Studies On Electrode Materials For Na-Ion Stationary Batteries

As the increasing aggravation of the energy crisis and the environmental pollution,the utilization of cleaner and renewable energy which is intermittent and random has caught much attention from researchers,but it is necessary to develop large-scale energy storage system?EES?.Lithium-ion batteries?LIBs?are most widely employed among various energy storage techniques,due to the high energy density and power density.However,the low abundance and non-uniform distribution of Li will limit their large-scale application in EES.Even though the energy density of Na-ion batteries?NIBs?is insufficient,the application of NIBs in EES which has modest requirement on energy density is quite promising because of the huge abundance and low cost of Na.Since the working principle of NIBs is similar to that of LIBs,the development of NIBs can borrow the mature principle of LIBs.The key is to find suitable anode and cathode materials.Among the various cathode and anode materials,sodium vanadium fluorophosphate cathode and disordered carbon-based anode materials are very promising,attracting great attention from scientific researchers and industrial promoters.However,the synthetic methods and electrochemical properties of these two kinds of electrode materials are not enough to meet the requirements of practical commercial applications,which need to be further optimized.Based on the above background,this thesis systematically investigated the preparation,structure and electrochemical performance of these two kinds of electrode materials,as well as the underlying problems and the optimization of electrochemical performance in full cells.In the first part,to solve the two problems derived from high-temperature sintering,including the large amount of energy consumption and the difficulty in obtaining phase-pure product due to the volatilization of VF₃,two kinds of low-temperature solution-based strategies were developed to prepare sodium vanadium fluorophosphate.Firstly,a low-temperature hydrothermal method was proposed.Based on this method,the effects of precursor,synthetic condition and pH on the purity,crystallinity,morphology,structure and electrochemical performance were systematically studied for the first time.The results showed that the precursor and the pH would significantly affect the crystallization rate,purity,crystallinity,morphology and electrochemical properties of the product.The acid conditions are favorable for the synthesis of products with high purity,special morphology and excellent electrochemical properties.Secondly,a large-scale room-temperature strategy was developed inspired by the hydrothermal synthesis.Based on this method,the pH range for the successful synthesis of the target product was measured to be around 2-9,and the morphology can be easily regulated by pH.To take as an example,multi-layered hollow microspheres were produced based on the in-situ generated bubbles as soft templates under acid condition.The results showed that the multi-layered hollow microspheres presented superior rate capability and long-term stability,with 81 mAh/g at 15C and 70%capacity retention after 3000 cycles.Different from the multi-phase transition of product prepared by traditional high-temperature solid-state method,the as-obtained product prepared by our room-temperature strategy underwent solid-solution reaction during charge/discharge process.The charge transfer mechanism during charge/discharge was firstly characterized by UV-Vis spectroscopy technique as V⁴⁺to V⁵⁺.In the second part,a slope-dominated carbon anode,an intrinsic improving strategy to prevent the graphitization of pitch,as well as the effect of the secondary low-temperature treatment on the microstructure and the charge curve shape were investigated based on the low-cost pitch precursor.Firstly,a"reverse low-temperature strategy"was proposed to obtain slope-dominated carbon anode.The results showed that the optimal sample obtained at 800 ^oC displayed a high and slope-dominated reversible specific capacity of 263 mAh/g at 0.15C with a high initial Coulombic efficiency?ICE?of 80%.When paired with NaNi_1/3Fe_1/3Mn_1/3O₂ cathode,the full cells exhibited superior rate capability with 75%capacity retention at 6C compared with that of 0.15C,and excellent stability with 73%capacity retention after 1000 cycles at3C.Secondly,a general strategy to inhibit the graphitization of pitch was put forward,by analyzing the origin of hard and soft carbon.The power of the general strategy was demonstrated by Mg?NO₃?₂·6H₂O.In addition,the effects of the Mg?NO₃?₂·6H₂O content and carbonization temperature on the microstructure and electrochemical properties of as-prepared carbon materials were systematically studied.The results showed that the sample obtained at 1400 ^oC with 50%Mg?NO₃?₂·6H₂O demonstrated best Na-storage performance with a high reversible capacity of 277.8 mAh/g and 97.7%capacity retention after 200 cycles at 0.1C.Thirdly,inspired by the"reverse low-temperature strategy",it is found that the secondary low-temperature treatment plays an important role in changing the microstructure and charge curve shape of carbon materials.The results showed that the reversible capacity of the above carbon anode can be elevated from 277.8 mAh/g to 313 mAh/g and that the exact temperature of the secondary treatment is of vital importance.In the third part,the full cells based on the sodium vanadium fluorophosphate cathode and carbon anode was investigated.It showed a low reversible capacity of only about 80 mAh/g in the first cycle and poor stability.It is concluded that the low reversible capacity and fast decay are resulted from the low Coulombic efficency of carbon and sodium vanadium fluorophosphate.The first reversible capacity can be significantly improved to about 120 mAh/g by adding 30%self-sacrificed salt Na₂C₄O₄,and the cycling performance can also be remarkably enhanced.In addition,improving the surface stability of cathode and anode materials is the key to further boost the performance of full cells.