Date Published: December 29, 2014
Publisher: John Wiley and Sons Inc.
Author(s): Nasir Mahmood, Yanglong Hou.
Lithium‐based batteries possessing energy densities much higher than those of the conventional batteries belong to the most promising class of future energy devices. However, there are some fundamental issues related to their electrodes which are big roadblocks in their applications to electric vehicles (EVs). Nanochemistry has advantageous roles to overcome these problems by defining new nanostructures of electrode materials. This review article will highlight the challenges associated with these chemistries both to bring high performance and longevity upon considering the working principles of the various types of lithium‐based (Li‐ion, Li‐air and Li‐S) batteries. Further, the review discusses the advantages and challenges of nanomaterials in nanostructured electrodes of lithium‐based batteries, concerns with lithium metal anode and the recent advancement in electrode nanostructures.
The huge consumption of fossil fuels by the increasing population is ringing the alarm bell for the environment. Thus, green energy has attracted intensive attentions since current “fossil fuels” are being depleted with the developing society and concerns about environment have become serious.1 To resolve these problems and to decrease the usage of fossil fuels, alternative renewable energy production technologies need to be utilized such as fuel cell and solar cell, hydro‐power and wind‐energy.2 These energy production systems need storage devices, thus it is highly desirable to develop efficient and cheaper energy storage devices to meet the requirement of advanced society.3, 4, 5, 6 Among various energy storage systems, rechargeable batteries are one of the most suitable and feasible options for the storage of electrical energy.7, 8, 9, 10 Several rechargeable battery systems like lead‐acid, nickel‐metal hydride, nickel‐cadmium and lithium‐based batteries are serving mankind from last one century. However, advancement in portable electronics and their high demands need faster and safer energy storage systems.5, 11, 12, 13, 14, 15, 16, 17, 18, 19 Furthermore, the dream of electrification of the road market and electricity storage from intermittent energy production systems needs advanced rechargeable batteries.20 Thus, several key parameters, e.g., cost, high energy and power, longer cyclic life, safety and environmental benignity, are needed to be considered in the development of advance rechargeable batteries to make them useful in hybrid electric vehicles (HEVs) and electric vehicles (EVs).21 It is noted that the low specific energy or energy density limit the most mass producible EVs to a short traveling distance (≈163 km) per charge.22 Furthermore, the complex systems result in high cost and make batteries thermodynamically unsafe. In this regard, the United States Advanced Battery Consortium (USABC) has introduced several standards and put forward long‐term objectives for batteries of EVs to lead the researchers in this field.23 The required specific energy and energy density values determined by USABC for rechargeable battery systems of EVs are 200 Wh kg–1 and 300 Wh L–1 at a discharge rate of C/3 (C/3 means a complete discharge in 3 hours).23 Furthermore, the battery should be able to cycle 1000 times with at least 80% capacity retention.24 From the recent development, it is considered that lithium‐based batteries (lithium ion battery, lithium sulfur battery, and lithium air battery) are capable of achieving these standard values and making the longer drives of EVs possible.25, 26, 27, 28, 29 Unfortunately, several problems are a big hurdle in the real application of lithium‐based batteries in EVs, e.g., poor cyclic life and power density.5, 30 The development of new high capacity electrode materials that possess long cyclic life than traditional materials is a key solution to above mentioned problems.31 The high capacity materials have ability to increase the energy contents per volume and weight at low cost. But these materials introduced new fundamental challenges both at their synthesis and operation as an electrode in batteries.12, 32, 33 Progress in lithium‐based batteries has been largely benefited by developing nanostructured electrodes in comparison to conventional electrode. The high active surface area of nanostructures significantly improves the efficiency to completely utilize the electrode material, resulting in enhanced performance of electrode.34, 35, 36 The conventional lithium ion (Li‐ion) batteries are made of graphite which acts as negative electrode and metal oxides or phosphates (LiCoO2, LiFePO4, etc.) as positive electrode.37 All these materials working as anode and cathode store lithium ions (Li+) through insertion process, thus resulting in limited energy storage due to limited available vacancies for Li+ in the host sites.38 However, the insertion mechanism brings good capacity retention because Li+ only resides at host sites and can be cycled easily without destroying the host structure. In contrast, to improve the specific capacity of Li‐ion battery new anode materials are introduced which store Li+ in different way.39, 40 These negative electrode materials (e.g., Si, Ge or Sn etc.) utilized the alloying or conversion reaction with Li+ by breaking the bonds between the host atoms, thus enhanced the capacitive performance of electrode.13, 41, 42, 43 In terms of theoretical capacity the Si‐Li, Ge‐Li and Sn‐Li alloy brings capacities as high as 4200, 1623, 994 mAh g–1, respectively but result in volume expansion up to 300%.44, 45, 46, 47, 48 Such drastic structural changes happen as result of bond breaking of host atoms during alloy formation that affect the capacity retention and cyclic life drastically.49, 50 The capacity decay could also arises due to electrical insulation of the fractured electrode material.51, 52 Thus a critical design at nanoscale and good control on structure is required to overcome volume changes, structural fracture and side reactions.
The working mechanism and structure of each type of cell is elaborated in the respective sections. In general all types of batteries (Li‐ion, Li‐air and Li‐S) have similar cell assembly that consists of two electrodes (anode and cathode/catalyst) and electrically insulating separator but are permeable for ions and electrolyte. However, keep in mind that Li‐air cell also needs a continuous oxygen supply. But the cell chemistries in all these lithium‐based systems are quite different from each other; this is explained below.
Future research demands nanostrucutured materials with high active surface areas which could offer large sites for intercalation of Li+ for both insertion and conversion processes ultimately providing oppurtunities to tailor high energy density/power density devices for practical applications. However, a great advancement in the field of lithium‐based batteries has overcome many of the challenges that are big obstacles in their application to the field of portable electronics or short distance traveling EVs. Despite these considerable advancements, there still remains a number of challenges to be addressed before the usage in long distance traveling or heavy duty storage systems, e.g., storage of electricity in grid station or from the intermittent energy productions systems such as solar cells.
It is well‐known fact that nanomaterials have a lot of advantages over their counter bulk materials, especially when we are talking about the electrode materials of lithium‐based batteries. Figure2 represents the behavior of two electrodes composed of nanostructure and bulk; it is obvious that both structures behave differently and improved results are obtained for nanomaterials.90 Why nanomaterials behave differently is explained below, but a few disadvantages of nanostructures are also listed here, so readers should pay attention to them while developing their nanostructures for lithium batteries electrodes. These challenges are easily overcome by good control over structure and fabrication method or by developing hybrid structures.
Nanotechnology has a revolutionary role in determining the new electrode materials and their unique nanostructures to bring breakthroughs in the lithium‐based batteries towards their real applications in EVs. The following sections highlight the recent advancements in the electrode field of lithium‐based batteries including Li‐ion, Li‐air, and Li‐S batteries.
Thanks to the high energy densities, long cyclic life, safety, and enviromental beniginity of lithium‐based batteries, the dream of a continuous supply of energy to portable electronices and short‐distance traveling EVs is coming true. A series of research has been done to improve the energy storage mechanisim of Li‐ion battery by introducing new nanostructures and their hybrids with graphene/carbon that can overcome several issues, such as structural changes, low conductivity and poor capacity, stability concerns of Li‐air battery by developing new breathing catalyst having the ability of ORR and OER to keep the air pathway clear and limited electronic flow caused by the deposition of reaction products. Further, development of electrolytes and several additives is also helpful to improve the cyclic life of the Li‐air battery. Moreover, polysulfide shuttles are also controlled by developing new hybrid structures of sulfur that limits the dissolution of polysulfide anions. The concerns using lithium metal anode are also resolved by introducing new lithiated anodes for Li‐air and Li‐S batteries. However, to achieve the standards set by USABC, new synthesis methods should be developed for electrode nanostructures that can be industrially acceptable. Definition of new stable designs and/or hybrids of nanostructures of electrodes is needed that can overcome the problems of large volume changes, electrical insulation, formation of homogenous SEI film and meanwhile brings higher packing density along with access to all redox sites for larger volumatric density. Porous breathing cathodes with larger surface area bi‐functional catalysts that can maintain higher electrical conductivity and successfully reduce and oxidize oxygen with ongoing discharge‐charge process. Sulfur containing closed structures should be developed for Li‐S battery that act as micro‐electrochemical reactors, inhibiting the dissolution of sulfur while control its volume change and enanhced its conductivity. Protection of lithium metal anode from side‐reactions to form surface dendrites and conversion to LiOH by reacting with hydroxal ions produced from decomposition of electrolyte at catalyst surface. Future work on lithium‐based batteries should explore the fundamental characterizations of reaction products, the battery chemistry and the stability concerns of electrodes and electrolytes inside the battery with ongoing charge‐discharge process.