【作者】 张海霞；

【导师】 段小川； 张鼎；

【作者基本信息】 太原理工大学 ， 化学工程与技术， 2024， 硕士

【摘要】 锂离子电池（LIBs）因其优异的能量密度和循环寿命被广泛应用于储能领域。然而,随着大规模储能需求不断增加,锂资源储量有限、地域分布不均等问题日益凸显。因此,寻找LIBs替代方案迫在眉睫。在众多储能器件中,钠离子电池（SIBs）由于其广泛分布的低成本钠资源和与LIBs相似的工作机制被认为是最有潜力的候选者之一。但目前SIBs的能量密度仍无法满足大规模储能要求,而正极材料是影响SIBs能量密度的重要因素,因此开发高能量密度正极材料是解决上述问题的关键。在众多SIBs正极材料中,锂锰基阴离子Redox型氧化物材料通过引入阴离子氧化还原过程以提供额外容量,是提高SIBs能量密度的可行方法。然而,由于锂锰基阴离子Redox型材料常在低电压下发生与Mn³⁺相关的Jahn-Teller畸变,不利于结构稳定。且在充电过程中没有足够的电化学活性元素进行电荷补偿,易发生严重不可逆的阴离子（氧）氧化还原反应过程以补偿电荷损失。此外,阴离子氧化还原所需的高电压会促进有害相变和不可避免的副反应发生,导致循环过程中容量快速衰减和严重电压衰减,这对电极过程极为不利。因此,针对上述问题,本论文以典型的锂锰基阴离子Redox型氧化物正极材料Na_0.75Li_0.25Mn_0.75O₂为研究对象,采用元素掺杂和相调控方法提高材料电化学过程稳定性。本论文主要研究内容如下:首先,采用溶胶凝胶法结合高温烧结过程制备了Na_0.75Li_0.25Mn_0.75O₂材料和一系列不同含量Cu掺杂的Na_0.75Li_0.25-2/3xMn_0.75-1/3xCu_xO₂（x=0、0.06、0.09和0.12）正极材料,尝试改善Na_0.75Li_0.25Mn_0.75O₂正极材料在充电过程中的相变、Mn³⁺相关的Jahn-Teller畸变和稳定阴离子氧化还原过程以实现高性能钠离子电池。实验结果表明,900℃条件下制备的P2-Na_0.75Li_0.25Mn_0.75O₂材料在0.1 C下,放电比容量为207.3m Ah g^-1。在引入Cu后,材料仍保持P2型结构,且当Cu掺杂量为0.09时,Na_0.75Li_0.19Cu_0.09Mn_0.72O₂材料获得了优异的电化学性能。在0.1 C时,放电比容量达到194.9 m Ah g^-1,在5 C的电流密度下,放电比容量仍高达118 m Ah g^-1。经过100次循环后,容量保持率可达到88.6%,且放电电压提升至≈2.81 V。通过Cu的引入,减缓了与Mn³⁺相关的Jahn-Teller畸变效应,抑制了高电压下与O₂释放相关的不可逆的阴离子氧化还原过程。此外,掺杂Cu后,材料的氧化还原动力学也得到了增强。Na_0.75Li_0.19Cu_0.09Mn_0.72O₂材料最终实现了高能量密度、稳定且无相变的电化学过程。其次,采用溶胶凝胶法合成了Na_0.75Li_0.25Mn_0.75O₂材料和一系列Cu、Ni、Fe、Co、Ti多元素掺杂的Na_0.75Li_0.05Cu_0.05Ni_0.1Fe_0.3-yCo_yMn_0.5-xTi_xO₂（x=0.025,0.05,0.1;y=0.1,0.15,0.2）正极材料。通过在P2-Na_0.75Li_0.25Mn_0.75O₂材料中掺入Cu、Ni、Fe、Co、Ti多种元素,形成P2/O3双相结构。并通过调控Ti和Co的含量,实现P2/O3相比例调控,以抑制Na_0.75Li_0.25Mn_0.75O₂材料相变和不可逆阴离子氧化还原过程。实验结果表明,当调控P2相比例为77.66%时,Na_0.75Li_0.05Cu_0.05Ni_0.1Fe_0.15Co_0.15Mn_0.45Ti_0.05O₂正极材料表现出优异的电化学性能。在0.1 C时,放电比容量高达203.7 m Ah g^-1,在5 C的电流密度下,放电比容量仍高达115.8 m Ah g^-1。且放电电压提升至≈3.1 V,有利于全电池能量密度的提升。通过电化学活性元素的引入,一定程度抑制Mn活性,从而减少Mn³⁺Jahn-Teller效应产生的不利影响。通过调控P2/O3双相结构,抑制了Na_0.75Li_0.25Mn_0.75O₂材料的相变过程和不可逆的阴离子氧化还原过程。显著提高了Na⁺扩散动力学,减小了电化学阻抗。Na_0.75Li_0.05Cu_0.05Ni_0.1Fe_0.15Co_0.15Mn_0.45Ti_0.05O₂正极材料与硬碳匹配的全电池,在0.1 C下可逆放电比容量为133 m Ah g^-1,循环200次后的容量保持率为75.2%,展示了良好的实用性。更多还原

【Abstract】 Lithium-ion batteries（LIBs）are widely used in energy storage due to their excellent energy density and cycle life.However,with the increasing demand for large-scale energy storage,the problems of limited lithium resource reserves and uneven geographical distribution are becoming more and more prominent.Therefore,the search for an alternative to LIBs is imminent.Among many energy storage devices,sodium-ion batteries（SIBs）are considered as one of the most promising candidates due to their widely distributed low-cost sodium resources and similar working mechanisms as LIBs.However,the energy density of SIBs is still unable to meet the requirements of large-scale energy storage,and the cathode material is an important factor affecting the energy density of SIBs,so the development of high-energy-density cathode materials is the key to solving the above problems.Among many cathode materials for SIBs,Li/Mn-based anionic redox type oxide materials are a feasible way to increase the energy density of SIBs by introducing an anionic redox process to provide additional capacity.However,the Li/Mn-based anionic redox type materials are not conducive to structural stabilization due to the Jahn-Teller distortion associated with Mn³⁺that often occurs at low voltages.Moreover,there are not enough electrochemically active elements for charge compensation during the charging process,which is prone to severe irreversible anionic（oxygen）redox reaction processes to compensate for the charge loss.In addition,the high voltage required for anion redox promotes the occurrence of deleterious phase transitions and unavoidable side reactions,leading to rapid capacity decay and severe voltage degradation during cycling,which is extremely detrimental to the electrode process.To deal with the above problems,this work takes a typical Li/Mn-based anionic redox type oxide cathode material Na_0.75Li_0.25Mn_0.75O₂as the research object,and adopts elemental doping and phase regulation methods to improve the stability of the material’s electrochemical process.The main research content of this thesis is as follows:Firstly,Na_0.75Li_0.25Mn_0.75O₂materials and a series of Cu-doped Na_0.75Li_0.25-2/3xMn_0.75-1/3xCu_xO₂（x=0、0.06、0.09 and 0.12）cathode materials with different contents were prepared by sol-gel method combined with high-temperature sintering process,in an attempt to improve the phase transition of cathode materials during charging,the Mn³⁺-related Jahn-Teller aberration,and stabilize anionic redox process to achieve a high-performance sodium-ion battery.The experimental results show that the P2-Na_0.75Li_0.25Mn_0.75O₂material prepared at 900℃has a specific capacity of discharge of 207.3 m Ah g^-1at 0.1 C.After the introduction of Cu,the material still maintains the P2-type structure,and when the Cu doping amount is 0.09,the Na_0.75Li_0.19Cu_0.09Mn_0.72O₂material obtains the excellent electrochemical performance.The discharge specific capacity reaches 194.9 m Ah g^-1at 0.1 C.At a current density of 5C,the discharge specific capacity is still up to 118 m Ah g^-1.The capacity retention rate reaches88.6%after 100 cycles,and the discharge voltage is increased to≈2.81 V.The Jahn-Teller aberration effect associated with Mn³⁺was mitigated by the introduction of Cu and the irreversible anionic redox process associated with oxygen release at high voltage was suppressed.In addition,Cu doping enhanced the redox kinetics of the material.The Na_0.75Li_0.19Cu_0.09Mn_0.72O₂material finally achieved high energy density,stable and no phase transition process.Secondly,Na_0.75Li_0.25Mn_0.75O₂materials and a series of Cu,Ni,Fe,Co,Ti multi-element doped Na_0.75Li_0.05Cu_0.05Ni_0.1Fe_0.3-yCo_yMn_0.5-xTi_xO₂（x=0.025,0.05,0.1;y=0.1,0.15,and 0.2）Positive electrode material.The P2/O3 biphasic structure was formed by doping various elements of Cu,Ni,Fe,Co,and Ti into the P2-Na_0.75Li_0.25Mn_0.75O₂material.And by regulating the content of Ti and Co,the phase ratio regulation was realized,for suppressing the phase transition and irreversible anion redox process of Na_0.75Li_0.25Mn_0.75O₂material.The experimental results showed that the Na_0.75Li_0.05Cu_0.05Ni_0.1Fe_0.15Co_0.15Mn_0.45Ti_0.05O₂cathode material exhibited excellent electrochemical performance when the P2 phase accounted for77.66%.The material has a discharge specific capacity of 203.7 m Ah g^-1at 0.1 C,and at a current density of 5 C,the discharge specific capacity still reaches 115.8 m Ah g^-1.And the discharge voltage is increased to 3.1 V,which is favorable for the full cell energy density.By introducing electrochemically active elements,the Mn activity is suppressed to a certain extent,which reduces the unfavorable effects produced by the Mn³⁺Jahn-Teller effect.The phase transition process and over-oxidation of anions in the Na_0.75Li_0.25Mn_0.75O₂material were suppressed by the introduction of a P2/O3 duplex.The Na⁺diffusion kinetics was significantly improved and the electrochemical impedance was reduced.Na_0.75Li_0.05Cu_0.05Ni_0.1Fe_0.15Co_0.15Mn_0.45Ti_0.05O₂cathode material matched with hard carbon demonstrated the practicality of the full cell with a reversible discharge specific capacity of133 m Ah g^-1at 0.1C and a capacity retention of 75.2%after 200 cycles.更多还原