Constructing protective pillaring layer by incorporating gradient Mn4+

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Energy, Environmental, and Catalysis Applications 4+

Constructing protective pillaring layer by incorporating gradient Mn to stabilize the surface/interfacial structure of LiNi Co Al O cathode 0.815

0.15

0.035

2

Chun-Liu Xu, Wei Xiang, Zhen-Guo Wu, Ya-Di Xu, Yong-Chun Li, Mingzhe Chen, Guo Xiaodong, Gen-Pin Lv, Jun Zhang, and Ben-He Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10372 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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ACS Applied Materials & Interfaces

structure, the cathode with gradient Mn4+ exhibits more stable cycling behaviour with a capacity retention of 80.0% after 500 cycles at 5.0 C. KEYWORDS:

lithium ion

batteries,

NCA,

surface

modification,

gradient

concentration, NiO-like phase, pillar effect, structure stability

Introduction To satisfy the requirements of electric vehicles and electrical energy storage systems, lithium-ion batteries (LIBs) with high energy density, excellent electrochemical performance and reliable safety have been demanded.1-2 The lack of cathode materials with reliable electrochemical performance has cause a significant bottleneck for the development of next generation LIBs.3 Among the numerous cathode materials, nickel-rich cathodes have been regarded as one of the most promising substitutes for LiCoO2 due to their high energy density, reduced cost and low toxicity1-3 However, their insufficient cycle life and poor thermal characteristics pose challenging hurdles for the commercial utilization. In general, structural degradation from the layered phase to spinel phase and eventually to rock salt phase firstly occur on the surface of particle and gradually spread to the bulk phase during lithiation process, especially at elevated temperature or high voltage.4 The structural degradation is stemmed from the fact that Ni2+ in the transition metal sites could migrate into lithium layer during delithiation and lithiation processes for the closed ionic radius between Li+ (0.76 Å ) and Ni2+ (0.69 Å ).5 As the occupation of Ni2+ on lithium slabs hinders the intercalation/deintercalation of Li+, high activation energy will be required for Li+ transportation in the disordered NiO-like structure layer, resulting in increased cell impedance and capacity fading.6 In addition, large amount of highly reactive Ni4+ predominated at high cutoff voltage will accelerate unexpected reactions between solid and solution, leading to the consumption of active material, capacity fading and gas evolution.7 In response, many efforts have been devoted to improving electrochemical performance of the nickel-rich cathode via decreasing cation mixing or modifying surface chemistry properties through surface coating8-16 lattice doping17-24, 2

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concentration gradient25-27, and composition or structure design of core-shell28-29. Although surface or bulk modification could effectively improve the electrochemical performance, most dopants and coating substances are electrochemical inactive, resulting in significant loss of the capacity. And the corresponding preparation approaches are usually time consuming with high cost of energy due to the two-step sintering process, which pose a big challenge for the commercialization of the materials with surface coating. Meanwhile, the structure of most electrochemical active coating substances reported is different form the layered phase of nickel-rich materials, resulting in low Li+ diffusion velocity in the interface layer. In the two typical materials of nickel-rich layered cathode, LiNi1-x-yCoxMnyO2 (x+y≤0.2, NCM) and LiNi1-x-yCoxAlyO2 (x+y≤0.2, NCA), the stabilizing roles of Mn and Al are not identical. The lighter Al3+ mostly substitutes for Ni3+ in the NCA cathode and facilitates Li+ diffusion. Whereas in the NCM cathode, the Mn4+ causes more Ni2+ in the structure and allows slight increase of capacities accompanying with more cation mixing.30 It has been confirmed that the structure degradation of nickel-rich cathode could be alleviated in the simultaneous presence of Al3+ and Mn4+. For instance, NCM doped by homogeneous Al3+ shows much better electrochemical performance than the pristine NCM.31 LiNi0.8Co0.15Al0.05O2 modified by in situ oxidizing-coating of KMnO4 shows enhanced cycling performance under elevated temperature and at high voltage as well as improved storage stability.32 Moreover, the NCM materials with Mn rich surface, such as core-shell structure and full-gradient structure above-mentioned have exhibited improved cycling and thermal stability for the stabilization role of the gradual increase of Mn in surface layer.25-28 However, there are very few reports to construct concentration gradient NCA material with Mn-rich surface. Considering the synergetic effects and similar O3-type structure of NCA and NCM, the incorporation of Mn in the NCA with a certain thickness stable layer would largely increase the structure stability and cycle life of NCA. Herein, Ni-Co-Al-Mn quaternary nickel-rich layered cathode materials with protective pillaring layer induced by gradient Mn4+ ions were prepared using Mn(Ac)2·4H2O coated porous (Ni0.815Co0.15Al0.035)OH2 as precursor. By virtue of high 3


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melting point of MnOx and low mobility of Mn ions, concentration gradients Mn4+ was formed in the primary particles of the sphere.33-34 The Mn4+ enriched in the outer surface region of grains could increase ratio of Ni2+/Ni3+ and form a NiO-like phase (Figure S1). The pillar role of Ni2+ in lithium slabs in the constructed NiO-like phase layer could inhibit the further migration of the Ni2+ from TM sites into lithium slabs, benefiting the maintenance of the crystal structure during charge/discharge processes. With the pillar effects of NiO-like phase and synergetic effects of Al and gradient Mn, the Mn incorporated NCA exhibits stabilized surface/interfacial structure, enhanced Li diffusion rate as well as improved electrochemical performance.

Experimental Section Synthesis of porous Ni0.815Co0.15Al0.035 (OH)2 precursor The Ni0.815Co0.15Al0.035(OH)2 precursor was obtained by a hydroxide coprecipitation method. The concrete details of synthesis process are shown in our previous work.16 Synthesis of the Ni-Co-Mn-Al quaternary cathode materials To prepare Ni-Co-Mn-Al quaternary cathode materials, a stoichiometric amount of manganese acetate was dissolved with distilled water, and then the porous precursor was added. Ethyl alcohol were added to disperse the precursor powder. The suspension was agitated at 100 °C to evaporate solvent and finally form a Mn(Ac)2·4H2O coated Ni0.815Co0.15Al0.035(OH)2 precursor. Next, the as-prepared precursor was calcined at 450 °C for 5 h. At last, the black powder was mixed and milled with a certain amount of LiOH·H2O and heated at 780 °C for 15 h under the oxygen flow. The Ni-Co-Mn-Al quaternary samples with different Mn content Li(Ni0.815Co0.15Al0.035)1-xMnxO2 (x=0.01, 0.02, 0.05) were synthesized, here denoted as NCAM1, NCAM2 and NCAM5, respectively. In contrast, pristine NCA is obtained via the similar synthesis procedure except the addition of Mn(Ac)2·4H2O (dentoted as NCAM0). Materials characterization Powder X-ray diffraction (XRD) was conducted to characterize crystal structure of materials. The diffractometer was adopted Cu Kα. The data were obtained in the range 4



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of 2θ between 10° and 70° with a 3° min-1 scan speed. Refine treatment was carried out via PDXL software. Ex XRD was carried out on the electrodes with Al foil after cycling. Inductively coupled plasma (ICP-AES) measurements were performed to determine chemical compositions of materials. X-ray photoelectron spectra (XPS) measurements were adopted a spectrometer of Kratos Analytical Lrd. The spectra were corrected with the standard C1s spectrum (284.8 eV). Spectra fitting was conducted with XPS Peak-fit software. Scanning electron microscopy (SEM, Hitachi S-4800) was performed to observe morphology of materials. Scanning transmission electron microscopy (STEM, Talos F200 X) was adopted to obtain Energy-dispersive X-ray spectroscopy (EDS) mapping images. Transmission electron micrograph (TEM) or high-resolution transmission electron microscopy (HRTEM) were performed to observe micro morphology or structures of samples (JEM-2100 ). The samples were dispersed by mesh sieve in alcohol solution under strict sonication before TEM measurements. The modified fast Fourier transform (FFT) or inverse fast Fourier transform (IFFT) were done using Digital Micrograph software. Electrochemical Measurements Prior to electrochemical performance measurements, the electrodes were assembled with separator (Celgard 2400), lithium plate and electrolyte (LiPF6 dissolved in ethylene carbonate and dimethyl carbonate (1:1vol)) using CR2025 cases to fabricate coin-type cells. The electrodes were consisting of cathode materials, conductive carbon and binder at a weight ratio of 80:13:7. To obtain reliable measurements data, the mass of cathode materials was fixed in 3.5±0.2 mg cm-2. Electrochemical

performance

of

as-prepared

electrodes

was

measured

by

galvanostatical battery test system through NEWARE software. The voltage window for half-cell is in the range of 2.7-4.3 V or 4.5 V (vs. Li/Li+) and 1 C refers to 180 mA g-1. The full cells were prepared by assembling as-prepared electrodes with commercial graphite. The voltage window for full cells is in the range of 2.5-4.25 V. Electrochemical impedance spectroscopy (EIS) was analyzed through electrochemical workstation (Zennium IM6) between 100 kHZ and 100 mHZ. Galvanostatic intermittent titration technique (GITT) was adopted to measure lithium ion diffusion 5


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coefficient.

Results and Discussion Morphology of Ni0.815Co0.15Al0.035(OH)2 precursor and all product samples were characterized by SEM (Figure S2). The loose and porous structure for secondary particle of Ni0.815Co0.15Al0.035(OH)2 (Figure S2a-b) could be beneficial for the impregnation of Mn ions solution into the inner part to ensure uniform Mn(Ac)2∙4H2O coating on the surface of primary grains. After mixing with LiOH·H2O and calcining at high-temperature, the product materials retain the sphere-like morphology with secondary particles (about 10 µm) composed of aggregated primary particles (about 500 nm) and no obvious morphological changes are observed (Figure S2c-f). And the EDS mapping images of transition metals (TM) for samples were displayed in the Figure S3. It could be obviously observed that these elements are evenly overlapped on the spherical particles. In addition, chemical compositions of as-prepared samples were measured by ICP-AES (Table S1). It could be observed that the actual chemical compositions for all samples are in basically consistent with the target value designed. Fig. 1 shows the XRD patterns and corresponding Rietveld refinements of the samples. As displayed in the Fig. 1a, all the materials show high crystallinity and are indexed to the layered R 3 m phase without any visible peaks belonging to impurity phase, suggesting the successful incorporation of Mn into the crystal. The clear splitting peaks for (006)/(012) and (018)/(110) indicate formation of ordered crystal structure.33-34 To obtain more details of the crystal structure, the XRD patterns are further analyzed by Rietveld refinement. The low value of Rwp indicates that the refinement results are reliable. As shown in Table S2, as Mn content increased, the value of I003/I004 decrease and lattice constants increase. The enlargement of lattice parameters for Mn contained samples is because the radius of Mn4+ (0.53 Å) is smaller than the value of Ni3+ (0.60 Å) and smaller Ni3+ (0.60 Å) transforms to larger Ni2+(0.69 Å) to make the charge balance after the incorporation of Mn4+ ions. And the content of Ni2+ is related to the amount of Mn in the quaternary cathode materials, as confirmed by the subsequent XPS results. Meanwhile, the decrease of I003/I004 for Mn 6



contained samples is owing to the aggravated cations mixing induced by higher amount of Ni2+. By the way, although quaternary cathode materials show more cationic mixing structure than NCAM0, the high ratio values of c/a (>4.948) and

I003/I104 (>1.28) indicate that the materials with incorporation of Mn already have well-ordered crystal structure.

Fig. 1 The XRD spectra (a) and Rietveld refinement results (b-e) for all the samples.

To investigate the valences of TM, the XPS measurements were carried out for as-prepared materials. Considering the stable valence of Al3+ ions, only the oxidation states of Mn, Co and Ni ions were investigated. Figure S4a-b display XPS spectra of Co 2p and Mn 2p after fitting based on full-scan XPS spectrum (Figure S5). The valence of Co and Mn for the samples are determined to be +3 and +4, respectively. However, the spectra of Ni 2p for all samples (Fig. 2) could be divided into four peaks located at 855.4 eV, 855.1 eV, 854.9 eV and 854.8 eV, indicating the valence of Ni is mixed with +2 and +3. As the valence state of Co remains unchanged, a certain quantity of Ni3+ transforms to Ni2+ to make the charge balance after the incorporation of high valence Mn4+. Besides, the semi-quantitative analysis of the relative content of Ni2+/Ni3+ based on the integral area of peaks shows that the content of Ni2+ increase with increasing the amount of Mn. It was reported that the Ni2+ could easily migrate into Li slab from TM slab and act as pillar ions to promote structure stability of 7


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layered transition metal oxide.35-37

Fig. 2 The XPS spectra of Ni 2p for NCAM0 (a), NCAM1(b), NCAM2 (c) and NCAM5 (d).

Considering the high melting point of MnOx and low mobility of Mn ions, it is worth to detecting the chemical component near surface of primary particle. XPS measurements of NCAM5 sample before and after etching were conducted to investigate valence of Ni and Mn in the surface and bulk. It could be observed that the Ni 2p3/2 peak (Figure S6a) shifts towards high binding energy with increase of etching depth, indicating the oxidation state of Ni element is increased from surface to bulk direction. However, in spite of gradually decreased Mn contents, no obvious changes in Mn valence state (+4) were observed between the outer part and bulk of the primary grain (Figure S6b). In the surface region (0~30 nm), gradient Mn4+ could induce reduction of partial Ni ions from Ni3+ to Ni2+, finally causing gradual change of nickel valence. However, in the core part, the chemical composition of NCAM5 is very closed to that of pristine NCA cathode, leading to the similar valence with that of NCAM0. The EDS line scan analysis was carried out for NCAM5 to obtain the profile of the element concentration vs. depth. In the Fig. 3a-e, the distribution of Mn 8



is overlapped with other metal elements in the hexagon-like shape primary particle, indicating that Mn element was successfully incorporated into the particle. Meanwhile, it can be observed that the concentration of Mn firstly decreases with increase of the depth in the range of 0-30 nm, subsequently reduces to zero and keeps almost unchanged. And the concentration of Ni, Co and Al maintain constant corresponding to designed molar ratio in the range of 30-300 nm. Thus, it can be inferred that Mn element was successfully incorporated into the particle and enriched in the outer region of the primary grain, forming gradient Mn doping with decreased concentration from the surface to the interior of grain.

Fig. 3 The TEM EDS mapping images of TM (a-d) and the HAADF image (e), the EDS line-scan profile of element atom ratio function as depth (f) and corresponding enlarged image (g) for NCAM5.

The HR-TEM analysis was carried out to detect the refined micro-structure in the interface layer of the cathodes. Fig. 4 exhibits the HR-TEM, FFT and IFFT images of NCAM0 and NCAM5. As shown in Fig. 4a, two types of structure could be found in different region of the particle for NCAM5. The bulk structure (site B) is well preserved as R 3 m phase, exhibiting regular layered lattice fringes. In the outer region (site A), a layer (~ 25 nm ) is observed as NiO like phase layer ( Fm 3 m), as confirmed by the FFT pattern (Fig. 4b). Fig. 4c and d show the IFFT images 9


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corresponding to site A and site B in the Fig. 4a. The bright TM slab was marked with red arrow, and invisible light Li element slab was marked with blue arrow. The area extruded with white dotted lines (Fig. 4c) show a clear white contrast in Li slabs. Meanwhile, no bright contrast of Li slabs could be found in the Fig. 4d. The bright spots of Li slabs and Fm 3 m space group in site A indicate migration of Ni2+ ions from TM sites to Li sites, forming NiO phase on the region close to the edge of the particle. However, in the NCAM0 sample, the phases near the edge and in the bulk were verified to be layered phase with a space group of R 3 m (Fig. 4f) with only a few cation mixing phases detected in the surface (Fig. 4g). Based on above results, it is can be confirmed that portion of Ni3+ ions near the surfaces is transformed to Ni2+ and partial Ni2+ ions in turn took in Li slabs as pillars ions, forming a nano-pillaring layer at the outer region of the Ni-Co-Al-Mn quaternary materials.5 By the way, the no detection of Fm 3 m phase from the XRD patterns is likely due to the overlap of the main diffraction peaks or the limit of detection.33 It is reported that the nickel-rich cathodes with the NiO-like (Fm 3 m)-layered (R 3 m) heterostructure could promote structure stability and reduce the side reaction with electrolyte. Therefore, it can be expected that the electrodes with quaternary cathode would possess more stable cycling performance.

10



Fig. 4 The HR-TEM (a, e), FFT (b, f) and IFFT (c-d, g-h) images of NCAM0 and NCAM5.

Fig. 5a shows the first charge/discharge profiles of the cells in cutoff voltage of 4.3 V at the current density of 0.1 C at 24 °C. NCAM0 displays the discharge specific capacity of 191.9 mAh g-1 with a Coulombic efficiency of 85.2%. While the quaternary cathode materials NCAM1, NCAM2 and NCAM5 show the discharge capacities of 191.1, 193.2 and 183.0 mAh g-1, corresponding to Coulombic efficiency of 86.5%, 83.7% and 83.5%, respectively. Such a tendency of discharge capacity may be the result of the comprehensive effect of the electrochemical inert Mn4+ and electrochemical active Ni2+. Particularly, the much lower discharge capacity for NCAM5 in comparison with NCAM0 is likely due to the dominated capacity reduction role induced by relatively large amount of inert Mn4+. Despite the lack of direct relationship with incorporation content of Mn4+ ions, the numerical value of the Coulombic efficiency are good consistent with results reported.7,38-39 Fig. 5b-f compare the rate performance of the NCAM0, NCAM1, NCAM2 and NCAM5 from 0.1 C to 10 C in the cutoff voltage of 4.3 V. It could be clearly observed that NCAM1, NCAM2 and NCAM5 display higher discharge capacities than NCAM0 after the current density is up to 3 C. At 10 C rate, NCAM0

exhibits only 77.2 mAh g-1, but

NCAM1, NCAM2 and MCAM5 exhibit discharge capacities of 107.1, 126.4 and 113.0 mAh g-1, respectively. The enhanced electrochemical behaviour of Mn incorporated materials could be ascribed to the well-maintained crystal structure and improved crystal structure stability during charge/discharge process. Meanwhile, the corresponding working voltage profiles of as-prepared samples (Figure S7) aslo indicate that NCAM2 undergoes the smallest polarization in charge/discharge processes.33

11


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Fig. 5 The first charge/discharge profiles (a), the rate performance (b) and the corresponding discharge curves (c-f) of the as-prepared electrodes.

Fig. 6a and Figure S8 show the cycle behaviour of the electrodes at 1.0 C over 100 cycles under 24 °C. As expected, quaternary cathodes exhibit significant improved cycling stability. The capacity of NCAM0 decreases rapidly after 100 cycles with only 79.9% of its first discharge capacity, while the capacity retention for NCAM1, NCAM2 and NCAM5 are 90.4%, 91.3% and 90.9%, respectively. Fig. 6b exhibits the differential capacity vs. voltage (dQ/dV) curves at 100th cycle. Three typical phase transition processes involved in inter-conversion between hexagonal phase and monoclinic phase are observed.4, 40 More importantly, the relatively smaller 12



potential differences (∆V) of NCAM1, NCAM2 and NCAM5 suggest that the quaternary cathode materials suffered from smaller polarization during cycling process.14,41 Furthermore, the electrodes were conducted long-term cycle tests at 5 C rate to demonstrate the superiority of the quaternary materials with the nanoscale protective interface layer. As exhibited in the Fig. 6c, NCAM1, NCAM2 and NCAM5 retain 53.6%, 80.0% and 67.0% of their initial discharge capacities after 500 cycles, which is significantly higher than the value of 40.1% for NCAM0. The good electrochemical behavior for the quaternary cathode materials can be attributed to the suppressed side reaction with electrolyte and stable structure provided by the protective interface layer with gradient Mn4+ ions and pillar layer.5 It is worth to noting that the best electrochemical behaviour for NCAM2 may arise from appropriate amounts of pillar ions (Ni2+ ions) and stability ions (Mn4+ ions).

Fig. 6 The cycle behaviour of electrodes at 1.0 C (a), the differential capacity curves of electrodes at 100th cycle (b) and the cycle behaviour of electrodes at 5.0 C (c).

To demonstrate the superior electrochemical behavior of the quaternary electrodes at the harsh conditions. The electrodes were cycled at high cut-off voltage or high temperature. Figure S9a shows the first charge/discharge profiles for NCAM0 and NCAM2 at 0.1 C in high cut-off potential of 4.5 V at 24 °C. At the initial cycle, 13


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NCAM0 and NCAM2 respectively deliver the discharge capacity of 209.0 and 212.2 mAh g-1, corresponding to Coulombic efficiency of 85.8% and 86.2%, which is consistent with the published works.5,39 However, the capacity of NCAM0 suffers from rapid loss in cycle processes (Figure S9b). Particularly, the capacity retention for NCAM0 after 100 cycles is only 60.0%, whereas the capacity retention for NCAM2 reaches 84.6%. The reason for the enhanced cycling behaviour at high cut off potential could be well explained by pillar effects that Ni2+ ions in Li slabs could offer an electrostatic repulsion force to inhibit further cation migration in the high state of charge and synergetic effects of stable Mn and Al.2 The improvement was also confirmed by cycle performance at elevated temperature. Compared with the severe capacity fade of NCAM0, NCAM2 exhibits relatively stable cycling behavior, corresponding to a capacity retention of 81.1% (Figure S9c). To further examine the structure stability and kinetic properties of the NCAM0 and NCAM2 electrodes, ex-situ XRD, GITT test and EIS measurement were conducted after 100 cycles at 1.0 C. In Figure S10, the shoulder peaks ((018)/(110) and (006)/(012)) of the XRD pattern for NCAM0 were merged into one peak after 100 cycles, confirming that the disordered crystal structure were formed during cycling process.35 However, the mergence of both shoulder peaks was obviously mitigated for NCAM2, indicating that crystal structure is well maintained by the nanoscale protective interface layer. Thus, it could be expected that the high reaction and kinetic activities could be retained for NCAM2 in the charge/discharge processes owing to well-maintained crystal structure. Figure S11a,d exhibit the GITT curves of discharge process at 0.1 C after cycling for 100 times, and both of samples are galvanostatically discharged for 20 min followed by a rest of 150 min to obtain a nearly steady state. The profiles of the single-step GITT experiment are shown in Figure S11b,e and the relevant parameters of τ, ∆Es, ∆Eτ and etc. are also schematically presented. Due to an approximate linear relationship between Eτ and τ1/2, the Li+ diffusion coefficient (DLi+) can be obtained by the Eq. (1)42-43:

14



D

Li +

=

4 mBVm 2 ∆E s 2 ( ) ( ) πτ M B S ∆Eτ

(τ

Constructing protective pillaring layer by incorporating gradient Mn4+

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