# Inorganics from kraft black liquor enable rapid oxidative crosslinking and morphology control in lignin derived hard carbons

Glen Pauls, Olli Sorsa, Antti Paajanen, Vaibhav Modi, Sebastien Leclerc, Younes Bouizi, Anthony Dufour, Anna Kalliola, Chamseddine Guizani

► To cite this version:

Glen Pauls, Olli Sorsa, Antti Paajanen, Vaibhav Modi, Sebastien Leclerc, et al.. Inorganics from kraft black liquor enable rapid oxidative crosslinking and morphology control in lignin derived hard carbons. Bioresource Technology, 2026, 447, pp.134177. 10.1016/j.biortech.2026.134177. hal-05512109

HAL Id: hal-05512109

https://hal.univ-lorraine.fr/hal-05512109v1

Submitted on 16 Feb 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
<figure><img src="image_2.png" /></figure>

# Inorganics from kraft black liquor enable rapid oxidative crosslinking and morphology control in lignin derived hard carbons☆

Glen Pauls<sup>a, ID</sup>, Olli Sorsa<sup>a</sup>, Antti Paajanen<sup>a, ID</sup>, Vaibhav Modi<sup>a, ID</sup>, Sebastien Leclerc<sup>b, ID</sup>, Younes Bouizi<sup>c, ID</sup>, Anthony Dufour<sup>d</sup>, Anna Kalliola<sup>a</sup>, Chamseddine Guizani<sup>a, *, ID</sup>

## HIGHLIGHTS

* Inorganics in kraft black liquor catalytically crosslink kraft lignin.

* Oxidative thermal stabilization by inorganics drastically reduces stabilization time.

* Experiments reveal organically bound Na-phenoxide groups as key catalytic centers.

* Morphology defined hard carbons without energy intensive and long pretreatment.

* Hard carbon particles with better delithiation capacity from low purity kraft lignin.

## GRAPHICAL ABSTRACT

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

- Kraft lignin

- Inorganics

- Thermal mobility

- Cross-linking

- Carbonization

- hard carbon

- anode

- Battery

## ABSTRACT

The conventional approach to converting kraft lignin (KL) into hard carbons is to start with highly purified, low-ash KL feedstocks and then rely on slow, energy-intensive oxidative stabilization and added crosslinkers to keep melting and foaming associated thermal challenges under control. Herein, we deliberately invert this paradigm. Instead of starting with highly purified KL, we retain pulping inorganics and use them as catalytic centers for oxidative crosslinking and melt suppression of KL. Spherical KL microparticles (KL-MP) were recovered from softwood black liquor by membrane filtration and spray-drying steps, intentionally retaining inorganic sodium (Na) salts as well as organically bound Na in KL-MP, and were compared to acid-precipitated, low-ash reference KL (KL-REF). During thermo-oxidative pretreatment (250 °C, 5 °C/min) in air, KL-MP undergoes inorganic-catalyzed rapid oxidative crosslinking that converts thermoplastic lignin into a rigid network, whereas KL-REF softens, foams, and fuses. Experimental analysis identifies organically bound Na-phenoxide type species as key catalytic sites. Proton magnetic resonance thermal analysis and molecular dynamics simulations reveal strongly reduced segmental mobility and Na-driven ionic clusters acting as physical crosslinking points. After pretreatment, inorganics are removed by a washing step, and the crosslinked KL-MP is carbonized, yielding low-surface-

E-mail addresses: glen.pauls@vtt.fi (G. Pauls), olli.sorsa@vtt.fi (O. Sorsa), antti.paajanen@vtt.fi (A. Paajanen), vaibhav.modi@vtt.fi (V. Modi), sebastien.leclerc@univ-lorraine.fr (S. Leclerc), younes.bouizi@univ-lorraine.fr (Y. Bouizi), anthony.dufour@univ-lorraine.fr (A. Dufour), anna.kalliola@vtt.fi (A. Kalliola), chamseddine.guizani@vtt.fi (C. Guizani).

<footnote><sup>a</sup> VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland</footnote>

<footnote><sup>b</sup> Université de Lorraine, CNRS, LEMTA, F-54000, France</footnote>

<footnote><sup>c</sup> Université de Lorraine, CNRS, CRM2, F-54000, France</footnote>

<footnote><sup>d</sup> Université de Lorraine, CNRS, ENSIC, LRGP, F-54000, France</footnote>

<footnote>* This article is part of a special issue entitled: 'Lignin Valorization' published in Bioresource Technology.</footnote>

<footnote>* Corresponding author.</footnote>
area hard carbons that retain their initial micron size and spherical morphology. As Li-ion battery anodes, the derived hard carbon shows better electrochemical performance than carbons derived from KL-REF. Overall, the work shows how otherwise undesirable inorganic impurities can simplify thermal conversion of KL, with potential for diverse applications where particle size and shape are critical.

# 1. Introduction

Lignin is the most abundant aromatic biopolymer on earth and a promising biobased precursor for renewable carbon materials, especially for electrochemical energy storage applications (Beaucamp et al., 2022). Lignin can be extracted from lignocellulosic biomass through various biorefining and pulping processes. Among these, the kraft process is the most adopted by the pulp and paper industry due to its efficiency and pulp quality (Sixta, 2006). The global volume of kraft lignin (KL) in black liquor form is estimated at 70 Mt/year, representing nearly 70% of the total lignin extracted by the chemical pulping industry (Correa-Guillen et al., 2025). Traditionally, black liquor is burned for calorific energy recovery primarily from lignin and regeneration of pulping chemicals. However, interests to improve value creation from lignin have incentivized research into its recovery. KL recovery technologies, such as Lignoboot (Tomani, 2010) and Lignoforce (Kouisi et al., 2016) has enabled the isolation of KL from black liquor at an industrial scale, and its valorization in higher value-added materials applications (Luo and Liu, 2023). In these recovery processes, lignin is typically precipitated from the black liquor using carbon dioxide (CO₂) followed by further precipitation and washing with sulphuric acid (H₂SO₄) providing lignin with a low ash content (Bertaud et al., 2023; Kienberger et al., 2021).

Carbonization of KL for battery anode fabrication has attracted considerable research attention owing to the ability to tailor hard carbon properties from KL and the promising electrochemical performance documented in recent studies (Kitchamsetti and Mhin, 2025). Despite the promise, challenges associated with its thermal processing must still be addressed before hard carbon production from KL can be effectively upscaled. The principal issues lie in the characteristic softening, fusing, and foaming behavior exhibited by KL particles during pyrolysis, which significantly impedes morphology control. These thermo-physical effects, arising from overlapping glass transition and depolymerization processes followed by crosslinking and solidification (Dufour et al., 2012; Shrestha et al., 2017), not only affect the final material properties but can also lead to complications such as fouling of pyrolysis equipment (Nowakowski et al., 2010), thus inhibiting continuous processing operations. Overcoming these obstacles is essential for advancing the production of hard carbons from KL.

Morphology control of micron sized KL derived carbon particles and fibers has been demonstrated by utilizing an oxidative thermal stabilization step prior to carbonization. This stabilization step involves heating KL in air at very slow heating rates (0.01–1 °C/min) up to moderate temperatures (200–300 °C) (Chatterjee & Saito, 2015; Köhnke et al., 2018; Zhao et al., 2021). Braun et al. (2005) reported that it is necessary to heat KL fibers at a rate of 0.06 °C/min or lower to prevent its fusibility by thermal stabilization under air atmosphere. During such slow heating rates, the KL glass transition temperature increases faster than the heat treatment temperature by the occurrence of crosslinking reactions, preserving material morphology. However stabilizing KL fibers or particles under air up to 300 °C at a heating rate of 0.06 °C/min would require a thermal treatment duration of ~ 83 h which is not practical in an industrial setting. A patent in the field outlines a crushing-compaction step in combination with oxidative thermal stabilization (Olsson et al., 2021). The mechanical action forms densified granules of KL particles, resulting in improved resistance to volume change during thermal conversion. Morphology is not controlled in this case, and particle size is dependent on subsequent grinding and sieving steps.

Inorganic impurities can adversely affect the electrochemical performance of hard carbon materials utilized as anodes in alkali-ion batteries. Consequently, there is a prevailing practice of producing hard carbon from extensively purified lignin feedstocks to minimize these inorganics (Matei Ghimbeu et al., 2024, Matei Ghimbeu et al., 2019). However, as outlined earlier, converting purified KL into a hard carbon necessitates the implementation of energy intensive stabilization procedures or external crosslinkers (Ren et al., 2022) to overcome its thermal challenges. Numerous studies have shown that inorganic compounds, especially alkali and alkaline earth metals found in or added to lignocellulosic feedstocks, significantly influence thermochemical processing (Ding et al., 2025). These substances are recognized for promoting charring during pyrolysis (Wang et al., 2006) and accelerating key heterogeneous reactions with oxygen, water, and carbon dioxide, all crucial steps in combustion and gasification (Dahou et al., 2021).

Inspired by this previous research in the field of biomass thermochemical conversion, we hypothesized that inorganic substances, particularly sodium (Na), which is prevalent in kraft black liquor could serve as catalysts to facilitate the oxidative cross-linking of KL molecules at temperatures below 250 °C. To evaluate this, we rely on an alternative KL recovery process based on successive membrane filtration, washing, and spray drying steps developed by VTT Finland (Fearon et al., 2024). This process yields spherical, micron-sized KL micro-particles (KL-MP), and allows for controlling inorganics content within these particles by adjusting process conditions.

This study employs KL-MP to demonstrate the catalytic function of inorganic constituents present in kraft black liquor to accelerate the crosslinking of KL particles during oxidative thermal stabilization. Upon completion of this catalytic crosslinking process, the inorganic materials can be efficiently removed through a mild acid washing procedure, resulting in low ash and crosslinked KL-MP. Subsequent carbonization of these crosslinked microparticles leads to the formation of spherical carbon microparticles. This approach enables the production of morphology and texture defined spherical carbon microparticles from KL without the need for external crosslinkers, mechanical compaction, or excessively prolonged thermal stabilization treatment. We investigate the catalytic crosslinking effect using thermo-analytical techniques, experimental work, and molecular dynamics simulations. Throughout the study, acid-precipitated, low-ash KL (KL-REF) is taken as a benchmark for comparison.

# 2. Materials and methods

## 2.1. Preparation and characterization of kraft lignin samples

Softwood KL-MP were prepared using industrial softwood black liquor provided by Metsä Fibre following a membrane filtration-based recovery process followed by spray drying (Fearon et al., 2024). In short, lignin was recovered from kraft black liquor using commercial polyethersulfone ultrafiltration membranes with a cut-off range of 1000–1200 Da in a sheer-induced rotation filtration set-up, typically operated at 60–80 °C and a transmembrane pressure of 6–8 bar. Polymeric lignin was retained in the concentrate while other components, such as process chemicals, were allowed to flow to the permeate. The recovered lignin was washed by diafiltration with water to reduce inorganics and impurities and then spray-dried in a rotary atomizer spray dryer with an inlet temperature of 170–190 °C, outlet temperature of 70–90 °C, and a feed rate of 20–30 kg/h, yielding KL-MP. The inorganics in KL-MP are derived from softwood black liquor. On average, about
60% of the Na in black liquor is organically bound, balancing the anionic functionalities such as carboxylic and phenolic groups in lignin polymer. The remaining Na exists as individual species, primarily as Na₂CO₃, followed by smaller amounts of Na₂S, Na₂SO₄, NaOH, and others (Hubbe et al., 2019). Concentrating of polymeric lignin by membrane filtration, followed by washing steps, results in relative abundance of organically bound Na species/ions in KL-MP, while the individual/free Na ions in the aqueous phase is reduced. The reference softwood KL-REF is an industrial KL precipitated from softwood black liquor and previously used in Widsten et al. (2020).

KL samples were analyzed using complementary techniques to determine their composition (proximate analysis, combustion elemental analyzer and inductively coupled plasma optical emission spectroscopy (ICP-OES)), structure (Fourier transform infrared spectroscopy (FTIR), phosphorus nuclear magnetic resonance (³¹P NMR), size exclusion chromatography (SEC)), and morphology (scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX)). The thermal properties of the lignin samples were characterized using complementary thermo-analytical techniques including simultaneous thermal analysis coupled to mass spectrometry (STA-MS), gram-scale TGA experiments simulating oxidative pretreatment of the KLs in air, differential scanning calorimetry (DSC), and proton magnetic resonance thermal analysis (PMRTA). Detailed analytical protocols are given in the electronic supporting information (ESI).

## 2.2. Preparation of model mixtures of salts and purified kraft lignin

To investigate the role of inorganic species in oxidative crosslinking, model systems were prepared by mixing KL-REF with selected alkali salts. Six salts were studied: sodium carbonate (Na₂CO₃), sodium hydroxide (NaOH), sodium sulfate (Na₂SO₄), sodium chloride (NaCl), potassium carbonate (K₂CO₃) and potassium hydroxide (KOH) (all analytical grade).

KL-REF and the corresponding salt were weighed to have 10 wt% of the alkali metal (Na or K) in the final mixture, resulting in a total mass of approximately 2.5 g. The weighed amounts of KL-REF and salt were then added to deionized water and stirred magnetically at room temperature for over 24 h to obtain a well-mixed solution or dispersion. During this step, KL-REF dissolved in Na₂CO₃, NaOH, K₂CO₃ and KOH, whereas no dissolution was observed in NaCl and Na₂SO₄, in the latter cases, KL-REF remained dispersed as a suspended solid. The resulting solutions and suspensions were then dried in an oven at 80 °C to constant mass, yielding solid salt-KL-REF mixtures.

## 2.3. Oxidative thermal stabilization and carbonization

Carbonization of the KL samples was done following two protocols: i) direct carbonization (DC) and ii) pre-oxidation treatment followed by carbonization (PC). The DC step was conducted in a tubular furnace (Carbolite Gero, UK) under a controlled N₂ flow rate of 500 ml/min. The KL sample (5 g) was heated from room temperature up to 1200 °C at a rate of 5 °C/min and held for one-hour at the final temperature. The tube was flushed with N₂ at a rate of 500 ml/min for 30 min prior to carbonization to ensure an inert environment in the furnace. In this case, the produced hard carbons were named as KL-MP DC and KL-REF DC. The pre-oxidation treatment involves heating the sample (5 g) in a chamber furnace (Nabertherm, Germany) under atmospheric conditions from room temperature up to 250 °C at a rate of 5 °C/min and holding it for two hours before cooling it down. The goal of the thermo-oxidative pre-treatment is to crosslink the KL particles before the high-temperature carbonization step. After the thermo-oxidative pretreatment, the crosslinked lignin samples were stirred overnight in dilute HCl solution (5% w/w), then filtered and washed with distilled water until pH of filtrate matched that of distilled water. The washed samples were dried overnight and then carbonized following the same protocol used for the DC step, forming hard carbons by the PC step. In this case, the

produced hard carbons were named as KL-MP PC and KL-REF PC. To evaluate the impact of salts on the oxidative crosslinking phase, the model mixtures comprising KL-REF and various salts were subjected to pre-oxidation, following the pre-oxidation protocol outlined previously.

## 2.4. Characterization of the KL-derived hard carbons

Hard carbons from both KLs were characterized for nanostructure (X-ray diffraction (XRD), Raman spectroscopy), texture (N₂ physisorption), and morphology (SEM), detailed protocols are described in the ESI. Their electrochemical performance as Lithium Ion Battery (LIB) anodes was evaluated using coin-cells with lithium counter electrodes, tested in triplicate, detailed protocols are described in the ESI.

## 2.5. Molecular modelling of substitution effects on lignin chain dynamics

Molecular dynamics simulations were carried out to study the physical effects of Na substitution on lignin chain mobility at elevated temperatures. The simulations were carried out using the GROMACS software (Abraham et al., 2015) and the CHARMM lignin force field (Vermaas et al., 2019b). The LigninBuilder tool (Vermaas et al., 2019a) was used for creating the molecular geometries and topologies. The chemical structure of the lignin molecules was based on the statistical description of spruce milled-wood lignin described by Balakshin et al. (2020). Native-like softwood lignin was used as a surrogate for softwood KL, as structural statistics required for model-building were available for the former. This approximation was deemed sufficient for demonstrating the effect of Na substitution on lignin chain dynamics and for gaining insight into the underlying mechanisms. Further details of the used lignin model can be found in the work by Zitting et al. (2024).

Condensed-phase models were prepared by arranging 128 lignin chains of 25 monomeric units into a cubic simulation domain and then transforming the systems into liquid phase by consecutive heating (27 °C to 427 °C, over 20 ns) and annealing steps (427 °C for 200 ns). Estimates for the glass transition behavior were obtained by dilatometric analysis of stepwise cooling simulations from 427 °C to 27 °C (20 °C steps, 10 ns each). Diffusivity of the lignin monomeric units was determined at temperatures between 67 °C and 427 °C using 150 ns simulations. All the simulations were carried out in the isothermal-isobaric (NPT) ensemble at 1 atm external pressure. To study the effect of Na substitution, two models were used: i) an unmodified reference model (unsubstituted), and ii) a model with all phenols substituted by Na phenoxides (substituted). Parameters for the modified lignin molecules with Na phenoxide substructures were generated based on the validated parent and functional group parameters from CGenFF (Vanommeslaeghe et al., 2010) in combination with the modular nature of atomic charge distribution in CHARMM.

# 3. Results and discussion

## 3.1. Compositional, structural, and morphological characteristics of the KL samples

Compositional analyses of the KL samples are summarized in Table 1. The ash content in KL-MP is significantly higher than in KL-REF, specifically elemental Na which is the most abundant element among the KL-MP inorganic fraction as revealed by ICP-OES. This is expected as inorganic species already present in the kraft process black liquor are partially retained in KL-MP (Krotscheck & Sixta, 2006). The hygroscopic nature of the Na salts (Zhang et al., 2020) could be the reason for the high moisture content of KL-MP (8.4%). Because KL-MP has higher moisture and ash levels, its elemental carbon content is lower than that of KL-REF. Both KL samples have similar sulphur concentrations, but KL-REF shows relatively high iron content, possibly because of the strong acid used in the acid precipitation process, which may corrode processing equipment.
<table><caption>**Table 1**
Compositional analysis of KL samples.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Proximate analysis<br>(% a.r.b)</th><th colspan="3">Ultimate analysis<br>(%d.b)</th><th colspan="10">Minerals and sulphur analysis (ICP-OES)<br>(mg/kg d.b)</th></tr><tr><th>Moisture</th><th>Ash</th><th>C</th><th>H</th><th>N</th><th>Al</th><th>Ca</th><th>Cu</th><th>Fe</th><th>K</th><th>Mn</th><th>Na</th><th>S</th><th>Si</th></tr></thead><tbody><tr><td>KL-MP</td><td>8.4</td><td>21.4</td><td>48.8</td><td>4.7</td><td>0.1</td><td>94</td><td>202</td><td>3</td><td>76</td><td>5360</td><td>118</td><td>65,500</td><td>17,650</td><td>157</td></tr><tr><td>KL-REF</td><td>2.4</td><td>1.3</td><td>61.5</td><td>4.9</td><td>0.1</td><td>121</td><td>28</td><td>4</td><td>5330</td><td>499</td><td>42</td><td>1950</td><td>18,700</td><td>124</td></tr></tbody></table>

a.r.b – as received basis, d.b – dry basis.

Structural properties of the KL samples were analysed by SEC and ³¹P NMR techniques (Table 2). The number averaged ($M_n$) and weight averaged ($M_w$) molar mass results indicate that KL-MP has a higher molecular weight and a greater polydispersity than KL-REF. There is an increase in the molecular weight distribution of KL-MP compared to its original form in black liquor (initial BL). This shift, especially the increase in $M_w$ from 3250 Da to 6000 Da, can be explained by the gradual removal of shorter lignin chains during successive membrane filtration steps involved in its production (Fearon et al., 2024). The ³¹P NMR results reveal a higher concentration of phenolic hydroxyl groups in KL-REF in comparison to KL-MP. On the other hand, KL-MP shows higher concentration of aliphatic and carboxylic hydroxyl groups, which can indicate that carbohydrates in the black liquors might be retained by the membrane. The higher phenolic hydroxyl content in KL-REF can be due to acidification steps involved in the recovery which precipitates small lignin molecules having more phenolic hydroxyl groups (Zhu et al., 2014), whereas there is no acid used during the recovery of KL-MP. Production of KL-MP from the initial black liquor results in a reduction of overall hydroxyl content. This could again be due to the membrane filtration process where more functionalized smaller lignin chains pass the membrane while the larger chains are concentrated, thus agreeing with the SEC results.

FTIR spectra of the KL-MP, KL-REF and KL-REF mixed with Na salts are depicted in Fig. 1(a). The broad band at 3400 cm⁻¹ are assigned to the phenolic and aliphatic O—H vibration. The shape and intensity of this band are different for KL-MP compared to the KL-REF sample. The presence of inorganics in KL-MP makes it more hygroscopic and increases its moisture content, leading to a higher absorption in the OH band region (Xiao et al., 2019). The band in the range of ~ 3000–2840 cm⁻¹ corresponds to the C—H stretches in methyl and methylene groups. The band shape is also different for the two KLs. The KL-REF shows two distinct peaks in this region which can be hardly distinguished in the KL-MP spectra.

The two KLs show major differences in the fingerprint region, which can be mainly attributed to the high content of inorganics in the KL-MP. The KL-MP spectrum shows less resolved peaks in the 1800–700 cm⁻¹ region. The three peaks visible for the KL-REF in the range of ~ 1800–1500 cm⁻¹, corresponding to different C=O stretches and aromatic vibrations, have been replaced by a single broader peak in the KL-MP. Similarly, the overlapping peaks in the range of ~ 1500–1300 cm⁻¹ corresponding to different aromatic and aliphatic vibrations, were replaced by a high-intensity peak conferring the KL-MP spectra a very different signature compared to the purer KL-REF.

To understand the effects caused by inorganics, pure KL-REF samples were mixed with two Na salts that could be present in black liquor, Na₂CO₃ and Na₂SO₄. While both salts modified the FTIR spectra of KL-REF, modifications caused by Na₂CO₃ represented some of the unique features observed in the spectrum of KL-MP. Namely the significant increase in the peak at 1450 cm⁻¹ and detection of a new peak at 878 cm⁻¹, both of which are due to infrared active vibrational modes of CO₃²⁻ (Joshi et al., 2013). This clearly indicates the existence of mineral Na₂CO₃ in KL-MP, among other potential forms. XRD analysis (Fig. S4, see ESI) of KL-MP also reveals diffraction peaks of Na₂CO₃, supporting presence of Na₂CO₃ as indicated by FTIR.

SEM images (Fig. 1(b), (c), (d) and (e)) show the morphological traits of the KL samples. KL-MP shows distinct spherical particle morphology and a smooth surface, whereas KL-REF shows no regular particle morphology with rough and crumpled surface. The KL-MP particles are also much smaller compared to the KL-REF particles. These variations are due to the different recovery and drying technologies used in the production of both the KL samples. The KL-REF particles are recovered by a series of acidification and filtration steps leading to uncontrolled aggregation of lignin chains and formation of irregular particle shapes which are further physically modified during drying and additional particle size controlling steps (Tomani, 2010). In contrast, control over particle size and shape is achieved by a spray drying step in the production of KL-MP, leading to the self-assembly of lignin into spherical and micron sized particles (Fearon et al., 2024). Na salts appear evenly dispersed on the KL-MP particles as revealed by SEM-EDX chemical mapping of KL-MP (Fig. 1(f)-(i)).

## 3.2. Thermal characteristics of the KL samples

The carbonization dynamics of the two KL samples were investigated by STA-MS (Fig. 2(a)). It is important to note that evolved gas analysis by mass spectrometry employs electron impact ionization at 70 eV, which is known to induce fragmentation of primary pyrolysis products (Jia et al., 2016). Therefore, although the *m/z* signals referenced below are associated with specific species for comparative purposes, these signals may also result from overlapping fragmentation products or other compounds. This limitation is particularly relevant within the temperature range of 200–500 °C, where substantial formation of primary aromatic pyrolysis products occurs. At temperatures exceeding 500 °C, the evolved gases predominantly consist of permanent gases (Shen et al., 2010). Mass loss occurs rapidly in the KL-MP sample with a first derivative thermogravimetric (DTG) peak at ~ 120 °C which

<table><caption>**Table 2**
Structural composition of KL as determined by SEC and ³¹P NMR.</caption><thead><tr><th rowspan="2"></th><th colspan="3">SEC</th><th colspan="8">³¹P NMR</th></tr><tr><th>M<sub>w</sub><br>(Da)</th><th>M<sub>n</sub><br>(Da)</th><th>PD</th><th>Aliphatic OH<br>(mmol/g)</th><th>Carboxylic OH<br>(mmol/g)</th><th>Condensed PhOH<br>(mmol/g)</th><th>Guaiacyl PhOH<br>(mmol/g)</th><th>Catechol PhOH<br>(mmol/g)</th><th>p-hydroxy PhOH<br>(mmol/g)</th><th>Total PhOH<br>(mmol/g)</th><th>Total OH<br>(mmol/g)</th></tr></thead><tbody><tr><td>Initial BL</td><td>3250</td><td>760</td><td>4.3</td><td>2.4</td><td>0.7</td><td>1.5</td><td>1.1</td><td>0.6</td><td>0.2</td><td>3.4</td><td>6.5</td></tr><tr><td>KL-MP</td><td>6000</td><td>1320</td><td>4.7</td><td>2.4</td><td>0.6</td><td>1.3</td><td>0.8</td><td>0.5</td><td>0.1</td><td>2.8</td><td>5.7</td></tr><tr><td>KL-REF</td><td>5010</td><td>2280</td><td>2.2</td><td>2</td><td>0.4</td><td>2.1</td><td>2.5</td><td>—</td><td>0.2</td><td>4.8</td><td>7.2</td></tr></tbody></table>

SEC – Size exclusion chromatography, $M_w$ – weight averaged molar mass, $M_n$ – number averaged molar mass, PD – polydispersity, ³¹P NMR – phosphorus nuclear magnetic resonance, OH – hydroxyl group, PhOH – phenolic hydroxyl group.
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** (a) FTIR spectra of KL samples. SEM images of KL-MP ((b) and (c)) and KL-REF ((d) and (e)). The SEM-EDX images of KL-MP (f) showing elemental mapping of C (g), O (h), and Na (i).</figcaption></figure>

corresponds to the release of physically bound water (H₂O), a concomitant peak at *m/z* = 18 is observed in the MS analysis. Above 200 °C, thermal degradation reactions start to occur with two major DTG peaks at 280 °C and 340 °C. These mass loss peaks are accompanied by major emissions of H₂O (*m/z* = 18) and CO₂ (*m/z* = 44), and to a lower extent of methane (CH₄, *m/z* = 16) and carbon monoxide (CO, *m/z* = 28). Close to 600 °C, another set of degradation reaction occurs, releasing both CO₂ and CO. The CO₂ emission here reaches a maximum close to 700 °C and drops beyond, while CO emission increases beyond 700 °C and shows a maximum close to 800 °C. The mass loss observed at temperatures beyond 700 °C likely corresponds to the gasification of the carbon surface following different reactions involving the inorganics inherently present in the KL-MP. For example, chemical activation of the carbon surface can occur through a carbothermic decomposition of Na₂CO₃ (Kim & Lee, 2001). The CO₂ produced by the decomposition of lignin and inorganic carbonate species could act as a physical activation agent and react with the carbon surface following the Boudouard reaction: CO₂(g) + C(s) ⇌ 2CO (Guizani et al., 2015). This reaction is known to be catalyzed by Na and K species (Bouraoui et al., 2015) which are abundant in KL-MP. At approximately 1300 °C, the carbonized KL-MP exhibits a minor mass loss along with the emission of CO, CO₂, and CH₄. This observation likely indicates that the nanostructure of the carbonized lignin is undergoing rearrangement and ordering, attributable to the surface decomposition of its more thermally stable functional groups and linkages.

For KL-REF, the thermal decomposition profile exhibits reduced complexity, with the DTG signal beginning to rise around 150 °C and forming a shoulder that merges with a primary DTG peak at 380 °C. This shoulder indicates the onset of the initial pyrolysis stage, which subsequently overlaps with the secondary pyrolysis stage. Throughout both

stages, KL experiences depolymerization and the liberation of phenolic compounds, concurrently with cross-linking and charring reactions (Kawamoto, 2017; Shen et al., 2010). Although the KL-REF thermal decomposition appears simple, it involves complex emissions of H₂O, CH₄, CO, and CO₂, each occurring at distinct temperatures and with varying emission patterns.

The difference in the thermal decomposition profiles of the two KL samples is the direct result of the multiple catalytic effects by the inorganics, which are abundant in the KL-MP sample.

The thermo-morphological changes of the carbonized KL samples were investigated using optical and scanning electron microscopy imaging of the samples after carbonization at different heating rates (2.5–20 °C/min) up to 1000 °C in the STA device. The optical microscopy images in Fig. 2(b) show the higher tendency of the KL-REF particles to foam as the heating rate is increased. This is particularly evident during carbonization at 20 °C/min where foaming is significantly more pronounced compared to carbonization at 2.5 °C/min. These morphological differences result from competitive physical phenomena and chemical reactions that govern the thermo-morphological evolution of KLs during thermal treatment. Crosslinking reactions are favored at low temperatures (<300 °C), low heating rates, and long residence times. These reactions limit the thermal mobility of the lignin chains and help retain the lignin particle morphology (Braun et al., 2005). Increasing the heating rate favors softening and thermal decomposition over crosslinking reactions, hence promoting gas formation, foaming and eventually the formation of a fused monolith with no distinct morphological features (Shen et al., 2010). These thermo-physical changes observed with KL-REF are reported in numerous prior studies on lignin pyrolysis (Nowakowski et al., 2010; Sharma et al., 2004; Shrestha et al., 2017), and matches the visual recording observations of organosolv lignin
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** (a) STA-MS analysis of KL samples presents the mass loss profile alongside its derivative in the lower section, with emission mass spectra displayed above. (b) Optical microscopy images illustrate the appearance of STA crucibles containing KL samples following carbonization at 1000 °C across various heating rates. (c) Differential scanning calorimetry curves are provided for both the first (H1) and second (H2) heating cycles. (d) Proton magnetic resonance thermal analysis is conducted to assess proton mobility throughout the in-situ thermal treatment process.</figcaption></figure>

pyrolysis in the work by Tiarks et al. (2019). Conversely, KL-MP does not show any marked morphological changes when changing the carbonization heating rate. The carbonized particles remain dispersible and retain their micron-sized spherical morphology at heating rates up to 20 °C/min, which are close to industrially relevant conditions. Morphology retention at even higher heating rates appears feasible and will be examined in future work using equipment capable of faster heating profiles.

To better understand the observed thermo-morphological changes during carbonization, complementary thermo-analytical measurements were conducted to measure the glass transition temperatures ($T_g$) and

hydrogen mobility in the KL samples. The $T_g$ of the KL samples were measured by DSC (Fig. 2(c)). The KL-REF sample shows a $T_g$ at 142 °C during the first cycle, which increased further to 151 °C during the second cycle, which is typical of industrial KL (Cui et al., 2013). The rise in $T_g$ observed during the second cycle can be attributed to the evaporation of water in the initial heating cycle, as water may serve as a plasticizer (Hatakeyama et al., 2010). Additionally, crosslinking between lignin units can occur through various radical coupling reactions, especially at temperatures above the glass transition point (Cui et al., 2013).

The glass transition temperature could not be accurately determined
for the KL-MP sample, as no distinct baseline shift was observed during the initial heating cycle; instead, an endothermic event extended up to 200 °C. In the second heating cycle, the DSC curve remained nearly flat, exhibiting neither a glass transition nor any other discernible thermal event. Previous studies have indicated that the pronounced amorphous character of KL, combined with its intricate hydrogen bonding network, can result in substantial enthalpy relaxation during DSC analysis, which appears as a broad endothermic peak encompassing and surpassing the glassy region in the first scan (Cui et al., 2013). To overcome such a strong enthalpy relaxation and to properly measure the *T*<sub>g</sub>, annealing KL above the *T*<sub>g</sub> during the first cycle seems to be required (Rials & Glasser, 1984). However, as crosslinking reactions might differ between lignin samples, the annealing during the first cycle might cause drastic and irreversible changes in the lignin structure through radical coupling reactions.

The DSC signature of the KL-MP sample indicates significant cross-
linking during the initial heating cycle, likely attributable to radical
coupling reactions (Cui et al., 2013), potentially catalyzed by inorganic
species. This process results in a rigid structure characterized by
restricted lignin chain mobility. Vural et al. (2018) utilized molecular
dynamics simulations and polymer theory to analyze atomic motions
contributing to the lignin glass transition. Their research demonstrated
that, below T<sub>g</sub>, lignin primarily undergoes internal and localized

movements, whereas, above T<sub>g</sub>, segmental motions prevail, facilitating
increased chain mobility. Enhanced mobility above T<sub>g</sub> is observed in
segments composed of three to five lignin monomeric units. Based on
these findings, the mobility of such short chains would be especially
constrained in the KL-MP sample.

To further understand the DSC findings, the thermal mobility of the KL chains during heat treatment was investigated by PMRTA (Maroto-Valer et al., 1997). The thermal mobility of hydrogen atoms in the KL samples as a function of temperature is shown in Fig. 2(d). Some technical lignins exhibit gradual increase in chain mobility after heating above the T<sub>g</sub>, with almost 100% hydrogen mobility in between 300–400 °C (Dufour et al., 2012; Shrestha et al., 2017). The KL-REF sample shows that lignin chains remain highly mobile during heat treatment, reaching up to 97% hydrogen mobility around 360 °C, which is consistent with typical behavior observed in technical lignins. In contrast, KL-MP particles experience a significant decrease in chain mobility, with a maximum hydrogen mobility of only 26% near 330 °C. This reduced mobility in KL-MP may result from various inorganic actions, such as the previously mentioned catalytic crosslinking of lignin chains and potential physical restriction of chain movement during heat treatment.

Molecular simulations of lignin chain dynamics were used to com-
plement the thermal analyses and to support their mechanistic

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (a) Molecular model of softwood lignin with the lignin chains (green) visible on the left side and the phenol O (red) on the right side of the simulation domain. (b) Molecular model of Na substituted lignin with only the phenoxide O (red) and Na ions (yellow) visible. (c) Cumulative distribution function of the size (atom count) of clusters formed by the phenol/phenoxide O. (d) Virtual dilatometry of the two lignin models. (e) Diffusivity of the lignin monomeric units as a function of temperature.</figcaption></figure>
interpretation. Two models were used for this purpose, namely that of native-like softwood lignin (unsubstituted) and its Na substituted variant (substituted). Using native-like softwood lignin as a surrogate for softwood KL was based on practical considerations and was deemed sufficient to demonstrate substitution effects and to observe their underlying mechanisms. Fig. 3(a) shows a molecular visualization of the

unsubstituted lignin model (instantaneous) with the spatial distribution of the phenolic O shown on the right side of the simulation domain. Fig. 3(b) shows the spatial distribution of the phenoxide O and Na ions in the substituted lignin model. The visualizations show that Na substitution leads to a clustering of the formed phenoxide groups and their counter-ions. These clusters are stable throughout the considered

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) Physical responses of KL-MP (left) and KL-REF (right) under different treatment conditions. SEM images of (b) KL-MP DC, (c) KL-REF DC and (d) KL-MP PC. SEM images of KL-MP particles during different treatment conditions, (e) KL-MP, (f) KL-MP DC (not washed), (g) KL-MP DC acid washed, (h) Pretreated KL-MP (not washed), (i) Pretreated KL-MP (acid washed), (j) KL-MP PC.</figcaption></figure>
temperature range and link together multiple neighboring lignin chains, thus acting as physical crosslinks. The structural changes are confirmed by a cluster analysis of the phenol/phenoxide O, shown in Fig. 3(c). The effects on lignin chain dynamics are demonstrated by dilatometry (Fig. 3 (d)) and monomer diffusivity data (Fig. 3(e)). Fig. 3(d) shows the thermal contraction behavior of the two lignin models through temperature-density curves. The unsubstituted model displays glass transition at roughly 200 °C (at 2.5 K/ns cooling rate), as evidenced by the gradual transition between two regions of linear contraction (or expansion). In the substituted model, thermal contraction (or expansion) above 200 °C is significantly lower, indicating a stiffer response, and the existence of a transition point is not clear. This is qualitatively consistent with our DSC (Fig. 2(c)) and PMRTA (Fig. 2(d)) observations, where KL-MP shows extensive structural relaxation and strongly restricted proton mobility compared with KL-REF. Lastly, Fig. 3(e) shows the diffusivity of the lignin monomeric units as a function of temperature. Both models display the expected exponential dependence on temperature. However, the diffusivity of the unsubstituted chains begins to diverge from that of the substituted ones above 150 °C, ultimately reaching values larger by two orders of magnitude. The larger diffusivity in the unsubstituted model reflects faster stress relaxation and lower viscosity in the liquid state. It should be noted that the molecular simulations were based on a non-reactive force field and thus do not address the formation of chemical crosslinks. Our aim was to separate purely physical effects of Na substitution from those resulting from other chemical changes. The modelling results corroborate the findings from experimental, thermo-analytical and imaging measurements, each pointing towards a major effect caused by inorganics during the thermal conversion of lignin.

### 3.3. Oxidative thermal stabilization and carbonization of the KL samples

Carbonization of the KL samples overall showed a yield (dry basis) of about 40 wt%, except for KL-MP PC which showed a yield of 31.4 wt%. Digital and SEM images of the KL samples after the different thermal treatment steps are shown in Fig. 4. After the oxidative pretreatment, the KL-MP exhibits almost no changes apart from slight darkening in its color while KL-REF softens, foams, and appears burnt. Upon mechanical action, the KL-MP was easy to disperse. It retained its initial powdery consistency and spherical morphology (Fig. 4(e)). In contrast, the KL-REF turned into a fused bloc, which is hard and very resistant to crushing when applying a mechanical force. After direct carbonization, KL-MP turned into an easily dispersible powder with spherical particle morphologies (Fig. 4(b,g)) while KL-REF became a hard and fused monolithic structure (Fig. 4(a,c)). After PC treatment, the KL-MP retained its powdery consistency and spherical morphology (Fig. 4(d, j)). In the case of KL-REF, the usual degree of fusion is not observed after PC which is due to the thermal stabilizing effect during the pretreatment.

Although KL-REF demonstrates thermo-morphological properties similar to those of standard KL (Nowakowski et al., 2010; Sharma et al., 2004), KL-MP displays unique characteristics. During oxidative pretreatment and DC treatment, inorganic substances in both lignin-bound and mineral forms remain within KL-MP, which prevents fusion and loss of morphology. These inorganics are subsequently washed out from KL-MP after oxidative pretreatment. After carbonization (PC treatment), the particles retain their morphology, even though the inorganic components are removed. This effect is credited to the role of inorganic substances in catalyzing lignin chain crosslinking during oxidative pretreatment, as evidenced by thermo-analytical data.

Achieving similar morphology retention in low-ash KLs using oxidative thermal stabilization typically requires much longer pretreatment times (Braun et al., 2005). For example, Zhao et al. (2021) found that low-ash KL microparticles needed at least 6 h of oxidative treatment at 250 °C with a heating rate of 1 °C/min to avoid fusion during carbonization. In contrast, KL-MP required just over 2 h under

similar conditions. Tests at higher heating rates showed that KL-MP particles maintained their morphology even when carbonized at 20 °C/min, indicating that pre-oxidation duration can be four times lower without compromising the particle integrity, while also aligning with industrially relevant heat-treatment conditions. The data also suggest that Na salt dispersion accelerates KL thermal stabilization, significantly reducing time and energy requirements.

The behavior of inorganic components during thermal treatment and washing steps was analyzed with SEM-EDX. Prior to thermal treatment, KL-MP appears as spherical, micron-sized particles featuring smooth surfaces (Fig. 4(e)), with sodium-based species distributed uniformly (Fig. 1(i)). Following DC, the particle morphology remains consistent. SEM images of carbonized particles reveal additional smaller features on the carbon surface, manifesting as stripes or round and flat particles (Fig. 4(f)). These stripes are removed after the washing process, resulting in a smooth carbon particle surface (Fig. 4(g)). Notably, such stripes are absent on KL-MP samples after pretreatment (Fig. 4(h)) and subsequent carbonization (Fig. 4(j)).

Elemental mapping reveals that the stripes appearing on the surface of KL-MP after carbonization correspond to Na oxide particles (Fig. S5). It seems that during carbonization, the Na-based inorganics diffuse from within the KL-MP particles and finally concentrate on the carbon surface in the form of crystallized Na oxide stripes. Those can be removed (or recovered) by a simple washing step leaving behind pure carbon particles. XRD analysis of KL-MP after carbonization also reveals the presence of these inorganics which disappear after the washing step (Fig. S4). The formation of the inorganic stripes occurs after a certain temperature threshold as the pretreated KL-MP sample does not exhibit this phenomenon. The absence of these inorganic particles after carbonization of pretreated and washed samples implies those can be readily removed by dilute acid washing. The removal of inorganics after the washing steps was almost quantitative, with almost 99% Na removed as determined by trace minerals analysis (Table S3, see ESI). STA-MS analysis of the pretreated and washed KL-MP sample (Fig. S3(a)) shows no activation reactions and closely matches the pretreated KL-REF sample (Fig. S3(b)), confirming the washing step's effectiveness in removing inorganics.

Altogether, the approach proposed in this work leverages the catalytic action of inorganic species inherently present in black liquor to crosslink KL during oxidative thermal stabilization, drastically shortening the stabilization time and bringing it closer to industrially relevant conditions. Unlike conventional strategies based on added organic crosslinkers, it does not require additional chemicals, stringent control of reaction stoichiometry, or separate synthesis and curing steps prior to carbonization. For example, phenol-formaldehyde chemistry can indeed produce a well-crosslinked lignin network (Ren et al., 2022), but at the expense of using phenol and formaldehyde and introducing extra processing stages. In contrast, our concept exploits existing kraft inorganics as in situ catalytic centers, simplifying the process while enabling effective melt suppression and morphology control.

### 3.4. Possible mechanism of inorganics-assisted oxidative thermal stabilization of kraft lignin

To better understand why KL-MP can be oxidatively stabilized without melting while KL-REF cannot, we combined FTIR spectroscopy, thermogravimetric analysis in air, and thermal stabilization experiments of model salt/KL-REF mixtures.

FTIR analysis (Fig. S3(d)) shows that, upon oxidative pretreatment, both KL-REF and KL-MP undergo general degradation and condensation (overall band intensity loss). However, KL-MP exhibits a much stronger increase of the carbonyl band (~1700 cm⁻¹) and retention or slight enhancement of the ether-related C—O—C band (~1220 cm⁻¹), whereas this band decreases markedly in KL-REF. This indicates more extensive oxidation and ether-forming condensation in KL-MP under identical conditions. A gram-scale thermogravimetric experiment in air reproducing the oxidative thermal pretreatment conditions further shows that
the mass-loss rate of KL-MP during the 250 °C step is about four times higher than that of KL-REF (Fig. S3(c)) consistent with accelerated re-action kinetics. Model salt experiments with KL-REF reveal that catalytic crosslinking and morphology retention occur only when KL dissolves in Na₂CO₃, NaOH, K₂CO₃ or KOH (conditions under which alkali

phenoxide/carboxylate species can form), but not with NaCl or Na₂SO₄ (Fig. S2). Taken together, these observations support a mechanism in which organically bound alkali species, especially Na enhance the reactivity of lignin functional groups toward oxygen and condensation, thereby catalyzing oxidative crosslinking and suppressing melting. A

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Properties of KL-derived hard carbon samples (a) Raman spectroscopy, (b) XRD and (c) Nitrogen physisorption. Electrochemical characteristics of KL-derived hard carbon samples (d) Specific delithiation capacity at different current (C) rates, (e) Average delithiation voltage at different C rates, (f) Initial cycle lithiation/delithiation profile of KL-derived hard carbon samples, (g) 10th cycle (end of 0.2C) lithiation/delithiation profile of KL-MP PC and commercial hard carbons, (h) 25th cycle (end of 2C) lithiation/delithiation profile of KL-MP PC and commercial hard carbons. (i) Stability tests, specific delithiation capacity over 200 cycles at 0.5C with three cycles at 0.2C at the start and after every 50 cycles.</figcaption></figure>
plausible explanation, consistent with the known higher reactivity of phenoxide versus phenol, is that sodium phenoxide sites are more readily oxidized and more prone to enter condensation pathways. However, this remains a working hypothesis that requires further investigation by more advanced characterization techniques to clarify the existing forms of Na in KL and its conversion products during and after crosslinking. In addition, while the present results clearly identify organically bound Na species as key enablers of melt-suppressing oxidative crosslinking, a systematic mapping of Na content versus crosslinking rate and morphology retention remains an important subject for future work aimed at optimizing the recovery process towards hard carbon application.

### 3.5. Properties of hard carbons derived from the KL samples

The nanostructure of hard carbons was investigated using Raman spectroscopy and XRD. Diffraction techniques are the most suitable methods for the structural characterization of highly ordered solid materials (crystalline long-range order). However, for highly disordered materials such as carbonized lignin, Raman spectroscopy is a more adapted since it is sensitive not only to crystal structures, but also to molecular structures (short-range order) (Sadezky et al., 2005). The Raman spectra of lignin-derived hard carbons (Fig. 5(a)) shows the characteristic D and G bands having peak maxima close to 1360 and 1580 cm⁻¹, respectively. The G band arises from in-plane bond-stretching vibrations by pairs of sp² hybridized carbon atoms (E₂g symmetry) at any site i.e., not restrictive to sixfold rings. The D band arises by breathing mode vibrations (A₁g symmetry) linked to sp² hybridized carbon atoms strictly in sixfold ring structures, this mode is forbidden in perfectly ordered graphite and is activated in presence of structural disorder (Ferrari & Robertson, 2000). Hence, in the case of hard carbons, the I₉/Iₐ peak intensity ratio increases with the extent of carbonization as the number of sixfold rings increases reflecting the level of nanostructural organization (Rouzaud et al., 2015). The I₉/Iₐ values (Table 3) indicate that KL-REF derived hard carbons have higher degree of nanostructural ordering, and that the pretreatment has little to no effect in this regard. The low I₉/Iₐ value for KL-MP DC indicates that the inorganics impede nanostructural evolution during carbonization. When the inorganics are removed during the pretreatment, the I₉/Iₐ ratio increases as seen for KL-MP PC.

The hard carbon samples were analyzed by XRD (Fig. 5(b)) to further understand their bulk structural ordering. XRD patterns of all samples are typical of non-graphitizing carbons, with broad 002 and 001 peaks around 24° and 43°, respectively. The broad nature of these peaks is typical for hard carbons due to its disordered structure and presence of nano-graphitic domains with a few numbers of stacked graphene layers (Iwashita, 2016). The calculated interlayer spacing (d₀₀₂) values are similar for all the samples (Table 3). No sample approaches the interlayer spacing of crystalline graphite.

The textural properties of the hard carbon samples were investigated by N₂ physisorption (Fig. 5(c), Table 3). KL-MP DC shows a total adsorbed volume of 283 cm³/g and a BET surface area of 903 m² g⁻¹, almost two orders of magnitude higher than the other three samples, which display very low N₂ uptake and Aₐ₉₆ values of 1–2 m² g⁻¹. For KL-

MP DC, the total pore volume is 0.40 cm³/g, with ~82% attributed to micropores and 17% to mesopores; ultramicropores account for 53% of the micropore volume. These features are consistent with the STA–MS results, which indicate activation of KL-MP above 700 °C by its inorganic fraction.

N₂ physisorption provides key textural metrics (BET area, total pore volume and isotherm shape) that clearly distinguish the strongly activated KL-MP DC from the low-surface-area carbons obtained via the PC route. For the latter, the very low N₂ uptake means that the absolute adsorbed amounts are close to the instrumental detection limit, and the available isotherms do not extend into the very low relative-pressure regime that most strongly constrains micropore filling in standard PSD models. In combination, these factors make pore-size-distribution analysis highly uncertain and noise-sensitive (Madani et al., 2015a, 2015b; Thommes et al., 2015). Moreover, gas sorption (with N₂ or other adsorptive molecules) probes only pores that are accessible to the gas. A rigorous quantification of closed pore volume would require complementary techniques such as small-angle scattering combined with dedicated modelling approaches (Saurel et al., 2019), which is beyond the scope of this study.

Overall, the elimination of inorganic components via oxidative pretreatment and acid washing effectively suppresses activation, as demonstrated by KL-MP PC, whose textural properties are comparable to those of KL-REF-derived hard carbons. Raman spectroscopy, XRD, N₂ physisorption and STA–MS consistently show that inorganics govern both nanostructural ordering and activation behaviour during carbonization, and that their removal after pretreatment yields morphology-retentive, low-surface-area hard carbons.

### 3.6. Electrochemical performance

The electrochemical performance of the hard carbons as the anode active material was determined by galvanostatic cycling experiments using coin cells with metallic lithium as the counter electrode. The specific delithiation capacity, average delithiation voltage, and voltage vs capacity plots are shown in Fig. 5(d-h), and the main electrochemical

<table><caption>**Table 4**
Electrochemical properties of KL and commercial hard carbons.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">ICE (%)</th><th colspan="2">Specific delithiation capacity (mAh/g)</th><th rowspan="2">Average delithiation voltage at 2C (V)</th><th rowspan="2">Retention (%)</th></tr><tr><th>at 0.2C</th><th>at 2C</th></tr></thead><tbody><tr><td>KL-MP DC</td><td>30</td><td>134</td><td>110.1</td><td>0.41</td><td>98.8</td></tr><tr><td>KL-MP PC</td><td>60</td><td>164.9</td><td>146.5</td><td>0.44</td><td>99.3</td></tr><tr><td>KL-REF DC</td><td>54</td><td>140.3</td><td>118.7</td><td>0.42</td><td>100</td></tr><tr><td>KL-REF PC</td><td>53</td><td>135.7</td><td>111.6</td><td>0.46</td><td>100</td></tr><tr><td>SIB-BHC300</td><td>69</td><td>167.4</td><td>162.2</td><td>0.44</td><td>&gt;100</td></tr><tr><td>MTI HC</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>PO0199 MSE HC</td><td>65</td><td>163</td><td>150.8</td><td>0.41</td><td>99.8</td></tr></tbody></table>

ICE – Initial coulombic efficiency.

<table><caption>**Table 3**
Properties of hard carbons derived from KL samples.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Raman spectroscopy<br>I₉/Iₐ</th><th rowspan="2">XRD<br>d₀₀₂<br>(nm)</th><th colspan="4">N₂ Physisorption</th></tr><tr><th>Specific surface area(BET)<br>(m²/g)</th><th>Total adsorbed volume<br>(cm³/g STP)</th><th>Cumulative pore volume(NLDFT)<br>(cm³/g STP)</th><th>Micropore volume (NLDFT)<br>(cm³/g STP)</th></tr></thead><tbody><tr><td>KL-MP DC</td><td>0.909</td><td>0.381</td><td>903.5</td><td>283.1</td><td>0.4</td><td>0.32</td></tr><tr><td>KL-MP PC</td><td>0.944</td><td>0.388</td><td>1.2</td><td>3.3</td><td>–</td><td>–</td></tr><tr><td>KL-REF DC</td><td>0.968</td><td>0.381</td><td>2</td><td>3.9</td><td>–</td><td>–</td></tr><tr><td>KL-REF PC</td><td>0.963</td><td>0.383</td><td>1.6</td><td>3.3</td><td>–</td><td>–</td></tr></tbody></table>

ID/IG – Intensity ratio of Raman D band to G band, d₀₀₂ – Interlayer spacing.
results can be found in Table 4. For a LIB anode active material, the delithiation phase corresponds to discharge in full cell configuration. All the samples show irreversible capacity due to solid electrolyte interface (SEI) formation in the initial cycle, which appears to have stabilized from the subsequent cycle onwards (Fig. S7). Alvin et al. (2020) and Yang et al. (2020) evidence lithium storage in hard carbons through an adsorption-intercalation mechanism, where lithium adsorption at defect sites, edge sites, and micropore surfaces accounts for most of the capacity and gives rise to the sloping voltage region, while lithium intercalation into nano-graphitic domains contributes only to a small extent at lower voltages. These studies report no distinct low-voltage plateau (within the voltage range tested) and therefore no clear separation between adsorption and intercalation contributions. The electrochemical profiles of all hard carbons investigated in this work are consistent with this behaviour, exhibiting slopping voltage capacity and minimal low voltage plateau capacity. The KL-MP PC sample shows much better electrochemical characteristics compared to KL-MP DC sample, specifically the initial cycle coulombic efficiency (ICE) is significantly higher for KL-MP PC (59.8%) than for KL-MP DC (30.4%) but not as high as the ICE of commercial hard carbons (65–70%). The poor ICE of the KL-MP DC sample is a consequence of the high surface area possessed by this sample (see Table 3) leading to pronounced consumption of electrolyte to form the SEI during the initial cycle. This is also observed in the dQ/dV plot showing a broad peak between 0.8–0.4 V for KL-MP DC during the initial lithiation step (Fig. S7(a)), which is mitigated in case of KL-MP PC.

The KL-MP PC sample exhibits much better delithiation capacity in comparison to both KL-REF derived hard carbons samples. During the initial lithiation step, the dQ/dV plot reveals KL-REF-derived samples display more pronounced SEI-related losses in the 1.0–0.6 V region, which is much lower in KL-MP PC. Beyond SEI formation, the KL-MP PC sample exhibits a relatively higher capacity contribution at low voltages (<0.2 V), indicative of enhanced lithium storage capacity by intercalation. The capacity drop while increasing the delithiation rate is very similar for all the KL-derived hard carbons despite the difference in reversible capacity suggesting that the rate determining step is not morphology dependent. The long cycle stability tests for the best performing hard carbon from each KL sample (KL-MP PC and KL-REF DC) along with the commercial MTI hard carbon sample as reference is shown in the Fig. 5(i). KL-MP PC exhibits stable capacity over 200 cycles and consistently delivers higher delithiation capacity than KL-REF DC. The capacity retention values after 200 cycles at 0.5C were 88.7% for KL-MP PC, 88.4% for MTI HC, and 84.1% for KL-REF DC. Also, the SEI seems very stable as the average coulombic efficiency during the 200 cycles was >99.9% for all the studied materials. These results are consistent with the rate capability tests further showing that KL-MP PC provides higher delithiation capacity and superior long-term stability compared to KL-REF DC, with capacity retention comparable to that of the commercial hard carbon sample.

Taken together, the structural (Raman, XRD) and textural (N₂ physisorption) data indicate only modest differences between KL-MP PC and KL-REF derived hard carbons at the nanostructural level, whereas their particle-scale morphologies differ strongly. The characterization results suggest that the superior electrochemical performance of KL-MP PC compared with KL-REF-derived hard carbons under identical testing conditions is primarily linked to its morphological features rather than to major differences in structure or texture. KL-MP PC consists of smooth, spherical particles that can pack efficiently and are likely to promote more uniform SEI formation resulting in shorter Li⁺ diffusion pathways. In contrast, the KL-REF-derived hard carbons required additional grinding and sieving, resulting in less well-defined particle shapes and broader size distributions. Such spherical morphologies offer a high degree of compaction, which can facilitate the fabrication of homogeneous electrodes (Beda et al., 2021) with improved mass transport (Tang et al., 2012), and better buffering of volume changes during cycling (Wang et al., 2018), all of which can contribute to the better

electrochemical performance of KL-MP PC. Collectively, the current electrochemical findings establish a foundational proof-of-concept for employing these hard carbons in alkali-ion batteries. Subsequent research will focus on comprehensive analysis of long-term cycling stability and the optimization of plateau capacity across various alkali-ion battery chemistries, accomplished through systematic regulation of the catalytic oxidative cross-linking and carbonization processes.

## 4. Conclusion

We have shown that kraft black liquor inorganics, typically removed during the recovery stage to obtain high purity KL, can instead be exploited as catalytic centers to rapidly stabilize KL during thermo-oxidative treatment. By retaining Na based species in spray-dried KL microparticles and pretreating them at 250 °C, we promote inorganics-assisted oxidative crosslinking that accelerates the conversion of a thermoplastic lignin into a rigid network and suppresses its melting and foaming. Thermal analysis, FTIR, proton magnetic resonance thermal analysis, targeted Na salt additions and molecular dynamics consistently point to Na phenoxide-type sites and Na-driven ionic clusters as the key drivers of this melt inhibition. After pretreatment and removal of inorganics, the crosslinked lignin yields morphology-retentive, low-surface-area hard carbon microspheres that are promising anode candidates for alkali-ion batteries. More broadly, our results establish a new design principle, leveraging rather than eliminating kraft inorganics to access melt-resistant lignins to produce morphology-controlled carbons for a range of functional materials. Mapping the influence of KL-MP preparation conditions (inorganics composition, content and membrane filtration process) and optimizing carbonization conditions for targeted applications, thereby establishing detailed process-structure-property relationships will be the subject of future scale-up-oriented studies.

### CRediT authorship contribution statement

**Glen Pauls:** Writing – review & editing, Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis, Data curation. **Olli Sorsa:** Writing – review & editing, Validation, Supervision, Software, Methodology. **Antti Paajanen:** Writing – review & editing, Writing – original draft, Software, Methodology, Formal analysis. **Vaibhav Modi:** Software, Methodology. **Sebastien Leclerc:** Writing – review & editing, Investigation, Formal analysis. **Younes Bouizi:** Writing – review & editing, Investigation, Formal analysis. **Anthony Dufour:** Writing – review & editing. **Anna Kalliola:** Writing – review & editing, Validation, Resources. **Chamseddine Guizani:** Writing – review & editing, Writing – original draft, Validation, Supervision, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Glen Pauls, Olli Sorsa, Anna Kalliola and Chamseddine Guizani has patent issued by World Intellectual Property Organization. The other authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgments

The authors gratefully acknowledge the funding from the Research Council of Finland, project 357161, fellowship grant (2023-2027). The authors thank Metsä Fibre for providing the black liquor used for the membrane-based lignin recovery in VTT funded project called Nano-Lignin. The authors would like to thank the CPM NMR facility of
Université de Lorraine pour its contribution to the present publication. Part of the facilities used at VTT were provided by the Academy of Finland Research Infrastructure Printed Intelligence Infrastructure (PII-FIRI, Grant No. 32020). The authors wish to acknowledge CSC - IT Centre for Science, Finland, for computational resources. This work made use of Aalto University Bioeconomy Facilities. The authors also gratefully acknowledge all the experimental help received from Elina Raineva, Atte Mikkelson, Juha Heikkinen, Hanna Kyllönen and Teemu Ruuska.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.biortech.2026.134177.

### Data availability

Data will be made available on request.

## References

Abraham, M.J., Murtola, T., Schulz, R., Páll, S., Smith, J.C., Hess, B., Lindahl, E., 2015. GROMACS: high performance molecular simulations through multi-level parallelism from laptops to supercomputers. SoftwareX 1-2, 19-25. https://doi.org/10.1016/j.softx.2015.06.001.

Alvin, S., Cahyadi, H.S., Hwang, J., Chang, W., Kwak, S.K., Kim, J., 2020. Revealing the intercalation mechanisms of lithium, sodium, and potassium in hard carbon. Adv. Energy Mater. 10, 2000283. https://doi.org/10.1002/aemn.202000283.

Balakshin, M., Capanema, E.A., Zhu, X., Sulaeva, I., Potthast, A., Rosenau, T., Rojas, O.J., 2020. Spruce milled wood lignin: linear, branched or cross-linked? Green Chem. 22, 3985-4001. https://doi.org/10.1039/D0GC00926A.

Beaucamp, A., Muddasar, M., Amiinu, I.S., Moraes Leite, M., Culebras, M., Latha, K., Gutiérrez, M.C., Rodríguez-Padron, D., del Monte, F., Kennedy, T., Ryan, K.M., Luque, R., Titirici, M.-M., Collins, M.N., 2022. Lignin for energy applications – state of the art, life cycle, technoeconomic analysis and future trends. Green Chem. 24, 8193-8226. https://doi.org/10.1039/D2GC02724K.

Beda, A., Rabuel, F., Morcrette, M., Knopf, S., Taberna, P.L., Simon, P., Matei Ghimbeu, C., 2021. Hard carbon key properties allow for the achievement of high Coulombic efficiency and high volumetric capacity in Na-ion batteries. J. Mater. Chem. A Mater. 9, 1743–1758. https://doi.org/10.1039/D0TA07687B.

Bertaud, F., Ottenio, P., Aubigny, A., Dufour, A., 2023. Recovering lignin in a real-case industrial kraft pulp mill: pilot-scale experiment and impact on the mill commodities. ACS Sustain. Chem. Eng. 11, 6311–6318. https://doi.org/10.1021/ACSSUSCHEMENG.2C07678/SUPPL_FILE/SC2C07678_S1_001.PDF.

Bouraoui, Z., Jeguirim, M., Guizani, C., Limousy, L., Dupont, C., Gadiou, R., 2015. Thermogravimetric study on the influence of structural, textural and chemical properties of biomass char on CO2 gasification reactivity. Energy 88, 703–710. https://doi.org/10.1016/J.ENERGY.2015.05.100.

Braun, J.L., Holtman, K.M., Kadla, J.F., 2005. Lignin-based carbon fibers: oxidative thermostabilization of kraft lignin. Carbon n. y. 43, 385–394. https://doi.org/10.1016/j.carbon.2004.09.027.

Chatterjee, S., Saito, T., 2015. Lignin-derived advanced carbon materials. ChemSusChem 8, 3941–3958. https://doi.org/10.1002/CSSC.201500692.

Correa-Guillen, E., Henn, K.A., Osterberg, M., Dessbesell, L., 2025. Lignin's role in the beginning of the end of the fossil resources era: a panorama of lignin supply, economic and market potential. Curr. Opin. Green Sustain. Chem. 54, 101038. https://doi.org/10.1016/J.COGC.2025.101038.

Cui, G., Sadeghifar, H., Sen, S., Argyropoulos, D.S., 2013. Toward thermoplastic lignin polymers; Part II: thermal & polymer characteristics of kraft lignin & derivatives. BioResources 8, 865–886.

Dahou, T., Devoort, F., Khari, B., Labaki, M., Dupont, C., Jeguirim, M., 2021. Role of inorganics on the biomass char gasification reactivity: a review involving reaction mechanisms and kinetics models. Renew. Sustain. Energy Rev. 135, 110136. https://doi.org/10.1016/J.RSER.2020.110136.

Ding, L., Cheng, M.H., Lin, Y., Lin, K.T., Sale, K.L., Sun, N., Donohoe, B.S., Ray, A.E., Li, C., 2025. Understanding the impacts of inorganic species in woody biomass for preprocessing and pyrolysis–a review. Energy 322, 135697. https://doi.org/10.1016/J.ENERGY.2025.135697.

Dufour, A., Castro-Díaz, M., Marchal, P., Brosse, N., Olcese, R., Bouroukba, M., Snape, C., 2012. In situ analysis of biomass pyrolysis by high temperature rheology in relations with 1H NMR. Energy Fuels 26, 6432–6441. https://doi.org/10.1021/ef301310x.

Fearon, O., Kalliola, A., Kyllönen, H., Heikkinen, J., Kontturi, K., Siipola, V., Torvinen, K., Guizani, C., Pauls, G., Sorsa, O., 2024. A method for recovering lignin microparticles from spent pulping liquor, a method for preparing carbonized lignin, lignin microparticles, a coating composition, a resin, an electrochemical energy storage device carbonized lignin microparticles, and use of the lignin microparticles and the carbonized lignin microparticles. World Intellectual Property Organization. WO2024170829A1.

Ferrari, A., Robertson, J., 2000. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B 61, 14095. https://doi.org/10.1103/PhysRevB.61.14095.

Guizani, C., Louisnard, O., Sanz, F.J.E., Salvador, S., 2015. Gasification of woody biomass under high heating rate conditions in pure CO2: experiments and modelling. Biomass Bioenergy 83, 169–182. https://doi.org/10.1016/J.BIOMBIOE.2015.09.017.

Hatakeyama, H., Tsujimoto, Y., Zarubin, M.J., Krutov, S.M., Hatakeyama, T., 2010. Thermal decomposition and glass transition of industrial hydrolysis lignin. J. Therm. Anal. Calorim. 101, 289–295. https://doi.org/10.1007/S10973-010-0698-8/FIGURES/9.

Hubbe, M.A., Alén, R., Paleologou, M., Kannangara, M., Kihlman, J., 2019. Lignin recovery from spent alkaline pulping liquors using acidification, membrane separation, and related processing steps: a review. Bioresour 14, 2300–2351. https://doi.org/10.15376/BIORES.14.1.HUBBE.

Iwashita, N., 2016. Chapter 2 - X-ray Powder Diffraction. In: Inagaki, M., Kang, F. (Eds.), Materials Science and Engineering of Carbon. Butterworth-Heinemann, pp. 7–25, 10.1016/B978-0-12-805256-3.00002-7.

Jia, L., Le Brech, Y., Mauviel, G., Qi, F., Bente-Von Frowein, M., Ehlert, S., Zimmermann, R., Dufour, A., 2016. Online analysis of biomass pyrolysis tar by photoionization mass spectrometry. Energy Fuels 30, 1555–1563. https://doi.org/10.1021/ACS.ENERGYFUELS.5B02274/ASSET/IMAGES/LARGE/EF-2015-02274G_0009.JPEG.

Joshi, S., Kalyanasundaram, S., Balasubramanian, V., 2013. Quantitative analysis of sodium carbonate and sodium bicarbonate in solid mixtures using Fourier transform infrared spectroscopy (FT-IR). Appl. Spectrosc. 67, 841–845. https://doi.org/10.1366/12-06915.

Kawamoto, H., 2017. Lignin pyrolysis reactions. J. Wood Sci. 2017 63: 2 63, 117–132. https://doi.org/10.1007/S10086-016-1606-Z.

Kienberger, M., Maitz, S., Pichler, T., Demmelmayer, P., 2021. Systematic Review on Isolation Processes for Technical Lignin. Processes 9, 804–821. https://doi.org/10.3390/pr9050804.

Kim, J.W., Lee, H.G., 2001. Thermal and carbothermic decomposition of Na2CO3 and Li2CO3. Metall. Mater. Trans. B 32, 17–24. https://doi.org/10.1007/S11663-001-0003-0/METRICS.

Kitchamsetti, N., Mhin, S., 2025. Advances and Prospects of Lignin-Derived Hard Carbons for Next-Generation Sodium-Ion Batteries. Polymers 2025, Vol. 17, Page 2801 17, 2801. https://doi.org/10.3390/POLYM17202801.

Köhne, J., Rennhofer, H., Lichtenegger, H., Mahendran, A.R., Unterweger, C., Prats-Mateu, B., Gierlinger, N., Schwaiger, E., Mahler, A.K., Potthast, A., Gindl-Altmutter, W., 2018. Electrically conducting carbon microparticles by direct carbonization of spent wood pulping liquor. ACS Sustain. Chem. Eng. 6, 3385–3391. https://doi.org/10.1021/ACSSUSCHEMENG.7B03582/ASSET/IMAGES/LARGE/SC-2017-035826_0008.JPEG.

Kouisni, L., Gagné, A., Maki, K., Holt-Hindle, P., Paleologou, M., 2016. LignoForce system for the recovery of lignin from black liquor: feedstock options, odor profile, and product characterization. ACS Sustain. Chem. Eng. 4, 5152–5159. https://doi.org/10.1021/ACSSUSCHEMENG.6B00907/ASSET/IMAGES/LARGE/SC-2016-0007C0004.JPEG.

Krotsccheck, A.W., Sixta, H., 2006. Recovery, in: Sixta, H. (Ed.), Handbook of Pulp. Wiley, pp. 967–996. https://doi.org/10.1002/9783527619887.ch9.

Luo, J., Liu, T.L., 2023. Electrochemical valorization of lignin: Status, challenges, and prospects. J. Bioresour. Bioprod. 8, 1–14. https://doi.org/10.1016/J.JOBAB.2022.11.003.

Madani, S.H., Badalyan, A., Biggs, M.J., Pendleton, P., 2015a. Uncertainty in pore size distribution derived from adsorption isotherms: I. Classical methods. Microporous Mesoporous Mater. 214, 210–216. https://doi.org/10.1016/J.MICROMESO.2015.04.026.

Madani, S.H., Diaz, L.H., Biggs, M.J., Pendleton, P., 2015b. Uncertainty in pore size distribution derived from adsorption isotherms: II. Adsorption integral approach. Microporous Mesoporous Mater. 214, 217–223. https://doi.org/10.1016/J.MICROMESO.2015.04.030.

Maroto-Valer, M.M., Andrésen, J.M., Snape, C.E., 1997. In-situ 1H NMR investigation of particle size, mild oxidation, and heating regime effects on plasticity development during coal carbonization. Energy Fuels 11, 236–244. https://doi.org/10.1021/EF960129N/ASSET/IMAGES/LARGE/EF960129NF00011.JPEG.

Matei Ghimbeu, C., Beda, A., Réty, B., El Marouazi, H., Vizintin, A., Tratnik, B., Simonin, L., Michel, J., Abou-Rjeilly, J., Dominko, R., 2024. Review: insights on hard carbon materials for sodium-ion batteries (SIBs): synthesis – properties – performance relationships. Adv. Energy Mater. 2303833. https://doi.org/10.1002/AENM.202303833.

Matei Ghimbeu, C., Zhang, B., Martinez de Yuso, A., Réty, B., Tarascon, J.M., 2019. Valorizing low cost and renewable lignin as hard carbon for Na-ion batteries: impact of lignin grade. Carbon n. y. 153, 634–647. https://doi.org/10.1016/J.CARBON.2019.07.026.

Nowakowski, D.J., Bridgwater, A.V., Elliott, D.C., Meier, D., de Wild, P., 2010. Lignin fast pyrolysis: results from an international collaboration. J. Anal. Appl. Pyrol. 88, 53–72. https://doi.org/10.1016/J.JAAP.2010.02.009.

Olsson, V., Areskogh, D., Walter, S., 2021. PROCESS FOR PRODUCING CARBON FROM HEAT TREATED LIGNIN. World Intellectual Property Organization. WO2021250604A1.

Ren, Q., Xue, Y., Cui, Z., Lu, Y., Li, W., Zhang, W., Ye, T., 2022. Preparation of lignin based phenolic resin microspheres with controllable particle size and its application in capacitors. Diam. Relat. Mater. 125, 109000. https://doi.org/10.1016/J.DIAMOND.2022.109000.
Rials, T.G., Glasser, W.G., 1984. Engineering plastics from lignin. X. Enthalpy relaxation of Prepolymers. J. Wood Chem. Technol. 4, 331–345. https://doi.org/10.1080/02773818408070653

Rouzaud, J.-N., Deldicque, D., Charon, É., Pageot, J., 2015. Carbons at the heart of questions on energy and environment: a nanostructural approach. C. R. Geosci. 347, 124–133. https://doi.org/10.1016/j.crte.2015.04.004.

Sadezky, A., Muckenhuber, H., Grothe, H., Niessner, R., Pöschl, U., 2005. Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information. Carbon n. y. 43, 1731–1742. https://doi.org/10.1016/j.carbon.2005.02.018.

Saurel, D., Segalini, J., Jauregui, M., Pendashteh, A., Daffos, B., Simon, P., Casas-Cabanas, M., 2019. A SAXS outlook on disordered carbonaceous materials for electrochemical energy storage. Energy Storage Mater. 21, 162–173. https://doi.org/10.1016/j.ensm.2019.05.007.

Sharma, R.K., Wooten, J.B., Baliga, V.L., Lin, X., Chan, W.G., Hajaligol, M.R., 2004. Characterization of chars from pyrolysis of lignin. Fuel 83, 1469–1482. https://doi.org/10.1016/j.FUEL.2003.11.015.

Shen, D.K., Gu, S., Luo, K.H., Wang, S.R., Fang, M.X., 2010. The pyrolytic degradation of wood-derived lignin from pulping process. Bioresour. Technol. 101, 6136–6146. https://doi.org/10.1016/j.BIORTECH.2010.02.078.

Shrestha, B., Le Brech, Y., Ghislain, T., Leclerc, S., Carré, V., Aubriet, F., Hoppe, S., Marchal, P., Pontvianne, S., Brosse, N., Dufour, A., 2017. A multitechnique characterization of lignin softening and pyrolysis. ACS Sustain. Chem. Eng. 5, 6940–6949. https://doi.org/10.1021/ACSSUSCHEMENG.7B01130/ASSET/IMAGES/LARGE/SC-2017-01130A_0010.JPEG.

Sixta, H., 2006. Introduction, in: Sixta, H. (Ed.), Handbook of Pulp. Wiley, pp. 2–19. https://doi.org/10.1002/9783527619887.ch1.

Tang, K., Fu, L., White, R.J., Yu, L., Titirici, M.M., Antonietti, M., Maier, J., 2012. Hollow carbon nanospheres with superior rate capability for sodium-based batteries. Adv. Energy Mater. 2, 873–877. https://doi.org/10.1002/AENM.201100691; REQUESTED JOURNAL:JOURNAL:16146840;ISSUE:ISSUE:DOI.

Thommes, M., Kaneko, K., Neimark, A.V., Olivier, J.P., Rodriguez-Reinoso, F., Rouquerol, J., Sing, K.S.W., 2015. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 87, 1051–1069. https://doi.org/10.1515/pac-2014-1117.

Tiarks, J.A., Dedic, C.E., Meyer, T.R., Brown, R.C., Michael, J.B., 2019. Visualization of physicochemical phenomena during biomass pyrolysis in an optically accessible reactor. J. Anal. Appl. Pyrol. 143, 104667. https://doi.org/10.1016/J.JAAP.2019.104667.

Tomani, P., 2010. The lignob boost process. cellulose chemistry and technology Cellulose Chem. Technol 44, 53–58.

Vanommeslaeghe, K., Hatcher, E., Acharya, C., Kundu, S., Zhong, S., Shim, J., Darian, E., Guvench, O., Lopes, P., Vorobyov, I., Mackerell, A.D., 2010. CHARMM general force

field: a force field for drug-like molecules compatible with the CHARMM all-atom additive biological force fields. J. Comput. Chem. 31, 671–690. https://doi.org/10.1002/JCC.21367.

Vermaas, J.V., Dellon, L.D., Broadbelt, L.J., Beckham, G.T., Crowley, M.F., 2019a. Automated Transformation of Lignin Topologies into Atomic Structures with LigninBuilder. ACS Sustain. Chem. Eng. 7, 3443–3453. https://doi.org/10.1021/ACSSUSCHEMENG.8B05665/SUPPL FILE/SC8B05665_S1_002.MPG.

Vermaas, J.V., Petridis, L., Ralph, J., Crowley, M.F., Beckham, G.T., 2019b. Systematic parameterization of lignin for the CHARMM force field. Green Chem. 21, 109–122. https://doi.org/10.1039/C8GC03209B.

Vural, D., Smith, J.C., Petridis, L., 2018. Dynamics of the lignin glass transition. PCCP 20, 20504–20512. https://doi.org/10.1039/C8CP03144D.

Wang, J., Zhang, M., Chen, M., Min, F., Zhang, S., Ren, Z., Yan, Y., 2006. Catalytic effects of six inorganic compounds on pyrolysis of three kinds of biomass. Thermochim Acta 444, 110–114. https://doi.org/10.1016/J.TCA.2006.02.007.

Wang, W., Li, W., Wang, S., Miao, Z., Liu, H.K., Chou, S., 2018. Structural design of anode materials for sodium-ion batteries. J. Mater. Chem. A Mater. 6, 6183–6205. https://doi.org/10.1039/C7TA10823k.

Widsten, P., Liitiä, T., Immonen, K., Borrega, M., Jääskeläinen, A.-S., Wikberg, H., Ohra-aho, T., Tamminen, T., 2020. Potential of lignin as antioxidant for thermoplastics and other materials. Lignin 1, 11–19. https://doi.org/10.62840/lignin.1.0.11.

Xiao, T., Yuan, H., Ma, Q., Guo, X., Wu, Y., 2019. An approach for in situ qualitative and quantitative analysis of moisture adsorption in nanogram-scaled lignin by using micro-FITIR spectroscopy and partial least squares regression. Int. J. Biol. Macromol. 132, 1106–1111. https://doi.org/10.1016/j.ijbiomac.2019.04.043.

Yang, G., Li, X., Guan, Z., Tong, Y., Xu, B., Wang, X., Wang, Z., Chen, L., 2020. Insights into lithium and sodium storage in porous carbon. Nano Lett. 20, 3836–3843. https://doi.org/10.1021/acs.nanolett.0c00943.

Zhang, H., Gu, W., Li, Y.J., Tang, M., 2020. Hygroscopic properties of sodium and potassium salts as related to saline mineral dusts and sea salt aerosols. J. Environ. Sci. 95, 65–72. https://doi.org/10.1016/j.jes.2020.03.046.

Zhao, B., Borghei, M., Zou, T., Wang, L., Johansson, L.S., Majoinen, J., Sipponen, M.H., Österberg, M., Mattos, B.D., Rojas, O.J., 2021. Lignin-based porous supraparticles for carbon capture. ACS Nano 15, 6774–6786. https://doi.org/10.1021/ACSNANO.0C10307/ASSET/IMAGES/LARGE/NN0C10307_0005.JPEG.

Zhu, W., Westman, G., Theliander, H., 2014. Investigation and characterization of lignin precipitation in the lignob boost process. J. Wood Chem. Technol. 34, 77–97. https://doi.org/10.1080/02773813.2013.838267.

Zitting, A., Paajanen, A., Altgen, M., Rautkari, L., Penttilä, P.A., 2024. Role of lignin in moisture interactions of cellulose microfibril structures in wood. Small Struct. 2400167. https://doi.org/10.1002/SSTR.202400167.