Boosting the cycling stability of Ni-rich layered oxide cathode by dry coating of ultrastable Li3V2(PO4)3 nanoparticles

Dongdong Wang ab, Qizhang Yan a, Mingqian Li ac, Hongpeng Gao d, Jianhua Tian b, Zhongqiang Shan b, Ning Wang *ae, Jian Luo acdf, Meng Zhou g and Zheng Chen *acdf
aDepartment of NanoEngineering, University of California San Diego, La Jolla, CA 92093, USA. E-mail: zhengchen@eng.ucsd.edu; hebsjzwn@gmail.com
bSchool of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, P. R. China
cProgram of Chemical Engineering, University of California San Diego, La Jolla, CA 92093, USA
dProgram of Materials Science and Engineering, University of California San Diego, La Jolla, CA 92093, USA
eAnalysis and Testing Centre, Hebei Normal University, Shijiazhuang 050024, P. R. China
fSustainable Power and Energy Center, University of California San Diego, La Jolla, CA 92093, USA
gDepartment of Chemical and Materials Engineering, New Mexico State University, Las Cruces, NM 88003, USA

Received 21st November 2020 , Accepted 7th January 2021

First published on 7th January 2021


Abstract

Nickel (Ni)-rich layered oxides such as LiNi0.6Co0.2Mn0.2O2 (NCM622) represent one of the most promising candidates for next-generation high-energy lithium-ion batteries (LIBs). However, the pristine Ni-rich cathode materials usually suffer from poor structural stability during cycling. In this work, we demonstrate a simple but effective approach to improve the cycling stability of the NCM622 cathode by dry coating of ultrastable Li3V2(PO4)3-carbon (LVP-C) nanoparticles, which leads to a robust composite cathode (NCM622/LVP-C) without sacrificing the specific energy density compared with pristine NCM622. The optimal NCM622/LVP-C composite presents a high specific capacity of 162 mA h g−1 at 0.5 C and excellent cycling performance with 85.0% capacity retention after 200 cycles at 2 C, higher than that of the pristine NCM622 (67.6%). Systematic characterization confirms that the LVP-C protective layer can effectively reduce the side reactions, restrict the cation mixing of NCM622 and improve its structural stability. Moreover, the NCM622/LVP-C||graphite full cells also show a commercial-level capacity of 3.2 mA h cm−2 and much improved cycling stability compared with NCM622/LVP-C||graphite full cells, indicating the great promise for low-cost, high-capacity and long-life LIBs.


1. Introduction

The wide-spread applications of lithium-ion batteries (LIBs) in electronic devices, electric vehicles (EVs), and grid energy storage have driven an increasing demand for electrode materials with higher energy density, better rate capability, and longer cycling stability.1–5 Owing to the high capacity and reduced cost, layered Ni-rich LiNi1−xyCoxMnyO2 (NCM, 0 < x, y, z < 1, 1 − xy ≥ 0.6) is becoming a dominant cathode material in state-of-the-art LIBs.6–8 Among them, LiNi0.6Co0.2Mn0.2O2 (NCM622) displays a high reversible capacity of 170 mA h g−1 in the voltage range from 3.0 to 4.3 V (vs. Li/Li+) and shows reasonably good thermal stability, rendering it one of the most promising candidates for the next generation of advanced high-energy LIBs.9 However, the as-synthesized Ni-rich cathode materials usually suffer from Li/Ni mixing and high residual Li components (Li2CO3/LiOH) on the particle surface, reducing the initial coulombic efficiency and discharge capacity.10–12 Meanwhile, irreversible phase transformation and loss of lattice oxygen of the cathode materials during the delithiation/lithiation process, accompanied by side reactions with the electrolyte, often leads to poor cycling performance.13–15 These problems hinder the wider range of applications for these high-energy Ni-rich cathode materials. On-going efforts on improving the cycling performance of NCM622 cathodes have been focused on establishing a protective coating or doping that can stabilize the particle morphology and structure, such as Al2O3, TiO2, LiNbO3, and Li1.3Al0.3Ti1.7PO4 (LATP).16–20 While these coating materials have demonstrated improved cycling stability on NCM622, they do not contribute to charge storage and also sacrifice capacity or rate performance due to extra ionic resistance added onto the electrode particles.

Recent studies showed that building a surface coating layer of olivine phase LiMPO4 (M = Fe, Mn, etc.), such as LiMPO4 could be an effective approach to enhance the electrochemical properties of Ni-rich layered oxides.21,22 However, previous approaches on building such types of coating materials have been based on solution processes followed by a high-temperature annealing process, which potentially raises cost concerns due to the complicated synthesis. More importantly, for a high Ni cathode such as NCM622, a solution process with annealing commonly induces side reactions such as Li loss, cation (Li+/Ni+) mixing and surface passivation, which deteriorates the intrinsic properties of the high-Ni cathode materials.

On the other hand, monoclinic Li3V2(PO4)3 (LVP) has also attracted great attention as a promising cathode material owing to its excellent cycling performance, low cost, high thermal stability, good safety and higher operation voltage compared with LiFePO4.23–25 Although LVP-based cathode materials have a relatively low theoretical capacity of 133 mA h g−1 when charged up to 4.3 V (vs. Li/Li+),26 previous studies show that carbon-coated LVP cathodes could maintain ultra-long cycling stability without obvious capacity decay (e.g., 2000 cycles), comparable to LiFePO4 cathodes.27–29 However, the charge/discharge voltage curves of LVP cathodes usually present multiple redox plateaus during the cycling process, resulting in potential challenges in matching with anode materials and monitoring the state-of-charge (SOC) of the cells, which limits their commercial applications.30,31

Herein, we show a promising strategy to achieve stable and high-performance composite cathodes by combining nanoparticulate LVP particles and microspherical NCM622 via a facile dry coating process. In this composite cathode (NCM622/LVP), the surface of NCM622 microspheres was uniformly coated with LVP nanoparticles via a simple drying mixing process without destructing the secondary structure of NCM622 microspheres. This approach effectively mitigates the intrinsic stability problem of pristine NCM622 and the voltage and capacity limitations of the LVP cathode while maintaining the desired properties of the two constituents. Integrating the two cathode materials is effective in enhancing the cycling stability and rate capability of NCM622 cathode materials without sacrificing the energy density of the entire cathode. Owing to the protective effect of the LVP layer, the cation mixing and irreversible phase transformation of the NCM622 cathode during the cycling process are reduced and restricted, enhancing the structural stability. In addition, with the controlled amount of LVP, the NCM622/LVP cathode eliminates the multiple charge–discharge plateaus of LVP, making it easier for SOC monitoring. We also show that such a NCM622/LVP composite cathode can maintain higher specific capacity than the NCM622 cathode (85.0% vs. 67.6%) after 200 cycles at 2 C in 3.0–4.3 V (vs. Li+/Li). Moreover, NCM622/LVP||graphite full cells with a high capacity of 3.2 mA h cm−2 also showed a capacity retention of 89.1% after 100 cycles at 0.5 C, higher than that of NCM622||graphite full cells (56.4%). This work demonstrated a new avenue for developing low-cost, high energy, and long-life LIB cathodes.

2. Experimental section

2.1 Materials preparation

The LVP nanoparticles with conductive carbon coating were prepared via a sol–gel method.32 In a typical synthesis, 0.1819 g of V2O5 and 0.3781 g of H2C2O4·2H2O were dissolved in 5 mL of distilled water at room temperature. The solution was then heated and stirred at 80 °C in a silicon oil bath. After heating and stirring for 1 h, 0.1108 g of Li2CO3 and 0.3451 g of NH4H2PO4 were added to the previous solution. After another hour of heating, a gel was formed. When the water was completely evaporated, the dried LVP power was transferred into an agate mortar and phenolic resin solution (10 wt%) was added. After grinding for 20 min, the mixture was calcined at 800 °C for 8 h in an argon atmosphere at a heating rate of 5 °C min−1. Finally, the LVP nanoparticles with carbon coating (LVP-C) were obtained after the ball milling for 30 minutes.

To make the composite cathode, commercial NCM622 (BTR Corporation) and the as-synthesized LVP-C nanoparticles were placed in the mortar and ground for 0.5 h. Then the samples were transferred to 20 mL vials and mixed for 0.5 h via a XH-C Vortex mixer with 1000 rpm. NCM622/LVP-C composites with different LVP-C loading (5 wt% to 30 wt%) contents were obtained by adjusting weight ratios between the two components. The porosity of each sample was calculated by comparing the actual density and the bulk density of each constituent.

2.2 Materials characterization

The morphology of different samples was examined using a ultrahigh resolution scanning electron Microscope (UHR-SEM, FEI XL30). Microstructural information and crystal characteristics of the samples were examined using transmission electron microscopy (TEM, FEI Titan 80–300 kV S/TEM, America) images and selected area electron diffraction (SAED). High-resolution TEM (HR-TEM) was recorded on a field emission gun JEOL-2800 at 200 kV with Gatan OneView Camera (full 4K × 4K resolution). Thermogravimetric analysis (TGA) was performed on an SDTA851E thermoanalyzer between room temperature and 800 °C at a heating rate of 10 °C min−1. Crystallographic structures of the samples were confirmed by X-ray diffraction (XRD) using Cu Kα radiation in the region of 2θ = 10–80° at 30 kV and 10 mA (Bruker D2 Phaser, Germany).

2.3 Electrochemical measurement

The cathode electrodes were fabricated by casting the slurry of active materials (NCM622, LVP or NCM622/LVP composites), Super-P, and poly (vinylidene fluoride) (PVDF) binder at a mass ratio of 8[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1 in N-methyl-2-pyrrolidone (NMP) solvent on Al foil, and then dried at 120 °C for 12 h in a vacuum oven before use. The graphite electrodes (MTI corporation, Fig. S1) were fabricated by casting the slurry of active materials, Super-P, and PVDF in a mass ratio of 90[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]5 in NMP solvent on Cu foil, and then dried at 80 °C for 8 h in a vacuum oven before use. CR-2032 type coin cells were assembled to evaluate the electrochemical properties of all the samples. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC)/diethyl carbonate (DEC) with a volume ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]1. A Celgard 2400 microporous polypropylene membrane was used as the separator. For half cells, the mass loading of the cathode electrode was controlled at 5 mg cm−2. For full cells, the mass loading of the cathode electrode was controlled to be 20 mg cm−2. The negative to positive capacity ratio (N/P ratio) of the full cells was controlled at 1.2. Thus, the rate and specific capacity values of full cells were calculated based on the cathode mass. All cells were assembled in a glovebox filled with argon gas (O2 and H2O levels <0.1 ppm). NEWARE battery testers were used to perform constant current charge–discharge cycling in the voltage range of 3.0–4.3 V (vs. Li+/Li) for half-cells and 2.8–4.2 V for full cells. Cyclic voltammetry (CV) was measured using a Metrohm Autolab electrochemical workstation between 3.0 and 4.3 V (vs. Li+/Li) with different scan rates. Electrochemical impedance spectroscopy (EIS) measurements were acquired in the frequency range from 0.1–105 Hz and a perturbation voltage of 5 mV.

3. Results and discussion

The LVP nanoparticles with conductive carbon coating were first synthesized by a sol–gel method for the preparation of composite cathodes. Fig. S2a and b show that the carbon-coated LVP (LVP-C) nanoparticles have a relatively uniform grain size of 100–200 nm. The HR-TEM image of LVP-C shows an amorphous surface carbon layer with a thickness of about 8 nm. In the bulk material, the lattice fringes have a typical spacing of 0.365 nm, which can be assigned to the (211) plane of LVP (Fig. S2c).33 Furthermore, the SAED pattern confirms that the monoclinic Li3V2(PO4)3 phase is successfully produced (Fig. S2d). The XRD pattern displays that LVP-C nanoparticles can be well indexed to a monoclinic structure phase (JCPDS no. 97-009-6962) with a space group of P21/n. No diffraction peak of carbon appeared, indicating that the residual carbon is mainly amorphous (Fig. S3a).34 TGA also presents that the carbon content of LVP-C nanoparticles is 4.04% (Fig. S3b). The SEM morphologies of NCM622/LVP-C composites with different LVP-C contents are shown in Fig. S4. When the LVP-C content was 5 wt%, a significant portion of the NCM622 surface was still exposed, indicating the loading of LVP-C nanoparticles was too little to form a protective coating layer (Fig. S4a and 3b). When the LVP-C content increased to 20 wt% and 30 wt%, although the NCM622 surface was completely covered by LVP-C nanoparticles, the aggregation of additional LVP-C nanoparticles occurred on the surface of the NCM622 particles (Fig. S4c and 3f). Therefore, the optimal LVP-C content in the composite was determined at 10 wt%. As shown in Fig. 1, NCM622 materials were spherical particles with a diameter of 3–10 μm consisting of densely packed primary nanoparticles of 200–800 nm (Fig. 1a and b). After dry coating of LVP-C nanoparticles, the optimal NCM622/LVP-C composites still showed spherical micron-sized particles, which were covered uniformly with LVP-C nanoparticles on the surface without excess LVP-C nanoparticle agglomeration (Fig. 1c and d).
image file: d0nr08305d-f1.tif
Fig. 1 SEM images of (a and b) the NCM622 and (c and d) the NCM622/LVP-C composites with 10 wt% of LVP-C coating.

Fig. 2 shows the cross-sectional SEM images of the two different electrodes with a loading of 20 mg cm−2. The NCM622 electrode displays that the micron-sized particles are relatively uniformly mixed with super-P and the binder with a clear porosity (Fig. 2a and b). After coating with the LVP-C nanoparticles on the NCM622 surface, the stacking pores are more completely filled with some LVP-C nanoparticles, leading to a higher tap density of the NCM622/LVP-C electrode (Fig. 2c and d). Table 1 summarizes that the NCM622/LVP-C electrode has a smaller porosity than the NCM622 electrode (27% vs. 31%), leading to a slightly enhanced volumetric energy density due to the LVP-C coating.


image file: d0nr08305d-f2.tif
Fig. 2 Cross-sectional SEM images of the NCM622 electrode (a and b) and the NCM622/LVP-C electrode (c and d).
Table 1 Summary of porosity of two electrodes
Electrodes NCM622/LVP NCM622
Porosity 27% 31%


The electrochemical performance of different electrodes was first examined using a half-cell configuration using coin cells. Fig. 3a shows that while the NCM622/LVP-C electrode has a slightly lower specific capacity than the NCM622 electrode in the initial 50 cycles at 0.5 C charge/discharge, it can retain a higher capacity after 100 cycles owing to the LVP-C protective layer (97.2% vs. 91.0%). Furthermore, a high average coulombic efficiency (CE) of 99.7% was obtained during the 100 cycles. In addition, the cycling performance of NCM622/LVP-C composites with different loadings of LVP-C nanoparticles at 0.5 C was also compared (Fig. S5). The NCM622/LVP-C (9[thin space (1/6-em)]:[thin space (1/6-em)]1) electrode displayed the best cycling performance in terms of stability and capacity among all electrodes due to the appropriate content of LVP-C. Meanwhile, the long-term cycling stability of two NCM622 and NCM622/LVP-C electrodes at a rate of 2C was also compared (Fig. 3b). The NCM622 electrode delivered a capacity of 103.9 mA h g−1 after 200 cycles, retaining only 67.60% of its original value of 153.7 mA h g−1. In contrast, the NCM622/LVP-C electrode presented a better cycle performance for which an initial discharge capacity of 149.6 mA h g−1 was achieved, and 127.1 mA h g−1 was maintained after 200 cycles, corresponding to a capacity retention of 85.0%, further suggesting the protective effect of the LVP-C layer. To confirm the positive role of LVP-C, the electrochemical performance of the LVP-C nanoparticles-based electrode also was tested and is shown in Fig. S6. The electrode showed stable cycling with negligible capacity decay for 100 and 200 cycles at 0.5 and 2 C, respectively, which suggests it superior cycling stability.


image file: d0nr08305d-f3.tif
Fig. 3 (a) Cycling performance of the NCM622 electrode and NCM622/LVP-C electrode at a current density of 0.5 C; (b) cycling performance of the NCM622 electrode and NCM622/LVP-C electrode at a current density of 2.0 C; the charge and discharge curves of the (c) NCM622 electrode and (d) NCM622/LVP-C electrode after different cycles; the CV curves of (e) the NCM622 electrode and (f) the NCM622/LVP-C electrode after different cycles at a scan rate of 0.1 mV s−1 between 3.0–4.3 V.

The morphologies of the NCM622 and NCM622/LVP-C electrodes after 50 cycles at 0.5 C were also characterized by SEM. Due to the formation of a cathode electrolyte interphase (CEI), a common phenomenon in oxide cathodes, the intrinsic surface features of NCM622 and NCM622/LVP-C electrodes cannot be maintained. Nevertheless, different morphologies are still observed on the two samples. As shown in Fig. S7, some microcracks and high surface roughness were observed in cycled NCM622 electrodes. In contrast, NCM622/LVP-C can maintain a relatively denser structure without some destructions, which can be attributed to the effective protection of the LVP-C coating layer. This is likely due to the higher rate capability of LVP-C nanoparticles, which participate in lithiation and delithiation reactions earlier than NCM622 and serve as a buffer to release chemical and electrochemical stress built in the charge/discharge cycling.

Fig. 3c and d further compare the charge and discharge curves of the above two electrodes at 1, 10, 100, and 200 cycles. It can be observed that the NCM622/LVP-C electrode shows a similar curve shape to the NCM622 electrode, eliminating the multiple charge–discharge plateaus typically observed for LVP electrodes (Fig. S8). This feature makes LVP-based electrodes more applicable for real applications due to easier monitoring of the state of charge. The average discharge voltage of the NCM622 electrode declined faster than the NCM622/LVP-C electrode due to the stronger polarization possibly originating from surface side reactions or microstructure changes during cycling. To investigate the electrode reaction process, CV profiles of the two electrodes were compared at different cycles under 0.1 mV s−1 between 3.0–4.3 V. A pair of oxidation and reduction peaks at about 3.6–3.9 V can be found in all CV curves for the NCM622 electrode, indicating the Ni2+/Ni4+ transformation of NCM622.35 The current density became lower with the increase of cycle numbers due to the reduced electrochemical capacity (Fig. 3e). For the NCM622/LVP-C electrode, except the Ni2+/Ni4+ peaks, there are three couples of redox peaks in each CV profile, representing the extraction/insertion of two Li+ in Li3V2(PO4)3 by three steps (Fig. S5).36 Furthermore, there is no obvious decrease of current density with increased cycle numbers, demonstrating better reversibility and higher cycling stability (Fig. 3f). The NCM622/LVP-C electrode also displays the better rate capability than NCM622 electrodes, especially at high rates such as 5C and 10C, suggesting that the composite electrodes are of great potential for fast charging batteries (Fig. S9). EIS measurements were also performed for two electrodes after different cycles. As shown in Fig. S10 and Table S1, the NCM622/LVP-C electrode delivered a smaller ohmic resistance (Rs) and charge-transfer impedance (Rct) than the NCM622 electrode, leading to improved cycling stability due to the highly stable LVP-C coating.

To further confirm the improved rate performance of the composite electrode, the Li+ diffusion coefficient (DLi+) was calculated from the following eqn (1)

 
image file: d0nr08305d-t1.tif(1)
where R is the gas constant (8.314 J K−1 mol−1), T the absolute temperature (298 K), A the surface area of electrode, n the number of electrons transferred in the half-reaction for the redox reaction, F the Faraday constant (96[thin space (1/6-em)]500 C mol−1), C the concentration of lithium ions and σ is the Warburg factor, which is based on eqn (2), and can be obtained from the slope of Z′ ∼ ω−1/2 plot as depicted in Fig. S11.
 
Z′ = Rb + Rsei + Rct + σω−1/2.(2)

The calculated DLi+ values of the two electrodes are shown in Table S2. NCM622/LVP-C displays a higher DLi+ (4.07 × 10−11), which is about 3 times larger than that of NCM622 (1.23 × 10−11). These results are consistent with its better rate performance of the NCM622/LVP-C electrode.

To investigate the structural changes of the NCM622 and NCM622/LVP-C composite cathode after cycling, HR-TEM and fast Fourier transformation (FFT) were performed. Fig. 4a presents the HR-TEM image of the cycled NCM622 cathode particle, while Fig. 4b and c show FFT filtered TEM images recorded from different selected areas in Fig. 4a. FFT results from different regions indicate that the bulk of particles are layered phase (Fig. 4c, region 2), while the surface of particles shows some rock salt phase (Fig. 4b, region 1). The characteristic spots in the FFT pattern of the rock salt phase can be indexed to the (111) and (002) plane of NiO (Fig. 4d), and the layered phase refers to the (003) plane (Fig. 4e).37 However, for the NCM622/LVP-C composite, the structure of NCM622 remained to be its original layered structure in all regions (Fig. 4f–h). Similarly, the FFT results also exhibit that NCM in the composite belongs to the (003) planes of α-NaFeO2 layered structures (Fig. 4i and j).38 This microstructure analysis indicates that the LVP-C can effectively restrict undesired phase change of NCM622 during long cycling.


image file: d0nr08305d-f4.tif
Fig. 4 (a) HR-TEM image of the cycled NCM622 materials; FFT filtered TEM images recorded from (b) region 1 and (c) region 2 in (a); FFT results of (d) region 1 and (e) region 2 in (a); (f) HR-TEM image of the cycled NCM622/LVP-C composite; FFT filtered TEM images recorded from (g) region 1 and (h) region 2 in (f); FFT results of (i) region 1 and (j) region 2 in (f).

To further explore the microstructural evolution after cycling, the XRD patterns of pristine NCM622, cycled NCM622 and cycled NCM622/LVP-C are compared as shown in Fig. 5. All the cycled cathodes displayed a typical pattern of the α-NaFeO2 structure with the R[3 with combining macron]m space group (Fig. 5a). Due to the relatively small loading (10 wt%), the LVP characteristic peaks were not observed in the composite cathode. It was also observed that compared to the pristine NCM622, the (003) peak of cycled NCM622 and NCM622/LVP-C shifts to lower angles (Fig. 5b) due to the loss of Li.39 The cycled NCM622 cathode exhibited the lowest angles which corresponds to an increase in the c lattice parameter because of the electrostatic repulsion between the oxygen layers along c directions in the Li-deficient state.40 At the same time, the distance between the (108) peak and (110) peak increases after cycling, corresponding to the decrease in a lattice parameters due to the smaller effective ionic radii of Ni3+ than Ni2+ to compensate Li deficiency.41 The cycled NCM622 had a larger peak separation than cycled NCM622/LVP-C owing to the higher content of Ni2+. Rietveld refinement was carried out on all of the XRD patterns (Fig. S12), and the lattice parameters were compared as shown in Table 2. The refinement results further demonstrate that the cycled cathode particles have a smaller a lattice parameter and larger c lattice parameter than pristine NCM622. In addition, the cycled NCM622/LVP-C showed a lower Li/Ni mixing than cycled NCM622 (6.83% < 9.02%), which is in agreement with HR-TEM results. Based on the analysis above, it is concluded that the LVP-C protective layer can effectively reduce the side reactions, inhibiting cation mixing of NCM622 and improving its structural stability, which leads to the improved cycling performance.


image file: d0nr08305d-f5.tif
Fig. 5 XRD patterns of pristine NCM622, cycled NCM622 and cycled NCM622/LVP-C in the ranges of (a) 10–80°; (b) 18.2–19.4 and 64–66°.
Table 2 Summary of fitting lattice parameters of pristine NCM622, cycled NCM622 and cycled NCM622/LVP-C
Samples α c Li/Ni mixing R wp χ 2
Pristine NCM622 2.8627 14.1930 4.87% 1.42% 0.0086
Cycled NCM622 2.8578 14.3431 9.02% 3.61% 0.0496
Cycled NCM622/LVP-C 2.8601 14.2528 6.83% 3.43% 0.0256


To further demonstrate the potential application of the NCM622/LVP-C composites, full cells were assembled and tested with graphite as the anode. Fig. 6a shows the cycling performance of different full cells at 0.5C rate in the voltage range of 2.8–4.2 V. The full cell with the NCM622/LVP-C cathode displayed an initial discharge capacity of 160.2 mA h g−1 (based on the cathode mass) with a commercial-level areal capacity of 3.2 mA h cm−2. The NCM622||graphite full cell delivered slightly higher initial capacity but with a more rapid capacity loss with a capacity retention of only 56.4% after 100 cycles. However, the NCM622/LVP-C||graphite full cell presented a significantly improved cycling stability with a capacity retention of 89.1% after 100 cycles. Fig. 6b and c show the charge and discharge curves of full cells at different cycles. It can be clearly observed that the average voltage of NCM622 declines at a faster pace compared with NCM622/LVP-C due to the stronger polarization due to surface and/or structure instability induced during cycling. Without special electrolyte formulation or cathode treatment, the drying coating of NCM622 by LVP-C nanoparticles helped to improve the cycling stability without sacrificing cell energy density, suggesting its potential as an effective, low-cost strategy to develop high-performance cathodes.


image file: d0nr08305d-f6.tif
Fig. 6 (a) The cycling performance of NCM622||graphite and NCM622/LVP-C||graphite full cells at 0.5 C between 2.8–4.2 V; the charge and discharge curves of (b) NCM622/LVP-C||graphite and (c) NCM622||graphite full cells at different cycles. Areal capacity as 3.2mA h cm−2 for cathodes and the N/P ratio was controlled at 1.2.

4. Conclusions

In summary, we demonstrated a simple and effective drying coating method to synthesize NCM622/LVP-C composites, in which micron-sized NCM622 secondary particles are uniformly protected by LVP-C nanoparticles. Due to the intrinsic structure stability of LVP and the uniform surface coating, such a composite cathode showed much better cycling stability than the pristine NCM622 cathode. Structure and morphology characterization showed that the LVP-C protective layer can effectively reduce the side reactions, restrict the cation mixing of the NCM622 cathode and improve its structural stability. With an optimal loading amount of the LVP-C nanoparticles, the NCM622/LVP-C composites can deliver a high specific capacity and excellent cycling performance, demonstrating its great promise for low-cost and high-performance cathodes. This method can be also extended to develop other composite cathode materials to further improve capacity and rate performance, making them more attractive for practical applications.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

Z. Chen acknowledges the start-up fund support from UC San Diego. The support provided by China Scholarship Council during a visit of D. D. Wang to the University of California San Diego (No. 201706250088) and China Postdoctoral Science Foundation (2020TQ0183) is acknowledged. The transmission electron microscopy work by Q. Yan and J. Luo was supported as part of the Center for Synthetic Control Across Length-scales for Advancing Rechargeables (SCALAR), an Energy Frontier Research Center funded by the US DOE, Office of Science, BES under Award No. DESC0019381.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr08305d

This journal is © The Royal Society of Chemistry 2021