Blended Electrodes for High Performance Lithium-Ion Batteries
5.4. Results and Discussion
5.4.2. Electrochemical performance studies of LMR-NMC, LMP and blended materials
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Figure 5.2(h-i) shows the blend of LMR-NMC and C-LMP which has uniform distribution of both the blended materials.
Fig. 5. 2: (e) Raman spectrum of C-LMP, (f) High resolution SEM image of pristine CNFs, (g) Raman spectrum of CNFs, (h) blend of LMR-NMC and LiMnPO4 spot 1, and (i) blend of LMR- NMC and C-LMPspot 2
5.4.2. Electrochemical performance studies of LMR-NMC, LMP and
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crystal environment. C-LMP shows the voltage plateau at 4.2 V during charging and 4.0 V during discharging. LMR-NMC shows the activation of LiMn2O3 component above 4.4 V where oxygen is released as Li2O and forms the MnO2 [11]. LMR-NMC has an operating voltage of about 3.75 V. As the operating voltage of C-LMP matches with that of LMR-NMC, C-LMP is an ideal to blend with LMR-NMC. In the discharge voltage profile of blended material, the flat plateau corresponds to C-LMP and the later the sloppy curve is attributed to LMR-NMC. 20 wt. % of C-LMP blended electrode material delivers good electrochemical performance (C rate performance data shown in Fig. 5.5(b-c)). More than 20 wt. % of C-LMP was not chosen for blending as the capacity of blend LMR-NMC may drastically reduce.
Fig. 5.3: (a) 1st charge voltage profiles, (b) 5th discharge voltage profiles (maximum capacity obtained) pristine LMR-NMC, pristine C-LMP and blend of LMR-NMC and C-LMP as indicated in the figure
The 20 wt. % C-LMP blending gives a reasonable flat plateau at 4 V. Also literature reports 15-30 wt. % blended materials give best electrochemical performance [23-26]. One of the drawback of LMR-NMC is the high irreversible capacity in first cycle which is alleviated by the blend which shows very less irreversible capacity (9%) compared to pristine LMR-NMC (17%). This reduced irreversible capacity may be attributed to approximate 100% electrochemical efficiency of C-LMP and stabilization of interface of LMR-NMC by the C-LMP coating. This coating protects the interface from degradation due to deposition of electrolyte at higher voltages. Another possible reason for reduced irreversible
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capacity is due to the retention of oxide ion vacancies during de-lithiation which is due to surface coating of C-LMP to LMR-NMC [33]. The other major disadvantage of LMR-NMC is the rapid loss of energy density during progress of cycling due to decay of voltage which is limiting its application in powering the hybrid EVs and EVs. The blend of LMR-NMC and C-LMP shows improved electrochemical performance in terms of energy density compared to pristine LMR-NMC which is evident from Fig. 5.4(a) and (b) where the energy density of pristine LMR-NMC is declined from 870 Wh kg-1 (5th cycle) to 681 Wh kg-1(200th cycle). The blended material shows the energy density of 880 Wh kg-1 (5th cycle) to 750 Wh kg-1 (200th cycle).
Fig. 5.4: Discharge voltage profiles at 5th, 100th and 200th cycles of (a) LMR-NMC, and (b) blend of LMR-NMC-C-LMP
The improvement in energy density of the blend is accredited to symbiotic effect of high voltage, stable capacity and stable crystal system of C-LMP [36-37], and high capacity of LMR-NMC [4-5]. Partly, the surface protection of LMR-NMC by C-LMP also helps in the improvement of energy density of LMR-NMC. It should be noted that, blended electrode has almost no voltage decay, it has about 10 % of loss of capacity after 200 cycles. If the capacity loss is further improved, blended electrode material may have still higher in energy.
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Fig.5.5: Cycle life of: (a) LMR-NMC, blended LMR-NMC and C-LMP, (b) C-Rate performance of blended electrode, (c) Comparison of rate capability of LMR-NMC and blended electrode
Cycle life of blended material, LMR-NMC and C-LMP composite materials are shown in Fig. 5.5(a). All the electrodes shows stable cycling behavior. At the end of 200 cycles, the blended and LMR-NMC composite electrodes have 10 % loss in capacity. The carbon nano fiber (CNFs) additives (1.5 wt. %) used here to keep the conductivity of the sample, thereby the cells improve cyclability along the cycles. It has been reported that when CNF additives are not used the cells have rapid capacity fade after 100 cycles [4, 12]. The CNFs make a good electronic contact along the particle thereby reduce electrode impedance even though the electrodes have salt decomposition products on the surface upon cycling to 4.7 V. There is an enhancement in the rate capability (at 1C and higher, of the blended electrode compared to pristine LMR-NMC (Fig. 5.5(b-c)) which is attributed to the superficial intercalation of lithium ions in the C-LMP and carbon coating on to C-LMP also
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improves the overall conductivity of the blend. Moreover, the C-LMP on the surface of LMR-NMC also acts as excellent lithium ion conductor at high potentials (4.1 V vs. Li/Li+) which is also responsible for high rate capability of the blended material.
Fig. 5.6: Impedance spectra, represented as Nyquist plots for the LMR-NMC, C-LMP and blended composite electrodes measured under open circuit condition after 12 h assembly of cells in the frequency range 1 MHz and 10 mHz
Impedance spectra provide very useful information about various resistances associated with the cell. EIS data of LMR-NMC, C-LMP and blended composite electrodes are measured under similar conditions in open circuit condition are presented in Fig. 5.6. The result indicates that all the three composite electrodes show a small Ohmic resistances of 4.5, 7.63 and 14.7 Ohm cm2 for theC-LMP and blended and LMR- NMC composite electrodes, respectively. Similarly, the charge transfer resistances for the C-LMP, blended and LMR-NMC composite electrodes were 348, 722 and 1585 Ohm cm2, respectively. C-LMP blending with LMR-NMC reduces Ohmic resistance and charge transfer resistances by 50% thereby improves the rate capability, cycling performance.
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5.4.3. Mn dissolution studies of LMR-NMC, LMP and blended materials