One of the renewable energy sources, electricity, is proposed as a candidate for available energy sources because it does not pollute the air and has a high efficiency. Due to these advantages, the replacement of the car's internal combustion engine and adaptation to the energy storage system (ESS) is foreseen. However, large batteries used in ESS or vehicles that need high capacity and high voltage cannot be realized with the existing cathode.
The structure of LCO is unstable when more than half of its lithium resources are released. It is necessary to develop a new cathode because the existing electrode has a limit even if we use the coating method. The overlithiated layered oxides (OLO), lithium-rich layered oxides, are one of the promising candidates for high-voltage, high-capacity cathodes.
It is able to meet high capacitance and high voltage, but the phase transformation during cycling is a problem because it increases the irreversible capacitance. The salt type consists of lithium ions and anion pairs, just like other lithium salts. It regulates insulating materials such as lithium fluoride caused by the decomposition of lithium salt when building a layer on the surface and builds a durable and well ion permeable membrane that can withstand high temperatures and high powers.
SEM/EDX analysis of OLO/Li half-cells after pre-cycling with reference electrolyte, 1%WCA2, 5% FEC and 1%WCA2+5%FEC.
Introduction
The Lithium ion batteries
- The demand of battery
- The Li-ion battery
- Overlithiated layered oxides (OLO)
The electric vehicle that only uses electricity as an energy source requires a higher price because of the battery price, so they combined the battery with a combustion engine (hybrid electric vehicle, HEV), which batteries are charged when it uses a motor when driving. It is called plug-in hybrid vehicle (PHEV), which increases the usage ratio of electricity compared to HEV to increase the distance. The lithium-ion batteries are one of the rechargeable batteries that can be used more than once during the charging process, that is why they are also called a secondary lithium-ion battery.
They use lithium ion as the main electrical energy storage materials because the light weight and low standard reduction potential give us high energy density and high voltage. Lithium-ion batteries consist of four parts which are cathode, anode, electrolyte and separator18. Conductive materials are usually used in the cathode because its ionic conductivity is very low.
The anode, which is not based on carbon-like graphite, which has good ionic conductivity, must use conductive materials. The separator separates cathode and anode, so direct contact of both cathodes is prevented, resulting in short circuits18. Lithium ion has positive charge meets with electrons making neutrality and stored as chemical energy in anode during the charging process.
At first they use lithium metal instead of graphite, but it fails because of the safety problem. In overcharged states, the bond between oxygen and cobalt is broken and it melts into electrolytes in the form of ions26-29. One of the candidates, NCM, is a ternary system that currently uses cobalt, nickel and manganese5-7.
The lithium-nickel-manganese oxides have spinel structure and high voltage plateau around 4.7V, but it has serious capacity fading problems due to its Mn3+ ions13-17. Nickel cobalt aluminum, which increases the nickel ratio and adds a small amount of aluminum (NCA), is reported for its stability in cycling performance, even containing high nickel ratios of 8-9. In this paper, we use overlithiated layered oxides, which is one of the candidates, and confirm its durability.
The overlithiated layered oxides or lithium rich layered oxides are one of the candidates for high voltage, high capacity cathodes. In this paper, we use overlithiated layered oxide cathode with additives to improve its electrochemical stability and delay phase transformation.
- Theoretical capacity
- Electrolyte
- Solid Electrolyte Interphase (SEI) layer
- Molecular orbital (MO)
- Functional additives
It can cause shortening due to growth of lithium dendrites produced as a by-product of salt decomposition or irreversible loss of lithium ions. It is one of the side products produced, but it plays the role of a protective layer which is permeable to the lithium ion and stops the side reaction between the electrolyte and the electrode1. In this work we used additives, lithium difluoro bis(oxalato) phosphate, to modify the conformation of the SEI layer on the double-lithiated layered oxide cathode.
The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) determine the stability of electrolytes4. The additives have a higher HOMO energy and decompose sooner than the electrolyte at the cathode site and produce a layer because it gives up electrons first. The additives have a low LUMO energy, which means that electrolyte is more likely to accept electrons than electrolyte and decompose reductively.
The common function of the additives is to build a permeable layer for lithium ions to transport lithium ions and protect against side reactions between the electrode and the electrolyte. It puts out the fire in short, which is called non-flammable additions21 or trapping HF20, which is formed from the decomposition of H2O and LiPF6. In addition, some additives are used to protect the electrodes in overcharged states, resulting in the destruction of the structure.
The two oxalate rings form a stable layer, but due to the thickness, the high-speed capability is low. The LiBOB is known as an anode side additive22,23, but it has recently been announced that it also works at the cathode site[].
Experimental
- Electrolytes and electrode
- Assembling cathode half cell and full cell
- Electrochemical measurements
- Precycle
- Cycle performance
- Rate capability test
- Electrochemical Impedance Spectroscopy (EIS)
- Instrumental analysis
- X-ray photoelectron spectroscopy (XPS)
- Scanning electron microscopy (SEM)
- Energy-dispersive X-ray spectroscopy (EDX)
The pre-cycle is done galvanostatically with battery measurement system (WonATech, WBCS3000) after aging for 10 hours to properly permeate the electrolyte. The resting time of 2 minutes is given between constant voltage to discharge and after discharge to start charge to stabilize the cell state. The cycle performance at elevated temperature charge and discharge between 4.6V-2.0V without constant voltage as 0.5C rate.
The charge rate is fixed at 0.2C and charged to 4.6V with 0.05C CV and discharged at different C rates. The impedance was obtained on an Iviumstat (Ivium technologies, Netherlands) The impedance after pre-cycle is calibrated from 100MHz to 0.01Hz at 0.005V amplitude. Ex-situ field-emission scanning electron microscopy (Nova Nano230 FE-SEM) is used to measure surface morphologies.
Results and discussion
Effect of LiDFBP additive on electrochemical performance of overlithiated layered oxide cathode
- Impedance
- Rate capability test
- Cycling performance of OLO/Li half cells at 25 o C
- Cycling performance of OLO/Li half cells at 60 o C
- XPS analysis of OLO cathode after precycle
- SEM/EDX analysis of cathode after precycle
LiBOB additive, which also contains a boron core in its structure, decomposes and forms a durable but thick resistance layer thanks to its oxalate rings. Its retention capacity, which compares capacity at 7°C and 0.2°C, shows the effect of LiDFBP (44.4% of reference compared to 60.2% of LiDFBP). The oxalate-based SEI layer may be a reason for the increase in resistance, but phosphorus and two fluorines at the core site are expected to stimulate ion transport.
The SEI layer obtained from LiDFBP delays the capacity decay slowly compared to the reference and stabilizes the fluctuation of Coulombic efficiencies. Therefore, the SEI layer formed in the precycle would be broken due to the activation byproduct of oxygen evolution. The broken site recovers and consumes lithium resources from the cathode or lithium salt.
After several cycles of repeating this reaction, it depletes not only the lithium salt in the electrolyte, but also the carbonate-based solvents that were used in the fabrication of the SEI layer. Otherwise, it is possible to think that additive residues that do not participate in the formation of the SEI layer are degraded due to its difficult state, resulting in recovery. High-intensity metal oxides mean that the SEI layer formed on the cathode surface is thin enough to detect the bond between the transition metal and the oxygen in the cathode materials.
Decomposition of LiDFBP based on oxalate ring opening reaction is the same as LiBOB, but final product is different. The P 2p and F 1s spectra show how LiDFBP effectively suppresses dissolution of lithium salt on cathode surface. This result is consistent with XPS F 1s spectra which also show a high fraction of LiF on cathode surface.
In SEM/EDX analysis, it is clear that SEI layer formed by LiDFBP prevents dissolution of salt.
Conclusion
37.J, Zheng; J, Xiao; M, Gu; P, Zuo; C, Wang; J,-G, Zhang., Interface modifications by anion receptors for high energy lithium ion batteries. 38.Y, Zhu; M, D, Casselman; Y, Li; A, Wei; D, P, Abraham., Perfluoroalkyl substituted ethylene carbonates: New electrolyte additives for high voltage lithium-ion batteries.
