Introduction

LiNi0.8Co0.1Mn0.1O2 is regarded as a promising cathode material in lithium ion batteries (LIB) for its high specific capacity and low cost. While the short cycle life, severe cathode deterioration as well as the poor safety characteristic hinder its wide application. In this work, we introduce a novel additive, Bis(4-fluorophenyl)-methanone, which enhances the capacity retention from 75.19% to 83.04% after 100 cycles at the rate of 1C. Researchers present a novel sulfone-derived organic compound, Bis(4-fluorophenyl)-methanone, as high voltage additive. As shows, this compound contains sulfone, phenyl and fluoride group. According to the previous researches, the sulfone group is helpful for forming a protective layer on the positive electrode, protecting the electrolyte from decomposition, and alleviating the deterioration of cathode at high voltage. Meanwhile, it is recognized that the aromatic ring enhances the oxidation stability of the electrolytes at high voltage, as well as the tolerance for overcharge. We speculated that the Bis(4-fluorophenyl)-methanone can facilitate the formation of protective layer on the surface of cathode, and the Bis(4-fluorophenyl)-methanone-derived film can protect against not only the decomposition of the electrolyte solvent but also oxidation of the cathode. [1]Scientists confirmed this speculation, it is found that the half-cell containing 0.5% Bis(4-fluorophenyl)-methanone have a much better electrochemical performance, and the discharge capacities with and without Bis(4-fluorophenyl)-methanone are 157.78 (mAh/g) and 173.76 (mAh/g) after 100 cycles at the rate of 1 C respectively. Besides, the half-cell with 2% Bis(4-fluorophenyl)-methanone exhibited a better overcharge tolerance than that without Bis(4-fluorophenyl)-methanone.

Bis(4-fluorophenyl)-methanone as a High Voltage Additive

To demonstrate the function of Bis(4-fluorophenyl)-methanone on film formation, it is necessary to calculate the HOMO energy of the Bis(4-fluorophenyl)-methanone and the electrolyte solvents. Asshows, the HOMO energy value of Bis(4-fluorophenyl)-methanone is slightly higher than that of the carbonates. As is known to us all, the higher is the HOMO energy value, the easier loose the electrons. Therefore, it shows that Bis(4-fluorophenyl)-methanone will be oxidized prior to the carbonates, which can suppress the severe oxidation of the electrolyte solvents. Galvanostatic method was adopted to investigate the cycle performances and rate capacities of cells with different content of Bis(4-fluorophenyl)-methanone. As shows, these cells were charged and discharged at the rate of 0.3C for the first three cycles, and then operated at the rate of 1C for the rest 97 cycles. It is observed that, the initial charge-discharge efficiency of the cells with and without Bis(4-fluorophenyl)-methanone are almost same, while the discharge capacities of the cells with standard electrolyte and with electrolyte containing 0.5% Bis(4-fluorophenyl)-methanone are 157.78mAh/g and 173.76 mAh/g respectively, which indicates that the discharge capacity of cells with Bis(4-fluorophenyl)-methanone are higher than that of the cells with standard electrolyte. After 100 cycles, the blank cells declined rapidly. In contrast, the batteries with Bis(4-fluorophenyl)-methanone exhibit higher capacities, and have a better cycle stability, attributed to the preferable decomposition of Bis(4-fluorophenyl)-methanone.[2]

Linear sweep voltammetry (LSV) was carried out to investigate the feature of Bis(4-fluorophenyl)-methanone as preferential decomposition. As shown in, the electrolyte with 0.5% Bis(4-fluorophenyl)-methanone has a peak at about 3.9 V (vs. Li/Li+), indicating that the Bis(4-fluorophenyl)-methanone is oxidized prior to the base electrolyte. In addition, the blank electrolyte begins to decompose at the voltage of 4.4 V (vs. Li/Li+), but the electrolyte with 0.5% Bis(4-fluorophenyl)-methanone begins to decompose at the voltage of 5.2 V (vs. Li/Li+). This demonstrates that the decomposed products of Bis(4-fluorophenyl)-methanone passivates the cathode surface and hinder further decomposition of the electrolyte at high voltage.

HOMO Energy Calculation and Electrochemical Performance Analysis

The potential curves of NCM811 is 0.5%, Bis(4-fluorophenyl)-methanone, 0.7%Bis(4-fluorophenyl)-methanone and without Bis(4-fluorophenyl)-methanone. It is observed that, in the initial cycle s, these cells with additive and without additive perform similarly. After 100 cycles, they are 173.76 mAh/g, 174.98 mAh/g and 157.78 mAh/g, respectively. It suggests that the blank cell has a poorer capacity retention. Meanwhile, in contrast to the cells with additive, the discharge curve of the blank cell moves downwards much, and the charge curve of that moves upwards apparently. It demonstrates that the cells with Bis(4-fluorophenyl)-methanone contained electrolyte have less pronounced polarization behavior than the one without Bis(4-fluorophenyl)-methanone after cycling 100 cycles, which represents lower irreversible electrochemical behavior, leading to a better electrochemical performance.

Cyclic Voltammetry was tested to investigate the transition of lattice and polarization of the cells during the process of charge and discharge. As shown, the cell with Bis(4-fluorophenyl)-methanone represents a bit larger polarization than the blank one after 1 cycle. While the former shows slower polarization growth than the latter. Especially, after 50 cycles, the cell with Bis(4-fluorophenyl)-methanone has a relatively completive lattice transition. To compare the polarization of the cells, the value of the △E (the difference between oxidation peak and reductive peak) was analyzed. After 50 cycles, the △E of blank cell is 0.49 V, and the Bis(4-fluorophenyl)-methanone contained cell is 0.25 V, which demonstrates that Bis(4-fluorophenyl)-methanone can reduce the polarization of cells effectively. In addition, the three couple of anodic and cathodic peaks (3.89, 3.64; 4.07, 3.91; 4.25, 4.05 V vs. Li/Li) shown in CV curves are responding to the three phase transitions of NCM 811 lattice. They are the transition of hexagonal to monoclinic, monoclinic to hexagonal, and hexagonal to hexagonal, respectively. It is found that the blank cell’s peaks are less prominent, while the cell with Bis(4-fluorophenyl)-methanone shows clear peaks, demonstrating that the cell with Bis(4-fluorophenyl)-methanone can form a stable film on cathode to prevent the active material from corrosion, and maintain its lattice structural stability.

Insights from Linear Sweep Voltammetry (LSV) Analysis

To further understand the effect of Bis(4-fluorophenyl)-methanone on the stabilizing the cathode, the morphology of cathode surface after 50 cycles and 100 cycles are shown. After 50 cycles, an amorphous layer is covered on the surface of the electrode, which consists of lithium salt and electrolyte solvent. Comparing to the referent electrode, the electrode with Bis(4-fluorophenyl)-methanone has a cleaner layer, and has fewer cracks on the surface. It demonstrates that Bis(4-fluorophenyl)-methanone can prevent electrolyte from decomposition, and decrease the decomposition product inhering the surface. Meanwhile, after 100 cycles, it is obviously observed that the electrode without Bis(4-fluorophenyl)-methanone suffers a severe corrosion than the one with Bis(4-fluorophenyl)-methanone. Therefore, the results imply that Bis(4-fluorophenyl)-methanone can resist the corrosion of the electrode and the decomposition of electrolyte. The overcharge protection performance of Bis(4-fluorophenyl)-methanone additives was further investigated by charging the batteries with 0%, 1%, 2% Bis(4-fluorophenyl)-methanone to 5 V at the rate of 0.1C. As shows, the curse representing the cell with Bis(4-fluorophenyl)-methanone are steadier and smoother than the blank. In the cells without Bis(4-fluorophenyl)-methanone, the discharge voltage rises sharply to 5 V, while the cells with Bis(4-fluorophenyl)-methanone rise relatively more slowly. The time charging to 5 V are 12.15 h, 17.2 h and 21.62 h, respectively. The time of cell with 2% Bis(4-fluorophenyl)-methanone was almost 1.8 times than that of the cell without Bis(4-fluorophenyl)-methanone. It demonstrates that Bis(4-fluorophenyl)-methanone can alleviate the overcharge behavior, and improve the safety of batteries.[3]

Summary

Bi(4-fluorophenyl) sulfone was proposed to be a new high voltage additive for Ni rich lithium ion batteries. The HOMO and LUMO theoretical calculations of the additive and blank electrolyte suggested the sacrificial oxidized behavior of Bis(4-fluorophenyl)-methanone. When tested as the electrolyte additive for NCM811-Li half-cell, the cell with Bis(4-fluorophenyl)-methanone enhances the discharge capacity from 157.78 m·Ah/g to 173.76 mAh/g, which enhances the cycling stability much. Moreover, overcharging test suggests that Bis(4-fluorophenyl)-methanone also has a function to alleviate overcharge behavior, which contributes to the decrease the safety issues. These findings lead us to conclude that Bis(4-fluorophenyl)-methanone as a bifunctional additive, not only can decompose primarily and prevent the electrolyte from decomposition, but also can protect the cathode from severe deterioration, moreover, it can prolong the overcharge time and enhance the safety of cells. Therefore, we believe that further progress can be provided by this experiment. Scientist has contributed to find a functional additive, and the future study will focus on the application of Bis(4-fluorophenyl)-methanone in full cells and we will investigate its influence on graphite anode. Moreover, we will devote to mixing Bis(4-fluorophenyl)-methanone with other additives to investigate the comprehensive function on cells.

References

[1] J. Liu, X. Song, L. Zhou, S. Wang, W. Song, W. Liu, et al, Nano Energy, 46 (2018), pp. 404-414

[2] V. Etacheri, R. Marom, E. Ran, G. Salitra, D. Aurbach, Energy Environ. Sci., 4 (2011), pp. 3243-3262

[3] T. Yim, K.S. Kang, J. Mun, H.L. Sang, S.G. Woo, K.J. Kim, et al, J. Power Sources, 302 (2016), pp. 431-438