High-Energy Layered Oxide Cathodes with Thin Shells for Improved

Oct 6, 2014 - Department of Nano Engineering, Sejong University, Seoul 143-747, South Korea. ABSTRACT: ... applied to the outer surface of the FCG cor...
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High-Energy Layered Oxide Cathodes with Thin Shells for Improved Surface Stability Hyung-Joo Noh,† Seung-Taek Myung,‡ Yun Jung Lee,*,† and Yang-Kook Sun*,† †

Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea Department of Nano Engineering, Sejong University, Seoul 143-747, South Korea

‡

ABSTRACT: Core−shell, nickel-rich layered oxide materials with a full concentration gradient (FCG) core and thin shells with low nickel content have been investigated. Hierarchically structured core−shell materials have the same FCG core, where the composition gradually changes from Li[Ni0.86Co0.07Mn0.07]O2 to Li[Ni0.67Co0.09Mn0.24]O2 from the center to the outer surface. A thin shell composed of either Li[Ni0.48Co0.26Mn0.26]O2 or Li[Ni0.56Co0.18Mn0.26]O2 was applied to the outer surface of the FCG core. This hierarchical core−shell structure efficiently integrates the benefit of high energy from the Ni-rich core, structural stability and favorable transport of Li+ ions from the FCG core, and surface stability from the low-Ni and high-Mn shell. The core−shell cathodes demonstrate improved cycling performance at 55 °C even up to 4.5 V when compared to the FCG core-only cathode. Shells of low nickel content and a thickness of ∼300 nm provide sufficient surface stability, particularly at elevated temperatures. We suggest this novel core−shell structure as a suitable cathode for power sources such as electric vehicles, where safety and energy density are equally important.

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INTRODUCTION Among the available energy storage technologies, lithium-ion batteries have served as the primary power source in portable electronics such as cell phones and laptops. As interest in electric cars and energy grids increases; however, research into advanced, lithium-based batteries has shifted toward greater energy storage and high-energy electrode materials. Ni-rich layered oxides represent high-energy cathode materials for lithium-ion battery systems with high practical capacities (220− 230 mAh g−1) at high voltages (4.4−4.6 V), with the additional advantage of relatively low cost.1−3 Issues related to safety are widely recognized as key challenges in successfully implementing the necessary conditions for the practical application of this high-energy material: Ni-rich layered oxide has shown intrinsic thermal instability, and Ni4+ reacts aggressively with the electrolytes in charged states. These result in poor cycle life, particularly at elevated temperatures.4−9 Ni-rich surfaces also react with air-generating LiOH and Li2CO3.10 LiOH reacts readily with LiPF6 salt in the electrolyte, forming HF in the electrolyte, while Li2CO3 swells severely upon storage at high temperatures in the charged state: both LiOH and Li2CO3 are thus detrimental to electrode stability.11 Mn substitution can provide structural stability by pinning the structure with electrochemically inactive Mn4+.12−15 Compromises may be required between capacity and safety with the introduction of Mn4+ due to the inactive character of Mn4+ and the cation disorder of Ni2+, which this also introduces.16−18 An ideal nanostructure might utilize both the high-energy of the Ni-rich component and the enhanced safety of the Mn incorporation. © XXXX American Chemical Society

Core−shell structures are often employed to integrate two different functions into one entity.19 We showed that core− shell structures with a Ni-rich core (Li[Ni0.8Co0.1Mn0.1]O2) and an Mn-rich shell (Li[Ni0.5Mn0.5]O2) can achieve high capacity at high voltage while maintaining structural stability.20 However, structural mismatch and differences in volume change behavior between the core and shell can generate large voids at the interfaces during long-term cycling, leading to cell failure. Nanostructuring on the shell to achieve a concentration gradient partly relieved the structural mismatch;21 however, the surface manganese concentration was low due to a limited shell thickness and thereby the surface stabilization was also weak, particularly at high temperatures. Our recent work22 reported a novel structure, the full concentration gradient (FCG). In this novel structure, the concentrations of nickel and manganese change gradually from center to surface and the Ni concentration decreases, whereas the Mn concentration increases linearly. FCG structures have the additional advantages of being highly percolated and having an aligned nanorod morphology which provides a fast Li+ ion diffusion pathway that has been proven beneficial for high rate performance. The materials reported here have a hierarchically structured core−shell platform to integrate both structural stability and improve safety. The core is an FCG, Ni-rich, layered oxide and Received: July 28, 2014 Revised: September 29, 2014

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a 4.0 mol dm−3 solution (aq.) of NaOH and 15.0 mol dm−3 of NH4OH solution (aq.) (chelating agent) were also separately pumped into the reactor. The concentration, pH (pH 11.7), temperature (50 °C), and stirring speed (800 rpm) of the reaction mixture were carefully controlled in the CSTR. To synthesize the FCG core−shell structure, we first coprecipitated spherical FCG core particles with a diameter of approximately 11.4 μm in the CSTR. For the (Ni0.4Co0.3Mn0.3)(OH)2 and (Ni0.5Co0.2Mn0.3)(OH)2 shell formation on the FCG core precursor particles, shell content aqueous solution (Ni: Co: Mn = 0.4:0.3:0.3 or 0.5:0.2:0.3 in molar ratio) from the tank was slowly fed into the CSTR; the transition-metal-containing hydroxide slowly accumulated on the surface of the FCG core precursor. The coprecipitated precursor powders were filtered, washed, and dried at 110 °C. The FCG core and FCG core−shell cathode materials were prepared by thoroughly mixing the precursor powder with LiOH·H2O, followed by calcination at various temperatures for 10 h: 780 °C for the FCG core, and 820 °C for the FCG core−shells of Li[Ni0.48Co0.26Mn0.26]O2 and Li[Ni0.56Co0.18Mn0.26]O2. Characterization. Atomic absorption spectroscopy (AAS) (Vario 6, Analyticjena) was employed to determine the chemical compositions. The morphology of the powders was imaged with Scanning Electron Microscopy (SEM, S-4800, Hitachi). The crystalline structure of the powders was identified with powder Xray diffraction (XRD, Rint-2000, Rigaku, Japan) measurements using Cu−ka radiation from 10 to 130 2θ° with a step size of 0.03°. Cross sections were prepared for EPMA analysis by embedding the assynthesized particles in an epoxy and polishing with sand paper. Line scans for the FCG core and FCG core−shell structure materials were obtained with an electron probe X-ray microanalyzer (EPMA, JXA8100, JEOL). The amounts of total residual lithium were measured by titration method (T50, METTLER TOLEDO, Switzerland). For analysis via differential scanning calorimetry (DSC), the 2032 cointype cells were fully charged to 4.3 V and opened in an Ar-filled dry room. After the remaining electrolyte was carefully removed from the surface of the electrode, the cathode materials were recovered from the current collector. A stainless steel sealed pan with a gold-plated copper seal was used to collect 3−5 mg samples. Measurements were performed in a DSC 200 PC (NETZSCH, Germany) at a temperature scan rate of 1 °C min−1. Electrochemical Test. The synthesized cathode material was stored in a dry room. The cathode was fabricated with a mixture of prepared layered-oxide powder (85 wt %), carbon black (7.5 wt %), and polyvinylidene fluoride (PVDF) (7.5 wt %) in N-methyl-2pyrrolidone (NMP). The slurry was spread onto aluminum foil with loading density of 8 mg cm−2 and dried in a vacuum oven at 110 °C. All electrodes tested have similar loading density and thickness to exclude the effects of electrode quality on the battery performances. The electrochemical performance of the cathode materials was evaluated in a 2032 coin-type cell. The cell consists of a cathode and a lithium metal anode separated by a porous polypropylene film. The electrolyte used was 1.2 M LiPF6 in a 3:7 volume mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) (PANAX ETEC Co., Ltd., Korea). The cells were fabricated in Ar-filled glovebox

the shell is a low-Ni content, layered oxide with a thickness of approximately 300 nm (Figure 1). The FCG-structured core

Figure 1. Schematics showing the structure of core−shell high energy cathodes with FCG core and thin shell with low nickel contents.

guaranteed high energy without structural mismatch; though it is important to note that the surface nickel content cannot be too low to achieve high energy. Herein, we adopted a thin shell with low nickel content to further improve the surface stability of the FCG core. Structural mismatch is negligible due to the thin layer shell, and the usage of expensive cobalt is minimized. Although core−shell structures have been explored in many layered oxide materials, this novel nanostructure with thin shells of such features encasing high energy FCG core is demonstrated for the first time in this study. The resulting Nirich, FCG core, thin shell of low Ni content demonstrated high energy with improved cycle life at elevated temperatures.

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EXPERIMENTAL SECTION

Material Synthesis. We synthesized the FCG core and FCG core−shells of LiNi0.48Co0.26Mn0.26O2 and LiNi0.56Co0.18Mn0.26O2 precursors via coprecipitation methods. Two aqueous solutions of outer and inner content were prepared separately for the FCG core formation by mixing the required amounts of NiSO4·6H2O, CoSO4· 7H2O, and MnSO4·5H2O solutions with a concentration of 2 mol dm−3: the molar ratios of the outer and inner content aqueous solutions were Ni:Co:Mn = 0.63:0.11:0.26 and Ni:Co:Mn = 0.9:0.05:0.05, respectively. The outer content aqueous solution was slowly pumped into the stoichiometric amount of inner content solution, while the resulting homogeneously mixed solution containing Ni, Co, and Mn ions was simultaneously fed continuously into a stirred tank reactor (CSTR) under a N2 atmosphere. At the same time,

Figure 2. SEM images of hydroxide the precursors. (a) FCG core only and FCG core−shells of (b) Ni0.4Co0.3Mn0.3(OH)2 and (c) Ni0.5Co0.2Mn0.3(OH)2. B

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Figure 3. SEM images and EPMA results of layered oxides. EPMA line scans of the integrated atomic ratio of transition metals are shown as a function of distance from the particle center to the surface; (a, d) FCG core only, (b, c, e, f) FCG core−shell of (b, e) Li[Ni0.48Co0.26Mn0.26]O2 and (c, f) Li[Ni0.56Co0.18Mn0.26]O2. and tested at 25 and 55 °C between 2.7 and 4.3 or 4.5 V under various current densities.

[Ni0.67Co0.09Mn0.24]O2 at the surface, as shown in Figure 3d. The cobalt concentration remained nearly constant in the FCG. The concentration of Ni and Mn changes linearly, as designed. In panels e and f in Figure 3, the concentration profile of the core FCG region is identical to the FCG core-only material, while the outermost concentration changed abruptly: while the Mn concentration remained high, the nickel concentration dropped to 48 and 56%, and the cobalt concentration jumped to 26 and 18% in the shell region. Though there were marginal differences between the designed and actual compositions due to metal-ion diffusion between the core and shell, it is clear that the shells have a lower Ni content and higher Co content than that of the core. The amount of residual lithium on the layered oxide surface has been analyzed. The residual lithium in the form of LiOH and Li2CO3 is one of the possible routes to degradation in performance of Ni-rich layered oxide cathodes. Ni-rich layered oxides with nickel contents exceeding 60% are known to react readily with air, generating LiOH and Li2CO3.10 The amount of total residual lithium is summarized in Table 1, showing

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RESULTS AND DISCUSSION The schematics showing the structural and compositional changes within the high-energy core−shell materials developed in this study are presented in Figure 1. The core is a Ni-rich layered oxide with a full concentration gradient, where the nickel content linearly decreases and the manganese content linearly increases from the center to the surface. The content of cobalt is maintained low in the FCG core. The thin shell of constant composition is applied to the outer surface of the FCG. The shell is designed to improve the surface stability of the high-energy cathode materials with low nickel content, and also to retain the structural stability by minimizing the mismatch between the core and the shell with a thickness of approximately 300 nm. Because the shell is designed to be thin, the lithium ions needed for the electrochemical reaction can diffuse through the shell to the core region. Two shell compositions were tested in this study: Li[Ni0.48Co0.26Mn0.26]O2 and Li[Ni0.56Co0.18Mn0.26]O2. The structural features of the precursor hydroxides and the final lithiated layered oxides were observed via SEM, with with results shown in Figures 2 and 3. The synthesized precursor hydroxides in Figure 2 both with and without shells have homogeneous, regular morphologies of spherical aggregates with an average size of approximately 10 μm. The core−shell materials may have larger diameters, though the size difference is not noticeable with a shell thickness of only approximately 300 nm. In the insets of the enlarged view, needle-like smaller primary particles constitute the dense secondary morphology. The gross morphologies of the hydroxide precursors remain nearly intact in the final lithiated products, as shown in Figure 3a−c. Figure 3 also shows the results of the electron-probe Xray microanalysis (EPMA), performed to track compositional changes within the particles. The EPMA results in Figure 3d−f present an elemental distribution of Ni, Co, and Mn within a single particle. The FCG core has a concentration of Li[Ni 0 . 8 6 Co 0 . 0 7 Mn 0 . 0 7 ]O 2 at th e cen ter , an d Li-

Table 1. Total Residual Lithium Amounts (LiOH and Li2CO3) on the Surface of the Layered Oxides (unit: ppm) core only core-shell Li[Ni0.48Co0.26Mn0.26]O2 core-shell Li[Ni0.56Co0.18Mn0.26]O2

LiOH

Li2CO3

Total

7,345

1,830

9,175

4,812

1,367

6,179

5,054

1,486

6,540

significant reductions in shells with low Ni content. The lowest surface nickel content, and thus the lowest residual lithium, was found in the oxide with a shell composed of Li[Ni0.48Co0.26Mn0.26]O2, as expected. To confirm the role of thin shells with low Ni contents, battery performance was examined in the harsh condition (55 °C and 4.5 V cutoff) as well as at normal operational conditions (room temperature and 4.3 V cutoff). Panels a and c in Figure 4 C

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Figure 4. Electrochemical performances of the FCG core, FCG core−shells of Li[Ni0.48Co0.26Mn0.26]O2 and Li[Ni0.56Co0.18 Mn0.26]O2. (a) The first charge−discharge curves at a rate of 0.1 C in a voltage window of 2.7−4.3 V at 25 °C, (b) corresponding discharge capacity of (a) vs cycle life a rate of 0.5 C in a voltage window of 2.7−4.3 V at 25 °C, (c) the first charge−discharge curves at a rate of 0.1 C in a voltage window of 2.7−4.5 V at 55 °C, and (d) corresponding discharge capacity of c vs cycle life a rate of 0.5 C in a voltage window of 2.7−4.5 V at 55 °C.

present the initial discharge and charge curves, whereas panels b and d in Figure 4 display corresponding discharge capacity of cells employing cathodes with an FCG core only, and FCG core −shells of Li[Ni 0 . 4 8 Co 0 . 2 6 Mn 0 . 2 6 ] O 2 and Li[Ni0.56Co0.18Mn0.26]O2 at normal operational conditions (in a voltage window of 2.7−4.3 V at room temperature). The initial discharge capacities at 0.1 C were 203.1, 197.3, and 200.0 mAh g−1 for core only, core−shell: Li[Ni0.48Co0.26Mn0.26]O2, and core−shell: FCG core only, and FCG core−shells of Li[Ni0.48Co0.26Mn0.26]O2 and Li[Ni0.56Co0.18Mn0.26]O2. The capacities were slightly lowered with the application of thin shells because of the lowered content of nickel. No remarkable difference was observed in the capacity retention on cycling at 0.5 C between the core-only cathode and the core−shell cathodes, Figure 4b. Capacities were slightly lower with the presence of shells in the initial discharge and charge curves at the higher potential cutoff of 4.5 V and higher operational temperature of 55 °C, as shown in Figure 4c, whereas lowering was negligible in the shell composed of Li[Ni0.56Co0.18Mn0.26]O2. The initial discharge capacities at 0.1 C were 220.6, 211.8, and 218.2 mAh g −1 for core-only, core−shell: Li[Ni0.48Co0.26Mn0.26]O2, and core−shell: Li[Ni0.56Co0.18Mn0.26]O2, respectively. However, under severe test conditions charged 4.5 V and cycled 0.5 C at 55 °C, thin shells with low Ni content do make a difference (Figure 4d). Although the FCG core-only cathode shows moderate degradation during testing for 100 cycles (86.8% retention), shells with low the nickel content present improved retention performance in spite of their thinness; 91.1% for Li[Ni0.56Co0.18Mn0.26]O2 and 93.2% for Li[Ni0.48Co0.26Mn0.26]O 2, reflecting surface stability was achieved by the low Ni content in the outer shell. The rate capability of the cathodes were explored in a voltage window of 2.7−4.3 V, as shown in Figure 5. In our previous reports,22,23 full concentration gradient materials showed

Figure 5. Rate capability of the FCG core, FCG core−shells of Li[Ni0.48Co0.26Mn0.26]O2 and Li[Ni0.56Co0.18Mn0.26]O2 in a voltage window of 2.7−4.3 V. Electrodes were charged and discharged at same rates.

excellent rate performance due to the unique morphologies of the percolated, aligned, nanorod network, and could provide a short pathway for facile Li+ ion diffusion. Here, the core−shell cathodes with the FCG core and thin shells with low nickel contents showed good rate performances compared to the FCG core-only cathode in Figure 5. Shells with low nickel content for improved surface stability did not deteriorate the rate performances. The slightly better rate property might be due to the increase in cobalt content in the shells, because the shells are also rich in cobalt compared to the core FCG to lower the Ni content while keeping the Mn content. However, the detailed investigation on the effect of cobalt content in the D

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resistance (Rct) couple with a double-layer capacitance. In Figure 7, the charge transfer resistance (Rct), the resistance at the interface between the FCG core−shell cathodes and the electrolyte, was significantly lower than that of the core-only cathodes upon cycling. The FCG core-only cathode developed moderate Rsf and Rct during cycling. Both the Rct and Rsf were remarkably low following the first cycle, particularly for the shell with Li[Ni0.48Co0.26Mn0.26]O2 composition due to the surface stabilization with low nickel content. The shell with lower nickel content showed the lowest interfacial resistance at 55 °C cycling, which is consistent with the retention performance in Figure 4. To investigate the chemical stability at the surface of the core−shell structure, the transition metal dissolution was traced, as shown in Figure 8. The cathodes, first charged to

shell is beyond the scope of current study, and will be examined later. The superior performance of the FCG core−shell was verified based on long-term cycle performances in the voltage range of 3.0−4.4 V at 55 °C. The pouch-type full cells using three cathodes of FCG core only, FCG core−shells of Li[Ni0.48Co0.26Mn0.26]O2 and Li[Ni0.56Co0.18Mn0.26]O2 together with mesocarbon microbeads (MCMB, graphite) as the anode were fabricated and cycled at a current of 200 mA g−1 (1 C corresponds to 200 mA g−1). As shown in Figure 6, the capacity

Figure 6. Cycling performance of laminated-type Al-pouch cell (25 mAh) using mesocarbon microbead (MCMB) graphite as the anode, and FCG core and FCG core−shells of Li[Ni0.48Co0.26Mn0.26]O2 and Li[Ni0.56Co0.18Mn0.26]O2. The cells were cycled between 3.0 and 4.4 V at 55 °C with a constant current of 1 C (corresponding to 200 mA g−1).

retention of the FCG core−shell of Li[Ni0.48Co0.26Mn0.26]O2 improved after 1800 cycles: 43.3% versus 40.1% and 35.0% for the FCG core−shell of Li[Ni0.56Co0.18Mn0.26]O2 and the core only, respectively, which is consistent with both expectations and the capacity retention performances of half-type coin cell (Figure 4). We believe that the improved cycling stability of the FCG core−shell materials is ascribed to the increased Mn and reduced Ni concentration in the particle surface.21,23 The improved cycling stability at high temperatures was monitored by AC impedance measurement. Figure 7 presents the impedance spectra of the FCG core-only and FCG core− shells with low Ni content cathodes. In our previous reports,24 the high-to-medium frequency semicircle in the EIS measurement is assigned to the resistance of the surface film (Rsf), and the low-frequency semicircle is attributed to the charge transfer

Figure 8. Transition metal (Ni, Co, and Mn) dissolution of the FCG core, FCG core−shells of Li[Ni 0.48 Co 0.26 Mn 0.26 ]O 2 and Li[Ni0.56Co0.18Mn0.26]O2 cathodes charged to 4.3 V.

4.3 V, were stored in fresh electrolyte for 4 weeks at 60 °C. The amount of dissolved transition metal ions was below 100 ppm

Figure 7. Nyquist plots of (a) C/FCG core, (b) C/FCG core−shell Li[Ni0.48Co0.26Mn0.26]O2, and (c) C/FCG core−shell Li[Ni0.56Co0.16Mn0.26]O2 cells charged at state to 4.2 V at 55 °C with respect to cycle number: 1st, 100th, 250th, and 500th cycle. E

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for the first 2 weeks, increasing noticeably after 2 weeks of storage for all elements (Ni, Co, and Mn). The dissolutions of all elements are clearly dependent on the surface composition, increasing with surface nickel contents. Shells with low nickel content successfully stabilized the surface, and transition metal dissolutions from the core−shell cathodes were insignificant even after 4 weeks. The most vulnerable element, nickel, dissolved to ∼100 ppm after 4 weeks. For the reason, the increment of Rct was the smallest for the FCG core−shell with Li[Ni0.48Co0.26Mn0.26]O2 during cycling in Figure 7b, confirming the surface stabilization by the introduction of the low Ni shell. Figure 9 displays the differential scanning calorimetry profiles of the delithiated cathodes in the presence of electrolyte. The

[Ni0.48Co0.26Mn0.26]O2 shell. The novel structure developed in this study moves one step closer to the realization of lithium ion batteries with large energy storage capacity and the safety needed for electric vehicles.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was mainly supported by Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning and the partially support Human Resources Development program (No. 20124010203290) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

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Figure 9. DSC profiles of the FCG core, FCG core−shells of Li[Ni0.48Co0.26Mn0.26]O2 and Li[Ni0.56Co0.18Mn0.26]O2 cathodes with a scan rate of 1 °C min−1. The cells were charged to 4.3 V at a rate of 0.2 C.

exothermic peak temperature is the highest and the heat evolution is the lowest for the cathodes with FCG core−shell: Li[Ni0.48Co0.26Mn0.26]O2 with the lowest surface nickel content. With the addition of this shell, the exothermic temperature increased from 265.3 to 274.9 °C, and the generated heat was lowered from 732.1 to 689.5 J g−1, compared to the FCG coreonly cathode. The exothermic reaction temperature and amount of heat evolution would be affected by the surface nickel and manganese content of the outer shell. Hence, the design of the surface structure is of paramount importance to the thermal stability of the surface with the electrolyte.

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CONCLUSION A novel core−shell structure was developed in this study to synergistically combine the high energy of Ni-rich layered oxides with the high thermal stability of low-Ni oxides. The Nirich core with a FCG showed high energy, structural stability, and facile transport owing to the unique morphology. The shells applied in this study were designed with thicknesses as low as ∼300 nm to prevent degradation in capacity and structural stability, and have compositions with low nickel and higher cobalt. Though thin, the shell successfully provided enhanced surface stability without compromising the high energy, rate capability and structural stability. The stability endowed by a low surface-nickel content enabled an improved cycle life at 55 °C and a 4.5 V voltage cutoff: 91.1% for the Li[Ni 0 . 5 6 Co 0 . 1 8 Mn 0 . 2 6 ]O 2 and 93. 2% for t he LiF

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(24) Lee, S. H.; Yoon, C. S.; Amine, K.; Sun, Y. K. J. Power Sources 2013, 234, 201.

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