Structural Relaxation of Li_xNi_0.8Co_0.1Mn_0.1O₂ and Li_xNi_0.55Co_0.20Mn_0.25O₂ after Lithium Extraction to High Voltage Region (x ≤ 0.12) at 0.1 C-rate

Abstract

The structural relaxation analysis has been carried out on Li_xNi_0.8Co_0.1Mn_0.1O₂ (NCM-811) and Li_xNi_0.55Co_0.20Mn_0.25O₂ (NCM-552025) after the lithium extraction at 0.1 C-rate to the high-voltage region (x ≤ 0.12). For NCM-811, the H3 phase appeared as the major phase with a small amount of the H2 phase after the lithium extraction. At the relaxation period, no significant phase change was observed and the structural variation including Li- and Ni-interlayer distances was small. On the other hand, NCM-552025 exhibited the single phase of the H2 even at deep lithium extraction up to x = 0.06. In the H2 phase, a slight increase in c-length was observed during the relaxation, which is the result of an increase in the Li-interlayer distance with a decrease in the Ni-interlayer. The relaxation behavior of interlayer distances can be explained by the localization of a small number of mobile lithium-ions.

1. Introduction

LiNiO₂-based layered oxides are the major cathode materials of lithium-ion batteries for the application of electric vehicles (EVs) or portable devices due to the advantages of large theoretical capacity with a high working potential, in addition to the relatively low cost of fabrication.^1–3 LiNiO₂ undergoes a successive phase transition by charging through H1, M, and H2 phases, and finally transforms into the H3 phase, where H and M denote the hexagonal and monoclinic symmetries, respectively.^4–8 The formation of the H3 phase at deep lithium extraction is thought to lead to mechanical stress and resultant micro-cracks associated with a distinct change in the c-length. The partial substitution of Co and Mn for Ni forming LiNi_xCo_yMn_1−x−yO₂ (NCM) improves the structural and chemical stability as well as cathode performance.^9–14 In NCM systems, it has been explained that nickel contributes to a high capacity with increased energy density of batteries, while cobalt and manganese provide electrical conductivity and structural stability, respectively.^15,16 Although increasing the nickel content above x = 0.8 provides an improved specific capacity, the cathode performance rapidly degrades during charge/discharge cycles,^17,18 and reduces the thermal stability.^19,20 To prevent such drawbacks, a lot of investigations have been carried out, such as surface coating,^21,22 single-crystal particles,^23,24 and fabrication of core-shell or concentration gradient structures.^25,26 The capacity fading of the Ni-rich NCM has been ascribed to the phase transition accompanying micro-cracking,^6,27–29 the introduction of anti-site defects (cation mixing),³⁰ side reaction with the electrolyte at high voltage region³¹ or oxygen missing accompanied by an irreversible spinel (or rock-salt) phases,³² although the detailed mechanism of degradation is one of the focused issues in this field.^33–36

The trace of the structural variation after the termination of the charge/discharge process provides valuable information on structural stability, considering that the structure of the electrode materials changes from the kinetically favorable state for the charge/discharge process toward a thermodynamic equilibrium one. We have applied the structural relaxation analysis to various cathode or anode materials using X-ray diffraction coupled with the Rietveld analysis.^37–45 We have investigated LiNiO₂ (LNO) and Li(Ni,Co,Al)O₂ (NCA), and reported that an increased amount of cobalt and manganese substitution in NCA reduces the formation of the H3 phase and allows the H2/H3 rate close to the equilibrium.^40–43 In recent years, we have carried out the relaxation analysis on LiNi_0.8Co_0.1Mn_0.1O₂ (NCM-811) after the termination of lithium extraction with the current density of 0.01 C to compare the relaxation behavior of NCAs and LNO.⁴⁴ We reported that NCM-811 appears to follow the equilibrium mole fraction at charging rather than NCAs or LNO. In the present study, to clarify the contribution of cobalt and manganese substitution in the NCM system, we adopted the relaxation analysis technique on NCM-811 and Co- and Mn-rich NCM (LiNi_0.55Co_0.20Mn_0.25O₂; NCM-552025) after the delithiation to the high voltage region at 0.1 C-rate.

2. Experimental

The cathode materials of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM-811) and LiNi_0.55Co_0.20Mn_0.25O₂ (NCM-552025) were supplied by Sumitomo Metal Mining Co., Ltd. The mixture of NCM powder (NCM-811 or NCM-552025), acetylene black (AB), and polyvinylidene fluoride (PVDF) with a weight ratio of 8 : 1 : 1 was spread on an aluminum foil with the aid of NMP, which was then dried at 60 °C for 4 h and 120 °C for 24 h. A two-electrode metal-jacket cell (Hohsen Corp.) was assembled using lithium metal as a counter electrode and 1 mol dm⁻³ LiPF₆ in EC/DMC solution (2 : 1 v/v, Kishida Chemical Corp., Ltd.) as an electrolyte in an Ar-filled glove box. Lithium-ion was electrochemically extracted from the NCM-811 and NCM-552025 at a constant current of 0.1 C to various SOC (State of Charge), i.e. x = 0.12, 0.09, and 0.06 for Li_xNi_0.8Co_0.1Mn_0.1O₂ and Li_xNi_0.55Co_0.20Mn_0.25O₂. To avoid the local cell reaction between the electrode and current collector through the electrolyte,⁴⁶ the working electrode was separated from the cell after the termination of lithium extraction followed by washing in EC and DMC solvent. This treatment also restricts the lithium migration across the electrolyte to homogenize the lithium distribution.

The working electrode was set in a sealed holder (2391A201, Rigaku Corp., Ltd.) equipped with a beryllium window in an Ar-filled glove box. The sealed holder was then mounted on an XRD diffractometer (Ultima-IV, Rigaku Corp., Ltd.) using CuKα radiation (40 kV and 40 mA). The diffracted data were collected in the range from 15° to 75° in 2θ with a scanning speed of 2° min⁻¹ by 0.04° step. The data collection was performed from 0 h to 50 h after the termination of lithium extraction every 10 h step. The Rietveld refinement was carried out on each obtained XRD profile using the RIEVEC program⁴⁷ assuming the single-phase and two-phase co-existing (H2 and H3), both of which belong to $R\bar{3}m$ symmetry. At the structure refinement, nickel (cobalt and manganese) and oxide ions were placed at the 3b and 6c sites, respectively in a hexagonal axis as shown in Fig. 1 drawn by using VESTA,⁴⁸ ignoring the contribution of lithium ions.

Figure 1.

Schematic illustration of the structure of the hexagonal phase of NCM drawn using VESTA.⁴⁸

3. Results and Discussion

Figure 2 draws the charge curves of Li_xNi_0.8Co_0.1Mn_0.1O₂ (NCM-811) and Li_xNi_0.55Co_0.20Mn_0.25O₂ (NCM-552025) with the current density of 0.1 C. Whereas these two curves show essentially similar behaviors, NCM-811 possesses slightly higher charging potential than NCM-552025 and represents step-like characteristics. The corresponding capacities at x = 0.12, 0.09, and 0.06 are depicted in the enlarged plots, where the relaxation experiments have been carried out. Figures 3a–3c shows the X-ray diffraction patterns of NCM-811 collected at various relaxation times with the enlarged profiles around the 003 peak in the inset. Since the obtained c-parameter from the major peak is distinctly smaller than the H2 region, we supposed that the H3 phase mainly occurred with a small amount of H2 for all the lithium concentrations of x = 0.12–0.06. The peak profiles for 0 h of relaxation are quite broad and rather asymmetric, indicating that various conditions of the H3 phase coexist. However, they become sharper after 10 h of relaxation, even though the liquid electrolyte was removed at the relaxation. It is also seen that the 003 peak of the H3 phase shifts toward the lower diffraction angle with relaxation time as indicated by arrows. The Rietveld refinements have been carried out on these data to clarify the structural modification during the relaxation. A typical Rietveld-fitted pattern is represented in Fig. 4 for x = 0.12 of NCM-811 after 50 hours of relaxation. A good agreement was achieved between the measured and fitted profiles with adequately small R_wp assuming two-phase co-existence. Figure 5 plots the refined mole fraction of the H3 phase for x = 0.09 and 0.12, where the rest from the unity corresponds to that of the H2 phase. Since the diffraction peaks of the H2 phase are much weaker for x = 0.06, the mole fraction and the precise structure were not determined. Extracting the lithium-ions to x = 0.12 and 0.09, approximately 90 % of the sample has been transformed into the H3 phase, and this molar ratio is not largely altered during the relaxation. The initial (0 h) and finally relaxed (50 h) structural parameters including mole fractions are given in Table 1. Note that the oxide ion position of the H2 phase in NCM-811 is excluded in the table for much smaller intensities.

Figure 2.

Charge curve of Li_xNi_0.8Co_0.1Mn_0.1O₂ and Li_xNi_0.55Co_0.20Mn_0.25O₂ cathode material at a constant current density of 0.1 C. The enlarged profiles around the termination of charging are represented in the inset. The lithium content x in Li_x(NCM)O₂ is provided at each capacity of relaxation experiments for NCM-811 (●) and NCM-552025 (○).

Figure 3.

X-ray diffraction patterns of (a)–(c) Li_xNi_0.8Co_0.1Mn_0.1O₂ and (d)–(f) Li_xNi_0.55Co_0.20Mn_0.25O₂ obtained after various relaxation times. (a), (d) x = 0.12, (b), (e) x = 0.09 and (c), (f) x = 0.06. Diffraction patterns measured after the relaxation time up to 50 h are superimposed on each plot. Asterisk marks (*) indicate the diffraction peaks of the aluminum foil current collector. Diffraction profiles around 003 reflections are enlarged in the inset.

Figure 4.

Observed and the Rietveld refined diffraction patterns of Li_xNi_0.8Co_0.1Mn_0.1O₂ (x = 0.12) after 50 h of relaxation from the termination of lithium extraction. The vertical lines in the middle section show the positions of peaks calculated for Bragg's reflection. The trace (ΔY) in the bottom section represents the difference between observed and calculated patterns. The asterisk mark (*) indicates the diffraction peaks of the aluminum foil collector.

Figure 5.

Mole fractions of the H3 phase of Li_xNi_0.8Co_0.1Mn_0.1O₂ plotted versus relaxation time. Error bars in each plot are the estimated standard deviation of the Rietveld refinement.

Table 1. Refined structure parameters of LiNi_0.8Co_0.1Mn_0.1O₂ for initial state (0 h), and after 50 h of relaxation.

x in Lix(NCM)O2 Relaxation time/h	0.12		0.09		0.06
	0	50	0	50	0	50
Mole fraction of H3	0.88(2)	0.909(5)	0.90(3)	0.89(5)	—	—
H2 phase
a/Å	2.8108(4)	2.8195(1)	2.813(1)	2.805(1)	—	—
c/Å	14.399(1)	14.4065(5)	14.221(4)	14.2173(5)	—	—
H3 phase
a/Å	2.8143(1)	2.8218(8)	2.8112(1)	2.8149(1)	2.8111(0)	2.8160(2)
c/Å	13.6667(3)	13.7503(2)	13.6071(5)	13.6508(4)	13.5971(2)	13.6699(6)
z	0.2376(8)	0.2372(6)	0.2437(9)	0.2418(6)	0.2360(9)	0.2389(8)
Rwp	6.797	5.014	10.074	8.126	9.704	7.771

Figures 6a and 6b show the relaxation time dependence of the lattice parameters for NCM-811. All the obtained a-lengths for all lithium concentrations and even in both H2 and H3 phases fall into similar values. For the relaxation behavior, the a-length of the H3 phase seems to slightly increase with the relaxation time, while that of the H2 phase does not. On the other hand, the c-parameters of the H2 phase are larger than the H3, and only the former exhibits lithium concentration dependence, whereas the H3 phase does not. During the relaxation, the c-lengths of the H2 phase remain almost constant, although those of the H3 phase grow a little with the relaxation time.

Figure 6.

Change in lattice parameters of the a-axis (a), (c) and c-axis (b), (d) of Li_xNi_0.8Co_0.1Mn_0.1O₂ and Li_xNi_0.55Co_0.20Mn_0.25O₂ with relaxation time. Closed and open symbols correspond to the H2 and H3 phases, respectively. □: x = 0.12, ○: x = 0.09 and ◇: x = 0.06.

From the refined lattice parameters and oxide ion position (0 0 z), the Li- and Ni-interlayer distances as indicated by d_Li and d_Ni in Fig. 1 were evaluated. Figures 7a and 7b plot the relaxation time dependence of the d_Li and d_Ni for the H3 phase of NCM-811. Although the data are a little scattered, it appears that d_Li increases and d_Ni decreases with the relaxation time ignoring the initial data (0 h) for the highly delithiated sample (x = 0.06) as guided by a line. Such behavior was also observed in the previous study of NCM-811 extracted lithium ions with a 0.01 C-rate. Since the increased amount of d_Li is slightly larger than the decreased d_Ni, the c-length of the H3 phase increases with the relaxation time as Fig. 6b.

Figure 7.

(a) Li- and (b) Ni-interlayer distances of the H3 phase of Li_xNi_0.8Co_0.1Mn_0.1O, (c) Li- and (d) Ni-interlayer distances of the H2 phase of Li_xNi_0.55Co_0.20Mn_0.25O₂ plotted versus relaxation time. □: x = 0.12, ○: x = 0.09 and ◇: x = 0.06. Open and filled symbols are for the H2 and H3 phases, respectively. Dashed lines in (a) and (b) are guides obtained for x = 0.06 ignoring the initial data (0 h).

The diffraction patterns of NCM-552025 are represented in Figs. 3d–3f for x = 0.12–0.06. The 003 peak appears with a broadened profile just after the lithium extraction (0 h) and it shifts toward lower diffraction angles with the relaxation time for all lithium concentrations. Since the corresponding c-parameter is in the range of the H2 phase and no additional diffraction peak was detected, the structure analysis was conducted assuming a single phase of the H2 phase. Our previous study of Ni-rich NCAs indicated that H3 phase formation is restricted by increasing the cobalt content,⁴⁴ which is consistent with the present results that the H3 phase was observed only in the NCM-811. The relaxation time dependences of the lattice parameters are plotted in Figs. 6c and 6d for NCM-552025. The initial (0 h) and 50 h-relaxed data are listed in Table 2. The values of the lattice parameters a and c almost coincide among various lithium concentrations x, both of which grow larger with the relaxation time. From the oxide ion position, interlayer distances of d_Li and d_Ni are obtained as Figs. 7c and 7d. It can be seen that d_Li increases with the relaxation time while d_Ni decreases, and each of these interlayer distances does not show a significant difference among the various lithium concentrations. Although this relaxation behavior is observed in the case of the H3 phase for x = 0.06 of NCM-811, it is more apparent for the present H2 phase of NCM-55202. The increased amount of d_Li and the decreased d_Ni are approximately 0.1 Å and 0.04–0.07 Å, respectively during the relaxation time up to 50 h, which results in the total increase in c-length as shown in Fig. 6d.

Table 2. Refined structure parameters of LiNi_0.35Co_0.35Mn_0.30O₂ just after the termination of lithium extraction (0 h), and after 50 h of relaxation.

x in Lix(NCM)O2 Relaxation time/h	0.12		0.09		0.06
	0	50	0	50	0	50
H2 phase
a/Å	2.8212(1)	2.8235(1)	2.81081(9)	2.8165(5)	2.81606(9)	2.8279(5)
c/Å	14.1277(4)	14.1809(4)	14.3816(3)	14.412(2)	13.8544(2)	14.074(1)
z	0.2370(7)	0.2337(6)	0.2346(7)	0.2359(8)	0.237(1)	0.225(1)
Rwp	7.788	6.284	7.236	7.661	9.150	8.205

From the relaxation behaviors of the H2 phase of NCM-552025, the following relaxation model can be estimated. During the charging process, lithium ions are supposed to randomly migrate within the Li-interlayer attracting oxide ions homogeneously to reduce the interlayer spacing d_Li. Just after the termination of delithiation, lithium ions would be randomly distributed in the Li-interlayer. However, in the NCM system, all the lithium-ion sites are not completely equivalent, but the potentials depend on the neighboring transition metal ions, Ni, Co, and Mn. Then lithium-ions are thought to be localized at favorable positions during the relaxation period. Therefore, the d_Li expands with relaxation time for the repulsive force between the oxide ions, assuming that the lithium-ion concentration is much smaller and most of the lithium-ion sites are vacant in such a highly lithium-extracted region. Ex-situ NMR experiment by Märker et al. also suggested the local ordering around the lithium site,⁴⁹ which is consistent with the present model after the long-term relaxation time. However, they did not imply the in-situ conditions.

In terms of the Ni-interlayer, the lower valent nickel would occur around the localized lithium-ion sites while higher valent nickel does apart from such sites. Then, d_Ni is thought to shrink during the relaxation, considering that higher valent nickel is the major in this region (x = 0.06–0.12). Accordingly, d_Li increases and d_Ni decreases during the relaxation process as shown in Figs. 7c and 7d for NCM-552025. On the other hand, since the lithium concentration dependence of the structure of the H3 phase of NCM-811 is small, the relaxation behavior of the interlayer distances becomes unclear in comparison with the H2 phase of NCM-552025. Smaller d_Li is supposed to contribute to less variability of the interlayer distances.

In the present study, we compared the relaxation behavior of the NCMs with different cobalt and manganese content in the highly lithium-extracted region. In terms of the interlayer distances, characteristic structural relaxation behavior can be observed for the H2 phase of NCM-552025, although it is rather ambiguous for the H3 phase of NCM-811. The present result is thought to contribute to the application of Ni-rich NCM up to the high-voltage region.

4. Conclusion

We have carried out the relaxation analysis on NCM-811 and NCM-552025 to study the structural variation after the deep lithium-extracted region as 0.06 ≤ x ≤ 0.12 at 0.1 C-rate. NCM-811 exhibits the coexistence of the major H3 and the minor H2 phases after the delithiation for all the lithium concentrations, and the amount of phase change between the two phases is very small during the relaxation. The structure of the H3 phase of the major does not largely change during the relaxation. On the other hand, Co- and Mn-enriched NCM-552025 shows the H2 single-phase in the present lithium concentration range. The Li-interlayer distance increases while the nickel decreases with the relaxation time, which results in a slight increase in c-length. In the high-voltage region as in the present study, more amounts of cobalt and manganese content contribute to the restriction of the H3 phase, and the H2 phase easily changes the interlayer distances into the equilibrium showing clear relaxation.

CRediT Authorship Contribution Statement

Jian Kang: Data curation (Lead), Formal analysis (Lead), Investigation (Lead), Visualization (Equal), Writing – original draft (Lead)

Takeshi Yabutsuka: Resources (Lead), Supervision (Supporting)

Takeshi Yao: Conceptualization (Equal), Methodology (Lead)

Shigeomi Takai: Conceptualization (Equal), Supervision (Lead), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Footnotes

A part of this paper has been presented in the ECSJ fall meeting in 2021 (#1F09) and the 62nd battery symposium in Japan (#1A19).

T. Yao and S. Takai: ECSJ Active Members

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