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NASICON-type surface functional modification in core-shell LiNi Mn Co O@NaTi(PO) cathode enhances its high-voltage cycling stabilty and rate capacity towards Li-ion batteries 0.5

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Longwei Liang, Xuan Sun, Chen Wu, Linrui Hou, Jinfeng Sun, Xiaogang Zhang, and Changzhou Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15808 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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ACS Applied Materials & Interfaces

NASICON-type Surface Functional Modification in Core-Shell LiNi0.5Mn0.3Co0.2O2@NaTi2(PO4)3 Cathode Enhances Its High-Voltage Cycling Stabilty and Rate Capacity towards Li-Ion Batteries Longwei Liang,† Xuan Sun,† Chen Wu,† Linrui Hou,† Jinfeng Sun,† Xiaogang Zhang,‡ and Changzhou Yuan *,† †

School of Material Science and Engineering, University of Jinan, Jinan, 250022, P.

R. China ‡

College of Material Science and Engineering, Nanjing University of Aeronautics

and Astronautics, Nanjing, 210016, P. R. China

Correspondence: Professor CZ Yuan, School of Material Science and Engineering, University of Jinan, Jinan, 250022, P. R. China *E-mail: [email protected]; [email protected]

KEYWORDS:

LiNi0.5Mn0.3Co0.2O2@NaTi2(PO4)3;

core-shell

high-voltage cathodes; surface modifications; lithium-ion batteries

1

ACS Paragon Plus Environment

architectures;

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ABSTRACT: Surface modifications are established well as efficient methodologies to enhance comprehensive Li-storage behaviors of the cathodes, and play a significant role in cutting edge innovations towards lithium-ion batteries (LIBs). Herein, we first logically devised a pilot-scale coating strategy to integrate solid state electrolyte NaTi2(PO4)3 (NTP) and layered LiNi0.5Mn0.3Co0.2O2 (NMC) for smart construction of core-shell NMC@NTP cathodes. The NASICON-type NTP nanoshell with exceptional ion conductivity effectively suppressed gradual encroachment and/or loss of electroactive NMC, guaranteed stable phase-interfaces, and rendered small sur-/interfacial

eletron/ion-diffusion

resistance

meanwhile.

Benefitting

from

immanently promoting contributions of the nano-NTP coating, the as-fabricated core-shell NMC@NTP architectures were competitively endowed with superior high-voltage cyclic stabilities and rate capacities within larger electrochemical window from 3.0 to 4.6 V when utilized as advanced cathodes for advanced LIBs. More meaningfully, the appealing electrode design concept proposed here will exert significant impact upon further constructing other high-voltage Ni-based cathodes for high-energy/power LIBs.

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INTRODUCTION With overwhelmingly high volumetric/gravimetric energy density demands, lithium-ion batteries (LIBs) with innately superior properties including affordable price, long cycle life, and environmental friendliness, have been identified well as ideal power supplies for electric transportation, electric vehicles (EVs) and hybrid EVs.1 The electrochemical performance of LIBs is intensely dependent upon the properties of both the deployed electrodes, especially the cathodes. To this end, the cathode materials to fulfill high safety, large capacity, and long-cycle lifetime are being studied by collaborative efforts in universities, national labs and industries around the world.2, 3 In particular, one representative Ni-based cathode, the layered LiNi0.5Mn0.3Co0.2O2 (denoted as NMC), obviously contributes to the declining cost and enhanced security, due to the less use of elemental Co and more introduction of Mn as compared to the LiCoO2, which makes it successfully adopted in commercial power LIBs.4-6 Of note, a larger range of reversible Li+ extraction from the bulk NMC cathode is especially prerequisite to obtain even higher capacities. Moreover, striking charge/discharge behaviors under the high cut-off voltage (≥ 4.4 V) is highly demanded. Nevertheless, it unavoidably triggers off some adverse reactions comprising electrolyte decomposition, irreversible transformation of the surface lattice structures, the oxygen loss and Mn(II)-dissolution from the cathode surface.7, 8 The contribution from Kang et al.9 manifests that the degree of structural degradation from the cathode surface is dramatically affected by the applied cut-off voltages, which leads to the formation of an ionically insulating layer on the cathode surfaces, 3



thus deteriorating the capacity retention. With the aim to efficiently mitigate the critical issues mentioned above, surface modifications have been extensively conducted, and vigorously proved to be a simple, indispensable yet effective strategy.10-15 Up till now, various inorganic compounds, including Al doped ZnO,16 V2O5,17,

18

fluoride (LiAlF4),19 and lithium-containing

compounds (LiAlO2,20 LiNixMn2-xO4,21 and Li1.2Ni0.2Mn0.6O2 22), have been widely investigated as coating phases for enhancing the Li-storage performance of NMC-based cathodes. It is full of challenge none the less to develop a superior coating layer that simultaneously possesses high chemically stability, fast Li+ conductance, high uniformity and excellent electro-activities. Lately, the solid state electrolyte (SSE), thanks to its fast lithium transportation and high bulk ionic conductivity, has increasingly attracted wide attention, and been applied as an elegant coating phase for high-performance cathodes. To date, thin SSE films, such as Li0.125La0.625TiO3,23 Li0.5La0.5TiO3,24,

25

Li1.3Al0.3Ti1.7(PO4)3,26 LiPON,27 Li2S-P2S5,28

and Li4SnS429 have been successfully deposited on the cathode surface by various methods. In particularly, NaTi2(PO4)3 (NTP), as a unique NASICON-type material with isostructural to LiTi2(PO4)3, is identified as one remarkable material towards advanced Na/Li-ion batteries, benefitting from its three dimensional (3D) phosphate opened-framework of large interstitial spaces and small volume expansion over cycling.30-32 Furthermore, the outstanding stability of the NASICON framework of the ATi2(PO4)3 (A = Li and Na) greatly favors for its reversible Li+ insertion/extraction over cycling, as reported by Masquelier.33 4


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With all the considerations above in mind, in this investigation, a typical NTP SSE with good crystallity and structural/thermal stability as well as exceptionally high ion conductivity is homogeneously built upon the electroactive NMC surface to purposefully fabricate the core-shell NMC@NTP cathodes. The prominent contribution of the NTP nanoshell, as a stable and Li-penetrable interfacial layer for layered NMC, is illuminated in-depth by physicochemical and electrochemical investigations. As a result, the core-shell NMC@NTP cathode delivered superior Li-storage ability (~153.2 mAh g-1 at 10 C, 1 C = 180 mA g-1) and cycling stability (~6.8% capacity degradation at 1 C rate) after 150 continuous charge-discharge cycles within a larger electrochemical window of 3.0 − 4.6 V (vs. Li+/Li). EXPERIMENTAL SECTION Preparation of the core-shell NMC@NTP samples Synthesis of the NMC. All the chemicals used in the work were all analytic-grade reagents, and directly used without any further purification. The Ni0.5Co0.3Mn0.2(OH)2 precursor was obtained by a co-precipitation method,34 and then mixed thoroughly with Li2CO3 with a molar ratio of 1 : 1.06 in a high-speed mixer. The mixture was first calcinated at 750 °C for 8 h, then 930 °C for 10 h in air. Finally, the NMC sample was prepared. Synthesis of the core-shell NMC@NTP. Typically, titanium (IV) sulfate and trisodium phosphate anhydrous with the stoichiometric ratio were completely blended using the polyurethane balls in planetary mill and the PEG-400 as the dispersant. Then, the pre-prepared NMC powder was added to the formed pulped mixture for further 5



mixing. After that, the obtained viscous slurry was pre-treated at 120 °C for 6 h. The residual soluble inorganic salt was adequately washed with distilled water until the SO42- in the filtrate was not visually detected with 0.5 M BaCl2 solution. The NMC cathodes coated by crystalized NTP nanoshell were finally prepared after calcination at 700 °C for 8 h in air. The amounts of the NTP corresponding to the NMC were conducted in molar ratio of 2% or 5%, and the final obtained products were labeled as NMC@NTP and NMC@NTP-5, respectively. Materials Characterization Typical X-ray diffraction (XRD) patterns were recorded with the diffraction angle range from 10 ° to 90 ° with a scanning speed of 2 ° min-1 by a Rigaku-TTRШ type X-ray diffractometer (Cu Kα, 40 kV, 300 mA, Japan). Corresponding lattice parameters were refined and calculated with Rietveld method. Morphological features were captured by scanning electron microscopy (SEM) (JEOL, JSM-6360LV, Japan) equipped with an energy dispersive spectrometer (EDS). Transmission electron microscopy (TEM) and High resolution TEM (HRTEM) (Philips CM200 microscope) were utilized to differentiate lattice fingers. X-ray photoelectron spectroscopy (XPS) was performed on Kratos Model XSAM800 using Monochromatic X-ray generated from Al Kα (1486.6 eV). For XRD and HRTEM characterizations of the cycled cathodes, the powders were obtained by soaked and washed with dimethyl carbonate (DMC) in the argon box. N2 adsorption/desorption was determined by Brunauer-Emmett-Teller (BET) measurements using an TriStar II 3020 surface area analyzer. Particle size analysis was conducted by Mastersizer 2000. 6


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Electrochemical measurements Electrochemical evaluation was performed by galvanostatic cycling of the assembled CR2025 type button cell. The electrodes were made up of electroactive material, acetylene black (AB), and polyvinylidene ﬂuoride (PVDF) with a mass ratio of 8 : 1 : 1, and N-methyl-2-pyrrolidone (NMP) was applied as the dispersant. The typical weight density of cathode material in the electrode was ≈15 mg cm-2. After dried, the as-prepared electrode was pressed by using a roller press into a film with a thickness of ~50 µm (compacted density of 3.2 g cm-3 or so) on each polished Al foil. The electrolyte was 1 M LiPF6 in ethylene carbonate/DMC/ethyl methyl carbonate (EC-DMC-EMC) with a volume ratio of 1 : 1 : 1. Celgard 2400 microporous membrane was used as the separator and Li-metal foil as the negative electrode. For the charge and discharge testing, the charging rate was fixed equal to the discharging rate, setting at 0.2 C rate (36 mA g-1) for the first three cycles and then charged/discharged at 0.5 C or 1 C rate in the following cycles in the voltage of 3.0 − 4.5 or 4.6 V. The 8-channel Land test system (CT2001A, Wuhan China) was used to collect the electrochemical data. Cyclic voltammetry (CV) was carried out in a voltage range of 2.8 − 4.5 V (vs. Li/Li+) at a scan rate of 0.1 mV s-1. After 200 charge-discharge cycles, the cells were charged to 4.5 V (or 4.6 V) at 0.5 C for electrochemical impedance spectroscopy (EIS) test in frequency range from 100 to 0.005 Hz with an AC signal amplitude of 10 mV. Both the CV and EIS data were collected on the IVIUM electrochemical workstation (the Netherlands). RESULTS AND DISCUSSION 7



Physicochemical characterizations Various routes have been successfully carried out for synthesizing the NTP, such as high or low temperature solid-state reactions,35, 36 hydrothermal method,37 and sol-gel method

38, 39

. In order to simplify the coating route, a simple but extremely effective

coating process, as graphically illustrated in Figure 1a, was herein put forward for practical industrial application. Typical XRD patterns and corresponding X-ray Rietveld refinement for the as-obtained NMC and NMC@NTP samples are collectively shown in Figure 1b, c and Supporting Figure S1a. Evidently, sharp diffraction peaks and marked splitting in pair reﬂections of (006)/(102) and (108)/(110) as well as their negligible difference in lattice parameters (Table 1), manifest the same crystalline structure (JCPDF card No. 85-1969 with R m space group) of the NMC and NMC@NTP ,40 and no any variation in the bulk NMC structure with reasonable adding amount of the NTP. Nevertheless, the XRD pattern of the NMC@NTP-5 (Supporting Figure S1b) combines both layered α-NaFeO2 structure and discernible superlattice reflections with 2θ located at 19 ° – 34 °, and no other distinct peaks can be visually detected. This observation strongly verifies the existence of only stable interface interaction in the NMC@NTP-5, and no any newly generated composite. As widely verified, the Ni2+ ion, which has an ionic radius (0.69 Å) similar to that of Li+ (0.76 Å), can easily migrate to Li slabs, leading to so called “cation mixing”.41, 42 This phenomenon will weaken the intensity of the (003) peak, while the intensity of (104) peak is unchanged. Thus, the degree of cation mixing in the layered structure with a R m space group can be supported by the intensity ratio 8


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of (003) to (104) peak, that is the I(003)/I(104).43 Generally, when the value of I(003)/I(104) ratio is greater than 1.2, the materials have a good layered structure with small cation mixing, and a value of < 1.2 commonly suggests a high degree of cation mixing.44 The I(003)/I(104) ratios of the NMC and NMC@NTP are calculated as 1.56 and 1.48, respectively, which obviously suggests well-ordered layered structure just with tiny cation mixing for the two. The I(003)/I(104) ratio of the NMC@NTP is slightly lower than that of the NMC, which probably attributes to the occasionality or the measuring error. We conduct two parallel experiments to further check the change of the I(003)/I(104) ratio after NTP SSE coating. The obtained XRD patterns and corresponding I(003)/I(104) ratio of the NMC and NMC@NTP are displayed in Supporting Figure S2. The results show that there is no obvious variation of I(003)/I(104) ratio before and after coating. It further demonstrates that the NTP shell just adheres tightly on the bulk NMC surface rather than incorporate into the layered NMC host. In addition, these indisputable facts can be fully confirmed by no noticeable fluctuation in the reflection ratio of Ni amounts in Li sites of the two. For further illumination, the XRD pattern (Supporting Figure S3) of single-phase NTP is recorded, and finely conforms to standard file (JCPDF card No. 84-2009),30 which incontrovertibly implies the high feasibility of our coating route. In order to further verify the coating layer in the molecular formula of the NTP rather than other compounds, and to perceive whether the sur-/interfacial oxidation of the NMC is extrally disturbed by following NTP coating, typical XPS spectra of Ti 2p, Ni 2p, Co 2p and Mn 2p are characterized in detail accordingly. Obviously, the Ti 2p 9



spectra for the NMC@NTP, as shown in Figure 2a, show typical double-peak feature for the Ti 2p2/3 and Ti 2p1/2, which are located at the binding energy (BE) of 458.89 and 464.59 eV, respectively, strongly confirming the tetravalency of titanium species in the NTP.38 Typical Ni 2p spectra and corresponding fitted profiles for both NMC and NMC@NTP are depicted in Figure 2b, and corresponding valence information of the elemental Ni is summarized in Table 2. As distinctly shown, the almost identical BE and no noticeable diversity of Ni(II)/Ni(III) in total contents calculated from both the samples, as well as the negligible fading in relative intensity of the NMC@NTP, all indicate that the NTP coating does not alter the interfacial composition and disturb specific elemental oxidation states of the NMC core. This also can be well supported by the BE peaks of the Co 2p and Mn 2p in both NMC and NMC@NTP. The Co XPS spectra (Figure 2c) for the two both display a Co 2p3/2 main peak at 780.19 eV with a Ni (LMM) peak at 783.17 eV, and a Co 2p1/2 main peak at 795.17 eV, indicating merely trivalent cobalt can be observed. Figure 2d presents the Mn 2p spectra of the NMC and NMC@NTP, in which two broad main peaks are detected at 642.49 and 654.9 eV, corresponding to Mn 2p3/2 and Mn 2p1/2, which implies that the Mn is mainly in tetravalent state.20,

41, 42

As a result, systematic investigations and

discussions above can definitely verify the remarkable feasibility in combining the Nasicon-structured NTP SSE shell with the bulk layered NMC core. Representative morphologies of the NMC, NMC@NTP and NMC@NTP-5 specimen are detailedly characterized by SEM and EDS, as displayed in Figure 3. Noticeably, the higher amount of the NTP coating, the smoother surface and the more 10


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indistinguishable square-shaped primary particles can be observed in comparison with the single-phase NMC, as comparatively shown in Figure 3a-c. Similarly, the semblable phenomenon can be discerned evidently from the EDS results. Element mapping images of a separated NMC@NTP particle (Figure 3d-k) by TEM further signify the uniform distribution of the host Ni, Co, and Mn species and the homogeneous NTP coating shell. As noted, taking element Ti and P for example, corresponding dot-mapping images for the NMC@NTP-5 (Supporting Figure S4a, b) turn out to be much denser in contrast to those for the NMC@NTP (Supporting Figure S4c, d), which is in good line with the SEM observation above. For comparison, the EDS results (Supporting Figure S4e-h) of the NMC including several spherical particles are displayed without the detected Na, Ti, and P species. In order to probe composition changes in the bulk and the surficial region of the NMC@NTP, the cross-section constituent (Ni, Co, Mn, Na, Ti, and P) variations of a single particle of ~16.0 µm in diameter are collected by EDS electron probe signals linear sweeping in diametrical direction. Apparently, elements including Ni, Mn, and Co in bulk NMC are evenly distributed across the spherical secondary particles without obvious fluctuation, as exhibited in Figure 3l. However, it can be visually observed from Figure 3m that the distributions of elemental Na, Ti, and P are dominantly located on the surface region of the particle, and scarcely spread in the core zone. Moreover, distinct segregation cannot be found here. Conceivably, the synchronous, recognizable gradient-increased distributions of Na, Ti, and P elements in both terminal sections can be responsible well for the formation of interlinked tunnels for convenient Li+ 11



transportation. With in-depth SEM and EDS analysis above, therefore, we can incontrovertibly conclude that nano-sized NTP coating shell not only exists but homogeneously anchors on each NMC particle surface. For further conducting the detailed characterizations of surface/interfacial structure, TEM images (Supporting Figure S5a, b) for a single separated NMC@NTP particle, the low-magnification SEM images (Supporting Figure S6a, b) and corresponding particle size analysis (Supporting Figure S6a, b) of the NMC@NTP particles are all provided. It can be observed from Figure S6c that the minimum grain diameter is ~3.8 μm, which is considerably larger than the nanoscale coating layer (Supporting Figure S5b). Therefore, as observed in Figure S5a, it is difficult to detect and check the uniformity of the NTP SSE coating layer from the separated NMC@NTP particle with overall perspective. However, as obviously seen from Figure S5b, a continuous and nano-sized NTP SSE shell of ~20 nm in thickness tightly covers on the surface of the NMC core. Figure 4a shows the TEM image of the NMC sample. Clearly, a solid structure with smooth and clean surface is presented. As a contrast, a legible, transparent and unintermittent coating layer emerges and tightly adheres to the bulk NMC core for both NMC@NTP (Figure 4b) and NMC@NTP-5 (Figure 4c) samples. The thickness of the NTP nanoshell in the NMC@NTP is around 20 nm, while it becomes > 50 nm in thickness for the NMC@NTP-5. HRTEM and homologous fast Fourier transform (FFT) are next accomplished to explicitly illustrate the NMC-NTP interfaces. The bulk NMC, as shown in Figure 4d, f, presents a typical layered structure, where its 12


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(104) plane of lattice spacing of ~2.06 Å is single, linear and uninterrupted until the boundary of the particle.43 While two representative HRTEM images derived from the edge of NMC@NTP particles are observed, and provided in Figure 4e, j. Apparently, the lattice fringe with a plane spacing of ~2.07 Å in the bulk zone, as shown in Figure 4g, clearly supports that the layered structure is well maintained after coated with the NTP. And an evenly nano-sized coating layer located in the marked h and i regions (Figure 4e) can be overtly perceived. Moreover, as shown in Figure 4h and i, this outer coating layer is well-crystallized and its lattice fringes with two interplanar spacings of ~2.43 and ~2.20 Å are plainly visible, which extremely conforms to the (300) and (208) planes of the NASICON-type NTP, rather than the layered structure.30, 37

One note that the examination here is in good agreement with the aformentioned

XRD results (Figure 1b). The distinguishable interfacial region (marked in the green rectangle) located between the bulk NMC and external NTP nanoshell is displayed in Figure 4j. Of particular note is that the (003) plane of layered structure and (226) crystal plane of the NTP phase are evident in the interfacial region.30, 44 Accordingly, it is rationally speculated that metal ions just can interdiffuse with even higher external energy, such as, higher-temperature annealing, which thus results in the intact integration of the cladding layer and bulk body. Therefore, we have well-founded reasons

to

believe

that

the

well-crystalized

NTP

nanoshell

with

high

structural/thermal stability adheres closely to the outer surface along with the interlinked Li-diffusion channels between the core NMC and NTP nanoshell. It will make the unique NMC-NTP interface possess superior Li+ penetrability. Besides this, 13



the NTP nanoshell of exceptionally high ion conductivity guarantees the unhindered shuttling back and forth of Li+ ions in the electrolytes, as vividly depicted in Figure 4k. Therefore, undoubtedly, this unique core-shell feature will exert a significant influence on the ultimate electrochemical Li-storage properties of the NMC@NTP cathodes. Electrochemical evaluaiton We firstly investigate the influence of the NTP shell thickness upon the initial discharge capacities (Supporting Figure S7 and Table S1). As expected, the initial discharge capacities of the NMC@NTP at 0.2 C and 0.5 C are both slightly higher than those for the NMC with upper voltage limits of 4.4 or 4.5 V. However, as for the NMC@NTP-5, the initial discharge capacities are evidently declined. This phenomenon can be reasonably put down to its excessively thick coating shell, as observed in Figure 4c, which probably prolongs the Li+ diffusion pathways and enlarges electrochemical polarization accordingly. As a consequence, the NMC@NTP with even better electrochemical Li-storage behaviors will be compared in detailed with the NMC in the following section. Figure 5a and b show representative charge-discharge profiles at 0.2 C for one cycle, and then followed at 0.5 C rate for the 1st, 10th, 20th, 30th, 40th, 50th···150th, 200th cycle in the voltage of 3.0 − 4.5 V (vs. Li/Li+) for the NMC and NMC@NTP, respectively. Obviously, the NMC@NTP cathode delivers an initial discharge capacity of ~197.2 mAh g-1 at 0.2 C, higher than the NMC (~195.9 mAh g-1). The nearly undifferentiated discharge potential platforms located approximately at 3.8 V reveal 14


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that the intrinsic electrochemical feature is not affected by our implemented coating strategy at all. However, the plots reflected from the NMC@NTP cathode, no matter in charge or discharge process, are concentrated upon cycling while the voltage gap of the NMC is increasingly aggravated. In other words, the drop of the initial voltage, working voltage platform, and discharge capacities of the NMC@NTP sample are all pronouncedly superior to those for the NMC counterpart. More appealingly, the NMC@NTP cathode still can display attractive Li-storage performance even with higher upper voltage of 4.6 V. Typical charge-discharge plots at 0.2 C for one cycle and followed at 1.0 C rate for the 1st, 10th, 20th, 30th, 40th, 50th···100th, 150th cycle in the voltage of 3.0 − 4.6 V (vs. Li/Li+) for the NMC and NMC@NTP are collected in Figure 5c, d. Apparently, the average working voltages for the NMC@NTP drop slowly with further cycling, and the charge and discharge curves intersect well even after 150 cycles, implying the substantially diminished electrochemical polarization. By contrast, the average working voltage of the NMC dramatically declines. The charge and discharge profiles of the NMC are completely detached from the 70th cycle. Also note that the working voltage platform nearly disappears for the NMC, while the NMC@NTP still remains acceptable voltage platform after 150 cycles. Strikingly, the NMC@NTP cathode can yield a capacity of ~206.5 mAh g-1 at 0.2 C in the larger electrochemical window of 3.0 − 4.6 V (vs Li/Li+), higher than that of the NMC (~204.8 mAh g-1). It further confirms that the NTP SSE coating layer still can work well under higher cut-off voltage and C rates, benefiting from its high ionic conductivity. 15



Thanks to inherently high Li+ conductivity of the NTP nanoshell, the NMC@NTP sample is highly anticipated to possess outstanding Li+ insertion and extraction kinetics. Typical rate properties of both NMC and NMC@NTP from 0.2 C to 10 C, and then back to 0.2 C with 5 cycles per step in the voltage of 3.0 − 4.6 V are characterized, and presented in Figure 5e and f, respectively. As obviously shown in Figure 5e, the discharge voltage platform decline of the NMC@NTP cathode, which is caused by electrochemical polarization and charge transfer resistance under the mounting current rates, shows a comparatively stable trend, whereas the NMC cathode exhibits an aggressive lapse. As evidently demonstrated in Figure 5f, it is amazingly found that the NMC@NTP can yield a discharge capacity of ~169.5 and ~153.2 mAh g-1 at 5 and 10 C rates, respectively, while the NMC just reserves as ~146.3 (5 C) and ~115.1 (10 C) mAh g-1. Meanwhile, a discharge capacity of ~192.5 mAh g-1, occupying ~93.5% of the original value (0.2 C) of the NMC@NTP, is retained when returning back to 0.2 C again, but ~141.5 mAh g-1 and ~68.7% are merely achieved for the NMC. Such significant enhancement is certainly ascribed to the surface protection of the existing NTP nanoshell. In general, another interface located between the core and coating materials appears due to the introduction of the coating layer, which probably brings about additional resistance to Li+ migration.7, 21 However, this potential disadvantage is evidently compensated by the constructed high-speed way and interdiffused tunnels for fast Li+ transportation, as observed in Figure 4k, j. Furthermore, it is generally recognized that the high operating voltage of the Ni-rich cathodes, such as NMC, will result in the decomposition of the LiPF6 salt, 16


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generating HF in the electrolyte.19 Consequently, for uncoated NMC sample, the surface would be directly exposed to the electrolyte containing the generated HF, which renders the unavoidable dissolution of the electroactive materials because of the HF etching, thus the poor rate capability deserves. Nyquist plots and corresponding fitted profiles of the NMC and NMC@NTP at the charge state of 4.5 V after 200 cycles and 4.6 V after 150 cycles are comparatively displayed in Figure 5g and h, respectively. The adopted equivalent circuit model (Supporting Figure S8) is delineated. The high-frequency intercept refers to the ohmic resistances (Rs). The high-/medium-frequency semicircles match with the surface-film resistances (Rf) including Li containing impurities layer, the coating layer and probable solid electrolyte interface (SEI) layer, and charge-transfer impedances (Rct) in the sur-/interface between electrolytes and electrodes. And the straight tail in the low-frequency region represents the Warburg impedance (Zw) in consequence of the Li+ diffusion in layered host cathodes.45,

46

Corresponding fitted values are

summarized in Supporting Table S2. Apparently, the Rct value of the NMC@NTP yields merely ~97.7 Ω for 4.5 V after 200 cycles and ~128.5 Ω for 4.6 V after 150 cycles, which is considerably smaller than that of the NMC (~429.3 Ω for 4.5 V, and ~431.8 Ω for 4.6 V). Furthermore, the NMC@NTP delivers the lower Rf, as compared to the NMC, persuasively demonstrating that the existence of the unique NTP nanoshell can efficiently cut down the detrimental reactions under high cut-off voltages, and thus prevent the bulk NMC from being corroded by organic electrolytes. As a consequence, the sur-/interfacial impedances are dramatically decreased and 17



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meanwhile the electrochemical kinetics is substantially enhanced. This should be responsible well for the enhanced discharge capacities and rate capabilities of the core-shell NMC@NTP cathode, particularly when operated at higher voltages. The Li+ diffusion coefficient (DLi+) of as-prepared cathodes can be calculated according to the following equation: 47 DLi+ = R2T2/(2A2n4F4C2σ2) Z´ = Rs + Rct + σω-1/2

(1) (2)

where R stands for gas constant (8.314 J K-1 mol-1), T is temperature (298.15 K), A corresponds to the efficient work area of the cathode. Here, the effective area is estimated as half of the BET surface area of the electrode, the BET surface area of the pressed cathode is ~0.46 m2 g-1, thus the efficient area in is ~23.3 cm2 for each electode (0.78 cm2), n is the number of electrons at charged state (n = 1 − 235/278 = 0.182), F is the Faraday constant (96485 C mol-1), C is the concentration of Li+ ions at delithiated state (4.5 or 4.6 V, the pellet density of our NMC electrode is ~3.2 g cm-3, here, C is estimated as below: C = 3.2 × 0.8 ÷ 96.555 × (1 − 235/278) = 0.0041 mol cm-3. The Warburg factor σ associated with Z´ can be calculated by the linear fitting results of Z´ against ω-1/2 (here, ω is angular frequency) in the oblique line region, as shown in Figure 6. The Warburg factor and DLi+ of the NMC and NMC@NTP after 200 cycles at 4.5 V and after 150 cycles at 4.6 V are all calculated by above equation and exhibited in Table S2. As a result, the integration of NTP SSE has a remarkable effect on the DLi+ of the NMC. The NMC@NTP electrode possesses a much higher DLi+ value (6.414 × 10-10 cm2 s-1 for 4.5 V and 2.733×10-10 cm2 s-1 for 4.6 V) than that 18


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of NMC (2.471×10-11 cm2 s-1 for 4.5 V and 4.746×10-11 cm2 s-1 for 4.6 V), due to the Rs and Rct values of NMC@NTP are much lower than that of pristine NMC. Although the NTP phase is known to have modest electronic conductivity, its good crystallity, high structural/thermal stability, and superior ion conductivity tremendously diminish side reactions at the electrode-electrolyte interface.33, 44 The interface of pristine NMC however is facilely covered by an insulative Li-based residue layer, which is indeed fatal to its electronic and ionic transportation during charge and discharge processes.7, 48

The higher DLi+ of the NMC@NTP demonstrates that the core NMC material

maintains a well-ordered layered structure during repeated charge/discharge process, ensuring unimpeded Li+ migration and upgraded Li+ storage properties. Figure 7a presents the cyclic stabilities and corresponding coulombic efficiency (CE) profiles of the NMC and NMC@NTP cathodes performed at 0.5 C rate in the voltage of 3.0 − 4.5 V. As clearly plotted, the discharge capacity of the NMC drops drastically from ~189.8 to ~98.1 mAh g-1 after 200 cycles at 0.5 C, with a capacity retention of only ~51.7%. This dramatical capacity loss can be reasonably ascribed to the fragile layered structure originating from the surface region when contacted intimately to the electrolyte. Conversely, the capacity loss observed for the NMC@NTP is significantly slowed down from ~191.9 to ~174.8 mAh g-1, that is, merely a capacity loss of ~8.9% undergoing the same cycle at 0.5 C. The NMC@NTP electrode also demonstrates robust cycling performance under higher rate, as shown in Figure 7b. At 4.5 V and 1 C, the capacity of NMC@NTP starts at a higher value of ~181.8 mAh g-1 owing to the aforementioned enhanced kinetics, and preserves ~155.6 19



mAh g-1 after 500 charge-discharge cycles, leading to a capacity retention of ~85.6%. while the capacity of NMC starts from ~180.1 mAh g-1 and ends at ~74.5 mAh g-1 after 400 cycles, corresponding to ~40.9% capacity retention. The same case can be clearly found in the cycling performance after 150 cycles at 1 C rate between 3.0 − 4.6 V. As shown in Figure 7c, there is no obvious difference of the initial discharge capacities, ~194.3 mAh g-1 (1 C) for the NMC and ~195.5 mAh g-1 (1 C) for the NMC@NTP cathode. Nevertheless, the results distinctly demonstrate that the capacity retention even under severe high cut-off potentials is pronouncedly improved. Of note, the NMC sample is retained as ~50.8% of the initial capacity after 150 cycles, and encouragely, ~93.2% for the NMC@NTP sample. For comparison, typical methods employed to improve the high-voltage performance of the NMC and corresponding electrochemical properties under 4.5 V and/or 4.6 V are summarized in Table 3. It can be clearly observed that the electrochemical properties under high cut-off voltages achieved in our work shows obvious superiority in comparison with the previous researches on the conventional spherical NMC.7, 20, 49-51 Even if one dimensional (1D) multi-shelled porous NMC fibers or nanorods could provide short pathway for electron and Li+ diffusion,52, 53 the hybrid NMC@NTP cathode still can be endowed with higher initial discharge capacities and higher cyclic stabilities. We therefore can deduce that the rational design and feasible synthetic strategy in this investigation jointly contribute to the excellent cyclic stability or rate capacity of the NMC@NTP cathode. In specific, the sturdy NASICON-structured NTP nanoshell can preserve the layered bulk from 20


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gradual encroachment of electrolytes, and suppress the loss of electroactive NMC core. Besides this, the outmost NTP nanoshell with exceptional ion conductivity renders smooth and fast Li+ ions diffusion between the electrolyte and the core-active NMC. Finally, the interlinked channels for Li+ ions formed between the NMC core and NTP nanoshell further substantially decrease the interfacial resistance, and thus make the core-shell NMC@NTP own much higher Li+-storage capacity to the extreme after long-term cycles. CV measurement is further executed to exhaustively comprehend the significant contribution of the NTP nanoshell to the Li+ insertion/extraction kinetics. CV curves for the initial three cycles and the 200th cycle for the NMC and NMC@NTP, as described in Figure 8, are implemented in a voltage range of 2.8 − 4.5 V (vs. Li/Li+) at a sweep rate of 0.1 mV s-1. Apparently, no more than one pair of redox peaks, corresponding to the oxidation/reduction between Ni2+/Ni3+ and Ni4+, can be observed at 3.6 − 4.0 V (vs. Li/Li+) for both the two.49, 54 With detailed comparisons, we can easily figure out that the cathodic peaks of the NMC shift to the lower potentials from the outset, whereas the anodic cathodic peaks of the 1st, 2nd and 3th curves for the NMC@NTP are highly overlapped, as observed in the inset in Figure 8. Remarkably note that the redox potential separation of the NMC, standing for the polarization depth, drastically augments to ~323 mV after 200 cycles, synchronously with the unsymmetrical and compressed CV profiles, which signifies the sluggish kinetics and aggravating obstacle of lithiation/de-lithiation. In stark contrast, the depolarization function is sufficiently embodied in the CV curve of the NMC@NTP after 200 cycles, 21



where the anodic peak shifts mildly to higher potential and the cathodic peak remains at the initial potential except for the peak-current intensity fading, merely with a polarization of ~177 mV. In addition, sharp and symmetrical peaks also can be distinctly observed for the NMC@NTP. The distinct depolarization should be ascribed to the boosted kinetics of electrochemical reactions, which results from the outmost NTP nanoshell and the interfacial interdiffused channels for Li+ ions. As a result, the NMC@NTP cathode exhibits high cyclic stability and excellent rate capability here. Characteristics of the cycled electrodes To further understand the immanent origin of the improved electrochemical properties contributed by the NTP nanoshell, representative XRD patterns of the cycled NMC and NMC@NTP electrodes are displayed in Figure 9a. And the homologous regions of (003), (101), (006)/(102) and (108)/(110) peaks are further magnified and exhibited in Figure 9b-e, respectively. As evident in Figure 9a, the bulk layered structure is well preserved for the two after 200 charge-discharge cycles. Meanwhile, no significant sign of structural distortion and/or formation of any other new phases can be noticed. However, one should particularly note that there exists much more severe fading in diffraction intensities for the NMC in comparison with that for the NMC@NTP, which implies that the bulk or surficial structure sustains a greater degree of destruction. The fluctuated magnitude of (003), representing the variation in the c axis,9, 55 are legibly shown in Figure 9b. Admittedly, the deviation of the (003) peak to lower angle is still somewhat observed in the NMC@NTP, which means the inevitable damage of crystal structure even with the NTP coating. It is 22


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therefore easy to conclude that although the NTP nanoshell cannot absolutely protect the bulk structure from collapse, it greatly inhibits the deviation degree of (003) peak when compared to that for the NMC. Moreover, as plotted in Figure 9c, the (101) peak of the NMC shifts to higher angles, indicating the lattice expansion along the c-axis and the concurrent contraction along the a- and b- axises.43 The semblable circumstance also can be demonstrated by the pair peaks (006)/(102) and (108)/(110) peculiar to hexagonal layered structure,55, 56 where the peak position and diffraction intensity of the NMC show varying degrees of deviation and decay, as shown in Figure 9d and e. In comparison, the XRD pattern of the cycled NMC@NTP remains acceptable under the same case. Reportedly, the emerging Li deficiency on the surface caused by the charged high cut-off voltage, which will trigger the phase transformation from layered to spinel and/or rock salt, can be responsible for the aforementioned peak position deviation.6, 9 As a consequence, the XRD investigation in cycled electrodes forcefully confirms the structural stability whether in the bulk or on the surface of the NMC is tremendously protected by the NTP nanoshell, accounting for the outstanding cyclic stability and rate capability of the NMC@NTP. To further intensively probe the local structural transformation, Figure 10a and b intuitively compare the SEM images of the NMC and NMC@NTP cathodes after consecutive 200 cycles at 0.5 C in a voltage of 3.0 − 4.5 V (vs. Li/Li+). Evidently, the densely agminated, micrometer-sized, block-shape primary particles observed on the surface of original NMC (Figure 3a) are severely destroyed, as shown in Figure 10a. As well known, LiPF6-based liquid electrolyte unavoidably containing trace amounts 23



Page 24 of 46

of water will generate the hazardous HF, which would give rise to the dissolution of electroactive elements from the surficial regions.57 Besides, due to various slip planes and crystallographic orientation of the primary particles, repeated expansion and contraction of lattice volume occurring in the lithiation and de-lithiation processes easily lead to the emergence of microcracks, which in turn serve as fresh reaction sites with the electrolytes.58 Thus, the surficial morphology, suffering from these iterative processes without any remedial measure, is horribly damaged as expected. In sharp contrast, the intact spherical secondary particles coupled with well-defined and visible primary particles are maximumly preserved for the NMC@NTP cathode, as displayed in Figure 10b. More significantly, representative HRTEM studies of the cycled NMC and

NMC@NTP

electrodes

are

implemented

to

evaluate

the

caused

phase-transformation degree and range, as clearly contrasted in Figure 10c and d. In sharp comparison with the original HRTEM image (Figure 4a), the surface of the cycled NMC is wrapped by an insulative, amorphous layer with a thickness of ~5 nm, as shown in Figure 10c, resulting from the uninterrupted accumulation of side-reaction byproducts containing LixPOFy, LiF, and so on,57, 59 which is extremely against the migration of Li+ ions. Moreover, careful observation from the adjacent interior lattice fringes visually confirms that the cation disordered phase, as marked by the red dotted line, occupies a large proportion of the region, which composes of spinel and/or cubic phase,9 and only some fragmental regions can be detected as the layered phase. According to the previous studies,58, 60, 61 the prolonged Li deficiency starting from the surface and the transposition of transition metal ions into the Li layer 24


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undergoing long-term charge/discharge process, particularly under high cut-off voltages, roughly account for the plentiful emergence of cation-mixing phase. This is well responsible for the steep capacity decline of the NMC cathode, as exhibited in Figure 5 and Figure 7. Conversely, we can amazingly find out from the HRTEM images of the cycled NMC@NTP (Figure 10d) that the bulk region firmly continues to be the rhombohedral layered phase with eye-catching (003) plane, and sparse disordered phase can be observed meanwhile. In addition, a NTP SSE coating shell with distinguishable lattice fringes, in good line with the observations in Figure 4e, j, is well retained. Expectedly, only a negligible insulative amorphous layer can be detected in the outermost zone. Thus, the NMC@NTP electrode exhibits superior electrochemical behaviors even after long-term cycle under high cut-off potentials, compared with the bare NMC (Figure 10e), thanks to the promoting role, i.e., a conductive and protective phase, of the NTP SSE in the NMC@NTP, as vividly depicted with straightaway schematic illustration (Figure 10f). In terms of the comprehensive and in-depth investigations above, rational integration of the layered NMC and Nasicon-structured NTP we developed is extremely conducive to the transportation of Li+ ions no matter in the layered bulk and at the NMC-NTP sur-/interfaces. As a result, excellent cyclic stability and rate capability are simultaneously merited for the NMC@NTP. CONCLUSIONS In conclusion, we purposefully designed and developed a novel core-shell NTP@NMC cathode, in which the uniform NASICON-type NTP nanoshell was 25



coated tightly upon the layered NMC core towards high-energy/power LIBs. The NTP SSE nanoshell not only preserved the bulk layered structure from destructive corrosions caused by the side-reactions with the electrolytes but also functioned as high-speed way for Li+ transportation between the electroactive NMC and electrolytes. Furthermore, the uinique interdiffused tunnels between the NMC core and NTP nanoshell are hugely beneficial for the fast Li+ shuttling back and forth. As a consequence, the cyclic stability (1 C) of the NMC@NTP was improved considerably with superior capacity retention of ~93.2% after 150 cycles at the depth charge/discharge rate within higher cut-off voltage range from 3.0 to 4.6 V. Furthermore, the outstanding rate capacities were expectedly observed for the NMC@NTP as well as the superior capacity retention especially at high rates. More importantly, we greatly anticipate the promising design and pilot-scale synthesis strategy we developed can eventually fulfill the advanced LIBs that maximumly boost the development of new energy industry.

ASSOCIATED CONTENT Supporting Information XRD patterns, Rietveld refinement, EDS dot-mapping results, SEM and TEM images, charge-discharge profiles, Equivalent-circuit model, discharge capacities at various C rates with cut-off voltages of 4.4 and 4.5 V, and EIS simulated results

AUTHOR INFORMATION 26


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Corresponding Author *E-mail: [email protected]; [email protected] ORCID Orcid.org/0000-0002-6484-8970

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51772127 and 51772131).

27



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(42) Kosova, N. V.; Devyatkina, E. T.; Kaichev, V. V. Optimization of Ni2+/Ni3+ Ratio in Layered Li(Ni,Mn,Co)O2 Cathodes for Better Electrochemistry. J. Power Sources 2007, 174, 965-969. (43) Li, S.; Fu, X.; Zhou, J.; Han, Y.; Qi, P.; Gao, X.; Feng, X.; Wang, B. An Effective Approach to Improve the Electrochemical Performance of LiNi0.6Co0.2Mn0.2O2 Cathode by an MOF-Derived Coating. J. Mater. Chem. A 2016, 4, 5823-5827. (44) Wu, C.; Kopold, P.; Ding, Y.-L.; van Aken, P. A.; Maier, J.; Yu, Y. Synthesizing Porous NaTi2(PO4)3 Nanoparticles Embedded in 3D Graphene Networks for High-Rate and Long Cycle-Life Sodium Electrodes. ACS Nano 2015, 6, 6610-6618. (45) Liang, L. W.; Wu, C.; Sun, X.; Sun, X. F.; Hou, L. R.; Sun, J. F.; Yuan, C. Z. Sur-/Interface Engineering of Hierarchical LiNi0.6Co0.2Mn0.2O2@LiCoPO4@Graphene Architectures as Promising High-Voltage Cathodes toward Advanced Li-Ion Batteries. Adv. Mater. Interfaces 2017, 4, 1700382. (46) Mao, S.; Huang, X.; Chang, J.; Cui, S.; Zhou, G.; Chen, J. One-Step, Continuous Synthesis of a Spherical Li4Ti5O12/Graphene Composite as an Ultra-Long Cycle Life Lithium-Ion Battery Anode. NPG Asia Mater. 2015, 7, 1-8. (47) Zheng, F.; Ou, X.; Pan, Q.; Xiong, X.; Yang, C.; Fu, Z.; Liu, M. Nanoscale Gadolinium Doped Ceria (GDC) Surface Modification of Li-Rich Layered Oxide as a High Performance Cathode Material for Lithium Ion Batteries. Chem. Eng. J. 2018, 334, 497-507. (48) Son, I. H.; Park, J. H.; Park, S.; Park, K.; Han, S.; Shin, J.; Doo, S. G.; Hwang, Y.; Chang, H.; Choi, J. W. Graphene Balls for Lithium Rechargeable Batteries with Fast Charging and High Volumetric Energy Densities. Nat. Commun. 2017, 8, 1561. (49) Wang, J.; Yu, Y.; Li, B.; Fu, T.; Xie, D.; Cai, J.; Zhao, J. Improving the Electrochemical Properties of LiNi0.5Co0.2Mn0.3O2 at 4.6 V Cutoff Potential by Surface Coating with Li2TiO3 for Lithium-Ion Batteries. Phys. Chem. Chem. Phys. 2015, 17, 32033-32043. (50) Wang, D.; Li, X.; Wang, Z.; Guo, H.; Xu, Y.; Fan, Y. Co-modification of LiNi0.5Co0.2Mn0.3O2 Cathode Materials with Zirconium Substitution and Surface Polypyrrole Coating: towards Superior High Voltage Electrochemical Performances for Lithium Ion Batteries. Electrochim. Acta 2016, 196, 101-109. (51) Yan, G.; Li, X.; Wang, Z.; Guo, H.; Wang, C. Tris(trimethylsilyl)phosphate: A Film-Forming Additive for High Voltage Cathode Material in Lithium-Ion Batteries. J. Power Sources 2014, 248, 1306-1311. (52) Zou, Y.; Yang, X.; Lv, C.; Liu, T.; Xia, Y.; Shang, L.; Waterhouse, G. I.; Yang, D.; Zhang, T. Multishelled Ni-Rich Li(NixCoyMnz)O2 Hollow Fibers with Low Cation Mixing as High-Performance Cathode Materials for Li-Ion Batteries. Adv. Sci. 2017, 4, 1600262. (53) Noh, H. J.; Ju, J. W.; Sun, Y. K. Comparison of Nanorod-Structured Li[Ni0.54Co0.16Mn0.30]O2 with Conventional Cathode Materials for Li-Ion Batteries. ChemSusChem 2014, 7, 245-252. (54) Wu, Z.; Han, X.; Zheng, J.; Wei, Y.; Qiao, R.; Shen, F.; Dai, J.; Hu, L.; Xu, K.; Lin, Y.; Yang, W.; Pan, F. Depolarized and Fully Active Cathode based on Li(Ni0.5Co0.2Mn0.3)O2 Embedded in Carbon Nanotube Network for Advanced 31



Batteries. Nano Lett. 2014, 14, 4700-4706. (55) Yoon, W.-S.; Chung, K. Y.; McBreen, J.; Yang, X.-Q. A Comparative Study on Structural Changes of LiCo1/3Ni1/3Mn1/3O2 and LiNi0.8Co0.15Al0.05O2 during First Charge Using In Situ XRD. Electrochem. Commun. 2006, 8, 1257-1262. (56) Li, D.-C.; Muta, T.; Zhang, L.-Q.; Yoshio, M.; Noguchi, H. Effect of Synthesis Method on the Electrochemical Performance of LiNi1/3Mn1/3Co1/3O2. J. Power Sources 2004, 132, 150-155. (57) Cho, D.-H.; Jo, C.-H.; Cho, W.; Kim, Y.-J.; Yashiro, H.; Sun, Y.-K.; Myung, S.-T. Effect of Residual Lithium Compounds on Layer Ni-Rich Li[Ni0.7Mn0.3]O2. J. Electrochem. Soc. 2014, 161, A920-A926. (58) Kim, H.; Kim, M. G.; Jeong, H. Y.; Nam, H.; Cho, J. A New Coating Method for Alleviating Surface Degradation of LiNi0.6Co0.2Mn0.2O2 Cathode Material: Nanoscale Surface Treatment of Primary Particles. Nano Lett. 2015, 15, 2111-2119. (59) Kim, D.; Lim, J.-M.; Lim, Y.-G.; Yu, J.-S.; Park, M.-S.; Cho, M.; Cho, K. Design of Nickel-rich Layered Oxides Using d Electronic Donor for Redox Reactions. Chem.Mater. 2015, 27, 6450-6456. (60) Kalluri, S.; Yoon, M.; Jo, M.; Liu, H. K.; Dou, S. X.; Cho, J.; Guo, Z. Feasibility of Cathode Surface Coating Technology for High-Energy Lithium-ion and Beyond-Lithium-ion Batteries. Adv. Mater. 2017, 1605807. (61) Gu, M.; Belharouak, I.; Genc, A.; Wang, Z.; Wang, D.; Amine, K.; Gao, F.; Zhou, G.; Thevuthasan, S.; Baer, D. R.; Zhang, J. G.; Browning, N. D.; Liu, J.; Wang, C. Conflicting Roles of Nickel in Controlling Cathode Performance in Lithium Ion Batteries. Nano Lett. 2012, 12, 5186-5191.

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Captions, Tables and Figures

Table 1 Lattice constants of the NMC and NMC@NTP calculated from XRD and X-ray Rietveld refinement

Samples

NMC

NMC@NTP

a / (Å)

2.8722

2.8704

c / (Å)

14.2435

14.2441

c/a

4.9501

4.9624

I(003) / I(104)

1.56

1.48

Ni in Li Site (%)

1.68

1.71

33



Page 34 of 46

Table 2 Peak positions and relative contents of the elemental Ni2+ and Ni3+ in the NMC and NMC@NTP samples

Ni 2p3/2

Samples Peak position (eV) NMC

NMC@NTP

At.%

854.23 (2+)

37.57

855.43 (3+)

28.93

858.70 (2+)

25.97

861.23 (3+)

7.53

854.23 (2+)

33.06

855.43 (3+)

33.64

858.70 (2+)

26.17

861.23 (3+)

7.14

34


Page 35 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Method 24 25 26Nano-LFP 27 coating 28 29 LiAlO2 30 coating 31 Li2TiO3 32 33 coating 34 Zr doping & 35 PPy coating 36 37 TMSP 38 coating 39 40 Multishelled 41 42 43 Nanorod 44 45 46NTP SSE 47 coating 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3 Typical method employed to improve the high-voltage performance of the NMC and the corresponding comparasion in electrochemical properties under 4.5 V and/or 4.6 V 4.5 V

4.6 V Ref.

Initial discharge capacity

Cyclic

Rate

Initial discharge capacity

Cyclic

Rate

181.5 mAh g-1 (1/3 C)

92.8% 150 cycles (1/3 C)

─

189.4 mAh g-1 (1/3 C)

91.5% 150 cycles (1/3 C)

─

7

─

─

─

202.0 mAh g-1 (1 C, 2.7-4.6 V)

91.0% 100 cycles (1 C)

158.5 mAh g-1 (5 C)

20

─

─

─

178.0 mAh g-1 (1 C)

92.4% 100 cycles (1 C)

136.5 mAh g-1 (5 C)

49

─

─

─

182.0 mAh g-1 (1 C)

89.8% 100 cycles (1 C)

135.5 mAh g-1 (5 C)

50

175.6 mAh g-1 (1 C)

90.9% 100 cycles (1 C)

─

─

─

─

51

~190.5 mAh g-1 (0.5 C, 2.5-4.5 V)

94.2% 200 cycles (0.5 C)

─

─

─

─

52

~183 mAh g-1 (0.5 C)

91.7% 100 cycles (0.5 C)

─

─

─

─

53

93.2% 150 cycles (1 C)

168.5 mAh g-1 (5 C) 153.2 mAh g-1 (10 C)

This work

191.9 mAh g-1 (0.5 C)

91.1% 200 cycles (0.5 C)

195.5 mAh g-1 (1 C)

─

Note: LFP for LiFePO4; PPy for polypyrrole; TMSP for tris(trimethylsilyl) phosphate

35



Figure 1. (a) Schematic illustration of the NTP coating over the NMC, (b) XRD patterns of the NMC (dark grey line) and NMC@NTP (orange line) and (c) Rietveld refinement of the NMC@NTP

36


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Figure 2. XPS spectra of (a) Ti 2p for the NMC@NTP, and (b) Ni 2p, (c) Co 2p and (d) Mn 2p for the NMC and NMC@NTP, respectively

37



Figure 3. SEM images of the (a) NMC, (b) NMC@NTP and (c) NMC@NTP-5 under low (displayed in each insets) and high magnifications; TEM images (d-e) and the corresponding EDS dot-mapping results (f-k) of the single NMC@NTP particle; Cross-sectional SEM image of the NMC@NTP and the corresponding elemental change trend of (l) Ni, Co, and Mn and (m) Na, Ti, and P

38


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Figure 4. TEM images of the (a) NMC, (b) NMC@NTP and (c) NMC@NTP-5; (d - j) HRTEM images taken from the white rectangles in panel (a) and (b), respectively; (f i) Enlarged HRTEM images taken from the white rectangles in panel (d) and (e) as indicated; (k) Interfacial schematic diagram for the NMC@NTP cathode

39



Figure 5. Typical charge-discharge profiles performed in the voltage of (a, b) 3.0 − 4.5 V and (c, d) 3.0 − 4.6 V for the (a, c) NMC and (b, d) NMC@NTP. (e, f) Rate behaviors between 3.0 and 4.6 V. Nyquist plots taken from the cells after (g) 200 cycles at 4.5 V and (h) 150 cycles at 4.6 V 40


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Figure 6. Liner fitting of Z´ vs. ω-1/2 in the low-frequency region for the NMC and NMC@NTP electrodes at 4.5 V after 200 cycles and 4.6 V after 150 cycles

41



Figure 7. (a) Cyclic stability and corresponding CE plot performed at 0.2 C for the first three cycles and then at 0.5 C for 200 cycles at 3.0 − 4.5 V. (b) Typical long cycle stability performed at 1 C for 500 cycles at 3.0 − 4.5 V. (c) Cyclic stability performed at 0.2 C for the first three cycles and then at 1 C for 150 cycles at 3.0 − 4.6 V

42


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Figure 8. CV plots (0.1 mV s-1) of the (a) NMC and (b) NMC@NTP electrodes after different cycles. Notedly, the fresh assembled cells are tested from 2.8 to 4.5 V for three cycles to obtain the first three curves, then cycled for another 196 charge-discharge cycles to obtain the 200th CV curve

43



Figure 9. (a) XRD patterns collected from the cycled NMC and NMC@NTP electrodes after 200 cycles at 0.5 C rate in the voltage of 3.0 − 4.5 V, and the homologous magnified diffraction peaks (b) (003), (c) (101), (d) (006)/(102), (e) (108)/(110), respectively

44


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Figure 10. (a, b) SEM and (c, d) HRTEM images of the (a, c) NMC and (b, d) NMC@NTP after 200 cycles at 0.5 C between 3.0 and 4.5 V. (e, f) Schematic illustration of the contribution of NTP SSE as a conductive and/or protective layer to restrain undesired interfacial side reactions

45



Elegant core-shell LiNi0.5Mn0.3Co0.2O2@NaTi2(PO4)3 cathodes were fabricated by a pilot-scale coating strategy, and competitively exhibited superior high-voltage cyclic stability and rate capacity, thanks to remarkable contribution from NASICON-type NaTi2(PO4)3 nanoshell


Page 46 of 46

3 Cathode Enhances Its High-Voltage Cycling ... - ACS Publications

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