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Application Of Layered Ni-Co-Mn-based Cathode Materials In Electrochemical Energy Storage

Posted on:2015-02-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z H HanFull Text:PDF
GTID:1311330428475245Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Along with the rapid progress in technology, the application of lithium ion batteries (LIBs) has been extended from the portable electronics to some power products even energy storage. However, current LIBs can't completely meet all the requirements generated from the new applications, especially the continuously increasing demands for lithium ion battery with high energy density, high power density and excellent thermal safety. For lithium ion battery, the cathode materials play a significant role which greatly influences even determines the electrochemical properties of the whole cell. So, searching for new cathode materials or modification of the conventional cathode materials is a big issue for the development of LIBs. Comparing with LiCoO2, Li-Ni-Co-Mn-O layered cathodes have earned more and more interests because of their merits of high energy density, excellent cycling stability, low cost and good safety.In this work, some practical issues of the layered Ni-Co-Mn compounds have been addressed. The study has been focused on the rate capability, thermal stability and other drawbacks of the layered Ni-Co-Mn compounds, and some strategies towards these problems have been proposed. Additionally, another analogous-NaxNi1/3Co1/3Mn1/3O2layered cathode is synthesized and its cathode application in sodium ion battery has been investigated. The main work and results are summarized as follows:(1) Investigation of LiNixCoyMn1-x-y02The layered stoichiometric materials LiNi1/3Co1/3Mn1/3O2, LiNio.6Coo.2Mn0.2O2and LiNi0.4Co0.2Mn0.4O2are synthesized by rheological phase method. Sb2O3-mixed phases are further obtained by mechanical ball milling the above phases with Sb2O3-The influence of Sb2O3-modification is exemplified by the comparison between LiNi1/3Co1/3Mn1/3O2and Sb2O3-mixed LiNi1/3Co1/3Mn1/3O2. The structure and morphology of the bare and mixed samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Charge/discharge tests indicate that the mixed phases all show an improved cycling stability, rate capability and thermal safety. Careful comparison of the charge/discharge profiles reveals that the polarization increment in cycling is significantly suppressed in the Sb2O3-mixed LiNi]/3Co1/3Mn1/3O2electrodes. AC impedance shows that Sb2O3/LiNi1/3Co1/3Mn1/3O2electrode has smaller charge transfer resistance value Rct and SEI resistance value Rf. Further analysis proves that Sb2O3hinders the reaction between electrolyte and cathode during charge/discharge process and helps to stabilize the SEI. Other experiments prove that the utilization of Sb2O3-coated separator can achieve similar positive effect on layered LiNixCoyMn1-x-yO2cathodes.(2) Investigation on Li1.182Ni0.182Co0.09iMn0.54502Solid solution material Li[Li0.182Ni0.182Co0.09iMno.545]02is synthesized by rheological phase method at different temperatures (700-900?). The structure and morphology of Li[Li0.182Ni0.182Co0.091Mn0.545]02powder are characterized by XRD and SEM. The results show that Li[Li0.182Ni0.182Co0.091Mno.545]O2sample possesses a layer hexagonal structure with very small cation disorder. With the calcination temperature increasing, the particle size gradually increases, and the particle aggregation apparently reduces. According to the charge/discharge tests, the initial discharge capacity decreases when the synthesis temperature is high. However, the cycle stability gets improved. Furthermore, we try to obtain the solid solution material Li[Li0.182Ni0.182Co0.091Mn0.545]O2through a step-wise calcination from600?to850?. It is found that the adoption of the stepwise calcination can suppress the particle size overgrowing and particle aggregation. Thus the obtained Li[Li0.182Ni0.182Co0.091Mno.545]02phase shows a good electrochemical property.(3) The effect of binder on Li1.182Ni0.182Co0.091Mno.54502Solid solution material Li[Li0.182Ni0.182Co0.091Mno.545]O2is synthesized by rheological phase method under stepwise calcination program. The effect of the binder on the property of Li[Li0.182Ni0.182Co0.091Mno.545]02cathode has been investigated. Four different binders including sodium carboxymethyl cellulose (CMC), polyvinylidene fluoride (PVDF), sodium alginate (SA) and polytetrafluoroethylene (PTFE) binder have been used. Electrochemical performances such as cyclability, rate capacity, voltage decay and coulomb efficiency of the cathodes with different binders are compared. The results show that the Li[Li0.182Ni0.182Co0.091Mno.545]O2electrode with SA binder has the best electrochemical properties. Experiments reveal that the Li[Li0.182Nio.i82Co0.091Mno.545]O2electrode with PVDF binder swells seriously in all the electrodes, which causes the appearance of electrode cracks and even electrode peeling off from the current collector. It is believed that this effect will eventually lead to the degraded electrical contact between material particle and conductive agent or electrode and current collector, and thus deteriorate the cycle stability.(4) Improvement of Li1.2Ni0.16Co0.08Mn0.56O2Solid solution material Li1.2Ni0.16Co0.08Mn0.56O2is obtained through rheological phase method and further treated in ammonium persulphate solution at different concentration. The obtained phases all show an improved electrochemical performance comparing with the pristine Li1.2Ni0.16Co0.08Mn0.56O2. The effect of the treatment has been investigated in terms of XRD, Raman spectrometer and ICP-AES. The results show that the structure change after treatment is very insignificant and the treated phases still maintain the layered structure. The content of Li+in the treated phases decreases with the increasing of (NH4)2S2O8concentration used in the treatment. Meanwhile, it is further found that the lost Li+mainly comes from the Li2MnO3component in the Li2MnO3-LiMO2solid solution. XPS data and charge/discharge curves of the treated Li1.2Ni0.16Co0.08Mn0.56O2reveal that the valences of transition metal cations keep unchanged after the (NH4)2S2O8treatment. Cycling tests indicate that the initial discharge capacity of the treated Li1.2Ni0.16Co0.08Mn0.56O2is lower than the pristine phase. However, the cycle stability is much better. In summary, the optimum content of (NH4)2S2O8used in the treatment is30%-40%, thus obtained samples exhibit very stable cycling behavior, good capacity performance and excellent rate capability.(5) Investigation of NaxNi1/3Co1/3Mn1/3O2NaxNi1/3Co1/3Mn1/3O2materials are synthesized by rheological phase method at different temperatures (700-900?). The structure and morphology characteristic of the result powders are determined by XRD and SEM. It is found that with the calcination temperature increasing, the particle-size becomes larger and the particle shape changes from block-like into flake-like. The results of the charge/discharge tests indicate that the initial discharge capacity also increases with the calcination temperature increasing and the sample calcined at900?shows the best electrochemical property. According to the XRD and initial charge/discharge result, we think the obtained NaxNi1/3Co1/3Mn1/3O2phase is non-stoichiometric in which x is less than1.
Keywords/Search Tags:Lithium ion battery, sodium ion battery, rheological phase method, layered cathode material, brinder, modification
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