Comparative Study of Ni-Rich Layered Cathodes for Rechargeable

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Comparative Study of Ni-Rich Layered Cathodes for Rechargeable Lithium Batteries: Li[Ni0.85Co0.11Al0.04]O2 and Li[Ni0.84Co0.06Mn0.09Al0.01]O2 with Two-Step Full Concentration Gradients Byung-Beom Lim,†,⊥ Seung-Taek Myung,‡,⊥ Chong S. Yoon,*,∥ and Yang-Kook Sun*,† †

Department of Energy Engineering and ∥Department of Materials Science and Engineering, Hanyang University, Seoul 04763, South Korea ‡ Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, South Korea S Supporting Information *

ABSTRACT: Compositionally graded Li[Ni0.84Co0.06Mn0.09Al0.01]O2 (TSFCGAl) with two-step concentration gradients was prepared as a high-capacity cathode for Li batteries. The concentration gradients were introduced within a single particle to maximize the Ni fraction; this increased the discharge capacity while ensuring cyclic stability with a Mn-enriched surface layer. The concentration gradients also produced a unique morphology, in which rodshaped primary particles were radially aligned in a spoke-like pattern. The fundamental electrochemical performance of TSFCG-Al is compared against that of commercial Li[Ni0.85Co0.11Al0.04]O2 (NCA). The TSFCG-Al cathode exhibits a higher discharge capacity and better cycling stability than the NCA cathode, even when they are charged to 4.5 V. Structural analysis of the cycled TSFCG-Al and NCA cathodes shows that the TSFCG-Al keeps its original structural integrity, while the NCA particles undergo serious particle degradation due to the accumulation of strain in the grain boundaries upon cycling.

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hexagonal 2 phases when large amounts of Li+ are extracted from the host structure.8,9 The finding of Abraham et al.10 from a LiNi0.8Co0.2O2 with 43% of the power faded was formation of a NiO-like or LixNi1−xO-like (rock-salt structure) byproduct on the surface of a LiNi0.8Co0.2O2 particle. NiO is known to have poor conductivities of lithium ions and electrons, and the resistance layer formed on the primary particles. This surface change would entail an increase in charge transfer resistance during cycling. This was confirmed by neutron and synchrotron XRD studies. Mori et al.11 correlated the power fade and cationic disordering of the NCA electrode. According to their XANES analysis, the valence of the Ni ion did not vary with degradation, and the structural change from the layer structure to disordered cubic structure was located near the surface of the NCA materials. Sasaki et al.12 and Muto et al.13 found that an inactive NiO-like phase was detected near the grain surface and boundaries in NCA by a spatial distribution map. Consecutive

ecent environmental concerns have led to an increased amount of interest in clean renewable energy technologies. Lithium-ion batteries (LIBs) have become crucial for harnessing new energy sources because they enable portability with high-energy density. Currently, LIBs are the dominant energy power sources for various applications ranging from portable electronic devices to electric vehicle applications.1−4 In particular, for plug-in hybrid and electric vehicles (P-HEVs and EVs), high capacity (i.e., high energy density) is the key requirement for the wide deployment of LIBs; hence, much effort has been spent to increase the capacity of cathodes. For instance, Ni-rich layered cathodes are known to deliver a large discharge capacity, reasonable rate capability, and low material cost due to their limited use of relatively expensive Co. Specifically, Ni-rich Li[Ni 1−x−y Co x Mn y ]O 2 (NCM) and Li[Ni 1−x−y Co x Al y ]O 2 (NCA), where Ni ≥ 80%, are typically able to deliver capacities as high as 200 mAh g−1 at a cut-off voltage of 4.3 V.5−7 However, these cathodes suffer from gradual capacity fading upon cycling due to their structural instability, which is ascribed to the successive phase transitions from hexagonal 1 to © 2016 American Chemical Society

Received: May 16, 2016 Accepted: June 19, 2016 Published: June 20, 2016 283

DOI: 10.1021/acsenergylett.6b00150 ACS Energy Lett. 2016, 1, 283−289

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Figure 1. SEM images of (a) Li[Ni0.85Co0.11Al0.04]O2 NCA and (b) Li[Ni0.84Co0.06Mn0.09Al0.01]O2 TSFCG-Al; Rietveld refinement results of the XRD patterns of (c) Li[Ni0.85Co0.11Al0.04]O2 NCA and (d) Li[Ni0.84Co0.06Mn0.09Al0.01]O2 TSFCG-Al; (e) EPMA line scan results from the particle center toward the surface and a summary of the data; and (f) scheme of Li[Ni0.84Co0.06Mn0.09Al0.01]O2 TSFCG-Al.

microscopic work by Zheng et al.14 demonstrated that the region with a disordered rock-salt structure, presumably a NiOlike or LixNi1−xO-like phase, is about 5 nm in width, while the transition region with a partially ordered structure is about 20 nm in width at the first cycle. The important thing is that a substantial volume of material with the transitional structure formed near the grain boundary. They further confirmed the same result using a conducting agent and binder-free NCA electrode.15 Conventional NCA and NCM materials have a spherical morphology with an average particle size of 10 μm; in particular, NCA sometimes shows large primary particles that are ∼1 μm.6,16,17 As a result, particle cracking or breakup of particles progressed gradually into the particle interior during cycling when 100% depth of discharge (DOD) was used.14,17 The byproducts produced on the surface of the cathode particle, as well as particle rupture, lead to electrochemical inactivity; thus, the observed capacity fading is unavoidable in both Ni-rich NCM and NCA cathodes. Another concern is oxidation of electrolyte at high voltage. Selection of proper electrolyte by addition of additives significantly enhances electrochemical performances such as less swelling, metal dissolution, and gas generation at the fully charged state for a graphite/Li[Ni0.8Co0.1Mn0.1]O2 full cell.18 Thin and less formation of LiF in the surface film of

Li[Ni1/3Co1/3Mn1/3]O2 achieved by addition of trimethylboroxine (TMB) could have excellent cycling performance when cycled in the voltage range of 3−4.5 V.19 Hence, it is likely that a significant breakthrough in the high-voltage electrolyte20,21 and ionic liquid22 widens the operating voltage window of the NCM cathodes. To resolve the aforementioned drawbacks, we recently proposed a new concept for Ni-rich NCM cathodes by confining stable Mn4+ in the outer surface region. Additionally, electroactive Ni2+ and Ni3+ species in the core region were responsible for activating the cathodes and delivering a high capacity, reaching approximately 200 mAh g−1 or higher, in the voltage range of 2.7−4.3 V.23 We also reported Ni-rich Li[Ni0.8Co0.06Mn0.14]O2 with a two-slope full concentration gradient (TSFCG), which was composed of radially aligned, nanorod-shaped primary particles. This structure delivers a discharge capacity of 210 mAh g−1 at a rate of 0.1 C with a capacity retention of 94% after 100 cycles.24 Because EVs require a capacity that is considerably higher than 210 mAh g−1, the possibility of increasing the rechargeable capacity by increasing the Ni content in the TSFCG cathode was explored. In addition to increasing the Ni content, Al was incorporated into the TSFCG cathode to form Li[Ni0.84Co0.06Mn0.09Al0.01]O2 (hereafter referred to as TSFCG-Al) via coprecipitation. This 284

DOI: 10.1021/acsenergylett.6b00150 ACS Energy Lett. 2016, 1, 283−289

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ACS Energy Letters was done because the presence of Al3+ in the transition metal layer tends to lower the cationic disorder.25,26 It is also assumed that the Gibbs energy for the formation of Al2O3 at 25 °C (−1582.3 kJ mol−127) is sufficiently low to stabilize the crystal structure (from a thermodynamic viewpoint), which helps stabilize the crystal structure. Scanning electron microscopy (SEM) images of the assynthesized Li[Ni0.85Co0.11Al0.04]O2 (hereafter referred to as NCA) and TSFCG-Al in Figure 1a,b show that both cathodes possess a spherical morphology, in which much smaller primary particles are tightly packed into a spherical shape. Although the differences in the size and morphology of the secondary particles were negligible, the primary particle size appeared to be smaller for the TSFCG-Al. It is notable that voids on the surface are less prominent in the TSFCG-Al particles as compared to those for the NCA particles. Both NCA and TSFCG-Al were crystallized into an O3-type structure with R3̅m space groups (Figure 1c,d). Despite the spatial variation of the composition, the XRD data for TSFCG-Al were refined based on the average composition in order to compare the cation mixing in the Li layer for both compounds (Figure 1c,d and Table 1). The refinement results indicated that the

TSFCG-Al as compared to those for NCA. The average oxidation states of Ni, Co, and Mn in Ni-rich NCM compounds are usually 3+ for each transition metal element in Li[Ni0.8Co0.1Mn0.1]O2.28 Also, Ni2+ (0.69 Å) and Mn4+ (0.53 Å) could be partially produced in the surface region, of which the chemical composition was expressed as Li[Ni0.800Co0.052Mn0.148Al0.010]O2. However, the larger lattice parameters are likely due to the prevalence of Mn3+ (0.645 Å) and Ni2+ (0.69 Å), though a small amount of Mn4+ could be formed near the TSFCG-Al surface, which has a substantially larger ionic radii in TSFCG-Al relative to the ionic radius of Al3+ (0.535 Å) in NCA, which does not have Mn in the compound. Measured tap densities were approximately 2.47 g cm−3 for NCA and 2.51 g cm−3 for TSFCG-Al. More physical properties of both NCA and TSFCG-Al are shown in Figures SI1 and SI2. TSFCG-Al was analyzed with EPMA to determine the local compositional change from the particle center to the surface. The concentrations of transition metals are plotted as a function of the distance from the particle center to the surface (Figure 1e). We carefully checked the concentration of transition metal elements using EPMA, and the data are highly reproducible. The most important fact is selection of the heattreatment temperature that does not show interdiffusion of transition metals. That is why low temperature is preferred to minimize such interdiffusion. For this reason, we used lithium hydroxide as the lithium source to impregnate lithium at low temperature, here 750 °C, instead of lithium carbonate, which requires high temperature to yield a single phase above 900 °C, to avoid the interdiffusion of transition metal elements. As designed, the concentration of Ni decreased gradually from 90.0 atom % (at the center) to 88.1 atom % (up to 3.3 μm away

Table 1. Rietveld Refinement Results of XRD Data for AsSynthesized Li[Ni0.85Co0.11Al0.04]O2 NCA and Li[Ni0.84Co0.06Mn0.09Al0.01]O2 TSFCG-Al NCA TSFCG-Al

Ni2+ in Li layer/%

a/Å

c/Å

Rwp/%

2.8 2.6

2.8688(1) 2.8743(1)

14.1709(2) 14.2124(2)

13.3 13.1

difference in the occupation of Ni2+ in the Li layer was negligible; however, the lattice parameters were greater for

Figure 2. (a) Initial charge−discharge curves at a current rate of 20 mA g−1 (0.1 C rate) and (b) differential capacity vs voltage curves at a current rate of 100 mA g−1 (0.5 C rate) for Li[Ni0.85Co0.11Al0.04]O2 NCA and Li[Ni0.84Co0.06Mn0.09Al0.01]O2 TSFCG-Al in the voltage range of 2.7−4.3 and 2.7−4.5 V. 285

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Figure 3. Discharge capacity vs cycle number of (a) Li[Ni0.85Co0.11Al0.04]O2 NCA and (b) Li[Ni0.84Co0.06Mn0.09Al0.01]O2 TSFCG-Al in the voltage range of 2.7−4.3 and 2.7−4.5 V at a current of 100 mA g−1 (0.5 C rate).

Figure 4. (a) SEM image and (b) TEM image of the Li[Ni0.85Co0.11Al0.04]O2 NCA electrode after 100 cycles with an upper cut-off voltage of 4.5 V. (c) SEM image, (d,e) TEM images, and (f) high-resolution TEM in the 100 zone of the Li[Ni0.84Co0.06Mn0.09Al0.01]O2 TSFCG-Al electrode after 100 cycles with an upper cut-off voltage of 4.5 V; the inset in (f) shows the Fourier-filtered image of the marked region.

from the center), which corresponds to the first gradient slope. Alternatively, the variations of the Co and Mn compositions were negligible. A constant level of Al was also detected throughout the particle. The concentration variation then

became steeper for Ni and Mn; this corresponds to the second gradient slope. Hence, TSFCG-Al is regarded as a continuous solid solution of Li[Ni 0.90 Co 0.05 Mn 0.04 Al 0.01 ]O 2 − Li[Ni0.79Co0.05Mn0.15Al0.01]O2, as graphically described in Figure 286

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microscopy (TEM), and Rietveld refinement in order to understand the reasons for the capacity fade in both cathodes. As shown in Figure 1a,b, both cathodes in the fresh state showed a spherical morphology composed of primary particles. The NCA cathode cycled to 4.3 V (100 cycles) underwent particle cracking, and the intimate contact between primary particles was lost (Figure SI1b); this is similar to other NCA cathodes that have been discussed in the literature.14,17,31 The degradation was much worse when NCA was cycled up to 4.5 V, demonstrating progress of particle rupture of the spherical morphology (Figures 4a,b and SI3). The broken parts shown in these figures are not likely to participate in the electrochemical reaction, resulting in an increase in the cell resistance (Figure SI4 and Table S1), which explains the poor capacity retention of the NCA cathode. The Rietveld refinement results also indicate that, although the crystal structure seemed to be maintained, the resulting cation mixing (i.e., occupation of Ni2+ in the Li layer) increased to 6.8% (Figure SI5 and Table S2), as compared to just 2.8% in the fresh state. Alternatively, TSFCGAl exhibited a smaller cation mixing change from 2.6 to 4.1% (Figure 1c and Table 1). In comparison of the c-axis parameters before and after cycling test, the c-axis was elongated from 14.1709 to14.2459 Å, approximately Δc = 0.075 Å for the cycled NCA to 4.5 V, whereas the Δc was calculated to be 0.0122 Å for the cycled TSFCG-Al to 4.5 V. Even in comparison with Al-free TSFCG, which showed c-axis expansion of 0.0158 Å when cycled to 4.3 V,29 the c-axis for the TSFCG-Al varied less, ∼0.0061 Å at the same operation condition. The markedly reduced c-axis variation after the cycling indicates that the crystal structure supported by the presence of a strong Al−O bond delays the crack propagation at the grain boundary, hence enabling substantial improvement in the cyclability of the TSFCG-Al, as shown in Figure 3. Surprisingly, for TSFCG-Al, the original spherical morphology was maintained, even after cycling at 4.3 and 4.5 V, as can be seen in SEM images of the TSFCG-Al cathode (Figures SI1b and 4c). A cross-sectional TEM image of a single TSFCGAl particle cycled at 4.5 V (Figure 4d) contained only a small number of cracks in the particle interior. The presence of the long rod-shaped primary particles demonstrates that the original radial spoke-like morphology of the TSFCG-Al cathode (Figure 4e) was maintained, which is a unique feature of the compositionally graded cathodes.32,33 A high-resolution image taken from the 100 zone showed no noticeable damage or localized structural transition; this was accomplished by minimizing microcrack formation within particles upon cycling, as shown in the Fourier-filtered image in the inset (Figure 4f). The SEM and TEM results clearly demonstrate that the TSFCG-Al cathode experiences minimal structural damage and that the structural integrity and overall particle morphology are preserved upon extensive cycling at 4.5 V. The capacity loss observed at 4.5 V for the TSFCG-Al cathode can be partially ascribed to the breakdown of the electrolyte at the high voltage;34,35 hence, it is surmised that if one used an improved high-voltage electrolyte that was specifically designed for Nirich NCM cathodes, the capacity retention for the TSFCG-Al cathode at 4.5 V would be greatly enhanced. It has been reported that capacity fading is related to surface degradation of the cathode particle surface.10−17,31 To verify this, electrochemical impedance spectroscopy (EIS) was used for both cathodes, as shown in Figure S4. The smaller increase in the impedance of TSFCG-Al is understandable; this impedance rise was lower than that of NCA because TSFCG-

1f, in which the concentrations of Ni and Mn are varied in two steps. The electrochemical properties of the NCA and the TSFCGAl cathodes were compared using 2032 coin-type half-cells in the voltage ranges of 2.7−4.3 and 2.7−4.5 V at 30 °C (Figure 2). The first charge and discharge data indicate that the TSFCG-Al cathode delivered a higher discharge capacity with improved Columbic efficiency: 221 mAh g−1 with 96.4% efficiency up to 4.3 V and 241 mAh g−1 with 95.0% efficiency up to 4.5 V (Figure 2a bottom). The NCA cathode delivered a discharge capacity of 214 mAh g−1 with 93.4% efficiency up to 4.3 V and 224 mAh g−1 with 92.3% efficiency up to 4.5 V, as shown in the top of Figure 2a. Large polarization between charge and discharge is observed for both cells when charged to 4.5 V compared to the cells charged to 4.3 V. The behavior is ascribed to an increase in impedance due to electrolyte decomposition when the cells are charged to 4.5 V at the first cycle. The electrochemical reactions of both the NCA and TSFCGAl cathodes are related to the successive phase transitions from hexagonal 1 to hexagonal 2 phases in a similar way,8,9 and no apparent variation was observed in the derivative curves of the TSFCG-Al upon cycling (when cycled to 4.3 and 4.5 V), as shown in the bottom of Figure 2b. Alternatively, the intensities of the peaks became polarized and shifted apart for the NCA cathode upon cycling; this was presumably due to the degradation of the cathode, which was found to be particularly serious when cycled at 4.5 V (the top of Figure 2b). In consideration of the chemical compositions for both NCA (Li[Ni0.85Co0.11Al0.04]O2) and TSFCG-Al (Li[Ni0.84Co0.06Mn0.09Al0.01]O2), more than 96% of Ni, Co, and Mn moieties are available for participation in the redox reaction. Accordingly, both cathodes should deliver similar discharge capacities. In particular, for the NCA cathode, the incorporated Al3+ element may significantly assist the stabilization of the crystal structure, which should impart good cycling performance. However, the cycling performance of the NCA cathode measured at a rate of 0.5 C (100 mA g−1) was very disappointing and showed a capacity retention of only 73.4% (146 mAh g−1) at 4.3 V and 65.3% (128 mAh g−1) at 4.5 V after 100 cycles (Figure 3a). Our prior work indicated that Ni-rich Li[Ni0.8Co0.06Mn0.14]O2 TSFCG material could deliver 210 mAh g−1 at a rate of 0.1 C with capacity retention of 94% after 100 cycles.24 Also, Li[Ni0.85Co0.05Mn0.10]O2 TSFCG maintained 92% of its initial capacity for 100 cycles at the same operation window.29 For the present TSFCG-Al cathode, the cycling performance was markedly improved (Figure 3b) compared to the NCA (Figure 3a) and our earlier TSFCG materials that contain no less than 80% Ni in the compounds. Specifically, the cathode was able to deliver a high capacity, even after 100 cycles; this value was 95.1% (197 mAh g−1) at 4.3 V and 88.9% (202 mAh g−1) at 4.5 V (Figure 3b). Even the Coulombic efficiency at the first cycle reached approximately 96.4% at the 4.3 V cut-off voltage; such a high efficiency has never been achieved as far as we know. These data substantialize cycling stability of TSFCG, especially when substituted with Al. The role of the Al substitution in improvement in capacity retention of the NCM cathodes has been previously documented for Li[Ni0.8Co0.1Mn0.1]O226 and Li[Ni1/3Co1/3Mn1/3]O2.30 The difference in the cycling performance was prominent when the upper cut-off voltage was raised to 4.5 V. Hence, the cycled electrodes were analyzed by SEM, transmission electron 287

DOI: 10.1021/acsenergylett.6b00150 ACS Energy Lett. 2016, 1, 283−289

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ACS Energy Letters Al has a lower specific surface area (0.336 m2 g−1) than NCA (0.491 m2 g−1). Once a cathode particle is exposed to the electrolyte, infiltration of the electrolyte proceeds via the capillary effect along the grain boundaries of the primary particles. The reduced contact area with the electrolyte leads to the smaller degree of degradation of the surface of primary particles. Furthermore, balancing the Ni and Mn contents toward the surface also helps improve the performance by reducing the amount of Ni from 90 to 80% and by increasing the amount of Mn from 5 to 16% (particle surface composition: Li[Ni0.79Co0.05Mn0.15Al0.01]O2). These synergistic effects prevented destructive particle degradation upon cycling, despite the fact that both cathodes contained a similar total average amount of Ni. The thermal stabilities of electrochemically delithiated, wet TSFCG-Al and NCA were evaluated by differential scanning calorimetry (DSC), as shown in Figure S4. For the TSFCG-Al cathode, the exothermic decomposition was initiated at 200 °C, and the amount of generated heat was maximized at 220 °C. Here, the reaction temperature and heat generation were somehow delayed, as compared with those of NCA. The thermal behavior difference appears to be related to the unique rod-shaped primary particle morphology of TSFCG-Al, which has a smaller surface area; less contact with the electrolyte can generate less heat, even in a highly delithiated state. In summary, we developed a compositionally graded Li[Ni0.84Co0.06Mn0.09Al0.01]O2 with a two-step concentration gradient of Ni and Mn throughout the particle. This cathode can deliver high reversible capacities of 221 (4.3 V cut-off) and 241 mAh g−1 (4.5 V cut-off) with outstanding capacity retentions of 95.1 and 88.9%, respectively, after 100 cycles. SEM and TEM analyses revealed that no significant structural changes were observed between the pristine and cycled TSFCG-Al cathodes due to the unique rod-shaped primary particle morphology. We believe that the present two-step full concentration gradient cathode represents a method to safely harness the high capacity of Ni-rich cathodes.

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ICT & Future Planning and a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2014R1A2A1A13050479).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00150. Experimental details, additional SEM images, XRD patterns, electrochemical impedance spectroscopy, and differential scanning calorimetry data for the synthesized NCA Li[Ni 0.85 Co 0.11 Al 0.04 ]O 2 and TSFCG-Al Li[Ni0.84Co0.06Mn0.09Al0.01]O2 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.S.Y.). *E-mail: [email protected] (Y.-K.S.). Author Contributions ⊥

B.-B.L. and S.-T.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was mainly supported by the Global Frontier R&D Program (2013M3A6B1078875) of the Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, 288

DOI: 10.1021/acsenergylett.6b00150 ACS Energy Lett. 2016, 1, 283−289

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DOI: 10.1021/acsenergylett.6b00150 ACS Energy Lett. 2016, 1, 283−289