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Sustainable Materials and Technologies 37 (2023) e00654
Available online 12 June 2023
2214-9937/© 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
Towards environmentally sustainable battery anode materials: Life cycle
assessment of mixed niobium oxide (XNO™) and
lithium‑titanium-oxide (LTO)
Lígia da Silva Lima
a
,
*
,
1
, Jianshen Wu
b
,
1
, Erasmo Cadena
a
, Alexander S. Groombridge
b
,
Jo Dewulf
a
a
Sustainable Systems Engineering (STEN), Department of Green Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, B,
9000, Ghent, Belgium
b
Echion Technologies LTD, Unit 9, Cambridge South, West Way, Sawston, Cambridge CB22 3FG, United Kingdom
ARTICLE INFO
Keywords:
Niobium-based anode
Lithium titanium oxide
Lithium-ion batteries
Life cycle assessment
Industrial anode production
Sustainability
ABSTRACT
Electric mobility has proven to be essential for the carbon neutrality of the transport sector. However, several
studies have demonstrated the environmental costs linked to the supply of rechargeable batteries, which should
not be overlooked. The supply of some elements has raised concerns, either because they are associated with
environmental and social risks, or because they are considered critical raw materials due to their concentrated
geographical supply. It is therefore important to look for innovative technologies capable of reducing the demand
for traditional battery raw materials and technologies, but that also have lower environmental impacts linked to
their supply. Niobium has been reported to improve the performance of battery components and could (partially)
replace some traditional battery materials, but little is known about the environmental impacts of niobium-based
battery materials. This study compares two commercial lithium-ion battery anode materials, namely lithium-
titanate (LTO) and an innovative mixed niobium oxide anode material (ECA-302, a formulation of XNO
TM
).
Life cycle assessment is employed to quantify the environmental impacts of both technologies, taking into ac-
count impacts on global warming potential (GWP), acidication, ozone depletion, photochemical ozone for-
mation (POF) and the use of fossil resources. The impacts were quantied by mass (1 kg anode material) and
functionality (1 kWh delivered/cycle life), using primary industrial data for ECA-302 and literature-adapted data
for the LTO. Results show that ECA-302 performs better than LTO considering both the material mass and energy
delivery per cycle levels. The GWP for the supply of the ECA-302 was 51% lower than the LTO, but the most
remarkable differences were observed for POF, for which ECA-302 had an impact about 72% lower than LTO at
the production stage and 77% lower at the energy delivery. The results also indicate that 20% less ECA-302
material is needed to deliver 1 kWh over the cycle life of the battery compared to LTO.
1. Introduction
Batteries have become essential for the reduction of carbon emis-
sions in the transport sector, being the key technology in electric vehi-
cles (EVs). They are considered a critical technology for achieving the
goals dened in the European Green Deal, which aims at a 55% reduc-
tion of greenhouse gases (GHG) emissions within the European Union by
2030 compared to 1990 levels, as well as ensuring that Europe becomes
the rst climate-neutral continent by 2050 [1]. Therefore, the European
Commission has encouraged the transition towards e-mobility as part of
the Climate Action Plan and, as a result, the battery sector for EVs has
expanded considerably in recent years. Different types of batteries are
available for EVs on the market, and among them, lithium-ion batteries
(LIBs) are one of the market leaders and the demand for this technology
is expected to increase [2–5]. There are different types of LIBs,
depending on their anode and cathode compositions and physical format
(shape). The most widely used cathode composition nowadays is the
lithium‑nickel‑manganese‑cobalt oxide (NMC) umbrella group along
* Corresponding author at: Research Group Sustainable Systems Engineering (STEN), Department Green Chemistry and Technology, Faculty of Bioscience Engi-
neering, Ghent University, Campus Coupure, Building B, Coupure Links 653, 9000 Ghent, Belgium.
E-mail address: ligia.lima@ugent.be (L. da Silva Lima).
1
Lígia da Silva Lima and Jianshen Wu contributed equally to the study. They are co-rst authors.
Contents lists available at ScienceDirect
Sustainable Materials and Technologies
journal homepage: www.elsevier.com/locate/susmat
https://doi.org/10.1016/j.susmat.2023.e00654
Received 17 March 2023; Received in revised form 19 May 2023; Accepted 10 June 2023
Sustainable Materials and Technologies 37 (2023) e00654
2
with lithium‑nickel‑cobalt-aluminium (NCA) and lithium‑iron-phos-
phate (LFP), whereas commercial anodes include graphite, lithium
titanate (Li
4
Ti
5
O
12
or LTO) and silicon/carbon [5–7].
An important parameter of LIBs for EVs is the battery capacity, but
also the charge and discharge speed plays a key role. Battery manufac-
turers have invested in the development of fast-charging battery mate-
rials to address the demand for this technology. The LTO-based batteries
have shown satisfactory results in this respect, being a successful tech-
nology in the fast-charging battery market, and this anode is often
combined with an NMC cathode in the battery cell [4]. Although LTO
batteries have been reported to have a low energy density, their positive
aspects, such as safety, extended lifetime, fast charging and slow
discharge have enabled their use in the automotive sector, even for large
vehicles such as buses [4,8–10]. LTO has been reported to be safer than
graphite, which is nowadays the most widely used anode material, even
though it does not have fast-charging properties [11].
Because of their potential to support the energy transition and the
achievement of ambitious goals related to the minimization of climate
change impacts (e.g. the European Green Deal goals), rechargeable
batteries are seen as environmentally friendly technologies. However,
their supply has unavoidable environmental burdens, which should not
be overlooked. Moreover, the transition to low-carbon transport results
in an increased demand for battery raw materials, such as lithium, co-
balt, graphite and titanium [12]. It has been estimated that the demand
for these materials in 2030 will be more than 2000% higher than the
2015 values [13], which may intensify environmental issues in the
supply phase. Between 2015 and mid-2018, the prices of lithium
increased by over 250%, as a result of expectations of higher future
demand for LIBs, which led to new exploration activities focused on this
metal [14]. Therefore, it is important to consider other raw materials
that could be used for battery technologies, thus, helping to diversify the
market.
In this context, niobium (Nb) appears as an alternative to reduce the
pressure on some raw materials. The development of Nb-based
rechargeable battery components has been investigated already for a
few years with positive results [15–19]. Nevertheless, these studies
focused on the electrochemical performance of Nb-based electrodes and
did not consider the environmental impacts to produce these battery
components or the burdens related to their use in LIBs. An innovative
class of Nb-based anode materials has been developed by Echion Tech-
nologies, Mixed Niobium Oxides (XNO™) [20]. It is an emerging anode
candidate and has received increasing attention, especially compared to
LTO in terms of electrochemical performance. From the XNO™ material
group, a specic formulation, “ECA-302” has been further developed
and tested, exhibiting close electrochemical potential to the LTO (1.65 V
for the ECA-302 and 1.55 V for the LTO) but higher reversible capacity
(208 mAh/g for the ECA-302 and 160 mAh/g for the LTO within the
voltage window of 1.1–3.0 V) and higher electrode volumetric energy
density (ca. 600 mAh/cm
3
for the ECA-302 and ca. 300 mAh/cm
3
for the
LTO) (Echion Technologies, personal communication). However, the
environmental impacts related to the production of this anode material
should also be evaluated and compared to LTO in order to evaluate if
ECA-302 is a more environmentally friendly technology or not.
Life cycle assessment (LCA) is a widely used methodology to quantify
the environmental impacts of a product or service over its life cycle,
being a well-established methodology, which also allows the compari-
son between different alternatives of the same product or service.
Several LCA studies for LIBs have been developed, but most of them
focus on the cathode composition of the battery. For instance, the NMC
batteries are often investigated, as this LIB chemistry has evolved and
demonstrated positive results for use in EVs [21–27]. In addition, the
LCA studies focusing on LTO-based LIBs do not provide detailed infor-
mation on the production of the battery and its components, i.e. the
material and energy input to produce the LTO anode [28–33]. Given the
growing battery market and the need for LCA studies assessing the po-
tential environmental impacts of those technologies, a detailed
inventory of the materials and processes to manufacture battery com-
ponents becomes essential. The currently available inventories for this
LTO anode material in the literature only provide limited information,
which makes it difcult to use them directly in an LCA study.
The objective of this study is to perform a comparative LCA study of
two LIB cells composed of two different anode active materials: an
emerging Nb-based active material, XNO™, formulation ECA-302 and
its well-known alternative, LTO, which is widely used in LIBs. This
assessment includes: i) the elaboration of a life cycle inventory (LCI)
describing the production of the LTO and ECA-302 anode active mate-
rials; ii) the quantication of the environmental impacts and their
comparison for ECA-302 and LTO at the production and energy delivery
simulation tests, considering the global warming potential, ozone
depletion, ozone formation, acidication and the use of fossil resources
impact categories; iii) the identication of the environmental hotspots
along the cradle-to-gate stages of the anode active materials production;
iv) the comparison of the global warming potential of the two fast
charging anode materials assessed, LTO and ECA-302, to a non-fast
charging alternative, the graphite anode material.
2. Materials and methods
2.1. Anode active materials technologies
To perform a fair comparison of ECA-302 and LTO, a similar LIB cell
design was assumed for both materials, both cells corresponding to the
LIB cells prototype validated in a Battery Design Model, described in
detail in Section 1.1 of the supplementary information (SI). Except for
the anode active material, all the other cell components were similar,
including the cathode, which was an NMC cathode with high nickel
content (NMC 811), as illustrated in Fig. 1. Since the anode active ma-
terials were different in composition, the anode slurry recipe and the
thickness of the anode coating (72.3
μ
m, excluding the thickness of
aluminium foil) were xed for the two cells, so that an equivalent cell
design was achieved to make a comparison as equivalent as possible
between the different chemistry systems. In this way, only the thickness
and mass loading of the cathode is varied within a xed volume of cell,
to match the changing capacity of the anode coating. The cell parame-
ters for both LTO and ECA-302 cells are listed in Table 1. The LTO-based
cell requires a slightly lower mass of active material (21.42 g) than the
ECA-302 (22.92 g) for operation. On the other hand, the ECA-302-based
cell has a larger capacity (considering the same cycling rate of 0.5C),
which results in more energy delivered over a cycle life (63.94 kWh)
compared to the LTO-based cell (47.59 kWh), wherein a cycle life is
dened as the number of charge and discharge cycles the cell can un-
dergo in a standardized battery design until the battery capacity has
dropped to 80% of its nominal value. Because of this difference in total
energy delivered, the ECA-302-based cell requires about 20% less anode
material (0.36 g) to deliver 1 kWh of energy over a cycle life, as
compared to the mass required by the LTO anode (0.45 g), to deliver the
same amount of energy.
2.2. Life cycle assessment methodology overview
Life cycle assessment is a standardized methodology used to quantify
the environmental impacts of a product or service along its life cycle,
which is developed in four stages adapted to describe the product or
service to be assessed [36,37]. These stages are: i) goal and scope de-
nition; ii) life cycle inventory (LCI); iii) life cycle impact assessment
(LCIA); and iv) interpretation. The application of the LCA methodology
to compare the environmental impacts of LTO and ECA-302, in accor-
dance with the International Standards Organization (ISO) recommen-
dations, is described in more detail in the following sections.
2.2.1. Goal and scope of the study
This study consists of a comparative cradle-to-gate LCA, focused on
L. da Silva Lima et al.
Sustainable Materials and Technologies 37 (2023) e00654
3
the production of two LIB anode active materials and their energy de-
livery over their cycle life (tests), but the end-of-life (EoL) of the tech-
nologies is not assessed. “Cycle life” is dened as the number of charge
and discharge cycles the cell with the tested anode active materials can
undergo in a standardized battery design until the battery capacity has
dropped to 80% of its nominal value [35]. This is sometimes dened as
service life, but the use of service life or service lifespan may lead to
confusion, as these terminologies have been reported to present different
meanings, in general referring to the period when a product still func-
tions, regardless of its efciency, which includes the duration of its
distribution to next users [38]. Therefore, in this study, cycle life will be
used to report the total amount of cycles of the cell, within a cell capacity
retention of 80% or more. The functional unit (FU) used to calculate the
environmental impacts of both materials was the mass of anode active
material (1 kg of LTO and 1 kg of ECA-302), while the functionality of
the materials was considered in a separate FU, based on the energy
delivered over the cycle life of the battery cells containing the anode
active materials (1 kWh delivered over a cycle life). To calculate the
total energy delivered by a cell over its cycle life, the following equation
was used, which is adapted from Swart et al. [35]:
The factor 0.9 is used to model the capacity decrease from 100% to
80% over the cycle life, assuming this drop occurs linearly. In the
equation proposed by Swart et al. [35], the voltage of the cell is also
included. In this study, the voltage of the cell is already taken into
Fig. 1. Schematic representation of a lithium-ion battery cell and its components. The battery component of focus in this comparative study is the anode active
material (LTO or ECA-302), the other cell components were considered to be the same, including the cathode. The LTO anode has a spinel conguration, as illustrated
in this gure, whereas the ECA-302 has a Wadsley-Roth structure (gure adapted from Zhao et al. [34]).
Table 1
Characteristics of LTO- and ECA-302-based lithium-ion battery cells. Results from the Battery Design Model. More information can be referred to in Section 1.1 in
Supplementary Information.
Cell parameter LTO ECA-302
Anode active material in a 2170 cell (g)
1
21.42 22.92
Nominal cell energy (Wh/cycle) 6.61 8.88
Cycle life at 0.5C (number of cycles until cell capacity retention drops to 80%)
2
8000 8000
Cell voltage 2.33 2.29
Energy delivered over a cycle life (kWh)
3
47.59 63.94
Required mass of anode active material to deliver 1 kWh over a cycle life (g/kWh)
4
0.45 0.36
1
The 2170 refers to a standard cell conguration in which the cell has 21 mm diameter and is 70 mm long. The cathode of the cell has a nickel‑manganese‑cobalt
composition with metal ratios of 8:1:1 (NMC 811). Anode active material mass based on an anode coating thickness of 72.3
μ
m for both anodes (excluding aluminium
foil) for equivalent cell design;
2
Cycle life is the number of charge and discharge cycles the cell can undergo in a standardized battery design until the battery capacity has dropped to 80% of its
nominal value. Herein a more conservative cycle life of 8000 cycles has been assumed based on available test data for commercial LTO cells, and cycle life forecasts for
ECA-302 from ongoing tests. Both technologies are potentially capable of cycle life in excess of 10,000 cycles. The C-rate relates to the discharge period of the battery
cell, a 0.5C (or C/2) means the cell will be fully discharged in 2 h;
3
The energy delivered over a cycle life is calculated by multiplying the cell capacity, cycle life and the 0.9 factor (adapted from Swart et al. [35]);
4
The mass of the anode active material required to deliver 1 kWh over a cycle life is calculated by dividing the mass of the active material in a cell by the total energy
delivered over the cycle life.
Energy delivered over the cycle life (kWh) = capacity(kWh
cycle)*cycle life (cycles)*0.9
L. da Silva Lima et al.
Sustainable Materials and Technologies 37 (2023) e00654
4
account in the calculation of the nominal cell energy (Wh/cycle) and
therefore, it is not included in the equation.
Based on the information available in the literature about the pro-
duction process of the LTO active material [28–30,39], which is similar
to the production process employed by Echion Technologies for the ECA-
302 active material, the same production processes were considered for
both anode active materials. This production process takes place via a
dry-powder approach, which involves a powder homogenization step,
Fig. 2. System boundaries of the comparative life cycle assessment. The dashed line indicates the boundaries of the material supply and industry where the anode
materials are produced: A) lithium titanate active material (LTO); and B) niobium-based active material (ECA-302). Arrows indicate input and output ows from
nature and technosphere to the systems and emissions from the systems to the technosphere and environment, including the nal products (anode active materials).
L. da Silva Lima et al.
Sustainable Materials and Technologies 37 (2023) e00654
5
high-temperature solid-state synthesis (calcination) and de-
agglomeration steps. The production scheme was adapted to include
the inputs and outputs reported in the literature for the LTO manufac-
ture and was validated with a Manufacturing Process Model (MPM)
(Section 1.2 of the SI),; the production scheme illustrated in Fig. 2A was
considered for LTO, which is very similar to the scheme for the ECA-302,
illustrated in Fig. 2B. The schemes consist of inputs from the techno-
sphere (i.e. energy and auxiliary materials), as well as emissions to the
technosphere (solid residues) and to the environment (atmospheric
emissions). The LCA study includes the aforementioned inputs and
outputs, required to produce the LTO and ECA-302 anode active
materials.
2.2.2. Life cycle inventory
The LCI consists of the inputs (e.g. raw materials, energy and aux-
iliaries) and outputs (e.g. products and emissions to the technosphere
and the environment) and their respective quantities. Typically, some
LCA evaluations re-used LCI from previous works, which may be out-of-
date, not validated or unsuitable for scenario analysis, leading to inac-
curate results and even misleading conclusions. To mitigate the un-
certainties, it is a good practice to adapt and validate previous LCI
datasets to a realistic scenario under assessment using a self-developed
manufacturing process model. This combined process modelling-LCA
approach has been widely used in the LCA community [40–42] and
thus, similarly implemented for the LTO in this study. The LCI describing
the LTO production process (foreground data) was adapted from the
inventory reported by Dai [28], being the latter elaborated using the
LTO production process described by Ohzuku et al. [39] as a reference.
The production parameters, such as feedstock, energy consumption and
reaction temperature, were simulated using the MPM, the same one used
to model the ECA-302 production. This was done because both
manufacturing processes share a high degree of similarity, and after
modelling the LCI data for LTO, very similar values as the ones reported
by Dai [28] were found, implying the validity of the model to simulate
the LTO production process. All the required datasets to describe the
inputs and outputs in the LTO production (background data) were
available in the ecoinvent v3.8 database, except for the alumina saggar,
for which the same dataset modelled for the ECA-302 anode production
was used.
The LCI for the ECA-302 production (foreground) was mainly based
on primary data provided by Echion Technologies, compiled based on
the production of 1 ktonne of anode in a year and utilizing the MPM,
which was associated with representative datasets available in ecoin-
vent v3.8 (background). However, due to the unavailability of some
datasets in ecoinvent describing certain inputs, datasets were modelled
based on studies available in the literature or information provided by
the material supplier, describing the production of these materials. This
was the case for the niobium oxide (Nb
2
O
5
), for which only one study
with a detailed LCI was found in the literature, and therefore, it was
assumed that the niobium oxide input for the ECA-302 was similar to the
material described in the published study [43]. Datasets also had to be
modelled for alumina saggar and the mixed metal oxides datasets on
information provided by the suppliers and related information pub-
lished in the literature. Due to condentiality, more detailed informa-
tion about the alumina saggar and the metal oxides mixture cannot be
provided (Echion Technologies, personal communication).
The production site of both LTO and ECA-302 anode materials was
assumed to be located in Brazil, with the use of local Brazilian electricity
(South-eastern Brazilian mix), as this is localized to the majority source
of Nb-based raw materials (CBMM). To enable a fair comparison and
because the production schemes of ECA-302 and LTO were assumed to
be similar, the local datasets, such as the energy matrix used in the
production of the LTO, were assumed to be similar to the ECA-302.
However, this may not be a realistic representation of the geograph-
ical production of the LTO material. Except for the local electricity and
created datasets, all the inputs/outputs were selected with the “Global
(GLO) market” option, unless the datasets did not have this geographical
coverage possibility. In this case, the “Rest of the World (RoW) market”
was chosen. For solid wastes generated from the production process,
waste treatment processes available in ecoinvent, such as incineration
and landll, were employed.
2.2.3. Life cycle impact assessment
A total of ve impact categories were considered and quantied for
the production of both anode active materials, as well as for the tests
simulating the energy delivery of the LIB cells. The selected impact
categories were global warming potential, acidication, ozone deple-
tion, photochemical ozone formation and fossil resources use. For this
selection, it was taken into account that the main material input for both
anode materials are metal-based oxides and hydroxides, and therefore,
most of the impact categories recommended in the literature for
harmonization of metal-related LCA were considered, except for eutro-
phication [44]. Instead of eutrophication, fossil resource use was cho-
sen, considering that the purpose of the anode materials is to support the
replacement of internal combustion engine vehicles (ICEV), which are
fossil-based, with EVs. In view of the potential that both LTO and ECA-
302 technologies have to reduce the consumption of fossil resources (i.e.
compared to ICEV), it is also important to look into the fossil-based in-
puts required for their production.
The LCIA method used to calculate the global warming potential for
100 years time horizon (GWP100) was the IPCC 2021, the most recent
one elaborated by the IPCC and based on their latest assessment report
[45]. The remaining impact categories were assessed using the Product
Environmental Footprint (PEF) calculation method, developed by the
European Commission as an outcome of the Integrated Product Policy
and its framework to improve products' environmental performance
Table 2
Life cycle inventory for the production of 1 kg lithium titanate (LTO) anode active material.
Flow Amount ecoinvent dataset or material details
Inputs
Titanium dioxide 0.90 kg Titanium dioxide {RoW}| market for | Cut-off, U
Lithium hydroxide 0.22 kg Lithium hydroxide {GLO}| market for | Cut-off, U
Alumina saggar 0.05 kg Dataset created, available in supporting information (Table S4)
Liquid nitrogen 0.75 kg Nitrogen, liquid {RoW}| market for | Cut-off, U
Electricity 3.25 kWh Electricity, medium voltage {BR-South-eastern grid}| market for electricity, medium voltage | Cut-off, U
Outputs
Lithium titanate (LTO) anode material 1.00 kg Reference product
Oxygen gas 0.08 kg Oxygen to air
Heat waste 1.97 MJ Wasted heat from the process to the atmosphere
Nitrogen gas 0.75 kg Nitrogen gas released into the atmosphere
Waste to treatment (incineration) 0.04 kg Hazardous waste, for incineration {RoW}|treatment of hazardous waste, hazardous waste incineration|Cut-off, U
Hazardous waste, for underground deposit {RoW}|treatment of hazardous waste, underground deposit|Cut-off, U
Waste to treatment (landll) 0.04 kg
L. da Silva Lima et al.
Sustainable Materials and Technologies 37 (2023) e00654
6
throughout their life cycle. The PEF has been used to guide European
policies and investments to achieve environmental sustainability goals,
such as the ones dened in the European Green Deal [46]. The PEF has a
set of 16 impact categories, from which acidication, ozone depletion,
photochemical ozone formation and fossil resources use were selected.
The life cycle was modelled and the impact assessment was calcu-
lated using SimaPro 9.4 software. The calculated impacts include
infrastructure requirements, such as energy and resources used in the
production of feedstock, electricity and auxiliaries. An uncertainty
analysis was performed for both anode materials (LTO and ECA-302) to
identify potential results deviations related to the datasets used to
calculate the impacts. The Monte Carlo simulation function available in
the SimaPro software was applied to the production models of both
anode materials. This is further described along with the results in the SI
(Section 6).
3. Results and discussion
3.1. Life cycle inventory
The LCI for the production of 1 kg LTO anode material is listed in
Table 2 and the LCI for the production of 1 kg ECA-302 anode material is
given in Table 3. Some of the inputs have been assumed to be the same
for both materials, such as electricity and the alumina saggar used in the
calcination process. The LTO anode requires a higher amount of elec-
tricity and demands liquid nitrogen, which is not needed in the pro-
duction of ECA-302. The main material inputs for the LTO are TiO
2
and
LiOH, as reported in the synthesis process by Ohzuku et al. [39]. In the
case of the ECA-302, the metal oxide mixture is the main input in terms
of mass, with a majority component being Nb
2
O
5
, as this is the matrix
constituent in this anode material chemistry, serving to incorporate
other metal elements. The specic percentage of Nb
2
O
5
, as well as the
composition of the other metal oxides cannot be published due to the
condentiality of the ECA-302 production process.
In the calcination process, an alumina saggar is used as a consumable
in both LTO and ECA-302 production schemes. This saggar has a total
weight of 4.8 kg and it is estimated to have a total of 20 thermal cycles
on average before reaching the end of life. Each production cycle results
in 5 kg of anode material, which means that one alumina saggar (4.8 kg)
can be used for the production of 100 kg of anode material. Therefore, it
translates into 0.048 kg (or approximately 0.05 kg) of alumina saggar
used in the calcination process to produce 1 kg of anode material. Also in
the calcination process, 30% of the electricity supplied for heating
purposes is assumed to be lost as heat, due to thermal convection and
heat carried to the cooling zones of the saggars [47]. This energy loss
was also considered in the emissions to the environment. The oxygen gas
(O
2
) emissions originate from the stoichiometric oxygen surplus present
in the chemical composition of the metal oxides after they undergo
chemical conversion at an elevated temperature.
In the MPM applied to LTO, nitrogen gas (N
2
) was assumed to be
emitted, since liquid nitrogen is used at a temperature of 800 ◦C for 12 h,
as reported by Ohzuku et al. [39] for the same process. No N
2
emissions
are reported in the LCI of Dai [28], which is also based on the process
described by Ohzuku et al. [39]. However, from a mass balance view-
point, it is expected that the N
2
added to the system gets released in some
way. It is also assumed that no nitrogen oxides (NO
x
) are formed,
meaning that no reaction between N
2
and O
2
is considered at 800 ◦C, as
it has been reported that the formation of nitrogen oxides occurs mainly
at temperatures above 1300 ◦C and around 800 ◦C hardly any nitrogen
oxides are formed [48].
In each of the production stages, solid residues are produced, con-
sisting of the metal oxides and lithium hydroxide (LiOH), but also the
alumina saggar. These solid wastes are currently collected and sent to
waste treatment, with the possibility of recycling waste fractions being
under investigation. Although the waste production is considered low
(about 8% of the total mass input), it is rich in metals that could be
reintegrated into the production of new anode materials. A recent study
reported the economic and environmental benets of recycling metal
saggars used in the production of LIBs, such as Ni-Co-containing saggars
used in the production of cathodes. The authors of the study concluded
that besides reduced resource consumption for the production of new
battery components, recycling also reduces costs of waste treatment
with a rate of return of 3.16, considering the resource input, environ-
mental cost and the value of products [49]. Since the recycling of
alumina saggar and other solid waste materials has not yet been
implemented, it was assumed that half of the solid waste is sent to
incineration and the other half is sent to landll, because according to
Mazari et al. [50], metal oxides such as those used for the production of
LTO and ECA-302 production are eventually incinerated or recycled.
3.2. Life cycle impact assessment and interpretation
3.2.1. Environmental impacts at the anode material level
The results from the environmental impacts assessed to produce 1 kg
of LTO and 1 kg of ECA-302 are illustrated in Fig. 3, which clearly shows
that ECA-302 anode production results in lower environmental impacts
compared to LTO, for all impact categories. In fact, the impacts of the
LTO are higher than the ECA-302 by a factor of at least 1.6 (ozone
depletion) and up to 3.5 (photochemical ozone depletion), as listed in
Table S11 (SI).
The metal oxides/hydroxides used for the production of 1 kg of LTO
material (TiO
2
and LiOH) are the main sources of impacts in all cate-
gories, being also the main reason for the higher impacts of the LTO
compared to the ECA-302. This is related to their share in the material
composition, which corresponds to 47.2% for TiO
2
and 11.3% for LiOH.
Looking at their contributions, TiO
2
is responsible for 56.4% of the
impacts on fossil resource use and 62.4% on acidication, its lowest and
highest impact shares, respectively. The LiOH shows overall lower
contributions but they are still remarkable. The lowest share is observed
for fossil resource use, with 30.3%, and the highest one for ozone
Table 3
Life cycle inventory for the production of 1 kg niobium-based (ECA-302) anode active material. Some material ows are presented in an aggregated form for
condentiality reasons.
Flow Amount ecoinvent dataset or material details
Inputs
Metal oxides 1.03 kg Niobium oxide (Nb
2
O
5
) and metal oxides mix supply. Nb
2
O
5
is the dominant compound (above 75% by weight)
and its dataset is available in the supporting information (Table S5)
Alumina saggar 0.05 kg Dataset created, available in supporting information (Table S4)
Electricity 1.61 kWh Electricity, medium voltage {BR-South-eastern grid}| market for electricity, medium voltage | Cut-off, U
Outputs
Niobium-based active anode material (ECA-302) 1.00 kg Reference product
Oxygen gas 3.52 10
−4
kg Oxygen to air
Heat waste 0.82 MJ Wasted heat from the process to the atmosphere
Waste sent to treatment (incineration) 0.04 kg Hazardous waste, for incineration {RoW}|treatment of hazardous waste, hazardous waste incineration|Cut-off, U
Hazardous waste, for underground deposit {RoW}|treatment of hazardous waste, underground deposit|Cut-off, U Waste sent to treatment (landll) 0.04 kg
L. da Silva Lima et al.
Sustainable Materials and Technologies 37 (2023) e00654
7
Fig. 3. Environmental impacts of the production of 1 kg anode material, impacts per input/output. The colors represent the inputs and outputs of the production
process and in the columns, they indicate the share of the total impact to which they contribute. The total impact value is reported above each of the stacked columns
and the unit of the quantied impact is indicated below the columns (e.g. kg CO
2
eq./kg anode). LTO stands for lithium titanate (Li
4
Ti
5
O
12
), whereas ECA-302 is a
niobium-based anode material.
Fig. 4. Environmental impacts of the production of 1 kg anode material, LTO and ECA-302, with impacts per production stage. The colors represent the stages of the
anode material production, including the supply of the raw materials and each of the processing stages. LTO stands for lithium titanate (Li
4
Ti
5
O
12
), whereas ECA-302
is a niobium-based anode material.
L. da Silva Lima et al.
Sustainable Materials and Technologies 37 (2023) e00654
8
depletion, with 32.4%. In the production of 1 kg of ECA-302 material,
Nb
2
O
5
results in most of the impacts, with impact shares from 84.5% on
photochemical ozone formation up to 88.1% on ozone depletion. This
was expected, as this material consists of more than 75% of the materials
input to produce the ECA-302. The other metal oxides together show
much lower impacts, having the highest impact share of 5.8% in
photochemical ozone formation. Besides the metal oxides/hydroxides,
electricity is another input resulting in signicant impacts in all the
categories for both materials, showing the highest impact shares in fossil
resource use, with 8.5% of the LTO impacts and 7.8% of the ECA-302.
These results are in line with the global warming potential quantied
by Dai [28], who reported that the material inputs were related to most
of the impacts (about 80%), followed by energy inputs.
Another way to illustrate the impacts is by looking at the production
stages and associated waste treatments, to identify the hotspots in terms
of impacts. This is represented in Fig. 4, where the anode production is
divided into four stages: i) metal oxides/hydroxides supply, consisting of
the background impacts for the production of the raw materials required
for the anode materials; ii) powder homogenization; iii) solid-state
calcination; and iv) deagglomeration. The latter three stages take
place within anode active material production facilities. Once more, the
supply of the raw materials is responsible for most of the impacts, with
more than 85% of the impacts for both LTO and ECA-302, in all impact
categories assessed. Within the active material manufacturing process,
the calcination is the one with the largest share, 5.0% to 10.1% of the
LTO impacts and 5.2% to 7.6% of the total impacts of ECA-302. This is
because this process needs more electricity than the others, as the ma-
terials have to be heated from room temperature to at least 800 ◦C to
complete the reaction. This is more prominent for the LTO, as additional
energy is needed to heat the nitrogen, which is an input not needed for
the ECA-302. Moreover, it is the stage when more solid waste is pro-
duced, including the residual alumina saggar.
Although the supply of metal oxides results in more than 85% of the
impact per category (Fig. 4), other inputs and outputs also have signif-
icant contributions that should not be overlooked. However, in the re-
sults in Fig. 3, the shares of these inputs and outputs are not clearly
visible. To allow a better interpretation of the results and comparison
between ECA-302 and LTO, the same LCIA results are presented in a
normalized stacked columns chart, in Fig. S2 of the SI, where the ows
with rather minor impact contributions are more visible. A detailed
discussion about the main sources of impact per impact category is also
provided in the SI (Section 4).
3.2.2. Environmental impacts at the energy delivery level
The environmental impacts of the cells containing the LTO- and ECA-
302-based anodes to deliver 1 kWh over a cycle life are represented in
Fig. 5. Compared to the anode production (Fig. 3), a similar distribution
of the impacts per input and output is observed, with most of the impacts
originating from the supply of the main anode materials compounds
(Nb
2
O
5
, TiO
2
, and LiOH).
The difference in the overall impacts between the LTO and ECA-302
is more pronounced for the energy delivery (per 1 kWh over the cycle
Fig. 5. Environmental impacts of cells containing the LTO- and ECA-302-based anodes to deliver 1 kWh over a cycle life, with impacts per input/output. The colors
represent the inputs and outputs of the production process and in the columns, they indicate the share of the total impact to which they contribute. The total impact
value is reported above each of the stacked columns and the unit of the quantied impact is indicated below the columns (e.g. kg CO
2
eq./kWh delivered). LTO stands
for lithium titanate (Li
4
Ti
5
O
12
), whereas ECA-302 is a niobium-based anode material.
Table 4
Comparative global warming potential for three anode active materials including a niobium-based material (ECA-302), lithium titanate (LTO) and graphite. The results
for the LTO and ECA-302 are from this study, whereas the results for graphite include the one reported by Engels et al. [51].
Impact category LTO
(this study)
ECA-302
(this study)
Graphite
(reported by Engels et al. [51])
Graphite
(calculated using data from Engels et al. [51])
Global warming potential
(kg CO
2
eq./kg material)
10.19 4.95 9.62 13.70
L. da Silva Lima et al.
Sustainable Materials and Technologies 37 (2023) e00654
9
life) than for the material production (per 1 kg material). Looking at the
results from the comparative impacts reported in Fig. 3 and Fig. 5, it
becomes clear that the ECA-302 has improved environmental perfor-
mance if the cycle life is considered, showing further reduced impacts
compared to the LTO, in all categories. For instance, the relative impact
of ECA-302 compared to LTO for ozone depletion drops from 63% at the
material production level (6.23 ⋅10
−7
divided by 9.94 ⋅10
−7
) to 50% at
the energy delivery assessment (2.23 ⋅10
−10
divided by 4.47 ⋅10
−10
), a
difference of 13%. This is because a lower mass of ECA-302 is required to
deliver 1 kWh, more specically, about 20% less ECA-302 is needed than
LTO (Table 1) for equivalent cell designs.
3.2.3. Comparative global warming potential for LTO, ECA-302 and
graphite
Taking into account that both LTO and ECA-302 are fast-charging
anode technologies, a comparative exercise was done between these
two materials and graphite, which is a non-fast-charging technology that
typically has a lower battery lifetime (500–3000 cycles, depending on
design). This was intended to evaluate the potential benets of the fast-
charging technologies over the non-fast charging one in terms of tech-
nological performance (charging time), but also environmental burdens.
Moreover, considering that graphite is nowadays the most widely used
anode material for LIBs, it becomes relevant to compare the environ-
mental impacts of this anode to its alternatives. This comparison was
focused on global warming potential and, although datasets were
available in ecoinvent v3.8 for battery-grade graphite, it has been re-
ported that these datasets are outdated and lead to an underestimation
of the impacts [51]. Therefore, the results reported by Engels et al. [51]
were used, as they quantied the global warming potential for graphite
anode production, based on primary industrial data. The authors pro-
vide an LCI, which could allow an assessment focused on several impact
categories besides GWP100. However, after using their LCI data to
model the impacts of graphite, an overestimated global warming po-
tential was found compared to the value reported by the authors (about
42.5% higher). This could be because the authors calculated the results
using a different database and software (GaBi), as well as a different
calculation method (CML), or because the datasets selected in ecoinvent
v3.8 (Table S6 to Table S10 in SI) were not as representative as they
should have been, or most likely, a combination of all these factors. The
value reported by the authors of the study as well as the one calculated
based on their LCI are listed in Table 4, but only the result reported by
the authors was considered in the comparison, which consists of the
global warming potential related to the production of graphite.
Besides the results listed in Table 4, the graphite anode production
process available in ecoinvent v3.8 (‘graphite, battery grade (CN)’) was
also calculated, which resulted in 2.02 kg CO
2
eq./kg material. How-
ever, it has been pointed out by Engels et al. [51] that this ecoinvent
dataset is not representative, as the authors of the reference study used
by ecoinvent seem to have underestimated the energy consumption,
besides having used approximations for some of the production stages (i.
e. mining, crushing and milling) due to lack of data. Engels et al. [51]
also emphasize the importance of transparent primary industrial data for
a realistic quantication of the impacts, including direct CO
2
emissions
from combustion processes. In their assessment, Engels et al. [51] used
2019-year production data provided by a Chinese graphite producer and
validated by experts in the eld (industry and LCA practitioners), and
therefore, their result seems to be the most reliable global warming
potential value for graphite, quantied in 9.62 kg CO
2
eq./kg material.
Most of these impacts (73.8%) are related to the high embodied energies
used across the different stages of graphite production, with the most
energy-demanding stages being coating (38.4%) and spheronization
(20.2%). Surprisingly and in contrast to the results from LTO and ECA-
302 the raw materials supply, i.e. mining, only accounts for 4.0% of the
total global warming potential of graphite, which is also in contradiction
to the ndings of a study cited by Engels et al. [51], where the authors
indicate the mining stage as the most energy-consuming. The global
warming potential reported by Dai [28] for the production of graphite
shows an intermediate result, with energy input being the main source
of impacts, but with a signicant share coming from the material inputs,
higher than 4% of the total impact for this category. According to Engels
et al. [51], the high electricity consumption in their study is related to
the carbonization furnace used during the coating, which is considered
old and inefcient by the graphite suppliers. Once more, the availability
of industrial scale data from the producers allows not only the quanti-
cation of accurate resource consumption but also the identication of
bottlenecks and opportunities for improvement, like establishing a green
material sourcing chain and robust carbon neutrality strategies. Besides
Engels et al. [51], other researchers have recently raised the need and
importance of transparent and up-to-date primary industrial data for
reliable LCA studies of LIBs. Erakca et al. [52] indicate that currently,
little primary data about the manufacturing of LIB components are
available and that often, the available information is not transparent,
which compromises the traceability and comparability of results.
Overall, the ECA-302 is considered the most environmentally
friendly anode active material in terms of GHG emissions at the supply
stage. Although graphite takes the second position, in the long-term and
looking at the functionality of the material, LTO might still be a pref-
erable technology, as it has a much longer life at the expense of a global
warming potential only 5.9% higher than graphite. Some aspects that
make graphite a less interesting option include its ammability, its non-
recyclability, at least with the current technologies, and reduced cycles,
1000 compared to 8000 for ECA-302 and LTO (Echion Technologies,
personal communication). However, it is also important to mention that
these results are based on the impacts of the anode materials production
(on a kg of material basis), and will change if the energy delivery over a
cycle life is also considered for graphite. It is also relevant to consider
that the production data for ECA-302 and graphite were provided by
producers of these materials, so data for LTO has a higher degree of
uncertainty due to assumptions made. To account for these aspects, a
data quality assessment (DQA) was performed and the results are re-
ported in the SI (Section 5). The DQA framework used is an adaptation of
the pedigree matrix to assess the LCI of processes and ows, which also
includes two additional indicators [53]. According to this quality rating,
different process aspects are assessed using scores from 1 to 5, with 1
being the best and 5 the worst value. From the DQA results it becomes
clear that ECA-302 and graphite have similar quality rates, 1.6 and 1.3,
respectively, which are much better than the 2.7 calculated for LTO.
Looking at these results from a material substitution viewpoint,
replacing LTO as an active material with ECA-302 enables to spare part
of the lithium used in the battery, which is important as it is considered a
critical raw material for the EU [54]. Lithium is required in several
lithium-ion battery technologies (NMC, NCA, LFP and others), and given
the increasing demand for these technologies, the diversication of
materials will play an important role to match supply and demand.
Despite niobium being also considered a critical raw material [55], the
possibility of partial substitution of lithium by niobium in LIBs is
considered positive and helps in reducing the criticality of these mate-
rials [54]. In addition, the criticality of niobium is mainly related to
geopolitical aspects, such as its concentrated supply (Brazil). In terms of
the physical availability of this material, it has been reported that the
global niobium resources are sufcient for the current and projected
demand for at least the upcoming 500 years [56,57]. An LTO anode has
approximately 21.42 g of active material in a 2170 cell design (Table 1),
from which about 1.3 g (6.0%) is lithium, considering the stoichiometry
of its chemical formula (Li
4
Ti
5
O
12
). In an LTO-NMC 2170 cell, the total
lithium content is distributed among the anode (44.9%), the cathode
(53.0%) and the electrolyte (2.1%) (Echion Technologies, personal
communication). This means that making use of ECA-302 to replace LTO
in LIB manufacturing will minimize the demand for lithium by almost
45%, as well as it will lead to lower environmental impacts of anode
production.
L. da Silva Lima et al.
Sustainable Materials and Technologies 37 (2023) e00654
10
3.3. A reection on the importance of raw materials diversication and a
proper end-of-life management
Investments in new battery raw materials are essential for a sus-
tainable transition to carbon neutrality. The demand for batteries con-
tinues to increase and the supply of several raw materials required in
LIBs results not only in environmental burdens but also in social risks.
Studies assessing social risks in the supply of battery raw materials have
indicated that serious social rights and human violations occur in the
production of some of these materials, such as child labor, forced labor,
forced migration due to conicts, lack of healthy and safe working
conditions, among others [58,59]. Replacing battery raw materials with
alternatives that entail less environmental burdens and social risks is not
the only important initiative from a sustainability perspective, it is also
essential to look at the EoL of LIBs and implement recovery strategies, to
keep the battery raw materials that already exist in society in use, but
also to support a circular economy and resource use. Recent research
studies have investigated LIB recycling, looking at the available tech-
nologies, future outlook and their benets from environmental and
economic perspectives [4,5,60–68]. Nevertheless, LIB recycling is still
considered to be in its early stages [61,62].
Hydrometallurgy, pyrometallurgy, biometallurgy and direct recy-
cling are the currently available recycling technologies, with the two
rst ones being the most well-established ones [62,63,66,69]. Pyro- and
hydrometallurgy are designed to recover valuable metals in the elec-
trodes, especially cobalt, nickel, manganese and lithium, although
lithium is only recoverable through hydrometallurgy [61–63,66,69].
Considering the recycling of a 253 kg NMC automotive traction LIB,
recovery rates of about 93% have been reported for cobalt, nickel, and
manganese with both pyro- and hydrometallurgy, and in the case of
lithium, the same recovery rate was reported for hydrometallurgy
whereas no recovery was possible with pyrometallurgy [66]. In pyro-
metallurgy, lithium typically ends up in slags in the form of alloys
formed at elevated temperatures. Because niobium-based battery ma-
terials, such as the ECA-302, are rather novel technologies, no infor-
mation is available in the literature about their recycling and recovery
rates. However, because Nb
2
O
5
has very distinct acid-dissolution prop-
erties from other traditional LIB materials (e.g. cobalt, lithium, nickel), it
is expected that the battery recycling owsheet can be simply adapted
by including additional separation steps to target the recovery of Nb
2
O
5
with positive outcomes (Echion Technologies, personal communica-
tion). Despite recovery rates for Nb
2
O
5
from batteries being currently
unavailable, it is anticipated that it could be more than 95% through
selective solvent extraction which is the most common step used in
commercial Nb
2
O
5
extraction [70,71], and will also most likely be used
in Nb
2
O
5
recycling.
Direct recycling allows the recovery of a wider range of materials,
offering also the benet of anode materials recovery, such as graphite
[61–63,66,72]. Another advantage of direct recycling is that it preserves
the crystal morphology of the electrodes in the recycling process [69],
and although recovery results are reported mainly for the cathode ma-
terials [63,69], recovery and regeneration of the anode are also possible
[72]. Therefore, this emerging recycling technology could be highly
important to recover LTO and ECA-302 anode materials without
destroying their crystal structure. The recovery of anode materials has
been proven to not only lead to reduced environmental burdens, as
fewer fresh materials are needed but also to be protable in the case of
graphite [61]. Considering that both LTO and ECA-302 are made of raw
materials more valuable than graphite, their recycling would also lead to
reduced damage to the environment, avoided social issues and most
likely, economic benets.
4. Conclusions and future perspectives
Lithium-ion batteries (LIBs) and their raw materials have become
essential for the energy transition and to mitigate the adverse impacts of
climate change. This study focused on a comparative environmental
performance assessment of two long-life and fast-charging anode tech-
nologies for LIBs, namely lithium titanate (LTO) and an innovative
niobium-based active material (ECA-302), with the assessment based on
primary industrial data. Besides the comparative life cycle assessment
(LCA) including ve impact categories, a comparison between the global
warming potential of these materials with a lower lifetime and non-fast
charging technology, graphite, was also performed. Overall, the ECA-
302 demonstrated outstanding results compared to LTO in all cate-
gories evaluated, not only regarding lower detrimental environmental
impacts at the manufacturing stage, but also at the energy delivery
simulation tests. The results for global warming potential for the ECA-
302 were 2.1 times lower than the LTO considering the material pro-
duction, and this difference increased to 2.6 when the energy delivery
was considered. Even better results were obtained for photochemical
ozone formation, for which the ECA-302 had up to 3.5 times lower
impacts than the LTO at the material level, and up to 4.4 times lower for
the energy delivery tests. The improved performance of ECA-302 at the
energy delivery over a cycle life indicates that this material is more
efcient in terms of resource use to store and deliver renewable elec-
tricity in relation to the LTO, with a mass requirement 20% lower to
deliver 1 kWh. Compared to graphite, ECA-302 showed a global
warming potential about 2 times lower at the anode material
manufacturing stage, and potentially signicantly lower when the bat-
tery lifetime is considered.
For both ECA-302 and LTO, most of the impacts (at least 85%) are
related to the supply of the raw materials required for the composition
and functionality of the active materials. However, an opportunity to
reduce the share of the impacts of their supply would be to ensure that
the feedstock is sourced from suppliers compromised with environ-
mentally friendly industrial practices, such as the use of renewable re-
sources and energy. At the manufacturing plant, lower impacts could be
achieved through reduced electricity consumption by minimizing en-
ergy losses (e.g. if more efcient equipment is available), as well as by
making use of fully renewable electricity and secondary materials,
wherever possible. Although the generation of solid waste is rather low
(8% of the total mass input), because these residues are rich in valuable
materials, a recovery process could be implemented.
Given the overall results, it can be concluded that the niobium-based
anode material ECA-302 contributes to diversifying the battery mate-
rials market, helping to reduce or to balance the demand for battery raw
materials such as lithium. In addition, compared to LTO and graphite,
ECA-302 has proven its higher potential to support the achievement of
the European Green Deal goals, which are focused on reducing GHG
emissions in technology supply and energy delivery. In addition, as
compared to the LTO and graphite, which end up in either slags or
incineration at EoL, the dissimilar chemical properties of niobium to
other valuable battery elements pave its way to be efciently recycled
and hence create a closed-loop recycling. Moreover, the use of primary
industrial data was essential to properly quantify environmental impacts
and production bottlenecks, which can be improved. Thus, collabora-
tions between industries and researchers should be promoted to provide
reliable and transparent datasets to support the development of LCAs in
the battery sector.
The ndings of this study provide relevant information for the
battery-related raw materials sector, which can be used to guide in-
dustrial development and policymaking (e.g. diversication of battery
raw materials to achieve the EU Green Deal goals). However, a more
holistic assessment, also including the use phase and end-of-life of the
anode materials could provide additional important results to quantify
the overall environmental impacts, which are sometimes under-
estimated. This type of assessment can provide useful information for
other important European and global environmental concerns, such as
the criticality and circularity of raw materials.
L. da Silva Lima et al.
Sustainable Materials and Technologies 37 (2023) e00654
11
CRediT authorship contribution statement
Lígia da Silva Lima: Conceptualization, Methodology, Software,
Investigation, Visualization, Writing – original draft. Jianshen Wu:
Conceptualization, Methodology, Resources, Validation, Investigation,
Writing – original draft. Erasmo Cadena: Methodology, Investigation,
Visualization, Validation, Writing – original draft. Alexander S.
Groombridge: Resources, Validation, Writing – review & editing. Jo
Dewulf: Conceptualization, Supervision, Resources, Writing – review &
editing.
Declaration of Competing Interest
The authors declare the following nancial interests/personal
relationships which may be considered as potential competing interests:
This study consists of an independent assessment of the environ-
mental impacts resulting from the production of two anode active ma-
terials. The study was co-funded by Ghent University and Echion
Technologies LTD. Jianshen Wu and Alexander S. Groombridge declare
competing interests, both being employees of Echion Technologies LTD,
a company that develops and supplies advanced anode active materials –
in particular the class of Mixed Niobium Oxide materials, XNO™, for
which the company further holds intellectual property. The technical
contributions from Echion were through the provision of life cycle
inventory data, information about the production processes, and assis-
tance in the writing and reviewing process.
Data availability
Part of the data is condential. The non-condential data is shared in
the article or supplementary material.
Acknowledgements
The authors are grateful for the production data provided by Echion
Technologies during the development of this study and to Andrew Pauza
for his valuable contributions. The authors also appreciated the support
provided by the Brazilian Mining and Metallurgy Company (CBMM),
which is an important raw materials supplier for one of the technologies
assessed, especially to Thiago de Souza Amaral for his support
throughout the study.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.susmat.2023.e00654.
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