Novel Charging Protocols in Lithium Ion Battery

Transcription

1 Novel Charging Protocols in Lithium Ion Battery Objective:-: To develop a protocol which could optimize the charging time and the capacity fade. Approach: The Constant voltage charging process yields to a significant loss in the capacity with cycling. The CC-CV protocol can be used to attain high capacities but only at the expense of maintaining the constant voltage part for a longer time, which is detrimental to the battery. Thus a protocol of varying instantaneous current was tried to achieve the optimization between the charging time and the capacity fade. It () a ct 1/2 = 1/2 1 + bt + Experimental studies: The trend for the current as a function of time was chosen so as to closely trace the conditions existing in the constant voltage charging. This is done to achieve maximum utilization in a lesser time. The current function used was 1/2 a ct It () = where a,b c,d are constants and t is the instantaneous 1/2 1 + bt + dt time. Using the above current expression there was a initial loss in time until the voltage shoots to the cut off value. This was overcome by using a high constant current charging until the cut off potential was reached and then the ensuing charge process is facilitated by the new protocol.(fig1) dt

2 Optimization of the initial constant current charging time: The initial CC time time was optimized by using the CC part along with the novel protocol for varying CC time, thereby estimating the limiting condition under which there was no overcharging but a good utilization was chosen as the initial optimized time. Charging mode Utilization % 85 seconds CC-new protocol seconds CC-new protocol seconds CC-new protocol seconds CC-new protocol 94.5 Above 85 seconds overcharging was observed. Comparison studies of the new protocol with the conventional CC-CV and CV protocol: To study the comparison of the new protocol with that of the conventional CC- CV and CV protocol cells were cycled through CC-CV mode using the average current that was used in the new protocol and through CV mode at the cut off potential under the same external conditions. The average current for the new protocol was.9a and thus was the current used in the CC part of the CC-CV charging. The cells were cycled using the three modes of charging. Charging Curve for the New protocol(fig1) Utilization Comparison(Fig2) Current(A) Current voltage Volatge(V) Utilization (%) CC-CV Protocol CV Protocol charging time (sec) Time(sec)

3 Results: Rate Capability Studies This is done to analyze the performance of the battery under different discharge rates. The various discharge rates that were used were C/1,C/8,C/6,C/4,C/2,C, 1.5C, and 2C rates. The charge after each discharge is done at 1A. The graphs are plotted for the fresh cell and the cells cycled for 1 cycles and a plot of discharge capacity against various discharge rates was drawn. Discharge Capacity (Ah) Discharge Rate Fresh Cell CC-CV (1 CV (1 cycles) cycles) (1 cycles) C/ C/ C/ C/ C/ C C C

4 Cyclic Voltammetry Studies The cells are cycled between 4.2 V and 2.5 V at a scan rate of.1mv/sec and the studies are done for cells which have been cycled for 1 cycles (using all the three modes of charging) and they are compared with the fresh cell. The reversibility of the reaction can be confirmed from the distinct lithiation and delithiation peaks. It can be seen that the peak current in the case of cycled cells are lesser than that of the fresh cell and a small distinction in the peak currents between the three modes of charging was observed. Impedance The completely charged cell shows shows a much lesser impedance than the completely discharged cell. The new protocol did not showed a very little increase in the overall impedance. Comparison of Impedance at 1 SOC between cycled and fresh cell The impedance data shows a significant increase in the value of the resistance of the cycled cells when compared to that of the fresh cell however among the cycled cells using different protocol a significant change cannot be observed at the end of 1 cycles.

5 Capacity Fade Protocol Capacity Fade % Cycle number CC-CV CV Charge Capacity against cycle number: Protocol Charge Capacity Cycle number CC-CV CV Utilization against cycle number: Protocol % Utilization Cycle number CC-CV CV

6 Experimental For CC-CV cycling, the current used for CC part is the average current used in the newly developed protocol. The following studies were made for both fresh and for the cells cycled with conventional CC-CV, CV protocol and the newly developed protocol. vrate Capability Studies were done after 1 and 15 cycles, where all cells are charged using CC-CV protocol with 1A DC and discharged at different rates namely C/1, C/2, 1C, 3C/2 and 2C. CV's were obtained at the scan rate of.1 mv/s within the voltage range of V. Impedance measurements were done at fully charged and fully discharged states for fresh and cycled cells. Charge Curves Current (A) cycle1 cycle 5 cycle 1 cycle 15 Current (A) cycle 1 cycle 5 cycle 1 cycle Time(sec) CC-CV-Protocol Time(sec) CV-Protocol Current (A) cycle 1 cycle 5 cycle 1 cycle Time(sec)

7 Variation of Charge Capacity with Cycling 1.4 Charge Capacity(Ah) Cycle Number CC-CV Protocol CV Protocol Discharge Curves Voltage(V) cycle 1 cycle 5 cycle 1 cycle 15 Voltage(V) cycle 1 cycle 5 cycle 1 cycle Discharge Capacity(Ah) CC-CV-Protocol Discharge Capacity(Ah) CV-Protocol Voltage(V) cycle1 cycle5 cycle1 cycle Discharge Capacity(Ah)

8 Variation of Discharge Capacity with Cycling 1.4 Discharge Capacity(Ah) Cycle Number CC-CV protocol CV Protocol Rate Capability after 15 cycles 1.4 Discharge capacity(ah) CC-CV charging CV charging Fresh Cell Discharge Current (A)

9 Nyquist Plot for Fresh Cell: 3 SOC 1 SOC Z Im (Ω cm 2 ) Z Re (Ω cm 2 ) After 1 Cycles at SOC After 15 Cycles SOC 3 3 Z Im (Ω cm 2 ) CC-CV protocol(1 cycles) (1 cycles) Fresh Cell CV Protocol(1 cycles) Z Im (Ω cm 2 ) CC-CV Protocol CV Protocol Fresh Cell Z Re (Ω cm 2 ) Z Re (Ω cm 2 )

10 Cyclic Voltammetry after 15 Cycles 1.4 Current(A) CC-CV protocol CV Protocol Fresh Cell Voltage(V) Conclusions:- New charging protocol shows promising results in terms of charging performance and capacity fade when compared with CC-CV and CV charging protocols. Cell cycled using the new protocol shows excellent rate capability at high rates of discharge when compared with CC-CV protocol. For the first 1 cycles, there is not much difference in the impedance for the cells charged with different protocols. Overall cell resistance was the same for cells charged with CC-CV and the new protocol and this was higher when compared with resistance of cells charged in CV mode after 15 cycles. Comparison of performance between the three protocols would need at least another 3 cycles.