Chemical Communications Blog

Increasing the volumetric energy densities of batteries is essential for improving the durability of portable electronics and the operating ranges of electric vehicles. One way to improve energy density is to enlarge the mass fraction of active materials in battery electrodes; however, the degree of enhancement remains limited. This limitation results from the densification of the electrodes when the mass fraction increases, making electron transport and ion diffusion throughout the electrodes sluggish. These drawbacks lower the utilization efficiency of the overall electrode materials.

A team of scientists from China and the United States has recently addressed the aforementioned challenges. Specifically, they synthesized a 3D cathode of carbon-coated Li₂MnSiO₄ (Li₂MnSiO₄/C) with a structure mimicking a honeycomb filled with bee pupas (Fig. 1). This lithium-ion battery cathode possesses a high mass fraction of 90% (of overall electrode mass) as well as a volumetric energy density as high as 2443 Wh/dm³.

The uniquely structured electrodes were prepared through a hard-template method (Fig. 1). Using polystyrene particles, silica surface coating, and Li₂MnSiO₄ precursor infiltration, the authors synthesized a carbon-coated Li₂MnSiO₄ honeycomb scaffold with each cavity filled with a carbon-coated Li₂MnSiO₄ particle. This architecture differed from previously reported 3D structures, which typically had a large portion of voids, and enabled an ultrahigh active-material mass loading of 90 wt.%. Additionally, the gaps between the scaffold and the particles functioned as ion-diffusion channels, and the carbon coatings served as electron-transport expressways. These characteristics effectively addressed the problem of sluggish ion diffusion and electron transport.

Figure 1. The synthesis procedures of the BPFH-shaped Li2MnSiO4/C electrode. The green particles and yellow scaffold represent polystyrene spheres and the silica coating, respectively.

Due to the facilitated electron transport and ion diffusion, the Li₂MnSiO₄/C electrode with a bee pupa-filled honeycomb (BPFH) structure (Fig. 2a) exhibited an outstanding charge-storage performance. Specifically, it delivered a high volumetric capacity of 643 mAh/cm³ at a current density of 0.1 C, corresponding to a volumetric density of 2443 Wh/dm³. This volumetric capacity was approximately two times higher than that of a Li₂MnSiO₄/C honeycomb lattice without any Li₂MnSiO₄ particles (Fig. 2b). After 100 consecutive charge-discharge cycles, the BPFH-shaped Li₂MnSiO₄/C electrode retained a volumetric capacity of 328 mAh/cm³ (Fig. 2c).

Figure 2. (a and b) Scanning electron microscopy images of (a) the BPFH-shaped Li2MnSiO4/C electrode and (b) the Li2MnSiO4/C scaffold. (c) The capacities and the Coulombic efficiencies of the two electrodes during 100 charge-discharge cycles.

The demonstrated BPFH architecture could be extended to other materials for the synthesis of battery electrodes with both high mass fractions of active materials and outstanding volumetric energy densities.

To find out more please read:

A Bee Pupa-Infilled Honeycomb Structure-Inspired Li₂MnSiO₄ Cathode for High Volumetric Energy Density Secondary Batteries

Jinyun Liu, Xirong Lin, Huigang Zhang, Zihan Shen, Qianqian Lu, Junjie Niu, Jinjin Li and Paul V. Braun

Chem. Commun., 2019, 55, 3582-3585

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

Organic compounds with conjugated electron structures are emerging as promising Li-ion battery cathodes due to their high capacity and environmental benignity. To make these cathodes practically feasible, organic electrodes are typically incorporated with metal ions to boost their energy densities. The addition of metal ions, however, usually jeopardizes the structural integrity of the electrodes and shortens battery lifetime.

Recently, three groups of Chinese researchers demonstrated that increasing the electrolyte concentration could effectively prolong the lifespan of metal-incorporated organic cathodes. The researchers studied cuprous tetracyano-quinodimethane (CuTCNQ), a Cu²⁺-containing organic Li-ion battery cathode, and observed its significantly improved cycling stability in a 7 M LiClO₄ electrolyte compared to a 1 M electrolyte. This work was published recently in ChemComm.

CuTCNQ in a typical diluted electrolyte of 1 M LiClO₄ exhibited unsatisfactory stability. Its first-cycle charging capacity reached ~180 mAh/g, but it dropped appreciably to 23 mAh/g after the first discharging process (Figure 1a). Concurrently, the electrolyte turned from clear to yellow (Figure 1b), due to the dissolution of TCNQ. These observations unequivocally showed the rapid disintegration of CuTCNQ in diluted electrolytes.

Figure 1. (a) The first-cycle charge-discharge profile of CuTCNQ in a liquid electrolyte containing ethylene carbonate (EC), propylene carbonate (PC) and 1 M LiClO4 (1 M LiClO4-EC/PC). (b) Photographs showing the electrolyte color before and after the first charge-discharge cycle.

CuTCNQ was found to be more stable in electrolytes with concentrations higher than 1 M. When the LiClO₄ concentration increased to 3 M, 5 M and 7 M, the specific capacities of CuTCNQ retained after 50 consecutive charge-discharge cycles were ~25 mAh/g, ~70 mAh/g, and ~110 mAh/g, respectively (Figure 2a). All of these capacities were higher than that of CuTCNQ in 1 M LiClO₄ after the same cycle number (<10 mAh/g). Additionally, the electrolytes experienced nearly no color change, suggesting little TCNQ was dissolved (Figure 2b).

The elevated stability of CuTCNQ correlates to the formation of Li⁺-ClO₄^– ion pairs in concentrated electrolytes (Figure 2c). With increasing LiClO₄ concentration, Li⁺ and ClO₄^– tend to form ion pairs that coordinate with solvent molecules. Solvent-coordination reduces the number of free solvent molecules that can dissolve TCNQ, thus minimizing the dissolution of TCNQ.

Figure 2. (a) The cycling stability performances of CuTCNQ with electrolytes with different LiClO4 concentrations. (b) Photographs showing the electrolyte color before and after 50 charge-discharge cycles at different LiClO4 concentrations. (c) Li+ and ClO4- form solvent-coordinated ion pairs in super-concentrated electrolytes (e.g., 7 M).

This work provides a facile approach to mitigate the capacity fading of CuTCNQ. The strategy may be extended to stabilize other metal-incorporated organic cathodes in Li-ion batteries.

To find out more please read:

Sustainable Cycling Enabled by A High-Concentration Electrolyte for Lithium-Organic Batteries

Ying Huang, Chun Fang, Wang Zhang, Qingju Liu and Yunhui Huang

Chem. Commun., 2019, 55, 608-611

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

Synthesizing nanomaterials consisting of two-particle pairs, or dimers, is no longer a headache. Hongyu Chen and coworkers from Nanjing Tech University, China recently developed a protocol that can produce gold dimers with a record high yield. This breakthrough is published in Chem. Commun.

Dimers are suitable platforms to study the effects of particle-particle interactions on the electrical and optical properties of the constituent materials. Unfortunately, no conventional synthesis methods to exclusively produce dimers from single particles have been successful. This is because of the uncontrollable particle-aggregation rate that leads to the formation of multi-particle clusters. Therefore, how to couple single particles into dimers without triggering their further aggregation has become a tough nut to crack.

Chen and coworkers found a solution by developing a polymer-assisted method that generates gold dimers with high yield. Firstly, they encapsulated individual gold nanoparticles with polymer shells made of polystyrene-b-poly(acrylic acid). Under optimized conditions, the gold nanoparticles were mostly coupled into dimers (Figure 1), achieving a dimer yield of 65%. This is the highest dimer yield achieved for one-step synthesis methods.

Figure 1. (a) A transmission electron microscopy (TEM) image of the polymer-encapsulated gold single particles. (b) A TEM image and (c) a scanning electron microscopy image of the synthesized gold dimers. All scale bars are 200 nm.

The key to this success is due to three factors: temperature, solvent composition and acid concentration. All these factors can change the strength of the repulsion force among the polymer shells. The force must be meticulously tuned to a level that is weak enough to induce 1-to-1 coupling, but strong enough to prevent 1-to-multiple or multiple-to-multiple aggregation. Through a set of control experiments, the authors identified the optimal conditions to be 60 ^oC, dimethylformamide/water (v/v)=6:1 and 5 mM of hydrochloric acid.

The method demonstrated herein could be extended to other particles. It may also inspire versatile synthesis strategies towards complex nanostructures with high selectivity.

To find out more please read:

Controllable Oligomerization: Defying Step-Growth Kinetics in the Polymerization of Gold Nanoparticles

Xuejun Cheng, Gui Zhao, Yan Lu, Miao Yan, Hong Wang and Hongyu Chen

Chem. Sci., 2018, DOI: 10.1039/C8CC03424A

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

Researchers in China recently developed a new Pd-based catalyst that outperformed Pt, the benchmark catalyst for electrochemical oxidation of ethanol. This catalyst, synthesized by a one-pot chemical reduction method, consists of branched Pd nanocrystals coated with thin Pt₃Ni shells.

The ethanol oxidation reaction (EOR) is a typical anode reaction that drives the energy output from fuel cells. Due to its intrinsically slow kinetics, the reaction requires proper EOR catalysts to facilitate the oxidation. Pt-based materials are highly active in promoting EOR, but the scarcity of Pt leads to high costs and demands efficient methods to recycle these materials. In addition, the instable catalytic activity of Pt significantly reduces the lifetime of EOR catalysts containing Pt. Clearly, developing inexpensive EOR catalysts with comparable performance to Pt is meaningful for the affordability and durability of fuel cells.

A research team led by Shuifen Xie at Huaqiao University and Shenzhen Research Institute of Xiamen University in China, have demonstrated a one-pot chemical reduction method of a novel EOR catalyst. This catalyst is made of Pt₃Ni-coated Pd nanocrystals as shown in Figure 1. There are three main advantages for this catalyst over the benchmark Pt: Firstly, the core material Pd is more affordable. Secondly, the ultrathin Pt-alloy coating, Pt₃Ni, contains relatively less Pt and is reported to exhibit high EOR catalytic activity. Thirdly, the little lattice mismatch between Pt and Pd allows seamless integration of the two metals that is beneficial to preserve the structural integrity and ensure excellent durability.

Figure 1. (a) The schematic illustration showing the key steps of the one-pot chemical reduction method. The catalyst is formed via consecutive reduction of Pd²⁺, Pt²⁺ and Ni²⁺ to Pd nano-branches, Pt nanoparticles and Pt₃Ni coatings, respectively. (b) A TEM image of a representative morphology of a branched nanocrystal. (c) Elemental mappings depict that Pt and Ni elements exist mainly in the shell while Pd is in the core.

Electrochemical characterizations revealed that the catalytic performance of the Pt₃Ni-coated Pd nanocrystals outperformed those of two commercial catalysts: Pt/C (Pt particles supported on activated carbon) and Pd black (a fine powder elemental Pd). Figure 2a compares the linear-sweep voltammograms of the three samples. The synthesized catalyst showed appreciably enhanced oxidation current at potentials beyond 0.4 V vs. RHE. The histograms in Figure 2b clearly display that the mass activity and the specific activity of the synthesized nanocrystals are the highest. The authors ascribed the superior performance to the high surface area (42.50 m² g^-1), the ultrathin Pt₃Ni coating with its {111} crystal planes exposed, and the core-shell configuration.

Figure 2. (a) Linear-sweep voltammograms of the synthesized catalyst (Pd@Pt₃Ni/C), Pt/C and Pd black. (b) The comparison of mass activity (i.e. oxidation current normalized to the masses of the catalysts) and specific activity (i.e. oxidation current normalized to the areas of the catalysts) of Pd@Pt₃Ni/C, Pt/C and Pd black.

This work signifies the feasibility of Pd-based nano-catalysts as alternatives to Pt towards catalyzing EOR. It is also expected to encourage the effort in developing a diverse array of inexpensive and high-performance catalysts for other reactions pertaining to fuel cells, including but not limited to oxidation of fuels other than ethanol and oxygen reduction reactions.

To find out more please read:

One-Pot Synthesis of Pd@Pt₃Ni Core-Shell Nanobranches with Ultrathin Pt₃Ni{111} Skins for Efficient Ethanol Electrooxidation

Yuanyuan Wang, Wei Wang, Fei Xue, Yong Cheng, Kai Liu, Qiaobao Zhang, Maochang Liu and Shuifen Xie

Chem. Commun., 2018, DOI: 10.1039/c8cc02816h

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in the United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.