Enhancing High-Voltage Performance of Ni-Rich Cathode by Surface

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Energy, Environmental, and Catalysis Applications

Enhancing High-Voltage Performance of Ni-Rich Cathode by Surface Modification of Self-Assembled NASICON-Fast Ion Conductor LiZr2(PO4)3 Jia-feng Zhang, Jianyong Zhang, Xing Ou, Chunhui Wang, Chunli Peng, and Bao Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00389 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Materials & Interfaces

Enhancing High-Voltage Performance of Ni-Rich Cathode by Surface Modification of Self-Assembled NASICON-Fast Ion Conductor LiZr2(PO4)3 Jiafeng Zhang1, Jianyong Zhang1, Xing Ou1,*, Chunhui Wang1, Chunli Peng2, Bao Zhang1 1. School of Metallurgy and Environment, Central South University, Changsha 410083, PR China 2. School of Energy Science and Engineering, Central South University, Changsha 410083, PR China Email: [email protected] Abstract: Coating methodology is commonly employed in the enhancement of Ni-rich cathode for Li-ion batteries (LIBs) as an efficient approach, while its strategy and effect are still great challenges to achieve the success of surface modifications for comprehensive electrochemical properties. In this work, the surface of Ni-rich cathode LiNi0.82Co0.15Al0.03O2 (NCA) is modified by intimately coating of NASICON-type solid electrolyte LiZr2(PO4)3 (LZP) via a facile approach involving electrostatically attraction. With the assistant of well-design architecture, that a uniform NASICON-type LZP nanolayer wrapping over the NCA microsphere, the entire electrode demonstrates exceptional Li+ diffusion and conductivity, and suppresses the side reaction between electrolyte and electroactive NCA, stabilizing the phase interface with less Li+/Ni2+ cation mixing. As a result, the NCA@LZP can deliver high reversible capacity of 182 mAh g-1 at 1C in 2.7-4.3 V, maintaining the capacity retention of 84.6% after 100 cycles. More importantly, the structure stability of NCA is enhanced substantially by surface modification of LZP at high cutoff voltage. It achieves the reversible capacity of 204 mAh g-1 and keeps 100.4 mAh g-1 after 500 cycles at 1C in the potential range of 2.7-4.5 V. This effective strategy of using NASICON-fast ion conductor like LZP as a coating layer, may provide a new insight to modify surface of Ni-rich electrode, improving the rate capability and cyclic performance under high voltage. Key word: surface modification; NASICON-type LiZr2(PO4)3; high voltage; rate capability; Ni-rich cathode; lithium ion battery 1

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1. INTRODUCTION As the world energy system rapidly turns to rely upon electronic system (electric vehicles, EVs) for its daily application, lithium-ion batteries (LIBs) are considered as promising power supplies to satisfy the increasingly demand with volumetric or gravimetric energy, owing to its overwhelming merits of reasonable price, excellent performance and friendly environment.1 Recently, the Ni-rich cathode material LiNixCoxAlzO2 (NCA, x>0.6, x+y+z=1) attracts immense attentions for its ultra-high capacity and low cost, which possesses extraordinary potential to replace the conventional LiCoO2 cathode material for commercial LIBs application.2 Unfortunately, the poor cycling and rate performance restrain the further development of NCA, especially under large rate and high voltage. Specifically, the problem about lithium residual (Li2O or LiOH) coating on the surface of NCA, derived from the chemical adsorption of H2O or CO2 in air, will induce the formation of cathode gel degrading the performance.3-4 Moreover, the side reaction between highly delithiated NCA cathode and organic electrolyte inevitably results in the loss of active material, finally capacity fading and increased interfacial resistance especially under high-voltage condition.5-6 Besides, the intrinsic citation mixing will easily cause highly instability by structural deterioration. Hence, it is emergency to explore a novel strategy to tackle with the practical application of NCA. Currently, numerous works had been applied in the enhancement of NCA. Ion doping as a typically method can suppress the Li+/Ni2+ disorder and improve the structural stability, yet the dissolution of active materials is still needed to address.7 Thus, surface modification is extensively investigated to be another effective approach to mitigate the structure degradation and improve its cycling performance.8-9 Commonly, various coating materials, for instance, Al2O3,10 TiO2,11 SiO2,12 ZrO2,13 La2O3,14 SnO2,15 have been intensively studied to alleviate the side reaction between electrolyte and host materials. However, most modified materials are attributed to the Li-ion insulators with merely formation of uniform coting skins, which exhibit low Li+/electron transport or restricted self-structural stability, resulting in the finite modification effect to poor rate 2


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properties.16-17 It is a great challenge to explore excellent modification layer simultaneously possessed with rapid Li+ conductive and high chemically stability. Thus, the fast lithium ion conductor as prominent coating materials is introduced to modify Ni-rich cathode materials, such as LiTi2(PO4)3,18 Li2SiO3,19 NaTi2(PO4)3,20 Li2TiO3,21 Li2ZrO3,22 LiMnPO4.23 It is noted that lithium ion conductors, NASICON type solid electrolyte LZP, can enhance the Li+ diffusion between electrode and electrolyte, and prevent the side reactions with air and electrolyte, demonstrating the superior structural stability. Particularly, the NASICON-structured LiZr2(PO4)3 (LZP),24-26 as a superior solid electrolyte, presents ultrahigh Li+ conductivity, benefitting from its unique phosphate structure with large interstitial spaces in an open framework and high stability with small volume variation. Therefore, LZP is expected to be an ideal coating material to modify Ni-rich materials with enhanced stability and superior electrochemical performance. However, the majority of coating layers are just adhered or attached insignificantly on the surface of electrode as previous report, which easily exfoliate from the host materials by any inter mechanical force during the cycling, presenting a nonuniform or broken coating film. Furthermore, some coating materials are more likely to agglomerate with uneven distribution, resulting in the unsatisfied modification effect. Herein, with above consideration, the NASICON-fast ion conductor LiZr2(PO4)3 with high stability and exceptional ion conductive is homogeneously wrapped on the NCA surface with a typical core-shell structure by charge attraction. It is well-known that positive charge and negative charge attract each other with strong force by electrostatic charge attraction. It is worthwhile that the surface of LZP precursor endowed with negative charges can be spontaneously attracted by NCA microsphere intrinsically with positive charges. It can make full use of electrostatic charge attraction that LZP nanoparticles are adhered on NCA surface to form the uniformly coating layer by self-created significant force. As a result, this unique strategy advances the reaction kinetic of NCA and enhances the structure stability, resulting in the outstanding electrochemical performance under 3



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high cut-off voltage condition.

2. EXPERIMENTAL SECTION 2.1 Material Synthesis The commercial Ni0.82Co0.15Al0.03(OH)2 fabricated by a co-precipitation method, was utilized as the precursor to obtain LiNi0.82Co0.15Al0.03O2 (NCA) cathode material. Typical, precursor Ni0.82Co0.15Al0.03(OH)2 was mixed with LiOH in a molar ratio of 1:1.05. Then the mixture was firstly calcined at 480°C for 5 h and subsequently sintered to 750°C for 12 h under oxygen atmosphere with a heating rate of 5°C min-1 and finally cooled to room temperature automatically. To obtain NCA@LZP material, the desirable mole ratio of LiH2PO4 and Zr(NO3)4 were dissolved into ethanol solution, respectively, then mixed together to obtain the ivory sol precursor by slowly agitation. Next, NCA powder was introduced into the sol mixture with optimum liquid-solid ratio of 10:1. Then the mixture was agitated at 80°C for 1.5 h until converted into dry slurry with evaporation of ethanol. Then the powder was sintered at 750°C in oxygen atmosphere for 1 h at the heating rate of 5°C. The contents of LZP with 0wt%, 0.5wt%, 1wt%, 3wt% are short for NCA, [email protected]%, NCA@LZP-1%, NCA@LZP-3% respectively. 2.2 Material Characterizations X-ray diffraction (XRD) patterns were recorded with a scanning speed of 10° min−1 by a Rint-2000, Rigaku type X-ray diffractometer, while the refined results were calculated with Rietveld method by the GSAS software.18,27 Morphological characters were analyzed by scanning electron microscopy (SEM, JEOL, JSM-5612LV, 20 kV) equipped with an energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM, JEOL-2100F, 200 kV). X-ray photoelectron spectroscopy (XPS) was taken by Thermo Fisher ESCALAB 250Xi. The NCA particles and LTP precursors were dispersed into the ethanol solution, which was directly measured by Zetasizer Nano ZS (Malvern Instruments) to obtain the Zeta potential of material surface. 2.3 Electrochemical evaluation 4


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Electrochemical performances were measured in the typical CR2025 coin cells for all below. The cathode material was prepared via pasting the mixture of active material, carbon black and PVDF with the weight ratio of 8:1:1. After that, the appropriate N-methyl-2-pyrrolidone (NMP) was added into mixture to form a homogeneous slurry by milling, then immediately filmed on current collector (aluminum foil) followed by drying at 120°C in vacuum to remove the NMP. The current collector was then cut into a circle of 12 mm with the mass loading of 3-4 mg cm2, then assembled in a glovebox with argon atmosphere protection. The metallic Li tablet is served as counter electrode, which is normalized and tested under the same condition. The electrolyte was 1 M LiPF6 dissolved in EC+DMC+EMC (1:1:1 in volume) solution. The separator (celgard2325) was sheared into 18mm circle. The galvanostatic charge/discharge was measured between 2.7 and 4.3 V (versus Li/Li+) on a Land CT2001A instruments. A CHI660E electrochemical workstation was used to characterize the electrochemical impedance spectroscopy (EIS) over the frequency range of 100000 Hz - 0.01 Hz at fully discharge states of 2.7 V and cyclic voltammetry (CV) in the potential window of 2.7-4.3 V.

3. RESULTS AND DISCUSSION The specific procedure of NCA@LZP preparation is explicitly illustrated in Figure 1. It is well-known that two opposite charges (negative and positive) will attract each other through the electrostatic attraction, which is suitable for nanoparticles applied in synthesis of self-assembled materials,28-29 as showed in Figure 1a. According to the Zeta potential measurement, it is found that the Zeta potential value of NCA and LZP precursor are -47.5 mV and +26.1 mV, respectively (Figure S1 and Figure S2). It is obvious that the surfaces of pristine NCA and LZP precursor particles display opposite charges, providing a strong effect of leading LZP precursor to NCA surface by electrostatic attraction (Figure 1b), resulting in a robust attach of LZP precursor on NCA surface and the formation of a thin shell layer (Figure 1c). During the whole process, electrostatic 5



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attraction can facilitate to form a uniform coating LZP layer on the NCA surface continuously with even dispersion of LZP particle and accurate coating amount, constructing a chemically stable core-shell structure with high conductivity.

Figure 1. Schematic synthesis of LZP cladding NCA cathode material by charge attraction. The XRD test was used to investigate the coating effects of LZP on NCA crystal structure. As displayed in Figure 2b and Figure S3b, the XRD patterns for all samples demonstrate similar profiles and can be matched well with hexagonal α-NaFeO2 structure with R3̅m space group.18,30 It is noted that there no distinctive difference between LZP-modified NCA and pristine NCA, suggesting that surface modification of LZP has little influence on crystal structure of NCA. This result indexes that coating with LZP maintain the pristine layered structure of NCA,27 which might be attributed to the low concentration of LZP thin layer wrapping on the NCA without phase transformation. Additionally, the individual synthesis procedure of LZP is analyzed by TGA test (Figure S4),

6


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confirming the formation of NASICON-type fast ion conductor LiZr2(PO4)3 with high crystalline structure, as presented by XRD (Figure S5).

Figure 2. (a) The layer structure of LiNi0.82Co0.15Al0.03O2 (NCA), (b) XRD patterns for pristine NCA and NCA@LZP-1%. XRD Rietveld refinement results of (c) pristine NCA and (d) NCA@LZP-1%. As verified, the ionic radius of Ni2+ (0.69 Å) is similar to Li+ (0.76 Å), thus Ni2+ ions can easily migrate to Li slabs, accompanied with the generation of cation mixing,31 which is distinctly revealed by the ratio of I(003)/I(104). It is confirmed that the higher ratio of I(003)/I(104) than 1.2 indicates a well-ordered layer structure for NCA cathode.20 Apparently, the value of I(003)/I (104) ratio for all samples are higher than 1.2, as listed in Table S1, whereas the value of I

(003)/I (104)

ratio for

modified NCA@LZP (>1.6) is much higher than that of pristine NCA (1.56). Moreover, the Rietveld refinement results are displayed in Table S2 and Figure 2c, d. The low error factors (Rp 7



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and Rwp) confirm the convince of calculated results. It is observed that the occupancy of Ni2+ in Li site for NCA@LZP-1% is only about 0.89%, much lower than that of pristine NCA sample (3.32%), further revealing that LZP-modification obviously decreases the cation mixing and maintains the well-ordered layer structure, which is in accordance with the result of I

(003)/I (104)

ratio. It is found

that the (003) peak slightly shifts to high angles with the increasing ratios of LZP coating (Figure S3a), indicating the variation of cell phase. This phenomenon may be assigned to the trace doping of Zr4+ or PO43- ions into the crystal lattice of LiNi0.82Co0.15Al0.03O2 during the coating process.32 The improved performance may be assigned to the decreasing of lithium residual by uniform distribution of LZP,33 which can reduce the side reaction between air and NCA, effectively mitigating the cation mixing of Ni2+/Li+ and preserving the layered structure. The well-ordered layer structure provides an excellent Li+ diffusion path and increases the structure stability. To better observe the shell layer of LZP on NCA surface and demonstrate the coating effect of charge attraction, the morphology of pristine NCA and LZP-modified NCA samples are investigated by SEM and TEM. As depicted in Figure 3a-e and Figure S6, the spherical secondary particle with average diameter of 15 μm is aggregated by numerous primary particles with size of around 300 nm in diameter. Particularly, the rough surface of pristine NCA particle gets smoother with the higher content of LZP coating materials. It is obvious that the pristine NCA sample demonstrates a rough skin as presented in magnified SEM image (Figure 3b). As a contrast, the modified NCA@LZP-1% sample exhibits an exquisite surface with a smooth shell, owing to the formation of a uniform LZP outer layer on the rough surface of NCA particles.

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Figure 3. Full and magnified SEM image for (a, b) pristine NCA and (c, d) NCA@LZP-1%. (e, f) EDS mapping image of NCA@LZP-1%. TEM, HRTEM and SAED images for (g-i) pristine NCA and (j-l) NCA@LZP-1%. Furthermore, the elemental distribution of NCA@LZP-1% is analyzed by EDS mapping (Figure 3f). Obviously, the distribution of Zr and P elements are evenly overlapped with the element of Ni, 9



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Co and Al, indicating that the uniform LZP layer is homogeneous coated on the surface of NCA. In order to further investigate the surface modification, TEM test is performed to confirm the coating layer of LZP. For pristine NCA sample, it exhibits a clean and bare surface with obvious crystallinity at edges (Figure 3g). As magnified TEM at edge area, the clearly lattice spacing of 0.472 nm corresponds to the (003) plane of NCA (Figure 3h), which is also confirmed by SAED patterns (Figure 3i). In contrast, a thin cladding layer with a thickness of approximate 10 nm is obviously observed on surface of NCA@LZP-1% sample (Figure 3j). Besides, the Figure 3k clearly depicts the modification layer, which can be discerned by different lattice fringes between cladding layer and NCA material. Specifically, the lattice fringe in the out layer with 0.318 nm is corresponding to (400) lattice plane of LZP. Whereas a set of lattice space of 0.235 nm is fitted with (012) lattice plane of R-3m layered structure in the inner part of NCA particle. Additionally, the modification layer can also be recognized by two different SAED images (Figure 3l). The marked spot in the outer shell is matched with (600), (111), (42-1), (33-2) plane of LZP, confirming a uniformly outer layer on NCA surface. The SEM/TEM results illustrate the integrally construction of core-shell structure by charge attraction, forming a homogeneous and stable layer of LZP on the inner NCA cathode, which can significantly facilitate the Li+ diffusion and maintain the structural stability of entire electrode. To further verify the existence of LZP, the XPS spectra measurement was conducted to analyze the chemical states and surface valence. There are mainly four elements of Ni, Co, Al and O detected in pristine NCA, while additional Zr and P elements are probed on surface of NCA@LZP-1% as shown in Figure 4. The Co XPS spectra for both samples display the binding energy of Co 2p1/2 (795.0 eV) and Co 2p2/3 (779.8 eV), indicating the trivalent valence of Co3+ (Figure 4b).34 Moreover, 10


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a broad peak of Al spectrum at approximately 73 eV is corresponded to Al 2p3/2, confirming the trivalent state for Al (Figure 4c).35 Besides, the two peaks at 856.8 and 873.4 eV are attributed to the Ni 2p3/2 and Ni 2p1/2, while the Ni 2p1/2 can be further deconvoluted into two peaks centered at 855.4 eV and 855.7 eV,36 corresponding to the different valence Ni2+ and Ni3+, respectively. It is known that the area of peak intensity is associated with the quantity of various Ni2+/Ni3+valences.37 As calculated from Figure 4d, the content of Ni2+ of 19.74% for NCA@LZP-1% is much lower than that of pristine NCA of 31.66%, which is in agreement with the XRD refinement，suggesting that the higher ratio of Ni3+ keeps the electrochemical activity of NCA and retards the cation mixing for similar ionic radius of Li+/Ni2+(0.76 Å/0.69 Å).38 The cation mixing of Li+/Ni2+ caused by the side reaction of lithium residual is inhibited after LZP modification, which is confirmed by low amount of Ni2+, indicating that the LZP coating layer effectively reduce the Ni2+ ratio. Additionally, the O1s peak of pristine NCA at 531.6 eV and 529 eV are related to the lattice oxygen for Li2CO3 and NCA (Figure 4e). In contrast, the O 1s peak of NCA displays a sharper peak at ~531.6 eV, much higher than that of pristine NCA@LZP-1% at 529 eV, revealing the existence of more oxygen on the surface of pristine NCA materials,39 which can be reacted with CO2 and formed Li2CO3 based on lithium residual. Furthermore, to confirm the existence of LZP on NCA surface, the Zr 3d peaks of 181.95 eV and 184.4 eV (Figure 4f), and P 2p peak of 133.4 eV (Figure 4g) are clearly detected,40 indicating that the valence state of Zr 3d and P 2p are assigned to Zr4+ and P3+, respectively, which are in accordance with LiZr2(PO4)3. Therefore, the XPS spectra reveal that the obtained NCA@LZP composite displays a high ratio of Ni3+ with less lithium residual, which is wrapped by the ultrathin coating layer of LZP. 11



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Figure 4. XPS spectrum of (a) full pattern, (b) Ni 2p, (c) Co 2p, (d) Al 2p, (e) O 1s for pristine NCA and NCA@LZP-1%, (f) Zr 3d and (P) 2p for NCA@LZP-1%. Through the unique modified strategy, LZP-modified NCA composite is expected to present advanced electrochemical properties. CV curves during the initial three cycles at 2.7-4.3 V for pristine sample and LZP-modified [email protected]%, NCA@LZP-1%, NCA@LZP-3% samples are displayed in Figure 5a, b and Figure S7. Three anodic peaks located at 3.774, 4.010 and 4.213 V, accompanied with the corresponding cathodic peaks at 3.671, 3.959 and 4.121 V, are assigned to the phase transformation of H1-M, M-H2 and H2-H3,36,41 respectively, which is caused by the phase transition from hexagonal to monoclinic (H1 to M), monoclinic to hexagonal (M toH2) and hexagonal to hexagonal (H2 to H3) during the Li-ions extraction process.41 Furthermore, the intervals between oxidation peak (around 3.9 V) and the reduction peak (around 3.7 V) during the initial three cycles are calculated as listed in Table S3, which illustrates the potential variation and polarization degree, indicating the electrochemical kinetics.42 As displayed in Figure 5a and b, the 12


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intervals during the first cycle (Δ1) for pristine NCA is 0.261 V, which is larger than that of modified NCA@LZP-1% of 0.165 V. Remarkably, the potential variation of NCA@LZP-1% during the second (Δ2) and third (Δ3) cycle drastically decreases to 0.009 V and 0.017 V, respectively, while the subsequent cycles exhibit highly overlapped, suggesting the lower polarization after LZP-modification. The distinct depolarization of NCA@LZP-1% with symmetrical and sharp CV peaks can be assigned to the enhanced reaction kinetics and minimized transfer resistance during continuous lithiation/delithiation, resulting from the uniform wrapping layer of LZP with interconnected diffusion channels of Li+ ions, which facilitates the Li+ transport at interface and LZP stabilizes the layered structure of NCA. As results, the NCA@LZP-1% cathode is anticipated to achieve better cycling stability and exceptional rate performance. Additionally, Figure 5c compares the charge/discharge profiles for pristine NCA and LZP-modified NCA at 1C after electrode activation with two cycling at 0.1C (1C = 200 mA g-1) in the potential range of 2.7-4.3 V. It is clearly observed that the discharge capacity of pristine NCA (185.7 mA h g−1) and the 183.2 and 182.0 mA h g−1 correspond for [email protected]% and NCA@LZP-1%, respectively, while the NCA@LZP-3% delivers the lowest discharge capacity of 172.2 mA h g−1 owing to the high contents of LZP wrapping. This is attributed to the electrochemical inactive of LZP under high voltage range, leading to the decrease of active materials relatively.

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Figure 5. The CV curves for (a) pristine NCA and (b) NCA@LZP-1% during the initial 3 cycles. (c) The charge-discharge curve at 1C and (d) rate performance for pristine NCA, [email protected]%, NCA@LZP-1%, NCA@LZP-3%. (e) The cycling properties for NCA and NCA@LZP-1% at 1C. Additionally, the rate capabilities of pristine NCA and LZP-modified NCA are also investigated as shown in Figure 5d. The cells are tested with climbed rate from 0.1C to 5C in 2.7-4.3 V, accompanied with the gradually decreased discharge capacity in terms of electrode polarization.14 It is clear that the pristine NCA exhibit inferior rate performance with sharply dropped reversible 14


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capacity, especially at ultrahigh rate of 5 C (130.5 mA h/g), resulting from the side reaction of electrolyte with degradation structure and increased resistance. In contrast, NCA@LZP-1% sample demonstrates the best rate ability with highest discharge capacity (151.7 mA h/g) at 5 C. Furthermore, when returned back to 0.1C, the discharge capacity of NCA@LZP-1% can recover to 202.8 mA h/g, which is levered off the pristine capacity, suggesting the superior rate tolerance. To confirm the impact of LZP-modification for enhancing the structure stability, the long-cycling performance was also measured. The significant distinction between pristine NCA and modified NCA is the capacity retention, which is effectively improved by the surface modification. When tested at 0.1C (Figure S8b), although displaying the same initial capacity, the NCA@LZP-1% can maintain the capacity of 196.4 mA h g−1 with capacity retention of 94.8% after 35 cycles, efficiently higher than pristine sample (90.4%). The enhancement of cycling stability is more obvious as measured at 1C in 2.7-4.3 V (Figure 5e), the pristine NCA@LZP preserves a discharge capacity of 153.9 mA h g−1 after 100 cycles, with capacity retention of 84.6%, much higher than that of pristine NCA (just 69.4%). Moreover, it maintains 120 mA h g−1 after long-term cycling of 300 cycles at voltage of 2.7-4.3 in 1C. In contrast, the pristine NCA merely retains 120 mA h g−1 merely with 150 cycles, further proving the robust function by surface modification of LZP, which offers electro/ion diffusion paths with better electrical/ionic conductivity and decrease the Li-residue layer with enhanced stable structure.20 Surprisingly, the NCA@LZP-1% demonstrates outstanding electrochemical properties even at long-term cycling with high rate of 5C in Figure S6. It obtains the reversible capacity of NCA@LZP-1% can achieve a high capacity of 101.7mA h g−1 after cycling of 300 cycles, but bare one only reaches 42.7 mA h g−1 after 300 cycles. Compared with other reported 15



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modified NCA in literatures as listed in Figure S9, it is obviously LZP-modified NCA presents superior performance in the long-term cycling with high rate in this work.43-50 More importantly, the high-voltage properties of NCA are also significantly improved by LZP-modification. Figure 6a displays the cycling property of pristine NCA and NCA@LZP-1% electrodes at 1C after activation with three cycles at 0.1C in 2.7-4.5 V. It is noted that the initial discharge capacities for both samples are improved after raise the cut-off voltage. The pristine NCA can reach 230.9 and 206.3 mA h g-1 at 0.1C and 1C, respectively, while it rapidly degrades to 36.2 mA h g-1 after 500 cycles at 1C in 2.7-4.5 V, suggesting the poor cycling stability of NCA at high voltage. On the contrary, the NCA@LZP-1% exhibits a superior stability with high discharge capacity of 100.4 mA h g-1 after long cycling for 500 cycles, demonstrating the prominent structure stability after LZP-modification. In addition, the charge/discharge curves at 1C in voltage from 2.7 to 4.5 V for pristine NCA and NCA@LZP-1% at different cycles of 1st, 4th, 50th, 100th and 200th, respectively. It is clearly observed that the working voltage plateau for pristine declines rapidly, with only 3.44 V after 200 cycles, as presented in Figure S10a. However, the LZP-modified NCA still maintain the high voltage plateau of 3.64 V (Figure S10b), with degradation of 1.56% compared with the pristine state.51 This result demonstrates that the voltage decrease of NCA is faster than that of NCA@LZP-1%, confirming the trouble of voltage drop is effectively mitigated by surface-modification of LZP, which results in the high energy density under high-voltage condition. It is common believed that voltage decay is associated with the phase transition from layered to spinel-like phase by the migration of transition metal ions.52 The main reason for mitigation of voltage fading of 16


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NCA@LZP-1% is attributed to the bulk structure effect of LZP coating, which effectively inhibits the formation of the spinel-like phase caused by migration of transition metal ions, resulting in the enhanced structural stability.53 Hence, it can be calculated to be 744.1 and 766.8 Wh kg-1 for the specific energy of pristine NCA and NCA@LZP-1% electrodes at 4th cycle (Figure S11). With the long-term cycling, the energy density of pristine NCA exhibits obvious attenuation with 353.0 Wh kg-1, much lower than that of NCA@LZP-1% with 566.8 Wh kg-1 after 200 cycles. All the superior electrochemical performance is attributed to the uniform LZP-modification layer, especially at high voltage and ultrahigh rate.

Figure 6. (a) The cycle performance of pristine NCA and NCA@LZP-1% at 1C in voltage range of 2.7-4.5V, and their corresponding charge/discharge profiles at 1st, 4th, 50th, 100th and 200th cycle for (b) pristine NCA and (c) NCA@LZP-1%. To better comprehend the remarkable rate characters at high voltage, the interfacial 17



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electrochemistry properties are further studied by electrochemical impedance spectroscopy (EIS). As displayed in Figure 7a and 7b, both samples after various cycling present the EIS plot, which can be delineated by employed equivalent circuit model (Figure 7g).10,54 The Nyquist plots include an oblique line at low-frequency and semicircles at high-intermediate frequency regions, respectively, assigning to the charge-transfer resistance (Rct) for electrochemical reactions at the interface of electrolyte/electrodes, and Warburg impedance (Zw) for Li+ ions diffusion.55 Besides, Rs and Rsf are attributed to the solvent electrolyte resistance and surface-film resistances, while the fitted results are listed in Table S4. It is observed that the interface resistance values of Rsf+Rct for pristine NCA is rapidly increasing from 79.3 Ω (4th) to 902.7 Ω (300th), suggesting the upsurge resistance along with cycling, On the contrary, NCA@LZP-1% shows a tardy increase about resistance value. Besides, it exhibits a relatively small (Rsf+Rct) value of 85.0 Ω, after 300 cycles, which is only one tenth of pristine NCA, indicating the superior charge transfer rate after long-term cycling. Furthermore, the Li+ ion diffusion coefficients (DLi+) are estimated by the following formula 14,18,30.

DLi+ = R2T2/2A2n4F4C2σ2

(1)

Where R, T, A, n, F, C represent the gas constant, absolute temperature (298 K), electrode surface area (1.13 cm2), number of electrons (n=1, Li+), Faraday constant (96500), concentration of Li+ (0.001 mol cm-3 in this work). While σ is Warburg coefficient calculated from the formula below (Figure 7c): Z´ = Rs + Rct + σω-1/2

(2)

Where Z´ and ω stand for the real impedance and angular frequency. According to the 18


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aforementioned formulas, the calculated DLi+ of NCA@LZP-1% (4.03×10-16 cm2 S-1) is much higher than that of pristine NCA (1.94×10-17 cm2 S-1) even after 300 cycles as presented in Table S4, revealing the high Li+ diffusion coefficient and excellent reaction kinetics after LZP-modification.

Figure 7. EIS Nyquist plots, the liner fitting of Z’ vs ω−1/2 after various cycles, CV curves at the different scan rates and the liner fitting relationship of Ip and v1/2 for (a, c, d, f) pristine NCA and (b, c, e, f) NCA@LZP-1% composite. (g) The equivalent circuit model derived from the Nyquist plots, (h) schematic illustration of superior Li+ diffusion path for NCA@LZP-1%. Additionally, to further confirm the enhanced Li-diffusivity, the CV tests of pristine NCA and NCA@LZP-1% at different scan rates are performed in Figure 7d and 7e. As the increase of scan rate, the cathodic and anodic peaks shift to the lower and higher potentials, respectively, indicating 19



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the behavior of diffusion-controlled at high rate. According to the relationship of scan rates (ν1/2) versus the peak current (Ip), the DLi+ of NCA and NCA@LZP-1% can be estimated as followed Randles–Sevcik equation56,57: Ip =2.68× 105× n3/2 ×A×DLi1/2×ν1/2×C0

(3)

Ditto, the DLi+ can be calculated according to these parameters, as presented in Table S5. The shift of corresponding redox peaks is assigned to the effects of polarization and irreversible behaviors, where NCA@LZP-1% (Figure 7d) displays the small variation than that of pristine NCA (Figure 7e). Therefore, the NCA@LZP-1% indicates the higher lithium diffusion rate than pristine NCA, no matter lithiation or delithiation process, which is in agreement with EIS results. As illustrated in Figure 7h, LZP, as an outstanding lithium ion transfer solid electrolyte, exhibits the excellent ionic conductivity. While the uniform LZP coating layers on NCA surface will significantly modify the surface interface, effectively diminishing the resistance of electrode and accelerating lithium transfer rate. This result is further demonstrating that LZP-modification offers a high-speed Li+ diffusion path, leading to the prominent stability at high potential with high rate, which is accounted for the improved performance at high rate under high-voltage condition. As previous reported, the NCA microsphere consisted of densely agminated primary particles are easily destroyed by the electrolyte corrosion,58 especially under the high-voltage condition. Specifically, small trace of water in the LiPF6-based electrolyte will unavoidably produce the hazardous HF, resulting the dissolution of electro-active materials. Moreover, the irreversible phase transformation will lead to the structural degradation as well as emergence of microcracks, suffering from the severe fade of electrochemical performance. To better probe the influence of 20


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LZP-modification, the pristine NCA andNCA@LZP-1% after 200 cycles under 2.7-4.5 V at 1C, are exhaustively tested to investigate the intrinsic structural transformation under the high-voltage condition. As displayed in SEM images (Figure 8e and 8g), it is distinctly observed that the spherical microsphere of aggregated particles is pulverized into several fragments, which will easily disperse into electrolyte or lost connection with current collector, leading to the poor conductive and electrochemical activity. Moreover, as presented in TEM images (Figure 8a and 8b), it exhibits a disordered structure in the mixing area and amorphous residue phase in the NCA surface, illustrating the severe structure collapse during continuous cycling under sharp high-voltage condition.59 It is confirmed that the increase of Ni2+/Li+ disorder and interfacial residues in Ni-rich materials will lead to the augment of conductive resistance, loss of reversible capacity and poor rate capability.58-60 It is proved that the surficial morphology and intrinsic structure of pristine NCA suffers from the severe damage, attributing to the intrinsic properties of NCA. However, it is found that the NCA@LZP-1% sample still maintains its original microsphere morphology without physical separation and isolation of grains (Figure 8f and 8h) after the intensive charge/discharge process. The outer layer of LZP, serving as a robust shell to protect from HF attack and facilitate the Li+ transport with reduced resistant, which is well corresponded to the EIS result. Meanwhile, the NCA@LZP-1% displays a well-ordered structure with a uniform coating layer of approximate 10 nm, which can be indexed into LZP (Figure 8c and 8d). Compared with pristine NCA, it is hardly observed the formation of rock-salt phase in NCA@LZP-1%, which results in the poor stability and deterioration performance. Additionally, based on the results of XRD patterns after 200 cycles (Figure 8j), the I(003)/I (104) ratio of NCA is much lower than the NCA@LZP-1%, further 21



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confirming the well-maintaining of layer structure for NCA@LZP-1%. It is noted that the infusion of solid electrolyte LZP in the grain boundary will enhance the fast Li+ diffusion, as well as mitigate the side reaction between electrolyte and NCA, preventing the transformation from layered structure to spinel phase and maintaining the structure integrity, especially under the high cut-off cycling condition.9,61

Figure 8. The HRTEM, SEM images and XRD patterns of (a, b, e, g, j) pristine NCA and (c, d, f, h, j) NCA@LZP-1% after 200 cycles under 2.7-4.5 V at 1C. (i) Schematic illustration of structure collapse with side reaction caused by electrolyte and lithium residual. 22


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Hence, the uniform coating layer of LZP can effectively mitigate the detrimental HF etching by side reactions between electrolyte and active electrode, maintaining the integral configuration with high chemical activity, as illustrated in Figure 8i. Besides, the LZP layer can alleviate the dissolving of active elements in electrolyte and remain its stability at high voltage during long-term cycling, acting like a shell to protect NCA cathode from structural deterioration. The existence of LZP-layer can reduce electrode polarization and voltage reduction, keeping the advantages of Ni-rich materials. More importantly, the NASICON-type fast ion conductor can facilitate Li+ migration rapidly by increasing the ionic conductivity, especially at the electrolyte/LZP interface at NCA surface, effectively reduce the transport resistance of entire electrode. As a result, NCA@LZP delivers the exceptional electrochemical performance, particularly the rate capability and cycle stability under high-voltage simultaneously. 4. CONCLUSIONS In summary, the LZP-modified NCA cathode materials are successfully fabricated by unique charge attraction method, ensuring the uniform coating of LZP-nanolayers on the NCA surface. The impact of LZP coating on the physicochemical characters and electrochemical performances are investigated. As expected, LZP, served as a robust protective layer, can mitigate the side reaction and formation of corrosive by-product, which benefits the LZP-modified NCA sample from possession of distinguished endurance and adamant stability under high voltage condition. Furthermore, LZP as the superior ionic conductivity simultaneously, can facilitate the Li+ diffusion at the electrolyte/LZP interface at NCA surface, significantly enhancing the reaction kinetic. Additionally, the electrode polarization and voltage degradation of NCA electrodes are also effectively alleviated owing to the 23



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LZP-modification. The electrochemical tests indicate that the LZP-modification with appropriate amounts (NCA@LZP-1%) can improve cycling stability (from 69.8% to 84.6% after 100 cycles at 1C), and strengthen the rate capability (151.1 mA h g-1 at 5C). Particularly, it delivers a highly cycling stability at high rate of 1C under cut-off voltage of 2.7-4.5 V (100.4 mA h g-1 after 500 cycles). Hence, the strategy of charge attraction promisingly provides a novel approach that simultaneous removal of surface lithium residues and formation of NASICON-fast ion conductor coating layer, which is highly desirable for Ni-rich cathode materials. ASSOCIATED CONTENT Supporting information Additional details of Zeta potential, XRD pattern, TG-DTA curves of LZP precursor and the cycling performance for pristine NCA, [email protected]%, NCA@LZP-1%, NCA@LZP-3% at 1C. Tables for comparison of rate capability with others reported in the literatures and results of electrochemical impedance. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(X. Ou). Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51472272，51772334, 51778627, 51822812) 24


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of

LiNi0.6Co0.2Mn0.2O2

Cathode

at

High

29


Cutoff

Voltage

by

Modifying


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Graphic abstract

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Enhancing High-Voltage Performance of Ni-Rich Cathode by Surface

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