K and Cl Co-Doping for Enhanced Ionic Conductivity and Structural Stability in Li-rich Cathode Materials

Min Seo Choi; J. Hyuk Kim; JM One; SH Lee; Jeong H. Cho; Yun Hyeok Choi; San Kang; Jong-Tae Son

doi:10.33961/jecst.2025.00605

Abstract

K and Cl were co-doped to improve the structural stability and ionic conductivity of Lithium-rich layered oxide (LLO) cathodes. This co-doping improved the initial discharge capacity, cycling performance, and impedance characteristics. X-ray diffraction analysis confirmed an increase in the c-axis and an improvement in the crystallinity of the layered structure owing to the doping effect. In contrast, energy-dispersive X-ray spectroscopy mapping showed that the elements, including K and Cl, were uniformly distributed on the surface of the cathode material. Electrochemical analysis revealed that the cathode material co-doped with K and Cl exhibited improvements compared with the conventional LLO cathode. After 100 cycles, the discharge capacity increased from 167.91 to 174.24 mAh/g. The capacity retention rate also improved, increasing from 85.55 to 95.39%. Furthermore, the C-rate performance was enhanced. At a 5C discharge rate, the capacity increased from 104.68 to 123.89 mAh/g. Electrochemical impedance spectroscopy analysis revealed that the charge transfer resistance improved from 55.25 to 29.83 Ω. The lithium diffusion coefficient also increased from 8.47×10^–14 to 2.31×10^–13 cm²/s. These results confirm the enhancement of the ionic conductivity and cycling performance owing to the doping effects of K and Cl.

Keywords: Li-rich cathode, Co-free, co-doping, Li^⁺ diffusion paths, electrochemical properties

INTRODUCTION

Lithium (Li)-ion batteries for electric vehicles require high capacity, stability, and cost-effectiveness, driving ongoing research and development in this area [1–4]. High-Ni NCM, which is one commercially available Li-ion battery, offers a high energy density as the Ni content increases. However, it has a theoretical capacity limit of 280 mAh/g [5–8]. Li-rich layered oxide (LLO) is a composite of Li₂MnO₃ and Li (Ni_xCo_yMn_z) O₂ phases, containing more than 1mol of Li, which gives it a higher energy density than High-Ni NCM. Furthermore, layered NCM oxides with increased Li and Mn contents and a reduced Co content offer significant cost advantages. Consequently, these materials have attracted significant attention as next-generation cathode materials [9–14]. However, commercialization has been hindered by several issues. First, the Coulombic efficiency is low during the initial charge-discharge cycles. Second, the C-rate performance is poor. Third, rapid voltage and capacity decay occur during cycling [15,16]. Various strategies have been explored to address these electrochemical challenges, including controlling the particle size and morphology, surface coatings, and heteroatom doping [17–19]. Heteroatom doping involves the introduction of cationic or anionic species into the lattice of cathode materials, which enhances their structural stability. Representative studies have shown that doping the transition-metal layer with Al, Ti, or Mg suppresses phase transitions. Doping the Li layer with Na or K facilitates Li-ion (Li^⁺) mobility. Furthermore, F and Cl doping at oxygen sites has been reported to enhance the structural stability of cathode materials [20–28]. In this study, K and Cl were co-doped into LLO cathode materials to improve their ionic conductivity and structural stability. The electrochemical properties of the doped materials were investigated.

EXPERIMENTAL

Precursor Synthesis

Spherical (Ni_0.25Mn_0.75) CO₃ was synthesized using a carbonate co-precipitation method. Aqueous solutions of NiSO₄·6H₂O (98.5%–102%, SAMCHUN) and MnSO₄·H₂O (98%, SAMCHUN) were prepared at an Ni:Mn molar ratio of 1:3, with concentrations of 2 mol/L. Subsequently, these solutions were fed into a 10 L reactor. An Na₂CO₃ solution (99%, SAMCHUN) at 2 mol/L was simultaneously introduced into the reactor as a pH regulator. An ammonia solution (25%–30%, SAMCHUN) at 0.3 mol/L was also fed in as a chelating agent. The pH inside the reactor was maintained at around 8, and the reaction temperature was kept at 55°C. Stirring was performed at 1000 rpm. The reaction time for the co-precipitation reaction was 8 hours. The synthesized (Ni_0.25Mn_0.75) CO₃ was filtered and washed. Thereafter, it was dried at 120°C for 24 h [7,12].

Cathode Material Synthesis

Li_1.2 (Ni_0.2Mn_0.6) O₂ was synthesized using a solid-state method. The precursor ((Ni_0.25Mn_0.75) CO₃) and Li₂-CO₃ (99.9%, Sigma-Aldrich) were mixed at an Li:Me molar ratio of 1.2:0.8. The mixture was then calcined at 500°C for 5 h. Finally, the mixture was calcined at 900°C for 12 h. To synthesize the K-doped material (LLO-KCl), 0.02 mol% of Li₂CO₃ was replaced with KCl (99.0%, Sigma-Aldrich) [9,10,23,25].

Material Characterization

The particle size distribution of the precursor was measured using a PSA (Sinco 1090). The crystal structure and crystallinity of the cathode material were analyzed by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with Cu Kα radiation. Scans were carried out from 10° to 80° (2θ) at a rate of 3°/min. The morphological characteristics of the synthesized materials were examined using field-emission scanning electron microscopy (FE-SEM). An FEI QUANTA 400 instrument was used for the analysis. The surface elemental compositions of the synthesized materials were analyzed using energy-dispersive X-ray spectroscopy (EDS) mapping. An FEI QUANTA 400 microscope equipped with an EDS detector was used for the analysis.

Electrochemical Characterization

Electrodes were fabricated to evaluate the electrochemical properties of the synthesized cathode materials. The cathode active material, conductive additive (Super-P, MMM Carbon Co.), and binder (PVDF, Sigma-Aldrich) were mixed at a weight ratio of 8:1:1. The mixture was dispersed in N-methyl-2-pyrrolidone to prepare a slurry. The slurry was coated onto aluminum foil using a doctor blade to a thickness of 25 μm. The mass of cathode materials for the electrode was manufactured to be 4.29 mg/cm². Thereafter, the electrodes were dried in a vacuum oven at 120°C. The dried electrodes were roll-pressed to secure the coated materials. The samples were then further dried in a vacuum oven at 120°C. The prepared electrodes were punched into disks using a 14-mm-diameter punch. Li metal was used as the anode for CR2032 coin cell assembly, and a polypropylene separator was used. The electrolyte consisted of 1M LiPF6 dissolved in an EC/DEC (3:7 by volume) solvent mixture. The coin cells were assembled in an argon-filled glove box according to standard procedures. The electrochemical properties of the assembled cells were evaluated at room temperature (25°C) using a PNE galvanostatic charge-discharge tester. Charge-discharge cycling was carried out between 2.0 and 4.8 V at 0.5 C for 100 cycles. Charge-discharge tests were performed at current densities of 0.1, 0.2, 0.5, 1, 2, and 5 C to evaluate the rate capability. Electrochemical impedance spectroscopy (EIS) was conducted using an IviumStat instrument (HS Technology) to determine the cell resistance and Li^⁺diffusion coefficient (D_Li). Measurements were carried out over a frequency range of 0.01 to 100 kHz with a perturbation amplitude of 10 mV [7]. Fig. 1 shows a schematic of the material synthesis and electrochemical characterization processes.

RESULTS AND DISCUSSION

Fig. 2 shows the XRD patterns of the LLO and LLO-KCl. Both materials exhibited a layered structure with the R-3m space group. In the 20–23° 2θ region, peaks corresponding to monoclinic Li₂MnO₃ with the C2/m space group were identified [9–14]. The lattice parameters of the LLO and LLO-KCl samples are listed in Table 1. Compared with LLO, the c-axis lattice constant of LLO-KCl increased from 14.2390 to 14.2422 Å. This expansion is a%ributed to the larger ionic radius of K^⁺ (1.33 Å) compared with Li^⁺ (0.76 Å). The resulting increase in the la-ce parameter is expected to extend Li^⁺ migration pathways, and thus, enhance the ionic conductivity. The I(003)/I(104) ratios, which reflect cation mixing within the lattice, were 1.8852 and 1.6227 for the LLO and LLO-KCl, respectively. Both values exceeded 1.2, confirming a low degree of cation mixing in the lattice. The R-factor [(I(006)+I(102))/I(101)] was calculated to assess the hexagonal crystallinity. The LLO-KCl exhibited a decrease in the R-factor from 0.4021 to 0.2301 compared with the LLO. This approximately 1.75-fold reduction indicates a significant improvement in crystallinity [23–26].

Fig. 3 shows the FE-SEM images and EDS mapping results for the LLO and LLO-KCl. Both samples exhibited particles with a diameter of approximately 8 μm. The average primary particle size increased from 271.15 nm for LLO to 409.62 nm for LLO-KCl, representing an approximately 1.5-fold increase. Morphological differences between the two materials have also been observed in Na-doped LLO cathodes. These reports indicated that alkali ions doped into the Li layer increase the crystallinity of the layered structure. As a result, the crystal growth tends to follow the layer orientation [23,24]. In addition, EDS mapping was performed to examine the surface elemental distribution. The EDS mapping confirmed that all elements, including K and Cl, were uniformly distributed across the surface of the cathode material.

Fig. 4 shows the dQ/dV curves for the LLO(a) and LLO-KCl(b). Using these curves, voltage fading and phase transformations during the cycling were analyzed [15,16]. During discharge, the LLO and LLO-KCl underwent three reduction reactions. Each reaction occurred through a distinct process. Re1 corresponds to the reduction in Li at the tetrahedral sites in the C2/m phase. Re2 corresponds to the reduction in Li at the octahedral sites in the R-3m phase, which was associated with Ni oxidation. Re3 corresponds to the reduction in Li at the octahedral sites in the R-3m phase, which was associated with Mn oxidation. Re3 is also linked to the phase transition of the LLO cathode towards the spinel structure [15,16]. During cycling, the Re3 peak of the LLO cathode continuously shifted towards lower potentials. After 50 cycles, a sharp peak appeared at 2.62 V, which is characteristic of the spinel structure. This confirms the occurrence of phase transition. In contrast, the LLO-KCl exhibited an Re3 peak at 3.18 V. This indicates that less phase transition to the spinel structure occurred during cycling [15,16].

Fig. 5 shows the initial charge-discharge profiles of the LLO and LLO-KCl. These measurements were carried out at a rate of 0.1 C. Both the LLO and LLO-KCl exhibited a smooth voltage rise below 4.5 V. At 4.5 V, each material exhibited a high-voltage plateau. The sloping region below 4.5 V arose from the oxidation of Ni^2⁺ to Ni^4⁺ in the LiMO₂ (M=Ni, Mn) phase and the concurrent extraction of Li^⁺ from the layered structure. The high-voltage plateau at 4.5 V corresponded to the irreversible removal of Li₂O from the Li₂MnO₃ phase [9–11]. Compared with the LLO, the LLO-KCl exhibited an increase in discharge capacity from 183.83 to 200.11 mAh/g. This improvement was attributed to the enhanced ionic conductivity of the cathode material resulting from the K doping of the Li layer. Several studies have reported that such enhanced conductivity facilitates Li^⁺ deintercalation [23–25].

Fig. 6 shows the cycling performances of LLO and LLO-KCl. Tests were performed at a rate of 0.5 C. After 100 cycles, the discharge capacity of the KCl-doped cathode increased from 167.91 to 174.24 mAh/g, compared with the undoped LLO material. The capacity retention also improved, rising from 85.55% to 95.39%, representing an increase of 9.84%. These improvements can be attributed to the anionic Cl doping. Chlorine, which has an electronegativity of 3.16, is less electronegative than oxygen (3.44), and exhibits strong polarizing effects. As a result, Cl⁻ donates electron density more readily compared with O²⁻. During Li^⁺ intercalation and deintercalation, this characteristic reduces electron repulsion within the O–M–O framework, thereby enhancing the structural stability of the lattice [27,28].

Fig. 7 shows the C-rate performance of LLO and LLO-KCl. The cells were cycled at various rates from 0.1 C up to 5 C. They were then returned to 0.1 C to evaluate the recovery of the cathode materials rate capability. A gradual increase in capacity was observed during the first 1–5 cycles, which is attributed to the activation of the Li₂MnO₃ phase. The activation of the Li₂MnO₃ phase has been reported to be accelerated by doping with hetero-elements [13–14]. The LLO cathode delivered discharge capacities of 198, 184, 170, 159, 133, and 105 mAh/g at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C, and 5 C, respectively. In contrast, the LLO-KCl cathode exhibited capacities of 212, 205, 191, 175, 153, and 124 mAh/g at the same rates. Overall, the electrochemical results indicate that K doping in the Li layer induce an expansion of the c-axis in the Li slab [23–25]. This structural modification enhances lattice stability and, consequently, delivers superior electrochemical performance.

Fig. 8 and Table 2 present the electrode resistance characteristics of the LLO and LLO-KCl, as determined by the impedance analysis. The equivalent circuit used for the fitting is shown in Fig. 9 [22]. R_s represents the SEI resistance of the electrode. R_ct denotes the charge-transfer resistance at the electrode-electrolyte interface. The R_s values of the cathode materials exhibited minimal changes. In contrast, R_ct decreased markedly in the LLO-KCl sample compared to the LLO sample. This behavior is attributed to the continuous structural degradation during cycling [15,16]. After 100 cycles, R_ct was 55.25 Ω for the pristine LLO and 29.83 Ω for the LLO-KCl sample. This decrease confirms that the KCl-doped material maintained a more stable electrode-electrolyte interface than the undoped LLO. The D_Li values were calculated based on the impedance data using Equation 1 [26]. A:er 100 cycles, D_Li for the pristine LLO was 8.47×10^⁻14cm²/s. For the LLO-KCl sample, D_Li increased to 2.31×10^–13 cm²/s, representing an approximately threefold enhancement. These results are consistent with the XRD analysis, suggesting that the c-axis expansion and improved layered crystallinity induced by the addition of KCl facilitated Li^⁺ mobility.

(1)

DLi+=RT22×A×n2×F2×CLi×σ2

CONCLUSION

In this study, KCl was doped into Li_1.2Ni_0.2Mn_0.6O₂ cathode materials to investigate the effects of K and Cl on their electrochemical properties. XRD analysis revealed that the c-axis of the layered structure expanded from 14.2390 to 14.2422 Å following KCl doping. Moreover, the R-factor, which indicates the crystallinity of the hexagonal layered phase, improved from 0.4021 to 0.2301, corresponding to an approximately 1.75-fold enhancement. SEM and EDS mapping were employed to analyze the morphologies and elemental compositions of the cathode materials. Both materials exhibited particle diameters of approximately 8 μm. The average primary particle size increased from 271.15 to 409.62 nm, representing an approximately 1.5-fold increase. Electrochemical analyses revealed that the K- and Cl-substituted materials exhibited a higher initial discharge capacity of 200.11 mAh/g. In addition, the cycling stability was enhanced, with a capacity retention of 95.39% after 100 cycles at 0.5 C. Furthermore, it showed significantly improved C-rate performance, delivering 123.89 mAh/g at 5C. Moreover, EIS analysis demonstrated that the LLO-KCl sample suppressed the transition from the layered to spinel structure. Following 100 cycles, it exhibited a lower R_ct of 29.83 Ω compared with 55.25 Ω for the pristine LLO. The D_Li value in the LLO-KCl was 2.31×10^–13 cm²/s, which was approximately three times higher than the 8.47×10^–14 cm²/s observed for the LLO. This superior electrochemical performance can be attributed to the combined effects of K and Cl doping. K doping expands the c-axis of the Li slab, enhancing the Li^⁺ ionic conductivity. Cl doping replaces O sites, which reduces electron repulsion between O–M–O bonds during Li^⁺ insertion and extraction, thereby improving the structural stability.

Notes

ACKNOWLEDGEMENTS

This research was supported by the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (RS-2022-KI002562, HRD Program for Industrial Innovation), and by the Ministry of Trade, Industry and Energy (MOTIE) and the Korea Institute for Advancement of Technology (KIAT) through the “Support for Middle Market Enterprises and Regional Innovation Alliances (R&D, RS-2025-02633071)” program.

Fig. 1.

Process schematic diagram of material synthesis and electrochemical characterization.

Fig. 2.

XRD patterns of LLO and LLO-KCl.

Fig. 3.

FE-SEM images of LLO(a) and LLO-KCl(c); EDS mappings of LLO(b) and LLO-KCl(d).

Fig. 4.

dQ/dV plots corresponding to 1^st, 25^th, 50^th, 100^th charge-discharge profiles at 0.5 C for LLO(a) and LLO-KCl(b).

Fig. 5.

Initial charge & discharge curves for LLO and LLO-KCl.

Fig. 6.

Cycle performance for LLO and LLO-KCl.

Fig. 7.

C-rate performance for LLO and LLO-KCl.

Fig. 8.

Nyquist plots (a) and Z’ versus ω^-0.5 plots (b) after 100 cycle for LLO and LLO-KCl.

Fig. 9.

Equivalent circuit models used to fit the electrochemical impedance spectroscopy (EIS) data of the cell.

Table 1.

Lattice parameters for LLO and LLO-KCl.

Sample	a (Å)	C (Å)	Cell Volume (V3)	(003)/(104)	R-factor
LLO	2.8556	14.2390	100.6204	1.8852	0.4021
LLO-KCl	2.8541	14.2422	100.4573	1.6227	0.2301

Table 2.

EIS and Li⁺ diffusivity of LLO and LLO-KCl cathode after 100 cycles.

Sample	LLO		LLO-KCl
Temperature		25°C
SEI Resistance	5.95 Ω		6.32 Ω
Charge transfer Resistance	55.25 Ω		29.83 Ω
Li^⁺ diffusivity (cm²/s)	8.47×10^–14		2.31×10^–13

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