Investigations Of Interphacial Structures And Processes Of Lithium Anode By AFM Imaging And Force Curve Method

Lithium metal anode which has the highest theoretical capacity and lowest electrochemical potential is regarded as the most promising anode candidate for the next-generation high-energy-density secondary batteries.The application of lithium metal anode in Li-S and Li-O2 batteries can boost theoretical energy densities up to 2600 Wh kg-1 and 3500 Wh kg-1,respectively,which far exceeds that of lithium-ion batteries.However,the commercialization of lithium metal batteries has been impeded by many problems,among which dendrite growth and unstable solid electrolyte interface(SEI)are most serious and intertwined ones on the anode side.Dendrite growth will lead to thermal runaway and bring serious safety hazards to the battery,while uneven and unstable SEI can promote the growth of dendrite and degrade the lifetime of the batteries.To alleviate the problems of dendrite growth and unstable SEI,the interphases of anode were characterized in detail by atomic force microscopy(AFM)combining with other complementary techniques in this thesis.On the one hand,through in-depth analysis of the mechanical properties of SEIs and their correlations with electrochemical properties,the force curve method of lithium surface SEI has been established,which can be used to rapidly evaluate the performance of SEI and predict the lifetime of Li anodes.Meanwhile,the mechanical behaviors of SEIs formed in different electrolytes and on different substrates in the practical battery systems were compared by applying the force curve method,and the high-quality SEIs were screened to improve the cycling performance and Coulombic efficiency of anodes.On the other hand,fundamental investigation on the initial nucleation and growth of lithium and influencing factors were carried out.The differences in nucleation behavior of lithium in between ester-based and ionic liquid(IL)electrolyte were compared.At the same time,the induction effects of crystal plans of substrate and types of SEI on the nucleation mode and shape of lithium nucleus were analyzed to further understand the mechanism of lithium deposition.The main conclusions are outlined as follows:(1)Agilent 5500 AFM test instrument for the characterization of lithium anode has been constructed in glove box with inert argon circulating atmosphere.By suspending the test base and keeping least equipment components in the glove box,the effect of external vibration and electromagnetic noise interference on the measurement can be minimized,thus improving the quality of AFM tests and the accurate collection of signals.(2)Development of AFM force curve method for characterization of SEI on Li anodes.Three basic types of single-layered SEIs on Li surfaces,which are mechanically stiff,moderate-stiff and soft,respectively,are prepared by chemical soaking and anodic stripping.For the stiff SEI with I-rich structure,the AFM force curve displays sawtooth feature,indicating rupture of the SEI during the measurement of force curve,and both the Young’s modulus and thickness of such an SEI scatter largely.On contrary,for the soft SEI with O-rich components,a small force can piere the SEI,meaning unable to resist dendrite growth and leaving a basin-like feature on the AFM force curve.While for the moderate-stiff SEI,its O-I hybrid composition can afford the external force without being pierced and AFM force curve shows a ramping-slope feature.The average modulus and thickness are between those of stiff and soft SEIs.In combination with surface flatness and thickness,the nanoindentation features reflecting the differences in hardness of SEIs can be further used to infer the structure of unknown SEIs and predict the performances of anodes.Multi-layer SEIs with more complex structures are constructed on lithium surface by applying multi-step electrochemical procedures,among which the I-O-I SEI with smooth surface and both flexible and rigid mechanical properties shows stable cycling number of 800 in symmetric battery.By systematically analyzing the nanoindentation features,thickness and Young’s modulus of various SEIs and combining with the performances of related batteries,a standard for rapidly evaluating of SEI is established,which can be used for prediction of cycling performances of batteries.(3)Application of the AFM force curve method to characterization SEIs of practical battery system.The differences of SEIs on Li formed in conventional electrolyte of 1 M LiTFSI/DME-DOL and high concentration electrolyte of 7 M LiTFSI/DME-DOL by electrochemical aging are compared.The surface of SEI formed in conventional system has many defects,and the force curve features indicate multilayer structure combining inorganic inner layer and organic outer layer with the outer modulus of about 300 MPa;while the SEI formed in high concentration system displays smooth and granular morphology and ramping-slope nanoindentation feature,and the modulus of the outer layer is up to 2.8 GPa guaranteeing the more stable cycling performance of battery.Changing the composition of electrolyte will also cause differences in configuration and mechanical properties of SEIs:the SEI formed in 1 M LiFSI/DME-DOL by electrochemical polishing shows sawtooth feature of I-rich structure,and the modulus can reach 15 GPa;while the SEI formed by polishing in 1 M LiPF6/EC-DMC displays structure of organic outer layer and inorganic inner layer,which is similar to the conventional mosaic model,and the SEI is thicker with a small Young’s modulus of outer layer.Free-standing like O-I and I-O-I SEIs showing smooth surfaces and mechanical properties of both elasticity and rigidity are prepared on Cu substrate by electrochemical control method.The Li‖Cu half cell with I-O-I SEI can stably cycle for 300 cycles with high Coulombic efficiency of 99%.Therefore,the characterization method of force curve can be applied to evaluation and screening of SEIs formed in different electrolyte systems and on different substrates and further to predict the performances of batteries.(4)In-situ EC-AFM investigations of the formation of SEI and Li deposition on Cu single crystal electrode.The effects of different electrolyte systems,crystal planes of Cu electrode and types of SEI on the nucleation and growth of lithium are compared.In ester-based electrolyte systems(EC-DMC),the lithium tends to form heterogeneous particles with few nucleation sites on the surface.On the contrary,dense and homogeneous nucleation sites of lithium appear on Cu single crystal electrodes in pyrrole-based ionic liquid(IL)system.In the mixed IL system(Py14TFSI-Py14FSI),the lithium nuclei show small triangular morphology with high nucleation density on Cu(100)electrode,while large triangular morphology and relatively low nucleation density of lithium nuclei are observed on Cu(111)electrode.Although there are difference in the shape and size of lithium nuclei on Cu(100)and Cu(111),the nuclei distribute relatively uniformly in a 3D progressive nucleation.The SEI formed by potential control in high concentration ether-based electrolyte has smooth surface and a thickness of about 12 nm.The SEI showing uniform distribution of surface modulus has a structure of inorganic outer layer and hybrid inorganic and organic inner layer.Spherical lithium nuclei can be induced to form in LiTFSI/Py14FSI electrolyte presenting 3D instantaneous nucleation behavior.However,the SEI in-situ formed in LiTFSI/Py14FSI electrolyte has larger overall roughness and a thickness of about 95 nm.Triangular lithium nuclei are induced to form by SEI which has wide distribution of modulus during overpotential deposition,presenting 3D progressive nucleation behavior.(5)Investigation of the intrinsic properties of LiCoO2 cathode materials by single dispersed electrode and single particle electrode.On one hand,nanoscale LiCoO2 particles with good crystal shape are synthesized by melting salt at high temperature,and reversible intercalation/deintercalation peaks can been observed when characterized by cavity microelectrode.However,the particles cannot be fixed to the electrode surface effectively even by the method of diazonium salt reduction,and further improvement are needed.On the other hand,efforts are devoted to establish AFM-based single particle method for characterization of electrode material.Pt AFM probes are coated are obtained through depositing 10 nm Al2O3 by ALD to electrochemically insulate the side walls with only exposure of the tip.Since the current signal of intercalation/deintercalation of Li+in LiCoO2 is masked by the large current of the double layer when measured with such coated Pt probe,a new probe encapsulation method is still needed.