Cathode Material with Nanorod Structure—An Application for

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Cathode Material with Nanorod StructureAn Application for Advanced High-Energy and Safe Lithium Batteries Hyung-Joo Noh,† Zonghai Chen,‡ Chong S. Yoon,§ Jun Lu,‡ Khalil Amine,*,‡ and Yang-Kook Sun*,† †

Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States § Department of Materials Science and Engineering, Hanyang University, Seoul 133-791, South Korea ‡

S Supporting Information *

ABSTRACT: We have developed a novel cathode material based on lithium−nickel−manganese−cobalt oxide, where the manganese concentration remains constant throughout the particle, while the nickel concentration decreases linearly and the cobalt concentration increases from the center to the outer surface of the particle. This full concentration gradient material with a fixed manganese composition (FCG−Mn-F) has an average composition of Li[Ni0.60Co0.15Mn0.25]O2 and is composed of rod-shaped primary particles whose length reaches 2.5 μm, growing in the radial direction. In cell tests, the FCG−Mn-F material delivered a high capacity of 206 mAh g−1 with excellent capacity retention of 70.3% after 1000 cycles at 55 °C. This cathode material also exhibited outstanding rate capability, good low-temperature performance, and excellent safety, compared to a conventional cathode having the same composition (Li[Ni0.60Co0.15Mn0.25]O2), where the concentration of the metals is constant across the particles. KEYWORDS: coprecipitation, nanorod, concentration gradient, cathode, lithium batteries

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INTRODUCTION In the past decade, worldwide attention has been focused on the development of lithium-ion batteries with high energy density and long cycle life for automotive applications, such as plug-in hybrid vehicles (PHEVs) and electric vehicles (EVs). Today’s conventional lithium-ion batteries have been introduced in the automotive industry with limited success, because of their high cost and low energy density. To reduce the cost of the battery system and increase the driving range for the EVs, batteries with very high energy density and improved safety are needed.1−4 In the past, most of the work has been centered on developing high-energy layered cathode materials. Among these materials, LiNi0.80Co0.15Al0.05O25−7 and Li[Ni1−x−yCoxMny]O2 (where 1 − x − y ≥ 0.6)8−10 have been investigated intensely over the past 10 years, because they offer higher capacity than existing cathode materials. However, they exhibit poor cycling and thermal characteristics, especially at high voltage and elevated temperatures, because of oxygen release from the charged electrode, which oxidizes the electrolyte and leads to a quick capacity fade and, in some cases, severe thermal runaway of the cell.11−13 To overcome the inherent problems of nickelrich layered materials, we have developed functional layered oxides with core−shell (or concentration gradient) cathode materials.14,15 More recently, we reported a high-energy full concentration gradient cathode material, where the concentration of nickel decreases gradually from the center toward the © XXXX American Chemical Society

outer layer of the particle, while the concentration of manganese increases gradually so that the manganese-rich and nickel-poor outer layer can stabilize the material, especially during high-voltage cycling.16 This material shows high capacity at high-voltage cycling, outstanding cycle life, and excellent safety performance. This new approach has opened a huge opportunity for designing materials with different gradient compositions and slopes across the particle. It has been well-known that the electrochemical performance of cathode materials is strongly influenced by their microstructure, especially during long-term cycling at elevated temperatures.17−19 All the cathode materials reported so far were composed microscale secondary particles consisting of spherical or long rectangular primary particles with size ranging from 0.2 μm to 1 μm.20 Hence, it is natural that Li+ ions should move through the phase boundary of each primary particle as well as diffuse throughout the primary particle, the capacity of which, as well as the rate capability, decreases at low temperature. Also, the exposed surface area of conventional cathode materials to the electrolyte is large, which leads to increased reactivity at the interface and thus poor cycling performance and inferior safety of the batteries. Received: February 27, 2013 Revised: April 22, 2013

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cell tests were performed with a 2032 coin-type cell using lithium metal as the anode. Long-term cycle-life tests were performed in a laminated-type full cell (35 mAh) wrapped with an Al pouch. Mesocarbon microbead graphite (MCMB, Osaka Gas) was used as the anode. The electrolyte solution was 1.2 M LiPF6 in ethylene carbonate−ethyl methyl carbonate (3:7 in volume). The cells were charged and discharged at 25 and 55 °C and between 3.0 and 4.4 V by applying a constant 1 C current (35 mA corresponds to 180 mA g−1). Thermal Properties. For the differential scanning calorimetry (DSC) experiments, the FCG−Mn-F and CC electrodes were charged to 4.3 V versus Li and disassembled in an argon-filled drybox. A stainless steel sealed pan with a gold-plated copper seal (which can withstand 150 atm of pressure before rupturing and has a capacity of 30 μL) was used to collect 3−5 mg samples. The measurements were carried out in a Pyris 1 differential scanning calorimeter (Perkin− Elmer) using a temperature scan rate of 1 °C min−1. The weight was constant in all cases, indicating no leaks during the experiments. For the overcharge experiment, the Al pouch-type full cell (280 mAh) containing FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2 was continuously charged until the charge capacity reached 250% of rated capacity with a constant charge current of 0.2 C (equivalent to 37.5 mA g−1 = 60 mA). For the nail penetration test, the full cell (280 mAh) charged to 4.2 V was penetrated by a sharp stainless-steel nail at a constant speed of 1 mm s−1 controlled by a motor. The cell temperature and cell voltage were monitored during the test.

Here, we report a novel cathode material with full concentration gradients of Ni and Co ions and a fixed composition of Mn (FCG−Mn-F) extending throughout each secondary particle, which has long rod-shaped primary particles that span from the center to the surface. The constant Mn concentration across the particle provides outstanding cycle life and safety, and the linear decrease in Ni concentration toward the particle outer surface results in high capacity. At the same time, the gradual increase of the Co concentration from the particle center to the surface increases the electronic conductivity, yielding excellent rate capability. This unique FCG−Mn-F morphology, reported here for the first time, could lead to (i) a highly densified particles, which can lead to high volumetric energy density at the cell level, (ii) fast lithium diffusion across the particle, which improved the rate capability of the material, especially at low temperature, and (iii) high thermal stability, because of the limited surface reactivity with the electrolyte.

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EXPERIMENTAL SECTION

Synthesis of Conventional Cathode Li[Ni0.60Co0.15Mn0.25]O2. [Ni0.60Co0.15Mn0.25](OH)2 precursor was synthesized via coprecipitation of NiSO4·6H2O, CoSO4·7H2O, and MnSO4·5H2O (60:15:25 in molar ratio) as starting materials. Details of the preparation procedures are given in a previous report.21 The obtained [Ni0.60Co0.15Mn0.25](OH)2 hydroxide precursor was mixed with LiOH·H2O and calcined at 850 °C for 10 h in air. Synthesis of FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2. To prepare the FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2, a Ni-poor aqueous solution (Ni:Co:Mn = 0.50:0.25:0.25 in molar ratio) from tank 2 made of NiSO4·6H2O, CoSO4·7H2O, and MnSO4·5H2O was slowly pumped into a Ni-rich (Ni:Mn = 0.75:0.25 in molar ratio) stock solution in tank 1, after which the homogeneously mixed solution was fed into a continuously stirred tank reactor (CSTR). During the early stage of coprecipitation reaction, spherical [Ni0.75Mn0.25](OH)2 particles were formed first, and a transition-metal-containing hydroxide with different composition gradually accumulated on the surface of the spherical [Ni0.75Mn0.25](OH)2 particles via layer-by-layer assembly with a precipitation process. In this case, unlike in the previous report,16 we increased the concentration of the ammonium hydroxide (molar ratio of ammonium hydroxide to transition metal = 1.0) and sodium hydroxide amounts (molar ratio of sodium hydroxde to transition metal = 2.0) to favor the growth the primary particles. The obtained FCG−Mn-F [Ni0.60Co0.15Mn0.25](OH)2 was mixed with LiOH·H2O, and the mixture was calcined at 825 °C for 10 h in air. Analytical Techniques. The chemical compositions were determined by atomic absorption spectroscopy (AAS) (Vario 6, Analyticjena). The crystalline phase of the synthesized powders was identified by powder X-ray diffraction (XRD) (Rigaku, Model Rint2000) using Cu Kα radiation. The morphology of the prepared powders was observed by scanning electron microscopy (SEM) (JEOL, Model JSM-6340F). To obtain the localized composition of the FCG−Mn-F materials, cross sections of the particles were prepared by embedding the particles in an epoxy and grinding them flat. Line scans of the polished surfaces for the prepared FCG−Mn-F hydroxide and lithiated Li[Ni0.58Co0.17Mn0.25]O2 powders were analyzed via electron probe microanalysis (EPMA) (JEOL, Model JXA-8100). X-ray photoelectron spectroscopy (XPS) (Perkin−Elmer, Model PHI 5600) measurements were performed to investigate the electronic state of Ni, Co, and Mn for the concentration-gradient material. Transmission electron microscopy (TEM) samples were prepared by focused ion beam and examined in a JEOL, Model JEM 2100F instrument. Electrochemical Test. For fabrication of the cathodes, the synthesized powders were mixed with carbon black and poly(vinylidene fluoride) (85:7.5:7.5) in N-methylpyrrolidinone. The obtained slurry was coated onto Al foil and roll-pressed. Preliminary

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RESULTS AND DISCUSSION We synthesized the proposed novel FCG−Mn-F cathode material via layer-by-layer assembly with a precipitation process (see the Experimental Section). This FCG−Mn-F powder has a center composition of Li[Ni0.70Co0.05Mn0.25]O2 and a surface composition of Li[Ni0.58Co0.17Mn0.25]O2. The average composition was determined to be Li[Ni0.60Co0.15Mn0.25]O2 via atomic absorption spectroscopy (AAS). The average particle size of the synthesized FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2 powder was spherical, 12 μm in diameter (see Figure S1 in the Supporting Information), and the tap density was as high as 2.4 g cm−3. Figure 1a shows a cross-sectional scanning electron microscopy (SEM) image of a single lithiated FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2 oxide particle and the corresponding elemental distribution of Ni, Co, and Mn. The elemental mapping of Mn shows a uniform contrast throughout the entire particle, as expected. The nickel was depleted at the particle surface and became gradually enriched toward the particle center, whereas the opposite concentration gradient was observed for cobalt. The compositional change within a particle was quantitatively characterized using EPMA with a probe diameter of 100 nm. As can be seen from Figure 1b, the Mn concentration remained constant, in agreement with the mapping data. The nickel concentration decreased linearly from 70 at. % to 58 at. % toward the particle surface, whereas the Co concentration increased from 5 at. % to 17 at. %. The Mn concentration remained constant from the center to the outer surface of the particle. The concentration variation of the metals almost matched the initial designed compositions, as the interdiffusion of transition-metal ions within the particle during the hightemperature calcination seemingly had little effect on the final concentration gradient. In addition to the continuously varying concentration gradient of Ni and Co, unique intraparticle morphology was produced by the layer-by-layer assembly in the precipitation process. Figure 1c shows a TEM image of a single FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2 particle that had been sectioned with a focused ion beam. Near the center, the particle was composed of equiaxed primary particles, ∼200 nm in B

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aligned and provided extended fast channels for Li+ ion diffusion. The observed crystallographic texture should enhance the Li+ ion transport and, thus, the electrochemical kinetics. As for the mechanism of formation of this microstructure, it appears that the directional growth of the elongated primary particles was inherited from the (NiCoMn)(OH)2 precursor of the full concentration gradient (FCG) material, including FCG−Mn-F. To confirm this assumption, we synthesized FCG precursor varying Ni, Co, and Mn concentration. Figures 2a

Figure 2. Cross-sectional TEM images of the hydroxide precursor to the FCG cathode material: (a) outer region, demonstrating the continuously varying Co/Ni atomic ratio along the particle length in the precursor hydroxide particle determined by EDS, and (b) center region.

Figure 1. (a) Cross-sectional SEM and EPMA mapping of Ni, Co, and Mn within a single lithiated FCG−Mn-F particle; (b) integrated atomic ratio of transition metals as a function of the distance from the center of the particle for the lithiated FCG−Mn-F material; (c) crosssectional TEM image of a single FCG−Mn-F particle; (d) TEM image, along with EDS data along the single elongated FCG−Mn-F primary particle; and (e) TEM image and the corresponding electron diffraction pattern from a FCG−Mn-F primary particle, illustrating the crystallographic alignment of the primary particle in the radial direction.

and 2b show TEM images of the outer region ([Ni0.65Co0.10Mn0.25](OH)2) and the center ([Ni0.90Co0.05Mn0.05](OH)2) of the corresponding FCG hydroxide precursor (average composition of [Ni0.79Co0.08Mn0.13](OH)2) particle, respectively. The center region was composed of thin platelets of randomly oriented crystals, whereas the outer part consisted of long (∼2 μm in length) primary particles emanating from the center, as observed in the lithiated particle in Figures 1c and 1d. The intended concentration gradient within a primary particle was also confirmed by EDS, as shown by the EDS data in Figure 2a. Such a rod-shaped morphology is believed to have developed as a result of the coprecipitation of the metal hydroxide during which the transition-metal concentrations were continuously varied. Similar microstructures (radial growth and concurrent texturing) have been observed during the electrodeposition of zinc leaves at the butyl acetate/zinc surface solution interface.22 The rod-shaped particles were observed at high current densities and filmlike deposits at low current densities. It was conjectured that, at high current densities, local changes in the Zn ion concentration contributed to the formation of the rodshaped structure. Similarly, during the growth of calcium carbonate without additives, the supersaturation level dictated the particle shapes, as spherulitic growth was preferred at high supersaturation levels while faceted morphologies were observed at low supersaturation.23 Similar to the cited examples, in the case of the graded hydroxide growth, the artificially introduced concentration gradient in the solution maintained a high supersaturation level, allowing the primary particle to grow in a pseudo-dendritic manner. The concentration gradient in the solution was, in fact, prerequisite for the formation of the rod-shaped textured microstructure. With identical experimental setup, the particles synthesized at a fixed concentration of the metal sulfate solution exhibited vastly

diameter, from which elongated primary particles whose length reached up to 2.5 μm (∼100 nm in width). These long rodshaped primary particles emanated radially from the center (see Figure S2 in the Supporting Information for detailed TEM images). The compositional variation of a single elongated primary particle was estimated by energy-dispersive X-ray spectroscopy (EDS); the result is shown superimposed on the TEM image in Figure 1d. The EDS data indicate that the Co/ Ni atomic ratio decreased from 0.291 to 0.246 along the 0.9-μm length, as determined by partial probing of the primary particle toward the particle core. The concentration gradient observed along the 0.9-μm-long primary particle generally agreed with the EPMA data in Figure 1b. It is remarkable that the concentration gradient was established within a single primary particle, which is a unique structural feature of the FCG−Mn-F particles produced by our layer-by-layer precipitation process. We speculate that the long diffusion length provided by the large (micrometer-sized) elongated primary particles radiating from the core limit the intermixing of the constituent atoms and enable the concentration gradient to remain stable during the calcination. Another unique microstructural feature of the FCG−Mn-F particle is the crystallographic texture developed in the primary particles. All of the observed primary particles have their c-axis aligned in the transverse direction (i.e., normal to the radial direction), as shown in Figure 1e. The directional growth of the primary particles allowed the (003) crystallographic layers in individual primary particles to be centrally C

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differently morphology. When the concentration was fixed, instead of the rod-shaped morphology, large equiaxed primary particles with no apparent crystallographic texture were consistently produced. An example of fixed composition of Li[Ni0.5Co0.2Mn0.3]O2 shown in (Figures 3a and 3b) contains faceted large primary

Figure 3. Cross-sectional TEM images of the primary particles: (a, b) fixed composition of Li[Ni0.5Co0.2Mn0.3]O2 without the compositional gradient, and (c, d) FCG cathode having the core composition of Li[Ni0.86Co0.07Mn0.07]O2 continuously varied to Li[Ni0.67Co0.09Mn0.24] O2 at the surface.

Figure 4. X-ray photoelectron spectroscopy (XPS) data for the lithiated FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2: (a) Ni 2p, (b) Co 2p, and (c) Mn 2p.

particles with no directional alignment. Figures 3c and 3d present TEM images of the primary particles from lithiated FCG material that was designed so that the center composition of Li[Ni0.86Co0.07Mn0.07]O2 was continuously varied to Li[Ni0.67Co0.09Mn0.24]O2 at the surface. Similar to Figure 1, radially aligned primary particles emanating out from the center region can be clearly discerned from Figures 3c and 3d. Hence, the introduction of a forced concentration gradient in the solution during the particle growth not only produces a functionally graded material in terms of electrochemical properties, but also a directionally aligned microstructure that is optimized for Li+ diffusion. Powder XRD patterns of the FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2 and conventional composition (CC) Li[Ni0.60Co0.15Mn0.25]O2 showed that both powders had a welldefined layer structure based on a hexagonal α-NaFeO2 structure with a R3̅m space group and no impurity phases (see Figure S3 in the Supporting Information). In addition to the microstructural analysis, the oxidation state of each transition metal in the synthesized FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2 particles was investigated by X-ray photoelectron spectroscopy (XPS), as shown in Figure 4. The oxidation state of the Ni from the surface to 2 μm toward the center was found to be slightly higher than 2+ (Figure 4a). The Mn and Co spectra matched well with Mn4+ and Co3+, respectively (see Figures 4b and 4c). The oxidation state of

each transition metal in the particle center (Li[Ni 0.7 Co 0.05 Mn0.25 ]O2 ) is expected to be preferentially approaching the trivalent state.8 We believe that the linear variation of the Ni and Co concentrations led to the gradual change in their respective oxidation states from the particle surface to the center. We characterized battery performance by voltage profile and cycling performance in cell tests with cathodes composed of FCG−Mn-F and CC Li[Ni0.60Co0.15Mn0.25]O2 materials. As seen in Figure 5a, the FCG−Mn-F material delivered a high discharge capacity, 206 mAh g−1, whereas the CC material showed a reduced capacity of 194 mAh g−1. Even though the FCG−Mn-F and CC Li[Ni0.60Co0.15Mn0.25]O2 materials have the same average composition, the resulting higher Ni2+ concentration of the FCG−Mn-F for the outer layer delivers higher capacity, because of the two-electron reaction of Ni (Ni2+ → Ni4+), shown in Figure 4a. To further appraise the FCG−Mn-F performance, the cells were tested under a severe condition by cycling to 4.5 V at 55 °C under the 0.5 C rate (114 mA g−1). As shown in Figure 5b, the capacity retention of FCG−Mn-F was much improved over the CC cathode, with a capacity retention of 91% while still maintaining a discharge capacity of 185 mAh g−1 after 100 cycles at 55 °C. By contrast, the CC cathode exhibited a rapid decrease of capacity, with a retention of only 83% over the same cycling period and temperature. D

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corresponds to 200 mAh g−1). As shown in Figure 5c, the cell showed good capacity retention (70.3%) after 1000 cycles, even at 55 °C. Also, the pouch-type full cells cycled to 4.2 V at the 1C rate exhibited excellent cycling performance with capacity retention of 92.5% and 78.5% at 25 and 55 °C, respectively (see Figure S5 in the Supporting Information). We believe that the enhanced cycling performance of FCG−Mn-F material is mainly due to the uniform and stable Mn4+ distribution and the reduced fraction of active and unstable Ni4+ ions on the particle surface after charging, in agreement with our previous report of the increased Mn and reduced Ni concentration in the particle surface of Li[Ni0.46Co0.23Mn0.31]O2.15,16 We checked the interdiffusion of transition-metal ions within the particle after 1000 cycles at both 25 and 55 °C. Long-term cycling did not give rise to any significant compositional change at the particle level of the concentration gradient, compared to pristine material (see Figure 1b) before cycling (see Figure S6 in the Supporting Information). A potential advantage of the rod-shaped morphology with (003) crystallographic texture is that it provides a smooth and quick pathway for Li+ ion diffusion under severe cycling conditions at ambient temperature and lower. Figure 6a shows

Figure 5. Electrochemical performance of CC and FCG−Mn-F Li[Ni0.60Co0.15Mn0.25]O2 cathodes (a) initial charge−discharge curves at 25 °C obtained from a 2032 coin-type half cell using lithium metal as the anode (a current density rate of 0.1 C corresponds to 22.2 mA g−1), (b) cycling performance of half cells between 2.7 V and 4.5 V at 55 °C by applying a constant current rate of 0.5 C (114 mA g−1), and (c) cycling performance of laminated-type Al-pouch cell (35 mAh) using mesocarbon microbead (MCMB) graphite as the anode and FCG−Mn-F as the cathode at a rate of 1 C, corresponding to 200 mAg−1 (upper cutoff voltage of 4.4 V).

Figure 6. Electrochemical performances of CC and FCG-Mn-F Li[Ni0.60Co0.15Mn0.25]O2 cathodes from a 2032 coin-type half cell between 2.7 V and 4.3 V (a) rate capabilities and (b) low temperature performance. In rate capability test, each cell was charged at a rate of 0.2 C before each discharge test.

the rate capability of the FCG−Mn-F and CC cathodes. Each cell was discharged galvanostatically at different C rates, ranging from 0.2 C to 5 C (40−1000 mA g−1). As anticipated, the FCG−Mn-F cathode showed higher discharge capacity than that of the CC cathode at all tested rates. The capacity difference between FCG−Mn-F and CC cathodes increased with the C rate. At a rate of 5 C, the FCG−Mn-F cathode delivered a discharge capacity of 155 mAh g−1, while that of the CC cathode was 136 mAh g−1. The low-temperature performance further highlights the benefit of the unique rodshaped morphology of the primary particle, as shown in Figure 6b. The FCG−Mn-F cathode delivered higher capacity retention than the CC cathode at temperatures from 0 °C to −20 °C. At −20 °C, the FCG−Mn-F cathode showed high capacity of 120 mAh g−1, which is much greater than 101 mAh g−1 and 81 mAh g−1 for the CC cathode and commercialized Li[Ni1/3Co1/3Mn1/3]O2, respectively (see Figure S7 in the Supporting Information).

The specific surface area of the FCG−Mn-F materials is 0.65 m2 g−1 (pore volume of 2.06 × 10−3 g cm−1), whereas that of the CC materials is 1.87 m2 g−1 (pore volume of 5.87 × 10−3 g cm−1). The lower specific surface area of the FCG−Mn-F materials leads to a decrease in the exposed contact area with electrolyte at the interface, thus enhancing the cycle life.24 Further study on the variation of upper cutoff voltage (4.3 and 4.6 V) at 25 and 55 °C strongly supported the improvement of the FCG−Mn-F cathode, in terms of capacity and cycle life, compared to the CC cathode (see Figure S4 in the Supporting Information). To investigate the long-term cycle life, an aluminum pouchtype full cell (35 mAh) with FCG−Mn-F material as the cathode and mesocarbon microbead (MCMB) graphite as the anode was fabricated and cycled between 3.0 V and 4.4 V at 25 and 55 °C with a specific current of 195 mA g−1. (Note: 1 C E

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To study the contribution of the electronic conductivity and chemical Li+ diffusivity on the electrochemical performances (rate capability and low temperature performance), we measured the electronic conductivities by a four-probe method and the chemical Li+ diffusivities by the galvanostatic intermittent titration technique for the FCG−Mn-F and CC materials.18 The electronic conductivity for the FCG−Mn-F material (6.3 × 10−5 S cm−1) is one order of magnitude higher than that of CC material (8.1 × 10−6 S cm−1). We believe that the increase in the electronic conductivity of the FCG−Mn-F material is due to the Ni2+/Ni3+ mixed valence effect caused by higher Ni2+ concentration on the particle surface and the rodshaped primary particle morphology. As can be seen in (Figure 7), the Li+ diffusivity for FCG−Mn-F material is ∼10 times

Figure 7. Chemical diffusion coefficient, DLi+, of FCG−Mn-F and CC Li[Ni0.60Co0.15Mn0.25]O2, as a function of the state of charge.

higher (10−11 cm2 S−1) than that for CC material (10−12 cm2 S−1). From the above results, we believe that the enhanced electrochemical performance of the FCG−Mn-F material is strongly correlated with both the rod-shaped microstructure and the oxidation state of Ni ions. Figure 8a shows the differential scanning calorimetry profiles of the FCG−Mn-F and CC electrodes charged to 4.3 V in the presence of electrolyte. The CC electrodes exhibited large exothermic peaks at 266 °C. In contrast, the exothermic reaction temperature for the FCG−Mn-F electrode was much higher (280 °C) with reduced heat generation. Also, the onset temperature was delayed by 14 °C. Figures 8b and 8c show the variation of cell voltage and temperature with time after overcharge and nail penetration tests, respectively. After overcharge (at 250% state of charge), the C/FCG−Mn-F cell showed a voltage increase to only 5.5 V, with the temperature remaining below 20 °C. The highest temperature of the cell after nail penetration was 70 °C. Photographs of the C/FCG− Mn-F cell after overcharge and nail penetration testing are shown as insets in Figures 8b and 8c, respectively. The cell showed no thermal runway or black smoke, and it did not explode.

Figure 8. (a) Differential scanning calorimetry (DSC) traces, showing heat flow from the reaction of the electrolyte with FCG−Mn-F and CC Li1−δ[Ni0.60Co0.15Mn0.25]O2 electrodes charged to 4.3 V. (b, c) Voltage and cell temperature plots with time for laminated-type Alpouch MCMB/FCG−Mn-F cell (280 mAh), as a function of time during (b) overcharge test charged to either 250% state of charge or 12 V and (c) nail penetration test at 100% state of charge (4.2 V). Insets show images of C/FCG−Mn-F cells after overcharge test and nail penetration test.

improved thermal stability compared to CC Li[Ni0.60Co0.15Mn0.25]O2. In cell tests, a FCG−Mn-F cathode delivered a high capacity of 206 mAh g−1 and retained 70.3% of the capacity under an aggressive test condition (55 °C between 3.0 V and 4.4 V). The excellent electrochemical and safety performance of FCG−Mn-F suggests that the concentration gradient approach will be very helpful in the development of advanced cathode materials with high capacity, long calendar life, and outstanding safety for lithium-ion batteries.

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CONCLUSION A novel cathode material composed of Li[Ni0.60Co0.15Mn0.25]O2 with full concentration gradient of Ni and Co ions at fixed Mn content (FCG−Mn-F) throughout each particle was successfully synthesized via layer-by-layer assembly with a precipitation process. The microsized FCG−Mn-F secondary particles are composed of rod-shaped primary particles grown in radial directions with crystallographic texture, resulting in a high rate capability, increased low temperature performance, and

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ASSOCIATED CONTENT

S Supporting Information *

Detailed characterization methods. This material is available free of charge via the Internet at http://pubs.acs.org. F

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(19) Myung, S.-T.; Ogata, A.; Lee, K.-S.; Komaba, S.; Sun, Y.-K.; Yashiro, H. J. Electrochem. Soc. 2008, 155, A374. (20) Cheralathan, K. K.; Kang, N. Y.; Park, H. S.; Lee, Y. J.; Choi, W. C.; Ko, Y. S.; Park, Y.-K. J. Power Sources 2010, 195, 1486. (21) Lee, M.-H.; Kang, Y.-J.; Myung, S.-T.; Sun, Y.-K. Electrochim. Acta 2004, 50, 939. (22) Tamamushi, R.; Kaneko, H. Electrochim. Acta 1980, 25, 391. (23) Andreassen, J.-P.; Beck, R.; Nergaard, M. Faraday Discuss. 2012, 159, 247. (24) Xia, Y.; Kumada, N.; Yoshio, M. J. Power Sources 2000, 90, 135.

AUTHOR INFORMATION

Corresponding Author

*E-mail addresses: [email protected] (Y.-K.S.), amine@anl. gov (K.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Human Resources Development program (No. 20124010203310) of the Korea Institute of Energy Technology Evaluation of Planning (KETEP) grant, funded by the Korea Government Ministry of Trade, Industry and Energy and the National Research Foundation of Korea (NRF) grant, funded by the Korea Government (MEST) (No. 2009-0092780). J. Lu was supported by the Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award under the EERE Vehicles Technology Program administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE contract number DE-AC05-06OR23100. Research at Argonne National Laboratory was funded by U.S. Department of Energy, FreedomCAR and Vehicle Technologies Office. Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC, under Contract No. DEAC02-06CH11357.

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dx.doi.org/10.1021/cm4006772 | Chem. Mater. XXXX, XXX, XXX−XXX