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Enhanced mechanical strength and electrochemical performance of core–shell structured high–nickel cathode material
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Enhanced mechanical strength and electrochemical performance of core–shell structured high–nickel cathode material Sangjin Maeng a, 1, Youngmin Chung b, 1, Sangkee Min a, **, Youngho Shin b, * a b
School of Mechanical Engineering, University of Wisconsin, Madison, WI, 53706, USA Materials Engineering Research Facility, Applied Materials Division, Argonne National Laboratory, Lemont, IL, 60439, USA
H I G H L I G H T S
� NMC811 with LiNi0.9Mn0.05Co0.05O2 core and LiNi1/3Mn1/3Co1/3O2 shell is prepared. � Mechanical strength and binding force are investigated via nanoindentation. � Core–shell NMC811 shows 37% better capacity retention than commercial NMC811. � Core–shell NMC811 improves thermal stability significantly. � After 200 cycles, core–shell NMC811 maintains clearer initial morphology. A R T I C L E I N F O
A B S T R A C T
Keywords: Lithium-ion batteries Core-shell High-nickel Cathode Mechanical failure Stress-strain curve
Improving capacity retention during cycling and the thermal–abuse tolerance of layered high–nickel cathode material, LiNi0.8Mn0.1Co0.1O2 (NMC811), is a significant challenge. A series of core–shell structured cathode materials with the overall composition of LiNi0.8Mn0.1Co0.1O2 was prepared via a coprecipitation method in which the nickel–rich composition (LiNi0.9Mn0.05Co0.05O2) is the core and the manganese–rich composition (LiNi0.33Mn0.33Co0.33O2) is the shell. In terms of achieving a higher nickel content (more than 80%) of hetero geneous material, this core–shell structured material is a more practical approach because it has a larger nick el–rich core region and a thicker manganese–rich shell than the full–concentration gradient material, not to mention being more feasible for continuous mass production. Analysis of mechanical strength through nano indentation shows that the core–shell structured NMC811 has higher stiffness and compressive stress–strain than the commercial homogeneous NMC811 and retains the mechanical strength and the binding force strong enough to prevent crack formation even after 200 cycles. The prepared core–shell structure NMC811 exhibits a greatly improved capacity retention of 76.6% compared to the commercial homogeneous NMC811 with a capacity retention of 39.6% after 200 cycles. This material also exhibits significantly improved thermal stability over the commercial homogeneous NMC811.
1. Introduction
lithium–ion batteries. To improve energy density, industry and re searchers have increased the nickel content of layered transition metal oxide materials. For example, high–nickel layered cathode materials are known to provide high capacity, low production cost, and reasonable rate capability because of the limited use of cobalt, which is relatively toxic and expensive. In particular, lithium nickel manganese cobalt oxide (LiNixMnyCozO2, referred to as NMC) materials, with a nickel content of � 80%, are able to deliver a high discharge capacity of 200
Lithium–ion batteries have been researched since 1976 and have become a general power source for portable electronic devices and electric vehicles [1,2]. Although research and commercialization of battery material have been going on for more than 40 years, the main concerns of energy–storage related companies are improving energy density, cycle life, and thermal stability and lowering the price of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Min), [email protected] (Y. Shin). 1 Contributed equally. https://doi.org/10.1016/j.jpowsour.2019.227395 Received 16 August 2019; Received in revised form 18 October 2019; Accepted 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Sangjin Maeng, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227395
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mAh g 1 at 4.3 V [3–6]. However, as a high–nickel layered cathode material, NMC811 has some drawbacks, such as fast capacity fading, short cycle life, and low thermal and structural stability. These disadvantages are due to the high nickel content, which results in more reactive surface oxygen, which in turn reduces the stability of the material when it is exposed to the electrochemical charge–discharge extended cycles [7–9]. Various methods have been proposed to solve these problems. In the most commonly attempted method, a high–nickel cathode particle surface is coated with a metal oxide, composed of components such as sodium, potassium, magnesium, calcium, strontium, nickel, cobalt, silicon, tita nium, boron, aluminum, tin, manganese, chromium, iron, vanadium, zirconium, germanium, gallium, and the like. In this approach, materials coated on the surface of the particles are often electrochemically inac tive, thus reducing the capacity. Furthermore, since the surface coating is formed with a very thin layer, it is difficult to completely protect the internal material from side reactions with the electrolyte. Capacity loss and thermal stability are still issues, and low rate capacity and imped ance growth problems in charge–discharge cycles must be considered as well. Full–concentration gradient cathode material, in which the nickel concentration decreases and the manganese concentration increases linearly toward the particle surface, has received considerable attention because of its high capacity and improved structural stability. The ma terial design of a nickel–rich center and manganese–rich surface of a particle with linear compositional change leads to high capacity and better thermal stability, compared with normal NMC cathode materials [10,11]. However, it is challenging to synthesize a well–functioning full–concentration gradient cathode with an overall composition of more than 80% nickel. It is difficult to increase the nickel content of the overall composition to 80% or more when the goal is to form sufficiently stable manganese–rich surface such as NMC333 while maintaining a linear compositional change from the particle center to the particle surface. Because the occupied volume increases in proportion to the third power of the radius as the particle radius increases, a linear in crease of the manganese composition to the particle surface does not increase the overall nickel content, or it is difficult to provide a man ganese–rich composition of sufficient thickness. Therefore, in the case of a material with an overall composition of 80% or more nickel, the cor e–shell particle structure is more effective in optimizing electrochemical performance and enabling mass production than the full–concentration gradient particle structure. It has been reported that, compared to the full–concentration gradient material, the core–shell material can cause cracks inside the layered oxide particle by volume changes on electrochemical cycling because of the significant compositional difference between the core and the shell interface and the structural mismatch [12]. This leads to impeded lithium ion diffusivity and deteriorates the electrochemical cycling performance [13]. The physical deformation of cathode particles occurs primarily in the application of strong pressure to increase the electrode’s loading amount, and secondarily in the delithiation and lithiation charge–discharge cycle by the volume change of primary particles, which weakens the bonding force between the primary par ticles and lowers the mechanical strength of the secondary particles. Through these processes, when the microstructural change of cathode particles occurs, the reactive area of the electrode gradually increases and the capacity fading begins. During charge–discharge cycling, highly reactive impurities such as water or carbon oxides induce surface im purities of cathode particles, which can be a defect in the lithium–ion batteries [14–18]. To investigate the effect of the physical deformation of cathode particles, Kim et al. studied the relationship between the mechanical strength of secondary particles and the capacity fading of the cathode with respect to cycle time [19] and found that lithium ion insertion and extraction result in mechanical stress on the high–nickel particles and lead to micro–cracks. Recently, the failure mechanism of a high–nickel cathode with a nickel content of 88%
(LiNi0.88Mn0.09Co0.03O2) was investigated with compressive tensile strength tests and internal pressure measurement [20]. However, a systematic and statistical study on the mechanical strength and binding force of core–shell (LiNi0.9Mn0.05Co0.05O2–LiNi0.33Mn0.33Co0.33O2) structured NMC811 particles has not been reported. In this work, a series of core–shell structured cathode materials was prepared at various calcination temperatures; the nickel–rich composi tion (LiNi0.9Mn0.05Co0.05O2) was the core, and the manganese–rich composition (LiNi0.33Mn0.33Co0.33O2) was the shell. The overall composition of the prepared core–shell materials was controlled as LiNi0.8Mn0.1Co0.1O2 by adjusting the ratio of the core–shell composition to have the same content of nickel, manganese, and cobalt as commer cially available homogenous NMC811. Those core–shell structured NMC811 materials were synthesized to improve the electrochemical performance of the commercial homogenous NMC811, such as capacity loss, low thermal stability, and impedance growth, as well as to inves tigate the effect of morphology of primary particle and structural het erogeneity of secondary particle on particle mechanical strength, which has not yet been studied. Thus, a series of core–shell structured NMC811 materials prepared at calcination temperatures of 700, 740, 760, 780, and 800 � C and the commercial homogeneous NMC811 material were tested to identify the electrochemical performance by coin half cell and to investigate mechanical failure and compressive stress–strain curve by a nanoindenter. This study observed fairly good mechanical strength as well as a substantial improvement in the electrochemical performance of the prepared core–shell structured NMC811, compared to the commer cial homogeneous NMC811. 2. Experiments 2.1. Synthesis of core–shell structured NMC811 cathode A precursor of core–shell structured NMC811 cathode materials, consisting of [Ni0.90Co0.05Mn0.05](OH)2 in the core and [Ni0.33C o0.33Mn0.33](OH)2 in the surface, was prepared via a coprecipitation method using a 20 L batch reactor. A nickel–rich aqueous solution, as a core composition consisting of NiSO4⋅6H2O, CoSO4⋅7H2O, and MnSO4⋅5H2O (molar ratio of Ni:Co:Mn, 90:5:5), from the first storage tank was fed into the batch reactor, which was filled with a certain amount of deionized water, ammonium hydroxide (NH4OH) solution (aq.), and sodium hydroxide (NaOH) solution (aq.) in a nitrogen atmo sphere. Simultaneously with the injection of the nickel–rich aqueous solution, a 10 mol L 1 NaOH solution (aq.) for pH adjustment (molar ratio of NaOH to transition metal, about 1.5) and a 3.8 mol L 1 NH4OH solution (aq.) for chelating purposes (molar ratio of NH4OH to transition metal, 1.0) were fed into the reactor separately. Through the first–stage coprecipitation process, the precursor with the core composition grew to the desired particle size. As the second–stage coprecipitation process, a manganese–rich aqueous solution, as a shell composition consisting of NiSO4⋅6H2O, CoSO4⋅7H2O, and MnSO4⋅5H2O (molar ratio of Ni:Co:Mn, 33:33:33), from the second storage tank was fed into the reactor instead of the nickel–rich aqueous solution from the first storage tank. In this process, the shell precursor composition, [Ni0.33Co0.33Mn0.33](OH)2, was deposited on the core precursor particle, [Ni0.90Co0.05Mn0.05](OH)2, formed through the first–stage coprecipitation process. The obtained precursor, which is [Ni0.8Co0.1Mn0.1](OH)2 as an overall composition, was filtered, washed, and dried for 20 h at 100 � C. In the next step, the dried precursor was mixed with LiOH⋅H2O, and portions of the mixture were calcined at 700, 740, 760, 780, and 800 � C for 20 h, each under oxygen flow. The core–shell structured NMC811 cathode materials thus obtained were named CS700, CS740, CS760, CS780, and CS800 ac cording to their clacination temperatures. 2.2. Material characterization The particle morphologies of commercial homogeneous NMC811 2
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and synthesized core–shell NMC811 materials were observed with scanning electron microscopy (SEM) (Phenon XL), and elemental map ping was performed with an energy dispersive X–ray spectroscopy (EDX) with a probe diameter of 1 μm and an acceleration voltage of 15 kV. Samples for cross–sectional element mapping and line scanning were prepared by embedding the powders in an epoxy and polishing them to investigate the compositional change of nickel, cobalt, and manganese within a particle qualitatively and quantitatively. EDX line mapping profiles for nickel, cobalt, and manganese were obtained. Note that the cross–sectional EDX mapping cannot provide actual element composi tion of the detected area. However, the cross–sectional EDX line scan ning shows the element ratio of the synthesized particle from the core to the outer shell.
The optical microscope measured particle size and classified mechanical failure as shown in Fig. 1(c). 3. Results and discussion 3.1. Particle structure and morphology characterization A series of core–shell structured NMC811 materials of overall composition LiNi0.8Mn0.1Co0.1O2, consisting of nickel–rich core composition LiNi0.9Mn0.05Co0.05O2 and manganese–rich shell composi tion LiNi0.33Mn0.33Co0.33O2, were synthesized through a coprecipitation reaction and a calcination process between 700 and 800 � C. Material characterization was conducted to confirm that the prepared powder had a heterogeneous particle structure with a core–shell shape as designed. First of all, for visual confirmation, EDX spectral mapping of nickel, cobalt, and manganese was performed to determine whether the elemental distribution of the core and shell of the particle matches the designed schematic diagram of the particle, shown in Fig. 2(a). Fig. 2(b) shows the starting and ending points set for the cross–sectional line scan from the particle center to the particle surface. The line scan results clearly show the presence of core and shell with dramatic compositional changes within the synthesized core–shell structured NMC811 particle [Fig. 2(c)]. The cross–sectional line scan results for the selected single particle demonstrate that the nickel concentration on the particle sur face is lower than that in the center of the particle, while the manganese and cobalt concentrations on the particle surface tend to be higher than those in the center of the particle. The center composition of the particle, consisting of 90% nickel, 10% manganese, and 10% cobalt, is main tained constant up to the vicinity of the surface, then shows a steep step change in composition, exhibiting a rapid movement toward the surface composition, consisting of 33% nickel, 33% manganese, and 33% cobalt, of the particle. The concentration of each element corresponds to the tendency of the schematic particle design shown in Fig. 2(a). However, closer examination reveals that the concentration of nickel in the synthesized particle is 90% at the center of the particle, then decreases to about 80% near the surface of the particle, and rapidly decreases to 33% at the shell region. The concentration of manganese and cobalt in the synthesized particle is also 5% at the center of the particle and increases to about 10% near the particle surface and to 33% in the shell area. This means that the concentration distributions of nickel, manganese, and cobalt inside the synthesized particles do not exactly match the schematic particle design of Fig. 2(a), which was planned for the synthesis of the spherical core–shell structured NMC811. This structural and compositional difference between the designed particle and the actually synthesized particle is mainly a result of incomplete coprecipitation process. This is because the synthesis of coreshell structured particles with uniform size and spherical shape is challenging due to too many process variables such as coprecipitation reactor geometry, impeller shape, stirring speed, reaction pH, reaction temperature, residence time, feed concentration, feed flow rate, and feed injection type. Due to these unoptimized process variables that lead to incomplete coprecipitation, core-shell particles have a non-spherical shape and size distribution, resulting in an Inconsistent shell thickness at each particle and at each location on the particle surface. Although this technical challenge makes it difficult to achieve the desired uniform spherical core-shell particles in realistic coprecipitation process, the result of the cross–sectional element map of all crowded particles in Fig. 2(d–g) shows that nickel–rich cores of all particles are well stacked with manganese–rich shells. Because of the limitation of the equipment used for cross–sectional element mapping, the increase in the concen tration of the cobalt at the particle surface in Fig. 2 (g) is not as obvious as in the case of manganese. However, cobalt–rich shells are formed, as shown in the line scan result inside a single particle [Fig. 2 (c)]. In addition, the overall chemical composition of the core–shell structured CS700 (calcined at 700 � C) was identified as LiNi0.796Mn0.101Co0.102O2 by inductively coupled plasma mass spectroscopy (ICPMS), which
2.3. Electrochemical test The commercial homogeneous NMC811 and the synthesized core– shell structured NMC811 powders were made into laminates comprising 90 wt % active material, 5 wt % super P carbon black, and 5 wt % pol yvinylidene fluoride (PVDF) binder dissolved in N–methyl–2–pyrrolidone (NMP). The prepared slurry was spread onto aluminum foil and then dried in a vacuum oven at 80 � C. Prior to coin cell assembly, the punched electrodes were dried in a vacuum oven to remove moisture contained in the electrodes. The dried electrodes were calendered with a press and then assembled into a type 2032 half–coin cell in argon–filled glove box using lithium foil as a counter electrode and Celgard 2325 as a separator. The electrolyte used was 2 M LiPF6 in a mixture solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with a volume ratio of 3:7. The assembled coin cells were then tested using a MACCOR cycler (Series 4000) between 3.0 and 4.4 V at a constant current of 0.1 C rate (20 mA g 1) and room temperature for 200 cycles. The thermal stability was measured by differential scanning calorimetry (DSC). The type 2032 half–coin cells containing the positive electrode materials were charged at a constant voltage of 4.4 V and a constant current of 0.1 C rate (20 mA g 1) and then disassembled in argon–filled glove box. The 4–5 mg positive active material samples, including the aluminum foil and the electrolyte, were injected into a high–pressure nickel–steel DSC pan with a gold–plated copper seal. The measurement was carried out using an STA 449 F3 Jupiter (NETZSCH, Germany) with a temperature scanning rate of 5 � C min 1. 2.4. Nanoindenter Nanoindentation tests were performed by using a commercial nanoindentation system (Hysitron TI 950, Bruker, USA). The indenter uses a capacitive transducer capable of operating with a load and displacement resolution of 1 nN and 0.1 nm and reaching a maximum of 18 mN in load and 5 μm in displacement. The precision of load feedback allows an indentation test in which the particles are subjected to a constant load increment per time, 0.3 mN s 1. A high–resolution optic microscope with a color charge coupled device (CCD) camera was incorporated into the machine for visualizing the particles and selecting the indenting position. A conical diamond tip with a 50 μm radius was installed on the transducer. The indentation experiments were conducted 15 times for 5 types of the core–shell structured NMC811 and 40 times for the commercial homogeneous NMC811 because the commercial one has a large distri bution in particle size. All indentation experiments were performed in air at ambient temperature (24 � C). The particles were spread out on a tungsten carbide holder, which has high hardness and good surface roughness. Indented particles were randomly selected and aligned with the center of the indenter tip by using the optical microscope. Once the diamond tip approaches and contacts the particle, the transducer detects reaction force and indentation measurement starts, as shown in Fig. 1 (b). Measuring the load in terms of displacement, the nanoindenter presses the particle with the diamond tip until reaction force releases. 3
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Fig. 1. (a) Schematic diagram of experiment approach with nanoindenter; (b) load–displacement (L–D) curves for indentation phases; and (c) NMC811 particles before and after a mechanical failure.
Fig. 2. Material characterization of the core–shell structured NMC811 (CS700): (a) schematic particle deign; (b) SEM; (c) cross–sectional line scan results of single particle; (d) cross–sectional elemental maps showing the distribution of (e) nickel, (f) manganese, and (g) cobalt in crowded particles.
indicates that it was successfully synthesized with a composition consistent with the schematic particle design. Fig. 3(a–e) show SEM images of the core–shell structured NMC811 cathode materials (CS700, CS740, CS760, CS780, and CS800) obtained by calcining a core–shell structured NMC811 precursor material at 700, 740, 760, 780, and 800 � C, respectively, after mixing with the lithium source. For comparison of the morphology of the synthesized core–shell structured NMC811 materials, the SEM of a commercially available homogeneous NMC811 material is shown in Fig. 3(f). The final products maintain the morphology, particle size, and particle size distribution of their spherical precursors, despite the different calcination tempera tures. The particle sizes of all synthesized core–shell structured NMC811 materials are about 5–10 μm as secondary particles; much smaller
primary particles are tightly packed into the secondary particles. The difference in size and shape of the secondary particles of all the core– shell structured NMC811 materials is negligible, but there are other characteristics about the primary particles. In the case of low–temperature calcination at 700 � C, the morphology of the primary particles has a sharp rod–like shape, its long axis is about 1 μm, and its short axis is about 200 nm [Fig. 3(a)]. As the calcination temperature increases to 800 � C, the length of the long axis of the primary particles is still about 1 μm, but the length of the short axis gradually increases to about 600 nm and the particle edge sharpness tends to decrease [Fig. 3(e)]. Thus, the formation of larger primary particles at higher calcination temperatures shows the possibility that the transition metal migrates during the heating process, resulting in a 4
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Fig. 3. SEM images of the core–shell structured NMC811 materials and (f) a commercial homogeneous NMC811 calcined at (a) 700 � C, (b) 740 � C, (c) 760 � C, (d) 780 � C, and (e) 800 � C.
slight change in particle morphology at the edge [21]. For this reason, in order to synthesize cathode particles in a more complete form of cor e–shell structure and to achieve improved electrochemical performance, the calcination process must be optimized to minimize the movement of the transition metal in the radial direction of the particles. Interestingly, the shape of the primary particles of the commercial homogeneous NMC811 [Fig. 3(f)] is similar to that of CS760 or CS780 [Fig. 3(c) or 3 (d)]. This means that the calcination temperature of the commercial homogeneous NMC811 is estimated to be between about 760 and 780 � C.
breaking force of the core–shell structured NMC811 materials are concentrated at 4.7–5.2 μm and 4.1–4.3 mN, respectively, as listed in Table 1. The commercial homogeneous NMC811 material with a similar particle average size of 4.9 μm has a lower value of 3.7 mN. The me chanical strength of the core–shell structured NMC811 materials is equivalent to or better than that of the commercial homogeneous NMC811. While calcination temperature influences the morphology of the primary particle, the mechanical strength of the secondary particle is consistent regardless of calcination temperature. It turns out that het erogeneity and binding force between the core and the shell do not degrade mechanical strength. The nominal compressive stress–strain behaviors were calculated to compare the mechanical property of the cathode particles with respect to particle size. The volume and Poisson’s ratio of the particles would be continuously varied in deformation of particles under compression because of the geometry and large defor mation [22]. According to the classical Hertz theory [23,24], the contact of the two elastic spheres with radii R1 and R2 can be derived by Eq. (1):
3.2. Mechanical properties Mechanical failure and stiffness of the core–shell structured and commercial homogeneous NMC811 particles were measured from L–D. curves. Fig. 4 shows the L–D curves for the particle size of the six types of NMC811 materials. Once the diamond tip contacts a particle, the reac tive load rises sharply along with the displacement. Mechanical failure of the particle occurs beyond the breaking force, and the reactive load becomes constant. Fig. 4 clearly indicates that the particle size is a dominant factor determining the breaking force. The breaking force is proportional to the particle size regardless of the type of NMC811 ma terial. This result means that heterogeneity of the core–shell structured NMC811 does not influence mechanical strength. The slope on the L–D curves represents the stiffness of the secondary particle. Therefore, it is clear that the stiffness of the core–shell structured NMC811 materials is higher than that of the commercial homogeneous NMC811. The particle with high stiffness will more likely show small displacements under the stress in calendaring process or the internal stress induced by particle volume change while charging and discharging. The small displacement would reduce the possibility of creating micro–cracks in particles. Fig. 5 shows a comparison of the breaking force of the core–shell structured and the commercial homogeneous NMC811 materials with respect to particle size. The breaking force of the commercial homoge neous NMC811 particle is linearly proportional to the particle size. The linear asymptote of the commercial homogeneous NMC811 particle presents 1.20 kN m 1, which is lower than the breaking forces of the core–shell structured NMC811 materials. The mean of particle size and
1
3
P ¼ Kc R2c h2c
(1)
where Kc, Rc, and hc are the effective modulus, the effective curvature, and the contact deformation, respectively. The effective modulus and curvature are calculated by Eqs. (2) and (3): � 1 � 4 1 v21 1 v22 Kc ¼ þ (2) 3 E1 E2 Rc ¼
R1 R2 R1 þ R2
(3)
where v1, v2, E1, E2, R1, and R2 are Poisson’s ratio, Young’s modulus, and the principal radii of the two spherical objects, respectively. The contact problem of two sphere objects, Eq. (1), becomes Eq. (4) as stress–strain form:
σc ¼
Kc
π
where 5
3
ε2c
(4)
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Fig. 4. L–D curves for the core–shell structured and commercial NMC811 materials: (a) CS700; (b) CS740; (c) CS760; (d) CS780; (e) CS800; and (f) a commercial homogeneous NMC811 material with respect to particle size. Table 1 Comparison of breaking force of the commercial homogeneous and the core– shell structured NMC811 materials with similar particle sizes. Type Commercial CS700 CS740 CS760 CS780 CS800
σc ¼
P
πR2c
Particle size (μm)
Breaking force (mN)
Average
Standard deviation
Average
Standard deviation
4.866 4.938 5.147 5.213 4.724 5.127
0.738 1.025 0.479 0.648 0.662 0.783
3.696 4.363 4.127 4.268 4.299 4.176
1.223 1.122 0.962 0.955 1.139 0.855
and εc ¼
hc Rc
Are the nominal compressive stress and strain, respectively. As the effective modulus, Kc, is only determined by the material property, Young’s modulus, and Poisson’s ratio, the compressive stress–strain behavior can be one of the constitutive mechanical properties of the particles. The slope in the stress–strain curve specifies the effective modulus of the tested particle. Fig. 6 shows that the compressive stress–strain curves are indepen dent of particle size regardless of the type of NMC811 material. What is remarkable is that the commercial homogeneous NMC811 has the
Fig. 5. Breaking force of the core–shell structured and commercial homoge neous NMC811 materials with respect to particle size.
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Fig. 6. Compressive stress–strain curves for the core–shell structured and commercial NMC811 materials: (a) CS700; (b) CS740; (c) CS760; (d) CS780; (e) CS800; and (f) a commercial homogeneous NMC811 with respect to particle size.
smoothest slope, indicating the softest particles. These results show that the core–shell structured NMC811 particles have better mechanical strength, resist breakage during the calendaring process, and are more likely to mitigate the formation of micro–cracks or fractures of the particles during charge–discharge cycles than the commercial homo geneous NMC811.
NMC811, during the initial 40 cycles, the coulombic efficiency decreases and then increases again. To understand this phenomenon, it is worth noting that the mechanical strength of this material is 15% lower than the CS700 [Table 1]. This results in a higher surface area due to the more easily caused particle damage, which leads to an irreversible reaction with electrolyte during charge and discharge processes. The CS700 shows a significantly improved lithium ion intercalation stability with a capacity retention of 76.6% after 200 cycles, and the commercial ho mogeneous NMC811 exhibits capacity retention of only 39.6% after the same cycling period. In addition, as shown in the charge-discharge voltage profiles [Fig. 7(b)–(c)], the CS700 demonstrates improved voltage stability as well as long-term capacity retention. The CS700 demonstrates this encouraging capacity retention and voltage stability result because it is covered with more stable manganese–rich composi tion, LiNi0.33Mn0.33Co0.33O2, which prevents direct contact with the electrolyte of the core composition, LiNi0.9Mn0.05Co0.05O2, during cycling and protects the particle surface. The heterogeneous character of the core–shell structured NMC811 makes the initial discharge capacity lower than that of the commercial homogeneous NMC811. But enables a higher discharge capacity after 30 cycles and improves cycle perfor mance by excellent surface stability. In particular, the coulombic effi ciency of the CS700 is superior to that of the commercial homogeneous
3.3. Electrochemical performance The cycling performance of the commercial homogeneous NMC811 and the core–shell structured CS700 materials was measured at a rate of 0.1 C (20 mA g 1) for 200 cycles by type 2032 half–coin cells employing lithium metal as the counter anode electrode, as shown in Fig. 7(a–c). For the initial discharge capacity, the commercial homogeneous NMC811 delivers a high initial discharge capacity of 202 mAh g 1 at the voltage window between 3.0 and 4.4 V, while the CS700 exhibits a reduced initial discharge capacity of 175 mAh g 1 because of insuffi cient initial activation of the cathode. This late activation, due to structural inconsistency in the interfacial region of the two compositions that make up the heterogeneous structure of the particles, increases the capacity of the CS700 and causes a change in coulombic efficiency during the initial 15 cycles. In the case of the commercial homogeneous 7
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Fig. 7. Cycling performance of the CS700 and commercial homogeneous NMC811 materials: (a) discharge capacity and coulombic efficiency; (b) voltage profile of CS700; and (c) voltage profile of commercial homogeneous NMC811. Type 2032 half–coin cells were tested at the voltage window between 3.0 and 4.4 V at 30 � C by applying a constant current density of 0.1 C rate (20 mA g 1) for 200 cycles. The rest time between charge and discharge was set to 5 min. Results were averaged with 2 sigma standard deviation.
NMC811 in the initial 50 cycles, as well as up to 200 cycles. Given this electrochemical performance, the core–shell structured cathode is a practical and effective approach to increasing the cycle life of high –nickel NMC materials of the same overall composition. The prepared core–shell structured NMC demonstrated superior cycling performance in the type 2032 half–coin cell test, as well as higher stiffness and compressive stress–strain in nanoindentation tests compared with the commercial homogeneous NMC811. To reconfirm the consistency of these results, the cross–sectional SEM images of the cathode electrodes were examined by disassembling the type 2032 half–coin cells used in the cyclability test shown in Fig. 7(a–c) after 200
cycles. Fig. 8(a–d) shows the morphology and microstructure of particles from cathode electrodes of the commercial homogeneous NMC811 and CS700 before cycling and after 200 cycles. As shown in Fig. 8(a) and (c), the electrodes of both of the commercially available homogeneous NMC811 and CS700 exhibit well–maintained spherical secondary par ticles without cracking, even though the laminate formation process includes a calendaring process that can trigger the breakage of particles. Considering the noncontinuity of the coprecipitation process and the compositional heterogeneity of the two compositions forming the cor e–shell particle structure, the shell of the core–shell structured CS700 can be broken or separated by calendaring pressure during laminate
Fig. 8. Cross–sectional SEM images of the commercial homogeneous NMC811 electrodes (a) before cycling and (b) after 200 cycles, the CS700 electrodes (c) before cycling and (d) after 200 cycles, and (e) DSC curves for heat flow of the commercial homogeneous NMC811 and CS700 samples from the electrodes charged to 4.4 V at a constant current density of 0.1 C rate in the presence of electrolyte. 8
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formation. However, as shown in Fig. 8(c), the secondary particles of CS700 remain on the electrode while maintaining their initial morphology. This result is consistent with the breaking force of the nanoindentation test shown in Fig. 5 and reconfirms that the commercial homogeneous NMC811 and CS700 have similar particle strength to resist cracking. Fig. 8(b) and (d) are the cross–sectional SEM images of the com mercial homogeneous NMC811 and the CS700 electrodes after 200 cy cles. Nearly all the cyclized particles of the commercially homogeneous NMC811 electrode were completely pulverized after 200 cycles, as ex pected from its relatively poor capacity retention. As shown in Fig. 8(b), the bond between the primary particles inside the secondary particles was significantly weakened and separated. Thus, a major cause of the capacity loss of commercial homogeneous NMC811 appears to be elec trode deterioration due to secondary particle fracture during the long –term charge–discharge cycle. On the other hand, as shown in Fig. 8(d), the core–shell structured CS700 electrode maintains the initial morphology of the particles even after 200 cycles, which is one reason why it shows solid capacity retention. Therefore, the formation of micro–cracks in the layered oxide particles by the volume change during electrochemical cycling, due to the important compositional difference between the core and the shell interface, is not significant within at least 200 cycles compared to the homogeneous one. In conclusion, for several hundred charge–discharge cycle levels, particle breakage due to the core–shell structure does not occur seriously, and the core–shell struc tured material has superior capacity retention compared to the homo geneous one.
4. Conclusion The high–nickel core–shell structured NMC811 materials were suc cessfully synthesized via a coprecipitation method composed of the core LiNi0.9Mn0.05Co0.05O2 and the shell LiNi0.33Mn0.33Co0.33O2; the overall composition is LiNi0.8Mn0.1Co0.1O2, which is like a commercial homo geneous NMC811. The SEM–EDX cross–sectional mapping and line scanning revealed that the prepared core–shell structured particle has the local compositional change from the particle center to the surface consistent with the designed particle structure. As the calcination tem perature increases from 700 to 800 � C, the primary particles of the core–shell structured NMC811 materials become thicker, but all the secondary particle breaking forces are similar to those of the commercial homogeneous NMC811 in nanoindentation testing. The stiffness and compressive stress–strain of the core–shell structured NMC811 are higher than those of the commercial homogeneous one. In the cathode laminate formation process to measure electrochemical performance, the morphologies of secondary particles of both commercial homoge neous NMC811 and core–shell structured CS700 electrodes are well –maintained without breakage after calendaring. On the other hand, after 200 cycles, the secondary particles of the commercial homoge neous NMC811 are completely pulverized, while the core–shell struc tured NMC811 particles are mostly maintaining their initial spherical morphologies. Thus, at the level of several hundred cycles, the heterogeneity of the core–shell structured NMC811 does not weaken the mechanical strength of the secondary particle, and the binding force between the core and the shell is strong enough to prevent micro–crack formation inside the particle. The core–shell structured NMC811 has a lower initial discharge capacity than the commercial homogeneous NMC811 but shows dramatically improved cycle stability (76.6% capacity retention) over that of the commercial homogeneous NMC811 (39.6% capacity reten tion) after 200 cycles. The DSC results also show that the onset tem perature of the core–shell structured NMC811 is 23 � C higher and represents 22.8% less heat generation than the commercial homoge neous NMC811. These results indicate that the core–shell structured heterogeneous particle composed of core LiNi0.9Mn0.05Co0.05O2 and shell LiNi0.33Mn0.33Co0.33O2 demonstrates improved thermal stability, sufficiently strong mechanical strength, and high capacity retention, as well as a high possibility of continuous manufacturing process for high volume production which is more difficult to achieve in the case of full–concentration gradient particle.
3.4. Thermal properties When cathode electrode materials are being considered for use in commercial applications, especially electric vehicles, the thermal sta bility as well as capacity retention of these materials is of critical importance. Better thermal stability improves the cycle performance and reduces the cooling requirement of the battery pack. Fig. 8(e) il lustrates DSC curves for the heat flow of cells with the commercial ho mogeneous NMC811 and CS700 cathode electrodes charged to 4.4 V with 0.1 C rate in the presence of the electrolyte. The onset temperature of the commercial homogeneous NMC811 electrode is 229 � C, while that of the CS700 is increased to 252 � C, 23 � C higher. The heat generation side of the commercial homogeneous NMC811 electrode is 1603 J g 1, whereas that of the CS700 is decreased to 1238 J g 1, 365 J g 1 less. In a typical NMC material, the lower the nickel content, the higher the onset temperature and the lower the heat generation. Therefore, both mate rials are expected to be similar in onset temperature and heat generation because the overall composition is the same. However, the onset tem perature of the core–shell structured CS700 is 23 � C higher and repre sents approximately 22.8% less heat generation. As can be seen in Fig. 7 (a–c), for the initial discharge capacity, the commercial homogeneous NMC811 provides a high initial discharge capacity of 202 mAh g 1, while the CS700 exhibits an initial discharge capacity of 175 mAh g 1, which is 13% lower. This means that the CS700, which has an overall composition of LiNi0.796Mn0.101Co0.102O2, has less lithium compared to the commercial homogeneous NMC811 after discharging, resulting in less overcharge and more structural stability. But the CS700’s 22.8% less heat generation and higher onset temperature results clearly reveal that the thermal stability of the outer shell LiNi0.33Mn0.33Co0.33O2 is responsible for the significant improvement of the core–shell structured CS700 by protecting the core LiNi0.9Mn0.05Co0.05O2. In other words, the improved thermal stability of the core–shell structured NMC811 is caused by the thermally stable outer shell that prohibits the highly delithiated core from contacting liquid electrolyte directly and thereby suppresses the oxygen release from the host structure of the delithiated core. The manganese–rich composition of the shell improves the thermal stability as well as lithium intercalation stability during the electro chemical cycle, thereby achieving higher capacity retention.
Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Support from the U.S. Department of Energy Office of Vehicle Technologies, particularly from Peter Faguy and Dave Howell, is gratefully acknowledged. This work was partially carried out at the Materials Engineering Research Facility at Argonne National Labora tory, which is supported within the core funding of the Applied Battery Research for Transportation Program. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory. Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE–AC02–06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid–up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. 9
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Glossary CCD: charge coupled device CS700: core–shell structured NMC811 precursor material calcined at 700 � C DSC: differential scanning calorimetry EC: ethylene carbonate EDX: energy dispersive X–ray spectroscopy EMC: ethyl methyl carbonate ICPMS: inductively coupled plasma mass spectroscopy L–D: load–displacement (curve) NMC: lithium nickel manganese cobalt oxide (LiNixMnyCozO2) NMC811: high–nickel cathode material (LiNi0.8Mn0.1Co0.1O2) NMP: N–methyl–2–pyrrolidone PVDF: polyvinylidene SEM: scanning electron microscopy
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Enhanced mechanical strength and electrochemical performance of core–shell structured high–nickel cathode material Sangjin Maeng a, 1, Youngmin Chung b, 1, Sangkee Min a, **, Youngho Shin b, * a b
School of Mechanical Engineering, University of Wisconsin, Madison, WI, 53706, USA Materials Engineering Research Facility, Applied Materials Division, Argonne National Laboratory, Lemont, IL, 60439, USA
H I G H L I G H T S
� NMC811 with LiNi0.9Mn0.05Co0.05O2 core and LiNi1/3Mn1/3Co1/3O2 shell is prepared. � Mechanical strength and binding force are investigated via nanoindentation. � Core–shell NMC811 shows 37% better capacity retention than commercial NMC811. � Core–shell NMC811 improves thermal stability significantly. � After 200 cycles, core–shell NMC811 maintains clearer initial morphology. A R T I C L E I N F O
A B S T R A C T
Keywords: Lithium-ion batteries Core-shell High-nickel Cathode Mechanical failure Stress-strain curve
Improving capacity retention during cycling and the thermal–abuse tolerance of layered high–nickel cathode material, LiNi0.8Mn0.1Co0.1O2 (NMC811), is a significant challenge. A series of core–shell structured cathode materials with the overall composition of LiNi0.8Mn0.1Co0.1O2 was prepared via a coprecipitation method in which the nickel–rich composition (LiNi0.9Mn0.05Co0.05O2) is the core and the manganese–rich composition (LiNi0.33Mn0.33Co0.33O2) is the shell. In terms of achieving a higher nickel content (more than 80%) of hetero geneous material, this core–shell structured material is a more practical approach because it has a larger nick el–rich core region and a thicker manganese–rich shell than the full–concentration gradient material, not to mention being more feasible for continuous mass production. Analysis of mechanical strength through nano indentation shows that the core–shell structured NMC811 has higher stiffness and compressive stress–strain than the commercial homogeneous NMC811 and retains the mechanical strength and the binding force strong enough to prevent crack formation even after 200 cycles. The prepared core–shell structure NMC811 exhibits a greatly improved capacity retention of 76.6% compared to the commercial homogeneous NMC811 with a capacity retention of 39.6% after 200 cycles. This material also exhibits significantly improved thermal stability over the commercial homogeneous NMC811.
1. Introduction
lithium–ion batteries. To improve energy density, industry and re searchers have increased the nickel content of layered transition metal oxide materials. For example, high–nickel layered cathode materials are known to provide high capacity, low production cost, and reasonable rate capability because of the limited use of cobalt, which is relatively toxic and expensive. In particular, lithium nickel manganese cobalt oxide (LiNixMnyCozO2, referred to as NMC) materials, with a nickel content of � 80%, are able to deliver a high discharge capacity of 200
Lithium–ion batteries have been researched since 1976 and have become a general power source for portable electronic devices and electric vehicles [1,2]. Although research and commercialization of battery material have been going on for more than 40 years, the main concerns of energy–storage related companies are improving energy density, cycle life, and thermal stability and lowering the price of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Min), [email protected] (Y. Shin). 1 Contributed equally. https://doi.org/10.1016/j.jpowsour.2019.227395 Received 16 August 2019; Received in revised form 18 October 2019; Accepted 1 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Sangjin Maeng, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227395
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mAh g 1 at 4.3 V [3–6]. However, as a high–nickel layered cathode material, NMC811 has some drawbacks, such as fast capacity fading, short cycle life, and low thermal and structural stability. These disadvantages are due to the high nickel content, which results in more reactive surface oxygen, which in turn reduces the stability of the material when it is exposed to the electrochemical charge–discharge extended cycles [7–9]. Various methods have been proposed to solve these problems. In the most commonly attempted method, a high–nickel cathode particle surface is coated with a metal oxide, composed of components such as sodium, potassium, magnesium, calcium, strontium, nickel, cobalt, silicon, tita nium, boron, aluminum, tin, manganese, chromium, iron, vanadium, zirconium, germanium, gallium, and the like. In this approach, materials coated on the surface of the particles are often electrochemically inac tive, thus reducing the capacity. Furthermore, since the surface coating is formed with a very thin layer, it is difficult to completely protect the internal material from side reactions with the electrolyte. Capacity loss and thermal stability are still issues, and low rate capacity and imped ance growth problems in charge–discharge cycles must be considered as well. Full–concentration gradient cathode material, in which the nickel concentration decreases and the manganese concentration increases linearly toward the particle surface, has received considerable attention because of its high capacity and improved structural stability. The ma terial design of a nickel–rich center and manganese–rich surface of a particle with linear compositional change leads to high capacity and better thermal stability, compared with normal NMC cathode materials [10,11]. However, it is challenging to synthesize a well–functioning full–concentration gradient cathode with an overall composition of more than 80% nickel. It is difficult to increase the nickel content of the overall composition to 80% or more when the goal is to form sufficiently stable manganese–rich surface such as NMC333 while maintaining a linear compositional change from the particle center to the particle surface. Because the occupied volume increases in proportion to the third power of the radius as the particle radius increases, a linear in crease of the manganese composition to the particle surface does not increase the overall nickel content, or it is difficult to provide a man ganese–rich composition of sufficient thickness. Therefore, in the case of a material with an overall composition of 80% or more nickel, the cor e–shell particle structure is more effective in optimizing electrochemical performance and enabling mass production than the full–concentration gradient particle structure. It has been reported that, compared to the full–concentration gradient material, the core–shell material can cause cracks inside the layered oxide particle by volume changes on electrochemical cycling because of the significant compositional difference between the core and the shell interface and the structural mismatch [12]. This leads to impeded lithium ion diffusivity and deteriorates the electrochemical cycling performance [13]. The physical deformation of cathode particles occurs primarily in the application of strong pressure to increase the electrode’s loading amount, and secondarily in the delithiation and lithiation charge–discharge cycle by the volume change of primary particles, which weakens the bonding force between the primary par ticles and lowers the mechanical strength of the secondary particles. Through these processes, when the microstructural change of cathode particles occurs, the reactive area of the electrode gradually increases and the capacity fading begins. During charge–discharge cycling, highly reactive impurities such as water or carbon oxides induce surface im purities of cathode particles, which can be a defect in the lithium–ion batteries [14–18]. To investigate the effect of the physical deformation of cathode particles, Kim et al. studied the relationship between the mechanical strength of secondary particles and the capacity fading of the cathode with respect to cycle time [19] and found that lithium ion insertion and extraction result in mechanical stress on the high–nickel particles and lead to micro–cracks. Recently, the failure mechanism of a high–nickel cathode with a nickel content of 88%
(LiNi0.88Mn0.09Co0.03O2) was investigated with compressive tensile strength tests and internal pressure measurement [20]. However, a systematic and statistical study on the mechanical strength and binding force of core–shell (LiNi0.9Mn0.05Co0.05O2–LiNi0.33Mn0.33Co0.33O2) structured NMC811 particles has not been reported. In this work, a series of core–shell structured cathode materials was prepared at various calcination temperatures; the nickel–rich composi tion (LiNi0.9Mn0.05Co0.05O2) was the core, and the manganese–rich composition (LiNi0.33Mn0.33Co0.33O2) was the shell. The overall composition of the prepared core–shell materials was controlled as LiNi0.8Mn0.1Co0.1O2 by adjusting the ratio of the core–shell composition to have the same content of nickel, manganese, and cobalt as commer cially available homogenous NMC811. Those core–shell structured NMC811 materials were synthesized to improve the electrochemical performance of the commercial homogenous NMC811, such as capacity loss, low thermal stability, and impedance growth, as well as to inves tigate the effect of morphology of primary particle and structural het erogeneity of secondary particle on particle mechanical strength, which has not yet been studied. Thus, a series of core–shell structured NMC811 materials prepared at calcination temperatures of 700, 740, 760, 780, and 800 � C and the commercial homogeneous NMC811 material were tested to identify the electrochemical performance by coin half cell and to investigate mechanical failure and compressive stress–strain curve by a nanoindenter. This study observed fairly good mechanical strength as well as a substantial improvement in the electrochemical performance of the prepared core–shell structured NMC811, compared to the commer cial homogeneous NMC811. 2. Experiments 2.1. Synthesis of core–shell structured NMC811 cathode A precursor of core–shell structured NMC811 cathode materials, consisting of [Ni0.90Co0.05Mn0.05](OH)2 in the core and [Ni0.33C o0.33Mn0.33](OH)2 in the surface, was prepared via a coprecipitation method using a 20 L batch reactor. A nickel–rich aqueous solution, as a core composition consisting of NiSO4⋅6H2O, CoSO4⋅7H2O, and MnSO4⋅5H2O (molar ratio of Ni:Co:Mn, 90:5:5), from the first storage tank was fed into the batch reactor, which was filled with a certain amount of deionized water, ammonium hydroxide (NH4OH) solution (aq.), and sodium hydroxide (NaOH) solution (aq.) in a nitrogen atmo sphere. Simultaneously with the injection of the nickel–rich aqueous solution, a 10 mol L 1 NaOH solution (aq.) for pH adjustment (molar ratio of NaOH to transition metal, about 1.5) and a 3.8 mol L 1 NH4OH solution (aq.) for chelating purposes (molar ratio of NH4OH to transition metal, 1.0) were fed into the reactor separately. Through the first–stage coprecipitation process, the precursor with the core composition grew to the desired particle size. As the second–stage coprecipitation process, a manganese–rich aqueous solution, as a shell composition consisting of NiSO4⋅6H2O, CoSO4⋅7H2O, and MnSO4⋅5H2O (molar ratio of Ni:Co:Mn, 33:33:33), from the second storage tank was fed into the reactor instead of the nickel–rich aqueous solution from the first storage tank. In this process, the shell precursor composition, [Ni0.33Co0.33Mn0.33](OH)2, was deposited on the core precursor particle, [Ni0.90Co0.05Mn0.05](OH)2, formed through the first–stage coprecipitation process. The obtained precursor, which is [Ni0.8Co0.1Mn0.1](OH)2 as an overall composition, was filtered, washed, and dried for 20 h at 100 � C. In the next step, the dried precursor was mixed with LiOH⋅H2O, and portions of the mixture were calcined at 700, 740, 760, 780, and 800 � C for 20 h, each under oxygen flow. The core–shell structured NMC811 cathode materials thus obtained were named CS700, CS740, CS760, CS780, and CS800 ac cording to their clacination temperatures. 2.2. Material characterization The particle morphologies of commercial homogeneous NMC811 2
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and synthesized core–shell NMC811 materials were observed with scanning electron microscopy (SEM) (Phenon XL), and elemental map ping was performed with an energy dispersive X–ray spectroscopy (EDX) with a probe diameter of 1 μm and an acceleration voltage of 15 kV. Samples for cross–sectional element mapping and line scanning were prepared by embedding the powders in an epoxy and polishing them to investigate the compositional change of nickel, cobalt, and manganese within a particle qualitatively and quantitatively. EDX line mapping profiles for nickel, cobalt, and manganese were obtained. Note that the cross–sectional EDX mapping cannot provide actual element composi tion of the detected area. However, the cross–sectional EDX line scan ning shows the element ratio of the synthesized particle from the core to the outer shell.
The optical microscope measured particle size and classified mechanical failure as shown in Fig. 1(c). 3. Results and discussion 3.1. Particle structure and morphology characterization A series of core–shell structured NMC811 materials of overall composition LiNi0.8Mn0.1Co0.1O2, consisting of nickel–rich core composition LiNi0.9Mn0.05Co0.05O2 and manganese–rich shell composi tion LiNi0.33Mn0.33Co0.33O2, were synthesized through a coprecipitation reaction and a calcination process between 700 and 800 � C. Material characterization was conducted to confirm that the prepared powder had a heterogeneous particle structure with a core–shell shape as designed. First of all, for visual confirmation, EDX spectral mapping of nickel, cobalt, and manganese was performed to determine whether the elemental distribution of the core and shell of the particle matches the designed schematic diagram of the particle, shown in Fig. 2(a). Fig. 2(b) shows the starting and ending points set for the cross–sectional line scan from the particle center to the particle surface. The line scan results clearly show the presence of core and shell with dramatic compositional changes within the synthesized core–shell structured NMC811 particle [Fig. 2(c)]. The cross–sectional line scan results for the selected single particle demonstrate that the nickel concentration on the particle sur face is lower than that in the center of the particle, while the manganese and cobalt concentrations on the particle surface tend to be higher than those in the center of the particle. The center composition of the particle, consisting of 90% nickel, 10% manganese, and 10% cobalt, is main tained constant up to the vicinity of the surface, then shows a steep step change in composition, exhibiting a rapid movement toward the surface composition, consisting of 33% nickel, 33% manganese, and 33% cobalt, of the particle. The concentration of each element corresponds to the tendency of the schematic particle design shown in Fig. 2(a). However, closer examination reveals that the concentration of nickel in the synthesized particle is 90% at the center of the particle, then decreases to about 80% near the surface of the particle, and rapidly decreases to 33% at the shell region. The concentration of manganese and cobalt in the synthesized particle is also 5% at the center of the particle and increases to about 10% near the particle surface and to 33% in the shell area. This means that the concentration distributions of nickel, manganese, and cobalt inside the synthesized particles do not exactly match the schematic particle design of Fig. 2(a), which was planned for the synthesis of the spherical core–shell structured NMC811. This structural and compositional difference between the designed particle and the actually synthesized particle is mainly a result of incomplete coprecipitation process. This is because the synthesis of coreshell structured particles with uniform size and spherical shape is challenging due to too many process variables such as coprecipitation reactor geometry, impeller shape, stirring speed, reaction pH, reaction temperature, residence time, feed concentration, feed flow rate, and feed injection type. Due to these unoptimized process variables that lead to incomplete coprecipitation, core-shell particles have a non-spherical shape and size distribution, resulting in an Inconsistent shell thickness at each particle and at each location on the particle surface. Although this technical challenge makes it difficult to achieve the desired uniform spherical core-shell particles in realistic coprecipitation process, the result of the cross–sectional element map of all crowded particles in Fig. 2(d–g) shows that nickel–rich cores of all particles are well stacked with manganese–rich shells. Because of the limitation of the equipment used for cross–sectional element mapping, the increase in the concen tration of the cobalt at the particle surface in Fig. 2 (g) is not as obvious as in the case of manganese. However, cobalt–rich shells are formed, as shown in the line scan result inside a single particle [Fig. 2 (c)]. In addition, the overall chemical composition of the core–shell structured CS700 (calcined at 700 � C) was identified as LiNi0.796Mn0.101Co0.102O2 by inductively coupled plasma mass spectroscopy (ICPMS), which
2.3. Electrochemical test The commercial homogeneous NMC811 and the synthesized core– shell structured NMC811 powders were made into laminates comprising 90 wt % active material, 5 wt % super P carbon black, and 5 wt % pol yvinylidene fluoride (PVDF) binder dissolved in N–methyl–2–pyrrolidone (NMP). The prepared slurry was spread onto aluminum foil and then dried in a vacuum oven at 80 � C. Prior to coin cell assembly, the punched electrodes were dried in a vacuum oven to remove moisture contained in the electrodes. The dried electrodes were calendered with a press and then assembled into a type 2032 half–coin cell in argon–filled glove box using lithium foil as a counter electrode and Celgard 2325 as a separator. The electrolyte used was 2 M LiPF6 in a mixture solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with a volume ratio of 3:7. The assembled coin cells were then tested using a MACCOR cycler (Series 4000) between 3.0 and 4.4 V at a constant current of 0.1 C rate (20 mA g 1) and room temperature for 200 cycles. The thermal stability was measured by differential scanning calorimetry (DSC). The type 2032 half–coin cells containing the positive electrode materials were charged at a constant voltage of 4.4 V and a constant current of 0.1 C rate (20 mA g 1) and then disassembled in argon–filled glove box. The 4–5 mg positive active material samples, including the aluminum foil and the electrolyte, were injected into a high–pressure nickel–steel DSC pan with a gold–plated copper seal. The measurement was carried out using an STA 449 F3 Jupiter (NETZSCH, Germany) with a temperature scanning rate of 5 � C min 1. 2.4. Nanoindenter Nanoindentation tests were performed by using a commercial nanoindentation system (Hysitron TI 950, Bruker, USA). The indenter uses a capacitive transducer capable of operating with a load and displacement resolution of 1 nN and 0.1 nm and reaching a maximum of 18 mN in load and 5 μm in displacement. The precision of load feedback allows an indentation test in which the particles are subjected to a constant load increment per time, 0.3 mN s 1. A high–resolution optic microscope with a color charge coupled device (CCD) camera was incorporated into the machine for visualizing the particles and selecting the indenting position. A conical diamond tip with a 50 μm radius was installed on the transducer. The indentation experiments were conducted 15 times for 5 types of the core–shell structured NMC811 and 40 times for the commercial homogeneous NMC811 because the commercial one has a large distri bution in particle size. All indentation experiments were performed in air at ambient temperature (24 � C). The particles were spread out on a tungsten carbide holder, which has high hardness and good surface roughness. Indented particles were randomly selected and aligned with the center of the indenter tip by using the optical microscope. Once the diamond tip approaches and contacts the particle, the transducer detects reaction force and indentation measurement starts, as shown in Fig. 1 (b). Measuring the load in terms of displacement, the nanoindenter presses the particle with the diamond tip until reaction force releases. 3
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Fig. 1. (a) Schematic diagram of experiment approach with nanoindenter; (b) load–displacement (L–D) curves for indentation phases; and (c) NMC811 particles before and after a mechanical failure.
Fig. 2. Material characterization of the core–shell structured NMC811 (CS700): (a) schematic particle deign; (b) SEM; (c) cross–sectional line scan results of single particle; (d) cross–sectional elemental maps showing the distribution of (e) nickel, (f) manganese, and (g) cobalt in crowded particles.
indicates that it was successfully synthesized with a composition consistent with the schematic particle design. Fig. 3(a–e) show SEM images of the core–shell structured NMC811 cathode materials (CS700, CS740, CS760, CS780, and CS800) obtained by calcining a core–shell structured NMC811 precursor material at 700, 740, 760, 780, and 800 � C, respectively, after mixing with the lithium source. For comparison of the morphology of the synthesized core–shell structured NMC811 materials, the SEM of a commercially available homogeneous NMC811 material is shown in Fig. 3(f). The final products maintain the morphology, particle size, and particle size distribution of their spherical precursors, despite the different calcination tempera tures. The particle sizes of all synthesized core–shell structured NMC811 materials are about 5–10 μm as secondary particles; much smaller
primary particles are tightly packed into the secondary particles. The difference in size and shape of the secondary particles of all the core– shell structured NMC811 materials is negligible, but there are other characteristics about the primary particles. In the case of low–temperature calcination at 700 � C, the morphology of the primary particles has a sharp rod–like shape, its long axis is about 1 μm, and its short axis is about 200 nm [Fig. 3(a)]. As the calcination temperature increases to 800 � C, the length of the long axis of the primary particles is still about 1 μm, but the length of the short axis gradually increases to about 600 nm and the particle edge sharpness tends to decrease [Fig. 3(e)]. Thus, the formation of larger primary particles at higher calcination temperatures shows the possibility that the transition metal migrates during the heating process, resulting in a 4
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Fig. 3. SEM images of the core–shell structured NMC811 materials and (f) a commercial homogeneous NMC811 calcined at (a) 700 � C, (b) 740 � C, (c) 760 � C, (d) 780 � C, and (e) 800 � C.
slight change in particle morphology at the edge [21]. For this reason, in order to synthesize cathode particles in a more complete form of cor e–shell structure and to achieve improved electrochemical performance, the calcination process must be optimized to minimize the movement of the transition metal in the radial direction of the particles. Interestingly, the shape of the primary particles of the commercial homogeneous NMC811 [Fig. 3(f)] is similar to that of CS760 or CS780 [Fig. 3(c) or 3 (d)]. This means that the calcination temperature of the commercial homogeneous NMC811 is estimated to be between about 760 and 780 � C.
breaking force of the core–shell structured NMC811 materials are concentrated at 4.7–5.2 μm and 4.1–4.3 mN, respectively, as listed in Table 1. The commercial homogeneous NMC811 material with a similar particle average size of 4.9 μm has a lower value of 3.7 mN. The me chanical strength of the core–shell structured NMC811 materials is equivalent to or better than that of the commercial homogeneous NMC811. While calcination temperature influences the morphology of the primary particle, the mechanical strength of the secondary particle is consistent regardless of calcination temperature. It turns out that het erogeneity and binding force between the core and the shell do not degrade mechanical strength. The nominal compressive stress–strain behaviors were calculated to compare the mechanical property of the cathode particles with respect to particle size. The volume and Poisson’s ratio of the particles would be continuously varied in deformation of particles under compression because of the geometry and large defor mation [22]. According to the classical Hertz theory [23,24], the contact of the two elastic spheres with radii R1 and R2 can be derived by Eq. (1):
3.2. Mechanical properties Mechanical failure and stiffness of the core–shell structured and commercial homogeneous NMC811 particles were measured from L–D. curves. Fig. 4 shows the L–D curves for the particle size of the six types of NMC811 materials. Once the diamond tip contacts a particle, the reac tive load rises sharply along with the displacement. Mechanical failure of the particle occurs beyond the breaking force, and the reactive load becomes constant. Fig. 4 clearly indicates that the particle size is a dominant factor determining the breaking force. The breaking force is proportional to the particle size regardless of the type of NMC811 ma terial. This result means that heterogeneity of the core–shell structured NMC811 does not influence mechanical strength. The slope on the L–D curves represents the stiffness of the secondary particle. Therefore, it is clear that the stiffness of the core–shell structured NMC811 materials is higher than that of the commercial homogeneous NMC811. The particle with high stiffness will more likely show small displacements under the stress in calendaring process or the internal stress induced by particle volume change while charging and discharging. The small displacement would reduce the possibility of creating micro–cracks in particles. Fig. 5 shows a comparison of the breaking force of the core–shell structured and the commercial homogeneous NMC811 materials with respect to particle size. The breaking force of the commercial homoge neous NMC811 particle is linearly proportional to the particle size. The linear asymptote of the commercial homogeneous NMC811 particle presents 1.20 kN m 1, which is lower than the breaking forces of the core–shell structured NMC811 materials. The mean of particle size and
1
3
P ¼ Kc R2c h2c
(1)
where Kc, Rc, and hc are the effective modulus, the effective curvature, and the contact deformation, respectively. The effective modulus and curvature are calculated by Eqs. (2) and (3): � 1 � 4 1 v21 1 v22 Kc ¼ þ (2) 3 E1 E2 Rc ¼
R1 R2 R1 þ R2
(3)
where v1, v2, E1, E2, R1, and R2 are Poisson’s ratio, Young’s modulus, and the principal radii of the two spherical objects, respectively. The contact problem of two sphere objects, Eq. (1), becomes Eq. (4) as stress–strain form:
σc ¼
Kc
π
where 5
3
ε2c
(4)
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Fig. 4. L–D curves for the core–shell structured and commercial NMC811 materials: (a) CS700; (b) CS740; (c) CS760; (d) CS780; (e) CS800; and (f) a commercial homogeneous NMC811 material with respect to particle size. Table 1 Comparison of breaking force of the commercial homogeneous and the core– shell structured NMC811 materials with similar particle sizes. Type Commercial CS700 CS740 CS760 CS780 CS800
σc ¼
P
πR2c
Particle size (μm)
Breaking force (mN)
Average
Standard deviation
Average
Standard deviation
4.866 4.938 5.147 5.213 4.724 5.127
0.738 1.025 0.479 0.648 0.662 0.783
3.696 4.363 4.127 4.268 4.299 4.176
1.223 1.122 0.962 0.955 1.139 0.855
and εc ¼
hc Rc
Are the nominal compressive stress and strain, respectively. As the effective modulus, Kc, is only determined by the material property, Young’s modulus, and Poisson’s ratio, the compressive stress–strain behavior can be one of the constitutive mechanical properties of the particles. The slope in the stress–strain curve specifies the effective modulus of the tested particle. Fig. 6 shows that the compressive stress–strain curves are indepen dent of particle size regardless of the type of NMC811 material. What is remarkable is that the commercial homogeneous NMC811 has the
Fig. 5. Breaking force of the core–shell structured and commercial homoge neous NMC811 materials with respect to particle size.
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Fig. 6. Compressive stress–strain curves for the core–shell structured and commercial NMC811 materials: (a) CS700; (b) CS740; (c) CS760; (d) CS780; (e) CS800; and (f) a commercial homogeneous NMC811 with respect to particle size.
smoothest slope, indicating the softest particles. These results show that the core–shell structured NMC811 particles have better mechanical strength, resist breakage during the calendaring process, and are more likely to mitigate the formation of micro–cracks or fractures of the particles during charge–discharge cycles than the commercial homo geneous NMC811.
NMC811, during the initial 40 cycles, the coulombic efficiency decreases and then increases again. To understand this phenomenon, it is worth noting that the mechanical strength of this material is 15% lower than the CS700 [Table 1]. This results in a higher surface area due to the more easily caused particle damage, which leads to an irreversible reaction with electrolyte during charge and discharge processes. The CS700 shows a significantly improved lithium ion intercalation stability with a capacity retention of 76.6% after 200 cycles, and the commercial ho mogeneous NMC811 exhibits capacity retention of only 39.6% after the same cycling period. In addition, as shown in the charge-discharge voltage profiles [Fig. 7(b)–(c)], the CS700 demonstrates improved voltage stability as well as long-term capacity retention. The CS700 demonstrates this encouraging capacity retention and voltage stability result because it is covered with more stable manganese–rich composi tion, LiNi0.33Mn0.33Co0.33O2, which prevents direct contact with the electrolyte of the core composition, LiNi0.9Mn0.05Co0.05O2, during cycling and protects the particle surface. The heterogeneous character of the core–shell structured NMC811 makes the initial discharge capacity lower than that of the commercial homogeneous NMC811. But enables a higher discharge capacity after 30 cycles and improves cycle perfor mance by excellent surface stability. In particular, the coulombic effi ciency of the CS700 is superior to that of the commercial homogeneous
3.3. Electrochemical performance The cycling performance of the commercial homogeneous NMC811 and the core–shell structured CS700 materials was measured at a rate of 0.1 C (20 mA g 1) for 200 cycles by type 2032 half–coin cells employing lithium metal as the counter anode electrode, as shown in Fig. 7(a–c). For the initial discharge capacity, the commercial homogeneous NMC811 delivers a high initial discharge capacity of 202 mAh g 1 at the voltage window between 3.0 and 4.4 V, while the CS700 exhibits a reduced initial discharge capacity of 175 mAh g 1 because of insuffi cient initial activation of the cathode. This late activation, due to structural inconsistency in the interfacial region of the two compositions that make up the heterogeneous structure of the particles, increases the capacity of the CS700 and causes a change in coulombic efficiency during the initial 15 cycles. In the case of the commercial homogeneous 7
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Fig. 7. Cycling performance of the CS700 and commercial homogeneous NMC811 materials: (a) discharge capacity and coulombic efficiency; (b) voltage profile of CS700; and (c) voltage profile of commercial homogeneous NMC811. Type 2032 half–coin cells were tested at the voltage window between 3.0 and 4.4 V at 30 � C by applying a constant current density of 0.1 C rate (20 mA g 1) for 200 cycles. The rest time between charge and discharge was set to 5 min. Results were averaged with 2 sigma standard deviation.
NMC811 in the initial 50 cycles, as well as up to 200 cycles. Given this electrochemical performance, the core–shell structured cathode is a practical and effective approach to increasing the cycle life of high –nickel NMC materials of the same overall composition. The prepared core–shell structured NMC demonstrated superior cycling performance in the type 2032 half–coin cell test, as well as higher stiffness and compressive stress–strain in nanoindentation tests compared with the commercial homogeneous NMC811. To reconfirm the consistency of these results, the cross–sectional SEM images of the cathode electrodes were examined by disassembling the type 2032 half–coin cells used in the cyclability test shown in Fig. 7(a–c) after 200
cycles. Fig. 8(a–d) shows the morphology and microstructure of particles from cathode electrodes of the commercial homogeneous NMC811 and CS700 before cycling and after 200 cycles. As shown in Fig. 8(a) and (c), the electrodes of both of the commercially available homogeneous NMC811 and CS700 exhibit well–maintained spherical secondary par ticles without cracking, even though the laminate formation process includes a calendaring process that can trigger the breakage of particles. Considering the noncontinuity of the coprecipitation process and the compositional heterogeneity of the two compositions forming the cor e–shell particle structure, the shell of the core–shell structured CS700 can be broken or separated by calendaring pressure during laminate
Fig. 8. Cross–sectional SEM images of the commercial homogeneous NMC811 electrodes (a) before cycling and (b) after 200 cycles, the CS700 electrodes (c) before cycling and (d) after 200 cycles, and (e) DSC curves for heat flow of the commercial homogeneous NMC811 and CS700 samples from the electrodes charged to 4.4 V at a constant current density of 0.1 C rate in the presence of electrolyte. 8
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formation. However, as shown in Fig. 8(c), the secondary particles of CS700 remain on the electrode while maintaining their initial morphology. This result is consistent with the breaking force of the nanoindentation test shown in Fig. 5 and reconfirms that the commercial homogeneous NMC811 and CS700 have similar particle strength to resist cracking. Fig. 8(b) and (d) are the cross–sectional SEM images of the com mercial homogeneous NMC811 and the CS700 electrodes after 200 cy cles. Nearly all the cyclized particles of the commercially homogeneous NMC811 electrode were completely pulverized after 200 cycles, as ex pected from its relatively poor capacity retention. As shown in Fig. 8(b), the bond between the primary particles inside the secondary particles was significantly weakened and separated. Thus, a major cause of the capacity loss of commercial homogeneous NMC811 appears to be elec trode deterioration due to secondary particle fracture during the long –term charge–discharge cycle. On the other hand, as shown in Fig. 8(d), the core–shell structured CS700 electrode maintains the initial morphology of the particles even after 200 cycles, which is one reason why it shows solid capacity retention. Therefore, the formation of micro–cracks in the layered oxide particles by the volume change during electrochemical cycling, due to the important compositional difference between the core and the shell interface, is not significant within at least 200 cycles compared to the homogeneous one. In conclusion, for several hundred charge–discharge cycle levels, particle breakage due to the core–shell structure does not occur seriously, and the core–shell struc tured material has superior capacity retention compared to the homo geneous one.
4. Conclusion The high–nickel core–shell structured NMC811 materials were suc cessfully synthesized via a coprecipitation method composed of the core LiNi0.9Mn0.05Co0.05O2 and the shell LiNi0.33Mn0.33Co0.33O2; the overall composition is LiNi0.8Mn0.1Co0.1O2, which is like a commercial homo geneous NMC811. The SEM–EDX cross–sectional mapping and line scanning revealed that the prepared core–shell structured particle has the local compositional change from the particle center to the surface consistent with the designed particle structure. As the calcination tem perature increases from 700 to 800 � C, the primary particles of the core–shell structured NMC811 materials become thicker, but all the secondary particle breaking forces are similar to those of the commercial homogeneous NMC811 in nanoindentation testing. The stiffness and compressive stress–strain of the core–shell structured NMC811 are higher than those of the commercial homogeneous one. In the cathode laminate formation process to measure electrochemical performance, the morphologies of secondary particles of both commercial homoge neous NMC811 and core–shell structured CS700 electrodes are well –maintained without breakage after calendaring. On the other hand, after 200 cycles, the secondary particles of the commercial homoge neous NMC811 are completely pulverized, while the core–shell struc tured NMC811 particles are mostly maintaining their initial spherical morphologies. Thus, at the level of several hundred cycles, the heterogeneity of the core–shell structured NMC811 does not weaken the mechanical strength of the secondary particle, and the binding force between the core and the shell is strong enough to prevent micro–crack formation inside the particle. The core–shell structured NMC811 has a lower initial discharge capacity than the commercial homogeneous NMC811 but shows dramatically improved cycle stability (76.6% capacity retention) over that of the commercial homogeneous NMC811 (39.6% capacity reten tion) after 200 cycles. The DSC results also show that the onset tem perature of the core–shell structured NMC811 is 23 � C higher and represents 22.8% less heat generation than the commercial homoge neous NMC811. These results indicate that the core–shell structured heterogeneous particle composed of core LiNi0.9Mn0.05Co0.05O2 and shell LiNi0.33Mn0.33Co0.33O2 demonstrates improved thermal stability, sufficiently strong mechanical strength, and high capacity retention, as well as a high possibility of continuous manufacturing process for high volume production which is more difficult to achieve in the case of full–concentration gradient particle.
3.4. Thermal properties When cathode electrode materials are being considered for use in commercial applications, especially electric vehicles, the thermal sta bility as well as capacity retention of these materials is of critical importance. Better thermal stability improves the cycle performance and reduces the cooling requirement of the battery pack. Fig. 8(e) il lustrates DSC curves for the heat flow of cells with the commercial ho mogeneous NMC811 and CS700 cathode electrodes charged to 4.4 V with 0.1 C rate in the presence of the electrolyte. The onset temperature of the commercial homogeneous NMC811 electrode is 229 � C, while that of the CS700 is increased to 252 � C, 23 � C higher. The heat generation side of the commercial homogeneous NMC811 electrode is 1603 J g 1, whereas that of the CS700 is decreased to 1238 J g 1, 365 J g 1 less. In a typical NMC material, the lower the nickel content, the higher the onset temperature and the lower the heat generation. Therefore, both mate rials are expected to be similar in onset temperature and heat generation because the overall composition is the same. However, the onset tem perature of the core–shell structured CS700 is 23 � C higher and repre sents approximately 22.8% less heat generation. As can be seen in Fig. 7 (a–c), for the initial discharge capacity, the commercial homogeneous NMC811 provides a high initial discharge capacity of 202 mAh g 1, while the CS700 exhibits an initial discharge capacity of 175 mAh g 1, which is 13% lower. This means that the CS700, which has an overall composition of LiNi0.796Mn0.101Co0.102O2, has less lithium compared to the commercial homogeneous NMC811 after discharging, resulting in less overcharge and more structural stability. But the CS700’s 22.8% less heat generation and higher onset temperature results clearly reveal that the thermal stability of the outer shell LiNi0.33Mn0.33Co0.33O2 is responsible for the significant improvement of the core–shell structured CS700 by protecting the core LiNi0.9Mn0.05Co0.05O2. In other words, the improved thermal stability of the core–shell structured NMC811 is caused by the thermally stable outer shell that prohibits the highly delithiated core from contacting liquid electrolyte directly and thereby suppresses the oxygen release from the host structure of the delithiated core. The manganese–rich composition of the shell improves the thermal stability as well as lithium intercalation stability during the electro chemical cycle, thereby achieving higher capacity retention.
Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Support from the U.S. Department of Energy Office of Vehicle Technologies, particularly from Peter Faguy and Dave Howell, is gratefully acknowledged. This work was partially carried out at the Materials Engineering Research Facility at Argonne National Labora tory, which is supported within the core funding of the Applied Battery Research for Transportation Program. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory. Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE–AC02–06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid–up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. 9
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Glossary CCD: charge coupled device CS700: core–shell structured NMC811 precursor material calcined at 700 � C DSC: differential scanning calorimetry EC: ethylene carbonate EDX: energy dispersive X–ray spectroscopy EMC: ethyl methyl carbonate ICPMS: inductively coupled plasma mass spectroscopy L–D: load–displacement (curve) NMC: lithium nickel manganese cobalt oxide (LiNixMnyCozO2) NMC811: high–nickel cathode material (LiNi0.8Mn0.1Co0.1O2) NMP: N–methyl–2–pyrrolidone PVDF: polyvinylidene SEM: scanning electron microscopy
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