Study On Stability Of The Graphite Anode |Electrolyte Interface In Li-ion Battery

With the continuous environmental deterioration and the gradual exhaustion of natural resources,human society urgently needs to transform from the current high-energy-consuming lifestyle to a more sustainable way.The revolution of energy technology has become the consensus of the whole society.The replacement of fuel vehicles by electric vehicles is an extremely important step in this great change.The development of electric vehicles also proposes higher standards for the lithium-ion battery technology.Lithium-ion battery is the most successful commercial secondary battery technology.It has the merits of high practical energy density and long cycle life.However,this technology still has problems in narrow operating temperature range,low security and bad rate performance especially under new application scenarios such as electric vehicles and energy storage power stations.Improving the performance of Li-ion batteries needs an in-depth understanding of the electrochemical reactions in Li-ion batteries.How to achieve the interfacial stability between the graphite anode and the electrolyte is the primary problem that worries the researchers in this area.This paper consolidates the studies from three independent research to discuss the topic of the stability of the interface between graphite anode and electrolyte:(1)A novel ether-ester mixture electrolyte that can stabilize the graphite-electrolyte interface was developed.We have developed new electrolytes that can form stable interfaces with graphite anodes.The solvents of this new electrolyte are propylene carbonate(PC)and diethylene glycol dimethyl ether(G2),which are originally incompatible with the graphite anode.The lithium salt for this new electrolyte is lithium nitrate,which is one of the cheapest lithium salts.We found that although the concentrated salt electrolyte of pure lithium nitrate and G2 can achieve lithium intercalation,the cycle performance declines rapidly.Combining the dq/dv curve and XRD data,we found that this is because the electrolyte can only partially passivate the graphite surface,and there is still some co-intercalation of G2 molecules.After adding PC,the stability of the electrolyte and the graphite anode was significantly improved.As a result,the reversible capacity improved.Analysis of the electrolyte structure by infrared spectroscopy and Raman found that PC accounted for a small proportion of the solvated structure of lithium ions.XPS and NMR characterization indicated that the Lithium propylene decarbonate(LPDC)generated by PC decomposition constituted an organic part in the solid electrolyte interlayer(SEI),playing an important role in suppressing solvent co-intercalation.This new model electrolyte not only provides us with a new type of inexpensive electrolyte free from expensive and toxic fluorine-containing lithium salts,but also give us new insights on stabilizing the interface between graphite anode and electrolyte.(2)The poor compatibility between the flame retardant co-solvent and the graphite anode is the main reason why the carbonate electrolyte in the conventional lithium-ion battery cannot be completely replaced by the flame retardant electrolyte.This accordingly hinders the progress of lithium-ion batteries to higher safety standards.We found that the stability between the graphite anodes and the carbonate-based electrolyte containing 20%vol trimethyl phosphate(TMP)can be significantly improved by adding only a small amount of lithium nitrate,while the security of the self-extinguishing electrolytes is not affected.The analysis of the solvation structure of the electrolyte by Raman show that the addition of lithium nitrate will not affect the coordination of TMP and lithium ions,but it will make the interaction between Li+and other components(i.e.,PF6-and carbonate)weaker.However,the XPS result show that the decomposition products of PF6-and carbonate(i.e.,LiF and ROCO2Li)increase with the LiNO3 addition,which is beneficial to the interfacial stability.It is well known that the solvation shell of Li ion is the precursor of SEI.Obviously,there is a disagreement between the Raman result and the XPS result.We confirmed a new mechanism at the interface by means of UV-Vis experiments and DFT calculations.That is,the nitrate anion generates active oxygen when it is electrochemically reduced,and the active oxygen attacks other components in the electrolyte(i.e.,PF6-and carbonate),so that the amount of LiF and ROCO2Li,beneficial for the SEI stability,rise.Then the new SEI with great robustness completely inhibited the co-intercalation of TMP.(3)Lithium ion and ether solvent co-intercalation graphite anode system is a new type of anode system emerging in recent years.However,this new system that does not produce SEI also has the problem on the stability between the electrolyte and the graphite anode.Specifically,when the G2-based electrolyte works with the graphite electrode,the irreversible capacity in the first cycle is quite large,and the coulombic efficiency of the first cycle is low.Two representative lithium salts,LiBr and lithium bis(trifluoromethane sulfonyl)imide(LiTFSI)were selected to combine with G2 as model electrolytes.After a series of characterizations on their electrochemical behavior,solvated structure and the decomposition products on the graphite surface,the source of the irreversible capacity is identified to be the decomposition of the G2 solvent itself below 0.6 V vs Li/Li+.There are two approaches found to solve this problem.One is to improve the reduction stability of G2 by reducing the interaction between G2 and lithium ions.The other is to change the working potential range of the graphite anode to avoid the irreversible decomposition,thereby making it closer to practical use while sacrificing a small amount of capacity.This work proposes a new solution to the problem of this novel system from the aspect of solvation,and shed light on its further development.