Improving Li+ Kinetics and Structural ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 5653−5661

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Improving Li+ Kinetics and Structural Stability of Nickel-Rich Layered Cathodes by Heterogeneous Inactive-Al3+ Doping Peiyu Hou,† Feng Li,‡ Yanyun Sun,‡ Meiling Pan,# Xiao Wang,† Minghui Shao,† and Xijin Xu*,† †

School of Physics and Technology, University of Jinan, 336 Nanxinzhuang West Road, Jinan 250022, China School of Materials Science and Engineering, National Institute for Advanced Materials, Nankai University, 94 Weijin Road, Tianjin 300350, China # Department of Basic, Hebei University of Water Resources and Electric Engineering, 1 Chongqing Road, Cangzhou 061001, Hebei Province, China ACS Sustainable Chem. Eng. 2018.6:5653-5661. Downloaded from pubs.acs.org by UNIV OF NEW ENGLAND on 08/20/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: To improve the Li+ kinetics and structural stability of high-capacity nickel-rich layered oxides, but not at the cost of reducing reversible capacity, a heterogeneous inactive-Al3+ doping strategy is proposed to build an Al3+-rich surface within a low doping amount. As anticipated, the heterogeneous inactive-Al3+ doped nickel-rich LiNi0.7Co0.15Mn0.15O2 shows a large reversible capacity of ∼215 mAh g−1, corresponding to a high energy density of ∼850 Wh kg−1. Moreover, it also exhibits longterm cycle lifespan, capacity retention of ∼90% after 200 cycles even at a high upper cutoff voltage of 4.5 V (vs Li/Li+), and improved thermal stability. Surprisingly, the heterogeneous inactive-Al3+ doped electrode shows a high capacity of ∼145 mAh g−1 even at a high rate of 10C, which corresponds to ∼70% capacity retention at 0.1C, due to the enhanced Li+ kinetics. Also this heterogeneous inactive-ion doped approach is capable of being readily expanded to other types of layered, spinel and olivine cathodes to enhance their structural stability and Li+ kinetics. KEYWORDS: Lithium-ion batteries, Nickel-rich cathodes, Heterogeneous doping, Li+ kinetics, Structural stability

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(M= Mn, Co, Mg, Al, etc., x ≤ 0.5), are regarded as the most promising candidates for the next-generation cathode in that they have high reversible capacities (200−250 mAh g−1), reasonable rate capability, and low cost.5,21−29 Ni-rich cathodes Li[Ni1−xMx]O2, derived from pure LiNiO2 together with various transition metal ions substitutions, have a typical rhombohedral crystal structure with an R3̅m space group. It is generally known that the Ni2+/3+/4+ redox-active couples provide the majority of the reversible capacity of the Ni-rich electrodes.21,30 However, high oxidation state and associated unstable thermodynamics of the Ni4+ ions tend to produce a NiO-type Fm3̅m rock-salt phase together with oxygen species releasing on the surface of Ni-rich electrodes, especially during the highly delithiated state.31−34 Then, this electrochemically inactive rock-salt phase results in a drastic increase in the electrochemical impedance, and it also blocks

INTRODUCTION In contrast to silicon/carbon composites as anode materials that have large reversible Li+ capacities exceeding 1000 mAh g−1 and low cost, the now broadly applied LiCoO2 cathode delivers only ∼140 mAh g−1 due to the unwanted phase transition when the Li+ deintercalation ratio (x) exceeds 0.5 in Li1−xCoO2.1−7 This condition has inhibited the attractiveness of lithium-ion batteries (LIBs) for application in electric vehicles (EVs).5,8−11 The development of high-capacity cathodes with long life, high rate capability, and good safety has been the primary driving force in LIB development for the past three decades.9,11 Three main classes of cathode materials have been at the center of this development such as layered oxides including lithium-stoichiometric Li[NixCoyMn1‑x‑y]O2 and lithium-rich Li1+zM1−zO2 (M = Mn, Ni, Co, Ru, Sn, Ir, etc.),8,11−16 spinel LiM2O4 (M = Ni, Mn),17,18 and olivine LiMXO4 (M = Fe, Mn, Co; X = P, Si).19,20 By contrast with these spinel- and olivine-type cathodes, layered-type cathodes, particularly the nickel-rich (Ni-rich) oxides Li[Ni1−xMx]O2 © 2018 American Chemical Society

Received: February 25, 2018 Published: March 16, 2018 5653

DOI: 10.1021/acssuschemeng.8b00909 ACS Sustainable Chem. Eng. 2018, 6, 5653−5661

ACS Sustainable Chemistry & Engineering

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the Li+ ion diffusion path, finally leading to the decline of reversible capacity and rate capability.35 In addition, the released active oxygen atoms appear to react with the organic electrolyte and graphite anode, which produces a significant safety hazard.29,30,36 As a result, the Ni-rich electrodes usually exhibit a gradual fading of the reversible capacity, rate capability, and thermal stability during cycling, which has limited wide practical applications of this material for use in LIBs. Electrode doping using electrochemically inactive cations such as Na+, Mg2+, Al3+, Ti4+, etc. is generally used to stabilize the layered structure and suppress oxygen loss in Ni-rich positive electrodes as reported in previous studies.37−40 This approach is also employed to enhance the performance of LIB anodes, such as hydrogen ion doping and the creation of oxygen vacancy.41,42 Even though inactive cation doping has been shown to improve Ni-rich electrode performance, many intractable and irreconcilable problems persist in this approach. In particular, a low doping ratio of inactive ions cannot stabilize the host structure, whereas a high doping ratio of inactive ions will reduce the available capacity. Simultaneously obtaining high capacity, enhanced Li+ kinetics, and structural stability in Ni-rich cathodes via inactive cation doping is an intriguing course that has yet to be realized. In this work, to solve the currently recognized shortcomings of inactive-ion doping in Ni-rich cathodes, a heterogeneous inactive-Al3+ doped strategy is proposed to produce an Al3+-rich surface within a low doping amount (∼1.5 mol %), as depicted in Scheme 1. The formed Al3+-rich surface is expected to

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EXPERIMENTAL SECTION

Preparation of Precursors and LiNi0.7Co0.15Mn0.15O2. The schematic diagram of preparing the heterogeneous inactive-Al3+ doped LiNi0.7Co0.15Mn0.15O2 is depicted in Scheme S1. To synthesize the aimed spherical precursors, stoichiometric NiSO4·5H2O (368.01 g), CoSO4·6H2O (84.35 g), and MnSO4·H2O (50.70 g) were added into 1 L of deionized water (Tank 1) to form the 2 M first solution. Here, 11.25 g of Al(NO)3·9H2O and 22.88 g of 5-sulfosalicylic acid were dissolved in 100 mL of deionized water in Tank 3 as the second solution. Additionally, 7.63 g of 5-sulfosalicylic acid was dissolved into 100 mL of deionized water in Tank 2 as the third solution. At the initial stage of reaction, the solution in Tank 1 was incessantly dropped into the continuously stirred-tank reactor (CSTR, 10 L) at 40 mL h−1. Meanwhile, the formed Al3+ complex solution (Tank 3) was added into Tank 2 at a rate of 4 mL h−1 to form increasing Al3+ ions, which was also dropped into CSTR at 8 mL h−1. During CSTR, a NaOH solution (5M) as the precipitation agent and ammonia (1.0 M) as the ligand as well as for constant pH (11.7) were optimized to prepare the target micron-sized spherical precursors. After 25 h, the solutions in Tanks 1−3 were completely consumed, finally forming the said designed heterogeneous inactive-Al 3 + doped precursors [Ni0.7Co0.15Mn0.15](OH)2. The undoped (normal) precursors were also prepared through a similar method. The normal precursors with stoichiometric Li2CO3 (Li/M = 1.03) were calcined at 800, 820, and 840 °C for 12 h under pure oxygen ambience to optimize the preparation conditions of this layered Ni-based oxides LiNi0.7Co0.15Mn0.15O2. Materials Characterization. The energy density of cathode materials can be defined as the product of median voltage and reversible capacity. The Ni, Co, and Mn contents of precursors were detected by using inductively coupled plasma-atomic emission spectrometry (ICP-AES, SPS 7800, Seiko Instruments). A tap-density instrument (ZS-201) is utilized to measure the tap density of Ni-rich cathode materials. The morphology, distribution of transition-metal elements, and structure were measured by scanning electron microscopy (SEM, FEG250, FEI QUANTA), X-ray diffraction (XRD, Advance D8, Bruker), and transmission electron microscopy (TEM, FEI Tecnai F20). Fullprof soft with the WinplotR package was utilized for the Rietveld refinement of XRD data. Electrochemical Measurement. The preparation of Ni-rich electrodes can refer to our previous studies.43,44 Note that these positive electrodes have a similar active oxide loading (3.0−3.5 mg cm−2). The electrolyte is 1 M LiPF6 dissolved into ethylene carbonate and dimethyl carbonate solvent (1:1 by volume). A LAND instrument (CT-2001A) was used for electrochemical tests. An electrochemical workstation (CHI660e, Chenhua) is used for cyclic voltammetry (CV) testing. A differential scanning calorimetry (DSC) test was similar to our previous literature except that the electrodes were charged to 4.5 V (vs Li/Li+) at 0.1C (1C = 200 mA g−1) in this study.44

Scheme 1. Schematic Illustration of Heterogeneous InactiveAl3+ Doping Ni-Rich Layered Lithium Transition-Metal Oxide LiNi0.7Co0.15Mn0.15O2

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RESULTS AND DISCUSSION To produce a heterogeneous inactive-Al 3+ doped LiNi0.7Co0.15Mn0.15O2 (HI-Al3+ doped NCM), the aimed precursors were synthesized using a developed hydroxide coprecipitation route. The resulting heterogeneous inactive-Al3+ doped precursors appeared to be monodispersed spherical particles that were assembled as spindle-shaped primary grains as shown in Figure 1(a−c). The undoped (normal) precursors also exhibited a spherical morphology in their secondary particles; however, they were assembled as plate-shaped primary grains in Figure S1(a−c). The particle size of both precursors had a Gaussian distribution in Figure 1(d) and Figure S1(d), where the average particle size (D50) of the undoped precursors was slightly larger than the heterogeneous inactive-Al3+ doped sample. As shown in Figure 1(e), the D50 of heterogeneous inactive-Al3+ doped precursors was plotted as a

enhance the Li+ kinetics and structural stability of Ni-rich cathodes but not at the cost of reducing reversible capacity. As a consequence, the heterogeneous inactive-Al3+ doped nickel-rich LiNi0.7Co0.15Mn0.15O2 delivers a large reversible capacity of ∼215 mAh g−1, superior cycling stability, and oustanding high rate capability owing to the enhanced structural stability and Li+ kinetics. 5654


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Figure 1. (a−c) SEM images, (d) particle size distribution, and (e) growth curve of heterogeneous inactive-Al3+ doped precursors. (f) XRD patterns of both as-prepared hydroxide precursors.

Figure 2. (a−c) SEM images at various magnifications of the HI-Al3+ doped NCM. (d) Cross section of a single spherical particle. (e) Ni, Co, Mn, and Al concentrations from center to surface on this cross section. (f) HRTEM image of the HI-Al3+ doped NCM.

function of reaction time, and the fitted growth function is given as follows: D = 0.9506 + 2.7751T1/3

Ni(OH)2-type layered phases in Figure 1(f). However, a weak layered double hydroxide (LDH) was observed in the heterogeneous inactive-Al3+ doped precursors as a result of the rapid precipitation of Al3+ with OH−.45 ICP-AES measurements further demonstrated that the Ni−Co−Mn molar ratio was in agreement with the target of 0.7/0.15/0.15

(1)

where, D indicates D50 and T refers to reaction time. XRD results for the products confirmed that both precursors were β5655


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Figure 3. XRD results and Rietveld refinements of (a) the normal NCM and (b) the HI-Al3+ doped NCM.

Table 1. Refined Lattice Parameters, Degree of Li+/Ni2+ Mixing, and R Factors for Both NCM Cathodes before Charging Lattice parameters Samples 3+

HI-Al doped NCM Normal NCM

a [Å]

c [Å]

c/a

V [Å3]

Li+/Ni2+ mixing

χ2

Rp [%]

Rwp [%]

2.8693 2.8722

14.2243 14.2158

4.9574 4.9494

101.749 101.564

2.56 4.21

1.31 1.58

8.54 7.28

12.1 9.79

Figure 4. (a) Initial cycling curves, (b) cycle lifespan of capacity, (c) cycling stability of discharged potential and energy density, (d,e) continuous curves from fifth to 200th cycle at 25 °C, and (f) cyclability of capacity at 50 °C for the normal NCM and the HI-Al3+ doped NCM. Note that the performed current density is 20 mA g−1 (0.1C).

from Table S1. In addition, the total Al3+ doping content reached 1.6 mol % in the heterogeneous inactive-Al3+ doped precursors To optimize the preparation conditions of the Ni-rich LiNi0.7Co0.15Mn0.15O2, the lithium precursor materials with the stoichiometric ratio of Li2CO3 were calcined at 800, 820, and 840 °C for 12 h. As shown in Figure S2, the sample calcined at 820 °C exhibited the highest initial Coulombic efficiency and reversible capacity. Therefore, based on this result, the heterogeneous inactive-Al3+ doped precursors were prepared with a stoichiometric ratio of Li2CO3 and were also sintered at 820 °C for 12 h to form the HI-Al3+ doped NCM. It was confirmed from SEM analysis that the spherical secondary particles were maintained after the solid-state reaction (Figure 2(a−c) and Figure S3). But the primary grains of both NCM

were significantly different from the grains of the pristine precursors. In the latter case, the material appeared to be a 500 nm polyhedral powder. The close stacking of the monodispersed, microsized spherical powder produced a high packing density of 2.46 g cm−3 for the undoped NCM and 2.39 g cm−3 for the HI-Al3+ doped NCM, which improved the volumetric energy density in practical LIBs.46,47 After lithiation, the elemental composition on the cross section of a single spherical particle was analyzed using EDS, and the corresponding results are depicted in Figure 2(d,e). It was detected that the Al3+ concentration increased from the interior (∼0.5 mol %) to the surface (∼2.5 mol %) in a single particle, which confirmed the behavior of the heterogeneous inactive-Al3+ doping. Additionally, the primary grains were very compact, and very little porosity was found inside the particle as 5656


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Figure 5. (a) Rate discharge capacity, (b) power density, and (c,d) charge/discharge curves for the normal NCM and the HI-Al3+ doped NCM electrodes from 0.1C to 20C.

HI-Al3+ doped NCM electrode even after 200 cycles. By contrast, the undoped NCM electrode suffered from rapid decay of capacity. It delivered a low capacity (137.2 mAh g−1) and a poor capacity retention of 62.7%. As shown in Figure 4(b,c), owing to high capacity of above 210 mAh g−1 and high potential of ∼3.8 V, both Ni-rich NCM showed high energy densities of ∼850 Wh kg−1; however, the HI-Al3+ doped NCM exhibited an enhanced retention of 88.1% for the improved reversible capacity and discharge potential compared to the 58.8% energy density retention for the undoped NCM. It is evident from Figure 4 (d,e) that the HI-Al3+ doped NCM exhibited remarkably reduced electrochemical polarization. To further evaluate the cycling stability of the Ni-rich NCM that resulted from the heterogeneous inactive-Al3+ doping, similar cycling tests at high temperature (50 °C) were conducted as shown in Figure 4(f). These results confirmed that a higher capacity retention of 90.6% was obtained for the HI-Al3+ doped NCM electrode after 100 cycles at 50 °C. A capacity retention of 68.5% was observed for the undoped NCM, indicating that the heterogeneous inactive-Al3+ doping produced enhanced structural stability under severe working conditions. Therefore, it can be concluded that fabrication of an inactive-Al3+-rich surface in the Ni-rich layered transition-metal oxide cathode appeared to stabilize the structural stability. It is speculated that the Al−O bonding in the material has a stronger binding energy (512 kJ mol−1) than the Ni/Co/Mn−O bonding, which has been reported to be 391.6, 368, and 402 kJ mol−1, respectively.52,53 Apart from cycling lifespan, high rate capabilty is also critical for energy storage devices.54−59 Consequently, the rate

shown in Figure 2(d). From the HRTEM images, as shown in Figure 2(f), a continuous interference fringe spacing of the (003) crystal plane with a ∼ 0.47 nm distance was detected for the HI-Al3+ doped NCM, which may have been available for Li+ migration during the redox reactions. XRD patterns and the corresponding Rietveld refinement results for the undoped NCM and the HI-Al3+ doped NCM are shown in Figure 3 and Table 1. The XRD diffraction peaks indicate both NCM are indexed to a well-defined α-NaFeO2type layered structure (space group of R-3m).48 However, a decreased lattice parameter a value and an increased lattice parameter c value were found for the HI-Al3+ doped NCM. Furthermore, it should be noted that the HI-Al3+ doped NCM exhibited a lower degree of Ni2+ occupancy in Li+ sites compared to the pristine material. Therefore, it can be concluded that the doped inactive-Al3+ cations formed a solid solution with Ni3+ and Co3+ in the transition-metal 3a sites and facilitated the Li+ diffusivity as confirmed by these current results and those of previous investigations.49−51 Figure 4 reveals the cycling curves, exhibiting the cycle performance of the HI-Al3+ doped NCM electrode and the undoped NCM electrode between 3.0 and 4.5 V (vs Li/Li+). The HI-Al3+ doped NCM showed a high capacity of 213.6 mAh g−1 compared to the 218.9 mAh g−1 for the undoped NCM electrode at 25 °C as shown in Figure 4(a). Both NCM electrodes exhibited similar initial Coulombic efficiencies of around 90%. As anticipated, the HI-Al3+ doped NCM electrode exhibited significantly improved cycling stability over that of the normal NCM at 25 °C as shown in Figure 4(b,c). Surprisingly, an outstanding capacity retention of 89.5% was achieved for the 5657


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Figure 6. Cyclic voltammetry (CV) curves of (a) Li/normal NCM half-cells and (b) Li/HI-Al3+ doped NCM half-cells at various scanning rates from 0.1 to 0.4 mv s−1. Linear fitting results of (c) peak 1 and peak 2 and (d) peak 3 and peak 4.

To understand the kinetics properties during redox, the Li+ diffusion constant (D) is extracted by the Randles−Sevcik equation60

capability of both types of Ni-rich NCM electrodes was determined from 0.1C to 20C, and the results are shown in Figure 5. Though both Ni-rich electrodes exhibited similar capacities from 0.1C to 1C as shown in Figure 5(a), the HI-Al3+ doped NCM electrode delivered a higher specific rate capacity than the undoped NCM electrode as the C rate was increased, particularly above 5C. Surprisingly, the HI-Al3+ doped NCM exhibited high capacities of ∼145 and 115 mAh g−1 at rates as high as 10C and 20C, which corresponded to ∼70% and 55% capacity retention of 0.1C. This was much better than the results for the undoped Ni-rich NCM electrode. In addition, the HI-Al3+ doped NCM electrode also exhibited a higher power density than the undoped NCM electrode, especially at rates above 1C as shown in Figure 5(b). Increased electrochemical polarization was observed for both of the Ni-rich NCM electrodes as the rate was increased from 0.1C to 20C as shown in Figure 5(c,d). Nevertheless the HI-Al3+ doped NCM experienced much less polarization, which produced a higher discharge capacity and average cell voltage on discharge. In general, the Ni2+ occupied the Li layer block Li+ migration during charge/discharge processes, while the large interlayer spacing within c axis facilitates Li+ migration. Note that the HIAl3+ doped NCM has a lower Li+/Ni2+ cation mixing and an expanded c axis (Table 1), which probably makes it enhanced Li+ kinetics.

i p = 2.686 × 105n3/2A D1/2 Cv1/2

(2)

in which ip corresponds to the peak current in amps, n indicates the number of electrons transferred in the redox, A is the electrode area in cm2, υ refers to the scanning rate in V s−1, D is the Li+ diffusion coefficient in cm2 s−1, and C is the Li+ concentration in the electrode in mol cm−3. To determine the Li+ diffusion coefficient, cyclic voltammetry was perfomed on both types of electrodes in a half cell configuration at various scanning rates from 0.1 to 0.4 mv s−1. CV results and the linear fitting results for the peak current (ip) of redox reactions versus the square root of the scan rate (υ1/2) are depicted in Figure 6. These results show that a good linear relationship existed between ip with υ1/2. The calculated Li+ diffusion coefficients based on the slope of the fitted results are given in Table 2. As the results indicate, the Li+ diffusion constant for cathodic/anodic redox reactions was found to be 1.56 × 10−10 and 3.81 × 10−11 cm2 s−1 for the HI-Al3+ doped NCM electrode, which are nearly twice the values of the undoped NCM electrode (6.54 × 10−11 and 1.97 × 10−11 cm2 s−1). Thus, the enhanced Li+ intercalation/deintercalation 5658


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ACS Sustainable Chemistry & Engineering Table 2. Calculated Li+ Diffusion Coefficient of Normal NCM Electrode and HI-Al3+ Doped NCM Electrode during Redox Reactions

cathode. These improved electrochemical properties of the heterogeneous inactive-Al3+ doped material was probably due to the enhanced structural stability and Li+ kinetics. This proposed approach also gives a new insight into methods for improving the electrochemical performance of other-type layered spinel or olivine cathodes for advanced LIBs.

Li+ diffusion coefficient (cm2 s−1) Samples

Charge

Discharge

HI-Al3+ doped NCM Normal NCM

1.56 × 10−10 6.54 × 10−11

3.81 × 10−11 1.97 × 10−11

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ASSOCIATED CONTENT

S Supporting Information *

kinetics produced a higher rate capability for the HI-Al3+ doped layered transition-metal oxide cathodes. Ni-rich layered cathodes generally present thermal instability in that the released oxygen probably will accelerate thermal runaway by reacting with the organic electrolyte.29,36 To estimate the thermal characteristics of both Ni-rich NCM electrodes, DSC analyses after the electrodes initially charged to 4.5 V were utilized, and the test results are shown in Figure 7.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00909. Schematic diagram of preparing the HI-Al3+ doped NCM; SEM, XRD, ICP-AES and charge/discharge curves. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X. Xu). ORCID

Peiyu Hou: 0000-0003-0476-5812 Xijin Xu: 0000-0002-3877-6483 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Shandong Provincial Natural Science Foundation (ZR2017BEM010), the National Natural Science Foundation of China (51672109), and the Natural Science Foundation of Shandong Province for Excellent Young Scholars (ZR2016JL015).

Figure 7. DSC results of both delithiated Ni-rich electrodes that are charged to 4.5 V (vs Li/Li+) at 0.1C.

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Both DSC profiles show that the exothermal curves nearly follow a normal distribution, in which a sharp peak pattern was presented for the normal delithiated electrode while the HI-Al3+ doped electrode exhibited a broadened curve. Notably, in contrast with the sharp peak with drastic heat generation, the latter one corresponds to a slow heat generation process, indicating a good safety property. In addition, the delithiated HI-Al3+ doped Ni-rich electrode revealed elevated peak temperature and reduced amount of heat release, which stands for the delayed thermal runaway reaction in full batteries. Thus, the improved thermal stability was also achieved for the HI-Al3+ doped Ni-rich electrode, which was probably attributed to the stronger Al−O bonding than that of Ni/Co/Mn−O bonding.30 These results suggest the HI-Al3+ doped Ni-based layered oxides with gradually increased Al concentration will be very promising cathode materials for long life, high rate capability, and safe LIBs.

REFERENCES

(1) Goodenough, J. B.; Park, K. S. The Li-ion Rechargeable Battery: a Perspective. J. Am. Chem. Soc. 2013, 135 (4), 1167−1176. (2) Li, W.; Song, B.; Manthiram, A. High-voltage Positive Electrode Materials for Lithium-ion Batteries. Chem. Soc. Rev. 2017, 46 (10), 3006−3059. (3) Eftekhari, A. Lithium-Ion Batteries with High Rate Capabilities. ACS Sustainable Chem. Eng. 2017, 5 (4), 2799−2816. (4) Myung, S. T.; Maglia, F.; Park, K. J.; Yoon, C. S.; Lamp, P.; Kim, S. J.; Sun, Y. K. Nickel-rich Layered Cathode Materials for Automotive Lithium-ion Batteries: Achievements and Perspectives. ACS Energy Lett. 2017, 2 (1), 196−223. (5) Manthiram, A.; Song, B.; Li, W. A Perspective on Nickel-rich Layered Oxide Cathodes for Lithium-ion Batteries. Energy Storage Mater. 2017, 6, 125−139. (6) Reimers, J. N.; Dahn, J. R. Electrochemical and In Situ X−Ray Diffraction Studies of Lithium Intercalation in LixCoO2. J. Electrochem. Soc. 1992, 139, 2091−2097. (7) Hou, P.; Chu, G.; Gao, J.; Zhang, Y.; Zhang, L. Li-ion Batteries: Phase Transition. Chin. Phys. B 2016, 25 (1), 016104. (8) Ko, M.; Oh, P.; Chae, S.; Cho, W.; Cho, J. Considering Critical Factors of Li-rich Cathode and Si Anode Materials for Practical Li-ion Cell Applications. Small 2015, 11, 4058−4073. (9) Lee, J. H.; Yoon, C. S.; Hwang, J.-Y.; Kim, S.-J.; Maglia, F.; Lamp, P.; Myung, S.-T.; Sun, Y.-K. High-energy-density Lithium-ion Battery using a Carbon-nanotube−Si Composite Anode and a Compositionally Graded Li[Ni0.85Co0.05Mn0.10]O2 Cathode. Energy Environ. Sci. 2016, 9 (6), 2152−2158. (10) Hou, P.; Yin, J.; Ding, M.; Huang, J.; Xu, X. Surface/Interfacial Structure and Chemistry of High-energy Nickel-rich Layered Oxide Cathodes: Advances and Perspectives. Small 2017, 13, 1701802.

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CONCLUSIONS In summary, a heterogeneous inactive-Al3+ doped strategy is proposed to produce an Al3+-rich surface within a low doping amount and to solve the currently recognized shortcomings of inactive-ion doping in Ni-rich LIB cathodes As anticipated, the heterogeneous inactive-Al3+ doped Ni-rich electrode showed a high capacity of around 215 mAh g−1, which corresponded to a high energy density of ∼850 Wh kg−1. In addition, this novel material also exhibited a long lifespan, enhanced safety, and outstanding rate capability compared with the undoped Ni-rich 5659


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