| Although lithium-ion batteries have been widely used in a wide range of fields,their energy densities have still been unsatisfactory to meet people’s needs.Cathode material is one of the key components of lithiumion batteries,which has an important impact on the energy density of lithium-ion batteries.Li-rich manganese-based layered oxide cathodes(Lirich Mn-based)are expected to be the high-performance cathode materials for the next-generation lithium-ion batteries because of their theoretical high specific capacity and low costs.However,the lattice oxygen oxidation of Li-rich Mn-based occurs during the initial cycle and is lost in the form of O2,inducing detrimental phase transition,which makes its structural unstable.The poor structural stability makes lithium-ion diffusion difficult,resulting in low initial coulombic efficiency(ICE)and poor rate capability.The continuous detrimental phase transition results in a rapid voltage decay of the lithium-rich Mn-based.These issues seriously hinder the further application of Li-rich Mn-based in battery products.In this thesis,we modify the properties of Li1.2Mn0.54Ni0.13Co0.13O2(LR)by exploiting different approaches including epitaxial growth modification layer,coating conductive material,material compounding and multiple interface design.All these modifications are targeted at a more stabilized structure of LR,resulting in enhanced ICE,improved rate performance,and effectively suppressed voltage decay.To improve the structure stability of LR,the LaMn O3-modified LR was prepared by epitaxially growing a phase-compatible layer.The impact of contents of LaMnO3 on the structure of the material and the epitaxial growth mechanism were studied intensively.The results showed that the La2O3 formed during sintering could react with the surface Mn species of LR to form a modified layer of LaMnO3,thanks to the fact that the bond length of Mn-O bonds in LaMnO3 is identical to that in LR.At the same time,a small amount of La diffuses into the lattice during sintering to form bulk doping,which effectively prevents the occurrence Li/Ni intermixing.The La-O bond,which is stronger than the Mn-O bond,effectively suppresses the irreversible release of lattice oxygen onto the LR surface.Furthermore,the good chemical stability of LaMnO3 suppresses side reactions at the electrode/electrolyte interface.Therefore,the LaMnO3-modified LR displayed an improved initial coulombic efficiency(87.28% vs.76.99%),and meanwhile,delivered a discharge capacity of 155.4 m Ah g-1 at 5 C rate,much higher than 110.8 m Ah g-1 of the original samples.In addition,the modified samples exhibited better capacity retention.The capacity retention rate of the modified sample was 85.04% after 200 cycles at 1 C rate,and 83.04% after 100 cycles at 5 C rate,much higher than the 75.87% and 63.78% retention rates of the original samples.To solve the problem that the modified layer has no electrochemical activity,a few-layer MXene sheet(Ti3C2)is used to coat the LR.The LR was coated with Ti3C2 with different mass ratios,and the effect of Ti3C2 on the structure,morphology and electrochemical performance of the material was studied individually.The results show that the excellent electrical conductivity of the few-layer Ti3C2 sheets reduce the electrochemical polarization of materials and improve the rate capability and cycle durability.In addition,the Ti3C2 sheets wrapped on the surface inhibit the side reactions at electrode/electrolyte interface,thereby slowing down the voltage decay.There is capacitive interfacial space charge storage between the few-layer Ti3C2 sheets and LR,showing an additional voltage plateau at 2.25 V.The capacity provided by this voltage platform not only improves the ICE and rate performance,but also improvs cycle stability.The 3.0 wt.%Ti3C2 modified materials show best electrochemical performance,with a discharge capacity of 268.7 m Ah g-1 at 0.1 C,an initial coulombic efficiency of 93.20%,and a capacity of 158 m Ah g-1 even at 5 C.Moreover,after 200 cycles at 1 C,the capacity retention of the modified material is 92.78%.Even after 300 cycles at 5 C,the capacity retention is still as high as 87.31%.To study the synergistic mechanism of electrochemically active components,Li Fe PO4(LFP)was used to composite with LR to form LRLFP,and its impacts on structure,morphology and electrochemical performance of the electrode material was carefully analyzed.The results showed that LRLFP combines the merits of LR and LFP.LRLFP has a higher specific capacity than LFP at low rates,and meanwhile maintains good capacity retention at high rates.Additionally,LRLFP exhibits excellent capacity retention at high temperatures.The LFP component in the composite is more stable,and the LFP contacting closely with the surface of the LR can improve the corrosion resistance of LR towards electrolyte,thereby maintaining the structural integrity.The LR in the component has a high electrical conductivity,which forms a synergistic shunt mechanism with LFP in the composite.When high-rate test is performed,the current passes through the LR preferentially,so that the composite material manages to obtain a higher specific capacity.Moreover,there is a pseudocapacitive effect between the LR and the LFP,which further enhances the rate capability of composite materials.Therefore,LRLFP has a stable structure,low charge transfer resistance and high lithium-ion diffusion coefficient.LRLFP deliver a specific discharge capacity of 139.6 m Ah g-1 at 5 C rate,with a capacity retention rate of 92.55% after 400 cycles at 1 C.To design a fully functional modified layer,solid acid [Zr(HPO4)2·H2O,ZHP] was wrapped on the surface of LR,and a ZHPmodified LR was obtained after sintering.Spinel phases,oxygen vacancies and an inert protective layer of zirconium phosphate were successfully introduced into the LR surface.The irreversible release of lattice O was effectively reduced due to the low O-2p energy band stabilizes the surface structure of the material well,thus improving the initial coulombic efficiency.Second,due to the lower energy level of the O 2p non-bonding band and electrons enriched in the layered/spinel phase,thus stabilizing the surface structure and improving the ICE of materials.In addition,due to the existence of spinel phase and O vacancies,the diffusion rates of ions and electrons in the modified material was significantly increased,which reduced the charge transfer resistance thereby significantly improving the rate performance.Meanwhile,the outermost zirconium phosphate inert layer acts as a buffer,hindering the side reaction of the highly active On-species with the electrolyte,thereby enhancing the stability of the surface structure and improving its cycle performance.As a result,the modified cathode achieved a high ICE of 88.40%.It delivers a discharge capacity of 188.2 m Ah g-1 at 5 C,which is much higher than that of the bare LR cathode materials(83.12% and 133 m Ah g-1).This multifunctional interface design is suitable for a variety of surface failure scenarios.There are 72 figures,6 tables,and 175 references... |
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