Development for 300Wh/Kg-Class High Energy Li-Ion Battery for EV
Authors: Kimura Naoki , Seki Eiji , Shin Seog-Chul , Takahashi Shin , Makino Shigeki 
Published: February 2020

Abstract

 High energy density lithium-ion secondary batteries must be developed to extend the driving range of battery electric vehicles (BEV). We have been focusing on LiMO2-type layered oxides for the high capacity cathode, and Si-based materials for the high capacity anode. We manufactured a prototype 30 Ah-class cell using a cathode of Ni-rich layered oxide and an anode of Si-alloy mixed with graphite to prove the achievement of our development. The cell demonstrated an energy density of 335 Wh/kg and power density of 1600 W/kg when charged up to 4.4 V. The short cycle life shown by the cells was improved by coating on the cathode active material particles and optimized electrolyte.

1．Introduction

 Hitachi, Ltd. and Hitachi Automotive Systems, Ltd. have been participating in the national project supported by New Energy and Industrial Technology Development Organization (NEDO) of Japan: “Applied and Practical LIB Development for Automobile and Multiple Application”. Hitachi, Ltd. is in charge of developing cell chemistry and basic design of the single cell for the target of a 300 Wh/kg-class lithium-ion secondary battery. Such a high energy density battery naturally demands us to introduce high capacity density material both for the cathode and anode. Based on our preliminary study, we chose Ni-rich layered oxide and lithium-rich layered oxide for the cathode and Si-based anode.

2．Experimental

 The cathode was manufactured using a Ni-rich layered oxide, conductive carbon, and binder at a coating amount of 335 g/m2 and density of 3.0 g/cm3. The anode consisted of a mixture of Si-alloy and graphite and was coated with polyamide-imide binder to make the capacity ratio 1.1 and density 2.1 g/cm3. We assembled 30 Ah-class pouch cells, which were charged up to 4.4 V or 4.2 V. The cells charged up to 4.4 V and 4.2 V are denoted as Cell-4.4 and Cell-4.2. Thee capacity was measured by discharge current 1/20 CA and 1/3 CA down to 2.0 V. The direct current internal resistance (DC-IR) was calculated on the basis of the 10-second voltage drop and discharge current of 1/3 CA at 50% SOC.

 We also manufactured 1 Ah-class small cell denoted as Cell-imp. which has the same cell chemistry as the 30 Ah-class ones, but the active materials were treated with coating and the electrolyte was optimized. Cell-4.4, Cell-4.2, and Cell-imp. were cycled at 1/2 CA between 20 and 100% SOC. After the cycle test, the cells were disassembled and analyzed.

3．Results and Discus

 Figure 1 shows discharge curves of prototype 30 Ah-class cells, and Table 1 summarizes the results. The capacities of the Cell-4.4 and Cell-4.2 were 30.5 Ah and 27.7 Ah, which were almost the same as designed. Since the average voltages were 3.46 V and 3.43 V and the cell weights were as shown in the table, the energy densities were 335 Wh/kg in Cell-4.4 and 290 Wh/kg in Cell-4.2. DC-IR and OCV (Open circuit voltage) was used to calculate output power density. The results were 1,600 W/kg and 1,270 W/kg.

 The cycle life test of Cell-4.4, Cell-4.2 resulted in poor cycleability: they lost all the discharge capacity after 100 cycles. On the contrary, the Cell-imp. showed high capacity retention of 90% even after 100 cycles. Cell-4.4 and Cell-4.2 revealed growth of a cubic layer and SEI film on the surface of cathode active material, and detachment of the mixed materials layer of the anode, while the Cell-imp. showed reduced growth and detachment.

Acknowledgement

 This work was supported by NEDO of Japan.

Figure 1