Investigations On The Solid Electrolyte Interface And Solid-state Battery

Due to its high energy density and superior cycling performance,the application of lithium ion batteries has penetrated into all aspects of modern society,such as transportation network and portable mobile electronic devices,making people’s life more convenient.At the same time,through the unremitting efforts of scientists,the performance of lithium-ion batteries has been further improved.However,with the progress of The Times and the rapid development of science and technology,people’s requirements to energy storage devices are becoming more and more stringent,which requires us to continue to invest a lot of time to solve the problems existing in the lithium-ion battery systems,so as to further improve the electrochemical performance.The poor interfacial stability and compatibility between anode and liquid electrolyte or solid-state electrolyte is one of the problems that need to be adressed.Due to the high reactivity of anode materials,continuous side reactions will be triggered at the interface when in contact with electrolyte,resulting in formation of the solid electrolyte interface（SEI）.The growth of SEI films plays an important role in stabilizing the interface between anode and electrolyte.However,the self-growing SEI films often have the following two drawbacks:（1）poor stability that makes it easy to rupture and regenerate during the electrochemical cycling;（2）poor kinetic performance that hinders the transport of lithium ion at the interface.Therefore,in order to solve the interface problem between the anode and the electrolyte,we need to find an appropriate approach to construct the SEI films with excellent kinetic performance and stability.Accordingly,in this dissertation,we first focused on the regulation of the SEI films between anode and liquid electrolyte or solid-state electrolyte,specifically involving the application of LiNO₃ and LiI.Firstly,considering that LiNO₃ can be decomposed into Li₃N and LiN_xO_y with high ionic conductivity,we applied it to construct the SEI films on the surface of graphite anode.LiNO₃ was uniformly dispersed into the anode electrode directly by means of the dissolution and precipitation principle in the slurry coating process.The subsequent electrochemical tests showed that the rate performance of the graphite electrode was improved significantly by adding a small amount of LiNO₃.When the current increased from 68 m A g^-1 to 680 m A g^-1,the charge specific capacity of the graphite anode with LiNO₃ decreased from 340.0 m Ah g^-1 to 280.0 m Ah g^-1,and the retention rate was 82.4%.However,the graphite anode without LiNO₃ was directly reduced from 330.0 m Ah g^-1 to 75.0 m Ah g^-1,and the retention rate was only 22.7%.Subsequently,the decomposition of LiNO₃ was verified successfully by X-ray powder diffraction（XRD）.In the meantime,a large amount of Li₃N and LiN_xO_y were detected on the surface of the anode electrode by X-ray photoelectron spectroscopy（XPS）,which indicated that the constructed SEI film was rich in the components with high ionic conductivity.The cyclic voltamogram test also showed that the lithium-ion diffusion coefficient of the graphite electrode was significantly increased(from 8.55×10^-9 cm² s^-1 to 2.33×10^-8 cm² s^-1)after adding LiNO₃,and the corresponding interface impedance was obviously reduced as well.Therefore,from these results,it is clear that the SEI films with high ionic conductivity can be constructed successfully on the surface of graphite electrode by in-situ decomposition of LiNO₃,which can effectively improve the rate performance of graphite anode.Then,considering that the interface stability of the Si-based anode was improved by the general electrolyte additives containing F,which could construct Li F-rich SEI films,but the high content of Li F also reduced the dynamic characteristics of the interface,we continued to modify the SEI films of Si-based anode using LiNO₃.After LiNO₃ was added into SiO_x/Graphite anode,the electrochemical tests were carried out in the liquid electrolyte containing vinyl fluoride carbonate（FEC）additive.The results showed that the rate performance of Si-based anode was significantly improved.At 40 m A g^-1,the specific capacities of the modified SiO_x/Graphite anode and the original anode were both close to 450 m Ah g^-1.When the current was continuously increased to 200,600,and 800 m A g^-1,the capacity retention rate of the modified anode showed 92.8%,72.5%,and 60.1%,respectively;but that was only76.1%,32.6%,and 22.2%for the original anode electrode.Subsequent in-situ/ex-situ XRD and XPS tests also showed that LiNO₃ was successfully decomposed during the electrochemical cycling.XPS test further indicated that in addition to a large amount of Li F,the SEI film also contained Li₃N and LiN_xO_y components with high ionic conductivity,which further stabilized the interface and improved the interfacial dynamics.In conclusion,this simple method can be used to construct a stable SEI film with high ionic conductivity on the surface of the anode electrode,which can effectively improve the ionic conductivity of the interface.It is expected to provide some new ideas for the interface design of the anode electrode,including the traditional liquid battery systems and solid-state battery systems.In addition to LiNO₃,we also focused on LiI.The interfacial reactions between lithium metal anode and PEO-based electrolytes not only caused the loss of active materials,but also seriously damaged the stability of the interface,leading to the uneven deposition of lithium.In the meantime,considering that LiI possesses the characteristic of electronic insulation and can transfer lithium ions,we applied it into the PEO-based solid-state lithium metal battery to build a uniform and stable passivation film for the lithium metal anode.In the specific experimental process,a small amount of LiI was uniformly added into the cathode electrode（Li Fe PO₄）via slurry coating process.The electrochemical tests showed that the coulombic efficiency and cycle performance of the lithium-metal battery were significantly improved after LiI modification.After modification,the average Coulombic efficiency of 160 cycles（0.2 C）was 99.92%,while that of unmodified battery was only 99.26%.In addition,the capacity of the modified battery was maintained at150.7 m Ah g^-1 with a retention rate of 93.1%after 500 cycles at 0.2 C;and maintained at 135.5 m Ah g^-1（87.8%）after 1000 cycles at 1 C.However,the capacity of the unmodified battery was only maintained at 103.3 m Ah g^-1 after 500 cycles at 0.2 C with a retention rate of 64.5%;and only maintained at 100.8 m Ah g^-1（65.5%）after1000 cycles at 1 C.In the meantime,obvious fluctuations of coulombic efficiency and specific capacity were observed in the unmodified solid-state battery.However,no significant performance fluctuations were observed after modified.Then,the surface morphology of the lithium metal was characterized by scanning electron microscope（SEM）.In the battery without modification,a large number of white reduction products were observed on the surface of lithium metal,and there was no regular interface morphology.After modification,the uniform morphology like leaf veins was observed on the surface of lithium metal,indicating that the uniform deposition of lithium metal was realized.Through XPS test,a large amount of LiI could be detected on the surface of lithium metal for the modified battery,indicating that the LiI added in the cathode electrode was successfully transferred to the anode surface and participated in the construction of SEI film.In addition,a lager number of Li F was also detected on the surface of the modified lithium anode,thus the presence of LiI might promote the production of Li F at the interface.The LiI and Li F-rich SEI film promoted the uniform deposition of lithium and improved the electrochemical performance of the solid-state battery.Considering the successful application of LiI in PEO-based batteries,we continued to apply LiI to the all-solid-state batteries based on sulfide electrolyte.The high reactivity of lithium metal anode doomed the violent interfacial reaction between lithium metal anode and sulfide solid-state electrolyte,which not only lost the active materials,but also produced a large number of by-products with low ionic conductivity.Our studies found that when Li₇P₃S₁₁ was in close contact with lithium metal,a large number of Li₂S and P-S-Li_x were generated at the interface.Among the by-products,the content of Li₂S with low ionic conductivity was very high（the peak area ratio in the S 2p spectrum reached 81.9%）,indicating that the solid-state electrolyte at the interface was almost reduced by lithium metal.However,after doping with LiI,the interface between solid-state electrolyte(Li₇P₃S₁₁-LiI)and lithium metal became more stable.Accordign to the results,the generated Li₂S was significantly reduced,and no obvious P-S-Li_x composition was observed.In addition,LiI possesses a higher ionic conductivity than Li₂S,thus a large amount of LiI at the interface after modification can also promote the effective transport of lithium ions compared with the unmodified electrolyte.In conclusion,our studies showed that LiI can provide a good protective effect on the surface of lithium metal to inhibit the deterioration of the interface and promote the transport of lithium ions,no matter in the PEO-based solid-state battery systems or the solid-state battery systems based on inorganic sulfide electrolyte.Then,considering that Li₂S cathode has a very high charge-discharge polarization due to low activity,especially in solid-state battery systems,and LiI has been reported as a catalyst in lithium air batteries.Therefore,we further used LiI as the catalyst for Li₂S cathode to study the electrochemical performance of all-solid-state batteries by using Li₁₀GeP₂S₁₂（LGPS）and Li₇P₃S₁₁-LiI as electrolyte.We found that LiI possessed an excellent catalytic effect on electrochemical decomposition of Li₂S.After using carbon nanotubes（CNT）and Super-P to construct a good electron transport network,and using LGPS to construct an ion transport channel,the discharge capacity of the all-solid-state battery at 20 m A g^-1（60℃）deliveried 1070.2 m Ah g^-1,and the discharge capacity higher than 2.2 V accounted for78.8%of the total capacity.However,under the same preparation and testing conditions,the specific capacity of Li₂S anode without LiI was less than 200 m Ah g^-1,and the discharge voltage platform was much lower than 2.2 V.In addition,the discharge specific capacities of 771.0 and 522.7 m Ah g^-1 at 40 and 100 m A g^-1 were achieved by adjusting the content of conductive additives and solid-state electrolyte within composite cathode,and a stable cycling of 20 cycles with the discharge capacity over 900 m Ah g^-1 was obtained at 20 m A g^-1.In conclusion,in this dissertation,we carried on the exploration of LiNO₃ and LiI.These two additives are expected to achieve the superior modification to the surface of anode.According to the results,LiNO₃ was helpful to construct the Li₃N and LiN_xO_y-rich SEI film,and to improve the interfacial dynamic performance and stability;LiI effectively passivated the surface of lithium metal and inhibited the interfacial reactions between lithium metal and solid-state electrolyte.Meanwhile,LiI can transport lithium ions well and will not deteriorate the dynamic performance of the interface.In addition,LiI possessed an excellent catalytic effect on the electrochemical conversion between Li₂S and S in all-solid-state lithium-sulfur batteries,which is expected to become one of the important components in all-solid-state lithium-sulfur batteries in the future.