High performance LiNi_0.5Mn_1.5O₄ cathode material with a bi-functional coating for lithium ion batteries

Abstract

LiPO₃, one of the compounds from the Li₂O–P₂O₅ binary phase diagram, is successfully coated on LiNi_0.5Mn_1.5O₄ particles as a bifunctional layer with respect to its good ionic conductivity and chemical passivation properties. The coating layer with a thickness of 1 nm is identified by X-ray diffraction (XRD) and high resolution transition electron microscopy (TEM). Fourier transform-infrared spectrometer (FT-IR) and Raman spectra reveal that LiPO₃ coated LiNi_0.5Mn_1.5O₄ (LiPO₃/LiNi_0.5Mn_1.5O₄) possesses a cubic spinel structure with a space group of Fd3̄m. The electrochemical properties of synthesized materials are evaluated in both Li ion half cells and full cells. LiPO₃/LiNi_0.5Mn_1.5O₄ exhibits significantly enhanced rate performance and superior cyclability compared with non-coated LiNi_0.5Mn_1.5O₄. Impedance analysis indicates that the LiPO₃ coating dramatically reduces the LiPO₃/LiNi_0.5Mn_1.5O₄ cell impedance, especially the resistances of the lithium ion migration compared with non-coated LiNi_0.5Mn_1.5O₄. In addition, the LiPO₃ coating can effectively act as a passivation layer to minimize electrolyte–electrode interface side reactions and thus improve the long-term cyclability.

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Introduction

Lithium ion batteries as light-weight power sources are in tremendous demand for consumer electronics and electric vehicle applications these days. Among battery performance evaluation criteria, energy density, the amount of energy stored in a given system per unit volume or mass, is of prime importance. To improve battery energy density, cathode materials with high capacity and/or high potential are desirable.^1–3 Compared with current commercial cathode materials, such as LiFePO₄ (LFP), LiCoO₂ (LCO), LiNi_1/3Mn_1/3Co_1/3O₂ (NMC), LiMn₂O₄ (LMO) or LiNi_0.8Co_0.15Al_0.05O₂ (NCA), spinel-type LiNi_0.5Mn_1.5O₄ is regarded as an advanced cathode material with the promise of higher energy density with respect to its high discharge voltage (∼4.7 V). In addition, its relatively high rate performance and high thermal stability are attractive in terms of power density and safety respectively.^4–7 However, the high operation voltage creates challenges for the battery components, such as binder instability, electrolyte decomposition, cathode structure degradation, metal dissolution from the cathode, metal deposition on the anode, etc. Eventually, systematic problems resulting from high voltage lead to insufficient cycling that can't meet the application needs.^8–10

To mitigate problems, various strategies have been developed from different perspectives. For instance, electrolyte solvents with a high oxidation potential or additives that can decompose to form protecting layer on cathode have been proposed and investigated to address electrolyte–electrode interface problems.^11–15 However, there is no doubt that the majority of research efforts have been made to advance stability of LiNi_0.5Mn_1.5O₄. In this regard, doping the third cation in LiNi_0.5Mn_1.5O₄ has been proved to be one of approaches to stabilize its structure and thus to enable a better cycling performance. A variety of M-dopants (M = Fe, Cu, Cr, Co, Mg, Al, Zn, Ti, Ru, Zr, Mo, etc.) has been intensively investigated for this purpose.^16–28 A concentration gradient design with LiNi_0.5Mn_1.5O₄ core materials and Ni deficient surface layer is another approach to improve performance as a result of preventing of catalytic Ni⁴⁺ from contact of electrolyte.^29,30 In addition, better performance of LiNi_0.5Mn_1.5O₄ materials can be achieved with well-designed structures, such as hollow microspheres or nano-structures.^31–36

Compared with technologies mentioned above, coating is regarded as the most effective approach to enhance the performance by protecting the active material surface from electrolyte, alleviating side reactions and suppressing transition metal dissolutions. Different materials including organic and inorganic compounds have been studied as coating materials in terms of their functionalities. Polymer coating has been investigated as a coating material for LiNi_0.5Mn_1.5O₄ due to their good flexibility, processability, and scalability.^37–39 However considering polymers' stability and compatibility with electrolyte at high voltage, not many candidates can meet the criteria. Compared with rarely studied soft coating materials, metal oxides and metal halides, such as Al₂O₃, Bi₂O₃, ZnO, SiO₂, ZrO₂, Y₂O₃, RuO₂, and AlF₃, as stabilized layer have been extensively studied.^40–46 In addition to the stabilization role, advanced coating materials with more functionalities can further benefit the high voltage system. From this perspective, Li₂O–P₂O₅ binary phase edge compounds are attractive because of their favorable Li⁺ conducting properties.^47–49 Thus, Li₃PO₄, Li₄P₂O₇, and Li_0.1B_0.967PO₄ have been demonstrated as good coating materials for LiNi_0.5Mn_1.5O₄ to improve its electrochemical performance.^44,50–53

In this work, we choose LiPO₃, a Li₂O–P₂O₅ binary phase edge compound, as a coating material for LiNi_0.5Mn_1.5O₄. To our best knowledge, it has not been studied for high voltage cathode materials. LiPO₃ coated cathode material (LiPO₃/LiNi_0.5Mn_1.5O₄) was prepared by solid-state reaction. Physical properties of synthesized materials were characterized with XRD, TEM, FT-IR and Raman spectroscopy. Electrochemical properties were investigated in half and full cells. As illustrated in Scheme 1, we expect that electrochemical performance of LiPO₃/LiNi_0.5Mn_1.5O₄ can be significantly improved compared with non-coated LiNi_0.5Mn_1.5O₄ due to the bi-functional surface coating layer of LiPO₃.

Scheme 1 Schematic depiction of LiPO₃ coating effects on LiNi_0.5Mn_1.5O₄.

Experimental section

Preparation and characterization of materials

Solid-state reaction was used to prepare LiPO₃/LiNi_0.5Mn_1.5O₄ and non-coated LiNi_0.5Mn_1.5O₄. To synthesize LiPO₃/LiNi_0.5Mn_1.5O₄, 7.8 g of Li₂CO₃ (99.0%, Alfa Aesar), 11.9 g of NiCO₃ (99.0%, Alfa Aesar), 26.1 g of MnO₂ (metals basis, 99.9%, Alfa Aesar), 1.2 g of NH₄H₂PO₄ (98.0%, Alfa Aesar) and Al₂O₃ beads (total volume of 150 mL) were put into a 500 mL high density polyethylene bottle. The mixture was ball milled in 180 mL of acetone for 24 hours at 300 rpm. In the reaction, the molar feed ratio to result in LiPO₃ : LiNi_0.5Mn_1.5O₄ was controlled to 0.05 : 1. After ball milling, acetone was removed with stirring at 60 °C. The resulting powders were collected and pelletized using a disk-shaped die with 25 kN cm⁻² pressure. Finally, the resulting pellets were heat treated at 760 °C for 200 h in box furnace in air followed by naturally cooling to room temperature. Preparation of non-coated LiNi_0.5Mn_1.5O₄ was reported previously.^51,52

Philips X'Pert Pro Multipurpose X-ray Diffractometer using Cu Kα radiation (λ = 0.15406 nm) at 45 kV and 40 mA, with 0.02° per step and a 80 s step time was used to identify the phase of synthesized powders. The morphology of prepared materials was examined by JEOL JSM-7500F field emission scanning electron microscope (SEM) and 200 kV FEI monochromatic F20 UT Tecnai high-resolution transmission electron microscopy (TEM). Particle size distribution was measured with an LS230 particle characterization instrument. Raman and FT-IR spectra of samples were collected with Labram integrated Raman microscope system and Thermo Scientific Nicolet 6700 Fourier transform-infrared spectrometer, respectively.

Fabrication and testing of electrodes and cells

Electrode slurries were made by mixing 80 wt% active material, 15 wt% acetylene black (AB), and 5 wt% polyvinylidene difluoride (PVDF) binder in anhydrous N-methylpyrrolidone (NMP) solvent. Battery-grade acetylene black was acquired from Denka Singapore Private Limited. PVDF no. 1100 binder was purchased from Kureha, Japan. Anhydrous NMP was purchased from Sigma-Aldrich. Polytron PT 10-35 homogenizer was used to obtain uniform slurry at 3000 rpm. The slurries were coated on aluminium current collector by using a doctor blade. The porosity of laminates was compressed to about 30%. The electrodes for cells were pouched out from laminates with 1.27 cm diameter and the electrodes were dried in vacuum at 140 °C for 16 h before use.

Coin cells were fabricated in Ar filled dry box with CR2325 coin cell hardware for electrochemical measurements. 1 M LiPF₆ in EC : DEC (1 : 1 by mass) was used as electrolyte and Celgard 2500 was used as separator. The rate capability of materials was tested in Li half cells by using a constant charge rate (0.1C) and various discharge rates. The cycling performance was tested in full cells with CGP-G8 graphite as anode. The 0.5C charge/discharge rate was applied after 10 cycles of 0.1C charge/discharge formations. The cut-off potential for the cells is 3.0–5.0 V. Rate and cycling measurements were performed with a Maccor 4000 battery tester at 30 ± 2 °C in an environmental chamber from Testequity (model TEC1). Electrochemical impedance spectroscopy was measured using a Schlumberger 1286 Electrochemical Interface analyzer, Solartron 1260 Impedance/Gain-Phase Analyzer, and CorrWare software from 100 kHz to 0.01 Hz.

Results and discussion

XRD was used to identify the structure of prepared materials. To acquire high resolution XRD spectra, a slow scanning rate with extended step time 80 s was applied. As shown in Fig. 1, both of coated and non-coated materials exhibit the characteristic LiNi_0.5Mn_1.5O₄ peaks at 18.7, 36.4, 38.0, 44.2, 48.5, 58.6, 64.4 and 67.7° associated with phases of (111) (311) (222) (400) (331) (511) (440) and (531) respectively. For non-coated LiNi_0.5Mn_1.5O₄, no impurity peaks were observed in the XRD pattern. As comparison, extra peaks were present for LiPO₃/LiNi_0.5Mn_1.5O₄ powders. Among them, three relatively obvious peaks at 37.3, 43.6 and 63.3° can be assigned to Ni_1−xLi_xO rock salt phase impurity.⁵⁴ With Ni_1−xLi_xO impurity, the presence of Mn³⁺ is expected to be observed due to the reduction of small portion of Mn⁴⁺ during synthesis process, which was confirmed by the existence of 4 V plateau (Mn^3+/4+) in LiPO₃/LiNi_0.5Mn_1.5O₄ potential curves (Fig. 4). In addition, phase of LiPO₃/LiNi_0.5Mn_1.5O₄ can be assigned to Fd3̄m with disordered structure due to the absence of obvious peaks at 15.3, 39.7, 45.7 and 57.5° that are characteristic peaks for cation ordered P4₃32 space group.^55,56 Other minor peaks in the range of 20–40° are associated with LiPO₃ coating that shows short distance ordered but long distance disordered properties.^57–59

Fig. 1 XRD patterns of LiPO₃/LiNi_0.5Mn_1.5O₄ and non-coated LiNi_0.5Mn_1.5O₄ powders.

In addition to XRD, the degree of cation ordering of LiPO₃/LiNi_0.5Mn_1.5O₄ was further characterized by FT-IR and Raman spectroscopy. Fig. 2a shows Raman spectra of LiPO₃/LiNi_0.5Mn_1.5O₄, in which, band at 637 cm⁻¹ can be assigned to symmetric Mn–O stretching vibration of MnO₆ octahedra and peaks at 404 cm⁻¹ and 497 cm⁻¹ are associated with Ni²⁺–O stretching. Disordered structure of LiPO₃/LiNi_0.5Mn_1.5O₄ can be confirmed by two peaks located at 588 and 609 cm⁻¹, which are not obviously split, typical features for disordered structure.⁵⁶ Additionally, the absence of featured bands of ordered structure at 202 and 240 cm⁻¹ also confirmed its disordered Fd3̄m phase.^26,56 In high wavenumber region, there are two Raman shifts appeared at 1077 and 1172 cm⁻¹ representing symmetric and asymmetric stretching modes of P-non-bridging oxygen in LiPO₃ phase.^58,60,61 Fig. 2b shows the FT-IR spectra of LiPO₃/LiNi_0.5Mn_1.5O₄. Characteristic peaks at 558 and 625 cm⁻¹ can be assigned to Mn–O and two peaks at 592 and 507 cm⁻¹ are associated with Ni–O.⁶² In addition, FT-IR can be used to identify the cation ordering by comparing the intensity of two peaks at 586 and 625 cm⁻¹. As clearly shown in Fig. 2b, peak at 625 cm⁻¹ prominently dominates, validating its cation disordering properties. A series of shifts appeared in high wavenumber region at 980, 1045 and 1097 cm⁻¹ are features for symmetric and asymmetric vibration of PO₃⁻.^63,64

Fig. 2 Raman (a) and FT-IR (b) spectra of LiPO₃/LiNi_0.5Mn_1.5O₄.

Morphology of LiPO₃/LiNi_0.5Mn_1.5O₄ was investigated by SEM. Fig. 3a shows a representative SEM image of LiPO₃/LiNi_0.5Mn_1.5O₄ materials, in which LiPO₃/LiNi_0.5Mn_1.5O₄ is composed of different sized primary particles and some primary particles were sintered together during synthesis to form secondary particles. The diameters of majority particles were determined to be 2 μm by particle size analysis (PSA) (Fig. 3c), which is consistent with SEM observations. TEM was employed to further examine the microstructure of prepared materials. As shown in Fig. 3b, the inspected particle exhibited clear crystal lattice features with around 1 nm amorphous LiPO₃ coating layer.

Fig. 3 SEM (a) and TEM (b) images of LiPO₃/LiNi_0.5Mn_1.5O₄ and particle size distribution (c).

Electrochemical properties of LiPO₃/LiNi_0.5Mn_1.5O₄ were evaluated in half cells with charge (delithiation) first followed by discharge (lithiation). The first cycle charge/discharge profiles were examined to understand the electrochemical formation behavior. As shown in Fig. 4, two major potential plateaus were observed in first charge and discharge curves of LiPO₃/LiNi_0.5Mn_1.5O₄. In addition to that, 4 V plateau associated with Mn^3+/4+ is present. dQdV plot (inset of Fig. 4) shows two couples of peaks at (4.686, 4.724 V) and (4.726, 4.754 V) respectively, which are consistent with its potential profiles. At 4 V, there is a hump indicating the redox couple of Mn^3+/4+. As previously studied, obvious noise was observed for first charge potential curve at round 5 V for non-coated LiNi_0.5Mn_1.5O₄, corresponding to vigorous electrolyte decomposition on the material bare surface.⁵¹ With LiPO₃ coating, clean potential curves were found indicating that LiPO₃ as a protective layer effectively minimized the electrolyte–electrode side reactions by blocking the access of electrolyte to active material surface.

Fig. 4 The first cycle voltage profile and dQdV plot (inset) of LiPO₃/LiNi_0.5Mn_1.5O₄ at 0.1C in the range of 3–5 V.

Fig. 5 shows the rate performance for LiPO₃/LiNi_0.5Mn_1.5O₄ and non-coated LiNi_0.5Mn_1.5O₄. The active material loading level was controlled to about 5 mg cm⁻² for both materials to ensure a fair comparison. At 0.1C rate, we noticed that the reversible capacity of LiPO₃/LiNi_0.5Mn_1.5O₄ is 130 mA h g⁻¹, slightly lower than non-coated LiNi_0.5Mn_1.5O₄, which may be because inactive LiPO₃ coating added up the active material mass of LiPO₃/LiNi_0.5Mn_1.5O₄. As C rate increased, LiPO₃/LiNi_0.5Mn_1.5O₄ exhibited significantly enhanced rate performance compared with non-coated materials. There was no obvious decay observed for LiPO₃/LiNi_0.5Mn_1.5O₄ up to 10C. Even at higher C rates, LiPO₃/LiNi_0.5Mn_1.5O₄ shows prominent rate capability. For example, 85% of capacity retention can be achieved at 20C and 77% capacity can be retained at 30C. Correspondingly, capacity of non-coated materials decayed to below 50% of initial capacity at 30C. We believe that good ionic conductivity of LiPO₃ coating can efficiently lower the cell impedance and thus result in the superior rate performance.

Fig. 5 The rate performance of LiPO₃/LiNi_0.5Mn_1.5O₄ and non-coated LiNi_0.5Mn_1.5O₄ electrodes charged at 0.1C then discharged at various C rates.

Electrochemical impedance spectra (EIS) was used to study the cell electrochemical behaviors at the interface of electrolyte/electrode for a better understanding of rate performance. Both LiPO₃/LiNi_0.5Mn_1.5O₄ and non-coated LiNi_0.5Mn_1.5O₄ were charged and discharged at 0.1C. Before AC impedance was measured, the cells were rested for 10 h at 50% of discharge state of the 10th cycle. Impedance spectra of experimental and fitting results along with equivalent circuits are shown in Fig. 6. As illustrated, Nyquist plots of LiPO₃/LiNi_0.5Mn_1.5O₄ and non-coated LiNi_0.5Mn_1.5O₄ exhibited different features. Non-coated LiNi_0.5Mn_1.5O₄ possesses 3 semicircles corresponding to the resistance of Li⁺ diffusion in the surface film layer (HFS), resistance of electronic property change of active material (MFS), resistance of the charge transfer reaction (LFS), respectively and a slope associated with resistance of Li⁺ diffusion in solid state of active materials (LFL) (Fig. 6b).⁶⁵ As a comparison, LiPO₃/LiNi_0.5Mn_1.5O₄ only shows two semicircles of HFS and LFS (Fig. 6a). The absence of MFS indicates that electronic conductivity of LiPO₃/LiNi_0.5Mn_1.5O₄ was improved with LiPO₃ coating. As we know that LiPO₃ is not electronically conductive, our speculation is that LiPO₃ coating may change the surface local crystal structure of LiNi_0.5Mn_1.5O₄ resulting in the electronic conductivity improvement.⁶⁶ In addition, it is obviously noted that the resistance of Li⁺ diffusion in the surface film layer (HFS) in high frequency region decreased from 47 Ω cm² for non-coated materials to 24 Ω cm² for LiPO₃/LiNi_0.5Mn_1.5O₄, validating that good ionically conductive LiPO₃ can significantly facilitate the migration of Li⁺ through surface layer of materials and this can account for the high rate performance of coated materials. In Fig. 6, we can also observe that the resistances of LFS for both materials are close but coated materials exhibit much smaller resistance of Li⁺ diffusion in solid state of active materials (LFL), which also attributes to its better rate capability.

Fig. 6 Electrochemical impedance spectra and equivalent circuits of (a) LiPO₃/LiNi_0.5Mn_1.5O₄ and (b) non-coated LiNi_0.5Mn_1.5O₄. HFS (resistance of Li⁺ diffusion in the surface film layer); MFS (resistance of electronic property change of active material); LFS (resistance of the charge transfer reaction); LFL (resistance of Li⁺ diffusion in solid state of active materials).

To evaluate LiPO₃/LiNi_0.5Mn_1.5O₄ as an advanced cathode material for lithium ion batteries, its cycling performance was tested in a full cell configuration with CGP-G8 as anode. The cells were cycled at 0.5C rate between 3.0 V and 5.0 V after 10 cycle formation at 0.1C rate with standard 1 M LiPF₆ in EC : DEC (1 : 1 by mass) electrolyte. To validate the LiPO₃ coating effect on cycling performance, non-coated LiNi_0.5Mn_1.5O₄ was tested under same condition. The capacity retention as a function of cycle number of LiPO₃/LiNi_0.5Mn_1.5O₄ and non-coated LiNi_0.5Mn_1.5O₄ was shown in Fig. 7. It is obvious that capacity of non-coated LiNi_0.5Mn_1.5O₄ dramatically decayed in first 50 cycles. This is because that transition metal can leach from LiNi_0.5Mn_1.5O₄ crystal structure and migrate to anode leading to the cathode material structure degradation and anode impedance increase. Also, at high voltage, electrolyte can be oxidized on cathode surface and Ni⁴⁺ from cathode materials can catalytically accelerate the side reactions on the electrolyte/electrode interface which is also part of reasons causing the quick cycle life decay. If moisture was present in the cell, water can react with LiPF₆ to generate HF that can attack cathode materials to make the situation become worse. As a comparison, with LiPO₃ coating, LiPO₃/LiNi_0.5Mn_1.5O₄ showed a superior cycling performance with 80% capacity retention up to 670 cycles. As all testing condition are same for coated and non-coated materials, we believed that the huge improvement of cyclability must derive from LiPO₃ coating which can act as a protection layer to effectively prevent side reactions happening on the surface of LiNi_0.5Mn_1.5O₄ and cathode metal dissolution can be significantly suppressed. There is still an ongoing research effort towards deep understanding of passivation effect of LiPO₃ and the relevant findings will be reported elsewhere.

Fig. 7 The cycling performance of LiPO₃/LiNi_0.5Mn_1.5O₄ and non-coated LiNi_0.5Mn_1.5O₄.

Conclusions

In this study, we reported LiPO₃ coated high voltage LiNi_0.5Mn_1.5O₄ materials prepared by solid-state reaction. Physical and electrochemical properties of resulting materials were characterized. LiPO₃ coating was identified about 1 nm by high resolution TEM. LiPO₃/LiNi_0.5Mn_1.5O₄ has a cubic spinel structure with a space group of Fd3̄m confirmed by Raman and FT-IR spectra. Compared with non-coated LiNi_0.5Mn_1.5O₄, LiPO₃/LiNi_0.5Mn_1.5O₄ showed much better rate capability and superior cyclability as a result of bi-functional LiPO₃ coating layer. Although the capacity fade of high voltage LiNi_0.5Mn_1.5O₄ is a complication concerning electrolyte decomposition, metal dissolution from cathode and metal deposition on anode, cathode–electrolyte interface is considered as a major cause. We believe that LiPO₃ surface coating effectively mitigated the electrode/electrolyte interface problems and thus terminated the corresponding consequences to achieve stable cyclability for high voltage LiNi_0.5Mn_1.5O₄. Beyond conventional surface protection role, as a good ionic conductor, LiPO₃ lowered the cell impedance and thus significantly increased the rate performance of LiPO₃/LiNi_0.5Mn_1.5O₄. Our study demonstrated that advanced coating can be the key to tailor the cathode material electrochemical properties and thus enable the high performance, especially for high voltage cathode materials with metal leaching problems and it should also be a reliable strategy for stabilizing high Ni content NMC materials that are enthusiastically pursued in lithium ion batteries industry to improve energy density.

Acknowledgements

This work was supported by National High Technology Research and Development Program of China (2013AA110103) and the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The National Center for Electron Microscopy at LBNL is acknowledged for the TEM experiments.

Notes and references

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Footnote

† Current address: DuPont Central Research and Development, Wilmington, DE, 19803, USA.

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