Highly Safe Electrolyte Systems For Lithium-ion Batteries

Lithium-ion batteries are now widely used as energy storage devices for portable electronic devices such as laptop computers, cellular phones and digital cameras due to their high-energy density. Large-sized lithium-ion batteries and battery packs are also very attractive for electric vehicles (EV) and hybrid electric vehicle (HEV) applications. However, safety concerns have become the most important issue for the development of lithium-ion batteries because quite a few serious accidents of fire and explosion can be traced back to the malfunction of lithium-ion batteries. In this doctoral dissertation, the author mainly pays attention to the highly safe electrolyte systems for the lithium-ion batteries, including flame-retardant or even nonflammable electrolytes, room temperature ionic liquid (RTIL) based electrolytes. The thermal stability and compatibility with electrodes are focused aspects for these electrolytes. As a spin-off research result, submicron graphitic sheets are also synthesized by the flame-retardant electrolyte via an electrochemical route.In Chapter 1, a general introduction is given on following aspects: the development and status of lithium-ion batteries, structure, components and working mechanism of lithium-ion batteries. Then, the safety concern of lithium-ion batteries is discussed in detail, including the reasons for thermal runaway and the ways to improve the safety characteristic.In Chapter 2, the physicochemical properties of dimethyl methyl phosphonate (DMMP) are first introduced. Then DMMP is used as a flame-retardant additive in the electrolyte of Li-ion batteries. It is found that DMMP is friendly for the cathode material, e.g. LiCoO2, but it is incompatible with the graphite anode. Fortunately, the DMMP-containing electrolyte, i.e. 10% DMMP added into the baseline electrolyte, has good compatibility with both mesocarbon microbead (MCMB) anode and surface-modified graphite (SMG) anode.In Chapter 3, an"absolutely"nonflammable electrolyte was prepared, and it exhibits low-temperature performance better than that the baseline electrolyte. Although a nonflammable electrolyte (with a high content of DMMP) has the poor compatibility with carbonaceous anodes, even with MCMB and SMG, the compatibility can be improved significantly by adding 5% vinyl ethylene carbonate (VEC), a film-formation additive, into the nonflammable electrolyte. Different with carbonaceous materials, Li4Ti5O12 anode has good compatibility with the nonflammable electrolyte without VEC. The novel LiNi0.5Mn1.5O4/Li4Ti5O12 cell with the nonflammable electrolyte is a promising candidate to resolve the safety concerns of lithium-ion batteries.In Chapter 4, the effect of activation temperature on compatibility between graphite anode and flame-retarded electrolytes containing DMMP and trimethyl phosphate (TMP) is investigated respectively. It is found that activation at elevated temperature promotes the formation of a stable solid electrolyte interface layer on the graphite electrode, which may significantly suppress the reductive decomposition of DMMP and TMP and avoid graphite exfoliation. But fierce oxidation of the electrolytes on the LiCoO2 electrode at elevated temperature is harmful to the cell performance. A procedure of so-called altered temperature activation (ATA) is adopted for LiCoO2/graphite full-cells. It can compromise the contradictive effects on the separate electrodes at the elevated temperature. High capacity and good rate capability are obtained for the cells with the flame-retardant electrolytes, especially for the TMP-containing electrolyte.Many factors could affect the compatibility between the flame-retardant electrolyte and carbonaceous anodes, such as the proportion of components in the electrolyte, the type of the carbonaceous material, binder and conductive additive. To investigate these factors, too many experiments would be done, which is fatiguesome and unnecessary. In Chapter 5, the orthogonal arrays for experimental design are used to evaluate all the factors above, in a simpler and faster manner.Room temperature ionic liquids (RTILs) have been extensively studied as new solvents in electrochemical batteries and supercapacitors due to their extremely low vapor pressure and good flame resistance, which make RTILs attractive candidates for the electrolytes of lithium-ion batteries with excellent safety characteristics. In Chapter 6, the RTIL consisting of N-methyl-N-propyl -piperidinium (PP13) cation and bis(trifluoromethanesulfonyl)imide (TFSI) anion is synthesized. The effect of the content of Li salt in the binary LiTFSI-PP13TFSI electrolytes on the ionic conductivity and cell performance is investigated. In addition, it is found that 20% diethyl carbonate (DEC) as a cosolvent introduced into the RTIL-based electrolyte can obviously improve the rate capability and low-temperature performance, with a negligible damage to safety characteristic of the Li-ion batteries.In Chapter 7, the effect of the capacity matchup between cathode and anode in the LiNi0.5Mn1.5O4/Li4Ti5O12 cell system on cycling property, choice of electrolyte, high voltage and overcharge tolerances is investigated. The cells with Li4Ti5O12 limiting capacity exhibit better safety characteristic and cycling performance than the cells with LiNi0.5Mn1.5O4 limiting capacity.In Chapter 8, thermal stability of LiPF6-based electrolyte is studied by in-situ FTIR spectroscopy and C80 calorimetry. It is found that the electrolyte undergoes furious polymerization and decomposition reactions. The thermal stability of the electrolyte in contact with seven delithiated cathodes (LixCoO2, LixNi0.8Co0.15Al0.05O2, LixNi1/3Co1/3Mn1/3O2, LixMn2O4, LixNi0.5Mn0.5O2, LixNi0.5Mn1.5O4 and LixFePO4) is also investigated by C80 calorimetry. The results show that the cathode materials except for LixFePO4 usually have an enhancement effect on the decomposition of the electrolyte, but LixFePO4 exhibits a suppression effect on the reactions of the electrolyte. LixFePO4 is found to be with excellent thermal stability.Most flame-retardant electrolytes have poor compatibility with the graphite anode, owing to the graphite exfoliation phenomenon accompanied by reductive decomposition of the electrolytes. Thus, in literature, extensive efforts were made to avoid the exfoliation of graphite. However, in Chapter 9, the author manages to make use of the graphite exfoliation to prepare novel submicron graphitic sheets material via the electrochemical route.Finally, the author gives an overview on the achievements and the deficiency in this thesis. Some prospects and suggestions of the possible future research directions are pointed out.