How Do We Make a Mechanically Flexible Li-Ion Micro Battery for Wearable Electronics

How do we make a mechanically flexible Lithium-ion microbattery for wearable electronics?

KHANG LEˆ

6 February 2015

Abstract

Electronic devices with high-technological functions such as wearability, deformabiliy and opti- cal transparency have received significant and growing interest these past couple of years. The development of flexible LIBs as power sources is therefore an important topic of research. In this review, recent advances, processing methods, mechanical and electrochemical performances of various flexible CNT-based, graphene-based and composite electrode materials and of some shape-deformable solid-state electrolytes are summarized.

1 Introduction

1.1 Mechanism of a typical lithium- ion battery

The rechargeable lithium-ion battery, which is vastly employed in today’s consumers electronic market, was first presented commercially by Sony in the early 1990s after two decades of research1 .

Figure 1: Schematic of a typical discharging LIB with a LiCoO2 ⊕ electrode, a graphite ⊖ electrode and with aluminium and copper current collectors respectively2.

A LIB consists of one or more power-generating lithium-ion based cells which can convert stored chemical energy into electrical energy. Each cell

is made of three main components: a positive electrode, a negative electrode and an organic solvent electrolyte which allows ionic movement in between2,3. As can be seen in figure 1, when a LIB cell is discharging, Li+ ions move from the negative electrode to the positive one through the electrolyte. Electrons flow simultaneously through an external circuit from the negative electrode to the positive one generating an elec- tric current in the opposite direction2,3. When a LIB is charging, the reverse process occurs.

The half reactions occurring at each electrode and the overall redox reaction are as follows2:

at the ⊕ electrode:

LiCoO2 ←−→ Li1−xCoO2 + xLi+ + xe− (1)

at the ⊖ electrode:

xLi+ + xe− + C6 ←−→ LixC6 (2)

overall reaction:

LiCoO2 + C6 ←−→ Li1−xCoO2 + LixC6 (3)

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1.2 Flexible high-technological appli- cations.

Flexible and portable electronics are a rising and resourceful technology for diverse future applications such as rollable displays, touch screens, wearable sensors and implantable med- ical devices4−6. Nowadays, lithium-ion batteries (LIBs) are the most common sources of energy in portable electronic devices due to their electri- cal properties: a relatively high energy density, high output voltage, long-term stability and no memory effect7.

To make the devices function, LIBs should be able to store a sufficiently high amount of en- ergy and to deliver a certain level of capacity of a few thousands of mAh8. Moreover they must be thin, light and able to withstand multiple deformations - bending, compression, stretch- ing, etc. - without degrading the batteries performance to satisfy the devices mechanical applications8. This means that all the various components of the batteries - their electrodes, electrolyte/separator, current collectors at each electrode terminal and packaging - have to be deformable and compatible with one another. Hence there has been ongoing research for a suitable selection and integration of materials in flexible LIBs in order to meet both the electri- cal and mechanical requirements of the portable electronic products8,9.

2 Materials selection and as- sembly for flexible LIBs

2.1 Flexible materials for electrodes

The positive and negative electrodes in a tradi- tional LIB are built by coating metal foil current collectors with a mixture of conductive carbon additives, polymer binders and the powdered active materials: transition metal oxide con- taining lithium such as LiCoO2 and graphite respectively9. As the materials employed in cur- rent LIBs electrodes show very little flexibility, the use of intrinsically flexible active materi- als as free-standing electrodes or of composite electrodes with flexible foundations for fabricat- ing deformable electrodes has been under study these past couple of years10. A comparison be-

tween conventional electrode components and flexible ones in LIBs is depicted in figure 2.

Figure 2: Schematic of (a) conventional electrode com- ponents and (b) flexible electrode components in LIBs9.

2.1.1 Free-standing electrode materials

Alternative carbon-based materials to graphite such as carbon nanotubes (CNTs) and graphene have shown favourable results as free-standing electrodes materials due to their electrochemi- cal and mechanical properties10−12. As can be seen in figure 313, these carbon allotropes con- sist of large surface area networks with multiple porous channels and low resistance that enable great lithium storage capability and fast ion transport. In result, they display high electrical conductivity, electrochemical stability and rel- atively good energy density and capacity14,15. Moreover their low mass density, mechanical flexibility and robustness which allow them to undergo multiple deformations without degrada- tion, make CNTs and graphene-based materials promising structures for light, durable and sta- ble flexible LIBs electrodes9,16. The main issues with CNT- and graphene-based electrodes are their complex fabrication processes and high cost.

Figure 3: Graphene as the basic unit of 0D fullerene, 1D carbon nanotubes, and 3D graphite13.

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Cnt-based electrodes

CNT paper is a thin sheet of assembled CNTs17 that can be fabricated by various methods such as casting CNT dispersion18, dip-coating, spin- coating19 , self-assembly20 , dry-drawing21 or vac- uum filtration17,22.

The electrochemical properties of CNT papers are influenced a lot by the CNT type and the processing technique. For example, vacuum fil- tration in a solvent with a surfactant, mechanical pressing and the control of the CNTs orientation can produce higher quality CNT papers17,22,23.

Free-standing electrodes composed of single- walled carbon nanotubes (SWNTs), double- walled CNTs (DWNTs) and multi-walled CNTs (MWNTs) exhibit