鋰離子二次電池中鋰錳氧正極材料之表面改質

Surface Modification of LiMn2O4 Cathode Material in Li-ion Secondary Battery

詹宏偉(Hong-Wei Chan)

指導教授 ： 杜正恭

國立清華大學/工學院/材料科學工程學系/博士(2006年)

https://doi.org/10.6843/NTHU.2006.00682

摘要

在鋰離子電池之鋰錳氧正極材料中，一般研究皆著重於降低尖晶石相鋰錳氧的電容衰退率以提高其電性表現，特別是在高電流密度下的表現。相較於其他正極材料，鋰錳氧正極材料具有低成本，環保以及安全性高的特點。利用正極材料表面改質的技術，可用於抑制電極及電解液之間的反應。
本論文將針對表面改質的鋰錳氧正極材料做做微結構及電性的分析。藉由固態法或化學溶液法鍍覆鋰硼氧化物以及鋰銅錳氧化物至鋰錳氧的表面。根據場發射掃描式電子顯微鏡的觀察，前述兩種材料的每一顆粉末都是由數百奈米的小粉末所組成。透過高解析穿透式電子顯微鏡更進一步的觀察，可以證實鋰硼氧化物以及鋰銅錳氧化物兩種表面改質的形式係為兩種不同的形態。此外，藉由高解析穿透式電子顯微鏡的的分析，可以得知在鋰銅錳氧化物鍍覆的尖晶石鋰錳氧正極材料中，銅是佔在16d的位置。
另外電性方面的量測是採用兩極式的鈕扣電池。首先，表面改質技術確實可以抑制電容量的衰退。以市售鋰錳氧km110，經0.4 wt％的鋰硼氧鍍覆量在經過10圈的充放電後，仍可維持93％的初始電容量。而鋰硼氧鍍覆量為0.3 wt％的鋰錳氧正極材料經過20圈的充放電測試後，只損失了7％的初始電容量，比未經表面改質的鋰錳氧的15％要低的很多。在經過鋰硼氧鍍覆後，其阻抗值亦比未鍍覆的鋰錳氧小，可以推論其原因為電極與電解液界面間的反應減緩而使阻抗縮小。此外，經過鋰銅錳氧的鍍覆之後，表面改質的鋰錳氧正極材料的電容量率退率在0.2C電流速率下充放電10圈，較未改質的鋰錳氧可減少2.25％。在更高的0.5C電流速率下充放電25圈後，其電容量衰退率更可減少5.16％。
經鋰銅錳氧鍍覆以及未經鍍覆的鋰錳氧，在0.1C，0.5C和1C等不同電流速率下於電壓3到4.5伏特間充放電之相變化可藉由同步輻射之繞射光譜來鑑定。結果顯示，未經鍍覆及鋰銅錳氧鍍覆之鋰錳氧的電位平台之電位差為50mV。其原因為經鋰銅錳氧表面改質後，鋰錳氧的內部傳導速率提高所致。從吸收光譜的分析中，可以得知在鋰銅錳氧鍍覆之鋰錳氧尖晶石結構裡面，銅與錳的價數分別較為接近正二價及正四價。在充放電過程中，其錳價數的增加（鋰遷出）及減少（鋰遷入）為可逆程序。錳-氧鍵結及錳-錳（或銅）鍵結在充放電過程中的變化趨勢亦與價數變化的趨勢符合，係為一可逆程序，其中隨著鋰遷出尖晶石結構其鍵結隨之縮短，反之則鍵結隨之拉長。藉由吸收光譜的分析，亦可證實透過表面改質技術，尖晶石相中之錳會偏向正四價，進一步抑制了楊泰勒晶格收縮(Jahn-Teller distortion)，而達到改善電性表現的良好結果。

關鍵字

鋰錳氧 ； 表面改質 ； 鋰電池

並列摘要

The surface-modified cathode material in Li-ion battery was synthesized to decrease the side reactions at the interface between the cathode electrode and electrolyte. Among all cathode materials, LiMn2O4 exhibits lower cost, acceptable environmental characteristics and better safety property than other cathode materials. The research focus is aimed to reduce the capacity fading and to enhance the electrochemical performance of spinel LiMn2O4, particularly at high C rate.
In this study, the microstructure and electrochemical property in the surface-modified LiMn2O4 were examined and probed. The Li2O-2B2O3 (LBO)-coated LiMn2O4 and LiCuxMn2-xO4-coated LiMn2O4 were synthesized by either solid-state method or chemical solution method. From the cross section view of LiCuxMn2-xO4-coated LiMn2O4 and LBO-coated LiMn2O4 observed with FE-SEM, it was demonstrated that the lager particles consisted of many smaller ones in the sub-micrometer range. It was argued that LiCuxMn2-xO4-coated LiMn2O4 and LBO-coated LiMn2O4 exhibited two distinct types of surface modification on the basis of the detailed analysis of HRTEM. In addition, the location of Cu in spinel LiCuxMn2-xO4-coated LiMn2O4 was at 16d site revealed by HRTEM.
In addition, the electrochemical behavior was examined by using two-electrode coin cells. First of all, the capacity fading can be reduced by the technique of surface modification. The 0.4 wt％ LBO-coated km110 powder retained 93％ of its original discharge capacity after 10 cycles. Furthermore, the capacity fading of 0.3 wt％ LBO-coated Li1+xMn2O4 cathode material was 7％ after 20 cycles, showing much better cycleability than the un-coated one of 15％. The resistance of the LBO-coated Li1+xMn2O4 was also smaller than the un-coated one, indicating that the side reaction at the interface between the cathode and electrode could be diminished. Besides, for the LiCuxMn2-xO4-coated LiMn2O4, the fading rate of LiMn2O4 at 0.2 C was reduced 2.25％ after 10 cycles by surface modification. At higher C rate of 0.5 C, the decrease of fading rate was more obvious at 5.16％ after 25 cycles.
The phase transformation of both base LiMn2O4 and LiCuxMn2-xO4-coated LiMn2O4 during charging at 0.1 C, 0.5 C and 1C rate from 3 V to 4.5 V was confirmed by the in situ synchrotron X-ray diffractometer (in situ XRD). The plateau potential difference between the base LiMn2O4 and LiCuxMn2-xO4-coated LiMn2O4 composite was 50 mV. The decrease of the plateau can be related to the fact that the kinetics of the LiCuxMn2-xO4-coated LiMn2O4 composite cathode material was faster than that of the uncoated material. The XANES of Cu and Mn K-edge spectrum for LiCuxMn2-xO4-coated LiMn2O4 showed that the valence of Cu and Mn was close to Cu2+ and Mn4+, respectively. Furthermore, the oxidation state of Mn was reversibly increased and decreased during charge. The EXAFS was further revealed that the trend of the variation for the bonding length of Mn-O and Mn-M (M=Mn or Cu) was in agreement with the oxidation state of Mn, which was decreased with Li deintercalation, while increased with Li intercalation during cycling. On the basis of the in situ XAS data, it was evidenced that Mn transferred toward Mn4+ to minimize the Jahn-Teller distortion by the technique of surface modification, and thus the better electrochemical property was achieved.

並列關鍵字

LiMn2O4 ； Surface Modification ； Li-ion Battery

參考文獻

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