Method for engineering hard carbon from kraft lignin rich in inorganics

Abstract

The present invention provides a method for engineering a hard carbon material from kraft lignin, including the steps of - obtaining a kraft lignin starting material rich in inorganic species from a kraft pulping process, - drying the thus obtained kraft lignin starting material, - carrying out a thermal pre-treatment step on the dried kraft lignin, - purifying the pre-treated kraft lignin by removing the inorganic species therefrom, and - carbonizing the obtained pre-treated and purified lignin material.

Description

METHOD FOR ENGINEERING HARD CARBON FROM KRAFT LIGNIN RICH IN INORGANICSFIELD

[0001] The present invention relates to a method for engineering a hard carbon material, suitable for use in battery electrodes, from kraft lignin that is rich in inorganic species, such as inorganic cations. Further, the invention relates to the thus engineered material as well as its use in electrodes.BACKGROUND

[0002] Lignin is produced as a by-product in various pulp making processes, such as the kraft process, and numerous applications for utilizing this by-product have been proposed over the years. Notably, several attempts have been made to use lignin-based carbon to replace conventional carbon in various products, aiming to develop renewable carbon alternatives.

[0003] The thermoplastic and fusing behaviour of lignin particles make their thermal conversion into dispersed carbonized particles with controlled morphology and size highly challenging. Likewise, the injection and thermal conversion of thermally fusible lignin particles in the state-of-the-art thermochemical conversion reactors remains challenging due to lignin softening, melting, and swelling upon heating, which causes fouling and agglomeration problems in the reactor injection systems.

[0004] Energy and time-consuming thermal stabilization protocols could be used to cross-link the lignin structure, suppress its fusibility, and maintain its original morphological features during subsequent carbonization. However, the scalability of such a thermal stabilization method can be highly unpractical due to the excessively long thermal stabilization time and high energy demand. A recent publication, W02021250604A1, suggests using macroscopic agglomerates of a purified kraft lignin, having a very low content of inorganics, as starting material in a method involving thermal stabilization and carbonization. The suggested purification uses concentrated sulphuric acid. In contrast, also such lignin pre-treatment strategies are possible that are based on additional method steps, where the lignin is reacted with further chemicals. These options all require a complexmethod with a number of added reactants and pre-treatment steps required before the stabilization and carbonization steps can be carried out.

[0005] Thus, there is a need for a simple, yet effective, method for preparing a hard carbon material from a lignin precursor.SUMMARY OF THE INVENTION

[0006] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0007] According to a first aspect of the present disclosure, there is provided a method for engineering a new type of hard carbon material suitable for use in battery electrodes.

[0008] According to a second aspect of the disclosure, there is provided a hard carbon material prepared using this method.

[0009] According to a further aspect, there is provided the use of such a hard carbon material in preparing electrodes for alkali metal ion batteries, such as lithium-ion (Li-ion) or post-lithium-ion batteries.

[0010] The present invention thus relates to a method for engineering a hard carbon material from kraft lignin, including the steps of- obtaining a kraft lignin starting material rich in inorganic species from a kraft pulping process,- drying the thus obtained kraft lignin starting material,- carrying out a thermal pre-treatment step on the dried kraft lignin,- purifying the pre-treated kraft lignin by removing the inorganic species therefrom, and- carbonizing the obtained pre-treated and purified lignin material.

[0011] Further, the invention relates to the hard carbon material prepared using said method, and to the use of this material in preparing electrodes for lithium-ion or post-lithium-ion batteries.

[0012] The invention is based on the idea of allowing the inorganics carried to the kraft lignin starting material from its source to remain in the material during the drying and stabilization steps. While the presence of inorganics is perceived detrimental for hard carbon performance, it has now been shown to be beneficial during the stabilization step. The presence of inorganics significantly lowers the fusibility and thermal mobility of the kraft lignin particles and catalyses their cross-linking during the thermal stabilization. Thus, the thermal stabilization time is shortened and requires less energy input compared to stabilization of conventional pure kraft lignin, such as the kraft lignin obtained by the LignoBoost process or alike process, which includes the step of purifying the lignin from inorganics.

[0013] Several advantages are achieved using the present method. Among others, the presence of inorganic species, such as inorganic cations, in the lignin starting material causes a faster catalytic cross linking during thermal stabilization, reduced thermal mobility of the lignin chains upon heating, and suppressed particle fusing and morphology change, as compared to the use of a purified lignin starting material.

[0014] Further, good electrochemical performance can be achieved for the hard carbons prepared according to the present method and used as electrode material in lithium-ion batteries, on par with commercial hard carbons, and better than for hard carbons prepared from LignoBoost kraft lignin.

[0015] Due to the presence of inorganic cations and other inorganic species in the kraft lignin starting material, the proposed method solves the problems relating to feeding the kraft lignin starting material to the thermal stabilization step. This can be problematic with conventional lignin, such as the LignoBoost lignin, due to lignin foaming and fusing. In conventional processes, the lignin is typically processed before the thermal steps of the process. In the present method there is no need for mechanical or extensively long stabilization step, or a multi-step stabilization. The presence of inorganic species catalyzes the cross-linking reactions during the stabilization step. Since the inorganic species needed for this effect are brought into the method with the lignin starting material, there is no need for further reagent additions or pre-treatment reactions. Likewise, there is no need for removing these inorganics before the thermal stabilization step, which is done with concentrated sulfuric acid in the LignoBoost process.

[0016] Finally, with the optional re-slurring and spray drying of the lignin used as starting material, the method also provides a control of the size and morphology of both the kraft lignin starting material and of the hard carbon material derived from the method described herein, as it reduces the heterogeneity of the particle size and form, and reduces the energy needed for grinding. This allows spherical carbon microparticles to be obtained.

[0017] Thus, the present disclosure, or at least embodiments thereof, solves the following problems related to the engineering of hard carbon materials from lignin precursors:- Problem 1 : Challenges relating to the control over the size and morphology of kraft lignin (KL) and the derived hard carbons.- Problem 2: Foaming and fusing of the KL during thermal treatment steps. - Problem 3 : The time and energy consuming thermo-mechanical stabilization steps needed to stabilize and cross link the lignin structure. - Problem 4: Preventing a reduction of the electrochemical performance of hard carbons through a purification step, particularly when used in lithium (Li)-ion and sodium (Na)-ion batteries.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIGURE 1 illustrates a process configuration encompassed by an embodiment of the method described herein;

[0019] FIGURE 2 illustrates a process configuration of some preferred embodiments, showing among others a preferred precipitation of the lignin starting material using carbon dioxide (CO2), some preferred solution circulation options, a preferred acidic purifying procedure, and the optional re-slurring and spray-drying of the lignin starting material, while further indicating with a box marked using a dotted line the steps of some preferred configurations that advantageously could be carried out using existing units of a kraft pulp mill;

[0020] FIGURE 3 shows SEM images of kraft lignin samples: a) CCL-precipitated kraft lignin (CO2-KL), and b) CCE-precipitated kraft lignin dried by spray drying yielding microspherical particles of CO2-KL (CO2-KL-MP), and c) LignoBoost kraft lignin (LB-KL).

[0021] FIGURE 4 shows photos of lignin samples before (left) and after (right) direct carbonization at 1200 °C: a) CO2-KL, b) CO2-KL-MP, and c) LignoBoost kraft lignin (LB-KL);

[0022] FIGURE 5 shows SEM images of carbonized lignin samples a) CO2-KL, b) CO2-KL-MP, and c) LB-KL after direct carbonization at 1200 °C;

[0023] FIGURE 6 shows photos of lignin samples before (top row) and after (bottom row) pre-oxidation in air for 2h at 250 °C: a) CO2-KL, b) CO2-KL-MP, and c) LB-KL;

[0024] FIGURE 7 shows SEM images of hard carbon (HC) particles from CO2-KL (left CO2-KL, right CO2-KL-MP) after pre-oxidation at 250 °C, dilute acid purifying, and carbonization at 1200 °C;

[0025] FIGURE 8 is a graphical illustration of a thermogravimetric analysis of CO2-KL along with LB-KL. (a) Mass loss curve (b) First derivative of mass loss showing rate of mass of loss;

[0026] FIGURE 9 illustrates the specific delithiation capacities of HCs derived from CO2-KL;

[0027] FIGURE 10 illustrates the specific delithiation capacities of HCs derived from CO2-KL in comparison to commercial HCs and HCs derived from LB-KL following the same carbonization protocols;

[0028] FIGURE 11 is a graphical illustration of the particle size distribution of HCs from CO2-KL;

[0029] FIGURE 12 is a graphical illustration of the N2 adsorption isotherms of HCs derived from CO2-KL;

[0030] FIGURE 13 is a graphical illustration of the correlation between volumetric delithiation capacity and DR micropore volume for HCs derived from CO2-KL;

[0031] FIGURE 14 shows the Raman spectra of HCs derived from CO2-KL and LB-KL; and

[0032] FIGURE 15 illustrates the correlation between volumetric delithiation capacity, Raman ID / IG ratio and second order Raman scattering band area for HCs derived from CO2-KL.EMBODIMENTS

[0033] DEFINITIONSIn the present context, the term “hard carbon” is intended to define a solid carbon material that will not be converted to graphite by heat-treatment at temperatures around 3000 °C. The manufacture of hard carbon materials from their carbonaceous precursors typically takes place by heating to a somewhat lower temperature, such as a temperature around 1000-1500 °C. Such hard carbon materials are used as carbon electrodes in various types of batteries, such as in lithium-ion batteries and post-lithium-ion batteries, wherein the latter can be defined as modified lithium-ion batteries where the material of either one or both of the electrodes typically used in lithium-ion batteries have been changed. Current examples of post-lithium-ion batteries include sodium-ion batteries.The term “inorganic species” is intended to encompass the ions of inorganic salts, i.e. cations and / or anions. Typically, these inorganic species are thus present as inorganic salts or bound organically to lignin as positive counter ions (cations). Particularly, the inorganic cations are thus of interest for the herein described purposes.A “thermostabilization” or “thermal pre-treatmenf ’ or “thermal treatment” is a method step used to provide further stability of a material at high temperatures, particularly to stabilize the lignin particle morphology. Also, this stabilization typically takes place at increased temperatures, but is a milder heating than for example a carbonization. The stabilization typically takes place in an oxidizing atmosphere.A “carbonization” is, in turn, a high-temperature step used to provide the final above-defined hard carbon structure of a precursor material, such as the herein mentioned lignin.

[0034] The present invention thus relates to a method for engineering a hard carbon material from kraft lignin, including the steps of- obtaining a kraft lignin starting material rich in inorganic species from a kraft pulping process,- drying the thus obtained kraft lignin starting material,- carrying out a thermal pre-treatment step on the dried kraft lignin,- purifying the pre-treated kraft lignin by removing the inorganic species therefrom, and- carbonizing the obtained pre-treated and purified lignin material.

[0035] These method steps are shown in Fig. 1. Various alternatives of some embodiments are shown in Fig. 2.

[0036] The kraft lignin is preferably obtained by precipitation from a black liquor, more preferably by using carbon dioxide (CO2). Using this procedure, a kraft lignin is typically obtained that is rich in inorganic cations, particularly being sodium (Na) or potassium (K) ions, most suitably Na ions. Most suitably, the kraft lignin starting material will have a concentration of inorganic species of 2-25 w-%.

[0037] In existing processes, the lignin has typically been purified to separate these ions and obtain a pure lignin material. However, if these cations are not separated from the precipitated lignin, and the lignin is used, as such in unpurified form, the cations can be utilized as internal catalyst in subsequent cross-linking reactions. Thus, the obtained precipitated kraft lignin is preferably used as such, without purification steps, such as redispersions, re-crystallizations, or the commonly utilized purification using sulphuric acid and sequential filtration steps.

[0038] The drying step is typically carried out by thermal drying, drum drying, flash drying or spray-drying, preferably by spray-drying. The drying step can be convenientlycombined with a compacting step, wherein the kraft lignin starting material is pressed into a more compact form, preferably into an increased bulk density of 500 - 700 kg / m3.

[0039] In a preferred embodiment, the CCh-precipitated kraft lignin (i.e. the CO2-KL) is re-slurred into water before the thermal pre-treatment is carried out, into a solids content of 5-40 w-%, before drying, preferably by spray drying (see Fig. 2). This optional re-slurring step provides the further advantage of improving the control of the particle size of the lignin. The final particles of the stabilized and carbonized material will have a better controlled size distribution, preferably in the size range of 1-100 pm, more preferably 10-30 pm, and a spherical morphology. Although the morphology control through re-slurring and spray drying does not directly improve the electrochemical performance compared to the direct use of the kraft lignin starting material described above, it does ease the preparation of electrochemical slurries due to its ideal particle size distribution, avoiding the need for particle size controlling steps such as mechanical grinding and sieving.

[0040] In another embodiment, the thermal pre-treatment step is carried out in the presence of the inorganic cations present in the dried kraft lignin (i.e. the internal catalysts obtained with the lignin starting material), under an inert atmosphere or in an oxidative atmosphere, by heating, preferably to a temperature of 200 - 500 °C. A suitable atmosphere for the thermal pre-treatment step can be achieved for example by using air or CO2, or by using N2 as an inert atmosphere. The duration of the thermal pre-treatment step is preferably 1-5 h, more preferably 1-2 h.

[0041] After the thermal pre-treatment step has been carried out, the inorganic cations that were allowed to remain in the kraft lignin during the drying and thermal pre-treatment steps can finally be removed.

[0042] Thus, in an embodiment, the thermal pre-treatment step is followed by a purification step, preferably carried out by removing inorganics by using water or acidified water, more preferably at a pH of 3-7.5, and most suitably at a temperature within the range of 20-150 °C, thus producing a purified lignin material and a spent purifying solution, which can be separated from each other. For the acidified water it is possible to use e.g. a dilute acid-containing aqueous solution, such as a dilute solution of sulfuric acid (H2SO4), or in another alternative hydrochloric acid (HC1), or a mixture of H2SO4 and HC1. As shown in anoption of Fig. 2, the acid can be diluted for example using a condensate obtained from the drying step of the method. Neither the use of concentrated sulfuric acid nor a very low pH value is needed at this stage, or at any other stage of the present method.

[0043] The spent purifying solution containing inorganics is preferably reused in e.g. a kraft pulping process (as shown in Fig. 2), preferably at an evaporation plant in such a kraft pulping process and / or in a chemical recirculation therein.

[0044] In a further embodiment, the carbonization step that is carried out on the pretreated and purified lignin material preferably involves heating said pre-treated and purified lignin to a temperature of > 300 °C, more preferably 300-1500 °C, and most suitably 900-1500 °C, to obtain a hard carbon material.

[0045] Thus, the present invention also relates to a hard carbon material engineered using the method described above, the hard carbon material being based on kraft lignin.

[0046] Due to the catalytic crosslinking effect imparted by black liquor inorganics on kraft lignin particles during heat treatment by utilizing the engineering methods described above, the prepared hard carbon material typically exhibits a volumetric expansion of no more than 20%, during or immediately following the carbonization step, relative to the volume of the kraft lignin starting material used. Even a volumetric expansion of less than 10%, or less than 5%, can be achieved, since there in practice occurs no volumetric expansion during the herein described method. More specifically, the prepared hard carbon material exists as easily dispersed particles rather than conventional fused monoliths following carbonization of the non-fusing lignin starting material. Furthermore, the obtained hard carbon typically has an average particle size below 50 pm when a kraft lignin starting material produced using the above-described methods involving spray-drying techniques is used. When the kraft lignin starting material is produced by the above described method without spray drying and is instead dried using alternative techniques, the hard carbon typically has an average particle size below 100 pm. Thus, the hard carbon material obtained in the method described above can exhibit an average particle size below 100 pm, specifically between 1-50 um, more specifically 15 ± 10 pm.

[0047] The particle size and the non-fusing character of the particles of the obtained material can be seen particularly from Figs. 3, 5, 7 and 11. Fig. 3 shows a comparison of the original precipitated starting material and a spray-dried material, and indicates particularly that spray drying, combined with a re-slurrying step prior to spray drying, reduces the particle size and homogenizes the spherical particle geometry, giving them the above mentioned average particle size and a spherical and regular geometry. Fig. 5 shows the non-fusing and easily dispersable character of the particles obtained by precipitation as described herein. Fig. 7, in turn, shows that the obtained particles carbonized according to the method described herein typically have a particle size below 10 pm, whereas Fig. 11 confirms the particle size distribution, and specifies that the peak value of the particle sizes of the spray-dried microparticles is even below 10 pm, or around 8 pm.

[0048] This lignin-based hard carbon material has similar performance in Li-ion batteries and other similar alkali metal ion batteries as compared to commercial fossil-based carbon electrode materials and better performance than a hard carbon derived from conventional kraft lignin materials, such as from LignoBoost lignin, following the same carbonization protocol.

[0049] Thus, the invention also relates to a use of the hard carbon material described herein, or engineered using the method described herein, for energy storage applications, or particularly in preparing electrodes for alkali metal ion batteries, such as lithium-ion or postlithium-ion batteries.

[0050] In such batteries, the anode is typically made of carbon, or more particularly of hard carbon, and can thus advantageously be substituted with the above described hard-carbon lignin.

[0051] It is to be understood that the embodiments of the present disclosure are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0052] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0053] As used herein, a plurality of items, structural elements, compositional elements, and / or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present disclosure may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present disclosure.

[0054] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments described herein. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0055] While the forgoing examples are illustrative of the principles of the present disclosure in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0056] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwiseexplicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.EXAMPLESExample 1 - Preparation and characterization of a kraft lignin starting material of the invention

[0057] Kraft lignin (KL) was precipitated from black liquor using carbon dioxide (CO2). The CCh-precipitated sample is used as such (CO2-KL) or re-slurried in water (the pH of the slurry remaining alkaline) and spray dried into micro spherical particles (CO2-KL-MP). The CO2-KL particles are coarse and have irregular geometries. Re-slurrying and spray drying reduces the particle size and homogenizes it into a spherical particle geometry (see Fig. 3).

[0058] Some sample properties of CCh-precipitated kraft lignin (CO2-KL) and LignoBoost kraft lignin (LB-KL) used as reference sample are compared in Table 1 below.Table 1. Sample comparison, dm = dry matter* Na and S contents do not change after re-slurrying and spray dryingExample 2 - Carbonization of the kraft lignin samples

[0059] The CO2-KL samples obtained from Example 1 were carbonized following two protocols:• Direct carbonization (DC) of the samples at 1200 °C followed by sample washing and drying.• Catalysed oxidative thermal pre-treatment (P) in air at 250 °C to cause stabilization and to cross-link the lignin structure, followed by sample washing to purify the sample and remove the inorganics, drying, and subsequent carbonization (C) at 1200 °C. The sample is washed again after the final carbonization step to purify the carbonized material.

[0060] The CO2-KL samples were compared with a reference LignoBoost KL (LB-KL) with regards to their propensity to fuse and foam upon thermal treatment. Photos of the lignin samples before and after direct carbonization are shown in Fig. 4, as well as SEM images after carbonization at 1200 °C in Fig. 5.

[0061] Overall, the Na-rich CO2-precipiated KL samples showed a much lower propensity to fuse and foam compared to the LB-KL sample. The CO2-precipiated KL kept a dispersed form after carbonization, while the LB-KL particles fused into a formless monolith. These results help to demonstrate and explain the effects of inorganic species, especially Na on the reduction of lignin chain thermal mobility during thermal treatment. Therefore, CO2-KLS are easier to process in thermochemical reactors compared to LB-KL, and do not require any special thermomechanical treatment to stabilize their structure and particle morphology.

[0062] Also, the stabilization of the CCL-precipiated KL and LB-KL structure and morphology after thermal oxidative stabilization in air at 250 °C for 2 h was studied. While fusing and foaming of LB-KL could still be observed during the pre-oxidation step, the CO2-precipiated KL samples did not fuse or foam and preserved their dispersed state, as shown in Fig. 6.

[0063] The fusing and foaming of LignoBoost kraft lignin, which has been purified and has a low content of inorganic species, is essential when considering volumetric expansion.When used directly, this type of lignin melts and foams during heat treatment. As a result, the original particles melt, foam and coalesce into a large monolithic structure. This behaviour is shown when comparing Lignoboost KL in Figure 3c (room temperature) and Figure 5c (after direct carbonization, where a large block is observed relative to the other samples). The same phenomenon is shown in Figure 4c (before and after direct carbonization) and Figure 6c (before and after pretreatment). This volumetric expansion of the lignin material is undesirable, since it prevents an efficient dispersion of the lignin and thus reduces the reaction rates in any subsequent process steps.

[0064] The hard carbon (HC) particles obtained after the carbonization of thermostabilized and washed CO2-KL samples are shown as SEM images in Fig. 7. The retention of individual, spherical and micron-sized particle morphology of HC derived from CO2-KL-MP in this case demonstrates the stabilization effects by inorganics.

[0065] Thermogravimetric analysis was performed on the CO2-KL and LB-KL to reproduce the oxidative thermal stabilization step and evaluate the reactivity of the samples with oxygen. The samples were heated stepwise from room temperature up to 250 °C under air (Fig. 8). The mass loss rates were much more pronounced in the temperature range of 110-250 °C for the Na-rich KL compared to the LB-KL (Fig. 8b). The significantly higher reactivity for the CO2-KL during the oxidative thermal stabilization can be explained by the catalytic effects of the inorganics, especially Na.

[0066] The results also show that the reactivity of CO2-KL-MP is significantly higher than the reactivity of CO2-KL. This difference can be explained by the smaller particle size, the larger surface area, and a homogenous distribution of the inorganic catalytic species in the spray dried spherical particles, which can all influence the rate of oxidative reactions with oxygen.

[0067] Altogether, those results show that the presence of Na catalyses the crosslinking reactions during oxidative thermal stabilization under air, which leads to particles with stabilized morphologies after removal of inorganics and additional carbonization thermal treatment at 1200 °C. This result could not be achieved with the LB-KL, which likely needs a longer and more demanding thermal pre-oxidation treatment. The catalytic action of inorganic species, especially Na during pre-oxidation leads to faster cross-linkingand stabilization of the particle structure and morphology. It reduces the time and energy needs in this step.

[0068] The gravimetric yields after direct carbonization and peroxidationcarbonization are shown in Table 2. The gravimetric yields of the directly carbonized CO2-precipiated samples are quite similar and close to 40%. However, after peroxidation treatment the gravimetric yield falls to 29.9 and 18.4%, respectively for the CO2-KL, CO2-KL-MP due to the reaction with the atmospheric O2. The lower yield obtained for the CO2-KL-MP can be explained by a higher reactivity with O2 due to smaller particle size and more homogenously dispersed Na catalyst.Table 2. Gravimetric yields of the starting lignin material after direct carbonization and preoxidation-carbonization experiments

[0069] The electrochemical (EC) performance of the HCs derived from the CO2-KL samples depend on the lignin pre-processing step (morphology control) and carbonization protocol (see Table 3 and Fig. 9). The oxidative thermal stabilization and removal of inorganics has a major impact on the EC performance, while the morphology control via reslurring-spray drying appear to be less critical.

[0070] Most importantly, the HC derived from CO2-KL shows a better EC performance compared to HC derived from LB-KL, and a similar performance to commercial hard carbons (Figure 10). Altogether, those results show that applying the abovedescribed methods to control the morphology and the properties of HCs from Na-rich CO2-KL can result into HCs with better controlled morphology and better EC performance compared to HCs from LB-KL.

[0071] Overall, the results show that the method of the invention leads to HC with higher performance compared to HC obtained from LB-KL. The differences between HCs performance are discussed considering their multiscale properties in the following paragraphs.Table 3. Electrochemical properties of HC samplesExample 3 - Hard carbon properties-performance relationships

[0072] The particle size distributions of the HCs obtained from the CCL-precipiated samples of the above Examples are shown in Fig. 11. The figure shows a substantial reduction in particle size and a better size uniformity for the HC samples obtained from spray-dried KL. This step can potentially avoid grinding requirements and impart the particles with a better controlled and uniform spherical morphology.

[0073] The average particle sizes of the HCs at different volume fractions are given in Table 4. As a comparison example, 50% of the HC particles obtained from CO2-KL-MP have sizes lower than 8.8 pm, while 50% of the HC particles obtained from CO2-KL have sizes lower than 42.8 pm. This four-fold average size decrease after re-slurring and spraydrying of CO2-KL indicates that particle size controlling steps can be completely avoided in this case. The HCs from LB-KL in all conditions had to be grinded and sieved and were not suitable for particle size analysis due to the use of particle size controlling steps.Table 4. Average particle size at different volume fractions

[0074] The tap densities of the commercial and lignin derived HCs is given in Table 5. The values observed for the HCs derived from the CO2-KL are slightly higher than the values observed for the HC derived from the CO2-KL-MP. As spray drying can make some CO2-KL-MP particles hollow, their tap density decreases and so does the tap density of their derived hard carbons. The tap density values are nonetheless close to each other, which will not induce substantial negative impacts on the volumetric density of the derived HC. The HCs from LB-KL show better tap densities than those derived from CO2-KL however these values are obtained after use of particle size controlling steps and do not represent the HC in its original fused state.Table 5. Tap densities of commercial HCs and HCs derived from CO2-KL and LB-KL.

[0075] The nitrogen physisorption isotherms show that highly microporous HCs are obtained from direct carbonization of CO2-KL and CO2-KL-MP (Fig. 12). The HCs from direct carbonization exhibit BET surface areas close to 500 m2 / g (Table 6). The high surface area is the result of carbon gasification reactions caused by the presence of inorganic species during carbonization, especially Na, in its organic and inorganic forms. The HCs obtained from direct carbonization of CO2-KL-MP show slightly higher surface area and micropore volume compared to the raw CO2-KL. This might be caused by a better and more homogeneous redistribution of the Na species after re-slurring and spray drying, leading to more homogenous gasification of the carbon surface in the carbonizing particle. The HCs obtained through direct carbonization show overall similar pore size distributions with minor differences in the proportions of ultramicropores and supermicropores (Table 6). This indicates that the re-slurring and spray drying step influence little the development of the nanoporosity during carbonization.

[0076] The removal of inorganics after the oxidative thermal stabilization step drastically reduced the occurrence of carbon gasification reactions and consequently decreased the surface area and pore volume of the HCs by almost 9 folds. More specifically, the removal of inorganics suppressed the formation of small ultramicropores and decreased the supermicropore volume (Table 6).Table 6. Textural properties of the hard carbons from CO2-KL.<

[0077] We calculated the volumetric delithiation capacities of the HCs by dividing their specific capacities with their respective tap densities. We observed a good correlation between the HC volumetric delithiation capacity and their DR micropore volume, which indicates that low HC micropore volume is preferred to increase its volumetric delithiation capacity (Fig. 13).

[0078] The nanostructure of the HCs was measured by Raman spectroscopy. The Raman spectra are shown in Fig. 14.

[0079] The ID / IG ratios directly calculated from the spectra without deconvolution are shown in Table 7. The ID / IG ratios show differences according to the type of KL and thermal process conditions, indicating differences in their nanostructures.Table 7. Raman ID / IG ratios for hard carbons derived from CO2-KL.

[0080] We observed correlations between Raman nanostructural parameters and electrochemical performance (Fig. 15). The volumetric and gravimetric specific capacitances increase respectively with the ID / IG ratio and the integral area of the second order region. Those correlations reflect relationships between nanostructural properties and HC delithiation capacities.INDUSTRIAL APPLICABILITY

[0081] The present invention provides a method for engineering a new type of hard carbon material suitable for use in electrodes.

[0082] Particularly, the invention provides a method that can utilize the inorganic species obtained with a lignin starting material obtained from a kraft pulping process. These inorganic cations are utilized in a thermal pre-treatment step carried out when engineering hard carbon materials to cause a faster catalytic cross linking during said pre-treatment step, as well as to cause reduced thermal mobility of the lignin chains upon heating, and suppressed particle fusing and morphology change, as compared to the use of a purified lignin starting material. This will further result in easier operation of the process (no clogging) and energy savings during the shorter thermal stabilization stage. In addition, due to lower fusing of lignin less energy will be required during grinding operation.

Claims

Claims1. Method for engineering a hard carbon material from kraft lignin, including the steps of- obtaining a kraft lignin starting material from a kraft pulping process by precipitation from a black liquor,- drying the thus obtained kraft lignin starting material,- carrying out a thermal pre-treatment step on the dried kraft lignin,- purifying the pre-treated kraft lignin by removing inorganic species therefrom, and- carbonizing the obtained pre-treated and purified lignin material.

2. The method of claim 1, wherein the kraft lignin is precipitated by using carbon dioxide (CO2).

3. The method of claim 1 or 2, wherein the kraft lignin starting material is rich in inorganic species, being ions of inorganic salts, particularly being sodium (Na) ions, in that it has a concentration of said inorganic species of 2-25 w-%.

4. The method of any preceding claim, wherein the kraft lignin starting material is reslurried into water into a solids content of 5-40 w-%, before drying, preferably by spray drying to produce spherical microparticles with controlled size and morphology, preferably in the size range of 1-100 pm, more preferably 10-30 pm.

5. The method of any preceding claim, wherein the drying step is carried out by thermal drying, drum drying or spray-drying, preferably by spray-drying.

6. The method of any preceding claim, wherein the drying step is combined with a compacting step.

7. The method of any preceding claim, wherein the thermal pre-treatment step is carried out in the presence of the inorganic species present in the dried kraft lignin, under an inert atmosphere or in an oxidative atmosphere, by heating, preferably to a temperature of 200-500 °C.

8. The method of claim 7, wherein the thermal pre-treatment step is carried out in air or in CO2 or in an inert atmosphere of N2.

9. The method of any preceding claim, wherein the duration of the thermal pretreatment step is 1-5 h, preferably 1-2 h.

10. The method of any preceding claim, wherein the purification step for removing inorganic species from the pre-treated kraft lignin is carried out by using water or acidified water, preferably at a pH of 3-7.5, and most suitably at a temperature within the range of 20-150 °C, thus producing a purified thermally pre-treated kraft lignin and a spent purifying solution, which can be separated from each other.

11. The method of claim 10, wherein the spent purifying solution containing inorganic species is recirculated to the kraft pulping process, preferably to an evaporation plant in said kraft pulping process and / or to a chemical recirculation therein.

12. The method of any preceding claim, wherein the carbonization step is carried out by heating to a temperature of > 300 °C, typically 300-1500 °C, and preferably at 900-1500 °C, to obtain a hard carbon material.

13. A hard carbon material based on kraft lignin, characterized by being engineered using the method of any preceding claim.

14. The hard carbon material of claim 13, exhibiting volumetric expansion of no more than 20% following the carbonization step, relative to the volume of the kraft lignin starting material.

15. The hard carbon material of claim 13 or 14, having an average particle size below 100 pm, specifically between 1-50 um, more specifically 15 ± 10 pm.

16. Use of the hard carbon material of any of claims 13 to 15, or of the hard carbon material engineered using the method of any of claims 1 to 12, in preparing electrodes for alkali metal ion batteries.

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