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Article

Facile SEI Improvement in the Artificial Graphite/LFP Li-Ion System: Via NaPF6 and KPF6 Electrolyte Additives

by
Sepehr Rahbariasl
and
Yverick Rangom
*
Department of Chemical Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada
*
Author to whom correspondence should be addressed.
Energies 2025, 18(15), 4058; https://doi.org/10.3390/en18154058 (registering DOI)
Submission received: 3 April 2025 / Revised: 21 July 2025 / Accepted: 28 July 2025 / Published: 31 July 2025
(This article belongs to the Special Issue Research on Electrolytes Used in Energy Storage Systems)
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Abstract

In this work, graphite anodes and lithium iron phosphate (LFP) cathodes are used to examine the effects of sodium hexafluorophosphate (NaPF6) and potassium hexafluorophosphate (KPF6) electrolyte additives on the formation of the solid electrolyte interphase and the performance of lithium-ion batteries in both half-cell and full-cell designs. The objective is to assess whether these additives may increase cycle performance, decrease irreversible capacity loss, and improve interfacial stability. Compared to the control electrolyte (1.22 M Lithium hexafluorophosphate (LiPF6)), cells with NaPF6 and KPF6 additives produced less SEI products, which decreased irreversible capacity loss and enhanced initial coulombic efficiency. Following the formation of the solid electrolyte interphase, the specific capacity of the control cell was 607 mA·h/g, with 177 mA·h/g irreversible capacity loss. In contrast, irreversible capacity loss was reduced by 38.98% and 37.85% in cells containing KPF6 and NaPF6 additives, respectively. In full cell cycling, a considerable improvement in capacity retention was achieved by adding NaPF6 and KPF6. The electrolyte, including NaPF6, maintained 67.39% greater capacity than the LiPF6 baseline after 20 cycles, whereas the electrolyte with KPF6 demonstrated a 30.43% improvement, indicating the positive impacts of these additions. X-ray photoelectron spectroscopy verified that sodium (Na+) and potassium (K+) ions were present in the SEI of samples containing NaPF6 and KPF6. While K+ did not intercalate in LFP, cyclic voltammetry confirmed that Na+ intercalated into LFP with negligible impact on the energy storage of full cells. These findings demonstrate that NaPF6 and KPF6 are suitable additions for enhancing lithium-ion battery performance in the popular artificial graphite/LFP system.

1. Introduction

Lithium-ion batteries have garnered significant attention over the past few decades. Over the past 30 years, the applications for these rechargeable, lightweight, and effective batteries have grown significantly, from our phones, computers, and video cameras to electric vehicles [1]. However, when it comes to electric vehicle batteries, higher energy density, lower production costs, and fast charging features are still needed [2]. Currently, battery electric vehicles (BEVs) are heating up as one of the last technological hurdles to displace traditional internal combustion engine vehicles (ICEVs), as charging is still less convenient than refueling [3,4]. Batteries gradually lose capacity as they age, which lowers power output, reduces driving range, and degrades user experience. Therefore, increasing capacity is essential to preserve the long-term dependability and performance of EV batteries [5].
One way to develop and enhance the fast-charging capability of lithium-ion batteries is to reduce the impedance of SEI layers [6]. During the initial charge-discharge cycle, a layer called the solid electrolyte interface (SEI) forms on the anode. This layer permits Li ions to transport across it but obstructs electron flow and inhibits solvent from migrating to the electrode surface. It also controls the kinetics of Li ions, which is crucial to fast-charging capabilities. Utilizing electrolyte additives is the most cost-effective and scalable method for enhancing the SEI layer’s characteristics [7]. The high resistance of the SEI would hinder the performance of the battery by raising the resistance of Li+ transfer. Thus, to reduce these difficulties, a strong, thin, and stable SEI with high ionic conductivity on the graphite anode is required [8]. However, as far as we know, no studies demonstrate the effects of sodium and potassium additives in a full cell system. This is a fundamental research gap, as the popular NMC is not compatible with these additives. NMC cathodes can intercalate sodium electrochemically with poor capacity retention; they also require sodium doping to improve reversible electrochemical intercalation of potassium, making NMC less directly compatible with sodium and potassium electrolyte additives [9,10]. However, lithium iron phosphate (LFP) is compatible with both. Lithium Iron Phosphate (LFP), on the other hand, is one of the main choices for EV batteries due to its low cost of raw materials, low toxicity, environmentally friendly nature, excellent safety properties, cycling performances, and long cycle life, making it a promising candidate for next-generation advanced high-energy lithium-ion batteries [11]. LFP has been demonstrated to reversibly intercalate sodium or potassium, making them suitable candidates for cycling in mostly lithium electrolytes with sodium and potassium additives [12,13]. Additionally, compared to chemistries based on nickel and cobalt, LFP cathodes are safer for demanding applications because of their greater thermal stability and resistance to thermal runaway under short-circuit and abuse circumstances [14]. Sodium additives have been efficient electrolyte property modifiers to improve battery safety and performance. Komaba et al. observed that adding NaClO4, an electrolyte additive in 1 M LiClO4 electrolyte, would decrease the resistivity of the SEI layer. Additionally, they confirmed that irreversible capacity during the first cycle was reduced [15]. Li et al. explored a sodium-added electrolyte. They concluded that Li-rich/graphite full cells can retain 86% capacity after 50 cycles with a negligible (0.1 wt%) addition of NaClO4 [16].
Many studies have shown that the use of electrolyte additives can improve rate performance considerably and stop lithium batteries from degradation. Their improved conductivity, which lowers charge transfer resistance and encourages the development of a stable SEI, is largely to blame for this [17]. For instance, Son et al. studied three kinds of additives: ethylene carbonate, fluoroethylene carbonate, and vinylene carbonate, as anode SEI formers. They discovered that LiNi0.6Co0.2Mn0.2O2/graphite complete cells with fast-charging capabilities can demonstrate outstanding capacity retention of 79% after 1000 cycles at a high charging current with the combination of fluoroethylene carbonate and dimethyl carbonate in the electrolyte [18]. In another study, Burns et al. investigated the effect of varying vinylene carbonate (VC) content in lithium-ion cells. They found that increasing the VC concentration enhanced coulombic efficiencies with at least 4% VC and reduced charge endpoint slippage by up to 6% VC [19]. It has been demonstrated that adding 2 weight percent of fluoroethylene carbonate (FEC) to a LiPF6-based electrolyte results in around 65% capacity retention after 100 cycles. This is because a LiF-rich, thinner SEI is formed, which lowers resistance [20]. Adding 1%wt lithium bis(oxalate)borate (LiBOB) to borate-based electrolytes improves capacity retention by 66.7% after 100 cycles by forming a mechanically stable SEI that lessens degradation [21].
Artificial SEIs, on the other hand, have gained attention over the past few years. The objectives of artificial SEI formation are to enhance the SEI’s mechanical and thermal stability, decrease its irreversible capacity by halting the electrochemical breakdown of the electrolyte, increase its reversible capacity, and allow for faster rates of charge and discharge. The reaction chemistry of SEI formation during battery operation can be altered by surface coating, leading to different SEI compositions or structures. This coating itself is occasionally described as an artificial SEI. Menkin et al. introduced the idea of an artificial SEI using sodium carboxymethyl cellulose (NaCMC) and poly (ethylene-co-acrylic acid) (PEAA) coatings. By electroplating a graphite anode and vacuum-plugging it into a nano-tin-alloy anode, homogenous, continuous coatings of the artificial SEI were formed on the anode surface, which resulted in five times more cycle life. Li et al. implemented Lithium phosphorus oxynitride (LIPON) as artificial SEI because of its strong electrochemical stability at low voltage and adequate ionic conductivity (2 × 10−6 S cm−1). The coulombic efficiency rises significantly to 99+% with LIPON artificial SEI thicker than 50 nm, with no discernible electrolyte reduction peak. In another study led by Winter et al., to obtain desirable electrode/artificial SEI/electrolyte interfacial characteristics, such as accelerated ion transfer rate, a novel idea for creating a polymeric artificial SEI based on the logical design of a multifunctional polymer-blend coating was presented using polyether called polyethylene glycol tert-octylphenyl ether (PEGPE; C14H22O-(C2H4O)n, n = 9–10). The significant increase in delithiation performance, which is equivalent to the anode’s discharging rate during full-cell operation, was the most noticeable effect. The natural graphite electrode demonstrated a capacity of 336 mAh g−1 (i.e., 95% of full-capacity) even at a 10 C rate, and its specific delithiation capacity remained constant when the current was increased 50 times, from 0.1 to 5 C rate. In contrast, only half of the capacity (168 mAh g−1, 48% retention) was given by the uncoated electrode [22,23,24,25]. However, artificial SEIs require production techniques that are both expensive and non-scalable, preventing their wide use in industry despite their good electrochemical properties.
Another approach would be to form the SEI at different C-rates. It has been shown that by controlling the SEI formation at high current densities, the charging time of the graphite anode may be greatly reduced [26,27]. Rangom et al. demonstrated that when compared to the SEI layer formed at 100 C, the SEI layer formed at the current density used in industry, which is 0.1 C, decreases the overall Li+ diffusivity by at least 23% [28].
Although the use of electrolyte additives such as NaPF6 in half-cell designs has been studied in the past [15], few researchers have used commercially relevant electrodes to investigate their effects in full-cell systems. Interestingly, the combination of LFP with an artificial graphite is still not well studied in this area. Our work provides new insights into additive behavior under full-cell settings by introducing this novel combination and methodically analyzing SEI production and electrochemical performance.
Our goal in this work is to use sodium and potassium electrolyte additives to enable the successful and scalable formation of a solid electrolyte interphase (SEI) layer in graphite/LFP full cells. The main objective is to assess the effects of these additives on cycling performance and SEI formation in both full-cell and half-cell setups. We predict that the addition of potassium and sodium ions will change the interfacial behavior and SEI composition on graphite electrodes, improving performance metrics and lowering irreversible capacity loss. This research aims to learn more about how mixed-ion electrolytes impact the interface between electrode and electrolyte materials, which will help design next-generation lithium-ion batteries.

2. Methods and Materials

2.1. Materials

The base electrolyte solution was bought from Sigma-Aldrich (St. Louis, MO, USA) and contained 1 M LiPF6 in a 1:1 (v/v) mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). Sodium hexafluorophosphate (NaPF6), Potassium hexafluorophosphate (KPF6), and Lithium hexafluorophosphate (LiPF6) salts were also obtained from Sigma-Aldrich. Lithium Iron Phosphate (LFP) electrodes and single-sided artificial graphite electrode sheets on Cu were purchased from the NEI Corporation (Somerset, NJ, USA).
Every electrode was punched into a 12-mm-diameter disc, which was equivalent to a 1.13 cm2 electrode area. In full-cell experiments, the theoretical areal capacities of approximately 1.24 mAh/cm2 and 1.94 mAh/cm2 were related to the areal loadings of the LFP cathode and graphite anode, which were 7.32 mg/cm2 and 5.22 mg/cm2, respectively. As a result, the cathode-to-anode (N/P) ratio remained constant at about 1.56. For half-cell experiments, the working electrode was graphite, the reference electrode was lithium metal, and the counter electrode was LFP, with an active area capacity of 3 mAh/cm2 (or around 16.98 mg/cm2). The LFP counter electrode was the same in all half-cell tests. The loadings of the graphite electrodes used in the half-cell experiments varied according to the C-rate employed for SEI formation and the electrolyte composition. For half-cells with SEI formed at 0.1 C, the loadings of graphite were 2.77 mg/cm2 (1.22 M LiPF6), 3.04 mg/cm2 (1 M LiPF6 + 0.22 M NaPF6), and 3.52 mg/cm2 (1 M LiPF6 + 0.02 M KPF6). Graphite loadings for half-cells with SEI formed at 1 C were 3.06 mg/cm2 (1.22 M LiPF6), 2.96 mg/cm2 (1 M LiPF6 + 0.22 M NaPF6), and 3.80 mg/cm2 (1 M LiPF6 + 0.02 M KPF6).

2.2. Electrode and Electrolyte Preparation

Four formulations of the electrolyte were made. There was 1.22 M LiPF6 in sample 1 (control). Sample 2 included 0.22 M NaPF6 and 1 M LiPF6. In Sample 3, there was 0.22 M KPF6 and 1 M LiPF6. In Sample 4, there was 0.02 M KPF6 and 1 M LiPF6. Table 1 summarizes the electrolyte formulations. A glove box filled with argon (H2O and O2 < 0.8 ppm) was used to weigh and dissolve the extra salts in the base electrolyte. This was done to avoid moisture contamination. For cell assembly, the produced electrolytes were utilized right away.

2.3. Characterization

X-ray photoelectron spectroscopy (XPS) was utilized to analyze the surface chemistry and chemical composition of SEIs formed on graphite electrodes with regard to different electrolyte additives. XPS was performed using a Thermofisher Scientific Nexsa G2 Surface (Thermofisher Scientific, Waltham, MA, USA) analysis system with a monochromatic Al Kα. EIS was performed on the Biologic SP-150 instrument (Biologic, Claix, France). SEM was conducted using a Tescan VEGA TS-5130 (Tescan, Brno, Czech Republic) at an accelerating voltage of 20 kV.

2.4. Electrochemical Methods

Swagelok cells were assembled in an Argon-filled glove box with O2 and H2O below 0.8 ppm. Using Biologic’s SP-150 (BioLogic, Seyssinet-Pariset, France) galvanostatic charge/discharge cycling, studies were performed in the potential range of 2.6 V to 3.6 V versus Li/Li+ for full cells and 2 V to 0.02 V for half cells. The formation of SEI layers occurred at C-rates of 0.1 C and 1 C, respectively. Cycling tests were performed at 2 C, 4 C, and 6 C for half cells and 0.5 C for full cells. Moreover, the CV experiment was carried out for both anode (from 2 V to 0.02 V) and cathode (from 2.7 V to 4.2 V) at a scan rate of 0.05 mVs−1. The symmetric graphite cells were used for EIS measurements. The measurements were made between 1 MHz and 0.1 Hz at open-circuit voltage with an amplitude of 10 mV AC.

3. Results and Discussion

The initial lithiation curves, which correspond to the SEI formation of all three electrolytes, are illustrated in Figure 1c. The control cell, with 1.22 M LiPF6, achieves a specific capacity of 607 mA·h/g (this high value includes irreversible reaction due to SEI formation, not reversible graphite lithiation), following the SEI formation, with an initial coulombic efficiency of 70.84% and an irreversible capacity of 177 mA·h/g. In contrast, the cells with NaPF6 and KPF6 additives yield lower specific capacities of 520 mA·h/g and 517 mA·h/g, respectively. However, these additives show enhanced initial coulombic efficiencies, with NaPF6 reaching 79.23% and KPF6 reaching 78.72%, corresponding to lower irreversible capacities of 108 mAh g−1 and 110 mAh g−1, respectively. Less specific capacity during the first lithiation shows that less SEI has been formed on the anode material, resulting in a likely thinner SEI layer but less irreversible loss of lithium. NaPF6 and KPF6 appear to have different effects on SEI formation compared to commonly used organic additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC), which have been demonstrated to improve long-term stability but frequently produce thicker SEI layers and do not significantly reduce initial irreversible capacity. This is consistent with research by Komaba et al. [15], which found that sodium-based additives lower the SEI’s resistivity and minimize early capacity loss. Our cells’ enhanced performance can be attributed to the altered SEI composition brought about by Na+ and K+ ions, which are thought to encourage the development of more inorganic-rich, ionically conductive SEI. Cycle rate and CV experiments have been conducted to better understand the effect of electrolyte additives on the battery performance.
The cyclic voltammetry (CV) profiles for both LFP cathode and graphite anodes conducted at a sweep rate of 0.05 mV/s are presented in Figure 1a,b. The characteristic peaks corresponding to the staging phenomena during lithium intercalation and deintercalation can be seen in these graphs. For the graphite anodes, as illustrated in Figure 1a, all three electrolyte additives show peak currents at about 0.2 V, according to the anodic scans for the graphite anode. This finding implies that the intercalation potentials for the various electrolyte additions are not significantly altered. However, during the cathodic scan, a little shift in the presence of the NaPF6 additive can be seen, but it is negligible. Additionally, potassium plating is observed for the electrolyte added with 0.22 M KPF6. However, no evidence of potassium plating can be found in the electrolyte with 0.02 M KPF6. This can happen for multiple reasons. For instance, at lower concentrations of KPF6, the deposition potential of potassium is relatively low. This means that the graphite anode does not reach the necessary potential for potassium to deposit as metal during regular cycling, especially at lower electrolyte concentrations. In contrast, higher concentrations of KPF6 result in higher deposition potentials, making it easier for the anode potential to reach these values during cycling, thus triggering potassium deposition. Moreover, at lower concentrations, fewer potassium ions participate in the deposition process. This reduces the likelihood of reaching the conditions necessary for potassium plating. Another reason for this would be that, at lower concentrations, the presence of K+ ions allows the battery to operate normally without significant polarization or resistance build-up. Higher concentrations, however, may cause the bigger K+ ions to impede the flow of Li+ ions, increasing internal resistance and changing the anode potential. Potassium plating may be made easier by this shift, which may bring the anode potential closer to the potassium deposition potential [29].
According to the LFP cathode CV curves (Figure 1b), it appears that the peaks for the control cell with 1.22 M LiPF6 and electrolytes with KPF6 additives occur at the same voltage, which corresponds to no intercalation of KPF6. However, the results indicate that the NaPF6 additive has a slight shift for both anodic and cathodic scans. This suggests that sodium ions intercalate and deintercalate into and out of the LFP structure. However, the use of NaPF6, as an electrolyte additive, results in increased polarization, as evidenced by the shift. Specifically, during the cathodic scan, the potential of the NaPF6-added cell increases from 3.8 V (observed with both LiPF6 and KPF6 additives) to 3.9 V, and during the anodic scan, it decreases from approximately 3 V to slightly below 3 V. This increased polarization indicates a larger voltage difference between lithiation and delithiation processes, which could negatively impact the energy efficiency of the cell. Thus, using NaPF6 as an electrolyte additive would make a slight negative impact on the energy efficiency of the battery. It is crucial to recognize that thermodynamic irreversibility and entropy formation have a substantial impact on ion transport behavior in electrochemical systems, even though in-depth b-value or scan-rate-dependent investigations were outside the purview of this study. This is especially important in mixed-ion environments, such as the one Li et al. examined in their study of entropy and ion dynamics in supercapacitor systems [30].
Figure 2a and Figure S1 provide details about the cycle rate of control LiPF6, NaPF6-added sample, and 0.02 M KPF6 additives half cells with the formation of SEI at 0.1 C (Figure 2a) and 1 C (Figure S1) over various C-rates. The capacity is normalized to the 10th cycle. It can be seen from Figure 2a that, when the SEI is formed at 0.1 C, the control cell with LiPF6 shows a superior performance in terms of capacity across various C-rates, especially at 4 C and 6 C. The SEI formed in samples with NaPF6 and KPF6 additives exhibits higher resistance. However, Figure S1 reveals that this resistance is eliminated when the SEI is formed at 1 C. The stability of the cells is also improved when they return to lower C-rates when NaPF6 and KPF6 are added to the electrolyte (4 C-r, 2 C-r, and 1 C-r). The Coulombic efficiency for the three electrolytes during 70 cycles at different C-rates is shown in Figure 2b. Throughout the experiment, every cell maintained a high Coulombic efficiency, remaining above 99%, which suggests robust cycling performance and little adverse effects. Nevertheless, during the high-rate 6 C cycle and the ensuing 4C-return phase, the cell with 0.02 M KPF6 additive displayed a discernible decrease in Coulombic efficiency, most likely as a result of enhanced polarization or unstable SEI behavior at high current densities. The efficiency notably improved in the later phases, indicating that the SEI created under high-rate cycling conditions somewhat stabilized. Figure 2c represents the EIS data for cells with and without SEI formed at 0.1 C. The equivalent circuit model R1 + (Q2‖R2) + (Q3‖R3) + Q4 that was utilized to fit the EIS data is shown in Figure 2d. A fitted graph has been plotted in Figure S4 in the SI, and the parameters of the fitting have been summarized in Table S1. R1 represents ion impedance in the solvent, Q2/R2 represents the electronic interface and is fitted to the high frequency part of the elongated semi-circle, while Q3/R3 represents the ion interface and is fitted to the lower frequency part of the semi-circle. Constant phase element Q4 represents the overall capacitance of these symmetric capacitor test cells. Among the samples with SEI formed at 0.1 C, the control sample with 1.22 M LiPF6 shows the highest solution impedance (R1) (2.41 Ω·cm2). However, the cell with 1 M LiPF6 and 0.22 M NaPF6 shows the lowest impedance in the low to mid impedance range (2.01 Ω·cm2). This suggests that the addition of NaPF6 enhances the SEI formation. However, the sample with 1.22 M LiPF6 and 0.02 M KPF6 shows slightly higher impedance (2.13 Ω·cm2) compared to the NaPF6 added sample. The ionic conductivity of the electrolytes was barely affected by the addition of NaPF6 and KPF6 additives, as shown by the impedance spectra of samples without SEI formation in Figure 2c. In the case of the control electrolyte with LiPF6, the extracted solution resistance (Rs) values were 2.10 Ω·cm2, 1.94 Ω·cm2 for the electrolyte that had NaPF6 added, and 2.09 Ω·cm2 for the electrolyte that had KPF6 added. These slight variations—within around ±4%—indicate that the enhancements in electrochemical performance are caused mainly by these additives’ effects on SEI characteristics, rather than modifications in ionic conductivity. The ion interface level (Q3 and R3) showed notable changes, confirming the claim that SEI formation alters this area in particular. With values more than doubling in the case of LiPF6, R3 shows a significant rise. This rise is in line with the extra ionic transport resistance at the interface caused by a resistive SEI layer. The electrochemical signature of SEI formation—a more resistive, less capacitive interface, which is a feature of passivating SEI films—is further supported by the notable decrease in Q3 and the rise in R3.
Using the same electrolyte compositions, full cells were assembled and cycled for 50 cycles at 0.5 C between 2.6 V and 3.6 V, after the electrolyte effects were confirmed in half-cell experiments (Figure 3a). Although LFP had an initial loading of 1.25 mAh/cm2, different electrolytes had different measured capacities, following SEI formation because of differing levels of irreversible capacity loss (ICL). According to Figure 3a, after SEI formation, the capacity decreased to 0.69 mAh/cm2, and the 1.22 M LiPF6 electrolyte showed the highest ICL (44.8%). By comparison, the electrolyte containing 1 M LiPF6 + 0.22 M NaPF6 had the lowest ICL (33.6%) and retained 0.83 mAh/cm2, whereas the electrolyte containing 1 M LiPF6 + 0.02 M KPF6 had an intermediate ICL (37.6%) and a capacity of 0.78 mAh/cm2.
The trends in capacity retention further deviated over the first 50 cycles. By the 50th cycle, the LiPF6 electrolyte’s capacity had dropped to 0.32 mAh/cm2, indicating severe degradation. On the other hand, the electrolyte containing KPF6 demonstrated a moderate capacity retention of 0.47 mAh/cm2, whereas the electrolyte with NaPF6 maintained the maximum capacity at 0.67 mAh/cm2. According to these findings, NaPF6 and KPF6 improve long-term cycling performance by reducing irreversible capacity loss and SEI formation. Our previous half-cell results, which showed that NaPF6 and KPF6 affected SEI composition and interfacial stability, are consistent with the full-cell data. Lower capacity retention and higher lithium consumption are the outcomes of the baseline LiPF6 electrolyte’s facilitation of larger SEI formation. Compared to LiPF6, KPF6 reduces initial capacity loss; nevertheless, it is less effective over the long term than NaPF6. In addition to the capacity data in Figure 3a, Figure 3b shows the Coulombic efficiency of the full cells across 50 cycles. With values remaining close to 99% during the cycle, all three electrolyte systems demonstrated continuously high Coulombic efficiency, indicating low parasitic reactions and good reversibility of the charge-discharge processes in all formulations. Additional tests were carried out utilizing sophisticated electrodes that could generate SEI at greater current densities in order to confirm the effectiveness of the NaPF6 and KPF6 additives. These studies demonstrate that, under both 1 C and 0.1 C SEI formation circumstances, NaPF6 consistently provides improved capacity retention when compared to the baseline and KPF6-containing electrolytes, as illustrated in Figure S3a,b in the Supplementary Information. Interestingly, under 1 C formation circumstances, NaPF6 inhibited lithium plating, whereas the baseline electrolyte showed early indications of plating. These results support NaPF6′s ability to stabilize SEI and improve long-term cycling performance, particularly when fast-formation methods and high-conductivity electrode designs are used.
XPS was performed on the graphite anodes to examine the surface chemistry of the solid electrolyte interphase (SEI) that forms in various electrolyte systems. The presence and impact of Na and K in the SEI were verified by high-resolution scans for Na 1s, F 1s, Li 1s, and C 1s on three separate samples. The Na 1s scan (Figure 4c) was employed to verify sodium incorporation in the SEI for the NaPF6-containing electrolyte (sample 2). Successful sodium incorporation was demonstrated when NaPF6 was added to the electrolyte, since sample 2 had a noticeable Na peak at 1072 eV, whereas the control sample (1.22 M LiPF6) did not exhibit it.
High-resolution F 1s, Li 1s, and C 1s scans were performed on sample 1 (control) and sample 4 (1 M LiPF6 + 0.02 M KPF6) in order to assess the role of Potassium in SEI formation. Figure 4a,b show the F 1s spectrum for samples 1 and 4. The C-F bond contribution increased from 10.09% in the control sample to 12.79% in sample 4, indicating that K affects surface species associated with fluorine. Additionally, the Li 1s spectrum, which is depicted in Figure 5a,b, provides additional evidence that K promotes LiF formation in the SEI by demonstrating a notable rise in LiF content, which increased from 19.78% in the control sample to 47.61% in sample 4. The observed trend in F 1s was further supported by the C 1s spectra (Figure 5c,d), which likewise show an increase in C-F bonding, going from 10.84% in the control sample to 15.35% in sample 4. These findings align with the previous literature that suggests K+ may not exist as free K-containing species but instead contribute mainly to the production of LiF and C-F bonds [31,32].

4. Conclusions

This study shows that industrially viable and reasonably priced NaPF6 and KPF6 electrolyte additives greatly improve capacity retention and reduce irreversible capacity loss when compared to traditional LiPF6 electrolyte.
Key accomplishments:
  • Irreversible capacity loss was significantly reduced by 38.98% and 37.85%, respectively, when KPF6 and NaPF6 were incorporated into graphite half-cells.
  • In the graphite/LFP full cell system, the presence of KPF6 and NaPF6 significantly enhanced reversible capacity by limiting lithium consumption during SEI formation.
  • The electrolyte with KPF6 showed a 30.43% improvement, while NaPF6 retained 67.39% more capacity than the baseline LiPF6 after 20 cycles.
These results show that capacity loss can be decreased by adding KPF6 and NaPF6. Of these, KPF6 provided less early-stage capacity loss, but NaPF6 showed more effective and long-lasting performance enhancement.
Furthermore, we showed that optimizing SEI formation at higher C-rates (1 C), using high electrical conductivity electrodes, further increases the gains in initial coulombic efficiency obtained at 0.1 C and shortens charging times without sacrificing SEI stability. Therefore, further research is warranted as SEI formed at 1 C further decreased SEI resistance in cells treated with KPF6 and NaPF6 additives compared to the ordinary reference LiPF6 electrolyte. Future research will concentrate on creating electrode materials and electrolyte formulations that allow for stable SEI formation at high C-rates to improve the lifespan and performance of fast-charging lithium-ion batteries.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en18154058/s1, Figure S1: 2 Cycle rate performance of half-cells with three different electrolytes, 1.22 M LiPF6 (red), 1 M LiPF6 + 0.22 M NaPF6 (green), and 1 M LiPF6 + 0.02 KPF6 (blue)-SEI formed at 0.1 C; Figure S2: SEM images of graphite anodes after the formation of SEI at 0.1 C at 20 μm and 50 μm scales. Sample 1 (a,b), sample 2 (c,d), sample 3 (e,f), and sample 4 (g,h). All images were captured using an accelerating voltage of 20 kV; Figure S3: Cycling performance of full cells with three electrolytes, 1.22 M LiPF6 (Orange dotted line), 1 M LiPF6 + 0.22 M NaPF6 (dashed green line), and 1 M LiPF6 + 0.02 M KPF6 (dashed and dotted blue line) with SEI formed at 1 C using advances electrodes (a) and cycling performance of the cells with advanced electrodes with SEI formed at 0.1 C; Figure S4: Fitted Nyquist plots of symmetrical graphite cells with no SEI, SEI formed at 0.1 C. (orange line: 1.22 M LiPF6 with SEI at 0.1 C), (dashed and dotted line: 1 M LiPF6 + 0.22 M NaPF6 with SEI at 0.1 C), (dashed dark blue: 1 M LiPF6 + 0.02 M KPF6 with SEI at 0.1 C), (black line: 1.22 M LiPF6 without SEI), (black dashed and dotted: 1 M LiPF6 + 0.22 M NaPF6 without SEI), and (black dashed line: 1 M LiPF6 + 0.02 M KPF6 without SEI). Table S1: Parameters of the fitting of the EIS data obtained from cells with and without SEI.

Author Contributions

Conceptualization, Y.R.; Data curation, S.R.; Formal analysis, S.R. and Y.R.; Investigation, S.R.; Methodology, S.R. and Y.R.; Supervision, Y.R.; Validation, Y.R.; Visualization, S.R.; Writing—original draft, S.R.; Writing—review and editing, Y.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Cyclic voltammetry (CV) curves of (a) the graphite anode, (dashed orange: 1.22 M LiPF6, green line: 1 M LiPF6 + 0.22 M NaPF6, dashed and dotted dark blue: 1 M LiPF6 + 0.22 M KPF6, and dotted red: 1 M LiPF6 + 0.02 M KPF6), (b) CV curves for LFP cathodes with same electrolytes, and (c) initial lithiation curves of three electrolyte additives, control LiPF6 (orange), NaPF6 (green), and (0.02) KPF6 (dotted blue), conducted at 0.1 C.
Figure 1. Cyclic voltammetry (CV) curves of (a) the graphite anode, (dashed orange: 1.22 M LiPF6, green line: 1 M LiPF6 + 0.22 M NaPF6, dashed and dotted dark blue: 1 M LiPF6 + 0.22 M KPF6, and dotted red: 1 M LiPF6 + 0.02 M KPF6), (b) CV curves for LFP cathodes with same electrolytes, and (c) initial lithiation curves of three electrolyte additives, control LiPF6 (orange), NaPF6 (green), and (0.02) KPF6 (dotted blue), conducted at 0.1 C.
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Figure 2. Cycle rate performance of half-cells with three different electrolytes: 1.22 M LiPF6 (orange), 1 M LiPF6 + 0.22 M NaPF6 (green), and 1 M LiPF6 + 0.02 KPF6 (blue)—SEI formed at 0.1 C. (a) Coulombic efficiency of graphite half-cells cycled at different C-rates with various electrolyte formulations, which correlates to the capacity retention data displayed in (b) and electrochemical impedance spectroscopy (EIS) data for cells with and without SEI formed at 0.1 C—(orange line: 1.22 M LiPF6 with SEI at 0.1 C), (dashed and dotted line: 1 M LiPF6 + 0.22 M NaPF6 with SEI at 0.1 C), (dashed dark blue: 1 M LiPF6 + 0.02 M KPF6 with SEI at 0.1 C), (black line: 1.22 M LiPF6 without SEI), (black dashed and dotted: 1 M LiPF6 + 0.22 M NaPF6 without SEI), and (black dashed line: 1 M LiPF6 + 0.02 M KPF6 without SEI) (c), two parallel combinations of constant phase elements and resistances that reflect interfacial processes, and a series resistance make up the equivalent circuit model that was utilized to match the EIS data (d).
Figure 2. Cycle rate performance of half-cells with three different electrolytes: 1.22 M LiPF6 (orange), 1 M LiPF6 + 0.22 M NaPF6 (green), and 1 M LiPF6 + 0.02 KPF6 (blue)—SEI formed at 0.1 C. (a) Coulombic efficiency of graphite half-cells cycled at different C-rates with various electrolyte formulations, which correlates to the capacity retention data displayed in (b) and electrochemical impedance spectroscopy (EIS) data for cells with and without SEI formed at 0.1 C—(orange line: 1.22 M LiPF6 with SEI at 0.1 C), (dashed and dotted line: 1 M LiPF6 + 0.22 M NaPF6 with SEI at 0.1 C), (dashed dark blue: 1 M LiPF6 + 0.02 M KPF6 with SEI at 0.1 C), (black line: 1.22 M LiPF6 without SEI), (black dashed and dotted: 1 M LiPF6 + 0.22 M NaPF6 without SEI), and (black dashed line: 1 M LiPF6 + 0.02 M KPF6 without SEI) (c), two parallel combinations of constant phase elements and resistances that reflect interfacial processes, and a series resistance make up the equivalent circuit model that was utilized to match the EIS data (d).
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Figure 3. Cycle rate performance of full cells, including 1.22 M LiPF6 (orange), 1 M LiPF6 + 0.22 M NaPF6 (green), and 1 M LiPF6 + 0.02 M KPF6 (blue) at 0.5 C after SEI formation at 0.1 C for 50 cycles (a), and columbic efficiency of the full cells over 50 cycles (b).
Figure 3. Cycle rate performance of full cells, including 1.22 M LiPF6 (orange), 1 M LiPF6 + 0.22 M NaPF6 (green), and 1 M LiPF6 + 0.02 M KPF6 (blue) at 0.5 C after SEI formation at 0.1 C for 50 cycles (a), and columbic efficiency of the full cells over 50 cycles (b).
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Figure 4. XPS F 1s spectra for control sample 1 (1.22 M LiPF6) (a) compared to sample 4 (1 M LiPF6 + 0.02 M KPF6) (b), and Na 1s spectra for sample 2 (1 M LiPF6 + 0.22 M NaPF6) compared to the control (sample 1, 1.22 M LiPF6) (c).
Figure 4. XPS F 1s spectra for control sample 1 (1.22 M LiPF6) (a) compared to sample 4 (1 M LiPF6 + 0.02 M KPF6) (b), and Na 1s spectra for sample 2 (1 M LiPF6 + 0.22 M NaPF6) compared to the control (sample 1, 1.22 M LiPF6) (c).
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Figure 5. XPS spectra of Li 1s (a,b) and C 1s (c,d) for sample 1 (a,c) and sample 4 (b,d), showing SEI composition.
Figure 5. XPS spectra of Li 1s (a,b) and C 1s (c,d) for sample 1 (a,c) and sample 4 (b,d), showing SEI composition.
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Table 1. Electrolyte formulations used in this study.
Table 1. Electrolyte formulations used in this study.
Sample No.Electrolyte Composition
Sample 1 (Control)1.22 M LiPF6
Sample 21 M LiPF6 + 0.22 M NaPF6
Sample 31 M LiPF6 + 0.22 M KPF6
Sample 41 M LiPF6 + 0.02 M KPF6
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Rahbariasl, S.; Rangom, Y. Facile SEI Improvement in the Artificial Graphite/LFP Li-Ion System: Via NaPF6 and KPF6 Electrolyte Additives. Energies 2025, 18, 4058. https://doi.org/10.3390/en18154058

AMA Style

Rahbariasl S, Rangom Y. Facile SEI Improvement in the Artificial Graphite/LFP Li-Ion System: Via NaPF6 and KPF6 Electrolyte Additives. Energies. 2025; 18(15):4058. https://doi.org/10.3390/en18154058

Chicago/Turabian Style

Rahbariasl, Sepehr, and Yverick Rangom. 2025. "Facile SEI Improvement in the Artificial Graphite/LFP Li-Ion System: Via NaPF6 and KPF6 Electrolyte Additives" Energies 18, no. 15: 4058. https://doi.org/10.3390/en18154058

APA Style

Rahbariasl, S., & Rangom, Y. (2025). Facile SEI Improvement in the Artificial Graphite/LFP Li-Ion System: Via NaPF6 and KPF6 Electrolyte Additives. Energies, 18(15), 4058. https://doi.org/10.3390/en18154058

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