Study of High Energy Density Li-Mn Rich (LMR) Ni-Mn-Co Oxide (NMC)
3.4. Results and Discussion 1. Structural characterization
3.4.3. Electrochemical performance studies
3.4.3.2 Electrochemical assessment of LMR and F-LMR-NMC
Figure 3.9 shows the galvanostatic charge-discharge cycling plots of F-LMR-NMC (Li1.2Ni0.15Mn0.55Co0.1O2-zFz) and pristine LMR-NMC (Li1.2Ni0.15Mn0.55Co0.1O2) at various C rates between voltages of 2.5 to 4.7 V on CF current collector. As shown in Fig. 3.9(a), on charging above 4.4 V, Li2MnO3 componentis getting activated and converted to Li2O and MnO2. The voltage profiles presented in Fig. 3.9(a) clearly indicates that F-LMR-NMC shows low charge voltage and hence more charge capacity at low voltages (less than 4.4 V) than pristine LMR-NMC. This is due to partial decrease of average oxidation state of metal ions and suggests that there is partial substitution of O2- by F- which is evident from increase in a-lattice parameter as it is the measure of M-M bond length [63]. This helps in getting the charge capacity at low voltages (>200 mAh g-1 capacity below 4.4 V for F-LMR-NMC) which minimizes the decomposition of electrolyte on electrode surface thus increasing the interfacial stability. Besides, high stable capacity of LMR-NMC could be obtained in the F-doped sample without adding any high voltage electrolytes. Higher discharge
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voltage plateau is also obtained for F-doped LMR-NMC compared to pristine LMR- NMC on CF as presented in Fig. 3.9(b). F-LMR-NMC shows high initial discharge capacity of ~300 mAh g-1 (Fig. 3.9(b)) as compared to pristine material of ~250 mAh g-1 at low discharge rates of C/10 (Fig. 3.9(b)). The irreversible capacity is significantly low, about 5% for the F-doped LMR-NMC compared to pristine LMR- NMC ~25% on CF as presented in charge-discharge voltage profiles in Fig. 3.9(a-b).
The high capacity of F-doped LMR-NMC is attributed to high stability of structure.
Fig. 3.9: (a) Voltage vs. charge capacity during charge of pristine LMR-NMC and F-LMR-NMC (1:50 wt.%) on CF at C/10 rate. (b) Voltage vs. discharge capacity of pristine LMR-NMC and F- LMR-NMC on CF at C/10 rate
Both pristine LMR-NMC and F-LMR-NMC on CF shows good cycling stability as shown in Fig. 3.10(a) compared electrodes prepared on Al current collector. The electrodes prepared on Al current collector shows gradual decrease in capacity where as both pristine and F-LMR-NMC on CF retain their capacity for long cycles. This is because CF provides good electrical contact to the material in a 3D electrode architecture compared to Aluminum in a 2D structure. Hence, in this work we focus the electrochemical studies of pristine and F-LMR-NMC on CF current collector only. The fluorination of Mn, Ni and Co based layered oxide cathode material has led to significant improvement in cycle life and power capability of the lithium cells. Besides, the specific energy calculated from the discharge voltage profiles presented in Fig. 3. 9(a) are 875 and 1050 Wh kg-1 for LMR-NMC and F- LMR-NMC of CF current collector respectively.
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Fig. 3.10: (a) Capacity vs. cycle number of pristine LMR-NMC, F-LMR-NMC (1:50 wt.%), on carbon fiber and Al-foil current collector at C/10 and C/5 rate (as indicated). 1st cycle data is not shown here, (b) Capacity vs. cycle number of different weight ratios of LiF and LMR-NMC (F- LMR-NMC) composite electrode at C/5 rate (as indicated) and 25 oC
The 1:50 weight ratio of LiF: LMR-NMC shows good electrochemical performance in terms of high capacity, good cycling stability compared all other weight ratios as shown in Fig. 3.10(b). So 1:50 wt. % is the optimized ratio for the better electrochemical performance of the LMR-NMC. Whereas, other ratios are showing good cycling stability but less capacity compared to 1:50 wt. % ratio (LiF:
LMR-NMC). The 1:25 wt. % is showing less capacity because of high LiF content induces formation strong M-F bonds which will hinder the lithium ion migration.
Besides, high amount of LiF will form a thick layer on the surface which also hinders the migration of Lithium ion. Cycle life data of 1:75 wt. % ratio (LiF: LMR-NMC) is not shown here as this composition has very similar capacity of 1:100 wt. % ratio. C rate performance of pristine LMR-NMC, F-LMR-NMC (1:50 wt. %) on CF are shown in Fig. 3.11(a) indicate a high reversible capacity of 15-20 % more can be achieved at low discharge rates and 10-15 % for high C rates (1C and 3C) for F-LMR-NMC compared to pristine LMR-NMC. The coulombic efficiency of both composite cathodes are over 99%. The cycling performance presented in Fig. 3. 11(a-b) indicates a very good capacity retention of F-LMR-NMC during 200 cycles. F-LMR-NMC electrodes delivers high 1C rate capacity of >200 mAh g-1. Besides, cycling performance of various composition F: LMR-NMC carried out at C/5 rate (Fig.
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3.10(b) indicates 1:50 wt. % of F: LMR-NMC (2% F in LMR-NMC) electrodes delivers high capacity compared to the 1:100, 1:75 and 1:25 wt. % compositions.
Fig. 3.11: (a) C rate performance of pristine LMR-NMC, F-LMR-NMC (1:50 wt. %), on carbon fiber, (b) Capacity vs. Cycle number of F-LMR-NMC (1:50 wt. %) on carbon fiber during 200 cycles (1-5 at C/10; 6-100 at C/5 and 101-200 at 1 C rate)
There is significant improvement in electrochemical performance of F-LMR- NMC (1:50 wt. %) compared to pristine LMR-NMC in terms of cycling stability, capacity retention and minimized voltage fade as shown in the 5th-50th discharge cycle in Fig. 3.12(a-b). Most of the capacity are obtained at 3 V region in LMR-NMC whereas average operating voltage for F-LMR-NMC has been increased close to 3.5 V. The capacity vs. voltage profiles during 110th and 200th cycles of F-LMR-NMC (1:50 wt. %) at 1C rate is presented in Fig. 3.12(c). The figure shows that there is good capacity retention and very less voltage fade even at high rate of 1C which is due to stability of the crystal structure, minimized surface degradation of electrode surface by LiF coating and high conductivity and corrosion resistance by CF. From the Fig.
3.12(c), it is very clear that there is almost no voltage fade during long term cycling (110th and 200th cycle). Besides, there is very less capacity fade during long cycling.
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Fig.3.12: Voltage vs. capacity of pristine LMR-NMC and F-LMR-NMC (1:50 wt. %) on carbon fiber during: (a) 5th cycle, (b) 50th cycle, at C/5 rate (the energy loss is calculated from these voltage profiles), and (c) Voltage profile of 110th and 200th cycle of F-LMR-NMC (1:50 wt. %) at 1C rate
The minimized voltage fade in F-LMR-NMC is further supported by the dQ/dV vs. V plots of LMR-NMC and F-LMR-NMC (1:50 wt. %) during 10th, 20th, 30th, and 50th cycles as shown in Fig. 3.13(a-b), respectively. Significant change in voltage dQ/dV vs. V plots of LMR-NMC is observed whereas voltage fade in F-LMR- NMC is minimized. The minimized voltage fade in F-LMR-NMC is attributed to structure stability and decreased inter layer transition metal ion migration which can be explained in the following way, the partial substitution of O2- by F- leads to M-F bond formation which is more stronger than M-O bond and hence stabilizes the structure during cycling. Moreover, the ionic radii of transitional metal ions increases as evident from the increase in ‘a’ lattice parameter. So, the movement (migration) of transition metal ions from transition metal layer to lithium layer decrease is due to high ionic size of transition metal ions which minimizes the cation mixing and in return it minimizes the layered to spinel transformation and finally it minimizes the
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voltage fade. The decrease in the average oxidation state of metals on the surface helps in minimizing the voltage fade of the material during long term cycling thus maintaining stable energy density. The specific energy loss for pristine and F-LMR- NMC were ~12.5 % and 2% at C/5 rate, respectively.
Fig. 3.13: Comparison of dQ/dV vs. V plots of: (a) LMR-NMC, and (b) F-LMR-NMC (1:50 wt.
%) during 10th, 20th, 30th, and 50th charge-discharge cycles
The EIS data provides very useful information about Ohmic resistance and charge transfer resistance associated with the cell. In order to get understanding of enhanced capacity and good capacity retention, we present the Nyquist plots of LMR- NMC and F-LMR-NMC on CF current collector and F-LMR-NMC on Al foil current collector during 50th cycles in Fig. 3.14(a-b). The impedance spectra in Fig. 3.14(a-b) shows a comparison of impedances of LMR-NMC and F-LMR-NMC on CF current collector and F-LMR-NMC on Al foil current collector under similar conditions. The data shows both LMR-NMC and F-LMR-NMC on CF have identical Ohmic resistances of about 10 Ohm cm2 whereas F-LMR-NMC on Al foil current collector shows 18 Ohm cm2 in 50th cycle. The charge transfer resistance for F-LMR-NMC on CF is much lower (482 Ohm cm2) in relation to LMR-NMC (902 Ohm cm2) on CF.
However, the charge transfer resistance for F-LMR-NMC (2068 Ohm cm2) on Al foil current collector is significantly higher in relation to F-LMR-NMC and LMR-NMC on CF. The increase in Ohmic resistances and charge transfer resistance reduces the capacity (Fig. 3.10(a)) during long cycling for F-LMR-NMC on Al foil current collector. From this, it can be explained that both LiF coating and CF current collector
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are synergistically helping in decreasing the impedance and thereby improving the rate capability, cycling performance and storage capacity of the electrode thus decreasing the polarization.
Fig. 3.14: (a) Comparison of impedance spectra of LMR-NMC and F–LMR-NMC on CF current collector and F-LMR-NMC on Aluminum foil current collector after 50 cycles in discharged condition (SoC 0) at equilibrium potential of 3.1 V, (b) Is the zoomed image in the high frequency region of (a)