| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 100 === Recently, there is a great demand for lithium ion battery to change from high energy density toward high power density owing to the rise of electronic vehicle market. LiNi0.5Mn1.5O4, an extended cathode material of spinel LiMn2O4 which possesses the advantages of high power density, safety and long cycle life, is intensively investigated for next generation high power supply.
For realistic applications, pristine LiNi0.5Mn1.5O4 still has some bottlenecks to overcome, including: (i) the degraded cyclability due to impurity contents, (ii) poor rate capability owing to a relatively low conductivity, (iii) severe capacity fading at high temperature caused by material dissolutions, and (iv) the delayed phase transition during charge-discharge processes. Therefore, a modified co-precipitation method was developed in this study. The impurity-free spinel LiNi0.5Mn1.5O4 cathode material with high-crystallinity was successfully fabricated. A significantly improved cyclability at both room temperature and elevated temperature was revealed. The best capacity retention of as-fabricated LiNi0.5Mn1.5O4 cathode at 25 oC and 55 oC are 97 % and 90% respectively after 100 cycles. The serious capacity decay associated with material dissolutions in conventional cathode was also suppressed by applying the modified co-precipitation method, which produces a highly-stoichiometric LiNi0.5Mn1.5O4 with a better structural stability.
In order to enhance the rate capability, the polymer-assisted method was incorporated to control the particle size of pristine LiNi0.5Mn1.5O4. The excellent capacity retention around 88 % was derived by nano-sized LiNi0.5Mn1.5O4 at a high cycling rate of 7 C, which significantly increased about 40 % as compared to micro-sized one. Besides, the rate capability of LiNi0.5Mn1.5O4 was also promoted by controlling Mn3+ contents during material calcinations. Through adjusting the calcination temperature, the oxygen non-stoichiometric LiNi0.5Mn1.5O4-x was derived above 700 oC, accompanying the formation of Mn3+ and crystallographic structure transformations. The superior capacity retention about 83 % was achieved in non-stoichiometric LiNi0.5Mn1.5O4-x (x=0.033) cathode with higher Mn3+ contents cycled at 7 C.
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