Progress in High-Capacity Core–Shell Cathode Materials for

Jan 30, 2014 - Biography. Seung-Taek Myung is an Associate Professor of Nano Engineering at Sejong University, South Korea. He received his Ph.D. in ...
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Progress in High-Capacity Core−Shell Cathode Materials for Rechargeable Lithium Batteries Seung-Taek Myung,*,†,§ Hyung-Joo Noh,‡,§ Sung-June Yoon,‡,§ Eung-Ju Lee,‡,§ and Yang-Kook Sun*,‡ †

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

‡

ABSTRACT: High-energy-density rechargeable batteries are needed to fulfill various demands such as self-monitoring analysis and reporting technology (SMART) devices, energy storage systems, and (hybrid) electric vehicles. As a result, high-energy electrode materials enabling a long cycle life and reliable safety need to be developed. To ensure these requirements, new material chemistries can be derived from combinations of at least two compounds in a secondary particle with varying chemical composition and primary particle morphologies having a core−shell structure and spherical cathode-active materials, specifically a nanoparticle core and shell, nanoparticle core and nanorod shell, and nanorod core and shell. To this end, several layer core−shell cathode materials were developed to ensure high capacity, reliability, and safety.

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safety. By contrast, Li[Ni0.5Mn0.5]O215−17 delivers a low reversible capacity of approximately 140 mA h g−1 in the same voltage range because tetravalent Mn does not join an electrochemical reaction but does stabilize the layer structure.

mong existing energy conversion and storage systems, lithium ion batteries (LIBs) are the most suitable energy storage system because of their high energy density, long life cycle, light weight, and compact design. In addition, reduction of carbon dioxide release is another emerging global issue to further motivate the recent development of reliable, highenergy lithium batteries to substitute fossil fuels. For these reasons, improvement in the cell chemistry is required to meet the performance and safety requirements for those applications. Two main components, a cathode and anode, comprise the LIBs. Carbonaceous materials allowing repetitive Li ion intercalation are the most representative anode-active materials and have been commonly used in most small- to large-format LIBs. After the discovery by Goodenough et al.,1 LiCoO2 has been evaluated as the best cathode material in terms of capacity, retention, high rate capability of Li ion intercalation, ease of synthesis, and handling during electrode fabrication. In contrast, structural instability, attributed to the structural transition from hexagonal to monoclinic phases2,3 and Co dissolution at a highly oxidized state,4,5 limits the practical use of LiCoO2 to 140 mA h g−1 so that the material is suitable only for small applications such as portable devices. Factors such as price, stability of the active material, electrode performance, and thermal properties of the cathode materials should be thoroughly considered for various applications. Unfortunately, existing materials cannot fully satisfy the abovementioned criteria. Capacity and safety have a complementary relationship; Li[Ni0.8Co0.1Mn0.1]O2,6 Li[Ni0.8Mn0.2]O2,7−9 and Li[Ni0.8Co0.15Al0.05]O210−12 exhibit reversible capacity as high as 200 mA h g−1 in the range of 2.5−4.3 V, while they suffer from poor thermal stability associated with exothermic oxygen evolution above 200 °C resulting from the phase transformation to the cubic spinel phase,13,14 which threatens cell © XXXX American Chemical Society

Coprecipitation enables the formation of homogeneous multiple transition-metal hydroxides with spherical morphology on the micrometer scale.

These two conflicting properties necessitate hybridization of the two materials in a particle level to form of a core−shell particle consisting of a high-capacity material as the core and a highly stable material as the shell. To make this concept a reality, the most plausible approach is to synthesize a micrometer-scale spherical particle, for which the particle size should be close to what is commercially available, which is a particle 10−12 μm in diameter. The difficulty is to determine the most appropriate synthetic method to homogeneously produce micrometer-scale spherical powders through mass production. Inhomogeneous reaction and irregularity in particle size would result in serious agglomeration or segregation of transition-metal concentrations in the spherical morphology. To produce the core−shell particles, coprecipitation in the form of metal hydroxide, M(OH)2 (M: transition metal) using Received: December 13, 2013 Accepted: January 30, 2014

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Figure 1. (a) A schematic drawing of the coprecipitation reactor (reproduced with permission from ref 20, Copyright 2005, American Chemical Society), (b) SEM image of (Ni1/3Co1/3Mn1/3)(OH)2 powders (reprinted from ref 19, Copyright 2004, with permission from Elsevier), and (c) the resulting powder XRD pattern for the as-synthesized (Ni1/3Co1/3Mn1/3)(OH)2 powders (reproduced with permission from ref 21, Copyright 2006, American Chemical Society).

the core−shell hydroxide. Calcination of the core−shell hydroxide and lithium carbonate led to the formation of a spherical core−shell Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 compound, with Li[Ni0.8Co0.1Mn0.1]O2 as the core and Li[Ni0.5Mn0.5]O2 as the shell (Figure 2c and d). The size and core−shell morphology remained before and after the calcination because the heat treatment was performed below the interdiffusion temperature of the transition metal elements. The core material, Li[Ni0.8Co0.1Mn0.1]O2, showed a high capacity of approximately 200 mA h g−1 (Figure 2e). Compared to the core material, the core−shell delivered a slightly lower capacity because the shell, Li[Ni0.5Mn0.5]O2, showed a reversible capacity of approximately 140 mA h g−1 in the voltage range.15−17 The Li[Ni0.5Mn0.5]O2 shell increased the operation voltage during charge, while a similar voltage variation was perceived for the core and the core−shell electrode upon discharge.20−22 This trend is related to the intrinsic property of Li[Ni0.5Mn0.5]O2 because the material shows higher charge voltage than that of the core material. Although the capacity was slightly reduced by the incorporation of stable tetravalent Mn in the shell part expensing the capacity, the core−shell electrode demonstrated superior cyclability in full cell tests (Figure 2f), which is associated with the stabilized structure derived by the presence of tetravalent Mn in the Li[Ni0.5Mn0.5]O2 shell. Similar tendencies shown above were also observed for other core−shell compounds, such as Li[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O222 and Li[(Ni1/3Co1/3Mn1/3)0.8(Ni0.5Mn0.5)0.2]O2,23 which emphasizes the choice of core and shell materials in terms of the high capacity and retention. Provided that the core material was present in the shell, the extraordinary electrode performance would not be achieved due to the structural instability of the core material. LiNiO2 and Ni-rich compounds suffer from poor thermal instability. 6 − 1 2 For deeply delithiated compounds,

a continuous stirred tank reactor (CSTR) was the most suitable method according to the following equation18,19 1 1 1 2+ Ni (aq) + Co2 +(aq) + Mn 2 +(aq) + x NH4OH(aq) 3 3 3 → [Ni1/3Co1/3Mn1/3(NH3)n2 + ](aq) + nH 2O + (x − n)NH4OH(aq)

(1)

[Ni1/3Co1/3Mn1/3(NH3)n2 + ](aq) + yOH− + z H 2O → Ni1/3Co1/3Mn1/3(OH)2 (s) + z NH4OH + (n − z)NH3 (2)

The above reaction occurred in the CSTR reactor (Figure 1a), and the as-coprecipitated resultant, (Ni1/3Co1/3Mn1/3)(OH)2, formed microscale spherical powders with high uniformity (Figure 1b). As a result, the powder X-ray diffraction (XRD) pattern was consistent with the typical fingerprint of a M(OH)2 (M: metal) structure (Figure 1c). Through this process, homogeneous metal hydroxides with a narrow particle size distribution were obtained with high tap density, harmonizing the two conflicting materials in a core− shell particle via coprecipitation; specifically, a material with high capacity but poor structural stability as the core and a material with low capacity but good structural stability as the shell were produced. Through this designation, a synergetic effect in which a high capacity is derived from the core and structural stability is realized from the shell of the core−shell particle can be anticipated. Ni-rich hydroxide was first formed as the core with a particle diameter of several micrometers (top XRD pattern of Figure 2a). The core was then completely encapsulated by the objected [Ni0.5Mn0.5](OH)2 with a thickness of less than 2 μm (middle XRD pattern of Figure 2a and b). The resulting XRD pattern exhibited two hydroxides of [Ni0.8Co0.1Mn0.1](OH)2 and [Ni0.5Mn0.5](OH)2 comprising 672

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Figure 2. (a) XRD patterns of as-prepared core [Ni0.8Co0.1Mn0.1](OH)2 (top), core−shell [(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2](OH)2 (middle), and shell [Ni0.5Mn0.5](OH)2 (bottom). The synthesis of the hydroxide was carried out using the coprecipitation method; (b) SEM image of fractured core−shell [(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2](OH)2; (c) XRD patterns of as-synthesized core Li[Ni0.8Co0.1Mn0.1]O2 (top) and core−shell Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 powders (bottom); (d) SEM image of fractured core−shell Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2; (e) the initial charge−discharge curves of Li/Li[Ni0.8Co0.1Mn0.1]O2 and Li/Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 cells at the voltage range of 3.0−4.3 V; (f) cyclability of both carbon/Li[Ni0.8Co0.1Mn0.1]O2 and carbon/Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 cells at the voltage range of 3.0−4.3 V (a−f reproduced with permission from ref 20, Copyright 2005, American Chemical Society); (g) DSC profiles of the Li1−δ[(Ni0.8Co0.1Mn0.1)1−x(Ni0.5Mn0.5)x]O2 (x = 0−1) electrodes charged to 4.5 V versus Li (reproduced with permission from ref 21, Copyright 2006, American Chemical Society); (h) voltage versus cell surface temperature plots in the Li ion cell of the core−shell C/ Li[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O2 cells during nail penetration tests at 4.3 V (the inset shows Li ion cell (200 mA h) images of the spherical C/core−shell Li1−δ[(Ni0.8Co0.2)0.8(Ni0.5Mn0.5)0.2]O2 cells after the nail penetration at 4.3 V (reproduced with permission from ref 22, Copyright 2006, American Chemical Society).

Li1−δ[Ni0.8Co0.1Mn0.1]O2, exothermic decomposition reactions are commonly initiated at approximately 190−200 °C accompanied by a significant oxygen release from the structure (Figure 2g).24 As the oxygen was removed from the structure, the original structure was progressively transformed to a cubic spinel phase.25,26 The exothermic reaction occurred violently in a broad temperature range, threatening the cell safety during

the thermal event. For Li1−δ[Ni0.5Mn0.5]O2; the primary exothermic temperature began at approximately 300 °C,27 supported by the stable tetravalent Mn and lower oxidation state of Ni at a charged state (3+) due to the delivered capacity (∼140 mA h g − 1 ) t hat was less than tha t of Li1−δ[Ni0.8Co0.1Mn0.1]O2 (3.6+, ∼220 mA h g−1). These effects were observed during the exothermic reaction for the core− 673

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shell Li0.28[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 electrode (Figure 2g). The exothermic onset temperature increased to 235 °C, and the primary exothermic reaction temperature was observed at 250 °C. The exothermic reaction temperature varied depending on the shell thickness; the reaction temperature gradually increased to 280 °C as the shell was further thickened.21 The improved safety was further evidenced in the nail penetration tests (Figure 2h). Once the cell was internally shorted by the conducting metallic nail, the charged electric energy was converted into heat immediately. Although the nail directly contacted the highly charged electrode, the cell did not ignite during the nail penetration tests, and its maximum temperature was below 50 °C. This coincided with the delay of exothermic decomposition up to 280 °C. By contrast, the charged Li[Ni0.8Co0.1Mn0.1]O2/graphite cell exhibited a sudden temperature increase to 300 °C as the nail caused a short circuit,22 and the cell abruptly ignited. Two different kinds of cathode materials could be hybridized by the formation of micrometer-scale spherical particles via the coprecipitation technique. These unprecedented electrochemical and safety data indicate that the concept of a core−shell particle is substantially effective in the development of safe advanced LIBs.

The core, Li[Ni0.8Co0.1Mn0.1]O2, is responsible for the high-capacity delivery, and the shell, Li[Ni0.5Mn0.5]O2, provides thermal stability. The above-mentioned core−shell material achieved a high reversible capacity and excellent cycle life, greatly enhancing the thermal stability attributed to the outer Li[Ni0.5Mn0.5]O2 layer. The capacity of the core−shell is less than that of the core material by approximately 20 mA h g−1. Furthermore, a structural mismatch between the core and shell appeared in the core−shell powders.28 More significantly, the core material underwent a volume change of approximately 9−10%,29 whereas the shell material changed by approximately 2−3% during deintercalation of Li ions.30 The different shrinkage ratios within the sample particle may induce gradual separation of the core from the shell because the core can lose the Li ions and electron conduction pathways. This difficulty can be solved by encapsulation of the core with a concentration gradient shell, varying the chemical composition continuously from the interface region to the outermost surface of the shell (Figure 3b), which minimizes the structural inconsistency between the core and shell. Additionally, a slight amount of trivalent Co during the shell formation while maintaining the Mn valence at 4+ seems to be helpful in increasing the capacity. A Ni-rich hydroxide was first coprecipitated using the Ni-rich solution from tank 1, and the precipitation was continued by changing the Ni, Co, and Mn concentrations from tank 2 (Figure 3a). Through this process, the chemical composition of the bulk was maintained, and the concentration of the transition metals varied from the interface to the surface in which the Ni concentration decreased abruptly from 80 to 40% toward to the outermost surface, whereas the concentrations of Co and Mn increased from 10 to 30% (Figure 3c and d). The condition was maintained even after thermal lithiation with lithium salt (Figure 3e and f) though the concentration slope of

Figure 3. (a) Schematic diagram of the CSTR in preparing the core− shell with a concentration gradient hydroxide precursor (from ref 34, reproduced by permission of The Royal Society of Chemistry, Copyright 2011); (b) schematic diagram of the positive electrode particle with a Ni-rich core surrounded by the concentration gradient outer layer (adapted from ref 32, reproduced by permission of Nature Publishing Group, Copyright 2009). Scanning electron microscopy (SEM) and electron-probe X-ray microanalysis (EPMA) results: (c) SEM photograph and (d) EPMA line scan of the precursor hydroxide and (e) SEM photograph and (f) EPMA line scan of the final lithiated oxide Li[Ni0.64Co0.18Mn0.18]O2. In both cases, the gradual concentration changes of Ni, Mn, and Co in the interlayer are clearly evident. The Ni concentration decreases, and the Co and Mn concentrations increase toward the surface (adapted from ref 32, Nature Publishing Group, Copyright 2009). 674

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Figure 4. (a) Cycling performance of half cells based on Li[Ni0.8Co0.1Mn0.1]O2, concentration gradient material Li[Ni0.64Co0.18Mn0.18]O2, and Li[Ni0.46Co0.28Mn0.31]O2 cycled between 3.0 and 4.4 V at 55 °C (adapted from ref 32, reproduced by permission of Nature Publishing Group, Copyright 2009); (b) discharge capacity versus cycle number of the core Li[Ni0.90Co0.05Mn0.05]O2 and concentration gradient Li[Ni0.83Co0.07Mn0.10]O2 between 2.8 and 4.3 V at 55 °C (from ref 34, reproduced by permission of The Royal society of chemistry, Copyright 2011); (c) differential scanning calorimetry traces showing heat flow from the reaction of the electrolyte with Li1−δ[Ni0.8Co0.1Mn0.1]O2, concentration gradient material Li1−δ[Ni0.64Co0.18Mn0.18]O2, and Li1−δ[Ni0.46Co0.23Mn0.31]O2 charged to 4.3 V (adapted from ref 32, reproduced by permission of Nature Publishing Group, Copyright 2009); (d) voltage and cell surface temperature plots for laminated-type Li ion batteries with an Al-pouch full cell (120 mA h) MCMB/concentration gradient Li1−δ[Ni0.64Co0.18Mn0.18]O2 during nail penetration tests at 100% SOC (4.2 V) (adapted from ref 32, reproduced by permission of Nature Publishing Group, Copyright 2009).

concentration gradient Li[Ni0.83Co0.07Mn0.10]O2 was remarkable as the decreased Ni concentration and the increased Mn concentration, leading to the formation of tetravalent Mn in the outer shell layer, play an important role in stabilizing the surface, which minimizes the formation of the LixNi1−xO phase on the surface and reactivity with the electrolyte, thus suppressing the increase in interfacial impedance with cycling. Additionally, the concentration gradient within the particle prevents the formation of microcracks and the segregation that can occur at the interface between the bulk and outer layer, where there was a sharp variation. As shown in the cycling data, the concentration gradient Li[Ni0.64Co0.18Mn0.18]O2/graphite full cell (capacity: 75 mA h) showed excellent capacity retention greater than 96.5% during 500 cycles.32 Similar to the core−shell particles, the concentration gradient cathodes have practical potential in terms of thermal and safety requirements. In particular, the onset temperatures of exothermic decomposition were dependent on the concentration of Mn in the outermost surface, 215 °C for Li1−δ[Ni0.83Co0.07Mn0.1]O2 (surface: Li1−δ[Ni0.68Co0.12Mn0.20]O2),34 260 °C for Li1−δ[Ni0.67Co0.15Mn0.18]O2 (surface: Li1−δ[Ni0.57Co0.15Mn0.28]O2),36 265 °C for Li1−δ[Ni0.72Co0.18Mn0.10]O2 (surface: Li1−δ[Ni0.55Co0.15Mn0.30]O2),35 and 275 °C for Li1−δ[Ni0.64Co0.18Mn0.18]O2 (surface: Li1−δ[Ni0.46Co0.23Mn0.31]O2).32 The fully charged 120 mA h full cell did not show a thermal event for the Li[Ni0.64Co0.18Mn0.18]O2 cathode. The superior safety properties were also observed in the fully charged 180 mA h cell utilizing a Ni-rich Li[Ni0.72Co0.18Mn0.10]O2 cathode, as demonstrated by the

transition metal ions (Ni, Co, and Mn) was less steep due to slight interdiffusion of Ni, Co, and Mn between the bulk and the outer layer, producing an average chemical composition of Li[Ni0.64Co0.18Mn0.18]O2. Although the chemical composition was variable for the outer layers (2 μm toward the surface from the interface), Mn contents of the outermost surfaces were designed to be greater than 20% to ensure the formation of tetravalent Mn that directly affects the structural and thermal stability.31 As a result, all surface data analyzed by X-ray photoelectron spectroscopy (XPS) indicated the presence of tetravalent Mn on the outermost surface.32,33 By varying the concentration of transition metals in both the bulk and outer layer, the resulting chemical composition of the concentration gradient products ranged from Li[Ni0.83Co0.07Mn0.10]O2 to Li[Ni0.64Co0.18Mn0.18]O2.32,34−36 In comparison with bulk materials, the concentration gradient particles enhanced the electrode performances at 25 °C and elevated temperatures (Figure 4a and b). The resulting cycling data were outstanding, even at a high-temperature cycling test. Surprisingly, the capacity retention of the concentration gradient Li[Ni0.64Co0.18Mn0.18]O2, which contained the Li[Ni0.46Co0.23Mn0.31]O2 surface composition, was close to that of the conventional bulk Li[Ni0.46Co0.23Mn0.31]O2 at 55 °C (Figure 4a). As shown in Figure 4b, for Li[Ni0.83Co0.07Mn0.10]O2 with a surface composition of Li[Ni0.68Co0.12Mn0.20]O2, the concentration gradient electrode was approximately 213 mA h g−1 and was stable, showing a capacity loss of only 7.9%. Compared to the bulk Li[Ni0.8Co0.1Mn0.1]O2 (Figure 4a), the cyclability of the 675

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Figure 5. (a) Schematic diagram of the full concentration gradient lithium transition-metal oxide particle with the nickel concentration decreasing from the center toward the outer layer and the concentration of manganese increasing accordingly (adapted from ref 37, reproduced by permission of Nature Publishing Group, Copyright 2012); (b) TEM image of the local structural feature near the edge of the particle showing highly aligned nanorods for the full concentration gradient Li[Ni0.75Co0.10Mn0.15]O2 (adapted from ref 37, reproduced by permission of Nature Publishing Group, Copyright 2012) Cross-sectional TEM images of the primary particles: (c) fixed composition of Li[Ni0.5Co0.2Mn0.3]O2 without the compositional gradient and (d) full concentration gradient material having the core composition of Li[Ni0.70Co0.05Mn0.25]O2 continuously varied to Li[Ni0.58Co0.17Mn0.25]O2 at the surface (reproduced with permission from ref 33, Copyright 2013, American Chemical Society).

accelerating rate calorimetry test.35 These properties confirm that the developed concentration gradient cathode materials followed the general performance trend of conventional layered materials; an increase in Ni content improved the capacity, and more Mn delivered better capacity retention and thermal properties at the expense of the capacity. The above-mentioned cathode materials are composed of microscale, spherical particles with polygonal primary particle shapes. Electrolytes penetrate small voids among the primary particles, which allow diffusion of Li ions along the grain boundary of each primary particle. Additionally, the exposed surface area of the reported materials to the electrolyte is large, which, in turn, results in increased reactivity at the interface and can cause deterioration of the electrode performance and inferior safety of the cell. The introduction of a concentration gradient in the outer layer greatly mitigated the structural mismatch, leading to the enhancement of the capacity retention. This motivated evaluation of a full concentration gradient, primarily the concentration on the outer layer and from the center of the core to the outermost surface of the outer layer (Figure 5a). An unusual distribution of Co and Mn also occurred, primarily at the center that was Co- (10%) and Mn-free and a surface that was Co- (10%) and Mn-rich (22%) in the hydroxide, in which the concentration of Ni also gradually declined from the center (90%) to the surface (68%). The architecture was maintained in the micrometer-scale spherical particle even after the thermal lithiation. Interestingly, the nanorods point from the center toward the surface (Figure 5b). In comparison with conventional core−shell particles in

which the faceted primary particles have no directional aliment (Figure 5c), it is possible to align these nanorod primary particles radially for a full concentration gradient particle (Figure 5b). Furthermore, changing synthetic conditions of hydroxide precursors that increase the concentration of ammonium hydroxide and sodium hydroxide favor the growth of primary particles, making nanorod particles thick and long (Figure 5d). This configuration of nanorods reduces voids in the spherical particle to minimize the contact area with the electrolyte, which decreases the side reaction with the electrolyte when cathode materials are highly oxidized. These trends are confirmed in cycling performances (Figure 6a and b). In comparison with the prior core−shell (Figure 2) and concentration gradient core−shell (Figures 3 and 4) materials, both full concentration gradient particles exhibited extraordinary cycling performance during 1000 cycles at 25 and 55 °C in full cell tests using a carbon anode. The capacity retention was approximately 95% at 25 °C and 71% at 55 °C for the short nanorod full concentration gradient particle (Figure 6a).37 Increasing the upper cutoff voltage to 4.4 V versus carbon (corresponding to 4.5 V versus Li/Li+ for a half cell), the long nanorod full concentration gradient particle showed an unprecedented high-capacity retention of approximately 88% during 1000 cycles (Figure 6b). As predicted in Figure 5b−d, this behavior would be related to the radial distribution of the long nanorods, which minimizes the presence of voids among primary nanorod particles. Even though the concentration gradient particle was composed of short nanorods, these vicinities appeared to not be favored due 676

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Figure 6. (a) Discharge capacity of MCMB/FCG Li[Ni0.75Co0.10Mn0.15]O2 cathode full cells at room and high temperature. The cells were characterized between 3.0 and 4.2 V with a constant current of 1 C (adapted from ref 37, reproduced by permission of Nature Publishing Group, Copyright 2012); (b) discharge capacity of the MCMB/FCG core composition of Li[Ni0.70Co0.05Mn0.25]O2 continuously varied to the Li[Ni0.58Co0.17Mn0.25]O2 cathode full cell at room and high temperature. The cells were characterized between 3.0 and 4.4 V with a constant current of 1 C (reproduced with permission from ref 33, Copyright 2013, American Chemical Society). (c) Differential scanning calorimetry (DSC) traces, showing heat flow from the reaction of the electrolyte with the FCG core composition of Li[Ni0.70Co0.05Mn0.25]O2 continuously varied to Li[Ni0.58Co0.17Mn0.25]O2 cathode electrodes charged to 4.3 V; (d) voltage and cell temperature plots with time for the laminated-type Al pouch MCMB/FCG core composition of Li[Ni0.70Co0.05Mn0.25]O2 continuously varied to the Li[Ni0.58Co0.17Mn0.25]O2 cathode cell (280 mA h) as a function of time during the overcharge test charged to either 250% of the state of charge or 12 V and (e) the nail penetration test at 100% of the state of charge (4.2 V). The insets show images of C/FCG core composition of Li[Ni0.70Co0.05Mn0.25]O2 continuously varied to Li[Ni0.58Co0.17Mn0.25]O2 cathode cells after the overcharge test and nail penetration test (reproduced with permission from ref 33, Copyright 2013, American Chemical Society).

peak temperature after the internal short circuits with the metallic nail was approximately 70 °C. During these safety tests, the full cells did not show thermal runway, smoke, or explosion. As observed over the past 2 decades, LIBs are a leading power source for portable devices to energy storage systems. Batteries that satisfy the high energy density, long cycle life, and good thermal stability are indispensable for supporting daily life. Among components constituting the batteries, the roles of cathode materials are very important, in particular, due to safety concerns. In a deeply charged state, oxygen evolution from the host structure is unavoidable to form a cubic spinel structure, where a violent exothermic reaction accompanies the reaction, threatening safety. This study introduces the development of cathode-active materials over the last 10 years that meet the high energy density, long cycle life, and good thermal stability achieved by the introduction of microscale spherical core−

to the larger contact area with the electrolyte, resulting in a faster capacity fade than the long nanorod, full concentration gradient particles. In addition, an advantage of the long nanorod, full concentration gradient particle is that it provides a quick pathway for Li ions of approximately 10−11 S cm−1, which is approximately 10-fold greater than that of the conventional core−shell material (10−12 S cm−1). This unique structure is also favored for thermal stability at a highly delithiated state, Li0.16[Ni0.60Co0.15Mn0.25]O2. The primary exothermic reaction appeared at 280 °C with less heat generation than any other core−shell materials (Figure 6c). An overcharge to 250% of the state of charge (280 mA h full cell) increased the voltage to 5.5 V and decreased the temperature to less than 20 °C (Figure 6d). A nail penetration test demonstrated significant thermal stability of the long nanorod, full concentration gradient particle (Figure 6e). The 677

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This full concentration gradient particle comprised of radially aligned nanorods could reduce the contact area with electrolyte, and this, in turn, affects highcapacity retention during extensive cycling. Also, the outer surface decorated with tetravalent Mn is responsible for the outstanding thermal properties.

REFERENCES

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shells to full concentration gradient particles supported by long nanorods distributed radially. In addition, surface modifications using inorganic oxides or organic compounds are applicable to further improve the electrode performances of these materials. The evolution to our latest product is believed to aid the development of advanced cathode materials with requirements for rechargeable lithium batteries with a satisfactory capacity, long calendar life, and outstanding safety in applications from portable devices to energy storage systems, including (hybrid) electric vehicles.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 82 2 3408 3454. Fax: 82 2 3408 4342(S.-T.M.). *E-mail: [email protected]. Tel: 82 2 2220 0524. Fax: 82 2 3282 7329 (Y.-K.S.). Author Contributions §

S.-T.M, H.-J.N., S.-J.Y., and E.-J.L. contributed equally to this work. Notes

The authors declare no competing financial interest. Biographies Seung-Taek Myung is an Associate Professor of Nano Engineering at Sejong University, South Korea. He received his Ph.D. in Chemical Engineering from Iwate University, Japan, in 2003. His research covers development of electroactive materials and corrosion of current collectors of Li ion batteries. Website: http://dasan.sejong.ac.kr/ ~smyung. Yang-Kook Sun obtained his Ph.D. from Seoul National University, South Korea, and is a Professor of Energy Engineering at Hanyang University, South Korea. His research interests include metal fluoridecoated cathodes, lithium transition-metal oxides, olivine-related cathodes and core−shell concentration gradient materials for advanced lithium ion batteries, and lithium metal-free, lithium sulfur, and lithium air batteries. Website: http://escml.hanyang.ac.kr/new/index.html.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2009-0092780). This work was also supported by the Human Resources Development program (No. 20124010203310) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. 678

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