Stable Interface Construction Of Solid Polymer Electrolyte/Cathode Towards Improvement Of PEO Based Solid State Battery At High Cut-off Voltage

Solid-state batteries are considered as one of the candidates for the next generation of high energy density and high safety energy-storage devices.Comparing with other solid-state batteries,solid-state polymer batteries are more suitable for manufacturing high-capacity and high-density power batteries.Among the many polymers,PEO-based solid polymers electrolytes have attracted widespread attention due to the excellent Excellent processability and good lithium-metal interfacial compatibility.However,the cathode/electrolyte interface reacts violently at high cut-off voltages,leading to high interfacial impedance and rapid capacity decay,which limits its application in the field of high energy density all-solid-state batteries.In this dissertation,we build a stable cathode/electrolyte interface layer with atomic layer deposition technology to improve the cycling stability for 4.5V LiCoO₂.For the narrow electrochemical window of PEO-based electrolyte,a composite electrolytes are prepared to increase the electrolyte electrochemical stability window and make it suitable for the coated LCO cathode.The physicochemical properties of PEO-based composite electrolytes are tested..Subsequently,the solid-state batteries were assembled and the electrochemical performance as well as the interfacial stability of the battery were investigated.The main studies are as follows.（1）Al₂O₃ is deposited on the surface of LiCoO₂particles by atomic layer deposition to establish a protective barrier layer,which inhibits the phase transition of the material at high potentials,prevents Co dissolution and side reactions caused by direct contact between electrolyte and electrode,and improves the electrochemical performance and cycling life of the battery.The results show that the battery using coated LCO has more stable interfacial impedance,and the oxide layer effectively reduced the oxidation and reduction of the electrolyte to LCO and inhibited the increase of the interface impedance.Meanwhile,the battery assembled with coated LCO cathode has better cycling performance and rate performance at high potentials.The initial discharge specific capacity was 175 m Ah g^-1at4.5V and 0.2C current density,and after 300 cycles,the discharge specific capacity was122.6 m Ah g^-1with an average Coulombic efficiency of 99.8%.The results show that the coated LCO improves the cathode/electrolyte interface stability and improves the capacity retention and working life of the battery.The atomic layer deposition technology can coat the surface of cathode material uniformly and precisely,and can control the coating thickness,which is a very promising technology for cathode material modification.（2）In this section PEO-based polymer electrolytes were prepared,the solid-state battery was assembled,and the electrochemical performance was tested.The ionic conductivity of PEO-LiDFOB_20%is 2.1×10^-5S cm^-1and the lithium ion transference number of PEO-LiDFOB_20%electrolyte is about 0.19.The electrochemical stability window is 4.0V vs.Li⁺/Li.The reversibility of lithium ion deposition and dissolution is good in the range of-1～5V.PEO-LiDFOB_20%electrolyte has good compatibility with lithium metal electrode.The Al₂O₃@LCO/PEO-LiDFOB_20%/Lisolid-state batteries were tested at 50°C,and the results showed good rate performance,charge and discharge performance and long cycling performance.The specific discharging capacity was 180 m Ah g^-1at 0.05C,86 m Ah g^-1at0.4C and 131.5 m Ah g^-1at 4.5V for 100 cycles,with a capacity retention of 75.6%and an average Coulombic efficiency of 99.7%.SEM observations showed that the surface of the Al₂O₃@LCO cathode was covered completely,which was verified by the disappearance of the Co signal peak in XPS.At a cut-off voltage of 4.5V,the PEO electrolyte undergoes oxidative decomposition,and the single oxygen radical is oxidized to form a carbon base.In the C 1s spectrum,the area ratio of R-C=O/C-O for the LCO cathode is 0.26,Al₂O₃@LCO cathode is 0.084.The same conclusion can be obtained from the O 1s spectrum.The protective effect of the Al₂O₃layer was further confirmed.Through XPS and EIS analysis of different cycles,it is shown that the coating layer can effectively isolate the contact between the electrolyte and the electrode,inhibit the oxidative decomposition of PEO by strong oxidizing Co⁴⁺at high cut-off voltage,slow down the deterioration of the interface layer impedance,and improve the electrolyte/electrode interface layer problem.In addition,the coating layer can also stabilize the LiCoO₂structure.（3）In order to better match with high voltage lithium cathode and give full play to the role of high-voltage cathode,a high voltage electrolyte was designed and prepared to improve the electrochemical stability of the electrolyte at high cut-off voltage.Starting from the design of composite solid electrolyte,PEO-based composite electrolyte was obtained by pouring PEO solution containing LATP on cellulose.LATP itself has strong oxidation resistance,and after fully compounding with PEO,it complements each other to achieve the effect of’1+1>2’,so as to enhance the electrochemical stability of the overall composite electrolyte in high cut-off voltage.The ionic conductivity of PEO-LiDFOB_20%-LATP-7.5%is1.2×10^-4S cm^-1and the lithium ion transference number is 0.38.The composite electrolyte has a oxidation decomposition voltage of 4.5V,which can work at high cut-off voltage and has high power charging and discharging ability.The surface of the lithium metal electrode after cycling was observed by SEM.The surface was covered by PEO electrolytes,and lithium dendrites were non-existent,indicating that the cathode had good compatibility with the electrolyte.The initial discharge capacity is 178.5 m Ah g^-1and the Coulombic efficiency is 98.1%at 0.1 C.After 200 cycles,the discharge specific capacity is 145.7 m Ah g^-1,the Coulombic efficiency is 99.7%,and the capacity retention rate is 81.6%.The composite electrolyte solid-state battery exhibits high energy density,low-capacity decay rate and excellent Coulombic efficiency.XPS results show that the interface layer of the composite electrolyte contains O-C=O,R-C=O,C-O,C-C,LiCO₃and LiF,among which LiCO₃and an appropriate amount of LiF are beneficial to the stability of the interface and the stable cycle of the solid-state battery.In summary,the electrolyte can be well match with cathode and is expected to be used in future high energy density all-solid-state electrolyte batteries.