Pyrolysis is a process for thermal decomposition of materials at elevated temperatures in an inert atmosphere, which has been intensively studied in recent years (Patwardhan, 2010; Li et al., 2017; Li W et al., 2020). Based on the heating rates, pyrolysis can be categorized into fast pyrolysis and slow pyrolysis. Fast pyrolysis, carried out at enhanced temperatures (around 500°C), high heating rate (up to 2000°C/s) and short reaction times (<2 s), has been demonstrated as an effective method for bio-oil production from biomass (normally around 60 to 70 wt%) in addition to a small amount of biochar (20%) and gases (15%); while slow pyrolysis, remaining a slow heating rate and long reaction times (up to days), primarily converts biomass to high yield of biochar (around 50%) at the expense of the volatile products (Li W et al., 2020).

The behavior of lignin in the pyrolysis process is affected by several factors, including the lignin origin (i.e., softwood, hardwood and grass lignin) (Li et al., 2018b), lignin extraction/pretreatment method (Zhou et al., 2016), pyrolysis heating rate and reaction temperature (Ferdous et al., 2002), and selection of catalyst (Yang et al., 2007). The main products acquired from the lignin pyrolysis includes gaseous compounds (e.g., H₂, CH₄, CO, and CO₂), phenolic monomers and other poly-substituted phenols. Besides the abovementioned liquid and gaseous compounds, depending on pyrolytic temperature and heating rate, a fraction of lignin is converted to a thermally stable solid product, usually referred to as biochar. With plenty of surface functional groups (e.g., C-O, C=O and OH), these carbon materials have potentials to be applied to functional materials when subjected to various functionalization processes.

Despite that, there is an agreement that lignin pyrolysis involves with lignin depolymerization and phenoxyl free radicals chain reactions (Hu et al., 2012), the formation mechanism of monomeric and oligomeric products evolved during pyrolysis has not been fully elucidated (Demirbaş, 2000; Murwanashyaka et al., 2001; Branca et al., 2003; Ben and Ragauskas, 2011; Zhou, 2013; Bai et al., 2014; Li et al., 2017). Some researchers believe that lignin pyrolysis will first be cracked into numerous phenolic oligomers. Those oligomers will be further depolymerized into a variety of phenolic monomers during a series of secondary reactions (Piskorz et al., 1999). Afterwards, a competing model suggested that the oligomers were formed from secondary reactions through repolymerization of monomers derived from the primary reactions (Patwardhan et al., 2011).

Compared to cellulose and hemicellulose pyrolysis, the reaction mechanism of pyrolysis carbonization of lignin is more complex due to the heterogeneity in lignin composition. It is believed that lignin pyrolysis is involved with multiple reaction phases, including prime and secondary reactions (Liu et al., 2015; Li et al., 2017). The prime reactions of lignin pyrolysis are related with the cleavage of β-O-4 linkages to generate vinylphenols (Li et al., 2017; Li W et al., 2020). The primary products undergo a series of secondary reactions to produce a variety of H, G and S type monomers. There is a general agreement that lignin pyrolysis involves with free radical chain reactions (Kim KH et al., 2015) and the monomer products are presented as free radicals (Li et al., 2017; Li et al., 2020b). Since free radical reaction are chain reactions, it would not terminate as long as the free radicals are present. Hence, the originally volatilized H, G and S type would subsequently undergo repolymerization and condensation into oligomers and finally form solid fractions, namely char and coke (Li W et al., 2020). With the heating rate reduced in slow pyrolysis, the free radicals are more likely to be repolymerized to form polycyclic aromatic hydrocarbons (PAHs) and coke (Li W et al., 2020).

Hydrothermal carbonization is a wet biomass thermochemical conversion technology, which mimics the natural process of coal formation but in a much shorter period of time. Being placed in a closed reactor, such as an autoclave, the biomass or biomass-based precursors are surrounded by water and treated at approximately 130°C–280°C under self-generated steam pressure. The final products include solid residue, referred to as hydrothermal biochar, soluble organic compounds and gaseous products, mainly composed of CO₂. Compared to pyrolysis carbonization, HTC generates more biochar while less gases (Sugimoto and Miki, 1997). In addition, with higher H/C and O/C ratios, the chemical structure of hydrothermal biochar is more analogous to natural coal rather than pyrolytic biochar (Van Krevelen, 1993). The primary advantage of HTC over pyrolysis is that it can directly deal with the high-moisture biomass feedstock without energy-intensive preprocessing/drying (Pandey et al., 2015). However, the high pressure of HTC reaction requires special reactor and process design, leading to high capital investments. Additionally, continuous feeding solid biomass into reactors is challenging under high pressure, which hinders the scaling up of HTC process (Pandey et al., 2015).

Although it is believed that HTC process is generally governed by dehydration and decarboxylation (Hoekman et al., 2011), the complex reaction networks are not fully understood yet. So far, only a series of separate reaction mechanisms are proposed and identified, which include hydrolysis, dehydration, decarboxylation, polymerization and aromatization (Funke and Ziegler, 2010). Despite it is difficult for lignin to be completed hydrolyzed under the typical hydrothermal temperatures, partial hydrolytic reactions may start from the cleavage of the ester and ether bonds and end up into phenolics. Chemical dehydration significantly lowers the O/C and H/C ratios (Bergius, 1928). HTC causes partial elimination of carboxyl groups (Blazsó et al., 1986; Lau et al., 1987). When reaction temperature increases over 150°C, both carbonyl and carboxyl groups degrade rapidly and generate CO and CO₂ (Murray and Evans, 1972). Depending on the severity of HTC condition, elimination of carbonyl and carboxyl groups during decarboxylation leaves plenty of unsaturated compounds that are very reactive and prone to repolymerization (Terres, 1952). The repolymerization contributes to formation of hydrothermal char on the surface of the non-hydrolyzed lignin or polyaromatic char. The rate of carbonization is determined by the degree of aromatic condensation. Alkaline condition appears to promote the aromatization (Nelson et al., 1984). It should be noted that all of the separate reactions mentioned above are not consecutive steps but rather a parallel pathway network, and that the extent of each reaction associated to the formation of the hydrothermal products primarily depends on the type of feedstocks and HTC conditions (Funke and Ziegler, 2010).

Laser pyrolysis is a novo technology developed for the synthesis of nanostructured materials, which involves complex photothermal and thermochemical processes. It attracts increasing attention, especially for the synthesis of carbide nanoparticles (e.g., SiC, ZrC, TiC) and ceramic nanoparticles (e.g., SiO2, TiO2) (Wang and Gao, 2019). During laser pyrolysis, a continuous wave CO2 laser is used to heat flowing reactant gases, resulting in lignin decomposition. It is estimated that a temperature of 4,000°C–10,000°C can be obtained within microseconds for a typical laser pyrolysis (Fanter et al., 1972). In addition to the ultra-fast temperature rise, the cooling rate of primary pyrolysis products generated by laser pyrolysis is reported as fast as its heating rate so that minimizer secondary reactions (Fanter et al., 1972).

As a byproduct of bio-oil production, highly crystalline flake graphite was synthesized from mixture of Fe metal and biochar derived from a variety of biomass (sawdust, wood flour, corn cob, cellulose and lignin) through a laser pyrolysis process. The size of a “potato”-shaped flake graphite was in a range of 1–5 μm, which is determined by the physical dimensions of the metal particles. The flake graphite is reported essentially indistinguishable from high-grade commercial Li-ion grade graphite, though electrochemical tests were not been provided (Banek et al., 2018). In a separate study, laser pyrolysis was applied to synthesize a Si/C nanocomposite that used as negative electrode of lithium-ion batteries. Silane and ethylene were the precursors of Si and C, respectively. The merit of the laser pyrolysis in this study is its high efficiency since it allows precursor gases pass through the reactor and grow at the same time with a one-step synthesis process (Sourice et al., 2015), as shown in Figure 1.

However, laser pyrolysis highly depends on the optical properties of precursor materials (Mbonane, 2018). It might be necessary to introduce photosensitizers, such as sulfur hexafluoride, ethylene, silicon tetrafluoride, and ammonia, into carrier gas to initiate particle nucleation (Yefimov, 2009). Additionally, the confined area that laser beam focuses and limited layers that laser is able to transmit significantly restrict sample size, which propose challenges for large-scale production as well as difficulties to generate homogeneous electrode materials. Take the previous mentioned study as example (Banek et al., 2018), the diameter of laser beam was 2 mm each pellet had to be laser pyrolyzed individually and rotated at around 1.2 rpm. The laser exposed surface layer needed to be cut off from the unexposed core to obtain the result graphite flake. It is hard to obtain a preferred material in large scale and with a well-controlled quality via the method.

Plasma is ionized gas. When inert gas is presented in a strong electromagnetic field, free electrons in the inert gas will oscillate, which cause the free electrons to collide with atoms of the inert gas and remove electrons so that ionize the inert gas and form plasma. Thermal plasma generated by carbon electrodes can be dated back to 1960s. However, the short longevity of the electrodes was a big challenge that hindered wide application of the technology (Nema and Ganeshprasad, 2002). As the advances in more reliable and efficient torches for thermal plasma generation in the recent years, the use of thermal plasma as a heating source for pyrolysis and gasification has received increasing interests (Tang et al., 2013), especially for converting high calorific plastic waste into the syngas. The process of thermal plasma pyrolysis can be described as carbonaceous solid precursor reacts with limited amount of oxygen at the high temperature of highly reactive plasma region, where a plenty of electrons, ions and excited molecules generated under high energy radiation (Tang et al., 2013).

A microwave assist plasma carbonization process was developed to fabricate carbon fiber from polyacrylonitrile (Kim SY et al., 2015). When compared with conventional pyrolysis, plasma carbonized carbon fiber exhibited a higher surface roughness, which lead to an improvement of increase the mechanical properties of the CF Reinforced Polymer.

A supercapacitor is a high-power energy storage device widely used in transportation vehicles, power grids and consumer electronics (Gu and Yushin, 2014). Compared to batteries, such as lithium-ion batteries (LIBs), supercapacitors are favored because of their high-power density and long lifespan, which are suitable for short-term energy storage and burst power delivery. As shown in Figure 2, a typical supercapacitor is made up of two conductive electrodes with high surface area, separated by an electron-insulating but ion permeable membrane and fully soaked in electrolyte. As the ions of electrolyte spontaneously transfer toward/apart the surface of electrodes with electrons moving between the two electrodes but no charge transfer occurring across the electrode and electrolyte interface during charging and discharging, the capacitance acquired is called electric double-layer (EDL) capacitance. The EDL capacitance is simply achieved by physical adsorption of ions without chemical reactions involved on the interface between electrolyte and electrodes during charging and discharging. Although EDL is mainly attributed to the primary capacitance originated from the electrode made of carbon materials, many carbon materials contain functionalities or have modification on their surface, which contribute to extra capacitance obtained via redox reactions between the electrolyte and electrode, referred as electrochemical pseudocapacitance (Winter and Brodd, 2004).

Based on the principles of EDL capacitance, porous materials, especially activated carbon, have received the most attention as electrode materials of supercapacitor not only due to their impressive surface area and electric conductivity but also their tailorable pore structure (shape, pore size distribution) (Zhai et al., 2011). Lignin has been considered as a favorable precursor for porous carbon materials owing to its high carbon content, highly branched and cross-linked structure, and low feedstock cost (Li et al., 2018b). Additionally, plenty of oxygen-containing functional groups formed on the surface of prepared carbon materials offer extra pseudocapacitance to the total capacitance (Saha et al., 2014). Therefore, lignin-derived carbon materials have received extensive investigation for the potential application as supercapacitor’s electrode in the last decade, with some literature summarized in Table 1. To obtain higher specific surface area and conductivity and tailored surface functionality for overall performance, various synthesis strategies have been examined in the lignin-derived carbon materials for supercapacitor application. Reviewed here are lignin-derived activate carbon and carbon fibers, surface modification of the lignin-derived carbon materials and lignin-derived carbonaceous composite materials.

Batteries, such as LIBs, are the most widely used energy storage technology for electric transportation and portable consumer electronics (Hadjipaschalis et al., 2009). The important criteria to evaluate an electric energy storage material are energy density, power density, efficiency, life span and costs (Musolino et al., 2010). Energy density is measured in watt-hours per kilogram by determining the amount of energy that a device can store in a given volume, while power density is measured in watts per hour by determining the amount of power that a device can generate in a given volume. LIBs have a higher energy density than supercapacitors, while a supercapacitor has a higher power density than a LIB. This difference makes LIBs capable of storing more energy; while supercapacitors release energy much faster. Another important characteristic that must be considered when comparing these two devices are the life span during charging/discharging cycles. Supercapacitors exhibit a cycle life that is two orders of magnitude larger than that of lithium batteries under full charging/discharging (Hadjipaschalis et al., 2009). Therefore, rate performance, as well as cycling performance are among the most importance characteristics to evaluate the electrochemical performance of a lithium-ion battery.

Lithium is the lightest metal as well as the most electropositive element LIBs (Hadjipaschalis et al., 2009). The configuration of a LIB consists of a cathode (Li⁺ host material) and an anode (with Li⁺ accessible inter-atomic sites), which are immersed in electrolyte and isolated by a separator, as shown in Figure 4. The intercalation and deintercalation of lithium-ions causes electrons transfer between cathode and anode during charging and discharging, which fulfill the conversion between chemical energy and electrical energy (Winter and Brodd, 2004).

Unlike carbon precursors from fossil fuels, technical lignins contain a variety of elemental impurities derived from ash content and fractionation processes. For example, kraft lignin may contain elemental impurity of Al, Ca, Cu, Fe, K, Mg, Mn, Mo, Na, P and S in forms of inorganic minerals and organic constituents (Fahmi et al., 2007). The presence of the elemental impurities has been reported adversely affecting the bio-oil yield during fast pyrolysis of biomass. Ash forming elements, even at trace levels (<0.1%), can alter both the thermal degradation rate and chemical pathways during biomass pyrolysis (Evans and Milne, 1987). The alkali metals potassium (K) and sodium (Na), and the alkaline Earth metals calcium (Ca) and magnesium (Mg), are known to catalyze the thermal degradation of biomass to light gases rather than the preferred liquids (Patwardhan, 2010). Other studies have demonstrated negative effects of higher bulk ash content on the pyrolysis process yield (Das et al., 2004; Fahmi et al., 2008).

However, the study associated with the impact of impurities in lignin on the reaction chemistry of the carbonization (slow pyrolysis) process and the inconsistency in physiochemical, structural and electrochemical properties of the resulting carbon materials is rare. It was found that the ash content (mainly Na₂CO₃ and Na₂SO₄) in the kraft lignin exhibited a high reactivity during CO₂ activation, which caused rapid collapse or blockage of micropores of resulting activated carbon (Carrott et al., 2008). However, the influence of impurities on the development of porous structure and thus the electrochemical performance remains to be further investigated.

Despite thermochemical conversion processes, including slow pyrolysis and hydrothermal carbonization, play a critical role to the synthesis of carbonaceous functional materials, the study that correlates process conditions, such as carbonization temperatures, heating rates, residence time, choice of inert environment, activation temperatures, activation methods (agents) and ratios, etc., with physiochemical properties and electrochemical performance of synthesized materials is rare. Furthermore, contradictory results have been reported in the limited literature regarding how process conditions affect the performance of lignin-derived carbon material for energy storage applications. For example, with black liquor as carbon precursor and KOH as activation agent, Zhang et al. systematically examined the effect of operation conditions, including KOH/lignin ratio (1:1 to 1:5), carbonization temperature (500°C and 800°C) and activation temperature (700, 800°C and 900°C) on electronical performance of supercapacitors. It was found that the electrode carbonized at 500°C and activated at 800°C with KOH/lignin ratio of 4:1 exhibited highest specific capacitance of 286.7 F/g at a current density of 0.2 A/g and retained 207.1 F/g at a high current density of 8 A/g (Zhang et al., 2015b). However, in a separate study, under the same conditions (black liquor as precursor, KOH as activation agent, carbonization temperature: 500°C, activation temperature: 800°C), the reported specific capacitance and SSA were 35 F/g and 2,694.5 m²/g, respectively at a low current density of 10 mA/g (Zhao et al., 2010). Take another study as example, Chen et al. reported a lignin-derived C/Si composite electrode through a slow pyrolysis process. When lignin and Si nanoparticles were copyrolyzed at 800°C, the synthesized anode material delivered a specific capacity of 1,390 mAh/g, as compared to a much higher specific capacity of 2,378 mAh/g obtained at a pyrolysis temperature of 600°C. Therefore, in addition to exploring the potentials of numerous novel carbon precursors, investigating novel structure, such as core-shell, hollow, hierarchy, etc., establishing a comparable quantitative analysis method to correlate the process-structure-property-performance based on deep understanding of lignin pyrolysis chemistry process is necessary for lignin-derived carbonaceous materials.

Gas chromatography–mass spectrometry (GC-MS) is an analytical technology that unites the features of gas-chromatography and mass spectrometry to identify and quantify different substances within a single test. By coupling a micro-pyrolyzer with a GC-MS, volatiles evolved from pyrolysis can be separated by GC column and then transfer to the MS and flame ionization detector (FID) to be further identified and quantified. The use of GC-MS as an analytical tool to investigate pyrolysis of lignin is dated back to the late of 1970s, when Kraft lignin was first attempted to be collected in a “captive sample”, only five phenolic compounds (phenol, guaiacol, cresol, 4-methylguaiacol, and 4-ethylguaiacol) were successfully quantified due to the limitations of analytical technology of the time (TJiang et al., 2010). Afterwards, numerous studies have been conducted aiming to obtain a mechanistic insight into lignin depolymerization during pyrolysis. However, all of those studies were focusing on fast pyrolysis, which was favored for its capacity of maximized converting biomass into liquid fuel, namely bio-oil (Bridgewater and Peacocke, 2000; S; Czernik and Bridgewater, 2004; Patwardhan, 2010; TJiang et al., 2010; Li et al., 2017).

The increasing demands for low costs while high performance materials attract attention to converting lignin into diverse carbonaceous functional materials. However, unlike fast pyrolysis of lignin, which has been extensively investigated, the process of lignin slow pyrolysis has not been quantitatively examined, primarily because 1) the multi-phased volatiles were hard to be recovered and tracked over longtime course during slow pyrolysis; 2) process conditions with a very low heating rate are hard to be well controlled. Until recently, with assistance of a cryo-trap that installed at the head of GC column so that selectively recover volatiles evolved in a well-controlled condition (temperature regions, heating rates), lignin slow pyrolysis was, for the first time, quantitatively investigated via a combination of EGA-MS and HC-GC-MS analyses (Li W et al., 2020). The configuration of pyrolysis-GC-MS for the EGA-MS and HC-GC-MS is shown in Figure 5. This study provides critical insights into the slow pyrolysis chemistry of lignin and the properties of the resulting carbon materials. In a separate study, the process-structure-property-performance relationship of a 3-dimensional, interconnected lignin-derived carbon/silicon (C/Si) composite electrode for lithium-ion battery application was investigated via EGA-MS and HC-GC-MS analysis (Li et al., 2021). It was found the elemental Si and C of the C/Si NPs were linked via O rather than direct Si-C bond. There is a subtle balance between the Si-O-C bond and more stable and stronger Si=O, which can be tailored by controlling thermal conversion conditions (Li et al., 2021). The balanced bonding overlayer on the surface of Si NPs may serve to alleviate the mechanical degradation through either restricting excessive volume change during electrochemical cycling or forming a shell to protect the Si NP core from redundant lithiation. This study establishes a link between synthesis condition, properties of C/Si nanocomposite materials and their electrochemical performance and durability as electrodes in the next-generation LIBs. Furthermore, the Py-GC-MS analytical method has potential to be extended into studies of other functional material synthesis, including but not limited to lithium-ion battery applications (Li et al., 2021).

It is believed the extensive volume change of Si nanoparticles leads to the pulverization and delamination of electrode layer, which significantly impairs the mechanical contact between the electrode materials and current collector (Ryu et al., 2004). Therefore, in order to assure electronic conductivity and sufficient mechanical integrity of electrode material, a number of polymeric binders were assessed, such as carboxymethyl cellulose (CMC), Nafion, sodium alginate (SA) and polyvinylidene fluoride (PVDF) (Liang et al., 2004; Li et al., 2007; Kovalenko et al., 2011b). However, the correlations between electromechanical stability and binder properties have not been fully understood. Some researchers believe that a superior electronic conductivity and mechanical integrity are able to be achieved by introducing a highly extensible elastomeric binder which is tolerant of huge volumetric changes (Chen et al., 2003); while others proposed that stiffer binders serve better in maintaining conductivity and mechanical integrity of Si-based composite electrodes than those elastomeric alternatives (Li et al., 2007; Kovalenko et al., 2011b). Nevertheless, a recent study found that both the flexible Nafion and the stiffer SA demonstrated better electrochemical performance than PVDF did when served as binder material of Si composite electrodes (Xu et al., 2017), though both hardness and elastic modulus of PVDF are in between those of Nafion and SA. Additionally, an adhesion property between electrode material and current collector is also believed critical to electrochemical performance of Si-based composite electrodes (Chen et al., 2006). However, PVDF exhibited an much inferior electrochemical performance when used as binder material of Si composite electrodes, despite that PVDF-Cu interface has a much stronger adhesion strength than SA-Cu, (Hu et al., 2018). Therefore, it is believed that neither the mechanical property of binder material, nor the adhesive strength, but the cohesive strength between the binder and active materials affects the electrochemical performance the most (Hu et al., 2018).

In order to assess the cohesive strength of a lignin-derived C/Si composite electrode for lithium-ion battery application, a set of scratch tests were conducted, as shown in Figure 6. The scratch test results demonstrated a superior cohesion strength between Si NPs and C, which can be attributed to the binding interactions formed among Si, C, and O during lignin slow pyrolysis. It is because the unique chemical bonding structure formed at a specific process condition (around 600°C) significantly improved the electrochemical properties of the C/Si composite electrodes.

It is challenging to measure mechanical properties of the electrodes after electrochemical tests because, 1) some of the SEI components, such as LiOH and R-CHOLi, are highly reactive when exposed to air and moisture (Malmgren et al., 2013); 2) crystalline intermetallics of Li and Si are thermodynamically unstable, it is easy to be oxidized in air (Gu et al., 2013). Therefore, to perform mechanical measurement, as well as sample preparation in an inert environment offers a solution to overcome those challenges. By installing a nanoindentation system inside an argon-filled glovebox, mechanical properties of Si composite electrodes before and after electrochemical tests under dry and wet condition (liquid electrolyte) were determined (Wang et al., 2018). With the same environmental nanoindentation method, the viscoplastic properties of pure Li metal were also successfully characterized (Wang and Cheng, 2017), as shown in the Figure 7. As an effort to establish a correlation among thermochemical process, mechanical property and electrochemical performance of lignin-derived C/Si composite electrode for lithium-ion application, mechanical properties (elastic modulus and hardness) of electrodes synthesized at 600°C and 800°C (pyrolyzed at 2°C/min in argon environment) were measured (Chen et al., 2016a). The composite electrodes synthesized at 600°C demonstrated a significantly stiffer property in comparison to those at 800°C. The environmental nanoindentation method has potential to be extended to a wide variety of energy storage materials.

Recently, extensive efforts have been made to obtain real time observations and measurements toward the evolution of lithiation and de-lithiation processes on atomic and molecular scale structure, morphology, chemical composition and mechanical properties and acquire a deeper mechanistic insight on SEM and stress development during electrochemical cycling. By using an in-situ X-ray transmission microscopy (TXM) (Chao et al., 2010) and an in-situ transmission electron microscope (TEM) (Liu and Huang, 2011) and in-situ atomic force microscopy (AFM), the evolution of the interior microstructure and morphology changes of working composite anodes for Li-ion battery have been successfully observed. In-situ NMR were developed to examine the electrochemical intercalation of lithium ions into three different anodes, including graphite, corannulene, and sepiolite clay-derived carbon (Gerald Ii et al., 2000), Si (Key et al., 2009). Lithiation-induced mechanical properties play a critical role to the performance and durability of lithium-ion batteries. In-situ mechanical properties and stress evolution of thin film graphite electrode during electrochemical cycling were measured (Mukhopadhyay et al., 2011). Recently, a mathematical model was proposed, based on the measurement of curvature evolution in respect with modulus and stress of Si composite electrode during lithiation and de-lithiation (Li D et al., 2020).

Molecular dynamics simulation was used to develop atomistic models associated with interactions among the amorphous and crystalline domains of lignin-derived non-graphitic carbon LIB anodes. Lignin-derived non-graphitic carbon LIB anodes are composed of amorphous and nanocrystalline domains. The nanocrystalline carbon with size of 5–17 Å in radius are dispersed within the amorphous carbon matrix. Molecular dynamics simulation mechanistically explains an experimental observation that spacing between planes of nanocrystalline carbon is inversely correlated with crystallite size and suggests that the structural configuration of crystallites is a function of entropy (McNutt et al., 2014b). Through molecular dynamics simulation and neutron diffraction, the relationship structural properties, including crystallite size, intracrystalline d spacing, crystalline volume fraction and composite density, was built up by pair distribution functions (McNutt et al., 2014a). Based on molecular dynamics simulation, a novel mechanism about lithiation of lignin-derived hierarchical carbon LIB anode was proposed. It is believed that the Li ions are actually intercalated at the interface of crystalline carbon domains rather than between layers of carbon as they do in graphite carbon LIBs anode, as shown in Figure 8, because the edges of the crystalline and amorphous carbon of the lignin-derived non-graphitic carbon composites are terminated by hydrogen atoms, which play a crucial role in adsorption (McNutt et al., 2017b). As an attempt to establish correlation among process, structure, property and performance of carbon composite LIBs, in a separate study, energetics of Li-ion binding is examined through associating the changes in energy and charge distribution with variation in structural properties. It was found that the carbon composite with more volume of crystallites has higher Li ions storage capability due to higher interfacial area between the amorphous and crystalline domains. With more volume of crystallites and stable H atoms at the edge of interfaces, therefore, graphite carbon can offers more reactive sites to absorb Li ions (McNutt et al., 2017a).