Design Of Phosphorus-Containing Flame Retardant Separators And Electrolytes For Thermal Safety Lithium Batteries

With increasing environmental awareness and rapid technological advances,lithium batteries have developed into one of the most mainstream clean energy storage devices.However,while lithium batteries provide convenience to people,the fire hazards associated with their high energy density are also a source of fear.In recent years,lithium batteries have caused numerous fire and explosion accidents,and the safety of lithium batteries has become the focus of public attention.The extremely combustible separator and liquid electrolyte,and uncontrollable lithium dendrites are considered to be the real culprits behind the safety accidents of lithium batteries.Therefore,the development of safety separators and electrolytes with excellent electrochemical performance and flame retardant properties are expected to solve the fire hazard of lithium batteries from the roots.For the manufacture of safety separators,electrostatic spinning,as a new fiber material production technology with wide applicability,high process controllability and simple equipment,has shown great potential in developing high-performance lithium battery separators.At the same time,the surface coating treatment technology based on commercial polyolefin separators is also considered to be a simple and easy-to-operate separator modification method with strong commercial application capability.For the development of safe electrolytes,phosphate esters with high flame retardant efficiency,low electrochemical toxicity and low cost show great promise in the development of flame retardant electrolytes,and the precise regulation of the solvation structure of the Li+enables the effective suppression of lithium dendrites,thus achieving simultaneous improvement of safety and electrochemical performance in lithium metal batteries(LMBs).Therefore,a series of researches based on the above research ideas and design concepts are carried out in this thesis,and the summary is as follows:(1)To address the safety concern of lithium dendrites,a polyvinyl alcohol/silica(PVA@SiO2)composite separators with a necklace-like structure and high surface Young’s modulus have been prepared by electrospinning technology.The separators display excellent electrolyte wettability,high ionic conductivity and thermal stability.LiFePO4//Li cells with composite separators show superior cycling and C-rate performance than commercial separator,exhibiting higher discharge specific capacity and lower voltage polarization at the same current density.Notably,the PVA@SiO2 separators are able to effectively suppress lithium dendrites through a "three lines of defense" mechanism.As a result,the PVA@SiO2 separator exhibits a high Coulombic efficiency of 91.77%(1.0 mA·cm-2)after 100 cycles of repeated lithium platting/stripping in a Li-Cu cell with carbonate based electrolyte.(2)In view of the above-mentioned disadvantages of the flammability of the separator,ammonium polyphosphate(APP),which has high flame retardant efficiency for polyvinyl alcohol(PVA)matrix,has been introduced.A microcapsule structure of APP surface modified by SiO2(APP@SiO2)has been synthesized,and then a polyvinyl alcohol/ammonium polyphosphate@silica nanocomposite fiber separators(PVA@AS)have been prepared by electrospinning technology.The composite separators show excellent flame retardant effect,and the self-extinguishing of the separator can be achieved at 10 wt%of APP@SiO2 addition.The PVA@AS composite separators also exhibit excellent lithium dendrite inhibition performance.Li-Cu cells with PVA@AS separator(1 mA·cm-2)can operate at an average Coulomb eficiency of 90.80%for more than 100 cycles.DFT calculations show that the binding energy of APP and SiO2 to the electrolyte is 4-6 times higher than that of polypropylene,therefore,the ionic conductivity of the PVA@AS can be as high as 1.475 mS·cm-1 when APP@SiO2 particles are added at 10 wt%,which is about 8 times higher than that of commercial separator.Owing to the remarkable ionic conductivity and commendable lithium dendrite suppression effect,the PVA@AS cell demonstrates excellent electrochemical performance,outperforming commercial separator cells in both cycle and C-rate performances.(3)Considering the good performance and low price of current commercial polyolefin separators,the above synthesized APP@SiO2 particles have been coated onto the surface of the commercial separator using a simple scraping method to form a sandwich-shaped separator.The experimental results show that the APP@SiO2 separator with electrolyte(about 4.45 μl·mm-2)shows excellent flame retardant properties and is rapidly self-extinguishing with no melt dripping after the igniter leaves.Furthermore,with the introduction of a high electrolyte affinity coating,the ionic conductivity of the sandwich separator has been increased to 0.8361 mS·cm-1.The electrochemical performance of cells with APP@SiO2 separator has also been significantly improved.At the same current density,APP@SiO2 cells can demonstrate higher initial discharge specific capacity and cycle stability than cells with commercial separator.The C-rate performance of the corresponding cell has also been significantly improved.More importantly,the APP@SiO2 separators show excellent litthium dendrite suppression properties because the high ionic conductivity of the modified separator promotes uniform Li+deposition and the lithium dendrites are consumed by SiO2 through alloying reactions to form LixSiy.Consequently,it guides the dendrites to grow along the cross-section of the separator.(4)To address the problem of highly flammable electrolytes,a low-cost and chemically compatible of triethyl phosphate(TEP)has been adopted as a flame retardant additive for commercial carbonate electrolytes.TEP(with a higher Gutmann donor number of Gn=26)has better solubility with LiNO3.Therefore,it can be used as a co-solvent to introduce LiNO3,which is poorly soluble in carbonate-based electrolytes.Additionally,the fluorinated ethylene carbonate(FEC)is added to improve the structural stability of the solid electrolyte interphase(SEI)protective layer,thereby improving the suppression of lithium dendrite formation.The results show that the modified electrolyte(base electrolyte:TEP:FEC=8:1:1,named as E-FT)exhibits good flame retardant effect compared with the high self-extinguishing time(SET)of 72.2 s·g-1 in commercial electrolytes,and the SET is significantly reduced to 11.1 s·g-1.The E-FT electrolyte also shows dendrite-free performance due to the excellent SEI film formation effect of LiNO3 and FEC.The Li-Cu cell can run stably for over 100 cycles with a Coulomb efficiency of 98.76%(1.0 mA·cm-2).At the same time,the electrochemical performance of the E-FT electrolyte is also significantly improved,in LFP//Li cell,the cells with E-FT electrolyte are hardly significant capacity degradation after 300 cycles,and matching the NCM811 cathode also has a good stabilizing effect on cycling,showing a high capacity retention rate of over 90%after 300 cycles.(5)In order to further improve the flame retardant properties of electrolytes,the flame retardant of TEP and FEC have been selected as co-solvents of electrolytes.A new intrinsically nonflammable electrolyte(TF31-1.5 M)has been formulated by using LiNO3 as the only lithium salt.In this electrolyte system,metallic lithium deposits tightly in a bulk form,exhibiting characteristics of dendrite-free morphology.Compared to commercial electrolytes,high Coulombic efficiency of the TF31-1.5 M electrolyte can reach 98.58%in Li-Cu test at 1 mA·cm-2 of 1.0 mAh·cm-2.The electrolyte also demonstrates significantly better electrochemical cycling stability than commercial electrolytes,with capacity retention rates of 96.39%and 83.74%after 1000 cycles for the LFP//Li and NCM811 systems,respectively.In order to further investigate the potential application of the electrolyte,the TF31-1.5 M electrolyte demonstrates superior cycling stability compared to commercial electrolytes under high LFP loading(15.5 mg·cm-2)and poor electrolyte(3.22μl·mg-1),as well as high NCM811 loading(8.6 mg.cm-2)with poor electrolyte(4.65 μl·mg-1).A combination of molecular dynamics simulations,DFT calculations and experiments reveal that TF311.5 M electrolyte has a unique solvation structure,which promotes the formation of SEI films with high ionic conductivity and structural stability during cycling,thus ensuring superior electrochemical performance.(6)The above safety separators(PVA@AS,APP@SiO2)and safety electrolytes(E-FT,TF31-1.5 M)with excellent electrochemical performance have been combined and integrated to construct a fully flame-retardant soft pack batteries.Compared to the cells with commercially formulated,the fully flame-retardant formulated cells(PVA@AS+TF3 1-1.5 M)demonstrate higher discharge specific capacity and lower cycle capacity decay rates in both the LFP//Li and NCM811//Li battery systems.The fully charged soft pack batteries with a capacity of approximately 1.0 Ah have been subjected to continuous external heating until ignition,and important indicators such as combustion phenomena,heat of combustion and smoke emission have been analyzed to investigate the fire hazard of the fully flame-retardant soft pack batteries.Compared to commercially available batteries that are highly prone to ignition,flame-retardant batteries do not exhibit intense combustion behavior caused by electrolyte and separator during the initial heating process.Instead,a substantial amount of white smoke with flame-retardant effects is generated until the eventual combustion of metallic lithium occurs.Compared with commercial batteries,the fully flame-retardant batteries delay the ignition of the LFP//Li and NCM811//Li batteries by 143 s and 105 s,respectively.The peak heat release rate of the LFP//Li battery has been reduced by 37.73%and the total heat has release by 56.43%,while the peak heat release rate of the NCM811//Li battery has been reduced by 41.60%and the total heat has release by 52.53%,Finally,the analysis of CO emission during combustion of lithium metal soft pack batteries shows that the total CO emissions of LFP//Li and NCM811//Li batteries,using the flame retardant formulation,have been reduced by 23.83%and 37.13%respectively.Thus,the formulation containing PVA@AS separator with TF31-1.5 M electrolyte has excellent electrochemical performance and fire safety,which is an important guideline for the commercial application of high performance and high safety LMBs.