Ming Xiea,
Tao Hub,
Liu Yangc and
Yun Zhou*c
aWuhan ATMK SuperG Technologies, Inc., Room E3186 Road, Wuhan, 430073, China
bKey Laboratory for Anisotropy and Texture of Materials of Ministry of Education, Northeastern University, Liaoning 110004, Shenyang, China
cCollege of Chemistry, Chongqing Normal University, Chongqing, 401311, China. E-mail: yunzhou@cqnu.edu.cn
First published on 13th June 2016
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.
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 Li1−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 coating8–11 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 LiAlxCo1−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).
We employed a simple, well-known ALD process utilizing trimethylaluminium (TMA) and H2O as precursors:23
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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.
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 LiAlxCo1−xO2 alloy. This peak is close to that of LiAlO2, but at a higher binding energy than LiAl0.1Co0.9O2 (72.4 eV) reported by Appapillai et al.25 Therefore, the surface of the LCO-16A may contain a layer of LiAlxCo1−xO2 alloy (0.1 < x < 1).
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 2p1/2 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 2p1/2 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 LiAlxCo1−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 Rm). 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.
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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 LiAlxCo1−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
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 |
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.
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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.
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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.
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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.
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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. |
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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