A review of hard carbon anode materials for sodium-ion batteries and their environmental assessment

Sodium-ion batteries are increasingly being promoted as a promising alternative to current lithium-ion batteries. The substitution of lithium by sodium offers potential advantages under environmental aspects due to its higher abundance and availability. However, sodium-ion (Na-ion) batteries cannot rely on graphite for the anodes, requiring amorphous carbon materials (hard carbons). Since no established market exists for hard carbon anode materials, these are synthesised individually for each Na-ion battery from selected precursors. The hard carbon anode has been identified as a relevant driver for environmental impacts of sodium-ion batteries in a recent work, where a significant improvement potential was found by minimising the impacts of the hard carbon synthesis process. In consequence, this work provides a detailed process model of hard carbon synthesis processes as basis for their environmental assessment. Starting from a review of recent studies about hard carbon synthesis processes from different precursors, three promising materials are evaluated in detail. For those, the given laboratory synthesis processes are scaled up to a hypothetical industrial level, obtaining detailed energy and material balances. The subsequent environmental assessment then quantifies the potential environmental impacts of the different hard carbon materials and their potential for further improving the environmental performance of future Na-ion batteries by properly selecting the hard carbon material. Especially organic waste materials (apple pomace) show a high potential as precursor for hard carbon materials, potentially reducing environmental impacts of Na-ion cells between 10 and 40% compared to carbohydrate (sugar) based hard carbons (the hard carbon material used by the current reference work). Waste tyres are also found to be a promising hard carbon precursor, but require a more complex pre-treatment prior to carbonisation, why they do not reach the same performance as the pomace based one. Finally, hard carbons obtained from synthetic resins, another promising precursor, score significantly worse. They obtain results in the same order of magnitude as the sugar based hard carbon, mainly due to the high emissions and energy intensity of the resin production processes.


Introduction
Sodium-ion batteries (SIB) are a recent development in the field of post-lithium batteries. These aim at overcoming limitations of existing lithium-ion batteries (LIB) in terms of resource availability, costs or performance. Following the same working principle as LIB (Fig. 1), they are considered a drop-in technology, based on similar electrochemical processes, materials and manufacturing processes. For SIB, the motivation for their development is the possibility of substituting lithium by sodium for the cathode and electrolyte, and of substituting copper by aluminium for the anode current collector (unlike lithium, sodium does not alloy with aluminium at the anode) [1]. The use of very abundant sodium promises advantages in terms of cost (sodium salts are a very cheap raw material), environmental impacts (lithium salt mining is more complex and requires higher inputs than sodium salt mining), resource supply (concentration of lithium in earth's crust is 20 ppm compared to 2.4% for sodium) and safety (SIB are, unlike most LIB, not prone to thermal runaway and explosion) [2]. However, due to the higher specific weight of sodium in comparison with lithium, the achievable energy densities are lower. This was identified as a major drawback in terms of economic competitiveness with LIB, requiring further research for obtaining better performance and further reducing costs [3]. In terms of environmental impacts, SIBs are considered promising, but only when achieving comparable performance regarding lifetime and energy density [4]. One of the key drivers for environmental impacts is the hard carbon (HC) anode. Currently, no hard carbon market exists for SIB anode material, why research projects and innovation companies use individual solutions for HC sourcing. These include HC synthesis from biogenic material like sugar or starch [5,6], organic waste fractions like husks or peels [7][8][9], but also fossil precursors like synthetic resins or petroleum coke [10][11][12]. For sugar based HC, significant environmental impacts are associated with the anode production, mainly due to low yields during carbonisation and significant energy demand for the process [4]. However, the underlying works that identified HC as one of the key contributors to the overall environmental impacts of SIB manufacturing modelled the HC production process in a very simple way, pointing out the need for future studies on the potential impacts of HC manufacturing including a more detailed process modelling [4]. The present study closes this gap by proving a comprehensive literature review of HC synthesis processes for developing a process model of HC synthesis from selected precursors as basis for their environmental assessment: in the following section (literature review), the results of the literature review are provided, including a tabulated overview of the key process parameters for HC synthesis and precursor treatment for all investigated feedstocks (the corresponding Table is located in the appendix at the end of the paper for better readability). Subsequently, the section "process modelling" describes the procedure for scaling up the underlying laboratory processes for three promising HC production pathways to a hypothetical industrial size, thus obtaining realistic energy and material balances for a hypothetical commercial production process. In the third section (life cycle assessment), an environmental assessment of the different hard carbon materials is done, providing information about the potential impacts of the different hard carbon materials and their relevance for the environmental performance of sodium-ion battery cells. The conclusion section finally provides a brief discussion of the main findings and an outlook for future works.

Literature review
A wide variety of hard carbon precursors are being investigated regarding their suitability for SIB anodes. In order to provide an overview of the most recent activities and the hard carbon properties obtained from the different materials, we reviewed the latest scientific works in the field. This was done via Science direct, Scopus and Google search, using the search terms "hard carbon", "sodium-ion", "anode" and "electrode". In order to provide a recent picture, the search was limited to 5 years timespan i.e., considering studies from 2014 onwards. Additionally, only studies that provide data about the electrochemical performance of the obtained material were considered.
A total of 30 studies on hard carbon synthesis were identified, with a wide bandwidth of both precursor materials and carbonisation temperatures used for the preparation. The results of the literature review including process parameters, pre-and post-treatment and key physical and electrochemical properties of the produced hard carbons are provided in Table A1 of the appendix. The variety of used feedstocks is huge, and so is the variety of the preparation processes. Acid or caustic pre-treatment is usually applied for improving the characteristics of the HC material (increased porosity/activation of the carbon). Twenty-four of the works rely on biogenic precursors (from wood over cotton or sugar until organic waste materials like apple peels or rice husks). Final carbonisation temperatures range between 700 and 1600°C, with 10 studies working with comparably low pyrolysis temperature (700-900°C), while the remaining ones apply temperatures of between 1000 and 1600°C with a peak around 1150°C ( Fig. 2; left). The reversible capacity of the obtained HC anode materials ranges between 175 and 390 mAh/g, with an uneven distribution and two peaks around 175-200 mAh/g and around 300 mAh/g ( Fig. 2; right). A correlation between severity of the carbonisation temperature and the electrochemical performance of the carbons cannot be identified. Also, no clear correlation between process conditions, type of precursor and electrochemical performance of the HC can be derived from the available studies. Additionally, only nine studies measure the performance of the HC material in full SIB cells. Therefore, no sufficiently solid basis for determining the most promising HC precursor for SIB anode material is given and the selection of the processes considered further is based on expert judgement and information from existing previous works. The selected precursor materials are (i) waste apple pomace, a biogenic waste material that attracted significant attention as promising precursor [13][14][15]; (ii) waste tyres, a non-biogenic waste material widely available [16][17][18][19], and (iii) synthetic resin, a nonwaste material available widely on the market at high quality [10,12,20].

Process modelling
For determining the input of energy and auxiliaries and emissions, detailed spreadsheet-based process models are created for the hard carbon synthesis from the selected feedstocks. These are based on the laboratory processes as described by the underlying publications, scaled up to a hypothetical industrial production for obtaining energy balances that would be representative for a commercial production process [21]. The assumed output of the hypothetical HC synthesis plant is 10 t/day, representing a small industrial size installation. The processes are modelled bottom-up on a component level in an Excel spreadsheet, with energy demand, auxiliary input and emissions of each component being individually determined based on chemical engineering plant design guidelines and reference processes [22,23]. While this approach is simpler than a full process simulation, it nevertheless yields reasonable approximations for a first evaluation and is frequently used for determining inventory data for life cycle assessments [4,21,24]. Also, for unconventional compounds like the given hard carbon precursors the usefulness of process simulation tools is very limited, while requiring extremely high modelling effort [25,26]. The core components common to all hard carbon processes (independent of the feedstock) are the pyrolysis reactor, associated compressors and pumps, gas combustion components and the exhaust gas cleaning. Bigger discrepancies exist in the pre-treatment of the precursors (complex process for the waste tyres, while synthetic resins do not require any further treatment prior pyrolysis due to their high purity (low content of contaminants). The processes are depicted as a flowsheet in Figures 3 and 4 and described in the following. Each component/processing step depicted in the flowsheets is represented by an individual process and dimensioned according to the assumed plant throughput. With the process parameters (temperature, reagents, residence time, yields etc.) from the laboratory processes, the energy inputs are then determined based on thermodynamic and stoichiometric calculations, reference processes and plant design guidelines [22,23,27]. Table 1 provides a more detailed picture of the principal parameters and the main literature sources used for estimating the inputs required in each process step.

Hard carbon production from waste tyres
For obtaining HC from waste tyres, a series of pretreatment steps are required. First, the rubber fraction is separated by side wall cutting, where tyres are cut and stripped off the scrap steel inside. The energy demand for this size reduction step is approximated based on technical reports on tyre recycling and equipment manufacturer datasheets [28][29][30]. The second step is shredding, where tyres are reduced to an average particle size of 300 mm, followed by solid state shear extrusion, where the particle size is reduced down to an average 300 mm [31]. For the former, required energy input is obtained from scientific publications [30,[32][33][34], while for the latter data from the corresponding patent is used [31]. For the intermediate transport steps, screw conveying is assumed, with the corresponding energy demand calculated based on general chemical plant design guidelines [23,35]. Tables 1 and 2 list the key design parameters for each of the process steps used for calculating the corresponding energy inputs. The finely ground tyres are then treated with concentrated sulphuric acid at 120°C overnight [36]. This yields sulfonated tyres powder that is then washed and filtered off. A serious metallurgic challenge facing the process model is designing a tank that can handle such severe process conditions. Under such conditions, an austenitic steel tank would be exposed to severe intergranular corrosion [37].
Thus, compromised process parameters are selected durable by stainless steel, with a sulphuric acid concentration of 30% kept at a temperature of 35°C [37]. The energy requirements for maintaining the acid bath at temperature are also obtained from chemical plant design guidelines [23,38]. The sulfuric acid is assumed to be regenerated every four cycles of hard carbon production. The regeneration process is modelled based on a comparable reference process, namely spent hydrochloric acid from pickling process (steel production) [39]: acid is pumped into   a roasting column where acid evaporates and goes through a cyclone for solids separation. The roasting column operates at 109°C, the boiling temperature of the acid. This is followed by an absorber column, where acid again is concentrated to 30% before being recycled back to the process. Efficiencies and energy inputs (electricity and natural gas) are taken from the reference process [39], adjusted by the specific heat capacity and boiling point of sulphuric acid [27,38]. The final pyrolysis temperature for waste tyres is 1100°C with a 4 h residence time in inert nitrogen environment [16]. While the highest initial coulombic efficiency is reported with hard carbon produced at 1600°C and prolonged residence times, such high temperature presents a metallurgic challenge for process and would require high grade steel. Thus, a pyrolysis temperature of 1100°C is considered an economic compromise, with lower capital investment and energy demand [36]. Reported yields are around 35%, requiring 3.2 tons of waste tyres per ton of hard carbon [29]. Apart from solid carbon, the process yields around 45% tar and 20% gases [17,18,36]. The pyrolysis process itself requires activation energy and elevated temperature, but the pyrolysis reaction is only slightly endothermic [26,40]. Thus, the energy input for the reactor can be approximated by the energy demand for heating of the vessel and auxiliaries, without accounting for the pyrolysis reaction enthalpy, thus simplifying the estimation significantly. Apart from heating the reactor (assumed to be fired with steam generated with natural gas), also other auxiliary equipment associated with the reactor is considered. This includes the nitrogen compressor for the reactor bed, and the steam (water) feed pump. Both are dimensioned according to standard chemical plant design guidelines with the parameters provided in Table 1.
After pyrolysis, a scrubber cleans the gas from sulphur components prior to combustion. Sodium hydroxide (NaOH) with 4% concentration is used for this purpose [16]. NaOH reacting with hydrogen sulphide is fully converted to sodium sulphide, with excess NaOH being filtered from the solids and recycled to a scrubbing column. The amount of sodium sulphide leaving the scrubbing column is calculated according to reaction stoichiometry [41]. Pyrolysis tar is not condensed as a side product, but burned entirely together with the gases, reducing the total energy demand. Although pyrolysis tar could also be condensed and recovered as a by-product, this is considered not to be interesting under economic aspects due to its recalcitrant properties (e.g., polymerisation and formation of resins, corrosiveness, abrasiveness) and low economic value [25]. A regenerative burner recovers the energy content of the gaseous pyrolysis products, modelled based on technical reports via thermodynamic estimations (stoichiometry and reaction enthalpy) [25,42]. This includes preheating the feed air with hot volatiles from pyrolysis bed prior to entering fired heater, combustion and final exhaust gas treatment (filtering of particles). An 80% efficiency is assumed for all heat transfer processes within. The recovered heat from combustion of pyrolysis is used for heating the pyrolysis bed, while excess heat is driving a Rankine cycle that generates electricity with an efficiency of 20%, thus reducing the demand for grid electricity [25,43]. The Rankine cycle is not modelled explicitly, but simply converts excess heat into electricity with the assumed efficiency.
With combustion of both tar and gas, and using the recovered energy for heat and electricity generation, the whole process would require a total net energy input of 1.77 MWh per ton of hard carbon obtained. Electricity demand makes up 29% of the total energy input (0.52 MWh) and heat the remaining 1.25 MWh. Highest demand for thermal energy stems from the pyrolysis reactor and the acid pre-treatment (acid bath heating and acid recovery), while electricity demand is driven mainly by shredding and pulverization processes. Internally generated heat and electricity cover the major share of the total plant energy demand (71%). Table 2 provides the corresponding break-down of the total energy demand to individual components.

Hard carbon production from apple pomace
Dou et al. updated their previous work for hard carbon synthesis from waste apples [13] to pectin free apple pomace [51]. Hard carbon yield from this feedstock is lower than from waste tyres (30% i.e., 3.3 tons of dry pomace feedstock per ton of hard carbon). Thus, bigger reactors and bigger conveying systems are required. Since no quantitative information on the yields of the pyrolysis byproducts (gas and tar) is provided in the underlying publication, tar and pyrolysis gas are assumed to have equal yields at 35% each. Since both are completely burned within the plant, the exact yield distribution is only of minor relevance. Apple pomace can be directly carbonized after grinding, unlike waste tyres that require extensive pre-treatment, simplifying the process flow significantly. Also, pectin extraction from the apple pomace already includes a phosphoric acid pre-treatment, why no further acid bath is required prior pyrolysis (repeating this process after pectin extraction does not improve HC yields or properties [51]). In consequence, the as-delivered pectin free apple pomace is simply dried with hot air and then ground to an average particle size of 50 mm [45,51]. A vacuum dryer is used for this purpose, with the required airflow and heat for drying the feedstock being estimated based on the key process parameters as given in Table 1 [52][53][54]. The final pyrolysis temperature for the pomace feedstock is 1100°C, with a residence time of four hours. An inert environment is achieved by a nitrogen atmosphere. The post pyrolysis system (combustor and exhaust gas cleaning) is designed in the same configuration as in the waste tyre process, though with varying throughput due to different feedstock and product properties (see Tab. 1). A flowsheet of the process is shown in Figure 4, while Table 2 provides the energy demand estimated for each process step. Due to the low inputs for pre-treatment and the higher amount of gas and tar produced (and thus the amount of heat and electricity generated), the process is energetically self-sufficient. It even generates a small amount of excess heat, which could be used for other purpose like e.g., space heating or generating electricity. However, since this amount is very small and might vary with feedstock composition, it is further considered in the assessment.

Hard carbon production from phenolic resins
The process model for hard carbon from phenolic resins is based primarily on the work of Wang et al. [10]. It shows the highest yield of hard carbon (40%) among the assessed processes, requiring 2.5 tons of resin per ton hard carbon produced. Due to the absence of data for yields of the remaining pyrolysis products, equal yields of 30% are assumed for both tar and gas [55][56][57]. Again, the exact percentages are of minor relevance since both fractions are burned within the process for producing process energy. The resin precursor is prepared from phenol, formaldehyde, water, and nitrogen [10]. Detailed inventory for this production process is available in the ecoinvent database [58], why it is not further modelled in detail. Since, due to their synthetic nature and corresponding high purity, the resins are fed as-delivered to the pyrolysis reactor, the drying and grinding steps depicted in Figure 4 do not apply for this feedstock. For the resin based HC, the pyrolysis residence time is four hours, with a final pyrolysis temperature of 1250°C [10]. Since the phenolic resins are a pure chemical product, they contain no sulphur and therefore no further post pyrolysis treatment (e.g., scrubbing) is required. In addition, no acid pre-treatment of the feedstock is required and thus no acid generation and -recovery. The remaining components of the HC production plant are identical with the previous processes. When looking at the energy demand per process step as provided in Table 2, again the pyrolysis reactor is the main driver for the process energy demand, requiring 4.2 MWh/t [59], while electricity demand for auxiliary components hardly adds up to 0.15 MWh. In consequence, the electricity demand can be covered internally, while additional fuel is needed for providing sufficient process heat.
4 Life cycle assessment 4

.1 Framework
The environmental assessment is following a cradle-to-gate approach i.e. quantifying the impacts associated with the hard carbon production until the final product [60,61]. The use and disposal of the product are disregarded because the information about electrochemical performance and cycle life is considered highly uncertain for the laboratoryproduced experimental hard carbons. Including these performance parameters might create misleading results. In consequence, the functional unit defined for comparing the environmental performance of the different hard carbon materials is 1 kg of hard carbon produced. For determining the influence of the different hard carbons on the impact of SIB manufacturing, a second assessment is done, using 1 kg of SIB cell. Also here, the different HC are assumed to be used in an identical amount and with comparable performance in the SIB pouch cell, and an assessment of the battery cells on a mass basis (without considering the specific energy density) is meaningful. Calculations are made in openLCA considering the cumulative energy demand [62] and the global warming potential (GWP) and resource depletion potential (RDP) according to the ILCD midpoint impact assessment method [63].

Inventory data
From the above described mass and energy balances, inventory tables are generated as input for the life cycle assessment. The obtained inventories as used for the following calculations are provided in Tables 3-5. For assessing the HC within SIB cells, inventory data from a previous life cycle assessment of SIB is used [4]. The inventory data for a single battery cell is taken from there and modified for modelling a pouch cell with increased energy density [64]. The inventory for the SIB pouch cell and the corresponding amount of hard carbon contained in the anode is provided in Tables 6 and 7.

Impacts of hard carbons
The most favourable results are obtained for the applepomace derived hard carbon (Tab. 8). It shows the lowest impacts in all three assessed categories. Especially for CED and GWP large differences can be observed. Like waste tyres, apple pomace is a waste product and (except transport) no environmental burdens or energy inputs are associated with its sourcing, but in addition it requires significantly less energy and chemicals input for pre-treatment than the waste tyres. Resin based hard carbon on the other hand shows clearly the highest impacts in all three categories, majorly due to the energy intensity of the precursor production. Resin synthesis requires significant inputs of chemicals and energy, leading to very high energy demand and thus unfavourable results in the assessed categories.
Breaking down the impacts to different compartments along the production chain allows for a more detailed analysis of the results. Figure 5 provides the environmental impacts obtained for the manufacturing of hard carbon from the three different precursors (waste tyres, apple pomace and resins) modelled in this work in comparison with hard carbon from sugar and petroleum coke. The impacts for the latter two are calculated based on the inventories provided by Peters et al. [4], the current reference work for SIB. Additionally, battery grade graphite is also included in the comparison, taken directly from the ecoinvent database [65]. Although not applicable for SIB, graphite nevertheless provides a certain benchmark, representing graphite-based LIB the current state of the art. The contribution of sub-processes is broken down to material precursors, chemicals required for the production process, direct process emissions, process energy and discussed in the following.

CED
Under CED aspects, huge differences can be observed between HC from precursors that have to be produced explicitly (sugar and resin), and from waste feedstocks or those from mined fossil precursors (tyres and pomace or graphite and coke). For the former, the total CED is dominated by the precursor preparation; mainly from agriculture and biomass cultivation, or for resin synthesis from crude oil-based precursors. The same applies in principal also to petroleum coke, which is, like resins, also based on crude oil. However, coke is a low value product and only a minor share of the energy demand is allocated to the coke product in the underlying ecoinvent dataset [66], thus obtaining more favourable results. Also, coke has similar properties to HC, why the conversion losses are lower and less precursor is needed for a given amount of HC [4]. In case of graphite, its energy content is not counted towards the CED in the applied LCIA methodology, Battery cell, Na-ion, NMMT-HC, pouch cell 1.00 kg considering graphite a pure non-energy product. However, this could be questioned, being graphite, like coal, also a carbonaceous energy-bearing mineral.

GWP
Highest GHG emissions are associated with sugar and resin-based HC. In both cases, the production of the material precursor (sugar and resin) is the main driver for GHG emissions, though due to different reasons. While for the sugar-based HC the direct emissions of CO 2 are significantly higher than for the resin based one, this is majorly biogenic CO 2 . This is reflected by the large negative emissions from feedstock production (CO 2 absorption from air during biomass growth). In sum, a  net GWP only slightly below that of resin-based HC is obtained. The same applies also to other biogenic materials, in this case the apple pomace. Accounting for the carbon uptake (negative emissions) during apple growth and allocating the corresponding share to the pomace residue would further reduce the corresponding net emissions. However, this would require modelling the whole apple production and apple juicing process, including the carbon uptake during biomass growth and emissions along the process chain, what is out of the scope of this work. This is coherent with the chosen cut-off process model approach, where all environmental impacts of a process are allocated to the products, leaving products from waste treatment to be available free of burden for further use [67]. Also for the resins, their production is energy and GHG intense, leading to the high impacts from feedstock production. Waste pomace and -tyres on the other hand show a good performance, since no environmental burdens are associated with the feedstock production.

RDP
Under resource depletion aspects, sugar and synthetic resins seem to be the least promising precursors, mainly due to the large amount of chemical products required for raw material production/agriculture (sugar) and for the raw material itself (fossil petroleum). However, unlike other impact categories, also natural graphite shows significant impacts in this category. Mining of natural graphite essentially is a mining activity aiming at extracting fossil and/or mineral resources, causing corresponding depletion of those. The same would apply also to petroleum coke, being crude oil a fossil resource as well. However, the ecoinvent approach allocates only a minor share of the RDP of crude oil to the coke by-product [66], why the contribution of the coke precursor to the RDP impacts of the HC are negligible (around 0.5%).

Impacts of SIB cells
For determining the relevance of the anode hard carbon material for the total environmental impact of the SIB cells, the manufacturing of SIB pouch cells is assessed with the different HC materials. The inventory data for the battery cells is based on a previous LCA of SIB, modified for pouch cells with a cell pouch mass share of 3% of the total cell mass [4,64]. Figure 6 displays the relative environmental impacts of battery cell manufacturing (1 kg of SIB cells; thus disregarding potentially different electrochemical performances of the HCs) with the different HC anodes, while Table 9 gives the corresponding numerical values obtained for the four assessed impact categories.
There is significant potential for reducing the total environmental impact of SIB production by carefully choosing the hard carbon anode material. Biogenic waste materials seem promising in this regard, as can be observed by the good results obtained for the apple pomace hard carbon, in line with the expectations by Peters et al. [4]. In  comparison, biogenic precursors that need to be produced explicitly like e.g., sugar seem to be less recommendable due to the comparatively high inputs required for their cultivation and lower yields during carbonisation. The same applies to non-biogenic precursors like resins. Their production is associated with significant burdens, driving up the environmental impacts of the SIB. However, electrochemical performances have not been considered, although these might vary notably. The environmental performance of the SIB cells based on hard carbon made from biogenic waste (apple pomace) is comparable to that estimated by Peters et al. for those using petroleum-or biowaste-based HC. The significant improvement potential expected by them for HC from biogenic waste material can thus be confirmed by the present assessment.

Conclusions
The three different hard carbon (HC) anode materials assessed in this prospective study show very different environmental profiles. The best results are obtained for HC made from biogenic waste material (apple pomace), confirming the expectations from a previous study [4]. For the assumed hypothetical pouch cell SIB, this would reduce the resource depletion potential by roughly 10%, GHG emissions by over 20%, and the cumulative energy demand by almost 40% compared to the reference hard carbon made from sugar precursors. The alternative non-biogenic waste raw materials, waste tyres, show similar performance in two of the three assessed categories, but slightly higher GHG emissions. The latter is caused mainly by the higher energy demand of the process, driven by the more complicated pre-treatment phase. Resin-based HC show a less promising performance, roughly comparable to that of the sugar-based HC, and would therefore not reduce the environmental impact of SIB significantly in comparison with the current reference. The results can serve as "guideline" for selecting promising HC precursors, with waste feedstocks showing a favourable performance, especially when no complex pretreatment is required. This seems reasonable, since converting waste into anode materials is also a way for creating added value from materials available at minimum costs. Another promising option seems to be fossil precursors like e.g., coke that already have characteristics similar to those of the final product. These show high conversion efficiency and thus low inputs per kg of HC produced. Less promising are feedstocks that are produced explicitly and with significant inputs (e.g., chemical products and/or energy). The corresponding upstream inputs and -emissions add to the total environmental burden and the obtained HC potentially show higher environmental impacts than graphite (the equivalent for LIB), thus not contributing to the environmental competitiveness of SIB.
Graphite, used as a reference material for comparison, in general shows a very good environmental performance. This can be attributed to the modelling approach in ecoinvent, where natural graphite is mined and only a comparably small amount of additional energy assumed to be required for its conversion into battery grade graphite, leading to low impacts except for resource depletion. Also, the intrinsic energy content of natural graphite is not counted towards the CED with the used impact assessment methodology. This is certainly questionable from a methodological point of view, since also the energy content of crude oil is considered for the CED in case of other nonenergy products like chemical products or plastics. Synthetic graphite, produced similar to hard carbons, would probably show a different environmental profile than the natural (mined) one. Current assessments of LIB are based majorly on natural graphite as modelled by ecoinvent, might therefore underestimate the actual environmental impacts of the anode.
However, also the limitations of this study need to be considered. It uses a simple spreadsheet model based on reference processes and generic chemical engineering guidelines and has to be considered as a first rough approximation. Also, the assessment does not consider possible problems with contamination of waste products, varying composition or quality, and thus potentially varying quality of the obtained hard carbons. Waste feedstock available at small scale might become valuable products with higher production volumes and increasing demand, leading to potential shortages in supply and maybe substitution effects. For a thorough assessment, also the current fate of the waste materials and such consequential effects should be considered (e.g., use of apple pomace previously used for animal fodder might cause increase in demand for other fodder feedstock and corresponding negative environmental impacts, etc.). The same applies to sugar as feedstock, where competence with the food sector might arise, analogue to the biofuel sector [68]. However, such consequential effects are out of the scope of this study and would require a dedicated separate assessment. Moreover, the batteries are assessed on a perkg basis, and the electrochemical performance of the different HC is not considered, though important. A more thorough assessment of the battery cells with the different HC anodes would be required for this purpose, taking into account also other performance parameters like energy density, cycle stability/lifetime, or efficiency under a life cycle perspective. Otherwise, results might be misleading, since all of these have significant influence on the lifetime environmental impact of battery cells [69]. Future works targeting the manufacturing of pilot scale battery cells with promising HC anodes and the experimental determination of these parameters would be required for overcoming these limitations. Appendix A  Table A1.