Study On The Preparation Of Al-based Composites As Anode Materials For Lithium Ion Batteries And Its Electrochemical Performance

Rechargeable lithium-ion batteries with high energy density and long lifetime have been regarded as promising energy storage and conversion devices for application in electric vehicles（EVs）and smart grids.Graphite is the commerically used anode material for the lithium-ion batteries.While graphite is low cost and has highly reversible lithium insertion/extraction reactions with facile kinetics,its application is limited by low theoretical storage capacity（372 mAh/g）.Seeking for high performance anode materials to replace the role of graphite is ongoing for years.Among all the non-carbonaceous anode materials,metal anode is the most promising one.Compared with other metal anodes,Al is paid much attention to because of its wide resource,high capacity（2234 mAh/g for Al4Li9）,stable voltage plateu,relatively small volume change（97%）.However,the intrinsic problem of Al can not be avoided.Firstly,the volume changes cause repeated breaking and re-formation of the solidelectrolyte interphase（SEI）film coating the active material,making coulombic efficiency（CE）≠100%in a cycle and converting cycleable or ’live’ lithium in the electrodes and electrolyte to ’dead lithium’ in SEI,and eventually the battery dies out of lithium exhaustion.Secondly,the active material（Al-Li）could pulverize or be pushed away during cycling,thus losing electrical contact from the current collector.Various effective strategies have been developed to solve the problem by surface coating,designing nanomaterials with special morphologies,dispersing Al into an inactive/active matrix as well as using different electrolyte system.The report of the work based on these respectives is few and all the modifications by the researches are based on the nanometer Al.Therefore,searching a theoretical ground of using a simple method to prepare the large-scale Al-based composites with high capacity and high cycle ability is of utmost importance.In this paper,through comparing the effect of the two methods of carbon coating and providing matrixes to Al-based materials,we can conclude that carbon coating is much better.To solve the oxidation problem and capacity degradation to a large extent,a more effective matrix Fe₃C@CNCs/EG should be introduced by improving the conductivity of the composite based on carbon coating.Taking the preparation of Fe₃C@CNCs/EG as reference and introduce the micrometer Al during the preparing process to obtain Al/Fe₃C@C/EG.Then optimize the preparation systems,Al content in the Al/Fe₃C@C/EG,calcination temperature,calcination time to obtain the best performance and production parameters according to their electrochemical performances.Study the effects of different metal salts（FeSO₄、CaSO₄、NiSO₄）to Al/EG composites and dig out the mechanism of the best one.On the basis of the best metal salt’s modification,coating a carbon layer of span20 and obtain Al/Fe₃C/C with a better electrochemical performance.（1）Using the carbon coating and matrix method to modify micrometer Al and obtain the respective composites.The electrochemical performances of composites are evaluated with HR-8315A electrolyte as a form of 2025 coin lithium batteries.According to the cycle abilities under 50 mA/g,two methods are not fitable for Al.Both show a worse electrochemical performance than that of Al.The Al@C with span 20 as carbon source delivers an initial capacity of 432.8 mAh/g and 135.7 mAh/g after 30 cycles.Al/AC and Al/EG composites display 150 mAh/g and 378.4 mAh/g after 30 cycles.But if we compare the two methods,carbon coating method is better.Considering the initial discharge-charge curves of both composites,carbon coating can fundermentally maintain its alloy-dealloy process.In the initial discharge-charge curve,the typical dealloying plateau appears at 0.5V in the charge curve.On the contrary,the composites can not maintain this typical plateau.（2）Fe₃C@CNCs/EG was prepared with EG and ferrocene as raw materials through simple mix step and following heating treatment.It can work as a high conductive matrix with high capacity.It was tested as anode materials for lithium-ion batteries with HR-8315A electrolyte in coin lithium batteries.From cyclic voltammetry curve,we can conclude that Fe₃C encased in CNCs can modify SEI film,enable a reversible reaction between the formation and decomposition of SEI film and improve its conductivity and reversible capacity.It maintains the reversible capacity of 1226.2 mAh/g after 75 cycles under 66 mA/g.Compared with EG,Fe₃C@CNCs/EG has a higher capacity.According to the electrochemical impedance spectroscopy（EIS）,Fe₃C@CNCs/EG has smaller resistance value than that of EG,demonstrating its smaller plorization and better conductivity.（3）Taking the preparation of Fe₃C@CNCs/EG as a reference,Al was introduced during the preparation to obtain Al/Fe₃C@C/EG Al/Fe₃C@C/EG composite delivers a better electrochemical performance than that of pure Al.Then the optimization is performed in four aspects:synthesis methods,Al content in the whole composite,calcination temperature,calcination time.The one with the best electrochemical performance should be conducted under 550℃ for 30 min and the best Al content is 33%.The initial reversible capacity is 865.5 mAh/g under 50 mA/g with an appropriate capacity retention of 77.0%.Even after 50 cycles,the capacity retention is 75.9%.During the optimization,the longer the calcination time is,the worse the electrochemical performance is.It is because the longer calcination time may increase the oxidation possibility.Similary,the higher the calcination temperature is,the more likely to be oxidated.And the higher the Al content is,the more likely to be oxidated.But the lower the Al content is,the lower reversible capacity is.（4）Add different metal salts（FeSO₄、CaSO₄、NiSO₄）into Al/EG composites.Only FeSO₄ treated Al/EG composites can maintain the electrochemical alloy and dealloy process.On the basis of FeSO₄ treating,another carbon layer（from span20）is covered on the precursor and form A/VFe₃C/C.In the Al/Fe₃C/C composite,carbon layer on the surface of Fe₃C particles appear part graphitization,which can be attributed to the catalysis of Fe.With Al/Fe₃C/C as anode materials for lithium-ion batteries,the electrochemical performance is evaluated in the form of 2025 coin lithium batteries.It can deliver a high initial coulumic efficiency of 70.0%under 100 mA/g,which even can maintain 960 mAh/g after 90 cycles with the capacity retention of 70.9%after 30 cycles.