However, along with the improvement in the electrochemical properties in Ni-based cathode materials, the thermal stability has been a major concern, and thus violent reaction of the cathode with the electrolyte must be avoided. However, when a voltage range of Ni-based cathode materials was increased to > 4.5 V, phase transitions occurring above 4.3 V led to accelerated formation of trigonal phase (P-3m1) and NiO phases, leading to and pulverization of the cathode during cycling at 60 °C. Role of Mn dissolution in LiMn2O4 spinel on the capacity dilution. mechanisms of the Mn3þ decomposition and electrolyte decomposition.
Introduction
Nevertheless, most lithium-ion cells currently use graphite electrodes as this gives a flatter discharge curve. So, in terms of voltage alone, one lithium-ion cell is equivalent to three nickel-cadmium or three nickel-metal hydride cells. Consequently, much research is being directed towards the development of a fully satisfactory manganese-based positive electrode for use in lithium-ion cells.
Theory and Literature Survey
Electrode Thermodynamics
The standard cell voltage (∆E0), i.e. the reversible cell voltage at standard temperature and pressure can be calculated from tables of thermodynamic data. The free energy of each of the reactants and products in (units of RJ) is shown below the reactants and products for the reaction in a lead-acid battery. The cell voltage arising from the electrode reactions is a thermodynamic quantity and is affected by the temperature, as expressed by.
Electrode Reaction Kinetics
The concentration or availability of the reactants in the electrolyte at the surface is controlled by their diffusion in the cell electrolyte. These resistive elements are essentially constant during discharge, except for the resistivity of the active mass. The ohmic loss in voltage is given by the total deviation from reversible operation is the combination of the two types of overpotential and the IR drop.
Cathode materials for Lithium ion batteries
- Layered oxide cathodes
- LiCoO 2
- LiNiO 2
- LiCo 1/3 Ni 1/3 Mn 1/3 O 2
- Spinel LiMn 2 O 4
- Olivine LiFePO 4
This can play an important role in improving the cycle performance of the cathode material. This results in severe particle agglomeration, non-stoichiometry and structural defects due to lithium volatilization. Much research effort has been devoted to improving the electrochemical performance of spinel LiMn2O4.
Role of surface chemistry for cathode materials
- Surface coating as electronic and ionic conducting media …
- Physical protection layer
Carbon coating on the cathode surface has been investigated to increase electron conductivity, especially for phosphate-based materials such as LiFePO4. The role of carbon during thermal decomposition is twofold: it aids in the reduction of Fe3+ to Fe2+ and the remaining contents form a thin carbon film on the surface of the crystalline phase of LiFePO4. On the other hand, the carbon from the additional source does not significantly improve electron conductivity.
Carbonization of sucrose on the surface of the single-crystal particles resulted in the formation of an electrical network within the secondary particles. The amorphous nature of the coating removes the anisotropy of the surface properties and increases the distribution of Li+ on the (010) facet of LiFePO4 where it can be embedded. It is also plausible that the disordered nature of the coating material modifies the lithium surface potential to facilitate Li+ adsorption from the electrolyte by providing different lithium sites with a wide range of energies that can match the lithium energy in the electrolyte. .
It is also worth noting that electrolyte decomposition occurs easily on the surface of LixCoO2 due to the highly oxidized Co4+. The improvement in cyclability was considered to be attributable to the suppression of the c-axis expansion of LixCoO2 and the cobalt solution in the electrolyte during cycling between 2.75 and 4.4 V65. a) The formation of a monoclinic phase with non-uniform lattice constant expansion (2.6%) in bare LiCoO2 during charging, and (b) the suppression of phase transition from hexagonal to monoclinic phase by a fracture-hardened thin film metal-oxide coating. Among these, capacity fading is considered to be mostly affected by Mn dissolution, because the dissolved Mn ions are re-precipitated on the surface of LiMn2O4 or diffuse into the anode surface, accompanied by a reduction of Mn ions and irreversible deintercalation of Li+- ions (Figure) 12).
Role of Een solution in LiMn2O4 spinel on the capacity increase. a) Schematic mechanisms of Mn3þ dissolution and electrolyte degradation.
Experiment
Spinel-Layered Core-Shell Cathode Materials for Li-ion Batteries
- Introduction
- Experiment
- Results and Discussion
- Conclusions
The surface of the samples is covered with gold thin film to avoid sample loading problems. A pre-spray of the surface was performed for 30 minutes before measurement to eliminate gold film and the surface contamination. The disassembled electrodes were washed several times with dimethyl carbonate (DMC) solvent and dried in the dry room and then attached to the sample holder of the oven.
14a and b display the elemental mapping of normalized Mn relative to Ni of the cross-section pristine LiNi0.7Co0.15Mn0.15O2 and core-shell particles. Electrode density of the tested cathodes was 3.3 g cc-1 when using respective 5 wt.% carbon black and binder. In addition, the first charge capacity of the core-shell sample is 234 mAh g-1, which is slightly lower than the pristine one with 237 mAh g-1.
Increased irreversible capacity ratio of the core-shell is 85%, which is 4% lower than the pristine one due to the spinel shell. Although more detailed work for such a difference of the core-shell sample is needed, the shell layer is believed to eventually suppress the phase transition. It has been reported that a main reason for the structural instability of the Ni-based cathode at elevated temperatures is associated with the formation of Ni2+ species from side reactions with the electrolytes and spontaneous reduction.81 Figure 20 shows the relative area ratio between Ni2+ and Ni3+ in Li [Ni0.7Co0.15Mn0.15]O2 and reference Li[Ni0.5Co0.2Mn0.3]O2 and core-shell before and after cycling at 60oC for 40 cycles using XPS analysis (Figure 21).
The structural stability of the charged electrodes (intact and core–shell) was investigated using in situ XRD analysis under argon atmosphere up to 325 °C ( Figure 23 ). The pristine sample shows two (108) and (110) diffraction peaks merged at 200 °C, indicating that the layered phase (Li1-zNiO2 (R-3m)) has decomposed into a cubic spinel phase LizNi2O4 (Fd-3m ).83 However, the core–shell cathode shows the merged peak at 250 °C, and therefore the structural stability of the core phase is greatly enhanced by the spinel shell. Most importantly, the discovery was made in the thermal instability of the Ni-rich phase (> 50 mol%) and this material with two heterostructures demonstrated a significant reduction in heat production, compared to Li[Ni0.5Co0.2Mn0.3] O2.
New Protective Surface Layer for High Capacity Ni-based Cathode Materials
- Introduction
- Experiment
- Results and Discussion
- Conclusions
Electrochemical impedance spectroscopy (EIS) of the samples cycled at 60 °C was measured in the charged state of 4.5 V at certain cycles (1st, 50th and 100th). In this study, we newly observed that a nanoscale columnar layer could be introduced in bare LiNi0.7Co0.15Mn0.15O2 (denoted as BC) when the amount of precursor Mn was reduced by 10 mol%. When Mn ions are substituted in Ni-cathode materials, Ni3+ ions are reduced to Ni2+.
Simultaneously, Ni2+ ions partially tend to migrate to the Li site in the space group R-3m, thus pushing the Li plates. It can be assumed that this mixture of cations can be related to pillar formation in Li plates. Consequently, the doping of Mn elements in BC leads to the increase of the pillar region on the surface, which means that the Ni3+ ions on the surface are effectively reduced to Ni2+ and, in turn, a part of the Ni2+ ions took to the Li plates as pillars . a) and (e) STEM images of a primary particle on the surface of BC and PC.
Therefore, we could predict the structural stability of the cathode materials in the full charge process. A particularly noticeable difference in phase transitions from R3 to T or M phase is observed in the samples, and PC shows much reduced such phase transitions in contrast to BC and RC. The electrochemical impedance of the cyclically processed samples at 60 °C was measured in the charged state of 4.5 V after the 1st, 50th and 100th cycle (Figure 33).
The first circle in the high frequency is related to the resistance of Li+ ion migration through the surface film (Rf) and film capacitance, while the second in the low frequency with charge transfer resistance (Rct). This result indicates that cation mixing repeatedly occurs in the structure during the cycle, resulting in the formation of NiO phase. The electrochemical impedance (Rf and Rct) of cyclic samples at 60 °C was measured in the charged state at 4.5 V after the 1st, 50th and 100th cycle.
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Yang, X.-Q., Combining in situ synchrotron
Acknowledgements
