3.1. The Total Value of the Four Cathodes’ Environmental Impact
It is necessary to qualify the total environmental impact of these four cathode materials before a deeper investigation, because all midpoint and endpoint indicators are divided from the total environmental impact. Unlike Gong’s research on evaluating environmental, economic, and electrochemical performance indicators by footprint family and Peters’s summary focusing on energy demand and warming gas emissions for LiCoO2
/C and LiFePO4
/C, we make a pure and further environmental assessment for these four cathode materials, and do not consider the economic benefits, electrochemical properties, energy demand, etc. This study aims to make a comprehensive assessment of these four typical cathodes directly. The total values for these four cathode materials in three LCAs are shown in Figure 2
As Figure 2
shows, we sort the four cathode types in descending order of environmental sustainability potential: D, C, B, and A. Although different LCAs have their own calculation standards, the trend of these four cathodes is the same. As Gong [23
] confirms, D has the best environmental performance compared with B and C. However, the study does not explain the differences in specific indicators between B, C, and D. More detailed indicators that are important for each of the three cathodes are found in this study. In addition, material A is added to the study as another major market cathode model. No matter what LCA we choose, the new cathode model D always presents the best potential for environmental sustainability, while material A performs the worst. The environmental sustainability of material B is close to that of material C. Methodological emphasis on environmental assessment is only reflected in quantitative values, rather than the qualitative environmental sustainable potential among these four cathodes.
3.2. Endpoint Level
To distinguish concrete environmental impacts of different cathode materials, we calculate all endpoint indicators in Table 1
. In any LCA, material A always has the highest endpoint value among these four materials. D is the smallest. Except the resource consumption in ReCiPe, 0.449 Pt of C larger than 0.393 Pt of B, other values for material B are slight larger than those for material C. IMPACT 2002+ and EI-99 have less impact on these four cathode materials’ ranking of environmental sustainable potential. The resource consumption value between B and C calculated by ReCiPe is different to that calculated by the other two LCAs.
The total value consists of the values of three or four endpoint indicators. To make a more intuitive observation, we calculated the contribution rates of different endpoint indexes to the total value, as shown in Figure 3
. In three LCAs, the maximum contribution proportion of these four cathodes comes from human health. For material A, the contribution rate of ecosystem quality is relatively large in IMPACT 2002+.
Obviously, in three LCAs, the environmental sustainability capacity for these four cathodes is related to human health, which means human health is a common problem for these four cathode materials to solve. In fact, Wanger [34
] has confirmed that the effect of LIBs on human health is a common problem for LIBs. For the cathode, the effect on human health remains a major concern. Its existence may require a major technical improvement to overcome. For these individual problems in different cathodes, we can learn from the strengths and weaknesses of different cathodes. For example, material A always shows the largest environmental load in these four cathodes. Reducing its yield or finding alternative models, elements, or mechanisms is a feasible way to reduce its impact on ecosystem quality. As we know, IMPACT 2002+ and EI-99 have the same ranking for the environmental sustainability among these four cathode materials. Three LCAs show that the impact on human health is a common problem for these four cathode materials. However, in ReCiPe, material C consumes more resources compared with B. In IMPACT 2002+, material A’s impact on ecosystem quality makes a relative contribution to its total environmental impact.
3.3. Midpoint Level
Similarly, each endpoint indicator can be divided into a number of midpoint indicators. In order to avoid interference from different units and magnitude, we choose all values of material B as the benchmark and normalize all values of the other three cathode materials. These indicators with an extreme value always show great disadvantages and advantages for different cathodes, and these normalized values, obviously larger or less than 1, are more meaningful for individual cathode improvement. In IMPACT 2002+, the difference values between all normalized values and B’ normalized value (1) are shown in Figure 4
. In order to consider the impact of different LCA, we still give the unit of each midpoint indicator, reflecting their evaluation criteria. As we can see, there are 14 midpoint indicators, each of which has a different unit of measurement. To some extent, IMPACT 2002+ is more suitable for characterization evaluation. For example, we can use unit kg PO4
p-lim to express the land use problem. Moreover, because of the presence of phosphorus, the data are meaningful for eutrophication.
As Figure 4
shows, when we regarded all normalized values of B as the baseline, except for the value of ionized radiation, other normalized values of A are larger than those of B. In particular, the eco-toxicity values of water body and surface are much larger, about 10.722 and 12.824, respectively. The high toxicity of cobalt may be the main reason for this situation. In fact, water problems and surface problems are difficult to completely separate, e.g., the toxicity of surface water. For water toxicity of A, water footprint assessment [35
] may be a great method to quantify its water problems and reflect its toxicity from another perspective. For material D, except for the value for mineral refinement, other values are less than for B. Though D shows the best environmental sustainability, a green mineral refinement process is needed for material D. Finally, all normalized values for material C are slightly less than those for B. Material C, as an improved cathode to B by slight manganese substitution, has similar environmental sustainability potentiality to material B. The close element composition between material B and C, as the common formula shows, LiFeX
/C (x = 0.98 in this study), accounts for the similar environmental sustainability. IMPACT 2002+ shows a sensitive assessment for cathode A, especially on water and surface ecotoxicity.
In EI-99, normalized values are shown in Figure 5
. These indicators have the same cells separated from the same endpoint indicators. Compared with the IMPACT 2002+, these indicators cannot reflect specific substances due to their common units.
As Figure 5
a–c shows, except for the radiation value, the normalized value of A is greater than that of B, and the respiratory inorganic matter, land occupation, and mineral resource problems of A are obvious, at 2.286, 2.247, and 1.982, respectively. For D, its mineral resource value is far larger, about 0.938. The ecological toxicity value is slightly larger. In addition to the mineral problems noted in IMPACT 2002+, the ecological toxicity of material D in EI-99 also becomes a low environmental sustainability index. Finally, all values of C are close to 1, as IMPACT 2002+ shows.
In ReCiPe, all normalized values are shown in Figure 6
. The midpoint indicators in ReCiPe are more detailed than EI-99. The unit for these indictors is different.
As Figure 6
a–c shows, the normalized values of nature land transformation and urban land occupancy for material A are far larger, at 2.975 and, 2.014 respectively. Actually, material D performs very well on these two indicators, with −0.586 and −0.702, respectively. Land occupation is an important part of the assessment of ecosystem quality [36
]. That is a key difference between material A and D. For material C, not all normalized values are close to B, especially metal resource consumption, which is as high as 1.752. The method ReCiPe concentrates more on the economic costs ($
) in resource consumption [37
], which means the economic cost in the slight manganese substitution process for material B needs to be cut down. To reduce the consumption of metal resources in the manganese substitution process must be a key issue for the development of material C. Finally, material D had low environmental sustainability in human toxicity, freshwater toxicity, and metal consumption compared with material B.
In addition, the problem of resource consumption deserves our attention. This endpoint in three LCAs is divided into two common midpoints, renewable and non-renewable resource consumption. We calculated the ratio of non-renewable resources to renewable ones. Mineral refinement in IMPACT 2002+, the mineral resource in EI-99 and the metal resource in ReCiPe are regarded as the renewable resource consumption. Likewise, the non-renewable energy, the fossil fuels and the fuel exhaustion are divided into the non-renewable resource consumption. The ratio is the non-renewable resources consumption per unit renewable resources consumption.
As Figure 4
b, Figure 5
d and Figure 6
d show, material B always has the highest ratio in three LCAs, 626.614 (MJ primary/MJ surplus) in IMPACT 2002+, 41.359 (MJ/MJ) in EI-99, 8.780 ($
) in ReCiPe. That means every unit renewable resource consumption needs more non-renewable resource in the whole life cycle of material B. Material B has the lowest environmental sustainability among these four cathodes. More green processes with low non-renewable resource consumption are needed for material B. As worldwide concern about fossil fuels grows, efforts at non-renewable resource protection are urgently required [38
]. These high ratios need to be reduced. Integrating the renewable resources in a small isolated power system, an isolated and complete battery [39
], and improving the capacity for cathodes [40
] are promising directions to achieve this goal.
Among the four cathode materials, the emphasis in the three LCAs is different. For material A, three LCAs all think that iron radiation is not a serious issue. The main problem in IMPACT 2002+ is ecotoxicity. On the contrary, EI-99 and ReCiPe think that the land issue is a serious issue. For materials B and C, the values are mostly close to each other except for the metal resource consumption. For material D, three LCAs all show its low environmental sustainability in terms of mineral resource consumption, and its toxicity is noted in EI-99 and ReCiPe.