Studies On The Behaviors Of Non-aqueous Electrolytes For Lithium Ion Batteries

The development of electrolytes for lithium-ion batteries was reviewed. The non-aqueous electrolytes were prepared with different additives such as SEI film formation additive, overcharge additive and flame retardant additive. The behaviors of electrolyte were characterized by various electrochemical methods in combination with Differential Scanning Calorimeter (DSC), Infrared (IR) Spectroscopy, High-temperature Experiment, Overcharge Experiment, Non-flammability Experiment, and so on.Lithiated molecular sieve was successfully prepared according to cation exchange approach combined with heat treatments using molecular sieve, alcohol and LiClO4 as starting materials. A purification technique of industrial dimethyl carbonate (DMC) was studied by using lithiated molecular sieve and P2O5. The DMC with 6 ppm H2O and without organic impurities according to the analysis of gas chromatograph was obtained.The behaviors of impurity water of electrolyte for lithium ion batteries were studied through DSC, FTIR and Cyclic Voltammetry approaches. Results show that the higher the water concentration of electrolyte was, the lower thermal stability of electrolyte and the higher content of Li2CO3 and ROCO2Li in the solid electrolyte interface film (SEI film).The behaviors of Li2CO3 as solid additive of electrolyte were studied. Results show that the cycleability of GBL-based electrolyte was improved by Li2CO3 additive. The electrolyte of 1 mol·L-1 LiPF6 EC-DMC-GBL (4:4:3, mass ratio)-4%VC (mass percent)-0.05 mol·L-1 Li2CO3 can deliver good cycleability and electrochemical performances at high temperature or low temperature. For example, the Li/C battery with this electrolyte delivered a reversible capacity of 341.8mAh·g-1 during the first cycle, and retained 98.5% of the reversible capacity after 50 cycles at 0.2C rate. The soft pack lithium-ion battery delivered a reversible capacity of 144.6mAh·g-1 during the first cycle and retained 91.2% of the reversible capacity after 200 cycles at 1 C rate.The effects of VC and Li2CO3 additives on the components of SEI film were studied by IR Spectroscopy. The possible mechanisms that the reduction decomposition of GBL was suppressed by VC-Li2CO3 additives were proposed as following. Firstly, the HF acid concentration of electrolyte was decreased by the reaction of Li2CO3 + HF = LiF + CO2. Secondly, the Li2CO3 and LiF deposited on the carbon electrode at the first charge process. At last, the resultants of the electrochemical polymerization reaction among VC molculars deposited on the carbon electrode. Then, the SEI film was almost formed before the reduction decomposition of GBL. The interface process of lithium insertion into carbon electrode was studied by A.C impedance and the equivalent circuit was obtained according to different A.C impedance spectrum at different potentials.Electrochemical behaviors of polymerization additives such as biphenyl (BP), cyclohexybenzene (CB), diphenyleneoxide (DPO) and hydrogenated diphenyleneoxide (H-DPO) for overcharge protection of lithium-ion batteries were studied. Results show that CB delivered the best integration electrochemical performance though the 4 kinds of additives all improved the safety properties of lithium ion batteries. Comparing to the batteries with the standard electrolyte of 1 mol·L-1 LiPF6 EC-DMC-EMC(1:1:1), the safety properties of lithium-ion batteries were remarkably improved by CB additives because the time of battery overcharged to 10V at 3 C rate was shortened and the highest surface temperature of battery was redued by about 12 ℃. The reseason was that the polymerization reaction of CB at the potential of 4.72-4.85 V (vs Li/Li+) increased the internal resistance and polarization of batteries.The prismatic lithium-ion batteries with the standard electrolyte including 3.5% of CB had 88% of capacity ratio and 60.3% of 3.6 V capacity efficiency after 150 cycles at 1C rate. With addition of 3.5%CB-4%VC-0.05 mol·L-1 Li2CO3 to standard electrolyte, the prepared lithium-ion battery delivered a reversible capacity of 145.5 mAh·g-1 and 88.2% of 3.6 V capacity efficiency during the first cycle, and had 94% of capacity ratio and 72.2% of 3.6 V capacity efficiency after 100 cycles at 1 C rate.The non-flammable electrolytes based on trimethyl phosphate (TMP) were prepared with (1-y%)(ECn-DMC1.0-EMC1.0)-y% TMP for mixture solvents and with LiPF6 for solute. The influences of carbonate solvents and the additives of forming SEI film on the reduction decomposition of non-flammable electrolyte were studied. Results show that the reduction decomposition of TMP was completely suppressed by adding 4%VC-0.05 mol·L-1 Li2CO3 to the non-flammable electrolyte of n>1.5 and y<39. The non-flammable electrolyte with 61%(EC1.5-DMC1.0-EMC1.0)-39%TMP-4%VC-0.05 mol·L-1 Li2CO3 as mixture solvents delivered good integration electrochemical properties. The lithium-ion batteries delivered a reversible capacity of 142.0 mAh·g-1 during the first cycle and of 124.1 mAh·g-1 after 50 cycles at 0.1C rate. The possible decomposition mechanisms of TMP were proposed and the resultants were considered as CH4, CH3CH3 and H3PO4.The DSC was used to perform the reactions of electrolyte with Li0.5CoO2 electrode or LiC6 electrode at elevated temperature. In the presence of sufficient electrolyte the decomposition reactions of Li0.5CoO2 proceeded in a clear stepwise manner through solid phase as a function of temperature as 6Li0.5CoO2→3LiCoO2+Co3O4→3LiCoO2+3CoO. Accompanying these changes were the electrolyte combustion reaction due to the evolved oxygen. If there was only a small amount of electrolyte relative to the amount of Li0.5CoO2, the second step of the decomposition reaction was difficult to occur. On the other hand, in the presence of sufficient electrolyte the reactions of LiC6 electrode at elevated temperature included the decomposition reaction of SEI film at around 154℃ and the complicatedreactions among the electrolyte, binder and the lithium of LiC6 compound at more than 200 ℃. Furthermore, the reaction heat of SEI film was considerably reduced by electrolyte additives of VC and Li2CO3.The method of elevated temperature experiments was used to characterize the effects of electrolyte additives on the thermal stability of lithium-ion batteries. Results show that the thermal stabilities of lithium-ion batteries were improved by VC, Li2CO3 or TMP additive. The lithium-ion battery with nonflammable electrolyte based on TMP delivered the highest thermal stability. In addition, the studies on the battery possible explosion mechanisms show that the main reason of the batteries explosion was the elevated temperature reactions between flammable electrolyte and Li0.5CoO2 or non-flammable electrolyte and LiC6 electrode.The influences of the electrolyte additives on the kinetics behaviors of carbon electrode were studied by means of Tafel polarization measurements. It was found that the exchange current densities (io) were reduced by GBL, CB or TMP additives and increased by VC or Li2CO3 additives. The i0 were more than 0.31 mA·cm-2 when adding 4% VC-0.05 mol·L-1 Li2CO3 to electrolytes with the following mixture solvents: EC-DMC-GBL(4:4:3), 61%(EC1.5-DMC1.0-EMC1.0)-39%TMP, or EC-DMC-EMC(1:1:1) -3.5%CB.