Nanomaterials for Energy Storage in Lithium-ion Battery Applications

Introduction

Rechargeable Lithium-ion (Li-Ion) battery technology has come a long way since its introduction in the early 1990s. Over the last two decades, they have become the technology of choice for powering portable electronic devices such as cellular phones and laptop computers. Currently, Lithium-ion batteries are steadily replacing Nickel-Cadmium (NiCd) and Nickel-Metal Hydride (NiMH) battery technologies in portable power tools. In the future, Lithium-ion batteries are poised to power a new generation of hybrid electric vehicles (HEV), plug-in hybrids (PHEV) and electric vehicles (EV). Another emerging application for Lithium-ion technology is in battery electrical energy storage systems for smart grids that are powered by traditional energy sources like coal, as well as intermittent renewable energy sources like solar and wind.1

The optimum combination of long run time (high energy density at the desired power) and long life (recharge characteristics) sets Lithium-ion battery technology apart from the competition. Needless to say, safety and cost expectations of the application also have to be satisfied. While all of these requirements are being met for portable electronics, the technology is just beginning to move up the optimization curve for emerging applications such as electric automotives, power tools and storage systems. For example, one factor that distinguishes the portable electronics and the electric vehicle application is power density. The latter requires much higher charge and discharge rates compared to the former. While higher power can be achieved to some extent by redesigning the way the battery cell is constructed, nanomaterials are also expected to play a key role in the achievement of high-power capability. Nanomaterial strategies are also being employed to provide better strain accommodation in high-capacity electrodes so that this storage capacity can be extracted reversibly with minimum compromise in cycle life. Alternatively, the stable cycle life and storage life characteristics of “zero-strain” electrode materials can be improved even further by utilizing nanosized versions of these electrodes, thereby providing a new generation of electrical energy storage options for smart grids and back-up power systems.

Overview of Lithium-ion Electrode Materials Chemistries

Conventional Lithium-ion battery materials typically start as 10-50 micron sized particles, which are then coated onto aluminum or copper current collectors along with conductivity enhancers and binders. The work-horse cathode chemistry for the last couple of decades has been Lithium Cobalt Oxide, LiCoO2, (Prod. No., 442704) a layered compound with a distorted rock-salt (α-NaFeO2) structure.2 Some alternative cathodes, like the three dimensional spinel Lithium Manganese Oxide,2 LiMn2O4, (Prod. 725129) have been commercialized only in niche applications, because of performance limitations.2 Carbon-based materials have been the preferred choice for anodes, with some version of graphite being utilized in a majority of the commercially available batteries.3 Battery developers choose electrode materials with the intent to optimize performance from the standpoints of energy, power, cycle life, cost and thermal stability. Until recently, much of this optimization has taken place with the portable electronic application in mind. More recently however, a significant effort is also being spent on new and emerging applications like power tools, electric vehicles and battery electrical energy storage systems. New chemistries have emerged for cathodes including a variety of mixed metal oxides, like LiMn1.5Ni0.5O4 (spinel, Prod. No. 725110 ), LiNi0.33Mn0.33Co0.34O2 (layered) and LiNiCoAlO2 (layered) and metal phosphates, like LiFePO4, LiCoPO4 (Prod. No. 725145 ) and LiMnPO4, (olivine).2,4 For anodes, new materials include oxides such as Lithium Titanates (Li4Ti5O12) & Tin Oxide (SnO2, Prod. No. 549657 ), elemental silicon (Si, Prod. No. 267414) & tin (Sn, Prod. No. 265640 ) and many carbon-based materials.3,5 In many of these new chemistries, having the materials in nanoparticle form or as a nanostructured particle or film is critical to achieving the desired performance dictated by the end application.6

Advantages of Nanomaterials for Lithium-ion Applications

From the battery application perspective, the incentive for implementing a nanomaterial electrode as a Lithium-ion storage material would be to derive significant improvement in energy, power, cycle life or some combination of the same. Nanoparticles or nanopowder electrode materials, i.e., ultrafine versions of the conventional micron-sized electrode powders, are the earliest implementation of nanomaterials science in the Lithium-ion battery application. Indeed, carbon-black, a nanomaterial that has been around for several decades, has been used in Lithium-ion batteries since its early days.7 While carbon-black is used in the electrode, it does not store electrical energy and merely acts as a “passive” conductivity enhancer to improve power capability. However, by designing the “active” energy storage component of the electrode as a nanoparticle, significant performance improvements can be realized for two reasons:

1. shorter diffusion lengths for the Lithium-ion travelling from the particle core to the surface where it transfers to the electrolyte and
2. higher electrode-electrolyte contact area arising from the inherently high surface areas characteristic of the particles.6

Reducing electrode particle size into the nanoscale regime is also believed to substantially reduce the mechanical stresses caused by volumetric expansion and contraction during charge and discharge. A recently developed model suggests that particles must have a size under a certain critical radius so that the strain produced by intercalation can be accommodated elastically, rather than by plastic deformation, allowing for the full recovery of the original, stable structure.8 Examples of nanomaterials either in nanopowder form or as nanostructured films with wire, rod, whisker or columnar morphologies are actively being pursued to maximize cycle life with minimum compromise in energy density.5,9

Some processing advantages may also be realized while working with nanosized Li-ion storage electrode materials. For example, most Lithium-ion cathode materials are made from precursors containing lithium and other transition metals, which are combined and then heat treated under a variety of conditions to arrive at the desired oxide or phosphate composition. Heat treatment processes can be tedious and energy-intensive, particularly for large particle aggregates where the surface of the aggregate may experience a different thermal profile compared to the bulk. Improper heat treatment leads to nonhomogeneous composition in the material and hence deterioration in performance. While heat treating nanomaterials, it is easier to maintain a homogenous thermal profile throughout the material and thereby produce homogenous compositions without having to resort to energy-intensive processing methods.

Nanomaterials could also enable ultrathin and flexible electrode geometries, which may spawn a new generation of high-rate battery formats for low profile components such as sensors, RFID and flexible devices for consumer and medical applications. Dispersions and inks made using nanopowder electrodes could be used to fabricate low-cost printed batteries via roll-to-roll processing or to integrate thin-film batteries with other devices in printed electronic assemblies made via ink-jet processes.10

Classification and Production of Nanomaterials

The term nanomaterial is typically used to refer to materials that have at least one dimension of less than 100 nm.11 In using the term, there is also an underlying implication that the material has some enhanced property or characteristic compared to larger particle size versions of the same composition. Nanomaterials are not a new class of materials, even though the recent attention that they have been garnering may suggest otherwise. Some of these materials, like volcano dust, have existed in nature forever. Other man-made materials, like carbon black and fumed titania (TiO2), have been around for several decades. One way to classify nanomaterials is to categorize them according to the method by which they are produced, i.e., physical or chemical. The physical method can be further sub-divided into mechanical or phase-change methods. In the physical-mechanical method, particles in the nanometer size range are produced by milling or grinding larger particles of the target composition without any accompanying chemical change.12 This method is commonly referred to as a “top-down” approach to making nanomaterials. In physical-phase change methods, the nanomaterial is created via a phase change process. Examples include direct precipitation, in which a material in solution is precipitated as a solid nanomaterial, and thermal, plasma or laser ablation processing in which vaporized material is condensed into solid nanoparticles. In using either of the physical processes, no chemical change is necessary to arrive at the target nanomaterial composition. Chemical methods include processes where the nanomaterial is synthesized from an initial material that is chemically different from the target composition. The target composition is arrived at via chemical synthesis of a solid that is formed directly in the nanoscale. Examples include flame pyrolysis, spray pyrolysis and wet-chemical methods such as sol-gel and solvo-thermal synthesis.12 The chemical method and the physical-phase change method are both examples of the “bottom-up” approach to making nanomaterials.

An example of a chemical process for making nanomaterials is NanoSpray Combustion, invented by one of the authors, where nanoparticles or nanopowders are produced via combustion chemical vapor condensation (nCCVC).13 In this process, nanoparticles are produced by the combustion of a solution nanospray that consists of precursors that contain the elements that eventually make up the nanoparticle. A schematic of this process is shown in Figure 1. nGimat′s proprietary Nanomiser® device is critical to generating the nanospray from which vapor species are formed in the flame. Condensation of particles can take place either in dry form nanopowders or into a liquid medium to form dispersions. By tailoring precursor formulations and process parameters, nCCVC can be also be used to produce nanomaterials with dopants and surface carbon-coatings to improve performance in the end application.