Enhanced Cyclability and High-rate Capability of LiNi0.88Co0

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Enhanced Cyclability and High-rate Capability of LiNi0.88Co0.095Mn0.025O2 Cathode by Homogeneous Al3+ Doping Xing Yang, Yiwei Tang, Guozhi Shang, Jian Wu, Yanqing Lai, Jie Li, Yaohui Qu, and Zhian Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10558 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Applied Materials & Interfaces

Enhanced Cyclability and High-rate Capability of LiNi0.88Co0.095Mn0.025O2 Cathode by Homogeneous Al3+ Doping Xing Yang,† Yiwei Tang,† Guozhi Shang,† Jian Wu,† Yanqing Lai,† Jie Li,† Yaohui Qu,§ and Zhian Zhang*,† †

School of Metallurgy and Environment, Central South University, Changsha, Hunan

410083, China §

School of Physics, Communication and Electronics, Jiangxi Normal University,

Nanchang, Jiangxi 330022, China KEYWORDS: Nickel-rich materials; Lithium-ion batteries; Homogeneous Al3+ doping; Radial primary grains.

ABSTRACT: To suppress capacity fading of nickel-rich materials for lithium-ion batteries, a homogeneous Al3+ doping strategy is realized through tailoring Al3+ diffusion

path

from

bulk

surface

to

interior.

Specifically,

layered

LiNi0.88Co0.095Mn0.025O2 cathode with radial arrangement of primary grains is successfully synthesized through optimization design of precursor. The Al3+ follows the radially oriented primary grains into the bulk by introduction of nano-Al2O3 during the sintering process, realizing the homogeneous Al3+ distribution in the whole material. Particularly, a series of nano-Al2O3 modified LiNi0.88Co0.095Mn0.025O2 are investigated. With the 2 % molar weight of Al3+ doping, capacity retention ratio of cathode is 1 ACS Paragon Plus Environment

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tremendously improved from 52.26 % to 91.57 % at 1 C rate after 150 cycles. Even at heavy current density of 5 (&10) C for LiNi0.88Co0.095Mn0.025O2-Al2% cathode, a high reversible capacity of 172.3 (&165.7) mAh g-1 can be acquired, which amount to the 84.46 (&81.25) % capacity retention at 0.2 C. Moreover, voltage deterioration is significantly suppressed by homogeneous Al3+ doping from the results of median voltage and dQ/dV curves. Therefore, homogeneous Al3+ doping benefited from radial arrangement of primary grains provides an effective and practical way to prolong lifespan, as well as improve rate performance and voltage stability of nickel-rich ternary materials.

1. INTRODUCTION Lithium-ion batteries (LIB) have been broadly used for electronic equipment with portability and are gradually applied to electric vehicles (EVs) and hybrid electric vehicles (HEVs) aiming to reduction of greenhouse gas emissions originated from fuel vehicle.1-4 Unsatisfactory cost, calendar life and energy densities have been regarded as hurdles for its widely industrial applications on EVs and HEVs, which are mainly attributed to the laggard development of cathode.5-6 Among current commercialized cathode candidates, high-nickel ternary cathodes (LiNixCoyMnzO2, x ≥ 0.6, y ≥ 0, y ≥ 0) have been regarded as promising and industrialized materials.7-9 However, increasing Ni content negatively impacts the cycling capability and structural stability of the cathode. The reasons mainly come down to the following points: Ⅰ) Li+/Ni2+ mixing lead to off-design stoichiometric proportion as well as degradation of layered structure.5, 9

Ⅱ) a mass of reactive Ni4+, formed when the cathode is in high delithiated degree, 2 ACS Paragon Plus Environment

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causes irreversible structural transformation by forming NiO rock salt on surface as well as side reactions with electrolyte.10 Ⅲ) microcracks, originated from polyphase transition and nonisotropic volume expansion/contraction in the cycle, will bring about secondary grains breakage, resulting in fast capacity attenuation.11-13 To circumvent the drawbacks of capacity fading and structural deterioration, many strategies, containing lattice doping,14-19 surface modification20-26 and shape control2731

have been come forward, in which lattice doping have been considered as the most

practical and cost-effective solution by introduction of foreign elements. Among the dopants, Al3+ has been widely investigated with the successful commercialization of LiNi0.8Co0.15Al0.05O2. Tailoring the distribution of Al3+ in the bulk by formation of homogeneous,32-35 concentration-gradient16,

36

and surface enriched distribution37-38

does enhance the cycling capability and structural stability of NCM cathode. Guo33 et al. synthesized homogeneous Al3+ doped Ni0.8Co0.1Mn0.09Al0.01(OH)2 precursor through using AlO2- solution as Al sources, and the Al doped cathode exhibits enhanced cycling property and storage stability. Sun34 et al. designed Ni0.7Co0.15Mn0.15(OH)2 precursor with concentration gradient distribution of inactive-Al3+ where aluminum is concentrated on the surface and progressively decreased from shell to core. The heterogeneous Al3+ doped LiNi0.7Co0.15Mn0.15O2 cathode shows robust calendar life and excellent performance even at high charged voltage due to enhanced Li+ kinetics and structural stability. However, due to Al3+ preferentially precipitated on account of lower Ksp value of Al(OH)3 than Ni(OH)2, Co(OH)2 and Mn(OH)2, aluminum is highly susceptible to uneven distribution in the bulk during in situ co-precipitation reaction. 3 ACS Paragon Plus Environment


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Moreover, complex co-precipitation process, especially among the synthetic process of concentration gradient cathode, will result in cost increase, which is not conducive to practical application. Therefore, it is necessary to explore a feasible and industrialized way to tailor the distribution of Al3+. Herein, a special structure of LiNi0.88Co0.095Mn0.025O2 material with radially arranged primary grains is designed from the perspective of precursor. The formation mechanism, particle size distribution and XRD of Ni0.88Co0.095Mn0.025(OH)2 precursors at different reaction times are analyzed. Strips made up of laminar primary grains are upright embedded into the Ni0.88Co0.095Mn0.025(OH)2 precursor, which are transformed into radially arranged primary grains after sintering. By the introduction of nano-Al2O3 during sintering, Al3+ spread along the radial primary grains resulting in homogeneous Al3+ distribution. It is found that 2 % (mole weight) Al3+ doped LiNi0.88Co0.095Mn0.025O2 cathode shows superior cycling and rate capability. Structure and voltage deterioration are significantly suppressed by homogeneous Al3+ doping from the results of SEM image after cycling, median voltage and dQ/dV curves. 2. EXPERIMENT SECTION 2.1.

Preparation

of

Ni0.88Co0.095Mn0.025(OH)2

precursors.

Ni0.88Co0.095Mn0.025(OH)2 precursors were fabricated based on our previous report39. Firstly, NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O were dissolved in ultrapure water by formation of solution 1 with a concentration of 2 mol L-1, in which mole ratio of Ni, Co, Mn was 88: 9.5: 2.5. Secondly, to obtain crystalline grain of high quality, 10 mol 4 ACS Paragon Plus Environment

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L-1 NaOH solution and 8 mol L-1 NH3·H2O along with the solution 1 were respectively pumped into reactor with ammonium ion concentration of 5.0 g L-1 and pH value of 11.2 under the stirring rate of 200 rpm. After 2 hours of reaction, ammonium ion concentration and pH value regulated to 9.0 g L-1 and 11.8 under the stirring rate of 250 rpm. After reacted to 25 h, suspension solution was aged for 2 h. After removed the sulfate radical and alkali by NaOH solution and ultrapure water with 60 oC, and then dried in vacuum oven with 120 oC overnight, Ni0.88Co0.095Mn0.025(OH)2 was collected. 2.2. Preparation of LiNi0.88Co0.095Mn0.025O2 cathode. The LiNi0.88Co0.095Mn0.025O2 microspheres were fabricated by calcining Ni0.88Co0.095Mn0.025(OH)2 and LiOH·H2O with Li/M(Ni+Co+Mn)=1.05:1 (mole ratio) at 760°C for 16 h under oxygen atmosphere. 2.3. Preparation of Al+ doped LiNi0.88Co0.095Mn0.025O2 cathodes. To prepared the Al3+ doped LiNi0.88Co0.095Mn0.025O2 materials, nano-Al2O3 was firstly mixed with Ni0.88Co0.095Mn0.025(OH)2, and then dispersed into an alcohol solution with stirring. After ultrasonic treatment and evaporation, Ni0.88Co0.095Mn0.025(OH)2 precursors with surface-dispersed nano-Al2O3 were obtained. The Al doped LiNi0.88Co0.095Mn0.025O2 microspheres were prepared by calcining Ni0.88Co0.095Mn0.025(OH)2 with surfacedispersed nano-Al2O3 and LiOH·H2O with Li/M(Ni+Co+Mn+Al)=1.05:1 (mole ratio) under the same condition with LiNi0.88Co0.095Mn0.025O2 material preparation. LiNi0.88Co0.095Mn0.025O2 materials with 1.0 %, 2.0 %, 3.0 % molar quantities of Al3+ doping were defined as LNCMO-Al1.0%, LNCMO-Al2.0%, LNCMO-Al3.0% cathodes. 5 ACS Paragon Plus Environment


2.4. Material characterization. Crystalline structure of powder was investigated by X-ray diffractometer (XRD, MiniFlex 600, Rigaku) between the 2θ range of 10o to 80° at a step size of 0.02o. Structural parameters of unit cell were refined by GSAS/EXPGUI program. Inductively coupled plasma (ICP-OES) was used for element content measurement. Tap density of materials was tested by powder density tester (HY-100). Bulk morphology was examined by a scanning electron microscopy (NovaNanoSEM 230). Material sections were prepared by argon-ion polishing cutter (PECS685, Gatan). Particle distribution of as-prepared materials was measured by particle size analyzer (Mastersizer 3000). Weight and heat change during sintering process was acquired by Mettler Toledo. Elemental oxidation states on material surface were measured by the X-ray photoelectron spectroscopy (XPS, Thermo, k-Alpha). 2.5. Electrochemical measurement. Cathode materials, carbon black and polyvinylidene fluorid (PVDF) (weigh ratio = 8:1:1) were firstly mixed and then placed in a container. After added appropriate N-methyl-2-pyrrolidone (NMP), the container was in high speed rotation with 2000 rpm for 6 min. The well mixed slurries were quickly spread on an Al foil current collector with mass loading of ~ 60 g m-2 and put it in a vacuum oven heated at 100 oC for 24 h. 2025 coin-type cells were assembled in dry Ar-filled glove box with fabricated electrode, electrolyte, separator and lithium metal sheet (counter electrode). A mixed solvent of ethylene carbonate and ethylmethyl carbonate (3:7 in volume) with 1.2 M LiPF6 dissolved and addition of 2.0 % (mass fraction) vinylene carbonate was used as electrolyte. A constant current/constant voltage charge-discharge measurement was conducted using a LAND CT2001A 6 ACS Paragon Plus Environment

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battery-testing instrument under the temperature of 27 oC. Cyclic voltammetry (CV) and Nyquist plots were carried out on PARSTAT 2273 electrochemical measurement system.

3. RESULTS AND DISCUSSIONS 3.1. Synthesis and characterization of Ni0.88Co0.095Mn0.025(OH)2 precursors The formation mechanism of Ni0.88Co0.095Mn0.025(OH)2 precursors is discussed by recording the particle morphology at the reaction time of 2 h, 6 h, 12 h, 20 h, 25 h (Figure S1, Supporting Information). There are three main processes involved: Ⅰ. generation of nanoplates with anisotropy; Ⅱ. aggregation of nanoplates by formation of irregular microspheres; Ⅲ. Polishing and growth of irregular microsphere by formation of close-grained microspheres. The results of ICP and tap density (Table S1, Supporting Information) manifest that element proportion of Ni, Co, Mn is in accord with experimental design and Ni0.88Co0.095Mn0.025(OH)2 precursors have a high tap density of 2.60 g cm-3. Figure S2a display the section SEM images of precursors treated by argon-ion polishing cutter, demonstrating that the precursor possesses a structure with unconsolidated interior (Figure S2b) and compact exterior (Figure S2c). Unconsolidated interior should be ascribed to unordered arrangement of nanoplates during agglomeration process. Compact exterior structure should be assigned to slow and steady particle growth during the co-precipitation process. it can be seen from elementary-line scanning of precursor section (Figure S2d) that elements of oxygen, nickel, cobalt, manganese are homogeneously distributed from core to shell.

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Moreover, particle size distribution and XRD patterns in different reaction times are collected. As shown in Figure S3a-e, precursors show D50 particle sizes of 1.357, 1.725, 2.430, 3.050, 3.293 μm in reaction time of 2h, 6 h, 16 h, 20 h, 25 h, respectively. It is found in Figure S3f that a linear relationship between particle size of D10, D50, D90 and reaction time after 6 h, demonstrating stable solution environment as well as slow and steady particle growth in the co-precipitation process. Figure S4 shows the XRD patterns of Ni0.88Co0.095Mn0.025(OH)2 precursors in reaction time of 2h, 6 h, 16 h, 20 h, 25 h. The peaks located at 19.28o, 33.07o, 38.66o, 52.09o, 59.12o, 62.82o, 69.66o, 72.91o are assigned to (001), (100), (101), (102), (110), (111), (200), (112) lattice plane of β-Ni(OH)2 with P3m1 space group. No other diffraction patterns can be found, implying that Ni0.88Co0.095Mn0.025(OH)2 precursors are individually generated during the reaction process. The intensities of (001), (100), (101) plane and values of I(001)/I(100), I(001)/I(101) in the reaction time of 2h, 6 h, 16 h, 20 h, 25 h are shown in table S2. The densities of (001), (100) and (101) increase with extended reaction time. The value of I(001)/I(100) has a growth tendency from 2 h to 12 h, and remains relatively stable from 12 h to 25 h. The value of I(001)/I(101) keeps growth tendency from start to finish. The result demonstrates that the precursor has growth with (001) plane orientation, which is consistent with previous report.40 3.2. Synthesis and characterization of LiNi0.88Co0.095Mn0.025O2 cathode and Al3+ doped LiNi0.88Co0.095Mn0.025O2 cathodes.


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Scheme 1 Preparation process of homogeneous Al3+ doped LNCMO cathodes.

Preparation process of homogeneous Al3+ doped LNCMO cathodes is displayed in scheme 1. Based on the structural design of Ni0.88Co0.095Mn0.025(OH)2 precursors, different doses of nano-Al2O3 well distributed on the surface of precursors through a simple process where mixture of nano-Al2O3 and precursors disperses in alcohol solution and follows on ultrasonic treatment and evaporation. After sintered the mixture with LiOH·H2O, homogeneous Al3+ doped LNCMO cathodes are obtained. The details will be discussed in the following.



Figure 1 SEM images of prepared samples: (a) precursor; (b) LNCMO cathode; (c)LNCMO-Al1% cathode; (d) LNCMO-Al2% cathode; (e) LNCMO-Al3% cathode; (f) LNCMO-Al2% cathode section. (g-l) Elemental mapping and (m) elementary-line scanning of LNCMO-Al2% section.

High magnification scanning electron microscopy (SEM) images of the prepared precursors, LNCMO and Al doped LNCMO cathodes are shown in Figure 1(a-f). Asprepared precursors (Figure 1a) have a special structure where strips made up of laminar primary grains are upright embedded into the bulk. Figure 1b displays that these strips 10 ACS Paragon Plus Environment

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are transformed into rod-like primary grains after sintering. The cathodes with Al3+ molar quantities of 1.0 %, 2.0 %, 3.0 % show the same morphological features (Figure 1b-e), suggesting that the addition of Al2O3 have a negligible impact on cathode morphology. To further understand the inner structure of cathodes, LNCMO-Al2% cathode is polished by argon-ion. As shown in Figure 1f, LNCMO-Al2% cathode, inherited the structural characteristics of precursors, possesses an interior structure with disordered arrangement of primary grains and an exterior structure with ordered arrangement of radial rod-like particles. In order to characterize the distribution of Al3+ in the bulk, surface and sectional elemental mapping of LNCMO-Al2% cathode is carried out. Figure S5 displays the external element distribution of LNCMO-Al2% cathode, in which the mapping regions of nickel, cobalt, manganese, aluminum are overlapped. It turns out that the nano-Al2O3 is evenly distributed around the precursor resulting in uniform distribution of aluminum on LNCMO-Al2% cathode during sintering process. Figure 1g-l displays inner element distribution of LNCMO-Al2% cathode. Similarly, overlap region of O, Ni, Co, Mn, Al demonstrates that the Al3+ diffuses into the bulk due to the ordered arrangement of radial rod-like particles. Moreover, energy dispersive spectrometer (EDS) is measured from shell to core. As shown in Figure 1m, the element contents of nickel, cobalt, manganese are approximately equal from shell to core. The atomic percent of Al are about 1.95, 1.86, 1.81, 1.82 % from point 1 to point 4, while the point 5 have lower Al atomic percent of 1.63 %. The result demonstrates that the ordered arrangement of radial rod-like particles



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is beneficial to Al3+ diffusion from exterior to interior during sintering process, resulting in homogeneous Al3+ doping in the bulk.

Figure 2 (a) Powder XRD patterns of LNCMO and Al doped LNCMO cathodes, (b) corresponding (104) plane and (c) (003) plane patterns.

Table 1 Rietveld Refinement results for LNCMO and Al doped LNCMO cathodes.

Samples

LNCMO

LNCMO-Al1%

LNCMO-Al2%

LNCMO-Al3%

a (Å)

2.874635

2.873081

2.872607

2.871737

c(Å)

14.186884

14.186968

14.189713

14.190898

V(Å3)

101.5243757

101.4152397

101.4013956

101.3484469

c/a

4.935194903

4.937893502

4.939663866

4.941572992

I(003)/I(104)

1.086

1.094

1.265

1.288

Ni in Li (%)

4.48

4.29

3.57

3.26

Rwp (%)

1.21

1.25

1.15

1.18

Rp (%)

0.75

0.74

0.7

0.72


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To investigate the elementary composition and crystalline structure of LNCMO and Al doped LNCMO cathodes, the inductively coupled plasma (ICP) and X-ray diffractometer (XRD) are carried out. As shown in table S1, the cathode without Al3+ doping basically conforms to molecular formula of LiNi0.88Co0.095Mn0.025O2. Specially, element content of Co and Mn in Al doped LNCMO cathodes remain relatively unchanged, while Ni content decreases with the increment introduction of Al3+. It demonstrates that the Al3+ substitutes the position of Ni2+/Ni3+ in the sintering process, which is consistent with the previous reports41-43 in which Al substitution is prove to prefer at Ni sites over Co and Mn sites. The tap densities of LNCMO, LNCMO-Al1%, LNCMO-Al2%, LNCMO-Al3% cathode materials are 2.93, 2.92, 2.94, 2.95 g cm-3, respectively, which is higher than that of precursor. It can be seen from Figure 2a that all the diffraction patterns of LNCMO and Al doped LNCMO cathodes can be indexed to characteristic rhombohedral α-NaFeO2 with a space group of R-3m. Apparent splitting peaks of (006)/(102) and (108)/(110) manifest as-prepared cathodes with a well-regulated layered structure.44-46 Besides, 003 (Figure 2b) and 104 (Figure 2c) peaks of Al doped LNCMO cathodes shift to high angle with increment introduction of Al3+, indicating the reduction of unit volume.47 The phenomenon should be ascribed to the lower ion radius of Al3+ (0.51 Å) than that of Ni2+/Ni3+ (0.69/0.56 Å). Figure S6 displays the result of the Rietveld refinement for LNCMO and Al doped LNCMO cathodes. Structural parameters obtained from Rietveld refinement are shown in table 1. Values of Rwp and Rp for LNCMO and Al doped LNCMO cathodes are minor, manifesting that the structural parameters are reliable. The lattice parameters along the 13 ACS Paragon Plus Environment


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a(b)-axis decrease by introduction of Al3+, while the lattice parameters along the c-axis have a slight increase. Expansile c-axis is beneficial to Li+ transportation during cycling. Due to apparent reduction of a(b)-axis, the cell volume is reduced as increment introduction of Al3+, which is in accord with the right deviation of (104) and (003) plane. The values of c/a also increase with the increment introduction of Al3+, suggesting Al3+ doping is conducive to formation of layered structure. Moreover, the value of I(003)/I(104) shows the degree of Li+/Ni2+ mixing, and the higher value means lower Li+/Ni2+ mixing. As displayed in table 1, the value of I(003)/I(104) increases with the increment introduction of Al3+, demonstrating that the Li+/Ni2+ mixing is restrained by Al3+ doping. Cation occupancy of Ni in Li derived from Rietveld Refinement also manifests that the cation mixing is reduced by introduction of Al3+.

Figure 3 TGA&DSC curves of LiOH·H2O and Ni0.88Co0.095Mn0.025(OH)2 mixture (a), LiOH·H2O, Ni0.88Co0.095Mn0.025(OH)2 and Nano-Al2O3 mixture (b).

In order to investigate composition change during the sintering process, TGA&DSC measurement is carried out. Figure 3 shows the TGA&DSC curves of LiOH·H2O

&

Ni0.88Co0.095Mn0.025(OH)2

mixture

and

LiOH·H2O

&

Ni0.88Co0.095Mn0.025(OH)2 & Nano-Al2O3 mixture. Both of the TGA&DSC curves have 14 ACS Paragon Plus Environment

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four weight changes. The first weight drop below 150 oC owes to evaporation of crystal water in LiOH·H2O and free moisture in the precursors. The second weight loss between 150 and 300 oC is related to majority of water loss derived from decomposition of precursor. The third weight loss 300 and 470 oC should be interpreted as the residual water loss from precursor. The last weight loss after 470 oC should be ascribed to decomposition of LiOH to Li2O as well as chemical reaction between Ni0.88Co0.095Mn0.025O2 and Li2O. It's worth noting that the total weight losses of the two TGA&DSC curves are about 28.49 and 28.59 %, respectively, from room temperature to 700 oC and the total heat losses of the TGA&DSC curves are about 1209.85 and 1177.37 J/g. Approximate weight and heat losses demonstrate that the introduction of nano-Al2O3 have a limited influence on the synthesis process of materials.



Figure 4 XPS curves of of (a) Ni 2p, (b) Co 2p, (c) Mn 2p, (d) Al 2p.

Elemental composition and oxidation states on cathode surface are analyzed by XPS measurement (Figure 4). Figure 4a shows the Ni XPS spectrum of LNCMO and LNCMO-Al2% cathodes. Two separated peaks, derived from fitting result of Ni 2p3/2 peak, can be observed at location of 855.64 eV and 854.54 eV (Figure 4a), which belong to Ni3+ and Ni2+ oxidation states of nickel.48-50 The area (36.3 %) of Ni2+ peak separation in LNCMO-Al2% cathode is larger than that of LNCMO cathode (31.51 %), which is assigned to charge balance where Ni3+ transforms to Ni2+ due to increased positive charge by introduction of Al3+. Co 2p3/2 and Co 2p1/2 peaks of LNCMO and modified LNCMO cathodes are located at 779.53 and 794.34 eV, demonstrating cobalt has a 16 ACS Paragon Plus Environment

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valent state of +3.37, 51 Three characteristic peaks located at 653.8, 642.7 and 637.4 eV in Mn XPS spectrum (Figure 4c) are related to Mn 2p1/2, Mn 2p3/2 and Ni (LMM), respectively. 52 Otherwise, Al 2p peak of LNCMO-Al2% cathode is centered at 67.66 eV (Figure 4d), indicate that aluminum has a valent state of +3 in the bulk.16, 47 3.3. Electrochemical performance of homogeneous Al3+ doped LNCMO cathodes

Figure 5 Electrochemical properties of LNCMO and Al doped LNCMO cathodes in 4.3 V cutoff voltage at 27 oC. (a) Cyclic voltammogram curves of LNCMO cathode at scan rate of 0.2 mV s-1, (b) first charge/discharge curves at a rate of 0.2 C (1 C=200 mA g-1), (c) cyclical capability at 1C rate, (d) rate capability under different C-rates.

Figure 5a displays the cyclic voltammogram curves of LNCMO cathode in the voltage range of 2.8 - 4.3 V. In the first cycle, an apparent anodic peak of 4.06 V and three cathodic peaks located at 3.672, 3.964, 4.127 V can be found in CV curves of 17 ACS Paragon Plus Environment


LNCMO cathode. These three cathodic peaks are assigned to H1 (hexagonal phase) to M (monoclinic phase), M to H2 (hexagonal phase) and H2 to H3 (hexagonal phase).53-55 In the following cycles, the anodic peak in the initial cycle transforms to three different andic peaks located at 3.745, 4.402, 4.228 V. Difference about anodic peak in the first two laps is assigned to the generation of solid electrolyte interphase (SEI) in the initial cycle.44,

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It's worth noting that the location of cathodic peaks remained basically

unchanged within the five cycles. Moreover, CV curves of LNCMO-Al2% cathode also tested and compare with that of LNCMO cathode. As shown in Figure S7, the LNCMOAl2% cathode shows similar curves but different location of redox peaks compared with LNCMO cathode, suggesting that the introduction of nano-Al2O3 have limited influence on chemical transformation. The potential differences of redox peaks for LNCMO and LNCMO-Al2% cathodes are displayed in Table S3. It can be found that the △V1, △V2, △V3 values of LNCMO-Al3% cathode are lowers than that of LNCMO cathode, demonstrating the introduction of Al3+ is beneficial to reduction of electrode polarization during the charge/discharge process. Figure 5b shows the first charge/discharge curves of LNCMO and Al doped LNCMO cathodes at 0.2 C-rate. It can be found that LNCMO and Al doped LNCMO cathodes show similar charge/discharge curves, which is consistent with the result of CV curves. The LNCMO cathode and Al3+ doped LNCMO cathode with aluminum molar quantities of 1.0 %, 2.0 %, 3.0 % exhibit discharge capacities of 210.7, 205.3, 203.5 and 201.8 mAh g-1 in the first cycle with coulombic efficiencies of 88.76, 85.92, 89.67 and 87.41 %, respectively. Descending initial specific capacities should be attributed to the 18 ACS Paragon Plus Environment

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incorporation of inactive Al3+. Figure 5c shows the cycling capability of LNCMO and Al doped LNCMO cathodes. The LNCMO cathode exhibits an initial discharge capacity of 195 mAh g-1 at 1 C-rate, but it undergoes a severe capacity fading in the subsequent cycles and discharge capacity decays to 101.88 mAh g-1 after 150 cycles. By comparison, the LNCMO cathodes with Al3+ doping show enhanced cyclability. The LNCMO-Al1%, LNCMO-Al2%, LNCMO-Al3% cathodes show lower initial discharge capacities of 192.2, 190.0 and 188.3 mAh g-1, respectively. But reversible discharge capacities can be maintained at 160.4, 174.0 and 163.0 mAh g-1 after 150 cycles, and corresponding capacity retention ratios reach up to 83.45, 91.58, 86.56 %, respectively. The results demonstrate that the LNCMO with 2 % (mole weight) Al3+ doping shows superior cycling capability. Moreover, the rate capability of LNCMO and Al doped LNCMO cathodes is tested and shown in Figure 5d. Rate measurement is carried out at incremental current densities from 0.2 C to 10 C every five cycles and current rate finally returns back to 0.2 C. At a low rate of 0.2, 0.5 and 1.0 C, the LNCMO cathode shows approximate discharge capacities compared to Al doped LNCMO cathodes. However, when the crates increase to 2, 3, 5 and 10 C, the LNCMO cathode shows obvious capacity decline, while Al doped LNCMO cathodes remain relatively high reversible capacity. Specially, LNCMO-Al1%, LNCMO-Al2% and LNCO-Al3% cathodes show reversible capacities of 156.8, 165.7, 162.7 mAh g-1 at the rate of 10 C, respectively. Enhanced cycling stability and rate capabilities should be attributed to homogeneous Al3+ doping benefited from 19 ACS Paragon Plus Environment


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radial distribution of primary particle, which can enhance structural stability and shorten the transport path of lithium ions.

Figure 6 (a) Median voltage curves of LNCMO cathode and Al doped LNCMO cathodes; Differential capacitance vs. voltage curves of (b) LNCMO and (c) LNCMO-Al2% cathodes; (d) Equivalent circuit and Nyquist plots of LNCMO and LNCMO-Al2% cathodes (e) in the first cycle and (f) after 150 cycles at 1.0 C.

Table 2 Fitting results of equivalent circuit from Nyquist plots for LNCMO and LNCMO-Al2% cathodes.

Samples

LNCO

LNCMO-Al2%

Rf (Ω)

Rct (Ω)

Rf (Ω)

Rct (Ω)

1st

3.172

13.74

2.644

7.537

150th

7.766

311.324

4.758

166.1

To further understand the improved electrochemical properties of homogeneous 20 ACS Paragon Plus Environment

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Al3+ doping cathodes, morphology of cycled electrodes, median voltage & differential capacitance curves, along with Nyquist plots before and after cycling are investigated. Figure S8 shows the morphology of LNCMO cathode electrode and LNCMO-Al2% cathode electrode before and after 150 cycles. Apparently, LNCMO cathode undergoes secondary grains breakage, which could be originated from anisotropic volume expansion and irreversible structural transformation by forming NiO rock salt on surface.12-13 By contrast, LNCMO-Al2% cathode still keep intact morphology after cycling. Therefore, homogeneous Al3+ doping can restrain secondary particle breakage due to the structural stabilization by strong Al-O bonding energy.8 Figure 6a shows the median voltage curves of LNCMO and Al doped LNCMO cathodes. Initial median voltages of the four cathodes are 3.8244, 3.8275, 3.8340, 3.8405 V, respectively, indicating that the initial median voltages are improved by the introduction of Al3+. Median voltage of LNCMO cathode decrease to 3.4967 V after 150 cycles at 1 C, while the median voltage of LNCMO-Al1%, LNCMO-Al2% and LNCO-Al3% cathodes can be maintained at 3.7466, 3.7962, 3.7779 V under the same condition. Differential capacitance vs. voltage curves are analyzed to explain the voltage decline of LNCMO cathode. As shown in Figure 6b-c, dQ/dV curves of LNCMO and LNCMO-Al2% cathodes show consistent cathodic peaks with CV curves in the initial cycle. It's worth noting that the dQ/dV curves of LNCMO-Al2% cathode have huge changes where the location of redox peaks has remarkable shifts to low voltage during cycling. Incremental initial median voltages and enhanced median voltages stability during cycling should be ascribed to improved symmetry of MeO6 octahedra by Al3+ 21 ACS Paragon Plus Environment


doping.8, 57 Figure 6e-f displays the Nyquist plots of the LNCMO and LNCMO-Al2.0% cathodes in the first cycle and 150th cycle under the charged state. To obtain the resistance parameters, Nyquist plots are fitted by equivalent circuit shown in Figure 6d. Re is assigned to the electrolyte resistance at high frequency; Rf represents the resistance of the surface SEI (solid electrolyte interface) film at middle frequency; Rct belongs to charge transfer resistance, and W is referred to Warburg impedance for Li+ diffusion.15, 58-60 As shown in Table 2, LNCMO-Al2.0% cathode get Rf and Rct values of 2.644 and 7.537 Ω in the first cycle, while the LNCMO cathode have higher Rf and Rct values 0f 3.172 and 13.74 Ω. In the 150th cycle, the Rf and Rct values for the LNCMO-Al2.0% cathode increases to 4.758 and 166.100 Ω, respectively. By comparison, the Rf and Rct values for the LNCMO cathode face a tremendous growth after 150 cycles, in which Rf value increase from 3.172 to 7.766 Ω and Rct value increase from 13.740 to 311.324 Ω. Augmented values of c-axis and lower Li+/Ni2+ mixing by incorporation of Al3+ are beneficial to Li+ transportation which result in lower Rf and Rct values during cycling.

4. CONCLUSION In general, Ni-rich LiNi0.88Co0.095Mn0.025O2 cathode with radial distribution of primary grains is successfully synthesized through design of precursor. Benefited from radially oriented primary grains, homogeneous Al3+ distribution in the bulk is realized by introduction of nano-Al2O3 during the sintering process. Homogeneous Al3+ doped cathodes exhibit enhanced cycling and rate capability, as well as improved voltages 22 ACS Paragon Plus Environment

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stability. Particularly, with the 2 % molar weight of Al3+ doping, capacity retention ratio of cathode is tremendously improved from 52.26 % to 91.57 % at 1 C rate after 150 cycles. Even at heavy current density of 5 (&10) C, a high reversible capacity of 172.3 (&165.7) mAh g-1 for LiNi0.88Co0.095Mn0.025O2-Al2% cathode can be acquired. Therefore, homogeneous Al3+ doping benefited from radial arrangement of primary grains provides an effective and practical way to prolong lifespan, as well as enhance rate capability and voltage stability of nickel-rich ternary materials.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Additional SEM images of precursors precursor section, Particle size distribution curves, XRD patterns of precursor, Surface elemental mapping, Rietveld refinement analysis, CV curves, SEM images of cycled electrode, ICP&tap density and Plane Intensity, (PDF).

■ AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel. +86 731 88830649 ORCID Zhian Zhang: 0000-0002-8691-6006 Yanqing Lai: 0000-0001-9510-5953 23 ACS Paragon Plus Environment


Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT This work is supported by the financial support of Guangdong Provincial Applied Science and Technology Research and Development Program (No.2017B010121004) and National Natural Science Foundation of China (No. 51674297 & No.51704134).

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Enhanced Cyclability and High-rate Capability of LiNi0.88Co0

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