Does high voltage mean poor cycle performance?
We Said: NO!
In recent years, in order to meet the increasing demand for high energy density of lithium-ion batteries, cathode materials are also developing in two directions:
1) high nickel, higher Ni content can bring higher capacity;
2) high voltage, The increase in voltage on the one hand increases the capacity of the material and on the other hand increases the voltage platform of the material. However, as the charging voltage increases, the stability of the positive electrode material/electrolyte interface decreases, causing an increase in side reactions, which seriously affects the cycle performance of the lithium ion battery. In order to solve the problem of poor stability under high voltage of the ternary material, the surface Coating, single crystal and electrolyte additives are the most commonly used methods.
Recently, EunJi Park and Kuk Young Cho of Hanyang University in South Korea added VC and sulfur-based compounds. BDTT significantly improved the cycling performance of NMC532/graphite cells at high voltage (4.5V), reducing impedance and polarization.
The molecular orbital energies of several electrolyte additives are shown in the following table, where LUMO is the lowest unoccupied orbital energy. The lower the energy, the easier it is to be reduced. HOMO is the highest occupied orbital energy. The higher the energy, the easier it is to be oxidized. It is seen that the LUMO of VC and EC are relatively low, so it is easier to reduce on the surface of the negative electrode, and the LUMO energy of BDTT is higher, so the reduction does not occur on the surface of the negative electrode, but the HOMO energy of BDTT is the highest, so it is easier. Oxidation occurs at the positive electrode.
From the above data, it can be seen that VC is more suitable as a negative electrode additive, and BDTT is more suitable as a positive electrode additive. In order to verify the effect of the two additives on the NMC532/graphite battery, EunJi Park designed additive-free, 2% VC, 1% BDTT and 2% VC + 1% BDTT four electrolyte experiments.
The figure below shows the dQ/dV curve of the four groups of electrolytes in the whole battery. It can be seen that the blank control group starts to rise at about 2.4V and then peaks at around 3.0V. It is generally believed that this peak is due to solvent EC decomposition. of. The battery with VC additive electrolyte showed a reduction decomposition peak around 2.4V, which indicated that VC additive began to decompose earlier than EC, thus reducing the decomposition of electrolyte solvent on the surface of the negative electrode. After adding 1% BDTT additive, the battery began to decompose at about 2.6V, but the decomposition peak intensity of the battery at 3.0V showed a significant decrease, indicating that the addition of BDTT can also inhibit the decomposition of EC solvent on the surface of the negative electrode to some extent. We can see from the following figure that the combination of VC and BDTT can effectively stabilize the negative SEI film and reduce the decomposition of EC on the surface of the negative electrode.
It can be seen from the following figure b that the conductivity of the electrolyte is reduced after the addition of the additive to the electrolyte, but it is basically at about 10 mS/cm, which satisfies the application requirements, and the viscosity of the electrolyte also increases after the addition of the additive.
The following figure shows the 0.5C cycle curve of a battery with several different additives in the range of 2.7-4.5V (vs Li/Li+). From the following figure a, it can be seen that increasing the charge cut-off voltage significantly increases the gram capacity of the NMC532 material. The NMC532 material with BDTT had a gram capacity of 169.3 mAh/g, which was higher than the blank electrolyte control group of 167.3 mAh/g. After 200 cycles, the capacity retention rate of the cells with BDTT and VC was 11.9% higher than that of the blank group. At 60 ° C high temperature, BDTT + VC additive for lithium-ion battery cycle performance is more obvious (as shown in Figure b below), the author believes that this is mainly due to BDTT formation of a stable interface film on the positive electrode, and VC forms a stable interfacial film on the surface of the negative electrode.
The following figure shows the charge-discharge voltage curves of different electrolyte batteries at 1, 50, 100, 150 and 200 cycles. From the following figure a, it can be seen that the battery with no additive has a rapid decay in the discharge voltage curve, and BDTT is added. The discharge voltage decay during the subsequent battery cycle is significantly reduced, which indicates that the addition of BDTT can effectively suppress the side reaction at the positive electrode interface and reduce the increase of the positive electrode interface impedance, thereby reducing the voltage drop of the battery during charge and discharge. .
At the same time, we can compare the voltage difference during charging and discharging to analyze the polarization of the battery. From the following figure, we can see that the polarization of the battery after adding VC+BDTT additive is significantly reduced, which indicates that the addition of VC and BDTT can be simultaneously The interface between the positive and negative electrodes is stabilized, thereby reducing polarization during charging and discharging.
AC impedance is the most effective and most intuitive method for analyzing the interface impedance of lithium-ion batteries. The following figure shows the AC impedance data of batteries with different electrolytes. From the figure we can see that the AC impedance spectra of these batteries are mainly composed of two semicircles. The semicircle in the higher frequency range mainly reflects the impedance Rf of the Li+ through the SEI film, and the semicircle in the low frequency region mainly reflects the charge exchange impedance Rct, which can be seen in the first cycle from the following figure a. The SEI membrane impedance of the additive battery was significantly higher than that of the blank control group, mainly because the additive promoted the formation of a more stable SEI film at the interface between the positive and negative electrodes, but we observed the AC impedance spectrum after 200 cycles (Fig. b below). The charge exchange resistance of the battery without additives has a significant increase, which is much higher than that of the battery containing BDTT additive. This is mainly because BDTT can form a protective layer on the surface of the positive electrode, thereby reducing the surface of the positive electrode at high voltage. Side reaction.
In order to further verify the above inference, the author analyzed the elements of the positive surface of NMC532 after 200 cycles by XPS tool. In the C1s diagram, the characteristic peak of PVDF is near 284.8eV. We can find the battery with different electrolytes. The peak intensity of the battery with BDTT electrolyte of 284.8eV is higher, indicating that BDTT can effectively reduce the decomposition of the electrolyte on the surface of the positive electrode, thereby obtaining a thinner interface film.
LiF is an important electrolyte decomposition product. From the following figure b, it can be seen that the addition of BDTT to the electrolyte has a significantly lower LiF content than the blank control group, indicating that the interface film formed by BDTT on the surface of the positive electrode is more stable. It can better inhibit the decomposition of the electrolyte on the surface of the positive electrode.
In order to prove that the above effect is achieved by the formation of a more stable SEI film on the surface of the positive electrode, the authors analyzed the S element on the surface of the positive electrode and found that Li2SO3 (169.1eV and 168.6eV) and RSO3Li (167.9) were present on the surface of the positive electrode to which BDTT was added. eV) and ROSO2Li (167.8eV) and other components, which is also a strong evidence that BDTT additives form a stable interface film on the surface of the positive electrode.
The following figure shows the fresh NMC532 positive electrode and graphite negative electrode, and the SEM image of the positive and negative electrodes after 100 cycles of different electrolytes. From the figure, we can see that there are significant differences between different electrolytes, from the following figure b and g We can see that the positive and negative surfaces of the blank control group are covered by a large amount of electrolyte decomposition products. At the same time, we noticed that the additive can significantly reduce the decomposition of the electrolyte on the positive and negative surfaces. Although BDTT and VC respectively stabilize the interface between the positive and negative electrodes, in fact, the two additives have a certain effect on improving the stability of the interface between the positive and negative electrodes. The electrolyte decomposition products of the extreme surface are relatively small.
High voltage is an important direction for the development of cathode materials for lithium-ion batteries in the future. Eun Ji Park's research indicates that the S-containing compound BDTT can form a stable interfacial film on the surface of the positive electrode, inhibiting the decomposition of the electrolyte on the surface of the positive electrode, thereby significantly improving The cycle stability of the NMC532 material at high voltages, especially when used with VC, can significantly improve the cycle performance of NCM/graphite batteries.
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