New Frontiers in Lithium-Ion Batteries
by Xin Su
July 11, 2016
Since their discovery in 1970 and commercialization in the 1990s, lithium-ion batteries (LIBs) have become one of the most popular energy storage solutions. Consumer electronics and electric vehicles are among their major applications.
Despite limitations such as degradation, safety problems, and strong competition from emerging alternatives, LIBs continue to be the primary choice for many applications. In the meantime, scientists continue to seek ways to improve the overall performance of LIBs, especially in terms of energy capacity, durability, and cost.
Many studies focused on the intercalation of lithium ions into electrodes because its efficiency directly affects LIB performance. In contrast, the intercalation of counterions (anions) has been overlooked. Recently, Yongbing Tang, Chun-Sing Lee, and colleagues at Shenzhen Institutes of Advanced Technology at the Chinese Academy of Sciences and the City University of Hong Kong maximized both intercalation processes to develop a high-performance aluminum–graphite dual-ion battery (AGDIB) at low cost.
The AGDIB consists of a graphite cathode, an aluminum anode, and a LiPF6–ethyl methyl carbonate electrolyte. The authors believe that this is the first use of an aluminum anode in a dual-ion battery. During the charging process, Li+ cations deposit on the aluminum anode to form an Al–Li alloy; and PF6– anions intercalate into the graphite cathode. The anode also takes on the role of the current collector, which greatly reduces the dead load and dead volume.
The AGDIB delivers much higher energy density and power density than conventional LIBs. Using low-cost, widely available electrode materials should ensure a highly competitive future for large-scale production and commercialization of these batteries. (Adv. Energy Mater. DOI: 10.1002/aenm.201502588)
LIB cathode materials usually have lower specific capacities than their anode counterparts, which leads to undesirable mismatches. Closing this capacity gap will eliminate a pervasive bottleneck that limits the overall performance of LIBs.
Creating highly porous micro- and nanostructures with ample free space is a promising solution because it would increase the lithium storage capability of cathode materials. A team led by Ranbo Yu, Yu Zhang, and Dan Wang at three Beijing campuses of the Chinese Academy of Sciences and Gold Coast Campus, Griffith University (Australia) developed multishelled vanadium oxide hollow microspheres that exhibit much higher capacity and better cycling stability than bulk vanadium oxide.
The researchers found that anions such as VO3– can adsorb onto negatively charged carbonaceous microspheres because their binding to hydroxyl groups overcomes electrostatic repulsion. This anion-adsorption process allows them to prepare metal oxides of multishell hollow microspheres (MO-MS-HMSs) by removing the carbonaceous template. The MO-MS-HMSs, especially MS-V2O5-HMSs, combine features of 3-D hollow microspheres and 0-D (“zero-dimensional”) nanoparticles to create more lithium storage sites with shorter diffusion paths for ions and electrons. According to the authors, “one can anticipate that cathodes based on MS-V2O5-HMSs will significantly reduce the mismatch between the capacities of anode and cathode materials, opening up an avenue for next-generation LIBs.” (Nat. Energy DOI: 10.1038/nenergy.2016.50)
In LIBs, the cathode materials are often lithium–transition metal oxides, in which the transition metals are present in excess of lithium. Lithium-excess materials are expected to provide higher energy densities. Demonstrating this, however, requires a thorough understanding of their electrochemical behavior, especially the role of oxygen, which when it oxidizes the transition metal, yields additional electrons that contribute to excess capacity.
Using ab initio calculations, Gerbrand Ceder and coauthors at MIT (Cambridge, MA), the University of California, Berkeley, and Lawrence Berkeley National Laboratory show how labile oxygen electrons are correlated with local lithium-excess environments; and they identify disordered local structural components that promote the extraction of these electrons. Unhybridized oxygen states in lithium-intercalation cathodes compete with the transition metal states for oxidation, making it possible to reduce the content of transition metals. The results provide an exciting, new alternative for increasing the capacity and energy density of LIBs. (Nat. Chem. DOI: 10.1038/nchem.2524)
Durability and safety
In addition to higher energy density, LIBs should benefit from being more durable. A. Basile, A. I. Bhatt*, and A. P. O’Mullane* at RMIT University (Melbourne), CSIRO (Clayton), and Queensland University of Technology (Brisbane, all in Australia) report a simple, practical, method to pretreat lithium metal electrodes with ionic liquids to improve the life cycle and safety of LIBs.
The authors’ method involves immersing the lithium metal anodes in lithium ion–containing ionic liquid electrolytes before the battery is assembled. During this process, a lithium ion–permeable solid-electrolyte interphase layer forms (see Figure 1). In the figure, SEI is the solid–electrolyte interphase; RTIL is a room-temperature ionic liquid; and DMC is dimethyl carbonate.
The interphase layer effectively prevents dendrite formation, thermal runaway, and electrolyte consumption. For Li|electrolyte|LiFePO4 batteries, the layer allows safe cycling for 1000 cycles with Coulombic efficiencies >99.5%, which are significantly higher than with untreated anodes. The simple, effective pretreatment is readily applicable to industrial manufacturing and should create safer, more durable LIBs. (Nat. Commun. DOI: 10.1038/ncomms11794)
The present and the future
There are many competing innovations in battery research, but LIBs are still the winning player. Because new electrode and electrolyte materials are being discovered, and the fundamental mechanisms are becoming better understood, LIBs will continue to improve and remain competitive. But future demands will require battery designs that accommodate flexible and wearable materials.