Synthesis of high-voltage (4.7 V) LiCoO₂ cathode materials with Al doping and conformal Al₂O₃ coating by atomic layer deposition

Abstract

The electrochemical properties of high-voltage (4.7 V) LiCoO₂ cathode materials with Al doping and a conformal Al₂O₃ coating by atomic layer deposition were studied in this paper. It was found that at ≤4.5 V, the Al₂O₃ ALD coating is sufficient to sustain the stability of LiCoO₂, with capacities of 190 mA h g⁻¹ at 0.1C and 175 mA h g⁻¹ at 1C. However, at 4.7 V, Al doping is more effective at maintaining the cycling life than the Al₂O₃ coating. LiCoO₂ with an Al₂O₃ ALD coating loses 34% of its initial capacity, but LiCoO₂ with Al doping preserves 81% of its initial capacity after 110 cycles, indicating a more pronounced effect on retaining the structural stability of LiCoO₂.

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Introduction

Currently, consumer electronics is the largest market for lithium ion batteries (LIBs), especially with the rising demands from smart phones and tablets. LiCoO₂ (LCO) is the most widely used cathode material for this application due to its easy synthesis, high operating voltage and high theoretical specific capacity.^1,2 Although LiNi_xMn_yCo_zO₂ (NMC, x + y + z = 1) has advantages of lower cost and better thermal stability, it has only taken over a small proportion of the market. One of the reasons for this is that LCO possesses an unprecedented volumetric energy density (W h L⁻¹) due to its much higher compacted density (4.1–4.3 g cm⁻³) compared to NMC (3.6–3.8 g cm⁻³). In addition, NMC-based pouch cells often suffer from a severe gassing problem at elevated temperatures, and it is difficult for them to pass a thermal safety test when fully charged and stored at 85 °C for 4 hours. For some low-end applications, a mixture of NMC and LCO has been adapted to accommodate the factors of cost, performance and compacted density. However, LCO will still dominate the high-end consumer electronics market until more progress can be made on NMC.

However, LCO suffers a relatively large fade in capacity after extended cycling.^3,4 This is due to anisotropic expansion and contraction during cycling which causes structural degradation of the material.⁵ The structural stability of Li_1−xCoO₂ can be maintained only for a limited compositional range of 0 < x < 0.5, while its stability rapidly deteriorates for x > 0.5, i.e., excessive removal of Li ions from LiCoO₂.³ This phenomenon is attributed to the structural transition from a hexagonal to monoclinic phase at x > 0.5 (4.1 to 4.2 V vs. lithium metal).^6,7 Recently, significant progress has been made with high-voltage LCO, which allows more than 0.5 Li⁺ to be extracted. With coating^8–11 and doping,^12–15 LCO can work at higher cut-off voltages (∼4.3 V vs. Li/Li⁺), offer a higher capacity (>150 mA h g⁻¹), and still maintain a reasonable cycling performance (>500 cycles with >80% capacity retention). However, the development of high energy density batteries is significantly slower than that of electronic hardware. In order to accommodate the high demand from battery manufacturers all over the world to look for thinner and lighter batteries but with a longer operation time, the battery industry needs better cathodes and anodes with higher capacity and operating voltages. With respect to cathodes, one solution is to extract more lithium from LCO at a higher voltage (≥4.5 V) to achieve a discharge capacity higher than 190 mA h g⁻¹ at 0.1C.

Recently, atomic layer deposition (ALD), as an advanced coating method for a variety of Li-ion battery electrodes, has been demonstrated.^16–19 ALD utilizes sequential and self-limiting surface reactions that enable tailored conformal coatings with Å-level thickness control.²⁰ However, the mechanism of battery performance improvement with ALD is not fully understood yet. There is an argument whether Al₂O₃ stays as an artificial SEI on the surface of LCO or forms a LiAl_xCo_1−xO₂ alloy during cycling. In this work, we studied the effects of Al₂O₃ ALD as a coating material and also as a doping source on the capacity and cycling retention of LCO at 4.3 V, 4.5 V and 4.7 V, respectively. At 4.3 V, uncoated LCO shows the best rate performance. Two cycle Al₂O₃ ALD coated samples show a very stable cycling performance and the highest capacity at 4.5 V. Heat-treatment doped LCO shows a stable but lower capacity. At 4.7 V, heat-treatment Al₂O₃ coated LCO, to achieve Al doping, exhibits the best cycling stability compared with that of Al₂O₃ ALD coated LCO, and a discharge capacity of ∼200 mA h g⁻¹ at 0.1C (14 mA g⁻¹).

Experimental

Al₂O₃ ALD coated LCO

LCO was used as received from Sigma-Aldrich (99.8% trace metals basis). Al₂O₃ ALD films were grown on LCO powders using a rotary ALD reactor.^21,22 The rotary reactor agitates the powders during ALD and prevents particle aggregation.

We employed a simple, well-known ALD process utilizing trimethylaluminium (TMA) and H₂O as precursors:²³

where the stars represent the substrate surfaces. The performance of both of these steps constitutes one ALD cycle. For the Al₂O₃ ALD, TMA (97%) and HPLC (high performance liquid chromatography) grade H₂O was obtained from Sigma-Aldrich.

The Al₂O₃ ALD reaction sequence was: (i) dose TMA to 1.0 Torr; (ii) evacuate reaction products and excess TMA; (iii) dose N₂ to 20.0 Torr; (iv) evacuate N₂; (v) dose H₂O to 1.0 Torr, (vi) evacuate reaction products and excess H₂O; (vii) dose N₂; and (viii) evacuate N₂ and any entrained gases. This sequence constitutes one AB cycle of Al₂O₃ ALD. The Al₂O₃ ALD was conducted at 180 °C. LCO with 2 and 25 cycles of Al₂O₃ were labeled as LCO-2ALD and LCO-25ALD, respectively.

Al doped LCO

Al doped LCO was obtained by post-annealing the LCO-25ALD sample in air at 700 °C for 4, 8 and 16 hours; the resulting materials were labeled as LCO-4A, LCO-8A and LCO-16A, respectively.

Structure characterization

X-ray diffraction (XRD) measurements were performed using a PANanalytical X-ray diffraction system. X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5000 Versa Probe system. Scanning electron microscopy (SEM) images were taken using a Carl Zeiss Ultra 1540 Dual Beam FIB/SEM System.

Electrochemical test

The electrodes were made by mixing the active materials with polyvinylidene fluoride (PVDF) and carbon black with a weight ratio of 80 : 10 : 10 in 1-methyl-2-pyrrolidinone (NMP) solvent. The slurry was coated onto aluminum foil and dried under vacuum at 80 °C. All of the cells were assembled in an argon-filled glove box with Li metal as the counter electrode. A Celgard separator 2340 and 1 M LiPF₆ electrolyte solution in 1 : 1 w/w ethylene carbonate : diethyl carbonate (Novolyte) were used. The galvanostatic charge/discharge characteristics were analyzed using an Arbin BT-2143 Battery Station. The electrode mass loading is about 2–3 mg cm⁻².

Results and discussion

In this paper, LCO coated with 2 and 25 cycles of Al₂O₃ are labelled as LCO-2ALD and LCO-25ALD, respectively. Al doped LCO samples were obtained by post-annealing the LCO-25ALD sample in air at 700 °C for 4, 8 and 16 hours, and are labelled as LCO-4A, LCO-8A and LCO-16A, respectively.

The surface chemical properties of uncoated LCO, LCO-2ALD, LCO-25ALD, and LCO-16A were investigated by XPS, as shown in Fig. 1. The single Al 2p peak at 75.8 eV for LCO-25ALD is in good agreement with that of Al₂O₃,²⁴ which indicates that Al₂O₃ remained as a coating on the surface of LCO after ALD. However, the Al 2p peak at 73.3 eV for LCO-16A is much lower, indicating that annealing at 700 °C promotes the formation of a LiAl_xCo_1−xO₂ alloy. This peak is close to that of LiAlO₂, but at a higher binding energy than LiAl_0.1Co_0.9O₂ (72.4 eV) reported by Appapillai et al.²⁵ Therefore, the surface of the LCO-16A may contain a layer of LiAl_xCo_1−xO₂ alloy (0.1 < x < 1).

Fig. 1 XPS spectrum of (a) Al 2p (b) O 1s (c) Co 2p.

The surface layer was further investigated by the O 1s spectra. The O 1s peaks of LCO-2ALD and LCO-16A both moved to a higher binding energy due to the existence of the Al₂O₃ coating.^25,26 Moreover, the higher binding energy of the O 1s line for LCO-16A can be attributed to the alloying of Al with LCO, which makes the cobalt–oxygen bonds more covalent and shifts the O 1s peak to a higher binding energy.^25,27,28 As shown in Fig. 1(c), both the uncoated LCO and LCO-2ALD display a single Co 2p_1/2 peak at 780.5 eV, further confirming that Al₂O₃ stays as a surface coating on LCO. In contrast, LCO-16A shows a shift in the Co 2p_1/2 peak by 2.0 eV toward a higher binding energy, which is probably due to the different bonding environment of Co³⁺ in pyramidal sites induced by the Co binding environments in LiAl_xCo_1−xO₂ with high levels of Al substitution approaching LiAlO₂ on the surface of the LCO-16A particles.²⁵

Fig. 2 exhibits XRD patterns of uncoated LCO, LCO-2ALD, LCO-25ALD, and LCO-16A, respectively. All of the samples show a single phase, which corresponds to the quasi-layered α-NaFeO₂ type structure (space group R_3̄m). This structure belongs to the rhombohedral system in which Li⁺, M³⁺ (M = Co, Ni, Al) and O²⁻ occupy 3a, 3b and 6c sites (Wyckoff notations), respectively.²⁹ A hexagonal α-NaFeO₂ type layered structure is observed for all of the samples, indicating that the ALD coating or post-annealing does not change the layer structure of LCO.

Fig. 2 X-ray diffraction patterns of uncoated LiCoO₂, LCO coated with 2 and 25 ALD Al₂O₃, and LCO after 16 hour annealing.

Rietveld refinement results are summarized using the X’Pert Highscore plus software program and are presented in Table 1. The c/a ratios are all greater than 4.9, indicating a well-developed layered structure.^29,30 The a-axis and c-axis increases slightly after the Al₂O₃ ALD coating on the surface of LCO. However, for LCO-16A, the c-axis increased dramatically compared to that of uncoated LCO, indicating that the Al dopant enters the structure of LCO forming an alloy of LiAl_xCo_1−xO₂.¹⁴ The lattice parameters obtained by Rietveld refinements show significant changes upon Al substitution, revealing that the major phase is a solid solution containing increasing amounts of the dopant. In this context, the c parameter is seen to increase slightly upon doping. This result can be understood on the basis of the two competing and opposite effects that this doping produces on the structure: (a) the polarizing effect of the Al³⁺ ion in the [MO₂] layers will tend to distort the structure and increase the interlayer distance along the c-axis; and (b) the reduction in the size of the cation will tend to shrink the structure.^14,31,32

Table 1 Refined lattice parameters of uncoated LCO, 2 and 25 ALD Al₂O₃ coated LCO and Al doped LCO after 16 hours annealing

Samples	2θ (003)	2θ (101)	a (Å)	c (Å)	c/a
LCO	19.05	37.52	2.793	13.98	5.005
LCO-2ALD	19.05	37.51	2.797	13.97	4.995
LCO-25ALD	19.03	37.49	2.795	13.99	5.005
LCO-16A	19.01	37.47	2.796	14.01	5.011

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Fig. 3 shows the surface and size of uncoated LCO, LCO-2ALD, LCO-25ALD, and LCO-16A, respectively. After 2 and 25 cycles of Al₂O₃ ALD, the surface and size of LCO particles do not change compared to the uncoated LCO. Ideally, ALD should give a conformal coating on the surface of LCO wherever an –OH group is readily available. The real scenario is probably that ALD starts growth at defects, edges and –OH groups due to nucleation issues, and then with more cycles, Al₂O₃ covers the rest of the surface. We believe those nanoparticles in Fig. 3(c) cover the defects on the surface of LCO, and the rest of surface should be conformally coated with a Al₂O₃ ALD film. Interestingly, after annealing at 700 °C for 16 hours, the size of LCO still remained the same. It is well-known that the average crystallite size of materials increases with annealing.^32–34 We believe that the Al₂O₃ coating helps prevent grain size growth due to its extremely high sintering temperature (>1200 °C).³⁵ Meanwhile, the conformal Al₂O₃ coating will form homogenous Al substitutions with LCO, helping improve its stability during cycling.^{13–15,30,36} When carefully controlling the annealing temperature and time, one can decide the doping level and coating thickness. In industry, metal dopants for LCO, such as Al, Mg and Ti, are usually achieved by mixing a cobalt precursor with metal oxide nanoparticles, followed by high-temperature annealing. The reaction temperature is required to be least 900 °C, where grain growth is unavoidable. Therefore, a second grinding stage is required to downsize sintered LCO particles. On the other hand, ALD forms a conformal coating on LCO, preventing individual LCO particles from sintering and growing.

Fig. 3 SEM images of (a) uncoated LiCoO₂, LCO coated with (b) 2 and (c) 25 cycles of ALD Al₂O₃, and (d) LCO after 16 hours annealing.

Uncoated LCO, LCO-25ALD and LCO-4A were firstly cycled under different current densities at 3.3–4.3 V, as shown in Fig. 4. The uncoated LCO showed the highest capacity and excellent rate capability. In this voltage window, LCO is stable without the need for coating or doping. LCO with 25 cycles Al₂O₃ ALD (LCO-25ALD) has almost no capacity due to the conformal and thick (∼5 nm) Al₂O₃ coating on surface. After annealing at 700 °C for 4 hours, LCO-4A shows a better capacity than LCO-25ALD, although still lower than uncoated LCO at all rates. The higher the rate is, the more significant the capacity difference is between the uncoated LCO and LCO-4A. This indicates that the Al₂O₃ coating layer is getting thinner after annealing compared to LCO-25ALD so that lithium can diffuse through the coating at low rates. However, at high rates, the post-annealed Al₂O₃ coating on LCO-4A is still too thick to allow lithium to diffuse freely.

Fig. 4 Discharge capacities of uncoated LiCoO₂, LCO coated with 25 cycles ALD Al₂O₃, and LCO after 4 hours annealing at different current densities between 4.3 and 3.3 V.

LCO-2ALD, LCO-4A and LCO-8A were further studied at 3.3–4.5 V with 3 formation cycles at 0.1C (14 mA g⁻¹), followed by subsequent cycling at 1C (140 mA g⁻¹), as shown in Fig. 5. LCO-2ALD shows the highest capacity of 190 mA h g⁻¹ at 0.1C, compared to LCO-8A with 170 mA h g⁻¹, and LCO-4A with 150 mA h g⁻¹. At 1C, LCO-2ALD still preserves a discharge capacity of 175 mA h g⁻¹, which is ∼92% of its capacity at 0.1C. However, LCO-4A and LCO-8A only present rate retentions of ∼73% and 82% of their capacity at 0.1C, respectively. This result indicates that at 4.5 V, the ALD coating is more effective than doping in achieving a balance between capacity, rate and cycling. In addition, a longer annealing time will drive more Al on the surface into the bulk of LCO to form a solid solution. Therefore, LCO-8A has higher capacity than LCO-4A at high rates due to there being a higher doping level and thinner coating.

Fig. 5 Lifetime of LCO coated with 2 cycles ALD Al₂O₃, LCO after 4 hours and 8 hours annealing at 0.1C and 1C between 4.5 and 3.3 V.

As Fig. 6 shows, when the cut-off voltage is increased to 4.7 V, LCO-2ALD shows the highest initial discharge capacity of ∼250 mA h g⁻¹ at 0.1C (14 mA g⁻¹), almost reaching its theoretical value of 274 mA h g⁻¹. However, this capacity is not sustainable. After the third cycle, its capacity has already dropped down to 225 mA h g⁻¹. LCO-8A and LCO-16A both have initial discharge capacities of ∼200 mA h g⁻¹ at the same rate of 0.1C. When the rate increases to 1C, both LCO-2ALD and LCO-8A show dramatic capacity losses with a capacity retention of ∼66% after 110 cycles. In contrast, LCO-16A still preserves ∼81% of its capacity after 110 cycles. To our best knowledge, our work gives the best capacity retention of LCO at such an extreme high voltage (>4.5 V) among all of the literature, data for which is given in Table 2. The reason that LCO-2ALD decays rapidly is probably due to its irreversible structural change at such a high voltage, even with a conformal Al₂O₃ coating by ALD. When Al₂O₃ coated LCO is annealed at 700 °C for only 8 hours, too much Al₂O₃ still remains on the surface. Without sufficient doping, lightly doped LCO cannot stand such a high voltage as well. Although LCO-8A has a lower capacity than LCO-2ALD, LCO-8A has a much slower capacity decay. LCO-16A has a higher doping level and a thinner Al₂O₃ layer than LCO-8A due to the longer annealing time. Therefore, although the initial capacities for LCO-16A and LCO-8A are almost identical at 0.1C, when the current is increased to 1C, LCO-8A has a discharge capacity of only 100 mA h g⁻¹, which is much lower than 150 mA h g⁻¹ for LCO-16A. We recognize that the annealing time and temperature may not be optimized yet, even for LCO-16A. In addition, we would like to point out that in this work, we did not use a high-voltage electrolyte. Conventional carbonate electrolytes suffer from decomposition above 4.5 V.^37,38 At 4.7 V, electrolyte decomposition becomes extreme severe, which could cause degraded cycling and low coulombic efficiency.

Fig. 6 Lifetime of LCO coated with 2 cycles ALD Al₂O₃, LCO after 8 hours and 16 hours annealing at 0.1C and 1C between 4.7 and 3.3 V.

Table 2 Performance of LCO at high voltages (1C = 140 mA g⁻¹)

	Voltage	Capacity	Cycling retention	Reference
AlPO4-coated LCO	4.6 V	210 mA h g^−1 at 0.1C	80.6% after 50 cycles	39
		180 mA h g^−1 at 1C
AlPO4-coated LCO	4.8 V	230 mA h g^−1 at 0.1C	78.9% after 50 cycles	39
		190 mA h g^−1 at 1C
Al2O3-coated LCO	4.8 V	220 mA h g^−1 at 0.1C	68.6% after 50 cycles	39
		175 mA h g^−1 at 1C
AlPO4-coated LCO	4.7 V	250 mA h g^−1 at 0.1C	83% after 30 cycles	25
		230 mA h g^−1 at 0.2C
Al2O3 ALD coated LCO	4.7 V	250 mA h g^−1 at 0.1C	74.9% after 50 cycles, 62.6% after 100 cycles	This work
		195 mA h g^−1 at 1C
LCO with Al doping and Al2O3 coating by ALD	4.7 V	200 mA h g^−1 at 0.1C	91.7% after 50 cycles, 83.4% after 100 cycles	This work
		150 mA h g^−1 at 1C

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Conclusions

In summary, we have examined the effects of doping and coating on LCO at 4.3 V, 4.5 V and 4.7 V. ALD coating is very effective to suppress LCO degradation up to 4.5 V and still maintains excellent rate capability. However, at 4.7 V, doping is more efficient than coating. ALD coating as a dopant resource allows a lower sintering temperature, prevents boundary growth and offers homogeneous doping. Optimization of dopant levels and different dopants in combination with ALD surface coating and high-voltage electrolytes will be further studied in the future.

Acknowledgements

The work was supported by the Natural Science Foundation of China (No. 21301199), Chongqing Municipal Education Commission (KJ130601) and the Natural Science Foundation of Chongqing Municipality (cstc2014jcyjA50035), and the Key Laboratory of Green Synthesis and Applications, College of Chemistry, Chongqing Normal University. This work at Wuhan ATMK Super EnerG Technologies Inc. is supported by the 3551 Recruitment Program of Global Experts by Wuhan East Lake Hi-Tech Development Zone, China.

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