稀有金属(英文版) 2020,39(09),1019-1033

Recent progress in plant-derived hard carbon anode materials for sodium-ion batteries:a review

Peng Yu Wei Tang Fang-Fang Wu Chun Zhang Hua-Yun Luo Hui Liu Zhi-Guo Wang

School of Chemistry and Materials Science,Hunan Agricultural University

作者简介：*Hui Liu received her B.S.and Ph.D.degree in Material Science and Engineering from South China University of Technology. She visited NSERC/3M Canada Industrial Research Lab at Dalhousie University from 2014 to 2015 as an assistant researcher. Now.she works in School of Chemistry and Materials Science,Hunan Agricultural University.Her research focuses on energy storage materials, including Li-ion batteries,Naion batteries and K-ion batteries.e-mail:liu.hui@hunau.edu.cn;*Zhi-Guo Wang received his B.S.in Metallurgical Engineering in 2007 and Ph.D.in Metallurgical Physics and Chemistry in 2015 from Central South University.He works in School of Chemistry and Materials Science,Hunan Agricultural University.His research interests focus on energy materials,including Li-ion batteries, Na-ion batteries and electrocatalysis.He possessed over 10 publications.e-mail:wangzhiguo@hunau.edu.cn;

收稿日期：17 February 2020

基金：financially supported by the Key Research and Development Project of Hunan Education Department (No.18A114);the Joint Natural Science Project of Hunan-Changde (No.2018JJ4001);the Youth Fund of Hunan Agricultural University (No.18QN01);the Funding for the Major Scientific Research and Innovation Team Cultivation at Hunan Agricultural University (No. 2018);

Recent progress in plant-derived hard carbon anode materials for sodium-ion batteries:a review

Peng Yu Wei Tang Fang-Fang Wu Chun Zhang Hua-Yun Luo Hui Liu Zhi-Guo Wang

School of Chemistry and Materials Science,Hunan Agricultural University

Abstract：

Sodium-ion batteries(SIBs) have been considered as a promising alternative to the commercialized lithium ion batteries(LIBs) in large-scale energy storage field for its rich reserve in the earth.Hard carbon has been expected to the first commercial anode material for SIBs.Among various of hard carbon materials,plant-derived carbon is prominent because of abundant source,low cost and excellent electrochemical performance.This review focuses on the recent progress in the development of plantderived hard carbon anodes for SIBs.We summarized the microstructure and electrochemical performance of hard carbon materials pyrolyzed from different parts of plants at different temperatures.It aims to present a full scope of plant-derived hard carbon anode materials and provide indepth understanding and guideline for the design of highperformance hard carbon for sodium ion anodes.

Keyword：

Sodium-ion battery; Anode material; Hard carbon; Plant derived; Electrochemical performance;

Received： 17 February 2020

1 Introduction

Energy and environmental problems are critical to human development.Traditional non-renewable fossil energy will be exhausted in the near future.At the same time,the massive consumption of fossil fuels contributes to global warming and ocean acidification [ 1] .Therefore,the development and utilization of clean and renewable energy sources are multiplying,such as wind energy,solar energy,tidal energy and biomass energy.At the same time,it is of extreme importance to develop energy storage to modulate these intermittent renewable resources.Among various energy storage equipment,such as supercapacitors [ 2, 3] ,lithium ion batteries (LIBs) [ 4] ,sodium-ion batteries(SIBs) [ 5] and potassium ion batteries (PIBs) [ 6, 7, 8, 9] ,the former two have been commercialized,and the latter two have great development prospects.Table 1 compares main performance parameters for energy storage devices.LIBs and SIBs have higher energy density than supercapacitors.At present,LIBs have been widely used in portable electronic products and electric vehicles.However,the development of the LIBs is limited resulted by the lacking and uneven distribution of lithium resources.Current cost comparison indicates that the cost of battery does not be directly reduced when replacing lithium with sodium.However,the reserve of sodium resources in the earth’s crust is up to 2.09%,much richer than 0.0065%of lithium.Therefore,it should be believed that the use of sodium would bring considerable cost advantages in the case of lithium shortages and associated price increases in future.Moreover,the replacement of current collectors from copper to aluminum in SIBs would also have a positive impact on the final price of battery [ 10] .The research and development of LIBs provides rich experiences to SIBs for their physicochemical properties are similar.Most of cathode materials for SIBs,such as oxides,poly anions and organic compounds,have a similar reproducible development to that used in LIBs [ 11] .Similarly,the main anode materials for SIBs are pided into metal oxide [ 12, 13, 14, 15] ,carbon [ 16, 17, 18, 19, 20, 21] ,alloy compound [ 22, 23, 24, 25, 26, 27] ,as shown in Fig.1.Furthermore,carbon materials have become the preferred target for researching anode for SIBs.Carbon materials are mainly classified into soft carbon,hard carbon,graphite and graphene [ 5, 28] .Except graphite,the dominant anode of commercial LIBs,it can only deliver specific capacity of 31 mAh·g^-1 for sodium storage [ 29] .It should be related to that Na⁺cannot be inserted in the interlayer spacing of graphite for its radius is 0.102 nm,greater than that of Li⁺(0.076 nm) [ 11] .Previous experiments and simulation results show that other carbon material with an ordered layer structure and an interface distance slightly larger than 0.37 nm can function as Na⁺similar to graphite inserted by Li⁺.

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Table 1 Comparison of main performance parameters of energy storage devices

Fig.1 The main anode materials for SIBs

Among various carbon materials,hard carbon is the most suitable for sodium storage.Hard carbon is less graphitized,and its average spacing is about 0.41 nm,which is larger than that of graphite [ 30] .Plus,the disordered structure of hard carbon can offer more defects and then provide more active sites for intercalation/deintercalation of sodium ions compared with soft carbon.Therefore,hard carbon shows excellent electrochemical properties for SIBs.Hard carbon materials are mainly derived from the pyrolysis of synthetic organics and biomass.The process of pyrolysis from the former has already been mature.But the synthetic organics are high molecular polymer,such as polyacrylonitrile [ 4] ,poly aniline [ 31, 32] ,polydopamine [ 16] ,phenolic resins [ 33] and polyimide [ 34] ,so the cost of carbon source materials is relatively high.Compared with synthetic organic matter,biomass is much more suitable for carbon source materials,for it has unique characteristics of perse types,physicochemical properties,environmental friendliness and considerable economic value [ 35] .Most of biomass-derived hard carbon can inherit porous micros true ture and layered structure from rich natural microstructure,which facilitates electrolyte penetration and the diffusion of sodium ions.In addition,natural biomass usually contains heteroatoms such as nitrogen [ 36, 37, 38] ,fluorine [ 39] ,sulfur [ 40, 41] ,boron [ 42] and potassium [ 43] ,which have complex effects on the properties of biomass-derived hard carbon.Many studies have shown that heteroatoms doping can increase conductivity,additional defects or the layer spacing of carbon materials,which is beneficial to improve sodium storage performance [ 44, 45] .The biomass can be roughly pided into three categories to obtain hard carbon:(1) microbial biomass,such as bacterial cellulose [ 46] ;(2) animal biomass,such as waste shrimp skin [ 47] and cuttlebones [ 48] ;(3) plant biomass,such as lotus petioles [ 39] ,ramie fibers [ 49] ,switchgrass [ 50] and peanut shell [ 51] .The microbial biomass needs to be cultured artificially,resulting in a long process cycle and complicated process.And similarly,the animal biomass is prone to metamorphism and long-term preservation.Therefore,the microbial biomass and animal biomass are not good candidates for commercial production of hard carbon.The plant biomass has a wide range of sources and is relatively easy to obtain and store.Therefore,plant-derived carbon materials have been widely studied.

In this review,we systematically summarize the recent progress in the development of plant-derived hard carbon materials for SIBs.The micros true ture and electrochemical performance of hard carbon materials prepared from different parts of plants are mainly discussed.In addition,the applicated problems of plant-derived hard carbon for SIBs are provided.The opportunities and challenges in future are also predicted.We aim to present a full scope of the plantderived hard carbon anode materials for SIBs.

2 Preparation of plant-derived hard carbon materials

Pyrolysis is the main method to prepare hard carbon materials derived from plants.Firstly,it is crucial to select the plant precursors,which must follow some principles.The plant precursors should be easy to collect and store.The corn cob [ 52] ,rice husk [ 53] ,old loofah [ 54] and kelp [ 55] are favorable precursors for plant-derived hard carbon materials.Secondly,the selected plant precursors should have high carbon content and low ash content.The main components of plants are lignin,cellulose,hemicellulose and other substances with high carbon content.The average carbon content of plant precursors can be up to 30% [ 56, 57] .Thirdly,the selected plant precursors should have porous or multilayered microstructure.The plant-derived hard carbon materials can inherit the natural microstructure of the precursor,which affects electrochemical performances of the resulting materials.For instance,a large number of natural pores and channels of apricot shells serve as the transport channel for water and nutrients.The microstructure could be well connected and inherited after carbonization [ 58] .The shaddock peel has a white sponge-like layer,which has a rich porous structure and closely arranged pores.It can play a good channel for the organic macromolecules to enter the interior,which is conducive to the formation of adsorption storage space [ 59] .The complex and perse microstructure of plants can provide more sodium storage sites,but also bring some disadvantages to the final materials,such as high initial irreversible capacity and low initial Coulomb efficiency induced by the high specific surface area.In general,the choice of plant precursors affects the sodium storage properties of the resulting plant-derived hard carbon materials.

Secondly,temperature is also an important factor of pyrolysis to obtain hard carbon.A temperature of1000℃or higher temperature is usually needed.Generally,the appearance of hard carbon would not change,but the spacing of carbon-carbon layers would vary along with pyrolysis temperature change,thereby asprepared hard carbon shows different sodium ions storage performance.Table 2 [ 50, 52, 56, 57, 58, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80] lists preparation,microstructure and performances of plantderived hard carbon materials studied in recent years.It is worth mentioning that the hard carbon materials are synthesized from a variety of plant precursors,and the corresponding pyrolysis temperatures are different.More importantly,the hard carbon materials synthesized from different plant precursors exhibit different microstructures after carbonization,and there are also large differences about sodium storage properties,which will be discussed in detail next.Certainly,the properties of some plant-derived hard carbon materials need to be improved.In particular,various efforts have been attempted,such as adjusting the surface area,the interlayer spacing,the number of defects and the pore size of the hard carbon materials [ 81, 82] .

3 Structure of plant-derived hard carbon materials

As described above,researchers would like to maintain the natural microstructure of plants after pyrolysis to obtain high-performance hard carbon materials.Understandably,the useful parts of a plant are the transport tissues and nutrition tissues.The transport tissues include roots,stems and leaves,whose function is to transport water,inorganic salts and nutrients for the plants.The typical feature is that the cells are elongated and often connected up and down to form a conduit suitable for transport.The nutrition tissues include flowers,fruits and seeds,which often have thin cell walls,and large vacuoles for storing nutrients.

Owing to many channels and pores inherited from the transport tissues of plants,hard carbon materials usually exhibit excellent diffusion kinetics of sodium ions as anode materials.Among the transport tissues of plants,the studies on hard carbon derived from stems and leaves are widely conducted because stem and leaf resources are easier to collect than root resources.Stem,as an important supporting organ and transport organ of plants,mainly consists of high carbon material,namely lignin,cellulose and hemicellulose.Figure 2 shows scanning electron microscopy (SEM) images of hard carbon materials derived from the stems of different plants [ 50, 65, 66, 73] .It could be observed that the original microstructure of the stems is retained after pyrolysis,which is mostly tubular porous structure.The Brunauer-Emmett-Teller (BET) surface area was usually provided as representative of the electrochemical active surface area.The poplar hard carbon was evaluated to be about 5.8 m²·g^-1,delivering a high specific capacity of 330 mAh·g^-1 and an initial Coulombic efficiency of 88.3%in half cells [ 73] .The dandelion hard carbon has a specific surface area of 4.692 m²·g^-1.It performs a capacity of 250 mAh·g^-1 as the current density increases from 0.05 to 0.20 A·g^-1.Even up to 5 A·g^-1,it still performs 94 mAh·g^-1 [ 65] .The lotus stem carbons show a high specific surface area of 24.37 m²·g^-1 and deliver the best rate capability delivering namely reversible capacities of 351,290,240 and 150 mAh·g-1 at 40,100,200 and 500 mA·g^-1,respectively [ 66] .Switchgrass hard carbon has a specific surface area of 23.1 m²·g^-1 and shows less capacity decay of 160,130,120 and1 10 mAh·g^-1 with an increasing current density of 0.2,0.5,1.0 and 5.0 A·g^-1,respectively [ 50] .In summary,the hard carbon produced by pyrolysis of the transport tissues has a relatively small specific surface area and good rate performance.

Figure 3 shows SEM images of hard carbon materials derived from nutrient tissues of different plants after carbonization.Three-dimensional porous structure and multilayer structure could be observed,which can provide excellent channels for sodium ions diffusion.Zhang et al.prepared pine pollen-derived hard carbon with a specific surface area of 171.54 m²·g^-1.When it was applied as anode material for SIBs,the discharge capacity maintains203.7 mAh·g^-1 at 0.1 A·g^-1 after 200 cycles,and it also exhibits outstanding rate performance at 5 A·g^-1 with reversible capacity of 87 mAh·g^-1 [ 83] .The residual hardcarbon of ganoderma lucidum inherits the open channel formed by the porous fungus,which provides a fast transport channel for sodium ions.The sample shows a high surface area of 339 m²·g^-1 and enables a capability of124 mAh·g^-1 at 5 A·g^-1 [ 42] .The specific surface area of lychee seeds is 6.61 m²·g^-1 [ 84] ,while the specific surface area of apricot shell is 46.1 m²·g^-1 [ 58] .It could be concluded that the specific surface area of vegetative tissuederived hard carbon is generally larger than that from transport tissue.

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Table 2 Preparation,microstructure and performances of plant-derived hard carbon materials in recent years

Fig.2 SEM images of hard carbon materials derived from stems of different plants:a poplar wood [73] Copyright Elsevier Ltd.All rights reserved;b dandelion [65] Copyright Elsevier Ltd.All rights reserved;c lotus stem [66] Copyright Elsevier Ltd.All rights reserved;and d switchgrass [50] Copyright Elsevier Ltd.All rights reserved

Fig.3 SEM images of hard carbon materials derived from nutrient tissues of different plants:a pine pollen [83] Copyright ACSPublications.All rights reserved;b ganoderma lucidum residue [42] Copyright ACS Publications.All rights reserved;c lychee seeds [84] Copyright Elsevier Ltd.All rights reserved;and d apricot shell [58] Copyright Elsevier Ltd.All rights reserved

Additionally,it is worth noting that the three-dimensional porous structure of plant-derived hard carbonmaterials by pyrolysis has good stability.Figure 4 shows SEM and transmission electron microscopy (TEM) images of the hard carbon material derived from rice husk pyrolysis at 1300℃.The results show that the overall structure of the hard carbon materials has no obvious change before and after charging and discharging.In addition,the surface of the electrode coated with the hard carbon material derived from rice husk pyrolysis at 1300℃has a relatively uniform SEI film with the thickness of approximately85 nm,which is thinner than the electrodes coated with the hard carbon materials pyrolysis at 1100 and 1500℃ [ 56] .

As already listed in Sect.2,hard carbon materials were pyrolyzed from different plants at different optimal temperatures.Previous research indicates that pyrolysis temperature has a great influence on the structure of plantderived hard carbon materials,such as pore size,specific surface area and degree of graphitization,which further affect electrochemical performances of the hard carbon anode materials for SIBs.Figure 5 shows the relationship between carbonization temperature and the structural properties of the hard carbon materials derived from different plant precursors listed in Table 2.It can be concluded that the interlayer spacing decreases with the increase in carbonization temperature for all precursors.In a large number of literatures,it is reported that the interlayer spacing of the prepared hard carbon materials is mostly greater than 0.37 nm,which just fulfills the minimum size of interlayer spacing for sodium ions intercalation [ 85, 86, 87] .Figure 5b shows the relationship between the interlayer spacing and the highest reversible specific capacity of the plant-derived hard carbon materials as anode materials for SIBs.It is found that the electrochemical performances of the prepared materials are improved with the increase in the interlayer spacing.However,when the interlayer spacing is greater than0.40 nm,the highest reversible specific capacity decreases [ 88] .The effect of carbonization temperature on the specific surface area of the plant-derived hard carbon materials and the effect of specific surface area on the initial Coulombic efficiency are plotted in Fig.5c,d.It has to be noted that the specific surface area of the prepared materials decreases with the increase in carbonization temperature,and the initial Coulombic efficiency of the prepared materials increases when the specific surface area decreases.It is mainly related to the decrease in the surface porosity and the formation of the solid electrolyte interface(SEI) layer [ 89] .Therefore,it is necessary to reduce the specific surface area of hard carbon.Then electrolyte decomposition and consumption on the surface would be reduced to improve the initial Coulomb efficiency of SIBs.Zhang et al. [ 50] prepared hard carbon material with a hollow three-dimensional structure by tube furnace pyrolysis and Joule heating.The switchgrass-derived hard carbon material exhibited an interlayer spacing of 0.376 nm by pre-carbonizing at 1000℃and Joule heating treatment at 2050℃.It was proved to be able to improve the initial Coulombic efficiency (from 42%to 64%) and the reversibility of electrode reaction.Zheng et al. [ 73] reported a poplar wood-derived hard carbon material,which is a"honeycomb-like"morphology with a hole,exhibiting the high specific capacity of 330 mAh·g^-1 and the initial Coulombic efficiency of 88.3%in half cells.In summary,hard carbon with excellent electrochemical performance could be achieved by adjusting the pyrolysis temperature to tailor the microstructure of electrode materials.

Fig.4 a SEM image and b TEM image of fresh electrode coated with hard carbon material derived from rice husk pyrolysis at1300℃;c SEM image and d TEM image of electrode after 100cycles [56] Copyright Elsevier Ltd.All rights reserved

Fig.5 Micro structure and capacity of different plant-derived hard carbon:a plots of d₀₀₂ along with carbonization temperature;b plots of highest reversible specific capacity versus d₀₀₂,c plots of specific surface carbonization along with carbonization temperature;d plots of initial Coulombic efficiency versus specific surface area

4 Electrochemical performances of plant-derived hard carbon materials

Figure 6 shows the electrochemical performances of hard carbon derived from different plants.The hard carbon materials derived from the transport tissues (reed straw and dandelion) and the nutrient tissues (apricot shell and water caltrop shell) of plants by pyrolysis at 1300℃exhibit excellent rate performance (Fig.6a) [ 57, 58, 65, 72] .These materials deliver a specific capacity of about 350 mAh·g^-1at the current density of 0.05 A·g^-1 and a specific capacity of about 100 mAh·g^-1 at the current density of 1.00 A·g^-1.As can be seen from Fig.6b [ 57, 60, 65, 66, 72, 74] ,the ratio of platform capacity to the total capacity increases when the pyrolysis temperature increases except the dandelion-derived and reed straw-derived materials.However,the interlayer spacing of the hard carbon material would decrease when the pyrolysis temperature was too high,as shown in Fig.5a.It is difficult for sodium ions insertion and deinsertion,which would lead to poor electrochemical performance.Figure 6c shows voltage platform of sodium storage for the hard carbon materials obtained from different plants by direct pyrolysis [ 42, 50, 52, 56, 57, 58, 61, 64, 65, 66, 69, 70, 71, 72, 74, 75, 76, 77, 78] .It should be an assurance for high working voltage of SIBs that most of hard carbon shows a platform voltage of about 0.1 V during the charge and discharge process.

Cyclic voltammetry (CV) test is commonly used to characterize the electrochemical performances of the plantderived hard carbon materials.Most of the results show that a pair of sharp oxidation and reduction peaks appear around 0.1 V,representing the insertion and deinsertion of sodium ions during the charge and discharge process.In the initial cycle,all of the hard carbon anode materials showed a loss of irreversible capacity derived from the formation of SEI layer,mainly related to electrolyte decomposition on the electrode surface.As the carbonization temperature increases,the irreversible capacity decreases.It indicates that the electrolyte is less decomposed and the initial Coulombic efficiency is higher,which is consistent with the lower specific surface area [ 90] .In addition,the contribution of redox tantalum capacitor to sodium storage performance was analyzed by CV test,separation of capacitance and diffusion control capacitance.Analysis of the relationship between scan rate and peak current in CV test [ 91] :

where i is the peak current,v is the voltage sweep rate,a and b are constants associated with the reaction mechanism.In particular,the b value of 0.5 means that the current is controlled by semi-infinite linear diffusion.The prepared SIBs are the behavior of battery,and the main mechanism is intercalation.The b value of 1.0 indicates that the current is controlled by the surface,and the battery is mainly capacitive behavior and adsorption.The area enclosed by the CV curve represents the total amount of stored charge produced by the Faraday and illegal pull reaction processes.a and b are constants related to the reaction mechanism.For example,the cherry petals-derived hard carbon material was fitted with a CV curve and a peak current at different scan rates,and the resulting b value was0.484,which indicates that the sample has been controlled by semi-infinite linear diffusion in the low voltage region as anode material for SIBs [ 67] .Yang et al. [ 92] reported that the pristine plant-derived carbon materials exhibited all battery behavior,but the hard carbon materials modified by heteroatom doping or activators exhibited completely different CV curves.For the cotton-derived hard carbon material modified by nitrogen and sulfur doping,the ratio of capacitance can reach 57.7%.At a sweep speed of10 mV,the ratio of capacitance is 96.9%.The sodium storage behavior is mainly capacitive behavior.

Fig.6 a Rate performance of hard carbon derived from transport tissues (reed straw and dandelion) and nutrient tissues (apricot shell and water caltrop shell);b ratio of platform capacity to total capacity for hard carbon;c voltage platform of sodium storage for hard carbon

Electrochemical impedance spectroscopy (EIS) measurement was used to study the kinetics of electrochemical reaction.Similarly,galvanostatic intermittent titration technique (GITT) was used to detect the sodium ion diffusion coefficient.According to Fick's second law [ 93, 94] ,the formula is as follows:

where m_B is the weight of hard carbon materials coated on the current collector,V_m represents the molar volume of carbon,M_B represents the molecular mass of carbon,S represents the area of the active materials on a single pole piece,τrepresents the pulse time,E_τrepresents the voltage at the pulse time,ΔE_S represents the voltage difference at the time when the voltage reaches the steady state in a single GITT process,and D_Na+is expressed in logarithmic form.Through the GITT test,the general sodium ion diffusion coefficient of the current hard carbon materials used as anode materials for SIBs is 1×10^-9-1×10^-10 cm²·s^-1.When the voltage is above than 0.1 V,the diffusion coefficient is 1×10^-9 cm²·s^-1.When the voltage is lower than 0.1 V,D_Na+drops sharply and is approximately1×10^-10 cm²·s^-1,indicating that the diffusion of sodium ions has changed,and the storage mechanism has also changed [ 57, 79] .The difference in the coefficient of diffusion coefficient between the slope zone and the plateau indicates that the sodium-carbon interaction has different binding energies.In combination with the previous reports about lithium ion diffusion in graphite,the curves are very similar,indicating that in the low pressure range of less than 0.1 V,graphite carbon combines with sodium ions to form Na-graphite intercalation compounds(NaGICs),which is a reversible structural phase transition [ 95, 96] .The sharp change in D_Na+may be due to a large amount of sodium ions leave the surface at the beginning of charge process,the D_Na+increased rapidly.With the reaction proceeding,the sodium ions on the surface became less and less,so the sodium ions embedded in the intergranular graphite crystallites began to detach and the D_Na+decreased.However,Hu et al.found that D_Na+dropped sharply in the platform area through GITT,but they did not think that sodium ions were embedded in graphite crystallites [ 64] .It was founded that the interlayer spacing did not increase after sodium insertion,measured by TEM and X-ray photoelectron spectroscopy (XPS).The edge of carbon layer and the nanopore have changed,and there is no high change in binding energy at 0.1 V.Combined with above analysis,they concluded that the mechanism of sodium storage is that sodium ions filled into the holes.

5 Sodium storage mechanism of plant hard carbon

Hard carbon is non-graphitizable and presents a disordered structure that is much more complex than the long-range ordered layer structure of graphite.The special structure causes hard carbon to have more types of storage sites,which can be used for the adjustment of sodium ion reversible capacity.Unlike graphite,the sodium storage mechanism of hard carbon is still elusive and controversial.The hard carbon shows a charge-discharge curve different from the other carbon-based materials,which is mainly pided into a low potential plateau region of 0.01-0.10 V and a high potential ramp zone above 0.1 V.It is crucial to define the sodium storage mechanism of hard carbon for the development of high-performance hard carbon materials.

Until now,there are mainly the following sodium storage mechanisms of hard carbon:(1) deintercalation between graphite layers,(2) storage of defective layers,(3)surface adsorption and (4) nanopore filling.However,there are still a lot of controversies about the way of sodium storage in the platform and slope zone (Fig.7).(i) Dahn et al.first proposed the"insertion-filling"mechanism when studying the sodium storage mechanism of glucosederived hard carbon [ 97] .The slope region is designated as sodium ions embedded in the graphite layer,and the platform region is filled with sodium ions and electroplated into the nanovoids.Komaba et al.reported that the hard carbon material carbonized from argan shell exhibited a reversible specific capacity of 330 mAh·g^-1.Through X-ray diffraction (XRD) and pore size distribution analysis,it was proved that the parallel relationship between the slope and the graphene layer spacing (d₀₀₂) with the pore size increasing,the platform capacity increases linearly.The mechanism of sodium storage in hard carbon was described as two steps,one is the slope region (0.15-1.20V) when the reduction reaction of sodium ions insertion in graphene layers,and the other region (0-0.15 V) due to sodium ions insertion in micropores [ 68] .(ⅱ) Cao et al. [ 98, 99] proposed the"adsorption-insertion"mechanism,that is,the low potential region corresponds to the deintercalation behavior of sodium ions in the graphite layers,while the high potential slope region corresponds to the adsorption behavior of sodium ions on the surface of hard carbon.The sodium storage mechanism of most plantderived carbon materials conforms to"adsorption-insertion"mechanism.The sodium storage mechanism of the hard carbon materials obtained from banana peel [ 60] ,water chestnut shell [ 57] ,cherry petals [ 67] and reed straw [ 72] was explained as:the platform region for sodium ions embedded in the graphite layer was below 0.1 V,and the slope region for sodium ions adsorbed in micropores,defects or holes was higher than 0.1 V.The capacity in the slope region is mainly attributed to the micropores and defect sites inside the material,and the capacity in the platform region is closely related to the size of the graphite-like nanocrystals.When the pyrolysis temperature increases,the capacity in the platform region that plays an important role in the total capacity increases significantly.Zhang et al. [ 66] deepened the understanding of the sodium storage mechanism of lotus stems-derived hard carbon.In the platform region,sodium ions were not only inserted in the graphite layer,but also deposited in the micropores.At the end of the platform at about 0.01 V,sodium ions were filled into the layered nanocrystallites.The rice huskderived hard carbon has the same sodium storage mechanism.As the carbonization temperature increases,the initial discharge capacity in the slope region decreases gradually,indicating that the slope region is related to the oxygen-deficient defect for hard carbon [ 56] .Xu et al.used ginkgo leaves as the precursor,a series of hard carbon materials with different microstructures were prepared by regulating the pyrolysis temperature in a wide temperature range of 600-2500℃,presenting an improved"adsorption-insertion"mechanism [ 88] .When the layer spacing is greater than 0.4 nm,sodium ions can enter freely in it,showing the behavior of"quasi-adsorption"of sodium storage,similar to the traditional"hole,edge,heteroatom"and other defects,which is reflected in the oblique area on the charge-discharge curve.The layer spacing is between0.36 and 0.40 nm,suitable for the insertion/ejection of sodium ions.It is the"interlayer insert"sodium storagemechanism,which is reflected in the platform area on the charge and discharge curve.When the layer spacing is less than 0.36 nm,sodium ions cannot enter due to the too small layer spacing,so there is no platform area and only a small amount of defective adsorption and storage of sodium.The improved"adsorption-insertion"model can well explain the evolution process of hard carbon micro structure and sodium storage behavior with pyrolysis temperature.(ⅲ) Tarascon et al.reported that the platform at the low voltage was due to neither sodium ions insertion or sodium ions adsorption,and the"adsorption-filling"mechanism was proposed.The slope region between 0.1and 1.0 V was related to the adsorption of defects by hetero atoms,which was caused by the adsorption of sodium ions on the surface of graphene sheet.The plateau region around 0.1 V was attributed to"nanohole filling" [ 100] .Li et al. [ 64] prepared the hard carbon microtube materials from the natural cotton precursors and explored the sodium storage mechanism.The adsorption of sodium ions in the disordered layer corresponds to the slope region at the high potential above 0.12 V in the charge-discharge curve,while the filling of nanopore by sodium ions corresponds to the plateau region at the low potential closed to 0 V.

Fig.7 Schematic diagrams of sodium storage mechanisms for hard carbon materials:a"intercalation-adsorption”mechanism;b”adsorption-insertion",mechanism [99] Copyright Wiley-VCH.All rights reserved;and c"adsorption-filling,"mechanism [100] Copyright Wiley-VCH.All rights reserved

6 Modification of plant-derived hard carbon materials

To meet the high performances,such as high reversible capacity,high initial Coulombic efficiency and long cycle life for the practically applications of SIBs,it is essential to modify the current plant-derived hard carbon materials.The pristine materials typically have a porous or multilayered structure of more functional groups and active sites,which may be destroyed when the carbonization temperature is too high.However,synthesis of hard carbon must occur at relatively high temperature.Therefore,various modification methods have been attempted to improve the electrochemical performance or reduce the carbonization temperature of the plant-derived hard carbon materials,including heteroatom doping and activator modification.

It is reported that the relatively common and effective modification method is heteroatom doping,which can increase the content of organic functional groups,defects on the surface and the carbon layer spacing of the pristine hard carbon materials.These conditions can change the local electronic environment around the carbon atom and the electron distribution on the surface of the hard carbon material,thereby increasing reactivity and electronic conductivity [ 101] .It further affects electrode storage capacity,surface wettability,electrode/electrolyte interaction and charge transfer [ 102] .The common heteroatoms used in hard carbon doping are mainly N [ 103] ,S [ 92] ,P [ 104, 105] ,etc.The doped heteroatoms may be single or multiple.The pyrolysis temperature of heteroatomic doping is obviously lower than that of the hard carbon prepared without doping,which is generally below 900℃,it can reduce energy consumption.

Gaddam et al. [ 106] used ethylenediamine to dilute mango dry powder with nitrogen to prepare hard carbon materials.The cell fabricated with nitrogen-doped hard carbon as anode material exhibited excellent capacity and cycling stability.The reversible specific capacity was about520 mAh·g-1 at the current density of 20 mA·g-1 along with a high rate performance.Zhao et al.investigated the potential of hierarchical nitrogen-doped porous hard carbon derived from jackfruit rags through a facile pyrolysis without any chemical or physical activation as anode material for SIBs [ 107] .

Compared with N,S has larger size and less electronegativity.Doping S helps to further enlarge the interlayer spacing,generate active sites and improve the electrochemical performance of hard carbon materials.Zhao et al. [ 108] prepared the durian shell-pretreated carbon with high specific surface area,which was further employed as carrier and template for the synthesis of S-doped carbon materials with outstanding sodium storage performance.In addition,S doping can induce the enlargement of carbon interlayer spacing,which not only increases the amount of stored sodium ions but also accelerates the speed of sodium ions insertion or deinsertion.That is,the electrode kinetics was enhanced,which insures the high initial Coulombic efficiency of 56.02%and the reversible capacity of 264 mAh·g^-1 at 0.1 A·g^-1 after200 cycles and 100.2 mAh·g^-1 at 5 A·g^-1 even after 4500cycles.

Also,many heteroatoms are also present in plants.The plant-derived hard carbon materials can be modified by self-doping.For instance,Wu et al. [ 43] chose naturally K-rich coconut endocarp as precursor to prepare K-doped hard carbon material.The results show that K-doping can improve the electrochemical performance of the hard carbon material,including a high initial reversible capacity of314 mAh·g^-1 and a high capacity retention of 92.1%after200 cycles.It mainly depended on the effectively increase in interlayer spacing in carbon layers after K-doping.

On the other hand,some researchers have also studied the effect of multiple heteroatom co-doping.Qin et al. [ 109] used carbon flakes derived from corn stalk and ammonium dibasic phosphate ((NH₄)2HPO₄) as precursors to synthesize N and P co-doped hard carbon by hydrothermal reaction.The resulting carbon sheet with a loose sheet-like morphology was obtained.It exhibited the specific capacity of 277 mAh·g^-1 at 0.25 C rate after 100cycles,the specific capacity of 202 mAh·g^-1 at 1.00 C rate after 200 cycles (Fig.8b) and the specific capacity of 105mAh·g^-1 even at 5.00 C rate after 2000 cycles,indicating good cycle stability and superior rate capability (Fig.8a).The cotton was pretreated with magnesium nitrate(Mg(NO₃)2) and carbonized at high temperature.Then,the obtained carbon was mixed with thiourea solution to prepare N and S co-doped hard carbon.C0,C15 and NS-C15represent samples treated in 0,15 mmol Mg(NO₃)2 solutions and sulfur co-doped C15 sample.NS-C15 had a specific surface area of 1595 m²·g^-1 and a layer spacing of0.415 nm and shows a much higher initial Coulombic efficiency of 83.5%than the other two samples.As the current density increases (Fig.8d),NS-C15 still shows a better cycling stability than CO and C15 with a reversible specific capacity of 324.1 mAh·g^-1 at 10.0 A·g^-1 (Fig.8c) [ 92] .

It is reported that the plant-derived hard carbon materials can also be modified by activators,which may adjust the pore structure,increase the specific surface area and defects of the material.KOH is the commonly used chemical activator.The activation method is mainly to directly mix KOH with the precursor or to soak the precursor in solution and then carry out high temperature pyrolysis in the inert gas.Hydrogen and carbon dioxide produced during the reaction process of KOH and carbon at high temperature promote the formation of porous structure in the resulting materials.Fu et al. [ 110] employed apple pomace as precursor to synthesize hard carbon anode materials via an activation-carbonization method for SIBs.The material exhibited a promising areal capacity of1.91 mAh·cm^-2 (208 mAh·g^-1) with a high mass loading after 200 cycles,indicating a good material utilization.KOH activation method can prepare hard carbon with excellent sodium storage performance by pyrolysis at800℃,not above 1000℃,which effectively decreases the pyrolysis temperature and reduces energy consumption.KOH was used to treat the porous structure of orange peel [ 111] ,peanut shell [ 112] and pine nut shell [ 113] to obtain the hard carbon material with a specific surface area of400 m²·g^-1 or even 1500 m²·g^-1.The prepared hard carbon materials contained a large amount of microporous structure,which was favorable for electrolyte penetration and provided a place for sodium storage.H₃PO₄ is another common activator,which can not only promote the formation of pore structure,but also form phosphorous functional groups on the surface of carbon materials.Hong et al. [ 104] prepared microporous citron peel-derived hard carbon material modified by H₃PO₄ activator.The obtained material had a three-dimensional porous structure with a specific surface area of 1272 m²·g^-1.It was also confirmed the presence of oxygen and phosphorus functional groups.Owing to the unique three-dimensional porous structure and special functional groups,the carbon material exhibited excellent cycle and rate performance.However,the initial Coulombic efficiency is only 27%,which was mainly caused by the formation of SEI film and side reactions with functional groups.Thus,the activator can promote the porous structure of hard carbon materials,which is beneficial to the cycle and rate performance,but can also lead to some adverse factors,such as an increase in specific surface area and the initial irreversible capacity.

Fig.8 a Long-terrn cycling performance of N,P-CS (N,P dual doping carbon sheets named as N,P-CS);b cycling performance of N,P-CS anode for initial 100 cycles at a rate of 0.25 C and following 200 cycles at a rate of 1.00 C [109] Copyright Elsevier Ltd.All rights reserved;c rate performance of CO,C15 and NS-C15 electrodes;d cycling performance of CO,C15 and NS-C15 at 1.0 A·g-1 [92] Copyright Elsevier Ltd.All rights reserved

7 Conclusion and prospects

SIBs are an ideal replacement of LIBs as power resources for low-speed electrical vehicles and grid-scale energy storage.Like graphite anode for commercial LIBs,hard carbon is the most promising anode for SIBs.In this review,we focus on the recent progress in plant-derived hard carbon as anodes for SIBs.Preparation,micros tructure,electrochemical performances and modification of plant-derived hard carbon materials have been systematically summarized.

Pyrolysis is a simple method to prepare plant-derived hard carbon.The natural compact porous morphology of plant precursors could be maintained in hard carbon during pyrolysis.At the same time,the layer spacing of carboncarbon could be tailored to an optimal value of0.38-0.40 nm by adjusting pyrolysis temperature.This unique microstructure of hard carbon would shorten diffusion path of Na⁺and benefit the extraction and insertion of Na⁺.Porous micros true ture could provide more storage sites to enhance capacity for hard carbon as sodium ion anodes.Nevertheless,low ICE would be induced by electrolyte decomposition on large specific surface area of porous structure.Hence,an excellent anode of plantderived hard carbon could be prepared via accurate selection of plant precursors and suitable pyrolysis process setting.Besides,heteroatoms doping or activators modification of hard carbon could remarkably improve its specific capacity and high C rate performance.But it also would increase defect voids,resulting in large specific surface area and low ICE.Therefore,the amount of heteroatoms or activators added should be controlled to balance electrochemical properties.It is expected that this review would provide in-depth understanding and guideline for the design of high-performance hard carbon anode materials from plant resources at low cost.Finally,the comprehensive utilization of high value-added waste resources will be realized.

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