Review on Anode Materials of Rare Metal Based Lithium-Ion Batteries
Lithium-ion batteries are clean energy to solve the shortage of fossil energy such as coal and oil,and are widely used in production,life,military and other fields.High energy density,small memory effect,long cycle life and low environmental pollution are obvious advantages of lithium-ion batteries.Lithium-ion batteries have been widely used in small mobile devices,medical micro-electronic devices,electric vehicles and other fields.Lithium-ion battery anode materials are divided into carbon and non-carbon an-ode materials,and rare metals area class of non-carbon anode materials.Compared with traditional graphite anode materials,volume capacity of tin-based materials is higher.Circulate,magnification and safety performance of titanium-based materials are higher.Mo-lybdenum-based material compounds are conducive to rapid charge and discharge.Diffusion rate and conductivity of Li+in germanium-based anode materials are higher.Theoretical specific capacity of antimony-based anode materials is higher,and reaction voltage is saf-er.Germanium-based anode materials is simpler than silicon,germanium and tin reaction process,moreover volume change is less.So rare metal is more applicated as the anode material of lithium-ion battery.However,the volume of rare metals is greatly varied during charging and discharging,which results in unstable structure of anode materials and reversible capacity of the lithium-ion battery is re-duced.For example,volume of tin-based,germanium-based,and antimony-based anode materials are expanded during charging and discharging,which unstable solid electrolyte interface(SEI)film is formed.Electronic conductivity and volumetric energy density of titanium-based anode material are low,which reversible capacity of the battery is decreased.Electrical conductivity of molybdenum-based anode materials is poor because of electrode is easy to crush.In this review,typical rare metals structural characteristics of tin,titanium,molybdenum,germanium,antimony or their chemical compound and mechanism of lithium storage as lithium-ion battery an-ode materials were expounded.The existing problems in the process of charge and discharge were summarized,and the research prog-ress and research trends of the modification methods for anode materials were analyzed.The structure analysis results showed that ru-tile SnO2 unit cells were body-centered orthogonal parallelepipeds,the crystal structure of Li4Ti5O12 was tetrahedral and octahedral structure,MoS2 was composed of three layers of S-Mo-S atoms.The unit cell of Ge was face-centered cubic unit cell,the structure of Sb was two-dimensional folded layered structure.The mechanism analysis of lithium storage showed that conversion of SnO2 to Sn was an irreversible reduction process,and this irreversible chemical conversion led to a large initial irreversible capacity loss of SnO2 elec-trode.There was a reversible alloying/dealloying process between Sn and Li,and 4.4 mol of Li could be stored in 1 mol of Sn.The vol-ume expansion of the reversible process was about 200％,resulting in large internal stress in the electrode material,and the capacity decayed rapidly after cycling.During the discharge process,3 mol Li+was first embedded in 1 mol spinel Li4Ti5O12 structure,and face-centered cubic structure was formed,resulting in a voltage drop to 1.0 V,60％of Ti4+in Li4Ti5O12 was reduced to Ti3+at the same time.According to first-principles calculations,the final composition of Li4Ti5O12,which was completely lithiated,and it should be Li8 5Ti5O12.Under different voltages,the chemical reaction process of MoS2 lithium-ion battery anode material was different.Conversion and alloying reaction mechanism were followed between Ge storing lithium,and Ge could be irreversibly reacted with lithium to gener-ate Li2O.Cyclic stability of GeO2 particles was added because of joining Ge or other nanomaterials.Ge in GeO2/Ge/C composites could be used as an electrocatalyst to promote the decomposition of Li2O.So reversible capacity of was composites improved.The volume ex-pansion of Sb reacting with less Li+was about 135％,which was relatively small among alloy anode candidates.Compared with the con-gener element P,Sb was exhibited higher stability and conductivity.Before the formation of Li3Sb,the crystalline phase Li2Sb ap-peared,and the Li2Sb intermediate phase would no longer be generated during the entire charging process.Final discharge product Li3Sb would be reduced to the crystalline phase Sb with the deinsertion of Li+,and Li2Sb was appeared again during the next discharge.Modification method analysis of negative electrode materials showed that nanoification,composite modification,and structural regula-tion of rare metal matrix composites were effective means to improve electrochemical performance of lithium-ion batteries.A large num-ber of studies had shown that although the structure and reaction mechanism of different rare metal-based anode materials were differ-ent.The reasonable design of nanostructures,composite and alloying with new carbon materials were effective ways to improve the ca-pacity and cycle stability of lithium-ion batteries.Nanostructure was conducive to the rapid redox reaction at the interface of electrode and electrolyte,and was easy to transfer mass and transfer charge.The small size was also beneficial to adapt to the strain generated by volume expansion and contraction when lithium ions were embedded and removed.Alloying and nanocomposites were formed to alter the reaction energy barrier,promote charge transfer,and adapted to volume changes.The great progress had been made in the modifi-cation of electrical properties of rare metal-based anode materials,and such theories and methods could be applied to other electrode materials such as lithium-ion batteries,sodium-ion batteries,and lithium-sulfur batteries.Optimization of nanostructure,reduction of materials cost,construction of porous or core-shell structure were the future research trends.
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