Capacity fade studies of Lithium Ion cells

Transcription

1 Capacity fade studies of Lithium Ion cells by Branko N. Popov, P.Ramadass, Bala S. Haran, Ralph E. White Center for Electrochemical Engineering, Department of Chemical Engineering, University of South Carolina Columbia, SC 29208

2 Part I Comparison of Pulse and DC mode of Charging

3 Objectives To cycle Li-ion cell using D.C. and pulse currents To compare the capacity fade of Li-ion cells cycled using D.C. and pulse charging protocols To measure experimentally the AC impedance response of individual electrodes of Li-ion batteries To correlate capacity fade to change in resistance To study change in electrode morphology and structure with cycling

4 A Typical Pulse Charging Protocol (on) T on = 10 ms ( off ) T off = 1 ms T on I avg =I x T on +T off Pulse Protocol for charging a Lithium-ion cell

5 Experimental Charge and discharge capacities of SONY US18650S cells charged with DC and pulse charge were estimated galvanostatically at constant current of 0.5 A with cutoff voltage of V. CV's were obtained at the scan rate of 0.1 mv/s within the voltage range of V. For half-cell studies, the working electrodes were discs with diameter of 1.2 cm cut from the original Sony electrodes. EIS measurements were carried out for both full cells and also for the electrode materials. The frequency of the AC signal ranged from 100 khz to 5 mhz.

6 Charge-Discharge Characteristics Voltage (V) Rate: 0.5 A Cutoff voltage: V pulse charge Capacity (mah) DC charge

7 Cyclic Voltammograms on Cycled Sony Cells After 800 Cycles Current (A) DC charge pulse charge Cell voltage (V)

8 Cycle Life Studies pulse current, 3 hr charge; 1 A discharge Discharge capacity (Ah) 0.50 DC, 3 hr charge, 1 A discharge Cycle number

9 Change in Impedance of Li-ion Cell with SOC Imaginary Z (W) % (Charged) 30% 15% 4% 0% (Discharge) Real Z (W)

10 Change in Impedance of Complete Cell with Cycling Z Im (W cm 2 ) DC Fresh 800 Pulse Z Re (W cm 2 )

11 Change in Impedance of LiCoO 2 with Cycling Z Im (W cm 2 ) pulse 800 DC Fresh Z Re (W cm 2 )

12 Change in Impedance of Carbon Electrode with Cycling Pulse 800 DC Z Im (W cm 2 ) Fresh Z Re (W cm 2 )

13 Control Parameters Electrode R W, W R 1, W C 1, F R 2, W C 2, F LiCoO 2 Fresh x x10-3 Carbon Fresh x x10-3 LiCoO2 800 Pulse x x10-3 Carbon 800 Pulse x x10-3 LiCoO DC x x10-4 Carbon 800 DC x x10-4

14 EPMA on Carbon Electrodes Fresh carbon Oxygen concentration 4.97% 800 DC cycled carbon Oxygen concentration % 800 Pulse cycled carbon Oxygen concentration %

15 Concentration profiles during constant Concentration (dimensionless) current charging of Li-ion anode 1.0 hour 0.8 hour 0.7 hour 0.5 hour 0.3 hour 0.1 hour 0.0 hour Radius (dimensionless)

16 Concentration profiles during pulse reversal mode of charging Li-ion anode Concentration (dimensionless) Time = 0 s Time = 2 s Time = 5 s Time = 10 s Time = 15 s Time = 20 s Time = 30 s Time = 40 s Radius (dimensionless)

17 Concentration profiles during pulse charging of Li-ion anode Concentration (dimensionless) Time = 0 s Time = 5 s Time = 10 s Time = 15 s Time = 20 s Time = 30 s Time = 40 s Radius (dimensionless)

18 Conclusions Pulse charging increases the discharge capacity of the cell. The lithium concentration at the particle interface controls the cell impedance. Pulse charging decreases the cell impedance by increasing the utilization of Li. Discharge capacities evaluated at different cycle numbers indicated that pulse charged batteries retain more capacity.

19 Part II Capacity Fade of Lithium Ion Cells cycled at Elevated Temperatures

20 Capacity Fade of Li-ion ion Cells Cycled at Elevated Temperatures by P.Ramadass, Bala S. Haran, Ralph E. White and Branko N. Popov Center for Electrochemical Engineering, Department of Chemical Engineering, University of South Carolina Columbia, SC ECS Centennial Meeting, Philadelphia, May 12-17, 2002 Center for Electrochemical Engineering University of South Carolina

21 Objectives Study the change in capacity of commercially available Sony Cells cycled at different temperatures. Develop a methodology to determine the cause of capacity fade in Li-ion cells: Perform rate capability studies on cycled cells. Perform half-cell studies on individual electrodes. Use impedance spectroscopy to analyze the change in cathode and anode resistance with cycling. Study structural and phase changes at both electrodes using XRD. Quantify capacity fade using experimental data.

22 Characteristics of a Sony Li-ion ion cell Characteristics Mass of the electrode material (g) Geometric area (both sides-cm 2 ) Loading on one side (mg/cm 2 ) Total Thickness of the Electrode (mm) Specific Capacity (mah/g) Cell Capacity (mah) Positive LiCoO Negative Carbon

23 Experimental Cycling Studies Cells cycled using Constant Current-Constant Potential (CC-CV) protocol. Charged at 1A current till potential reaches 4.2 V Hold potential at 4.2V till current decays to 50 ma. Cells were discharged at a constant current of 1 A. Batteries were cycled at four temperatures: RT(25 o C), 45 o C, 50 o C and 55 o C. Rate capability studies done after 150, 300 and 800 cycles Cells charged at 1 A and discharged at different rates (C/9 to 1C). EIS measurements were done for fresh and cycled cells. (100 khz ~ 1 mhz ±5 mv)

24 Half Cell Studies: T-Cell T LiCoO 2 orcarbon inert material porous electrode separator LithiumFoil reference electrode -lithium foil current collector Swagelok TM Three Electrode Cell Impedance analysis khz ~ 1 mhz ±5 mv. Specific capacity measurement (~C/20 rate) Material characterization - XRD studies

25 1.90 Capacity Fade as a Function of Cycle Life 1.55 Capacity (Ah) o C 25 o C 45 o C o C Cycle Number Temperature % Capacity Fade (cycle number) o C o C o C o C

26 Discharge Curves at Various Cycles Voltage (V) Voltage (V) deg C 4.20 Capacity (Ah) Capacity (Ah) deg C Voltage (V) Voltage (V) deg C Capacity (Ah) Capacity (Ah) 55 deg C

27 Charge Curves at Various Cycles Current (A) deg C Time (h) Current (A) deg C Time (hrs)

28 Change in Charging Times with Cycling CC Time (h) Constant Current RT CV Time (h) Constant Voltage RT

29 Change in Cell Voltage with Cycling Cell Voltage Discharge (V) Cell Voltage-Charge (V) Cycled at 50 o C Capacity (Ah)

30 Possible Reasons for Capacity Fade Increase in cell resistance Increase in anode and cathode resistance Increase in electrolyte resistance Loss in secondary active material Irreversibility in cathode material Changes at the anode surface Loss in primary active material Film formation at both electrodes Perform Impedance and Rate Capability Measurements Low Rate Charge / Discharge Studies using T-cells Perform a Charge Balance

31 Rate Capability with Cycling Discharge Capacity (Ah) 2.00 Fresh deg C 2.00 Applied Current (A) Discharge Capacity (Ah) 2.00 Fresh Applied Current (A) deg C Discharge Capacity (Ah) Fresh Discharge Capacity (Ah) Fresh deg C Applied Current (A) Applied Current (A) 55 deg C

32 Nyquist Plots for Cycled Cells RT-0 SOC SOC SOC SOC RT-0 SOC SOC SOC Z Im (Ω) 0.2 Z Im (Ω) Z Re (Ω ) Z Re (Ω ) After 300 Cycles After 800 cycles

33 Electrode Resistance (Ω-cm 2 ) Comparison of Electrode Resistances LiCoO2 Carbon Electrode Resistance (Ω-cm 2 ) LiCoO2 Carbon 0 RT RT Cycles 300 Cycles Electrode Resistance (Ω-cm 2 ) LiCoO2 Carbon 800 cycles 800 cycles cycles 800 Cycles 0 RT 45 50

34 Specific Capacity of Positive and Negative Electrodes at Various Cycles and Temperature Temperature 25 o C 45 o C 50 o C 55 o C Fresh Cycle Number Specific Capacity (mah/g) LiCoO Carbon

35 Possible Reasons for Rapid Capacity Fade at Elevated Temperatures The SEI layer formed on a graphite electrode changes in both morphology and chemical composition during cycling at elevated temperature. The R-OCO 2 Li phase is not stable on the surface and decomposes readily when cycled at elevated temperatures that creates a more porous SEI layer and also partially exposes the graphite surface, causing loss of charge on continued cycling. The LiF content on electrode surface increases with increasing storage temperature mainly due to decomposition of the electrolyte salt. SEI and electrolyte (both solvents and salt) decomposition have a more significant influence than redox reactions on the electrochemical performance of electrode materials at elevated temperatures.

36 Capacity Fade Balance Q: Total Capacity Loss Q = Q 1 + Q 2 + Q 3 Q 1 : Q 2 : Q 3 : Capacity Fade due to cell resistance Difference in capacity between C/9 and C/2 rate discharges. Capacity Fade due to loss of secondary active material (LiCoO 2 and Carbon) Measurement done on T-cells Capacity Fade due to loss of primary active material (Li + ) and other losses.

37 Capacity Fade - Quantified Temperature Cycle Number Total Capacity Loss Q (mah) Resistance Loss Q 1 (mah) Secondary Active Material Loss Q 2 (mah) Primary Active Material and Other Losses Q 3 (mah) o C o C o C o C

38 Capacity Fade - Quantified % of Total Capacity Loss Q1 Q2 Q o C Cycle Number % of Total Capacity Loss Q1 Q2 Q3 50 o C Cycle Number

39 Conclusions Capacity fade increases with increase in temperature. Negative Electrode (Carbon) No changes were found in the carbon structure with cycling. The resistance increases with cycling due to continuous film formation. Lithiation Capacity decreases with cycling. Positive electrode (LiCoO 2 ) Resistance increases with cycling and remains higher than negative electrode when cycled at 25 o C. Lithiation capacity decreases with cycling and is comparable to that of anode. XRD reveals a decrease in Li stoichiometry at the positive electrode with cycling.

40 Conclusions With increase in temperature, anode resistance increases than that of cathode. Rate of film formation of anode increases with temperature. A capacity fade balance has been developed based on our experimental study.

41 Acknowledgements This work was carried out under a contract with the National Reconnaissance Office for Hybrid Advanced Power Sources # NRO-00-C Center for Electrochemical Engineering University of South Carolina Thank You!!

42 Part III Capacity Fade Estimation & Cycle Life Prediction

43 APPROACH Capacity Fade Analysis of Li-ion ion Cells Supported by National Reconnaissance Office (NRO) OBJECTIVES To characterize the capacity fade phenomena of Li-ion batteries and to develop theoretical model to account for capacity fade. Study capacity fade using Constant Current- Constant Voltage charging protocol. Half cell studies for the cycled cathode and anode material using the T-cell assembly. Electrochemical and material characterization of electrodes from cycled cells. Develop theoretical model to account for capacity fade. A Typical Li-ion ion Cell (top) and T-Cell used to study cycled electrode materials (bottom)

44 Experimental Cycling Studies Cells were cycled using Constant Current-Constant Voltage (CC- CV) protocol for charging and constant current for discharging. Batteries were cut open in a glove box after several cycles and disc electrodes were made for T-cell studies Electrochemical characterization studies includes Impedance analysis, Cyclic Voltammetry and Rate capability studies. Material characterization includes XRD studies and SEM, EPMA analysis. 42-Channel Arbin BT-2000 charger

45 Capacity Fade Loss of Li (Primary Active Material) Degradation of C, LiCoO 2 (Secondary Active Material) SEI Formation Electrolyte Oxidation Salt Reduction e + 3Li 3 PF 2 LiF + PF CH CHOCO CH Solvent Reduction 3 Overcharge + + 2e + Li CH CHCH + Li CO Structural Degradation

46 Proposed Reasons for Capacity Fade Loss of Primary Active Material (Li + ) Loss of Secondary Active Material Increased Ohmic Loss with Cycling Decrease in Rate Capability with Cycling

47 Capacity Fade Balance Q = Q 1 + Q 2 + Q 3 Q:Capacity Fade from Full Cell Cycling Q 1 :Capacity Fade from Rate Capability Measurement Q 2 :Capacity Fade due to loss of secondary active material Q 3 :Capacity Fade due to loss of primary active material (Li + )

48 Parameters Considered for developing Correlations Both primary and secondary active material losses are lumped together as loss of Lithium ion available for intercalation. The variables to be considered for active material loss is the State Of Charge (SOC) of positive and negative electrode. Film resistances for both cathode and anode are included as a part of over potential in the Butler- Volmer equation.

49 Model Equations Governing Equations Solid Phase Potential (φ 1 ): ( σ φ ) = Solution Phase Potential (φ 2 ): 1 j 0 ( ) ( ) κ φ2 + κ D ln c + j = 0 Solution Phase Li-ion Concentration (C): c ε t ( D c) = + ( + 1 t ) Solid Phase Li-ion Concentration (C s ): cs Ds 2 cs = r 2 t r r r F j

50 α a, jf α c, jf where i2 = j = ai j 0, j exp η j exp η j, j = n, p RT RT j η j = φ1 φ 2 Uref j R f where, Uref fn ( SOC) t = 0, c = c, c = c 0 0 e e s s Model Equations Initial and Boundary Conditions φ the ve end, φ 1 = = = y y 2 0, 0, 0 1 the + veend, σ = iapp, = 0, = 0 j φ φ c y y all other boundariesall derivatives are zero cs cs r = 0, = 0 r = Rs, D = r r af

51 Description of Symbols & Parameters s k k D j e D Diffusion coefficient in a phase, m 2 /s t + Transference number of Li + ions in solution i Local current density in a phase, A/m 2 2 I app Applied Current (A) a R f Specific area of an electrode, m -1 Film Resistance (Ω-m 2 ) i Exchange current density at an interface, A/m 2 o h Conductivity of the matrix phase, S/m Conductivity of the electrolyte, S/m Diffusional conductivity of the electrolyte, A/m Local volumetric transfer current density due to charge transfer, A/m 3 Volume fraction of a phase Surface Over-potential, V

52 U ref A Function of SOC ref n θn θn θn θn U =1.32 exp(-30 )+exp(0.001e ) exp( ) LiCoO 2 Uref p Uref n SOC n Carbon SOC p U ref p = θ p θ p θ p θp θp p p p p p θ θ θ θ θ

53 Why to Consider θ po and θ no? Specific Capacity decreases for both positive and negative electrode materials with cycling. Eventually State of Charge varies in accordance with varying capacity of electrode materials. Since reference potentials are expressed as functions of SOC, OCP(Uref p -Uref n ) changes with cycling.(θ po =1-Q p /Q theo and θ no = Q n /Q theo ) θ po and θ no are the initial values of SOC of positive and negative electrodes respectively for a completely charged full cell at any cycle.

54 Phase-I Our Approach in developing correlations Parameters Considered: SOC of negative electrode (θ o n) and Film resistance of electrodes(r f ). Discharge curves of several cycle numbers obtained experimentally were fitted by varying these two parameters. Empirical correlations were developed for SOC of negative electrode and the film resistance as a function of cycle number.

55 Discharge Curves from Simulation These discharge curves were obtained by varying SOC of negative electrode and the film resistance of both electrodes.

56 Variation of Li ion Resistance and SOC With Cycling Cycle C s /C smax R f (Ω-m 2 )

57 Comparison of Discharge Curves Model Expt Correlations for SOC(n) and R f : 0 3 θ = n a ln( ) 1 bx 1 c1 x 2 R = a + bx+ cx f xcyclenumber :

58 Phase-II Our Approach in developing correlations Parameters Considered: SOC of negative and positive electrode (θ o n& θ o p) and Film resistance of electrodes (R f ). The exact values of θ o n,θ o p and R f obtained from full cell and half-cell experiments were used to get the discharge curves for different cycle numbers.

59 Variation of Electrode Resistances with Cycling Electrode Resistance (Ω-cm 2 ) Positive Electrode Negative Electrode Variation of Specific Capacity of Electrodes Temperature RT Fresh Cycle Number Specific Capacity (mah/g) LiCoO 2 Carbon % % % % % %

60 SOC s and Li-ion Cell Resistance from Experiment Cycle SOC n SOC p R f (W-m 2 )

61 Semi Empirical Correlation for SOC of Negative and Positive Electrodes SOC (negative) θ = ( ) o 3 n a3 bx 3 c3 xln x Cycle Number (x) SOC (Positive) o θ p = a4 b4 xln( x) Cycle Number (x)

62 Change of R Ω and R p with Cycling (from Impedance Measurement) Resistance (Ω) Electrolyte Resistance (R Ω ) R W =a 1 +b 1 x Polarization Resistance (R Ω ) R p =a 2 +b 2 x+c 2 x 2 Cycle Number (x)

63 Comparison of Discharge Curves Error (θ o p ): 3.38% Model Expt Error (θ o p ): 14.81% This difference can be accounted for rate capability

64 Variation of Li ion Resistance and SOC With Cycling Cycle C s /C smax R f (Ω-m 2 )

65 Comparison of Discharge Curves cycle-experiment 800 th cycle-experiment 500 th cycle-experiment 1cycle-model curve 800 th cycle-model curve 500 th cycle-model-curve Cell Voltage (V) Discharge Capacity (Ah)

66 Rate capability with Cycling 2.00 Discharge Capacity (Ah) RT RT Fresh Applied Current (A) From Graph the capacity difference at the 800 th cycle(rt) due to the rate capability is c.a 9%

67 Capacity Fade Balance Q = Q 1 + Q 2 + Q 3 Q:Capacity Fade from Full Cell Cycling Q 1 :Capacity Fade from Rate Capability Measurement (influence of diffusion coefficient and transference number) Q 2 :Capacity Fade due to loss of secondary active material (LiCoO 2 and Carbon) Q 3 :Capacity Fade due to loss of primary active material (Li + ) Cell Q (mah) Q 1 (mah) Q 2 (mah) Q 3 (mah) 150-RT

68 Acknowledgements This work was carried out under a contract with the National Reconnaissance Office for Hybrid Advanced Power Sources # NRO-00-C Center for Electrochemical Engineering University of South Carolina

69 Conclusions Empirical correlations has been developed for SOC and film resistance of electrode materials that accounts for capacity loss with continuous cycling. For cells cycled at RT, Li-ion diffusion model has been used to verify the correlation between lithium loss with cycle number. Specific capacity of the positive electrode needs to be measured accurately in order to match the initial portions of the discharge curve. The capacity obtained through simulation doesn t match with that got from experiment and this difference could be attributed to the capacity loss due to rate capability.