High performance screen printable lithium-ion battery cathode ink based on C-LiFePO₄

Highlights

• C-LiFePO₄ paste was been prepared for screen-printing technique.

• The inks produced have a Newtonian viscosity of 3 Pa.s for this printing technique.

• C-LiFePO₄ inks present a 48.2 mAh.g⁻¹ after 50 cycles at 5C.

• This ink is suitable in the development of printed lithium ion batteries.

Abstract

Lithium-ion battery cathodes have been fabricated by screen-printing through the development of C-LiFePO₄ inks. It is shown that shear thinning polymer solutions in N-methyl-2-pyrrolidone (NMP) with Newtonian viscosity above 0.4 Pa s are the best binders for formulating a cathode paste with satisfactory film forming properties. The paste shows an elasticity of the order of 500 Pa and, after shear yielding, shows an apparent viscosity of the order of 3 Pa s for shear rates corresponding to those used during screen-printing. The screen-printed cathode produced with a thickness of 26 μm shows a homogeneous distribution of the active material, conductive additive and polymer binder. The total resistance and diffusion coefficient of the cathode are ∼ 450 Ω and 2.5 × 10⁻¹⁶ cm² s⁻¹, respectively.

The developed cathodes show an initial discharge capacity of 48.2 mAh g⁻¹ at 5C and a discharge value of 39.8 mAh g⁻¹ after 50 cycles. The capacity retention of 83% represents 23% of the theoretical value (charge and/or discharge process in twenty minutes), demonstrating the good performance of the battery. Thus, the developed C-LiFePO₄ based inks allow to fabricate screen-printed cathodes suitable for printed lithium-ion batteries.

Introduction

The growing demand of energy produced by renewable sources such as solar, wind or biomass, together with the constant development and use of portable electronic devices, such as mobile-phones and computers [1], [2], leads to an increasing need of efficient energy storage systems for increasing autonomy and life time of the products [3].

Nowadays, the most important energy storage systems are rechargeable batteries [4], [5] and, among them, lithium ion batteries represent 75% of the global rechargeable battery market as they show high energy density, flexible and lightweight design, and longer lifespan than other competing battery technologies [6], [7], [8], [9].

Lithium-ion battery main constituents are cathode, anode and a separator membrane [10] and thus, a wide range of materials are being developed for those components in order to improve specific energy, power, safety and reliability [7].

Cathodes are typically composed by a polymer binder, conductive additive and active material, and is responsible for the cell capacity and cycle life [11].

Among the different active materials used for lithium-ion batteries, such as lithium iron phosphate (LiFePO₄), lithium nickel manganese cobalt oxide ((LiNiMnCoO₂), lithium nickel cobalt aluminum (LiNiCoAlO₂), lithium titanate oxide (LiTiO₂) and lithium manganese spinel oxide (LiMn₂O₄), among others, LiFePO₄ stands out due to its moderately low cost, high rate performance, slow reaction with electrolyte and increased safety (no oxygen release) [12], [13]. It also shows low density (3.6 g/cm³) and a high theoretical capacity of 170 mAh/g (2.0–4.0 V) [14], [15].

The increasing trend towards miniaturization, integration and flexibility of lithium ion batteries for the development of smart cards, RFID tags and remote sensors, leads to the consideration of printing technologies [16] for the fabrication of batteries.

Printing technologies allow to obtain batteries which are thinner and lighter in comparison to conventional batteries, the printing process allowing also cost reduction and efficient large scale production [16].

Independently of the printing technique, one of the most important parameters affecting the final performance of the printed device is the ink rheology, which mainly depends on particle size, solid loading concentration and solvent type [17], [18].

The most popular printing technique for battery production is screen printing, which is among the fastest and more versatile printing technologies [19].

Thus, screen printing has been used for printing cathodes based on lithium cobalt oxide (LiCoO₂) with a discharge curve close to the theoretical value [20], [21]. This technique was also used for printing all solid state lithium ion batteries using garnet-type oxide and Li₃BO₃ solid electrolytes [22], electrode layers and gel polymer electrolyte layers [23]. Li-O₂ battery cathodes have been also produced by screen printing [24].

Ink-jet printing has been also used for the fabrication of micro-batteries based on LiFePO₄ [25], [26] and LiCoO₂ [27] as cathode based material and tin oxide (SnO₂) for anode [28]. Finally, spray coating has been used for pilot-scale lithium-ion battery production [29].

Thus, taking into account both that screen printing is among the most interesting technologies for lithium-ion battery fabrication and that LiFePO₄ is among the most promising active materials, this work reports on the development of an ink formulation based on carbon coated LiFePO₄, suitable for screen-printing.