Synthesis And Electrochemical Study Of Vanadium-Based Electrode Materials For Lithium Ion Batteries

The rapid development of new energy electric vehicles poses great challenges to current lithium ion batteries?LIBs?.It is highly desired to exploit new electrode materials with high performance and low cost.Vanadium-based materials have attracted tremendous attention,which can be attributed to the following advantages.Firstly,more than one Li-ion per formula is able to intercalate into the crystal structure,leading to large theoretical specific capacity.Secondly,abundant vanadium resource in nature can lower application cost.Thirdly,vanadium owns rich valence states.When used as electrode materials,the potential can be regulated by adjusting the valence state of vanadium,making it can be used as both cathodes and anodes.Among vanadium-based cathode materials,Li₃V₂?PO₄?₃ with high working potential,long lifespan and especially superior low temperature performance is regarded as a perfect low temperature cathode.Rhombohedral Li₃V₂?PO₄?₃ with single potential plateau and faster ionic conductive structure is expected to be more suitable as a cathode material than monoclinic counterpart.Unfortunately,it is hard to obtain rhombohedral Li₃V₂?PO₄?₃ because of its less thermodynamic stability.Among vanadium-based anode matherials,Li₃VO₄ with high specific capacity,safer potential than graphite and lower potential than Li₄Ti₅O₁₂ is considered to be a promising anode.However,Li₃VO₄ suffers from poor electronic and ionic conductivity and slow reaction kinetics.To radically improve the electrochemical performance,we modify and optimize Li₃V₂?PO₄?₃cathode and Li₃VO₄ anode from the point of crystal structure.For Li₃V₂?PO₄?₃,rhombohedral Li_3-xNa_xV₂?PO₄?₃ is successfully obtained by introducing Na-ions into the crystal structure and exhibits outstanding low temperature performance.For Li₃VO₄,the ionic conductivity and electronic conductivity are improved by ion-doping.Specifically,we carry out the following work:?1?To stabilize the rhombohedral structured Li₃V₂?PO₄?₃,we directly replace the inactive Li sources in raw materials with Na sources.The introduction of Na-ions changes the product from pure monoclinic phase Li₃V₂?PO₄?₃ to hybrid phase Li₂NaV₂?PO₄?₃consisting of rhombohedral as major phase and monoclinic as minor phase,suggesting that Na-ions are able to stabilize the rhombohedral structure.Then,the low temperature performance of hybrid phase Li₂NaV₂?PO₄?₃ is studied,exhibiting high rate capability in a temperature range from 25 ^oC to-20 ^oC.The results indicate the advantage of rhombohedral phase in low temperature LIBs.?2?Since the above direct method is not able to obtain pure rhombohedral phase,we conduct a Na-Li ion-exchange process from the rhombohedral Na₃V₂?PO₄?₃ precursor and pure rhombohedral Li_3-xNa_xV₂?PO₄?₃ is successfully obtained.The residual small amount of Na-ions plays the important role of stabilizing the rhombohedral structure.Then,the low temperature performance of rhombohedral Li_3-xNa_xV₂?PO₄?₃ is systematically studied for the first time.Benefiting from the intrinsic lithium superionic conductor structure,rhombohedral Li_3-xNa_xV₂?PO₄?₃ exhibits superior high rate capability and long cycle stability in a wide temperature range from 25 ^oC to-30 ^oC,which outperforms currently reported LIBs cathodes.Finally,EIS measurements are performed to evaluate the impact of temperature on reactions kinetics.The results suggest that both the increased charge transfer resistance and decreased Li-ion diffusion coefficient are responsible for the descending electrochemical performance at low temperatures.?3?To improve the ionic conductivity and electronic conductivity of Li₃VO₄,ion-doping is employed.Firstly,GITT and EIS are used to study the impact of Si-doping on Li-ion diffusion process and charge transfer process in Li₃VO₄ electrode.The results show that the ionic conductivity and reaction kinetics are improved after Si-doping.Then,Ge-doping is further curried out.It is found that Ge-doping can also transform Li₃VO₄ from?-phase to?-phase.Preliminary attempts of anion-doping suggest that the product of S-doping is still?-Li₃VO₄ without changing the phase structure.However,S-doping is helpful to improve the electronic conductivity,leading to a better rate performance.