Porous carbon nanofibers facilitate the design of practical K Metal batteries

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As an alkali metal electrode for new electrolytic energy storage devices, potassium (K) is an attractive replacement for lithium. Unfortunately, potassium utilization is hampered by dendrite formation and volumetric fluctuations during the cycle.

Study: Co-doped porous carbon nanofibers as potassium metal host for non-aqueous K-ion batteries. Image Credit: Troggt/Shutterstock.com

A study published in the journal Nature Communication have proposed the fabrication and use of highly porous nitrogen (N) and zinc (Zn) co-doped carbon nanofibers as potassium hosts to avoid these problems.

K-Ion batteries

Lithium-ion batteries (LIBs) have the disadvantages of lithium scarcity and high extraction costs, which limits their use for storage.

K-ion batteries are attracting increasing interest as a low-cost alternative to LIBs. Potassium is a practically inexhaustible and thermodynamically stable natural resource. Accordingly, potassium offers great prospects for scalable electrochemical energy storage applications.

It is expected that K-ion batteries may have higher operating voltages than their competitors, including LIBs.

Figure 1. Synthesis and physico-chemical characterization of MSCNFs. a synthetic route of MSCNFs and the corresponding composite K-anode. b PXRD models of MSCNFs. c MSCNF pore distribution and corresponding N2−77 K adsorption isotherms (inset). d–f SEM images of MSCNFs. The red dotted lines highlight the porous structure of the MSCNFs. g TEM image and h HRTEM image of MSCNFs. I Elementary EDS mapping of MSCNFs in HAADF mode.© Li, S., Zhu, H. et al. (2022).

Benefits of Potassium Anodes in K-Ion Batteries

Various materials have been explored for use as the anode in K-ion batteries. Potassium metal is the ideal option due to its minimal redox potential and high specific capacity compared to other anode material choices.

Additionally, the inclusion of metallic potassium may allow the use of potassium-free cathodes in the manufacture of K-ion batteries with high specific energies.

The problem of dendrite growth

Unfortunately, metallic potassium anodes, like those of lithium and sodium, suffer from dendrite formation during electroplating and pickling. This phenomenon greatly limits their practical application in K-ion batteries.

Exposure to potassium dendrites with large surface areas can lead to continuous reactions with the electrolyte, resulting in low Coulomb efficiency. Moreover, the stripping mechanism usually takes place at the dendritic roots, which causes electrical insulation of the dendrites, producing “dead potassium”.

Dendrites can also form on the opposite electrode, causing internal shorts, K-ion battery failure, and even safety hazards like explosion or fire.

Generally, the volatile solid electrolyte (SEI) interphase is the main source of potassium dendrites.

Methods of removing potassium dendrites

By modifying the interphase surface of metal or solid electrolyte, electrolyte compositions or electrode surface design can effectively prevent the formation of potassium dendrites.

While a better SEI can protect the potassium anode against dendrite growth to some extent, this protective layer is rendered useless when the electrode experiences significant volume fluctuation during potassium plating and etching.

A conductive host matrix can also be used to stabilize the potassium anode.

Physicochemical study of metallic potassium deposition for various metallic potassium hosts.  In situ optical microscopy observation of the deposition of K on a sheet of Cu and b MSCNFs at a current density of 6 ?mA ?cm-2.  Contour curves of XRD operando models for deposition of K on c Al and d MSCNF foil at a current density of 0.5 mA cm-2.  e Voltage profiles of K deposition on MSCNFs at a current density of 0.5 mA?  cm-2 and f the corresponding SEM images of MSCNFs deposited at different states of discharge.  All electrochemical measurements were performed at a temperature of 25 ± 2°C.

Figure 2. Physicochemical study of metallic potassium deposition for various metallic potassium hosts. Observation in situ optical microscopy of the deposit of K on a Cu sheet and b MSCNF at a current density of 6 mA cm−2. Contour plots of XRD operando patterns for K deposition on vs Aluminum foil and D MSCNF at a current density of 0.5 mA cm−2. e Voltage profiles of K deposition on MSCNFs at a current density of 0.5 mA cm−2 and F the corresponding SEM images of MSCNFs deposited at different states of discharge. All electrochemical measurements were performed at a temperature of 25 ± 2°C. © Li, S., Zhu, H. et al. (2022).

Benefits of Using Conductive Hosts to Stabilize Potassium Anodes

The use of a conductive host allows the encapsulation of potassium metal within the host matrix, thereby reducing the possibility of unwanted reactions between the electrolyte and the alkali metal.

Additionally, by attenuating volumetric changes, a host matrix can increase the structural integrity of the potassium anode, provide conductive networks that promote rapid transfer of ions and electrons, and lower localized current density, thereby preventing the dendrite formation.

Metal hosts are difficult to use due to their high density and limited use of space. Carbonaceous host materials, on the other hand, are better suited to host potassium due to their low weight and electrolytic stability, which can promote higher energy and specific energy densities.

Carbon nanofibers that are both strong and electrically conductive hold great promise for hosting potassium in K-ion batteries. Therefore, the combination of carbon nanofibers with various potassiophiles is a viable approach to improve potassium plating/stripping electrochemistry.

Metallic nanoparticles are also of interest due to their high affinity for potassium and their ease of production. Gold, silver and zinc nanoparticles have already been shown to be beneficial in promoting the heterogeneous seeded growth of lithium on host carbon nanofibers.

The use of metal nanoparticles capable of creating potassium alloys represents an effective technique to increase the potassiophily of carbon nanofiber hosts.

Ex situ physico-chemical characterizations of various metallic potassium-based electrodes.  Ex situ SEM images of a–d and e–h MSCNF-K bare K anodes from symmetric cells at different cycle numbers.  Ex situ XPS spectra of i C 1?s, j O 1?s and k F 1?s of symmetric cells based on Cu-K anodes at different cycle numbers (Fresh, after 1st charge, and after 20th charge ).  Ex situ XPS spectra of l C 1?s, m O 1?s and n F 1s of symmetric cells based on MSCNF-K anodes at different cycle numbers (fresh, after 1st charge and after 20th charge).  The ex situ electrodes were taken at an intermediate potassium state.  All electrochemical measurements were performed at a temperature of 25 ± 2°C.

Picture 3. Ex situ physico-chemical characterizations of various metallic potassium-based electrodes. Ex situ SEM images of aD nu K and eh MSCNF-K anodes of symmetric cells at different cycle numbers. Ex situ XPS spectra of I C 1s, I O 1 sec and k F 1 s from symmetric cells based on Cu-K anodes at different cycle numbers (Fresh, after 1st charge, and after the 20e charge). Ex situ XPS spectra of I C 1s, m O 1 sec and not F 1 from symmetric cells based on MSCNF-K anodes at different cycle numbers (fresh, after 1st charge, and after the 20e charge). The ex situ electrodes were taken at an intermediate potassium state. All electrochemical measurements were performed at a temperature of 25 ± 2°C. © Li, S., Zhu, H. et al. (2022).

Main results of the study

In this work, the team produced and hierarchically characterized porous carbon nanofibers containing monodisperse binary active spots and clusters of nitrogen and zinc.

The porous carbon nanofiber host, combined with zinc-bearing binary potassiophilic zones, provided a strong potassiophilic nature, resulting in greater nucleation of potassium atoms, low weight, and ample room for accommodation of metallic potassium .

By using the carbon nanofiber host, efficient induction of a homogenized electric field and smooth potassium plating were also achieved.

The developed carbon-potassium nanofiber composite anodes, having a large potassium content and specific discharge capacity, can be fabricated using a rapid thermal infusion technique.

In K-PPB and KS symmetric-cell and full-cell tests, the developed carbon-potassium nanofiber composite electrodes retained excellent throughput capability and cycle stability.

Reference

Li, S., Zhu, H. et al. (2022). Co-doped porous carbon nanofibers as a potassium metal host for non-aqueous K-ion batteries. Communication Nature, 13. Available at: https://www.nature.com/articles/s41467-022-32660-y

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