Optimization of Layered Cathode Material with Full Concentration

Article pubs.acs.org/JPCC

Optimization of Layered Cathode Material with Full Concentration Gradient for Lithium-Ion Batteries Jin-Wook Ju,† Eung-Ju Lee,† Chong S. Yoon,‡ Seung-Taek Myung,§ and Yang-Kook Sun*,†,∥ †

Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea Department of Materials Science and Engineering, Hanyang University, Seoul 133-791, South Korea § Department and Institute of Nano Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul 143-747, South Korea ∥ Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, 21589, Saudi Arabia ‡

ABSTRACT: Li[NixCoyMn1−x−y]O2 cathode materials were synthesized with varying concentration gradients of Ni and Co ions from the particle center (0.62−0.74 mol % for Ni and 0.05 mol % for Co) to the surface (0.48−0.62 mol % for Ni and 0.18 mol % for Co), i.e., full concentration gradient (FCG) with fixed Mn concentrations. In particular, the Mn concentration (20, 25, and 33 mol %) was controlled to optimize electrode performance. The average chemical compositions of lithiated products were Li[NixCo0.16Mn0.84−x]O2 (x = 0.64, 0.59, 0.51). These cathode materials with concentration gradients followed the general performance trend of conventional layered materials; an increase in Ni content improved the capacity, whereas a higher amount of Mn delivered better capacity retention and thermal properties at the expense of capacity. As a result, we determined an optimal level of Mn concentration among the tested FCG cathodes, which maximized the discharge capacity of 188 mAh g−1 and had an excellent capacity retention of 96% over 100 cycles operated up to 4.3 V at 25 °C, with a composition of FCG Li[Ni0.59Co0.16Mn0.25]O2.

1. INTRODUCTION Rechargeable lithium batteries are the most widely used power source for portable devices. Because of the lightweight and high energy density of lithium batteries, reduction in size has been made possible, which has led to miniaturization of batteries and subsequently hand-held devices. At present, lithium batteries are being developed as power sources for plug-in hybrid electric and electric vehicles, and for these applications, high energy cathodes are necessary to reduce battery size because the mounting space is significantly limited.1−3 Hence, layered lithium nickel-cobalt-manganese oxide, Li[NixCoyMnz]O2 (x + y + z = 1), which delivers a high discharge capacity of over 170 mAh g−1, has been intensively studied for high energy density lithium-ion batteries.4−6 However, these cathodes exhibit some serious problems, such as capacity fade and poor thermal stability, which hinder their use in vehicle applications. In particular, oxygen evolution from the delithiated cathode can cause serious safety concerns. These limitations necessitate the development of new materials to resolve the above-mentioned difficulties. Recently, we introduced unique materials, which have a core−shell (core with gradient shell) or full concentration gradient (FCG) structure in a particle level.7−9 The structure is basically composed of a Ni-rich core that delivers high capacity and a Mn-rich shell that provides outstanding thermal stability. Our latest report included a new type of FCG material composed of long rod-shaped primary particles approximately 2.5 μm in length,10 which demonstrated good electrochemical performance and excellent thermal properties due to lower grain boundary resistance ascribed to lower specific surface area © 2013 American Chemical Society

contacting with the electrolyte during cycling. However, the Mn concentration could not be varied because of the difficulty of the rod-shaped particle synthesis. Here, we report the design of FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.64, 0.59, and 0.51) cathode materials with fixed Mn contents of 20, 25, and 33% in the transition-metal layer. We also report the effects of Mn concentration in terms of structural, electrochemical, and thermal characteristics of the FCG materials.

2. EXPERIMENTAL SECTION Spherical FCG [NixCo0.16Mn0.84−x](OH)2 (x = 0.51, 0.59, and 0.64) precursors were synthesized by a coprecipitation method.11 A Ni-poor aqueous solution (molar ratio of Ni:Co:Mn = 0.47:0.20:0.33, 0.55:0.20:0.25, 0.60:0.20:0.20,), consisting of NiSO4·6H2O, CoSO4·7H2O, and MnSO4·5H2O as starting materials, was slowly pumped from tank 2 into a Nirich (molar ratio of Ni:Mn = 0.67:0.33, 0.75:0.25, 0.80:0.20) stock solution in tank 1. The homogeneously mixed solution was then fed into a continuously stirred tank reactor (CSTR, 4 L) in a replenished N2 atmosphere. Simultaneously, a 3.0 mol L−1 NaOH solution (aq.) and the desired amount of a NH4OH chelating agent solution (aq.) were pumped separately into the reactor. The concentration of the solution, pH, temperature, and stirring speed of the mixture in the reactor were carefully controlled. During the early stage of the coprecipitation Received: October 2, 2013 Revised: November 15, 2013 Published: November 20, 2013 175

dx.doi.org/10.1021/jp4097887 | J. Phys. Chem. C 2014, 118, 175−182

The Journal of Physical Chemistry C

Article

obtained data were refined using the Rietveld program, FULLPROF.12 Cells consisting of a cathode and lithium metal anode separated by a porous polypropylene film were fabricated. Positive electrodes were prepared by blending Li[NixCo0.16Mn0.84−x]O2 active materials, Super P carbon black, and polyvinylidene fluoride (85:7.5:7.5). The resulting slurry was cast onto aluminum foil and dried at 110 °C for 10 h in a vacuum oven. Electrochemical characterization was conducted using a 2032 coin-type cell with 1 M LiPF6 solution in an ethylene carbonate (EC)−ethylmethyl carbonate (EMC) mixture (3:7 volume ratio, PANAX ETEC Co. Ltd., Korea). Cycling tests were performed at a constant current with an upper cutoff voltage of 4.3 V at 25 and 55 °C. The dc electrical conductivity was measured by the direct volt-ampere method (CMT-SR1000, AIT Co.), in which disk samples were contacted with a four-point probe. The chemical Li+ diffusivity was measured by the galvanostatic intermittent titration technique (GITT) (Table 1). The GITT measurements consisted of a step of constant current applied to the cointype half-cell at the charge process, each followed by a 50 min relaxation period for new open-circuit cell potential.13 To calculate the diffusion coefficient (Dk) for the each FCG materials using equation (1), we obtained the values of the electrode potential with Li-ion concentration (dε/dδ) and the electrode potential with time (dε/dt1/2) during the constant current of 0.1-C rate applied.

Table 1. Symbol Description for GITT Analysis symbol

symbol description

Dk z F A Vm I0 ε δ t L

chemical Li+ diffusivity valence of species Faraday’s constant electrode area unit cell volume applied constant electric current galvanic cell potential ideal stoichiometry time diffusion length

process, Ni-rich hydroxide (center composition) first precipitated. Nickel-cobalt-manganese hydroxide of different compositions then gradually layered onto the initially formed [NixCo0.16Mn0.84−x](OH)2 (x = 0.51, 0.59, and 0.64) particles, resulting in a linear composition change of Ni and Co toward the outer surface of the particles. Precursor powders were obtained through filtering, washing, and drying at 100 °C overnight. The obtained FCG [NixCo0.16Mn0.84−x](OH)2 (x = 0.51, 0.59, and 0.64) was mixed with LiOH·H2O, and the mixture was calcined at various temperatures for 10 h in air: 780 °C for Li[Ni0.64Co0.16Mn0.20]O2, 845 °C for Li[Ni0.59Co0.16Mn0.25]O2, and 920 °C for Li[Ni0.51Co0.16Mn0.33]O2. To obtain the localized composition of the FCG 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 of the prepared FCG hydroxide and lithiated FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64) powders were analyzed by an electron probe microanalyzer (EPMA) (JXA-8100, JEOL).The chemical compositions were determined via atomic absorption spectroscopy (AAS) (Vario 6, Analyticjena). The morphologies of the prepared powders were observed using scanning electron microscopy (SEM) (JSM-6340F, JEOL). Powder X-ray diffraction (XRD) (Rint-2000, Rigaku) using Cu Kα radiation was used to identify the crystalline phases of the prepared powders. The XRD data were obtained in the 2θ range of 10−110° with a step size of 0.03° (2θ), and the

dε 2⎛ π⎛ V ⎞ Dk = ⎜ m ⎟ ⎜I 0 ddδε 4 ⎝ zFA ⎠ ⎜ ⎝ d t

⎞2 2 ⎟ for t ≪ L ⎟ Dk ⎠

(1)

The chemical delithiation procedure consisted of mixing a stoichiometric amount of nitronium tetrafluoroborate as a reducing agent and the as-prepared powder in 200 mL of acetonitrile. The solution was stirred for 2 days, then filtered, and washed several times with acetonitrile. The resulting powder was dried at 30 °C under vacuum for 8 h. Thermal gravimetric analysis (TGA) (TG 209 F3, NETZSCH) of the chemically delithiated powder was carried out under nitrogen at a scan rate of 5 °C min−1 in the temperature range of 50−600

Figure 1. EPMA line scan of the integrated atomic ratio of transition metals as a function of the distance from the center to the surface for FCG [NixCo0.16Mn0.84−x](OH)2, (a) x = 0.64, (b) x = 0.59, and (c) x = 0.51, and for FCG Li[NixCo0.16Mn0.84−x]O2, (d) x = 0.64, (e) x = 0.59, and (f) x = 0.51. 176



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Figure 2. SEM images of as-prepared FCG Li[NixCo0.16Mn0.84−x]O2 powders for (a) x = 0.64, (b) x = 0.59, and (c) x = 0.51 (scale bar = 10 μm).

°C. For differential scanning calorimetry (DSC) analysis, the electrodes were fully charged to 4.3 V and opened in an Arfilled dry room. After the electrolyte was carefully removed from the surface of the electrode, the cathode material was 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 using a DSC (200 PC, NETZSCH) at a temperature scan rate of 1 °C min−1.

Table 2. Lattice Parameters Obtained from Rietveld Refinements of the Powder XRD Pattern of FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.64, 0.59, and 0.51) Li[Ni0.64Co0.16Mn0.20]O2 atom

site

Li1 3a Ni2 3a Ni1 3b Co 3b Mn 3b Li2 3b O 6c crystal system space group a axis/Å c axis/Å Rwp/% Rp/%

3. RESULTS AND DISCUSSION Full concentration gradient (FCG) is a unique concept, which correlates with the continuous, gradual change in composition

x

y

0 0 0 0 0 0 0 0 0 0 0 0 0 0 rhombohedral R3̅m 2.8662(1) 14.2131(14) 10.8 10.1

z

g

B/Å2

1/2 1/2 0 0 0 0 0.258(2)

0.974 0.026(2) 0.614(1) 0.16 0.20 0.026(2) 1

0.8 0.8 0.6 0.6 0.6 0.6 0.8

Li[Ni0.59Co0.16Mn0.25]O2 atom

site


Figure 3. Rietveld refinements of XRD data of (a) FCG Li[Ni0.64Co0.16Mn0.20]O2, (b) FCG Li[Ni0.59Co0.16Mn0.25]O2, and (c) FCG Li[Ni0.51Co0.16Mn0.33]O2.

x

y

0 0 0 0 0 0 0 0 0 0 0 0 0 0 rhombohedral R3m ̅ 2.8675(2) 14.2206(10) 10.3 9.72

z

g

B/Å2

1/2 1/2 0 0 0 0 0.258(1)

0.969 0.031(1) 0.559(1) 0.16 0.25 0.031(1) 1

0.8 0.8 0.6 0.6 0.6 0.6 0.8

Li[Ni0.51Co0.16Mn0.33]O2

of transition metals from the particle center to its surface. This structural characteristic is available only for micrometer-sized spherical particles. Our optimized coprecipitation synthetic conditions enable such spherical particles to be prepared.8,9 To confirm that the distribution of each transition metal was realized as designed, the precursor and lithiated materials were analyzed by EPMA with a probe diameter of 100 nm. As shown in Figure 1, the Mn content across a single precursor particle was constant at 20, 25, or 33 mol % throughout the entire precursor particle regardless of the sample design. The measured Ni and Co concentration profiles were also reproducible and exhibited the intended concentration variation. For the sample with Mn fixed at 20 mol %, the composition at the center, Ni0.80Mn0.20(OH)2, gradually changed with a linear gradient to Ni0.60Co0.20Mn0.20(OH)2 at the particle surface (Figure 1a). The same linear concentration gradient was observed for precursor samples with Mn fixed at 25 and 33 mol % (Figure 1b,c). The composition gradually

atom

site


x

y

0 0 0 0 0 0 0 0 0 0 0 0 0 0 rhombohedral R3̅m 2.8705(2) 14.2367(11) 10.9 10.3

z

g

B/Å2

1/2 1/2 0 0 0 0 0.258(1)

0.959 0.041(1) 0.469(1) 0.16 0.33 0.041(1) 1

0.8 0.8 0.6 0.6 0.6 0.6 0.8

varied from Ni 0 . 7 5 Mn 0 . 2 5 (OH) 2 at the center to Ni0.55Co0.20Mn0.25(OH)2 at the surface for 25 mol % Mn and 177



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Figure 4. Initial charge−discharge curves of the FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64) (a) between 2.7 and 4.3 V at 25 °C obtained from a 2032 coin-type half-cell using Li metal as the anode (current density of 0.1 C-rate corresponds to 20.0 mA g−1), and (c) between 2.7 and 4.3 V at 55 °C (current density of 0.1 C-rate corresponds to 20.6 mA g−1). Corresponding cycling performance of half-cells (b) between 2.7 and 4.3 V at 25 °C by applying a constant current of 0.5 C-rate (100.0 mA g−1), and (d) between 2.7 and 4.3 V at 55 °C by applying a constant current of 0.5-C rate (102.8 mA g−1).

Mn0.20]O2, Li[Ni0.59Co0.16Mn0.25]O2, and Li[Ni0.51Co0.16Mn0.33]O2 powders. These products exhibited a typical layered structure with the R3̅m space group. Differences between the observed and calculated XRD patterns were negligible. Because of the gradual change in the chemical composition of the FCG particles, it was not possible to refine the crystal structure from the XRD patterns. Since the shapes and intensities of the XRD patterns were very similar for the samples, Rietveld refinement of the XRD patterns was performed based on the average chemical compositions of Li[Ni 0.64 Co 0.16 Mn 0.20 ]O 2 , Li[Ni0.59Co0.16Mn0.25]O2, and Li[Ni0.51Co0.16Mn0.33]O2. Because of the large amount of Mn in the shell, we speculate that the oxidation state of Mn was close to tetravalent, and the oxidation state of Ni was close to Ni2+, presumably slightly higher than 2+ based on our previous results for Li[Ni0.50Co0.25Mn0.25]O2 and Li[Ni0.6Co0.2Mn0.2]O2.15 We considered two possibilities for cation mixing: (i) Li layer occupation of Ni2+ from the transition-metal layer and (ii) site exchange of Li+ and Ni2+. The latter case consistently resulted in a smaller intensity difference between the observed and calculated patterns and better reliability factors (Rwp and Rp values). It is notable that divalent Ni occupation of the Li layer increased as the added amount of Mn increased: 2.6% for Li[Ni0.64Co0.16Mn0.20]O2, 3.1% for Li[Ni0.59Co0.16Mn0.25]O2, and 4.1% for Li[Ni0.51Co0.16Mn0.33]O2 (Table 2). These trends are also found in conventional Ni-rich layered materials.16 The lattice parameters were also affected by the concentration of incorporated Mn in the FCG materials. According to our prior reports,15,16 the oxidation state of Ni was dependent on the amount of added Mn; i.e., the oxidation state of Ni became lower as the amount of Mn increased. On the basis of this fact, it is understandable that a larger amount of divalent Ni formed in Li[Ni0.51Co0.16Mn0.33]O2 relative to Li[Ni0.64Co0.16Mn0.20]O2. This fact reflects

from Ni0.67Mn0.33(OH)2 to Ni0.47Co0.20Mn0.33(OH)2 for 33 mol % Mn. It is notable that the EPMA result exactly matched the predicted compositions. The slope of the concentration gradient of Ni in the hydroxide precursors was 0.033 mol μm−1 for all three FCG precursor materials. However, the higher lithiation temperature (>780 °C) caused a slight interdiffusion of Ni and Co for the lithiated oxide during calcination, as confirmed by the somewhat flattened gradient (Figure 1d−f), compared to those of the hydroxides (Figure 1a−c). Consequently, the respective slopes of the Ni concentration gradient dropped to 0.018, 0.022, and 0.024 mol μm−1 for the lithiated FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.64, 0.59, and 0.51). As a result, the Ni content at the center in the lithiated oxide decreased to 74, 70, and 62 mol %, while the surface Ni content increased to 62, 57, and 48 mol % from the respective initial precursor compositions. For a similar reason, the Co concentration at the center increased from 0 to 5−6 mol %, whereas the Co concentration at the surface decreased by 1−2 mol %, resulting in surface compositions of Li[Ni0.62Co0.18Mn0.20]O2, Li[Ni0.57Co0.18Mn0.25]O2, and Li[Ni0.48Co0.19Mn0.33]O2. The final average composition of the FCG materials was found to be Li[Ni0.64Co0.16Mn0.20]O2, Li[Ni0.59Co0.16Mn0.25]O2, and Li[Ni0.51Co0.16Mn0.33]O2, as confirmed by AAS. SEM images of the as-synthesized FCG powders are shown in Figure 2. The average diameter of the secondary particles was approximately 12 μm for all three samples, and these particles were composed of micrometer-sized (1−3 μm) elongated primary particles. The primary particles appeared to become larger with increasing Mn content, which is in agreement with previous results.14 Figure 3 compares the Rietveld refinement results for the XRD patterns of the as-synthesized FCG Li[Ni0.64Co0.16178



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Table 3. Electronic Conductivities of the FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.64, 0.59, and 0.51) electronic conductivity (S cm−1) Li[Ni0.64Co0.16Mn0.20]O2 Li[Ni0.59Co0.16Mn0.25]O2 Li[Ni0.51Co0.16Mn0.33]O2

1.07 × 10−4 5.84 × 10−5 1.07 × 10−5

materials follow the general trends observed for conventional Ni-rich layered materials. Figure 4a shows the first charge/discharge curves of FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64) cells cycled between 2.7 and 4.3 V at a constant current density of 0.1 Crate (20 mA g−1) at 25 °C. The initial discharge capacity of the FCG Li[Ni0.64Co0.16Mn0.20]O2 cell delivered the highest capacity of the three electrodes, 193.5 mAh g−1, whereas the capacities of the Li[Ni0.59Co0.16Mn0.25]O2 and Li[Ni0.51Co0.16Mn0.33]O2 cells were 188.2 and 180.3 mAh g−1, respectively. Figure 4b shows the capacity retention of the FCG electrodes cycled between 2.7 and 4.3 V at 25 °C with a constant current density of 100 mA g−1 (0.5 C-rate). Although the FCG Li[Ni0.64Co0.16Mn0.20]O2 delivered the highest discharge capacity, its retention was limited to 94.1% after 100 cycles, whereas Li[Ni0.51Co0.16Mn0.33]O2, which had the lowest initial discharge capacity, had excellent retention, maintaining 96.5% of the initial capacity after 100 cycles. It appears that the discharge capacity more or less increased with the average Ni concentration, while at the same time, the stability of the electrode material during extended cycling deteriorated, which holds true for all Li[NixCoyMn1−x−y]O2 cathodes.14 Nevertheless, in terms of discharge capacity and cycle retention, the FCG materials tested here outperformed cathodes of similar composition, but without a concentration gradient.14−16 To judge the relative merits of the FCG Li[NixCo0.16Mn0.84−x]O2 cathodes, coin-type half-cells were tested at 55 °C and C-rates of 0.1 and 0.5 (20.6 and 102.8 mA g−1) with a cutoff voltage of 4.3 V, and the results are presented in Figure 4c,d. The trend observed at 25 °C was similarly found for cycling at the elevated temperature. The Li[Ni0.64Co0.16Mn0.20]O2 cell again delivered the highest initial capacity of 201.7 mAh g−1 and exhibited the worst capacity retention, whereas the Li[Ni0.51Co0.16Mn0.33]O2 cathode with the lowest discharge capacity of 184.8 mAh g−1 best retained its initial capacity after 100 cycles, as seen in Figure 4d. It is generally known that a high Mn content in Li[NixCoyMn1−x−y]O2 is detrimental to the rate capability of the Li-ion battery.16,17 To assess the Mn effect on the rate capability of the FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64) cathodes, the respective cells were charged and then discharged at different C-rates ranging from 0.2 to 5 C (40−1000 mA g−1). As seen in Figure 5, the rate capability deteriorated faster with electrodes containing an increasingly higher Mn content, in agreement with previous results of Li[NixCoyMn1−x−y]O2 cathodes.16−18 The capacity retention of Li[Ni0.64Co0.16Mn0.20]O2 and Li[Ni0.59Co0.16Mn0.25]O2 at 5 C compared to that at 0.2 C was 73.0% and 69.2%, respectively, whereas Li[Ni0.51Co0.16Mn0.33]O2 exhibited a considerably lower retention of 58.3%. Judging solely from the discharge capacity and cycle retention, the FCG Li[Ni0.59Co0.16Mn0.25]O2 would be the optimal composition with its relatively high discharge capacity and excellent capacity retention both at 25 and 55 °C up to 90%. In addition, the rate capability of the FCG Li[Ni0.59Co0.16Mn0.25]-

Figure 5. Discharge curves of the Li/FCG Li[NixCo0.16Mn0.84−x]O2 cells with (a) x = 0.64, (b) x = 0.59, and (c) x = 0.51 as a function of the C-rates.

Figure 6. Variation of the chemical diffusion coefficient, DLi+, of FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64) as a function of the state of charge.

the gradual increase in lattice parameters with increasing Mn content. Furthermore, this phenomenon would promote site exchange due to the similarity of the ionic radii between Li+ (coordination number: 6, 0.76 Å) and Ni2+ (coordination number: 6, 0.69 Å). These results indicate that even FCG 179



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Figure 7. Before and after 100 cycled electrodes XRD patterns of (a) FCG Li[Ni0.64Co0.16Mn0.20]O2, (b) FCG Li[Ni0.59Co0.16Mn0.25]O2, and (c) FCG Li[Ni0.51Co0.16Mn0.33]O2. (d) Variation of the lattice parameters (Δa and Δc) and volume of the FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64).

Figure 8. Thermal stability test of FCG Li1−δ[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64): (a) TGA curves of the chemically delithiated FCG, (b) DSC trace results of the charged state FCG.

magnitude higher than that of Li[Ni0.51Co0.16Mn0.33]O2 (2.95 × 10−10 cm2 S−1). The general behavior of the Li+ diffusivity as a function of the state of charge was also in good agreement with data from previous conventional single concentration Li[NixCoyMn1−x−y]O2 cathodes,14 indicating that the electrochemical properties and related phase transition at different states of charge were distorted by the concentration gradient. The four-probe method was used to measure the electronic conductivity of FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64), and the results are listed in Table 3. Similar to the ionic diffusivity, the electronic conductivity progressively

O2 compares favorably with that of Li[Ni0.64Co0.16Mn0.20]O2, as shown in Figure 5. To study the contribution of chemical Li+ diffusivity and electronic conductivity on the rate capability, chemical Li+ diffusivity for FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64) was measured by the galvanostatic intermittent titration technique (GITT), shown in Figure 6 as a function of the state of charge. The Li+ diffusivity increased with decreasing Mn content as the average Li+ diffusivities for Li[Ni0.64Co0.16Mn0.20]O2 and Li[Ni0.59Co0.16Mn0.25]O2 were 2.37 × 10−9 and 1.14 × 10−9 cm2 S−1, which were approximately 1 order of 180



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increased with decreasing Mn content as previously observed.19 Both the Li+ diffusivity and electronic conductivity data verify and support the rate capability of the three cathodes shown in Figure 5. To investigate the structural stabilities of the three FCG cathodes, XRD measurements were carried out with the electrodes after 100 cycles at 25 °C. The XRD results in Figure 7a−c show no drastic change in the overall crystal structures of the cycled electrodes except for minor peak shifts. For detailed analysis, the variation of the lattice parameters (Δa and Δc) and the unit cell volumes (ΔV) of the cathodes before and after 100 cycles were calculated, and the results are shown in Figure 7d. The data clearly demonstrate increasingly large deviations of lattice parameters and unit cell volume during cycling with decreasing Mn content,14 which suggests that structural stability is enhanced with increasing Mn content. The improved structural stability brought about by higher Mn content correlates well with cycling data in which Li[Ni0.51Co0.16Mn0.33]O2 exhibited the best capacity retention at both ambient and elevated temperatures. To assess the thermal safety of the FCG cathodes, the amount of oxygen released from the chemically delithiated FCG Li[NixCo0.16Mn0.84−x]O2 cathodes during thermally induced phase transitions was analyzed using TGA, and the results are shown in Figure 8a. The FCG cathodes, similar to conventional single concentration electrodes, underwent twostep phase transitions from layered (R3̅m) → M3O4 spinel (Fd3̅ m ) to M 3 O 4 spinel (Fd3̅ m ) → NiO rock-salt (Fm3m).20−23 Oxygen is released during each phase transition, resulting in a measurable weight loss. The phase transition temperature shifted to higher temperature in the order of FCG Li0.24[Ni0.64Co0.16Mn0.20]O2, FCG Li0.25[Ni0.59Co0.16Mn0.25]O2, and Li0.27[Ni0.51Co0.16Mn0.33]O2. The amount of oxygen released from the FCG Li0.25[Ni0.59Co0.16Mn0.25]O2 and the FCG Li0.27[Ni0.51Co0.16Mn0.33]O2 (14.5 and 7.1 wt %) was smaller than that from Li0.24[Ni0.64Co0.16Mn0.20]O2 (17.5 wt %). The FCG Li[Ni0.51Co0.16Mn0.33]O2, which has a higher concentration of Mn ions on the particle surface in contact with the electrolyte, was structurally stable due to the presence of inactive Mn4+ ions.8,9,15 To further evaluate the thermal stability of electrochemically delithiated FCG Li1−δ[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64), the electrodes were charged to 4.3 V and were analyzed by DSC in the presence of the electrolyte. As seen in Figure 8b, the FCG Li0.32[Ni0.59Co0.16Mn0.25]O2 and Li0.37[Ni0.51Co0.16Mn0.33]O2 showed exothermic peaks at temperatures of 287.2 and 304.1 °C with a heat generation of 698.9 and 528.9 J g−1, respectively. On the other hand, the FCG Li0.30[Ni0.64Co0.16Mn0.20]O2 exhibited an exothermic peak at a lower temperature of 266.3 °C with a larger heat generation of 802.3 J g−1. Hence, the thermal stability of FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.51, 0.59, and 0.64) increased as the Mn content increased.24,25

results reaffirmed the trend in battery performance generally observed in conventional single concentration Li[NixCoyMn1−x−y]O2 cathodes by changing the relative fraction of each transition-metal ion. Increasing Ni content raised the capacity, whereas increasing Mn content improved battery safety and cycle retention. Therefore, the optimal level of Mn from the tested FCG cathodes would be near 25 mol % Mn, which provides excellent cycle retention with reasonable structural and chemical stability. This material represents a specifically designed electrode material based on Li[NixCoyMn1−x−y]O2 that takes full advantage of each transition-metal ion present in the material.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (82)222200524. Fax: (82)222827329. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2009-009278) and by the Global Frontier R&D Program (2013-073298) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning.

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REFERENCES

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4. CONCLUSIONS We previously demonstrated the full concentration gradient (FCG) concept of cathode materials to be superior to conventional single composition materials. In this study, we compared the characteristics of the FCG materials with varying amounts of Mn in the cathodes. The FCG Li[Ni0.64Co0.16Mn0.20]O2 delivered the highest capacity (201.7 mAh g−1) when tested to 4.3 V at 55 °C, but showed a poor capacity retention of 88.1% after 100 cycles, and also exhibited comparatively less structural and thermal stability. The FCG 181



Article

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Optimization of Layered Cathode Material with Full Concentration

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