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Morphological Evolution of High-Voltage Spinel LiNi0.5Mn1.5O4 Cathode Materials for Lithium-Ion Batteries: The Critical Effects of Surface Orientations and Particle Size Haidong Liu,† Jun Wang,† Xiaofei Zhang,† Dong Zhou,‡ Xin Qi,† Bao Qiu,∥ Jianhui Fang,§ Richard Kloepsch,† Gerhard Schumacher,‡ Zhaoping Liu,∥ and Jie Li*,† †
MEET Battery Research Center & Institute of Physical Chemistry, University of Muenster, Corrensstr. 46&28/30, 48149 Muenster, Germany ‡ Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany § Department of Chemistry, College of Sciences, Shanghai University, 200444 Shanghai, China ∥ Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences, 315201 Ningbo, China S Supporting Information *
ABSTRACT: An evolution panorama of morphology and surface orientation of high-voltage spinel LiNi0.5Mn1.5O4 cathode materials synthesized by the combination of the microwave-assisted hydrothermal technique and a postcalcination process is presented. Nanoparticles, octahedral and truncated octahedral particles with different preferential growth of surface orientations are obtained. The structures of different materials are studied by X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption near edge spectroscopy (XANES), and transmission electron microscopy (TEM). The influence of various morphologies (including surface orientations and particle size) on kinetic parameters, such as electronic conductivity and Li+ diffusion coefficients, are investigated as well. Moreover, electrochemical measurements indicate that the morphological differences result in divergent rate capabilities and cycling performances. They reveal that appropriate surface-tailoring can satisfy simultaneously the compatibility of power capability and long cycle life. The morphology design for optimizing Li+ transport and interfacial stability is very important for high-voltage spinel material. Overall, the crystal chemistry, kinetics and electrochemical performance of the present study on various morphologies of LiNi0.5Mn1.5O4 spinel materials have implications for understanding the complex impacts of electrode interface and electrolyte and rational design of rechargeable electrode materials for lithium-ion batteries. The outstanding performance of our truncated octahedral LiNi0.5Mn1.5O4 materials makes them promising as cathode materials to develop long-life, high energy and high power lithium-ion batteries. KEYWORDS: cathode material, LiNi0.5Mn1.5O4, lithium-ion batteries, microwave synthesis, morphology and surface orientation
1. INTRODUCTION Modern society has taken great efforts to explore the shifting from fossil-fuel economy to one based on renewable and sustainable technologies; there is clearly a pressing need to improve significantly our ability to store energy prior to its implementation.1 Therefore, the worldwide demand for energy storage components records a sharp rise with the fast technological development over recent years. Lithium-ion batteries (LIBs) are acknowledged as one of the most suitable and auspicious energy storage systems and are being intensively investigated for the application in portable electronic devices, transportation applications, and stationary storage of renewable energy sources like solar and wind. However, the present LIBs do not fully meet the performance, stability and safety requirements for widespread usage. Consequently, it is being actively searched and pursued to develop a reliable and durable high-performance battery with the features of the low cost, superior safety, high specific capacity, high operation voltage, © XXXX American Chemical Society
and long cycle life within a wide range of working temperatures.2−4 The essential step for the realization of this challenge is required to find advanced cathode materials, which is generally recognized as a performance-determining component of LIBs.5 Lithium nickel manganese oxide LiNi0.5Mn1.5O4 (LNMO) with the spinel structure is considered as one of the most promising cathode materials for next generation lithium-ion batteries, attributed to its abundance of the raw materials, superior rate capability due to the rapid three-dimensional lithium ion conduction, and most importantly its high energy density because of the high discharge plateau of about 4.7 V vs Li/ Li+.6−8 The additional benefit of the high-voltage spinel LNMO is that the safety of battery packs for practical application is Received: November 30, 2015 Accepted: January 29, 2016
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DOI: 10.1021/acsami.5b11389 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
potentials, leading to low Coulombic efficiency and dramatic capacity decay.31 On the other hand, the large-size LNMO, e.g., on the micron-scale, was adopted because the lower surface area originating from the large size which can not only effectively decrease the activity of the side reactions to enhance cycling performance, but also result in a higher tap density meaningful for increasing energy density in the practical application. The ionic diffusivity of large-sized bulky material could be enhanced by tailoring the appropriate crystal surface orientations for supporting Li+ transport to satisfy fast Li+ insertion-extraction kinetics for high-power requirements.23,24 Besides, the intrinsically high Li+ diffusion coefficient in the range of 10−9 to 10−8 cm2 s−1 was also predicted by Ma et al. for micrometer-scaled LNMO particles.32 So, it is essential for LNMO to make a good compatibility between power output depending on the fast kinetics of Li+ insertion/extraction and cycling stability originating from benign interfacial behaviors under high voltage. In light of these issues, extensive studies of the LNMO cathode material have been performed, but noticeable improvement on the electrochemical performance was rarely reported.33,34 Clearly, to improve further the cycling stability of LNMO, it is still crucial and desirable to design and develop new strategies for its synthesis. Among the various synthesis methods adopted, microwave−assisted heating method as a promising synthesis technique has been employed to prepare electrode materials for LIBs because of fast volumetric heating, which leads to higher reaction rates and selectivities, reduction of reaction time by orders of magnitude, and higher product yields compared with conventional heating methods.35−37 Previously, microwave solid-state sintering reactions have been reported for the synthesis of LNMO cathode material, the electrochemical performance of LNMO was improved.38,39 In contrast to microwave solid-state heating, the structure and morphology of the primitive product prepared by microwave hydrothermal process are relatively easy to be controlled by adding templates or surfactants into the reaction solution, or by varying post thermal treatment conditions. So, it is more favorable to use microwave-assisted method for regulating the surface orientations and morphologies. In this work, an evolution of morphology and surface orientation of LNMO materials from nanoparticle to large-sized truncated octahedron is successfully controlled by a microwaveassisted hydrothermal method with varying the temperature of thermal post-treatment in the absence of any template and surfactant. Strikingly, compared with the previous reports in the literature (more details and discussion can be found in Table S1, Supporting Information),40−42 this morphological evolution is based on the same precursor without preparing the various precursors for different morphologies in advance, which can reduce experimental deviations induced by different morphological surfactants, synthesis methods and processes, because different technologies adopted in the preparation of electrode materials the topology of particle wiring certainly differs.43 The microwave-assisted hydrothermal process gives rise to the agglomerated nanosized particulate precursor with low crystallinity, which can be transformed to rounded nanoparticles, octahedrons and truncated octahedrons in sequence after the postheating at the temperatures of 700 °C, 800 °C, 900 and 1000 °C, respectively. Thereinto, two types of truncated octahedrons calcined at 900 to 1000 °C with different proportions of {100} and {111} surfaces. The resultant LNMO materials show the different morphologies
enhanced because fewer cells are needed to achieve the required voltage.9 However, high-voltage spinel LNMO suffers from limited cycle life, particularly at elevated temperature, which is triggered by degradation of electrolyte at high operation voltage,10−12 or structure-related Mn/Ni dissolution.13−16 These two problems involve various factors including the cation ordering related to Mn 3+ content, particle morphology, surface modification and the surface crystalline planes in contact with the electrolyte, whereas, all these factors are profoundly influenced by the different synthesis routines and conditions. One approach is to stabilize the LNMO crystal structure for overcoming these issues, e.g., cationic doping of the bulk structure. The substitution of Mn/Ni by cations, such as Cr3+, Fe3+ and Al3+, could eliminate the formation of rock-salt impurity phase and stabilize the disordering of Ni2+ and Mn4+ ions in the 16d octahedral sites of the Fd3m ̅ space group in the LNMO spinel structure.17,18 Another approach has been focused on is to alleviate the surface and interfacial interactions, which can be achieved through coating the surface and tailoring specific morphology. Surface coating could prevent direct contact between the LNMO and the electrolyte for suppressing the corrosion reaction. For instance, as the coating layer, various kinds of metal oxides and Li+ conductor materials, has been proved to enhance the resistance of LNMO against hydrogen fluoride attack from the electrolytes.18−20 Improving the interfacial stability by the specific morphological control could also positively take effect in hindering the side reactions and transition-metal dissolution on the electrode/electrolyte interface which are largely dependent on the lattice orientations of surface planes based on the arrangement of atoms.21,22 It was suggested that regular morphologies, especially the crystallographic planes on single-crystal surface facets in contact with the electrolyte, have a significant effect on the electrochemical properties of the spinel material.23−25 The stability of crystal surfaces is directly related to the dissolution of transition metals, for example, Mn is lost from spinel electrode surfaces into the electrolyte undergoing a disproportionation reaction of Mn3+: 2Mn3+→ Mn4+ and Mn2+, and the {110} surfaces whose orientations are aligned to Li diffusion channels were approved to be the most vulnerable to the dissolution Mn2+ into the electrolyte.22 So, it is necessary to inhibit the growth of {110} planes to effectively mitigate the Mn dissolution. As reported in the spinel system, compared with {111} and {110} family of surface planes, {100} surfaces are more stable and take positive effects on the electrochemical performance, which was proved through atomistic simulations by Benedek and Thackeray and de With et al.26,27 Thus, it is probably beneficial to obtain a stable interfacial environment by truncating a high proportion of {100} surfaces in spinel structure. The particle size is another important factor for the interfacial stability. Nanostructured and large-sized LNMO materials were advocated, respectively. On one hand, nanoscale LNMO is declaimed because it has been widely demonstrated that the power output of LIBs can be notably increased by reducing the particle size due to the shorter Li+ diffusion pathways.28,29 Nevertheless, the structure and phase stability in nanoscale system may become unstable, because the surface contributions to the total energy become increasingly important as the particle size decreases.30 Additionally, large active surface area of the nanostructures might accelerate the side reactions with the electrolytes under high operating B
DOI: 10.1021/acsami.5b11389 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
100 °C over a frequency range from 100 mHz to 10 MHz at an AC voltage of 1 V. The temperature control was assured by the Quatro Cryosystem and performed within ±0.5 K (all modules supplied by Novocontrol). 2.3. Electrochemical Measurements. The electrodes consisted of 80:10:10 wt % active material, super C65 conductive carbon (Imerys), and polyvinylidene difluoride (PVdF) binder were prepared from a slurry with N-methyl-2-pyrrolidone as solvent that was cast on Al foil and dried at 120 °C under vacuum and then pressed. The mass loading of the electrode was about 2 mg cm−2. 2016-type coin halfcells were assembled with a borosilicate microfiber filter (Whatman GF/D, pore size is 2.7 μm and thickness is 675 μm) as separator, and 1 M LiPF6 in a 1:1 weight mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolyte. Lithium metal was used as counter electrode. The galvanostatic measurements were conducted on a Maccor Series 4000 battery testing unit (Maccor Inc., USA) in a voltage range of 3.5−5 V at various current rates (1 C = 147 mA g−1) under different temperatures of 20, 40, and 60 °C, respectively. Cyclic voltammetry (CV) was performed using a VSP electrochemical workstation (Biologic, France) at different scanning rates of 0.05 to 2 mV s−1 between 3.5 and 5 V. The electrochemical impedance spectroscopy (EIS) measurements were also applied on the same VSP system with sinusoidal signal of 10 mV over a frequency range from 100 kHz to 0.1 Hz. All the EIS measurements of the cycled cells were conducted after being fully discharged to 3.5 V. Herein, all voltages quoted in the text refer to the Li counter electrode in two-electrode cell arrangement. The specific energy E (Wh kg−1) could be calculated
and preferential growth orientation of surface facets under the different sintering temperature, resulting in divergent high-rate capability and long-term cycling performance. The crystal chemistry and kinetics properties of lithium transport are compared, and the influence of morphology and surface orientations on electrochemical performance in spinel is demonstrated.
2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. The as-prepared nanostructured precursor was synthesized by microwave-assisted hydrothermal method, and the different morphological LiNi0.5Mn1.5O4 crystalline materials were prepared by heating the above precursors at different temperatures. In a typical process, a 200 mL solution of MnSO4·H2O (3.6 mmol) and NiSO4·6H2O (2.0 mmol) was added dropwise into another 200 mL solution of KMnO4 (2.4 mmol) and LiOH·H2O (40 mmol), then kept stirring for 30 min. The molar ratio between the lithium and the total transition metals was fixed to 5. The mixture was transferred into a Teflon-lined microwave reactor (UltraCLAVE, Milestone) with the temperature controlled at 200 °C for 30 min. The reaction process can be described by eq 1, see below. After the reaction was complete, the resulting brown precipitate was centrifuged and washed several times with deionized water and ethanol. The precursor was obtained after being dried at 100 °C overnight. The as-prepared precursor was heated at 700, 800, 900, and 1000 °C for 3 h in a muffle furnace, respectively, and then cooled down naturally to get LNMO materials with different morphologies. The four prepared materials are named as 700C, 800C, 900C, and 1000C, respectively.
t I
by E = ∫ mo V (t )dt , where t is the discharge time, I0 (A) is the 0 constant current, m (kg) is the mass of the active material, and V(t) is the time-dependent voltage in the dimension of V. The specific power P (W kg−1) can be calculated by P = E/t.
6KMnO4 + 9MnSO4 + 5NiSO4 + 32LiOH → 10LiNi 0.5Mn1.5O4 + 3K 2SO4 + 11Li 2SO4 + 16H2O
(1)
3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. In the following, the prepared LNMO samples synthesized in this study will be referred to as 700C, 800C, 900C, and 1000C, respectively. The X-ray diffraction (XRD) patterns of the four LNMO materials 700C, 800C, 900C, and 1000C are shown in Figure 1a. All materials display the typical profiles of the spinel phase (JCPDS No. 80-2162) with Fd3̅m cubic space group. On the right-hand side, two magnified views of Figure 1a present that there are visible weak peaks at 2θ = 37.5°, 43.6°, and 63.5° which could be assigned to a rock-salt impurity phase (NixO or Li1−xNixO), it is a common concomitant impurity during the heating treatment over 650 °C.6,45 It indicates that fraction of Mn in LNMO is reduced from Mn4+ to Mn3+ to maintain charge balance with the formation of disordered phase, because the lattice oxygen tends to be released during thermal treatment process.46 The impurity peaks become more apparent with the temperature rising, which indicates that higher temperature promotes the loss of oxygen and the Ni/Mn-disorder within LNMO materials. Also the presence of Mn3+ ions is also related to the Li loss and/or minor decomposition during the synthesis at high temperature.47 The composition of the four materials (Table 1) was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) measurement. The amount of Li slightly decreases with the increase of calcination temperature may due to the volatilization of Li. The Rietveld refinement patterns of the four LNMO materials are displayed in Figure S1 (Supporting Information). The refinement results and other physical parameters are summarized in Table 1. In general, with the increase of heating temperature, the lattice parameter shows an increasing trend from 8.1649 to 8.1783 Å. The lattice parameter of LNMO is further plotted as a function of the synthesis temperature in
2.2. Materials Characterization. The crystal structure of the synthesized LNMO materials was characterized by powder X-ray diffraction (D8-Advance, Bruker) in a 2θ range of 10° to 90° and preset time of 5 s with Cu Kα radiation (40KV, 40 mA). The powder diffraction patterns were refined by Rietveld analysis implemented in the General Structure Analysis System (GSAS) program.44 The morphology and structure of the materials were studied by scanning electron microscope (SEM, Zeiss Auriga) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010F, 200 kV). Selected-area electron diffraction (SAED) was performed to study the crystallinity and structure of the materials. Raman spectra were measured using a Bruker SENTERRA dispersive Raman microscope, equipped with a 532 nm argon-ion laser. The chemical composition of each powder was determined by inductively coupled plasma opticalemission spectrometry (ICP-OES, Spectro Arcos). The specific surface area was evaluated by the Brunauer−Emmett−Teller (BET) method (Micromeritics ASAP 2020). X-ray absorption spectroscopy (XAS) experiments were carried out in the transmission mode at Mn and Ni K-edge of various LNMO materials at beamline KMC-2 of the BESSY-II synchrotron light source at Berlin, Germany, equipped with a graded Si−Ge (111) double crystal monochromator. High harmonics were rejected by detuning the monochromator such that the intensity of the beam on the samples was 65% of the maximum possible intensity. A reference spectrum of each element was simultaneously collected for energy calibration. Besides, the manganese reference compounds such as Mn2O3 and MnO2 were also measured. X-ray absorption near-edge structure (XANES) data were analyzed with the ARTEMIS software package. The electronic conductivity was evaluated by impedance spectroscopy with a Novocontrol AN-alpha analyzer as an alternating current (AC) frequency generator and a POT/GAL 20/11 electrochemical test station. Approximately 0.4 g of each material was pressed with a force of 100 kN cm−2 into pellets of 13 mm diameter and a thickness in the range from 0.8 to 0.9 mm. Each pellet was sputter coated with Au on both sides to create blocking electrodes. Conductivity measurements were carried out at the temperature range of −40 to C
DOI: 10.1021/acsami.5b11389 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) XRD patterns of the four LNMO materials 700C, 800C, 900C, and 1000C, the standard phase of disordered spinel LNMO is located at the bottom (JCPDS card NO. 80-2162). The asterisk (*) indicates the impurity rock-salt phase. The right-hand side figures with expanded view show the rock-salt phase impurities. (b) Changes of lattice parameters and Mn3+ content dependent on the heating temperature.
Figure 2. (a) Raman spectroscopy of the four LNMO materials. (b) Mn Ni K edge XANES data for the four LNMO materials with Mn2O3 and MnO2 standards. (c) Ni K edge XANES data for the four LNMO materials.
Figure 1b, along with the content of Mn3+ calculated from the first discharge curves which will be shown later. Obviously, the lattice constant and Mn3+ content are both proportional to the synthesis temperature. The increased amounts of Mn3+ which have a larger ionic radius than that of Mn4+ ions also lead to the expansion of the unit cells and a larger lattice parameter. The lattice expansions at higher temperatures of 900 and 1000 °C are intensified, which suggests that the degree of disordering is also enhanced. The quantity of disordered phase in each material could be directly reflected by the lattice Mn3+ concentration for they are closely related with each other and change concurrently.48 The structure and the degree of Ni/Mn ordering of the four LNMO materials were also investigated by Raman spectroscopy. Figure 2a presents typical Raman spectra of well-crystallized LNMO materials. Both bands at about 400 and 490 cm−1 are associated with the Ni−O stretching mode within the spinel structure. The peak at around 630 cm−1 is attributed to the
symmetric Mn−O stretching vibration of MnO6 octahedron. The observation of a single peak without split around 600 cm−1 indicates that the phase is disordered structure of the Fd3m ̅ type. With the increase of heating temperature, the intensity ratio of F2g(2) and A1g peaks gradually increases from 700C to 1000C materials. It can be noticed that some small characteristic peaks of ordered LNMO in space group P4332 are detected between 200 and 260 cm−1 in the 700C material, but another character of the ordered phase is missing that the peak around 600 cm−1 should split into two peaks.28,49 This means that the 700C material is a mixture of the ordered and disordered phase. Based on the structural analysis above, the 700C material has the lowest Mn3+ content and lowest degree of Ni/Mn-disorder, so the Ni/Mn arrangement in the structure of the 700C material is probably partly ordered. In fact, the case of a combination of two phases (ordered and disordered) has also been reported.50−52
Table 1. Physical Properties of the LNMO Materials 700C, 800C, 900C, and 1000C samples 700C 800C 900C 1000C
ICP-OES (Li:Ni:Mn) 1.11:0.49:1.50 1.11:0.50:1.50 1.10:0.49:1.50 1.07:0.49:1.50
a (Å)a 8.1649(3) 8.1658(8) 8.1700(4) 8.1783(8)
Rp (%) 3.0 2.5 2.9 4.7
Mn3+ (%)b 3.9 5.9 9.8 12.1
BET surface area (m2 g−1) 12.3 4.9 3.9
