Effects of Discharge Rates on the Capacity Fade of Li-ion Cells

1
Effects of Discharge Rates on the Capacity Fade
of Li-ion Cells

• Gang Ning, Bala S. Haran, B. N. Popov

2
Objectives

• To determine the capacity fade of Li-ion cells
  cycled under different discharge rates
• To break down total capacity fade of Li-ion cells
  into separate parts
• To analyze the mechanism of the capacity fade
• To provide experimental data for the capacity
  fade model under high discharge rate

3
Background

• Capacity fade is a key factor in determining the
  life of the battery in a specific application.
• Generally there are two ways to analyze this
  phenomenon
• calendar/shelf life study ( under no applied
  current)
• cycling study (under a specific chargedischarge
  protocol)
• Many papers regarding charge protocols and the
  capacity fade can be found in current literature.
  Performance of Li-ion cells cycled at higher
  discharge rate is scarcely reported.

4
Capacity fade as a function of cycle No.

• CCCV charge (1.0A4.2 V50 mV)
• Discharge Rates 1C, 2C, 3C
• Frequency once/50 cycles
• Capacity Measurement Rate 0.7 A
• Temperature 25 0C

5
Discharge Profile of fresh Li-ion cell and cells
cycled after 300 times
6
Rate capability study

• Cells were fully charged with CC-CV protocol and
  discharged subsequently with C/10, C/4, C/2, 1C,
  2C and 3C rates

7
DC resistance Rdc as a function of depth of
discharge (DOD)

• Internal DC resistance of the whole-cell was
  determined by intermittently interrupting the
  discharge current in the process of discharge
• Rdc (Discharge Voltage Open Circuit Voltage
  (0.1 second after the pulse rest))/ Discharge
  Current (1A)

8
Impedance Spectra of fresh cell and cells cycled
up to 300 cycles

• (a) 0 SOC
  (b) 100 SOC

9
Half Cell Study (T-cells)
10
Half-cell analysis of capacity fade (in
percentage) of negative Carbon electrode and
positive LiCoO2 electrode

• The percentage loss of capacity is calculated
  based on the capacity of fresh electrode material.

11
Breakdown of the total capacity fade of the whole
lithium-ion battery

• Q total capacity loss of the whole lithium-ion
  cell
• Q1 capacity correction due to rate capability
• Q2 capacity fade due to the loss of secondary
  material (Carbon or LiCoO2)
• Q3capacity fade due to the loss of primary
  material (Li)

Cell cycled at 1C rate Cell cycled at 2C rate Cell cycled at 3C rate
Total capacity fade of Li-ion Battery 9.5 13.2 16.9
Q1 3.5 2.9 2.8
Q2 (Carbon) NA 8.4 10.6
Q2 (LiCoO2) 3.8 NA NA
Q3 2.3 2.0 3.4
QQ1 Q2 Q3
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Typical Nyquist plots of Carbon half-cell
obtained at 25 0C (a)

• potential ranging from 0.913 to 1.730 V vs.
  Li/Li

13
Typical Nyquist plots of Carbon half-cell
obtained at 25 0C (b)

• potential ranging from 0.126 to 0.773 V vs.
  Li/Li

14
Equivalent circuit of the EIS spectra

• Relect resistance of electrolyte
• Rf resistance of surface film
• Rct resistance of charge transfer

• Re resistance of bulk material
• Zw Resistance of Warburg Diffusion
• Cintintercalation capacitance
• Q constant phase elements

15
Data Fitting
Rf 6.87 ? Re 110 ? Rct 40.37 ? Cint 1.5
F Log(D) -9.7
16
Parameter comparisons
Rf
Re
Rct
17
SEM images of the electrode surface

• SEM (X1000/30 ?m) of Carbon materials cycled at
  different discharge rates.
• (A) Carbon cycled at 1C
• (B) Carbon cycled at 2C discharge rate
• (C)(D) Carbon cycled at 3C discharge rate

18
Mechanism of Property Changes
Initial SEI film
Carbon Particles
Binder particles
Current collector
2Li 2e- 2(CH2O) CO (EC) ? CH2 (OCO2Li)
CH2OCO2Li ? CH2CH2 ? 2Li 2e- (CH2O) CO
(EC) ? Li2CO3 ? C2H4 ?
Li e- CH3OCH2CH3 (DMC) ?
CH3 OCO2Li ? CH3
19
Conclusion

• The negative Carbon electrode deteriorates much
  faster than the positive LiCoO2 electrode when
  the Li-ion cell was cycled under higher CC
  discharge rate.
• Increase of the internal impedance,
  (predominantly resulting from the thicker SEI
  film of carbon) is the primary cause of the
  capacity fade of the whole Li-ion battery.
• High internal temperature due to high discharge
  rates probably leads to the cracks of initial SEI
  film and more electrolyte will take part in the
  side reactions. As a consequence, the products of
  those side reactions will make the SEI film
  become thicker and thicker.