Design,Preparation And Energy Storage Mechanism Of High Energy Density Electrode Materials

The last two decades saw the establishment of Li-ion batteries（LIBs）as a leading technology for energy storage in portable and automotive applications.In the meantime,other storage technologies are also developping,trying to reach the performance of LIBs and to surpass it in some aspect.Compared with lithium ion battery,sodium ion battery has a greater application prospect in large energy storage.Because sodium resources are abundant and distributed evenly on the earth surface,which makes up for the low abundance and uneven distribution of lithium resources,thus reduces the cost of the entire energy storage system.In recent years,it has become a research hotspot.However,the ion radius of sodium ions is 55%larger than that of lithium ions,which means that lithium ion electrode materials cannot be simply applied directly to sodium ion batteries.On the other hand,sodium ion batteries have a lower energy density than lithium ion batteries due to their higher weight and lower reducibility.Therefore,the preparation and study of high energy density sodium ion electrode materials has become a very meaningful research topic.1.In this paper,polyacrylonitrile was selected as the base,and cyano group on the surface was hydrolyzed to amide group under alkaline conditions,and then modified polyacrylonitrile with a large number of amide group functional groups on the surface was obtained.The modified polyacrylonitrile was applied to high energy density electrode materials.Firstly,in the preparation of anode materials,due to a large number of amide groups distributed at the atomic level on the surface of the modified polymer,some nitrogen-containing functional groups still remain in the carbon materials obtained after carbonization.Therefore,amine-functionalized carbon nanosheets constructed three dimensional porous carbon material with high energy density were successfully prepared.Similar to other heteroatom doping,the addition of amino functional groups to carbon materials can also broaden the graphite layer spacing and provide more active sites,which is conducive to the realization of highly reversible sodium-storage reactions.In addition,open tank-like nanopores formed by the ultrathin nanosheets can transform the electrochemical reaction active site from the surface to the whole,so as to make full use of the functional functional groups inside the material.The method achieves both the uniform mixing of high quality amine functional groups and the construction of the unique morphology of the open tank-like nanopores,which significantly improves the fast charge storage capacity of sodium ions.The carbon material applied to the sodium ion battery shows excellent rate performance and cycle stability.According to the theoretical calculation results,the capacity improvement of this electrode is also confirmed to benefit from the enhancement of Na-absorption with amine groups.On the other hand,due to their high charge-discharge voltage and specific capacity,layered transition metal oxides are the most promising high energy density cathode materials for lithium ion batteries and sodium ion batteries.We also applied the modified polyacrylonitrile to such cathode materials.Herarchical LiNi_1/3Co_1/3Mn_1/3O₂polyhedron assemblies were obtained through in-situ chelation of transition metal ions(Ni²⁺,Co²⁺and Mn²⁺)with amide groups uniformly distributed along the backbone of modified polyacrylonitrile chains to achieve intimate mixing at the atomic level.Due to its superior hierarchical structure,such as exposed active crystal facets provide more channels for Li⁺diffusion,and meso/macropores serve as access shortcuts for fast migration of electrolytes,Li⁺and electrons,which enables the cathode to exhibit superior rate capability,high volumetric energy density,and espeically ultralong high-rate cyclability.Besides,we also use the amide group on the backbone of modified polyacrylonitrile chain to in situ bond the transition metal manganese ion,and then successfully prepared Na_0.7MnO_2.05 cathode material.Its special morphology significantly alleviated the deformation stress caused by sodium ion de-intercalation,the capacity retention can reached as high as 89.3%and 88.8%after 300 cycles at 5 C and 10 C,respectively.As far as we know,it has the best the high rate cyclic stability among the reported Mn-based layered metal oxides cathode materials.It can be seen that the modified polyacrylonitrile as chelating agent proposed in this paper is conducive to the atomic-level mixing of transition metal ions and provides an effective way for the synthesis of other such cathode materials.2.Na₃V₂（PO₄）₂F₃ is considered as one of the most promising positive electrode materials for sodium ion batteries due to its high theoretical capacity(128 mAh g^-1)and average operating potential.Its specific energy of up to 500 Wh g^-1 is an important milestone to reach the level of commercial LIBs cathode materials.However,similar to other metal-phosphate cathode materials cathode materials,it also has low electronic conductivity,which hinders its further industrial development.Herein,we successfully prepared Na₃V₂（PO₄）₂F₃/rGO&C cathode material.The Na₃V₂（PO₄）₂F₃ nanoparticles uniformly dispersed in the three dimensional continuous conductive network formed by rGO&C structure,resulting in the fast and efficient electron transfer,and flexibility of its unique three-dimensional continuous conductive network structure alleviates the volume change during the long time cycling.The electrode material demonstrated excellent rate performance(discharging a capacity of 120,90,and 80 mAh g^-1 at 0.5 C,10 C,and 30 C,respectively)and cycling stability（with a capacity retention of 81.9%after 1000 cycles at 30 C）.However,the lack of cathode materials with sufficient energy density and good cycle life is still a key problem hindering the development of sodium ion batteries.Although Na₃V₂（PO₄）₂F₃ is currently reported as a promising cathode material,vanadium source is very expensive and difficult to be commercialized.In this paper,a new type sodium super ion conductor Na₃FeV（PO₄）₃ cathode material was successfully prepared for the first time by replacing part of vanadium with cheaper iron.This positive electrode material can discharge a high capacity up to 175 mAh g^-1 at a low current density（0.2 C）,with a Coulombic efficiency almost close to 100%,indicating that the redox reaction in the electrochemical reaction process has a good reversibility.In addition,this cathode material also shows good rate performance and cycling stability,and its energy density can be up to 450 Wh kg^-1,which is expected to be one of the most promising cathode materials for sodium ion batteries.Furthermore,the XPS characterization method and first principle density functional theory（DFT）were used to understand the underlying Na ion storage mechanisms.3.The structure or morphology of the electrode material plays a crucial role in its electrochemical properties,among which the two-dimensional nanosheet structure has a shorter ion transmission path and a larger open exposure surface,so that the block electrode material formed by it generally has excellent rapid charging and discharging performance.NASICON cathode materials have attracted researchers’attention due to their three-dimensional ion transport channels and small volume expansion,among which vanadium based Na₃V₂（PO₄）₂F₃,NaVPO₄F and Na₃V₂（PO₄）₃ are the most widely studied,However,its components are complex,and the preparation process usually requires a long time and high temperature calcination,so the particles are extremely easy to grow and agglomerate,and it is difficult to form a special two-dimensional nano-sheet structure.In this paper,a series of vanadium based NASICON cathode materials（Na₃V₂（PO₄）₂F₃,NaVPO₄F and Na₃V₂（PO₄）₃）/C composite nano-sheets were successfully prepared by using the low-cost amylopectin as organic carbon source and structural guidance agent.They all shows good electrochemical performance in the half cell test.In particular,its excellent electrical contact,two-dimensional fast ion transport channel and outstanding structural stability are closely related to its excellent high rate performances.Among them,Na₃V₂（PO₄）₂F₃/C was further applied to the full battery,which was matched with amine-functionalized carbon nanosheets constructed three dimensional porous carbon anode material and assembled into the full battery.The battery exhibits high energy density(94.8 Wh kg^-1),high power density(7295 W kg^-1)and has good cycle stability.It can be seen that the method proposed in this paper is a general method for the preparation of vanadium based 2D nanosheet structure under high temperature,which has certain guiding significance for the synthesis of this kind of materials.