Understanding of thermal stability of lithium ion batteries

Understanding of Thermal Stabilities of Components in Li-ion Batteries

Luu Van Khue

Department of Applied ChemistryHanbat National University

2013, February, 19

Outline of cathode materials

Electrochemical performance

LiFePO4, LiMn2O4

LiCoO2, LiNi0.8Co0.15Al0.05O2, LiNi1/3Co1/3Mn1/3O2.

V. Etacheri, Ener. & Env. Scie., 4, 3243 (2011).

Material LiFePO4 LiMn2O4 LiCoO2 LiNiO2 NMCCrystal Struc-ture

Olivine Spinel Layered Layered Layered

Discharge Voltage

3.4 4.0 3.9 3.8 3.8

Capacity 155 (170) 110-148 140-274 180-274 140-277

Density (g/cm3) 3.6 4.29 5.05 4.76 4.75

Energy density (Wh/g)

530 440 550 680 570

Energy Density (Wh/L)

1900 1880 2770 3230 2700

Electronic Conductivity (S/cm)

10-8 10-5 10-3 10-2 10-3

Transition metal deposits

106< 430 7 62 -

Relative Cost 1 2.2 45 10 19

ARC Analysis

E. P. Roth et al., Journal of Power Sources, 101, 375 (2001).

EC:PC:DMC1.2M LiPF6

Decreased Cathode Reactions Associated with Decreasing Oxy-

gen Release

Charged State

dT/

dt

Introduction

LiCoO2 → Li1-xCoO2 + xLi+ + e- 6C + xLi+ + xe- → LixC6

Theoretical: 274mAhg-1x = 1 Practical: 140-160 mAhg-1

x ~ 0.5-0.6

Concept (1980) ⇔ Commercialization: Sony (1990)

J.-M. Tarascon and M. Armand, Nature, 414, 359–67 (2001).

Specific capacity = Number of e- or Li+

Molecular weight

LiCoO2LiM-n2O4LiFePO4

GraphiteLi4T5O12Silicon

Positive Materials

• LiCoO2

• NCA (LiNi0.8Co0.15Al0.05O2) and NCM (LiNi1/3Co1/3Mn1/3O2)

• LiMn2O4

• LiFePO4

- Good electrochemical perfor-mances- Relatively high working voltage (4.2V)

- High cost- Toxicity

- Good electrochemical performances- High working voltage (4.3V)

- Fast intercalation process- Electrochemically and thermally stable- Low cost- Environmental friendliness

- Low capacity (110-120mAh/g)- Mn ions dissolution

- Relatively high capacity (170mAh/g)- Most stable positive material- Low cost- Environmental friendliness

- Low ionic and electronic conductivity- Low working voltage (Fe2+/Fe3+ vs. Li/Li+ = ~3.5V)- Dissolution ??

First generation of cathode ma-terial for portable electronic de-vices: mobile phones, laptops,

digital cameras

First cathode genera-tion

for vehicular applica-tions

L. Lu, Journal of Power Sources, 226, 272–288 (2013).

LiFePO4

Comparison of LiFePO4 nanoplates with thick plates

Saravanan et al., J. Mater. Chem., 19 (2009) 605

LiO6 octahedra arranged following the b-axis → Li diffusion directionFeO6 octahedra is not continuous due to the corner shared with PO4 tetra-hedra→ Low electronic conductivity

⇒ Reduce to nanosize and coating with car-bon

Considered as second generation of positive material for vehicular applications

Thermal stability of Lithium ion batteries

Q. Wang et al., Jour. of Pow. Sour., 208, 210 (2012).

Possible Thermal Reactions of Cathode Materi-als

LixCoO2 xLiCoO2 + Co3O4 + O2

Thermal behavior of cathode itself

Co3O4 　→ 3CoO + O2

CoO → Co + O2

• During charging process– Li ion is removed from cathode left vacant sites inside the material– To stabilize the structure ⇒ partial structural change

Possible reactions with electrolyte

Li0.5CoO2 + 0.1C3H4O3 (EC) → 0.5LiCoO2 + 0.5CoO + 0.3CO2 + 0.2H2O

1. Thermal reactions of solvent with positive material

O2 + C3H4O3 (EC) →3CO2 + 2H2O2. Combustion reaction of solvents

J. R. Dahn, Solid State Ionics, 69, 265–270 (1994).

V. Etacheri, Ener. & Env. Scie., 4, 3243 (2011).

More exactly, is thermal degradation

DSC Measurements

Thermal Stability Battery’s Components

• SEI is thermally decomposed at around 100-140oCThe first exothermic reaction occurring in LIB

D. D. MacNeil, Jour. of The Electro. Soc., 150, A21 (2003).

Improved Cathode Stability Results in Increased Thermal Runaway Temperature

Solid Electrolyte Interface/interphase (SEI)

• Products of redox reactions of electrolyte, reactions of elec-trolyte-electrodes, etc.

– Inorganic species: Li2Co3, LiOH, LiF, Li2O etc.– Organic species: Alkyl carbonates, (CH2OCO2Li)2, ROCO2Li, etc.

– Polymer species: polycarbonates, PEO-like polymers, etc.• Anode

– Reduction reactions take place as low as 0.5-1.5 V vs. Li/Li+

– Surface activity such as graphite • Cathode

– Oxidation reactions at potential of as high as >3V vs. Li/Li+

The SEI on negative electrode is considered more resistive than the one on cathode

K. Xu, J. of Mat. Chem., 21, 9849 (2011).

P. Verma, Electrochimica Acta, 55, 6332 (2010).

Understanding of SEI

D. Aurbach et al., Journal of Materials Chemistry, 21, 9938 (2011).

Possible reactions of EC in electrolyte systems

Effect of LiPF6

Lithium salts

• LiPF6

LiPF6(s) → LiF(s) + PF5(g)PF5 + H2O → 2HF + PF3O

LiPF6

Melting

Decomposition

Thermally decomposed at 270oC

S. E. Sloop, Journal of Power Sources, 119-121, 330–337 (2003).

Formation of the PEO-like polymers upon cathodesas a oxidative products of EC⇒ Increase the thermal stability of cathode materials

(Exceptions for LiMn2O4 and LiFePO4)

-e-

LiBOB

Decomposition

LiBOB

• LiBOB

Thermally decomposed at 320oC

K. Xu, Electro. and Sol. Let., 6, A144 (2003).

Reduction mechanism and product of LiBOB

Additives

• Polymerizable additives: VC, VEC (vinyl ethylene carbonate), FEC, etc. – Containing double bonds that can be polymerized

• Retardant additives – To prevent capability of solvents combustion

Mechanism of additive polymerization

• Normally, the additives are added to make a more stable SEI layer on the anode material

S. S. Zhang, Jour. of Pow. Sour., 162, 1379 (2006).

– Containing functional groups: e.g. LiBOB

Conclusions

• Basically, most studies on the thermal stability of Li-ion bat-teries based on:

– The nature of materials– The thermal stability of the SEI layer: new additives, or electrolyte solu-

tions, which is how to improve the stability of the SEI.• Works on thermal stability

– LiFePO4 is considered as the best candidate for near future vehicular applications

– Dissolution of carbon coated-LiFePO4 (capacity fading) at high working temperature (60oC)

– Salts or Additives (LiBOB, VC, FEC)

Effect of LiPF6 based electrolyte to electrochemical performances of LiFePO4

• LiFePO4– Thickness: 40 – Density: 2.0 g/cm3

• Testing – Precycling

• Formation: 0.1C• Stabilization: 0.5C for 4 cycles

– Cycling• 100 cycles at room temperature• 100 cycles at 60oC

Top

Spring

Spacer

LiFePO4

Separa-tor

Li-metal

gasket

Bottom

LiFePO4 and 0.75 M LiPF6 in EC/DEC= ½ (v/v)

Thickness: 40mDensity: 2.0g/cm3

Cycle Form 2nd 3rd 4th 5th

Eff. 88.6598 36.8679 53.0879 76.3025 90.4

0 20 40 60 80 100 120 1402.5

3

3.5

4

4.5Precycling

Form. 0.1C2nd 0.5C3rd 0.5C4th 0.5C5th 0.5C

Capacity (mAh/g)

Volt

age

(V)

LiFePO4 and 1.0 M LiPF6 in EC/DEC= ½ (v/v)

0 20 40 60 80 100 120 140 1602.5

3

3.5

4

4.5Precycling

Form 0.1C2nd 0.5C3rd 0.5C4th 0.5C5th 0.5C

Capacity (mAh/g)

Volt

age

(V)

Thickness: 40mDensity: 2.0g/cm3

Cycle Form 2nd 3rd 4th 5th

Eff. 90.51383 94.26854 94.47853 94.4898 94.09369

LiFePO4 and 1.2 M LiPF6 in EC/DEC= ½ (v/v)

0 20 40 60 80 100 120 140 1602.5

3

3.5

4

4.5Precycling

Form.0.1C2nd 0.5C3rd 0.5C4rd 0.5C5th 0.5C

Capacity (mAh/g)

Pote

ntia

l (V)

Thickness: 40mDensity: 2.0g/cm3

Cycle Form 2nd 3rd 4th 5th

Eff. 90.51383 94.26854 94.47853 94.4898 94.09369

LiFePO4 and 1.0 M LiPF6 in EC/DEC= ½ (v/v)

0 20 40 60 80 100 120 140 1602.5

3

3.5

4

4.5Precycling

Form. 0.1C2nd 0.5C3rd 0.5C4th 0.5C5th 0.5C

Capacity (mAh/g)

Volt

age

(V)

Adding 2% VC Thickness: 40mDensity: 2.0g/cm3

Cycle Form 2nd 3rd 4th 5th

Eff. 54.47667 93.73737 96.70103 97.1134 96.3039

Vision

• Cycling at high temperature• Additives: FEC, LiBOB