Tailoring defects in hard carbon anode towards enhanced Na storage performance

Currently, lithium-ion batteries (LIBs) have achieved worldwide attention as advanced energy storage systems for commercial electronics and electric vehicles. Nevertheless, low abundance and uneven distribution of lithium resource have aroused the awareness of sustainable accessibility of LIBs to fulfill ever-increasing demand of grid-scale electrochemical energy storage systems. To spur more research into a more abundant and much cheaper candidates, alternatively, researchers in China assessed the current advances of research and proposed sodium-ion batteries (SIBs) as the imperative candidates of LIBs, and offered novel hard carbon anodes for more sustainable and cost-effective batteries.

They published their work on Mar. 10 in Energy Material Advances.

“Sodium-ion batteries (SIBs) have come under the spotlight toward replacing lithium-ion batteries,” said paper author Ying Bai, professor at School of Materials Science and Engineering in Beijing Institute of Technology. “Thanks to cost effectiveness, inexhaustible Na resources, similar chemical nature of Na with Li, and similar operation mechanism to LIBs, the development of SIBs is highly desirable. Accordingly, it is critical to develop advanced electrode materials with excellent rate performance, cycling stability and high-energy densities.”

Bai explained that among various anode materials, carbon materials may be the most likely commercialized candidates due to multiple superiorities, such as low cost, easily attainable, high cycling performance.

“Graphite represents the most promising carbon-based anodes that have been commercially used in LIBs for decades. However, graphite is not an appropriate choice for SIBs, because it is not energetically stable to form sodium-graphite intercalation compounds (Na-GICs).” Bai said. “Soft carbon anodes also show relatively lower sodium storage capacities be due to the insufficient interlayer distance for ion intercalation. Compared with soft carbon and graphite, hard carbons (HCs) possess relatively higher Na storage capacity because of the more heterogeneous structure that contains curved graphitic domains with large interlayer spacing for Na insertion and massive nanopores and edge terminations for Na adsorption.”

However, some obstacles hinder commercialization of HCs. According to Bai, the key challenges for HCs, especially their low ICE, poor cyclic stability, and poor rate capability, are still demand of deep exploration. Low ICE means existing nonreciprocal Na⁺ loss in side, requiring enough Na⁺ supply extracted from super-proportional cathodes when packing full batteries, which can decrease overall energy density and cycling performance of the full batteries. The poor rate capability limits their applications in high-power electronic devices, and poor cycling performance significantly hinders the practical realizations of SIBs. Therefore, advanced material engineering strategies are highly demand for boosting SIB performances of HC anodes.

To enhance the Na storage performance of HCs, Bai said many studies focus on amorphous carbons with large specific surface area (SSA) or heteroatom-doped carbon materials with more defects, such as porous carbon, nanosized carbon, or heteroatom-doped carbon by anions. Bai and her team overviewed the advances of material engineering strategies for HC anodes. First, nanostructure design provides controllable processing advantages in constructing carbons with hierarchical and complicated architectures, morphologies, and dimensionalities. Second, pore engineering with interconnected micro/meso/macropores can improve ion diffusion and the utilization of inner active sites. Third, defect engineering is effective to promote electrochemical activity of carbons to contribute high storage capacity. Unfortunately, Bai said both large SSA and excessive defects in the carbon structure tend to induce uncontrollable decomposition of electrolyte and formation of uneven and unstable solid electrolyte interphase film (SEI), resulting in low ICE, poor cyclic stability, and decreased sodium diffusivity. Therefore, according to Bai, novel material optimization strategies are highly desirable.

“Different from heteroatom doping with anions and engineering pore architecture,” Bai said. “Introducing cations can also regulate the microstructure of the HCs, such as interlayer spacing, electronic conductivity, graphite microdomains, and rebuild surface functionality, etc., while no extra active defects or pores formed. Therefore, cation-doping is imperative to optimize HCs with desirable physiochemical properties for high-performance anodes. In this work, we prepared K-doped HCs by annealing of K⁺ chemically-preabsorbed carbon resources. The K⁺ was selected to be preabsorbed on oxygen functional groups and some defects in HCs to deactivate these active sites, contributing to high ICE and high cycle stability.”

“Potassium has low ionization energy and can bind with negatively charged oxygen-containing functional groups with large electrostatic attraction, forming a stable structure.” Bai said. “The oxygen functional groups such as carbonyls and hydroxyls and some defect sites on carbon can work as anchoring sites for K⁺.” According to Bai, the K⁺ is chemically adsorbed on the oxygen functional groups by forming C-O-K bonds and occupying some defect sites.

“Therefore, irreversible adsorption of Na⁺ by oxygen functional groups and other defects can be reduced, thus leading to an improved ICE”. Bai said. “Meanwhile, the preabsorbed K⁺ can lead to carbon structural rearrangement during the carbonization process at high temperature, resulting in enlarged interlayer spacing and improved graphitization extent.” According to Bai, these structural evolutions lead to fast Na⁺ diffusion and higher conductivity, so that the rate capabilities of K⁺-preabsorbed hard carbon also get promoted, and a better ICE and an outstanding cycling stability can be also obtained.

This work put forward a novel, efficient, and low-cost way to improve the electrochemical performances of HCs. Bai said, such method is suitable for large-scale production, thus promoting the commercial application of HCs for SIBs.

“Although great achievements have been made, the development of practical HC anodes for SIBs is still facing massive challenges, such as complicated fabricating steps, complex microstructure of HCs, unclear Na storage mechanism, etc.,” Bai said. “Each type of carbon material has its own development bottlenecks, which are demand of advanced strategies to mitigate these issues. In addition, in-depth understanding of the electrochemical reaction mechanisms of carbon materials for SIBs is also equally important for high-performance material design. Overall, there is a long way to go for the commercialization of HCs and SIBs.”

Other contributors include Ruiqi Dong, Feng Wu, Yu Li, Qiao Ni, and Chuan Wu, Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and Engineering, Beijing Institute of Technology; and Qinghao Li, Xiqian Xu, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academic of Science.

The present work was supported by the National Natural Science Foundation of China (Grant No. 21975026). This work used resources of the Advanced Light Source, DOE Office of Science User Facility, under contract no. DEAC02-05CH11231.

##

Reference

Authors: Ruiqi Dong,¹ Feng Wu,¹ Ying Bai,¹ Qinghao Li,² Xiqian Yu,² Yu Li,¹ Qiao Ni,¹ and Chuan Wu,¹

Title of original paper: Tailoring Defects in Hard Carbon Anode towards Enhanced Na Storage Performance

Journal: Energy Material Advances

DOI: 10.34133/2022/9896218

Affiliations: 

¹Beijing Key Laboratory of Environmental Science and Engineering, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

²Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academic of Science, Beijing 100190, China

About the author:

Ying Bai is currently a professor at Beijing Institute of Technology (BIT). Her research interests focus on electrochemical energy storage and conversion technology. Dr. Bai earned her bachelor degree from Applied Chemistry Division at Harbin Institute of Technology, China (HIT) in 1997. She completed her Ph.D. from School of Chemical Engineering and Materials Science at BIT in 2003. In 2013, she was awarded New Century Excellent Talents in University from the Chinese Ministry of Education. She hosted and is hosting the projects of the National Natural Science Foundation of China as a principal investigator.