Study On Interfacial Modification Of Anode Materials With High Specific Capacity In Lithium Batteries

Energy crisis and environmental deterioration have always been threatening the historical course of human civilization.The effective use of new energy will determine the sustainable development of human society.Therefore,the research and application of lithium batteries,one kind of chemical energy storage power source,plays a vital role.With the development of society,in the field of large-scale energy storage power stations,intelligent mobile terminals and electric vehicles,higher requirements have been put forward for the performance of lithium batteries,especially in the related indexes of energy density and safety performance.Within,the development of high-specific capacity materials on the negative side has become a key link.The specific capacity of traditional graphite anode has reached the upper limit.In order to continue to improve the energy density of lithium batteries,it is necessary to develop silicon-based anode and lithium metal anode with low discharge potential and high specific capacity.However,both of these two types of anode materials have huge volume changes during the electrochemical cycling,leading to serious interface instability.For lithium metal anode,the growth of lithium dendrites may even cause safety problems.The combined effect of these factors will lead to low coulombic efficiency and cycling stability of the electrode.Therefore,it is critical to modify the interface of anode materials with high specific capacity.Silicon-carbon(Si/C)composites,as one of the development directions,represent one promising class of anode materials for next-generation lithium-ion batteries and are expected to be widely used commercially in the short term.To achieve high performances of Si-based anodes,it is critical to control the surface oxide of Si particles,so as to harness the chemomechanical confinement effect of surface oxide on the large volume changes of Si particles during lithiation/delithiation.Here a systematic study of Si@SiOx/C nanocomposite electrodes consisting of Si nanoparticles covered by a thin layer of surface oxide with a tunable thickness in the range of 1-10 nm is reported.It is shown that the oxidation temperature and time during annealing not only quantificationally control the thickness of the surface oxide,but also change the compositions,structure and valence state of Si in the surface oxide.These factors can have a strong influence on the lithiation/delithiation behavior of Si nanoparticles,leading to different electrochemical performances.By combining experimental and modeling studies,based on the competing effects of self-limiting lithiation in the bulk Si and oxide fracture on the surface of Si under different thickness of oxide layer,an optimal thickness of about 5 nm for the surface oxide layer of Si nanoparticles is identified,which enables a combination of high capacity and long cycle stability of the Si@SiOx/C nanocomposite anodes.This work provides an in-depth understanding of the effects of surface oxide on the Si/C nanocomposite electrodes.Insights gained are important for the design of high-performance Si/C composite electrodes.Considering a medium/long-term development strategy,the attention towards rechargeable lithium(Li)metal anodes,an ultimate aim in the process of searching for alternative anode materials,has been rekindled in recent years as it would boost the energy density of new-generation Li batteries.However,the issue of interface instability is prominent,especially when cycled in traditional carbonic ester electrolytes that exhibit a wide voltage window and are compatible with most of the cathode materials.Herein,lithium difluorophosphate(LiDFP)and vinylene carbonate(VC)are combined,and demonstrated to be synergistic in constructing in situ a mechanically stable and highly Li-ion conducting surface film on the Li metal anode.This results in uniform and compact Li deposition largely suppressing the formation of Li dendrites,dead lithium and irreversible Li-species as revealed by operando neutron depth profiling(NDP)and scanning electron microscope(SEM).This enables enhancement of the Coulombic efficiency and long-term cycling stability for rechargeable Li metal anodes.By combining solid state nuclear magnetic resonance(SSNMR)and spectroscopic studies like X-ray photoelectron spectroscopy(XPS)and fourier transform infrared spectroscopy(FTIR),the origin of the influence of composite additives on lithium metal deposition/dissolution process is analyzed.It is demonstrated that VC slows down the LiDFP reduction,yet promoting the breaking of the P-F bonds,which leads to a protective film.This film is rich in LiF-Li3PO4 inorganic compounds,distributed homogeneously,that is embedded in a matrix of P-O-C species and macromolecular organic compounds like lithium ethylene dicarbonate.This composition is responsible for the improved ionic conductivity and mechanical stability of the protective film during extended cycles.The detailed insight in the additive interaction provides new opportunities for the design of rational surface films necessary for realizing high-performance lithium metal batteries.