Studies On The Intercalation Between Graphite And Electrolytes And Functional Electrolytes For Lithium Secondary Batteries

The electrochemical behaviors of graphite anode greatly depend on the kind of the electrolytes.During the initial charge and discharge process,the electrolyte would inevitably react with graphite to form a solid electrolyte interphase?SEI?.The stable SEI film can further enables reversible lithium ion?Li⁺?intercalation/de-intercalation within graphite,as well as suppresses the electrolyte from decomposition.Otherwise,the Li⁺-solvent complexes would co-intercalate into the graphite and subsequently lead to the exfoliation of graphite.In this paper,the compatibility between graphite and electrolyte is investigated from the point of their intercalations.On one hand,the co-intercalation of the Li⁺-solvent complexes was used to exfoliate graphite into high quality graphene,which was further employed to modify various electrode materials.On the other hand,the suppression of graphite exfoliation is systematically investigated to develop advanced electrolytes for lithium ion batteries.In addition,due to the limitation of the theoretical specific capacity of graphite anode,it is difficult to further improve the energy density of lithium ion batteries;therefore high energy density lithium secondary batteries have attracted much attention.In order to suppress the growth of lithium dendrites of lithium secondary batteries,functional electrolytes for lithium secondary batteries have also been studied.Following are the main contents of this paper:?1?High quality boron-doped graphene sheets?B-EEG?are prepared via electrochemical exfoliation of graphite in lithium bis?oxalato?borate/dimethyl methylphosphonate?LiBOB/DMMP?electrolyte.The preparation process is simple and easy to scale up.The obtained B-EEG possesses high quality with a large planar size of?11?m,few layers?5 layers?,few structure defects and low oxygen content of 7.93%.When B-EEG was used as a host material for sulfur cathode,large-sized few-layer B-EEG facilitates encapsulating sulfur efficiently and thus reduces the dissolution of long-chain polysulfides via physical adsorption.Specifically,the doped boron atoms could also create more active sites,which can in turn help trap polysulfides through chemical adsorption.Meanwhile,the good conductivity can supply sufficient electrons to anchored polysulfides so that the polysulfides can be reduced in time.Additionally,the good mechanical property of B-EEG is also beneficial for accommodating the volume expansion and shrinkage of sulfur.?2?By directly contact graphite and lithium metal powders in lithium hexafluorophosphate/propylene carbonate?LiPF₆/PC?electrolyte,tremendous Li||graphite micro-cells under short-circuit conditions are formed.Subsequently,the co-intercalation of Li⁺-?PC?₄ complexes in the electrolyte would spontaneously exfoliate graphite into high quality graphene.Since there is no electric energy consumption,this obtained graphene is donated as“non-electrified electrochemical exfoliation graphene?NEEG?”.This non electrified approach can avoid the using of graphite rod as raw material and is beneficial for achieving continuous graphite exfoliation,which is helpful for large-scale production of high quality graphene.?3?An advanced lithium ion sulfur battery?LISB?is configured based on non-electrified electrochemical exfoliation and lithiation of graphite in electrolytes.On one hand,the high quality graphene,obtained via spontaneously electrochemical exfoliation of graphite in PC based electrolyte,works as a matrix to improve the conductivity of sulfur and trap the polysulfides.On the other hand,the lithiated graphite anode,obtained by spontaneously electrochemical lithiation of graphite in ethylene carbonate-based electrolyte,increases the safety coefficient of LISB.?4?The effect of the spatial configuration of Li⁺-?PC?_n?1?n?4?complexes on the electrochemical behaviors of graphite electrode in lithium bis?trifluoromethanesulfonyl?imide/PC?LiTFSI/PC?electrolyte is investigated.At a low concentration of 1.3 M and2.1 M,the graphite electrode is exfoliated.By contrast,at a high concentration of 2.5 M and 3.3 M,the graphite exfoliation can be suppressed.According to the density functional theory?DFT?calculations,the electrochemical behaviors of the graphite anode is tightly associated with the special spatial configuration of Li⁺-?PC?_n?1?n?4?complexes and the corresponding SEI formation mechanism at various LiTFSI concentration.Upon increasing the concentration of LiTFSI,the spatial configuration of Li⁺-?PC?_n?1?n?4?complexes gradually changed from tetrahedron?occupied space of10.19??to planar?occupied space of 3.05??,which is beneficial for suppress the graphite from exfoliation.Meanwhile,the affinity between Li⁺-?PC?_n?1?n?4?complexes and TFSI^-was increased,subsequently greatly contributed to the decomposition of TFSI^-anions to form a SEI on the surface of graphite.?5?A highly concentrated lithium bis?fluorosulfonyl?imide/trimethyl phosphate?LiFSI/TMP?solvent is prepared by tuning the solvation structures of electrolyte.The highly concentrated LiFSI/TMP electrolyte is totally nonflammable,which is beneficial for alleviating the potential safety hazards of batteries caused by the combustion of electrolytes.Meanwhile,the highly concentrated LiFSI/TMP electrolyte is helpful for suppressing graphite exfoliation,preventing the corrosion of LiFSI toward Al and the growth of lithium dendrites.Besides,the concentrated LiFSI/TMP electrolyte can achieve good electrochemical performances both in lithium metal batteries(Li||LiFePO₄and Li||LiNi_1/3Co_1/3Mn_1/3O₂)and lithium ion batteries(graphite||LiNi_1/3Co_1/3Mn_1/3O₂).?6?Lithium difluorophosphate?LiDFP?is employed as an electrolyte to suppress the growth of lithium dendrites.The LiDFP-derived SEI is dense,stable and highly ion conductive.Besides,the SEI is rich in LiF and P-O phosphate compounds,which is beneficial for suppressing the side reactions between lithium and the electrolyte.After adoption 0.15 M LiDFP,the Li||Cu cell shows a highest coulombic efficiency of 95.2%.Meanwhile,the optimal 0.15 M LiDFP electrolyte allows the Li||Li symmetric cells to cycle stably for more than 500 and 200h at 0.5 and 1.0 mA cm^-2,respectively,much longer than the control electrolyte without LiDFP additive.?7?We summarized this dissertation;put forward the possible suggestions and research plan.