| Lithium metal as a battery cathode has a very high theoretical specific capacity(3860 mAh·g-1),more than 10 times of carbon material;therefore the lithium metal as negative electrode for the new generation of rechargeable battery,is called"holy grail"of energy.However,the problems of dendrite and low coulomb efficiency of lithium metal electrode seriously restrict the practical application of lithium electrode.In this work,LiPF6-PC?LiPF6-EC/DEC?electrolyte system widely used in lithium ion batteries is selected to inhibit dendrite growth of lithium metal by adding different crown ether,boric acid and organic additive diethylenetriamine,and the effect of additive on coulombic efficiency and electrode cycling stability of lithium electrode are investigated.A simple and feasible electrodeposition technique is used to simulate the charge and discharge behavior of lithium metal on the electrode surface.In the three-electrode system,copper is used as the collector fluid and lithium as the constant current electrodeposition on the electrode.The influence of electrolyte additives on negative electrode electrodeposition behavior of lithium is evaluated by means of cyclic volt-ampere characteristic curve,electrodeposition curve,deposition morphology of lithium metal,impedance spectrum,etc.The results show that a certain amount of crown ether?15crown 5,12 crown 4?complexes can preferentially coordinate with lithium ions in the electrolyte to form ion groups,reduce the lithium ion concentration of hot spot in the lithium cathode,inhibit the advantageous growth of lithium synapses,and play a role in leveling the electrodeposition morphology of lithium metal to a certain extent.The experimental results show that at the current density of 2.0 mA/cm2 and the deposition amount of 1.0 mAh/cm2,the uniform and dense nano-columnar lithium layer could still be obtained under the action of crown ether.The optimal concentration of 15 crown 5 is 0.3 M,and the optimal concentration of 12 crown 4 is 0.2 M.A certain amount of boric acid was used as the film forming additive of SEI film,and SEM show that even at 10.0 mA/cm2 current density and 5.0 mAh/cm2deposition amount,uniform and dense nano-columnar lithium layer could still be obtained.The optimal concentration of boric acid is 0.02 M.XPS component test shows that Li-B-O is present on the surface of lithium metal electrode,which proves that boric acid can effectively participate in the construction of SEI passivation layer on the surface of lithium metal electrode.The coulombic efficiency and cycling stability of lithium metal electrodes are characterized by Li/Cu half cell and Li/Li symmetrical cell.We found that the coulombic efficiency decrease with the increase of crown ether concentration after the addition of crown ether,which may lead to the preferential solvation of lithium ions and crown ether weaken the formation of carbonate passivation film on the surface of lithium metal.After the addition of boric acid with the optimal concentration,the cycle life of lithium metal cathode is slightly extended,and the polarization is delayed after multiple deposition and stripping.However,its coulombic efficiency is not ideal.We also use organic compounds diethylenetriamine?DETA?as additive to regulate electrolytes in lithium metal deposition.Experiments show that in 1 M LiPF6-PC,0.02 vol%diethylenetriamine can better restrain the growth of the lithium dendrite,get more uniform density of lithium metal layer,and lithium basically grow in the form of bulk and short bar.Under the condition of 0.5 mA/cm2,0.5mAh/cm2,the efficiency of 200th cycle remains about 78%;under the condition of 1.0 mA/cm2 and1.0 mAh/cm2,the CE of 45th cycle is about 66%,which are better than the control group.In 1 M LiPF6-EC/DEC,when the concentration of diethylenetriamine is 0.01 vol%,the growth of lithium dendrites can be better inhibited,relatively uniform and dense lithium metal layer can be obtained,and the filamentous lithium dendrites on lithium Bulk also disappear.Under the condition of 0.5mA/cm2 and 0.5 mAh/cm2,the efficiency remain about 84%after 250 cycles;under the condition of1.0 mA/cm2 and 1.0 mAh/cm2,the CE of 55th cycle is about 88%,which are better than the control group.CV and XPS tests also show that diethylenetriamine have chemical and electrochemical reactions with lithium metal,and the products R-N-Li and LiF are involved in the construction of SEI passivation layer on the surface of the electrode,which could better protect the active lithium metal electrode.In conclusion,the three electrolyte additives studied in this paper can effectively inhibit the dendrite growth on the surface of Li metal cathode,but the inhibition of dendrites does not correspond to the increase of coulombic efficiency.The improvement of coulomb efficiency is the reflection of the comprehensive optimization of electrode,electrolyte and diaphragm system,which needs to be further studied. |
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