【作者】 史洁；

【导师】 李平；

【作者基本信息】 北京科技大学 ， 材料科学与工程， 2023， 博士

【摘要】 开发具有高安全性、高能量密度、长循环寿命的全固态锂电池已成为电池领域研究的热点。基于高比容量的富镍氧化物正极和高锂离子电导率的硫化物固态电解质组装的硫化物基全固态锂电池有望实现电池性能的提升。但正极与硫化物固体电解质之间存在的空间电荷层效应、界面副反应及物理接触失效等问题制约其发展应用。包覆、掺杂改性等策略可提高界面稳定性,但高性能低成本的涂层材料及制备工艺有待探究。本论文针对硫化物全固态锂电池中的正极界面问题展开研究,从材料合成设计角度,探索新型涂层材料及低成本、高效的制备工艺,调控成分比例、剖析合成机制,提出定制化的正极协同共改性策略,实现高性能硫化物全固态锂电池的同时阐明其改性作用机理。具体研究内容如下:(1)在材料制备前端加入过量锂源,调控原材料比例,提出并采用原位固相烧结法,一步合成正极材料LiNi0.8Co0.1Mn0.1O2(NCM811)的同时实现了薄且均匀Li2O涂层的原位包覆。该Li2O材料作为涂层成分被提出,因其固有的点缺陷而具有传输Li+的能力。Li2O涂层与正极和硫化物固态电解质Li6PS5Cl(LPSC1)双向兼容且稳定存在,降低了电极极化和界面电阻的同时提高了界面Li+的传输动力学,进而提升了硫化物全固态锂电池的电化学性能。通过调控包覆层厚度(最佳为2～4nm),获得最优性能为:室温0.1C放电比容量为189.66 mAh g-1,首效81.26%,1C可稳定循环2584圈,容量保持率为81.47%,即每圈平均容量衰减率仅为0.0073%。(2)为进一步设计合成高性能的界面改性材料,提高硫化物全固态锂电池的首效和长循环稳定性,在制备Li2O涂层的基础上,添加钽源Ta2O5,调控成分比例,合成了一种新型Li7TaO6涂层原位包覆及Ta5+梯度掺杂的核壳结构正极材料。结合理论计算,研究该涂层对界面反应能垒的影响。通过电池失效测试分析,阐述协同共改性机制对电化学性能提升的作用机理。分析表明Li7TaO6缓冲层可抑制界面化学/电化学副反应,减少电极极化,提高锂离子扩散系数;Ta5+掺杂扩大正极基体层间距,降低Li+/Ni2+混排度,增强H2→H3相变的可逆性,有助于硫化物全固态锂电池长循环稳定性的提升。全电池0.1C初始放电比容量为203 mAhg-1,首效高达85.42%,1C稳定循环长达8200圈,容量保持率为57.1%,每圈的容量衰减率低至0.0053%。(3)针对硫化物全固态锂电池在超高电压和极端温度条件下的正极界面接触物理失效问题,合理设计涂层成分及掺杂元素,将Li2O与低成本BPO4结合,原位一步固相烧结制备了 Li3PO4涂层和B/P梯度共掺杂的NCM811正极材料。通过构筑包覆-掺杂共改性界面工程,测试分析改性对全电池高电压性能的影响,阐明改善界面稳定性的作用机理。Li3PO4涂层减少界面阴阳离子的交换的同时避免电解质氧化分解,有效降低了界面阻抗。P5+以磷氧四面体的形式掺杂到基体内部,扩大了层间距,提高了 Li+的输运。B3+和P5+共掺杂通过锚定正极在高电压过度脱锂态下易逸出的晶格O,提高正极层状结构的稳定性,减少微裂纹的产生。通过材料微观结构及涂层设计改性显著提高了全电池高电压电化学性能,4.8 V(vs.Li+/Li)下具有208.26 mAh g-1初始放电比容量,1C可稳定循环1986圈,每圈容量衰减率为0.02%,-20℃/60℃温度下同样具有良好的循环稳定性。更多还原

【Abstract】 The development of all-solid-state lithium batteries(ASSLBs)with high safety,high energy density and long cycle life has become a hot spot in the field of batteries.Sulfide-based ASSLBs based on high specific capacity nickel-rich oxide cathodes and sulfide solid electrolytes(SEs)with high lithium-ion conductivity are expected to improve battery performance.However,the spatial charge layer effect,interfacial side reactions and physical contact failure between the cathodes and SEs restrict its development and application.Strategies such as coating and doping modification can improve the interface stability,but their high-performance and low-cost coating materials and preparation processes need to be explored.In this paper,we study the cathode interface problem in sulfide-based ASSLBs,explore new coating materials and low-cost and efficient preparation processes from the perspective of material synthesis design,adjust the composition ratio,analyze the synthesis mechanism,and propose a customized cathode synergistic co-modification strategy to realize high-performance sulfide-based ASSLBs and clarify their modification mechanism.The specific research contents are as follows:(1)In the front end of material preparation,an excess lithium source is added,the proportion of raw materials is adjusted,and the in-situ solid-phase sintering method is proposed and used to synthesize the cathodes LiNi0.8Co0.1Mn0.1O2(NCM811)in one step,and the in-situ coating of the thin and uniform Li2O coating is realized.This Li2O material is proposed as a coating component with the ability to transport Li+due to its inherent point defects.The Li2O coating is bidirectionally compatible with the cathodes and Li6PS5Cl(LPSCl),which reduces the electrode polarization and interfacial resistance and improves the transport kinetics of the interfacial Li+,thereby improving the electrochemical performance of the sulfidebased ASSLBs.By adjusting the buffer layer thickness(the best is 2～4 nm),the optimal performance is obtained:the 0.1C discharge specific capacity is 189.66 mAh g-1,the initial Coulomb Efficiency(CE)is 81.26%.The 1C large rate can stabilize 2584 cycles with capacity retention rate of 81.47%,and the average capacity decay rate per cycle is 0.0073%.(2)In order to further design and synthesize high-performance interface modified materials and improve the CE and long-cycle stability of sulfide-based ASSLBs,a new type of core-shell cathodes with in-situ Li7TaO6 coating and Ta5+gradient doping are synthesized on the basis of the Li2O coating preparation,the addition of tantalum source Ta2O5 and the adjustment of the composition ratio.Combined with theoretical calculations,the effect of the coating on the energy barrier of the interfacial reaction is studied.Through the batteries failure test and analysis,the mechanism of co-modification mechanism on the improvement of performance is illegitated.The results show that the Li7TaO6 buffer layer can inhibit the interfacial side reactions,reduce the polarization of the electrode,and improve the Li+ diffusion coefficient.Ta5+ doping expands the cathodes layer spacing,reduces the degree of Li+/Ni2+ mixing,and enhances the H2→H3 phase transition reversibility,which is conducive to the improvement of the sulfide-based ASSLBs stability.The 0.1C initial discharge specific capacity is 203 mAh g-1,the CE is as high as 85.42%,the 1C stable cycle is as long as 8200 cycles,the capacity retention rate is 57.1%,and the capacity attenuation rate per cycle is as low as 0.0053%.(3)Aiming at the interface problem of the ASSLBs under ultra-high voltage and extreme temperature conditions,the coating composition and doping elements are reasonably designed,and the Li2O and low-cost BPO4 are combined with insitu solid-state sintering to prepare the Li3PO4 coating and B/P gradient co-doped NCM811 cathodes.By constructing a coating-doping co-modified interface engineering,the effect of modification on the high-voltage performance of the cell is tested and analyzed,and the mechanism of improving the interface stability is clarified.The Li3PO4 coating can reduce the interface anion exchange,avoid the SEs oxidation decomposition,which effectively reduces the impedance of the interface.P5+is doped into the matrix in the form of phosphorus tetrahedron,which expands the layer spacing and improves the Li+ transport of.B3+ and P5+ co-doping improves the cathode stability and reduces the generation of microcracks by anchoring the lattice O that is easy to escape from the cathode under high voltage and excessive delithiumization.The cell high-voltage performance is significantly improved by the modification of the material microstructure and coating design,and it have an initial discharge specific capacity of 208.26 mAh g-1 at 4.8 V(vs.Li+/Li),1986 cycles at 1C,with a capacity attenuation rate of 0.02%per cycle,and good cycle stability at-20℃/60℃.更多还原