electrochemical properties of materials for electrical ... · pourbaix diagram many...

Electrochemical Properties of Materials for Electrical Energy Storage Applications

Lecture Note 6

November 18, 2013

Kwang Kim

Yonsei Univ., KOREA

[email protected]

39Y

88.91

8O

16.00

53I

126.9

34Se

78.96

7N

14.01

Galvanic cell vs. Electrolytic cell

Electric load

Zn Zn2+ + 2e

Electron generation

Anodic oxidation

Cu2+ + 2e Cu

Electron consumption

Cathodic reduction

Electron flow Electron flow

+ve‐ve

Current flow

DC power supply

Zn2+ + 2e Zn


Cathodic reduction

Cu Cu2+ + 2e

Electron generation

Anodic oxidation


+ve‐ve

Current flow

Galvanic cell Electrolytic cell

Electrochemical cell determines the polarity of electrodes.

DC power supply determines the polarity of electrodes.

Discharge Charge


DC power supply

Zn2+ + 2e Zn


Cathodic reduction

Cu Cu2+ + 2e

Electron generation

Anodic oxidation


+ve‐ve

Current flow

Electrolytic cell

DC power supply determines the polarity of electrodes.

Charge

Current

potential

EoCu2+/Cu

Eo Zn2+/Zn

Change in electrode potential with current during galvanostatic discharge

‐veCathodicpolarization

+veAnodicpolarization

Cell voltage

Inet = ic ‐ ia


Electric load

Zn Zn2+ + 2e

Electron generation

Anodic oxidation

Cu2+ + 2e Cu


Cathodic reduction


+ve‐ve

Current flow

Galvanic cell

Electrochemical cell determines the polarity of electrodes.

Discharge

potential

EoCu2+/Cu

Eo Zn2+/Zn

Change in electrode potential with current during galvanostatic discharge

+veAnodicpolarization

‐veCathodicpolarization

Cell voltage

Inet = ic ‐ ia

Current

Polarization

- Activation Polarization : activation

Butler Volmer Equation

- Concentration Polarization : concentration

Diffusion or Mass Transfer ControlledActivation Controlled

Exchange Current Densities in 1 M H2SO4

Electrode Material

‐log10(A/cm2)

Palladium 3.0Platinum 3.1Rhodium 3.6Nickel 5.2Gold 5.4Tungsten 5.9Niobium 6.8Titantium 8.2Cadmium 10.8Manganese 10.9Lead 12Mercury 12.3

The exchange current density for the hydrogen reduction reaction on metals is useful because it give an indication of the speed of this reaction on various metals.

Concentration Polarization

• Sometimes the mass transport within the solution may be rate determining – in such cases we have concentration polarization

• Concentration polarization implies either there is a shortage of reactants at the electrode or that an accumulation of reaction product occurs

O2H4e4HO 22

Mass Transfer Control

- If dissolved O2 in the O2 reduction is in short supply, mass transferof O2 can become rate limiting.

- The cathodic charge-transfer reaction at the metal/solution interface is fast enough to reduce the concentration of the reagent at interface (cathodic sites) to a value less than that in the bulk solution.

- This sets up a concentration gradient and the reaction becomes diffusion controlled.

- Under steady state, mass transfer rate = reaction rate

- Maximum transport and reaction rate are attained when C0approaches zero and the current density approaches the limiting current density:

(1) 10 3dxdcD

dtdn

(3) 10 30

CCDnFi B

(4) 10 3CDnFiL

B

- Fick’s Law:

- Under steady state, mass transfer rate = reaction rate

(1) 10 3dxdcD

dtdn

(3) 10 30

CCDnFi B

- Maximum transport and reaction rate are attained when C0 approaches zero and the current density approaches the limiting current density:

(4) 10 3CDnFiL

Overpotential due to concentration polarization

• If copper is made cathode in a solution of dilute CuSO4 in which the activity of cupric ion is represented by (Cu+2 ), then the potential φ1 , in absence of external current, is given by the Nernst equation:

)log(Cu320.337)(Cu

1log320.337 221

nFRT.

nFRT.

• When current flows, copper is deposited on the electrode, thereby decreasing surface concentration of copper ions to an activity (Cu2+ )s . The potential φ2 of the electrode becomes:

S2

S22 )log(Cu320.337

)(Cu1log320.337

nFRT.

nFRT.

Since (Cu2+ )s is less than (Cu2+ ), the potential of the polarized cathode is less noble, or more active, than in the absence of external current. The difference of potential, φ2 − φ1 , is the concentration polarization , equal to:

)(Cu)(Culog32

2S

2

12

nFRT.

Pourbaix diagram

Many electron-transfer reactions involve hydrogen ions and hydroxide ions.

Because multiple numbers of H+ or OH– ions are often involved, the potentials

given by the Nernst equation can vary greatly with the pH.

It is frequently useful to look at the situation in another way by considering what

combinations of potential and pH allow the stable existence of a particular species.

This information is most usefully expressed by means of a E-vs.-pH diagram, also

known as a Pourbaix diagram.

Stability of water

As was noted in connection with the shaded region, water is subject to decomposition

by strong oxidizing agents such as Cl2 and by reducing agents stronger than H2.

The reduction reaction can be written either as

2 H+ + 2 e– → H2(g)

or, in neutral or alkaline solutions as

H2O + 2 e– → H2(g) + 2 OH–

These two reactions are equivalent

and follow the same Nernst equation.

E H+/H2= Eo

H+/H2+ (RT / 2F) ln {[H+]2 / PH2

}

at 25°C and unit H2 partial pressure reduces to

E = E° – 0.059 pH = –0.059 pH

Stability of water

Similarly, the oxidation of water

H2O → O2(g) + 4 H+ + 2 e–

is governed by the Nernst equation.

EO2/H2O = EoO2/H2O + (RT/4F) ln {PO2

[H+]4}

at 25°C and unit H2 partial pressure reduces to

E = 1.23 – 0.059 pH

1 gram‐equivalent weight : 96485C or 26.8 Ah

[Coulomb] = [Ampere] x [second] = [Ampere] x [h/3600]

Zn(s) Zn2+(aq) + 2e‐

Atomic weight : 65.4 g

g‐equi. weight : 65.4 / 2 = 32.7g

Capacity : 26.8Ah/32.7g = 0.82Ah/g

1.22 g/Ah

Cu2+(aq) + 2e‐ Cu(s)

Atomic weight : 63.5 g

g‐equi. weight : 63.5 / 2 = 31.75g

Capacity : 26.8Ah/31.75g = 0.84Ah/g

1.19 g/Ah

Theoretical Capacity (Ah/g or mAh/g) of LiCoO2

1 gram‐equivalent weight : 96485C or 26.8 Ah

[Coulomb] = [Ampere] x [second] = [Ampere] x [h/3600]

CoO2 + Li+ + e‐ LiCoO2 ; discharge

Molecular weight of LiCoO2 : 97.871 g

g‐equi. weight : 97.871 / 1 = 97.871 g

Capacity : 26.8 Ah / 97.871 g = 0.274 Ah/g

= 274 mAh/g

납축전지 생산(1944, 한국전지)

1950년 1970년 1990년

건전지 생산(1946, 로켓트전지)

니켈카드뮴전지 생산

(1986, 로켓트전지)

니켈수소전지생산

(1996, 로켓트전지)

납축전지(40Wh/kg)

니켈카드뮴(60Wh/kg)

니켈수소(80Wh/kg)

리튬이온(150Wh/kg) 나트륨황

(120Wh/kg)

리튬이차전지 생산(1999, LG화학,

삼성SDI)

2000년 2005년 2013년

전기차용리튬폴리머전지생산

(2005 LG화학)

전기차용리튬이차전지생산 (2011,

삼성SDI)

레독스흐름(35Wh/kg)

슈퍼커패시터(40Wh/kg) 공기아연

(300Wh/kg)

11년 리튬이차전지세계시장 점유율1위

‘04년 처음으로수출이 수입량 추월

소재소재

양극양극

음극음극

전해질전해질

분리막분리막

원료원료

리튬망간흑연황

리튬망간흑연황

코발트니켈수은아연

코발트니켈수은아연

재산업소재산업

원 산업원료산업

전지산업IT산업

수송기계산업수송/기계산업

에너지산업에너지산업( 율향상)

신재생에너지(효율향상)

Tablet PC / 노트북(Computer)

(Communication)스마트폰

(Communication)

/디카MP3/디카(Consumer)

전기자전거

전기자동차

지능형로봇

ESS (저장·활용)

[후방산업] [전방산업]

전지산업의 파급효과로 본 융∙복합산업

후방산업의 원천 기술개발과 전방산업의 미래시장 선점 등 산업 장벽을 뛰어넘는 시너지 효과 발생

① 녹색성장의 핵심산업 ② 핵심소재의 중요성 ③ 전·후방 산업에 대한 연쇄효과가 큰 산업

이차전지 시장 전망

(출처 : B3, 후지경제)

(억불)

이차전지시장은、12년 601억불에서、20년 1,149억불로 연평균 8% 성장

리튬이차전지는、12년 157억원에서、20년 659억불로 연평균 19.6% 고성장 전망

전체 이차전지 중 리튬이차전지의 비중이 큰 폭으로 증가할 전망

전체이차전지중리튬의비중의변화전망은12년26% →、20년57%

-

200

400

600

800

1,000

1,200

2012년 2015년 2020년

444 461 490

157 301

659 리튬이차전지

납축전지601

762

1,149

이차전지(전체) : 8%

리튬이차전지 : 19.6%

연평균 성장률

26.1%

73.9% 60.5%

39.5%

42.6%

57.4%

전체

리튬이차전지 시장 전망

(출처 : B3, 후지경제)

향후 리튬이차전지 시장은 중대형 중심으로 연 40% 이상 급속히 성장

향후 자원고갈, 환경규제에 따라 xEV, ESS등 중대형시장 중심으로 재편될 전망

소형전지는 완만한 성장이 예상되며, 당분간 세계 1위는 지속 전망

2003년~2010년까지는 소형전지가 시장 주도(연평균20.2%성장)

-

100

200

300

400

500

600

700

`12년 `20년

128 138

12

257

17

264 ESS용 LIB

xEV용 LIB

소형 LIB

(억불)

81.5%

7.6%10.8%

40.1%

39.0%

20.9%

소형IT : 0.9%

전기차 : 46.7%

E S S : 40.9%

연평균 성장률

157

659

전체

주요국별 산업 현황

생 산

국산화율

수 출

구분(%) `09 `10 `11 `12

양극재 41 46 62 69

음극재 0 0 0 1

분리막 15 12 15 21

전해액 54 66 70 80

(억불)

0

10

20

30

40

50

60

`08년 `09년 `10년 `11년 `12년

10 8 11 13 12

24 3335

40 46 리튬전지

납축전지

(억불)

(출처 : 무역협회 & 업계조사자료)(출처 : 업계조사자료)

0% 50% 100%

2008

2009

2010

2011

2012

22%

31%

35%

40%

43%

51%

43%

41%

35%

30%

23%

20%

20%

20%

21%

2%

3%

4%

5%

5%

한국

일본

중국

기타

점유율(전지)

출처 : B3 Report

0% 50% 100%

2008

2009

2010

2011

2012

12%

13%

16%

21%

24%

63%

55%

52%

47%

41%

14%

20%

22%

24%

27%

12%

12%

10%

9%

8%한국

일본

중국

기타

점유율(소재)

출처 : Yano

: ’12년 약 87억불 수준 : ’12년 약 58억불 수준

(출처: 녹색기술선도형이차전지기술개발, 한국전지산업협회재구성)

* 양극: 한국유미코아(벨기에)의국산화율37% 포함* 전해액: 첨가제전량수입의존

0

50

100

150

200

250

10년 11년 12년 15년 20년

16 20 20 22 2440 48 67

160

224리튬전지

납축전지

기업현황

L&F 신소재, 삼성코닝정밀소재, 에코프로한국유미코아, 삼성정밀화학, 코스모신소재한화케미칼, 포스코ESM, 대정이엠

양극

GS에너지, 포스코켐텍, 일진전기 애경유화, 모간코리아, 세진이노테크

음극

후성, 에코프로, 파낙스이텍, 솔브레인

전해액

SK이노베이션, SKC, 씨에스텍

분리막

제조

부품∙소재

장비

국내 10대 그룹 중 7대 그룹이 이차전지 산업에 투자

이차전지의 응용 분야

모바일 디바이스의 전원의 발전 Trend(삼성전자)

Energy Storage Systems (ESS)

Smart grids provide enhanced, real-time data on demand patterns, which although useful to utilities and customers, cannot be efficiently managed without integrated energy storage. More efficient use of generated electricity reduces dependency on fuel imports and cuts CO₂ emissions. With $32bn spent on smart grids worldwide, many governments are finally investing in the benefits of smart grid management.

Batteries

Battery Electrode Reactions

The Li electrode in Figure B is discharged by oxidation. The formed Li+ cation is going into solution. The reaction is reversible by redeposition of the lithium. However, like many other metals in batteries, the redeposition of the Li is not smooth, but rough, mossy, and dendritic, which may result in serious safety problems.

Here, the formed Pb2+ cation is only slightly soluble in sulfuric acid solution, and PbSO4 precipitates at the reaction site on the electrode surface. This solution‐precipitation mechanism is also working during the charge reaction, when PbSO4 dissolves and is retransformed into metallic Pb.


Figure D shows a typical electrochemical insertion reaction. The term “electrochemical insertion” refers to a solid‐state redox reaction involving electrochemical charge transfer, coupled with insertion of mobile guest ions (in this case Li+ cations) from an electrolyte into the structure of a solid host, which is a mixed, that is, electronic and ionic, conductor (in this case graphite). The insertion electrodes (Figure D) have the capability for high reversibility, due to a beneficial combination of structure and shape stability. Many secondary batteries rely on insertion electrodes for the anode and cathode. A prerequisite for a good insertion electrode is electronic and ionic conductivity.

However, in those materials with poor electronic conductivity, such as MnO2, good battery operation is possible. In this case, highly conductive additives such as carbon are incorporated in the electrode matrix, as in Figure E. The utilization of the MnO2 starts at the surface, which is in contact with the conductive additive and continues from this site throughout the bulk of the MnO2 particle.


Lead-Acid Battery

Pb

PbSO4

Lead - Acid Battery

Lead-acid batteries• Positive electrode: Lead dioxide (PbO2)

• Negative electrode: Lead (Pb)

• Electrolyte: Solution of sulfuric acid (H2SO4) and water (H2O)

PbPbO2

H2O

H2O

H2O

H2O H2O

Lead-acid batteries

PbO2

H2O

H2SO4

• Chemical reaction (discharge)

Pb2+

O22-

2H+

SO42-

2e-

Pb2+

PbSO4

2-2H+

H2SO4

PbSO4

2e-

PbSO4

H2O

H2O

2H2O

H2OH2O

Lead-acid batteries

PbO2 + 4H+ + 2e- Pb2+ + 2H2O

• Chemical reaction (discharge)

• Negative electrode

• Electrolyte

• Positive electrode

•Overall

• The nominal voltage produced by this reaction is about 2 V/cell. Cells are usually connected in series to achieve higher voltages, usually 6V, 12 V, 24 V and 48V.

Pb Pb2+ + 2e-

2H2SO4 4H+ + 2SO42-

Pb2+ + SO42- PbSO4

Pb2+ + SO42- PbSO4

Pb + PbO2 + H2SO4 2PbSO4 + 2H2O

GRIDS

• Each positive and negative plate in a battery is constructed on a framework, or grid, made primarily of lead.

• The positive plates have lead dioxide placed onto the grid framework.

• The negative plates are pasted with a pure porous lead, called sponge lead, and are gray in color.

• The positive and the negative plates must be installed alternately next to each other without touching.

• Many batteries use envelope-type separators that encase the entire plate and help prevent any material that may shed from the plates from causing a short circuit between plates at the bottom of the battery.

FIGURE 16-2 The grid provides support for the plate active material.

• Cells are constructed of positive and negative plates with insulating separators between each plate.

• Most batteries use one more negative plate than positive plate in each cell.

FIGURE 16-5 Two groups are interlaced to form a battery element.

Lead-acid batteries

• Each cell is separated from the other cells by partitions,which are made of the same material as that used for the outside case of the battery.

FIGURE 16-6 A cutaway battery showing the connection of the cells to each other through the partition.

Lead-acid batteries

-

+-

+-

+-

+

(+) and (-) plates are connected to make a 2 volt cell.

All 6 cells are connected inside the box to make a 12 volt battery

The case is filled with electrolyte (sulfuric acid & water)

Electrolyte must always cover the battery plates (but don’t fill to top).

Figure Depiction of the components of a lead acid battery showing the differences between theoretical and practical energy density of a lead acid battery and source of the differences.

Both the Pb and PbO2 electrode reaction mechanisms follow the solution‐precipitationmechanism and the cell reaction shown in this Figure. In addition to the lead and lead oxide electrodes, sufficient amounts of sulfuric acid and water have to be provided for the cell reaction and formation of the battery electrolyte. For ionic conductivity in the charged and discharged states, an excess of acid is necessary. Considering the limited mass utilization and the necessity of inactivecomponents such as grids, separators, cell containers, etc., the practical value of specific energy (Wh/kg) is only 25% of the theoretical one for rechargeable batteries. Due to the heavy electrode and electrolyte components used, the specific energy is low.

Additional Reactions of Significance

• Oxygen Reaction Cycle:: ½O2 + Pb PbO

PbO + H2SO4 PbSO4 + H2O

Note: Oxygen reaction cycle is a benchmark characteristic of VRLA batteries. It is more pronounced with AGM than with gel constructions.

• Severe Overcharge Reaction: 2H2O O2 + 4H+ + 4e-

Note: This results in water loss due to venting of O2 and can be life limiting.

• Positive Grid Corrosion: Pb + 2H2O PbO2 + 4H+ + 2e-

Note: This results in water loss and can be life limiting.

C

C

C

C

Battery Basics-Cell Chemistry

electrochemical properties of materials for electrical ... · pourbaix diagram many...

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