Determining Factors for Charge-Discharge Performances of Composite Electrode in Lithium-Ion Battery
Authors: Orikasa Yuki , Yamashige Hisao , Tanida Hajime , Katayama Misaki , Kitada Koji , Chen Ke Zheng , Inada Yasuhiro , Ohta Toshiaki , Siroma Zyun , Kato Shiro , Kinoshita Hajime , Arai Hajime , Ogumi Zempachi , Uchimoto Yoshiharu 
Published: February 2020

Composite electrodes containing active materials, carbon and binder are widely used in Li-ion batteries. It is well known that their morphology influences the electrochemical performance of batteries and we have tuned the parameters of composite electrodes such as porosity, compounding ratio and thickness.1
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)However, the tuning of composite electrodes has been performed on an empirical basis because decision factors for the improved performance in tuning composite electrodes have not clearly understood. From a practical standpoint, it is important to scientifically develop the design guide for composite electrodes. There are a number of resistances in composite electrodes. Since the electrode reaction proceeds in regions with lower resistance, reaction distribution is happened within the composite electrode. It is required to understand the relationship among battery performance, reaction distribution and electronic/ionic conductivity in composite electrodes. Previously some researchers have reported the distribution model from the theoretical aspect.3
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) Recently there are reports made to directly observe the reaction distribution of composite electrodes.5-8
) Unfortunately these advanced studies just observed distribution phenomena without addressing the decision factor for the distribution and electrode performance. This study aims to explain their relationship from the experimental results. To investigate the cross-sectional reaction distribution occurring in composite electrodes, X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) technique was applied. For the measurement of electronic / ionic conductivity in composite electrodes we have developed a method for simultaneous measurement of electronic and ionic conductivities in composite electrodes.9) First, we examined the reaction distribution of LiFePO4 and LiCoO2 electrodes. The reaction distribution in mono-phasic materials is relaxed by the potential gradient inside composite electrodes. On the other hand, the reaction distribution using LiFePO4 composite electrodes is remained under the open circuit condition.10) Because our XAS method detects the static information, we selected LiFePO4as the active materials. Composite electrodes using LiFePO4active material with low porosity cause a large polarization in high rate discharge reaction and decrease their capacity. In these electrodes, the discharge reaction occurs preferentially at the top surface of the electrode near electrolyte side. Further, low porosity results in a low effective ionic conductivity. This study clearly reveals the ionic conduction in the composite electrodes is the governing factor of lithium-ion battery performance. Control of ionic conductivities in composite electrodes is important to further improve the performance of lithium-ion batteries. The knowledge is useful to understand the decision factor for charge-discharge performance in Lithium-ion batteries and design principle of composite electrodes. 

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