MXene-based materials have emerged as a promising class of anodes for lithium-ion (Li-ion) and sodium-ion (Na-ion) batteries due to their unique structural and electrochemical properties. These two-dimensional transition metal carbides, nitrides, and carbonitrides exhibit high electrical conductivity, tunable surface chemistry, and a layered architecture that facilitates rapid ion diffusion. Their performance in energy storage applications is closely tied to their synthesis methods and post-processing functionalization, which can be tailored to enhance capacity, cycling stability, and rate capability.

The conductivity of MXenes is one of their most notable attributes, often exceeding 10,000 S/cm for certain compositions. This high electronic conductivity stems from their metal-rich composition and the presence of transition metals like titanium, vanadium, or molybdenum in their structure. Unlike graphite, the conventional anode material, MXenes do not rely solely on intercalation mechanisms; instead, they combine intercalation and surface redox reactions, enabling higher charge storage capacities. For instance, Ti3C2Tx, one of the most studied MXenes, demonstrates a theoretical capacity of approximately 320 mAh/g for Li-ion batteries, with experimental values reaching up to 400 mAh/g when optimized. In Na-ion systems, MXenes show slightly lower but still competitive capacities, typically in the range of 100–250 mAh/g, depending on the composition and electrolyte used.

The layered structure of MXenes is another critical factor contributing to their performance. MXenes are produced by selectively etching the A-layer (typically aluminum) from MAX phases, leaving behind a stack of two-dimensional sheets with abundant surface terminations (Tx), such as -O, -F, or -OH. These terminations influence the interlayer spacing and ion transport kinetics. The interlayer spacing can be further adjusted through intercalation of cations, polymers, or other spacers, which helps mitigate restacking—a common issue that reduces accessible surface area and ion diffusion rates. For example, inserting hydrazine or dimethyl sulfoxide (DMSO) between MXene layers can expand the interlayer distance, improving Na+ diffusion coefficients by an order of magnitude compared to untreated MXenes.

Synthesis of MXenes primarily involves etching MAX phases using hydrofluoric acid (HF) or fluoride-containing salts like LiF and HCl. The HF-based method is the most established, selectively removing the A-layer while preserving the MXene sheets. However, the use of HF poses safety and environmental concerns, prompting research into milder etching agents. The minimally intensive layer delamination (MILD) method, which employs a mixture of LiF and HCl, has gained traction as a safer alternative, producing high-quality MXenes with fewer defects. After etching, delamination is often achieved via sonication or mechanical agitation, yielding single- or few-layer MXene flakes dispersed in colloidal solutions.

Functionalization strategies play a pivotal role in optimizing MXene anodes. Surface modifications, such as covalent bonding with organic molecules or doping with heteroatoms like nitrogen or sulfur, can enhance electrochemical performance. For instance, nitrogen-doped Ti3C2Tx exhibits improved Li+ storage capacity due to increased active sites and electronic conductivity. Similarly, sulfonation of MXene surfaces has been shown to facilitate Na+ adsorption and diffusion. Another approach involves creating MXene composites with conductive polymers (e.g., polyaniline) or carbonaceous materials (e.g., graphene, carbon nanotubes). These composites mitigate restacking while providing additional conductive pathways, leading to higher rate capabilities. For example, a Ti3C2Tx/graphene hybrid anode demonstrated a stable capacity of 350 mAh/g at 1C for over 500 cycles in a Li-ion battery.

In terms of electrochemical performance, MXene anodes exhibit several advantages over traditional materials. Their pseudocapacitive behavior contributes to fast charge-discharge rates, making them suitable for high-power applications. Unlike graphite, which suffers from sluggish kinetics in Na-ion systems due to the larger ionic radius of Na+, MXenes can accommodate Na+ ions more effectively due to their adjustable interlayer spacing. However, challenges remain, including irreversible capacity loss in the first cycle due to side reactions with electrolytes and long-term cycling stability at high charge rates. Strategies such as pre-lithiation/sodiation or electrolyte optimization (e.g., using ether-based electrolytes) have been explored to address these issues.

Recent advancements in MXene anode research focus on tailoring their properties for specific battery chemistries. For Li-ion batteries, efforts are directed toward maximizing capacity while minimizing volume changes during cycling. In Na-ion batteries, the emphasis is on enhancing ion accessibility and reducing diffusion barriers. Computational studies have been instrumental in predicting optimal MXene compositions and surface terminations for these purposes. For example, simulations suggest that Mo2CTx may outperform Ti3C2Tx in Na-ion storage due to its higher binding energy for Na+ ions.

Despite their promise, scalability and cost remain hurdles for widespread MXene adoption. The synthesis of high-purity MAX phases and the etching process require precise control, and the yield of delaminated MXenes is often low. Researchers are investigating scalable production techniques, such as molten salt etching or electrochemical etching, to address these limitations. Additionally, the environmental impact of MXene production and disposal is an area of ongoing study, particularly concerning fluorine-containing byproducts.

In summary, MXene-based anode materials offer a compelling combination of high conductivity, tunable layered structures, and versatile functionalization options for Li-ion and Na-ion batteries. Their synthesis and modification strategies directly influence their electrochemical performance, with ongoing research aimed at overcoming challenges related to cycling stability, scalability, and cost. As understanding of these materials deepens, MXenes are poised to play a significant role in advancing next-generation energy storage technologies.