Hard carbons are promising anode candidates for sodium‐ion batteries due to their excellent Na‐storage performance, abundant resources, and low cost. Despite the recent advances in hard carbons, the interpretation of the Na‐storage mechanism in hard carbons remains unclear, with discrepancies over a general model describing the corresponding structure–property relationship. For the rational structural design of high‐performance hard carbon anodes, a thorough understanding of the charge storage mechanism and the relationship between microstructure and Na‐storage performance is critical. This review provides a comprehensive overview of the known models to describe the Na‐storage mechanism in hard carbons with a discussion focused on Na‐storage active sites such as interlayer space, pores, heteroatoms, and defects. The present models, including the “insertion‐filling”, “adsorption‐filling”, “adsorption‐insertion”, and “multistage mechanism” and the associated analytical characterization, are elaborately discussed with a view to the present challenges and the prospects for unveiling Na‐storage mechanism in hard carbons. The review serves to clarify the Na‐storage mechanism in hard carbons with guidance toward the design of high‐performance anodes for sodium‐ion batteries.

Understanding of Sodium Storage Mechanism in Hard Carbons: Ongoing Development under Debate

Development of energy storage systems is a topic of broad societal and economic relevance, and lithium ion batteries (LIBs) are currently the most advanced electrochemical energy storage systems. However, concerns on the scarcity of lithium sources and consequently the expected price increase have driven the development of alternative energy storage systems beyond LIBs. In the search for sustainable and cost-effective technologies, sodium ion batteries (SIBs) and potassium ion batteries (PIBs) have attracted considerable attention. Here, a comprehensive review of ongoing studies on electrode materials for SIBs and PIBs is provided in comparison to those for LIBs, which include layered oxides, polyanion compounds and Prussian blue analogues for positive electrode materials, and carbon-based and alloy materials for negative electrode materials. The importance of the crystal structure for electrode materials is discussed with an emphasis placed on intrinsic and dynamic structural properties and electrochemistry associated with alkali metal ions. The key challenges for electrode materials as well as the interface/interphase between the electrolyte and electrode materials, and the corresponding strategies are also examined. The discussion and insights presented in this review can serve as a guide regarding where future investigations of SIBs and PIBs will be directed.

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Active material and interphase structures governing performance in sodium and potassium ion batteries

Potassium-ion batteries using KFSI in monoglyme electrolytes: Implications of cation solvation on the K+-graphite (co-)intercalation mechanism

Confining nano FeSb2S4 in carbon nanotube/oxide graphene 3D porous networks for high-capacity sodium ion battery anode

The co‐intercalation of solvent molecules along with Na+ into the crystal lattice of electrode materials is an undesired process in sodium batteries. An exception is the intercalation of ether solvated alkali ions into graphite, a fast and highly reversible process. Here, reversible co‐intercalation is shown to also be possible for other layered materials, namely titanium disulfide. Operando X‐ray diffraction and dilatometry are used to demonstrate different storage mechanisms for different electrolyte solvents. Diglyme is found to co‐intercalate into the TiS2 leading to a change in the voltage profile and an increase in the interlayer spacing (≈150%). This behavior is different compared to other solvents, which expand much less during Na storage (24% for tetrahydrofuran [THF] and for a carbonate mixture). For all solvents, specific capacities (2nd cycle) exceed 250 mAh g−1 whereas THF exhibited the best stability after 100 cycles. The solvent co‐intercalation is rationalized by density functional theory and linked to the stability of the solvation shells, which is largest for diglyme. Finally, the TiS2 electrode with diglyme electrolyte is paired with a graphite electrode to realize the first proof‐of‐concept solvent co‐intercalation battery, that is, a battery with two electrodes that both rely on reversible co‐intercalation of solvent molecules.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/aenm.202202377

Co‐Intercalation Batteries (CoIBs): Role of TiS2 as Electrode for Storing Solvated Na Ions

The electrochemical intercalation/deintercalation of solvated sodium ions into graphite is a highly reversible process, but leads to large, undesired electrode expansion/shrinkage ("breathing"). Herein, two strategies to mitigate the electrode expansion are studied. Starting with the standard configuration (-) sodium | diglyme (2G) electrolyte | graphite (poly(vinylidene difluoride) (PVDF) binder) (+), the PVDF binder is first replaced with a binder made of the sodium salt of carboxymethyl cellulose (CMC). Second, ethylenediamine (EN) is added to the electrolyte solution as a co-solvent. The electrode breathing is followed in situ (operando) through electrochemical dilatometry (ECD). It is found that replacing PVDF with CMC is only effective in reducing the electrode expansion during initial sodiation. During cycling, the electrode breathing for both binders is comparable. Much more effective is the addition of EN. The addition of 10 v/v EN to the diglyme electrolyte strongly reduces the electrode expansion during the initial sodiation (+100% with EN versus +175% without EN) as well as the breathing during cycling. A more detailed analysis of the ECD signals reveals that solvent co-intercalation temporarily leads to pillaring of the graphite lattice and that the addition of EN to 2G leads to a change in the sodium storage mechanism.

Strategies for Alleviating Electrode Expansion of Graphite Electrodes in Sodium-Ion Batteries Followed by In Situ Electrochemical Dilatometry.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/admi.202100596

In Situ (Operando) Electrochemical Dilatometry as a Method to Distinguish Charge Storage Mechanisms and Metal Plating Processes for Sodium and Lithium Ions in Hard Carbon Battery Electrodes

Ines Escher

Papers

Strategies for Alleviating Electrode Expansion of Graphite Electrodes in Sodium-Ion Batteries Followed by In Situ Electrochemical Dilatometry.

In Situ (Operando) Electrochemical Dilatometry as a Method to Distinguish Charge Storage Mechanisms and Metal Plating Processes for Sodium and Lithium Ions in Hard Carbon Battery Electrodes