The influence of the voltage plateau on the coulombic efficiency and capacity degradation in LiNi0.5Mn1.5O4 materials

Journal of Alloys and Compounds 820 (2020) 153443

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The influence of the voltage plateau on the coulombic efficiency and capacity degradation in LiNi0.5Mn1.5O4 materials Xiaoling Cui a, b, Tongtong Geng a, b, Feilong Zhang a, Ningshuang Zhang a, b, Dongni Zhao a, b, Chunlei Li a, b, Shiyou Li a, b, * a b

College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou, 730050, China Gansu Engineering Laboratory of Cathode Material for Lithium-ion Battery, Lanzhou, 730050, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 February 2019 Received in revised form 1 December 2019 Accepted 17 December 2019 Available online 19 December 2019

High-voltage LiNi0.5Mn1.5O4 materials with working voltage of 4.7 V vs. Li/Liþ can be used in lithium-ion batteries to meet the demands of high-voltage applications. However, these materials tend to undergo capacity degradation with cycling, and exhibit low coulombic efficiency. The redox reactions of the transition metals in LiNi0.5Mn1.5O4 are related to voltage plateaus, which have an important influence on the electrochemical performance of lithium-ion batteries. In this work, we investigated the effects of two voltage plateaus for LiNi0.5Mn1.5O4 cathode (4.0 V vs. Liþ/Li and 4.7 V vs. Liþ/Li) on the capacity degradation and coulombic efficiency, through the charging of half-cells to different cut-off potentials. We find that the redox reactions of manganese at the 4.0 V plateau play a negligible role in the capacity loss, while the increase in manganese content in the cathode material can significantly affect the coulombic efficiency. By contrast, when the cell was charged to cut-off potentials 4.8 V with a plateau at 4.7 V for nickel, the cell capacity degraded rapidly. Interestingly, when the cells were charged to 5.0 V, the content of P, F, Ni, and Mn increased with the thickness of the SEI film, indicating accelerated decomposition of the electrolyte due to the contribution of nickel. Thus, this work verifies the dependence of the capacity degradation and coulombic efficiency on the voltage plateau, and provides important information for further investigation of the effect of voltage plateaus on the characteristics and behavior of cathode materials. © 2019 Elsevier B.V. All rights reserved.

Keywords: LiNi0.5Mn1.5O4 Voltage plateau Coulombic efficiency Capacity degradation Cut-off potential

1. Introduction Lithium-ion batteries (LIBs) are considered one of the most suitable energy storage systems, and are widely used in a variety of applications, from portable electronic products and all the way up to electric vehicles, merited by a high energy density, high power density, long life, and nearly no memory effect [1e4]. However, the currently available LIBs suffer from capacity and power fade, and are therefore incapable of meeting the stability and safety requirements necessary for long-term use. Many cathode materials have been investigated towards the goal of improving the performance of LIBs in terms of rate capability, cycle stability, and other metrics of operational longevity. High-voltage spinel LiNi0.5Mn1.5O4 is one of the most promising

* Corresponding author. College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou, 730050, China. E-mail address: [email protected] (S. Li). https://doi.org/10.1016/j.jallcom.2019.153443 0925-8388/© 2019 Elsevier B.V. All rights reserved.

cathode materials for LIBs, due to its high voltage plateau at 4.7 V, high energy density, high theoretical capacity, and superior rate capability [5e7]. However, it suffers from low coulombic efficiency (CE) and rapid capacity degradation, which remain major obstacles hindering the use of this material as battery cathode [8e10]. A higher coulombic efficiency indicates better cycle life and higher energy efficiency. A coulombic efficiency approaching 100% indicates that minimal undesired side reactions, such as active material loss and electrolyte decomposition, occur during the charging and discharging of the device. In other words, maximizing coulombic efficiency depends on the success in minimizing side reactions [11], because low efficiency is indicative of irreversible consumption of recyclable Liþ caused by these reactions. However, the relationship between the coulombic efficiency and cycle life of a half cell is not necessarily simple, as low efficiency may not necessarily result from rapid capacity degradation [12]. In general, lithium insertion/extraction is accompanied by changes in the chemical valence of the transition metals in the

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cathode, which is related to the voltage plateau of these materials. The redox reactions of transition metals have been studied in the literature through cyclic voltammetry (CV) and differential capacity curves (dQ/dV), to understand the electrochemical film formation behavior of the cathode material [7,13e15]. For LiNi0.5Mn1.5O4, the charging and discharging profile of the battery consists of a redox voltage plateau for nickel at approximately 4.7 V, and may also comprise an additional plateau at 4.0 V, originating from the redox reaction of manganese that is inevitably introduced during the material synthesis process, and can also influence battery performance. It has been reported that a small amount of manganese can increase the structural disorder in the material, and thus improve the rate capability [16e18]. However, manganese dissolution, and the Jahn-Teller effect, can on the other hand affect both the cyclic performance and the coulombic efficiency. Furthermore, modifications arising due to metal doping, surface layers coating, and electrolyte additives, which are adopted to improve the cycle life for high-voltage systems, will also affect the battery performance through the capacity and coulombic efficiency [13,19e21]. To the best of our knowledge, studies investigating the relation between the voltage plateau and the electrochemical performance of lithium-ion batteries remain scarce. In this work, we investigated that relation for LiNi0.5Mn1.5O4 cathode, through charging/discharging to different charge cut-off potentials (4.0 V vs. Liþ/Li and 4.7 V vs. Liþ/Li), and examined the corresponding changes in the coulombic efficiency and capacity degradation with cycle number. We also evaluated the impact of charging time, which is related to the Mn content, on the resulting coulombic efficiency. 2. Experimental 2.1. Electrode preparation and cell assembly The active material for the cathode (LiNi0.5Mn1.5O4) was a commercial product purchased from Sichuan Xingneng New Material Co., China. The electrode used for half cells was fabricated by mixing LiNi0.5Mn1.5O4 powder (80 wt%), acetylene black (10 wt%), and PVDF binder (10 wt%), then dissolving the mixture in N-methyl pyrrolidone (NMP) solvent. The resulting slurry was spread onto Al foil, then dried at 100  C for at least 12 h. The half cells were assembled in an Ar-filled glovebox, with Li foil as both the counter electrode and reference electrode, and a sheet of Celgard polypropylene 2400 as the separator. The electrolyte system was 1 M LiPF6 dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) with a ratio of 1:1 in volume. The electrochemical cells were kept at room temperature for 24 h prior to the measurements, to ensure complete permeation of the electrolyte through the electrodes and the separator. 2.2. Electrochemical testing Galvanostatic charge/discharge measurements of the assembled half cells were performed at room temperature, using a battery test system (LAND CT-2001A instrument, Wuhan, China). The charge cut-off potentials ranged from 4.0 V to 5.0 V, while the discharge cut-off potential was set to 3.5 V. To study the effect of the two plateaus (4.0 V and 4.7 V) on the performance of LiNi0.5Mn1.5O4, all the assembled cells were divided into two groups: group I, which was charged at cut-off potentials below 4.7 V (4.0e4.7 V), and group II that was charged at potentials above 4.8 V (4.8 Ve5.0 V). The detailed settings for the cycle tests are shown in Table 1. Electrochemical impedance spectroscopy (EIS) was carried out in a three-electrode configuration, using an electrochemical workstation (CHI660D, Shanghai, China), and within a frequency range of 0.01 Hze100 kHz, with all the measurements performed at room temperature.

Table 1 The settings of the cycle tests. Group

Charging potential/V

plateau voltage/V

Group I Group II

4.0, 4.3, 4.5, 4.6, 4.65, 4.7 4.8, 4.9, 5.0

4.0 4.7

2.3. Characterization The electrodes were charged and discharged for 150 cycles at 1.0 C, after which they were washed several times with dimethyl carbonate (DMC), and dried in an Ar atmosphere. X-ray diffraction (XRD) was performed using a D/max-2400 from Rigaku (Japan) in the 2q range from 10 to 90 , with a step size of 0.02 , and a step time of 15 s/step. Scanning electron microscopy (SEM) (JSM-5600, Japan) was used to image the surface morphology of the electrodes charged at different cut-off potentials. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used to obtain information about the elemental composition on the surface of the electrode.

3. Results and discussion 3.1. Electrochemical performance of LiNi0.5Mn1.5O4 under various cut-off potentials The voltage profiles for LiNi0.5Mn1.5O4 electrodes in the cells of groups I and II are shown in Fig. 1a and Fig. 1b, respectively. Fig. 1a shows that the discharge capacity and plateau potential show visible increase with increasing the potential during the initial discharge process. This increase may be attributed to the contribution from added recyclable Liþ in the battery system, and also from the redox reaction of manganese. To verify the discharge capability of the cells in group I, with a 4.0 V plateau, the contribution of the charge capacity in the constant-current charging stage was ignored, as shown in Fig. 1a. The charge/discharge curves for group II, measured after three formation cycles at 0.1 C, show a negligible difference from group I. The curves consist of two plateaus corresponding to the redox reactions of the nickel and manganese. The cells of group II deliver discharge capacities of approximately 122e131 mAh g1. Brutti et al. reported the low efficiency in first-cycle of LiNi0.5Mn1.5O4 in three voltage ranges [8]. However, our results suggest that the initial coulombic efficiency increases with increasing the charging potentials, up to the value of 4.8 V, then decreases with further increase in the potential. Interestingly, the cells charged at potentials from 4.6 V to 4.7 V show different behavior, as shown by the red circle (Fig. 1c). We also observed that the coulombic efficiency was always less than 90%, irrespective of the cut-off potentials. Based on these phenomena, the accumulated coulombic efficiencies can be explained as follows: (i) the low efficiencies at cut-off potentials from 4 V to 4.5 V can be attributed to the insufficient driving forces for Liþ intercalation, which are associated with kinetic control, (ii) the abnormal coulombic efficiencies obtained for the cells charged to potentials from 4.6 V to 4.7 V, can be ascribed to the limited kinetics due to low charging potentials, and to the decomposition of LiPF6, which inhibits the transportation of Liþ ion via diffusion, (iii) the low efficiency for a cut-off potential over 4.8 V could result from the growth and regeneration of an interfacial film, which is accelerated by the decomposition of the electrolyte. The interfacial film restricts the number of removable lithium ions, and increases the barrier for Liþ diffusion.

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Fig. 1. (aec) Electrochemical performances of two groups; (d) dQ/dV plots of LiNi0.5Mn1.5O4 measured at the initial cycle of group II; (e) dQ/dV plots cycled from 3.5 to 5.0 V for three cycles.

To further investigate the low efficiency at cut-off potentials over 4.8 V, especially 5.0 V (group II), we performed differential capacity dQ/dV analysis for the first charge-discharge process, as shown in Fig. 1d and e. The plot of spinel electrode shows two redox peaks at approximately 4.0 V and 4.7 V, with the LiNi0.5Mn1.5O4 lattice undergoing Mn3þ/Mn4þ or Ni2þ/Ni3þ and Ni3þ/Ni4þ oxidation via the deintercalation of Liþ ions [22]. In addition, the intensities and areas of the charging and discharging peaks at approximately 4.7 V, are much stronger than those of the peaks around 4.0 V, as shown in the inset of Fig. 1d and e, suggesting that the redox reactions of Ni significantly affect the capacity [9] and the

capacity loss. However, prolonged cycling of LiNi0.5Mn1.5O4 at 5.0 V leads to an increased degree of polarization, with the peaks shifting to higher potential, and also results in sluggish kinetics for Liþ transportation, due to the instability of the electrolyte at high voltage. We attempted to shed light on these factors by measuring EIS spectra, in order to track the electrochemical processes that will affect the capacity and the coulombic efficiency of the electrode. Fig. 2 shows the changes in the impedance spectra of a LiNi0.5Mn1.5O4 cathode in the delithiation state, at various cut-off potentials. In Fig. 2a, the impedance spectrum at 4.0 V shows two separated semicircles, which can be ascribed to Liþ diffusion

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Fig. 2. (aeb) The EIS of the samples during the first cycle for group I and group II. All impedance spectra are fitted by (c) the equivalent circuit.

through the interfacial film (Rfilm), and the charge-transfer process corresponding to film impedances (Rct) [23]. The linear part of the plot is associated from the diffusion of Liþ traveling through the solid structure of the electrode [24]. With the open-circuit voltage charging to 4.3 V or higher, the two semicircles turn into one semicircle, and the diameter of the semicircles increases first, then starts to decrease. This phenomenon occurs due to the initial charging stage, as the electrochemical polarization of the material increases with the increase in the amount of intercalated lithium, and the impedance increases. During electrochemical charging/ discharging, the material is gradually activated, and impedance begins to decrease. However, when the cells are charged to higher voltages (group II), as shown in Fig. 2b, the impedance begins to increase and become more prominent, due to the accelerated decomposition of the electrolyte at the electrode/electrolyte interface under high-voltage. All impedance spectra were fitted by the equivalent circuit shown in Fig. 2c, and the detailed impedance values are given in Table 2. The variations in the values of Rfilm and Rct in the potential region from 4.0 to 5.0 V can be attributed to the reversible breakdown of the interfacial film, and the instability of the interfaces [25]. The cycling performance for the two groups of cells is shown in Fig. 3a and b. For group I, the discharge capacities are lower than 20 mAh g1, which means that the number of recyclable Liþ ions in the material is small. In addition, all cells maintained high capacity retention above 95% at all potentials, except 4.7 V with little fluctuation. By contrast, the discharge capacity was recovered to normal levels when the cut-off potentials increased to 4.8 V or higher, with reasonable capacity retention (group II), as shown in Fig. 3b. Unfortunately, these cells suffered from critical capacity

fade, especially at 5.0 V. It can be concluded from Fig. 1aeb and Fig. 3aeb, that the redox reaction of manganese at the 4.0 V plateau contributes to the capacity of the cells when charged below 4.7 V, while the redox reaction of Ni dominates the capacity in cells charged over 4.7 V. Because of the smaller capacity loss of the former compared to the latter, we conclude that the primary reason for the capacity degradation of cells is associated with the redox reaction of Ni. The coulombic efficiencies of cells charged at various cut-off potentials during long-term cycling are shown in Fig. 3c and d. All voltages appear to achieve a relatively high coulombic efficiency and reasonable capacity retention, except for the lower coulombic efficiency value obtained at 4.0 V, due to the sluggish kinetic process. It should be noted that the low efficiencies achieved during the first several cycles are related to the consumption of recyclable Liþ as a result to the formation of the solid electrolyte interface (SEI). However, for the cells in group I that are charged below 4.7 V (Fig. 3c), the coulombic efficiency ranged from 90% to 99%, and were unstable with cycling, which is consistent with the change in the coulombic efficiencies in Fig. 1c. For the cells with higher charge potentials (group II) in Fig. 3d, all the values shift to 98% or higher, which shows the effect of the cut-off potentials on the coulombic efficiency during cycling. Moreover, these low values imply the occurrence of side reactions at the electrode/electrolyte interface. It is worth mentioning that to display the smaller efficiency range that emphasizes the change of the efficiency as a function of the cut-off potential, the initial coulombic efficiencies are not shown in Fig. 3c and d (the initial coulombic efficiency is shown in detail in Fig. 1c). Furthermore, the fluctuations in the overall capacity and coulombic efficiency with cycling are caused by small temperature variations in the environment.

Table 2 The fitting impedance values of Rfilm and Rct during the initial cycle under various potential conditions.

3.2. Structural changes

Group I Potential Rfilm/U Rct/U

4.0 V 105.6 234.8

Group II 4.3 V 28.19 253.3

4.5 V 58.26 271.7

4.6 V 15.71 373.2

4.65 V 31.42 330.6

4.7 V 42.87 310.1

4.8 V 148.4 328.4

4.9 V 293.6 346.7

5.0 V 330.4 435.1

Fig. 4 shows the XRD patterns obtained for LiNi0.5Mn1.5O4 electrode samples that had been cycled to various cut-off potentials. To identify the structural changes in the two groups of cells, representative samples were selected. The observed diffraction pattern of pristine spinel LiNi0.5Mn1.5O4 can be indexed to a cubic

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Fig. 3. Specific discharge capacities and coulombic efficiency at 1.0 C over 150 cycles for cells operated with different cut-off potentials.

Fig. 5. Discharge medium voltage of cells cycled at various cut-off potentials. Fig. 4. XRD diffraction pattern obtained from the pristine LiNi0.5Mn1.5O4 electrode samples and those representative samples with 4.3, 4.6, 4.8 and 5.0 V in two groups. The unlabeled peaks represent carbon.

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structure with the space group Fd-3m. However, the peak positions for these cycled samples do not show any differences compared to the pristine sample, which indicates that the cubic spinel structure of the LiNi0.5Mn1.5O4 material is retained without collapse. The only change is the peak intensity with increasing the cut-off potential, which indicates the contraction of the unit-cell dimension, leading to the dissolution of transition metals and the corrosion caused by side-reaction products, such as HF that is formed from the decomposition of the electrolyte. This causes the cells of group II to suffer from serious deterioration in both the coulombic inefficiency and capacity.

3.3. Mechanisms for capacity fade and coulombic inefficiency

Fig. 6. Relative coulombic efficiency variation as a function of manganese content. Note that the charging times correspond to the voltage ranging from 4.0 V to 4.1 V, which represent the manganese content during this charge process.

Although the electrochemical performance of the two groups of cells in the initial and subsequent cycles has already been evaluated, and discussed in relation to the effect of the two plateau voltages (4.0 V and 4.7 V) on capacity fade and coulombic efficiency, some of the negative effects remain unclarified. Therefore, and to

Fig. 7. SEM image of (a) the pristine LiNi0.5Mn1.5O4 electrode and (bee) the electrodes at different cut-off potentials after the cycling test.

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elaborate on the phenomenon of capacity fade, we examined the changes in the discharge medium voltages, defined as the voltage values at which the cells are discharged to half of their capacity [26], as shown in Fig. 5. For cells cycled below 4.7 V (group I), negligible differences in the medium voltages were observed during long-term cycling, especially for the cells discharged at maximum potential of 4.7 V. By contrast, there are evident changes in the discharge medium voltages at charging potentials of 4.8 and 4.9 V. A significant drop from 4.5457 V to 4.2968 V after 150 cycles can be observed when the cells are charged up to 5.0 V. These changes suggest the occurrence of significant polarization and side reactions in cells operated above 4.7 V (group II), which eventually results in rapid capacity loss. The above results demonstrate that the capacity fade is mainly due to the plateau where Ni redox reactions take place. However, it is widely acknowledged that manganese dissolution is one of the main factors underlying the deterioration of spinel structure material [27,28]. In addition, when cells are charged at high potentials (>4.7 V), two plateaus appear, and correspond to the redox reactions of the nickel and manganese (Fig. 1b). Therefore, the contribution of manganese to the capacity loss is not negligible. As a result, it is necessary to stress the effect of manganese on the electrochemical performance of batteries with LiNi0.5Mn1.5O4 as the cathode material. The main valence state of manganese in LiNi0.5Mn1.5O4 is tetravalent. The introduction of trivalent manganese during material synthesis is unavoidable [29], and once the potential reaches the value corresponding to the redox reaction of manganese, these reactions will affect the battery performance. Considering the disproportional reaction of Mn3þ, which can occur at low electrode potentials, the dissolution of Mn can also be associated with the onserved capacity and the coulombic efficiency [30,31]. Fig. 6 shows the effect of the amount of manganese on the coulombic efficiency. The charging time corresponding to the voltage range from 4.0 V to 4.1 V was used to evaluate the manganese content. As shown above, the coulombic efficiency obtained from ten half cells is consistent for three different charging times. The average coulombic efficiency increased from 98.4% to 99.5% with decreasing the charging time, indicating that the manganese content is one of the main causes of the low coulombic efficiency of LiNi0.5Mn1.5O4.

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In Fig. 8a and b, the cells are charged at 4.5 V (group I) and 5.0 V (group II) to verify that the capacities are attributed to the different voltage plateaus discussed above. It can be seen that there is no difference in the elemental species in both cases, but a difference in peak intensities is observed, especially for the Ni and Mn elements, demonstrating the different degrees of electrolyte decomposition occurring at the electrode/electrolyte interface. Fig. 8c shows the relative amounts of elements for a fresh electrode, and for electrodes that underwent 150 cycles at 4.3, 4.5, 4.9, and 5.0 V. For the fresh cathode electrode, C, O and F are the typical composition elements, and originate from acetylene black and polyvinylidene difluoride. In addition, the sample contains Ni and Mn transitional metals. However, for the cycled electrode, P and F are formed on the surface, and their elemental content, which is marked by the pink and yellow areas, increases with the increase in the charging voltage, indicating accelerated decomposition of LiPF6 at the cathode/electrolyte interface [32]. The phosphorous compounds and fluorine-containing compounds are known as high-impedance impurities that can block intergranular spaces, and increase the resistance for Liþ diffusion, resulting in the low coulombic efficiencies observed in Fig. 1c. In addition, XPS analysis suggests notable increase in the elemental content of Mn and Ni when the cell is cycled at 5.0 V, compared to other voltage values, as indicated by the green and blue areas. This change in the content of the two metals is attributed to the dissolution of the two metals from the cathode lattice, due to the excess removal of Li ions, and the subsequent redeposition of manganese-containing compounds on the cathode surface

3.4. Surface morphology To verify our conclusions, subsequent characterization tests were needed. The morphology differences for electrode particles after charging at various charge cut-off voltage values were characterized by SEM, as shown in Fig. 7, which shows the changes in the surface morphology of LiNi0.5Mn1.5O4 electrodes with different potential values. Fig. 7a presents a pristine electrode with a smooth surface and clear outline of the spinel structure. The surface of the particles after the electrode was charged at 4.3 V (Fig. 7b) is covered with a thin layer of the so-called SEI film, and the particles’ outline became indistinct. When the cut-off potential was increased to 4.8 V or higher, the SEI film grew thicker and became porous, due to the acceleration of the oxidation of the electrolyte, which can be ascribed to the contribution of nickel to the 4.7 V plateau. Detailed surface information is provided below. 3.5. Investigation of the SEI film composition Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were employed to investigate the elemental composition and distribution in the SEI layer formed at the cathode surface, and their results are shown in Fig. 8 and Fig. 9, respectively.

Fig. 8. EDS spectrum of cells cycled at (a) 4.5 V and (b) 5.0 V. (c) Element compositions (wt %) of SEI layers formed in different operation potentials after 150 cycles.

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Fig. 9. XPS spectra of (a) C 1s, (b) F 1s, (c) Mn 2p and (d) Ni 2p for LiNi0.5Mn1.5O4 cycled at 4.65 V and 4.8 V.

[33]. However, the elemental content of C and O decreased at the voltage 5.0 V. This result can be explained by the increased generation of H2O, EtOH, Et2O, and CO2 [34e37], caused by the instability of the LiNi0.5Mn1.5O4/electrolyte interface under extreme operation conditions, where surface carbon and oxygen can be consumed. This causes the battery to suffer from poor electrochemical performance, and rapid capacity fading. To confirm the influence of the redox reactions of the transition metals’ voltage plateaus on the surface composition of the cathode material, XPS was used to evaluate the components of the mixture adhering to the surface, and the form of the transition metal formed. Fig. 9 shows the C1s, F1s, Mn2p, and Ni2p spectra for a cycled cathode, charged at 4.65 and 4.8 V (identified as the redox voltages of the two transitional metals). For both potential operations, there are four main peaks corresponding to the C1s binding energy, and appear dominated by the CeH bond in hydrocarbons (284.8 eV), and also Li2CO3 (290.1 eV), C]O (289 eV), and CeO

(286.4 eV) in ROCO2Li. However, the spectra also show significant differences between the composition of the electrodes. The CeO content of the electrode charged at 4.8 V is higher than that of the electrode charged at 4.65 V, indicating the generation of more organic compounds at higher charging voltage. Visible differences are also observed on the F1s spectra, as shown in Fig. 9b. The purple arrows suggest that the peak corresponding to LiF (at binding energy of 685 eV) on the spectra of the electrode cycled at 4.65 V, had much stronger intensity compared to the peak on the spectra of the electrode cycled at 4.8 V. It is possible that the decomposition of LiPF6 (which occurs at approximately 4.5 V) observed for the cathode cycled at 4.65 V, is the reason behind the low content of LiPF6 and the high content of inorganic compound on the cathode surface. The spectra for the Mn2p and Ni2p orbitals were also compared to determine the changes in the chemical states and content as a function of voltage, as shown in Fig. 9c and d. The Mn2p spectra

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show the presence of Mn on the surface of the electrode regardless of the voltage. The two peaks corresponding to Mn2p3/2 at approximately 641 eV and 643 eV, demonstrate the coexistence of Mn3þ and Mn4þ in the probed sample [38]. In addition, the peak intensities for the electrodes cycled at 4.8 V are much stronger than those for the electrodes cycled at 4.65 V, which is in good agreement with the EDS results presented in Fig. 8. Similar to the Mn2p spectra, the Ni2p spectra in Fig. 9d demonstrate that there is still significant atomic content of Ni on the surface of the cycled cathode. The most obvious difference is the content and peak position emphasized with pink circles. In conclusion, these phenomena suggest a different influence for the two voltage plateaus on the structure of the cathode surface. 4. Conclusions 5. In summary, we investigated the effects of the charging voltage plateaus on the coulombic efficiency and capacity degradation of LiNi0.5Mn1.5O4 cathode, cycled at different cut-off voltage values. The cells charged below 4.7 V with a 4.0 V plateau, had a capacity of less than 20 mAh g1, with high capacity retention, indicating that the 4.0 V plateau, related to the redox couple of Mn3þ/Mn4þ, contributes to a low capacity, and has little influence on the capacity degradation. However, the high amount of manganese in the material during the oxidation reaction, caused significant reduction in the coulombic efficiency, due to manganese dissolution and other undesired side reactions. Moreover, the 4.7 V plateau, which is attributed to the reaction of Ni2þ/Ni4þ, provides the main contribution to the capacity in the cells charged at voltage higher than 4.7 V. However, the 4.7 V plateau caused rapid capacity loss, especially at 5.0 V, due to the decomposition of the electrolyte at high-voltage. Our study shows that there may not be necessarily a direct relationship between the capacity degradation and coulombic efficiency. A low coulombic efficiency does not indicate rapid capacity fade, as this phenomenon depends on the transition metal content, which in turn is related to the voltage plateaus of the electrode materials. Therefore, our findings can provide useful information for further investigation regarding the capacity loss and the coulombic inefficiency of LiNi0.5Mn1.5O4. Author contribution statement Xiaoling Cui: Conceptualization, Methodology, Software; Tongtong Geng: Data curation, Writing- Original draft preparation. Conceptualization, Methodology; Feilong zhang: Supervision; Ningshuang Zhang: Supervision; Dongni Zhao: Writing- Reviewing and Editing; Chunlei Li: Funding acquisition; Shiyou Li: corresponding author, who is responsible for ensuring that the descriptions are accurate and agreed by all authors. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 21566021 and 51962019), the Gansu Province Science and Technology Major Project (17ZD2GC011) and the Hongliu first-class discipline construction plan of the Lanzhou University of Technology.

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