This document summarizes the understanding of thermal stabilities of components in Li-ion batteries. It outlines various cathode materials used such as LiFePO4, LiMn2O4, and LiCoO2. It discusses the solid electrolyte interface and how additives can improve its stability. Thermal degradation of electrolyte components like LiPF6 and LiBOB are also covered. The document concludes that most studies on thermal stability are based on the nature of materials used and improving the stability of the solid electrolyte interface through new electrolytes or additives.

1. Understanding of Thermal Stabilities
of Components in Li-ion Batteries
Luu Van Khue
Department of Applied Chemistry
Hanbat National University
2013, February, 19

2. Outline of cathode materials

3. Electrochemical performance
LiFePO4,
LiMn2O4
LiCoO2,
LiNi0.8Co0.15Al0.05O2,
LiNi1/3Co1/3Mn1/3O2.
V. Etacheri, Ener. & Env. Scie., 4, 3243 (2011).

4. Material LiFePO4 LiMn2O4 LiCoO2 LiNiO2 NMC
Crystal Structure 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

5. ARC Analysis
E. P. Roth et al., Journal of Power Sources, 101, 375 (2001).
EC:PC:DMC
1.2M LiPF6
Decreased Cathode Reactions
Associated with Decreasing
Oxygen Release
Charged State
dT/dt

6. Introduction
LiCoO2 → Li1-xCoO2 + xLi+ + e-
6C + xLi+ + xe- → LixC6
Theoretical: 274mAhg-1
x = 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 =
26.8 × ∆𝑥
𝑀
Number of e- or Li+
Molecular weight
LiCoO2
LiMn2O4
LiFePO4
Graphite
Li4T5O12
Silicon

7. Positive Materials
• LiCoO2
• NCA (LiNi0.8Co0.15Al0.05O2) and NCM (LiNi1/3Co1/3Mn1/3O2)
• LiMn2O4
• LiFePO4
- Good electrochemical performances
- 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 material
for portable electronic devices:
mobile phones, laptops, digital
cameras
First cathode generation
for vehicular applications
L. Lu, Journal of Power Sources, 226, 272–288 (2013).

8. 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 direction
FeO6 octahedra is not continuous due to the corner shared with PO4 tetrahedra
→ Low electronic conductivity
⇒ Reduce to nanosize and coating with carbon
Considered as second generation of positive material for vehicular applications

9. Thermal stability of Lithium ion batteries
Q. Wang et al., Jour. of Pow. Sour., 208, 210 (2012).

10. Possible Thermal Reactions of Cathode
Materials
LixCoO2 → xLiCoO2 +
(1−x)
3
Co3O4 +
(1−x)
3
O2
Thermal behavior of cathode itself
Co3O4 → 3CoO +
1
2
O2
CoO → Co +
1
2
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
5
2
O2 + C3H4O3 (EC) →3CO2 + 2H2O
2. 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

11. DSC Measurements

12. Thermal Stability Battery’s Components
• SEI is thermally decomposed at around 100-140oC
The 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

13. Solid Electrolyte Interface/interphase (SEI)
• Products of redox reactions of electrolyte, reactions of electrolyte-
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).

14. Understanding of SEI
D. Aurbach et al., Journal of Materials Chemistry, 21, 9938 (2011).
Possible reactions of EC in electrolyte systems
Effect of LiPF6

15. 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 cathodes
as a oxidative products of EC
⇒ Increase the thermal stability of cathode materials
(Exceptions for LiMn2O4 and LiFePO4)
-e-

16. LiBOB
Decomposition
LiBOB
• LiBOB
Thermally decomposed at 320oC
K. Xu, Electro. and Sol. Let., 6, A144 (2003).
Reduction mechanism and product of LiBOB

17. 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

18. Conclusions
• Basically, most studies on the thermal stability of Li-ion batteries
based on:
– The nature of materials
– The thermal stability of the SEI layer: new additives, or electrolyte solutions,
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)

19. Effect of LiPF6 based electrolyte to
electrochemical performances of LiFePO4
• LiFePO4
– Thickness: 40 𝜇𝑚
– 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
Separator
Li-metal
gasket
Bottom

20. LiFePO4 and 0.75 M LiPF6 in EC/DEC= ½ (v/v)
Thickness: 40𝜇m
Density: 2.0g/cm3
Cycle Form 2nd 3rd 4th 5th
Eff. 88.6598 36.8679 53.0879 76.3025 90.4
2.5
3
3.5
4
4.5
0 20 40 60 80 100 120 140
Voltage(V)
Capacity (mAh/g)
Precycling
Form. 0.1C
2nd 0.5C
3rd 0.5C
4th 0.5C
5th 0.5C

21. LiFePO4 and 1.0 M LiPF6 in EC/DEC= ½ (v/v)
2.5
3
3.5
4
4.5
0 20 40 60 80 100 120 140 160
Voltage(V)
Capacity (mAh/g)
Precycling
Form 0.1C
2nd 0.5C
3rd 0.5C
4th 0.5C
5th 0.5C
Thickness: 40𝜇m
Density: 2.0g/cm3
Cycle Form 2nd 3rd 4th 5th
Eff. 90.51383 94.26854 94.47853 94.4898 94.09369

22. LiFePO4 and 1.2 M LiPF6 in EC/DEC= ½ (v/v)
2.5
3
3.5
4
4.5
0 20 40 60 80 100 120 140 160
Potential(V)
Capacity (mAh/g)
Precycling
Form.0.1C
2nd 0.5C
3rd 0.5C
4rd 0.5C
5th 0.5C
Thickness: 40𝜇m
Density: 2.0g/cm3
Cycle Form 2nd 3rd 4th 5th
Eff. 90.51383 94.26854 94.47853 94.4898 94.09369

23. LiFePO4 and 1.0 M LiPF6 in EC/DEC= ½ (v/v)
2.5
3
3.5
4
4.5
0 20 40 60 80 100 120 140 160
Voltage(V)
Capacity (mAh/g)
Precycling
Form. 0.1C
2nd 0.5C
3rd 0.5C
4th 0.5C
5th 0.5C
Adding 2% VCThickness: 40𝜇m
Density: 2.0g/cm3
Cycle Form 2nd 3rd 4th 5th
Eff. 54.47667 93.73737 96.70103 97.1134 96.3039

24. Vision
• Cycling at high temperature
• Additives: FEC, LiBOB