Improved cyclic stability of LiNi_0.8Mn_0.1Co_0.1O₂ cathode enabled by a novel CEI forming additive

Abstract

The undesired side reactions at electrode/electrolyte interface as well as the irreversible phase evolution during electrochemical cycling significantly affect the cyclic performances of nickel-rich NMCs electrode materials. Electrolyte optimization is an effective approach to suppress such an adverse side reaction, thereby enhancing the electrochemical properties. Herein, a novel boron-based film forming additive, tris(2,2,2-trifluoroethyl) borate (TTFEB), has been introduced to regulate the interphasial chemistry of LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathode to improve its long-term cyclability and rate properties. The results of multi-model diagnostic study reveal that formation lithium fluoride (LiF)-rich and boron (B) containing cathode electrolyte interphase (CEI) not only stabilizes cathode surface, but also prevents electrolyte decomposition. Moreover, homogenously distributed B containing species serves as a skeleton to form more uniform and denser CEI, reducing the interphasial resistance. Remarkably, the Li/NMC811 cell with the TTFEB additive delivers an exceptional cycling stability with a high-capacity retention of 72.8% after 350 electrochemical cycles at a 1 C current rate, which is significantly higher than that of the cell cycled in the conventional electrolyte (59.7%). These findings provide a feasible pathway for improving the electrochemical performance of Ni-rich NMCs cathode by regulating the interphasial chemistry.

Graphical abstract

Keywords

NMC811 / cathode electrolyte interphase / film forming additives / cyclic stability

1 Introduction

Lithium-ion batteries (LIBs), as a promising secondary battery technology, are extensively utilized as power source for portable electronics, electric vehicles, and grid energy storage systems [1–5]. The growing demand for high performance LIBs with high energy and power density as well as long cycle life has spurred significant research on new electrode materials, electrolyte formulations, and interface engineering strategies [6–10]. Recently, nickel-rich layered oxide (NMC, Ni > 0.6) cathodes have been widely investigated because of their impressive energy density and relatively low cost [11–14]. Compared to other cathode materials, Ni-rich NMC offers enhanced capacity and improved voltage stability, positing it as an ideal candidate for next-generation LIBs. However, Ni-rich NMCs encounter challenges such as poor cycling stability, poor thermal stability, capacity degradation, and increased impedance at the electrode-electrolyte interface [15–18]. Various strategies have been utilized for enhancing bulk and interphasial stabilities of NMCs including elemental doping [19,20], surface coating [21,22], electrolyte engineering [23,24], as well as monitoring and regulating the state of health of batteries through advanced methods such as machine learning [25]. One prevalent approach is the introduction of surface coating layer to regulate the electrode-electrolyte interface [26–29]. Artificial protective layer with fluorine-containing species enables Ni-rich layered oxides to demonstrate improved cycle life and reduced voltage decay during prolonged cycling [30–32]. Such an enhanced electrochemical performance is attributed to formation of a stable cathode electrolyte interphase (CEI), which serves as a protective layer to inhibit undesired reactions and improves the overall electrochemical performance [33–35]. Another effective approach is the inclusion of in situ film forming additives to form stable and robust CEI, further suppressing parasitic reactions. Recent studies have demonstrated the potential of electrolyte additives, such as N,O-bis(trimethylsilyl) trifluoro acetamide (BTA), lithium difluoro oxalate borate (LiDFOB), in optimizing interphasial chemistry, thereby improving cycling stability and overall battery performance [36–38].

Fluorinated borate compounds are highly effective anion acceptors [39] that enhance the dissociation of Li salts in solvents and further improve the ionic conductivity. Moreover, functional groups such as -F, -CH₂CF₃, and -CF₂CF₃ in these compounds participate in oxidative and reductive decomposition during battery charging and discharging, resulting in the formation of LiF-rich CEI. Therefore, fluorinated borates are typically utilized as an electrolyte additive to regulate both the lithium dissociation behavior and interphasial chemistry.

In this study, the effectiveness of three different boron-based electrolyte additives, triethyl borate (TEB), tris(2,2,2-trifluoroethyl) borate (TTFEB), and tris(trimethylsilyl) borate (TMSB), on CEI stability formed at LiNi_0.8Mn_0.1Co_0.1O₂(NMC811) surface was comprehensively investigated. The electrochemical measurement results demonstrated that TTFEB significantly improved the cyclic and rate properties of the NMC811 cathode. The comprehensive analysis suggested that the electrolyte side reaction and surface reconstruction were mitigated by the TTFEB additive. As a result, a high reversible capacity with 72.8% retention after 350 electrochemical charging/discharging cycles at 1 C rate was achieved, surpassing the performance of NMC811 cycled in baseline electrolyte (BE). Multi-model diagnostic techniques provided insights into the interphasial chemistry of TTFEB, revealing a thin and uniform LiF-rich CEI with homogenously distributed boron species on the NMC811 cathode surface. In addition, the TTFEB additive effectively suppressed transition metal dissolution, thus enhancing the structural stability of NMC811. The results provide a fundamental understanding of the degradation mechanism at the NMCs interphase and offer important guidance for designing electrolyte components for high energy battery systems.

2 Experimental methods

2.1 Li/NMC811 cell assembling

The TTFEB, TMSB, and TEB were purchased from Shanghai NMR Biotechnical Co., Ltd. and used as received, without further purification. The battery-grade ethylene carbonate (EC), ethyl methyl carbonate (EMC), and LiPF₆ were purchased from Shanghai Songjing New Energy Technology Co. The BE was prepared by dissolving 1 mol/L LiPF₆ in EC/EMC solvents (3:7, wt.%)

Hefei Kejing Materials Technology Co., Ltd., and the NMC811 electrodes (areal loading of approximately 5 mg/cm²) were prepared by coating the mixture slurry of NMC811, Super-P, and polyvinylidene difluoride (PVDF, 8:1:1, wt.%) in NMP onto Al foil and dried under vacuum at 80 °C for 15 h. The Li foil anode with a thickness of 450 µm was also provided by Hefei Kejing Materials Technology Co., Ltd. The CR2032-type lithium symmetric and Li/NMC811 coin cells were assembled in an argon (Ar)-filled glove box. Each electrochemical cell was filled with 70 µL of electrolyte.

2.2 Electrochemical measurements

The Li/NMC811 cells were activated at a lower current density of 20 mA/g for three cycles and cycled at 200 mA/g to compare the long-term cyclic stability. The rate capability of Li/NMC811 cells was measured at 0.1, 1, 2, 3, 5, and 10 C current rates (1 C = 200 mAh/g). The electrochemical measurements were conducted using Neware Battery Test System instruments at room temperature. Electrochemical impedance spectroscopy (EIS) measurement were conducted for Li/NMC811 cells before and after electrochemical cycles within the frequency range of 100 kHz to 100 mHz on Gamary Interface 1010E electrochemical workstation. The Li/Li symmetric cells were cycled at 0.5 mA/cm² with a deposition/stripping capacity of 0.5 mAh/cm².

2.3 Material characterizations and computation

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of electrolyte solvents and additives were calculated using the density functional theory (DFT) with GAUSSIAN 09 software package. The molecular structures of electrolyte solvents and additives were fully optimized using the B3LYP method at 6-31 + G (d, p) basis set.

The chemical composition of CEI formed on NMC811 were measured by X-ray photoelectron spectroscope (XPS) with Al Kα radiation (hv = 1486.6 eV) source. Depth etching profiling was conducted using 2 kV Ar⁺ ion gun with Ta₂O₅, achieving etch rates of 0.06 nm/s for 30, 60, and 90 s to access different depths of detection. All samples were prepared in an Ar-filled glove box and sealed in pouch bags to avoid air exposure. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurement was conducted with a TOF-SIMS 5 spectrometer (ION-TOF GmbH). The depth distribution of related chemical species in CEI was achieved by high lateral resolution mapping. The morphology of NMC811 particles after cycling and the thickness of CEI layer formed in different electrolytes were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The transition metal dissolution was assessed by measuring the dissolved metals in the electrolyte using X-ray fluorescence (XRF).

3 Results and discussion

3.1 Screening of additives

Fig.1(a) shows the molecular 3D structure of three additives alongside EC and EMC solvents. It also presents the calculated LUMO and HOMO energy levels for different components. Formation of CEI typically occurs by electrolyte oxidation decomposition at the cathode side during charging. However, a recent study by Zhang et al. [40] suggested that the CEI is also formed during discharging through the reductive decomposition of the electrolyte. Therefore, functional additive with lower LUMO energy level than the solvents can be used for CEI regulation. The LUMO energy levels of three additives (−0.478 eV of TEB, −1.470 eV of TTFEB, and −1.225 eV of TMSB) are lower than the LUMO energy levels of EC (0.816 eV) and EMC (0.822 eV) solvents, indicating that the CEI is formed by decomposition of electrolyte additives other than the electrolyte solvents during discharging.

The effectiveness of various film-forming additives on the electrochemical stabilities and Coulombic efficiencies of Li/NMC811 cells was evaluated within the voltage range of 2.5 to 4.6 V, as shown in Fig.1(b) and S1. It is evident that the discharge specific capacity of Li/NMC811 cell containing TTFEB additive is about 184.79 mAh/g after 50 electrochemical cycles, with a capacity retention of 87.2%. This performance is significantly better than that of BE (178.0 mAh/g, 86.5%), and TEB (171.3 mAh/g, 84.9%), as well as TMSB additive (160.6 mAh/g, 79.3%)-containing electrolytes. In addition, the Columbic efficiency of Li/NMC811 cycled in TTFEB electrolyte is obviously higher than that in BE, TEB, and TMSB, which is attributed to the formation of better protective layer on the NMC811 surface by decomposition of TTFEB. Therefore, TTFEB has been selected for further study to reveal the CEI forming mechanism.

To investigate the influence of additive concentration on battery performance, electrochemical properties of the NMC811 cathode have been measured in electrolytes with different concentrations (0.5%, 1%, and 2%) of TTFEB additive. A higher current density of 1 C and a voltage range of 2.5 to 4.4 V were used to compare the long-term cycling stability. Fig.1(c) presents the 1st and 200th charge/discharge curves of Li/NMC811 cells cycled with BE and different concentrations of TTFEB additives. The electrochemical curves and initial charge capacities show negligible differences. However, the initial reversible capacity cycled in TTFEB electrolyte is higher than that in BE. After 200 cycles, a significant difference is observed that both the capacity fading and overpotential of Li/NMC811 cells in TTFEB electrolyte are much lower compared to those in BE electrolyte. Fig.1(d) is a comparative study of the long-term cyclic stability of Li/NMC811 cells across various electrolyte systems. After 200 cycles, the discharge specific capacities for the electrolyte systems with an additive concentration of 0.5%, 1%, and 2% are 158.2, 162.8, and 157.7 mAh/g, respectively. However, the capacity of the Li/NMC811 cell cycled in BE maintained only 139.2 mAh/g. In contrast, the cyclic performance of NMC811 in the electrolyte with a TTFEB additive concentration of 1% demonstrated a significant improvement. Furthermore, the Coulombic efficiencies of cells cycled in the electrolyte containing TTFEB additive after 200 cycles are 99.82%, 99.83%, and 99.79%, respectively, surpassing that of the BE (99.01%). These results confirm the role of the TTFEB additive in enhancing the cycling stability of NMC811, particularly with a concentration of 1%.

3.2 Electrochemical properties

To confirm the effectiveness of TTFEB in enhancing the cycling stability in NMC811, the long-term cyclic performance and assessed rate capabilities are measured. The capacity retention of NMC811 after 350 cycles and selected charge/discharge curve are demonstrated in Fig.2(a) and Fig.2(b) and S2. The initial discharge capacities of NMC811 cycled in BE and 1% TTFEB-containing electrolyte (TTFEB) are 203.9 and 207.2 mAh/g, respectively. After 350 charge/discharge cycles, the NMC811 cathode cycled in TTFEB electrolyte still maintains a reversible capacity of 140.2 mAh/g, with a capacity retention of 72.8%. In contrast, when cycled in BE, it exhibits a lower reversible capacity of 115.5 mAh/g with a capacity retention of only 59.7%. Moreover, the NMC811 cathode cycled in TTFEB-containing electrolyte (solid red circles) delivers a higher Columbic efficiency of over 350 cycles than that in BE (solid blue circles), indicating that the side reaction of electrolyte is effectively suppressed by the TTFEB additive. These results suggest that the CEI formed by decomposition of TTFEB enhances the cyclic stability of the NMC811 electrode.

The rate performance of the NMC811 cathode was tested at various current rates ranging from 0.1 to 10 C, with a charging cutoff voltage set at 4.4 V (Fig.2(c)). At the lower current rate of 0.1 C, the reversible capacity of the NMC811 cathode cycled in electrolyte with TTFEB is significantly higher than that of BE. As the current rate increased, the reversible capacity of NMC811 gradually decreased, reaching only 130.0 mAh/g at 10 C. In comparison, the NMC811 cycled in TTFEB electrolyte exhibits a higher capacity retention and a reversible capacity of 142.8 mAh/g achieved at a current rate of 10 C. Upon returning to the rate of 0.1 C, the NMC811 cathode cycled with TTFEB electrolyte still maintains a higher specific capacity, indicating a better reversibility. The improved rate performance can be attributed to two main factors: first, TTFEB forms “beneficial” CEI to facilitate the Li⁺ diffusion at cathode side; next, as an anion receptor, TTFEB facilitates the dissociation of lithium salts, thereby accelerating lithium-ion diffusion in bulk electrolyte.

The Nyquist plots of Li/NMC811 cells after the 1st and 100th cycles are measured to comprehensively analyze the lithium diffusion behavior within CEI with the results shown in Fig.2(d). The interfacial resistance R_f, and the charge-transfer resistance R_ct are obtained by fitting and analyzing the equivalent circuit diagrams based on these plots as shown in Fig.2(e). It can be clearly seen that, the TTFEB additive reduces the initial R_ct of the cell as well as its accumulation, which is attributed to the reduced inorganic insulating lithium salts, such as Li₂CO₃, in the CEI. The TTFEB and BE electrolytes underwent linear sweep voltammetry (LSV) testing within the voltage range of 0–3 V vs. Li/Li⁺(Fig.2(f)). During the discharge process, due to the lower LUMO energy level of TTFEB, a small peak was located at about 2.27 V associated with the decomposition of TTFEB, indicating that TTFEB would be preferentially reduced to produce CEI. To investigate the effectiveness of the TTFEB additive on lithium deposition behavior of lithium metal anode, a Li–Li symmetric cell was assembled and measured in two different electrolytes (Fig. S3). The results show that the TTFEB does not significantly enhance the cycling stability of lithium metal anode, suggesting that the TTFEB does not affect the SEI stability. Therefore, the improvement in the long-term cycling stability of the Li/NMC811 cell contributes to the formation of stable CEI on NMC811.

3.3 Formation mechanism of CEI with TTFEB additive

Multi-model characterizations including XPS, TEM, SEM, TOF-SIMS, and HRTEM can be used to analyze the chemical composition, homogeneity, and thickness of CEI formed in the electrolyte with and without TTFEB additive [22,24,41]. To analyze the composition of CEI at varying depth on NMC811, depth-etched XPS (etching rate 0.06 nm/s) is utilized to characterize the cathode surface after 10 cycles, as shown in Fig.3(a) and (b).

The C 1s spectra distinctly display five characteristic peaks at 284.8, 286.2, 288.4, 290.8, and 292.8 eV, corresponding to the C–C/C–H, C–O, C–O, CO₃^2–, and C–F bonds, respectively. Among them, the C–C peak is primarily due to the super P in the electrode. The C–F and C–H bonds are indicative of the PVDF binder. In addition, C–O is associated with the organic oligomers, while C–O comes from alkyl lithium carbonates (e.g., ROCO₂Li and Li₂CO₃). The XPS data clearly shows that the peak intensity of the C–O species in CEI formed in BE gradually decreases as the etching depth increases, indicating that the organic polymers in CEI are dominant at the surface, while solvent decomposition is dominant during the electrochemical process. By introduction of the TTFEB additive, the peak intensity of C–O decreases and remains relatively stable, regardless of the etching depth. The comparison suggests that TTFEB effectivity mitigates electrolyte solvent decomposition and promotes the homogenous CEI layer. After introducing TTFEB, a new peak corresponding to C–F species emerges in the C 1s spectra, which is mainly from the binder, indicating the formation of thinner CEI. The peak intensity of C–O and CO₃^2– species reveals a significant decrease in TTFEB-containing electrolyte, suggesting that TTFEB induces less inorganic lithium-salt-containing CEI, such as lithium carbonate and ROCO₂Li. Inorganic lithium salts, mainly formed by decomposition of carbonate solvents, are not conductive, thus hinders Li⁺ diffusion. The O 1s spectrum shows that the M–O peak (530.4 eV) almost disappears with TTFEB, which indicates that the TTFEB induced CEI inhibits the transition metal dissolution. Meanwhile, the O 1s spectra shows a strong B–O peak located at 533.7 eV in the CEI formed in the TTFEB containing electrolyte, and the relative peak intensity decreases with deepening of the etching depth. As shown in the O 1s spectra, the intensity changes of the C–O (531.7 eV) and C–O peaks (532.4 eV) have the same trends as the corresponding peaks in the C 1s spectra, supporting the findings in the C 1s spectra. The F 1s spectrum shows that two different fluorine-containing species in CEI appear as two different peaks located at 688.1 and 685.6 eV, in which the peak at 688.1 eV represents F in organic compounds as well as Li_xPF_y and Li_xPO_yF_z, etc., that formed the following dissociation of lithium salts in the electrolyte. The peak intensity of F-containing components decreases as etching depth increases, indicating an enhanced formation of P-containing compounds at the surface of CEI contacted with the electrolyte. More importantly, the LiF content (685.6 eV) in BE increases with a greater etching depth, indicating a lower concentration of LiF at the surface of CEI. In contrast, a higher concentration of LiF extending from the surface to bulk of CEI is observed in TTFEB. Two different lithium-containing species in CEI can be observed in Li 1s XPS: Li₂CO₃ and LiF. The CEI formed by TTFEB exhibits an increased LiF content compared with BE, effectively enhancing the interphasial stability. Furthermore, the content of Li₂CO₃ is lower than that of BE, resulting in a CEI with a smaller interfacial impedance. The formation of B-containing components is also clearly demonstrated in Fig.4(a), confirming that CEI is formed through the decomposition of TTFEB. The XPS results suggest that introducing TTFEB additive results in a more uniform and thinner CEI with F-rich components, whereas solvent decomposition is predominant in the BE electrolyte.

TOF-SIMS was used to systematically investigate the distribution of CEI component [42]. Fig.4(b) displays both the surface and the 3D spatial distribution of B within the interphase layer on the NMC811 electrode after charge/discharge cycling in the TTFEB. It can be observed from the spatial elemental distribution map that B-containing species are homogeneously distributed within CEI without aggregation. Similarly, the 2D surface element distribution map demonstrates a homogenous distribution of B elements on CEI surface, without any enriched or less-B regions. The TOF-SIMS results suggest that the B-containing species serve as a skeleton to connect different components and enhance the structural and mechanical stabilities of CEI to accommodate volume changes during cycling.

Fig.4(c) depicts the spatial 3D distribution of LiF within the interphase layer formed on the NMC811 cathode after 10 cycles in BE and the electrolyte containing the TTFEB additive. Initially, the spatial 3D imaging reveals a significant increase in the overall LiF content in the CEI derived from the decomposition of TTFEB compared to that from BE, as evidenced by the stronger LiF signal intensity. Additionally, the 2D distribution plots illustrate notable differences in the surface distribution of LiF within the CEI generated from the two electrolyte systems. Specifically, adding TTFEB results in a higher and more uniform distribution of LiF within the CEI, as indicated by the even distribution in the 2D mapping. In contrast, uneven distribution, and agglomeration of LiF with enriched and less-LiF regions can be observed from the 2D mapping of the CEI formed in BE.

Based on the XPS and TOF-SIMS results, the schematic diagram of the CEI formed in BE and the TTFEB electrolytes are shown in Fig.4(d). The CEI formed in BE primarily consists of a small amount of LiF, organic oligomers, lithium carbonate, and phosphorus-containing compounds Li_xPO_y. In contrast, the CEI formed in TTFEB based electrolyte demonstrates a more uniform distribution of organic oligomers, resulting in a thinner CEI. In addition, the concentrations of P-containing compounds and lithium carbonate are reduced, while the content of LiF is increased obviously compared to BE. Moreover, B–O species are evenly distributed throughout the CEI, serving as a structural framework that enhances uniformity and density.

3.4 Effectiveness of CEI formed by TTFEB

The thickness and uniformity of CEI on NMC811 after electrochemical cycling were analyzed using TEM, as shown in Fig.5(a) and S4. The non-uniform distribution of CEI with the thickness of 6.5–14.3 nm is clearly observed for the NMC811 cathode cycled in CE. In contrast, a thin and uniform interfacial film with a thickness of approximately 4.6 nm was detected at the surface of cathode cycled in the electrolyte containing TTFEB additive. The morphology of the initial and cycled NMC811 cathode particles in different electrolytes is shown in SEM images in Fig.5(b) and S5 while Table S1 present the energy dispersive spectroscopy (EDS) map of the initial NMC811 cathode particles. The secondary particles of the initial NMC811 sample exhibit regular shapes with complete and smooth surfaces. However, after 100 cycles, the particles cycled in BE exhibit evident cracks, attributed to stress accumulation from cathode-electrolyte side reactions and volume changes during the high-voltage phase transition of the cathode material. In contrast, negligible changes in particle shape can be observed for NMC811 cycled in TTFEB-1%, indicating that the CEI generated by the TTFEB-containing electrolyte effectively protects the cathode surface and suppresses parasitic reactions at the interface, resulting in the good mechanical stability to accommodate volume changes during electrochemical cycling.

To evaluate the effectiveness of TTFEB on suppressing the TM dissolution, XRF was utilized to measure the TM content on the separator harvested from the cycled Li/NMC811 (Fig.5(c), Tables S2 and S3). The content of Ni, Co, and Mn on the separator harvested from the cell cycled in BE are 4.154%, 0.576%, and 0.892%, respectively. In comparison, adding TTFEB significantly reduces TM dissolution with average concentrations of Ni, Co, and Mn at 2.875%, 0.275%, and 0.383%, respectively. The XPS and XRF results demonstrate that the LiF-rich CEI film generated by the TTFEB additive effectively suppresses the HF attack and further inhibits the TM dissolution, which contributes to the enhanced cycling performance of the NMC811 cathode. To reveal the effectiveness of TTFEB additive on suppressing the surface reconstruction, HRTEM has been measured for the NMC811 cathode after 100 cycles in the electrolyte with and without additives as shown in Fig. S6. It can be clearly seen that layered structure turns into rock-salt phase after cycling in BE, while the NMC811 cathode surface cycled in the TTFEB electrolyte still maintains a well-developed layered structure with a uniform CEI layer. These observations further explain that enhanced long-term cyclic stability of NMC811 in TTFEB is attributed to the formation of protective robust CEI and suppression of aggressive side reactions as well as structure destruction. As discussed above, the LiF-rich, thin, and uniform CEI produced by the 1% TTFEB electrolyte additive effectively enhances the electrochemical properties of the NMC811 cathode. This improvement is achieved by the forming of the thin and homogenous CEI layer, which reduces the parasitic reactions between the cathode material and the electrolyte, thereby improving the long-term cyclic stability and rate performance of the NMC811cathode.

4 Conclusions

In this work, a novel boron-based film forming additive, tris(2,2,2-trifluoroethyl) borate (TTFEB), has been introduced to regulate the interphasial chemistry of LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathode to improve its long-term cyclability and rate properties. It can be concluded that the introduction of the novel film-forming TTFEB electrolyte additive enhances the electrochemical stability of the Ni-rich NMC811 cathode. Specifically, TTFEB fosters the formation of a LiF-rich interphase layer that regulates the interphasial chemistry and boosts the overall electrochemical performance of NMC811. The XPS results demonstrate that the relative amount of organic polymer in CEI formed with TTFEB is much lower than that formed in BE, facilitating the formation of a thicker and denser CEI layer while the presence of B–O species in CEI acts as a structural scaffold, promoting uniformity. Additionally, the CEI generated by the TTFEB also suppresses the TM dissolution and surface reconstruction. As a result, even at a high current rate of 1 C, the NMC811 cathode cycled with 1% TTFEB-containing electrolyte delivers a high reversible capacity with a retention of 72%. These results indicate that B-based TTFEB additive enriched with F-containing functional groups have a great potential for enhancing the overall performances of Ni-rich NMCs and provide valuable information for developing high performance electrolyte systems for Ni-rich NMCs.

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