The Ni_0.8Mn_0.2(OH)₂ precursor was synthesized via a co-precipitation method based on previous studies, using a 10 L reactor under controlled pH conditions [19–22]. A mixed aqueous solution of NiSO₄·6H₂O (98.5–102.0%, SAMCHUN) and MnSO₄·H₂O (98.0%, SAMCHUN) at a molar ratio of Ni:Mn = 8:2 was injected into the reactor. NaOH solution (30%, SAMCHUN) was used as the precipitating agent, whereas NH₄OH solution (25.0–30.0%, SAMCHUN) served as the chelating agent. Subsequently, they were fed into the reactor at a controlled flow rate. To prevent oxidation of the materials, a nitrogen atmosphere was maintained inside the reactor, and the temperature was maintained at 55°C using a thermostatic bath. The stirring speed was fixed at 1000 rpm, and the synthesis was conducted for 8 h within a pH range of 11–12. The resulting precursor was washed with distilled water to remove residual ions such as Na⁺, SO₄^2-, and NH₃⁺, followed by drying in a vacuum oven at 120°C for 24 h. The dried powder was then sieved to obtain the final Ni_0.8Mn_0.2(OH)₂ precursor.

LiNi_0.8Mn_0.2O₂ was synthesized based on previous studies using the Ni_0.8Mn_0.2(OH)₂ precursor and LiOH·H₂O (99.9%, Aldrich). The precursor and Li salt were mixed at a molar ratio of 1:1.07 to provide excess Li. The mixture was blended using a mortar mixer for 1h and subjected to thermal treatment in a muffle furnace. The heating rate was set to 3°C/min under an oxygen atmosphere. The first calcination was performed at 500°C for 5 h, which was followed by a second calcination at 750°C for 12 h. The material was naturally cooled to room temperature, resulting in the final LiNi_0.8Mn_0.2O₂ product, which is hereafter referred to as bare.

To synthesize LiNi_0.8Mn_0.2O₂ coated with Li₃BO₃, 2 wt% of H₃BO₃ (99.99%, Aldrich) relative to the cathode material was dissolved in 100 mL of ethanol to prepare the coating solution. The solution was dispersed using an ultrasonic cleaner (Power Sonic 505, HSt) for 1 h. LiNi_0.8Mn_0.2O₂ powder was then added to the solution, and the mixture was stirred at 80°C until the ethanol was fully evaporated. The solvent was further removed using vacuum filtration, and the sample was heat-treated at 120°C for 2 h to eliminate residual solvent. Subsequently, the powder was calcined at 500°C for 5 h under an oxygen atmosphere at a heating rate of 3°C/min to form the Li₃BO₃ layer [23,24]. The final product was ground using a mortar to obtain Li₃BO₃-coated LiNi_0.8Mn_0.2O₂, which is hereafter referred to as 2-LBO (Fig. 1). All samples were stored in a reagent chamber at 10°C. The residual Li content after calcination was measured in ppm using Warder’s method.

The particle size distribution of the synthesized precursor was analyzed using a particle size analyzer (Sinco 1090). The surface morphologies of the coated cathode materials were examined using scanning electron microscopy (SEM, JEOL JSM-7610F). The elemental distribution of the coated cathodes was investigated using energy-dispersive X-ray spectroscopy (EDS) mapping. The crystal structure and crystallinity of the cathode materials were analyzed using X-ray diffraction (XRD, D2 Phaser, Bruker) with Cu Kα radiation (λ = 1.5405 Å), scanned at 3°/min over a 10–80° 2θ range.

To analyze the electrochemical properties, electrodes were fabricated according to previous studies [19–22]. The cathode was composed of active material, a conductive agent (Super P, MMM Carbon Co), and a binder (PVDF, Aldrich) at a weight ratio of 8:1:1 and N-methyl-2-pyrrolidone was used as the solvent to form a slurry using a mechanical stirrer. The slurry was coated onto 15 μm aluminum foil to a thickness of 25 μm using a doctor blade and dried in a vacuum oven at 120°C for 24 h. The dried electrodes were then rolled using a roll-press.

For coin cell (CR2032) assembly, Li metal was used as the anode, polypropylene was used as the separator, and the electrolyte was composed of 1.0 M LiPF₆ in ethylene carbonate/diethyl carbonate (3:7 wt%). The coin cells were assembled in an argon-filled glove box under vacuum conditions.

The electrochemical performance of the fabricated coin cells was evaluated within a voltage range of 3.0–4.5 V at 25°C under 0.1 C using a galvanostatic charge-discharge system (PNE). The cycling performance was measured at 1 C within the same voltage and temperature ranges. The C-rate capability was evaluated at 0.1, 0.2, 0.5, 1, 2, 5, and 10 C. Electrochemical impedance spectroscopy (EIS) was performed using an Ivium Stat (HS Technology) over a frequency range of 0.01 Hz to 100 kHz with an AC amplitude of 5 mV. All measurements were carried out at room temperature. Prior to EIS testing, the cells were stabilized for 24 hours, and after cycling, an additional stabilization time of 12 hours was applied before impedance measurements to ensure reliable data acquisition.

Fig. 2. shows the residual Li contents in the bare and 2-LBO cathodes immediately following synthesis and after 14 days of exposure to air at 25°C and 33% relative humidity, as measured using Warder’s method. Compared with the bare sample, the 2-LBO sample exhibited a 69.7% and 71.0% reduction in LiOH and Li₂CO₃, respectively, suggesting that residual Li was effectively controlled through the reaction with H₃BO₃. Following exposure, both cathodes showed increased residual Li, but 2-LBO still showed an 80.6% and 75.0% reduction in LiOH and Li₂CO₃, respectively, compared with the bare sample. Furthermore, the increase in Li₂CO₃ content following exposure was 120.9% for bare and 90.8% for 2-LBO. These results indicate that the Li₃BO₃ layer, formed through reaction with residual lithium, effectively serves as a protective barrier against ambient moisture and CO₂, thereby inhibiting the formation of surface decomposition products such as Li₂O, LiOH and Li₂CO₃ [25,26]. This effect is expected to improve the cycle life and electrochemical stability.

Table 2 shows the particle size distribution and span value of the Ni_0.8Mn_0.2(OH)₂ precursor that was synthesized by co-precipitation. The D_n values represent the particle sizes corresponding to the cumulative percentages. A lower span indicates a narrower distribution. The mode indicates the most frequent particle size. A reaction time of 8 h yielded smaller particles, and a span value of 1.363 indicated some non-uniformity. To achieve a more uniform distribution, higher stirring speeds and reaction times of over 15 h are recommended [27–29].

Fig. 3. shows the microstructures observed by SEM. All samples showed particle sizes in the 2–3 μm range, consistent with the particle size analysis. The bare samples had rough and irregular primary particles, whereas the 2-LBO samples showed smooth and uniform particles, which is attributed to H₃BO₃ surface modification. Previous studies have reported that such a uniform morphology may suppress electrolyte corrosion [30,31]. EDS mapping confirmed uniform distribution of Ni, Mn, O and B elements. The presence of B on the surface indicated successful coating. Further analyses, such as ICP, are required to confirm the compositional distribution.

Fig. 4. and Table 3 present the XRD results and lattice constants. All samples showed a single phase with an α-NaFeO₂ (R-3m) structure and no impurities. The clear separation of the (006/102) and (108/110) peaks indicates well-ordered layered structures. The higher I(003)/I(104) ratio in 2-LBO indicates suppressed cation mixing owing to excess Li. Lower R-factor values confirm high crystallinity. A shift of the (003) peak near 18.8° to lower angles is observed in the 2-LBO sample, indicating a slight expansion along the c-axis.

According to recent studies (Zhang et al., 2021; Chen et al., 2024; Li et al., 2022), this peak shift may result from interfacial strain induced by the Li₃BO₃ coating or partial incorporation of B³⁺ ions into tetrahedral interstitial sites in the layered lattice. Such incorporation has been reported to enlarge the lattice parameters and reduce cation mixing, ultimately enhancing Li^⁺ ion transport. While our current results align with these observations, additional structural and spectroscopic analyses are necessary to confirm this mechanism conclusively [32–34].

Fig. 5 shows the initial charge-discharge curves at 0.1 C and 25°C. The discharge capacities were 192.46 mAh/g (bare) and 206.89 mAh/g (2-LBO). The increased capacity for 2-LBO is attributed to the high Li-ion conductivity of Li₃BO₃ and improved Li-ion diffusion from B³⁺ substitution [35,36].

Fig. 6 shows the charge-discharge profiles at 1 and 25°C for 100 cycles. After 100 cycles, the capacity retentions were 94.98% for the bare sample and 98.03% for the 2-LBO sample, indicating significantly improved cycling stability. This enhancement is attributed to the Li₃BO₃ coating, which forms a coherent and elastic interface layer that effectively suppresses microcrack propagation caused by mechanical stress during cycling. According to the study by Chen et al. (2024), such a coherent interface not only enhances Li⁺ diffusion but also alleviates particle breakage by reducing interfacial stress [33]. Furthermore, Zhang et al. (2021) reported that the Li₃BO₃ coating acts as a robust protective layer, isolating the cathode from HF and other reactive electrolyte species, thereby preventing surface degradation and structural transformation [34]. These mechanisms collectively contribute to the improved long-term electrochemical performance.

Fig. 7 shows the rate performance at 1 C. 2-LBO exhibited the highest discharge capacity of 208.43 mAh/g. While the performance of bare degraded at higher rates, 2-LBO maintained excellent performance. Table 4 shows the follow-up recovery at 0.1 C: 96.13% (bare) and 98.76% (2-LBO), confirming that the Li₃BO₃ layer reduces direct electrolyte contact and suppresses particle cracking [33–36].

Fig. 8 shows the dQ/dV plots after 1, 50, and 100 cycles at 1 C. The peak near 3.8 V corresponds to Ni²⁺ oxidation and phase transformation from hexagonal (H1) to monoclinic. The peaks from 4.1–4.3 V represent further phase transitions to hexagonal (H2) and (H3). The voltage difference (ΔV) at one cycle was 0.05 V for both samples. After 50 cycles, ΔV was 0.04 V (bare) and 0.17 V (2-LBO); after 100 cycles, the values were 0.15 and 0.21 V, respectively. While 2-LBO exhibited higher energy density, polarization increased owing to the electronic insulating nature of Li₃BO₃. A shift in the reduction peak to lower voltages for 2-LBO indicates surface reconstruction. The greater suppression of the H2–H3 transition by 2-LBO suggests that the coating layer effectively prevents rock salt phase formation.

Fig. 9 and Table 5 present the electrode resistance characteristics of the bare and 2-LBO electrodes, as determined by impedance analysis. The equivalent circuit used for the fitting is also shown in Fig. 9. In this circuit, Rs represents the SEI resistance of the electrode, while R_ct denotes the charge-transfer resistance at the electrode–electrolyte interface. All fitting parameters were obtained using Zview software with high reliability. EIS measurements in this study were performed using a two-electrode configuration, and therefore the reported R_ct and D_Li⁺ values may include contributions from the lithium counter/reference electrode. As highlighted in previous studies (Raccichini et al., 2019), (Cengiz et al., 2021) such a configuration inherently combines the impedance responses of both electrodes, which can lead to ambiguities in interpreting the charge-transfer resistance and Li⁺ diffusion behavior [37,38]. This limitation is acknowledged in this study, and future work will adopt a three-electrode configuration to separate the working electrode’s response more accurately and further refine the electrochemical analysis. As shown in Table 5, the R_ct value of the bare electrode decreased significantly from 423.6 Ω to 42.47 Ω after 50 cycles at 1 C, a reduction of 381.15 Ω. Similarly, the 2-LBO electrode exhibited a decrease in R_ct from 395.23 Ω to 35.33 Ω, a reduction of 359.90 Ω. These marked reductions demonstrate that the Li₃BO₃ coating effectively suppresses interfacial degradation and facilitates Li^⁺ migration during cycling. Furthermore, the lithium-ion diffusion coefficients (D_Li⁺) were calculated from the Warburg region according to Equation (1), which incorporates the absolute temperature (T), the gas constant (R), the number of electrons (n) involved in the redox reaction, Faraday’s constant (F), the Li^⁺ concentration in the active material, the electrode contact area (A), and the slope (δ) of the Z′ versus ω^–0.5 line in the low-frequency region, with the slope (δ) values obtained from Fig. 9(c,d) [39]. The calculated values show that after 50 cycles the bare electrode exhibited a D_Li^⁺ of 2.44 × 10^–15 cm²/s, whereas the 2-LBO electrode reached 3.23 × 10^–13 cm²/s, confirming enhanced Li⁺ transport and improved electrochemical performance, consistent with the enhanced discharge capacity and c-axis expansion observed in the XRD [40,41].