Multifunctional approaches for safe structural batteries

Highlights

• Review of approaches for multifunctional structural batteries.

• Structural multifunctionality and safety are considered.

• Multiscale design approaches are summarized.

Abstract

Recent advancements in Li and Li-ion based energy storage resulted in development of novel electrode materials for higher energy density which are finding their applications in transportation. There appears to be a limitation in improvement of specific energy of the system based solely on design of material compositions for multivalent intercalation compounds. In addition, higher energy stored by the system implies need for addressing safety concerns especially when it comes to large automotive battery packs. New approaches for improvement of both energy density and safety of batteries are emerging, where multifunctionality of the materials and/or architectures is utilized. This article presents a review for such approaches from multifunctional current collectors to design of batteries capable of supporting mechanical loads and thus possessing ability to be used as a structural component.

Introduction

The concept of an electrochemical energy storage device has been perceived for a long time as a separate system with the sole role of providing electrical current to the external load. Such system would require protective measures in terms of physical barrier against any mechanical impacts as well as cooling and battery management systems (BMS). Such perception dominated the research since the first introduction of Li-ion batteries to the market by SONY [1]. Significant efforts undertaken by the research community have led to development of multiple new materials for electrodes in addition to progress in processing and manufacturing [2]. This resulted in incremental increase of specific energy of cathodes, which are the limiting electrodes in terms of potential and capacity of the cell. A good example is the progress made with the layered transition metal oxide compounds where increase in nickel content LiNi_1/3Mn_1/3Co_1/3O₂ (NMC333) [3] - LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532) [4] - LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) [5]- LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) [6] has resulted in the overall increase in usable capacity from 150 mAh/g (NMC333) to 200 mAh/g (NMC811) [5], [7]. A lithium-rich cathode such as layered–layered xLi₂MnO₃ (1-x)LiMO₂ (M=Mn, Ni, Co) has also been developed which can deliver capacity of above 250 mAh/g [8], [9]. Novel materials and processing methods for the negative electrodes have also been developed [7], [10], [11], [12].

While materials research and discovery resulted in development of electrodes with increased capacity and fast charging capabilities [19], the cell and system level energy density still needs improvement. The structure of the typical cell sandwich, a unit block of a cell and battery, involves significant amounts of inactive components in form of metal current collectors (copper on the negative side and aluminum on the positive side of the electrode pair) and separators. This reduces both volumetric and gravimetric energy densities of a single electrode pair. With the addition of cell packaging material (pouch of pouch cells, or cans used for prismatic and cylindrical cells), module enclosures, battery enclosures and protective structures, the useful volumetric energy density decreases even further. Implementation of multifunctional concepts and materials in batteries can eliminate some of the inactive components in battery structure. Developments in this area are expected to provide significant improvement in performance of energy storage systems in addition to discovery of new Li (or other) ion host materials for electrodes.

Multifunctionality has been an actively pursued concept in engineering and covers a wide range of applications [20], [21]. In this regard, it is rather more suitable to utilize the notion of multifunctional materials systems, which covers materials and composites as well as structures [20]. Usually the multifunctional materials applications combine intrinsic material properties of responding to external stimulus with some degree of integration into structures. The response to stimulus can have different forms, which most frequently include: response to temperature (shape memory alloys), response to mechanical stress (piezoelectric), response to electric current (piezoelectric, dielectric elastomers), and response to magnetic field (magnetorheological fluids). These properties are utilized by embedding the devices in structures for sensing, vibration damping, energy harvesting, heating and cooling, etc. Excellent reviews can be found in [20], [21]. Multifunctional systems can span different length scales, depending on the target properties and application. As a good example, carbon nano-tubes (CNTs) can serve as reinforcement for composites (meso- and macro-scale) utilizing their high strength. Such composites in turn serve as structural components. Other properties of CNTs, such as high electrical and thermal conductivity, allow expansion of multifunctionality and usage of CNT-based composites in applications for heat dissipation [22] and mechanical damage sensing [23].

While multifunctionality in composite structures has been explored relatively well, mostly from the mechanics view point, multifunctionality applied to energy storage is a rather new subject. One of the earlier examples is the structural battery introduced by Liu et al. [24] and Ekstedt et al. [25]. Most recent reviews can be found in [26], [27]. In addition to load bearing capabilities, multifunctionality, when applied to batteries should result in improved safety and performance without sacrifice of stored energy per unit mass and volume. In this report we summarize achievements in this area that cover both multifunctional materials and multifunctional structures utilized for energy storage purpose. An overview of multifunctional energy storage approaches is given in Fig. 1. The structure of this review will follow Fig. 1 with the discussion of multifunctional materials and concepts for battery cell components, battery cells, embedded batteries, and finally battery safety.