【作者】 张洁；

【导师】 杨军； 王久林；

【作者基本信息】 上海交通大学 ， 应用化学， 2015， 博士

【摘要】 锂离子电池以其高能量密度和长循环寿命等优点,已广泛应用于多种便携器件中。电动汽车和储能电站的开发和应用对未来二次电池提出了更高的要求,可归纳为“三高和两低”:高能量密度、高功率特性、高安全性能、低成本和低(无)污染。以钴酸锂为代表的传统正极材料,其可逆容量一般在200 mAh g-1以下,不能完全满足人们对高性能锂离子电池的迫切需求。因此,研究开发高容量、大倍率的正极材料对进一步提高锂离子的能量密度和功率密度至关重要。近年来,富锂锰基正极材料(简称富锂材料),因其高比容量(>250 mAh g-1)和低廉的价格受到广泛关注,被认为具有广泛应用前景的下一代锂离子电池正极材料之一。但是,富锂材料较大的首次不可逆容量损失、较差的循环和倍率性能以及电压衰减等问题阻碍了其应用进程。针对这些关键问题,本论文以Li1.2Ni0.13Mn0.54Co0.13O2(简写为LNMCO)材料为研究对象,从表面包覆、电解液成膜添加剂和复合尖晶石相等方面对富锂材料表面优化调控,提高其电化学性能;结合扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线衍射(XRD)、X射线光电子能谱(XPS)、电感耦合等离子体发射光谱(ICP)红外光谱(IR)、交流阻抗(EIS)和恒电流间歇滴定(GITT)等方法,详细研究了富锂材料表面组成、结构与电化学性能之间的关系。具体内容如下:1.在富锂材料表面包覆聚酰亚胺(PI)保护层。利用聚酰胺酸(PAA)与金属氧化物间较强的相互作用力在富锂材料表面包覆一层PAA,再经亚胺化热处理将PAA包覆层转变为PI。1.0 wt.%PAA溶液可在富锂材料表面均匀地包覆一层约3纳米的薄膜,而较高浓度PAA溶液(2.5 wt.%和5.0 wt.%)包覆的复合材料颗粒之间有片状高分子膜存在。450℃热亚胺化处理后,PI-LNMCO-450复合材料的红外光谱图中出现三个新峰且XPS结果显示复合材料中Mn的结合能降低,表明亚胺化热处理过程中LNMCO材料与包覆层之间存在电荷转移反应,即材料中+4价Mn吸收PI膜中C=O或苯环的电子,形成Mn(III)···O和Mn(III)···苯环相互作用。电化学测试表明,PI-LNMCO-450材料的循环性能和倍率性能均明显提高,其原因主要包括两方面:(1)PI包覆层将富锂材料和电解液隔离,有效地稳定了电极/电解液界面;(2)材料表面部分+4价锰被还原为+3价锰,有利于锂离子在材料表面迁移。2.采用三(三甲基硅烷)磷酸酯(TMSP)作为电解液添加剂。线性扫描伏安曲线显示,TMSP可先于碳酸酯电解液发生氧化分解反应,有效地提高了电解液在高电压(>4.5 V)下的稳定性。XPS和TEM结果表明,在充放电过程中TMSP参与在富锂材料表面形成稳定的SEI膜。在添加1.0 wt.%TMSP电解液中,LNMCO/Li电池表现出稳定的循环性能,50次循环容量保持率达90.8%。3.采用水合肼蒸汽处理富锂材料,调控颗粒表层结构。与高浓度水合肼溶液处理会严重破坏富锂材料的晶体结构相比,水合肼蒸汽处理相对比较温和,并且处理层厚度可控。ICP和XRD结果表明,通过Li+/H+交换反应,水合肼蒸汽可从LNMCO材料Li2MnO3组分中脱出部分锂。TEM照片显示,LNMCO-HV材料内部层状结构得以保持,表面形成一层约3纳米的贫锂层。与LNMCO材料相比,LNMCO-HV材料的首次充电4.5 V平台变短,放电容量和库仑效率均有所提高。300℃热处理可将贫锂层中H+脱出留下锂空位,过渡金属离子迁移并占据锂空位,从而在材料表面形成尖晶石Li1-xM2O4相。LNMCO-HV-300材料的首次放电容量和库仑效率分别高达295.6 mAh g-1和89.5%,其微分电容曲线上~2.8 V的还原峰,对应于锂离子嵌入到尖晶石相八面体16c位。4.探讨了LNMCO材料在钠离子电解液中的离子脱嵌反应。线性扫描伏安曲线显示,1 M NaClO4/PC电解液在4.0 V即开始发生轻微的氧化分解反应,4.75 V以后剧烈分解。在1.7~4.5 V电压范围内,LNMCO/Na电池的循环性能比较差,这与高电压循环时电解液不稳定有关。GITT分析可知,LNMCO/Na电池在首次充电过程中锂离子从富锂材料中脱出,而首次放电过程中则发生锂离子和钠离子共同嵌入。XRD结果表明,NMCO/Na电池首次放电后有富钠相生成,而LNMCO/Na电池,随着循环次数的增加逐渐生成富钠相。添加5.0 vol.%FEC,可以显著地提高1 M NaClO4/PC电解液在高电压下的稳定性。LNMCO/Na电池在该电解液中表现出稳定的循环性能。更多还原

【Abstract】 Rechargeable lithium-ion batteries have been widely used in portable devices due to their high energy density and long cycle life. Recently, they are considered as crucial power sources for electric vehicles(EVs) and green grid. These potential applications in the new energy fields put forward higher requirements for lithium ion batteries, summarized as "three high and two low" : high energy density, high power density, high safety, low cost and low(no) pollution. Traditional cathode materials, represented by lithium cobalt oxide, usually show reversible capacities of less than 200 mAh g-1, which cannot fully meet the demands of high-performance lithium ion batteries. Therefore, it is very important to develop high capacity and high rate cathode materials to improve the energy density and power density of lithium ion battery.Lithium-rich manganese-based materials have been currently investigated as promising cathode candidates for lithium ion batteries due to their high capacity(> 250 mAh g-1) and low cost. However, some notable drawbacks of these materials, such as the initial large irrevisible capacity loss, poor cyclic stability and rate capability, and severe voltage decay during cycling, hinder its practical applications. To overcome these shortcomings, several approaches including surface coating, film-forming electrolyte additive and modification of surface layer structure, were proposed in this thesis to improve the electrochemical performance of the Li1.2Ni0.13Mn0.54Co0.13O2(LNMCO) material. Combined with scanning electron microscope(SEM), transmission electron microscope(TEM), X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS), Inductively Coupled Plasma Optical Emission Spectrometer(ICP-OES), infrared spectroscopy(IR), electrochemical impedance spectroscopy(EIS) and galvanostatic intermittent titration(GITT) methods, the relationship between surface structure, composition and electrochemical performance was carefully studied. The detailed contents are summarized as follows:1. A polyimide(PI) protective layer was coated on the surface of LNMCO material. The polyamic acid(PAA) was used to coat on the surface of lithium rich material, then the PAA coating layer was transferred to PI by themal treatment. By 1.0wt.% PAA solution, a ~ 3 nm coating layer is evenly coated on the surface, while higher concentration solutions(2.5 wt.% and 5.0 wt.%) results in the existence of polymer membrane between composite particles. Three new peaks appear in the infrared spectrum of PI-LNMCO-450 material and the binding energy of Mn in PI-LNMCO-450 material is slight lower than that in LNMCO material, indicating the existence of charge transfer between LNMCO material and coating layer, Mn(IV) of LNMCO material absorbing electrons from C=O or phenyl of the PI coating layer and formation of the Mn(III)…O and Mn(III)…benzene interactions, during the thermal imidization process. Electrochemical performances including cycling stability and rate capability are evidently improved by the PI coating layer. PI coating layer separates cathode material from electrolyte and effectively stabilizes the electrode/electrolyte interface at high voltage. Moreover, the reduction of partial tetravalent manganese to trivalent manganese is benefical to lithium ion migration in the material surface.2. Tris(trimethylsilyl)phosphate(TMSP) was investigated as a electrolyte additive. Linear sweep voltammetry results show that the decomposition voltage of TMSP additive is lower than that of reference electrolyte. And the high voltage stability of reference electrolyte is greatly improved by the addition of TMSP. TEM and XPS results clearly illustrate the partcipation of TMSP additive in the formation of a stable solid electrolyte interphase(SEI) layer on the LNMCO material surface. The LNMCO/Li cell cycled in 1.0 wt.% TMSP-containing electrolyte demonstrates a stable cycling performance with a capacity retention of 90.8% after 50 cycles.3. The LNMCO material was treated by hydrazine vapor to modify the surface structure of the LNMCO particle. Differing to the treament by high concentration hydrazine hydrate solution which causes serious structure damage, the hydrazine vapor treatment is relatively mild and the thickness of treatment layer is controllable. The ICP and XRD results indicate that lithium ions are leached out from the Li2MnO3 component of LNMCO through Li+/H+ exchange process by the hydrazine vapor. TEM images of LNMCO-HV material indicate that the inner particle maintains the layered structure, while a ~3 nm Li-deficient layer forms on the surface. Compared with LNMCO material, the discharge capacity and coulombic efficiency of LNMCO-HV material are enhanced with a shorter plateau at ~4.5 V plateau in the initial charge process. After the thermal treament at 300 oC, the removal of the incorporated protons and migration of transition metal ions into the lithium vacancy result in the formation of spinel Li1-xM2O4 phase on the surface of LNMCO-HV-300. The LNMCO-HV-300 sample exhibits high initial discharge specific capacity(295.6 mAh g-1) and coulombic efficiency(89.5 %) with an obvious 2.8 V cathodic peak in the corresponding dQ/d V curve, which is a characteristic of spinel-like phase corresponding to the insertion of Li+ into the empty 16 c octahedral sites.4. The ionic intercalation reaction of LNMCO electrode in Na-ion electrolyte was initially incestigated. Linear sweep voltammetric curve shows that the sodium ion electrolyte(1 M NaClO4/PC) starts to slightly decompose at 4.0V, and violently over 4.75 V. The specific capacity of LNMCO decreases significantly when cycling in the voltage range of 1.7~4.5 V, due to the decomposition of electrolyte at high voltage. From the GITT results, lithium ions are leached out from the LNMCO material in the initial charge process of LNMCO/Na cell, and both sodium and lithium ions are inserted during the following discharge process. The XRD results indicate that a Na-excess phase is formed after the initial discharge of NMCO/Na cell, while in the LNMCO/Na cell, the Na-excess phase is gradually formed during cycling. Adding 5.0 vol.% FEC, the stability of 1 M NaClO4/PC electrolyte is greatly improved. Thus, LNMCO/Na cell exhibits a stable cycling performance.更多还原