Infineon-WhitePaper-Industrial-Battery-formation-WP

White Paper

03-2019

www.infineon.com/batteryformation Please read the Important Notice and Warnings at the end of this document v2.0

Efficient battery formation systems with energy

recycling

Overview of Infineon devices for battery formation - from AC grid to

transforming the chemistries in a useable format. Therefore, a battery formation system requires high

power density in order to increase the charge and discharge channels, and efficient power conversion with

energy recycling capability, i.e., bidirectional power processing. In this white paper, we begin with a brief

tour of the lithium-ion battery manufacturing process and a short overview of different types of formation

systems. After some background understanding, we move to key design challenges in formation systems:

power density, reliability, and energy recycling. Some essential power device characteristics will also be

discussed. The core stages of the formation system, i.e., power factor correction (PFC) stage, isolated

DC-DC and non-isolated DC-DC stages, topologies and Infineon recommended power devices will be

presented. Finally, we make suggestions on practical solutions for each stage as reference.

Table of contents

1.1 What is battery formation? 3

1.2 Types of formation systems 4

4.1 Power factor correction converter 14

4.2 Isolated DC-DC converter 15

4.3 Non-isolated DC-DC converter 16

Efficient battery formation systems with energy recycling

Overview of Infineon devices for battery formation - from AC grid to battery

1 Introduction to battery formation

Our everyday lifestyle trends towards having more and more wireless and battery powered devices in our

surroundings. The worldwide battery production capacity is estimated to increase substantially year by

year. This trend is driven by the growing demand of battery powered devices and the rapidly rising number

of EVs, targeting high capacity batteries of 500-700 watt-hour per kg. The essential stage every battery

needs to undergo in the manufacturing process is called battery formation [1]. The purpose of battery

formation is to activate battery chemistries and also to determine the characteristic of the battery [2]. In

this process, every newly assembled battery is initially charged and discharged with high accuracy.

Figure 1 shows the lithium-ion battery manufacturing process that includes electrode preparation,

assembly, and formation. The battery formation stage has two key functions; on one hand to create the

solid electrolyte interphase (SEI) on the anode and cathode electrolyte interphase (CEI) [1-2]. On the other

hand, during the formation process battery cell performance measurements, such as impedance and

current capacity, are collected and recorded for battery pack process or quality analysis. SEI and CEI

layers are created by deposition in the formation process when the cells take their first charge. Since the

electrolyte lithium salt dissolved in organic solvent, it reacts vigorously with carbon anode during the initial

formation charge and builds a thin SEI layer by the moderated charge rate with 0.1 C rated current.

1 C rate means that the discharge current will discharge the entire battery in one hour. To complete the

formation power supply to provide stable charging and discharging current.

Efficient battery formation systems with energy recycling

Overview of Infineon devices for battery formation - from AC grid to battery

1.2 Types of formation systems

Figure 2 shows the functional block diagram of the formation of the power supply from the AC grid to the

formatted battery. As depicted, the system includes a PFC stage as an interface to the AC grid, an isolated

DC-DC stage for galvanic isolation and voltage step down, and a non-isolated DC-DC stage to provide

tight charge and discharge voltage together with well-controlled charge and discharge current. All the

stages are based on switching converter technologies rather than linear regulators. The switching

converter approach allows the formation system to increase energy efficiency, power density, and gives

the possibility to use the same hardware for energy recycling, thus reducing battery manufacturing cost.

Figures 3, 4 and 5 show different types of energy recycling formation systems in which the isolated DC-

DC converter usually supports more than one channel in the non-isolated DC-DC converter for battery cell

formation. The traditional and simple switching converter approach uses synchronous (SR) buck-boost

converter for bidirectional power flow. The PFC stage and the isolated DC-DC stage have unidirectional

power flow from the AC grid to the buck-boost input as shown in Figure 3. This energy recycling is based

on controlling the charging and discharging time of each or a set of buck-boost converters, therefore the

Efficient battery formation systems with energy recycling

Overview of Infineon devices for battery formation - from AC grid to battery

A full bidirectional energy flow battery formation system is shown in Figure 4. Compared to the traditional

approach, the discharge energy can transfer from the formatted batteries to the grid due bidirectional

power flow design of the isolated DC-DC and the PFC stages. The system’s charge and discharge power

level is usually a few kilowatts and the connected buck-boost converters should charge and discharge at

the same time to maximize recycling energy efficiency. This configuration allows to measure the high

power battery pack characteristics, but designing the isolated DC-DC stage is required to provide a certain

range of variable output voltages.