A Highly-Stabilized Ni-Rich Cathode Material with Mo Induced

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A Highly-Stabilized Ni-Rich Cathode Material with Mo Induced Epitaxially Grown Nanostructured Hybrid Surface for High Performance Lithium Ion Batteries Chunliu Xu, Wei Xiang, Zhen-Guo Wu, Yadi Xu, Yongchun Li, Yuan Wang, Yao Xiao, Xiaodong Guo, and Benhe Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03403 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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ACS Applied Materials & Interfaces

A Highly-Stabilized Ni-Rich Cathode Material with Mo Induced Epitaxially Grown Nanostructured Hybrid Surface for High Performance Lithium Ion Batteries Chunliu Xu,†, ‡ Wei Xiang,*,†,⊥ Zhenguo Wu, ‡ Yadi Xu, ‡ Yongchun Li, ‡ Yuan Wang, ‡

Yao Xiao, ‡ Xiaodong Guo*,‡,§ and Benhe Zhong‡

†College of Materials

and Chemistry & Chemical Engineering, Chengdu University of Technology,

Chengdu, 610059, PR China ‡School

of Chemical Engineering, Sichuan University, Chengdu, 610065, PR China

§Institute

for Superconducting and Electronic Materials, Australian Institute for Innovative

Materials, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2522, Australia. ⊥Post-doctoral

Mobile Research Center of Ruyuan Hec Technology Corporation, Guangdong,

Ruyuan, 512000, PR China * Corresponding authors: *E-mail addresses: [email protected] (W. Xiang), [email protected] (X.-D. Guo). Tel: +86-28-85406702; Fax: +86-28-85406702.

Abstract Capacity fading induced by unstable surface chemical property and intrinsic structural degradation is a critical challenge for the commercial utilization of Ni rich cathodes. Here, a highly-stabilized Ni-rich cathode with enhanced rate capability and cycle life is constructed by coating molybdenum compound on surface of LiNi0.815Co0.15Al0.035O2 secondary particles. The infused Mo ions in the boundaries not only induce Li2MoO4 layer in the outermost, but also form an epitaxially grown outer surface region with NiO-like phase and enriched content of Mo6+ on bulk phase. The Li2MoO4 layer is expected to reduce residential lithium species and promote the Li+ transfer kinetics. The transition NiO-like phase, as a pillaring layer, could maintain integrity of crystal structure. With the suppressed electrolyte-cathode interfacial side reactions, structure degradation and intergranular cracking, the modified cathode with 1

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1% Mo exhibits a superior discharge capacity of 140 mA h g-1 at 10 C, a superior cycle performance with a capacity retention of 95.7% at 5 C after 250 cycles and a high thermal stability. Key words: surface modification, NiO-like phase, heterostructure, Ni-rich, gradient

Introduction Ni-rich cathode materials have been widely studied due to their relatively high energy density and acceptable cost.1-3 However, their unstable surface chemical property and intrinsic structural degradation give rise to challenging barriers for the further commercial application.1 Residual lithium oxide species (Li2O2, Li2O) remaining on the surface of particle could react with CO2 and H2O when exposed to the air, leading to the formation of insulating or nonconducting lithium compounds layer and thereby increased impedance for Li+ transport.2 Also, the high alkalinity of lithium compounds puts forward a great challenge for the following slurry coating process. During charging/discharging, phase degradation from the layered to rock-salt structure usually occurs at Ni-rich electrode surface due to the inter-migration between lithium and transition metal (TM) ions. The existence of disordered rock-salt like structure not only hinders the intercalation of Li+ during discharging for the occupation of TM ions on Li+ site, leading to severe irreversible capacity loss, but also increases the energy barrier for Li+ diffusion due to its smaller distance between slabs, resulting in low Li+ diffusion rate. In addition, the continuous anisotropic lattice volume change of the primary particle during lithiation/delithiation could easily cause intergranular cracks due to the different crystallographic orientation of primary particles in the aggregate, aggravating the interfacial side reactions and the sluggish electrical conduction.1-3 What’s worse, highly oxidative Ni4+ produced at delithiated state accelerates the reactions responding to the decomposition of electrolyte and the generation of solid electrolyte interface layer on the cathode surface. In response, extensive efforts, such as surface coating4-14, bulk doping15-26 and structure design27-31, have been carried out to overcome these above-mentioned problems. Considering that the interfacial/surface chemistry and structure exert strong 2


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influence on the electrochemical properties of the cathode, surface coating is one of the most effective techniques since the surface layer can physically protect the active material from the electrolyte. Among the various coating materials, electrically and electrochemically active materials with stable physic-chemical characteristics in organic electrolyte are broadly used as coating substances for their fast electron/Li+ conduction. To simultaneously reduce the surface residual lithium species, lithium contained compounds produced by the reaction between surface residual lithium species and coating precursors have been considered as potential coating candidates. In the near past, lithium-reactive coating materials, LixCoPO4, Li3PO4, LixAlPO4 and LiV3O8 were adopted to regulate the interfacial/surface chemistry and structure.11,14,22 Apart from the composition of coating materials, the architecture between surface layer and host material has important effect on the ionic transportation and inhibition of phase transition.27 To substantially minimize the mismatch and promote the interface bonding, the coating layer needs to ensure a similar chemical affinity between the surface layer and host phase. Recently, rock salt like structure which has similar packing array of oxygen with layered structure, was integrated on the layered Ni-rich cathode by using co-precipitated spherical W-doped [NixCoyMn1-x-y](OH)2 precursor.32,33 With the segregation of rock salt phase at the surface, the detrimental reactions between cathode and organic electrolyte are substantially inhibited due to the exclusion of Ni4+ in the rock salt structure.34-36 However, the strategy of introduction rock salt using doped precursor cannot decreases the contents of residual lithium compounds and only constructs a single phase on the granular surface. Thus, it is highly desirable to simultaneously introduce lithium-reactive coating materials and control the interface architecture between surface layer and host material by exploiting coating precursor. In this work, we construct a Mo-modified LiNi0.815Co0.15Al0.035O2 cathode via a wet chemical coating and subsequent calcination process using (NH4)2MoO4 as coating precursor. The introduction of Mo6+ ions at calcination process not only reacts with residual lithium to form Li+ ion conducting Li2MoO4 compound layer, but also enables the formation of an epitaxially grown NiO-like phase in the surface region for the redundant amount of Ni2+ reduced from Ni3+ after the incorporation of Mo6+ into the 3



bulk phase. Thus, the Mo modified Ni rich cathode material exhibits an architecture of epitaxially grown nanostructured hybrid surface composing Li2MoO4 (LMoO), NiOlike phase (Fm-3m) and layered structure (R-3m) with Mo gradient concentration from shell to core (Figure 1). The lithium-reactive coating material LMoO is expected to promote the Li+ transfer kinetics and inhibit the side reaction with electrolyte, while the transition NiO-like layer, as a protective pillar layer, could maintain integrity of crystal structure thus enhancing structure stability and cycle life. Moreover, the increased content of Ni2+ endows the cathode with increased discharge capacity.

Figure1 Schematic diagram of Mo modified LiNi0.815Co0.15Al0.035O2 cathode material.

Experiment Section Preparation of LiNi0.815Co0.15Al0.035O2 powder The LiNi0.815Co0.15Al0.035O2 (NCA) powder was prepared by a high-temperature lithiation process using Ni0.815Co0.15Al0.035(OH)2 precursor synthesized from conventional co-precipitation method. The detailed procedure was presented in our previous research works.5,37 Preparation of Mo-modified NCA cathode To prepare Mo-modified NCA sample, desirable amount of ammonium molybdate was added into suspension consisted of NCA powder. Ethyl alcohol was used as solvent and dispersant. After magnetic stirring, the mixture was heated in thermostat water bath to evaporate solvent. Finally, the obtained powder was calcined at 450 °C for 6 h under air atmosphere after strictly grinding. According to the content of molybdenum (1 or 5% mol based on TM) added, the Mo-modified samples are recorded as NCA-Mo1 and NCA-Mo5, respectively. For comparison, the pristine NCA was subjected to the same 4


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treatment process except the addition of molybdenum, and is denoted as NCA-0. Physical characterizations X-ray diffraction (XRD) was used to detect the details of crystal phase. The data was collected in a scan speed of 3° min-1 with Cu Kα radiation. Refine treatment was performed by PDXL software. X-ray photoelectron spectroscopy (XPS) spectrum were obtained by a spectrometer of Kratos Analytical Ltd. with Al Kα radiation. The spectrum data was fitted with XPS Peak-Fit software. Scanning electron microscope (SEM), transmission electron microscope (TEM) were used to evaluate morphology or microstructure of the materials. Energy dispersive X-ray spectroscopy (EDS) mapping images were obtained by SEM. Selected area electron diffraction (SAED) patterns were obtained by HR-TEM. The fast Fourier transform (FFT) images were obtained via Digital-Micrograph software. To obtain the SEM images on cross section, focused ion beam (FIB) was used to cut the particles. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was carried out to obtain the chemical composition of samples. Differential scanning calorimetry (DSC) measurements were carried out with electrodes soaking in the electrolyte after charging to 4.5 V. Electrochemical testing To investigate the electrochemical performance of the materials, the cell in a coin configuration (CR2025) was assembled with the cathode, separator, lithium plate (or graphite) and electrolyte under Ar atmosphere. The electrode was prepared by casting the slurry composing cathode material, carbon black and binder (80:13:7 by weight) on the Al foil. The loading mass of active material was fixed at 3.5 ± 0.2 mg to ensure the veracity of measurement results. The as-prepared cells were subjected to electrochemical measurements via Neware software. For half cells, the range of cell potential is between 2.7-4.3/4.5 V, while that for full cells is between 2.5-4.25 V. Specially, the current density of 1 C is 180 mA g-1. Electrochemical impedance spectroscopy (EIS) results (100 kHZ-0.1 HZ) were obtained by an electrochemical workstation. The data was fitted via Zview software. Galvanostatic intermittent titration technique (GITT) was conducted to evaluate kinetic properties of electrodes. 5



Results and discussion Figure 2 shows typical SEM images of the samples, which clearly exhibits that the spherical secondary particles for all materials are assembled with hexagon primary grains. It could be found that the secondary particles of NCA-0 NCA-Mo1 and NCAMo5 are all distributed between 6 and 26 µm, and exhibit average particle sizes of 13.1, 13.4 and 13.4 µm, respectively, with negligible differences (Figure S1a-c). More importantly, with increase of Mo content, the primary particles are more tightly packed and possess smoother surface, which could be ascribed to the decreased residential lithium species and formation of LMoO coating layer on the surface after ammonium molybdate modification. NCA-0 and NCA-Mo1 samples exhibit the similar sizes of primary grain. However, the primary grain of NCA-Mo5 with excessive Mo ions shows decreased sizes, which could be attributed to the change of surface energy for primary particles due to the diffusion of Mo ions during calcination.38 The EDS mapping images (Figure S1d-f) indicate that TM and Mo were uniformly overlapped on the spherical particle. The ICP results (Table S1) manifest that the chemical composition of asprepared samples is basically aligned to the designed values.

6


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Figure 2 SEM images of NCA-0 (a), NCA-Mo1 (b) and NCA-Mo5 (c) at different magnifications.

Figure 3a exhibits the XRD patterns of NCA-0, NCA-Mo1 and NCA-Mo5. All diffraction peaks for the samples could be indexed to layered α-NaFeO2 phase with R3m space group, suggesting that the host structure is not significantly affected after Mo modification.22,39 With increase of Mo content, additional tiny peaks belonging to Li2MoO4 which is a fast ion conductor, are observed, especially for NCA-Mo5 sample. The existence of Li2MoO4 verifies that the incorporation of Mo ions could reduce the residual lithium species on the surface of particle. Rietveld refinement results (Figure 3b-d and Table S2) exhibit that lattice parameters and unite cell volume have changed after Mo modification, confirming partial Mo ions have diffused into the bulk after calcination. The enlargement of lattice parameters and volume is attributed to comprehensive effect induced by Mo ions. On one hand, the radius of Mo ion in the valence state of +6 (as confirmed by XPS results), is larger than that of Ni3+, Co3+ and Al3+ (0.59 Å vs. 0.56, 0.55 and 0.54 Å). On the other hand, the existence of Mo6+ in the lattice induces transformation of Ni3+ (0.56 Å) into larger Ni2+ (0.69 Å) to make valence 7



balance. Due to the increased amounts of Ni2+, the values of I(003)/I(104) gradually decrease with the increase of Mo content, indicating the aggravated cation mixing for Mo modified samples. According to our previous work,37 it could be presumed that the cation mixing is related to the generation of NiO-like phase.

Figure 3 The XRD patterns of samples (a) and the Rietveld refinement results of NCA-0 (b), NCAMo1 (c) and NCA-Mo5 (d).

XPS measurements were carried out to investigate influence of Mo modification on valence state of TM ions. The peaks of 235 and 232 eV in Mo 3d spectrum (Figure 4a) indicate presence of Mo6+ for NCA-Mo1 sample.39 And the oxidation states of Co (+3) and Al (+3) are both unchanged after Mo6+ modification (Figure 4b), predicting a reduction in the oxidation state of Ni ions to make charge balance. Figure 4c shows Ni 2p XPS spectra for NCA-0 and NCA-Mo1, which clearly exhibits Ni 2p3/2 peak of NCA-Mo1 shifts to lower binding energy, implying increased content for Ni2+ after incorporating Mo6+ ions. These Ni2+ ions would partially migrate to Li+ sites to form NiO-like phase as a protective layer, pillaring Li slabs to inhibit further migration of TM ions during lithiation/delithiation, especially at high cutoff voltage. Consequently, to further investigate physical-chemical characteristics in the outer surface region of Mo modified particles, XPS analysis within the etching depth of 100 nm for NCA-Mo5 8


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sample was performed. The quantitative analysis results (Figure 4d) indicate the concentration of all elements except Mo are nearly flat near surface of particle. While the concentration of Mo shows a gradient distribution, which gradually decreases from the surface to bulk. The formation of Mo-rich phase on the surface could be attributed to the partial penetration of Mo ions into the bulk during post calcination process. As etching depth increased, Ni 2p3/2 peaks shift towards high binding energy while valence state of Mo almost maintains unchanged (Figure 4e-f), suggesting Ni2+ ions are dominant near surface of particle for Mo modified samples. Combining with XRD analysis, it could be inferred that Mo ions enriched near the surface cause the reduction of Ni ions from +3 to +2, so that partial Ni2+ ions migrate from transition metal slabs to Li slabs, generating a cation mixing structure related to NiO phase near the surface.33,37 The further evidence for the formation of NiO-like phase is provided by following HRTEM analysis.

Figure 4 Mo 3d (a), Co 2p and Al 2p (b) and Ni 2p (c) XPS spectra for NCA-0 and NCA-Mo1. The atomic concentration depth profile based on XPS data for NCA-Mo5 (d). Ni 2p (e) and Mo 3d (f) spectra for NCA-Mo5.

Figure 5 presents the microstructural information of samples obtained by HR-TEM. It could be observed that the detailed structure near the surface of particle for NCAMo1 (Figure 5a) is epitaxially changed from coating layer, transition phase to bulk structure towards core direction. The SAED patterns (Figure 5b) indicate presence of 9



mixed structure consisted of LMoO, Fm-3m and R-3m phase for NCA-Mo1 particle. The local HR-TEM images and corresponding FFT images for site I-III (Figure 5c-h) confirm that coating layer, transition phase and bulk structure belong to Li2MoO4, Fm3m and R-3m phase, respectively. The formation of NiO-like transition phase is mainly ascribed to the migration of partial Ni2+ from TM slabs to Li slabs induced by diffusion of some Mo6+ ions. The epitaxially grown architecture, involved in NiO-like and LMoO layers is conductive to interfacial stability and structure integrity of cathode.33 However, for pristine NCA-0 particle (Figure 5i-k), except a tiny cation-mixing phase is detected on the surface, the whole region is indexed as the layered phase with a space group of R-3m. Overall, combined with XPS and XRD analysis mentioned, it could be concluded that an epitaxial nano layer sequential containing LMoO and NiO-like layer with a higher Mo concentration is localized in the surface region of Mo modified sample.

Figure 5 (a) HR-TEM image of NCA-Mo1 sample. (b) SAED patterns for NCA-Mo1. (c-e) The local HR-TEM images of NCA-Mo1. (f-h) The local FFT images of NCA-Mo1 particle. (i) The HR-TEM image of NCA-0 particle. (j-k) The local FFT images of NCA-0.

Figure 6a shows the initial charge/discharge profiles at 0.1 C in cell potential range of 2.7-4.3 V at 27 °C, which exhibits that NCA-0, NCA-Mo1 and NCA-Mo5 deliver 10


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discharge specific capacity of 194.3, 190.3 and 191.7 mAh g-1 with Coulombic efficiency of 85.8, 84.3 and 84.8%, respectively. The slight difference in capacity values could be ascribed to comprehensive results of electrochemical active Ni2+ and inert Mo6+ ions. However, when the current is larger than 0.5 C, Mo modified electrodes exhibit superior rate capabilities to pristine one (Figure 6b and Figure S2). Specially, NCA-Mo1 shows discharge capacity of 140.5 mAh g-1 at 10 C, which is far higher than 104.0 mAh g-1 of NCA-0. Although NiO phases reported in literature are not conducive to the transportation of Li+, the gradient Mo ion doping only generates a few nanoscaled NiO-like structures on the surface, which could minimize the disadvantages. Meanwhile, the LiMoO fast ionic conductor introduced in the outer layer could exert a positive effect on kinetics of Li+ transportation, resulting in a superior rate performance. Cycle test results of three samples at 1.0 C (Figure 6c) show that NCA-0, NCA-Mo1 and NCA-Mo5 exhibit 137.1, 162.5 and 155.5 mAh g-1 with capacity retention of 79.8, 95.1 and 90.0% after 100 cycles, respectively. The corresponding discharge profiles and dQ/dV curves (Figure S3) indicate that Mo modified samples suffer from smaller cell potential degradation and polarization during cycling. Figure 6d further demonstrates the cycle performance conducted at a rate of 3.0 C at 27 °C, which shows NCA-Mo1 could retain a discharge capacity of 144.5 mAh g-1 with capacity retention of 89.0% after 200 cycles. However, NCA-Mo5 only delivers 75.2% capacity retention, which is even lower than that of NCA-0 (75.3%). This result indicates excess amount of Mo would largely increase the degree of cation mixing, leading to the hindrance for Li+ migration and aggravated electrochemical performance. The long cycle performance at 5.0 C (Figure 6e) displays NCA-Mo1 could deliver 143.8 mAh g-1 discharge capacity corresponding to 95.7% retention after 250 cycles, which is significantly superior to NCA-0 (101.4 mAh g-1, 72.5%). More impressively, a discharge capacity of 126.5 mAh g-1 with capacity retention of 90.1% at 10 C is obtained for NCA-Mo1 after 300 cycles (Figure 6f). The excellent electrochemical properties of NCA-Mo1 are mainly benefited from boosted structural stability ensured by the epitaxial protective layer consisted of nanoscaled LMoO coating layer and NiOlike pillaring layer. To further investigate the stabilization role induced by Mo 11



modification, the NCA and NCA-Mo1 electrodes were cycled at elevated temperature or high cutoff cell potential. The first charge and discharge profiles at 0.1 C within 2.74.5 V (Figure 6g) show discharge capacities of 212.7 and 216.3 mA h g-1 for NCA-0 and NCA-Mo1, corresponding to Coulombic efficiency of 82.1 and 86.0%, respectively. After 100 cycles at 1.0 C, compared with the fast capacity decay for NCA-0, NCAMo1 still exhibits a relatively stable behavior with a capacity of 162.0 mA h g-1 corresponding to retention rate of 84.5% (Figure 6h). Moreover, the cycle stability of NCA-Mo1 is significantly improved when cycled in 2.7-4.3 V at 1.0 C at 55 °C (Figure 6i). NCA-Mo1 shows a capacity of 161.2 mAh g-1 with retention of 85.0% after 100 cycles, which is significantly higher than 112.4 mA h g-1 (60.2% retention) for NCA-0. What’s more, compared with Ni-rich cathodes with similar Ni content reported in published works (Table S3), the electrochemical performance of NCA-Mo1 do outperform most reported results.

Figure 6 (a-b) Initial charging/discharging profiles at 0.1 C and rate performance for all electrodes in 2.7-4.3 V at 27 ℃. (c-d) Cycle performance at 1 C and 3 C for all electrodes in 2.7-4.3 V at 27 ℃. (e-f) Cycle performance for NCA-0 and NCA-Mo1 at 5 C and10 C in 2.7-4.3 V at 27 ℃. (g) Initial 12


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charging/discharging profiles for NCA-0 and NCA-Mo1 at 0.1 C in 2.7-4.5 V at 27 ℃. (h-i) Cycle performance for NCA-0 and NCA-Mo1 at high cut-off cell potential or elevated temperature.

The ex-situ XRD was carried out during the second cycle at 0.1 C to observe the structural change of NCA-Mo1. Typical diffraction peaks belonging to hexagonal phase (Figure 7a and Figure S4) could be observed, which is similar with the reports published.40-43 The dQ/dV curves of NCA-Mo1 (Figure 7b) are served to clearly clarify the (003) peak shift, which exhibits NCA-Mo1 electrode undergoes continuous phase transition involved in the first hexagonal phase (H1), monolinic phase (M), the second hexagonal phase (H2) and the third hexagonal phase (H3).44-45 At the beginning, an irreversible structural transition from H1 to M with the enlargement of c-axis occurs in the range of 3.0 to 3.80 V. Meanwhile, the (003) peak slightly shifts to a lower angle in XRD pattern. Upon further charging (from 3.80 to 4.00 V), the electrode exists as a solid solution phase containing M and H2, accompanying the further expansion of c axis and the shift of (003) to a lower angle in the XRD pattern. Toward the end of the charge (from 4.0 to 4.3 V), the phase transition of H2 to H3 induced by the collapse of LiO6 layers due to the deep delithiation results in the contraction of the c-axis. Thus, the (003) peak shifts back to higher angles in XRD pattern.46During discharging process, the position of the (003) peak nearly shows a symmetrical change trend with the charging process, suggesting excellent structural reversibility for NCA-Mo1 electrode. The lattice parameters obtained via least square refinement are plotted in the Figure S5. It indicates that the unit cell volume of NCA-Mo1 has no obvious change during the charging/ discharging process, further confirming the favorable structural stability.

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Figure 7 (a) Ex-situ XRD patterns of NCA-Mo1 in selected 2θ region during the second charging/discharging process at 0.1 C. (b) The corresponding charging/discharging curves.

SEM images on cross section of the electrodes before and after 100 cycles at 4.5 V are shown in Figure 8. Both samples show densely aggregated primary particles before cycling (Figure 8a-b). However, significant microcracks inside the particle of NCA-0 are observed after 100 cycles (Figure 8c, e). Those cracks could aggravate side reactions between electrolyte and particles as well as electrical insulation, thus accelerating structural degradation and capacity fade.33,47 However, the pulverization phenomenon is effectively suppressed for NCA-Mo1 electrode (Figure 8d, f), which is mainly benefited from the pillar effect of NiO-like layer. Due to the further inhibited migration of transition-metal ions to the lithium slab, the volume change of the grains is significantly suppressed during cycling process. Ni 2p XPS spectra of NCA-0 and NCA-Mo1 electrodes after cycling (Figure S6a-b) both show peaks assigned to NiF2, Ni2O3 and NiO.48 The Ni spectra of the materials before cycling indicate that the content of Ni2+ for NCA-0 and NCA-Mo1 is 25% and 42%, respectively. After 100 cycles, the content of Ni2+ for NCA-0 electrode suddenly increases to 71%, while that for NCAMo1 only shows a slight increase of 17% (from 42% to 59%), indicating that NCAMo1 electrode have a lower Li/Ni mixing level after cycling. EIS Nyquist plots of samples (Figure S7a, c) show that NCA-Mo1 exhibits a slightly smaller value of surface film impedance (Rf) and charge transfer resistance (Rct) than NCA-0 before cycling, whereas the difference for values of Rf and Rct between NCA-0 and NCA-Mo1 is significantly enlarged after 100 cycles (Figure S7b, d). The increased Rf and Rct for NCA-0 could be ascribed to the formation of additional SEI layer induced by side reaction in the new generated microcracks during cycling process. Additionally, GITT measurements after cycling (Figure S8) indicate that NCA-0 shows a lager overpotential response than NCA-Mo1 during charging and discharging, suggesting a favorable kinetic characteristic for NCA-Mo1.30 To investigate the positive role of the epitaxial protective layer in inhibiting formation of SEI film, XPS surface analysis was performed for the electrodes after cycling. The peaks corresponding to C−C, C−H, C−O, C=O and OCO2 bonds could be 14


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observed in the C 1s spectrum of both electrodes (Figure 8g, j). The presence of C-C and C-H bonds is mainly assigned to binder and conductive substance in the electrodes, while C-O, C=O and OCO2 bonds are related to decomposition of electrolyte during electrochemical reaction process.17 The larger peaks of C-O, C=O and OCO2 bonds for NCA-0 electrode, in contrast, implies the formation of an additional SEI film caused by decomposition of carbonate solvent. The F 1s spectra of both electrodes (Figure 8h, k) are divided into five peaks, assigned to LiF, NiF2, LixPOyFz, PVDF and LiPxFy, respectively. It is reported that the generation of LiF and LixPOyFz is ascribed to side reactions between the LiPF6 and the moisture.44 The significantly reduced peaks of LiF and LixPOyFz for NCA-Mo1 electrode further indicates that the growth of SEI layer is effectively suppressed due to the epitaxial protective layer constructed on the surface of particle. In O 1s spectra (Figure 8i, l), typical peaks attributed to M−O (M = Ni, Co and Al), Li2CO3 and ROCO2Li (R represents alkyl) could be observed.17,49 The smaller peak of ROCO2Li for NCA-Mo1 electrode suggests the decomposition of electrolyte and formation of SEI layer could be inhibited after Mo modification.

Figure 8 (a-f) SEM images on cross section of the electrodes before and after 100 cycles at 4.5 V. (g-l) XPS pattern of C 1s, F 1s, and O 1s for NCA-0 and NCA-Mo1electrodes after 100 cycles at 2.7 V.

The ex situ XRD patterns at selected cycles show that split peaks such as (006)/(012) 15



and (018)/(110) for NCA-0 electrode (Figure S9a) are gradually merged into one peak with the increase of cycles, indicating destruction of layered structure.32 Whereas NCAMo1 still exhibits clear peak split for both of (006)/(012) and (018)/(110) even after 100 cycles (Figure S9b), which suggests the crystal structure could be well retained during cycling process. The thermal characteristics of electrodes were investigated by DSC analysis (Figure S10). The higher onset temperature (215 °C) and less heat generation (707 J g-1) of the exothermic peak for NCA-Mo1 electrode compared to those of NCA-0 electrode (185 °C, 1501 J g-1) indicate the thermal stability of NCA cathode could be significantly enhanced after Mo modification. Furthermore, ex situ XRD patterns of electrodes after heat treatment (Figure S11) show (018) and (110) peaks of NCA-0 electrode become one merged peak of (440) at 215 °C, indicating a transformation of structure from layered phase to cubic spinel phase.50 However, NCAMo1 cathode still exhibits a typical layered phase at 215 °C, which further confirms an enhanced thermal stability of NCA-Mo1 cathode. To further verify the outstanding lithium storage capacity of Mo stabilized material, full cell, in a coin type, was assembled with NCA-Mo1/NCA-0 as cathode and commercial graphite as anode (discharge capacity of 370 mAh g-1at 36 mA g-1, Figure 9b). The initial charging/discharging profiles (Figure 9c) show NCA-Mo1 could exhibit a capacity of 189.3 mAh g-1 (based on cathode mass) with coulombic efficiency of 87.3%, which is larger than those of NCA-0 (183 mAh g-1, 84.5%). More impressively, compared with fast capacity fading of NCA-0, NCA-Mo1 exhibits an initial capacity of 161 mA h g-1 with the capacity retention of 94.2% after 150 cycles at 1 C in full cell test. (Figure 9d).

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Figure 9 (a) Schematic model of the full cells. (b) Charging/discharging profiles of the initial three cycles for commercial graphite anode at a current density of 36 mA g-1. (c) Charging/discharging profiles of the first cycle at 0.1 C for NCA-0 and NCA-Mo1 between 2.5 - 4.25 V in full cell test. (d) Cycle performance of NCA-0 and NCA-Mo1 between 2.5 - 4.25 V in full cell test.

Conclusions In summary, an epitaxial grown nanoscaled hybrid layer, which consists of Li2MoO4 layer and NiO-like transition layer, was successfully constructed on the surface region of LiNi0.815Co0.15Al0.035O2 cathode via a wet-chemical coating method and subsequent calcination process using (NH4)2MoO4 as coating precursor. Benefited from a gradient distribution of Mo6+ and formation of Li2MoO4 fast ion conductor, Mo-modified LiNi0.815Co0.15Al0.035O2 cathode shows superior rate performance (140 mA h g-1 at 10 C). The NiO-like transition phase, as a protective pillaring layer, inhibits structural collapse and corrosion of HF thus promoting the structural and thermal stability of materials. Thus, the NCA-Mo1 electrode delivers not only an excellent cyclic performance (a capacity retention of 95.7% after 250 cycles at 5 C) but also a high thermal stability. This strategy to boost structural stability of cathode is very promising 17



and could be further extended to other layered oxide cathodes materials.

Acknowledgement This work was funded by the following organizations: 1. The National Natural Science Foundation of China (21878195, 21805018, 21506133). 2. The National Undergraduate Training Programs for Innovation and Entrepreneurship (201810616085).

Supporting information SEM and EDS mapping images; Rate discharge curves; Discharge profiles and dQ/dV curves at selected cycles; ex stiu XRD patterns at different potentials; Variations of the lattice parameters during charge/discharge process; XPS spectra after cycles; EIS Nyquist plots; GITT testing curves; Ex situ XRD patterns after different cycles; DSC profiles; Ex situ XRD patterns at different temperatures; ICP-AES results; Rietveld refinement results; The comparison of electrochemical performance between other materials reported and NCA-Mo1 cathode.

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A Highly-Stabilized Ni-Rich Cathode Material with Mo Induced

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