Synthesis of Micron-Sized LiNi_0.8Co_0.1Mn_0.1O₂ and Its Application in Bimodal Distributed High Energy Density Li-Ion Battery Cathodes

Abstract

The uniform and smaller-sized (~3 μm) LiNi_0.8Co_0.1Mn_0.1O₂ (SNCM) particles are prepared via a fast nucleation process of oxalate co-precipitation, followed by a two-step calcination procedure. It is found that the fast nucleation by vigorous agitation enables us to produce oxalate nuclei having a uniform size which then grow into micron-particles in less than a few minutes. The impacts of solution pH, precipitation time, calcination temperature, and surface modification with ZrO₂ on the structural, morphological, and electrochemical properties of SNCM are systematically examined to identify the optimal synthetic conditions. A novel bimodal cathode design has been highlighted by using the combination of the SNCM particles and the conventional large (~10 μm) LiNi_0.83Co_0.12Mn_0.05O₂ (LNCM) particles to achieve the high volumetric energy density of cathode. The volumetric discharge capacity is found to be 526.6 mAh/cm³ for the bimodal cathode L80% + S20%, whereas the volumetric discharge capacity is found to be only 480.3 and 360.6 mAh/cm³ for L100% and S100% unimodal, respectively. Moreover, the optimal bi-modal cathode delivered higher specific energy (622.4 Wh/kg) and volumetric energy density (1622.6 Wh/L) than the L100% unimodal (596.1 Wh/kg and 1402.1 Wh/L) cathode after the 100th cycle. This study points to the promising utility of the SNCM material in Li-ion battery applications.

1. Introduction

In the past decades, stimulated by the increasing awareness of the energy crisis and eco-friendly conservation around the world, research with great physio-electrochemical energy storage devices such as lithium-ion batteries (LIBs), supercapacitors, etc., become a vital issue [1,2,3,4,5,6]. In comparison with the commercial LiCoO₂ cathode, Ni-rich layer-structured Li(Ni, Co, Mn)O₂ (NCM) has received noteworthy attention and is considered a suitable cathode for next-generation LIBs because of its lower cost, less toxicity, and high energy density. Abundant studies in the literature focused on challenges that obstruct the performance of the Ni-rich NCM cathodes, which include Li⁺/Ni²⁺ cation disordering during the synthesis, stress-induced microcrack formation triggered by substantial volume deviation through Li⁺ intercalation/extraction, and instability in air and moisture during storage. Significant efforts from the viewpoint of particle synthesis have been dedicated to enriching the structural parameters, as well as electrochemical properties of the Ni-rich NCM electrodes, including those focusing on bulk doping [7,8,9,10,11,12,13,14,15,16], surface modification [17,18,19], gradient concentration design [20,21,22], synthetic condition [23], calcination temperature [24,25], and calcination atmosphere [26]. Recently, Arun et al. [27] reported a review article based on the modification strategies used to improve the specific capacity and cycling stability of the NCM cathode materials in detail. So far, relatively less attention has been given to the issue of enhancing the volumetric energy and/or capacity density of the NCM cathode in the existing literature.

Currently, the hydroxide co-precipitation process in a continuously stirred tank reactor (CSTR) has most widely been applied for producing the precursors of commercial NCM cathode powders, which are secondary particles predominantly larger than 10 μm [28]. Although a large particle size offers the advantage of high tap density, it leads to inferior rate performance due to a long Li-ion diffusion path upon lithiation/delithiation. In addition, large particles are less capable of accommodating the stress arising from the volume variations associated with lithiation–delithiation, and the variation is known to increase with the Ni content of NCM [29]. As a result, microcracking along grain boundaries between primary grains within the interior has become one major fading mechanism for NCMs with Ni contents exceeding 80% [30]. The microcracking causes electrical isolation of part of the particle interior, leading to capacity fade.

Recently, there has been an increase in interest in the use of a few micron-sized single-crystal Ni-rich NCM particles, which are known to mitigate microcracking [31,32]. Moreover, the buildup of side reaction products and unpredicted phase transformations owing to the restricted electrolyte/electrode area can be alleviated by the sole structure of single crystal NCM [33]. However, the smaller size of around 3~4 µm is crucial for single crystal NCM particles to retain the rate capabilities due to the smaller surface area for Li insertion/exertion when compared with the conventional polycrystalline NCM. Moreover, the growth mechanism and methodology, such as the molten salt method, are not compatible with the existing synthesis processes, adding difficulties for industrialization. On the other hand, the formation of smaller polycrystalline NCM (SNCM) particles with a larger surface area will lead to the higher rate capability of the cathode via the shorter lithium diffusive pathway. Furthermore, the internal cracking on NCM particles will be minimized during the cycling mechanism.

Herein, a novel oxalate-based co-precipitation approach is developed to prepare uniform and micron-sized SNCM precursors by inducing fast nucleation. Other than the benefits of feasible preparation, the formation of metal constituents breaks down the precipitation rate and creates the nucleation, as well as the growth of the particles more controllable. The novel fast agitation mechanism is developed to control the uniformity and size of the nuclei formed during the oxalate-based co-precipitation approach for the first time. It is found that the fast nucleation by vigorous agitation enables producing oxalate nuclei having a uniform size, which then grow into micron-particles in less than a few minutes. The uniform and smaller particles with a diameter of around 3 μm are synthesized by optimizing the pH value, precipitation time, calcination temperature, and ZrO₂. The results show that ideal calcination temperature provides enhanced cycle life, which is highly viable compared to the conventional hydroxide-based co-precipitation route. Then, the novel bimodal cathode distributions are created with the blending combination of SNCM particles and commercially available larger NCM (LNCM) particles to achieve the high volumetric capacity density of the cathode, where the smaller SNCM particles are embedded into the vacancies between the larger LNCM particles. Remarkably, from the electrochemical analysis, a synergistic effect is found in particular blended ratios that intensely encourage the rate capability and cycle life, and this is favorable for fast-charging batteries. This novel research thoroughly investigates the mechanism and principle in the processing of SNCM, which unlocks a new way for synthesizing bimodal layered Ni-rich cathode materials in LIBs.

2. Materials and Methods

2.1. Material Synthesis

2.1.1. Preparation of Ni_0.8Co_0.1Mn_0.1C₂O₄·2H₂O Precursor

The precursor (Ni_0.8Co_0.1Mn_0.1C₂O₄·2H₂O) particles were produced via an oxalate co-precipitation route by using a batch reactor. Initially, the stoichiometric ratio of NiSO₄, CoSO₄, and MnSO₄ with a total amount of 0.2 M was dissolved in 200 mL of deionized (DI) water and denoted as Solution A. The base Solution B was separately made by mixing 0.22 M oxalic acid and 0.2 M NaOH with 0.05 M NH₄OH in 200 mL of DI water. Solution A and Solution B were heated at 50 °C, and the base solution was quickly poured into the metal ions solution under vigorous stirring at 600 rpm for 20 s. Then the mixed solution was stirred under 60 rpm for 3 h to make sure that the complete co-precipitation reaction. The precursors were gradually precipitated from 3 min to 3 h, under the constant pH = 2.0. The precipitates were thoroughly washed with DI water to eliminate the residual ions and then dried at 100 °C for 12 h in a vacuum oven.

2.1.2. Preparation of Smaller-Sized LiNi_0.8Co_0.1Mn_0.1O₂ Cathode

LiNi_0.8Co_0.1Mn_0.1O₂ (SNCM) was prepared via the two-step solid-state calcination procedure. The as-synthesized Ni-rich precursors were stoichiometrically mixed with LiOH (3 wt.% excess) and were kept in calcination at 700 °C for 12 h in an O₂-atmosphere horizontal quartz-tube furnace with a heating rate of 2 °C/min. Then, before the second-step calcination process, the oxide powders were ground well to avoid agglomeration of particles and then calcined at 750 °C for 12 h in an O₂ atmosphere with a heating frequency of 5 °C/min. The powder was cooled down naturally to room temperature and then finely sieved with a 400-mesh screen.

2.1.3. Preparation of ZrO₂-Treated and Bimodal Cathode

To make zirconium surface-treated SNCM, four different amounts of ZrO₂ (2.0, 1.0, 0.5, and 0.25 wt.% vs. the precursor) are selected and labeled as NCM-Zr_2.0%, NCM-Zr_1.0%, NCM-Zr_0.5%, NCM-Zr_0.25%, respectively. The precursors of SNCM were mixed with the appropriate amount of ZrO₂, and the mixtures were ground well for 10 min. Then the hand-mixed precursors were mechanically mixed for 20 min, using the Planetary Inter Mixer (Model: IMX-150). The well-mixed powder was transferred to the tube furnace, and the preparation temperature of 750 °C for 12 h was elected as the optimum temperature. The larger-sized commercial LiNi_0.83Co_0.12Mn_0.05O₂ (LNCM) was received from Industrial Technology Research Institute (ITRI). The different weight ratios of SNCM and LNCM (10:90, 20:80, 30:70, and 40:60 of SNCM to LNCM, denoted as L90% + S10%, L80% + S20%, L70% + S30%, and L60% + S40%) were blended well for at least 15 min to make the homogenous distribution of powders. The aforementioned process was performed in the Ar-filled glove box to avoid the effect of the ambient environment.

2.2. Physical Characterization

XRD measurements were performed on an X-ray diffraction instrument with Cu-K_α radiation (λ = 1.5418 Å) (Rigaku Ultima, Tokyo, Japan), with an accelerating voltage of 40 kV and at 40 mA of current. The scanning rate was typically fixed as 10 °/min in the range of 10 ° to 80 °. The inductively coupled plasma–mass spectrometry (ICP–MS, Agilent 7500ce, Santa Clara, CA, USA) was an analytical technique used for semi-quantitative analysis and isotope measurement. The surface morphology of the specimen was revealed by field-emission scanning electron microscopy (JSM) operated with 10 kV of accelerated voltage. Particle size distribution was performed by Horiba-Partica-Laser analyzer (Model: LA-950-V2, Horiba, Kyoto, Japan), and the detective range was from 40 nm to 2000 μm. A thermal analysis of SNCM-precursors was performed by using thermal gravimetric and differential scanning calorimetry (TGA–DSC) to realize the thermal behavior under an air atmosphere (Model: SDT-650, TA Discovery Instruments, New Castle, DE, USA) at a heating rate of 5 °C/min.

2.3. Electrochemical Characterizations

The electrode slurry was made of 90 wt.% SNCM powders, 5 wt.% super P (carbon black), and 5 wt.% PVDF binder (polyvinylidene fluoride) cast on the Al-foil current collector. After drying at 120 °C for 12 h, the dried electrodes were calendered by a roll-press machine to about 70% of the original value and then pierced into discs that are 12 mm in diameter. The average mass loading of the electrodes ranged from 8.5 to 15.2 mg/cm². The coin cell CR-2032 consisted of the active materials, such as a cathode, a lithium foil as anode, a 16 μm–thick polypropylene (PP, ND416Z) as a separator, and a 1.2 M LiPF₆ in ethylene carbonate/dimethyl carbonate (EC/DMC) in the proportion of 1:2 by vol.% with 4 wt.% fluoroethylene carbonate (FEC) as an electrolyte. The cells were fabricated in an Ar-filled glove box, of which the humidity and oxygen levels are lower than 0.01 and 0.5 ppm, respectively. Galvanostatic charge/discharge experiments, cycling stability, and rate performance profiles were performed from 2.8 to 4.3 V at ambient temperature on a Neware-battery analyzer.

3. Results and Discussion

3.1. Impact of pH Value

Figure 1a depicts the SEM image of the oxalate precursors under a pH value of 2. The SEM images and particle size values of another pH (ranging from 3 to 9) are given in Supplementary Figures S1 and S3a, respectively. To optimize the pH value, the constant co-precipitation time of 3 h was fixed for all the samples. As seen from the SEM photos, the particles have a uniform oval shape, with an average particle size of about 3–5 µm. There are no obvious differences between the shapes for the pH value below 6, whereas, beyond the pH value of 6, some irregular shapes appear with the increase of particle size, which can be ascribed to the slow reaction kinetics that leads to an inhomogeneous nucleation rate.

3.2. Effect of Co-Precipitation Time

Apart from altering the pH value, the oxalate precipitates are collected at different periods of co-precipitation time (3, 5, 10, 20, 30, 60, 120, and 180 min), as given in Supplementary Figure S2. To optimize the co-precipitation time, the constant pH value of 2 is fixed for all the samples. The SEM image of the oxalate precursors with the co-precipitation time of 3 h is shown in Figure 1b. At the initial stage (3 min), almost all precipitates reveal an identical shape to the optimized condition (pH = 2) in Supplementary Figure S2a, and the subsequent results also display a similar trend (from 5 to 180 min). Since the resemblance of average particle size at every testing stage (Supplementary Figure S3b), the co-precipitation time of 3 h is fixed to obtain a higher yield fraction. It is found that the fast nucleation by vigorous agitation enables the production of oxalate nuclei that have a uniform size (Figure 1c,e) and can grow into micron-particles in less than a few minutes, whereas the slow agitation leads to the formation of nuclei with an irregular shape with larger sizes (Figure 1d,f). Hence, considering the uniform morphology with smaller particle size and the high yield, the optimal synthetic parameter for the oxalate precursor is pH = 2, and co-precipitation time is 3 h.

3.3. Effect of Calcination Temperature

The DSC–TGA analysis of the as-prepared SNCM precursor in the temperature range from room temperature to 600 °C under air atmosphere is revealed in Figure 2a. As can be seen, TGA curves have two significant weight losses around 180 and 270 °C. The former indicates the release of crystal water, while the latter implies the oxidation of oxalate, which transforms into carbon dioxide. The DSC curve also displays a similar trend at 180 and 270 °C, which suggests an endothermic (release of crystal water) and exothermic (oxidation of oxalate) phenomenon, respectively. The entire reaction is demonstrated below:

$${\mathrm{Ni}}_{0.8}{\mathrm{Co}}_{0.1}{\mathrm{Mn}}_{0.1}{\mathrm{C}}_2{\mathrm{O}}_4·2{\mathrm{H}}_2\mathrm{O}\stackrel{\mathsf{\Delta }}{\to }{\mathrm{Ni}}_{0.8}{\mathrm{Co}}_{0.1}{\mathrm{Mn}}_{0.1}{\mathrm{O}}_{\mathrm{x}}+2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{CO}}_2$$

(1)

Consequently, according to the DSC–TGA result, the heating rate for the sintering process from 180 to 350 °C is adopted as 0.2 °C/min to decompose crystal water, as well as to oxidize oxalate in a much slower manner to prevent the internal force generated from particles, resulting in the breakage of particles. Moreover, owing to the reaction between LiOH and SNCM precursor (Figure 2b) to form a layered structure, the heating rate from 350 to 500 °C is also reduced to 0.5 °C/min to prevent local nucleation problems. Then the rise of temperature to 700 °C for 12 h is used to enhance the particle strength. After cooling down, the SNCM powder will be calcined to obtain a well-ordered layered structure.

Six different temperatures were chosen from 725 to 850 °C (with an increment of 25 °C), for 12 h, to study the effect of calcination temperature on SNCM materials. The stoichiometric compositions of synthesized SNCM powders are well-matched with the experimental scheme obtained from the ICP–MS result (Supplementary Table S1). The observed lower-stoichiometric-level lithium in the SNCM calcined at 825 °C and 850 °C are attributed to the desertion of lithium during high-temperature synthesis.

Figure 3a exhibits the powder XRD patterns of SNCM cathodes synthesized at various calcination temperatures from 725 to 850 °C. The corresponding samples are denoted as NCM-725, NCM-750, NCM-775, NCM-800, NCM825, and NCM-850. The characteristic diffraction pattern of the phase-pure NCM was detected without any notable secondary phases. The recorded XRD patterns could be matched to the α-NaFeO₂ layered structure with space group $R\overline{3}m$, except for NCM-725. It is found that the lower calcination temperature of 725 °C is not sufficient for the complete formation of the fcc lattice structure. The perfect splitting of (006)/(012) and (108)/(110) planes in Figure 3b,c (indicated by arrows) specifies that a formation of well-arranged hexagonal-lattice structure [34,35,36]. The above results confirmed that the main crystallinity of layer-SNCM is formed at calcination temperature above 725 °C. The integrated intensity ratio [I₍₀₀₃₎/I₍₁₀₄₎] is a significant factor to decide the degree of cation ordering owing to the resemblance between ionic radii of Ni²⁺ (0.69 Å) and Li⁺ (0.76 Å). Generally, the value of I₍₀₀₃₎/I₍₁₀₄₎ higher than 1.2 guarantees lower cation mixing [37,38,39], and SNCM calcined between 750 and 850 °C displays similar values of around 1.5. The cation mixing problem is minimized, and the intensity ratio slightly increases at higher sintering temperatures due to their high crystallinity.

In addition, with the rise of synthesis temperature, the value of FWHM (full width at half maximum) of (003) plane drops, indicating a growth of crystalline-size SNCM particles. The crystalline size is calculated from the Debye–Scherer formula and given in

Table 1. Crystallographic data and BET surface area of SNCM cathodes.

Temperature	725 °C	750 °C	775 °C	800 °C	825 °C	850 °C
Grain size (nm)	36.3	41.9	68.6	95.9	170.7	213.6
I(003)/I(104)	1.23	1.49	1.46	1.56	1.58	1.57
BET (m2/g)	4.11	3.23	2.96	1.49	1.29	0.83
Pore volume (cm3/g)	0.01304	0.01254	0.01084	0.0039	0.0029	0.0018
Pore diameter (nm)	10.73	15.01	14.44	14.64	11.28	22.97

. The average crystallite size is found to be increased from 37 to 213 nm for the sample calcined at 725 °C to 850 °C, respectively. The larger growth in grain size will prolong the length of the Li⁺/electron diffusive pathway, resulting in inferior rate performance and electrochemical kinetics, as discussed later. The N₂ desorption/adsorption curves and pore-size-distribution curves are displayed in Figure S4a,b, respectively. The SEM images of the SNCM powders prepared from 725 to 850 °C are given in Supplementary Figure S5. The higher calcination temperature inclines to give a smaller specific surface area on account of the larger primary particles, as well as an agglomeration of secondary particles. The observed trend is quite consistent with BET results, and the relevant results are tabulated in Table 1.

As given in Figure 4a, the primary oxidation-reduction curves at 0.1 C in the voltage window of 2.8–4.3 V for all the SNCM cells exhibit similar capacities and voltage profiles. The specific discharge capacities are found to be 197.8, 194.8, 200.2, 196.6, and 199.3 mAh/g for NCM-750, NCM-775, NCM-800, NCM-825, and NCM-850, respectively. Moreover, an initial coulombic efficiency is found to be 85.83, 82.98, 82.94, 82.37, and 81.98% for NCM-750, NCM-775, NCM-800, NCM-825, and NCM-850, respectively. The results validate that a higher annealing temperature provides better crystallinity, which could be the probable reason for the slight increase in initial capacity. Among all the samples, NCM-750 displays preferably high coulombic efficiency due to the high BET and larger pore-size distribution, which induces efficient Li⁺ diffusion.

All the SNCM cells show clear variations in terms of cycle number vs. specific discharge capacity at 0.3 C, as shown in Figure 4b. It is of great interest that the observed capacity retention at the end of 200 cycles decreases constantly with the raising of the preparation temperature. The specific discharge capacities are found to be 112.9, 126.9, 118.9, 107.8, 88.5, and 86.1 mAh/g for cells made with NCM-725, NCM-750, NCM-775, NCM-800, NCM-825, and NCM-850, respectively. The capacity retentions are found to be 72.84, 75.76, 70.90, 63.11, 52.27, and 50.68% for cells made with NCM-725, NCM-750, NCM-775, NCM-800, NCM-825, and NCM-850, respectively. Moreover, the cells made with the samples calcined at 750 and 775 °C display notable reversible capacity during the redox process and are still proficient in providing discharge capacities of 126.9 and 118.9 mAh/g at the end of 200 cycles, making it eye-catching for large-scale utilization. Comprehensive cycling data are demonstrated in Supplementary Table S2. The observed serious capacity fading above NCM-775 should be ascribed to the rise of electrode resistance attributed to the inner stress generated during the repetitive oxidation/reduction process, which results in harming the electrode materials. The calcination temperature of 750 °C is selected as the ideal temperature for the preparation of SNCM-active materials based on the specific reversible capacity and cycle stability.

3.4. Effect of ZrO₂ Modification

The powder XRD patterns of bare (NCM-750) and ZrO₂-surface-treated SNCM samples are presented in Figure 5a. All the characteristic planes can be matched with the typical structure of hexagonal α-NaFeO₂ (space group: $R\overline{3}m$) without a secondary phase. The obvious doublet of (006)/(012) and (108)/(110) planes show that all of the samples have a well-organized layered crystalline structure. The slight deviation of the (003) and (104) diffracted peaks to the lower angle side is due to the larger ionic radii of Zr⁴⁺ (0.72 Å) compared to the Ni²⁺/Ni³⁺ (0.69 Å/0.56 Å), Co³⁺ (0.545 Å), and Mn⁴⁺ (0.53 Å). Furthermore, from the value of I₍₀₀₃₎ to I₍₁₀₄₎, we can calculate the degree of cation ordering, which is 1.49 for the NCM-pristine and 1.56 for the NCM-Zr_0.5%, meaning that Li⁺/Ni²⁺ cation mixing decreases after the modification of ZrO₂. In terms of charge compensation, the minor parts of Ni³⁺ might be reduced to Ni²⁺ due to the incorporation of zirconium ions. However, they may act as a pillar in the lithium layer owing to the similar ionic radii between Li⁺ (0.76 Å) and Zr⁴⁺ (0.72 Å). It alleviates the subsequent relocation of Ni²⁺ to the Li layer and, thus, reduces the cation mixing and enhances the overall structural properties.

The existence of ZrO₂ on the particle surface is further identified by TEM, as shown in Supplementary Figure S6. An obvious ZrO₂ layer with a thickness around 5 nm can be observed in NCM-Zr_0.5% in comparison with NCM-pristine. From Supplementary Figure S7, we can see that the Ni-2p_3/2 spectra display two characteristic peaks, which are allotted to Ni³⁺ and Ni²⁺. The broad and bigger area of the Ni²⁺ peak in NCM-Zr_0.5% cathode compared to that of NCM-pristine determines the higher content of Ni²⁺ on the NCM-Zr_0.5% cathode’s surface. That is favorable for the layered crystal structure of NCM, which also confirms the results of XRD analysis. Otherwise, a characteristic peak of Zr-3d_5/2 appears at 180.98 eV, along with a Zr-3d_3/2 peak detected at 183.34 eV in the Zr- spectra, standing for the valence state of Zr, which is presented as +4 [9,40], further certifying that the ZrO₂ protective layers are effectively coated on the outer surface of the NCM-Zr_0.5% cathodes.

The first cycle charge–discharge curves of cells made with pristine and ZrO₂-modified SNCM cathodes are shown in Figure 5b. The initial discharge capacity values are found to be 197.8 and 194.2 mAh/g for the pristine and NCM-Zr_0.5% cathode, respectively. The long-term cyclability of cells made with bare and ZrO₂-surface modified cathodes is tested and given in Figure 5c. It shows that all the modified cells have a significant improvement in capacity retention except for NCM-Zr_2.0%, which shows a reversible capacity of only 126.73 mAh/g with 79.02% of retention. Meanwhile, the reversible capacities and capacity retention are found to be 146.75, 166.26, and 159.37 mAh/g and 85.86, 89.01, and 88.03%, for cells made with NCM-Zr_1.0%, NCM-Zr_0.5%, and NCM-Zr_0.25% cathodes, respectively. On the contrary, the NCM-pristine sample shows the reversible specific capacity of 139.51 mAh/g with 83.44% of retention at the end of the 100th cycle. The observed electrochemical data are compared with the literature reports, and the comparative table is demonstrated in Supplementary Table S3. Notably, it is concluded that the amount of ZrO₂ should be carefully optimized to avoid the insulating properties that would potentially deteriorate the performance of NCM cells. Based on the above results, the 0.5 wt.% ZrO₂–modified cathode is chosen as optimized and used for the investigations of bimodal studies.

3.5. Synergistic Effect of Bimodal Behavior

The preparation of bimodal cathode materials is carried out by directly mixing with the different blend ratios of SNCM and LNCM, as discussed in the experimental section. The schematic for the preparation of the bimodal cathode is given in Supplementary Figure S8a, in which the SNCM and LNCM particle sizes are found to be 3–4 μm and 10 μm, respectively. The SEM photos of other blended cathodes are given in Supplementary Figure S9. To investigate the variation of particle size of unimodal and bimodal materials, a particle-size-distribution analysis was applied to examine the diversification of L100% and one of the representatives of blended samples, L80% + S20%. From Supplementary Figure S8b, we can observe the broad peaks at about 14.94 μm for L100%, while for blended cathodes (L80% + S20%), the peak at 7.08 μm is obtained owing to the combination of different size NCM powders.

The SEM images of the electrode plane and cross-section mode are also observed to verify the homogeneity of blended materials after the preparation of slurry, as well as serve as the target for the measurements of electrode density. Figure 6 displays the normal and cross-sectional images of L100% and L80% + S20%, and it can be seen that the particles are well blended with carbon conductive and binder (Figure 6a,c). The electrode thickness is found to be 65.25 μm and 50.63 μm for the cell L100% and L80% + S20%, respectively (Figure 6b,d). As seen, the electrode thickness is decreased abruptly when even a trace amount of SNCM is blended with LNCM. The L100% shows an electrode density of 2.33 g/cm³, whereas the value gradually enhances and reaches up to 2.61 g/cm³ for the L80% + S20% sample, indicating that suitable amounts of blended cathodes would have a positive effect on increasing the electrode density.

Figure 7a reveals the long cycle behavior (0.3 C at room temperature) according to specific and volumetric discharge capacity, and the relevant specific/volumetric profiles are illustrated in Supplementary Figure S10. Because of the identical trend of specific and volumetric capacities, the latter explanation is given in detail. The initial volumetric capacity of L100% (black) is 443.5 mAh/cm³ and then decreases to 366.1 mAh/cm³ in the 100th cycle, with the retention equal to 82.56%, whereas the volumetric capacity retention of the L80% + S20% blended sample is found to be 88.09%, which uncovers the possibility of a bimodal cathode that goes through a different declination rate through the interaction with the surroundings. The difference in volumetric capacity is ascribed to the intrinsic nature between large and small particles, in which the larger particle provides a higher tap density, as well as electrode density, but the smaller one guarantees the ability of cell operation under high rates.

As demonstrated in Figure 7b, it is obvious that a similar specific capacity can be acquired regardless of pure or blended cathodes, and the deviation is approximately less than 10% from 0.1 to 5 C, standing for the identical ability for the cell during “discharge” process. Furthermore, at 5 C, the specific discharge capacities overlap, which have 135.2, and, 138.1 mAh/g for L100%, and L80% + S20%, respectively. The fact causing rise in capacity at elevated C-rate is attributed to the ability of SNCM for rapid charge/discharge owing to the shorter diffusion track of lithium-ion compared to LNCM. The volumetric discharge capacity possesses 526.6, 498.9, 476.2, 454.4, 427.3, and 363.8 mAh/cm³ in comparison with L100%, which only exhibits 480.3, 462.2, 435.9, 419.1, 396.6, and 340.3 mAh/cm³ under different C-rates from 0.1 to 5 C. The superior performance of these bimodal materials could be explained by the optimized blended ratio that allows the small particle to insert into the space of large particles, thus helping to facilitate the overall performance.

To understand the prevailing advantage of bimodal design, the volumetric charge capacity versus time under 5 C (high rate) is further shown in Figure 8a. Interestingly, the optimal blended ratio (L80% + S20% and L70% + S10%) displays a higher value than others. It demonstrates an extraordinary performance in terms of mass loading and electrode thickness. To achieve the rate capability under higher C-rates for commercialization, the pellet density should be supposed to reduce to its 75% original value for large particles since their overall resistance originated from the bulk electrolyte diffusion onto the particle surface. Nevertheless, the introduction of a bimodal system that embeds the small particle into the space of a large particle could not only enhance the electrode density and volumetric capacity but simultaneously increase the rate performance. The blend of SNCM with LNCM has a porous structure, promoting the infiltration of electrolytes and high rates of performance. The histogram illustrated in Figure 8b represents the available increase in volumetric capacity, using the bimodal method. Among these materials, L80% + S20% also demonstrates the best volumetric energy density after cycling (1622.6 Wh/L) compared to those L100%, mainly because of its higher electrode density (2.61 g/cm³), as well as better capacity retention (88.09%) that is considered to offer the largest amount of energy density, which make it preferable for commercialization utilization.

4. Conclusions

In summary, uniform SNCM particles are prepared by optimal synthesis conditions (pH = 2 and co-precipitation time of 3 h) through a fast nucleation process during the oxalate co-precipitation process. For the first time, the novel fast agitation mechanism is developed to control the uniformity and size of the nuclei formed during the oxalate-based co-precipitation approach. It is found that the fast nucleation by vigorous agitation enables producing oxalate nuclei with a uniform size that then grow into micron-particles in less than a few minutes. The SNCM cathode materials prepared at 750 °C for 12 h are found to be the optimal calcination condition in terms of intact morphology, smaller primary particle, specific capacity, and long-term cycle stability. The existence of bulk Zr⁴⁺ doping, as well as surface ZrO₂ coating on SNCM particles, is further confirmed by XRD and XPS analyses. After 100 cycles at 0.3 C, the capacity withholding is 89.01% for NCM-Zr_0.5%, as opposed to 83.44% of NCM-pristine material. Moreover, the discharge capability at 5 C can exhibit nearly 150 mAh/g for NCM-Zr_0.5%, which is nearly twice as much as NCM-pristine. The novel bimodal cathode distributions were successfully created with the blending combination of as-prepared smaller NCM and commercial larger NCM. The bimodal cathode behavior between SNCM and LNCM particles was investigated in detail. Owing to the inset of smaller particles into the void open space of larger particles, the electrode density and the volumetric capacity were increased dramatically in bimodal samples. More importantly, from the observation of voltage profiles, the outstanding performance of smaller particles under the 5 C rate further supports the fast charging capability for bimodal cathodes. This uncomplicated but important analysis unveils the feasibility of promoting specific/volumetric energy density for practical utility, shedding light on the possibility of commercialization, and simultaneously opens up an innovative concept of using a bimodal cathode system.