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Improved cycling stability of LiNi0.6Co0.2Mn0.2O2 through microstructure consolidation by TiO2 coating for Li-ion batteries
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Improved cycling stability of LiNi0.6Co0.2Mn0.2O2 through microstructure consolidation by TiO2 coating for Li-ion batteries Yan Mo a, Lingjun Guo a, Hongfei Jin b, Baodong Du b, Bokai Cao b, Yigao Chen a, De Li b, Yong Chen b, * a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, PR China State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Laboratory of Research on Utilization of Si-Zr-Ti Resources, College of Materials and Chemical Engineering, Hainan University, 58 Renmin Road, Haikou, 570228, China
b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A thin TiO2 layer in-and outside of a LiNi0.6Co0.2Mn0.2O2 secondary particle is fabricated. � TiO2 modification mitigates the me chanical degradation and side reaction on cycling. � Excellent cyclability with 79% after 500 cycles is achieved in the full-cell testing.
A R T I C L E I N F O
A B S T R A C T
Keywords: Li-ion battery LiNi0.6Co0.2Mn0.2O2 cathode TiO2-Coating Surface degradation
The commercial deployment of Ni-rich layered oxide cathodes is hampered by both contaminating species on the surface and mechanical degradation associated to the microcracks. To resolve both issues, a series of TiO2modified LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode materials is constructed and studied. As expected, TiO2 could trap the residual lithium on the surface, minimizing the undesired side reaction. In addition, infiltrative layer with Ti gradient concentration from the outer to inner is also acquired, which strengthens the primary particles, reduces the gaps between randomly oriented grains and stabilizes the structure of NCM622 during charge discharge cycles. As a result, the NCM622 with TiO2 incorporation thus displays a high capacity of 168.7 mAh g 1, with 96% retention after 200 cycles, which is 18% higher than that of the bare NCM622. Structural and compositional characterization (i.e., in-situ Raman, EIS and depth XPS) further reveal the electrochemical mechanism and kinetics of the designed NCM622-TiO2 cathode.
1. Introduction With the rapid development of electric transportation fleets, the requirement for high-performance Li-ion batteries (LIBs) has promoted
great efforts to search for cathode materials with high energy density and long cycling life [1–3]. Layered transition metal oxides LiNi1-x- yCoxMnyO2 (NCM) are the most practical candidates for electric vehicles (EVs), on account of their high practical capacities (above 200 mAh g 1)
* Corresponding author. E-mail address: [email protected] (Y. Chen). https://doi.org/10.1016/j.jpowsour.2019.227439 Received 28 August 2019; Received in revised form 19 October 2019; Accepted 9 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yan Mo, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227439
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at high voltages (4.4–4.6 V) [4,5]. Among them, LiNi0.6Co0.2Mn0.2O2 (NCM622) have been widely investigated and used in commercial cells [6–11]. However, surface degradation that occurred at higher voltage and accelerated at elevated temperature seriously compromises the cycling lifetime. On one hand, the reactivity of charged NCM622 with the electrolyte which may lead to consumption of active material and gas generation [9,10]. Furthermore, microcracks initiated by the uneven pore network hinder the electrical conductivity, as well as cause the uncontrollable side reaction because of the newly formed surface [6,11, 12]. Recently, a few reports have proposed effective strategies to inhibit the surface degradation of NCM materials which comes from both parasitic reaction and intergranular cracks [13–17]. For instance, spinel-typed Li-reactive coating can extend the cycle life of LiNi0.91 Co0.06Mn0.03O2 from 84% to 92% [16]. This was attributed to the reduced amount of residual lithium and intrinsically isotropic property of the coating layer that alleviates the microcracks inside. In addition, integrating LixCoO2 into the precursor considerably increased the functionality of NCM622, which resulted 80% capacity retention after 150 cycles [11]. Similarly, improved cycling stability of LiNi0.8 Co0.15Al0.05O2 can also be achieved by infiltrative coating with LixCoO2 [17]. It was believed that the LixCoO2 can penetrate into the inner voids, reinforcing the intergranular adhesion and mitigating the microcracks formation. These studies offer a huge opportunity for the advanced modification of NCM cathodes. Besides, TiO2 as a competitive coating layer has been applied to various cathodes for its unique properties, involving abundance, structural stability and the electrochemically inertness [18,19]. It was reported that Ti-based doping is also conducive to the structural stability of NCM cathodes [20–23]. However, to the best of our knowledge, there is no report about the presence of TiO2 at both the outer surface and the bulk of host materials. In this study, nanoscale coating layer composed of TiO2 is introduced into the inside and outside of the secondary particles of the NCM622, (labeled as NCM@TO2-x (x ¼ 1, 2 and 3)) through a simple wet chemical approach. By applying the coating layer, contaminating species on the surface and microcracks inside the particles are effectively alleviated. Besides, the provided coating layer also facilities the incorporation of Ti4þ ions into the bulk NCM622, which is expected to stabilize the structure. In addition, in-situ Raman is used to track subtle structural consequences of TiO2 modification. Scanning electron microscope (SEM) images and depth X-ray photo electronic spectra (XPS) are con ducted to detect the mechanical stability and surface configuration of the electrode after cycling.
2.2. Material characterization The crystalline phase of the samples was characterized by the X-ray diffraction (XRD, Bruker D2), in which 1% graphite was added into the prepared powder to calibrate the peak position. The particle morphol ogies were determined with transmission electron microscope (TEM, Tecnai G2 F30, S-TWIN) and scanning electron microscope (SEM, Hitachi S-4800SEM). To obtain the composition and morphology changes from the surface to the center, cross-sections of the particles were prepared by embedding the particles in an epoxy and grinding them flat. The concentration gradient of titanium metal within the crosssections for the NCM@TO2-x samples was analyzed by electron micro probe analysis (EMPA, Shimadzu EPMA-1720). Point scans of the pol ished surfaces of the NCM622 and NCM@TO2-2 samples were analyzed by Raman (Thermo Fisher DXRxi). The structural evolution during charge-discharge was measured by in-situ Raman (Thermo Fisher DXRxi). Cells for in-situ Raman studies were specially designed. The positive side of the coin cell was punched with a hole with 4 mm in diameter, which was covered by a piece of quartz and held by the thermoplastic film. X-ray photoelectron spectroscopy (XPS, Thermo Escalab250) measurements were performed for evaluation of elemental species. 2.3. Electrochemical measurement The NCM622 (or NCM@TO2-x) electrodes were prepared by slurring the active material, 10 wt% Ketjen Black (KB), and 10 wt% poly vinylidenefl uoride (PVDF) binder in N-methyl-2-pyrrolidinone (NMP) solvent, and coating the mixture with 3–5 mg cm 2 active material onto the aluminum. The working electrodes for normal study were assembled in a 2025-type coin cell using Li foil as the counter electrode. Full cells test was constructed by using the as-prepared NCM622 (or NCM@TO2x) as cathode and graphite as anode in a 2032 coin-type cell. The ma terial loading amount of cathode film was controlled about 15 mg cm 2. The capacity ratio of the anode to the cathode was selected to be 1.08. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volumetric ratio of 1:2. The chargedischarge measurements were performed in constant current mode using Neware-BTS9 battery testing systems. Cyclic voltammograms (CV) curves were recorded at a scanning rate of 0.1 mV s 1. Electrochemical impedance spectroscopy (EIS) was collected in the frequency range from 1 mHz to 100 kHz. 3. Results and discussion
2. Experimental
The synthetic process of NCM@TO2-x (x ¼ 1, 2 and 3) is presented in Fig. 1a, in which the precursor Ni0.6Co0.2Mn0.2(OH)2 is first surfacemodified by TiO2∙xH2O through a slowly hydrolyzing induced by evenly dispersed Ti(OC4H9)4, followed by sintering with the lithium source and realizing two purposes in the subsequent lithiation process. One is that the TiO2 could react with the residual lithium and act as a resistance layer, minimizing the undesired side reaction. Another is that the provided coating materials can penetrate into the particle interior, filling the voids between the grains and preventing the particle pulver ization. Meanwhile, the Ti4þ can diffuse into the crystal lattice of NCM622, thus expanding the lattice making subsequent Li-ion migra tion easier. The morphology of the particle surface is conducted by TEM char acterization for the NCM@TO2-2 sample. It can be seen from Fig. 1b, the NCM@TO2-2 sample involves a coating layer, which is distinct from the host with a clear boundary and shows the thickness of 14 nm. Besides, the elemental mapping (Fig. 1c) shows that the Ti and O are distributed homogeneously throughout the surface of the particle, while most of the Ni, Co and Mn elements are distributed on the bulk and a few on the layer part of the particle. This suggests that Ni, Co and Mn would diffuse from the bulk to the Ti-concentrated layer and react with the reduction
2.1. Sample preparation The samples modified with different contents of TiO2 designated as NCM622, NCM@TO2-1, NCM@TO2-2 and NCM@TO2-3, corresponding to 0, 1.6, 3.2 and 4.8 mol% TiO2, respectively, were prepared through a hydrolysis process method. The Ni0.6Co0.2Mn0.2(OH)2 was synthesized as our previous study [24]. Stoichiometric amounts of Ti(OC4H9)4 and Ni0.6Co0.2Mn0.2(OH)2 particles, and an appropriate amount of poly vinylpyrrolidone were dissolved into absolute ethanol to generate a suspension under vigorous stirring at room temperature. After stirring for 12 h, deionized water (DI water) was added into the above solution with a volume ratio of DI water: ethanol ¼ 1:10, and the solution was kept stirring for 12 h to trigger the hydrolysis reaction completely. The obtained products were rinsed with ethanol and water successively, and collected by centrifugation. The mixture of LiOH and modified Ni0.6C o0.2Mn0.2(OH)2 with an atomic ratio of Li/(Ni þ Co þ Mn) ¼ 1.05 were annealed at 780 � C for 10 h.
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Fig. 1. Schematic illustration of the synthetic route of the NCM@TO2-x samples (a); TEM images (b) and corresponding EDS results (c) of the NCM@TO2-2 sample; the Ti content change obtained from EPMA of a single particle of the NCM@TO2-x samples (d).
of residual lithium on the surface during the sintering process, forming Li–Ti-M-O on the surface. Furthermore, the EPMA of the cross-section shows that the Ti content increases from the center to the edge (Fig. 1d), indicating that the TiO2 infiltrates into the nano-sized gap and generates a Ti gradient material, which would help preserve the volume change during cycling. The surface structure of the NCM622 and NCM@TO2-x samples are also characterized by SEM (Fig. 2). Each particle for all samples exhibits comparably spherical in shape as seen from Fig. 2a–d with an average diameter from 10 to 13 μm. Subtle differences are detected from magnified images (Fig. 2e–h), in which the primary particles of the
NCM@TO2-x samples arrange more tightly and their boundaries become blurred due to the successful formation of coating layer. The crosssectional SEM images (Fig. 2i-l) further indicate that the microstruc ture of the NCM@TO2-x samples is inherently changed by TiO2 coating. In contrast to the NCM622 sample containing a porous core, the voids for the NCM@TO2-x samples on the inner are less prominent. The X-ray diffraction (XRD) patterns for the NCM622 and NCM@TO2-x samples are shown in Fig. 3. The NCM622 sample exhibits layered α-NaFeO2 structure corresponding to the R3m space group [25, 26]. In comparison, the NCM@TO2-x samples are free from impurity phases and also have good crystallinity, as evidenced by the narrow and
Fig. 2. SEM images of the NCM622 (a, e and i), NCM@TO2-1 (b, f and j), NCM@TO2-2 (c, g and k) and NCM@TO2-3 (d, h and l). 3
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NCM@TO2-2 samples is compared from 2.8 to 4.6 V at 5 C (Fig. 4d). The dramatic capacity decline of the NCM622 sample can be observed after 50 cycles, and the discharge capacity drops to 84.4 mAh g 1 after 200 cycles. On the contrary, the NCM@TO2-2 sample shows a higher initial discharge capacity (161.8 mAh g 1) and better capacity retention (80.3%) after 200 cycles. CV is applied for further exploration of the electrochemical behavior along with deintercalation/intercalation, as shown in Fig. 4e and f. During the second lithiation process, two peaks centered at 3.81 V/ 3.65 V are clearly observed for the NCM622 sample, which is respec tively assigned to the oxidation/reduction of Ni2þ/Ni3þ. The potential gap (ΔV) between them is known to indicate the electrode polarization [29–31]. In the subsequent CV curves, the ΔV increases from 0.16 V (2nd cycle) to 0.21 V (4th cycle), followed by an obvious intensity drop in both oxidation and reduction peaks. This is presumably because of the degradation of the electrode. In the case of the NCM@TO2-2 sample, the ΔV varies less, is 0.13 V at 2nd cycle and 0.15 V at 4th cycle. The markedly decreased ΔV suggests that the structural stability of each primary particle is supported by TiO2 infiltrative layers, giving rise to a substantial enhancement in the cycleability of NCM622. Electrochemical characterization of the NCM622 and the NCM@TO2-2 samples are also conducted by using natural graphite as the anode in coin-type full cells (Fig. 4g and h). Because 8% excess ca pacity is designed for anode in full cells, the capacity and energy density of full cells is evaluated based on the active materials of the NCM622/ NCM@TO2-2 cathodes. The discharge capacity of NCM622 sample drops from 132 to 72 mAh g 1 after 500 cycles with capacity retention of only 54%. By contrast, the NCM@TO2-2 sample with an initial capacity of 141 mAh g 1 delivers a better value of 120 mAh g 1 over 500 cycles (Fig. 4h). Moreover, the decay of average voltage plateau is well sup pressed after surface modification. As illustrated in Fig. 4g, the central voltage plateau of the NCM622 sample declined to below 3.0 V after 500 cycles, while that of the NCM@TO2-2 sample varies from the initial 3.6 V–3.3 V after 500 cycles. As a result, the energy density of NCM@TO2-2 retains 400 Wh kg 1 after 500 cycles with a retention of 79%, much larger than that of the NCM622 sample (226 Wh kg 1). The in-situ Raman spectra and corresponding contour plots of the NCM622 and NCM@TO2-2 samples (Fig. 5) explicitly illustrate the structural evolution during one deintercalation/intercalation process. For both cases, two major bands located at 488 cm 1 and 556 cm 1 in open circuit voltage (OCV) of layered LiMO2 family (R3m space group) are assigned to the Eg mode and the A1g mode, respectively. Upon charging, the intensity of A1g markedly drops to a minor signal induced by the removal of the lithium ions, which reflects the progressive change of the short-range local environment [32]. As for Eg mode of the NCM622 sample, it moves toward higher wavenumber (3.63–4.05 V) and then shifts back toward lower wavenumber (4.05–4.34 V), which attributed to an initial steady expansion of lattice c with a subsequent contraction [33]. The Eg mode of NCM@TO2-2 sample exhibits the same trend but its movement amplitude (4.07–4.32 V) related to the lattice contraction is lower than that of NCM622 sample, increasing by 3 cm 1 (from 465 to 468 cm 1) as compared to the 9 cm 1 (from 465 to 474 cm 1) for the NCM622 sample. This further demonstrates that the infiltrative coating layer of the NCM@TO2-2 sample reduces the level of mechanical stress and relieves the undesired volume contraction, which reflects the better stability of the crystal structure of the NCM@TO2-2 sample [33,34]. In addition, a new peak at around 545 cm 1 (Fig. 5a and b), ascribing to the active modes of LiNi3þO2, grows since 3.9 V, reflecting the oxidation of Ni2þ to Ni3þ at charge state [33,35]. It can be found that the LiNi3þO2 peaks for NCM622 decays abruptly above 4.35 V (Fig. 5a), which attributed to the highly reactive environment that triggers the disorder ionic arrangement by the reduction of Ni4þ to Ni2þ [33,36]. However, the LiNi3þO2 peak in the case of NCM@TO2-2 could be observed even till the end of the charge. This observation can be attributed to that Ti4þ doping would provide extra electrons in
Fig. 3. XRD patterns of the NCM622 and NCM@TO2-x samples.
sharp peaks. As observed from the enlarged region of I, the (003) peak of NCM@TO2-x samples gradually shifts to lower angle with increasing the amount of TiO2 as compared with that of NCM622, while the position of (002) of the graphite is well-kept at 26.60� in all the NCM622 and NCM@TO2-x samples. This subtle structural variations after TiO2 incorporation are observed due to the larger size of the Ti4þ in an octahedral environment (rTi4þ ¼ 0.60 Å) as compared to that of Ni3þ (rNi3þ ¼ 0.56 Å), Co3þ (rCo3þ ¼ 0.54 Å) and Mn4þ (rMn4þ ¼ 0.53 Å), which suggests the partial Ti4þ incorporated into the NCM622 lattice [27,28]. Further peak fitting has been carried out by using GSA S/EXPGUI (Fig. S1) and the Rietveld refined results are presented in Table 1. The calculated a and c of the NCM@TO2-x samples grow with increasing the amount of TiO2. These values are 2.8778 Å and 14.2611 Å for the NCM@TO2-3 sample, respectively, while that of the NCM622 sample are 2.8627 Å and 14.1989 Å, respectively. This elongation was also reported previously in Li–Ti–O coated cathodes, which is also a clear evidence of the presence of Ti4þ in the NCM622 lattice [27,28]. The cycling performance of the investigated samples is compared in Fig. 4. As shown in Fig. 4a, all samples display prominent layered-like plateaus in the ~3.7 V regions. The corresponding initial capacity is reduced with TiO2 modification from 177.2 mAh g 1 (NCM622), to 172 mAh g 1 (NCM@TO2-1), 168.7 mAh g 1 (NCM@TO2-2) and 166.2 mAh g 1 (NCM@TO2-3). Despite the decrease in the discharge capacity, the followed cycling tests confirm the advantageous perfor mance of the NCM@TO2-x samples: capacity retention of 92% for the NCM@TO2-1 sample, 96% for the NCM@TO2-2 sample and 91% for the NCM@TO2-3 sample after 200 cycles are achieved in comparison with only 78% for the bare NCM622 sample. It is believed that the capacity fading mainly depends on the electrolyte decomposition for parasitic reactions and the intergranular structural cracks, the filling layer of TiO2 can alleviate both two effects. Additionally, the rate capability for the NCM622 and NCM@TO2-2 samples tested at current rates of 0.2, 0.5, 1, 2, 5, 10 C and ending with 0.2C are summarized in Fig. 4c. The NCM@TO2-2 sample exhibits less fading than the NCM622 sample with the increased rate. It delivers 160.4, 148.2 and 108.4 mAh g 1 at the rate of 2 C, 5 C and 10 C, while the NCM622 sample can maintain only 141.7, 118.2 and 10.2 mAh g 1 at the same rate, respectively. The high operating-voltage stability of batteries is also an important parameter in practical application. The cycling performance of the NCM622 and Table 1 Results of XRD Rietveld refinements of the NCM622 and NCM@TO2-x samples. Sample NCM622 NCM@TO2-1 NCM@TO2-2 NCM@TO2-3
Lattice parameters a (Å)
c (Å)
V (Å3)
2.8627 2.8647 2.8765 2.8778
14.1989 14.2073 14.2509 14.2611
100.769 100.975 101.955 102.302
Rwp (%)
I(003)/I(104)
1.65% 1.70% 1.68% 1.70%
1.57 1.63 1.66 1.68
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Fig. 4. The initial charge and discharge profiles and cycleabilities at 1 C in the voltage range of 2.8–4.4 V (a–b); rate capability in the voltage range of 2.8–4.4 V (c); the cycleabilities at 5 C in the voltage range of 2.8–4.6 V (d); CV at 0.1 mV s 1 (e–f); comparison of discharge curves (g) and cycling performance (h) of the full coin cell between 2.6 and 4.2 V at 2 C, 25 � C.
Fig. 5. Raman spectra and corresponding contour plots of the NCM622 (a, c) and NCM@TO2-2 (b, d) samples. 5
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retarding the oxidation of Ni3þ to Ni4þ and thereby structural distortion, which leads to lower capacity but more sustainable intercalation reac tion [20]. Therefore, the corresponding intensities and shapes in the contour plots (Fig. 5c and d) for NCM@TO2-2 are more reversible than that of NCM622 after a full charge/discharge cycle, providing un equivocal proof that the TiO2 modification improves the cycling stability of the NCM622 cathode. To directly observe the surface degradation, the cycled cathodes for the NCM622 and NCM@TO2-2 samples are detected by SEM (Fig. 6). Apparently, the surface pulverization for the NCM622 electrode is more serious than that for NCM@TO2-2 electrode. We find many cracks on the surface for NCM622 sample along with extensive fracture of particles, but few for the NCM@TO2-2 sample. Upon magnification, the original aggregated primary particle of the NCM622 sample is smashed, leading to various microcracks. The cracks of the electrode are also observed in NCM@TO2-2 sample. But the generation of microcracks during the cycling is significantly suppressed in the NCM@TO2-2 sample. The XPS results for cycled electrodes of the NCM622 and NCM@TO22 samples are performed within the depth of 40 nm (Fig. 7). As displayed in Fig. 7a, the peak for C 1s is fitted and divided into four peaks. One of – O of Li2CO3 (located at 290.1 eV) of the them assigned to the C– NCM622 and NCM@TO2-2 samples are quite different [37]. It shows high intensity at each depth in the NCM622 sample (Fig. 7a). But after – O signal on the outer surface incorporating TiO2, there is only a weak C– (0–20 nm) of the NCM@TO2-2 sample (Fig. 7b). We believe this differ ence is because the residual lithium species are consumed by reacting with the TiO2 particles in the form of Li–Ti-M-O coating layer during sintering process. Furthermore, as described in Fig. 7a, the proportion of LiF (684.2 eV) increases gradually as increasing depth and becomes dominant after a depth of 30 nm [37]. With the increased amount of LiF, the peaks of LixPOyFz (686.4 eV), LiPxFy (688.1 eV) and PVDF (687.4 eV) continuously drop and finally approach to an invisible signal [37,38]. By contrast, the amount of LiF of NCM@TO2-2 sample is considerably reduced as its peak ratio decreases gradually as increasing depth. Since the LiF is the byproduct from decomposition of surface impurity and electrolyte during repeated cycling, the severe side reactions occurring on the electrode/electrolyte interface is significantly mitigated in the NCM@TO2-2 sample [39]. Besides, the peaks of TMFx, which are the byproducts of undesired electrode consumption, observed from the two samples are both very pronounced. However, it is clear that the surface damages of the NCM@TO2-2 sample are not as serious as that of the NCM622 sample (Fig. 6). As a result, the dominant of the TMFx for the NCM@TO2-2 sample can be mainly attributed to the formation of Ti–F
species, which effectively scavenges the HF and prevents the electrode form the dissolution of TM ions into the electrolyte [40]. The afore mentioned results further confirm that the TiO2 modification is helpful for the interface stability and promotion of easier charge transfer during cycling. EIS is employed to further investigate different reaction kinetics between the NCM622 and the NCM@TO2-2 samples (Fig. 8). All the plots involve two semicircles in the high and medium frequency, respectively, and a tail in the low frequency. This indicates that elec trochemical process is determined by the surface film resistance (Rsf), the charge transfer resistance (Rct), and Warburg element (W) that related to Li-ion migration (Fig. 8a and b) [41]. As shown in Fig. 8d, the NCM@TO2-2 sample exhibits lower Rsf and Rct at every given cycle, suggesting the better Li-ion intercalation kinetics. This is particularly obvious after 150th cycle, the Rsf and Rct of the NCM622 sample respectively reach to 28.7 Ω and 144.3 Ω, four times larger than that at the 50th cycle (6.6 Ω and 37.9 Ω), respectively. But the NCM@TO2-2 sample shows a significantly reduced increment, maintaining a far less resistances 19.9 Ω for Rsf and 73.7 Ω for Rct after 150th cycles. The significantly reduced Rct reflects the smaller surface degradation of the NCM@TO2-2 sample during Li-ion insertion/extraction, further leading to a superior cycling stability. Meanwhile, the smaller value of Rsf is the result of removed residual species (Li2CO3 and LiF) on the surface, confirming that the TiO2 modification facilitates the Li-ion migration, which are mainly responsible for the high rate capability of the NCM@TO2-2 sample. Furthermore, the DLiþ of the two samples is calculated from Fig. 8a-b [42]. The DLiþ of the NCM@TO2-2 sample decreases from 10.9 to 3.3 cm2 s 1 after 150 cycles but that of the NCM622 sample shows sharp reduction from 8.3 to 0.9 cm2 s 1, further reflecting the inherent advantage of the NCM@TO2-2 sample in the Li-ion storage process. 4. Conclusion In this work, TiO2 is introduced to modify the outer and inner surface of the NCM622 via the wet-chemistry coating method. Compared to the bare NCM622, the NCM@TO2-2 sample exhibits improved capacity retention (96% after 200 cycles vs. 78% for the NCM622 sample) and better rate capability. The coating layer effectively reduces the un wanted side reaction and stabilizes cathode-electrode interface; The mechanical strength of the cathode could be reinforced by minimizing the voids between the primary particles. It also found that the lattice contraction of the NCM@TO2-2 sample is smaller than that of the
Fig. 6. SEM image of the cycled electrode for the NCM622 (a–c) and the NCM@TO2-2 (d–f) samples after 100 cycles at the rate of 5 C. 6
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Fig. 7. XPS of the C 1s and F 1s for the both cycled NCM622 (a) and NCM@TO2-2 (b) electrodes in the fully discharged state after 100 cycles at the rate of 5 C.
Fig. 8. EIS results of NCM622 (a) and NCM@TO2-2 (b); the relationship plot between Z0 and ω circuit (inset of Fig. 8b) for impedance spectra (d).
NCM622 sample; moreover, the relative intensities and shapes of the NCM@TO2-2 sample are more reversible than that of the NCM622 sample during charge/discharge process. In addition, depth XPS analysis indicates that the residual species (Li2CO3 and LiF) on the surface are significantly reduced after TiO2 modification. These results further demonstrate the enhanced mechanical strength and interface stability,
1/2
(c); simulated parameters and DLiþ fitted with the equivalent
thus resulting improved the cycling stability of the NCM@TO2-2 sample. Finally, when the NCM@TO2-2 sample is tested in a full cell, the cathode retained 85% of its initial capacity after 500 cycles, which considerably outperforms the NCM622 sample.
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Acknowledge
[20] I.M. Markus, F. Lin, K.C. Kam, M. Asta, M.M. Doeff, Computational and experimental investigation of Ti substitution in Li1(NixMnxCo1-2x-yTiy)O2 for lithium ion batteries, J. Phys. Chem. Lett. 5 (2014) 3649–3655. [21] K.C. Kam, A. Mehta, J.T. Heron, M.M. Doeff, Electrochemical and physical properties of Ti-substituted layered nickel manganese cobalt oxide (NMC) cathode materials, J. Electrochem. Soc. 159 (2012) A1383–A1392. [22] Q.Q. Qiu, Z. Shadike, Q.C. Wang, X.Y. Yue, X.L. Li, S.S. Yuan, F. Fang, X.J. Wu, A. Hunt, I. Waluyo, S.M. Bak, X.Q. Yang, Y.N. Zhou, Improving the electrochemical performance and structural stability of the LiNi0.8Co0.15Al0.05O2 cathode material at high-voltage charging through Ti substitution, ACS Appl. Mater. Interfaces 11 (2019) 23213–23221. [23] M.A. Cambaz, B.P. Vinayan, H. Euchner, R.E. Johnsen, A.A. Guda, A. Mazilkin, Y. V. Rusalev, A.L. Trigub, A. Gross, M. Fichtner, Design of nickel-based cationdisordered rock-salt oxides: the effect of transition metal (M ¼ V, Ti, Zr) substitution in LiNi0.5M0.5O2 binary systems, ACS Appl. Mater. Interfaces 10 (2018) 21957–21964. [24] X. Chen, D. Li, Y. Mo, X. Jia, J. Jia, C. Yao, D. Chen, Y. Chen, Cathode materials with cross-stack structures for suppressing intergranular cracking and highperformance lithium-ion batteries, Electrochim. Acta 261 (2018) 513–520. [25] E. Zhao, L. He, B. Wang, X. Li, J. Zhang, Y. Wu, J. Chen, S. Zhang, T. Liang, Y. Chen, X. Yu, H. Li, L. Chen, X. Huang, H. Chen, F. Wang, Structural and mechanistic revelations on high capacity cation-disordered Li-rich oxides for rechargeable Li-ion batteries, Energy Storage Mater 16 (2019) 354–363. [26] F. Wu, J. Tian, N. Liu, Y. Lu, Y. Su, J. Wang, R. Chen, X. Ma, L. Bao, S. Chen, Alleviating structural degradation of nickel-rich cathode material by eliminating the surface Fm3m phase, Energy Storage Mater 8 (2017) 134–140. [27] E. Zhao, X. Liu, Z. Hu, L. Sun, X. Xiao, Facile synthesis and enhanced electrochemical performances of Li2TiO3-coated lithium-rich layered Li1.13Ni0.30Mn0.57O2 cathode materials for lithium-ion batteries, J. Power Sources 294 (2015) 141–149. [28] N.H. Vu, J.C. Im, S. Unithrattil, W.B. Im, Synergic coating and doping effects of Timodified integrated layered–spinel Li1.2Mn0.75Ni0.25O2þδ as a high capacity and long lifetime cathode material for Li-ion batteries, J. Mater. Chem. A 6 (2018) 2200–2211. [29] J. Zhang, Z. Yang, R. Gao, L. Gu, Z. Hu, X. Liu, Suppressing the structure deterioration of Ni-rich LiNi0.8Co0.1Mn0.1O2 through atom-scale interfacial integration of self-forming hierarchical spinel layer with Ni gradient concentration, ACS Appl. Mater. Interfaces 9 (2017) 29794–29803. [30] C. Wang, D. Du, M. Song, Y. Wang, F. Li, A high-power Na3V2(PO4)3-Bi sodium-ion full battery in a wide temperature range, Adv. Energy Mater. 9 (2019) 1900022. [31] X. Sun, G.P. Hao, X. Lu, L. Xi, B. Liu, W. Si, C. Ma, Q. Liu, Q. Zhang, S. Kaskel, O. G. Schmidt, High-defect hydrophilic carbon cuboids anchored with Co/CoO nanoparticles as highly efficient and ultra-stable lithium-ion battery anodes, J. Mater. Chem. A 4 (2016) 10166–10173. [32] Y. Mo, L. Guo, B. Cao, Y. Wang, L. Zhang, X. Jia, Y. Chen, Correlating structural changes of the improved cyclability upon Nd-substitution in LiNi0.5Co0.2Mn0.3O2 cathode materials, Energy Storage Mater 18 (2019) 260–268. [33] C. Venkateswara Rao, J. Soler, R. Katiyar, J. Shojan, W.C. West, R.S. Katiyar, Investigations on electrochemical behavior and structural stability of Li1.2Mn0.54Ni0.13Co0.13O2 lithium-ion cathodes via in-situ and ex-situ Raman spectroscopy, J. Phys. Chem. C 118 (2014) 14133–14141. [34] T. Ohzuku, A. Ueda, Solid-state redox reactions of LiCoO2 (R3m) for 4 volt secondary lithium cells, J. Electrochem. Soc. 141 (1994) 2972–2977. [35] G. Singh, W.C. West, J. Soler, R.S. Katiyar, In situ Raman spectroscopy of layered solid solution Li2MnO3–LiMO2 (M ¼ Ni, Mn, Co), J. Power Sources 218 (2012) 34–38. [36] J.X. Huang, B. Li, B. Liu, B.J. Liu, J.B. Zhao, B. Ren, Structural evolution of NM (Ni and Mn) lithium-rich layered material revealed by in-situ electrochemical Raman spectroscopic study, J. Power Sources 310 (2016) 85–90. [37] X. Zuo, C. Fan, J. Liu, X. Xiao, J. Wu, J. Nan, Effect of tris (trimethylsilyl) borate on the high voltage capacity retention of LiNi0.5Co0.2Mn0.3O2/graphite cells, J. Power Sources 229 (2013) 308–312. [38] P. Hou, H. Zhang, X. Deng, X. Xu, L. Zhang, Stabilizing the electrode/electrolyte interface of LiNi0.8Co0.15Al0.05O2 through tailoring aluminum distribution in microspheres as long-life, high-rate, and safe cathode for lithium-ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 29643–29653. [39] P. Hou, J. Yin, M. Ding, J. Huang, X. Xu, Surface/Interfacia structure and chemistry of high-energy nickel-rich layered oxide cathodes: advances and perspectives, Small 13 (2017), 1701802-1701802. [40] C. Qin, J. Cao, J. Chen, G. Dai, T. Wu, Y. Chen, Y. Tang, A. Li, Y. Chen, Improvement of electrochemical performance of nickel rich LiNi0.6Co0.2Mn0.2O2 cathode active material by ultrathin TiO2 coating, Dalton Trans. 45 (2016) 9669–9675. [41] Y. Zou, X. Yang, C. Lv, T. Liu, Y. Xia, L. Shang, G.I. Waterhouse, D. Yang, T. Zhang, Multishelled Ni-rich Li(NixCoyMnz)O2 hollow fibers with low cation mixing as high-performance cathode materials for Li-ion batteries, Adv. Sci. 4 (2017) 1600262. [42] X. Jia, M. Yan, Z. Zhou, X. Chen, C. Yao, D. Li, D. Chen, Y. Chen, Nd-doped LiNi0.5Co0.2Mn0.3O2 as a cathode material for better rate capability in high voltage cycling of Li-ion batteries, Electrochim. Acta 254 (2017) 50–58.
This study was financially supported by the Hainan povincial natural science foundation of China (2018CXTD332 and HD-SYSZX-201802), Science and technology development special fund project (ZY2018HN09-3), National Natural Science Foundation of China (Nos. 51362009 and 21603048). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227439. References [1] Y.K. Sun, Z. Chen, H.J. Noh, D.J. Lee, H.G. Jung, Y. Ren, S. Wang, C.S. Yoon, S. T. Myung, K. Amine, Nanostructured high-energy cathode materials for advanced lithium batteries, Nat. Mater. 11 (2012) 942–947. [2] Y.K. Sun, S.T. Myung, B.C. Park, J. Prakash, I. Belharouak, K. Amine, High-energy cathode material for long-life and safe lithium batteries, Nat. Mater. 8 (2009) 320–354. [3] Z. Luo, L. Liu, J. Ning, K. Lei, Y. Lu, F. Li, J. Chen, A microporous covalent-organic framework with abundant accessible carbonyl groups for lithium-ion batteries, Angew. Chem. Int. Ed. 57 (2018) 9443–9446. [4] S.T. Myung, F. Maglia, K.J. Park, C.S. Yoon, P. Lamp, S.J. Kim, Y.K. Sun, Nickelrich layered cathode materials for automotive lithium-ion batteries: achievements and perspectives, ACS Energy Lett 2 (2016) 196–223. [5] A. Manthiram, B. Song, W. Li, A perspective on nickel-rich layered oxide cathodes for lithium-ion batteries, Energy Storage Mater 6 (2017) 125–139. [6] Y. Ruan, X. Song, Y. Fu, C. Song, V. Battaglia, Structural evolution and capacity degradation mechanism of LiNi0.6Mn0.2Co0.2O2 cathode materials, J. Power Sources 400 (2018) 539–548. [7] F. Schipper, M. Dixit, D. Kovacheva, M. Talianker, O. Haik, J. Grinblat, E. M. Erickson, C. Ghanty, D.T. Major, B. Markovsky, D. Aurbach, Stabilizing nickelrich layered cathode materials by a high-charge cation doping strategy: zirconiumdoped LiNi0.6Co0.2Mn0.2O2, J. Mater. Chem. A 4 (2016) 16073–16084. [8] L. Liang, K. Du, Z. Peng, Y. Cao, J. Duan, J. Jiang, G. Hu, Co–precipitation synthesis of Ni0.6Co0.2Mn0.2(OH)2 precursor and characterization of LiNi0.6Co0.2Mn0.2O2 cathode material for secondary lithium batteries, Electrochim. Acta 130 (2014) 82–89. [9] S. Sun, C. Du, D. Qu, X. Zhang, Z. Tang, Li2ZrO3-coated LiNi0.6Co0.2Mn0.2O2 for high-performance cathode material in lithium-ion battery, Ionics 21 (2015) 2091–2100. [10] S. Li, X. Fu, J. Zhou, Y. Han, P. Qi, G. Xing, F. Xiao, W. Bo, An effective approach to improve the electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode by an MOF-derived coating, J. Mater. Chem. A 4 (2016) 5823–5827. [11] H. Kim, M.G. Kim, H.Y. Jeong, H. Nam, J. Cho, A new coating method for alleviating surface degradation of LiNi0.6Co0.2Mn0.2O2 cathode material: nanoscale surface treatment of primary particles, Nano Lett. 15 (2015) 2111–2119. [12] Q. Liu, X. Su, D. Lei, Y. Qin, J. Wen, F. Guo, Y.A. Wu, Y. Rong, R. Kou, X. Xiao, F. Aguesse, J. Bare~ no, Y. Ren, W. Lu, Y. Li, Approaching the capacity limit of lithium cobalt oxide in lithium ion batteries via lanthanum and aluminium doping, Nat. Energy 3 (2018) 936–943. [13] K.J. Park, H.G. Jung, L.Y. Kuo, P. Kaghazchi, C.S. Yoon, Y.K. Sun, Improved cycling stability of Li[Ni0.90Co0.05Mn0.05]O2 through microstructure modification by boron doping for Li-ion batteries, Adv. Energy Mater. 8 (2018) 1801202. [14] B.B. Lim, S.T. Myung, C.S. Yoon, Y.K. Sun, 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, ACS Energy Lett 1 (2016) 283–289. [15] Y. Cho, P. Oh, J. Cho, A new type of protective surface layer for high-capacity Nibased cathode materials: nanoscaled surface pillaring layer, Nano Lett. 13 (2013) 1145–1152. [16] K. Min, C. Jung, D.S. Ko, K. Kim, J. Jang, K. Park, E. Cho, High-performance and industrially feasible ni-rich layered cathode materials by integrating coherent interphase, ACS Appl. Mater. Interfaces 10 (2018) 20599–20610. [17] H. Kim, S. Lee, H. Cho, J. Kim, J. Lee, S. Park, S.H. Joo, S.H. Kim, Y.G. Cho, H. K. Song, S.K. Kwak, J. Cho, Enhancing interfacial bonding between anisotropically oriented grains using a glue-nanofiller for advanced Li-ion battery cathode, Adv. Mater. 28 (2016) 4705–4712. [18] H. Liu, W. Li, D. Shen, D. Zhao, G. Wang, Graphitic carbon conformal coating of mesoporous TiO2 hollow spheres for high-performance lithium ion battery anodes, J. Am. Chem. Soc. 137 (2015) 13161–13166. [19] J.M. Zheng, J. Li, Z.R. Zhang, X.J. Guo, Y. Yang, The effects of TiO2 coating on the electrochemical performance of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode material for lithium-ion battery, Solid State Ion. 179 (2008) 1794–1799.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Improved cycling stability of LiNi0.6Co0.2Mn0.2O2 through microstructure consolidation by TiO2 coating for Li-ion batteries Yan Mo a, Lingjun Guo a, Hongfei Jin b, Baodong Du b, Bokai Cao b, Yigao Chen a, De Li b, Yong Chen b, * a
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, PR China State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Laboratory of Research on Utilization of Si-Zr-Ti Resources, College of Materials and Chemical Engineering, Hainan University, 58 Renmin Road, Haikou, 570228, China
b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� A thin TiO2 layer in-and outside of a LiNi0.6Co0.2Mn0.2O2 secondary particle is fabricated. � TiO2 modification mitigates the me chanical degradation and side reaction on cycling. � Excellent cyclability with 79% after 500 cycles is achieved in the full-cell testing.
A R T I C L E I N F O
A B S T R A C T
Keywords: Li-ion battery LiNi0.6Co0.2Mn0.2O2 cathode TiO2-Coating Surface degradation
The commercial deployment of Ni-rich layered oxide cathodes is hampered by both contaminating species on the surface and mechanical degradation associated to the microcracks. To resolve both issues, a series of TiO2modified LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode materials is constructed and studied. As expected, TiO2 could trap the residual lithium on the surface, minimizing the undesired side reaction. In addition, infiltrative layer with Ti gradient concentration from the outer to inner is also acquired, which strengthens the primary particles, reduces the gaps between randomly oriented grains and stabilizes the structure of NCM622 during charge discharge cycles. As a result, the NCM622 with TiO2 incorporation thus displays a high capacity of 168.7 mAh g 1, with 96% retention after 200 cycles, which is 18% higher than that of the bare NCM622. Structural and compositional characterization (i.e., in-situ Raman, EIS and depth XPS) further reveal the electrochemical mechanism and kinetics of the designed NCM622-TiO2 cathode.
1. Introduction With the rapid development of electric transportation fleets, the requirement for high-performance Li-ion batteries (LIBs) has promoted
great efforts to search for cathode materials with high energy density and long cycling life [1–3]. Layered transition metal oxides LiNi1-x- yCoxMnyO2 (NCM) are the most practical candidates for electric vehicles (EVs), on account of their high practical capacities (above 200 mAh g 1)
* Corresponding author. E-mail address: [email protected] (Y. Chen). https://doi.org/10.1016/j.jpowsour.2019.227439 Received 28 August 2019; Received in revised form 19 October 2019; Accepted 9 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yan Mo, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227439
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at high voltages (4.4–4.6 V) [4,5]. Among them, LiNi0.6Co0.2Mn0.2O2 (NCM622) have been widely investigated and used in commercial cells [6–11]. However, surface degradation that occurred at higher voltage and accelerated at elevated temperature seriously compromises the cycling lifetime. On one hand, the reactivity of charged NCM622 with the electrolyte which may lead to consumption of active material and gas generation [9,10]. Furthermore, microcracks initiated by the uneven pore network hinder the electrical conductivity, as well as cause the uncontrollable side reaction because of the newly formed surface [6,11, 12]. Recently, a few reports have proposed effective strategies to inhibit the surface degradation of NCM materials which comes from both parasitic reaction and intergranular cracks [13–17]. For instance, spinel-typed Li-reactive coating can extend the cycle life of LiNi0.91 Co0.06Mn0.03O2 from 84% to 92% [16]. This was attributed to the reduced amount of residual lithium and intrinsically isotropic property of the coating layer that alleviates the microcracks inside. In addition, integrating LixCoO2 into the precursor considerably increased the functionality of NCM622, which resulted 80% capacity retention after 150 cycles [11]. Similarly, improved cycling stability of LiNi0.8 Co0.15Al0.05O2 can also be achieved by infiltrative coating with LixCoO2 [17]. It was believed that the LixCoO2 can penetrate into the inner voids, reinforcing the intergranular adhesion and mitigating the microcracks formation. These studies offer a huge opportunity for the advanced modification of NCM cathodes. Besides, TiO2 as a competitive coating layer has been applied to various cathodes for its unique properties, involving abundance, structural stability and the electrochemically inertness [18,19]. It was reported that Ti-based doping is also conducive to the structural stability of NCM cathodes [20–23]. However, to the best of our knowledge, there is no report about the presence of TiO2 at both the outer surface and the bulk of host materials. In this study, nanoscale coating layer composed of TiO2 is introduced into the inside and outside of the secondary particles of the NCM622, (labeled as NCM@TO2-x (x ¼ 1, 2 and 3)) through a simple wet chemical approach. By applying the coating layer, contaminating species on the surface and microcracks inside the particles are effectively alleviated. Besides, the provided coating layer also facilities the incorporation of Ti4þ ions into the bulk NCM622, which is expected to stabilize the structure. In addition, in-situ Raman is used to track subtle structural consequences of TiO2 modification. Scanning electron microscope (SEM) images and depth X-ray photo electronic spectra (XPS) are con ducted to detect the mechanical stability and surface configuration of the electrode after cycling.
2.2. Material characterization The crystalline phase of the samples was characterized by the X-ray diffraction (XRD, Bruker D2), in which 1% graphite was added into the prepared powder to calibrate the peak position. The particle morphol ogies were determined with transmission electron microscope (TEM, Tecnai G2 F30, S-TWIN) and scanning electron microscope (SEM, Hitachi S-4800SEM). To obtain the composition and morphology changes from the surface to the center, cross-sections of the particles were prepared by embedding the particles in an epoxy and grinding them flat. The concentration gradient of titanium metal within the crosssections for the NCM@TO2-x samples was analyzed by electron micro probe analysis (EMPA, Shimadzu EPMA-1720). Point scans of the pol ished surfaces of the NCM622 and NCM@TO2-2 samples were analyzed by Raman (Thermo Fisher DXRxi). The structural evolution during charge-discharge was measured by in-situ Raman (Thermo Fisher DXRxi). Cells for in-situ Raman studies were specially designed. The positive side of the coin cell was punched with a hole with 4 mm in diameter, which was covered by a piece of quartz and held by the thermoplastic film. X-ray photoelectron spectroscopy (XPS, Thermo Escalab250) measurements were performed for evaluation of elemental species. 2.3. Electrochemical measurement The NCM622 (or NCM@TO2-x) electrodes were prepared by slurring the active material, 10 wt% Ketjen Black (KB), and 10 wt% poly vinylidenefl uoride (PVDF) binder in N-methyl-2-pyrrolidinone (NMP) solvent, and coating the mixture with 3–5 mg cm 2 active material onto the aluminum. The working electrodes for normal study were assembled in a 2025-type coin cell using Li foil as the counter electrode. Full cells test was constructed by using the as-prepared NCM622 (or NCM@TO2x) as cathode and graphite as anode in a 2032 coin-type cell. The ma terial loading amount of cathode film was controlled about 15 mg cm 2. The capacity ratio of the anode to the cathode was selected to be 1.08. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volumetric ratio of 1:2. The chargedischarge measurements were performed in constant current mode using Neware-BTS9 battery testing systems. Cyclic voltammograms (CV) curves were recorded at a scanning rate of 0.1 mV s 1. Electrochemical impedance spectroscopy (EIS) was collected in the frequency range from 1 mHz to 100 kHz. 3. Results and discussion
2. Experimental
The synthetic process of NCM@TO2-x (x ¼ 1, 2 and 3) is presented in Fig. 1a, in which the precursor Ni0.6Co0.2Mn0.2(OH)2 is first surfacemodified by TiO2∙xH2O through a slowly hydrolyzing induced by evenly dispersed Ti(OC4H9)4, followed by sintering with the lithium source and realizing two purposes in the subsequent lithiation process. One is that the TiO2 could react with the residual lithium and act as a resistance layer, minimizing the undesired side reaction. Another is that the provided coating materials can penetrate into the particle interior, filling the voids between the grains and preventing the particle pulver ization. Meanwhile, the Ti4þ can diffuse into the crystal lattice of NCM622, thus expanding the lattice making subsequent Li-ion migra tion easier. The morphology of the particle surface is conducted by TEM char acterization for the NCM@TO2-2 sample. It can be seen from Fig. 1b, the NCM@TO2-2 sample involves a coating layer, which is distinct from the host with a clear boundary and shows the thickness of 14 nm. Besides, the elemental mapping (Fig. 1c) shows that the Ti and O are distributed homogeneously throughout the surface of the particle, while most of the Ni, Co and Mn elements are distributed on the bulk and a few on the layer part of the particle. This suggests that Ni, Co and Mn would diffuse from the bulk to the Ti-concentrated layer and react with the reduction
2.1. Sample preparation The samples modified with different contents of TiO2 designated as NCM622, NCM@TO2-1, NCM@TO2-2 and NCM@TO2-3, corresponding to 0, 1.6, 3.2 and 4.8 mol% TiO2, respectively, were prepared through a hydrolysis process method. The Ni0.6Co0.2Mn0.2(OH)2 was synthesized as our previous study [24]. Stoichiometric amounts of Ti(OC4H9)4 and Ni0.6Co0.2Mn0.2(OH)2 particles, and an appropriate amount of poly vinylpyrrolidone were dissolved into absolute ethanol to generate a suspension under vigorous stirring at room temperature. After stirring for 12 h, deionized water (DI water) was added into the above solution with a volume ratio of DI water: ethanol ¼ 1:10, and the solution was kept stirring for 12 h to trigger the hydrolysis reaction completely. The obtained products were rinsed with ethanol and water successively, and collected by centrifugation. The mixture of LiOH and modified Ni0.6C o0.2Mn0.2(OH)2 with an atomic ratio of Li/(Ni þ Co þ Mn) ¼ 1.05 were annealed at 780 � C for 10 h.
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Fig. 1. Schematic illustration of the synthetic route of the NCM@TO2-x samples (a); TEM images (b) and corresponding EDS results (c) of the NCM@TO2-2 sample; the Ti content change obtained from EPMA of a single particle of the NCM@TO2-x samples (d).
of residual lithium on the surface during the sintering process, forming Li–Ti-M-O on the surface. Furthermore, the EPMA of the cross-section shows that the Ti content increases from the center to the edge (Fig. 1d), indicating that the TiO2 infiltrates into the nano-sized gap and generates a Ti gradient material, which would help preserve the volume change during cycling. The surface structure of the NCM622 and NCM@TO2-x samples are also characterized by SEM (Fig. 2). Each particle for all samples exhibits comparably spherical in shape as seen from Fig. 2a–d with an average diameter from 10 to 13 μm. Subtle differences are detected from magnified images (Fig. 2e–h), in which the primary particles of the
NCM@TO2-x samples arrange more tightly and their boundaries become blurred due to the successful formation of coating layer. The crosssectional SEM images (Fig. 2i-l) further indicate that the microstruc ture of the NCM@TO2-x samples is inherently changed by TiO2 coating. In contrast to the NCM622 sample containing a porous core, the voids for the NCM@TO2-x samples on the inner are less prominent. The X-ray diffraction (XRD) patterns for the NCM622 and NCM@TO2-x samples are shown in Fig. 3. The NCM622 sample exhibits layered α-NaFeO2 structure corresponding to the R3m space group [25, 26]. In comparison, the NCM@TO2-x samples are free from impurity phases and also have good crystallinity, as evidenced by the narrow and
Fig. 2. SEM images of the NCM622 (a, e and i), NCM@TO2-1 (b, f and j), NCM@TO2-2 (c, g and k) and NCM@TO2-3 (d, h and l). 3
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NCM@TO2-2 samples is compared from 2.8 to 4.6 V at 5 C (Fig. 4d). The dramatic capacity decline of the NCM622 sample can be observed after 50 cycles, and the discharge capacity drops to 84.4 mAh g 1 after 200 cycles. On the contrary, the NCM@TO2-2 sample shows a higher initial discharge capacity (161.8 mAh g 1) and better capacity retention (80.3%) after 200 cycles. CV is applied for further exploration of the electrochemical behavior along with deintercalation/intercalation, as shown in Fig. 4e and f. During the second lithiation process, two peaks centered at 3.81 V/ 3.65 V are clearly observed for the NCM622 sample, which is respec tively assigned to the oxidation/reduction of Ni2þ/Ni3þ. The potential gap (ΔV) between them is known to indicate the electrode polarization [29–31]. In the subsequent CV curves, the ΔV increases from 0.16 V (2nd cycle) to 0.21 V (4th cycle), followed by an obvious intensity drop in both oxidation and reduction peaks. This is presumably because of the degradation of the electrode. In the case of the NCM@TO2-2 sample, the ΔV varies less, is 0.13 V at 2nd cycle and 0.15 V at 4th cycle. The markedly decreased ΔV suggests that the structural stability of each primary particle is supported by TiO2 infiltrative layers, giving rise to a substantial enhancement in the cycleability of NCM622. Electrochemical characterization of the NCM622 and the NCM@TO2-2 samples are also conducted by using natural graphite as the anode in coin-type full cells (Fig. 4g and h). Because 8% excess ca pacity is designed for anode in full cells, the capacity and energy density of full cells is evaluated based on the active materials of the NCM622/ NCM@TO2-2 cathodes. The discharge capacity of NCM622 sample drops from 132 to 72 mAh g 1 after 500 cycles with capacity retention of only 54%. By contrast, the NCM@TO2-2 sample with an initial capacity of 141 mAh g 1 delivers a better value of 120 mAh g 1 over 500 cycles (Fig. 4h). Moreover, the decay of average voltage plateau is well sup pressed after surface modification. As illustrated in Fig. 4g, the central voltage plateau of the NCM622 sample declined to below 3.0 V after 500 cycles, while that of the NCM@TO2-2 sample varies from the initial 3.6 V–3.3 V after 500 cycles. As a result, the energy density of NCM@TO2-2 retains 400 Wh kg 1 after 500 cycles with a retention of 79%, much larger than that of the NCM622 sample (226 Wh kg 1). The in-situ Raman spectra and corresponding contour plots of the NCM622 and NCM@TO2-2 samples (Fig. 5) explicitly illustrate the structural evolution during one deintercalation/intercalation process. For both cases, two major bands located at 488 cm 1 and 556 cm 1 in open circuit voltage (OCV) of layered LiMO2 family (R3m space group) are assigned to the Eg mode and the A1g mode, respectively. Upon charging, the intensity of A1g markedly drops to a minor signal induced by the removal of the lithium ions, which reflects the progressive change of the short-range local environment [32]. As for Eg mode of the NCM622 sample, it moves toward higher wavenumber (3.63–4.05 V) and then shifts back toward lower wavenumber (4.05–4.34 V), which attributed to an initial steady expansion of lattice c with a subsequent contraction [33]. The Eg mode of NCM@TO2-2 sample exhibits the same trend but its movement amplitude (4.07–4.32 V) related to the lattice contraction is lower than that of NCM622 sample, increasing by 3 cm 1 (from 465 to 468 cm 1) as compared to the 9 cm 1 (from 465 to 474 cm 1) for the NCM622 sample. This further demonstrates that the infiltrative coating layer of the NCM@TO2-2 sample reduces the level of mechanical stress and relieves the undesired volume contraction, which reflects the better stability of the crystal structure of the NCM@TO2-2 sample [33,34]. In addition, a new peak at around 545 cm 1 (Fig. 5a and b), ascribing to the active modes of LiNi3þO2, grows since 3.9 V, reflecting the oxidation of Ni2þ to Ni3þ at charge state [33,35]. It can be found that the LiNi3þO2 peaks for NCM622 decays abruptly above 4.35 V (Fig. 5a), which attributed to the highly reactive environment that triggers the disorder ionic arrangement by the reduction of Ni4þ to Ni2þ [33,36]. However, the LiNi3þO2 peak in the case of NCM@TO2-2 could be observed even till the end of the charge. This observation can be attributed to that Ti4þ doping would provide extra electrons in
Fig. 3. XRD patterns of the NCM622 and NCM@TO2-x samples.
sharp peaks. As observed from the enlarged region of I, the (003) peak of NCM@TO2-x samples gradually shifts to lower angle with increasing the amount of TiO2 as compared with that of NCM622, while the position of (002) of the graphite is well-kept at 26.60� in all the NCM622 and NCM@TO2-x samples. This subtle structural variations after TiO2 incorporation are observed due to the larger size of the Ti4þ in an octahedral environment (rTi4þ ¼ 0.60 Å) as compared to that of Ni3þ (rNi3þ ¼ 0.56 Å), Co3þ (rCo3þ ¼ 0.54 Å) and Mn4þ (rMn4þ ¼ 0.53 Å), which suggests the partial Ti4þ incorporated into the NCM622 lattice [27,28]. Further peak fitting has been carried out by using GSA S/EXPGUI (Fig. S1) and the Rietveld refined results are presented in Table 1. The calculated a and c of the NCM@TO2-x samples grow with increasing the amount of TiO2. These values are 2.8778 Å and 14.2611 Å for the NCM@TO2-3 sample, respectively, while that of the NCM622 sample are 2.8627 Å and 14.1989 Å, respectively. This elongation was also reported previously in Li–Ti–O coated cathodes, which is also a clear evidence of the presence of Ti4þ in the NCM622 lattice [27,28]. The cycling performance of the investigated samples is compared in Fig. 4. As shown in Fig. 4a, all samples display prominent layered-like plateaus in the ~3.7 V regions. The corresponding initial capacity is reduced with TiO2 modification from 177.2 mAh g 1 (NCM622), to 172 mAh g 1 (NCM@TO2-1), 168.7 mAh g 1 (NCM@TO2-2) and 166.2 mAh g 1 (NCM@TO2-3). Despite the decrease in the discharge capacity, the followed cycling tests confirm the advantageous perfor mance of the NCM@TO2-x samples: capacity retention of 92% for the NCM@TO2-1 sample, 96% for the NCM@TO2-2 sample and 91% for the NCM@TO2-3 sample after 200 cycles are achieved in comparison with only 78% for the bare NCM622 sample. It is believed that the capacity fading mainly depends on the electrolyte decomposition for parasitic reactions and the intergranular structural cracks, the filling layer of TiO2 can alleviate both two effects. Additionally, the rate capability for the NCM622 and NCM@TO2-2 samples tested at current rates of 0.2, 0.5, 1, 2, 5, 10 C and ending with 0.2C are summarized in Fig. 4c. The NCM@TO2-2 sample exhibits less fading than the NCM622 sample with the increased rate. It delivers 160.4, 148.2 and 108.4 mAh g 1 at the rate of 2 C, 5 C and 10 C, while the NCM622 sample can maintain only 141.7, 118.2 and 10.2 mAh g 1 at the same rate, respectively. The high operating-voltage stability of batteries is also an important parameter in practical application. The cycling performance of the NCM622 and Table 1 Results of XRD Rietveld refinements of the NCM622 and NCM@TO2-x samples. Sample NCM622 NCM@TO2-1 NCM@TO2-2 NCM@TO2-3
Lattice parameters a (Å)
c (Å)
V (Å3)
2.8627 2.8647 2.8765 2.8778
14.1989 14.2073 14.2509 14.2611
100.769 100.975 101.955 102.302
Rwp (%)
I(003)/I(104)
1.65% 1.70% 1.68% 1.70%
1.57 1.63 1.66 1.68
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Fig. 4. The initial charge and discharge profiles and cycleabilities at 1 C in the voltage range of 2.8–4.4 V (a–b); rate capability in the voltage range of 2.8–4.4 V (c); the cycleabilities at 5 C in the voltage range of 2.8–4.6 V (d); CV at 0.1 mV s 1 (e–f); comparison of discharge curves (g) and cycling performance (h) of the full coin cell between 2.6 and 4.2 V at 2 C, 25 � C.
Fig. 5. Raman spectra and corresponding contour plots of the NCM622 (a, c) and NCM@TO2-2 (b, d) samples. 5
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retarding the oxidation of Ni3þ to Ni4þ and thereby structural distortion, which leads to lower capacity but more sustainable intercalation reac tion [20]. Therefore, the corresponding intensities and shapes in the contour plots (Fig. 5c and d) for NCM@TO2-2 are more reversible than that of NCM622 after a full charge/discharge cycle, providing un equivocal proof that the TiO2 modification improves the cycling stability of the NCM622 cathode. To directly observe the surface degradation, the cycled cathodes for the NCM622 and NCM@TO2-2 samples are detected by SEM (Fig. 6). Apparently, the surface pulverization for the NCM622 electrode is more serious than that for NCM@TO2-2 electrode. We find many cracks on the surface for NCM622 sample along with extensive fracture of particles, but few for the NCM@TO2-2 sample. Upon magnification, the original aggregated primary particle of the NCM622 sample is smashed, leading to various microcracks. The cracks of the electrode are also observed in NCM@TO2-2 sample. But the generation of microcracks during the cycling is significantly suppressed in the NCM@TO2-2 sample. The XPS results for cycled electrodes of the NCM622 and NCM@TO22 samples are performed within the depth of 40 nm (Fig. 7). As displayed in Fig. 7a, the peak for C 1s is fitted and divided into four peaks. One of – O of Li2CO3 (located at 290.1 eV) of the them assigned to the C– NCM622 and NCM@TO2-2 samples are quite different [37]. It shows high intensity at each depth in the NCM622 sample (Fig. 7a). But after – O signal on the outer surface incorporating TiO2, there is only a weak C– (0–20 nm) of the NCM@TO2-2 sample (Fig. 7b). We believe this differ ence is because the residual lithium species are consumed by reacting with the TiO2 particles in the form of Li–Ti-M-O coating layer during sintering process. Furthermore, as described in Fig. 7a, the proportion of LiF (684.2 eV) increases gradually as increasing depth and becomes dominant after a depth of 30 nm [37]. With the increased amount of LiF, the peaks of LixPOyFz (686.4 eV), LiPxFy (688.1 eV) and PVDF (687.4 eV) continuously drop and finally approach to an invisible signal [37,38]. By contrast, the amount of LiF of NCM@TO2-2 sample is considerably reduced as its peak ratio decreases gradually as increasing depth. Since the LiF is the byproduct from decomposition of surface impurity and electrolyte during repeated cycling, the severe side reactions occurring on the electrode/electrolyte interface is significantly mitigated in the NCM@TO2-2 sample [39]. Besides, the peaks of TMFx, which are the byproducts of undesired electrode consumption, observed from the two samples are both very pronounced. However, it is clear that the surface damages of the NCM@TO2-2 sample are not as serious as that of the NCM622 sample (Fig. 6). As a result, the dominant of the TMFx for the NCM@TO2-2 sample can be mainly attributed to the formation of Ti–F
species, which effectively scavenges the HF and prevents the electrode form the dissolution of TM ions into the electrolyte [40]. The afore mentioned results further confirm that the TiO2 modification is helpful for the interface stability and promotion of easier charge transfer during cycling. EIS is employed to further investigate different reaction kinetics between the NCM622 and the NCM@TO2-2 samples (Fig. 8). All the plots involve two semicircles in the high and medium frequency, respectively, and a tail in the low frequency. This indicates that elec trochemical process is determined by the surface film resistance (Rsf), the charge transfer resistance (Rct), and Warburg element (W) that related to Li-ion migration (Fig. 8a and b) [41]. As shown in Fig. 8d, the NCM@TO2-2 sample exhibits lower Rsf and Rct at every given cycle, suggesting the better Li-ion intercalation kinetics. This is particularly obvious after 150th cycle, the Rsf and Rct of the NCM622 sample respectively reach to 28.7 Ω and 144.3 Ω, four times larger than that at the 50th cycle (6.6 Ω and 37.9 Ω), respectively. But the NCM@TO2-2 sample shows a significantly reduced increment, maintaining a far less resistances 19.9 Ω for Rsf and 73.7 Ω for Rct after 150th cycles. The significantly reduced Rct reflects the smaller surface degradation of the NCM@TO2-2 sample during Li-ion insertion/extraction, further leading to a superior cycling stability. Meanwhile, the smaller value of Rsf is the result of removed residual species (Li2CO3 and LiF) on the surface, confirming that the TiO2 modification facilitates the Li-ion migration, which are mainly responsible for the high rate capability of the NCM@TO2-2 sample. Furthermore, the DLiþ of the two samples is calculated from Fig. 8a-b [42]. The DLiþ of the NCM@TO2-2 sample decreases from 10.9 to 3.3 cm2 s 1 after 150 cycles but that of the NCM622 sample shows sharp reduction from 8.3 to 0.9 cm2 s 1, further reflecting the inherent advantage of the NCM@TO2-2 sample in the Li-ion storage process. 4. Conclusion In this work, TiO2 is introduced to modify the outer and inner surface of the NCM622 via the wet-chemistry coating method. Compared to the bare NCM622, the NCM@TO2-2 sample exhibits improved capacity retention (96% after 200 cycles vs. 78% for the NCM622 sample) and better rate capability. The coating layer effectively reduces the un wanted side reaction and stabilizes cathode-electrode interface; The mechanical strength of the cathode could be reinforced by minimizing the voids between the primary particles. It also found that the lattice contraction of the NCM@TO2-2 sample is smaller than that of the
Fig. 6. SEM image of the cycled electrode for the NCM622 (a–c) and the NCM@TO2-2 (d–f) samples after 100 cycles at the rate of 5 C. 6
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Fig. 7. XPS of the C 1s and F 1s for the both cycled NCM622 (a) and NCM@TO2-2 (b) electrodes in the fully discharged state after 100 cycles at the rate of 5 C.
Fig. 8. EIS results of NCM622 (a) and NCM@TO2-2 (b); the relationship plot between Z0 and ω circuit (inset of Fig. 8b) for impedance spectra (d).
NCM622 sample; moreover, the relative intensities and shapes of the NCM@TO2-2 sample are more reversible than that of the NCM622 sample during charge/discharge process. In addition, depth XPS analysis indicates that the residual species (Li2CO3 and LiF) on the surface are significantly reduced after TiO2 modification. These results further demonstrate the enhanced mechanical strength and interface stability,
1/2
(c); simulated parameters and DLiþ fitted with the equivalent
thus resulting improved the cycling stability of the NCM@TO2-2 sample. Finally, when the NCM@TO2-2 sample is tested in a full cell, the cathode retained 85% of its initial capacity after 500 cycles, which considerably outperforms the NCM622 sample.
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Acknowledge
[20] I.M. Markus, F. Lin, K.C. Kam, M. Asta, M.M. Doeff, Computational and experimental investigation of Ti substitution in Li1(NixMnxCo1-2x-yTiy)O2 for lithium ion batteries, J. Phys. Chem. Lett. 5 (2014) 3649–3655. [21] K.C. Kam, A. Mehta, J.T. Heron, M.M. Doeff, Electrochemical and physical properties of Ti-substituted layered nickel manganese cobalt oxide (NMC) cathode materials, J. Electrochem. Soc. 159 (2012) A1383–A1392. [22] Q.Q. Qiu, Z. Shadike, Q.C. Wang, X.Y. Yue, X.L. Li, S.S. Yuan, F. Fang, X.J. Wu, A. Hunt, I. Waluyo, S.M. Bak, X.Q. Yang, Y.N. Zhou, Improving the electrochemical performance and structural stability of the LiNi0.8Co0.15Al0.05O2 cathode material at high-voltage charging through Ti substitution, ACS Appl. Mater. Interfaces 11 (2019) 23213–23221. [23] M.A. Cambaz, B.P. Vinayan, H. Euchner, R.E. Johnsen, A.A. Guda, A. Mazilkin, Y. V. Rusalev, A.L. Trigub, A. Gross, M. Fichtner, Design of nickel-based cationdisordered rock-salt oxides: the effect of transition metal (M ¼ V, Ti, Zr) substitution in LiNi0.5M0.5O2 binary systems, ACS Appl. Mater. Interfaces 10 (2018) 21957–21964. [24] X. Chen, D. Li, Y. Mo, X. Jia, J. Jia, C. Yao, D. Chen, Y. Chen, Cathode materials with cross-stack structures for suppressing intergranular cracking and highperformance lithium-ion batteries, Electrochim. Acta 261 (2018) 513–520. [25] E. Zhao, L. He, B. Wang, X. Li, J. Zhang, Y. Wu, J. Chen, S. Zhang, T. Liang, Y. Chen, X. Yu, H. Li, L. Chen, X. Huang, H. Chen, F. Wang, Structural and mechanistic revelations on high capacity cation-disordered Li-rich oxides for rechargeable Li-ion batteries, Energy Storage Mater 16 (2019) 354–363. [26] F. Wu, J. Tian, N. Liu, Y. Lu, Y. Su, J. Wang, R. Chen, X. Ma, L. Bao, S. Chen, Alleviating structural degradation of nickel-rich cathode material by eliminating the surface Fm3m phase, Energy Storage Mater 8 (2017) 134–140. [27] E. Zhao, X. Liu, Z. Hu, L. Sun, X. Xiao, Facile synthesis and enhanced electrochemical performances of Li2TiO3-coated lithium-rich layered Li1.13Ni0.30Mn0.57O2 cathode materials for lithium-ion batteries, J. Power Sources 294 (2015) 141–149. [28] N.H. Vu, J.C. Im, S. Unithrattil, W.B. Im, Synergic coating and doping effects of Timodified integrated layered–spinel Li1.2Mn0.75Ni0.25O2þδ as a high capacity and long lifetime cathode material for Li-ion batteries, J. Mater. Chem. A 6 (2018) 2200–2211. [29] J. Zhang, Z. Yang, R. Gao, L. Gu, Z. Hu, X. Liu, Suppressing the structure deterioration of Ni-rich LiNi0.8Co0.1Mn0.1O2 through atom-scale interfacial integration of self-forming hierarchical spinel layer with Ni gradient concentration, ACS Appl. Mater. Interfaces 9 (2017) 29794–29803. [30] C. Wang, D. Du, M. Song, Y. Wang, F. Li, A high-power Na3V2(PO4)3-Bi sodium-ion full battery in a wide temperature range, Adv. Energy Mater. 9 (2019) 1900022. [31] X. Sun, G.P. Hao, X. Lu, L. Xi, B. Liu, W. Si, C. Ma, Q. Liu, Q. Zhang, S. Kaskel, O. G. Schmidt, High-defect hydrophilic carbon cuboids anchored with Co/CoO nanoparticles as highly efficient and ultra-stable lithium-ion battery anodes, J. Mater. Chem. A 4 (2016) 10166–10173. [32] Y. Mo, L. Guo, B. Cao, Y. Wang, L. Zhang, X. Jia, Y. Chen, Correlating structural changes of the improved cyclability upon Nd-substitution in LiNi0.5Co0.2Mn0.3O2 cathode materials, Energy Storage Mater 18 (2019) 260–268. [33] C. Venkateswara Rao, J. Soler, R. Katiyar, J. Shojan, W.C. West, R.S. Katiyar, Investigations on electrochemical behavior and structural stability of Li1.2Mn0.54Ni0.13Co0.13O2 lithium-ion cathodes via in-situ and ex-situ Raman spectroscopy, J. Phys. Chem. C 118 (2014) 14133–14141. [34] T. Ohzuku, A. Ueda, Solid-state redox reactions of LiCoO2 (R3m) for 4 volt secondary lithium cells, J. Electrochem. Soc. 141 (1994) 2972–2977. [35] G. Singh, W.C. West, J. Soler, R.S. Katiyar, In situ Raman spectroscopy of layered solid solution Li2MnO3–LiMO2 (M ¼ Ni, Mn, Co), J. Power Sources 218 (2012) 34–38. [36] J.X. Huang, B. Li, B. Liu, B.J. Liu, J.B. Zhao, B. Ren, Structural evolution of NM (Ni and Mn) lithium-rich layered material revealed by in-situ electrochemical Raman spectroscopic study, J. Power Sources 310 (2016) 85–90. [37] X. Zuo, C. Fan, J. Liu, X. Xiao, J. Wu, J. Nan, Effect of tris (trimethylsilyl) borate on the high voltage capacity retention of LiNi0.5Co0.2Mn0.3O2/graphite cells, J. Power Sources 229 (2013) 308–312. [38] P. Hou, H. Zhang, X. Deng, X. Xu, L. Zhang, Stabilizing the electrode/electrolyte interface of LiNi0.8Co0.15Al0.05O2 through tailoring aluminum distribution in microspheres as long-life, high-rate, and safe cathode for lithium-ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 29643–29653. [39] P. Hou, J. Yin, M. Ding, J. Huang, X. Xu, Surface/Interfacia structure and chemistry of high-energy nickel-rich layered oxide cathodes: advances and perspectives, Small 13 (2017), 1701802-1701802. [40] C. Qin, J. Cao, J. Chen, G. Dai, T. Wu, Y. Chen, Y. Tang, A. Li, Y. Chen, Improvement of electrochemical performance of nickel rich LiNi0.6Co0.2Mn0.2O2 cathode active material by ultrathin TiO2 coating, Dalton Trans. 45 (2016) 9669–9675. [41] Y. Zou, X. Yang, C. Lv, T. Liu, Y. Xia, L. Shang, G.I. Waterhouse, D. Yang, T. Zhang, Multishelled Ni-rich Li(NixCoyMnz)O2 hollow fibers with low cation mixing as high-performance cathode materials for Li-ion batteries, Adv. Sci. 4 (2017) 1600262. [42] X. Jia, M. Yan, Z. Zhou, X. Chen, C. Yao, D. Li, D. Chen, Y. Chen, Nd-doped LiNi0.5Co0.2Mn0.3O2 as a cathode material for better rate capability in high voltage cycling of Li-ion batteries, Electrochim. Acta 254 (2017) 50–58.
This study was financially supported by the Hainan povincial natural science foundation of China (2018CXTD332 and HD-SYSZX-201802), Science and technology development special fund project (ZY2018HN09-3), National Natural Science Foundation of China (Nos. 51362009 and 21603048). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227439. References [1] Y.K. Sun, Z. Chen, H.J. Noh, D.J. Lee, H.G. Jung, Y. Ren, S. Wang, C.S. Yoon, S. T. Myung, K. Amine, Nanostructured high-energy cathode materials for advanced lithium batteries, Nat. Mater. 11 (2012) 942–947. [2] Y.K. Sun, S.T. Myung, B.C. Park, J. Prakash, I. Belharouak, K. Amine, High-energy cathode material for long-life and safe lithium batteries, Nat. Mater. 8 (2009) 320–354. [3] Z. Luo, L. Liu, J. Ning, K. Lei, Y. Lu, F. Li, J. Chen, A microporous covalent-organic framework with abundant accessible carbonyl groups for lithium-ion batteries, Angew. Chem. Int. Ed. 57 (2018) 9443–9446. [4] S.T. Myung, F. Maglia, K.J. Park, C.S. Yoon, P. Lamp, S.J. Kim, Y.K. Sun, Nickelrich layered cathode materials for automotive lithium-ion batteries: achievements and perspectives, ACS Energy Lett 2 (2016) 196–223. [5] A. Manthiram, B. Song, W. Li, A perspective on nickel-rich layered oxide cathodes for lithium-ion batteries, Energy Storage Mater 6 (2017) 125–139. [6] Y. Ruan, X. Song, Y. Fu, C. Song, V. Battaglia, Structural evolution and capacity degradation mechanism of LiNi0.6Mn0.2Co0.2O2 cathode materials, J. Power Sources 400 (2018) 539–548. [7] F. Schipper, M. Dixit, D. Kovacheva, M. Talianker, O. Haik, J. Grinblat, E. M. Erickson, C. Ghanty, D.T. Major, B. Markovsky, D. Aurbach, Stabilizing nickelrich layered cathode materials by a high-charge cation doping strategy: zirconiumdoped LiNi0.6Co0.2Mn0.2O2, J. Mater. Chem. A 4 (2016) 16073–16084. [8] L. Liang, K. Du, Z. Peng, Y. Cao, J. Duan, J. Jiang, G. Hu, Co–precipitation synthesis of Ni0.6Co0.2Mn0.2(OH)2 precursor and characterization of LiNi0.6Co0.2Mn0.2O2 cathode material for secondary lithium batteries, Electrochim. Acta 130 (2014) 82–89. [9] S. Sun, C. Du, D. Qu, X. Zhang, Z. Tang, Li2ZrO3-coated LiNi0.6Co0.2Mn0.2O2 for high-performance cathode material in lithium-ion battery, Ionics 21 (2015) 2091–2100. [10] S. Li, X. Fu, J. Zhou, Y. Han, P. Qi, G. Xing, F. Xiao, W. Bo, An effective approach to improve the electrochemical performance of LiNi0.6Co0.2Mn0.2O2 cathode by an MOF-derived coating, J. Mater. Chem. A 4 (2016) 5823–5827. [11] H. Kim, M.G. Kim, H.Y. Jeong, H. Nam, J. Cho, A new coating method for alleviating surface degradation of LiNi0.6Co0.2Mn0.2O2 cathode material: nanoscale surface treatment of primary particles, Nano Lett. 15 (2015) 2111–2119. [12] Q. Liu, X. Su, D. Lei, Y. Qin, J. Wen, F. Guo, Y.A. Wu, Y. Rong, R. Kou, X. Xiao, F. Aguesse, J. Bare~ no, Y. Ren, W. Lu, Y. Li, Approaching the capacity limit of lithium cobalt oxide in lithium ion batteries via lanthanum and aluminium doping, Nat. Energy 3 (2018) 936–943. [13] K.J. Park, H.G. Jung, L.Y. Kuo, P. Kaghazchi, C.S. Yoon, Y.K. Sun, Improved cycling stability of Li[Ni0.90Co0.05Mn0.05]O2 through microstructure modification by boron doping for Li-ion batteries, Adv. Energy Mater. 8 (2018) 1801202. [14] B.B. Lim, S.T. Myung, C.S. Yoon, Y.K. Sun, 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, ACS Energy Lett 1 (2016) 283–289. [15] Y. Cho, P. Oh, J. Cho, A new type of protective surface layer for high-capacity Nibased cathode materials: nanoscaled surface pillaring layer, Nano Lett. 13 (2013) 1145–1152. [16] K. Min, C. Jung, D.S. Ko, K. Kim, J. Jang, K. Park, E. Cho, High-performance and industrially feasible ni-rich layered cathode materials by integrating coherent interphase, ACS Appl. Mater. Interfaces 10 (2018) 20599–20610. [17] H. Kim, S. Lee, H. Cho, J. Kim, J. Lee, S. Park, S.H. Joo, S.H. Kim, Y.G. Cho, H. K. Song, S.K. Kwak, J. Cho, Enhancing interfacial bonding between anisotropically oriented grains using a glue-nanofiller for advanced Li-ion battery cathode, Adv. Mater. 28 (2016) 4705–4712. [18] H. Liu, W. Li, D. Shen, D. Zhao, G. Wang, Graphitic carbon conformal coating of mesoporous TiO2 hollow spheres for high-performance lithium ion battery anodes, J. Am. Chem. Soc. 137 (2015) 13161–13166. [19] J.M. Zheng, J. Li, Z.R. Zhang, X.J. Guo, Y. Yang, The effects of TiO2 coating on the electrochemical performance of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode material for lithium-ion battery, Solid State Ion. 179 (2008) 1794–1799.
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