Bis(2,2,2-trifluoroethyl) ether and fluoroethylene carbonate hybrid additives to improve the electrochemical performance of Ni-rich cathode

In the case of the anode, the introduction of silicon (Si, ~4200 mAh/g) into the commercialized graphite anode (~372 mAh/g) is the most efficient way to upgrade the performance. The addition of fluoroethylene carbonate (FEC) is considered one of the most effective Si-containing anodes. Schematic representation of the phase transition of the structure and possible migration path of TM cations in charged NMC cathode materials.

A comparison of transition metal solution of 1% BTFE+2% FEC and 3% FEC containing electrolyte at 45oC for 7 days. The SEM images of the surface of gra-SiC-Si alloy anode of LiNi0.8Co0.1Mn0.1O2/gra-SiC-Si alloy full cell after cycle performance with (a) baseline electrolyte after 150. and both with (b ) 1%. The discharge capacity retention and DC-IR during the cycling test at 45 °C of the LiNi0.8Co0.1Mn0.1O2/gra-SiC-Si alloy full cells according to the electrolyte.

Lithium-ion battery

Cathode materials are a source of lithium ions (Li-ions) and emit Li-ions when oxidation reactions occur during their charging and absorb Li-ions to reduce the discharge of electrical energy. Anode materials absorb Li-ions and electrons when charged, unlike cathode materials, and release Li-ions and electrons during the discharge process. Separators are porous polymer films that separate the cathode and anode materials of the battery to prevent an internal short circuit during the passage of the Li-ions so that the charging and discharging process can occur.

The electrolytes act as the channel to provide a path for Li ions to move between the anode and cathode materials. LIBs use the difference in chemical energy between the anode and cathode materials, and during the discharge process, the Li ions in the anode materials are released and in the cathode active material, which has a relatively low level of chemical energy, by electrolyte, when the electrons flow through the external wires to perform their power functions. Charging is an opposite process in which Li ions are stored on the anode active materials by making the energy level of the cathode material high by external power sources.

Figure 2. Schematic illustration of the Lithium-ion battery.

For the high capacity of LIBs

Introduction of Ni-rich cathode

These lead residual Li2O on the Ni-rich cathode surface to inevitably react with moisture and CO2 in the air to form residual lithium as LiOH and Li2CO3.[18]. This leads to the loss of active materials, being lithium remaining on the surface of the cathode. The lithium remaining on the surface reacts with the LiPF6 salt and causes it to form acidic substances such as hydrogen fluoride (HF), which causes the cells to deteriorate.

Also, this reduced nickel ion reacted with HF and formed nickel fluoride (NiF2) on the cathode surface. Furthermore, the NiF2 on the surface can act as a resistive layer which can block the Li-ion path. Thus, surface chemistry and side reactions are the main factors that significantly affect the performance of LIBs using Ni-rich materials as cathode.

Figure 4. Mechanisms of Mn 3+ dissolution. [11]

Introduction of Silicon as anode

Making anode material with the composition of Si and graphite can be a way to reduce the side reaction and obtain the advantages of the capacitance side. In this article, we introduce the graphite-silicon coated graphite and Si alloy composite (gra-SiC-Si alloy). However, the Si breathing problem persisted with repeated cycling and led to the pulverization of the Si alloy and coated silicon, causing repeated damage to the surface.

Another problem with Si, the greater affinity between water and HF than that of graphite, and the reaction is easier, forming an irreversible combination, which also causes loss of Si active material.

Figure 9. Regarding problems with using Si as the anode. [33]

Improvement strategies with electrolyte

EXPERIMENTAL

Electrochemical measurements

Li/Li+ at 25oC at a constant current of C/10 using a computerized battery test equipment (WonATech WBCS 3000). At the first cycle of pre-cycle, after 2 hours of charging, stop charging and cut off the gas space in the bag cell to degas the cell by degraded electrolyte forming the SEI layer on the electrodes. By sealing the bag with a vacuum sealing machine (AZC-010, airzero, INTRISE), the remaining preceding process continued.

After precycling charging, the constant voltage condition was applied until the current was lower than C/20 and 2 cycles ensured the stabilization and formation of the SEI layer and CEI layer on both sides of the electrodes. After the rest time, first discharge for 10 seconds at C/10 and take the remainder of 10 seconds and then charge for 10 seconds at C/10. discharge-rest-charge for every 10 seconds). After these processes we can obtain the voltage of each process and the current with C-rate.

Using ohm's law (R = V/I), with all the information from the above process, we can calculate and get the cell resistance.

Characterization

De-lithiated cathodes were stored in the same type and amount of electrolyte (baseline electrolyte, 2g), only to control the effect of the CEI layer. The retention and DC-IR during the cycling test at 45 °C of the LiNi0.8Co0.1Mn0.1O2/gra-SiC-Si alloy cells according to the electrolyte. These reactions of HF cause depletion of electrolyte and sudden drop in the discharge capacity of the cell.

To identify the degree of microcrack formation in the LiNi0.8Co0.1Mn0.1O2 cathode, cross-sectional SEM images were taken after ion milling with 1% BTFE+2% FEC and 3% FEC. Interestingly, it shows that the BTFE additive was reduced before the EC and formed the SEI layer on the gra-SiC-Si alloy anode which reduced the EC reduction peak. To understand the electrochemical enhancement behavior with 1% BTFE + 2% FEC in LiNi0.8Co0.1Mn0.1O2 /gra-SiC-Si alloy whole cells, the dQ/dV plot of cells after precycling was obtained with and without 1% BTFE +2% FEC.

A computational approach to determine the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels (b) Differential capacitance versus voltage (dQ/dV) curves during precycling at 25oC for the formation for LiNi0.8Co0. 1Mn0.1O2/gra-SiC-Si alloy whole cells with and without BTFE, (c) dQ/dV plots of LiNi0.8Co0.1Mn0.1O2/gra-SiC-Si alloy whole cells with 1%BTFE+2% FEC and 3% FEC in the base electrolyte during the first Li intercalation of the formation cycle. On the other hand, in the case of the 1% BTFE+2% FEC formulation, although a slight deterioration of the edge was observed, there was no deposition of lithium metal on the surface. Therefore, there was much electrolyte decomposition in the anode cycled with base electrolyte and 3% FEC, which reported the high intensity of the corresponding peak.

The surface of the graphite and Si alloy particles was observed to identify the deterioration of the particles in each electrolyte composition. The thickness of the gra-SiC-Si alloy anode was verified after cycling, volume expansion occurred in all three electrolytes compared to the intact cathode. In addition, ICP-OES revealed features that CEI-derived BTFE+FEC additives prevented transition metal dissolution and severe intergranular cracking of secondary particles and smoothed the phase transition on the surface of primary LiNi0.8Co0.1Mn0.1O2 particles.

These components of the SEI layer enable to accommodate the volume expansion of the gra-SiC-Si alloy anode and this is the reason why there is little post-cycle electrolyte decomposition (XPS) and Li metal deposition on the anode.

Figure 10. The mechanism of the LiPF 6 hydrolysis existed ROH in the electrolyte. (a) OPF 3 generation reaction with ROH and PF 5 and (b) further mechanisms by the reactivity of OPF 3

RESULTS AND DISCUSSION

With the above results, we compared the effects of BTFE with FEC additive, which is considered as one of the most effective additives with Si-containing anode. By comparing the discharge capacity with each electrolyte, it was certain that the introduction of the FEC additive reported more improved capacity retention and lower resistance of the cell during the cycle at 45 oC. FEC additives contributed to making the SEI layer on the Si-containing anode, while the charging process and FEC-derived SEI layer effectively sustained the volume expansion of the anode and prevented further Li-ion utilizing reaction, thus improving the cycle performance and reducing the cell resistance .

These because the FEC additive effectively protected the Si-containing anode interface by forming the LiF-based SEI polymer layer preventing the cracking of the SEI layer and further decomposition of the electrolyte between the anode and the electrolyte. The discharge capacity retention value of full cells with 3% FEC is obtained only 39.1% compared to cells introducing BTFE, 1% BTFE+2%. All had electrolytic decomposition on the cathode surface and can be checked to compare the C-F peak at 687.9 eV which represented high intensity while maintaining the surface with PVDF binder.

TEM/STEM measurements were performed to closely examine the phase transition of the primary cathode particle after cycling. In 1% BTFE+2% FEC electrolyte, the thickness of the edge-side rock salt phase is about 2 nm, while in the case of 3% FEC it was more than 7 nm. 44] This could be one of the reasons why 3% FEC electrolytes suddenly decreased in capacity.

The implied intensity of the peaks corresponding to phosphate (P-O) around 137 eV in the P 2p peak (Figure 24(b) and (c) is remarkably reduced for the anode with FEC-containing composition. Even with the naked eye, it is verified that there was severe deterioration on the surface of the anode as deposition of Li metal and traces of tearing on the baseline after cycle 150. and FEC after cycle 250. With 1% BTFE+2% FEC electrolyte the smallest volume expansion occurred, which thanks to having a larger amount of LiF compared to 3% FEC, making the film robust and more volumetric expansion acceptable as (Figure 18(c)) g) The schematic illustration of the Si particle during the repeated cycling process.

This may be the key to why the 1% BTFE+2% FEC electrolyte had a higher 250 cycle capacity and lower cell resistance.

Figure 15. Cycle performance and DC-IR resistance of LiNi 0.8 Co 0.1 Mn 0.1 O 2 /gra-SiC-Si alloy full cells with baseline, 1% BTFE, 1% BTFE+2% FEC, 1% BTFE+3% FEC and 3% FEC additive at 45 o C, at a rate of C/2

Effects of BTFE+FEC on the cathode surface

• Surface modification of LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode….…
• The effect of BTFE+FEC-derived CEI

Effects of BTFE+FEC on the anode surface

• Surface modification of gra-SiC-Si alloy anode
• The effect of BTFE+FEC-derived SEI

CONCLUSION