1 / 19

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

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells. Gang Ning, Bala S. Haran, B. N. Popov. 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

talib
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 charge&discharge 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. • CC+CV charge: (1.0A+4.2 V+50 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) Carbon Half-cell LiCoO2 Half-cell

  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. Capacity Fade (in percentage) Fresh 1C 300 Cycles 2C 300 Cycles 3C 300 Cycles Carbon 0.00% 2.77% 8.30% 10.59% LiCoO2 0.00% 3.98% 4.38% 5.18%

  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) Q3:capacity fade due to the loss of primary material (Li+) Q:=Q1 + Q2 +Q3

  12. 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 Re: resistance of bulk material Zw: Resistance of Warburg Diffusion Cint:intercalation capacitance Q: constant phase elements Qe Qf Qct Cint Relect Rf Rct Re Zw • Relect: resistance of electrolyte • Rf: resistance of surface film • Rct: resistance of charge transfer

  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. A B C D 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. Thicker SEI film 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.

More Related