Preparation And Properties Of NaTi₂（PO₄）₃ As Anode Materials For Aqueous Rechargeable Na-ion Batteries

Sodium ion battery has the advantage of resource and cost,which is becoming a hotspot of new generation technology.For large scale fixed energy storage occasions,the aqueous sodium ion battery is more safe,reliable,inexpensive and environmentally friendly,has a promising application prospects in theory.Olivine type NaTi₂（PO₄）₃ with typical sodium fast ion conductor structure,the size of the tunnel allows sodium ions to move freely.NaTi₂（PO₄）₃ has certain insert sodium capacity and good cycling performance in aqueous battery system.The theory capacity is 133 mAh/g,become an ideal sodium ion battery anode materials.Firstly,pure phase NaTi₂（PO₄）₃ materials were synthesized by solid-state method.The optimal sintering condition is sintering at 850 ℃ for 8 h.The capacity of the synthesized samples in aqueous and non aqueous electrolytes was 67.8 mAh/g and 68.2 mAh/g at 0.1 C.Pechini method was used to synthesize Pure phase NaTi₂（PO₄）₃materials..The capacity was84.5 mAh/g and 102.5 mAh/g at 0.1 C in aqueous solution and non aqueous electrolyte.Modifying the pure phase synthesized by Pechini method because of better electrochemical properties.Secondly,NTP/C composites were synthesized using citric acid retained as carbon sources by modified Pechini method.Graphite coated NTP/G were prepared by ball milling technology.The electrochemical performance of NTP/C material prepared under optimal conditions was obvious better than NTP/G by comparison.A capacity of 107.9mAh/g was retained after charge-discharge at 2 C for 100 cycles.The reversible capacity was 75.5 mAh/g at 20 C,and the capacity of 69.7 mAh/g was maintained after 100 cycles.Thirdly,Na₂FePO₄F/C crystal materials were prepared by rheological phase method.The sodium insertion/emersion reaction is in two step during the range of-0.3-0.5 V（vs.Ag/AgCl electrode）.The capacity of Na₂FePO₄F/C crystal materials is 51.4 mAh/g in aqueous solution,the capacity retention rate is 79.3% after 30 cycles,means Na₂FePO₄F/C crystal materials could charge-discharge in aqueous solution.At last,NaTi₂（PO₄）₃/C/Na₂FePO₄ F battery system was assembled,the capacity is115.2 mAh/g at 1 C under room temperature.Temperature has an effect on the charge anddischarge performance of the battery system.When charging and discharging in the environment of 35 ℃,capacity performance and cycle performance are improved,the capacity is 130 mAh/g at1 C.The structure change of Na₂FePO₄ F material in charge and discharge is larger,which is the reason of the unstable electrochemical performance in aqueous solution.