Preparation And Properties Of Lithium-ion Battery Nanomaterials And Thin Film Electrodes

With the fast development of microelectronic system and very large-scale integrations,there is an increasing requirement of micro driving power with excellent electrochemical performance.In the fields of micro power source, all-solid-state thin film lithium ion batteries has been viewed as one of the most promising micro powers due to its high specific capacity and excellent cycling performance.But the large scale fabrication of thin film electrode brings a big challenge in its application.In the thin film preparation field,ink-jet printing technique has its special advantages of economy,fast and high efficiency,which will be a promising way to large scale fabrication of thin film electrode in all solid state lithium ion batteries.Recently,the spinel Li₄Ti₅O₁₂ has been viewed as a promising alternative of graphite to be an anode material in lithium ion batteries.But,its' application in high rate lithium ion batteries was hindered by its low electronic conductivity.Therefore, how to increase the electronic conductivity of Li₄Ti₅O₁₂ is a key solution for its high-rate application.This paper consists of two parts.In the first part,the sol-gel method was used to synthesize nano cathode/anode materials for lithium ion batteries.With these materials the thin film electrodes were prepared by ink-jet printing technique.Also, their electrochemical properties were investigated in detail.In the second part,the rate performance of Li₄Ti₅O₁₂ was improved by its structure modification or composing with carbon nano fibers.The main results were listed as following:1.With the aim to improve the quality of thin film electrode and to simply the processes,a new high efficient dispersant(Lomar D) was used to suspense the particles for preparing electrode printing 'ink'.Compare with our previous method it can save the wet ball milling process,and the consequent heat treatment process.2.Through a sol-gel process and the calcination at 750℃in O₂ atmaspher, nano-LiCoO₂ particles with the diameter of 93nm could be synthesized.Using ink-jet printing technique LiCoO₂ thin film with the thickness of 1.16μm was prepared after 30 iterative printings and hot rolling process.The loading amount of LiCoO₂ on the substrate of aluminum sheet was 0.30mg/cm².The influence of thermal rolling process on the electrochemical performance of thin film electrode was investigated.The thin film without thermal rolling has a rough surface and loose structure,and bad cycle capability.XRD and Raman spectra showed that the LiCoO₂ crystal structure maintained very well in thin film even after the ultrasonic and printing processes.After 100 cycles at 5 C it still kept stable.This thin film electrode presented a good rate capability and capacity retention.At current of 64μA/cm² (about 1.8 C),the discharge capacity was 132mAh/g.It even could be charged-discharge at 384μA/cm²(about 10C) with a stable discharge capacity of 105 mAh/g. At the current rate of 192μA/cm²(5.4 C),after 100 cycles the discharge capacity kept at 118 mAh/g,which was only 5%loss compare with the value in initial cycle. The good electrochemical performance of thin film LiCoO₂ electrode could be attributed to its extremely thin thickness,low inner resistance by carbon doping, nano particle size of LiCoO₂ and the stable film structure.3.Spinel LiMn₂O₄ was synthesized by a sol-gel method,the influence of calcination temperature on LiMn₂O₄ properties was investigated.The LiMn₂O₄ treated at 750℃with a particle size of about 100 nm was chosen for preparing printing 'ink'.The jet printed thin film electrode with thickness of about 1.8μm has a smooth surface.The loading amount of LiMn₂O₄ was 0.48mg/cm².Its electrochemical performance was investigated by CV,EIS and chronoampermetry methods.The Li⁺ diffusion behavior was performed as a semi-infinite diffusion process with the diffusion efficiency of 1.15×10^-11cm²/s.LiMn₂O₄ thin film electrode showed a good rate capability.At discharge current density of 100μA/cm²(2C),the electrode presented a discharge capacity of 104mAh/g.After 54 cycles,the capacity loss was only 0.1%for per cycle.This good electrochemical performance could be attributed to its extremely thin thickness,good crystal structure,and the nano particle size of LiMn₂O₄.4.Spinel Li₄Ti₅O₁₂ was synthesized through a sol-gel method.Under O₂ atmosphere the influence of calcination temperature on the morphologies,size and the electrochemical properties of Li₄Ti₅O₁₂ was investigated.According to the results of XRD,SEM and charge/discharge tests,Li₄Ti₅O₁₂ prepared at 650℃with the diameter of about 120nm was selected as the raw material for preparing printing ink. The jet printed Li₄Ti₅O₁₂ thin film electrode with the thickness of about 1μm showed a smooth surface and a porous structure.The performance of all solid battery of Li/PEO(LiN(CF₃SO₃)₂)/Li₄Ti₅O₁₂(about 1μm ) was tested.The charge/discharge plateau was about 1.5 V.At the current density of 20μA/cm²,its discharge capacity was 22μAh/cm² at 35℃.5.An all solid thin film polymer electrolyte battery was prepared in first time. The battery consisted of jet printed thin film electrodes and a PEO(LiN(CF₃SO₃)₂ separator:Li4_Ti₅O₁₂(about 1μm ) / PEO(LiN(CF₃SO₃)₂(27μm)/LiMn₂O₄(1.8μm). This thin film battery with the thickness of less than 30μm presented a work voltage about 2.5 V.At 35℃,it can be charge-discharged at current density of 20μA/cm² with a capacity of 18μAh/cm².6.Hollow spherical Li₄Ti₅O₁₂ was prepared by a macro emulsion method using P123 as emulsifier.Its frameworks built from many nano Li₄Ti₅O₁₂ particles with the size of 100nm.Different morphologies from hollow sphere to marco porous structure of material could be synthesized by adjusted the amount of Ti(OC₄H₉)₄ and LiAC in the two phases n-heptane/ethanol solvent system.Among them the mono layer hollow spherical material showed the best electrochemical performance.It can be charge-discharged at 20 C(3.4A/g) with the specific capacity of 95mAh/g. Besides its excellent rate capability,this material also presented good capacity retention:over 500 cycles at the charge and discharge rate of 2 C the specific capacity kept very stable as 140mAh/g,and only loss 0.01%for per cycle.The excellent electrochemical performance of hollow spherical Li₄Ti₅O₁₂ is mainly due to its stable hollow structure and the nano scale particles.7.CNTs(carbon nano tubes)/Li₄Ti₅O₁₂ composite was prepared by sol-gel method using Ti(OC₄H₉)₄,LiCH₃CO0·2H₂O and the n-heptane containing CNTs. The CNTs amount in the composite was about 8.2%.The characters of CNTs/Li₄Ti₅O₁₂ composite were determined by XRD,SEM,and TG methods.Its electrochemical properties were measured by charge-discharge cycling and impedance tests.TEM image shows that CNTs dispersed homogenously in CNTs/Li₄Ti₅O₁₂ samples and the nano Li₄Ti₅O₁₂ particles stickled on the surface of CNTs.Compared with spinel Li₄Ti₅O₁₂ prepared by similar way,the size of Li₄Ti₅O₁₂ particles in CNT-Li₄Ti₅O₁₂ composite was smaller.Experimental results showed that the CNTs/Li₄Ti₅O₁₂ composite presented an excellent rate capability and capacity retention.At 0.5 C it demonstrated a discharge capacity of 150mAh/g,if considered the weight content of Li₄Ti₅O₁₂ in composite,the nominal capacity of Li₄Ti₅O₁₂ should be 163mAh/g,which is close to the theoretical capacity of 175 mAh/g.At the charge-discharge rate of 5 C and 10 C,the discharge capacities of Li₄Ti₅O₁₂/CNTs were 145 and 135 mAh/g respectively.After 500 cycles at 5 C,the discharge capacity retained as 142mAh/g.Even it can be discharged at 20 C,the discharge capacity was 106mAh/g,which was equal to 70.1%of that at 0.5 C.The excellent electrochemical performance of CNTs/Li₄Ti₅O₁₂ electrode could be attributed to the improvement of electronic conductivity and the porous electrode structure caused by adding of CNT fibers and the nano size of Li₄Ti₅O₁₂ particles in the CNTs/Li₄Ti₅O₁₂ composite.