Polycrystalline nickel (Ni)-rich cathode materials face challenges including surface reconstruction, transition metal dissolution, and interparticle cracking during charge–discharge cycles, which degrade battery performance [1–3].

To address these issues, researchers have focused on single-crystal cathode materials by aligning grain boundaries into single particles to improve structural stability [4–6]. However, single crystallization lengthens lithium-ion (Li-ion) diffusion paths, which results in slow diffusion rates and poor rate performance than their polycrystalline counterparts [7,8]. Doping elements into polycrystalline cathodes is an extensively accepted strategy used to enhance both structural and electrochemical properties [9–15].

In this study, doping was employed to improve the electrochemical performance of Ni-rich single-crystal cathodes. Specifically, yttrium (Y) was introduced during the synthesis of single-crystal LiNi_0.78Mn_0.2Y_0.02O₂. Y has a higher binding energy with oxygen (−1816.65 kJ/mol) compared to Ni (−211.7 kJ/mol) and manganese (Mn) (−465.1 kJ/mol), which enable it to substitute transition metals in the transition metal (TM) layer [16]. This substitution improves structural stability, expands the lithium pathway, enhances the ionic conductivity, and ultimately boosts the battery's high-rate performance [17–18].

The synthesized precursor Ni_0.8Mn_0.2(OH)₂ was combined with LiOH·H₂O and dopant C₆H₉O₆Y for the solid-state synthesis of SC-NMY. XRD analysis (Fig. 1) showed that both SC-NM82 and SC-NMY possess layered structures corresponding to the R-3m space group (α-NaFeO₂-type). Y substitution in the TM layer increased the c-axis length and shifted the I(003) peak to a lower angle, thereby indicating lattice expansion [5,6].

SEM and EDS mapping (Fig. 2) revealed that SC-NMY and SC-NM82 formed single-crystal particles of 2–3 μm [16,22]. The grain boundaries of the particles appear to be well-aligned indicating a single-crystalline morphology of the particles [21–23]. Elemental mapping confirmed the uniform distributions of Ni, Mn, and Y.

Initial charge-discharge profiles at 0.1 C (Fig. 3, Table 2) showed that SC-NMY had a higher initial discharge capacity (181.9 mAhg^–1) and coulombic efficiency (89%) than SC-NM82 (157.3 mAhg^–1, 87%). In addition, the bare material exhibited a steeper voltage increase, whereas SC-NMY showed a more stabilized voltage rise.

This indicates that voltage stabilization occurred in SC-NMY, which is expected to be advantageous for high-rate charge/discharge operations.

The dQ/dV curves (Fig. 4) revealed structural transitions during cycling at the 1st, 30th, 50th cycles: H1 ⇄ M ⇄ H2 ⇄ H3. SC-NMY exhibits sharper redox peaks than SC-NM82, thereby suggesting a more uniform electrochemical reaction. In Fig. 4a, the oxidation during the first cycle of the bare material increased from 3.82 V to 3.89 V, with a broadly distributed oxidation peak. In contrast, SC-NMY showed an oxidation peak rising from 3.80 V to 3.89 V, but the peak was sharper compared to that of the bare material. This suggests that although the voltage increased due to Y doping, the electrochemical reaction occurred more intensively around a specific voltage. Compared to the bare material, the redox reactions in SC-NMY were more uniform and homogeneous [24].

Cycling performance shown in Fig. 5 was measured at 1C with a cut-off voltage range of 3.0 to 4.3 V up to the 100^th cycle. SC-NMY exhibited an initial discharge capacity of 163.15 mAhg^–1 at the 1st cycle, with a value approximately 20 mAhg^–1 higher than that of the bare material (143.5 mAhg^–1). After 100 cycles, the bare material delivered a capacity of 127.68 mAhg^–1 with a retention of 88.9%, while SC-NMY retained 146.23 mAhg^–1, corresponding to a retention of 89.6%. The minor difference in capacity retention was attributed to the structural stability of the single-crystalline cathode material, which is one of its key advantages. Based on this observation, a C-rate performance test was subsequently conducted.

C-rate performance (Fig. 6) applied current densities were set in the following order: 0.1 C (17mAhg^–1), 0.2 C (34mAhg^–1), 0.5 C (85 m Ah g^–1), 1 C (170 mAhg^–1), 2 C (340 mAhg^–1), 5 C (850 mAhg^–1) and finally back to 0.1C (17 mAhg^–1) [19]. The capacity retention results are shown in Table 3. The bare material showed an initial capacity of 169.59 mAhg^–1 at 0.1 C, and retained 120.68 mAhg^–1 at the high rate of 5C, corresponding to an efficiency of 71.5%. In contrast, SC-NMY exhibited a discharge capacity of 183.90 mAhg^–1 at 0.1 C, and retained 140.67 mAh g^–1 at 5C, achieving an efficiency of 76.5%, with a value roughly 5% higher than that of the bare material. When the current density was returned to the low rate of 0.1 C, the doped SC-NMY delivered a capacity of 179.40 mAhg^–1, showing a reversibility of 97.4%, which is approximately 11% higher than that of the bare material (146.53 mAhg^–1). This improvement is attributed to the enhanced Li-ion diffusion during charge/discharge, which is a result of the expanded Li-ion pathway caused by Y substitution in the transition metal (TM) layer, as also observed in the XRD results [21]. Moreover, the substitution of Y in the TM layer contributed to structural stabilization due to its high binding energy, which led to the observed high capacity reversibility [23].

EIS results (Fig. 7, Table 4) showed that SC-NMY had significantly lower charge transfer resistance before (7.3 Ω) and after cycling (19.2 Ω) than that of SC-NM82 (25.1 Ω and 72.4 Ω, respectively). Moreover, the Li-ion diffusion coefficient of SC-NMY (2.85×10^–13 cm²/s) was 1.5× higher than that of SC-NM82 (1.92×10^–13 cm²/s), which is attributed to the expanded Li pathways from Y substitution., consistent with the c-axis expansion in the XRD.

This study synthesized Y-doped single-crystal LiNi_0.78Mn_0.2Y_0.02O₂ to address low lithium-ion diffusivity in single-crystal cathodes. Y, with its high oxygen binding energy, was successfully substituted into the TM layer. XRD analysis confirmed c-axis expansion(14.225→14.231 Å), indicating the expansion of the Li-ion pathway, and EIS and rate performance data validated enhanced ionic conductivity and structural stability. SC-NMY demonstrated a 3-fold reduction in charge transfer resistance, with R_ct values decreasing from 25.1 Ω to 7.3 Ω before cycling and from 72.4 Ω to 19.2 Ω after 100 cycles, compared to the bare material. In addition, lithium-ion diffusivity improved from 1.92 × 10^–13 cm²/s to 2.85 × 10^–13 cm²/s, representing a 1.5× enhancement. During the C-rate test, this improvement in Li ion diffusivity contributed to superior high-rate performance, with SC-NMY showing 76.5% efficiency at 5 C, compared to 71% for the bare material. Furthermore, in the recovery test following high-rate cycling, SC-NMY delivered a discharge capacity of 179.4 mAhg^–1, corresponding to 97.4% reversibility, which is approximately 11% higher than that of the bare material. These results confirm that doping with Y, which has a high binding energy with oxygen, is a promising strategy to enhance both diffusivity and structural stability in single-crystal cathode materials.