Hierarchically Structured Lithium-Rich Layered ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Hierarchically Structured Lithium-Rich Layered Oxide with Exposed Active {010} Planes as High-Rate-Capability Cathode for Lithium-Ion Batteries Ruizhi Yu, Xiaohui Zhang, Tao Liu, Xia Xu, Yan Huang, Gang Wang, Xianyou Wang,* Hongbo Shu, and Xiukang Yang National Base for International Science & Technology Cooperation, National Local Joint Engineering Laboratory for Key Materials of New Energy Storage Battery, Hunan Province Key Laboratory of Electrochemical Energy Storage & Conversion, School of Chemistry, Xiangtan University, Yuhu District, Xiangtan 411105, Hunan, China

ABSTRACT: Lithium-rich layered oxide is an attractive candidate for high-energy-density cathodes in lithium-ion batteries. However, the low cycling performance and poor rate capability severely impede its commercial applications, among which rate capability is a serious barrier, because it will lead to serious energy fading during fast charging/discharging. To cure this issue, we report an efficient strategy to fabricate high-rate and cycling-stable hierarchically structured lithium-rich layered oxide, Li1.2Mn0.54Ni0.13Co0.13O2+δ, by using the evolutionary coprecipitate method and subsequent high-temperature calcination technique. The structure and electrochemical performance of this cathode are systematically investigated, and the results reveal that the hierarchically structured microsphere is self-assembled with nanoscaled grains and radial nanoplates with exposed active {010} planes, which favor Li+ transport kinetics and structural stability during the charge/discharge process. Benefiting from the unique architecture, this cathode material reciprocates a high initial reversible capacity (282.5 mA·h·g−1) and excellent cycling performance (93.6% capacity retention after 150 cycles at 0.5 C and 83.8% capacity retention after 200 cycles at 5 C). Moreover, it exhibits an outstanding rate capability and can achieve about 55.2% (155.8 mA·h·g−1) of the capacity at 0.1 C within about 4.7 min of ultrafast charging/discharging (10 C). The favorable results provide a feasible route to enhance the electrochemical performance of lithium-rich layered oxide for constructing high-energy and high-power lithium-ion batteries. KEYWORDS: Hierarchically structured microspheres, Exposed active planes, Lithium-rich layered oxide, Lithium ion batteries, High rate capability

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INTRODUCTION

insufficient to meet the growing demands of EVs due to the low voltage plateaus and limited specific capacities, below 190 mA· h·g−1.6 Thus, the exploration of advanced cathode materials becomes important. One potential cathode material that can meet the high-energy-density requirements is lithium-rich layered oxide (LLO).7 LLO, either as a solid solution or as a nanocomposite of rhombohedral LiTMO2 (space group R3̅m) and monoclinic Li2MnO3 (space group C2/m), can deliver ultrahigh specific capacities (>250 mA·h·g−1) with an average discharge voltage of >3.5 V.8−10 Numerous studies concluded

Lithium-ion batteries (LIBs) now have been applied as the main power source for electric vehicles (EVs) and hybrid electrical vehicles (HEVs) because LIBs own the highest energy densities compared with all secondary batteries.1−5 The present EV batteries have a specific energy of 200−250 W·h·kg−1, but the Battery 500 Consortium proposes a very challenging target of 500 W·h·kg−1 with service life over 10 years and total mileage of ∼150 000 miles.5 As is known to all, the performance of LIBs is predominantly restricted by the cathode material. 3 However, the resulting specific energies of commercially adopted cathode materials, such as layered structures (LiCoO 2 , LiNi 0 . 5 Co 0 . 2 Mn 0 . 3 O 2 , and LiNi0.8Co0.15Al0.05O2), olivine LiFePO4, and spinel LiMn2O4, are © 2017 American Chemical Society

Received: June 4, 2017 Revised: August 13, 2017 Published: August 30, 2017 8970

DOI: 10.1021/acssuschemeng.7b01773 ACS Sustainable Chem. Eng. 2017, 5, 8970−8981

Research Article

ACS Sustainable Chemistry & Engineering

phase, which showed high discharge capacity and remarkable cycling performance and rate capability. In our previous work,34 we have rationally designed and successfully prepared a new type of fusiform porous micro/nanostructured 0.5Li2MnO3· 0.5LiNi1/3Co1/3Mn1/3O2 cathode material through a facile coprecipitation strategy, and it delivered a remarkable capacity of 139.5 mA·h·g−1 at 10 C and a high capacity retention of 87.1% after 200 cycles at 0.5 C. More recently, it has been widely reported that Li+ preferably migrates two-dimensionally along the (001) planes in αNaFeO2 structural LLO.35−39 That is, only the {010} lattice planes, including (010), (1̅10), (1̅00), (01̅0), (11̅0), and (100) facets, can afford unimpeded paths for Li+ diffusion during the charge/discharge process. Thus, exploring LLO nanoplates with exposed {010} planes is feasible to facilitate Li + deintercalation/insertion and enhance the rate capability of LLO. For example, platelike hierarchical Li1.2Mn0.54Ni0.13Co0.13O2 with exposed {010} planes prepared by Zeng et al.39 presented excellent capacity of 182 mA·h·g−1 after 100 cycles at 5 C between 2.0 and 4.8 V. Wu and coworkers37,38 synthesized hierarchical Li1.2Ni0.2Mn0.6O2 nanoplates with exposed {010} planes and sphere-shaped hierarchical Li1.2Ni0.13Mn0.54Co0.13O2 with enhanced growth of nanocrystal planes, these cathode materials display outstanding electrochemical properties, especially in the case of high-rate charge/discharge. On this basis, the combination of both hierarchical structure and electrochemically active {010} planes is essential to significantly improve the integral electrochemical performance of LLO. However, structural and morphological control of LLO is complex because the intricate growth processes involved have dissimilar microscopic mechanisms and reaction rates.21 Additionally, the high-surface-energy facets, such as (001) and (010) planes, are prone to vanish during synthesis.37,40 As a consequence, the controllable fabrication of LLO possessed of both hierarchical structure and exposed {010} planes remains a significant challenge. Herein, we successfully fabricated porous hierarchical structured Li1.2Mn0.54Ni0.13Co0.13O2+δ microspheres self-assembled with nanoscaled grains and radial nanoplates via a unique approach; a schematic illustration is shown in Scheme 1. It is demonstrated that the nanoscaled primary grains and nanoplates are self-assembled into micrometer-sized secondary particles, which can guarantee the structural stability. Meanwhile, a porous framework can be formed, and the gaps between nanoscaled grains allow much easier penetration of the electrolyte into the electrodes. Furthermore, the nanoplates can constitute 3D divergent paths span from the center to the surface of microspheres, and the lateral planes of the nanoplates are exposed electrochemically active {010} planes, which affords excellent Li+ intercalation/deintercalation kinetics. In brief, this unique architecture may bring us a step closer to the kinetic requirements for long-term life and fast charging/ discharging of an ideal electrode material. The influence of morphology and structure on the electrochemical performance of as-prepared cathodes is systematically investigated in detail.

that high capacity of the LLO is divided into two mechanisms: one is the activated manganese redox process (Mn3+/Mn4+), and another is reversible anionic redox reactions (O2−/O22− or O2−/O2n−, 1 < n < 3) accompanying an irreversible oxygen loss.11−14 Recent studies indicate that reversible participation of the distinct anionic redox reaction of O2− is the origin of the ultrahigh capacity beyond the redox of transition metals.10,15 With significant enhancement in capacities and cycling performance for the past few years, LLO can reach considerable energy densities of ∼800 W·h·kg−1 after >100 cycles.3 Furthermore, LLO is economically attractive due to the high content of manganese, which is much cheaper and less toxic than cobalt. Thus, LLO shows great promise as a nextgeneration cathode material for EVs. However, despite these technological advantages, LLO encounters a series of obstacles in commercial development, such as large first-cycle irreversible capacity loss, limited longterm performance, severe voltage fading, and intrinsic poor rate capability.8,16 Three main reasons are responsible for these issues: (i) irreversible extraction of Li+ from the Li2MnO3 matrix and the accompanying oxygen loss when first charged above 4.5 V; (ii) rearrangement of surface structure during the initial activation and structural transformation during cycling; and (iii) low conductivities and sluggish kinetics itself.16−21 To minimize these issues, some promising approaches have been explored. Surface coating technologies of LLO with various compounds, such as metal fluoride (AlF3),22 metal oxide (ZnO),23 metal phosphate (FePO4),24 or conductive material (reduced graphene oxide),25 are widely used to prevent the LLO from being etching by the electrolyte and enhance the surface electronic conductivity. These methods can effectively improve the cycling performance and Coulombic efficiency of LLO.26−28 However, most of these works involve tedious synthetic processes; moreover, since most of the surfacecoating layers are electrochemically inactive during the charge/ discharge process, the advanced structural stability comes at the cost of reducing the specific capacity and energy density of the cathode.29 Given that the properties of cathode materials are highly dependent on the morphology and structure, rational architecture design has been proposed.30 Many researchers have made lots of efforts to improve electrochemical performance (especially rate capability) of LLO through reducing particle size to nanoscale to shorten the lithium-ion diffusion length. Nevertheless, these nanoscaled materials are vulnerable to severely undesired side reactions at the electrode/electrolyte interface due to the excessively high surface area, which may induce irreversible capacity loss and sacrifice the structural stability of the materials.30−32 Another problem encountered with nanoscaled cathodes is low volumetric energy density, because of their relatively low taping density.6 Recently, a category of morphological design of hierarchical architecture, assembly of nanosized primary particles, has acted as a demonstration to integrate the merit of both nanosized grains and microsized matrix with enhanced Li+ diffusion and reinforced structural stability. Moreover, hierarchical structures can deliver higher volumetric energy density than nanoscaled materials. Fu et al.21 reported that a hollow porous hierarchicalstructured 0.5Li2MnO3·0.5LiMn0.4Co0.3Ni0.3O2 can deliver a high rate capability (162.6 mA·h·g−1 at 10 C) and a capacity retention of 87% after 100 cycles at 0.5 C. Wang and coworkers33 reported nanotube-assembled three-dimensional (3D) Li-rich hierarchitectures with a layered-spinel composite

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EXPERIMENTAL SECTION

Material Preparation. The hierarchical lithium-rich layered oxide Li1.2Mn0.54Ni0.13Co0.13O2+δ was synthesized by an evolutionary coprecipitate method and subsequent high-temperature calcination technique, as illustrated in Scheme 1. All the chemicals (analytically pure) were purchased from Guanghua Sci-Tech Co., Ltd. (Guangdong, China) and used without further purification. In a 8971


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of equal volumes of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the electrolyte, and porous polypropylene-based membrane (Celgard 2325) as the separator in a glovebox filled with argon, where the oxygen and moisture contents were maintained below 5 ppm. Galvanostatic charge/discharge tests were performed on a battery tester (CT-3008, Neware Co., Ltd.) with a voltage window of 2.0−4.6 V (vs Li+/Li) at selected current rates. The current density of 1 C was based on a capacity of 200 mA·g−1. For the galvanostatic intermittent titration technique (GITT) test, batteries were charged/discharged at 0.05 C for activation in the voltage range 2.0−4.6 V and then charged/ discharged at 0.1 C for an interval τ of 10 min. Subsequently, the batteries were kept in open-circuit voltage (OCV) for 60 min to allow the voltage to relax to the steady-state value Es.

Scheme 1. Schematic Diagram of Synthetic Route for Porous Hierarchically Structured Lithium-Rich Layered Oxide with Exposed Active {010} Planes

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RESULTS AND DISCUSSION The atomic composition of H- and C-LMNCO samples was measured by ICP-OES, and the results are tabulated in in Table Table 1. Atomic Composition of Obtained Samples Examined by ICP-OES Analysis normalized element contenta sample Li1.2Mn0.54Ni0.13Co0.13O2+δ (HLMNCO) Li1.2Mn0.54Ni0.13Co0.13O2+δ (CLMNCO)

typical synthesis, 16 mmol of MnCl2·4H2O, 4 mmol of NiCl2·6H2O, and 4 mmol of CoCl2·6H2O were first dissolved in 120 mL of deionized water, named solution A. Synchronously, a sodium dodecyl sulfate (SDS; 0.1 mol·L−1, 120 mL) solution was added into solution A at vigorous stirring. Then Na2CO3 solution (0.2 mol·L−1, 120 mL) was dropped slowly into the above mixed solution at room temperature under vigorous stirring to obtain a suspension. After stirring of the suspension for 2 h, the precipitate was collected by filtration and washed with deionized water and ethanol several times. Subsequently, the washed sample was sonicated in an ultrasonic cleaning bath for 10 min and then dried in an oven at 80 °C for 12 h to obtain the hierarchically structured precursor (H-precursor). Thereafter, Hprecursor was preheated at 600 °C for 6 h to convert into hierarchically structured oxide powder (H-oxide) in a muffle furnace. To form the final hierarchically structured Li1.2Mn0.54Ni0.13Co0.13O2+δ (H-LMNCO) material, the H-oxide was mixed with an appropriate amount of Li2CO3 and calcined at 750 °C for 12 h in air. The synthetic steps for the conventional precursor (C-precursor), oxide powder (Coxide), and Li1.2Mn0.54Ni0.13Co0.13O2+δ (C-LMNCO) material were identical to those for the H-precursor, H-oxide, and H-LMNCO samples except that SDS was not added as a structure-directing agent. Materials Characterization. Chemical composition of the asprepared materials was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 7300 DV, PerkinElmer Co.). A scanning electron microscope (SEM, JSM6100LV, JEOL, Japan) was implemented to analyze the size and morphology of the samples. Surface area (SBET) of the samples was determined by the Brunauer−Emmett−Teller (BET) method (TriStar II 3020, Micromeritics USA) with nitrogen as adsorption/desorption gas. X-ray diffraction (XRD) patterns of samples were collected on an X-ray diffractometer (Bruker AXS D8, Bruker AXS GmbH), equipped with Cu Kα radiation at 40 kV and 40 mA. The XRD data were collected in the scattering angle (2θ) range 10°−80° at a step size of 0.1° and a counting time of 1 s/step. Further structural characterization and elemental distribution of the materials were carried out on a transmission electron microscope (TEM, JEM-2100F, JEOL, Japan) equipped with an energy-dispersive X-ray (EDX) spectroscope. Electrochemical Measurements. The positive electrode was composed of 80 wt % active material, 10 wt % binder [poly(vinylidene fluoride), PVDF], and 10 wt % conductive additive (acetylene black) with an aluminum foil current collector. Generally, the loading mass of active material was 4−5 mg·cm−2. CR2025 coin cell assembly was carried out with lithium foil as the anode, 1 mol·L−1 LiPF6 in a mixture

a

Li

Mn

Ni

Co

1.180

0.540

0.128

0.132

1.190

0.540

0.131

0.129

Mn = 0.540 was used for all samples.

1. It can be seen that the exact ratios of Li:Mn:Ni:Co in the samples are approximately consistent with the expected Li1.2Mn0.54Ni0.13Co0.13O2+δ stoichiometry within experimental error. Figure 1a exhibits the XRD patterns of H- and C-LMNCO cathode materials. In the XRD patterns, all peaks are welldefined and sharp, indicating that both H- and C-LMNCO samples are universally well-crystallized. Apart from a number of peaks between 20° and 25°, the main diffraction patterns are in accordance with the typical hexagonal α-NaFeO2-type structure (space group R3̅m), which is similar to the typical layered structure of LiMn1/3Ni1/3Co1/3O2 illustrated in Figure 1b. The inset in Figure 1a shows weak superlattice peaks between 20° and 25°, corresponding to the internal cation arrangements of LiMn6 in the transition metal layers and characteristic of the integrated monoclinic Li2MnO3 (Figure 1c).41−43 In addition, clear separations of the adjacent peaks of (006)/(012) and (018)/(110) for as-prepared H- and CLMNCO samples imply the formation of ordered layered structure.23 Usually, the intensity ratios I(003)/I(104) and (I006 + I012)/I101 are used to evaluate the degree of cation mixing. The larger value of I(003)/I(104) and smaller value of (I006 + I012)/I101 indicate lower cation mixing and better hexagonal ordering.44,45 The I(003)/I(104) values for as-prepared H- and C-LMNCO samples are 1.28 and 1.27, and the corresponding (I006 + I012)/ I101 values are 0.378 and 0.382, respectively, implying that HLMNCO has the better cation arrangement and more favorable layered structures. The morphologies and microstructures of as-obtained precursors, oxides, and their corresponding cathode materials were evaluated by SEM. Figure 2exhibits SEM images of (a) Hprecursor and (g) C-precursor. The precursors appear as homogeneous spheriform secondary particles assembled from randomly oriented nanoparticles. However, the H-precursor and C-precursor do not share the same surface morphology and 8972


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Figure 1. (a) XRD patterns of H- and C-LMNCO cathode materials. (b, c) Crystal structures of (b) layered LiTMO2 (space group R3̅m) and (c) monoclinic Li2MnO3 (space group C2/m).

Figure 2. SEM images of (a) H-precursor and (d) magnified image, (b) H-oxide and (e) magnified image, (c) H-LMNCO cathode material and (f) magnified image; (g) C-precursor and (j) magnified image, (h) C-oxide and (k) magnified image, and (i) C-LMNCO cathode material and (l) magnified image.

the difference between H-oxide and C-oxide becomes more evident. The primary grains of C-oxide display obvious aggregations (Figure 2h,k), whereas those of H-oxide are well-defined nanoscaled particles on the surface of microspheres (Figure 2b,e). Furthermore, as-prepared H-LMNCO (Figure 2c,f) and C-LMNCO (Figure 2i,l) cathode materials essentially maintain their respective morphology after the high-

diameter. It can be seen that the H-precursor (Figure 2d) possesses a coarser surface morphology and a smaller diameter (1−2 μm) than the C-precursor (diameter 2−4 μm, Figure 2j). This is because a certain amount of SDS as a structure-directing agent can effectively slow down growth velocity of particles and can regulate crystallization of the precursor. After preheating at 600 °C for 6 h, the precursors convert into oxide powder, and 8973


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Figure 3. Nitrogen adsorption/desorption isotherms and pore-size distributions of (a) H-LMNCO and (b) C-LMNCO samples.

Figure 4. (a) TEM image of a single H-LMNCO microsphere, (b) HR-TEM image and FFT pattern of frontal plane in region 1, (c) HR-TEM image and FFT pattern of frontal plane in region 2, (d) HR-TEM image and FFT pattern of lateral plane in region 3, and (e) EDX spectra of HLMNCO sample.

which is about twice that of C-LMNCO sample (4.658 m2·g−1). This may be ascribed to the reduced agglomeration of primary grains and the pores existing among particles. Besides, the calculated total pore volume of H-LMNCO sample is 5.951 × 10−2 cm3·g−1, which is much larger than that of C-LMNCO sample (1.968 × 10−2 cm3·g−1). Additionally, the tap densities of H- and C-LMNCO materials are about 1.57 and 1.61 g·cm−3, respectively. In general, appropriate specific surface area and porous structure can directly enlarge the solid−liquid interface in the final electrodes and simultaneously facilitate travel of Li+ from the electrolyte to the solid material. Furthermore, the maintained connected points between the primary particles result in a facile transfer of Li+ in the solid active material.46 These advancements are expected to promote the rate capability of LLO. Moreover, this type of porous structure is supposed to effectively buffer and suppress large volume swings during the repeated Li+ insertion/extraction process, thus maintaining structural integrity in the charge/discharge process.47 TEM and high-resolution (HR) TEM analyses were used to further elucidate accurate structural characteristics of H-

temperature lithiated reaction. Especially for the H-LMNCO sample, a few radial nanoplates are interspersed in the sample and some pores between the primary grains can be clearly observed, which may enlarge the specific surface area of the cathode material. The appropriate specific surface area can generate more contact areas between electrode and electrolyte, and thus more reactive points may be afforded during the charge/discharge process.46 Overall, the secondary particles of H-LMNCO sample have homogeneous hierarchical morphologies, and porosity exists distinctly among the nanoscaled primary grains. To further verify the porosity of H-LMNCO sample, nitrogen adsorption/desorption measurements were carried out. Figure 3 shows the N2 adsorption/desorption isotherms and Barrett−Joyner−Halenda (BJH) pore-size distribution (insets) analysis of H- and C-LMNCO samples. As seen in Figure 3, both samples appear to be a type III category, demonstrating mesoporosity. Correspondingly, the average pore sizes of H- and C-LMNCO samples are 2.72 and 7.75 nm. It is noteworthy that the results show that H-LMNCO sample exhibits a BET specific surface area of 11.048 m2·g−1, 8974


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Figure 5. (a) Cycling performance of H- and C-LMNCO cells at 0.5 C after activation by initial cycling at 0.1 C between 2.0 and 4.6 V. (b, c) Continuous charge/discharge profiles of the first, second, and 151st cycles for (b) H-LMNCO and (c) C-LMNCO cells.

Figure 6. SEM images of H-LMNCO electrode at (a) low and (b) high magnification after 150 cycles at 0.5 C.

concluded that the frontal planes of nanoplates are parallel to the (001) plane of layered structure, and the lateral planes that are perpendicular to (001) plane logically correspond to {010} planes, which are normal to the facile pathway for Li+ migration and electrochemically active. To present direct evidence to support this conclusion, one of the nanoplates’ lateral plane is evaluated. The HR-TEM image and FFT pattern of the lateral plane (region 3, Figure 4d) show an interplanar distance of 0.475 nm assigned to the distance of the transition metal layers of rhombohedral LiTMO2 along the [001] direction.38 This confirms that the lateral planes assuredly correspond to the {010} planes. Combined with the morphology displayed in Figure 2, it can be concluded that the H-LMNCO microspheres

LMNCO sample, especially for the radial nanoplates in the sample (Figure 4). It can be clearly seen from Figure 4a that the H-LMNCO matrix consists of many interconnected nanoplates and nanoparticles with porous structure, which is in good agreement with the previous SEM results. Figure 4b,c shows HR-TEM images of the frontal planes from two different nanoplates (regions 1 and 2) and their corresponding fast Fourier transform (FFT) patterns (insets). The FFT image confirms that the appearing reflections of the nanoplates are uniquely indexed on the basis of the monoclinic structure of Li2MnO3, and two sets of lattice fringes of 0.409 and 0.426 nm, corresponding to the lattice spacing of (110) and (020) planes of Li2MnO3, are clearly presented. Therefore, it can be 8975


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Figure 7. (a) Rate capabilities of H- and C-LMNCO cells. (b, c) Discharge curves of (b) H-LMNCO and (c) C-LMNCO cells at various rates in the voltage range 2.0−4.6 V. (d) Discharge capacity vs cycle number for H- and C-LMNCO cells at 5 C between 2.0 and 4.6 V.

possess exposed {010}-oriented planes on the surface. Thus, porous hierarchically structured Li1.2Mn0.54Ni0.13Co0.13O2+δ microspheres with exposed {010} planes are verified, which match well with our original design. With both the unobstructed divergent paths and increased exposed active planes at the surface, superior Li+ transport kinetics is expected for H-LMNCO sample. Besides, EDX spectra of the HLMNCO sample (Figure 4e) demonstrates that the molar ratio of Mn, Ni and Co elements is close to the designed ratio of 0.54:0.13:0.13, which is compatible with the above ICP-OES results. To estimate the electrochemical behavior of as-prepared cathode materials, H- and C-LMNCO samples were evaluated

as the cathode active material in coin cells and then tested at 0.5 C between 2.0 and 4.6 V after activation by initial cycling at 0.1 C. Figure 5 shows the cycling performance and continuous charge/discharge profiles of H-LMNCO and C-LMNCO cells. During the initial cycling, both H- and C-LMNCO cells reveal a typical initial charge/discharge characteristic of the LLO cathode. The initial charging process can be divided into two regions: (1) a gradual increase of voltage up to 4.4 V, corresponding to the oxidation of Ni2+/Ni4+ and Co3+/Co4+, accompanied by Li+ extraction from layered LiTMO2 structure; and (2) a long plateau of voltage higher than 4.4 V, in which Li2MnO3 is activated.48 During region 2, Li+ derived from the component Li2MnO3 is further extracted from the layered 8976


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ACS Sustainable Chemistry & Engineering Table 2. Performance Comparison of This Work with Some Other Similar Composites structure and morphology

current density (mA·g−1)

hierarchically structured Li1.2Mn0.54Ni0.13Co0.13O2+δ with exposed active {010} planes

specific capacity (mA· h·g−1)

2000

155.8

hierarchical Li[Li0.2Ni0.2Mn0.6]O2

1000 1600

192.7 ∼160

hierarchically spherical Li1.2Mn0.56Ni0.16Co0.08O2

2000

131.2

hierarchical nano/microspherical Li1.2Ni0.2Mn0.6O2

1000

122.6

Li1.2Ni0.2Mn0.6O2 nanoplates with {010} planes

1500

113.8

orthogonally arranged {010}-oriented Li-rich layered oxide nanoplates

1000

structure, whereas oxygen loss occurs with Li+ extraction. Meanwhile, partial Mn4+ can be activated electrochemically, leading to the rearrangement of crystal structure.49 During the first discharging processes, sloping discharge voltage profiles are observed in the voltage range applied (4.6−2.0 V), which are correlated with the intercalation of Li+ into the generated layered LixMO2 frameworks. However, the extractions of Li+ and oxygen from the Li2MnO3 component are difficult to reinsert into the structure due to structural rearrangement, which can result in low initial Coulombic efficiency. Simultaneously, the decomposition of electrolyte and the formation of solid−electrolyte interface (SEI) film can further increase the initial irreversible capacity.47−49 The ideal charge/ discharge process in the first cycle can be expressed by eqs 1−3:47,49

(1)

x Li 2MnO3 ·(1 − x)MO2 → x MnO2 ·(1 − x)MO2 + x Li 2O

ref this work

∼96.4 (50 mA·g−1, 200 cycles) 80.7 (200 mA·g−1, 100 cycles) 94.4 (100 mA·g−1, 150 cycles) ∼80% (30 mA·g−1, 50 cycles) 92 (30 mA·g−1, 100 cycles)

6 50 51 52 53

To investigate the excellent structural stability of H-LMNCO sample, the H-LMNCO cell after 150 cycles was dissembled and the H-LMNCO electrode was rinsed with organic solvent in a glovebox, and then the electrode was collected and further demonstrated by SEM (Figure 6). As shown in Figure 6, it can be clearly observed that H-LMNCO electrode is composed of spherical cathode materials and other residue, which may be PVDF and acetylene black. It is noteworthy that the hierarchically structured microspheres are well maintained without significant agglomeration and pulverization. Only the surface morphology of the particles shows some small changes due to the side reactions at electrode/electrolyte interface. Maintenance of the hierarchical porous structure ensures that the high reversible capacity can be obtained after long-term cycling, and this result is consistent with the electrochemical performance described. In addition to cycling performance, the rate capability of cathodes is another especially important property for practical application as a power battery for EVs. To deliberate the electrochemical behavior of as-obtained cathodes, rate capabilities of both H- and C-LMNCO cells are presented in Figure 7a−c. The cells were tested at different rates ranging from 0.1 to 10 C between 2.0 and 4.6 V. It can be seen that discharge capacities of both samples decrease with increasing C-rate due to increased polarization at high current density. However, the magnitude of capacity decrease for the two cells is different, and the H-LMNCO cell always shows a higher discharge capacity than that of C-LMNCO cell at each current rate. For example, the discharge capacities of the H-LMNCO cell at 5 and 10 C are 192.7 and 155.8 mA·h·g−1, respectively, corresponding to 68.2% and 55.2% of that at 0.1 C. However. the C-LMNCO cell can only deliver 103.4 and 57.8 mA·h·g−1 at 5 and 10 C, respectively. When the current density switches back from 10 to 0.1 C, the discharge capacity of the H-LMNCO cell can rapidly return to 288.1 mA·h·g−1, which indicates the favorable structure stability of the H-LMNCO sample even at a high rate. In contrast, the capacity of the C-LMNCO cell cannot be recovered quickly. To further study the cycling performance of the samples at a high rate, both H- and C-LMNCO cells were cycled at 5 C between 2.0 and 4.6 V, as presented in Figure 7d. The H-LMNCO cell shows a distinct advantage, and the capacity gap between H- and C-LMNCO cells progressively increased with increasing number of cycles. After 200 cycles, the H-LMNCO cell can maintain discharge capacity of 160.6 mA·h·g−1 with capacity retention of 83.8%, whereas the CLMNCO cell suffers a fast capacity decay and can only afford a relatively low discharge capacity of 39.7 mA·h·g−1 with capacity

x Li 2MnO3 ·(1 − x)LiMO2 → x Li 2MnO3 · (1 − x)MO2 + (1 − x)Li

∼190

capacity retention (%) 93.6 (100 mA·g−1, 150 cycles)

(2)

x MnO2 ·(1 − x)MO2 + Li → x LiMnO2 ·(1 − x)LiMO2 (3)

As shown in Figure 5, excellent initial discharge capacity of 282.5 mA·h·g−1 and high initial coulombic efficiency of 85.5% are obtained for the H-LMNCO cell. In contrast, the CLMNCO cell delivers a poor initial discharge capacity of 257.3 mA·h·g−1 with an initial Coulombic efficiency of 77.1%. The high discharge capacity and Coulombic efficiency obtained at 0.1 C for the H-LMNCO cell might be associated with the better structure formed, such as porous hierarchical framework and exposed active planes, which can afford a large electrochemical active surface and allow a large Li+ flux across them. After initial cycle, the specific capacities of both H- and CLMNCO cells degrade substantially with increased cycling, but the cycling performance of H-LMNCO cell is much better than that of C-LMNCO cell. For the H-LMNCO cell, it can deliver a discharge capacity of 264.6 mA·h·g−1 at 0.5 C and maintains a stable reversible capacity of 247.7 mA·h·g−1 after 150 cycles, with a high capacity retention of 93.6%, whereas for the CLMNCO cell, capacity retention is only 74.2%. The superior cycling performance of H-LMNCO cell may benefit from stable hierarchical architecture, which can not only resist electrolyte corrosion but also maintain continuous Li+ pathways during repeated cycling. 8977


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Figure 8. Galvanostatic intermittent titration technique (GITT) curves of (a) H-LMNCO and (b) C-LMNCO cells in the voltage range 2.0−4.6 V. (c) dE/dx and (d) dE/dt1/2 as a function of stoichiometry x. (e) Diffusion coefficients of Li+ for H- and C-LMNCO cells.

literature. Li et al.51 designed hierarchical nano/microspherical Li1.2Ni0.2Mn0.6O2 and demonstrated good capacity retention of 94.4% after 150 cycles at a current density of 100 mA·g−1, but the capacity of 122.6 mA·h·g−1 can only be achieved at a rate of 1000 mA·g−1. Xu et al.53 reported orthogonally arranged {010}oriented Li-rich layered oxide nanoplates, which exhibited a discharge capacity of ∼190 mA·h·g−1 at a rate of 1000 mA·g−1; however, when cycled at a rate of 30 mA·g−1, the nanoplates can only deliver capacity retention of 92% after 100 cycles. Moreover, a comparison of electrochemical performance of asprepared H-LMNCO with some other similar composites for applications of LIBs is given in Table 2. Apparently, the H-

retention of 37.2%. The remarkably enhanced rate capability of H-LMNCO is related to its unique architecture. The nanosized primary grains and radial nanoplates with exposed active planes can maintain unobstructed divergent paths for Li+ and abundant reactive points during the charge/discharge process. Meanwhile, the matrix can provide excellent structural integrity. Additionally, the porous structure not only facilitates penetration of electrolyte but also mitigates volume swelling during repeated Li+ insertion/extraction processes, thus improving rate capacity and consolidating the stability of the host structure. The comprehensive electrochemical properties of HLMNCO are clearly higher than those in the recent reported 8978


Research Article

ACS Sustainable Chemistry & Engineering ORCID

LMNCO presents clearly excellent cycling stability and rate capability. Galvanostatic intermittent titration technique (GITT) test, as a reliable method to evaluate the Li+ chemical diffusion coefficient (DLi+) in the cathode, was performed to confirm the accelerated Li+ insertion/extraction kinetic of H-LMNCO sample. Figure 8a,b shows GITT patterns of H- and CLMNCO samples at 0.1 C between 2.0 and 4.6 V. Here, the DLi+ is calculated according to eq 4: D Li+ =

2 2 4 ⎛ Vm ⎞ ⎛ dE /dx ⎞ ⎜I ⎟ ⎜ ⎟ 0 π ⎝ FS ⎠ ⎝ dE /dt 1/2 ⎠

Xianyou Wang: 0000-0001-8888-6405 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the National Science Foundation for Post-Doctoral Scientists of China (2015M570682), the Key Project of Strategic New Industry of Hunan Province (2016GK4005 and 2016GK4030), and the Hunan Provincial Innovation Foundation for Postgraduate (CX2016B229).

(4)

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for t ≪ L /DLi . Here I0 (amperes) is the applied current; Vm (cubic centimeters per mole) represents the molar volume of compound, which is deduced from crystallographic data; F (coulombs per mole) is the Faraday constant; S (square centimeters) refers to the surface area of the electrode; and L (centimeters) represents the diffusion length. The plots of dE/dx and dE/dt1/2 as a function of stoichiometry x during the charge process are exhibited in Figure 8c,d. Based on eq 4 and the data from GITT tests, the calculated DLi+ are patterned in Figure 8e, and the results exhibit similar rules as those of normal LLO. For the HLMNCO cell, DLi+ can retain at an almost constant value (∼1.59 × 10−10 cm2·s−1) when the stoichiometry x is in the range from 0.3 to 1.0. However, the average value of CLMNCO cell is only about 2.26 × 10−11 cm2·s−1. The revealed excellent electrochemical performance of H-LMNCO sample from the results of GITT can be attributed to the hierarchical porous structure of the exposed {010} planes, which extends the reactive interface between the active material and electrolyte and broadens the Li+ diffusion channel. 2

+

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CONCLUSIONS In summary, the Li1.2Mn0.54Ni0.13Co0.13O2+δ microsphere selfassembled with nanoscaled grains and radial nanoplates (HLMNCO) has been successfully synthesized by an evolutionary coprecipitate method. The unique hierarchical architecture that combines the advantages of hierarchical porous structure and electrochemically active {010} plane, generates both stable matrix and efficient Li+ diffusion channel. Noticeably, this new cathode material for lithium-ion batteries can reciprocate preeminent electrochemical performance and deliver a high specific capacity of 282.5 mA·h·g−1 at 0.1 C and excellent cycling performance with 93.6% capacity retention after 150 cycles at 0.5 C. Moreover, the H-LMNCO cathode possesses outstanding rate capability and cycling stability at a high current density; it can deliver high specific capacity of 155.8 mA·h·g−1 even at 10 C and shows remarkable capacity retention of 83.8% after 200 cycles at a high rate of 5 C. Such extraordinary electrochemical performance can be mainly ascribed to its unique configuration. More importantly, the approach not only can be significant for the development of power batteries with high energy density for application in EVs or HEVs but also will be referential for controllably synthesizing other functional electrode materials with high rate and stable cycling performance.

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REFERENCES

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*Telephone +86 731 58293377; fax +86 731 58292052; e-mail [email protected]. 8979


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