TECHNICAL FIELD

The invention relates to the field of lithium batteries, and in particular, to a lithium metal anode protection method improving lithium utilization efficiency.

BACKGROUND

In the process of the recycling of the lithium metal anode, there are problems such as unevenness of lithium deposition and dissolution, and high side reaction activity between electrolyte and lithium metal, which result to the growth of lithium dendrites, low coulombic efficiency and low cycle capacity retention of lithium batteries, etc.

SUMMARY

The purpose of the present invention is to reduce the contact of components other than lithium ions in the lithium battery electrolyte with the lithium metal anode, so as to avoid the occurrence of continuous side reactions, and to make the lithium deposition smaller and more uniform. Thereby to slow down the formation of lithium dendrites and achieve a high lithium utilization efficiency of the lithium metal anode. Wherein the high lithium utilization efficiency refers to the improvement of coulombic efficiency and full battery performance. And further includes the use of a very small amount of lithium to complete the long-term stable cycle of the battery, with a high cycle capacity retention rate of the lithium battery. To achieve the forgoing objective, the present invention adopts the following technical solutions.

The present invention provides a lithium metal anode protection method improving lithium utilization efficiency. The lithium metal anode protection method comprises steps of depositing lithium metal on a current collector as an anode of the lithium battery and adding a high molecular polymer configured as an additive to an ester electrolyte.

The high molecular polymer being prepared by a polymerization reaction of monomer A being acrylonitrile or derivatives thereof, monomer B being perfluoroalkyl ethyl methacrylate or derivatives thereof, and monomer C being alkyl alcohol diacrylate or derivatives thereof.

Because the surface of lithium metal is negatively charged, the —CN and —CF3in the polymer are strong electron-withdrawing groups, which promote the preferential adsorption of electrolyte additives on the surface of lithium metal. At the same time, the battery with carbon paper as the cathode, and lithium sheet as the anode is tested by CV (cyclic voltammetry) method with an open circuit voltage test at a scan rate of 50 mV/s. The half-cell with electrolyte additives does not produce a new signal peak for the decomposition of electrolyte components. It is proved that the high molecular polymer will not decompose in the battery test environment and will not participate in the electrochemical reaction. The adsorption effect of electrolyte additives can reduce the contact of other components in the electrolyte with lithium metal, thereby avoiding the occurrence of continuous side reactions.

Lithium metal is very active, and it will react with electrolyte to form SEI film, which will consume a large amount of active lithium and electrolyte. This is also one of the factors that cause the long-term stable operation of lithium metal batteries. By adsorbing, a layer of organic molecules that will not decompose on the surface of lithium metal, the contact between lithium salt and solvent molecules in the electrolyte can be reduced, and the probability of decomposition can be reduced. At the same time, the polymer molecules are capable of conducting lithium ions, thus will not have a great impact on the transmission of lithium ions. Therefore improves the Coulomb efficiency.

Through the CV test, it is known that when the polymer is added as an electrolyte additive, the potential of lithium overpotential deposition negative shifts, indicating that the electrolyte additive has a blocking effect on lithium deposition. And the overpotential of lithium deposition is inversely proportional to the size of lithium nuclei. Therefore, the addition of electrolyte additives makes lithium deposition smaller and more uniform, thereby slowing the formation of lithium dendrites.

Preferably, the current collector is carbon paper. Carbon paper is more beneficial to the improvement of the coulombic efficiency of the lithium metal anode. By depositing a small amount of lithium on the surface of the current collector as the anode of the lithium battery, the safety performance of the battery can be improved while improving the lithium utilization efficiency.

Preferably, the cathode of the lithium battery is lithium iron phosphate.

Preferably, an addition amount of the high molecular polymer is 2 wt % of that of the ester electrolyte. The addition of electrolyte additives with a mass fraction of 2% is more conducive to the improvement of coulombic efficiency and the long-term stable cycle of the battery.

Preferably, the ester electrolyte is a 0.8˜1.2 mol/L LiPF6solution, wherein the solvent is selected from ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate, and the LiPF6solution contains fluoroethylene carbonate with a mass ratio of 3 to 7%.

Preferably, the step of preparing the high molecular polymer from monomer A, monomer B and monomer C through polymerization reaction includes: under the protection of inert gas, dispersing monomer A, monomer B, and monomer C in water with a molar ratio of 1:1:0.01-2; heating to 50-80° C. and stirring for 5-15 minutes; and then adding an initiator potassium persulfate; performing the polymerization reaction at 50-80° C. for 5-24 hours to obtain a polymerization solution; wherein a mass ratio of the initiator potassium persulfate to the monomer C is 0.01-0.2:1; drying the polymerization solution to obtain the high molecular polymer.

Preferably, the monomer B is selected from perfluoroalkylethyl methacrylate, (meth)acrylic acid fluorine-containing alkyl ester, (meth)acrylic acid heteroatom-containing perfluoroalkyl ester, (meth) acrylic acid perfluoroamide and (meth) perfluorosulfonamide acrylate.

Preferably, the monomer C is selected from diethylene glycol diacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, tripropylene glycol diacrylate, and neopentyl glycol diacrylate.

Preferably, the step of depositing the lithium metal on the current collector of the lithium battery as the anode of the lithium battery comprises assembling a half-cell where a cathode of the half-cell is a current collector and an anode of the half-cell is a lithium sheet; depositing a predetermined amount of lithium metal on the current collector under a predetermined current density in a predetermined time; and disassembling the current collector deposited with the lithium metal from the half-cell to act as the anode of the lithium battery; wherein an amount of the lithium metal deposited on the current collector is 0.1˜5 mAh/cm2.

Beneficial effects of the present invention lie in that, the contact of components other than lithium ions in the lithium battery electrolyte with the lithium metal anode are reduced, thereby avoiding the occurrence of continuous side reactions, and making the lithium deposition smaller and more uniform, so as to slow down the formation of lithium dendrites, and achieve high lithium utilization efficiency of lithium metal anodes. Wherein high lithium utilization efficiency refers to the improvement of coulombic efficiency and full battery performance. Further includes the use of a very small amount of lithium to complete the long-term stable cycle of the battery, and the lithium battery has a high cycle capacity retention rate.

