Phosphate Modification On Spinels For Lithium-ion Batteries

With the decline of fossil fuel reserves and the gradual intensification of the greenhouse effect,the international community is paying more and more attention to the production,storage and use of clean and sustainable energy.Making full use of renewable energy such as wind,tidal energy and solar energy requires efficient energy storage techniques.By far,Lithium-ion battery has become the most popular energy storage technique,which could be widely used in electric cars and power grids in the future.It has the advantages of high volumetric and gravimetric energy density,excellent cycle performance,and good reliability.However,the poor power capability due to sluggish charge and discharge of lithium-ion batteries cannot satisfy the power requirements for the next generation of electric devices.To further increase power capability of lithium-ion batteries,electrode materials with improved Li⁺diffusion need to be developed for fast charge and discharge.The most widely used spinel cathode material is Li Mn₂O₄ with the discharge voltage plateau of 4.05 V（vs.Li/Li⁺）.Its theoretical specific capacity is 148 m Ah/g,while the practical specific capacity is 100～120 m Ah/g.Li Mn₂O₄ has been considered as the ideal cathode material for lithium-ion batteries due to its low cost,high voltage and environmental friendliness.However,the cycle performance of Li Mn₂O₄ is limited especially at elevated temperature because of structural instability and Mn dissolution during repeated charge and discharge processes.In addition,although the spinel-type anode Li₄Ti₅O₁₂ has the advantages of good thermal stability and long cycle life,its electronic conductivity and Li⁺diffusion coefficient are very low,which greatly limit its rate performance.To improve the electrochemical performance of these two spinels,numerous methods such as morphology control,surface coating and foreign atom doping have been extensively investigated.However,these methods usually involve complex processes with high costs,and cannot be applied for industrial production.Therefore,low cost and efficient modification strategies for the spinel cathodes need to be further explored to develop high performance and sustainable lithium-ion batteries.In this thesis,we developed a facile phosphate modification method by adding phosphate（Na H₂PO₂）to the precursor of Li Mn₂O₄ with following solid-state sintering.By adjusting the ratio of phosphate in the precursor,we prepared phosphate modified Li Mn₂O₄ with greatly improved cycle and rate performance.Through XRD,SEM,TEM,XPS,Raman analysis,the effect of phosphate modification was systematically investigated.After electrochemical measurements,we found that when 5%sodium hypophosphite is added to the precursor,the prepared Li Mn₂O₄ has the best cycle and rate performance.After 1200 cycles at a current density of 1C,the electrode can still retained 80%of its initial capacity.Even at a high current rate of 20 C,the electrode can still deliver a high specific capacity of 100.1 m Ah/g.The EIS,CV and GITT results reveal that the phosphate modified Li Mn₂O₄ has much larger Li⁺diffusion coefficients than those of the pure Li Mn₂O₄ at different electrode potentials.At a low temperature of-30°C,the LMO-P5%sample can still deliver a specific discharge capacity of94.5 m Ah/g,further demonstrating its superior electrode kinetics.For Li₄Ti₅O₁₂ spinel anode,We used the similar method to investigate the influence of phosphate modification on its electrochemical performance.By adjusting the proportion of phosphate in the precursor,we prepared phosphate modified Li₄Ti₅O₁₂ with excellent cycle and rate performance.The structural modification of the spinel has been well characterized by XRD,SEM,XPS,TEM on different samples.The electrochemical test results showed that when 2%sodium hypophosphite is added to the precursor,the prepared Li₄Ti₅O₁₂ has the best cycle and rate performance.After 400 cycles,high capacity retention of about 96.3%can be achieved for the modified spinel anode,and a reversible capacity of 77.7 m Ah/g can be retained even at a high current rate of 20C.