Modification Of Na₃V₂（PO₄）₃ Cathode For Sodium Ion Batteries

The lithium-ion batteries（LIBs）have been the major commercial energy device accounting for the lack of traditional energy systems.However,the soaring cost caused by the deficiency in lithium resources has greatly restricted its further research and applications.Simultaneously,the sodium ion batteries（SIBs）emerged as a great replace candidate due to the abundant sodium reserves and the similar electrochemical mechanism of rocking chair battery.Generally,cathodes provide many de/insertion ions and high-voltage redox pair so that directly decide the electrochemical performance of batteries.Among many kinds of cathode materials,Na₃V₂（PO₄）₃ have been greatly developed by its high stability and 3D open framework.However,the low intrinsic electronic and ionic conductivity have limited its property.Especially,the serious volume collapse caused by Na⁺de/intercalation seriously impacts the structure of NVP,leading to decrease of capacity.Thus,researcher adopted the strategies of“bulk phase doping”and“compositing carbon materials”to modify NVP to improve the property of NVP through the efficient Na⁺and electrons transport pathway.Specific research contents and achievements of this thesis are as follows:1.A series of K_0.1Na_2.9+xV_2-xCo_x（PO₄）₃ cathode materials with synergy modified by K/Co co-doping are produced via sol-gel method.Significantly,K⁺ions,partly occupy the Na1 sites,could support the crystal structure of cathode materials and expand the inner channels for Na⁺transport without the capacity loss of the reversible Na⁺de/insertion in Na2sites.Simultaneously,Co²⁺located in V³⁺sites could not only further increase the inner space for Na⁺migration,but also generate amount of hole carriers to improve the electronic conductivity of cathode materials.This article explores the influence of different doping amount on electrochemical performance,K_0.1Na_2.95V_1.95Co_0.05（PO₄）₃display with the best rate and cyclic property.Its initial capacity under 10 C is 98.8 m A h g^-1 and the corresponding retention with 400 cycles is high of 76%.2.K_0.1Na_2.95V_1.95Co_0.05（PO₄）₃/CNTs cathode materials which possess double-redox pairs(V³⁺/V⁴⁺and Co²⁺/Co³⁺)are obtained by the combined modified strategy of doping K/Co and compositing CNTs.Significantly,introducing K⁺and Co²⁺into the Na1 and V sites can efficiently stabilize the crystal structure and expand the channels for Na⁺transportation.The tubular CNTs are beneficial to restricting the growth of active grains,effectively downsizing the grain size and providing a shortened pathway for the electronic/ionic migration.Moreover,the amorphous carbon layers combining with the conductive CNTs construct a favorable network for the accelerated electronic transportation.Furthermore,the ex-situ XPS characterization reveals that an extra redox reaction pair of Co²⁺/Co³⁺is successfully activated at high voltage range,resulting in superior capacity and energy density property for K_0.1Na_2.95V_1.95Co_0.05（PO₄）₃/CNTs composite.Its capacities are 113.2 and 105.8 m A h g^-1 at5 and 10 C,maintaining capacities of 105.8 and 100.8 m A h g^-1after 500 cycles,means an outstanding retention of 96.6%and 95.3%.Notably,it releases a capacity of 99.8 and 84.5m A h g^-1 at 50 and 100 C and remains 66.2 and 58.8 m A h g^-1 after 2500 cycles.3.Zn-doped Na_3.07V_1.93Zn_0.07（PO₄）₃@CNFs cathode material with fiber morphology is prepared via electrospinning method.Specifically,Zn²⁺doping could support the crystal to improve structural stability and increase the inner space to promote Na⁺migration.And the carbon nanofibers（CNFs）produced by electrospinning have promoted the generation of nano-sized crystal grains,what’s more,the obtained material possess a special morphology similar to a pearl necklace.Simultaneously,MS simulation calculations have proved that the partial substitution of Zn in the V sites increases the density of states and reduces the energy barrier of Na⁺migration.The Na_3.07V_1.93Zn_0.07（PO₄）₃@CNFs electrode material exhibits excellent sodium storage property.Comprehensively,cathode material exhibited the retention of 91%and 90%for 400 cycles under 5 C and 10 C.Furthermore,under the rate of 50 C and100 C,the cathode could still deliver a reversible capacity of 93.1 and 92.3 m A h g^-1.