Linden D., Reddy T.B. (eds.) Handbook of batteries

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LITHIUM-ION BATTERIES 35.9

100 150 200 250 300 350 400

Temperature (°C)

-40

120

Heat Flow (W/g)

200 250

pure LiNiO

LiNi

0.7

0.2

0.05

DSC 10°C/min

LiNi

0.8

0.2

LiCoO

LiNiO

LiNi

0.8

0.2

LiCoO

FIGURE 35.7 DSC data showing the onset temperature and energy released from

charged cathode materials upon heating. Electrode ﬂoat charged to 4.5 V for 40 hours.

(Courtesy of FMC.)

To reduce the energy evolved during positive material decomposition, new materials have

been developed that offer either less evolved energy or higher onset temperatures. Exam-

ples include Al or Ti and Mg doped lithium nickel cobalt oxides. DSC data for

LiNi

0.7

0.2

0.005

is included in Fig. 35.7. As shown, it evolves much less energy

than even LiCoO

, however its reversible capacity is 25 mAh/g less than LiNi

0.8

0.2

Others have developed magnesium oxide coated LiNi

⫺

type materials that offer elec-

trochemical performance comparable to un-modiﬁed materials, but with a higher self-heating

temperature (219

⬚C versus 211⬚C) and a reduced exotherm when heated in a DSC.

Electrical Properties of Positive Electrode Materials. The voltage characteristics of

LiMn

, LiCoO

, and LiNi

⫺

, when cycled versus a Li counter electrode are illus-

trated in Fig. 35.8 (Charge) and Fig. 35.9 (Discharge). As indicated, LiMn

offers the

highest voltage (4.0 V) but the lowest capacity (

⬃120 mAh/g), LiNi

⫺

the lowest

average voltage (

⬃3.75), but the highest capacity (⬃205 mAh/g), LiCoO

is intermediate

(3.88 V,

⬃155 mAh/g). Spinel is unique in that it exhibits two distinct voltage plateaus on

charge and discharge, more evident in the differential capacity plot shown in Fig. 35.10.

Also shown in Fig. 35.10 is the differential capacity for LiNi

0.8

0.2

. As shown,

LiNi

0.8

0.2

reversibly incorporates lithium over the range of 3.7 V to 4.25 V on charge

(deintercalation), and 3.4 V to 4.25 V on discharge (intercalation). Shown in Fig. 35.11 is

the differential capacity for LiCoO

when cycled versus Li. It intercalates the majority of its

capacity at 3.93 V with small peaks at 4.07 V and 4.20 V that correspond to small peaks

observed in the case of the LiNi

0.8

0.2

material.

A comparison of three different LiNi

⫺

materials, LiNi

0.7

0.3

, LiNi

0.8

0.2

and LiNi

0.9

0.1

, is illustrated in Fig. 35.12, which shows ﬁrst cycle charge curves, and

Fig. 35.13, which shows ﬁrst cycle discharge curves. On charge, these LiNi

⫺

mate-

rials offer similar voltage, except that as the cobalt content is reduced, higher capacity results,

up to 222 mAh/ g. This trend in capacity is also observed on discharge. The synthesis,

properties and electrochemical performance of LiNi

⫺

(x ⫽ 0, 0.1, 0.2, 0.3) materials,

including charge, discharge, and irreversible capacities, and their thermal stability have been

reported.

35.10 CHAPTER THIRTY-FIVE

050100150200

3.2

3.4

3.6

3.8

4.0

4.2

4.4

1+x

(118mAh/g)

LiCoO

(158mAh/g)

LiNi

0.8

0.2

(205mAh/g)

Voltage (V vs. Li)

Specific Capacity (mAh/g)

FIGURE 35.8 Voltage and speciﬁc capacity of common positive electrode

materials on the ﬁrst charge at 25⬚C. All materials were charged at approx-

imately C / 20, the LiCoO

at 7.2 mA/ g, the LiNi

⫺

materials at 0.16

mA/cm

, and the LiMn

at 0.09 mA / cm

.(Courtesy of FMC and Yardney

Technical Products, Inc.)

050100150200

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

LiMn

(106 mAh/g)

LiCoO

(136 mAh/g)

LiNi

0.8

0.2

(190 mAh/g)

Voltage (V vs. Li)

Specific Capacity (mAh/g)

FIGURE 35.9 Voltage and speciﬁc capacity of common positive electrode

materials on the ﬁrst discharge at the C / 20 rate. The LiNi

⫺

materials

were discharged at 0.16 mA / cm

, the LiCoO

at 7.5 mA / g, and the LiMn

at 0.09 mA/ cm

.(Courtesy of FMC and Yardney Technical Products, Inc.)

LITHIUM-ION BATTERIES 35.11

3.00 3.25 3.50 3.75 4.00 4.25

-30

-20

-10

Discharge

Charge

Differential Capacity (dQ/dV)

Voltage (V vs. Li)

LiMn

LiNi

0.9

0.1

FIGURE 35.10 Differential capacity on the ﬁrst cycle for LiMn

and

LiNi

0.9

0.1

when cycled versus Li at the C / 15 rate. (Courtesy of Yardney

Technical Products, Inc.)

3.0 3.2 3.4 3.6 3

Voltage (V vs. Li)

.8 4.0 4.2 4.4

-40

-20

Differential Capacity (dQ/dV)

FIGURE 35.11 Differential capacity on the ﬁrst cycle for LiCoO

when cy-

cled versus Li at the C/ 20 rate. (Courtesy of FMC and Yardney Technical

Products, Inc.)

35.12 CHAPTER THIRTY-FIVE

0 50 100 150 200

3.2

3.4

3.6

3.8

4.0

4.2

4.4

LiNi

0.9

0.1

(222 mAh/g)

LiNi

0.8

0.2

(205 mAh/g)

LiNi

0.7

O.3

(190 mAh/g)

LiNi

0.7

0.2

0.005

(183 mAh/g)

Voltage (V vs. Li)

Specific Capacity (mAh/g)

FIGURE 35.12 Charge curves for various LiNi

⫺

materials charged at

the C / 20 rate. (Courtesy of Yardney Technical Products, Inc. and FMC.)

0 50 100 150 200

2.50

2.75

3.00

3.25

3.50

3.75

4.00

4.25

LiNi

0.9

0.1

(203 mAh/g)

LiNi

0.8

0.2

(190 mAh/g)

LiNi

0.7

0.3

(172 mAh/g)

LiNi

0.7

0.2

0.005

(165 mAh/g)

Voltage (V vs. Li)

Specific Capacity (mAh/g)

FIGURE 35.13 Discharge curves for various LiNi

⫺

materials at the C / 20 rate

except LiNi

0.7

0.2

0.005

which was discharged at the C / 7 rate. (Courtesy of

Yardney Technical Products, Inc. and FMC.)

LITHIUM-ION BATTERIES 35.13

The LiNi

0.7

0.2

0.005

material, engineered to offer improved safety through

reduced exotherm on decomposition, offers slightly lower capacity, 183 mAh/g on the ﬁrst

deintercalation, and 165 mAh/ g reversibly, but with voltage similar to the other LiNi

⫺

materials described.

Synthesis of Lithiated Metal Oxides. The synthesis of lithiated metal oxides, including

LiCoO

and LiMn

, has be achieved through a wide variety of routes, although those

practiced commercially use inexpensive starting materials, such as lithium carbonate, lithium

hydroxide and the metal oxide. The physical and electrochemical properties of the materials

may be controlled by the choice of starting materials and the preparation conditions.

The easiest to prepare is LiCoO

. Its thermodynamic stability results in the desired phase

after treatment of a range of lithium and cobalt reagents, including carbonates,

oxides,

hydroxides, nitrates,

and organic acid complexes,

in the appropriate ratio at high tem-

perature, 600

⬚C to 1100⬚C, in air. LiCoO

can be prepared in bulk at lower temperatures,

400

⬚C, from either cobalt nitrate

or the acetates.

Preparation of LiCoO

at 300⬚C has

been achieved from hydroxide mixtures.

LiCoO

has also been prepared via nonaqueous

routes from nitrates,

under hydrothemal conditions,

and as thin ﬁlms by laser ablation

or spray pyrolysis.

An overview of the preparation and properties of lithium cobalt oxides

is available.

Lithium nickel cobalt oxides can be prepared using routes similar to those used in the

preparation of LiCoO

, although the properties of the material are more sensitive to the

preparative method. Preparations of lithium nickel cobalt oxides are designed to achieve

molecular mixing of the cobalt and nickel materials prior to their reaction. Lithium cobalt

nickel oxides have been prepared from lithium, nickel and cobalt hydroxide co-precipitates

from nitrate solutions, treated between 400

⬚C and 800⬚C after removal of excess water.

Another preparation includes treatment of Li

, CoCO

and Ni(NO

)

 6H

O at 400⬚C.

The electrochemical properties of LiMn

materials are sensitive to the preparative

method, motivating the development of preparations that yield single phase material with

controlled Li, Mn, and O stoichiometry with the desired structure. The effect of preparation

conditions on the properties of spinels prepared from Li

and MnO

, such as electrolytic

MnO

(EMD), at 600⬚Cto900⬚C,

has been the subject of numerous investigations.

Ma-

terials have also been prepared from LiOH and

␥

-MnO

(CMD),

or Mn

towards

spinels with improved capacity retention.

35.2.3 Capacity Fade in C/ LiMn

Cells

Manganese-based materials continue to receive industrial and commercial interest because

of their low cost, benign environmental qualities,

good electrochemical properties, and

excellent safety properties, despite the higher capacity and improved high temperature sta-

bility possible when cobalt- or nickel-based materials are used. The complex chemistry of

the lithium manganese oxides has been the focus of many academic

and industrial studies.

The most signiﬁcant differences in performance between typical manganese- and cobalt-

based cells are their lower capacity, their higher rate of capacity fade when cycled or stored

in the charged or discharged state, and the ability of spinel cells to sustain abuse, attributed

to the stability of

␭

-MnO

to deoxygenation, relative to NiO

or CoO

Capacity fade in spinel cells is the result of multiple processes,

including those related

solely to the spinel material and others involving the interaction of spinel with the electrolyte

and negative electrode materials. Most fade mechanisms can be attributed to three factors:

1. Dissolution of Mn

⫹

into the electrolyte after disproportionation of Li

⫹

2Mn → Mn ⫹ Mn

(solid) (solid) (solution)

35.14 CHAPTER THIRTY-FIVE

Dissolution of spinel into the electrolyte,

41,42

is promoted by acid-induced delithiation,

resulting in disproportionation and formation of

␭

-MnO

. Li-ion electrolytes that employ

LiPF

are acidic, as the salt reacts with adventitious water to form HF. This reaction has

a secondary effect of reducing electrolyte conductivity:

4H O

⫹ LiPF → LiF ⫹ 5HF ⫹ HPO

26 34

However, studies of the material losses from spinel electrodes, and the resulting Mn

⫹

concentration in the electrolyte, found dissolution can account for only a fraction of the

capacity fade observed in spinel cells.

For example, when spinel cells were cycled at

⬚C, analysis of the electrolyte for manganese found spinel dissolution could account

for only 34% of the capacity fade observed.

2. Instability of the electrolyte

Decomposition of the electrolyte includes reaction of the solvent at electrode sur-

faces

42,44,45

to yield a passivation layer (Solid-Electrolyte Interphase or SEI), resulting in

an increase in electrode resistance, cell polarization, and apparent capacity loss.

The presence of lithium alkoxides within the SEI formed on the surface of graphite

negative electrodes, as well as LiOH, Li

, and lithium alkyl carbonates, has been

established.

45,46

One proposed spinel degredation mechanism

starts with the reaction of

lithium alkoxides at the graphite negative with adventitious water to yield alcohol:

LiOMe

⫹ HO→ MeOH ⫹ LiOH

The alcohol can then react at the positive electrode where it is oxidized to H

O and CO

the spinel electrode serving as a source of oxygen:

MeOH

⫹ 1/

␦

Mn O → CO ⫹ 2H O ⫹ 1/

␦

Mn O

24 2 2 24

⫺

␦

This water and CO

may then return to the negative electrode and react with Li

yield immobilized Li in the form of LiOH or Li

2H O

⫹ 1/

␦

LiC → 2LiOH ⫹ 1/

␦

Li C ⫹ H

26 1

⫺

␦

HCO ⫹ 1/

␦

LiC → Li CO ⫹ 1/

␦

Li C ⫹ H

23 6 23 1

⫺

␦

Any H

generated may be oxidized at the positive electrode to form more water:

⫹ 1/

␦

Mn O → HO⫹ 1/

␦

Mn O

2242 24

⫺

␦

This mechanism does not require positive electrode weight loss, Jahn-Teller distortion,

or lattice contraction. Consistent with this mechanism, analysis of C/LiMn

cells cycled

at 45

⬚C found the majority (75%) of the immobilized lithium within the graphite negative

electrode, the remainder (25%) was found within the positive electrode, implicating in-

volvement of the negative electrode in spinel cell capacity fade.

3. Jahn-Teller distortion in discharged cells (Li

)

Jahn-Teller distortion

occurs in LiMn

at 7⬚C (280 K).

This phase transition results

in a transformation from the cubic space group Fd3m to the tetragonal group I4

/amd.

The structural distortion results from interaction of the Jahn-Teller active species Mn

⫹

-e

), in contrast, Mn

⫹

-e

) and Mn

⫹

-e

) are not Jahn-Teller active. Because

of the low temperature of this transition, and that modiﬁed spinels have in general lower

transition temperatures, this mechanism may not be as relevant as others for currently

used spinel materials. Related mechanisms can cause strain and structural failure,

re-

sulting in electrically disconnected particles.

LITHIUM-ION BATTERIES 35.15

01020304050

100

110

120

130

140

Specific Capacity (mAh/g)

Cycle

25°C

55°C

FIGURE 35.14 The speciﬁc capacity of a manganese spinel cycled at 55⬚C and 23⬚C at the C / 2 rate

between 4.3 and 3.5 V. (Courtesy of Carus Chemical.)

Capacity fade in spinel cells is prominent when cells are stored in either the charged or

discharged state.

An unmodiﬁed spinel will lose over 20% of its capacity when stored for

three weeks at 50

⬚C. Coated spinels, such as those protected by an inorganic coating of

or LiCoO

, offer improved stability, loss of 1% to 3% per week at 50⬚C is typical.

The coatings inhibit electrolyte decomposition and acid formation, thus are most effective

in mitigating capacity loss in the charged state.

Modiﬁed spinels that contain excess lithium, and preferably an admetal

(Li

⫹

⫺

,M⫽Al

⫹

,Cr

⫹

,Ga

⫹

), offer improved storage stability in the discharged

state as manganese disproportionation is inhibited when the Mn

⫹

:Mn

⫹

ratio is reduced.

Materials with admetal content offer fade rates at 55

⬚C of 0.05%/cycle (0.05 mAh/g per

cycle),

whereas current coated materials offer irreversible capacity loss on storage of less

than 1% per week at 55

⬚C in the discharged state, or 20% capacity loss after 500 cycles at

⬚C.

Alternatively, improved electrolytes, in particular those with additives to reduce water and

HF impurity levels, such as hexamethyldisilazane,

permit spinel cells to offer improved

performance. To illustrate the ability of the improved spinel materials to cycle at elevated

temperature (55

⬚C), Fig. 35.14 shows the speciﬁc capacity of a spinel material when cycled

versus lithium at 23

⬚C and 55⬚C. As shown, at 23⬚C the material demonstrated a fade rate

of 0.04% /cycle and at 55

⬚C, 0.15% /cycle. While these fade rates are higher than those

demonstrated for LiCoO

, they are viable for applications that value the low cost and benign

safety properties of the manganese oxides.

Since reaction with the electrolyte or dissolution processes occur at the particle surface,

materials with lower surface area and specially coated surfaces have been developed, such

as LiCoO

or Li

coated LiMn

. To illustrate the ability of these materials to sustain

cycling at 55

⬚C, Fig. 35.15 shows the speciﬁc capacity of un-coated and LiCoO

coated

LiMn

materials cycled at C/ 2 at 23⬚Cor55⬚C. As illustrated, at 55⬚C the un-coated

LiMn

material initially provides 126 mAh/ g but the fade rate increased after 10 cycles

to 1.3% /cycle. In contrast, the LiCoO

coated material demonstrated lower capacity fade at

55⬚C, 0.6% /cycle.

35.16 CHAPTER THIRTY-FIVE

0 5 10 15 20 25

100

110

120

130

140

Specific Capacity (mAh/g)

Cycle

55°C, coated

23°C, coated

55°C, un-coated

FIGURE 35.15 Speciﬁc capacity of LiCoO

coated and un-coated spinels cycled at the C/ 2 rate

between 4.2 and 3.5 V at 23⬚Cor55⬚C. (Courtesy of Carus Chemical.)

35.2.4 Negative Electrode Materials

Historical Overview. Since the early 1970s, intercalation compounds have been considered

as electrode materials for secondary lithium batteries. However, secondary lithium battery

development effort throughout the 1970s and early 1980s focused on the use of lithium metal

as the negative electrode because of the high speciﬁc capacity of the metal. Cells with

impressive performance were developed and some were commercialized, however safety

issues with lithium metal batteries

caused the industry to concentrate on using lithium

intercalation into carbon at the negative electrode instead of lithium metal.

The safety issues

with lithium metal have been attributed to the changing morphology of lithium as a cell is

cycled. As described in Chapter 34, the safety properties of negative electrodes may be

correlated to their surface area, thus while the properties of lithium metal negative electrodes

change with use, carbon electrodes offer stable morphology resulting in consistent safety

properties over their useful life.

By utilizing low surface area carbons, electrodes with

acceptable self-heating rates may be fabricated.

The ﬁrst Li-ion batteries marketed by Sony utilized petroleum coke at the negative elec-

trode. Coke-based materials offer good capacity, 180 mAh/g, and are stable in the presence

of propylene carbonate (PC)-based electrolytes, in contrast to graphitic materials. The dis-

order in coke materials is thought to pin the layers inhibiting reaction or exfoliation in the

presence of propylene carbonate.

In the mid-1990s most Li-ion cells utilized electrodes

employing graphitic spheres, in particular a Mesocarbon Microbead (MCMB) carbon.

MCMB carbon offers higher speciﬁc capacity, 300 mAh/g, and low surface area, thus pro-

viding low irreversible capacity and good safety properties. Recently, a wider variety of

carbon types has been used in negative electrodes. Some cells utilize natural graphite, avail-

able at very low cost, while others utilize hard carbons that offer capacities higher than

possible with graphitic materials.

Types of Carbon. Many types of carbon materials are industrially available and the struc-

ture of the carbon greatly inﬂuences its electrochemical properties, including lithium inter-

calation capacity and potential. The basic building block for carbon materials is a planar

sheet of carbon atoms arranged in a hexagonal array, as shown in Fig. 35.16. These sheets

are stacked in a registered fashion in graphite. In Bernal graphite, the most common type,

ABABAB stacking occurs, resulting in hexagonal or 2H graphite. In a less common poly-

morph, ABCABC stacking occurs, termed rhombohedral or 3R graphite.

LITHIUM-ION BATTERIES 35.17

FIGURE 35.16 The hexagonal structure of a carbon layer and the structures of hexagonal (2H) and rhom-

bohedral (3R) graphite.

Artificial Graphite

Petroleum Coke

Coal Tar Coke

Liquid Phase Precursor

Vapor Grown Carbon Fiber

Acetylene Black

Vapor Phase Precursor

Resin Pyrolytic Graphite

Solid Phase Precursor

Carbon

FIGURE 35.17 Carbons classiﬁed by the precursor phase.

Most real materials contain disorder, including the 2H and 3R stacking orders as well as

random stacking, thus a more precise way to identify a graphite is to specify the relative

fractions of 2H, 3R and random stacking. Forms of carbon have been developed with a range

of stacking disorders and different morphologies. Stacking disorders include those where the

graphitic planes are parallel but shifted or rotated, termed turbostratic disorder,

or those in

which the planes are not parallel, termed unorganized carbon.

Particle morphologies range

from the ﬂat plates of natural graphites, to carbon ﬁbers, to spheres.

Carbon materials can be considered as different aggregations of a basic structural unit

(BSU) consisting of two or three parallel planes with a diameter of 2 nm.

The BSU’s may

be oriented randomly, resulting in carbon black, or oriented to a plane, axis or point, resulting

in a planar graphite, a whisker or a spherule.

The types of carbon may alternatively be organized based on the type of precursor ma-

terial, as illustrated in Fig. 35.17 as the precursor material, and the processing parameters

determine the nature of carbon produced. Materials that can be graphitized by treatment at

high temperature (2000

⬚C to 3000⬚C) are termed soft carbons. Upon graphitization, the

turbostratic disorder is removed and strain in the material relieved.

Hard carbons, such as

those prepared from phenolic resin, cannot be readily graphitized, even when treated at

3000

⬚C. Coke type materials are prepared at ⬃1000⬚C, typically from aromatic petroleum

precursors.

35.18 CHAPTER THIRTY-FIVE

Voltage (V)

FIGURE 35.18 The voltage of a Li/ graphite cell illustrating

Li staging upon intercalation of graphite. (Reproduced with

permission from Physical Review B. From Ref. 59,)

Graphite

123

Carbon laye

Lithum laye

Stage

FIGURE 35.19 Schematic diagram of lithium staging in graphite.

Staging and Electrochemical Intercalation into Carbon. When lithium is intercalated into

graphite, the ABAB structure transforms to an AAAA structure and distinct voltage plateaus

are observed. This is illustrated in Fig. 35.18, which shows the voltage of a Li /graphite cell

over one cycle at low rate for a highly ordered graphite. Voltage plateaus are observed upon

lithium intercalation as distinct phases (stages) are formed.

A classical model of lithium

staging is illustrated in Fig. 35.19. As shown, lithium forms ‘‘islands’’ within graphite instead

of distributing homogeneously. The most lithium rich stage, LiC

, is termed stage 1 and is

formed at the lowest voltage, as indicated in Fig. 35.18. As lithium is removed from the

graphite, higher stages are formed, as indicated in the ﬁgure.

In graphites used in Li-ion cells, less distinct stages are observed and a ﬂat discharge

proﬁle results. In contrast, when petroleum coke or another disordered material is used, a

continuous, sloping voltage proﬁle is observed. This is illustrated in Fig. 35.20, which shows

the ﬁrst intercalation (charge) and deintercalation (discharge) for coke and artiﬁcial graphite.

As shown, the coke material does not exhibit distinct stages and has a higher average voltage,

0.3 V versus lithium.