Air Storage Impact on Surface Evolution of Stoichiometric and Li-Rich NMC811

Abstract

In recent years, a type of layered oxide, LiNi _x Mn _y Co _z O₂ (NMC) where x+y+z = 1, has become the preferred cathode material for electric vehicle (EV) batteries. Despite some disorder in the crystal structure due to Li⁺/Ni²⁺ cation mixing, the composition offers a high specific capacity of up to 200 mAh g^-1 at 4.3 V vs Li|Li⁺. The objective of this study is to comprehensively evaluate the structural and electrochemical changes in NMC811 after storage in ambient conditions. In this report, we study stoichiometric and Li-rich NMC811 in terms of their structural, morphological, and electrochemical differences. Following literature reports, a rigorous aqueous washing procedure was used alternatively to remove a possible lithium excess from the NMC surface. The findings of this study hold immense significance as they focus on the potential challenges that may arise due to the remaining lithium content or Li⁺ extraction from the near-surface NMC811 materials. There is no consensus in the literature on whether excess lithium can harm the material's structural and electrochemical properties, reduce performance and safety concerns, or be beneficial regarding its protective properties, for Ni-rich NMC. Proper treatment of as-synthesized Ni-rich NMCs helps to develop procedures to address the residual lithium compounds issues, leading to enhanced performance and safety. Here with this report, we show another aspect not being considered in the literature before, regarding morphological NMC811 reshaping and a mechanism of LRC transition and growth due to aging. In addition, we linked the selected structural parameters to the electrochemical performance of various NMC811 materials. We discuss the well-known structural factors and their limitations and introduce a doublet resolution criterion that can help in predicting electrochemical performance.

Figure 1

Electrochemical C-rate test results for fresh and aged NMC 811 samples (LR – stands for Li-rich, ST – stoichiometric, A – aged, W – washed, WA – washed and aged). Tests were carried out at room temperature for 10 cycles at 0.1C, 5 at 0.2C, 5 at 0.5C, 5 at 1C, and 10 at 0.1C.

Figure 2

SEM images of all of the prepared NMC811 samples. A morphology change is visible upon aging of the unwashed stoichiometric samples with altered shape of observed particles. For Li-rich NMC811 aged samples surface becomes covered with LRC particles (LR-NMC811-A and LR-NMC811-WA). For stoichiometric washed samples (ST-NMC811-W and ST-NMC811-WA), no significant changes in surface morphology are observed after aging. The scale bar for all images is 1 μm.

Figure 3

XRD patterns for all NMC811 samples with α-NaFeO₂-type layered structure of R3m space group symmetry (ICDD card no. 96-152-0790) with indexed reflexes,^,, and zoom of 2θ 20–36° angle range of LRC with identified phases of Li₂CO₃ (blue-yellow), Li₂O (turquoise), and LiOH (purple) with ICDD cards no. 96-900-8284, 96-151-4093, and 96-10-0302, respectively; the signal at the zoom image was multiplied to magnify reflexes.

Figure 4

Raman spatial maps of fresh and aged NMC811 samples before and after the washing procedure: the red area is related to the intensity of NMC811 at A_1g mode, the blue area–to LiOH at 328 cm^–1, and the green area to Li₂CO₃ at 1088 cm^–1 mode.

Figure 5

Raman spectra of investigated materials: (a) Raman spectra of the LR-NMC811 sample with marked modes related to impurities: LiOH and Li₂CO₃. (b) Raman spectra of all aged samples show a Li₂CO₃ impurity signal. (c) A comparison of NMC811 spectra of fresh and aged samples; light-violet zone – E_g mode, light-red zone – A_1g mode, dot-dash line – new Raman peaks from Li-deficient surface region at 470 and 545 cm^–1.

Figure 6

Schematics of LRC formation process: (Stage I) Li₂O postsynthetic residue quickly reacts with H₂O after cooling; (Stage II) LiOH created from Li₂O residue reacts with H₂O creating monohydrate; (Stage III) LiOH· H₂O further reacts with CO₂; (Stage IV) Water molecules released from Li₂CO₃ formation might further react with Li-ion out-diffusing to the top surface of NMC creating thicker LRC deposits and Li-deficient NMC grain shells.

Figure 7

Comparison of structural parameters derived from XRD data through Rietveld refinement and electrochemical performance: unit cell volume V(Å^–3), intensity ratio of I(006 + 012)/I(101), (006–012) and (018–110) doublets resolution, crystallite size (nm), and 35th and 1st discharge cycle capacities. Detailed parameters are presented in Table S6, pages S-13 (light gray lines are for eye-catching only).