Phase analyses of the obtained products through the three synthesis methods were carried out via X-ray diffraction (XRD) technique and the results confirmed the formation of pure NMCP phase, as reported in previously published literature by the author (Hassanzadeh et al. 2016b, a, 2018; Hassanzadeh and Sadrnezhaad 2021), in which the fabrication methods were exactly the same as those adopted in the current study. The morphological investigations revealed that the ball-milled sample is constituted of micron-sized particles formed from a myriad of needle-like primary nanoparticles with diameters of ∼15 nm, as seen in the FESEM image in Fig. 4a (Hassanzadeh et al. 2016b). Producing extremely fine NMCP nanoparticles via ball milling would result in the enhancement of electronic and ionic conductivities of the prepared material and consequently higher discharge capacities could be achievable. In contrast, the NMCP sample prepared by the hydrothermal method showed a wide range of particle sizes (25 nm to \(\tilde4 \mu m\)), as observed in Figs. 4b and d, and the average particle size was larger than that of the other two samples (Figs. 4a and c). Noted to mention that FESEM and TEM images are used in this figure according to their availability in literature.
Moreover, the performance of sodium-ion batteries constructed from the prepared NMCP cathodes was reported earlier (Hassanzadeh et al. 2016b, a, 2018; Hassanzadeh and Sadrnezhaad 2021), and is here summarized in Table 4. As presented in this table, the NMCP material prepared via ball milling delivered the highest discharge capacity, amongst the three samples. The NMCP cathodes fabricated by hydrothermal and stirring assisted hydrothermal methods delivered discharge capacities of 76 and 94 mAh/g, respectively. The difference in the electrochemical performance of the samples could mainly be attributed to the various morphologies of the obtained materials, as compared in Fig. 4. In conclusion, the findings revealed that the stirring-assisted hydrothermal method resulted in the occurrence of mild conditions in both morphological and specific capacity features, as seen in Fig. 4c and Table 4 (Hassanzadeh et al. 2016b, a, 2018; Hassanzadeh and Sadrnezhaad 2021).
Table 4
Comparison of the electrochemical performance of NMCP materials prepared by different synthesis methods.
Material
|
Synthesis method
|
Current rate
|
First discharge capacity (mAh/g)
|
Ref.
|
NMCP
|
Ball milling
|
C/100
|
126
|
(Hassanzadeh et al. 2016b)
|
NMCP
|
Ball milling
|
C/30
|
73
|
(Hassanzadeh et al. 2016b)
|
NMCP
|
Hydrothermal
|
C/100
|
76
|
(Hassanzadeh et al. 2018)
|
NMCP
|
Hydrothermal
|
C/30
|
67
|
(Hassanzadeh et al. 2016a)
|
NMCP
|
Stirring assisted hydrothermal
|
C/100
|
94
|
(Hassanzadeh and Sadrnezhaad 2021)
|
In the current study, the comparative LCA study was performed to evaluate the sustainability of the production phase of NMCP nanoparticles via three representative synthesis strategies including ball milling, hydrothermal, and stirring assisted hydrothermal. Table 5 shows the comparative life cycle impacts in ten environmental categories for the production of 1 kg of NMCP through three different methods of ball milling (B), hydrothermal (H), and stirrer-assisted hydrothermal (S). Comparing the data in columns B, H, and S of the table reveals that there is no obvious winner between the three different synthesis methods in terms of environmental footprint.
For better comparison, Fig. 5 illustrates the findings of Table 5. In this figure, the maximum value of three synthesis methods in each of the ten environmental categories has been set as the base for comparison. Therefore, the environmental footprint of the three synthesis routes can be compared in each category. The hydrothermal method has much impacts in Non-carcinogenics and Respiratory effects categories, while the stirring-assisted hydrothermal method has the most impact in the categories of ozone depletion, global warming, smog, and fossil fuel depletion. The ball milling method shows significant impacts in three categories of acidification, eutrophication, and carcinogenics, while its impact in the other categories is slightly less than hydrothermal-based methods. The results suggest that among all the three reported methods for the synthesis of NMCP, ball milling demonstrates the lowest environmental performance in most environmental categories. Furthermore, the hydrothermal (H) method shows the lowest environmental impacts in ozone depletion, global warming, fossil fuel depletion, and smog, while the stirrer-assisted hydrothermal method exhibits the best environmental performance in the rest of environmental categories.
Table 5
Overall results for production of 1 kg NMCP via different routes
Impact Category
|
Unit
|
Ball milling (B)
|
Hydrothermal (H)
|
Stirrer-assisted Hydrothermal (S)
|
B/H
|
H/S
|
S/B
|
Ozone depletion
|
Kg CFC 11 eq
|
1.54E-06
|
1.29E-06
|
2.04E-06
|
120%
|
64%
|
133%
|
Global warming
|
Kg CO2 eq
|
1.53E+01
|
1.42E+01
|
2.00E+01
|
108%
|
71%
|
131%
|
Smog
|
Kg O3 eq
|
8.61E-01
|
9.07E-01
|
1.04E+00
|
95%
|
88%
|
121%
|
Acidification
|
Kg SO2 eq
|
5.33E-01
|
1.95E-01
|
1.15E-01
|
273%
|
170%
|
22%
|
Eutrophication
|
Kg N eq
|
9.21E-01
|
2.89E-01
|
1.08E-01
|
319%
|
245%
|
13%
|
Carcinogenics
|
CTUh
|
3.23E-05
|
2.12E-05
|
1.80E-05
|
153%
|
118%
|
56%
|
Non-carcinogenics
|
CTUh
|
4.63E-06
|
6.40E-06
|
5.73E-06
|
73%
|
112%
|
124%
|
Respiratory effects
|
Kg PM2.5 eq
|
1.82E-02
|
1.90E-02
|
1.82E-02
|
96%
|
105%
|
100%
|
Ecotoxicity
|
CTUe
|
7.07E+02
|
6.88E+02
|
6.15E+02
|
103%
|
112%
|
87%
|
Fossil fuel depletion
|
MJ surplus
|
2.54E+01
|
1.74E+01
|
3.39E+01
|
146%
|
52%
|
134%
|
To detect the main source of the environmental impacts in each synthesis process, the contribution of the system inputs and outputs in all three processes was analyzed and the results are shown in Fig. S1. This figure indicates the detailed LCA results for each input of the three synthesis processes and also for the total emissions to air and water. Noted that the inputs are the same in all three methods. In order to know the percentage impact contribution of each component of the process, the 100% columns histogram was used. The three methods’ impact cannot be compared according to the column scale.
As demonstrated by Fig. S1, for the ball milling method, electricity contributed much of the total impacts in ozone depletion, global warming, and fossil fuel depletion categories. Whereas, direct emissions contributed more than 80% of the total impact for the acidification and eutrophication categories. Manganese nitrate contributed 50%, 60%, and more than 90% of the total impact for the respiratory effects, ecotoxicity, and carcinogenics categories, respectively.
In the hydrothermal method, sodium carbonate as an input material, demonstrates the highest contribution ranging from 40% to about 80% in 7 categories. The reason is due to the required amount of sodium carbonate to synthesize pure NMCP through hydrothermal-based methods, which is about 10 times higher than the stoichiometric value. And finally, the main contributors to the stirring-assisted hydrothermal process are electricity and sodium carbonate.
Figures 6a, b, and c show the relative contribution of each unit process in the impact categories for fabricating 1 kg of NMCP via ball milling, hydrothermal, and stirring assisted hydrothermal methods, respectively. As seen in these figures, deionized water, stirring, and oven drying exhibit small environmental footprints in all three fabrication methods. Besides, direct emissions are the main contributor to the impact categories of acidification and eutrophication, and manganese nitrate is the main environmental weakness in the carcinogenics category, in almost all three synthesis routes.
Since the starting materials are the same in all three methods, a comparison of the techniques shows that the hydrothermal process itself, had little environmental footprint while both ball milling and S-hydrothermal processes contributed more than 50% of the total impact for the ozone depletion and fossil fuel depletion categories. In categories of non-carcinogenics, respiratory effects, and ecotoxicity, more than 85% of the environmental impacts originated from the use of raw materials including manganese nitrate, disodium phosphate, and sodium percarbonate.
According to the literature (Hassanzadeh et al. 2016b, a, 2018; Hassanzadeh and Sadrnezhaad 2021) and as aforementioned, the sample synthesized by the ball milling and hydrothermal processes showed the highest and lowest specific capacities, respectively. In contrast, the LCA results suggested that among all the three reported methods for the synthesis of NMCP material, ball milling demonstrates the highest environmental footprint in most environmental categories.
As aforementioned, the best attainable electrochemical performance is not the only determining factor for choosing the appropriate synthesis route for the large-scale production of electrode materials. Instead, the best appropriate route should be adopted according to both electrochemical performance and environmental footprint. Consequently, the stirring-assisted hydrothermal method could be introduced as the most appropriate route for large-scale production of NMCP as the cathode material in sodium-ion batteries.