Recent progress on biomass-derived ecomaterials toward advanced rechargeable lithium batteries

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing, China Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing, China Advanced Research Institute of Multidisciplinary Science, Beijing Institute of Technology, Beijing, China Department of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, China College of Materials and Energy, South China Agricultural University, Guangzhou, China Department of Chemical Engineering, Inha University, Incheon, Republic of Korea Department of Chemical Engineering, Imperial College London, London, UK

mechanism have featured great advantages of high safety and long lifespan, and become the powerful promotor of the intelligent economy. 1,2 Despite these tremendous strides, commercially visible LIBs are approaching their theoretical limit and uncapable of satisfying the growing demand of high-end storage devices for higher energy density.
The next-generation advanced lithium batteries such as lithium-sulfur (Li-S) and lithium-oxygen (Li-O 2 ) batteries have been regarded as the "beyond LIBs" owing to extra-high theoretical energy density (eg, 2600 Wh kg −1 for Li-S batteries and 5210 Wh kg −1 for Li-O 2 batteries in comparison to 387 Wh kg −1 for graphite LIBs), which are generally calculated by the total mass of cathode and anode. [3][4][5][6][7][8] However, the application of these advanced batteries on the basis of lithium metal chemistry and conversion chemistry behind the LIB technologies are long deserted by practical devices due to uncontrollable lithium dendrite growth, large electrode volume variation, poor charge conductivities of active species, as well as dissolution and diffusion of electrochemical intermediates, which considerably result in low Coulombic efficiency, rapid capacity degradation, inferior cycling stability, and severe safety hazards.
Oriented material designs have been deemed as one of effective strategies to address the aforementioned challenges and promote the development of advanced lithium (Li) batteries for practical application. [9][10][11][12][13][14][15] Naturally biomass materials always exist in the form of biological macromolecules or carbohydrate compounds in plant or animal cells, which endow themselves with diverse microstructures, complex compositions, and abundant functional groups. These unique physicochemical properties offer an emerging opportunity to compatible with high-energy advanced lithium batteries. 2,[16][17][18][19][20][21][22][23][24] On one hand, macromolecular natural biological polymers are fantastic candidates to replace the industrial fossil-derived polymeric materials fabricating electrodes, electrolytes or separators. In general, natural polymers are water-soluble and have abundant polar functional groups, which provide significant possibilities to boost the electrochemical performance and enhance the environmental and manufacturing safety. 25,26 On the other hand, raw biomass usually possesses intrinsic specific porous structures and thus can be used as a natural template to orient the fabrication of carbon materials with desired functionalities and structures. [27][28][29] Additionally, the features of biomass materials in green and circular economy, due to their superiorities of lowcost, renewability, biodegradability, and environmental benefits, can also accord well with the sustainable development path of coordinating nature, economy, and society. [30][31][32][33] In this contribution, we highlight how biomassderived materials (eg, natural biological polymers and bio-derived oriented carbonaceous materials) with special properties improve the interfacial and bulk problems in lithium batteries, as well as promote the creation of energy devices from alternative, abundant, and renewable sources for increasing their overall sustainability. An appropriate guideline of the combination between sustainable natural materials and protective strategies are provided, highlighted especially for advanced Li batteries. Finally, perspectives and recommended research directions for the further development of biomass materials in Li batteries are proposed.

| Functional aqueous binders
Since the commercialization of LIBs in 1991, the Li battery industries have flourished by the increasing development of 3C electronics and electric vehicles. 34,35 In general, fluorinated vinyl polymers (such as polyvinylidene fluoride, PVDF) as binder and organic solvents (such as N-Methyl-Pyrrolidone, NMP) as dispersant are adopted in electrode fabrication of Li batteries, which cause not only the considerations on the health and environment by the poisonousness of organic solvents but also the safety hazards by their inflammability at elevated temperature. 36,37 Natural biological polymers have attracted numerous scientific attentions due to their structural and functional diversities. Particularly, abundant hydrophilic polar functional groups in the skeletons intensify the affinity with water molecules, which endows natural polymers with a good water-solubility and therefore improves the compatibility with aqueous solutions. With the increasing concerns on human health and environment protection, natural polymers have been integrated into green manufacturing of batteries, in which harmful organic solvents are seldom or even not used. 38,39 To date, plenty of natural polymers (for instance, cellulose, chitosan, alginate, gum, cyclodextrin, lignin and their derivates) have been served as water-soluble binders for fabricating green electrodes toward advanced Li batteries.
In addition, as a critical component in electrode, the primary responsibility of polymer binders is to maintain the structural stability and integrity, which is the premise of constructing effective conductive framework and achieving lithiation/delithiation of active materials. 37 Natural polymers are generally insoluble in organic electrolyte solutions. The implementation of water-soluble natural polymer-based binders can mitigate and even avoid the instability and collapse of electrode structure resulted from the dissolution and swelling of conventional binders in organic electrolyte. Furthermore, sufficient polar groups that widely presented in natural biological polymer structure, such as ─OH, ─COOH, ─NH 2 , also propel the intramolecular or intermolecular interaction between them and themselves. The formed intermolecular cross-linking framework could contribute to the structural or interfacial stability and the electrolyte solution reservation in electrode. [40][41][42] Especially in a working Li-S battery, sulfur conversion chemistry involves phase transform and migration. The dissolution and migration of polysulfide intermediates lead to low active species utilization and reduced Coulombic efficiency, seriously limiting the practical application of Li-S batteries. [43][44][45][46] In 2013, Cui and coworkers demonstrated that the electronegative heteroatoms or groups have the ability of trapping electropositive Li ions. 47 Due to the presence of abundant polar heteroatom-containing groups, natural polymers used as functional binders exhibited great superiority in surface electronegativity ( Figure 1A-D). The polar functional groups can be served as affinity sites to anchor dissolved polysulfides, thus significantly mitigating the shuttle effect and improving energy efficiency in a working Li-S battery ( Figure 1E,F). 40 Meanwhile, the natural polymers are also enriching with many electronegative functional groups. They have the ability to coordinate with electropositive multivalent metal ions, which act as efficient chemical binding agents for immobilizing intermediates. 48,49 Owing to the electrostatic interaction between sodium alginate (SA) and Cu 2+ ions, an ionically crosslinking network binder with robust mechanical propriety was recently proposed to not only trap polysulfide anions through strong chemical coupling of electropositive Cu 2+ ions but also accommodate electrode stress during cycling. 48 By incorporating the dual-function binder, a highly stable sulfur electrode with superior cycling stability and capacity retention were observed.
Recently, a facile in situ self-polymerization of bioderived polymer monomers to form polymer resin serving as functional binders have attracted fascinating interests. 50 On one hand, the in situ polymerization process of monomers was triggered inside electrode, which can be served as electrode additives to fill in the inner gaps and pores of electrode, and thus reduced the amount of electrolyte required for the electrode. On the other hand, this strategy offered the skinned binders with remarkable mechanical strength and long-range order structure, which not only averted the electrode structure destruction but facilitated electronic/ionic transportation, consequently rendering an excellent electrochemical performance of battery systems. The electrode fabrication based on in situ selfpolymerization strategy opens a new way on the revolution of electrode process from environmentally benign raw biomass to biopolymer monomers and eventually to polymeric bio-derived electrode materials.

| Gel polymer electrolyte or separator
To date, most of commercial Li batteries are based on organic electrolyte. The fluidity and low flash point of organic solution easily result in the leakage and volatilization of electrolyte as well as the burning of battery devices, which deviate from the tenet of pursuing higher security of Li battery in future development direction. 51,52 Especially in Li metal batteries, Li dendrites are one of major risks for battery safety. Li dendrites can react with and consume limited liquid electrolyte, resulting in infertile and finally dry electrolyte condition in a working Li metal battery. Moreover, the generation and growth of Li dendrites have the possibility of permeating across separator and lead to battery short circuit, causing thermal runaway and final combustion or explosion of batteries. 37 Gel polymer electrolytes that combine the advantages of highly interfacial infiltration of liquid electrolyte and high solidification strength of solid electrolyte have been regard as a promising strategy to mitigate safety hazards. 53 Liquid organic electrolyte can be fixed and reserved in micro gel polymer framework structure, which can refrain from the leakage of electrolyte and alleviate their exhaustion with Li metal. In addition, high mechanical flexibility and elasticity of gel electrolyte can also function as a physical barrier to not only separate from the positive and negative electrodes but also prevent the growth of Li dendrites, thus enhancing battery safety.
Natural polymers have intrinsic superiority in forming ionically cross-linked networks owing to intramolecular and intermolecular interaction in water solution. Generally, the interaction between polar hydrophilic functional groups is dynamic and changeable. In water electrolytes, carboxymethylcellulose can be gelled to form a self-healing aqueous gel polymer network. 54 The aqueous Li battery with carboxymethylcellulose gel electrolyte exhibited better safety, stability, and reliability in comparison with organic electrolytes. 55 Other biological polymers, such as tamarind seed polysaccharide, can also be solidified by gelation in aqueous solution. 56 However, owing to the insolubility in organic solvents, natural polymers are generally difficult to use directly as gel polymer skeletons to reserve organic liquid electrolytes in conventional Li batteries. Modification strategies through grafting or combining with other macromolecules have also been demonstrated. 51,[57][58][59][60][61][62][63] For instance, by combining starch with γ-(2,3-epoxypropoxy)propyltrimethoxysilane, organic electrolyte can be gelled via the chemical substitution reaction between ─OH groups and ─Si─(OCH 3 ) 3 (Figure 2A,B). 57 Recently, Dong et al reported a new kind of bio-based poly (methyl vinyl ether-alt-maleic anhydride) composite electrolyte layer supported by natural bacterial cellulose and demonstrated its effectiveness in Li metal anode protection. 58 In addition, electrolyte gelation can be also achieved by the doping oleic acid and glycerol plasticizer into carboxymethyl cellulose, 60 the reinforcement of gelatin protein toward polyethylene oxide (PEO) electrolyte, 61 the cross-linking polymerization of functionalized lignin-derivatives, vanillyl alcohol, and gastrodigenin with multifunctional thiol monomers, 62 and the recombination of inorganic molybdenum disulfide (MoS 2 ) nanoflakes with surfactantoxidized cellulose nanocrystal (OCNC). 64 Particularly, beneficial from the well-designed three-dimensional ion transportation channels and high mechanical modulus enabled by well-dispersed MoS 2 and OCNC, this plastic crystal composite polymer electrolyte displayed excellent capability of suppressing Li dendrites in Li metal batteries ( Figure 2C,D).
Similar to most of conventionally industrial polymer macromolecules, naturally biological polymers have the same ability of casting membranes and electrospinning fibers to serve as a separator in Li batteries. 65,66 Owing to the tensile strength and thermal stability, cellulose-based microporous membranes have been wildly investigated. [67][68][69] Cui and coworkers prepared cellulose-based nonwoven nanofiber membranes as an advanced LIB separator with low cost, renewability, and environmental benefits. 68 Compared to the commercialized polyolefin separator, the cellulose-based nanofibrous separator displayed not only higher ionic conductivity, but also improved thermal-resistance property with no shrinkage up to 200 C, rendering the LIBs higher rate capability and better capacity retention. After that, the cellulose/ polysulfonamide composite membrane was obtained by the same research group via a facile papermaking process, exhibiting excellent electrolyte wettability and thermal endurance. 69 Besides, a series of cellulose-based battery nanofiber membrane enhanced with alkalitreated polysulfonamide fibers or functionalized with polypyrrole and polyaniline have been successfully implemented and exhibit improved electrolyte wettability and superior electrochemical performances. 43,70,71 Notably, in addition to excellent heat tolerance, cellulosebased separators hold one of prominent advantages in narrow nanopore distribution. When used for Li protection in a Li metal battery, cellulose-based membranes can effectively regulate the Li ion flux on the surface of Li anode and contribute to the safe Li plating with a dendrite-free morphology. 72,73

| Artificial solid electrolyte interphase layers
Li metal has been regarded as the most promising anode alternative owing to its ultrahigh theoretical capacity and negative electrode potential. However, instable solid electrolyte interphase (SEI) as well as the resultant Li dendrite growth seriously cause safety concerns, once Li metal is implemented as an anode in a battery. 74 In the view of being able to manipulate the mechanical properties by macromolecular chains as well as regulate Li ion flux and distribution by lithiophilic functional groups, biomass materials (eg, cellulose, starch, protein, lignin, chitin, polysaccharide) have been integrated into artificial SEI protective films for restraining Li plating and stripping to suppress the dendrite propagation. [75][76][77][78][79] Recently, a natural agarose biopolymer film (AG) was reported as an enabling SEI protective layer. The assembled Li-Cu half cell with AG protective layer displayed higher Coulombic efficiency of 98% even after 100 cycles, much better than that with bare Cu foils (< 40% after 50 cycles), indicative of the excellent structure stability of bio-based AG films against Li metal anode. More importantly, the AG film was also capable of high ionic conductivity and good elasticity, which significantly promoted fast Li-ion transfer and effectively accommodated Li dendrite growth during repeated discharging/ charging processes ( Figure 3A,B). 80 A kind of cellulose fiber paper membranes was also successfully adopted to protect Li metal. In most cases, due to the evident protuberances on electrodes surface, the electric field intensity around these tips was enlarged, intensifying the inhomogeneous Li ion distribution, which usually was called "tip effect" ( Figure 3C). 82,83 For the cellulose paper protective layer, there were abundant lithophilic polar groups such as ─OH and ─C─O─C─ at the cellulose backbone, which inhibited the movement of Li ions toward the protrusions of electrode, mediating the distribution of Li ion and finally the homogeneous electric field on the surface of Li metal ( Figure 3D). Owing to eliminated nonuniform electric field, Li deposition at local area as a result of tip effect was suppressed and thus smooth Li plating without Li dendrite growth was achieved. 81 Owing to the advantages of improving the mechanical properties and enhancing the electrolyte reserve by intermolecular or intramolecular cross-linking, natural polymers with low cost, renewability, and environmental friendliness have been widely used as binders, gel electrolyte matrix, or separators in advanced Li batteries. Particularly, when considering the regulating distribution of Li ion and thus the electric field by surface polar groups, natural polymers have the ability of mediating the Li deposition behavior and inhibiting the growth of Li dendrites. However, the compatibility of natural polymers with organic electrolyte solutions should be considered. The use of functional natural polymers, natural polymers-inorganic composites, electrolyte additives, and concentrated electrolytes, are considered to be effective strategies to enhance compatibility and thus stabilize the natural polymersbased protective layers/electrolyte interface. Furthermore, the relative chemical/electrochemical stability of natural

| Conductive carbon host
The fundamental challenges, including huge volume deformation of electrode, low electrical conductivity, and dissolution/diffusion of electrochemical intermediates, as well as dendrite propagation on metallic Li anode, plague advanced Li batteries with severe electrode pulverization, poor electron conduction, active materials loss and unstable electrolyte/electrode interface, which lead to poor Coulombic efficiency and rapid capacity decay, and therefore limit their practical realization. Nanostructured carbonaceous materials reveal significantly potentials to mitigate the above problems in a Li battery owing to their unique structural merits in nano spatial confinement, long/short-range conductivity, as well as surface/interfacial property. [84][85][86][87] Biomass, holding the traits of high carbon content has been regarded as a very suitable raw precursor to prepare porous carbon materials for advanced Li-based batteries. [88][89][90][91][92][93][94][95] In fact, biomass materials are endowed with unique structures, which consequently confer biomassderived carbonaceous materials with versatile microstructures and special characteristics. 4,96

| Structure-oriented host
Host materials featuring highly porous structure and large surface area can allow sufficient space to encapsulate the electrode active phase and abundant active electrode/electrolyte interface for promoting reversible electrochemical conversions. Meanwhile, high porosity can enhance Li ion transport kinetics within the electrode, which should also be emphasized in the design of conductive carbon matrix. [96][97][98][99] Zhong and coworkers developed a puffing process to produce porous carbon (PRC) as a sulfur cathode host derived from rice ( Figure 4A). 100 The resultant materials exhibited a unique porous microcellular structure, providing enough space to homogeneously confine active More interestingly, the inherent structure and morphology in biomass sources can always be well retained after treatment. In nature, some special biomaterials, such as cotton, kapok, bacterial cellulose, and so forth, intrinsically preserve their 3D cross-linked fibrous structure. When used as raw materials, their carbon-derived analogues can inherit the interconnected fiber structure, and thus benefit from high porosity as well. Recently, 3D hollow carbon fibers (3D-HCF) containing metallic lithium hosts were demonstrated inherited from fibrous cotton ( Figure 4D,F). 101 Such special structure of 3D-HCF effectively regulated Li ion deposition behavior and confined Li growth within the interspace among the fibers, consequently realizing smooth deposition without discernable dendrites at high deposition capacity of 4 mAh cm −2 ( Figure 4G).
Tortuosity of pores in carbon hosts also play a critical role for electrolyte permeation or ion transportation in the electrodes. The randomly distributed pores without interconnected and perforative structure give rise to a long ion diffusion distance or decreased ion transport channel, which lead to high ion migration resistance. 89,[102][103][104][105] Especially in electrodes with high loading, this adverse effect will be amplified owing to the higher response to ion transportation. 106,107 Therefore, the oriented design of interconnected architecture with low tortuosity electrode should be strongly encouraged. Enlightened by well-aligned channel structure within natural wood for water and ion transport, Hu and coworkers developed a 3D aligned porous carbon matrix derived from natural wood to accommodate cathode electrocatalysts in Li-O 2 batteries ( Figure 5A). 108 This carbonized wood (CA-wood) perfectly inherited the well-distributed channel structure with low tortuosity from wood, which considerably shortened the ion transport pathways and facilitated oxygen diffusion into electrode ( Figure 5B). With such design, the assembled Li-O 2 battery with wood-based cathode demonstrated a high specific area capacity of 8.58 mAh cm −2 at 0.1 mA cm −2 and superior cycling performance. Besides, oriented design strategy of porous electrode has also been implemented in a working Li metal battery, which not only alleviates volume expansion and enables a uniform Li ion flux, but also guides the Li deposition within channels and impedes dendrite growth. 109 To further rationally construct electrodes with interconnected conductive network, bio-derived carbon frameworks combined with other low-dimensional (1D and 2D) conductive fragments (such as graphene, carbon nanotube, etc.) are desired. [110][111][112][113][114] The 2D nanosheet or 1D nanotube morphology are beneficial for fast electron transfer; and meanwhile, the interconnected framework promotes rapid ion transport, thus facilitates reversible electrochemical redox.

| Affinity-oriented host
The dissolution/diffusion of electrochemical intermediates into electrolyte, such as the shuttle effect of Li polysulfides in a working Li-S battery, generally results in low Coulombic efficiency and poor cycling stability. The shuttle of electrochemical intermediates can be hindered by the confinement inside a porous carbon host. Factually, the diffusion of intermediates driven by concentration gradient cannot be completely avoided thermodynamically. Thus, reducing the concentration of intermediates in the electrolyte should be one of effective strategy to mitigate the shuttle effect. However, in conventional Li-S batteries, the host material of sulfur is usually a conductive carbon. The great difference in polarity leads to the weak chemical interaction between nonpolar carbon and polar Li polysulfides, which reduces the retention effect of conductive skeleton on Li polysulfides and consequently strengthens the shuttle effect of polysulfide within electrolyte. Recently, researchers found that introducing heteroatoms (eg, nitrogen, oxygen, or sulfur) into graphene can render tunable electronic properties and thereof enhance the affinity of carbon surface toward polar polysulfides. [115][116][117] Therefore, the regulation of surface affinity by heteroatom-doped has been widely implemented to the design of host materials for trapping the dissolved electrochemical intermediates.
Normally, biomass raw materials possess abundant heteroatom-containing functional groups in macromolecular backbone, which endow their carbonaceous derivative with the possibilities of in situ doping of functioned heteroatoms during carbonization. 118-120 Such a selfdoped strategy alters the surface properties of the carbon host, which ensures strong electrochemical affinity toward electrochemical intermediates and therefore increases the utilization of active phase and final the capacity retention. To date, a mass of biomass-based heteroatom-contain carbon materials derived from hair ( Figure 6A), 121 soybeans, 124 red algae ( Figure 6B,C), 122 coffee waste, 125 chrysanthemum, 126 and so forth, have been developed and exhibit enhanced electrochemical performances. Additionally, in a working Li metal battery, the heteroatom-containing polar carbon surface demonstrates also affinity toward metallic Li, which can guide a uniform nucleation of lithium. 127,128 Besides heteroatom-doping, the incorporation of biomass-derived carbon hosts with affinity species (such as metal, metal oxide, and metal sulfide) is also an effective strategy to trap electrochemical intermediates or Li ions within the electrolyte. [129][130][131] In a typical example, Tao et al demonstrated a bamboo-derived 3D hierarchical porous carbon host decorated with lithiophilic ZnO quantum dots (ZnO@HPC), which can induce preferential Li deposition within the porous scaffold, rendering a dendrite-free Li metal anode ( Figure 6D-F). 123 Such multifunctional carbon host derived from biomass are anticipated to be the most available avenue for advanced Li-based batteries. However, different energy devices have different critical issues and challenges, thus the requirements of carbonaceous hosts with respect to structure or physiochemical properties may be completely different. Therefore, understanding of the basic relationships between the electrochemical performance of batteries and structure and functions of conductive carbon hosts is imperative, which is the precondition for guiding the rational design of the multifunctional-oriented biomass-based carbon hosts in advanced Li-based batteries.

| Functional interlayers
Except the aforementioned electrode hosts, bio-based carbonaceous materials can also be used as interlayers to address the critical issues in Li-based batteries, especially to retard the diffusion of polysulfide intermediates in Li-S systems. In general, carbonaceous interlayers are established to contact with cathode, which function as not only conductive matrixes to improve the conductivity of whole cathode but also active surfaces to facilitate the reversible electrochemical conversion of active species in electrolyte. As the bio-based interlayers indicate similar effects as electrode conductive scaffolds, the oriented strategies toward electrode hosts can also be implanted into the interlayers engineering. Enlightened by inherent moisture retention properties of natural leaves, Manthiram group reported a separator interlayer based on a carbonized leaf (CL, Figure 7A). 132 After carbonization, the CL inherited the pore-size gradient structure: The macroporous network composed of pores and stomata can serve as reservoirs for electrolyte ( Figure 7B,C); the dense layer with micro/ mesopores can function as a blocking layer to impede the diffusion of dissolved polysulfides across separator ( Figure 7D). In view of the superiorities of bio-derived functional materials in immobilize polysulfides, lotus plumule, 134 fructose, 135 banana peel, 136 and so forth have also been highlighted as promising resources of producing porous carbonaceous interlayer to suppress polysulfides intermediates in Li-S batteries.
Other structure-oriented biomass-based carbonaceous materials are also considered as effective functional interlayers toward polysulfide immobilization. Gu et al proposed an interwoven carbon fiber (BCF) membrane obtained from carbonized bamboo as a functional interlayer ( Figure 7E). 133 The BCF membrane offered ample macro/micropore structures for fast transportation of electrolyte and ion, and sufficient active surface and efficient conductive networks for the deposition of sulfur active materials ( Figure 7F,G). A series of conductive bacterial cellulose-based nanofibers membrane, 137 cassavaderived high conductive carbon sheet, 138 and porous carbonized graphene-embedded fungus membrane 139 were also implemented for achieving high stable Li-S batteries. Meanwhile, heteroatom-doped carbon within interlayer can also play an important role in improving sulfur electrochemical behavior as the heteroatom dopants function as active sites for polysulfide adsorption and electrocatalysis. [140][141][142] Thanks to high-content sulfur-and oxygen-containing groups, Zhou and coworkers reported a sulfur-doped microporous carbon (SMPC) interlayer by the carbonization of luffa sponge. 143 Unique microporous framework and in situ S-doping in SMPC effectively enabled rapid ion transport and powerful adsorption for dissolved polysulfides, consequently rendering superior rate capability and cycling stability in Li-S batteries.

| CONCLUSIONS AND PERSPECTIVES
Sustainable biomass materials hold advantages in terms of versatile microstructures, diverse composition, ample function groups, water-solubility as well as environmental benefits, which render multifunctional biopolymers and carbonaceous derivatives suitable for high-energy Libased batteries.
In this perspective, on account of the special properties and functionalities of biomass materials, we highlighted the design principles of biological derivates in addressing critical problems in Li battery systems and promoting the sustainability of energy devices: (a) accommodating volume changes; (b) suppressing dendrite growth; (c) improving charge transportation; and (d) capturing electrochemical intermediates. Based on the breakthrough progresses achieved, future research directions on biomass materials for boosting advanced Libased batteries are suggested herein (Figure 8).

Fundamental understanding of biopolymers: Owing
to the complexity of composition in raw biomass precursors, the knowledge of biological materials parameters such as macromolecular structure, molecular weight, and polymerization degree as well as composition are key to achieve reproducible materials. 2. Consistency of raw biomass material: Biomass is a mixture of analogues with different molecular weights, so it is difficult to guarantee the consistency of biomass materials used in each experiment. Until now, the composition, structure, and properties of raw materials can only be qualitatively analyzed due to the difficulties in obtaining a pure substance. As such, determination and quantitative analysis of the key components should be particularly important. In addition, physicochemical stability of raw biomass materials and their detection methods also need to be taken into consideration. 3. Rational design of functional materials: Different battery systems need different functional structured materials. Rational design of appropriate functional structure materials is key to boost the electrochemical performance of advanced Li-based battery systems. However, current scientific research mainly relies on empirical strategies of trial and error, which is insufficient and time-consuming. More importantly, due to the species diversities of biomaterials, the screening criteria of available bio-based precursors is still difficult to be defined accurately, and become one of the restrictive problems in this field. Therefore, artificial intelligence assistance on achieving an efficient screening of structure-oriented, morphology-oriented, surface/interface chemistry-oriented, and functionoriented biomass precursors, and their derived functional materials should be considered. 4. Facile preparation processes: The refinement or conversion of biomass resources into available bio-based materials has low efficiency and yields. Generally, highly conductive bio-derived carbonaceous materials are obtained at an excessively elevated carbonization temperature (normally >800 C), which is energyconsuming and environmentally inefficient. More importantly, the frequently applied methods can only generate materials with uncontrollable pore-size distribution. Therefore, exploiting facile, cost-effective, as well as controllable preparation methods is imperative. The incorporation of pretreatment process or template strategies, development of advanced thermal management systems with gradient heating protocols, and regulation of appropriate heat treatment atmosphere such as reducing or inert gases, are promising methods to manufacture the biomass-derived ecomaterials. 5. Advanced characterization techniques: At present, there are lack of powerful and effective characterization techniques to track the evolution behaviors of bio-based materials in a working battery that easily lead to inscrutable understanding of how the biological polymer or bio-derived materials functions in a working battery. In addition, the diverse structure and complex properties of biomass materials further exacerbate the difficulties in detecting their active state under battery operation. Advanced in situ characterization measurements, such as X-ray absorption, microscopy, and spectroscopy, are required to discover the working states and evolution behaviors of biobased functional materials, which will give fresh insight to the oriented-design of biomass materials toward high-energy Li-based batteries.
F I G U R E 8 The further research directions on biomass materials for boosting advanced Li-based batteries 6. Greenness and sustainability of energy devices: Green energy and devices are the ultimate goal of society development. It is worthy to consider the greenness degree of batteries including all battery components during the whole cycling life, which is of significance to improve the greenness in energy storage process. Moreover, artificial intelligence design of energy environmental materials is also an effective avenue for resourcilization of energy sources and devices.
To conclude, the implantation of biomass sources into battery systems is helpful to not only push forward green energy at the resource but address the critical issues in Li-based batteries, consequently rendering high-energydensity and high-safety battery systems. Future research efforts should be focused on the efficient bio-based precursors screening, multifunction-oriented biomass materials, as well as in-depth structure evolution and fundamental mechanism analyses. Furthermore, the development strategies of biomass materials from single battery component to whole battery components have received great interests for promoting sustainability of energy devices. In light of these, we hope that this perspective can guide an oriented-strategy of battery materials based on biomass resources, and also give new inspirations on rational design of functional biomass materials toward high-energy-density Li-based batteries and other advanced energy storage systems.