Structural and electrochemical characteristics of morphology-controlled Li[Ni0.5Mn1.5]O4 cathodes

doi:10.1016/j.electacta.2015.01.027

Electrochimica Acta

Volume 156, 20 February 2015, Pages 29-37

https://doi.org/10.1016/j.electacta.2015.01.027 Get rights and content

Highlights

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Two types of spinel Li[Ni_0.5Mn_1.5]O₄ (LNMO) were prepared by a facile two-step approach.
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Both the LNMO structure and morphology were easily controlled to nanorods and octahedral shape along with ordered (P4₃32) and disordered (Fd-3 m) phases, respectively.
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Overall battery performances of the octahedral LNMO particles are superior to those of the LNMO nanorods.
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The octahedral LNMO consisting of mixed particle sizes exhibits the best battery performances with high cycle stability and excellent high-rate capability.

Abstract

Two types of structure- and morphology-controlled spinel Li[Ni_0.5Mn_1.5]O₄ (LNMO) are prepared and systematically investigated as 5 V, high-rate, and long-life cathode materials for rechargeable Li-ion batteries. The octahedral LNMO particles (1 μm or 1–5 μm mixed sizes) are prepared through a heat-treatment at 850 °C after a hydrothermal reaction, and their performance is compared with that of one-dimensional LNMO nanorods (100–200 nm and 1–3 μm in diameter and length, respectively), which are synthesized via a two-step method consisting of a hydrothermal reaction followed by solid-state Li and Ni implantation. They show high single crystallinities with an ordered (P4₃32) and disordered (Fd-3 m) phase for the nanorods and octahedral particles, respectively. Rietveld refinement of X-ray and neutron diffraction, FT-IR, SEM, and TEM are employed to study their phases and microstructures. Galvanostatic studies reveal that overall battery performances of the octahedral LNMO particles are superior to those of the LNMO nanorods. In particular, the disordered octahedral LNMO particles that are composed of mixed particle sizes ranging from of 1 to 5 μm show not only the best rate capability and specific discharge capacity but also an excellent cycle stability with a capacity retention of 89% (corresponding to specific discharge capacity of 105 mA h g⁻¹) at a 10 C cycling rate, even after 1000 cycles. This remarkable performance is attributed to the structural stability, while the highest electrode tap density (1.59 g cm⁻³) in combination with efficient packing resulted in the coexistence of various particle sizes that can provide a shortened pathway for lithium ions between particles.

Graphical abstract

Introduction

With the growing interest to employ lithium-ion batteries (LIBs) in large-scale applications such as electric vehicles (EVs), hybrid EVs (HEVs), plug-in hybrid electric EVs (PHEVs), or stationary energy storage for smart grids, cathode materials that operate at high-voltage and deliver higher energy and power densities have attracted much attention in recent years [1], [2]. A high battery energy density can be accomplished either by high-voltage or high-capacity, and in this context, the use of a high-voltage cathode has been well recognized as an effective way to increase the voltage of LIBs, since the working voltage of the anode has almost reached the working potential of lithium metal [3].

So far, several compounds have been investigated because of their high-voltage characteristics [4], [5], [6]. One of the top contenders is spinel Li[Ni_0.5Mn_1.5]O₄ (LNMO), which can work with a conventional carbonate-based electrolyte, although there are some accompanying side reactions [7], [8], [9], [10]. The material possesses fast three-dimensional Li⁺ diffusion channels, a theoretical capacity of 147 mA h g⁻¹ (equivalent to an energy density of 700 Wh kg⁻¹), and access to a rare two-electron transition from the double redox couples Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ that are responsible for the high-voltage charge-discharge potential accompanying the Li-insertion and extraction at two voltage plateaus around 4.7 V (vs. Li⁺/Li), where a relatively high-capacity can be obtained. Because of the high-operating voltage and a comparable capacity (around 140 mA h g⁻¹) to LiCoO₂ (LCO, ∼620 Wh kg⁻¹) and LiFePO₄ (LFP, ∼591 Wh/kg⁻¹), spinel LNMO (∼658 Wh kg⁻¹) gives a higher specific energy density than many commercialized compounds [7], [11], [12], [13]. It is also well-known that spinel LNMO exists in two polymorphs: the cubic spinel with a P4₃32 space group, which is called “ordered” LNMO, and the so-called “disordered” LNMO with an Fd-3 m space group, depending on the oxygen stoichiometry/ordering of the Ni/Mn cations at the octahedral sites. The ordered structure provides very flat charge-discharge profiles at around 4.7 V (vs. Li⁺/Li), whereas discrete curves result in the case of a disordered structure. Most researchers have shown disordered LNMO to have a better rate capability and cyclability than ordered LNMO, though the difference in performance between the phases varies [14], [15], [16]. One explanation for this is that the disordered phase shows higher electronic conductivity than the ordered one owing to an existence of Mn³⁺ caused by Ni/Mn disordering. It was also reported that the P4₃32 phase undergoes a transformation to an intermediate phase exhibiting Fd-3 m symmetry during delithiation, which may suggest a lower structural reversibility of the ordered compared to the disordered form, especially at high-rates [14], [16], [17], [18]. However, this comparison of the electrochemical performances of spinel LNMO with different structures is still controversial. As previously reported by the Amatucci group [16], [19], it is believed that disordered spinel LNMO exhibits a better cycle performance owing to its higher electronic and lithium-ion conductivity. On the other hand, Ma et al. [17] have reported with experimental and computational methodologies that ordered spinel LNMO could show superior cyclability along with a high-rate capability.

Apart from the inherent properties of the material, other attributes, including particle size, morphology, pore structure etc., are also important and should be optimized. Several methods such as the solid-state, molten salt, sol-gel, and co-precipitation method amongst others [20], [21], [22], [23], [24], have been reported to prepare spinel LNMO, and various morphologies and particle sizes ranging from nanometers to microns have been synthesized by these techniques. In particular, spinel LNMO nanostructures with different morphologies, e.g., nanoparticles, nanorods, hierarchical micro/nanostructures, porous nanorods, and hollow microspheres [25], [26], [27], [28], [29], have received much interest recently, as they lead to an improved kinetic performance by reducing the transport path lengths of lithium-ions and electrons. However, although nanostructured LNMO shortens the transport distance for lithium-ions and assures a large specific surface area that offers advantages for electrochemical performance enhancements, it also increases the risk of surface side reactions. Besides, its low tap density renders a lower volumetric energy density that is more detrimental. It has been shown that spinel LNMO with micro-sized secondary particles outperforms nano-sized samples in both cycling capacity retention and rate capability [30], [31], [32].

Therefore, in an effort to optimize the performance and stability of spinel LNMO cathodes, much research has focused on the correlation between their electrochemical performance and physical properties, such as morphology, particle size, crystal structure, and Mn³⁺ content. In this regard, a systematic study of the influence of various factors on the electrochemical performance is still both challenging and lacking. Herein, we report a facile design and comparative studies of two different types of spinel LNMO, which are controlled by both structure and morphology, and their application as high-operating cathode materials for LIBs. The effect of electrochemical performance on the structural and morphological properties, including the different shapes and particle sizes, are systematically investigated and compared. The results showed that the overall battery performances, such as the specific capacity, the cycle and high-rate capability, and the long-life stability of the disordered octahedral LNMO particles are superior to those of the ordered LNMO nanorods.

Section snippets

Materials

For the synthesis of two types of spinel Li[Ni_0.5Mn_1.5]O₄, commercial LiOH∙H₂O, KMnO₄, MnSO₄∙H₂O, MnCl₂∙4H₂O, NiSO₄∙6H₂O, and NiCl₂∙6H₂O were directly used as sources of Li, Mn, and Ni, and (NH₄)₂S₂O₈ and P123 (poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol)) were used as the oxidizing agent and surfactant, respectively. All analytical grade reagents were used as received without further purification and were purchased from Sigma-Aldrich (USA).

Synthesis of disordered octahedral spinel Li[Ni_0.5Mn_1.5]O₄ particles

Disordered octahedral spinel

Morphological and structural control of Li[Ni_0.5Mn_1.5]O₄

Fig. 1 presents typical FE-SEM images of the two LNMO spinels showing the distinct morphological features, which are accomplished by applying different two-step approaches: a hydrothermal reaction followed by i) heat-treatment at 850 °C (Fig. 1a) or ii) solid-state Li and Ni implantation (Fig. 1b). As seen in Fig. 1a, the octahedral shapes for a typical spinel structure (hereafter, LNMO-OHs) display relatively uniform particle sizes of approximately 1 μm, which are obtained through the

Conclusions

In summary, highly crystalline ordered and disordered spinel Li[Ni_0.5Mn_1.5]O₄ (LNMO) were successfully prepared with different forms of nanorods and octahedral particles, respectively. With a facile two-step approach for the creation of each product, both the LNMO structure and morphology could be easily controlled, and in the case of the LNMO-OHs, the particle size could also be controlled by varying the heating time after the hydrothermal reaction. In particular, the disordered LNMO-OHs-m

Acknowledgements

This research was performed as a cooperation project supported by the Korea Research Institute of Chemical Technology. This work was also supported by the National research Foundation of Korea (2009-0094046) and by Korea Small and Medium Business Administration (S2230272).

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More than just a protection layer: Inducing chemical interaction between Li<inf>3</inf>BO<inf>3</inf> and LiNi<inf>0·5</inf>Mn<inf>1·5</inf>O<inf>4</inf> to achieve stable high-rate cycling cathode materials
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Citation Excerpt :
EDS mapping images in Fig. 1e and f reveal homogeneous distribution of the elements B, Ni and Mn, confirming the presence of the LBO coating layer. TEM analyses of the bare LNMO and LBO2%@LNMO samples were shown in Fig. 1g and h. Consistent with the XRD analysis, both samples exhibit high crystallinity and lattice fringe with a lattice spacing value of 0.46 nm, corresponding to (111) plane of LNMO spinel structure [27]. Moreover, shown in the Fig. 1h, the coating layer displays lattice stripes with a lattice spacing of about 0.28 nm, which matches with the (110) plane of LBO structure, as also found in the pure LBO sample depicted in Fig. S1.
Spinel LiNi_0·5Mn_1·5O₄ (LNMO), though being considered as a promising cathode material because of their high energy and power density, always suffers from rapid capacity deterioration due to the Jahn-Teller effect and severe leakage of Mn ions into electrolytes at elevated temperatures. In this work, Li₃BO₃ (LBO) formed from slow hydrolysis of triethyl borate was coated on the surface of LiMn_1·5Ni_0·5O₄ materials prepared through a modified wet-chemistry method. Our results put an emphasis on the strong chemical interactions between the external LBO coating layer and the internal LNMO core induced by a high-temperature thermal treatment; owing to the favorable solid-solid interfacial interaction, the presence of the relatively oxidative Li₃BO₃ layer was found to diminish the population of the oxygen vacancy and minimize the unstable Mn³⁺ species in the LNMO active materials, which thus could alleviate the adverse Jahn-Teller distortion and enhance the structural stability. Moreover, as a coating layer, the LBO could prevent direct interaction between the cathode active materials and the electrolyte; as such, the etching of the LNMO cathode materials by the decomposition products of electrolytes such as HF and the leakage of active metal species were inhibited effectively. Besides, the LBO itself is a highly conductive ionic conductor, which thus could enable fast Li⁺ cation diffusion on the cathode. On this basis, a significant improvement was observed in the high-rate cycling performance for thus coated LNMO cathode materials. The bare LNMO electrode failed within merely 300 cycles. In contrast, our optimal sample LBO_2%@LNMO with 2 wt% of LBO delivers a high discharge specific capacity of 127 mAh g⁻¹, which accounts for 92% of its initial capacity after 500 cycles at 1C in the range of 3.5–4.99 V; when cycling at 10 C, it displays an initial discharge specific capacity of 111 mAh g⁻¹ and maintains 85 mAh g⁻¹ after 1000 cycles. Our work demonstrates that in addition to behaving as a physical protection layer, the introduction of the ionic conductive Li₃BO₃ coating layer could also modify the local composition of the spinel-type LNMO through strong interfacial chemical reaction, which thus could profoundly enhance the structural integrity and high-rate cycling stability.
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LiNi_0.5Mn_1.5O₄ (LNMO) with a spinel crystalline structure exhibits excellent properties such as a high voltage of ~4.7 V and a fairly high theoretical capacity of ~147 mAh g⁻¹ as well as low cost. However, LNMO cathode materials exhibit a deteriorated high-rate performance and capacity fading because of their low structural stability caused by dissolution of Mn ions into the electrolyte via the Jahn–Teller effect during the Li⁺ ion insertion/desertion process. Herein, to obtain electrochemical/structural stabilities and to prevent dissolution of Mn ions from pristine LNMO, we designed M_t-doped LNMO cathode materials (LiNi_0.4Mn_1.5M_tO₄) with different transition metal elements (M_t = Co and Fe) having a chemical valence electron of M_t³⁺ using a hydrothermal method. All samples exhibited that the M_t-doped LNMO structures were homogeneously doped with Co and Fe elements. Furthermore, compared with the undoped LNMO materials, the M_t-doped LNMO cathode materials showed superior electrochemical properties in terms of high discharge capacities (121.1 mAh g⁻¹) at 120 mA g⁻¹ and good cycle retentions (over 99.7%) after 200 cycles as well as improved high-rate performance, because of the well-engineered valance and disordered structure for the doping of M_t³⁺, preventing the dissolution of Mn ions via the Jahn–Teller distortion.
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Citation Excerpt :
However, the high operating potential of LNMO can lead to electrolytic performance deterioration during the charge/discharge process. Surface modification is an efficient way to suppress the side reactions and decomposition and dissolution of Mn ions, which in turn, improve electrochemical performance [7–9]. Numerous coating materials, such as Al2O3 [10], RuO2 [11], TiO2 [12], Li2TiO3 [13], ZrO2 [14], LiAlSiO4 [15], and CoAl2O4 [16] have been utilized to cover the surface of LNMO particles.
The high-voltage spinel LiNi_0.5Mn_1.5O₄ (LNMO) is a potential cathode material for lithium-ion batteries with outstanding energy density and power density. Here, we document a facile approach to prepare Al₂O₃-modified LNMO cathode materials. The Al₂O₃-modified LNMO materials were synthesized via a one-step solid-state reaction and then modified with Al₂O₃ via a wet chemical technique. The impacts of Al₂O₃ modification on the structure and electrochemical properties of LNMO materials were examined by X-ray diffraction, Fourier-transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, charge-discharge tests, cyclic voltammetry measurements, electrochemical impedance spectroscopy, and aging tests. Throughout the modification process, several Al³⁺ were noted to substitute for Ni²⁺, resulting in a decrease of Mn⁴⁺ to Mn³⁺; this increased the electronic conductivity and lowered the electrochemical polarization of the LNMO material. An amorphous Al₂O₃ coating layer developed on the surface of the LNMO particles in the modification, and this could alleviate the strike of HF caused by electrolyte decomposition as well as the development of a solid electrolyte interphase. Thus, the 0.5 wt% Al₂O₃-modified LNMO material had decreased R_sf and R_ct and greater D_Li values with a rate capability and cycling stability better than LNMO. The rate capability was 105.6 and 83.3 mAh g^–1 at high C rates of 5 C and 7 C, as opposed to 83.3 and 54.9 mAh g^–1, respectively; the room temperature (25 °C) capacity retention was 92.6% at 1 C after 200 cycles, as opposed to 87.0%. The high-temperature (55 °C) capacity retention was 90.9% at 1 C rate after 200 cycles as opposed to 86.5%. Thus, this is an easy and feasible method to improve the electrochemical performance of LNMO cathode materials for industrialization.

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Structural and electrochemical characteristics of morphology-controlled Li[Ni0.5Mn1.5]O4 cathodes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Synthesis of disordered octahedral spinel Li[Ni0.5Mn1.5]O4 particles

Morphological and structural control of Li[Ni0.5Mn1.5]O4

Conclusions

Acknowledgements

J. Power Source

J Power Source

J. Power Source

J. Power Source

J. Power Source

Electrochim. Acta

J. Power Source

Electrochim. Acta

J. Power Source

J. Power Source

Electrochim. Acta

J. Electroanal. Chem.

J. Cryst. Growth

Mater. Lett.

Electrochim. Acta

Solid State Ionics

Electrochim. Acta

J. Power Source

J. Power Source

J. Power Source

Adv. Energy Mater.

Adv. Energy Mater.

Chem. Mater.

Ionics

Structural and electrochemical characteristics of morphology-controlled Li[Ni_0.5Mn_1.5]O₄ cathodes

Synthesis of disordered octahedral spinel Li[Ni_0.5Mn_1.5]O₄ particles

Morphological and structural control of Li[Ni_0.5Mn_1.5]O₄