Na₃V₂（PO₄）₃@C正极材料的结构优化与储钠机理研究

【作者】 刘小红；

【导师】 钟本和；

【作者基本信息】 四川大学 ， 化学工艺， 2021， 博士

【摘要】 清洁能源的使用是解决当前能源危机和环境问题的可行方案,而这些可持续资源的使用与大规模储能技术密切相关。与有限的锂资源相比,钠离子电池因资源和成本优势在规模储能领域应用前景明朗。然而,受限于钠离子本身较大的原子质量和半径,脱嵌型正极材料的能量密度和循环稳定性并不理想。聚阴离子系化合物由于阴离子基团的强诱导效应和稳定性通常表现出较高的电压平台和良好的热力学稳定性。快离子导体（NASICON）型Na₃V₂（PO₄）₃因其离子扩散通道宽、热力学稳定性优异和能量密度高等优势,吸引了研究者的广泛关注,但其低电子导电率问题严重阻碍了它的实际应用。本文以Na₃V₂（PO₄）₃作为研究对象,通过合成工艺优化、碳包覆改性和体相掺杂等策略完善其电化学性能。主要研究内容及研究结论如下:（1）研究钠盐种类和烧结温度对Na₃V₂（PO₄）₃的影响。选择不同钠盐（氢氧化钠和碳酸钠）通过固相法合成Na₃V₂（PO₄）₃@C材料,对比分析不同钠盐制备Na₃V₂（PO₄）₃@C的结晶性、颗粒形貌和电化学性质。研究结果表明以两种钠盐为原料均可合成磷酸钒钠,但由于前驱混合物中不同原料之间存在的反应,导致Na₃V₂（PO₄）₃@C的形貌和性能有所差异。其中以氢氧化钠为原料制备的Na₃V₂（PO₄）₃@C团聚严重,在20 C下容量为0。而以碳酸钠为原料制备的材料尺寸较小,且表面有约8 nm的碳包覆层,具有较高的比容量和较为稳定的循环寿命:在20 C下的容量为71.6 mAh g^-1和5 C下循环300周后的容量保持率为93.5%。在择定碳酸钠为优选钠盐的基础上,研究煅烧温度（650℃、750℃和850℃）对Na₃V₂（PO₄）₃@C的影响。发现伴随煅烧温度的提升,材料的结晶性有所提升,同时颗粒的团聚行为加剧。合适的煅烧温度应同时得到结晶性良好且颗粒尺寸较小的产品。综合电化学测试结果分析,750℃为优化煅烧温度。（2）为了进一步降低原料成本,以廉价易得的糖类化合物为碳源,对比不同聚合度碳源（葡萄糖、蔗糖和可溶性淀粉）对所得Na₃V₂（PO₄）₃@C的影响。研究表明有机物裂解的碳基质对Na₃V₂（PO₄）₃的结晶性影响不大,Na₃V₂（PO₄）₃纳米颗粒会镶嵌在形成的碳网络中。不同聚合度糖类化合物通过影响碳复合形式而影响所得材料性能,其中高聚合度的化合物由于本征的连接属性容易形成整体导电性较高的碳基质。以葡萄糖作为碳源容易获得单颗粒碳包覆的纳米粒子;以蔗糖作为碳源倾向于获得片状颗粒;而以可溶性淀粉作为碳包覆原料,能够在机械混合阶段在前驱体颗粒表面形成膜状物质,在后续高温煅烧过程中形成电子导电率较高的均匀碳包覆层,有效抑制颗粒在高温烧结过程中的晶体生长和缓冲钠离子脱嵌带来的体积应变。半电池和全电池性能测试结果表明,随着糖类化合物聚合度的提升,电化学性能逐渐增加。其中以可溶性淀粉合成的Na₃V₂（PO₄）₃@C表现出最优的储钠性能:40 C下具有72 m A h g^-1的放电比容量,1 C下循环1000周后可以保持初始容量的82.8%,与硬碳匹配的全电池在1 C下循环100周后容量留存率为80.1%。钠离子扩散动力学分析进一步佐证了这一规律,并且该类材料的电化学反应受扩散过程控制。（3）在通过碳包覆改善材料导电性的基础上,引入高价态Nb⁵⁺和碳包覆协同改性。采用溶胶凝胶法制备Na₃V_2-xNb_x（PO₄）₃@C（x=0、0.1、0.2、0.3）,系统研究Nb⁵⁺掺杂对晶体结构、元素价态组成、颗粒形貌、电化学性能以及钠离子扩散动力学的影响规律。由于Nb⁵⁺较小的离子半径和高价态Na₃V_2-xNb_x（PO₄）₃@C的晶胞参数略有减小并且V³⁺的含量由于电荷平衡的结果有所增加。此外,Nb的引入会导致颗粒尺寸略微减小,Na₃V_1.8Nb_0.2（PO₄）₃@C表现出较大的比表面积(34.62 m² g^-1),并且颗粒表面带有一层均匀的碳包覆层。电化学测试结果表明,由于引入的Nb为电化学惰性元素,可在结构中起着支撑性原子的作用。Na₃V_2-xNb_x（PO₄）₃@C样品随着Nb掺杂量的增加,首周放电比容量逐渐下降,但在高电流密度下的倍率性能和循环稳定性逐渐提高,其中Na₃V_1.8Nb_0.2（PO₄）₃@C样品在50 C下表现出81.6 m A h g^-1的比容量。扩散动力学分析表明优化后的Na₃V_1.8Nb_0.2（PO₄）₃@C材料具有较高的钠离子扩散系数,同时该类材料的电化学反应过程受扩散行为控制,改性后样品具有较高的赝电容贡献值。（4）在单元素掺杂改性Na₃V₂（PO₄）₃@C的基础上,同时采用结构调控、形貌设计和碳层石墨化程度调节三种方法进行改性。以草酸亚铁作为掺杂源设计并合成了Na₃V_2-xFe_x（PO₄）₃@C（x=0、0.05、0.15、0.25）。材料为多孔结构,且表现出较好的导电性和电化学性能。这主要源于Fe C₂O₄的分解在一定范围内可以诱导多孔结构的产生,且铁掺杂改性后的样品颗粒尺寸明显减小。固体核磁结果表明部分铁元素进入晶体结构内部,打破原来规则的原子排布情况;部分铁元素进入外部碳化层,调节外层石墨化程度。经验公式和DFT理论计算证实铁元素应占据晶格中的过渡金属位点并调节体相结构,改性后样品电子传输性能提升。半电池和全电池测试结果表明,铁掺杂的Na₃V_1.85Fe_0.15（PO₄）₃@C表现出不错的倍率性能(20 C的电流密度下表现出94.05 mAh g^-1的比容量),循环寿命（在1 C下循环1200周后的容量保持率为91.52%）和全电池初始容量(0.1C下的首周容量为105.07 mAh g^-1)。充放至不同电下的XRD结果证实电化学过程完全可逆,循环200周后,材料的晶体结构和多孔结构仍旧可以保持;钠离子扩散动力学测证实铁掺杂样品具有较小的电荷转移阻抗值、较高的钠离子扩散系数和赝电容贡献值,并且钠离子扩散能垒下降。综上,本文以钠离子电池Na₃V₂（PO₄）₃@C正极材料作为研究对象,针对材料的基础合成和碳复合、离子掺杂、形貌调控等多种改性策略开展逐级递进式研究,涉及基础合成工艺的认知、碳复合材料的优化设计、离子掺杂的基础改性原理和多重效应合一的协同设计,对聚阴离子化合物正极材料的设计和发展具有指导意义。更多还原

【Abstract】 Utilization of clean energy is a feasible solution to the current energy crisis and environmental problems,and the use of these sustainable resources is closely related to large-scale energy storage technology.Compared with limited lithium resources,sodium-ion batteries have a brighter application prospect in the field of large-scale energy storage due to their resource and cost advantages.However,due to the large atomic mass and radius of sodium ions,the energy density and cycle stability of the deintercalation type cathode material are not ideal.Polyanionic compounds usually show a higher voltage plateau and good thermodynamic stability due to the strong induction effect and stability of the anionic group.The fast ion conductor NASICON Na₃V₂（PO₄）₃ has attracted wide attention of researchers due to its wide ion diffusion channel,excellent thermodynamic stability,and high energy density.However,its low electronic conductivity has seriously hindered its practical application.In this thesis,NASICON-type Na₃V₂（PO₄）₃ is selected as the research object,and the electrochemical performance of Na₃V₂（PO₄）₃ is improved by optimizing synthesis parameters,carbon modification and bulk doping.The main research content and conclusions are as follows:（1）Study the influence of sodium salt type and sintering temperature on properties of Na₃V₂（PO₄）₃.Choose different sodium salts（sodium hydroxide and sodium carbonate）to synthesize Na₃V₂（PO₄）₃@C by solid-phase method,and compare and analyze the crystallinity,morphology evolution and electrochemistry performance of Na₃V₂（PO₄）₃@C prepared from different sodium salts.The relevant research results show that sodium vanadium phosphate can be obtained from both sodium salts as raw materials,owing to the reaction among the different raw materials,inducing the morphology and performance difference of Na₃V₂（PO₄）₃@C products.Among them,Na₃V₂（PO₄）₃@C prepared with sodium hydroxide as the raw material display serious agglomeration and there is no capacity observed at 20 C,while the material prepared with sodium carbonate as the raw material has a smaller particle size and a carbon coating layer of about 8nm on the surface.Compared with the sodium storage performance,Na₃V₂（PO₄）₃@C synthesized with sodium carbonate as raw material has higher specific capacity(the capacity at 20 C is 71.6 mAh g^-1)and better cycling stability（the capacity retention rate cycled at 5C after 300 cycles of circulation is 93.5%）.Based on choosing sodium carbonate as the optimized sodium salt,the influence of calcination temperature（650℃,750℃and 850℃）on the physical and chemical properties and electrochemical properties of Na₃V₂（PO₄）₃@C was studied.It could be found that with the increase of the calcination temperature,the crystallinity of the material is improved,and the agglomeration behavior of the particles is intensified.Appropriate calcination temperature should simultaneously obtain products with good crystallinity and smaller particle size.Comprehensive analysis of electrochemical test results,750℃is the optimized calcination temperature.（2）In order to further reduce the cost,cheap and easily available carbohydrates were used as carbon sources to compare the effects of carbon sources with different polymerization degrees（glucose,sucrose and soluble starch）on the resulting Na₃V₂（PO₄）₃@C.Studies have shown that the carbon matrix of organic matter cracking has little effect on the crystallization of Na₃V₂（PO₄）₃,and sodium vanadium phosphate nanoparticles will be embedded in the formed carbon network.Carbohydrate compounds with different degrees of polymerization affect the final performance by affecting the form of carbon composite,and polymers originated from its intrinsic combination properties tends to obtain high conductive carbon matrix in the whole integrity.It is easy to obtain single-particle carbon-coated nanoparticles with glucose as the carbon source,and sucrose as the carbon source tends to obtain flaky particles,and soluble starch as the carbon-coated raw material can form a film on the surface of the precursor particles during the mechanical mixing stage.In the subsequent high-temperature sintering process,a uniform carbon coating layer with higher electronic conductivity is formed,which effectively inhibits the crystal growth of particles during the high-temperature sintering process and buffers the volumetric strain caused by the deintercalation of sodium ions.Half-cell and full-cell performance test results show that as the degree of polymerization of sugar compounds increases,the electrochemical performance gradually increases.Among them,Na₃V₂（PO₄）₃@C synthesized from soluble starch shows the best sodium storage performance:a specific discharge capacity of 72 m A h g^-1 at 40 C,and can maintain 82.8%of initial capacity after 1000 cycles at 1 C.And the capacity retention rate of a full battery matched with hard carbon is 80.1%after 100 cycles at 1 C.The analysis of sodium ion diffusion kinetics further supports this rule,and the electrochemical reaction of these type of materials is controlled by the diffusion process.（3）On the basis of improving the conductivity of the material through carbon coating,this section introduces the synergistic modification of high-valence Nb doping and carbon coating.Na₃V_2-xNb_x（PO₄）₃@C（x=0,0.1,0.2,0.3）was prepared by sol-gel method,and the effect of Nb⁵⁺doping on the crystal structure,element valence state composition,particle morphology,electrochemical properties,and sodium transfer dynamics were systematically studied.The unit cell parameters of Na₃V_2-xNb_x（PO₄）₃@C are slightly reduced and the content of V³⁺increases due to the result of charge balance due to the smaller ion radius and high valence of Nb⁵⁺.In addition,the introduction of Nb will slightly reduce the particle size,resulting in Na₃V_1.8Nb_0.2（PO₄）₃@C shows a large specific surface area(34.62 m² g^-1).The electrochemical test results show that,since the introduced Nb is an electrochemically inert element,it could act as a pillar atom in the structure.The Na₃V_2-xNb_x（PO₄）₃@C sample with the increase of Nb doping,the initial discharge specific capacity gradually decreased,but the rate performance and cycle stability under high current density gradually improved,of which Na₃V_1.8Nb_0.2（PO₄）₃@C sample showed a specific capacity of 81.6 m A h g^-1 at50 C.Diffusion kinetic analysis shows that the optimized Na₃V_1.8Nb_0.2（PO₄）₃@C material has a high sodium ion diffusion coefficient,and the electrochemical reaction process of this type of material is controlled by the diffusion behavior,and the modified sample has a high contribution value of pseudocapacitance.（4）On the basis of the single element doping modification Na₃V₂（PO₄）₃@C in previous chapter,this part combines three strategies of structure modification,morphology design and carbon layer graphitization adjustment at the same time.Ferrous oxalate was used as the doping source to synthesize Na₃V_2-xFe_x（PO₄）₃@C（x=0,0.05,0.15,0.25）,which possesses porous structure,improved electrical conductivity and electrochemical performance.This is mainly because the decomposition of Fe C₂O₄ can induce the generation of porous structure within a certain range,and the particle size of the sample modified by iron doping is significantly reduced.The solid-state NMR results show that some iron atoms enter the crystal structure,breaking the original regular atomic and rearrangement;some iron atoms would diffuse into the outer carbonized layer to adjust the degree of graphitization.The empirical formula and DFT theoretical calculations confirmed that the iron element should occupy the transition metal sites in the crystal lattice and adjust the bulk structure and the electron transport performance after doping is improved.Half-cell and full-cell test results show that the iron-doped Na₃V_1.85Fe_0.15（PO₄）₃@C shows good rate performance(it shows a specific capacity of 94.05 mAh g^-1 at 20 C),improved cycling stability（the capacity retention rate is 91.52%after 1200 cycles at 1C）and high capacity of full cell(initial capacity at 0.1C is 105.07 mAh g^-1).XRD patterns under different various voltages in process of charging/discharging confirm that the electrochemical process is highly reversible.After cycling,the crystal structure and porous structure of the material can still be maintained.In addition,sodium ion diffusion kinetics test confirms that the iron-doped sample has a small charge transfer resistance value,higher sodium ion diffusion coefficient and pseudocapacitance contribution value,and decreased sodium ion diffusion energy barrier.In short,this thesis chosen Na₃V₂（PO₄）₃@C cathode material of sodium-ion battery as research object,and carries out gradual progress for the basic synthesis of materials and various modification strategies such as carbon recombination,ion doping,and morphology control.The research involves the knowledge of the basic synthesis process,the optimal design of carbon composite materials,the basic modification principle of ion doping and the coordinated design of multiple effects,which would have guiding significance for the design and development of polyanionic compound cathode materials.更多还原