【作者】 黄德权；

【导师】 王红强； 胡思江；

【作者基本信息】 广西师范大学 ， 物理化学， 2023， 博士

【摘要】 追求高能量密度是可充电电池的发展方向。锂离子电池具有能量密度高、安全性好等特点,广泛应用于便携式移动设备和新能源汽车等领域。目前,基于石墨负极的锂离子电池已无法满足人们对更高能量密度的要求。锂金属具有高的理论比容量(3860 m Ah g^-1)和最低的还原电位（相对于标准氢电极为-3.040 V）,是高能量密度可充电电池的理想负极。然而,锂金属电池在循环过程中存在严重的体积膨胀、界面副反应、锂枝晶生长等问题,导致电池库伦效率低、循环寿命短及安全隐患,阻碍了锂金属电池的大规模应用。针对以上问题,本论文通过构筑亲锂性界面结构和设计新型电解液等策略,获得了高库伦效率和长循环寿命的锂金属负极。主要研究内容及结论如下:（1）三维多孔铜网（Cu mesh）电极存在亲锂性差、Li⁺不均匀沉积等问题。通过化学镀方法,在铜网电极上均匀沉积银纳米粒子活性位点。亲锂性银纳米粒子可引导Li⁺在镀银铜网（Ag-Cu mesh）上均匀沉积,有效降低锂的成核过电位并抑制锂枝晶的生长,显著提高锂金属电池的库伦效率及循环寿命。通过原位X射线粉末衍射（XRD）和原位拉曼光谱等表征证实了Li⁺在镀银铜网电极上的均匀沉积过程。与未镀银铜网电极相比,镀银铜网电极在1.0 m A cm^-2电流密度下能实现150次稳定循环和98.6%的库伦效率,同时呈现出最小的极化电压。Ag-Cu mesh/Li||Ag-Cu mesh/Li对称电池在1.0 m A cm^-2电流密度下能实现超过1200h的循环寿命。在全电池测试中,Ag-Cu mesh/Li||Li Fe PO₄（LFP）和Ag-Cu mesh/Li||Li Ni_0.8Co_0.1Mn_0.1O₂（NCM811）全电池均表现优异的电化学性能,Ag-Cu mesh/Li||LFP在1.0 C循环300次后容量保持率为98.5%,Ag-Cu mesh/Li||NCM811在1.0C循环500次后容量保持率为84.8%。（2）锂金属负极在循环过程中存在锂枝晶生长和非活性锂堆积等问题。基于此,将适量的碘化钾（KI）功能添加剂引入到双（三氟甲基）磺酰亚胺锂（Li TFSI）/醚基电解液和六氟磷酸锂（Li PF₆）/酯基电解液中。低浓度下K⁺可自发附在锂金属负极的尖端处形成电荷屏蔽,引导Li⁺在远离尖端处沉积,有效抑制锂枝晶生长。同时,I^-/I₃^-的氧化还原可以快速激活锂金属电池在循环过程中所产生的非活性锂和氧化锂（Li₂O）。通过原位拉曼、X射线光电子能谱（XPS）等表征证实了Li⁺在镀银铜网电极上的均匀沉积过程以及电解液中I^-/I₃^-的存在。与未加KI添加剂的1 M Li TFSI基础电解液相比,Li||Ag-Cu mesh电池在1 M Li TFSI+0.01 M KI电解液中能实现200次稳定循环和98.5%的库伦效率。Li||Li对称电池在同样测试条件下能实现超过1400 h以上的稳定循环及最低的极化电压。全电池测试中,Ag-Cu mesh/Li||LFP全电池在1 M Li TFSI+0.01 M KI电解液中表现出优异的电化学性能,0.1 C的首次放电比容量为167.1 m Ah g^-1,在2.0 C下稳定循环800次仍然具有91.7%的容量保持率。在1 M Li PF₆+0.01 M KI电解液中,Li||NCM811全电池在0.1 C展现出高达203.9 m Ah g^-1的初始放电容量,1.0 C下循环500次后具有53.4%的容量保持率。（3）高浓度电解液存在生产成本高、离子导电性差、粘度大及润湿性差等问题。通过引入一种新型1,1,2,2-四氟乙基-4-甲基苯基醚（TFTT）氟化醚作为含氟稀释剂,设计了一种低浓度、高离子导电性及润湿性的新型局部高浓度电解液。一方面,TFTT的加入可以较好保留高浓度电解液中的溶剂化结构,增强Li⁺-FSI^-离子对之间的相互作用。另一方面,TFTT可以降低电解液的浓度、粘度及成本,提高电解液的电导率。此外,低LUMO能级的TFTT及强相互作用的Li⁺-FSI^-可以促进形成富含氟化锂（Li F）的固体电解质界面（SEI）膜,有效抑制锂枝晶生长。在LHCE-3电解液中,Cu||Li半电池在0.5 m A cm^-2和1.0 m A cm^-2的电流密度下分别实现近700次循环及99.1%高库伦效率和600次循环及98.8%高库伦效率。Li||Li对称电池能实现超过1000 h的稳定长循环及低极化电压。在LHCE-3电解液中,Li||LFP全电池在2.0 C下循环800次容量保持率为88.7%,Li||NCM811在1.0 C下循环500次容量保持率为82.8%。相比之下,在LCE基础电解液中,Li||LFP全电池在2.0 C下循环800次容量保持率为81.8%,Li||NCM811在1.0 C下循环500次容量保持率为54.0%。（4）利用Si-O键的高键能,设计了一种新型弱溶剂化的硅氧烷基电解液。Si-O键的存在使硅氧烷基电解液比DME醚基电解液具有更好的稳定性,在电池中展现出更出色的性能。同时低溶剂化的DMMS可以有效调控Li⁺的溶剂化结构,诱导更多的FSI^-阴离子在锂金属负极表面形成富含Li F的SEI膜。进而使SEI膜具有高界面能、高机械强度及较低的Li⁺扩散势垒,从而实现均匀稳定的Li沉积,有效抑制锂枝晶生长,延长锂金属电池的循环寿命。在3 M Li FSI+DMMS电解液中,Cu||Li电池在0.5 m A cm^-2和1.0 m A cm^-2电流密度下分别能实现超过1000次循环及99.4%的高库伦效率和超过800次循环及98.8%的高库伦效率。Li||Li对称电池在1.0 m A cm^-2和2.0 m A cm^-2的电流密度下也能分别实现超过2000 h和1200 h的稳定长循环和低极化电压。在3 M Li FSI+DMMS电解液中,Li||LFP在2.0 C下循环800次依旧具有96.68%的容量保持率,Li||NCM811全电池在1.0 C下循环500次容量保持率为89.8%。更多还原

【Abstract】 There is an ever-growing demand for developing rechargeable batteries with high energy density.Due to their high energy density and better security,lithium-ion batteries（LIBs）have been widely used in portable electronics and electric vehicles.Currently,LIBs powered by graphite-based anodes cannot meet the requirement of high energy density.The Li metal batteries（LMBs）can achieve the target energy density of>350Wh kg^-1 thanks to the Li metal anode（LMA）with the highest theoretical specific capacity(3860 m Ah g^-1)and the lowest redox potential（-3.04 V vs.the standard hydrogen electrode）among all possible anodes.However,it suffers from significant volume change,severe interface side reactions,and Li dendrite growth.Therefore,low Coulombic efficiency,poor cycle life,and safety issues have become a huge bottleneck for the commercial implementation of Li metal anode.To address the above challenges,we propose pragmatic approaches,including constructing lithiophilic interfaces and electrolyte engineering.Such feasible strategies ensure selective deposition and stable encapsulation of lithium metal to eliminate dendrite formation.Consequently,these approaches enable higher Coulombic efficiency and improved cycling.The results achieved are as follows:（1）The three-dimensional porous copper mesh（Cu mesh）electrode suffers from poor lithiophilic and uneven Li deposition,which leads to the growth of lithium dendrites,serious polarization,rapid decay of cycle life,and even short circuit of batteries.Uniform deposition of silver nanoparticle as active sites on copper mesh electrodes by electroless plating.The lithiophilic silver nanoparticles can guide the uniform deposition of Li⁺on the Ag-Cu mesh,effectively reduce the lithium nucleation overpotential and inhibit the growth of lithium dendrites,significantly improving the Coulombic efficiency and cycle life of lithium metal batteries.The uniform deposition process of Li⁺on Ag-Cu mesh electrode was investigated through characterization methods such as in-situ XRD and in-operando Raman spectroscopy.Compared with Cu mesh electrode,Ag-Cu mesh electrode can achieve 150 stable cycles with a Coulombic efficiency of 98.6%at 1.0 m A cm^-2 current density,while exhibiting the lower polarization voltage.Ag Cu mesh/Li||Ag Cu mesh/Li symmetric batteries can achieve a cycle life of over 1200 hours under current density of 1.0 m A cm^-2.Under the testing of full cells,both Ag Cu mesh/Li||LFP and Ag Cu mesh/Li||NCM811 showed excellent electrochemical performance.The Ag Cu mesh/Li||LFP cells with a capacity retention rate of 98.5%after 300 cycles at 1.0 C and the Ag Cu mesh/Li||NCM811 cells with a capacity retention rate of 84.8%after 500 cycles at 1.0 C.（2）The growth of lithium dendrites and the accumulation of inactive lithium will lead to low Coulombic efficiency and short cycling life during the cycling process of lithium metal anode.Based on this,an appropriate amount of potassium iodide（KI）functional additive was introduced into Li TFSI/ether based electrolyte and Li PF₆/ester based electrolyte.On the one hand,the low concentrations of K⁺ can spontaneously attach to the tip of the lithium metal anode to form charge shielding,and guiding deposition of Li⁺away from the tip,effectively inhibiting the growth of lithium dendrites.On the other hand,the oxidation-reduction of I^-/I₃^-can quickly activate the inactive Li and Li₂O generated during the cycling process and achieving long cycling life of lithium metal batteries.The uniform Li deposition process on Ag-Cu mesh electrode and the presence of I^-/I₃^-species in the electrolyte were demonstrated through in-operando Raman spectroscopy,XPS characterization methods.Compared with 1 M Li TFSI basic electrolyte without KI additives,Li||Ag-Cu mesh cells can achieve 200stable cycles in 1 M Li TFSI+0.01 M KI electrolyte.Li||Li symmetric cells can achieve stable cycles of over 1400 hours and the lowest polarization voltage under the same testing conditions.During the testing of full cells,the Ag Cu mesh/Li||LFP full cells exhibited excellent electrochemical performance in 1 M Li TFSI+0.01 M KI electrolyte,with a first discharge specific capacity of 167.1 m Ah g^-1 at 0.1 C and the capacity retention rate of 91.7%after 800 cycles at 2.0 C.Li||NCM811 full cells also exhibits excellent electrochemical performance in 1 M Li PF₆+0.01 M KI electrolyte.The cell exhibits an initial discharge capacity of up to 203.9 m Ah g^-1 at 0.1 C,and still maintains a capacity retention rate of 53.4%after 500 cycles at 1.0 C.（3）High concentration electrolytes suffer from high production costs,poor ionic conductivity,high viscosity and poor wettability.Based on this,a new type of fluorinated ether TFTT（1,1,2,2-tetrafluoroethyl-4-methylphenyl ether）was introduced as a fluorinated diluent,and a new local high concentration electrolyte with low density,high ionic conductivity,and high wettability was designed.On the one hand,the addition of TFTT can well retain the solvation structure in high concentration electrolyte and enhance the interaction between Li⁺-FSI^-ion pairs.On the other hand,it can reduce the concentration,viscosity,cost,and improving the conductivity of the electrolyte.Additionally,the low LUMO level of TFTT and enhanced interaction between Li⁺-FSI^-can promote the formation of Li F-rich SEI films,effectively inhibiting lithium dendrite growth.In LHCE-3 electrolyte,Cu||Li half cells can achieve nearly 700 cycles with 99.1%high Coulombic efficiency and 600 cycles with 98.8%high Coulombic efficiency under current densities of 0.5 m A cm^-2 and 1.0 m A cm^-2,respectively.Long stable cycles and low polarization voltage exceeding 1000 hours can be achieved in Li||Li symmetric cells.The full cells also exhibit excellent electrochemical performance in LHCE-3 electrolyte.The Li||LFP full cells had a capacity retention rate of 88.7%after 800 cycles at 2.0 C,and the Li||NCM811 had a capacity retention rate of 82.8%after 500 cycles at 1.0 C.In contrast,in LCE basic electrolyte,the Li||LFP full cell has a capacity retention rate of 81.8%after 800 cycles at 2.0 C,and the Li||NCM811 full cell has a capacity retention rate of 54.0%after 500cycles at 1.0 C（4）A novel weakly solvated siloxane-based electrolyte was designed by utilizing the higher bond energy of Si-O bonds.The siloxane electrolytes with Si-O bonds have higher redox stability than the DME ether based electrolyte with C-O bonds.Meanwhile,low solvation DMMS can effectively regulate the solvation of the electrolyte,induce more FSI^-anions to form a Li F rich SEI film on the lithium metal anode surface.The Li F rich SEI film has high interface energy,high mechanical strength,and low Li⁺diffusion barrier,thereby achieving uniform and stable Li deposition,effectively inhibiting the growth of lithium dendrites,and extending the long cycle life of the lithium metal batteries.In 3 M Li FSI+DMMS electrolyte,Cu||Li half cells can achieve over 1000 cycles with 99.4%high Coulombic efficiency and 800 cycles with 98.8%high Coulombic efficiency under current densities of 0.5 m A cm^-2 and 1.0 m A cm^-2,respectively.Li||Li symmetric cells can achieve long stable cycles and low polarization voltages exceeding 2000 hours and 1200 hours under current densities of 1.0 m A cm^-2and 2.0 m A cm^-2,respectively.The Li||LFP full cells with 3 M Li FSI+DMMS electrolyte has a capacity retention rate of 96.68%after 800 cycles at 2.0 C.The Li||NCM811 full cells also exhibit excellent cycling performance with a capacity retention rate of 89.8%after 500 cycles in 3 M Li FSI+DMMS electrolyte at 1.0 C.更多还原