Battery Spray Pyrolysis

Abstract

Energy is one of the major challenges of the 21st century for applications including power generation and storage. Developing state of the art battery would facilitate the implementation of renewable energy sources as well as significantly reduce the carbon footprint of the transportation sector. A low cost, robust synthesis process with high reproducibility is required to produce nanostructured lithium-ion battery cathode materials. Lithium-ion batteries are considered an attractive power source for portable devices, electric and hybrid electric vehicles, and large renewable power facilities. LiNi0.6Mn0.2Co0.2O2 composite materials with layered structures have received attention as high-capacity, low cost, and safe cathode materials for lithium-ion batteries. The conventional synthesis method for these materials is co-precipitation, which has challenges associated with uniformity and process times. Therefore, a spray pyrolysis synthesis was developed by LACER lab as a scalable, low cost production method. Due to the formation mechanism of the particles, the product contains hollow spheres, which cause low bulk density. Recently, the Laboratory for Advanced Combustion and Energy Research (LACER) developed a scaled-up spray pyrolysis process for the synthesis of non-hollow, solid lithium transition metal oxide materials. The method at present can produce high quality battery materials around 50 grams per hour scale. In the present study layered LiNi0.6Mn0.2Co0.2O2 material is produced and the electrochemical properties will be discussed.

Introduction

Energy supply has become a critical component to maintain sustainable development in the 21st century. With the rapid growth in economics and human population comes an increase in global demand for energy. Energy is mainly derived from variations of the combustion reaction, but has come with many negative consequences. The resulting emissions of carbon dioxide and other greenhouse gases have driven global climate change. If a new energy economy is to develop, it must be based on a cheap and clean energy supply. Cleaner and more sustainable sources are already developing and beginning to replace conventional sources. However, these sources are not without their own problems, as the rise of hybrid and electric vehicles, power grid storage for effective use of renewable energies, and even portable electronics have brought attention to a growing need for reliable and compact rechargeable power sources. Lithium-ion batteries have presented themselves as good candidates to fulfill this need, based on their significant energy density, flexibility, and lifespan.1

A battery is composed of two electrodes connected by an ionically conductive material called an electrolyte. The two electrodes have different chemical potentials based on the chemistry that occurs at each one. When the electrodes are connected via an external device, the electrons in the battery spontaneously flow from the more negative electrode (anode), to the positive electrode (cathode). Ions are transported through the electrolyte, which contains dissociated salts to enable ion transfer between the two electrodes to maintain the charge balance. A schematic of this process in a lithium-ion battery is shown in Fig. 1.2 The electrical energy that results can be tapped from an external circuit. For a secondary, or rechargeable battery, a voltage applied in the opposite direction causes the battery to recharge.

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The amount of electrical energy per mass that a battery can deliver is a function of the cell’s potential (or voltage) [J C-1] and capacity [A h kg-1], which are linked directly to the chemistry of the system.3 Among various existing technologies (Fig. 2), Li-based batteries outperform other systems and currently accounts for 63% of worldwide sales values in portable batteries.4

The cost of batteries is driven by the cost of the cathode material. High energy density cathode materials are required for large scale commercial implementation of energy applications that require robust energy storage systems. Desired characteristics of cathode materials include being safe, non-toxic, inexpensive, that is provides a high-power density and a long-life cycle.

For different cathode material groups shown in Fig. 3, LiNixCOyMn1-x-yO2, also known as nickel-rich NCM (or NMC), have received significant attention because of their high energy capacity, relatively low cost and improved safety.5 The material offers the best balance between desired characteristics relative to other materials. This is mainly due to the transitional metal properties and ratios in the compound. Compared to traditional LiCoO2, its low cobalt content makes it safer and more affordable.6 Excess nickel, which is the main active redox species (Ni2+ <--> Ni4+) in the structure, allows NCM to deliver a higher capacity than that of LiNi1/3Co1/3Mn1/3O2 (150 mAh g-1).7 However, excess nickel can also decrease the cycle life and thermal stability of the cathode material. Overall, nickel provides a high capacity but poor thermal stability.6 Manganese offers cycle life and safety.7 The high electronic conductivity of cobalt results in excellent rate capability.7 However, Layered Li-excess materials suffer from a layered-spinel phase change. This leads to a voltage fade over extended cycling, which needs to be addressed before commercial implementation is feasible.

Among the various nickel-rich NCM materials, LiNi0.6Co0.2Mn0.2O2 (x = 0.6, also called NCM 622) has been widely studied, and is considered one of the most balanced materials between the properties of the three transitional metals present in the cathode.8 Table 1 below displays different studies on 622 NCM and their results.

NCM 622 can be synthesized via co-precipitation or solid-state reaction, but these processes require extensive heat treatment to obtain crystalline materials, and they have difficulties in obtaining homogeneous products due to challenges of process control. To reduce energy consumption during production, and improve homogeneity of the product, an aerosol based synthesis method called spray pyrolysis can be utilized to synthesize multi-component metal oxides. Spray pyrolysis is an attractive method for generation of non-agglomerated powders with micron particle size. It is a rapid, robust process that can be scaled up to industrial production of nanostructured metal-oxide materials. Comparison of different cathode production methods are summed up in Table 2.12

Aerosol means small particulates suspended in gas phase. Compared to other processes, aerosol-based methods provide a scalable approach to produce nanostructured powders with narrow size distribution reproducibly in an inexpensive method. The process of spray pyrolysis involves the atomization