| Reducing the weight of lithium-ion batteries,improving their cycling durability and minimizing costs while remaining safe have been top priorities for researchers.The powdering,cracking,spallation and safety problems of electrode materials have become a important problem for developing next generation high performance LIBs and wide applications.The mechano-chemical degradation and failure mechanisms of multilayered electrode materials under charging-discharging cycling have become the key scientific problems in the development of new electrode materials and the improvement of specific capacity of batteries.In this thesis,SiO@C composite anodes and doped LiNi0.5Mn0.3Co0.2O2 cathodes were prepared.The corresponding electrochemical properties were tested and analyzed.A set of experimental system for in-situ characterization of electrode deformation under charge-discharge condition was designed and developed to realize the dynamic monitoring of electrode surface deformation.A thermo-mechano-chemical coupling constitutive model was established to predict the evolution of lithium ion concentration and stress characterizations of electrode.The proposed analytical and experimental methods would be useful for understanding the mechano-chemical coupling mechanisms of LIBs under electrochemical cycles.The main works was as follows,Firstly,the in-situ deformation observation and stress analysis of SiO@C composite electrodes under charge-discharge condition were studied.The SiO@C composite electrodes were firstly prepared.The composition,microstructure,thermogravimetric and electrochemical properties of SiO@C composite materials were tested and analyzed,respectively.Based on the non-contact digital image correlation technique,a set of testing system is designed and assembled to in situ monitor the strain variation on the active coating surface during the analysis of mechano-chemical coupling performance.The strain evolution of SiO@C composite electrodes was successfully measured by using a special speckle during electrochemical cycling.The monitoring results showed that with the increase of cycle times,the residual lithium ion concentration on the surface and interface of the active layer increased continuously,resulting in mismatched residual strain and stress in the active layer.At the end of the sixth cycle,the residual strain reached the maximum value of 3.0%.The SiO@C composite electrodynamic constitutive model of the two-layer structure was established to reveal a sine form strain/stress evolution law,and it was found that there was a large gradient distribution of lithium ion concentration in the cycling process.At the same time,it was also found that the electrochemical part contributed about 3 times more to the stress of the active layer than the mechanical part.Secondly,the diffusion-induced strain and stress analysis of two-layered SiO@C electrode under charge-discharge condition were studied.Two-layered SiO@C electrode materials with different active layer thicknesses were prepared.The effect of different current densities on its electrochemical cycling performances was analyzed.The larger the current density is,the slower the specific capacity of the battery becomes.But the thicker the active layer is,the faster the electrochemical performance decreases.An analytical solution was deduced to predict the evolution of mechano-chemical performances of two-layered SiO@C electrodes during 200 charging-discharging cycles,including lithium ion concentration,strain and stress.It was found that different current densities had a great influence on the variation amplitude of strain and stress in the electrode,but had no obvious influence on the residual strain/stress of the electrode after circulation.During the cycling process,the thickness of the active layer had a significant influence on the strain/stress distribution in the electrode.The thickness of the active layer increased by one time,resulting in a nearly three-fold increase in the stress of the active layer.Thirdly,the effects of graphene oxides and graphene on the thermal runaway of lithium-ion batteries under mechanical abuse were studied.The graphene oxide and graphene doped LiNi0.5Mn0.3Co0.2O2 electrode materials were prepared.The morphology and structure of the electrode materials were characterized and the electrochemical performances were tested.The effects of mechanical abuse on the thermal runaway of LiNi0.5Mn0.3Co0.2O2 electrode materials with different doping ratios were studied by means of impact,scratch and nail penetration tests,respectively.The results show that the addition of graphene oxide and graphene are useful for forming the crack network in the active layers.In nail penetration tests,the maximum temperature increment of graphene oxide modified electrode reached about 7.5℃.However,the peak temperature of graphene modified electrode can reach about 11.2℃.It is explained that graphene oxide and graphene can effectively inhibit the short-circuit heat generation of LIBs,which provides experimental basis for improving the safety design of LIBs.Fourthly,the mechanical properties of LiNi0.5Mn0.3Co0.2O2 composite materials modified with different doping ratio of graphene and multi-walled carbon nanotubes were studied.The graphene and multi-walled carbon nanotubes were selected as two different dopants with three kinds of 1.0 wt.%,3.0 wt.%and 5.0 wt.%ratio,respectively.The LiNi0.5Mn0.3Co0.2O2 electrode materials were prepared by using the traditional method.The morphology and structure of the materials were characterized and the electrochemical properties were tested.An in-situ testing system applied for non-contact strain measurements based on digital image correlation technique was presented in this work.A modified shear lag model was proposed to analyze the failure mechanism of double-layer active layer/current collector system in LIBs.The tensile fracture failure processes of two kinds of LiNi0.5Mn0.3Co0.2O2(NMC532)electrode materials were systematically studied in detail.The fracture strength,fracture toughness and fracture energy of the composite active layer materials were obtained.It is found that the multi-walled carbon nanotubes are more useful for improving the mechanical properties of the NMC532 electrode at the same doping ratio,compared with the influence of grapheme.The results would provide an effective experimental method to understand the rapid characterization of the strength parameters of the active layer of LIBs in the future. |
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