Synthesis of Novel Carbonophosphate Na3MPO4CO3(M = Fe, Mn, Ni, and Co) and the CathodeProperties in Na-ion Battery

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九州大学学術情報リポジトリ

Kyushu University Institutional Repository

Synthesis of Novel Carbonophosphate Na3MPO4CO3 (M = Fe, Mn, Ni, and Co) and the Cathode

Properties in Na-ion Battery

謝, 宝偉

https://doi.org/10.15017/4060202

出版情報：九州大学, 2019, 博士（工学）, 課程博士バージョン：

権利関係：

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（様式３）Form 3

氏名：謝宝偉（

Xie Baowei

）

Name

論文名： Synthesis of Novel Carbonophosphate Na3MPO4CO3 (M = Fe, Mn, Ni, and Co), and the Cathode Properties in Na-ion Batteries

(新規炭酸リン酸塩Na3MPO4CO3 (M = Fe, Mn, Ni, and Co)の合成とそのNaイオン電池における正極特性)

Title

区分：甲

Thesis Summary

From portable device to large-scale energy storage system (such as electric vehicles and grid system), the cost performance and safety become more important, compared with high energy density. As a charge carrier, sodium has a larger abundance in crust than lithium, which makes Na-ion batteries more promising compared with Li-ion batteries. Moreover, from the viewpoint of safety and environment friendly, the cathodes consisted with the rare-metal-free elements are expected as one of the next-generation chooses. However, for Na-ion batteries, the standard potential of Na metal is higher than Li metal, and the atomic weigh is heavier than Li metal, which lead to a lower operation voltage and lower discharge capacity of Na-ion batteries.

For this study, we focus on the cathode materials of carbonophosphate compounds.

Na3MPO4CO3 (M = Fe, Mn, Ni and Co) attract a lot of attention for its high theoretical capacity of

~192 mAh/g involving two-electrons reaction, and the redox pair of Fe²⁺/Fe⁴⁺ and Mn²⁺/Mn⁴⁺ are under the stable electrochemical window of electrolytes. From viewpoint of safety, aqueous electrolytes are nonflammable and have higher ionic conductivity, and thus they are considered a better choice for large-scale energy storage. Therefore, we also investigated the cathode properties of carbonophosphate in highly concentrated aqueous electrolyte of 17 m NaClO4. In this paper, we will present the synthesis of novel carbonophosphate Na3MPO4CO3 (M = Fe, Mn, Ni, and Co),and the cathode properties in Na-ion batteries with organic and aqueous electrolytes.

In chapter 1, we introduced the general information of rechargeable batteries, brief description of cathodes used for Li-ion batteries and Na-ion batteries, and the objectives of this study.

In chapter 2, Na3MnPO4CO3 was synthesized by mechanical ball milling method its

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electrochemical performance was investigated in an organic electrolytes of 1 M NaPF6/EC:DMC (1:1 v/v) and in an aqueous electrolyte of 17 m NaClO4. Owing to the formation of nano-size particles as observed in the particle distribution analysis and SEM images, Na3MnPO4CO3(MnCO3) showed excellent initial discharge capacities of 134 mAh/g and 116 mAh/g in an organic electrolyte and kept better cyclabilities of 97 mAh/g and 78 mAh/g after 30 cycles at 1/30 C and 1/10 C, respectively. Due to the decomposition of electrolyte after fully charging, the formation of passivation film on cathode surface is obtained by XPS analysis, which leads to a degradation during cycling. The reversible structure exchange of Na3MnPO4CO3 and valance changes of Mn were confirmed by ex-situ XRD and ex-situ XANES analysis. In addition, in an aqueous electrolyte of 17 m NaClO4, Na3MnPO4CO3 (MnCO3) could deliver a large discharge capacity of 134 mAh/g even at a high current density of 2 mA/cm² (1/2 C), and the retention capacity was 74 mAh/g after 30 cycles. This is the first investigation of the electrochemical performance of Na3MnPO4CO3 in an aqueous electrolyte.

In chapter 3, Na3FePO4CO3 was synthesized by the mechanical ball milling method.

Na3FePO4CO3 delivered a specific discharge capacity of 124 mAh/g and 159 mAh/g at 25°C and 60°C at the first cycle between 1.5 and 4.5 V at 0.4 mA/cm², and it maintained good capacities of 91% and 88% after 30 cycles, respectively. Meanwhile, a full cell Na3FePO4CO3 // NaTi2(PO4)3

using an aqueous electrolyte of 17 m NaClO4 was tested for the first time. It delivered a discharge capacity of 161 mAh/g at the first cycle and remained at 105 mAh/g after 30 cycles. From the results of ex-situ XANES analysis, the formation of Fe⁴⁺ after charging up to 4.5 V and the return of Fe²⁺

after discharging down to 1.5 V were confirmed. Reversible structure evolution and an Fe K-edge position shift during the charge-discharge process contributed to the excellent capacity retention of Na3FePO4CO3.

In chapter 4, we successfully synthesized the carbonophosphate compounds of Na3CoPO4CO3

and Na3NiPO4CO3 by a hydrothermal method. For the first time, the cathode properties of these compounds were evaluated for Na-ion batteries. For Na3CoPO4CO3, the first discharge capacity was improved from 30 mAh/g to 70 mAh/g after high-speed ball milling with AB at 400 rpm for 4 h, due to the reduction in particle size. However, for Na3NiPO4CO3, a high-speed ball milling treatment did not the improve the cathode properties. This result may be associated with this compound’s high theoretical operation voltage. The electrochemical performance of Na3NiPO4CO3 could be activated with a new electrolyte with a wide electrochemical potential window.

In chapter 5, we summarized the results of this study.