Study Of Graphene-based Modification Layers On The Surface Of LiNi_0.5Mn_1.5O₄ Electrode And Lithum Sulfur Batteries Separators

LiNi_0.5Mn_1.5O₄(LNMO)high-voltage lithium-ion battery and lithium sulfur(Li-S)battery are candidate powers in future electric vehicles,but they all suffer from poor cycle performance.To overcome this issue,ultra-thin and ultra-light graphene-based layers were deposited on the surface of electrodes and separators by electrophoretic deposition(EPD)and vacuum filtration.The electrochemical performances of LNMO electrode and Li-S battery were improved without reducing the mass and volume specific energy.The mechanisms of the performance enhancement were deeply analyzed.Firstly,p-phenylene diamine decorated rGO(pPD-rGO)nanosheets were deposited on the surface of LNMO electrode by EPD.The areal loading of pPD-rGO layer was controlled by adjusting the deposition voltage and time.An new electrophoretic filtration deposition method was developed from routine EPD technology.The key point in the new method was placing a separator in the middle of the electrophoretic electrodes without contact with two electrophoretic electrodes.Under the driving force of electric field,graphene nanosheets were filtered and deposited on the separator.Based on this new method,a continuous electrophoretic device was designed.Two kinds of graphene/polypropylene composite separators,pPD-rGO/3401 separator and LDG/3401 separator,were successfully prepared by using this device.The areal loading of graphene layers on pPD-rGO/3401 and LDG/3401 separators were only 0.017 mg/cm² and 0.1 mg/cm²,with mass percentages less than 1%and 10%in the composite separators,respectively.Additionally,TiO₂ coated less defects graphene(TiO₂-LDG)nanosheets were synthesized.TiO₂-LDG/3401 composite separator was prepared by vacuum filtration.The lowest areal loading of TiO₂-LDG layer that completely covered the separator was only 0.09 mg/cm².Secondly,the effects of the areal loading of pPD-rGO layer,the charge-discharge potential range and the chemical structure of graphene on the cycle performance of LNMO electrode were studied.After optimizing the areal loading of pPD-rGO layer and the charge-discharge potential range,it was found that when the areal loading of pPD-rGO layer was 20?g/cm² and the charge-discharge potential range was 4.2-5.2 V,the pPD-rGO layer modified LNMO electrode had the best cycle performance.After 1000 cycles at 1 C,the remaining specific capacity was100 m Ah/g,the capacity retention rate was 81.7%,and the average decay rate per cycle was as low as 0.018%.Raising the deposition voltage or prolonging the deposition time led to the higher areal loading of pPD-rGO layer,which could hinder the transport of lithium ion and reduce the specific capacity of LNMO.Due to alkaline pPD molecules adsorbing on pPD-rGO nanosheets,the pPD-rGO layer could consume acidic F radicals through neutralization reaction,reducing the number of opened EC ring.Therefore,long-chain PEC and more Li F were prone to form on the surface of pPD-rGO layer,making CEI layer between pPD-rGO/electrolyte interface more compact.The compact CEI layer prevented the diffusion of PF₆^-ions and the formation of new F radicals,thus stabilizing CEI layer and improving the cycle performance of LNMO eletrode.Furthermore,the pPD-rGO/3401 composite separator was used with LNMO electrodes and the cycle performance was also improved,indicating the pPD-rGO layer on the surface of the separator could also stabilize CEI layer.Thirdly,LDG/3401 composite separator was used to improve the cycle performance of Li-S battery.The reason was that the LDG layer increased the conductivity of sulfur electrode and reduced the charge transfer impedance.Moreover,the LDG layer inhibited the diffusion of polysulfide ions to lithium anode,and reduced the irreversible loss of active sulfur.Finally,TiO₂-LDG/3401 composite separator was used to improve the cycle performance of Li-S battery.The optimal areal loading of TiO₂-LDG layer was 0.09mg/cm²,with which the battery showed the best cycle performance.Using this TiO₂-LDG/3401 composite separator,sulfur electrodes with sulfur loading of 1.4mg/cm² and 3.5 mg/cm² delivered the specific capacity of 704 m Ah/g and 686m Ah/g after 150 cycles at 0.2 C,and the average decay rates per cycle were only0.09%and 0.2%,respectively.Additionally,the effects of different using methods of TiO₂-LDG/3401?0.09 separator on the cycle performance of Li-S battery were studied.When the TiO₂-LDG layer faced the sulfur cathode in the battery,TiO₂-LDG layer improved the cycle performance by capturing the polysulfide ions dissolved in the electrolyte,inhibiting the the diffusion of polysulfide ions and reducing the charge transfer impedance.When the TiO₂-LDG layer faced the the lithium anode,TiO₂-LDG layer improved the cycle performance by inhibiting the growth of lithium dendrite.