| Since the commercialization in 1991,lithium ion batteries have been widely used in various portable electronic devices.However,with the development of electric vehicles,people have higher requirements on energy density of energy storage devices.At present,the most commercial cathode material is LiCoO2,which suffers from severe structural instability and low specific capacity.Moreover,the use of precious metal Co also increases the cost of lithium ion batteries,making it difficult to realize the large-scale application in the field of electric vehicles.As the most promising alternative cathode for LiCoO2,LiNiO2has attracted much attention because of its low cost and high specific capacity(theoretical capacity275 mAh g-1).Nevertheless,there are still many problems to be solved in LiNiO2 system,such as difficult synthesis,short cycling life and low thermal stability.One possible method to overcome the performance degradation of LiNiO2 cathode is to replace other metal ions at Ni sites partially.Among these substituting ions,a small amount of Co2+ can i1hibit the cation mixing and improve the electrochemical performance;A13+ is proved to enhance the structural stability and improve the thermal stability.LiNio.8Co0.15Al0.05O2(NCA)materials,which have large reversible capacity(-220 mAh g-1)and modest cyclic stability,have been extensively studied and already used in Tesla for electric vehicle applications.However,as a commercial material,the preparation of NCA is still immature.They are usually prepared by co-precipitation method with ammonia as the chelating agent.However,Al3+is not easy to complex with ammonia,so it precipitates faster to form a lot of small particles.This will retard the particle growth and result in an inhomogeneous element distribution for NCA products.Ni0.8Co0.15Al0.05(OH)2 precursor was prepared via a modified co-precipitation method by using a5L continuous stirred tank reactor.NiSO4·6H2O?CoSO4·7H2O and Al2(SO4)3·12H2O were used as the starting materials,NaOH was used as the precipitation agent,NH3·H2O and EDTA were used as two chelating agents to slow down the reaction rate of three metal cations simultaneously.The precursor was thoroughly mixed with LiOH·H2O and then sintered under a flowing oxygen atmosphere.We optimized the synthesis process and obtained the best NCA products at the stirring speed of600rpm.The obtained secondary particle exhibits a quasi-spherical shape with uniform element distribution.To confirm the structure of the as-prepared NCA material,we conducted powder X-ray diffraction(XRD)and the product has well-defined hexagonal structure.The NCA electrode was assembled to lithium ion batteries with lithium foil as the counter electrode.It delivers a discharge capacity of more than200mAh g-1at0.1C and retains a capacity retention of 97.9%after 100 cycles at 0.2 C.The capacity is almost not decaying.The phase transition mechanism and exothermic mechanism of NCA positive electrode during the charge and discharge process were investigated by in situ X-ray diffraction and thermal activity monitor.We used homemade shells to assemble the in-situ batteries and monitor the real-time change of NCA electrode.There are no resolved features suggesting a second phase,but only some reversible shift of peak position such as the(003)?(101)and(113)diffraction peaks,which suggest a solid-solution reaction mechanism of NCA electrode during lithium extraction/insertion.The crystal structure was refined using the GSAS software and the evolutions of lattice parameters(a,c,and V)are obtained.The change of unit-cell volume before and after lithium extraction is only 2.13%,which gives NCA electrode superior structural stability during the cycling process.We demonstrate a practical employment of Thermal Activity Monitor(TAM)technique in monitoring the heat change for NCA coin-type cells for the first time.The total generated heat of NCA prepared with modified co-precipitation method showed a low heat generation of 298 J g-1.The results show that the optimized preparation method can obviously improve the thermal stability of NCA materials.Moreover,the application of microcalorimeter technology in the field of batteries is also a great inspiration for the follow-up research on the exothermic performance of lithium ion batteriesSodium ion batteries have the similar working principle with lithium ion batteries,their cathode side is related to the insertion and extraction of ions.Therefore,it is thought to apply the excellent layered structure of lithium ion batteries in the field of sodium ion batteries.In this paper,we investigate the sodiation and desodiation behavior of LixNi0.8Co0.15Al0.05O2 layered structure for the first time.Electrochemical ion exchange method was used to replace the lithium ion of LiNi0.8Co0.15Al0.05O2with sodium ion,and the cathode material of NaxNi0.8Co0.15Al0.05O2(Na-NCA)was successfully prepared.The obtained material was assembled to sodium ion batteries for the charge and discharge testing.It delivers a discharge capacity of 140 mAh g-1 at0.1 C in the voltage range of 2.0-4.0 V and retains a capacity retention of 96.8%after 100cycles at 2C.Ex-situ XRD was used to characterize the layered structure.The sodium ions could be reversibly inserted and extracted,and the layered structure remained stable throughout the whole process.These data prove that the layered structure of LixNi0.8Co0.15Al0.05O2 has good characteristics of sodiation and desodiation.When used as cathode material for sodium ion batteries,it not only has a high discharge specific capacity,but also has excellent cyclic stability. |
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