Electrochemical Properties At Elevated Temperature Of LiFePO₄F Li-ion Batteries

The lithium-iron-fluorophosphate?LiFePO₄F?cathode material in a relative lithium-ion battery is with a tavorite structure.Compared to the olivine-structured lithium iron phosphate?LiFePO₄?,the electronegativity difference between F^-and O^2-makes its structure more stable.The ion channels change from one-dimension into three-dimension,then its ionic conductivity could be improved by more than two orders of magnitude?vs.LiFePO₄?.These benefit the preparation of Li-ion batteries which behave excellent cycle performance and rate capability at room temperature?RT?and can keep stable at elevated temperature.In this work,on the basis of the early work in our group,synthesizing process for highly-pure LiFePO₄F has been optimized and the relative batteries with high-temperature performance been valuated.The in-situ carbon-coating is usually an effective technique to modify cathode materials and was then studied firstly in this work.Three forms of carbon source such as graphite??-SP?,glucose?C6H12O6?and graphene oxide?GO?was used.Unfortunately,all of the resultants were LiFePO₄ rather than LiFePO₄F phases.This confirms that the in-situ carbon-coating is unsuitable for the modification of LiFePO4F-type Li-ion batteries.Two kinds of commercial conductive carbons such as nano-sized spherical super conductive carbon?nano-SP?and micron-sized flake-like super conductive carbon??-SP?were used to prepare LiFePO4F batteries which are stable at RT/high temperature.It shows be that nano-SP is obviously better than?-SP for the ex-situ coating of micron-sized LiFePO₄F.The initial discharge specific capacity is 130mA·h/g at the rate of 0.1 C for the battery by using nano-SP,but only 55 mA·h/g by using?-SP.This indicates that nano-SP is more suitable for the ex-situ carbon coating in LiFePO₄F-type Li-ion batteries.Two kinds of electrolytes were used to evaluate the RT/high temperature performance of LiFePO₄F-type Li-ion batteries.The cyclic voltammetry proved that the LFPF-n-E₀ battery without vinylene carbonate?VC?additive exhibited poor cycle performance at elevated temperature.Its redox potential difference??Ep?in the 3^rdd scan increased sharply form 0.53 V at 40?to 1.53 V at 70?.However,the?E_p of LFPF-n-EVC battery with VC changed only from 0.69 V to 0.53 V.The discharge-charge cycle performances of LFPF-n-EVC and LFPF-n-E0 batteries are almost the same at RH,indicating that VC additive doesn't affect electrochemically the nano-carbon coated cathode materials at RT.But the cycle performance of LFPF-n-EVC is much better than that of LFPF-n-E0 at elevated temperature?40?,50?,60?,70?and 80??with rates of 0.2 C/1 C.For the LFPF-n-E_VC battery,the initial discharge capacity at 0.2 C and at 40?,50?,60?,70?and 80?is 125 mA·h/g,132 mA·h/g,129 mA·h/g,140 mA·h/g and 103 mA·h/g,respectively.The capacity retention after 60 cycles is 78.4%,62.9%,65.1%,57.9%and 33.2%,respectively.The initial discharge capacity at 1 C and at40?,50?,60?and 70?is 83 mA·h/g,101 mA·h/g,116 mA·h/g and 108mA·h/g,respectively.The capacity retention after 60 cycles is 120.8%,101%,87.9%,and 97.2%,respectively.For the LFPF-n-E₀ battery,the initial discharge capacity at 0.2 C and at 40?,50?,60?,70?and 80?is 119 mA·h/g,130 mA·h/g,141 mA·h/g,143 mA·h/g and 109 mA·h/g,respectively.It can't be cycled up to 60 times in the above temperature range.The initial discharge capacity at 1 C and at 40?,50?,60?and 70?is 97 mA·h/g,86 mA·h/g,121 mA·h/g and 110 mA·h/g,respectively.It can be cycled up to 60 times at 40?/50?and the capacity retention after 60cycles is 95.9%/102.3%.But it can't be cycled up to 60 times at 60?/70?.The cyclic voltammetry,galvanostatic charge-discharge performance,rate capability,the corresponding impedance and electrolyte dissolve-elements results prove that VC additive would significantly at high temperature inhibit the formation of resistive SEI film between cathode materials and electrolyte interface,and prohibit the electrolyte decomposition which could destroy the structure of LiFePO₄F cathode material.This is the reason that LFPF-n-EVC battery works stablely at elevated temperature.