Revealing the Effect of Ti Doping on Significantly Enhancing Cyclic

May 24, 2019 - When LiNi0.8Co0.15Al0.05O2 cycles under a high cutoff voltage, the appropriate ... Lithium-ion batteries (LIBs) have been extensively a...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10661−10669

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Revealing the Effect of Ti Doping on Significantly Enhancing Cyclic Performance at a High Cutoff Voltage for Ni-Rich LiNi0.8Co0.15Al0.05O2 Cathode Da Liu,† Siyang Liu,† Congcong Zhang,‡ Longzhen You,† Tao Huang,‡ and Aishui Yu*,†,‡ †

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Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200438, China ‡ Laboratory of Advanced Materials, Fudan University, Shanghai 200438, China S Supporting Information *

ABSTRACT: As a kind of Ni-rich cathode material, LiNi0.8Co0.15Al0.05O2 undergoes severe phase transition when cycling under a high cutoff voltage, causing a sharp decline in reversible capacity. In this study, we synthesize LiNi0.8Co0.15Al0.05O2 with a range of Ti doping contents through a facile solid state approach. When the doping content is 1 mol %, it can still deliver a discharge capacity of 179.6 mAh g−1 after 200 cycles at 3.0−4.5 V, with a capacity retention of 97.4%, in comparison with 167.3 mAh g−1 and 89.2%, respectively, for pristine LiNi0.8Co0.15Al0.05O2. Various morphological and structural characterizations are performed to thoroughly comprehend the excellent cyclic performance of Ti-doped LiNi0.8Co0.15Al0.05O2. High resolution transmission electron microscopy images in combination with powder X-ray diffraction patterns illustrate that Ti dopant effectively suppresses the undesirable phase transition. Electrochemical impedance spectroscopy results confirm a relatively low increase of charge transfer impedance, and differential capacity versus voltage curves show a more moderate polarization during cycling. When LiNi0.8Co0.15Al0.05O2 cycles under a high cutoff voltage, the appropriate amount of Ti doping plays a role in stabilizing the structure and relieving the surface deterioration, which proves to be the key to the extremely superior cyclic performance even at 4.5 V. KEYWORDS: Lithium-ion battery, LiNi0.8Co0.15Al0.05O2, Doping, Cyclic performance, High cutoff voltage

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coating and doping are the most extensive and effective.15−18 In terms of coating, it is difficult to form a sufficiently uniform coating layer to act as a barrier against erosion from the electrolyte, and the complex coating process limits its largescale application. By comparison, doping strategy is easier to achieve industrialization and is expected to enhance the electrochemical performance through stabilizing the surface structure and facilitating the migration of electrons and ions. Considerable research has reported on various doping strategies and corresponding enhanced electrochemical properties, based on metal cations, anions, and even polyanions, such as Na+1,19 Mg2+,20 V4+,21 F−,22 and PO43−.23 It has also been shown that the improved electrochemical properties can be obtained for layered cathode materials after Ti doping.24−29 However, the enhanced performance is still not close to meeting commercial requirements, and the mechanism remains ambiguous how doped substances affect the structural evolution during repeated charge−discharge process. Hence, it

INTRODUCTION Lithium-ion batteries (LIBs) have been extensively applied to electric vehicles (EVs) and hybrid electric vehicles (HEVs) to cope with the environmental pollution and energy shortages worldwide.1−3 Since the cathode is the dominant restriction on the performance of an integrated battery system, much research has been carried out to develop cathode materials with high capacity, long cycle life, and low cost.4−6 Ni-rich layered oxide LiNi0.8Co0.15Al0.05O2 (NCA) has been industrialized on a large scale and applied to LIBs of EVs due to its outstanding advantage of high capacity.7,8 Unfortunately, NCA suffers from drastic capacity fading during the repeated charge−discharge cycles, resulting from an irreversible phase transition and deteriorating surface state.9−11 This deterioration can be more serious especially under a high cutoff voltage.12 Studies have shown that the degradation rises from the surface to the bulk of the material particles, and the surface structure goes through a transition from the layered phase to the disordered spinel phase and finally to the rock-salt phase.13,14 To stabilize the structure and inhibit the deterioration, multiple optimization strategies have been proposed, of which © 2019 American Chemical Society

Received: March 6, 2019 Revised: May 14, 2019 Published: May 24, 2019 10661

DOI: 10.1021/acssuschemeng.9b01312 ACS Sustainable Chem. Eng. 2019, 7, 10661−10669

Research Article

ACS Sustainable Chemistry & Engineering

performed on an electrochemical workstation (Biologic, VSP300) over a frequency range of 100 kHz to 10 mHz.

is essential to focus on the structural evolution and interfacial change during cycling by means of multiple characterizations. For this report, we prepared a series of LiNi0.8Co0.15Al0.05O2 with different contents of Ti doping through dry ball milling and solid state calcination methods. The results of charge− discharge tests indicated that the NCA with 1 mol % Ti doping showed a substantially elevated capacity retention after 200 cycles at 1 C and 25 °C. In view of this, comprehensive morphological and structural comparisons were developed between the pristine and modified NCA to elucidate the effects of Ti doping on stabilizing the surface structure and optimizing the electrode/electrolyte interface.

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RESULTS AND DISCUSSION Figure 1 shows SEM images of pristine NCA [Figure 1a] and NCA with Ti dopant [Figure 1b−d], which reveals that all

EXPERIMENTAL SECTION

Material Synthesis. Commercial precursor Ni0.8Co0.15Al0.05(OH)2 (Zhongwei Group Co., Ltd., China) was purchased to synthesize pristine LiNi0.8Co0.15Al0.05O2 through a solid state method. Ni0.8Co0.15Al0.05(OH)2 and LiOH·H2O with a molar ratio of 1:1.03 were mixed in a ball-milling machine at 150 rpm for 2.5 h to ensure thorough and homogeneous mixing. The mixture was preheated at 500 °C for 6 h in a muffle furnace. The acquired substance was mixed again and then annealed at 750 °C for 15 h under an atmosphere of flowing oxygen. The Ti doping materials were obtained by a similar procedure and the percentage of titanium doping is for the total amount of transition metal in the precursor. To be specific, for the synthesis of LiNi0.8Co0.15Al0.05O2 with 1 mol % Ti doping, Ni0.8Co0.15Al0.05(OH)2, LiOH·H2O and TiO2 (with particle size of 20 nm, J&K Scientific Ltd.) with a stoichiometric ratio of 1:1.03:0.01 were mixed and sintered by the procedure described above. We named pristine LiNi0.8Co0.15Al0.05O2 without Ti doping as NCA and the 0.5, 1, and 2 mol % Ti doping LiNi0.8Co0.15Al0.05O2 samples were named as 0.5%Ti-NCA, 1%Ti-NCA, and 2%Ti -NCA, respectively. Characterization. Powder X-ray diffraction (XRD, Bruker D8 Discover) with Cu Kα radiation was used to detect the crystalline structure of synthesized powder and electrodes after charge− discharge cycles. The microstructures, morphology, and distribution of elements were observed by high resolution transmission electron microscopy (HRTEM, JEOL 2010F) and scanning electron microscopy (FESEM, S-4800, Hitachi Co., Japan) coupled with an energy dispersive X-ray spectrometer (EDS). X-ray photoelectron spectroscopy (XPS) analysis was carried out on a RBD upgraded PHI5000C ESCA system (PerkinElmer Co., USA) with Al Kα radiation (hν = 1486.6 eV) to evaluate the relative content and valence state of the surface compositions, and all results were calibrated with the C 1s peak at 284.8 eV. To obtain the cycled NCA and 1%Ti-NCA cathodes for analysis, the cycled coin-type cells at a discharged state were disassembled, washed with dimethyl carbonate solvent and dried naturally in an argon-filled glovebox. Electrochemical Measurements. We evaluated the electrochemical performance of prepared samples using CR2016 coin-type cells. The active materials, Super P and polyvinylidene fluoride (PVDF) with a mass ratio of 8:1:1, were dissolved in N-methyl-2pyrrolidinone (NMP) and mixed thoroughly to obtain a wellproportioned slurry, which was spread on aluminum foil and dried at 80 °C for 12 h in a vacuum oven. Then, the thoroughly dried electrodes were cut into wafers with a diameter of 12 mm, and the loading of the active material was about 3.82 mg cm−2. Finally, the cathode film was assembled into coin-type half cells (CR2016) in an argon-filled glovebox, using 1 M LiPF6 in ethylene carbonate (EC)/ dimethyl carbonate (DMC)/ethylmethyl carbonate (EMC) (1:1:1 by volume) as the electrolyte, lithium metal as the counter electrode and microporous film (Celgard 2300) as the separator. Galvanostatic charge−discharge tests (1 C = 200 mA g−1) were carried out on a battery test system (Land CT2001A,Wuhan Jinnuo Electronic Co. Ltd., China) with a voltage window of 3.0−4.5 V (vs Li+/Li) at 25 °C. Electrochemical impedance spectroscopy (EIS) measurements were

Figure 1. SEM images of (a) NCA, (b) 0.5%Ti-NCA, (c) 1%Ti-NCA, and (d) 2%Ti-NCA. SEM images of (e) 1%Ti-NCA and corresponding EDS mapping results of (f) Ni, (g) Co, (h) Al, and (i) Ti.

samples are in the form of spherical morphology and these secondary spherical particles are formed by primary particles with a size of several hundred nanometers. The primary particles have a smaller size and more compact accumulation after Ti doping, which is thought to be beneficial to rate performance to some extent. However, there is no obvious difference in the surface state, which possibly suggests that Ti has been doped well into the crystal structure.29 To further prove this conjecture, HRTEM analysis was conducted, with results as displayed in Figure S1, which illustrates that both NCA and 1%Ti-NCA samples contain a continuous interference fringe spacing of (003) crystal plane, but nothing else. That is to say, there is no scaled lithium titanium oxide coating on the surface, and the variation of NCA after modification could be attributed to Ti doping. To further confirm the consequence of doping, EDS spectra was implemented to monitor the distribution of elements. As shown in Figure 1e−i, all elements, including titanium, are uniformly distributed. The powder XRD patterns and Rietveld refinement results of pristine NCA and NCA with Ti doping are displayed in Figure 2. The diffraction peaks of all samples can be indexed to a well-defined layered hexagonal structure of α-NaFeO2 with the R3̅m space group. There is no additional diffraction peak in the XRD pattern of Ti-doped NCA samples, even when the Ti doping content increases to 2 mol %, which indicates that no extra phase generates after Ti doping. This may be ascribed to the result that Ti has been calcined into the crystal structure rather than wrapped around the surface as a coating layer, which is consistent with SEM and TEM results.29,30 The obvious split of (006)/(012) and (108)/(110) peaks indicates that all samples are formed into a highly ordered layered structure.31 To further quantify the effect of Ti dopant on the crystal structure of NCA, Rietveld refinement was carried out, and the results are displayed in Figure 2b−e and Table 1. The calculated results coincide well with observed curves, and the 10662

DOI: 10.1021/acssuschemeng.9b01312 ACS Sustainable Chem. Eng. 2019, 7, 10661−10669

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. (a) XRD patterns for all samples. Rietveld refinement results of (b) NCA, (c) 0.5%Ti-NCA, (d) 1%Ti-NCA, and (e) 2%Ti-NCA.

strongly related to the degree of Li+/Ni2+ disordering: The higher the ratio, the lower the mixing.34 Interestingly, the degree of Li+/Ni2+ mixing is even higher than that of pristine NCA when the doping amount of Ti is too low or too high, as shown in Table 1. This may be explained by charge compensation theory and the fact that part of Ti4+ is doped into the Li layer and has an impact on Li+/Ni2+ disordering.32,35 Ti doping has two opposite effects on the degree of Li+/Ni2+ mixing. On one hand, part of the Ti4+ occupies the positions of Li+ and the migration of Ni2+ from the transition metal layer to the Li layer was suppressed, with lower cation mixing obtained. On the other hand, limited by electrical neutrality, as the amount of doping Ti increases, there will be more Ni2+ converted from Ni3+, leading to the more serious cation mixing.32 Combining these two issues, the doping content of Ti should be appropriate to balance two opposite effects to obtain the lowest degree of Li+/Ni2+ disordering. When NCA is modified with 1 mol % Ti, the larger lattice parameters can provide expanded channels for the extraction and insertion of Li+, and the lower cation mixing can maintain the stability of the crystal structure, jointly leading to better electrochemical performance. On account of this, we focus below on 1%Ti-NCA, with pristine NCA as a comparison.

Table 1. Lattice Parameters Obtained by Rietveld Refinement of All Samples properties

NCA

a (Å) c (Å) unit volume (Å3) I(003)/I(104) Ni2+ in Li+ layer (%) Rwp (%)

2.8645 14.1740 100.72 1.80 1.61 2.145

0.5%Ti-NCA 1%Ti-NCA 2%Ti-NCA 2.8639 14.1763 100.69 1.76 1.87 2.190

2.8646 14.1794 100.77 1.90 1.36 1.944

2.8681 14.2044 101.19 1.52 4.17 2.280

values of Rwp are low, which can attest to the reliability of the calculation. As shown in Table 1, the lattice parameters a and c and the unit volume increase with increased amount of Ti, which is considered as the result of charge compensation and ion radius variation.32 To be specific, as the content of doping Ti increases in the crystal, part of Ni3+ turns into Ni2+ with lower valence in order to keep the charge balance, and simultaneously, the ionic radii of Ti4+ (0.68 Å) and Ni2+ (0.69 Å) are both larger than those of Ni3+ (0.56 Å), Co3+ (0.55 Å), and Al3+ (0.54 Å), leading to the increase of lattice parameters and unit volume. The value of c-axis is associated with the migration of Li+ and the increase of c-axis implies an expanded path.33 It is widely accepted that the ratio of (003)/(104) is 10663

DOI: 10.1021/acssuschemeng.9b01312 ACS Sustainable Chem. Eng. 2019, 7, 10661−10669

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Figure 3. XPS spectra and fitted results of (a) Ni 2p, (b) Ti 2p, (c) O 1s, and (d) C 1s of the NCA and 1%Ti-NCA samples.

Figure 4. (a) Initial charge and discharge curves for all samples at 0.1 C; (b) cyclic performance between 3.0 and 4.5 V at 25 °C under 1 C for all samples; dQ/dV curves of (c) NCA and (d) 1%Ti-NCA under various cycles.

which indicates the coexistence of Ni3+ and Ni2+ for both samples.36 The fitted main Ni 2p3/2 peak of 1%Ti-NCA is at 855.2 eV, while that of NCA is at 855.5 eV. The shift to a lower binding energy demonstrates that more Ni2+ converted from Ni3+ exists on the surface as a result of charge compensation effect after high valence ions doping, which is

X-ray photoelectron spectroscopy (XPS) was performed to further investigate the surface compositions and valence states of elements, and the results fitted by XPSPEAK are displayed in Figure 3. As shown in Figure 3a, Ni 2p3/2 peak can be divided into two resolved peaks, whose locations are at 855.9 and 854.6 eV, corresponding to Ni3+ and Ni2+, respectively, 10664

DOI: 10.1021/acssuschemeng.9b01312 ACS Sustainable Chem. Eng. 2019, 7, 10661−10669

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Figure 5. Nyquist plots of (a) NCA and (b) 1%Ti-NCA after various cycles; (c) equivalent circuit used for both samples.

consistent with previous study.37 Further quantitative analysis also conforms the ratio of Ni2+/Ni3+ increases from 1.097 to 1.203 after Ti modification. Figure 3b shows the Ti 2p spectra of 1%Ti-NCA, and the main peak (2p3/2) is located at around 457.8 eV, which is the characteristic of Ti4+.38 In addition, there are no shifts of the Co 2p and Al 2p peaks after Ti doping (Figure S2), indicating that Ti doping does not affect their valence states. The O 1s peak at 529.3 eV corresponds to the lattice oxygen bound to metals, and peaks at 531.7 and 531.4 eV are related to active oxygen species (Li2CO3/ LiOH). The C 1s peaks at 289.8 and 284.8 eV belong to Li2CO3 and hydrocarbon, respectively. 39 Estimating the content of substances by the area under the curves, the O 1s spectra in Figure 3c and C 1s spectra in Figure 3d reveal a significant reduction of residual lithium on the surface after Ti doping. As a kind of Ni-rich cathodes, inevitable air exposure after synthesis make NCA react actively with H2O and CO2 in ambient air to form Li2CO3 and LiOH on the surface, which are the main component of residual lithium. Ti doping effectively inhibits this unfavorable reaction through a robust lattice structure facilitated by the strong Ti−O covalent bonds (with the bonding energy of 666.5 kJ mol−1). Therefore, the surface residual lithium of 1%Ti-NCA is significantly reduced compared with that of pristine NCA.39,40 Residual lithium is not only electrochemically inactive due to poor electronic conductivity and low Li+ conductivity but easily causes gas evolution during battery operation, which may cause serious safety incidents.36 Less residual lithium will effectively alleviate the side reaction between the cathode and electrolyte to protect the electrode from erosion, which obviously will improve the cyclic performance. To characterize the electrochemical properties of pristine NCA and NCA with Ti doping, charge−discharge tests were performed with a voltage window of 3.0−4.5 V at 25 °C, and profiles are presented in Figure 4. Initial charge and discharge curves at 0.1C (1C = 200 mA g−1) are shown in Figure 4a. Just as some doping elements with no electrochemical activity lead to a lower proportion of active material, the initial discharge capacity and corresponding Coulombic efficiency (CE) decrease with the increase of Ti dopant.35 To be specific, the initial discharge capacity of NCA without modification is 205 mAh g−1 and the CE is 88.7%. However, the discharge capacities drop to 203.3, 203, and 193.8 mAh g−1 in terms of

0.5%Ti-NCA, 1%Ti-NCA, and 2%Ti-NCA, respectively, with corresponding reduced CE of 86.9%, 86.8%, and 83.4%. Interestingly, 2%Ti-NCA sample shows very low initial capacity. This can be interpreted as the result of excessive Ti doping in the Li layer, which blocks the migration path of Li+. In addition, combined with the XRD results above, higher initial cationic disordering after the synthesis limits the insertion and extraction of Li+, leading to a low initial discharge capacity. It is worth noting that the initial charge plateau rises with the increase of Ti doping amount. The rise is ascribed to the larger electrochemical polarization caused by the different interfacial properties after in-active Ti doping.35 Figure 4b demonstrates the cyclic performance of all samples between 3.0 and 4.5 V at 25 °C under 1C. The capacity retention of 1%Ti-NCA after 200 cycles is 97.4%, much higher than that of pristine NCA, at 89.2%, although the initial capacity of 1%Ti-NCA is slightly lower. To further accelerate the difference, we evaluated the cyclic performances of pristine NCA and 1%Ti-NCA at an elevated temperature (55 °C), and the results are shown in Figure S3. Modified with Ti, NCA exhibits excellent capacity retention during the charging and discharging cycles at 55 °C, and the difference in cyclic performance between pristine and Ti-doped NCA is more significant. Given that the normal cutoff voltage for NCA is 4.2 V, we also tested cyclic performance between 3.0 and 4.2 V at 25 °C under 1 C and the results are shown in Figure S4. Consistent with the performance at the cutoff voltage of 4.5 V, the capacity retention of 1%Ti-NCA at 4.2 V is significantly higher than that of pristine NCA. For 1%Ti-NCA, a part of Ti is doped into the transition metal layer, which will stabilize the structure due to the stronger bonding energy of Ti−O (666.5 kJ mol−1), and the remaining Ti occupies crystal sites of Li to act as pillar ions, which will prevent layered structures from collapsing during Li+ insertion/extraction.41,42 In addition, less residual lithium on the surface of 1%Ti-NCA sample helps maintain the surface state and reduce byproducts. The unique advantages in crystal structure and surface state combine to lead to excellent cyclic performance. Moreover, the rate properties of all samples are shown in Figure S5. The higher the doping amount of titanium, the poorer the rate performance, especially at 4 C. This is consistent with the previous report which explains that the pillar ions (Ti4+ in the Li+ slab) block the Li+ transport, resulting in poor rate 10665

DOI: 10.1021/acssuschemeng.9b01312 ACS Sustainable Chem. Eng. 2019, 7, 10661−10669

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samples change very slightly during the long cycling. However, the variation trends of Rct are highly differentiated, i.e., Rct of NCA increases dramatically from 33.52 Ω after 50 cycles to 106.2 Ω after 200 cycles, while Rct of 1%Ti-NCA increases only from 19.13 Ω to 23.48 Ω. Combining the above two observations, the increase of Rct, rather than Rsl, assumes the main responsibility for capacity fading. It is quite clear that Rct of NCA grows sharply and consistently, while that of 1%TiNCA increases extremely slowly. In this aspect, Ti doping effectively facilitates charge transfer process and mitigates the increase of Rct especially during long cycling, so that 1%TiNCA exhibits excellent cyclic performance. To further clarify the improvement in interface stability and structural stability after Ti doping, the cycled coin-type cells were disassembled to obtain recycled cathode material for further characterization. Figure 6 displays the SEM, HRTEM images and the corresponding fast Fourier transform (FFT) images of the selected regions for electrodes of NCA and 1% Ti-NCA after 200 cycles at 1C and 25 °C. Figure 6a,c shows SEM images of NCA and 1%Ti-NCA, respectively, and significant differences between their surface states can be seen. A large number of apparent byproducts exist on the electrode surface of NCA, which are produced by the side reaction between the cathode and the organic electrolyte.39 Moreover, severe cracks can be observed for pristine NCA in Figure 6a, which is due to iterative volume expansion and contraction during repeated charge and discharge processes. The cracks could cause poor contact between primary particles of NCA, resulting in capacity fading of NCA.44 On the contrary, the morphology of the compact accumulation of primary particles remains for 1%Ti-NCA [Figure 6c], and the relatively clean surface indicates that the erosion of the electrode surface by the electrolyte is inhibited. After Ti doping, a stable crystal structure can inhibit volume change during cycling to avoid cracks, and less residual lithium on the electrode surface is good for reducing the side reaction between the electrode and the electrolyte. These observations are consistent with the XRD and XPS results above. HRTEM and the corresponding fast Fourier transformation (FFT) were performed to provide an intuitive proof of phase

performance; thus, the optimization of the amount of doped Ti is crucial.32,43 Differential capacity versus voltage (dQ/dV) curves of pristine NCA and 1%Ti-NCA are used to characterize structural evolution under cycling. As we can see in Figure 4c,d, the difference between them is pronounced under various cycles. For pristine NCA, the voltage of the anodic peak shows a sharp increase, while the cathodic peak shifts to a significantly lower voltage. By contrast, the voltage positions of anodic and cathodic peaks for 1%Ti-NCA have changed much less under cycling. This observation indicates that the polarization is alleviated significantly during the cycles after Ti doping, resulting primarily from the structural stability and the suppressed phase transition after Ti doping. (discussed later) Electrochemical impedance spectroscopy (EIS) measurements were applied to determine reaction kinetics and the improvement of electrochemical performance. The Nyquist plots of NCA and 1%Ti-NCA samples at a charge state of 4.0 V after various cycles are shown in Figure 5, which are both composed of two semicircles and an inclined line, corresponding to the solid−electrolyte interface film resistance (Rsl), charge transfer resistance at the electrolyte/electrode interface (Rct), and Warburg impedance, respectively. Fitting results are listed in Table 2. There is a huge divergence between the initial Table 2. Fitted Impedance Parameters for NCA and 1%TiNCA under Various Cycles NCA after 50 cycles after 100 cycles after 200 cycles

1%Ti

Rsl

Rct

Rsl

Rct

14.22 13.77 16.51

33.52 60.42 106.2

41.43 41.83 46.55

19.13 16.01 23.48

Rsl values of two samples, indicating the distinguishing interfacial composition and structure. As an element with no electrochemical activity, doping Ti changes the interfacial properties, leading to a solid−electrolyte interface film which is different from that of pristine NCA, further resulting in a higher Rsl of 1% Ti-NCA.35 Still, the values of Rsl for both

Figure 6. SEM images for cycled electrodes of (a) NCA and (c) 1%Ti-NCA; HRTEM images for cycled electrodes of (b) NCA and (d) 1%TiNCA, and the corresponding selected area FFT images (I, II). 10666

DOI: 10.1021/acssuschemeng.9b01312 ACS Sustainable Chem. Eng. 2019, 7, 10661−10669

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Figure 7. (a) XRD patterns for NCA and 1%Ti-NCA before and after cycles and (b) the corresponding partial enlargement with 2-theta range from 30° to 50°.

performance under a high cutoff voltage make it possible to apply optimized LiNi0.8Co0.15Al0.05O2 to commercial power LIBs with long cycle life.

transition near the surface after 200 cycles. For NCA, there are noticeable corrosion holes near the surface, caused by the attack from HF.11 Figure 6b-I shows that the inner region (10−15 nm deep) changes from the layered phase to the spinel phase after repeated cycles. Figure 6b-II demonstrates typical diffraction patterns of the rock salt phase detected in the surface region, indicating serious structural deterioration. Conversely, (003) planes corresponding to the layered structure can still be observed in the inner region (10−15 nm deep) of 1%Ti-NCA after long cycles, as shown in Figure 6d-I. Even in the surface region [Figure 6d-II], the crystal structure is seen as a mixture of layered and spinel phases rather than the rock salt phase. These results are in accord with previous studies, finding that the surface of Ni-rich materials will experience a phase transition from the layered structure to the spinel structure and finally to the rock-salt structure during cycling.45 Ti doping effectively inhibits the phase transition which is the main cause of irreversible capacity loss, and higher capacity retention and better cyclic performance are obtained as a result. The XRD analysis of the electrodes before and after the cycles also supports this conclusion and XRD results are plotted in Figure 7. An obvious peak located around 43.362° can be observed for pristine NCA sample after 200 cycles in Figure 7b, corresponding to an NiO-like rock salt phase (PDF#75-0197). This result means that the surface phase transition of NCA after long cycles has been extremely serious, which is bound to cause a sharp reduction in discharge capacity.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01312.

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TEM images of NCA and 1%Ti-NCA before cycles, XPS spectra and fitted results of Co 2p and Al 2p of the NCA and 1%Ti-NCA samples, cyclic performance between 3.0 and 4.5 V at 55 °C under 1 C for pristine NCA and 1%Ti-NCA, cyclic performance between 3.0 and 4.2 V at 25 °C under 1 C for pristine NCA and 1%Ti-NCA, and rate performance between 3.0 and 4.5 V at 25 °C for all samples (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-21-51630320. E-mail: [email protected]. ORCID

Aishui Yu: 0000-0002-8135-5123 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Science & Technology Commission of Shanghai Municipality (No. 08DZ2270500), China.

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CONCLUSIONS A simple but effective strategy was developed through Ti doping to optimize the structure and enhance the cyclic performance of LiNi0.8Co0.15Al0.05O2 under a high cutoff voltage. After doped with 1 mol % Ti, LiNi0.8Co0.15Al0.05O2 showed markedly superior cyclic performance, with a capacity retention of 97.4% after 200 cycles at 3.0−4.5 V, resulting from the particularly stable structure and excellent surface state. The crystal structure was confirmed by XRD and TEM analysis, and a lower degree of Li+/Ni2+ disordering and suppressed phase transition during cycling are detected. The results of XPS and SEM hint to less residual lithium on the surface and fewer byproducts from decomposition of the electrolyte after cycles, representing an improved surface state. Electrochemical tests prove that the polarization and the increase of charge transfer impedance are inhibited during cycling. After Ti doping, such a sturdy structure and prominent cyclic

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DOI: 10.1021/acssuschemeng.9b01312 ACS Sustainable Chem. Eng. 2019, 7, 10661−10669

Research Article

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DOI: 10.1021/acssuschemeng.9b01312 ACS Sustainable Chem. Eng. 2019, 7, 10661−10669