1 Introduction

Due to their superior energy density, Li-ion batteries (LIBs) are increasingly utilized as the preferred energy sources for a verity of purposes, such as portable electronics and electric vehicles [1,2,3,4,5,6,7]. Despite numerous attempts to enhance the energy density of these batteries, there are still problems in meeting the energy needs of the world. The nickel-rich layered transition metal (TM) oxide cathode compounds with great capacity and high operating voltage, for instance, (LiNixCoyMnzO2 (x + y + z = 1, NCM, x ≥ 0.5) have been attracted many attentions to enhance the energy density of commercial LIBs[8,9,10,11]. Nevertheless, using NCM cathode especially at a high potential (≥ 4.3 V vs Li/Li+) is limited by rapid capacity fade caused by the electrolyte’s oxidative degradation, which produces larger quantities of a resistive passivation coating on the cathode [12,13,14,15,16,17]. Furthermore, transition metal dissolution into the electrolyte, which consequently deposit on the negative surface causes capacity fading [1, 11]. In this regard, many efforts have been accomplished to address these problems such as adding functional film-forming additives into the base electrolyte which pre-oxidize and create CEI layer on the cathode [18,19,20,21]. This has been suggested as an affordable method to promote the cycling stability of the cathodes. A stable electrolyte–cathode interphase, which forms a uniform coating on the electrode surface, hinders parasitic reactions, occurring at the electrolyte/electrode interphase and protect the cathode surface [22,23,24,25]. Han et al. reported that using 0.5-wt% 1,4-Dicyanobutane (ADN) in the NCM523 /graphite full cell revealed significant cycling performance at high voltage, which is due to the creation of a homogeneous CEI layer resulting from the interfacial reaction between the electrolyte and the NCM electrode[26]. Li’s group has investigated 2,4,6-Triphenyl Boroxine as a solid electrolyte interphase (SEI) forming agent for high-voltage LIBs. It is revealed that a low impedance film induced by 2,4,6-Triphenyl Boroxine can be constructed on a High-Voltage LiNCM811 cathode and improved the cyclic stability upon 200 cycles at 1 C within 3.0–4.35 V[22]. Bio-based antioxidant electrolyte additives are also being explored because of their high resource availability, environmental benignity, and low cost. Kong et al. have proposed utilizing L-Tryptophan (TrP) antioxidant as a proper SEI creating additive to enhance the capacity retention from 67% for STD to 77% for TrP containing electrolyte after 300 cycles in a lithium-rich layered oxide cathode [27]. According to Y. M. Lee et al., it has been discovered that dopamine can serve as a useful electrolyte additive for LIBs when operating at elevated temperatures [28]. Kim et al. have employed Quercetin antioxidant as an additive for electrolyte to prolong the cycle behavior of LiNi0.5Mn1.5O4 (LNMO) at high voltage and high temperature [29]. Their finding suggest that the application of quercetin can remarkably extend the cycle durability of LNMO cathode, operating under high temperature.

This article describes the first instance of utilizing bio-based Curcumin antioxidant with high oxidation activity as an additive to create a protective CEI layer on the NCM523 surface. The electrochemical characteristics of NCM523 cathode operating within the 2.7–4.4 V were analyzed in Curcumin-containing electrolyte. Curcumin is a natural antioxidant with a polyphenolic structure, capable of scavenging free radicals by providing proton hydrogen through its phenolic hydroxyl and methylene groups, displaying significant antioxidant activity. Moreover, it has lower oxidation potential, which can be preferentially oxidized on the electrode surface, forming a very robust protective layer on the NCM surface, which tremendously enhances the interface stability of the NCM/electrolyte. The scanning electron microscopy (SEM) verified the creation of this layer, and the optimal quantity of Curcumin needed to achieve the best performance was identified.

2 Experimental section

2.1 Materials and methods

A mixture was prepared using N-methyl-2-pyrrolidone (NMP), the active material (NCM523, 80 wt.%), polyvinylidene fluoride (PVDF, 10 wt%), and conductive carbon black (10 wt%) to provide a slurry. Afterward, the coating slurry was applied onto aluminum foil, and the resulting electrodes were then subjected to drying in a vacuum oven at 80 °C for a duration of 12 h. Lithium hexafluorophosphate (LiPF6) and the electrolyte solvents were prepared from EXIR Co., Ltd and Sigma-Aldrich, respectively. The base electrolyte was obtained by dissolving 1-M LiPF6 in a solution of EC/DMC solvents in a volumetric ratio of 50:50. Curcumin was purchased from Sigma-Aldrich. To create the Curcumin-containing electrolyte, 0.02-wt%, 0.04-wt%, and 0.06-wt% Curcumin were dissolved in the base electrolyte. The assembly of NCM/Li half cell, was conducted inside a glove box filled with argon and H2O/O2 levels below 5 ppm, using Celgard 2400 as the separator. In this study, various techniques were employed to assess the electrochemical characteristic of the NCM523 cathode. An electrochemical workstation (Autolab) with a platinum as working electrode was used to conduct LSV. During the experiment, both the reference and counter electrodes were composed of lithium, and the scan rate was adjusted to 0.1 mV s−1.

CV was performed on Galvanostat/Potentiostat Autolab (PGSTAT 302N) with a scan rate of 0.1 mV s−1 in the voltage amplitude of 2.2–4.6 V. An eight-channel battery cycler (Neware, China) was applied to evaluate the performances of NCM/Li half cells. Both the NCM523/Li half cells in presence and absence of Curcumin were subjected to a formation step consisting of three cycles with a rate of 0.2 C in the cut-off voltage of 2.7 V–4.2 V, and 2.7–4.4 V continued by 100 cycles at 1 C (1 C = 160 mAh g−1). Furthermore, the C-rate capabilities were assessed by changing the charge/discharge current densities from 0.1 to 0.2, 0.5, 1.0, and 2 C and finally returned back to 0.1 C in the potential range from 2.7 to 4.4 V. Electrochemical impedance spectroscopy measurements were taken, while the cell was in discharge state, covering a frequency amplitude from 105 to 0.01 Hz. In order to examine the physical characteristics of the cycled electrodes, the cells were disassembled within glove box filled with argon. The electrodes underwent a rinsing process with DMC solvent for at least three repetitions to remove any remaining lithium salt and carbonate solvents and were subsequently dried at room temperature under vacuum conditions for duration of 10 h. Surface characteristics of cycled NCM electrodes with and without Curcumin were examined through a FESEM (MIRA3), and TEM (Carl Zeiss-EM10C-100 kV-Germany). To obtain more thorough understanding of the crystal structure of the electrodes after cycling, XRD (EQUINOX3000, Intel) spectra were gathered. The surface composition of cycled electrodes was evaluated by XPS (Bes Tec XPS system, Germany with an Al kα X-ray source (hν = 1486.6 eV). The transition metal quantity were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) (Varian VISTA-PRO).

2.2 Theoretical study

Theoretical computations were performed to optimize the molecular configuration of EC, DMC, and Curcumin, utilizing DFT in the Gaussian 09 package with the B3PW91/6–311 +  + G (d,p) method[19]. The effect of the surrounding solvent, with a dielectric constant of 50.5, was studied applying polarized continuum models (PCM). The oxidation potential (Eox) was determined using the equation [30] (1):

$${\text{E}}_{{\text{ox}}} \left( {{\text{Li}}^+ /{\text{Li}}} \right) \, = \, [{\text{G}}({\text{M}}^+ ) \, - {\text{ G}}\left( {\text{M}} \right)] \, /{\text{ F }} - { 1}.{\text{4 V}}$$
(1)

where G(M) represents the free energies of the solvated complex M, G(M+) represents the free energies of its solvated oxidized form M+ at temperature of 298.15 K, and F is the Faraday’s constant [31]. The natural bond orbital (NBO) theory was employed to calculate the atomic charge distributions.

3 Results and discussion

3.1 DFT outcomes

A proper film-forming additive shows higher oxidation activity than the carbonated electrolyte, undergoing preferential oxidative degradation on the electrode to create a CEI film [32,33,34]. Molecules that possess a higher HOMO energy are typically more prone to oxidation [35,36,37]. To investigate the tendency for electrochemical oxidation of the Curcumin in compare with EC and DMC, DFT calculations were used to calculate the HOMO energy and oxidation potentials of EC, DMC, and Curcumin. As depicted in Fig. 1a, it can be inferred that Curcumin exhibits the highest HOMO energy level at − 7.26 eV when compared to the molecules of EC and DMC. In contrast, DMC possesses a HOMO energy level of − 7.95 eV, whereas EC displays the lowest HOMO energy level at − 8.50 eV. As well as, according to Eq. (1) Curcumin exhibits a lower oxidation potential (4.35 V) compared to EC (6.80 V) and DMC (6.38 V) as listed in Table 1.

Fig. 1
figure 1

Optimized structures of Curcumin, EC and DMC molecules and corresponding HOMO/ LUMO energies (a); LSV of the electrolyte using 1 M LiPF6-EC/DMC (50:50 by volume) with and without Curcumin at 0.1 mV s.−1 scan rate (b)

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Table 1 Calculated HOMO energies (eV) and oxidation potentials (V vs Li+ /Li) of EC, DMC, and curcumin
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According to results, Curcumin demonstrates a higher propensity for oxidation when compared to EC and DMC. Therefore, it can take part in creating a protective layer on the surface of the cathode. In order to evaluate these results, the oxidation behavior of Curcumin was tested by linear sweep voltammetry (LSV). As depicted in Fig. 1b, a distinct anodic peak around 4.12 V is related to the oxidation decomposition of Curcumin. Moreover, the electrolyte with Curcumin began to be oxidized at about 5.5 V, which was later than the blank electrolyte of 5 V. This elucidates Curcumin can be preferentially oxidized and create a CEI film on NCM electrode, thus hinder the further decomposition of electrolyte which is consistent with the calculated outcomes.

The oxidation of molecule happens in the HOMO of the molecule. The HOMO of Curcumin is placed in C33-C35, C36-C40, C40-C38, and C46-C47 of the O-methoxy-phenolic group as depicted in Fig. 2a Therefore, the atomic charges of the aromatic ring in neutral state, calculated by natural bond orbital (NBO) methods, was − 0.172 a.u. and then became positive 0.074 a.u. after oxidation of Curcumin molecule (Fig. 2b). Moreover, the bond length of C33–C35, C36–C40, C40–C38, and C46–C47 bonds increased (Fig. 2c).

Fig. 2
figure 2

Computed DFT structure of Curcumin, where red = oxygen, gray = carbon, white = hydrogen (a) NBO atomic charges of the Curcumin at neutral and oxidation states (b) Bond lengths of the Curcumin at neutral and oxidation states (c)

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3.2 Influence of curcumin additive on the NCM523/Li cell

To evaluate the electrochemical characteristics of the electrolytes cyclic voltammetry was conducted on NCM523 in both a base electrolyte and an electrolyte containing Curcumin. The cyclic voltammograms of the first and second cycles are represented in Fig. 3a and b, respectively. It must be noted that all the CV measurements were conducted in 1.0-M LiPF6-EC:DMC (50:50, by volume) in absence and presence of Curcumin. The appearance of redox peaks at approximately 4.36 V and 3.60 V is associated with process of lithium insertion/de-insertion coupled with the redox reaction related to Ni2+/3+ and Ni4+ at the voltage between 2.2 and 4.6 V. As it can be seen from Fig. 3a, in the Curcumin-containing electrolyte, an oxidation current is observed around 3.2 V, which can be attributed to the Curcumin preferential oxidation. It is noted that the oxidation current associated with Curcumin vanished in the second CV curve, as represented in Fig. 3b, elucidating that the oxidation reaction of Curcumin predominantly occurs during the initial charging process. Moreover, there is no additional oxidation current observed from NCM523 in electrolyte without additive. Hence, the Curcumin molecules are more prone to be oxidized on the cathode prior to the degradation of base electrolyte, which may hinder the decomposition of the oxidized electrolyte, so as to boost performance of the cathode in the high voltage.

Fig. 3
figure 3

CV curves of NCM523/Li half cells at a scan rate of 0.1 mV s−1 applying 1 M LiPF6-EC/DMC (50:50, by volume) electrolyte with and without additive (a) first cycle; b second cycle; Electrochemical performance of the NCM523/Li half cells in electrolyte without and with Curcumin at room temperature between 2.7 and 4.2 V; c cyclic performance, coulombic efficiency; d retention profiles;

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To further evaluate how Curcumin affects the NCM electrode and find the optimal amount of Curcumin in the pristine electrolyte, NCM/Li half cells were cycled at 1 C rate with cut-off voltage of 2.7–4.2 V after undergoing 3 formation cycles at a rate of 0.2 C. The electrolyte composed of 1-M LiPF6 dissolved in a mixture of EC and DMC in a 50:50 volumetric ratio containing various amounts of Curcumin namely 0 wt%, 0.02 wt%, 0.04 wt%, and 0.06 wt%, respectively. As depicted in Fig. S1, when 0.02-, 0.04-, and 0.06-wt % Curcumin are added into the pristine electrolyte, the capacity retention of the Li/NCM cell enhances from approximately 85 to approximately 88, 95, and 87% after 50 cycles, respectively. Incorporating excessive amount of Curcumin into the pristine electrolyte can lead to reduced capacity retention due to the involvement of some Curcumin decomposition products in forming a thicker layer.

For further investigation, the NCM/Li half cell’s AC impedance was carried out in electrolytes without and with different percentages of Curcumin after 50 cycles. As can be seen from Fig. S2a both the charge transfer impedance and surface film impedance are higher for the cell with pristine electrolyte than that of 0.02-, 0.04-, and 0.06-wt% Curcumin-containing electrolyte. Notably, when the Curcumin concentration was 0.06 wt%, the cell’s charge transfer impedance emerged as the highest compared to other electrolytes containing Curcumin. This is likely due to the formation of a thicker surface layer resulting from the increased concentration of Curcumin. This occurrence could explain the decreased cycling performance observed at elevated concentrations of added Curcumin, as depicted in Figure S1.

The higher interfacial impedance of the cell with pristine electrolyte is linked to the significant growth of surface layer generated on electrode surface, implying additional electrolyte decomposition during cycling. This indicates that the surface film formed through the oxidation of the pristine electrolyte is unstable and continues to grow throughout the cycling process.

The EIS data are applied to determine the diffusion coefficients of Li ions (DLi+) according to Eq. (2) [38]:

$${\text{D}} = {\text{ T}}^{2} {\text{R}}^{2} /{\text{2A}}^{2} {\text{F}}^{4} {\text{n}}^{4} {\text{C}}^{2} \sigma^{2}$$
(1)

where T, R, A, F, n, C, and σ represent the thermodynamic temperature (298 K), gas coefficient (8.314 Jk−1mol−1), area of the electrode (A = πr2), Faraday coefficient (96,500 Cmol−1), transferred electron number through insertion/de-insertion of Li-ion (for Li+ n = 1), Li+ ion concentration in solid, and the slope (σ) of the linear part in Z′ ∼ ω−1/2 (Warburg factor), respectively. σ (Ω·s−1/2) can be obtained from Eq. (3) [39]:

$${\text{Z}}{\prime} \, = {\text{ R}}_{\text{S}} + {\text{ R}}_{{\text{ct}}} + \, \sigma \omega^{ - {1}/{2}}$$
(2)

Fig. S2b shows the Warburg factor (σ) by fitting line slopes for the cell without and with different amounts of Curcumin. The DLi+ of the cell with 0.02-, 0.04-, and 0.06-wt% Curcumin and additive free electrolyte is 1.21 × 10−13, 1.59 × 10−13, 0.90 × 10−13, and 0.87 × 10−13 cm2 s−1, respectively, indicating greater DLi+ of the cell with 0.04% Curcumin. For this reason, we selected the electrolyte which includes 0.04-wt% Curcumin for further investigations in the subsequent discussion.

Fig. S3 depicts the first charge/discharge profiles of the half cells at 0.2 C and cut-off voltage of 2.7–4.2 V. The NCM/Li cell with pristine electrolyte exhibited the specific charge/discharge capacities of ~ 147 and ~ 136 mAh g−1, respectively, with Coulombic efficiency of 92.5%. However, upon the incorporation of 0.04%, Curcumin to the pristine electrolyte, the NCM/Li cells illustrated the first charge capacity of ~ 154 mAh g−1 and discharge capacity of ~ 140 mAh g−1, respectively, with corresponding Coulombic efficiency of 91%. It can be concluded that, unlike increase in the first charge/discharge capacity of curcumin containing electrolyte, the initial Columbic efficiency of NCM523 with Curcumin is slightly lower than that of without curcumin, which should be related to the oxidation of Curcumin, creating a cathode–electrolyte interface on the NCM523 electrode [40].

Following three initial cycling at 0.2 C, the NCM523/Li cells underwent 100 subsequent cycles at a rate of 1.0 C, ranging from 2.7 to 4.2 V. Figure 3(c, d) illustrates the cyclic performance accompanied with Coulombic efficiency and capacity retention of NCM523/Li cells employing both the pristine and Curcumin-containing electrolyte. The cell that utilizes an electrolyte with 0.04-wt % Curcumin exhibits a notable improvement in the cyclic performance, reaching 109.80 mAh g−1 with 90% capacity retention after completing 100 cycles, while it is only 98.45 mAh g−1 with 82% capacity retention for the pristine electrolyte. Moreover, Curcumin-containing electrolyte demonstrates a higher Coulombic efficiency than the pristine electrolyte.

The impact of Curcumin additive at higher voltage of 4.4 V in both cells with and without Curcumin additive was also investigated. As depicted in Fig. 4, the pristine cell demonstrates a dramatic capacity fade from ~ 133 mAh g−1 to 105 mAh g−1 after 100 cycles. It retains only ~ 79% of its original capacity and maintains an efficiency of ~ 94%. On the other hand, the cell with 0.04-wt.% Curcumin exhibits a capacity of 122 mAh g−1 at the 100th cycle, retaining 91% of its initial capacity, while maintaining an efficiency of ~ 98%. This observation suggest that the CEI layer developed with the Curcumin-containing electrolyte demonstrates reduced polarization effects compared to the one without Curcumin after 100 cycles. This signifies a decrease in irreversible electrochemical behavior, resulting in improved electrochemical performance. This conclusion is further supported by the results of the subsequent electrochemical impedance spectroscopy (EIS) test. The findings indicates that the NCM cathode cannot undergo consistent cycling in the base electrolyte while maintaining a high voltage of 4.4 V. The inclusion of a minute quantity of Curcumin additives (0.04 wt %) can notably enhance the cycle stability of the cathode at high voltages.

Fig. 4
figure 4

Electrochemical performance of the NCM523/Li half cells in electrolyte without and with Curcumin at room temperature between 2.7 and 4.4 V: Charge–discharge curves at different cycles (a) without; and b with Curcumin; c cyclic performance, coulombic efficiency after 100 cycles in a rate of 1 C; d rate performance

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The rate performance of the NCM523/Li cathode was evaluated from 0.1 to 2 C and then returned to 0.1 C with each current rate for fore cycles in both electrolyte with and without Curcumin. As depicted in Fig. 4(d), the discharge capacity of the battery cycled in the electrolyte with 0.04-wt% Curcumin exhibits comparable discharge capacity to that in the additive-free electrolyte at low current densities. Nevertheless, as the current density increases, the battery cycled in the electrolyte containing 0.04-wt% Curcumin demonstrates significantly higher discharge capacity in comparison to the battery in the pristine electrolyte. When the current density is raised from 1 to 2 C, the discharge capacity in the electrolyte containing 0.04-wt% Curcumin decreases to ~ 105 mAh g−1, experiencing a capacity decay of 26 mAh g−1. In contrast, in the base electrolyte, the discharge capacity sharply declines to ~ 82 mAh g−1, accompanied by a capacity decay of 48 mAh g−1. Furthermore, upon reducing the rate back to 0.1 C, Curcumin-containing electrolyte indicates higher discharge capacity than that of pristine electrolyte. In conclusion, the incorporation of 0.04-wt% Curcumin into the pristine electrolyte brings about a noteworthy improvement in the rate capability of the NCM523 cathode, particularly at current densities exceeding 1 C.

To explore more, the electrochemical impedance spectra (EIS) was accomplished on NCM523/Li cells in an electrolyte with and without Curcumin, before and after cycling in the voltage amplitude of 2.7–4.4 V. It can be observed from Fig. 5a that the resistance of electrolyte (Rs) represented by the intercept at high frequency and the diameter of the semicircle, indicating the interphase resistance which is influenced by the electrolyte, remained relatively same for both cell with and without additive before cycling. However, after cycling the Nyquist plot of both cells changed to have two semicircles in low and medium frequencies which are related to charge transfer and SEI film resistances, respectively [42,43,44,45,46]. The cell with no additive shows significant increment for the total resistance (film and charge transfer resistances). However, the electrolyte with Curcumin indicates much smaller charge transfer and SEI resistances after 100 cycles (Fig. 5b). These results are numerically specified through their fitted values represented in Table 1. The finding indicates that the CEI film produced from Curcumin on NCM523 cathode surface demonstrates a lower charge transfer resistance (94 Ω) at the interphase between electrode and electrolyte compared to the base electrolyte (250 Ω), facilitating the Li+ ions motion and leading to a substantial enhancement in the cells’ cycle performance. It can be concluded that the addition of Curcumin can remarkably decease total resistance of the cells thanks to the creation of a conductive and protective CEI film constructed between the cathode and the electrolyte (Table 2).

Fig. 5
figure 5

Nyquist plots of NCM523/Li half cells (a) before cycling; and (b) after 100 cycles

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Table 2 Equivalent circuit data of the NCM523/Li half cells without and with 0.04 wt% Curcumin after 100 cycles
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In order to understand how Curcumin affects the NCM523 electrode morphology, the FESEM was applied to characterize the both electrodes with and without Curcumin before and after cycling. Figure 6a shows the clean and smooth surface morphology for the NCM electrode prior to cycling. Nevertheless, the surface of electrode has disintegrated and left cracks after undergoing cycling in the electrolyte without additive. The demolition of the balk structure is related to the degradation of the electrolyte, which subsequently leads to the production of HF and finally accelerates the transition metals dissolution from the electrode surface to the electrolyte. On the contrary, the electrode’s morphology can be effectively conserved in the electrolyte containing Curcumin, where the electrode particles are able to maintain their structural integrity without cracking. This image corroborates that Curcumin can generate a protective film on NCM523, leading to the enhanced cycling stability and low impedance. TEM is also applied to more investigate the effect of Curcumin on the NCM523 cathode. It is obvious that the electrode surface is clean and smooth before cycling. Following the cycling process, deposition layers resulting from electrolyte decomposition can be observed on both NCM523 electrodes with and without Curcumin (Fig. 6d-f). The CEI layer formed with Curcumin is dense and uniform, while in the blank electrolyte, the layer appears thick and exhibits irregular coating, aligning with the observations obtained from the SEM analysis.

Fig. 6
figure 6

The SEM and TEM images of NCM523 electrodes; before cycling (a,d), after cycling in the pristine electrolyte (b,e), with 0.04% Curcumin (c,f)

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To further investigate the structural damage of NCM523 caused by cycling, the XRD test was carried out on the recovered electrodes after cycling in the electrolytes with and without Curcumin. As depicted in Fig. 7a there is a noticeable decrease in the intensity of all peaks when cycled in the additive-free electrolyte. This elucidates that NCM523 structure undergoes significant damage in the pristine electrolyte. Conversely, the cycled electrode with Curcumin demonstrated a mitigating effect on this detrimental outcome.

Fig. 7
figure 7

XRD spectra of the fresh NCM523 electrode and cycled electrodes in electrolyte with and without Curcumin (a); concentration of transition metal elements deposited on lithium metal after cycled in the base and Curcumin containing electrolyte (b)

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The demolition of cathode is led to dissolving of transition metal elements (TMs = Ni, Co, and Mn), which consequently can deposit on the negative surface. The abundance of these elements present in the lithium anode could serve as an indicator to determine the extent of damage. Hence, the NCM523/Li cells after cycling were disassembled and the lithium foil was removed and subsequently dissolved in deionized water. Then the quantity of Ni, Mn, and Co ions were measured by ICP-OES. According to the test results represented in Fig. 7b, the amounts of TMs, namely Ni, Co, and Mn in a Lithium foil cycled in the pristine electrolyte was found to be 30, 30, and 20 ppb, respectively. While, the concentration of these elements in a Lithium foil cycled in electrolyte with 0.04% Curcumin was determined to be 10, 20, and 10 ppb, respectively. It is clear that the 0.04% Curcumin additive remarkably hinders dissolution of these elements from the NCM523 electrode and maintain the crystal structure of the cathode.

3.3 Electrochemical behavior of the graphite/Li cells

To further conduct a more detailed examination of the effect of Curcumin on graphite electrode, the first charge/discharge curves and cyclic performance of graphite/Li cells were obtained using both the pristine electrolyte and an electrolyte containing 0.04-wt. % Curcumin, and the results are displayed in Fig. S4 and S5. Both cells exhibit comparable lithium intercalation and deintercalation plateaus. The initial charge specific capacity of cells using the pristine electrolyte and an electrolyte containing 0.04 wt% Curcumin are 337.72 and 342.64 mAh g−1 with initial Coulombic efficiency of 90% and 92%, respectively, which is quite similar. Moreover, as can be seen from figure S5, the cyclic performance of both samples with and without curcumin are comparable and there is no noticeable difference in cyclic performance. This results demonstrate that the introducing of Curcumin has no adverse impact on the cyclic performance of the cell.

Surface analysis utilizing XPS was conducted after cycling to determine the effect of Curcumin on the chemical composition of CEI. According to research studies, LiPF6 has been well defined as a source of fluorides when contacted to a trace amount of water (LiPF6 + H2O = 2HF + POF3 + LiF). As well as, due to its thermodynamic instability, LiPF6 can undergo a conversion to a potent Lewis acid called PF5, leading reactions that ultimately produce HF or other P–F compounds [40, 46, 47]. These reactions can result in the dissolution of transitional metals. The XPS analysis of the cycled cathodes, as illustrated in Fig. 8, revealed the presence of two prominent peaks in F 1 s. The first peak corresponds to the C − F bond, which can be attributed to PVDF component. The second peak corresponds to the LiF ingredient, which originates from the oxidative degradation of PF6− anions [48]. The weaker intensity of LiF peak and the more intense C − F peak observed in the Curcumin-containing electrolyte elucidates that the passivation layer with Curcumin was relatively thinner and more effective in preventing decomposition of PF6− anions compare to the electrolyte without Curcumin. The obtained results are consistent with the impedance data mentioned earlier. As a result, the Curcumin-containing cell exhibited lower resistances in terms of passivation film and charge transfer. After cycling, the intensity of Li2CO3 peak did not exhibit significant enhancement in the Curcumin-containing electrolyte. The P2p spectra exhibit two clear peaks at 136 and 134 eV corresponding to LixPFy and LixPOyFz, respectively [49]. Notably, the intensities of LixPFy and LixPOyFz peaks are lower in the cathode with curcumin additive compared to the cathode without additive. This finding aligns with the observations from F1s spectra. This implies that the cathode electrode interphase generated on the Curcumin-induced electrolyte did not exhibit increased thickness upon cycling, as evidenced by the O 1 s spectra

Fig. 8
figure 8

XPS spectra of C 1 s, F 1 s, O 1 s and P 2p on the surface of the NCM523 electrodes after cycling in the electrolyte with and without 0.04-wt % curcumin

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Typically, electrolyte additives for LIBs undergo a complex evaluation process and also are sold at very high price. It is important to consider incompatibility issues before utilizing additives. On the other hand, Curcumin is an inexpensive and non-toxic substance that can be readily acquired. It has the ability to inhibit resistance build up during cycling, thereby enhancing the safety of LIBs without compromising their capacity and cycling performance.

4 Conclusion

In this study, Curcumin as a novel bio-based additive is incorporated for the first time in LiNi0.5Co0.2Mn0.3O2/Li cells at high voltage. The DFT calculation showed that Curcumin can be preferentially decomposed via oxidation and take part in CEI forming. In addition, through LSV, CV, XRD, SEM, and TEM analyses, it has been observed that the preferential oxidation of Curcumin leads to the creation of a CEI layer adhering to the cathode surface. This CEI is advantageous in preventing both the electrolyte degradation and the TMs leaching from the cathode. The investigation revealed that the electrolyte containing 0.04-wt % Curcumin has higher cyclic stability with the capacity retention of 91% after 100 cycles, while it was only 79% for the pristine electrolyte. This finding suggests that Curcumin has potential as a CEI-forming electrolyte additive for the high-voltage cathode for Li-ion batteries.