Study On Electrode Materials And Interphases Of Electrode-Electrolyte In Li-ion Batteries

Safe Li batteries with high specific energy show promising potential applications in the energy storage system,electrical vehicles,and portable electronics.Currently commercial Li-ion battery system accompanied with Li transition metal oxide cathode(?200 mAh/g)and graphite anode(370 mAh/g),however,is approaching its specific energy bottleneck(?300 Wh/kg).Replacing the graphite anode with high specific capacity anodes,such as Si or Li is a promising method to reach higher specific energy(500 Wh/kg).The huge volume changes of Li and Si anode during charge and discharge induces particles fracture,loss of electrical contact and unstable solid electrolyte interphases(SEI),consequently inducing fast capacity degradation.To address the above drawbacks,nano silicon with surface and interfacial engineering was studied.Within a two-step electrochemical etching method,a thick porous silicon film with control pore diameter(?10 nm-?1?m)and thickness(?100 nm-?100 ?m)was successfully fabricated.To improve the electric conductivity and to stabilize the interface with liquid electrolyte,amorphous carbon is coated.A covalent interfacial Si-N-C bond is in-situ built to further enhance the adhesion and mechanical stability between a-C and silicon.In addition,the inactive Si-N-C layer is demonstrated to reduce the sub-reactions with liquid electrolyte.The effects of surface coating and interfacial engineering on the electrical conductivity enhancement and stabilizing SEI were evaluated.For the first time,a remarkable Si anode with interfacial Si-N-C layer shows 2000 cycle performance without capacity loss.We also introduce ultra-thin a-Ge(?5 nm)to modulate the potential barrier between NiSix and a-Si nanowires.The resultant NiSix/a-Ge/a-Si shows excellent rate performance(40 A/g)and improved interfacial adhesion.To overcome the intrinsic disadvantages of nano Si,such as large surface area,low areal capacity and low coulombic efficiency,we turn to the micron-meter-sized polycrystalline Si particles.The lithiation mechanism of MSi/mG is revealed.Within heavily boron doped silicon,we scalable prepared micron-meter sized porous silicon particles,the MSi electrode shows high initial coulombic efficiency(92.4%),high areal capacity(4 mAh/cm2)and stable cycling.To further improve the coulombic efficiency of MSi,an ALD/CVD-based technique is developed to in-situ grow mG on MSi/Al2O3 particles.The growth mechanism and its effects on the electrochemical performances are investigated.On the other hand,the usage of Li metal and liquid carbonated electrolyte in conventional batteries causes serious safety issues.Using inflammable ceramic solid electrolyte has been considered as a promising candidate for safe Li battery.The usage,however,is limited by its large interfacial resistances with most of the cathode materials,due to the chemical potential incompatibility as well as the thermodynamic instability for high temperature sintering that is usually needed to achieve high density,as well as the limitation of integration technique.Herein,enabled by liquid phase sintering with a low melting point B2O3 additive,the sintering temperature of LATP was successfully reduced from 900 ? to 750?.The B2O3 is also helpful to form thin,percolative,and mixed conductive interphases between LATP and LCO.In addition,a LATP/LCO-LATP double layer structure is achieved through one-step dry process.These mixed conductive interphases drastically improve the kinetics,leading to high-loading solid LATP/LiCoO2 cathodes up to?6 mAh cm-2.The technique is also applicable to Ni-rich NMC cathode materials based on several key innovations:exceptional thick,mechanically robust,and dense cathodes(>90 vol.%,up to several mm)with a high active materials loading(98 wt.%)and a high conductivity(1.5×10-2 S cm-1).After sputtering a thin film LATP(?350 nm)on the cathode,the LiNi0.6Mn0.2Co0.2O2(NMC622)demonstrates up to?10 mAh cm-2,which is expected to achieve a 400 Wh kg-1 SSB.Our composite cathodes show a ten-times and three-times area capacity improvement over the state-of-the-art using oxides and sulfides,respectively.