Sacrificial Template Strategy toward a Hollow LiNi1/3Co1/3Mn1/3O2 Nanosphere Cathode for Advanced Lithium-Ion Batteries.

In this work, a hollow LiNi1/3Co1/3Mn1/3O2 (H-NCM) nanosphere cathode with excellent electrochemical performance is developed for lithium-ion batteries. Preparation of the H-NCM nanospheres involves the sacrificial template method, in which carbon nanospheres work as the template and polyvinylpyrrolidone works as an additive. Structural and morphological analyses show that the as-prepared H-NCM nanospheres are highly uniform with diameters of approximately 50 nm and wall thicknesses of 10 nm. Electrochemical tests demonstrate that the H-NCM cathode not only manifests outstanding rate performance in the potential window of 2.5-4.5 V with high reversible specific capacities of 205.6, 194.9, 177.8, 165.9, 151.7, 126.0, and 115.3 mA h g-1 at 0.1, 0.2, 0.5, 1, 2, 5, and 10 C, respectively, but also delivers excellent stability with a capacity retention of 60.1% at 10 C after 2000 cycles. The superior electrochemical performance of the H-NCM cathode can be put down to the distinctive hollow interior structure with thin nanostructured walls, which can synergistically benefit the significantly enhanced rate capability and cycling stability.

Introduction

Inspired by the high gravimetric and volumetric energy density of lithium-ion batteries (LIBs), electrification devices and electrical vehicles based on LIBs have aroused considerable interest.[1−4] Nevertheless, current cathode materials (e.g., LiCoO2, LiNiO2, or LiFePO4) cannot meet the ever-increasing demand of high energy and high power; therefore, development of high-performance cathode materials has proven to be challenging. Among these materials, LiNi1/3Co1/3Mn1/3O2 (NCM), a layered lithium transition metal oxide, has drawn much attention mainly on account of its high reversible capacities, high energy density, moderate voltage platform, thermal stability, and inexpensive price.[5−8] Although implemented in numerous studies, certain drawbacks of the NCM hinder their sustainability and high-power applications, including phase deterioration that exists during the lithium-ion (Li+) insertion/extraction and morphological changes of the active electrode materials during the charging–discharging process.[9]

To solve above problems, hollow-structured materials have been proposed and received considerable attention.[10] Benefitting from the merits of large specific surface area, great “breathability”, large void space, and good monodispersity, hollow spheres of the nanometer or micrometer dimensions, by virtue of their unique morphology with controllable size, shell composition, and internal structure, manifest physical and chemical advantages compared with conventional solid counterparts.[11] They also present promising application prospects in diverse fields, such as drug delivery, energy storage devices, catalysis, nanoreactors, and sensors.[12,13] Generally, nanostructures with hollow interiors could augment the electrochemical performances of electrode materials through various mechanisms, such as by reducing the effective diffusion distance for Li+ and by affording a large surface area that could in turn act as a larger reactive interface for the Li+ storage.[14−17]

Recently, considerable progress has been engaged in the exploration of synthesis of hollow micro-/nanostructures based on templates to improve properties.[18−23] Wu et al. devised a nanoporous LiNi1/3Co1/3Mn1/3O2 cathode material via vapor-grown carbon fibers as templates, which presented ultrafast charge capability at 180 C as well as good cycling stability.[24] Qian et al. designed an in situ template-sacrificial route to prepare LiNi1/3Co1/3Mn1/3O2 hollow microspheres with porous spinel Mn1.5Co1.5O4 hollow microspheres as the template, which achieve a high discharge capacity of 120.5 mA h g–1 at 0.5 C after 200 cycles.[8] Cao and co-workers reported LiNi1/3Co1/3Mn1/3O2 hollow nano-/micro-hierarchical microspheres (NCM-HS) and cube-shaped hierarchical LiNi1/3Co1/3Mn1/3O2 (CH-NCM) that were synthesized using MnCO3 microspheres and cubes as self-templates.[7,25] Both NCM-HS and CH-NCM cathodes displayed augmented electrochemical performances. The unique hollow nanostructures endow the electrode material with a short transport length for Li+ insertion/extraction and good structural stability, yielding outstanding rate capability and cycling stability. Although great progress has been made in hollow NCM (H-NCM) materials, it is highly desirable to prepare a nano-sized H-NCM cathode with significantly enhanced electrochemical performances for LIBs, which still remains a grand challenge.

Herein, we demonstrate a scalable strategy to prepare uniform H-NCM nanospheres with carbon nanospheres as the template. The carbon nanospheres are saturated with desired metal salts in a unified polyol system and a “deep penetration–calcination” process for the formation of uniform H-NCM nanospheres. In the calcination process, the template serves to not only form a cavity but also effectively avoid particle aggregation. When evaluated as a cathode material for LIBs, the unique H-NCM electrode exhibits outstanding rate capability, long-term cycling stability, and high discharge capacity, achieving 205.6 mA h g–1 at 0.1 C. At higher rates, the reversible capacities of the H-NCM cathode are, respectively, 194.9 (0.2 C), 177.8 (0.5 C), 165.9 (1 C), 151.7 (2 C), 126.0 (5 C), and 115.3 mA h g–1 (10 C).

Results and Discussion

The overall procedure for the synthesis of H-NCM nanospheres is illustrated in Figure . First, carbon nanospheres were prepared from glucose under hydrothermal conditions according to a reported procedure.[26] The hydrothermal process resulted in the formation of a large amount of hydroxyl and carboxyl groups on the carbonization particles. The presence of carboxyl groups renders the carbon nanospheres negatively charged, which possess a strong bonding ability with cationic metal ions by electrostatic attractions. Second, polyvinylpyrrolidone (PVP), as a noted coordination agent via −N and/or C=O functional groups, combines metal ions and can self-assemble into micelles, which can be used as soft templates.[27] Subsequently, carbon nanospheres were well-dispersed in a PVP–ethylene glycol (EG) solution of acetate solutions of different metal ions. The solution was heated up to 120 °C for 12 h and 170 °C for another 2 h under reflux to enable deep penetration. In the initial stage of refluxing, the acetate groups of M(CH3COO)2 precursors were replaced gradually by EG units through the formation of M–O– covalent and M←OH coordination bonds.[28] Therefore, M-glycolate (MG) would be observed in both the internal and external surfaces of carbon nanospheres during this process.[29] Afterward, lithium acetate was introduced to the carbon nanospheres@MG hybrid by grinding. The thus-formed mixture was treated at 550 °C for 4 h and finally calcined at 850 °C for another 8 h in air. The use of carbon nanospheres as hard templates and of PVP as soft templates as well as the formation of MG plays central roles in the generated process of H-NCM nanospheres. Apart from the carbon nanospheres and its oxygen-containing functional groups that constitute the carbon nanospheres@MG hybrid, there are a large fraction of organic species including PVP, CH3COO–, and partially polymerized EG.

Figure 1

Schematic illustration of the preparation of H-NCM nanospheres.

Indeed, the compositional changes associated with calcination are also traced using the TGA. Figure shows the TGA curve recorded under a flow of air at 20 °C min–1 in the temperature range between 30 and 900 °C, indicating a significant weight loss of around 50.6%. A three-stage pattern of weight loss is noted in the temperature ranges of 25–200, 200–340, and 340–900 °C. The first stage of weight loss (7.5%) could be ascribed to physically adsorbed water and evaporation of volatile organic components. The second relatively large weight loss (29.4%) could be ascribed to the burnout of inorganic carbon and decomposition of most of the organic groups, which contained PVP, CH3COO–, and partially polymerized EG in the carbon nanospheres@MG hybrid. The third stage of weight loss (13.7%) is present during the formation of an intermediate product and the decomposition of residual organics.

Figure 2

Thermogravimetric analysis (TGA) curve of the H-NCM nanospheres under air flow with a temperature ramp of 20 °C min–1.

The structural evolution mechanism of the hollow structure during the thermal disintegration can be explained as the heterogeneous contraction caused by the nonequilibrium heating process. Of particular note is a large contraction force (donated as Fc) that is induced by the oxidative degradation of the carbon nanospheres, combined with an opposite adhesive force (donated as Fa) produced during the formation of metal oxide nanocrystallites cooperatively that drives the separation between metal oxide nanocrystallite shell and shrinkage of carbon nanosphere templates, thus resulting in the formation of a hollow architecture. In the presintering process, controlling the slow heating rate is found to be vital for maintaining the uniformity of the formed hollow nanoscaled precursors: an overly high ramping rate of 5 °C min–1 would lead to collapse of the hollow structure. Therefore, the well-defined H-NCM precursors are acquired after annealing at 550 °C for 4 h with a precise control of heating rate of 1 °C min–1. Last, the precursors were calcined at 850 °C for 8 h in air and resulted in the formation of H-NCM nanospheres.

Further elucidation of the H-NCM formation process was performed through more in-depth observation and characterization to support our crafted route. A TEM panoramic view of the pristine carbon nanospheres showed high uniformity with a diameter of approximately 100 nm (Figure A). As can be inspected from Figure B, a MG layer can be uniformly grown on the surface of carbon nanospheres to produce a carbon nanosphere@MG hybrid structure after refluxing in the metal acetate PVP–EG solution at an elevated temperature (170 °C). The average diameters of about 110 nm of the resultant H-NCM precursors are slightly larger than those of the carbon nanosphere templates. After being pretreated at 550 °C for 4 h in air, the resultant H-NCM precursors have diameters of about 80 nm (Figure C). Compared with the carbon nanospheres@MG hybrid structure, the diameters of these precursors are reduced, which is caused by overall shrinkage during annealing. Estimations from the enlarged TEM image (Figure D) reveal that the shells of the hollow nanospheres are made up of numerous nanocrystals. There are two prerequisites for the formation of a hollow structure: (1) a deep penetration of metal ions within the carbon nanosphere templates and (2) the two rates of precursor shell formation and of carbon nanosphere decomposition must match with each other. When the heating temperature is low, the disintegration rate of the carbon species is slow, allowing the metal atoms to gather within the carbon templates and interconnect to form shell layers. Figure E reveals that each highly porous hollow nanosphere is made of interconnected nanocrystallites and that the hollow morphology is well-preserved after calcination at 850 °C for another 8 h. As shown in Figure F, the nanosphere diameter, shell thickness, and nanocrystallite size are estimated to be 50, 10, and 5 nm, respectively. A representative HRTEM image (Figure G) taken from H-NCM shows the same interplanar distance to be 2.459 Å, which matches well with the (010) and (100) planes. The interlayer spacing of 1.431 Å can be identified as (110) planes, corresponding to the layered structure of NCM in the SAED pattern.[30] The SAED pattern in Figure H proves that these nanocrystallites are of hexagonal symmetry, which is consistent with the hexagonal structure of NCM.[31] We further detect the elemental mapping via EDX spectroscopy, which is shown in Figure I. It is noted that the distribution of Co, Mn, Ni, and O elements is uniform within the hollow structure and that no elements in the middle area originate from the hollow interiors. The precise elemental analysis can be checked as Li1.002Mn0.338Ni0.327Co0.335O2 by inductively coupled plasma investigations, which maintains good consistency with the expected ratio of 1.00:0.334:0.333:0.333 for Li/Mn/Ni/Co.

Figure 3

Transmission electron microscopy (TEM) images of (A) carbon nanospheres, (B) carbon nanospheres@MG hybrid, (C,D) well-defined H-NCM precursors, (E,F) panoramic and detailed views of the H-NCM hollow spheres, (G) high-resolution TEM (HRTEM) image at the edge of the H-NCM nanospheres in (E); inset is a magnified image of the area marked by the white square, (H) corresponding selected-area electron diffraction (SAED) pattern at the regions indicated as the white square in (G), and (I) energy-dispersive X-ray (EDX) spectroscopy elemental mappings of an individual H-NCM nanosphere in (F).

XRD in Figure demonstrates the crystallographic structures of H-NCM and B-NCM nanocomposites. No significant crystallographic difference in the XRD patterns is observed between H-NCM and B-NCM. All peaks can be easily indexed to the hexagonal α-NaFeO2 phase with a space group of R3̅m (JCPDS 82-1495), indicating that there is no impure phase. The peaks of (006)/(102) and (108)/(110) have a good splitting, manifesting that the as-formed NCM materials have a well-ordered layered structure. The ratios of the intensities for (003) and (104) peaks are found to be 1.74 and 1.61 for H-NCM and B-NCM, respectively, which are greater than 1.2, implying no evident cation mixing.

Figure 4

X-ray diffraction (XRD) patterns of the as-prepared H-NCM nanospheres and bulk NCM (B-NCM).

The electrochemical measurements of the H-NCM and B-NCM cathodes were evaluated by both cyclic voltammetry (CV) and galvanostatic charge–discharge tests. Figure A demonstrates the CV curves of H-NCM and B-NCM cathodes for the first cycle within the voltage range of 2.5–4.5 V vs Li+/Li at a scan rate of 0.1 mV s–1. The anodic peak of the H-NCM cathode occurs at 3.854 V and the cathodic peak at 3.64 V, whereas the anodic peak of the B-NCM cathode occurs at 3.878 V and the cathodic peak at 3.638 V. Thus, the potential intervals (ΔV) of the two samples are, respectively, 0.214 and 0.240 V. It is well-known that the smaller the value of ΔV, the better the reversibility of Li+ insertion and extraction. For the H-NCM cathode, the oxidation peak decreased from 3.854 V and stabilized at 3.801 V with an increasing number of cycles. The different area of the first CV curves originates from the different peak currents (Ip) in the same scan rate and voltage range. According to the Randles–Sevcik equationwhere n is the number of electrons per reaction species (it is 1 for Li+), A is the electrode area, DLi is the diffusion coefficient of Li+, CLi* is the bulk concentration of Li+ in the electrode, and the scan rate (v) is 0.1 mV s–1. DLi is proportional to Ip based on the above equation. It is worth noticing that the H-NCM nanostructure and the average 5 nm size of the primary nanoparticles as well as the approximately 10 nm permeable walls will provide a short pathway, which largely shortens the diffusion path for Li+, thus enhancing the DLi of H-NCM. Accordingly, the higher DLi, the larger Ip, and the larger area of the first CV curves are acquired. The CV profiles from the second cycle show insignificant changes in subsequent sweeps (Figure S1), suggesting outstanding reversibility and good cycle stability of electrochemical reactions in the well-defined H-NCM electrode. Figure B shows the galvanostatic charge–discharge curves of H-NCM nanospheres for the 1st, 2nd, 5th, and 50th cycles at 0.1 C (1 C = 278 mA g–1). All of the charge and discharge profiles of H-NCM are similar to those reported previously,[7,18] indicating a similar underlying electrochemical pathway and the stable electrode structures in the voltage range between 2.5 and 4.5 V. The first discharge capacity of 205.6 mA h g–1 is obtained with an irreversible capacity loss of 20.1%, which corresponds to an initial Coulombic efficiency of 79.9% (Figure C). The capacity decay may be ascribed to the generation of a solid electrolyte interlayer (SEI) and the partial deintercalation of Li+ from the electrode material. The discharge capacities of the 2nd, 5th, and 50th cycles are 203.7, 201.8, and 188.5 mA h g–1, respectively. Compared with the first discharge capacity, there is only about 8.5% capacity decay after 50 cycles. The comparison cycling performances and Coulombic efficiencies of H-NCM and B-NCM at 0.1 C are depicted in Figure C. The initial discharge capacity and cycling stability of H-NCM are better than that of the B-NCM electrode. Impressively, the capacity retention ratios for the two electrodes are 89.5 and 77.1% after 100 cycles, respectively. Their Coulombic efficiencies immediately resume around 98.1 and 95.5% for the 2nd cycle and maintain a value above 99% after a few charge–discharge cycles. More importantly, the H-NCM electrode can be cycled with excellent stability in high rates of 1 C and 10 C (Figure D). By contrast, severe capacity degradation was observed to have occurred within the B-NCM cathode after 100 charge–discharge cycles at 1 C and 10 C. In addition, the H-NCM composite manifests excellent 2000 continuous cycles with the initial capacity preservation of 60.1% at 10 C (Figure S2). Furthermore, a corresponding postmortem morphological analysis elucidated that hollow nanosphere architectures could be preserved after 2000 cycles, although certain hollow nanospheres were cracked and deformed (Figure S3). These demonstrate that such a distinctive hollow nanoarchitecture for the spheres affords good cycling stability.

Figure 5

Electrochemical measurements of the H-NCM and B-NCM cathodes: (A) first CV curves in the voltage range of 2.5–4.5 V at the scan rate of 0.1 mV s–1, (B) charge–discharge curves of the H-NCM cathode for the 1st, 2nd, 5th, and 50th cycles at 0.1 C (1 C = 278 mA g–1), (C) comparative cycling performances and Coulombic efficiencies at 0.1 C, (D) comparative cycling performances at 1 C and 10 C, (E) comparative rate capabilities, and (F) electrochemical impedance spectroscopy (EIS) profiles and equivalent circuit on the discharged states after five charge–discharge cycles at frequencies ranging from 0.1 Hz to 100 kHz.

Considering the significance of the rate capability for practical applications, initial charge and discharge curves from 0.1 to 5 C in the potential window of 2.5–4.5 V (Figure S4) for the H-NCM cathode were measured. The H-NCM sample shows high reversible specific capacities of 205.6, 194.9, 177.8, 165.9, 151.7, and 126.0 mA h g–1 at 0.1, 0.2, 0.5, 1, 2, and 5 C, respectively. Conversely, the discharge capacities of B-NCM are virtually all much weaker than those of H-NCM at any rate (Figure E). Besides, after the high rate of 5 C cycling, a discharge capacity near its primal value of 199.8 mA h g–1 can be restored when the rate is resumed to 0.1 C; however, the B-NCM cathode only recovers to 146.3 mA h g–1. It is worth noting that the H-NCM cathode have a remarkable capacity, reversibility, and rate capability.

As is known to all, EIS has been measured to investigate the electrode kinetics, which was performed on the discharged status of H-NCM and B-NCM electrodes after five galvanostatic charge–discharge cycles at frequencies ranging from 0.1 Hz to 100 kHz (Figure F). The semicircle can be assigned to the combination of the SEI resistance and the charge-transfer resistance (Rct) at the interface between the electrolyte and the electrode, specifically, the cell impedance is mainly ascribed to Rct. From the simulated equivalent circuit (in the inset of Figure F), the Rct of H-NCM is calculated to be 60 ± 3 Ω, whereas that of B-NCM is 195 ± 5 Ω. This measurement suggests that this distinctive interior void space of LiNi1/3Co1/3Mn1/3O2 hollow spherical nanostructure can endow plentiful contacts between the primary particles and the electrolyte, improve charge transport, and decrease the kinetic impedance.

The augmented cycling stability and rate performance of H-NCM nanospheres could shed light on the following meritorious features. Impressively, the structure is noteworthy: the microscopic structure with the hollow cavity favors rapid electrolyte flooding and infiltration; the void space in the interior could withstand the volume variation during repeated lithiation and delithiation, which might contribute to the excellent cycling stability. Moreover, the average 5 nm size of the primary nanoparticles together with the approximately 10 nm permeable walls will provide a short pathway, which largely shortens the diffusion path for Li+. Meanwhile, the empty space is favorable for serving as a reservoir for efficient penetration of the electrolyte to boost active materials and the electrolyte contact interface, hence leading to good rate capability.

Conclusions

To summarize, we have designed and demonstrated H-NCM nanospheres by using carbon nanospheres as hard templates and PVP as an additive. The carbon nanosphere templates serve to not only form a cavity but also effectively avoid particle aggregation during calcination. As cathodes for LIBs, the H-NCM electrode manifests enhanced specific capacities and rate performances with the discharge capacities of 205.6 mA h g–1 at 0.1 C and 113.6 mA h g–1 at 10 C between 2.5 and 4.5 V. Moreover, the as-synthesized H-NCM cathode material also enables a superior cycling performance, especially, long-term cycling stability has been demonstrated; the capacity retention ratios are as high as 60.1% after 2000 continuous cycling at 10 C. Collectively, the satisfactory discharge capacity, remarkable cyclic stability, and desirable rate capability result from the synergism between the small average size of nanocrystallites in a thin nanostructured wall, which offer a short Li+ transport pathway, and the sufficient void space, which buffers the volume expansion. This paper presents the synthesis and characterization of hollow architectures of multicomponent cathode materials in LIBs, which, given their desirable features, may find wide practical utility.

Experimental Section

Materials

Mn(CH3COO)2·4H2O (99%, AR, Aladdin), Co(CH3COO)2·4H2O (99.5%, AR, Aladdin), Ni(CH3COO)2·4H2O (99%, AR, Aladdin), LiCH3COO·2H2O (99%, AR, Aladdin), glucose (≥99.5%, GC, Aladdin), PVP (Mw ≈ 55 000, Sigma), and EG (HOC2H4OH, absolute for analysis) are obtained from Shanghai LingFeng Chemical Reagent Co. Ltd. All chemicals are directly used without further purification. Deionized double-distilled water is used for making all solutions.

Preparation of Carbon Nanospheres

In a typical synthesis, carbon nanospheres were first obtained through emulsion–polymerization of glucose under hydrothermal conditions. A 100 mL Teflon-lined stainless steel autoclave contains 10.8 g of glucose and 60 mL of distilled water, which was maintained at 180 °C for 8 h. The puce precipitation was separated by filtration, alternately washed six times with distilled water and ethanol, and dried at 80 °C in an oven.

Synthesis of H-NCM Nanospheres

A total of 1.0 g of PVP was dissolved in 100 mL of EG to form a clear solution with the aid of ultrasonication. Afterward, an equivalent molar ratio of Ni(CH3COO)2·4H2O, Co(CH3COO)2·4H2O, and Mn(CH3COO)2·4H2O was added to the PVP–EG solution under vigorous stirring. Subsequently, freshly prepared carbon nanosphere powder (0.5 g) was dispersed into the above mixture solution via sonication for 30 min. The resultant solution was transferred to a 250 mL three-necked flask equipped with a condenser, kept at 120 °C for 12 h, and allowed to heat to 170 °C for another 2 h to ensure sufficient penetration of metal ions into the carbon nanospheres. The precipitation was harvested by centrifugation and alternately washed with ethanol and double-distilled water for seven times. The product was dried at 80 °C overnight. The powder thus obtained was ground manually with LiCH3COO·2H2O for 30 min, preheated at the temperature of 550 °C for 4 h in air to form the hollow precursors, and finally calcined at 850 °C for another 8 h with a slow heating rate of 1 °C min–1 to yield the H-NCM nanospheres.

To compare with the H-NCM cathode, nanoscaled B-NCM was prepared by the similar coprecipitation strategy as that previously reported.[32] The B-NCM precursor was sintered in air at 2 °C min–1 up to 850 °C for 8 h.

Characterization

The crystal structures of the prepared composites were carried out on a Bruker D8 ADVANCE DA VINCI X-ray polycrystalline diffractometer with Cu Kα radiation (λ, 1.5405 Å). A JEOL JEM-2100F bright-field TEM (200 kV accelerating voltage) was used to identify the crystallite size, morphology, and phase. TGA (TGA-209F3, NETZSCH, Germany) was recorded from 30 to 900 °C at a ramping rate of 20 °C min–1 under a flow of air to investigate the material behavior during annealing.

Electrochemical Measurements

According to the literature reported,[33−35] the electrochemical measurements of the as-prepared materials were tested using CR2025 coin-type half-cells with Li foil serving as both counter and reference electrodes at room temperature. The working electrode consists of 80 wt % as-synthesized active materials, 10 wt % conductive carbon black, and 10 wt % binder (polyvinylidene fluoride). The mixture was dispersed in N-methyl-2-pyrrolidone to form a uniform slurry and was casted on an aluminum foil current collector, followed by drying at 120 °C for 12 h in a vacuum oven. Cells were assembled in an argon-filled glovebox with the concentrations of moisture and oxygen below 0.1 ppm. Celgard 2400 films were used as the separator, and a composition of 1 M LiPF6 in a 1:1 v/v mixture of ethylene carbonate/dimethyl carbonate was used as the electrolyte. The cell electrochemical performances were conducted on a LAND CT2001A battery test system (Wuhan JINNUO Electronics, Ltd., China). CV and EIS were performed via a CHI760e electrochemical workstation (Chenhua Instrument Co., Shanghai, China). EIS measurements were detected in the frequency range of 0.1 Hz to 100 kHz and an ac voltage with 5 mV amplitude.