the high-voltage electrospinning device mainly consists of three parts,namely,a propeller for storing and transporting the polymer precursor,a high-voltage power supply for providing a high-voltage electric field capable of stretching the precursor droplets into fibers,and a collector for collecting nanofibers(Figure 1)^[9]。 under the action of surface tension,the precursor liquid forms a droplet on the needle,and with the increase of the high-voltage electric field,the electrostatic force makes the droplet become conical and form a Taylor cone.When the electric field increases to a certain critical value,the electrostatic repulsion acting on the surface of the droplet is greater than the surface tension of the droplet.the droplet is ejected from the Taylor cone,and the liquid with a large number of charges extends continuously Under the action of the high-voltage electric field.Finally,it reaches the surface of the collector and evaporates and solidifies to form nanofibers^[10]。

the selection of electrospinning parameters,including the viscosity,surface tension and conductivity of the polymer solution,is very important to obtain continuous,uniform and high-quality nanofibers with controllable diameter;Setting of instrument parameters such as applied voltage,distance between the needle and the collector,liquid flow rate,etc.;Environmental factors such as temperature and humidity。

When the viscosity of polymer solution is too high,the viscoelastic resistance is too large,which is not conducive to the formation of jets and fibers;When the viscosity is too small,the interaction between polymer chains becomes weak,the jet stretching ability is reduced,and beaded fibers are easy to form.Therefore,in order to obtain nanofibers with excellent properties,it is necessary to select polymer solutions with moderate viscosity;the morphology of nanofibers can be controlled by adjusting the conductivity and surface tension of the solution.the diameter of nanofibers decreases with the increase of the conductivity of the solution,and the formation of beaded fibers can be avoided by reducing the surface tension^[11,12]。

as far As the selection of instrument parameters is concerned,the voltage directly affects the electric field force acting on the surface of the droplet.When the electric field force exceeds a certain value,the droplet can overcome the surface tension and be ejected from the Taylor cone.the high voltage can stretch the droplet with a large number of charges continuously,and finally form thinner fibers.But on the other hand,if the voltage is too high,it is easy to form uneven morphology with beads or break the fiber^[13]。 In addition,the solution flow rate and the distance between the needle and the collector can also affect the morphology of the nanofibers.When the liquid flow rate is increased or the collection distance is shortened,the fiber size will increase,but When the flow rate is extremely high or the collection distance is extremely short,the fiber on the collector may not have time to evaporate and solidify;When the flow rate is too small or the collection distance is too long,it is easy to form discontinuous fibers^[13,14]。

For temperature and humidity,too high temperature and too low humidity will accelerate the evaporation rate of solvent,which may lead to needle blockage,but when the temperature is too low,the evaporation rate of solution on the collector will be reduced,which will affect the preparation efficiency^[15]。

Therefore,the final fiber quality is the result of the interaction of all parameters,and all parameters need to be designed and adjusted in the experiment to obtain high-quality fiber materials。

One-dimensional materials with different morphologies and specific properties can be obtained by adding appropriate additives to the electrospinning precursor solution or using different electrospinning technologies.For example,when preparing a fiber material with a porous structure,a polymer which is easy to decompose or a metal which is easy to etch can be added to the electrospinning precursor solution;Nanofibers with hollow structure,core-shell structure,tubular structure and multi-channel structure can be synthesized by different electrospinning technologies.These structures can not only provide enough voids to alleviate the volume expansion of anode materials during cycling,but also accelerate the speed of electron transfer and ion diffusion,thus improving the cycling performance and rate performance of anode materials.At the same time,compared with the traditional anode materials,the self-supporting anode materials prepared by electrospinning can avoid the use of conductive additives and binders,thus improving the energy density of the battery.In addition,electrospinning technology has the advantages of low cost and simple operation^[16⇓~18]。

Carbon-based anode materials are the most widely studied and used anode materials for lithium-ion batteries due to their low redox potential,abundant reserves,and environmental friendliness;however,as one of the main anode materials for commercial lithium-ion batteries,graphite has a theoretical specific capacity of only 372 mAh·g^-1,which limits its further application^[19,20]。 Compared with graphite,carbon nanofibers(CNFs)prepared by electrospinning technology can not only provide higher capacity than traditional graphite electrodes,but also have excellent electronic/ionic conductivity and structural stability,which has attracted great interest^[21]。 Zhang et al.Prepared CNFs by electrospinning using polyacrylonitrile(PAN)as a precursor,used them as free-standing anode materials for lithium-ion batteries,and studied the effect of carbonization temperature(500-950℃)on the electrochemical properties of PAN-derived carbon nanofibers^[22]。 The results show that the CNFs obtained at medium carbonization temperature have the highest specific capacity,and the capacity is 555 mAh·g^-1after 105 cycles at 550℃.This is because with the increase of carbonization temperature,the graphitization degree of CNFs increases and the conductivity increases,but at the same time,the increase of carbonization temperature also reduces the defect content in CNFs,which affects the storage of lithium ions,while the medium carbonization temperature can achieve a balance between conductivity and lithium storage sites,which makes CNFs have the highest capacity 。

to further increase the electronic/ionic conductivity of electrospun CNFs,the introduction of heterogeneous elements such as N and P into CNFs is an effective modification strategy,in which nitrogen is the most representative doping element To adjust the conductivity and electrochemical reactivity of electrospun CNFs^[23,24]。 Liu et al.Prepared CNFs with high nitrogen content(denoted as NRPP1)by electrospinning using nitrogen-rich pitch(NRP)and PAN as precursors.Compared with the carbon nanofibers containing only PAN in the precursor(denoted as PAN-700),the lithium-ion battery with NRPP1 as the anode material showed excellent electrochemical performance(Figure 2),with a capacity of 644.3 mAh·g^-1at a current density of 0.1 A·g^-1[25]; And the capacity retention was 88.0%after 1000 cycles at a current density of 2 A·g^-1,while the capacity retention of PAN-700 was only 61.5%.This is because the nitrogen-rich carbon clusters introduced by NRP can change the surface microstructure of anode materials,form a thin and dense SEI layer,inhibit the continuous decomposition of electrolyte,and reduce the diffusion resistance of Li⁺ 。

In addition to introducing heterogeneous elements,structural design is also an effective modification method for CNFs.porous and hollow structures can not only increase the specific surface area of CNFs and provide more reactive sites to promote the reaction,but also increase the contact area with the electrolyte,shorten the ion diffusion distance and accelerate the ion transport.In order to obtain CNFs with Porous and hollow structures,some polymers which are easy to decompose or metals which are easy to etch are often added to the precursor solution^[26⇓~28]。 Chen et al.Introduced Zn(Ac)₂into the PAN-based electrospinning precursor solution,and prepared the CNFs electrode with porous structure through the processes of electrospinning,pre-oxidation and carbonization.The porous structure not only increased the contact area between the electrolyte and the electrode,but also shortened the diffusion distance of Li⁺and accelerated the transmission of Li^+[29]。 In addition to introducing a porous structure,Zn(Ac)₂can also improve the flexibility of PAN nanofibers by reducing their crystallinity.Therefore,the lithium-ion battery using the porous CNFs as the anode material shows excellent electrochemical performance 。

The electrochemical performance of CNFs can be improved by heterogeneous element doping or structural design,because heterogeneous element doping can not only improve the conductivity of CNFs,but also increase the number of surface active sites;However,the design of CNFs with porous and hollow structures can further increase the contact area between CNFs and electrolyte and accelerate the transmission of Li⁺.Therefore,both element doping and structure design have been used in CNFs to make the lithium-ion batteries with CNFs as anode materials have more excellent electrochemical performance^[30,31]。 Tong et al.Used coaxial electrospinning technology to prepare nitrogen-containing porous CNFs(denoted as NCFs-CW)with walnut-like structure(Fig.3)by using the interaction of inner layer precursor polyethylene oxide(PEO),outer layer precursor polyvinylpyrrolidone(PVP)and PAN during pyrolysis.Because N doping improves the conductivity of CNFs,The unique porous structure enhances the speed of electron transfer and ion diffusion,and it shows excellent electrochemical performance when used as an anode material for lithium-ion batteries,with an initial discharge capacity of 965.3 mAh·g^-1at a current density of 0.2 A·g^-1,remaining at 819.7 mAh·g^-1after 500 cycles,and a capacity of 260.5 mAh·g^-1after 1000 cycles even at a high current density of 5 A·g^-1[32]。

The TiO₂anode material has a high lithium intercalation potential,which can avoid the deposition of lithium dendrites during cycling,so it is safer than graphite,which is widely used at present.In addition,TiO₂has the advantages of stable structure,small volume change during circulation,abundant reserves and environmental friendliness.However,the low electronic conductivity of TiO₂limits its further application^{[33⇓⇓~36]}。

A common method for improving the conductivity of the TiO₂by using the electrospinning technology is to obtain the TiO₂nanofiber with a porous or hollow structure by adding an additive into an electrospinning precursor solution or using different electrospinning technologies,These structures can not only shorten the diffusion distance of lithium ions,but also increase the contact area between active materials and electrolyte,which significantly improves the electrochemical performance of TiO₂^[37]。 Zhang et al.Used microemulsion electrospinning technology to prepare several groups of nanofibers with multi-channel structure by adjusting the proportion of TiO₂and paraffin oil in the electrospinning precursor solution(Fig.4).Scanning electron microscopy(SEM),transmission electron microscopy(TEM)and N₂adsorption-desorption isotherms show that with the increase of the ratio of TiO₂to paraffin oil,the surface area of nanofibers increases and the number of axial channels increases,which can make them have better lithium storage performance^[38]。 When the ratio of TiO₂to paraffin oil is 2.25,the initial discharge capacity is 634.7 mAh·g^-1at a current density of 40 mA·g^-1,and after 100 cycles,the discharge capacity is 264.56 mAh·g^-1.However,Han et al.Fabricated TiO₂nanofibers with hollow structure by coaxial electrospinning technology,which greatly improved the rate performance of TiO₂^[39]。

the results show that the incorporation of heterogeneous atoms such as N and S into carbon nanofibers can make the nanofibers have higher conductivity^[40]。 Hu et al.Embedded TiO₂into N-doped carbon nanofibers prepared by electrospinning to prepare TiO₂@C/N composite nanofibers,which were used as free-standing electrode materials in lithium-ion batteries.With the synergistic effect of TiO₂,N atoms and stable carbon nanoframes,the material showed excellent electrochemical performance,and the capacity（388 mAh·g^-1）after 400 cycles was still higher than the theoretical capacity of graphite when the current density was 0.3 A·g^-1[41]。

Compositing TiO₂with metals or metal compounds can also improve the conductivity of materials.First-principles calculations show that Ta doping can reduce the diffusion barrier of lithium ions and increase the diffusion rate of lithium ions.Su et al.Doped different proportions of Ta into the TiO₂/CNF through electrospinning and subsequent heat treatment,and compared with the TiO₂/CNF without doped Ta,it showed more excellent cycle performance and rate performance.When the molar ratio of Ti to Ta was 95∶5,the（TiO₂-5-Ta/CNF）performance was the most excellent,and it could still maintain a high specific capacity of 300 mAh·g^-1after 1000 cycles at a current density of 2 A·g^-1[42]。

In addition,coating conductive layer on the surface of TiO₂is also one of the strategies to improve the conductivity of TiO₂.Han et al.Prepared TiO₂hollow nanofibers coated with highly conductive layers of TiO_xN_yand TiN by electrospinning and subsequent nitridation treatment,and one-dimensional nanofibers shortened the lithium ion diffusion distance and enhanced the ion diffusion kinetics^[39]; The conductive layer enhances the electron transport properties and prevents the formation of an SEI layer on the electrode surface.Under the synergistic effect of the nanofiber and the conductive layer,the capacity of the nitrided hollow TiO₂nanofiber is twice that of the ordinary TiO₂nanofiber even when the current density is 5 C 。

The titanium-based anode materials prepared using the electrospinning technique and their lithium storage properties are summarized in Table 1.It can be seen from the table that the structural design of TiO₂nanofibers by electrospinning,such as hollow structure and porous structure,can improve the electrochemical performance of the material to a certain extent,while coating the surface of TiO₂with a conductive layer on the basis of structural design can further improve the performance of the material.However,compared with these modification methods,TiO₂showed the most excellent electrochemical performance when Ta element was doped into TiO₂nanofibers.This is because with the increase of Ta-doped content,the crystal structure of TiO₂changes from anatase to rutile,which may be due to the increase of atomic kinetic energy or the rearrangement of Ti-O ions.When the molar ratio of Ti and Ta is 95∶5,TiO₂shows excellent cycle performance and rate performance due to its single rutile structure and increased specific surface area^[42]。 Therefore,the electrochemical performance of the material can be significantly improved by controlling the structure and composition of the TiO₂nanofibers.However,the specific modification mechanism and influencing factors still need to be further studied and explored to further optimize the properties of TiO₂nanofibers 。

In addition to TiO₂,lithium titanate（Li₄Ti₅O₁₂）is also used as an intercalation titanium-based anode material in lithium-ion batteries.Although the theoretical specific capacity of Li₄Ti₅O₁₂is only 175 mAh·g^-1,as a zero-strain material,it can maintain a high degree of structural stability in the process of lithium ion insertion and extraction;In addition,the potential of Li₄Ti₅O₁₂with respect to lithium metal is 1.55 V,which can prevent the formation of solid electrolyte film,making Li₄Ti₅O₁₂more safe^[43⇓~45]。 However,the conductivity of Li₄Ti₅O₁₂is low and needs to be further improved^[46]。 At present,there are several methods to improve the conductivity of Li₄Ti₅O₁₂,such as nanocrystallization of materials,compounding with excellent conductive materials such as carbon or metal/metal compounds,and forming conductive layers on the surface 。

Nanocrystallization of Li₄Ti₅O₁₂can shorten the lithium ion diffusion path and electron transmission distance,increase the contact area between the electrolyte and the active material,and obtain Li₄Ti₅O₁₂with excellent electrochemical performance^[47,48]。 Bian et al.Prepared carbon-free Li₄Ti₅O₁₂porous nanofibers with high rate and long cycle life by electrospinning and subsequent calcination,and the nanostructured Li₄Ti₅O₁₂shortens the distance of lithium ion diffusion and electron transfer during charge-discharge process,which promotes the rapid transport of ions and electrons^[49]。 The porous structure not only increases the contact area between the electrode material and the electrolyte,but also increases the active sites for electrochemical reactions.When it is used as an anode material for lithium ion batteries,the initial discharge capacity is 168.8 mAh·g^-1at a current density of 0.1 C,and after 100 cycles,the discharge capacity is 165.3 mAh·g^-1,and the capacity retention rate is as high as 97.9%;In addition,it also showed excellent rate capability with an initial discharge capacity of 149.1 mAh·g^-1at a current density of 10 C 。

In addition to the nanocrystallization of Li₄Ti₅O₁₂,the formation of a conductive layer on the surface of Li₄Ti₅O₁₂is also an effective way to improve its conductivity.The formation of a conductive layer on the surface of active materials can accelerate the ion and electron reaction kinetics and improve the electrochemical performance of materials.To improve the low rate performance due to the low electronic conductivity of Li₄Ti₅O₁₂,Park et al.Heat-treated the Li₄Ti₅O₁₂precursor prepared by electrospinning in NH₃atmosphere,and obtained Li₄Ti₅O₁₂nanofibers uniformly coated by conductive layer TiN/TiO_xN_y(Fig.5 )^[50]。 Because TiN/TiO_xN_yis the inactive phase of lithium,the initial charge-discharge capacity and the capacity retention rate after 100 cycles of Li₄Ti₅O₁₂nanofibers are reduced after coating the conductive layer of TiN/TiO_xN_y,but it shows excellent rate performance,and the discharge capacity is about 1.35 times of that without the conductive layer at a current density of 10 C.Huang et al.Used electrospinning and in-situ polymerization to synthesize one-dimensional Li₄Ti₅O₁₂@PANI composite materials for lithium-ion battery anode materials,which showed high specific capacity,excellent cycle performance and rate performance due to the synergistic effect of conductive polymer PANI and one-dimensional nanostructures to shorten the lithium-ion diffusion distance and improve the electronic conductivity^[51]。

As far as Li₄Ti₅O₁₂anode materials are concerned,it can be seen from Table 1 that both PANI and TiO_xN_y/TiN can improve the cycle performance and rate performance of the materials,and PANI has a better effect than TiO_xN_y/TiN.However,compared with surface coating,the porous Li₄Ti₅O₁₂nanofibers prepared by electrospinning and subsequent calcination show more excellent electrochemical performance due to the shortened diffusion distance of ions and electrons and the full penetration of electrolyte 。

As one of the most promising alloying anode materials,silicon has the advantages of high theoretical capacity,（3579 mAh·g^-1,Li₁₅Si₄）,low discharge potential,（0.5 V vs Li/Li⁺）,abundant reserves and low cost.However,the intrinsic conductivity of silicon is low,and the silicon will have a serious volume expansion(280%)in the process of lithium intercalation,which may expose more silicon to the electrolyte and form an unstable solid electrolyte film,eventually leading to the pulverization and cracking of silicon during cycling,affecting the cycle stability of the battery^[52,53]。

In order to solve these problems,the common solution is to composite silicon and carbon materials(graphene,carbon nanotubes,carbon nanofibers,etc.)Through electrospinning technology^[54]。 Carbon nanofibers are the most widely used,because they can not only provide one-dimensional conduction paths and shorten the transmission distance,accelerate electron transfer and ion diffusion,but also enhance the mechanical properties of materials^[55,56]。

in addition,studies have shown that nanocrystallization of silicon materials can alleviate their volume expansion during cycling,but active silicon nanoparticles with small size and large surface area tend to agglomerate In carbon materials.This will not only lead to stress concentration and instability of the solid electrolyte membrane,but also reduce the lithium storage performance of the material due to the reduction of silicon nanoparticles on the surface^[57][58]。 in order to improve the dispersion of silicon nanoparticles in the electrospinning precursor solution,surfactants or dispersants are usually Introduced into the precursor solution.Wang et al.introduced polyether(F127)as a surfactant and porous template into the electrospinning precursor solution,and prepared the Si/PCNF composite by electrospinning(Fig.6a).the study showed that silicon could be uniformly dispersed in the carbon nanofiber even when the mass fraction of silicon nanoparticles was 50%^[59]。 Park et al.Made commercial silicon nanoparticles uniformly dispersed in nitrogen-doped carbon nanofibers(w-Si@N-CNFs)by adding dispersant polyethylene glycol in the precursor solution to avoid the aggregation of silicon nanoparticles^[60]。 Compared with the wo-Si@N-CNFs without the addition of polyethylene glycol in the precursor solution(wo-Si@NCNFs),the wo-Si@N-CNFs showed excellent cycle stability and excellent rate capability,with a reversible specific capacity of 1045 mAh·g^-1at a current density of 0.2 A·g^-1and a capacity retention of 66%after 150 cycles;And the capacity is 640.8 mAh·g^-1when the current density rises to 2 A·g^-1 。

Although the addition of surfactants and dispersants can improve the performance of silicon-based anode materials to a certain extent,the cycle performance of silicon-based materials still needs to be further improved.Mechanical stress and material pulverization caused by the volume expansion of silicon in the process of lithium insertion/extraction are one of the most important factors affecting the cycle performance of silicon-carbon composite anode materials.the design of silicon-carbon composites into various structures(such as core-shell structure,porous structure,multi-core core-shell structure,hollow structure,yolk-shell structure)by electrospinning can provide effective pores to adapt to the volume expansion of silicon during cycling.Compared with other structures,the multi-core core-shell structure can provide more space to alleviate the volume expansion of silicon,and ZIF-8 is an ideal template for the introduction of multi-core core-shell structure because Zn is easy to evaporate at high temperature after the reduction of ZnO to Zn in ZIF-8^[61][29]。 Zeng et al.Introduced Si-ZIF-8 into the electrospinning precursor solution to prepare nitrogen-doped porous nanofiber-coated Si/C composites(Si/C-ZIF-8/CNFs)(Fig.6 B)^[62]。 Under the synergistic effect of ZIF-8 and PAN,there is a large number of pore structures in the material,which can not only alleviate the volume expansion of Si,but also promote ion diffusion and electron transfer,and provide more active sites for chemical reactions.Therefore,the Li-ion battery assembled with Si/C-ZIF-8/CNFs as the anode material has a long cycle stability with a capacity of 538.6 mAh·g^-1after 500 cycles at 0.5 A·g^-1 。

in addition to the above research ideas,researchers have also found that compounding silicon with metals,metal compounds and other materials can further improve the conductivity of materials and thus improve the cycle performance and rate performance of batteries.Liu et al.First prepared Si-NiO-C nanomaterials by electrospinning,and then heat-treated them In argon atmosphere to form hollow Si-Ni-C nanofibers^[63]。 Among them,the hollow structure alleviates the volume expansion of silicon during cycling,while the nickel atoms increase the conductivity of the nanofibers,and the lithium-ion batteries using them as free-standing anode materials demonstrate excellent cycling performance and rate capability.Du et al.Combined silicon and metal sulfide Co₉S₈.They first prepared Si@ZIF-67 particles by hydrothermal method,and then encapsulated Si@Co₉S₈with yolk-shell structure in one-dimensional carbon nanofibers through electrospinning,vulcanization,carbonization and other processes(Fig.7 )^[64]。 The electronic conductivity of the composite material is improved and the electron transfer is accelerated through the synergistic effect of the one-dimensional carbon nanofiber and the Co₉S₈;A large number of pores in the yolk-shell structure effectively solve a series of problems caused by the volume expansion of silicon.The lithium-ion battery assembled with the Si@Co₉S₈carbon fiber as the anode material has excellent rate performance,and the reversible capacity after 250 cycles remains at 743 mAh·g^-1even at a current density of 1 A·g^-1 。

The silicon-based anode materials prepared using the electrospinning technique and their lithium storage properties are summarized in Table 2.Electrospinning can directly compound silicon and carbon materials,for example,by spinning silicon nanoparticles and carbon precursors together to form Si-C composite nanofibers.This composite structure is conducive to alleviating the volume expansion of silicon anode during cycling and improving the conductivity of the material.The addition of active agent F127 and dispersant PFG in the precursor solution of electrospinning can avoid the agglomeration of silicon nanoparticles and further improve the cycle performance and rate performance of the material.In addition,compared with PEG,F127 can also be used as a porous template to prepare porous carbon nanofibers,thereby increasing the surface area and ion transport channels of the material.In addition,the introduction of ZIF-67,ZIF-8,Ni(CH₃COO)₂₄and other substances into the precursor solution can realize the composition of silicon and metal or metal compounds,while improving the conductivity of silicon-based anode,these substances can also form nanofibers with specific structures,such as hollow structure,core-shell structure and so on.It can further alleviate the volume expansion of silicon-based materials during cycling,and improve the stability and cycle life of the materials.Although this method can prepare silicon-based anode materials with excellent cycling performance and rate capability,the formation mechanism of these structures still needs to be further studied to further optimize the performance of silicon-based anode materials 。

In addition to silicon,silicon oxides are also used as anode materials for lithium-ion batteries.Compared with silicon,silicon oxides have smaller volume change during cycling,but some capacity loss is caused by the formation of Li₂O and Li₄SiO₄phases during lithiation.In addition,low electronic conductivity limits their further application^[65,66]。

Compounding SiO_xand carbon nanofibers is currently one of the most common solutions to improve the performance of SiO_x.Due to the large specific surface area,abundant pores and good conductivity,the recombination of CNF and SiO_xcan not only alleviate the volume expansion of SiO_xduring cycling,but also shorten the ion/electron transport path and promote ion diffusion and electron transfer^[67]。 Belgibayeva and Taniguchi prepared SiO₂/C composite nanofiber mats by electrospinning using PVP and TEOS as carbon and silicon sources,respectively,and studied the effect of PVP concentration in the precursor solution on the structure and electrochemical properties of the materials^[68]。 The study showed that when the mass concentration of PVP in the precursor solution was 5%,the specific surface area of the SiO₂/C composite nanofiber mat was as high as 642 m²·g^-1,which contributed to its excellent electrochemical performance.When the current density is 0.1 A·g^-1,the initial discharge and charge capacities are 1800 mAh·g^-1and 984 mAh·g^-1,respectively,and the capacity remains at 754 mAh·g^-1after 200 cycles with a Coulombic efficiency close to 100% 。

In order to further improve the conductivity of SiO_x,a layer of conductive polymer is usually coated on its surface.Conductive polymer can not only improve the conductivity of SiO_x,but also avoid the contact between SiO_xand electrolyte,and prevent the occurrence of side reactions^[69]。 Thirugnanam et al.Coated conductive polymer PPy on the surface of porous CNF-SiO₂composites by electrospinning and in-situ polymerization,which improved the cycle performance and rate performance of the materials^[70]。 When the current density is 0.05 A·g^-1,the initial reversible specific capacity is 1070 mAh·g^-1,and the reversible specific capacity is stably maintained at 610 mAh·g^-1during the second cycle and beyond;In addition,it also demonstrated excellent rate capability,with the reversible specific capacity still up to 300 mAh·g^-1after 300 cycles when the current density was increased to 0.5 A·g^-1.Although the coating of conductive polymer can obtain SiO₂anode materials with excellent performance,the process is usually complex and the cost is high,so it is still necessary to explore new modification methods to solve the problems of low conductivity and capacity loss during cycling faced by silicon oxides 。

Tin can theoretically combine with 4.4 lithium ions to form a Li_4.4Sn,and the capacity can reach 994 mAh·g^-1.In addition,its lithium storage platform has a high（0.6 V vs Li/Li⁺）,and its conductivity is good compared with other alloy anode materials such as Si and Ge.Therefore,tin has been widely studied as an anode material.However,the huge volume expansion of tin during alloying makes the material easy to agglomerate and form an unstable solid electrolyte film,which hinders its practical application.In order to solve these problems,the current research ideas mainly focus on the nanocrystallization of tin or the composite of tin with carbon-based materials,metals and metal oxides.In addition,the morphology and structure design of tin to introduce pores is also one of the effective methods to improve the properties of materials^[71,72]。 The modification of tin-based anode materials using electrospinning technique and their corresponding electrochemical properties are summarized in Table 3。

the results show that the nanocrystallization of elemental tin can inhibit the pulverization of materials caused by the volume effect of tin in the cycle process,shorten the diffusion distance of lithium ions,and accelerate the reaction kinetics.However,the agglomeration of nanomaterials makes the reaction active sites of materials reduce and the lithium storage performance decline.Electrospinning technology can be used to prepare uniformly dispersed nanomaterials to avoid the agglomeration of nanomaterials^[73,74]。 Therefore,the researchers combined tin and carbon materials and designed their structures,which can not only inhibit the agglomeration of tin nanoparticles,but also provide sufficient space to alleviate the volume expansion caused by Sn in the alloying process^[75]。 carbon materials with porous structure are usually obtained by etching,which will reduce the mechanical properties of the materials.As a self-supporting substrate for tin,Carbon materials need to have both excellent electrochemical and mechanical properties^[76]。 Zhu et al.introduced Sn-MOF into interconnected one-dimensional carbon nanofibers by electrospinning,taking advantage of the large specific surface area and porous structure of MOF structure.Then Sn@C@CNF with layered porous structure was obtained by carbothermal reduction,which not only promoted the diffusion and charge transfer of electrolyte,but also alleviated the volume expansion of tin nanoparticles during cycling^[77]。 When it is used in a soft-pack battery,the battery shows excellent cycle stability even after 210 times of repeated bending,which indicates that the Sn@C@CNF with a layered porous structure has excellent mechanical properties。

the interface strength between carbon materials and tin is weak,while the combination of tin and other metals or metal oxides can enhance the interface strength and improve the structural stability of materials,so the combination of tin and metals or metal oxides has attracted more and more attention.Compared with other material composite methods,Lu et al.Obtained MnO-Sn@CNF with excellent lithium storage performance by embedding cubic MnO-Sn into nitrogen-doped carbon nanofibers through simple and controllable electrospinning^[78]。 the combination of Sn and metal oxide MnO improves the structural stability and electrochemical stability of Sn,while the application of CNF alleviates the volume expansion of MnO-Sn in the cycle process,and the mesoporous surface shortens the ion diffusion path and solves the problem of slow MnO ion kinetics。

Tin-based oxide anode materials include SnO₂,SnO,and SnO_x,among which SnO₂has been widely studied due to its high theoretical capacity and low electrode potential relative to lithium,However,SnO₂will react with lithium ions to form LiO₂during the first charge process,resulting in irreversible capacity loss.In addition,SnO₂also have some problems,such as low intrinsic conductivity and pulverization caused by volume expansion during cycling^[79][80,81]。

One of the modification methods of SnO₂is to reduce the particle size of SnO₂and make it nano-sized.Hu et al.Showed that the coarsening of tin particles would affect the energy conversion efficiency of SnO₂and the diffusion kinetics of lithium ions,and the tin particles in tin oxide with a size of less than 11.3 nm could ensure sufficient diffusion kinetics between Sn/Li₂O interfaces,and the nanocrystallization of SnO₂could also alleviate its volume expansion during alloying^[82]。 However,usually the nano-sized SnO₂particles tend to agglomerate.In order to solve this problem,Liang et al.First prepared nitrogen-doped carbon nanofibers by electrospinning,and then successfully grew the nano-sized SnO₂nanoflowers on the carbon fibers by hydrothermal method.The nanosized SnO₂alleviated the volume expansion of the material,while the N-doped carbon nanofibers prepared by electrospinning not only prevented the agglomeration of SnO₂nanoflowers,but also provided a one-dimensional conductive network to increase the conductivity of the material and provide a supporting framework for the growth of nanoflowers^[83]。

Electrospinning can not only improve the dispersion of SnO₂nanomaterials,but also easily produce nanofibers with structural diversity,so it is also an effective modification strategy to design the structure of SnO₂to build pore space.Among the various structures,the hollow structure can not only provide void space to adapt to the volume change of SnO₂,but also provide more active sites for chemical reactions,shorten the ion/electron transmission path,and improve the cycling stability and rate performance of materials.However,the volume energy density of the hollow structure is greatly reduced compared with that of the solid structure^[84]。 In order to solve this problem,Gao et al.Prepared SnO₂nanofibers with tube line structure by adjusting the heat treatment parameters,and then coated them with polypyrrole(PPy)and calcined them in N₂to obtain multi-wall Sn/SnO₂@CNF（with hollow structure(Fig.8 )^[85]。 The Sn/SnO₂@CNF with a multi-walled hollow structure exhibited excellent electrochemical performance,and the capacity was still maintained at 986.3 mAh·g^-1（1 A·g^-1）and 508.2 mAh·g^-1（5 A·g^-1）after 2000 cycles 。

The composite nanomaterials composed of different metal oxides can effectively alleviate the volume expansion of electrode materials during cycling by adjusting the local stress and prolong the cycle life of the materials;therefore,compounding metal oxides such as ZnO,TiO₂,and MoO₃with SnO₂is also an effective modification strategy^[86]。 Zhou et al.Synthesized SnO₂/MoO₃@Graphene composite by electrospinning,in which the three-dimensional network structure formed by porous nanotubes provided more transmission channels for electrons and ions,and increased the contact area between electrolyte and electrode materials,while the introduction of graphene not only improved the electronic conductivity of the material,but also provided a large number of active sites for electrochemical reactions^[87]。 Under the combined action of the three,SnO₂/MoO₃@Graphene has an initial discharge capacity up to 1014 mAh·g^-1,and after 300 cycles at 1 A·g^-1,it still has a capacity of 798 mAh·g^-1 。

it can be seen from Table 3 that the modification method of compounding metal oxides with high theoretical capacity with tin-based anode materials shows excellent electrochemical performance compared with other modification methods,but the conductivity of metal oxides is poor,and It is usually necessary to add materials with good conductivity such as graphene to make them have more excellent performance。

In addition to tin and tin-based oxides,SnS and SnSe have also received extensive attention as tin-based anode materials,because SnS and SnSe not only have high theoretical capacity,but also have large interlayer spacing,which is conducive to the rapid transport of ions and electrons.Compared with SnS,SnSe has higher conductivity and better electrochemical reversibility,but they also have the common problem of tin-based anode materials,that is,huge volume expansion during alloying^[88⇓~90]。 At present,compounding SnS or SnSe with carbon materials by electrospinning is an effective strategy to alleviate the volume expansion of materials.Xia et al.Prepared SnSe/CNF composite by electrospinning and subsequent high temperature heat treatment,which has excellent mechanical flexibility without any fracture even when bent 180°,in addition to its excellent electrochemical performance when used as a self-supporting anode material for lithium-ion batteries,which can maintain a discharge capacity of 405 mAh·g^-1after 500 cycles at a current density of 1 A·g^-1[90]。

metal compound anode materials include metal oxides,metal sulfides,metal selenides,metal nitrides and so on,which have been widely studied due to their high theoretical specific capacity,abundance,low price and environmental friendliness.However,the low electronic conductivity of metal compounds and the pulverization and shedding of materials caused by volume expansion limit their further application^{[91⇓⇓~94]}。

At present,one of the common modification strategies is to compound metal compounds with conductive materials.Wu et al.Prepared MoS₂/C composite nanofibers（MoS₂/C/C）with surface coated double-layer carbon materials by hydrothermal method and electrospinning,with uniform fiber diameter and smooth surface(Fig.9 )^[95]。 Because the coating of the double-layer carbon material effectively prevents the stacking and agglomeration of MoS₂nanosheets,the volume expansion of MoS₂in the lithiation/delithiation process is relieved,and the conductivity and structural stability of MoS₂are enhanced.The MoS₂/C/C fiber electrode had an initial capacity of 1275 mAh·g^-1at a current density of 0.2 A·g^-1and a capacity retention of 92.6%after 800 cycles at a high current density of 1 A·g^-1.In addition to carbon materials,MXene is also used to compound with metal compounds to improve the conductivity of materials because of its high electronic conductivity and abundant functional groups^[96]。 Guo et al.Embed hollow Fe₃O₄nanospheres encapsulated by MXene material Ti₃C₂into nitrogen-doped carbon nanofibers through electrospinning and subsequent carbonization processes to obtain Fe₃O₄@MXene/CNFs.MXene and nitrogen-doped carbon nanofibers not only provide double electron transmission channels and accelerate electron transmission,But also relieves the volume expansion of the Fe₃O₄in the charge-discharge process,and the lithium ion battery assembled by using the lithium ion battery as the negative electrode material shows excellent cycle performance and rate performance,and can still maintain a reversible capacity of 806 mAh·g^-1after 500 cycles when the current density is 2 A·g^-1[97]; And a reversible capacity of 613 mAh·g^-1can still be obtained at a current density of 5 A·g^-1 。

in addition to compounding with conductive materials,designing the structure and morphology of metal compounds is also an effective modification method.One-dimensional hollow nanofibers can not only shorten the diffusion path of lithium ions and accelerate the reaction kinetics,but also provide enough space to adapt to the volume expansion In the cycle process,avoid the pulverization and shedding of materials caused by stress,and improve the cycle performance of materials^[98,99]。 Chaudhari et al.Prepared one-dimensional hollowα-Fe₂O₃fibers by calcining the Fe（acac）₃-PVP composite nanofibers obtained by electrospinning in air,and first used them in lithium-ion battery anode materials.The initial capacity is 1795 mAh·g^-1at a current density of C/16,and the reversible capacity of 1293 mAh·g^-1can be maintained after 40 cycles,and the coulombic efficiency is as high as 95%,because the hollow structure alleviates the volume change ofα-Fe₂O₃during charge and discharge^[100]; In addition to measuring the cycling performance of hollowα-Fe₂O₃,the charge transfer impedance at the initial cycle and after 5 cycles was measured by electrochemical impedance spectroscopy,and it was found that the initial impedance was 41.84Ω,while the impedance was 29.7Ωafter 5 cycles,because the hollow structure increased the contact area between the electrolyte and the active material and accelerated the electron transport.Oh et al.Prepared one-dimensional hollow NiO nanofibers(fig.10)by virtue of the movement of camphene to the center of the jet due to the viscosity gradient during electrospinning and the sublimation of camphene during subsequent calcination.Compared with solid NiO nanofibers,The lithium ion diffusion path is shortened,and the lithium storage performance of the material is enhanced,so the material shows excellent cycling performance and rate capability,and the reversible capacity of 956 mAh·g^-1is still maintained after 400 cycles when the current density is 2 A·g^-1[101]。

In order to further improve the electronic conductivity of metal compounds and alleviate the volume expansion of materials during cycling,more and more studies have applied conductive materials such as carbon or graphene to metal compounds with hollow structures.Lee et al.Dispersed hollow NiO nanospheres on hollow carbon nanofibers by electrospinning,which not only accelerated the speed of electron transfer and ion diffusion,alleviated the stress concentration caused by volume expansion,but also made the electrolyte more fully infiltrated into the electrode.When mixed with a binder conductive agent as a battery anode material,it showed excellent electrochemical performance,with an initial capacity of 1298 mAh·g^-1at a current density of 1 A·g^-1,a capacity retention of 66%and a coulombic efficiency of 99.8%after 250 cycles^[102]。 Luo et al.Uniformly anchored hollow Co₃S₄/C nanoparticles with a size of 30 nm on N-doped carbon nanofibers for（Co₃S₄/CNF）through electrospinning,carbonization,and vulcanization^[103]。 The intrinsic void of hollow Co₃S₄/C nanoparticles alleviates its volume expansion during cycling,and the large specific surface area and pore volume increase the contact area with the electrolyte and shorten the ion diffusion distance.The composite of the carbon nanofibers and the carbon nanofibers effectively alleviates the characteristic that the nanoparticles are easy to agglomerate.The lithium-ion battery with Co₃S₄/CNF as the free-standing negative electrode exhibited excellent cycling performance,which could still show a high reversible capacity of 742 mAh·g^-1after 200 cycles at a current density of 0.2 A·g^-1,and the capacity was still as high as 436 mAh·g^-1after 500 cycles even at a high current density of 1 A·g^-1.Considering that the carbon nanofibers obtained by using PAN as the electrospinning precursor are fragile,Ni et al.Selected the copolymer of acrylonitrile andβ-monomethyl itaconate[P(AN-co-MHI)]as the precursor to prepare nitrogen-doped carbon nanofibers(NHMCFs)with hollow and multi-channel structures by electrospinning technology.And ultrathin MoSe₂nanosheets were grown on both the inner and outer surfaces of carbon fibers by hydrothermal method(Fig.11 )^[104]。 The multi-channel structure and the hollow structure can not only shorten the ion diffusion distance and alleviate the volume expansion of the material during cycling,but also increase the loading capacity of MoSe₂and the contact area between MoSe₂and electrolyte.Therefore,NHMCFs/MoSe₂and N-doped carbon nanofiber（NCFs/MoSe₂）,N-doped carbon nanofiber with multi-channel structure（NMCFs/MoSe₂）showed the most excellent electrochemical performance,and the capacity remained at 586.7 mAh·g^-1after 400 cycles at a current density of 1 A·g^-1 。