ny cc ion rt linh ng. iu kin lm vic ca in cc tu thuc vo phn cc m cc ion c kch thc nh nh Li+ c th vo/ra. y l bn cht tch / thot ca vt liu LiMn2O4. Nhng khng gian trng trong mng spinel LiMn2O4 c th sp xp thnh ng hm hay di m hnh xp cu (hnh 7):

Hnh 7. M hnh cu trc xp cu vt liu LiMn2O4.


C ch phng in ca vt liu ny c nhiu nh khoa hc cho l nh sau (hnh 8) :

Hnh 8. S c ch phng np ca pin LiMn2O4 Phng trnh phn ng:


np Li+ + e- + 2MnO2 LiMn2O4 phng

Tng qut vi acquy ion Li+ in cc m l Cacbon, ta c th vit:

np C6 + Li Mn2O4 LixC6 + Li1-xMn2O4 phng

C 2 phng php c bn iu ch vt liu LiMn2O4:

- Tng hp bng phn ng pha rn nhit cao. Khi ta nghin, trn hp cht ca Mn nh MnO2, MnO3, MnCO3... vi hp cht cha Li nh LiNO3,

LiOH, LiCO3... theo t l s mol nht nh ri nung nhit cao t 750 9000C trong nhiu gi. Qu trnh nung to s khuch tn cc vt liu vo nhau. y l phng php d lm, tit kim thi gian nhng thnh phn khng ng u.

- Tng hp bng phng php Sol-gel. Dng cc mui cha Mn v Li nh Mn(NO3)2, LiNO3, Li2CO3... khuy trn to sol. Sau a vo cc cht to gel nh PVA, PEG, PAA ri em sy kh, thiu kt v nung c sn phm. Phng php ny cho ta sn phm ng nht nhng qui trnh thng rt phc tp v cht ch.

Vt liu in cc LiMn2O4 dng trong nghin cu ca lun n c iu ch theo phng php Sol-gel. Vt liu ny c tng hp bi lun n trc, chp X-Ray khng nh. Tuy nhin cha c iu kin nghin cu v hnh vi in ho. Lun n ny l s tip tc kho st v loi vt liu LiMn2O4 trong dung dch in ly nc.


III. Phng php nghin cu

III.1. Phng php qut th tun hon CV(Cyclicvoltammetry) Phng php qut th tun hon xc nh h s khuch tn D v xem xt

s bin thin thun nghch (kh nng c th phng v np) ca vt liu nghin cu. in th y bin thin tuyn tnh theo thi gian.

Hnh 9. th qut th vng cyclicvoltametry

Bin thin in th theo thi gian c th xc nh theo cc cng thc sau: = v. khi 0 < <

= v.+v(-) khi > Trong : v Tc qut th 0,000 V/s 1000 V/s - Thi im i chiu qut th, s - Thi gian, s

- in th ban u (V) III.1.1. Vi h thng thun nghch

Khi qut CV cho b mt in cc nghin cu, th ph thuc ca in th v dng in c dng:

,s


Hnh 10. Quan h gia in th v dng in trong qut th vng

Dng cc i: ip,R = -2,69.105.n3/2.Do1/2.Co.v1/2 Trong : n - S in t tng i Do - H s khuych tn (cm2/s) Co - Nng ban u ca cht O (mol/cm3)

298K p,R p.2,R59 mVn

= (Khng ph thuc vo tc qut th) v p.Op.R

I1

I=

III.1.2. Vi h thng bt thun nghch Dng in cc i: Ip,R = -2,99.105n((1-)n)1/2ACoDo1/2v1/2

Trong : n - s in t trao i n - s in t trao i trong giai on khng ch - h s chuyn in tch (0 < < 1)

Ip,R

Ip,Oi


A - Din tch in cc (cm2) Do- H s khuych tn (cm2/s) Co - Nng ban u ca cht O (mol/cm3).

III.2. Phng php o ph tng tr Trong ch to v nghin cu vt liu, thng thng phi quan tm n nhiu thng s nh thnh phn ho hc, t chc pha, cu trc tinh th, cu trc di nng lng, cc tnh cht quang, t in xc nh hot tnh in ho ca vt liu, phng php tng tr l mt trong nhng k thut p ng c cc yu cu ni trn, nn khng th thiu c i vi mt phng th nghim in ho. Trung tm ca h thng nghin cu in ho l mt t bo in ho bao gm: in cc nghin cu M, trong M l mu vt liu m ta quan tm, in cc i Pt, in cc so snh calomel bo ho. in cc i Pt l kim loi tr ch lm nhim v dn in. Hai in cc M v Pt c nhng trong mt mi trng in ly (nc hoc khng nc) c dn , hai u ra c ni vi mch o. Khi ti b mt M xy ra mt phn ng in ho (qu trnh trao i in tch): M ne- < > Mn+ (1) io,ko phn ng in ho (1) phn nh hot tnh in ho ca vt liu. Cng ca hot tnh c o bng tc ca phn ng (1), i lng o c gi l mt dng trao i io, c hng s tc ko tng ng, Mun nghin cu ng hc phi bit c io v ko.

III.2.1. Nguyn l ca ph tng tr Khi ta cho mt dao ng bin nh xoay chiu hnh sin Uo, tn s gc =2f i qua mt h in ho cn nghin cu. Trong mch s xut hin mt dng in p ng hnh sin c bin Io cng tn s gc nhng lch pha mt gc


so vi in th a vo. V bin ca dao ng nh nn c th tuyn tnh ho cc phng trnh. Uo Bnh in Ut, It Ho Hnh 11. S khi m phng nguyn l o tng tr Trong = Uo sin t = Io sin ( t+ ) Theo nh lut Ohm c th nh ngha tng tr Z() nh sau: Z() = / = (Uo/ Io) exp(-j) = Z. exp(-j) Z() l mt hm s phc,c th biu din trong h to in cc bi bin

Z v gc pha , hoc trong ta cc:

Z() = ZRe + j.ZIm Trong ZRe l phn thc ca tng tr ZIm l phn o ca tng tr Ta c: Z2() = Z2Re + Z2Im ZRe = Z cos ZIm = Z sin tg () = (ZIm/ZRe) ZIm Z .cos Z( j ) Z .sin

ZRe


Hnh 12. Biu din hnh hc cc phn t phc

Tng tr Z() ca bnh in ho bao gm cc thnh phn nh: tng tr ca qu trnh Faraday Zf , in dung lp kp coi nh mt t in Cd v in tr dung dch R. K thut x l ton hc cho ta tnh c cc gi tr Cd, Rdd, Zf v cho cc thng s ng hc cui cng ca h in ho (io, ko, D), kt qu nhn c thng c biu din di dng th thng gi l ph (Nyquits, Bode).

III.2.2. Mch tng ng trong ph tng tr

S mch tng ng th hin hnh vi ca bnh in ho c th hin trn s mch:

Cd

R

Zf Hnh 13. Mch tng ng ca mt bnh in phn

R l in tr dung dch Zf l tng tr qu trnh Faraday Cd l in dung ca lp kp Xy dng th quan h -ZIm theo ZRe s thu c mt cung bn nguyt vi bn knh bng (Rp Rdd )/2. C th xc nh Rp, Rdd (R) bng cch ko di hnh bn nguyt ct vi trc honh ZRe. in dung Cd c th xc nh t Rp v gi tr tn s fmax ti im cc i trn vng bn nguyt i vi trc o ZIm.

Tng tr khuch tn Warburg, Zw


Zw l tng tr khuch tn hay cn gi l tng tr Warburg c cho bi: Zw = (1-j) -1/2

Trong : = oDCnF

RT2)( 0*2

l hng s Warburg

Mch tng ng ca tng tr Warburg gm mt t in mc ni tip nh hnh sau: Rw Cw Hnh 14. Mch tng ng tng tr khuch tn Warburg

Tng tr Randles Trong trng hp phn ng in cc b khng ch bi c giai on chuyn in tch v khuch tn ta c tng tr Ersler Randles:

ZRe = Rct + (1-j) dOx Re+

Trong :

Ox = oo DCnF

RT2)( 22 v dRe

= Rd DCnF

RT2)( *2

t = Ox + Red cui cng ta c : ZRe = Rct + (1-j) -1/2 S tng qut ca tr Randles c m t nh sau:


Clk Rdd Rct Hnh15. S tng ng ca bnh in phn

Trong in tr chuyn in tch Rct thng c xc nh bng cch ngoi suy tng tr tn s thp v ZIm 0 Rdd l gi tr ZRe ti tn s rt cao III.2.3. Biu din tng tr trn mt phng phc Nu h thng bnh in phn tho mn s Randles th tng tr bnh in phn s l:

Z = Rdd + ( )[ ] 12/1ct 1R.1

++ jCj Z = ZRe j.ZIm Tch phn thc vi phn o phng trnh tng tr bnh in phn trn, ta c :

ZRe = Rdd + 22/12222/1

2/1

)()1(

++++

ctdd

ct

RCCR

ZIm = 22/12222/1

2/1222/1

)()1()(

++++++

ctdd

dctd

RCCCRC

1. Khi tn s 0 th : ZRe = Rdd + Rct + -1/2 v ZIm = --1/2 2.2 Cd

W


Ph nhn c tu theo cch biu din s liu c 2 dng vi tn gi l ph Nyquits hoc ph Bode. ng biu din ZRe theo ZIm (ph Nyquits) s l ng thng vi dc bng 1 v ngoi suy s ct trc thc ZRe ti (Rdd + Rct - 22Cdd). ng thng ny tng ng vi khng ch khuch tn v tng tr Warburg c lch pha l /4. 2. Khi : tn s cao phn ng ch b khng ch ng hc v Rct >> Zw

ZRe = Rdd + 2221 ctdct

RCR

+

ZIm = 222

2

1 ctdctd

RCRC

+

-ZIm, Khng ch ng hc Khng ch khuch tn

max=(Rct.Cd)-1

Rdd Rdd +Rct/2 (Rdd + Rct) ZRe, Rdd +Rct- 22Cd

Hnh 15. Tng tr trn mt phng phc Cui cng ta c :


(ZRe - Rdd Rct/2)2 + (ZIm)2 = (Rct/2)2 chnh l biu thc ca vng trn bn knh Rct/2 ct trc ZRe ti Rdd khi v ti Rdd + Rct khi 0. Khi qu trnh in cc gm nhiu giai on th ta c th thy cc na vng trn lin tip xut hin. Khi c s hp ph cn thy na vng trn di trc ZRe. Khi 0 v khi c s th ng cn thy gi tr in tr m.

III.3. Phng php o ng cong phng in

xt kh nng phng np ca in cc LiMn2O4 ta dng cc phng php nh phng vi dng khng i, phng vi tr khng i

III.3.1. Phng vi dng khng i Cho vt liu phng vi dng khng i (I=const) v kho st s bin thin

ca in th theo thi gian cho n khi in th ca ngun in st n mt gi tr Ec nht nh. Khi thu c ng cong c dng hnh 17.

E(V)

Ed Ec 0 tc t(h)

Hnh 17. ng cong phng in E = f(t) vi dng khng i


Dung lng l i lng ch lng in m ngun in tch tr c tnh bng tch ca cng dng phng in I nhn vi thi gian phng tc . Dung lng ca in cc LiMn2O4 c xc nh theo cng thc: Q = I.tc mAh Hay dung lng ring: Qr = I.tc /m mAhg-1 Trong : I : dng in phng khng i, mA tc : thi gian phng in ti in th cui, h III.3.2. Phng vi in tr khng i y l phng php kho st s bin thin ca in th v dng in theo thi gian. Cc in cc c phng qua mt in tr c gi tr xc nh khng i. ng cong thu c c th l I = f (t) hay E = f (t). Khi dung lng ca vt liu c xc nh theo cng thc: Q = Itb . tc Hay Q = Etb . tc / R trong Itb , Etb l cc gi tr dng in v in th trung bnh c th xc nh t th in th thi gian. tc : thi gian phng in ti in th cui, h Trong phm vi n ta ch kho st vi phng vi dng khng i.


Phn II

Thc nghim v tho lun kt qu I. Qut th tun hon CV I.1. Qut trong dung dch LiClO4 - Mc ch kho st phn ng in ho xy ra trn mt in cc spinel LiMn2O4 trong dung dch LiClO4. - Cch tin hnh: Ta tin hnh qut CV cc tc qut khc nhau trong cng mt iu kin: Dung dch in ly LiClO4 1M Khong in th -1V 1V/SCE in cc i li Pt in cc so snh Calomel bo ho C sc kh tr N2 ui ht O2 ho tan trong dung dch. Ngm trong dung dch LiClO4 1M trc 3h - Kt qu v tho lun:

a. Tc qut th 50mV/s


Hnh 18. Ph qut CV in cc LiMn2O4 tc qut 50mV/s

b. Tc qut 10mV/s


0.25

-1.50E-02

-1.00E-02

-5.00E-03

0.00E+00

5.00E-03

1.00E-02

1.50E-02

2.00E-02

-1.5 -1 -0.5 0 0.5 1 1.5

E(V)/SCE

I(A

)

0.25

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

0.01

-1.5 -1 -0.5 0 0.5 1 1.5

I(A

)

E(V)/SCE


a. Tc qut 2mV/s

Hnh 20. Ph qut CV in cc LiMn2O4 tc qut 2mV/s d. Tc qut 1mV/s


-0.8 0.25 0.51

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

-1.5 -1 -0.5 0 0.5 1 1.5

E(V)/SCE

I(A

)

-0.8 0.25 0.51

-0.008

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

0.01

-1.5 -1 -0.5 0 0.5 1 1.5

E(V)/SCE

I(A

)


e. Tc qut 0.5mV/s

Hnh 22. Ph qut CV in cc LiMn2O4 tc qut 0.5mV/s Ta thy vi tc qut kh nhanh 50mV/s trn ph CV (hnh 18) cha hin r cc pic oxh/kh nhng bt u dn hnh thnh 1 pic oxy ho ti in th E = 0,25V/SCE. tc qut 10mV/s (hnh 19) pic ny th hin r nt hn tuy nhin vn cha xut hin pic kh ti tc ny. Khi gim tc qut xung 2mV/s (hnh 20) trn th ph CV bt u hnh thnh thm 2 pic kh cc in th E1K = 0,5V/SCE v E2K = -0,8V/SCE, nhng cc pic ny cn kh t. tc qut rt chm 1mV/s v 0,5mV/s (hnh 21, 22) ta thy cc pic oxh/kh th hin rt r nt. Cc pic oxh/kh ny tng ng vi qu trnh oxh/kh ca in cc LiMn2O4 v cng ng thi l qu trnh ci v kh ci Li+ trong mng tinh th spinel LiMn2O4. c 2 tc trn v tr cc pic l gn ging nhau v tc 0,5mV/s th cc pic oxh/kh r rng hn. iu ny chng t qu trnh ci v kh ci ca Li+ vo trong in cc spinel LiMn2O4 xy ra rt chm. V vy ch

A1 -0.8 0.5 0.15 C2 C1

-0.008

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

0.01

0.012

-1.5 -1 -0.5 0 0.5 1 1.5

E(V)/SCE

I(A

)


c th kho st tnh cht b mt ca n nhng tc chm. Tuy nhin do iu kin lm vic v thi gian c hn nn trong phm vi n ta ch xt n tc 0,5mV/s. Trn th qut CV(hnh 22) xut hin 3 pick gm 1 pick oxi ho EA = 0,15V/SCE v 2 pick kh E1K = -0,8V/ SCE v E2K = 0,5V/ SCE. Ti EA =0,15V/SCE xy ra qu trnh oxy ho Mn+3 Mn+4 ng thi din ra qu trnh kh ci Li+ khi b mt in cc : LixMn2O4 x Li+ + 2MnO2 + x e- (0 < x 1) Ti E1K = 0,5V/SCE xy ra qu trnh kh Mn+4 Mn+3 ng thi l qu trnh ci Li+ vo b mt in cc: x Li+ + 2MnO2 + x e- LixMn2O4 (0 < x 1) E2K = -0,8 V/SCE xy ra qu trnh kh Mn+3 Mn+2 ng thi l qu trnh tip tc ci Li+ vo b mt in cc : y Li+ + LixMn2O4 + y e- Lix+yMn2O4 (0 < y 1) Ta thy rng y vi pic kh tng ng vi qu trnh kh Mn+3 Mn+2 th dng nh khng c pick oxy ho tng ng Mn+2 Mn+3. iu c ngha l khng c qu trnh bin i theo chiu nghch t Mn+2 Mn+3, nn c th v th m lm cho dung lng ca in cc gim dn. C th ni ti E2K = -0,8V/SCE cc. T y cn ch rng trong qu trnh lm vic ca Pin ta phi quan tm n gi tr gii hn ca in th lm vic v khng c phng np pin vt qu vng in th gii hn ca n. T ng CV (hnh 22) ta thy t l cng gia pic C1 v pic A1 l : ip,C1 / ip,A1 0.7 nn c th d on phn ng in cc thun nghch khong 70%. Do phn ng oxh/kh ca in cc LiMn2O4 trong dung dch LiClO4 c th coi l qu trnh thun nghch. Chnh v vy c th s dng in cc LiMn2O4/ dung dch LiClO4 lm pin np li c


T y c th suy ra rng trong mng tinh th spinel LiMn2O4, ion Li+ nm 2 dang lin kt: - Lin kt bn cht ca Li+ trong cc hc t din hay bt din trng. - Lin kt km bn ca Li+ khng gian trng gia cc mt phng ca cc lp nguyn t. Trong Li+ ch yu tn ti khng gian trng gia cc mt phng ca cc lp nguyn t. n lin kt yu vi khung cu trc ca tinh th spinel LiMn2O4 nn c th ra vo d dng. I.2. Tnh ton h s khuch tn ca Li+ Qua php o ph CV ta c th xc nh c h s khuch tn ca Li+ trong dung dch n b mt in cc hoc ngc li t b mt in cc ra ngoi dung dch. p dng cng thc: ipC = - 2,69.105. n3/2.A. Do1/2. Co .v1/2 Trong : ipC : l cng dng in catot cc i, A n : s in t trao i A : din tch in cc , cm2

Do : h s khuch tn, cm2/s Co : nng ban u ca cht oxh, mol/cm3

v : tc qut th, V/s Ti pick C1: ipC1 = -5,5.10-3 A, n =1, A= 1,33cm2, ta c: -5,5.10-3 = -2,69. 105. 13/2. 1,33. Do1/2. 10-3. (0,5.10-3)1/2

suy ra Do 4,72.10-7 cm2/s. I.3. Qut th tun hon CV trong dung dch KClO4


Hnh 23. Ph qut CV in cc LiMn2O4 trong dung dch

KClO4 tc qut 0.5mV/s Ch qut : Dung dch in mi KClO4 0,05M Tc qut 0.5mV/s Khong in th -0,7V 1V /SCE in cc i li Pt in cc so snh Calomel bo ho C sc kh N2 ui kh O2 ha tan trong dung dch Ngm trong dung dch KClO4 trc 3h. Cng ging nh khi qut CV trong dung dch LiClO4, khi qut th CV trong dung dch KClO4 i hi phi qut tc rt chm. y ta kho st vi tc 0,5mV/s, thy rng cc pic khi qut trong 2 dung dch kh khc nhau. ng cong CV trong dung dch KClO4 (hnh 23) c pic oxy ho EA = 0,1V/SCE, c 2 pic kh E1K = -0,07V/SCE v E2K = 0,7V/SCE. Ta thy cng dng ti cc pic ny nh hn nhiu so vi khi o trong dung dch LiClO4 v hnh dng cc pic t

0.7 -0.07 0.1

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

-1 -0.5 0 0.5 1 1.5

I(A

)

E(V)/SCE


hn. iu ny c th gii thch l do trong dung dch khi thay th ion Li+ (c RLi+ = 0,6) bng ion K+ c bn knh nguyn t ln hn (RK+ = 1,96 ), nn khng th ci vo mng tinh th LiMn2O4 nh ion Li+. Chng t qu trnh oxh/kh ca LiMn2O4 trong dung dch KClO4 l khng thun nghch. V vy khng th s dng KClO4 lm pin th cp (np li c).

II. o ph tng tr II.1. o tng tr trong dung dch LiClO4 II.1.1. Kho st tng tr mt s gi tr in th Nh trnh by trong phn trn, khi qut th tun hon CV vi in cc LiMn2O4 trong dung dch LiClO4 1M tc qut 0.5mV/s ta c th hnh 21. Trn th CV xut hin 3 pic ti in th : EA = 0.15V/SCE ; E2K = -0.8V/SCE v E1K = 0.5V/SCE; - nghin cu phn ng in cc ti cc pic ta s kho st ph tng tr ti in th ca 3 pic ny. -Tin hnh o ph tng tr c o trong dung dch LiClO4 1M, in cc i Pt, in cc so snh calomel bo ho. Dng xoay chiu t vo c bin Uo= 5mV, tn s bin thin f = 10Hz 20mHz.

a. Khi o ti in th E = 0.15V/SCE Ph tng tr th hin trn hnh 24.


Hnh 24. Ph tng tr ti in th E = 0.15V/SCE

Trn th ph tng tr hnh 23 ta thy: ti vng tn s cao 10Hz 1Hz th tng tr c dng hnh bn nguyt, cn ti vng tn s thp 1Hz 20mHz th c dng ng thng nghing. Vng bn nguyt th hin qu trnh khng ch ng hc. Ti qu trnh chuyn in tch quyt nh tc phn ng. Vng ng thng nghing th hin qu trnh khng ch khuch tn Warburg, c trng cho s khuch tn ca ion Li+ vo trong vt liu in cc. H in ho c th biu din bi m hnh mch in tng ng nh trong hnh 25. Clk R Rct

20mHz 100Hz 2Hz 10Hz

-18

-16

-14

-12

-10

-8

-6

-4

-2

0 80 85 90 95 100 105 110 115

R'(Ohm)

R

"(O

hm)

W


Hnh 25. S mch tng ng ca h thng in ho

T th ph tng tr bng cch Fitting mch tng ng ta thu c kt qu sau: Rdd= 85,21 Rct = 12,48 Clk = 6,6 mF

w = 7,694 DW max =8,19 rad/s Trong : - Rdd l in tr thun ca dung dch. T y ta c th tnh c dn in ca vt liu theo cng thc:

= 1

= ddRS

l 1. = 21.851

33,1101 4

= 8,824.10-7 (-1.cm-1)

- l in tr sut ca dung dch - S l din tch in cc - l l khong cch gia in cc nghin cu v u mao qun - Rct l in tr chuyn in tch - Clk l in dung t in. Trong mch tng ng, Clk chnh l in dung ca lp in tch kp ti b mt in cc v dung dch in ly. - W l tng tr khuch tn hay cn gi l tng tr Warburg chnh l t hp ca Rw v Cw, c trng cho qu trnh khuch tn. - w l hng s Warburg Theo th ta c : Rdd + Rct - 22Cd = 98,4

w = Rct))/2Cd (Rdd-(97.4 + = 7,33 DW


gi tr tnh ton ny xp x gi tr Fitting mch tng 7,694 DW Tnh h s khuch tn ca h w = RT/(nF)2 Co*2Do 2Do =(RT/w (nF)2Co*)2 Do = 4,782 x 10-15 m2/s = 4,782 x 10-11 cm2/s Ta thy Do nm trong khong 10-10 10-13cm2/s ph hp vi kt qu ca mt s ti liu khoa hc mi y.[] Phn ng in cc ng vi in th ny nh sau: LixMn2O4 2MnO2 + x Li+ + x e- LixMn2O4 v MnO2 l cc cht hot ng in cc, nm c nh trong cu trc khung rn. Li+ ng vai tr l cht tham gia phn ng in cc, nh l 1 cht xc tc. Qu trnh khuych tn ca ion Li+ bao gm 2 giai on: - Khuch tn t trong in cc ra b mt - Khuch tn t b mt in cc vo dung dch Cn phi xt xem giai on no quyt nh tc qu trnh khuych tn. So snh kt qu tnh Do theo phng php qut th tun hon CV v phng php o tng tr ta c: Do= 4,782.10-11


Hnh 26. Ph tng tr ti in th E = -0.8 V/SCE

Trn th ph tng tr hnh 26 ta thy: ti vng tn s cao 10Hz 50mHz tng tr c dng hnh bn nguyt th hin qu trnh khng ch ng hc. Ti vng tn s thp 50mHz 20mHz c dng ng thng nghing th hin qu trnh khng ch khuch tn Warburg, c trng cho s khuch tn ca ion Li+

ra/vo trong vt liu in cc. Sau khi Fitting mch tng ng ta thu c kt qu sau: Rdd = 85.1 Rct = 78.05 Clk = 80 mF w = 49.05 DW fmax = 25mHz max =0.16 rad/s w = 49.05 DW H s khuch tn :

Do = 3.21 x 10-17 m2/s = 3.21 x 10-13 cm2/s

c. Khi o in th E = 0.51V/SCE Ph tng tr th hin trn hnh 27.

20mHz 50mHz 3Hz 10Hz

-70

-60

-50

-40

-30

-20

-10

0

0 50 100 150 200 250

R'(Ohm)

R''(

Ohm

)


Hnh 27. Ph tng tr ti in th E = 0.51V/SCE Trn th ph tng tr hnh 27 ta thy rng: ti vng tn s cao 10Hz 100mHz tng tr c dng hnh bn nguyt th hin qu trnh khng ch ng hc. Ti vng tn s thp 100mHz 20mHz c dng ng thng nghing th hin qu trnh khng ch khuch tn Warburg, c trng cho s khuch tn ca ion Li+ ra v vo trong vt liu in cc. Sau khi Fitting mch tng ng ta thu c kt qu sau: Rdd= 85.81 Rct = 7.168 Clk = 50.58 mF w = 3.75 DW max =2.71 rad/s Tnh h s khuch tn:

w = RT/(nF)2 Co* o2D

2Do =(RT/w (nF)2Co*)2. Do = 1.71 x 10-15 m2/s = 1.71 x 10-11 cm2/s

20mHz 0.5Hz 100Hz 10Hz

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0

75 80 85 90 95 100 105 110

R'(Ohm)R

"(O

hm)


Qua vic kho st c th kt lun rng ti vng tn s cao, tng tr c dng hnh bn nguyt. Ti qu trnh chuyn in tch ng vai tr quyt nh tc phn ng. Ti vng tn s thp hn ph tng tr c dng ng thng nghing, ti qu trnh khuch tn ng vai tr quyt nh tc phn ng.

II.1.2. Kho st s ph thuc ca cc thng s phn ng vo in th a. S ph thuc ca h s khuch tn D vo in th

T cc s liu xy dng th quan h gia h s khuch tn D v in th in cc th hin trn hnh 28.

1.00E-13

1.00E-12

1.00E-11

1.00E-10

1.00E-09

1.00E-08

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1E(V)/SCE

D(c

m2/

s) Series1

Hnh 28. th mi quan h D & f(Ec) T th hnh 28 ta thy gi tr h s khuch tn dao ng trong khong 10-13 10-9 ph hp vi kt qu nghin cu mi y[]. H s khuch tn D ca Li+ ra/vo in cc lm vic vng in th -0.7V -0.3V rt nh, trong khong gi tr 10-13 10-12cm2/s. Ti vng in th +0.2 +1V th h s khuch tn tng khi tng in th ca in cc, gi tr ca h s khuch tn nm trong vng t 10-11 10-9 cm2/s.

b. S ph thuc ca hng s Warburg w vo in th


T cc s liu xy dng th quan h gia w v in th in cc th hin trn hnh 29.

0

20

40

60

80

100

120

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

E(V)/SCE

w

(D

W)

Hnh 29. th mi quan h w & f(Ec)

Hng s w c n v l (DW) hay .s-1/2 c quan h nghch o vi D1/2. V vy w c xem nh l in tr cn tr qu trnh khuch tn ca ion Li+ vo trong cu trc pha rn ca in cc. Thng thng th gi tr ca w rt hu ch cho vic nh lng hnh vi khuch tn ca vic ci ion Li+ trong iu kin c bit, v d trong iu kin phn ng ch c 2 pha. Nhng thc t th khng th tnh ton chnh xc c h s khuch tn i vi phn ng xy ra tai vng c nhiu pha cng tn ti hoc cc pha lun bin i thnh pha mi trong thi gian ngn. Trn th ta thy ti cc vng in th -0.8V -0.75V, 0V +0.1V v +0.4 V +0.5V hng s w c gi tr nh. iu c ngha l trong vng in th ny qu trnh khuch tn ca Li+ b cn tr t nht. Kt qu ny ph hp vi khi qut th CV, ti nhng vng in th ny thun li cho phn ng xy ra (xut hin cc pic oxh/kh).


b . S ph thuc ca Rct vo in th in cc T cc s liu xy dng th quan h gia Rct v in th in cc th hin trn hnh 30.

0

50

100

150

200

250

300

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

E(V)/SCE

Rct

(Ohm

)

Hnh 30. th mi quan h Rct & f(Ec)

T th trn cho thy in tr chuyn in tch tng khi tng in th in cc t -1V/SCE -0.4V/SCE. Sau in tr gim dn trong vng t -0.3V +1V. Gi tr Rct t khong 5 200 . Trong vng in th t -0.6V/SCE n -0.3VSCE (ln cn in th cn bng Eo= 0,36V), gi tr Rct c gi tr khong 150 260 . Chnh ti in th cn bng Eo th Rct t gi tr cc i, Rct = 256 . Gi tr dng trao i io l :

io= (RT / nF)/Rct = (8,314 . 298/ 1. 96500)/ 256 io = 1,0.10-4 A = 0.1mA.

c. S ph thuc ca Clk vo in th T cc s liu xy dng th quan h gia Clk v in th in cc th hin trn hnh 31.


4

6

8

10

12

14

16

18

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

E(V)/SCE

Clk

(mF

)

Series1

Hnh 31. th mi quan h Clk & f(Ec)

T th trn ta thy rng in dung lp kp ti b mt in cc tng khi

tng in th trong vng in th -0.9V 0V/SCE v t +0.2V +0.9V/SCE. S bin i in dung ca lp kp l kt qu ca hiu ng ci ion Li+ vo trong b mt in cc. S bin i in dung lp kp l kt qu ca hiu ng ci ion Li+ vo trong b mt in cc. Trong cc vng in th -0,5V 0V/SCE v 0,4V 0,8V/SCE th dung lng lp kp c gi tr ln nht iu ny c th thy rng ti kh nng ci Li+ vo trong in cc l ln nht.

II.2. o tng tr trong dung dch KClO4 II.2.1. Kho st tng tr mt s gi tr in th Khi qut CV in cc spinel LiMn2O4 trong dung dch KClO4 0.05M trong vng in th -0,7V 1V/SCE vi tc qut 0,5mV/s ta c th hnh 23.

Trn ph qut CV xut hin 3 pic ti cc in th: EA = 0,1V/SCE ; E1K = 0,7V/SCE v E2K = -0,07V/SCE;


Kho st ph tng tr ca in cc LiMn2O4 trong dung dch KClO4 0,05M vi in cc i Pt, in cc so snh calomel bo ho ti cc pic trn .in th xoay chiu t vo l Uo = 5mV, tn s bin thin trong khong f = 5Hz 20mHz :

a. Ti E = 0,1V/SCE Ph tng tr th hin trn hnh 32.

-40

-35

-30

-25

-20

-15

-10

-5

080 100 120 140 160 180 200

R'(Ohm)

R''

(Ohm

)

Hnh 32. Ph tng tr ti E = 0,1V/SCE Trn th hnh 32 ta thy ph tng tr ca in cc LiMn2O4 trong dung dch KClO4 khc hon ton khi qut trong dung dch LiClO4. y ch c vng khng ch ng hc ng vi dng hnh bn nguyt, khng c vng khng ch khuch tn. iu ny chng t trong dung dch KClO4 khng c qu trnh khuch tn ca ion K+ vo trong in cc.

H in ho c th biu din bi m hnh mch in tng ng nh trong hnh 33.


Clk R Rct

Hnh 33. S mch tng ng ca h thng in ho Bng cch Fitting mch tng ng ta thu c kt qu sau: Rdd = 107,1 Rct = 102,48 Clk = 10,78 mF max = 0.48 rad/s b. Ti E = -0,07V/SCE

Ph tng tr th hin trn hnh 34.

-60

-50

-40

-30

-20

-10

080 100 120 140 160 180 200 220 240

R'(Ohm)

R''

(Ohm

)

Hnh 34. Ph tng tr ti E = -0,07V/SCE Tong t phn trn sau khi fitting mch tng ng ta cng thu c cc kt qu sau: Rdd= 108,1


Rct = 160,48 Clk = 3,6 mF max = 0.188 rad/s

c. Ti E = 0,7V/SCE Ph tng tr th hin trn hnh 35.

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0106 108 110 112 114 116 118 120 122

R'(Ohm)

R''(

Ohm

)

Hnh 35. Ph tng tr ti in th E = 0,7V/SCE

Sau khi fitting mch tng ng ta cng thu c cc kt qu sau: Rdd= 107,5

Rct = 29,48 Clk = 13,7 mF max = 0.253 rad/s T cc kt qu o trn ta thy rng mi gi tr in th ph tng tr u c dng hnh bn nguyt, trong c vng tn s cao n nhng vng tn s rt thp. Khng thy biu hin xut hin vng khng ch khuch tn. iu ny chng t


ion K+ vi kch thc ln hn nhiu so vi ion Li+ khng c kh nng ci vo trong cu trc mng ca in cc spinel LiMn2O4. II.2.2. Kho st s ph thuc ca cc thng s phn ng vo in th

a. S ph thuc ca Rct vo in th in cc T cc s liu xy dng th quan h gia Rct v in th in cc th hin trn hnh 36.

0

50

100

150

200

250

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

E(V)/SCE

Rct

(Ohm

)

Hnh 36. th mi quan h Rct = f(Ec)

T th hnh 36 ta thy trong vng in th -0,7V -0,1V/SCE th in tr Rct c gi tr kh ln 150 200 v t gi tr cc i 198 ti Eo =- 0,4V/SCE (in th cn bng). Gi tr dng trao i ng vi ti in th cn bng

l:

io= (RT / nF)/Rct = (8,314 . 298/ 1. 96500) / 198 io 1,3.10-4 A = 0,13mA. Ti vng in th - 0,1V/SCE 0,8V/SCE th Rct c gi tr gim dn theo chiu tng ca in th, tc l khi p in th dn ln cao th in tr qu trnh


chuyn in tch cng nh tc qu trnh chuyn in tch ca mangan cng thun

li hn.

b. S ph thuc ca Clk vo in th in cc T cc s liu xy dng th quan h gia Clk v in th in cc th hin trn hnh 37.

0

5

10

15

20

25

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

E(V)/SCE

Clk

(mF

)

Hnh 37. th mi quan h Clk = f(Ec)

Trn th hnh 37 ta thy gi tr Clk gim nh trong vng in th -0,8V -0,2V/SCE, sau tng nhanh cng vi chiu tng ca in th in cc trong

khong in th -0,1V 0,5V/SCE, ri gim dn trong khong in th 0,7V 1V/SCE.


III. ng cong phng np ca vt liu in cc LiMn2O4 Th nghim xc nh dung lng thc ca in cc LiMn2O4 Ta thit lp mt h pin gm: - Anot l l Zn kim loi - Catot l vt liu in cc cn nghin cu LiMn2O4 - H in dch l dung dch LiOH bo ho S h pin nh sau: (-) Zn dd LiOHbh LiMn2O4 (+) Kt qu thu c sau khi phng vi dng 1mA v np vi dng 5mA iu kin nhit thng vi cc chu ki` khc nhau ta thu c ghi trong bng 6. Trong tnap - thi gian np in cho pin, h tphong - thi gian phng in ca pin, h Qn, Qrn - dung lng v dung lng ring ca qu trnh np, mAh Qp, Qrp - dung lng v dung lng ring ca qu trnh phng, mAh % - hiu sut phng np: % = Qp / Qn x 100(%) y s dng in cc LiMn2O4 c : din tch b mt: S = 1,33 cm2 khi lng: m = 0,27 g Bng 6. Dung lng thc ca in cc LiMn2O4 sau cc chu k


T bng s liu trn ta xy dng th quan h dung lng v hiu sut vi s chu k kho st.

Chu K

tnap (h)

Qn

(mAh)

Qrn

(mAh/g)

tphong (h)

Qp

(mAh)

Qrp

(mAh/g)

(%)

1 4.750 23.750 87.963 16.333 16.333 60.492 68.771

2 4.500 22.500 83.333 13.583 13.583 50.307 60.371

3 3.833 19.167 70.989 9.917 9.917 36.730 51.742

4 3.583 17.915 66.352 9.333 9.333 34.555 52.096

5 3.333 16.665 61.722 8.667 8.667 32.100 52.014

6 3.250 16.250 60.185 8.500 8.500 31.481 52.312

7 3.200 16.000 59.259 8.333 8.333 30.852 52.063

8 3.133 15.665 58.018 8.083 8.083 29.937 51.654

9 3.033 15.165 56.166 7.916 7.916 29.318 52.199

10 3.000 15.000 55.555 7.833 7.833 29.011 52.222

11 2.833 14.165 52.463 7.500 7.500 27.778 52.948

12 2.766 13.833 51.222 7.716 7.716 28.577 55.790

13 2.733 13.665 50.611 7.250 7.250 26.852 53.054

14 2.693 13.465 49.870 7.250 7.250 26.852 53.842

15 2.666 13.333 49.370 7.166 7.166 26.541 53.760

16 2.583 12.915 47.833 7.166 7.166 26.541 55.846

17 2.466 12.33 45.667 7.083 7.083 26.233 57.444

18 2.333 11.665 43.203 6.833 6.833 25.307 58.577

19 2.033 10.165 37.648 6.250 6.250 23.148 61.485

20 1.666 8.300 30.741 5.750 5.750 21.296 69.275

21 1.333 6.651 24.629 4.250 4.250 15.741 63.912


a. th quan h dung lng - s chu k

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25

f(chu k)

Q(m

Ah/

g)

Qrn(mAh/g)

Qrp(mAh/g)

Hnh 38. th quan h dung lng - s chu k T th hnh 38 ta nhn thy dung lng ca pin t l nghch vi s chu k phng np ca pin. nhng chu k u tin dung lng gim rt nhanh, nhng sau dn dn n nh. S gim dung lng c th l do mt s nguyn nhn sau: - Khi phng in Mn+3 bin thnh Mn+2, nhng do qu trnh ny khng thun nghch nn trong khi np khng c qu trnh ngc li to Mn+3. V th dung lng ca pin gim i. - Trong iu kin th nghim, dung dch LiOHbh tip xc vi khng kh nn trong dung dch xut hin lp cabonat kt ta. Mt phn lp kt ta ny bm trn b mt in cc to thnh mng mng lm gim kh nng hot ng ca in cc, do lm gim dung lng in cc.


b. th quan h hiu sut phng np - s chu k

0

10

20

30

40

50

60

70

80

0 5 10 15 20 25

f(chu k)

%

Hnh 39. th quan h (%) - f(chu k) T th hnh 38 ta thy rng nhng chu k u tin hiu sut phng np gim kh nhanh, nhng sau khong 5 chu k th hiu sut n nh v tng nh dn sau mi chu k. Gi tr hiu sut ny nm trong khong 50 70 % (theo l thuyt l khong 80%), tng i ph hp vi d on kt qu trong phn qut th CV (khong 70%).

Phn III


Kt lun T kt qu nghin cu hnh vi in ho ca vt liu in cc spinel LiMn2O4 rt ra mt s kt lun sau:

1. Ph CV ca in cc LiMn2O4 qut trong dung dch LiClO4 tc qut chm th hin r cc pic oxy ha/ kh hn tc qut nhanh. Chng t rng qu trnh ci v kh ci ca ion Li+ vo trong in cc LiMn2O4 l rt chm.

2. T hp qu trnh oxy ha kh ca ion Mn+2 v Mn+3 v qu trnh ci / kh ci ca ion Li+ vo mng li LiMn2O4 l qu trnh thun nghch, t khong 80%. ng gp vo phn bt thun nghch 20% l do giai on ci chm ca ion Li+ vo cc hc bt din hay t din ca mng LiMn2O4.

3. Kt qu o CV v ph tng tr ca in cc LiMn2O4 trong dung dch KClO4 khng nh khng c s ci ion K+ vo mng ca in cc nghin cu. l do ion K+ c bn knh ion ln hn bn knh ion Li+. (RLi+ = 0,6 , RK+ = 1,96 )

4. Tnh ton c cc thng s ca phn ng in cc LiMn2O4 nh: Rct, D, Rdd, Clk

5. Dung lng phng ca in cc l tng i n nh v c th phng np c nhiu chu k.

6. Qu trnh phng np ca in cc LiMn2O4 trong dung dch in ly nc l kh phc tp nn cn phi tip tc nghin cu.

Documents

Hung K46 LiMn2O4