Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 1 Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Gang Ning, Bala S. Haran, B. N. Popov

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 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

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 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.

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 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 0 C

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 5 Discharge Profile of fresh Li-ion cell and cells cycled after 300 times

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 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

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 7 DC resistance R dc 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 R dc = (Discharge Voltage – Open Circuit Voltage (0.1 second after the pulse rest))/ Discharge Current (1A)

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 8 Impedance Spectra of fresh cell and cells cycled up to 300 cycles (a) 0% SOC (b) 100% SOC

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 9 Half Cell Study (T-cells) Carbon Half-cellLiCoO 2 Half-cell

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 10 Half-cell analysis of capacity fade (in percentage) of negative Carbon electrode and positive LiCoO 2 electrode Capacity Fade (in percentage) Fresh 1C 300 Cycles 2C 300 Cycles 3C 300 Cycles Carbon0.00%2.77%8.30%10.59% LiCoO %3.98%4.38%5.18% The percentage loss of capacity is calculated based on the capacity of fresh electrode material.

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 11 Breakdown of the total capacity fade of the whole lithium-ion battery Q: total capacity loss of the whole lithium-ion cell Q 1 : capacity correction due to rate capability Q 2 : capacity fade due to the loss of secondary material (Carbon or LiCoO 2 ) Q 3: capacity 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% Q1Q1 3.5%2.9%2.8% Q 2 (Carbon) NA8.4%10.6% Q 2 (LiCoO 2 )3.8%NA Q3Q3 2.3%2.0%3.4% Q:=Q 1 + Q 2 +Q 3

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 12 Typical Nyquist plots of Carbon half-cell obtained at 25 0 C (a) potential ranging from to V vs. Li + /Li

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 13 Typical Nyquist plots of Carbon half-cell obtained at 25 0 C (b) potential ranging from to V vs. Li + /Li

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 14 Equivalent circuit of the EIS spectra R elect RfRf R ct ReRe ZwZw C int QfQf Q ct QeQe R e : resistance of bulk material Z w : Resistance of Warburg Diffusion C int :intercalation capacitance Q: constant phase elements R elect : resistance of electrolyte R f : resistance of surface film R ct : resistance of charge transfer

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 15 Data Fitting R f : 6.87  R e : 110  R ct :=40.37  C int := 1.5 F Log(D) := -9.7

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 16 Parameter comparisons R f R e R ct

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 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 AB D C

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 18 Mechanism of Property Changes Carbon Particles Initial SEI film Binder particles Current collector 2Li + + 2e - + 2(CH 2 O) CO (EC) → CH 2 (OCO 2 Li) CH 2 OCO 2 Li ↓+ CH 2 CH 2 ↑ 2Li + + 2e - + (CH 2 O) CO (EC) → Li 2 CO 3 ↓ + C 2 H 4 ↑ Li + + e - + CH 3 OCH 2 CH 3 (DMC) → CH 3 OCO 2 Li ↓ + CH 3 Thicker SEI film

Effects of Discharge Rates on the Capacity Fade of Li-ion Cells Department of Chemical Engineering University of South Carolina 19 Conclusion The negative Carbon electrode deteriorates much faster than the positive LiCoO 2 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.