Sourav Ghosh for their constant support throughout in learning various experimental techniques and conceptual understanding which really helped me throughout the project. 2.8(a) Emission profile of LMR-NMC (b) Emission profile of Mg-F doped LMR-NMC (c) Emission profile of both LMR-NMC and Mg-F LMR-NMC for the 30th cycle. The LMR NMC cathodes deliver a capacity of > 250 mAh/g when operated between 2.5 V and 4.8 V.
However, LMR-NMC suffers from some drawbacks that limit its application in electric vehicles such as poor conductivity, less interfacial stability, structural instability, which leads to poor cycle life. The energy loss due to the structural transformation from layer to spinel, which leads to stress breakdown, has not been properly addressed so far. Mg doped LMR-NMC was synthesized by combustion process followed by LiF coating/doping with LMR NMC by solid state synthesis.
Structural and physical properties of doped and pristine LMR-NMC were studied by XRD, SEM, EDX and (Rietveld refinements). The role of Mg doping in LMR-NMC is to stabilize the crystal structure during cycling. Addition of Fluorine in crystal structure leads to change in oxidation state of transition metal ions, also strongly.
Mg- and F-doped LMR NMC samples show stable reversible capacities >300 mAh/g and capacity retention >260 mAh/g at 1C rate, good cycle life and minimized voltage decay compared to LMR-NMC pristine.
Motivation
Why LIB Among batteries
Introduction
Working Principle of LIB
Current Status of LIB
Introduction
Due to the combination of nickel, cobalt and manganese, NMC offers several advantages over LiCoO2, such as lower cost, less toxic, mild thermal stability, better high temperature cycling stability and high reversible capacity. In NMC, lithium extraction takes place as a result of oxidation/reduction of Ni2+/Ni4+, Co3+/Co4+ and Mn4+. The problem with NMC is that cation mixing occurs between lithium and nickel ions, which degrades its electrochemical performance.
It was found that increasing Co content can suppress cation mixing but at the same time decrease in capacity can be observed, while increasing Ni content can increase capacity but increase cation mixing as a result. Thus, Ni, Mn, Co contents were optimized and the contents of Co, Mn and Ni in LMR-NMC were found as promising cathode materials. Thackeray and coworkers suggested that LMR-NMC is a two-component composite between Li2MnO3 and LiMO21.
Lithium rich NMC can deliver capacity more than 250 mAh/g with an operating voltage higher than 4.5V. Li2MnO3 phase in LMR-NMC is considered the best stabilized component as it is electrochemically inactive over a wide potential range (2.2-4.5V), but above 4.4V it is electrochemically active and can contribute to irreversible capacity loss in the first cycle2 -5.
Structure of LMR-NMC
Why LMR-NMC
Mechanism for High-Voltage and Large Capacity
LMR-NMC provides high capacity >250 mAh/g and voltage ~4V is considered to be an attractive candidate for LIB. The reason for poor rate performance of LMR-NMC is associated with poor electron transfer due to insulating Li2MnO3 component (with low electronic conductivity) and formation of thick SEI (solid-electrolyte interface) layer formed during charge/discharge at the reaction surface of the cathode and electrolyte at >4.4 V. The rapid energy fading of LMR-NMC during cycling is also due to increase in impedance, which may be due to occupation of lithium atoms in various oxide sites during charge/discharge.
The weakening of the voltage occurs due to the transformation of the structure from layer to spinel (Fig. 2.3). The reason for the poor cyclability is due to the low electronic and ionic conductivity of LMR-NMC and interfacial instability. Why Mg is so important: i) Doped Mg will block tetrahedral sites which is the pathway for transition metal ions to migrate from the transition metal layer to the lithium layer. ii).
The electrochemical properties of the cathode material containing LMR-NMC as the active mass were measured using two electrodes of Swage-lok cells with Li as the counter electrode. The crystal structure of LMR-NMC and LMR-NMC doped with Mg and F was investigated by XRD. Thus, based on these results, it is clear that the doping of Mg and F in LMR-NMC increased the lattice parameter of the crystal structure due to the increase of both cellular parameters (a and c).
This result indicates that Mg and F are fully incorporated into the crystal lattice of LMR-NMC. The EIS spectra of Mg and F doped LMR-NMC and pristine LMR-NMC are shown in Fig 2.6. For LMR-NMC the EIS data over the full SOC range are complicated due to several factors which are: (i) due to the activation of the Li2MnO3 phase at 4.5V, the change in the crystallographic phase (ii) the constant voltage fading during cycling represents the major structural transition (i) iii) SEI formation due to electrolyte decomposition at high voltage.
-F doped LMR-NMC shows high initial capacity, ~300 mAh/g and that of pristine LMR-NMC is ~250mAh/g during 1st cycle. a) Discharge profile of LMR-NMC. The discharge capacity of Mg-F doped LMR-NMC is >260 mAh/g at C/10 rate for 30th cycle and corresponding pristine LMR-NMC is 200 mAh/g. The high capacity is attributed to extra lithium added in the form of LiF in Mg-F doped LMR-NMC.
Also the voltage decay is smaller in the case of the doped sample compared to the pristine LMR-NMC. Mg- and F-doped LMR-NMC shows stable cycle life at different C rates (> 260 mAh g-1 at C/5), while pristine LMR NMC shows rapid capacity decay during cycling (200 mAh g- 1 at speed C/20) .
Challenges & Strategies
Experimentation
- Material Preaprtion-Mg & F Doped LMR-NMC
- Structural & Morphological Characterisation Techniques
- Electrochemiacal Impedance Spectroscopy (EIS) Studies
- Electrochemical Performance
Small peaks appearing at 2θ from 21◦ to 23◦ indicate the signature peak for the lithium-rich material, due to the (monoclinic) Li2MnO3 phase42. This may be due to the partial reduction of TM ions by fluorine doping, also due to the longer Mg-O bond length in the layered oxide than that of Ni-O. The result obtained by Reitevled's refinement reveals that the introduction of Mg and F neither affects the crystal structure of the precursor nor produces unwanted secondary phases.
The second semicircle in the mid-frequency region is associated with the charge transfer resistance (Rct) between the SEI and the cathode active material due to the Li insertion reaction. This may be due to the fact that Rf is associated with SEI formation which is mainly caused by cycling. Thus, the semicircle is mainly due to the charge transfer resistance (Rct) for both samplesdue to the charge transfer resistance (Rct) for both samples.
Incorporation of Mg and F into material lowered the electrode impedance and LiF coating on material reduced the polarization. Due to successful doping of both Mg and F, it leads to high capacity, this is due to structure stabilization by doping. Since Mg and F help to blunt the strain, it helps to reduce the structural transformation from layered to spinel.
Conclusion
The doped sample shows stable cycle life and stable capacity, almost no voltage decay indicating the structural transition from layered to spinel is softened to a certain extent.
