LiNi1/3Mn1/3Co1/3O2 nanoparticles produced by flame spray pyrolysis with crystallinity characteristics similar to commercial NMC particles

Abstract

To achieve higher energy densities in lithium-ion batteries, improvements in the battery cathode performance are crucial. As cathode materials, nickel-rich layered transition metal oxides play an important role in the market. However, they suffer from surface degradation which contributes to the capacity fade. Using nanoparticles, which offer a large surface to volume ratio, these surface degradation reactions can be better understood. But to do so, nanoparticles with properties similar to those of primary particles in commercial NMC are necessary. In this work, we present the synthesis of sub-100 nm of LiNi_1/3Mn_1/3Co_1/3O₂ (NMC111) nanoparticles through a flame spray pyrolysis and post-calcination. We study the phase purity and electrochemical performance of the NMC111 nanoparticles as a function of the calcination temperature and demonstrate that optimizing the calcination temperature enables us to achieve a pure layered phase and electrochemical performance on par with commercial NMC111 particles. Mild acid treatment can be used to remove surface impurities that develop with air exposure and improve the long-term stability.

Fig. 1. Depiction of the LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) nanoparticle synthesis. Flame spray pyrolysis yields nanopowder (a and b), which is then calcined (c and d) to achieve a high-performance NMC nanopowder. The calcined powder inevitably encounters air exposure, which degrades the surface (e). Surface impurities can be removed via acid wash and subsequent drying (f).

Fig. 2. Structure and morphology of NMC111 nanoparticles. (a) XRD patterns of the from flame and after calcining at different temperatures from 600 to 800 °C. XRD patterns of LiNi_0.5Mn_1.5O₄ (spinel phase, Fd3̄m, ICSD-8408 (ref. 46)) and LiNi_0.33Mn_0.33Co_0.33O₂ (layered phase, R3̄m, ICSD-171750 (ref. 47)) are shown as a reference. (b) Zoomed-in view of XRD Region I (35–40° 2θ), highlighting evolution of (006)/(102) peaks. (c) Zoomed-in view of XRD Region II (62–68° 2θ), showing changes in (008)/(110) peaks. (d–f) TEM images of NMC111 nanoparticles calcined at 800, 700, and 600 °C, and their corresponding size distribution histograms.

Fig. 3. Electrochemical behavior of as-synthesized NMC111 nanoparticles. (a) Charge and discharge profiles of the 1st cycle at C/10. (b) dQ/dV plot for the 3rd cycle at a rate of C/10. (c) Rate capability test. The error bars represent the standard deviation calculated from three cells. The detailed standard deviations of the commercial NMC cells can be found in Table S7. (d) Cycling performance at C/3 in the voltage range from 2.8 V to 4.3 V. Four formation cycles at C/10 were performed before the long-term cycling at C/3 to form a stable CEI (cathode electrolyte interphase) layer.

Fig. 4. (a–c) FTIR spectra of the NMC111 nanoparticles calcined at 600 °C before and after acid washing. (d) Scheme of the acid-washing process. (e) Comparison of dQ/dV plot (at a rate of C/10) among the NMC111 nanoparticles calcined at 600 °C and 800 °C before and after washing. (f) The capacity degradation after 100 cycles at a rate of C/3 versus the size of the synthesized NMC111 nanoparticles w/wo acid washing. The error bars represent the standard deviation calculated from three cells.