Preparation And Optimization Of LiFePO₄ Based Lithium-ion Batteries For High And Low Temperatures

To explore the new energy and to develop the efficient energy storage systems have become the main strategy to solve the increasing energy crisis.Lithium-ion batteries as one of the most stable energy storage systems,have found extensive applictions in electronic devices and electrical vehicles.However,the poor low-temperature performances and unsatisfied high-temperature performances have severely limited their applications in cold areas and special fields and increased the risks in the charging/discharing process.The rate-determining step of low-temperature performance is the chemical reaction on the electrode/electrolyre interface.The essence of the improvement of low-temperature performance is to enhance the interfacechemical reaction.Thus,the optimization of electrole materiasl and solid-electrolyre interface（SEI）film is important.The SEI properties are correlated to the materials of electrode and electrolytes.The optimization of electrolytes and electrode should be the first step to improve the low-temperature performance.The degradation of battery starts form45℃.High thermal stability for each part of batteris is necessary to the remarkable high-temperature performances.Separator may be the most vulnerable part for next generation high-temperature batteries exploitation.Becaue the electrodes and electrolytes with high thermal stability have been reported.However,sepatator with high thermal stability and high performances is rare in the previous studies.Therefore,the optimization of separator should be the first step of to improve the high-temperature performances.In this work,the low-temperature performance and high-temperature performances of lithium-ion batteries are improved by the optimization of electrolytes and separator.According to the experiments and analysis,we conclude as follows:（1）The replacement of traditional electrolytes by 1 M Lithium oxalyldifluoroborate/Ethyene Catrbonate-Propylene carbonate-Methyl butyrate（LiODFB/EC-PC-MB）significant reduces the charge-transfer resistence(R_ct)and increases the ionic conductivity,result in superior low-temperature-performances.The capacity retension of lithium iron phosphate（LFP）/Celgard 2325/Li cell at-20℃with LiODFB/EC-PC-MB reaches 39%from 11%with traditional electrolytes.Then,we replace the commercial separator with PVDF-HFP/SiO₂ separator,which improve the capacity retension to 71%in comparison to the commercial one.This is caused by the remarkabe electrolyte uptake and wettability of PVDF-HFP/SiO₂ separator.（2）We obtain a high-performance PVDF-HFP-LFP separator by the introduction of LFP into PVDF-HFP.Calculations combined with Fourier-transform infrared spectroscopy analysis suggest that there exists a strong interaction between the-CF₃ group in PVDF-HFP and the LiO surface of LFP.A more stable PVDF-HFP-LFP separator structure formed in the form of chemical bond such as Fe-C bond and Li-F bond.The in-situ Raman spectroscopy and electrochemical test of aluminum foil/PVDF-HFP-LFP/Li cell prove that the LFP in separator takes part in the electrochemical reaction and contribures 2.7%capacity in comparison to the LFP cathode.In addition,the atomic-level integrity of the separator-cathode interface expedites lithium-ion transfer efficiency.Thus,PVDF-HFP-LFP separator owns pronounced ionic conductivity,micro-pore structure,thermal stability,wettability,and electrolyte uptake.LFP/PVDF-HFP-LFP/Li cell exhibits a room temperature capacity up to 162 mAh g^-1 at0.1 C,a capacity up to84 mAh g^-1 at 5 C.It also delivers a capacity up to 146 mAh g^-1 at80 ^oC at 0.5 C,only 5.4%capacity degradation was observed after 10 charge/discharge cycles.（3）PVDF-HFP-LiMn_0.5Fe_0.5PO₄（LMFP）separator was obtained by the uniform addition of LMFP into PVDF-HFP.Calculations combined with Fourier-transform infrared spectroscopy analysis suggest that there also exists a strong interaction between the-CF₃ group in PVDF-HFP and the LiO surface of LMFP.The chemical activity of Fe and Li in LMFP did not affected by the doping of Mn2⁺.Moreover,the active materials in LMFP also take part in the charge/discharge electrochemical reaction and contribute to the capacity.LMFP/PVDF-HFP-LMFP/Li cell exhibits a room temperature capacity up to 150 mAh g^-1 at 0.1 C.It also delivers a capacity up to 140 mAh g^-1 at 80 ^oC at 0.5C,only 2.1%capacity degradation was observed after 10 charge/discharge cycles.These prove that the PVDF-HFP-LMFP separator not only improves the pore structure,thermal stability,but also enhances the cathode-separator synergistic and energy density of battery.Here,we provide a new avenue to the battery energy density and thermal stability improved.（4）The crystallinity of separator is fine-tuned by the incorporation of graphene oxide（GO）into PVDF-HFP.GO occupys the crystal note of the PVDF and breaks its ordered structure result in a reduction of crystallinity of separator.The addition of 1 wt%GO into PVDF-HFP exhibits the lowest crystallinity.The introduction of GO in PVDF-HFP gives separator a high electrolyte uptake,wettability,mechanincal properties,and thermal stability.LFP/PVDF-co-HFP/GO/Li cell exhibits a remarkable electrochemical performance and high tempetature performance.It delivers a room temperature capacity up to 160 mAh g^-1 at 0.5 C with only 5.0%capacity degradation after 200 cycyles,and a capacity of 100 mAh g^-1 at 10 C.It also delivers a capacity up to 148 mAh g^-1 at 80 ^oC at 0.5 C,with only 2.7%capacity degradation after 10 charge/discharge cycles and with106%capacity retension of inicial capacity.