High-voltage Phosphate-based Positive Electrode Materials For Lithium/Sodium Ion Batteries

Ever-growing demand for advanced rechargeable lithium-ion batteries in portable electronics and electric vehicles has spurred intensive research efforts over the past decade.The key to sustaining the progress in Li-ion batteries lies in the quest for safe,low-cost positive electrode materials with desirable energy,and power capabilities.One approach to boost the energy and power density of batteries is to increase the output voltage while maintaining a high capacity,fast charge-discharge rate,and long service life.However,with the increase of the overall market share of lithium-ion batteries,a large amount of lithium resources will be required to meet the demand and supply chains will be challenged.Sodium has a large abundance,wide gloabal distribution and suitable electrochemical potential.Thus,rechargeable batteries based on sodium electrochemistry are considered as promising alternatives for grid-scale electrical energy storage applications,which put specific requirements on the cost and sustainable resource supply of the battery.Therefore,to explore new high-voltage cathode materials,modulate structure and morphology,optimize electrochemical performance,and understand the corresponding mechanisms are highly desirable for both basic researches and practical applications.In this paper,we mainly focused on the methods of synthesis,general composition-morphology-property relationships and corresponding mechanisms of the high-voltage phosphate cathode materials,including LiCoPO₄,α-Li₃V₂（PO₄）₃ and Na₃V₂（PO₄）₂O₂F.Some useful results and understanding were obtained,listed as follows:1.Highly[010]-oriented self-assembled LiCoPO₄/C nanoflakes were prepared through a simple and facile solution-phase strategy at low temperature and ambient pressure.The formation of5-hydroxylmethylfurfural and levoglucosan via the dehydration of glucose during the reaction played a key role in mediating the morphology and structure of the resulting products.LiCoPO₄ highly oriented along the（010）-facets exposed Li⁺ion transport channels,facilitating ultrafast lithium ion transportation.In turn,the unique assembled mesoporous structure and the flake-like morphology of the prepared products benefit lithium ion batteries.The tested batteries constructed using two-dimensional LiCoPO₄/C nanoflakes selfassembles as cathodes and commercial Li₄Ti₅O₁₂2 as anodes provide high capacity of 154.6 mAh g^-11 at0.1 C(based on the LiCoPO₄ weight of 1 C=167 mAh g^-1)and good cycling stability with 93.1%capacity retention after 100 cycles,which is outstanding compared to other recently developed LiCoPO₄ cathodes.2.NASICON-structured Li₃V₂（PO₄）₃/C nanoparticles were synthesized by a low temperature reluxing route using two different organic solvnets,including dimethyl sulfoxide（DMSO）and ethylene glycol（EG）.The effect of the organic solvents on particle size,morphology,nature of carbon coating,and electrochemical property of the resulting Li₃V₂（PO₄）₃ was investigated.It was confirmed that the carbon coated LVP-DMSO nano-flakes obtained from DMSO delivered high initial discharge capacity（130.1 mAh g^-11 at 0.2 C）,superior rate performance（113.5 mAh g^-11 at 10 C）and exellent cyclic stability（92.8%of capacity retention at 5 C after 500 cycles）.The improved electrochemical characteristics are attributed to the morphology of 2D nanoflake which allows facile electrical conductivity.3.Unique hierarchical mulberry-shaped Na₃V₂（PO₄）₂O₂F@C nanocomposite was fabricated by a rapid microwave-assisted low-temperature refluxing strategy.The V（acac）₃ reverse micelle systems in the water-in-oil microemulsions played key roles in forming the self-assembly architectures.The prepared Na₃V₂（PO₄）₂O₂F@C nanoparticles with the anisotropic growth along the[002]direction were in-situ encapsulated in carbon shells,which greatly contribute to fast Na⁺/e^-transfer in electrodes.And the self-assemblies with high structure stability help to improve the cycle performance and mitigate voltage fading.The initial discharge capacity of Na₃V₂（PO₄）₂O₂F@C as cathode for sodium ion batteries is about127.9 mAh g^-11 at 0.1 C.Besides,a high rate performance with a capacity of 88.1 mAh g^-11 at 20 C has been achieved,and the capacity retains 82.1%after 2000 cycles.In addition,the reaction kinetics and Na⁺transportation mechanism of Na₃V₂（PO₄）₂O₂F@C are preliminarily investigated by the ex situ X-ray diffraction,X-ray photoelectron spectroscopy and galvanostatic intermittent titration technique.More interestingly,when coupled with Li,the fabricated hybrid Li/Na-ion batteries also exhibit excellent rate and cycling performances.The proposed rapid refluxing strategy to synthesize mulberry-shaped Na₃V₂（PO₄）₂O₂F@C opens up a new opportunity to develop high-performance electrode materials for the energy storage systems.In this paper,high-voltage phosphate cathode materials,including self-assembled LiCoPO₄/C nanoflakes,NASICON-structured Li₃V₂（PO₄）₃/C and hierarchical mulberry-shaped Na₃V₂（PO₄）₂O₂F@C were synthesized,and the composition-morphology-property relationships with corresponding mechanisms were investigated.Based on the research results,the following conclusions are acquired:（a）The precursors of the phosphate compounds with the hierarchical self-assembled structure can be obtained by the low-temperature refluxing strategy.The organic groups from the co-solvent system exhibit an important effect on the composition and morphology of the products.（b）Nano-sized primary particles effectively shortern ions diffusion distance in crystal materials and expanded the electrode-electrolyte contact area.（c）In the hierarchical configurations,the building blocks of primary nanoparticles could effectively avoid self-aggregation and structure degradation upon cycling.（d）The inter-particle pore network in the complex hierarchitectures could facilitate electrolyte penetration and simultaneously permit accormmodation of large volume variation during charging-discharging processes.（e）The homogeneous in-situ carbon-coating layer establishes an electronic conduction network for fast electron transport between the primary nanoparticles,and the carbon layer can also help to mitigate strain effects caused by volume change during Li⁺/Na⁺insertion/extraction processes,thereby improving cycle stability of the cathode.Notably,the low-temprature refluxing synthesis strategy to prepare phosphate cathode materials can provide a new opportunity for the development of high-performance electrode materials for the energy storage systems.